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

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 h 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.