Synthesis, characterization and hydrogenation behaviour of Mg-ξwt.%FeTi(Mn) and La2Mg17-ξwt.%LaNi5-new hydrogen storage composite alloys

Abstract

New hydrogen storage materials with higher capacity and better suited for applications have been successfully synthesized. The hydriding behaviour of the new composite materials, Mg-ξwt.%FeTi(Mn) and La2Mg17-ξwt.%LaNi5, were studied for various values of ξ (ξ = 10, 20, 30, 40 and 50). The Mg-ξ-%FeTi(Mn) materials were activated under a hydrogen atmosphere (about 33 kgf cm-2) and an optimum storage capacity of about 3.5 wt.% corresponding to room temperature hydrogenation was established for ξ=40. This high storage capacity-almost double the storage capacity of the well-known FeTi(Mn)-has been observed under ambient conditions. The La2Mg17-ξ-%LaNi6 materials were activated at higher temperatures (about 360 °C) in a hydrogen atmosphere. An optimum storage capacity of 4 wt.% in terms of pressure and composition was observed for La2Mg17-20%LaNi5 at 350 °C. In comparison with the native ingredient La2Mg17, much faster (nearly three times) kinetics were found. In order to understand the hydrogenation behaviour and the high storage capacity, structural-microstructural and chemical analyses of these composite materials were carried out. From the structural investigations it has been found that all the synthesized materials are multiphase. The composite material Mg-FeTi(Mn) was found to contain FeTi1-ξ1 magnesium, titanium and Ti-Mg phases. The higher storage capacity (about 3.5 wt.%) in the case of Mg-40%FeTi(Mn) is probably due to FeTi-Mg complexes. The hydrogen molecule is split at the FeTi surface and diffuses into the magnesium matrix via FeTi. In the case of La2Mg17-ξ%LaNi5, the composite material consists of La2Mg17, MgNi2, nickel and LaNi3 phases. Because of the presence of nickel and nickel-containing phases (e.g. Mg-Ni), it is assumed that the dissociation of hydrogen is easier and hence the system La2Mg17-ξ%LaNi5 has better kinetics than its counterpart La2Mg17 alone. © 1992.