Study On Modification Of Nitrogen-doped Carbon Nanofibers And Their Catalytic Activity For Oxygen Reduction Reaction

Fuel cells as clean, highly efficient and sustainable new energy system havebecome one of the most effective ways to resolve the global energy crisis andenvironmental pollution problems. However, the large-scale commercialapplications of fuel cells have still been hampered by the utilization of expensivePt-based electrocatalysts for both cathodic oxygen reduction reaction (ORR) andanodic hydrogen oxidation reaction (HOR) which occupy more than half of the costof a whole fuel cell stack. Additionally, compared with HOR, ORR is kineticallysluggish and the produced exchange current density is small and usually needs moreprecious Pt-based catalysts to satisfy the whole exchange current density. Therefore,it is more important to explore low cost and highly active non-noble metalelectrocatalyst for cathodic ORR to substitute Pt-based catalysts and thus promotethe further commercialization of fuel cells.Nitrogen-doped carbon nanomaterials as a new ORR electrocatalyst systemusually have good electrocatalytic activity in alkaline media while their activity inacid media is much poor, which greatly limits their wide applications in protonexchange membrane fuel cells (PEMFC). In this dissertation, we devote to improvethe electrocatalytic ORR activity of them especially in acid media.Electrospun PAN-based NCNFs were modified by different treatment methodsincluding oxidation by concentrated acid mixture, NH3etching and the combinationto of the two to enhance their electrocatalytic activity. It was demonstrated that,NCNFs treated by concentrated H2SO4/HNO3mixture with a volume ratio of3/1at60oC for3h and then treated by NH3etching at900oC showed the best ORRactivity in both acid and alkaline media. The surface modified NCNFs also showedexcellent stability in alkaline media. Not very high as the stability in acid media was,it was nevertheless better than that of commercial Pt/C catalyst.Polymer bicomponents were used as precursors to prepare porous NCNFswhich displayed further improved electrocatalytic activity in both acid and alkalineelectrolytes after surface modification. For polymer bicomponent of PAN with otherthermoplastics such as CA, PVP, PMMA or PVDF, when experiencing carbonizationat high temperature in inert gas, the thermoplastics would be pyrolyzed into gaseousspecies and leave the micropore network in the PAN-converted carbon nanofibersubstrate which possibly could promote the surface the process of concentrated acidoxidation and NH3etching, and thus ultimately resulted in the enhancement ofelectrocatalytic performance of NCNFs. It was demonstrated that the surfacemodified NCNFs from PAN/CA and PAN/PVP were more active than that from PAN/PMMA and PAN/PVDF.Fe-N co-doped CNFs (Fe-N/CNFs) were also prepared by heat treating theelectrospun FeC2O4/PAN nanofiber precursor in different atmosphere. Fe-N/CNFsprepared by second heat treating in NH3at high temperature presented highlypositive ORR onset potential which was only40mV negatively lower than that ofcommercial Pt/C catalyst, the similar kinetic current density to the later, and thelonger durability than the later in acid media. Furthermore, such Fe-N/CNFelectrocatalyst also showed similar activity to that of commercial Pt/C catalyst inalkaline electrolyte. Fe-N/CNFs treated in N2atmosphere showed more positiveonset potential but with relatively smaller current density. The onset pontetial ofFe-N/CNFs treated in N2/H2was negatively shifted30mV compared with that of theNH3-treated Fe-N/CNFs. It was found that, in the range of60to300mg, the highercontent of FeC2O4achieved the higher activity of the as-prepared Fe-N/CNF catalyst.Additionally, the Fe-N/CNFs prepared using different process showed greatlydifferent morphology, however, the ORR activities of them were comparable.By comparing and analyzing the relationship between electrochemicalperformance with fiber micromophology, structure and surface electronic propertiesof various NCNFs and Fe-N/CNFs, we found that Fe-N/CNFs or surface modifiedNCNFs with higher activity usually presented higher density of porous structure inboth surface and inner part of fiber, and the nanofiber was much loose. Structurecharacterization indicated that many irregular and bended graphitic carbon layerscould be found in these porous nanofibers, especially for Fe-N/CNFs which showedmore graphitic layers and some of them presented unique hollow onion-likegraphitic nanoshell structure. It was inferred that the highly enhancedelectrocatalytic activities of NCNF and Fe-N/CNF electrocatalysts were probablycaused by these loose porous structure and bended graphitic layer. On the one hand,these unique porous structure and bended graphitic carbon as well as openonion-like graphitic nanoshells could provide higher contact area for O2during ORRprocess. On the other hand, they could provide more edge planes and thus benefitthe formation of more active sites for ORR. Additionally, the electron clouds in thebended carbons are more exposed, which could promote the adsorption of O2duringthe ORR. The surface modifications and heat treatment under special atmosphere aswell as the introduction of Fe all promoted the formation of these loosly porousstructure and unique graphitic carbon and thus resulted in the enhancement of theactivity of the as-prepared electrocatalysts.