Cyclic Voltammetry of Ion-Coupled Electron Transfer Reactions in Permeable Redox Films: The MnO 2 Case

The methods of Nicholson and Shain and Randles−Ševcik are the paradigms of voltammetry of redox species. However, as they were originally developed for aqueous redox couples, they cannot be directly applied to solid redox films such as those of battery materials. Herein, for the first time, we present a cyclic voltammetry model based on semi-infinite linear diffusion for ion-coupled electron transfer reactions. The simulated CVs contain parameters such as capacity (q, C/cm2), ion activity, formal potential of proton-coupled electron transfer, and scan rate that are physically more meaningful than those of the current models in characterizing energy storage materials. We apply this model to the ε-MnO2 cathode material as a proof of concept and discuss thesignificance of the simulation parameters for determining the charge storage mechanism, kinetics, and energetics in aqueous solutions. Our model suggests a quasireversible proton-coupled electron transfer reaction mechanism for the ε-MnO2 film in both alkaline and acidic solutions. According to the present model, two linear regression lines can be established from relatively simplevoltammetry experiments for characterizing energy storage materials: (1) the regression line of the CV midpeak potential vs log of ion activity (or pH), in which the slope and the intercept provide information on the type of charge-carrier ions and their solvation state, respectively; and (2) the regression line of the capacity vs ν−1/2, where the slope and intercept indicate the contributions of bulk and surface capacities in thin redox films, respectively.