Optimization of Pulsed Polarization Condition for Efficient Electrochemical CO2 Reduction

Abstract

Introduction Massive emission of CO2 gas from fossil fuel combustion causes the severe global greenhouse effect. Electrochemical reduction of CO2 using renewable energy has been attractive due to its simple methodology, while the conversion efficiency may be lessened due to undesirable side reactions such as water decomposition. In this study, CO2 was covered to CO on Ag electrode in aqueous KHCO3 solution using potentiostatic pulse polarization, and optimal pulse condition to improve the conversion efficiency was explored. Experiment An electrochemical cell was composed of a cathode compartment containing an Ag working electrode (W.E.) and an Ag/AgCl reference electrode and an anode compartment containing a Pt counter electrode. Two compartments were separated by a Nafion membrane. An Ag-W.E. was mechanically polished and potentiostatically pulse-polarized in a CO2-saturated 0.1 M KHCO3 solution to form CO. After some polarization period, the conversion efficiency ( η=[CO]/([CO]+[H2]) ) of CO2 to CO was evaluated by using gas chromatography as a function of various parameters such as cathodic polarization potential (E c) and time (t c), rest potential (E a) and time (t a), KHNO3 concentration, etc. Result and discussion When E c was varied, the highest η was obtained around –1.5 to –1.6 V. When t c was varied at E c = –1.6 V, the highest η was obtained around t c = 3 s, as shown in Fig. 1. The t a did not affect η. These results reveal that the t c influences the conversion efficiency viacontrolling the diffusion layer thickness of reactants such as CO2 molecular. The highest η at the optimal pulse condition was ca. 0.9 and is comparable to the value obtained by the previous work using ionic liquid solution (T. Oguma et al., Electrochemistry, 88 (2020) 451–456). Fig. 1. Relationship between the cathodic polarization time t c and reduction efficiency Figure 1