Direct Detection of the Superoxide Anion as a Stable Intermediate in the Electroreduction of Oxygen in a Non-Aqueous Electrolyte Containing Phenol as a Proton Source

The non-aqueous Li-air (O₂) battery has attracted intensive interest because it can potentially store far more energy than today′s batteries. Presently Li-O₂ batteries suffer from parasitic reactions owing to impurities, found in almost all non-aqueous electrolytes. Impurities include residual protons and protic compounds that can react with oxygen species, such as the superoxide (O₂^-), a reactive, one-electron reduction product of oxygen. To avoid the parasitic reactions, it is crucial to have a fundamental understanding of the conditions under which reactive oxygen species are generated in non-aqueous electrolytes. Herein we report an in situ spectroscopic study of oxygen reduction on gold in a dimethyl sulfoxide electrolyte containing phenol as a proton source. It is shown directly that O₂^-, not HO₂, is the first stable intermediate during the oxygen reduction process to hydrogen peroxide. The unusual stability of O₂^- is explained using density functional theory (DFT) calculations. Stable superoxide found in gold ORR: An in situ spectroscopic study of the oxygen reduction reaction (ORR) on gold in a DMSO electrolyte containing phenol as a proton source shows that the ORR can begin with 1e^- transfer to O₂. Thus O₂^-, not HO₂, is the first stable intermediate during the ORR to hydrogen peroxide. The unusual stability of O₂^- is explained using DFT calculations.