A Comparative Study of MnO2 and Composite MnO2–Ag Nanostructures Prepared by a Hydrothermal Technique on Supercapacitor Applications

Pure MnO2 and composite MnO2–Ag electrodes with four different structures were synthesized via a hydrothermal process. Tube, urchin, rod and wire/sphere-like structures were obtained from the addition of HCl, H2SO4, (NH4)2S2O8 or CO(NH2)2 reagents into potassium permanganate solutions, respectively. The crystal structure of the MnO2 particles was examined using X-ray diffraction and transmission electron microscopy, revealing an α-phase MnO2. Specific capacitance values of 74.5, 111.7, 103.4 and 204.1 F g−1 at a charge/discharge current density of 0.3 A g−1 were obtained for tube, urchin, rod and wire/sphere-like pure MnO2 structures, respectively. The wire/sphere-like structure delivered the highest specific capacitance owing to its largest specific surface area (164.60 m2 g−1). The specific capacitances were further increased to 96.6, 210.9 and 186.4 F g−1, respectively, for tube, urchin and rod-like structures after Ag addition. Additionally, the capacitance retention of the rod and wire/sphere-like composite MnO2–Ag films were also prolonged because Ag nanoparticles prevented the aggregation and/or decomposition of MnO2.