Activation characteristics of multiphase Zr-based hydrogen storage alloys for Ni/MH rechargeable batteries

AB(2) type Zr-based Laves phase alloys have been studied for possible use as negative electrodes of Ni/MH batteries with high hydrogen storage capacity. However, these alloys have the serious problem of slow activation owing to the formation of surface oxide films. To overcome this problem, alloys with multiphase microstructures have been developed. These alloys become electrochemically active via the creation of micropores by the dissolution of soluble oxide components such as vanadium oxide. However, this phenomenon has been described based only on changes in the chemical composition of the oxide layer. In the present study, this phenomenon is approached with respect to interactions between the constituent phases. An electrochemical analysis of constituent phases showed that the second phase dissolved into the electrolyte by a galvanic coupling effect between the matrix phase and the second phase, resulting in localized Ni-rich pits on the alloy surface. The presence of microcracks at the periphery of the Ni-rich pits after 30 h exposure to KOH electrolyte implies that hydrogen is absorbed preferentially at Ni-rich pits, thereby forming a large active surface area. However, such multiphase alloys have poor cycle durability due to the persistant dissolution of components in the second phase. Through Cr substitution, we have developed a family of durable alloys to prevent this unwanted dissolution from the second phase. (C) 1999 The Electrochemical Society. S0013-4651(99)04-077-5. All rights reserved.