Experimental and Theoretical Investigation of AB3 and A3B Alloys for Hydrogen Storage
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AB3 and A3B intermetallic compounds, with Li, K, Ca, Ti, Zr or Hf as element A, and Fe or Ni as element B were studied both experimentally and computationally to explore their potentials for hydrogen storage at the moderate temperatures and pressures compatible with fuel cells. The motivation for this research was progress with promising AB3 compounds based on LaNi3. Attempts were made to synthesize compounds with stoichiometries AB3 and A3B, where A = Li, K, Ca, Ti, Zr, Hf and B = Fe, Ni. Among these, TiNi3, Zr3Fe, ZrNi3 and HfNi3 were known to exist. The synthesis techniques used were argon-arc melting, furnace melting under a pressure of argon and ball milling. The produced materials were examined with x-ray diffraction and in some cases electron microscopy. None of the attempts to synthesize a new phase was successful, although a number of unknown phases were observed in x-ray diffraction patterns from the multi-phase products. Partially substituted Zr3Fe1-xNix with x = 0.3, 0.5 were successfully prepared. The hydrogen absorption properties of the produced materials were measured using a commercial Sieverts-type hydrogenator. Only Zr3Fe was a known hydrogen absorber and its measured properties agreed with literature reports. Both Zr3Fe0.7Ni0.3 and Zr3Fe0.5Ni0.5 absorbed as much hydrogen as the parent compound. Moreover, the temperature range in which hydrogen was desorbed was considerably lower in Zr3Fe0.7Ni0.3, demonstrating that the properties of Zr3Fe were improved without loss of hydrogen capacity. Most of the new materials did not absorb a useful amount of hydrogen, although the materials with nominal stoichiometries “KNi3”, “Ca3Fe” and “TiFe3” absorbed enough hydrogen to make further study to isolate the hydrogen-absorbing component worthwhile. The known compounds TiNi3, HfNi3 did not absorb hydrogen at room temperature, but could be investigated under more extreme conditions of temperature and pressure. A comparative study of Zr3Fe, Zr3Fe0.7Ni0.3 and Zr3Fe0.5Ni0.5 under deuterium with in-situ neutron diffraction showed that the crystal structure of Zr3Fe0.7Ni0.3 its deuteride were the same as those of the parent compound. However, while Zr3Fe suffered from disproportionation during hydriding, and Zr3Fe0.5Ni0.5 did not. Theoretical calculation was the other important part of this work. Computation, based on density functional theory, was done with Zr3Fe compounds and with some reference systems. Firstly, the USPEX (Universal Structure Predictor: Evolutionary Xtallography) calculation was conducted to predict the structure of Zr3Fe hydride. Although the crystallographic structure of the Zr3Fe hydride was already reported, the successful application of the USPEX code to this compound might introduce a new, important and promising alternative approach for discovery of new hydride systems. From the USPEX trials, the fixed composition USPEX calculation was demonstrated as being able to obtain the real Zr3Fe hydride structure. In order to verify the stability of the predicted hydride structures at room temperature, phonon calculations were then done with the predicted structures. The results predicted that the experimentally-determined structure of Zr3Fe hydride was not the most stable one. An issue that needs to be resolved is that the predicted stability of hydrides increased with added hydrogen, which is contrary to experience. Secondly, after TOPOS calculations to predict interstitial sites, the basic VASP (Vienna Ab initio Simulation Package) relaxation was made on fully hydrided Zr3Fe, LaNi5, Fe and Pd lattices. This was done to explore the way to calculate hydride structures from the basic metallic lattice. The trial was successful for Zr3Fe, Fe and Pd but not for LaNi5. The reason may be that the wrong space group for LaNi5H6 was used, despite it being generally accepted. TOPOS was employed to obtain the possible H interstitial sites and H migration paths in the metallic structures. The nudged elastic band (NEB) method was then used to calculate the energy barriers encountered along the different migration paths, in both the real hydride structures and the postulated non-existent hydride structures. The results showed much higher energy barriers in the migration paths through empty interstitial sites in the real hydrides or compounds. Phonon calculations were also conducted with these real hydride structures and the non-existent hydride structures to verify the findings in the above step, and to deduce some fundamental mechanisms of the formation of hydrides. Thirdly, based on the neutron diffraction results of Zr3Fe and the Ni-substituted Zr3Fe hydrides, NEB and MD (molecular dynamics) calculations were made to study the effects of partially substituting Ni for Fe atom in Zr3Fe. The results were consistent with higher energy barriers to migration and slower diffusion in Zr3Fe0.7Ni0.3H7 compared to Zr3FeH7, and also consistent with the experimentally slower kinetics of hydrogen absorption by Zr3Fe0.7Ni0.3.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Natural Sciences
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