ELECTROCHEMICAL PROPERTIES OF OVER-STOICHIOMETRIC ZRMN1-XVXNI1.4-PHASE(Y ALLOYS WITH C15 LAVES)

The hydrogen storage performance and electrochemical properties of ZrM n1-xVxNi1.4+y (x = 0.5, 0.7; y = 0.0, 0.2, 0.4, and 0.6) alloys, one t ype with a high amount of manganese, i.e., x = 0.5, and the other with a small amount of manganese, i.e., x = 0.7, were investigated. The re lationship between discharge performance and alloy characteristics suc h as crystallographic parameters, surface morphology, reaction surface area, and exchange current density is also discussed. All of these al loys were found to have mainly a C15-type Laves phase structure by x-r ay diffraction analysis. The equilibrium hydrogen pressure of the allo y increased as the amount of Ni increased in both types of alloys. In the case of ZrMn0.5V0.5Ni1.4+y (y = 0.0, 0.2, 0.4, and 0.6) alloys, th e discharge efficiency and the rate capability decreased as the amount of Ni increased, while these values increased for the ZrMn0.3V0.7Ni1. 4+y (y = 0.0, 0.2, 0.4, and 0.6) alloys. After measuring the specific reaction surface area and exchange current density to identify the rea son why the electrode properties had a reverse tendency between the tw o systems, we confirmed that for the ZrMn0.5V0.5Ni1.4+y (y = 0.0, 0.2, 0.4, and 0.6) alloys the major factor controlling the electrode prope rties was the specific reaction surface area, not the exchange current density, while, in the case of ZrMn0.3V0.7Ni1.4+y (y = 0.0, 0.2, 0.4, and 0.6) alloys, mainly the exchange current density, not the specifi c reaction surface area, controlled the electrode properties.