Cation Effect‐Engineered Electrocatalytic Interfaces Boost Pure‐Water CO ₂ Electrolysis with Optimized Ion Dynamics

Electrochemical CO₂ reduction in zero-gap electrolyzers using pure water provides a sustainable pathway for fuel synthesis, avoiding salt precipitation-induced stability degradation. However, inefficient ion transport limits the efficiency and scalability for CO₂ electroreduction under such conditions. Here, we address the limitation through cation effect-engineered electrocatalytic interfaces that optimize hydroxide ion dynamics. By integrating a quaternary ammonia poly(N-methyl-piperidine-co-p-terphenyl) (QAPPT) structural layer with a cation-engineered poly(N-methyl-piperidine-co-biphenyl) (QAPPB) overlayer, we construct a double-layer membrane (DLM) with optimized ion-conduction pathways. The QAPPB overlayer, with ultrahigh ion-exchange capacity, enhances CO₂ reduction selectivity by accelerating hydroxide mobility under high current densities. This architecture achieves 93% Faradaic efficiency for CO production at 500 mA cm^-2 in pure water, sustaining stable operation for over 100 h. Scaling the system to a 100 cm² electrolyzer achieves a CO production rate of 344 mL min^-1 at 50 A, highlighting system-level robustness. By coupling cationic polymer design with ion-transport kinetics, our work advances electrocatalytic interfaces for efficient pure-water CO₂ electrolysis.