Effects of annealing and stoichiometry to (Nd, Mg)(Ni, Al)3.5 metal hydride alloys

The structures, gaseous phase hydrogen storage, and electrochemical properties of a series of (Nd0.83Mg0.16Zr0.01)(Ni0.953Al0.046Co0.001)(alpha) alloys, where alpha = 3.3, 3.4, 3.5, 3.6, and 3.7, before and after annealing (900 degrees C and 5 h in argon) were studied. Besides the main Nd2Ni7 phase, other secondary phases, such as MgNdNi4, NdNi5, NdNi3, NdNi, and CeNi3, were present in most of the samples and influenced the hydrogen storage properties. After annealing, several changes happened: the stoichiometry of the main Nd2Ni7 phase remained constant at B/A = 3.3 and its abundance increased; the abundances of the major secondary phases decreased but were not totally eliminated (which helped preserve the catalytic effects); both the gaseous phase hydrogen storage and electrochemical capacity increased; the high-rate dischargeability decreased slightly; and the activation became more difficult. A stoichiometry of AB3.5 showed the best compromise among electrochemical capacity, high-rate dischargeability, and ease of activation. (C) 2012 Elsevier B.V. All rights reserved.