Hydrogen storage materials with focus on main group I-II elements. Preparation and characterization

A future hydrogen based society, viz. a society in which hydrogen is the primary energy carrier, is viewed by many as a solution to many of the energy related problems of the world { the ultimate problem being the eventual depletion of fossil fuels. Although, for the hydrogen based society to become realizable, several technical difficulties must be dealt with. Especially, the transport sector relies on a cheap, safe and reliable way of storing hydrogen with high storage capacity, fast kinetics and favourable thermodynamics. No potential hydrogen storage candidate has been found yet, which meets all the criteria just summarized. The hydrogen storage solution showing the greatest potential in fulfilling the hydrogen storage criteria with respect to storage capacity, is solid state storage in light metal hydrides e.g. alkali metals and alkali earth metals. The remaining issues to be dealt with mainly concerns the kinetics of hydrogen uptake/release and the thermal stability of the formed hydride. In this thesis the hydrogen storage properties of some magnesium based hydrides and alkali metal tetrahydridoaluminates, a subclass of the so called complex hydrides, are explored in relation to hydrogen storage. After briefly reviewing the major energy related problems of the world, including some basic concepts of solid state hydrogen storage the dehydrogenation kinetics of various magnesium based hydrides are investigated. By means of time resolved in situ X-ray powder diffraction, quantitative phase analysis is performed for air exposed samples of magnesium, magnesium-copper, and magnesium-aluminum based hydrides. From kinetic analysis of the different samples it is generally found that the dehydrogenation kinetics of magnesium hydride is severely hampered by the presence of oxide impurities whereas alloying with both Cu and Al creates compounds significantly less sensitive towards contamination. This leads to a phenomenological explanation of the large variations in the observed apparent activation energies of hydrogenation/ dehydrogenation of magnesium based systems, as generally found in the literature. Further, concurrent changes apparent prefactors i.e. a compensation effect (CE) is found. A detailed analysis leads to the general conclusion that any observed CE based on an Arrhenius analysis is false and a direct consequence of the data analysis. The effect of both particle/crystallite size reductions along with the effect of Ti-doping on the two-step dehydrogenation kinetics of lithium aluminum hydride is investigated. It is found that only the kinetics the first reaction step is sensitive to a reduction in the crystallite size. In order to achieve improved kinetics of the second reaction step as well, Ti-doping is found to be very effective. The main results of these investigations are; i) the first dehydrogenation step is subject to transport limitations probably diffusional limitations ii) the apparent activation energy of both dehydrogenation steps is insensitive to Ti-doping, suggesting that a prefactor effect is responsible for the kinetic improvements i.e. the number of reaction sites is probably increased e.g. by creation of lattice defects such as atomic vacancies. Finally, the hydrogen mobility in sodium aluminum hydride, potentially limiting the overall kinetics of hydrogenation/dehydrogenation, is studies with neutron scattering experiments. Both the hydrogen jump frequency and the mean square atomic displacement of hydrogen atoms are estimated.