Synthesis and characterization of Mg-Al-Ni alloys and Li-Mg borohydrides for hydrogen storage

Two important classes of hydrogen storage materials were considered: metallic alloys and molecular borohydrides. Each of them has advantages and disadvantages. Alloys absorb and release hydrogen reversibly and promptly, but in relatively small amount, whereas borohydrides decompose irreversibly yielding over 10 wt% of H2. Two systems of suitable chemical composition were selected from such classes, and materials belonging to them were synthesized and characterized from the structural and thermal point of view; the detailed and thorough experiments of dehydrogenation were performed on them, in order to determine the thermodynamic and kinetic aspects of the hydrogen release. Within metallic alloys, the Mg-Al-Ni was selected to study the addition of Al to the Mg-Ni system. Samples with MgmAlNin composition (m,n <= 3) were synthesized by ball milling; by X-ray di raction studies, they were found to be a basically single phase with substitutionally disordered CsCl-type cubic structure. Only compositions with m large and n small proved to segregate minor quantities of Mg and Mg2Ni. Hydrogenation experiments on the Mg2AlNi2 sample by the PCI technique (Sievert apparatus) showed a reversible absorption/desorption of 1.4 wt% H2 with formation / decomposition of the MgH2 and Mg2NiH4 hydrides. A particularly favourable dehydrogenation temperature (T >= 531 K for p >= 1 bar) was observed, by comparison with those of the single phase hydrides. Also the kinetics of the gas release proved to be satisfactory, indicating that addition of Al improves the H-storage performance of the Mg-Ni alloy substantially. The mixed LiBH4-Mg(BH4)2 borohydride system was investigated, to determine its possible better performance as hydrogen storage material with respect to the end-members pure borohydrides. Several composites were synthesized by ball milling, namely xLiBH4-(1-x)Mg(BH4)2 with x = 0, 0.10, 0.25, 0.33, 0.40, 0.50, 0.60, 0.66, 0.75, 0.80, 0.90, 1. The physical mixture was investigated by using X- ray powder di raction and thermal analysis. Interestingly, already a small amount of LiBH4 proved to make the alpha to beta transition of Mg(BH4)2 reversible, which had not been reported before. The eutectic composition was found to exist at 0.50 < x < 0.60, exhibiting a eutectic melting at 180 °C. A phase diagram was built based on the data obtained in this study. Furthermore, the decomposition of the material begins right after the melting; thus the decomposition temperature of the composite is much lower than those of the pure borohydrides. At 270 °C the x = 0.50 composite releases about 7.0 wt% of hydrogen. A full thermodynamic study of the dehydrogenation reaction was performed (Sievert apparatus) on the eutectic mixture, 0.6LiBH4-0.4Mg(BH4)2, and on the end members LiBH4 and Mg(BH4)2. Both the dynamic technique (constant pressure, temperature ramp vs. time) and the equilibrium mode (constant temperature, variable pressure with waiting time for pressure equilibration) were employed to measure the wt% of H2 released. It was found that the decomposition behavior of the eutectic composite is quite similar to that of Mg(BH4)2, but the starting temperature of the process is substantially lower, as shown by DSC measurements, opening the way to possible applications. With the help of Van't Hoff plots and by comparison with literature data, it was possible to analyze the dehydrogenation mechanism of the eutectic composite in terms of four steps, implying the intermediate formation of MgH2. Lithium borohydride proved to play an important role in assisting the fi nal decomposition of MgH2.