Electrochemical Impedance Spectroscopic Investigation of Sodium Ion Diffusion in MnO₂Using 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 MnO 2 . 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 (MnO 2 )/multiwall carbon nanotube (MWCNT) composites on the MnO 2 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 MnO 2 loading amounts in the composites.