Investigation of microstructure and electrochemical properties of Ti0.8Zr0.2V2.7Mn0.5+xCr0.8Ni1.5-x hydrogen storage alloys

The effects of partial substitution of Mn for Ni on the structural and electrochemical properties of Ti0.8Zr0.2V2.7Mn0.5+xCr0.8Ni1.5-x (x=0.0-0.4) hydrogen storage alloys have been systematically investigated. It is found by XRD and Rietveld analysis that all of the alloys consist of a C14 Laves phase with a hexagonal structure and a V-based solid solution phase with a BCC structure. With increasing x, the abundance of the C14 Laves phase increases, but the abundance of the V-based phase decreases progressively. The electrochemical measurements indicate that the maximum discharge capacity of the alloy electrode increases first from 324.8 mAh/g (x = 0.0) to 356.4 (x = 0.2) and then decreases to 310.0 mAh/g (x = 0.4). But the cyclic stability of the alloy electrode deteriorates with increasing Mn content. Moreover, the results of electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization and potentiostatic discharge tests indicate that the exchange current density I-0, the limiting current density I-L and the hydrogen diffusion coefficient D all increase first and then decrease with increasing x. The high rate dischargeability (HRD) of the alloy electrode varies in the same manner. An optimum overall performance is obtained when x = 0.1. (c) 2006 Elsevier B.V. All rights reserved.