What It Takes for Imidazolium Cations to Promote Electrochemical Reduction of CO(2)

[Image: see text] Imidazolium cations enhance the performance of several electrodes in converting CO(2) to CO in non-aqueous media. In this publication, we elucidate the origin of the function of imidazolium cations when exposed to Au electrodes in anhydrous acetonitrile in CO(2) atmosphere. We demonstrate that imidazolium cations lead to unprecedentedly low overpotentials for CO(2) reduction to CO on Au, with ∼100% Faradaic efficiency. By modification of the N(1) and N(3) functionality of the imidazolium cation, we show a direct correlation between the performance in CO(2) reduction and the C(2)–H acidity of the cation. Based on NMR analyses, DFT calculations, and isotopic labeling, showing an inverse kinetic isotope effect, we demonstrate that the mechanism involves a concerted proton–electron transfer to the electrode-adsorbed CO(2) intermediate. The demonstrated mechanism provides guidelines for improvement in the energy efficiency of non-aqueous electrochemical CO(2) reduction, by a tailored design of electrolyte cations.