Preparation And Properties Of Electrode Materials For Graphene-based Supercapacitors

Due to the excessive consumption of global fossil energy and the deterioration of the environment,the development and use of sustainable energy has become a major issue facing the world today.How to construct an effective energy storage and release system,so that the rational use of renewable and sustainable energy is a hot topic in this field.Supercapacitor is a new type of green energy storage device that can meet this demand.It has high power density,excellent cycle stability and good safety,and the electrode material is the key factor affecting the performance of supercapacitor.Graphene is a new type of two-dimensional material discovered in recent years.Due to its high specific surface area,high conductivity and good chemical stability,graphene is considered to be a promising electrode material for supercapacitors.However,graphene is prone to lamellar stacking during preparation and use,resulting in reduced utilization of specific surface area and affecting its application performance in supercapacitors.Therefore,how to start from the preparation of graphene and adjust the pore structure and different doping levels of graphene to improve the effective utilization of specific surface area is the key to the application of graphene in high performance supercapacitors.In this thesis,around the scientific problem of pore structure regulation and surface doping degree regulation of electrode materials,three kinds of graphene electrode materials with different pore structure and different functional group modification degree were designed and synthesized,and the morphology,structure,physical and chemical properties of the materials and the correlation with the performance of supercapacitors were explored.This study is expected to promote the development of high performance supercapacitor electrode materials.The main research contents and progress are as follows:1.A structure controllable graphene-based electrode material(LTRGO)was prepared by a low temperature thermal reduction method.Due to the inconsistent degree of reduction of different sizes of graphite oxide precursors during low temperature thermal reduction,a series of graphene-based electrode materials with different pore structure and functional group modification degree can be obtained by adjusting the size of the precursor.The larger the size of the precursor,the lower the reduction degree of graphite oxide during the thermal reduction exfoliation process,and the product has lower pore volume and higher modification degree of oxygen-containing functional groups.The LTRGO electrode material was used to construct the supercapacitor,and the electrochemical test analysis showed that the graphene prepared by using graphite oxide with larger precursor size could reach the specific gravimetric capacitance of 199.7 F g^-1 in the alkaline electrolyte at low current density.In the neutral electrolyte system,the device exhibits the gravimetric energy density of 13.3 Wh kg^-1and high volumetric energy density of 15.1 Wh L^-1.After 9000 cycles,the capacitance retention rate of the cell is 94.2%,indicating excellent cycling stability.2.In order to obtain a supercapacitor with a high volumetric energy density,it is necessary to construct graphene-based electrode materials with high particle density.In this section,the thermal reduced graphene(TGO)with oxygen-containing functional groups on the sheet was dispersed in NaOH solution by pulsed ultrasound according to electrostatic adsorption of electric charge.Based on the electrostatic adsorption of sodium ions and oxygen-containing functional groups,and the mechanism of increasing the proportion of oxygen-containing functional groups by treating carbon materials in an alkaline solution,a functionalized graphene-based electrode material(TGOP-Na)with high particle density was prepared.The material exhibits a graphite-like surface with a dense topography,and its particle density is nearly twice as high as that of TGO.In the electrochemical test,the TGOP-Na electrode exhibits the specific gravimetric capacitance of 310 F g^-1 and the volumetric capacitance of 469 F cm^-3 at low current density under the condition of alkaline electrolyte.Subsequently,the device was packaged and tested under a neutral electrolyte system.The TGOP-Na material can reach the gravimetric energy density of 16.0 Wh kg^-1 and the volumetric energy density of 24.2 Wh L^-1,which is a leading result in the carbon-based aqueous supercapacitors.3.In order to construct the hierarchical pore structure of graphene to facilitate the rapid migration of organic liquid ions between graphene sheets,this chapter mainly introduces the solvothermal method by mixing ethanol and hydrogen peroxide solvent to etch low temperature thermal reduction graphene,and the porous graphene(HTGO)was prepared.By controlling the concentration of hydrogen peroxide in the solvent,hydrogen peroxide radicals are generated during the reaction to etch the pores of the graphene sheets,and a series of graphene with different pore structures are constructed.After the ascorbic acid reduction,the gravimetric capacitance of the electrode(RHTGO-5)at 1 A g^-1 can reach 297 F g^-1.In the alkaline electrolyte and organic electrolyte system,the gravimetric energy density of RHTGO-5 supercapacitor can reach 7.2 Wh kg^-1 and 26.5 Wh kg^-1,respectively.After 5000 cycles under 5 A g^-1,the capacitance retention of the device in alkaline electrolyte and neutral electrolyte was remained 94.8%and 84.7%,respectively,indicating excellent cycling stability.