ASSESSING NON-AQUEOUS PROTON-COUPLED ELECTRON TRANSFER REACTIONS THROUGH ELECTROCHEMICAL METHODS Brian D. McCarthy A dissertation submitted to the faculty of the University of North Carolina at Chapel Hill in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry. Chapel Hill 2016 Approved by: Jillian Dempsey Simon Meek Cynthia Schauer Joseph Templeton Mark Wightman ©2016 Brian D. McCarthy ALL RIGHTS RESERVED ii ABSTRACT Brian D. McCarthy: Assessing Non-Aqueous Proton-Coupled Electron Transfer Reactions Through Electrochemical Methods (Under the direction of Jillian L. Dempsey) The multi-decade proliferation of electrochemical hydrogen evolution catalysts has resulted in a relatively small handful of excellent catalysts. Thus, research has turned towards understanding catalytic mechanism in the hope of rationally guiding the next generation of catalysts. Specifically, recent focused effort has sought understanding of how homogeneous catalysts mediate the combination of two electrons from the electrode and two protons from solution-based sources. The individual proton and electron movements are frequently coupled such that the movement of one triggers the movement of the other, sometimes resulting in simultaneous transfer. Intentional catalyst designs that favor stronger coupling have shown impressive catalytic rates. This realization has promoted increased scrutiny of proton-coupled electron transfer (PCET) reactivity. PCET reactions have both kinetic and thermodynamic components; this dissertation focuses on thermodynamic aspects of electrochemical PCET through the development of relationships between applied potential and non-aqueous acidity. Non-aqueous potential-pKa theory is demonstrated through the construction of two experimental diagrams. A third candidate example of a potential-pKa diagram is discussed in the context of the challenges and opportunities of gaining thermodynamic information from irreversible electrochemical data. Two challenges were encountered during this research. First, direct reduction of acids by the electrode can obscure the desired PCET reactivity. Electrochemical analysis of over twenty acids in acetonitrile yielded a dataset of direct acid reduction potentials, information that was iii used to guide later PCET studies. This work additionally summarized unique considerations associated with acid-base behavior in non-aqueous solvents. Second, metal complexes used for PCET and hydrogen evolution studies can degrade at the electrode. Successful identification of when decomposition/transformation occurs allows accurate interpretation of electrochemical data and provides a guide for selecting metal complexes likely to be more robust. This guidance, coupled with better understanding of PCET mechanism, will help enable economical and efficient catalysts for solar fuel production. iv ACKNOWLEDGEMENTS Thank you to all to those who offered kind words, friendship, lent me chemicals, advice, and your time during my graduate studies. UNC-CH Chemistry Department, your friendliness really made this experience for me. Kita, Kate, Goldfogel, Andrew, Melissa, Alex Venning, and Nate – thank you for your friendship and the fun we had together. You were some of my first best friends here and I will remember our time fondly. I hope we will get future opportunities to spend time together. Many people passed through the Dempsey group while I was here. I am grateful to you all, however, I want to thank a few specific individuals who had outsized positive impacts. Daniel Martin, thank you. You helped me collect and analyze endless data and gave me an opportunity to learn how to better lead new scientists. You also took a real interest in my later projects. I appreciate your sincerity and am amazed that I was lucky enough to first mentor you and later work alongside you. Robin Knauf and Thomas Eisenhart, you were first Dempsey lab members. You helped me find my way at UNC-CH, and you were instrumental in helping get this lab running and were two of my first friends at UNC! To the younger students in the Dempsey lab, thank you for putting up with us, the occasionally obnoxious “first four.” It has been a privilege to see you grow as scientists and leaders in the group. Chris, I wish we had roomed together sooner as I delight in our esoteric conversations and your brimming enthusiasm for learning. Dan, there was a time we sat next to each other and while I moaned about how not sitting next to Eric was the “dark times,” I am grateful we did and for your subsequent contributions to lab and to my own research. Katherine, I have enjoyed working with you and I am going to miss your music video narration. You are v becoming an expert electrochemist and I am glad the future of our subgroup will be in your hands. Noémie, thank you for helping me grow as an electrochemist. The Dempsey lab and I have benefited from your electrochemistry expertise and I have enjoyed discussing science with you. Eric, thank you in particular for perhaps the most positive influence of everyone I have had the privilege to meet at UNC-CH. In science, I turned to you most – I truly believe that you are one of the current world electrochemistry experts. However, I learned so much more through in our conversations about life and I came to deeply value your perspectives. I will miss being able to just turn in my chair to talk to you. Jillian, I came to Chapel Hill because of your reputation as a scientist and as a kind person. I found these to be very true and I am grateful for your steady mentorship and guidance. You challenged me as a scientist and pushed me to do the best work I could and gave me opportunities I did not dream of having. Thank you for your support – I will miss very much working with and learning from you. Finally, thank you Mom, Dad, Alex, and Kelly. You are my closest loved ones, core support network, and best friends. Alex, thank you for all the Skype conversations and board games – and for help with that one literature meeting. Kelly, thank you for being there for me – and for the incredible amount of fun we had while somehow also doing PhDs. Mom and Dad, without your commitment to education, I would not be writing this. This dissertation is dedicated to you. vi TABLE OF CONTENTS LIST OF TABLES ................................................................................................................ xii LIST OF FIGURES ............................................................................................................. xiii LIST OF ABBREVIATIONS AND SYMBOLS .................................................................. xxviii CHAPTER 1 . Introduction .................................................................................................... 1 1.1 Need for Large Scale Fuel Synthesis ...................................................................... 1 1.2 Fuel Synthesis by Electrolysis ................................................................................. 2 1.3 Proton-Coupled Electron Transfer Underpins H2 Evolution ..................................... 4 1.4 Studying PCET by Cyclic Voltammetry ................................................................... 7 1.4.1 Protonation Kinetics Can Shift Potential ........................................................... 8 1.4.2 Interpreting PCET within Catalysis ................................................................... 9 1.4.3 Total catalysis gives rise to a second peak .................................................... 10 1.5 Dissertation Overview ........................................................................................... 12 CHAPTER 2 . Electrochemical Reduction of Acids by Glassy Carbon ................................ 13 2.1 Introduction ........................................................................................................... 13 2.2 Results .................................................................................................................. 15 2.2.1 Reference Electrode ...................................................................................... 15 2.2.2 Electrode Fouling ........................................................................................... 16 2.2.3 Acid Reduction Measurements ...................................................................... 17 2.2.4 Reduction Potentials ...................................................................................... 19 2.2.5 Influence of Water .......................................................................................... 22 vii 2.3 Discussion ............................................................................................................ 24 2.3.1 Acetonitrile as Solvent ................................................................................... 24 2.3.2 Homoconjugation ........................................................................................... 26 2.3.3 Electrode and Solvent Fouling ....................................................................... 29 2.3.4 Acid Reduction ............................................................................................... 30 2.3.5 Reduction Potentials ...................................................................................... 33 2.3.6 Influence of Water .......................................................................................... 39 2.4 Conclusion ...........................................................................................................