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 (R_s), electron-transfer resistance (R_et), degree of frequency dispersion in semi-infinite diffusion, and effective diffusion length is increased with an increase in MnO₂ loading amounts in the composites.