Study On Synthesis And Supercapacitive Properties Of Graphene Based Composites

To face the challenge of the storage and transportation of new energy,there is an urgent need to design and develop new types of energy conservation and storage devices,which have high power density,high energy density and long lifecycle.Supercapacitor has attracted significant attention,due to its longer lifecycle,higher power density and less environmental pollution,compared to batteries.Graphene,the typical 2D materials,becomes an ideal electrode material for supercapacitor because it has a large theory specific surface area,excellent electrical conductivity and stable electrochemical ability,etc.In this dissertation,modified Hummers method was used to prepare the graphite oxide（GO）,and graphene materials（24h reduced graphene oxide（24h r GO）,curved graphene（CG）,and porous graphene（PG））were synthesized by chemical reduction method and hydrothermal method,respectively.MnO₂ was obtained through solution and hydrothermal method,respectively.Composites,consisted of curved graphene and MnO₂,were prepared through hydrothermal method.The microstructure and morphology of prepared materials were characterized by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,and Scanning electron microscopy（SEM）,and Nitrogen absorption-desorption isotherms and so on.Supercapacitive properties of graphene materials,MnO₂,and composites were examined by cycling voltammetry and electrochemical impedance spectroscopy.The difference of crystal structure between GO and graphite could be found through the examination of XRD.Variety of carbon-oxygen functional groups of prepared GO could be observed in the spectrum of FTIR.Different types of chemical reduced graphene oxide（CrGO）,having different degree of reduction,could be obtained through the change of reduction time.Results of XRD and FTIR indicate that 24 hours are the best bet to get CrGO with well reduced.SEM and Nitrogen adsorption-desorption isotherms prove that the sample of 24h rGO has a crumpled morphology with slightly wrinkles and micro-/meso-porous structure.The result of CV curve for 24h rGO indicates good cycling performance if the scanning speed is less than 60 mv/s.The result of AC impedance spectroscopy for 24h rGO shows that the contract resistance is 1.53Ω。Hydrothermal method was also employed to prepared graphene with different morphologies,and different acid concentrations and cooling-down methods adopted during the reaction procession were used to form curved or porous structure on graphene surface.The result of SEM indicates that low concentration of H₂SO₄,cooled in the oven,could obtain CG;while high concentration of H₂SO₄,cooled in the oven,could obtain PG.Restults of Nitrogen adsorption-desorption isotherms show that there are micro-/meso-porous structure on the surface of CG and PG.Results of CV curves for hydrothermal reduced graphene show a larger cycling area of PG than that of CG under the same window voltage.Results of AC impedance spectroscopy demonstrate the contract resistance and charge-transfer resistance of the electrode/electrolyte interface for PG are both better than that of CG.MnO₂ was prepared through two different ways,solution method and hydrothermal method.Results of XRD and SEM show that both methods could get MnO₂,but crystallinity and diversity of MnO₂ prepared by hydrothermal method are better than that prepared by solution methods.With curved graphene as a carbon substrate,curved graphene/MnO₂composites were synthesized by adding KMnO₄ into the mixture solution of GO and H₂SO₄.The result of FTIR shows that functional groups ofα-MnO₂ could be observed in the composites.Results of SEM and Energy dispersive spectroscopic（EDS）indicate that the distribution of MnO₂ is uniform throughout the surface of carbon substrate at the low concentration of H₂SO₄.The result of CV curve shows that MnO₂ possesses the good properties in the capacitance and resistance if it is pressed under a pressure of 5 MPa.It is showed that properties in the capacitance and resistance of the composite,with the addition of graphene,are both better than that of pure MnO₂.