Investigation On Supercapacitors Properties Of Flexible Carbon Nanofibers Composite

Increasing power and energy density demands for next-generation portable and flexible electronics,such as roll-up displays,photovoltaic cells and wearable devices,have spurred the development of flexible,lightweight and environmentally friendly energy storage devices.Compared with conventional capacitors,flexible all-solid-state supercapacitors have attracted increasing interest due to their light weight,good deformability,wide operating temperature and high power density.However,there are still problems such as slow transmission rate of electron or electrolyte ions,insufficient deformability and low energy density.This dissertation focuses on the preparation and characterization of flexible electrode materials with high energy density and suitable conductive porous structures,and the important role these flexible materials play in improving the performance of supercapacitors.Firstly,polyacrylonitrile(PAN)and polyvinyl pyrrolidone(PVP)were used as the shell precursor,while polymethyl methacrylate(PMMA)and graphene nanosheets(GNs)were used as the core precursor.A hierarchical porous nonwoven of hollow N-doped carbon nanofibers embedded with graphene nanosheets(HN-CNFs/GNs)was synthesized by coaxial electrospinning,followed by pre-oxidation and carbonization.PVP is used as a nitrogen source and a pore former to generate micropores during thermal decomposition,while macropores are formed by the network structure of CNFs to obtain a hierarchical porous structure.It was found that GNs were curled inside the electrospun fibers,but freely expanded and connected adjacent CNFs after carbonization,increasing the conductivity and contact area with the electrolyte ions.When the weight ratio of GNs is 2 wt.%,the symmetric all-solid-state supercapacitor delivers a remarkable cycling stability with a capacity retention of 90%after 2000 cycles and high rate capacity.In addition,the device exhibits a maximum energy density of 6.3 Whkg-1 and a power density of 7431.0 Wkg-1.The device also displays outstanding flexibility with no obvious capacitance decay even when bent to 180°.Secondly,based on the above work,nickel hydroxide@carbon nanofibers(Ni(OH)2@CNFs)composites were prepared by acid oxidation and electrodeposition.The highly conductive network structure of the hierarchical porous CNFs facilitates the growth of the Ni(OH)2 nanosheets,the Ni(OH)2 nanosheets provide pseudocapacitance and shorten the ion diffusion path while increasing the contact area between the active materials and the electrolyte.When the electrodeposition time is 30 minutes,the composite exhibits a high specific capacitance of 1333 F g-1 at a current density of 1 A g-1 in a three-electrode configuration,and a high specific capacitance of 139 F g-1 at a current density of 1 Ag-1 in an asymmetric two-electrode system and good rate performance.The prepared flexible all-solid-state asymmetric supercapaciter exhibits a maximum energy density of 26.2 W h kg-1 and power density of 7549.3 W kg-1.The hierarchical porous structure,nitrogen doping,high conductivity network and uniform distribution of active materials proposed in this paper effectively improve the electrochemical performance of carbon nanofiber composites.The constructed flexible all-solid-state supercapacitor has great application potential in the field of wearable microelectronic devices.