Metal hydrides (MHx) provide a promising solution for the requirement to store large amounts of hydrogen in a future hydrogen-based energy system. This requires the design of alloys which allow for a very high H/M ratio. Transition metal hydrides typically have a maximum H/M ratio of 2 and higher ratios can only be obtained in alloys based on rare-earth elements. In this study we demonstrate, for the first time to the best of our knowledge, that a high entropy alloy of TiVZrNbHf can absorb much higher amounts of hydrogen than its constituents and reach an H/M ratio of 2.5. We propose that the large hydrogen-storage capacity is due to the lattice strain in the alloy that makes it favourable to absorb hydrogen in both tetrahedral and octahedral interstitial sites. This observation suggests that high entropy alloys have future potential for use as hydrogen storage materials.

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Superior hydrogen storage in high entropy alloys.

A high-entropy alloy (HEA) of HfNbTiVZr was synthesized using an arc furnace followed by ball milling The hydrogen absorption mechanism was studied by in situ X-ray diffraction at different temperatures and by in situ and ex situ neutron diffraction experiments The body centered cubic (BCC) metal phase undergoes a phase transformation to a body centered tetragonal (BCT) hydride phase with hydrogen occupying both tetrahedral and octahedral interstitial sites in the structure Hydrogen cycling of the alloy at 500 °C is stable The large lattice strain in the HEA seems favorable for absorption in both octahedral and tetrahedral sites HEAs therefore have potential as hydrogen storage materials because of favorable absorption in all interstitial sites within the structure

Structure and Hydrogenation Properties of a HfNbTiVZr High-Entropy Alloy.

Evidence of near-ambient superconductivity in a N-doped lutetium hydride

https://hal.archives-ouvertes.fr/hal-03201481/document

Hydrogen storage in MgAlTiFeNi high entropy alloy

The hydriding/dehydriding (HID) rates and pressure-composition isotherms (PCTs) of Mg-Pd, Mg-Nd and Mg-Pd-Nd alloys with ∼ 90 at% Mg have been measured, and the characteristics of these hydrogen-storage properties have been discussed in relation with the phase structures and microstructures of the alloys studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). All these alloys exhibit a hydrogen absorbency of ∼ 5 mass% at 573 K. The PCTs of the alloys containing Pd exhibit three plateau-like regions. A complementary examination of the phase structures, microstructures and PCT characteristics at 523-623 K shows that the HID kinetics is strongly influenced by the following three factors: (1) A catalytic effect of NdH 2.5 and NdH 3 which assist hydriding and dehydriding of Mg matrix; (2) the disproportionation reaction of Mg 6 Pd to form Mg 5 Pd 2 and MgH 2 , which retards the overall reaction kinetics; and (3) the refinement of the microstructure which enhances the kinetics of all the H/D reactions. A melt-spun and crystallized Mg-Pd-Nd alloy which shows a fine-grained (∼ 1 μm) microstructure exhibits the best H/D kinetics, especially the desorption kinetics, among the three alloys investigated, and it can be a promising candidate for an efficient hydrogen storage material in the future. The formation of Mg 5 Pd 2 H ∼5 hydride is suggested from the PCTs of the alloys containing Pd, although it has not been evidenced by XRD and SEM.

Hydrogen storage properties and phase structures of Mg-rich Mg-Pd, Mg-Nd and Mg-Pd-Nd alloys

The rare earth (RE) metals lanthanum, cerium, praseodymium, neodymium and samarium form hydrides REH≈233 and deuterides RED≈233 with a tetragonally distorted fluorite structure X-ray investigations show a facecentred tetragonal arrangement of the rare earth cations and a lattice parameter ratio c a > 1  Neutron diffraction studies of NdD236 yield a tetragonal superstructure with c a > 2 and space group I41md The structure is related to the REH2 (CaF2) cell; some of the tetrahedral sites are vacant, but half or almost half of the octahedral sites are occupied by deuterium atoms in an ordered manner These results are compared with those of other workers

Hugo W. Mayer

Papers

Tetragonal rare earth hydrides REH(D)≈2.33 (RE La, Ce, Pr, Nd, Sm) and a neutron diffraction study of NdD2.36