Research On Preparation And Electrochemical Performance Of Graphene Based Composites For Supercapacitor

Graphene which has a two dimensional carbon plane with one-atomicthickness is considered as a new carbon material for supercapacitors, due to itsextraordinary electrical and high surface-to-volume. However, the lowcapacitance of graphene hinders its practical application. Additionally,pseudo-capacitor materials have high theoretical capacitances, but the poorelectric conductivity and low specific surface area of these materials reduce theutilization of active materials in the electrode, rate capability and cycleperformance. With the special chemical and physical properties of graphene, thecombination of graphene with a second phase material can further obtain thecomposites with excellent electrochemical properties. Herein, graphene wasprepared through Hummer method followed by thermal treatment. AndMnO₂/graphene composites, MnOOH/graphene composites, Co/graphenecomposites and Co₃O₄/graphene composites were synthesized, and theirelectrochemical performance was studied.Researches on the preparation of graphene show that the graphene preparedfrom graphite powers with regular morphology possesses small and regulargraphene clusters. However, the electrochemical characterization of grapheneindicates that this kind of carbon material exhibits different electrochemicalperformance in the terms of the specific capacitance and rate performance inNa₂SO₄electrolytes and KOH electrolytes, respectively. In Na₂SO₄electrolytes,specific capacitances of graphene are56.3F g^-1and18.1Fg^-1at CV scan rates of5mV s^-1and500mV s^-1, respectively. Moreover, in KOH electrolytes, specificcapacitances of graphene are146.0Fg^-1and95.8F g^-1at CV scan rates of5mVs^-1and500mV s^-1, respectively. The results show that graphene has a higherspecific capacitance in KOH electrolytes rather than that in Na₂SO₄electrolytes.The specific capacitance of the symmetrical capacitor consisting of graphene asthe active materials is steady when the cell potential range of symmetricalcapacitor is changed, indicating that graphene has ideal electrochemical stabilityin Na₂SO₄electrolytes and KOH electrolytes. Through hydrothermal reaction,the number of carbonyl groups within graphene was decreased. The results fromFT-IR and EIS show that the reduction of carbonyl groups can damage theconjugate structure of graphene and increase its hydrophobic, which reducedspecific capacitances of graphene.In the researches on the graphene based composites, MnO₂particles wereintroduced to the surface of graphene through the redox reacton between KMnO4 and graphene. And then, MnO₂was transformed to MnOOH nanowire under thehydrothermal condition. The electrochemical characterizations ofMnO₂/graphene and MnOOH/graphene show that the addition of MnO₂canenhance obviously specific capacitances of the electrode materials forsupercapacitor in Na₂SO₄electrolytes; aqueous ammonia added into the reactionsystem can decrease the electrochemical impedance of as preparedMnOOH/graphene and increase the specific capacitance.Co（OH）₂nanoplates can be inserted into the intercalated structure of graphenethrough the precipitation method. Co₃O₄nanoplates/graphene were preparedthrough thermal treatment of Co（OH）₂nanoplates within graphene-basedcomposites. Moreover, under the hydrothermal condition, Co particles can beobtained within graphene-based composites and dispersed well on the surface ofgraphene. The research on electrochemical performance indicates that specificcapacitances of Co₃O₄nanoplates and Co₃O₄nanoplates/graphene are203F g^-1and338F g^-1at a current density of0.2A g^-1; specific capacitances of Co andCo/graphene are460F g^-1and1373F g^-1at a current density of1.5A g^-1. Theelectrochemical reaction of Co/graphene occurs at-0.70V and^-1.07V （vs.Hg/HgO）. Hence, this composite material can be used as an anode material forsupercapacitors. The specific capacitance of the asymmetry capacitor usingCo/graphene as the anode electrode and graphene as the cathode electrode is197F g^-1at a current density of0.5A g^-1.