Heteroatom-Containing Carbon Nanostructures As Oxygen Reduction Electrocatalysts for PEM and Direct Methanol Fuel Cells
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Heteroatom-containing Carbon Nanostructures as Oxygen Reduction Electrocatalysts for PEM and Direct Methanol Fuel Cells Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Dieter von Deak, B.S.ChE Graduate Program in Chemical Engineering * * * * The Ohio State University 2011 Dissertation Committee: Professor Umit S. Ozkan, Advisor Professor David Wood Professor James Rathman Copyright by Dieter von Deak 2011 2 ABSTRACT The main goal of this work was to undertake a fundamental investigation of precious metal-free carbon catalysts nano-structure modification to enable their use as oxygen reduction reaction (ORR) catalysts in proton exchange membrane (PEM) fuel cells. The sluggish ORR is accelerated by fiscally prohibitive loadings of Pt catalyst. The expense and availability of platinum motivate the development of non-precious metal carbon-nitroge-based ORR catalysts (CNx). The project targets the nature of oxygen reduction reaction active sites and exploring ways to create these sites by molecular tailoring of carbon nano-structures. CNx grown with phosphorous had a significant increase in the ORR active site density. CNx catalyst growth media was prepared by acetonitrile deposition over a Fe and P impregnated MgO. Rotating Ring Disk Electrode (RRDE) Activity and selectivity showed a significant increase in oxygen reduction current with CNx grown with less than a 1:1 molar ratio of P:Fe. Selectivity for the full reduction of dioxygen to water trended with increasing ORR activity for phosphorous grown CNx catalysts. Phosphorus growth altered the morphology of carbon-nitride graphite formed during pyrolysis. The role of the transition metal used to form non-noble metal electrochemical oxygen reduction CNx catalysts was investigated through sulfur and carbon monoxide ii treatments of the CNx and Pt/carbon electrocatalysts. The intent of poisoning was to show the existence of a non-iron containing electrocatalytic active site in CNx. The sulfur treatment increased the overpotential on a platinum catalyst, but enhanced the current density of the CNx catalyst while leaving the CNx iron phase unchanged. CO in the present of oxygen was found to strongly adsorb to platinum and completely eliminate all oxygen reduction. Under identical conditions, CNx showed a displacement of oxygen due to CO and no oxygen reduction poisoning effect. This suggests that either iron-based active site is sulfur and CO tolerant or that this active site does not participate in the electrocatalytic reduction of oxygen in CNx catalysts. Density functional theory (DFT) calculations of small polycyclic aromatic hydrocarbons (PAHs) that have a similar electronic structure to carbon-nitride catalyst materials were preformed. A strong correlation between B3LYP method N 1s energies and experimental N 1s energies was established for the PAHs studied. Additionally, experimental ionization potentials that would correspond to electron donation trended strongly with the DFT adiabatic and vertical ionization potentials. The testing and setup of fuel cell test station was accomplished. Bench scale membrane electrode assemblies (MEAs) were fabricated cell and achieved comparable performance to a commercial MEA constructed from similar materials. A MEA was constructed with a CNx cathode and was found to have fuel cell performance of the same order of magnitude as other graphitic carbon-nitrogen catalysts heat-treated in the presence of a transition metal. iii Vulcan carbon and CNx catalysts were compared in accelerated carbon corrosion by examining the current of the electrochemically active surface species hydroquinone/quione with cyclic voltammetry after extended potential holds. CNx was found to be more corrosion resistant than Vulcan carbon that is the most commonly used support in fuel cell electrodes. iv DEDICATION To my friends and family, without your support this would not be possible. v ACKNOWLEDGMENTS I gratefully acknowledge the teaching, guidance, and camaraderie I have received in the laboratory from all the past and present heterogeneous catalysis research group members. I am especially indebted to those who began this project: Paul Matter, PhD and his undergraduate assistants Eugenia Wang and Maria Arias. A special thanks is given to Eugenia Wang and Maria Arias for leading me to graduate school and towards Dr. Ozkan's research group. Thank you to those who I have worked with on the PEM Team: undergraduates Doug Knapke, Jesaiah King, Katie Luthman, Hilary Marsh, Dan Valco, and Judi Keys along with high school student Kate Baker, and of course, fellow graduate students Elizabeth Biddinger, who pushed me to organize and think critically, and Deepika Singh, who will carry on great work after me. This coherence of a great research group would not be possible without the guidance and dedication of my advisor Professor Umit S. Ozkan. Thank you for taking me into your care and helping me to grow. Finally, a deep thanks to my family and friends for supporting me as a graduate student. I thank my grandmother von Deak for softening the financial burden of my undergradute degree which allowed me to focus on my studies. I am indebted to Troy Vogel and Adam Burley who accompanied me through undergraduate and graduate school and Katie Richards for her love and companionship. vi VITA March 31, 1983 ………………. Born, Richmond, Virginia June 2001 ………………………… H.S. Diploma, Buckeye Sr. High School, Medina, Ohio Winter 2005-Spring 2006………… Chemical Engineering Internship, Entrotech, Columbus, Ohio June 2006 …………………………. B.S. Chemical Engineering, Ohio State University, Columbus, Ohio June 2011 …………………………. M.S. Chemical Engineering, Ohio State University, Columbus, Ohio Septemper 2011 ……………………Ph. D. Chemical Engineering, Ohio State University, Columbus Ohio vii PUBLICATIONS Invited Papers E.J. Biddinger, D. von Deak, U.S. Ozkan, “Nitrogen-Containing Carbon Nanostructures as Oxygen-Reduction Catalysts,” Topics in Catalysis, 52 (2009), 1566-1574. Journal Articles E.J. Biddinger, D.S. Knapke, D. von Deak, U.S. Ozkan, “Effect of Sulfur as a Growth Promoter for CNx Nanostructures as PEM and DMFC ORR Catalysts,” Applied Catalysis B – Environmental, 96 (2010), 72-82. D. von Deak, D. Singh, E.J. Biddinger, J.C. King, B. Bayram, J.T. Miller, U.S. Ozkan, "Investigation of sulfur poisoning of CNx oxygen reduction catalysts for PEM fuel cells," Journal of Catalysis, (2011), submitted. D. von Deak, E.J. Biddinger, K.A. Luthman, U.S. Ozkan, "The effect of phosphorus in CNx catalysts for the oxygen reduction in PEM fuel cells," Carbon, 48(12) (2010), 3637-3639. D. von Deak, E.J. Biddinger, U.S. Ozkan, "Carbon corrosion characteristics of CNx nanostructures in acidic media and implication for ORR performance," Journal of Electroanalytical Chemistry, (2011) accepted. viii E.J. Biddinger, D. von Deak, H. Marsh, U.S. Ozkan, "RRDE catalyst ink aging effects on selectivity to water formation in ORR," Electrochemisty Solid-State Letters, 13 (2010), B98-B100. E.J. Biddinger, D. von Deak, D. Singh, H. Marsh, B. Tan, D.S. Knapke, U.S. Ozkan, "Examination of catalyst loading effects on the selectivity of CNx and Pt/VC ORR catalysts using RRDE," Journal of Electrochemistry Society, (2011) accepted. X. Bao, D. von Deak, E.J. Biddinger, U.S. Ozkan, C.M Hadad, "A computational exploration of the oxygen reduction reaction over a carbon catalyst containing a phosphinate functional group," Chemistry Communications, 46(45) (2010), 8621- 8623. FIELDS OF STUDY Major Field: Chemical Engineering Area of Interest: Heterogeneous Catalysis ix TABLE OF CONTENTS ABSTRACT ........................................................................................................................ ii DEDICATION .................................................................................................................... v ACKNOWLEDGMENTS ................................................................................................. vi VITA ................................................................................................................................. vii LIST OF FIGURES ........................................................................................................ xvii LIST OF TABLES .......................................................................................................... xxv CHAPTER 1. Executive Introduction of Carbon-based Oxygen Reduction Electrocatalysts ................................................................................................................... 2 CHAPTER 2. Research Objectives..................................................................................... 3 CHAPTER 3. Literature Review of Oxygen Reduction Catalysts in Fuel Cells ................ 7 3.2 Introduction to Polymer Electrolyte and Direct Methanol Fuel Cell Technologies . 7 3.2.1 Motivation for Fuel Cell Research ......................................................................... 7 3.2.2 Introduction of Polymer Electrolyte and Direct Methanol Fuel Cell ..................... 9 3.3 Fuel Cell Testing and Performance ......................................................................... 13 3.3.1 Polarization Performance Curves ......................................................................... 13 x 3.3.2 Resistance Correction with Impedance ................................................................ 15 3.4 Current State of Technology for PEM and Direct Methanol