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
Trang 1Original Article
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
Article history:
Received 13 July 2016
Received in revised form
21 July 2016
Accepted 21 July 2016
Available online 28 July 2016
Keywords:
Electrocatalyst
CO 2 reduction
Cu 2 O
Co 3 O 4
a b s t r a c t
The reduction of carbon dioxide (CO2) to products electrochemically (RCPE) in 0.5 M NaHCO3and 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 re-action during reduction of CO2to 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 electro-catalyst successfully to remove atmospheric CO2in 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 Na2CO3electrolyte 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 (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
The gradual increase in atmospheric CO2concentrations due to
large scale utilization of fossil fuels leads to global warming effect
[1] A recent study reveals that the concentrations of CO2in the
atmosphere have reached to 400 ppm from preindustrial period
and goes on increasing[2] In order to reduce these CO2
concen-trations, scientists were working from past few decades in both
fundamental and practical point of view[3,4] Several methods are
in existence to reduce CO2from air throughout the world However,
the reduction of CO2to 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
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 recog-nized as best appropriate catalyst in reducing CO2to suitable hy-drocarbons 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 to-wards high selectivity, catalyst stability and activity Kaneco et al reported the effect of RCPE at the copper electrode in aqueous NaHCO3solution 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 syn-thesized by the reduction of cuprous oxidefilms are able to reduce
CO2to CO, HCOOH in 0.5 M NaHCO3at 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
Co3O4as a replacement electrocatalyst for H2O oxidation in oxygen based reactions[15e18] Existing Pt as anode may be replaced with
Co3O4to study the CO2reduction In this work, the main focus is on reduction of CO2to liquid products only The role of electrocatalyst
Co3O4for water oxidation, Cu2O for CO2reduction in 0.5 M NaHCO3 and Na2CO3 solutions for different applied voltages has been
* Corresponding author Fax: þ91 361 2582291.
E-mail address: shyam.kumar@iitg.ernet.in (V.S.K Yadav).
Peer review under responsibility of Vietnam National University, Hanoi.
Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d
http://dx.doi.org/10.1016/j.jsamd.2016.07.006
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
Trang 2studied The electrocatalysts used for RCPE here was synthesized by
electrodeposition method and confirmation was done by
charac-terizing 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
2 Experimental
2.1 Materials
Graphite plates (1.5 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 cm2electrodes were prepared using
Co3O4 and Cu2O electrocatalysts by brush coating on graphite
plates 200ml 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
100C for 2 h in oven to get the fullyfinished electrode[19,20]
2.3 Carbondioxide (CO2) electroreduction
A 2-electrode cell was used in study of CO2reduction, in which
Co3O4/graphite as anode and Cu2O/graphite as cathode were used
The schematic setup used for RCPE is shown inFig 1 80 ml of 0.5 M
electrolyte solutions were prepared to which CO2is bubbled up to
50 min to get CO2saturated 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
respec-tively[21,22]
2.4 Analysis of products from reduction of CO2
Different products formed from RCPE were detected using
ultra-fast liquid chromatography (UFLC), Shimadzu LC-20AD and
UV-detector of deuterium lamp (SPD-20A) at 205 nm wavelength
The solution of 20 ml is injected to the C-18 column of size
(10 4 mm), 5 mM Tetrabutyl ammonium hydrogen sulfate was used as mobile phase at 1 ml min1flow rate
3 Results and discussion 3.1 Reduction of CO2electrochemically 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 cm2were obtained
Results in Fig 2depicts that different quantities of products formed with reaction time and applied voltages Products for-mation is mainly based on proton availability at the cathode sur-face However, proton liberates at anode Co3O4due 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 CO2molecules to form products Possible reactions at the anode and cathode are shown inFig 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 formal-dehyde (20 min) were observed as reaction products for applied voltages Acetic acid was identified at reaction times of 10 and
20 min and shown inFig 2b Ren et al studied the reduction of
CO2electrochemically to ethanol and ethylene on Cu2O electro-catalyst using Pt as anode[23] Ethanol is mainly obtained at 2 V along with some quantities of formic acid and propanol at reac-tion 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 for-mation of multiple products with different concentrations during the RCPE at altered voltages and reaction time was reported[22] Reduction of CO2to different products like formic acid, methanol, acetic acid, ethanol, formaldehyde and was reported using copper based electrocatalysts[24e26] Ethanol and propanol were wit-nessed at higher applied voltages of 3 and 3.5 V with higher ethanol quantities inFig 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 CO2molecule to form its free radical The formed radical takes extra electron from cathode to form carbon mon-oxide/adsorbed CO and further adsorbed CO participates in reac-tion by accepting protons and electrons to get different products was reported[19]
3.1.2 Influence of time on Faradaic efficiency of product formed in NaHCO3electrolyte solution
The influence of Faradaic efficiency on product formed with time for RCPE was shown inFig 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 ef fi-ciency of 47.7% at reaction time 25 min was observed as an
V.S.K Yadav, M.K Purkait / Journal of Science: Advanced Materials and Devices 1 (2016) 330e336 331
Trang 3optimized condition for ethanol formation Results for RCPE
to-wards ethanol and formic acid formation were reported on copper
electrocatalyst In 0.5 M KHCO3is 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
for-mic 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
con-dition 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
elec-trocatalysts 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 respectivelyFig 3c
How-ever, 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 H2evolution[29] Results of applied experimental conditions show the fact that electrocatalysts and electrolyte plays a major role in CO2reduction Ethanol is observed as main product for all applied voltages along
Fig 2 a) Voltage vs current density Formation of various products with time at constant voltage in NaHCO 3 electrolyte solution: b) 1.5 V, c) 2 V, d) 2.5 V, e) 3 V, f) 3.5 V.
Trang 4with 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
elec-trocatalyst with cobalt oxide Quantities of product formed are not
same with respective time interval may be due to oxidation or
reduction of formed products inFig 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 Na2CO3are presented inFig 4a It is observed from thefigure
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 cm2were 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 CO2to ethanol and formic acid Kuhl et al reported the sixteen products from CO2 reduction on copper and Pt based electro-catalysts[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 CO2to different products on copper electro-catalyst was reported [4] FromFig 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 con-centration 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
Fig 3 Effect of time on the Faradaic efficiency of products formed at various applied voltages for RCPE in NaHCO 3 electrolyte solution a) 1.5 V, b) 2 V, c) 2.5 V, d) 3 V, e) 3.5 V.
V.S.K Yadav, M.K Purkait / Journal of Science: Advanced Materials and Devices 1 (2016) 330e336 333
Trang 5the 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 CO2to products was observed at 3.5 V and
shown inFig 4f Acetic acid, formaldehyde and formic acid were
obtained as main products for the reaction time of 5 min
Inter-estingly, 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
Na2CO3electrolyte solution
The effect of reduction of CO2to different products, upon the
applied voltage and the Faradaic efficiency of products formed with
time of the reaction in Na2CO3 solution is shown inFig 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 ef
fi-ciencies of 55.21 and 53.51% of the reaction time of 15 and 10 min
which are the optimum conditions for reduction of CO2with high
efficiency Hori et al studied the CO reduction to different products
in aqueous phase on coppere Pt electrocatalyst[31] The Faradaic
efficiencies of the reduced CO2products with time at 2 V are shown
inFig 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 for-mation 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 ficiency of 42.34% for reaction of 5 min for ethanol and 31.76% ef-ficiency for methanol at reaction time of 10 min were found to be the best reaction condition Yano et al reported the CO2reduction
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 KHCO3solution[11] At 3 V, the RCPE was shown inFig 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
Fig 4 a) Voltage vs current density Formation of products with time in Na 2 CO 3 electrolyte b) 1.5 V; c) 2 V; d) 2.5 V; e) 3 V and f) 3.5 V.
Trang 6the 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 inFig 4reveals the fact that formation of
products depends on the applied voltage and time of reaction and
respective Faradaic efficiencies with time inFig 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 NaHCO3and Na2CO3
electrolyte solutions on CuO electrocatalyst is explained with
respect to the applied voltage and reaction time This study was able tofind 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 Na2CO3solution 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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