Solid solubility of Mg in Ca2Ni7 and hydrogen storage properties of (Ca2−xMgx)Ni7 alloys

The phase relations and hydrogen storage properties of the (Ca_2−xMg_x)Ni₇ alloys were investigated. It was found that the maximum solid solubility of Mg in the (Ca,Mg)₂Ni₇ phase is about x = 0.5 in the present study. The ‘inter-block-layer’ type stacking faults exist in the (Ca,Mg)₂Ni₇ phase when Mg content is very low. However, the density of stacking faults decreases and the lattice parameters reduce as Mg content increases to its maximum solid solubility. Thus the (Ca_1.5Mg_0.5)Ni₇ alloy has a good reversibility of hydrogen absorption–desorption.     

Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg₂Ni, Mg₂Co as well as a small amount of Mg₁₂Ce, and convert into the MgH₂–CeH_2.73-Mg₂NiH₄–Mg₂CoH₅ composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH₂, Mg₂Ni/Mg₂NiH₄ and Mg₆Co₂H₁₁/Mg₂CoH₅. Among them, the transformation between Mg₂Ni and Mg₂NiH₄ triggers the “spill-over” effect which promote the decomposition of MgH₂ phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg₆Co₂H₁₁ to Mg₂CoH₅ phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H₂ within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H₂ within 60 min at the temperature of 260 °C.     

Improvement in the hydrogen storage properties of Mg by mechanical grinding with Ni, Fe and V under H2 atmosphere

Mg-5wt%Ni-2.5wt%Fe-2.5wt%V (named Mg-5Ni-2.5Fe-2.5V) powder was prepared by reactive mechanical grinding using a planetary ball mill. The activation process, the changes in phase and microstructure with hydriding-dehydriding cycling, and the variations in the hydriding and dehydriding rates with temperature were investigated. The rate-controlling step for the dehydriding reaction of Mg-5Ni-2.5Fe-2.5V was analyzed by using a spherical moving boundary model. As the temperature increased from 473 K through 623 K, the initial hydrogen absorption rate under 12 bar H₂ decreased, while the hydrogen desorption rate under 1.0 bar H₂ increased.     

Synthesis of Mg2FeH6 by hydrogenation of Mg/Fe powder mixture prepared by cold roll milling in air: Effects of microstructure and oxygen distribution

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.