Effect of V and Zr on the electrochemical properties of La-based AB(5)-type metal hydride electrodes

Abstract

In this paper, investigation has been carried out to find the effects of V- or Zr-addition and fluorination on the discharge capacity of AB(5)-type alloy with a multicomponent La-Ni-Al-Co-Mn-M-y (M=V, Zr; y=0.0, 0.02, 0.05, 0.1, 0.2, 0.3) system. Spectral studies have been made by EDS and SEM, and the crystal structure was also determined by XRD. In order to calculate the hydrogen storage capacity, the pressure-composition isotherms (PCT curves) were utilized. The metal hydride electrode was also used as a galvanostatic cycling test. As the amount of added V or Zr is increased, the portion of the second phase is also increased gradually, but the crystal structure, CaCu5-type Laves phase, is not changed. But, in the matrix, the main component is Ni, in one of the major phases the main component is La and in another minor phase the main component is V or Zr. It has been confirmed that the addition of V or Zr causes improvement in the discharge capacity (Zr, yless than or equal to0.05; V, yless than or equal to0.1) and the cyclability (Zr, yless than or equal to0.05; V, yless than or equal to0.1) up to 200 cycles. However, adding more than y=0.2 has some significant effect on these properties. Generally, V-containing alloys may have many second phases of a round shape appearing in dispersed form in the matrix, while Zr-containing alloys' second phases appear with a plate or needle-like shape. La-rich particles precipitated along boundaries of the second phases which can act as sites of preferred hydrogen absorption, to increase the discharge capacity. In addition, many micro-cracks are formed around the La-rich particles at the surface of the La-rich particle itself. Therefore, such second phases are considered as catalytic sites for hydrogen penetration, surface reaction, initial activation and change transfer properties. (C) 2002 Elsevier Science B.V. All rights reserved.