Ab Initio Free Energy Surfaces for Coupled Ion-Electron Transfer

TL;DR

This paper develops a formalism to compute free-energy surfaces for coupled ion-electron transfer processes in anisotropic environments, revealing strong coupling effects and different activation barriers compared to traditional models.

Contribution

It introduces a new ab initio approach to calculate coupled free-energy surfaces considering anisotropy, extending Marcus theory to more complex electrochemical systems.

Findings

Strong coupling between ion-electron transfer and environmental anisotropy.

Significantly different activation barriers predicted by the new formalism.

Application to CO2 reduction on gold shows the method's effectiveness.

Abstract

The Marcus theory of electron transfer assumes that diabatic energy gaps are sampled from a single ensemble. This assumption can break down in spatially anisotropic environments, such as Faradaic reactions at electrochemical interfaces, where distinct solvent ensembles arise along a collective variable describing the anisotropy. Treating this collective variable as an additional reaction coordinate linearly independent from the Marcus reaction coordinate, we develop a formalism that enables calculation of the resulting Coupled Ion-Electron Transfer (CIET) free-energy surface directly from constrained ab initio trajectories. Applied to CO2 redox on a gold electrode, this method reveals strong coupling to the anisotropy, predicting significantly different activation barriers compared to either coordinate alone.