Synthesis Of Three-dimensional Sponge-like Manganese Dioxide/Composite And Its Supercapacitor Properties

The supercapacitor is a novel energy storage device appearing in late twentieth century, its energy storage properties fall between the traditional static capacitor and the electrochemical power source. Compared with the battery, the supercapacitor has the characteristics of higher power density, shorter charging time, bigger working current, longer life, and better performance under low temperature etc., thereby is considered as the "green energy". At present, materials used in supercapacitors study are mainly carbon material, transition metal oxide and conductive polymer, while the transition metal oxide supercapacitors have been widely studied but still at the laboratory level. As an important electrode material, MnO2 is suitable for storage material due to its low prices, large specific surface area, a wide variety of sources, environmentally friendly, electrochemical stability, high specific capacitance etc. However, MnO2 is a semiconductor material and has various crystal forms with which capacitance performance varies; moreover, the low conductivity of MnO2 is the other problem against its application.In this paper, the three-dimensional structure of manganese dioxide and carbon composite materials have been successfully synthesized using three-dimensional porous aluminum oxide as template. Several techniques such as XRD, SEM, TEM, cyclic voltammetry (CV), EIS were used to study the structural properties and electrochemical performances of the as-synthesized materials. Firstly porous sponge-like alumina is synthesized via sol-gel method, then three-dimensional structure of MnO2 is prepared by copying alumina template. The effects of different preparative conditions of template and MnCl2 concentrations on MnO2 structures and its electrochemical performances are investigated. And morphological features have been examined by transmission electron microscopy (TEM). The results show that manganese dioxide consists of numerous nanowires and has a good pore structure like a sponge. The MnO2 prepared via two kinds of Al2O3 templates and manganese chloride concentration of 0.02mol/L has good electrochemical performance and high specific capacitance, The CV of manganese dioxide electrode is tested in 1 mol L-1 Na2SO4 aqueous solution at a scan rate of 2mV s-1. The results show that specific capacitances of two types of manganese dioxides are about 215.3F/g and 205.5F/g respectively, and this value decreases with increasing MnCl2 concentrations. When manganese chloride concentration is up to lmol/L, their specific capacitances are only 202F g-1 and 193.5F g-1. The main reason for this change is that the specific surface area and electrical conductivity of manganese dioxide decrease with increasing manganese chloride concentrations.In order to improve the capacitive properties of MnO2, manganese dioxide/carbon composite materials are prepared based on the successful synthesis of the three-dimensional manganese dioxide. Compared with the simple MnO2, the composite electrode greatly improves manganese dioxide capacitance and conductivity. The CV of manganese dioxide/carbon composites electrode prepared with manganese chloride concentration of 0.02mol/L and two carriers is tested in 1 mol L-1 Na2SO4 aqueous solution at a scan rate of 2mV s-1. The results show that specific capacitances of manganese dioxide/carbon composites reach 295.OF/g and 274.1F/g, respectively. However, when manganese chloride concentration is up to lmol/L, their specific capacitances are only 227.4F g-1 and 210.4F g-1, the capacitance value of manganese dioxide/carbon composites with increasing manganese chloride concentrations tends to decrease. The main reason for this change is that the specific surface area and electrical conductivity of manganese dioxide/carbon composites decrease with increasing manganese chloride concentrations. Capacitance retention rates of manganese dioxide and manganese dioxide/carbon composites electrodes are 91% and 97% respectively after 1000 cycles. The results show that the three-dimensional porous manganese dioxide and carbon composite materials have a high specific capacitance and good cycle stability and are suitable as the active materials for supercapacitors.