Effect of partial substitution of nickel by tin, aluminum, manganese and palladium on the properties of LaNi5-type metal hydride alloys

Abstract

This work reports studies on the electrochemical behavior of AB 5-type hydrogen storage alloys, formed by LaNi(5-x)Z x, where Z is a metallic element partially replacing Ni, which included Sn, Al, Mn, and Pd. In the case of Mn, some AB6-type structures were also considered. Substitution of a small fraction of Ni by Al, Sn, and Mn (x ≅ 0.3) leads to an increase of the hydrogen storage capability (HSC), while for Pd there is a decrease of this property. Generally all alloys presenting larger initial HSC exhibit lower stability. A decrease of the hydrogen equilibrium pressure as a function of Mn content is observed for the AB5 alloys and this is related to an increase of the crystalline unit cell volume. Electrochemical impedance measurements show a significant increase of the hydration/dehydration reaction kinetics due to a raise on the active area as a function of the charge/discharge cycle number. It is also seen that the alloys presenting larger HSC are those showing smaller activation energies for the hydrogen oxidation reaction.