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

Abstract Electrochemical CO 2 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 CO 2 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 CO 2 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 cm 2 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 CO 2 electrolysis.