Effect of annealed treatment on microstructure and cyclic stability for La–Mg–Ni hydrogen storage alloys

The influence of annealed treatment on the microstructure and the cyclic stability for La–Mg–Ni alloys are systematically investigated by FESEM-EDX, XRD, PCT and electrochemical measurements. It is demonstrated that Ni and La are almost evenly dispersed, but Mg is nonuniform distribution in these alloys. Rietveld analysis results illustrate that the alloys mainly consist of LaNi5, Pr5Co19-type, Ce2Ni7-type and PuNi3-type phases. The mass fraction of each phase in these alloys varies after annealed. The 3d annealed alloy shows the better hydriding/dehydridng capacity and electrochemical cyclic stability. As annealed time is continually prolonged, the hydrogen storage capacity slightly increases, but the electrochemical discharge capacity decreases. Estimating from the pressure-composition isotheral curves, it is considered that the hydriding of these alloys is governed by two processes: the initial process can be expressed by the hydriding of the phases with stacking of [LaMgNi4] and [LaNi5] slabs and the second is controlled by LaNi5 phase.

► The results demonstrate that Ni and La are almost evenly dispersed, but Mg is nonuniform distribution in these alloys.
► The mass fraction of LaNi5, Pr5Co19, Ce2Ni7 and PuNi3-type phase in these alloys varies after annealed.
► The hydriding/dehydridng capacity and electrochemical cyclic stability are better than others after annealed 3d.
► The hydriding is governed by two processes: the initial process can be expressed by the hydriding of the phases with stacking of (LaMg)2Ni4 and LaNi5 slab and the second is controlled by LaNi5 phase.