Preparation And Electrochemical Performance Of Mo₂C/C Composites And MnO₂ Nanowires

As a new kind of energy storage devices, supercapacitors have large capacity and high power density, so they have wide reality and potential applications. The specific capacity of carbon materials which are used in electrochemical double layer capacitors(EDLC) cannot be highly improved, therefore, the research and development of new carbon materials with high capacity have become very important. As a supplementary form, pseudocapacitors also have aroused wide concern among the researchers. Their capacity comes from the redox reaction of oxide thin films. The applications of pseudocapacitors are largely limited because of their high price. In view of the rich resources and extremely similar capacitance behaviors with noble metals such as RuO2, MnO2 become the most promising cheap metal oxides to substitute for RuO2. Mo2C/C composites and MnO2 nanowires were synthesized and characterized with XRD, SEM and electrochemical tests. The main contents are as follows.Molybdenum carbides/carbon composites (MCCs) with high specific surface area were prepared using the direct carbonization. By regulating the amount of K2CO3 added during the preparation process, high BET specific surface area(>900m2/g) could be gained. X-ray diffraction patterns showed the formation ofα-Mo2C with small amount of Mo3C2 in carbon matrix. SEM micrographs showed the size of Molybdenum carbides was between 1-3μm and they were well dispersed in carbon. There were many micropores on the surface of the composites which resulted in high specific surface area. Electrochemical tests showed that MCCs was a kind of ideal electrode materials with high capacity and long cycling stability. When the amount of K2CO3 was 0.3g, the prepared sample showed favorable electrochemical properties. Its specific capacity was 238F/g at the current density of 50mA/g and could still retain a high capacity (>150F/g) at lA/g during the galvanostatic charge-discharge. The obtained high capacity of Mo2C/C composites was attributed to the superposition of EDLC capacity provided by porous carbon and pseudocapcity provided by transition metal oxides.MnO2 nanowires were successfully prepared from the reaction of MnS04 and KMnO4 by hydrothermal synthesis method. SEM characterization showed that the MnO2 nanowires are 3-10μm in length and 10-50 nm in diameter. The influences of the proportion of the reactants, reaction temperature, reaction time and the kind of electrolytes on the morphology electrochemical performance were also discussed. The best preparation conditions of MnO2 nanowires were as follows:the molar ratio of MnSO4 and KMnO4 was 3:2, the temperature of reaction was 120℃and reaction time was 9h. Specific capacity as high as 582.8 F/g was obtained at the current density of 50mA/g and a capacity of 100.9F/g could still retain at 500 mA/g during the galvanostatic charge-discharge in 1 mol/L KOH. However, the electrochemical performance was not very ideal because of high resistance of MnO2 nanowires. Doping certain metals such as Ce or Cr could be considered in the future research so as to improve the electrochemical performance of MnO2 nanowires.