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

Area of Science:

• Electrochemistry
• Physical Chemistry
• Materials Science

Background:

• Electrochemical stability of aqueous electrolytes is limited by the hydrogen evolution reaction (HER).
• The molecular origin of HER onset potential's dependence on salt concentration is unresolved.
• Previous studies suggested a link between redox potential and local electrostatic potential.

Purpose of the Study:

• To investigate the molecular origin of the concentration dependence of the hydrogen evolution reaction (HER) onset potential in aqueous electrolytes.
• To establish a link between local electrostatic potentials and bulk electrochemical observables.
• To develop a predictive model for concentration-dependent HER onset potentials.

Main Methods:

• Voltammetric measurements of HER onset potential in aqueous NaNO2.
• Molecular dynamics simulations to determine local electrostatic potential at water oxygen atoms (V_OW).
• Analysis of the correlation between V_OW and HER onset potential (U_HER).

Main Results:

• Water reduction is controlled by the local electrostatic potential at water oxygen atoms (V_OW).
• A quadratic concentration dependence was observed for both V_OW and U_HER, indicating a linear correlation.
• The study successfully predicted concentration-dependent U_HER, linking electrostatic potentials to a bulk electrochemical observable.
• Electrochemical stability was found to be complementary and anticorrelated with pKw.

Conclusions:

• The local electrostatic potential at water oxygen atoms is the key factor governing HER onset potential in aqueous electrolytes.
• The developed model provides a molecular handle on the electrochemical stability window of aqueous electrolytes.
• This approach requires only classical sampling and is extensible to other salts, offering a simpler alternative to DFT or ab initio MD.