Utilization of multiple graphene layers in fuel cells. 1. An improved technique for the exfoliation of graphene-based nanosheets from graphite

An improved, safer and mild method was proposed for the exfoliation of graphene like sheets from graphite to be used in fuel cells. The major aim in the proposed method is to reduce the number of layers in the graphite material and to produce large quantities of graphene bundles to be used as catalyst support in polymer electrolyte membrane fuel cells. Graphite oxide was prepared using potassium dichromate/sulfuric acid as oxidant and acetic anhydride as intercalating agent. The oxidation process seemed to create expanded and leafy structures of graphite oxide layers. Heat treatment of samples led to the thermal decomposition of acetic anhydride into carbondioxide and water vapor which further swelled the layered graphitic structure. Sonication of graphite oxide samples created more separated structures. Morphology of the sonicated graphite oxide samples exhibited expanded the layer structures and formed some tullelike translucent and crumpled graphite oxide sheets. The mild procedure applied was capable of reducing the average number of graphene sheets from 86 in the raw graphite to nine in graphene-based nanosheets. Raman spectroscopy analysis showed the significant reduction in size of the in-plane sp2 domains of graphene nanosheets obtained after the reduction of graphite oxide

Facile synthesis of polypyrrole/graphene nanosheet-based nanocomposites as catalyst support for fuel cells

The integration of catalyst metals into the graphene-based composites can be a new way to ensure thermal and electronic conductivities of the catalyst support materials in polymer electrolyte membrane fuel cells. In this work, graphene nanosheets were synthesized via a mild and safer chemical route including three major steps: graphite oxidation, ultrasonic treatment and chemical reduction. Then, polypyrrole was coated on graphene nanosheets by in-situ polymerization to fabricate polypyrrole/graphene nanosheet-based nanocomposites as the catalyst supports. Pt nanoparticles were uniformly dispersed on the surface of nanocomposites by sonication technique

Fluorinated nanofibers for potential biomedical applications

The application of supercritical carbon dioxide has been attracting more attention in the synthesis of biodegradable polymers. Highly pure products without residues can be recovered after the polymerization in supercritical carbon dioxide. In the present work, three types of block poly(L-lactide-co-s-caprolactone), with a central fluorinated segment and polylactide/polycaprolactone side chains were synthesized by sequential ring-opening polymerization in supercritical carbon dioxide. Perfluoro polyethers can be used as blood substitutes to deliver oxygen to tissues so that these materials are promising for biomedical applications. In the first part of the work, fluorinated reactive stabilizers (prepolymers) with inner fluorinated segment and polylactide or polycaprolactone side chains were synthesized in bulk from three different fluorinated alcohols. The prepolymers were then utilized for the synthesis of copolymers in supercritical carbon dioxide, where polylactide segments were successively incorporated to the ends of the prepolymer, forming a block structure with polyester side chains. Solubility tests of the prepolymer and the pentablock copolymer in supercritical carbon dioxide showed effective solubilization at the reaction temperature and pressure. In the second part of the work, with the process of electrospinning, nanofiber webs were prepared from these biodegradable materials. Material characterization was carried out by nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC), gel permeation chromatography (GPC), optical microscopy and scanning electron microscopy (SEM)

New fuel cell electrodes made from graphene nanosheets and their nanocomposites

The production of novel catalyst support materials could open up new ways to enhance the catalytic activity by reduced catalyst loadings. Nanocomposites composed of conducting polymers reinforced with graphene nanosheets (GNS) or graphite oxide (GO) sheets can be potential fuel cell electrodes as an alternative to commercial fuel cell electrodes

Application of multiple graphene layers as catalyst support material in fuel cells

The fuel cell electrode layer is a significant part of a fuel cell. The electrode layer is composed of the catalyst and porous electrode or gas diffusion layer. Catalyst has critical importance due to the influence on the cost and durability of fuel cells. The production of novel catalyst support materials could open up new ways in order to ensure the catalytic activity by lowering the amount of catalyst loaded [1]. At this point, utilization of multiple graphene layers as catalyst support material increase thermal and electronic conductivities of the membrane electrolyte in fuel cells. Graphene is the flat monolayer of carbon atoms in sp2 hybridization and it has exceptional electronic conductivity, high chemical and mechanical stability, and high surface area [2]. In the present work, we propose an enhanced, safer and mild technique for the separation of graphene layers from graphite to be used in the production of low-cost and durable catalyst support for polymer electrolyte membrane fuel cells. All samples were characterized in details by Scanning Electron Microscopy (SEM), XRay Diffraction (XRD), Thermal Gravimetric Analyzer (TGA), Atomic Force Microscope (AFM) and Raman Spectroscopy. [1] Y. Shao, J. Liu, Y. Wang, Y. Lin, Novel catalyst support materials for PEM fuel cells: current status and future prospects, J. Mater. Chem. 19 (2009) 46–59 [2] M. I. Katsnelson, Graphene: carbon in two dimensions, Materials Today 10 (2007), 20-2

Graphene nanosheet and carbon nanotube based nanocomposites as an electrode support for fuel cells

An electrode support material in fuel cells has great influence on catalyst dispersion, charge transport, and stabilization of the catalyst particles. Graphene and carbon nanotubes have been considered as a fuel cell electrode material due to their high specific surface area, exceptional electronic and mechanical properties. The mesopores in graphene nanosheets and carbon nanotube electrodes are interconnected, providing a continuous charge distribution that uses nearly all of the available surface area. In present work, for the production of advanced type of electrode materials, the distinguished properties of graphene nanosheets and multi walled carbon nanotubes were combined with the structural properties of conducting polymers (polypyrrole) by the incorporation of graphene and carbon nanotubes into a polymer matrix. Graphene nanosheets were exfoliated from graphite by a mild chemical treatment including graphite oxidation using sulphuric acid and potassium dichromate, ultrasonic treatment, and chemical reduction by hydroquinone. Pyrrole was coated on graphene nanosheets and carbon nanotubes by in situ polymerization by different feeding ratios. Graphene nanosheet and carbon nanotube based nanocomposites were compared according to their structural properties, thermal stabilities and electrical conductivities. Samples were analyzed in detail by SEM, XRD, TGA, AFM, TEM, FTIR and Raman Spectroscopy

Fabrication of multilayer graphene oxide reinforced high density polyethylene nanocomposites with enhanced thermal and mechanical properties via thermokinetic mixing

High density polyethylene (HDPE) was compounded with thermally exfoliated graphene oxide (TEGO) by thermokinetic mixing in a short time. High shear rates during the compounding process provided high exfoliation and proper dispersion of graphene layers in the polymer matrix. Different TEGO/polymer ratios were used to get efficient melt mixing. Structural analysis by spectroscopic techniques confirmed the exfoliation of TEGO sheets and the coverage of their surfaces by HDPE chains. Furthermore, homogeneous dispersion of graphene sheets in the matrix led to the enhancement in the mechanical and thermal properties of HPDE-based nanocomposites. Especially stress concentration sites were significantly reduced by preventing the agglomeration and restacking of graphene sheets in the matrix. Therefore, the tensile modulus and strength of HDPE nanocomposite increased about 36.5% and 45.7%, respectively, with the incorporation of 2 wt% TEGO. Microscopy analysis showed the separation of graphene layers in the cross-sectional area of composite specimens. TEGO-reinforced HDPE nanocomposites showed high thermal stability compared to the neat sample

Enhanced exfoliation technique for the separation of graphene nanosheets

Graphene sheets are carbon-based materials which combine exceptional electron conductivity, mechanical strength and optical transparency. Graphene nanosheets were fabricated by an enhanced, safer and mild technique in a shortened processing time. Samples were characterized by SEM, XRD, TGA, AFM and Raman Spectroscopy

An improved technique for the exfoliation of graphene nanosheets and utilization of their nanocomposites as fuel cell electrodes

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

Surface modifications of graphene-based polymer nanocomposites by different synthesis techniques

With the appropriate surface treatments, graphene sheets can be separated from graphite material and the layer-to-layer distance can be extended. In the present work, 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. For the production of advanced polymer nanocomposites, the distinguished properties of GNS were combined with the structural properties of conducting polypyrrole by the proposed simple and low-cost fabrication technique. The changes in surface morphologies and surface functional groups were estimated by controlling the polymer coating on graphite oxide (GO) sheets, expanded GO and GNS