Product Selectivity Affected by Cationic Species in Electrochemical Reduction of CO2 and CO at a Cu Electrode

Abstract Electroreduction of CO2 and CO was studied with a copper electrode in hydrogencarbonate solutions of various cations. Product selectivity was greatly affected by cationic species as well as HCO3− concentration. H2 evolution prevailed over CO2 reduction in Li+ electrolyte, whereas CO2 reduction was favorable in Na+, K+, and Cs+ solutions. C2H4 formation became greater than CH4 with increase of the cation size, i.e. with sequence of Li+, Na+, K+, Cs+. The variation of the product selectivity caused by cationic species is rationalized by the difference of outer Helmholtz plane (OHP) potential. The OHP potential will vary with cationic species in accordance with the degree of specific adsorption. The concentration of H+ at the electrode will depend upon the OHP potential, since H+ is a charged species. This difference of pH at the electrode will lead to the variation of product selectivity.