Electrochemical Proton-Coupled Electron Transfer Free Energy Surfaces from Ab Initio Molecular Dynamics

Proton-coupled electron transfer (PCET) is an elementary step in many electrocatalytic reactions, yet molecular-level descriptions of solvent reorganization energetics in these reactions remain limited. Herein, we describe a first-principles computational platform to calculate free energy surfaces and quantify reorganization energies for interfacial electrochemical PCET. Hydrogen electrosorption at Pt(111)/water interfaces is used as a model reaction to derive PCET free energy surfaces using ab initio molecular dynamics simulations in an explicit solvent. The reaction is described as a non-adiabatic transition between reactant and product states defined in terms of localization of electrons and the transferring quantized proton. Free energy surfaces are calculated from statistical fluctuations of the vertical energy gap. This computational approach directly incorporates explicit solvent dynamics into the calculated reorganization energies that are obtained from parabolic fits to the reactant and product free energy surfaces. The reorganization energies are quantitatively similar to the mean vertical energy gap near the free energy minima of the reactant and product state free energy surfaces. The calculated PCET reorganization energy of 3.19 eV compares favorably with previous semi-empirical estimates. This work establishes a robust computational platform to assess the role of interfacial solvent properties on intrinsic electrochemical kinetics.