Overcoming Salt Precipitation in CO2 Reduction Membrane Electrode Assembly Electrolyzer Enables Over 4200 Hours Stable Operation.

Alkali metal cations (AMCs) are widely recognized as critical promoters of CO2 electroreduction. Yet, salt precipitation severely undermines membrane electrode assembly (MEA) electrolyzer stability, while limited mechanistic understanding of how the interfacial cation is sustained hinders efforts to address this challenge. Here, we reveal that sustained AMC-enabled CO2 reduction in MEA systems relies on a dynamic interfacial cation replenishment mechanism, in which continuous cation transport from the anolyte to the cathode compensates for the rapid depletion of interfacial cations, a process particularly pronounced in CEM-MEA systems. Guided by these insights, we demonstrate that (CH3)4N+ can serve as an effective alternative to AMCs, achieving CO2RR performance comparable to that of Cs+ in MEA systems. Crucially, the high solubility and strong hygroscopicity of (CH3)4NHCO3, together with the reduced crossover of (CH3)4N+, effectively suppress salt precipitation. Integrating this precipitation-free cation system with a continuous cation recovery unit enabled long-term CO2 electrolysis: an AEM-MEA operated continuously for over 4200 h without disassembly at 100 mA cm-2, with the CO Faradaic efficiency (FECO) maintained above 90%, and a CEM-MEA operated intermittently for a cumulative 380 h at 50 mA cm-2 with FECO above 80%. These results open a promising route toward durable, precipitation-free CO2 electrolysis.