| With the serious environmental degeneration and gradual exhaustion of fossil fuels,it has become an urgent need for human development and social progress that designing and producing highly efficient renewable energy conversion and storage devices.Supercapacitors are considered as one of the potential candidates in the domain of electrical energy storage devices owing to their high rate performance,superior power density,good cycle stability and rapid charging-discharging rate.In addition,the widespread use of supercapacitors has been taking place in wearable electronic devices,aerospace industry,weapons manufacturing,traffic and transportation engineering,home and office intelligent systems,etc.Among various electrode materials,carbon nanomaterials(e.g.,zero-dimensional carbon dots,one-dimensional carbon nanotubes,two-dimensional graphene and three-dimensional porous carbon materials,etc.) have triggered an explosion of interest for supercapacitor application due to their unique features of long cycle life,high porosity,good electrical conductivity and chemical stability.However,the practical use of carbon nanomaterials is impeded by their configurational anisotropy,severe agglomeration,poor interfacial wettability and structural control problems.To solve these issues,the design and construction of carbon nanohybrids has been realized via reasonable hybridization and efficient dispersion between individual carbon nanomaterial.The resultant carbon nanohybrids with controllable synthesis and hierarchical structures have been considered as promising materials to realize the high energy and power densities,which fully utilizes excellent physical and chemical properties of carbon nanomaterials.There are still many challenges in the preparation of high-performance carbon nanohybrids with three-dimensional hierarchical structures by simple and novel methods,and there are some key issues that need to be solved ungently.For example,it is different to overcome the severe agglomeration of carbon nanomaterials and realize their effective combination and homogeneous dispersion.The interfacial interaction between the individual component in carbon nanohybrids materials need to be further modified for construction of efficient conductive networks.Besides,the practical applications of carbon nanohybrids have been limited by absence of simple and efficient synthesis methods.In this regard,we construct different carbon nanohybrids with three-dimensional hierarchical structures in molten-salt pyrolysis through reasonable hybridization and efficient dispersion between individual carbon nanomaterial.It is worth mentioning that the composition,structure,preparation condition,properties and applications of carbon nanohybrids can be tuned in this novel pyrolysis process.Considering their different microstructure,conductivity,specific surface area,and porosity,the electrochemical energy storage properties of as-fabricated carbon nanohybrids have been investigated.The main results obtained in this dissertation are as follows:(1)The interwoven carbon nanotube-carbon nanosheet(CNT-CNS)hybrids can be prepared by a simple pyrolysis of mixtures of powders consisting of cheap biomass glucose,commercially available CNTs,and eutectic molten salts.The molten salts act as renewable and recyclable solvents of the carbonization reaction for the preparation of 3D carbon homostructure.The effective hybridization between one-dimensional carbon nanotubes(CNTs)and two-dimensional carbon nanosheets(CNSs)has been realized.The presence of CNTs within CNT-CNS hybrids have severed as the interconnected conductive frameworks,and incorporation of CNSs into CNTs provides appropriate surface areas and well-balanced micro/mesoporosity.Furthermore,the regulation role of the molten salt/precursor weight ratios in the evolution process of morphology,ion-electron transport rate,specific surface area,pore size distribution and functional groups for CNT-CNS hybrids is evaluated,and the relationship between the structure of hybrids and electrochemical energy storage performance has been explored.The CNT-CNS hybrids with large surface areas and hierarchical porosity enable effective electron/proton transport pathways,which therefore exhibits a large specific capacitance of 370 F -1 at 1 Ag-1,a high energy density of 23.6 Whkg-1 at 2 kWkg-1 and an excellent cycling life of?100% retention after 5000 cycles.(2)A hierarchical hybrids of carbon nanotube-backboned microporous carbon(CNT-MPC)can be prepared by a simple pyrolysis of powder mixture of pristine CNTs,carbohydrates(glucose) and metal chloride salts.The molten salts provide a confined environment to regulate the activation process.The effective hybridization between one-dimensional carbon nanotubes(CNTs) and microporous carbon nanomaterials(MPCs) has been realized.CNTs bridge the separated MPCs together to build effective conductive pathway,and abundant micropores within MPCs provide highly active sites for electrochemical energy storage.Furthermore,the regulation role of the type of molten salts and molten salt/precursor weight ratios in the evolution process of microstructure,conductivity,hierarchical porosity,specific surface area and element composition for CNT-MPC hybrids is investigated,and the relationship between the composition/structure of hybrids and the supercapacitor performance has been explored.In the NCP@CNS,the CNT backbone provided a high electrical conductivity,while the MPC sheath possessed integrated meso-/micro-pores for efficient ion reservoirs,which therefore exhibits a superior specific capacitance of 320 Fg-1,an outstanding energy density of 22.2 Whkg-1 at 2 kWkg-1 and a long cycling life with 101.8%capacitance retention after 5000 cycles.(3)An innovational competitive carbonization-etching strategy was presented to prepare the nitrogen-doped carbon polyhedron@carbon nanosheet(NCP@CNS)hybrids by a simple pyrolysis of ZIF-8 in the eutectic molten salts.The molten salts provide a confined environment to regulate the activation process.The effective hybridization between nitrogen doped carbon nanopolyhedrons(NCPs) and two-dimensional carbon nanosheets(CNSs) has been realized.Their hierarchically porous NCPs serve as ion-buffering reservoirs for offering fast ion transport channels,and the CNSs within hybrids not only link the NCPs together to build electron transfer pathways but also restrict the volume fluctuation of electrodes during charging and discharging process.Furthermore,the morphological evolution,efficient heteroatom doping and construction of hierarchical conductive networks for CNT-CNS hybrids can be well-manipulated by tuning molten salt/precursor weight ratios,and the relationship between these parameters and electrochemical energy storage performance has been explored.Benefiting from synergy of components in supercapacitors,the NCP@CNS electrode exhibits a superior specific capacitance of 320 Fg-1,an outstanding energy density of 22.2 Whkg-1 and a long cycling life with 101.8%capacitance retention after 5000 cycles.(4)An interwoven carbon nanofiber/nanosheet network(CNF/CNS)has been produced by the pyrolysis of the biomass BC in a renewable molten-salt solvent,which can be employed as a conductive 3D carbon framework for uniformly immobilizing a PANI nanorod array(PANI@CNF/CNS).The effective hybridization between one-dimensional carbon nanofibers(CNFs)and two-dimensional carbon nanosheets(CNSs)is realized.The incorporation of CNSs into CNFs for consecutive CNF/CNS hybrids can further promote the mechanical strength,electrolyte contact area and ion/electron transport of carbon skeletons.The initial feedings of molten salts and precursors are tailored to investigate confined self-assembly,structural evolution and construction of interconnected electron/ion transport pathways of CNF/CNS hybrids.In addition,the relationship between surface wettability,ion/electron mobility and electrochemical performance of resultant hydrophilic PANI@CNF/CNS is further explored.Benefiting from high conductivity of the hierarchical carbon network and fast ion transport within the PANI@CNF/CNS,an asymmetric supercapacitor using the PANI@CNF/CNS and CNF/CNS as cathode and anode,respectively,exhibited a high energy density of 65.3 Whkg-1 at a power density of 800 Wkg-1and an excellent cycling life of 98%retention after 5000 cycles at 10 Ag-1. |
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