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