Elucidating the Role of Bicarbonate in CO ₂ Electroreduction on Au via in Situ Sum Frequency Generation Vibrational Spectroscopy

Due to the complexity of electrochemical interface analysis and the lack of direct probing techniques, the reaction mechanism of the carbon dioxide reduction reaction (CO₂RR) on gold electrodes in aqueous solutions has remained controversial. In this study, sum frequency generation (SFG) spectroscopy was employed to investigate the specific roles of bicarbonate ions and metal cations at the gold electrode/electrolyte interface during the CO₂RR. It has revealed that bicarbonate ions (HCO₃⁻) can be directly reduced to adsorbed CO intermediates (*COad) on the electrode surface during the CO₂RR. The active sites occupied by these *COad species differ from those occupied by adsorbed CO originating from the diffusion of dissolved CO gas (dCOad). Furthermore, the cation layer formed by potassium ions at the electrode surface is found to hinder the approach of dCOad to the surface, while it does not impede the access of *COad. These findings provide detailed and direct molecular-level spectroscopic evidence for the CO₂RR mechanism, which will facilitate the development of highly efficient electrocatalysts for carbon dioxide reduction.