The hydrogenation and dehydrogenation behavior of LaNi 5 , LaNi 4.75 Co 0.25 and LaNi 3 Co 2 was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1 to 5 MPa in the temperature range from 323 to 473 K with the heating and cooling rates of 2 to 30 K min −1 . In the heating run of the hydride of LaNi 5 , two endothermic peaks were observed. One was the peak for the transformation from the γ phase (full hydride LaNi 5 H 6 ) to the β phase (hydride LaNi 5 H 3 ). The other was the peak for the transformation from the β phase to the α phase (solid solution). In the cooling run, one exothermic peak for the transformation from the α phase to the γ phase was observed. These endothermic and exothermic peaks shifted to higher temperatures with the increase in hydrogen pressure. In the heating and cooling runs of the LaNi 4.75 Co 0.25 -H 2 system the PDSC curves similar to those of the LaNi 5 -H 2 system were observed. However, in the heating run of the hydride of LaNi 3 Co 2 only one endothermic peak was observed. Using Ozawa's method, the activation energies for dehydrogenation of the hydrides were estimated. The activation energy for the γ -β transformation was higher than that for the β-α transformation. Substitution of cobalt for a part of nickel in LaNi 5 increased the activation energies for the phase transformations.

The hydrogenation and dehydrogenation behavior of LaNi 5−x Al x (x = 0-0.5) was studied by the pressure differential scanning calorimetry (PDSC) at the hydrogen pressure range of 1 to 5 MPa in the temperature range from 323 to 573 K with the heating and cooling rates of 2 to 30 K min −1 . In the heating runs of the hydride with x ≤ 0.1 two endothermic peaks were observed. With the increase in the aluminum content, the first peak at lower temperature decreased, while the second peak at higher temperature increased. Furthermore, the difference in the temperatures of the peak top decreased with the increase in the aluminum content. In the heating runs of the hydride with x > 0.1 only one endothermic peak was observed. These endothermic peaks shifted to higher temperatures with the increase in the hydrogen pressure. Using Ozawa's method, the activation energies for dehydrogenation and hydrogenation processes were estimated. The activation energy for the dehydrogenation process of the hydrides increased with the increase in the hydrogen pressure and the aluminum content. However, the dependence of the activation energy on the aluminum concentration in the range of x ≥ 0.1 was different from that of x < 0.1. The activation energy for the hydrogenation process was estimated only in the range of x > 0.1 at the hydrogen pressure of 5 MPa. The value of activation energy for the hydrogenation was lower than that for the dehydrogenation.

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