Dual‐Role of Polyelectrolyte‐Tethered Benzimidazolium Cation in Promoting CO2/Pure Water Co‐Electrolysis to Ethylene.

Electrochemical CO2 reduction reaction (CO2RR), 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 CO2 electrolyzers. Herein, by studying the organic cation effect on CO2RR, we find that benzimidazolium cation (Beim+) significantly outperforms other commonly‐used nitrogenous cations (R4N+) in promoting C2+ (mainly C2H4) production over copper electrode. Cyclic voltammetry and in situ spectroscopy studies reveal that the Beim+ can synergistically boost the CO2 to *CO conversion and reduce the proton supply at the electrocatalytic interface, thus facilitating the *CO dimerization toward C2+ formation. By utilizing the homemade APE ionomer, we further realize efficient C2H4 production at an industrial‐scale current density of 331 mA cm−2 from CO2/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 CO2RR through the structural design of polyelectrolytes. [ABSTRACT FROM AUTHOR]