Hydrolysis of Electrolyte Cations Enhances the Electrochemical Reduction of CO2 over Ag and Cu

Electrolyte cation size is known to influence the electrochemical reduction of CO 2 over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the p K a for cation hydrolysis decreases and is sufficiently low for hydrated K +, Rb +, and Cs + to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO 2 . The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H 2 and CH 4, and an increase in Faradaic efficiencies for CO, C 2 H 4, and C 2 H 5 OH, in full agreement with experimental observations for CO 2 reduction over Ag and Cu.