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 possess 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. Schlapbach, J. Alloys Compounds, 203 (1994) 235-241); however, the electrode 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 (1991) 922). The oxide layer on the alloy grains can be dissolved in strong alkaline or acid solutions and a high nickel concentration remains on the surface (A. Zuttel, F. Meli and L. Schlapbach, J. Alloys Compounds, 209 (1994) 99-105). In this paper, we report the effect of the partial substitution of zirconium by titanium. The formation of a compact zirconium oxide film can be prevented by an increase in the defect density, i.e. by the presence of titanium. Titanium is also oxidized on the surface; however, a mixture of titanium and zirconium oxides should be much less of a barrier to hydrogen penetration compared with a pure zirconium oxide layer. Increasing titanium content in TiyZr1-y(VxNi1-x)(2) alloys results in an increased plateau pressure and a lower capacity. Differences in the kinetics can be explained by the significant changes in the composition of the surface layers (approximately 30 Angstrom).