Preparation And Application In Supercapacitor Of Flexible Graphene Based Composite Film Electrode Material

With people's attention to environmental issues and the development of green energy storage devices,the energy supply for human life has gradually shifted from traditional fossil energy to clean,sustainable development of new energy.As a new type of energy storage device,supercapacitors have many advantages such as high power density,acceptable energy density,long cycle life,fast charge and discharge rate,wide operating temperature range,and high reliability,thus received extensive research and attention.On the other hand,with the rapid development of flexible electronic processes and new materials,a large number of lightweight,portable and foldable flexible electronic devices are entering our lives,and how to develop flexible energy storage matching these new types of electronic devices have become a hot area of current research.At present,the development of the flexible supercapacitor is mainly limited by the performance of the flexible electrode,and the flexible electrode material must satisfy both the good electrochemical performance and the excellent flexibility.Therefore,graphene and its composite thin film materials with ultra-high conductivity,mechanical properties,and self-supporting properties are considered to be ideal materials for flexible supercapacitor electrodes.Based on this,in this thesis,graphene is used as the core material,and a graphene-based composite film is prepared by vacuum assisted flow-filtration,which is used as a highly conductive flexible current collector both side coated with manganese dioxide,and further assembled into a flexible all-solid asymmetric Super capacitor.A series of characterization methods and test methods were used to analyze and discuss the microstructure and electrochemical properties of the prepared materials.The main research contents are summarized as follows:(1)Preparation and Basic Properties research of Graphene/Carbon Fiber Composite Films.Carbon fiber is pre-oxidized and de-slurry,then mixed with mesophase pitch toluene solution,carbon fiber membrane skeleton is prepared by vacuum assisted flow-filtration,secondary filtration graphite oxide is filled between carbon skeletons.A self-supporting G-CF-MP composite film with a three-dimensional network structure was obtained after annealing.The effects of different heat treatment temperatures of carbonation and graphitization on the morphology,electrical conductivity,and thermal conductivity of the thin film materials were explored and analyzed.Through structural characterization,it was found that carbon fibers form a carbon skeleton with high mechanical properties.Carbon fiber surfaces and the spaces between the fibers are evenly coated and filled with graphene.The mesophase pitch exhibits fluidity and viscosity after reaching the softening point can fully wetting the gap between carbon fiber and graphene,the three carbon materials act synergistically to obtain a high mechanical strength and high conductivity G-CF-MP composite film material.Conductivity test found that graphitization can effectively improve the conductivity of the material.The square resistance of G-CF-MP composite film after carbonization at 900? is 2.853 ?/sq,and the square resistance after graphitization is 0.229?/sq.The thermal conductivity test results showed that the thermal conductivity of G-CF-MP(900?)is 475.2 W/(m·k),and the thermal conductivity of G-CF-MP(2300?)is 532.8W/(m · k).(2)Preparation and Capacitance Properties of MnO2-ENGP Flexible Electrode.The dicyandiamide is used as a nitrogen source,and is mixed with the graphene oxide solution and then directly filtered to form a film,and then the nitrogen-doped graphene paper is obtained through high temperature treatment.The surface treatment process for NGP was performed with electrolytic etching method and loaded with MnO2 by electrochemical deposition to obtain a MnO2-ENGP self-supporting electrode.The MnO2-ENGP-2 electrode exhibited excellent electrochemical performance under the three-electrode test and reached a capacity of 368.3 F/g at a current density of 0.2 A/g and high cycle-life stability with capacitance retention of 98.4%at 5 A/g after 3000 cycles.(3)Assembly and Electrochemical Performance of Flexible All-solid Asymmetric Supercapacitors.The MnO2-ENGP flexible electrode was used as the positive electrode,the activated carbon was used as the negative electrode,and the Na2SO4/PVA gel was used as the solid electrolyte to successfully assemble the all-solid asymmetric supercapacitor with good flexibility.Electrochemical test results show that the device has a voltage window of 0 to 1.8 V and a specific capacity of 23.3 F/g at a high current density of 2 A/g(calculated based on the total mass of the positive and negative active materials),and the maximum energy density of 10.5 Wh/Kg(with a power density of 1800 W/Kg)and a maximum power density of 9000 W/Kg(with a energy density of 1.75 Wh/Kg).The CV curves tested under bending over 90° are highly coincident,which proves that the flexible all-solid-state device not only retains the excellent electrochemical performance of the supercapacitor but also has the ability to work normally under the state of mechanical deformation.