Electrochemical Impedance Spectroscopic Investigation of Sodium Ion Diffusion in MnO2 Using a Constant Phase Element Active in Desired Frequency Ranges

Herein, we report an equivalent circuit model for electrochemical impedance spectroscopic (EIS) investigation of the Na+ ion diffusion process in MnO2. We combine two constant phase elements (CPEs) active in the desired frequency ranges as the element associated with the diffusion reaction of ions in the solid active materials in a modified equivalent circuit model. Adoption of these CPEs instead of a finite-space Warburg impedance allows for the more precise analysis of measured impedance data for a cavity microelectrode (CME). The dependence of electrochemical properties of manganese oxide (MnO2)/multiwall carbon nanotube (MWCNT) composites on the MnO2 content is then investigated using this EIS analysis. According to EIS analyzes, series resistance (Rs), electron-transfer resistance (Ret), degree of frequency dispersion in semi-infinite diffusion, and effective diffusion length is increased with an increase in MnO2 loading amounts in the composites.