(79a) How Do Electrolyte Cations Affect Activity and Selectivity of the Electrochemical Reduction of CO2 over Ag and Cu?

The electrochemical conversion of water and CO₂ to renewable fuels offers a sustainable route to support future energy demands and store intermittent solar energy. The efficiency of such conversion over metals is strongly affected by electrolyte cations. Previous attempts to explain this phenomenon have not been successful. We show here that cation size affects the activity and selectivity of Ag and Cu for CO₂ reduction, and we propose that the observed effects can be attributed to hydrolysis of hydrated cations in the immediate vicinity of the cathode. With increasing cation size, the pKa for hydrolysis decreases and is sufficiently low for K⁺ Rb⁺, and Cs⁺ for these hydrated cations 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. The present study further reveals that hydrolysis of hydrated cations can be used to increase the activity and selectivity of any proton-transfer reaction.