The mechanism of specific capacitance improvement of supercapacitors based on MnO2 at an elevated operating temperature

Amorphous nanostructured MnO₂ film was anodically deposited onto economical duplex stainless steel substrate. The obtained MnO₂ film was characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy for microstructural, morphological, and compositional studies. The capacitive behavior was systematically investigated by cyclic voltammetry, charge-discharge cycling and electrochemical impedance spectroscopy (EIS) in 1 M Na₂SO₄ electrolyte at different operating temperatures ranging from 20 to 60 °C. The specific capacitance (SC) was improved with an increase of operating temperature, and the highest SC of 398 F/g was achieved at a scan rate of 10 mV/s and operating temperature of 60 °C. The mechanism of SC improvement at elevated operating temperature was investigated using EIS. With an increase of operating temperature, the conductivity of electrolyte was improved, and the charge-transfer resistance (R_ct) was decreased. The temperature dependence of 1/R_ct follows an Arrhenius equation. The MnO₂ film was electrochemically activated at 60 °C due to the formation of Na_yMnO₂ after discharging.