Synthesis, characterization and hydrogenation behaviour of composite hydrogen storage alloys, LaNi5/La2Ni7, LaNi3

Abstract

The present study deals with investigations on the composite hydrogen storage materials LaNi5/La2Ni7, LaNi3, for efficient, high energy density Ni-MH batteries. To achieve this goal extensive investigations were carried out to study the effect of presence of secondary phases on the main phase LaNi5 with reference to hydrogenation behaviour and structural-microstructural characterizations using XRD, TEM and SEM techniques. Alloys were synthesized through melt-casting with varying concentrations of secondary phases like La2Ni7, LaNi3, Ni (minor phases), i.e., 55%, 30%, 5% and 0% by a special technique of pellet encapsulation. Structural characterizations using the XRD technique confirmed the formation of secondary phases together with the major phase LaNi5. The diffraction patterns after hydrogenation did not exhibit secondary phase diffraction peaks, instead the resulting phase was found to be single phasic, exhibiting LaNi5 like diffraction patterns with an unusual intensity distribution. TEM investigations showed the presence of two types of modulated phases; one of which corresponds to a≈1.5o, c≈co and the other to a≈1.3ao, c≈co, together with the parent phase (LaNi5), in the as-synthesized and hydrogenated/dehydrogenated versions, corresponding to alloy with secondary phase concentration upto 30%, (ao and Co being lattice constants of the parent LaNi5 structure). The signature of the modulated phases were not present in XRD patterns, but could be seen in the selected area electron diffraction patterns, suggesting that modulation has taken place in local areas of the basal plane, due to ordering of hydrogen atoms. These modulated unit cells can be modeled in terms of the original lattice of LaNi5, by inserting and ordering of hydrogen atoms at viable interstitial sites. Micro structural evaluations through SEM revealed that secondary phases suppressed pulverization. Investigations of the hydrogenation behaviour of these alloys showed that hydrogen storage capacity and desorption kinetics did not decrease for minor concentrations of secondary phases, but a decrease was found for the alloys having higher concentrations of secondary phases. Similar results were obtained for alloy MmNi5, where secondary phases corresponded to Mm2Ni7 and MmNi. © 1999 Elsevier Science S.A. All rights reserved.