Binder-free Hybrid Architectures Based On Graphene,carbon Nanotubes And Carbon Nanocoils/NiCo Compounds As Electrodes For Supercapacitors

Supercapacitor,one of the promising energy storage devices,has the capabilities of large charge storage,and rapid distribution rate.Efficacious electrode materials with excellent physical,chemical and morphological properties are great significance to the development of next-generation high-performance supercapacitors.Carbon nanomaterials,such as graphene(Gr),carbon nanotubes(CNTs),carbon nanocoils(CNCs)and their composites have remarkable electric conductivity,high specific surface area,high stability,easy processing and low cost,are the promising candidates as electrode materials.However,stacking of Gr layers and agglomeration of CNTs limit their specific surface areas and consequently limit the uniform growth of pseudocapacitive nanostructure on their surfaces.Additionally,the conductive additives and binders also limit the electrochemical performance of the electrodes.In order to solve these issues,the research works in this thesis focuses on synthesizing binder free sandwich structures of Gr and CNTs to increase their specific surface areas and improve the performance of the prepared electric double layer capacitors,and uniform growth of NiCo compounds with high pseudocapacitive activity on the three-dimensional(3D)CNC conductive networks to improve the electrochemical performances by utilizing their synergistic effect.Major achievements in this dissertation are summarized below.1)A novel sandwich structure composed of CNTs and reduced holey graphene oxide film(RHGOF)was fabricated by a chemical vapor deposition technique.The CNTs grown inside the graphene layers act as spacers and bridges to increase the conductivity of RHGOF,while the CNTs grown on the surfaces of the graphene contribute to increase its specific surface area.The resultant composite has faster ion diffusion rate,longer diffusion distance and more abundant charge storage sites as compared to the pristine RHGOF.As a result,a high galvanostatic charge-discharge specific capacitance of 557 F g-1 at a current density of 0.5 A g-1,and excellent rate capability and superior cyclic stability have been achieved.2)Nickel cobalt oxide(NiCo2O4)was grown on highly conductive substrates of CNCs/nickel foam(NF)via a solvothermal method to prepare binder free NiCo2O4/CNCs/NF electrodes,the porous feature of NiCo2O4 nanosheets and unique 3D spiral morphology of CNCs increase the electrochemically active sites in the composite electrodes.Moreover,the conductive network of CNCs/NF aids the electrolyte ions in diffusing deeply within the architecture.On these bases,NiCo2O4 nanosheet arrays and CNCs/NF conductive network play a synergistic effect to promote the redox reactions.The optimized electrode has a specific capacitance of 2821 F g-1 at the current density of 1 A g-1,and after 3000 cycles at the current density of 5 A g-1,the capacitance retention is 91.7%.3)Binder free and highly conductive NiCoNx/CNCs/NF composite electrodes were synthesized by annealing of NiCo hydroxide at different temperatures(400,500,600 and 700℃)in NH3 atmosphere.It is found that the composite annealed at 600℃ yields the best electrochemical performance.The specific capacitance is as high as 5235 F g-1 at the current density of 1 A g-1,and the rate capability is 86%at 50 A g-1.After 3000 cycles,the capacitance retention is 95.6%.The composite architecture,with NiCoNx nanoparticles anchored on CNCs/NF conductive network,has a promising prospect for high performance supercapacitors.4)In order to make full use of the nanosizes,high specific surface areas and conductive bridging effect of CNTs,a new binder free hybrid composite of CNTs/NiCoSx/CNCs/NF was constructed by filling CNTs in the voids of NiCoSx/CNCs composite with porous feature and abundant electrochemically active sites using a vacuum filtration technique.The introduction of CNTs effectively increases the specific surface area of electrode,provides sufficient ion storage sites and charge transfer channels,and cooperates with the pseudocapacitive NiCoSx.The optimized CNTs/NiCoSx/CNCs/NF hybrid electrode has specific capacitance of 3184 F g-1 at the current density of 1 A g-1,and capacitance retention of 97.2%after 3000 cycles.