Access Type

Open Access Dissertation

Date of Award

January 2012

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemical Engineering and Materials Science

First Advisor

Simon Ng

Second Advisor

Steve O. Salley

Abstract

Compared to the mish metal-based AB5 MH alloy commonly used in Ni/MH batteries, the transition metal-based AB2 MH alloy not only reduces the rare earth dependency, it also has higher specific energy. In order to further improve the performance of AB2 MH alloy, it's crucial to full understand its multi-phase nature, which includes the main C14/C15 Laves phases and the secondary non-Laves phases.

In order to optimize the gaseous phase and electrochemical advantages of both the C14 and C15 Laves phases, a study was established to recognize the factors that affect the C14/C15 phase abundance. Average electron density (e/a) was proven to be an influential parameter in determining the C14/C15 phase abundance: as e/a increased, C14/C15 became less/more dominant, respectively. However, with different A-site composition, a shift in e/a was observed in the C14/C15 phase abundance vs. e/a relationship. The average chemical potential for electronic charge of A atoms (Φ * A) was found to show a nearly perfect linear correlation to the C14/C15 threshold with various selections of A-site elements. The combination of e/a and Φ * A can be used to predict the C14/C15 phase abundance and assist future AB2 MH alloy design process.

Four non-Laves phase alloys, Zr8Ni21, Zr7Ni10, Zr9Ni11, and ZrNi, commonly seen in AB2 MH alloys were studied. Annealing treatment was adopted on each alloy to change the abundances of various phases. Annealing suppressed secondary phases except for the case of Zr9Ni11, where its secondary ZrNi phase increased. As the Zr/Ni ratio increased, the maximum gaseous phase hydrogen storage capacity increased but maximized at Zr : Ni = 9 : 11. Comparing the properties before and after annealing, it was clear that the natures of constituent phases influenced the gaseous phase storage. The highest full discharge capacity was obtained at Zr : Ni = 7 : 10, which is a compromise between the hydrogen desorption rate and the theoretical maximum gaseous phase hydrogen storage. As the Zr/Ni ratio increased, the amount of metallic Ni in the surface oxide decreased, therefore the high-rate dischargeablity decreased. Among all alloys, the unannealed Zr7Ni10 demonstrated the best gaseous phase hydrogen storage and electrochemical capacities, and the unannealed Zr8Ni21 showed excellent HRD and activation.

Zr8Ni21 alloy was then chosen based on its promising performance to be further modified for the purpose of developing alternative MH alloys for Ni/MH batteries. Zr8Ni19 X 2 alloys (X = Ni, Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf) were prepared and studied. The effect of annealing on these alloys was also investigated. Only the main phase of the annealed Sn-substitution remained Zr8Ni21-structured while those of other substitutions turned into Zr7Ni10 or Zr2Ni7. Annealing generally suppressed secondary phases except for the case of Zr8Ni19Sn2, where the major phase transformed from Zr2Ni7 to Zr8Ni21. Both the maximum gaseous phase hydrogen storage and electrochemical full discharge capacities followed the increasing order of B/A ratio of the main phase. After annealing, all alloys except for the Sn-substituion showed degradation in full discharge capacity due to the reduction in number and abundance of the catalytic secondary phases. Among all alloys, the as-cast Hf-substituted Zr8Ni21 alloy demonstrated the best overall gaseous phase hydrogen storage and electrochemical properties.

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