The effect of partial substitution of Zr for Ti on the electrochemical properties and surface passivation film of Mg35Ti10-xZrxNi55 (x=1, 3, 5, 7, 9) electrode alloys

In this paper, the substitution of different amounts of Zr for Ti in the hydrogen storage electrode alloy Mg35Ti10Ni35 has been carried out for the purpose of further improving the electrochemical performance of the electrode alloy. The electrochemical properties were tested on the purposely prepared Mg35Ti10-xZrxNi55 (x = 1, 3, 5, 7, 9) electrode alloys. The results showed that all alloys substituted with Zr possessed a higher cycling capacity retention and a larger initial discharge capacity (except M35Ti1Zr9Ni55) than the original alloy Mg35Ti10Ni55. Among these alloys M35Ti5Zr5Ni55 exhibited the highest discharge capacity and cycling stability. It was found from X-ray Photoelectron Spectroscopy (XPS) surface analysis that the Zr content reduced the degree of oxidation of Mg. The higher the Zr content, the lower is the degree of oxidation of Mg. Auger electron spectroscopy (AES) analysis revealed that the thickness of the passive oxide film coating on the alloy surfaces became first reduced as the Zr content increased but subsequently became augmented as the Zr content exceeded 5 at.%. The study indicated that the corrosion inhibiting effect of Zr alone. without the presence of Ti was weak. The synergetic effect of Zr and Ti made the passive film more compact, less pervious and more protective than by Ti or Zr alone. The thickness of the passive film agreed well with the polarization resistance of the alloys obtained from electrochemical impedance spectroscopy (EIS) Nyquist diagrams, suggesting that the surface polarization resistance R, being closely related to the film thickness and the exchange current density on the surface of the alloy varied accordingly. The conclusion of this study is that 5 at.% Zr substitution in Mg35Ti10Ni55, leads to the formation on the surface of a good and thin passive film, which retards the corrosion of the alloy and at the same time leads to the least polarization resistance and highest exchange current in this series of alloys studied. (C) 2002 Elsevier Science B.V. All rights reserved.