Synthesis and Hydrogen Storage Properties of Mg(AlH4)2

Metal alanates are considered to be a very promising group of materials for high-density solid-state hydrogen storage. Hence, new alanates with sufficiently fast reaction kinetics of absorption and desorption at moderate temperatures and high hydrogen contents are still a very active area of research.

Modified Synthesis Procedure

In this study, Hiden Analytical developed a modified synthesis procedure for Mg(AlH₄)₂ with high purity. Mg(AlH₄)₂ submicron rods were successfully achieved as the resultant product by ball milling the NaAlH₄ and MgCl₂ mixture (molar ratio: 2:1) in Et₂O followed by Soxhlet extraction, and the mixture’s purity was determined to be a maximum of 96.1%. On heating, ~ 9.0 wt% of hydrogen was released from the as-prepared Mg(AlH₄)₂ with a three-step reaction. This was calculated by combining MS (m/z 2, Hiden QIC-20) and volumetric analyses. As shown in Figure 1 (a), Mg(AlH₄)₂ initially decomposed at 125-200 °C to release hydrogen and produce MgH₂ and Al. As the temperature is increased to 320 °C, the newly generated MgH₂ reacts with Al to develop the Al_0.9Mg_0.1 solid solution along with the release of hydrogen. When the temperature was increased further to 440 °C, the reaction occurred between the Al_0.9Mg_0.1 solid solution and the remaining MgH₂ to produce additional hydrogen and form Al₃Mg₂. As shown in Figure 1 (b), DSC measurement revealed that the initial step dehydrogenation is exothermic, whereas the last two steps are endothermic.

Figure 1. H₂-TPD, volumetric release (a) and DSC (b) curves of the as-prepared Mg(AlH₄)₂.

Examination of the isothermal and non-isothermal behaviors demonstrated a diffusion-controlled kinetic mechanism for the initial step dehydrogenation, and its apparent-activation energy was measured to be around 123.0 kJ/mol. However, only ~2.3 wt% of hydrogen could be recharged at a temperature of 140-210 °C and 100 bar of hydrogen pressure. Hence, additional improvement on hydrogen storage reversibility of Mg(AlH₄)₂ must be carried out to make it a potential material for hydrogen storage.

Project Summary by:

Y. Liu, Y. Pang, M. Gao & H. Pan
State Key Laboratory of Silicon Materials,
Key Laboratory of Advanced Materials & Applications for Batteries of Zhejiang Province &
Department of Materials Science and Engineering, Zhejiang University
Zhejiang Province, Hangzhou 310027 China

Paper Reference: Liu et al. (2012) “Synthesis and hydrogen storage thermodynamics and kinetics of Mg(AlH₄)₂ submicron rods” International Journal of Hydrogen Energy, 37, (23), 18148-18154

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

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