Electrochemical impedance and deterioration behavior of metal hydride electrodes

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

Hydrogen absorption in nanocrystalline Mg2Ni formed by mechanical alloying

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

Self restoring of the active surface in LaNi5

The formation of MgH2 directly from magnesium and molecular hydrogen is extremely slow; however, reasonable hydrogenation kinetics have been reported in compounds and alloys of magnesium with nickel, copper and rare earth metals. Using X-ray photoemission spectroscopy and Auger electron spectros-copy, we analysed the chemical states and the concentrations of magnesium, nickel and copper at the surface of the eutectic alloy MgMg2Ni and of the intermetallic compounds Mg2Ni and Mg2Cu. Air-exposed samples are greatly magnesium enriched on their surface, the magnesium being oxidized. Surfaces cleaned in ultrahigh vacuum show segregation of magnesium under the influence of oxygen even at room temperature. The segregated magnesium oxidizes and keeps the nickel and copper in the metallic state. Molecular hydrogen can be dissociated at the metallic nickel or copper particles or at the metallic subsurface. The catalytic activity of Mg2Ni and Mg2Cu in the hydrogenation of magnesium observed by Reilly and Wiswall is therefore due to a surface decomposition which continues the supply of atomic hydrogen.

A. Seiler

Papers

Surface effects and the formation of metal hydrides

Self restoring of the active surface in LaNi5

Surface analysis of Mg2NiMg, Mg2Ni and Mg2Cu☆

Magnetism and hydrogen storage in LaNi5, FeTi and Mg2Ni

A new mechanism for lengthening the lifetime of hydrogenation catalysts