Synthesis Of Vanadium Oxide/Carbon Composites By Micelle Anchored Method

Supercapacitors are new energy storage devices, complementary totraditional capacitors and secondary batteries. They have the advantages of high power density, fast charging/discharging rates and long cycle life. The performance of supercapacitors greatly depend on their electrode materials. At present, electrode materials for supercapacitors are carbonaceous materials, transition-metal oxides and conducting polymers. Transition-metal oxides are promising electrode materials due to their high theoretical specific capacitances. However, they always suffer from poor electrochemical performance because of low conductivity. Therefore, in this thesis, vanadium oxide composites with increased conductivity were prepared via the micelle anchored method, in which, vanadate ions were anchored onto the surface of carbonaceous materials by micelles due to electrostatic force. The micelle anchored method established in this thesis is applicable to all transition-metal oxides.V2O3 nanoflakes@C core/shell composites were prepared by the micelle-anchoring method in combination with in-situ carbonthermal reduction. Hexadecyl trimethyl ammonium bromide(CTAB) micelles assembled to solubilize activated carbon and anchor vanadate ions of the precursor, NH4VO3, ontothe carbon surface. During drying and calcination, CTAB and NH4VO3 decompose to produce V2O5, which is in-situ carbon thermally reduced to V2O3. The structure and morphology analysis indicate that monodisperse V2O3 nanoflakes stand edge-on the carbon surface, forming a carbon core with a shell layer of edge-on standing V2O3 nanoflakes. Because of the increased electric conductivity and high specific surface area, the electrochemical performance show that V2O3 nanoflakes@C composites exhibit a specific capacitance of 205 F/g at 0.05 A/g, which surpasses those of their individual counterparts(67 F/g and 159 F/g at 0.05 A/g for activated carbon and bulk V2O3, respectively). The composites also showed good cycling stability due to structure support of the inner carbon cores(76% capacity retention after 500 cycles).Graphene were parpared by the improved Hummers method and were exfoliated by microwave irradiation, after which V2O3/graphene composites in layered structure were parpared by hydrothermal method in combination with micelle-anchoring method. Intrinsically hydrophobic graphene were covered CTAB micelles, which attract vanadate ions onto the surface via electrostatic force, forming the V2O3/graphene composites with layered structure. The structure and morphology characterization indicate that V2O3 particles in diameter of 50 nm are ramdomly dispersed on the surface and into interlayers of graphene. The substrate of graphene not only improves the conductivity of the composites but also reduces volume expansion of V2O3 during charging/discharging. The electrochemical performance show that V2O3 nanoparticles /graphene composites exhibit a specific capacitance of 180 F/g at 0.05 A/g, which surpasses those of their individual counterparts(83 F/g and 159 F/g at 0.05 A/g for graphene and bulk V2O3, espectively), with highest energy density of 22.5 Wh/kg.