© 2020 Yongrui Yang ALL RIGHTS RESERVED i FLEXIBLE SUPERCAPACITORS WITH NOVEL GEL ELECTROLYTES A Thesis Presented to The Graduated Faculty of The University of Akron In Partial Fulfillment of the Requirement for the Degree Master of Science Yongrui Yang May 2020 i FLEXIBLE SUPERCAPACITORS WITH NOVEL GEL ELECTROLYTES Yongrui Yang Thesis Approved: Accepted: _______________________________ ___________________________ Advisor Department Chair Dr. Xiong Gong Dr. Mark D. Soucek _______________________________ ___________________________ Committee Member Dean of the College Dr. Kevin A. Cavicchi Dr. Ali Dhinojwala _______________________________ ___________________________ Acting Dean, Graduate School Committee Member Dr. Marnie Saunders Dr. Weinan Xu ___________________________ Date ii ABSTRACT In this thesis, we study flexible supercapacitors. Recent progress in flexible supercapacitors is reviewed in chapter I. Ionic liquid gel electrolytes used for approaching high-energy density supercapacitors are investigated in chapter II. In chapter III, we study the double-network hydrogel with redox additives as the solid- state electrolytes for flexible supercapacitors In Chapter I, after a brief introduction to the current situation of flexible supercapacitors and the mechanisms of different supercapacitors, we summarize the recent progress in both electrode materials and electrolyte materials for flexible supercapacitors, which includes carbon materials, conducting polymers, transition metal oxides/chalcogenides/nitrides, MXenes, metal-organic frameworks (MOFs), and covalent-organic frameworks (COFs) for electrode materials and the polymer-based gel electrolytes with different supporting electrolytes. In the end, we give a brief discussion on current challenges and an outlook for the future directions. In Chapter II, a novel 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4) : poly(vinyl alcohol) (PVA) : tetrabutylammonium tetrafluoroborate (TBABF4) (BMIMBF4:PVA:TBABF4) is developed for enhancing the energy density of supercapacitors. It is found that the BMIMBF4:PVA:TBABF4 gel electrolytes exhibit an ionic conductivity of 21.7 mS/cm, which is much higher than that of BMIMBF4:PVA gel electrolyte. The quasi-solid-state asymmetric supercapacitors assembled by using the BMIMBF4:PVA:TBABF4 gel as electrolyte, and MnO2 coated carbon cloth as the positive electrode and reduced graphene oxide coated on carbon cloth as the negative electrode, respectively, exhibit a wide operational window of 3V, iii an energy density of 61.2 Wh/kg at the power density of 1049 W/kg, an operational temperature ranging from 20 oC to 90 oC, and more than 80% capacitance retention after 3000 charge/discharge cycles. In Chapter III, we developed pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes for approaching high energy density and flexibility of supercapacitors. The pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes exhibit superior mechanical strength, good flexibility, and fast self-recovery performances. The symmetric supercapacitors (SSCs) constructed by using pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel as the quasi-solid-state electrolytes and carbon cloth as electrodes show an energy density of 0.07 mWh/cm3 (~34 Wh/kg) at the power density of 3.8 mW/cm3 (~1800 W/kg), and more than 95% capacitance retention after 3000 charge/discharge cycles. iv ACKNOWLEDGEMENT First of all, I am grateful to my advisor Prof. Xiong Gong for his kindness and continuous support. His unselfish and wise suggestions teach me not only how to do research and study but also to be a good person. Then I would like to show my extreme gratitude to my beloved parents for their love, support, and enouncement all the time. Then I would like to give my appreciation to my group mates: Mr. Zhihao Ma, Mr. Luyao Zheng, Mr. Tao Zhu, Mr. Xiuyuan Zhang, Mr. Baosen Zhang, Mr. Suyuan Zhou Mr. Cheng Chi, and Mr. Sian Xiao, especially for Mr. Zhihao Ma, Mr. Luyao Zheng, and Mr. Tao Zhu for their selfless help and encouragement. Finally, I want to express my gratitude to all the faculty members and my classmates in the Department of Polymer Engineering for their sincere guidance and suggestions. v TABLE OF CONTENTS LIST OF TABLES…………………………………………………………………….ix LIST OF FIGURES……………………………………………………………………x LIST OF SCHEMES………………………………………………………………...xvi CHAPTER I. RECENT PROGRESS IN SOLID-STATE FLEXIBLE SUPERCAPACITORS 1.1 Introduction ......................................................................................................... 1 1.2 Mechanisms of supercapacitors ............................................................................ 3 1.2.1 Electric double-layer capacitors .................................................................... 3 1.2.2 Pseudocapacitors ........................................................................................... 4 1.2.3 Hybrid-capacitors .......................................................................................... 5 1.2.4 Device performance parameters used for evaluation of supercapacitors ......... 6 1.3 Materials for high-performance flexible electrodes .............................................. 9 1.3.1 Carbon materials ......................................................................................... 10 1.3.2 Conducting polymers................................................................................... 13 1.3.3 Metal oxides/chalcogenides/nitrides ............................................................ 17 1.3.4 MXenes ....................................................................................................... 24 1.3.5 Metal-organic frameworks (MOFs) ............................................................. 28 1.3.6 Covalent-organic frameworks (COFs) ......................................................... 30 vi 1.4 Polymer based solid-state electrolyte materials for flexible supercapacitors ....... 32 1.4.1 Aqueous gel electrolytes .............................................................................. 32 1.4.2 Organic gel electrolytes ............................................................................... 36 1.4.3 Ionic liquid gel electrolytes.......................................................................... 39 1.5 Summary and Outlook ....................................................................................... 43 II. QUASI-SOLID-STATE ASYMMETRIC SUPERCAPACITORS WITH NOVEL IONIC LIQUID GEL ELECTROLYTES 2.1 Introduction ....................................................................................................... 45 2.2 Experimental section .......................................................................................... 46 2.2.1 Materials ..................................................................................................... 46 2.2.2 Synthesis of GO .......................................................................................... 47 2.2.3 Synthesis of TBABF4:PVA:BMIMBF4 gels ............................................... 47 2.2.4 Preparation of the electrodes ........................................................................ 48 2.2.5 Fabrication of ASCs .................................................................................... 48 2.2.6 Characterization of ASCs ............................................................................ 48 2.3 Results and discussion ....................................................................................... 49 2.4 Conclusions ....................................................................................................... 55 III. FLEXIBLE SUPERCAPACITORS WITH DOUBLE-NETWORK HYDROGEL REDOX-ELECTROLYTES vii 3.1 Introduction ....................................................................................................... 57 3.2 Experimental section .......................................................................................... 59 3.2.1 Materials ..................................................................................................... 59 3.2.2 Preparation of the pHEAA-gelatin-chitosan Na2MoO4 DN hydrogel electrolytes ........................................................................................................... 59 3.2.3 Tensile measurement ................................................................................... 60 3.2.4 Hysteresis measurement .............................................................................. 60 3.2.5 SEM-EDX measurement ............................................................................. 60 3.2.6 FTIR-ATR Spectroscopy ............................................................................. 60 3.2.7 Preparation and electrochemical characterization of SSCs ........................... 60 3.3 Results and Discussion ...................................................................................... 62 3.4 Conclusion ......................................................................................................... 74 REFERENCES ........................................................................................................ 76 APPENDICES ......................................................................................................... 81 viii LIST OF TABLES Table 1.1 Recent progress in CPs based supercapacitors…….……………………….17 Table 1.2 Recent progress in supercapacitors with metal oxides/chalcogenides/nitrides electrodes……………………………………………………………………………..23 Table 1.3 Recent progress in supercapacitors with MXene electrodes……………..…27