The evolution of α-MnO2 from hollow cubes to hollow spheres and their electrochemical performance for supercapacitors

The evolution of α-MnO2 from hollow cubes to hollow spheres was achieved by using MnCO3 as the template. The as-obtained α-MnO2 crystals were characterized by using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET). The as-synthesized α-MnO2 hollow cubes (with the side length of about 2 µm) and hollow spheres (with the diameter of about 2 µm) were uniform particles. The as-prepared α-MnO2 hollow spheres have a large specific surface area (417 m2 g−1). A process has been proposed for the evolution of MnCO3 templates from cubes to spheres. Then, the evolution of α-MnO2 was achieved by two-step mechanism with the treatment of previously obtained MnCO3 templates. Cyclic voltammetry (CV), galvanostatic charge–discharge and electrochemical impedance spectroscopy (EIS) measurements were used to characterize the electrochemical performances of the as-synthesized α-MnO2. The initial specific capacitance at a current density of 1 A g−1 of the as-prepared α-MnO2 hollow spheres is 203 F g−1, which is higher than that of α-MnO2 hollow cubes (152 F g−1). In addition, the α-MnO2 hollow cubes retain 93% and the α-MnO2 hollow spheres retain 80% of the initial specific capacitance after 2000 charge/discharge cycles at 2 A g−1. The α-MnO2 hollow spheres-based supercapacitors exhibit 38.7 W h kg−1 at a power density of 1000 W kg−1 and maintain 7.8 W h kg−1 at a high power density of 10028 W kg−1.