Synthesis And Electrochemical Characterization Of Transition Metal Oxides/Graphene Nanocomposites

With the economic development, the clean energy and ecological environmenthave become increasingly concerned. As a new kind of energy storage device,supercapacitor has been thought as the most important energy storage device becauseof high power density, excellent cycle ability and environment protection.Meanwhile, lithium-ion batteries （LIBs） have been used widely in portable electronicdevices such as cell phones, digital camera and laptop, because of their high energydensity, high voltage and non-pollution. Furthermore, they have a greatapplication-potential in the fields of energy storage for renewable energy sources andelectric vehicles.Graphene, a monolayer of carbon atoms packed into a dense honeycomb crystalstructure, has attracted tremendous attention from both the experimental andtheoretical scientific communities since it was found in2004. Due to the uniquenanostructure and extraordinary properties, graphene based materials have shownpromising applications in electronics and energy storage.In this work， we designed various novel graphene/transition metal oxidecomposites with suitable nanostruetures for supercapacitors and rapidinsertion/desertion of lithium ions by utilizing the confinement effect and interfacialinteraction between metal oxide and graphene sheets （GNS）. Then, the formationmechanisms of these nano-structured composites were studied. And the effects ofmorphologies on the electrochemical performances for these graphene/transitionmetal oxide composites were investigated in detail.1. Graphene oxide is prepared from nature graphite via a way of modifiedHummer’s method, and then reduced by pyrolytic deoxidation method. GNS withdifferent reduction levels have been produced through thermal reduction of grapheneoxide in the temperature range of200–900°C. The effects of interlayer spacing, oxygen content, BET specific surface area and disorder degree on their specificcapacitance are explored systematically. The variation of oxygen-containing groupsis shown to be a main factor influencing the electrochemical double layer capacitorsperformances of the pyrolytic graphene. The highest capacitance of260.5F/g at acharge/discharge current density of0.4F/g is obtained for the sample thermallyreduced at about200°C.2. The layer-by-layer porous CuO/graphene nanocomposites have beensuccessfully synthesized by the oriented attachment mechanism based on a facilehydrothermal method. The as-prepared composite is characterized using XRD,Raman, SEM, TEM and nitrogen adsorption/desorption. The growth mechanism isdiscussed by monitoring the early growth stages. It is shown that the CuOnanoleaves are formed through oriented attachment of tiny Cu（OH）2nanowires.Electrochemical characterization demonstrates that the leaf-like CuO/graphene arecapable of delivering specific capacitance of331.9and305F/g at the current densityof0.6and2A/g, respectively. A capacity retention of95.1%can be maintained after1000continuous charge-discharge cycles, which should be attributed to theimprovement of electrical contact by graphene and mechanical stability bylayer-by-layer structure.3. Series of NiO/graphene composites have been synthesized by many differentmethods.（1） Co-precipitation method was used to synthesize mono-graphene/NiOnanocomposite, in which graphene conduct as conductive matrix and NiO with adiameter of3-5nm were grown on the two sides of graphene. The two-dimensionstructure can prevent the aggregation of nanomaterials, enlarge specific surface areaand improve wetting ability of the composite. The test results reflect that thecomposite with novel structure shows high capacity and excellent cyclic ability, thespecific capacitance is525F/g at current density of200mA/g and the capacityretention is95.4%after1000cycles.（2） The flower-like NiO/graphene compositeswere synthesized by a refluxing method in the presence of graphene, the amount of NiO loaded is adjusted by controlling the refluxing times. The three-dimensionalcomposites possessed improved electrochemical performance and high specificcapacitance. At the electric current density of200mA/g, the discharge specificcapacitance of the obtained composite materials was about687.6F/g. Thiscapacitance is very impressive by comparison with pure NiO, which owns only276.4F/g at the same conditions. The capacity retention of90.8%can be maintainedafter2000continuous charge-discharge cycles, demonstrating its promising potentialto be used for supercapacitors.（3） The hydrothermal method is an important methodfor nanomaterials. The flower-like porous NiO/graphene was synthesized byhydrothermal method with mixed glycerol and DI-water solvents, and characterizedby SEM, TEM, XRD, BET and so on. Then the electrochemical performances of theobtained composite materials were studied by CHI660C. It shows that the dischargespecific capacitance was about413F/g at the current density of200mA/g. Whengalvanostatic charge/discharge at current density of1A/g, the capacity retention is89.8%after2000cycles.4. A nanorod-like Fe2O3/graphene nanocomposite is synthesized by a faciletemplate-free hydrothermal method and a following calcination in the air at300°Cfor2h. The Fe2O3nanorods with diameter of20-30nm and length of150-250nmare homogenous distributed on both sides of graphene. The morphologies ofintermediates at different hydrothermal reaction times are investigated by TEMcharacterization and a possible growth mechanism of this one-dimensional structureis proposed. It is shown that the α-FeOOH rod-like precursors are formed through arolling-broken-growth （RGB） model, which are then transformed into α-Fe2O3nanorods during calcinations, preserving the same rod-like morphology.Electrochemical characterizations demonstrate that the nanorod-like Fe2O3/graphenecomposites exhibit a very large reversible capacity of1063.2mAh/g at currentdensity of100mA/g as well as good cycling performance.5. The Li₃V₂（PO₄）₃material modified by graphene and citric acid shows high specific capacity and excellent cycling stability. The3D network graphene andLi₃V₂（PO₄）₃primary nanoparticles embedded in micro-sized spherical secondaryparticles interlaced with each other. And this special nanostructure facilitatedelectron migration throughout the secondary particles. Meanwhile, the citricacid-derived amorphous carbon species are covered on the surface of Li₃V₂（PO₄）₃,which would diminish particle growth and improve the conductivity. The dischargecapacity of the composite could severally achieve131.4and181.5mAh/g in thevoltage range3.0–4.3V and3.0–4.8V at0.1C discharge rate, both of them showedfaint capacity decay when cycled for100times.