Construction Of Carbon Nanofiber Based Nanocomposites And Their Performance As Electrode Materials In Supercapacitor

With the continuous development of global economy,social resources are becoming increasingly scarce.How to realize the social sustainable development and seek new,clean,stable,efficient and low-cost renewable energy have become a hot research topic in the field of scientific research.Supercapacitor is a highly efficient and stable energy storage device with the ability to convert chemical energy into electric energy.Its advantages including quick charge-discharge capability,good cycle stability,no special requirement for the environment temperature,good safety,and easy to carry make it become the promising energy storage device.At present,the researchers mainly focus on finding new electrode materials and controlling its structure characteristics,as well as improving the original carrier characteristics to enhance the capacitance and cycling stability of the electrode materials.Carbon nanofibers（CNF）with one-dimensional structure have attracted wide attention in capacitors due to their unique physical and chemical properties,such as high conductivity,anisotropy of morphological structure,and controllable fiber surface structure.All these unique material properties play an important role in the construction of ideal electrode materialIn this paper,carbon nanofibers with one-dimensional structure were successfully prepared by using electrostatic spinning technology.By regulating the composition of spinning solution,optimizing spinning technology,and the following pre-oxidation and carbonization processes,the porous carbon nanofibers（PCNFs）,hollow porous carbon nanofibers（HPCNFs）,cobalt modified hollow porous carbon nanofibers（Co-HPCNFs）and polyacrylonitrile/lignin based hollow carbon nanofibers（PAN/lignin based HCNFs）were well synthesized,which were chosen as the matrix.Then PANI and metal oxide/hydroxide nanomaterials were uniformly loaded in the matrix,and finally,the pseudo-capacitance nanocomposites PANI/PCNFs and MnO₂/Co-HPCNFs,and the battery-like electrode material Ni（OH）₂/HPCNFs,NiCo₂O₄/HCNFs and Co₂（OH）₃Cl-MnO₂ composite were obtained.Then the capacitance property of these single electrode materials and the device assembled by these anode materials and the activated carbon（AC）were measured.In order to broaden the application of the CNF in new energy field,the MnO₂/Co-HPCNFs composite materials was also applied in zinc air battery.Its electrochemical activity toward oxygen reduction reaction（ORR）was measured.The main contents are as follows:（1）PAN/PMMA dissolved in DMF was as an electrostatic spinning precursor,where PMMA was a perforated agent.Followed by pre-oxidation and carbonation,the PCNFs was obtained.Then PANI is loaded on the surface of PCNFs through in-situ aggregation,with hydrochloric acid（HCl）and sodium dodecyl sulfate（SDS）as a dopant,and SDS also as an emulsifier at the same time to improve the distribution and stability of PANI on the surface of CNFs.The combination of CNFs with high conductivity,and PANI with good electrochemical stability makes the nanocomposite as an attracting electrode material for supercapacitor.The measurements showed that PANI uniformly grew on CNFs.Electrochemical analysis illustrated that the specific capacitance of the CNFs/PANI nanocomposites was much better than pure materials.The high specific capacitance of 501 F/g at a current density of 1 A/g and capacitance retention of 78%after 3000 cycles at a current density of 10 A/g were obtained due to the special hybrid structure（the performance was much better than the pure PANI）,indicating that the material has a great potential as supercapacitor electrode.The excellent capacitive performances could be ascribed to unique structural advantages of PANI nanofiber array enhanced by synergetic effect between PANI and PCNFs.（2）PAN/PMMA dissolved in DMF was as coaxial electrostatic spinning outer solution,PMMA dissolved in DMF was as inner solution,then followed by pre-oxidization and carbonization,HPCNFs was obtained.Nickel hydroxide nanobristles were fabricated on the surface of HPCNFs through a mild and effective bio-mediated technology,where the eggshells with eggshell membrane were selected as multifunctional reactors to separate the components of the precursor solution and control OH^-ion diffusion in order to formα-Ni（OH）₂ instead ofβ-Ni（OH）₂.The as prepared Ni（OH）₂/HPCNFs displayed high capacity（283 mAh/g at 1 A/g）and a remarkable cycle life（82%specific capacitance retained after 5000 cycles）.Moreover,a hybrid supercapacitor was fabricated,in which Ni（OH）₂/HPCNFs and activated carbon were as positive and negative electrode,respectively（Ni（OH）₂/HPCNFs//AC ASC）.The prepared supercapacitor also displayed excellent performance with a maximum capacitance（106 F/g at 0.5 A/g）,high energy density（31.3?W?h?/kg at300.5?W?/?kg）,and outstanding cycle life（87%retention of the capacitance after 5000cycles）.The enhanced electrochemical performance of composite performance is due to theα-Ni（OH）₂ formation by eggshell membrane-assisted preparation method.The prepared Ni（OH）₂/HPCNFs nanomaterials exhibit excellent electrochemical properties is due to synergetic effect betweenα-Ni（OH）₂ nanostribles and highly conductive HPCNFs.The results also demonstrate that the bio-mediated method for achieving Ni（OH）₂/HPCNFs nanomaterials is becoming a promising technology in supercapacitors.（3）PAN/PMMA/Co（CH₃COO）₂ dissolved in DMF was as coaxial electrostatic spinning outer solution,PMMA dissolved in DMF was as inner solution,then followed by pre-oxidization and carbonization,the cobalt-doped hollow porous carbon nanofibers（Co-HPCNFs）was finally obtained.A honeycomb double-layered MnO₂nanosheets were successfully deposited on both inner and outer surface of Co-HPCNFs by the hydrothermal method to form MnO₂/Co-HPCNFs.Benefitting from the distinctive hollow nanostructure of Co-HPCNFs,and honeycomb double-layered nanosheets of MnO₂,MnO₂/Co-HPCNFs nanocomposites exhibited an enhanced electrochemical performance in supercapacitor and zinc-air battery,including a high specific capacitance（446?F/g at 0.5?A/g in 1?M Na₂SO₄ electrolyte）,and a preferable ORR catalytic activity(E_1/2=0.842 V compared to commercial 20 wt%Pt/C catalyst of E_1/2=0.830V)and long-term stability.Notably,a primary zinc-air battery with MnO₂/Co-HPCNFs as the air cathode was fabricated and had better performance than the Pt/C catalyst in regard to long-term stability and power density.The as-prepared MnO₂/Co-HPCNFs show excellent capacitive and electrocatalytic properties,due to the hollow structure of Co-HPCNFs benefit the formation of double-layer MnO₂,double-layer MnO₂ nanosheets improve the surface area of composite materials,increase the reaction activity site,thereby showing a certain advantage in energy storage materials and energy conversion system.（4）PAN/lignin dissolved in DMF was as coaxial electrospinning outer solution,PMMA dissolved in DMF was as inner solution,then followed by pre-oxidation and carbonization,the PAN/lignin based HCNFs were fabricated.The results showed that the NiCo₂O₄ nanosheets were uniformly deposited on the inner and outer layers of HCNFs by a facile hydrothermal method without any toxic reagents are.The electrochemical results indicated that NiCo₂O₄@HCNFs composite had excellent electrochemical properties,including a considerable capacity of 1998.18 mF/cm²（1665F/g）at 2 mA/cm²（the mass loading of 1.2 mg/cm²）and an excellent cycling stability with 87%specific capacitance retention after 5000 cycles under the current density of10 mA/cm².In addition,an asymmetric supercapacitor（NiCo₂O₄@HCNFs//AC）was assembled with NiCo₂O₄@HCNFs and AC.The device exhibited high capacitance of111 F/g at 1 A/g,excellent energy density of 40.3 Wh/kg at 810?W/kg,and superior cycling stability（92%capacitance retained after 5000 cycles at 5 A/g）.NiCo₂O₄@HCNFs show better capacity performance is due to the hollow structure of the PAN/lignin-based HCNFs,which benefit the double-layer loading of the NiCo₂O₄nanosheets.The composites show good capacity performance is due to the synergy between the double-layer NiCo₂O₄ nanosheets and HCNFs.（5）Three-dimensional（3D）flower-like Co₂（OH）₃Cl-MnO₂ nanostructures were fabricated inside eggshell through a facile and effective method.Inspired by semipermeable membranes,a shell membrane was selected as an interface for ion diffusion.In specific,an eggshell with shell membrane was employed as a multifunctional reactor to separate the components of the precursor solution.OH^-ion diffusion was performed through porous eggshell membrane.The electrochemical measurements demonstrated that the hybrid composite achieved high capacity of4533mAh/m² at 1 mA/cm²（251 mAh/g with the mass loading of 1.8 mg/cm²）and an excellent cycling stability（71%specific capacitance retained after 5000 cycles）,which were much better than pure Co₂（OH）₃Cl or MnO₂.Moreover,a hybrid supercapacitor was assembled with Co₂（OH）₃Cl-MnO₂-2 and AC as positive and negative electrode,respectively（Co₂（OH）₃Cl-MnO₂-2//AC ASC）,which exhibited high capacitance（134.8 F/g at 0.2 A/g）,excellent energy density（42.2 Wh/kg at 150.3 W/kg）,and remarkable cycling stability（80%capacitance retained after 5000 cycles）.Benefited from eggshell membrane-assisted synthesis controlling the velocity of ion diffusion and the synergy between Co₂（OH）₃Cl and MnO₂,3D flower composite exhibits good electrochemical properties and is becoming a promising supercapacitor electrode material.