Microfluidic Platforms for the Investigation of Fuel Cell Catalysts and Electrodes

MICROFLUIDIC PLATFORMS FOR THE INVESTIGATION OF FUEL CELL CATALYSTS AND ELECTRODES BY FIKILE R. BRUSHETT DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemical Engineering in the Graduate College of the University of Illinois at Urbana-Champaign, 2010 Urbana, Illinois Doctoral Committee: Professor Paul J.A. Kenis, Chair Professor Rich I. Masel Professor Andrzej Wieckowski Professor Edmund G. Seebauer Abstract A clear need exists for novel approaches to producing and utilizing energy in more efficient ways, in light of society’s ever increasing demand as well as growing concerns with respect to climate change related to CO2 emissions. The development of low temperature fuel cell technologies will continue to play an important role in many alternative energy conversion strategies, especially for portable electronics and automotive applications. However, widespread commercialization of fuel cell technologies has yet to be achieved due to a combination of high costs, poor durability and, system performance limitations (Chapter 1). Developing a better understanding of the complex interplay of electrochemical, transport, and degradation processes that govern the performance and durability of novel fuel cell components, particularly catalysts and electrodes, within operating fuel cells is critical to designing robust, inexpensive configurations that are required for commercial introduction. Such detailed in-situ investigations of individual electrode processes are complicated by other factors such as water management, uneven performance across electrodes, and temperature gradients. Indeed, too many processes are interdependent on the same few variable parameters, necessitating the development of novel analytical platforms with more degrees of freedom. Previously, membraneless microfluidic fuel cells have been developed to address some of the aforementioned fuel cell challenges (Chapter 2). At the microscale, the laminar nature of fluid flow eliminates the need for a physical barrier, such as a stationary membrane, while still allowing ionic transport between electrodes. This enables the development of many unique and innovative fuel cell designs. In addition to addressing water management and fuel crossover issues, these laminar flow- based systems allow for the independent specification of individual stream compositions (e.g., pH). Furthermore, the use of a liquid electrolyte enables the simple in-situ analysis of individual electrode performance using an off-the-shelf reference electrode. These advantages can be leveraged to develop microfluidic fuel cells as versatile electro-analytical platforms for the characterization and optimization ii of catalysts and electrodes for both membrane- and membraneless fuel cells applications. To this end, a microfluidic hydrogen-oxygen (H2/O2) fuel cell has been developed which utilizes a flowing liquid electrolyte instead of a stationary polymeric membrane. For analytical investigations, the flowing stream (i) enables autonomous control over electrolyte parameters (i.e., pH, composition) and consequently the local electrode environments, as well as (ii) allows for the independent in-situ analyses of catalyst and/or electrode performance and degradation characteristics via an external reference electrode (e.g., Ag/AgCl). Thus, this microfluidic analytical platform enables a high number of experimental degrees of freedom, previously limited to a three-electrode electrochemical cell, to be employed in the construct of working fuel cell. Using this microfluidic H2/O2 fuel cell as a versatile analytical platform, the focus of this work is to provide critical insight into the following research areas: Identify the key processes that govern the electrode performance and durability in alkaline fuel cells as a function of preparation methods and operating parameters (Chapter 3). Determine the suitability of a novel Pt-free oxygen reduction reaction catalyst embedded in gas diffusion electrodes for acidic and alkaline fuel cell applications (Chapter 4). Establish electrode structure-activity relationships by aligning in-situ electrochemical analyses with ex-situ microtomographic (MicroCT) structural analyses (Chapter 5). Investigate the feasibility and utility of a microfluidic-based vapor feed direct methanol fuel cell (VF-DMFC) configuration as a power source for portable applications (Chapter 6). In all these areas, the information garnered from these in-situ analytical platforms will advance the development of more robust and cost-effective electrode configurations and thus more durable and commercially-viable fuel cell systems (both membrane-based and membraneless). iii Acknowledgements I would like to express my gratitude to several people without whom this work would not have been possible. First and foremost, I am profoundly grateful to my advisor, Professor Paul J.A. Kenis, for his patience, guidance, and support. I am also deeply indebted to Dr. Jayashree Ranga, Dr. Wei-Ping Zhou, and Dr. Michael Mitchell for their invaluable mentorship in fuel cell and electrochemical analyses. Also, I am thankful to my thesis committee, Professors Rich Masel, Andrzej Wieckowski, and Edmund Seebauer, for their support during the faculty and postdoctoral job searches. In addition to my mentors, I would like to thank my co-workers in the Kenis group. Many thanks go to Matt Naughton and Molly Jhong with whom I worked most closely. Indeed, much of this work would not be possible without their excellent contributions. In addition, thanks go to the talented undergraduate students, namely Desmond Ng, Alix Schmidt, Raymond Grieshaber, and Nicolas Lioutas, whom I have had the pleasure of mentoring. I am also grateful to my colleagues in the electrochemistry section of the Kenis group, namely Adam Hollinger, Pedro Lopez-Montesinos, Michael Thorson, and Devin Whipple, for their scientific insight. Finally, I am grateful to the other Kenis group members, particularly Joshua Tice, Benjamin Schudel, Jerrod Henderson, Sudipto Guha and Sarah Perry, for their scientific insight, editorial assistance, and much needed camaraderie during the completion of this work. Beyond the Kenis group, I would like to thank my collaborators. I am grateful to Professor Andrew Gewirth and members of his research group, namely Matt Thorum and Claire Tornow, in the Chemistry Department for their help in synthesizing and characterizing the novel catalyst described in Chapter 4. I am also grateful to Leilei Yin, Darren Stevenson, Scott Robinson, and Cate Wallace in the Imaging Technology Group at the Beckman Institute for their assistance in materials imaging and analysis. Finally, I am appreciative of the generous financial support provided by the Graduate Engineering for Minorities (GEM), the Support for Under-Represented Groups in Engineering (SURGE), and the Harry G. Drickamer fellowship programs. iv To my immediate and extended family who taught me there is no substitute for hard work v Table of Contents Chapter 1: Introduction ...................................................................................................................1 1.1 Addressing Global Energy Challenges ...............................................................................1 1.2 Overview of Current Acidic Fuel Cell Technology ............................................................2 1.3 Overview of Current Alkaline Fuel Cell Technology.........................................................7 1.4 Key Remaining Challenges in the Development of Low-Temp Fuel Cells .....................10 1.5 References .........................................................................................................................11 Chapter 2: Membraneless Microfluidic Fuel Cells: Literature Overview and Remaining Opportunities .................................................................................................................................14 2.1 Introduction .......................................................................................................................14 2.2 Multi-stream Laminar Flow Concept................................................................................15 2.3 Key Benefits of Membraneless Microfluidic Fuel Cell Design........................................16 2.4 Overview of Current Membraneless Microfluidic Fuel Cell Technology ........................17 2.5 Microfluidic Hydrogen-Oxygen Fuel Cells as Electro-Analytical Platforms ...................24 2.6 Topics Studied in this Thesis ............................................................................................27 2.7 References .........................................................................................................................27 Chapter 3: Analysis of Pt/C-based Electrode Performance in an Alkaline Fuel Cell ...................31 3.1 Introduction .......................................................................................................................31 3.2 Experimental .....................................................................................................................33 3.2.1 Gas Diffusion Electrode Preparation .......................................................................33 3.2.2 Fuel Cell Assembly and Testing ..............................................................................33 3.3 Results and Discussion .....................................................................................................35

Load more