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

Understanding the reactive site/CO 2 /electrolyte interfacial behaviors is very crucial for the design of an advantageous CO 2 electrocatalytic reduction (CO 2 ER) system. One important but unrevealed question is how the CO 2 ER process is influenced by the high concentration of HCO 3 –, which is deliberately added as electrolyte or from the inevitable reaction between dissolved CO 2 and OH – . Here, we provide unambiguous in situ spectroscopic evidence that on Ag-based catalysts, HCO 3 – is apt to facilitate *OCO – generation and therefore rebalances CO 2 ER pathways. By employing an alternative acid electrolyte to restrict the exchange between CO 2 and HCO 3 – and eliminating the effect of solution pH, we reveal that HCO 3 – can decrease the onset potential of *OCO – and promote further formate production. Theoretical calculations indicate HCO 3 – can stabilize the adsorption of *OCO – instead of *COOH. The renewed understanding of the role of HCO 3 – could facilitate the judicious selection of electrolytes to regulate the CO 2 ER pathway and product distribution.