Effect of copper oxide electrocatalyst on CO2 reduction using Co3O4 as anode 2016 Journal of Science Advanced Materials and Devices

Journal of Science: Advanced Materials and Devices 1 (2016) 330e336

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Journal of Science: Advanced Materials and Devices
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Original Article

Effect of copper oxide electrocatalyst on CO2 reduction using Co3O4 as
anode
V.S.K. Yadav*, M.K. Purkait
Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 13 July 2016
Received in revised form
21 July 2016
Accepted 21 July 2016
Available online 28 July 2016

The reduction of carbon dioxide (CO2) to products electrochemically (RCPE) in 0.5 M NaHCO3 and Na2CO3
liquid phase electrolyte solutions was investigated. Cobalt oxide (Co3O4) as anode and cuprous oxide
(Cu2O) as the cathode were considered, respectively. The impacts of applied potential with time of reaction during reduction of CO2 to products were studied. The anode and cathode were prepared by
depositing electrocatalysts on the graphite plate. Ultra-fast liquid chromatography (UFLC) was used to
analyze the products obtained from the reduction of CO2. The feasible way of reduction by applying
voltages with current densities was clearly correlated. The results illustrate the capability of electrocatalyst successfully to remove atmospheric CO2 in the form of valuable chemicals. Maximum Faradaic

efficiency of ethanol was 98.1% at 2 V and for formic acid (36.6%) at 1.5 V was observed in NaHCO3. On the
other hand, in Na2CO3 electrolyte solution maximum efficiency for ethanol was 55.21% at 1.5 V and 25.1%
for formic acid at 2 V. In both electrolytes other end products like methanol, propanol, formaldehyde and
acetic acid were formed at various applied voltage and output current densities.
© 2016 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi.
This is an open access article under the CC BY license ( />
Keywords:
Electrocatalyst
CO2 reduction
Cu2O
Co3O4

1. Introduction
The gradual increase in atmospheric CO2 concentrations due to
large scale utilization of fossil fuels leads to global warming effect
[1]. A recent study reveals that the concentrations of CO2 in the
atmosphere have reached to 400 ppm from preindustrial period
and goes on increasing [2]. In order to reduce these CO2 concentrations, scientists were working from past few decades in both
fundamental and practical point of view [3,4]. Several methods are
in existence to reduce CO2 from air throughout the world. However,
the reduction of CO2 to products electrochemically (RCPE) appears
to be a potentially efficient method [5]. Some challenges remain, as
the RCPE goes with slow reaction kinetics due to the deactivation of
electrocatalyst used in the reaction and a possible cause for this
deactivation was elaborated in the literature [6].
Many researchers have studied the effect of a catalyst and
electrolyte on RCPE, but studies are still going on to improve the
properties of electrocatalyst. The catalyst activity was decreasing
due to surface reactions of RCPE which deactivates the catalyst.

* Corresponding author. Fax: þ91 361 2582291.
E-mail address: (V.S.K. Yadav).
Peer review under responsibility of Vietnam National University, Hanoi.

However, multiple products were observed in the reduction of CO2,
which mainly depends on several experimental conditions like
electrocatalyst, electrolyte and applied voltages. Copper is recognized as best appropriate catalyst in reducing CO2 to suitable hydrocarbons at significant current densities [4]. Dependency of
supporting salts used in RCPE on product efficiency was reported
well [7]. The main purpose of present research is to concentrate on
electrocatalyst in order to advance the reaction rate of RCPE towards high selectivity, catalyst stability and activity. Kaneco et al.
reported the effect of RCPE at the copper electrode in aqueous
NaHCO3 solution at 273 K, methane is formed with high Faradaic
efficiency of 46% at 2 V. It was clearly reported that, temperature
plays a key role in hydrogen formation [8]. Copper particles synthesized by the reduction of cuprous oxide films are able to reduce
CO2 to CO, HCOOH in 0.5 M NaHCO3 at low over potentials with
high Faradaic efficiency [9].
From the literature, it is envisaged that Pt is used mainly as an
anode electrocatalyst in RCPE [10e14]. Some researchers have used
Co3O4 as a replacement electrocatalyst for H2O oxidation in oxygen
based reactions [15e18]. Existing Pt as anode may be replaced with
Co3O4 to study the CO2 reduction. In this work, the main focus is on
reduction of CO2 to liquid products only. The role of electrocatalyst
Co3O4 for water oxidation, Cu2O for CO2 reduction in 0.5 M NaHCO3
and Na2CO3 solutions for different applied voltages has been

/>2468-2179/© 2016 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license
( />

V.S.K. Yadav, M.K. Purkait / Journal of Science: Advanced Materials and Devices 1 (2016) 330e336

331

studied. The electrocatalysts used for RCPE here was synthesized by
electrodeposition method and confirmation was done by characterizing the synthesized electrocatalyst [19]. The effect of Faradaic
efficiency towards RCPE is explained in detail. Co3O4, Cu2O coated
on a graphite plate were used as the anode and cathode in the
present work.

(10 Â 4 mm), 5 mM Tetrabutyl ammonium hydrogen sulfate was
used as mobile phase at 1 ml minÀ1 flow rate.

2. Experimental

3.1.1. Effect of time on mole of product formed in NaHCO3
electrolyte solution
Fig. 2 illustrates the effect of current density with applied
voltages towards RCPE by electrocatalysts used. Fig. 2a displays
that increase in current density reflects the voltage increase which
resembles rate of reaction. Various products were detected for
applying voltages at different time intervals, however, ethanol
was observed as main product for all applied voltages along with
reasonable quantities of propanol, formic acid and methanol.
Based on applied voltages (1.5, 2, 2.5, 3 and 3.5 V) respective
current densities of 0.84, 3.5, 13.6, 26.5 and 56.9 mA cmÀ2 were
obtained.
Results in Fig. 2 depicts that different quantities of products
formed with reaction time and applied voltages. Products formation is mainly based on proton availability at the cathode surface. However, proton liberates at anode Co3O4 due to oxidation
reaction which in turn depends on the activity of catalyst and
applied voltages [21]. Formed protons reach to cathode surface via
electrolyte medium react with CO2 molecules to form products.

Possible reactions at the anode and cathode are shown in Fig. 1. At
1.5 V, ethanol is observed as main product at 5, 15, 25 reaction
time. Formic acid (10, 15 min), methanol (5, 15 min) and formaldehyde (20 min) were observed as reaction products for applied
voltages. Acetic acid was identified at reaction times of 10 and
20 min and shown in Fig. 2b. Ren et al. studied the reduction of
CO2 electrochemically to ethanol and ethylene on Cu2O electrocatalyst using Pt as anode [23]. Ethanol is mainly obtained at 2 V
along with some quantities of formic acid and propanol at reaction times of 15 and 20 min. Quantities of product formed are
more compared with 1.5 V at this voltage show that rise in current
density increases the product formation (Fig. 2c). Fig. 2d depicted
that products like formic acid, propanol, ethanol, and methanol
were observed at 2.5 V with ethanol as main product. The formation of multiple products with different concentrations during
the RCPE at altered voltages and reaction time was reported [22].
Reduction of CO2 to different products like formic acid, methanol,
acetic acid, ethanol, formaldehyde and was reported using copper
based electrocatalysts [24e26]. Ethanol and propanol were witnessed at higher applied voltages of 3 and 3.5 V with higher
ethanol quantities in Fig. 2e, f. Different products were formed by
reduction of CO2, mainly ethanol is observed as product for all
applied voltages. The reaction mechanism for the formation of
multiple products, primarily for ethanol was due to the electron
acceptance by CO2 molecule to form its free radical. The formed
radical takes extra electron from cathode to form carbon monoxide/adsorbed CO and further adsorbed CO participates in reaction by accepting protons and electrons to get different products
was reported [19].

2.1. Materials
Graphite plates (1.5 Â 2.5) cm2, Sodium bicarbonate (NaHCO3),
potassium bicarbonate (KHCO3), Sodium carbonate (Na2CO3),
(Merck, India), Nafion (5 wt.% from DuPont, USA) and DC source
(Crown, India) was used for the experiment. All the experiments
were performed using deionized water.
2.2. Preparation of anode (Co3O4) and cathode (Cu2O) electrodes

The active areas of 2 mg cmÀ2 electrodes were prepared using
Co3O4 and Cu2O electrocatalysts by brush coating on graphite
plates. 200 ml binder solution was prepared by adding the 1:5 ratios
(nafion þ IPA (iso propyl alcohol)) solutions. Further, 7.5 mg
(electrocatalyst) was added to binder solution and further 30 min
sonication. The catalyst ink is coated on the graphite plate at the
80  C to get electrodes and these electrodes were dried at the
100  C for 2 h in oven to get the fully finished electrode [19,20].
2.3. Carbondioxide (CO2) electroreduction
A 2-electrode cell was used in study of CO2 reduction, in which
Co3O4/graphite as anode and Cu2O/graphite as cathode were used.
The schematic setup used for RCPE is shown in Fig. 1. 80 ml of 0.5 M
electrolyte solutions were prepared to which CO2 is bubbled up to
50 min to get CO2 saturated solution. The solution is taken in a glass
cell and RCPE is accompanied by connecting the DC source to two
electrodes in electrolyte solution. The reactions were done at
different applied voltages of with variable reaction times respectively [21,22].
2.4. Analysis of products from reduction of CO2
Different products formed from RCPE were detected using ultrafast liquid chromatography (UFLC), Shimadzu LC-20AD and UVdetector of deuterium lamp (SPD-20A) at 205 nm wavelength.
The solution of 20 ml is injected to the C-18 column of size

Fig. 1. Schematic diagram for the setup used in RCPE.

3. Results and discussion
3.1. Reduction of CO2 electrochemically

3.1.2. Influence of time on Faradaic efficiency of product formed in
NaHCO3 electrolyte solution
The influence of Faradaic efficiency on product formed with
time for RCPE was shown in Fig. 3. At 1.5 V, efficiencies of 14.8, 39.6

and 47.7% were obtained for ethanol at reaction times 5, 15, 25 min,
respectively. Formic acid (10, 15 min) 36.6 and 3.5%, acetic acid (10,
20 min) 42.5, 32.9%, methanol (5, 15 min) 5.3, 20.7%, formaldehyde
(20 min) 10.5%, respectively, were obtained (Fig. 3a). Faradaic efficiency of 47.7% at reaction time 25 min was observed as an


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V.S.K. Yadav, M.K. Purkait / Journal of Science: Advanced Materials and Devices 1 (2016) 330e336

Fig. 2. a) Voltage vs current density. Formation of various products with time at constant voltage in NaHCO3 electrolyte solution: b) 1.5 V, c) 2 V, d) 2.5 V, e) 3 V, f) 3.5 V.

optimized condition for ethanol formation. Results for RCPE towards ethanol and formic acid formation were reported on copper
electrocatalyst. In 0.5 M KHCO3 is ethanol (2.6%) at 1.55 V (10 min)
and formic acid (11.5%) was reported [27]. At 2 V, ethanol was
observed as main product. With reaction time 5, 10, 15, 20, 25 min,
Faradaic efficiencies of 4.2, 59.8, 31.6, 21.4 and 45.1% respectively,
were observed. Lower Faradaic efficiencies were observed for formic acid (15 min) 2.3%, propanol (20 min) 0.3% of this voltage.
Higher Faradaic efficiency for ethanol was obtained at reaction time
of 10 min is 59.7%, which were accepted to be an optimized condition towards reduction to CO2 to ethanol. The mechanism for
different products formation and change in product concentration
change with reaction time was reported [22]. using copper electrocatalysts of RCPE at 2.5 V illustrates the formation of ethanol
with reaction time (5, 10, 15, 25 min) with Faradaic efficiencies of
43.1, 7.3, 30.5 and 0.59% were observed respectively Fig. 3c. However, lower Faradaic efficiencies of formic acid were observed at all
reaction times by 0.8, 0.06, 0.34, 0.6 and 0.05%. Other products like
methanol (25 min), propanol (25 min), acetic acid (20 min) were
formed with Faradaic efficiencies in 0.3, 1.3, and 3.9%. This voltage

gives the most feasible results towards ethanol formation with
reasonably good efficiencies. The optimized condition for the

ethanol formation was 5 min reaction time with Faradaic efficiency
of 43.1%. Chi et al. studied the reduction of CO2 to ethanol and
propanol at cuprous oxide at Pt electrocatalyst [28]. RCPE at 3 V,
ethanol and propanol, was observed as products at all reaction
times with Faradaic efficiencies of ethanol (38.01, 7.1, 36.8, 29.6 and
13.1%) and propanol (1.8, 10.9, 3.5, 0.14, 0.24%), respectively. Ethanol
formation with 38.01% Faradaic efficiency for reaction time of
5 min, which accepted to be a most optimized condition towards
ethanol formation reaction. RCPE at 3.5 V showed very low faradic
efficiencies (Fig. 3e). Ethanol and propanol were the main products
observed in applying voltage. Faradaic efficiencies were observed to
be 3.45, 4.2, 13.2, 13.5 and 7.1% for ethanol, 0.2, 0.36, 0.34, 0.35, and
0.17% for propanol. However, at these voltage maximum current
densities towards RCPE was observed for low Faradaic efficiencies
of product formed which is due to high H2 evolution [29].
Results of applied experimental conditions show the fact that
electrocatalysts and electrolyte plays a major role in CO2 reduction.
Ethanol is observed as main product for all applied voltages along


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333

Fig. 3. Effect of time on the Faradaic efficiency of products formed at various applied voltages for RCPE in NaHCO3 electrolyte solution. a) 1.5 V, b) 2 V, c) 2.5 V, d) 3 V, e) 3.5 V.

with propanol, formic acid, acetic acid and methanol. The effect of
Co3O4 for water oxidation towards RCPE has been studied. The
results clearly show the impact of electrocatalyst towards RCPE
which can be used as a replacement for high cost platinum electrocatalyst with cobalt oxide. Quantities of product formed are not

same with respective time interval may be due to oxidation or
reduction of formed products in Fig. 2.
3.1.3. Influence of time on mole of product formed in Na2CO3
electrolyte solution
RCPE depends on the observed current density for the applied
different voltages. The effects of current density at altered voltages
in Na2CO3 are presented in Fig. 4a. It is observed from the figure
that the current density increases with applied voltage. This
depicted the high reaction rate. The current densities 0.456, 3.31,
18.9, 42.2 and 69.3 mA cmÀ2 were obtained for the applied voltages
1.5, 2, 2.5, 3 and 3.5 V. Different products were observed at various
applied voltages and reaction time.
The effect of time on the amount of product formed at a constant
1.5 V is shown in Fig. 4b. Mainly ethanol is observed as main
product for reaction time 15, 20 and 25 min along with the formic
acid at time 10, 15 and 20 min. However, acetic acid was witnessed

in the reaction of 5, 10 min along with minute amount of formic
acid at the reaction time of 10 min. From the figure it may be
concluded that applied voltage is more favorable for the reduction
of CO2 to ethanol and formic acid. Kuhl et al. reported the sixteen
products from CO2 reduction on copper and Pt based electrocatalysts [30]. The RCPE at 2 V (Fig. 2c) shows that this voltage is
favorable for formic acid for the reaction time of 10e25 min. With
the increase of reaction time, the product formation increases along
with some quantity of methanol is observed at reaction time of
20 min, formaldehyde at 15 min. However, acetic acid and minor
quantities of formic acid is observed at 5, 15 min reaction. A review
for the reduction of CO2 to different products on copper electrocatalyst was reported [4]. From Fig. 4d it may be observed that
ethanol is only product formed after 5 min and higher quantities of
methanol is observed at 10 min reaction. Formic acid is observed at

reaction time of 15 and 20 min along with formaldehyde in 20 min
reaction. Minor quantities of formaldehyde and ethanol are
observed at the reaction of 25 min. The variation in product concentration with time is by the oxidation of formed product was
reported [22]. The products formed at 3 V are shown in Fig. 4e.
Small quantities of formic acid are observed at all the reaction times
except for 25 min. Higher quantities of acetic acid are observed in


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V.S.K. Yadav, M.K. Purkait / Journal of Science: Advanced Materials and Devices 1 (2016) 330e336

Fig. 4. a) Voltage vs current density. Formation of products with time in Na2CO3 electrolyte b) 1.5 V; c) 2 V; d) 2.5 V; e) 3 V and f) 3.5 V.

the reaction of 25 min but quantity is decreased at reaction time of
10 min. Ethanol is observed at the reaction of 15 min. However, this
voltage is feasible for the formation of formic acid at all reaction
times. The reduction of CO2 to products was observed at 3.5 V and
shown in Fig. 4f. Acetic acid, formaldehyde and formic acid were
obtained as main products for the reaction time of 5 min. Interestingly, major quantities of ethanol are observed for the reaction
upto10 min with some amount of formic acid Same quantities of
formic acid and formaldehyde were obtained in the reaction of
15 min. Acetic acid is identified as product for the reaction of 20,
25 min along with some amount of formic acid for the reaction of
20 min.
3.1.4. Effect of time on Faradaic efficiency of product formed in
Na2CO3 electrolyte solution
The effect of reduction of CO2 to different products, upon the
applied voltage and the Faradaic efficiency of products formed with
time of the reaction in Na2CO3 solution is shown in Fig. 5. The

products observed at 1.5 V (Fig. 5a) are formic acid, ethanol and
acetic acid. Ethanol is formed at reaction time of 15, 20 and 25 min
with Faradaic efficiencies of 55.21, 32.1, 46.2%, acetic acid (5, 10 min)
15.95, 53.51%, formic acid (10, 15, 20 min) with efficiencies of 17.76,
14.13, 8.92% were observed. At this voltage, high Faradaic efficiencies of 55.21 and 53.51% of the reaction time of 15 and 10 min
which are the optimum conditions for reduction of CO2 with high
efficiency. Hori et al. studied the CO2 reduction to different products

in aqueous phase on copper e Pt electrocatalyst [31]. The Faradaic
efficiencies of the reduced CO2 products with time at 2 V are shown
in Fig. 5b. Mainly, formic acid is formed with Faradaic efficiencies of
8.44, 6.27, 5.03 and 21.14% at 10, 15, 20, 25 min time of reaction, and
the efficiencies of acetic acid (5 min), methanol (20 min) and
formaldehyde (15 min) is 25.1, 6.59 and 1.436%. At this applied
voltage the maximum Faradaic efficiencies are observed by 21.14%
for ethanol at reaction time of 20 min and 25.1% for formaldehyde
at the reaction of 15 min which is the optimized reaction for the
reduction of CO2. The mechanism for these multiple product formation at different applied conditions was reported [22]. At 2.5 V
(Fig. 5c), different products like formaldehyde (20, 25 min) 0.59,
0.19%, methanol (10 min) 31.76%, formic acid (15, 20 min) 0.79,
4.18%, ethanol (5, 25 min) 42.349, 0.736% and acetic acid (15 min)
0.183% Faradaic efficiencies were observed. Maximum Faradaic efficiency of 42.34% for reaction of 5 min for ethanol and 31.76% efficiency for methanol at reaction time of 10 min were found to be
the best reaction condition. Yano et al. reported the CO2 reduction
to different products on copperePt electrocatalyst in acid solution
and shown that the reaction at 2.4 V for ethanol to be 0.1% in 0.5 M
KHCO3 solution [11]. At 3 V, the RCPE was shown in Fig. 5d. Formic
acid is formed at the reaction time of 5, 10, 15 and 20 min with
efficiencies of 1.07, 0.19, 0.11 and 0.15%. For acetic acid at reaction
time of 10, 25 min with 0.46, 4.66%, ethanol (15 min) 5.43%,
methanol (5 min) 2.88% was observed. Low Faradaic efficiencies

were obtained though the high current density is attained due to


V.S.K. Yadav, M.K. Purkait / Journal of Science: Advanced Materials and Devices 1 (2016) 330e336

335

Fig. 5. Effect of time on the Faradaic efficiency of products formed with applied voltages for RCPE in Na2CO3 electrolyte solution. a) 1.5 V, b) 2 V, c) 2.5 V, d) 3 V, e) 3.5 V.

the hydrogen formation favors than CO2 reduction. The effect of
Faradaic efficiency with time of the reaction at 3.5 V is given in
Fig. 5e. Different products like formic acid with reaction of 5, 10, 15,
20 min with Faradaic efficiencies of 1.07, 0.19, 0.11 and 0.15%.
Ethanol (20 min) 5.43%, methanol (5 min) 2.89% and acetic acid (10,
25 min) with Faradaic efficiency of 0.46, 4.66% were observed.
However, the Faradaic efficiencies are very low with lower applied
voltages. The results show that time of reaction also depends on the
Faradaic efficiency.
The result shown in Fig. 4 reveals the fact that formation of
products depends on the applied voltage and time of reaction and
respective Faradaic efficiencies with time in Fig. 5. All the products
formed depends on the time of reaction, ethanol and formic acid
are observed at low voltages of 1.5 and 2 V. Though the reaction is
happening at same voltage, the product formation is changing with
time of reaction that may be due to the oxidation or reduction of
forming products after certain time of reaction due to the decrease
in concentration of CO2.
4. Conclusion
The effect of RCPE was studied in 0.5 M NaHCO3 and Na2CO3
electrolyte solutions on Cu2O electrocatalyst is explained with

respect to the applied voltage and reaction time. This study was
able to find the optimum time and voltage for the products formic
acid and ethanol. Maximum Faradaic efficiency for ethanol was
observed at 2 V with 98.1% after 5 min reaction which is the most
optimum condition for ethanol formation in NaHCO3 solution.
Formic acid was formed at 2 V and ethanol at 1.5 V along with other
products like methanol, formaldehyde, acetic acid based on the
applied voltage with different time of reactions in Na2CO3 solution.
The present study can be used in future work by replacing electrical
energy by solar energy in order to make the process economically
viable.
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