High-performance CO2 electrolysis in membrane electrode assemblies: the role of non-alkali cations.

The electrocatalytic CO2 reduction reaction (CO2RR) in membrane electrode assemblies (MEAs) represents a pivotal technology for carbon neutrality, yet its industrial deployment is severely restricted by the carbonate dilemma. Traditional alkali-cation systems (e.g., K+ and Cs+) suffer from salting deposition and carbonate crossover, which lead to gas diffusion electrode flooding and abrupt mass-transport failure. This review provides a comprehensive and timely analysis of non-alkali cationic systems, including inorganic ammonium salts, quaternary ammonium salts, and cationic polyelectrolytes, as a superior alternative for high-performance CO2 electrolysis. We systematically elucidate the multifaceted roles of cations based on their molecular level involvement: (1) indirect mediators, cations modulate local pH and exert electrostatic repulsion on H+ (protons); (2) energetic modulators, cations modify the energy of intermediates and reorganize the hydrogen-bonding network; and (3) reaction participants, cations serve as proton sources and co-catalytic species in the reaction transition state. For each category, we assess their functional role in the reaction, highlighting unique advantages and inherent limitations. Furthermore, we summarize multidimensional characterization techniques and multiscale theoretical simulation tools employed to unravel the complex kinetic processes at these interfaces. Finally, we propose a strategic roadmap for transitioning from nanoscale molecular design to macroscopic, facilitating the eventual commercialization of CO2 valorization.