The stability (high stability = high desorption temperature and vice versa) of a metal hydride consisting of two or more metal species is usually intermediate of the respective binary hydrides (consisting of one metal species and hydrogen). The ideal metal hydride has a high content of hydrogen, and both absorb and desorb hydrogen fast at low temperatures and pressures.

Alanates

At IFE research is focused on aluminium- and magnesium-based metal hydrides. The aluminium-based hydrides in question are called alanates, which consist of hydrogen, aluminium and one or more alkali- or earth alkali metal. Aluminium is bound to hydrogen in a tetrahedral (pyramid shape) structure, with hydrogen ions at the corners and aluminium at the centre. These tetrahedral units of aluminium and hydrogen carry a negative charge, which is balanced by alkali- or earth alkali metal cations (ions with positive charge).

The high content of hydrogen (7.4 and 10.5 weight% of hydrogen in NaAlH₄ and LiAlH₄ respectively) makes alanates a very interesting type of material for hydrogen storage, especially in the transport sector. In addition, some alanates are reversible at quite low temperatures and acceptable pressures (150 – 200 ^oC at 70 – 150 bar for NaAlH₄).

However, the hydrogen sorption rate (absorption and desorption) usually must be increased with the addition of certain catalysts. The effects of catalysts on alanates are not understood, and much research is performed worldwide to better understand them. In addition much work is carried out to find new and better catalysts.

At IFE the research is mainly focusing on the alanates LiAlH₄, NaAlH₄ and the intermediate hydrides such as Na₂LiAlH₆, and on the effect of various catalysts on the hydrogenation properties of these alanates

Magnesium Hydrides

Magnesium hydride, MgH₂, is a relatively cheap material. It contains as much as 7.6 weight% of hydrogen, which is desorbed in one step. These facts combined make magnesium hydride a promising material for hydrogen storage. However, the hydrogen desorption rate is slow, and temperatures of at least 250^oC are needed for desorption to start. Previously, to manipulate the sorption  rate and the stability (which governs the desorption temperature) of magnesium hydride, magnesium has been alloyed with other elements. A wide range of magnesium-alloys exists, whereof several have good hydrogen sorption properties.

At IFE the research on magnesium is focusing on the hydrogen sorption properties of different magnesium alloys and magnesium complex hydrides (such as Mg₂FeH₆), and also on possible new and better methods to synthesise these magnesium alloys and complex hydrides.

The principal objectives are to lower the desorption temperature of magnesium hydride, and to improve its hydrogen sorption rate. The hydrogen sorption rate, which is slow because of the poor diffusion of hydrogen in the MgH₂ phase, can be improved if the structure of MgH₂ is modified. Moreover, the addition of certain catalysts can facilitate the reaction of H₂ with Mg atoms on the surface of the magnesium metal, and thus increase the sorption rates.

The desorption temperature, on the other hand, can only be lowered when the chemical properties of the hydride are changed. Therefore, one or more magnesium-metal alloys will be synthesised, with a subsequent absorption of hydrogen.