Controlled Synthesis Of Three Dimensional Graphene-based Materials And Their Energy Storage And Electrosorption Properties

Graphene possesses many excellent physicochemical properties such as large theoretical specific surface area,high conductivity,excellent mechanical properties,good chemical stability and thermal stability,making it has a wide range of applications in energy storage and conversion,enbiomedical,anti-corrosion coating,sensing and other fields.However,the restacking vironmental management,or irreversible agglomeration of graphene layers in practical applications,caused by the strong van der Waals forces and ?-? attraction between the layers,can greatly affect the intrinsic performance of graphene and thus seriously affect its practical application.The three-dimensional(3D)graphene structure can not only solve this problem to some extent,but also provide a large active area,micro-reaction environment,porous and connected structure.By compounding graphene with carbon materials,such as porous carbon,hollow carbon spheres and carbon nanotubes,3D graphene-based composites were obtained.It can combine their advantages to build a more practical 3D connected network structure.This thesis aims to construct 3D graphene-base structure through its own self-assembly or combination with other carbon materials(such as hollow carbon spheres,carbon nanotubes)by using graphene oxide as a base material to reduce the restacking and irreversible agglomeration between graphene sheets and provide large active surface area,a large number of pore and connected structures.Finally,it uses synergies between assembly structures or materials to construct excellent supercapacitors,Li-S batteries or electroadsorption electrodes.In this thesis,a series of 3D graphene and its composites with different morphology and structure were prepared by the method of assembly or in-situ generation under set conditions.By changing the condition of preparation,the effects of different components,dosage or calcination conditions on the morphology and structure of products were investigated.The controlled synthesis of 3D graphen-based assembly structure was realized by adjusting key parameters in the preparation to further explore the relationship between assembly structure and performance in applications.Subsequently,the 3D graphene-based materials were designed for electrode to explore their performance in supercapacitors,Li-S batteries and organic dye removal.This research work mainly includes the following four contents:(1)Scalable synthesis of high-density N-doped graphene and its application for supercapacitors.The thesis presents a facile strategy for the preparation of high-density N-doped graphene(HNG).The success of this work depends on the use of polyethyleneimine(PEI)as crosslinker to realize fast cross-linking of graphene oxide(GO)under ambient conditions and thus rapid sedimentation,followed by thermal treatment of the sediment.The introduction of PEI could effectively prevent the restacking of GO sheet and the volume expansion caused by the intense thermal expansion of GO during the process of thermal treatment,allowing maximal volume shrinkage.The HNG shows a good balance between specific surface area and tapped density.By adjusting the amount of PEI,the tapped density can be tuned in the range of 0.90-1.20 g cm-3.While the tapped density reaches a higher value,the material still maintains a moderate specific surface area(changed from 457.0 to 119.6 m2 g-1).By weighing the factor of tapped-density and specific surface area,the product with the tapped-density of 1.15 g cm-3 and specific surface area of 195.5 m2 g-1 shows higher volumetric specific capacitance.The volumetric specific capacitance is up to 547.8 F cm-3 at a scan rate of 10 mV s-1 and 317.3 F cm-3 at a current density 0.2 A g-1.The product also shows good rate performance and long cycle stability.Compared to the volumetric capacitance of rGO obtained without PEI and graphene-based materials reported in literature,the as-prepared HNG also display excellent volumetric capacitive performance.In addition,the method can be easily scalable for large manufacturing of high-density N-doped graphene-based materials by simply increasing the amount of GO and PEI.(2)Size-controlled preparation of urchin-like reduced graphene oxide microspheres with high density and their application in supercapacitorBy using a simple emulsion assisted method,urchin-like reduced graphene oxide microspheres(UrGOMSs)were assembled in situ.It achieved a high-density assembly of reduced graphene oxide.Polyethyleneimine,as a macromolecule,could effectively stabilize emulsion system.Ethylenediamine with rich amines could induce the assembly of GO in spherical droplets,allowing the formation of UrGOMSs with high density.The UrGOMSs possess crumpled surface and urchin-like structure,which could provide large amount of efficient ion diffusion pathways and accessible surface area.The size of UrGOMSs can be tuned in the range of 2.30-4.58 ?m by regulating the water-oil ratio.The specific surface area of UrGOMSs reaches up to 356.33 m2 g-1 with a high density of 1.37 g cm-3.The specific surface area and pore structure of product are greatly affected by the calcination conditions.By optimizing the calcinated conditions and size,the product exhibit high volumetric capacitance of 527.6 F cm-3 at 10 mV s-1 and 452.1 F cm-3 at 0.2 A g-1.When used in symmetric supercapacitor coin-cell,the products show good cycling stability,higher volumetric energy density and power density.At 2 A g-1,the retention rate of the specific capacitance is 97.4%after 5000 cycles.At 0.2 A g-1,the volumetric energy density is 10.0 Wh L-1 and its volumetric power density is 137.1 W L-1.The above results indicate that the novel strategy for designing densely assembled graphene microspheres to improve the practical application of supercapacitors is feasible.(3)In-situ synthesis of reduced graphene oxide/hollow carbon microsphere composites and its application in lithium-sulfur batteriesA composite structure of reduced graphene oxide/hollow carbon microsphere(rGO/HCSs)was prepared by simple method of metal catalyzed in-situ thermolysis.The success of this work relies on the use of nickel to catalyze solid carbon source for in-situ growing hollow carbon microspheres in rGO networks,followed by the acid washing to remove metal catalyst.The resultant rGO/HCSs show a 3D interconnected porous structure by connecting the hollow carbon microspheres with rGO,which are beneficial for rapid ion diffusion and electron transport.The hollow carbon microspheres distributed homogeneously in rGO networks can provide macropore to allow ultra high sulfur loading.Benefiting from such architectures,an ultra high sulfur loading of 91.9 wt%can be realized in S/rGO/HCSs composite electrode.By controlling the size of PS spheres,the optimization of meso-/macroporous structure,specific surface area and the efficiency of charge transfer in sulfur host material can be realized,which will further improve the overall electrochemical properties of the materials.After the optimization,rGO/HCSs exhibit a high specific capacity,favourable rate capabilities and good cycling stability with high sulfur loading.At the current rate of 0.2 C,the discharge capacity is up to 1374 mAh g-1.After 100 cycles,the discharge capacity can still remain at 860 mAh g-1 with a Coulombic efficiency of above 99%.(4)Synthesis of porous reduced graphene oxide/single-walled carbon nanotubes film for enhanced removal of organic dye by electrosorptionElectrosorption is an electrochemically enhanced adsorption phenomenon,which is mainly used in desalination or removal of heavy metal ion.However,only few researches reported the application of this technology for enhanced removal of organic pollutants.In this thesis,a kind of porous reduced graphene oxide/single-walled carbon nanotubes(prGO/SWCNTs)film was prepared by simple directional flow assembly method as a flexible electrode for the electroadsorption of organic dyes.Polystyrene served as templates were introduced into the sheets of GO to generate macropores,which could prevent the restacking of the GO sheets.SWCNTs were interspersed among the film,which could not only further improve the specific surface area of the film,but also provide an efficient electronic conduction pathway and improve the conductivity of the film.In addition,the introduction of SWCNTs and porous structures can improve the hydrophilicity of the film,which facilitates the penetration of electrolyte and the transport of electrolyte ions across the film.Upon used as freestanding electrode,the film exhibits extraordinary adsorption capacity and good recyclability for removal of methylene blue(MB)from water.The maximum adsorption capacity reaches up to 13014.3 mg g-1.After 5 recycles,the capacity retention is around 103%of initial value.These results indicate the feasibility of the novel strategy for designing graphene-based freestanding films to create flexible electrode for enhanced removal of organic dye.