Electrochemical reduction of CO2 in ionic liquid: Mechanistic study of Li–CO2 batteries via in situ ambient pressure X-ray photoelectron spectroscopy

Abstract In-depth mechanistic understanding of CO2 reduction reaction (CRR) and CO2 evolution reaction (CER) is the prerequisite to develop stable and efficient Li–CO2 batteries. In this study, we investigate the redox reaction of CO2 on the porous carbon surface in ionic liquid electrolyte via synchrotron-based in situ ambient pressure X-ray photoelectron spectroscopy (APXPS) technique. We used ionic liquid to facilitate CO2 capture and stabilize CRR intermediate anions owing to its strong solvation for Li+. We demonstrate that pure CO2 reduction is not electrochemically active at room temperature on porous carbon electrode and its kinetics can be remarkably improved by H2O, which could be responsible for the discrepancy on CRR kinetics in the literature. However, in the presence of H2O, the formed LiOH gradually converted to Li2CO3 leading to severe electrolyte degradation in charging reaction. With the assistance of O2 in the ionic liquid-based electrolyte, we show, for the first time, CRR yields a low-valence amorphous carbon and Li2O2/Li2O in the Li–CO2 batteries. The formed amorphous carbon, Li2O2 and Li2O exhibit higher rechargeability and faster kinetics compared with Li2CO3-oxidation. These new findings provide direct evidence of CRR mechanism and insights in resolving the discrepancy on CRR kinetics in Li–CO2 batteries.