the effect of metal oxides additives on the absorption
TRANSCRIPT
The Effect of Metal Oxides Additives on the Absorption/Desorption of MgH2 for Hydrogen Storage Applications
Author
Alsabawi, Khadija, Gray, Evan
Published
2019-07-15
Thesis Type
Thesis (PhD Doctorate)
School
School of Environment and Sc
DOI
https://doi.org/10.25904/1912/1452
Copyright Statement
The author owns the copyright in this thesis, unless stated otherwise.
Downloaded from
http://hdl.handle.net/10072/386537
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i
The Effect of Metal Oxides Additives on the
Absorption/Desorption of MgH2 for Hydrogen
Storage Applications
Khadija Alsabawi
BScHons
School of Environment and Science
and
Queensland Micro- and Nanotechnology Centre
Griffith University, Queensland, Australia
Submitted in fulfilment of the requirements of the degree of
Doctor of Philosophy
FEBRUARY 2019
To my parents
My loving Husband Youssef
And my wonderful kids
Naomi & Ameer
STATEMENT
This work has not previously been submitted for a degree or diploma in any university.
To the best of my knowledge and belief, the thesis contains no material previously
published or written by another person except where due reference is made in the thesis
itself.
Khadija Alsabawi
i
ACKNOWLEDGEMENT
This project was completed under the supervision of Dr Jim Webb. I am immensely
grateful for his patience, invaluable guidance and the support he has given me
throughout the project. The lessons he’s taught me have sculpted me into an
independent and critical minded researcher and for this I am very grateful. I feel very
fortunate to have worked with him and believe that I can still learn much from him in
our future projects.
I would like to express my gratitude to Prof Evan Gray for his indispensable assistance
and suggestions throughout the years.
To all the PhD candidates in my group, many thanks for making my time a forever
enjoyable and memorable experience. Special thanks go to Tim, Donya, Lubna,
Nazanin, and Samaneh who shared their PhD experience with me.
In addition, I would like to acknowledge the financial support received from the
Australian government through the Australian Government Research Training Program
Scholarship. Another big thanks to the members of Griffith University Higher Degree
Research department, the school of Environment and Science and the Queensland Micro
and Nanotechnology Centre (QMNC).
Finally, a tremendous thanks to my family for taking this journey with me, without their
support and encouragement this would not have been possible.
ii
ALL PAPERS INCLUDED ARE CO-AUTHORED
Acknowledgement of Papers included in this Thesis
Included in this thesis are papers in Chapters 2, 4, 5 and 6 which are co-authored with
other researchers. My contribution to each co-authored paper is outlined at the front of
the relevant chapter. The bibliographic details for these papers including all authors, are:
Chapter 2:
Alsabawi K, Gray EM and Webb CJ. ‘A review of the effect of metal oxide additives on
the sorption kinetics of MgH2 for hydrogen storage’. International Journal of Hydrogen
Energy. (To be submitted)
Chapter 4:
Alsabawi K, Gray EM and Webb CJ. ‘A Sieverts apparatus with TPD/TDS for accurate
high-pressure hydrogen uptake and kinetics measurements’. International Journal of
Hydrogen Energy. (Submitted)
Chapter 5:
Alsabawi K, Webb TA, Gray EM and Webb CJ. ‘Kinetic enhancement of the sorption
properties of MgH2 with the additive titanium isopropoxide’. International Journal of
Hydrogen Energy. 2017;42(8):5227-34;10.1016/j.ijhydene.2016.12.072.
Chapter 6:
Alsabawi K, Gray EM and Webb CJ. The effect of ball-milling gas environment on the
sorption kinetics of MgH2 with/without additives for hydrogen storage. International
Journal of Hydrogen Energy. 2019;10.1016/j.ijhydene.2018.12.026.
Chapter 7:
Alsabawi K, Gray EM and Webb CJ. ‘The effect of transition metal oxide additives on
the sorption kinetics of MgH2 for hydrogen storage.’ International Journal of Hydrogen
Energy. (Submitted)
iii
Appropriate acknowledgements of those who contributed to the research but did not
qualify as authors are included in each paper.
(Signed) _____________________(Date)__14/02/2019____________
Khadija Alsabawi
(Countersigned) ______________ (Date)__ 14/02/2019________________________
Supervisor: Jim Webb
iv
ABSTRACT
Magnesium is considered as one of the more promising candidates for hydrogen
storage, primarily because of its abundance and the high hydrogen capacity of MgH2,
magnesium dihydride (7.6 wt%). Unfortunately, practical applications of MgH2 are
limited by poor hydrogen sorption kinetics and high thermodynamic stability, resulting
in slow charging and the need for high temperatures to release the hydrogen. Methods to
improve the sorption kinetics of MgH2, through ball-milling, alloying and introducing
small amounts of additives are currently under investigation. The aim of this work was
to investigate the effect of different transition metal oxides, as well as other catalysts, on
the hydrogen sorption kinetics, hydrogen release temperature and cycling stability of
MgH2.
The rate of MgH2 absorption is determined by the physisorption and dissociation of
molecular hydrogen, diffusion through the hydride layer and then nucleation of the
hydride. Whereas desorption is determined by nucleation of metal phase, diffusion of
atomic hydrogen through the metal and hydride, and recombination to form molecular
hydrogen at the surface before dissociation of the molecule form the surface.
The addition of small amounts of additives (< 10 mol%) to MgH2 during milling have
been shown to have a significant effect on the kinetics of absorption and desorption of
hydrogen. In addition, the effect of oxygen as a component of these additives has been
extensively studied. Although a surface layer of MgO is known to slow the diffusion of
hydrogen into metal, niobium oxide, Nb2O5, is one of the best additives for kinetic
improvement of MgH2. It has been suggested that higher valance oxides have a greater
effect on the kinetics; also recent studies have indicated that the formation of
magnesium-niobium ternary oxide compound may be responsible for the enhancement,
however the exact mechanism by which Nb2O5 enhances the kinetics is still unclear.
Various transition metal oxides have proved to be effective additives for hydrogen
absorption/desorption of Mg-based hydrides e.g., Nb2O5 and TiO2. In this work organo-
metallic additives based on transition metal oxides (Ti and V) and halides have been
chosen, some of which are liquid at room temperature. These additives were chosen to
extend the oxide valency to higher values, provide a comparison between liquid and
solid oxide additives, and compare a non-oxide transition metal additive (Ti-based
chloride). Nb2O5 was used as the benchmark for comparisons.
v
A range of different amounts of transition metal (Ti and V) oxides and other additives,
both liquid and solid, were ball milled with pre-milled MgH2 under different gas
environments and the hydrogen sorption behaviour of the ball milled composite samples
was investigated using a TPD/TDS Sieverts apparatus. A key difference from previous
work in this area was the use of organo-metallic compounds, some of which were liquid
at room temperature. Samples were characterized using X-Ray Diffraction, and, where
feasible, Scanning Electron Microscopy and Raman Spectroscopy before and after
recording the hydrogen sorption behaviour.
A Sievert apparatus was improved, then modified to include TPD and TDS capability to
determine the hydrogen uptake and release and to perform all these measurements in
one consistent environment.
With the aim of understanding what effect the ball milling gas environment might have
on the MgH2 sorption kinetics, two different gases, Ar and H2, were used. MgH2, with
and without additives (C60, TTIP (Titanium TetraIsoPropoxide) and Nb2O5) were milled
under the two different gases. In most cases the milling gas had little to no effect on
desorption kinetics, but a noticeable effect in the absorption uptake was observed - by
up to 2 wt% - depending on the gas used.
To understand the role of an organo-metallic oxide liquid additive on MgH2 sorption
kinetics, a systematic survey was performed by milling up to 2 mol% of TTIP with
MgH2 and the results compared to composite samples with Nb2O5 as the additive. TTIP
was found to be equally as effective as Nb2O5 with superior hydrogen capacity, and, just
as for Nb2O5, only a small amount of additive, 0.5 mol% of TTIP was found to be
sufficient for kinetics enhancement. Interestingly, 0.5 mol% TTIP hand-mixed with the
pre-milled MgH2 was also found to be effective for desorption, but not for the
absorption kinetics.
To further investigate the effect of organo-metallic liquid oxides, particularly transition
metal oxides, on the enhancement of the sorption kinetics of MgH2, a range of liquid
oxides (Ti-based and V-based oxides) were milled with MgH2 and compared to powder
oxides. Ti-based oxides were found to have superior desorption enhancement, with the
liquids performing better than the powders. The V-based oxides (all liquids) showed
faster absorption and higher uptake when compared to Ti-based oxides. The Ti-chloride
based organo-metallic additive was investigated to compare a non-oxide transition metal
additive to the oxide additives. It was found that a small amount of 1 mol% of additive
vi
milled for short time (1 h) had the best desorption of all the additives investigated in this
work - but with quite poor absorption.
Overall, this project established that the use of transition metal oxides as additives has a
great impact on improving the sorption kinetics of MgH2. The TTIP additive produced
results at least as good as the benchmark additive Nb2O5, but with the significant
advantage of being able to be mixed with MgH2 without ball-milling. The Ti and V
based oxides additives were also shown to be effective, Ti-based achieving better
desorption enhancement, whereas V-based oxide samples had faster absorption and
higher uptake. It was also determined the ball milling gas environment can have a
significant effect on the sorption kinetics, but the effect depended on the additive. The
difference in the effect of additives on desorption and absorption cycles confirms the
need to study combinations of additives for optimal overall benefit.
CONTENTS
ABSTRACT .................................................................................................................... iv
CONTENTS .......................................................................................................................
CHAPTER ONE ............................................................................................................... 1
1. Introduction ............................................................................................................... 1
1.1. Renewable energy .............................................................................................. 1
1.2. Hydrogen energy system (closed cycle) ............................................................ 1
1.3. Hydrogen properties .......................................................................................... 2
1.4. Hydrogen production ......................................................................................... 4
1.5. Hydrogen storage ............................................................................................... 4
1.5.1. Gaseous storage .......................................................................................... 7
1.5.2. Liquid storage ............................................................................................. 7
1.5.3. Solid state materials .................................................................................... 7
1.5.3.1 Physisorption on carbons: ................................................................... 8
1.5.3.2 Simple and Intermetallic Metal Hydrides: .......................................... 8
1.5.3.3 Complex hydride: .............................................................................. 11
1.6. Magnesium hydride MgH2 ............................................................................... 11
1.6.1 The effect of additives ....................................................................... 12
1.6.1.1 Transition metals and Metal Oxides: .......................................... 13
1.7. Thesis structure ................................................................................................ 14
1.8. List of publications and presentations ............................................................. 15
1.9. References ........................................................................................................ 16
CHAPTER TWO ............................................................................................................ 24
Abstract ........................................................................................................................... 24
2. Introduction ............................................................................................................. 25
2.1. Kinetic processes in magnesium hydride ......................................................... 26
2.2. Mechanical ball milling and additives ............................................................. 27
2.3. Oxide additives ................................................................................................ 28
2.3.1. Magnesium and Metal Oxide additives: ................................................... 28
2.3.2. Zirconium dioxide (ZrO2) ......................................................................... 33
2.3.3. Cerium (IV) oxide (CeO2) ........................................................................ 34
2.3.4. Vanadium oxide ........................................................................................ 35
i
2.3.5. Titanium Oxide based additives ............................................................... 38
2.3.6. Magnesium oxide: .................................................................................... 42
2.3.7. The effect of Cr2O3 on MgH2 ................................................................... 44
2.3.8. Niobium Pentoxide Nb2O5: ...................................................................... 48
2.4. Summary .......................................................................................................... 61
2.5. Reference ......................................................................................................... 63
CHAPTER THREE ........................................................................................................ 74
3. Introduction ............................................................................................................. 74
3.1. Synthesis .......................................................................................................... 74
3.1.1. Ball-milling ............................................................................................... 74
3.1.1.1. Mechanical grinding (MG) ................................................................... 75
3.1.1.2. Mechanical alloying (MA) .................................................................... 75
3.1.1.3. Reactive ball milling (RBM) ................................................................ 75
3.2. Preparation of milled hydride/additive mixtures ............................................. 76
3.3. Characterisation techniques ............................................................................. 78
3.3.1. X-Ray Diffraction (XRD) analysis ........................................................... 78
3.3.1.1. Bragg’s Law .......................................................................................... 78
3.3.2. Scanning Electron Microscopy (SEM) ..................................................... 80
3.3.3. Synchrotron Radiation X-Ray Powder Diffraction (SR-XRPD) .............. 82
3.3.4. Kinetics characterisation........................................................................... 83
3.3.5. Temperature Programmed Desorption and Thermal desorption spectroscopy ............................................................................................................ 84
3.4. References ........................................................................................................ 86
CHAPTER FOUR .......................................................................................................... 89
4. Introduction ............................................................................................................. 89
4.1. References ........................................................................................................ 91
4.2. Paper Sieverts instrument ................................................................................ 91
Abstract ....................................................................................................................... 93
Introduction .................................................................................................................... 94
Uptake measurements ................................................................................................. 95
The Sieverts technique ................................................................................................ 97
Errors and uncertainties using the Sieverts method .................................................... 98
Temperature Programmed Desorption (TPD) ............................................................ 99
Thermal Desorption Spectroscopy (TDS)/ (TDS-MS) ............................................... 99
Hydrogen pressure amplification.................................................................................... 99
ii
Instrument design ......................................................................................................... 102
Temperature control .................................................................................................. 102
Leak management ......................................................................................................... 103
Preparation section .................................................................................................... 104
Reference section .......................................................................................................... 104
Sample section .............................................................................................................. 104
Sample environment ................................................................................................. 105
Experimental procedure ............................................................................................ 105
Volume calibration ................................................................................................... 105
Empty-cell isotherm ..................................................................................................... 106
Divided Volume Calibration ..................................................................................... 109
Instrument validation: ................................................................................................... 112
Standard Sample measurements ............................................................................... 112
TPD ............................................................................................................................... 114
TDS ............................................................................................................................... 115
Conclusion .................................................................................................................... 116
References ................................................................................................................. 118
CHAPTER FIVE .......................................................................................................... 125
5. Introduction ........................................................................................................... 125
5.1. References ...................................................................................................... 126
5.2. TTIP paper: .................................................................................................... 127
CHAPTER SIX ............................................................................................................ 137
6. Introduction ........................................................................................................... 137
6.1. : Milling Environment Paper ......................................................................... 138
CHAPTER SEVEN ...................................................................................................... 144
7. Introduction ........................................................................................................... 144
7.1. References ...................................................................................................... 146
7.2. Paper .............................................................................................................. 146
Abstract ......................................................................................................................... 148
Introduction .................................................................................................................. 149
Experimental ................................................................................................................. 152
Results and discussion:(1 mol%) Ti-based oxide additives milled for 2 h ................... 153
Different amount of Ti-ethoxide milled for 2 h ............................................................. 155
1 mol% V-based oxide additives milled for 2 h ............................................................ 157
iii
BisTiCl2 (Bis(isopropylcyclopentadienyl) titanium dichloride) additive ..................... 159
Comparison of all additives .......................................................................................... 161
Conclusion .................................................................................................................... 162
Acknowledgements ...................................................................................................... 163
References: ................................................................................................................... 164
CHAPTER EIGHT ....................................................................................................... 170
8. Summary and Conclusion ..................................................................................... 170
8.1. Future work .................................................................................................... 174
8.2. Reference ....................................................................................................... 176
iv
List of Figures
2 Fig 1-1:Typical operating systems for various hydrogen storage technologies and their operating temperatures[22] ............................................................................................... 5 Fig 1-2: (a) Pressure-Concentration-Temperature (PCT) curve and (b) van ’t Hoff plot [31]. .................................................................................................................................. 9 Fig 1-3: Table of the binary hydrides and the Allred-Rochow electronegativity [32]. .. 10 Fig 1-4:Lennard-Jones potential of hydrogen approaching a metallic surface [34]. ...... 10 Fig 2-1:Transition metal oxides and their effect on hydrogen desorption kinetics of MgH2 [31] ....................................................................................................................... 28 Fig 2-2: Desorption rates for metal oxides milled with MgH2, from [29]...................... 29 Fig 2-3: Decomposition temperature of MgH2 milled with different metal oxides [32].32 Fig 2-4: (a) A side view (b) top view of (1*3) V2O5 (001) surface. (c) the optimized MgH2 clusters. red (O), grey (V), green (Mg) and white balls (H)[49]. ........................ 36 Fig 2-5: Crystal structure of TiO2 a) anatase b) rutile [57]. ........................................... 38 Fig 2-6: Correlation between desorption temperature achieved upon oxide addition and electronegativity of the oxide additives [68]. ................................................................. 43 Fig 2-7: The selected structural models of MgH2 on (a) Nb(1 0 0), (b) NbO(1 0 0), (c) NbO(1 10), (d) NbO(1 1 1) and (e) Nb2O5(1 0 0). Atomic colour codes: Mg, green; H, white; Nb, blue; O, red. [106] ......................................................................................... 59 Fig 3-1: Bragg diffraction in a 2D lattice ....................................................................... 79 Fig 3-2: A schematic view of a laboratory X-ray Diffractometer (XRD) (image form PANAlytical) [13]. ......................................................................................................... 80 Fig 3-3: Electron beam sample interaction [14]. ............................................................ 81 Fig 3-4: Schematic layout of the 10-BM-1 Powder Diffraction beamline “Australian synchrotron [17]” ............................................................................................................ 82
List of Tables
Table 1-1: Physical properties of molecular hydrogen ................................................................. 3 Table 1-2: Heating values of some common fuels [13] ................................................................ 3 Table 1-3:basic hydrogen storage methods and phenomena, [23] modified from table 1. ........... 6 Table 3-1:Chemicals and materials used in this project .............................................................. 77 Table 3-2: The Australian Synchrotron Parameters. ................................................................... 83
1
CHAPTER ONE
1. Introduction
1.1. Renewable energy
The increasing world population continues to grow energy demand. As of 2017, three
quarters of the world’s energy was provided by fossil fuels [1], which are forecast to be
limited in the near future and when combusted they release greenhouse gases into the
atmosphere posing a threat to the global environment and climate change [2]. Profound
changes are needed regarding the production, distribution and the consumption of
energy. Renewable energy is an emerging solution for a clean, sustainable, affordable
and long-term source of energy for the future. Sources of renewable energy include
solar, wave, tide, wind, geothermal and biomass, however, practical implementation of
most of these sources is constrained by their intermittent and unpredictable nature, such
as wind, for example. Therefore, the use of an energy storage system is necessary to
store energy during excess production and re-supply the energy when needed.
Electricity is an energy carrier which can be produced from different sources and is
technically easy to transport, although this can be quite expensive, especially over long
distances [3]. It has a relatively low environmental impact when produced from
renewable sources. Hydrogen is another efficient and versatile energy carrier, and
together with electricity they may satisfy all the energy requirements, and form a clean
energy system when sourced using renewable energy [4]. Jules Verne in 1874 was the
first to suggest that hydrogen derived from water electrolysis will be used in place of
coal for energy purposes - “water will be the coal of the future”. In the 1930s Rudolf
Erren suggested the use of hydrogen as a transportation fuel, and, after another 30 years,
the concept of a hydrogen economy was introduced as a future energy system to replace
depleting fossil fuels [5].
1.2. Hydrogen energy system (closed cycle)
Hydrogen is the most abundant element accounting for 75% of the mass of the universe
[6]. However, elemental or molecular hydrogen does not occur naturally as a gas on
earth but is found in compounds combined with other elements such as oxygen, carbon
and nitrogen including many organic compounds such as hydrocarbons. Hydrogen has
2
relatively high specific energy content (140 MJ/kg), with liquid hydrogen being used by
NASA since the 1970s to propel the space shuttle and other rockets into orbit. Hydrogen
also has the advantage over hydrocarbons that hydrogen can be generated from water,
and, when combusted or used in a fuel cell to generate electricity, combines with
oxygen to turn back into water [7]. This closed cycle produces little to no pollutants,
whereas hydrocarbons convert mainly to carbon dioxide, a greenhouse gas. In its
molecular form (H2), hydrogen is a gas that burns in oxygen following the simple
reaction:
2 2 2
12
H O H O heat+ → + (1)
Hydrogen is an ideal fuel for the fuel cell which was invented in 1839 [8], although
solid oxide fuel cells for ammonia [9] and methane [10] exist. The fuel cell is an
electrochemical device that converts chemical potential energy directly into electrical
energy and since the ideal fuel cell is reversible, it is not limited by the Carnot
efficiency imposed by the 2nd law of thermodynamics on heat engines. A PEM (Proton
Exchange Membrane) fuel cell can use hydrogen gas and oxygen as fuel, where they are
converted directly into electricity, water and heat [11].
1.3. Hydrogen properties
Hydrogen is the simplest atomic element, consisting of one proton and one electron
only, in its simplest form. Hydrogen has three isotopes, H1 (protium), H2 (deuterium),
both stable isotopes, and H3 (tritium), which has a half-life of 12.32 years. It is a
colourless, odourless gas at ambient temperature and pressure with no toxic effects. It is
the lowest density element and has the lowest melting and boiling points after helium
(see Table 1-1) for physical properties of hydrogen).
3
Table 1-1: Physical properties of molecular hydrogen
Parameter Hydrogen
Molecular mass 2.01588 g/mole
Melting point 13.99 K
Boiling point 20.271 K
Critical point 32.938 k, 12.85 bar
Density (as gas) 0.089 g/L
Density (as liquid) 0.07 g/ cm3
Hydrogen is very reactive element due to its single valence electron and usually
combines two atoms to form the molecule H2. It is found in a wide range of inorganic
and organic chemical compounds, but rarely in its gas form. Combined with carbon,
hydrogen has the ability to form long chains and complex molecules, and this has a
major role in organic life (hydrocarbons, carbohydrate). Hydrogen, as an energy carrier,
plays an important part in the metabolism of plants, animals, and humans [12].
Table 1-2: Heating values of some common fuels [13]
Fuel High Heating value (MJ/kg) Low Heating value (MJ/kg)
Hydrogen 142 120
Gasoline 46 44
Diesel 45 43
Methane 55 50
As seen in Table 1-2, hydrogen has the highest energy to weight ratio when compared
to other common fuels; since the other fuels listed all contain carbon atoms which are
significantly heavier than hydrogen. By weight, hydrogen produces energy during a
reaction of about 2.5 times the heat of combustion of common hydrocarbon fuels.
4
1.4. Hydrogen production
Hydrogen can be produced from fossil fuel based hydrocarbons, such as methanol,
gasoline, propane and natural gases, by applying heat in the presence of a catalyst, in a
process known as ‘reforming’ [14]. Currently 96 % of all the hydrogen produced is
derived from fossil fuels, with an estimated 49 % derived from natural gas, 29 % from
liquid hydrocarbons, 18 % from coal and about 4 % from electrolysis [15]. Electrolysis
of water produces hydrogen when an electrical current is passed through water
separating it into oxygen and hydrogen.
2 2 22 2H O electricity H O+ → + (2)
Other research currently exploring new methods for producing hydrogen from
renewable resources includes the use of sunlight, biomass and biological organisms
(bacteria and algae). Semiconductor or biological organisms can be used to absorb
sunlight, split water and produce hydrogen. Some biological organisms produce
hydrogen naturally through their normal metabolic function, and semiconductors can
generate an electric current that splits water to produce hydrogen. NREL researchers
have developed a device (PEC system) that uses photovoltaic cells immersed in acidic
electrolyte to initiate a water electrolysis reaction. This system converts about 16.2 % of
available sunlight into hydrogen [16].
1.5. Hydrogen storage
Hydrogen gas has a very low volumetric energy density which makes it inefficient to
store in small vessels. The density of hydrogen in a storage container can be increased
by applying work to either compress the gas (high pressure gas cylinder), and/or
decrease the temperature below the critical point (liquid hydrogen) or finally, reduce the
repulsion between hydrogen molecules by utilizing an interaction between hydrogen
and another material (solid state storage). It is important for a hydrogen storage system
to reversibly store and release the hydrogen, and for this to be practical, at temperatures,
pressures and efficiencies able to be employed in power stations, homes and motor
vehicles (Fig 1-1). Commercial hydrogen powered cars such as the Toyota Mirai,
Honda Clarity and Hyundai ix35 fuel cell vehicles are already available [17-19], but due
to its low volumetric energy density, 0.5 kWh/dm3 for 350 bar H2 [12], compared to
gasoline (~7 kWh/dm3), large storage tanks of compressed hydrogen are required in
order to have a sufficient driving range. High pressure storage increases the energy
density, but then requires stronger, more expensive storage tanks [20] to be safe. Liquid
5
hydrogen cryogenic tanks need costly and energy consuming cooling systems [21]. In
addition, any compression system requires parasitic energy use that reduces the capacity
of the total system. A viable alternative is solid state storage.
Fig 1-1:Typical operating systems for various hydrogen storage technologies and
their operating temperatures[22]
Züttel [23] described the possible methods for reversible hydrogen storage with high
volumetric and gravimetric density, (see Table 1-3). Each of the listed methods have
their advantages and disadvantages. ρm is the gravimetric capacity, ρv the volumetric
capacity, T the working temperature and P the pressure. RT: room temperature (25°C).
6
Table 1-3:basic hydrogen storage methods and phenomena, [23] modified from
table 1.
Storage method ρm(mass%) ρV(kg
H2m−3
)
T(°C) p(bar) Phenomena and remarks
High-pressure
gas cylinders
13 <40 RT 800 Compressed gas
(molecular H2) in
lightweight composite
cylinder (tensile strength
of the material is
2000 MPa)
Liquid hydrogen
in cryogenic
tanks
Size
dependent
70.8 −252 1 Liquid hydrogen
(molecular H2),
continuous loss of a few
% per day of hydrogen at
RT
Adsorbed
hydrogen
≈2 20 −80 100 Physisorption (molecular
H2) on materials, e.g.
carbon with a very large
specific surface area,
fully reversible
Absorbed on
interstitial sites
in a host metal
≈2 150 RT 1 Hydrogen (atomic H)
intercalation in host
metals, metallic hydrides
working at RT are fully
reversible
Complex
compounds
<18 150 >100 1 Complex compounds
([AlH4]− or [BH4]−),
desorption at elevated
temperature, adsorption
at high pressures
7
1.5.1. Gaseous storage
Gaseous hydrogen is still the most commonly-used method to store hydrogen for a
range of applications, despite having a low energy density compared to other methods.
The gaseous hydrogen has to be compressed to high pressures, in high-pressure gas
cylinders, to improve its density [24]. However, the very high pressure needed to store
the gas (up to 800 bar, Table 1-3) imposes a safety concern, and together with the
relatively low hydrogen density and energy losses in compressing the gas, makes this an
unsuitable method for storing hydrogen for the future of most applications.
1.5.2. Liquid storage
Storing hydrogen in a liquid form requires a large amount of energy to compress, cool
and liquefy the gas. The liquefaction process requires the following steps: the gas is
compressed then cooled in a heat exchanger, then undergoes an isenthalpic Joule-
Thompson expansion by passing through a throttle valve, producing liquid hydrogen.
This process (named the Joule-Thompson cycle) has been used for space projects and is
considered one of the simplest methods to liquefy gases. The liquefied hydrogen is
stored in well-insulated tanks at very low temperature. However, liquid hydrogen
suffers from boil-off due to heat leaks and para-ortho H2 conversion [25], as well as a
low ignition threshold, which poses safety threats [26].
1.5.3. Solid state materials
Solid state materials such as metal hydrides (including elemental metal hydrides such as
MgH2, and alloys, such as LaNi5) and complex hydrides (e.g. amides, borohydrides and
alanates) [27], as well as physisorption on high surface area materials (e.g. graphite,
graphene, polymers, metal-organic frameworks and carbons), are all capable of storing
hydrogen with varying gravimetric and volumetric densities. The physisorption process
has a relatively low operating pressure, the price of materials involved is affordable, and
the simplicity of the storage system design, are all advantages for using this process.
However, physisorption has drawbacks such as the low amount of hydrogen adsorbed
and the cryogenic temperatures needed to attain a suitable capacity.
Metal hydrides and complex hydrides can store a large amount of hydrogen in a safe
and compact way. Reversible intermetallic hydrides are available that work at ambient
temperature and atmospheric pressure, but their gravimetric hydrogen density is limited
to <3 mass% [23]. Chemical hydride compounds such as the simple metal hydrides and
8
the complex metal hydrides often have high formation energies and therefore require
high temperatures to release the hydrogen, which makes them unsuitable for many
practical applications [28].
1.5.3.1 Physisorption on carbons:
Physisorption is the adsorption a solid surface by a molecular fluid via the same
intermolecular forces as those involved in the condensation of vapours and real gas,
such as van der Waals forces [29]. During absorption and desorption the electronic
orbital patterns of the species involved are not significantly affected by the
intermolecular forces, preserving the chemical nature of the solid/fluid [29]. Hydrogen
can be stored via physisorption in light-weight nanoporous materials, such as carbon,
where the interaction between hydrogen and the material are mainly weak van der
Waals forces. The key to maximising hydrogen storage lies in having a high surface
area, high characteristic binding energy of the hydrogen molecule with the material, and
appropriate pore size [30]. Nanostructured carbon, activated carbon and carbon
nanotubes (CNTs), have large surface areas which make them possible structures for
physisorption of H2.
The operating temperature for a hydrogen storage system based on physisorption on
solid state materials, is determined by the binding energy. The hydrogen binding
energies for pure carbon materials are in the range of 4-15 kJ/mol, where the lower is
typical for graphite and activated carbon, and the upper for internal and interstitial sites
of single walled nanotubes (SWNT) and SWNT bundles. The confined geometry effects
of the SWNT sites are associated with higher binding energy [30]. As seen in Table 1-3,
most carbons need a very low temperature (<–80 ºC) to enable reasonable adsorption
capacity.
1.5.3.2 Simple and Intermetallic Metal Hydrides:
Hydrogen storage in metal hydrides has been extensively researched and explored, due
to their capability to reversibly absorb large amounts of hydrogen. Pressure-
Composition isotherms (PCT) seen in Fig1-2 are used to describe the thermodynamic
properties during the formation of hydride from gaseous hydrogen.
9
Fig 1-2: (a) Pressure-Concentration-Temperature (PCT) curve and (b) van ’t Hoff
plot [31].
Initially some hydrogen will dissolve into the host metal as solid solution (α-phase).
With increasing hydrogen pressure and concentration in the host metal the α-phase will
change to metal hydride (β-phase). The isotherm ideally shows a flat plateau when the
α-phase coexists with the β-phase, where its length determines the amount of H2 that
can be reversibly stored at the equilibrium dissociation pressure. The critical point TC
occurs when the temperature is high enough that there is no difference between the α-
and β-phases. Since the equilibrium pressure ‘plateau’ depends strongly on temperature,
it is related to the changes in the enthalpy, ∆H, and entropy, ∆S, by the van ’t Hoff
equation:
0
ln p H S
p RT R
∆ ∆= −
(3)
Entropy loss occurs in the change of molecular hydrogen gas to dissolved solid
hydrogen, and this enthalpy loss characterises the energy stability of the metal hydrogen
bond. The metal hydride entropy of formation leads to a significant heat evolution ∆Q =
T·∆S during the hydrogen absorption (exothermal reaction) and consequently this heat
must be provided for desorption (endothermal reaction) [23].
10
Fig 1-3: Table of the binary hydrides and the Allred-Rochow electronegativity
[32].
Metal-hydrogen compounds can be formed via reaction of hydrogen gas with a metal,
metal alloy or intermetallic compound. According to the Allred-Rochow
electronegativity (Fig 1-3), all these compounds exist as either ionic, volatile covalent,
polymeric covalent or metallic hydrides. The reaction of hydrogen gas with metal can
be described by a one-dimensional Lennard-Jones potential energy curve (see Fig1-4 )
[33].
Fig 1-4:Lennard-Jones potential of hydrogen approaching a metallic surface [34].
11
The van der Waals force is the first attractive interaction of the hydrogen molecule,
leading to the physisorbed state (EPhys ≈ -5 kJ/molH). When hydrogen approaches the
surface of a metal, it has to overcome an activation barrier for the dissociation of the
molecule before formation of the bonds between the resultant hydrogen atoms and the
surface material. The chemisorbed state is reached when the hydrogen atoms share their
electrons with atoms at the surface of the metal (EChem ≈-50 kJ/molH). Then the
chemisorbed hydrogen atom is able to jump into the subsurface layer and the host metal
lattice enables the atom to diffuse on interstitial sites [34].
1.5.3.3 Complex hydride:
Elements in group 1, 2 and 3 e.g. Li, Mg, B and Al are the building blocks for a large
range of metal hydrogen complexes. Their light weight and the high number of
hydrogen atoms per metal atom makes them interesting for hydrogen storage media
[34]. A large number of complex metal hydrides, such as alanates [AlH4]−, amides
[NH2]−, imides and borohydrides [BH4]−, have been investigated [35]. In such systems
hydrogen is often located in the corners of a tetrahedron with B or Al in the centre. The
alanates and borohydrides may have potential for further investigation due to the very
high capacity, for example, Al(BH4)3 has the highest volumetric density (150kg/m3) and
LiBH2 has the highest gravimetric density at room temperature (18 mass%) [36],
however, these materials are either not reversible or require extremely high temperature
to release the hydrogen.
1.6. Magnesium hydride MgH2
Magnesium hydride (MgH2) is a promising candidate for a solid state hydrogen storage
material. The metal hydride bond, crystal structure and properties of MgH2 make it
unique among metal hydrides. Under normal conditions, α-MgH2 has a tetragonal
crystal structure of rutile type and under high pressure undergoes a polymorphic
transformation to form β-MgH2 with hexagonal structure and γ-MgH2 with
orthorhombic structure [37, 38]. The large hydrogen storage capacity (up to 7.6 wt%)
was first observed in the late nineteenth century [39]. MgH2 is thermodynamically
stable (ΔH=75 kJ.mol-1H2), which corresponds to unfavourable operating temperatures
(of the order of 573–673 K). In addition, the Mg-H system has slow hydrogen
absorption/desorption kinetics [40]. For practical use it is crucial to find a convenient
way to overcome the high stability and slow kinetics.
12
The slow kinetics of MgH2 is due to multiple factors, including a passivation layer
(MgO) covering the surface of Mg particles, slow dissociation of hydrogen at the
magnesium surface, a slow hydrogen diffusion rate in bulk Mg and particularly in the
forming hydride layer, MgH2. The MgO layer is not transparent to H2 and prevents
penetration into the metal; therefore, for Mg to absorb H2 the oxide layer must be
perforated or cracked. Annealing at 673 K in vacuum is known to help crack the oxide
layer [40], as does ball-milling. The slow dissociation rate can be overcome by the
presence of a catalytic metal e.g. Pd or Ni.
Ball milling has been used to reduce the grain size of magnesium metal, however this is
difficult due to the high reactivity and cold welding (re-agglomeration) of the
magnesium nanoparticles [41]. More recently, milling magnesium hydride rather than
Mg metal, or milling Mg under a hydrogen atmosphere, has been more successful at
producing a material with much faster desorption kinetics. This improvement is due to
the small particle size, introduction of defects and the increase of the surface area during
milling, which increases the nucleation site density and reduces the diffusion lengths.
However, a degree of re-agglomeration of the material with cycling can partially
remove these benefits [42].
Changing the enthalpy is much more difficult and is not achieved through defect
enhancement or catalysts. Attempts to lower the enthalpy have included alloying with
other metals, confinement in other materials and reducing the particle size to very low
nano-sizes. Wagemans et al.[43] investigated the thermodynamic stability of MgH2, by
determining the effect of decreasing the crystal grain size of magnesium to nanoscale.
The change of enthalpy during hydriding and dehydriding of magnesium was found to
decrease significantly provided the particle size was less than ~1.3 nm [43].
1.6.1 The effect of additives
Generally, the addition of comparatively small amounts of additives or catalysts has
been shown to be highly effective in enhancing the kinetic performance of the MgH2/
Mg system. Many additive materials have been introduced into MgH2, including various
types of carbons, halides, oxides, metal and non-metal elements, intermetallics, hydrides
and others (carbides, nitrides and borides) [44]. Most of these additives were reported to
improve the hydrogen storage performance [45]. The rate of absorption is controlled by
the following factors: hydrogen dissociation rate at the surface, hydrogen ability to
penetrate from the surface which is typically covered by an oxide layer, and the
13
hydrogen diffusion rate into the bulk metal and through the hydride. The addition of
additives could improve the kinetics by facilitating hydrogen dissociation at the surface,
increasing the available surface area, and by introducing defects that promote hydrogen
diffusion. The additives investigated in this study mainly fall into the category of
transition metal oxides.
1.6.1.1 Transition metals and Metal Oxides:
Over the past few decades, numerous transition metals have been investigated as
additives (i.e. Ti, Nb, V, Co, Mo, Fe, Mn, Ni and Ce) and their compounds, on the kinetics
of MgH2 and found to be effective in reducing the operating temperature and improving the
rate of absorption and desorption [46]. Among the investigated transition metals, niobium,
vanadium and titanium have shown superior results in kinetic enhancement of MgH2 [47].
The addition of transition metals was found to reduce the nucleation barrier and thus
improve the sorption properties of MgH2 [48].
Different types of metal oxides have been used as an additives to improve the sorption
kinetics of MgH2, including Nb2O5 [49-53], MgO [54-56], TiO2 [57-59], V2O5 [60, 61],
Cr2O3 [62-64] and many others. Niobium pentoxide, Nb2O5, is considered as a bench
mark for improving MgH2 kinetics, although the reasons for the kinetic enhancement of
this and other additives are less well-known. There are different explanations in the
literature for the superiority of Nb2O5, including that it acts as a dispersive during
milling, which yields a smaller average particle size of MgH2 [65], that it can act as an
active reservoir that helps with the association and disassociation of H2 in Mg/MgH2
system [66], or that it partially reduces during milling to produce catalytic compounds,
including a ternary Mg-Nb oxide, suggested to facilitate the diffusion through the MgO
layer [67, 68].
It has also been proposed that the metal’s valence state in the oxides plays an important
role in improving the sorption kinetics [69], as well as the high defect density
introduced during milling at the surface of the metal oxide particles [70]. It has been
found that milling MgH2 with a metal oxide yields a smaller average particle size than
milling with metals [65]. However, the exact mechanism of the kinetic enhancement is
still under investigation.
14
1.7. Thesis structure
The overall objective of this project was the enhancement of the absorption/desorption
kinetics of MgH2 using different types of additives with ball-milling methods. The
choice of additive was guided by the literature on high oxygen containing transition
metal compounds. A number of organic compounds, some of which are liquids at room
temperature as well as others containing little to no oxygen or halides, were chosen for
comparison.
The history of the use of additives for kinetic improvement of MgH2 is detailed in the
literature survey in Chapter 2. It provides a critical review for a range of metal-based
oxide additives used to improve the kinetics of MgH2 through ball milling.
Chapter 3, reports on the experimental techniques and materials preparation used
during this work, along with the explanation of the different characterization techniques
used in studying the composite materials. Chapter 4, describes the experimental method
used – in particular the Sieverts apparatus, including the design, performance, operation,
data acquisition and calculation.
Chapters 5, 6 and 7, contain the experimental results of the various additive materials
used to modify the sorption kinetics of MgH2. Chapter 5, is a published paper on the
effect of titanium tetraisopropoxide on the sorption kinetics of MgH2, Chapter 6, is
another published work on the effect of the ball milling environment on the kinetics of
MgH2 with and without additives, and Chapter 7 is a submitted paper showing the effect
of different transition metal oxide additives (powder vs liquid) on the kinetics of MgH2.
Conclusions are presented in Chapter 8 and suggestions for future work are given.
15
1.8. List of publications and presentations
Journal papers
1) Alsabawi K, Webb TA, Gray EM and Webb CJ. The effect of C60 additive on
magnesium hydride for hydrogen storage. International Journal of Hydrogen
Energy. 2015;40(33):10508-15;10.1016/j.ijhydene.2015.06.110.
2) Alsabawi K, Webb TA, Gray EM and Webb CJ. ‘Kinetic enhancement of the
sorption properties of MgH2 with the additive titanium isopropoxide’.
International Journal of Hydrogen Energy. 2017;42(8):5227-
34;10.1016/j.ijhydene.2016.12.072.
3) Alsabawi K, Gray EM and Webb CJ. The effect of ball-milling gas
environment on the sorption kinetics of MgH2 with/without additives for
hydrogen storage. International Journal of Hydrogen Energy.
2019;10.1016/j.ijhydene.2018.12.026.
4) Alsabawi K, Gray EM and Webb CJ. ‘The Effect of Metal Oxides on the
Sorption Kinetics of MgH2 for Hydrogen Storage, a review’. International
Journal of Hydrogen Energy. (To be submitted)
5) Alsabawi K, Gray EM and Webb CJ. ‘A Sieverts apparatus with TPD/TDS
for accurate high-pressure hydrogen uptake and kinetics measurements’.
International Journal of Hydrogen Energy. (submitted)
6) Alsabawi K, Gray EM and Webb CJ. ‘The effect of transition metal oxide
additives on the sorption kinetics of MgH2 for hydrogen storage.’ International
Journal of Hydrogen Energy. (submitted)
Conferences
Alsabawi K, Gray EM and Webb CJ, ‘Kinetic enhancement of the sorption
properties of MgH2 with the additive titanium isopropoxide’. The
16th International Symposium on Metal-Hydrogen Systems (MH2018),
Guangzhou, China, October 28-November 2, 2018. (Oral presentation)
K. Alsabawi, T.A. Webb, E.MacA. Gray and C.J. Webb, ‘Kinetic Enhancement
of the Sorption Properties of MgH2 with The Additive Titanium Isopropoxide’.
2017 International Conference on BioNano Innovation, Brisbane, Australia, Sept
2017. (Poster)
16
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68. Friedrichs O, Sanchez-Lopez JC, Lopez-Cartes C, Klassen T, Bormann R and
Fernandez A. Nb2O5 "pathway effect" on hydrogen sorption in Mg. The journal
of physical chemistry B. 2006;110(15):7845-50;10.1021/jp0574495.
69. Pukazhselvan D, Nasani N, Correia P, Carbó-Argibay E, Otero-Irurueta G,
Stroppa DG, et al. Evolution of reduced Ti containing phase(s) in MgH 2 /TiO 2
22
system and its effect on the hydrogen storage behavior of MgH 2. Journal of
Power Sources. 2017;362:174-83;10.1016/j.jpowsour.2017.07.032.
70. Walker G. Solid-state hydrogen storage: materials and chemistry: Elsevier;
2008.
23
STATEMENT OF CONTRIBUTION TO CO-AUTHORED PUBLISHED PAPER
This chapter includes a co-authored paper. The bibliographic details of the
co-authored paper, including all authors, are:
Alsabawi K, Gray EM and Webb CJ.
‘The Effect of Metal Oxides on the Sorption Kinetics of MgH2 for Hydrogen Storage,
A review’. International Journal of Hydrogen Energy. (To be submitted)
My contribution to the paper involved:
1) Literature review
2) Collection of information
3) Manuscript preparation
(Signed) _________________________________ (Date)_12/02/2019___________
Khadija Alsabawi
(Countersigned) __________________________(Date)12/02/2019______________
Corresponding author of paper: Khadija Alsabawi
(Countersigned) ___________________________ (Date)_12/02/19_____________
Supervisor: Colin Jim Webb
24
CHAPTER TWO
A review of the effect of metal oxide additives on the sorption kinetics of MgH2 for
hydrogen storage
K. Alsabawi*, E.MacA. Gray, C.J. Webb
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, 4111
Brisbane Australia
Abstract
Magnesium hydride is a most promising hydrogen storage material, due to its high
hydrogen capacity and low cost of production. However, slow kinetics and a high
enthalpy precludes most practical applications. One strategy to improve the hydrogen
sorption kinetics of the Mg–H system that has met with considerable success is ball-
milling with small amounts of additives, which facilitate faster absorption, desorption or
both. In particular, the metal oxide additives, notably niobium pentoxide, but also many
others, have been shown to be particularly successful. This review details the progress
in the improvement of the sorption properties of MgH2 by the addition of different
metal oxides and in terms of their effect on the activation energy, kinetics and
thermodynamic properties.
Keywords: Magnesium hydride; Oxides; Additives; Review; Surface properties.
* Corresponding author: Tel.: +61 7 37353625 E-mail address: [email protected]
25
2. Introduction
Hydrogen is considered to be a viable energy carrier that, when produced from
renewable primary energy sources, can be used to generate clean, affordable energy [1].
Although not a primary energy source, as it is naturally found in compounds with other
elements, it can readily be produced from water through electrolysis or other means and
converted back to heat or electricity when required. When used in a fuel cell to generate
electricity, or combusted in oxygen or air, the emission is predominantly water. With
the increase in power generation from renewable energy sources, the requirement to
store large amounts of energy, to buffer the differences between supply and demand, is
increasing. This is due to the intrinsically intermittent nature of some renewable
sources, such as solar photovoltaic arrays and wind turbines. Hydrogen is an enabling
energy vector with a minimal environmental impact.
Although there are established methods for producing and regenerating energy from
hydrogen, storing hydrogen still presents a major challenge, because of its low boiling
point (20.4 K at 1 atm) and low density in the gaseous state (90 g/m3) [2]. Compressed
hydrogen gas, as currently used in commercial fuel cell vehicles [3] still has a
significantly lower volumetric energy density than gasoline, and presents higher
containment costs and safety issues. Storage as a cryogenic liquid improves the
volumetric density but introduces parasitic energy and economic costs to maintain the
temperature as well as additional safety requirements. Storage in a solid state material
has the advantages of high energy density (higher than liquid hydrogen density), lower
containment pressures than compressed gas, and long term stability compare to batteries
[4, 5].
In solid state material the hydrogen can be stored through physical or chemical
absorption. Materials such as zeolites, carbons, porous polymers and metal organic
frameworks adsorb molecular hydrogen on the surface and in pores through
physisorption. The weak van der Waals forces between the molecular hydrogen and the
surface of the material necessitate low temperatures to achieve sufficient capacity.
Metal and complex hydrides store ionic or atomic hydrogen through chemisorption,
often with covalent or ionic chemical bonds with the opposite problem – the strong
interaction due to the high enthalpy of formation requires high temperatures to release
the hydrogen. The addition of a non-fuel storage material to the actual energy vector
increases the overall weight, and consequently light materials are preferable as solid
state materials for hydrogen storage [2, 6]. The light metal hydrides are low weight,
26
simple hydrogen containing materials with a relative high hydrogen capacity.
Unfortunately, some of the light metal hydrides have high enthalpies, such as LiH
(−90.65 kJ/mol) making it difficult to retrieve the hydrogen. In addition, most metal
hydrides have poor sorption kinetics, making the hydriding and de-hydriding processes
impractically slow. While it is more difficult to overcome thermodynamic limitations,
there has been considerable success in improving the kinetics of light metal hydrogen
sorption, through high energy ball milling and the use of suitable additives that facilitate
the absorption/desorption processes. In particular, Mg-based hydrides have been shown
to have dramatically improved kinetics after ball-milling with small amounts of
additives, and consequently MgH2, with high volumetric and gravimetric capacities of
~0.1g/cm3 and 7.6 wt%, respectively, remains a potential candidate for hydrogen
storage applications [7-9].
A vast range of additives have been tested for kinetic enhancement of MgH2 [10]. While
there is still no clear understanding of what role the additive plays in improving the
absorption and desorption, it is clear that transition metal-based additives are
particularly successful [11-13] and, in particular, the oxides of transition metals. While
transition metals are known to facilitate the dissociation of molecular hydrogen, which
could increase the rate of absorption provided the dissociation step is a rate-limiting
step, the presence of oxygen in Mg is largely regarded as impeding absorption, because
of the very low diffusivity of H in MgO.
2.1. Kinetic processes in magnesium hydride
In order to phase transition from solid magnesium to solid magnesium dihydride in the
presence of molecular hydrogen gas, a number of sequential steps must occur. A
hydrogen molecule must first adsorb via physisorption to the surface of the material.
The molecule must then dissociate into hydrogen atoms which then diffuse through the
bulk material before nucleating the new phase. Desorption starts with nucleation of the
metal phase, followed by diffusion of hydrogen atoms to the surface where they can
associate to form molecular hydrogen, and finally leave the surface.
Slow kinetics is expected to be due to one or more of these sequential steps occurring at
a particularly slow rate. For hydrogenation: (i) the ready formation of a oxide or
hydroxide layer on the surface can prevent or slow the dissociation of molecular H2 as
well as the diffusion of atomic H into the bulk; (ii) the low diffusion coefficient of
MgH2 compared to Mg, means it is much slower for the H to diffuse through a MgH2
27
layer formed on the surface of the Mg [14, 15]; and (iii) the rate of nucleation may
depend on the presence or otherwise of different phases. Slow dehydrogenation may be
cause by: (i) the strong Mg-H bond; (ii) low diffusion rate of H in MgH2; (iii)
nucleation of Mg on the surface of MgH2 requiring a high energy; and (iv) a low
recombination rate of hydrogen atoms at the surface [16, 17].
Extensive research has been devoted to exploring ways to decrease the desorption
temperature and enhance the kinetics and cycle life of MgH2. Possible methods include
changing the microstructure of the hydride by ball milling, and the addition of materials
that reduce the stability of the hydride or catalyse the sorption reactions [18-22].
Zaluska et al. [23] assumed that the crystallite size of ball milled samples is one
important factor in improving the hydrogen sorption kinetic, however other reports
disagree and relate the improvement to the particle size. Dornheim et al. [2] stated that
the effect of crystallite size on sorption kinetics is clear when the particle size is larger
than 700 nm and Dehouche et al. [24] showed that when the crystallite size is smaller
than 80 nm, the influence of the crystallite size is not substantial.
2.2. Mechanical ball milling and additives
Mechanical ball milling of metal powders under reactive gas is widely used to synthesis
metal hydrides [2,25]. The resulting hydrides are usually nanocrystalline materials,
which have a fast hydrogen sorption kinetics, due to the introduction of defects and
fresh surface areas as well as the reduction of particle size, due to the milling [25, 26].
Ball milling pure Mg is often associated with cold welding and reagglomeration of the
magnesium, resulting in large particles and crystallite size [27]. However, ball-milling
MgH2, which is a brittle material, successfully produces quite small particle and gain
sizes. Huot et al. [28] indicated that the specific surface area of the milled β- MgH2 had
increased of about 10 fold compared to the un-milled. The milling destroys the oxide
layer on the surface of Mg by repeatedly breaking the particles and producing fresh
metal surfaces for interaction with hydrogen accelerating dissociation of the molecules.
This also reduces the diffusion lengths for hydrogen in the bulk material [14, 28].
Nanocrystalline ball-milled MgH2 has reasonable absorption kinetics at 300 ºC,
however, this is still too high for practical applications [29]. The ability of metallic Mg
to dissociate hydrogen molecule is poor, due to the low probability of adsorption of the
hydrogen molecule to the Mg-surface of 10-6 [30]. Transition metal oxides are widely
used in small mole fractions as catalysts, as transition metals are known to facilitate
28
hydrogen dissociation, and have proven successful in improving the sorption kinetics of
MgH2 [29, 31, 32]. Furthermore, oxides are typically cheap and only small amounts are
required. Among all the transition metal oxides, see Fig 2-1, niobium pentoxide (Nb2O5)
is still considered one of the most effective additives.
Fig 2-1:Transition metal oxides and their effect on hydrogen desorption kinetics of
MgH2 [31]
During the complex process of milling MgH2 with metal oxides, the metal oxides acts
as a refinement agent for MgH2 powder which in turns facilitate the formation of
nanocrystalline magnesium hydride, that exhibit faster hydrogen sorption behaviour [32,
33]. Oxides with small particle size and a higher initial surface area have been found to
have better dispersion in MgH2 during milling and subsequently provide a greater
number of active sites for enhancing dehydrogenation [34].
2.3. Oxide additives
2.3.1. Magnesium and Metal Oxide additives:
The influence of (Sc2O3, TiO2, V2O5, Cr2O3, Mn2O3,Fe3O4,CuO,Al2O3 and SiO2) metal
oxides on the sorption kinetics of nanocrystalline Mg-based systems was investigated
by Oelerich et al. [29] in 2001. MgH2 was milled for 20 h, and then it was further milled
with the addition of 5 mol% metal oxides for 100 h. The comparison of the results of
MgH2 with and without the additives showed a noticeable enhancement in the
absorption and desorption kinetics for the composite samples. The ball-milled MgH2
composite samples with additives V2O5 and Fe3O4 had the fastest desorption kinetics
29
(Fig 2-2), whereas SiO2 impeded the kinetics even in comparison to pure
nanocrystalline MgH2. Cr2O3 had the highest rate for hydrogen absorption.
Fig 2-2: Desorption rates for metal oxides milled with MgH2, from [29].
As well as 5 mol%, different amounts of additive (0.2, 1 and 5 mol%) were investigated
using the MgH2/Cr2O3 system as an example, since Cr2O3 gave the fastest hydrogen
absorption. The results indicated that the absorption kinetics was independent of the
catalyst amount, for the amounts tested. They showed that only transition metal oxides
catalysts lead to a significant enhancement of hydrogen sorption kinetics and proposed
that the reason for the enhancement by transition metals was due to their different
valence states. This could indicate that the ability of the metal atom to take different
electronic states is a factor in the improvement of the kinetics of the Mg-H2 reaction.
The catalytic effect of Nb2O5 on the sorption kinetics of MgH2 was first studied in 2003,
by Barkhordarian et al. [35] and compared it to pure Nb metal catalysts [36, 37] and
metal oxides. They suggested that the oxides of metals with multiple valence states have
greater catalytic effect, through the electronic exchange reactions with hydrogen
molecules, and accelerating the gas-solid reaction.
In 2004, Castro et al.[38] mechanically milled 10 wt% tungsten oxide (WO3) with
MgH2 under hydrogen pressure for a range of times (1, 2, 4, 8, 12 and 16 h) to study the
30
hydrogen sorption properties of the composite samples. Tungsten (W) was chosen due
to its multivalence state and it was expected that the addition of WO3 would improve
the hydrogen sorption of MgH2. After milling the composite sample for 16 h, the XRD
still showed the reflections of WO3, indicating that oxide did not completely reduce
during milling. With increasing milling time to 8 h, the crystallite size of the MgH2
decreased to 12 nm and a uniform distribution of Mg and WO3 could be seen, little
improved was observed for longer milling times. Absorption studies at 300 ºC and
desorption studies at 330 ºC were performed and it was found that the addition of WO3
was substantially accelerated – at least double the absorption and desorption rates of
pure Mg. For the sample milled for 8 h, it was shown that there was no difference
between the absorption and desorption for first and the 15th cycle. However, the authors
were unable to explain how WO3 improved the sorption kinetics of Mg.
The effect of milling 10 wt% of Fe2O3 in Mg was investigated in 2005 by Kwon et al.
[33]. Samples were milled under hydrogen pressure at various revolution speeds and
times. It was concluded that the post-milling hydriding-dehydriding cycles played a role
in increasing the hydrogen sorption rates, by facilitating nucleation and introducing
cracks on the surface of Mg particles and reducing the particle size that shorten the
diffusion paths of hydrogen atoms, similar to milling. Their optimal milling parameters
were 250 rpm for 24 h.
In 2006, Jung et al .[39] synthesized composites of MgH2 and a range of metal oxides
(V2O5, Cr2O3, Fe3O4, and Al2O3) by ball milling, to investigate their hydriding
properties. This work was similar to the previous work by Oelerich et al. [29] but used
shorter milling times (1 and 2 h). They found that at 300 ºC the composite sample
MgH2/Al2O3 had the highest hydrogen absorption capacity of 4.09 wt%. At lower
temperatures (250-200 ºC) the MgH2/V2O5 composite sample showed even faster
kinetics with absorption capacities up to 3.2 and 2.25 wt%, which was in agreement
with the previous work [29].
Barkhordarian et al.[31] also investigated the mechanism of the catalytic behaviour of
non-metallic and metallic transition-metal doped samples. They argued that electronic
structure of transition metal ions had an important role in improving their catalytic
effect on MgH2. They discussed four factors that could have a role in changing the
electronic properties of the transition metal. The first was the introduction of structural
defects and increasing fresh surface area by high energy ball milling. The second was
31
the thermodynamic stability of the transition metal compound, which should be low
enough to allow a chemical interaction with magnesium but high enough to avoid a
complete reduction of the transition metal compound. From their results they showed
that oxides with low stability had the highest catalytic activity, but when the stability is
too low as in the Re2O7 the compound reacted completely with magnesium and lost its
catalytic ability. The third factor was the nature of the transition metal compound,
where they proposed that it should have a high valence state. This was established when
they compared the catalytic efficiency of different niobium oxides (NbO, NbO2 and
Nb2O5) and found that the catalytic activity increased with the valence state of the
niobium. Lastly, the fourth factor was a high interaction energy of the transition metal
compound with hydrogen.
In 2007, Aguey-Zinsou et al. [32] studied the effect of nanosized transition and non-
transition metal oxides on hydriding/dehydriding kinetics of MgH2. They classified the
non-transition metal oxides as additives with no catalytic potential and the transition
metal oxides as having potential catalytic effects on chemisorption, confirming previous
studies [29]. However, they found that the only non-TMO that exhibit similar kinetics to
TMO was Al2O3 (Fig 2-3). They suggested TMO partial or total reduction to the
elemental metal during mechanical milling could have a catalytic effect on hydrogen
absorption/desorption and the presence of MgO also contributed in improving the
sorption kinetics of MgH2. However, this could not be proven since XRD studies
showed that the TMOs were not reduced during milling. They argued that the oxides
with more ionic than covalent metal-oxygen bonds build up charge on the surface of the
sample during milling as well as balance the surface forces generated by electrostatic
charges on MgH2 particles. They suggested this could be the key parameter to stabilize
the size of MgH2 nanoparticles and prevent cold welding and agglomerations.
32
Fig 2-3: Decomposition temperature of MgH2 milled with different metal oxides
[32].
In 2007, Varin et al. [40] studied the hydrogen desorption properties of MgH2 milled
with 5 wt% of (Al2O3 and Y2O3). They found that doping with Al2O3 had no beneficial
effect on hydrogen desorption kinetics contradicting previous studies [32,39].
Conversely, Y2O3 had a limited effect on the hydrogen desorption kinetics. They were
able to overcome this problem by doping the nanocomposites with specialty Ni which
reduced the desorption temperature and increased the desorption kinetics.
Polanski et al. [41] in 2009 studied the influence of different nano-sized metal oxides
(TiO2, Cr2O3, In2O3, Fe3O4, Fe2O3 and ZnO) on the hydrogen sorption of MgH2. They
employed a simplified geometrical model to calculate the amount of metal oxide needed
to cover the surface of micro-sized particles, considering their geometrical parameters
and densities. They found that as the catalyst particle radius decreased, a smaller
amount was needed to cover the same area on the surface of the sample. A significant
difference between the oxide covering ratios was observed, depending on the oxide
density. Denser oxides had a lower covering ratio and number of active centres on the
surface of MgH2. For a catalyst with nanoparticle size less than 20 nm diameter, they
observed a sharp increase in the number of particles and active centres on the MgH2
surface. Absorption/desorption studies showed that the addition of Cr2O3 and TiO2 had
a significant improvement on the hydrogen sorption properties. Both Fe3O4 and Fe2O3
showed a slight improvement in the desorption process with almost no influence on the
33
absorption process, whereas both In2O3 and ZnO acted as inhibitors rather than catalysts
in the hydrogenation process.
Bellemare et al. [42], in 2012 compared the kinetics after ball milling and cold rolling
MgH2 with transition metal oxides. Samples were milled for 30 min under argon or cold
rolled for five rolling passes to obtain a nanocrystalline size. 2 at.% of OX was mixed
with MgH2 where OX= (Co3O4, Cr2O3, Cu2O, Fe2O3, MnO, Nb2O5, NiO, TiO2, and
V2O5). They found that the sorption kinetics of the samples prepared by the cold rolling
method were slightly slower than that of the ball milled samples. The NiO and Nb2O5
additives gave the fastest desorption kinetics irrespective of the mixing technique,
whereas ball milled V2O5 composite sample showed faster kinetics than the cold rolled
sample. As per the authors recommendation, this comparison is not entirely valid, since
the cold rolling could have been performed under an inert atmosphere rather than in air,
and for a longer time.
In 2017, the catalytic effect of the iron (Fe, Fe2O3 and FeF3) and niobium group (Nb,
Nb2O5, NbF5) additives on MgH2 were examined by Floriano et al. [43]. 2 mol% of
each additive was cryomilled with MgH2 at 77 K for 3 h under Ar, then the hydrogen
desorption properties were investigated by DSC and in a manometric PCT apparatus.
SEM and XRD analysis were performed to determine the microstructure and
morphology of the samples. They found that the fluoride composite samples had the
smallest crystallite size and a very heterogeneous particle size distribution followed by
the oxide composites. These results translated to the absorption/desorption kinetics
where the fluoride containing samples had the fastest absorption and the best
desorption. It was also noticed that the niobium group additives were superior to the
iron additive samples.
2.3.2. Zirconium dioxide (ZrO2)
In 2012 Zeng et al. [44], studied the effect of ZrO2 on the dehydrogenation kinetics of
MgH2 through milling. Different amounts of MgH2 (100 mg- 300 mg) were milled with
ZrO2 ball-mill balls for 20 h, and the kinetics were compared to MgH2 + 1 mol% ZrO2
milled with steel balls. They found the desorption peak for the sample with the smallest
amount of MgH2, milled with ZrO2 balls, to be at around 240 ºC, which was similar to
the sample with 1 mol% ZrO2 milled with the same amount of MgH2. These results
indicated contamination with ZrO2 occurred during milling, which was also confirmed
by the ICP-AES analysis revealing Zr in the samples milled with ZrO2 balls. They have
34
concluded that the presence of ZrO2 helped improving the kinetics hydrogenation and
dehydrogenation behaviours of MgH2.
In 2016, Chen et al. [45] studied the catalytic effect of milling (1 and 5 mol%) ZrO2
with. TEM images showed the grain size of the 5 mol% ZrO2 composite sample was
smaller than the composite with 1 mol%. The 5 mol% sample absorbed 6.73 wt% in 100
s at 150 ºC. At room temperature this sample was able to absorb 4 w% whereas the
sample with 1 mol% was able to absorb 3 wt%. These results indicated that milling
ZrO2 with MgH2 helped to in refining the grain of the composite which was beneficial
in enhancing the hydrogen absorption/desorption capacity of the system.
2.3.3. Cerium (IV) oxide (CeO2)
Singh et al. [46] 2013, prepared different sizes of CeO2 nanoparticles by ball milling
and then studied their effect as additives on the absorption kinetics and decomposition
temperature of MgH2. The synthesis of the nanoparticle was done by varying the ball
milling time (5- 50 h) where 30 h was the optimal time for obtaining nano-crystalline
CeO2. For their studies they used the synthesised CeO2 that was milled for 30 h and 50
h. Different amounts (0.5- 5 wt%) were milled with MgH2 and it was found that the 2
wt% for the 30 h ball milled CeO2 had a 45 ºC decrease in the desorption temperature
when compared to MgH2 and in the first 10 min it absorbed 3.8 wt%, compared to 2.5
wt% for MgH2.
The catalytic effect of CeO2 nano-powder on MgH2 was investigated by Mustafa et al.
[47] 2017, where 5 wt% CeO2 was ball milled with MgH2 for 1 h under Ar, and a
Sievert apparatus used to perform the sorption measurements. Isothermal
dehydrogenation and rehydriding measurements were performed on as-received and as-
milled MgH2 and the composite sample at different temperatures 300 and 320 ºC where
the higher temperature had better kinetics. They indicated that the addition of 5 wt%
nano-powder CeO2 improved the kinetics of MgH2, during dehydrogenation, with the
composite releasing 3.6 wt% hydrogen after 30 min and absorbed 3.95 wt% after 5 min
at 320 ºC. The activation energy for MgH2 of 133.62 kJ/mol also decreased to 108.65
kJ/mol after the addition of CeO2. The XRD patterns showed formation of new phases
of CeH2, and the authors believed the enhancement in the kinetics was due to the
combination of CeO2 and the formation of CeH2.
35
2.3.4. Vanadium oxide
The catalytic effect of high-purity V and different vanadium containing compounds on
the sorption kinetics of MgH2 were investigated by Oelerich et al. [29] in 2001. A low
content (1 mol%) of each of V2O5, VN and VC was milled with MgH2. Negligible
enhancement in the kinetics of MgH2 was found when using high-purity V, whereas
(V2O5, VN, VC) showed a significant enhancement. They found that V2O5 was the most
effective for desorption – at 8 times faster than MgH2. To further extend their
understanding of the kinetic improvement, they exposed the (MgH2)0.99(V2O5)0.01 to air
for 17 h at room temperature, which improved the sorption kinetics. Oelerich et al.
argued that the enhancement of the V2O5, despite its low content, would be due to the
milling behaviour of metal oxides which produce a finer particle distribution and finer
microstructure. In addition, they suggest a stepwise exposure to small amounts of air
also enhances the kinetics. This added to the evidence that transition metal oxides have
a favourable catalytic effect on the sorption kinetics of MgH2 [48].
Du et al. 2008 [49] theoretically investigated the effect of V2O5 on the hydrogenation
and dehydrogenation of MgH2, by performing ab initio density functional theory (DFT)
calculations. They characterized the V2O5 (001) surface as having single, two- and
three-coordinated oxygen sites (O1, O2 and O3) and two types of fourfold hollow sites
(Hole I, Hole II).
36
Fig 2-4: (a) A side view (b) top view of (1*3) V2O5 (001) surface. (c) the optimized
MgH2 clusters. red (O), grey (V), green (Mg) and white balls (H)[49].
The Mg-H bond length in the MgH2 clusters positioned on the V2O5 (001) surface, were
found to be elongated and weakened and the hydrogen molecule was formed at the hole
site on the V2O5 (001) surface (Fig 2-4). The activation barrier for the hydrogen
dissociation on the top vanadium was found to be 1/5 of that of pure Mg (001). They
established that oxygen and hole sites on the V2O5 (001) surface are catalytically active,
which could help accelerate the dissociation of H2.
Grigorova et al. [50] 2013, studied the hydrogen sorption properties of MgH2 + 10 wt%
V2O5 composite samples using the manometric method. At 300 ºC, an absorption
capacity of 6.3 wt% H2 was achieved, but the desorption was slow, after 1 h less than
0.5 wt% was desorbed. By increasing the temperature to 350 ºC desorption finished in
25 min with ~5.4 wt% H2 released. TEM studies showed that the particle size of the
composite decreased further after cycling. They have observed that after 10 cycles the
V2O5 did not completely reduce to V.
In 2015, the kinetics of MgH2 milled with transition metal hydrides, chlorides and
oxides was investigated by Korablov et al. [51]. The oxide sample was 5 mol% (V2O5),
milled with MgH2 under argon. Diffraction peaks for V2O5 were not observed in the in
situ SR-PXD, suggesting a redox reaction during milling. V2O5 was found to readily
reduce with hydrogen or during milling with MgH2, as also observed for Nb2O5 [39,52].
After heating the sample to 450 ºC and cycling twice, XRD patterns for V2H and V
were clearly observed. This suggested that V2H/V system acts as an active catalyst [53],
37
since it does not form an alloy or intermetallic compound with Mg. They observed that
the MgH2 /V2O5 sample had the fastest hydrogen desorption and absorption kinetics at
320 ºC. A drawback of using V2O5 is the formation of considerable amount of MgO
during its reduction in the MgH2 -V2O5 system.
2016, Milosevic et al. [54] studied the hydrogen sorption kinetics of MgH2 milled with
(5 and 15 wt%) VO2 which had been hydrothermally synthesized from V2O5. XRD
showed that after cycling, the composite MgH2-VO2 sample had ~46% of MgO, as also
reported by other authors [51]. The oxidation could be due to high temperature reaction
with oxygen impurities and/or from the reactions:
2 2MgH Mg H→ + (1)
22 2Mg VO MgO V+ → + (2)
H2 and V interaction could lead to formation of an interstitial solid solution, VH~0.8, or
VH2 depending on the pressure and temperatures. XRD analysis confirmed the
formation of a VH2 phase in both samples after cycling. The XRD also showed that in
the 5 wt% VO2 sample, most of the VO2 had been reduced to VH2, whereas the sample
with 15 wt% was only partially reduced, and this result was confirmed with Raman
spectroscopy. The 5 wt% composite was able to desorb 4.9 wt% in 120 s, while the 15
wt% composite desorbed 4.3 wt% after 85 s at 350 ºC, - in agreement with Grigorova
et al. [50].
The hydrogen storage properties of MgH2 doped with (5, 10 and 15 wt%) as-
synthesized VNbO5 was investigated in 2018, by Valentoni et al. [55]. VNbO5 was
prepared by a prolonged annealing of a mixture of V2O5 and Nb2O5 in a molar ratio 1:1.
TPD experiments using a Sievert instrument and Pressure-Composition-Isotherm (PCI)
measurements, as well as calorimetric measurements, were performed to study the
sorption properties of the samples, and structural and microstructure characterization
was performed with XRD. The desorption temperature of the MgH2 was reduced to 280
ºC due to the addition of the VNbO5 catalyst. The composite with 15 wt% VNbO5 was
found to be the best performing absorbing 5.5 wt% hydrogen in 10 min at 160 ºC under
20 bar of hydrogen and full reversibility at 275 ºC for more than 50 cycles. XRD
patterns showed that the catalyst was dispersed in the whole system. Activation energies
were lowered to 99 kJ/mol from the 115 kJ/mol values obtained for the undoped
sample.
38
2.3.5. Titanium Oxide based additives
Wang et al. [56], 2000, employed the reaction ball milling (RBM) method to prepare
Mg-TiO2 composite, using rutile TiO2, in order to study the hydriding properties at
moderate temperatures. XRD studies showed that the high intensity milling resulted in
nanostructured TiO2 and nanocrystalline MgH2 and γ-MgH2. The composite sample
showed superior kinetics at 350 ºC under 20 bar H2 and high hydrogen capacity. There
was no evidence of oxide reduction and they considered the existence of the n-TiO2 was
the reason for the favourable performance. They also mentioned that the distribution
state of the catalyst played an important role. By performing RBM, a brittle Mg hydride
phase was formed which enabled the TiO2 to be highly dispersed. This helped the
hydrogen diffusion through the TiO2 to the Mg-TiO2 interface and into the Mg bulk,
improving the sorption kinetics.
Fig 2-5: Crystal structure of TiO2 a) anatase b) rutile [57].
High energy milling of MgH2 with various types of TiO2 (rutile, anatase (Fig 2-5) and
commercial P25) was investigated by Jung et al. [58] in 2007, to determine the
hydrogen storage capacity and hydriding kinetics of Mg. Different concentrations of the
rutile (3, 5, 7 and 10 mol%) were milled with MgH2, and by performing Selected Area
Diffraction (SAD) patterns they found that by increasing the concentration of the rutile,
MgH2 became increasingly single-crystal-like and the diffraction intensity of the rutile
ring decreased. The formation of nanocomposite MgH2-(rutile)0.05 was observed. It was
indicated that a uniform distribution of the nanocomposite only occurred when the
concentration of the rutile TiO2 was below 5 mol%. The nanocomposite MgH2-
39
(rutile)0.05 exhibited the best hydrogen absorption kinetics, absorbing 4.4 wt% hydrogen
at 300 ºC which was 10 times faster than that of ball milled MgH2.
In 2009, the valence state and the local structure of the metal oxides (V2O5, Nb2O5 and
TiO2nano) ball milled with MgH2 was investigated by Hanada et al. [59] to determine the
precise structures of the catalysts. MgH2 was ball milled for 20 h and then further milled
with 1 mol% (V2O5, Nb2O5 and TiO2nano) for 20 h. Below 250 ºC, all composite samples
desorbed more than 6 mass% of hydrogen, and after rehydrogenation the desorbed
hydrogen in all samples decreased by less than 1 mass%. XRD profiles for the
composite samples showed a growth of the MgO phase in addition to Mg phase after
rehydrogenation at 450 ºC. Based on the Nb, V and Ti K-edge XANES spectra they
were able to confirm that the samples were reduced during ball milling to NbO, VO and
Ti2O3. The K-edge EXAFS spectra for the samples were performed after ball milling,
dehydrogenation and re-hydrogenation with references of Nb and NbO for niobium and
V, VO for vanadium and Ti, TiH2 and Ti2O3 for titanium. The authors analysed the
Fourier transformation of the EXAFS spectra and found that two peaks corresponded to
metal-metal and metal-oxygen bonds. They concluded that the catalytic effect of the
metal oxides on the hydrogen sorption of MgH2 occurred due to the lower oxidation
state and the disarranged local structure.
The catalytic effect of titanium oxide based additives was further investigated by
Croston et al. [34] in 2010. The additives used for this study had been prepared from
alkoxide precursors using a sol-gel route and calcined at temperatures from 70 ºC to 800
ºC. The amorphous oxide structures were calcined at 70 ºC and 120 ºC and the anatase
and rutile structures of the sample were calcined at 400 ºC and 800 ºC respectively. The
addition of titanium oxide based additives to the ball milled MgH2 showed a significant
reduction in the dehydrogenation onset temperature. The dehydrogenation of MgH2-
Ti70 started at 257 ºC, MgH2-Ti120 at 291 ºC and for MgH2-Ti400 started at 263ºC,
however for the MgH2-Ti800 sample, the hydrogenation started at the higher
temperature of 310 ºC. These authors were the first to report the influence of oxide
calcination temperature on the dehydrogenation temperature of MgH2. They also
emphasized that the anatase was more effective than the rutile at lowering the
dehydrogenation temperature of MgH2. They suggested that the Ti+4 in TiO2 samples
were reduced to metallic Ti0 during milling and further reduced after dehydrogenation.
Both anatase and rutile TiO2 were reduced at similar rate, but the higher surface area of
40
the anatase resulted in higher dispersion of active sites throughout the MgH2, compared
to the lower surface area rutile.
Pitt et al. [60] in 2012, studied the effect of synthesized nanoscopic Ti based
compounds on MgH2 and compared it to the effect of Nb2O5. By ball milling
MgH2+0.02 TiO2 for 30 min, they noticed that it was reduced by ca. ¾ of the original
TiO2 forming MgO. Since they were not able to detect residual Ti from the TiO2
reduction in the XRD diffraction pattern, it was assumed that the Ti was
inhomogenously dispersed and only weakly covered large regions of the MgH2 with
distinctly concentrated particles on the outer MgH2 surface. For the reduced additives
such asNb2O5 and TiO2, the authors noticed that a Mg/transition-metal exchange
occurred forming a tertiary structure. These tertiary structures e.g., MgxNb1-xO are
considered to be hard phases that could act as effective grinding agent to reduce MgH2
external powder dimensions.
In 2015, Jardim et al. [61] compared the enhancement of the hydrogen uptake of MgH2
milled with 5 wt% of two different one dimensional TiO2 based additives. The samples
were titanate nanotubes (TTNT-Low) which were synthesized by an alkaline
hydrothermal method and annealed at 100 ºC. This sample was further heated to 550 ºC
to produce the second sample, TiO2 nanorods (TTNT-550 C). The TTNT-550 C was
able to desorb 5.2 wt% at 300 ºC after 10 min compared to the TTNT-Low sample that
only desorbed 2.5 wt%. The TTNT-550, also showed better absorption kinetics
attaining 5.5 wt% of H2 at 350 ºC while the TTNT-Low sample only absorbed 1.7 wt%.
TEM and STEM were used to study the effect of milling on the composite sample,
where they found that the crystal structure and the morphology of TTNT-550 C were
preserved, whereas the TTNT-Low was partially destroyed. They concluded that it is
essential to preserve the morphology of the 1 D TiO2 based nanomaterials in order to
improve the kinetics of the MgH2.
MgH2 was milled with rutile and anatase TiO2 by Vujasin et al. [57], 2016, to study the
desorption properties. XRD, particle size (PSD) analysis and SEM were used to
investigate the role of the additive in improving the kinetics of MgH2. XRD patterns
showed four phases: tetragonal β-MgH2, anatase and rutile TiO2 as well as metallic
magnesium. The PSD results showed that around 10% of particles have average size
around 0.5 mm while around 90% are sized of ~20 μm. Isoconversional kinetic analysis
of the differential thermal analysis (DTA) spectra together with TDS were employed to
41
investigate the mechanism of the hydrogen desorption of the composite samples. The
hydrogen desorption for the composite samples was found to proceed in three steps,
where they suggest that at low temperature H is released and OH is formed, at
intermediate temperature peaks were formed which relates to the catalytic effect of the
composite and at high temperature H2 was desorbed from non-catalysed MgH2. They
found that the desorption activation energy was decreased when using rutile TiO2
additives but the anatase TiO2 had negligible effect. They also found that the Avrami–
Erofeev model for pure MgH2 was changed from n=3 to n=4 for the composite material.
This increase in the Avarmi parameter, n, refers to an increase in nucleation and/or
multidimensional growth of nuclei, reducing the effect of diffusion such that it is no
longer the rate-limiting step.
In 2017, titanium tetraisopropoxide (TTIP), a Ti-based organo-metallic material which
is liquid at room temperature, was used for the first time as an additive to improve the
sorption kinetics of MgH2 alone (Alsabawi et al. [62]). Different amounts of TTIP (0.5-
2 mol%) were ball milled with MgH2 for various times. The effect of TTIP on the MgH2
was compared with the bench mark Nb2O5, and it was found that both have a similar
effect on the absorption and desorption kinetics. For both samples, 0.5 mol% of additive
was sufficient. A hand mixed version of the composite sample also showed excellent
desorption kinetics, but poorer absorption compared to the ball milled MgH2.
The catalytic reaction mechanism of TiO2 additive with MgH2 was investigated by
Pukazhselvan et al. [63] in 2017. They analysed the different compositions of TiO2
milled with MgH2 using XRD, XPS, DSC, HRTEM and chemical mapping. The XPS
study confirmed the existence of Ti3+and Ti2+ valance states and the DSC study showed
a reduction of the Ti oxide phase. The reaction of TiO2 additives with MgH2 was found
to produce a mono-oxide rock salt phase. XRD, HRTEM/SAED and chemical mapping
studies suggested that this phase contain Ti and metal dissolved MgO. They also found
the mono-oxide rock salt phase to be coexisting with Ti2+ cations, which the authors
suggested plays a critical step in the catalysis of the TiO2/MgH2 hydrogen storage
system.
Pukazhselvan et al. [64] 2017, investigated the chemical interaction between MgH2 and
TiO2 by performing a quantitative XRD studies and to explore the formation of a new
phase. MgH2 was ball milled with xTiO2 (x=0.25, 0.33, 0.5 and 1) for 30 h under N2
atmosphere. XRD patterns showed that all composite samples contain a rock salt phase
42
(MgxTiyOx+y), believed to be the product of the chemical interaction between the TiO2
and MgH2, which may promote the dehydrogenation of MgH2. The authors also studied
the cycling life of the composite samples, where they speculated that MgxTiyOx+y salt
phase varies during cycling with a negative effect on the desorption kinetics after ten
cycles. They concluded that the titania additives are not suitable for long term MgH2
cycling.
Following their previous studies [64] Pukazhselvan et al. [65] 2018, chose to investigate
the effect of two synthesized ternary oxides (Mg2TiO4 and MgTiO3) on MgH2. 3 wt% of
each of these oxides was milled with MgH2 for 5 h and DSC profiles were recorded. It
was found that the Mg2TiO4 composite sample released hydrogen at a slightly lower
temperature than the corresponding Mg2TiO3 sample. An XRD study of 10 wt%
additive samples found that during the interaction with MgH2 both additives reduced
and formed an active in-situ MgxTiyOx+y, however, this had slower kinetics than using
titania additives. This work confirmed their previous work [63] and showed that the
reduction and the formation of in-situ MgxTiyOx+y is the most important step in order for
the titania based additive to improve the MgH2.
2.3.6. Magnesium oxide:
In 2006, Aguey-Zinsou et al [66] used MgO as an additive to improve the (de)hydriding
properties of Mg. MgH2 was milled with 10 wt% MgO for milling times from 20 to 100
h, then analyses of the MgH2 particles morphology, crystalline structure,
thermodynamic and H-sorption properties were conducted. The hydrogen kinetics were
improved due to the decrease in the particle size of MgH2. MgO has lubricant and
dispersing properties which reduce the agglomeration and cold welding of MgH2 during
milling. However, according to the XRD patterns there were no changes to the
structural properties. They also found that MgO does not modify the thermodynamic
properties of MgH2.
Usually surface metal oxide layers are not transparent to hydrogen molecules, so a MgO
layer prevents or shows hydrogen penetration [23, 67]. In 2007, Aguey-Zinsou et al.
[32], studied the effect of milling microsized MgO (MgOmicro) and nanosized MgO
(MgOnano) with MgH2 on the sorption kinetics of MgH2. A decrease of the
decomposition temperature was observed for the MgOmicro while the decomposition
temperature of the MgOnano milled for 50 h was found to be higher than pure MgH2
milled for 200 h. They also analysed the morphology of MgOnano and MgOmicro
43
additives and found that smaller particles of MgH2 are formed for MgOmicro than for
MgOnano, although agglomeration occurred for both additives.
Ares-Fernandez et al. [68] presented a detailed analysis in 2012 on the kinetic effects of
modifying MgH2 with MgO. They observed a significant decrease in the hydrogen
desorption temperature for the milled MgH2+10 wt% MgO sample, as the desorption
peak shifted from 336 ºC for milled MgH2 to 262 ºC for the composite sample. It was
observed that hydrogen absorption or desorption could be achieved in less than 100 s at
300 ºC. They also observed that full absorption was completed at 250 ºC in less than
200 s and desorption in less than 800 s. They indicated that the absorption process was
limited by diffusion of hydrogen into MgH2 in a one dimensional process. However, for
desorption the rate limiting process was shifted from nucleation and growth, to be
controlled by the Mg/MgH2 interface as the MgH2 core is shrinking. The rate limiting
steps for MgH2 milled with MgO were comparable to those observed with Nb2O5
addition [69]. However, since MgO was unable to catalyse the reaction of hydrogen
with magnesium, the authors considered the effect of MgO was to reduce the size of the
MgH2 during the milling.
Fig 2-6: Correlation between desorption temperature achieved upon oxide addition
and electronegativity of the oxide additives [68].
Fig 2-6 shows the correlation between the potency of a metal oxide in improving the
hydrogen kinetics and the electronegativity. The higher the oxide electronegativity, such
as MgO, the lower the hydrogen desorption temperature.
44
2.3.7. The effect of Cr2O3 on MgH2
In 2001, Oelerich et al. [29] investigated the influence of metal oxides, such as Cr2O3,
on the sorption kinetics of nanocrystalline Mg-based systems. XRD studies for the
composite sample MgH2/(Cr2O3)0.05 showed that the sample composition did not change
and both Cr2O3 and MgH2 were present. They also found that Cr2O3 did not reduce
during milling, which could be due to using MgH2 instead of pure Mg during the
milling process. Different amounts of Cr2O3 (0.2, 1 and 5 mol%) were used to
determine their effect on the hydrogen sorption property. They found that for the lowest
content of Cr2O3 the hydrogen capacity reached 6.7 wt% and for the highest it was 4.7
wt% within 2 min at 300 ºC.
Song et al. [70] studied the effect of reactive ball-milling Mg with 10 wt% Cr2O3, Al2O3
and CeO2 under H2 on the hydrogen sorption properties in 2002. The sample with Cr2O5
had the best hydriding rate at 300 ºC, with 5.87 wt% of hydrogen absorbed in 60 min;
whereas Al2O3 and CeO2 absorbed 5.66 and 3.43 wt% respectively. The Cr2O5 also had
the highest dehydriding rate at 300 ºC, desorbing 4.44 wt% of hydrogen over 60 min;
whereas Al2O3 and CeO2 desorbed 0.58 and 0.88 wt% respectively. All samples showed
a decrease in the amount of hydrogen desorbed and absorbed during 60 min after the 5th
cycle. XRD studies showed that the Cr2O5 in the Mg + 10 wt% Cr2O5 powder was
reduced during cycling, concluding that it was due to the higher chemical affinity of Mg
than Cr for oxygen.
In 2002, Dehouche et al. [24] investigated the thermal and cycling stability (up to 1000
cycles), of nanostructured MgH2 milled with 0.2 mol% Cr2O3. Based on the PCT
measurements and dynamic hydrogen absorption and desorption curves, they observed
an increase in the reversible hydrogen storage capacity from 5.9 to 6.4 wt%, between
the first and the 500 or 1000 cycle at 300 ºC. Conversely, the desorption kinetics was
reduced by about a factor of four which was attributed to crystallite growth and
structural relaxations. They also reported that the change of sorption properties during
cycling was due to the reduction of Cr2O3 to Cr metal.
A mixture of crystalline Mg and 5 wt% nanocrystalline Cr2O3, synthesized by the
supercritical fluid process was prepared by, Bobet et al. [71] in 2003 by reactive ball-
milling, to compare their morphology, crystallinity, chemical composition and hydrogen
sorption properties with Mg + crystalline Cr2O3 They hypothesized that differences
between the mechanical properties and the particle size of the nano and micro-sized
45
additives may lead to the formation of finer particle size when using the crystalline
Cr2O3. After 2 min the hydrogen sorption capacity was higher for the FSC Cr2O3. Zeta
potential measurement was performed on both samples milled for different times; for
the nanocrystalline Cr2O3 mixture a decrease in zeta potential was observed indicating
that some Cr3+ may have been reduced to Cr metal. They concluded that the presence of
Cr atom clusters or atoms may have an important role in improving the sorption
properties.
In 2004, Bobet et al. [72], following their previous work [71], investigated the effect of
milling parameters on the chemical composition, morphology and crystallinity of Mg +
5 wt% Cr2O3. The XRD patterns obtained after different milling duration, for vibratory
and planetary mill, and for different rotation speeds indicated that the higher rotation
speed allowed for a higher formation rate of MgH2. They concluded that the ball to
powder weight ratio parameter did not affect the efficiency of the milling, since the
efficiency is linked to the total energy of milling and the shock probability. A
subsequent work, Bobet et al. [73] in 2005 focused only on the effect of reactive ball-
milling on hydrogen sorption properties of Mg + Cr2O3 mixture. They showed that by
increasing the injected power of the milling, the hydriding rates increased. To achieve
high hydrogen sorption properties, the authors noted that one must use a planetary ball
mill with high rotation speed and high numbers of balls in the milling vials.
In 2006, Vijay et al. [74] ball-milled nanocrystalline Mg-x wt% Cr2O3 (x=5-20)
composites to study the absorption/desorption kinetics at 100, 200 and 300 ºC as well as
the Cr2O3 distribution and grain size. The absorption rate increased with increasing
Cr2O3 content at each temperature but decreased with decreasing temperature. The
reason for the near-instantaneous absorption was linked to the saturation of hydrogen
atoms on surface and grain boundary of Mg by adsorption and surface/grain boundary
diffusion. At 100 ºC the composite Mg-15 wt% Cr2O3 showed the highest absorption
capacity, suggested to be due to a smaller interparticle distance of Cr2O3 on Mg
surface/grain boundaries. For 200 and 300 ºC the Mg-5 wt% Cr2O3 composite showed
the highest absorption. For desorption, it was observed that it was incomplete in all the
composites at 300 ºC and marginal at 200 and 100 ºC. The desorption rates increased
with increasing the content of Cr2O3 and a similar pattern was observed for the
desorption capacity till 15 wt% then decreased with further increase of Cr2O3 at 300 ºC.
46
In 2006 Pranzas et al. [75] investigated the structural changes of high-energy ball-
milled MgHx and MgH2/10 wt% Cr2O3 with small and ultra small-angle neutron
scattering (SANS/USANS). Different milling parameters were investigated, such as
milling time, and vial and ball material (ZrO2 balls and Al2O3 vials), to gain an insight
into the hydrogen sorption mechanisms for the MgH2/Cr2O3 composite. Crystallite
coarsening was found to take place during cycling at 300 ºC, in agreement with the
literature [24]. They also found that during cycling, particles with radii larger than 10
μm began to break up. They compared the scattering curves of MgH2 and samples
containing Cr2O3 using SANS/USANS, and found that Cr2O3 lead to much smaller
Mg/MgH2 particle sizes, indicating Cr2O3 acts as an agent to reduce particle size during
the milling process [76].
In 2007, Aguey-Zinsou et al. [32] studied the effect of milling micro- and nanosized
Cr2O3 (Cr2O3micro and Cr2O3
nano) with MgH2 on the sorption kinetics. Their results
showed a decrease in the decomposition temperature for both nano and micro
Cr2O3 samples with only slight differences, and there was greater improvement with
increased milling time. SEM and XRD studies showed that the nano and micro
Cr2O3/MgH2 samples had similar particle size and XRD patterns. However, the time for
Cr2O3nano to desorb hydrogen was about half of the time taking the Cr2O3
micro.
In 2008, Polanski et al. [77] investigated microstructural and catalytic activity of nano-
Cr2O3 particles on the hydrogen sorption properties of nanocrystalline MgH2. After
milling MgH2 with 10 wt% Cr2O3 for 20 h, after only 1 min 6 wt% of hydrogen was
absorbed at 300 ºC and 10 bar, and the material was completely desorbed within 5 min
at 325 ºC under 1 bar, in agreement with previous work [32]. The SEM analysis
revealed that Cr2O3 particles were embedded in the MgH2 matrix forming a
nanocomposite structure and were distributed heterogeneously and closely spaced,
forming brittle chains. STEM studies confirmed that the nano-Cr2O3 particles were
present both on the surface and within the micrometric particles of MgH2. They
performed BET specific surface area on the MgH2 powder after milling with nanosized
Cr2O3 and found that it decreased from 43.9 m2/g without catalyst to 9.0 m2/g. Based on
this, they concluded that the distribution of nano-Cr2O3 and the morphology of MgH2
were important but not crucial factors in enhancing the hydrogen sorption properties.
Patah et al. [78] in 2009, milled MgH2 + Nb2O5 + Cr2O3 to investigate the hydrogen
sorption kinetics, activation energy for hydrogen desorption and the morphology
47
changes of the hydride. They prepared a composite sample with 1 mol% Cr2O3 which
desorbed 1.5 wt% of hydrogen in 20 min at 300 ºC, whereas the composite sample with
1 mol% Nb2O5 desorbed 3.6 wt%, indicating the superiority of the Nb2O5 additive.
They then compared MgH2+1 mol% Nb2O5+0.2 mol% Cr2O3 and MgH2+1 mol%
Nb2O5+1 mol% Cr2O3 samples. They found that the small addition of Cr2O3 improved
the desorption kinetics up to 5.1 wt% in 20 min and absorbed 6 wt% in 5 min, but
increasing the amount of Cr2O3 had a detrimental effect on the hydrogen capacity,
desorbing 4.3 wt% and absorbing 5.3 wt%. This suggests that the type of the oxide is
not the only factor in improving the hydrogen capacity, but the amount of the oxide is
also important. The activation energy of MgH2 decreased from 206 kJ mol-1 to 136 kJ
mol-1 after the addition of both oxides. XRD showed that the addition of Cr2O3 together
with Nb2O5 increased the formation of the ternary Mg-Nb-oxide, thereby decreasing the
formation of MgO. The formation of the ternary oxide is linked to improving the
hydrogen desorption kinetics and hydrogen capability of MgH2.
In 2011, Polanski et al. [79] studied the cyclic behaviour (desorption/absorption) and
the microstructure of ball milled MgH2 + 10 wt% Cr2O3. Scanning transmission electron
microscopy (STEM) showed that Cr2O3 was uniformly distributed in the MgH2. The
composite sample was cycled 150 times at 325ºC where a systematic loss of hydrogen
capacity was observed. In the first cycle the sample hydrogen capacity was ~6.7 wt%,
reducing to ~ 4.6 wt% after 75cycles. For the following 75 cycles the hydrogen capacity
did not change, suggesting that most of the microstructural changes responsible for the
catalytic effect occurred during the first 75 cycles. XRD, STEM and XPS studies
showed that the major microstructural changes upon cycling were sintering of
individual powder particles into agglomerates, Cr2O3 particle segregation at the
interfaces between the sintered Mg particles, grain/crystallite growth to larger than 100
nm in the agglomerated particles, and the reduction of Cr2O3 to form Cr and MgO.
Song et al. [80] 2011, used spray conversion to prepare nano-structured Cr2O3 and used
it as an oxide additive to MgH2. The effect of ball milling Mg with 10 wt% of both the
synthesized and as-purchased Cr2O3, for 2 h under 10 bar of hydrogen, on the hydrogen
absorption/desorption kinetics of MgH2 was investigated. SEM images showed that the
addition of Cr2O3 during milling facilitated pulverization of Mg, created multiple
defects and reduced the Mg particle size. The nano-structured synthesized Cr2O3 had a
superior effect on the hydrogen absorption/desorption kinetics when compared to the
48
other additives - it absorbed 4.57 wt% H for 5 min at 593 K and desorbed 0.55 wt% H
over 10 min at 603 K.
2.3.8. Niobium Pentoxide Nb2O5:
Niobium pentoxide (Nb2O5) is known as a promoting agent and an acid catalyst that
enhances the catalytic activity of other catalysts such as molybdenum used for
dehydrogenation of alkanes [81-83]. Nb2O5 is considered one of the best additives for
enhancing the kinetics of MgH2 present [10]. However, the mechanism of hydrogen
sorption enhancement by Nb2O5 is still unclear. When milling MgH2 and Nb2O5
together, Nb2O5 is increasingly embedded in the MgH2 matrix covered by MgH2 and an
outer surface of oxide layer forms during the milling process. This layer prevents further
oxidation of the sample, but it hinders hydrogen diffusion, as does the MgH2 phase [84].
It has been reported that MgH2 milled with Nb2O5 improves the kinetics of the system
due to formation of a ternary Mg-Nb-O phases during the hydrogen sorption cycles
[78,85].
In 2003, Huot et al. [37] investigated the dehydrogenation mechanism of MgH2-Nb
nanocomposites, and its effect on the crystal structure. They performed in-situ XRD
under 1 bar of hydrogen pressure, to gain insight into the structural changes in the NbH
phase. They concluded that the change in the lattice parameters of NbH phase was due
to the hydrogenation/dehydrogenation of the magnesium and not to the change in
temperature. This demonstrated that niobium hydride was transformed into a metastable
NbH0.6 phase before dissociating to form Nb. The formation of an ordered pattern of H
vacancies was found in the hydrogen sub-lattice of the β phase. This indicated that
hydrogen diffusion out of magnesium hydride has to pass through niobium [36,37].
In 2003, Barkhordarian et al. [35] used Nb2O5 as a catalyst for the first time, and tried
to understand its catalytic effect. They compared the use of pure Nb metal catalysts
[36,37] and metal oxides, and they found considerable differences with respect to the
amount of catalysts required for similar kinetics. They suggested that the oxides of
metals with multiple valence states have greater catalytic effect, which in turns improve
the electronic exchange reactions with hydrogen molecules, thus accelerating the gas-
solid reaction. They assumed that Nb2O5 reduced the kinetic barriers and suggested that
the thermodynamic driving force dominates the reaction speed and compensates for
changes in the kinetic barriers. They also found that the absorption kinetics were
negatively influenced by decreasing temperature and that this decreased the
49
thermodynamic driving force, whereas increasing temperature had an opposite effect on
the desorption kinetics, increasing the rate of desorption.
In 2004, Barkhordarian et al. [69] compared different amount of Nb2O5 ranging from
0.05 to 1 mol%, to study the kinetics of the nanocomposite. Adding 0.5 mol % Nb2O5 to
MgH2 and milling for 100 h, the sample desorbed 7 wt% of hydrogen gas within 90 s
under vacuum at 300 ºC and then absorbed ~7 mass% hydrogen within 60 s. The
absorption kinetics were largely independent of the amount of the catalyst used,
suggesting that 0.5 mol% was sufficient, and the activation energy decreased
exponentially with the addition of the catalysts.
In 2005, Fatay et al. [86] studied the effect of milling time of nanocrystalline MgH2
with Nb2O5 on the H-desorption kinetics and desorption temperature. They performed
XRD studies on the milled samples and suggested that the mixing between the hydride
and the catalyst particles occurs on the nanoscale, without any observable reaction
between the hydride and the catalyst. In addition, they found that when the milled
hydride was subsequently milled with a catalyst, the grain size refinement in the second
milling was negligible. The addition of Nb2O5 decreases the activation energy for short
milling times, however at longer milling times subsequent increase in activation energy
was found. They suggested that the difference in the activation energy could be due to
penetration of the catalytic particles into the MgH2 rather than the surface where it was
more active reducing the activation energy.
In 2005, Huhn et al. [87] studied the thermal stability of nanocrystalline MgH2 with
and without Nb2O5. The samples were annealed for 5 h at temperatures between 300
and 400 ºC. The sorption properties for the 20 h milled MgH2 samples annealed at 300
and 310 ºC showed a distinct deterioration; whereas at 310 ºC up to 370 ºC the samples
were quite similar. The absorption and desorption kinetics showed a slight deterioration
for samples annealed at 380 and higher temperatures for 5 h. For the catalysed samples
the kinetics were unchanged at 300 ºC up to the annealing temperature of 380 ºC.
However, the reaction kinetics slowed down and the storage capacity decreased for
temperatures above 380 ºC. Based on their results they indicated that Nb2O5 is stable up
to temperatures below 380 ºC, they also found that the catalysed sample annealed at 400
ºC was worse than the uncatalyzed sample. They concluded that at high temperatures,
380 ºC, Mg reduces the Nb2O5 to form Nb and MgO.
50
In 2005, Friedrichs et al. [85] studied the chemical, structural and kinetic behaviour of
MgH2 and Nb2O5 composite samples with heating. Two mol% of Nb2O5 was milled
with MgH2 for different times. The samples then underwent a series of characterization
tests: structural analysis using XRD with in-situ heating to analyse the cycled samples
and kinetics sorption measurement using a volumetric Sieverts apparatus. Based on
their XRD characterization they hypothesized the existence of a ternary magnesium-
niobium oxide (MgNb2O3.67) and thus a reduction of the Nb2O5 by the magnesium. As a
result of the Nb2O5 reduction, a large amount of MgO is formed in the sample which
exceeds the oxidation level of the ‘pure’ MgH2 sample.
During their in-situ heating experiment, the diffraction patterns showed that Mg reduces
Nb2O5 to form MgO and a ternary oxide with niobium, as well as metallic niobium. An
XPS experiment was performed to study the surface chemical composition of the
catalysed samples, however they were not able to detect any catalyst on the surface of
the milled sample. The cycled sample showed metallic niobium and niobium oxide
phases, which were initially dispersed through the sample and then emerged on the
surface after the first cycle. Their kinetics sorption measurement showed that the initial
capacity of the catalysed MgH2 was 6.5 wt% compared to 7.5 wt% for pure MgH2, due
to the non-absorbing Nb2O5. However, the kinetics of the catalysed samples both in
absorption and desorption of hydrogen was much faster than the pure MgH2 sample
[35,69] which is in agreement with previous studies.
In 2006, Friedrichs et al. [88] studied the effect of Al2O3 and Nb2O5 nanoparticles as
milling additives for MgH2. 10 wt% of additives were milled with MgH2 for different
times, the kinetics sorption measurements were carried out using volumetric Sievert
apparatus and the crystalline characterization was done using XRD. For long milling
times, both additives had improved kinetics when compared to pure MgH2. However,
when samples were milled for shorter times, only Nb2O5 additives had better kinetics,
suggesting that Al2O3 effect on the MgH2 is only due to the reduction in the particle size
rather than a catalytic one. They also compared the kinetic differences between Nb2O5
nanoparticles and microparticles. They found that the MgH2/Nb2O5-nano kinetics
behaviour for shorter milling time is about 10 times faster than the MgH2/Nb2O5-micro
sample which enabled them to analyse the sorption kinetics without long term milling.
This confirmed the catalytic effect of Nb2O5 .
51
The ternary oxide found previously was confirmed [85]. They detected Mg-Nb oxide
with an increased magnesium oxide signal in the diffraction pattern of the cycled MgH2/
Nb2O5 sample. Nb2O5 reduced completely during the hydrogen cycling which was
confirmed by XRD and X-ray photoelectron spectroscopy (XPS) studies for the
MgH2/Nb2O5-micro sample. Therefore, they concluded that the Nb2O5 by itself had no
catalytic effect on the samples. They suggested different niobium phases with lower
oxidation states or the non-stoichiometric magnesium-niobium oxides were responsible
for the catalytic effect on the samples.
In 2006, Friedrichs et al. [84] proposed a pathway model that facilitates hydrogen
transportation through a sample, which was based on their characterization of the
MgH2/ Nb2O5 nano-powder system. By taking into account their previous finding [88]
regarding the reduction of the milling time, they were able to exclude long term-milling
effects when analysing the hydrogen sorption kinetics. When they compared a
physically mixed sample and a sample ball milled for short time, the results indicated
that milling the sample for short time improved the sorption kinetics of the sample.
However, the XRD patterns for both samples showed identical results, where the Nb2O5
peaks disappeared after cycling and a new peak was observed. The new peak could be
due to magnesium oxide phase with larger lattice spacing or by the formation of a
ternary magnesium oxides [85,88]. The ternary magnesium oxide phase was confirmed
by using electron diffraction (ED) where the d spacing values were close to the d
spacing of the ternary oxide found by the XRD. After the hydrogen was desorbed, the
Nb2O5 reacted with the liberated Mg and their product penetrated the outer oxide
surface and formed a network of pathways inside the MgH2 phase.
In 2007, Friedrichs et al. [89] performed an in-situ energy-dispersive XAS and XRD
study on MgH2/Nb2O5. They demonstrated that Nb2O5 partially reduced when milled
with MgH2 then during the first desorption a fast decrease of energy and further
reduction of the sample was observed. Since Nb2O5 has a very low redox potential, the
freed Mg led to faster reduction of the niobium oxide at the initial desorption, after
which it slowed and the oxidation state of the Nb2O5 approached a minimum l. Based
on this work and their previous works [84, 85, 88], they concluded the following; Nb2O5
reduced during milling and was further reduced during heating in hydrogen and the first
absorption process. The crystalline content of Nb2O5 decreased and an apparent MgO
peak growth shifted to lower diffraction angles, indicating the formation of ternary
oxide MgxNbxO.
52
In 2006, Bhat et al. [90] compared the effect of, Nb2O5, with its counterpart halide
NbCl2 and a non-transition metal halide CaF2, in order to understand the role of the
oxygen content in the oxides in improving the hydrogen desorption in MgH2. They
showed that both transition and non-transition metal halides were able to desorb 5.2
wt% and 5.8 wt% of hydrogen in 50 min respectively, whereas Nb2O5 desorbed nearly
5.2 wt% in 80 min. They had concluded that all three samples had similar catalytic
properties. They had prepared Nb2O5-based catalyst using the precipitation technique to
increase the surface area to determine the relation between surface area and the
improvement in desorption kinetics. When they compared the desorption properties of
the commercial and the prepared Nb2O5, they found that the higher surface area of the
prepared sample played a strong role in improving the kinetics even at low temperature
(200 ºC). Then they went on to determine the role of the transition metal cations, Nb+5
and the anion O-2, by performing hydrogen sorption of MgH2 in the presence of the
three catalysts. It was found that the catalytic activity for NbCl2 and CaF2 were higher
than that for Nb2O5, which indicate that neither transition metal cation nor oxide ion
were crucial for hydrogen sorption.
In 2006, Hanada et al. [91] studied the kinetics of hydrogen sorption on the MgH2
composite doped with Nb2O5 by ball milling. They examined the chemical bonding
state of milled MgH2/Nb2O5 sample by XAFS measurement and found that Nb2O5 was
partially reduced by MgH2 during milling, which is in agreement with previous studies
[89]. They considered the H-absorption kinetics had been controlled by a thermally
activated process like the Arrenius process. They reported that 1 mol% Nb2O5+ MgH2
milled for 20 h, was able to absorb ~ 4.5 wt% hydrogen after full desorption under 1.0
MPa within 15 s at ambient temperature. A Kissinger plot was used to determine the
activation energy of H-desorption for the catalysed MgH2, which was deduced from the
first order reaction. The plots indicated that the activation energy was sufficiently
decreased by the catalytic effect of Nb2O5. In the same year, Hanada et al. [92]
confirmed their previous finding [91] by examining the catalytic effect of Nb2O5 and
found that the Nb2O5 was reduced by MgH2 to other Nb compounds, with a valence
state of less than 5+ for the Nb atom.
In 2007, Hanada et al. [93] examined the hydrogen absorption kinetics of MgH2
catalysed with Nb2O5 under various pressure and temperature conditions. For 1 mol%
Nb2O5 milled with MgH2, they found that the absorption reaction rate decreased with
increasing temperature above 150 ºC under any pressure, whereas the hydrogen
53
absorption amount increased significantly with increasing pressure at lower
temperatures. They argued that the reason for this behaviour could be due to the
variation of surface coverage of hydrogen atoms on the surface of Mg with reaction
temperature. However, this assumption does not explain the behaviour of the sample at
high pressure. Therefore, they had to consider the heat generation effect of Mg caused
during the absorption reaction. They proposed that the high reaction speed at low
temperature under high pressure in the short time range was initiated in the temperature
increase by heat generation due to hydrogenation. Also, they found that the high
pressure leads to high reaction speed due to increasing the temperature of the sample
from low to equilibrium temperature (400 ºC).
In 2007, Aguey-Zinsou et al. [83] mechanically milled 17 wt% of Nb2O5 with MgH2 to
investigate the effects on crystalline structure, particle morphology, thermodynamic and
hydrogen sorption properties. Based on their analysis of the crystalline structure they
observed that Nb2O5 was not reduced and the γ-MgH2 was not detected. Also, they
found that the amount of MgO and Nb2O5 stayed constant during the first cycle at 300
ºC, indicating no significant decomposition of Nb2O5. When they performed SEM on
the composite sample, they found that the average particle size of MgH2 milled with
Nb2O5 appeared to be significantly smaller than pure MgH2. Based on their sorption
measurements they found that the addition of the Nb2O5 reduced the decomposition
temperature of MgH2. However, when the composite sample was cycled, they found
that Nb2O5 did not change the thermodynamic properties of MgH2. As a result of their
study they demonstrated that Nb2O5 which is considered as a lubricant, dispersing and
cracking agent during milling, helped in reducing the MgH2 particle size.
In 2007, Dolci et al. [94] investigated the hydrogen uptake for the MgH2/Nb2O5 system.
They used different techniques to explain the features of the interaction of hydrogen
with Nb2O5 system, including: XRD, Electron Paramagnetic Resonance (EPR), Diffuse
Reflectance UV-Vis Spectroscopy, Thermal Desorption Spectroscopy –Mass
Spectrometry (TDS-MS), Differential Scanning Calorimetry and Thermal Programmed
Desorption (TPD). They performed three different types of experiments: treatment in
molecular hydrogen at 673 K, interaction with atomic hydrogen and interaction with
‘nascent’ hydrogen generated by metal-acid reaction in solution. The resulting EPR
spectrum suggested that due to a chemical action of hydrogen, reduction of the material
had occurred where the released electrons were either localized by Nb+5 ions, which
reduced to Nb+4 or delocalized in the conduction band of the solid. The effect of
54
hydrogen treatment resulted in a wide absorption band, spanning form the UV to the
Near Infrared region. They found that the oxide treated with ‘nascent’ hydrogen in
hydrochloric acid solution was the most effective oxide in desorbing hydrogen based on
their observation of the (TDS-MS), confirming that Nb2O5 can act as an ‘active-
reservoir’ of hydrogen and might have a role in the kinetic activation of hydrogen
uptake in mixed system.
Dolci et al. [95] in 2008, investigated the hydrogen uptake in Mg-Nb-O ternary phases
following on the from work of Friedrichs et al. [84, 88] that proposed a ‘reactive
pathway’ mechanism caused by the presence of the ternary oxide (Mg3Nb6O11) which
improved the hydrogen sorption kinetics. The purpose of their study was to understand
the interaction of hydrogen with several mixed Mg-Nb-O compounds. They were able
to obtain three individual ternary phases MgNb2O6, Mg4Nb2O9 and Mg3Nb6O11. Based
on thermal desorption mass spectrometry (TDMS) mixtures with the first two oxides
had similar results, where they underwent irreversible interaction with H2, a slow
reduction of the oxides occurred when contacted with H2 molecule forming water.
However, the third mixture with Mg3Nb6O11 showed a reversible interaction with
molecular hydrogen. They hypothesized that the reason for this reactivity was due to the
occurrence of niobium octahedral clusters inside the oxidic lattice. A similar structure
was observed for NbO, which has similar reactivity with molecular hydrogen.
Therefore, they concluded that both Mg3Nb6O11 and Nb2O5 were active in absorption
and desorption of molecular hydrogen.
In 2009, Dolci et al. [96] compared the interaction of hydrogen with different oxidic
promoters (Nb2O5, WO3 and a ternary Mg/Nb/O oxide), in order to understand the role
of oxides as promoters for hydrogen storage in MgH2. To test the reactivity of the
oxides they used either “nascent” hydrogen or molecular hydrogen. TDMS experiments
were performed on the oxides and it was found that water was the only product
desorbed by WO3 after contact with hydrogen. In comparison Nb2O5 released molecular
hydrogen after reacting with nascent hydrogen and the Mg/Nb/O multiphase mixture
showed reversible uptake of nascent hydrogen, but even better uptake when reacted
with molecular hydrogen.
55
In 2008, Yang et al. [97] applied the Chou model to understand the effects of particle
size and the ball milling time on dehydriding kinetic mechanism of MgH2- Nb2O5 1. The
results of using these calculations indicated that the rate-limiting step is the surface
penetration of hydrogen atoms. Based on their linear regression method they found that
the dehydriding reaction rate of the ball-milled nano particle composite was 1-9 times
faster than that for the micro particle.
Porcu et al. [98], 2008 used TEM to study the microstructure of a MgH2-Nb2O5
nanocomposite formed by ball milling. They compared the particle size of MgH2 milled
for 30 h with MgH2-Nb2O5 milled for the same amount of time. They found only a
slight decrease of particle size in the mixed sample, which is attributed to the presence
of oxides that facilitate the cracking of the MgH2 particles. They indicated the
possibility of an interaction between MgH2 and Nb2O5 during ball milling due to the
iconic character, which promotes the diffusion of hydrogen through the Mg matrix.
They suggested that the defective and highly dispersed structure of Nb2O5 may provide
easy pathways for hydrogen atoms to diffuse through the Mg particle core and the
surface area of the catalyst was increased due to ball milling to provide a wider area for
the Mg/MgH2 reaction to occur. Also, they indicated that Nb2O5 partially reduced to
Nb2O forming MgO during desorption process. Then when Nb2O5 inter-diffused with
MgO, a new mixed oxide MgNb2O3.67 was formed which enhanced the kinetics.
In 2009, Aurora et al. [99] performed kinetic studies and microstructural investigation
to distinguish the difference between the catalytic effect of nanometric and micrometric
Nb2O5 on MgH2. A morphological refinement of the micro Nb2O5-MgH2 composite was
shown as well as a progressive reduction of the dimensional distribution. Isothermal
hydrogen absorption/desorption measurements were carried out and MgH2 milled with
nano Nb2O5 showed an improved kinetic performance when compared to the micro
Nb2O5. They showed that using nano Nb2O5 reduced the milling time requirement to
one third of the milling time and halved the desorption time, under the same
experimental condition. XRD confirmed the formation of ternary oxides (MgNbxOy) in
the cycled MgH2/ Nb2O5 milled sample, which was also found in previous studies,
1 ‘Chou model involves a series of analytic equations for various kinds of rate-controlling steps. These
formulae express the reacted fraction ξ of hydriding/dehydriding reaction as an explicit function of time
t, temperature T, hydrogen partial pressure p(��) and particle size ��. Among them, the reacted
fraction (or transformation fraction) ξ is a key parameter to describe the kinetic behavior’98. Liu Y,
Li Q and Chou K-c. Dehydriding reaction kinetic mechanism of MgH2-Nb2O5 by Chou model. Trans
Nonferrous Met Soc China. 2008;18(Copyright (C) 2015 American Chemical Society (ACS). All Rights
Reserved.):s235-s41;10.1016/s1003-6326(10)60209-9..
56
however its role in the sorption process is still unclear [84,89]. Based on electron
microscopy observations, they found that after milling, the catalyst particle size for the
nano Nb2O5 was lower than the micro Nb2O5, this corresponded to a higher volume of
MgH2 phase in contact with the catalyst. Hence, this increased the surface of the
interface between the catalyst and MgH2, which in turns increased the number of
nucleation sites of the reacted phase (Mg). The authors found that per unit time there
was a higher number of nucleation sites of the reacted phase, which explains the faster
kinetics of nano Nb2O5 respect to micro Nb2O5 sample.
In 2011, Rahman et al. [100] following the steps of Dolci et al.[95] investigated the
hydrogen sorption and desorption in MgH2 ball milled with 1 mol% MgNb2O6,
Mg4Nb2O9 and Mg3Nb6O11. These three distinct phases were formed during ball milling
MgH2 with Nb2O5. Using XRD to characterize the crystal structure after milling, they
found that the relative amounts of additives were unchanged and hence there was no
interaction with MgH2. They suggested that the occurrence of nanostructured phase
after milling together with thermodynamic effects had an important role on the reaction
kinetics by reducing the release time of hydrogen under isothermal conditions. During
the desorption reaction, a significant decrease in the amount of the additives was
observed, for the MgNb2O6 mixture, whereas the reduction was strong for Nb2O5 and
limited for Mg4Nb2O9 mixture. For the Mg3Nb6O11 mixture they were able to detect a
progressive decrease in the amount of additive as function of desorption time, which
may suggest a chemical interaction with the Mg/MgH2 matrix or an interaction of the
oxide phase with MgO. Based on these observations, they confirmed that the additives
play an active role in improving the kinetics which is in agreement with suggested
“pathway effects” for hydrogen sorption in Mg [84]. The presence of the octahedral
niobium clusters in the structure of Mg3Nb6O11 additive, act as an insertion site for
hydrogen uptake and release and had a role in the reversible interaction of hydrogen and
improved kinetics.
In 2011, Rahman et al. [101] wanted to understand the synthetic route for the
preparation of MgNb2O6, Mg4Nb2O9 and Mg3Nb6O11 phases, by facilitating solid state
reactions between MgO, Nb2O5 and/or metallic Nb precursors. The synthesis of these
ternary compounds were reported previously by Pagola et al. [102] using a conventional
solid state reaction based on a simple annealing of commercial precursor materials.
However, the formation mechanisms of theses phases were unclear and further
investigations were needed. Rahman et al. characterized the pure ternary phases in their
57
reactivity with molecular hydrogen. The samples were prepared at different
temperatures of 600 ºC, 800 ºC and 1000 ºC, then the samples at the highest temperature
were tested in hydrogen absorption/desorption experiments. A negligible amount of
water was desorbed by the three phases, but for the Mg3Nb6O11 sample it was observed
that the H2 desorption drastically changed. They concluded that both MgNb2O6 and
Mg4Nb2O9 were totally inert, whereas Mg3Nb6O11 had an active reversible interaction
with molecular hydrogen occurring at around 395 ºC.
In 2011, Ma et al. [103] studied the hydrogen desorption properties, microstructure,
kinetics and chemical bonding states of catalyst surfaces for MgH2 and 1 mol% Nb2O5
composites milled for different times under 10 bar H2 atmosphere. They found during
ball milling the catalytic effect of Nb2O5 was gradually activated and improved with
increasing the milling time. The desorption activation energy (Ea) was calculated by the
Kissinger method and Ea decreased with increasing ball milling times. They suggested
that the better distribution and refined size of Nb2O5 was responsible for improving the
desorption kinetics especially for the sample milled for 20 h.
In 2012, Nielsen et al. [52] investigated the MgH2-Nb2O5 composite by in situ
synchrotron X-ray diffraction in order to further understand the effect of Nb2O5 for
hydrogen release and uptake in Mg/MgH2 system. They also were trying to explore the
chemical reactions in the Mg-MgH2-Nb2O5 system. Based on their SR-PXD data and
Rietveld refinement analysis, they proposed that a reaction occurred during ball milling
between MgH2 and Nb2O5, and at elevated temperatures Nb2O5 reacted with Mg to
produce the ternary oxide MgxNb1-xO. They also showed that the formation of the
ternary oxide MgxNb1-xO lead to a decrease in the gravimetric hydrogen storage
capacity after ball milling and cycling the sample at elevated temperature (>300 ºC). In
this work they suggested that the ideal ternary composition was Mg0.6Nb0.4O which
considered the active material for the prolific kinetic properties for the hydrogen release
and uptake in the system.
In 2013, da Conceicao et al. [104] compared three Nb materials, commercial oxide (c-
Nb2O5), high surface area synthesized oxide (s-Nb2O5) and metallic Nb in order to study
their catalytic capability on hydrogen sorption of MgH2. The hydride phase stability for
the three samples were examined by using a Differential Scanning Calorimeter (DSC)
and based on their results they found that the hydride decomposition temperature
decreased when adding higher amounts of Nb2O5. They compared the absorption
58
kinetics of the composite samples and found that all samples absorbed the same amount
of 5.2 wt% hydrogen. However, the s- Nb2O5 was 6 times faster than c-Nb2O5 sample
and the metallic Nb sample showed the slowest kinetics. Also, they found that
increasing the temperature from 300 to 350 ºC had a negative effect on the absorption
kinetics as indicated in previous study [35]. They had suggested that increasing the
amount of Nb2O5 improved the desorption kinetics however the metallic Nb showed
slower kinetics compared to the other samples. They concluded that the high surface
area improved the kinetics as well as other factor such as the amount of the catalyst and
the experimental conditions.
In 2013, Ma et al. [105] investigated the Nb-contained catalysts (Nb, NbO and Nb2O5)
in MgH2 nanocomposites during full cycling. They found that the Nb2O5 reduces during
milling with MgH2 to form NbH2 and MgO, with the reaction equation written as:
�� +�
��� = ��� +
�� +
�
↑ (3)
Nb also reacted with MgH2 during milling to form NbH
�� + �� = �� + �� (4)
However, NbO did not react during milling which was confirmed by the XRD profiles.
After one cycle for the Nb-doped and Nb2O5-doped samples the compositions were the
same, except for the presence of MgO in the Nb2O5-doped sample. They then
investigated the role of MgO by comparing Nb-doped sample and Nb-MgO-doped
sample and found that both exhibited the same dehydrogenation peak-temperature,
indicating that the presence of MgO provided no contribution to the desorption
property. They concluded that the catalytic effect in MgH2- Nb2O5 and MgH2-Nb
nanocomposites were attributed to Nb, which is based on the Nb-gateway model, where
Nb facilitates the hydrogen transportation from MgH2 to the outside. They compared
the size of the samples using the Scherrer equation:
� =��
����� (5)
and their morphology by SEM and found that the Nb2O5 doped sample grain size was
distinctly smaller than the other samples. Since Nb2O5 is considered as a ceramic, it
exhibits a hard and brittle character which helps refine the composite sample during
milling, while Nb metal and NbO, which have metallic properties are ductile and
difficult to mill. They suggested that the smaller size had a partial effect on the
59
dehydrogenation property. The Nb-gateway model was validated in MgH2- Nb2O5
system, when Nb and NbH2 with 10-20 nm in size were observed in the 1 mol% Nb2O5-
doped sample. Finally, they concluded that Nb crystals which were highly dispersed in
the composites act as the essential catalyst and followed the Nb-gateway model in the
enhancement of desorption properties.
Fig 2-7: The selected structural models of MgH2 on (a) Nb(1 0 0), (b) NbO(1 0 0),
(c) NbO(1 10), (d) NbO(1 1 1) and (e) Nb2O5(1 0 0). Atomic colour codes: Mg,
green; H, white; Nb, blue; O, red. [106]
A density functional theory study was conducted by Takahashi et al.[106] in 2013, to
investigate the catalytic effect of Nb, NbO and Nb2O5 with different surface planes (Fig
2-7) on the dehydrogenation of MgH2. They showed that the adsorption site of Mg and
H are different depending on the surface planes and found that among all the surface
planes NbO (1 1 1) had the best kinetics. NbO (1 1 1) electron pairing was responsible
for providing very high adsorption energy instead of the charge transfer, also they
indicated that there was a strong interaction between the s-state of H and d-state of Nb
based on the electronic structure of NbO (1 1 1).
60
Pukazhselvan et al.[107] 2015, studied the formation of Mg-Nb combined active rock
salt (RS) nanoparticle MgxNbyOx+y. They tried to clarify the chemical interaction
between MgH2 and Nb2O5, by monitoring ball milled MgH2+ n Nb2O5 (n=0.083-1.5) for
different time intervals (1, 10 and 30 h). By performing DSC studies, they confirmed
that the in-situ formed RS structures were able to enhance the hydrogen storage
properties of MgH2 system. Based on the XRD patterns they suggested that the initially
formed RS had a high concentration of Nb, but with increasing the milling time to more
than 10 h, the Mg concentration increased. An interesting outcome of milling MgH2+
0.5 Nb2O5 for 10 h and 30 h was the coexistence of two different Mg-Nb-O phases,
Mg4Nb2O9 and MgxNbyOx+y Rs, which they relate to the characteristic of the specific
stoichiometry. The crystallographic structure of the RS was found to be similar to the
MgO structure but they possessed higher lattice parameters, at least 1-4% [108], and
cell volume than MgO.
In 2016, Pukazhselvan et al.[109] and following their previous study [107] they have
noticed that during milling a considerable amount of Fe impurities were present as Nb
dissolved in the RS to form MgxNbyOx+y. since Fe was used before as an additive, they
had to investigate the Fe impurities by using different milling medium, such as steel and
zirconia. 6MgH2+Nb2O5 was milled for 30 h using a steel milling medium and a
zirconia milling medium. The steel milled sample had ~2.5 mol% Fe, and monophasic
MgxNbyOx+y, which suggested to be a Mg0.75Nb0.25O composite based on some
occupational analysis. Whereas the zirconia milled sample had ~4.5 mol% zirconia and
more than one Mg-Nb-O rock salt phase, where one phase was closer to NbO and the
other was MgxNbyOx+y.
Then in 2016, Pukazhselvan et al. [108] investigated this further, expanding their
research , by choosing one concentration of Nb2O5 (n = 0.167) and ball-milled MgH2+
0.167 Nb2O5 for different time intervals (2, 5, 15, 30 and 45 min, as well as 1, 2, 5, 10,
15, 20, 25 and 30 h). They confirmed the formation of catalytically active Nb-doped
MgO nanoparticles, by transforming the MgH2+0.167 Nb2O5 composite through an
intermediate phase typified by Mg m-xNb 2n-yO 5n-(x+y). The Mg0.75Nb0.25O composite
exhibited an outstanding catalytic effect on the hydrogen storage performance of MgH2.
They noted that the Nb-dissolved MgO significantly enhanced the sorption kinetics of
MgH2, whereas MgO by itself provide an undesirable hydrogen diffusion barrier to the
MgH2 system.
61
2.4. Summary
• Additives such as metal oxides shown to help in reducing the activation energy of
the MgH2 by providing a dissociation and recombination sites for hydrogen
absorption and desorption.
• During ball milling Mg or MgH2 powder particles break down providing fresh un-
oxidized surfaces which in turns reduces the activation energy. Ball milling with
hard/brittle additives, increases the surface area to the volume ratio of the material.
• Higher surface area increases the rate of hydrogen contacts which leads to a
reduction in the required pressure drive. The increased surface/volume ratios lead to
faster diffusion of the hydrogen through the bulk material due to the shorter path.
• The most effective oxide additives were the ones with multivalent metals, which
appeared to enhance the kinetics compared those with a single oxidation state that
had no effect, The structure of the oxides was found to be important in providing
additional sites for the dissociation of the hydrogen [24, 71].
• The brittle nature of the oxides together with ball milling causes structural
refinement that help in breaking down the hydride into smaller particles [77, 105]
• The effect of metal oxides catalysts might be due to many factors other than its
electronic structure. Other mechanisms that may affect this are: catalyst acting as
milling aid, high defect density, surface area and size effect, crystal structure and
availability of sites for OH groups [110].
• Currently, considering all the additives studied, no materials have been found to
have the required combination of fast reaction kinetics, ability to be cycled and high
H2 capacity for practical applications.
• Nb2O5: has been repeatedly reported as the best metal oxide additives in improving
the hydrogen sorption of MgH2 [31, 35, 69, 86, 88]
• Barkhordarian et al. and Fatay et al. suggested that Nb2O5 acts as a catalyst for
hydrogen desorption and found no evidence for any structural transformation of
Nb2O5 phase.
• Using XPS Friedrichs et al. showed a reduction in the Nb species after cycling
and observed a new phase MgNb2O3.67 using XRD. Therefore, they suggested
that the Nb2O5 is not the active catalyst and proposed a reaction mechanism.
• These two studies, and others, contradicted each other. For most studies, the
difficulty determining the nature of the oxide phase,
62
• TiO2: it was found that the TiO2 reduces during ball milling to Ti2O3 which has a
lower oxidation state, which has been linked to improving the sorption kinetics of
MgH2 [34,59,60]. A uniform distribution of TiO2 on the surface of MgH2 occurs
when the concentration of TiO2 is below 5 mol%, and this nanocomposite was able
to absorb hydrogen 10 times faster than that of ball milled MgH2 [56,58]. By using
the calcination technique it was found to improve the dehydrogenation temperature
and the anatase form of TiO2 was more effective than the rutile in improving the
dehydrogenation of MgH2[34].
• ZrO2: was found to enhance the sorption kinetics by reducing the grain size of the
composite during the milling [45].
• CeO2: was found that after the addition of CeO2 the activation energy of MgH2
decreased, which help enhancing the kinetics of MgH2, and it was observed that the
formation of a new phase CeH2 together with CeO2, also improved the kinetics [47].
• MgO: was found to decrease the desorption temperature, due to its high
electronegativity as well as the ability to decrease the MgH2 particle size when
milled together [32,66,68].
• Cr2O3: was observed that Cr2O3 reduces after cycling to form Cr atom or clusters
which have an important role in speeding the sorption kinetics of MgH2 [70, 71, 79].
The reversible H2 storage capacity increased after cycling for 500 to 1000 cycles but
due to the reduction of the additive, the desorption kinetics slowed [24]. At
temperature as low as 100 ºChigh H2 absorption was observed which was ascribed
to the small interparticle distance of Cr2O3 on Mg surface boundary [74]. When
comparing the micro and nano Cr2O3, it was found that they have similar particle
size and both are able to decrease the decomposition temperatures; however, the
nano sample desorbed hydrogen in half time taken for the micro Cr2O3[32].
• V2O5: was found to be readily reduced in content with hydrogen or milled with
MgH2. TheV2H/V system act as an active catalyst [53] [54], since it does not form
alloy or intermetallic compounds with Mg. A drawback with using V2O5 is the
formation of considerable amount of MgO during its reduction [51]. The
enhancement with V2O5, is attributed to the milling behaviour of metal oxides which
produce a finer particle distribution and finer microstructure [29].
63
2.5. Reference
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103. Ma T, Isobe S, Morita E, Wang Y, Hashimoto N, Ohnuki S, et al. Correlation
between kinetics and chemical bonding state of catalyst surface in catalyzed
73
magnesium hydride. International Journal of Hydrogen Energy.
2011;36(19):12319-23;10.1016/j.ijhydene.2011.07.011.
104. da Conceição MOT, Brum MC, dos Santos DS and Dias ML. Hydrogen sorption
enhancement by Nb2O5 and Nb catalysts combined with MgH2. Journal of
Alloys and Compounds. 2013;550(Copyright (C) 2015 American Chemical
Society (ACS). All Rights Reserved.):179-84;10.1016/j.jallcom.2012.09.094.
105. Ma T, Isobe S, Wang Y, Hashimoto N and Ohnuki S. Nb-Gateway for Hydrogen
Desorption in Nb2O5 Catalyzed MgH2 Nanocomposite. The Journal of Physical
Chemistry C. 2013;117(20):10302-7;10.1021/jp4021883.
106. Takahashi K, Isobe S and Ohnuki S. The catalytic effect of Nb, NbO and Nb2O5
with different surface planes on dehydrogenation in MgH2: Density functional
theory study. Journal of Alloys and Compounds. 2013;580(Copyright (C) 2015
American Chemical Society (ACS). All Rights Reserved.):S25-
S8;10.1016/j.jallcom.2013.02.044.
107. Pukazhselvan D, Bdikin I, Perez J, Carbó-Argibay E, Antunes I, Stroppa DG, et
al. Formation of Mg–Nb–O rock salt structures in a series of
mechanochemically activated MgH 2 + nNb 2 O 5 (n = 0.083–1.50) mixtures.
International Journal of Hydrogen Energy. 2016;41(4):2677-
88;10.1016/j.ijhydene.2015.12.077.
108. Pukazhselvan D, Perez J, Nasani N, Bdikin I, Kovalevsky AV and Fagg DP.
Formation of Mgx Nby Ox+y through the Mechanochemical Reaction of MgH2
and Nb2O5, and Its Effect on the Hydrogen-Storage Behavior of MgH2.
Chemphyschem. 2016;17(1):178-83;10.1002/cphc.201500620.
109. Pukazhselvan D, Otero-Irurueta G, Pérez J, Singh B, Bdikin I, Singh MK, et al.
Crystal structure, phase stoichiometry and chemical environment of
MgxNbyOx+y nanoparticles and their impact on hydrogen storage in MgH2.
International Journal of Hydrogen Energy. 2016;41(27):11709-
15;10.1016/j.ijhydene.2016.04.029.
110. Walker G. Solid-state hydrogen storage: materials and chemistry: Elsevier;
2008.
74
CHAPTER THREE
Synthesis and Characterisation
3. Introduction
This chapter describes the synthesis of the materials investigated in this work as well as
the methods used to characterise the materials produced and their hydrogen sorption
kinetics. Different ball-milling techniques used to prepare commercial and research
materials are described and the synthesis methods used in this work are detailed. X-Ray
Diffraction (XRD) was used to determine the structure, size and composition of the
composite samples using Rietveld refinement for the analysis. The measurement
methods used to investigate the sorption kinetics and maximum uptake of the modified
magnesium hydride samples are detailed. In addition, the handling and the preparation
techniques for the samples are discussed.
3.1. Synthesis
Composite samples, consisting of MgH2 with small amounts of additive (up to 10
mol%) were synthesised by ball-milling. The different additives included Ti-based and
V-based transition metal oxides in liquid and powder forms as well as a Ti-based
chloride. The benchmark Nb2O5 was used for comparisons, see Table 3-1 for the full list
of additives. The purpose of the range of additives was to explore the role of transition
metal oxides in enhancing the sorption kinetics of MgH2. The effect of different
additives, different amounts of additives and the role of the ball-milling process were
investigated. Ball-milling was performed for different amounts of time and under
different gases to provide a systematic evaluation of these parameters.
3.1.1. Ball-milling
Ball-milling is a mechanical process that is widely used to combine materials as well as
reduce the particle and crystallite size of most metal hydride materials create fresh
surfaces with desired properties. In addition, it serves to homogeneously distribute
different materials throughout the sample. Preparing Mg-based materials by ball
milling, at low temperature and short times, has been widely developed over the past
twenty years [1]. Milling parameters such as mill type (vibratory, planetary, attritor,
high energy, shakers/mixers), milling medium (steel, ceramic, zirconia, etc.) [2], and
process variables (milling time and speed, ball-to-powder weight ratio, temperature,
75
atmosphere) [3], can be combined and varied to achieve desirable milling outputs [4].
According to the targeted microstructure and composition of the desired product, ball-
milling may be classified into three types mechanical grinding, mechanical alloying and
reactive ball milling [5].
3.1.1.1. Mechanical grinding (MG)
In MG the material is nanostructured without any change to its initial compositional
phase. In this method the particle size of the material and other phases can be reduced,
due to the balls impact on the powder, to micrometric size but it is difficult to reduce
the size further by milling for longer, while the crystallites size can reach nanometer
scale [5]. For Mg-based materials this technique can be used to incorporate catalyst
additives during grinding, where the additives are homogeneously distributed at the
nanometer scale.
3.1.1.2. Mechanical alloying (MA)
Alloys can be synthesized from combinations of metallic elements by ball-milling
which is usually referred to as mechanical alloying. In this process, an alloy is formed
by violently deforming mixtures of different powders. The material is nanostructured
with a change in the composition of the starting material and the synthesized samples.
For Mg powder, metals in the desired ratios are milled together and nanostructured
alloys are rapidly obtained at room temperature [4, 6]. The sorption kinetics of samples
synthesized by the MA method is better than kinetics of the samples obtained by
conventional metallurgy [7]. This improvement is due to the presence of defects
introduced during plastic deformation and the high density of the active surface
available for hydrogen absorption.
3.1.1.3. Reactive ball milling (RBM)
In the RBM method the starting materials are milled under reactive atmosphere, i.e.
hydrogen or deuterium gas, instead of an inert atmosphere, such as argon. This
technique is used to synthesize hydrides of metals and alloys in a single step process at
low temperature and under hydrogen pressures from 1 to 150 bar [1]. The fresh metal
surfaces created during the milling process provide reaction sites for hydrogen, as it
breaks the particles’ oxide surface layer which often obstructs or slows the formation of
hydrides. Modern milling vials can be equipped with sensors to record the temperature
76
and pressure during the synthesis process, which allows for in situ monitoring of
hydrogen absorption [8, 9].
All the ball milling techniques above can produce nanosized materials with defects and
increased grain boundaries that provide pathways for hydrogen diffusion and this can
improve the kinetics of absorption and desorption [1]. This improvement is due to the
reduction of the hydrogen diffusion path, increased surface area and formation of
defects into the crystal lattice which favours nucleation of the hydride phase.
3.2. Preparation of milled hydride/additive mixtures
MgH2 can react in the air due to the presence of water (humidity) and oxygen typically
forming Mg(OH)2 preferentially, Because of this, exposure to the atmosphere must be
avoided to prevent sample contamination. All sample handling, including loading the
ball mill vial, transferring to sample cells (for kinetics measurements) or capillaries (for
XRD) as well as subsequent storage were carried out in an Ar glove box (MBRAUN
Unilab), with O2 and H2O levels maintained at less than 0.1 ppm. Milling vials were
initially loaded and sealed inside the Ar glove box, then, depending on the milling
environment, were either transferred directly to the ball-mill or connected to a gas-
handling apparatus to evacuate the Ar and load with 10 bar of H2 before sealing and
transferring to the ball-mill.
All sample mixtures used in this project were produced using a PQ-N04 planetary mill
(Across International) to mill the MgH2 (ABCR) and MgH2/additive mixtures. The
milling vial and balls used were zirconia to reduce the possibility of contaminating the
samples with ball-mill materials. Contamination from ball-mill is a problem that has
affected sample synthesis and distorted metal hydride measurements in a number of
instances [1]. This hard ceramic is less likely to deteriorate with the constant high
energy impact of the balls during use, compared to steel milling equipment, for
example. In addition, any zirconium in the sample could be readily detected using the
lab source XRD with a silver X-ray generating tube, as zirconium fluoresces at this
wavelength (0.68901 Å) [10].
MgH2 was initially ball-milled for 20 h using 10 zirconia balls 10 mm diameter with a
ball to powder ratio of 2:1. This pre-milled MgH2 was prepared in 5 g batches and
usually milled under 10 bar of hydrogen pressure or 1 bar of Argon. The mill was
running at speed of 600 rpm for 30 min followed by a rest of 6 min this cycle was
77
repeated to give a total of 24 h of milling time. The pre-milled MgH2 was further milled
with different amounts of the desired additives (0.5 - 10 mol%), see Table 3-1 for a list
of additives, for time durations from 0.5 -20 h. Mixtures were milled using six zirconia
balls at a speed of 600 rpm with a ball to powder ratio 10:1, under different gas
environments. After milling the vial was then transferred to the glove box to store the
samples for further investigation.
Table 3-1: Chemicals and materials used in this project
Materials/ chemicals Formula Purity
(%)
State Supplier
Magnesium hydride MgH2 98 Powder ABCR
Niobium pentoxide Nb2O5 (20 nm) _ powder NOVacentrix
Titanium isopropoxide Ti(OCH(CH3)2)4 97 Liquid Sigma-Aldrich
Titanium(IV) methoxide Ti(OCH3)4 95 powder Sigma-Aldrich
Titanium(IV) ethoxide Ti(OC2H5)4 97 Liquid Sigma-Aldrich
Vanadium(V) oxytriisopropoxide OV(OCH(CH3)2)3 _ Liquid Sigma-Aldrich
Vanadium(V) oxytripropoxide OV(OC3H7)3 98 Liquid Sigma-Aldrich
Vanadium(V) oxytriethoxide OV(OC2H5)3 95 Liquid Sigma-Aldrich
Bis(isopropylcyclopentadienyl)
titanium dichloride
C16H22Cl2Ti 95 powder Sigma-Aldrich
Activated Carbon FiltraSorb-400 powder
Fullerene C60 99.9 powder SES
Hydrogen H2 99.999 Gas BOC (purified by
MH reservoir)
Argon Ar 99.997 Gas BOC
Helium He 99.999 Gas BOC
78
3.3. Characterisation techniques
3.3.1. X-Ray Diffraction (XRD) analysis
XRD is an analytical technique used to investigate material properties such as crystal
structure, phase composition, and texture of powder, solids and some liquid samples.
Typically, XRD is a non-destructive characterisation technique, which makes it suitable
for in situ studies. The XRD diffracted intensity is proportional to the element electron
number, therefore the higher the atomic number the greater the intensity. Most
crystalline materials have unique diffraction patterns, and databases of these can be used
to identify a variety of chemical compounds. The purity of the sample and the
composition of any impurities present may also be determined based on diffraction
patterns for crystalline materials.
3.3.1.1. Bragg’s Law
X-rays are partially scattered by atoms when they strike the surface of a crystal. The
formation of diffraction patterns occurred when the wavelength of the radiation is of the
same scale as the atomic spacing in the crystal, such that the diffraction gives rise to a
set of well-defined beams arranged with a characteristic geometry. XRD patterns are the
result of the relative intensities for each reflection within the set of planes in the crystal,
known as the miller indices (h, k, l), accompanied by the corresponding scattering angle
for that reflection. Fig 3-1 shows Bragg diffraction where a peak is observed when a
constructive interference occurs from scattering of X-rays by the atomic planes in a
crystal [11]. The positions and intensities of the diffracted rays are a function of the
arrangements of the atoms in space and other atomic properties. By recording position
and intensity of the diffracted beam, the nature and arrangement of the atoms in the
crystal can be deduced.
79
Fig 3-1: Bragg diffraction in a 2D lattice
The condition for constructive interference from planes is given by Bragg’s law:
nλ= 2dhklsinθ (1)
where
n = an integer, for peak order,
� = wavelength of the radiation (X-ray),
� =Bragg angle or the incidence of the X-ray angle and
!�" =interplanar spacing of the (h k l) planes in the crystal lattice.
A schematic of a laboratory X-ray Diffractometer (XRD) is shown in Fig 3-2. An
electron gun in the X-ray tube is used as a source of electrons fired at a target ‘sample
holder’. The core electrons of the targeted sample are ejected from the atoms by the
high energy incident electrons. The atoms’ electrons in higher energy orbitals drop back
into the low-lying orbitals in the target, which results in emission of X-ray photons. The
emitted radiation is a characteristic wavelength for the element [12]. The XRD
apparatus used in the project, a PANalytical Empyrean, was fitted with a silver tube to
produce X-rays with an average energy of 22 keV.
80
Fig 3-2: A schematic view of a laboratory X-ray Diffractometer (XRD) (image
form Malvern Panalytical) [13].
XRD patterns were collected before and after sample synthesis as well as after
desorption/absorption measurements. Samples were loaded into glass capillaries of 0.5
or 0.8 mm internal diameter in the glove box and sealed with super glue gel to avoid
oxidation during XRD measurements.
3.3.2. Scanning Electron Microscopy (SEM)
A scanning electron microscope (SEM) can be used to obtain quantitative and
qualitative information about the composition and morphology of a sample. This
technique provides a reproducible characterisation of image features such as particle
size, external morphology (texture) and shape of a solid structure. In SEM a high energy
electron beam (from 0.3 to 30 keV) is focused into a fine probe, in vacuum, which is
rastered over the sample surface. When the electrons penetrate the surface, multiple
interactions occur which result in the emission of electrons or photons from or through
the surface, which are then detected. The principal images produced in an SEM are of
three types: backscattered electron images, secondary electron images and elemental X-
ray maps. The most common imaging technique is the one produced by secondary
electrons (SE). SE result from inelastic interactions between the sample surface atoms
and the primary electron beam. They are used to observe the morphology and
topography of a surface. Due to their low energy; only those emitted within few
nanometres from the sample surface are detected. Backscattered electrons (BSE) are
emitted from elastic interactions between the sample atom and the incident electron
beam. Large atoms have a large cross sectional area and therefore have high probability
of generating a collision with primary electrons. In consequence, the backscattering
81
intensity is proportional to the atomic number Z of the sample. Thus, the signal due to
backscattering and the image brightness increase with increasing atomic number. This
method can probe a sample to a depth of about 1 μm (check Fig 3-3).
Fig 3-3: Electron beam sample interaction [14].
The Energy dispersive X-ray (EDX) technique is a complementary tool to SEM that
uses the unique X-ray emission characteristics of each element to determine the relative
atomic composition of the sample. The inelastic collisions of the incident electron beam
with electrons in the inner atomic orbits excite those electrons to higher levels. The
excited atom decays to the ground state emitting an Auger electron or a characteristic X-
ray photon. The photons for a given element have a fixed wavelength, which is related
to the difference in energy levels of electrons in different shells, enabling different
elements and their relative abundance to be determined.
A JEOL-JSM 6510 SEM was used to examine surface topography and morphology of
the samples. MgH2 samples were placed on a carbon tape and then coated with
palladium, which was done outside the glove box. However, this arrangement was not
ideal and the absence of a preparatory stage inert environment, accelerated the oxidation
of the samples. In addition, MgH2 was unstable under the electron beam and visible
degassing was observed, resulting in the conclusion that these results were unreliable.
82
3.3.3. Synchrotron Radiation X-Ray Powder Diffraction (SR-XRPD)
Synchrotron Radiation X-Ray Powder Diffraction (SR-XRPD) is a characterisation
technique used to investigate the microstructure of materials using X-rays generated by
a synchrotron. SR-XRD are accessed in large facilities, where electrons are produced in
electron gun and accelerated to near light speed to reach multi-GeV level energy. The
electrons are then injected into an ultra-high vacuum storage ring up to several
kilometres in circumference where they travel at relativistic speeds while in the storage
ring. At each instrument, the electron direction is changed by either an undulator or a
bending magnet, and the resulting acceleration causes electromagnetic radiation to be
emitted in a narrow cone. Typically the collimating mirror is a toroid used to improve
the energy resolution by collimating the beam in the vertical direction into parallel light
[15]. The monochromators are typically Si<111> which is used to create a tunable
monochromatic X-ray beam focused in the horizontal direction [16].
Fig 3-4: Schematic layout of the 10-BM-1 Powder Diffraction beamline
“Australian synchrotron, Diagram courtesy of ANSTO” [17].
Fig 3-4, shows a schematic illustration of the Australian SR-beamline, where the main
components are the collimating mirror, monochromators, and focusing mirror.
83
Table 3-2: The Australian Synchrotron Parameters.
Parameter Values
Source AS Bending Magnet
Energy range 3-20 keV
Monochromator Type Si <111> flat and Si<311> sagittal focus
Resolution dE/E 1.4 × 10-4
Flux (photons/sec) 1 × 1011 at 10 keV
Beam size focused (H×V) 0.5 × 0.5 mm
Beam size unfocused (H× V) 5 × 2 mm
The Australian Synchrotron is a relatively small synchrotron with a storage ring
circumference of 216 m. The powder diffraction instrument is mounted on a bending
magnet and the beamline parameters are presented in Table 3-2.
This instrument was used to investigate ball-milled MgH2, mixed with 10 mol% TTIP
additive, where the higher additive content was needed to increase the diffracted
intensity from any crystalline products formed during mixing. Ball milling reduces the
particle size to ~20 nm, and the small particle size limits the ability to resolve small
peaks from any new compounds formed due to peak broadening, so relatively long
collection times were needed to provide sufficient signal. Composite samples were
prepared by both hand-mixing and ball-milling, and then samples were mounted in
quartz capillaries and diffraction scans were taken first at room temperature and then in
situ while heating to 400°C under vacuum to desorb the hydrogen. Finally, a pressure of
30 bar H2 was applied at 250°C and scans taken as the sample absorbed. The energy
range used for the investigated hydrogen storage materials was 16 keV.
3.3.4. Kinetics characterisation
The Sieverts or ‘manometric’ technique and the gravimetric technique are the two main
methods to measure hydrogen uptake by a solid state host. Pressure measurements,
together with temperature and volume values are used to determine the uptake in the
84
Sieverts technique, whereas the gravimetric directly measures the change in the weight
of the sample during absorption or desorption. In this work, the Sieverts method was
used to load the samples with a single shot of hydrogen. The hydrogen sorption for the
sample can be calculated from the non-ideal gas law,
( ),PV
nZ T P RT
= (2)
where n is the number of moles of gas in the volume V, P is the pressure, R is the gas
constant, T the temperature and Z is the compressibility of the gas. Z is a function of
both temperature and pressure (calculated using the NIST REFPROP database2) and
represents the variation from an ideal gas (for which Z=1).
A custom manometric Sieverts apparatus constructed from commercial Sitec and VCR
components was used to perform the hydrogen desorption and absorption kinetics
measurements. This instrument was improved during this project with the addition of
active temperature control, a MH hydrogen reservoir as well the development of TPD
and TDS capabilities. Full details on the equipment information can be found in [18]
which is provided here as chapter 4.
3.3.5. Temperature Programmed Desorption and Thermal desorption
spectroscopy
The kinetics of a hydrogen storage material - how fast the material can absorb and
desorb hydrogen - can be characterized using Temperature Programmed Desorption
(TPD) [19, 20]. TPD involves heating the sample at a constant linear temperature rate
while monitoring the pressure of the H2 released from the desorbing sample. Thermal
desorption spectroscopy, enables the determination of the gases released during
temperature desorption. The addition of TDS complements the TPD technique; while
the sample is being heated at a known rate any decomposition products are carried to a
mass spectrometer to identify the evolved atomic or molecular species [21], and this
might help to understand the decomposition of the sample or additive.
For this project TPD studies were performed on all the samples with the same
temperature program. A Platinum Resistance Thermometer (PRT) was used to record
the internal temperature of the sample cell during the experiment. TPD studies was
2 (NIST) National Institute of Standards and Technology (REFPROP) Reference Fluid Thermodynamic and
Transport Properties Database, https://www.nist.gov/srd/refprop
85
initiated at room temperature, and then sample was heated by a furnace controlled by a
Eurotherm 2404 temperature controller, which is used to set the temperature ramping
rate, under a starting vacuum of typically 3 ×10-6 mbar. A MicroPirani 925 vacuum
gauge was used to record to the flow of the hydrogen pressure released by the sample
while heating. All data needed for calculating the hydrogen sorption kinetics
(temperature and pressure of various parts of the instrument) were recorded using a
laptop computer running the control and logging software Kheiron. TDS measurements
were conducted simultaneously with TPD studies, to account for all the gases released
with respect to temperature. A quadrupole mass spectrometer (HIDEN Isochema
DSMS) was connected to the instrument via a (ID= 5 mm) PTFE pipe, where all the
data was recorded using MASsoft software.
86
3.4. References
1. Huot J, Ravnsbæk DB, Zhang J, Cuevas F, Latroche M and Jensen TR.
Mechanochemical synthesis of hydrogen storage materials. Progress in Materials
Science. 2013;58(1):30-75;10.1016/j.pmatsci.2012.07.001.
2. Pukazhselvan D, Otero-Irurueta G, Pérez J, Singh B, Bdikin I, Singh MK, et al.
Crystal structure, phase stoichiometry and chemical environment of
MgxNbyOx+y nanoparticles and their impact on hydrogen storage in MgH2.
International Journal of Hydrogen Energy. 2016;41(27):11709-
15;10.1016/j.ijhydene.2016.04.029.
3. Polanski M, Bystrzycki J and Plocinski T. The effect of milling conditions on
microstructure and hydrogen absorption/desorption properties of magnesium
hydride (MgH2) without and with Cr2O3 nanoparticles. International Journal of
Hydrogen Energy. 2008;33(7):1859-67;10.1016/j.ijhydene.2008.01.043.
4. Suryanarayana C. Mechanical alloying and milling. Progress in Materials
Science. 2001;46(1-2):1-184;10.1016/s0079-6425(99)00010-9.
5. Shao H, Xin G, Zheng J, Li X and Akiba E. Nanotechnology in Mg-based
materials for hydrogen storage. Nano Energy. 2012;1(4):590-
601;10.1016/j.nanoen.2012.05.005.
6. Koch CC, Scattergood RO, Youssef KM, Chan E and Zhu YT. Nanostructured
materials by mechanical alloying: new results on property enhancement. Journal
of Materials Science. 2010;45(17):4725-32;10.1007/s10853-010-4252-7.
7. Prasad Yadav T, Manohar Yadav R and Pratap Singh D. Mechanical Milling: a
Top Down Approach for the Synthesis of Nanomaterials and Nanocomposites.
Nanoscience and Nanotechnology. 2012;2(3):22-48;10.5923/j.nn.20120203.01.
8. Oghenevweta JE, Wexler D and Calka A. Understanding reaction sequences and
mechanisms during synthesis of nanocrystalline Ti2N and TiN via magnetically
controlled ball milling of Ti in nitrogen. Journal of Materials Science.
2017;53(4):3064-77;10.1007/s10853-017-1734-x.
9. Doppiu S, Schultz L and Gutfleisch O. In situ pressure and temperature
monitoring during the conversion of Mg into MgH2 by high-pressure reactive
ball milling. Journal of Alloys and Compounds. 2007;427(1-2):204-
8;10.1016/j.jallcom.2006.02.045.
10. Bearden JA. X-ray Wavelength and X-ray Atomic Energy Levels: National
Bureau of Standards, for sale by the Superintendent of Documents …; 1967.
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11. Encyclopedia of Materials Characterization: Surfaces, Interfaces, Thin Films.
Brundle CR, Charles A. Evans J, Wihon S, editors. Greenwich: Manning
Publication Company; 1992.
12. Wang Q, Sun Q, Jena P and Kawazoe Y. Theoretical Study of Hydrogen Storage
in Ca-Coated Fullerenes. J Chem Theory Comput. 2009;5(2):374-
9;10.1021/ct800373g.
13. PANalytical. Heavy Mineral Sand Analysis for Industrial Process Control,
http://www.usedscreeners.com/news/heavymineralsandanalysisforindustrialproc
esscontrol?tmpl=%2Fsystem%2Fapp%2Ftemplates%2Fprint%2F&showPrintDi
alog=1 ; 2012
14. Secondary Ion TESCAN Detector, https://www.tescan.com/en-
us/technology/detectors/sitd; 2019
15. Strocov VN, Schmitt T, Flechsig U, Schmidt T, Imhof A, Chen Q, et al. High-
resolution soft X-ray beamline ADRESS at the Swiss Light Source for resonant
inelastic X-ray scattering and angle-resolved photoelectron spectroscopies. J
Synchrotron Radiat. 2010;17(5):631-43;10.1107/S0909049510019862.
16. Isshiki M, Ohishi Y, Goto S, Takeshita K and Ishikawa T. High-energy X-ray
diffraction beamline: BL04B2 at SPring-8. Nuclear Instruments and Methods in
Physics Research Section A: Accelerators, Spectrometers, Detectors and
Associated Equipment. 2001;467-468:663-6;10.1016/s0168-9002(01)00441-7.
17. Technical information (Powder), https://www.ansto.gov.au/user-
access/instruments/australian-synchrotron-beamlines/powder-
diffraction/technical-information ; 2019
18. Alsabawi K, Gray EM and Webb CJ. A Sieverts apparatus with TPD/TDS for
accurate, high-pressure hydrogen uptake and kinetics measurements.
International Journal of Hydrogen Energy. 2019;Submitted.
19. Kowalczyk P, Kaneko K, Terzyk AP, Tanaka H, Kanoh H and Gauden PA. New
approach to determination of surface heterogeneity of adsorbents and catalysts
from the temperature programmed desorption (TPD) technique: one step beyond
the condensation approximation (CA) method. J Colloid Interface Sci.
2005;291(2):334-44;10.1016/j.jcis.2005.05.029.
20. Bhattacharya M, Devi WG and Mazumdar PS. Determination of kinetic
parameters from temperature programmed desorption curves. Applied Surface
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88
21. Broom DP. Hydrogen storage materials: the characterisation of their storage
properties: Springer Science & Business Media; 2011.
89
CHAPTER FOUR
A reliable, high accuracy Sieverts instrument with TPD and TDS
(As a Journal paper submitted to International Journal of Hydrogen Energy)
4. Introduction
Accurate measurement of hydrogen uptake and release for investigation of future
hydrogen storage materials is vital to determine suitability for practical use, as well as
understand the properties and nature of the materials in order to assist with the
development of new materials. This chapter describes the design and implementation of
a Sieverts type apparatus for characterising hydrogen sorption kinetics; with pressure
capability to 340 bar and sample environment temperatures from 77-773 K. This
instrument, which existed at the start of the project, was improved and then further
developed during the course of this project to enhance the temperature stability,
improve the ratio of reference and cell volumes and establish a single environment to
perform sorption kinetics measurements. The sorption kinetics are measured by the
addition of temperature programmed desorption (TPD) and temperature desorption
spectroscopy (TDS).
During the TPD measurement process, the sample is heated at a constant rate using a
temperature controller to achieve the required temperature (maximum of 773 K), while
monitoring the pressure or flow of the released gases. The addition of a mass
spectrometer to provide TDS capability enabled the determination of the atomic masses
of the gases released during the TPD processes. A number of experiments to validate
the operation and accuracy of this instrument were conducted by measuring hydrogen
uptake and release, kinetics and decomposition for different, representative samples.
An important facility in this instrument is the use of a Metal-Hydride (MH) compressor
as a reservoir for local hydrogen storage. This enabled recycling of hydrogen desorbed
from the sample, a higher hydrogen purity and higher pressure capability than most
commercial cylinders. Commercial/ industrial MH compressors often use more than one
stage/material to achieve sufficient compression of a large quantity of hydrogen,
suitable for gas cylinder replacement or a hydrogen fuel station, for example. In such
cases, combinations of metal hydrides with different enthalpies are used to increase the
final compression ratio while maximizing the absorption pressures of each stage [2].
90
However, for the simpler application of providing hydrogen to a single experimental
apparatus, a small single stage MH compressor was used, housed in a 150 cc Swagelok
stainless steel volume with a rated pressure of 340 bar, incorporating approximately 400
g of the AB5 intermetallic MmNi4:2Co0:8 (Japan Metals and Chemicals Co.), where
Mm represents mischmetal, a combination of naturally occurring rare earth metals. This
reservoir material was capable of generating hydrogen pressure of at least 340 bar when
heated manually using a heat gun and could absorb hydrogen back into the reservoir
with cooling from liquid nitrogen.
The Sieverts technique is vulnerable to errors and uncertainties when calculating the
amount of hydrogen taken up by the sample. The measurement accuracy of the pressure,
temperature and volumes involved is critical in the determination of the amount of gas
in a volume, which is the basis of the Sieverts technique. The choice of sensors is
discussed as well as the volume calibrations for room temperature calibration and non-
ambient sample temperatures. A new volume calibration procedure, using isotherm
measurements with no sample is described which enables a more accurate determination
of the sample cell volume. A number of validation measurements for the instrument are
shown to demonstrate the uptake, kinetics and decomposition capabilities with different
materials including LaNi5, MgH2, and a low density carbon material (FiltraSorb-400).
Full details of the instrument design and validation are in the accompanying paper.
91
4.1. References
1. Reilly Jr JJ and Wiswall Jr RH. Method of storing hydrogen. Google Patents;
1970.
2. Hopkins RR and Kim KJ. Hydrogen compression characteristics of a dual stage
thermal compressor system utilizing LaNi5 and Ca0.6Mm0.4Ni5 as the working
metal hydrides. International Journal of Hydrogen Energy. 2010;35(11):5693-
702;10.1016/j.ijhydene.2010.03.065.
3. Cieslik J, Kula P and Filipek S. Research on compressor utilizing hydrogen
storage materials for application in heat treatment facilities. Journal of Alloys
and Compounds. 2009;480(2):612-6.
4. Rajalakshmi N, Dhathathreyan KS and Ramaprabhu S. Investigation of a Novel
Metal Hydride Electrode for Ni-Mh Batteries. In: Grégoire Padró CE, Lau F,
editors. Advances in Hydrogen Energy. Boston, MA: Springer US; 2002. p. 121-
30.
4.2. Paper Sieverts instrument
92
STATEMENT OF CONTRIBUTION TO CO-AUTHORED PUBLISHED PAPER
This chapter includes a co-authored paper. The bibliographic details of the
co-authored paper, including all authors, are:
Alsabawi K, Gray EM and Webb CJ.
A Sieverts apparatus with TPD/TDS for accurate high-pressure hydrogen uptake and
kinetics measurements’. International Journal of Hydrogen Energy (Submitted).
My contribution to the paper involved:
1) Re-Building, and help in the modification of the instrument
2) Perform experiment
3) Calculations and analysis of the results
4) Manuscript preparation
(Signed) _________________________________ (Date)__12/02/2019___________
Khadija Alsabawi
(Countersigned) _________________________(Date) 12/02/2019______________
Corresponding author of paper: Khadija Alsabawi
(Countersigned) _________________________(Date)____ 12/02/2019__________
Supervisor: Colin Jim Webb
93
A Sieverts apparatus with TPD/TDS for high-pressure hydrogen uptake and
kinetics measurements
K. Alsabawi*, E.MacA. Gray, C.J. Webb
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, 4111
Brisbane Australia
Abstract
A Sieverts (manometric gas absorption) apparatus has been designed,
constructed and used to measure hydrogen absorption and desorption,
pressure-composition-temperature (PCT) isotherms at pressures up to 340
bar and temperatures to 773 K. The hydrogen pressure is provided using a
metal-hydride reservoir which removes the requirement for a high
pressure cylinder and also enables the hydrogen to be recycled. The
addition of temperature programmed desorption (TPD) and temperature
desorption spectroscopy (TDS) using a mass spectrometer enhances the
capabilities of this instrument and allows measurements of the absorption
and desorption kinetics of potential hydrogen storage materials.
Key words:
Sieverts apparatus, Hydrogen storage, MH compressors, PCI curve, TDS,
TPD.
* Corresponding author: Tel.: +61 7 37353625. E-mail address: [email protected]
94
Introduction
Hydrogen has the highest energy density of all common fuels by weight and can be used
as an energy carrier or storage system. Combustion of hydrogen in air to produce heat,
or in a fuel cell to produce electricity, produces little to no pollutants, with the primary
product being water [1]. Where the production of hydrogen is achieved using a
renewable source of energy such as wind or solar energy and the source of hydrogen is
water, this forms a closed energy cycle without any detrimental environmental
consequences [2, 3]. As a mechanism for storing energy generated by inherently
intermittent renewable energy sources, hydrogen offers a way of maintaining a balance
between energy supply and demand [4].
Commercial hydrogen powered cars such as the Toyota Mirai, Honda Clarity and
Hyundai ix35 fuel cell vehicles are already available [5-7]. However, hydrogen gas has
a low volumetric energy density, 0.5 kWh/dm3 for 350 bar H2 [8], compared to gasoline
(~7 kWh/dm3) which requires large storage tanks of compressed hydrogen in order to
have a sufficient driving range. High pressure storage increases the energy density, but
requires stronger, more expensive storage tanks [9] to be safe. Liquid hydrogen
cryogenic tanks need costly and energy consuming cooling systems [10]. A viable
alternative is solid state storage.
Solid state materials such as metal hydrides (including elemental metal hydrides such as
MgH2, and alloys, such as LaNi5) and complex hydrides (e.g. amides, borohydrides and
alanates) [11], and physisorption on high surface area materials (e.g. graphite, graphene,
metal-organic frame works and aerogels) are all capable of storing hydrogen with
varying gravimetric and volumetric densities. Physisorption has a relatively low
operating pressure, the price of component materials is affordable and the storage
system design can be relatively simple. However, the drawbacks include low capacity
and a requirement for low temperatures.
Metal hydrides and complex hydrides can store large amounts of hydrogen in a safe and
compact way. Many intermetallic alloys reversibly store hydrogen and operate at
ambient temperature and pressure, but the gravimetric hydrogen capacity is <3 wt%
[12]. Chemical hydride compounds typically often have high formation energies and
therefore require high temperatures to release the hydrogen which make them unsuitable
for many practical applications [13].
95
The search for new hydrogen storage materials requires the accurate measurement of
hydrogen uptake [14-16] as well as other characterisations such as the sorption kinetics,
activation energy and the changing sample composition during the sorption processes.
Reversible hydrogen capacity as well as enthalpy and entropy changes for potential
storage materials are commonly measured using the manometric technique (also called
Sieverts or volumetric) or the gravimetric method [17]. Temperature programed
desorption (TPD) [18-20] is used to determine the sorption kinetics and activation
energies and temperature desorption spectroscopy (TDS) [21] can be employed to
investigate gases released during the desorption process.
Uptake measurements
Pressure-Composition-Temperature isotherms (PCT) are typically used to characterise
the thermodynamics of metal hydrides and adsorption materials [17]. PCT isotherms
describe the dependence between the hydrogen equilibrium pressure and the hydrogen
concentration in the material at fixed temperatures [22]. To establish an isotherm using
a manometric instrument, aliquots of hydrogen gas are added or removed from a system
with known volume. For many binary metals and intermetallic alloys, hydrogen at low
pressure first forms a solid solution where the uptake rises quickly with the pressure, as
shown in Fig 1, plot A, as equilibrium pressure vs uptake. At first hydrogen forms a
dilute solid solution in the metal, typically called alpha (α) phase at increasing pressure,
ideally following Sieverts' Law. At a critical concentration, the concentrated hydride
phase (typically called beta (β)) nucleates. Ideally, the equilibrium between the three
phases present (H2, α, β) has only one degree of freedom according to the Gibbs Phase
Rule, so that the concentration becomes independent of pressure while the alpha and
beta phases coexist, leading to a plateau (ideally horizontal) in the pressure-composition
isotherm. Once the alpha phase is fully converted to beta phase, hydrogen dissolves in
the beta phase at increasing pressure.
The equilibrium pressure logarithm derived from PCT isotherms conducted over a range
of temperatures, is used to construct a van ’t Hoff plot, from which thermodynamic
parameters such as the enthalpy, ∆H, and entropy, ∆S, of the hydride can be obtained
[22]. For physisorption using porous materials, the PCT isotherm can be classified as
IUPAC type I – VI isotherm, depending on the porosity [23]. An example of a type I
absolute isotherm (uptake vs equilibrium pressure) is shown in Fig 1, plot B.
96
Fig 1: (A) PCT absorption isotherm, (B) absolute adsorption isotherm, (C) excess
adsorption isotherm.
An absolute adsorption isotherm is difficult to determine experimentally as it requires
an assumption about the density of the adsorbate [24]. As gas adsorbs onto the material,
the increasing layer of adsorbate occludes more of the volume occupied by the gas.
Since the manometric method requires knowledge of the volume available to the gas in
order to calculate the uptake, this leads to an under-estimation of the amount of uptake,
which increases with increasing pressure – an excess isotherm. To convert the measured
excess isotherm to an absolute isotherm, the volume of the adsorbate must be known,
however, there is no accurate experimental method to determine this [25]. It is often
assumed that the adsorbate is of uniform density at the bulk liquid density [26]. Since
this assumption cannot be verified, the measured excess isotherm is frequently
published (Fig 1, plot C).
In the gravimetric technique the hydrogen uptake for an isotherm is determined by the
weight change of the sample following a step change in the hydrogen pressure. In the
manometric technique, hydrogen sorption is determined from measurements of the
hydrogen pressure in a system of fixed known volume, Both Sieverts and gravimetric
techniques have advantages and disadvantages [27], as they both rely on accurate
knowledge of the volume of the sample and the adsorbed phase to measure the absolute
adsorption or capacity [24, 28, 29]. In the manometric case, the volume of the sample is
needed to determine the void volume in the sample cell in order to calculate the amount
97
of hydrogen in the gas phase. For gravimetric measurements, the sample volume is
required for the buoyancy correction [25].
The Sieverts technique
Fig 2: Minimal Sieverts apparatus for determining the uptake of hydrogen by a
sample.
A simplified Sieverts apparatus is shown in Fig 2, which is used to determine the
hydrogen uptake by a solid sample. The reference volume (Vref) and the sample cell
volume (Vcell), separated by a valve (Vs), are used to progressively increase the pressure
on the sample by introducing known increments of gas into the sample cell volume. For
accurate results, both volumes must be carefully calibrated.
At each pressure step in the construction of a PCT isotherm, hydrogen is introduced into
the reference volume and the amount of gas in this volume is calculated using the non-
ideal gas law,
( ),PV
nZ T P RT
= (1)
where n is the number of moles of gas in the volume V, P is the pressure, R is the gas
constant, T the temperature and Z is the compressibility of the gas. Z is a function of
both temperature and pressure and represents the variation from an ideal gas (for which
Z=1).
98
The valve Vs is then opened to allow the hydrogen to react with the sample in the
sample cell volume Vcell. After a suitable time to allow the sample to reach equilibrium
with the hydrogen pressure and temperature and for any temperature excursions to
equilibrate, the pressure throughout the hydrogenator and the temperatures in each
volume are recorded. The amount of gas sorbed (desorbed, adsorbed or absorbed) by the
sample is determined from any difference in the calculated total gas in the system before
and after opening the valve Vs [17]. The process is then repeated, introducing more gas
into the reference volume and increasing the sample pressure in a stepwise fashion.
Following the procedure in [17], for any step k in the procedure, the absolute mass of
hydrogen sorbed by the sample in this step can be calculated.
, ,
1, ,=
( )
k b k a
ref ref refk k
abs absk a k a k
cell cell cell x
Vm M n
V V
ρ ρ
ρ ρ−
− ∆ ∆ =
+ − − (2)
Where
,,
, ,
k a
refk a
ref k a k a
ref ref
MP
Z RTρ = is the gas density in the reference volume at step k after the
valve has been opened and similarly for the other densities. k
xV is the volume of the
sample plus adsorbate at step k, M is the molecular mass of the gas used and the
superscripts b and a refer to before and after the valve Vs is opened, respectively.
The total amount sorbed after n steps is therefore:
, ,
1, ,1 1
= ( )
k b k an n
ref ref refn k
abs absk a k a k
k kcell cell cell x
Vm M n
V V
ρ ρ
ρ ρ−= =
− ∆ =
+ − − (3)
Errors and uncertainties using the Sieverts method
The Sieverts technique is vulnerable to errors and uncertainties when calculating the
amount of hydrogen taken up by the sample [15, 16, 24, 27, 29]. The hydrogen uptake is
calculated by subtracting two relatively large numbers, the amount of hydrogen in the
system before and after the valve is opened. For the same sample, lower amounts of gas
before and after reduces the relative uncertainty – hence, where practical, smaller
volumes for the reference and sample cell can moderate this problem [29].
99
To achieve accurate results when using a manometric instrument, volume calibration of
each component is required. The strong temperature dependence of any pressure reading
necessitates careful control of the temperature of the valve manifold and the reference
volumes. Depending on the nature of an experiment, the sample temperature may differ
from the rest of the system. Therefore, extra care is needed when modelling the
temperature of each volume of the system [17].
Temperature Programmed Desorption (TPD)
TPD can be used to determine the kinetics of the desorption processes or decomposition
reactions. The TPD process involves heating the sample at a constant rate while
monitoring the pressure of the released gases [30]. For hydrogen desorption studies this
can determine the maximum amount of hydrogen released as well as indicating the
temperature range over which the desorption takes place. Where the desorption process
occurs in discrete reaction steps or involves phase changes, this can also be observed.
TPD allows the calculation of the activation energies of the desorption process by
measuring the desorption spectra for different temperature ramp rates [31].
Thermal Desorption Spectroscopy (TDS)/ (TDS-MS)
TDS adds the capability of determining the nature of the gases released during
temperature desorption. As the sample is heated any decomposition products are carried
to a mass spectrometer or other device which can determine the atomic or molecular
species [27]. This technique can be used to monitor the hydrogen signal to gain more
accurate results for the characterization of small samples [32]. Similarly to TPD, TDS
can be used to determine the activation energies using the same method [33].
Hydrogen pressure amplification
If the experimental conditions require a hydrogen pressure greater than that supplied
from the available compressed gas cylinders (typically 100-200 bar depending on local
regulations), amplification of the pressure is needed. Pressure amplification of hydrogen
can be performed using mechanical compression, metal hydride (MH) compression,
thermal compression and electrochemical compression [34-36]. Mechanical
compression includes syringe pumps [37], reciprocating compressors, rotary
compressors and ionic compressors [38]. A syringe pump reduces the volume
containing the gas in order to increase the pressure. Thermal compression uses the
temperature dependence of a gas to put more gas into a fixed volume at a low
100
temperature, which increases the pressure when the temperature is raised.
Electrochemical compressors use proton exchange membranes (PEM) to dissociate pure
hydrogen at the anode and then recombine as highly pressurised hydrogen at the
cathode [35]. MH compression uses the thermodynamic properties of an intermetallic
metal alloy which can absorb hydrogen at relatively low temperature and release at a
higher pressure when the hydride is heated.
Advantages of MH compressors
Non-mechanical compressors, including metal hydride compressors, have advantages
over mechanical compressors such as no moving parts, low maintenance, little to no
noise and minimal energy use [36], although diaphragm compressors are twice as
efficient [39] and syringe pumps are more easily automated. Metal hydride compressors
release high purity hydrogen [39-41] since typical contaminants such as water and
oxygen are adsorbed by the MH and not desorbed until the material is heated to a much
higher temperature than required for hydrogen release. MH compressors for research
applications are typically compact, which reduces the requirements and simplifies the
compressor system [36].
MH compressor operation
The first periodically operated MH compressor was patented in 1970 by Wiswall and
Reilly [42]. At low temperature and low pressure, hydrogen is absorbed into the metal,
due to the low equilibrium pressure, during which heat generated by the exothermic
reaction must be removed. Subsequent heating of the MH increases the thermodynamic
equilibrium pressure, releasing hydrogen at a higher pressure.
To achieve sufficient compression of a large quantity of hydrogen, suitable for gas
cylinder replacement or a hydrogen fuel station, for example, industrial MH
compressors often use more than one stage/material, where combinations of different
metal hydrides are used to increase the final compression ratio while maximizing the
absorption pressures of each stage [43]. For example, using ZrNi alloy, hydrogen can be
compressed at very low pressure (0.00013 MPa) [44]. Subsequent stages can use higher
pressure AB5 materials, which can be tailored by substitution of the A or B element
depending on the desirable properties [45]. In order for the alloys to reach and maintain
temperatures that correspond to the required equilibrium pressure, it is essential to cool
and heat the alloys during absorption and desorption cycles respectively.
101
For the relatively small quantity of hydrogen required for a laboratory instrument,
amplifying the pressure of hydrogen from several bar to several hundred bar can readily
be performed with a manually operated single stage reservoir of suitable MH material.
The performance of single stage MH compressors can be optimised by finding a MH
material that has a maximum reversible sorption capacity, in order to use less material
and to reduce heat loss due to thermal swing. Hysteresis, where, for the same
temperature, absorption occurs at a higher equilibrium pressure than desorption, reduces
the efficiency of the compression cycle. Low hysteresis and low plateau slope optimise
the compressor capacity and efficiency [34].
The thermodynamics of the metal-hydrogen interaction determines both the heat
generated/needed during absorption/desorption and the pressure amplification for fixed
absorption and desorption temperatures. The low temperature condition required for
absorption and the maximum temperature required for maximum desorption are usually
determined by limitations of the laboratory equipment and ease of use. For example, the
maximum desorption temperature used for the laboratory MH compressor described
here is restricted to 473 K due to the use of Teflon in the seals and this is easily supplied
manually using a hot air gun. Liquid nitrogen is readily available to cool the compressor
to attain a very low absorption temperature. Any MH material which absorbs down to
less than 1 bar at the absorption temperature and desorbs to the desired maximum
pressure at 473 K would be suitable.
In this paper we present an experimental apparatus for the study of the hydrogen
sorption kinetics of hydrogen storage material based on the manometric technique and
TPD/TDS. The instrument uses a metal hydride compressor for hydrogen pressure
amplification and allows the determination of the pressure-composition-temperature
isotherms (PCT), activation energies and kinetics of solid–hydrogen systems in the
temperature range from 77-773 K and hydrogen pressures up to 340 bar.
102
Instrument design
Fig 3: Instrument schematic showing the main components for the preparation,
reference and sample sections.
A schematic of this instrument is shown in Fig 3. Conceptually, the instrument is
divided into three main sections. The preparation section is physically separated for
thermal isolation from the reference section and has connections for vacuum and
hydrogen gas supply as well as any external attachments such as a volume calibration
apparatus and the mass spectrometer. The reference section includes an empty volume
with an internal platinum thermometer, and the pressure transducer. This section is
isolated from the preparation section by valve Vp and from the sample section by valve
Vs. The sample section includes the sample cell, thermocouple and any sample
environments (e.g. furnace, liquid nitrogen bath).
Temperature control
For accurate measurements, the temperature in all parts of the apparatus must be known
[16, 29]. Where the gas temperature is measured at one point in a particular volume, it is
essential to ensure that the rest of the volume is at the same temperature, such that all
103
measured temperatures are in isothermal volumes. In addition, the temperature should
not vary significantly with time, requiring a temperature controlled environment.
Whenever gas is released from one volume to another, temperature changes occur due
to the adiabatic gas expansion and the temperature must be allowed to equilibrate before
measurements can be made. The use of stainless steel, with relatively low thermal
conductivity, for valves, pipe and fittings can make temperature equilibration time
consuming.
In this instrument, temperature is measured separately in the main gas volume of the
reference section and inside the sample cell, where the sample is contained. To maintain
a constant temperature and to remove or re-distribute heat generated in the gas handling
processes, the laboratory is controlled at 293 ± 0.1 K and constant temperature water
circulating through the reference volume backplane is used for active thermal
management. The physical layout of the instrument helps to reduce the effect of heating
or cooling the MH compressor on the reference and cell volumes.
Leak management
Leak integrity in manometric instruments is vital to the accuracy of resulting
measurements. For example, any leak out of the sample cell not only affects the current
measurement, but continues while the reference volume is being prepared again and will
affect the next measurement. A drop in pressure in the sample cell may cause desorption
– which for a sample exhibiting hysteresis will make the measurements non-
equilibrium. In addition, the successive accumulation of both data and errors in the
Sieverts technique amplifies any errors due to leaks in a single measurement [29].
The use of components with metal seals tightened to the correct specifications reduces
the possibility of leaks. Leak testing prior to the commencement of a measurement is
typically performed by filling the system with hydrogen gas to ~50 bar, and using a
hydrogen leak detector (Hydrogen Leak Locator H1000, AS90100, with a resolution of
0.00001 cc/s) to determine the existence of any leak to the atmosphere. In addition, each
section is individually checked by applying a hydrogen gas pressure close to the
maximum pressure to be used and monitoring the pressure over time duration similar to
the experiment to be performed. A pressure drop of less than the accuracy of the
pressure transducer (ie no experimentally observable leak) over that period ensures no
additional measurement error is introduced. This ensures that any leak is too small to
affect the experiment.
104
Preparation section
The preparation section includes the MH compressor, connections for gas input and
vacuum and the requirements for TPD and TDS.
Vacuum is provided by a Pfeiffer turbo molecular pump (model TMH 071 P). The flow
of the gas to the vacuum system is regulated by a needle valve (VN), which is used to
reduce vacuum capacity when a small back-pressure is desirable. A vacuum gauge, 925
MicroPirani Transducer, is mounted in the preparation section to monitor the release of
the hydrogen gas out of the sample.
A MH compressor employing an AB5 alloy in a 150cc Swagelok stainless steel volume
provides hydrogen during absorption measurements and recycles hydrogen during
desorption (except for TPD/TDS experiments). The volume has a rated pressure of 340
bar and contains approximately 400 g of MmNi4.35Co0.8Al0.05 (Japan Metals and
Chemicals Co.). Hydrogen is absorbed into the MH compressor alloy by cooling the
reservoir in a liquid nitrogen bath, reducing the pressure below 2 bar and is released by
heating the volume with a heat gun. Hydrogen pressures of 340 bar are readily
achievable.
Reference section
A custom made chrome-plated CuBe volume forms the primary volume in the reference
section. Gas temperature in this volume is measured using a platinum resistance
thermometer (PRT) silver soldered into a plug in the base. An NPT to Sitec adapter
connects the volume to the backplane. A Quartzdyne pressure gauge (model DSB301-
05-C85) with accuracy 0.015% FSR (340 bar) provides accurate pressure measurements
in this volume.
Sample section
The sample cell consists of a male-male VCR4 union. A VCR4 cap with a silver-
soldered thermocouple seals the bottom of the cell using a silver plated stainless steel
gasket to withstand high temperatures (to 773 K). The cell connects to the sample stick
using a silver plated stainless steel filter gasket (500 nm filter) to contain the sample
during pressure changes and avoid contaminating the system.
The components for all sections of the instrument, excluding the MH reservoir, are rated
to a pressure of at least 1000 bar. The pressure gauge is chosen to be the lowest pressure
105
range that satisfies the requirements of the experiment in order to maintain the highest
accuracy [25] and this determines the pressure limit of the current instrument.
Sample environment
For different sample temperatures above ambient, the sample cell can be immersed in a
custom furnace with a maximum temperature of 773 K. The furnace temperature is
maintained by a programmable PID controller (Eurotherm 2404), and has a uniform
temperature zone of 140 mm and a stability of ± 0.5 K. For measurements at a
cryogenic temperature, a liquid nitrogen bath is available.
Experimental procedure
For loading and unloading the sample, the sample cell is detached above the valve Vstick
(see Fig 3) to isolate the sample from the atmosphere during transport to and from the
glovebox. Prior to an experiment the sample (and whole instrument) is evacuated to
~10-6 bar, and any sample preparation, including outgassing, is performed. The sample
cell is then immersed in the required sample environment (furnace or liquid nitrogen).
Temperature and pressure measurements are continuously recorded using a PC-based
custom designed data acquisition program with a graphical interface [37].
Volume calibration
The volumes of the reference volume, Vref, and cell volume, Vcell, must be known
accurately to calculate reliable hydrogen sorption isotherms [17, 46, 47]. To calibrate
the reference volume, the gas expansion method was used [17]. An external, known
calibration volume with a high precision Paroscientific 31K-101 pressure transducer
(accuracy of 0.01 % FSR 70 bar) was connected via the preparation section. The
calibration volume is then loaded with ultra-high purity helium gas and the pressure and
temperature in the calibrated volume is recorded. The mass of gas in this volume is:
i
ca l ca lM n Vρ= (4)
where, Vcal is the calibration volume, ρi is the initial gas density in the calibration
volume and M is the molecular mass of the gas used. The valve is then opened to the
previously evacuated target volume and sufficient time (typically 15 minutes) is allowed
for thermal equilibrium. With the assumption that there is no adsorption on internal
walls of the system components and no leaks, the total amount of free gas is constant
and the unknown volume can be calculated from:
106
i f f
cal cal cal cal T TM n V V Vρ ρ ρ= = + (5)
where the superscript ‘f’’ refers to after the valve is opened and subscript ‘T’ specifies
to the target (unknown) volume. Rearranging Eqn 5 for the preparation volume gives
[ ]i f
cal calT
f f
cal T T
V
V
ρ ρ
ρ ρ= − (6)
from which the unknown volume, VT, can be determined. The procedure is repeated
multiple times to ascertain the uncertainty in the measurement. The effect of any
internal adsorption can be checked by performing the calibration at different pressures.
Subsequent unknown volumes can also be determined by allowing the calibration gas to
expand into each volume in turn, although the uncertainty increases with each additional
volume. Applying the calibration procedure to this instrument using helium gas
determined the values for the preparation and reference volumes shown in Table 1. The
cell volume was also determined in this fashion, but greater accuracy is achieved by
using the empty-cell isotherm method.
Empty-cell isotherm
Although the process of volume calibration is very similar to the process of determining
an isotherm, the differences in the expansion gas (helium) and the pressure transducer
used may result in slightly different measurements. If the same pressure transducer as
used in the actual experiments is used with hydrogen to determine the cell volume, these
differences can be avoided.
Table 1: Ambient temperature volume calibration
Volume Measured (cm3)
preparation 9.45 ± 0.03
Reference 12.33 ± 0.06
Sample cell 2.61 ± 0.08
107
By performing the equivalent of a PCT isotherm at room temperature, but with no
sample present, it is possible to determine both the volume of the empty cell and the
volume occupied by the valve, Vs. Eqn 3 describes the uptake after n pressure steps in
an isotherm calculation. In this equation, the value of volume Vref is determined from
the previous helium volume calibration and is necessarily the volume with the valve Vs
closed. The volume Vcell is also defined as the volume of the sample section with the
valve Vs closed.
For an empty cell, and in the absence of leaks, the mass uptake of hydrogen should be
zero, and Eqn 3 becomes:
, ,
1, ,1 1
0
k b k an n
ref ref refn k
abs absk a k a
k kcell cell cell
Vm M n
V
ρ ρ
ρ ρ−= =
− = ∆ = =
+ − (7)
Rearranging Eqn 7 gives:
( ) ( ),
1
, 1, ,
1
k b k a k a k a cellref ref cell cell
re
n n
k fk
V
Vρ ρ ρ ρ
= =
−= −−− (8)
Plotting ( ),
1
,k b k a
ref ref
n
k
ρ ρ=
− as a function of ( )1
1, ,k a k a
cell c
k
ell
n
ρ ρ=
− − yields a straight line with
intercept zero and slope cell
ref
V
V− , from which Vcell can be calculated.
In order to use values for Vref and Vcell which assume the valve Vs is closed, it is
necessary for the measurements above to have the valve closed at the end of each
isotherm step before recording the pressure. When closing the valve the gas currently
occupying the valve volume is displaced to either side changing the pressure on both
sides and, requiring a short wait for pressure and temperature stabilisation. For this
reason the valve should be closed as slowly as possible allowing the gas pressure to
equalise as much as possible before passage through the valve is stopped.
If the pressure is also measured before the valve is closed, it is possible in principle to
determine the volume of the valve between the reference and cell volumes, as the total
volume occupied by the gas must be the reference and cell volumes plus the valve
volume.
108
( )
( )
( )
( )
, , 1, ,
, ,
1 1
1 1
k b k o k a k on n
k k
ref ref cell cell
cell v
k o k o ref refv v
n n
k k
V V
V V
ρ ρ ρ ρ
ρ ρ
=
=
−
=
=
= −
− −
(9)
Where the superscript o designates the valve is open, such that ,k o
cellρ represents the
density of hydrogen in the cell at the end of the kth isotherm step while the valve is still
open, and ,k b
refρ is the density of gas in the reference volume before the valve has been
opened. For constant cell, valve and reference volumes, plotting ( )
( )
,
1
,
,
1
n
k
k b k o
ref ref
k on
k
v
ρ ρ
ρ
=
=
−
as a
function of ( )
( )
1, ,
1
1
,
k a k on
ce
k
n
k
ll cell
k o
v
ρ ρ
ρ
−
=
=
−
yields a straight line with slope cell
ref
V
V and intercept v
ref
V
V− .
An empty-cell isotherm was performed and the results of plotting Eqn 9 are shown in
Fig 4 The straightness of the line (R2 = 0.999997) confirms the validity of the technique
and the slope and intercept have been used to determine the cell and valve volume
shown in Table 2. The value for the valve volume Vv, has a high uncertainty due to the
uncertainty in the intercept and the small volume, and may not provide useful
information, but the slope, which yields the cell volume, is more accurate.
Table 2: Ambient temperature empty cell isotherm
Volume Measured (cm3)
Vcell 2.606 ± 0.004
Vv 0.014 ± 0.045
109
Fig 4: a straight line generated from the isotherm for hydrogen adsorption of an
empty cell @296 K and to 180 bar.
Divided Volume Calibration
The calibration techniques described above provide ambient temperature values for the
two critical volumes, Vref and Vcell. Temperature uniformity in Vref is maintained at
ambient by the room temperature control and the temperature controlled backplane. The
sample cell is often used at a variety of non-ambient temperatures, which can result in a
temperature gradient in the sample cell. As this is difficult to accurately measure, the
divided volume calibration technique [17] is used.
110
Fig 5: Divided-volume schematic of temperature distribution in the sample cell
volume.
Fig 5, shows the sample cell volume conceptually divided into two volumes, Vamb at
temperature Tamb (normally the room temperature) and Vsam at Tsam (the sample working
temperature as provided by a furnace or other heating or cooling facility) such that:
cell amb samV V V= + (10)
The divided volume calibration procedure starts with a known amount of hydrogen gas
at ambient temperature in the reference and cell volumes. The mass of hydrogen in the
system is given by:
( )i
H ref cellMn V Vρ= +
(11)
The temperature of the sample cell is then increased in steps from ambient to the highest
temperature to be used (in this case 773 K) and after the temperature has equilibrated
the pressure is measured. As the total amount of gas does not change, Eqn 11 becomes:
( )i
H ref cell ref ref sam sam amb ambMn V V V V Vρ ρ ρ ρ= + = + +(12)
111
Rearranging Eqn 12 yields the equation for the ratio of the sample volume to the
ambient volume:
i
sam cell amb
amb sam amb sam
V V
V V
ρ ρ
ρ ρ
= −
(13)
Using Eqns 10 and 13 together with the calibrated ambient temperature value for the
cell volume (Vcell) found from the empty cell calibration, Vsam and Vamb can be
determined. This procedure is repeated several times to ensure a reliable result and to
determine any uncertainty. This technique relies on the accuracy of the assumption that
the temperature gradient in the sample cell can be modelled as two discrete volumes,
one at ambient temperature and one at the sample temperature and that the boundary
between them occurs at the same place, independent of the temperature. For best results,
that part of the volume which is physically subject to the thermal gradient is best kept as
small as possible – such as a pipe with a small internal diameter. If the equipment is
such that the model is invalid, this will show in a poor fit of a straight line to the
measurements at different temperatures. The divided volume calibration was performed
for this instrument for temperatures from 300 K to 773 K, and the results for the
volumes are shown Table 3. The coefficient of determination (R2) was 0.99977.
The divided volume technique works well, provided the identical arrangement and
circumstances apply for both the volume calibration and the isotherm experiments, so it
is important that the same conditions (such as furnace type and positions, liquid
nitrogen bath shape and nitrogen level) are maintained, especially for liquid nitrogen
baths [16].
Table 3: Divided volume calibration in furnace environment
Volume Measured (cm3)
Vamb 1.027 ± 0.008
Vsam 1.580± 0.007
Vcell 2.608± 0.014
112
Instrument validation:
Standard Sample measurements
One method to validate the instrument and the experimental procedure, is to run a
standard material [25] such as LaNi5, to check that the isotherm, is in agreement with
the literature [48].
Fig 6: Absorption (triangles) and desorption (circles) isotherms for LaNi5 at
ambient temperature (296 K).
The LaNi5 isotherm (Fig 6) illustrates the classic phase transformation in AB5
intermetallic compounds, starting with the solid solution α phase followed by a plateau
region during which β hydride phase is formed, and finally a sharp increase in pressure
once the sample is all β-phase [49].
LaNi5 has a fast and reversible sorption of hydrogen at room temperature with a
moderate equilibrium pressure of a few bars and exhibits hysteresis. It has been
extensively studied [50, 51], and modelled [52-54] and provides a suitable material for
initial validation isotherms. However, the high density prevents any check of the
sensitivity of the instrument to inaccurate or uncertain sample density and the fast
kinetics of reversible H2 absorption and desorption of comparatively large amount of H2
limits its ability to validate the instrument [16, 55]. High uptake also fails to adequately
check the sensitivity of the instrument to low-capacity samples. It can, however,
indicate the absence of leaks, the accuracy of the volume calibrations and the suitability
113
of the data processing methods. Low-capacity and low-density materials such as the
light metals [56], graphene [57] and other carbons are needed to further validate the
instrument.
Porous materials such as polymers [58] and the porous carbons are potential low density
hydrogen storage materials [24] and have previously been involved in erroneous
measurements of hydrogen uptake [59]. As such, these provide suitable materials to
perform more challenging validation tests of an instrument.
A guide to the amount of sample required for accurate measurements can be obtained
from the Figure of Merit, η [55].
kS
Pη
δ= (14)
where Sk is the slope of the isochore, which is determined by the volumes and the
quantity of sample, divided by the resolution of the pressure transducer ( Pδ ) . Using
the volumes above and the pressure transducer specification, the LaNi5 sample above
gives a Figure of Merit of 145.11 which is substantially above the value of 100,
suggested by the authors as a rule of thumb for accurate isotherm measurements [55].
The hydrogen adsorption excess isotherm for a commercial activated carbon FiltraSorb-
400 with a density of 1.96 g/cm3 [37], was measured (Fig 7) at ambient temperature
(296 K) to 300 bar. Using the Figure of Merit in Eqn 14, a sample quantity of 0.145 g is
needed for an accurate measurement.
114
Fig 7: H2 adsorption isotherm for a commercial activated carbon(FiltraSorb-400)
obtained at ambient temperature (296 K) and pressures up to 300 bar.
TPD
The apparatus was also tested by running a TPD experiment for MgH2 milled for 20 h
under (10 bar) of H2 pressure (Fig 8). Mg is a light metal which has been investigated
for hydrogen storage applications [60-64]. The TPD studies were carried out following
the procedure in [65].
115
Fig 8: Desorption curve for MgH2 milled 20 h under H2
The MgH2 sample started to desorb at 493K, where the desorption peak occurred at 598
K, which is a typical desorption curve for MgH2 milled for 20 h [18].
TDS
TDS offers the additional advantages of monitoring the release of the gases with respect
to temperature. To demonstrate the use of this system for TDS measurements, a
Dynamic Sampling Mass Spectrometer (DSMS) supplied by HIDEN Analytical was
connected to the instrument, below the vacuum valve seen in Fig 3, to perform a TDS
experiment where a sample of MgH2 + (1 mol%) titanium isopropoxide (TTIP) milled
for 2 h under Ar, was heated to 673 K. This TDS experiment helped to evaluate the
quantities of hydrogen and other gases desorbed from this sample with respect to
temperature.
116
Fig 9: TDS spectra for MgH2 + (1 mol%) TTIP. A, hydrogen (2 amu) , B, oxygen
(16 amu), C, water (18 amu).
Fig 9, shows the TDS spectra of this sample where H2 gas (A) started to desorb at
around 423 K, with a desorption peak at 553 K, confirming earlier desorption than the
MgH2 sample without additives shown in Fig 8 [18]. Other gases such as O2 (16 amu)
(B) had a minimal presence and desorbed in the temperature range (473-573 K). A gas
(C) with mass 18 amu (assumed to be water vapour) was also released in a similar
temperature range of H2. The presence of H2 is expected due to the presence of H2 in
both MgH2 and TTIP (C12H28O4Ti), as well as O2, with other combinations of C, H and
O also possible.
Conclusion
The construction of a Sieverts (manometric gas sorption) apparatus that can measure
PCT isotherms to 340 bar at temperatures from 77 K to 773 K has been reported.
Control of the apparatus opening and closing valves to either evacuate or to introduce
gas to each part of the system is achieved manually and a computer program has been
used to monitor and record data, as well as control the sample environment temperature,
as experiments are performed. Absorption and desorption kinetics measurements of
potential hydrogen storage materials were performed using this apparatus. A
vulnerability of Sieverts technique for errors was mitigated by performing multiple
volume calibrations. A full volume calibration accounted for the different physical
volumes of the apparatus, an empty cell isotherm was performed at ambient temperature
to more accurately confirm the sample cell calibration, and a divided-temperature
117
volume calibration accounted for the temperature changes between the components of
the apparatus in the sample environment.
TPD experiments were performed using this Sievert apparatus, using a temperature
controller to ramp the sample temperature to 773 K. TDS experiments were also
performed by linking the Mass Spectrometer while a TPD experiment is running to
evaluate the desorbed gases from the samples. These validation experiments
demonstrate the applicability of this instrument to measurements of hydrogen uptake
and release, capacity, kinetics and decomposition.
Acknowledgements
KA acknowledges receipt of an Australian Government Research Training Program
Scholarship.
118
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124
STATEMENT OF CONTRIBUTION TO CO-AUTHORED PUBLISHED PAPER
This chapter includes a co-authored paper. The bibliographic details of the
co-authored paper, including all authors, are:
Alsabawi K, Webb TA, Gray EM and Webb CJ.
‘Kinetic enhancement of the sorption properties of MgH2 with the additive titanium
isopropoxide’. International Journal of Hydrogen Energy. 2017;42(8):5227-
34;10.1016/j.ijhydene.2016.12.072.
My contribution to the paper involved:
1) Perform the full experiment
2) Calculations and analysis of the results
3) Manuscript preparation
(Signed) ____________________(Date)_ 12/02/2019________________________
Khadija Alsabawi
(Countersigned) _____________ (Date) 12/02/2019_________________________
Corresponding author of paper: Khadija Alsabawi
(Countersigned) ______________ (Date) 12/02/2019_________________________
Supervisor: Colin Jim Webb
125
CHAPTER FIVE
The effect of TTIP as an additive on the hydrogen sorption properties of MgH2
(Published as a Journal paper in the International Journal of Hydrogen Energy)
5. Introduction
The practical application of MgH2 as a hydrogen storage material for renewable energy
demand/supply smoothing, or for automotive or remote operation, is limited by its
relatively high thermodynamic stability and very slow kinetics. However, the sorption
kinetics of MgH2 can be significantly improved by ball-milling or other severe
deformation techniques and by mixing with additives such as metal oxides and other
materials. In particular, the use of transition metal oxides has been shown to enhance
the kinetics of the Mg-H reaction, by facilitating the dissociation of the hydrogen
molecule into atomic hydrogen, thus increasing the absorption desorption rates. Nb2O5
is the classic oxide additive, which demonstrably enhances both absorption and
desorption as well as reducing the re-agglomeration of Mg/MgH2 with repeated cycling.
The use of a small amount of Nb2O5 – 0.5 mol% has been shown to be sufficient -
milled for short time has been extensively studied and the kinetic enhancement
confirmed [1, 2]. It has been reported that the beneficial effect of Nb2O5 into MgH2 was
due to the formation of new ternary oxide phase MgNbxOy [3, 4], although other
additives, including TiN, TiC [5] , vanadium oxides [6, 7] , and chlorides [8, 9] and
fluorides [10, 11], and many more [12], have also been effective, which are not
expected to form ternary oxides.
In this chapter, a titanium-based oxide liquid precursor, titanium isopropoxide (TTIP)
as an additive to MgH2 was investigated. TTIP is similar to Nb2O5, in that both are
transition metal-based and contain a large number of oxygen atoms per transition metal
atom (O/Nb ratio of 2.5, cf O/Ti ratio of 4). The differences – a different transition
metal with a different interaction with magnesium as well as the liquid property –
provide an opportunity to study the properties which contribute to the kinetic
enhancement, and may provide a commercial advantage if the additive is employed in a
practical application. TTIP had not been used previously for the purpose of enhancing
the kinetics of MgH2. A study of the kinetic effects of using the organo-metallic oxide
126
liquid, TTIP, as a catalytic additive on the sorption kinetics of MgH2 was made.
Different amounts of TTIP up to 2 mol% milled with MgH2 for different ball milling
times (0.5- 10 h), as well as hand mixing, were investigated and compared to the
benchmark Nb2O5. A Sievert apparatus (described in chapter 4) was used to perform the
kinetics measurements. The desorption kinetics were measured by performing a TPD
experiment and this was followed by a single pressure aliquot of hydrogen to measure
the absorption kinetics. It was found the TTIP was just as effective as Nb2O5. The
question of how this material assists the kinetic processes in both absorption and
desorption of MgH2 remains. Full details of the experiment and the results and
conclusions are given in the attached paper.
5.1. References
1. Barkhordarian G, Klassen T and Bormann R. Fast hydrogen sorption kinetics of
nanocrystalline Mg using Nb2O5 as catalyst. Scripta Materialia. 2003;49(3):213-
7;10.1016/s1359-6462(03)00259-8.
2. Friedrichs O, Klassen T, Sanchez-Lopez JC, Bormann R and Fernandez A.
Hydrogen sorption improvement of nanocrystalline MgH2 by Nb2O5
nanoparticles. Scripta Materialia. 2006;54(7):1293-
7;10.1016/j.scriptamat.2005.12.011.
3. Friedrichs O, Agueyzinsou F, Fernandez J, Sanchezlopez J, Justo A, Klassen T,
et al. MgH with NbO as additive, for hydrogen storage: Chemical, structural and
kinetic behavior with heating. Acta Materialia. 2006;54(1):105-
10;10.1016/j.actamat.2005.08.024.
4. Patah A, Takasaki A and Szmyd JS. Influence of multiple oxide (Cr2O3/Nb2O5)
addition on the sorption kinetics of MgH2. International Journal of Hydrogen
Energy. 2009;34(7):3032-7;10.1016/j.ijhydene.2009.01.086.
5. Pitt MP, Paskevicius M, Webb CJ, Sheppard DA, Buckley CE and Gray EM.
The synthesis of nanoscopic Ti based alloys and their effects on the MgH2
system compared with the MgH2 + 0.01Nb2O5 benchmark. International Journal
of Hydrogen Energy. 2012;37(5):4227-37;10.1016/j.ijhydene.2011.11.114.
6. Oelerich W, Klassen T and Bormann R. Metal oxides as catalysts for improved
hydrogen sorption in nanocrystalline Mg-based materials. Journal of Alloys and
Compounds. 2001;315(1-2):237-42;10.1016/s0925-8388(00)01284-6.
127
7. Grigorova E, Khristov M, Peshev P, Nihtianova D, Veliehkova N and Atanasova
G. Hydrogen sorption properties of a MgH2 - V2O5 composite prepared by ball
milling. Bulgarian Chemical Communications. 2013;45(3):280-7.
8. Ivanov E, Konstanchuk I, Bokhonov B and Boldyrev V. Hydrogen interaction
with mechanically alloyed magnesium–salt composite materials. Journal of
alloys and compounds. 2003;359(1-2):320-5.
9. Korablov D, Nielsen TK, Besenbacher F and Jensen TR. Mechanism and
kinetics of early transition metal hydrides, oxides, and chlorides to enhance
hydrogen release and uptake properties of MgH2. Powder Diffraction.
2015;30(S1):S9-S15.
10. Malka I, Bystrzycki J, Płociński T and Czujko T. Microstructure and hydrogen
storage capacity of magnesium hydride with zirconium and niobium fluoride
additives after cyclic loading. Journal of Alloys and Compounds.
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11. Jin S-A, Shim J-H, Cho YW and Yi K-W. Dehydrogenation and hydrogenation
characteristics of MgH2 with transition metal fluorides. Journal of Power
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storage material. Journal of Physics and Chemistry of Solids. 2015;84:96-
106;10.1016/j.jpcs.2014.06.014.
5.2. TTIP paper:
Alsabawi K, Webb TA, Gray EM, Webb CJ. Kinetic enhancement of the sorption
properties of MgH2 with the additive titanium isopropoxide. International Journal of
Hydrogen Energy, 2017; 42(8): 5227-34)
Kinetic enhancement of the sorption properties ofMgH2 with the additive titanium isopropoxide
K. Alsabawi, T.A. Webb, E.MacA. Gray, C.J. Webb*
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan 4111, Brisbane, Australia
a r t i c l e i n f o
Article history:
Received 7 September 2016
Received in revised form
12 December 2016
Accepted 15 December 2016
Available online 12 January 2017
Keywords:
Magnesium hydride
Hydrogen storage
Catalyst
Sorption kinetics
a b s t r a c t
The effect of titanium isopropoxide (TTIP) on the kinetics of absorption and desorption of
magnesium hydride was compared to the benchmark additive niobium oxide (Nb2O5). A
systematic survey was performed for ball-milled MgH2 mixed with up to 2 mol% TTIP or
Nb2O5 additive for different ball-milling times. It was found that TTIP was just as effective
as Nb2O5 for the enhancement of the kinetics of absorption and desorption and that 0.5 mol
% was sufficient. The TTIP composite sample had superior hydrogen capacity compared to
the Nb2O5 composite sample. As TTIP is a liquid at room temperature, 30 min ball-milling
with the magnesium hydride was sufficient time for mixing. Hand mixing the TTIP with
MgH2 demonstrated excellent desorption kinetics but the absorption kinetics were poorer
compared to ball-milling.
Crown Copyright © 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publica-
tions LLC. All rights reserved.
Introduction
Magnesium is a potential hydrogen storage material, primar-
ily because of a moderate H-storage capacity of 7.6 wt% and
110 g H/l. The abundance of magnesium in the form of salts in
the earth's crust and oceans enables future high volume
implementation of magnesium-based technologies [1]. Un-
fortunately, the practical use of Mg is limited by poor
hydrogen sorption kinetics and the high dissociation tem-
perature of MgH2 [2]. Attempts have been made to overcome
these problems through ball-milling, alloying and the use of
small amounts of additives [3e5].
Both the absorption of hydrogen by magnesium and
desorption of magnesium hydride occur through a number of
sequential steps, where the slowest process determines the
overall kinetic rate. The desorption of magnesium hydride
starts with nucleation of the metal phase, diffusion of
hydrogen atoms through the material, and subsequent
recombination at the surface to form molecular hydrogen
[6,7]. Finally, the hydrogen molecule must desorb from the
surface of the material back into the gas phase.
Similarly, the steps for absorption of hydrogen by magne-
sium are physisorption of hydrogen gas on to the surface of
the material, dissociation of the hydrogen molecule, diffusion
of atomic hydrogen into the bulk material and ultimately
nucleation of the hydride phase. As the absorption proceeds,
hydrogen atoms must diffuse through more of the hydride
layer to reach the magnesium. Diffusion of hydrogen through
magnesium hydride is known to be slow (�2.5 � 10�9 cm2/s at
350 �C) [8], approximately three orders of magnitude slower
than hydrogen diffusion through magnesium [4,9] and diffu-
sion is often considered to be the rate-limiting step for both
desorption and absorption [2,10,11].
Mechanical milling and severe plastic deformation tech-
niques have been shown to improve the kinetics of the
* Corresponding author. Fax: þ61 7 3735 7656.E-mail address: [email protected] (C.J. Webb).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier .com/locate/he
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 4
http://dx.doi.org/10.1016/j.ijhydene.2016.12.072
0360-3199/Crown Copyright © 2016 Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC. All rights reserved.
absorption and desorption reactions through reduction of
particle size, mixing of the surface oxide layer, and introduc-
tion of defects [4,12]. The reduction in external particle size as
well as crystallite size reduces diffusion path lengths through
the bulk material. Reducing the surface oxide layer and the
introduction of defects and grain boundaries allow faster
diffusion paths for hydrogen [2,5].
The hydrogen sorption kinetics of the MgeH system have
been shown to significantly improve further with the combi-
nation of small amounts of additives, usually <10 mol%. Of
these, compounds of transition metals, particularly oxides,
but also halides have been shown to be effective [2,7]. In
particular Nb2O5 [13e15] is generally recognised as a bench-
mark material for the best enhancement of absorption and
desorption kinetics. Barkhordarian et al. were the first to
report the superior catalytic effect of Nb2O5 in improving the
kinetics of MgH2, compared to other metal-oxide additives
[15,16]. It has been reported that the Nb2O5 additive was
partially reduced during milling with MgH2 and then further
reduced after cycling [17e19]. The partially reduced Nb2O5
forms MgO and other Nb compounds, and Nb2O5 reacts with
MgO during cycling, forming new ternary oxides [20e22]. It
was suggested that the reduced Nb-containing compounds
decreased the activation energy for hydrogen desorption of
the catalysed MgH2 and that the activation energy decreased
with increasing milling time [18,23,24]. The mechanism by
which the kinetic enhancement takes place is not well un-
derstood and knowledge of this mechanism would guide the
development of new additives.
Barkhordarian et al. [25] compared the effect of adding
catalyst (Nb2O5) to MgH2 on the rate-limiting step of the ab-
sorption/desorption reaction by fitting the respective kinetic
equations.They found that the rate-limiting step for desorption
changedwith increasingNb2O5contentormilling time, fromre-
association to interface velocity of the transformed phase.
In subsequent desorption studies, the effects of nano- and
micro-sized Nb2O5 were compared and it was found that for
short milling times the desorption kinetic behaviour of the
nanoNb2O5 catalysedMgH2was about 10 times faster than the
micro [19]. The smaller nano-sized Nb2O5 particles increased
the specific surface area of the interface between the additive
and MgH2, which increased the number of nucleation sites for
the reacted Mg phase [26]. Ball milling also helped to create
defects and disperse Nb2O5 through the Mg matrix, providing
easy pathways for hydrogen atoms to diffuse through the Mg
particle core [20]. Dolci et al. [27] compared three different
MgeNbeO phases and found that the Mg3Nb6O11 phase
showed a reversible interaction with molecular hydrogen.
They then hypothesized that this was due to the occurrence of
octahedral niobium clusters which were also observed for
NbO. Following this work, Rahman et al. [28] confirmed the
presence of an octahedral structure and suggested that it acts
as an insertion site for hydrogen uptake and release. In a later
study they showed that Mg3Nb6O11 had an active reversible
interaction with molecular hydrogen occurring at around
395 �C [29]. A more recent study [30] has suggested that the
presence of MgO after milling Nb2O5eMgH2 had no effect in
improving the sorption kinetics of the sample. The improve-
ment was attributed to Nb, supporting the Nb-gateway model
[31], where Nb clusters facilitate hydrogen transportation out
of the MgH2 particles.
Titanium based additives have also been successfully used
to improveMgH2 hydrogen sorption kinetics, exhibiting a high
affinity towards hydrogen at moderate temperatures. The
desorption of composite samples synthesised from mechan-
icallymilled Ti andMgH2 showed a rapid hydrogen desorption
at temperatures above 250 �C and absorption at temperatures
as low as 29 �C, when compared to other additives such asMn,
Fe, and Ni [32]. A comparison between the addition of Ti and
NieTi catalysts toMgH2was conducted by Lu et al. [33], finding
that Ti significantly decreased the desorption temperatures
but that the Ni/Ti-doped MgH2 composites had the lowest
desorption temperature (ca 200 �C).Wang et al. [34] used ball milling to produce a MgeTiO2
composite which showed superior kinetics at 350 �C under
20 bar of H2 and a high hydrogen capacity attributed to
effective dispersion of the catalyst in the Mg matrix as well as
the existence of n-type TiO2 rutile. Different forms of TiO2
(rutile, anatase and commercial P25) were investigated by Jung
et al. [35] to determine the hydriding kinetics of Mg as well as
the hydrogen storage capacity. They found that, of the TiO2
samples studied, the nano-composite MgH2-(rutile)0.05showed the greatest kinetic improvement [36] absorbing
4.4 wt% hydrogen at 300 �C which was 10 times faster than
that of ball milled MgH2 without additives.
Dobrovolsky et al. [37] mechanically milled (50 wt%) TiB2
with (50 wt%) MgH2 and found that the addition of TiB2
decreased the dissociation temperature of the MgH2 by about
50 �C. This decrease in the temperature was ascribed to the
catalytic effect of TiB2 surface particles on hydrogen desorp-
tion occurring on the surface of the MgH2 particles.
Pitt et al. [38] investigated the effect of nanoscopic Ti-based
additives (Ti, TiC, TiN, TiB2 and TiO2) on MgH2 sorption ki-
netics and hydrogen release temperature. They concluded
that all the Ti-based additives studied produced a strong effect
on hydrogenation kinetics, with fast (ca 20 s) absorption to 90%
of capacity in all samples. The effects of various titanium
compounds (TiF3, TiCl3, TiO2, TiN and TiH2) on the hydrogen
sorption kinetics of MgH2 were examined by Ma et al. [39].
They found that among these compounds the addition of TiF3had the most pronounced improvement on both absorption
and desorption rates. The kinetics of the TiF3 doped sample
was almost double that of TiCl3, TiO2 and TiN samples and
over 10 times that of adding TiH2.
Hence, small amounts (~1 mol%) of both Nb2O5 and Ti
compounds are known to contribute significantly to the
enhancement of the kinetics of the MgeH reaction. Transition
metals facilitate the dissociation of the hydrogen molecule
into atomic hydrogen promoting higher rates for absorption
and desorption, however dissociation and recombination are
not usually the rate-limiting steps [13].
The oxide layer on magnesium is expected to retard the
kinetics of absorption due to the slow diffusion of hydrogen
through surfaceMgO layers [40]. One of the advantages of ball-
milling is to provide fresh non-oxided surfaces for hydrogen
reaction [41]. Despite this, oxides such as Nb2O5 and TiO2 are
among the best additives for improvement ofMgH2 absorption
and desorption kinetics. MgOwas used as an additive by Ares-
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 45228
Fernandez et al. [42] and this showed improvement of the
desorption kinetics but not absorption.
It has been suggested that metal oxides with multiple
valence states have a greater catalytic effect due to improve-
ment of the electronic exchange reaction with hydrogen
molecules [15], however when Nb2O5 was added to MgH2 that
had been ball-milled to grain sizes less than 1.5 nm, no
enhancement was found [43]. These seemingly contradictory
findings indicate that the underlying mechanisms by which
the kinetics are improved are not yet understood.
Titanium isopropoxide, also called titanium tetraisoprop-
oxide (TTIP) is an organic alkoxide of titanium containing Ti
and oxygen as well as carbon and hydrogen (Ti[OCH(CH3)2]4)
which is a liquid at room temperature. For each Ti atom, there
are four oxygen atoms. This material has been used as an
additive to improve the hydrogen sorption kinetics of the
LiBH4eMgH2 reactive hydride composite [44e46], but has not
been used previously with MgH2. TTIP is unlikely to form a
ternary oxide (as suggested for Nb2O5) as Ti and Mg are
immiscible elements in thermodynamic equilibrium due to
their positive enthalpy of mixing, and do not form any ther-
modynamically stable alloy or compound. The enthalpy of
mixing for Mg0.5Ti0.05 mixture is DHmix z 20 kJ/g-atom [47,48].
This work compares the effects of titanium isopropoxide
and Nb2O5 on the sorption kinetic properties of MgH2. A sys-
tematic study of the amount of additive and the milling time
was employed to determine the existence of any link between
these parameters and the desorption properties. The choice of
an additive with some similar properties to Nb2O5 (transition
metal and oxygen) but with marked differences (liquid) may
enable comparisons which facilitate a better understanding of
the underlying mechanisms.
Experimental
Magnesium hydride (MgH2) powder (98% purity, AB109434)
was purchased from ABCR GmbH & Co. KG. Titanium iso-
propoxide (99.999% purity) from ALDRICH and 20 nm Nb2O5
from NovaCentrix were used for the additives. The MgH2 was
pre-milled in a PQ-N04 planetary mill (Across International)
for 20 h using a 50 ml zirconia vial and 10 zirconia balls
(10 mm) with a ball to powder ratio (bpr) of 21:1 and a vial
filling factor [49] of 17.4%. Samples were prepared in 5 g
batches under 1 bar of Ar. Different amounts of TTIP from
0.5 mol% to 2 mol% were subsequently milled with the pre-
milled MgH2 for 2 h. In addition, the 1 mol% TTIP was milled
with the pre-milledMgH2 for different times from 0.5 h to 10 h,
(1 g sample, 6 ZrO2 balls, 600 rpm, 21:1 bpr, vial filling factor
7.7%).
Similar compositions andmilling times were employed for
the Nb2O5 composite samples for comparison purposes. All
materials were handled in a dry Ar atmosphere MBraun glo-
vebox with O2 and H2O levels below 1 ppm.
Hydrogen cycling was performed on a modified Sieverts
apparatus constructed from commercial VCR and Sitec com-
ponents. Samples were loaded under Ar into a stainless steel
sample cell, sealed and transferred to the gas apparatus. A K-
type thermocouple internal to the sample cell was used to
monitor the temperature of the sample. A MicroPirani 925
vacuum gauge was used to record the pressure of the released
hydrogen. Temperature Programmed Desorption (TPD)
studies were carried out on the as-prepared sample under
dynamic vacuum from room temperature to 400 �C with a
temperature ramp-rate of 2 �C/min. Baseline starting vacuum
was typically 3 � 10�6 mbar. The sample remained at 400 �Cfor 2 h to ensure complete desorption. Absorption kinetics
data were recorded at 230 �C, after a single hydrogen pressure
aliquot to reach an initial pressure of 50 bar on the sample. A
Quartzdyne DL series pressure sensor with an overall accu-
racy of 0.015% FSR (340 bar) was used to measure the subse-
quent pressure reduction during absorption.
A PANalytical Empyrean X-ray diffractometer was used to
characterize the samples before and after cycling. Samples
were loaded into glass capillaries to collect powder X-ray
diffraction (XRD) data. A silver source with a wavelength of
0.5594�Awas used, with the diffractometer in DebyeeScherrer
geometry. Topas Academic v5 fitting software was used to
analyse the diffraction data. The crystallite size and strain
analysis was determined using Topas Academic's built in
models, which are based on the Scherrer equation and strain
models from Balzar [50].
Results and discussion
The prepared samples were first desorbed in the TPD appa-
ratus under vacuum and subsequently absorbed hydrogen
under 50 bar at 230 �C. Samples with 1 mol% additive were
prepared with a range of ball-milling times, and samples with
a range of additive amounts were ball-milled for 2 h in order to
investigate the effect of ball-milling duration and additive
amount. Previous studies have shown that maxima in the
enhancement of kinetics of MgH2 desorption can be observed
for both of these parameters [51].
Effect of amount of additive
The desorption kinetics of MgH2with varying amounts of TTIP
additive ball-milled for 2 h are shown in Fig. 1 together with
data for the Nb2O5 composite samples. It can be seen that all
the additives produce a substantial improvement (~80 �C) indesorption kinetics compared to the MgH2 ball milled for 20 h
without additives, although the desorption is spread over a
greater range of temperatures. For both TTIP and Nb2O5
composite samples, 0.5 mol% is sufficient to produce the
improvement in desorption kinetics. In addition, within the
experimental uncertainties, the results for TTIP and Nb2O5 are
essentially the same. Some gas release can be seen for the
TTIP samples as low as 130 �C. This was only observed for the
first cycle of TTIP composite samples and is assumed to be
decomposition of the TTIP.
Effect of ball-milling time
In order to investigate the effect of milling time on the
desorption kinetics, 1 mol% of TTIP was milled with MgH2 for
times from0.5 h to 10 h. Nb2O5 samplesweremilled for 2 h and
10 h to provide a comparison.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 4 5229
Fig. 2 shows the desorption kinetics for these samples.
Again the addition of 1 mol% of additive to the pre-milled
MgH2 produces a substantial improvement in the desorption
kinetics as shown by the earlier onset of the desorption
compared to theMgH2 without additives. There is only a small
improvement with additional milling for both additives and
thus 2 h is sufficientmilling time for bothNb2O5 and TTIP. This
agrees with previous findings for Nb2O5 [52]. The desorption
kinetics for both TTIP and Nb2O5 at both 2 h and 10 h milling
are effectively the same.
Also shown in Fig. 2 is a samplewhere theTTIP additivewas
hand mixed with ball-milled MgH2 (TTIP HM). The desorption
result for this sample is at least asgoodasanyof theball-milled
samples. This sample was made by adding the TTIP liquid to
the pre milled MgH2 in a mortar and pestle and mixing for
about 10 min, when the composite sample appeared uniform.
Fig. 1 e Desorption kinetics of pre-milled MgH2 ball-milled for 2 h with varying amounts of TTIP and Nb2O5.
Fig. 2 e Desorption kinetics of pre-milled MgH2 with 1 mol% additive milled for 0.5e10 h. Results for Nb2O5 are shown for
comparison.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 45230
Absorption
Fig. 3 shows the effect on the absorption kinetics of the
different amounts of additives milled as well as different ball-
milling times, along with the pre milled MgH2 without addi-
tives. The pre-milled MgH2 without additives has the highest
hydrogen capacity but the slowest kinetic rate. All the sam-
ples with additives have significantly improved kinetics but
with hydrogen capacity lower than the pure MgH2 sample,
showing that the additives used (TTIP or Nb2O5) make a sig-
nificant enhancement to the absorption kinetics. The reduc-
tion in the gravimetric hydrogen capacity for the composite
samples is partly due to the addition of material which does
not contribute to the hydrogen absorption. All of the com-
posite samples with ball-milled additives produce similar re-
sults, other than themaximum capacity and, in particular, the
Fig. 3 e Absorption kinetics of all the composite samples (pre-milled MgH2 with TTIP or Nb2O5). Ball-milling times varied
from 0 to 2 h and the amount of additive from 0.5 mol% to 1.5 mol%.
Fig. 4 e Desorption kinetics of cycled pre-milled MgH2 with 1 mol% TTIP milled for 0.5 h compared to 1 mol% Nb2O5 milled
for 2 h and pre-milled MgH2 without additives over five desorption/absorption cycles.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 4 5231
uptake slopes for the first minute of the TTIP and Nb2O5 ad-
ditive samples are not distinguishable, indicating that the
TTIP additive performs equally well to the Nb2O5 additive for
absorption. In addition, the absorption kinetics are not
improved with greater quantities of the additive e for both
cases, 0.5 mol% of additive is sufficient to produce the kinetic
enhancement. Interestingly, the hand-mixed TTIP sample
showed a poorer result than the ball-milled additives, despite
improvement in the desorption kinetics.
The effect of cycling
Two TTIP composite samples with one of the best combina-
tions of milling and quantity of TTIP (1 mol% TTIP milled for
0.5 h and 1 mol% TTIP hand mixed) were cycled a total of five
times and the results of the desorption kinetics were
compared to the 1 mol% Nb2O5 composite sample.
The cycling desorption data for MgH2þ 1 mol% TTIP com-
posite sample milled for 0.5 h compared to MgH2þ 1 mol%
Nb2O5 are shown in Fig. 4. Pre-milled MgH2 without additives
is also shown for comparison. While the desorption kinetics
formagnesiumhydridewithout additivesworsenwith cycling
(primarily on the first cycle), both Nb2O5 and TTIP show at
worst only a small deterioration. This has been previously
observed for Nb2O5 [4,53].
Without additives, MgH2 desorption kinetics deteriorate
significantly due to re-crystallisation, increasing the grain size
as shown in Table 1 fromX-ray diffraction data ofMgH2milled
under hydrogen for varying times and then cycled.
The cycling desorption data for handmixed TTIP compared
to (1 mol%) Nb2O5 are shown in Fig. 5. There is a small dete-
rioration for both additives and little difference between the
two additives after the first two cycles. The absorption for the
hand-mixed sample, which was poorer than the ball-milled
samples, also showed little change after cycling.
Conclusion
The effect of ball milling of small amounts of TTIP (0.5e2 mol
%), with 20 h pre-milled MgH2 was systematically surveyed by
milling the samples for different times from 0 to 10 h under
1 bar of Ar pressure. To investigate the kinetics of absorption
and desorption of the samples, thermal desorption under
vacuum followed by a single absorption pressure step at 50 bar
of H2 at 230 �C was employed. The results were compared to
the benchmark Nb2O5 additive. It was found that TTIP had a
positive effect on the absorption as well as the desorption
kinetics.
The use of Nb2O5 additives improved the sorption kinetics
of the MgH2, whichmatches the body of literature, confirming
that Nb2O5 is one of the best additives for MgH2. Equivalent
results were observed when comparing the effect of TTIP on
MgH2 with the effect of Nb2O5. As for Nb2O5, the addition of
only 0.5 mol% of TTIP is found to be sufficient to enhance the
kinetics of MgH2. TTIP liquid mixes readily with the MgH2 and
the handmixed sample produced the best desorption kinetics,
Table 1 e Crystallite size of milled MgH2 and before andafter cycling: determined from X-ray diffraction.
Timeh
Samples gMgH2
wt%bMgH2
wt%Scherrer size
nm
20 MgH2 12.4 85.7 13.4
22 MgH2 12.1 85.7 13.1
25 MgH2 12.2 86.2 12.1
30 MgH2 11.7 85.6 11.4
40 MgH2 11.1 86.8 11.4
20 MgH2 cycled 5 times e 89.03 102
MgH2 “as received” 225.5
Fig. 5 e Desorption kinetics of cycled pre-milled MgH2þ 1mol% TTIP handmixed (HM) compared to 1 mol% Nb2O5 milled for
2 h over five desorption/absorption cycles.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 45232
but the absorption kinetics were poorer compared to ball-
milling. TTIP had higher capacities than the Nb2O5 due to
the lower amount of additive required and the lower weight of
Ti compared to Nb.
While this study has shown that TTIP produces equally
good results as Nb2O5 when these compounds are used as
additives for the enhancement of kinetics in MgH2, it is hoped
that the real value of such a study is in increasing the infor-
mation available which may facilitate a future understanding
of the mechanism by which the kinetics are enhanced.
The underlying reason for enhancement of the absorption
and desorption kinetics remains unclear. Studies of the effect
of particle size reduction on Nb2O5 additive suggest that
dispersion throughout the MgH2 is important and TTIP, as a
liquid above 17 �C, may disperse more readily than most
powders. However, different ternary oxides such as
Mg3Nb6O11 or other phases that facilitate H2 pathways or
reduce activation energies are less likely in the case of the
TTIP additive. Further studies with different additives as well
as in-situ studies are needed to help establish the mecha-
nisms by which small amounts of additive significantly
improve the kinetics of absorption and desorption.
Acknowledgements
KA acknowledges receipt of an Australian Postgraduate
Award scholarship.
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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 5 2 2 7e5 2 3 45234
136
STATEMENT OF CONTRIBUTION TO CO-AUTHORED PUBLISHED PAPER
This chapter includes a co-authored paper. The bibliographic details of the
co-authored paper, including all authors, are:
Alsabawi K, Gray EM and Webb CJ.
The effect of ball-milling gas environment on the sorption kinetics of MgH2
with/without additives for hydrogen storage. International Journal of Hydrogen Energy.
2019;10.1016/j.ijhydene.2018.12.026.
My contribution to the paper involved:
1) Perform the full experiment
2) Calculations and analysis of the results
3) Manuscript preparation
(Signed) _______________________________(Date)_ 12/02/2019_____________
Khadija Alsabawi
(Countersigned) ________________________(Date)_ 12/02/2019_____________
Corresponding author of paper: Khadija Alsabawi
(Countersigned) ________________________(Date) 12/02/2019______________
Supervisor: Colin Jim Webb
137
CHAPTER SIX
The effect of the milling environment gas on the hydrogen sorption properties of
MgH2+additives
Published as a Journal paper in the International Journal of Hydrogen Energy,
6. Introduction
The ball milling of metal hydride materials is known to have a significant effect on their
sorption kinetics, through the introduction of fresh surfaces, defects and reduced grain
size, which aids in faster diffusion of hydrogen into the bulk material. However, there
are many aspects to ball-milling, such as the type of mill, gas environment, vial and ball
material, ball to powder ratio, pressure and temperature in the vial, sample volume ratio,
rotation speed and milling/rest time ratios, all of which can have an effect on the final
milled sample. However, authors don’t usually provide all the necessary information
with regard to the ball-milling parameters that would enable a direct comparison.
This chapter reports on an investigation as to whether the gas environment in the ball-
mill vial during milling has an effect on the kinetic enhancement of magnesium hydride
with different additives. To do this, kinetic experiments were conducted on composite
materials ball-milled under argon (1 bar) and hydrogen (10 bar) atmospheres.
Conceptually, argon, being an inert gas, might be expected to play no role during the
repeated high energy and high pressure ball-ball and ball-vial impacts with intervening
particles of the composite sample. Molecular hydrogen might be expected to react with
exposed metal surfaces, or even oxygen if available, but the original materials are
already stoichiometric compounds, and the magnesium metal is already hydrided as
MgH2, so some degree of dissociation of these materials would be required before the
hydrogen gas environment would have a possible reaction path.
In this work, the ball milling process was performed in two stages. Firstly, the MgH2
was ball-milled without additive for 20 h to introduce defects and grain boundaries and
reduce the average crystallite size. This pre-milled MgH2 was then further ball-milled
with 1-2 mol% of the additives involved in this study (titanium isopropoxide, niobium
oxide and C60, carbon buckyballs). In both ball-milling stages, the gas environment
could be either hydrogen or Ar. TPD experiments were carried out to determine the
138
desorption kinetics, followed by a single pressure aliquot of hydrogen for absorption to
determine the sorption kinetics. It was found that the milling environment had a
negligible effect on desorption kinetics of most composites, however, depending the on
the gas used, the absorption uptake of some composites was changed. However, the
changes were different for different composite samples, with the same gases leading to
an improvement on one sample, but not others and vice versa. To understand the effect
of the ball-milling gas environment and the relationship to the additives used, requires
further investigation. The following paper provides the full details on the materials,
methods and results.
6.1. : Milling Environment Paper
Alsabawi K, Gray EM and Webb CJ. The effect of ball-milling gas environment on
the sorption kinetics of MgH2 with/without additives for hydrogen storage.
International Journal of Hydrogen Energy. 2019;44(5):2976-
80;10.1016/j.ijhydene.2018.12.026.
The effect of ball-milling gas environment on thesorption kinetics of MgH2 with/without additivesfor hydrogen storage
K. Alsabawi*, E.MacA. Gray, C.J. Webb
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, 4111, Brisbane, Australia
a r t i c l e i n f o
Article history:
Received 8 October 2018
Received in revised form
4 December 2018
Accepted 5 December 2018
Available online 6 January 2019
Keywords:
Magnesium hydride
Hydrogen storage
Catalyst
Milling gas
Sorption kinetics
a b s t r a c t
A study of the effect of the ball-milling gas environment on the kinetic enhancement of
MgH2 with different additives was conducted using argon and hydrogen. The as-sourced
MgH2 was milled for 20 h and then milled for a further 2 h after adding 1e2 mol% of one
of the additives titanium isopropoxide, niobium oxide or carbon buckyballs, varying the
gas environment for both ball-milling stages. The milling environment had little or no
effect on the desorption kinetics in most cases. However, in some cases, the absorption
uptake differed by up to 2 wt%, depending on the gas used. This effect was not consistent
among the composite samples surveyed, demonstrating the importance of reporting all
information about the ball-milling processes used, including the gas environment.
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction
Magnesiumhydride (MgH2) has gravimetric and volumetric H-
storage capacities of 7.6 wt% and 110 g H/l respectively, which
makes it a hydrogen storagematerial of interest. However, the
high operating temperature, due to the thermodynamic sta-
bility of the hydride, limits its application [1]. Ball-milling
MgH2 enhances the sorption kinetics at 300 �C compared to
the non-milled material [2].
Ball-milling metal hydride materials helps create fresh
metal surfaces, increases the surface area, forms micro/
nanostructures and introduces defects (such as dislocations,
stacking faults, vacancies, extra grain boundaries, depending
on the material used) on both the surface and in the interior
of the sample [3,4]. The exposed metal surface aids the
dissociation of hydrogen molecules and increased grain
boundaries promote faster diffusion of hydrogen into the
bulk material [4].
Milling parameters such as milling speed, time, tempera-
ture, atmosphere, ball-to-powder ratio [2], milling medium
(ceramic, steel, zirconia, etc.) [5] and ball-mill type (planetary,
high energy, shakers/mixers), all potentially change the final
material. Some of these parameters are interdependent; for
example, optimum milling time is dependent on the speed
and temperature of themill, size of container and type of mill.
Ball-milling may also affect the sample through the intro-
duction of impurities, the generation of higher temperatures
and the increased handling [3].
* Corresponding author.E-mail address: [email protected] (K. Alsabawi).
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier .com/locate/he
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 2 9 7 6e2 9 8 0
https://doi.org/10.1016/j.ijhydene.2018.12.0260360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Ball-milling MgH2 with small amounts (up to 10 mol%) of
additives such as carbons, metal oxides, metal halides and
many others, has been shown to significantly enhance the
MgH2 sorption kinetics [6,7]. Previous studies have shown that
the amount of additives used could significantly affect the
hydrogen sorption kinetics of MgH2 [8e11]. Ball-milling addi-
tives with MgH2 helps to create a homogeneous distribution
throughout the composite material, improving the contact
between the metal hydride and additive.
MgH2 milled with additives has been studied by several
authors using different milling parameters [7,12], including
the type ofmill, milling time [1], number and size of balls, ball-
to-powder ratio, rotational speed, milling atmosphere and
hydrogen pressure [1]. Many authors do not provide all the
necessary information with regard to the ball-milling to
enable repeat experiments or to make comparisons. Also,
while changes to the additive, such as decomposition and
interactionwith the Mg or MgH2 may be detailed, there is little
understanding of the role of additives in improving the ki-
netics of MgH2. Previous studies suggested that the gas envi-
ronment used during ball-milling can make a difference
[13e15], with the most commonly used gases being Ar
[16e18] and H2 [19,20] and in some cases under N [21], He [22]
and even under vacuum [23].
Varin et al. [24] studied the effect of ball-milling MgH2
under hydrogen and argon on the desorption rates. They
found that activation energy for desorption was lower for
hydrolysed MgH2 powder, i.e. covered with a layer of Mg(OH)2,
that was milled under H2 when compared to an argon milled
sample. However, they concluded that the amount of
hydrogen desorbed was related to the milling time and was
independent of the milling atmosphere. Asselli et al. [14]
showed that milling (2 Mg þ Fe) and (2MgH2 þ Fe) under
different environments did not affect the kinetics of absorp-
tion and desorption, but milling under hydrogen resulted in
decreasing the particles sizes and doubled the amount of
Mg2FeH6 formation.
Spassov et al. [13], produced nanocrystalline
Mg87Ni3Al3Mm7 hydrides by reactive mechanical milling.
They studied the effect of the gas atmosphere on the micro-
structure of the hydrides by milling one sample under H2 gas
only, and another sample first under Ar and then under H2.
The amount of MgH2 phase was found to be slightly larger for
the sample milled under H2 only. The H-absorption rate was
found to be higher for the sample milled under Ar than H2.
Ershova et al. [15], investigated the effect of milling under
different gases on the kinetics and thermodynamics of MgH2þ10 wt% Al mixtures, where they found that milling under
different environments influenced the crystalline phases.
Their first samplewasmilled under 12 bar of H2 (MA1) for 10 h,
second sample (MA2) under Ar for 17 h then H2 for 10 h, third
sample (MA3) under Ar for 20 h and the last sample (MA4) was
pure MgH2 milled under the same condition as MA1 for com-
parison. XRD patterns were obtained for all the samples after
milling. For MA1, three phases were found to be present,
metallic Mg, MgH2 and crystalline phase Mg17Al12. For MA2
there was only MgH2 andmetallic Mg, whereas MA3 consisted
of metallic Mg and Mg17Al12. Their results showed that
regardless of themilling atmosphere, MgH2 phase was formed
from which the aluminum was absent or present in a minute
content. Their results were in agreement with [25], in which
the Al was found to be rejected from Mg(Al) solid solution
during hydriding.
As an additive for MgH2, Nb2O5 has been studied exten-
sively to determine its catalytical effect on the hydrogen
sorption kinetics of MgH2 [6]. Nb2O5 has been milled with
MgH2 under Ar [26,27], H2 [28,29] or N [5,30] atmospheres, with
the majority of users using Ar. There has been no direct
comparison for Nb2O5 milled under different ball-milling
atmospheres.
Francke et al. [31], studied themodification of the structure
and specific surface area of graphite by high-energy milling
under argon and hydrogen atmospheres. TEM studies showed
that after milling, graphite “stacking packages” of particles
appeared. These “stacking packages” disappeared after 10 h of
milling under argon but stayed even after 15 h ofmilling under
hydrogen. The authors suggested that these packages influ-
ence the specific surface area rather than the structure. The
hydrogen-milled samples hadmore small particles (�15 nm in
length and �3 nm in height) than argon-milled samples,
which indicated that the hydrogen atmosphere prevented
agglomeration. Ong et al. [32], milled natural graphite under
inert and oxygen atmospheres. They found that the oxygen
preserved the crystal structure and suppressed the specific
surface area growth, but the sample was found to be highly
anisotropic with large stacking faults.
Rietsch et al. [33] used a planetary ball mill to evaluate the
influence of the gas atmosphere on the evolution of carbon
properties. They found that the atmosphere inside themilling
vial affects themillingmodes (sliding or shockmode)which in
turn affect the evolution of the graphite composition. The
sliding mode is considered to be gentle on the graphite
structure whereas the shock mode degrades the graphite
structure. The lubricating nature of graphite prevents the
shock mode from occurring, but only in the presence of oxy-
gen which annihilates the fresh reactive sites by forming
functional groups. However, under an inert atmosphere, or
when the oxygen is consumed, the lubricating layer is
destroyed resulting in shock mode. This establishes a link
between graphite structure evolution and the milling
atmosphere.
Xiao et al. [34] milled Ti-doped NaAlH4 complex hydrides
under atmospheres of H2 and Ar, and found thatmilling under
H2 improved the reversible hydrogen storage properties of the
hydrides, resulting in a 0.28 wt% higher hydrogen capacity
with a faster hydrogen desorption rate.
Recently, the kinetics of composite samples of MgH2 and
additive titanium isopropoxide, also called titanium tetraiso-
propoxide (TTIP) were compared to Nb2O5, finding that TTIP
produced equivalent results for both desorption and absorp-
tion [35].
The studies above indicate that the gas environment while
ball-milling can potentially affect both the magnesium hy-
dride and the additive. This work investigated the sorption
kinetics of MgH2 milled with different additives under argon
and hydrogen atmospheres, where theMgH2was initially ball-
milled without additive and then further ball-milled with
additive. Temperature Programmed Desorption (TPD) fol-
lowed by a single pressure aliquot of hydrogen for absorption
was carried out to determine the sorption kinetics of the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 2 9 7 6e2 9 8 0 2977
milled samples. Care was taken to control the milling condi-
tions so that comparisons could be made between samples,
altering only one aspect at a time. The two best additives for
MgH2 enhancement, Nb2O5 and TTIP were chosen as test
cases, together with C60.
Experimental
Magnesium hydride powder, 98% purity, (AB109434) was
purchased from ABCR GmbH & Co. 99.999% purity TTIP from
SigmaeAldrich, 20 nm Nb2O5 from NovaCentrix and 99.9%
fullerene C60 (SES Research) were used for the additives. The
as-supplied MgH2 was pre-milled in a PQ-N04 planetary mill
(Across International) for 20 h using a 50 ml zirconia vial and
10� 10 mm zirconia balls with a ball-to-powder ratio (bpr) of
21:1 and a vial filling factor [2] of 17.4%. Pre-milled MgH2
samples (preMgH2) were prepared in 5 g batches under either
1 bar of Ar or 10 bar of H2. Additives were subsequently milled
with the preMgH2 for 2 h (1 g sample, 6 ZrO2 balls, 600 rpm, 21:1
bpr, vial filling factor 7.7%) also under both Ar and H2 envi-
ronments. All materials were handled in a dry Ar atmosphere
MBraun glovebox with O2 and H2O levels below 1 ppm.
Hydrogen cycling was performed on a modified custom
Sieverts apparatus. TPD studies were carried out on the as-
prepared samples under dynamic vacuum from room tem-
perature to 400 �C with a temperature ramp rate of 2 �C/min
[8,35]. Baseline starting vacuum was typically 3� 10�6 mbar.
The sample remained at 400 �C for 2 h to ensure complete
desorption. Absorption kinetics data were recorded at
230 �C, after a single hydrogen pressure aliquot to reach an
initial pressure of 50 bar on the sample. XRD was performed
on each 5 g batch of pre-milled MgH2 as well as the 1 g
composite samples to ensure uniformity of the starting
material. No variation was observed in the crystalline
structure between Ar and H2 milled samples. All desorption/
absorption experiments were undertaken at least twice to
ensure repeatability.
No additives: MgH2 milled for 20 h under Ar andH2
Absorption/desorption data for MgH2 milledwithout additives
for 20 h under Ar or H2 are shown in Fig. 1. It can be seen that
the desorption kinetics are similar within experimental error.
The sample milled under H2 has faster absorption initially
than the Ar-milled sample, but does not reach the same ca-
pacity by 400 �C.
PreMgH2 milled with 1 mol% Nb2O5 for 2 h
Two batches of PreMgH2 were prepared by ball-milling for 20 h,
one under 10 bar of H2 and the other under 1 bar of Ar. 1 mol%
of Nb2O5 additive was added to each batch and thenmilled for
2 h under either 10 bar H2 or 1 bar of Ar, resulting in 4 samples.
The desorption kinetics for the composite samples are shown
in Fig. 2. The sample milled under H2 both times finished
Fig. 1 e Absorption/desorption of MgH2 milled for 20 h
under H2 (dashed line) and under Ar (unbroken line).
Fig. 2 e Desorption kinetics of pre-milled MgH2 ball-milled
for 2 h with (1mol%) Nb2O5 under different gas
environments.
Fig. 3 e Absorption kinetics of pre-milled MgH2 ball-milled
for 2 h with (1mol%) Nb2O5 under different environment.
i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 2 9 7 6e2 9 8 02978
desorbing at a slightly lower temperature than the sample
where the additive was milled under Ar. The onset of
desorption for all of the samples is roughly the same. Samples
that were milled fully or partially under Ar show a greater
spread of desorption with temperature, often with a more
obvious second peak.
The absorption kinetics for the four samples are shown in
Fig. 3. The composite samples where the final milling envi-
ronment was Ar have very similar kinetics, which suggests
that the pre milling environment of MgH2 had no effect on the
absorption kinetics. However, the composite sample that was
milled under H2 for both MgH2 and additives had the slowest
and the least uptake between the four samples, whereas the
sample where MgH2 was milled under Ar then the composite
was milled under H2 had the highest hydrogen capacity and
the fastest kinetics. Although the differences are small, these
results suggest that milling under Ar for either pre-milling
alone or for the total milling procedure is preferable, con-
trary to the desorption results. The TPD experiments were
repeated for these four samples, confirming the same results
within experimental uncertainties.
PreMgH2 milled with 1 or 2 mol% TTIP for 2 h
1 mol% of TTIP was added to the H2-milled MgH2 and the
compositewasmilled underH2 environment and another 1mol
% TTIP was added to Ar-milled MgH2 and thenmilled under Ar.
Fig. 4 shows both the absorption and desorption kinetics of the
composite samples. It can be seen that the desorption kinetics
for the samples are similar, but the Ar-milled sample desorbs
the majority of hydrogen at a slightly lower temperature than
the H2-milled sample. Conversely, the H2 milled sample has
superior absorption. The same results were also found for a
2 mol% TTIP composite sample (not shown).
PreMgH2 milled with 1 mol% C60 for 2 h
Fig. 5 shows the absorption/desorption kinetics of thepremilledMgH2 with 1mol% C60 carbon buckyballs, either both
milled under H2 or both under Ar. While there is little to no
change in the desorption kinetics between the Ar and H2
milled samples, the Ar milled sample had significantly better
absorption kinetics and absorbed ~2 wt% more H2 in 20 min
than the sample milled under H2
Conclusion
The effect of ball-milling MgH2 with and without additives
under different gases (Ar, H2) was surveyed. MgH2, pre-milled
for 20 h under either 10 bar of hydrogen or 1 bar of argon was
milled for an extra 2 h with 1 mol% TTIP, Nb2O5 and C60, also
under either H2 or Ar. The kinetics of absorption and desorp-
tion of the composite samples was investigated by thermal
desorption under vacuum, followed by a single absorption
pressure step of 50 bar of H2 at 230 �C.The milling environment had little to no effect on the
desorption kinetics inmost cases, with only the TTIP composite
sample showing marginal improvement. However, the ab-
sorption results demonstrate a significant difference between
Ar and H2 milling with the additives. Unfortunately, the dif-
ference reverses between the TTIP and other samples, with
the TTIP composite sample showing better absorption for H2
milling whereas the Nb2O5 and C60 composite samples had
superior absorption with Ar milling.
While this study has shown there is an effect of the milling
environment on the absorption kinetics of MgH2, it does not
suggest a simple consistent answer as to which gas produces
the best results as it varies according to additive used.
Acknowledgements
K.A acknowledges receipt of an Australian Government
Research Training Program Scholarship.
Fig. 4 e Absorption/desorption of H2-milled preMgH2 milled
with (1 mol%) TTIP for 2 h under H2 (dashed line) and under
Ar (unbroken line).
Fig. 5 e Absorption/desorption of preMgH2 milled with
1 mol% C60 for 2 h under H2 (dashed line) and under Ar
(unbroken line).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 2 9 7 6e2 9 8 0 2979
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[29] da Conceicao MOT, Brum MC, dos Santos DS, Dias ML.Hydrogen sorption enhancement by Nb2O5 and Nb catalystscombined with MgH2. J Alloy Comp 2013;550:179e84.Copyright (C) 2015 American Chemical Society (ACS). AllRights Reserved.
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i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 4 ( 2 0 1 9 ) 2 9 7 6e2 9 8 02980
144
CHAPTER SEVEN
The effect of transition metal oxide additives on the sorption kinetics of MgH2 for
hydrogen storage
(As a Journal paper submitted to International Journal of Hydrogen Energy)
7. Introduction
The discovery that the transition organo-metallic oxide, TTIP, was just as effective as
the bench mark Nb2O2 (chapter 5) [1] on the sorption kinetics of MgH2 raised a number
of questions. Was the kinetic enhancement due or partly due to the use of the transition
metal titanium? Did the oxygen content of TTIP also play a role in improving the
kinetics? Was there a contribution from the carbon and/or hydrogen content of the
organic? Or was it simply that TTIP is a liquid at room temperature and was therefore
better dispersed throughout the MgH2 during milling? Or various combinations of these
reasons.
These questions relate to the role of the additive in the kinetic enhancement of the Mg-
H interaction, and, although it was shown that TTIP was another effective (transition
metal based high valence oxide) additive, it did not help explain the role this, or other
additives, play in the Mg-H system.
Understanding the role of additives in enhancing the kinetics of MgH2 is still a
challenge. Most published kinetic experiments involve several or many parameters
simultaneously, including ball milling parameters (gas environment, pressure and
temperature in the vial, milling time, vial and ball material, ball to powder ratio, sample
volume ratio [2], type of mill, rotation speed, milling/rest time ratios), the nature of the
additive and the MgH2 (liquid/solid, powder size, crystalline/amorphous, contaminants,
especially oxides) and the experimental procedures (sample cell materials, sample size,
sample temperature, especially during hydrogenation/dehydrogenation due to
exo/endothermic reactions, quality of vacuum, temperature ramp rate, back pressure
during desorption) – some of which are not measured or not communicated.
As part of a larger plan to relate the properties of additives to their measured
enhancement in order to evaluate which properties are effective and which properties do
145
not help, another investigation into similar but slightly different additives (liquid/solid,
titanium/vanadium, oxide/non-oxide) was undertaken to establish which additives work,
and under what conditions.
This chapter consists of an experimental paper which has been submitted to the
International Journal of Hydrogen Energy. Six different new additives, not previously
employed in the sorption kinetics of MgH2, were used as well as TTIP and Nb2O2 for
comparison purposes. Altogether, the eight additives included three powder samples
and five liquids, and the transition metals Nb, Ti and V. All but one additive were
oxides, with the other being an organo-metallic titanium-based chloride. Apart from the
Nb2O2 all the samples were organo-metallics, containing carbon, hydrogen and oxygen
(except the chloride additive which did not contain oxygen in the chemical
composition).
The same experimental procedure was used as in the previous TTIP investigation.
Different amounts of additives (0.5- 10 mol%) were ball milled with MgH2 for different
times, where the milling parameters and the apparatus used for the kinetics
measurements were similar to that used previously (the full details are in the submitted
paper). Overall both Ti and V based additives enhanced the sorption kinetics of MgH2,
where the V-based oxides had faster absorption and higher uptake when compared to
Ti-based oxides. A small amount of 1 mol% Ti-chloride based additive milled for short
time (1 h) had better desorption than all additives investigated in this work but with very
poor absorption.
Summarising, this investigation also raises further questions. While it did not
specifically address the physical or chemical role of the additive in the
adsorption/desorption process, it has established the general benefit of a liquid additive
over comparable solid additives. However, contradicting this, the chloride additive, a
solid, had the best desorption kinetics. This additive also suggests that an oxide additive
is not necessary for fast desorption. Interestingly, the extremely poor absorption results
for the chloride additive, to the extent that all of the composite chloride additive
samples absorbed more slowly than the ball-milled MgH2 without additive, support the
idea that different additives can enhance absorption and desorption differently, and that
combinations of additives might be an effective avenue to pursue [3].
146
7.1. References
1. Alsabawi K, Webb TA, Gray EM and Webb CJ. Kinetic enhancement of the
sorption properties of MgH2 with the additive titanium isopropoxide.
International Journal of Hydrogen Energy. 2017;42(8):5227-
34;10.1016/j.ijhydene.2016.12.072.
2. Polanski M, Bystrzycki J and Plocinski T. The effect of milling conditions on
microstructure and hydrogen absorption/desorption properties of magnesium
hydride (MgH2) without and with Cr2O3 nanoparticles. International Journal of
Hydrogen Energy. 2008;33(7):1859-67;10.1016/j.ijhydene.2008.01.043.
3. Webb CJ. A review of catalyst-enhanced magnesium hydride as a hydrogen
storage material. Journal of Physics and Chemistry of Solids. 2015;84:96-
106;10.1016/j.jpcs.2014.06.014.
7.2. Paper
147
STATEMENT OF CONTRIBUTION TO CO-AUTHORED PUBLISHED PAPER
This chapter includes a co-authored paper. The bibliographic details of the
co-authored paper, including all authors, are:
Alsabawi K, Gray EM and Webb CJ.
The effect of transition metal oxide additives on the sorption kinetics of MgH2 for hydrogen
storage. International Journal of Hydrogen Energy. (submitted)
My contribution to the paper involved:
1) Perform the full experiment
2) Calculations and analysis of the results
3) Manuscript preparation
(Signed) _______________________________(Date)_ 12/02/2019_____________
Khadija Alsabawi
(Countersigned) ________________________(Date)_ 12/02/2019_____________
Corresponding author of paper: Khadija Alsabawi
(Countersigned) ________________________(Date) 12/02/2019______________
Supervisor: Colin Jim Webb
148
The effect of transition metal oxide additives on the sorption kinetics of MgH2 for
hydrogen storage
K. Alsabawi*, E.MacA. Gray, C.J. Webb
Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, 4111
Brisbane Australia
Abstract
The effect of a number of additives on the kinetics of absorption and desorption of
magnesium hydride were studied using TPD, TDS and single aliquot absorption in a
Sieverts instrument. The additives were mainly transition metal oxides (3 x Ti-based
organo-metallic oxides, 3 x V-based organo-metallic oxides and Nb2O5 as the
benchmark for comparison). The purpose was to investigate each additive to determine
the relative effect of the different transition metals, the effect of the organo-metallic
components, and the difference between liquid and powder additives. In addition, a Ti-
based organo-metallic chloride (C16H22Cl2Ti) was also tested to compare an oxide and
non-oxide additive. A systemic survey was performed for ball-milled MgH2 mixed with
various amounts of additives for different times. Most of the additives used were liquids
and it is possible to conclude that liquids are effective at improving the sorption kinetics
of MgH2, possible due to better dispersion during milling. The Ti-based chloride
(BisTiCl2) had the best desorption kinetics but the slowest absorption, raising questions
about the role of additives and differences between absorption and desorption kinetics.
* Corresponding author: Tel.: +61 7 37353625 E-mail address: [email protected]
149
Introduction
Magnesium Hydride (MgH2) remains an attractive material for hydrogen storage, due
its high volumetric capacity (~0.1g/cm3) and gravimetric (7.6 wt%) capacity and low
cost [1-3]. Unfortunately, the high thermodynamic stability (∆H= -75 kJ mol-1) [4],
requiring high temperatures for hydrogen release, hinders practical use of magnesium
for hydrogen storage and poor kinetics makes the rate of the hydrogen absorption and
desorption slow under moderate conditions [5-7].
Research has shown that decreasing the crystallite size of MgH2 through ball milling or
other forms of severe deformation [8] helps to improve the sorption kinetics. Ball
milling introduces additional grain boundaries which reduce the diffusion lengths for
hydrogen [9, 10] and creates fresh surfaces free from oxide layers that slow
hydrogenation.
The addition of small amounts (up to 10 mol%) of various materials added to MgH2,
sometimes described as catalysts, during milling, enhances the sorption kinetics [11-15].
Transition metal oxides have been of a particular interest, although other materials,
including the transition metal halides and elemental transition metals, have also
significantly improved the kinetics of MgH2 absorption and desorption [16-18].
Elemental transition metals such as V and Nb do not form intermetallic compounds
with magnesium [19], which makes them heterogeneous catalysts [20]. The influence of
a wide range of metal oxides on the sorption kinetics of nanocrystalline Mg-based
systems were investigated, by Oelerich et al. [21] where they showed that only
transition metal oxides additives lead to a significant enhancement of hydrogen sorption
kinetics. One of the earliest, and still one of the most effective additives, is Nb2O5[22],
which is regarded as the benchmark for Mg-H system kinetics. Other materials
producing similar results include TiO2 [23, 24], V2O5 [21, 25, 26] and titanium tetra-
isopropoxide TTIP [27].
Transition metals, thought to enable dissociation and re-association of hydrogen on the
magnesium surface, may also facilitate the reduction of MgH2 particle size during
milling [7, 28]. Several authors have also suggested that the electronic structure of
transition metal ions determines their catalytic effect on MgH2, where metal oxides with
higher valence state have greater catalytic effect [21, 22]. Different niobium oxides
(NbO, NbO2 and Nb2O5) additives were investigated by Barkhordarian, and it was
found that the catalytic efficiency increased with the valence state of the niobium [29].
150
However, the exact catalytic mechanism of transition metal oxides on the sorption
properties of MgH2 is still undetermined.
Metal oxide surfaces tend to passivate the metal, preventing reactive gases from
reaching the pure metal; hence the slow diffusion of hydrogen through a MgO layer
slows the hydrogenation of Mg [6, 30]. However, Aguey-Zinsou et al. [28, 31] showed
that MgO acts as a ‘process control agent’ during milling, by preventing cold welding
and reducing particle agglomeration, and thus helps to decrease the particle size of
MgH2, which increases H diffusion and improves the rate of H-sorption in magnesium.
Identifying the nature of the oxide phase after milling, and then after cycling, is still a
challenge, in the case of Nb2O5 some studies have found no structural transformation of
the Nb2O5 phase and that it only acts as a catalyst [22, 32], whereas others have shown
that Nb2O5 acts as an additive where they report a reduction in the Nb oxide species
after cycling and a new ternary oxide phase MgNbxOy observed by XRD [33-36].
One problem with understanding the role of additives is that many factors are
(sometimes necessarily) incorporated into each experiment. Most studies involve ball-
milling, but the parameters vary and are not always well-reported. Sometimes negative
results help to indicate what is effective and what isn’t. TiN is also an effective additive,
suggesting oxides are not essential [37]. There are studies that show some additives
enhance desorption but not absorption (and vice versa), suggesting that they support a
rate mechanism involved in one process, but not the other. For this reason, it has been
suggested [7] that strategic combinations of additives may be effective.
Jung et al .[38] in 2006 synthesized composites of MgH2 with a range of metal oxides
(V2O5, Cr2O3, Fe3O4, and Al2O3) by ball milling where they followed the steps of
Oelerich et al. [21] but milled for shorter time, to investigate the hydriding properties.
They confirmed the work done previously [21], and found that at lower temperatures
(250-200 ºC) the MgH2/V2O5 sample showed even faster kinetics and higher absorption
capacities up to 3.2 and 2.25 wt% compared to unmodified MgH2. Korablov et al. [39],
found that V2O5 reduces readily on contact with hydrogen and during milling with
MgH2. V2O5 composite samples were cycled twice, and XRD patterns collected,
showing V2H and V peaks, which suggest that V2H and V act as active catalysts, since
they did not form intermetallics or alloys.
151
Another oxide that has been widely studied is TiO2. Wang et al.[40] showed that ball
milling MgH2 with TiO2 improved the sorption kinetics, where they claimed that the
improvement was due to absence of oxide reduction and the existence of n-TiO2, which
was also found later [23], while others reported reduction of TiO2 during milling [24,
41-43]. Jung et al. [23] showed that a uniform distribution occurs during milling
provided the rutile TiO2 concentration is below 5 mol%. Croston et al. [42] prepared
TiO2 through calcination at different temperatures from 70 to 800 ºC, where they found
that the calcination temperature has an effect on the dehydrogenation temperature of
MgH2 - with 70 ºC calcination temperature having the best result. They also found that
anatase was more effective than the rutile at lowering the dehydrogenation temperature
of MgH2. However, Vujasin et al. [44] showed that rutile TiO2 additives decreased the
desorption activation energy while anatase TiO2 had a negligible effect. Alsabawi et al.
[45] used TTIP as an additive for MgH2 and found that it has similar effect on the
sorption kinetics as the bench mark Nb2O5. The fact that TTIP is a liquid may have
facilitated a more homogeneous distribution and therefore higher surface contact with
smaller well-distributed particles, thus improving the kinetics of MgH2.
Although the mechanisms by which additives enhance the sorption kinetics of the Mg-H
system are still unknown, and despite some contradictory results, some aspects of
additives have been confirmed. The success of transition metal compounds is well-
established, although this is not a requirement. The suggestion of higher order oxides
has been supported by the initial results of Nb2O5 and the recent success of TTIP
(Ti(OCH(CH3)2)4). With these results in mind, and including the prospect of better
dispersion for a liquid additive, the following compounds have been chosen for this
study:
• Titanium Ethoxide, Ti(OC2H5)4, a Ti based organo-metallic liquid for comparison
with TTIP as another liquid with Ti and 4 oxygens,
• Titanium(IV) methoxide Ti(OCH3)4 as a powder Ti based additives to compare
with the liquid additives,
• Vanadium(V) oxytriisopropoxide OV(OCH(CH3)2)3,
• Vanadium(V) oxytripropoxide OV(OC3H7)3,
• V-oxytriethoxide OV(OC2H5)3
(All the vanadium based additives in this work are liquids at room temperature, with
different hydrocarbon chain lengths, for comparison with each other as well as the Ti
based organo-metallic liquid.)
152
• Bis(isopropylcyclopentadienyl) titanium dichloride C16H22Cl2Ti, is a powder, Ti-
based chloride additive used to compare the effect of a Cl-based to the oxide based
additives investigated.
• TTIP,
• and Nb2O5 were repeated as benchmarks and for comparison.
Table 1: A list of material and chemical used for this studies, indicating suppliers, state, purity and
chemical formula
Materials/ chemicals Formula Purity
(%)
Amount
(mol%)
Milling
time
(h)
State Supplier
Magnesium hydride MgH2 98 20 Powder ABCR
Niobium pentoxide Nb2O5 (20 nm) _ 1 2 powder NOVacentrix
Titanium isopropoxide Ti(OCH(CH3)2)4 97 1 2 Liquid Sigma-
Aldrich
Titanium(IV) methoxide Ti(OCH3)4 95 1 2 powder Sigma-
Aldrich
Titanium(IV) ethoxide Ti(OC2H5)4 97 0.5-
10
2 Liquid Sigma-
Aldrich
Vanadium(V)
oxytriisopropoxide
OV(OCH(CH3)2)3 _ 1 2 Liquid Sigma-
Aldrich
Vanadium(V) oxytripropoxide OV(OC3H7)3 98 1 2 Liquid Sigma-
Aldrich
Vanadium(V) oxytriethoxide OV(OC2H5)3 95 1 2 Liquid Sigma-
Aldrich
Bis(isopropylcyclopentadienyl
) titanium dichloride
C16H22Cl2Ti 95 1-2 1-10 powder Sigma-
Aldrich
Experimental
As-sourced MgH2 was pre-milled in a PQ-N04 planetary mill (Across International) for
20 h using a 50 ml zirconia vial and 10×10 mm zirconia balls with a ball-to-powder
ratio (bpr) of 7:1 and a vial filling factor [46] of 17.4%. Pre-milled MgH2 samples
153
(preMgH2) were prepared in 5 g batches, as per our previous studies [45, 47, 48]. The list
of additives studied, as well as chemical formula and source, is presented in Table 1.
Additives were subsequently milled with the preMgH2 (1 g sample, 6 ZrO2 balls, 600
rpm, 21:1 bpr, vial filling factor 7.7%).
For most additives, 1 mol% was added to the MgH2 and ball-milled for 2 h, as this has
been shown to be sufficient for Nb2O5 and TTIP [45, 49].
Bis(isopropylcyclopentadienyl) titanium dichloride samples were prepared with a range
of ball milling times (1- 10 h) and additive quantity in order to investigate the effect of
these parameters on the sorption kinetics. Different quantities of the Titanium (IV)
ethoxide additive were also used to determine the optimal amount. All materials were
handled in a dry Ar atmosphere MBraun glovebox with O2 and H2O levels below 1
ppm.
A modified custom Sieverts apparatus was used to perform the hydrogen cycling. The
as-prepared hydride samples started under dynamic vacuum, 3×10-6 mbar, before
initiating the TPD studies, where the samples were heated from room temperature to
400 °C at a ramp rate of 2 °C/min [45, 50]. To ensure complete desorption, the sample
temperature was kept at 400 °C for a further 2 h. Then the sample temperature was
reduced to 230 °C, and a single hydrogen pressure aliquot was introduced to reach an
initial pressure of 50 bar on the sample. The pressure of the sample was constantly
monitored by a Quartzdyne DL series pressure sensor with an overall accuracy of
0.015% FSR (340) bar.
Results and discussion:(1 mol%) Ti-based oxide additives milled for 2 h
Different Ti-based oxide additives were milled with preMgH2 in order to compare their
effect on the sorption kinetics of MgH2. Fig 1 shows the desorption kinetics of the Ti-
based oxides, where it can be seen that all additives have a positive effect on the
kinetics when compared to the preMgH2 without additive. The best performing additive
is TTIP confirming our previous studies [45]. This composite released hydrogen in the
temperature range (120-310 ºC) with a maximum peak at 250 ºC. Ti-ethoxide was
slightly less effective than TTIP, starting desorption at a later temperature (166 ºC) and
peaking at 270 ºC. The Ti-methoxide composite sample required a higher temperature
before hydrogen was released (200 ºC), and for all the additives the desorption was
complete by 310 ºC (ie before the ball-milled MgH2 without additive reached its peak in
the hydrogen release). Both TTIP and Ti-ethoxide are liquid at room temperature while
154
Ti-methoxide is in powder form, lending some support to the suggestion that liquids are
better dispersed during milling, helping to improve the sorption kinetics of MgH2.
Fig 1:Desorption kinetics of Pre
MgH2 + 1 mol% Ti based-oxides milled for 2h
under Ar.
The absorption kinetics of the Ti-based oxides are shown in Fig 2. It can be seen that all
composite samples reached their maximum capacity in the first 10 min. Ti-methoixde
and Ti-ethoxide additives have the highest uptake ~6.5 wt%, however the latter was
faster. The preMgH2 without additives has the highest hydrogen uptake but the slowest
kinetic rate. The TTIP composite sample has the lowest uptake which could be due to
its higher molecular weight compared to the other additives. These results show that the
additives make a significant enhancement to the absorption kinetics of MgH2, as well as
the desorption kinetics. Due to the fast kinetics and high capacity, the Ti-ethoxide
composite sample was further investigated to determine the optimal amount of additive.
155
Fig 2: Absorption kinetics of Pre
MgH2 + 1 mol% Ti based-oxide milled for 2 h
under Ar.
Different amount of Ti-ethoxide milled for 2 h
The effect of different amounts (0.5- 10 mol%) of Ti-ethoxide additive on the
desorption kinetics is shown in Fig 3. It can be seen that desorption kinetics differ
significantly depending on the amount added and an optimal amount was found to be 1
mol%. Decreasing the amount of Ti-ethoxide to 0.5 mol% yielded poorer results; with
broadening of the peak shape indicating the release of hydrogen by the composite
sample over a greater range of temperature. For the higher quantities, 2 and 10 mol%,
there appears to be two overlapping processes where the second peak of 10 mol% is
matches the pre-milled MgH2 peak, suggesting a portion of the composite sample is not
affected by the additive, even though there is more of it.
156
Fig 3: Desorption kinetics of Pre
MgH2 + X mol% Ti-ethoxide milled for 2h under
Ar
Fig 4 shows the effect of different amounts of Ti-ethoxide on the absorption kinetics
together with pre-milled MgH2 without additives for comparison. The 0.5 mol% Ti-
ethoxide sample has the highest hydrogen capacity and fastest kinetic rate, whereas the
10 mol% has the lowest capacity and slowest kinetics – significantly worse than the
ball-milled pure MgH2. There is little difference between the 0.5 mol% and 1 mol%
composite samples, other than the final capacity, which is simply due to the amount of
non-absorbing additive used. Overall, these comparative results match the
corresponding desorption kinetics.
157
Fig 4: Absorption kinetics of Pre
MgH2 + X mol% Ti-ethoxide milled for 2h under
Ar
1 mol% V-based oxide additives milled for 2 h
In order to further investigate the effect of liquid oxides on the kinetic enhancement, V-
based liquid oxide additives were milled under the same conditions to draw a
comparison between the Ti- and V-based additives. Fig 5 shows the desorption kinetics
of 1 mol% V-based organo-metallic oxide composite samples milled for 2 h under Ar.
All samples showed similar kinetics, covering approximately the same temperature
range. The broadening of the peak shape indicates that hydrogen is released over larger
temperature range, in the case of V-oxytriisopropoxide there appear to be overlapping
processes, evidenced by a small peak/plateau at about 250 ºC. While the V-oxide liquid
additives improved the desorption kinetics when compared to the preMgH2 without
additives, they did not achieve the same desorption enhancement as TTIP or Nb2O5
which is in agreement with the literature [51].
158
Fig 5: Desorption kinetics of Pre
MgH2 + 1 mol% V based-oxide milled for 2h under
Ar. (Scaled by a factor 3)
The effect of the V-based oxide additives on the absorption kinetics of MgH2 is shown
in Fig 6. All additives show fast kinetics, both V-oxytriethoxide and V-oxytripropoxide
reached their maximum capacity in the first minute, whereas V-oxytriisoprpoxide
reached its maximum after 8min but had the highest capacity of 6.68 wt%. While the
capacity is largely determined by the weight of the 1 mol% of the additive, since the
composite samples were close to fully absorbed after 20 minutes, the slower result for
V-oxytriisopropoxide confirms that additives affect the absorption and desorption
process differently.
159
Fig 6:Absorption kinetics of Pre
MgH2 + 1 mol% V-oxide milled for 2 h under Ar.
BisTiCl2 (Bis(isopropylcyclopentadienyl) titanium dichloride) additive
To compare the effect of a Cl-based additive to the oxide based additives investigated,
BisTiCl2 was also studied. This additive contains chlorine, where various halides have
been shown to kinetically enhance MgH2, but no oxygen. Fig 7, shows the effect of 1
mol% BisTiCl2 milled for different times (1 – 10 h) under Ar, and 2 mol% milled for 5
h, in order to investigate the effect of different amounts of additive and milling time.
The best desorption kinetics were observed for the 1 mol% of BisTiCl2 sample milled
for 1 h, with the hydrogen released in the temperature range (200 – 280 ºC), whereas the
2 mol% milled for 5 h had poor kinetics and a very wide peak over a large temperature
range. In terms of when the desorption finished, it did not show a significant
improvement over the un-modified preMgH2. Overall, the longer milling times appear to
have significantly reduced the kinetic enhancement, pointing to a possible
decomposition of the additive during milling.
160
Fig 7: Desorption kinetics of Pre
MgH2 + X mol% BisTiCl milled for different times
under Ar
Fig 8 shows the absorption kinetics of the BisTiCl composite samples milled for
different times. All the composite samples have significantly poorer absorption kinetics
than the Pre MgH2 without additives, indicating the additive has inhibiting absorption
rather than facilitating it. The sample with 1 mol% milled for 10 h is the best of the
composite samples; the shorter milling times produce poorer results. The composite
sample with 1 mol% milled for 1 h, which had the best desorption kinetics, had the
slowest absorption kinetics and the least hydrogen uptake.
161
Fig 8: Absorption kinetics of Pre
MgH2 + X mol% BisTiCl milled for different times
under Ar
Comparison of all additives
In order to make comparisons with the literature and to put the improvement of the
desorption kinetics of organo-metallic liquid oxides milled with MgH2 in perspective,
the desorption kinetics of 1 mol% of all additives used in this study milled for 2 h,
except for BisTiCl which was milled for 1 h, are plotted in Fig 9. The best composite
sample for desorption was the BisTiCl2, a powder additive for which the desorption was
finished by 270 ºC. This was followed by the TTIP and Nb2O5 composite samples.
162
Fig 9: Desorption kinetics of 1 mol% all additives, milled for 2 h (except BisTiCl 1
h) with MgH2
Conclusion
The effect of various amounts of different liquid oxide (Ti-based and V-based)
additives milled with MgH2, as well as two Ti-based powder additives, one an oxide
and the other a chloride, was systematically surveyed by measuring the desorption and
absorption kinetics. TPD studies were performed under vacuum followed by a single
absorption pressure aliquot at 50 bar H2 at 230 ºC. The benchmark Nb2O5 additive was
used to compare the effect of these additives on MgH2, as well as the recently studied
TTIP. It was found that when comparing the effect of the Ti and V based oxide
additives on the kinetics of MgH2, that Ti-based oxide additives improved the desorption
kinetics whereas the V-based oxide have a positive effect on the absorption kinetics.
The 1 mol% BisTiCl2 composite sample, milled for 1 h, which was used as a non-oxide
additive to compare with the oxide samples, had the best desorption kinetics when
compared to all the additives surveyed in this work, but, interestingly, it had the slowest
absorption and lowest hydrogen uptake, lower than the undoped MgH2, demonstrating
the different effect of additives on the two different sorption processes.
Part of this investigation was to examine the difference between liquid and powder
additives, and, generally the liquid additives perform well, as expected if the
163
distribution of additive through the magnesium/hydride is an important consideration.
However, while a number of additives compare favourably, or even equally, with
Nb2O5, a powder, they are not significantly better, questioning whether the liquid state
is of benefit, or whether Nb2O5 has other characteristics that outweigh poorer
distribution due to being in powder form.
The sole chloride sample, with excellent enhancement of desorption and significant
inhibition of absorption, provides further evidence that additives can influence the
different absorption and desorption processes in different ways, suggesting the possible
use of two or more additives that complement each other and maximize the net effect on
the sorption kinetics.
Acknowledgements
K.A acknowledges receipt of an Australian Government Research Training Program
Scholarship.
164
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170
CHAPTER EIGHT
8. Summary and Conclusion
Hydrogen is considered to be a readily available energy vector that can be produced
from renewable energy sources and used to generate clean, affordable energy [1, 2].
One of the main barriers in establishing a strong hydrogen economy is a safe,
economical and practical storage method. To eliminate this barrier, new designs for
hydrogen storage materials must consider weight, volume, safety and cost as well as
good reversibility for release and uptake of hydrogen in practical applications. Several
methods have been utilised or suggested to store hydrogen, including compressed and
liquid hydrogen, liquids such as ammonia and methylcyclohexane, and solid state
storage materials that can readily take up and release large quantities of hydrogen. Solid
state materials, including simple metal and intermetallic hydrides, complex hydrides,
metal organic materials, such as MOFs, and adsorptive materials such as carbons,
zeolites and polymers, are all capable of storing hydrogen with different volumetric and
gravimetric densities, but none of them fulfil all the requirements for facilitating
hydrogen storage for applications such as hydrogen fuel cell vehicles or smoothing
supply/demand requirements for intermittent renewable energy generators.
This project focussed on the synthesis and characterisation of composite materials to
investigate the kinetic properties of the Mg-H system, by milling MgH2 with different
additives. Magnesium is a promising hydrogen storage material containing 7.6 wt% of
hydrogen as magnesium dihydride, MgH2. Practical applications of MgH2 for hydrogen
storage are limited by poor absorption/desorption kinetics and the high dissociation
temperature of MgH2, due to the thermodynamic stability of the hydride (approx. 75
kJ/mol.H2). Additives in small amounts have been shown to have a considerable effect
on the kinetics of absorption and desorption of hydrogen, although the effect on the
thermodynamics is minimal or negligible. The additives chosen for this work have not
previously been investigated for their effect on hydrogen sorption of MgH2. However,
the choice of additive was based on results of previous studies.
Many studies have shown the beneficial effect of transition metal oxides as additives for
the enhancement of hydrogen sorption kinetics e.g., Nb2O5 and V2O5. The dramatic
kinetic improvement by addition of Nb2O5 was found in 2003 [3] and this material is
171
now considered a benchmark for comparing additives. Following an exploration of
different Nb [3-6] and V oxides [7-9] it was suggested that the higher valence oxides
have a greater effect on the sorption kinetics [10, 11]. Transition metal halides have also
been successfully used as additives for improving the kinetics e.g., VCl3 [12]and TiF3
[13] as well as many other materials [14].
Finally, an important aspect of the synthesis of composite samples is ensuring a good
interaction between the hydrogen sorption material and the additive. The use of ball-
milling is common in studies of the effect of additives on MgH2, and this can decrease
the size of the MgH2 and additive particles, creating more surface area for interaction,
and also disperses the additive throughout the MgH2. A liquid may more readily
disperse throughout the MgH2 powder, and for this reason a number of liquid (at RT)
additives were chosen for this work.
Since Nb2O5 and V2O5 have already been reported in the literature, organo-metallic
materials based on oxides of transition metals offer another set of additives worthy of
investigation. While the role of oxygen in MgH2 is still unclear, some of the organo-
metallic oxides used contain more oxygen per transition metal atom than Nb2O5. In
addition, some of these are liquid at RT, providing another set of comparisons - between
that of powder and liquid additives. The additives chosen for this work were transition
metal organo-metallic oxides in liquid and powder forms as well as a Ti-based organo-
metallic chloride. The transition metals were Ti and V. Nb2O5 was used as the
benchmark for comparisons.
The composite sample materials were prepared through ball milling under different
conditions to determine the optimal parameters. Ball milling time varied from 0.5h to 20
h depending on the additive used, to explore the effect of milling time on the composite.
In addition, composite samples were ball milled under different gases (H2 and Ar), in an
attempt to understand the effect of the milling environment on the sorption kinetics.
The positive effect of ball milling with additives on enhancing the temperature onset
and rates of hydrogen sorption for the MgH2 system has been widely studied, and the
studies reported here confirm the previous articles. This work resulted in a new set of
information, extending the knowledge regarding transition metal oxide additives for
MgH2, including the use of novel additives and may help to understand some of the
mechanisms by which the additives influence the sorption kinetics of MgH2.
Comparisons of Ti and V oxides, Nb oxide with Ti and V organo-metallic oxides, liquid
172
v solid additives and ball-mill gas environments all contribute to the knowledge in this
area.
Sievert apparatus 300 bar:
In chapter 3, a detailed description was given of the design and implementation of a
Sieverts type apparatus for determining hydrogen uptake and release at pressures up to
340 bar and temperatures from 77-773 K. An existing instrument was re-developed
during this project to enhance the temperature stability, improve the ratio of reference
and cell volumes and add TPD and TDS capabilities, in order to perform all the required
kinetic and capacity measurements in a single environment.
As part of the re-development, characterisation and verification of the apparatus was
performed. Multiple volume calibrations (full volume calibration, empty cell isotherm
and divided-volume calibration) were made, to fully characterise the instrument
volumes. The performance of the instrument for isotherm measurements and
calculations was validated by running standard materials such as LaNi5 and a low
density material (FiltraSorb-400). Details of the instrument design and validation were
written as a journal paper and submitted to IJHE.
Milling MgH2 with additives under different environment
In this work MgH2 was milled with additives under Ar (1 bar) and H2 (10bar)
atmospheres (Chapter 6). The as-sourced MgH2 was milled first for 20 h without
additives and then milled for another 2 h with the addition of 1-2 mol% of the additives
TTIP, Nb2O5 and C60, while varying the gas environment for both ball-milling stages.
TPD experiments were carried out to determine the desorption kinetics, which was then
followed by a single pressure aliquot of hydrogen for the absorption kinetics
measurement. The milled composites revealed that the milling environment had little to
no effect on the desorption kinetics, although, the Ar milled TTIP composite desorbed
most of the hydrogen at a slightly lower temperature than that milled under H2.
Importantly, significant differences in the absorption kinetics were observed for Nb2O5
sample milled under Ar then H2. Both Nb2O5 and C60 composites samples milled under
Ar had superior absorption than those milled under H2. Conversely, the TTIP composite
sample milled under Ar showed poorer absorption kinetics than that milled under H2.
Based on these findings, the milling environment has been shown to have a significant
effect on the sorption kinetics, but there is no easy formula to determine which gas
173
produces the best results as it was dependent on the additives used. This work has been
published in IJHE [15].
Milling MgH2 with TTIP
A systematic survey of the effect of ball milling MgH2 with TTIP (organo-metallic
liquid) additive on the absorption and desorption kinetics was conducted. TTIP has been
used previously as an additive to improve the sorption kinetics of LiBH3-MgH2 system,
but had not been used before to improve the kinetics of MgH2 alone. Chapter 5 shows a
comparison between the effect of TTIP and the benchmark additive Nb2O5 on the
sorption kinetics of MgH2. Various amounts (0.5 – 2 mol%) of both additives were
milled with MgH2 for different times (0.5- 10 h), as well as hand mixing 1 mol% TTIP
with the pre-milled MgH2. A TPD experiment followed by a single pressure aliquot of
hydrogen were performed to measure the desorption and absorption kinetics,
respectively. TTIP was found to be equally effective as Nb2O5 as an additive and 0.5
mol% of TTIP was sufficient to enhance the kinetics of MgH2. In addition, the hand
mixed TTIP composite sample produced the best desorption results, but had poorer
absorption compared to the milled samples. The underlying mechanism by which the
additive TTIP enhances the sorption kinetics is still unclear. One of the possible
contributing reasons proffered is that the liquid state of TTIP above 290 K facilitates
good dispersion through the MgH2. This work has been published in the International
Journal of Hydrogen Energy [16]
Kinetics of MgH2 with different oxide/non-oxide Ti/V based liquid and powder
additives
To gain further information which might improve the understanding of the role of
transition metal oxides in enhancing the sorption kinetics of MgH2, a number of
selected transition metal organo-metallic oxide (Ti and V) additives as well as a
titanium-based organo-metallic chloride (C16H22Cl2Ti) were investigated and compared
to Nb2O5. In order to follow up the favourable results of using the liquid TTIP additive
(chapter 5), as well as to compare the relative effect of liquid and solid additives, the
majority of these additives investigated were also liquid at room temperature.
From the kinetics measurements, a number of conclusions can be drawn from this study.
For Ti-based additives it was found that TTIP and Ti-ethoxide which are liquid at room
temperature had greater improvement of the desorption kinetics compared to Ti-
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methoxide which is in powder form. The V-based oxides, all in liquid form, were found
to improve the kinetics compared to the preMgH2 without additives and all showed
similar enhancement of the desorption kinetics, but their absorption kinetics differed.
Finally, the Ti-chloride additive (1 mol% of BisTiCl2 in powder form milled for 1 h)
had the best desorption kinetics compared to Ti-based and V-based oxides, but,
interestingly, showed significantly poorer absorption kinetics than even the Pre MgH2
without additives. Overall the Ti-based oxides achieved better desorption enhancement
than the V-based oxides, whereas, V-based oxides had faster absorption and higher
uptake.
8.1. Future work
In this work different composite materials were synthesised to investigate their effect on
the sorption kinetics of MgH2. Different transition metal-based additives were chosen
(organo-metallic liquid/solid oxides, non-oxides and bench mark Nb2O5 for
comparison) and it was clear that each composite synthesised shared some common and
some different effects on the sorption kinetics of MgH2. In total, all the additives used in
this work (except for the Ti-chloride), made a significant improvement to the sorption
kinetics when compared to MgH2 without additives. However, to determine the
underlying mechanism by which these additives improve the kinetics needs more
research. Even with the benchmark Nb2O5, which has been extensively studied for more
than a decade, the debate on the exact mechanism is still open. Similarly, some of the
experimental work can be questioned and there are contradictions from one study to
another. Some studies have found that Nb2O5 does not undergo any structural
transformation during milling or cycling and only acts as a catalyst, and others have
found that Nb2O5 reduces during milling and/or further reduces during cycling with a
new ternary oxide phase MgNbxOy observed with XRD.
Generally, the existence of a thin surface oxide layer (MgO) covering the magnesium
metal particles, slows the kinetics of MgH2, due to poor diffusion rates of hydrogen
through MgH2, and yet some studies showed that milling MgO with MgH2 showed an
improvement in the hydrogen sorption of MgH2 by acting as dispersive agent during
milling reducing the particle size. Considering the difficulty of completely eliminating
oxygen contamination, and the excellent kinetic enhancement by various oxides, the
role of oxygen in the Mg-H system may be critical to the understanding of how
additives work.
175
The kinetics of absorption and desorption have different pathways with different rate
limiting steps– so different additives may perform differently for absorption and
desorption. An interesting result presented in this work, when Ti-chloride additive, a
solid, was compared with other transition metal oxides, it showed the best desorption
kinetics, but had extremely poor absorption results which was even slower than the ball-
milled MgH2 without additive. This outcome supports those studies that report that
some additives enhanced absorption and desorption differently. Future work might
combine different additives to optimally enhance the sorption kinetics of MgH2.
The search for a hydrogen material that meets the target capacities and practical
operating conditions, including moderate temperatures, such as could be integrated with
a fuel cell for on board vehicle applications is still ongoing. The materials investigated
in this study do not meet the optimal targets, but the studies provide additional
information which may assist in further understanding of their properties and their
effect on the sorption kinetics of MgH2.
176
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