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Investigation of Ca-Doped Yttrium Iron Garnet for Solid Oxide Fuel Cells

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Investigation of Ca-Doped Yttrium Iron Garnet for Solid Oxide Fuel Cells University of Calgary PRISM: University of Calgary's Digital Repository Graduate Studies The Vault: Electronic Theses and Dissertations 2020-04-28 Investigation of Ca-doped Yttrium Iron Garnet for Solid Oxide Fuel Cells Zhang, Zheyu Zhang, Z. (2020). Investigation of Ca-doped Yttrium Iron Garnet for Solid Oxide Fuel Cells (Unpublished master's thesis). University of Calgary, Calgary, AB. http://hdl.handle.net/1880/111927 master thesis University of Calgary graduate students retain copyright ownership and moral rights for their thesis. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission. Downloaded from PRISM: https://prism.ucalgary.ca UNIVERSITY OF CALGARY Investigation of Ca-doped Yttrium Iron Garnet for Solid Oxide Fuel Cells by Zheyu Zhang A THESIS SUBMITTED TO THE FACULTY OF GRADUATE STUDIES IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE GRADUATE PROGRAM IN CHEMISTRY CALGARY, ALBERTA APRIL, 2020 © Zheyu Zhang 2020 Abstract Solid oxide fuel cells (SOFCs) are among the next-generation electrochemical energy conversion devices with higher energy efficiency. A major drawback hindering the mass commercialization of SOFCs is the slow oxygen reduction reaction kinetics at the cathode when the operating temperature is lowered from a high temperature (~1000 °C) to an intermediate temperature (500–750 °C). While many perovskite materials have been considered for use in novel SOFC cathodes, there remains a number of issues, such as large thermal expansion coefficient of Ba0.5Sr0.5Co0.8Fe0.2O3-δ due to the presence of Co and chemical instability under humidity and CO2- containing atmosphere of various perovskites. Therefore, the search, characterization and optimization of new cathode materials is of vital importance to the development of SOFCs. In this work, garnet-type Y3-xCaxFe5O12-δ was investigated as a promising candidate for a novel SOFC cathode. Structural variations from Ca-substitution in the parent phase Y3Fe5O12 was studied using powder X-ray diffraction. Different charge compensation mechanisms because of aliovalent doping of Ca were studied and validated through iodometric titration and X-ray absorption spectroscopy. It was suggested that formation of oxide ion vacancy and electron-hole co-exist in Ca-doped garnet samples. The maximum electrical conductivity was found in the x = 0.1 garnet composition, whose ionic transport number was revealed to be about 1.82×10-5 at 750 °C. Symmetrical cells with garnet and La0.8Sr0.2Ga0.8Mg0.2O3-δ composite electrode were fabricated and evaluated for their electrochemical properties at 600–900 °C in air and at 750 °C in various oxygen partial pressures. At 750 °C, the lowest area-specific resistance in air was obtained by x = 0.3 garnet composition, with dissociation and partial reduction of adsorbed oxygen identified as rate-limiting steps. ii Acknowledgements First and foremost, I would like to express my sincere appreciation and deepest gratitude to Dr. Venkataraman Thangadurai, my supervisor, for his excellent and constant support, guidance and encouragement throughout my M.Sc. study at University of Calgary. It was a privilege to be in his lab and pursue this degree under his supervision. His expertise, personality and the attitude towards scientific research will always inspire me. I would like to extend my thanks to my supervisory committee, Drs. Todd Sutherland and Simon Trudel for their helpful comments and advices on my research. I would also like to thank Dr. Michelle Dolgos for joining my examination committee. I would like to thank Dr. Viola Birss for her insightful electrochemistry course. I am grateful to every former and present member of our research group and members in Dr. Birss’ group whom we share office with in comradery. I want to thank Dr. Kalpana Singh for her tremendous help – this work would not have been possible without her input. I would like to have a special thank to Jialang Li. I would also like to thank my colleagues and friends: Dr. Xia Tong, Chengtian Zhou, Miao Wang, Bin Pan, Taozhe Wu and Lei Wang. There will always be many things that I need to learn from each of you. I want to extend my gratitude to Drs. Yan Jiang, Guangwei Wang, Tao Wang and Xiaoan Li. I am thankful to Sanoop Kammampata and Kyle Hofstetter for their generous help with lab instruments and a lot of constructive discussions. I would like to thank Drs. Senthil Venkatesan, Arpita Nandy and Anand Singh for their useful suggestions on experimental designs. I am grateful to Drs. Sourav Bag, Alfred Samson, Scott Paulson and Jason Young for their valuable advices and assistance. In addition, I thank Marwa Atwa, Chengying Ai, Haris Ansari, Orrsam Abubaker and Samantha Luong. It is you all mentioned above who have made this learning environment so friendly and supportive to me. iii I would like to thank Janice Crawford and all other chemistry department staffs who helped me during the course of this degree program. I extend my thanks to Drs. Jigang Zhou and James Dynes at Canadian Light Source for the collaboration on synchrotron-based X-ray absorption spectroscopy experiments. I would like to thank Dr. Yoed Tsur at Technion – Israel Institute of Technology for his support of using impedance spectroscopy genetic programming (ISGP). I want to thank Dr. Wang Hay Kan at China Spallation Neutron Source for sharing his view of performing Rietveld refinement. Finally, I would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Chemistry Department, Faculty of Science at University of Calgary for their financial support. iv Dedication I dedicate this thesis to my parents. v Table of Contents Abstract .......................................................................................................................................... ii Acknowledgements ...................................................................................................................... iii Dedication ...................................................................................................................................... v Table of Contents ......................................................................................................................... vi List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Symbols ............................................................................................................................ xii List of Abbreviations ................................................................................................................. xiii Chapter One: Introduction .......................................................................................................... 1 1.1 Project background ............................................................................................................. 1 1.2 Project objectives ................................................................................................................ 3 1.3 Thesis organization ............................................................................................................. 4 Chapter Two: Background .......................................................................................................... 5 2.1 Solid oxide fuel cell (SOFC)............................................................................................... 5 2.1.1 Basic principle of SOFCs........................................................................................... 6 2.1.2 Reversible potential and cell efficiency ..................................................................... 7 2.1.3 Electrochemical performance .................................................................................... 9 2.2 Materials of SOFCs........................................................................................................... 12 2.2.1 Electrolytes .............................................................................................................. 12 2.2.1.1 Oxide ion conducting electrolytes for O-SOFCs ............................................ 13 2.2.1.2 Proton conducting electrolytes for H-SOFCs ................................................. 16 2.2.2 Anodes ..................................................................................................................... 20 2.2.3 Cathodes ................................................................................................................... 22 2.2.4 Interconnects and sealants........................................................................................ 25 Chapter Three: Experimental Methods.................................................................................... 27 3.1 Material preparation .......................................................................................................... 27 3.1.1 Synthesis of garnet-type Y3-xCaxFe5O12-δ ................................................................ 27 3.2 Symmetrical cell fabrication ............................................................................................. 28 3.3 Characterization
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