Electrode Reactions in Slowly Relaxing Media

TL;DR

This paper explores how slowly relaxing media affect electrode reaction kinetics, revealing nonergodic effects that alter reorganization energy and reaction rates, especially in ionic liquids with stretched exponential relaxation.

Contribution

It introduces a model for electrode reactions in slowly relaxing media, highlighting the impact of nonergodic dynamics on reorganization energy and reaction rate behavior.

Findings

Reorganization energy varies with overpotential, transitioning from thermodynamic to nonergodic limits.

Sharp drops in reorganization energy occur in media with Debye relaxation.

In ionic liquids, solvation energy is significantly below the thermodynamic limit.

Abstract

Standard models of reaction kinetics in condensed materials rely on the Boltzmann-Gibbs distribution for the population of reactants at the top of the free energy barrier separating them from the products. While energy dissipation and quantum effects at the barrier top can potentially affect the transmission coefficient entering the rate preexponential factor, much stronger dynamical effects on the reaction barrier are caused by the breakdown of ergodicity for populating the reaction barrier (violation of the Boltzmann-Gibbs statistics). When the spectrum of medium modes coupled to the reaction coordinate includes fluctuations slower than the reaction rate, such nuclear motions dynamically freeze on the reaction time-scale and do not contribute to the activation barrier. Here we consider the consequences of this scenario for electrode reactions in slowly relaxing media. Changing electrode overpotential speeds electrode electron transfer up, potentially cutting through the spectrum of nuclear modes coupled to the reaction coordinate. The reorganization energy of electrochemical electron transfer becomes a function of the electrode overpotential, switching between the thermodynamic value at low rates to the nonergodic limit at higher rates. The sharpness of this transition depends of the relaxation spectrum of the medium. The reorganization energy experiences a sudden drop with increasing overpotential for a medium with a Debye relaxation, but becomes a much shallower function of the overpotential for media with stretched exponential dynamics. The latter scenario characterizes electron transfer in ionic liquids. The analysis of electrode reactions in room-temperature ionic liquids shows that the magnitude of the free energy of nuclear solvation is significantly below its thermodynamic limit.