K-O₂ electrochemistry at the Au/DMSO interface probed by in situ spectroscopy and theoretical calculations

The reaction mechanism underpinning the operation of K-O₂ batteries, particularly the O₂ reactions at the positive electrode, is still not completely understood. In this work, by combining in situ Raman spectroelectrochemistry and density functional theory calculations, we report on a fundamental study of K-O₂ electrochemistry at a model interface of Au electrode/DMSO electrolyte. The key products and intermediates (O₂^-, KO₂ and K₂O₂) are identified and their dependency on the electrode potential is revealed. At high potentials, the first reduction intermediate of O₂^-* radical anions (* denotes the adsorbed state) can desorb from the Au electrode surface and combine with K⁺ cations in the electrolyte producing KO₂via a solution-mediated pathway. At low potentials, O₂ can be directly reduced to on the Au electrode surface, which can be further reduced to at extremely low potentials. The fact that K₂O₂ has only been detected in the very high overpotential regime indicates a lack of KO₂ disproportionation reaction both on the Au electrode surface and in the electrolyte solution. This work addresses the fundamental mechanism and origin of the high reversibility of the aprotic K-O₂ batteries.