Electrochemical properties of over-stoichiometric ZrMn1-xVxNi1.4+y alloys with C15 Laves phase

The hydrogen storage performance and electrochemical properties of ZrMn1-xVxNi1.4+y (x = 0.5, 0.7; y = 0.0, 0.2, 0.4, and 0.6) alloys, one type 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 relationship between discharge performance and alloy characteristics such as crystallographic parameters, surface morphology, reaction surface area, and exchange current density is also discussed. All of these alloys were found to have mainly a C15-type Laves phase structure by x-ray diffraction analysis. The equilibrium hydrogen pressure of the alloy 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, the 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 reason why the electrode properties had a reverse tendency between the two 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 properties 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 specific reaction surface area, controlled the electrode properties.