Preparation Of Hierarchically Electrospun Carbon Nanofibers Based Hybrid Composites For High-Performance Supercapacitors

As one of the key part of supercapacitors,electrode materials play a crucial role in electrochemical energy storage.At present,the greatest barrier in the development of supercapacitors is that their energy density is lower than that of secondary batteries,which seriously restricts the wide application of supercapacitors.The development of new electrode materials with high energy density and stable structure has always been a hot,difficult point of research.In this paper,carbon nanofiber-based three-dimensional composites with hierarchical structure were prepared by electrospinning,hydrothermal synthesis and in situ growth.We investigate the properties of these electrode materials for supercapacitors.Firstly,polyacrylonitrile nanofiber was prepared by electrospinning technology,and carbon nanofibers（CNF）was obtained after pre-oxidation and high-temperature carbonization.Then,TiO₂ nanorod arrays arrays were"planted"on the surface of the CNF to prepare CNF/TiO2 material.Finally,the nano-sheet Ni（OH）2 was grown between CNF/TiO₂ by secondary hydrothermal method,and the three-dimensional CNF/TiO₂@Ni（OH）₂ composite was obtained.Electrochemical tests demonstrate that the specific capacitance of the composite is 308.4 F g^-1 at a current density of 2 A g^-1,which is nearly 6 times higher than that of CNF/TiO₂,indicating that the nanostructure can greatly improve the specific capacity of the composite.In addition,after 3000 cycles of charge and discharge cycle test at 6 A g^-1,the electrode material still has an initial specific capacitance retention of 63%.Molybdenum disulfide（MoS₂）has a flake structure like grapheme.Nanostructured MoS₂ has a large specific surface area and good double-layer capacitance effect,so it is one of the potential candidates as electrodes of supercapacitor.In this paper,CNF/TiO₂ material was used as the substrate,and the CNF/TiO₂@MoS₂ core-shell hierarchical material like“cable”have been successfully prepared by growing MoS₂ nano-layered structure on the surface of CNF/TiO₂.The test results of galvanostatic charge and discharge show that the specific capacitance is as high as 510.4 F g^-1 at a current density of 0.5 A g^-1,which proves that the CNF/TiO2@MoS2 core-shell heterogeneous material has excellent capacitance performance.And the cycle life test was carried out at a current density of 3.5 A g^-1.After3000 cycles of continuous charge and discharge,the initial capacitance retention rate was as high as 95.7%,indicating excellent cycle stability of the electrode material.The metal-organic framework materials such as ZIF-8 have a porous structure,and there have been a lot of research reports in the fields of catalysis,gas storage and so on.However,due to its poor electrical conductivity,up to now,there are not too many reports on ZIF-8 material in the field of energy storage.In this paper,CNF/TiO₂@ZIF-8 composites were prepared by in situ growth of ZIF-8 nanospheres on the surface of CNF/TiO₂.Then,the composite was carbonized at 800℃in nitrogen atmosphere,and the surface of the CNF/TiO₂ was uniformly covered with a layer of cracked nano-sheet structure,mainly containing carbon and nitrogen compounds,zinc oxide and other residues.The carbonized composites were tested by electrochemical method.When the current density is 0.5 A g^-1,the mass specific capacity of the composite is 293.2 F g^-1.And after 2500 times of galvanostatic charge and discharge at 3 A g^-1,the initial capacity retention rate is 87.4%.These results show that the composite has good supercapacitor performance and cycle stability.