A nanostructured composite material for hydrogen storage: design & analysis

Abstract

Hydrogen has long been considered an ideal energy carrier for a sustainable energy economy, for both direct combustion and as a fuel for polymer-electrolyte fuel cells. One of the main challenges associated with the use of hydrogen is to find efficient methods of storage. Any method must be safe, reversible, cost-effective and practical. In this thesis, a general introduction to hydrogen energy and the hydrogen economy is provided, together with descriptions of incumbent and emerging storage methods. A mathematical framework for simulating sorption isotherms in microporous materials is developed. This framework provides explicit expressions for the excess, condensed, compressed and absolute hydrogen masses. Furthermore, key parameters such as the surface area and adsorption volume can be estimated (for the first time) using a single-step nonlinear regression analysis, with the use of any isotherm model. Values are derived for three classes of porous materials, showing consistency with experimental data. A novel composite consisting of titanate nanotubes decorated with nanostructured metal cyanide frameworks, e.g., cadmium ferricyanide (Cd3[Fe(CN)6]2), are synthesised. The equilibrium and kinetic hydrogen sorption properties of the titanate-nanotube/Cd3[Fe(CN)6]2 composite are studied at low, intermediate and high pressure (up to 150 bar), revealing uptake values of ca. 14 weight %, which compare favourably with known materials for hydrogen storage. The role of mass transport in the sorption process is investigated, including the effects of boundary-layer diffusion and intraparticle diffusion. The results suggest that the composite possesses good hydrogen mass transfer characteristics. The effects of the reaction environment during synthesis are explored and the samples are thoroughly characterised. Significant differences in the loading of Cd3[Fe(CN)6]2 on the titanate nanotubes are seen. Hydrogen and nitrogen sorption analyses reveal the role of the pore size distribution on the effective surface area for adsorption and, therefore, the hydrogen uptake.

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