Cyclic Voltammetry of Ion-Coupled Electron Transfer Reactions for Diagnosing Energy Storage Materials

Abstract

The methods of Nicholson and Shain and Randles-Sevcik 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, 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 MnO2 cathode material as a proof of concept and discuss the significance of the simulation parameters for determining the energetics of the underlying phase transitions and the charge storage mechanisms. According to the present model, two linear regression lines can be established from relatively simple voltammetry experiments for characterizing energy storage materials: 1) The regression line of the CV mid-peak 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. 2) The regression line of the capacity vs. the inverse of the square root of scan rate, where the slope and intercept indicate the contributions of bulk and surface charges in thin redox films, respectively.