| The ever worsening energy depletion and global warming issues call for not only urgent development of clean alternative energies and emission control of global warming gases, but also more advanced energy storage and management devices. As a green and friendly new energy storage device, supercapacitors emerge as the times require. Supercapacitors have been applied in many fields because they possess high power density, high energy density, long cycle-life, and so on. In the research of supercapacitors, the development of electrode materials, with high-rate performance and high specific capacity, has significant realistic meaning and theoretical value. Carbon-encapsulated metal oxides is a new kind of composite electrode material, which posses the high power density of carbon and the high energy density of metal oxide. As a result, they will become one of the optimal choice of electrode material in supercapacitors. In this thesis, carbon-encapsulated metal oxides nanoparticles(CEMONP) were synthesized by carbon-encapsulated metal nanoparticles(CEMNP) via direct oxidation or hydrothermal oxidation methods and were applied as the electrode materials for supercapacitor. The morphologies and structures of CEMONP were characterized by Scanning Electron Microscope(SEM), Transmission Electron Microscope(TEM), high resolution TEM(HRTEM), X-ray Diffraction (XRD), Fourier transform Infrared spectroscopy (FTIR). Electrochemical performances were investigated by constant current Galvanostatic charge/discharge test, cyclic voltammetry and alternating current impedance in 30wt.% KOH electrolyte.The results show that the particle size of carbon-encapsulated nickel oxide nanoparticles(NiO@C) synthesized from direct oxidation at 300℃is 10～20 nm. According to the different time of oxidation, the product oxidized at 300℃for 10 hours showed the best electrochemical performance with a high specific capacitance of 193 F/g under the current density of 0.1 A/g and good power performance. Besides, carbon-encapsulated cobalt oxide nanoparticles synthesized by this method at 270℃possessed uniform particle size of 30～40nm and hollow structure (Co3O4@C-HNPs), and the sample oxidized 24 hours at 270℃showed the best electrochemical performance, which exhibited the specific capacitance of 108 F/g under the current density of 0.1 A/g.In this thesis, we also founded a new approach called hydrothermal oxidation method to synthesize CEMONP, in which CEMNP was used as the starting material,30% H2O2 as the oxidant and oxidation took place in a Teflon-lined autoclave at the tempreture of 200℃for different time. By this way, carbon-encapsulated NiO hollow nanoparticles were obtained and the product oxidized 15 hours showed the specific capacitance as high as 966 F/g under the current density of 0.1 A/g and still keeping 402 F/g even under the higher current density of 2 A/g. Obviously, the electrochemical performance of NiO@C synthesized by this method was better than the former method. The reasons should be attributed to the hollow structure which provide more active sites for the redox reaction of nickel oxide. At the same time, the functional groups formed at the surface of carbon shell improved the pseudocapacitance. |
