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Improvement of Electrochemical Reduction of CO2 Using Received: March 17, 2020 Electrochemistry Accepted: June 21, 2020 Published online: August 18, 2020 The Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.20-00037 Article Electrochemistry, 88(5), 451–456 (2020) Improvement of Electrochemical Reduction of CO2 Using the Potential-Pulse Polarization Method Toshi OGUMAa,* and Kazuhisa AZUMIb a Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kitaku, Sapporo, Hokkaido 060-8628, Japan b Graduate School of Engineering, Hokkaido University, Kita 13, Nishi 8, Kitaku, Sapporo, Hokkaido 060-8628, Japan * Corresponding author: [email protected] ABSTRACT Electrochemical conversion of CO2 gas emitted to the atmosphere to useful chemicals has been expected to suppress the global greenhouse effect and to conserve the natural resources. For the electrochemical reduction of CO2 on an Ag electrode, the effect of the −3 addition of 1-ethyl-3-methylimidazolium ethyl sulfate (EMISE) ionic liquid to the aqueous solution of 0.1 mol dm K2CO3, and the effect of the polarization methods, i.e., the potentiostatic polarization and the potential-pulse polarization, on the reduction efficiency were investigated. From the gas chromatography measurement, CO as the main product of CO2 reduction and H2 as a by-product of water decomposition were obtained. Faraday efficiency of CO formation obtained by the potential-pulse polarization was considerably higher than that obtained by the potentiostatic polarization. The addition of EMISE could add further improvement in the efficiency of CO formation higher than 70%. Such improvement provided by the potential-pulse polarization method and the addition of EMISE was interpreted by the depletion and recovery of the reactants on the electrode in the pulse cycle and promotion of CO2 molecular formation − from HCO3 ions in the dense adsorption layer of EMIM cations on the electrode surface. © The Author(s) 2020. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium provided the original work is properly cited. [DOI: 10.5796/electrochemistry.20-00037]. Uploading "PDF file created by publishers" to institutional repositories or public websites is not permitted by the copyright license agreement. Keywords : Pulse Polarization, Electrochemical Reduction, Ionic Liquid, Silver Electrode 1. Introduction solvent. Recently, ionic liquids (ILs) have been used as electrolyte solutions for electrochemical processes, owing to their attractive Since the industrial revolution in the 18th century, the drastic properties such as high ionic conductivity, wide potential window, increase in the consumption of coal and oil has considerably high thermal stability, and negligible volatility. Some ILs also has increased the concentration of carbon dioxide in the atmosphere as the ability of CO2 absorption. For example, 1-butyl-3-methylimida- 400 ppm in 2016 and recognized nowadays as the dominant causes zolium hexafluorophosphate (BMIMPF6) can absorb CO2 molecular 1 14 of temperature rise. The reduction of CO2 gas emission is, physically with a molar fraction of 0.6 at a pressure of 8 MPa and therefore, the highest-priority issue from the sustainable develop- 1-ethyl-3-methylimidazolium acetate (EMIM Ac) can absorb CO2 15 ment perspective. The CO2 capturing and storage to underground chemically with a molar fraction of 0.2. Extensive research has (CCS) have been investigated to reduce CO2 emission to the been, therefore, conducted on the electrochemical reduction of CO2 2,3 16 atmosphere. The electrochemical reduction of CO2 has also been using IL solvents. Zhao et al. investigated the electrochemical attempted because of its simple facility comprises of electrodes reduction of CO2 using BMIMPF6 at various CO2 pressure and and electrolyte, mild operation conditions under the atmospheric revealed that the Faraday efficiency of the CO formation increased 17 pressure, and room temperature. Although the electrochemical CO2 by increasing the CO2 pressure. Pardal et al. reported that the reduction by using electricity produced by fossil fuels does not Faraday efficiency of the CO and H2 formation in an EMIM match the energetic benefit, use of renewable energies is beneficial trifluoromethanesulfonate-based electrolyte solution changed with to leveling the energy consumption and suppress the CO2 emission. the Zn content of the Zn-Cu electrode. The effects of the addition of 17 Several products of CO2 reduction, such as carbon monoxide, ILs to the aqueous solution were also investigated. methane, ethylene, and methanol, were obtained respectively on the In this paper, the electrochemical reduction of CO2 in the mixed 4,5 electrode catalysts of Au, Ag, Cu, and Zn. Reduction products are electrolyte of aqueous K2CO3 solution and 1-ethyl-3-methylimida- also affected by the polarization conditions such as the electrode zolium ethyl sulfate (EMISE) was investigated. Since EMISE was potential or polarization current, the cell temperature, and the gas reported to suppress the hydrogen evolution reaction (HER),18 it is pressure.5–8 Among the molecular products, CO is useful as a quite attractive for the optimizing the electrolyte solutions for the reforming gas and a raw material for organic synthesis (C1 electrochemical CO2 reduction. The-potential pulse polarization chemistry) to produce various compounds.9,10 method was also examined because it has been used to control the Since the CO2 molecule is thermodynamically quite stable and kinetics of the electrochemical process in various electrochemical requires a considerably less-noble polarization potential for its processes such as electrolysis and plating. In the pulse polarization electrochemical reduction, a side reaction of the electrolyte applied to the electroplating of metals, the evolution of the depletion decomposition takes place simultaneously to reduce the Faraday layer of the metal ions in the electrolyte solution vicinity of the efficiency of CO2 reduction. Improvement of the Faraday efficiency substrate surface is mitigated, and thus uniform electrodeposition of CO2 reduction has been intensively investigated, such as the can be achieved. Such a strategy seems to be useful for efficient 11 suitable crystallographic orientation of the electrode surface, the electrochemical CO2 reduction, although the effect of the potential- 12 modification of the electrode surface with halides, the adjustment pulse polarization on electrochemical CO2 reduction in IL-contains of the electrode morphology13 and a choice of suitable electrolyte electrolyte has not yet been reported. 451 Electrochemistry, 88(5), 451–456 (2020) sampled against the total gas phase of 10 cm3 and supplied to the gas chromatography (GC; L.C. Science Co., model INORGA, Molecu- lar Sieve column). In this work, only CO as a product of CO2 reduction and H2 as a by-product of water decomposition were detected. The Faraday efficiencies of CO and H2, fCO and fH2 [%], were calculated from the amount of the product obtained by the GC and the net cathodic charge Q from the equations; fH2 ¼ 100 nH2zF=Q ð3Þ fCO ¼ 100 nCOzF=Q ð4Þ where n [mol] is the amount of each product, z is the valency of reaction, and F is the Faraday constant. All experiments were performed a few times to confirm reproducibility. The solubility of CO2 in a mixed solution of water and EMISE 3 was roughly estimated as follows. First, 15 cm CO2 gas was sucked into a 20 cm3 syringe, then 5 cm3 of a degassed solution was added Figure 1. Schematic diagram of two compartment electrochemical to the syringe. The syringe was shaken several times and left for 30 cell. minutes at 293 K. From the volume loss of the CO2 gas phase in the syringe, ¦VCO2, caused by the dissolution in the solution phase, the solubility of dissolved CO2 was calculated as, 2. Experimental sCO2 ¼ 200PÁVCO2=RT ð5Þ Figure 1 shows a two compartments electrochemical cell made where P is the atmospheric pressure, R is the gas constant, T is the by machining from a poly(chlorotrifluoroethylene) (Daiflonμ) block experimental temperature (293 K), with the ideal gas approximation in which the anodic and the cathodic reactions were separated by a and the assumption that the partial pressure of water in the gas phase TM 19 cation exchange membrane (Nafion 117). Potassium carbonate was negligible compared to the CO2 gas. (>99.5%, Junsei Chemical Co., Ltd.) and 1-ethyl-3-methyl- imidazolium ethyl sulfate (EMISE) (>95.0%, Sigma-Aldrich, Inc.) 3. Results and Discussion were used to prepare the aqueous catholyte of 0.1 mol dm¹3 of ¹3 K2CO3 + x mol dm of EMISE (x = 0, 1.0). The anolyte was an Time-transition of the current density i of Ag electrode during the ¹3 ¹3 aqueous solution of 0.01 vol% of H2SO4 (>95.0%, Junsei Chemical potentiostatic polarization in 0.1 mol dm K2CO3 + x mol dm Co., Ltd.). The EMISE was vacuum dried at 105 °C for 24 h before EMISE (x = 0, 1) solution is shown in Fig. 2(a). The current the experiment. The solution in the cathode compartment was curves show the initial drop and a subsequent steady current at all deaerated by bubbling with the Ar gas (>99.99 vol%)orCO2 gas potentials. Figure 2(b) shows the averaged current density iav at the 3 ¹1 (>99.5 vol%) with a flow rate of 0.1 dm min for more than 1 h. three polarization potentials. This result shows that the iav increased The pH of the catholyte after CO2 gas saturation was ca. 7.1. with lowering the polarization potential, and the addition of EMISE A working electrode of Ag plate (Ag-WE, >99.98%, Nilaco Co., (x = 0) caused an increase in iav at ¹1.4 V and ¹1.5 V but not at Ltd.), a counter electrode of Pt plate (>99.98%, Nilaco Co., Ltd.), ¹1.6 V. and an Ag/AgCl (saturated KCl) reference electrode were used for Figure 3(a) shows the time-transition of the current density i in the three electrodes system.
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