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 fo r nickel on the electrochemical cycle life of LaNi5-based alloys. Latt ice expansion during hydriding was measured by means of X-ray diffract ion for the alloys LaNi4Co, LaNi3.5CoAl0.5 and LaNi4.5Al0.5. The surfa ce composition of the alloy grains was analysed by means of X-ray phot oelectron spectroscopy (XPS). The XPS-depth-profiles are mentioned in this paper. The mechanical and electrochemical properties of these all oys and additionally of Zr0.2La0.8Ni4.5Al0.5 and Er0.2La0.8Ni4.5Al0.5 were also measured. We observed a strong correlation between the hardn ess of these alloys and the cycling stability. Harder alloys lose capa city more rapidly with cycling. Cobalt appears to lower the hardness a nd therefore increase the cycle life of these alloys. It is well known that alloys which show a large lattice expansion during hydriding, pu lverize faster with cycling. This behaviour was clearly observed in ou r measurements. The combination of a minimal lattice expansion and a l ow hardness seem to have a synergetic effect in increasing the cycle l ife.