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A Study of Oxides for Solid Oxide Cells

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A Study of Oxides for Solid Oxide Cells NORTHWESTERN UNIVERSITY AStudyofOxidesforSolidOxideCells ADISSERTATION SUBMITTED TO THE GRADUATE SCHOOL IN PARTIAL FULFILLMENT OF THE REQUIREMENTS for the degree DOCTOR OF PHILOSOPHY Field of Materials Science and Engineering By Olivier Comets EVANSTON, ILLINOIS December 2013 UMI Number: 3605699 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMI 3605699 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, MI 48106 - 1346 2 c Copyright by Olivier Comets 2013 All Rights Reserved 3 ABSTRACT AStudyofOxidesforSolidOxideCells Olivier Comets As the world energy consumption increases, it is a question of global health to increase energy production efficiency and to reduce CO2 emissions. In that respect, solid oxide cells are solid state devices that convert directly fuel into electricity, or vice versa. In fact, when run in fuel cell mode, such devices produce electricity with efficiency up to twice that of current natural gas power plants. However, systems equipped with them have only seen limited commercialization owing to issues of cost, durability, and performance. In this thesis, three di↵erent aspects of solid oxide cells are studied. First, the e↵ects of stress on the properties of mixed ionic electronic conducting oxides are considered. Such oxides can be used as electrode materials, where they are often subject to large stresses, which can, in turn, a↵ect their performance. Hence, understanding the rela- tionship between stress and properties in such materials is crucial. Non-stoichiometry in strontium substituted lanthanum cobaltite is found to increase under tension and to decrease under compression. 4 Then, degradation taking place when the cell is run in electrolysis mode is discussed. A high current allows for a high production rate of hydrogen gas. However, this can also lead to oxygen bubble nucleating in the electrolyte and subsequent degradation of the cell. The analysis conducted here shows that such nucleation phenomenon can be avoided by keeping the overpotential at the oxygen electrode below a critical value. Finally, the growth and coarsening of catalyst nanoparticles at the surface of an oxide is studied. Scientists have developed new oxides for anodes in which a catalyst material is dissolved and exsolves under operating conditions. As the performance of the cell is controlled by the surface area of the catalyst phase, understanding the kinetics of the growth is critical to predict the performance of the cell. An approach is developed to study the growth of one particle, in the limiting case where only bulk transport is allowed. 5 Acknowledgements As I reflect back at my time in the Department of Materials Science and Engineering at Northwestern, I realize how much I have learned, how many great people I have met, and how many amazing experiences I have lived. Undeniably, this department and the people I have met through it have played a major role in my scientific development, personal fulfillment, and my integration in the US. First and foremost, I would like to thank my advisor, Peter Voorhees, for his knowl- edge and guidance while confronting me with such exciting and stimulating projects. Knowing that a graduate school experience is both of academic and human nature, he encouraged me to develop my soft skills through various projects unrelated to work. Peter, thank you for everything! I would like to thank Scott Barnett, who played the role of a second advisor given the overlap of my research and his expertise, for insightful discussions and thrilling collaboration work. I would also like to thank my committee members Thomas Mason, Kenneth Poeppelmeier, and Chris Wolverton for thoughts, suggestions, and insight. I’m very fortunate to be part of such an amazing and complementary group as the Voorhees Research Group and I would like to individually thank each one of you: Kuo-An, Tony, Thomas, Begum, Larry, Ian, Megna, Alanna, Eddie, Anthony, John T., Tom, John G., Kevin, Quentin, and Ashwin. I leave the group with memories of great scientific discussions, help in dire situations, and with great friendships. I am also very 6 grateful to the Barnett group for their thoughts and insights on Solid Oxide Cells, and namely to: David B., Scott, Kyle, Gareth, Ann, Beth, and David K. This work would not have been possible without the many challenging and en- lightening discussions with our collaborators: professors Jason Nicholas, Stuart Adler, Katsuyo Thornton, Dr. Hui-Chia Yu, and T. J. McDonald as well as with Prof. Anil Virkar and Prof. Junichiro Mizusaki. The English in this thesis wouldn’t have been as good without the help of John, Alex, Ahmed and Kyle. Finally, I would like to thank my loving family and friends for all their support during the process. I am grateful to my parents for the education they provided me with and to my parents, Aude, and Antoine for their constant encouragements. I am very glad to Dave Herman, Ahmed Issa, Carlos Alvarez and Begum Gulsoy for valuable friendships, great advice and much fun Ive had during grad school. Last but not least, IwouldliketoacknowledgemyfriendPierreGarreauforhisconstantsupport,an infallible friendship, a lot of fun during grad school and many essential conversations weve had together. This work was financially supported by the US Department of Energy (DOE) and the National Science Foundation (NSF). 7 Contents ABSTRACT 3 Acknowledgements 5 List of Figures 11 List of Tables 14 Chapter 1. Introduction 15 Chapter 2. Background 17 2.1. ElectricityproductionintheUS 18 2.2. Solid Oxide Cells 20 2.2.1. Fuel cell mode 21 2.2.2. Electrolysis mode 22 2.2.3. Materials 23 2.2.4. Features 25 Chapter 3. The E↵ects of Stress on the Defect and Electronic Properties of MixedIonicElectronicConductors 27 3.1. Introduction 27 3.2. Thermodynamics 29 3.2.1. Thermodynamic description of the system 29 8 3.2.2. Equilibrium conditions 31 3.2.3. NewfreeenergyfunctionandMaxwell’sequation 39 3.2.4. Chemical potential of oxygen under stress 41 3.3. E↵ects of stress on the non-stoichiometry 43 3.4. E↵ects of stress on the vacancy formation energy 45 3.5. E↵ectsofstressonthechemicalcapacitance 48 3.6.Comparisonsandpredictions 51 3.6.1. E↵ects of a hydrostatic stress on the properties of La0.8Sr0.2CoO3 δ 52 − 3.6.2. Thin Films 56 3.7. Discussion 69 3.7.1. LSC thin films 69 3.7.2. Generalization to other mixed conductors 72 3.8. Conclusion and future work 73 Chapter 4. Oxygen Bubble Formation in Solid Oxide Electrolysis Cells 76 4.1. Introduction 76 4.2. Thermodynamics of nucleation 78 4.2.1. Thermodynamic model 82 4.2.2. Internal energies 84 4.2.3. Constraints 86 4.2.4. Equilibrium conditions 92 4.3. Driving force 94 4.3.1. Value of the oxygen potential 94 4.3.2. Expression of the oxygen potential 95 9 4.3.3. Expressions of the grand potentials 99 4.3.4. Change in the grand potential 103 4.3.5. Free energy change of nucleation 107 4.4. Results and discussion 112 4.4.1. Critical radius 112 4.4.2. Homogeneousandheterogeneousnucleation 114 4.4.3. E↵ects of parameters on the nucleation polarization 117 4.4.4. Critical current 118 4.4.5. Vacancy concentration 121 4.5. Conclusion and future work 122 Chapter 5. Growth and Coarsening of Nanoparticles on the Surface of an Oxide 125 5.1. Introduction 125 5.2. Background 127 5.2.1. Coarsening in 3D 128 5.2.2. Coarsening in 2D 129 5.3. Modeling considerations 129 5.4. Mathematicalformulationofthesystem 131 5.4.1.Governingequation 131 5.4.2. Boundary conditions 131 5.4.3. Particle growth rate 132 5.4.4. Undimensionalizingtheequations 133 5.5. Approach 135 5.5.1. Green’s function 135 10 5.5.2. Green’s theorem 140 5.5.3. Solving the equations 143 5.6. Extension of the model and future work 143 5.7. Conclusion 144 Chapter 6. Conclusion 146 References 148 11 List of Figures 2.1 World energy consumption in the world as predicted by the United StatesEnergyInformationAdministrationin2011. 18 2.2 CompositionoftheelectricityproducedintheUSbyresources. 19 2.3 Projections for added electricity generation capacity as a function of sources through 2040. 20 2.4 Schematic of a solid oxide cell running in fuel cell mode on hydrogen gas. 22 2.5 Schematic of a solid oxide cell running in electrolysis mode on water. 23 3.1 System under consideration for the derivation of the equilibrium conditions: oxide and gas phase delimited by an arbitrary interface @ .33 V 3.2 Thought experiment to understand the e↵ect of stress on the non-stoichiometry. 44 3.3 Non-stoichiometry as a function of the trace of the stress in LSC-82. 56 3.4 Schematic of the change in non-stoichiometry in a coherent and dislocation-free thin film due to lattice mismatch with the substrate. 61 12 3.5 Schematic of the change in non-stoichiometry in a thin film grown on asubstrateunderthermalstress. 62 3.6 Chemical capacitance versus oxygen partial pressure at T =873K as estimated for bulk La0.6Sr0.4CoO3 δ,asreportedina1.5µm-thick − LSC film on GDC and according to the model. 64 3.7 Chemical capacitance versus oxygen partial pressure at T =793K evaluated for bulk La0.8Sr0.2CoO3 δ,asreportedfora45nm-thick − LSCfilmonYSZandaccordingtothemodel.
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