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

Nanostructured mesoporous manganese oxides were easily prepared by mixing KMnO 4 with ascorbic acid in an aqueous solution under ambient conditions. The obtained manganese oxides were identified as having an α-MnO 2 tunnel structure composed of an edge-shared network of [MnO 6 ] octahedra. TEM observations revealed that the obtained MnO 2 materials had three-dimensional frameworks which consisted of homogeneous nanoparticles with sizes of ca. 5 nm. Nitrogen sorption analyses showed that these MnO 2 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 α-MnO 2 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 MnO 2 with three-dimensional frameworks exhibit a high capacitance up to ∼200 F g −1 . Furthermore, a hybrid supercapacitor was assembled by using MnO 2 mixed with a small amount of activated carbon as the positive electrode and activated carbon as the negative electrode in a 0.5 M Na 2 SO 4 electrolyte. By balancing the mass of MnO 2 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 10.4 Wh kg −1 and power density is 14.7 kW kg −1 . Thus, the obtained α-MnO 2 nanoparticles are suitable for use as supercapacitor electrode materials.