Spectroscopic Investigation for Oxygen Reduction and Evolution Reactions on Carbon Electrodes in Li-O₂ Battery

The electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on two typical carbon electrodes, a glassy carbon electrode and a porous carbon electrode made of Ketjenblack, have been systematically evaluated in a DMSO-based electrolyte solution to develop a lithium-oxygen (Li-O₂) battery with a high specific energy. The galvanostatic cycling was employed to characterize the electrochemical properties on these carbon electrodes during the ORR and OER processes. The ex situ far/mid-infrared spectroscopy and in situ UV-vis absorption spectroscopy were employed to quantitatively analyze the reaction products adsorbed on the electrode surface and dissolved in solution, respectively. Our results demonstrated that lithium peroxide (Li₂O₂) is a major ORR product on the porous carbon electrode, while the lithium superoxide (LiO₂) becomes a major ORR product on a glassy carbon electrode. On the contrary, a large amount of byproducts, such as lithium carbonate (Li₂CO₃), was also observed only on the porous carbon electrode surfaces at the end of the ORR. Li₂CO₃ gradually decomposed with the increasing potential but remained on the electrode surface, depending on the upper potential limit of the OER. The accumulation of the byproducts during the ORR/OER results in electrode passivation and capacity fading. A better understanding of the ORR/OER mechanism and possible reason for poor reversibility on these electrodes has been achieved in this study.