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.