Electrochemical reduction of CO2 to HCOOH on a synthesized Sn electrocatalyst using a Co3O4 anode

Article abstract of DOI:10.1039/c5ra12369k

The present work investigates the electrocatalytic effect of tin (Sn) and cobalt oxide (Co₃O₄) on the reduction of CO₂ to products electrochemically (RCPE). Electrocatalytic (Sn) powder was synthesized by an electrodeposition method from a SnCl₂·2H₂O solution. Experiments were conducted using Co₃O₄ and Sn electrodes as the anode and cathode, respectively, in 0.5 M potassium and sodium carbonate and bicarbonate solutions. The electrodes were prepared by coating the electrocatalysts onto a graphite plate surface. The experiments were conducted with different applied voltages (1.5 to 3.5 V) and time intervals (5, 10, 15, 20 and 25 min) in the various electrolyte solutions. It was observed that HCOOH acid was the only product formed for all the applied conditions. At 1.5 V and 2 V, maximum Faradaic efficiencies of 74.04% and 92.6% for HCOOH production were obtained from 20 and 5 min reactions in a KHCO₃ electrolyte solution. The ability of Co₃O₄ to perform water oxidation and the Sn electrocatalyst to reduce CO₂ to HCOOH was established from the obtained results. The optimized reaction conditions to achieve high Faradaic efficiencies are explained in detail.

Full text of DOI:10.1039/c5ra12369k

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Electrochemical reduction of CO2 to HCOOH on a synthesized Sn electrocatalyst using
Co3O4 anode
V. S. K. Yadav and M. K. Purkait*
Department of Chemical Engineering
Indian Institute of Technology Guwahati
Guwahati - 781039, Assam, India
* Corresponding author
Tel: + 91 - 361 - 2582262
Fax: +91 - 361 - 2582291
E. Mail: mihir@iitg.ernet.in
1

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Abstract
The present work investigates the electrocatalytic effect of tin (Sn) and cobalt oxide (Co3O4)
towards reduction of CO2 to products electrochemically (RCPE). Electrocatalyst (Sn) powder
was synthesized by electrodeposition method from SnCl2. 2H2O solution. Experiments were
conducted using Co3O4 and Sn electrodes as anode and cathode, respectively, in 0.5M carbonates
and bicarbonates of potassium and sodium solutions. Electrodes were prepared by coating
electrocatalysts on graphite plate surface. The experiments were conducted at different applied
voltages (1.5 to 3.5 V) and time intervals (5, 10, 15, 20 and 25 min) at various electrolyte
solutions. It was observed that HCOOH acid was the only product formed for all the applied
conditions. At 1.5 V and 2 V, maximum Faradaic efficiencies of 74.04 % and 92.6 % for
HCOOH were obtained at 20 and 5 min reaction in KHCO3 electrolyte solution. The ability of
Co3O4 for water oxidation and Sn electrocatalyst towards reduction of CO2 to HCOOH was
established from the obtained results. Optimized reaction conditions to get high Faradaic
efficiencies were explained in detail.
Keywords: Faradaic efficiency; Reduction of CO2 to products electrochemically (RCPE); Sn;
Electrodeposition; Co3O4.
1. Introduction
Fossil fuels are used as world’s primary energy source. Since energy produced from
fossil fuel is cheap, most of the energy generating in present’s technology is by using fossil fuels.
The process of energy generation deals with combustion of fossil fuels like coal, natural gas and
petroleum. Mainly CO2 and some other gases like nitrous oxide, methane and other halocarbons
2

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were evolved into the atmosphere as greenhouse gases during combustion. Utilization of fossil
fuel energy is drastically increasing due to rapid rise in population. The emissions of greenhouse
gases mainly CO2 is proportional to the energy generation and is the main cause for global
warming.1-3 Most of the environmentally challenging issues arise due to CO2 in all the countries.
Hence, it is highly recommended to control this greenhouse gases before its release into the
environment. Many methods were in existence for the reduction of CO2 to various products, but
reduction of CO2 to products electrochemically (RCPE) is promising method because of its
several advantages. Firstly, RCPE in the aqueous systems has yielded high selectivity at low cost
by using heterogeneous catalyst.4-7 Secondly, the entire reaction process can be done at ambient
temperature and pressures.8 Thirdly, it can be done using solar energy as a source.9-11 Fourthly,
energy generated for RCPE can be stored in the form of fuels.12-13 Fifthly, generation of the
proton source of wastewater decreases the overall chemical consumption. Finally, RCPE systems
have compact design and easy to scale up for industrial application.
It is envisaged from the above literatures that the RCPE is going to be a promising
technology for the reduction of CO2 to produce several products. Reduction of CO2 mainly
depends on electrocatalyst used in the reaction and applied potentials. However, work is going
on to improve the reduction rates by improving stability and efficiency of electrocatalyst. The
main attention is focused to study the extent of catalyst deactivation in order to find developed
electrocatalyst of high stability.14 Electrolytes15, pH16 and catalyst structure17-19 plays a major
role for product selectivity in RCPE. Copper is well known electrocatalyst in RCPE as various
products like; methane, ethylene and other hydrocarbons are formed with significant efficiencies.
However, due to formation of multiple products, process becomes more complex.20 In order to
reduce this complexity; CO2 has to be reduced to single product with high Faradaic efficiency. If
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liquid product is formed in RCPE with high Faradaic efficiencies, this could be sustainable
approach for future liquid fuel production. Some studies were done for RCPE at Pb and Sn
electrocatalysts to form HCOOH.21-25 In most of the RCPE processes platinum (Pt) was used as
anode for oxidation of water.26-27 However, few authors investigated the effect of water oxidation
using Co3O4 electrocatalyst for hydrogen evolution reactions.28–31
From the extensive literature survey, it was observed that Pt was used as anode
electrocatalyst for the oxidation reaction in most of the cases. Use of Co3O4 as electrocatalyst in
place of Pt for RCPE is scant and might be a better alternative. Present work explores the use of
Co3O4 as anode and Tin (Sn) as cathode material in RCPE for the reduction of CO2 to HCOOH
as a single liquid product. The role of electrocatalyst for an anode (Co3O4) and cathode (Sn)
towards RCPE is explained with respect to rate of reaction for all applied voltages using a 2-
electrode glass cell. To the best of our knowledge, no studies have been reported using Co3O4 as
anode for RCPE on Sn electrocatalyst. Therefore, data generated in this investigation might be of
useful in the field of RCPE during selection of low cost and appropriate electrocatalyst.
2. Experimental
2.1. Materials, characterization and product analysis
Materials: Graphite plates (1.5 × 2.5) cm2 were purchased from Sunrise enterprises,
Mumbai. Sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3), sodium carbonate
(Na2CO3), potassium carbonate (K2CO3), tin chloride dihydrate (SnCl2.2H2O), acetone
(CH3COCH3) and isopropyl alcohol ((CH3)2CHOH) were procured from Merck, India. Nafion (5
wt %) solution was obtained from DuPont, USA and direct current (DC) source was purchased
4

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from Crown, India. All the chemicals were used without any further purification and deionized
water was used in all experiments.
Characterization: Sn electrocatalyst was characterized using Fourier Transform Infrared
Spectrophotometer (FTIR) (make: Shimadzu; model: IR Affinity-1). Peaks were recorded in the
range of 500 - 4000 cm-1 by crushing the sample with KBr (IR grade). X-ray diffraction (XRD)
analysis was done using X-ray diffractometer (make: Bruker; model: D8 advance) between 10o to
80o 2θ. Particle size analysis of synthesized electrocatalyst was done using Delsa nano (make:
Beckman coulter; model: Delsa nano C) particle size analyzer.
Product analysis: Product from CO2 reduction was analyzed by ultra-fast liquid chromatography
(UFLC, Shimadzu LC-20AD, UV-detector of deuterium lamp SPD-20A). The product was
detected at 205 nm wavelengths by injecting 20 µl of reacting sample to the C-18 Column (10 X
4 mm). Tetrabutyl ammonium hydrogen sulfate (5 mM) was used as mobile phase at flow rate of
1 ml/min.
2.2. Synthesis of Sn powder electrochemically
Tin (Sn) powder was synthesized by electrodeposition method.32 Schematic for the
synthesis of Sn powder is shown in Figure 1. Metal powder was extracted from the solution of
0.1M SnCl2.2H2O by supplying energy between the metallic copper plate and graphite plate in an
electrolytic cell. A constant current of 0.2 A for 3 min was applied by which the Sn deposition
takes place on graphite plate. The deposited Sn powder was removed with using acetone.
Further, catalyst solution was heated at the 100 oC for 1h to obtain Sn powder.
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Figure 1. Schematic representation for the synthesis of Sn powder and its mechanism
2.3. Electrodes preparation
Electrodes were prepared by taking solution of 1:5 ratio of 200 µl of nafion + isopropyl
alcohol (IPA) which acts as a binder. 7.5 mg of electrocatalyst (Co3O4 and Sn) was added to the
binder solution separately, and sonicated for 30 min. The solution was coated on the graphite
o
plate surface at 80 C to get the electrodes at active area 2 mg/cm2. Thereafter, electrodes were
dried for 2 h in oven at 100 oC.
2.4. Electrochemical studies towards CO2 reduction
Prepared electrodes were used in RCPE by using a 2-electrode homemade glass cell in
order to study the effect of CO2 reduction. Figure 2 shows the schematic of experimental setup
used for RCPE using a two electrode cell. The glass cell was filled with prepared 0.5 M
electrolyte solution of 80 ml which was saturated with CO2 by continuous bubbling for 50 min.
RCPE experiments were conducted in CO2 saturated solution by connecting the electrodes to the
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DC source. Experiments were conducted at potentials of 1.5, 2, 2.5, 3 and 3.5 V with reaction
time of 0-5, 10, 15, 20 and 25 min for every applied voltage.
Figure 2. Experimental setup for the reduction of CO2 to HCOOH on Sn electrocatalyst
3. Results and discussion
3.1. Characterization of Sn electrocatalyst and its mechanism
FTIR spectrum of synthesized Sn electrocatalyst was represented in figure 3a. The broad
band around 3000-3600 cm-1, 1642 cm-1 corresponds to O-H stretching vibrations and O-H
bending vibrations respectively, attributed to adsorbed water. Band at 1000-1250 cm-1 confirms
the presence of Sn.32 XRD pattern of synthesized Sn electrocatalyst was shown in figure 3b. Peak
positions at 30.7o, 32.08o, 43.97o, 45.0o, 55.46o 62.67o, 63.93o 64.73o, 72.59o, 73.34o and 79.7o
,
,
are matched closely to Sn structure.33 The particle size analysis was performed using Delsa nano
(make: Beckman coulter; model: Delsa nano C) particle size analyzer and the particle size
distribution data for synthesized catalyst is shown in Figure 3c. The particle size of Sn powder
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was found in the range of 156.7 – 278.9 nm. The distribution median size (Dv50) of Sn
electrocatalyst particle was found to be 192.5 nm.
Figure 3. Characterization of Sn electrocatalyst (a) FTIR, (b) XRD, and (c) particle size analysis
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A mechanism for the formation of Sn electrocatalyst is shown in figure 1. Oxidation of Copper
Cu 2+ + 2e – (0.34 V)) and reduction of Tin (Sn2+ + 2e -
( Cu(s)
Sn (0.14 V)) is
taking place at anode and cathode, respectively. Sn ion from the electrolyte solution was
deposited on the cathode surface by accepting electrons generated at anode due to oxidation of
Copper. Copper ion formed at anode had a driving force for the deposition of Sn ion on the
surface of graphite plate. The deposition rate was proportional to the formation of a new copper
chloride molecule in solution. Further, upon heating the deposited Sn powder on graphite gives
Sn electrocatalyst.
3.2. RCPE studies at Sn electrocatalyst in different electrolyte solutions
3.2.1. Variation of current density with applied voltage during CO2 reduction on Sn
electrocatalyst
Experimental results for the variation of current density with applied voltage were shown
in figure 4 for all the electrolytes. It was observed that the current density increased with increase
in applied voltages for all the cases. Reduction of CO2 signifies the increase of current density.
Increase in current density dictates the high reaction rate for CO2 reduction or hydrogen
generation.
Figure 4. Variation of current density with applied voltage during CO2 reduction using Sn
electrocatalyst.
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Again, figure 4 show that potassium carbonate electrolyte gives high current density compared to
bicarbonates.This confirms that the rate of reaction is more for carbonates of potassium and
sodium. It is also found that the potassium electrolytes show high current densities than sodium
electrolytes. In summary, the increase in current density signifies the rate of reaction towards
CO2 reduction occurs at the surface of cathode in all conditions applied.
3.2.2. Reduction of CO2 to HCOOH in KHCO3 solution (time vs Faradaic efficiency)
In order to find the applied energy utilized towards CO2 reduction, Faradaic efficiency
was calculated with time for applied voltages in 0.5 M KHCO3 electrolyte solution and shown in
figure 5a.
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Figure 5. Variation of Faradaic efficiency with time during reduction of CO2 to HCOOH using
a) KHCO3, b) K2CO3, c) NaHCO3, and d) Na2CO3 electrolyte solutions.
It was observed that the only product formed was HCOOH for the applied conditions. This
signifies that synthesized electrocatalyst was able to reduce CO2 to single products. Maximum
Faradaic efficiencies were obtained at low applied voltages of 1.5 V and 2 V compared to others
considered here. Faradaic efficiencies for the reduction of CO2 to HCOOH at 1.5 V with reaction
time of 5, 10, 15, 20 and 25 min were observed to be 8.28, 13.54, 52.2, 74.04 and 40.61%,
respectively. Overall, high efficiencies (74%) were observed for the applied voltage after 20 min
reaction. Similar results were shown when Pt was used as the anode (Table 1) after a reaction
time of 30 min.22 High Faradaic efficiencies (92.6, 67.36, 10.17, 0.91 and 14.51% respectively)
were obtained (figure 5a) at 2 V. RCPE at 2 V shows significant results for HCOOH with high
Faradaic efficiency of 92.6% for 5 min reaction. However, at this applied voltage maximum
energy was utilized in reducing CO2 to HCOOH. At 2 V a Faradaic efficiency of 47% was
reported22 and maximum Faradaic efficiency of 92.3% was observed at Sn/Pt in 0.1M KHCO3
solution.34 Faradaic efficiencies at 2.5 V decrease with time to be 16, 5.33, 3.45, 7.77 and 1.09
%, of which maximum efficiency of 16 % was observed at reaction time of 5 min. Low Faradaic
efficiencies of 0.10, 0.83, 2.87, 1.22 and 0.23 % at 3 V and 0.43, 0.22, 0.21, 0.73 and 0.69 at 3.5
V, respectively, were found (figure 5a). Considering the above results it may be envisaged that
low efficiencies were obtained at 3 V and 3.5 V, which may be due to other side reactions,
especially hydrogen evolution due to high proton generation at Co3O4 anode. The synthesized
electrocatalyst was able to reduce CO2 to HCOOH at all applied conditions. However, high
efficiencies were obtained at low potentials compared with high potentials. The optimized
conditions for RCPE were observed to be 1.5 V (20 min) and 2 V (5 min) towards HCOOH
formation.
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3.2.3. Reduction of CO2 to HCOOH in K2CO3 solution
Reduction of CO2 to HCOOH using Sn and Pt as cathode and anode material in 0.1 M
K2CO3 solution was investigated and reasonable Faradaic efficiencies were obtained for different
applied voltages.22 Faradaic efficiencies towards HCOOH formation from RCPE using Co3O4 as
anode at different applied voltages in presence of 0.5M K2CO3 electrolyte solution was
investigated in this work and shown in figure 5b. Significant results were obtained towards
RCPE at low applied voltages, particularly at 2 V than 1.5 V. However, comparing with KHCO3
solution (Table 1), the efficiencies obtained were low that may be due to high hydrogen
formation.23 For RCPE at 1.5 V, Faradaic efficiencies for HCOOH with reaction time of 5, 10,
15, 20 and 25 min were 64, 43, 8, 47.3 and 0.60 %, respectively. Faradaic efficiency of 39 % was
reported at 1.5 V.22 Overall, applied voltage is able to reduce CO2 to HCOOH with good
efficiencies and the optimized reaction condition at this potential is 64 % for 5 min reaction.
RCPE at 2 V observed that efficiency increases with time and the obtained Faradaic efficiencies
were 3, 10, 3, 4 and 36 %. Faradaic efficiency of 36 % for 25 min reaction was observed as
optimized reaction at this particular applied voltage. The RCPE at 2.5 V shows low Faradaic
efficiencies of 7.1, 0.62, 0.90, 0.15 and 0.12 %. Low efficiencies were observed with high
current density, which may be due to other side reactions (figure 5b). At 3 V and 3.5 V, very low
Faradaic efficiencies were obtained (for 5, 10, 15, 20 and 25 min reaction) as 0.73, 0.75, 1.36,
1.08 and 0.1 % and 1.14, 0.31, 0.02, 0.02 and 0.03 %, respectively. The RCPE results were also
confirmed the formation of HCOOH in all the experimental conditions considered here.
However, low efficiencies were obtained for carbonate compared to bicarbonate solutions. The
high current densities so obtained may be due to competition in hydrogen evolution reaction. The
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synthesized catalyst Sn and effect of Co3O4 as anode in reducing the CO2 to HCOOH is proven
to be an efficient method.
3.2.4. Variation of Faradaic efficiency with time during reduction of CO2 to HCOOH
The experimental results on reduction of CO2 to HCOOH in 0.5M NaHCO3 electrolyte
solution is shown in figure 5c. High Faradaic efficiencies were observed at 1.5 V and low for the
remaining applied voltages (figure 5c). However, the only product formed at all the applied
conditions is HCOOH. At 1.5 V, the Faradaic efficiencies of 14.3, 27.2, 67.8, 32.2 and 68.7 %,
respectively, were obtained after 5, 10, 15, 20 and 25 min. Decrease in efficiency at 40 min
reaction may be due to the oxidation of HCOOH at Co3O4 anode.25 However, maximum Faradaic
efficiency of 68.7 % (25 min) and 67.8 % (15 min) reaction was observed as an optimum
condition. 19 % Faradaic efficiency was reported in 0.5M NaHCO3 solution for Sn/SnOx catalyst
at 0.7 V.23 Faradaic efficiencies of 6.8, 19.8, 13.9, 13 and 11.8 % were obtained for RCPE at 2
V. This potentially reduces CO2 with low faradaic efficiency compared with applied voltage of
1.5 V in these experimental conditions. For reaction at 2.5 V low efficiencies were observed to
be 7.02, 4.3, 12.26, 2.60 and 2.97% (figure 5c). Lower efficiencies were observed as hydrogen
generation favors the reaction (figure 5). The reduction of CO2 to HCOOH at 3 V and 3.5 V were
observed for which Faradaic efficiencies were 10.6, 5.4, 1.2, 1.1 and 1.05 % and 0.03, 0.67, 1.07,
0.26 and 0.06 %, respectively after the said time interval. It may be concluded from this section
that RCPE favors at all applied voltages and maximum efficiency is observed at 1.5 V which is
the most optimum potential towards HCOOH formation.
3.2.5. Reduction of CO2 to HCOOH in Na2CO3 solution
RCPE study using Sn as electrocatalyst in 0.5M Na2CO3 solution towards HCOOH
formation is shown in figure 5d. Formation of HCOOH at all applied conditions shows that low
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applied voltages are more favorable for high Faradaic efficiencies. Faradaic efficiencies of 32.8,
24.3, 22.2, 9.8 and 32.3 % were obtained at 1.5 V. Sudden decrease in efficiency after 15 min
reaction was due to HCOOH oxidation at anode.25 However, this voltage was more favorable for
the reduction of CO2 to HCOOH with efficiency of 32.3 % after 25 min. For the reaction at 2 V,
Faradaic efficiencies of 9.5, 21.4, 9.2, 2.9 and 3.4 % were observed (figure 5d). It may be noted
that efficiencies were low when compared with RCPE in NaHCO3 electrolyte solution. RCPE in
Na based electrolyte solution was able to reduce effectively in Sn/SnOx electrode.23 Faradaic
efficiencies of 8, 6, 6.9, 0.91 and 7.8 % were observed to be less at 2.5 V compared with above
applied voltages. Low efficiencies were obtained at 3 V (0.20, 1.55, 2.98, 1.14 and 0.1%) and 3.5
V (0.49, 1.40, 0.49, 0.76 and 0.10%). This may be due to high proton generation at Co3O4 which
leads to more hydrogen formation at the cathode surface. RCPE using this electrolyte was able to
form HCOOH from CO2 at all applied voltages. However, optimum conditions for maximum
efficiencies were low applied voltages towards RCPE. Finally, application of Co3O4 as anode for
the reduction of CO2 to HCOOH was proven. Comparison of HCOOH Faradaic efficiency at
different experimental conditions with literature was shown in Table 1. From the table it may be
concluded that the use of Co3O4 as an alternative to Pt for anode during RCPE is economically
favorable. Experimental conditions to get high HCOOH Faradaic efficiency with time in RCPE
is also shown with respect to the applied voltages. It is concluded that bicarbonates were able to
reduce more CO2 than carbonates electrolyte solutions.
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Table 1: Comparison of HCOOH Faradaic efficiency at different experimental conditions with
literature.
Applied Reaction
Faradaic efficiency
Electrode
voltage
(V)
1.5
2
time
(min)
30
(HCOOH)
Electrolyte
References
Anode Cathode
(%)
74
47
20
20
-
Pt
Sn
Sn
0.5 M
KHCO3
30
1.5
2
120
120
30
[22]
Pt
0.1 M
K2CO3
1.5
2
30
14.2
-
1.5
2
120
120
13
Pt
Pt
Sn
Sn
Sn
0.1 M
KHCO3
1.39
-
92.3
[34]
[25]
0.1 M
KHCO3
1.8
1.5
60
20
91
Co3O4
0.5 M
74.08
KHCO3
2
1.5
2
5
92.6
0.5 M
K2CO3
0.5 M
NaHCO3
0.5 M
5
64.03
36.18
68.72
19.83
32.82
21.44
25
25
10
10
10
Present
work
1.5
2
1.5
2
Na2CO3
4. Conclusion
A study on RCPE was done by synthesizing Sn electrocatalysts via electrodeposition
method. Result showed that Co3O4 was able to oxidize H2O efficiently for CO2 reduction.
However, high Faradaic efficiencies were obtained for bicarbonates than carbonate based
electrolyte solutions at low applied voltages. The synthesized electrocatalyst was able to reduce
CO2 to HCOOH effectively at all applied voltages. A maximum Faradaic efficiency of 92.6%
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was observed at 2 V for reaction time of 5 min and 74.06 % at 1.5 V for 20 min in KHCO3
solution. For NaHCO3, at 1.5 V high Faradaic efficiencies of 68.72% were observed at reaction
time of 25 min which is the most optimum condition for RCPE. This preliminary study will be
helpful towards findings of cheap and effective electrocatalyst for RCPE (alternate to expensive
Pt).
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DOI: 10.1039/C5RA12369K
Graphical abstract
Reduction of CO2 to HCOOH on Sn electrocatalyst
Research highlights
• Co3O4 is used as an alternative to Pt for the reduction of CO2 to HCOOH using Sn
electrocatalyst.