Modeling of Solid Oxide Fuel Cells by Won Yong Lee B.S. Mechanical Engineering Seoul National University, 2001 SUBMITTED TO THE DEPARTMENT OF MECHANICAL ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE IN MECHANICAL ENGINEERING AT THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY SEPTEMBER 2006 C 2006 Massachusetts Institute of Technology. All rights reserved. The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Signature of A uthor .......................................................... Department of Mechanical Engineering August 19, 2006 Certified by ............................... Ahmed F. Ghoniem Professor Thesis Supervisor A ccepted by................................................................. Lallit Anand Chairman, Department Committee on Graduate Students Modeling of Solid Oxide Fuel Cells By Won Yong Lee Submitted to the Department of Mechanical Engineering on 19 August 2006 in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Abstract A comprehensive membrane-electrode assembly (MEA) model of Solid Oxide Fuel Cell (SOFC)s is developed to investigate the effect of various design and operating conditions on the cell performance and to examine the underlying mechanisms that govern their performance. We review and compare the current modeling methodologies, and develop an one-dimensional MEA model based on a comprehensive approach that include the dusty- gas model(DGM) for gas transport in the porous electrodes, the detailed heterogeneous elementary reaction kinetics for the thermo-chemistry in the anode, and the detailed electrode kinetics for the electrochemistry at the triple-phase boundary. With regard to the DGM, we corrected the Knudsen diffusion coefficient in the previous model developed by Multidisciplinary University Research Initiative[1]. Further, we formulate the conservation equations in the unsteady form, allowing for analyzing the response of the MEA to imposed dynamics. As for the electrochemistry model, we additionally analyzed all the possibilities of the rate-limiting reaction and proposed rate-limiting switched mechanism. Our model prediction agrees with experimental results significantly better than previous models, especially at high current density. Thesis Supervisor: Ahmed F. Ghoniem Title: Professor 2 Acknowledgements First, I would like to thank Professor Ghoniem for his guidance and support through the past two years. I owe a great deal to my colleagues, Daehyun Wee, Jean-Christphe Nave, Murat Altay, Fabrice Schlegel and Raymond Speth. Daehyun Wee shared his valuable time for the discussion on the topics in this thesis. I also thank Sangmok Han, who has provided valuable help on formatting this thesis. I am very grateful to my parents, Hak-Joon Lee and Ok-Ja Kim for their love, support and guidance. I also thank other members of my family, including grandmother, my sister Myung-Sook Lee and brother-in-law Joon-Gu Kim for their incredible understanding and emotional support. Finally, special thanks to my right knee. Even though it suffered from three times surgeries, the right knee has supported me continuously with patient. Through the surgeries, I realized how valuable the health, the friendship, and the modesty are. Without the awareness of these, it was impossible to withstand everything that I have been through during the past two years. 3 Contents Contents ..............................................................................................................................4 List of Figures .......................................................................................................................6 List of Tables .........................................................................................................................8 Nom enclature ........................................................................................................................9 Chapter 1 Introduction ...................................................................................................... 12 1. 1. M otivation .................................................................................................................. 12 1.2. Introduction to Fuel Cells .......................................................................................... 13 1.2. 1. Definition ............................................................................................................ 13 1.2.2. Components ........................................................................................................ 14 1.2.3. Stacks .................................................................................................................. 15 1.2.4. Types ................................................................................................................... 16 1.3. Solid Oxide Fuel Cell ................................................................................................. 17 1.3. 1. Advantages and Disadvantage of SOFC ............................................................. 17 1.3.2. Physics ................................................................................................................ 18 1.3.3. M aterials ............................................................................................................. 20 1.3.4. Current SOFC Research foci .............................................................................. 22 1.3.5. M odels ................................................................................................................ 24 1.4. Conclusion ................................................................................................................. 25 Chapter 2 M odeling of SO FC M EA ................................................................................. 27 2.1. Equilibrium potential ................................................................................................. 27 2.2. Overpotentials ............................................................................................................ 29 2.2. 1. Concentration Overpotential ............................................................................... 31 2.2.2. Activation overpotential ..................................................................................... 48 2.2.3. Ohm ic overpotential ........................................................................................... 66 2.3. Conclusion ................................................................................................................. 66 Chapter 3 The Sim ulation of SO FC MEA ....................................................................... 68 3.1. The Current Anode M odels ........................................................................................ 68 3. 1. 1. M odel I ............................................................................................................... 68 4 3.1.2. M odel 2 ............................................................................................................... 69 3.1.3. M odel 3 ............................................................................................................... 70 3.1.4. M odel 4 ............................................................................................................... 70 3.1.5. Com parison of Current Anode M odels ............................................................... 71 3.2. An Im proved M EA model .......................................................................................... 76 3.2. 1. Concentration Overpotential ............................................................................... 77 3.2.2. Activation Overpotential ..................................................................................... 79 3.3. Sim ulation method ..................................................................................................... 82 3.4. Sim ulation Results ..................................................................................................... 85 3.4. 1. Single rate-lim iting ............................................................................................. 86 3.4.2. Rate-lim iting switch-over ................................................................................... 93 3.4.3. Contribution of Each Overpotentials .................................................................. 97 3.5. Conclusion ............................................................................................................... 101 C hapter 4 C onclusion ....................................................................................................... 103 4.1. Sum m ary .................................................................................................................. 103 4.2. Future Work ............................................................................................................. 103 R eference ..........................................................................................................................107 5 List of Figures Figure 1-1 The Schem atics of Fuel Cells ...................................................................................... 14 Figure 1-2 Fuel C ell Stacks ..............................................................................................................