Electrochemical Electron Transfer: Key Concepts, Theories, and Parameterization via Atomistic Simulations
Electron transfer (ET) at electrochemical interfaces is central to energy conversion and storage, yet its theoretical and computational modeling remain active research areas. This review elucidates key concepts and theories of ET kinetics, focusing on coupling between classical solvent fluctuations and quantum electronic states of metallic electrodes and redox species. We begin with fundamental rate theories, reaction coordinates, and electrochemical timescales, then explore weak, strong, and intermediate electronic coupling regimes. Special attention is given to solvent dynamics and the structure of the electrical double layer (EDL), which critically impact ET kinetics. Atomistic simulations, particularly density functional theory (DFT) and molecular dynamics (MD), are highlighted for testing linear response and determining solvent reorganization energy, electronic coupling strengths, and solvent relaxation dynamics. A central theme is linear response enabling tractable treatments across Marcus theory, empirical valence bond (EVB) models, the Anderson-Newns-Schmickler framework, and generalized Langevin dynamics. While linear response offers useful simplifications, we assess its limitations, particularly for strong solvation changes or inner-sphere ET at catalytic interfaces. We discuss advances, including mapping Hamiltonian-based EVB-MD, constrained DFT, and non-Gaussian free energy formulations, enabling rigorous tests and access to diabatic and adiabatic free energy surfaces. We outline opportunities to advance multiscale, quantum-classical models that integrate EDL effects, multiple reaction coordinates, solvent-controlled dynamics, and transitions between adiabatic and nonadiabatic regimes. This review serves as a conceptual guide and practical resource for researchers integrating theory and simulation in studying electrochemical ET across diverse systems.