J. R. Johnson

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.

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.

Effect of Zn additives to the electrolyte on the corrosion and cycle life of some AB{sub 5}H{sub x} metal hydride electrodes

Investigation on a series of ABs-type metal hydride electrodes reveals significant improvement of cycle life in the presence of zincate electrolytes (0.5 M ZnO in 6 M KOH) for alloys without substituents such as Ce. For alloys containing Ce, which are known to impart passive protection against corrosion no additional advantage was derived by using zincate electrolytes. X-ray absorption near-edge structure investigation at the Ni K edge, due to its relative high abundance and catalytic importance in the hydriding process, was used to elicit the corrosion characteristics as a function of cycling. The spectra averaged over the bulk of the sample (transmission mode) and those from the top 200 to 250 A (electron yield mode) indicate significant lowering of corrosion and buildup of Ni(OH)2 in non-Ce substituted alloys when using zincate electrolytes. In Ce substituted samples this effect was marginal.

Reaction kinetics and x-ray absorption spectroscopy studies of yttrium containing metal hydride electrodes

This was a study of electrode degradation mechanisms and the 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 (XRD), x-ray absorption (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. Alloying with Y decreased the corrosion rate. The results are consistent with corrosion inhibition by a Y containing passive film. The increase in the kinetics of the hydrogen oxidation reaction (HOR) 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.