Study On The Synthesis And Capacitance Performance Of Carbon-based Composites

Carbon-based materials including multiwalled carbon nanotubes and grapheme, are commendable electrode materials for electrochemical capacitor and have been widely studied in recent years. A modified Hummers method can be adopted to oxidize graphite and prepare graphene oxide or unzip carbon nanotubes ?CNTs? longitudinally to obtain culy graphene nanosheets ?CGN?. Since the obtained graphene oxide and curly graphene nanosheets are prone to agglomerate together, it is necessary to introduce transition metal oxides, conducting polymers or other carbon-based nanomaterials and prepare three-dimensional ?3D? composite materials which own better supercapacitive properties. In recent years, many studies indicated that nitrogen doping contributes significantly to the improvement of the electrochemical capacitive properties of carbon-based nanomaterials. The main research contents of this thesis are the preparation of curly graphene nanosheets/MnO₂ and graphene oxide/MnO₂ composites and the corresponding properties after the nitrogen dopping process. The results obtained are listed as follows:?1? A modified Hummers method and an aqueous solution synthesis method have been adopted to prepare curly graphene nanosheets/manganese oxide ?CGN/MnO₂? composite using multiwalled carbon nanotubes and KMnO₄ as precursors. The characterization of morphology and structure indicated that MnO₂ acicular crystals with about 5 nm in diameter and about 100 nm in length crisscross uniformly dispersed on the surface of CGN to form three-dimensional nanostructured CGN/MnO₂ composite. The electrochemical results show that the CGN/MnO₂ composite electrode delivered a specific capacitance of 224 F g^-1 at the current density of 1 A g^-1 in 1 M Na2SO₄ solution, which was about 2.7 times higher than that of pure CGN (83 F g^-1). Meanwhile, CGN/MnO₂ composite exhibits excellent high current charge-discharge capability and long-time cycling stability ?the capacitance remains 95% of the initial value after 10000 cycles?.?2? Taking melamine as nitrogen source and curly graphene nanosheets/MnO₂ as template, we successful prepared nitrogen-doped CGN/MnO₂ by calcination method at high temperature. The corresponding morphology and structure of the samples were characterized by means of scanning electron microscopy ?SEM?, X-ray diffraction ?XRD? and Raman spectrum. The electrochemical performances were measured using cycling voltammetry ?CV? and galvanostatic charge-discharge methods. The results showed that the specific capacitance of N-doped 10% CGN/MnO₂ was up to 264 F g^-1 at the current density of 1 A g^-1 in 1 M Na2SO₄ solution. Meanwhile, the specific capacitance of N-doped 20% CGN/MnO₂ was 132 F g^-1 and it still retained 235.7 F g^-1 after 1000 cycles, which was about 1.8 times higher than the initial value (132 F g^-1).?3? Graphene oxide/manganese dioxide ?GO/MnO₂? composite was synthesized at room temperature and N-doped GO/MnO₂ composite was then prepared by calcining GO/MnO₂ at high temperature using melamine as nitrogen source. The corresponding morphologies and structure of the samples were characterized by means of scanning electron microscopy ?SEM?, high resolution transmission electron microscopy ?HRTEM?, X-ray photoelectron spectroscopy ?XPS? and Raman spectrum. The electrochemical performances were measured using cycling voltammetry ?CV? and galvanostatic charge-discharge methods. The results show that the specific capacitance value of N-doped GO/MnO₂ was up to 360 F g'1 at the current density of 1 A g^-1 in two-electrode system.