Electrochemical Reduction of CO2 to Methane at the Cu Electrode in Methanol with Sodium Supporting Salts and Its Comparison with Other Alkaline Salts

The electrochemical reduction of CO2 at the Cu electrode was investigated in methanol-based electrolyte using various sodium supporting salts, such as acetate, chloride, bromide, iodide, thiocyanate, and perchlorate, at a low temperature (243 K). The main products from CO2 by the electrochemical reduction were methane, ethylene, carbon monoxide, and formic acid. The formation of methane from CO2 predominated in all sodium supporting salts, and the efficiencies were relatively high. A maximum faradaic efficiency of methane was 70.5% in NaClO4/methanol-based electrolyte at −3.0 V versus Ag/AgCl-saturated KCl. In the sodium salts/methanol-based electrolyte system, except for the case of acetate, the efficiency of hydrogen formation, being a competitive reaction against CO2 reduction, was depressed below 18%. Especially, the efficiency was below 1% when sodium thiocyanate salt was used as a supporting salt. On the basis of this work, the high-efficiency electrochemical CO2−methane conversion method is achieved.