Cation-Mediated Proton Transfer Enhances Acidic CO2 Electrolysis on a Molecular Catalyst

Electrochemical CO2 reduction reaction (CO2RR) in acidic media is attractive for mitigating carbonate formation, yet it typically relies on alkali metal cations to promote CO2RR over the competing hydrogen evolution reaction. Here we show that CH3NH3+, an alkylammonium cation with proton-donating capability, markedly enhances acidic CO2 electrolysis on immobilized cobalt phthalocyanine (CoPc), outperforming alkali metal cations. Compared with Na+, CH3NH3+ increases the CO partial current density by ∼10-fold at modest overpotentials while maintaining ∼95% Faradaic efficiency for CO production. Combined electrochemical analyses and grand-canonical density functional theory calculations reveal a distinct cation role beyond electrostatic stabilization of *CO2: CH3NH3+ serves as an interfacial proton-transfer mediator that enables directed proton transfer to adsorbed *CO2, thereby facilitating the rate-limiting protonation step. This cation-mediated mechanism enables CO partial current densities up to 600 mA cm-2 and single-pass CO2-to-CO conversion approaching 90% in acidic media. These findings expand the conventional view of electrolyte cations in CO2RR and establish cation-enabled interfacial proton transfer as a strategy for promoting protonation-limited electrocatalysis.