Construction Of Three-dimensional Graphene And Their Properties

Due to its excellent optical,electrical,mechanical and other properties,graphene has broad application in photoelectric conversion devices,energy storage electronic devices,biomedicine,catalysis,sensing,field emission,separation and purification and other fields,and has attracted extensive attention of researchers.However,due to the influence of van der Waals force,graphene crystal domains are prone to stacking in the process of splicing and continuous film formation,which greatly reduces the specific surface area of graphene and leads to the degradation of its double-layer electric-charge performance.The three-dimensional porous structure can not only effectively alleviate the stacking between graphene sheets,but also facilitate the transfer and shuttle of electrons and ions.Moreover,the open porous structure is an ideal conductive support,and the three-dimensional graphene with high porosity also has a large number of active edges.In this thesis,in order to increase the specific surface area of graphene and improve the transfer and shuttle efficiency of electrons and ions in graphene,we constructed two three-dimensional porous structures of graphene and used them as electrodes for supercapacitors.The research content of this paper mainly includes the following two aspects:（1）The carbon spheres were intercalated between the graphene layers to form a three-dimensional porous graphene/carbon spheres structure,and the composite was used as the electrode of the supercapacitor.Firstly,graphene oxide and carbon sphere dispersion were prepared by chemical redox and hydrothermal method respectively,and the carbon spheres were intercalated into the GO through mechanical ultrasound to form a graphene oxide/carbon spheres sandwich structure.The graphene/carbon spheres composite was obtained by high temperature reduction in an inert atmosphere.Another beneficial effect of high temperature calcination is to improve the pore structure of the composite.The results of nitrogen isothermal adsorption and desorption test combined with BJH calculation model show that the calcination temperature has a certain influence on the pore size distribution of the composite.Electrochemical test results show that the graphene/carbon spheres composite obtained by calcination at 800℃（GCS@800）has excellent double-layer capacitance behavior.The electrochemical properties of the materials were tested in 6M KOH.When the current density is 0.2 A/g,the specific capacitance of GCS@800 composite is 272.8 F/g,which is much higher than that of the original graphene,indicating that the formation of multi-level channels with the carbon spheres embedded can effectively promote the transport of electrolyte ions.（2）Vertical graphene was synthesized by plasma chemical vapor deposition and loaded with manganese dioxide as electrode material for supercapacitors.The morphologies and structures of vertical graphene were optimized by adjusting the position of substrates,gas composition,and temperature.With the unique three-dimensional porous structure of vertical graphene,it is used as double-layer electrode materials with high specific surface area and conductive frames to load manganese dioxide.The morphologies and structures of vertical graphene/Mn O₂ composite were optimized by changing the deposition rate of cyclic voltammetry process.SEM pictures showed that a thin layer of manganese dioxide（VGM500）was loaded on the vertical graphene sheet after one cycle of scanning when the deposition rate was 500 m V/s,and the porous framework of the vertical graphene was maintained.The electrochemical properties of the materials were tested in 1M Na₂SO₄.The specific capacitance of the composite was as high as 1110.35 F/g at a current density of 1 A/g,and the capacity retention rate was 83.4%after 3000 cycles（2 A/g）.When the power density was 612.8W/kg,the energy density attends to 34.48 Wh/kg.The performance of vertical graphene/Mn O₂ composite was better than that of Mn O₂ synthesized under the same conditions,indicating that vertical graphene plays an important role in improving the electrochemical performance of composite materials.