The electrode characteristics of over-stoichiometric ZrMn0.5V0.5Ni1.4+y (y=0.0, 0.2, 0.4 and 0.6) alloys with C15 Laves phase structure

Abstract

Some over-stoichiometric Zr-based Laves phase alloys were prepared and studied in view of rate capability for electrochemical application. After some careful selection of ZrMn2 Laves phase alloys to have suitable hydrogen equilibrium pressure and to be discharged in KOH solution, the ZrMn0.5V0.5Ni1.4, alloys met the above requirements reasonably well. The hydrogen storage performance and discharge characteristics of ZrMn0.5V0.5Ni1.4+y (y = 0.0, 0.2, 0.4 and 0.6) alloys were determined for variable Ni content, which is believed to have some influence on the rate capability. As the Ni content in the alloy increased, the hydrogen storage capacity decreased and the hydrogen equilibrium pressure increased. However, it was unexpectedly found that the rate-capability of these alloys decreased with increasing Ni content. In order to analyze the above phenomena, the reaction surface area, surface reaction kinetics and surface morphologies were examined by the BET method, measurements of the exchange current and SEM respectively. The decrease of reaction surface area with increasing Ni content in this alloy explains that the major factor affecting the rate capability is the reaction surface area and not the specific surface catalytic activity. After comparison of discharge characteristics such as discharge capacity and rate capability of ZrMn1-xVxNi1.4+y (x=0.5, 0.7; y=0.0, 0.2, 0.4 and 0.6) for variable Mn to V ratio, it is found that the more Mn rich alloy showed higher rate capability due to a larger reaction surface. On the basis of the above results, the alloy composition was optimized for a Mn to Ni ratio of 0.7:1.2, i.e. for ZrMn0.7V0.5Ni1.2. (C) 1998 Elsevier Science S.A.