Synthesis and supercapacitor characteristics of hydrothermally-deposited MnO2 films and chemically Co-deposited MnO2-polyaniline films on stainless-steel substrates

Abstract

MnO2 films were hydrothermally grown directly onto stainless-steel substrates and used as supercapacitor electrodes. MnO2 with a cube-like structure was observed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The measured sizes of the cubes were in the range of 270–820 nm. The specific capacitance (SC) of the 5-hour hydrothermallygrown MnO2 films was ~20.2 F/g, which was slightly higher than that of the 3-hour films (~13.8 F/g). The low values of the SC of the hydrothermally-deposited MnO2 films are attributed to their high series resistances of 1.73–2.06 Ω measured by using electrochemical impedance spectroscopy. However, the specific capacitance was greatly increased, up to ~226 F/g, after a polyaniline polymer had been added into the MnO2 hydrothermal reaction, there by producing a composite of the MnO2 and the polyaniline polymer. This SC improvement was attributed to presence of two active materials (polyaniline and MnO2) and the reduction of the electrode series resistance to 0.93 Ω.