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Electrode Surface Activation and Nanostructuring Effects ELECTRODE SURFACE ACTIVATION AND NANOSTRUCTURING EFFECTS ON FUEL CELL PERFORMANCE A DISSERTATION SUBMITTED TO THE DEPARTMENT OF MATERIALS SCIENCE AND ENGINEERING AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Jason David Komadina December 2010 © 2011 by Jason David Komadina. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/zq508hs6390 ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Friedrich Prinz, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Yi Cui I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Paul McIntyre Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract Fuel cells are an attractive clean energy technology due to the low or zero emissions from operation and the potentially high efficiency. However, several challenges exist that hinder the implementation of fuel cells as a part of portable power devices. Among these challenges are the losses inherent to fuel cell operation. This work presents the results of three studies that attempt to lower activation losses on the fuel cell anode by catalysis or increased reaction area. The first study focuses on the anode electrochemistry for a novel fuel cell device that utilizes the naturally occurring charge separation in photosynthesis as the electrolyte. The second study investigates the use of an oxide ion conducting electrolyte and a PtRu anode for a direct methanol fuel cell at temperatures much lower than previously considered feasible. The third study examines the fabrication of high-surface area mixed electronic and ionic conducting anodes and their impact on fuel cell performance. The latter two studies are motivated by interest in low-temperature direct methanol fuel cells for mobile devices, while the former was an exploratory investigation into the possibility of “bioelectricity”, which may more appropriately have been called a “photosynthetic fuel cell”. Photosynthesis energizes electrons obtained through hydrolysis in the thylakoid space and through a series of steps, reduces the charge-carrying protein ferredoxin (Fd). The charge on Fd is used in numerous processes throughout the cell. It is well established that Fd does not readily give up its charge to a bare metal electrode. Therefore, mediators or surface modifiers must be used to capture this high-energy electron. The bioelectricity investigation studied the effects of various chemical modifiers attached to gold electrodes on the electrooxidation of reduced Fd and found that poly-L-lysine covalently bound to a monolayer of mercaptoundecanoic acid on gold resulted in reasonable oxidation kinetics. iv Typical solid oxide fuel cells operate at temperatures above 600°C. It was found in the second study that a PtRu anode was effective in low temperature (250-450°C) direct methanol operation using yttria-stabilized zirconia (YSZ), (Y2O3)0.08(ZrO2)0.92, as the electrolyte. In this arrangement, methanol may be used without equimolar quantities of water, in contrast to typical direct methanol fuel cells (DMFCs). Electrochemical impedance spectroscopy (EIS) measurements were analyzed in an effort to better understand the rate-limiting processes. In the third study presented in this work, the fabrication of high-surface are mixed electronic and ionic conducting (MEIC) anodes is discussed, along with the results of fuel cell characterization with MEIC anodes of high surface area. In particular, fuel cells fabricated using yttrium-doped BaZrO3 (BYZ) electrolytes are studied with high surface area Pd anodes for use with H2 and methanol fuels. v Acknowledgements I must take a moment to thank all of the wonderful people who have helped me, personally or professionally, along the way. First, I would like to extend my deep thanks to my advisor, Fritz Prinz. Without his constant support and encouragement, this work would not have been possible. Fritz has been as good an advisor as I could ask for, and over the past six years has always been able to motivate me in the lab and remind me how exciting it can be to research. He has a real passion and gift as a mentor, and was always willing to help my progress in any way possible. My reading committee members, Dr. Paul McIntyre and Dr. Yi Cui, deserve my thanks for their feedback and dedication to reading my work. Along with Dr. Rainer Fasching and Dr. James Swartz, they also engaged in an excellent discussion of my research during my oral defense. The Nanoscale Prototyping Laboratory (NPL), formerly the Rapid Prototyping Laboratory (RPL), is a large group, and every member past and present has lent a hand in this work. Whether aiding with sample fabrication, testing, or discussing results and theories, the collaborative spirit in this lab is strong and is a phenomenal resource. My coauthors, collaborators, colleagues, and cohorts all deserve my thanks. Of course, I would not be at this point in my life were it not for the numerous wonderful educators I‟ve had the pleasure to learn from over the years. I have to start as far back as elementary school with my first grade teacher, John Lewis, at Judson Montessori in San Antonio, Texas, who was as happy to teach as I was to learn. Continuing to Lake Country in Minneapolis, I‟ve been lucky enough to encounter dedicated teachers willing to go the extra mile for their students. Through middle school and high school in Edina, Minnesota, there were always those ready to challenge and encourage me in my learning, and the same is true for the instructors in the University of Minnesota Talented Youth Math Program (UMTYMP), which I can‟t recommend enough for young math lovers who have the opportunity to join. I also only have good things to say about my time at Harvey Mudd College and vi Stanford University. All of the professors at these institutions are extremely bright individuals with a strong desire to help students produce their best work in both the classroom and the laboratory. Their drive and dedication are an inspiration to me as I finish my doctorate and prepare for my own experience as a professor. No education is complete without the friendships that develop in the process. I‟ve been fortunate to know a number of wonderful people as friends, many of whom I‟ve also had the pleasure of working with, in our studies or in the lab. Their support during the good weeks and bad weeks has always been appreciated, and it is truly a joy to know all of them. My heartfelt thanks go to my parents, Kevin and Jayne, and my brother, Greg, too. I don‟t get a chance to see them as often as I would like, but the door is always open, and the table always set. I am sincerely grateful for their, encouragement, love, and faith in my ability to succeed in whatever I set out to do. I‟ve saved the best for last, as there‟s also a wonderful woman, my fiancée, Rachel. She keeps me fun when I get too serious, and keeps me working when I get too distracted. There is no day so awful that she can‟t fix with one smile, and I could not be happier to have her in my life. To everyone, thank you. vii Table of Contents Abstract ............................................................................................................................... iv Acknowledgements .............................................................................................................. vi List of Tables ....................................................................................................................... xi 1 Introduction ................................................................................................................. 1 2 Fuel Cells ...................................................................................................................... 5 2.1 General operation................................................................................................... 5 2.2 Thermodynamic Potential ...................................................................................... 7 2.3 Reaction kinetics .................................................................................................... 9 2.4 Losses and efficiency ............................................................................................ 11 2.4.1 Activation losses ............................................................................................ 12 2.4.2 Ohmic losses ................................................................................................. 13 2.4.3 Concentration losses ..................................................................................... 14 2.4.4 Fuel cell efficiency........................................................................................
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