Hiroshi Miyamura

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].

Surface treatment of a LaNi5-type metal-hydride electrode with an alkaline solution dissolving cobalt(II) hydroxide

The effects of KOH aqueous solution saturated with Co(OH)2 on LaNi4.7Al0.3 electrodes were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The electrochemical activity of an LaNi4.7Al0.3 electrode was considerably improved by a treatment with Co(OH)2-KOH solution below the potential of −0.8 V (vs. Hg/HgO) after several cycles in pure KOH electrolyte. These electrodes exhibited higher metallic-nickel and lower lanthanum concentrations in the subsurface region of the alloy than those without any treatment. This metallic-nickel enrichment was concluded to be caused by oxidation of metallic lanthanum with water catalyzed by Co(OH)42− ions followed by a dissolution of lanthanum hydroxide. In contrast, the electrode treated with the Co(OH)2-KOH solution without regulation of potential before cycling in KOH solution exhibited lower electrochemical activity and metallic-nickel concentration than that without any treatment. In this case, Co(OH)42− ions were considered to act as an oxidant to the alloy.

Hydrogen storage alloys for nickel/metal-hydride battery

Abstract Efforts to improve performance of metal hydride electrodes such as substitution of alloy components, heat treatment, and surface treatment intended to change surface and bulk structure of hydrogen storage alloys, mainly LaN 5 based alloys, are reviewed. The importance of control of morphology is emphasized.

Methanation of carbon dioxide over LaNi4X type catalysts

Abstract The hydrogenation of carbon dioxide into methane proceeds over catalysts whose precursors are intermetallic compounds of LaNi4X (X = Ni, Cr, Al, Cu). Among them LaNi5 is significantly active at 250°C after several hours of the reaction, while the activity at the initial stage is very low. During the reaction LaNi5 is decomposed to give metallic nickel, and methanation activity is parallel to the amount of metallic nickel. The XPS analysis of the catalyst suggests the formation of new nickel species with a valence close to +1.

Methanation of carbon dioxide over LaNi4X-type intermetallic compounds as catalyst precursor

Abstract The hydrogenation of carbon dioxide is catalyzed over intermetallic compounds LaNi4X (X ue5fc Ni, Cr, Al, Cu) at a reaction temperature as low as 250 °C under 5 MPa, where LaNi5 and LaNi4Cr are significantly active. On the basis of X-ray diffraction analyses of the catalysts after the reaction, the catalytic activity seemingly depends on the amounts of metallic nickel formed by decomposition of the intermetallic compounds during the reaction, and the compounds preserving the alloy structure are less active.

Hydriding behavior of pseudobinary CaAl2−xMx (M = B, Si, 0 ⩽ x ⩽ 1) alloys

Abstract The hydriding behavior of pseudobinary CaAl 2− x M x (M = B, Si, 0 ⩽ x ⩽ 1) alloys with the C15 Laves phase structure is examined in the search for new metal hydrides for hydrogen storage. The initial activation for hydriding proceeded at an H 2 pressure of 3 MPa and at temperatures between 300 and 400°C without changing their phase structure or decomposing thermally. The CaAl 1.8 B 0.2 alloy absorbed the largest amount of hydrogen among these alloys, forming CaAl 1.8 B 0.2 H 0.6 at an H 2 pressure of 1 MPa at 60°C. For alloys with compositions around CaAl 1.8 M 0.2 , pressure plateaus were observed in the pressure-composition isotherms. Lattice contractions were also observed in these alloys.

Metal hydride electrodes with lamellar-type network structure

Abstract New types of metal hydride electrodes with three dimensional networks of conducting metal powder have been developed. Utilization efficiency of the alloy was successfully improved because of elimination of the usual non-conductive polymeric binder and also the material cost could be reduced because it was not necessary to use an expensively formed nickel substrate or an expensive surface coating process. The new electrodes are composed of hydrogen storage alloy and newly developed flake-type copper and/or nickel powder formed into a laminated network structure in the electrode, thus providing better binding ability and better electrode conductivity than a conventional dendritic powder. The mixture, in the weight ratio of Cu(Ni)/alloy=0.2, was press-bound on a nickel mesh current collector. The effect of flake powder was more pronounced for the Ti–Zr based AB 2 type alloy (less conductive surface) than the rare earth based AB 5 type alloy.

Thin Film Preparation of Hydrogen Storage Alloys by RF-Sputtering and Electrochemical Measurements of Their Hydrogen Storage Properties*

Thin films of hydrogen storage alloys (LaNi5, LaNi2.5C02.5) were prepared by rf-sputtering. The atomic configuration (amorphous or crystalline) and the hydrogen capacity of the prepared thin films depended on the kinds of targets and substrates and the sputtering conditions such as rf-power, temperature, and sputtering time. The crystalline films had the oriented-structure in which the c-axis is parallel to the substrate plane. The hydrogen storage properties, such as the pressure-composition isotherms and the durability, were examined electrochemically for the alloy films prepared on a nickel

Electrochemical activity enhancement of a LaNi4.7Al0.3 electrode treated with an alkaline solution containing H2O2

Abstract Enhancement of the electrochemical activity of a LaNi 4.7 Al 0.3 electrode was observed subsequent to some charge–discharge cycles in a KOH solution containing hydrogen peroxide. Metallic nickel is enriched at the alloy surface after the treatment, and the concentration of lanthanum decreased during the treatment. The change in the distribution of elements at the alloy surface is expected to lead to a different dispersion state of nickel particles in the region, resulting in improvement of the catalytic activity for the charge-transfer process on the alloy surface.

Discharge characteristics of Ti-Zr based multi-component alloy hydride electrodes

Charge and discharge characteristics were investigated for some Ti-Zr based multi-component metal hydride(MH) Electrodes. Both charge and charge overpotentials considerably increased at low hydrogen contents in the MH electrodes. The increase resulted in the decrease of discharge capacities in high-rate discharge. The dependence of the overpotentials on hydrogen content are discussed from the point of view of the formation of low hydrogen concentration region at the MH particle surfaces in the electrode.