Effects of rare-earth content and annealing on the electrochemical properties of Mm(NiCoMnAl)5 hydrogen storage alloys

The effects of the La and Ce contents and heat treatment on the crystal structure and electrochemical properties of Mm(NiCoMnAl)(5) alloys were studied. For the as-cast alloy, the electrochemical capacity increases and cycle lifetime decreases with increasing La content. The lattice distortion and defects decreased and grain size increased for all the specimens studied after heat treatment. However, these alloys show very different electrochemical properties after heat treatment. For the La-rich alloys, the cycle lifetime increases obviously after heat treatment. But the electrochemical capacity decreases. Especially the high rate charge/discharge ability decreases markedly. For the Ce-rich alloys, both the electrochemical capacity and the cycle lifetime decreased after heat treatment. (C) 2003 Elsevier B.V. All rights reserved.