Felix Meli

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.

Effects of pretreatment on the activation behavior of Zr(V0.25Ni0.75)2 metal hydride electrodes in alkaline solution

Metal hydrides used as negative electrodes in rechargeable batteries are covered with surface oxides during preparation of the electrode. They inhibit or slow down the absorption rate of hydrogen and have to be dissolved or made conductive in an activation process. In order to minimize the activation cycles we have investigated the influence of the pretreatments of the metal hydride powder Zr(V0.25Ni0.75)2 with KOH and fluoride-containing solutions on the surface composition of the alloy grains by means of X-ray photoelectron spectroscopy. The study establishes the relationship between the surface composition of the hydrogen-storing material and the electrode performance, e.g. discharge capacity and high-rate dischargeability. The speed of activation increased with increasing nickel content in the top surface layers (30 A) of the metal hydride.

Electrochemical and surface properties of low cost, cobalt-free LaNi5-type hydrogen storage alloys

Abstract For many applications AB 5 type hydrogen storage alloys are still too expensive. We report on the results of studies aimed at lowering the alloy costs by partial substitutions of A and B elements respectively. We have improved the capacity of silicon-containing cobalt-free AB 5 -type alloys by optimization of the La:Ce ratio in the A component and by the substitution of aluminium by manganese in the B component. The alloy Lm 0.5 Mm 0.5 Ni 4.2 Mn 0.2 Al 0.3 Si 0.3 (Mm, rare earth mischmetal; Lm, lanthanum-rich mischmetal) showed a maximum capacity of 270 mA h g −1 and good cycling stability, i.e. the optimum in price/performance ratio. Substitution of larger amounts of nickel with copper could further lower the price of such alloys; however, these copper-containing alloys showed low kinetics and a slow activation. Electrochemical measurements were carried out on ten different single-phase AB 5 alloys and on a two-phase AB 5.36 -type alloy. Up to 500 charge-discharge cycles were performed. Samples were characterized using X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray analysis and photoelectron spectroscopy. Photoelectron spectroscopic analysis of Lm 0.5 Mm 0.5 Ni 2.4 Cu 2 Mn 0.25 Si 0.35 after 30 electrochemical charge-discharge cycles points to a thick nickel oxide top surface layer, no enrichment of metallic nickel in near surface layers and to a slightly enhanced metallic copper subsurface layer. No passivating silicon surface oxide film could be found. We attribute the low electrochemical kinetics of this alloy to the absence of metallic nickel at the electrode-electrolyte interface.

Surface and bulk properties of LaNi5−χSiχ alloys from the viewpoint of battery applications

Abstract Because of their rather low hydrogen storage capacity, LaNi 5−χ Si χ alloys have not been studied well up to now and not considered for battery applications. We have found applications. We have found, however, that the alloy LaNi 4.5 Si 0.5 shows fast activation and an incredibly high stability upon electrochemical cycling (−0.07 m A h g −1 cycle −1 ) with a fairly high electrochemical capacity (245 m A h g −1 ). X-ray diffraction, pressure-composition isotherms and electrochemical measurements have been carried out on LaNi 4.7 Si 0.3 and LaNi 4.5 Si 0.5 . By means of photoelectron spectroscopy we have analysed the composition and thickness of surface layers formed on LaNi 4.5 Si 0.5 powder electrodes after various pretreatments. Electrochemical cycling in 6 M KOH electrolyte forms a sublayer rich in metallic nickel. Contrary to earlier suggestions, no passivating silicon surface oxide film could be found on cycled LaNi 4.5 Si 0.5 electrodes. We attribute the fast activation of these alloys to the very thin oxide layer and to metallic nickel just under the surface, and we attribute the high corrosion resistance to the low volume expansion upon hydriding and the compact surface layer which is therefore formed.

Electrochemical and surface properties of Zr(VxNi1-x)2 alloys as hydrogen-absorbing electrodes in alkaline electrolyte

Note: Times Cited: 53 Reference EPFL-ARTICLE-206083doi:10.1016/0925-8388(94)90741-2View record in Web of Science URL: ://WOS:A1994MR90800041 Record created on 2015-03-03, modified on 2017-05-12

Electrochemical and surface properties of iron-containing AB5-type alloys

LaNi5-type metal hydride electrodes can attain a long cycle life in rechargeable batteries by partial substitution of Ni by expensive Co. We have investigated the surface and electrochemical properties of (La,Mm)Ni5−x(Fe,Al,Mn,Cu)x-type alloys with regard to battery applications. We found that partial substitution of nickel by iron did not increase the durability of LaNi5. The initial capacity of LaNi4.5Fe0.5 was 320 mAh g−1, while after 200 charge-discharge cycles it decreased to 130 mAh g−1. With the combined substitution of iron and aluminium the durability increased. A more dramatic improvement to the stable alloys was attained for mischmetal-based alloys (e.g. MmNi3.6Fe0.7Al0.3Mn0.4). Deep discharges and cycling at 40°C caused only a minor capacity decrease for these alloys. Surface analysis using X-ray photoelectron spectroscopy (XPS) of LaNi4Fe and LaNi4.2Fe0.5Al0.3 showed significant changes in the surface composition of electrochemically cycled alloys. The surface changed from being lanthanum oxide rich to nickel rich. No aluminium or iron enrichment was found in the cycled alloys. The content of metallic nickel was lower than observed previously on the surface of LaNi1−xSix alloys. LaNi4.2Fe0.5Al0.3 with good cyclic stability also showed a higher metallic Ni content in the subsurface layer. For all analysed samples, lanthanum was oxidized almost throughout the entire sputtering depth, whereas nickel became metallic at a depth of 0 to 60Aand iron at a depth of 50 to 400A.

Surface and bulk properties of the TiyZr1−y(VxNi1−x)2 alloy system as active electrode material in alkaline electrolyte

Note: Times Cited: 32International Symposium on Metal-Hydrogen Systems - Fundamentals and ApplicationsNov 06-11, 1994Fujiyoshida shi, japan Reference EPFL-ARTICLE-206086doi:10.1016/0925-8388(95)01745-3View record in Web of Science URL: ://WOS:A1995TQ53600121 Record created on 2015-03-03, modified on 2017-05-12

AB2 AND AB5 METAL HYDRIDE ELECTRODES - A PHENOMENOLOGICAL MODEL FOR THE CYCLE LIFE

The variation of the capacities of different metal hydride electrodes as a function of the number of charge-discharge cycles, i.e. the cycle life curve, were quantified using a phenomenological model. This model allows a numerical fit of the measured discharge capacities as a function of the number of cycles. The model determines five independent parameters which describe the electrode cycle life. These parameters are the total and the active start-up capacity, and the activation, oxidation and reduction constants. These are the most interesting characteristics of the electrode for battery applications. This model was applied to the cycle life curve of different AB2 and AB5-type electrodes. We found that these parameters depend on the applied discharge current. We must be able to explain this effect. Using X-ray photoelectron spectroscopy depth profiles, we observed strong changes in the surface composition which, together with the increased surface area as a result of pulverization, is responsible for the activation.

Effects of electrode compacting additives on the cycle life and high-rate dischargeability of Zr(V0.25Ni0.75)2 metal hydride electrodes in alkaline solution

Abstract Materials used as a compacting agent in the production of metal hydride electrodes influence the electrode performance, e.g. cycle life and high rate dischargeability. We have tested several metal hydride electrodes compacted with different metal powders: copper, nickel, cobalt and gold as well as platinum on carbon. The hydrogen storage material of all these electrodes was ZrV 0.5 Ni 1.5 . The compacted electrodes were cycled in a 6 M KOH electrolyte. Electrodes using nickel as compacting powder showed the best activation, while copper compacted electrodes showed the best cyclic stability. The ratio of compacting material to the active material is also important as it affects the internal resistance of the electrode. This is especially true for an alloy to binder ratio higher than 25 wt.%. Cobalt shows a partly reversible reaction at the standard potential of −0.83 V vs. the Hg/HgO/OH − reference electrode; it can therefore increase the capacity of the cobalt compacted electrode.

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.