The authors review available experimental information on the existence, thermodynamic stability and physical properties of hydrides formed by the absorption of hydrogen gas in intermetallic compounds of two transition metals. The emphasis is on stability. It is shown that empirical models for the stability of ternary hydrides can be reconciled with ideas based on the results of band structure calculations of (binary) metallic hydrides. It is concluded that metallic hydrides can be looked upon as alloys of metallic hydrogen. In addition to the thermodynamic properties of ternary metallic hydrides the authors discuss experimental information on electronic properties (magnetic, super-conductivity, band structure features) and on crystallographic and metallurgical properties (neutron scattering, nuclear and electron spin resonance, Mossbauer spectroscopy, diffusion). Applications are briefly reviewed.

https://iopscience.iop.org/article/10.1088/0034-4885/45/9/001/pdf

Hydrides formed from intermetallic compounds of two transition metals: a special class of ternary alloys

Magnesium and magnesium alloy hydrides

Hydrogen is stored atomically in metal hydrides. Dissociative chemisorption and associative desorption are therefore important steps in the hydrogen absorption and desorption processes. We summarize the theoretical and experimental knowledge of hydrogen chemisorption on clean and precovered metal surfaces and correlate it with the techniques for preparing metal hydrides. At the surface of hydride-forming intermetallics, precipitates of d metals and a metallic subsurface are produced by surface segregation and decomposition. The subsurface and the precipitates are able to dissociate H2. Our recent work on the surface analysis of LaNi5, FeTi, Mg2Ni and ErFe2 is reviewed.

Surface effects and the formation of metal hydrides

High capacity hydrogen storage: Basic aspects, new developments and milestones

The processes involved in the first few hydriding-dehydriding cycles, referred to as activation, are discussed. The controversial activation of TiFe is reviewed and new H 2 -D 2 isotope scrambling results are presented which indicate that metallic iron does not play a catalytic role. The hydriding-dehydriding kinetics are resolved into three interacting processes: intrinsic kinetics, heat flow and mass flow. The intrinsic kinetics are further resolved into three processes occurring simultaneously and in series: a surface process, diffusion and phase transformation. The problem of determining the rate-limiting intrinsic process from the time-temperature-pressure ( t  T  P ) dependences of the different processes is discussed. The employment of surface modification techniques to complement the t  T  P analysis is emphasized. Most kinetics analyses that have identified an intrinsic process as rate limiting are criticized as being invalid because 1. (1) the rate theory used has been inappropriate. 2. (2) heat flow has not been adequately taken into account. The viewpoint adopted is that only nucleation and growth (NG) theory is applicable. The NG formalism is developed and is employed to treat the interplay between the simultaneous intrinsic processes which is generally ignored. NG theory is also employed to model kinetics behavior in order to discuss heat flow. The control of heat flow by the previously overlooked, but very useful, control of heat capacity is discussed.

Hydriding and dehydriding kinetics

A model is presented for the mechanism of absorption (or release) of hydrogen by LaNi/sub 5/. The surface of LaNi/sub 5/ is extensively coated by La/sub 2/O/sub 3/. The surface oxide forms in two ways: (1) by reaction of gaseous oxygen with LaNi/sub 5/ or (2) by diffusion of dissolved oxygen to the surface where it reacts with LaNi/sub 5/ to form oxide and precipitate Ni, some of which is also at the surface. Hydrogen reaches the underlying LaNi/sub 5/ by diffusion in monatomic form along the La/sub 2/O/sub 3/--Ni interface. The dissociation of H/sub 2/ at the point of entry into the interfacial region is rate controlling for absorption. Release of hydrogen involves diffusion of monatomic hydrogen along the interface followed by recombination to form molecular hydrogen at the top of the interfacial region. The latter is rate controlling. The model is in accord with the known surface features of the intermetallic and is consistent with the observed kinetics of absorption and desorption of H/sub 2/ by LaNi/sub 5/. It is also consistent with the kinetics of hydrogenation of C/sub 2/H/sub 4/ over LaNi/sub 5/ and LaNi/sub 5/H/sub 2/ /sub 4/. In the latter, interfacial diffusion of monatomic hydrogenmore » is rate controlling. The present model differs from earlier models which regarded either the phase transformation or mass transport through macroscopic cracks of fissures as the rate-controlling step. 2 figures.« less

H. Imamura

Papers

Mechanism of hydrogen absorption by lanthanum-nickel (LaNi5)