Non-aqueous metal-air batteries have generated strong research interest because they possess much greater theoretical gravimetric energy storage density than today’s rechargeable battery technologies.^1-4 This has re-kindled research into the fundamentals of oxygen reduction and evolution reactions (ORR and OER) in non-aqueous electrolytes because they are the central electrochemical processes at metal-air cathodes. Understanding the reaction mechanisms is critical to improving cycling efficiency and making rechargeable metal-air batteries a reality.

We will present the results of a series of electrochemical studies of the ORR/OER on a gold electrode in strictly dry, non-aqueous dimethyl sulfoxide (DMSO) electrolytes containing different cations, including H⁺, Li⁺, and Na⁺,^5,6 using in situ spectroelectrochemistry and first-principles calculations and theoretical modeling. Significantly the ORRs share a common characteristic, namely the superoxide anion (O₂^-), the one-electron reduction product of O₂, is the first O₂ reduction product to form. It has several important consequences: O₂^- thus controls the onset potential of the ORR, and its solubility in the electrolyte makes solution-phase reactions involving O₂^- an integral part of the electrode processes at low overpotentials, including the formation of superoxides and peroxides (via disproportionation of the superoxides) away from the cathode. On the other hand, there is a clear contrast between Li⁺ and Na⁺: Whereas the main reduction product is Li₂O₂ for Li⁺, which requires a high overpotential to be oxidized, the reduction product for Na⁺ is predominantly NaO₂, whose high solubility in DMSO alleviates solid buildup on the electrode and also facilitates the oxidation of NaO₂. Consequently the Na⁺ O₂ cathode is much more reversible than its Li⁺ counterpart. The energetic factors controlling the formation of O₂^-, solution vs. surface reaction pathways, and the lack of Na₂O₂ formation will be discussed. Overall, our studies reveal that the ORR/OER mechanisms depend intimately on the cation, potential, and electrolyte, and contribute new mechanistic understanding to the eventual realization of reversible metal-air battery technologies.

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6. Y. Zhang, X. Zhang, J. Wang, W.C. McKee, Y. Xu, Z. Peng, J. Phys. Chem. C 2016, 120, 3690.