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

The influence of CO₂partial pressure on electrochemical reduction of CO₂using 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 CO₂partial 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 Vvs.RHE. Such optimum in ethylene FE, ranging from ∼10-45%, is obtained at a variety of applied voltages (−0.7 to −1.1 Vvs.RHE), but only at relatively low concentrations of KHCO₃of less than 0.25 M. Since a low KHCO₃concentration induces only a low buffer capacity, we conclude that a rise of local pH induced by a decreased CO₂partial pressure explains improved selectivity towards ethylene. If the CO₂partial pressure decreases below ∼0.4 atm, not only the availability of CO₂limits 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 CO₂and 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 CO₂electroreduction products, and suggest high ethylene selectivity over oxide derived Cu electrodes can be obtained for diluted CO₂feed compositions if the electrolyte has a relatively low buffer capacity.