Hydrogen Storage Properties and Phase Structures of Mg-Rich Mg-Pd, Mg-Nd and Mg-Pd-Nd Alloys

Abstract

The hydriding/dehydriding (H/D) rates and pressure-composition isotherms (PCTs) of Mg–Pd, Mg–Nd and Mg–Pd–Nd alloys with ∼ 90 at% Mg have been measured, and the characteristics of these hydrogen-storage properties have been discussed in relation with the phase structures and microstructures of the alloys studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All these alloys exhibit a hydrogen absorbency of ∼ 5 mass% at 573 K . The PCTs of the alloys containing Pd exhibit three plateau-like regions. A complementary examination of the phase structures, microstructures and PCT characteristics at 523–623 K shows that the H/D kinetics is strongly influenced by the following three factors: (1) A catalytic effect of NdH_2.5 and NdH₃ which assist hydriding and dehydriding of Mg matrix; (2) the disproportionation reaction of Mg₆Pd to form Mg₅Pd₂ and MgH₂, which retards the overall reaction kinetics; and (3) the refinement of the microstructure which enhances the kinetics of all the H/D reactions. A melt-spun and crystallized Mg–Pd–Nd alloy which shows a fine-grained (∼ 1 \\micron) microstructure exhibits the best H/D kinetics, especially the desorption kinetics, among the three alloys investigated, and it can be a promising candidate for an efficient hydrogen storage material in the future. The formation of Mg₅Pd₂H_∼5 hydride is suggested from the PCTs of the alloys containing Pd, although it has not been evidenced by XRD and SEM.