Journal of Science: Advanced Materials and Devices (2016) 330e336 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd 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 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 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/) Keywords: Electrocatalyst CO2 reduction Cu2O Co3O4 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: ỵ91 361 2582291 E-mail address: shyam.kumar@iitg.ernet.in (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 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 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 (http://creativecommons.org/licenses/by/4.0/) V.S.K Yadav, M.K Purkait / Journal of Science: Advanced Materials and Devices (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 mm), mM Tetrabutyl ammonium hydrogen sulfate was used as mobile phase at ml minÀ1 flow rate Experimental 3.1.1 Effect of time on mole of product formed in NaHCO3 electrolyte solution Fig 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, 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 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 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 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 V along with some quantities of formic acid and propanol at reaction times of 15 and 20 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 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 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 (naon ỵ IPA (iso propyl alcohol)) solutions Further, 7.5 mg (electrocatalyst) was added to binder solution and further 30 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 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 80 ml of 0.5 M electrolyte solutions were prepared to which CO2 is bubbled up to 50 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 Schematic diagram for the setup used in RCPE 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 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 was observed as an 332 V.S.K Yadav, M.K Purkait / Journal of Science: Advanced Materials and Devices (2016) 330e336 Fig a) Voltage vs current density Formation of various products with time at constant voltage in NaHCO3 electrolyte solution: b) 1.5 V, c) V, d) 2.5 V, e) 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 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 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 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 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 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 V.S.K Yadav, M.K Purkait / Journal of Science: Advanced Materials and Devices (2016) 330e336 333 Fig 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) V, c) 2.5 V, d) 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 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, 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 along with the formic acid at time 10, 15 and 20 However, acetic acid was witnessed in the reaction of 5, 10 along with minute amount of formic acid at the reaction time of 10 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 V (Fig 2c) shows that this voltage is favorable for formic acid for the reaction time of 10e25 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 However, acetic acid and minor quantities of formic acid is observed at 5, 15 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 and higher quantities of methanol is observed at 10 reaction Formic acid is observed at reaction time of 15 and 20 along with formaldehyde in 20 reaction Minor quantities of formaldehyde and ethanol are observed at the reaction of 25 The variation in product concentration with time is by the oxidation of formed product was reported [22] The products formed at V are shown in Fig 4e Small quantities of formic acid are observed at all the reaction times except for 25 Higher quantities of acetic acid are observed in 334 V.S.K Yadav, M.K Purkait / Journal of Science: Advanced Materials and Devices (2016) 330e336 Fig a) Voltage vs current density Formation of products with time in Na2CO3 electrolyte b) 1.5 V; c) V; d) 2.5 V; e) V and f) 3.5 V the reaction of 25 but quantity is decreased at reaction time of 10 Ethanol is observed at the reaction of 15 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 Interestingly, major quantities of ethanol are observed for the reaction upto10 with some amount of formic acid Same quantities of formic acid and formaldehyde were obtained in the reaction of 15 Acetic acid is identified as product for the reaction of 20, 25 along with some amount of formic acid for the reaction of 20 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 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 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 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 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 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 and 25.1% for formaldehyde at the reaction of 15 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 for ethanol and 31.76% efficiency for methanol at reaction time of 10 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 V, the RCPE was shown in Fig 5d Formic acid is formed at the reaction time of 5, 10, 15 and 20 with efficiencies of 1.07, 0.19, 0.11 and 0.15% For acetic acid at reaction time of 10, 25 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 (2016) 330e336 335 Fig Effect of time on the Faradaic efficiency of products formed with applied voltages for RCPE in Na2CO3 electrolyte solution a) 1.5 V, b) V, c) 2.5 V, d) 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 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 reveals the fact that formation of products depends on the applied voltage and time of reaction and respective Faradaic efficiencies with time in Fig All the products formed depends on the time of reaction, ethanol and formic acid are observed at low voltages of 1.5 and 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 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 V with 98.1% after reaction which is the most optimum condition for ethanol formation in NaHCO3 solution Formic acid was formed at V and ethanol at 1.5 V along with other products like 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reduction of forming products after certain time of reaction due to the decrease in concentration of CO2 Conclusion The effect of RCPE was studied... the reaction of 15 Acetic acid is identified as product for the reaction of 20, 25 along with some amount of formic acid for the reaction of 20 3.1.4 Effect of time on Faradaic efficiency of product... 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