Itsuki Uehara

Synthesis and structure determination of a new series of hydrogen storage alloys; RMg2Ni9 (R=La, Ce, Pr, Nd, Sm and Gd) built from MgNi2 Laves-type layers alternating with AB5 layers

Abstract A number of new ternary magnesium based alloys, RMg2Ni9 (where R=La, Ce, Pr, Nd, Sm and Gd), have been synthesized by reacting a mixture of MgNi2 with RNi5 intermetallic compounds or by direct combination of the elements in the atomic ratio R:Mg:Ni=1:2:9. The crystal structure, determined by Guinier-Hagg X-ray powder diffraction, is related to the hexagonal PuNi3 type. All interatomic distances are close to the corresponding ones in (C15) MgCu2, (C14) MgZn2, (C36) MgNi2, RNi3 and RNi5 of metallic-type bonding.

Structural investigation and hydrogen capacity of YMg2Ni9 and (Y0.5Ca0.5)(MgCa)Ni9: new phases in the AB2C9 system isostructural with LaMg2Ni9

New ternary magnesium-based alloys, YMg2Ni9 and (Y0.5Ca0.5)(MgCa)Ni9 have been synthesized and their crystal structures were determined by Guinier–Hagg X-ray powder diffraction. The compounds are a hexagonal and isostructural with LaMg2Ni9 (AB2C9 type), in which Y is uniquely at the A site as in YMg2Ni9, and Ca partially occupies both A and B sites. The hydrogen absorption/desorption properties were determined by pressure–composition isotherms. YMg2Ni9 does not absorb hydrogen, whereas (Y0.5Ca0.5)(MgCa)Ni9 absorbs ∼2 wt.% H2 at ∼3.3 MPa and 263 K.

Electrochemical Impedance Spectra and Deterioration Mechanism of Metal Hydride Electrodes

Electrochemical impedance spectrum of a metal hydride electrode was interpreted by an equivalent circuit including a reaction resistance on the alloy surface, a contact resistance between the current collector and the pellet, one related to alloy particle‐to‐particle contact, and a Warburg impedance. According to the interpretation, deterioration of a metal hydride electrode using copper‐coated alloy powder was found to be caused by passivation of the alloy surface only. On the other hand, deterioration of an electrode using uncoated alloy was dominated by increase of the contact resistances.

Phase structure of V-based solid solutions containing Ti and Ni and their hydrogen absorption-desorption properties

Abstract Crystal structures and pressure-composition isotherms were investigated for the vanadium rich ternary TiVNi solid solution. The alloy containing more than 85at.% vanadium absorbed hydrogen of 1.6 in H/M ratio that amounted to 3.2 mass% H. The pressure-composition isotherm for Ti 22 V 66 Ni 12 in the hydrogen pressure range of 10 0 to 10 −8 MPa was obtained by the electrochemical method. Two pressure plateaus were found at hydrogen pressures of 10 −2 MPa and 10 −6 MPa. The upper plateau would be ascribed to the reaction from dihydride to monohydride and the lower plateau would be ascribed to the reaction from monohydride to solid solution of the alloy and hydrogen.

Hydrogen Storage Alloys for Nickel — Metal Hydride Battery*

— 2 times higher energy density per unit volume and longer cycle life than Ni —Cd cells with almost comparable rate capability and charge retention. A very unique feature of the hydrogen battery is a self-protective mechanism for overdischarge. The hydrogen produced on the nickel electrode is absorbed on the MH electrode and then oxidized to water, keeping the cell voltage around —0.2 V. No oxidation and degradation of alloy occurs during overdischarge if the recombination reaction is fast enough to keep up with the discharge rate. This good overdischarge protection is very advantageous for the series stacking of many cells, i.e., for high voltage applications such as electric vehicles (EV) [6]. Secondly, AB5 (A = rare earth, Zr, B = Ni, Co, Fe, Mn, Al) and AB2 (A = Ti, Zr, B = Ni, V, Fe, Co, Mn, Al) type alloys do not contain hazardous materials, being safe from the standpoint of environmental and safety regulations [7].

The state of research and development for applications of metal hydrides in Japan

Abstract This paper outlines Japans current activities in the research and development (R&D) and commercialization of metal hydrides for applications in hydride electrodes and batteries, hydrogen storage, hydrogen purification and effective utilization of thermal energy. Representative and notable activities and technologies promoted by private corporations and national projects are briefly covered.

X-ray diffraction peak broadening and lattice strain in LaNi5-based alloys

Abstract Peak broadening in X-ray powder diffraction (XRD) profiles of LaNi5-based alloys after hydriding and dehydriding processes was investigated in order to clarify the mechanism of formation of lattice strain in hydriding and dehydriding. The Rietveld method was used to evaluate the degree of peak broadening and to determine anisotropic peak broadening axis for LaNi5 and LaNi5−αMα (M: Mn, Fe, Cu, Al; α=0.25, 0.5) before hydriding, after activation and after 1000 hydriding–dehydriding cycles. All the alloys studied showed anisotropic broadening vectors of the same direction 〈110〉 after activation. The degree of the peak broadening, however, strongly depended on the substitution elements. Hydriding–dehydriding cycles did not influence the direction of the anisotropic peak broadening axis, while both anisotropic and isotropic peak broadening increased with number of cycles. It was found that the lattice strain analyzed from the peak broadening in X-ray diffraction profiles corresponded to dislocations with Burgers vectors 〈hk0〉 observed by transmission electron microscope.

The TiV3Ni0.56 hydride electrode: its electrochemical and cycle life characterization

Abstract Charge/discharge cycle tests for the TiV 3 Ni 0.56 electrode were conducted in order to investigate the processes of activation and deterioration. Cracking formation was observed to occur across the alloy grain during the activation process. After a few more cycles following the activation, smaller cracks were observed between the matrix and a secondary phase and also within the secondary phase. In the deterioration process titanium and vanadium were found to dissolve selectively from both phases. When the electrode lost its discharge ability, titanium and vanadium in the secondary phase had almost disappeared, leaving the nickel layer, while the matrix phase still retained the original composition and hydrogen storage ability.

Hydrogenation characteristics of ternary alloys containing Ti4Ni2X (X=O, N, C)

Abstract Ti 2 Ni-based alloys containing oxygen, nitrogen and carbon were investigated regarding their metallographic structures and hydrogenation characteristics. The alloy samples containing these non-metal elements had multi-phase structures composed of Ti 4 Ni 2 X, TiNi and TiX (X=O, N, C) phases. The ternary alloy samples reversibly absorbed and desorbed hydrogen under moderate conditions such as room temperature and atmospheric pressure. The hydrogenation properties of the alloy samples containing the non-metal elements were compared with those of the binary Ti 2 Ni compound.

Lattice expanding behaviour and degradation of LaNi5-based alloys

Abstract Lattice expansion, contraction and degradation were investigated in two types of LaNi5-based alloys to identify the dominant factors in the mechanism of intrinsic degradation. Alloys were subjected to hydriding–dehydriding cycling tests up to 1000 cycles in pure hydrogen. Lattice expanding behaviour was studied by in-situ X-ray powder diffractometry, focusing on discrete lattice expansion of the crystal lattice between the α phase and the β phase in the plateau region. The alloy with a wider plateau region showed larger discrete expansion and lattice strain. The difference in degradation behaviour between the two alloys was related to the discrete expansion, especially in the c axis direction. This phenomenon is considered to be the dominant factor in the intrinsic degradation mechanism.