Synthesis of Nanostructured Mesoporous Manganese Oxides with Three-Dimensional Frameworks and Their Application in Supercapacitors

Nanostructured mesoporous manganese oxides were easily prepared by mixing KMnO4 with ascorbic acid in an aqueous solution under ambient conditions. The obtained manganese oxides were identified as having an alpha-MnO2 tunnel structure composed of an edge-shared network of [MnO6] octahedra. TEM observations revealed that the obtained MnO2 materials had three-dimensional frameworks which consisted of homogeneous nanoparticles with sizes of ca. 5 nm. Nitrogen sorption analyses showed that these MnO2 nanoparticles exhibited a type IV isotherm, indicating a mesoporous character. Large surface areas up to 284 m(2) g(-1) were recorded. The electrochemical performances of the synthesized alpha-MnO2 nanoparticles as supercapacitor electrode materials were studied using cyclic voltammetry and galvanostatic charge-discharge cycling in a three-electrode system at a potential range from 0 to 1.0 V vs a saturated calomel electrode in 0.5 M sodium sulfate solution. The result showed that mesoporous MnO2 with three-dimensional frameworks exhibit a high capacitance up to similar to 200 F g(-1). Furthermore, a hybrid supercapacitor was assembled by using MnO2 mixed with a small amount of activated carbon as the positive electrode and activated carbon as the negative electrode in a 0.5 M Na2SO4 electrolyte. By balancing the mass of MnO2 and activated carbon, a practical cell voltage of 1.8 V could be obtained in aqueous medium with a capacitance of 23.1 F g(-1). After 1200 cycles, the maximum energy density is 104 Wh kg(-1) and power density is 14.7 kW kg(-1). Thus, the obtained alpha-MnO2 nanoparticles are suitable for use as supercapacitor electrode materials.