Study On The Electrochemical Properties Of Carbon Nanofiber Composite Transition Metal Sulfide/nitrid

Renewable energy storage and conversion technologies have attracted much attention in today’s world of energy depletion and environmental pollution.Supercapacitors（SC）bridge the power/energy gap between conventional capacitors and batteries/fuel cells due to their fast charging,ultra-long cycle life,and high power density.As an important component of supercapacitors,the structural design and combination of electrode materials have been inextricably linked to the electrochemical performance of supercapacitors and become a popular research object in recent years.Among them,transition metal sulfide/nitride is a promising pseudocapacitive material due to its low cost,simple preparation,and environmental friendliness.It has been widely studied owing to its high theoretical specific capacitance and excellent electronic conductivity.However,the low electrical conductivity and irreversible structural distortion of transition metal sulfide/nitride hinder their rate performance and cyclic stability.To overcome this limitation,composites of transition metal sulfide/nitride and carbon materials can improve their electrochemical properties.The carbon component increases the electrical conductivity while providing a backbone for the deposition of metal sulfide/nitride due to its high specific surface area.Carbon materials have good stability and power density,but their specific capacitance is insufficient.Therefore,combining their advantages can obtain supercapacitors with excellent electrochemical properties.This thesis focuses on transition metal sulfur/nitride and carbon nanofibers composite.The carbon nanofibers with high specific surface area can increases their electrical conductivity and provide a three-dimensional network skeleton for the uniform deposition of metal sulfide/nitride,thus enhancing the effective utilization of transition metal sulfide/nitride.（1）Synthesis of nickel-based sulfide/carbon nanofiber（Ni S/CNFs）composites.First,polyacrylonitrile nanofiber films were electrospun and carbonized into more stable carbon nanofibers（CNFs）by high temperature calcination.Then Ni S nanosheets were directly grown on the surface of CNFs using a one-step hydrothermal method.Among them,diethanolamine acts as a chelating agent and solvent,which can make Ni S nanosheets uniformly and densely immobilized on the surface of highly conductive CNFs skeleton.The prepared Ni S/CNFs composites have high specific capacity,excellent multivalence properties,and excellent cycling stability.The hybrid supercapacitor assembled with Ni S/CNFs cathode and activated carbon（AC）anode achieves a high energy density of 22.35 W h kg^-1at a power density of 750.8 W kg^-1.It maintains an energy density of 12.69 W h kg^-1even at a high power density of 7500W kg^-1.In addition,the specific capacitance of the device maintains 91.14%of the initial value after 1000 cycles at a current density of 2 A g^-1.（2）Synthesis of Fe₂N nanofibers（Fe₂N/CNFs）electrode material.Fibrous membranes of Fe OOH@PAN were prepared by electrospinning,and then carbonized into a three-dimensional skeleton structure with a more stable structure by high-temperature calcination with ammonia.The combination of carbon nanofibers with good electrical conductivity and iron nitride nanoparticles with high specific capacitance ensures the stability of the hybrid supercapacitor structure and excellent electrochemical performance.A significant pore-like structure appears at a calcination temperature of 650°C,which increased the specific surface area and electrochemical activity of the composite.Then a hybrid supercapacitor with Ni S/CNFs cathode and Fe₂N/CNFs anode was assembled,exhibiting a high energy density of 52.62 W h kg^-1at a power density of 750 W kg^-1.In summary,the high-performance transition metal sulfide/nitride and carbon nanofiber composites were successfully synthesized.The proposed synthetic method effectively compensates for the low electrical conductivity of transition metal sulfide/nitride.It also improves the electrochemical properties of carbon nanofibers composites and provides a new idea for the preparation of high-performance carbon composites.