Attempts to improve Mg2Ni hydrogen storage by aluminium addition

In an attempt to destabilize Mg2NiH4, the starting alloy, Mg2M, was doped with aluminium by ball milling before hydrogenation. The structure of Mg2NiH4 is based on an electron-rich d(10) [NiH4]-complex stabilized by a Mg ion framework and from theoretical calculations it had been suggested that a further destabilization of the hydride could be obtained by substituting some Mg for Al. After hydrogenation, the structure, color and phase transition temperature of Mg2NiH4 was found to be affected by Al addition. The conventional monoclinic low-temperature (LT) structure of Mg2NiH4 was not observed by X-ray diffraction for Mg2-xAlxNiH4, which instead could be indexed with a cubic unit cell, a = 6.519 Angstrom, that is very close to the dynamically disordered, cubic, high-temperature (HT) phase of Mg2NiH4 (a = 6.490 Angstrom). In contrast, the Al-doped Mg2NiH4 is stable at room temperature. Furthermore, the typical orange color of Mg2NiH4 was not observed and differential scanning calorimetric measurements (DSC) of the M-containing hydride showed a 10 degreesC decrease in phase transition temperature. This is interpreted as a transition from a statically disordered pseudo-HT structure to the dynamically disordered HT phase. However, the monoclinic structure of the LT phase of Mg2NiH4 could again be observed at room temperature after 10 hydriding/dehydriding cycles, indicating that the Al-doping of Mg2NiH4 is counteracted by cycling and only has a temporary influence on the properties. (C) 2002 Elsevier B.V. All rights reserved.