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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.

The nature of TiFe-based surface oxides and their evolution during conventional activation heat treatment were investigated in this study. The as-prepared TiFe alloy was found to possess an initial composite Ti and Fe amorphous surface oxide layer of about 6 nm. Depth profiling has shown that oxides steadily vanish with increasing depth, whereas metallic Fe contribution and mixed Ti oxides arise, before eventually being found in their metallic state at 7.5 nm. In situ XPS measurements, carried out to directly observe the evolution of oxides during the activation procedure, have indicated that the initial ternary oxide begins to transform to metallic Fe and mixed Ti oxides at a temperature as low as 200 °C. Consistent with the literature, the reduction of Ti oxides took a major turn at around 400 °C. Toward the end of the in situ measurements, oxygen was partially dissolved due to the limited measurement duration: TiO and metallic Fe remained beyond 400 °C. A similar overall reduction behavior was observed for the Ni- and Mn-substituted alloys, with a few subtle exceptions: Ni existed in its metallic state from 200 °C whereas Mn was reduced from Mn3+ to the Mn2+ state only beyond 400 °C due to a pronounced difference in the oxidation driving force of these two substitution metals.

In this study, we investigated the microstructures, hydrogen absorption kinetics, and surface oxides of a Ti0.2Zr0.2Nb0.2V0.2Cr0.17Fe0.03 high entropy alloy (HEA). The prepared HEA had a dual phase microstructure consisting of body-centered cubic (BCC) phase (32.2Ti-13.1Zr-30.3Nb-18.6 V-5.8Cr) and face-centered cubic (FCC) phase (11.3Ti-19.6Zr-13.2Nb-24.7 V-25.7Cr-5.5Fe). The HEA ingot absorbed hydrogen under a hydrogen pressure of 5 bar at room temperature without any thermal activation process. After hydrogenation, the FCC and BCC phases were transformed to a monohydride and a dihydride phase, respectively. To examine the hydrogen absorption behavior of each phase, two ingots having the same compositions as the BCC and the FCC phases were separately prepared. Though the BCC phase ingot did not react with hydrogen, the FCC phase ingot absorbed hydrogen, which could result from the formation of a highly reactive oxide layer on the FCC phase ingot. From the X-ray photoelectron spectroscopy results of the two ingots, although the BCC phase ingot contained Cr, no Cr was detected in the oxide layer. In contrast, the oxide layer on the FCC phase ingot displayed a high Cr concentration, and it seems that the reactivity of the oxide layer with hydrogen could be improved by the presence of Cr in the surface oxides.

This study examines the insulation coating technology of Fe–Si–Cr powder via selective oxidation annealing, which oxidizes elements selectively by controlling the oxidation potential. The study calculated the oxidation driving force of Fe, Si, and Cr, and conducted a thermodynamic analysis of oxidation and reduction conditions according to temperature and oxidation potential. Based on the results, a selective oxidation annealing was performed in an atmosphere in which Fe is reduced and only Si and Cr are selectively oxidized. The oxidation potential was controlled through the partial pressure ratio of hydrogen and water vapor. The XPS analysis results confirmed that a Si and Cr complex oxide layer formed on the powder surface after the selective oxidation annealing. Afterward, withstanding voltages were analyzed to evaluate the insulation property. Then, the withstanding voltage of the powder applying the selective oxidation annealing increased significantly compared to that of the initial powder. Further analysis showed that the powder annealed in an air atmosphere had a significantly lower saturation magnetic flux density than the initial powder, while the powder applying the selective oxidation annealing had only a slightly reduced saturation magnetic flux density.

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.

Solid-state hydrogen storage covers a broad range of materials praised for their gravimetric, volumetric and kinetic properties, as well as for the safety they confer compared to gaseous or liquid hydrogen storage methods. Among them, AxBy intermetallics show outstanding performances, notably for stationary storage applications. Elemental substitution, whether on the A or B site of these alloys, allows the effective tailoring of key properties such as gravimetric density, equilibrium pressure, hysteresis and cyclic stability for instance. In this review, we present a brief overview of partial substitution in several AxBy alloys, from the long-established AB5 and AB2-types, to the recently attractive and extensively studied AB and AB3 alloys, including the largely documented solid-solution alloy systems. We not only present classical and pioneering investigations, but also report recent developments for each AxBy category. Special care is brought to the influence of composition engineering on desorption equilibrium pressure and hydrogen storage capacity. A simple overview of the AxBy operating conditions is provided, hence giving a sense of the range of possible applications, whether for low- or high-pressure systems.

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.

In the aim of evidencing the relationship between Mg2FeH6 synthesis and the size of Fe particles, several specimens have been prepared by applying various milling energies (milling time) on a 2.1 Mg and 1Fe powder mixture doped with a small fraction of Unsaturated Fatty Amine (UFA). The resulting nano-crystalline composite structures display a broad Fe particle size distribution as a function of milling time. The hydrogenation of those complex powders has been conducted at temperatures lower than 400 °C under 60 bar of hydrogen pressure. As expected, the Fe particle size significantly influenced the hydrogenation kinetics. Also, the inevitable distribution of a minor fraction of oxides occurring during the milling process affected greatly the hydrogen storage capacity. Under the low pressure and temperature conditions selected in the frame of this study, lower than 100 bar and 500 °C conventionally used for synthesis of high purity Mg2FeH6, the hydrogenation reaction was demonstrated to be almost completed within 6 h, confirming the fast hydrogen absorption capability of the prepared materials. Plus, nearly 84 wt% of Mg2FeH6 was achieved under the afore mentioned moderate conditions and a minor fraction of unreacted Fe still remained due to diffusion constraints existing at low temperatures.

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.

Low temperature formation of Mg2FeH6 is demonstrated by hydrogenation of Mg-Fe elemental powder mixture at a temperature as low as 350 °C which is lower than the conventional process temperature, 500 °C. To enable the low temperature synthesis, the powder mixture of Mg and Fe has been prepared by high energy ball milling using different process control agents (PCAs). A systematic study on the ball milling and hydrogenation conditions has been carried out to maximize the yield of the ternary line compound. The hydrogenation conditions together with the particle size of the starting materials turn out to play a significant role in the hydrogenation kinetics of the system. An optimized condition has demonstrated a significant hydrogenation as well as a robust cycling ability at low temperature which suggests the strong potential of the process for practical applications. Keywords: Metal hydrides, Hydrogen solid-state storage, Thermal analysis, Transmission electron microscopy, Cycling ability

Different Cu-Zr alloys were hydrogenated under 100 bars of hydrogen pressure at different temperatures. The hydrogenation induced transformation of the initially amorphous phase into a polycrystalline structure characterized by its nanoscale ( 2 . Calorimetry measurements after hydrogenation showed a low temperature exothermic transformation occurring in the alloys hydrogenated at temperatures below 473 K, followed by multiple endothermic peaks at higher temperature attributed to dehydrogenation of different hydride phases. Activation barrier energies of the phase transformation were derived from Kissinger's method, and further characterization involving transmission electron microscopy revealed the existence of delta ZrH 2 with cubic structure contrasting with the typical epsilon ZrH 2 with tetragonal structure. This study evidences the effect of hydrogen pressure, temperature, and the alloy chemistry on the nature of the hydride formation in Cu-Zr binary amorphous alloys during the hydrogenation procedure.

International audience; The hydrogen-induced phase separation of Cu–Zr binary amorphous alloys during hydrogen gas charging at elevated temperature was demonstrated; the homogeneous binary alloy was decomposed into pure Cu and Zr-hydride by absorbing hydrogen into the structure. The decomposition, which is attributed to the opposed affinity to hydrogen of Cu and Zr, took place in nanometer scale. The hydrogen absorption kinetics was compared for the alloys with different compositions. The structure after hydrogen absorption was analyzed using x-ray diffraction, ultra-small and small angle neutron scattering, and electron microscopy.