Electrochemical and structural studies on nonstoichiometric AB2-type metal hydride alloys

This work reports the results of studies of some physical and electrochemical properties of three AB2-types non-stoichiometric metal hydride alloys, Ti0.1Zr0.9−xExNi1.1V0.5Mn0.6, where E=Y, La and Nb and x=0, 0.2 and 0.4. X-ray diffraction analysis shows that partial substitution of Zr by Y or La induces an increase in the unit-cell parameters of the alloy, while Nb has the opposite effect. X-ray absorption near-edge structure measurements show no variation in the energy and occupation of the 3d electronic levels of the Ni atoms, a decrease in the empty 3d density of states for Mn, and an increase for V, compared to the corresponding pure elements. In all cases, replacement of Zr resulted in a decrease of the maximum electrochemical charge storage capacity, but for Y and La the alloy activation time and the hydrogen equilibrium pressure are reduced. It is also seen that, within the experimental error, the activation energy of the charge transfer step of the hydriding/dehydriding process is independent of the alloy composition, meaning that the reaction kinetics is not substantially affected by Zr substitution.