Preparation And Performance's Research Of Graphene Flexible Supercapacitor Based On Redox Method

Supercapacitor is a new type of energy storage device that can quickly store and release energy.Because of its high energy density,high power density,fast charge and discharge process,long cyclic life,and strong environmental friendliness,it plays an important role in the field of energy storage.With the rise of portable and wearable electronic devices in recent years,flexible energy storage devices have received widespread attention.The performance of portable,wearable energy storage devices heavily relies on the research of flexible electrodes.In this thesis,prepare graphene oxide(GO)dispersion was prepared through modified Hummers method.Then GO was assembled and reduced into flexible graphene membrane and fiber electrodes,and the electrochemical energy storage properties were systematically studied and optimized.Flexible graphene membrane electrode:GO dispersion was prepared through modified Hummers method and then assembled into GO membrane by spin coating.After reducing process,reduced graphene oxide(RGO)membrane was produced.Then the resulted RGO was transferred onto a flexible substrate to obtain a flexible graphene electrode and used for electrochemical energy storage.On the one hand,the ratio of deionized water to absolute ethanol in GO dispersion,the rotation speed and time of spin coating to prepare a continuous and uniform GO membrane.On the other hand,the dependence of reducing level on the reducing process was studied.Then,the GO membrane was reduced by hydroiodic acid to prepare a reduced graphene oxide(RGO)membrane.The reduced graphene oxide graphene flexible membrane was used for assembling supercapacitor,and the electrochemical energy storage performance was studied.At a current density of 2?A/cm~2,the specific capacitance can reach 0.418m F/cm~2.Flexible graphene fiber electrode:Through capillary molding,being assisted with chemical reduction and hydrothermal reduction,a simple method for preparing high-performance graphene fiber has been developed.The obtained flexible graphene fiber electrode exhibited excellent electrochemical energy storage performances.Three key points have been applied.i)The structure of graphene oxide was properly controlled to form lyotropic nematic liquid crystals which can be easily dispersed in water and form ordered arrangement of the reduced graphene sheets in the produced graphene fiber.This ordered arrangement can effectively increase the mechanical and electrical properties of the fiber,and thus can improve the energy storage performance and flexibility of the fiber electrode.ii)Capillary was used for the shaping and reduction of graphene oxide fiber.GO dispersion was mixed with ascorbic acid and then sealed into capillary.Heating the sealed capillary to 180?,a hydrothermal reaction environment was formed.Assisted with chemical reduction,graphene oxide was reduced and uniform graphene fiber was produced.iii)The concentration of the GO dispersion and reducing time were fully studied and optimized to control the arrangement of graphene sheets and reduction degree of RGO.Through these strategies,a simple and low-cost method was developed and the produced graphene fiber with proper arrangement and reduction structure exhibited excellent energy storage performances and mechanical flexibility.Without being composited with any extra active materials,the graphene fiber electrode reached 246.01 F/g at current density of1 A/g(equal to 553.03 m F/cm~2at 2.248 m A/cm~2).The electrode also exhibited excellent cyclic stability with capacitance retention of 90.8%after 100 000 cycles at current density of 10 A/g.Being assembled into symmetric supercapacitor,the graphene's specific capacitance reached 89.8 F/g at a current density of 1 A/g.The energy density can reach 3.118 Wh/kg with power density is 0.25 k W/kg.After 100 000charge and discharge cycles at a current density of 10 A/g,93.88%of the specific capacitance can be maintained.The supercapacitor also has good flexibility.It can be bent at different angles,and the specific capacitance remains basically unchanged.After 10 000 bending cycles at a current density of 1 A/g,the capacitance deduction was only 1.06%.Composite fiber electrode:The RGO fiber was further reduced by thermal reduction with assistance of hydrogen.By electrochemical deposition method,polyaniline(PANI)is deposited onto the further reduced RGO fiber.PANI is a conductive polymer material that can provide pseudocapacitance.The dependence of the composite electrode performance on the PANI content in the composite electrode was mainly studied.When the deposited PANI content is small,the porous microstructure of the RGO fiber can be well maintained,no obvious PANI aggragation can be observed,and thus the energy storage performance of the graphenen fiber was improved.At a current density of 1 A/g,the composite fiber electrode with a PANI content of 8%was increased from 68.9 F/g to 108.5 F/g.When excessive PANI was deposited onto the graphene fiber,the energy storage performance was deducted because PANI aggragated and the pores of graphene fiber were sealed.