Rapid microwave synthesis of δ-MnO2 microspheres and their electrochemical property

The δ-MnO2 microspheres as well as the δ-MnO2 microspheres/α-MnO2 nanorods mixture were synthesized by reduction of KMnO4 under microwave irradiation. The morphology and structure of the as-prepared products were characterized by powder X-ray diffraction (PXRD), field-emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer–Emmett–Teller (BET) measurements and Raman spectra. Electrochemical performances of the samples were examined using cyclic voltammetry (CV) and galvanostatical discharge–charge test in Na2SO4 and Li2SO4 aqueous electrolyte, respectively. It was found that the nearly ideal rectangular shapes of the CV curves show that these samples exhibited good pseudo-capacitor properties in the voltage range from 0.0 to 1.0 V. The test of long-term cycling stability indicated that two samples all have a reasonably good stability. However, the δ-MnO2 microspheres possessed a lower specific surface area while a higher supercapacitance than those of the δ-MnO2 micropheres/α-MnO2 nanorods mixture did. The presence of a few fraction α-MnO2 impurities reduces the specific capacitance of δ-MnO2 electrode. Because of the crystal structure and morphologies are two such important parameters that have to be taken into account in this work.