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Development of Energy Conversion Systems for Carbon Dioxide
Utilization
BY
VENKATA ADITYA ADDEPALLI
B.E. Mechanical Engineering
Anna University, India, 2013
THESIS
Submitted as partial fulfillment of the requirements
for the degree of Master of Science in Mechanical Engineering
in the Graduate College of the
University of Illinois at Chicago, 2015
Chicago, Illinois
Defense Committee:
Amin Salehi-Khojin, Chair and Advisor
Jeremiah Abiade
Amid Khodadoust, Civil and Materials Engineering
ii
This thesis is dedicated to my dear family,
friends and well-wishers
iii
ACKNOWLEDGEMENTS
Firstly, I would like to express my gratitude to God for blessing me with good health and
capability to successfully complete my studies. Several people have provided me invaluable help
and support to complete this thesis and I would like to thank all of them. I am grateful to my
advisor, Dr. Amin Salehi-Khojin for giving me this wonderful opportunity to work with him. His
guidance, understanding, mentorship and support gave me the motivation to work on this
research topic in my graduate studies. I am also thankful to Dr. Amid Khodadoust and Dr.
Jeremiah Abiade for agreeing to be a part of my thesis defense committee.
Further, I would like to thank all my colleagues who have helped me. In particular I would like
to thank Mohammad Asadi, Dr. Bijandra Kumar and Dr. Kibum Kim for supervision and
guidance at every stage of my research. I am also grateful to Eric, Dave and Gary from the
machine shop for their advice and fabrication of various parts required for my experiments.
Finally, I am grateful to my family for their love, encouragement and constant support.
iv
TABLE OF CONTENTS
Chapter 1: Introduction ............................................................................................................ 1
1.1 Background ............................................................................................................ 3
1.2 Recent Efforts to Chemically Utilize CO2 ............................................................. 7
1.3 Global Energy Perspective ..................................................................................... 9
1.4 Technologies for Carbon Dioxide Reduction ........................................................ 11
1.5 Carbon Capture and Sequestration (CCS) ............................................................. 12
1.5.1 Components of CCS ............................................................................ 13
1.5.2 Challenges faced for CCS .................................................................... 13
1.6 Carbon Capture and Utilization (CCU) ................................................................. 17
Chapter 2: Electrochemical Reduction of CO2 .......................................................................................................... 21
2.1 Comparison of Proton Exchange Membrane Electrolysis and CO2 electrolysis ... 23
2.2 Electrochemical Reduction Mechanism ................................................................ 26
2.3 Terminology for Performance Matrix .................................................................... 30
2.4 Thermodynamic Considerations ............................................................................ 32
2.5 Trends in Electrochemical CO2 Reduction ............................................................ 34
2.5.1 Aqueous Solution .................................................................................... 34
2.5.2 Non-Aqueous Solution............................................................................ 36
Chapter 3: Electrochemical Flow Reactor and Electrode Design ............................................ 38
3.1 Introduction ............................................................................................................ 38
3.2 Transition from Static Flow Reactor to Dynamic Flow Reactor ........................... 40
3.3 Electrochemical Flow Cell Design ........................................................................ 42
3.4 Gas Diffusion Electrodes ....................................................................................... 45
3.5 Nafion Membrane .................................................................................................. 47
3.5.1 Preparation of Nafion Membrane ........................................................... 48
3.6 Reactor Operation .................................................................................................. 49
3.7 Ionic Liquid for CO2 Reduction ............................................................................. 51
Chapter 4: Results and Discussion ........................................................................................... 57
4.1 Experimental Procedure ......................................................................................... 58
4.2 Effect of Different Water Mole Fractions and Cathodic Cell Potential ................. 60
4.3 Performance of Transition Metal Dichalcogenides (TMDCs)............................... 68
4.4 Membraneless Electrochemical Flow Cell ............................................................ 72
4.5 Experimental Section ............................................................................................. 75
v
TABLE OF CONTENTS (continued)
4.5.1 Electrode Preparation .............................................................................. 75
4.5.2 Flow Cell Assembly and Testing ............................................................ 75
4.6 Effect of Water Mole Fraction ............................................................................... 76
Chapter 5: Photochemical Reduction of CO2 .......................................................................... 81
5.1 Introduction ............................................................................................................ 81
5.2 Experimental Arrangement .................................................................................... 86
5.2.1 Wireless Photochemical Cell Assembly .................................................. 86
5.3 Oxygen Evolution Catalyst .................................................................................... 89
5.3.1 CO-OEC Electro-deposition .................................................................... 89
5.4 MoS2 Drop Casting ................................................................................................ 91
5.5 Product Analysis .................................................................................................... 91
5.6 Preparation of Ionic Liquid .................................................................................... 92
5.7 Results and Discussion .......................................................................................... 93
5.8 PV Efficiency Measurement .................................................................................. 97
Chapter 6: Conclusion.............................................................................................................. 99
Cited Literature ........................................................................................................... 101
vi
LIST OF TABLES
TABLE PAGE
1. CO2 conversion into various products and their corresponding enthalpies ...........28
2. Variation of the electrical conductivity with respect to cation and
temperature; Anion is BF4 .................................................................................53
3. Solubility of CO2 in organic solvents, EMIM-BF4 and water ...............................54
4. The pH of EMIM-BF4 solution mixed with various amounts
and their corresponding pH values .. .....................................................................66
vii
LIST OF FIGURES
FIGURE PAGE
1. Times series graph measured at Scripp’s Mauna Loa Observatory from
1958 To 2014 ............................................................................................................... 3
2. Distribution of different Global Greenhouse Gas Emissions[6] .................................. 4
3. 2013 share of energy consumption at the source [7] ................................................... 5
4. 2013 CO2 emissions from fossil fuel combustion by sector and fuel type[71] ........... 6
5. Projection of Global energy scenario between 2000 and 2050, adopted from [5] ...... 10
Annual CO2 emission level from selected industries and partial pressures,
Adapted from [19]......................................................................................................... 15
6. A schematic diagram of the hydrogen fuel cell is described where an external
potential is applied to enable the flow of electrons and protons to produce
electrical energy. A solar energy utilizing reactor is shown in fig. b, here
the energy drives the reaction for the flow of protons and electrons. The solar
energy breaks the water molecule, thereby generating solar fuels H2 and O2 .............. 19
7. Electrolysis of water in proton exchange membrane electrolysis cell .......................... 25
8. Electrolysis of CO2 and water to produce synthetic gas ............................................... 26
9. Overview of electrochemical reduction of CO2 ............................................................ 27
10. Energy balance diagram for CO2 conversion and CO2 separation. The red
arrows show that energy is consumed for a particular process and the green
arrow for energy release ................................................................................................ 29
11. Electrode Materials and Reaction Products for electrochemical CO2 reduction
in aqueous medium ........................................................................................................35
12. Schematic diagram of a standard three electrode electrochemical cell ........................ 41
13. Schematic view of a Microfluidic Fuel Cell with a Nafion Membrane
separating the anode and the cathode for electrochemical CO2 reduction.
(1) Aluminium current collector(cathode) (2) Cathode gas diffusion electrode
(3) Cathode Catalyst (4) Liquid flow channel (catholyte) (5) Nafion Membrane
(6) Liquid flow channel(anolyte) (7) platinum catalyst (8)Anode gas
diffusion electrode (9) Aluminium current collector(anode) ........................................ 42
14. Sectional view of the microfluidic cell with flow of CO2, current collector plates,
Flowing electrolyte, electrode and membrane assembly. The flowing electrolytes
are separated by a nafion 117 ion exchange membrane. Two aluminium plates
serve as current collectors are placed on either side of the electrodes (GDE)
onto which catalyst are coated. A reference electrode placed in thee exit liquid
stream for monitoring individual electrode performance ............................................. 45
15. Cross-section Scanning Electron Microscopy (SEM) images of a cut
gas diffusion layer (a) front view at scalebar of 20μm (b) side view
viii
LIST OF FIGURES (continued)
at scalebar of 100μm .................................................................................................... 47
16. Structure of Nafion membrane ...................................................................................... 48
17. Schematic diagram of the Microfluidic Fuel Cell for electrochemical
reduction of CO2 ........................................................................................................... 50
18. Experimental arrangement of the Electrochemical Flow Cell ...................................... 51
19. Chemical Structure of EMIM – BF4 ionic Liquid ........................................................ 52
20. Schematic diagram proposing the free energy pathway for CO2 reduction
in aqueous solutions and in EMIM- BF4....................................................................... 55
21. The schematic diagram for the Electrochemical Reduction of CO2 to CO
using electrochemical flow cell. Ultra-high purity CO2 is supplied to the
reactor through the mass flow controller. The electrolyte for the
reduction process is supplied through the syringe pump and is collected
in a beaker after the reaction. The gases exiting the reactor are sent
to the Gas Chromatograph for gas detection ................................................................. 58
22. Effect of water mole fraction on the current density at a
cathode cell potential of -1.2V, - 1.4V, -1.6V and -1.8V with a MoS2
cathode and a platinum anode, EMIM – BF4 as a catholyte, 0.1M H2SO4
as an anolyte and 99.99% ultra-high pure CO2 gas stream inlet ................................... 61
23. Shows the comparison between the current density from an
uncoated gas diffusion electrode (GDE) and a GDE coated with
MoS2 nanoparticles ....................................................................................................... 62
24. Effect of different water mole fractions at different cathodic cell potentials
on the faradaic efficiency for Co2 conversion to CO using MoS2 cathode,
platinum anode, EMIM BF4 as a catholyte, 0.1M H2SO4 as an anolyte and
99.99% ultra-high pure carbon dioxide as a gas inlet ................................................... 63
26. CO peaks observed from the gas chromatograph as a function of cathodic
cell potential (CCP) at 90 mol% water mole fraction.
TCD -Thermal Conductivity Detector .......................................................................... 65
27. The photograph shows vials with 5 different concentrations of EMIM – BF4
and water mixtures used in the experiments to analyze the performance
of the MoS2 catalyst. From left, 0 mol% water, 10 mol% water,50 mol% water,
90 mol% water and 96 mol% water .............................................................................. 67
28. Current density curves for Tungsten Selenide (WSe2), Molybdenum
Selenide (MoSe2), Tungsten Disulphide (WS2), Molybdenum Disulphide (MoS2)
and Silver nanoparticles (Ag) which were coated onto the gas diffusion
electrode (GDL) and tested in a microfluidic fuel cell with
90 mol% EMIM – BF4 ionic liquid on the cathode side and 0.1M H2SO4
on the anode side with nafion membrane as a separation medium between
the two chambers. ......................................................................................................... 69
29. Picture showing the coating of five different catalysts on to the
gas diffusion electrode (GDE). From the left Ag NPS, MoS2, WS2,
MoSe2 and WSe2 ........................................................................................................... 71
30. Faradaic efficiency curves for Molybdenum Disulphide (MoS2),
Molybdenum Selenide (MoSe2), Tungsten Disulphide (WS2),
Tungsten Selenide (WSe2), and Silver nanoparticles (Ag)
ix
LIST OF FIGURES (continued)
31. Exploded view of single compartment microfluidic flow cell .................................... 72
32. Sectional view of Single compartment membraneless flow cell
with gas and electrolyte flow ...................................................................................... 73
33. Effect of adding water to the ionic liquid on the current density curves
for CO2 reduction to CO. These experiments were done at varying cell
potentials with platinum anode and MoS2 as cathode ................................................ 77
32. Faradaic Efficiency for CO production at different cell potentials
and varying electrolyte concentrations ....................................................................... 79
33. Schematic diagram utilizing solar energy to generate solar fuels ............................... 82
34. Structure of a triple-junction stainless steel substrate based solar cell ....................... 84
35. Schematic of the Photo-electrochemical Chamber. 1– 2 PV cells in
series connected by a copper tape, 2-Cobalt coated on ITO,
3-Potassium Phosphate buffer solution,4-Nafion (117) Membrane,
5-10% EMIM-BF4 solution, 6-Scotch tape to generate headspace,
7-WSe2 on Stainless Steel substrate(drop casting), 8-Gas bubbles
generated through chemical reaction .......................................................................... 88
36. (A) The solar cell dipped in cobalt(II) nitrate hexahydrate and
potassium based buffer solution (KPi). The working electrode (green)
connected to the ITO coated layer of the solar cell, Pt mesh (red) served
as counter electrode and Ag/AgCl (white) was used as reference electrode
(B) An uncoated solar cell with ITO substrate and (C) Solar cell
coated with cobalt through electrodeposition ............................................................. 90
37. MoS2 drop casted onto the stainless steel substrate of the solar cell .......................... 91
38. PV cells connected in tandem through a copper tape with
cobalt coated on the ITO substrate which serves as the photo anode
and initiates the water splitting reaction. The MoS2 is coated on
the stainless steel substrate and serves as the photo-cathode
which catalyzes the conversion of CO2 into syngas .................................................. 92
39. Pictures showing the experimental setup of the photo-electrochemical cell
using a 300W Xe arc lamp at 1 sun irradiation connected to the
gas chromatograph for gas sampling .......................................................................... 93
40. Variation of pressure as a function of solar irradiance ............................................... 94
41. Variation of Pressure change (Pa) and current density (mA/cm2) as a
function of external bias (V) ....................................................................................... 96
42. Gas samples curves from the gas chromatograph. (A) Calibration curve
for CO generation showing the different gas detected in the sample using
a thermal conductivity detector and (B) Gases detected from the sample
taken from the Photo-electrochemical cell ................................................................. 97
43. Gas samples curves from the gas chromatograph. (A) Calibration curve
for CO generation showing the different gas detected in the sample
using a thermal conductivity detector and (B) Gases detected from the sample
taken from the Photo-electrochemical cell ................................................................. 98
x
Summary
Growing concerns on greenhouse gas emissions due to industrial activity and decrease in
conventional energy resources present significant challenges to satisfy the world’s energy
demand. Minimizing CO2 emissions by developing efficient methods to capture and store or
convert CO2 into useful chemicals is indeed critical for environmental protection.
Electrochemical and photochemical reduction of CO2 have been identified as viable technologies
to recycle the CO2 to reduced forms and store energy in chemical form. Energy storage in
chemical form is highly desirable as it generates carbon neutral fuels for portable applications
which could replace those driven by fossil fuels maintaining an environmental stability.
Previously, works have shown the discovery of co-catalyst systems with different noble
metals such as Ag and ionic liquids such as EMIM-BF4 which have opened up several avenues
for electrochemical reduction of CO2. This thesis reports the development and validation of
Electrochemical Flow Cell as well as a Photo-Electrochemical (PEC) Cell for electrochemical
and photochemical reduction of CO2, respectively. These systems have been designed and
fabricated to exhibit the outstanding performance of inexpensive and earth-abundant transition
metal dichalcogenides (TMDCs) materials as electrocatalysts along with ionic liquid (EMIM
BF4) for conversion of CO2 into energy rich intermediates. The results of these experiments
suggest that the co-catalyst system (TMDCs and EMIM-BF4) could convert CO2 in a fast, energy
and cost effective way which could open doors for CO2 conversion in ambient conditions.
1
1. Introduction
This work will summarize the current status, future challenges and opportunities for
electrochemical and photochemical conversion of carbon dioxide into value added products at
ambient conditions. In this chapter a background of the various sources of the emissions of CO2
and their subsequent impacts, focusing mainly on the changes in climatic conditions will be
discussed. Furthermore, various possible alternatives and their feasibility with regards to the
generation of carbon neutral fuels will be discussed and a relationship between their generation
and CO2 reduction will be established.
During the last few decades, the levels of CO2 have been steadily on the rise and are
expected to rise significantly keeping in mind the population rise and the increase in energy
consumption. To meet this ever-increasing demand principally due to economic growth and
industrial development, particularly in the developing nations, our society has turned to the
combustion of fossil resource such as petrol, coal, natural gas and wood. The use of these
resources inevitably comes with the release of many compounds which have a negative impact
on the environment with the release of gases such as sulfur dioxide (SO2), nitrogen oxides (NOx)
and carbon dioxide (CO2). The supply of safe, ecological, and clean energy is an urgent
challenge facing humanity in the 21st century.
On the surface, CO2 seems to be harmless; it is colorless, odorless and non-toxic to
humans. With about 300 billion tons, CO2 is the fourth most abundant gas in the atmosphere,
after Oxygen, Nitrogen and Argon[1]. CO2 has been present in the atmosphere for most of the
geological time and is natural, fluctuating constituent. Since the beginning of the industrial
revolution the concentration levels of this gas have been constantly varying from about 280ppm
2
in the 1960s to about 380 in 2009. With concentration levels now averaging around 400 parts per
million by volume; it becomes a very important component of the biosphere[2].
The increase in the amount of CO2 emissions in the atmosphere is majorly attributed to
the burning of fossil fuels such as coal, natural gas and oil, which occupy a healthy 80 – 85% of
the world’s energy resources. Keeping in mind their ability to supply the needs of the rising
energy demand as well as being inexpensive energy carriers, these resources are here to satisfy
the majority of the energy needs at least for the next few decades. Therefore, a further rise in the
CO2 concentration levels of 50- 100% are envisaged by the year 2030.
These rapid changes in the concentrations of CO2 levels have started to cause many
unpredictable changes in the environment. Figure 1 shows the CO2 concentration level (red
curve), measured in terms of a mole fraction in dry air of CO2 over the past 50 years at the
Mauna Loa Observatory in Hawaii. The black curve represents the seasonally corrected data.
The problem is the long term rise in the concentrations in the atmosphere and the increased heat
effect impacting the climate system.
CO2 is a greenhouse gas and that is transparent to light, but rather impervious to heat
rays. Therefore, CO2 prevents the heat generated on the Earth back into Space leading to
“greenhouse effect”. Global warming, a phenomenon has received widespread attention from the
emission of the greenhouse gases. Hence the majority of the focus of today’s political and
scientific scenario is to address the emissions and lower the concentration.
3
1.1 Background
Carbon is a fundamental building block for the ecosystem and is very essential for many
processes. The regions that predominantly generate CO2 are known as sources, while those that
absorb CO2 are known as sinks. CO2 acts like a huge blanket over the planet, leading to the
trapping of the longwave radiation, which could otherwise lead to radiating the heat away from
the planet.
Figure 2, shows the contribution of different greenhouse gases emitted into the
atmosphere. Most of the energy consumption in today’s world comes from burning fossil fuel
resources, including petroleum, coal, natural gas and wood[3]. Despite their low cost and high
Figure 1: Time series graph measured at Scripp’s Mauna Loa observatory from1958 to
2014
4
quality, fossil fuels are coupled with the release of many compounds such as CO2, SO2, NOx
which causes adverse effects on the environment such as climate change[4]. Through various
research studies, it has been estimated that the energy consumption would rise by 2.3% yr-1
due
to population and economic growth[5]. The Intergovernmental Panel on Climatic Change (IPCC)
in its most recent report, provided extensive scientific models and results showing the rise in
global mean temperatures[6]. Figure 3 shows the 2013 data of energy distribution from different
energy sources as developed by the EPA to satisfy the rising demands. The pie chart shows that
there substantial dependence on fossil fuels to satisfy the estimated demand (89%).
Nitrous
Oxide
8%
Methane
14%
Carbon Dioxide
(Decay of
biomass,deforestrat
ion ,etc.,) 17%
Carbon Dioxide
(Other)
3%
Carbon Dioxide
(Fossil Fuels)
58%
Figure 2: Distribution of different Global Greenhouse Gas emissions [6]
5
Figure 4, shows the 2013 CO2 emissions data by sector and fuel type. It has been
predicted that in the coming few decades (2010 -2060) a cumulative amount of CO2 will increase
up to 496 gigatonnes due to combustion of fossil fuels with the existing infrastructure[2].
Further, the climatic responses to the current CO2 is uncertain and with the rising concentrations
this risk of uncertainty would be monumentally.
Simple analyzation of recent emission trends suggests that irrespective of the persistent
efforts to limit CO2 emissions, the CO2 concentrations would rise up exponentially by mid
twentieth century[7]. As a result there is an urgent need for development of alternative sources
and controlling the CO2 emission rates together. As discussed previously, with the rising
population and economic needs, it is not possible to find immediate alternative for replacing
Petrolum 39%
Natural Gas 30%
Renewable Energy 10%
Nuclar Electric Power 1%
Coal 20%
Figure 3: 2013 share of the energy consumption at the source [7]
6
fossil fuels. Thus recent efforts are focused on reducing the CO2 emissions through capture,
sequestration or conversion to useful energy rich products.
Among these approaches, CO2 utilization is very attractive as it offers green solutions for
generation of renewable hydrocarbon fuels and simultaneously reduces CO2 emissions. CO2 can
be mainly converted into value added products mainly through photo, electro-chemical reduction
and also by catalytic hydrogenation process. However, in the former case, hydrogen produced by
water-splitting reaction is an essential reactant for CO2 conversion which hampers the energy
and cost efficiency of the process. Here, it must be noted that CO2 is the highest oxidized state of
carbon. Hence, the central problem associated with photo and electro-chemical is the high
overpotential and low conversion rate due to the stability of the CO2 molecules. Not surprisingly,
the overpotential should be minimized and the reduction rate must be increased in order to make
this technology economically feasible. Converting CO2 into useful products such as synthetic
0
500
1000
1500
2000
2500
MM
T C
O2
Eq
Petroleum
Coal
Natural Gas
Figure 4: 2013 CO2 Emissions from fossil fuel Combustion by sector and fuel type. The
data was obtained from the following source[8].
7
gas, methanol or formic acid using solar energy as an initiator presents a convenient way to
generate renewable fuels.
1.2 Recent Efforts to Chemically Utilize CO2
The rise in the CO2 concentration levels has led to an increase level of awareness about
the trends in global warming, which could lead to severe global climatic change. Several
approaches are being reviewed so as to alleviate the problems associated with excess CO2 being
produced as a by-product of fossil fuel combustion. Using CO2 as a feedstock to produce a wide
array of usable organic compounds by using renewable energy sources as an input for processing
(solar, wind, geothermal, etc.) presents a sustainable and long term resource. The equations (1.1
– 1.5) show the different organic compounds to name a few such as urea, salicylic acid, dimethyl
carbonate and acetic acid[9][10].
Urea: CO2 + 2NH3 NH2CONH2 + H2O (1.1)
Salicylic Acid: C6H5OH + CO2 C6H4OHCOOH (1.2)
DMC: CO2 + 2CH3OH CH3COCOOCH3 + H2O (1.3)
CO (syngas) = CO2 + H2 CO + H2O (1.4)
Acetic Acid: CH4 + CO2 CH3COOH (1.5)
According to a recent report, 105 million tons of CO2 are used for the production of urea,
salicylic acid( 90000 tons), cyclic carbonates (80,000) and polypropylene carbonate ( 70000
tons) are being produced on industrial scale[11]. Industrial applications of CO2 have been
increasing in the recent years. It has a growing application as a fluid in the dry – cleaning
industry, food and agro-chemical industry, fire extinguishing applications and water treatment
8
applications[9]. “Carbon dioxide can be reacted directly with hydrogen to yield a mixture of
water and carbon monoxide via a reverse water gas shift reaction or produce methanol and water
directly”[1].
CO2 + H2 CO + H2O (1.6)
CO2 + 3H2 CH3OH + H2O (1.7)
The reversible gas shift reactions can difficult to drive to completion as they require high
energy for completion. The development of supercritical boiler has opened up a new avenue for
utilization of CO2 as a working fluid. Using CO2 as a working fluid can also be considered as
part of Carbon Capture and Storage (CCS). The gases emitted from a combustible source could
be captured and filtered using a membrane technology or other means. The captured CO2 could
then be transported to the end user such as power plants and utilized as a working fluid for their
respective applications[12]. The advantages and disadvantages of CCS would be discussed in
detail in the next section. Apart from the water treatment application, CO2 can be recovered at
the end of the application for all other applications.
Use of carbon dioxide as both a building block in the synthesis of chemicals as well as a
working fluid in technological applications minimizes the dependency on fossil fuels and leads
to a development of a useful product. With all these perspective advantages of CO2 as a useful
building block, a few questions need to be answered such as a) amount of CO2 required for each
process, b) expenditure involved, c) amount of control on global warming, etc. Implementation
of a life cycle assessment (LCA) would make way for answering the above questions[13].
9
1.3 Global Energy Perspective
Figure 5 shows the global energy consumption which was analyzed in a research study by
Lewis and Nocera [5]. The supply of clean, secure and sustainable energy is one of the daunting
tasks of the 21st century. The global population in 2050 is going to rise to about 9.1 billion, an
increase of 3 billion from 2000. This would mean a subsequent increase in the energy demand to
satisfy the needs of the rising population. The figure shows the distribution of energy generated
from different sources. In 2001, the average energy consumption was estimated to about 13.5
TW, out of which 86% was supplied through the fossil fuels(oil, gas and coal) and the remaining
coming from biomass and a small fraction from the renewable sources[14]. The demand in
energy requirement is expected to rise to about 40 TW by 2050 due to economic and population
growth. These calculations were done using the below equation which was based upon three
important factors:
“E = N. (GDP/N). ( E / GDP)” [5]
Global Population (N)
Globally Averaged Gross Domestic Product ( GDP)
The Energy consumed per unit of GDP
In order to satisfy this rising need of energy and meet the optimistic needs of the future (40 TW),
there needs to be a shift from carbon intensive energy generation such as power generation using
coal to a carbon free process as the predicted reserves for these fossil fuels are limited and can be
exhausted in certain period of time. With future predictions, the world carbon emission levels
would raise considerable from 6.6 GtC yr -1
in 2001 to11 GtC yr-1
by 2050[15].
10
Hence, significant contribution from the carbon free power to the energy – mix is very essential.
This approach would enable prolonged use of the available fossil fuel reserves as well as
stabilize the CO2 concentration levels[16]. Hence, a major part of this additional energy
requirement could be satisfied through:
1. Nuclear Energy
2. Carbon Capture and Sequestration
3. Utilizing Renewable energy as a source for the generation of clean energy
For nuclear energy, the process is pretty straight forward with regards to the steady
supply of power and also the infrastructure requirement. But the widespread implementation
would lead to exhaustion of the uranium reserves. Also, the construction of the power plants
would need to proceed at a rapid pace to be able to produce large volumes of power.
0
5
10
15
20
25
30
35
2000 2050
Ene
rgy
(TW
)
Other Renewables
Nuclear
Hydro
Biomass
Coal
Gas
Oil
Figure 5: Projection of Global energy scenario between 2000 and 2050, adapted from [5]
11
Carbon Capture and Sequestration (CCS) which will be discussed in the next section has
its own challenges with regards to transportation, separation and storage. Hence, CCS technically
would help in buying time, but isn’t a permanent solution for reduction of CO2 content in the
atmosphere.
The third approach would be to use renewable energy. However, the intermittent nature
of these sources reduces their capability to satisfy the current and future demand as a whole. Of
the available renewable technologies, solar energy has huge potential has the amount of sunlight
hitting the surface in 1 hour (4.3*1020
J) is greater the total energy consumption per year (4.1 *
1020
J)[5]. Rise in the use of solar and wind energy would require the development of energy
storage process, such as electrochemical and photochemical reduction of carbon dioxide.
Electrochemical reduction of CO2 is a very resourceful process as the generated products can be
utilized for power generation, transportation or as chemical feedstocks. Integrating the
photocatalytic H2O splitting to fix CO2 and produce fuels is a very attractive approach for
minimizing CO2 emissions.
1.4 Technologies for Carbon Dioxide Reduction
With the rising levels of CO2 in the atmosphere as well as the energy crisis booming
large, there is an immediate necessity to address these issues which has prompted international
and national agencies such as the Intergovernmental Panel for Climate Change (IPCC) and the
United Nations Framework Commission on Climate Change to design systems so as to reduce
the CO2 emission. Carbon capture and sequestration (CCS), Carbon Capture and Utilization
(CCU) have emerged as front runners for CO2 reduction systems. CCS has been developed and
integrated at a few large emission sources such as power plants, has proposed a strategy to
12
reduce the CO2 content in the atmosphere. In order to reduce the burden on fossil fuels, two other
technologies natural gas to liquid (GTL) and coal to liquid (CTL)[1] are also being researched as
they produce syngas(a mixture of carbon monoxide and hydrogen) an output which then would
be converted into liquid hydrocarbons through a series of chemical reactions through the Fisher –
Tropsch process[17][18]. CCS implementation would result in CO2 reduction in the atmosphere
but by utilizing this CO2 i.e., through GTL and CTL would generate a useful end product such as
liquid fuels subsequently will also reduce the GHGs emissions. Several other technologies are
being developed to generate carbon neutral fuels such as, production of biofuels from algae and
woody mass. Renewable technologies such as wind, solar, hydro, etc.., are also being scaled up
to generate carbon neutral fuels. As discussed in the previous section, CO2 is already being used
to produce urea, salicylic acid, acetic acid, etc. But the demand for these products isn’t as high as
the amount being produced through combustible related activities. At this stage, the
technologically feasibility and economic sense regarding the large scale implementation of these
technologies is still unclear and further development through life cycle analysis study (LCA)
would present an overall picture.
1.5 Carbon Capture and Sequestration (CCS)
The main objective of Carbon Capture and Sequestration(CCS) is to separate the CO2
from the industrial effluents derived from fossil fuel combustion, transporting them to a reservoir
and storing at these reservoirs for a period of time[19]. The rationale thinking behind CCS is to
persist with the use of fossil fuels and subsequently reduce CO2 usage. The storage facilities for
CCS are highly dependent on the geological location of the reservoir. Some reservoirs might be
able to store CO2 for about 500 years while the others might give out CO2 within 20 years.
13
Hence, CCS gives a temporary relief, but not a permanent solution. Also, there are other issues
related to scaling up CCS which will be discussed in the next section:
1.5.1 Components of CCS
Capture and separation: This is the most important component of CCS. Development of
systems for capture and separation, scrubbers, membranes, aquifiers, absorption and
adsorption based separation, are very necessary to be implemented at the source (e.g.:
Powerplants) which will help the separation of CO2 from the effluent streams. Capture is
generally required to economically store and transport CO2.
Transport: The separated CO2 is transported from the source to the reservoir. This
transportation is generally done using pipelines as the reservoirs are located in areas
inaccessible by general transport.
Injection: Injection relates to the injection or deposition of CO2 into the different storage
reservoirs. The storage reservoirs that are being used include deep oceans, ocean
sediments and mineralization.
Monitoring: The main purpose of monitoring is to ensure that the injected or deposited
CO2 remains within the reservoir. Also, since CO2 is neither flammable nor toxic, it
doesn’t pose any direct hazards.
1.5.2 Challenges faced for CCS
In the Fourth Assessment report of the IPCC, Carbon Capture and Sequestration (CCS)
has been referred to as the only way to be continually utilize fossil fuels. However, CCS by itself
has issues with scaling up and deployment that are yet to be addressed. Although a number of
CCS units are being integrated at the source, a very few units have been integrated in the United
14
States which indicates the need for development. As discussed in the previous section, the four
components of CCS have their own requirements and exhibit challenges at different situations.
Distance of the source from the sequestration site, dilution of the CO2 gas in the effluent stream
and the size of the source and the reservoir are some of the uncertain questions which need to be
found[19]. The Carbon sequestration technology has already been developed on Mt scale,
mainly for separating CO2 from Natural gas and coal.
To have an active CO2 capture mechanism, it must be made sure that the materials are
regenerative given the total magnitude of CO2 emissions worldwide. There are certain obstacles
which hinder the scaling up of CCS approach. Three separation issues are discussed in the
review [10] :
Separation of CO2 from flue gases
Separation of CO2 from natural gas wells
Separation of fuel gases
The separation of CO2 in a post combustion process is very suitable for implementation
in presently operating processes that generate flue gases. The flue gases come out at low
pressures and the CO2 concentration in the volume is comparatively low with flue gases having a
mixture of gases such as SO2, CH4, N2O, CO2, etc. Hence, separate sources must be designed to
handle large volumes and segregate the respective constituents.
15
Nevertheless, the ability to utilize CO2 mainly depends on the source, i.e., purity and
partial pressure. Figure 6, shows the CO2 emissions from selected industry sectors as well as
their respective partial pressures[20]. It can be seen that the partial pressure of the flue gases
produced through combustion of fossil fuels is at a minimum. Subsequently, separating CO2
from the largest stationary sources is economically not attractive.
Natural gas reserves have about 40% CO2 and N2, hence for utilization of these reserves
would be acceptable only when the CO2 capture and sequestration takes place at the source.
Therefore, this process requires separation at high pressure so as to separate CO2 from other
0
2
4
0
2000
4000
6000
8000
10000
CO
2 p
ress
ure
(b
ar)
Over
all
CO
2 e
mis
sion
s (M
t yea
r- 1)
Overall CO2 Emissions CO2 Pressure
Figure 6: Annual CO2 emission level from selected industries and partial pressures, adapted from
[19]
16
natural gas components. Separation with fuel gas (e.g., Synthetic gas) also takes place at high
pressure and high temperature (250 – 4500
C). Additionally, a key challenge for separating
materials is to differentiate between different gases as their properties (such as diameter) are
quite similar. Extensive research work was done by Krishna[21] regarding the development of
materials investigating the relationship between adsorption and diffusion selectivity. Capture of
CO2 from ambient air sources i.e., pre-combustible sources such as home furnaces presents
another challenge as the amount of CO2 present in the air is very minimal (0.04%) and separation
of large volumes would require state of the art absorption materials.
However, when designing a system to separate CO2 from air, several factors must be
considered. Some of these factors are a) the requirement of a high thermodynamic barrier due to
the low concentrations of CO2 in air b) the cost and energy for the materials required for moving
great quantities of air through a certain structure. “The energy required to separate the gases
scales as a log of partial pressure and if we consider the excess energy required for compression
of gases to pipeline pressures”[7], it is evident that thermodynamically higher excess energy is
required for air capture in comparison to post combustion capture. Hence, it is clear that air
capture (pre-combustion) will require higher economic investment when comparing with
designing of post-combustion systems if both facilities are designed and operated under identical
economic conditions[7]. The findings by national agencies like the U.S. Department of Energy
and the IPCC indicate uncertainties over the geological storage of CO2 ; the behavior of CO2
when injected into the subsurface, such as travel and storage[19].The seismic data collection
could give a trend about the structural formation in the subsurface which would be ideal in the
selection of the geological site, but practical challenges which depend on the properties of the
rocks such as pore space available for storage will essentially determine the storage efficiency.
17
When CO2 is injected into the storage sites, such as rocks, several physical processes
govern the storage at these sites. Raza (2009) did a research study on the leakage capacity, such
as leakage time, leakage amounts and leakage location of the geological sites. According to the
report, CO2 injected to a depth of 800m or more below the surface, was very appropriate for
storage as increased depth results in increased pressure for storage and also meant the CO2 was
in supercritical or dense phase. Also, a layer of impermeable rock “cap rock”, on top of the
formation ensured the CO2 would be stored and wouldn’t rise and escape to the surface. The
results of the report discussed that the leakage potential of the sites was very minimum, which
meant a well-chosen site would offer security for a long period of time[22].
1.6 Carbon Capture and Utilization (CCU)
The potential for carbon dioxide conversion is substantial. As discussed previously,
electrochemical researchers are trying to develop various pathways to efficiently utilize CO2 and
produce useful products such as fuels and chemicals[1][21]. Although this technology is yet to
be commercialized on a large scale, the interest in this topic has been increasing the past few
decades.
Arguably, one of the most recognizable ways for CO2 emissions is through the
combustion of fossil fuels for the generation of electricity. Renewable sources such as wind,
hydro, solar, etc. have been used for generation of clean electricity with generating any
combustible by-products. However, the electricity generated from these sources is not enough to
satisfy the rising demand. Hence, we need to develop an effective integration of renewable
systems with fossil fuel systems, to mitigate the excess CO2 being produced. This integration
18
must have the capability to fuel the energy requirement in the transportation segment as well as
increase the electrical energy output by using the carbon neutral fuels is the highlight of CCU.
Fuel cells enable efficient conversion of chemical energy into electrical energy without
the Carnot cycle limitations. Additionally, use of alternative fuels such as hydrogen, formic acid,
biofuels and production of energy on-site make fuel cells a very likable means to generate clean
energy. Figure 7 compares a conventional hydrogen fuel cell (PEMFC) where hydrogen and
oxygen are supplied as inputs to the cell and power is generated as an output through the reaction
between the hydrogen and oxygen ions.
Here, hydrogen is oxidized to H+ ions at the anode and oxygen reduction reaction (ORR)
takes place at the cathode. An external voltage is supplied to drive the reactions and an ion
exchange membrane is used so as to prevent gas crossover.
Figure 7 also shows a schematic diagram of an electrolyzer similar to a conventional fuel
cell but uses light as a driving force for the reactions to move uphill. Here, we borrow the storage
model from nature where energy is stored in chemical bonds and carbon neutral fuels are
generated in an artificial photosynthesis process. Artificial photosynthesis, where the spectrum of
light is used to fix carbon dioxide and generate useful intermediates could prove to be a feasible
path to reduce global warming as well as generate renewable fuels. The primary step of this
process involves the conversion of sunlight into wireless current which oxidizes water to oxygen
and generate two hydrogen protons. The cathodic current reduces the protons to hydrogen and
generates other organic compounds such as methanol, synthetic gas through fixation of the
captured CO2. Thus, through the photosynthesis process, the sunlight is stored in chemical bonds
and utilized to evolve oxygen and nature’s form of hydrogen.
19
Anode Membrane Cathode
Figure 7: A schematic diagram of the hydrogen fuel cell is described where an external potential
is applied to enable the flow of electrons and protons to produce electrical energy. A solar energy
utilizing reactor is shown in fig. b, here the solar energy drives the reaction for the flow of
protons and electrons. The solar energy breaks the water molecule, thereby generating solar fuels
H2 and O2.
The artificial leaf system could be observed as a charge separating assembly with macro
or nanoscale catalyst for enhancing the oxidation and reduction with a light collection source for
driving this photosynthesis reaction to generate the desired product. A solar PV assembly is used
to generate necessary electron – hole pairs by shining light onto the surface of the cell and the
catalysts on one side or either sides of the cell capture these ions. The energy stored in the bonds
lead the rearrangement of water to produce hydrogen and oxygen. Storing energy in chemical
H2 O2
e- e
-
4 H+
Solar PV Cell
H2O
OO
-
-
+
+
+
-
4H+
O2
2H2
Solar Energy Utilizing Reactor
Fuel Cell
20
form is very useful as it addresses two critical issues (1) Global Warming (2) Energy Crisis. A
detailed view regarding the science, advantages and disadvantages of the electrochemical
reduction of CO2 have been discussed in the next chapter.
21
2. Electrochemical Reduction of CO2
The amount of CO2 generated must be balanced out with the amount consumed; this
helps to maintain an environmental stability. Unfortunately, due to the increased human
industrial activity, CO2 balance in atmosphere has been disrupted, resulting in global warming
which is an urgent issue. Hence, development of CO2 mitigation techniques for reducing the CO2
production as well as generating useful products from CO2 has become a priority.
Different approaches have been proposed, namely CO2 capture and sequestration (CCS)
or CO2 capture and utilization to produce into useful products. The conversion of CO2 into useful
products is generally obtained through chemical processes such as photocatalytic and electro-
catalytic reductions[23][24]. A brief introduction for CCU was given in the previous chapter.
Also, a short summary regarding the components and challenges faced in CCS has been
discussed in the previous chapter. A detailed study about the need for electrochemical reduction
of CO2 and the challenges involved will be discussed in this chapter. This chapter also
summarizes the advances in developing materials and techniques for efficient CO2 reduction
reported in numerous literatures.
The primary focus of this research is to maximize the faradaic efficiency, towards the
generation of energy specific value added products[1]. The majority study on electrochemical
CO2 reduction has been done on laboratory scale so as to investigate and maximize product
yields before commercializing this technology. Most of the studies performed use water splitting
reaction at the anode as a reaction initiator with the generation of hydrogen, although hydrogen is
fed directly into the system in certain cases.
22
There are many barriers which hinder the process of CCU such as, (1) high costs of CO2
capture and separation techniques; (2) high energy requirements for CO2;conversion process (3)
lack of industrial motivation towards generation of CO2 based chemical; (4) limited number of
incentives for such approaches [1][4][7]. Despite such hindrances, Carbon capture and storage
(CCS), carbon capture and utilization (CCU) are viewed as potential sources for energy and
environmental research.
Electrochemistry is “a science that deals with the relation of electricity to chemical
changes and with the interconversion of chemical and electrical energy” [25]. “Electrochemistry
can be considered as having two kinds of reaction mechanisms: a chemical reaction actuated by
an external power supply, or voltage generated due to a chemical reaction”[26]. In this thesis I
will describe pros and cons of both the approaches.
Electrochemical chemical oxidations or reductions taking place at controlled potentials by
adding or withdrawing electrons has opened up new avenues for research in the field of
environmental engineering and renewable sciences. In the last few decades, electrochemical
reduction has attracted great attention due to its many advantages, (1) the product generation is
controlled by an electric potential; (2) the electricity used the drive this process is from a
renewable source, thereby reducing the scope of generation of any CO2; (3) the reactor designs
being small, modular, inexpensive and easy to scale up for industrial applications.
The electrochemical reduction of CO2 is of significant potential interest as a component
of the carbon energy cycle. The carbon energy cycle is briefly described below:
CO2 + Energy Methane CO2 + Energy
23
Also, electrochemical reduction is an outstanding prospect because CO2 is an crucial by-
product in any process involving oxidation of carbon compounds. Further, the reduction of CO2
leads to generation of carbon feedstock thereby becoming a source for the manufacture of
chemicals. The electrochemical reduction of CO2 on different metal electrodes yields different
organic compounds like CO, CH4, C2H6 and the other alcohols, etc. The formation of these
organic compounds is highly dependent on the type of electrolyte, the catalyst chosen and the
cathodic reaction. This approach must take into account the significant challenges for
overpotentials due to the kinetic barrier of reducing the CO2 molecule. Investigations on CO2
reduction could be categorized into two groups based on the catalytic system used:
1. Homogeneous Catalytic System
2. Heterogeneous Catalytic Sytem
This thesis study focusses on the development of electrochemical techniques for CO2
utilization at electrodes using a heterogeneous catalytic system.
2.1 Comparison of Proton Exchange Membrane Electrolysis and CO2 Electrolysis
The operation of a CO2 electrolysis cell is basically running a hydrogen fuel cell in
reverse[27]. In recent years, several research projects have been developed to enhance fuel cell
performances by developing efficienent catalysts, electrodes and cell configurations. These
parameters also apply to the development of CO2 electrolysis processes, but require different
certain aspects to be monitored and optimized separately. For example, both low temperature
fuel cells and CO2 electrolyzers are often limited by cathode performance. Thus, both these
setups need their cathodic reaction kinetics to be improved through an active catalyst. However,
for CO2 electrolyzer other than the activity, the catalyst must be highly selective so that the
24
formation of desired products are higly favored and other by—products are suppressed. Also,
removal of products from the catalyst layer so as to prevent blockage of active sites must be
thought off as it is important for both the fuel cell and CO2 electrolysis. Nevertheless, the
strategies must be very different for each cell configuration. A conventional hydrogen fuel cell
is dominated by oxygen reduction reaction (ORR) and generates water, thereby leading to water
management issues[28]. For a CO2 electrolysis cell, CO2 reduction takes place on the cathode
thereby leading to the formation of gasous products (eg., CO and H2) and liquid products[29].
Thus, efficient removal of these gaseous and liquid products is very important in boosting the
performace of the process. In the following paragraphs, a detailed diagramatic explaination of
the two cell configuarations will be discussed.
Figure.1 shows a schematic diagram of the PEM electrolysis. In PEM electrolysis, the
system operates by conducting protons through the proton exchange membrane and recombines
them to form gasous H2 at the cathode. Water is given as an input for the anode, where it is
absorbed on to the anode thereby releasing two protons and two electrons for the power supply.
The protons travel through the ion – exchange membrane and then recombine at the catalyst site
with the electrons to forming H2(g).
Anodic Half Reaction : H2O(l) ½ O2 (g) + 2H+ + 2e
- (2.1)
Cathodic Half Reaction : 2H+ + 2e
- H2(g) (2.2)
Overall Reaction : H2O(l) H2 (g) + ½ O2 (g) (2.3)
Figure 2 shows a schematic diagram of water – CO2 electrolysis cell that has almost
similar features as a water electrolysis cell explained previously. The anode side reaction remains
ideantical to the PEM cell with formation of H+ ions being the key part. The H
+ is transported to
the cathode side through the electrolyte which in most cases remains to be the proton exchange
25
membrane. Further description releated to the reaction mechanism and the type of materials used
will be discussed in the next chapters. The general mechanism has been that the adsorbed CO2
reacts with adsorbed H+ ions to form different formate related products or CO in the casse of
hydrocarbons[29][30][31].
Figure 1: Electrolysis of water in proton exchange membrane electrolysis cell
The selectivity for formation of various carbon related products such as CO, formic acid,
methane is highly dependent on the selectivity of the catalyst being used for reduction in the
system. In electrochemical reduction of CO2, any current that does not contribute towards the
reduction process is considered a waste. Therefore, formation of H2 as a by product in the
reduction is considered inefficient except when synthetic gas is the desired product.
26
Figure 2. Electrolysis of CO2 and water to produce synthetic gas
Anodic Reaction : 2H2O(l) 2O2 (g) + 4H+ + 4e
- (2.4)
Cathodic Reaction : 4H+
+ 4e- + CO2(g)
CO (g) + H2 (g) + H2O (l) (2.5)
Overall Reaction : H2O(l) + CO2(g)
CO (g) + H2(g) + O2(g) (2.6)
2.2 Electrochemmical Reduction Mechanism
Synthetic gas (CO + H2) is typically produced through combustion of coal or natural gas
and is a primary product required for the hydrocarbon fuel synthesis process. Figure 3, describes
an ideal cycle through implementation of an electrolyzer for CO2 reduction, where the CO2
emissions from the combustible sources will be captured and separated through various
separation available such as Amine separation and MEA.
27
The captured CO2 will then be injected into an electrochemical electrolyzer very similar
to Figure 2, where the electricity generated from a renewable energy source such as wind or
solar will provide the necessary electric potential to drive the dissociation of CO2 and H2O[1];
thereby generating synthetic gas. This synthetic gas is a building block for the chemical
feedstocks and hydrocarbon fuels synthesis( such as Fisher – Tropsch)[32].The profitability of a
particular and conversion process does not depend only on the variety of value added products
generated, but also on the energy input for respective conversion.
Figure 4, shows the energy balance for catalytic hydrogenation of CO2 as well as
electrochemical and photochemical based approaches[4]. For CO2 separation approaches, energy
Electrochemical CO2
Reduction
Syngas Generation
Generation of Useful Products
CO2 Capture
CO2 Emissions
Carbon Neutral
Cycle
Figure 3: Overview of Electrochemical Reduction of CO2
28
must be supplied so as to separate CO2 from the flue gas stream through several separation
techniques such as amine separation, membrane separation, etc. The minimum energy
requirement for CO2 separation from flue gases which is about 11 vol% is 7.8 KJ mol-1
( 177 KJ
/kg)[4]. For photochemical and electrochemical approaches, an additional driver is required to
drive the reaction uphill, i.e., solar energy or input from other renewable sources. Formation of
methane, methanol, carbon monoxide, formic acid which are typical products from CO2 also
require certain amount of energy to be spent on CO2 separation
Table 1: CO2 conversion into various products and their corresponding enthalpies[33]
Number Reaction ΔHO
298K
KJ mol-1
(CO2)
1. CO2(g) + 2H2O(l) CH4(g) + O2(g) 890.9
2. CO2(g) + 2H2O(l) CH3OH(l) + 1.5O2(g) 726.7
3. CO2(g) + H2O(l) CHOOH(l) + 0.5O2(g) 255.0
4. CO2(g) + 4H2(g) CH4(g) + 2H2O(g) -165.1
5. CO2(g) + 3H2(g) CH3OH(g) + H2O(g) -49.7
6. CO2(g) + 3H2(g) CHOOH(g) 14.9
7. CO2(g) + 3H2(g) -CH2-(g) + 2H2O(g) -110.8
8. CO2(g) CO(g) + 0.5O2(g) 283.2
29
For example, from Table 1; reaction 1, it is observed that the highest amount of energy is
required for methane generation. 6– 15% of this overall energy is used for CO2 separation. Also,
it is evident that the formation of formic acid requires minimum energy consumption; however,
55% of the overall energy is utilized for CO2 separation.
Here, it must be noted that CO2 hydrogenation reactions when compared to photo or
electrochemical reduction of CO2 are fairly exothermic. Table 1; reaction 4 illustrates the
exothermic nature of CO2 hydrogenation for producing methane. However, it must be realized
that the hydrogen required for this process is supplied mainly through renewable sources such as
wind, solar, etc. or through water electrolysis which consume certain amount of energy to
generate hydrogen[14][15]. For example, it is worth mentioning that the high temperature water
(1073 K) electrolysis requires 244 KJ mol-1
. Also, the hydrogen produced through electrolysis
H2 GenerationCO2
Separation
CO2
Conversion
CO2
Conversion
Value Added Products
Energy
Energy
Energy
Energy
CO2H2O H2
Figure 4: Energy balance diagram for CO2 conversion and CO2 separation. The red arrows show that
energy is consumed for a particular process and the green arrow for energy release.
30
could be utilized for storage and transport of solar energy. However, the energy density for
hydrogen is very less with 0.2 toe m-3
in comparison to diesel oil which has an energy density for
1.2 toe m-3
at 700 bar and 25oC[4]. Liquefaction and transport of the produced hydrogen could be
an issue with regards to energy requirement, “e.g., 20 – 30% of the energy is consumed for
liquefaction and about 10% for transport as compressed gas through pipelines”[34]. Hence, it can
be concluded that for photo or electrochemical CO2
reactions would need significant
improvements to be able to compete with hydrogenation reactions. Another limitation of going
for electro or photochemical reaction is the low solubility levels of CO2 in aqueous electrolytes,
resulting in mass transport issues. Possible improvements for counter such issues could be a)
using gas diffusion electrodes for improved reaction kinetics b) using better solvents with
improved conductivity levels c) maintaining the experimental conditions at elevated pressures
and temperatures. The main objective of this thesis is developing reactors where we use gas
diffusion electrodes for improved, EMIM – BF4 as an ionic liquid and try to observe the
synergistic effect of catalyst and the ionic liquid to attain high current densities, faradaic
efficiencies and subsequently a respectable amount of energy efficiency.
2.3 Terminology for Performance Metric
The development of a cost efficient process for electrochemical reduction of CO2 is
essential for a sustainable economy and chemical industry. For more efficient electrochemical
reduction, a highly active and selective electro-catalyst for CO2 reduction is necessary. Some key
parameters to access the aforementioned parameters and predict the economic feasibility are:
Energy Efficiency (EE) – Overall energy utilization for a process
Current Density (CD) – Reaction rate or measure of rate of conversion
31
Faradaic Efficiency (FE) – Measures the selectivity towards a particular
product
Catalyst Stability
Process Costs
Process costs include material consumption costs, design and manufacturing costs and other
subsidiary costs such as electricity costs. However, the main focus of many research groups is to
develop a relation between the three figures of merit – Energy Efficiency, Faradaic Efficiency
and Current Density as there are no sound conventions developed yet for durability test and any
cost models developed for electrochemical reduction of CO2.
For electrochemical conversion to be feasible, the system must satisfy two important
criteria, 1) high reaction rate and 2) high energy efficiency. Although energy efficiency is not
mentioned, it is a very important parameter as it defines the recoverable energy in a product;
specifically the energy cost for generating the desired product. High energy efficiency is highly
dependent on selectivity (faradaic efficiency) and low overpotential. The expression for energy
efficiency is shown in equation[23]
ɛenergetic = 𝐸0
𝐸0+ ɳ∗ ɛfaradaic (2.7)
Here, EO is the standard equilibrium potential for the reaction, ɛfaradaic is the faradaic efficiency
for the product and ɳ is the cell overpotential.
EO
cell = EO
cathode - EO
anode = -0.1V – 1.23V = -1.33V for converting CO2 to CO
EO
cell = -1.23V for water splitting reaction
32
Faradaic efficiency for a desired product is calculated by using equation (2.8), where z = number
of electrons exchanged for the process (e.g. n =2 for CO2 conversion to CO), n = number of
moles for a given product, F = faradays constant ( F = 96485 C/mol) and Q is the charge
passed[1][35].
ɛfaradaic = 𝑧 .𝑛.𝐹
𝑄 (2.8)
Current density as mentioned previously is used as a metric for measuring the rate of
conversion. The current density is defined as the current measured at a certain potential divided
by the active area of the cathode for catalyst interaction. The current density is a very important
metric as it helps determine the electrode area and correspondingly the electrolyzer size and
capital investment needed for generation of the desired product.
2.4 Thermodynamic Considerations
Carbon dioxide is considered to be one of the stable molecules among the carbonaceous
molecules available in the environment. Although the reactivity of CO2 is considerably low, the
electrochemical reduction of CO2 from a thermodynamic consideration is not difficult. The
equilibrium potential for CO2 reduction in an aqueous media and operating in a neutral pH
environment is identical to hydrogen evolution.
H+(aq) + H
+(aq) H2(g) -0.414V vs SHE at pH 7 (2.9)
CO2 + H2O + 2e- HCOO
- + OH
- -0.430V vs SHE at pH 7 (2.10)
CO2 reduction in an aqueous medium requires high overpotential, much more negative
than the equilibrium potential mentioned above. This is mainly due to the formation of the
(CO2)– intermediate. Hydrogen evolution at such high negative potentials in aqueous mediums
33
may be preferred[30]. The hydrogen evolution reaction (HER) is highly dependent on the pH of
the solution. With an increase in the pH there is a subsequent raise in the equilibrium potential
for CO2 reduction. As a result, hydrogen evolution reaction is thermodynamically preferred in
such situations over CO2 reduction.
Various reaction pathways exist for CO2 reduction in aqueous Medium. The below
mentioned reactions is a list of reaction pathways with respect to equilibrium potentials vs SHE
at pH 7 and 250C [9].
CO2 + H2O +2e-
CO + 2OH- -0.52V (2.11)
CO2 + H2O + 8e-
CH4 + 8OH- -0.25V (2.12)
2CO2 + 8H2O +12e- C2H4 + 12OH
- -0.34V (2.13)
2CO2 + 9H2O + 12e- C2H5OH + 12OH
- -0.33V (2.14)
3CO2 + 13H2O + 18e-
C3H7OH + 18OH- -0.32V (2.15)
The above mentioned reactions show hydroxide anion is generated as a product for CO2
reduction which mediates the pH of the solution. This rise in pH hinders the CO2 reduction
process as well as the selectivity of the catalyst [9]. Also, the equilibrium potential helps develop
a benchmark for the experimental analysis as comparisons of this value with the experimental
value will determine the overpotential. “Generally, the difference in the equilibrium potential and
actual potential for a particular reaction is called overpotential, and represents the lost energy
during a chemical conversion”[26]. Hence, the overpotential is always meant to be kept at a
minimum in order to make an electrochemical reaction economically viable.
34
The list of reaction pathways mentioned above indicates that the reduction process can
proceed through a two, four, six and eight electron charge transfer process to achieve the desired
product. The direct conversion of CO2 into liquid fuels (2.13 – 2.15) is challenging from a
reaction kinetic point of view. The electrochemical reduction described in the first pathway (2.8)
is the easiest conversion from a thermodynamic and kinetic point of view as the equilibrium
potential for the formation of CO is the least and also as it involves only 2 electrons for the
reaction.
Grouping metals according to their electronic configuration (s, p and d) and according to
their electrolyte type (aqueous or non-aqueous) is helpful as the products can be identified
uniquely according to the physical nature of the metal, electrolyte environment and electronic
property.
2.5 Trends in Electrochemical CO2 reduction
Electrocatalysts are required to bind and activate CO2 in order to reduce the large
overpotential generally associated with CO2 reduction. Significant developments have been
made in commercializing CO2 reduction electrochemically. The electrocatalytic behavior of
metals for CO2 reduction is highly dependent on their electronic configuration and grouped
according to s, p and d metal electrodes. Further work done by Hori[9], revealed the nature of
supporting electrolyte (aqueous or non- aqueous) on the metal electrode influences the catalyst
selectivity.
2.5.1 Aqueous Solution:
Figure 5 shows simple metal electrodes that are classified into four groups in aqueous
media which are dependent on the type of reaction products[36]. Copper belonged to the first
35
grouped showing exceptional selectivity towards the conversion of CO2 to hydrocarbons. Au,
Ag, Zn belonged to the second group as they had strong affinity towards formation of CO. Pb,
Sn, In and Cd are in the third group, which has been categorized through the generation of
formate ion as their major product. The fourth group of metals, Ni, Pt, Fe and Ti did not have
any product formation for CO2 reduction, while hydrogen evolution was exclusively observed
for this group of metals.
For group four, CO is adsorbed very strongly on metals which prevents further reduction
of CO2 and forms hydrogen in a stable state[36][37]. The metals in the fourth group (Pt, Ni, Fe
and Ti) have high catalytic activity in aqueous mediums as they have very low hydrogen
overpotential and hence do not produce any products for CO2 reduction.
CO
2
Cu
Au, Ag, Zn
Pb, Sn, In, Cd
Ni, Fe, Pt, Ti
Hydrocarbon
CO
HCOO-
H2
Electrode Product
Fig 5: Electrode Materials and Reaction Products for electrochemical CO2 r-
eduction in aqueous medium
36
Although several developments have been made in aqueous CO2 reduction field, some
key issues still need to be addressed such as (i) high overpotential (ii) low solubility of CO2 in
water (iii) catalyst selectivity (iv) the poisoning of catalyst due to adsorption of impurities[32].
2.5.2 Non-Aqueous Solutions
The equilibrium potential for CO2 reduction in aqueous medium is similar to that of
hydrogen evolution reaction (HER). However due to thermodynamic barriers, excess energy is
needed to drive the electrochemical conversion of CO2. Also, previous studies have shown that
CO2 reduction takes place at high over-potential mainly due to high activation energy for (CO2)-
intermediate formation[29]. This is the first step involved in the CO2 reduction process. The
excess voltage supplied is an energy loss, meaning a higher energy is consumed in the generation
of useful products than what is stored in the chemical bonds. The electrochemical reduction of
CO2 is quite significant in a non-aqueous medium as (i) the scope for hydrogen evolution
reaction can be suppressed due to low proton availability (ii) CO2 is highly soluble in organic
solvents such as methanol, which influences the efficiency for carbon dioxide reduction [9].
There are many non-aqueous solvents available but methanol is the most attractive solvent as it
is cheaper, and it can be produced as a by-product in large scale[38].
The reaction mechanism is mediated through one electron transfer to form the CO2-
intermediate. Here, due to the non-availability of H2O molecule, the intermediate CO2- is freely
present in the solution as it is not absorbed on the catalyst surface. This CO2- will then react with
CO2 molecule to form adduct of (CO2)2- which will lead to formation complex products. Hori, Y
[39] in his review studied that the (CO2)2- influenced the formation of different products on the
same metal electrodes.
37
2CO2 + 2e- (COO2)
2- (2.16)
(COO2)2-
+ 2H2O + 2e- HCO-COO
- + 3OH
- (2.17)
HCO – COO- + 2H2O + 2e
- H2C (OH) – COO
- + 2OH
- (2.18)
Thus in addition to the metal electrode, solvent, the electron mediator (electrolyte) plays a
major role towards the activity and selectivity of a catalyst. In this study, we found a unique
solution for continuous production of syngas as well as increasing the solubility in an electrolyte
by replacing conventional systems with non-aqueous ionic liquid (EMIM BF4). EMIM-BF4 is
used to prevent water hydrolysis in order to convert CO2 with high efficiencies even in the
presence of water. Further, ionic liquid can catalyze the formation of CO2 intermediate by
forming an intermediate complex with (CO2)-, which greatly reduced the energy requirement to
drive the reaction. Salehi et al., performed studies on EMIM-BF4 and found that CO2 reduction
rate was enhanced with water content being added to the EMIM BF4 solution[6][30]. This was
mainly because pH of the system was decreased by adding water to the dry EMIM BF4 solution,
which correspondingly increased the proton availability and thereby lowered the barrier of the
reaction and enhanced the rate. Mohammad et al., found a strong synergy between the MoS2 and
ionic liquid suggesting a novel co-catalytic activity for CO2 reduction into syngas[40]. This
establishment of co-catalyst system for electrochemical CO2 reduction has been a breakthrough
in CO2 reduction and establishes a benchmark for further research. A detailed discussion on
room temperature ionic liquids is also done in the next chapter.
38
3. Electrochemical Flow Reactor and Electrode Design
3.1 Introduction
CO2 is thought to be at present and in the future as a significant contributor to the change
in climatic conditions. Extensive efforts need to be put in place to control these emissions and
also to develop a reusable system wherein this greenhouse gas can be treated and used to produce
useful products. As discussed in the previous chapter, electrochemical reduction of CO2 is a very
viable and promising technology. The electrochemical reduction through the development of a
electrochemical flow cell is one such beneficial approach for utilizing CO2 and generating useful
products.
The need for portable devices that operate for prolonged durations without the need for
recharging has resulted in increased interest in need for micro power sources. To date,
microscale systems have been limited to miniaturization of functional components such as
MEMS, cell phones and portable electronics[41]. However, little research has been developed
towards miniaturization of long-term power sources. Here, we study a flow cell that utilizes fluid
delivery at microscale with multi-stream laminar flow regime.
“An electrochemical flow cell is defined as a fuel cell with fluid delivery and removal,
reaction sites and electrode structures all confined to a microfluidic channel”[27]. Ionic charge
transfer in such type of architecture is provided through the co-flowing electrolytes in laminar
regime.
The flow cell reactor is a very useful model for CO2 reduction as it combines the transfer
of charge, mass and momentum along with electrochemistry for both anode and cathode. Several
novel methods have been designed to be used as reactors for CO2 reduction, most prominent
39
ones being biofuel cells[42], micro fabricated fuel cells[1][3], solid oxide fuel cells[43] and
hydrogen fuel cells[44]. The proton exchange membrane fuel cells (PEMFCs) are one the most
developed fuel cells which have been successfully implemented in widespread stationary
applications for power generation and extra-terrestrial applications[45]. However, portability of
such devices is not viable because of the safety concerns and challenges related to the storage of
hydrogen fuel. The reaction mechanisms, scalability of the system and additional requirement
leading to complexity of the system lead to PEMFCs not being the primary choice to be the apt
reactors for such an application. Direct methanol fuel cells have been thought as another
alternative for reactors for CO2 reduction application, but issues such as flooding on cathode,
anode dry out and fuel crossover hamper the performance of such a reactor[46]. Due to the
absence of turbulent mixing of streams in a microfluidic environment, the utilization of
microfluidic reactor as a power source would be appropriate as issues associated with other
power sources can be minimized and ion transport between the anode and cathode can be
enhanced. Moreover, microfluidic fuel cell architecture presents great prospects for integration
across different fields.
The i-v curves achieved by testing the flow cell at different applied cell potentials and
electrolyte concentrations help to understand the reaction kinetics and generate the scope of
determining the kinetic parameters critical for the electrochemical reaction rate equations.
Significant improvements have to be done on the reactor design so as to optimize the
three important process parameters, i.e. current density, faradaic efficiency and energy efficiency
in order to have a high product output. The variation of the product selectivity with regards to
temperature, pressure, pH variation of the electrolyte and its composition cannot be observed in a
conventional three electrode cell[28][29][46]. Hence, design and demonstration of continuous
40
flow reactor is the purpose of this thesis. Study on this reactor with and without the ion-exchange
membrane have been performed and reported in this thesis with regards to effects of design and
operational parameters (variation in gas and electrolyte flow, applied potentials and electrolyte
concentrations).
3.2 Transition from Static Flow Reactor to Dynamic Flow Reactor
To study the analyte at an electrode/electrolyte interface, we need to monitor current and
voltage. For this purpose, we used a three electrode electrochemical cell - static type reactor to
study the catalytic activity of a particular catalyst. Cyclic voltammetry is a very useful diagnostic
tool to obtain broad knowledge on the electrochemical system. In this approach, potential is
swept between two defined potential limits for a specific number of cycles and at a particular
scan rate.
A three electrode cell system essentially consists of working, reference and counter
electrodes[40]. The working electrode applies the desired potential and moves the system away
from equilibrium. It makes contact with the analyte and controls the transfer of charge to and
from the analyte. The reference electrode is a half cell with known potential. It measures and
controls the potential delivered by the working electrode. At no point during the course of the
experiment does the reference electrode pass any current. Calomel electrode, Ag/AgCl is some
of the most commonly used reference electrodes. An auxiliary or counter electrode completes the
circuit to carry current. It makes sure that the reference electrode does not generate any current.
41
Figure 1: Schematic diagram of a standard three electrode electrochemical cell
A further study on optimizing numerous parameters such as varied potential ranges,
electrolyte concentrations, support materials and temperature of the active area is needed to
commercialize electrochemical reduction of carbon dioxide. The study cannot be performed in a
conventional three electrode cell but performed in a dynamic type reactor whose properties could
be varied with respect to time. Based on these promising catalytic results from a static three
electrode cell, a continuous flow reactor, which is more effective and versatile system was
modelled and built as shown in Figure 2. The development and demonstration of the continuous
flow reactor working under various experimental parameters is the primary purpose of this study.
42
Figure 2: Schematic view of a Microfluidic Fuel Cell with a Nafion Membrane separating the
anode and the cathode for electrochemical CO2 reduction. (1) Aluminum current
collector(cathode) (2) Cathode gas diffusion electrode (3) Cathode Catalyst (4) Liquid flow
channel (catholyte) (5) Nafion Membrane (6) Liquid flow channel(anolyte) (7) platinum catalyst
(8)Anode gas diffusion electrode (9) Aluminum current collector(anode)
3.3 Electrochemical Flow Cell Design
A microfluidic fuel cell consists of layers of parallel and rectangular channels of similar
widths, lengths and heights. The reactor borrows its architecture from the PEM fuel cells with
electrolytes flowing through the anode and cathode compartments of the cell to perform water
splitting and CO2 reduction, respectively. As discussed previously, there are two different
approaches in the design of the reactors for electrochemical reduction of CO2. One approach
being the use of a polymer exchange membrane (Nafion) to separate the anode and the cathode.
In the other approach, the liquid electrolyte serves as a separation medium between the cathode
and the anode, and it is for this reason the arrangement is called a microfluidic reactor. The
reactor designed, fabricated and utilized in this work is shown figures 2 and 3.
The reactor is characterized with a co-current flow of gaseous CO2 and catholyte on the
cathode side and an anolyte (bottom) on the anode. The microfluidic fuel cell setup consists of
43
an anode and cathode current collector plate. These plates are made of aluminum as it has
excellent corrosion resistance and is a good conductor of heat and electricity. The Cathode
current collector also serves as the gas channel for CO2 (Praxair, 99.9% UHP). Just below the gas
channel was a piece of Sigracet 35BC graphite gas diffusion electrode (GDE) on which the
catalyst ink for the cathode was coated using a paint brush. The gas diffusion electrodes are
Teflon-bonded carbon black matrix on which the catalyst particles are dispersed. The catalyst ink
is prepared by mixing 10 mg of MoS2 nanoparticles mixed with 600 μl of DI water and 600 μl of
Isopropyl alcohol and 10 μL of 1100 EW 5% Nafion solution (DuPont). The Nafion solution
helps in improving the binding of the catalyst ink to the Gas Diffusion Electrode (GDE). The
platinum ink for the anode side is also prepared with the same recipe but instead of the cathode
particles, 5 mg platinum particles are used. Below the cathode is a liquid channel made of
Teflon. This liquid channel has a slot in the middle of its cross section and this slot is the active
area of the fuel cell where the three phase (CO2, EMIM – BF4 and Nanoparticles) interaction
takes place and the necessary products are formed. The ionic liquid (EMIM-BF4 + H2O) flows
through this liquid channel and is called the catholyte. A 2 cm2 polymer exchange membrane
(Nafion 117) is sandwiched between the catholyte and the anolyte. This membrane acts as a
barrier separating the two electrolytes and permeating only H+ ions generated through OER to
transfer to the other compartment to participate in the CO2 reduction reaction. It also prevents
gas and fuel crossover thereby maintaining stability in the system. The separation of the anolyte
and catholyte by the membrane also prevents the deactivation of platinum by poisoning through
CO produced at the cathode[9][11]. The anolyte is a solution of 0.1 M H2SO4 which is flown at
the same flow rate as the ionic liquid. The H2SO4 solution maintains the conductivity of the cell.
44
The entire setup is held together by a squeeze action toggle plier clamp (McMaster).As
this is a microfluidic setup there are several advantages over the other reactors:
Minimum water management issues in the fuel cell. The design (GDEs + Catholyte +
Membrane + Anolyte) has its disadvantages by increase in contact resistance across the
cell. However, water management issues are very minimal as there is a constant removal
of liquid electrolytes from the reactor and is not stagnant.
Flexibility for the reactor. The fuel cell can be tested with different electrolyte
concentrations as well as with different electrodes.
The continuous flow design permits online sampling and product analysis. The fuel cell is
connected to a gas chromatograph and hence the products exiting the cell can be analyzed
in a convenient way.
Gas and fuel crossover is controlled. The ion-exchange membrane acts as a barrier
thereby preventing any CO generated during the reaction from crossing over and
poisoning the platinum catalyst on the anode side.
Ease of use. The individual electrode potentials of the anode and cathode can be
monitored separately by the reference electrode which helps to analyze the performance
of each electrode.
45
Figure 3: Sectional view of the microfluidic cell with flow of CO2, current collector plates,
flowing electrolyte, electrode and membrane assembly. The flowing electrolytes are separated by
a Nafion 117 ion exchange membrane. Two aluminum plates serve as current collectors are
placed on either side of the electrodes (GDE) onto which catalyst are coated. A reference
electrode placed in thee exit liquid stream for monitoring individual electrode performance
3.4 Gas Diffusion Electrodes
Gas diffusion electrodes (GDE) are Teflon-bonded carbon black matrix which generally
acts as a supporting material in every fuel cell application. This matrix provides significant
improvements in mass transfer rates to the catalyst surface. As the solubility of CO2 in water at
ambient conditions is very less, a technology based on gas diffusion electrodes has been
developed to minimize mass transport thereby enabling the achievement of high current densities
which are desirable for any electrochemical application[47]. The GDE contains a macroporous
support layer of carbon fibers which are treated with PTFE to make the layer hydrophobic. A
Flowing
Electrolyte
Current
Collector
Plates
Electrode
and
Membrane
46
microporous carbon layer is then topped over this PTFE treated layer The catalyst nanoparticles
are coated onto this microporous layer. The GDE is then placed in a gas phase and liquid
electrolyte environment which generates a three phase medium of the liquid electrolyte, gas
phase reactants and the solid catalyst particles on the GDE. The gaseous reactants permeate
through the backing carbon fiber layer to react with the catalyst and interact with the liquid
electrolyte. Hence, gas diffusion layer has three layers, a water repellent layer which does not
allow any interaction between the liquid, an electronic conductive layer for supplying electric
current and a fine reaction layer having the catalyst. Numerous experiments performed using the
GDEs have indicated that this backing layer have enhanced the current density with about 1-2
orders of magnitude[12][13].
Although there are several advantages in using GDEs as support materials, issues with
regards to long term stability and liquid accumulation in the macroporous regions have also been
reported[18]. Additionally, during the experiment, it was also observed that the liquid electrolyte
was able to completely penetrate through the electrode and gather in the gas channel. This blocks
the exit gases from the reactor into the gas chromatograph and represents a loss in the system.
Also, on a few occasions the gas products would form on the liquid electrolyte and would not be
able to permeate through the wetted catalyst. Hence, bubbles were observed in the collection
beaker which caused erratic cell behavior. A definite approach towards addressing these issues is
beyond the scope of this study but a temporary solution was found by adjusting the liquid and
gas flow rates and purging nitrogen gas through the cell after each run which removed any
stagnant liquid in the flow channel. Figure 4 shows SEM images of a cut gas diffusion electrode
on the front and side view at a scale bar of 20µm and 100µm, respectively.
47
Figure 4 : Cross-section Scanning Electron Microscopy (SEM) images of a cut gas diffusion
layer (a) front view at scalebar of 20μm (b) side view at scalebar of 100μm
3.5 Nafion Membrane
In fuel cell applications, a fuel is oxidized (water in our case), leading to the generation of
protons, electrons and reaction by-products. The electrons travel around an outer circuit
powering a load and the protons travel to a conductive electrolyte to participate in the reaction on
the cathode side. Nafion, a persulfonated polytetrafluoroethylene ionomer (synthetic polymer
with ionic properties) developed by DuPont, is most widely used as an ion conducting
electrolyte[48]. The equivalent weight (EW) and material thickness are used to describe the most
commercially available membrane. For example, Nafion 117 used in our experiments indicates a
material with 1100 g EW and 0.007 inches of thickness. The EW is defined as the weight of the
Nafion (in terms of molecular mass) per sulphonic acid group. It is used in wide variety of
applications from Energy storage devices (Fuel Cells and Batteries) to strong acid catalysis and
artificial muscles. The Nafion membrane has some dominating properties such as easy handling,
compact, amenable to mass transport and excellent resistance to gas crossover, which makes it a
suitable in applications where liquid electrolytes are involved. For low temperature application
such as the fuel cell system used for our experiments, the Nafion membrane has a multiphase
structure, a hydrophobic layer as a continuous phase, and sulphonic acid as a hydrophilic phase.
A B
48
The hydrophilic phase acts as a water reservoir is very essential for the proton conductivity of the
membrane. The degree of hydration is the main factor that governs the proton conductivity of the
Nafion membrane. The continuous hydrophobic phase of the Nafion membrane acts as a
backbone for the membrane structure[49]. The polymer has negatively charged species such as –
SO3-, -COO
-, -PO3
2-, -PO3H
-,etc., fixed to the membrane backbone allowing the passage of
cations and rejecting anions. The polymer has excellent chemical stability like Teflon but unlike
Teflon, it swells and adsorbs in water due to sulfonic acid terminated side chains on the
fluoropolymer backbone. Figure 5 shows a structural diagram of the nafion membrane.
Figure 4: Structure of Nafion membrane
3.5.1 Preparation of Nafion Membrane
A sheet of untreated 10 X 10 sheet of Nafion 117 membrane was bought from
FuelCellEtc. In order to use the membrane in the fuel cell process, it needed to be treated. Small
strips of the membrane were cut in dimensions slightly higher than that of the flow channel. The
treatment process was as follows:
1. Initially, the membranes were dipped in DI water so as to stay hydrated.
49
2. The membranes were then dipped into a 5% hydrogen peroxide solution which
was pre heated to a temperature of 800C and the membrane were kept in the
solution for one hour. This treatment process was very important as hydrogen
peroxide removes the impurities present on the membrane.
3. After hydrogen peroxide treatment, the membranes were heated again at 80 0C
and soaked them in DI water for 1 hour.
4. The membranes were then soaked in 0.5 M sulfuric acid solution. This treatment
was very important for the membrane usage in acidic environment as treatment
with H2SO4 activates the sulphonic acid (SO3-) groups necessary for cation
exchange in the membrane.
5. The membranes were once again soaked and heated for 1 hour in DI water.
For application of flow cell in basic environment (e.g., KOH, NaOH) the membrane treatment
process had to be slightly altered. Instead of treating with H2SO4 and activating the sulphonic
group, the membrane was treated with 5 wt% of KOH at room temperature for 20 minutes. This
was because treatment with KOH was a very fast ion exchange.
3.6 Reactor Operation
The microfluidic fuel cell operates on a redox mechanism with oxidation taking place on
the anode side which generates electrons. These electrons by mean of an external load travel to
the cathode side and participate in the reduction reaction. The liquid electrolyte acts as
interconnect between the two compartments (anode and cathode) by providing ions for the
reaction to proceed.
50
Figure 5 shows a clear schematic of the reactor design used in the study. This
microfluidic design is a modification of the hydrogen fuel cells[50]. The flowing electrolyte
provides flexibility in operation of the cell and adjusts the parameters according to the
concentration and pH of the electrolyte. As discussed in the previous chapter, electrochemical
reduction of CO2 is a highly energy consuming process, with one electron transfer step to form
CO2- intermediate being the rate limiting step in the reduction of CO2 .
A few years ago, a few researchers projected requirement of high overpotential for
conversion of CO2[51]. The equilibrium potential is hence very electronegative for the formation
of this complex in water or in any other organic solvent[20][21], which is not desirable as this is
highly energy intensive. Figure 6 shows the experimental arrangement of the electrochemical
flow cell. Rosen et al., proposed co-catalyst system using EMIM – BF4 and silver cathode to
lower the potential of this CO2- intermediate complex[10][12] and then react with H
+ ions on the
catalyst surface to produce CO. The objective of this work is to show the enhanced levels in
Figure 5: Schematic diagram of the Microfluidic Fuel Cell for electrochemical reduction of
CO2
51
production of CO with regards to current density and faradaic efficiency using such a system
(e.g. (EMIM – BF4 as electrolyte) and Transition Metal Dichalcogen (TMDCs) as
electrocatalysts) and compare their performance to that of noble metals such as Ag.
3.7 Ionic Liquid for CO2 Reduction
Ionic liquids are a great solution to minimizing the energy consumption necessary for
CO2 reduction due to their unusual properties as liquids.. Ionic liquids have relatively high levels
of solubility for carbon dioxide, which makes them promising prospect as absorbents for CO2
reduction. The Room Temperature Ionic Liquid (RTILs) electrolytes are completely dissociated
into ions and are not mixed with any bulk solvents. As the ionic liquids are generally composed
of only ions, they show high conductivity, non-volatility and flame retardance [52]. The RTILs
are generally composed of a bulky organic cation (e.g. imidazole) and an organic/inorganic
cation (e.g. tetrafluoroborate). These ionic liquids have numerous features which make them
attractive for such electrochemical applications[53] :
Figure 6: Experimental arrangement of the Electrochemical Flow Cell
52
Tunability : The properties of the ionic liquids can be adjusted by tailoring the structures
of the anion or cation
Fluid Properties: The melting points of the ionic liquids are below the ambient
temperature, and they also have low viscosity values which makes gas diffusion through
the ionic liquids a rapid process.
Non-volatile: Low vapor pressure at room temperature makes ionic liquids
environmentally friendly and also thermally and electrically stable. Also, since water is
volatile, it is difficult there exist a difficulty for applications across at wide temperature
range[52].
Conductivity: The ionic liquid are composed on ions which increases there conductivity
with rise in temperature
Table 1, shows the electrical conductivity of different ionic liquids for different cations and
temperature. Note from Table 1 that there is a rise in conductivity of the liquids with the rise in
temperature. This is due to the liquids having low viscosity and hence there is an increase in the
Figure 7: Chemical Structure of EMIM – BF4 ionic Liquid
53
mobility of the ions. The conductivity of any particular ionic liquid is related to the charge
density and mobility of the ions. The RTILs are also highly desirable for CO2 reduction due to
their high solubility and selectivity to several gases. Table 2, shows the solubility of CO2 in
various organic solvents, EMIM – BF4 and water.
Table 1: Variation of the electrical conductivity with respect to cation and temperature; Anion is
BF4. Data taken from [54]
Temperature
( 0C)
[EMIM][BF4] [BMIM][BF4] [HMIM][BF4] [OMIM][BF4]
κ/S.m-1
-35 0.00212 0.000405 0.0001524
-15 0.228 0.000778 0.001685 0.000650
5 0.703 0.1132 0,0332 0.014
25 1.546 .0352 0.1229 0.0579
55 3.51 1.150 0.483 0.252
85 6.20 2.52 1.214 0.688
Although high faradaic efficiencies have been reported (> 90% for formic acid and CO,
65 -70% for methanol and ethylene), high overpotential is always a hindrance when scaling up
technologies to an industry scale. The current density which indicates reaction rates is a vital
factor as it defines the size of the reactor and capital costs for the process. until now current
density in the range of 200 – 600 mA/cm2 have been reported on different substrates such as gas
diffusion electrodes and other reactors for fuel cells.
54
Table 2: Solubility of CO2 in organic solvents, EMIM – BF4 and water, Data taken from [53]
Solvent S CO2
CH3CN 7.1
Acetone 6.6
THF 6.2
MeAc 6.0
DMF 4.1
PC 3.9
MeOH 3.6
CHCl3 3.6
EMIM – BF4 1.8
Water 0.76
As mentioned previously, the CO2 reduction process is a two-step reduction process with
the rate determining step being the formation of the intermediate CO2- complex. This is mainly
because the energy required for formation the CO- intermediate is really high and hence it is
known as the rate determining step. This mechanism is shown in figure 6 with the bold line and
the corresponding energy requirement. Through numerous electrochemical experiments it was
found that the initial energy barrier could be reduced to a very minimum value (0.17 V) using
EMIM – BF4 ionic liquid, , although there would be additional barrier to form the final reaction
55
products[11]. This work is shown in Figure 6. Salehi et al., performed research to find ways for
CO2 reduction at less negative potentials. They found that performing CO2 reduction in water or
other aqueous solvents was highly energy inefficient. This is indicated through the bolded line in
figure 6. Hence, they developed a promising co-catalyst system (Ag + EMIM-BF4 ionic liquid)
for efficient conversion of CO2 at high current densities and high faradaic efficiency. They also
proposed that if a substance formed a product with CO2- on the metal surface, the reaction
pathway would follow the dashed line in Figure 6. Although a barrier may still exist, the initial
energy barrier would be reduced. The binding of CO2 to EMIM – BF4 is weaker than in any other
ionic liquid. This was another important aspect as strong bonding would affect the reactivity of
Figure 6: Schematic diagram proposing the free energy pathway for CO2
reduction in aqueous solutions and in EMIM- BF4
56
CO2. Also, Figure 6 shows the energy efficiency of a process could be enhanced through high
selectivity (faradaic efficiency) and low overpotential[1].
The use of this co-catalyst system (EMIM – BF4 + metal catalysts) can be considered as a
breakthrough as the energy efficiencies as high as 87% (current density and faradaic efficiency)
have been reported for CO2 reduction[10][12]. This serves to be a suitable candidate for possible
scale up applications in the future. This co-catalyst system has been used in all the energy
conversion systems that have been developed in this thesis. Next chapter, will demonstrate the
electrochemical reduction of CO2 in an electrochemical flow cell.
57
4. Results and Discussion
For the electrochemical experiments, the catalyst was coated onto the gas diffusion
electrode (GDE) for both the anode and cathode side and mounted in a two- compartment cell.
Both the anode and cathode had an active area of 1.25 cm2 for the catalyst to interact with the
liquid and gas molecules. Experiments on flow cell reactor were performed by running the cell at
different cell potentials using the chronoamperometry approach and at different water mole
fraction concentrations of the ionic liquid (EMIM – BF4). The flow cell was also tested in two
different arrangements, one arrangement incorporating an ion-exchange membrane and the other
arrangement was membraneless, utilizing an electrolyte as a barrier between the two
compartments. Here, the main objective is to observe if the flow cell was able to generate a
tunable range of syngas at different potentials applied and electrolyte concentrations. This
product variation is highly desired as syngas is an energy intermediate product that could be fed
into various fuel synthesis processes such as Fisher-Tropsch, Methanol/DME synthesis and
higher alcohol synthesis.
As discussed in the previous chapter, the equilibrium potential for formation of (CO2)-
intermediate is very negative. Hence, there is a requirement for a large overpotential for CO2
reduction which has been mainly attributed to the initial electron transfer for the formation of the
(CO2)- intermediate, which is poorly stabilized on most metal surfaces. EMIM-BF4 along with
the catalyst nanoparticles works well as a co-catalyst system as it reduces this large overpotential
of the CO2- intermediate which then reacts with the H
+ on the nanoparticles coated on the
cathode to produce CO.
58
4.1 Experimental Procedure
Figure 1 shows a schematic diagram of the experimental flow cell reactor system. CO2
was supplied through a 99.99% carbon dioxide cylinder which was purchased from Praxair. A
mass flow controller (Sierra Smart Trak 50 series) was used to regulate the flow of CO2 into the
system. Different concentrations of EMIM-BF4 ionic liquid were flown through the upper liquid
chamber of the flow cell as a catholyte at 0.5 ml/min using a Harvard Apparatus Dual Injection
Syringe Pump. The bottom liquid channel had 100mM H2SO4 flown through also at 0.5 ml/min.
The liquid injection rates would be adjusted in a few occasions, mainly to bring the system to
equilibrium (i.e. to have very minimum bubbles in the liquid electrolyte exiting the flow cell). A
CH instruments potentiostat was used for electrolysis characterization. The potentiostat was
Figure 1: The schematic diagram for the Electrochemical Reduction of CO2 to CO using a
Microfluidic Fuel Cell. Ultra-high purity CO2 is supplied to the reactor through the mass flow
controller. The electrolyte for the reduction process is supplied through the syringe pump and
is collected in a beaker after the reaction. The gases exiting the reactor are sent to the Gas
Chromatograph for gas detection.
59
connected to the computer through CH instruments software. A reference electrode (silver wire)
was placed in the exiting liquid stream. In this way, a control over the performance of individual
electrode could be maintained. Before running the experiments, argon was purged through the as
to remove any residual gases and relieve the system from any start up effects. As the CO2 gas
enters the flow cell, it diffuses through the Sigracet 35BC gas diffusion electrode onto which the
catalyst nanoparticles were coated using a paint brush. The electrochemical active area of the gas
diffusion layer (GDL) was used to calculate the current density (activity) and faradaic efficiency
(selectivity) of the catalyst. This was the region where the CO2 gas molecules interact with both
the solid catalyst particles and the liquid electrolyte. The flow cell was permitted to attain steady
state for 120s, after which individual electrode potentials were measured using the Fluke
multimeter connected to each electrode and a silver wire that served as a reference electrode
which was placed in the exit stream.
Reference electrode is very useful in understanding the performance of individual
electrodes in an operating fuel cell. A reference electrode is used to determine the performance at
each electrode, decoupling the effects from both anode and cathode. An Ag/AgCl or saturated
calomel electrode is used as a performance analyzer in a liquid electrolyte electrochemical cell
while the Proton exchange membrane (PEM) and Solid oxide fuel cell (SOFC) generally use the
reference electrode for proper alignment. The dependence on anode, cathode and relative
geometrical configurations has not been studied before and the position of reference electrode
with respect to the afore mentioned parameters could open up a new area of research[55].
The exiting liquid electrolytes were collected in vials and were filtered using vacuum
filtration technique and reused in the experiment. This led to a reduction in ionic liquid usage,
hence saving time and money. The setup was connected to an SRI 8610C Gas Chromatograph
60
into which 1mL of the effluent gas stream was sent for product analysis. A Gas Chromatography
system was “equipped with 72 * 1/8 inch stainless steel sieve packed column and a thermal
conductivity detector (TCD). 5.0 Ultra high purity helium (Praxair) was used as a carrier gas for
detection of H2 and CO”[40]. The oven temperature was maintained at 1000C, and the column
temperature was maintained at 1500C. The only cathode products detected were H2 and CO when
we used MoS2or other TMDCs as catalysts.
4.2 Effect of Different Water Mole Fractions and Cathodic Cell Potential
Initially, the flow cell was tested with an ion-exchange membrane to separate the anode
and cathode at different cell potentials between 3V and 5V so as to see the performance of the
catalyst with respect to the current density. Important observation have been previously reported
with respect to cathodic cell potential (CCP). This is because the reduction reaction takes place at
cathode and also there were certain losses in the system such as ohmic potential drop in the
electrolyte, losses in the membrane and contact resistance as the assembly involved several
components ( membrane + liquid channel + current collector). An initial instability was observed
in the system with currents fluctuating between 45mA/cm2
and 70 mA/cm2 with applied cathodic
cell potentials fluctuating between 1.3 and 2.1 V for an applied cell potential of 4.5V. This was
mainly because there was no consistency between the gas and liquid flow rates. Liquid flow rates
needed to optimize since this is a continuous flow system. Further, the ionic liquid needed to
have enough residence time to be completed saturated with the gas molecules leading to a higher
possibility for generation of the desired product. Adjusting the gas flow rate was also very
critical as there were instances when a lot of gas bubbles were observed in the exit liquid streams
indicating the higher gas flow. Excess gas flow was also undesirable as higher flow would affect
61
the catalyst loading on the surface of the GDE which brings another inconsistency into the
system.
Figure 2 shows the variation of current density with respect to different cathodic cell
potentials and water mole fraction concentrations of EMIM–BF4 as an electrolyte. The current
densities were observed to at a minimum when dry ionic liquid was flown through the cell. This
was mainly because the number of H+ ions participating in the reaction was at a minimum which
inhibited the CO2 reduction process. Also the viscosity of the solution is comparatively higher
which meant higher mass transport issues for the CO2 to be saturated in the ionic liquid solution.
0 20 40 60 80 1000
20
40
60
80
Cu
rren
t D
en
sit
y(m
A/c
m2)
Water Mole Fraction(%)
-1.8V CCP
-1.6V CCP
-1.4V CCP
-1.2V CCP
*CCP - Cathodic Cell Potential
Figure 2: Effect of water mole fraction on the current density at a cathode cell
potential of -1.2V, - 1.4V, -1.6V and -1.8V with a MoS2 cathode and a platinum
anode, EMIM – BF4 as a catholyte, 0.1M H2SO4 as an anolyte and 99.99% ultra-
high pure CO2 gas stream inlet
62
However, the current density continued to rise by the addition of water in measured
concentrations to the EMIM BF4 solution. This is shown in Figure 2 from the current density
curves where current density at -1.8V vs Ag wire, for 90 mol% water concentration was
measured to be at 82 mA/cm2, which was significantly higher than the value of 35mA/cm
2
measured at 0 mol% water concentration. So at this potential approximately 2.5 times higher
activities of the catalyst were observed in two different concentrations. Current density at any
given condition was averaged over the chronoamperometry experiment run time before stepping
up to the next potential.
To confirm that the maximum contribution towards current density was influenced by
MoS2 nanoparticles and not the carbon substrate on the gas diffuse electrode, control
-1.2 -1.4 -1.6 -1.8 -2.00
15
30
45
60
75
90
Cu
rre
nt
De
ns
ity
(m
A/c
m2)
Cathodic Cell Potential (V)
MoS2
GDE
Figure 3: Shows the comparison between the current density from an uncoated gas
diffusion electrode (GDE) and a GDE coated with MoS2 nanoparticles
63
experiments were designed using 90 mol% water and 10 mol% EMIM – BF4 ionic liquid
mixture. GDEs on which MoS2 particles were coated were compared with plain GDEs. Figure 3
shows the comparison between the two tests and it could be observed that the current density was
small for a plain uncoated gas diffusion electrode. Also, no peak in the gas chromatograph was
observed when the samples were injected for analysis. This shows that no activity and selectivity
were observed for CO2 reduction and CO production is influenced in the presence of catalyst
nanoparticles.
The faradaic efficiency curves are shown in Figure 4. Here, similar trend as that of the
current density curves was observed. The current density and the faradaic efficiency started to
0 20 40 60 80 100
0
20
40
60
80
100
Fa
rad
ic E
ffic
ien
cy
(%
)
Water Mole Fraction (%)
-1.2V CCP
-1.4V CCP
-1.6V CCP
-1.8V CCP
*CCP - Cathodic Cell Potential
Figure 4: Effect of different water mole fractions at different cathodic cell potentials
on the faradaic efficiency for Co2 conversion to CO using MoS2 cathode, platinum
anode, EMIM BF4 as a catholyte, 0.1M H2SO4 as an anolyte and 99.99% ultra-high
pure carbon dioxide as a gas inlet
64
increase when water was added to the dry EMIM-BF4 ionic liquid solution. The increasing of
both, faradaic efficiency and current density upon water addition to EMIM-BF4 signifies
acceleration of desired CO2 reaction, while the EMIM cation inhibits the H2 evolution even at
higher water concentration.
The performance of electrochemical reaction is significantly better when 90 mol% water
mole fraction (10mol% EMIM – BF4 electrolyte) was used. The faradic efficiency for CO
formation increased by the addition of water to the dry EMIM-BF4 solution with a maximum
92% being observed at 90% water mole fraction concentration. The pH of the solution was at a
minimum of 3.2 when the ionic liquid had 90% water mole fraction concentration. Apart from
the reduced pH, the mass transport losses were also minimized by the addition of water.
The onset potential for CO2 reduction to CO in a flow cell reactor is defined as the
minimum potential at which a peak for CO could be observed in the Gas Chromatograph. For the
experiments performed the onset potential varied depending on the water mole fraction of the
ionic liquid. This can be seen in Figure 4 where CO peaks weren’t observed for 10 mol% and 0
mol% water mole fraction concentration at -1.2V cathodic cell potentials indicating that CO2
reduction is dependent on the water concentration in the ionic liquid. The earliest onset potential
as well as the highest current density was observed for 90 mol% EMIM BF4 – H2O mixture at a
pH of 3.2. Very small peak were observed at 2.5V cell potential with cathodic potential being
around 1.2V. After repetitive tests and a consistent data were obtained, these results were
confirmed as the onset potential.
Figure 5 shows the GC detection CO peaks as a function of change in the cathodic cell
potential at 90 mol% water concentration. In these experiments the voltage was held constant for
300 seconds after which the samples were injected into the gas chromatograph for detection. The
65
flow cell was connected directly to the gas chromatograph through an injection valve which
enabled for continuous detection of samples. GC was initially calibrated with different
concentrations of CO. These calibration curves were essential to confirm that the obtained curves
were CO and not any other gas. As discussed previously, we began to detect CO -1.2V cathodic
cell potential (2.5V cell potential). This can be seen through the pink curve in figure 5 showing
very small amounts of CO. The CO peak started to grow with the rise in the applied cell
potential. However, the increase in the peaks was limited because most the cell potential was
being consumed to overcome anode polarization and membrane losses if any. The areas under
these GC peaks were used to calculate the faradaic efficiency which has already been discussed
in Figure 4. A similar process was followed for detection of hydrogen gas with only difference
being nitrogen used as carrier gas instead of helium.
100 200 300 400 500 600 700
-40
-20
0
20
40
60
80
TC
D S
ign
al fo
r C
O (
A.U
.)
Time(sec)
-1.8V CCP
-1.6V CCP
-1.4V CCP
-1.2V CCP
*CCP - Cathodic Cell Potential
Figure 5: CO peaks observed from the gas chromatograph as a
function of cathodic cell potential (CCP) at 90 mol% water mole
fraction. TCD -Thermal Conductivity Detector
66
”The current density represents CO formation rate and the faradaic efficiency indicates
the amount of current density that was consumed to generate the desired product (CO)”[40]. The
steady state current density curves also increased at higher cathodic cell potentials and increased
water mole fractions. The availability of electrons at higher potentials accelerates the reaction
rates, leading to a higher current density. The increase was mainly attributed to the hydrolysis of
tetrafluoroborate (BF4)-, which releases protons when mixed with water. These protons
accelerate the reduction rates of CO2 until a critical mole fraction was reached. The hydrolysis of
the BF4 anion altered the pH of the EMIM BF4 – H2O mixtures which had an effect on the
reduction rates of CO2. Table 1 shows the pH values for different ionic liquid and water mixture.
Table 1: The pH of EMIM – BF4 solution mixed with various amounts of water and their
corresponding pH values
Solution Composition pH
0 mol% H2O 5.50
10 mol% H2O 4.9
50 mol% H2O 3.75
90 mol% H2O 3.20
96 mol% H20 4.90
The half-cell reactions taking place in the flow cell reactor are shown below:
The water splitting reaction takes place at the anode:
2H20 4H+ + 4e
- + O2 E
0 = 1.23V (4.1)
At the Cathode:
CO2 + e - CO2
- E
0 = -1.9V (4.2)
CO2- + H
+ + e
- CO + OH
- E
0 = - 0.53V
(4.3)
67
The overall cell reaction for C02 reduction:
CO2 + 2H+
+2e-
CO + H2O E0
= -0.53V at pH 7 (4.4)
E0 = -0.11V at pH 0 (4.5)
The equation 4.2 shows that the formation of the intermediate complex, CO2- requires a high
potential which is the rate determining step of the reaction. The ionic liquid brings this high
potential down to a very low value by forming a complex with this intermediate product (EMIM
– CO2-) which was then catalyzed on the catalyst surface.
The reactions by utilizing the ionic liquid, EMIM – BF4:
EMIM+
+ CO2 + e - Complex
-(ad) (l) E
0 ~ 0 (4.6)
Complex-(ad) (l) + H
+ (l) + e
- CO + EMIM
+ + OH
- E
0 = - 0.53V (4.7)
OH- + H
+ (l) H2O (l) (4.8)
Figure 6 shows a photograph of the ionic liquids with 5 different concentrations of EMIM BF4
Figure 6: The photograph shows vials with 5 different concentrations of EMIM – BF4 and
water mixtures used in the experiments to analyze the performance of the MoS2 catalyst.
From left, 0 mol% water, 10 mol% water,50 mol% water, 90 mol% water and 96 mol%
water.
68
and water mixtures used in the experiments. In the experiments, we observed that water lowers
the overpotential for CO2 reduction. This is due to the hydrolysis of BF4- to form anions such as
[BF3OH]-, [BF2(OH)2]
-, [BF3(OH)3]
-[27][53]. Hence, these results show that addition of water to
EMIM – BF4 decreases the pH of the solution leading to a greater availability of protons and in
fact accelerate the reduction rate of CO2.
Different ratios of syngas (mixture of CO and H2) were produced by varying the cathodic
cell potential between 1.2V and 1.8 V and also varying the electrolyte concentration between 0
mol% to 96 mol% water with EMIM – BF4. This tunable process of syngas production range
makes this electrochemical flow cell very liable for practical applications and making MoS2 a
very energy efficient catalyst due to its ability to selectively produce CO (FE) with high current
densities.
Through the water mole fraction experiments, we had observed that the MoS2 – EMIM
BF4 co-catalyst system exhibited high reduction current density (82 mA/cm2
at -1.8V cathodic
cell potential), where CO2 was selectively converted to CO (FE ~ 93%) at 90 mol% water
concentration.
With these observed conditions, it was useful to compare the MoS2 catalyst performance
with other existing transition dichalcogenindes (TMDCs) such tungsten selenide (WSe2),
tungsten disulphide (WS2), molybdenum disulphide (MoSe2) and existing noble metal silver
(Ag).
4.3 Performance of Transition Metal Dichalcogenides (TMDCs)
Experiments performed on the MoS2 catalyst showed that the co-catalyst system
performed best when the reactor was made to run with 90 mol% water concentration solution.
Hence, we decided to do a comparative study on other TMDCs such as WS2, WSe2 and MoSe2
69
along with Ag nanoparticles on the selectivity for generation of CO as the desired product as
well as compare them based on their CO2 reduction current density. The experiments for all these
catalyst mentioned above had similar operational parameters, and they were all tested with
90mol% water and 10 mol% EMIM – BF4.
Figure 7 shows the current density curves all TMDCs NFs (WSe2, MoSe2, WS2 and
MoS2) as well as the Ag nanoparticles which were coated onto the gas diffusion electrodes
(GDE) and tested in the microfluidic reactor. The cell was tested as different cathodic cell
potentials -1.2V, -1.4V, -1.6V and -1.8V measured with respect to Ag wire as a reference
electrode which was placed inside the Teflon channel of the catholyte. The recorded current
density at every cathodic cell potential were significantly higher for the TMDCs NFs when
-1.2 -1.4 -1.6 -1.8
30
60
90
120
150
180
Cu
rre
nt
De
ns
ity
(m
A/c
m2)
Cathodic Cell Potential(V)
WSe2
MoSe2
WS2
MoS2
Ag
Figure 7: Current density curves for Tungsten Selenide (WSe2),
Molybdenum Selenide (MoSe2), Tungsten Disulphide (WS2), Molybdenum
Disulphide (MoS2) and Silver nanoparticles (Ag) which were coated onto
the gas diffusion electrode (GDL) and tested in a microfluidic fuel cell
with 90 mol% EMIM – BF4 ionic liquid on the cathode side and 0.1M
H2SO4 on the anode side with nafion membrane as a separation medium
between the two chambers.
70
compared to the Ag NPs. Among the TMDCs, the WSe2 NFs showed extremely high CO2
reduction activity
The current density for synthesized WSe2 NFs (160 mA/cm2
at -1.8V cathodic cell
potential) was almost twice as much as the current density produced from MoS2 (82 mA/cm2 at -
1.8V cathodic cell potential) and almost 2.5 times higher than the current density produced from
Ag NPs (60 mA/cm2 at 1.8V cathodic cell potential) which are supposedly the most active non-
noble and noble catalyst, respectively for CO2. These current density experiments confirm that
the WSe2 NFs are the most efficient and active catalyst materials for CO2 reduction. Figure 8
shows a picture of the 5 different catalysts that t studied in the flow cell for CO2 reduction.
Figure 9 shows the faradaic efficiency curves for all the TMDCs NF materials as well as
Ag nanoparticles. It can be seen that the faradaic efficiency of all the TMDCs are very similar to
each other with a difference of about 5% between each other at a particular cathodic cell
Figure 8: Picture showing the coating of five different catalysts on to the gas diffusion
electrode (GDE). From the left Ag NPS, MoS2, WS2, MoSe2 and WSe2
71
potential. The obtained faradaic efficiency was at least 15% greater at each cathodic cell
potential for the TMDCs NF in comparison to Ag NPs. The faradaic efficiency of CO kept
increasing and reached a maximum of 92% for MoS2 and 78% for Ag at -1.8V cathodic cell
potential. The data extracted from the Gas Chromatograph (GC) clearly shows hydrogen being
produced as a main by product which permits the formation of synthetic gas (H2 + CO) and
consequently be utilized for other development processes.
This is very advantageous in comparison to noble metals such as Au NPs which
produce formic acid (HCOO-) as a by-product under examined conditions[40]. The fact
that CO production reaches a maximum at a particular mole fraction shows the
absorption effect of CO on the surface of TMDC catalyst. It is difficult show which
-1.2 -1.4 -1.6 -1.8
40
60
80
100
Fara
dic
Eff
icie
ncy(%
)
Cathodic Cell Potential (V)
MoS2
MoSe2
WS2
WSe2
Ag
Figure 9: Faradaic efficiency curves for Molybdenum Disulphide (MoS2),
Molybdenum Selenide (MoSe2), Tungsten Disulphide (WS2), Tungsten Selenide
(WSe2), and Silver nanoparticles (Ag) as a function of varying cathodic cell
potential
72
among the four TMDCs is the most active catalyst for CO2 reduction solely based on
reaction kinetics. Studies such as Density functional theory calculations, work function
calculations and other characterization techniques must be performed to get a better
understanding of the science of these materials, which is beyond the scope of this thesis.
4.4 Membraneless Electrochemical Fuel Cell
To alleviate the losses from a two compartment cell, we performed similar experiments
with single compartment membraneless electrochemical flow cell and analyzed the variation
from a two compartment cell. Membraneless flow cells do not use any physical barrier such as
membranes to separate the two compartments i.e., the anode and the cathode. Hence, it enables a
Figure 10: Exploded view of single compartment microfluidic flow cell
73
continuous interaction between the electrolyte, anode and cathode with subsequent oxidation and
reduction reactions. Recently several reactor designs have been discussed in the literature for
electrochemical reduction of CO2 to CO[2][5][6]. Most of the designs were inspired from fuel
cells which utilized a proton exchange membrane to separate the anode and cathode. However,
these membranes have certain technical issues such as anode dry out, gas crossover, flooding on
the cathode side which hinders the performance of the system. “Anode dry out generally occurs
due to the osmotic drag of water molecules along with the protons across the membrane”.
Osmotic drag of water molecules causes flooding in cathode compartment which on occasions
hampers the interaction between the gas and the catalyst nanoparticles.
Figure 10 and 11 show exploded and sectional views of a single channel membraneless flow cell
with the flow of gas and electrolyte across the flow cell. Here, the laminar nature of flow
Electrolyte
Figure 11: Sectional view of Single compartment membraneless flow cell with gas and
electrolyte flow
74
essentially avoids the necessity of a physical barrier like a membrane, however allowing active
ion exchange between the two electrodes (anode and cathode). Recent studies have shown that
flowing electrolyte in laminar regime could be used as a separation medium to compartmentalize
the fuel cell system[56]. An aqueous stream of a variety of liquid fuels such as methanol, formic
acid or dissolved hydrogen could be used as a separation for the microfluidic system. Moreover,
the membraneless reactor is very flexible; the concentrations of the electrolyte can be varied
independently which improves the reaction kinetics.
Jayashree et al., published work where they developed a H2/O2 fuel cell where a flowing
liquid electrolyte separated the anode and the cathode[47]. Here, we use this fuel cell model to
report our work on CO2 reduction to CO using MoS2 as catalyst and observe the performance of
the reactor with varying concentrations of the liquid electrolyte (EMIM – BF4)
This membraneless structure overcomes the above mentioned membrane related issues:
(1) minimizing fuel crossover by reducing the electrode to electrode distance and liquid flow
rate; (2) water management issues such as anode dry-out and cathode flooding issues can be
avoided in this aqueous liquid stream system; (3) the electrolyte concentrations can be adjusted
independently thereby optimizing the cell at individual electrodes. To optimize the
membraneless flow cell design, a trade off existed between maximizing the cell performance
such as high current density and faradaic efficiency (selectivity) to fuel utilization. By reducing
the electrode to electrode distance, the interaction between the electrolyte and catalyst can be
enhanced leading to higher current density. However, reduction in the distance between the
electrodes also had a negative impact on the system as there were higher chances for gas
crossover which hampers the performance of the fuel cell such as poisoning of the platinum
catalyst with carbon monoxide (CO) produced during the chemical reaction. Increasing flow
75
rates minimized gas crossover as the fluid had minimum residence time and also minimized the
effect of mass transport arising due to the interaction between the electrolyte and metal catalyst.
Increasing the flow rates shortens the residence time for the electrolyte inside the reactor. This
resulted in the electrolyte having minimum time to diffuse through the electrode and precede the
corresponding oxidation and reduction reactions.
4.5 Experimental Section
4.5.1 Electrode Preparation
Catalyst inks for the cathode were prepared by sonicating a mixture of 10 mg MoS2 with
0.6 ml isopropyl alcohol, 0.6 ml Deionized water and 10μl of 5 wt% EW Nafion Solution for 5
minutes. The anode catalyst ink was prepared in an identical manner with platinum being used
instead of MoS2. The catalyst inks were coated onto a 1.25 cm2
active area of toray carbon paper
gas diffusion electrode (Sigracet 35-BC), followed by drying under a 300 W high temperature
halogen lamp.
4.5.2 Flow Cell Assembly and Testing
Both the gas diffusion electrodes, one coated with MoS2 and the other coated with
platinum were placed across a 1 mm thick Teflon channel with 2.5 mm long and 0.5 mm wide
window machined to allow EMIM – BF4 ionic liquid electrolyte to flow. The catalyst coated
sides of gas diffusion electrode were exposed to the flow of the electrolyte. A 1 mm thick
aluminum plate served as a current collector. CO2 was flown into the reactor through the
cathodic current collector plate. The whole assembly of the Teflon channel, the aluminum plates
and the gas diffusion electrodes were held together using a high pressure C Clamp. Prior to
starting the experiments, deionized water was pumped through the cell at a constant flow rate for
a few minutes. If any kind of leaks were observed, the cell was disassembled and realigned so as
76
to make the system leak proof. Also, prior to experimentation argon was purged through for first
10 minutes and was made to run in open circuit at 2.5, 2.0 and 1.5V. This was mainly to remove
any residual gases in the chamber and enable the detection of CO at very low cell potentials.
The electrochemical experiments on the cell were performed by using a potentiostat (CH
instruments). The polarization curves for the flow cell were acquired by chronoamperometry
measurements at different cell potentials. The anode and cathode potentials were monitored
independently using a silver wire as a reference electrode placed in the exit electrolyte stream
just after it exits the reactor. The current densities were calculated based on the exposed surface
area of 1.25 cm2 (2.5 * 0.5 cm
2). All the experiments were designed to be run at room-
temperature and design was flexible enough to be run with varying electrolyte concentrations.
The electrolyte and gaseous streams were guided through a fine – gauge stainless steel tubing.
This micro fine tubing also allowed the liquid stream to flow in a laminar flow regime without
causing any turbulent mixing inside the reactor.
4.6 Effect of Water Mole fraction
To analyze the performance of single compartment cell, a similar approach for to that of a
two compartment cell was followed. The only difference in operation was that a single
electrolyte was flown and the flow rate was increased to 0.7 ml/min in comparison to 0.5 ml/min
that was used for the two compartment cell. This was mainly to alleviate the cell from the mass
transport issues. Also, all experiments performed have been reported with respect to cell
potential. The ion-exchange membrane used in two compartment cell is replaced by a flowing
electrolyte. Hence, this simplified configuration will have no membrane losses and minimum
anode polarization losses. Initially, before the experiment began we purged argon through the
77
cell for about 10 minutes so as to remove traces of any other gases. The cell was made to run
from 1.4V cell potential to observe any CO formation with minimum overpotential. We started
to observe CO formation at 1.8V cell potential and the current density started to rise with rise in
the cell potential
The flow cell performance was investigated by varying the electrolyte concentration
between 90 mol% and 95 mol% water mole fraction in EMIM – BF4 ionic liquid. The CO2 gas
was supplied at through the Sierra Mass Flow Controller at 5 sccm to the cell. Figure 12 shows
how the current density varies with different cell potentials and also with change in the
electrolyte concentration. It can be observed that the current density linearly increases with rise
2.0 2.2 2.4 2.6 2.8 3.020
40
60
80
100
120
Cu
rren
t D
en
sit
y (
mA
/cm
2)
Cell Potential(V)
CD_90 mol% water mole fraction
CD_95 mol% water mole fraction
Figure 12: Effect of adding water to the ionic liquid on the current density curves for
CO2 reduction to CO. These experiments were done at varying cell potentials with
platinum anode and MoS2 as cathode
78
in cell potential. The current density increased from 90 mA/cm2 to 120 mA/cm
2 at the cell
potential of 3 V when the water mole fraction of the ionic liquid was reduced from 95% to 90%.
The rise in performance exhibits the need for proton availability on CO2 reduction rate. As
shown previously in Table 1, 90 mol% water mole fraction has a minimum pH (3.2) compared to
other electrolyte concentrations. Hence, these results show the improvement of CO2 reduction
reaction kinetics at lower pH. Also, the rise in performance with increase in cell potential for
both electrolyte concentrations shows that the applied potential (electrons) is utilized for CO2
reduction and oxygen evolution reaction (OER) and is not wasted in overcoming other undesired
side reactions which might arise in a two compartment cell. The reference electrode helps to
analyze the polarization effects on the individual electrode and determine the effects of
decreasing the pH. Clearly changing the pH does not affect the system as it generates higher
current densities in comparison to a two compartment cell where lesser current densities were
observed at identical experimental parameters.
Figure 13 shows the faradaic efficiency for CO formation with varying cell potential and
varying electrolyte concentration. Although the onset potential for CO production in both the
electrolyte concentrations appears to start at the same potential (2V), the faradaic efficiency
drastically varies. This can be seen in Figure 13 where the faradaic efficiency for CO formation
is 72% at 2V for 90 mol% water mole fraction concentration in comparison to 38% when the cell
was made to run with 95 mol% solution. The catalyst selectivity towards formation of CO is
further enhanced at lower pH as indicated through the faradaic efficiency curves. The data
reported here shows the amount of CO produced was close to 97% at 3V cell potential in 90
mol% water mole fraction ionic liquid. The ability to generate a wide range of CO with variation
79
of cell potential and the pH of the electrolyte solution is a very good indicator for the tunable
syngas production.
The electrochemical reduction of CO2 using a flow cell has clearly been
demonstrated in this chapter. However, research towards minimizing losses such as membrane d
has to be done. Development of inexpensive anodic materials for oxygen evolution, testing the
flow cell with different electrode support materials such as carbon nanofiber or graphite plates
could
be highly useful to bring the operational cost of this process and could promote large scale
2.0 2.2 2.4 2.6 2.8 3.020
40
60
80
100
Fa
rad
aic
Eff
icie
nc
y (
%)
Cell Potential (V)
FE_90 mol% water mole fraction
FE_95 mol% water mole fraction
Figure 13: Faradaic Efficiency for CO production at different cell potentials and varying electrolyte
concentrations
80
development of this technology. In the next chapter, development of photochemical approach
towards reduction of CO2 will be discussed.
81
5. Photochemical Reduction of CO2
5.1 Introduction
Significant improvements in the development of energy conversion and storage systems
are necessary to reduce our dependency on fossil fuel resources, while improving the standard of
living. Through various reviews, it has been widely accepted that utilizing the captured CO2
rather than storing it in a natural reservoir has many advantages[57][58]. Electrochemical,
Photochemical and Catalytic Hydrogenation approaches have been identified as mediums
through which we could develop energy rich products from the captured CO2. In this chapter a
discussion on the photochemical reduction technology and experimental set-up developed to
experiment this approach will be done.
A source that is abundant, widespread across various geographical locations,
environmental clean and highly efficient is considered ideal to satisfy the present day needs. A
review of the available options indicates that sunlight is the most attractive renewable and
sustainable source, which can be used to satisfy the rising energy needs. It has been estimated
that the sun provides 120,000 TW of energy to the Earth[5]. In other words, it means that the
average sunlight the Earth receives in a day is greater than global energy requirement in a year.
Conversion of carbon dioxide and water into fuels using solar energy is of great interest as it has
great potential to reduce greenhouse gases as well as produce carbon neutral fuels.
Figure 1 shows a schematic diagram of utilizing solar energy to generate solar fuels. The
feedstocks necessary to carry out this process such as CO2, H2O and solar energy are captured
on-site or transported to the site. CO2 is generally provided as feedstock through the carbon
capture techniques such as absorption, adsorption, and membrane based separation technology.
82
Solar energy provides photons which are used by the solar cells to convert CO2 and H2O
into fuels. CO2 and H2O are converted into fuels by solar CO2 activation to generate syngas (CO
and H2). Syngas is then utilized as an energy intermediate product to generate fuels through
catalytic conversion such as Fisher-Tropsch process.
Most of the research in solar photovoltaic (PV) cells has been towards enhancing
electricity generation. Here, we discuss an approach to generate fuels such as hydrogen, which
works as an excellent storage medium for sunlight as well as photo chemically reduce CO2 to
energy rich intermediates such as synthetic gas. Integrating silicon solar cell with oxygen
evolving catalyst in a neutral pH yields a photo-assisted anode for generation of solar fuels
through water splitting reaction. Using oxidized hydrogen through a membrane based
Figure 1: Schematic diagram utilizing solar energy to generate solar fuels
Absorption
Membrane
Adsorption
Cryogenic
CO2 Capture
Solar CO2
Activation
Catalytic
Conversion
Photo-electrochemical
Photo-Catalytic
WGS
RWGS
Methanol Synthesis
Fisher - Tropsch
Fuels
Photons
CO, CO2
H2, H2O
Fossil Fuel
Power Plant
Flue Gas
83
configuration, carbon dioxide can be reduced through a co-catalyst system to generate energy
rich intermediates such as synthetic gas.
A basic photodiode inside an amorphous silica-based solar cell consists of three layers
either in p-i-n or n-i-p arrangement. A triple junction solar cell essentially consists of a very thin
p-type layer (20 nm), a thick (a few hundred nanometers) intrinsic layer and a thin n-type layer
(20nm)[59]. In this photodiode the excess electrons generated are provided from the n-type
region to the p-type region which generates a corresponding electric field. When this type of
photodiode is exposed to a light source, most of the photons travel through the p-type layer and
are captured by the intrinsic layer. The absorbed photons lead to the emission of electron and
hole photocarriers. These generated photocarriers are swept away onto either side of the p-type
and n-type layers producing solar electricity.
Several variations have been employed in the design of multijunction solar cells using a-
Si. Figure 2 shows the structure of a triple-junction stainless steel substrate based solar cell.
Amorphous Silica based solar cells are very efficient as they absorb maximum intensity of
photons. The thicknesses of the absorbing layers are generally less than 1μm[59]. In the
fabrication process, a metal reflective layer is first deposited onto the substrate through
sputtering, which is followed by ZnO buffer layer. This metal layer deposition takes place at
high temperature (300 – 4000C) after which the samples are loaded into a Plasma-enhanced
chemical vapor deposition (RF PECVD) for semiconductor layer deposition. a-SiGe (1.45 eV)
intrinsic layer is first deposited upon which asecond SiGe layer (1.6 eV) is deposited and a third
layer of a-Si (1.8 eV) based layer is then deposited. A thick layer of Indium – tin – oxide (ITO,
70 nm) is deposited at the top of the semiconductor layer through sputtering. This ITO works as
top electrode and also as an antireflection coating.
84
Figure 2: Structure of a triple-junction stainless steel substrate based solar cell
Persistent exposure to an oxidizing atmosphere causes many anodic catalysts responsible
to initiate water splitting reactions to degrade. Development of an earth-abundant catalyst
responsible for water splitting at a low overpotential and operating around neutral pH based
environment still remains a challenge. Kanan et al., worked on this challenge and developed an
oxygen-evolving catalyst that operates in neutral phosphate solution containing Co2+
ions and
oxidizes water at a very low overpotential (1.29V)[60]. Through this study, they also found that
Top
Middle
Bottom
85
operating in a neutral phosphate environment aids the stability of the anode in comparison to an
neutral water, acidic (pH 2-5) or a basic medium (pH 14) where long-term stability would be big
challenge. We used this Cobalt oxygen evolving catalyst in a neutral phosphate solution as anode
in the photo-electrochemical cell. The novelty of this approach is to engineer a simple stand-
alone, robust photo-electro-chemical cell (PEC) architecture for photo-assisted CO2 reduction
which would be a significant step towards the development of inexpensive direct solar to fuel
conversion technologies. Photo-electrochemical system in our study integrates harvesting of
solar energy with electrolysis of water as well as CO2 reduction.
Previously, research on generating hydrogen through water-splitting in photo-
electrochemical cell was done in abundance[62]. Here, an alternative route to generate energy
rich intermediate from CO2 and H2O without forming hydrogen will be discussed. The photo-
electrochemical cell was designed comprising of Earth abundant element that operates around
neutral pH responsible to initiate water splitting reaction. Transition Metal Dichalcogenides
(TMDC) are interfaced with the solar cell for catalyzing the conversion of CO2 to CO. The
chamber consists of amorphous silica based triple junction photovoltaic cell (PV) interfaced with
catalysts for oxygen evolution and CO2 reduction.
The crucial phase in natural photosynthesis is the capture, absorption of sunlight and
subsequent conversion into electron and hole pair. To mimic this for an artificial photosynthesis
setup, sunlight is absorbed by a triple junction amorphous semiconductor which then generates
electron hole pairs. The holes generated are captured by the oxygen evolution catalyst (OEC) to
split H2O to produce O2 and an oxidized form of hydrogen (H+). This hydrogen permeates
through an ion-exchange membrane to participate in the reduction reaction on the cathode side.
86
A detailed discussion on the development of an experimental set-up to test this mechanism will
be done in the subsequent sections.
5.2 Experimental Arrangement
5.2.1 Wireless Photochemical Cell Assembly
The photo-electro-chemical chamber as shown in Figure 3 is an acrylic plastic
(CHEMCAST GP) based transparent chamber that was machined according to the required
dimensions and assembled together using acrylic glue. The transparent chamber is separated into
two compartments by two tandem amorphous silica based triple junction (a-Si/a-SiGe/a-SiGe)
photovoltaic (PV) cells connected through a copper tape and a piece of Nafion membrane. The
efficiency of PV – electrolysis system is comparatively less for a single bandgap Photovoltaic
cell when compared with multiple junction PV cell[61]. This is because the multiple junction PV
cell have the ability to absorb and preserve energy from the photons at wider range wavelength
of solar radiation in the form of increased voltages[63]. Silicon was chosen for the artificial leaf
experiment due to its earth abundance and application across various fields.
The efficiencies of real solar cells have many extrinsic limitations which can be reduced.
These extrinsic losses include reflection, series resistance, non-radiative recombination and
above all the ambient cell temperatures. Other than these extrinsic losses the solar cell also
suffers from a couple of intrinsic losses which cannot be avoided due to semiconductor material
properties. A few of these intrinsic losses are the inability of single energy band gap of the solar
cell to match the broad spectrum. Photons with energies (hν) less than energy gap Eg are not
absorbed while photons with hν>Eg generate the necessary electron hole pairs but are
immediately lost due to the excess energy. Hence, these issues can be avoided in PV cells
87
designed with multiple bandgap ultimately leading to efficient solar energy conversion and
generation of desired product.
The cell is composed of three major segments (i) multi-junctions solar cells to collect sun
energy, (ii) the MoS2/IL co-catalyst system on the cathode side for CO2 reduction and (iii) Cobalt
(Co2+
) catalyst on the anode side to perform the oxygen evolution reaction (OER)[57][58][61]. In
this platform, an amorphous silicon based triple junction photovoltaic cell (PV-a-si-3jn) was
employed to generate the required potential for the reaction upon exposure to a light source,
ranging from visible light to near infrared.
The illuminated side of the solar cell was electro-deposited with cobalt. Cobalt coated ITO (light
illuminated side) side of the solar cell forms a photo-anode and was immersed in a Potassium based
buffer solution (0.071 M KPi, pH=7), is responsible to initiate water splitting reaction. The back stainless
substrate of the solar cell was coated with 40nm MoS2 nanopowder using drop casting coating method.
This MoS2 coated stainless steel substrate forms the photocathode and is placed in a 10 mol% EMIM-BF4
in water electrolyte solution which is saturated with CO2. As discussed in previous chapters and also in
literature, MoS2/IL co-catalytic worked towards the conversion of CO2 to CO. Coating methods of cobalt
and MoS2 as an oxygen evolution catalyst as well as CO2 reduction catalyst have been discussed in detail
later on in this chapter.
A membrane in the PECs acts as a barrier and separates different liquids, thereby preventing them
from mixing. Hence we employ a Nafion 117 membrane in our system. Further, the Nafion membrane
permeates only H+
ions generated through the OER to transfer to the other compartment to participate in
CO2 reduction. The treatment of Nafion membrane has been discussed in detail in chapter 3. The only
significant difference in the activation of the Nafion membrane is that the ion transfer treatment in this
case was done in 5% KOH solution. This treatment was mainly done to keep a stable membrane in a basic
media.
88
Water electrolysis is a complex reaction, requiring a minimum thermodynamic potential
of 1.23V to drive the reaction uphill. Photochemical systems are far more complex than their
electrochemical counterparts as additional power is required to perform both water electrolysis
and CO2 reduction requires. Therefore the efficiency of PECs is significantly low as they require
significant over-potential to drive the reactions at anode and cathode. Output voltage yielded
from one PV cell was merely sufficient for driving both water splitting reaction and CO2
conversion; hence, we decided to use two PV cells in series using copper tape to develop enough
output voltage to drive the reaction. When the PV cells are exposed to sunlight, “they absorb two
photons and produce one separated electron-hole pair”[64].
Figure 3: Schematic of the Photo-electrochemical Chamber. 1– 2 PV cells in series connected by
a copper tape, 2-Cobalt coated on ITO, 3-Potassium Phosphate buffer solution,4-Nafion (117)
Membrane, 5-10% EMIM-BF4 solution, 6-Scotch tape to generate headspace, 7-WSe2 on
Stainless Steel substrate(drop casting), 8-Gas bubbles generated through chemical reaction.
89
One set of holes and electrons are recombined through the copper tape, and other holes
and electrons flow towards the illuminated surface (i.e. photoanode) and the back substrate (i.e.
photocathode), respectively. Holes and electrons captured by Co and MoS2 catalysts are
consumed at anode and cathode, respectively
5.3 Oxygen Evolution Catalyst
The illuminated side of the solar cell was electro-deposited with cobalt. Cobalt is an
excellent oxygen evolution catalyst which self assembles upon oxidation to Co2+
, self-heals[64]
and works well in buffered solutions at and around room temperatures[65][66]. This cobalt
coated surface of the PV cell acts as a photoanode as it initiates the oxygen evolution reaction.
Also, this surface is directly made to come in contact with potassium based buffer solution
(0.071 M KPi, pH=7). Nocera et al. have previously reported development of catalyst based
systems based on cobalt-phosphate (Co-Pi) and nickel-borate(Ni-Bi)[61]. These catalysts are
very interesting in application of photo-electrochemical (PEC) cells as they have the ability to
drive the OER at moderate potentials. Also, these catalysts are best to operate around neutral to
slightly basic pH solutions. The buffer salts in the solution are necessary to maintain the anode
and cathode pH and also help in carrying the protons from anode to the cathode. Different
characterizations for OER with cobalt electrodeposited on the solar cell have been performed and
reported and is not the primary focus in this work.
5.3.1 Co-OEC Electro-Deposition
Co – OEC was electrodeposited onto the ITO substrate of the solar cell from cobalt (II)
nitrate hexahydrate (Co(NO3)2.6H2O, Alfa Aesar). The electrodeposition was carried out using a
solution prepared by mixing 73 mg of cobalt nitrate hexahydrate (Co(NO3)2.6H2O, Alfa Aesar)
90
in 500ml of 0.087 M potassium phosphate (pH 7) using a 3–electrode cell configuration
comprising of ITO working, platinum mesh as counter and Ag/AgCl reference electrode. Figure
4 shows the electro deposition coating arrangement of the solar cell and also pictures of solar
cells before and after the deposition process.
The solar cell was placed in the Co – Pi solution and the deposition process took place for
10 minutes. The electrodeposition was done by running the three electrodes and solar cell
configuration in a chronoamperometry experimental setting at 1.5V vs Ag/AgCl without stirring
and without any iR compensation. New solutions were prepared for each experiment. The
stainless steel substrate of the solar cell wasn’t supposed to be coated with Co and hence was
wrapped up in scotch tape.
A
B
C
Figure 4: (A) The solar cell dipped in cobalt(II) nitrate hexahydrate and potassium based buffer
solution (KPi). The working electrode (green) connected to the ITO coated layer of the solar cell,
Pt mesh (red) served as counter electrode and Ag/AgCl (white) was used as reference electrode,
(B) An uncoated solar cell with ITO substrate and (C) Solar cell coated with cobalt through
electrodeposition.
91
5.4 MoS2 Drop Casting
After the solar cell was electrodeposited with cobalt, the back stainless steel substrate of
the solar cell was coated with MoS2 through drop casting. The catalyst ink weighed was based on
the active area being utilized for a particular reaction. For most of our experiments we had an
active area of 12 cm2 and hence we prepared catalyst ink by sonicating a mixture of 30 mg of
40nm MoS2 nanopowder with 4 ml iso-propanol and 40 μl of 5% Nafion solution. The stainless
steel surfaced was initially roughed up by using sand paper to enable appropriate adhesion of the
catalyst to the surface. This catalyst ink was continuously drop casted on the stainless steel
substrate and was allowed to dry until the isopropanol evaporates from the surface and a fine
uniform coating of MoS2 developed. Figure 5 shows a picture of the stainless steel substrate
coated with MoS2 which is deposited through drop casting.
Figure 5: MoS2 drop casted onto the stainless steel substrate of the solar cell
5.5 Product Analysis
The gases generated through the chemical reactions inside the headspace were injected
into the Gas Chromatograph for product analysis. The Gas Chromatography experiments were
92
done using a SRI 8610C GC system equipped with 72 * 1/8 inch stainless steel sieve packed
column and a thermal conductivity detector (TCD). 5.0 Ultra high purity helium (Praxair) was
used as a carrier gas for detection of H2 and CO. The oven temperature was maintained at 1000C
and the column temperature was maintained at 1500C.
5.6 Preparation of Ionic Liquid
Prior to starting any of the photo-reduction experiments, pure ionic liquid was diluted
with water in appropriate amounts to achieve 10 mol% EMIM–BF4 in water. CO2 (99.9% UHP,
Praxair) was flown at 1ml/min using a mass flow controller and was directed into the EMIM –
BF4 electrolyte for 30 minutes so as to saturate the ionic liquid with CO2. Figure 6 shows a direct
solar to fuel conversion using an assembly of 2 solar cells connected in tandem using a copper
tape with cobalt coated on ITO substrate and MoS2 coated onto the back stainless steel substrate.
Figure 6: Schematic diagram of the custom made photo-electrochemical cell setup
93
Figure 7 shows pictures of the photo-electrochemical cell connected to the gas chromatograph
under one side illumination.
5.7 Results and Discussion
The stand-alone operation of the PEC was done at different solar irradiances such as 1 sun, 2 sun,
3 sun and 4 sun and was simulated using 300W Xe arc lamp. These calibrations were done using a
photodiode (spec). All calculations performed using this PEC system were done in a wireless mode i.e.
no potentiostat was used to supply external potential for the cell. We used a barometric concept to analyze
and quantify the product volume generated through the chemical reaction. For this, we initially created a
tiny chamber which by using scotch tape. The headspace inside this tiny chamber was used to suck out
the generated gases and injected into the gas chromatograph for product analyzation. Initially, air inside
the headspace was sucked out using a syringe. This led to the level of ionic liquid inside the tiny chamber
to rise up. As the liquid level changes, the pressure between the headspace and outside of the chamber
was different as the height of the liquid level changed. The level of ionic liquid went down as the gas
bubbles were generated on the cathode surface due to the chemical reaction and accumulated in the
headspace of this tiny chamber.
A B
Figure 7: Pictures showing the experimental setup of the photo-electrochemical cell using a 300W Xe arc lamp at 1
sun irradiation connected to the gas chromatograph for gas sampling
94
With the gas generation observed on the cathode side being dependent on the solar irradiation
which subsequently influences the pressure change based on the change in the liquid level, we developed
a relation between pressure change and solar irradiance. The initial pressure change inside the chamber
was calculated by through equation (5.1)
𝑃𝑐ℎ𝑎𝑚𝑏𝑒𝑟 = 𝑃𝑎𝑡𝑚 − 𝜌𝑔𝐻 (5.1)
Where Pchamber = Pressure inside the tiny chamber, Patm = Atmosphere Pressure, H = Change in initial
height after air was sucked out, ρ = density of Ionic Liquid.
𝑃𝑐ℎ𝑎𝑚𝑏𝑒𝑟 = 𝜌𝑔𝛥𝐻 (5.2)
Where Pchamber = Pressure change due to the generated gases, ΔH = Change in height due to gas bubble
generation. Based on equation (5.1) and (5.2) it was clear that the height change in the chamber was
0.0 0.5 1.0 1.5 2.0 2.5
40
80
120
160
Pre
ss
ure
Ch
an
ge
(P
a)
Solar Irradiance (Sun)
Figure 8: Variation of pressure as a function of solar irradiance
95
dependent on the solar illumination and hence we plotted a graph between the solar irradiance and
corresponding pressure change. Figure 8 shows the variation of pressure change as a function of the solar
irradiance (sun).
For calculating the height change in equation (5.2) we ran the photochemical experiment for 5
minutes and placed the chamber at various positions as calibrated by the photodiode and observed the
corresponding gas generation. From the figure, it was clear that the solar illuminance definitely had an
impact on gas bubble generation. Highest amount of bubble generation was observed at 2 sun
illumination with a pressure change of almost 140 Pa in comparison to the 0.5 sun illumination where the
pressure change was at a minimum of 40 Pa. However, new materials and designs need to be researched
upon for performing experiments with 2 sun illumination. This is because solar cell tends to deteriorate at
a faster rate with 2 sun illumination due to temperature and oxidation effects.
Previously, solar to fuel efficiency (SFC) was calculated with a wired system configuration with
current density being an essential contributor to the calculation[6]. The current generated by a PV
integrated with catalyst type solar cell were done running experiments in a three electrode cell. However,
this approach to compute the SFE cannot be applied for a wireless system. Hence, we decided to develop
a similar setting to that of a three-electrode electrochemical cell. Copper wires were connected to the
cobalt and the MoS2 coated sides of the solar cell. These copper wires were connected to the leads of
potentiostat (CH instruments). This potentiostat supplied the necessary external bias to the solar cell and
the current was obtained by running the cell in chroamperometry setting between -0.5V and 1V vs Ag/Ag
wire. The pressure and the liquid level change as a function of gas bubble generation was monitored.
Figure 9 shows the relation between pressure change and current density as a function of applied external
bias.
96
The pressure change data was calculated in a similar approach to that of the wireless system. The
chroamperometry experiments were made to run for 300 seconds and change in the liquid level was used
as a metric to calculate the necessary pressure change. From figures 8 and 9, we derived a relation
between wired and wireless system in terms of pressure change and estimated the current density for the
wireless system through this relation.
We analyzed the gas yield generated through the chemical reaction inside the PEC by injecting
the samples into the gas chromatograph. Figure 10 shows the curve obtained through the gas
chromatograph analysis. This confirms that only two gas components (CO and H2) are generated inside
the PEC. The solar to fuel (SFE) of our system was determined in terms of gas product using the
analyzed data from the GC. In addition to being dependent on the intensity of the incident solar
irradiance, the gas generation rate is also proportional to the catalytic surface area (i.e., MoS2 doped
-1.0 -0.9 -0.8 -0.7 -0.6 -0.50
40
80
120
160
Pressure Change
Current Density
External Bias (V)
Pre
ssu
re C
han
ge (
Pa)
-8
-6
-4
-2
Cu
rren
t D
en
sit
y (
mA
/cm
2)
.
Figure 9: Variation of Pressure change (Pa) and current density (mA/cm2)
as a function of external bias (V)
97
surface area) and most importantly the SFE of the solar cell. The GC results indicate that H2 and CO are
the only products produced by the chemical reaction. Hence, SFE could be calculated by using the
equation 5.3,
(5.3)
Where, N1 and N2 = are number of moles of produced gas per unit of time (mol/s) measured using GC, E1
and E2 = are Energy Densities of the corresponding gas (kJ/mol), Acat = Catalytic surface available for the
reaction (m2), Ug = total solar irradiance (mW/cm
2).
5.8 PV Efficiency Measurement
The solar to electricity conversion efficiency of triple-a-Si PV cell coated with Co and MoS2
catalysts was measured under one sun simulated sunlight illumination. The voltage produced by a PV cell
was directly measured with a multimeter while the resistance of the circuit was changed using variable
resistors. An open circuit voltage (Voc), a short circuit current and the average fill factor of two PV cell in
series configuration were 2.12 V, 6.1 mA/cm2, 0.53 respectively. Therefore, the dry cell efficiency of
7.1% can be calculated by dividing the product of the above three parameters by the one sun illumination
ɳ = 𝑁1. 𝐸1 + 𝑁2. 𝐸2
𝑈𝑔. 𝐴𝑐𝑎𝑡
CON2
O2
CO
N2
O2
H2
A B
Figure 10: Gas sample curves from the gas chromatograph. (A) Calibration curve for CO generation
showing the different gas detected in the sample using a thermal conductivity detector and (B) Gases
detected from the sample taken from the Photo-electrochemical cell
98
(100 mW/cm2). Voc of two PV cell in series configuration was 3.6 V in a dry cell environment. However,
when this arrangement was placed in an electrolyte solution the voltage was observed to drop to 3V while
the short circuit current remained unchanged when the cell was brought to inside of electrolyte solution.
The fill factor was assumed to change rarely within the experimental range. Based on the measurement,
the change in Voc affected the PV efficiency that could be reasonably predicted as high as 6% in the
electrolyte. This obtained SFE was significantly higher than the hydrogen evolution system (2.5%)
reported previously for a similar architecture.
A
0.1
1
10
mo
le/s
( x
10
-7)
CO
H2
0.5 1.0 1.5 2.0
4
5
6S
FE
%
Sun illumination
PV Cell Eff.
B
C
A
B
Figure 11: (A) Number of moles of H2 and CO at
different illumination of sun (B) Solar to Fuel
Efficiency at different illumination of sun
99
6. Conclusion
Large scale development of energy conversion systems (electrochemical and
photochemical) still remains a critical challenge to address the greenhouse gas emissions and to
satisfy the energy demand. In this work, miniature models related for continuous generation of
syngas from CO2 were studied and demonstrated.
This thesis work resulted in several new findings. Firstly, an electrochemical flow cell
was developed in two different cell configurations (membrane and membraneless). Here,
inexpensive and non-noble earth abundant transitional metal dichalcogenides (TMDCs) were
used as cathodic catalysts which in combination with ionic liquid (EMIM-BF4) converted CO2 to
syngas with high current densities (82 mA/cm2 at -1.8V for MoS2 in two-compartment cell,
120mA/cm2
in a single compartment cell) and high faradaic efficiencies (92% at -1.8V for two
compartment cell and 98% at -1.8V for single compartment cell). Additionally, the
electrochemical flow cell produced a tunable range of syngas when the cell was tested at
different water mole fraction concentrations (0 mol%, 10mol%, 50 mol%, 90 mol% and 96
mol%) in EMIM-BF4 ionic liquid solution and different applied cell potentials. This study
exhibited the flexibility of the system.
Further, the performance of different transition metal dichalcogenides(TMDCs) along
with noble metal such as silver (Ag) was studied and compared on the basis of current density
and faradaic efficiency. WSe2 was the best performing catalyst for CO2 reduction to syngas at 90
mol% water mole fraction in EMIM-BF4 solution. The current density was observed to be 160
mA/cm2 at -1.8V cathodic cell potential with a faradaic efficiency close to 90%. The current
100
density was almost 2.5 times higher when compared with silver (60 mA/cm2 at -1.8V) which was
supposedly the most active noble catalyst for CO2 reduction.
Lastly, a photo-electrochemical cell (PEC) was developed to demonstrate photochemical
reduction of CO2. In this approach, amorphous silica based triple junction photovoltaic cell was
used to generate the necessary potential for the reaction upon exposure to the light source. The
co-catalyst concept of MoS2/IL used to on the cathode side for CO2 reduction and Cobalt (Co2+
)
in a neutral salt solution (potassium phosphate with a pH 7) was used as a photo-anode to
perform oxygen evolution reaction. Using this platform we were able to generate syngas
concentration in 10 mol% EMIM-BF4 solution similar to that observer in an electrochemical
flow cell or a three electrode cell. Further, solar to fuel efficiency was computed based on light
illumination (no. of suns). The PEC generated 4.6% SFE which is mainly limited due to the
maximum efficiency of solar cell (6%). This obtained SFE is first of its kind and is significantly
higher that of hydrogen evolution reaction (2.5%) reported previously using identical solar cell.
101
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VITA
Name: Venkata Aditya Addepalli
Education: B.E., Mechanical Engineering, Anna University, Chennai, India, 2013
M.S., Mechanical Engineering, University of Illinois at Chicago,
Chicago, Illinois, 2015
Experience: Graduate Research Assistant at the Nanomaterials and Energy System
Laboratory, Prof. Salehi-Khojin, Department of Mechanical and
Industrial Engineering, University of Illinois at Chicago, Chicago,
Illinois. 2013-2015
Teaching Assistant in Department of Mechanical and Industrial
Engineering, University of Illinois at Chicago, Chicago, Illinois. 2014 -
2015