Cation-Dependent Interfacial Structures and Kinetics for Outer-Sphere Electron-Transfer Reactions

The kinetics of aqueous outer-sphere electron-transfer (ET) reactions are determined in large part by noncovalent electrostatic interactions that originate from the surrounding electrolyte solution. In this work, we examine the role of spectator cations in modifying the rate of heterogeneous ET for an [Fe(CN)6]3-/[Fe(CN)6]4- redox pair. We combine the results of electrochemical measurement, in situ surface-enhanced infrared absorption spectroscopy (SEIRAS), classical molecular dynamics simulation, and theoretical modeling to demonstrate how changing the identity of the spectator cation species over a series that includes Li+, Na+, K+, Rb+ to Cs+ influences the solvation properties and ET kinetics of the redox species. By analyzing the results in the context of the Marcus-Hush-Chidsey (MHC) theory, we find that the solvent reorganization energy increases systematically as the cationic radius decreases. The trend can be attributed to cation-dependent coordination environments of the redox species, whereby more cations of less charge density such as Cs+ than Li+ are present in the redox solvation shell in bulk and at the electrified interface, promoting weaker hydrogen bonds and lowering the effective interfacial static dielectric constant. We discuss the implications of these findings for enabling the tunability of reaction thermodynamics and rates in electrochemical processes.