Study Based On Graphene Composite Electrode And Solid-state Capacitor

Supercapacitor is a new type of energy storage devices with high power density and excellent cycling stability. An amount of research results have shown that supercapacitors have potential applications in portable electronic equipment, hybrid electric vehicles,uninterruptible power supply ?UPS?,and weapons. Compared with lithium-ion batteries,supercapacitors have some important advantages, such as high power density, good cycling stability and low cost, but they have an unavoidable disadvantage, that is low energy density. To solve this problem, many researchers have tried to do lots of experiments on components of capacitor. However, the electrode materials are the most important factor which has played an important role in these components. So preparing nano materials with good performance has become an important way to overcome low energy density of supercapacitor.Graphene is a type of new 2D carbon material with excellent performance, which has an important application in supercapacitor. However, graphene sheets usually suffer from agglomeration and restacking due to the vander Waals interactions, which leads to a great loss of effective surface area and consequently a low capacitance. In order to enhance the performance of graphene based energy storage materials, researchers normally combine with other material to improve the specific capacitance by the synergistic effect in them. Cobalt oxide ?Co₃O₄? is an important transition metal oxides and has high theoretical capacitance ?3560 F/g?. Co₃O₄ has low conductivity that limits its application in supercapacitor. Thus, combing highly conductive materials with Co₃O₄ is an effective solution to this problem. Graphene is a typical candidate material for its high conductivity and other outstanding performance. Unfortunately graphene has low capacitance, so preparing the composite of Co₃O₄ and graphene has become a hot research. In this paper, firstly, using flake graphite as raw material, graphite oxide was prepared by modified Hummers' method. The characterization results show that graphite oxide has oxygen containing functional groups on the surface, which laid an important foundation for preparing composite. Then we explore the impacts of experimental conditions ?reactants' concentration, hydrothermal time, hydrothermal temperature, pH, calcining temperature? on the properties of materials and do a series of characterizations ?XPS, XRD, Raman, SEM?. The electrochemical properties of the composites were investigated by cyclic voltammetry, constant current charge and discharge, electrochemical impedance spectroscopy by three electrode system. The electrode material of optimal condition was assembled into solid state capacitor for testing. Finally, we prepared the composite in optimal conditions. The experimental results show that the composite achieve the specific capacitance of 1475 F/g at 0.1 A/g in the potential range of 0 to 0.8 V and the solid state capacitor exhibits an energy density of 13.97 Wh/kg at power density of 3.04 kW/kg. The main contents are as follows:1. We successfully prepared graphite oxide by Hummers' method and SEM, XRD,Raman were used to characterize the morphology and structure of graphite oxide.2. We study the impact of concentration of Co?NO3?2 and GO ?graphene oxide?,and obtain the optimal value are 0.01 M and 2 mg/mL, respectively.3. We investigate the impact of hydrothermal time and hydrothermal temperature,and obtain the optimal time of 15 h and the optimal temperature of 200?, respectively.4. We explore the impact of calcining temperature, and obtain the optimal temperature of 350?.5. We assemble a solid state capacitor, by using PVA-KOH as gel electrolyte,active carbon as negative electrode, graphen/Co₃O₄ as positive electrode which were prepared in optimal conditions,then electrochemical tests was perfomanced for solid state capacitor.