Facile synthesis of hollow sphere amorphous MnO2: the formation mechanism, morphology and effect of a bivalent cation-containing electrolyte on its supercapacitive behavior

Abstract

Nearly X-ray amorphous hollow sphere manganese oxides (hollow sphere MnO2) have been synthesized by a carboxylic acid-mediated system containing KMnO4 and Na2S2O4 under ambient conditions for supercapacitor applications. The product was characterized by powder XRD, Raman spectroscopy and thermal analysis. SEM and TEM were used to investigate the morphology of MnO2. The as-prepared MnO2 was X-ray amorphous and had particles in the size range 0.1–1 μm. A mechanism has been proposed for the formation of hollow sphere structures in the micro-emulsion medium. Upon annealing the sample at temperatures greater than 500 °C, the amorphous MnO2 transforms into Mn2O3. Cyclic voltammetry and galvanostatic charge–discharge cycling were used to evaluate the electrochemical performance. The initial discharge capacities were found to be 283 and 188 F g−1 in 0.1 M Ca(NO3)2 and 0.1 M Na2SO4, respectively, at a current density of 0.5 mA cm−2. The higher specific capacitance in the electrolyte with a bivalent cation is attributed to the reduction of two Mn4+ to Mn3+ by each of the bivalent cations present in the electrolyte.