Effect of partial pressure on product selectivity in Cu-catalyzed electrochemical reduction of CO2

The influence of CO(2)partial pressure on electrochemical reduction of CO(2)using oxide-derived electrodeposited copper surfaces in a conventional two compartment cell configuration, is discussed. Contrary to what has been reported in the literature for polished copper surfaces, demonstrating a linear decrease in the faradaic efficiency (FE) as a function of decreasing partial pressure, the (FE) and partial current density of both ethylene and methane are improved when the CO(2)partial pressure is decreased below 1 atm, and an optimized ethylene efficiency of similar to 45% is achieved in the range of similar to 0.4-similar to 0.6 atm at -1.1 Vvs.RHE. Such optimum in ethylene FE, ranging from similar to 10-45%, is obtained at a variety of applied voltages (-0.7 to -1.1 Vvs.RHE), but only at relatively low concentrations of KHCO(3)of less than 0.25 M. Since a low KHCO(3)concentration induces only a low buffer capacity, we conclude that a rise of local pH induced by a decreased CO(2)partial pressure explains improved selectivity towards ethylene. If the CO(2)partial pressure decreases below similar to 0.4 atm, not only the availability of CO(2)limits ethylene selectivity, but also a fall in local pH, associated with the decreasing partial current density in formation of ethylene. Calculations of local concentrations of CO(2)and the pH corroborate these hypotheses. These findings contribute to, and substantiate the current understanding of the significant role of local pH conditions on the selectivity of CO(2)electroreduction products, and suggest high ethylene selectivity over oxide derived Cu electrodes can be obtained for diluted CO(2)feed compositions if the electrolyte has a relatively low buffer capacity.