Preparation Of Electrospun PAN-based Carbon Nanofibers And Its Nanocomposites With Devisable Composition/Structure Applied In Supercapacitors

With the fast consumption of fossil fuels and increasing environmental issues,there is an urgent need for the development of effcient,sustainable and renewable energy sources from wind power,solar energy,tidal energy and so on.Since most of the above available energy sources are intermittent and regional in nature,high-performing electrochemical energy storage systems are currently the focus of most attention.Besides,the rapid development of electric vehicle and portable/wearable electronic devices also put forward the need for better electrochemical energy storage devices.Specifcally,lithium-ion batteries and supercapacitors are the most widely studied electrochemical energy storage devices,nowadays.Lithium-ion batteries have the advantage of high energy densities,while supercapacitors exhibit high power densities,superb cyclic lifetimes and fast charge/discharge rates.In order to meet the growing demand for the capacitance and endurance of energy storage devices,the improvement of the energy density of supercapacitors has become the key to their development.According to E=1/2CV²,the energy density of supercapacitors can be improved by increasing its capacity and operation voltage window.The capacity of supercapacitors is mainly influenced by electrode materials（including the composition,micro-nano structures,etc）and the structure of the electrode（including the stacking structure of electrode materials and the construction of conductive-binder network）,while the operation voltage window is affected by electrolyte and the symmetry of the positive and negative electrodes.Thus,preparation of high-performance electrode materials,optimization of electrode structures and broadening of the operating potential window are main tasks of this work.At first,to prepare high-performance electrode materials,a wise design of compositions and micro-nano structures of electrode materials is important.Electrospinning technique can be a proper choice to prepare fiber materials with devisable compositions and structures.Then,in order to optimize the conductive-binder network in the electrode,a type of carbon network composed of one-dimensional CNT-COOH and two-dimensional rHGO has been used to replace PTFE and acetylene black in the traditional electrode preparation.Finally,for the improvement of the operation voltage window,an asymmetric supercapacitor of PCNF/U-MnO₂//N,B-PCNFs has been assembled whose working voltage can reach about 2.2 V,and it also shows good flexibility and potential value of application.Main results of this work are as follows:1)At first,electrospun PAN fiber was prepared by electrospinning,and process conditions of pre-oxidation and carbonization of PAN were explored and optimized.It was revealed that the pre-oxidation process had an important effect on the morphology of the final CNF（PAN）,and the lower heating rate,such as 1 ^oC/min,was more conducive for the preservation of one-dimensional morphology of PAN fibers during the pre-oxidation process for formation of more trapezoidal structures in PAN fibers.The specific preparation process of CNF（PAN）after optimization was as follows:PAN fibers were pre-oxidated at 250 ^oC for 30 min with the heating rate of 1 ^oC/min in the air,and then carbonized at 800 ^oC for 60 min with the heating rate of 1 ^oC/min in 99.999%N₂.To improve the capacitive property of CNF（PAN）,MnO₂was introduced and CNF（PAN）/MnO₂ nanocomposite fibers were synthesized,which combined both advantages of pseudocapacitive and carbon materials.Finally,the specific capacitance of CNF（PAN）/MnO₂ was 178.6 F g^-1,much higher than that of CNF（PAN）which was only 3.0 F g^-1.2)N-doping porous carbon nanofiber was further prepared by electrospinning to improve the capacitive property of CNF（PAN）.The introduction of porous structures and N heteroatoms could help improve the electrical double-layer capacitance and pseudocapacitance,respectively.Besides,a three-dimensional aerogel stack structure of the porous carbon nanofibers was prepared to further enhance its capacitive performance.Specifically,ZIF-8 nanoparticles were used as the pore-forming agent to be co-electrospun with PAN to form the PAN/ZIF-8 fiber,and melamine as well as the organic ligands of ZIF-8（2-methylimidazole）was employed as N sources to form N doping on the surface of and in carbon nanofibers,respectively.The construction of the three-dimensional aerogel stack structure was based on ice-template method and freeze-drying technology.N-doping porous carbon nanofiber aerogel（N-PCNFAs）could be obtained after the carbonization of PAN/ZIF-8 fiber-based three-dimensional foam with GO and melamine in it.Finally,N-PCNFAs had excellent compressibility,elasticity and structural stability for its layer-by-layer structures and two types of mechanically reinforced structures.The specific capacitance of N-PCNFAs as the electrode material could reach 279 F g^-1 at0.5 A g^-1,and it also showed outstanding rate performance and cycling stability.3)Based on the synthesis of MnO₂and preparation of N-doping porous carbon fiber in the former two parts.a kind of PCNF/MnO₂ nanocomposite fiber was synthesized with hierarchical structures.Among PCNF/MnO₂,PCNFs was employed as the core and the interacted MnO₂sheets were as the shell.When served as electrode materials,PCNF/MnO₂ showed good electrochemical performance.At the current density of 0.5 A g^-1,its specific capacitance was 269 F g^-1 with a capacitance retention of 58%even at 10 A g^-1,and also shows superior cycling stability of 98%after 5000CV cycles.Thereafter,through in-depth study of the hierarchical of PCNF/MnO₂,it could be found that MnO₂of it showed higher specific capacitance(308 F g^-1)than that of CNF（PAN）/MnO₂(200 F g^-1)without well-designed porous structure,which indicated the superiority of the structure of PCNF/MnO₂.4)A PCNF/U-MnO₂flexible electrode was prepared through vacuum filtration with CNT-COOH/rHGO as the conductive-binder network on CNT-NC7000/PA filter.By the introduction of CNT-COOH/rHGO,PCNF/U-MnO₂ active materials were embedded tightly in the all-carbon network to form a strong electrode structure;the all-carbon network formed by one-dimensional CNT-COOH and two-dimensional rHGO endowed the electrode with good electrical conductivity;holey rHGO with in-plane nanopores and good electrolyte wettability of CNT-COOH/rHGO were both conducive to electrolyte penetration and ion diffusion.Thus,PCNF/U-MnO₂flexible electrode with CNT-COOH/rHGO showed good capacitance performance,having a specific capacitance of 250.3 F g^-1 at 0.5 A g^-1（mass of electrode materials consisted of PCNF/U-MnO₂,rHGO and CNT-COOH）.Furthermore,the specific capacitance of PCNF/U-MnO₂ alone in the electrode with CNT-COOH/rHGO was 289.8 F g^-1 at1 A g^-1,much higher than that of the electrode with traditional PTFE/C as the binder and conductor.The result showed the superiority of CNT-COOH/rHGO conductive-binder network.Moreover,the flexible substrate of CNT-NC7000/PA filter endowed the electrode with good mechanical deformation capacity.5)An asymmetric supercapacitor（ASC）of PCNF/U-MnO₂//N,B-PCNFs was assembled to improve the working voltage,with a flexible positive electrode of PCNF/U-MnO₂ prepared above and a flexible negative electrode of N and B doped porous carbon nanofiber（N,B-PCNFs）by electrospinning.The working voltage of the as-prepared ASC was 2.2 V.When 1 M Na₂SO₄ was used as the electrolyte,the specific capacitance of PCNF/U-MnO₂//N,B-PCNFs was 69.0 F g^-1 at 0.2 A g^-1,with the highest energy density tested of 45.8 Wh kg^-1 and corresponding power density of 218.7 W kg^-1.It also showed good cycling stability of 93.7%after 4500 cycles.When Li Cl/PVA hydrogel was used as the electrolyte,an all-solid ASC was prepared,with the highest energy density tested of 45 Wh kg^-1 and corresponding power density of 540.8 W kg^-1.Besides,when the all-solid ASC was in a bent state,its energy density could still reach 43.4 Wh kg^-1,remaining nearly the same as flat straight state as above.The result not only exhibited good capacitive property and high energy density of PCNF/U-MnO₂//N,B-PCNFs,but also showed its good ability of bending and deformation and the potentiality applied in the wearable device area.