Electronic friction and electron transfer rates at metallic electrodes

A theory is presented for the rate constant k for electron transfer between a metal electrode and a redox couple solute in solution, in or near the electronically adiabatic regime. The departure of k from its electronically adiabatic transition state theory limit kTST is described via Grote–Hynes theory, and includes two sources of friction. The electronic friction arises from excitation of electron hole pairs in the metal, i.e., electronic nonadiabaticity effects. The solvent friction arises from solvent dynamical effects. Both features can result in significant reduction of k below kTST, and their interplay can lead to interesting nonmonotonic variations with reaction overpotential.