Karl J. Gross

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 transition to a steady state composition

Magnesium alloys are potentially the best materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La2Mg17 + LaNi5 mechanically milled in a 2:1 weight ratio. Composites produced with varying durations and intensities of milling were tested. Those milled to the greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled composite were superior to those of La2Mg17. This composite absorbed 90% of its full hydrogen capacity (3.5 wt.% H2) in less than 1 min at 250°C and desorbed the same quantity of hydrogen in 6 min. Under the same conditions pure La2Mg17 took 2.5 h to absorb and 3 h to desorb 90% of its full hydrogen capacity (4.9 wt.% H2). Scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the mechanically milled powders before and after hydriding. The unhydrided powders consisted of LaNi5 grains surrounded by a fractured LaMg17 matrix. Hydrogen cycling, at temperatures up to 350°C, induced phase changes, segregation, and disintegration of the composites. The resulting fine powder (less than 1 μm) consisted primarily of Mg, Mg2Ni, and La phases.

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

Mg composites for hydrogen storage - The dependence of hydriding properties on composition

Abstract Magnesium alloys show great potential as materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of composites formed by mechanically milling La 2 Mg 17 with x wt.% LaNi 5 ( x =0, 10, 20, 30, 40, 50, 60 wt.%). The rate of hydrogen absorption and desorption was measured for all of these compositions over a wide range of temperatures. These composites were then characterized using a set of reaction rate equations. The composite La 2 Mg 17 +40 wt.% LaNi 5 showed the best overall kinetics. At 250°C it has an average absorption rate of 8.2 (wt.% min −1 ) and a desorption rate of 1.0 (wt.% min −1 ) with a final capacity of 3.7 wt.%. This is approximately 50 times faster than pure La 2 Mg 17 under the same conditions.

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.

On the possibility of metal hydride formation - Part II: Geometric considerations

Abstract A simple analysis based on crystal geometry is proposed as an aid in predicting the formation of hydrides from intermetallic compounds. It is derived from a slight variation Westlakes empirical geometric criterion that an interstitial site should have a radius ≥0.4 A to be occupied by hydrogen. For comparison this criterion has been applied to a compound which hydrides (LaNi5) and to a compound with a similar crystal structure (MgNi3B2) which does not. When applied to the LaNi5–H system, this criterion predicts the occupation of the 6m and 12n sites in the P6/mmm α solid solution phase LaNi5H

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.

CeMnA1Hx, a new metal hydride

Abstract In contrast to CeAl 2 , which does not form hydrides, and CeMn which does not form binary intermetallic compounds at all, CeMnAl absorbs considerable amounts of hydrogen. A stable hydride phase (β-phase) is formed at low pressures ( 10 mbar (at 300 K). In this work we show the absorption characteristics and structural properties of this new metal hydride. The intermetallic compound CeMnAl crystallizes in the cubic C15 Laves phase as observed by X-ray diffraction. Neutron scattering experiments on this intermetallic compound show that the Mn and Al atoms are distributed randomly on the B sites in this AB 2 -type C15 Laves phase compound (A = Ce; B = Mn, Al). Upon hydride (deuteride) formation the C15 structure is preserved. The deuterium atoms occupy only the 2A-2B-tetrahedral interstitial sites (96g-positions) over the investigated concentration range 0.0 x .

Hydriding properties of Ce(Mn, Al)(2) and Ce(Fe, Al)(2) intermetallic compounds

Abstract A major breaktrough in the developement of metal hydrides for hydrogen storage will require inexpensive intermetallic compounds that absorb large quantities of hydrogen at near ambient conditions (atmospheric pressure and room temperature). Unfortunately, in the last ten years there has been relatively little progress in this field. We investigated intermetallic compounds Ce(T1−yAly)2 with T=Mn, Fe in the range 0.5≤y 10 mbar. The alloys and their hydriding characteristics were analyzed by means of X-ray diffraction and pressure-concentration isotherms. The two different absorption behaviours are correlated to the occupation of 96g-positions with different surrounding atomic constitutions.

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.