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.
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