Multiple Design Of Electrospun Nanofibers Derived Carbon Nanofibers For Oxygen Electrocatalyst

Many new energy conversion devices are developed to resolve the serious energy crisis and environmental pollution caused by the large-scale use of fossil energy.Among them,fuel cells and metal-air batteries have attracted widespread attention due to their advantages of pollution-free and high energy density.However,there are severe shortcomings need to be overcome,the main problem is the low rate of the oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)of the cathode,which result in a large overpotential.In order to solve this problem,efficient catalysts are indispensable.Now,the most widely used electrocatalysts in the commercial are precious metals such as Pt,Ru,Ir,etc.,however,the development of this electrocatalysts are limited by scarcity,high cost,poor stability and poor methanol resistance.Therefore,the development of efficient,inexpensive,and stable non-precious metal catalysts is the key to promoting fuel cells and metal-air batteries.The works of this thesis are to.build one-dimensional non-noble metal catalyst with excellent performance by multiple designing of the structure and components of the electrospinning nanofibers to introduce metal/heterogeneous atom doping and a large number of microporous and mesoporous structures.The main research works are as follows:1.PAN/Ma/FeCl3@PT,a novel hybrid core-shell structure of fiber,was prepared by using electrospun organic-inorganic composite nano-fiber(PAN/Ma/FeCl3)as template and coating with sulfur-containing conductive polymer(polythiophene,PT)on the fiber surface by novel UV polymerization technology.The core-shell fibers are converted to N,S-codoped carbon fibers with a large number of metal nanoparticles by carbonization at high temperature.The temperature has critical influences on electrocatalytic performance.The ORR electrocatalytic properties of carbon fibers obtained at different temperatures were investigated.The results indicated that the catalyst obtained at 800? displayed the highest ORR electrocatalytic activity,which was comparable to commercial Pt/C catalysts.Meanwhile,the catalytic stability and methanol resistance of Fe-S/N-C-800 was better than that of commercial catalysts.2.In order to improve the specific surface area of the fibers and increase the density of active sites,the zeolitic imidazolate framework(ZIF-67),a kind of porous material,was used as a self-sacrificing template to obtain ZIF-67-L@PAN nanofiber by electrospinning.The porous Co/Coox-N-C-L fibers were fabricated by high-temperature heat treatment the electrospun nanofiber.The ZIF-67 crystal size was adjusted to study the effect of crystal size on the structure,composition and properties of the catalyst.The SEM and TEM results showed that when the average crystal size of ZIF-67 was 370 nm,the electrospun nanofiber could form a unique pod-like hollow structure after carbonization,which was conducive to improving the specific surface area.The results of ORR electrocatalytic performance proved that the pod-like fibers had excellent catalytic activity and stability.In addition,Co/CoOx-N-C-L was further applied as a cathode catalyst of the Zn-air battery.The related results indicated that the energy density of battery was 610 mAh/g,which was 8.93%higher than that of the commercial catalyst.This work provided a new strategy for the preparation of porous one-dimensional non-precious metal catalysts.3.In order to further control the pore structure in the fiber,the hard template SiO2 was used instead of ZIF-67 to mix with ferric chloride,nickel nitrate and PAN to prepared electrospun nanofiber.The bimetallic doped carbon fiber with controlled pore structure was prepared by high temperature carbonization,NaOH etching and reheating.The effect of two metals on ORR catalytic activity was studied by adjusting the ratio of metal Fe/Ni in the precursor.The ORR electrocatalytic measures suggested that Fe28/Ni2-NPCF exhibited excellent catalytic activity and stability.Meanwhile,it was found that doping trace metal Ni in Fe-based catalyst had no obvious effect on the initial potential of the catalyst,but it was beneficial to improve the half-wave potential and diffusion current of the catalyst.This is instructive for the preparation of excellent bimetallic catalysts.4.In addition,in order to enable catalysts to be used in reversible metal-air batteries,the structure of the Fe/Ni bimetallic doped carbon fiber was further adjusted to exhibit the bifunctional catalytic activity of OER and ORR.Electrospun FeCl3@PAN nanofibers were used as templates to grow Fe/Ni flake bimetal hydroxide in situ on the surface of the fiber,which was carbonized to obtain multi-stage nanofibers with a large number of carbon nanotubes grown on the surface.The structure and distribution of carbon nanotubes on the surface of the fiber are controlled by regulating the ratio of Fe/Ni in the precursor.The ORR and OER properties of the as-prepared catalyst were evaluated.The ratio of Fe to Ni in the precursor was adjusted to optimize the performance of the electrocatalyst.Electrochemical results showed that the ORR and OER catalytic performance of Ni2Fe1-FeNCF was excellent,the oxygen potential difference was 0.8 V and the cycle performance was stable.The difference in potential between the ORR current density at-3 mA/cm2 and OER current density at 10 mA/cm2 was 0.8 V,which was lower than that of noble metal electrocatalysts.In addition,the circulation of this electrocatalyst for ORR and OER was stable.This work showed that the electrospun fiber can be used as a precursor to prepare bifunctional catalyst by adjusting its composition and structure,which has great potential for application in reversible metal-air battery catalysts.