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Solar cell driven electrochemical process for the reduction of CO2 to HCOOH on Zn and Sn electrocatalysts
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ScienceDirect Solar Energy 124 (2016) 177–183 www.elsevier.com/locate/solener
Solar cell driven electrochemical process for the reduction of CO2 to HCOOH on Zn and Sn electrocatalysts V.S.K. Yadav, M.K. Purkait ⇑ Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Received 28 May 2015; received in revised form 4 November 2015; accepted 23 November 2015
Communicated by: Associate Editor Gion Calzaferri
Abstract This article presents the reduction of carbon dioxide (CO2) to products using solar energy on Zn and Sn electrocatalysts. Cobalt oxide (Co3O4) electrode was used as an anode for water dissociation reaction. Zinc (Zn) and tin (Sn) electrocatalysts were used as cathodes for photo electrochemical reduction of CO2. The extent of CO2 reduction was investigated in presence of various concentrations of bicarbonates of sodium and potassium electrolytes. The samples were collected for the reaction time of (10, 20, 30, 40, 50 and 60 min) and analyzed by using ultra fast liquid chromatography (UFLC). Formic acid (HCOOH) was the only product formed for both the electrocatalysts and respective results and optimized conditions were given in detail. Maximum HCOOH formed in 0.2 M KHCO3 for 10 min reaction was 430 lmol and 400 lmol using Sn and Zn electrocatalyst, respectively. On the other hand, 0.2 M NaHCO3 produced 494.6 lmol and 176 lmol of HCOOH after 10 min using Sn and Zn electrocatalyst, respectively. Therefore, the work is worthy for single product formation from CO2 and water using inexpensive and efficient electrocatalysts. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Solar cell; Photo electrochemical; CO2 reduction; HCOOH; Zn; Co3O4
1. Introduction The increase in CO2 concentrations in the atmosphere causes greenhouse effect. CO2 can be reduced to some useful products which solves the two problems, i.e., global warming and energy crisis (Robinson et al., 2007; Ganesh, 2011). This can be done by developing a process system to store solar energy in the form of fuel. The system should convert CO2, which is released during the combustion reaction of fossil fuels to energy rich products (Adachi and Ohta, 1994; Kumar et al., 2012; White et al., 2014; Yadav et al., 2012; Lyu et al., 2014). However, developing a best process for this conversion is a great challenge. Of ⇑ Corresponding author. Tel.: +91 361 2582262; fax: +91 361 2582291.
E-mail address: [email protected] (M.K. Purkait). http://dx.doi.org/10.1016/j.solener.2015.11.037 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
the available resources, solar energy is one of the best abundant inexhaustible resources available. Solar energy available on the earth’s surface is free from non-polluting and relatively best resource to utilize its energy for the conversion of CO2 to useful products. It was reported that total solar energy falls on the surface of the earth in 14 days as contained energy in world’s fossil fuel resources which is around 1016 kW. In India, the total annual incidence of this solar energy alone is about 107 kW (Ganesh, 2011). It will be a novel approach if the incident solar energy can be utilized for the conversion of CO2 to liquid products (AurianBlajeni et al., 1980; Halmann and Aurian-Blajeni, 1983; Mahmodi et al., 2013). However, reduction of CO2 by utilizing this energy is not so easy. The natural process of photosynthesis utilizes solar energy to convert into solid fuel, which is nothing but fossil fuels by transforming the plants
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accumulated for over years beneath the earth. Hence, an alternative method should be used in order to store this maximum amount of energy. It will be highly acceptable if the solar energy is utilized for the reduction of CO2 to form useful single product. The photo electrochemical reduction of CO2 to HCOOH was studied using pure Zn catalyst in NaHCO3 electrolyte solution at different temperatures (Jin et al., 2014). Studies were done to develop a best possible kinetics and selection of catalyst for H2O oxidation and CO2 reduction using solar energy (La Porte et al., 2014; Hull et al., 2012; Brendelberger and Sattler, 2015). The review toward the nano-materials and their structural properties for reduction of CO2 to methane by catalytic and photo catalytic processes were reported (Fechete and Vedrine, 2015). Similarly, reduction of CO2 to methanol using catalyzed p-GaP in a photochemical cell was reported (Barton et al., 2008). Studies were examined thermodynamically a two-step solar cycle for CO2 splitting by using Zn/ZnO and FeO/Fe3O4 redox reactions. Several studies have been done using titanium based electrocatalyst for photo catalytic reduction of carbon dioxide to fuels (Irvine et al., 1990; Adachi and Ohta, 1994; Anpo and Chiba, 1992; Kwak et al., 2015; Kaneco et al., 1999; Ohno et al., 2014; Dey et al., 2004; Liu et al., 2015). The photochemical reduction of CO2 was investigated on copper based electrocatalyst in various fuel generations (Wang et al., 2014; Li et al., 2013; Handoko and Tang, 2013; Won et al., 2014; Tseng et al., 2002). Reduction of CO2 using solar energy was studied in 0.1 M KHCO3 solution on copper catalyst as cathode and Pt as anode. Different products like methanol, methane formaldehyde and formic acid were reported (Peng et al., 2013). The photochemical reduction of CO2 on Zn based catalyst was also reported using Pt catalyst (Jin et al., 2014; Nunez et al., 2013). The present work focused on a simple process for the conversion of CO2 to HCOOH by photo electrochemical process on Sn and Zn electrocatalysts. Many researchers studied the effect of CO2 reduction electrochemically to HCOOH using Sn and Zn electrocatalysts using Pt as anode (Zhang et al., 2014; Hara et al., 1995; Hori et al., 1985; Lv et al., 2014). Pt was used as anode for most of CO2 reduction reactions reported till date. Low cost Co3O4 was used for water oxidation reaction for O2 evolution reactions (Yusuf and Jiao, 2012; Blakemore et al., 2013; Grzelczak et al., 2013; Dey et al., 2014). Use of cheap and easily available Co3O4 as anode in place of costly platinum (Pt) for CO2 reduction is scant. Again, photo electrochemical reductions of CO2 to HCOOH on Sn and Zn electrocatalysts have not been reported using Co3O4 as anode. It would become a novel approach for future liquid fuel production if reduction of CO2 able to form a single liquid product with higher conversion rates using solar driven energy. Therefore, in this work the effect of CO2 reduction by photo electrochemical process was studied in different concentrations of bicarbonate electrolyte solutions. The selected electrocatalyst were able to reduce
CO2 successfully to HCOOH as single liquid product. The performance of electrocatalyst for an anode (Co3O4) and cathode (Sn) toward CO2 reduction by photo electrochemical process is explained with respect to concentration of electrolyte solutions using a 2-electrode glass cell. Considering the importance of utilizing the solar energy for reduction of CO2, the results not only show the effect of Co3O4, Sn and Zn as photo catalysts but also the selective production of solar fuel HCOOH. 2. Experimental 2.1. Materials Solar panel (8.8 V 340 mA) was purchased from Waare Energies Pvt. Ltd, Surat, India. Graphite plates (1.5 2.5) cm2 were procured from Sunrise Enterprises, Mumbai. Sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3) and iso-propyl alcohol ((CH3)2CHOH) were purchased from Merck, India. Nafion (5 wt%) solution was kindly supplied by DuPont, USA. All the chemicals were used without any further purification and deionized water were used in all the experiments. 2.2. Preparation of electrocatalyst and electrodes for photo electrochemical reduction of CO2 Tin (Sn), Zinc (Zn) and Cobalt oxide (Co3O4) powders were synthesized by electrodeposition method. All these powders were extracted from 0.1 M solutions of their respective salts [(Tin chloride dihydrate (SnCl22H2O), Zinc chloride dihydrate (ZnCl22H2O) and Cobalt nitrate hexahydrate (Co(NO3)26H2O)] by applying current between the two electrodes (copper and graphite (G) plates) in an electrolytic cell. A constant current of 0.2 A was applied for 3 min due to which the respective metal deposits on graphite plate surface. Surface metal was removed with acetone solvent and dried at 100 °C for 1 h to get metal powders. Electrodes were prepared by coating electrocatalyst ink on the surface of graphite plate. The ink was prepared by preparing a binder at the ratio of 1:5 (nafion + iso-propyl alcohol (IPA) of 200 ll solutions. 7.5 mg of electrocatalyst (Co3O 4, Zn and Sn) was mixed separately with binder and sonicated for 30 min for respective catalyst. The ink was coated on the plate at the 80 °C to get the plate at a catalyst rate of 2 mg/cm2. The plates were dried for 2 h in oven at 100 °C to obtain completely formed electrode. 2.3. Photo electrochemical studies toward reduction of CO2 The experiments were conducted by using a 2-electrode homemade glass cell to study the solar driven CO2 reduction. Schematic for experimental setup used for the reduction of CO2 is shown in Fig. 1. The electrolyte solutions of 80 ml were prepared (0.2, 0.4, 0.6, 0.8, 1 M) using sodium and potassium bicarbonates and bubbled with CO2 for
V.S.K. Yadav, M.K. Purkait / Solar Energy 124 (2016) 177–183
Fig. 1. Experimental setup for photo electrochemical reduction of CO2.
1 h to get completely saturated solution. Experiments were conducted separately using prepared solutions by connecting two electrodes [Cobalt oxide coated on graphite plate (Co3O4/G) – Tin coated on graphite plate (Sn/G) or Co3O4/G – Zinc coated on graphite plate (Zn/G)] to the solar panel. Different concentrations of bicarbonate based sodium and potassium electrolytes were considered and the same solution was allowed for reduction in various time (0–10, 20, 30, 40 and 50 min) intervals. 2.4. Product analysis The product from electrochemical reduction of CO2 was analyzed by ultra fast liquid chromatography (UFLC; Make: Shimadzu LC-20AD; UV-detector of deuterium lamp SPD-20A). The analysis was done at 205 nm wavelength by injecting 20 ll of product sample to the C-18 column (10 4 mm). 5 mM tetrabutyl ammonium hydrogen sulfate was used as mobile phase at the flow rate of 1 ml/min. 3. Results and discussion The photo electrochemical reduction of CO2 was done using Co3O4/G as anode. Sn/G and Zn/G electrode was separately used as cathode. Experiments were performed using different concentrations of KHCO3 and NaHCO3 for both Sn and Zn electrode arrangement and results were discussed in subsequent sections 3.1. Photo electrochemical reduction of CO2 using Sn electrocatalyst
3.1.1. Considering KHCO3 as electrolyte solution The experimental results for the reduction of CO2 photo electrochemically in KHCO3 was shown in Fig. 2. It may be seen from the figure that HCOOH was the only product formed in all different electrolyte concentrations, which
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Fig. 2. Photo electrochemical reduction of CO2 to HCOOH (at 8.8 V 340 mA) on Sn electrocatalyst in KHCO3 electrolyte.
signifies that electrocatalyst was able to reduce CO2 to single product (HCOOH) selectively. Ohno et al., studied the effect of CO2 reduction for CH3OH and other minor products formation on silver or gold coated titanium dioxide (TiO2) electrocatalyst (Ohno et al., 2014). From the figure it is also observed that lower electrolyte concentration favors high HCOOH formation from CO2 reduction than high electrolyte concentrations. The reaction in 0.2 M solution gives 430, 319.2, 372.3, 271.8 and 267.9 lmol of HCOOH in reaction time of 10, 20, 30, 40 and 50 min. However, the reaction in 0.2 M shows the best performance of the selected electrocatalyst toward CO2 reduction to HCOOH using solar energy. The optimized condition for high CO2 conversion is 430 lmol for 10 min. It was reported that the reduction of CO2 to HCOOH using solar energy in 0.1 M KHCO3 electrolyte on Cu catalyst and different products like HCOOH, HCHO and CH3OH were reported with current efficiency of HCOOH is 0.69% after 1 h reaction (Peng et al., 2013). The CO2 reduction in 0.4 M solution shows better reduction rate, but less compared to reaction in 0.2 M electrolyte solution. Amount of HCOOH formed with respect to time was 16.8, 104.8, 206.7, 210.5 and 342 lmol. The reaction was increased with time and maximum conversion after 50 min was 342 lmol. Though the reaction in 0.6 M solution was good, but rate of reduction was low compared to 0.2 M. For 0.6 M KHCO3 solution, 210, 120.3, 50.8, 67.7 and 128.2 lmol HCOOH were formed for various time (Fig. 2). Lower concentrations of HCOOH were observed. This is because of the fact that hydrogen evolution reaction is more favorable than CO2 reduction (Corma and Garcia, 2013). The reduction in 0.8 and 1 M electrolyte solutions were low which may be due to high hydrogen evolution than CO2 reduction. The moles of HCOOH formed were reported to be 0.68, 8.3, 127.2, 88.3 and 127 lmol at 0.8 M and 5.7, 8, 49.9, 65 and 10.8 lmol at 1 M. It may be concluded that 0.2 M of KHCO3 is the optimum for efficient HCOOH formation in the electrocatalyst considered herein.
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3.1.2. Considering NaHCO3 as electrolyte solution The experimental results on the formation of HCOOH from photo electrochemical reduction of CO2 with time using NaHCO3 electrolyte solution is shown in Fig. 3. It may be seen from the figure that the selected electrocatalyst was capable to reduce CO2 to HCOOH effectively in presence of different concentrations of NaHCO3. The photo reduction of CO2 on composites of cuprous oxide and graphene oxide was studied for the formation of CH3OH (Wang et al., 2014). The reduction in 0.2 M NaHCO3 was reported to be 494.6, 452.8, 339.2, 470 and 330.2 lmol for the reaction times of 10, 20, 30, 40 and 50 min. The high conversion (494.6 lmol) after 5 min was the optimum one. Photo electrochemical reduction of CO2 to HCOOH in NaHCO3 electrolyte solutions using Zn electrocatalyst was reported (yield of 40%) after the reaction of 100 min (Jin et al., 2014). 0.4 M concentration solution gives a reduction of CO2 to HCOOH as 25.9, 33, 29.7, 162.2 and 13.61 lmol for different time. Initial reaction shows low conversion and at 40 min the conversion is highest. However, the rate of CO2 reduction is lowest in 0.6, 0.8 and 1 M electrolyte concentrations which may be due to high hydrogen evolution reaction at the cathode surface (Kaneco et al., 2009). Moles of HCOOH formed in 0.6 M solution is 3.1, 7, 6.4, 16.8 and 27.7 lmol for the said reaction times and the optimized condition to be a reaction after 50 min. The CO2 reduction in 0.8 M solution is 3, 2.3, 14.3, 15.7 and 18.6 lmol. The conversion rate increased with the increase in time. However the conversion rate is very low compared with that of in 0.2 M. This is because of faster hydrogen favorable reaction (Fig. 3). For 1 M solution, the formation of HCOOH were obtained to be 3, 18.7, 10, 14.3 and 0.76 lmol and the optimized reaction condition to be 18.7 lmol for reaction time of 20 min. The ability of the Co3O4 as anode and Sn as cathode toward photo electrochemical reduction of CO2 is confirmed by results discussed in the preceding section.
However, in both KHCO3 and NaHCO3 electrolyte solutions, low molar concentrations shows better conversion rates than a high concentrated electrolyte solution. The optimized reaction conditions for the photo electrochemical reduction of CO2 to HCOOH are shown in Table 1. 3.2. Photo electrochemical reduction of CO2 using Zn electrocatalyst
3.2.1. Reduction of CO2 photo electrochemically at Zn electrocatalyst in KHCO3 electrolyte solution Fig. 4 confirms that only product formed during the photo electrochemical reduction of CO2 on Zn electrocatalyst in different KHCO3 solutions is HCOOH. Won et al., shows the formation of HCOOH and CH3OH from Photo electrochemical CO2 reduction on copper based electrocatalysts (Won et al., 2014). The reduction of CO2 in 0.2 M electrolyte solution shows higher conversions than other molar concentration of KHCO3 solutions. The moles of HCOOH formed at this condition are reported to be 400, 358.9, 355, 309.7 and 138.9 lmol for the reaction time of 10, 20, 30, 40 and 50 min, respectively. This shows high conversion rates toward CO2 reduction at the applied experimental condition and the maximum efficiency is 400 lmol for 10 min reaction. Methane synthesis using Photo catalytic CO2 reduction by using nickel ion with TiO2 electrocatalyst and importance of nickel content on production formation was reported (Kwak et al., 2015). The photo electrochemical reductions of CO2 in 1 M KHCO3 electrolyte solutions using iridium based electrocatalyst and a high Faradaic efficiency of 67% was reported (white et al., 2014). The reaction in 0.4 M electrolyte solution shows the feasible conversion rate, but low compared to 0.2 M solution. The moles of HCOOH formed are reported to be 46.4, 36, 170.3, 208.4 and 74.2 lmol which confirms that electrolyte concentration affects the reduction rates. This was due to high hydrogen formation at the cathode. The optimized condition at this electrolyte concentration is 208.4 lmol for 40 min reaction. The variation in product concentrations with time may be due to the oxidation of formed product at anode and getting hydrogen evolution (Lv et al., 2014). Photo electrochemical reduction of CO2 to HCOOH in 0.6 M electrolyte solution shows that Table 1 Optimized experimental conditions for reduction of CO2 on Sn electrocatalyst. Molarity
Electrolytes KHCO3
Fig. 3. Photo electrochemical reduction of CO2 to HCOOH (at 8.8 V 340 mA) on Sn electrocatalyst in NaHCO3 electrolyte.
NaHCO3
M
lmol
(min)
lmol
(min)
0.2 0.4 0.6 0.8 1
430.04 342.01 210.11 127.21 65.01
10 50 10 30 40
494.60 162.21 27.75 18.64 18.73
10 40 50 50 20
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Fig. 4. Photo electrochemical reduction of CO2 to HCOOH (at 8.8 V 340 mA) on Zn electrocatalyst in KHCO3 electrolyte.
Fig. 5. Photo electrochemical reduction of CO2 to HCOOH at (at 8.8 V 340 mA) on Zn electrocatalyst in NaHCO3 electrolyte.
moles of product formed to be 27.1, 93.3, 22.6, 98.3 and 56.1 lmol which are low compared with reaction at 0.4 M. Optimized condition toward higher conversion is 98.3 lmol for 40 min. Similarly, the reaction in 0.8 M and 1 M have shown less conversions comparing with low molar concentrations which may be due to high proton participation at the cathode for hydrogen formation (Tahir and Amin, 2013). Reaction in 0.8 M electrolyte solution shows the formation of HCOOH to be 21.05, 19.25, 13.55, 47.53 and 85.31 lmol with high conversion at reaction time of 50 min is 85.31 lmol. The rate of HCOOH formation, with time in 1 M solution is 28.8, 21.6, 17, 25.3 and 24.6 lmol. For a reaction time of 5 min shows high conversion rate toward HCCOH formation is 28.8 lmol. The results in Fig. 4 confirm that low molar concentrated solutions are more feasible for CO2 conversion rates for the formation of HCOOH.
formation of carbon monoxide (CO) on different structures of copper based electrocatalysts was shown and effect of crystal morphologies on photo reduction of CO2 was clearly mentioned (Handoko and Tang, 2013). The photo electrochemical reduction of CO2 to HCOOH (25.3, 6.7, 19.3, 41.8 and 81.8 lmol) was observed in 0.6 M electrolyte solution. The optimized efficiency was 81.8 lmol for the reaction time of 50 min. Low conversions was observed in 0.8 M and 1 M solutions because of high hydrogen formation (Kaneco et al., 2009). The performances are 30.6, 34.4, 55.6, 42.6 and 57.1 lmol in 0.8 M solution and 42.6, 19.1, 8.7, 28.6 and 45.7 lmol in 1 M solution, respectively (Fig. 5). The maximum performance was 57.1 (50 min) and 45.7 (50 min) for 0.8 and 1 M solutions, respectively. Similar to KHCO3, lower molar concentration shows high CO2 conversions for NaHCO3. The photo electrochemical reduction of CO2 to HCOOH was studied using Co3O4 as anode and Zn as cathode. The selected electrocatalysts were capable of reducing CO2 by using solar energy. The low molar electrolyte solution showed better conversions in both KHCO3 and NaHCO3 electrolyte. The optimized reaction conditions are shown in Table 2. Elementary mechanism for HCOOH formation from CO2 reduction is represented in
3.2.2. Reduction of CO2 photo electrochemically at Zn electrocatalyst in NaHCO3 electrolyte solution Experimental results for the photo electrochemical reduction of CO2 to HCOOH with time in NaHCO3 electrolyte solution is shown in Fig. 5. The effect of CO2 reduction using solar energy was shown for the formation of HCOOH on Zn electrocatalyst in NaHCO3 electrolyte solution (Jin et al., 2014). The figure confirms the selected Co3O4 as anode and Zn as cathode is able to reduce CO2 to HCOOH effectively in all molar solutions of NaHCO3. For the reaction time of 10, 20, 30, 40 and 50 min the formation rates of HCOOH were reported to be 146.2, 176, 187, 254 and 169 lmol. The optimized experimental condition toward the high HCOOH formation is 254 lmol for 40 min reaction. Moles of HCOOH (51.6, 49, 37, 78.6 and 43 lmol) were formed from the reaction in 0.4 M electrolyte solution. The rate of reaction toward HCOOH was low compared with 0.2 M solution which may be due to high hydrogen evolution than CO2 reduction. The
Table 2 Optimized experimental conditions for reduction of CO2 on Zn electrocatalyst. Molarity
Electrolytes KHCO3
NaHCO3
M
lmol
(min)
lmol
(min)
0.2 0.4 0.6 0.8 1
400.02 208.41 98.38 85.31 28.81
10 40 40 50 10
254.01 78.62 81.85 57.12 45.71
40 40 50 50 50
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Fig. 6. Proposed elementary mechanism for photo electroreduction of CO2 to HCOOH.
Fig. 6. The adsorbed CO2 on cathode accepts electron to form radical. The radical reacts with proton and electron that are generated at anode due to water oxidation reaction to form formate. The new formate further accepts proton to form HCOOH. 4. Conclusion A simple method for solar driven CO2 reduction to HCOOH was studied using Sn and Zn electrocatalysts. The selected Co3O4 for H2O dissociation was effective for CO2 reduction toward HCOOH generation. However, high conversions were observed at low electrolyte concentrations for bicarbonates of sodium and potassium. For Sn based reduction, maximum product formed were 430 lmol (KHCO3) and 494.6 lmol (NaHCO3) for 10 min reaction in 0.2 M electrolyte solution. Whereas for Zn electrocatalyst, it was observed to be 400 lmol (KHCO3) for 10 min and 254 lmol (NaHCO3) for 40 min in 0.2 M electrolyte solution. The selected electrocatalysts were able to reduce CO2 to HCOOH effectively at all electrolyte concentrations. These preliminary studies will be helpful toward effective solar driven CO2 reduction for future applications. References Adachi, K., Ohta, K., 1994. Reduction of carbon dioxide to hydrocarbon using copper loaded Titanium dioxide. Sol. Energy 53, 187–190. Anpo, M., Chiba, K., 1992. Photo catalytic reduction of CO2 on anchored titanium oxide catalysts. J. Mol. Catal. 74, 207–212. Aurian-Blajeni, B., Halmann, M., Manassen, J., 1980. Photoreduction of carbon dioxide and water into formaldehyde and methanol on semiconductor materials. Sol. Energy 25, 165–170. Barton, E.E., Rampulla, D.M., Bocarsly, A.B., 2008. Selective solardriven reduction of CO2 to methanol using a catalyzed p-Gap based photo electrochemical cell. J. Am. Chem. Soc. Commun. 130, 6342– 6344. Blakemore, J.D., Gray, H.B., Winkler, J.R., Mu, A.M., 2013. Co3O4 nanoparticle water-oxidation catalysts made by pulsed-laser ablation in liquids. ACS Catal. 3, 2497–2500.
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ScienceDirect Solar Energy 124 (2016) 177–183 www.elsevier.com/locate/solener
Solar cell driven electrochemical process for the reduction of CO2 to HCOOH on Zn and Sn electrocatalysts V.S.K. Yadav, M.K. Purkait ⇑ Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India Received 28 May 2015; received in revised form 4 November 2015; accepted 23 November 2015
Communicated by: Associate Editor Gion Calzaferri
Abstract This article presents the reduction of carbon dioxide (CO2) to products using solar energy on Zn and Sn electrocatalysts. Cobalt oxide (Co3O4) electrode was used as an anode for water dissociation reaction. Zinc (Zn) and tin (Sn) electrocatalysts were used as cathodes for photo electrochemical reduction of CO2. The extent of CO2 reduction was investigated in presence of various concentrations of bicarbonates of sodium and potassium electrolytes. The samples were collected for the reaction time of (10, 20, 30, 40, 50 and 60 min) and analyzed by using ultra fast liquid chromatography (UFLC). Formic acid (HCOOH) was the only product formed for both the electrocatalysts and respective results and optimized conditions were given in detail. Maximum HCOOH formed in 0.2 M KHCO3 for 10 min reaction was 430 lmol and 400 lmol using Sn and Zn electrocatalyst, respectively. On the other hand, 0.2 M NaHCO3 produced 494.6 lmol and 176 lmol of HCOOH after 10 min using Sn and Zn electrocatalyst, respectively. Therefore, the work is worthy for single product formation from CO2 and water using inexpensive and efficient electrocatalysts. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Solar cell; Photo electrochemical; CO2 reduction; HCOOH; Zn; Co3O4
1. Introduction The increase in CO2 concentrations in the atmosphere causes greenhouse effect. CO2 can be reduced to some useful products which solves the two problems, i.e., global warming and energy crisis (Robinson et al., 2007; Ganesh, 2011). This can be done by developing a process system to store solar energy in the form of fuel. The system should convert CO2, which is released during the combustion reaction of fossil fuels to energy rich products (Adachi and Ohta, 1994; Kumar et al., 2012; White et al., 2014; Yadav et al., 2012; Lyu et al., 2014). However, developing a best process for this conversion is a great challenge. Of ⇑ Corresponding author. Tel.: +91 361 2582262; fax: +91 361 2582291.
E-mail address: [email protected] (M.K. Purkait). http://dx.doi.org/10.1016/j.solener.2015.11.037 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
the available resources, solar energy is one of the best abundant inexhaustible resources available. Solar energy available on the earth’s surface is free from non-polluting and relatively best resource to utilize its energy for the conversion of CO2 to useful products. It was reported that total solar energy falls on the surface of the earth in 14 days as contained energy in world’s fossil fuel resources which is around 1016 kW. In India, the total annual incidence of this solar energy alone is about 107 kW (Ganesh, 2011). It will be a novel approach if the incident solar energy can be utilized for the conversion of CO2 to liquid products (AurianBlajeni et al., 1980; Halmann and Aurian-Blajeni, 1983; Mahmodi et al., 2013). However, reduction of CO2 by utilizing this energy is not so easy. The natural process of photosynthesis utilizes solar energy to convert into solid fuel, which is nothing but fossil fuels by transforming the plants
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accumulated for over years beneath the earth. Hence, an alternative method should be used in order to store this maximum amount of energy. It will be highly acceptable if the solar energy is utilized for the reduction of CO2 to form useful single product. The photo electrochemical reduction of CO2 to HCOOH was studied using pure Zn catalyst in NaHCO3 electrolyte solution at different temperatures (Jin et al., 2014). Studies were done to develop a best possible kinetics and selection of catalyst for H2O oxidation and CO2 reduction using solar energy (La Porte et al., 2014; Hull et al., 2012; Brendelberger and Sattler, 2015). The review toward the nano-materials and their structural properties for reduction of CO2 to methane by catalytic and photo catalytic processes were reported (Fechete and Vedrine, 2015). Similarly, reduction of CO2 to methanol using catalyzed p-GaP in a photochemical cell was reported (Barton et al., 2008). Studies were examined thermodynamically a two-step solar cycle for CO2 splitting by using Zn/ZnO and FeO/Fe3O4 redox reactions. Several studies have been done using titanium based electrocatalyst for photo catalytic reduction of carbon dioxide to fuels (Irvine et al., 1990; Adachi and Ohta, 1994; Anpo and Chiba, 1992; Kwak et al., 2015; Kaneco et al., 1999; Ohno et al., 2014; Dey et al., 2004; Liu et al., 2015). The photochemical reduction of CO2 was investigated on copper based electrocatalyst in various fuel generations (Wang et al., 2014; Li et al., 2013; Handoko and Tang, 2013; Won et al., 2014; Tseng et al., 2002). Reduction of CO2 using solar energy was studied in 0.1 M KHCO3 solution on copper catalyst as cathode and Pt as anode. Different products like methanol, methane formaldehyde and formic acid were reported (Peng et al., 2013). The photochemical reduction of CO2 on Zn based catalyst was also reported using Pt catalyst (Jin et al., 2014; Nunez et al., 2013). The present work focused on a simple process for the conversion of CO2 to HCOOH by photo electrochemical process on Sn and Zn electrocatalysts. Many researchers studied the effect of CO2 reduction electrochemically to HCOOH using Sn and Zn electrocatalysts using Pt as anode (Zhang et al., 2014; Hara et al., 1995; Hori et al., 1985; Lv et al., 2014). Pt was used as anode for most of CO2 reduction reactions reported till date. Low cost Co3O4 was used for water oxidation reaction for O2 evolution reactions (Yusuf and Jiao, 2012; Blakemore et al., 2013; Grzelczak et al., 2013; Dey et al., 2014). Use of cheap and easily available Co3O4 as anode in place of costly platinum (Pt) for CO2 reduction is scant. Again, photo electrochemical reductions of CO2 to HCOOH on Sn and Zn electrocatalysts have not been reported using Co3O4 as anode. It would become a novel approach for future liquid fuel production if reduction of CO2 able to form a single liquid product with higher conversion rates using solar driven energy. Therefore, in this work the effect of CO2 reduction by photo electrochemical process was studied in different concentrations of bicarbonate electrolyte solutions. The selected electrocatalyst were able to reduce
CO2 successfully to HCOOH as single liquid product. The performance of electrocatalyst for an anode (Co3O4) and cathode (Sn) toward CO2 reduction by photo electrochemical process is explained with respect to concentration of electrolyte solutions using a 2-electrode glass cell. Considering the importance of utilizing the solar energy for reduction of CO2, the results not only show the effect of Co3O4, Sn and Zn as photo catalysts but also the selective production of solar fuel HCOOH. 2. Experimental 2.1. Materials Solar panel (8.8 V 340 mA) was purchased from Waare Energies Pvt. Ltd, Surat, India. Graphite plates (1.5 2.5) cm2 were procured from Sunrise Enterprises, Mumbai. Sodium bicarbonate (NaHCO3), potassium bicarbonate (KHCO3) and iso-propyl alcohol ((CH3)2CHOH) were purchased from Merck, India. Nafion (5 wt%) solution was kindly supplied by DuPont, USA. All the chemicals were used without any further purification and deionized water were used in all the experiments. 2.2. Preparation of electrocatalyst and electrodes for photo electrochemical reduction of CO2 Tin (Sn), Zinc (Zn) and Cobalt oxide (Co3O4) powders were synthesized by electrodeposition method. All these powders were extracted from 0.1 M solutions of their respective salts [(Tin chloride dihydrate (SnCl22H2O), Zinc chloride dihydrate (ZnCl22H2O) and Cobalt nitrate hexahydrate (Co(NO3)26H2O)] by applying current between the two electrodes (copper and graphite (G) plates) in an electrolytic cell. A constant current of 0.2 A was applied for 3 min due to which the respective metal deposits on graphite plate surface. Surface metal was removed with acetone solvent and dried at 100 °C for 1 h to get metal powders. Electrodes were prepared by coating electrocatalyst ink on the surface of graphite plate. The ink was prepared by preparing a binder at the ratio of 1:5 (nafion + iso-propyl alcohol (IPA) of 200 ll solutions. 7.5 mg of electrocatalyst (Co3O 4, Zn and Sn) was mixed separately with binder and sonicated for 30 min for respective catalyst. The ink was coated on the plate at the 80 °C to get the plate at a catalyst rate of 2 mg/cm2. The plates were dried for 2 h in oven at 100 °C to obtain completely formed electrode. 2.3. Photo electrochemical studies toward reduction of CO2 The experiments were conducted by using a 2-electrode homemade glass cell to study the solar driven CO2 reduction. Schematic for experimental setup used for the reduction of CO2 is shown in Fig. 1. The electrolyte solutions of 80 ml were prepared (0.2, 0.4, 0.6, 0.8, 1 M) using sodium and potassium bicarbonates and bubbled with CO2 for
V.S.K. Yadav, M.K. Purkait / Solar Energy 124 (2016) 177–183
Fig. 1. Experimental setup for photo electrochemical reduction of CO2.
1 h to get completely saturated solution. Experiments were conducted separately using prepared solutions by connecting two electrodes [Cobalt oxide coated on graphite plate (Co3O4/G) – Tin coated on graphite plate (Sn/G) or Co3O4/G – Zinc coated on graphite plate (Zn/G)] to the solar panel. Different concentrations of bicarbonate based sodium and potassium electrolytes were considered and the same solution was allowed for reduction in various time (0–10, 20, 30, 40 and 50 min) intervals. 2.4. Product analysis The product from electrochemical reduction of CO2 was analyzed by ultra fast liquid chromatography (UFLC; Make: Shimadzu LC-20AD; UV-detector of deuterium lamp SPD-20A). The analysis was done at 205 nm wavelength by injecting 20 ll of product sample to the C-18 column (10 4 mm). 5 mM tetrabutyl ammonium hydrogen sulfate was used as mobile phase at the flow rate of 1 ml/min. 3. Results and discussion The photo electrochemical reduction of CO2 was done using Co3O4/G as anode. Sn/G and Zn/G electrode was separately used as cathode. Experiments were performed using different concentrations of KHCO3 and NaHCO3 for both Sn and Zn electrode arrangement and results were discussed in subsequent sections 3.1. Photo electrochemical reduction of CO2 using Sn electrocatalyst
3.1.1. Considering KHCO3 as electrolyte solution The experimental results for the reduction of CO2 photo electrochemically in KHCO3 was shown in Fig. 2. It may be seen from the figure that HCOOH was the only product formed in all different electrolyte concentrations, which
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Fig. 2. Photo electrochemical reduction of CO2 to HCOOH (at 8.8 V 340 mA) on Sn electrocatalyst in KHCO3 electrolyte.
signifies that electrocatalyst was able to reduce CO2 to single product (HCOOH) selectively. Ohno et al., studied the effect of CO2 reduction for CH3OH and other minor products formation on silver or gold coated titanium dioxide (TiO2) electrocatalyst (Ohno et al., 2014). From the figure it is also observed that lower electrolyte concentration favors high HCOOH formation from CO2 reduction than high electrolyte concentrations. The reaction in 0.2 M solution gives 430, 319.2, 372.3, 271.8 and 267.9 lmol of HCOOH in reaction time of 10, 20, 30, 40 and 50 min. However, the reaction in 0.2 M shows the best performance of the selected electrocatalyst toward CO2 reduction to HCOOH using solar energy. The optimized condition for high CO2 conversion is 430 lmol for 10 min. It was reported that the reduction of CO2 to HCOOH using solar energy in 0.1 M KHCO3 electrolyte on Cu catalyst and different products like HCOOH, HCHO and CH3OH were reported with current efficiency of HCOOH is 0.69% after 1 h reaction (Peng et al., 2013). The CO2 reduction in 0.4 M solution shows better reduction rate, but less compared to reaction in 0.2 M electrolyte solution. Amount of HCOOH formed with respect to time was 16.8, 104.8, 206.7, 210.5 and 342 lmol. The reaction was increased with time and maximum conversion after 50 min was 342 lmol. Though the reaction in 0.6 M solution was good, but rate of reduction was low compared to 0.2 M. For 0.6 M KHCO3 solution, 210, 120.3, 50.8, 67.7 and 128.2 lmol HCOOH were formed for various time (Fig. 2). Lower concentrations of HCOOH were observed. This is because of the fact that hydrogen evolution reaction is more favorable than CO2 reduction (Corma and Garcia, 2013). The reduction in 0.8 and 1 M electrolyte solutions were low which may be due to high hydrogen evolution than CO2 reduction. The moles of HCOOH formed were reported to be 0.68, 8.3, 127.2, 88.3 and 127 lmol at 0.8 M and 5.7, 8, 49.9, 65 and 10.8 lmol at 1 M. It may be concluded that 0.2 M of KHCO3 is the optimum for efficient HCOOH formation in the electrocatalyst considered herein.
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3.1.2. Considering NaHCO3 as electrolyte solution The experimental results on the formation of HCOOH from photo electrochemical reduction of CO2 with time using NaHCO3 electrolyte solution is shown in Fig. 3. It may be seen from the figure that the selected electrocatalyst was capable to reduce CO2 to HCOOH effectively in presence of different concentrations of NaHCO3. The photo reduction of CO2 on composites of cuprous oxide and graphene oxide was studied for the formation of CH3OH (Wang et al., 2014). The reduction in 0.2 M NaHCO3 was reported to be 494.6, 452.8, 339.2, 470 and 330.2 lmol for the reaction times of 10, 20, 30, 40 and 50 min. The high conversion (494.6 lmol) after 5 min was the optimum one. Photo electrochemical reduction of CO2 to HCOOH in NaHCO3 electrolyte solutions using Zn electrocatalyst was reported (yield of 40%) after the reaction of 100 min (Jin et al., 2014). 0.4 M concentration solution gives a reduction of CO2 to HCOOH as 25.9, 33, 29.7, 162.2 and 13.61 lmol for different time. Initial reaction shows low conversion and at 40 min the conversion is highest. However, the rate of CO2 reduction is lowest in 0.6, 0.8 and 1 M electrolyte concentrations which may be due to high hydrogen evolution reaction at the cathode surface (Kaneco et al., 2009). Moles of HCOOH formed in 0.6 M solution is 3.1, 7, 6.4, 16.8 and 27.7 lmol for the said reaction times and the optimized condition to be a reaction after 50 min. The CO2 reduction in 0.8 M solution is 3, 2.3, 14.3, 15.7 and 18.6 lmol. The conversion rate increased with the increase in time. However the conversion rate is very low compared with that of in 0.2 M. This is because of faster hydrogen favorable reaction (Fig. 3). For 1 M solution, the formation of HCOOH were obtained to be 3, 18.7, 10, 14.3 and 0.76 lmol and the optimized reaction condition to be 18.7 lmol for reaction time of 20 min. The ability of the Co3O4 as anode and Sn as cathode toward photo electrochemical reduction of CO2 is confirmed by results discussed in the preceding section.
However, in both KHCO3 and NaHCO3 electrolyte solutions, low molar concentrations shows better conversion rates than a high concentrated electrolyte solution. The optimized reaction conditions for the photo electrochemical reduction of CO2 to HCOOH are shown in Table 1. 3.2. Photo electrochemical reduction of CO2 using Zn electrocatalyst
3.2.1. Reduction of CO2 photo electrochemically at Zn electrocatalyst in KHCO3 electrolyte solution Fig. 4 confirms that only product formed during the photo electrochemical reduction of CO2 on Zn electrocatalyst in different KHCO3 solutions is HCOOH. Won et al., shows the formation of HCOOH and CH3OH from Photo electrochemical CO2 reduction on copper based electrocatalysts (Won et al., 2014). The reduction of CO2 in 0.2 M electrolyte solution shows higher conversions than other molar concentration of KHCO3 solutions. The moles of HCOOH formed at this condition are reported to be 400, 358.9, 355, 309.7 and 138.9 lmol for the reaction time of 10, 20, 30, 40 and 50 min, respectively. This shows high conversion rates toward CO2 reduction at the applied experimental condition and the maximum efficiency is 400 lmol for 10 min reaction. Methane synthesis using Photo catalytic CO2 reduction by using nickel ion with TiO2 electrocatalyst and importance of nickel content on production formation was reported (Kwak et al., 2015). The photo electrochemical reductions of CO2 in 1 M KHCO3 electrolyte solutions using iridium based electrocatalyst and a high Faradaic efficiency of 67% was reported (white et al., 2014). The reaction in 0.4 M electrolyte solution shows the feasible conversion rate, but low compared to 0.2 M solution. The moles of HCOOH formed are reported to be 46.4, 36, 170.3, 208.4 and 74.2 lmol which confirms that electrolyte concentration affects the reduction rates. This was due to high hydrogen formation at the cathode. The optimized condition at this electrolyte concentration is 208.4 lmol for 40 min reaction. The variation in product concentrations with time may be due to the oxidation of formed product at anode and getting hydrogen evolution (Lv et al., 2014). Photo electrochemical reduction of CO2 to HCOOH in 0.6 M electrolyte solution shows that Table 1 Optimized experimental conditions for reduction of CO2 on Sn electrocatalyst. Molarity
Electrolytes KHCO3
Fig. 3. Photo electrochemical reduction of CO2 to HCOOH (at 8.8 V 340 mA) on Sn electrocatalyst in NaHCO3 electrolyte.
NaHCO3
M
lmol
(min)
lmol
(min)
0.2 0.4 0.6 0.8 1
430.04 342.01 210.11 127.21 65.01
10 50 10 30 40
494.60 162.21 27.75 18.64 18.73
10 40 50 50 20
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Fig. 4. Photo electrochemical reduction of CO2 to HCOOH (at 8.8 V 340 mA) on Zn electrocatalyst in KHCO3 electrolyte.
Fig. 5. Photo electrochemical reduction of CO2 to HCOOH at (at 8.8 V 340 mA) on Zn electrocatalyst in NaHCO3 electrolyte.
moles of product formed to be 27.1, 93.3, 22.6, 98.3 and 56.1 lmol which are low compared with reaction at 0.4 M. Optimized condition toward higher conversion is 98.3 lmol for 40 min. Similarly, the reaction in 0.8 M and 1 M have shown less conversions comparing with low molar concentrations which may be due to high proton participation at the cathode for hydrogen formation (Tahir and Amin, 2013). Reaction in 0.8 M electrolyte solution shows the formation of HCOOH to be 21.05, 19.25, 13.55, 47.53 and 85.31 lmol with high conversion at reaction time of 50 min is 85.31 lmol. The rate of HCOOH formation, with time in 1 M solution is 28.8, 21.6, 17, 25.3 and 24.6 lmol. For a reaction time of 5 min shows high conversion rate toward HCCOH formation is 28.8 lmol. The results in Fig. 4 confirm that low molar concentrated solutions are more feasible for CO2 conversion rates for the formation of HCOOH.
formation of carbon monoxide (CO) on different structures of copper based electrocatalysts was shown and effect of crystal morphologies on photo reduction of CO2 was clearly mentioned (Handoko and Tang, 2013). The photo electrochemical reduction of CO2 to HCOOH (25.3, 6.7, 19.3, 41.8 and 81.8 lmol) was observed in 0.6 M electrolyte solution. The optimized efficiency was 81.8 lmol for the reaction time of 50 min. Low conversions was observed in 0.8 M and 1 M solutions because of high hydrogen formation (Kaneco et al., 2009). The performances are 30.6, 34.4, 55.6, 42.6 and 57.1 lmol in 0.8 M solution and 42.6, 19.1, 8.7, 28.6 and 45.7 lmol in 1 M solution, respectively (Fig. 5). The maximum performance was 57.1 (50 min) and 45.7 (50 min) for 0.8 and 1 M solutions, respectively. Similar to KHCO3, lower molar concentration shows high CO2 conversions for NaHCO3. The photo electrochemical reduction of CO2 to HCOOH was studied using Co3O4 as anode and Zn as cathode. The selected electrocatalysts were capable of reducing CO2 by using solar energy. The low molar electrolyte solution showed better conversions in both KHCO3 and NaHCO3 electrolyte. The optimized reaction conditions are shown in Table 2. Elementary mechanism for HCOOH formation from CO2 reduction is represented in
3.2.2. Reduction of CO2 photo electrochemically at Zn electrocatalyst in NaHCO3 electrolyte solution Experimental results for the photo electrochemical reduction of CO2 to HCOOH with time in NaHCO3 electrolyte solution is shown in Fig. 5. The effect of CO2 reduction using solar energy was shown for the formation of HCOOH on Zn electrocatalyst in NaHCO3 electrolyte solution (Jin et al., 2014). The figure confirms the selected Co3O4 as anode and Zn as cathode is able to reduce CO2 to HCOOH effectively in all molar solutions of NaHCO3. For the reaction time of 10, 20, 30, 40 and 50 min the formation rates of HCOOH were reported to be 146.2, 176, 187, 254 and 169 lmol. The optimized experimental condition toward the high HCOOH formation is 254 lmol for 40 min reaction. Moles of HCOOH (51.6, 49, 37, 78.6 and 43 lmol) were formed from the reaction in 0.4 M electrolyte solution. The rate of reaction toward HCOOH was low compared with 0.2 M solution which may be due to high hydrogen evolution than CO2 reduction. The
Table 2 Optimized experimental conditions for reduction of CO2 on Zn electrocatalyst. Molarity
Electrolytes KHCO3
NaHCO3
M
lmol
(min)
lmol
(min)
0.2 0.4 0.6 0.8 1
400.02 208.41 98.38 85.31 28.81
10 40 40 50 10
254.01 78.62 81.85 57.12 45.71
40 40 50 50 50
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Fig. 6. Proposed elementary mechanism for photo electroreduction of CO2 to HCOOH.
Fig. 6. The adsorbed CO2 on cathode accepts electron to form radical. The radical reacts with proton and electron that are generated at anode due to water oxidation reaction to form formate. The new formate further accepts proton to form HCOOH. 4. Conclusion A simple method for solar driven CO2 reduction to HCOOH was studied using Sn and Zn electrocatalysts. The selected Co3O4 for H2O dissociation was effective for CO2 reduction toward HCOOH generation. However, high conversions were observed at low electrolyte concentrations for bicarbonates of sodium and potassium. For Sn based reduction, maximum product formed were 430 lmol (KHCO3) and 494.6 lmol (NaHCO3) for 10 min reaction in 0.2 M electrolyte solution. Whereas for Zn electrocatalyst, it was observed to be 400 lmol (KHCO3) for 10 min and 254 lmol (NaHCO3) for 40 min in 0.2 M electrolyte solution. The selected electrocatalysts were able to reduce CO2 to HCOOH effectively at all electrolyte concentrations. These preliminary studies will be helpful toward effective solar driven CO2 reduction for future applications. References Adachi, K., Ohta, K., 1994. Reduction of carbon dioxide to hydrocarbon using copper loaded Titanium dioxide. Sol. Energy 53, 187–190. Anpo, M., Chiba, K., 1992. Photo catalytic reduction of CO2 on anchored titanium oxide catalysts. J. Mol. Catal. 74, 207–212. Aurian-Blajeni, B., Halmann, M., Manassen, J., 1980. Photoreduction of carbon dioxide and water into formaldehyde and methanol on semiconductor materials. Sol. Energy 25, 165–170. Barton, E.E., Rampulla, D.M., Bocarsly, A.B., 2008. Selective solardriven reduction of CO2 to methanol using a catalyzed p-Gap based photo electrochemical cell. J. Am. Chem. Soc. Commun. 130, 6342– 6344. Blakemore, J.D., Gray, H.B., Winkler, J.R., Mu, A.M., 2013. Co3O4 nanoparticle water-oxidation catalysts made by pulsed-laser ablation in liquids. ACS Catal. 3, 2497–2500.
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