I. Uehara

Electrochemical impedance and deterioration behavior of metal hydride electrodes

Abstract Electrochemical impedance spectroscopy (EIS) was applied to metal hydride electrodes. Cole-Cole plots for the electrodes consisted of two obvious semicircles and a slope related to Warburg impedance. The semicircle in the high-frequency region was mainly related to the resistance and capacitance between the current collector and the pellet of alloy powder. The semicircle in the low-frequency region, which exhibited appreciable dependence on hydrogen content, was attributed to electrode reactions on the alloy particles and double-layer capacitance on the alloy particles. Resistance and capacitance between alloy particles in the electrodes also need to be taken into account. Deterioration processes of metal hydride electrodes using a mischmetal-based alloy, MmNi 3.5 Co 0.7 Al 0.8 , were also studied employing EIS. Deterioration of a metal hydride electrode using copper-coated alloy powder was dominated by a decrease in reactivity of the alloy surface. In contrast, an increase in the contact resistances and a decrease in the amount of electrochemically utilizable alloy particles were significant in the deterioration of electrodes using uncoated alloy powder. Deterioration of the electrodes was avoided to some extent by elevating the hot-press temperature during electrode preparation.

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

Abstract A new quaternary magnesium based alloy (La 0.65 Ca 0.35 )(Mg 1.32 Ca 0.68 )Ni 9 and its hydride have been synthesized and their crystal structures were determined by Guinier–Hagg X-ray powder diffraction. The compound has a hexagonal structure and is isostructural with LaMg 2 Ni 9 (AB 2 C 9 type), in which Ca partially occupies both A and B sites. The hydrogen absorption/desorption properties were determined by pressure–composition isotherms and compared with LaMg 2 Ni 9 . (La 0.65 Ca 0.35 )(Mg 1.32 Ca 0.68 )Ni 9 absorbs ∼1.87 wt.% H 2 at ∼3.3 MPa H 2 and 283 K.

Structural investigation and hydrogen capacity of CaMg2Ni9: a new phase in the AB2C9 system isostructural with LaMg2Ni9

Abstract A new ternary magnesium-based alloy CaMg 2 Ni 9 has been synthesized, and its crystal structure was determined by Guinier–Hagg X-ray and neutron powder diffraction. The compound has a hexagonal structure and is isostructural with LaMg 2 Ni 9 type, with Ca at the La atom site. The hydrogen absorption/desorption properties have been determined by thermal analysis and pressure–composition isotherms. The electrochemical properties of the alloy were also examined.

Hydriding properties of LaNi3 and CaNi3 and their substitutes with PuNi3-type structure

Abstract The PuNi 3 -type intermetallic compounds LaNi 3 , CaNi 3 , La 0.5 Ca 0.5 Ni 3 , LaCaMgNi 9 , La 0.5 Ca 1.5 MgNi 9 , CaTiMgNi 9 , LaCaMgNi 6 Al 3 and LaCaMgNi 6 Mn 3 have been prepared using a powder–metallurgy–sintering method. The hydrogenation behaviour of these materials has been studied through the gas–solid reaction. The as-prepared compounds were easily activated at room temperature under a hydrogen pressure of 3.3 MPa. The pressure–composition–temperature ( P – C – T ) curves show a single plateau region with the exception of LaNi 3 -H, which shows no plateau, and La 0.5 Ca 1.5 MgNi 9 -H, which shows two plateaus. All of these alloys can absorb/desorb hydrogen by 1.8 wt.% under the conditions studied. X-ray diffraction (XRD) analysis reveals that LaNi 3 H 4.5 is in the amorphous state, and the other hydrides are accompanied by different expansions of the unit cell volume of the host alloy.

Metal hydride electrodes based on solid solution type alloy TiV3Nix (0 ≦ x ≦ 0.75)

Abstract Microstructures, pressure-composition isotherms, and charge-discharge characteristics of TiV 2 Ni x (0 ≦ x ≦ 0.75) alloys were investigated. The alloys for x ≧ 0.25 composed of a vanadium rich (βTi, V) main phase and a TiNi based b.c.c. secondary phase. The secondary phase formed a three dimensional network, enhancing electrode kinetics of the alloys. The maximum discharge capacity was 420 A h kg −1 for x = 0.56.

Hydrogen storage alloys rapidly solidified by the melt-spinning method and their characteristics as metal hydride electrodes

Abstract Rapidly solidified LaNi5-based hydrogen storage alloys were prepared by a melt-spinning method. The prepared melt-spun alloy ribbon had very fine crystal grain of below 10 μm. The hydrogen absorption behavior and electrode properties of the alloys were greatly improved. Heat treatment at 400 °C which did not cause enlargement of the grain further improved these properties.

Nickel-metal hydride battery for electric vehicles

When a nickel-powder-mixed alloy electrode using MmNi3.5Co0.7Al0.8 (Mm = mischmetal) was held in a complete discharge state at 40 °C, a severe capacity decrease due to passivation was observed. This capacity lowering was prevented by a nickel (copper) coating or by a cobalt powder mixing. The alloy MmNi3.8Co0.5Mn0.4Al0.3 did not cause passivation even for the nickel-mixed alloy. Prismatic nickel-metal hydride batteries (30–60 A h) were constructed using the alloy electrodes and evaluated as an electric vehicle battery.

Characterization of metal hydride electrodes by means of electrochemical impedance spectroscopy

Abstract Resistive components of various metal hydride electrodes were evaluated by means of electrochemical impedance spectroscopy. Examples of applications of the technique are reported: characterization of (1) the difference in performance between a misch-metal-based alloy electrode and a titanium-based alloy electrode and (2) the influence of the storage period of misch-metal-based alloy powder on the alloy surface activity.

Rare-earth-based hydrogen storage alloys for rechargeable nickel-metal hydride batteries

Abstract The cycling lives of metal hydride electrodes using mischmetal (Mm)-based alloys were significantly impaired by a stoichiometric deviation to the Mm-deficient side, because elements such as manganese and aluminum were preferentially precipitated at grain boundaries. Heat treatment at 1000 °C was effective for improving the cycling lives.

Hydrogenation and phase structure of Ti-Fe-V alloys

Copyright (c) 1997 Elsevier Science S.A. All rights reserved.Hydrogenation behavior and phase structures of some Ti-Fe-V ternary alloys were studied. Three different crystal structures, namely, a bcc, a CsCl-type and a C14 Laves phase structure were found. The atoms in the C14 Laves phase TiFe 2 could be partially substituted by vanadium atoms without changing its structure. It was found that the C14 Laves phase structure region was expanded nearly to the center of the Ti-Fe-V ternary phase diagram. Lattice parameters of the alloys increased almost in proportion with increasing average atomic size of the constituent elements. It was also found that hydrogen storage capacity at a constant pressure increased with increasing the degree of vanadium substitution. The pressure-composition (P-C) isotherms of the C14 Ti-Fe-V alloys strongly sloped and they had no flat plateau. The storage capacity of C14 Laves phase Ti-Fe-V alloy was optimized to be 1.0 at the chemical composition of TiFeV 0.7 .