An improved technique for the exfoliation of graphene nanosheets and utilization of their nanocomposites as fuel cell electrodes
Saner Okan, Burcu (2011) An improved technique for the exfoliation of graphene nanosheets and utilization of their nanocomposites as fuel cell electrodes. [Thesis]
Official URL: http://192.168.1.20/record=b1378276 (Table of Contents)
Graphene nanosheets (GNS) were separated from graphite by an improved, safer and mild method including the steps of oxidation, thermal expansion, ultrasonic treatment and chemical reduction. With this method, the layers in the graphite material were exfoliated, and high-quality GNS were produced with higher yields. Scanning Electron Microscopy (SEM) images exhibited that GNS can exist by being rippled rather than completely flat in a free standing state. The mild procedure applied was capable of reducing the average number of graphene sheets from an average value of 86 in the raw graphite to 9 in GNS. Raman spectroscopy analysis confirmed the significant reduction in size of the in-plane sp2 domains of GNS obtained after the reduction of graphite oxide (GO). BET measurements by nitrogen adsorption technique showed that the surface area of GNS was 507 m2/g [square meters/gram]. The electrical conductivity of GNS was measured as 3.96 S/cm by the four-probe method. As the oxidation time was increased from 50 min to 10 days, stacking height of graphene sheets decreased and thus the number of graphene layers decreased. The variations in interplanar spacings, layer number, and percent crystallinity as a function of oxidation time indicated how stepwise chemical procedure influenced the morphology of graphite. The percent crystallinity of GO sheets decreased down to 2% due to the change of stacking order between graphene layers and the random destruction of graphitic structure after oxidation process. For the production of advanced type of catalyst support materials, the distinguished properties of GNS were combined with the structural properties of conducting polypyrrole (PPy) by the proposed simple and low-cost fabrication technique. A precise tuning of electrical conductivity and thermal stability was also achieved by controlling the polymer thickness of randomly dispersed GO sheets and GNS by a layer-by-layer polymer coating. However, non-uniform polymer dispersion on the surface of expanded GO occurred due to the removal of oxygen functional groups on the surface during thermal expansion of GO sheets. The shortest and most effective impregnation technique of Pt catalysts on the surface of GO, expanded GO and GNS based composites was achieved by a sonication process of 2 hrs. The C/O ratios of GO, expanded GO and GNS were measured as 2.3, 6.0, and 3.2, respectively. The characterization results showed that the presence of oxygen surface groups and the amount of PPy in nanocomposites favored the Pt dispersion and hindered the aggregation of Pt particles on the support surface. As GO content increased three times larger than the amount of PPy in nanocomposite, size distribution of catalyst particles was decreased into the range of 9 nm to 16 nm. Finally, novel fuel cell electrodes made of GO, GNS and their nanocomposites were fabricated in the form of thin-films by applying drop-casting method. Then, the performance of the prepared membrane electrode assemblies was tested in a single fuel cell. Comparably better fuel cell performance was obtained when GO sheet was used as the cathode electrode due to the large amount of oxygen surface groups on the surface of GO sheets.
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