On the mechanism of electrochemical ion transfer reactions

Abstract The Gibbs energy curve experienced by an ion approaching the surface of a metal electrode is investigated by molecular dynamics using the I − ion approaching a platinum electrode in an aqueous solution as an example. The maximum of the potential energy curve is determined by the partial desolvation of the ion before it is adsorbed. Electron exchange with the electrode does not occur at the maximum of the potential energy curve, but on the descending branch where the ion approaches the surface. A surface charge density on the electrode surface changes the potential energy curve both through the action of the resulting electric field and by changing the structure of the solvent near the surface. It is argued that ion transfer reactions differ in fundamental aspects from electron transfer reactions and cannot be interpreted in terms of the Marcus theory.