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

Abstract

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.

Formation of Solid–Electrolyte Interfaces in Aqueous Electrolytes by Altering Cation‐Solvation Shell Structure

“…Organic molecules with strong polarities often contain the elements O, N, or F, whose lone pairs of electrons can be coordinated with Li + , [ 18 ] while organic molecules must have a Brønsted acidity much lower than that of water to form dense SEI layers. Based on the proton theory of acids and bases, the Brønsted acidity of organic compounds should be much lower than that of water in the case of hydrogen evolution, [ 19 ] as if organic molecules with reactive hydrogen atoms coordinate to Li + , both water and organic molecules will contribute to hydrogen evolution at the anode. [ 20 ] After screening potential organic molecules, we finally chose urea, DMSO, and DMF molecules for use in the Li + solvation shell for three reasons: 1) they are strongly polar aprotic solvents, which are miscible with water and have good dissolution ability for salts; 2) their low proton activities intrinsically help to prevent hydrogen evolution; and 3) they contain functional groups that can take part in electrochemical reactions.…”

“…Therefore, the hydrogen evolution reaction (HER, H 2 O + e − = 1/2 H 2 + OH − ) is more likely to take place during battery operation. [ 19 ] By adding urea to the LiClO 4 solutions, we prepared three electrolytes of different concentrations with LiClO 4 ‐H 2 O‐urea ratio of 1‐5‐2, 1‐4‐2, and 1‐3‐2 (referred to as 1‐5‐2 electrolyte, 1‐4‐2 electrolyte, and 1‐3‐2 electrolyte, respectively). On addition of urea, the electrolyte pH values steadily increase, from pH = 4.6 in the normal 5 m LiClO 4 solution to almost pH = 6.8 in the 1‐3‐2 electrolyte, where more neutral solutions suppress hydrolysis more effectively.…”

“…Water decomposition is a two‐step reaction, with the first step involving the reduction of protons, determined by the pH of electrolyte, and the second event being the splitting of water molecules, characterized by a greater overpotential as a result of the energy penalty required to break the OH bonds of water molecules. [ 19 ] Strong coordination of water to Li + , results in very low reaction activity; [ 4 ] therefore, hydrogen evolution potentials shift down as the electrolyte concentration is increased.…”

Controlling palladium morphology in electrodeposition from nanoparticles to dendritesviathe use of mixed solvents

“…33 For example, in batteries it has already been shown that by moving to mixed-solvents, metal dendrite formation can be suppressed 34 and in electrocatalysis, water molecule reactivity can be increased through watersolvent co-ordination bonding. 35 Furthermore, many of the studies that work in a non-aqueous solvent system, be it organic, ionic liquid or deep eutectic solvents are in reality highly likely to be mixed solvent due to the uptake of trace water, which can be difficult to exclude completely. 36 In this work we investigate the electrodeposition of Pd, from its metallo-organic salt (Pd-acetate) in miscible organicaqueous solvents; here water and acetonitrile (MeCN), under organic rich (90% : 10% MeCN : water v/v), water rich (10% : 90% MeCN : water v/v) and equal contributions of both solvent by volume (50% : 50% v/v), conditions.…”

Water-in-Bisalt Electrolyte with Record Salt Concentration and Widened Electrochemical Stability Window

“…4 Very recent studies, however, suggest that the strong Lewis acidity of Li + promotes the hydrogen evolution reaction (HER) at the anode, especially at high salt concentrations. 5 If the high salt concentrations of WiSEs and WiBS electrolytes do not intrinsically hamper the HER, more attention must be given to the SEI formation and its influence in the overall ESW. Indeed, several homogeneous 6 and heterogeneous 7 Step-wise isothermal thermogravimetric analysis of the WiBS electrolyte.…”