Mechanochemical synthesis of a Mg-Li-Al-H complex hydride
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Mg-Li-Al alloy was prepared by ingot casting and then underwent subsequent reactive ball milling. A Mg-Li-Al-H complex hydride was obtained under a 0.4 MPa hydrogen atmosphere at room temperature, and as high as 10.7 wt% hydrogen storage capacity was achieved, with the peak desorption temperature of the initial step at approximately 65 C. The evolution of the reaction during milling, as well as the effect of Li/Al ratio in the raw materials on the desorption properties of the hydrides formed, were studied by x-ray diffraction and simultaneous thermogravimetry and differential scanning calorimetry techniques. The results showed that mechanical milling increases the solubility of Li in Mg, leading to the transformation of bcc b(Li) solid solution to hcp a(Mg) solid solution, the latter continues to incorporate Li and Al, which stimulates the formation of Mg-Li-Al-H hydride. A lower Li/Al ratio resulted in faster hydrogen desorption rate and a greater amount of hydrogen released at a low temperature range, but sacrificing total hydrogen storage capacity. I. INTRODUCTION
Hydrogen-storage materials have attracted much attention due to the development of hydrogen energy systems. To meet the practical requirements, especially the criteria on the volumetric and gravimetric energy densities, with particular reference to automotive hydrogen fuel cells, hydrogen-storage materials have to have high hydrogen content. To this end, magnesium and magnesium-based alloys are particularly interesting, because they can store more hydrogen by weight than most of the other currently known metal hydrides (MgH2 is equivalent to 7.6 wt% hydrogen). However, several problems remain to be solved with these systems because Mg and Mg-based alloys have the following obstacles: (i) poor hydrogen absorption and desorption kinetics; and (ii) they are too stable for most applications, with its operating temperature being above 350 C. A common method used hitherto to improve the hydrogen storage properties of Mg is to mechanically alloy (MA) Mg with one or more metals by high-energy ball milling.1–5 These metals include Ni, Ti, Al, Nb, Mn, V, Y, and Zr, either in the form of pure metals, metal hydrides, or metal oxides. In recent years, significant improvement in kinetics has been achieved by the addition of suitable catalysts.3,6 However, thermal stability is still a critical issue for technical applications. More recently, aluminium-based complex hydrides have also attracted attention as one of the prospective family materials. Compared with metal hydrides, complex metal hydrides have higher theoretical hydrogen contents a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0345
2880
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 9, Sep 2009 Downloaded: 27 Mar 2015
and lower desorption temperatures. These compounds have the potential to meet the required storage capacity for on-board hydrogen storage. Among those, alkali and alkaline earth alanates, typically LiAlH4 and Mg(AlH4)2 (the
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