Competitive Carbonate Binding Hinders Electrochemical CO₂ Reduction to CO on Cu Surfaces at Low Overpotentials

The electrochemical reduction of CO₂ to useful chemicals holds promise for a sustainable carbon cycle. However, the key factors that control the pathways to various desired products remain unresolved, partially due to the limited knowledge of reaction intermediates on the electrode surface. To address this, we utilize in situ electrochemical shell-isolated nanoparticle-enhanced Raman spectroscopy in combination with density functional theory calculation to examine the potential-dependent composition of adsorbed species during CO₂ reduction on polycrystalline copper. The results reveal that carbonate anion adsorption outcompetes other carbon-containing species, including adsorbed CO₂ activation intermediate *COO^- and *CO, which has the effect of anodically shifting the onset potential of *CO formation in electrolyte solutions with a lower carbonate concentration. These results suggest that the competitive binding of carbonate impedes the reduction of CO₂ on the Cu surface. Monte Carlo simulations show that both potential dependent electrode surface change and electrode-carbonate Coulomb interaction are key to understanding the competitive binding process. Our findings suggest that reducing the competitive binding of carbonate may be a promising route to improve the CO₂ reduction on Cu electrodes at low overpotentials.