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

Handle URI:
http://hdl.handle.net/10754/623669
Title:
First Principles Studies of Perovskites for Intermediate Temperature Solid Oxide Fuel Cell Cathodes
Authors:
Salawu, Omotayo Akande ( 0000-0003-0576-0204 )
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.
Advisors:
Schwingenschlögl, Udo ( 0000-0003-4179-7231 )
Committee Member:
Di Fabrizio, Enzo ( 0000-0001-5886-4678 ) ; Hussain, Muhammad Mustafa ( 0000-0003-3279-0441 ) ; Chroneos, Alexander
KAUST Department:
Physical Sciences and Engineering (PSE) Division
Program:
Materials Science and Engineering
Issue Date:
15-May-2017
Type:
Dissertation
Appears in Collections:
Dissertations

Full metadata record

DC FieldValue Language
dc.contributor.advisorSchwingenschlögl, Udoen
dc.contributor.authorSalawu, Omotayo Akandeen
dc.date.accessioned2017-05-22T06:14:12Z-
dc.date.available2017-05-22T06:14:12Z-
dc.date.issued2017-05-15-
dc.identifier.urihttp://hdl.handle.net/10754/623669-
dc.description.abstractFundamental 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.en
dc.language.isoenen
dc.subjectCathodeen
dc.subjectFirst-principlesen
dc.subjectCobaltitesen
dc.subjectFuel Cellen
dc.subjectPerovskiteen
dc.subjectDefectsen
dc.titleFirst Principles Studies of Perovskites for Intermediate Temperature Solid Oxide Fuel Cell Cathodesen
dc.typeDissertationen
dc.contributor.departmentPhysical Sciences and Engineering (PSE) Divisionen
thesis.degree.grantorKing Abdullah University of Science and Technologyen_GB
dc.contributor.committeememberDi Fabrizio, Enzoen
dc.contributor.committeememberHussain, Muhammad Mustafaen
dc.contributor.committeememberChroneos, Alexanderen
thesis.degree.disciplineMaterials Science and Engineeringen
thesis.degree.nameDoctor of Philosophyen
dc.person.id124297en
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