Dual‐Role of Polyelectrolyte‐Tethered Benzimidazolium Cation in Promoting CO 2 /Pure Water Co‐Electrolysis to Ethylene

Abstract Electrochemical CO 2 reduction reaction (CO 2 RR), as a promising route to realize negative carbon emissions, is known to be strongly affected by electrolyte cations (i.e., cation effect). In contrast to the widely‐studied alkali cations in liquid electrolytes, the effect of organic cations grafted on alkaline polyelectrolytes (APE) remains unexplored, although APE has already become an essential component of CO 2 electrolyzers. Herein, by studying the organic cation effect on CO 2 RR, we find that benzimidazolium cation (Beim + ) significantly outperforms other commonly‐used nitrogenous cations (R 4 N + ) in promoting C 2+ (mainly C 2 H 4 ) production over copper electrode. Cyclic voltammetry and in situ spectroscopy studies reveal that the Beim + can synergistically boost the CO 2 to *CO conversion and reduce the proton supply at the electrocatalytic interface, thus facilitating the *CO dimerization toward C 2+ formation. By utilizing the homemade APE ionomer, we further realize efficient C 2 H 4 production at an industrial‐scale current density of 331 mA cm −2 from CO 2 /pure water co‐electrolysis, thanks to the dual‐role of Beim + in synergistic catalysis and ionic conduction. This study provides a new avenue to boost CO 2 RR through the structural design of polyelectrolytes.