Electron transfer across the electrode/electrolyte interface: Influence of redox ion mobility and counterions

A microscopic model for electron transfer (ET) across an electrolyte/metal electrode interface is studied by molecular dynamics (MD) simulation. The Anderson-Newns Hamiltonian is used to describe the adiabatic ET for an Fe2+/Fe3+ ion in water at a Pt(111) electrode surface using an implicit representation of the electron transfer event. The diabatic and adiabatic solvent free energy curves for the electron transfer are calculated; however, the present study significantly expands upon our earlier simulations [Straus, J. B.; Calhoun, A.; Voth, G. A. J. Chem. Phys. 1995, 102, 529)]. In particular, the motion of the redox ion is now allowed, as well its corresponding image in the metal surface arising from the electrode polarization. Importantly, a counterion is also added to the simulations in order to study its effect on the ET free energy curves. In all cases, the effects are found to be significant.