Abstract
By examining the electrochemical and thermodynamic properties of Zr1−xA′x(VyNizMnuB′v)2+α (0 ⩽ x ⩽ 0.1; 0.10 ⩽ y ⩽ 0.15; 0.41 ⩽ z ⩽ 0.62; 0.28 ⩽ u ⩽ 0.39; 0 ⩽ v ⩽ 0.05; 0 ⩽ α ⩽ 0.02; y + z + u + v = 1;A′ ≡ Ti, Nb; B′ ≡ Co, Cr, Cu, Fe) alloy systems with C15-type Laves phase structure, the results described below were obtained. The alloy electrodes were completely activated even by one or two charge-discharge cycles at a current density of 17 mA g−1. Some of the alloys had discharge capacities larger than 400 mA h g−1 at 17 mA g−1. They also had capacities larger than 380 mA h g−1 even at 220 mA g−1. Moreover, the high rate capability for discharge increased as the absolute value of the partial molar enthalpy ΔH0 for hydride formation decreased. The ΔH0 values of such alloys with excellent electrochemical properties were found to range from −36 to −42 kJ mol H2−1. On the other hand, the substitution effect of the elements in the alloys was qualitatively and quantitatively evaluated as a function of the atomic radii and electronegativities of the constituent elements. ΔH0, which shows the stability of the absorbed hydrogen, had a good correlation individually with the average electronegativity ξav and the average atomic radius rav. The correlation was further improved when these two factors were considered together. The cell volume of C15 type had a correlation with ΔH0 and a good correlation with both the average atomic radius and the difference ΔξB−A in electronegativity between the A and B atoms. From the above description it can be concluded that the ranges of rav and ξav of the alloys with excellent capacity and high rate efficiency are 1.516 ⩽ rav ⩽ 1.525 and 1.621 ⩽ ξav ⩽ 1.640 respectively.
Published Version
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