Light-Metal Hydrides for Hydrogen Storage

Demands for zero greenhouse-gas emission vehicles have sharpened with today’s increased focus on global warming. Hydrogen storage is a key technology for the implementation of hydrogen powered vehicles. Metal hydrides can claim higher energy densities than alternative hydrogen storage materials, but a remaining challenge is to find a metal hydride which satisfies all current demands on practical usability. Several metals store large amounts of hydrogen by forming a metal hydride, e.g., Mg, Ti and Al. The main problems are the weight of the material and the reaction energy between the metal and hydrogen. Magnesium has a high storage capacity (7.6 wt.% hydrogen) in forming MgH2; this is a slow reaction, but can be accelerated either by minimizing the diffusion length within the hydride or by changing the diffusion properties. Light-metal hydrides have been studied in this thesis with the goal of finding new hydrogen storage compounds and of gaining a better understanding of the parameters which determine their storage properties. Various magnesium-containing compounds have been investigated. These systems represent different ways to address the problems which arise in exploiting magnesium based materials. The compounds were synthesized in sealed tantalum tubes, and investigated by in situ synchrotron radiation X-ray powder diffraction, neutron powder diffraction, isothermal measurements, thermal desorption spectroscopy and electron microscopy. It is demonstrated that hydrogen storage properties can be improved by alloying magnesium with yttrium or scandium. Mg-Y-compounds decompose in hydrogen to form MgH2 nano-structures. Hydrogen desorption kinetics are improved compared to pure MgH2. The influence of adding a third element, gallium or zinc has also been studied; it is shown that gallium improves hydrogen desorption from YH2. ScAl1-xMgx is presented here for the first time as a hydrogen storage material. It absorbs hydrogen by forming ScH2 and Al(Mg) in a fully reversible reaction. It is shown that the hydrogen desorption temperature of ScH2 is reduced by more than 400 °C by alloying with aluminium and magnesium