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This paper discusses the effect of AB2 (Ti(Cr, Fe)2) phase on the hydrogenation properties of a Ti–Fe–Cr alloy system. Five Ti–Fe–Cr based alloys were fabricated by varying the Cr content. The microstructural analysis results revealed that the fraction of the Ti(Cr, Fe)2 phase increased with the increasing Cr content. The first hydrogenation test results indicated that all the alloys could absorb a significant amount of hydrogen at room temperature (30 °C) without a separate activation process. This behavior improved when the Ti(Cr, Fe)2 phase existed in the AB phase; the kinetics of the first hydrogenation tended to increase with the fraction of Ti(Cr, Fe)2 phase. The enhancement in the first hydrogenation kinetics of the Ti–Fe–Cr based alloys was attributed to the synergetic effect of the interface between the AB and Ti(Cr, Fe)2 phases and the inherent fast hydrogenation of the Ti(Cr, Fe)2 phase. However, the total hydrogen storage capacity decreased when the fraction of Ti(Cr, Fe)2 phase increased.

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

We present that the equilibrium hydrogen pressure of titanium iron (TiFe) alloy, a room-temperature hydrogen storage material, can be tailored via vanadium alloying. While many 3d transition metal alloying elements (e.g., Mn, Cr, Co, and Ni) typically replace the Fe sublattice in TiFe, vanadium can replace both the Ti and Fe sublattices. Density functional theory calculation predicts that the substitution of Ti with V yields a unique effect: the equilibrium pressures of TiFe/TiFeH (P1) and TiFeH/TiFeH2 (P2) are closer, resulting in a decreased P2/P1 ratio. Experimental pressure-composition isotherms confirm this theoretical prediction. The lower P2/P1 is beneficial because the two-step TiFe hydrogenation reactions can be contained within a narrow pressure range. In contrast, the substitution of V for Fe lowers both P1 and P2, but lowers P1 more, resulting in a higher P2/P1 ratio. The contrasting effects contingent on the substitution site is a crucial factor in alloy design. It highlights the significance of vanadium as a versatile alloying element that modifies the hydrogen storage property of TiFe.