Abstract
Electrochemical CO(2) reduction (CO(2)R) in acid may enable high carbon utilization but faces selectivity challenges, particularly from the Hydrogen Evolution Reaction (HER). While the source of protons and cation concentrations play a role in this balance, the role of anions remains underexplored. Here, we combine in situ surface-enhanced Raman Spectroscopy during CO(2)R in acid with theoretical simulations to investigate the role of anionic species over copper gas diffusion electrodes at application-relevant current densities (up to 0.2 A·cm(-2)) and performance. Our observations reveal that sulfate adsorption inhibits CO(2)R at low pH and delays CO(2)R intermediate formation, which is enabled by hydroxyl species coadsorption. Such competition regulates *CO stabilization and the balanced *CO coverage needed to favor the formation of multicarbon products. These results shed light on how anion interactions govern CO(2)R selectivity under acidic conditions and their impact on overpotentials, offering guidance on catalyst-electrolyte interface design.