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This study aims to shed light on the unusual trend in the stabilities of Zr(MoxFe1−x)2, 0 ≤ x ≤ 1, hydrides. Both the rule of reversed stability and the crystal volume criterion correlate with the increased hydride stabilities from x = 0 to x = 0.5, but are in contrast with the destabilization of the end member ZrMo2 hydride. The pressure-composition isotherms of ZrMo2-H2 exhibit very wide solid solubility regions, which may be associated with diminished H–H elastic interaction, uelas. In order to discern this possibility, we measured the elastic moduli of Zr(MoxFe1−x)2, x = 0, 0.5, 1. The shear modulus, G, shows a moderate variation in this composition range, while the bulk modulus, B, increases significantly and monotonically from 148.2 GPa in ZrFe2 to 200.4 GPa in ZrMo2. The H–H elastic interaction is proportional to B and therefore its increase cannot directly account for a decrease in uelas. Therefore, we turn our attention to the volume of the hydrogen atom, v H , which usually varies in a limited range. Two coexisting phases, a Laves cubic (a = 7.826 Å) and a tetragonal (a = 5.603 Å, c = 8.081 Å) hydride phase are identified in ZrMo2H3.5, obtained by cooling to liquid nitrogen temperature at about 50 atm. The volume of the hydrogen atom in these two hydrides is estimated to be 2.2 Å3/(H atom). Some very low v H values, have been reported by other investigators. The low v H values, as well as the one derived in this work, significantly reduce uelas for ZrMo2-H2, and thus reduce the corresponding critical temperature for the metal-to-hydride phase transition, and the heat of hydride formation. We suggest that the bulk stiffening in ZrMo2 confines the corresponding hydride expansion and thus reduces the H-H elastic interaction. [ABSTRACT FROM AUTHOR]

The search for a “magic” intermetallic compound, absorbing reversibly and easily copious quantities of hydrogen, has been pursued since the discovery of the first intermetallic hydrides, forty–fifty years ago. Obviously, such intermetallic compounds should contain light metal elements like Li, Mg, Al, Ca, etc. We summarize in this work the experimental evidence that sheds light on the hydrogen absorption abstinence of Al-rich intermetallic compounds. The elastic moduli and/or bonding properties of Al-containing pseudobinary compounds initially decrease and then increase as a function of the Al composition. The shear and the bulk moduli behave differently in the Al-rich portion, where they present significantly higher and lower values, respectively, than the corresponding moduli in the Al-poor region. We claim that the extensive shear stiffening inhibits the hydrogen absorption in the Al-rich compounds. An explanation for the choice of the LaSn 0.22 Ni 0.78 compound as a sorption cryocooler in the Planck mission is suggested in view of its shear softening with respect to LaNi 5 . Recent attempts to find pseudobinary intermetallic hydrides, containing Li and Ca, are also presented. These attempts are conducted by considering some of the factors which regulate hydrogen absorption in intermetallic compounds. [ABSTRACT FROM AUTHOR]

Abstract Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications. With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus. In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized. In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles. In the frame of the "Hydrogen Storage Systems for Mobile and Stationary Applications" Group in the International Energy Agency (IEA) Hydrogen Task 32 "Hydrogen-based energy storage", different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundred kg for use in hydrogen storage applications. Highlights • Latest developments in the application of hydride technology. • Investigations on both stationary and mobile applications. • Special attention to metal hydride compressors. • Heat storage and H 2 compression especially noted in the Outlook. [ABSTRACT FROM AUTHOR]