Effects of annealing on Zr8Ni19X2 (X = Ni,Mg, Al,Sc, V,Mn, Co,Sn, La,and Hf): Hydrogen storage and electrochemical properties

The effects of 8-h annealing at 960 °C on the gaseous phase hydrogen storage and electrochemical properties of partial-Ni substituted Zr₈Ni₂₁ alloys were studied. The substituting elements included Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf. Only the main phase of the annealed Sn-substitution remained Zr₈Ni₂₁-structured while those of other substitutions turned into Zr₇Ni₁₀ or Zr₂Ni₇. The observed trend in maximum gaseous phase hydrogen storage capacity followed the increasing order of B/A ratio of the main phase as orthogonal Zr₇Ni₁₀ > tetragonal Zr₇Ni₁₀ > Zr₈Ni₂₁ > Zr₂Ni₇. After annealing, due to the increase in abundance of the main phase, the maximum gaseous phase hydrogen storage capacities of alloys with higher capacities before annealing increased while others decreased. The full discharge capacity also improved in the same increasing order of B/A ratio in the main phase. Hf-substitution showed the highest electrochemical discharge capacity at 200 mAh g⁻¹. After annealing, all alloys with the same main phase as the as-cast alloys showed degradation in full electrochemical capacity due to the reduction in both number and abundance of the catalytic secondary phases. All supplements assisted in improving surface exchange current from the base binary Zr₈Ni₂₁ alloy. Except for La- and Hf-substitutions, annealing reduced the surface exchange current density. The bulk hydrogen diffusion coefficient decreased with most of the supplements except for V- and Sn-substitutions. All supplements, except for Sc, showed improvement in the bulk diffusion after annealing. Furthermore, the maximum gaseous phase hydrogen storage capacity showed a strong correlation to the full electrochemical discharge capacity. Among all alloys in this study, the as-cast Hf-substituted Zr₈Ni₂₁ alloy demonstrated the best overall gaseous hydrogen storage and electrochemical properties.