Deriving effective electrode-ion interactions from free-energy profiles at electrochemical interfaces

TL;DR

This paper investigates ion adsorption at electrified metal-electrolyte interfaces using molecular dynamics with classical and machine-learned potentials, highlighting the importance of accurate force field parameterization for predictive electrochemical modeling.

Contribution

It introduces a systematic methodology for tuning Lennard-Jones parameters and validates machine-learned interatomic potentials for modeling ion-specific adsorption at electrochemical interfaces.

Findings

Classical force fields' predictions depend heavily on LJ parameters.

Machine-learned interatomic potentials agree with experimental trends.

Ion adsorption significantly impacts interfacial electrochemical properties.

Abstract

Understanding ion adsorption at electrified metal-electrolyte interfaces is essential for accurate modeling of electrochemical systems. Here, we systematically investigate the free energy profiles of Na$^+$, Cl$^-$, and F$^-$ ions at the Au(111)-water interface using enhanced sampling molecular dynamics with both classical force fields and machine-learned interatomic potentials (MLIPs). Our classical metadynamics results reveal a strong dependence of predicted ion adsorption on the Lennard-Jones parameters, highlighting that -- without due care -- standard mixing rules can lead to qualitatively incorrect descriptions of ion-metal interactions. We present a systematic methodology for tuning the cross-term LJ parameters to control adsorption energetics in agreement with more accurate models. As a surrogate for an ab initio model, we employed the recently released Universal Models for Atoms (UMA) MLIP, which validates classical trends and displays strong specific adsorption for chloride, weak adsorption for fluoride, and no specific adsorption for sodium, in agreement with experimental and theoretical expectations. By integrating molecular-level adsorption free energies into continuum models of the electric double layer, we show that specific ion adsorption substantially alters the interfacial ion population, the potential of zero charge, and the differential capacitance of the system. Our results underscore the critical importance of force field parameterization and advanced interatomic potentials for the predictive modeling of ion-specific effects at electrified interfaces and provide a robust framework for bridging molecular simulations and continuum electrochemical models.