Proton-donating cations enable efficient and stable acidic CO2 reduction in membrane electrode assemblies

Abstract Electrochemical CO2 reduction (CO2R) in acidic membrane electrode assemblies (MEAs) represents a promising pathway for sustainable chemical production, but achieving high selectivity, low cell voltage, and long-term stability remains challenging. Current approaches using alkali cations can promote selectivity through cationic effects, but relying on H2O as a weak proton donor results in high overpotential and severe precipitation, causing elevated cell voltage and poor operational stability. Here, we introduce NH4+ as a proton-donating cation that simultaneously addresses these challenges in acidic MEAs. As a cation, it electromigrates to the catalyst surface, stabilizing *CO2 intermediates and reducing localized H+ concentration for high selectivity. As a proton donor, it provides superior proton-donating ability compared to H2O when H+ mass transport is limited, which decreases the protonation barrier and reduces CO2R overpotential on CoPc@CNT, resulting in a lower cell voltage. Furthermore, NH4+ effectively donates protons to bicarbonate, promoting its decomposition at significantly lower temperatures compared to KHCO3, thereby enabling easy removal of precipitates through mild heating and maintaining an NH3/NH4+ recirculation system for operational stability. As a result, this approach achieves an average CO2-to-CO selectivity of 86% in acidic MEAs at 100 mA cm−2 and 60°C using CoPc@CNT-NH2 catalyst, with stable performance over 110 hours at an average cell voltage of 2.84 V, corresponding to a 40.6% energy efficiency. This strategy advances acidic MEA-based CO2R toward practical implementation by simultaneously achieving high selectivity, low overpotential, and stable operation.