Morphology-dependent electrochemical supercapacitive characteristics of nanostructured manganese dioxide

The dependence on morphology of the supercapacitive characteristics of manganese dioxide nanospheres (NSs) and nanorods (NRs) was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and a series of electrochemical techniques. Because the nanosized pores in MnO₂ NSs resulted in high surface area, MnO₂ electrodes made of NSs had higher specific capacitance (SC) than those made of NRs at current densities less than 2.0 A g⁻¹. However, at current densities over 2.0 A g⁻¹, the power density of MnO₂ electrodes composed of NRs was better than that of NSs. The high surface area and nanosized pores in MnO₂ NSs increase the number of redox active sites, which leads to high specific capacitance. On the other hand, the small pore size in MnO₂ NSs restricts the rates of charge and discharge, thus limiting their power density.