Effect of bicarbonate on CO2 electroreduction over cathode catalysts

CO2 electroreduction (CO2 ER) using renewable energy is ideal for mitigating the greenhouse effect and closing the carbon cycle. Bicarbonate (HCO3−) is most commonly employed as the electrolyte anion because it is known to facilitate CO2 ER. However, its dynamics in the electric double layer remains obscure and requires more in-depth investigation. Herein, we investigate the refined reduction process of bicarbonate by employing in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy. By comparing the product distributions in Ar-saturated KCl and KHCO3 electrolytes, we confirmed CO production from HCO3− in the absence of an external CO2 source. Notably, in contrast to an electric compulsion, negatively charged HCO3− anions were found to accumulate near the electrode surface. A reduction mechanism of HCO3− is proposed in that HCO3− is not adsorbed over a catalyst, but may be enriched near the electrode surface and converted to CO2 and react over Au and Cu electrodes. The dependence of the CO2 ER activity on the local HCO3− concentration was subsequently discovered, which was in turn dependent on the bulk HCO3− concentration and cathodic potential. In particular, the local HCO3− concentration was limited by the cathodic potential, leading to a plateau in the CO2 ER activity. The proposed mechanism provides insights into the interaction between the catalyst and the electrolyte in CO2 ER.