Hydrolysis of Electrolyte Cations Enhances the Electrochemical Reduction of CO_2 over Ag and Cu

摘要

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 pK_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_2H_4, and C_2H_5OH, in full agreement with experimental observations for CO_2 reduction over Ag and Cu.