Preparation And Electrochemical Properties Of Electrospun Carbon Nanofiber-based Flexible Electrodes For Supercapacitors

At present,with the increasing demand for portable or flexible electronic devices such as folding screens,wearable devices,and implantable electronic devices,it is particularly urgent and important to develop flexible energy storage devices with lightweight,environmentally friendly,high-performance characteristic.Flexible supercapacitors,which stand out due to their advantages of the fast charge-discharge process,high power density,and long cycle life,have great development potential in the field of flexible electronic devices.Carbon nanofibers,a commonly used electrode material for supercapacitors,have been widely studied in recent years due to their lightweight,electrical conductivity,and adjustable structure advantages.Although constructing pore structures on carbon nanofibers can significantly increase the specific surface area（SSA）for enhancing the specific capacitance of electrode materials,the formation of pores affects the conductivity and flexibility of carbon nanofibers.In addition,researchers have greatly improved the capacitance of carbon nanofiber-based electrodes by making holes and compounding pseudocapacitive materials,but they have not entirely overcome the contradiction between carbon nanofiber holes and the flexibility or conductivity of carbon nanofibers.Therefore,simultaneously improving the capacitance and flexibility of carbon nanofiber-based electrodes is the difficulty of preparing high-performance flexible electrodes.In this thesis,the internal cross-linked structure of carbon nanofibers was constructed by the self-synthesized phosphazene compound,and the polyaniline（PANI）decorated heteroatom-doped porous carbon nanofiber flexible electrodes were prepared via low temperature in situ polymerization;The interfiber cross-linked structure between carbon nanofibers was constructed by taking advantage of the difference in thermal stability of polymer materials,and nickel-cobalt bimetallic oxide decorated heteroatom-doped porous carbon nanofiber flexible electrodes were developed via in situ hydrothermal synthesis;Carbon nanotubes（CNTs）were used as the"bridge"inside the hollow porous carbon nanofibers to construct the internal bridge structure in the carbon nanofibers,and PANI decorated CNTs bridged hollow porous carbon nanofibers were prepared via in situ polymerization.The main research contents and the progress obtained include:（1）The mixture of polyacrylonitrile（PAN）,polymethylmethacrylate（PMMA）,self-synthesized phosphazene compounds（HAPCP）,and graphene oxide（GO）was electrospun and then carbonized to prepare flexible N,P doped porous carbon nanofibers（0.1GPHCNFs）.The study found that the HAPCP formed cross-linking networks with the carbon aromatic structure during the high-temperature carbonization process,thereby constructing the internal cross-linking of the carbon nanofibers（cross-linking within the nanofiber-microscale）,which effectively improved the flexibility（bent,twisted,and knotted）and electronic conductivity(331.6 S m^-1)of the carbon nanofibers.In addition,HAPCP in situ doped N and P atoms in carbon nanofibers;Thermal decomposition of PMMA created pore structures;The addition of conductive agent GO alleviated the conductivity loss.these improved the electrochemical performance of carbon nanofibers in different ways.In 1 M H₂SO₄ electrolyte,0.1GPHCNFs had a mass specific capacitance of 241.3 F g^-1 tested with a current density of 0.5 A g^-1.Inspired by the structure of Setaria Viridis,a hybrid electrode（400P@0.1GPHCNFs）was prepared by in situ polymerization of PANI nanofibers on the surface of 0.1GPHCNFs at low temperature.The 0.1GPHCNFs imitated the stalk of Setaria Viridis,providing electron and ion transport channels;while the PANI nanofibers grown on the surface of0.1GPHCNFs imitated the cilia of Setaria Viridis,which increased the contact area with the electrolyte,shortened the distance of ion transmission,and promoted the rapid diffusion of ions.Therefore,400P@0.1GPHCNFs obtained a high mass specific capacitance of 680.8 F g^-1 tested with0.5 A g^-1,and the capacitance retention rate reached 93.5%after 3000 charging and discharging cycles,indicating good charging-discharging cycling stability.In addition,the flexible symmetric supercapacitor assembled by 400P@0.1GPHCNFs as both positive and negative electrodes owned an energy density up to 27.7 W h Kg^-1,and successfully powered electronic devices such as electronic watches and electronic thermometers.（2）The eccentric special-shaped cross-section core-shell composite nanofibers with the mixture of PAN and polyvinylpyrrolidone（PVP）as the shell and PVP as the core were prepared by coaxial electrospun with a custom eccentric coaxial needle.Then,the N-doped cross-linked carbon nanofibers（NCNFs）were prepared by high-temperature carbonization.Taking advantage of the difference in thermal stability between PAN and PVP,the PVP in the core of composite nanofibers flowed out from one side and adhered to adjacent nanofibers（interfiber crosslinking-macroscale）.The construction of cross-linking between carbon nanofibers can effectively improve the flexibility of carbon nanofibers（bent,twisted,and knotted）on the one hand,and overcome the obstacles of large contact resistance and long charge transfer times caused from insufficient fiber-to-fiber contact in electrospinning membranes on the other hand.Moreover,PVP acted as N dopant and activator during the preparation of NCNFs simultaneously.Therefore,in the 2 M KOH electrolyte,NCNFs exhibited a high specific capacitance of 196.3 F g^-1 tested with 0.5 A g^-1.When the current density was increased by 20 times,the capacitance retention rate reached 53.5%(105.0 F g^-1).Subsequently,Ni Co₂O₄ nanoneedles were hydrothermally synthesized on the surface of NCNFs to prepare hybrid electrodes（Ni Co₂O₄@NCNFs）,and Ni Co₂O₄@NCNFs had a mass specific capacitance high to1474.2 F g^-1.And the mass specific capacitance was still as high as 1149.7 F g^-1,maintaining 78.0%,at the current density up to 10 A g^-1.The contributions of the Faradaic reaction current and the capacitive response current of Ni Co₂O₄@NCNFs during the electrochemical reaction were analyzed,revealing that the capacity generated by the diffusion process dominated the total capacitance of Ni Co₂O₄@NCNFs.The asymmetric supercapacitors assembled with the cathode of Ni Co₂O₄@NCNFs and the anode of NCNFs exhibited an energy density high to 53.0 W h kg^-1 along with a power density up to 402.8 W kg^-1.The assembled flexible asymmetric supercapacitor device could power a lamp group with a"DHU"pattern composed of 25 LEDs,and the device could continuously and stably power electronic devices under severe bending.（3）Inspired by the natural phenomenon of"the lotus root breaks,but the fibers hold together,"the carbon nanotube（CNTs）bridged hollow porous carbon nanofibers（HPCNFs@CNTs）were designed.Core-shell nanofibers composited with MWCNTs were prepared via coaxial electrospinning with PAN,PMMA,and PVP as the shell and PMMA and multiwalled carbon nanotube（MWCNTs）as the core.The MWCNTs in the core were oriented and partially entered the shell of core-shell nanofibers because of the vigorous whipping motions of the jet during the electrospinning process.If the prepared HPCNFs@CNTs were regarded as composite materials,MWCNTs,as the reinforcement of hollow porous carbon nanofibers（matrix）,endowed the HPCNFs@CNTs membrane with a tensile stress of 11.0 Mpa and bending stiffness of 1.4 m N mm².HPCNFs@CNTs had excellent flexibility and could be continuously bent,twisted,and bent without damage.The bridging structure of CNTs in HPCNFs@CNTs enhanced the electrical conductivity of carbon nanofibers,and the hollow and hierarchical porous structure of carbon nanofibers endowed HPCNFs@CNTs with high SSA.Therefore,HPCNFs@CNTs owned a specific capacitance as high as 461.0 F g^-1 in 1 M H₂SO₄ electrolyte with a current density of 0.5 A g^-1,which retained 98.7%of the initial capacity after 10,000 charging-discharging cycles.The hybrid electrode（300HPCNFs@CNTs）was fabricated by in situ polymerization of PANI on the surface of HPCNFs@CNTs,which exhibited a mass specific capacitance up to 629.1 F g^-1 at 0.5 A g^-1,maintaining 88.5%of the initial capacitance after 5000 charging-discharging cycles.The symmetric supercapacitor assembled with 300HPCNFs@CNTs exhibited good rate capability（76.7%capacitance after 20-fold expansion of current density）and excellent cycle stability（91.3%capacitance after 5000 charging-discharging cycles）,as well as a high energy density of 23.3 W h kg^-1 along with a power density of 202.7 W kg^-1.In addition,the assembled flexible symmetric supercapacitor could continuously power electronic devices in the continuous bending state,reflecting the value of 300HPCNFs@CNTs for practical applications.To sum up,in consideration of the shortcomings of the existing carbon nanofiber-based electrodes,such as low capacitance and poor flexibility,high-performance supercapacitor electrodes and devices were designed and prepared from four aspects:elemental composition（heteroatom doping）,structural design（crosslinking inside of nanofibers,inter-fiber crosslinking,hollow structure,porous structure,and CNTs bridging structure）,materials compositing（conductive polymer or metal oxide compounded with carbon nanofibers）,and assemblage of devices（symmetrical supercapacitors and asymmetrical supercapacitors）.Discussing the influence and action mechanisms of electrode composition,morphology,and structure on its electrochemical performance are analyzed and explored,as well as the capability of the assembled supercapacitor in practical application,according to the systematic characterization of material morphology,structure,and the electrochemical performance of electrodes and supercapacitors.The relevant results of this study provide some new ideas and methods for developing flexible electrode materials.