Electrochemical Properties Of Graphene Pores And Pore Graphene And Its Composite Electrode Materials

As a new type of energy storage systems, supercapacitor plays an important role in the modern society because it makes fast energy storage and release become possible. Electrode materials are one of the most important factors affecting the whole performance of the supercapacitor and they have attracted much attention during these years. Among various electrode materials, graphene has become the distinguished one due to its outstanding physical and chemical properties. However, its physical and chemical properties are limited by its irreversible aggregation tendency during the fabrication process, as well as, the long ion diffusion pathway between its different layers. This thesis focuses on the creation of nanopores on graphene sheet and the preparation of pesudocapacitive materials/holey graphene hybrid electrode materials. It is expected to improve the energy storage properties of graphene based hybrid electrode materials. The main research works are as follows:Through the low temperature hydrothermal reaction between graphene oxide ?GO? and hydrogen peroxide ?H₂O₂?, numerous nanopores with the pores size ranging from 0.5 to 6.0 nm can be successfully introduced into GO nanosheet, without sacrificing the dispersity of the obtained product. The as-obtained holey graphene oxide ?HGO? is reduced by hydrazine solution, and then the flexible holey graphene film can be assembled by simple vacuum filtration. The porosity of the obtained holey graphene is systematically investigated by optimizing the reaction conditions between GO and H₂O₂. In three-electrode system with 6.0 M KOH as aqueous electrolyte, the holey graphene film with optimum porosity exhibits high gravimetric capacitance (251 Fg^-1) and high volumetric capacitance ?216 F cm-3? at a current density of 1 A g^-1. When the current density increases from 1 Ag^-1 to 60 A g^-1, its capacitance retention reaches 73%, which is superior to the graphene film ?63%?. The holey graphene film with optimum porosity is employed to assemble a symmetric supercapacitor with 6.0 M KOH as aqueous electrolyte and its relaxation time constant ??o? is 0.67 s, which is much shorter than the graphene based symmetric supercapacitor ?1.51 s?. This is an eco-friendly and low cost method, which is favorable for the large-scale production of holey graphene.Vanadium pentoxide/holey graphene hybrid aerogel ?VHGA? has been prepared by hydrothermally treating the mixture of well-dispersed HGO, ammonium metavanadate ?NH₄VO₃?, and acetic acid at 180 ? for 24 h. The mass loading of vanadium pentoxide ?V₂O₅? can be controlled by adjusting the added amount of NH₄VO₃. When the added amount of NH₄VO₃ is 60 mg, V₂O₅ nanobelt with a diameter of 20-80 nm and a length of a few millimeters can be homogenously dispersed on the HRGO sheet, then VHGA-2 hybrid electrode is prepared. In three-electrode system with 0.5 M K₂SO₄ as aqueous electrolyte, the specific capacitance of VHGA-2 electrode is 264 F g^-1 at a current density of 0.25 A g^-1, which is much higher than HGA (130 F g^-1) and V₂O₅ nanobelt electrodes ?153 F g-'?. When the current density increases from 0.25 A g^-1 to 10 A g^-1, its capacitance retention reaches 77.3%, which is superior to the V₂O₅ nanobelt electrode ?45.8%?. In addition, VHGA-2 shows an enhanced cycling stability by remaining 85% of its initial specific capacitance after 1000 charging/discharging cycles under a moderate scan rate of 20 mV s^-1. The cycling stability of VHGA-2 electrode is much higher than V₂O₅ nanobelt electrode ?38%?, which clearly demonstrates the advantages of the composited material electrode. In contrast with VHGA-2, the specific capacitance of VGA-2 is only 213 F g^-1, and its capacitance retention decreases to 71.4% when the current density increases from 0.25 A g^-1 to 10 A g^-1, indicating that the numerous nanopores on graphene nanosheet are beneficial to enhance its electrochemical property. This simple preparation method can also be extended to the preparation of other pesudocapacitive materials/holey graphenre hybrid electrode materials, and it also provides a new way to improve the energy density and power density of supercapacitor.