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

The water reduction which 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 batteries electrolytes. Nevertheless, recent studies pointed towards 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<sup>+</sup> cations is more reactive than “free” water (or non-coordinating) water molecules.