Promoting CO 2 Electroreduction to Multi‐Carbon Products by Hydrophobicity‐Induced Electro‐Kinetic Retardation

Advancing the performance of the Cu‐catalyzed electrochemical CO 2 reduction reaction (CO 2 RR) is crucial for its practical applications. Still, the wettable pristine Cu surface often suffers from low exposure to CO 2 , reducing the Faradaic efficiencies (FEs) and current densities for multi‐carbon (C 2+ ) products. Recent studies have proposed that increasing surface availability for CO 2 by cation‐exchange ionomers can enhance the C 2+ product formation rates. However, due to the rapid formation and consumption of *CO, such promotion in reaction kinetics can shorten the residence of *CO whose adsorption determines C 2+ selectivity, and thus the resulting C 2+ FEs remain low. Herein, we discover that the electro‐kinetic retardation caused by the strong hydrophobicity of quaternary ammonium group‐functionalized polynorbornene ionomers can greatly prolong the *CO residence on Cu. This unconventional electro‐kinetic effect is demonstrated by the increased Tafel slopes and the decreased sensitivity of *CO coverage change to potentials. As a result, the strongly hydrophobic Cu electrodes exhibit C 2+ Faradaic efficiencies of ≈90 % at a partial current density of 223 mA cm −2 , more than twice of bare or hydrophilic Cu surfaces.