Synthesis Of Graphene-based Composite Materials And Their Application In Supercapacitor

Supercapacitors or ultracapacitors,also called electrochemical capacitors(EC),have attracted significant attention,mainly due to their high power density,long lifecycle,and bridging function for the power/energy gap between traditional dielectric capacitors and batteries/fuel cells.With their many advantages,EC have become very competitive choices for applications such as electric vehicles,electric hybrid vehicles,pulse laser techniques and storage of the energy generated by solar cells.Graphene,a two-dimensional single-atom-thick carbon material,is considered as the ideal electrode material for supercapacitors due to its excellent physical and chemical properties,such as tremendous specific surface area,high conductivity,and thermal stability.Regardless of the materials for EC electrodes,combining different materials to form composites should be an important approach because the individual substances in the composites can have a synergistic effect through minimizing particle size,enhancing specific surface area,inducing porosity,preventing particles from agglomerating,improving cycling stability,providing extra pseudocapacitance,and so on.As a result,the obtained composites can overcome the drawbacks of the individual substances and embody the advantages of all constituents.In this dissertation,the graphene/CNTs composites,graphene/Fe2O3 and graphene/PEDOT composites as supercapacitors electrode materials were prepared by different approaches and those composites were used as electrode materials to value the electrochemical properties for supercapacitor.The main contents are as follows:A carbon nanotubes(CNTs)and reduced graphene oxide(RGO)composite(RGO/CNTs)as electrode material were fabricated by combing freeze-drying with thermal reduced method.The influence of the carbon nanotubes addition and temperature of thermal reduced process on the morphology and electrical properties of the as-obtained composites were investigated.The results show that the three-dimensional conductive network with macroscopic pore structure was obtained by freeze-dried,and the nanotubes is evenly distributed on graphene matrix,which are easily accessible to the electrolyte ions,leading to a fast rate of charge transfer.These unique materials are found to provide high specific capacitance and good cycling stability.The RGO/CNTs composite(the F-RGO/CNTs=15 reduced at 400?)deliver a large specific capacitance of 415 F/g at the scan rate of 10 mV/s in the 6 M KOH electrolyte.The capacitance retention was maintained 98.7%after 1000 cycles.The graphene/Fe2O3 composites were synthesized by one-step hydrothermal method,by using the graphene oxide as the substrate,the ferric nitrate as iron source and the DMF as solvent.The results show that Fe2O3 nano-particles(about 50nm)were uniformly distributed in the surface of the reduced graphene oxide nano-sheets,the graphene oxide sheets were not only separated,but also the degree of fold was reduced.Benefits from the combined graphene and Fe2O3 particles are that the graphene/Fe2O3 composite electrode exhibits an ultrahigh specific capacitance of 799 F/g at the current density of 2 A/g.However,the cycling stability of the graphene/Fe2O3 composites are to be improved.For graphene/Fe2O3 composite,42%of the initial capacitance can be maintained after 100 cycles,and 22.9%after 500 cycles.RGO/PEDOT(Poly-3,4-ethylene dioxythiophene)composites was synthesized by the cyclic voltammetry method at the potential range of-1.0?1.2V.In this process,a free-standing graphene/PEDOT films was obtained by electropolymerization and electrochemical reduction.Graphene oxide was successful reduced to form the grapheme,while EDOT monomer successful polymerized in the working electrode surface.The effect of the scan rate and concentration of RGO on super-capacitance performance of the material were also studied.The optimum condition was achieved at the scan rate(30 mV/s)and grapheme oxide addition(1 mg/ml).The high specific capacitance of 124 F/g is obtained from cyclic voltammetry measurement at a scan rate of 10 mV/s in the 1 M Na2SO4 as the RGO/PEDOT composites.