Hydrogen storage alloys are of particular interest as a novel group in functional materials owing to their potential and practical applications in Ni/MH rechargeable batteries. This review is devoted to the specific alloy families developed for high-energy and high-power Ni/MH batteries in the last decades, especially for EV, HEV and PHEV applications. The scope of the work encompasses principles of Ni/MH batteries, electrochemical hydrogen storage thermodynamics and kinetics, prerequisites for hydrogen storage electrode alloys and recent advances in hydrogen storage electrode alloys. Rare earth AB5-type alloys, Ti- and Zr-based AB2-type alloys, Mg-based amorphous/nanocrystalline alloys, rare earth-Mg–Ni-based alloys and Ti–V-based alloys are highlighted. Additionally, the challenges met in developing advanced hydrogen storage alloys for Ni/MH rechargeable batteries are pointed out and some research directions are suggested.

Advanced hydrogen storage alloys for Ni/MH rechargeable batteries

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

Structural investigation and hydrogen storage capacity of LaMg2Ni9 and (La0.65Ca0.35)(Mg1.32Ca0.68)Ni9 of the AB2C9 type structure

Hole sizes and intersite distances were calculated at various hydrogen concentrations for the interstices in hydrides of TiCr 1.8 , ZrTi 2 , ZrV 2 , ZrCr 2 , ZrFe 2 , HfV 2 and TaV 2 with the cubic MgCu 2 C15 structure and of ZrMn 2 and ErMn 2 with the hexagonal MgZn 2 C14 structure. From a survey of other metallic hydrides, empirical criteria were chosen for the minimum hole size (0.40 A) and the minimum H-H distance (2.1 A) for hydrogen atom occupancy in stable hydrides. With these criteria, a model was developed for predicting which sites and how many of them could be occupied by hydrogen atoms in each of the intermetallic compounds listed. The resulting stoichiometries were determined by experimentally filling the interstices of model lattices constructed of Friauf polyhedra without violating either of the adopted geometric criteria. For each of the compounds rationales are presented to explain the experimental observations reported by other researchers.

Site occupancies and stoichiometries in hydrides of intermetallic compounds: Geometric considerations

Electrochemical behaviour of intermetallic-based metal hydrides used in Ni/metal hydride (MH) batteries: a review

A limited survey was conducted of hydrogen and deuterium sorption by 13 selected AB itx alloys (A  La, Ce, Pr, Er; B  Co, Ni). A structural analysis showed that AB 5 hydrides (PrCo 5 H 3.9 and LaNi 5 H 6 are paradigms) share essentially the same structure, with a maximum of nine sites per AB 5 unit available for hydrogen storage. To determine whether all nine sites could be filled at pressures higher than those previously studied, pressure-composition isotherms were obtained at 21 °C and up to 1500 atm for each of the alloys under study. The greatest hydrogen absorption occurred for LaCo 5 , La 0.45 Ce 0.55 Ni 5 and Lao.5Ce 0.5 Co 5 with 9, 8.3 and 8.5 hydrogen atoms per mole of alloy respectively. The effect on the pressure-composition isotherm of substituting one A or B element for another (neodymium or manganese) who also explored. The pressure plateaus were usually shifted by these substitutions.

Hydrogen and deuterium sorption by selected rare earth intermetallic compounds at pressures up to 1500 atm

High pressure hydrogen gas (up to 1550 atm) was used to study hydrogen absorption in YNi5, LaPt5 and ThNi5, which do not absorb hydrogen at moderate pressures and at room temperature. They are found to absorb a large quantity of hydrogen at high pressures. YNi5 and LaPt5 form hydride phases whereas ThNi5 dissolves hydrogen as a continuous series of solid solutions to high hydrogen concentrations. Comparing this behavior with the results obtained for CaNi5, LaNi5 and other compounds, the hydride formation in YNi5 and LaPt5 is thought to be controlled mainly by their lattice compressibility and not by the unit cell size. The anomalous behavior of ThNi5 is probably a result of the tetravalency of thorium.

High pressure hydrogen absorption study on YNi5, LaPt5 and ThNi5

Pressure-composition data obtained for 21 °C and pressures up to 1400 atm indicate four or perhaps five phases in the DyFe2-H2 system. These probably correspond to different ways of arranging the hydrogen atoms over the abundant tetrahedral interstices. The magnetic behavior of DyFe2H3.5 was established and was compared with that of other RFe2 hydrides (where R is a rare earth). Hydrogenation weakens the Dy-Fe exchange so that for T > 75 K the dysprosium sublattice is magnetically disordered. Between 100 and 300 K the magnetization originates with the ordered iron sublattice. The magnitude of the magnetization indicates an iron moment of approximately 2μB per atom. The field dependence of magnetization at 4.2 K suggests a non-collinear alignment of moments, perhaps a consequence of fanning of the dysprosium moments.

DyFe2-H2 system: Magnetism and pressure-composition isotherms to 1400 atm

A gas high-pressure cell for X-ray diffraction studies was developed. It is capable of 2700 atm hydrogen pressure at ambient temperature, when used in a Debye-Scherrer camera, and 1020 atm to at least 300°C, when used in a Guinier (Huber) camera. It is suitable for weakly scattering materials.

John F. Lakner

Papers

Hydrogen and deuterium sorption by selected rare earth intermetallic compounds at pressures up to 1500 atm

High pressure hydrogen absorption study on YNi5, LaPt5 and ThNi5

DyFe2-H2 system: Magnetism and pressure-composition isotherms to 1400 atm

X-ray diffraction cell for Debye-Scherrer and Guinier (Huber) cameras at pressure and temperature

X-ray diffraction cell for Debye-Scherrer and Guinier (Huber) cameras at pressure and temperature