MOLECULAR MODELING OF IONIC LIQUIDS: STRUCTURE, DYNAMICS AND ELECTROCHEMICAL PERFORMANCE IN SUPERCAPACITORS By Song Li Dissertation Submitted to the Faculty of Graduate School of Vanderbilt University In partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY In CHEMICAL ENGINEERING May, 2014 Nashville, Tennessee Approved: Peter T. Cummings, Ph.D. Clare M. McCabe, Ph.D. Kane G. Jennings, Ph.D. Kalman Varga, Ph.D. DEDICATION This dissertation is dedicated to my husband, Yuqiang Zhao, whose love and encouragement support me throughout my Ph.D. study, and to my encouraging parents and supportive friends in Vanderbilt University. ii ACKNOWLEDGMENTS I would never have been able to complete my dissertation without the guidance of my advisor, help from group members and support from my family. I would like to express my deepest and sincerest gratitude to my advisor, Professor Peter T. Cummings for his guidance, encouragement and support throughout the course of this work. His extensive knowledge, insightful vision and creative thinking inspired me to pursue not only my Ph.D. but also a future career in scientific research. I greatly appreciate the opportunities provided for me by Prof. Cummings to present my work and communicate with other researchers in international conferences. I would also like to thank my committee members: Prof. Clare McCabe, Prof. Kane G. Jennings and Prof. Varga Kalman. Their invaluable critiques and helpful suggestions are precious during this process. Special thanks go to Dr. Guang Feng, whose instruction and mentorship greatly improve my research skills and scientific thinking. I appreciate the time and effort he spent for inspiring discussion and helpful advices. His passion for scientific research and philosophy also boost my motivation to pursue an academic position. I also gratefully appreciate the discussion with several members in Cummings Group: Li Wan, Dr. William French, Dr. Mingjie Wei and Dr. Stepan Hlushak. I really enjoy the time in laboratory with their warmhearted help and friendship. I would like to sincerely thank Dr. David J. Wesolowski, the director of Fluid Interface Reactions, Structures and Transport (FIRST) center, an Energy Frontier Research Center. I cherish and appreciate the collaborating opportunities provided by FIRST center throughout my Ph.D. and the chance to meet and communicate with excellent researchers from different fields. A big thank you goes to several collaborators in FIRST center: Dr. José Leobardo Bañuelos (ORNL), Dr. Pasquale F. iii Fulvio(ORNL), Dr. Kee Sung Han (ORNL), Dr. Jianchang Guo (ORNL), Dr. Pengfei Zhang (ORNL), Katherine L. Van Aken (Drexel University), John K. McDonough (Drexel University). They performed the experiments to support the simulation results discussed in this dissertation. In addition, I would like to thank Dr. Sheng Dai, Dr. Edward Hagaman and Dr. Yury Gogotsi. Their encouragement and help make me bravely face the challenges and difficulties in research. I also own my gratitude to all of my friends in Vanderbilt University, especially those in Department of Chemical and Biomolecular Engineering. Their company and friendship make my life during Ph.D. study less stressful and full of happiness. Finally, I would like to thank my beloved husband, Yuqiang Zhao, my parents and parents-in-law. Without their love, patience, and sacrifice, I cannot complete this work and achieve my goals. iv Table of Contents Page DEDICATION ...................................................................................................ii ACKNOWLEDGMENTS..................................................................................iii LIST OF ABBREVATIONS ...........................................................................xiv I INTRODUCTION ...........................................................................................1 II BACKGROUND ...........................................................................................5 2.1 Ionic Liquids .......................................................................................... 5 2.2 Spatial Heterogeneity in Ionic Liquids................................................... 6 2.3 Ionic Liquids Electrolytes in Supercapacitors........................................ 8 III METHODOLOGY ......................................................................................13 3.1 Molecular Dynamics Simulation.......................................................... 13 3.2 Data Analysis ..................................................................................... 15 3.2.1 Structure Factor Calculation .......................................................... 15 3.2.2 Supercapacitor Capacitance Computation .................................... 16 IV STRUCTRAL ORGANIZATION OF BULK IONIC LIQUIDS....................22 4.1 Introduction ....................................................................................... 22 4.2 Simulation Details ............................................................................... 25 4.3 Monocationic Ionic Liquids [CnMPy][Tf2N]........................................... 27 4.3.1 Influence of Chain Length on Spatial Heterogeneity ..................... 27 4.3.2 Temperature Effects ...................................................................... 36 4.4.3 Conclusion..................................................................................... 40 4.4 Dicationic Ionic Liquids ....................................................................... 40 4.4.1 Chain Length Effects on Dicationic [Cn(mim)2](BF4)2..................... 40 4.4.2 Influence of Anion Types ............................................................... 49 4.4.3 Structure Models for DILs.............................................................. 50 4.4.4 Conclusion..................................................................................... 53 V IONIC LIQUIDS IN CONFINMENT ............................................................55 5.1 Introduction ....................................................................................... 55 5.2 Simulation Details ............................................................................. 58 v 5.3 Dynamics Properties of Ionic Liquids Confined in Silica and Carbon Mesopores ................................................................................................. 60 5.3.1 Influence of Temperature on Diffusion .......................................... 60 5.3.2 Interfacial Microstructure ............................................................... 64 5.3.3 Solid-Liquid Interaction Potential ................................................... 67 5.3.4 Conclusion..................................................................................... 71 VI IONIC LIQUIDS AS ELECTROLYTES IN SUPERCAPACITORS ...........73 6.1 Introduction ....................................................................................... 74 6.2 Simulation Details ............................................................................. 80 6.3 Capacitive Behavior of Monocationic Ionic Liquids............................. 83 6.3.1 Conductivity and Phase Transition Temperature of Mixture.......... 83 6.3.2 EDL Microstructure and C-V curve................................................ 85 6.3.3 Temperature Effects on Capacitance ............................................ 87 6.3.4 Conclusion..................................................................................... 93 6.4 Capacitive Behavior of Dicationic Ionic Liquids .................................. 93 6.4.1 Electrical Double Layer Structure .................................................. 93 6.4.2 Capacitive Performance of [Cn(mim)2](BF4)2 ................................. 96 6.4.3. C-V curves of [C6(mim)2](Tf2N)2 versus [C6mim][Tf2N] .............. 103 6.4.4 Conclusion................................................................................. 105 6.5 Capacitive Behavior of Dicationic Ionic Liquids in Organic Solvents 107 6.5.1 Conductivity of DILs at Varying Concentrations in the Presence of Organic Solvents ......................................................................................... 107 6.5.2 Electric Double Layer Structure in the Presence of Organic Solvents ..................................................................................................................... 108 6.5.3 Influence of Organic Solvents on Capacitance............................ 114 6.5.4 Conclusion................................................................................... 120 VII CONLUSION AND FUTURE WORK.....................................................122 7.1 Conclusion ........................................................................................ 122 7.2 Recommendation for Future Work.................................................... 126 7.2.1 Spatial Heterogeneity of RTILs in Confinement .......................... 126 7.2.2 Capacitive Performance of RTILs Electrolytes with Realistic Carbon Electrode Model........................................................................................... 126 7.2.3 Charging/discharging Dynamics of RTILs .................................... 127 vi REFERENCES .............................................................................................129 vii LIST OF FIGURES Figure Page 2.1. Potential applications of ionic liquids. Reproduced from Ref. 24. 24 ...................6 2.2. Spatial heterogeneity observed in (A) snapshots of MD simulation36 and (B) structure