Linking Local Water Electrostatic Potentials to Measured Hydrogen Evolution Onset in Aqueous Electrolytes

The electrochemical stability of aqueous electrolytes is limited by the hydrogen evolution reaction (HER), a parasitic water-reduction process whose onset potential shifts strongly with the salt concentration. The molecular origin of this dependence has remained unresolved, although studies of discrete molecular complexes have implicated a relationship between the redox potential and the local electrostatic potential. Here, we combine voltammetric measurements of the HER onset in aqueous NaNO₂ with molecular dynamics to show that water reduction is controlled by the local electrostatic potential at the water oxygen atoms (V_OW). This potential derives from the distribution of local environments set by each oxygen's nearest neighbors, where the systematic changes to the environment create a quadratic concentration dependence. U_HER exhibits the same quadratic dependence, and thus the two variables are linearly correlated. We explain the physical basis of this correlation and use it to predict concentration-dependent U_HER, for the first time linking the electrostatic potentials to a bulk electrochemical observable. Importantly, the redox stability of water in aqueous electrolytes is demonstrated to be complementary and anticorrelated with pK_w, described in a recent work, because the two reaction have the same physical dependence on V_OW. Unlike the density functional theory or ab initio molecular dynamics, the U_HER prediction needs only classical sampling and a few measured potentials. The approach is extensible to other salts and gives a molecular handle on the electrochemical stability window of aqueous electrolytes used in batteries, gas capture, and waste processing.