Tungsten, Boron And Nitrogen Doped Carbon Nanofibers And Their Activity For Oxygen Reduction Reaction

The electrocatalysts for ORR used in fuel cells are mainly platinum, but platinum catalysts are expensive and easily lose activity, which greatly limits their commercial application. The objective of this research is to develop a new type of electrocatalyst with low cost, high electrochemical activity and good stability for ORR in alkaline fuel cells.Some reports suggested that carbon materials have ORR activities and nitrogen doping could further improve their electrochemical activities. In this thesis, tungsten, boron and nitrogen doped CNFs were prepared by electrospinning. The morphology, composition and microstructure of the catalysts were characterized by SEM/EDX, XRD, Raman, FTIR and XPS. The electrochemical activities of these catalysts for ORR were evaluated by Cyclic Voltammetry (CV) and Rotating Disk Electrode (RDE) methods.Firstly, we investigated the effects of tungsten doping on the CNFs. The purpose of tungsten doping is to form tungsten carbide (WC) species in the CNFs. The crystal structure of WC is similar to platinum metal, and thus the formation of WC species due to the tungsten doping may improve the catalytic activities of the CNFs. The electrospun polyacrylonitrile (PAN) nanofibers were impregnated with ammonium metatungstate salt (AMT) solution and WC coated CNFs (WC/CNFs) were obtained after heat treatment. The WC/CNF catalysts showed ORR activity, although not very high.Secondly, this thesis focused on the effects of nitrogen doping on the CNFs. It was found that heating atmosphere has great effects on the structure and properties of CNFs. The heat treatment in NH3 caused a significant reduction of the diameter of the CNFs and the shrinkage rate could be over 90%. The average diameter of N-doped CNFs (N-CNFs) could be below 20nm. To our best knowledge, this value is the smallest diameter of the CNFs fabricated by electrospinning in open literatures reported by now. In contrast, the N-CNFs treated in N2 have an average diameter of about 150nm. It was found that the N-CNFs treated in NH3 showed excellent electrochemical activity for ORR. When the treatment temperature increased from 600oC to 1000oC, the peak current intensity of the catalysts for ORR increased by about 30 times. The activities of the N-CNFs treated in N2 are far lower than those of the N-CNFs treated in NH3. XPS results showed that the N-CNFs treated in NH3 have more pyridinic nitrogen while the nitrogen existed in the N-CNFs treated in N2 are mainly pyrrolic. The pyridinic nitrogen probably was the active sites for ORR. The best samples have a cyclic voltammogram (CV) curve with the peak potential of -0.17V, the initial potential of -0.03V, and the peak current density of 6.64mAcm-2 at the scanning rate of 10mV/s and using the electrolyte of oxygen-saturated 0.1M KOH solution.Finally, B- and N-codoped carbon nanofiber (BN-CNF) catalysts were studied. The BN-CNFs exhibited better electrocatalytic performance during ORR than undoped CNFs, which was a little worse than N-CNFs.In summary, tungsten, boron and nitrogen doped carbon nanofiber catalysts were prepared and the effects of the doping on the electrochemical activities to ORR were investigated. The N-CNFs treated in NH3 showed excellent activity and cycle stability due to their smaller diameter and higher ratio of pyridinic nitrogen.