Phase structure, crystallography and electrochemical properties of Laves phase compounds Ti0.8Zr0.2V1.6Mn0.8-xMxNi0.6 (M = Fe, Al, Cr, Co)

The effects of partial substitution of Mn by M (M = Fe, Al, Cr, Cc) on the phase structures, crystallography and electrochemical properties of Laves p hase compounds Ti0.8Zr0.2V1.6Mn0.8-xMxNi0.6 (M = Fe, Al, Cr, Cc) have been studied. It is found by X-ray powder diffraction and energy dispersive X-ra y spectrometer analysis that most alloys are composed of a AB(2)-type hexag onal structure C14 Laves phase matrix and a BCC structure TiNi-based second ary phase alloy and some of them contain another BCC phase. The optical pho tomicrograph shows that the C14 Lave phase forms a continuous matrix and th e TiNi-based secondary phase intersperses among it in the form of dendritic structures. With the substitution of Mn by Fe and Cr, the dendrites are la rger, while Cc and Al substitutions make the dendrites smaller. Electrochem ical studies indicate that Cr substitution improves the cyclic durability o f the alloys greatly despite the decrease of discharge capacity, which can be attributed to the suppression of surface segregation of Ti, Zr and V ele ments by the presence of Cr, which suppresses the oxidation and the dissolu tion of V and the oxidation of Ti and Zr greatly as evidenced by the XPS an alysis. Other element substitutions such as the substitution with Fe, Al an d Co do not have significant effect on the alloy electrode performance. (C) 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.