Composite And Electrochemical Properties Of Metal Carbides/sulfides And Carbon Nanofiber

In order to meet the growing demand for energy storage,researchers around the world have spared no efforts in researching various energy storage systems.As a new type of energy storage device,supercapacitors have better safety,excellent cycling stability and higher power density,which show unique advantages in more energy storage applications.Due to the existence of different energy storage mechanisms of electrode materials,the supercapacitor assembled by selecting suitable positive and negative materials can achieve excellent electrochemical performance.Based on this idea,this thesis focuses on two different types of electrode materials for energy storage and assembles them into hybrid supercapacitors as follows:（1）Design and synthesis of carbon nanofibers encapsulated iron carbide nanoparticles as anode materials.A mixed solution of iron acetylacetonate and polyacrylonitrile was electrospun into iron-containing fiber films.Then carbon nanofibers containing iron carbide nanoparticles were obtained after the fiber films were solidified in air and carbonized in nitrogen.The effects of different carbonization temperatures（500℃,650℃,and 800℃）on the microscopic morphology,crystal structure,and electrochemical properties of the products were investigated in detail.The electrochemical results show that the electrode material obtained by carbonization at 650℃has the best electrochemical performance,exhibiting a specific capacitance of 209 F g^-1 at a current density of 1 A g^-1.And the capacity retention is 92.5%after5000 cycles at a high current density of 10 A g^-1.In contrast,the electrode materials obtained by carbonization at 500°C and 800°C have specific capacitances of only141.89 F g^-1 and 123.7 F g^-1 at a current density of 1 A g^-1,respectively.（2）Construction of nickel-cobalt sulfide particles-coated carbon nanofiber composites as cathode materials.The electrospun carbon nanofibers were used directly as the backbone to hydrothermally grow nickel-cobalt sulfide compounds,followed by solidification and carbonization.To obtain stable electrode materials,the effects of different carbonization temperatures（550℃,650℃,and 750℃）on the microscopic morphology,crystal structure,and electrochemical properties of the composites were investigated detailly.The electrochemical results show that the specific capacitance of the composite obtained by carbonization at 650℃is 1551 F g^-1 at a current density of1 A g^-1,while the products carbonized at 550℃and 750℃are only 1211 F g^-1 and 935F g^-1,respectively.Moreover,the specific capacitance of the composite obtained by carbonization at 650℃is the highest at various current density.In this composite,Ni Co S nanoparticles are uniformly distributed on the surface of carbon nanofibers,and the nanofibers obtained after carbonization show a hollow structure.This unique structure can increase the active sites for electrolyte ion adsorption in electrochemical reactions and shorten and enrich the diffusion path of electrolyte ions.Finally,to explore the future application of the as-synthesized electrode materials,a hybrid supercapacitor was assembled by iron carbide/carbon nanofiber composites anode and carbon nanofiber/nickel-cobalt sulfide composite cathode.The supercapacitor achieves a high energy density of 43.2 Wh kg^-1 at a power density of800 W kg^-1 and maintains 20.67 Wh kg^-1 at a high power density of 8000 W kg^-1.Therefore,this thesis proposes an effective and feasible synthetic strategy to prepare carbon nanofiber-based positive and negative materials.The hybrid supercapacitors constructed from them have ultra-high electrochemical performance and show promising practical applications.