A Study On Nitrogen Doping Of Carbon Nanofibers And Their Electrochemical Properties

Carbon nanofibers (CNFs) are currently widely used in many fields, due to theirhigh aspect ratio, large specific surface area, high electrical and thermal conductivities.The electronic strcture and surface structure of the CNFs can be effectively improved bynitrogen doping, resulting in the improvement of the electrochemical properties. In thisthesis, the CNFs have been prepared by electrospinning and doped with differentnitrogen sources including melamine, aniline, urea and polyaniline. The effects ofnitrogen doping on the electrochemical properties of the CNFs and their application forfuel catalysts and supercapacitors were investigated. The results indicate that theelectrochemical properties of the CNFs can be improved effectively by appropriatedoping with nitrogen. The main work includes the following three aspects:The influence of carbonization temperature on the CNFs has been studied. Throughscanning electron microscopy and Raman spectroscopy, it can be found that the carbonnanofiber diameter diminishes while the crystallinity increases with the carbonizationtemperature. Electrochemical performance tests showed that the CNFs carbonized at1000℃exhibit the best performance for catalyzing oxygen reduction reaction in bothacid and alkaline medium. On the other hand, the CNFs carbonized at1100℃have thehighest specific capacitance in neutral electrolyte.The influences of nitrogen-doping on the CNFs have been studied. Scanningelectron microscopy and Raman spectroscopy tests demonstrate that the addition ofthese nitrogen sources including melamine, urea and polyaniline has little impact on themicrostructure of CNFs. However, the addition of aniline lead to the uniform straightCNFs into the continuous bead granular CNFs. Moreover, the addition of nitrogensources results in the crystallinity of CNFs decreases. Further study of theelectrochemical performance indicate that the addition of these nitrogen sourcesincrease the catalyzing activity for oxidation reduction reaction in both acid and alkalinemedium for the CNFs. The CNFs carbonized at1000℃in NH3atmosphere with20/3ANI/PAN (the mass ratio of aniline and polyacrylonitrile) precursor fibers has the bestcatalytic for ORR in O2saturated0.1mol/L KOH solution with the peak position up to-0.047V vs Hg/Hg2Cl2. In addition, the supercapacitor performance has been also improved by nitrogen doping. For the optimal nitrogen and doping concentration, theobtained nitrogen-doped carbon nanofibers (NCNFs) show increase specific capacitancedouble that of the undoped CNFs in1mol/L Na2SO4solution.Based on the optimal concentration of the nitrogen sources, the influence ofcarbonization temperature on the electrochemical properties of the NCNFs has beenstudied. As the carbonization temperature increases, the diameter of the NCNFs reduces,and the crystallinity increases. Electrochemical performance tests showed that theinfluence of carbonization temperature on the electrochemical properties of NCNFsexhibit a similar rule with that of undoped CNFs, however, the NCNFs have moreexcellent electrochemical performance.