Preparation And Properties Of Manganese Oxide Materials For Supercapacitors Electrodes

Supercapacitors have become the new energy storage devices in recent years because of their excellent properties such as high safety, high power density, fast charge/discharge rate and long cycle life and safe operation for the hot research spots in the world. According to the charge-storage mechanism, they are generally divided into two categories, the electrical double layer capacitors using carbon materials with high specific surface area, and the pseudo-capacitors using redox-active metal oxides. Because of the different storage mechanism, the pseudo-capacitance is10-100times of the electrical double layer capacitance in the same area. Therefore, the pseudo-capacitors electrode materials have attracted more research attention. Recently, for pseudocapacitors, there has been extensive interest in developing commercial attractive metal oxide especially cheap metal oxide electrodes, as their high theoretical capacitance, low cost, and environmental friendliness.In this paper, manganese oxides have been investigated by enhancing the conductivity and the active area to improve the capacitance and cycle stability. Recently, the manganese oxides with high specific capacitance are mainly electrodeposited on conductive matrix. These methods are difficult to apply on engineering due to their high cost and low productivity. Therefore, the superstructures and composites of MnO2were prepared by chemical methods in our work, and their electrochemical properties were measured. The main contents are as follows:(1) The silver nanoparticles were used to induce the growth of the manganese dioxide in the mixture solution of ammonium sulfate and manganese sulfate. The final morphology of the manganese dioxide is changed from nanosheets into nanowires dominated by the SLS growth mechanism, and obtains superstructures of MnO2micro-spheres (1-2μm) covered with nanowires (～10nm). The HRTEM and SEM images reveal that the Ag2O nanoparticles reside on the end of the MnO2nanowires. The crystal type of the manganese dioxide with superstructures is a-MnO2, and its crystallization is better than pure manganese dioxide. The manganese dioxide with superstructures has superior performance at high current. Its supercapacitance has reached417.2F/g at a fast scan rate of100mV/s, which is almost ten times of the pure manganese dioxide. Meanwhile, the manganese dioxide with superstructures has excellent cycle stability, it still has112.7%capacitance retention over2000cycles, which is much better than the pure manganese dioxide (50.9%).(2) The MnO2nanorods with uniform size had been synthesized by hydrothermal method. A thin layer of alkaline chloride grew on the surface of MnO2nanorods by sensitizing treatment. The alkaline chloride was decomposed into tin oxide by drying at high temperature. Finally, the MnO2@SnO2core-shell heterostructured nanorods were accquired. The composites have a superior specific capacitance of367.5F/g at50mV/s, which is four times of the MnO2nanorods. Regarding durability, the composites have91.3%capacitance retention over2000cycles, which is much better than the MnO2nanorods (74.6%). The heterostructure in MnO2@SnO2composites plays a key role for the enhancement of the SC and the structural stability.(3) A simple method is developed to synthesize MnO2nanorod/Au nanoparticles composites. The MnO2nanorods with uniform size had been synthesized by hydrothermal method. The Au nanoparticles were grown on the surface of the MnO2nanorods by the reduction of H[AuCl4]. The composites have superior supercapacitance of406.8F/g at a scan rate of50mV/s, which is almost five times that of the MnO2nanorods. Regarding long-term durability, the composites have93.3%capacitance retention over2000cycles, which is much better than the pure MnO2nanorods. The role of Au nanoparticles is not only to improve the electric conductivity, but also to enhance the structural stability.(4) A simple two-step procedure is developed to synthesize MnO2nanorod/Ag nanoparticles composites. The silver ions were tightly sticked on the surface of MnO2nanorods by the treatment of sodium polyacrylate. The Ag nanoparticles were grown on the surface of the MnO2nanorods when the sodium borohydride were added into the react system. Finally, the MnO2nanorod/Ag nanoparticles composites were accquired. The composites have superior supercapacitance of396.4F/g at a scan rate of50mV/s, which is almost five times that of the MnO2nanowires. Regarding long-term durability, the composites have95.2%capacitance retention over2000cycles, which is much better than the pure MnO2nanowires (74.6%). The role of Ag nanoparticles is not only to improve the electric conductivity, but also to enhance the cycle stability. The features of execellent performance and low price make the composites promising for supercapactior electrodes.