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.