Selective electrochemical reduction of CO2 to ethylene at a three-phase interface on copper(I) halide-confined Cu-mesh electrodes in acidic solutions of potassium halides

In the electrochemical reduction of CO2 on a Cu foil electrode in a neutral solution, the electrode suffers a decline in its catalytic activity for the reduction of CO2 to hydrocarbons. This is caused by the deposition of poisonous compounds such as graphitic carbon. To settle this problem, we have developed an electrolysis system in which CO2 is reduced at the three-phase (gas|liquid|solid) interface on a Cu-mesh electrode. In the present study, a Cu-mesh electrode is modified beforehand by copper(I) halides, and CO2 is reduced at a constant potential with these modified electrodes in an acidic solution of potassium halide. The Faradaic efficiency for C2H4 is considerably increased and reversely that for H2 is decreased by confining a copper(I) halide. In the case of CuBr, the conversion of CO2 to C2H4 and the hydrogen evolution are observed with Faradaic efficiencies of about 80% and 9%, respectively. Such a contribution of copper(I) halide to the CO2 reduction is related to its reversible combination with CO and C2H4. At the three-phase interface, CO2(g) is first reduced to CO(g). This gas is ready to adsorb to copper(I) halide with its π-bond perpendicular to the surface, and the CO is subjected to electron injection from the electrode to be reduced to the methylene radical. The coupling of methylene radicals results in the formation of C2H4 and this product is stabilized by adsorbing to copper(I) halide with its π-bond parallel to the surface.