Preparation And Properties Of Graphene And Graphene Composites For Supercapacitors

Supercapacitors, also known as electrochemical capacitors, are one of thepromising electrochemical energy storage devices. They have higher power densityand longer cycle life than batteries, and they are called hundred metres athletesbecause of their high speed of energy transfer. Recently, graphene has beenrecognized as one of most promising eletrode materials for supercapacitors becauseof its excellent electronic conductivity and high theoretical specific surface, ofwhich graphene composites have attracted much attention because they have canprovide higher energy density than other materials.In this paper, graphene and graphene/CNT/Ni（OH）₂composites were preparedas electrode materials for supercapacitor. The samples were characterized by XRD,BET, SEM and TEM tests, and their electrochemical properties were investigated byCV, EIS and galvanostatic charge/discharge tests.Graphite oxide was prepared by flake graphite via modified Hummers method,and chemical reduction method had been used to synthesize graphene. In thisexperiment, two different kinds of reducing agents were studied. The results showedthat the graphene prepared by using ascorbic acid（VC） had better performance thanusing sodium borohydride（NaBH4）. At a current density of0.2A/g, its dischargespecific capacitance could reach132F/g, and after300cycles at different currentdensity, the specific capacitance still kept in113F/g（2.0A/g）, which showed goodrate capability and cycling stability.Preparation of CNT/Ni（OH）₂composites were used a simple precipitation way,and the electrochemical properties of composites with different proportion wereinvestigated. The results indicated that the specific capacitance and rate capability ofNi（OH）₂had been effectively improved by introducing CNT. The composites withthe mass ratio of1:1had a good condition of capacity properties and powerperformance, at current density of2.0A/g, the maximum specific capacitance wasachieved350F/g. Therefore, this kind of composite was selected as precursor tosynthesize graphene/CNT/Ni（OH）₂composites.Graphene/CNT/Ni（OH）₂composites were synthesized by chemical reductionmethod and ultrasonic dispersion method. The experimental results revealed thatcomposites prepared by ultrasonic dispersion method exhibited more excellentperformance, and the introduction of graphene had further increased the specificcapacitance and rate capability of CNT/Ni（OH）₂composites. After300cycles atdifferent current density, the composite with the mass ratio of1:5provided higherspecific capacitance than other samples, it also hold460F/g at a current density of 2.0A/g. Furthermore, asymmetric supercapacitors using this composite as electrodematerials were studied and showed good electrochemical performance. The specificenergy density of this kind of asymmetric supercapacitors was almost close to15Wh/kg at a current density of0.5A/g, that was2times of graphene supercapacitors.And after500cycles the capacitance retention rate was still hold100%, whichexhibited a promising application prospect.