Zr, Ti-based AB2 metal hydride (MH) alloys are considered to be one of the most promising candidates to replace the misch metal based AB5 alloys presently used as the negative electrode in nickel/metal hydride (Ni/MH) batteries. Since most of the studies on AB2 MH alloys focus on multi-element and highly disordered families, the understanding of basic Ti-Ni and Zr-Ni intermetallic alloys is crucial for fully utilizing the synergetic effects between the main AB2 Laves phases and the non-Laves secondary phases.
For the best balance between large storage capacity (lower hydride heat of formation) and high discharge capability (higher hydride heat of formation), a stoichiometry of A:B close to 1:2 is desirable. However, there is no AB2 intermetallic alloy for either Zr-Ni or Ti-Ni binary systems. The Zr-Ni phase diagram shows several intermetallic alloys, and the closest intermetallic compounds to AB2 are Zr7Ni10 and Zr8Ni21. Zr7Ni10 has slightly stronger metal-hydrogen bond strength and solidifies congruently from liquid. Zr was replaced partially by Ti to reduce the metal-hydrogen bond strength. Other substituting elements were also studied in order to improve the electrochemical properties. Zr8Ni21 was chosen as the focus of this report. Zr8Ni21 does not solidify directly from the liquid as in the case of Zr7Ni10. Instead, the Zr2Ni7 phase is first solidified from the liquid with a Zr-to-Ni ratio of 8:21 and later reacts with the remaining liquid to form Zr8Ni21 alloy peritectically. If the condition of equilibrium is not reached during cooling, one or more secondary phases will precipitate out. Therefore, an annealing process was adopted for this study.
The effects of annealing at 960 °C for 8 hours on the phase structures of a series of partially substituted Zr8Ni21 alloys were studied. The substituting elements included Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf. In general, the abundance of dominant phase increased and the number of smaller secondary phases diminished after annealing. Similar to the choice of C14/C15 main phase in AB2 alloys, the number of outer-shell electrons plays an important role in determining the main phase of the A8B21samples. As number of outer shell electrons of the substituting element increases, the main phase evolves from tetragonal Zr7Ni10, to orthogonal Zr7Ni10, Zr2Ni7 and then Zr8Ni21.
The annealing effects on the gas phase and electrochemical hydrogen storage properties were studied. It’s observed that as the Zr-content in the main phase of the alloy increases (orthogonal Zr7Ni10 > tetragonal Zr7Ni10 > Zr8Ni21 > Zr2Ni7), the gas phase storage capacity increases. After annealing, due to the increase in the abundance of main phase, the capacities of alloys with higher capacities before annealing increase while others decrease. The sample with Hf-supplement shows the highest electrochemical discharge capacity of 200 mAh/g. After annealing, all samples with the same main phase as the as-cast ones show degradation in electrochemical capacity due to the reduction in number and abundance of secondary phases. All supplements show improvement in surface exchange current from the base alloy. Except for La and Hf supplements, annealing reduces the exchange current density. The bulk hydrogen diffusion constant decreases for most of the supplements except V and Sn. All supplements, excluding Sn, show improvement in bulk diffusion after annealing. The gas phase full capacity has strong correlation to the full electrochemical discharge capacity. Among all samples, as-cast La-supplement shows the best overall electrochemical properties.
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