Daniel Chartouni

The influence of cobalt on the electrochemical cycling stability of LaNi5-based hydride forming alloys

We present an investigation of the influence of cobalt substitution for nickel on the electrochemical cycle life of LaNi5-based alloys. Lattice expansion during hydriding was measured by means of X-ray diffraction for the alloys LaNi4Co, LaNi3.5CoAl0.5 and LaNi4.5Al0.5. The surface composition of the alloy grains was analysed by means of X-ray photoelectron spectroscopy (XPS). The XPS-depth-profiles are mentioned in this paper. The mechanical and electrochemical properties of these alloys and additionally of Zr0.2,La0.8Ni4.5Al0.5 and Er0.2, La0.8Ni4.5Al0.5 were also measured. We observed a strong correlation between the hardness of these alloys and the cycling stability. Harder alloys lose capacity more rapidly with cycling. Cobalt appears to lower the hardness and therefore increase the cycle life of these alloys. It is well known that alloys which show a large lattice expansion during hydriding, pulverize faster with cycling. This behaviour was clearly observed in our measurements. The combination of a minimal lattice expansion and a low hardness seem to have a synergetic effect in increasing the cycle life.

Mechanically milled Mg composites for hydrogen storage : the relationship between morphology and kinetics

Note: Times Cited: 73 Reference EPFL-ARTICLE-205956doi:10.1016/s0925-8388(97)00627-0View record in Web of Science URL: ://WOS:000074117500049 Record created on 2015-03-03, modified on 2017-05-12

Relationship between composition, volume expansion and cyclic stability of AB5-type metalhydride electrodes

Abstract Metal hydride alloys as electrode material for battery application contain up to 15 at.% cobalt. Alloys without cobalt show a much shorter cycle life compared to cobalt containing alloys. The mechanism of how cobalt influences the cycle life is still not well understood. The aim of this work is to investigate the influence of cobalt on the properties of the electrode. A series of alloys with different cobalt content and several other substituents for nickel (Fe, Cu,...) were prepared in two different ways. A set of samples was conventionally melted. A second set of samples was prepared by gas atomization. The volume expansion upon hydriding was analyzed by means of X-ray diffraction. Electrochemical measurements, e.g., discharge capacity as a function of cycle number, were performed. The volume expansion upon hydriding decreases with increasing cobalt content of the alloy. Cobalt substitution for nickel improves the cycle life of an electrode, especially at elevated temperatures (40°C). However, alloys where cobalt is partially substituted by iron show an even better cyclic stability and rate capability.

Pd-cluster size effects of the hydrogen sorption properties

Clusters are agglomerates of a few to some hundred atoms. A large fraction of the atoms find themselves on a surface site. The geometrical structure of a cluster, in contrast to the structure of crystalline bulk material, can not be described with a repeated, a space filling unit cell. Clusters appear often in regular geometrical polyhedrons e.g. cuboctahedron or icosahedron. The interaction between hydrogen and palladium clusters of different sizes was investigated. The sorption properties of the clusters are compared with the well known properties of Pd bulk material. The Pd clusters reversibly absorb and desorb hydrogen. The density of states (DoS) distribution of hydrogen in the Pd cluster becomes wide with decreasing cluster size. This is an indication for a lowering of the critical temperature for the hydride formation of small Pd clusters.

Properties of Zr(V0.25Ni0.75)(2) metal hydride as active electrode material

We have investigated the Zr(VxNi1−x)2 alloy series in the range of 0 ⩽ x ⩽ 0.4 and observed very pronounced absorption and desorption properties in Zr(V0.25Ni0.75)2. We have shown previously that Zr(V0.25Ni0.75)2 has a high reversible capacity of 364 mA h g−1 (C/30 rate) and a good electrochemical cycle stability (8%/100 cycles capacity loss, C/3 rate). However, large-scale production of this alloy seems to be difficult, and annealing deteriorates the high capacity and the good kinetics as well. In this study we have analyzed the microstructure of the alloy and the thermodynamics of hydride formation of the alloy electrodes. Two different phases were observed, Zr7Ni10 and Zr(V0.33Ni0.67)2.3; however, both of them are, as single phase alloys, not quite as good as their combination in Zr(V0.25Ni0.75)2.

Influence of the alloy morphology on the kinetics of AB5-type metal hydride electrodes

Abstract Three different cast LmNi 3.6 Co 0.7 Al 0.3 Mn 0.4 alloys (Lm=Lanthanum rich mischmetal) were used for the investigation. One alloy was rapidly cooled, a second was rapidly cooled and annealed and the third one was cast in a conventional way. Our main interest are the kinetic properties of these alloys, measured by means of Electrochemical Impedance Spectroscopy (EIS) and electrochemical charge discharge experiments. The rapidly cooled alloy sample showed better fast discharge behavior and smaller reaction resistance compared to annealed samples. In combination with X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) a possible explanation on the improved kinetics of the rapidly cooled electrode can be given: The smaller grain size and the high amount of grain boundary in the alloy leads to a faster activation and therefore to an enhanced alloy surface. This results in a smaller reaction resistance and an improved fast discharge behavior.

About the mechanism and the rate limiting step of the metalhydride electrode reaction

Abstract LaNi 5 alloy powder samples absorb in the gas phase 80% of their maximum hydrogen capacity (H/M=6) in less than 30 seconds. In the electrochemical discharge process, the charge transfer at the surface of the negative electrode should be the rate determining step because an electrochemical discharge reaction is about an order of magnitude slower than a gas phase desorption process. In this study the alloy system Lm(Ni 3.6 Co 0.7 Al 0.3 Mn 0.4 ) α with 0.96≤ α ≤1.12 was examined as a model system. The reaction resistance of the electrodes was directly measured by means of impedance spectroscopy as a function of the hydrogen concentration in the metalhydride. The high rate dischargeability as well as the resistance are related to the surface composition and surface morphology. The surface composition of a metalhydride electrode not only varies with the cycle number but also changes within a single cycle.

Bulk and surface properties of crystalline and amorphous Zr36(V0.33Ni0.66)64 alloy as active electrode material

Abstract The electrochemical hydrogen absorption/desorption behavior of a crystalline and an amorphous Zr 36 (V 0.33 Ni 0.67 ) 64 alloy sample was investigated. Electrodes made of the crystalline sample show a much lower resistance and consequently a higher discharge capacity compared to the electrodes made of the amorphous alloy. The XPS surface analysis did not explain the difference, we observed almost exactly the same depth profile on both samples. However the surface of the activated alloy compared to the non-activated was drastically changed. The oxide layer on an amorphous alloy seems to grow in a much more compact and stable structure than the oxide layer on a crystalline sample.

A new hexagonal Laves phase deuteride CeMn1.5Al0.5Dx (0<x<4): Investigated by in situ neutron diffraction

Abstract The crystal structure of the C14 ( P6 3 /mmc ) hexagonal Laves phase compound CeMn 1.5 Al 0.5 D x ( x real-time neutron scattering measurements were performed during the deuteriding of the compound from 0 to 170 ncm 3 D 2 /g-sample (3.9 D/fu). From these measurements we determined the critical concentrations at which phase transitions occurred. In this compound, the deuterium initially dissolved into the host-metal lattice forming a solid solution (α-phase). This phase exhibited an isotropic lattice expansion with increasing deuterium concentration. The nucleation of a deuteride (the β phase) occurred at a total deuterium content of 20 ncm 3 D 2 /g-sample. The transformation into the deuteride was complete at 145 ncm 3 D 2 /g-sample, giving a stoichiometry of CeMn 1.5 Al 0.5 D 3.0 for the β phase. With increasing pressure, this phase continued to absorb deuterium forming a β′ deuteride solid solution. The growth of the β phase is accompanied by the occurrence of a broad peak in the background. The position and diffuse character of this peak clearly indicates the presence of short-range deuterium ordering in the otherwise disordered occupation of interstitial sites. Finally, the crystal structures of all CeMn 1.5 Al 0.5 D x ( x 1.5 Al 0.5 , is a pseudo-binary AB 2 -type compound showing a statistical distribution of Mn and Al on two different B sites. In the CeMn 1.5 Al 0.5 D x phases, deuterium was located only on A2B2-type tetrahedral interstitial sites. The occupation of these specific interstitial sites can be explained in terms of diffusion paths, maximized D–D distances, as well as, the preference of deuterium for interstitial sites coordinated by the largest number of Ce second-nearest-neighbors atoms.

Phase Transitions in LaNi4Co during Electrochemical Cycling An In Situ X-Ray Diffraction Study

Phase and crystal structure changes during the electrochemical hydriding and dehydriding of LaNi 4 Co were investigated using in situ X-ray diffraction (XRD). A specially designed cell allowed dynamic XRD measurements during five charge-discharge cycles. This enabled the direct observation of the activation process. In addition, crystal lattice information derived from these measurements help to explain the long cycle life of cobalt containing AB 5 -type battery alloys. The formation of an intermediate γ-phase hydride between the hydrogen solid-solution α-phase and the fully hydrided β-phase was clearly observed during absorption and desorption. The volume expansion in the formation of the γ and β hydride phases is highly anisotropic. Lattice expansion in the α-phase to γ-phase transformation occurs mainly in the basal plane, whereas the transition from the γ-phase to the β-phase causes a lattice expansion in the c axis direction. It is believed that the two-step phase transition in this Co-substituted alloy generates less internal stress than the single-step volume expansion of the archetype LaNi 5 compound. This reduces the stress-induced pulverization that occurs during electrochemical cycling. Consequently, the metal hydride electrode maintains larger particle sizes and, thus, smaller surface areas subject to corrosion by the electrolyte, which is the principle cause of capacity loss.