Electrochemical Reduction of Aqueous Imidazolium on Pt(111) by Proton Coupled Electron Transfer

Recent electrochemical studies have reported aqueous CO₂ reduction to formic acid, formaldehyde and methanol at potentials of ca. -600 mV versus SCE, when using a Pt working electrode in acidic pyridine solutions. In those experiments, pyridinium is thought to function as a one-electron shuttle for the underlying multielectron reduction of CO₂. DFT studies proposed that the critical step of the underlying reaction mechanism is the one-electron reduction of pyridinium at the Pt surface through proton coupled electron transfer. Such reaction forms a H adsorbate that is subsequently transferred to CO₂ as a hydride, through a proton coupled hydride transfer mechanism where pyridinium functions as a Brønsted acid. Here, we find that imidazolium exhibits an electrochemical behavior analogous to pyridinium, as characterized by the experimental and theoretical analysis of the initial reduction on Pt. A cathodic wave, with a cyclic voltammetric half wave potential of ca. -680 mV versus SCE, is consistent with the theoretical prediction based on the recently proposed reaction mechanism suggesting that positively charged Brønsted acids could serve as electrocatalytic one-electron shuttle species for multielectron CO₂ reduction.