A Free-standing Nanocomposite Comprising Nanocarbon Materials And Graphene Nanosheets For Supercapacitor

Supercapacitors are electrochemical energy storage devices between secondary batteries and general physical capacitors. Because of their high power density and better cycling property, in recent years, supercapacitors have attracted tremendous interests in electrochemical energy storage applications. The intrinsic capacitance of single-layer graphene reaches ca. 21 μF cm-2 when the entire surface area is used. As the theoretical surface area of single layer graphene is too high ～ 2630 m2 g-1 it is expected to show Cs of 550 F g-1. However, solution processing results in aggregation of the obtained graphene nanosheets due to strong van der Waals forces of attraction(π～π interactions between adjacent sheets) resulting in lower values of surface area and lesser number of electrochemically active sites thereby giving lower than the theoretically expected value of Cs. In this thesis, the formation of graphene with nanocarbon materials were used to improve their electrochemical performance.(1) we demonstrate the design and simple fabrication of an ultralight weight and highly conductive graphene nanosheets/carbon nanotube paper(GN/CNTP) composites as high-rate free-standing flexible electrodes for electrochemical capacitors. The SEM images of GN/CNTP clearly exhibits that GN has been successfully deposited on the surface of CNTP. This structure alleviates agglomeration of the 2D graphene nanosheets and maximizes the utilization of the surface of the graphene nanosheets to guarantee the permeation of electrolyte ions. Highly conductive carbon nanotube papers(CNTP) are chosen as a flexible three-dimensional conductive substrate, can serve as an effective “spacer” to prevent the restacking of graphene nanosheets. The specific capacitances of the GN/CNTP composite with a mass fraction of 80% were 18.1 m F·cm-2 at a current density of 3.2 m A·cm-2 and 16.3 m F·cm-2 at 24 m A·cm-2, respectively. The specific capacitance of GN/CNTP composite remains at 16.7 m F·cm-2 after 10000 cycles at a current density of 6.4 m A·cm-2 with a capacitance retention of 93.1%.(2) A novel 1D porous carbon nanofibers decorated graphene was fabricated as an effective electrode material for supercapacitor through a rational self-assembled hierarchical nanostructure design. The porous carbon fibers layer served as an effective inhibitor of the aggregation between graphene sheets, while the graphene sheets acted as a highly conductive carbon frame. Compared to the graphene, the as obtained electrode materials exhibited higher surface area and more favourable pore size distribution. The as-made electrodes exhibit a reversible specific capacitance of 270 F·g-1 at the current density of 0.5 A·g-1in 6 M aqueous KOH electrolyte. The other electrochemical revealed that the as-made electrodes offered high rate capability and excellent cycling stability.(3) N-doping carbon nanofibers(NCNF) comprised by graphene hydrogels are prepared via one-step strategy. The three-dimensional(3D) network architecture composed of N-doping carbon nanofibers and graphene hydrogels(NCNF/GH) process high electrical conductivity and ion transfer efficiency. Nitrogen groups in the PNHCS not only improve the wettability of the carbon surface, but also enhance the capacitance by addition of a pseudocapacitive redox process.Cyclic voltammetry and galvanostatic charge-discharge are applied to investigate the performance of the NCNF/GH. Electrochemical tests show that NCNF/GH deliver a specific capacitance of 300 F·g-1 at a current density of 0.5 A·g-1. The improved performance is reasonably attributed to their unique 3D hierarchical structure.(4) We reported a new type of carbon foam/graphene hydrogels(CF/GH) hybrid material prepared through a facile two-step approach and explored its energy storage application as a free-standing electrochemical capacitors electrode. By simple hydrothermal and the subsequent thermal annealing of carbon foam/graphene hydrogel hybrid precursor, three-dimensional graphene aerogels with high mass, hierarchical porosity, and high conductivity were deposited on a carbon foam framework. The resulting free-standing CF/GH electrode exhibited satisfactory double-layer capacitive behavior with high rate capability, good electrochemical cyclic stability, and a high specific capacitance of 331 F g-1 at a current density of 0.5 A g-1. With advantageous features, such a facile and versatile fabrication technique shows great promise in the preparation of various types of carbon-metal hybrid electrodes.