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

The influence of CO2 partial pressure on electrochemical reduction of CO2 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 CO2 partial pressure is decreased below 1 atm, and an optimized ethylene efficiency of ∼45% is achieved in the range of ∼0.4–∼0.6 atm at −1.1 V vs. RHE. Such optimum in ethylene FE, ranging from ∼10–45%, is obtained at a variety of applied voltages (−0.7 to −1.1 V vs. RHE), but only at relatively low concentrations of KHCO3 of less than 0.25 M. Since a low KHCO3 concentration induces only a low buffer capacity, we conclude that a rise of local pH induced by a decreased CO2 partial pressure explains improved selectivity towards ethylene. If the CO2 partial pressure decreases below ∼0.4 atm, not only the availability of CO2 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 CO2 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 CO2 electroreduction products, and suggest high ethylene selectivity over oxide derived Cu electrodes can be obtained for diluted CO2 feed compositions if the electrolyte has a relatively low buffer capacity.