/3Al+1/2H2↔1/3Na3AlH6+2/3Al+H2↔NaAlH4 was studied in the temperature range of 80-170℃ and the pressure range of 8-12MPa. It was found that under the pressure of 12MPa, an increase in the hydrogenation temperature from 100 to 170℃ causes a reduction in the amount of absorbed hydrogen. Under the constant pressure of 10MPa, the initial rate of hydrogenation increases when the temperature increases from 80 to 120℃, and reaches an optimum at 120℃; further increase in temperature leads to a decrease in the rate. It was also observed that at a constant temperature, the rate increases with the increase of applied hydrogen pressure. The cycling studies of titanium doped NaAlH4 have shown that the initial capacity was about 3.6wt.%, but after a few dehydrogenation-rehydrogenation cycles, the capacity decreases to 3.0wt.%. And this capacity was retained with no further deterioration in the following consecutive 100 cycles. However, the rates of the two-step dehydrogenation processes were found to reduce kinetically with the ongoing cycling.

We propose a new method to prepare hydrogen absorbing alloys from single phase intermetallic ones. The alloys, (100-x) Zr0.5Ti0.5Cr1.5Ni0.51x LaNi5 (x=5-20 mass%) as prepared by this method showed dramatic enhancement of the hydrogenation properties. The Rietveld analysis on XRD data revealed that pure Cr precipitated from the Laves phase and its relative amount was proportional to the nominal LaNi amount. The PCT curves measured at 313K showed a large hydrogen capacity, 1.4wt%.

Zr-based AB, (B=V, Cr, Mn) Laves phases are studied for their ability to reversibly store hydrogen and are potential negative electrode materials in Ni-MH batteries. The hydrides formed are too stable for electrochemical use and their thermodynamicat properties can be adapted by Ni substitution on the B sublattice. However, measured electrochemical reversible capacities are very low compared to solid-gas results. This phenomenon is related to passivation and surface corrosion in electrolytic medium. To overcome this problem, the role of secondary phases has been studied following two routes: precipitation of controlled amounts of Zr-Ni binary intermetallic compounds or addition of a rare earth (R) element leading to precipitation of RNi compound in the matrix. The consequences on the electrochemical behaviour are discussed in terms of the microstructure of the alloys which lead to composite electrodes by taking advantage of the bulk properties of the Laves phase and the surface properties of the secondary phase (s).

Mg2Ni, Mg2Cu and MgZn2 were synthesized by means of ball-milling plus solid-state reactions. Firstly, the mixtures of the two elemental powders, corresponding to the compositions of Mg2Ni, Mg2Cu and MgZn2, were subject to ball-milling for 20 min. Then, they were pressed into pellets and sintered at temperatures below the melting points of the components. X-ray diffraction experiments show that the yields of the desired products are above 97wt%, higher than those prepared by the conventional metallurgical method. The hydrogenation properties of the Mg2Ni sample synthesized by solid-state reaction are in agreement with the previously published resultson alloys prepared by the conventional methods.

In situ X-ray diffraction (XRD) shows that LaMg and LaMg12 and thermally stable under argon upon heating to 330℃, while under hydrogen they decompose to LaH3 and MgH2 at about 290 and 270℃, respectively. The decomposition is believed to result from the large difference in enthalpy between the parent compounds and the resultants. Prior to the decomposition, the lattice parameters of LaMg12 and La2 Mg17 do not change, indicating that hydrogen does not dissolve into them to form solid solution of LaMg12 Hx and La2 Mg12 Hx. The enhanced mobility of La and/or Mg was observed in the presence of hydrogen, which might be related to the formation of the copious vacancy.