University of South Carolina Scholar Commons Theses and Dissertations 2016 Electrochemical Reduction Of Carbon Dioxide On Carbon Nanostructures: Defect Structures & Electrocatalytic Activity Pranav Parag Sharma University of South Carolina Follow this and additional works at: https://scholarcommons.sc.edu/etd Part of the Chemical Engineering Commons Recommended Citation Sharma, P. P.(2016). Electrochemical Reduction Of Carbon Dioxide On Carbon Nanostructures: Defect Structures & Electrocatalytic Activity. (Doctoral dissertation). Retrieved from https://scholarcommons.sc.edu/etd/3785 This Open Access Dissertation is brought to you by Scholar Commons. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. ELECTROCHEMICAL REDUCTION OF CARBON DIOXIDE ON CARBON NANOSTRUCTURES: DEFECT STRUCTURES & ELECTROCATALYTIC ACTIVITY by Pranav Parag Sharma Bachelor of Technology National Institute of Technology, Warangal, 2007 Master of Science Clarkson University, 2010 Submitted in Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy in Chemical Engineering College of Engineering & Computing University of South Carolina 2016 Accepted by: Xiao-Dong Zhou, Major Professor Bihter Padak, Committee Member John R. Regalbuto, Committee Member John W. Weidner, Committee Member Guiren Wang, Committee Member Lacy Ford, Senior Vice Provost and Dean of Graduate Studies © Copyright by Pranav Parag Sharma, 2016 All Rights Reserved. ii DEDICATION To my parents and my wife, Udita iii ACKNOWLEDGEMENTS I would like to express my gratitude towards my advisor Dr. Xiao-Dong Zhou, for his continuous guidance, support and encouragement during this doctoral work. He always encouraged me to think deeply and this has helped me improve as a researcher. I also thank my thesis committee members, Dr. Bihter Padak, Dr. John R. Regalbuto and Dr. Guiren Wang for their time and suggestions. I would like to thank Dr. Jingjie Wu at Rice University and Dr. Fu-Sheng Ke at Wuhan University for helping me throughout my doctoral study with valuable discussions. I am grateful to Dr. Perry J. Pellechia at the Department of Chemistry for helping me with nuclear magnetic resonance spectroscopy measurements. I would also like to acknowledge the help provided by my colleagues at Rice University, especially, Dr. Ram Yadav for synthesizing the N-doped carbon nanostructures and Mingjie Liu with density functional theory calculations. Finally, I thank all my friends and family members, especially, my wife Udita, for the support, care and encouragement. iv ABSTRACT The advantages of the electrochemical conversion of carbon dioxide to fuels using renewable energy sources are two-fold: (1) it has the potential to accomplish a carbon- neutral energy cycle and (2) it can provide an approach to tackle the environmental challenges caused by anthropogenic carbon dioxide emissions. Although thermodynamically possible, the kinetics of carbon dioxide reduction to fuels remains challenging and therefore, an efficient and robust electrocatalyst is needed to promote the reaction. The ideal catalyst for the electrochemical CO2 reduction must be capable of mediating multiple proton-coupled electron transfer reactions at low overpotentials, suppressing the concurrent hydrogen evolution reaction, converting CO2 to desired chemicals with high selectivity, and achieving long-term stability. Extensive research has been carried out on metallic electrocatalysts during the past three decades; however, none of these materials are simultaneously efficient and stable for practical purposes. This Ph.D. dissertation focuses on the investigation of the electro-reduction of CO2 on carbon nanostructures with a focus on understanding the relationship between defect structures and electrocatalytic activity. The initial focus of this work was to accomplish active performance and durability for electrosynthesis of fuels from CO2 using cost-effective catalysts. N-doped carbon nanotubes (NCNTs) were demonstrated as highly efficient, selective and more importantly, stable catalysts to achieve CO2 conversion to CO. The catalytic activity of v these NCNTs was further benchmarked against other metallic catalysts reported in literature (Chapter 2). Compared to noble metals Ag & Au, these NCNTs exhibited a lower overpotential to achieve similar selectivity towards CO formation. The second part of this work was a study of the dependence of catalytic activity, i.e., the overpotential and selectivity for CO formation on the defect structures (pyridinic, graphitic, pyrrolic-N) inside NCNTs. The presence of both pyridinic and graphitic-N was found to significantly decrease the absolute overpotential and increase the selectivity towards CO formation (Chapter 3). The third part of this thesis work was to investigate CO2 reduction on N- doped graphene, in order to explore morphology effects on catalytic activity of NCNTs towards CO formation (Chapter 4). Overall, pyridinic-N defects exhibited the highest catalytic activity; thereby suggesting the directions for developing carbon nanostructures as metal-free electrocatalysts for CO2 reduction. vi TABLE OF CONTENTS DEDICATION ....................................................................................................................... iii ACKNOWLEDGEMENTS ........................................................................................................ iv ABSTRACT ............................................................................................................................v LIST OF TABLES ....................................................................................................................x LIST OF FIGURES ................................................................................................................. xi LIST OF SYMBOLS ............................................................................................................. xvi LIST OF ABBREVIATIONS .................................................................................................. xvii Chapter 1: Introduction and Thesis Objectives....................................................................1 1.1 Rising CO2 Levels ................................................................................................1 1.2 Electrochemical Reduction of CO2 ......................................................................2 1.3 Thermodynamic Aspects: CO2 Electro-reduction ................................................2 1.4 Kinetic Aspects: CO2 Electro-reduction ...............................................................4 1.5 Thesis Objectives ..................................................................................................7 References ..................................................................................................................9 Chapter 2: Achieving Highly Efficient, Selective and Stable CO2 Reduction on Nitrogen- doped Carbon Nanotubes ...................................................................................................14 2.1 Introduction ........................................................................................................14 2.2 Experimental Methods ........................................................................................15 2.3 Results & Discussion ..........................................................................................18 2.4 Conclusions ........................................................................................................35 vii References .................................................................................................................36 Chapter 3: Nitrogen Doped Carbon Nanotube Arrays for High Efficiency Electrochemical Reduction of CO2: On the Understanding of Defects, Defect Density, and Selectivity ....................................................................................................................39 3.1 Introduction ........................................................................................................39 3.2 Experimental Methods ........................................................................................40 3.3 Results & Discussion ..........................................................................................43 3.4 Conclusions ........................................................................................................56 References .................................................................................................................57 Chapter 4: Incorporation of Nitrogen Defects for Efficient Reduction of CO2 via Two- Electron Pathway on Three-Dimensional Graphene Foam ...............................................61 4.1 Introduction ........................................................................................................61 4.2 Experimental Methods ........................................................................................63 4.3 Results & Discussion ..........................................................................................65 4.4 Conclusions ........................................................................................................76 References .................................................................................................................77 Chapter 5: Electrocatalytic Conversion of Carbon Dioxide to Fuels: A Review on the