REDUCTION OF ADSORBED CO ON A NI ELECTRODE IN CONNECTION WITH THE ELECTROCHEMICAL REDUCTION OF CO2

Electrochemical reduction of CO2 at Ni electrodes in aqueous media yields CH4, C2H4 and C2H6, with the simultaneous formation of H-2 as reported previously. The partial current density of CO2 reduction is ca. 0.2 mA cm-2 when the electrolysis is carried out at a constant current of 5 mA cm-2. During the electroreduction of CO2, CO is formed and strongly adsorbed on the surface of the Ni electrode. The adsorbed CO prevents hydrogen evolution since CO occupies most of the surface sites on which hydrogen may proceed. The extent of prevention of hydrogen evolution by the adsorbed CO may be estimated from the current density on a CO adsorbed electrode with reference to that on a CO free electrode at a constant potential. The present paper presents a new technique for estimating the coverage of adsorbed CO and the reduction rate of adsorbed CO from the extent of prevention of hydrogen evolution and the time course of the cathodic current in constant potential measurements. The rate of reduction of adsorbed CO obtained in this manner agrees quite well with that obtained by coulometric measurements reported previously. This agreement indicates the usefulness of the new estimation technique of surface coverage of adsorbed species, and verifies the hypothesis that the adsorbed CO is an intermediate species formed in the electroreduction of CO2 to hydrocarbons and alcohols at Ni electrodes.