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22345.Pdf (6.55 Processing and Properties of Nanocomposite Thin Films for Microfabricated Solid Oxide Fuel Cells A Dissertation submitted to the Graduate School Of the University of Cincinnati In partial fulfillment on the Requirements for the degree of Doctor of Philosophy In the Materials Science and Engineering program Of the College of Engineering and Applied Science By Michael A. Rottmayer M.S., University of Dayton, 2007 Committee Chair: Dr. Raj Singh 1 Abstract Microfabricated solid oxide fuel cells (mSOFCs) have recently gained attention as a promising technology, with the potential to offer a low temperature (as low as 300°C), reduced start-up time, and improved energy density for portable power applications. At present, porous Pt is the most common cathode being investigated for mSOFCs. However, there are significant technical challenges for utilizing pure metallic electrodes at the operating temperatures of interest due to their tendency towards Ostwald ripening, as well as no bulk ionic conductivity. Nanocomposite materials (e.g. Pt/YSZ) are a promising alternative approach for providing both microstructural and electrochemical stability to the electrode layer. The overall objective of this research was to explore the processing of nanocomposite metal / metal oxide materials (i.e. Pt/YSZ) for use as a high performance cathode electrode for mSOFCs. The Pt/YSZ nanocomposite cathodes were deposited through a co-sputtering process and found to be stable up to 600°C in air for extended periods of time through an exhaustive materials and electrochemical study. A percolation theory model was utilized to guide the design of the Pt/YSZ composition, allowing for a networked connection of ionic- and electron-conduction through the membrane, leading to an extension of the triple phase boundary (TPB). The Pt/YSZ composite deposition pressure was found to be a key in helping to stabilize the morphology of the film. By increasing the deposition pressure, this led to the formation of intergranular void spacing, or porosity, as well as a reduction of film strain in the post-annealed film. Surface analyses of the composite film demonstrated that the lower film strain led to a minimization of Pt hillock grain coarsening and de-wetting, even after exposure to high temperatures (600°C) for extended periods of time (tested up to 24hrs) in air. Analyses of the Pt/YSZ composite microstructure and composition by TEM confirmed an interconnected network of Pt and YSZ 2 was maintained through the film’s thickness after exposure to high temperatures in air. A significant improvement in the stability of the electrical conductivity was demonstrated relative to the Pt electrodes, tested under constant current measurement conditions for up to 24hrs at 600°C. mSOFC testing results revealed that interconnectivity or percolation of the Pt and YSZ through the composite cathode was achieved, effectively leading to an increase in the TPB length, or increase in reaction sites for the oxygen reduction reaction to occur. The activation energy associated with the oxygen reduction reaction charge transfer kinetics in the Pt/YSZ was shown to be lower than a pure porous Pt electrode, along with a significant improvement in stability of the morphology during extended mSOFC operation. Mass diffusion of oxygen through the cathode to the TPB was found to be the rate determining step in the oxygen reduction reaction process. A further increase in porosity in the Pt/YSZ cathode should result in more efficient oxygen diffusion and a higher performance mSOFC cathode. 3 4 Acknowledgements I would first and foremost like to recognize and thank my family, wife Kate and sons Benjamin and Andrew, for their continued support and encouragement to complete this work. Without them, none of this would be possible. I would also like to thank my faculty advisor, Dr. Raj Singh, for his guidance and support throughout my thesis work. The insight and experience was certainly appreciated and his willingness to help was admirable. I would, also, like to thank my colleagues at the Air Force Research Laboratory, too many to name specifically, who have bestowed their knowledge and experience upon me which has helped guide this research. I would like to thank my dissertation committee members, Dr. Buchanan, Dr. Roseman and Dr. Huang, for taking the time out of their busy schedules to provide insightful comments. Finally, I would like to show my appreciation for my parents and family who have always encouraged me and bestowed upon me the value of education. 5 Table of Contents 1.0 Introduction ..............................................................................................................................15 2.0 Literature Review.....................................................................................................................18 2.1 Significance of Microfabricated Solid Oxide Fuel Cells (SOFCs) .....................................21 2.2 Microfabricated SOFC Materials .........................................................................................22 2.2.1 Anode Materials ............................................................................................................23 2.2.2 Electrolyte Materials .....................................................................................................27 2.2.3 Cathode Materials ..........................................................................................................30 2.2.4 Substrate Materials ........................................................................................................35 2.3 Percolation Theory in SOFC Composite Electrodes ............................................................38 2.4 Microfabricated SOFC Design Configurations ....................................................................41 2.5 Microfabricated SOFC Cell Performance ............................................................................43 2.6 Summary ..............................................................................................................................45 3.0 Objective of Research ..............................................................................................................48 4.0 Experimental Procedures .........................................................................................................52 4.1 Thin Film Vacuum Deposition .............................................................................................52 4.2 Processing of the Composite Pt/YSZ Thin Film Cathode ....................................................54 4.3 Compositional Analyses of the Composite Pt/YSZ Thin Film ............................................55 4.4 Functionally-Graded Pt/YSZ Composite Thin Film Cathodes ............................................58 6 4.5 X-Ray Diffraction (XRD) Analyses of the Composite Pt/YSZ Thin Film ..........................60 4.6 Electron Microscopic Analyses of the Composite Pt/YSZ Thin Film .................................61 4.7 Electrochemical Analyses of the Composite Pt/YSZ Thin Film and Microfabricated SOFCs ........................................................................................................................................62 4.8 Microfabricated SOFC Cell Processing and Fabrication .....................................................66 4.8.1 Silicon-Supported Microfabricated SOFC Cell Fabrication ........................................66 4.8.2 Electrolyte-Supported Microfabricated SOFC Cell Fabrication ...................................73 4.9 Microfabricated SOFC Custom-Designed Test Stand and Cell Test Fixture ......................74 4.10 Microfabricated SOFC Cell Testing and Characterization ................................................76 4.10.1 Testing Results of Silicon-Supported Microfabricated SOFCs...................................78 5.0 Results and Discussions ...........................................................................................................80 5.1 Morphological and Electrical Stability Studies of Pt/YSZ Thin Film Cathodes .................80 5.2 Influence of Deposition Pressure on the Formation and Evolution of Film Strain and Particle Size of Pt/YSZ Thin Film Cathodes .............................................................................94 5.3 Influence of Porosity, Composition, and Thickness on the Performance of Microfabricated SOFCs Consisting of Uniform Pt/YSZ Cathodes .....................................................................98 5.4 Influence of Porosity and Composition on the Performance of Microfabricated SOFCs Consisting of Functionally-Graded Pt/YSZ Cathodes ............................................................108 6.0 Conclusions ............................................................................................................................113 7.0 Future Work ...........................................................................................................................117 7 Bibliography ................................................................................................................................119 8 List of Tables Table 1: Composite Pt/YSZ calibration table for various composition ratios ...............................55 Table 2: Summary of mSOFC cathode electrochemical performance ..........................................60 Table 3: Summary of mSOFC EIS measurements at 400°C. ........................................................66 9 List of Figures Figure 1: Schematic
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