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.