Understanding of the Electrogenerated Bulk Electrolyte Species in Sodium-Containing Ionic Liquid Electrolytes during the Oxygen Reduction Reaction

An understanding of the species generated in the bulk ionic liquid electrolyte in the presence of superoxide anion, O₂^•-, is of interest due to its close relationship to the nature of the electrode reduction products. Unlike conventional organic solvents, ionic liquids are composed entirely of ions, thereby requiring an understanding of the intermediates generated in the bulk electrolyte. The generation of a complex species, [O₂^•-][C₄mpyr⁺]_n [Na⁺]_m, is envisioned in the bulk sodium cation pyrrolidinium-based ionic liquid with a composition depending on the Na⁺ concentration. In this work, the superoxide anion, O₂^•-, has been considered in theoretical calculations regarding the oxygen reduction reaction in order to determine its average coordination number and also its dynamics in these mixtures. Most interestingly, the final reduction product can be tuned depending on the Na⁺ concentration, whereby a limited supply of Na⁺ favors the superoxide product while a sufficient excess of Na⁺ leads to the formation of the peroxide product. These findings have been identified using a pressure cell and corroborated by rotating ring-disk electrode measurements. Thus, the preferential generation of Na₂O₂ over NaO₂ could drastically improve the specific energy of the Na-air battery due to a higher number of electrons exchanged.