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    First Principles Studies of Perovskites for Intermediate Temperature Solid Oxide Fuel Cell Cathodes

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    Name:
    Omotayo Salawu - Dissertation - Final Draft.pdf
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    Type
    Dissertation
    Authors
    Salawu, Omotayo Akande cc
    Advisors
    Schwingenschlögl, Udo cc
    Committee members
    Di Fabrizio, Enzo M. cc
    Hussain, Muhammad Mustafa cc
    Chroneos, Alexander
    Program
    Material Science and Engineering
    KAUST Department
    Physical Science and Engineering (PSE) Division
    Date
    2017-05-15
    Embargo End Date
    2018-05-22
    Permanent link to this record
    http://hdl.handle.net/10754/623669
    
    Metadata
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    Access Restrictions
    At the time of archiving, the student author of this dissertation opted to temporarily restrict access to it. The full text of this dissertation became available to the public after the expiration of the embargo on 2018-05-22.
    Abstract
    Fundamental advances in cathode materials are key to lowering the operating temperature of solid oxide fuel cells (SOFCs). Detailed understanding of the structural, electronic and defect formation characteristics are essential for rational design of cathode materials. In this thesis we employ first principles methods to study La(Mn/Co)O3 and LnBaCo2O5+δ (Ln = Pr, Gd; δ = 0.5, 1) as cathode for SOFCs. Specifically, factors affecting the O vacancy formation and migration are investigated. We demonstrate that for LaMnO3 the anisotropy effects often neglected at high operating temperatures become relevant when the temperature is lowered. We show that this fact has consequences for the material properties and can be further enhanced by strain and Sr doping. Tensile strain promotes both the O vacancy formation and migration in pristine and Sr doped LaMnO3, while Sr doping enhances the O vacancy formation but not the migration. The effect of A-site hole doping (Mg2+, Ca2+ or Ba2+) on the electronic and magnetic properties as well as the O vacancy formation and migration in LaCoO3 are studied. All three dopants are found to facilitate O vacancy formation. Substitution of La3+ with Ba2+/Mg2+ yields the lowest O vacancy formation energy for low/intermediate spin Co, implying that not only the structure, but also the spin state of Co is a key parameter. Only for low spin Co the ionic radius is correlated with the O migration barrier. Enhanced migration for intermediate spin Co is ascribed to the availability of additional space at the transition state. For LnBaCo2O5+δ we compare the O vacancy formation in GdBaCo2O5.5 (Pmmm symmetry) and GdBaCo2O6 (P4/mmm symmetry), and the influence of Sr doping. The O vacancy formation energy is demonstrated to be smaller in the already O deficient compound. This relation is maintained under Sr doping. It turns out that Sr doping can be utilized to significantly enhance the O vacancy formation in both compounds. The observed trends are explained on a microscopic level. Furthermore, we consider antisite defects as they may modify the electronic and O migration properties but are rarely studied in double perovskite oxides. It turns out that O vacancy formation is significantly easier in PrBaCo2O5.5 than in GdBaCo2O5.5, the difference in formation energy being hardly modified by antisite defects. Finally, having established that the O vacancy formation energy is significantly lower in PrBaCo2O5.5 than in GdBaCo2O5.5, we study the O Frenkel energy and migration of O ions in PrBa(Co/Fe)2O5.5. The electronic structure and charge redistribution during defect formation are analyzed. We demonstrate that Co↔Fe substitution strongly affects the formation of defects and, consequently, the O migration. The low O Frenkel energy points to a high concentration of O vacancies. The migration of the O ions shows a distinct anisotropy.
    Citation
    Salawu, O. A. (2017). First Principles Studies of Perovskites for Intermediate Temperature Solid Oxide Fuel Cell Cathodes. KAUST Research Repository. https://doi.org/10.25781/KAUST-3362M
    DOI
    10.25781/KAUST-3362M
    ae974a485f413a2113503eed53cd6c53
    10.25781/KAUST-3362M
    Scopus Count
    Collections
    Dissertations; Dissertations; Physical Science and Engineering (PSE) Division; Material Science and Engineering Program

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