Multi-component ab2 metal hydride alloys for nickel metal hydride battery applications

<p>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.</p> <p>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 (<i>e</i>/<i>a</i>) was proven to be an influential parameter in determining the C14/C15 phase abundance: as <i>e</i>/<i>a</i> increased, C14/C15 became less/more dominant, respectively. However, with different A-site composition, a shift in <i>e</i>/<i>a</i> was observed in the C14/C15 phase abundance vs. <i>e</i>/<i>a</i> relationship. The average chemical potential for electronic charge of A atoms (<i>Φ</i> ^* _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 <i>Φ</i> ^* _A can be used to predict the C14/C15 phase abundance and assist future AB₂ MH alloy design process.</p> <p>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.</p> <p>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 <i>X</i> ₂ alloys (<i>X</i> = 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.</p>

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Open Access Dissertation

Discipline thesis:degree_discipline

Chemical Engineering and Materials Science

Year dc:date.available

2012

Author dc:creator

• Nei, Jean

Contributors dc:contributor

• Simon Ng
• Steve O. Salley

• Electrochemical reactions, Hydrogen absorbing materials, Metal hydride electrodes, Nickel metal hydride battery, Synergetic effect, Transition metal alloys
• Materials Science and Engineering
• Oil, Gas, and Energy

Repository record dc:identifier

https://digitalcommons.wayne.edu/oa_dissertations/465

OAI identifier oai:identifier

oai:digitalcommons.wayne.edu:oa_dissertations-1464