| Low-cost transition metal oxides can be choosed as electrode materialsfor electrochemical capacitors or lithium-ion batteries because theirreversible redox reactions occuring under certain conditions could give riseto significant pseudocapacitance or lithium storage capacity. Twotechnology barriers, one is the poor conductivity of transition metal oxidesleading to a high internal resistance of the energy storage devices, the otheris their unsatisfactory cycling performance originating from that thestructure of materials or electrodes could be damaged or collapsed in thecharge-discharge process because of the electrolyte ionsembedded/de-intercalation, have been depressing their applications as thecommodity electrode materials for energy storage devices. Dispersing themonto the surface of carbon materials with a good conductivity (such asgraphene, carbon nanotubes, porous carbons, etc.) to obtain compositematerials, is one of the methods to solve the above problems. Graphene as astorage material has shown a good prospect in the field of solar cells,supercapacitors and lithium-ion batteries, and also is an ideal substratematerial for dispersing/anchoring various of functional particles due to itstwo-dimensional structure and good electrical conductivity.In this paper, graphene/nickel oxide, graphene/vanadium oxidenanotubes (VOx-NTs) and graphene/Co3O4composites were synthesizedusing different hydrothermal processes. The physical and electrochemicalproperties of the materials were systematicly studied using a variety ofcharacterization methods. Compared with bare transition metal oxides, theelectrochemical performances of graphene/transition metal oxidecomposites were significantly improved. Concrete content as follows:(1) Graphene/nickel oxide composite was synthesized by ahydrothermal-heat treatment process in which graphene and nickel nitrate asprecursors，urea as precipitant, sodium dodecyl sulfate (SDS) as surfactant.Scanning electron microscopy (SEM) analysis shows that nickel oxidepresents a mesh-like morphology and is dispersed on the surface ofgraphene uniformly. The formation mechanism of nickel oxide with specialmorphology was discussed. The composite containing52wt%of nickel oxide exhibits a mass specific capacitance of360F/g in2M KOH, which issignificantly higher than that of graphene (113F/g) and nickel oxide(220F/g). In addition, electrochemical reversibility and cycle stability ofnickel oxide are also notablely improved by the specific roles of graphene.(2) Graphene/VOx-NTs composite was prepared through a simplehydrothermal process in which graphene and vanadium oxide solution asprecursor, hexadecylamine (HAD) as structure-directing agent. Vanadiumoxide exhibits a nano tubular morphology and diperses uniformly on thesurface of graphene. When the content of vanadium oxide was69wt%, thespecific capacitance of composite was188F/g in2M Na2SO4solution,significantly larger than that of the graphene (94F/g) and vanadium oxide(88F/g). The electrochemical impedance spectroscopy (EIS) simulationresults reveal that the improvement of quality utilization of vanadium oxideand the reduced charge transfer resistance for pseudo-capacitance occuringlead to the enhanced electrochemical capacitance performance of thecomposite.(3) Graphene/Co3O4composite was prepared by a low-temperatureliquid phase oxidation. Electrochemical capacitance and lithium-ion batteryanode performance of the composite were studied. The graphene facilitatesthe formation of Co3O4with small crystal grains and high electrochemicalactivity on account of its two roles, acting as a dispersing substrate andproviding abundant points of heterogeneous nucleation during theprecipitation of Co3O4. By comparing with bare Co3O4, it could be foundthat selective growth of Co3O4nanoparticles on graphene results in theimprovement of the mass specific capacitance and lithium storage capacityof the composite. |
