(Invited) Elucidating the Cation Effect in CO2 Electroreduction Using Non-Metal Cations

Electrochemical reduction of CO2 may enable a promising route for the recycling of CO2 and the sustainable production of chemicals and fuels. Due to the complex electrochemical environment, a deeper understanding of the reaction mechanism, particularly the role of cations in the process, is still needed. It was recently demonstrated that metal cations are essential for the CO2 reduction to CO, by stabilizing the CO2 – intermediate via electrostatic interaction. Here we investigate the cation effect in CO2 reduction using non-metal cations, particularly quaternary ammonium cations, whose size and shape symmetry can be tuned. We select CO2 reduction on Bi catalyst that produces both CO and formate, thus to reveal and compare the effect of cations on the two reaction pathways. Interestingly, the Faradaic efficiency for CO production increased apparently for the electrolyte with larger cations, while the formate Faradaic efficiency showed a weak dependence on the cation size, indicating a reaction-pathway-dependent effect of the cations. We further compared cations with different symmetries and observed a more profound effect of substituted ammonium cations on the CO2 reduction activity and selectivity, which was attributed to the charge distribution and weak hydration shells of the cations that help stabilize reaction intermediates. Our work elucidates the critical role of cations in the CO2 electroreduction catalysis and suggests a method to improve the electrocatalysis with optimized electrolytes.