Surface Modification And Microstructure Regulation Of 3D Nano-carbon Materials And Their Electrochemical Properties

Supercapacitors,as a new-style green energy storage and conversion devices,have attracted considerable attention due to their fast charge/discharge rate,long cycling stability and high power density.However,the energy density of supercapacitors,especially for volumetric performances,is still relatively low,restricting their further applications.Carbon materials,as one of the most widely used commercial electrode materials,have high surface area,porous structure and excellent electrochemical performances.However,carbon materials store the charges by the electric double layer,and show low bulk density,leading to low specific capacitance(especially for volumetric capacitance).Therefore,the specific capacitance improvement of carbon materials through control of the structure and process becomes one of the research hotspot in this field.In this thesis,the densely packed graphene electrode materials with high volumetric energy density was investigated in detial.The diffusion of the electrolyte ion within the dense graphene material was improved considerably by inserting of support materials between the graphene layers and introducing of pores on the graphene surface.Furthermore,the oxygen-containing groups,which gives additional pseudocapacitance,were also introduced on the edges of graphene to enhance the specific capacitance of dense graphene electrode materials.In order to further improve the specific capacitance of carbon materials,deeply graphitized hollow spherical carbon materials with surface chemical modification was prepared and filled with electrochemical active substances.As a result,not only the specific capactiance of electrode materials was improved,but also the loss of active substance was inhibited.The research contents were illustrated in detail as follow:Firstly,densely packed graphene nanomesh-carbon nanotube(CNT)hybrid film(GNCN)was prepared through a simple graphene etching process and subsequent vacuum-assisted filtration method.CNTs can not only inhibit the aggregation of graphene,but also efficiently improve the overall electrical and mechanical properties of the hybrid film.The ion diffusion between graphene layers was effectively converted from 2D model into 3D model due to the introduce of pores on the sureface of graphene,which can also significantly shorten diffusion distance of the electrolyte ion.This structure design make it possible that electrolyte ions rapidly diffuse and transmit from all directions.Thus the key problem,like the long ion diffusion distance between the layers due to the densification of graphene,could be solved.The results show that the GNCN film electrode displays a high specific capacitance of 294 F/g and ultra-high volumetric capacitance of 331 F/cm3 at 5 mV/s in 6 mol/L KOH due to its high surface area,fast ion diffusion and high film density(1.126 g/cm3).In addition,the assembled symmetric supercapacitor achieves a high energy density of 23.1 Wh/kg and volumetric energy density of 26 Wh/L in 1 mol/L Na2SO4.Secondly,ozone oxidation and heat treatment process were combined to generate oxygen-containing functional groups on the edges of the graphene,and broke graphene into fragments.As a result,dense and oxygen-doped self-support graphene materials(FPGF-200)was prepared.The diffusion rate of electrolyte ions could be improved by interlayer surpport structure.The overall electrochemical capacity of electrode materials was enhanced by introducing oxygen containing functional groups,which provides additional pseudocapacitance.The effect of the ozone oxidation degree and heat treatment temperature on the material microstructure as well as the doped content and the stability of oxygen containing functional groups was investigated.The results show that ozone treatment can generate a large amount of oxygen containing functional groups on the edges of graphene,and the stable oxygen-containing functional groups can be selectively retained by appropriate temperature control.The obtained FPGF-200 material by 200? thermal treatment has a dense structure and high bulk density,resulting in an ultrahigh volumetric capacitance of 400 F/cm3 in 1 mol/L KOH electrolyte.The assembled FPGF-200 symmetric supercapacitor achieves high energy density of 18 Wh/kg at power density of 180 W/kg,and high volumetric energy density of 27 Wh/L in aqueous electrolytes.Finally,deeply graphitized hollow spherical carbon materials(HC)with 3D structure was prepared with nano silica as the template and asphalt as carbon source.Then oxygen-containing functional groups were introduced on the surface of HC by chemical oxidation method.As a result,carbon-based composite electrode materials(HPFC-F)with high electochemical performance was prepared by filling K3Fe(CN)6 redox-active additive into the pores of chemically modificated HC.The "reservoir" constructed inside this carbon materials makes it possible that more electrochemical active additives could be filled.The strong bounding between oxygen containing functional groups and K3Fe(CN)6 as well as the narrow pores on "reservoir" wall can considerably inhibit the loss of active substance,and shorten the electrolyte ion diffusion distance.Moreover,the adsorbed Fe(CN)63-can not only serve as a counterion dopant to ensure electronic conduction of the carbon frameworks,but also give pseudocapacitance to enhance the whole capaitance of the electrode composite.The HPFC-F shows pseudocapacitance in partial positive voltage of-0.2 to 0.8 V(vs.SCE),and electric double layer capacitance in the potential range of-1 to 0 V(vs.SCE)in 1 mol/L Na2SO4 electrolyte.Therefore,A self-asymmetric supercapacitor,which exhibits high energy density of 53 Wh/kg at power density of 1.6 kW/kg and outstanding cycling stability,was assembled with the HPFC-F as both cathode and anode.