Research On The Fabrication Of Composite Materials Based On Two Kinds Of Nanofibers And Their Electrochemical Properties

With the rapid development of industrial society,scarcity of fossil fuels have become important issues that plague economic development.Development of low-cost and environmentally friendly renewable energy and reducing traditional energy consumption are the main goals for the energy structure transformation.In recent years,various electrochemical storage and conversion systems have received extensive attention of researchers.However,the lack of low-cost and efficient electrode materials limits the commercial application of various electrochemical energy storage systems.Meanwhile,since electronic devices tend to be portable,flexible,and wearable,it is difficult for devices such as traditional supercapacitors or lithium batteries to operate safely under bending or folding conditions.Therefore,portable and flexible electrode materials and devices have become a research focus.Nanofiber materials,especially ultrafine nanofibers,have certain flexibility and can be used as nano-filled or nano-element materials in various electrochemical energy storage systems.In this dissertation,starting from the source of nanofibers,two different types of nanofiber materials are selected:one is biomass,green bacterial cellulose（BC）natural nanofibers,and the other is synthetic aramid nanofiber（ANF）with extremely high mechanical properties.These two types of nanofibers have similar fiber sizes,uniform surface functional groups and weaved into three-dimensional network structure.The derived carbon fibers also have the characteristics of low density and high conductivity.Using BC and ANF as the nano-element,some electrode materials with high reactivity were constructed and optimized through different technical means,and the two types of nanofibers and their composite materials were studied in several representative electrochemical energy storage systems.With the help of a series of characterization methods and electrochemical tests,in-depth study of the structure-activity relationship between the structure of nanocomposite fiber electrode materials and its electrochemical properties provides new ideas for key electrode materials in electrochemical energy storage systems.The details are as follows:1.Application of nitrogen-doped carbonized bacterial cellulose supported platinum catalyst in the electrooxidation of methanolN-doped carbon nanofibrous with high specific surface area,high conductivity,and nitrogen doped networks are prepared by annealing a gel containing two inexpensive and ecofriendly precursors,that is,bacterial cellulose and urea,for the loading of Pt nanoparticles.Owing to the uniform oxygen groups on the BC surface,urea molecules could in-situ self-assembly attach through nanofibers,resulting uniform nitrogen-doped nanofibers.The effect of the annealing temperature on the performance of the catalysts is evaluated.The results show that the N doping and higher annealing temperature can improve the electron conductivity of the catalyst and provide more active sites for the loading of ultrafine Pt nanoparticles with a narrow size distribution.Meanwhile,nitrogen doping amount and bonding structure of carbon nanofibers will change with the carbonization temperature.When the carbonization temperature is 1000℃,the catalyst has best MOR electrocatalytic activity(627 m A mg^-1),CO poisoning resistance and chemical stability than commercial Pt/C catalysts.This work demonstrates an ideal Pt supporting material for the methanol oxidation reaction.2.Preparation of tin sulfide（SnS）coated carbon nanofibers based on bacterial cellulose for reversible lithium storageFree-standing SnS nanosheet-coated carbon nanofiber films（SnS/CBC）based on bacterial cellulose template were prepared by solvothermal and high-temperature heat treatment,which were used as anodes to improve lithium storage.The SnS/CBC composites possess three dimensional interconnected core-shell nanostructures,which is crucial for the high conductivity and high lithium storage capacity.The structure and composition of SnS/CBC were tunable by the carbonization temperature and load concentration.Therefore,lithium ion battery（LIB）using optimized SnS/CBC anode exhibits a reversible capacity of872 m Ah g^-1at 100 m A g^-1 after 100 cycles,and the capacity remains as high as 527 m Ah g^-1at 2000 m A g^-1 after 1000 cycles.The free-standing sulfide-based nanocomposites with unique nanostructure composition and flexibility could be utilized as promising electrode materials for future LIB systems.3.Nitrogen-doped carbon nanofibers derived from metal organic framework/aramid nanofiber for oxygen reduction catalysisNitrogen-doped porous carbon nanofiber（N-PCNF）was prepared based on ANF as the nanofiber template and metal organic framework（ZIF-8）as the N source,which was used as a non-metallic catalyst for oxygen reduction reaction（ORR）.The uniform amide groups on ANF promote the highly dispersed nucleation of ZIF-8 and thus these nanocrystals are evenly assembled and grown along the nanofibers.Meanwhile,the molecular chains of ANF form a three-dimensional network structure through the strong intermolecular interaction.Therefore,N-doped porous carbon nanofiber with high conductivity was obtained after carbonization process.Compared with the N-doped porous carbon obtained by carbonization of pure ZIF-8,the N-doped porous carbon nanofiber exhibits a larger specific surface area(851.04 m²g^-1)and pore volume.Electrochemical results show that the N-doped porous carbon nanofibers in alkaline electrolytes exhibit superior ORR catalytic performance,chemical stability and methanol tolerance than commercial Pt/C catalysts（the electron transfer number of 3.80 and half-wave potential of-0.201 V vs Ag/Ag Cl）.We believe that this preparation method provides an ingenious structure design idea for constructing low-cost and high-performance MOF-based electrode materials.4.Flexible electrode and separator based on ANF for symmetrical supercapacitorSymmetric flexible supercapacitors based on all-in-one monolithic films of exfoliated aramid nanofiber-graphene nanosheets film as electrode,highly ionic conductive boron aramid nanofiber-nitride nanosheets film as ultrathin separator integrated on single substrate.Aramid nanofiber can not only increase the interlayer spacing of the two-dimensional sheets（boron nitride or graphene）preventing from agglomeration,but also provide high-strength mechanical properties to obtain flexible films.Excellent thermal stability and structural stability enable the monolithic film to operate safely and stably at 100°C.Although the material needs to be optimized of capacitance performance,it exhibits a relatively stable cycle performance under high temperature conditions,providing a new idea of a simple and controllable flexible electrode and potential film device.