Effect of Zn on the hydrogen storage characteristics of multi-component AB(5)-type alloys

The mischmetal-based hydrogen storage alloy systems MlNi(3.8)Co(0.5)Mn(0.4) Al(0.3)Zn(x) (Ml, La-rich mischmetal; 0<x<0.2) have been successfully synth esized by the diffusion method (DM). The characteristics of these alloys ha ve been carefully investigated by means of XRD; PAT, PCT, EIS and measureme nt of electrochemical capacity. Experimental results show that the unit cel l volume becomes larger and the point defect density increases strongly due to the diffusion of Zn. The presence of Zn decreases the desorption platea u and prevents losing the capacity to a certain degree at high discharge cu rrent density. Electrochemical impedance spectra (EIS) results show that th e rate-determining step is the charge-transfer reaction on the alloy/electr olyte interface. The charge-transfer resistance of the electrode increases with increasing Zn concentration due to the formation of an oxide layer and the reduction of active sites on the electrode surface. So the concentrati on of Zn in the allay should be limited to below x=0.1. The Zn-free multico mponent AB(5) alloy has also been examined for comparison. (C) 2001 Elsevie r Science B.V. All rights reserved.