Effect of the KMnO4 concentration on the structure and electrochemical behavior of MnO2

Various MnO2 structures have been synthesized in this study by a facile method through the manipulation of KMnO4 concentration. The effect of KMnO4 concentration on the structure and morphology has been thoroughly investigated by X-ray powder diffraction, field-emission scanning electron microscopy, and nitrogen adsorption–desorption measurements. Birnessite-type MnO2 flower-like microsphere can be obtained at a lower concentration, while α-MnO2 nanorods can be obtained at a higher concentration. By increasing the KMnO4 concentration further, α-MnO2 nanorods can assemble into urchin-like microspheres and spherical aggregates. The possible formation mechanism based on the experimental results has been proposed to understand their growth procedures. The electrochemical properties of the synthesized materials evaluated by cyclic voltammetry and constant-current charge–discharge cycling techniques are dependent on their pore size distribution and nanostructures. Birnessite-type MnO2 microsphere showed excellent capacitive behavior with a maximum specific capacitance of 185 F g−1.