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 CO₂ 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 CO₂. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H₂ and CH₄, and an increase in Faradaic efficiencies for CO, C₂H₄, and C₂H₅OH, in full agreement with experimental observations for CO₂ reduction over Ag and Cu.