Effects of B, Fe, Gd, Mg, and C on the structure, hydrogen storage, and electrochemical properties of vanadium-free AB2 metal hydride alloy

The structural, gaseous phase hydrogen storage, and electrochemical properties of a series of vanadium-free AB₂ Laves phase based metal hydride alloys with various modifiers (Ti₅Zr₃₀Cr₉Mn₁₉Co₅Ni_32−xM_x, M = B, Fe, Gd, Mg, and C) were studied. While B and Fe completely dissolve in the main AB₂ phases, Gd, Mg, and C form individual secondary phases. The solubilities of Gd, Mg, and C in the AB₂ phases are not detectable, 0.3 at.%, and very low, respectively. The C14 crystallite sizes, C15 phase abundances, and Zr₇Ni₁₀ phase abundances of modified alloys are larger than those of the base alloy. All modified alloys show decreases in plateau pressure, reversible gaseous phase storage capacity, formation activity, electrochemical capacity, and cycle life. A small amount of boron (0.2 at.%) and carbon in the alloy improve the half-cell high-rate dischargeability and bulk hydrogen diffusion. All modifiers, except for boron, reduce the surface exchange reaction current densities of the alloys. Both Mg and C show improvement in charge retention. Full-cell high-rate performance is improved by adding only a small amount of boron (0.2 at.%). Fe, Gd and 0.2 at.% of boron improve the low-temperature performance of the sealed batteries.