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In this investigation, we report the cyclic performance, microstructure and thermal properties of near eutectic Mg–Ni alloys with different Ni contents (4.4, 11.3 and 16.3 at%). The starting cast ingots are mechanically chipped to flakes of about 400 μm, all displaying composite structures characterized by a typical eutectic microstructure with rather coarse features (1–5 μm). The flakes are cycled 1000 times at 330 °C under 30/1 bar H2 for the absorption/desorption processes. The hydrogen storage capacity is maintained throughout the cycling: 5.09, 4.46 and 3.49 wt% H2 for Ni16.3, Ni11.3 and Ni4.4 (at%), respectively. No significant microstructural change is observed, indicating the excellent stability of the alloys at elevated temperatures. Nevertheless, a marked porosity, and spheroidal Mg2Ni clusters can be noted after cycling, however their exact contribution to reaction kinetics has yet to be fully elucidated. An attempt is made to estimate the dehydrogenation activation energy of Ni16.3, and the calculated value seems comparable to that obtained for an early cycling stage (10 cycles). In the light of the superior stability under cyclic service and the low decomposition temperature, the Mg–Mg2Ni system is shown to possess an excellent potential for long-term hydrogen and heat storage applications.

This article reports the hydrogenation properties of several Mg–Mg2Ni composite specimens with increasing Ni content: 4.4, 11.3 (eutectic), and 16.3 (in at%). Mg–Mg2Ni composites were prepared by means of induction melting, followed by simple mechanical chipping of the casts. The hydrogenation and dehydrogenation reaction kinetics were studied, and reaction mechanisms were described by means of solid-gas reaction modeling. Absorption and desorption properties were evidenced to be diffusion controlled, with hydrogen diffusion through hydrided/dehydrided phases being the rate limiting step in most cases (the migration of metal/hydride interface at a constant velocity being rate-limiting in only few of them). The kinetics study was supported by a thorough thermal analysis to provide in-depth insights of the decomposition reaction. Hence, thermogravimetry (TG) and pressurized differential scanning calorimeter (PDSC) were combined to investigate the dehydrogenation properties such as hydrogen gravimetric density, reaction onset temperature, enthalpy and activation energy as a function of Ni content. Structural analysis included X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM) to discuss the structural stability and microstructural evolution as a function of cycles, notably during the activation procedure. Finally, cyclic performance was evaluated for 100 cycles, using a custom-made large-scale reactor to demonstrate scale-up feasibility.

Mg–23.5 wt%Ni–10 wt%Cu and Mg–23.5 wt%Ni–10 wt%La alloys were prepared by melt spinning and crystallization heat-treatment. The Mg–23.5Ni–10Cu alloy (5.18 wt%) after planetary ball milling has a higher hydrogen-storage capacity than the Mg–23.5Ni–10La alloy (4.98 wt%) at 573 K. The activated Mg–23.5Ni–10Cu alloy has a slightly lower hydriding rate in the beginning, but a higher hydriding rate after about 5 min than the activated Mg–23.5Ni–10La alloy at 573 K. The activated Mg–23.5Ni–10Cu alloy has a much higher dehydriding rate and a larger quantity of the decomposed hydrogen than the activated Mg–23.5Ni–10La alloy at 573 K.