Interfacial Adsorbate Competition Regulates Intermediate Stabilization and Onset Potential in Acidic CO 2 Electroreduction

Electrochemical CO2 reduction (CO2R) 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 CO2R 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 CO2R at low pH and delays CO2R 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 CO2R selectivity under acidic conditions and their impact on overpotentials, offering guidance on catalyst-electrolyte interface design.