SYNOPSIS OF

(a) Electrochemical Reduction of Carbon dioxide to Useful Chemicals (or)(b) Unmodified and modified Cu, Zn electrode towards Electrochemical

Reduction of CO2 in Halide Electrolytes (or)(c) Electrochemical Reduction of CO2 on modified Cu, Zn electrodes in Halide Electrolytes

A THESIS

to be submitted by

G. KEERTHIGA

for the award of the degree

of

DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMICAL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY MADRAS

CHENNAI – 600 036, INDIA

September -2014

0

1. Introduction

Combustion of solid, liquid and gaseous fuels in any form leads to environmental

pollution. Out of emissions of unburnt hydrocarbons, NOx, SOx and CO2 emission

alone contributes to nearly 75% of all other pollutants (Song, 2006). The upper safe

limit of CO2 in the atmosphere is 350 ppm, which has been reached long ago

(weblink1, CO2 now.org). Hence, stabilization of levels of CO2 is needed so as to

cope up with the Earth’s climate. Recovering CO2 to synthesize fuels can also be

thought as an alternative to solve the current energy problem.

CO2 is very stable, with a standard free energy of formation (G° = -394.36

KJ/mol) and it is known to be used in some limited applications like fire extinguishers

and beverages. CO2, the most oxidized form of carbon, needs chemical

transformations induced by energy source which is preferably renewable. Out of

biochemical, photochemical, electrochemical, solar thermo chemical and chemical

sequestration of CO2, electrochemical reduction of CO2 was practiced by absorbing

CO2 on metal surfaces and activating it by supplying electrons. Compared to other

methods of CO2 reduction, electrochemical method offers advantage like directing the

intermediates towards reduction, less formation of byproducts and quick industrial up-

gradation.

Figure 1 shows classification of the electrode materials and its reaction products.

On investigating metals towards CO2 reduction, massive literature study shows

significant hope towards medium hydrogen over voltage metals (Cu, Au, Ag, Zn); in

particular Cu as it will emerge as a new era of CO2 conversion catalyst.

Figure1: Electrode materials and reaction products (reproduced from Kondratenko et al., 2013)Moreover, the choice of aqueous electrochemical reduction of CO2 becomes

feasible with an electrolyte of good CO2 solubility and conductivity. Arriving at an

innovative electrocatalyst with its optimized parameters of reduction (electrolyte,

1

potential and time of reduction), will be a gateway in dealing with such complicated

process of reducing CO2.

2. Objectives

The main objective of this work is to understand the behavior of CO2 reduction

on medium hydrogen overvoltage metals. The unique property of these metals is to

adsorb and desorb the CO from its surface. Among which, Cu is unique for the

reduction of CO2 as it can catalyze the breaking of C-O bond in CO2 and hence further

form hydrocarbons.

Though several research work has focused on Cu along with variation in

electrolyte (Kaneco et al., 2002), lowering temperature (Kaneco et al., 2006 (a)),

increasing pressure (Hara et al., 1994), the efficiency towards CO2 reduction was not

satisfactory. Hence, attempts to increase the same were achieved by adopting suitable

modification of the electrode by deposition, oxidation, alloying with other metals etc.

Aim of the present investigation is to activate and thus to reduce CO2 efficiently, and

the objectives of the work are

To employ Cu electrode towards electrochemical reduction of CO2 and

investigated it in supporting electrolytes such as KCl and KHCO3.

Attempts were done to activate Cu electrode by oxidation (Oxidized Cu-I,

Oxidized Cu-II) and by electrodepositing Cu on Cu (Cu/Cu-L, Cu/Cu-H).

To employ the less exploited metal, Zn towards CO2 reduction and to modify

Zn by depositing it on a Cu substrate (Zn/Cu-H, Zn/Cu-L).

The above electrodes were characterized by Scanning Electrode

Microscopy (SEM), Electron dispersive X-ray analysis (EDX), X-ray

diffraction (XRD) and X-ray electron photoelectron spectroscopy

(XPS).

The products of CO2 reduction were analyzed by Gas Chromatography

(GC).

A comparative analysis of modified electrode with the unmodified Cu,

Zn electrode was done in terms of Faradaic efficiency (FE), Partial

current density (PCD).

2

To extend the reduction methodology to a continuous electrochemical

reduction of CO2 with the gas diffusion electrode (GDE) of Cu and modified

Cu as cathode in a full cell set up.

3. Electrochemical Reduction of Carbon dioxide on Cu and Modified Cu

Electrode

The electrochemical conversion of carbon dioxide (CO2) into useful chemicals

can be altered by variables like electrode material, electrolyte and applied potential. In

this work, the influence of supporting electrolytes over a range of electrode potential

for the electrochemical reduction of CO2 at Cu electrode in aqueous solution under

ambient condition was compared. The supporting electrolyte belonging to buffered

(KHCO3) and non-buffered (KCl) were investigated for CO2 reduction. KHCO3 helps

in buffering the solution during CO2 reduction (Kortlever et al., 2013), while KCl

suppresses the adsorption of protons thus promoting reduction (Ogura et al., 2010).

The electrochemical measurements were performed in EC epsilon potentiostat in

a divided H-type cell. Initial screening was done by voltammetric studies on the

chosen electrolyte and electroreduction of CO2 was performed for a period of 4 hrs

with the potential ranging from -0.7 V to -1.8 V vs. NHE. The products formed were

methane and ethane, with hydrogen as the byproduct analyzed.

Figure 2: Cyclic voltammogram of Cu electrode obtained at a scan rate of 10 mV s -1 in (a) 0.5 M

KCl and (b) 0.5 M KHCO3. Inset in Figure 2 (a) shows the enlarged voltammogram of forward

scan in the potential range -0.5 V to -1.2 V. Effect of reduction potential for the products formed

on Cu electrode in (c) 0.5 M KCl and (d) 0.5 M KHCO3.

Figure 2 shows the cyclic voltammogram (CV) of Cu electrode in 0.5 M KCl and

0.5 M KHCO3 supporting electrolyte with the inset showing enlarged voltammogram

of Cu-KCl system. The dotted line corresponds to CV of nitrogen (N2) saturated

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solution and solid line corresponds to that of CO2 saturated solution; while the onset

potential and current density were the parameters used to estimate the performance of

reduction.

From the inset in Figure 2 (a) showing the enlarged area from -0.5 V to -1.2 V for

Cu-KCl system, we can observe that the onset potential for CO2 saturated solution

starts at -0.64 V where current density of -0.1 mA cm-2 is observed (Kaneco et

al.,1999). An increased current density of 1.3 mA cm-2 was observed for CO2 saturated

solution compared to 0.45 mA cm-2 for N2 saturated solution, when measured at a

potential of -1.1 V. The difference in cathodic current confirms the reduction of CO2.

Figure 2 (b) shows the CV of Cu electrode in KHCO3 electrolyte. The onset potential

for CO2 saturated solution was observed at -0.89 V. An increase in current density of

4.34 mA cm-2 was observed at -1.1 V for CO2 saturated solution compared to 1.27 mA

cm-2 for N2 saturated solution which confirms the reduction of CO2. A small peak

observed around -0.02 V (in KCl) and -0.01 V (in KHCO3) could be due to the

successive reductions of Cu(II)/Cu(I) and Cu(I)/Cu(0) on the electrode surface

(Shaikh et al., 2011). The current increase at negative cathodic potential for N2

saturated solution could be due to the hydrogen evolution or could be both the

hydrogen evolution and CO2 reduction for CO2 saturated solution (Cardona et al.,

2001). In general, the onset potential is determined by cation and anion of the

supporting salt and pH change of catholyte (Kaneco et al., 1999, Kaneco et al., 2006b)

where the effects of these in the voltammetric studies are still being examined. In

particular for KCl electrolyte, on applying negative potential, the hydronium ions

orient towards the electrode and the chloride ions adsorb on the electrode surface

suppress the adsorption of protons (Ogura et al., 2010), which could have lowered the

onset potential (-0.64 V < -0.89 V). Whereas in the case of KHCO3 electrolyte,

equilibrium between H+ and HCO3- exist near to the electrode surface, which require

high negative potential for the reduction of CO2.

The effect of potential on the product formed in KCl and KHCO3 electrolyte was

compared in Figure 2 (c) and (d). In KCl electrolyte system, maximum hydrocarbon

formation was observed at -1.6 V with 3 mmol cm-2 of ethane, 0.58 mmol cm-2 of

methane and 114 mmol cm-2 of hydrogen. The maximum product formation in

KHCO3 was observed at -1.8 V with 7 mmol cm-2 of methane, 1.8 mmol cm-2 ethane

and 133 mmol cm-2 of hydrogen. The maximum product formation was observed at

lesser negative potential on KCl (-1.6 V) than in KHCO3 (-1.8 V). This is in

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coherence with CV result, where KCl (-0.64 V) showed lower onset potential than

KHCO3 (-0.89 V). A similar sequence was recently reported by Wu et al. (2010) with

maximum formic acid formation when Sn electrode was examined in KCl and

KHCO3 electrolyte. Further on plotting Faradic efficiency, it was found that KCl

electrolyte was selective towards ethane with a maximum Faradaic efficiency of

38.5% at -1.6 V and KHCO3 shows methane with a maximum Faradaic efficiency of

19.6% at -1.8 V. In order to improve the Faradic efficiency, metallic Cu was

modified by oxidation and deposition.

As it was established that oxidized derive electrodes can catalyze energy efficient

CO2 reduction (Le et al., 2011), modification by oxidation was carried out by two

different methods. The first is by conventional oxidation where the electrodes were

oxidized and annealed at a particular temperature in a furnace. The second is by

exposing it to a high temperature flame followed by quenching. These electrodes were

designated as oxidized Cu I and oxidized Cu II respectively and characterized by

SEM, XRD and XPS.

(a) (b) (c)Figure 3: Scanning electron micrographs of (a) Oxidized Cu I (b) Oxidized Cu II (c) Bare Cu.

Figure 3 shows microscopic image corresponding to oxidized Cu I, II and bare

Cu. Figure 3 (a) shows electrode oxidized at 500°C (oxidized Cu I) where pillar like

structures were formed on the Cu substrate. Build up cube like arrangement ensures

higher surface area than pure Cu electrode, which could form more active sites for

reduction. Figure 3 (b) of the oxidized Cu II shows humps of Cu oxide grown on the

metallic Cu while Figure 3 (c) shows scanning electron microscopic image of bare

Cu.

Figure 4 (a) compares the product formed on oxidized and bare Cu electrode. The

electrode namely oxidized Cu I and II was found to yield ethane selectively with an

Faradaic efficiency of 60% and 52% respectively which is in comparison with the FE

of ethane on bare Cu electrode (5%). In all the three electrodes studied, the Faradaic

5

efficiency of methane was found to be less than 10%, with significant average

hydrogen evolution of 34%.

(a) (b)Figure 4 (a):Comparison of FE of products formed on Cu and oxidized Cu I, II at -1.2 V vs. NHE. (b) Effect of Faradic efficiency vs. potential observed for oxidized Cu II electrode

Though the oxidized Cu I was selective towards ethane formation, it offers high

resistance to the reaction. The thickness and double layer capacitance values of the

oxidized Cu I (14.5 m, 12.5 mF/ cm2) was in comparison with the values of oxidized

Cu II (8.1 m, 4.9 mF/ cm2). The roughness of the oxidized Cu I is nearly double than

oxidized Cu II and hence later during electrochemical reduction of CO2 offers

resistance at the potentials greater than -1.2 V. But no such discrepancy was observed

in oxidized Cu II electrode; moreover, it could offer nearly same efficiency as

oxidized Cu I. Moreover, the preparation of oxidized Cu II electrode was less time

consuming than oxidized Cu I electrode. Furthermore, the individual characteristics of

the Cu and oxidized electrodes was examined by XPS which revealed the presence of

satellite structures for oxidized Cu II rather than oxidized Cu I. In the Figure 4 (b),

the effect of Faradaic efficiency on the potential was examnined on oxidized Cu II

electrode. As the potential increases, efficiency of methane decreases from 22% to

8% while the efficiency of ethane attains maximum of 52% at -1.2 V vs. NHE. The

efficiency of hydrogen increases and attains 44% at -1.6 V vs. NHE.

In order to correlate the selectivity results to the metal oxide interface; electrodes

were prepared with powders of CuO, Cu2O and tested for CO2 reduction. The

experiments were carried out at -1.2 V for a period of time and results were compared

with the bare Cu metal strip. From the Figure 5 (b), it can be seen that the metallic Cu

yielded higher methane than the corresponding powders. Pelletized oxides of Cu

(CuO, Cu2O) when tested for CO2 reduction gave only methane rather than ethane,

which brings out the fact that a metal oxide interface is required for facile formation

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Figure 5:Effect of electrolysis time on products formed by electro reduction of CO2 on Cu, CuO and Cu2O pellets in 0.5 M KCl at room temperature at a potential of -1.2 V vs. NHE.

of C-C coupling in CO2 reduction reaction. From the Figure 5 (b), the sequence

of methane formation, at 3 hour of the reaction varies in the order of metallic Cu (72

mol/cm2) > Cu2O (38 mol/cm2) > CuO (18 mol/cm2) > Cu (8 mol/cm2). Thus,

the modification effected by creating metal-oxide interface helps in activating CO2

towards electrochemical reduction.

The product distribution of electrochemical reduction of CO2 was altered by

modifying the surface of pure copper by deposition. In this study,

chronoamperometric deposition of Cu on Cu (Cu/Cu) was carried out with 0.025 M

and 0.25 M bath concentrations of CuSO4 and termed as Cu/Cu-L and Cu/Cu-H

respectively.

(a) (b)

0.0

0.4

0.8

1.2

1.6

2.0

Text

ure

coeffi

cien

t ( a

.u)

Crystal planes

111 200 220 311

(c)Figure 6: SEM images of (a) Cu/Cu-L and (b) Cu/Cu-H (c) Texture coefficient of Cu particles calculated from the XRD pattern. Cu (), Cu/Cu-L () and Cu/Cu-H ().

Figure 6 (a) and (b) shows SEM image of Cu/Cu-L and Cu/Cu-H respectively.

Figure 6 (a) shows spheres of Cu nanoparticles sparsely populated with uniform

distribution on the Cu substrate while Figure 6 (b) shows densely populated larger

cubes of Cu deposits followed by the small growing nanoparticles which completely

covers the underneath Cu substrate. The low and high metal concentration species

studied here follow two different mechanisms so as to lower surface energy and 7

initiate nucleation. The nucleation progresses till the reaction attain steady state and

further deposition occurs on already-established copper nuclei (Grujicic et al., 2002).

Figure 6 (c) shows texture coefficient of the facet {hkl} vs. crystal planes which

was calculated by using Halls method by the formula given below.

Here C(hkl) is the texture coefficient of the facet {hkl}, I (hkl) is the intensity of the

(hkl) reflection of the sample under analysis, Io(hkl) is the standard intensity of the

(hkl) reflection of a sample taken from the JCPDS data and ‘n’ is the number of

reflections taken into account. If the calculated C(hkl) is equal to unity, the facets

doesn’t have any preferential orientation. If a particular facet is greater than unity,

then it is preferentially oriented (Navaladian et al., 2009). Pure Cu shows orientation

towards the facet of (111) and (200) with the texture coefficient of 1.2 and 1.5,

respectively. Comparatively Cu/Cu-L shows orientation towards the facet of (220),

with texture coefficient of 1.7 followed by Cu/Cu-H of 1.5 as seen from Figure 6 (c).

-0.8 -1.0 -1.20

10

20

30

40

50

-0.8 -1.0 -1.20

20

40

60

80

-0.8 -1.0 -1.20

5

10

15

20

25

30

Ethane

Hydrogen

FE (

%)

Methane

Potential (V vs. RHE)

Figure 7:(a) Plot of FE vs. potential for the products formed on Cu and electrodeposited electrodes in 0.5 M KCl; Cu (), Cu/Cu L () and Cu/Cu H () (b) Plot of partial current density of the products formed at -1.2 V (vs RHE) on Cu and electrodeposited electrodes in 0.5 M KCl. Methane (), Ethane () and Hydrogen () and total current density (◊).

As can be seen from the Figure 7 (a), maximum Faradaic efficiency of 28% is

observed for methane on Cu electrode which has a dominating (111) plane, but for the

electrodeposited Cu electrodes where the texture coefficienct of (111) plane was near

to unity shows lower methane formation e.g. Faradaic efficiencies were 26% and 20%

for Cu/Cu-L and Cu/Cu-H electrodes, respectively. As depicted in Figure 7 (a),

Cu/Cu-L showed maximum Faradaic efficiency up to 43% for ethane compared to

Faradaic efficiencies of 10% and 28% on Cu and Cu/Cu-H, respectivley. As discussed

in the texture coefficient plots (Figure 6 (c)), Cu/Cu-L has crystal orientation towards

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(220) plane and hence favours ethane formation (Hori et al., 2003). Figure 7 (a) shows

the Faradaic efficiency plot of hydrogen evolution for Cu and electrodeposited Cu

electrodes; a decrease in Faradaic efficiency was observed with an increase in

electrode potential, which could probably due to the utilization of H+ ions towards

hydrocarbon formation (Gatrell et al., 2006). Moreover, the results for this behaviour

was analyzed in the PCD plots in Figure 7 (b).

Figure 7(b) shows the partial current density (PCD) and total current density plots

of products formed versus Cu and electrodeposited Cu electrodes at -1.2 V. The

partial current density for methane was higher on Cu electrode, followed by the partial

current density for ethane on Cu/Cu-L which is in agreement with the trend observed

in the texture coefficient plots (Figure 6 (c)). Enhancement in CO2 reduction by the

addition of steps and kinks to atomically flat Cu surface has been reported by Hori et

al.,(2003) and Takashi et al. (2012) reported higher yield of C2 when n = 2 on (110)

plane due to the presence of two neighboring step lines. In addition, as reported by

Hori et al., (2003) when a cyrstal surface of (220) plane has step lines adjacent to

(111) plane, it suppress the formation of methane and promotes the formation of the

intermediate species though C-C bonding (Hori et al.,(2003)).

4. ELECTROCHEMICAL REDUCTION OF CARBONDIOXIDE ON Zn

AND MODIFEID Zn ELECTRODE

Apart from widely studied Cu electrode, the hunt for new alternative metals

/materials for CO2 reduction are widely investigated. In this aspect, Zn metal which

exhibits similar properties like that of Cu are exploited for electrochemical reduction

of CO2 in KCl and KHCO3 electrolyte.

Figure 8 (a) and (b) shows the products formed on Zn electrode in KCl and

KHCO3 electrolytes at -1.2 V. The main products measured were methane, hydrogen,

while ethane and methanol were observed in trace. Over a period of 4 hours, methane

formation upto 35.6 µmol cm-2 and hydrogen formation upto 0.95 mmol cm-2 were

observed in Zn-KCl system. Similarly in Zn-KHCO3 system, methane up to 52.2

µmol cm-2 and hydrogen of 10 mmol cm-2 were observed. Ethane and methanol

formed in the Zn system was observed as trace.

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Figure 8: Effect of electrolysis time on the products formed during the reduction of CO2 on Zn at -1.2 V in (a) 0.5 M KCl and (b) 0.5 M KHCO3. Effect of reduction potential for the products formed on Zn electrode in (c) 0.5 M KCl and (d) 0.5 M KHCO3.

Figure 8 (c) and 8 (d) compares the effect of potential on the product formation

for Zn electrode in KCl and KHCO3 electrolyte, respectively. In Zn in KHCO3

system, the products evolved increases with potential and maximum product formed

were methane of 245 mol cm-2, ethane of 7.4 mol cm-2, methanol of 16 mol cm-2

and hydrogen of 138 mmol cm-2 was observed at potential of -1.8 V. Similarly, for Zn

in KCl system, maximum products of methane of 431.7 mol cm-2, ethane of 10.4

mol cm-2 and hydrogen of 144 mmol cm-2 was obtained at a potential of -1.8 V. On

comparing of Figure 8 (c), (d) it was observed that Zn in KCl exihibit higher methane

formation on (431.7 mol cm-2) than Zn in KHCO3 (245 mol cm-2) at a potential of

-1.8 V. In addition to this evaluation, we found bicarbonate electrolyte leads to the

formation of methanol. On accounting for the current applied to the system, a

maximum Faradaic efficiency of 7.3% and 12.7% for methane was observed on Zn-

KCl, Zn-KHCO3 system at a potential of -1 V respectively. As the efficiency obtained

are low than compared to Cu electrode, surface modification of the electrode to

improve the efficiency was investigated.

Hence, the efficiency of Zn in CO2 reduction was studied by depositing Zn on the

Cu substrate. Figure 9 depicts the micrographic image of Zn/Cu deposited from 0.6 M

and 6 M sodium zincate bath solution designated as Zn/Cu-L and Zn/Cu-H

respectively.

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(a) (b)

Figure 9: Scanning electron micrograph image of Zn/Cu-L and Zn/Cu-H.

The Figure portion 9 (a) depicts uniform hexagonal structures of Zn arranged in a

row like fashion with growing whiskers around them. The Figure portion 9 (b) depicts

clearly the dendritic arrangement of Zn grown along with intercalated, hexagonal

cubic structure of Zn deposited on the Cu electrode. These electrodes were

characterized by SEM, XRD and XPS. Later, the deposits of Zn on Cu electrode were

studied for electrochemical reduction of CO2 by varying the reduction potential as

shown in Figure 10 (a).

(a) (b)Figure10: (a) Effect of reduction potential for the products formed on Zn/Cu-H and Zn/Cu-L electrode in 0.5 M KCl.(b) Products formed methane, ethane and hydrogen vs. electrode potential on Zn on Cu, Cu and Zn electrode in KCl electrolyte. Products analyzed after 1 hr of electrolysis.

As inferred from the comparitive analysis, the hydrocarbon efficiency of Zn/Cu-L

samples was found to be less than Zn/Cu-H selectively favoring hydrogen evolution

upto 59%. Maximum methane of 52%, ethane of 24% and H2 efficiency of 8.7% was

observed at a potential of -1.6 V on Zn/Cu-H electrode. Due to the tendency of high

hydrogen evolution and low hydrocarbon formation on lower concentration deposit

(Zn/Cu-L), we restrict our further analysis to higher concentration deposit (Zn/Cu-H).

11

Moreover, the FE plots in the Figure 10 (b) compares the product formed

(methane, ethane and hydrogen) on bare electrodes (Cu, Zn) and electrodeposited

electrodes (Zn/Cu). The Faradaic efficiency of methane on Zn/Cu electrode was found

to be higher (average efficiency of 50%) than Cu and Zn electrodes. Say, at a

potential at -1.6 V, Faradaic efficiency of Zn/Cu was 52% which is nearly 13 times

higher than Cu (4%) and Zn (1%). Hence, it was observed that the modification

effected by depositing Zn on Cu electrode enhanced the selectivity of C1 product.

Formation of ethane was not observed till a potential of -1.2 V, then at a potential of

-1.6 V, 38% of ethane was formed on Cu, followed by 16% on Zn on Cu as shown in

the Figure 10 (b), while Zn yields negligible ethane efficiency throughout the

potential studied. Among the electrode studied, Cu has good hydrogenating property

and the similar trend was reflected in Faradaic efficiency plots of H2 where Cu

showed higher Faradaic efficiency than Zn and Zn/Cu electrode. At a potential of -1.8

V, Cu evolved 68% of H2 compared to 8% on Zn/Cu and 1% Zn. The total Faradaic

efficiency less than 100% signifies the possibility of some unidentified products.

Later, to overcome the limitations in a batch set up (Aeshala, et al., 2012),

continuous reduction of CO2 were performed in a solid polymer electrolyte reactor

enclosed full cell set up. The schematic of the setup and the preparation of the

membrane electrode assembly were discussed in detail. Cu-Gas Diffusion Layer

(GDL) was used cathode with commercial Pt/C (20 wt %) as anode. Reduction of

gaseous CO2 was performed in these assembly and the products formed were

analyzed by varying the potential and time.

5. CONCLUSIONS

The electrochemical reduction of CO2 was performed in two different electrolytes

i.e. KCl and KHCO3 using Cu as working electrode and their results were compared.

It was found that Cu in KCl electrolyte favors methane at lower potentials and ethane

at higher cathodic potentials, whereas on KHCO3, the efficiency of methane increases

with potential. Metal oxide film was developed on the Cu surface by two different

methods (oxidized Cu I and oxidized Cu II) and investigated for CO2 reduction.

Though the oxidized Cu I electrode yields higher Faradic efficiency of ethane than

oxidized Cu II, the former offers higher resistance to reaction when carried out at

potential higher than -1.2 V. These emphasize the need of thin metal oxide interface

12

for tuning the selectivity of C2 towards electrochemical reduction of CO2 as the

powders of oxide does not yield the expected results.

Yet again, when the surface of the bare Cu electrode was modified by

electrodeposition, it was found that Cu favours methane formation and

electrodeposited Cu/Cu favours ethane formation in KCl electrolyte. The reason could

be due to the alignment of expressed plane which was identified by the calculation of

the texture coefficient. Pure Cu was oriented towards (111) and (200) planes, in

contrast to electrodeposited Cu which was oriented towards (220) plane. Hence, the

selectivity of the electrode can be tuned by altering the texture coefficient of the

electrode which was achieved by electrodepositng Cu on Cu.

The performance of Zn electrode in KCl and KHCO3 electrolyte was studied by

varying the time and potential, which showed lower yield towards C1 with higher

hydrogen evolution. Further, to improve the selectivity of Zn towards CO2 reduction,

surface modification was attempted. Chronoamperometric deposition of Zn on Cu

yields two deposits namely, Zn/Cu-L and Zn/Cu-H by varying bath concentration.

Electron micrographic image showed uniform hexagonal structures for Zn/Cu-L and

dendrite-leaf like morphology for Zn/ Cu-H. Zn/Cu-H was selective towards C1

formation and more efficient than Zn/Cu-L. Moreover, on comparing Zn/Cu-H with

bare electrode, it was found that Faradaic efficiency of methane was enhanced twice

on Zn/Cu (52%) compared to bare Cu (23%) electrodes. It was accomplished that the

feasible reaction on Zn surface was protonation rather than C-C coupling of to get

higher hydrocarbons. Few initial studies on continuous reduction of CO2 were also

carried out in this study which may open up new opportunities in the electrochemical

reduction of CO2.

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Ogura, K, J.R. Ferrell, A.V. Cuginic, E. S. Smotkin and M. D. Villalpando (2010), CO2 attraction by specifically adsorbed anions and subsequent accelerated electrochemical reduction, Electrochimica Acta, 56, 381-386.

Shaikh, A., Badrunnessa, J. Firdaws, M.D.S. Rahman, N.A. Pasha and P.K. Bakshi (2011), A Cyclic Voltammetric study of the influence of supporting electrolytes on the redox behavior of Cu (II) in aqueous medium, Journal of Bangladesh Chemical Society, 24, 158-164.

Skofic, I. K., J. Kovac and N. Bukovec (2008), The Ion-Storage Capacity and Surface Characterization of Ce/Cu Thin Films, Acta Chimica Slovenica, 55, 897–903.

Song, C. S. (2006), Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing, Catalysis Today, 115, 2-32.

Takahashi, I., O. Koga, N. Hoshi and Y. Hori (2012), Electrochemical reduction of CO2 at copper single crystal Cu(S)-[n(111) ×/(111)] and Cu(S)-[n(110)×/(100)] electrodes, J. Electroanal. Chem. 533 135-143.

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Wu, J., F. G. Risalvato, F. Ke, P. J. Pellechia and X. D. Zhou (2012), Electrochemical Reduction of Carbon Dioxide I. Effects of the Electrolyte on the Selectivity and Activity with Sn Electrode, Journal of Electrochemical Society, 159, F353-F359.

Weblink; http://Co2now.org

6. Proposed Contents of the Thesis

A general introduction on the recent energy and environmental scenario is

addressed. A brief overview of CO2 emission with detail description of CO2 molcule

and the ways and means available for its mitigation is discussed (Chapter 1). Recent

advances in electrochemical reduction of CO2 on metals; especially copper and its

modification are reviewed in the literature part of the thesis (Chapter 2). Materials

and methods used for preparation of the modified electrode, pre-treatment of bare

electrode, their physical (Electron micrographs, X-ray diffraction, X-ray

Photoelectron spectroscopy) and electrochemical characterization (Cyclic

voltammetry, Linear sweep voltammetry, Constant potential electrolysis techniques)

are discussed in Chapter 3.

In first part of the thesis, Cu was employed for CO2 reduction in KCl and KHCO3

supporting electrolyte, and its comparative performance was studied (Chapter 4).

Surface modification of the metallic Cu surfaces was attempted by developing a metal

oxide film on the Cu surface (by two methods) and also by electrodepositing Cu on

metallic Cu. The efficiency of the oxidized Cu and electrodeposited Cu was studied in

comparison to the bare Cu electrode correspondingly in Chapters 5 and 6.

The later part of the thesis focuses on the metallic Zn towards electrochemical

reduction of CO2 in KCl and KHCO3 electrolyte. Effect of time, potential and Faradaic

efficiency of Zn, Cu was compared and discussed in KCl and KHCO3 electrolytes and

reported in Chapter 7. Subsequently, Chapter 8 deals with electrodeposited Zn on Cu

electrode, as an approach to activate CO2, and henceforth to enhance the performance

of the bare Zn electrode. Detailed discussion on its Faradaic efficiency in comparison

to bare Cu and Zn electrodes is reported in Chapter 8.

The continuous mode of electrochemical reduction of CO2 was attempted in

Chapter 9. Here we discuss on preparation of membrane electrode assembly (MEA)

and hence the operation of Solid Polymer Electrode (SPE) for CO2 reduction with Cu

and its modified electrode as cathode. At the end, overall conclusion of all chapters

with future work was detailed in Chapter 10.

PUBLICATIONS BASED ON THE PRESENT WORK

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Refereed journal papers

1. Keerthiga, G., B. Viswanathan and Raghuram Chetty (2014) Electrochemical Reduction of CO2 on Electrodeposited Cu Electrodes Crystalline Phase Sensitivity on Selectivity, Catalysis Today (doi: 10.1016/j.cattod.2014.08.008)

2. Keerthiga, G., B. Viswanathan, C. Alex Pulikottil and Raghuram Chetty (2012), Electrochemical Reduction of Carbon Dioxide at Surface Oxidized Copper Electrodes, Bonfring International Journal of Industrial Engineering and Management Science, 2, 41-43.

Conference proceedings

1. Keerthiga, G., B. Viswanathan, C. Alex Pulikottil and R. Chetty, “Electrochemical Reduction of Carbon Dioxide at Copper Electrodes, ChEmference’11- Annual Research Symposium’, IISc Bangalore, India, Sep, 2011. (Awarded 2nd Prize for the Best Poster Presentation).

2. Keerthiga, G., B. Viswanathan, C. Alex Pulikottil and R. Chetty, “Comparison of Supporting Electrolytes for Electroreduction of CO2on Cu and Zn electrodes” a poster presented in 15th National Workshop on Role of Materials in Catalysis (NCCR), IITM, Dec, 2011.

3. Keerthiga, G., B. Viswanathan, C. Alex Pulikottil and R. Chetty, “Comparison of Electrochemical Reduction of Carbon Dioxide at Copper and Zinc Electrodes” poster presentation at international conference on electrochemical power sources, “Asian Conference on Electrochemical Power Sources (ACEPS-2012)”, Chennai, India, Jan, 2012.

4. Keerthiga, G., B. Viswanathan, and R. Chetty, “Electrochemical Reduction of Carbon dioxide  at modified Copper electrodes” oral presentation at the International conference on “Control of Industrial Gaseous Emission”(CIGE 2012) Annamalai University, Chidambaram, India, Feb, 2012.

5. Keerthiga, G., B. Viswanathan, C. Alex Pulikottil and R. Chetty,“Effect of Supporting Electrolyte on the Electrochemical Reduction of Carbon Dioxide at Copper Electrode” oral presentation at the “National Symposium on Electrochemical Science and Technology (NSEST-12)”, Bengaluru, India, Aug, 2012.(Awarded 2nd Prize for the Best Paper Presentation).

6. Keerthiga, G., B. Viswanathan, C. Alex Pulikottil and R. Chetty, “Comparison of Cu and Zn electrodes for the Electrochemical Reduction of CO2” oral presentation at 10th International Oil and Gas conference and Exhibition (Petrotech-2012) New Delhi, India, October 2012. (Best Paper Award).

7. Keerthiga, G., B. Viswanathan and R. Chetty, “Electrochemical Reduction of CO2on Cu and Electrodeposited Cu Electrodes: Crystalline Phase Sensitivity on Selectivity”, oral presentation at 6thAsian Pacific Congress on Catalysis, APCAT-6, organized by Catalysis Society of Taiwan and National Taiwan University, Taipei, Taiwan, October, 2013.

8. Keerthiga, G., B. Viswanathan and R. Chetty, “Electrochemical reduction of CO2 on modified Zn/Cu electrodes”, oral presentation at International Conference on Electrochemical Science and Technology (ICONEST-14), IISc, Bengaluru, India, Aug, 2014.

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