Preparation And Electrochemical Properties Of Graphene / Vanadium Oxide Composites

Vanadium oxides have been widely investigated because they can be used in many fields such as optical, electrical, magnetic, biological sensors, and so on. Also vanadium oxides are considered as one of the most promising pseudocapacitance electrode materials due to its high specific capacitance, low cost and abundant resources. In the developing process of vanadium oxides as ideal electrode materials, one challenge still is how to improve its electron conductivity, cycle stability and the ion diffusion/penetration rate and to inhibit the degradation of electrode materials. Therefore, developing vanadium oxide hybrid materials and tuning/designing a desirable microstructure of electrochemically active materials may be the key to enhance their respective performances. By hybriding the vanadium oxides with graphene with good conductivity property and stability, graphene/vanadium oxides hybrid electrode material with good capacitance is expected to be prepared.This thesis consists of three sections, review, experiments and discussion and conclusion. The structure, property, the preparation method and the application of both the graphene and vanadium oxides are reviewed in Chapter1. The preparation method, characterization, and the electrochemical property of VOx·nH2O and graphene/VOx·nH2O hybris nanocomposites are described in the experimental section and discussion (Chapter2and3). The conclusion is conclused in Chapter4.The main research works are as follows:(1) Graphene/vanadium oxides hybrid electrode materials with different graphene amounts (GR/VOx·nH2O) are prepared in a suspension of bulk NH4VO3, NH2CSNH2and graphite oxide (GO) nanosheets by one-step simultaneous hydrothermal-reduction technology at180℃for18h. By using the similar procedure, VOx·nH2O with nanoplate morphology is also obtained, and its formation mechanism is also discussed. Graphite oxide (GO) with a basal spacing of0.73nm is prepared by improved Hummers method, the graphite oxide suspensions with different GO amounts are suspensed with ammonium metavanadate and thiourea, the obtained suspensions are hydrothermally treated at180℃for18h, graphene/vanadium oxides hybrid electrode materials with different graphene amounts are prepared. The structure and morphology of both VOx·nH2O and GR/VOx-nH2O hybrid nanocomposites are characterized by XRD, SEM, TEM, TG, and IR analyses.(2) In GR/VOx·nH2O hybrid nanocomposites with different graphene amounts, the thickness of VOx·nH2O is about200nm and its size is in the range of4-7μm. The graphene nanolayers are dispensed on the surface of VOx·nH2O nanoplates and further intimated contact between the graphene nanosheets and VOx·nH2O particles, and the agglomeration of both VOx·nH2O nanoplates and graphene nanolayers is obviously decreased in the GR/VOx·nH2O hybrid nanocomposites with different graphene amounts. GR(0.5)/VOx·nH2O hybrid nanocomposite shows uniform morphology and good dispersity.(3) The electrochemical property of GR/VOx·nH2O hynrid nanocomposites with different graphene amounts is investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy in0.5mol L-1K2SO4electrolyte within a potential window of-0.2to0.8V by using a three electrode system. GR (0.5)/VOx·nH2O electrode shows a specific capacitance of384F g-1at a scan rate of5mV s-1it is higher than that of VOx·nH2O (224F g-1). Moreover, the GR (0.5)/VOx·nH2O electrode possesses smaller internal resistance and diffused reistance. After1000cycles, the capacitance retention of GR(0.5)/VOx·nH2O electrode retains about60%of the initial capacitance value, suggesting that graphene addition can increase the cycle stability of VOx·nH2O electroactive material.