J.J. Reilly

The correlation between composition and electrochemical properties of metal hydride electrodes

This paper is concerned with an overview of the properties of metal hydride electrodes used for battery applications. The emphasis is on the properties of AB5 electrodes but others are treated as well. The review begins with a brief discussion of the pertinent chemistry of hydrogen in metals, the properties of intermetallic hydrides and their relation to electrochemical behavior. Systematic guidelines which permit the modification of such properties for electrochemical applications are discussed. The electrochemical behaviors of certain specific AB5 alloy electrodes are covered in detail, emphasizing the effects of composition changes with respect to both the A and the B components. The consequences of electrode expansion and contraction with respect to hydride formation and decomposition are discussed quantitatively. Novel alloy compositions and phases are noted and evaluated. The attractive properties of cobalt-free, non-stoichiometric AB5+x electrodes are noted.

Cerium Content and Cycle Life of Multicomponent AB 5 Hydride Electrodes

Multicomponent AB5 hydrides are attractive replacements for the cadmium electrode in nickel-cadmium batteries. The archetype compound of the AB~ alloy class is LaN%, but in a typical battery electrode mischmetal is substituted for La and Ni is substituted in part by various metals. While the effects of Ni substitution have been widely studied, relatively little effort has been focused on the effect of La substitution. Cerium is the predominant rare earth in normal misehmetal, and this paper deals with the effect on cycle life and storage capacity due to the increasing presence of Ce in the alloy series La1_x Ce~ Ni3.~5 Co0.7~Mn0.~Al0.3. Alloys were characterized by the determination of pressure-composition relationships, molar volume of H in the hydride phase and electrode cycle life. The effects due to lattice expansion are taken into account. It was concluded that the rate of loss of electrochemical capacity per charge-discharge cycle due to electrode corrosion was significantly decreased by the presence of Ce.

Function of cobalt in AB5Hx electrodes

Abstract The role of cobalt in the behavior of AB 5 H x electrodes has been investigated. Alloy compositions were as follows, LaNi 4.3− x Co x Mn 0.4 Al 0.3 ( x =0, 0.2, 0.4, 0.75) and MmNi 4.3− x Co x Mn 0.4 Al 0.3 ( x =0, 0.75). Cobalt was found to decrease the molar volume of hydrogen, V H , in the hydride phase. Both LaNi 4.3− x Co x Mn 0.4 Al 0.3 and MmNi 4.3− x Co x Mn 0.4 Al 0.3 alloy electrodes were subjected to repeated electrochemical cycling and corrosion rates measured. The corrosion rate was found to be inversely proportional to the cobalt content of the electrode.

In situ X-ray absorption spectroscopy studies of metal hydride electrodes

In situ x-ray absorption spectroscopy (XAS) studies were done on three metal hydride electrodes, LaNi~, LaNi~.sSns.2, La0.sCe02Ni48Sn0.2, in 6M KOH. Ex situ measurements were also made on dry uncycled electrodes and on material from an La0.sCe0.2Ni48Sn0.2 electrode that had been cycled 25 times. Comparison of the in situ XAS at the Ni K and at the La L3 edge of charged and discharged electrodes indicates large changes in the electronic and structural characteristics on introduction of hydrogen. Results at the Ce L2 edge in La~.sCe~.2Ni4.~Sn0.~ show a transition from a mixed valent c~ to a -/-like Ce state as the lattice expands during charge. Ex situ x-ray absorption near-edge structures (XANES) at the Ni K edge indicate that the additions of either Ce or Sn fill empty Ni 3d states. The Ni K edge extended x-ray absorption fine structures (EXAFS) for all three alloys in the dry uncharged state were similar, indicating that minor substitutions for either the A or B component do not substantially change the structure. The Sn substitution causes an increase both in a and c axis as evidenced from increase in the Ni-Ni and the Ni-La distances. Partial substitution of La by Ce causes a slight contraction in the Ni-La distance. The Ni XANES and EXAFS indicate that about 6 % of the Ni in the La0.sCe0.2Ni~.sSn02 corroded after 25 cycles. Ce XANES on the cycled electrode indicates some corrosion of Ce and the formation of Ce (III) state. The results indicate that XAS is a very useful technique for the study of alloy hydrides, particularly the role of electronic structure, the environment around minor constituents, and the corrosion of individual components. Recent advances in the development of stable metal hydride alloy electrodes have led to their use as a replacement for cadmium anodes in rechargeable alkaline batteries. I Present battery electrodes are either AB2 or AB5 type alloys. The performance and life of these alloys greatly depend on their composition. In the case the AB5 type alloys, substitution of either component in the prototype alloy, LaNi~, with small amounts of other alloying elements can have major effects in the performance and stability of the alloy. Previous results with Ce, Sn, and Co substitution have demonstrated promising results in battery electrodes. 2 Recent results 3 have shown that Sn and Co substitution for some of the Ni causes a lowering of the hydrogen plateau pressure. Partial substitution of La with Ce results in improved corrosion resistance and cycle life. The Ce substitution also causes an increase in the hydrogen plateau pressure. An understanding of the mechanism of these effects would help in optimizing metal hydrides for various hydrogen storage and battery applications.

Reaction Kinetics and X-Ray Absorption Spectroscopy Studies of Yttrium-Containing Metal Hydride Electrodes

This is a study of electrode degradation mechanisms and reaction kinetics of LaNi{sub 4.7}Sn{sub 0.3}, La{sub (1{minus}x)}Y{sub x}Ni{sub 4.7}Sn{sub 0.3}(x = 0.1, 0.2, and 0.3) and La{sub 0.7}Y{sub 0.3}Ni{sub 4.6}Sn{sub 0.3}Co{sub 0.1} metal hydride electrodes. Alloy characterization included X-ray diffraction. X-ray adsorption (XAS), hydrogen absorption in a Sieverts apparatus, and electrochemical cycling of alloy electrodes. The atomic volume of H was determined for two of the alloys. Electrochemical kinetic measurements were made using steady-state galvanostatic measurements, galvanodynamic sweep, and electrochemical impedance techniques. XAS was used to examine the degree of corrosion of the alloys with cycling. Allowing with Y decreased the corrosion rate. The results are consistent with corrosion inhibition by a Y-containing passive film. The increase in the exchange current density of the hydrogen oxidation reaction with increasing depth of discharge was much greater on the Y-containing alloys. This may be due to the dehydriding of the catalytic species on the surface of the metal hydride particles.

Effect of Ce, Co, and Sn Substitution on Gas Phase and Electrochemical Hydriding/Dehydriding Properties of LaNi5

A number of LaNi{sub 5}-based intermetallics with Ce, Sn, and Co substitutions were prepared by the arc-melting technique and tested as electrodes in Ni/MH{sub x} batteries. Sn and Co substitution expand the unit cell and lower the plateau pressures whereas the introduction of Ce result in the reverse effects. Ce and Co substitutions result in a decrease in the storage capacity of the intermetallics. Most of the Sn substituted compounds exhibit high hydriding capacities, though they widely differ in their electrochemical behavior, viz., cycle life and rate capabilities. Two such intermetallic compounds, with different amounts of substitution, show promise with high capacities (over 300 mAh/g), even at high rates of discharge. Intermetallics with Ce, Sn, and Co retain the high capacity upon long-term cycling.

Site Preference of Cobalt and Deuterium in the Structure of a Complex AB 5 Alloy Electrode: A Neutron Powder Deffraction Study

The site preference of cobalt and deuterium in the charged and discharged state of a commercial-type metal hydride anode was investigated by high resolution neutron powder diffraction using an alloy with an altered isotope composition. In the alloy La( 58 Ni 0.376 62 Ni 0.624 ) 3.55 Co 0.75 Mn 0.33 Al 0 30 the isotope ratio of 58 Ni to 62 Ni leads to a zero neutron scattering length for nickel. Rietveld refinement revealed that Al and Mn occupy the midplane 3g sites of the CaCu 5 structure, while cobalt has a preference for 3g but also occupies the basal plane 2c sites. The space group of the deuteride remains P6/mmm which differs from the parent LaNi 5 D 7 which is P6 3 mc.

Behavior of hydrided and dehydrided LaNi5Hx as an hydrogenation catalyst

Abstract The catalytic properties of varying phase compositions of LaNi 5 H x were studied using the hydrogenation of 1-undecene as a model reaction. All experiments were carried out with the catalyst in liquid suspension at 308 K under varying pressures of hydrogen. The H content of the catalyst particles was tracked during the course of each experiment. Four types of catalytic behavior were observed and are catalogued in turn. While the metal solid solution phase (α-LaNi 5 H x ) exhibits catalytic properties, catalytic activity is significantly enhanced by the presence of the hydride phase (β-LaNi 5 H x ) in the particle bulk even though the surface composition of the particle is likely unchanged. It was concluded that several factors contribute to this enhancement: (1) the hydride phase acts as a reservoir of H atoms which communicate with the surface via microcracks and grain boundaries; (2) the substantial increase in bulk volume upon the conversion of the metal to the hydride phase enlarges these conduits and facilitates said communication; (3) weak chemisorption of hydrogen on the surface, due to the high hydrogen atom activity when the hydride phase is present, increases the amount of labile hydrogen.

Decomposition kinetics of palladium nickel hydride

Abstract The kinetics of the decomposition of β-Pd 0.85 Ni 0.15 H x were determined under isothermal conditions. The experiments were carried out with a suspension of alloy particles in liquid undecane. The rate-limiting process is the solid state transformation taking place at the interface between the hydride and the hydrogen solid solution phase. The activation energy was 78 kJ mol −1 ; the pre-exponential in the Arrhenius expression is 4.38 × 10 9 s −1 . The results are discussed in terms of the Polanyi-Wigner expression and the activated complex theory. It is proposed that the decomposition proceeds by the conversion of a group of approximately ten whole β phase unit cells to a corresponding group of whole α phase unit cells.

Crystal structure and magnetic properties of DuY2BaCuO5 (u = 0.00, 0.61, 1.31)

Abstract Samples of D u Y 2 BaCuO 5 ( u =0.61, 1.31), were prepared by a direct reaction of the green phase (Y 2 BaCuO 5 ) with D 2 gas. The crystallographic structure of the three compounds ( u =0.00, 0.61, 1.31) were studied using X-ray and neutron powder diffraction. It was found that as D enters the green phase, a solid solution phase is formed with no symmetry change. Two sites partially occupied by D were determined. The occupancy of these sites is consistent with a maximum solubility of u =2. The temperature dependence of the magnetic susceptibility of the three samples obeyed the Curie-Weiss law with antiferromagnetic interaction. The paramagnetic Curie temperature decreased with deuteriding, while, the Cu + + magnetic moment remained unchanged.