Bicarbonate Rebalances the *COOH/*OCO– Dual Pathways in CO2 Electrocatalytic Reduction: In Situ Surface-Enhanced Raman Spectroscopic Evidence

Understanding the reactive site/CO2/electrolyte interfacial behaviors is very crucial for the design of an advantageous CO2 electrocatalytic reduction (CO2ER) system. One important but unrevealed question is how the CO2ER process is influenced by the high concentration of HCO3-, which is deliberately added as electrolyte or from the inevitable reaction between dissolved CO2 and OH-. Here, we provide unambiguous in situ spectroscopic evidence that on Ag-based catalysts, HCO3- is apt to facilitate *OCO- generation and therefore rebalances CO2ER pathways. By employing an alternative acid electrolyte to restrict the exchange between CO2 and HCO3- and eliminating the effect of solution pH, we reveal that HCO3- can decrease the onset potential of *OCO- and promote further formate production. Theoretical calculations indicate HCO3- can stabilize the adsorption of *OCO- instead of *COOH. The renewed understanding of the role of HCO3- could facilitate the judicious selection of electrolytes to regulate the CO2ER pathway and product distribution.