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.