Effect of bicarbonate on CO2 electroreduction over cathode catalysts

Abstract

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.