Influence of Water Content on Electrochemical CO₂ Reduction in Acetonitrile Solution on Cu Electrodes

The electrochemical reduction of CO₂ (CO₂RR) on copper electrodes in acetonitrile (MeCN) solutions offers a promising route for converting CO₂ into valuable products but competes with the hydrogen evolution reaction (HER). This study systematically explores the impact of varying water content in MeCN on the selectivity and efficiency of CO₂RR and HER. Cyclic voltammetry shows that increasing water content shifts onset potentials and Tafel slopes, indicating changes in reaction mechanisms and rate‐determining steps. In dry MeCN, CO₂RR predominates due to high CO₂ solubility and limited proton availability, but as water content increases, HER kinetics improve, eventually dominating the reaction at higher water concentrations. In situ FTIR spectroscopy and molecular dynamics simulations reveal that water preferentially adsorbs onto the copper electrode surface, enhancing stabilization of reaction intermediates and facilitating HER. These findings provide critical insights into optimizing electrochemical systems for selective CO₂ reduction by controlling water content, offering a pathway for improved electrocatalytic performance.