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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 study investigated how Fe substitution with Ni, Co, Cu, Mn, and Cr affected the first hydrogenation behavior of air-exposed TiFe-based hydrogen storage alloys. The alloy ingots were crushed into powders and exposed to air for 1 h to analyze the first hydrogenation kinetics. Although Fe was substituted with up to 30% of Ni, Co, and Cu, the alloys had a single TiFe phase. In addition, the TiFe0·7Ni0·2Co0.1 and TiFe0·7Co0·2Ni0.1 alloys also had a single TiFe phase in spite of the simultaneous substitution. The composition of the oxide layer changed by the addition of Ni, Co, and Cu, but the alloys did not absorb hydrogen. In the TiFe0·8Mn0.2 and TiFe0·8Cr0.2 alloys, a dual-phase microstructure consisting of TiFe and Mn/Cr-rich C14 Laves phase was formed, with a larger amount in TiFe0·8Cr0.2. Both samples absorbed hydrogen after air exposure without any thermal activation process. Comparing the first hydrogenation kinetics, TiFe0·8Cr0.2 had a shorter incubation time and faster hydrogen absorption rate than TiFe0·8Mn0.2.

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.

Herein, we describe the synthesis of Mg2FeH6 by hydrogenation of a 2.1 Mg:Fe (mol/mol) powder mixture prepared by cold roll milling (CRM) in air. The thickness of Fe layers and the amount and distribution of oxygen with number of CRM passes were systematically analyzed. CRM-induced microstructural changes were shown to play an important role in Mg2FeH6 formation. Although repeated CRM effectively decreased the Fe layer thickness to values sufficient for the fast formation of Mg2FeH6, too much CRM passes decreased the total degree of hydrogenation due to inevitable oxidation of Mg in air. Both microstructure refinement and minimal oxidation are the prerequisites for efficient Mg2FeH6 synthesis, with the former condition being achievable by optimizing the number of milling passes, and the latter one requiring CRM under an inert atmosphere.