Stoichiometry and annealing condition on hydrogen capacity of TiCr2-x AB2 alloys.

This study presents the effect of stoichiometry and annealing condition on Ti–Cr AB 2 -type hydrogen storage alloys. Prior to annealing the majority phase of the as-cast alloys was the C14 Laves phase, with separate Ti and Cr phases. Annealing treatment (1273 K/14 d) led to a transition from C14 to C15 Laves phase structure. Both C14 (as-cast) and C15 (annealed) cell size increased with Ti content, up to a ratio (Cr/Ti) of 1.6, due to B-site Ti substitution in the lattice up to a limit. Pressure composition isotherm (PCI) measurements demonstrated alloys containing a greater Ti content had a better maximum hydrogen storage capacity (1.5 vs. 1.03 wt%) and lower plateau pressure (9.4 vs. 15.8 bar) at 253 K. Annealing resulted in a lower storage capacity (1.05 vs. 1.49 wt%), greater plateau pressure (ca. 30 bar) and flatter plateau slope (25 % reduction in plateau slope). Reduction in hydrogen storage capacity of annealed alloys could be due to diffusion of residual Cr in the alloy into the C15 Laves phase during the annealing process, thereby changing the local composition as confirmed through X-ray diffraction (XRD). • The C14 and C15 cell size for as cast and annealed alloys increased with Ti content. • The increase in cell size indicates (limited) Ti substitution in the lattice. • Cr vacancies are present in smaller deviations away from AB2 stoichiometry. • Greater Ti content leads to high hydrogen storage capacity and low plateau pressure. • Capacity reduces post-annealing due to C15 B-site Cr diffusion and increased oxygen. [ABSTRACT FROM AUTHOR]