SURFACE AND BULK PROPERTIES OF THE TIYZR1-Y(VXNI(1-X))(2) ALLOY SYSTEM AS ACTIVE ELECTRODE MATERIAL IN ALKALINE ELECTROLYTE

Multicomponent Zr-Ni-based alloys of the AB(2) Laves phase structure p ossess promising properties as electrode materials in reversible metal hydride batteries. Previously, we have shown that ZrV0.5Ni1.5 reaches a reversible capacity of 345 mA h g(-1) (A. Zuttel, F. Meli and L. Sc hlapbach, J. Alloys Compounds, 203 (1994) 235-241); however, the elect rode requires approximately 30 activation cycles to reach the maximum capacity due to an oxide layer on the grain surfaces. Zirconium oxide has been reported to be a strong barrier to hydrogen penetration (J.O. Strom-Olsen, Y. Zhao and D.H. Ryan, J. Less-Common Met., 272-174 (199 1) 922). The oxide layer on the alloy grains can be dissolved in stron g alkaline or acid solutions and a high nickel concentration remains o n the surface (A. Zuttel, F. Meli and L. Schlapbach, J. Alloys Compoun ds, 209 (1994) 99-105). In this paper, we report the effect of the par tial substitution of zirconium by titanium. The formation of a compact zirconium oxide film can be prevented by an increase in the defect de nsity, i.e. by the presence of titanium. Titanium is also oxidized on the surface; however, a mixture of titanium and zirconium oxides shoul d be much less of a barrier to hydrogen penetration compared with a pu re zirconium oxide layer. Increasing titanium content in TiyZr1-y(VxNi 1-x)(2) alloys results in an increased plateau pressure and a lower ca pacity. Differences in the kinetics can be explained by the significan t changes in the composition of the surface layers (approximately 30 A ngstrom).