The Synthesis And Characterization Of The Carbon Based Nano-composites And Their Application In Fuel Cells

Carbon nanotubes (CNTs) and graphene have excellent physical and chemicalproperties which can be used in enormous applications such as hybrid materials,catalysis, lithium battery and nano-electron devices. In recent years, CNTs, grapheneand their hybrids were widely used as cathode catalysts in fuel cells. These hybrids canmaintain high performance without the loading of noble metal which may overcomeshortcomings in fuel cell such as high cost and toxicities to environment. Synchrotronradiation techniques are effective methods to study the electronic, atomic structure andchemical interaction of carbon-based nano-materials, and they have wonderfuladvantages to investigate oxygen reduction reaction mechanism in fuel cell.In this thesis, graphene and CNTs hybrids with various metals or metal oxides wereprepared by wet chemistry methods and used as catalysts of oxygen reduction reactionsin fuel cells. Various synchrotron radiation techniques (such as scanning transmissionX-ray microscopy-STXM, X-ray absorption spectroscopy) were used to probe thechanges of electronic structures which provided experimental and theoretical evidencefor the understanding of carbon-based nanohybrids with excellent performance. Maincontents of this thesis include:Basic electronic structures of graphene and CNTs were studied by STXM.Graphene and CNTs were sonically dispersed in ethanol and then supported on Cu gridfor TEM and STXM experiments. Various factors that may alter the STXM spectralfeatures in graphene and CNTs have been discussed. A uniform oxidation of thegraphene sheets in the flat area was observed regardless of the thickness, while in the folded area the result could be strongly affected by the geometry. Moreover, thick partsof the aggregation showed strong angle-dependence to the incident X-ray, while thinparts showed less angle-dependence, which might be related to the surface wrinkles andripples. The electronic structure differences due to the geometry and thickness suggest acomplicated situation in the aggregation of graphene sheets. Similar studies wereperformed on CNTs, The electronic structure of individual CNTs with differentdiameters has been investigated by STXM. Various factors that may alter the STXMspectral features in CNT bundles have been discussed. Tube diameter has been found tobe proportional to the spectral intensity, whereas it has negligible effect on the spectralshape. Disordered structure in CNTs introduced in the synthesis process was observed,which significantly affected the X-ray absorption spectrum by a reduction of the π*excitation.The formation of a carbon coating on CNTs was observed in X-ray microscopyexperiments. X-ray absorption near-edge structure (XANES) spectroscopy showed thatthe coating originated from the lacey carbon on the substrate, which was heated by theX-ray beam and then deposited on CNTs. The coating shows some fingerprint featuresaround the π*peak of the C1s XANES spectrum, which were widely observed inliterature but attributed to chemical modification or interfacial interaction in carbonnanomaterials. Our results suggest that the X-ray induced coating could be a possiblereason for those XANES features. CNTs decorated with Fe2O3nanoparticles or Runanoparticles were synthesised by wet chemistry methods, and the changes of electronicstructures of CNTs surfaces were studied by STXM. In these experiments, two kinds ofcarbon coating on CNTs after decoration with nanoparticles were observed by STXM,revealing the complex compositions in CNTs after chemical modification. Moreover, asandwich structure shown as CNT-nanoparticle-coating can be created with theexposure to X-ray for CNTs decorated with nanoparticle outside the wall. The coatingshows an effective way for site-selective modification of CNTs with various carbonstructures.The identification of effective components on the atomic scale in carbon nanomaterials which improve the performance in various applications remainsoutstanding challenges. The catalyst residues in individual CNT were clearly imagedwith a concurrent characterization of their electronic structure by STXM. Except forprominent catalyst nanoparticle at the tip, tiny catalyst clusters along the tube weredetected, indicating a migration of the catalysts with the growth of CNTs. Theobservation provides the direct evidence on the atomic metal in CNT for oxygenreduction reported in the literature. Interaction between catalysts (Fe, Ni) and CNTs atthe tip was also identified by comparing the X-ray absorption spectra. A deepunderstanding of catalyst residues such as Fe or Ni in carbon nanomaterials is importantto the growth mechanism and practical applications. The materials were also used ascatalyst of oxygen reduction reaction in fuel cells and the performance was tested.