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

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.