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 CO 2 reduction reaction (CRR) and CO 2 evolution reaction (CER) is the prerequisite to develop stable and efficient Li–CO 2 batteries. In this study, we investigate the redox reaction of CO 2 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 CO 2 capture and stabilize CRR intermediate anions owing to its strong solvation for Li + . We demonstrate that pure CO 2 reduction is not electrochemically active at room temperature on porous carbon electrode and its kinetics can be remarkably improved by H 2 O, which could be responsible for the discrepancy on CRR kinetics in the literature. However, in the presence of H 2 O, the formed LiOH gradually converted to Li 2 CO 3 leading to severe electrolyte degradation in charging reaction. With the assistance of O 2 in the ionic liquid -based electrolyte, we show, for the first time, CRR yields a low-valence amorphous carbon and Li 2 O 2 /Li 2 O in the Li–CO 2 batteries. The formed amorphous carbon, Li 2 O 2 and Li 2 O exhibit higher rechargeability and faster kinetics compared with Li 2 CO 3 -oxidation. These new findings provide direct evidence of CRR mechanism and insights in resolving the discrepancy on CRR kinetics in Li–CO 2 batteries. • Synchrotron-based in-situ APXPS is developed to probe the Li–CO 2 reaction chemistry in ionic liquid electrolyte. • Pure CO 2 reduction is not electrochemically active at room temperature and its kinetics can be significantly improved by H 2 O. • CO 2 reduction in ionic liquid yields amorphous carbon and Li 2 O 2 /Li 2 O which are more reversible than Li 2 CO 3 .