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.