The Fate of Water at the Electrochemical Interfaces: Electrochemical Behavior of Free Water Versus Coordinating Water.

The water reduction that produces hydrogen is one key reaction for electrochemical energy storage. While it has been widely studied in traditional aqueous electrolytes for water splitting (electrolyzers), it also plays an important role for batteries. Indeed, the reduction of water at relatively high potential prevents the practical realization of high-voltage aqueous batteries, while water contamination is detrimental for organic battery electrolytes. Nevertheless, recent studies pointed toward the positive effect of traces of water for Li-air batteries as well as for the formation of solid-electrolyte interphase. Herein, we provide a detailed understanding of the role of the solvation on water reduction reaction in organic electrolytes. Using electrochemistry, classical molecular dynamics simulations, and nuclear magnetic resonance spectroscopy, we were able to demonstrate that (1) the hydrophilicity/hydrophobicity of the species inside the electrochemical double layer directly controls the reduction of water and (2) water-coordinating strong Lewis acids such as Li cation are more reactive than free water (or noncoordinating) water molecules.