This thesis describes the preparation and characterisation of graphene-derived composites with ammonia borane or metal borohydride-based materials as a hydrogen store. Ammonia borane and light metal borohydrides can be seen as attractive candidates for hydrogen storage due to their relatively high hydrogen gravimetric and volumetric densities. The development of renewable energies is nowadays a necessity and hydrogen is seen as a promising alternative to fossil fuels. This thesis work was motivated by the intensive need for high-performing solid-state hydrogen stores and by an interest in assessing the potential of graphene-derived materials to fulfil such objectives. In this study, we describe a fresh approach to nanocomposite fabrication, first designing a novel porous host material from (partially reduced) graphene oxide ((r)GO) and subsequently engineering nanocomposites with the hydrogen store in a simple one-step process. New 3-dimensional porous scaffold materials have been fabricated by ice-templating sheets of graphene oxide (GO) or partially reduced graphene oxide (rGO). Aqueous suspensions of GO (or rGO) can be cast into monoliths or formed as beads on cooling and the solid matrices can be fashioned with either laminar or radial porosity as result. GO beads with radial lamellar porosity and typical diameters of ~ 2.8 mm and densities of ~ 8 mg.cm-3 were obtained. Further, ammonia borane (AB), itself with a gravimetric capacity of 19.6 wt.% hydrogen (ca. 13 wt.% at more workable dehydrogenation temperatures), can be integrated into the hierarchical structures in-situ in a one-step process without the requirement of melt infiltration or solution impregnation techniques. The ensuing self-assembled beads release hydrogen without volume expansion on heating, supressing the release of diborane and borazine, and significantly decreasing the ammonia release. The use of partially reduced graphene oxide (rGO) as a scaffold also demonstrated the elimination of CO/CO2 release from the carbonaceous matrix. Thermal analysis confirms that both the kinetics and thermodynamics of AB dehydrogenation are altered by its incorporation in the nanocomposites and shows improved dehydrogenation properties compared to that of neat AB. Metal borohydrides such as lithium borohydride LiBH4 (18.54 wt.% hydrogen), magnesium borohydride Mg(BH4)2 and its ammoniated equivalent Mg(BH4)2.6NH3 (respectively 14.96 wt.% and 16.8 wt.% hydrogen theoretical content) were also investigated. Nanocomposite formation occurs in-situ and requires no subsequent impregnation or infiltration step. Self-assembly is again driven by the growth of “ice” crystals during the templating process. The dehydrogenation temperature was significantly decreased, with for example an onset of ca. 70 °C for the magnesium-based systems. More than 5 wt.% hydrogen desorption was measured for the rGO-Mg(BH4)2.6NH3 composites on heating from 30 – 350 °C under flowing argon, with higher hydrogen purity than Mg(BH4)2.6NH3 itself. All systems were characterised principally by Powder X-ray diffraction (PXRD), Infrared and Raman spectroscopy, and electron microscopy to obtain information related to the sample structure, composition, and morphology. Simultaneous thermogravimetric analysis (TG-DTA) coupled with mass spectroscopy (MS) was also performed in order to investigate the thermal decomposition of the materials allowing an assessment of their onset and peak hydrogen release temperatures, the gravimetric hydrogen density, and the nature and amount of the gases evolved during the decomposition.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:754327 |
| Date | January 2018 |
| Creators | Champet, Simon |
| Publisher | University of Glasgow |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://theses.gla.ac.uk/30655/ |
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