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 Zr0.2La0.8Ni4.5Al0.5 and Er0.2La0.8Ni4.5Al0.5 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.