Electrospinning Nanomaterials For Supercapacitor Electrodes:Hierarchical Structural Design And Electrochemical Energy Storage Performance

The low energy density is the key factor that limits the application of the supercapacitors.Development of high-performance electrode materials is an effective way to increase the energy density.Electrospinning nanofibers with adjustable composition,adjustable wettability,good connectivity,and unidirectional electron flow is an excellent candidate as electrode materials for supercapacitors.This research combines electrospinning nanofibers with metal oxide and metal sulfides to prepare carbon nanofiber-based(CNF)two-dimensional flexible membranes and three-dimensional compressible and resilient aerogels as electrode materials.This work involves the design of flexible CNFs with hierarchical porous bionic reed structure,CNF aerogels with "welding" hierarchical structure,and multi-channel CNFs embedded with hollow metal oxide nanoparticles hybrid aerogel.The details are as follows:(1)Flexible reed-like structure nanofiber membrane and its composite electrodes with metal sulfide/oxide:In order to solve the problem of fragile characteristic and low capacity of electrospun carbon-based nanofiber membranes,in this chapter,CNF membranes with a reed-like structure are prepared through electrospinning,high-temperature carbonization,and SiO2 template etching.The CNF membranes exhibit flexibility due to the unique porous structure.Afterward,metal sulfides and metal oxides are loaded on the surface of the nanofiber membranes as the cathode and anode through electrochemical deposition,hydrothermal,polydopamine coating,and heat treatment,respectively.The cathode and anode exhibit high specific capacitance of 1728 and 221.5 F g-1(1A g-1),respectively.In addition,the assembled quasi-solid-state asymmetric supercapacitor with PVA/KOH gel both as the separator and electrolyte provides a high energy density of 44.9 W h kg-1(1549.7 W kg-1),excellent flexibility,and 94%capacitance retention at 180° bent.(2)Hierarchical structure CNF-CNT-Co3O4 composite aerogel:In this chapter,compressible and resilient CNF aerogel with "welded" structure between adjacent nanofibers is prepared and used as anode through electrospinning,shear homogenization,and heat treatment.It can withstand 80%reversible compression deformation,and there is no obvious plastic deformation under 50%compression strain after 100 compression-release cycles.Based on the "welded" structure,CNF-CNT-Co3O4 composite aerogel is prepared and used as cathode by chemical vapor deposition and heat treatment.The cathode and anode provide high specific capacitance of 2376 F g-1(1 A g-1)and 300 F g-1(0.3 A g-1),respectively.The assembled asymmetric supercapacitor provides a high energy density of 48.1 W h kg-1(780.2 W kg-1).(3)Hollow Co3O4 nano-particle embedded multi-channel CNFs hybrid aerogel:The goal of this chapter is to further improve the electrode material rate and cycle stability while increasing the specific capacity on the basis of the previous chapter.A hybrid aerogel with multi-channel CNFs embedded with hollow Co3O4 nanoparticles is prepared and used as cathode through PAN,PS,and ZIF-67 nanoparticles co-electrospinning,shear homogenization,and heat treatment,showing excellent integrated electrochemical performance,namely,high specific capacitance,high rate performance,and ultra-long cycle life.The specific capacitance rearches 1600.6 F g-1(1 A g-1),maintains 72%of the original specific capacitance when the current density is increased to 20 A g-1,and maintains 90.5%of the original specific capacitance after 30,000 charge-discharge cycles(20 A g-1).Furthermore,the Co3O4 nanoparticles are etched away to obtain N,O co-doped hierarchical porous aerogels based on multi-channel CNFs embedded with hollow vesicles and used as anode,showing excellent electrochemical performance.The specific capacitance reaches 362.5 F g-1(0.3 A g-1),and 95.5%of the original specific capacitance retains after 30,000 cycles(5 A g-1).The assembled asymmetric supercapacitor provides a high energy density of 51.9 W h kg-1(750.3 W kg-1),in addition,90.1%of the original specific capacitance retains after 5000 cycles(5 A g-1).