ELECTROCHEMICAL REDUCTION OF CO2 ON A CU ELECTRODE UNDER HIGH-PRESSURE - FACTORS THAT DETERMINE THE PRODUCT SELECTIVITY

The selectivity of the electrochemical reduction products of CO2 on a Cu electrode in aqueous KHCO, solution depended remarkably on the CO2 pressure (<60 atm), stirring condition, and the current density. As the flux of CO2 to the Cu electrode surface was increased by increasing CO2 pressure and/or by stirring, the main reduction product was changed in the order of H-2 --> hydrocarbons --> CO and/or HCOOH. On the other hand, the selectivity of the reduction products was changed in the order of CO and/or HCOOH --> hydrocarbons --> H-2 by increasing the current density. From these results, it was concluded that the balance between the flux of CO2 from the bulk solution to the electrode surface and the current density determines which reduction products are produced. Hydrocarbon formation was preferential when the current density was comparable to the maximum CO2 flux under given condition. The partial current density for maximum hydrocarbon formation occurred at 375 mA cm-2 under CO2 pressure at 30 atm.