Graphene-carbon Fiber Composite As Electrode In Supercapacitor

Due to its low production cost, capability for mass production, feature of adjustableperformance, etc., graphene oxide has become one of the most popular research ofgraphene and its applications. Because of its excellent performance in many aspects,carbon fibers have been widely used in our daily life. In this thesis, we combine grapheneoxide with carbon fibers through electrochemical deposition method to prepare graphene-carbon fiber composite materials applied as supercapacitor electrodes exhibitingexcellent electrochemical performance.Graphene coating with folded, wrinkled structure was prepared by synchronousdeposition-reduction method. During deposition process, graphene oxide can be reducedto some extent at the same time. Then, the graphene-carbon fiber hybrid materials canbe employed directly in the electrochemical test, and display excellent electrochemicalproperties in aqueous electrolyte which is over hundred times higher than that of purecarbon fiber. The specific capacitance of the composite materials is enhanced by anorder of magnitude, reaching at22.6μF/cm through further optimization, i.e. oxidationpretreatment of carbon fiber before electrochemical deposition of graphene coating. Itsspecific capacitance remains95%after5000charge-discharge cycles, suggestingexcellent capacitive stability. Such a gratifying capacitive performance benefits fromenhanced specific surface area by wrinkled graphene coating, thereby forming moredouble layer interface.A solid supercapacitor is prepared using graphene-carbon fibers electrode and gelelectrolyte made of polyvinyl alcohol (PVA) containing phosphate acid. Length specificcapacitance of~13.5mF/cm, energy density of1.2~1.9μWh/cm and power density ofup to0.7mW/cm are obtained. When the solid supercapacitor is bended to180°, itselectrochemical properties are not decreased. The solid device can be easily assembledin series when used showing potential application in flexible electronic devices.In order to increase the amount of electrochemical deposited graphene oxide oncarbon fibers, staged deposition-reduction method is applied followed by freeze-dryingand chemical reduction of graphene oxide to form a unique three-dimensional structureof graphene-carbon fibers hybrid materials. Length specific capacitance of such an electrode reaches at7mF/cm, and the corresponding mass specific capacitance isapproximately35.7F/g. To further improve electrochemical performance, polyaniline iselectro-polymerized on graphene surface from aniline. The diameter of as preparedpolyaniline-graphene-carbon fibers composite electrode is approximately1mm.Electrochemical performance of the hybrid electrode is enhanced by pseudo-capacitancebehavior of polyaniline nanoclusters. As a consequence, length specific capacitance ofthe ternary composite reaches at257mF/cm,36.7times higher than graphene-carbonfibers electrode, and mass specific capacitance of205F/g is obtained correspondingly.Besides, energy density and power density are measured to be~19.6Wh/kg and~2.1kW/kg.