Polymer–Ionic liquid Electrolytes for Electrochemical ... · PDF filePolymer–Ionic liquid Electrolytes for Electrochemical Capacitors ... 8 6 5.1 PEO–EMIHSO4 ... 7.1 Proton Activity

Polymer–Ionic liquid Electrolytes for Electrochemical Capacitors

by

Sanaz Ketabi

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Graduate Department of Materials Science and Engineering University of Toronto

© Copyright by Sanaz Ketabi, 2015

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ABSTRACT

Polymer–Ionic liquid Electrolytes for Electrochemical Capacitors

Sanaz Ketabi Doctor of Philosophy

Graduate Department of Materials Science and Engineering University of Toronto

2015

Polymer electrolyte, comprised of ionic conductors, polymer matrix, and additives, is one of

the key components that control the performance of solid flexible electrochemical capacitors

(ECs). Ionic liquids (ILs) are highly promising ionic conductors for next generation polymer

electrolytes due to their excellent electrochemical and thermal stability. Fluorinated ILs are

the most commonly applied in polymer–IL electrolytes. Although possessing high

conductivity, these ILs have low environmental favorability. The aim of this work was to

develop environmentally benign polymer–ILs for both electrochemical double-layer

capacitors (EDLCs) and pseudocapacitors, and to provide insights into the influence of

constituent materials on the ion conduction mechanism and the structural stability of the

polymer–IL electrolytes.

Solid polymer electrolytes composed of poly(ethylene oxide) (PEO) and 1‐ethyl‐3‐

methylimidazolium hydrogen sulfate (EMIHSO4) were investigated for ECs. The material

system was optimized to achieve the two criteria for high performance polymer–ILs: high

ionic conductivity and highly amorphous structure. Thermal and structural analyses revealed

that EMIHSO4 acted as an ionic conductor and a plasticizer that substantially decreased the

crystallinity of PEO.

Two types of inorganic nanofillers were incorporated into these polymer electrolytes. The

effects of SiO2 and TiO2 nanofillers on ionic conductivity, crystallinity, and dielectric

properties of PEO–EMIHSO4 were studied over a temperature range from −10 °C and 80 °C.

Using an electrochemical capacitor model, impedance (complex capacitance) and dielectric

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analyses were performed to understand the ionic conduction process with and without fillers

in both semi-crystalline and amorphous states of the polymer electrolytes. Despite their

different nanostructures, both SiO2 and TiO2 promoted an amorphous structure in PEO–

EMIHSO4 and increased the ionic conductivity 2-fold. While in the amorphous state, the

dielectric constant characteristic of the fillers contributed to the increased conductivity and

cell capacitance. Leveraging the fillers, the ionic conductivity of the environmentally friendly

polymer–ILs approached the level of the polymer–fluorinated IL at room temperature, and

exceeded the latter at high temperature.

Another approach to improve the performance of polymer electrolytes was undertaken

through the development of protic ILs (PILs) and polymer–PIL electrolytes for

pseudocapacitors. Binary eutectic systems of PILs were investigated, and the proton

conduction of the eutectic systems was characterized in both liquid and polymer states.

Devices enabled by PEO–EMIHSO4 and PEO–binary PILs demonstrated a comparable

energy density to that with polymer–fluorinated ILs.

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ACKNOWLEDGEMENTS

I wish to express my sincere gratitude to my supervisor Professor Keryn Lian. Thank you

for your support and guidance during these few years. I especially thank you for taking the

time to discuss the many aspects of this project and for being more than a supervisor.

I would like to thank the members of my advisory committee Professors C. Jia, H.

Naguib, and E. Sone for taking the time to meet with me and give insightful suggestions. I

would also like to thank Professor S. Thorpe for his advice and for attending my final exam. I

am also thankful to Professors M. Barati and Z. Chen for accepting to be on my final exam

committee.

My appreciation goes to Dr. D. Grozea for providing the access to DSC and IR at any

time and J. McDowell for help with the synthesis and helpful discussions. I would like to

thank the assistance of undergraduate students B. Decker, X. Liu, and Z. Le. I also thank the

former and present members of the Flexible Energy and Electronics Laboratory for their

support, especially those who I spend more time with them lately: H. Gao, M. Genovese, G.

Wu, and J. Li.

Financial assistance is also acknowledged and appreciated from: the Natural Science and

Engineering Research Council of Canada (NSERC CREATE) and the Ontario Research

Fund (ORF). The perfect administrative assistance from M. Fryman, J. Prentice, and F.

Strumas-Manousos let me complete my research.

Thanks also to B. Ting and J. Hsu for their help and accompany during experiments. A

special thank you to those who helped in so many ways with love and words of

encouragement, especially: Dad, Mom, and my sister and brother. To Mehran, thank you for

always being there for me, for making me realize things I wouldn’t have, and for bringing joy

into my life in Toronto.

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To Mom, Dad, and my grandmothers

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CONTENTS

ABSTRACT ......................................................................................................................................................... ii

ACKNOWLEDGEMENTS ................................................................................................................................iv

CONTENTS .........................................................................................................................................................vi

LIST OF TABLES ...............................................................................................................................................ix

LIST OF FIGURES ........................................................................................................................................... xii

NOMENCLATURE ....................................................................................................................................... xviii

INTRODUCTION ................................................................................................................................................ 1

1.1 Objectives ................................................................................................................................................... 4

1.2 Thesis Overview ......................................................................................................................................... 4

BACKGROUND ................................................................................................................................................... 9

2.1 Fundamentals of Electrochemical Capacitors ............................................................................................. 9

2.1.1 Types of electrochemical capacitors .................................................................................................. 9

2.1.2 Advances in electrochemical capacitors ........................................................................................... 11

2.2 Types of Liquid Electrolytes ..................................................................................................................... 13

2.3 Ionic Liquids ............................................................................................................................................. 15

2.3.1 Classes of ionic liquids ..................................................................................................................... 17

2.3.2 Properties of ionic liquids ................................................................................................................. 19

2.3.3 Ionic conductivity and conduction mechanism ................................................................................ 23

2.3.4 Application in ECs ........................................................................................................................... 26

2.4 Polymer Electrolytes ................................................................................................................................. 28

2.4.1 Classification of polymer electrolytes .............................................................................................. 29

2.4.2 IL-based polymer electrolytes .......................................................................................................... 30

2.4.3 Polymer network .............................................................................................................................. 31

2.4.4 Conduction mechanism in polymer electrolytes .............................................................................. 34

2.4.5 Effect of nanofillers on PEO-based electrolytes ............................................................................... 36

2.4.6 Polymer–IL electrolytes for ECs ...................................................................................................... 38

2.4.6.1 Application in EDLCs ......................................................................................................... 38

2.4.6.2 Polymer–PILs for pseudocapacitors .................................................................................... 39

2.5 Gap Analysis and Selection of Materials .................................................................................................. 42

2.6 Characterization Techniques ..................................................................................................................... 46

2.6.1 Electrochemical characterization ...................................................................................................... 46

2.6.1.1 Cyclic voltammetry (CV) .................................................................................................... 46

2.6.1.2 Electrochemical impedance spectroscopy (EIS) ................................................................. 48

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2.6.2 Structural characterization ................................................................................................................ 56

2.6.2.1 X-ray diffraction .................................................................................................................. 56

2.6.2.2 Differential scanning calorimetry ........................................................................................ 57

2.6.2.3 Infrared (IR) spectroscopy .................................................................................................. 59

EXPERIMENTAL METHOD AND CHARACTERIZATION ..................................................................... 61

3.1 Materials ................................................................................................................................................... 61

3.1.1 Ionic conductors ............................................................................................................................... 61

3.1.2 Polymers ........................................................................................................................................... 63

3.1.3 Additives .......................................................................................................................................... 63

3.2 Polymer Electrolytes Fabrication .............................................................................................................. 63

3.2.1 Preparation of PEO–EMIHSO4 ........................................................................................................ 64

3.2.2 Preparation of PVdF-HFP–EMIBF4 ................................................................................................. 65

3.2.3 Preparation of polymer–IL with filler .............................................................................................. 65

3.3 Device Fabrication .................................................................................................................................... 66

3.3.1 Electrodes ......................................................................................................................................... 66

3.3.2 Liquid cells ....................................................................................................................................... 67

3.3.3 Solid cells ......................................................................................................................................... 67

3.4 Characterization ........................................................................................................................................ 68

3.4.1 Structural characterization ................................................................................................................ 68

3.4.2 Electrochemical characterization ...................................................................................................... 69

IONIC LIQUID ELECTROLYTES ................................................................................................................. 72

4.1 Effect of Anion ......................................................................................................................................... 72

4.1.1 Ionic conductivity ............................................................................................................................. 73

4.1.2 Potential window .............................................................................................................................. 74

4.1.3 Electrode capacitance and device performance ................................................................................ 75

4.2 Effect of Cation ......................................................................................................................................... 79

4.2.1 Ionic conductivity of IL solutions .................................................................................................... 80

4.2.2 Device performance using IL solutions ............................................................................................ 82

4.3 Summary ................................................................................................................................................... 84

POLYMER–IONIC LIQUID ELECTROLYTES ........................................................................................... 86

5.1 PEO–EMIHSO4 and PVdF-HFP–EMIBF4 Electrolytes ........................................................................... 86

5.1.1 Ionic conductivity ............................................................................................................................. 87

5.1.2 Crystallinity and thermal characterizations ...................................................................................... 90

5.1.2.1 XRD analyses ...................................................................................................................... 91

5.1.2.2 DSC analyses ...................................................................................................................... 94

5.2 Interaction Between Polymer and IL ........................................................................................................ 98

5.2.1 Effect of crystallinity ........................................................................................................................ 99

5.2.2 Effect of interaction between PEO and HSO4− ............................................................................... 100

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5.3 Device Performance ................................................................................................................................ 103

5.4 Summary ................................................................................................................................................. 108

POLYMER–IONIC LIQUID ELECTROLYTES WITH FILLERS........................................................... 110

6.1 Effect of Fillers on Ionic Conductivity ................................................................................................... 110

6.2 Effect of Fillers on Crystallinity ............................................................................................................. 117

6.2.1 XRD analyses ................................................................................................................................. 118

6.2.2 DSC analyses.................................................................................................................................. 121

6.3 Effect of Fillers on Interaction Between PEO and EMIHSO4 ................................................................ 125

6.4 Impedance and Dielectric Analyses ........................................................................................................ 128

6.4.1 Complex capacitance and dielectric analyses ................................................................................. 128

6.4.2 Capacitance and dielectric response of polymer electrolytes ......................................................... 129

6.4.3 Effect of fillers ............................................................................................................................... 132

6.5 Effect of Fillers on Device Performance ................................................................................................. 137

6.6 Summary ................................................................................................................................................. 143

PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE ................................................................ 145

7.1 Proton Activity and Melting point .......................................................................................................... 145

7.2 Proton Conductivity of PIL Solutions ..................................................................................................... 148

7.2.1 Performance of RuO2 with PIL electrolytes ................................................................................... 148

7.2.2 Performance of carbon/POM in PIL electrolytes ........................................................................... 151

7.2.2.1 Electrode performance in PIL electrolytes ........................................................................ 152

7.2.2.2 Device performance in PIL electrolytes ............................................................................ 154

7.3 Binary Mixtures of PILs ......................................................................................................................... 156

7.3.1 MIHSO4-ImHSO4 binary system ......................................................................................... 158

7.3.2 EMIHSO4-ImHSO4 binary system ....................................................................................... 160

7.3.3 Performance of RuO2 in eutectic PILs ................................................................................. 162

7.4 Performance of Solid RuO2 Cell with Polymer–eutectic PILs ................................................................ 164

7.5 Summary ................................................................................................................................................. 167

CONCLUSIONS AND FUTURE WORK ...................................................................................................... 169

8.1 Contributions .......................................................................................................................................... 169

8.2 Conclusions ............................................................................................................................................. 170

8.3 Future Work ............................................................................................................................................ 173

LIST OF REFERENCES ................................................................................................................................. 176

APPENDIX A: PIL ELECTROLYTES AND POLYMER–IL SYSTEMS ................................................. 189

APPENDIX B: XRD, DSC, AND DIELECTRIC ANALYSES .................................................................... 193

APPENDIX C: MATERIALS WEIGHT DISTRIBUTION ......................................................................... 197

APPENDIX D: REPRODUCIBILITY OF CV MEASUREMENTS ........................................................... 198

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LIST OF TABLES

Table 2-1 Comparison between the characteristic properties of liquid and polymer electrolytes and the enabled devices....................................................................................... 13

Table 2-2 Comparison of properties of aqueous, organic, and ionic liquid electrolytes for ECs......................................................................................................................................... 15

Table 2-3 Some basic properties of ionic liquids................................................................. 17

Table 2-4 Some common polymer hosts with their corresponding chemical formula and thermal properties [65,66]................................................................................................. 32

Table 2-5 Polymer electrolytes used for activated carbon EDLCs and their electrochemical properties........................................................................................................ 41

Table 2-6 Cost of the materials for cells fabricated with the polymer–IL electrolytes (1 cm2 laminated pouch-type cells)......................................................................................... 45

Table 3-1 Structure of the studied ILs................................................................................... 62

Table 3-2 Properties of the polymer matrices....................................................................... 63

Table 3-3 Properties of the SiO2 and TiO2 fillers [117,118].............................................. 63

Table 3-4 Material components of the polymer electrolytes.............................................. 65

Table 3-5 Parameters of interest and the relationship between capacitor performance properties and the related electrolyte/polymer electrolyte properties................................ 71

Table 4-1 Conductivity, potential window, and viscosity of studied electrolytes (at room temperature)..................................................................................................................... 73

Table 4-2 Structure and melting temperature of the ILs with different cations............... 79

Table 5-1 Conductivity and activation energy of ionic conduction for ILs and polymer–ILs (viscosity of pure ILs is also listed)................................................................. 88

Table 5-2 Melting and recrystallization temperatures, and degree of crystallinity of PEO powder, PEO film, and the polymer electrolytes......................................................... 96

Table 5-3 Melting and recrystallization temperatures, and degree of crystallinity of PVdF-HFP film and PVdF-HFP–EMIBF4............................................................................. 97

Table 5-4 FTIR band positions and associated bonding modes for PEO–EMIHSO4 in (1:2) composition and its components.................................................................................... 102

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Table 6-1 Room temperature ionic conductivity of PEO–EMIHSO4 and PEO–EMIHSO4 electrolytes containing SiO2 and TiO2 nanofillers, and activation energy (Ea) of ionic conduction for the respective electrolytes at low and high temperatures.... 113

Table 6-2 Room temperature ionic conductivity of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, PVdF-HFP–EMIBF4−3% TiO2, and the activation energy (Ea) of ionic conduction for the respective electrolytes........................................... 114

Table 6-3 Melting temperature (Tm), recrystallization temperature (Trc), and crystallinity (Xc) of PEO film, PEO–EMIHSO4, and PEO–EMIHSO4–filler electrolytes.................................................................................................................................. 123

Table 7-1 Structure and melting temperature of PILs with different cations................... 147

Table 7-2 ESR and capacitance of RuO2 cells using PIL electrolytes and H2SO4.......... 150

Table 7-3 Proton concentration of PIL/MeOH electrolytes obtained from titration with 0.1 M NaOH...................................................................................................................... 150

Table 7-4 Capacitance of carbon/PMo12 cells in aqueous and the corresponding cells in PIL/solvent electrolytes at 100 mV s-1............................................................................... 156

Table 7-5 Thermal properties of MIHSO4-ImHSO4 binary system at different compositions............................................................................................................................... 159

Table 7-6 Thermal properties of EMIHSO4-ImHSO4 binary system at different compositions............................................................................................................................... 161

Table 7-7 Conductivity of pure EMIHSO4, eutectic EMIHSO4-ImHSO4, and eutectic MIHSO4-ImHSO4 and the capacitance of RuO2 cells enabled with respective PILs.............................................................................................................................................. 163

Table 7-8 Capacitance of RuO2 cells enabled with PVdF-HFP–EMIBF4, PEO–EMIHSO4-ImHSO4 (eutectic 70:30), and PEO–MIHSO4-ImHSO4 (eutectic 70:30)........................................................................................................................................... 166

Table A-1 PILs reported in the literature, and their conductivity, viscosity, and electrochemical window (using different electrodes) [34,43,115,152,153]...................... 189

Table A-2 Polymer–IL systems developed by polymerization in ILs, and their ionic conductivity and potential window [65,123,154-156].......................................................... 191

Table A-3 Polymer-IL systems developed by the incorporation of ILs into the matrix [85,96,138,139,157-160].......................................................................................................... 192

Table B-1 Intensity of the crystalline peaks of all samples and the ratio of crystalline peaks with respect to the amorphous baseline....................................................................... 193

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Table B-2 Melting temperature (Tm), recrystallization temperature (Trc), and crystallinity (Xc) of PVdF-HFP film, PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–SiO2, and PVdF-HFP–EMIBF4–3% TiO2 electrolytes......................................................... 194

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LIST OF FIGURES

Figure 1-1 Overview of the characteristics of ILs (top) and polymer–IL electrolytes (bottom) and the approaches undertaken to improve the respective properties..................................................................................................................................... 6

Figure 1-2 The components for developing electrochemical capacitors........................... 7

Figure 2-1 (left) a spiral configuration of ECs utilizing liquid electrolytes, and (right) a flexible and multi-stacking design of ECs enabled with polymer electrolytes.............. 12

Figure 2-2 Generic structures of common cations and anions for ionic liquids.............. 16

Figure 2-3 Synthesis of [EMI][TFSI].................................................................................... 18

Figure 2-4 Synthesis of [α-Pic][TFA].................................................................................... 18

Figure 2-5 Variation in melting point with alkyl chain length for ionic liquids containing 1-alkyl-3-methylimidazolium cations and different anions [30]..................... 20

Figure 2-6 Oxidation and reduction of [Pyr][HSO4] (structure 11)................................... 22

Figure 2-7 Classification Walden plot constructed from the literature data of some ILs................................................................................................................................................ 25

Figure 2-8 Schematic representation of proton transfer via (a) Grotthuss mechanism of dissociated imidazole or H3PO4 and (b) vehicular mechanism of imidazolium [1].... 26

Figure 2-9 Representation of ionic motion in a PEO-based polymer electrolyte (a) assisted by polymer chain motion for dissociated ions; (b) taking account of ion associated species [61].............................................................................................................. 35

Figure 2-10 (a) Cyclic voltammetry sweep, cyclic voltammogram profiles for (b) ideal and resistive double-layer capacitance, and (c) pseudocapacitance.......................... 47

Figure 2-11 Phasor diagram showing the relationship between alternating current and voltage signals at angular frequency ω [104]......................................................................... 49

Figure 2-12 (a) Equivalent circuit of an RC system (an ideal capacitor), (b) Nyquist plot, and (c) Bode plot for the series RC system................................................................... 49

Figure 2-13 Schematic of the real (solid line) and imaginary (dash dotted line) parts of the capacitance....................................................................................................................... 52

Figure 2-14 Schematic of the real (solid line) and imaginary (dash dotted line) parts of the complex dielectric function for a relaxation process and electrode polarization [108]............................................................................................................................................. 54

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Figure 2-15 Schematic illustration of XRD pattern of a semi-crystalline polymer displaying the broad amorphous peaks and the crystalline diffraction peaks................... 57

Figure 2-16 Schematic illustration of heating and cooling DSC thermograms including the thermal transitions, heat of crystallization, ΔHc, and heat of melting, ΔHm.............................................................................................................................................. 58

Figure 2-17 IR spectra (transmittance) of polyethylene displaying the main CH2 vibrations..................................................................................................................................... 60

Figure 3-1 Preparation of imidazolium hydrogen sulfate (ImHSO4) ionic liquid........... 62

Figure 3-2 Preparation steps of polymer–IL electrolytes.................................................... 64

Figure 3-3 Filler-free PEO–EMIHSO4, and PEO–EMIHSO4 containing SiO2 and TiO2 nanofillers.......................................................................................................................... 66

Figure 3-4 Schematic representation of device configuration for the (a) Liquid–1, and (b) Liquid–2 cells....................................................................................................................... 67

Figure 3-5 (a) Schematic representation of device configuration for the solid cells; (b) the resulting laminated cells............................................................................................... 67

Figure 4-1 Structure of EMIHSO4 and EMIBF4.................................................................. 73

Figure 4-2 Conductivity as a function of EMIHSO4 concentration in PC........................ 74

Figure 4-3 Voltammetric potential window recorded at a glassy carbon electrode at a sweep rate of 100 mV s-1 (due to the high viscosity of EMIHSO4, measurements were performed at a low sweep rate: 5 mV s-1)............................................................................... 75

Figure 4-4 Cyclic voltammograms of graphite cells tested with EMIHSO4 and EMIBF4 at a sweep rate of 100 mV s-1 (Liquid–1 beaker cells)......................................... 76

Figure 4-5 Real C′ and imaginary C″ part of the capacitance vs. frequency for graphite EDLCs with EMIHSO4 and EMIBF4 (Liquid–2 filter paper cells)..................... 77

Figure 4-6 Double-layer capacitance of glassy carbon electrode and conductivity as a function of EMIHSO4 concentration in PC............................................................................ 78

Figure 4-7 Conductivity of solutions of EMIHSO4 ( ), MIHSO4 ( ), and ImHSO4 ( ) in methanol (filled symbols) and acetic acid (empty symbols)........................................... 81

Figure 4-8 Cyclic voltammograms of graphite cells using EMIHSO4/PC, EMIHSO4/MeOH, MIHSO4/MeOH, ImHSO4/MeOH electrolytes at (a) 100 mV s-1 and (b) 1 V s-1............................................................................................................................. 83

Figure 5-1 Temperature dependence of the ionic conductivity of EMIHSO4 and PEO–EMIHSO4 in (1:2), (1:3), and (1:4) compositions...................................................... 88

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Figure 5-2 Temperature dependence of the ionic conductivity of PEO–EMIHSO4 and PVdF-HFP–EMIBF4................................................................................................................. 90

Figure 5-3 XRD patterns of (a) PEO powder, PEO film, and PEO–EMIHSO4 (1:2) electrolyte; (b) PEO–EMIHSO4 electrolytes in (1:1), (1:2), and (1:3) compositions...... 92

Figure 5-4 XRD patterns of PVdF-HFP powder, PVdF-HFP film, and PVdF-HFP–EMIBF4 electrolyte.................................................................................................................... 94

Figure 5-5 Heating and cooling DSC thermograms of PEO film, PEO–EMIHSO4 electrolytes in (1:2) and (1:3) compositions........................................................................... 95

Figure 5-6 Heating and cooling DSC thermograms for PVdF-HFP film and PVdF-HFP–EMIBF4 electrolyte.............................................................................................. 97

Figure 5-7 FTIR spectra of pure PEO film, pure EMIHSO4, and PEO–EMIHSO4 in (1:2) composition....................................................................................................................... 99

Figure 5-8 FTIR spectra of PEO–EMIHSO4 electrolytes in (1:1), (1:2), and (1:3) compositions in the range of 2000–650 cm-1......................................................................... 101

Figure 5-9 Cyclic voltammograms of graphite ECs with EMIHSO4 and PEO–EMIHSO4 electrolytes at sweep rates of (a) 100 mV s-1 and (b) 1 V s-1............................ 104

Figure 5-10 (a) Real part C′ and (b) imaginary part C″ of the capacitance and vs. frequency for graphite ECs with EMIHSO4 and PEO–EMIHSO4 electrolytes................ 106

Figure 5-11 Cyclic voltammograms of graphite EDLCs with PVdF-HFP–EMIBF4 and PEO–EMIHSO4 electrolytes at sweep rate of 1 V s-1.................................................... 107

Figure 6-1 Temperature dependence of the ionic conductivity of (a) PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2; and (b) PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2.............................................. 112

Figure 6-2 Temperature dependence of the ionic conductivity of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2............... 114

Figure 6-3 Temperature dependence of the ionic conductivity of PEO–EMIHSO4, PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2............................................ 116

Figure 6-4 The variation of ionic conductivity of PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2 over time......... 117

Figure 6-5 XRD patterns for (a) SiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2; and (b) TiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2.............................................. 119

Figure 6-6 XRD patterns of SiO2 and TiO2 nanofillers, PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2 electrolytes........... 120

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Figure 6-7 XRD patterns of PEO film, PEO–EMIHSO4, PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2.............................................................................................. 121

Figure 6-8 Heating and cooling DSC thermograms of (a) PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2; and (b) PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2........................................................ 122

Figure 6-9 DSC heating and cooling thermograms of PEO film, PEO–EMIHSO4, and PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2............................................ 124

Figure 6-10 FTIR spectra of SiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2.................................................................................... 126

Figure 6-11 FTIR spectra of PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2................................................................................................................ 127

Figure 6-12 Variation of (a) real part C′ and (b) imaginary part C″ of capacitance with respect to frequency for cells leveraging PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 at 30 °C.............................................................................. 130

Figure 6-13 Dielectric derivative vs. frequency for PEO film, PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 at 30 °C......................................................... 131

Figure 6-14 Dielectric derivative vs. frequency for (a) PEO–EMIHSO4, (b) PEO–EMIHSO4–SiO2, and (c) PEO–EMIHSO4–TiO2 at different temperatures: −10 °C ( ), 0 °C ( ), 10 °C ( ), 20 °C ( ), 30 °C ( ), 40 °C ( ), 50 °C ( ), 60 °C ( ), 70 °C ( ), 80 °C ( ).......................................................................................................................................... 133

Figure 6-15 Capacitance of cells leveraging PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2, respectively, at different temperatures (at 0.1 Hz)…........... 134

Figure 6-16 Electrode polarization time constant EP (empty symbols) and relaxation time constant R (filled symbols) for PEO–EMIHSO4 ( ), PEO–EMIHSO4–SiO2 ( ), and PEO–EMIHSO4–TiO2 ( )................................................................................................. 135

Figure 6-17 (a) Cyclic voltammograms of graphite EDLC devices with PEO–EMIHSO4, PEO–EMIHSO4–SiO2 at 1 V s-1; (b) variation of real part C′ and imaginary part C″ of capacitance with respect to frequency for the respective cells....... 139

Figure 6-18 (a) Cyclic voltammograms of graphite EDLC devices with PEO–EMIHSO4 and PEO–EMIHSO4–TiO2 at 1 V s-1; and (b) variation of real part C′ and imaginary part C″ of capacitance with respect to frequency for the respective cells....... 140

Figure 6-19 Cycle life test of graphite EDLC device with PEO–EMIHSO4–10% SiO2 electrolyte at 1 V s-1................................................................................................................... 141

Figure 6-20 Comparison between the ionic conductivity as a function of temperature of the starting PEO–EMIHSO4, the optimized PEO–EMIHSO4–SiO2 and PEO–EMIHSO4–TiO2, and PVdF-HFP–EMIBF4........................................................................... 142

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Figure 7-1 Cyclic voltammograms of RuO2 cells in ImHSO4/MeOH, MIHSO4/MeOH, EMIHSO4/MeOH, and EMIHSO4/PC electrolytes at (a) 100 mV s-1 and (b) 1 V s-1 (Liquid–2 configuration)................................................................................ 149

Figure 7-2 Cyclic voltammograms of bare carbon (dashed line) and carbon/PMo12 (solid line) electrodes in 0.5M H2SO4 at 100 mVs-1............................................................. 152

Figure 7-3 Cyclic voltammograms of bare carbon and carbon/PMo12 electrodes in (a) EMIHSO4/MeOH, (b) MIHSO4/MeOH, (c) ImHSO4/MeOH electrolytes, and (d) comparison of cyclic voltammograms of carbon/PMo12 electrodes in the three PIL electrolytes (sweep rate: 100 mV s-1)...................................................................................... 153

Figure 7-4 Cyclic voltammograms of (a) bare carbon cells and (b) carbon/PMo12 cells in EMIHSO4/MeOH, MIHSO4/MeOH, ImHSO4/MeOH, and EMIHSO4/PC electrolytes at 1 V s-1 (Liquid–2 configuration).................................................................... 155

Figure 7-5 DSC thermograms of pure EMIHSO4, MIHSO4, and ImHSO4 at heating and cooling scans of 10 °C min-1............................................................................................. 157

Figure 7-6 DSC thermograms of various compositions of MIHSO4-ImHSO4 binary mixtures at heating and cooling scans of 10 °C min-1.......................................................... 159

Figure 7-7 Phase diagram for MIHSO4-ImHSO4 binary system: () melting point; () solid-solid transition; ( ) glass transition......................................................................... 160

Figure 7-8 Phase diagram for EMIHSO4-ImHSO4 binary system: () melting point; () solid-solid transition; ( ) glass transition......................................................................... 162

Figure 7-9 Cyclic voltammograms of RuO2 cells using pure EMIHSO4, eutectic EMIHSO4-ImHSO4 (70:30), and eutectic MIHSO4-ImHSO4 (70:30) at 5 mV s-1........... 163

Figure 7-10 Cyclic voltammograms of solid RuO2 cells enabled with PVdF-HFP–EMIBF4, PEO–EMIHSO4-ImHSO4 (eutectic 70:30), and PEO–MIHSO4-ImHSO4 (eutectic 70:30) at (a) 5 mV s-1 and (b) 50 mV s-1................................................................ 165

Figure 7-11 Comparison of the specific energy and power density (per cm3 of stack cell) of solid ECs enabled with the polymer–ILs (volumetric energy and power densities are for the stack comprising the current collectors, the active material, and the polymer electrolyte)............................................................................................................ 167

Figure B-1 Heating and cooling DSC thermograms for PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2 electrolytes........... 194

Figure B-2 Variation of (a) dielectric permittivity ′ and (b) dielectric loss ″ with respect to frequency for PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 at 30 °C.......................................................................................................... 195

Figure B-3 DSC thermograms of EMIHSO4-ImHSO4 binary mixtures at heating and cooling scans of 10 °C min-1.................................................................................................... 196

xvii

Figure C-1 Materials weight distribution for solid cells enabled by PEO–EMIHSO4 (left) and PVdF-HFP–EMIBF4 (right) (1 cm2 laminated pouch-type cells)...................... 197

Figure C-2 Weight distribution of EC modules with soft-pack assembly for hybrid electric vehicles.......................................................................................................................... 197

Figure D-1 Cyclic voltammograms of pure EMIHSO4 at different potential intervals at 5 mV s-1................................................................................................................................... 198

Figure D-2 Cyclic voltammograms of EDLCs enabled by PEO–EMIHSO4–SiO2 at 1 V s-1.............................................................................................................................................. 199

xviii

NOMENCLATURE

Acronyms

AN Acetonitrile

AIBN Azobisisobutyronitrile

BPO Benzoyl peroxide

DMF N,N-dimethyl formamide

EC Ethylene carbonate

EP Electrode polarization

ESR Equivalent series resistance

MP Methyl-2-pentanone

NMP N-methyl-2-pyrrolidone

PC Propylene carbonate

PAN Poly(acrylonitrile)

PEO Poly(ethylene oxide)

PMMA Poly(methyl methacrylate)

PVA Poly(vinyl alcohol)

PVdF Poly(vinylidene fluoride)

PVdF-HFP Poly(vinylidene fluoride-co-hexafluoropropylene)

PVP Poly(vinyl pyrrolidine)

THF Tetrahydrofuran

TMS Sulpholane

English symbols

C capacitance (F)

C′ real part of capacitance (F)

C″ imaginary part of capacitance (F)

xix

E energy density (Wh kg-1)

Ea activation energy (kJ mol-1)

Ec Coloumbic attraction (J)

f0 characteristic frequency at −45° phase angle (Hz)

P power density (kW kg-1)

Tb boiling temperature (°C)

Tc crystallization temperature (°C)

Tg glass transition temperature (°C)

Tm melting temperature (°C)

Trc recrystallization temperature (°C)

Xc degree of crystallinity (%)

Z impedance (Ω)

Z′ real part of impedance (Ω)

Z″ imaginary part of impedance (Ω)

Greek symbols

Λ molar conductivity (S cm2 mol-1)

η dynamic viscosity (cP or mPa·s)

σ ionic conductivity (S cm-1)

θ phase angle (°)

2θ diffraction angle (°)

ε dielectric constant

ε′ real part of dielectric function (dielectric permittivity)

ε″ imaginary part of dielectric function (dielectric loss)

τ0 RC time constant (s)

τEP time constant of electrode polarization (s)

τR relaxation time constant (s)

ΔHc heat of crystallization (J g-1)

xx

ΔHm heat of melting (J g-1)

ΔHrc heat of recrystallization (J g-1)

1

CHAPTER 1

INTRODUCTION

As energy becomes more valuable, storage is essential for the sustainable and reliable use of

energy. In this respect, electrochemical capacitors (ECs) provide high power and energy

densities, long cycle life, and highly reversible charge/discharge characteristics, bridging the

gap between batteries and conventional capacitors. They can be used for various high power

applications in portable electronics, electric vehicles, and hybrid systems with batteries and

intermittent generators, including photovoltaics and windmills to complement other energy

sources for peak power.

Polymer electrolytes are key enablers for solid, thin, flexible, and portable

electrochemical energy storage devices. Acting as a separator and an ionic conductor,

polymer electrolytes allow lightweight designs that are safe from the leakage of liquid.

Polymer electrolytes for high performance electrochemical devices such as ECs should

possess: (i) high ionic conductivity for power capability, (ii) wide electrochemical stability

window for high operating voltage and capacitor energy, (iii) high thermal and

environmental stability for device safety and shelf life, (iv) good electrode-electrolyte contact

for low resistance and high capacitance, and (v) low cost.

A typical polymer electrolyte consists of an ionic conductor, a polymer matrix, and

additives. One of the most important properties of high performance polymer electrolytes is

CHAPTER 1. INTRODUCTION

2

ionic conductivity. Ionic conduction depends on the dissociation of the ionic conductor and

the structural characteristics of the polymer matrix.

Currently used aqueous-based and organic-based electrolytes have limitations for high

performance ECs. Despite the high conductivity of aqueous electrolytes, their

electrochemical stability window is limited and they have low thermal stability. In

comparison, organic electrolytes offer an acceptable ionic conductivity with a wider

operating voltage. However, organic solvents are volatile and flammable, affecting the safety

of the device.

In contrast to classical electrolytes that are obtained by dissolving salts into solvents,

ionic liquids (ILs) are salts consisting of ions with a relatively low melting temperature

(˂ 100 °C). This new class of ionic conductors are composed of organic cations and organic

or inorganic anions. Good conductivity together with low volatility and a wide potential

window make ILs promising alternatives to conventional organic electrolytes [1,2]. ILs can

also be incorporated into polymer electrolytes to act as ionic conductors and plasticizers [3-

5].

The shortcoming of ILs is usually their high viscosity. The challenge is to select room

temperature ionic liquids (RTILs) that feature wide electrochemical stability windows

combined with high ionic conductivity. The majority of ILs used in ECs are fluorinated ILs

owing to their wide potential window, low viscosity, and hence high ionic conductivity.

Although studying fluorinated ILs are necessary to understand the characteristics of these

new electrolytes, their practical applications is limited from an environmental standpoint.

The higher viscosity of non-fluorinated ILs could be less problematic in thin-film polymer

electrolytes, in which the ionic conduction is different from that in liquid electrolytes. The

motivation of this thesis is to investigate non-fluorinated ILs to develop high performance

polymer electrolytes for ECs.

Polymer electrolytes consisting of poly(ethylene oxide) (PEO) and a non-fluorinated IL,

1-ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4) were developed, and their


3

suitability for electrochemical double-layer capacitors (EDLCs) were investigated. PEO has

been extensively used as a polymer matrix due to its compatibility with a wide range of ionic

conducting salts while maintaining acceptable chemical and electrochemical stability.

However, PEO-based electrolytes tend to crystallize at low temperatures. This restricts the

segmental motion of the polymer chain and thus limits ion transport. A low degree of

crystallinity that provides more flexibility to the polymer backbone is desirable to achieve

high ionic conductivity.

One way to improve the performance of PEO-based electrolytes is to disperse inorganic

fillers such as SiO2, TiO2, or Al2O3 in the electrolytes [6-8]. Inorganic fillers have been

reported to: (i) prevent the recrystallization of the polymer, and (ii) promote the ionic

mobility and ionic dissociation through Lewis acid–base interaction between filler and

polymer or filler and ionic species [6,9,10]. Dielectric constant of inorganic fillers could also

play an important role in ionic conduction and intrinsic capacitance of polymer

electrolyte-enabled cells. The effects of SiO2 and TiO2 nanofillers on the electrochemical

performance and structural stability of a PEO–EMIHSO4 electrolyte was studied. Through

complex capacitance and dielectric analyses, the ion transport mechanism in the

filler-containing PEO–EMIHSO4 electrolytes was deduced and the influence of nanofillers

on the ionic conduction process was identified.

Another approach to improve the performance of the electrolyte is to develop proton

conducting polymer–IL electrolytes that not only contribute to double-layer capacitance, but

also can promote pseudocapacitance. Proton conducting ILs could be obtained by tweaking

the cationic structure of the HSO4-based IL, which adds additional functionality to the

polymer–IL electrolytes. The trade-off is that the non-fluorinated proton conducting ILs have

high melting temperatures. The melting point of ILs can be lowered by adding another IL,

disrupting the close packing of ions. Binary eutectic systems of proton conducting ILs were

investigated, and the proton conduction of the eutectic systems was characterized in liquid

and in polymer states.


4

Efforts were made to develop environmentally benign polymer–IL electrolytes and

improve their performance to the levels of fluorinated polymer–ILs. The understandings as

well as the approaches in this study are not limited to the applications in ECs. The insights

from this work can be extended to other electrochemical energy storage technologies and

beyond.

1.1 Objectives

A systematic approach is presented to develop high performance and environmentally benign

polymer−IL electrolytes for solid, lightweight, and flexible ECs. The specific objectives of

the study were to:

develop and optimize non-fluorinated polymer–IL electrolytes to reach the

performance of fluorinated polymer–ILs,

develop a fundamental understanding of the crystallinity of polymer electrolytes and

the interactions between the polymer, ILs, and additives on ionic conduction,

deduce the ionic conduction mechanism in the polymer electrolyte and examine the

effects of additives on the ion transport,

explore the structure of cations and anions of ILs to develop proton conducting

polymer–ILs in order to further enhance the performance of electrolytes, and

demonstrate the developed polymer–ILs in solid flexible ECs and leverage the

strength of ECs for high rate performance.

1.2 Thesis Overview

The approaches described in Figure 1-1 were undertaken, addressing the properties of the

liquid ILs and the polymer–IL systems. The top flowchart describes the characteristics


5

studied for both aprotic ILs and protic ILs in liquid electrolytes, which were carried out

before proceeding to develop polymer–ILs shown in the bottom half of the flowchart. In

parallel with PEO–EMIHSO4, a fluorinated IL, 1-ethyl-3-methylimidazolium

tetrafluoroborate (EMIBF4), and the enabled polymer electrolytes using poly(vinylidene

fluoride-co-hexafluoropropylene)(PVdF-HFP) matrix were also studied as a benchmark (not

shown in the flowchart).


6

Figure 1-1 Overview of the characteristics of ILs (top) and polymer–IL electrolytes (bottom) and the approaches undertaken to improve the respective properties

ionic liquid (IL)

(ionic conductor)

Aprotic IL

EMIBF4

EMIHSO4

Protic IL

protic functional group

EMIHSO4(no protic functionality)

MIHSO4(one protic functionality)

ImHSO4(two protic functionality)

eutectic binary systems

EMIHSO4-ImHSO4

MIHSO4-ImHSO4

PEO–EMIHSO4

polymer electrolyte

Polymer–aprotic IL

plasticizer

propylene carbonate

mixed solvent

inorganic fillers

amorphous SiO2

(low dielectric constant)

crystalline TiO2

(high dielectric constant)

Polymer–protic IL protic ILs

EMIHSO4-ImHSO4

(eutectic)

MIHSO4-ImHSO4

(eutectic)


7

ECs were fabricated with the optimized polymer electrolytes using double-layer and

pseudocapacitive electrode materials. This is schematically illustrated in Figure 1-2.

Figure 1-2 The components for developing electrochemical capacitors

The remainder of this thesis is organized as follows:

Chapter 2 begins with a review of the advances in flexible ECs and the polymer

electrolytes. It is followed by the introduction of ILs and their characteristics for application

in both double-layer capacitors and pseudocapacitors. Then, reviews of the state-of-the-art

polymer–IL electrolytes and criteria for selecting the IL and the polymer matrix are given.

The chapter ends with a summary of the characterization techniques used in this study.

Chapter 3 details the experimental procedures and characterization methods.

In Chapter 4, the electrochemical properties of liquid ILs are presented, specifically a

comparison of the effects of fluorinated and non-fluorinated anion, and cationic functional

groups.

Polymer electrolytes

ionic conductor polymer matrix additives

Electrode materials

double-layer capacitive pseudocapacitive

Symmetric solid electrochemical

capacitors


8

In Chapter 5, the optimization of polymer–IL electrolytes is discussed based on the ionic

conductivity and the degree of crystallinity. A comparison of the ionic conductivity of the

non-fluorinated and fluorinated polymer–ILs and the performance of the enabled devices are

presented.

Chapter 6 reports the studies of the impact of two types of inorganic fillers on the: (i)

ionic conductivity, crystallinity, and interactions between polymer and IL, (ii) ion transport

mechanism, and (iii) capacitance and rate performance of the enabled ECs. A method using

complex capacitance and dielectric analyses was used to correlate the dielectric properties of

polymer electrolytes and the performance of the enabled capacitors.

In Chapter 7, the proton activity and melting temperature of protic ILs is presented as a

function of cationic functional group. The proton conducting characteristics of two eutectic

binary IL systems is demonstrated in both liquid and polymer states.

Chapter 8 concludes the thesis and outlines recommendations for future work.

9

CHAPTER 2

BACKGROUND

2.1 Fundamentals of Electrochemical Capacitors

The rapid development of portable and miniaturized electronic devices for various

applications in sensors, microrobots and implantable medical devices, wearable and

self-powered smart tags relies on flexible and high-power energy systems [11-14]. While

batteries, especially Li-ion batteries, carry the most energy storage currently used in portable

electronics, they have low power capability and limited cycle life. Electrochemical capacitors

(ECs), also known as supercapacitors, are power devices that can be charged and discharged

in seconds with ultra-long cycle life. Although their energy density is lower than in batteries

(5 Wh kg-1), ECs have much higher power delivery or uptake (10 kW kg-1) for shorter time,

and can bridge the application gap between batteries and conventional capacitors.

2.1.1 Types of electrochemical capacitors

Similar to batteries, ECs are composed of electrodes and an electrolyte. Depending on the

charge storage mechanism as well as the active materials, ECs are classified into

electrochemical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors

[15,16].

CHAPTER 2. BACKGROUND

10

EDLCs store charge electrostatically through a reversible adsorption of ions of the

electrolyte onto the surface of electrodes. Charge separation at the electrode-electrolyte

interface produces the double-layer capacitance, C:

r 0ACd

, (2-1)

where εr is the electrolyte dielectric constant, ε0 is the dielectric constant of the vacuum, d is

the effective thickness of the double layer and A is the electrode surface area. Double-layer

capacitances at a smooth electrode are usually of the order of 15-40 μF cm-2 [17,18]

depending on the electrolyte. High surface area carbon materials are generally used for

EDLCs to reach high capacitance.

Charge storage in pseudocapacitors is faradaic in nature and originates from fast and

reversible redox reaction at the surface of the electrode material [15]. Transition metal oxides

(e.g., ruthenium oxide (RuO2) and manganese oxide (MnO2)) and electrically conducting

polymers (e.g., polyaniline, polypyrrole, and polythiophene) are examples of these

electroactive materials. Capacitance of pseudocapacitors is typically 10-100 times larger than

that of EDLCs (per real surface area). RuO2 is an example of well-known pseudocapacitive

material due to its fast and reversible electron transfer in acidic solutions. The continuous

change of the oxidation states during proton insertion or de-insertion on RuO2 leads to a

capacitive behaviour [17] according to Equation 2-2:

2 2 x xRuO xH xe RuO (OH) , 0 x 2

. (2-2)

Hybrid capacitors are based on a combination of a faradaic battery-type electrode coupled

with a capacitative electrode. This new category of ECs, also termed as asymmetric

capacitor, have the advantage of high power density of the capacitor-like electrode with the

good energy density of the battery-like electrode in one cell.


11

2.1.2 Advances in electrochemical capacitors

The wide applications of ECs range from small-scale portable, flexible, and wearable

electronics to large-scale back-up power supplies and hybrid electric vehicles [19,20].

Because of the electrostatic charge storage mechanism in EDLCs and highly reversible redox

processes in pseudocapacitors, ECs have long cycle life in the order of 105-106 cycles

[16,17]. However, their energy density is lower than batteries which is a key limitation that

must be overcome to meet the higher demands of future energy storage applications. Since

the energy density of ECs is proportional to the capacitance, C, or the operating voltage

window, V, according to Equation 2-3:

2

21 VE = CV and P =

2 4 ESR, (2-3)

increasing the capacitance of electrode materials or the operating voltage of electrolytes

result in higher energy density. A major effort has been devoted to develop high performance

electrode materials, including activated carbon, carbide-derived carbons, nanotubes, and

fibres. Thin-film carbon nanotube or graphene sheet are examples of flexible and

self-supported electrodes. Nano-architectured metal oxide/carbon or conducting

polymer/carbon composite electrodes have also been fabricated for on-chip micro-systems

[11,12]. Because of the high power output and long-term cycling ability of these micro-ECs,

they can replace or complement micro-batteries for energy storage or energy generation in

small and portable devices such as power buffer applications and memory back-up in

consumer electronics.

While the fabrication of novel nanostructured electrode materials and the development of

thin-film manufacturing techniques allow ECs to be integrated into small-scale devices

[14,21], their practical application could not be realized by only advancing electrode

materials. The performance of electrolyte is equally important in the overall performance of

ECs. As shown in Equation 2-3, both power and energy density of ECs are proportional to


12

V2. Increasing the operating voltage of electrolytes and reducing the equivalent series

resistance (ESR) can significantly improve energy and power density of ECs.

Relatively less attention has been directed toward developing advanced electrolytes.

Currently, liquid electrolytes or separator membranes impregnated with liquid electrolytes

are used in commercial ECs. Liquid electrolytes have drawbacks with possible leakage issues

that not only need excessive packaging, but also impose potential safety hazard. The

resulting ECs are not flexible and cannot be formed in different shapes and configurations.

Polymer electrolytes, acting as ionic conductors and separators, are therefore good

alternatives to liquid electrolytes.

While polymer electrolytes have 100 to 1000 times lower ionic conductivity than liquid

electrolytes, this shortcoming can be compensated by a number of factors shown in Table 2-

1. For example, Figure 2-1 shows a conventional spiral design for ECs in which the separator

is soaked with liquid electrolytes and the assembly is sealed. In comparison, solid polymer

electrolytes can be formed into thin films of large surface that significantly reduce the

packaging materials, resulting in high volumetric energy and power densities [22,23].

Flexible and multi-stacking configuration can also be obtained by thin-film polymer

electrolytes, so that multi-cell in one package can be achieved (Figure 2-1).

Figure 2-1 (left) a spiral configuration of ECs utilizing liquid electrolytes, and (right) a flexible and multi-stacking design of ECs enabled with polymer electrolytes


13

Table 2-1 Comparison between the characteristic properties of liquid and polymer electrolytes and the enabled devices. Property Liquid electrolytes Polymer electrolytes Conductivity High ionic conductivity Low ionic conductivity

Safety Leakage of liquid or gas formation No leakage

Function Ionic conductor with a porous separator (dead mass)

Ionic conductor and separator (active mass)

Design /Package

Protective case and packaging Thin, flexible, and space-efficient designs with different configurations

laminated packaging

Device performance

– Increase in volumetric energy and power densities

2.2 Types of Liquid Electrolytes

The type of electrolytes employed in ECs has a marked effect on the energy stored, and how

quickly it can be released. From Equation 2-3, it is clear that the operating voltage and the

ESR are important factors for the energy density and the power density of the devices. In

ECs, the decomposition voltage of the electrolyte determines the operating voltage. The

maximum operating voltage for aqueous electrolytes is theoretically limited by the

electrolysis of water to 1.2 V. The energy density of ECs increases when using organic

electrolytes, where the operating voltage could be up to 2.5 V. On the other hand, the ionic

conductivity of organic electrolytes is much lower than aqueous electrolytes, resulting in

higher ESR in ECs and lower power output. Another major disadvantage of organic

electrolytes is the volatility and flammability of the organic solvents which affects the safety

of the device. Acetonitrile (AN) appeared to be a good solvent for its low viscosity, and thus

high ionic conductivity. Due to the high volatility and toxicity of AN, most organic

electrolyte-based ECs available commercially utilize propylene carbonate (PC) [24].

Ionic liquids (ILs) have been investigated in the last decade as alternative electrolytes for

ECs. Compared to classical aqueous and organic electrolytes that are obtained by dissolution


14

of salts in solvents, ILs are organic salts composed of ions with a relatively low melting

temperature. The two main characteristics of ILs, wide potential window and high thermal

stability (i.e., low volatility), have made them potentially “greener” alternatives to

conventional organic electrolytes. The limitation is that the viscosity of ILs is typically

higher than organic and aqueous electrolytes, leading to ionic conductivities lower or equal to

that of organic electrolytes.

Table 2-2 shows a comparison of important properties of aqueous, organic, and IL

electrolytes for ECs. As ILs exhibit a wide operating voltage and an acceptable conductivity,

they have promising characteristics over volatile organic electrolytes. To reduce the viscosity

of ILs for practical applications in ECs, thermally stable solvents have been added. Also,

different types of ILs (e.g., protic ILs) have been developed to achieve high ionic

conductivities.


15

Table 2-2 Comparison of properties of aqueous, organic, and ionic liquid electrolytes for ECs. Property Aqueous Organic Ionic liquida Ref.

Conductivity High

H2SO4 (30 wt%) (730 mS cm-1) KOH (30 wt%) (540 mS cm-1)

Low

TEABF4b (0.65 M in PC)

(10.6 mS cm-1) TEABF4 (0.65 M in AN)

(49.6 mS cm-1)

Moderate

~10 mS cm-1 [EMI][TFSI] (8.4 mS cm-1) [EMI][BF4] (14 mS cm-1)

[2,25]

Operating voltage

1 V 2.5-3 V 1.5-4 V [2,25]

Thermal stability

Low Volatile

(132 °C)c

High

(150-400 °C)d [26]

Viscosity Low

H2SO4 (1.5-2 cP) KOH (3.7 cP)

Low

TEABF4 (0.65 M in PC) (2.5 cP)

TEABF4 (0.65 M in AN) (0.3 cP)

Highe

[EMI][TFSI] (28 cP) [EMI][BF4] (40 cP)

[2,25,27-29]

a values are given for some common pure ILs b tetraethylammonium tetrafluoroborate c flash point of propylene carbonate (PC) d range of decomposition of ILs e viscosity of some ILs can be higher than 500 cP

2.3 Ionic Liquids

ILs are salts composed of organic cations and organic and inorganic anions with a relatively

low melting point (˂ 100 °C) [30]. The first scientific report about ILs was in 1914 on the

preparation of ethylammonium nitrate [31]. The compound was a liquid at room temperature,

but it was sensitive to moisture which limited its use. Today, the increasing interest in ILs for

electrochemical applications is usually directed towards stable and room temperature ionic

liquids (RTILs) [32]. Since ILs are composed of organic ions, they can have unlimited

structural variations and combinations. ILs are “designable” or “fine-tunable” to meet the


16

requirements for specific applications. The generic structures of some common cations and

anions are shown in Figure 2-2.

Cations Anions

[BF4] [HSO4]

[TFSI]

[NO3]

[Tf]

[PF6]

Figure 2-2 Generic structures of common cations and anions for ionic liquids

Some of the basic properties generally known for ILs are summarized in Table 2-3. In

addition to electrochemical characteristics of ILs, their high thermal stability plays an

important role in the safety of energy storage devices. Different classes of ILs have been

prepared for various applications not only in electrochemical devices such as Li-ion batteries,

ECs, fuel cells, but also in diverse synthetic reactions, separations and extractions as well as

electrodeposition, nanotechnological, and biotechnological processes [26].


17

Table 2-3 Some basic properties of ionic liquids. Properties Advantage for electrochemical application

Low melting point Treated as liquid at RT Wide usable temperature range

Low vapor pressure Negligible under normal condition Thermal stability, usually non-flammable

Reasonable to high conductivity (10 mS cm-1)

Relatively high ion density

Wide operating potential window High electrochemical stability

Tunable/Designable Various kinds of salts

2.3.1 Classes of ionic liquids

ILs have been categorized in three classes by Ohno: aprotic ILs, protic ILs (PILs),

zwitterionic liquids (ZILs) [1,33]. Aprotic ILs and PILs have characteristics suitable for ECs

which will be discussed here. ZILs are characterized by a tethered cation and anion in an

intramolecular structure, and may be useful for other applications such as Li-ion batteries and

fuel cells. For application in ECs, aprotic ILs are defined as those ILs without protonated

ions and proton conducting characteristics, while protic ILs (PILs) contain protonated species

that are proton conductive and active in pseudocapacitive redox reactions (i.e., similar to

acidic solutions).

Aprotic ionic liquids. These ILs are usually synthesized in two steps. First the halide salt

with the required cation is prepared by alkylation. The halide anion is then exchanged with

the required anion [30,32]. For example, [EMI][TFSI] (structure 1) is prepared by the anion

exchange reaction between [EMI]Cl and Li[TFSI] (see Figure 2-3). The majority of ionic

liquids investigated for ECs, specifically for EDLCs, are aprotic ILs. Dialkylimidazolium and

alkylpyridinium (Figure 2-2) are examples of the commonly used cations, and more recently

tetraalkylammonium and alkylpyrrolidinium cations have been investigated. Among the most

common anions are PF6, BF4, Tf, and TFSI ions (Figure 2-2).


18

Figure 2-3 Synthesis of [EMI][TFSI]

Protic ionic liquids. PILs are formed by transferring a proton from a Brönsted acid to a

Brönsted base (Equation 2-4).

HA B HB A (2-4)

For example, [α-Pic][TFA] (structure 2) is prepared by the addition of trifluoroacetic acid to

2-methylpyridine (α-picoline) (see Figure 2-4). When either of the starting materials are

solid, PILs can be synthesized by two methods: (i) by neutralization of an aqueous solution

(or other solvents such as methanol) of the starting base with a suitable acid, or vice versa,

and then removing water by distillation [34], and (ii) by directly mixing the reagents and

heating the mixture above their melting points in an inert atmosphere (under solvent-free

conditions) [35]. Examples of PILs reported in the literature are summarized in Table A-1.

Figure 2-4 Synthesis of [α-Pic][TFA]

When HA and B are a strong acid and base, the transferred proton is attached strongly to

the base, and the process in Equation 2-4 is irreversible, resulting in full ionic dissociation. If

the free energy of proton transfer is small, the reaction is reversible, leading to lower ionic

dissociation. The lower dissociation as well as the presence of H-bonds can reform the

original acid and base.


19

2.3.2 Properties of ionic liquids

In this section, the properties of ILs that are important for electrochemical applications are

introduced, and the impact of their structure on such properties are discussed.

Melting point and thermal stability. The liquidus ranges exhibited by ILs can be much

greater than those for common molecular solvents. The lower temperature limit of most ILs,

either as glass transition (Tg) or melting (Tm), is governed by the structure of the cations and

anions and the Coulombic attraction (Ec) between the ions:

c0

Z ZE M

4 r

, (2-5)

where M is the Madelung constant, Z+ and Z− are the ion charges, and r is the inter-ion

separation. The ionic interaction and hence the melting point of salts depends on (i) the

distribution of charge on respective ions, (ii) ion-ion separation, and (iii) packing efficiency

of the ions (reflected in M, in Equation 2-5). Low-melting salts, as in the case of RTILs, are

obtained when the charges on the ions are small (i.e., ±1), and when the size of ions are large

(i.e., greater ion-separation (r)). Large ions also permit charge delocalization, further

reducing overall charge density. For example, for a given cation such as EMI, the melting

temperature of [EMI][BF4] (structure 3) is 15 °C, whereas it decreases to −3 °C for

[EMI][TFSI] (structure 1, Figure 2-3) with the larger and more complex anion.

The melting point of an IL also can be lowered by a reduction in the symmetry of cation

or distortion in ideal close-packing of the ions. For instance, the symmetry of cations can be

reduced by increasing the length of alkyl chain substitution on cations such as

diaalkylpyrrolidinium and diaalkylimidazolium (see Figure 2-2). It has been shown that for


20

several ILs with 1-alkyl-3-methylimidazoilum cations (structure 4), the melting point

decreased with increasing the chain length (R) up to 6 to 8 carbons, but progressively

increased with further increase of chain length (Figure 2-5). The van der Waals interactions

between the long hydrocarbon chains increase the viscosity and contribute to an ordered

structure which results in higher melting point [30].

Figure 2-5 Variation in melting point with alkyl chain length for ionic liquids containing 1-alkyl-3-methylimidazolium cations and different anions [30]

Alternatively, the melting point of an IL can be reduced by adding another IL to form a

eutectic mixture [30,32]. The mixtures of ILs have been reported using either the same type

of cation or anion as well as different types of cation and anion (i.e., ion mixtures) [36,37].

Studies on mixtures of pyrrolidinium-based ILs, [BMPyr][TFSI] (structure 5) and

[MPPyr][FSI] (structure 6), showed reduced melting points and enhanced liquidus range of


21

the IL mixture compared to the pure ILs. The ionic conductivity of the binary IL mixtures

exceeded those of the pure ILs at low temperatures. For example, at −40 °C, the conductivity

was up to 4 orders of magnitude higher than that of the respective pure ILs [38]. In a study of

binary mixtures of several pyrrolidinium-based ILs with FSI or TFSI anions, it was suggested

that the crystallization of these binary systems initiated by the crystallization of anionic

structure followed by the ordering of cations [39]. A promising performance of EDLCs were

reported by Lin et al. utilizing a eutectic mixture comprised of [MPPip][FSI] (structure 7)

and [BMPyr][FSI] (structure 8). The operating conditions were extended from −50 °C to

100 °C over a 3.5 V voltage window and high charge/discharge rates (up to 20 V s-1) were

obtained [40].

Since ILs have low vapor pressure, their upper liquidus limit is usually determined by

thermal decomposition rather than boiling or evaporation. However, the thermal stability of

organic salts depends largely on their structure, and it would be misleading to think that ILs

never vaporize. Nevertheless, most recently reported ILs are stable enough for use at

temperatures up to 200 °C to 300 °C [1].

Viscosity. ILs are more viscous than most common molecular solvents due to the high

ionic interaction. The ionic motion is inversely proportional to the viscosity of the liquid

electrolyte. To reduce the viscosity and to achieve ILs with high conductivity at room

temperature, increasing the size of ions with functional groups or with longer alkyl chains

would be one way to lower ionic attraction. However, the effect of van der Waals

interactions should also be controlled. The fluorinated anions are most commonly used in ILs

because of their unpolarizable nature that minimizes the van der Waals interactions. For


22

example, the room temperature viscosities of [HMI][BF4] (structure 9) and [HMI][NO3]

(structure 10) are 314 cP and 804 cP, respectively [32]. Large anions such as Tf and TFSI are

most frequently used for this reason.

Electrochemical stability. The electrochemical potential windows of ILs are usually in

the range of 2 to 4 V governed by the limiting potentials of the cation to reduction and the

anion to oxidation [1]. The reduction of protons in PILs usually narrows their potential

window compared to aprotic ILs (see Table A-1). For example, the oxidation and reduction

reactions of [Pyr][HSO4] with a potential window of 3 V are shown in Figure 2-6. The

hydrogen sulfate anions are oxidized at positive potentials (e.g., E = 1.8 V vs. Ag/AgCl)

giving persulfate. The reduction of pyrrolidinium cation proceeds by the deprotonation, and

then followed by the reduction of proton (E = −1.2 V vs. Ag/AgCl) [34].

Figure 2-6 Oxidation and reduction of [Pyr][HSO4] (structure 11)

Overall, the electrochemical stabilities of the ILs, based on the type of cation, increases in

the order of: pyridinium ˂ pyrazolium ≤ imidazolim ≤ sulfonium ≤ ammonium (Figure 2-2).


23

The stabilities of anions towards oxidation appear to be the highest for perfluorinated ions

due to the strong electron withdrawing nature of fluorine. For example, ILs based on a

quaternary ammonium and TFSI anion exhibit large potential windows up to 5.7 V at glassy

carbon electrode [1,25,30].

Miscibility with polymer. Although there are many studies on polymer–IL systems in

the literature, the selection of polymers and ILs and their compatibility are still based on

empirical approaches [41]. Ideally, the salts for polymer electrolytes should have a low Tg to

remain rubbery at room temperature [42]. ILs with low Tm and Tg meet the requirements of

plasticizing salts for polymers. The high thermal stability of ILs may also expand the

temperature range where polymer electrolytes can be used.

2.3.3 Ionic conductivity and conduction mechanism

The room temperature conductivity of RTILs, within a broad range of 0.1-18 mS cm-1 [25], is

lower than that of conventional aqueous electrolytes and organic electrolytes (see Table 2-2).

Generally, a conductivity of the order of 10 mS cm-1 is typical of ILs based on EMI cation.

The lower than expected conductivity of ILs is due to the ion-ion interaction and formation

of ion pairs that reduces the number of charge carriers, and the low ionic mobility resulting

from the large ion size.

The mechanism of ionic conduction in electrolytes is often studied through characterizing

the ionic conductivity as a function of temperature. Ionic conductivity of ILs usually exhibit

classical linear Arrhenius behavior above room temperature:

aEA exp

RT

, (2-6)

where A is a pre-exponential factor, R is the gas constant, and Ea is the activation energy of

ionic conduction. In some cases, as temperature approaches the glass transition temperatures

(Tg) of the ILs (i.e., below room temperatures), the conductivity deviates from linear


24

behavior. Their conductivity trend is better described by the empirical Vogel–Tammann–

Fulcher (VTF) relation [30]:

1

a2

0

Eexp

R(T T )

, (2-7)

where T0 is the temperature at which the conductivity reaches zero. T0 is ascribed to Tg –

50 K. A lower Ea denotes a facilitated ion transport mechanism. The facts that the activation

energy is higher than conventional electrolytes and that the conductivity deviates from

linearity at low temperatures suggest that viscosity controls the ionic motion of ILs.

The influence of viscosity on the conductivity of ILs is described by the Walden’s rule:

Λη = constant , (2-8)

which indicates the inverse relationship between molar conductivity (Λ) and viscosity (η).

This rule is commonly used to evaluate the ionic dissociation of ILs and is illustrated by

Walden plot (see Figure 2-7). For an ideal electrolyte (i.e., a complete ionic dissociation)

such as classical dilute KCl aqueous solution, the Walden product (Λη) remains constant.

When an IL is fully dissociated with no ion-ion interactions, it will correspond closely to the

ideal line (e.g., [EMI][BF4]). Equivalent conductivities that are higher than the ideal line

indicate the dissociation of a charge carrier with high mobility (e.g., proton) that is

independent from the ionic motion and hence viscosity (e.g., [Pyr][HSO4]). This is a

desirable characteristic which can be observed in PILs. Values that are below the ideal line

imply the pairing of the ions that can produce neutral species or ion aggregates.


25

Figure 2-7 Classification Walden plot constructed from the literature data of some ILs

A high degree of dissociation of ILs is therefore desired as electrolytes. Specifically, in

PILs, a high degree of proton dissociation could have additional contribution to ionic

conductivity. There are two typical mechanisms of proton conduction, namely proton

hopping (Grotthuss mechanism) and matrix transport (vehicular mechanism) illustrated in

Figure 2-8. The Grotthuss mechanism usually occurs between the proton donor and acceptor

sites on the structure (e.g., dissociated protons in imidazole or H3PO4). This leads to higher

proton conductivity than the vehicular mechanism due to the higher mobility of protons

compared to the diffusion of a large proton-containing ion (e.g., imidazolium cation in this

case).

-5

-4

-3

-2

-1

0

1

2

3

-5 -4 -3 -2 -1 0 1 2 3

log

Λ [

S c

m2

/mo

l]

log (1/η )[P-1 ]

[pyrr][Fm]

[pyrr][TFA]

[Pyrr][NO3]

[2-MePy][Tf] (1:2)

[2-MePy][Fm]

[EMI][BF4] (aprotic)

[4-MePy][TFA] (1:2)

[4-EtPy][TFA] (1:2)

[2-MePy][TFA] (1:2)

[3-MePy][TFA] (1:2)

[3-EtPy][TFA] (1:2)

[2-Etpy][TFA] (1:2)

[DEA][Fm]

[EMI][Tf] (aprotic)

[Pyrr][AC]

[DEA][OSA]

[2-pentylPy][TFA] (1:2)

[Pyrr][HSO4]

[EMI][HSO4] this work

KCl

low vapor pressure

Poor ionic liquids

Non ionic liquids

Super ionic liquids

high vapor pressures


26

(a)

(b)

Figure 2-8 Schematic representation of proton transfer via (a) Grotthuss mechanism of dissociated imidazole or H3PO4 and (b) vehicular mechanism of imidazolium [1]

2.3.4 Application in ECs

Despite the high viscosity of ILs, some RTILs have shown conductivities comparable to

organic electrolytes (up to ~10 mS cm-1) [43]. Due to the reasonable conductivity and wide

potential windows (up to 4 V) of ILs, much attention has been directed recently to their

application in EDLCs. Studies have been reported on EDLCs based on activated carbon

[44,45], vertically aligned nanotubes [2], and graphene [46] electrodes with various aprotic

ILs as electrolytes. Different types of ILs have been synthesized to achieve lower viscosity

and high conductivity. The addition of solvent to ILs has also been reported to lower the

viscosity while maintaining a wider operating voltage than organic electrolytes. In a study by

Lewandowski et al., properties of activated carbon cloth EDLCs with different ILs

([EMI][BF4], [EMI][TFSI], [MPPyr][TFSI], [MPPip][TFSI]) were compared to an organic

and aqueous electrolytes. The operating voltage of capacitors was up to 3.5 V with the ILs

3 V with the solution of ILs in PC 2.5 V with classical organic electrolyte [47]. The highest

specific energy was reported for the device working with the ILs, while the highest power

was the characteristic of the aqueous-based device. The impact of the thermal stability of ILs


27

on device performance was demonstrated by Arbizzani et al. for EDLCs with

pyrrolidinium-based ILs, where an operating voltage of 4 V and a cycling ability of 20 000

cycles were reported at 60 °C [48]. There are also studies which have focused on the

relationship between the ion size, its solvation shell, and the pore size of carbon materials by

analyzing the capacitance of carbon electrodes in solvent-free ILs such as [EMI][TFSI]

(structure 1) [49]. These studies have shown that materials with improved performance can

be developed by matching the pore size and the ion size [16]. An EC based on an aprotic IL

has been shown to have superior properties compared to ECs with conventional organic

electrolyte such as a solution of tetraethylammonium tetrafluoroborate (TEABF4) in PC [50].

A commercial pouch-type EC that utilizes this electrolyte is available from Japan Radio Co.,

Ltd. [51].

While aprotic ILs are mainly used in EDLCs, the interest in PILs is to leverage proton

conducting properties to replace aqueous-based electrolytes currently used in

pseudocapacitors. This could combine the benefits of a wider potential range (e.g., 1 V)

with the high specific capacitance of pseudocapacitive materials to achieve high performance

ECs. So far, there are only a few studies that demonstrated electrodes with pseudocapacitive

activities in PILs. Rochefort et al. reported pseudocapacitive behavior of RuO2 in PILs based

on trifluoroacetic acid (TFA) and various heterocyclic amines including 2-methylpyridine

(structure 2) [52,53]. Chang et al. found pseudocapacitive behavior of MnO2 (a cheaper

alternative to RuO2) in an aprotic IL, 1-ethyl-3-methylimidazolium thiocynate [EMI][SCN]

(structure 12), and suggested that MnO2 is not compatible with PILs [54,55]. Recently, Ruiz

et al. suggested that charge storage of MnO2 occurs in a PIL composed of


28

2-methoxypyridinium trifluoroacetate (TFA) (structure 13). However, the operating potential

of MnO2 in this PIL was limited to more positive potentials, so an asymmetric EC was

suggested leveraging this PIL with MnO2 as positive and carbon as negative electrodes,

respectively [56]. Also, carbon electrodes containing surface functionalities, with

pyrrolidinium nitrate [Pyr][NO3] (structure 14) and pyrroldinium formate [Pyr][HCOO]

(structure 15) as electrolytes, were investigated by Mysyk et al. They showed that the

capacitors could operate and maintain their capacitance at low temperatures (−10 °C) [57].

The development of PILs is highly promising for ECs although a greater understanding of the

fundamental operating mechanisms is yet to be achieved.

2.4 Polymer Electrolytes

In 1973, Wright [58] reported the first polymer electrolyte system based on poly(ethylene

oxide) and alkali salts. When Armand [59] proposed the use of polymer electrolytes for

Li-ion batteries, research on polymer electrolytes significantly increased in this area. Since

then, the term “polymer electrolyte” has been applied to different systems characterized by

conductivities higher than 10–4 mS cm–1. The polymer electrolytes for solid-state flexible

energy storage devices such as ECs should satisfy the following properties [60]:

Good ionic conductivity,

Low electronic conductivity,

High chemical, electrochemical, and thermal stability,

High mechanical strength and structural stability, and


29

Good film forming properties for easy processing.

2.4.1 Classification of polymer electrolytes

Polymer electrolytes can be divided into the classical categories: solvent-free, gel,

plasticized, ionic rubber polymer electrolytes, and ion conducting polyelectrolytes [61,62].

While these types are based on the composition and preparation method of polymer

electrolytes, from the conduction mechanism perspective, polymer electrolytes can be

classified into two main groups: salt-in-polymer and polymer-in-salt [42,62,63].

Salt-in-polymer. Salt-in-polymer electrolytes usually constitute of a precursor solution

of salt in a polymer. Examples are conventional PEO–Li salt polymer electrolytes. The ionic

conductivity of these systems are limited by two factors: (i) solubility or ionic dissociation of

the salt in the polymer system, and (ii) structure and crystallinity of the polymer which

influences the mechanism of ion transport. For example, the ionic motion in PEO-based

electrolytes is usually coupled with the local segmental motion of the polymer chains. To

improve the ionic conductivity of these polymer systems, plasticizers are often added [22].

Another approach is to obtain polymer gel electrolytes that include solvent molecules to

swallow (dissolve) the polymer matrix [62]. Nevertheless, solvent-free polymer electrolytes

remain important as the volatility and flammability of some organic solvents are undesirable.

Polymer-in-salt. In polymer-in-salt electrolytes, a small amount of a high molecular

weight polymer is added to the ionic conductor to act as a binder to provide flexibility and

mechanical stability. These polymer electrolytes are preferable since the mechanism of ionic

conduction of the salt is predominately preserved in the polymer electrolyte. The electrolyte

salt and the polymer matrix in such systems need to have some essential characteristics: (i)

The polymer and the salt should be compatible or the polymer should be soluble in the salt.

(ii) The salt (i.e., IL in this case) needs to have low melting and glass transition temperatures


30

to remain liquid at room temperature and retain high conductivity. (iii) The polymer network

should leverage the ion conduction paths of the salt so that ion transport is not hindered.

For applications such as Li-ion batteries or fuel cells, single-ion conducting polymer

electrolytes (i.e., Li+ or H+ conductors) are required. The polymer host and the ionic

conductor should have characteristics that meet the requirements for such applications. The

polymer electrolytes that are currently used for ECs are based on aqueous or organic ionic

conductors. Solid polymer electrolytes with higher thermal stability and wider

electrochemical stability are necessary for ECs. ILs possess properties that makes them

suitable candidates to develop polymer electrolytes for both EDLCs and pseudocapacitors.

2.4.2 IL-based polymer electrolytes

IL-based polymer electrolytes are developed through different methods:

(1) incorporation of an IL in a polymer matrix,

(2) polymerization of a vinyl monomer in an IL (as solvent and ionic conductor), and

(3) polymerization of a polycation or a polyanion (polymerizable ILs).

Stable solid gel electrolytes can be formed by the addition of ILs to polymer matrices.

These gel electrolytes provide the structural stability of a polymer while maintaining a

reasonable ionic conductivity. The polymer–IL electrolytes are obtained by casting

procedures. Due to its simplicity, this method has been extensively studied for polymer–ILs.

Examples of polymer–IL electrolytes and their ionic conductivity at room temperature are

shown in Table 2-5.

Since ILs can provide a medium for polymerization, in situ polymerization of monomers

in ILs is another approach [64]. Watanabe and coworkers have reported highly conductive

and mechanically stable polymer-in-salt electrolytes by in situ radical polymerization of

vinyl monomers in ILs. These systems were also called “ion gels” when the resulting

polymer network and IL were completely compatible. Ion gels based on poly(methyl


31

methacrylate) (PMMA) network in [EMI][TFSI] exhibited ionic conductivities in the order

of 10 mS cm-1 [42]. While this method is promising, the compatibility of monomers and ILs

could be a limiting factor in terms of the choice of ILs. Table A-2 summarizes some

polymer–IL systems developed by polymerization methods.

Polymerizable ILs are obtained by the introduction of a polymerizable group, such as the

vinyl group, in the cationic or anionic structure of an IL. The radical polymerization produces

a polymer chain on which one of the IL ions is immobilized, constituting single-ion polymer

electrolyte such as Li+ or H+ conducting electrolytes. The ionic conductivity of these polymer

electrolytes is usually low due to the decrease in the segmental motion of the polymeric

structure, or the distance that ions can travel. Introduction of ethylene oxide units into the

polymer chain has been shown to increase the chain flexibility and the ionic conductivity

[65].

The procedure of polymerization is generally not simple as it involves different chemical

reaction steps. The yield of polymerization and unwanted side reactions as well as the

interactions with the conducting ILs also need to be controlled. As such, integration of ILs

with polymer networks is the preferred method and feasible for the fabrication of polymer

electrolytes for ECs. Polymer–IL electrolytes prepared by this method potentially can be

directly cast onto the electrode, minimizing the electrode-electrolyte contact resistance.

2.4.3 Polymer network

A polymer network for polymer electrolytes should immobilize the ionic conductor while

providing conduction paths with a low barrier to ionic motion. The compatibility of the

polymer and the salt, polarity of the polymer to dissociate the salt (e.g., dielectric constant),

structural characteristics such as crystallinity, and electrochemical and thermal stability of the

polymer matrix are factors that influence the overall performance of the polymer electrolyte.

Ionic conductivity in polymer electrolytes is generally associated with the local motion of the


32

polymer chain. Polymers with a low glass transition that provide flexibility to the polymer

electrolyte at room temperature are preferred. Meanwhile, the polymer needs to provide

sufficient mechanical integrity to process thin-film polymer electrolytes. Among the common

polymer candidates, the ones that meet some of the criteria are summarized in Table 2-4

together with their properties.

Table 2-4 Some common polymer hosts with their corresponding chemical formula and thermal properties [65,66].

Polymer host Repeating unit Glass

transition Tg (°C)

Melting point

Tm (°C) Mw.

Poly(ethylene oxide) PEO

–(CH2CH2O)n– –64 65 900,000

Poly(vinylidene fluoride) PVdFb

–(CH2CF2)n– –62 160 180,000

Poly(vinylidene fluoride-co-hexafluoropropylene) PVdF-HFPb

–(CH2CF2)x–(CF2CF(–CF3))y– –62 140 400,000

Poly(acrylonitrile) PAN

–(CH2CH(–CN))n– 85 317 150,000

Poly(methyl methacrylate) PMMA

–(CH2C(–CH3)(–COOCH3))n– 105 -a 996,000

a amorphous polymer b properties obtained from Sigma-Aldrich

Despite the increasing interest in IL-based polymer electrolytes, the understandings of

solubilization of polymers in ionic liquids are mostly qualitative [41]. The major advances in

polymer electrolytes are seen for Li-ion batteries which have been adapted in ECs. The two

most common polymer hosts are: poly(ethylene oxide)(PEO) and poly(vinylidene fluoride-

co-hexafluoropropylene) (PVdF-HFP).

Poly(ethylene oxide)(PEO). PEO-based electrolytes have been extensively used in

Li-ion batteries due to their acceptable electrochemical and thermal properties [61]. PEO is

compatible with a variety of ionic conducting salts, including organic ionic salts [67]. The


33

polar C–O (structure 16) groups in the polymer chain can promote ionic dissociation and

provide ionic conduction through the polymer backbone. The ion transport in PEO-based

electrolytes predominately occurs through the amorphous state, where the movement of ions

is assisted by the local or segmental motion of the polymer. However, PEO is

semi-crystalline with a melting point of approximately 65 °C. Its restricted structural

movement and flexibility limits the ionic conductivity of PEO-based electrolytes at room

temperatures. There have been continuing efforts to reduce the crystallinity in PEO-based

electrolytes in order to increase the ionic conductivity.

One approach is the addition of small organic molecules (e.g., PC and ethylene carbonate

(EC)) to the polymer-salt systems. The main role of these solvents is to plasticize the host

polymer, improving the flexibility and the segmental motion of the chains [61]. While the

reactivity and volatility of such solvents in the polymer system is much lower than in liquid

organic electrolytes, it still remains a concern. PC is commonly used due to its high dielectric

constant (ε = 65) and thermal stability (Tb = 242 °C) [66]. Another promising approach is to

disperse inorganic nanofillers (e.g., SiO2, TiO2, and Al2O3) into the polymer electrolyte to

hinder the crystallization of the polymer chains. The effects of nanofillers on the properties

of PEO-based electrolytes are discussed in section 2.4.5.

Poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP). This semi-crystalline

polymer has recently gained attention for polymer electrolytes in both ECs and Li-ion

batteries. It consists of crystalline vinylidene fluoride (VdF) and amorphous

hexafluoropropylene (HFP) units (structure 17). The chemical stability of this polymer is

mainly attributed to the VdF units, while enhanced plasticity is obtained through the HFP


34

units which promote the ionic conduction [68]. The crystallinity remaining in the system

retains sufficient mechanical stability to allow it to act as a separator, and the amorphous phase

provides the conductive path.

PEO-based and PVdF-HFP-based polymer–IL electrolytes have shown ionic conductivities

up to 7 mS cm-1 and potential windows up to 4 V. Polymer–IL electrolytes based on other

polymers such as PAN and PMMA have also shown promising ionic conductivities and operating

voltages (see Table A-3). The electrochemical stability of such polymers may be limited due the

presence of the organic functional groups [61]. The side chains in the polymer may impede ion

transport as reported for PMMA–LiClO4 electrolyte [69]. Nevertheless, more detailed studies are

needed on chemical and electrochemical stabilities of these polymer electrolyte systems.

2.4.4 Conduction mechanism in polymer electrolytes

Polymer electrolyte materials are characterized by the presence of disordered structure

resulting from the polymer host. For high molecular weight polymer hosts, the movement of

chain is small and makes little contribution to mechanisms for ion transport. Above the glass

transition temperature, there is an additional dynamic disorder: segmental motion of polymer

chain. For instance, segmental motion arises from the local relaxation of dipoles in the PEO

chains under an excitation, such as an electrical field, which can be described as the

reorientation of polar segments (C–O bonds) in the polymer backbone. When an ionic

conductor is added into PEO, the segmental motion could be coupled with the ionic dipolar

relaxation or the motion of the charge carriers [61,70]. Figure 2-9 shows a simplified

schematic of this type of motion. Figure 2-9a shows the movement of dissociated ions (e.g., a

cation) between polar groups on one chain or between neighboring chains. In Figure 2-9b,

examples are depicted for the cases where ion pairs or ion associated species are present, and

how the dissociation of ions and conductivity is promoted by polymer chains.


35

Figure 2-9 Representation of ionic motion in a PEO-based polymer electrolyte (a) assisted by polymer chain motion for dissociated ions; (b) taking account of ion associated species [61]

The ionic conductivity of polymer electrolytes based on semi-crystalline polymers, such

as PEO-based electrolyte, has been more complicated by the presence of multiphases:

crystalline and amorphous states. At temperatures below the melting point of the crystalline

phase (ca. 65 °C), the motion of ions in polymer electrolytes is strongly dependent on the

segmental motion of polymer. At above the melting point, the polymer electrolyte is

amorphous where the polymer chains are flexible and can enhance ion transport. Thus, two


36

main aspects govern the ionic conductivity: the degree of crystallinity and the ionic

dissociation.

The conduction mechanism in polymer electrolytes is studied via characterizing the

conductivity as a function of temperature by Arrhenius function (2-6) or the empirical VTF

(2-7) approach. The activation energy of ionic conduction and the variation of conductivity

with temperature provide information on the transport mechanism in each state. Techniques

such as dielectric relaxation and loss have also been applied to polymer electrolytes to study

the frequency-dependent properties of polymeric and ionic species [61]. Dielectric relaxation

gives a measure of the dynamic and relaxation behavior of electric dipoles in the matrix and

ionic charge. Studies of the dielectric response are informative in: (i) distinguishing detailed

mechanisms for ion transport and differences in interactions between polymer and ions

[22,71], and (ii) identifying the contribution of dielectric properties from polymer electrolyte

materials and relating their impact to the resistance and capacitance characteristics of ECs.

The details on dielectric characterization method are explained in section 2.6.1.2.

2.4.5 Effect of nanofillers on PEO-based electrolytes

The main challenge associated with PEO-based electrolytes is the crystallization of polymer

chains at room temperature which reduces the chain flexibility and hence the ionic

conductivity. One way to improve the performance of these electrolytes is to disperse

inorganic fillers such as SiO2, TiO2, or Al2O3 in the electrolytes [6-8]. The addition of fillers

has been extensively studied for PEO–LiX electrolytes [6-8,72], but not for polymer–IL

systems. The functions of the fillers in PEO-based electrolytes have been reported as the

following: (i) prevention of crystallization: filler particles may act as cross-linking agents for

PEO segments thus inhibiting their reorganization [9,73], (ii) contribution to ionic

dissociation: the Lewis acid-base interaction between the surface polar groups of the

inorganic filler and the ionic species in the electrolyte may promote the dissociation [9,10],


37

(iii) enhancement of ionic mobility: the interaction between surface groups of filler and ionic

species provides additional sites creating a conducting interfacial region between the

particles and the polymer electrolyte [74], (iv) retention of the liquid electrolyte in the

polymer network: the porous morphology of fillers absorbs the electrolyte or the solvent,

maintaining them in the polymer network [75-77], and (v) improvement of the mechanical

strength of the polymer electrolyte [74,78].

These effects may vary with the ionic conductor, the size and type of the filler, and the

manufacturing conditions of the electrolytes. Studies by Scrosati et al. on nano-sized SiO2,

TiO2, and Al2O3 in PEO–LiClO4 electrolytes suggest that fillers can perform as solid

plasticizers by kinetically inhibiting the reorganization of polymer chains [9,79,80]. Agrawal

et al. and Pandey et al. showed that the crystallinity of PEO–NH4SCN and PEO–NH4HSO4

decreased with the addition of nano-sized SiO2 [81,82]. In contrast, Zhang et al. reported that

nano-sized fumed SiO2 increased the glass transition temperature, but had no effect on either

the crystallinity or the conductivity of PEO–LiClO4 [73]. In another study of PEO–LiClO4

with various fillers including micro-sized fumed SiO2 and TiO2, Choi et al. observed an

increase in crystallinity, but found no correlation between the crystallinity and the glass

transition or melting temperature of this system [83,84]. Jayathilaka et al. have shown that

the degree of enhancement in ionic conductivity of amorphous PEO–LiTFSI was dependent

on the nature of surface groups of Al2O3 which decreased in the order of acidic > basic >

neutral > weakly acidic groups [6].

Overall, in spite of some reported trends, the effect of fillers on the properties of

PEO-based electrolytes remains inconclusive. Other properties of inorganic fillers such as

their dielectric constant may also play an important role in ionic conduction.


38

2.4.6 Polymer–IL electrolytes for ECs

2.4.6.1 Application in EDLCs

Although there has been an increase in research on polymer–ILs over the last decade, studies

on polymer–IL electrolytes for EDLCs and pseudocapacitors are still in their infancy. Some

polymer–aprotic IL electrolytes reported for EDLCs are summarized in Table 2-5. For

comparison, the properties of polymer electrolytes based on TEABF4, a common salt for

organic electrolytes, is also reported. Overall, the ionic conductivities of polymer–IL

electrolytes are higher than that of TEABF4-based polymer electrolytes and their potential

windows are equal to or higher than that of organic baseline.

The capacitance values in Table 2-5 correspond to the specific capacitance of the carbon

materials. Although the gravimetric capacitance is not the most correct basis for comparison,

as different carbon materials exhibit different packing density (consider activated carbon vs.

graphene), the comparison in this table is based on the most relevant reported values using

activated carbon.

The capacitances of polymer–IL-based EDLCs are comparable to that of devices enabled

by the organic-based polymer electrolytes. The operating voltages of EDLCs are usually

somewhat lower than the intrinsic potential window of polymer–ILs measured using glassy

carbon or smooth electrodes. This is due to the different redox functional groups on the

surface of different carbon materials, especially in activated carbon, that affect the operating

window of the polymer electrolytes. The overall performance of EDLCs enabled by

polymer–ILs is equal to or better than organic-based devices. While further improvement is

desirable, polymer–ILs have great potential to replace the current organic electrolytes for

EDLCs.

It has been suggested that additives such as plasticizers can improve the ionic

conductivity and the mechanical strength of polymer–IL systems [85,86]. As shown in Table

2-5, the addition of sulpholane (TMS) as a plasticizer to PAN–EMIBF4 and PAN–EMITf


39

increased the ionic conductivities of the corresponding polymer electrolytes to even higher

than that of pure ILs. Since ion pairs exist in pure ILs, the polymer and the plasticizer act as

neutral diluents to dissociate the ions, increasing the number of mobile charge carriers and

hence the conductivity [86].

2.4.6.2 Polymer–PILs for pseudocapacitors

Proton conducting polymer electrolytes can enhance the performance of not only EDLCs, but

also pseudocapacitors. Increasing the operating voltage of pseudocapacitors warrants the

development of proton conducting polymer–PILs as alternatives to aqueous-based polymer

electrolytes. Currently, there is only one study on polymer–PIL electrolytes by Sellam and

Hashmi [87], where they developed a flexible pseudocapacitor comprised of an electrically

conducting polymer/RuO2 composite electrode and 1-ethyl-3-methylimidazolium hydrogen

sulfate ([EMI][HSO4]) immobilized in a blend of poly(vinyl alcohol) PVA and poly(vinyl

pyrrolidone) (PVP), which is an aqueous based system. The ionic conductivity of this

polymer–PIL was 6 mS cm-1, and the device performance was demonstrated up to 1 V s-1

over an operating voltage of 1.6 V.

The major advances in polymer–PIL electrolytes are seen in fuel cells, in which the

incorporation of PILs into the conducting polymer system enhances its performance at

elevated temperatures (above 120 °C). Various types of polymer networks and PILs have

been utilized to develop proton conducting polymer electrolytes. For example, proton

conducting electrolytes were obtained by incorporating N-ethylimidazolium

bis(trifluoromethanelsulfonyl)imide [EIm][TFSI] (structure 18) into a PVdF-HFP polymer


40

matrix [88], or by integrating diethylmethylammonium trifluoromethanesulfonate

([DEMA][Tf]) (structure 19) with sulfonated polyimides (SPI) as the polymer matrix [89].

Also, polymerization of a mixture of styrene and acrylonitrile in N-ethylimidazolium

trifluoromethanesulfonate ([EIm][Tf]) (structure 20) were reported to develop polymer–PILs

[90]. In another study, imidazole (i.e., proton containing) (structure 21) groups were tethered

in a polysiloxane backbone, in which the proton conduction was dominated by the diffusion

of proton rather than the molecular diffusion of imidazole [91].

So far, polymer–PILs have not been explicitly explored for pseudocapacitors, in spite of

the promising examples of PILs in fuel cells. Although the available polymer–PIL

electrolytes are demonstrated for operating conditions of fuel cells, the underlying principles

of proton conduction in these systems can be also applied to pseudocapacitors.


41

Table 2-5 Polymer electrolytes used for activated carbon EDLCs and their electrochemical properties. Composition Intrinsic property EDLC performance

Ref. Polymer IL/salt Solventa Plasticizera

Conductivity(mS cm-1)

Potential window

(V)b

Operating voltage

(V)

Capacitance (F g-1)c

Scan rate (mV s-1)

PVdF [TEA][BF4] PC+EC 2.5 - - - - [92]

PVdF-HFP [TEA][BF4] PC+EC 5 4 3 123 - [93] [EMI][Tf] NMP 6.9 4 - - - [86] [EMI][Tf] acetone

NMP Mg (Tf)2

d, PC+EC

6.3-8 4 - 84-150 500 [94]

[EMI][TFSI] Zeolite - - - 148e 20 [95]

PAN [TEA][BF4] DMF 0.2-4.5 3 2 33 100 [92] [EMI][BF4] DMF 6.6 3 - - - [96] [EMI][BF4] DMF TMS 15f 3-4 2.5 55-200 5 [85] [BMI][PF6] DMF TMS 6.3 - - 35-45 - [85] [EMI][Tf] DMF 6.7 4 3 130 - [86] [EMI][Tf] DMF TMS 16.2f 4 2.5 230 2 [86]

PEO

[BMPyr][TFSI] AN 0.3 3 - 35 - [85] [EMI][Tf] AN 2.6 3.7 - - - [86]

a NMP: N-methyl-2-pyrrolidone; DMF: N,N-dimethylformamide; TMS: sulpholane b electrochemical potential window measured at glassy carbon (GC) electrode c specific capacitance of activated carbon d magnesium trifluoromethansulfonate e estimated from the reported information in the literature

f conductivity of pure [EMI][BF4] and [EMI][Tf] were ca. 14 mS cm-1


42

2.5 Gap Analysis and Selection of Materials

As presented in this chapter, different classes of ILs with various combinations of cations and

anions can be obtained. Although not quite straightforward, the properties of ILs can be

tuned to meet the requirements of a specific application. Accordingly, the number of studies

on ILs in both liquid and solid state electrolytes is rapidly growing. There are still progress to

be made in the development of high performance polymer–ILs for ECs as well as in the

understanding of the relation between ionic and polymeric structures and the ion transport

mechanism. Some of the performance gaps and questions yet to be investigated are discussed

for both liquid ILs and polymer–ILs. A preliminary cost analysis is also presented for devices

enabled with polymer–ILs.

Liquid ILs:

The advance in aprotic ILs for EDLCs is highly focused on fluorinated IL systems. While the

low viscosity, high ionic conductivity, and wide potential window of fluorinated ILs are

desirable properties, their potential environmental issue and high cost [97] are equally

important for practical application. Also, among the few studies on PILs for

pseudocapacitors, those containing fluorinated anions are most commonly used.

The low or negligible vapor pressure of ILs is a major advantage over volatile organic

electrolytes that has significant positive environmental impact. To date, the information on

the environmental outcome, any potential instability, and toxicity issues of the ILs are not

fully clear. It has been suggested that ILs should be treated the same as other chemicals with

caution to avoid inappropriate experimental conditions. Studies have shown that typical ILs

with perfluorinated anions such as PF6 may decompose and produce toxic products (e.g., HF)

when contact with moisture [26,98]. Additional effort (e.g., sealing and packaging) will be

needed to avoid such reactions. It is necessary to develop environmentally benign ILs for

EDLCs and pseudocapacitors. There is also little research on PILs for pseudocapacitive


43

electrodes, and therefore a lack of understanding of the proton conduction mechanism in such

systems.

As alternatives to fluorinated ions, anions such as SO42−, PO4

3−, NO3−, acetate, and

methanesulfonate have been proposed to be environmentally benign [97,98]. In terms of

cation, ILs comprising of imidazolium cations have generally shown chemical stability and

high conductivity. Also, the planar imidazolium ring and its dangling alkyl groups constrain

the geometric packing. This together with the delocalization of charge over the N–C–N group

within the ring decreases the ionic interaction and lowers the melting points of these

compounds [33].

A non-fluorinated IL, 1-ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4) has

very interesting properties. This RTIL was also suggested as a proton conducting IL for fuel

cells and pseudocapacitors [87,99]. However, the viscosity of EMIHSO4 is noticeably high

(1650 cP) which leads to lower ionic conductivity than fluorinated ILs. The mechanism of

ion transport in liquid electrolyte relies on the motion of the ions and hence on the viscosity.

The ionic conduction mechanism in polymer electrolytes is different from that in liquid

electrolyte, and factors such as the ionic dissociation and the structural characteristics of the

polymer play important roles.

Polymer–ILs:

The use of fluorinated aprotic ILs was also dominant in polymer–ILs for EDLCs (see Table

2-5). There are only a few studies on non-fluorinated polymer–IL electrolyte for energy

storage devices. In a study by Sutto et al., solid state alkaline/acid batteries were developed

by utilizing PVA and EMIHSO4 [100]. A similar chemistry was used by Sellam and Hashmi

where they demonstrated the performance of the polymer–PIL for pseudocapacitors. This is

the only reported polymer–IL with proton conduction for ECs (see section 2.4.6.2).

The properties and ion transport mechanism of polymer–IL electrolytes are different from

available polymer–Li salt systems. Investigations on proton dissociation and proton

conduction mechanism of polymer–PILs are also scarce. There is a need to study the


44

dominating factors in the ionic conduction mechanism of polymer–IL electrolytes. It is

unclear whether the understandings of ion transport in liquid ILs are fully transferable to

polymer–ILs, i.e., in the presence of polymeric chains and their functional groups. While the

knowledge of ionic conduction in polymer–aprotic ILs can be applied to polymer–PILs, other

types of bonding (e.g., H-bonding) in PILs may affect the conduction process differently.

The other issue that is somewhat controversial is the effect of inorganic fillers. Most of

the studies in polymer–Li salts focus on the impact of fillers on the ionic conductivity and the

structural properties of polymer electrolyte. The intrinsic properties of fillers such as

nanostructure and dielectric constant can also lead to additional functionalities of the

polymer–IL systems. It is important to understand the relationship between such effects and

the overall performance of ECs enabled with these polymer–ILs.

Materials cost evaluation:

The weight distribution of materials in an EC as well as the cost of the devices depend

strongly upon its application and configuration. For instance, the performance of large-scale

ECs for power generation such as grid application requires bulk active materials, while safety

and volumetric energy and power densities of ECs are key factors for applications such as

consumer electronics and wearable devices. ECs enabled by polymer electrolytes are geared

towards the consumer and wearable applications and can be employed as hybrid systems

with batteries. In thin film EC configurations, the percentage of electrode materials is much

smaller than that of polymer electrolytes. A hypothetical weight distribution of EC cells

enabled by polymer–ILs is shown in Appendix C.

An estimated cost breakdown is given in Table 2-6 for EDLCs fabricated with the

polymer–ILs (i.e., using non-fluorinated and fluorinated ILs). The cost is also compared to a

device using the conventional liquid organic electrolyte. To focus on the cost of the

electrolytes, the weight distribution and the costs of current collector and electrode materials

of all cells were calculated for a common electrode material.


45

Table 2-6 Cost of the materials for cells fabricated with the polymer–IL electrolytes (1 cm2 laminated pouch-type cells).

Type of material Unit cost

($) Cost per cell ($)

Type of material

Unit cost ($)

Cost per cell ($)

EDLC with PEO–EMIHSO4 EDLC with PVdF-HFP–EMIBF4 Polymer electrolyte

0.078 Polymer electrolyte

1.483

IL (EMIHSO4) 640.00/kg IL (EMIBF4) 17460.00/kg Polymer (PEO)

446.00/kg Polymer (PVdF-HFP)

487.00/kg

Solvent (PC) 28.00/kg Solvent (NMP) 88.00/kg Filler (SiO2) 1252.00/kg - -

Electrodea 0.001 0.001 Aluminum current collector

1.00/m2

Graphite carbon, PVdF binder, NMP solvent

28.18/kg

Packaging 0.034 0.034 Sealing tape 72.38/m2 Aluminum lamination

4.07/m2

Total 0.113 1.518 a costs of electrode materials were obtained from [101] * the estimated cost of EDLC with liquid organic electrolyte (1 M TEABF4 in PC) was $0.38/cell

The analysis in Table 2-6 shows that the cost of devices enabled by polymer–IL

(HSO4-based) can be comparable to that using organic electrolyte. In general, the cost of ILs

can be reduced in bulk productions. Recently, the feasibility of large scale synthesis of ILs

has been demonstrated for HSO4-based ILs with ammonium and imidazolium cations. The

price of an imidazolium hydrogen sulfate IL has been estimated as $2.96-$5.88 kg-1 [102].

Considering an average of $4.00 kg-1, the price of the polymer electrolyte using EMIHSO4

can decrease by an additional 20% per cell.


46

2.6 Characterization Techniques

2.6.1 Electrochemical characterization

The electrochemical properties of electrolytes as well as their performance in EC devices are

characterized by both direct current (DC) and alternating current (AC) methods. In this

section, the two characterization techniques are introduced, and the principles for evaluating

the electrochemical properties are discussed.

2.6.1.1 Cyclic voltammetry (CV)

In a CV test, the applied voltage to the working electrode is varied linearly at a constant

sweep rate ±s = dV/dt and the resulting response current is measured (see Figure 2-10a). The

current vs. potential is represented by a cyclic voltammogram (see Figure 2-10b). To analyze

the performance of an EC, CV is used to characterize: (i) the amount of charge stored over

the operating potential window, (ii) the reversibility of the charge and discharge processes,

(iii) the different stages of the charge or discharge processes, and (iv) the rate performance of

the system with increasing the sweep rate.

For a given inert electrode material, CV can provide information on the performance of

the electrolyte. In this study, CV was used to determine the electrochemical potential window

of the electrolytes, and the capacitance and the rate capability of ECs. The characterization of

these electrochemical properties is discussed in the following.


47

Figure 2-10 (a) Cyclic voltammetry sweep, cyclic voltammogram profiles for (b) ideal and resistive double-layer capacitance, and (c) pseudocapacitance

a. Electrochemical potential window

The electrochemical potential window of an electrolyte is defined as the voltage range where

no electrochemical reaction (e.g., faradaic reaction of the electrolyte) is observed (from A to

B in Figure 2-10b). In a three-electrode cell configuration, the potential of an inert working

electrode (e.g., glassy carbon) is swept to the positive and negative limits of the electrolyte.

To evaluate the reduction or oxidation limiting potentials, a certain current density, cut-off

current, is selected. In studies of ECs, the cut-off current density has been selected to be

below 0.1 mA cm-2 [1].

b. Capacitance

A pure capacitor and the double-layer capacitor with a smooth electrode surface exhibit

ideally a “rectangular” current response or CV profile (Figure 2-10b). The capacitive current

response is independent of the potential, and the direction of current is immediately reversed

upon reversal of the potential sweep. In this case, the capacitance, C, is


48

iC =

s [F cm-2], (2-9)

where i is the capacitive current density in A cm-2 and s is the sweep rate in V s-1 [15]. The

capacitance value is also expressed as F g-1 of electrode material (i.e., active materials). The

focus of this work is on polymer electrolytes for solid thin and flexible ECs in which

thin-film electrodes are employed. Thus, the weight of the active materials is negligible

compared to the area [40]. The capacitance values reported in this work are mainly area

specific capacitance.

In practice, most ECs have an effective ESR which causes deviations from ideal

capacitive behavior. The ohmic component or the ESR arises from the electrolyte resistance,

Rs, the external lead contact resistance, and sometimes a distributed resistance because of

diffusion of ions into porous electrode materials. The resistive CV profile is also shown in

Figure 2-10b, where the current response can depend on the potential (or charge/discharge

rate) [103].

In the case of pseudocapacitive electrode materials, the current response is not constant as

the redox reactions occur over the potential range. The CV profile often shows reversible

peaks and a differential profile of C is generated. When a capacitor deviates from ideal

behavior, the capacitance is then calculated by

Δ qC =

ΔV [F cm-2], (2-10)

where Δq is the integrated stored charge in C cm-2 over the potential range ΔV (V) [15].

2.6.1.2 Electrochemical impedance spectroscopy (EIS)

In addition to DC methods, AC impedance measurement is another principal technique for

evaluating the electrochemical respond of ECs. In this method, the magnitude and the phase

relation of an AC current is recorded in response to an applied low-amplitude alternating

voltage. Considering a sinusoidal voltage shown in Figure 2-11, the current signal is


49

generally not in phase, and thus phase angle, θ, denotes the phase separation between the

voltage and the current.

Figure 2-11 Phasor diagram showing the relationship between alternating current and voltage signals at angular frequency ω [104]

The electrode-electrolyte and their interface can be expressed by equivalent-circuit

models, representing the capacitive and resistive behavior of the actual system. An

equivalent-circuit representation of a simple capacitor in series with a resistance (i.e., RC

system) is illustrated in Figure 2-12.

Figure 2-12 (a) Equivalent circuit of an RC system (an ideal capacitor), (b) Nyquist plot, and (c) Bode plot for the series RC system


50

The impedance as a function of frequency, f, is expressed in the complex notation as

Z( ) = Z ( ) jZ ( ) f f f , (2-11)

where Zʹ and Zʺ are the real and the imaginary parts of the impedance. The impedance

response is commonly represented by two types of diagrams (see Figure 2-12):

(1) complex-plane or the Nyquist plot, in which Zʺ (usually the capacitive one) is plotted

vs. Zʹ (ohmic one) over the frequency range, and

(2) Bode plot of the modulus of the impedance, |Z|, vs. log frequency and phase-angle

plot of θ vs. log frequency.

Considering the RC circuit shown in Figure 2-12a, at high frequencies, the resistive

component dominates and the system behaves as a pure resistor (Zʹ = R) with a phase angle

of 0° (Figure 2-12c). As the frequency decreases, the capacitive component contributes to the

impedance (Zʺ = 1/jωC), and reaches a phase angle of −90° for a pure capacitor. The

phase-angle in Bode plot demonstrates the transition of resistive to capacitive behavior. At

−45°, the resistance and capacitive reactance are equal, and hence as proposed by Miller, its

characteristic frequency can be used to evaluate the rate performance and the available

capacitor energy [105,106].

The Nyquist plot for the RC circuit is a straight line as shown in Figure 2-12b. In

practice, the angle obtained is often less than 90° mainly due to the diffusion and

accessibility of ions to the electrode which limits the formation of the double-layer

capacitance.

Accordingly, EIS enables the evaluation of: (i) resistance of the electrolyte or ESR of the

device, (ii) capacitance as a function of frequency, (iii) time constant or the rate of the

capacitive response of a capacitor, and (iv) various kinetic or associated electrical responses

(e.g., relaxation processes) over a wide frequency range. In the following, the

characterization methods of such properties are presented.


51

a. Ionic conductivity

The resistance of the electrolytes, Rs, or ESR is often measured by complex impedance

method using EIS. A two-electrode cell configuration is used where two smooth electrodes

(e.g., stainless steel blocking electrodes) with a fixed distance are immersed in the

electrolyte. For the polymer electrolytes, the sample is sandwiched between the electrodes,

and good contact is established by pressing [67]. A schematic of the cell setup for both

liquid-state and solid-state electrolyte is depicted in Chapter 3. The ionic conductivity of

electrolytes is obtained by calculating

d

σ =ESR×A

[S cm-1], (2-12)

where d is the distance between the two electrodes and A is the geometric area of the

electrodes. ESR (Ω) is directly extracted from the Nyquist plot at 0° phase angle, which

includes the electrolyte resistance and the external contact lead resistance.

b. Device capacitance and time constant

An electrochemical capacitor often oscillates in between two states: capacitive at low

frequencies and resistive at high frequency. Its frequency (f ) responses, obtained from the

impedance (Z), can be expressed as a complex capacitance function C(f ) = C′(f ) − jC″(f ).

C(f ) can be deconvoluted into the real C′(f ) and imaginary parts C″(f ) according to

Equations 2-13 and 2-14 [105], and shown by a schematic diagram in Figure 2-13.

2

ZC

(2π ) Zf

(2-13)

2

ZC

(2π ) Z

f (2-14)

The real part of the capacitance C′(f ) corresponds to the deliverable capacitance of the

cell. At low frequencies, C′(f ) reaches a plateau which is equivalent to the capacitance

obtained from DC measurements. The imaginary part of the capacitance C″(f ) is analogous


52

to energy dissipation by an irreversible process (e.g., dielectric loss). The rise in C″(f ) passes

through a maximum at a frequency f0 with a characteristic time constant τ0 = 1/f0. This time

constant, extracted at a phase angle (θ) of −45°, represents the transition from a capacitive

dominated behavior to a more resistive behavior. This characteristic time constant τ0 has been

defined as a dielectric relaxation time of each individual capacitor [105,107]. Between the

two states, capacitors behave similar to resistance-capacitance (RC) transmission line

circuits.

Figure 2-13 Schematic of the real (solid line) and imaginary (dash dotted line) parts of the capacitance

c. Dielectric Analyses

The dielectric properties of the polymer electrolytes were obtained from EIS. The variation

of dielectric response with frequency is expressed as the complex dielectric function

(f ) = ′(f ) − j″(f ), where the real part represents the dielectric permittivity of the system,

and the imaginary part represents the dielectric loss [71]. Since the entire two electrode

assembly is a capacitor, the complex dielectric function is described as (f ) = C(f )/C0, where

C0 = 0A/d is the vacuum capacitance (0 is the permittivity of free space, A is the geometric


53

surface area, and d is the thickness of the polymer electrolyte). Dielectric permittivity (′) and

dielectric loss (″) can be derived using Equations 2-15 and 2-16:

2

0

Zε =

(2π )C Zf

(2-15)

2

0

Zε =

(2π )C Zf

. (2-16)

The dielectric characteristics of a polymer electrolyte change under an external electric

field due to processes such as a reorientation of molecular dipoles, increase in mobile charge

carriers, and additional polarization from charge separation at its interfaces. A schematic

representation of the dielectric function vs. frequency of the electric field is shown in Figure

2-14. The dielectric permittivity (′) is attributed to the polarization of the polymer

electrolyte under the electric field, and the dielectric loss (″) corresponds to the fluctuations

of molecular dipoles and the motion of charge carriers. The profiles in Figure 2-14 reveal two

characteristic frequency regions: electrode polarization (EP) occurs at low frequencies and

dipolar relaxation occurs at high frequencies. EP is a process of ion accumulation at the

electrode-electrolyte interface, forming an electrical double layer as observed in a capacitor

[108]. Dipolar relaxation is due to the delay of the dipole response or the polymer segmental

relaxation under an electric field, which leads to an internal energy dissipation and dielectric

loss.


54

Figure 2-14 Schematic of the real (solid line) and imaginary (dash dotted line) parts of the complex dielectric function for a relaxation process and electrode polarization [108]

In capacitors enabled by polymer electrolytes, the dielectric property of the electrolyte

system and the electrode polarization influence the capacitance of the device. It will be

interesting to combine dielectric analyses of polymer electrolytes with the complex

capacitance of the enabled cell to understand the ionic conduction process.

d. Complex capacitance and dielectric analyses

In this method, the complex capacitance of capacitors enabled by polymer electrolytes was

analyzed together with the dielectric response of polymer electrolytes in the same capacitor

cell. The capacitors enabled by polymer electrolytes were subjected to two types of

impedance analyses: complex capacitance from EC point of view, and dielectric analyses

from conventional polymer approach. Then, the performance of capacitors was correlated to

the intrinsic dielectric properties of polymer electrolytes. It is necessary to connect the

schematics of capacitance and dielectric functions against the same frequency along with the

Bode plot.


55

Examining Figure 2-12, Figure 2-13, and Figure 2-14 the following relationships can be

obtained:

At low frequencies, the apparent dielectric constant ε′ (Figure 2-14) is proportional to

the capacitance of an electrochemical capacitor (C′) (Figure 2-13) in the EP region,

where the phase angle is approaching −90º in the Bode plot for a capacitor (Figure 2-

12c).

At the characteristic frequency where the phase angle is −45º, a sharp decrease in ε′

and a loss peak in ε″ appear, representing the transition of capacitive to resistive

behavior of the capacitor cells.

At high frequencies, ion transport is restricted due to the limited response of dipoles

in polymer chains and charge carriers. In this frequency region, the phase angle of a

capacitor is 0º, such that the contribution of capacitance reaches zero and the cell is

equivalent to a resistor.

Both EP and the dipolar relaxation processes are characterized by their respective time

constants (τEP and τR, τ = 1/f ). The time constant of EP (τEP) reflects the rate of charge and

discharge of the double layer. The relaxation time (τR) represents the response time of the

polymer motion. Also seen in Figure 2-14 is a derivative form of dielectric constant εder,

which can be used in the cases when the dielectric loss in the EP region overlaps with the

ionic conduction and obscures the dipolar loss peaks. Based on Equation 2-17 [5,109-111],

the derivative spectra often better reveals EP and relaxation peaks in situations where broad

peaks extend over several frequency decades.

der

2 ε ( )ε =

π ln

f

f (2-17)

In electrochemical capacitors, high capacitance and fast response are the key performance

attributes. When translated to the dielectric analysis of a solid capacitor based on a polymer

electrolyte, a high apparent dielectric constant ′ implies a greater amount of charge stored; a

shorter τ indicates fast rate capability (given the same capacitance). Specifically, a shorter τEP


56

suggests a faster capacitive response of the cells, while a shorter τR implies an easier polymer

segmental motion and hence fast ion transport and smaller resistance in the cells.

Consequentially, impedance and dielectric analyses should focus on C′ for EP and on R for τ.

2.6.2 Structural characterization

The structure of polymer–IL electrolytes is investigated to understand: (i) the effects of IL

and fillers on the crystalline and amorphous phases, (ii) the interaction between polymer, IL,

and filler, and (iii) the ion transport mechanism. The main techniques used in this study to

support the electrochemical analyses are: X-ray diffraction (XRD), differential scanning

calorimetry (DSC), and infrared (IR) spectroscopy.

2.6.2.1 X-ray diffraction

The diffraction of X-rays by crystals can be treated as reflections of X-rays by atomic lattice

planes characterized by spacing d. Reflection occurs when the condition of the Bragg law is

satisfied [112]:

2dsinθ = nλ , (2-18)

where d is the spacing of the atomic lattice planes, θ is the angle between the X-ray beam and

the planes, and λ is the wavelength of the X-ray. XRD studies provide structural information

of polymer electrolytes, namely the crystalline and the amorphous states.

The XRD patterns of semi-crystalline polymer materials are characterized by a few Bragg

reflections that are superimposed to the amorphous broad scattering (see Figure 2-15). The

diffused scattering originated from the amorphous contribution has a low-intensity profile

and is extremely broad and structureless due to the presence of structural disorder. The Bragg

peaks are the characteristics of the crystalline phase [113]. The difference in the diffraction

peaks of polymer crystals and normal crystals (e.g., salts) is that molecules, (i.e., polymer

chains) rather than atoms, construct the unit cell. As a result, the dimension of unit cells in


57

polymer crystals tend to be larger than that of normal crystals. The larger lattice parameters

and the increased interplanar spacings lead to diffraction angles generally much smaller than

for crystals. The amorphous and crystalline phases can be distinguished in the XRD pattern,

allowing monitoring the changes to the crystallinity as a function of salt or additive content.

Figure 2-15 Schematic illustration of XRD pattern of a semi-crystalline polymer displaying the broad amorphous peaks and the crystalline diffraction peaks

2.6.2.2 Differential scanning calorimetry

The structure and morphology of polymer electrolytes, which influence their properties such

as ionic conductivity, are strongly temperature-dependent. Differential scanning calorimetry

(DSC) is a technique well-suited for thermal analysis of both pure ILs and polymer

electrolyte systems. The heat input (ΔH) to the sample and a reference material is adjusted so

that sample and reference are kept at the same temperature. DSC measures the heat flow into

or from a sample during heating or cooling.


58

Figure 2-16 Schematic illustration of heating and cooling DSC thermograms including the thermal transitions, heat of crystallization, ΔHc, and heat of melting, ΔHm

A typical DSC thermogram is depicted in Figure 2-16 which illustrates the phase

transition processes during heating and cooling. In the case of semi-crystalline polymers and

pure ILs usually the following characteristics are observed:

During heating, the first significant heat capacity change is characteristics of the glass

transition (Tg) of the amorphous parts of the material.

At higher temperatures, the endothermic peak corresponds to the melting of the

crystalline phase of the material at the melting temperature (Tm).

On cooling, an exothermal peak indicates the recrystallization of the melted material

at a temperature (Trc) lower than Tm.

An exothermal crystallization (Tc) may occur on heating at above Tg where the

disordered amorphous domains become partially organized.

DSC provides information such as the melting temperature and the extent of crystalline

phase which directly impact the ionic conductivity of polymer electrolytes. Since the


59

polymer segmental motion occurs at above the glass transition, DSC is also useful to

determine the effect of ionic conductor or additives on this transition [71].

Similarly, DSC can be used to characterize the thermal properties of pure ILs, specifically

to determine their liquidus range. Most ILs form glasses at low temperatures and their lowest

liquid range is determined by either glass transition or melting point.

The analyses of DSC thermograms allow to: (i) identify the transition temperatures such

as Tg, Tm, and Trc, and (ii) quantify the degree of crystallinity. The percent crystallinity is

determined using the following equation [112]:

ο

m

m

ΔH% Crystallinity = ×100

ΔH (2-19)

The heat of melting (or heat of fusion), ΔHm, in J g-1 is obtained by integrating the areas

under the endothermic melting peaks (see Figure 2-16). The term ΔHm° is a reference value

and represents the heat of melting if the polymer was 100% crystalline. The reference heat of

melting has been established for the commonly used polymers. To account for the effect of

the remaining solvent on the crystallinity of the polymer electrolytes, the melting heat of

polymer films (i.e., without the ILs) were used as the reference.

2.6.2.3 Infrared (IR) spectroscopy

The mid-infrared IR spectra is usually acquired in the 4000-400 cm-1 range of the

electromagnetic spectrum. When the sample is radiated with infrared light, the chemical

bonds vibrate at specific frequencies that are characteristics of their molecular structure.

Absorbance occurs at different IR wavelengths, reflecting the connectivity of the atoms, the

surrounding molecules, and the type of vibration (e.g., stretching or bending). Thus, IR

spectroscopy probes the structure of a material through the molecular vibrations. For

example, the IR spectra of poly(ethylene) is shown in Figure 2-17. The CH2 groups in the

polymer can vibrate in different ways and the strong bands are assigned to asymmetric


60

stretching, bending, and rocking modes. In more complicated structures, double bonds (C=C)

or H-bonds (X–H) appear at different wavenumbers and usually higher than single bonds.

Figure 2-17 IR spectra (transmittance) of polyethylene displaying the main CH2 vibrations

In polymer electrolytes, IR spectroscopy is useful to study the interactions among the

electrolyte constituents, and to obtain chemical information about their structure. This

technique has been used to study: (i) interactions between the ions and the host polymer and

interactions between cations and anions [69], and (ii) the changes to the crystalline phase of

the polymer. The intensity and position of the corresponding peaks can be analyzed to

investigate the interactions between the polymer, IL, and fillers that provide information on

the ionic dissociation and crystallinity of the polymer electrolyte [114].

61

CHAPTER 3

EXPERIMENTAL METHOD AND CHARACTERIZATION

3.1 Materials

3.1.1 Ionic conductors

A non-fluorinated RTIL, 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4), was

selected for this study. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) was used as

the baseline fluorinated IL and to study the influence of anion on the properties of the ILs. In

addition, 1-methylimidazolium hydrogen sulfate (MIHSO4) and imidazolium hydrogen

sulfate (ImHSO4) were chosen to investigate the effect of the cationic substituent alkyl chain

and functional groups on the properties of the ILs. The structures of the ILs are depicted in

Table 3-1.

All the ILs but one were acquired from Sigma-Aldrich and Alfa Aesar. ImHSO4 was

synthesized using an acid-base reaction [34,115]. The reaction and the procedure is shown in

Figure 3-1, and the details are described in the following:

CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION

62

Table 3-1 Structure of the studied ILs. Ionic liquid Structure

1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4, 95% Sigma-Aldrich)

1-Methylimidazolium hydrogen sulfate (MIHSO4, 95% Sigma-Aldrich)

Imidazolium hydrogen sulfate (ImHSO4, synthesized) (Imidazole, 99% Alfa Aesar)

1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4, 98% Alfa Aesar)

(1) imidazole (i.e., the base) (24.94 gr, 0.37 mol) was first dissolved in water and then

introduced into a two-necked flask immersed in a dry ice bath and equipped with a

dropping funnel to add acid.

(2) an equimolar amount of sulfuric acid (97-98% in water) (35.88 gr, 0.37 mol) was

added dropwise to the base solution under stirring in about 1 hr. The temperature

during the reaction was maintained below 35 °C.

(3) the mixture was further stirred for 4 hr at room temperature, and then dried at 70 °C

under vacuum for 48 hr. The final product was a crystalline powder (55.41 gr, yield

91%).

Figure 3-1 Preparation of imidazolium hydrogen sulfate (ImHSO4) ionic liquid

+ H

H +


63

3.1.2 Polymers

The focus of this work was on polymer electrolytes constituted of PEO as the polymer matrix

and EMIHSO4 as the ionic conductor. The baseline polymer electrolyte was composed of

PVdF-HFP polymer matrix and EMIBF4. The structure of the polymer matrices and their

properties are summarized in Table 3-2.

Table 3-2 Properties of the polymer matrices.

Polymer matrix Repeating unit Mol. wt. Glass

transition Tg (°C)

Melting point

Tm (°C)

Ref.

Poly(ethylene oxide) PEO (Alfa Aesar)

–(CH2CH2O)n– 1,000,000 – 64 72 [66]

Poly(vinylidene fluoride-co- hexafluoropropylene) PVdF-HFP (Kynar Flex® 2801)

–(CH2CF2)x–(CF2CF(–CF3))y– 470,000 – 36 140 [116]

3.1.3 Additives

The effect of two types of inorganic nanofillers was investigated. For comparison, selected

SiO2 and TiO2 fillers had a similar particle size, but different nanostructures and dielectric

constants. The properties of the fillers are listed in Table 3-3.

Table 3-3 Properties of the SiO2 and TiO2 fillers [117,118].

Filler Structure Particle size

(nm)

Dielectric constant

(ε) SiO2 (Alfa Aesar) amorphous 10-20 3.8 TiO2 (Alfa Aesar) crystalline (anatase) 15 86

3.2 Polymer Electrolytes Fabrication

All polymer electrolytes were prepared by solution casting. The preparation of the polymer

electrolytes was carried out in a glove box under nitrogen atmosphere with trace moisture

less than 1 ppm. All the ILs were dried for 48 hr at 70 °C under vacuum to remove any trace


64

water. The general procedure is depicted in Figure 3-2, and the details are described in the

following sections.

Figure 3-2 Preparation steps of polymer–IL electrolytes

3.2.1 Preparation of PEO–EMIHSO4

PEO–EMIHSO4 electrolytes were prepared by the following procedure [119,120]:

(1) PEO was dissolved in propylene carbonate (PC) at 50 °C, and the mixture was stirred

for 7-8 hr,

(2) EMIHSO4 was added to the PEO gel, and mixed for 7-8 hr to obtain a homogeneous

solution,

(3) the resulting precursor solution was cast on a glass petri dish (the solution was spread

to ensure a uniform thickness), and


65

(4) the mixture was gradually heated from 50 °C to 80 °C during 8-10 hr and then dried

under vacuum to form a thin film.

The polymer electrolytes were flexible films with a thickness of 150−300 μm. The

polymer–IL composition is reported as a weight ratio of polymer to IL. PEO–EMIHSO4 were

prepared with different weight ratios of PEO:EMIHSO4: (1:1), (1:2), (1:3), and (1:4).

3.2.2 Preparation of PVdF-HFP–EMIBF4

PVdF-HFP–EMIBF4 electrolytes were prepared by a procedure similar to that for PEO–

EMIHSO4. In step (1), PVdF-HFP was dissolved in N-methyl-2-pyrrolidinone (NMP) to

form a solution. The time required to dissolve the polymer (step 2), and to dry the cast film

(step 4) was 4-5 hr. Table 3-4 summarizes the constituent materials for each polymer

electrolyte system.

Table 3-4 Material components of the polymer electrolytes. Electrolytes polymer Solvent IL PEO–EMIHSO4 PEO

Propylene carbonate

(PC, 99% Alfa Aesar) EMIHSO4

PVdF-HFP–EMIBF4 PVdF-HFP N-Methyl-2-pyrrolidinone (NMP, 99% Alfa Aesar)

EMIBF4

3.2.3 Preparation of polymer–IL with filler

Polymer electrolytes with inorganic fillers were prepared in a similar way to those without

filler. Prior to step (1), SiO2 and TiO2 powders were first dried (at 150 °C) and then mixed

with the respective polymers. The fillers were added at 3 and 10 wt% of the total weight (i.e.,

IL and polymer) excluding the solvent. As an example, the resulting electrolytes are referred

to as PEO−EMIHSO4–10% SiO2 or PEO−EMIHSO4−10% TiO2. The resulting PEO–

EMIHSO4 films without filler were translucent, and turned opaque with the addition of SiO2

and TiO2 fillers (see Figure 3-3).


66

Figure 3-3 Filler-free PEO–EMIHSO4, and PEO–EMIHSO4 containing SiO2 and TiO2 nanofillers

3.3 Device Fabrication

3.3.1 Electrodes

Three types of electrode materials were used to determine the performance of the electrolytes

and the EC devices:

(1) metallic electrode: stainless steel foil (50 μm thick, Type 304, McMaster-Carr),

(2) carbon electrodes: glassy carbon (3 mm diameter, Gamry Instruments) and graphite

ink (Alfa Aesar) coated on stainless steel (25-30 μm thick, ca. 1 cm2), and

(3) pseuodocapacitive electrodes: RuO2 on Ti foils (50 μm thick, ca. 0.5 cm2), the process

of manufacturing RuO2 electrodes is described in [121]. The loading of RuO2 was

about 1.5 mg cm-2, resulting in a capacitance of 150 to 170 mF cm-2 in an H2SO4

electrolyte for a single electrode. The composite carbon electrodes used in this study

were developed using multi-walled carbon nanotubes (MWCNT) which was

chemically modified by phosphomolybdate, PMo12O403− (PMo12) (25-30 μm thick, ca.

0.5 cm2). The modification method and the electrode fabrication are reported in [122],

and the pseudocapacitive performance of this composite electrode, referred to as

carbon/PMo12, has been demonstrated for ECs in aqueous electrolyte.


67

3.3.2 Liquid cells

To study the performance of the liquid electrolytes in a device, two types of EC cell

configurations were investigated, as illustrated in Figure 3-4:

(1) a beaker cell in which the distance between the electrodes was ca. 3 mm (referred to

as Liquid‒1), and

(2) a filter paper (Whatman® cellulose filter paper, medium porosity, thickness: 150 μm)

as separator was soaked with the electrolytes, and then sandwiched between the

electrodes (referred to as Liquid‒2).

Figure 3-4 Schematic representation of device configuration for the (a) Liquid–1, and (b) Liquid–2 cells

3.3.3 Solid cells

Solid EC devices were fabricated by stacking the polymer electrolytes between two

electrodes (see Figure 3-5a). The cells were further protected with electroplating tape and

then sealed by PET SelfSealTM lamination. Unless otherwise specified, all experiments were

carried out at room temperature inside a glove box.

Figure 3-5 (a) Schematic representation of device configuration for the solid cells; (b) the resulting laminated cells


68

3.4 Characterization

3.4.1 Structural characterization

Structural studies were conducted by XRD on a Philips PW 1830 HT diffractometer with a

Cu Kα radiation source (λ = 0.154 nm) operated at 40 kV and 40 mA to differentiate the

crystalline and amorphous states of the polymer electrolytes. The samples were sealed inside

glass vials in a glove box, and placed on the silicon substrate prior to the measurements.

Thermal analyses were performed using DSC on a DSC Q20 TA Instruments. The

samples for DSC measurements were sealed using hermetic aluminum pans in a glove box.

Thermograms were recorded during heating followed by a cooling scan at a rate of

10 °C min-1. All samples were initially equilibrated at −90 °C, then heated to 150 °C,

followed by cooling to −90 °C. This scan was repeated once for liquid samples, and thermal

properties were collected during the second scan, so that all liquid samples had the same

thermal treatment. The transition temperatures as well as the degree of crystallinity were

obtained from DSC analyses.

Information on chemical bonding, interactions between electrolyte components, and the

structure of polymer electrolytes were obtained via fourier transform infrared spectroscopy

(FTIR) on a Thermo Scientific Nicolet iD5 ATR spectrometer under a nitrogen purged

atmosphere in a glove bag. The IR spectra were recorded in the 4000-650 cm-1 frequency

range at a resolution of 2 cm-1. The advantage of using attenuated total reflectance (ATR)

technique is that the samples were examined without further preparation. This is particularly

useful for soft thin film polymers which were directly placed on the ATR crystal (diamond).

Similarly, for liquid samples (i.e., ILs) a small amount was simply placed onto the surface of

crystal. The main benefit of ATR sampling comes from the very thin sampling path length

and depth of penetration of the IR beam into the sample.


69

3.4.2 Electrochemical characterization

The electrochemical performance of the electrolytes and the enabled cells was evaluated by

CV and EIS using a Princeton Applied Research VersaSTAT 3 potentiostat. EIS had a

voltage amplitude of ± 10 mV and a frequency range from 0.1 Hz to 100 kHz.

The electrochemical potential window of liquid electrolytes was measured in a

conventional three-electrode cell with a glassy carbon as the working electrode (WE) and the

counter electrode (CE) and Ag wire as a quasi-reference electrode (QRE). The potential was

swept towards positive and then negative intervals to select the proper windows (see

Appendix D).

The conductivity of electrolytes was measured using stainless steel electrodes with a cell

assembly similar to Liquid‒1 for the liquid electrolytes and Liquid‒3 for the polymer

electrolytes (see Figure 3-4a and Figure 3-5a). The resistivity of the electrolytes was

extracted from the impedance analyses, and reported based on the average of a minimum five

cells. The cell resistance was recorded over a temperature range from −10 °C to +80 °C using

an Espec SH-241 temperature and humidity chamber. For these tests, the cell assembly was

further laminated with an aluminium-plastic film (thickness: 111 μm, MTI corporation) to

protect the solid cells (see Figure 3-5b).

The electrochemical analysis of electrodes was performed in a three-electrode cell in

which Pt mesh was used as CE. Saturated Ag/AgCl was used as the RE for aqueous

electrolytes, while AgNO3/acetonitrile (10 mM) RE was prepared for the IL electrolytes. The

electrochemical performance of EC devices with both liquid and polymer electrolytes were

measured in a two-electrode cell configuration (Figure 3-4b and Figure 3-5a). The

capacitance of EC devices were evaluated from both CV and EIS methods. The

measurements were repeated for at least five cells. The results are shown for the selected

cells representing each sample (see Appendix D). Table 3-5 summarizes the parameters


70

measured using each technique and the relationship between the performance of capacitor

and the properties of interest of liquid and polymer electrolytes.


71

Table 3-5 Parameters of interest and the relationship between capacitor performance properties and the related electrolyte/polymer electrolyte properties. Technique Measured parameter Capacitor properties Related electrolyte properties Equation

CV

potential window (E)

operating voltage stability of electrolyte (oxidation/reduction)

-

capacitance (C)

state of charge/discharge in EDLC and pseudocapacitor; rate

capability with increasing sweep rate

stability and compatibility towards electrodes

2-9,2-10

cycle life capacitance retention - -

EIS

high frequency cell impedance

(Z′)

equivalent series resistance (ESR)

ionic conductivity (σ)

2-12

low frequency cell capacitance

(C′)

capacitance corresponding to DC capacitance

low frequency dielectric constant (i.e., extent of electrode polarization)

(ε′,τEP) 2-13,2-15

RC time constant (τ)

rate performance (i.e., transition between resistive and

capacitive behavior)

dielectric relaxation of electrolyte (i.e., transition between dipolar relaxation and ionic conduction)

(ε″,τR)

2-14,2-16

CV & EIS energy density (E) and

power density (P) - - 2-3

72

CHAPTER 4

IONIC LIQUID ELECTROLYTES

This study begins with the investigation of the intrinsic properties of the ILs. The effects of

anion and cation on ionic conductivity, operating voltage, and the resultant double-layer

capacitance of the ILs were studied and presented in two parts: ILs with different anions

were investigated by comparing the properties of a non-fluorinated (HSO4−) imidazolium salt

to a common fluorinated (BF4−) salt with the same cation (section 4.1); the cations are

discussed in terms of the impact of the imidazolium substitution groups on the melting

temperature and the ionic conductivity (section 4.2). Most of the current commercial ECs use

organic electrolytes. The electrochemical properties of the studied ILs are compared to that

of a conventional organic electrolyte: tetraethylammonium tetrafluoroborate in PC

(TEABF4/PC) in order to assess their viability for capacitors. The performance of EDLCs

employing the ILs is also compared to serve as a baseline for the polymer–IL electrolytes.

4.1 Effect of Anion

A high performance EC requires a low cell resistance, a large capacitance, and a wide

operating voltage to achieve high energy density and power density. Ionic conductivity,

CHAPTER 4. IONIC LIQUID ELECTROLYTES

73

potential window, and resultant capacitance are key parameters for IL electrolytes in ECs.

EMIHSO4 was investigated in parallel with EMIBF4, a common RTIL composed of the same

imidazolium cation. Through such comparisons, the influence of anion on electrochemical

characteristics of ILs was studied (see Figure 4-1).

Figure 4-1 Structure of EMIHSO4 and EMIBF4

4.1.1 Ionic conductivity

The ionic conductivity of pure EMIHSO4 and EMIBF4 electrolytes are summarized in Table

4-1. Since the ionic conductivity is proportional to the mobility of charge carriers, and hence

inversely proportional to viscosity, the viscosity of the ILs is also included. For comparison,

the conductivity and viscosity of TEABF4/PC organic electrolyte with a concentration of

0.65 M (10.5 wt%) is also listed.

The high viscosity of EMIHSO4, shown in Table 4-1, is due to the more localized charge

on HSO4− compared to that on BF4

− anion and its tendency to form hydrogen bonding with

the cation. This increases the interaction between EMI+ and HSO4−, and consequently raises

the viscosity of the IL which in turn decreases the conductivity. Given that both ILs have the

same cation, the comparison of viscosities explains the effect of fluorinated anion on

reducing the viscosity and hence increasing the ionic conductivity of EMIBF4.

Table 4-1 Conductivity, potential window, and viscosity of studied electrolytes (at room temperature). Electrolyte σ (mS cm-1) E (V vs. Ag wire) η (cP) Ref. EMIHSO4 1.5 ± 0.1 2.2 1650b EMIBF4 18.6a 3.8 40c [32,43] EMIHSO4/PC solution 8.4 2.0 - TEABF4/PC solution 11.9a 2.4 2.5c [29] a values are consistent with the literature b provided by the manufacturer; c obtained from the literature


74

To improve the ionic dissociation of EMIHSO4 and compare its conductivity to organic

electrolytes, PC (as a solvent) was added to EMIHSO4. The conductivity at different

concentrations of EMIHSO4/PC solution is shown in Figure 4-2. The addition of PC reduced

the ionic attraction and the highest conductivity (8.4 mS cm-1) was obtained for 25 to 40 wt%

of EMIHSO4 which was in the same order as that of TEABF4/PC solution (see Table 4-1).

This indicates the presence of ion pairs in pure EMIHSO4 which was reduced with the

addition of solvent.

Figure 4-2 Conductivity as a function of EMIHSO4 concentration in PC

4.1.2 Potential window

Figure 4-3 illustrates the electrochemical stability of EMIHSO4, EMIBF4, EMIHSO4/PC, and

TEABF4/PC organic electrolyte measured with a glassy carbon electrode. The potential

windows were evaluated at a cut-off current of 0.1 mA cm-2 and their values are summarized

in Table 4-1. Both pure EMIHSO4 and EMIHSO4/PC exhibited a potential window of ca.

2 V, similar to that of organic electrolyte. Although the addition of PC slightly narrowed the


75

stability window of EMIHSO4, it significantly increased the ionic conductivity. Comparing

EMIHSO4 and EMIBF4, the wider potential window of EMIBF4 is due to the fluorinated

anion which usually exhibits higher stability towards oxidation [1,123].

Figure 4-3 Voltammetric potential window recorded at a glassy carbon electrode at a sweep rate of 100 mV s-1 (due to the high viscosity of EMIHSO4, measurements were performed at a low sweep rate: 5 mV s-1)

4.1.3 Electrode capacitance and device performance

The capacitance of a glassy carbon electrode in EMIHSO4 and EMIBF4 was estimated

through the integration of charge from their CV profiles (Figure 4-3). The capacitance of the

glassy carbon electrode was 54.0 ± 3.0 μF cm-2 in EMIHSO4 and 42.2 ± 1.6 μF cm-2 in

EMIBF4 which are within the expected range (i.e., microfarad) for double-layer capacitance

on a smooth electrode.

To characterize the performance of the pure ILs in a device, a two-electrode cell was used

to resemble a symmetrical EDLC (Liquid‒1, see Figure 3-4a). Figure 4-4 shows the CVs of

the cells using graphite electrodes and EMIHSO4 and EMIBF4 as respective electrolytes. The


76

capacitance of both devices was similar (ca. 1.4 mF cm-2) at 100 mV s-1. However, at 1 V s-1

the CV profile of EMIHSO4-based cell was resistive and the resulting capacitance was lower

than that of fluorinated EMIBF4-based cell. This suggests that the high viscosity of

EMIHSO4 affected the motion of ions and hence the high rate performance of the device.

While the operating voltage of graphite EDLC enabled with EMIHSO4 was 2 V, it was lower

with metallic electrodes such as stainless steel. Accordingly, 1.5 V was selected as the

operating voltage.

Figure 4-4 Cyclic voltammograms of graphite cells tested with EMIHSO4 and EMIBF4 at a sweep rate of 100 mV s-1 (Liquid–1 beaker cells)

The performance of the EC cells (Liquid–2 configuration) was further characterized by

impedance analyses, where the capacitance of a device C(f ) was deconvoluted into C′(f ) and

C″(f ) corresponding to the real and imaginary part of the capacitance, respectively

(Equations 2-13 and 2-14). Figure 4-5 shows Cʹ and Cʺ with respect to frequency for EDLC

devices using EMIHSO4 and EMIBF4. The deliverable capacitance of cells was obtained

from Cʹ at low frequencies, and τ (RC time constant) was derived from Cʺ plot. This time


77

constant, which is at the characteristic frequency where the phase angle is −45°, represents

the rate capability of the device to transit from a more resistive behavior at high frequency to

a capacitive dominated behavior at low frequency. Given the same capacitance, a smaller τ is

desired for high rate ECs.

Figure 4-5 Real C′ and imaginary C″ part of the capacitance vs. frequency for graphite EDLCs with EMIHSO4 and EMIBF4 (Liquid–2 filter paper cells)

At low frequency (0.1 Hz), the capacitance of EDLC using EMIHSO4 was 1.1 mF cm-2,

higher than that for EMIBF4-enabled device (0.75 mF cm-2). On the other hand, the charge

delivery in the device using EMIBF4 was much faster than that with EMIHSO4: 40 ms

compared to 6 s. Since they have similar capacitance, the large difference in τ is attributed to

the resistance of these two ILs. These results agree with those obtained from the CVs in

Figure 4-4: the capacitive current of EMIHSO4-enabled cell was slightly higher, whereas

EMIBF4-enabled cell performed faster charge/discharge. While EMIHSO4 shows promising

electrochemical performance, its high viscosity is a limiting factor for high rate applications.


78

The effect of viscosity on the capacitance was examined by analyzing the double-layer

capacitance of a glassy carbon electrode in EMIHSO4/PC solutions. Figure 4-6 shows the

electrode capacitance and the ionic conductivity vs. the concentration of EMIHSO4 in PC.

The capacitance of the electrode increased with the EMIHSO4 concentration. The highest

capacitance was obtained at 40 wt% EMIHSO4 which was also at the optimum concentration

with the highest ionic conductivity. The similar trend of capacitance and ionic conductivity

indicates that the number of dissociated ions increased up to 40 wt% EMIHSO4, resulting in

a high ionic conductivity and double-layer capacitance. With further addition of EMIHSO4,

both conductivity and capacitance decreased due to the increase of ion-pair formation and

hence high viscosity.

Figure 4-6 Double-layer capacitance of glassy carbon electrode and conductivity as a function of EMIHSO4 concentration in PC

Although EMIHSO4 has much lower ionic conductivity than EMIBF4, EMIHSO4-based

EDLC can store and deliver the same or even higher amount of charge as that of

EMIBF4-based cell at low sweep rate. In addition, the intended application of ILs in this

work is for polymer electrolytes, in which ion transport takes place through polymer chain

motions rather than the movement of ions in pure IL. The high viscosity of EMIHSO4 may


79

be less influential in a polymer matrix than in the liquid electrolyte. Therefore, developing

polymer electrolytes using non-fluorinated EMIHSO4 could be a viable approach for high

performance and environmental friendly solid ECs.

4.2 Effect of Cation

The electrochemical properties of EMIHSO4 can be adjusted by altering the constituent

cation or anion. Since the primary focus of this thesis is on the environmentally safe ILs, an

alternative to replace fluorinated ILs is to change the cation structure. Alkylimidazoliums are

among the most common cations used in ILs due to their reasonable conductivity and

electrochemical stability [30]. Imidazolium-based cations were investigated by studying the

impact of cationic substitution groups on the thermal and the electrochemical properties of

ILs. Specifically, introducing functionalities such as protons was expected to have additional

contribution to the ionic conductivity of the ILs. The cations were tailored by varying the

alkyl substitution groups of the imidazolium ring. The structures of the respective ILs are

depicted in Table 4-2 together with their melting points.

Table 4-2 Structure and melting temperature of the ILs with different cations.

Ionic liquid Structure Melting point

(°C) 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4)

24

1-Methylimidazolium hydrogen sulfate (MIHSO4)

47

Imidazolium hydrogen sulfate (ImHSO4)

85

+H

H+

H


80

1-Ethyl-3-methylimidazolium (EMI) cation has two alkyl substitutions at the nitrogen

atoms. In 1-methylimidazolium (MI) cation the ethyl group is replaced by a methyl group,

and the methyl on N-3 is substituted by a hydrogen. Imidazolium (Im) cation only has

hydrogen substituents on the nitrogen atoms. As shown in Table 4-2, by substituting the alkyl

chains of EMI cation with hydrogen, the proton activity may increase in MIHSO4 and further

in ImHSO4. On the other hand, the melting temperature of the ILs increased from 24 °C for

EMIHSO4 to 47 °C and 85 °C for MIHSO4 and ImHSO4, respectively. The increase in

melting point was expected as the removal of the alkyl substitution groups increased the

symmetry in the cation structure, permitting more efficient ion–ion packing and stronger

ionic bonding that raised the melting point of the IL system.

While MIHSO4 and ImHSO4 contain protons and can potentially act as proton conducting

ILs, their melting point should be lowered to allow the application as electrolytes for room

temperature operating conditions. Nevertheless, the electrochemical property and the

feasibility of these ILs as electrolytes for ECs can be examined for their solutions in certain

solvents which are discussed in the following sections.

4.2.1 Ionic conductivity of IL solutions

Solid MIHSO4 and ImHSO4 were dissolved in solvents to prepare electrolytes for

electrochemical characterizations. For comparison, solutions of EMIHSO4 were also

prepared. The effect of cationic functional groups of these ILs on the electrochemical

performance such as conductivity, potential window, and capacitance of enabled EC were

analyzed and compared.

Among various polar solvents, EMIHSO4, MIHSO4, and ImHSO4 were soluble in

methanol and acetic acid (i.e., polar protic solvents). The conductivity at different

concentration of IL/MeOH and IL/Ac acid solutions are shown in Figure 4-7. The

comparisons between the conductivity trends show the following:


81

(i) The overall trend shows the higher ionic conductivity of IL/MeOH compared to that

of IL/Ac acid solutions. It indicates the impact of the greater polarity of MeOH

( = 32) than that of Ac acid ( = 6), resulting in a higher ionic dissociation and thus

higher conductivity.

(ii) The highest ionic conductivities were obtained at a concentration of 40 wt%

IL/MeOH solutions. Within this concentration, ionic conductivity increased in the

order of EMIHSO4 < ImHSO4 < MIHSO4. This could be due to the additional

contribution of MI and Im cations to proton conduction.

(iii) The effect of solvent can be seen by comparing the ionic conductivity of EMIHSO4 in

PC (Figure 4-2), Ac acid, and MeOH. Conductivity increased in the order of

EMIHSO4/PC ˂ EMIHSO4/Ac acid ˂ EMIHSO4/MeOH. The much higher

conductivity of EMIHSO4/MeOH than that of EMIHSO4/PC reflects the higher ionic

dissociation in protic MeOH ( = 32) vs. aprotic PC ( = 64). Since the polarity of PC

is higher than MeOH, the significantly higher ionic conductivity of EMIHSO4/MeOH

may indicate the influence of protic solvent on promoting proton dissociation.

Figure 4-7 Conductivity of solutions of EMIHSO4 ( ), MIHSO4 ( ), and ImHSO4 ( ) in methanol (filled symbols) and acetic acid (empty symbols)


82

The methanol solutions of EMIHSO4, MIHSO4, and ImHSO4 at 40 wt% IL were used for

further electrochemical analyses. The performances of these three IL solutions were first

compared in enabled EDLCs.

4.2.2 Device performance using IL solutions

The electrochemical properties of the three ILs were examined to evaluate their respective

double-layer capacitance of ECs. Filter papers were impregnated with the methanol solutions

of EMIHSO4, MIHSO4, and ImHSO4 and graphite two-electrode cells were assembled

according to Liquid–2 configuration (see Figure 3-4b). A cell with the solution of 40 wt%

EMIHSO4/PC was also prepared as a baseline.

Figure 4-8 shows the device performance using EMIHSO4/PC, EMIHSO4/MeOH,

MIHSO4/MeOH, and ImHSO4/MeOH at 100 mV s-1 and 1 V s-1. EC cells with IL/MeOH

electrolytes exhibited a similar capacitive performance and maintained an operating voltage

of 1.5 V. This demonstrates the viability of MIHSO4 and ImHSO4 as ionic conductors for EC

applications. The capacitance of the cells with methanol solutions is slightly higher for

MIHSO4-based electrolyte followed by ImHSO4 and EMIHSO4-based electrolytes. This

trend is consistent with the ionic conductivity of the electrolytes observed in Figure 4-7

which may result from the higher ionic dissociation or the proton dissociation in MIHSO4

and ImHSO4 electrolytes.

The comparison between the CVs of cells with EMIHSO4/MeOH and EMIHSO4/PC

clearly shows the influence of methanol on improving the ionic dissociation. The greater

ionic dissociation in methanol led to a higher cell capacitance at both low and high sweep

rates as illustrated in Figure 4-8.


83

Figure 4-8 Cyclic voltammograms of graphite cells using EMIHSO4/PC, EMIHSO4/MeOH, MIHSO4/MeOH, ImHSO4/MeOH electrolytes at (a) 100 mV s-1 and (b) 1 V s-1


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The study of IL solutions enabled us to: (i) compare the effect of cationic functional

groups on the electrochemical properties of respective ILs, and (ii) determine the impact of

solvents on ionic dissociation. Despite the superior conductivity and electrochemical

performance of the ILs in MeOH to that in PC, application of MeOH is not practically

feasible and environmentally favorable for ECs due to its low flash point (11 °C) and boiling

point (65 °C).

Among the three ILs, EMIHSO4 remains liquid at room temperature and exhibited a

reasonable ionic conductivity. Thus, it can be incorporated into polymer network to develop

thin-film polymer electrolytes and warrants further investigation. The compatibility of

EMIHSO4 with PC was demonstrated at different concentrations (Figure 4-2). PC has been

used as a solvent in organic electrolytes and as a plasticizer in polymer electrolytes. The

majority of this work was focusing on developing polymer electrolytes based on EMIHSO4;

characterizing their structural and electrochemical properties; and optimizing the material

system for application in ECs.

To explore the proton conductivity of these three ILs, alternative approaches can be

employed to examine other protic solvents with a lower volatility, or to reduce the melting

temperature of MIHSO4 and ImHSO4 to obtain pure liquid ILs at low temperatures. The

latter approach was investigated which is presented in Chapter 7.

4.3 Summary

Ionic conductivity and potential window of the imidazolium-based ILs and capacitance of the

enabled devices were studied. The study was mainly on two aspects of ILs: (i) fluorinated vs.

non-fluorinated anion, and (ii) different cationic functional groups. The impact of the

fluorinated anion on the properties of ILs was characterized by the lower viscosity and the

higher conductivity of EMIBF4 compared to EMIHSO4. At low sweep rate, EDLC using

EMIHSO4 could store the same amount of charge as the EMIBF4-based cells. Addition of


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solvent can be used to reduce the ion-pair formation in EMIHSO4, increasing the ionic

conductivity to the level of organic electrolytes. However, viscosity may not be the

dominating factor on the ion transport in polymer electrolytes. This will be explored in the

next chapter.

The melting point of EMIHSO4 increased with substituting the cationic alkyl groups by

shorter alkyl chains and/or hydrogen in MIHSO4 and ImHSO4. The electrochemical

properties were studied for solutions of MIHSO4 and ImHSO4 and compared to EMIHSO4

solution. The ionic conductivity of the three ILs were higher in MeOH than in Ac acid and

PC, indicating that the polar protic solvent increased the ionic dissociation and likely

promoted proton dissociation. EDLCs enabled by MIHSO4/MeOH and ImHSO4/MeOH

exhibited a higher capacitance than that of EMIHSO4/MeOH-based cells in agreement with

the conductivity of the respective IL solutions, further suggesting the contribution of proton

conduction to the ionic conductivity and the double-layer capacitance.

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CHAPTER 5

POLYMER–IONIC LIQUID ELECTROLYTES

The feasibility of PEO–EMIHSO4 for ECs is investigated by examining the ionic

conductivity, structural characteristics, thermal stability, and the performance of enabled

devices. These properties are compared to both liquid EMIHSO4 and PVdF-HFP–EMIBF4.

The ionic conductivity is studied as a function of IL content and temperature. The influence

of IL on the crystallinity and the melting temperature of the polymer–ILs is presented, and

correlated to the ionic conductivity (section 5.1). The interactions between PEO and

EMIHSO4 are identified to assess its influence on crystallinity and conducting ions (section

5.2). Finally, the capacitance and rate performance of EDLCs enabled with PEO–EMIHSO4

are evaluated and compared to its liquid counterpart and to that of PVdF-HFP–EMIBF4

(section 5.3).

5.1 PEO–EMIHSO4 and PVdF-HFP–EMIBF4 Electrolytes

The electrochemical properties of EMIHSO4 and EMIBF4 were characterized and presented

in Chapter 4. Although the non-fluorinated EMIHSO4 had a high viscosity and hence a low

conductivity compared to fluorinated EMIBF4, the low-rate performance of EMIHSO4-based

device was comparable to that based on EMIBF4. To investigate the viability of incorporating

CHAPTER 5. POLYMER–IONIC LIQUID ELECTROLYTES

87

the IL into a polymer, PEO was integrated with EMIHSO4 to develop PEO–EMIHSO4

electrolytes. PVdF-HFP–EMIBF4 electrolytes were also developed and analyzed to serve as a

baseline for the structural and the electrochemical comparisons.

5.1.1 Ionic conductivity

The ionic conductivities of PEO–EMIHSO4 with different compositions of EMIHSO4 were

measured to optimize the polymer electrolytes. Addition of polymers to ionic conductors

generally decreases ionic conductivity [60,124]. However, the extent of reduction in the ionic

conductivity of PEO–EMIHSO4 compared to that of pure EMIHSO4 (1.5 mS cm-1) was not

as severe as those PEO–IL electrolytes reported in [85,86,96]. The average conductivity of

PEO–EMIHSO4 (1:2) was 0.7 mS cm-1 and increased to 0.8 mS cm-1 in PEO–EMIHSO4

(1:3). This is within the range reported for PEO-based electrolytes such as PEO–BMPyrTFSI

(0.3 mS cm-1 [85]) and PEO–MMPIBF4 (4.4 mS cm-1 [125]). A further increase in the IL

content in 1:4 composition had a negligible effect on conductivity, and negatively affected

the structural integrity of the polymer electrolyte. Thus, 1:3 was selected as the optimum

composition for further analyses.

Due to a different mechanism of ionic conduction in polymer electrolytes, the low

viscosity and the high conductivity of many fluorinated ILs may not have the same advantage

in polymer matrices. Despite the significantly lower viscosity of EMIBF4, a noticeable

decrease in its ionic conductivity was observed. Table 5-1 summarizes the ionic conductivity

of both polymer–IL electrolytes and the corresponding pure ILs. The viscosities of pure ILs

are also given for comparison. The ionic conductivity of PVdF-HFP–EMIBF4 decreased at

higher EMIBF4 content, and the resulting polymer films were too soft. Thus, PVdF-HFP–

EMIBF4 (1:2) was selected as the fluorinated polymer–IL baseline in this study.


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Table 5-1 Conductivity and activation energy of ionic conduction for ILs and polymer–ILs (viscosity of pure ILs is also listed).

Electrolytes η (cP) σ (mS cm-1) Ea (kJ mol-1)

Low T High T EMIHSO4 (liquid) 1650 1.5 ± 0.1 44 PEO–EMIHSO4 (1:2) - 0.7 ± 0.1 28 18 PEO–EMIHSO4 (1:3) - 0.8 ± 0.1 28 18 EMIBF4 (liquid) 40 18.6 PVdF-HFP–EMIBF4 (1:2) - 6.5 ± 0.3 24

The ionic conductivity of EMIHSO4 and PEO–EMIHSO4 electrolytes were measured as a

function of temperature to study the effect of polymer on the ionic conduction of the IL.

Figure 5-1 shows the ionic conductivities of pure EMIHSO4 from 25 °C to 80 °C as well as

PEO–EMIHSO4 compositions: 1:2, 1:3, and 1:4, at temperatures from –10 °C to +80 °C. The

activation energies of ionic conduction were calculated from Arrhenius relationship, and are

reported in Table 5-1.

Figure 5-1 Temperature dependence of the ionic conductivity of EMIHSO4 and PEO–EMIHSO4 in (1:2), (1:3), and (1:4) compositions


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Since EMIHSO4 is highly viscous, ionic mobility is expected to be limited in the liquid

IL electrolyte. Indeed, the estimated activation energy of 44 kJ mol-1 suggested that the

motion of the constituent ions (EMI+ and HSO4−) is controlled by the viscosity of EMIHSO4

[53,99].

The polymer–IL electrolytes had lower conductivities than the pure IL. The ionic

conductivity of PEO–EMIHSO4 increased with the increase of EMIHSO4 from 1:2 to 1:3

compositions, and remained relatively unchanged for 1:4 composition within the temperature

range. All three PEO–EMIHSO4 electrolytes exhibited two linear regions attributed to a

transition from a semi-crystalline state at low temperatures to an amorphous state at high

temperatures separated by the melting point of PEO (60–70 °C) [61]. The transition between

these states occurred at temperatures between 40 °C and 50 °C, lower than the melting point

of crystalline PEO. The decrease in the activation energy of PEO–EMIHSO4 from 28 kJ mol-1

at below melting point to 18 kJ mol-1 at temperatures above melting corresponds to the

structural transition of PEO–EMIHSO4 from semi-crystalline to amorphous [60,124]. The

lower activation energy at high temperatures implies that the ion transport is facilitated in the

amorphous PEO–EMIHSO4. The increase in the segmental motion of the polymer backbone

in the amorphous state promotes the movement of ions that is desirable for a polymer

electrolyte.

The comparison between the activation energies of PEO–EMIHSO4 electrolytes and pure

EMIHSO4 supports the notion that the conduction mechanism in the thin-film polymer

electrolyte is different from that in pure IL. The lower activation energy of ionic conduction

in PEO–EMIHSO4 suggests that addition of PEO did not add an extra energy barrier to the

ionic motion of EMIHSO4. Ionic conductivity at low temperatures can be achieved in the

polymer electrolytes, while application of pure EMIHSO4 is limited to temperatures above its

melting point (24 °C).

Although the ionic conductivity of PEO–EMIHSO4 is within the range reported for PEO–

ILs, the performance of PEO–EMIHSO4 requires improvement to a level comparable to


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fluorinated polymer–IL electrolytes (i.e., in the range of mS cm-1 or higher). Figure 5-2

shows the comparison between the ionic conductivity of PEO–EMIHSO4 and PVdF-HFP–

EMIBF4 over the temperature range. Accordingly, it is necessary to extend and stabilize the

amorphous phase of the polymer electrolyte in order to improve the ionic conductivity of

PEO–EMIHSO4 for applications at room temperature and ambient environment. To

understand the effect of IL on the crystallinity and thermal stability of the polymer

electrolytes, structural and thermal characterizations of the polymer–ILs were performed and

are discussed in the following section.

Figure 5-2 Temperature dependence of the ionic conductivity of PEO–EMIHSO4 and PVdF-HFP–EMIBF4

5.1.2 Crystallinity and thermal characterizations

As discussed in the previous section, the ionic conductivity of PEO–EMIHSO4 can be

enhanced by increasing the amorphous phase. As shown in Figure 5-1, the transition between

the semi-crystalline and the amorphous states of PEO–EMIHSO4 electrolytes was at


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temperatures lower than that of crystalline PEO, implying that addition of IL affected the

structure of polymer. The phase structure of polymer electrolytes was characterized using

XRD to investigate the impact of IL on the crystalline or the amorphous phase of polymer.

The crystallinity and the thermal properties of polymer–ILs such as melting point were

further examined through DSC analyses.

5.1.2.1 XRD analyses

XRD studies were performed to investigate the changes to PEO structure after it has been

impregnated with EMIHSO4. Figure 5-3a illustrates the XRD patterns of PEO powder, PEO

film, and PEO–EMIHSO4 (1:2) electrolyte. Two dominant peaks at 19.3° and 23.4° 2θ

related to the crystalline phase of PEO [7,126] were observed in both PEO film and PEO–

EMIHSO4. The intensity of these peaks decreased from PEO powder to PEO film and PEO–

EMIHSO4, suggesting a corresponding decrease in the crystalline structure of the

electrolytes. The peaks observed at 15.1°, 15.5°, 26.4°, and 27.1° 2θ for PEO powder were

hardly visible for the polymer electrolyte which also implies a decrease of the crystalline

phase. The peak intensities of the two dominant crystalline peaks are reported in Table B-1.

Figure 5-3b reveals the influence of the EMIHSO4 content on the polymer structure. The

XRD patterns depict a broad amorphous peak in the baseline between 15° and 25° 2θ

diffraction angles along with the dominant crystalline peaks. As the intensity of the

crystalline peaks substantially decreased with increasing IL content, the amorphous peak

became more dominant in PEO–EMIHSO4 (1:3). This indicates that the incorporation of

EMIHSO4 into PEO promoted the degree of disorder in the polymer chains and hence

improved the amorphous phase.


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Figure 5-3 XRD patterns of (a) PEO powder, PEO film, and PEO–EMIHSO4 (1:2) electrolyte; (b) PEO–EMIHSO4 electrolytes in (1:1), (1:2), and (1:3) compositions


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The structure of PVdF-HFP–EMIBF4 electrolyte was also analyzed parallel to that of

PEO–EMIHSO4. Figure 5-4 shows the XRD patterns of PVdF-HFP powder, PVdF-HFP film,

and PVdF-HFP–EMIBF4. Comparing this graph to Figure 5-3a, the effect of EMIBF4 on the

crystalline structure of PVdF-HFP is analogous to PEO–EMIHSO4 electrolyte. The XRD

pattern of PVdF-HFP shows the characteristic peaks of the crystalline structure of PVdF at

17.9°, 26.3°, and 38.0° 2θ corresponding to the large α-phase spherulites [127,128]. The peak

at 19.5° 2θ corresponds to a mixture of α-phase and γ-phase (i.e., small crystals or

spherulites) [128]. In the spectrum of PVdF-HFP film, the peaks at 26.3° and 38.0° 2θ were

not visible, and the intensity of the diffraction peak at 17.9° 2θ decreased. This peak

disappeared with the addition of EMIBF4 into PVdF-HFP. Comparing to the XRD pattern of

PVdF-HFP film, although the peak at 19.5° 2θ became sharper, the amorphous baseline was

more pronounced. This can be attributed to the incorporation of EMIBF4 which caused a

small change in the crystal structure of PVdF from α-phase to γ-phase [129], suggesting the

reduction of large crystals.

XRD analyses suggested that the integration of both EMIHSO4 and EMIBF4 with the

respective polymer matrices reduced the ordered and the crystalline structure. To determine

the degree of crystallinity of the polymer electrolytes as a function of IL content and its effect

on ionic conduction, thermal analyses were performed using DSC.


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Figure 5-4 XRD patterns of PVdF-HFP powder, PVdF-HFP film, and PVdF-HFP–EMIBF4 electrolyte

5.1.2.2 DSC analyses

Using DSC, the melting temperature and the crystallinity of PEO–EMIHSO4 electrolytes

were determined at different IL content. A PEO film was used as a reference. DSC

thermograms of PEO film and PEO–EMIHSO4 electrolytes are shown in Figure 5-5. An

endothermic peak upon heating the PEO film was found at 67 °C corresponding to the

melting point of the crystalline phase. During cooling, an exothermic peak was observed at

45 °C attributed to the recrystallization of PEO. When PEO was impregnated with

EMIHSO4, both melting and recrystallization peaks shifted to lower temperatures at 50 °C

and 42 °C for PEO–EMIHSO4 electrolytes in 1:2 and 1:3 compositions, respectively. This

indicates that the incorporation of EMIHSO4 into PEO changed the phase structure of the

polymer.


95

Figure 5-5 Heating and cooling DSC thermograms of PEO film, PEO–EMIHSO4 electrolytes in (1:2) and (1:3) compositions

The crystallinity (Xc) of the polymer electrolytes were calculated from the melting heat of

PEO film and are listed in Table 5-2 together with the melting (Tm) and the recrystallization

(Trc) temperatures. Observing the significant decrease in melting heat, it can be concluded

that the crystallinity of PEO–EMIHSO4 was considerably reduced by 48% with increasing IL

content in 1:3 composition. These results clearly demonstrate that the addition of EMIHSO4

into PEO promoted the disorder of the polymer chains and hence decreased the crystallinity

[130]. The melting temperature of PEO–EMIHSO4 electrolyte obtained from DSC supports

the conductivity trend shown in Figure 5-1 in which the transition from semi-crystalline

phase to amorphous states appeared at temperatures between 40 °C and 50 ºC. The decrease

of the melting temperature in PEO–EMIHSO4 electrolyte is advantageous as most

applications are at ambient conditions.


96

Table 5-2 Melting and recrystallization temperatures, and degree of crystallinity of PEO powder, PEO film, and the polymer electrolytes.

Samples Tm (°C) Trc (°C) Xc (%) PEO powdera 72 47 87 PEO filma 67 45 66 PEO–EMIHSO4 (1:2) 50.0 ± 6.5 21.3 ± 5.3 31.5 ± 6.0 PEO–EMIHSO4 (1:3) 42.3 ± 1.2 15.3 ± 0.5 17.7 ± 1.3

a values are consistent with the literature

In the XRD analyses of PVdF-HFP–EMIBF4, it was observed that with the addition of

EMIBF4, the increase of the amorphous phase was accompanied with an increase in the

intensity of crystalline peak at 19.5° (see Figure 5-4). The influence of EMIBF4 on the

crystallinity of PVdF-HFP was not as clear as that of PEO–EMIHSO4. Accordingly, DSC

analyses were performed to characterize the crystallinity of PVdF-HFP–EMIBF4. Similarly,

the percentage of crystallinity was calculated from the melting heat of PVdF-HFP film.

Figure 5-6 shows the DSC thermograms of PVdF-HFP film and PVdF-HFP–EMIBF4

electrolyte. While the melting temperature of PVdF-HFP film is higher (132 °C) than that of

PEO film, it exhibits a lower degree of crystallinity (see Table 5-3). The addition of EMIBF4

decreased both melting and recrystallization of PVdF-HFP film and reduced its crystallinity

to 41%.


97

Figure 5-6 Heating and cooling DSC thermograms for PVdF-HFP film and PVdF-HFP–EMIBF4 electrolyte

Table 5-3 Melting and recrystallization temperatures, and degree of crystallinity of PVdF-HFP film and PVdF-HFP–EMIBF4.

Samples Tm (°C) Trc (°C) Xc (%) PVdF-HFP film 132 79 49 PVdF-HFP–EMIBF4 (1:2) 108 46 41

Both XRD and DSC analyses indicated that while the crystalline phase was still present

in PEO–EMIHSO4 and PVdF-HFP–EMIBF4, the degree of crystallinity decreased with the

addition of IL in agreement with that reported in [131]. This suggests the plasticizing effect

of ionic conducting RTILs such as EMIHSO4 and EMIBF4. The impact of EMIHSO4 on

lowering the crystallinity of PEO was more pronounced which could be due to interactions

between polymer and IL. To characterize such interactions, FTIR was used to verify the

change to the crystalline phase as well as to identify any possible interactions between PEO

and EMIHSO4 which may affect the ionic conduction.


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5.2 Interaction Between Polymer and IL

FTIR analyses were performed to study the possible interaction between EMIHSO4 and PEO

through structural, compositional, and bonding characterizations of the polymer electrolyte.

PEO–EMIHSO4 was investigated along with pure EMIHSO4 and PEO film as references.

Figure 5-7 shows the FTIR spectra of the polymer electrolyte and its individual components

in the 4000-650 cm-1 range. The wavenumbers of the significant bands with their associated

bonding interactions for pure EMIHSO4, PEO film, and the PEO–EMIHSO4 (1:2) electrolyte

are summarized in Table 5-4. To study the structural changes in PEO–EMIHSO4, the

polymer electrolytes were analyzed as a function of EMIHSO4 composition. As shown in

Table 5-4, most of the cationic bands are located in the 4000-2000 cm-1 region. As expected,

the intensity of these characteristic bands increased with the increase of EMIHSO4; however,

there was no significant change in their wavenumber. Figure 5-8 is presented which shows

the effect of IL content on the FTIR bands of PEO–EMIHSO4 in the 2000-650 cm-1 region.

Comparing the FTIR spectrum of PEO–EMIHSO4 with the spectra of its individual

components, almost all bands in the PEO–EMIHSO4 spectrum can be accounted for (see

Figure 5-7). Although no additional bands were observed in the PEO–EMIHSO4 spectrum,

two main effects can be derived: crystallinity and interaction between the polymer and the

anion.


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Figure 5-7 FTIR spectra of pure PEO film, pure EMIHSO4, and PEO–EMIHSO4 in (1:2) composition

5.2.1 Effect of crystallinity

The crystalline phase of PEO can be observed and analyzed based on the FTIR spectra [114];

the intensity and position of the corresponding peaks can be used to explain the phase

structure of the polymer electrolyte. In Figure 5-7, the main characteristic bands for the

crystalline phase of PEO associated with CH2 and C–O–C vibration modes appeared as the

doublet peaks at 1358 and 1342 cm-1; the triplet absorption bands at 1144, 1100, and

1058 cm-1; and the band at 840 cm-1. Figure 5-8 shows that while the vibration modes of the

crystalline structure existed in the polymer electrolyte, their peak intensity, width, and

position changed after the incorporation of EMIHSO4. These changes, together with a

decrease in the intensity of the CH2 peak at 1342 cm-1 and a broadening of the C–O–C peak


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at 840 cm-1, suggest that the addition of EMIHSO4 resulted in a decrease in the degree of

crystallinity of the PEO structure.

5.2.2 Effect of interaction between PEO and HSO4−

Based on the overlap between the anion and polymer vibrational peaks, there was a stronger

interaction between HSO4− and PEO than between EMI+ and HSO4

− in the pure IL. The

fingerprint region of pure EMIHSO4 in Figure 5-7 shows the bands attributable to HSO4−

vibration modes at 1218 and 1020 cm-1 assigned to –O–S– stretching. The strong peak at

830 cm-1 together with the weak peak at 760 cm-1 corresponds to S–OH stretching mode. In

the PEO–EMIHSO4 spectrum, these peaks overlapped with the PEO vibration modes and

shifted to higher wavenumbers at 1226, 1040, 840, and 774 cm-1 as shown in Figure 5-7. At

the same time, in the PEO–EMIHSO4 spectrum, changes in the C–O–C and CH2 peak

intensity as well as the peak width of PEO are noticeable. The vibration modes of the PEO

film at 1240 cm-1 overlapped with the HSO4− peak at 1218 cm-1 and shifted to 1226 cm-1 in

the polymer electrolyte. A similar trend was observed for the C–O–C triplet vibration modes

of PEO, further supporting the interaction between HSO4− and PEO.

The most important interactions were observed in the 2000-650 cm-1 frequency range.

Figure 5-8 shows that the relative intensity of the bands at 1240, 1040, and 840 cm-1 (in

PEO–EMIHSO4 with 1:1 weight ratio) increased and slightly shifted with increasing IL

content. Since the dominant vibration modes of PEO within this range are C–O–C stretching,

the changes in the polymer structure reflect the interaction between HSO4− and the ether

oxygen of PEO. This interaction is most likely due to the formation of hydrogen bonds

between HSO4− and the oxygen atom. The presence of hydrogen bonds generally has a higher

impact on the donor group (HSO4−) than the acceptor group (C–O–C) [114]. Indeed, this can

be clearly seen in Figure 5-7. The strong interaction between the polymer and anion together


101

with the slight change in the position of cationic bands indicate that the structural changes are

the result of the HSO4− and PEO interaction.

Figure 5-8 FTIR spectra of PEO–EMIHSO4 electrolytes in (1:1), (1:2), and (1:3) compositions in the range of 2000–650 cm-1


102

Table 5-4 FTIR band positions and associated bonding modes for PEO–EMIHSO4 in (1:2) composition and its components.

Wavenumbers (cm-1) Band assignments Ref.

PEO film EMIHSO4 PEO–EMIHSO4

776 760 774 OC–CO vibrations, S–OH stretching

[126,132]

840 830 840 CH2 rocking and C–O–C deformation, S–OH stretching

[114,133]

944,960 - 946,962 CH2 symmetric and asymmetric rocking

[114,134]

1058,1100,1144 1020 1040,1100,1168 C–O–C symmetric and asymmetric stretching, –O–SO3 symmetric stretching

[114,126,133-136]

- 1162 1168 –N–CH2 and –N–CH3 stretching, S–O vibrations of SO4

[114,136-138]

1240 1218 1226 CH2 asymmetric twisting, –O–SO3 asymmetric stretching

[126,133,136,137]

1342,1358 - 1342,1354 CH2 symmetric and asymmetric wagging

[114,126,134]

- 1572 1572 C=N stretching [136,138]

2882 - 2882 CH2 stretching [126,135]

- 2982 2986 C–H stretching of methyl group

[137,138]

- 3104,3150 3108,3152 C–H stretching of imidazole ring

[136-139]


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5.3 Device Performance

The electrochemical performance of the developed PEO–EMIHSO4 and PVdF-HFP–EMIBF4

electrolytes was demonstrated in EDLCs. To understand the effect of the polymer on the

electrochemical behavior of the ILs, ECs with pure EMIHSO4 and EMIBF4 electrolytes were

also studied. In spite of its small capacitance, graphite electrodes were selected to minimize

the influence of porosity and to focus on the properties of the polymer electrolyte and the

high rate capability of the cells.

Figure 5-9 shows the voltammograms of all three types of EC devices illustrated in

Figure 3-4. Figure 5-9a compares the CVs of the Liquid–1 EC cell (EMIHSO4, Figure 3-4a)

with the solid EC cell (PEO–EMIHSO4, Figure 3-4c) at a sweep rate of 100 mV s-1. The solid

EC showed higher area specific capacitance than its liquid counterpart. To further verify this

observation, a Liquid–2 EC cell (EMIHSO4, Figure 3-4b) was also tested and its CV is

overlaid in Figure 5-9. Although this configuration increased the cell resistance, the distance

between the electrodes is reduced to the same order as that of the polymer electrolyte. The

CV of the Liquid–2 EC cell was identical to that of the Liquid–1 EC cell in terms of profile

and current response. Both liquid cells with pure EMIHSO4 electrolyte had a smaller

capacitance than the solid EC cell with PEO–EMIHSO4 electrolyte. This observation was

more pronounced when the devices were subjected to higher sweep rates. The CV profile in

Figure 5-9b shows that the solid EC cell exhibited a capacitive response even at 1 V s-1. In

contrast, both liquid EC cells appeared to be more resistive at high sweep rates. A similar

electrochemical performance was observed for solid EC cell with PVdF-HFP–EMIBF4 and

pure EMIBF4.


104

Figure 5-9 Cyclic voltammograms of graphite ECs with EMIHSO4 and PEO–EMIHSO4 electrolytes at sweep rates of (a) 100 mV s-1 and (b) 1 V s-1


105

The performance of the EC cells was further characterized by impedance analyses. Figure

5-10a shows the real part of the capacitance (C′) as a function of frequency. C′ at low

frequency represents the capacitance of a device, which at 3 mF cm-2 was significantly higher

for the solid EC cell than for the liquid EC cells (1 mF cm-2). This trend is in agreement with

that obtained from DC characterization shown in Figure 5-9b. As shown in Figure 5-10b, the

time constant τ estimated from the maximum of the C″ vs. frequency (at −45° phase angle)

was 4 s, representing the rate capability of the device to deliver the stored charge. Compared

to the cells with a liquid EMIHSO4 electrolyte, the solid cell with the polymer electrolyte

shows comparable rate response, demonstrating its improved performance. For the liquid

EMIHSO4 electrolyte, its high viscosity hinders the mobility of ions. However, the viscosity

of EMIHSO4 seemed to be less problematic when immobilized in a thin polymer matrix in

the solid state polymer electrolyte.

The capacitance of EC device with PEO–EMIHSO4 was similar to that with PVdF-HFP–

EMIBF4, but the latter had a time constant of 0.2 s which was the result of the higher

conductivity of PVdF-HFP–EMIBF4.


106

Figure 5-10 (a) Real part C′ and (b) imaginary part C″ of the capacitance and vs. frequency for graphite ECs with EMIHSO4 and PEO–EMIHSO4 electrolytes


107

The performance of EC device employing PVdF-HFP–EMIBF4 with operating potentials

of 1.5 V and 2 V is shown in Figure 5-11. For comparison, the CV profile of PEO–

EMIHSO4-based EC is superimposed on this graph. The more rectangular CV profile of

PVdF-HFP–EMIBF4-based device and its higher operating potential are due to the

characteristics of fluorinated IL (i.e., high conductivity and electrochemical stability).

Nevertheless, EC device with non-fluorinated PEO–EMIHSO4 electrolytes stored similar

charge to that with fluorinated polymer–IL over an operating potential of 1.5 V. By

improving the ionic conductivity and electrode-electrolyte interface, a lower time constant

for PEO–EMIHSO4 system is expected.

Figure 5-11 Cyclic voltammograms of graphite EDLCs with PVdF-HFP–EMIBF4 and PEO–EMIHSO4 electrolytes at sweep rate of 1 V s-1

The PEO–EMIHSO4 film has shown promising performance as an electrolyte for solid

ECs exceeding the performance of liquid EMIHSO4 electrolytes. This result may appear

counter-intuitive. But similar observations have also been reported in the literature for other


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IL-based electrolytes. Comparing the performance of polymer–IL-based ECs to their liquid

IL-based counterparts, Lu et al. reported a similar CV profile for liquid IL and polymer–IL

composites containing inorganic fillers at 20 mV/s [95]. In our work, both CV and EIS

analyses have confirmed a better performance of the polymer–IL-based EDLC over the pure

IL-based device at both low and high rates. A similar improvement in performance of

polymer electrolyte-based over liquid electrolyte-based EDLC devices was also reported for

proton conducting heteropoly acid polymer electrolyte systems at very high scan rates

(exceeding 10 V s-1) [140].

The presence of neutral ion pairs is expected in pure ILs [32].This may also be true for

highly viscous EMIHSO4, and could be one of the causes of its low conductivity. Integrating

a polymer network and a plasticizer (residual PC in this case) into the IL may promote the

dissociation of ion pairs and facilitate the movement of charge carriers as reported by

Lewandowski et al. [86,96] and Singh et al. [74]. The results in this study support the notion

of the impact of the high viscosity of liquid EMIHSO4 on its ionic conduction. Within the

same range of conductivity, the viscosity of IL is a key parameter that governs the overall

performance of the device. However, the ion motion in the PEO–EMIHSO4 polymer

electrolyte is less affected by the high viscosity of EMIHSO4. The polymer network provides

a much shortened conduction path for ions and a more stable electrode-electrolyte interface,

which is important especially at high rates when compared with liquid IL.

5.4 Summary

The viability of PEO–EMIHSO4 for application in ECs was studied in terms of ionic

conductivity, structural characteristics, and performance of enabled device. These properties

were compared to both pure EMIHSO4 and PVdF-HFP–EMIBF4. The thin and flexible PEO–

EMIHSO4 films showed an ionic conductivity of 0.8 mS cm-1 at room temperature.

Considering the much higher viscosity of EMIHSO4 compared to that of EMIBF4, the


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conductivity of EMIHSO4 was not significantly affected in polymer state. Thus, addition of

PEO did not hinder the ionic motion of EMIHSO4.

Structural and thermal analyses revealed that impregnating PEO with EMIHSO4 had a

positive impact on the structure of the polymer electrolyte: Addition of EMIHSO4 into PEO

not only decreased the crystallinity of the polymer, but also lowered the melting point of

PEO. Interactions between EMIHSO4 and PEO promoted the dissociation of ions as well as

reduced the crystalline state of polymer. Structural characterization of PVdF-HFP–EMIBF4

confirmed that ILs act as both ionic conductors and plasticizers in the polymer electrolytes.

The performance of the polymer electrolytes was examined for EDLCs. Devices

leveraging the PEO–EMIHSO4 electrolyte showed a capacitive behavior up to 1 V s-1. Within

a similar operating voltage, the capacitance of the device with PEO–EMIHSO4 was

comparable to that using PVdF-HFP–EMIBF4 which makes PEO–EMIHSO4 a promising

environmentally friendly electrolyte enabling solid ECs. Nevertheless, further improvement

in ionic conductivity of PEO–EMIHSO4 at room temperature is necessary to reach rate

performances in the orders of fluorinated polymer–ILs. To address this issue, in Chapter 6,

the incorporation of fillers into polymer electrolytes was studied to determine their impact on

the crystallization and ion transport in PEO–EMIHSO4 electrolytes, and to correlate the key

parameters to the performance of ECs.

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CHAPTER 6

POLYMER–IONIC LIQUID ELECTROLYTES WITH FILLERS

The effects of two types of inorganic nanofillers on the properties of the polymer–ILs are

presented. This chapter begins by examining the ionic conductivity of polymer electrolytes as

a function of filler content and temperature (section 6.1). Then, the impact of fillers on the

crystallinity, melting temperature (section 6.2), and interactions between the polymer and the

IL is presented (section 6.3). Following this, the influence of the fillers on the ionic

conduction process is discussed through impedance and dielectric analyses, and the results

are correlated to the ionic conductivity and structural properties of the polymer–ILs (section

6.4). The performance of graphite EDLCs enabled by filler-containing polymer electrolytes

are compared in terms of capacitance and rate response (section 6.5).

6.1 Effect of Fillers on Ionic Conductivity

As discussed in Chapter 5, PEO–EMIHSO4 electrolytes showed a reasonable conductivity of

0.8 mS cm-1 and the enabled EDLCs demonstrated high rate performance [120]. The

interaction between EMIHSO4 and PEO lowered the crystallinity of the electrolyte and

promoted ionic dissociation [119]. Since a highly amorphous structure of the polymer

CHAPTER 6. POLYMER–IONIC LIQUID ELECTROLYTES WITH FILLERS

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and a fully dissociated IL are the two most important factors for high performance polymer–

ILs, the aim was to promote these features at room temperature to further enhance the

performance of the polymer–ILs in order to reach the level of polymer–fluorinated ILs.

Accordingly, two types of nanofillers: amorphous SiO2 and crystalline TiO2 were

incorporated into PEO–EMIHSO4 to improve its performance and to determine their

dominating function in ionic conduction, structural crystallinity, and performance of device.

The PEO–EMIHSO4 composition, optimized at 1:3, was used as a baseline for

investigating the impact of the SiO2 and TiO2 nanofillers on the ionic conductivity and

structural properties of the electrolyte. To understand the influence of SiO2 and TiO2 fillers

on the ionic conductivity of PEO–EMIHSO4, the relationship between temperature and ionic

conductivity for PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2

electrolytes were examined and are shown in Figure 6-1a and Figure 6-1b. The average

conductivities of these electrolytes at room temperature and the activation energies of ionic

conduction are reported in Table 6-1.

Figure 6-1a shows the ionic conductivity of PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2,

and PEO–EMIHSO4–10% SiO2 as a function of temperature. Within the temperature range

of –10 °C to +80 °C, the conductivity of electrolytes containing SiO2 was higher than that of

the filler-free electrolyte. All three polymer electrolytes exhibited a transition from

semi-crystalline to amorphous states at temperatures above the melting point.

The trend of conductivity was different for the electrolytes with 3 wt% and 10 wt% SiO2.

At low temperatures, the highest conductivity was obtained for PEO–EMIHSO4–10% SiO2,

whereas above the melting point, PEO–EMIHSO4–10% SiO2 showed a lower conductivity

than that of PEO–EMIHSO4–3% SiO2.


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Figure 6-1 Temperature dependence of the ionic conductivity of (a) PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2; and (b) PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2


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The ionic conductivity of PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–

EMIHSO4–10% TiO2 as a function of temperature is shown in Figure 6-1b. The trend of

ionic conductivities of PEO–EMIHSO4–TiO2 electrolytes was somewhat different from that

of SiO2-containing electrolytes. The average ionic conductivity of PEO–EMIHSO4–3% TiO2

was similar to that of filler-free electrolyte which is also listed in Table 6-1. The ionic

conductivity increased over the entire temperature range with the addition of 10 wt% TiO2.

Table 6-1 Room temperature ionic conductivity of PEO–EMIHSO4 and PEO–EMIHSO4 electrolytes containing SiO2 and TiO2 nanofillers, and activation energy (Ea) of ionic conduction for the respective electrolytes at low and high temperatures.

Polymer electrolytes Conductivity (mS cm-1) Ea (kJ mol-1)

Low T High T PEO–EMIHSO4 0.8 ± 0.1 29 15 PEO–EMIHSO4–3% SiO2 1.5 ± 0.1 29 17 PEO–EMIHSO4−10% SiO2 2.1 ± 0.2 28 17 PEO–EMIHSO4–3% TiO2 1.0 ± 0.3 36 17 PEO–EMIHSO4−10% TiO2 1.8 ± 0.2 24 14

From the results in Figure 6-1, it seems that the addition of fillers in low quantity (i.e.,

3 wt%) has an inconsistent effect on the ionic conductivity of PEO–EMIHSO4: increased in

the case of SiO2, but unaffected in the case of TiO2. To examine whether the observed trends

are due to the PEO–IL system, or the small amount of fillers, the ionic conductivity of

PVdF-HFP–EMIBF4 electrolytes with 3 wt% SiO2 and with 3 wt% TiO2 was also studied.

Figure 6-2 shows the ionic conductivity of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–

3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2 in the temperature range from –10 °C to

+80 °C. The average conductivities of the respective electrolytes at room temperature, listed

in Table 6-2, show that the addition of 3 wt% SiO2 or TiO2 did not significantly change the

conductivity of PVdF-HFP–EMIBF4. This is also seen in Figure 6-2 where the ionic

conductivity of PVdF-HFP–EMIBF4–3% SiO2 is only slightly higher than that of

PVdF-HFP–EMIBF4 and PVdF-HFP–EMIBF4–3% TiO2. This trend was consistent with that

seen in PEO–EMIHSO4–3% SiO2 and PEO–EMIHSO4–3% TiO2.


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Figure 6-2 Temperature dependence of the ionic conductivity of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2

Table 6-2 Room temperature ionic conductivity of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, PVdF-HFP–EMIBF4−3% TiO2, and the activation energy (Ea) of ionic conduction for the respective electrolytes.

Polymer electrolytes Conductivity (mS cm-1) Ea (kJ mol-1) PVdF-HFP–EMIBF4 6.5 ± 0.3 24 PVdF-HFP–EMIBF4–3% SiO2 6.2 ± 0.6 24 PVdF-HFP–EMIBF4−3% TiO2 6.2 ± 0.8 27

So far, the comparison between the trends of ionic conductivity of filler-containing PEO–

EMIHSO4 and filler-containing PVdF-HFP–EMIBF4 leads to the following observations:

(i) The addition of 10 wt% SiO2 and TiO2 improved the conductivity of PEO–EMIHSO4

over the entire temperature range. In the case of PEO–EMIHSO4–SiO2, the 3 wt%

SiO2 at amorphous state is more effective than that of 10 wt% SiO2. This implies that

the higher SiO2 content may restrict the chain motion or block the ion transport

[7,73,141], which explains the slightly higher activation energy of the ionic

conduction for PEO–EMIHSO4–SiO2 electrolytes at high temperature region (see


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Table 6-1). The trends in conductivity of PEO–EMIHSO4–TiO2 at 3 wt% and 10 wt%

filler was parallel at both low and high temperature ranges, at which the conductivity

of 3 wt% TiO2 was lower.

(ii) The lower SiO2 and TiO2 content (i.e., 3 wt%) was insufficient to show a clear trend

in the ionic conductivity. While the addition of 3 wt% SiO2 increased the conductivity

of PEO–EMIHSO4, 3 wt% TiO2 had no significant impact. This difference could be

attributed to the structural characteristics of the SiO2 and TiO2 fillers, which could

affect the structure of PEO–EMIHSO4 and the interaction between PEO and

EMIHSO4.

(iii) Overall, the effect of 3 wt% filler on ionic conductivity of PVdF-HFP–EMIBF4 was

negligible compared to that of PEO–EMIHSO4. The effect of fillers also depends on

the polymer matrix, and it was more realized for the polymer with a higher

crystallinity.

The trends in conductivity of PEO–EMIHSO4–10% SiO2 and PEO–EMIHSO4–10% TiO2

are superimposed in Figure 6-3 together with that of filler-free PEO–EMIHSO4. The

transitions in PEO–EMIHSO4 and in PEO–EMIHSO4–10% TiO2 were much more

pronounced than the transition in PEO–EMIHSO4–10% SiO2. The ionic conductivity of

PEO–EMIHSO4–10% TiO2 was similar to that of PEO–EMIHSO4–10% SiO2 at low

temperatures, but significantly increased at temperatures above the melting point.


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Figure 6-3 Temperature dependence of the ionic conductivity of PEO–EMIHSO4, PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2

Figure 6-4 shows the ionic conductivity of the studied polymer electrolytes over a period

of 5 months. While all electrolytes showed a good shelf life, the addition of fillers increased

the conductivity of PEO–EMIHSO4. Since the increase of conductivity was only observed at

10 wt% TiO2 addition, the shelf life of PEO–EMIHSO4–10% TiO2 is shown. As summarized

in Table 6-1, the average conductivities of PEO–EMIHSO4–10% fillers were more than

double the conductivity of the filler-free electrolyte.

The presence of fillers is beneficial to the ionic conductivity of PEO-based electrolytes by

impeding the reorganization and recrystallization of PEO chains [9,73] and by promoting the

ionic dissociation via the interactions between polar nanofillers and the ionic species

[9,10,74,80]. In addition to structural effects, the observations in Figure 6-3 imply that TiO2

and SiO2 fillers may affect the ionic dissociation and the ion transport in PEO–EMIHSO4

differently, especially in the amorphous state. Structural, impedance, and dielectric analyses

were utilized to identify the origin of these effects in order to gain insights into how intrinsic


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properties of SiO2 and TiO2 nanofillers influence the performance of the PEO–EMIHSO4

electrolyte.

Figure 6-4 The variation of ionic conductivity of PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2 over time

6.2 Effect of Fillers on Crystallinity

In section 6.1, it was shown that ionic conductivity of filler-containing PEO–EMIHSO4

depend on the type and the amount of fillers. One of such dependence is related to the

changes in the crystallinity. To evaluate the impact of SiO2 and TiO2 fillers on the structure

of the polymer electrolytes, filler-containing PEO–EMIHSO4 and PVdF-HFP–EMIBF4 were

characterized by XRD and DSC.


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6.2.1 XRD analyses

Figure 6-5a shows the XRD patterns of amorphous SiO2 nanofiller, PEO–EMIHSO4, PEO–

EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2. As shown in Chapter 5, the

incorporation of EMIHSO4 into PEO increased the amorphous phase. The addition of 3 wt%

and 10 wt% SiO2 further decreased the intensity of the crystalline peaks of PEO at 19° and

23° 2θ, suggesting a progressive decrease in the crystallinity of PEO–EMIHSO4. Meanwhile,

the amorphous peak in the baseline of the PEO–EMIHSO4 spectrum became broader for

PEO–EMIHSO4−3% SiO2 and further for PEO–EMIHSO4−10% SiO2.

The XRD patterns of crystalline TiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3%

TiO2, and PEO–EMIHSO4–10% TiO2 are illustrated in Figure 6-5b. The XRD spectrum of

TiO2 shows characteristic peaks at 25° and 37° 2θ, corresponding to the anatase structure of

the nanofiller. The peak at 25° was also observed in the XRD patterns of both PEO–

EMIHSO4–3% TiO2 and PEO–EMIHSO4–10% TiO2. A similar trend to that of PEO–

EMIHSO4–SiO2 was observed: the intensity of crystalline peaks substantially decreased from

3 wt% to 10 wt% TiO2 (see Table B-1). The broad amorphous peak also indicated that

crystallinity was suppressed in PEO–EMIHSO4–10% TiO2.


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Figure 6-5 XRD patterns for (a) SiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2; and (b) TiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2


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From the XRD results, it appears that a low quantity of fillers (i.e., 3 wt% TiO2 or SiO2)

may not significantly change the structure of the polymer electrolytes. Moreover, The effect

of their crystalline structures at small quantity is not clear. A different polymer system was

also studied, where the structure of PVdF-HFP–EMIBF4 containing fillers was characterized.

Figure 6-6 shows the XRD spectra of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2,

and PVdF-HFP–EMIBF4–3% TiO2 as well as the XRD patterns of SiO2 and TiO2 nanofillers.

The comparison of the spectra revealed a small effect of 3 wt% fillers on the structure of

PVdF-HFP–EMIBF4, supporting the observations of PEO–EMIHSO4 (Figure 6-5).

Figure 6-6 XRD patterns of SiO2 and TiO2 nanofillers, PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2 electrolytes

Figure 6-7 compares the XRD spectra of PEO–EMIHSO4–10% SiO2 and PEO–

EMIHSO4–10% TiO2 to that of PEO–EMIHSO4 and PEO film. The decrease in the intensity

of the crystalline peaks and the increase of the amorphous phase clearly indicated that the

addition of 10 wt% SiO2 and TiO2 reduced the crystallinity of PEO–EMIHSO4 electrolytes.


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Figure 6-7 XRD patterns of PEO film, PEO–EMIHSO4, PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2

6.2.2 DSC analyses

The degree of crystallinity of PEO–EMIHSO4 containing fillers was quantified using DSC to

confirm the effect of filler content on the crystallinity, and hence on ionic conductivity. The

DSC thermograms of PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10%

SiO2 are depicted in Figure 6-8a. The degree of crystallinity (Xc) of the polymer electrolytes

was calculated from the melting heat of the PEO film and is summarized in Table 6-3. The

percentage of crystallinity of PEO–EMIHSO4 was reduced by 48% over that of the PEO film

due to the presence of a relatively large amount of EMIHSO4 which itself can act as a

plasticizer. With the addition of 3 wt% SiO2, the crystallinity of PEO–EMIHSO4 decreased

from 18% to 16%, which further decreased to 12% with 10 wt% SiO2. This shows the same

trend observed in XRD analyses (Figure 6-1a).


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Figure 6-8 Heating and cooling DSC thermograms of (a) PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2; and (b) PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2


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The DSC thermograms of TiO2-containing PEO–EMIHSO4 together with that of PEO–

EMIHSO4 are shown in Figure 6-8b. The crystallinity of the respective electrolytes is also

listed in Table 6-3. Different from that seen in PEO–EMIHSO4–SiO2, the trend of

crystallinity in PEO–EMIHSO4–TiO2 increased from 18% to 27% with 3 wt% addition, and

then decreased to 12% with 10 wt% TiO2, agreeing with the XRD results in Figure 6-1b.

Table 6-3 Melting temperature (Tm), recrystallization temperature (Trc), and crystallinity (Xc) of PEO film, PEO–EMIHSO4, and PEO–EMIHSO4–filler electrolytes.

Electrolytes Tm (ºC) Trc (ºC) Xc (%)

PEO filma 67 45 66 PEO–EMIHSO4 42.3 ± 1.2 15.3 ± 0.5 17.7 ± 1.3 PEO–EMIHSO4–3% SiO2 44.3 ± 1.2 17.0 ± 0.8 16.4 ± 0.4 PEO–EMIHSO4−10% SiO2 49.5 ± 0.5 22.8 ± 0.3 12.2 ± 0.1 PEO–EMIHSO4–3% TiO2 47.8 ± 0.8 21.5 ± 0.5 27.4 ± 0.5 PEO–EMIHSO4−10% TiO2 45.7 ± 2.1 17.3 ± 1.2 11.7 ± 0.9

a values are consistent with the literature

The effect of 3 wt% SiO2 and TiO2 on the crystalline structure of PVdF-HFP–EMIBF4

was also evaluated from DSC thermograms of the respective polymer electrolytes (shown in

Figure B-1). The analyses further confirmed the crystallinity trend observed in PEO–

EMIHSO4–3% filler, and substantiated the small impact of 3 wt% filler on the structure of

PVdF-HFP–EMIBF4.

Figure 6-9 shows the comparison between the DSC thermograms of PEO–EMIHSO4–

10% SiO2 and PEO–EMIHSO4–10% TiO2 which clearly reveals that the addition of 10 wt%

SiO2 and TiO2 decreased the crystallinity. It is evident from both XRD and DSC that the

addition of 10 wt% fillers prevented the growth of the crystalline phase in PEO–EMIHSO4.

This also agrees well with those reported in the literature [6,69,79], and suggests that fillers

may disrupt the crystallization of the polymeric chains.


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Figure 6-9 DSC heating and cooling thermograms of PEO film, PEO–EMIHSO4, and PEO–EMIHSO4–10% SiO2, and PEO–EMIHSO4–10% TiO2

Also shown in Figure 6-9 and Table 6-3, the filler-containing electrolytes showed a slight

increase in Tm over PEO–EMIHSO4. This small increase in Tm of 10 wt% filler-containing

electrolytes was only within the 12% of the crystalline phase of the electrolyte, which has

much smaller impact compared to the majority of amorphous phase. Indeed, the higher ionic

conductivity of the filler-containing electrolytes, especially below the phase transition (see

Figure 6-3), suggests that the main effect of the fillers at low temperatures is to suppress

crystallization. The fact that both crystalline TiO2 and amorphous SiO2 at 10 wt% reduced

the crystallinity of PEO–EMIHSO4 implies that the crystal structure of nanofillers plays little

role in the overall structural change of the polymer electrolyte. However, TiO2 and SiO2

fillers have different dielectric constants (see Table 3-3) than PEO (ε = 5) [142,143] which

may affect the interaction between polymer and IL.


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6.3 Effect of Fillers on Interaction Between PEO and EMIHSO4

The fillers with a polar surface can act as a “solvent” to promote ionic dissociation which

increases ionic conductivity, as suggested by Scrosati et al. [9]. When introducing fillers into

the PEO–EMIHSO4 system, an additional polarization is induced which can compete with

the existing interactions among the charged species and the PEO matrix. This may result in

additional free charge carriers. This effect has also been reported for PEO–LiX electrolytes

where the interactions between PEO and Li+ were disrupted by SiO2 or TiO2 fillers [7]. To

test this hypothesis, the filler-containing PEO–EMIHSO4 were characterized using FTIR and

compared to PEO–EMIHSO4 in order to investigate the filler-IL and filler-polymer

interactions.

Figure 6-10 shows the spectra of SiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3%

SiO2, and PEO–EMIHSO4–10% SiO2. The characteristic peaks of PEO were CH2 stretching

at 2882 cm-1, and the symmetric and asymmetric wagging as the doublet peaks at 1358 and

1342 cm-1, as well as the C−O−C stretching band shown as triplet peaks at 1168 cm-1,

1112 cm-1, and 1051 cm-1. As described in Chapter 5, this triplet peak of PEO–EMIHSO4

spectrum was assigned to a combination of individual C−O−C and HSO4− characteristic

modes, and the peak shifts reflected H-bonding between oxygen atoms on PEO chain and

HSO4− [119].


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Figure 6-10 FTIR spectra of SiO2 nanofiller, PEO–EMIHSO4, PEO–EMIHSO4–3% SiO2, and PEO–EMIHSO4–10% SiO2

The spectrum of PEO–EMIHSO4–3% SiO2 was similar to that of PEO–EMIHSO4 with

no significant change in the relative intensity of the peaks. The main asymmetric Si−O−Si

peak of the filler was at 1067 cm-1 consistent with other reports [7,77,144] which is

overlapped with the triplet bands. With the addition of 10% SiO2, while there was no

frequency change of the CH2 peaks, the relative intensity of the CH2 peaks at 2882 cm-1 and

1342 cm-1 slightly increased. The main difference in the spectra is the increase in relative

intensity of the triplet peaks, especially the C−O−C peak at 1112 cm-1. The stronger C−O−C

vibration implies that SiO2 may interact with the polar or the charged species in the PEO–

EMIHSO4 electrolyte. This in turn will reduce the attraction between PEO and HSO4−, and

lead to a higher ionic dissociation in PEO–EMIHSO4–10% SiO2. The increase in CH2 and

C−O−C intensities of PEO–EMIHSO4–10% SiO2 together with thermal analysis and XRD,

all suggest an increased polymer chain movement and vibration in the PEO matrix.


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The effect of TiO2 on the interaction between the polymer and IL is not visible as the IR

active vibrations of TiO2 usually occur at low wavenumbers (e.g., characteristic band of

Ti−O is at 512 cm-1 as a result of the anatase crystalline phase [128]). As shown in Figure 6-

11, the spectra of PEO–EMIHSO4–10% TiO2 and PEO–EMIHSO4 were similar and no

significant change in the peaks were observed. In the spectrum of PEO–EMIHSO4–3% TiO2,

the increase in the intensity of the peaks at 2882, 1342, and 1112 cm-1, which represent the

characteristic vibrations of crystalline phase, infers the higher crystallinity of PEO–

EMIHSO4–3% TiO2 in line with the DSC analyses.

Figure 6-11 FTIR spectra of PEO–EMIHSO4, PEO–EMIHSO4–3% TiO2, and PEO–EMIHSO4–10% TiO2

The structural characterization supported the effect of SiO2 and TiO2 fillers on the ionic

conductivity of PEO–EMIHSO4, leading to the following conclusions:


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(i) 10 wt% SiO2 and TiO2 additions suppressed the crystallinity of PEO–EMIHSO4,

increasing the ionic conductivity over the entire temperature. Despite their different

crystalline structures, both SiO2 and TiO2 had plasticizing effects.

(ii) There was little or no change on the ionic conductivity of PEO–EMIHSO4 with

3 wt% fillers. The low quantity of fillers was insufficient to noticeably decrease the

crystallinity.

(iii) The interactions between PEO and EMIHSO4 was affected with 10 wt% SiO2,

suggesting further ionic dissociation.

However, it appears that the lowered crystallinity may not be the only effect, since the

ionic conductivity of PEO–EMIHSO4–10% SiO2 and PEO–EMIHSO4–10% TiO2 also

increased at high temperatures (see Figure 6-3), where the polymer electrolyte is supposedly

amorphous. There could be additional functions of SiO2 and TiO2 in the electrolyte that

affects the interaction between polymer and IL and facilitates further ionic dissociation.

6.4 Impedance and Dielectric Analyses

6.4.1 Complex capacitance and dielectric analyses

The intrinsic properties of inorganic fillers such as its dielectric constant may also play an

important role in ionic conduction, which may not be differentiated in the ionic conductivity,

thermal, and structural analyses. The dielectric characteristics of the fillers were analyzed

using a capacitor configuration to understand their influence on the mobility of the polymer

chain and the ion transport in PEO–EMIHSO4. The electrochemical performance (complex

capacitance) was also characterized and correlated to the dielectric analyses in the same

capacitor cells.

The principles of each method and the relationship between the two techniques were

explained in section 2.6.1.2. This is the first attempt to combine these two approaches to


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correlate the capacitance, resistance, and rate performance (i.e., time constant) of an EC with

the intrinsic dielectric properties (i.e., dielectric constant and loss) of a polymer electrolyte

and fillers to establish the connections between the fundamental properties and the

performance output in a device.

6.4.2 Capacitance and dielectric response of polymer electrolytes

The concentration of SiO2 and TiO2 was held at 10 wt% in the polymer electrolytes for

impedance and dielectric characterizations. Three types of metallic cells were assembled

using PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 electrolytes. Since

the effect of porosity and electrode resistance is minimized in these metallic cells, the

performance of the capacitors is dependent on the ionic conductivity and the dielectric

characteristic of the polymer electrolytes. Figure 6-12a shows C′ vs. frequency for these cells.

The higher capacitance at low frequency (0.1 Hz) is equal to the DC capacitance of the cells

(25 to 30 μF cm-2). The transition at −45° phase angle is visible in Figure 6-12b, where a

peak at around 1 kHz can be observed. Thus, the AC responses of the cells are divided into

two frequency regions: below 1 kHz, the cells had dominating capacitive performance, while

above 1 kHz, they became more resistive. As shown in Figure 6-12, the transition occurred at

frequencies between 1 kHz and 3 kHz, at which the systems are controlled by a combination

of capacitive and resistive components.


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Figure 6-12 Variation of (a) real part C′ and (b) imaginary part C″ of capacitance with respect to frequency for cells leveraging PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 at 30 °C

The dielectric permittivity (′) and dielectric loss (″) of PEO–EMIHSO4, PEO–

EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 were extracted from the complex impedance

measurements using Equations 2-15 and 2-16, and are shown in Figure B-2. However, the EP

and dipolar relaxation were not as distinct as those illustrated in Figure 2-14, because of the

extended EP at 1 kHz. To minimize this overlapping effect, the derivative formalism was

used (Equation 2-17).

The dielectric derivative spectra of the three polymer electrolytes and PEO film at 30 ºC

are shown in Figure 6-13, where plateaus at low frequencies and relaxation peaks at high

frequencies are observed. The peaks at approximately 1 to 3 kHz for polymer electrolytes

coincide with τ0 at a phase angle of −45 of the cells (Figure 6-12b) and are associated with

the transition between the capacitive and resistive states. Since there is no clear characteristic

peak at high frequencies in Figure 6-13, the peak at the characteristic frequency (f0) is used to

evaluate the dipolar relaxation. Although a small contribution of capacitance from EP is still


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present at the relaxation peaks at 1 to 3 kHz, through the derivative formalism the

contribution of resistance to the transition peaks is clearly seen by correlating Figure 6-13

and Figure 6-12b.

The addition of the IL to PEO increased the structural flexibility of the polymer, and

hence significantly shifted the relaxation peak of PEO to a higher frequency, which is

consistent with other reports on PEO-salt electrolytes [145,146]. At frequencies above 3 kHz,

the response of the polymer segments and the ions cannot follow the change of the external

field, and hence ion transport is restrained with increased energy dissipation resulting in

resistive behavior. At below the transition at 1 kHz, the motion of ions assisted by local

movement of polymer chains promoted ionic conductivity and contributed to electrode

polarization. In accordance with Figure 6-12, the relaxation at 1 to 3 kHz can be ascribed to a

transition state between dipolar relaxation and ionic conduction. At lower frequencies, the

conducting ions had more time to polarize at the electrode interface and build up the double

layer at frequencies between 0.1 and 1 Hz.

Figure 6-13 Dielectric derivative vs. frequency for PEO film, PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 at 30 °C


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6.4.3 Effect of fillers

The filler-containing PEO–EMIHSO4 electrolytes showed a shift towards lower frequencies

in the EP region as well as a shift towards higher frequencies for relaxation when compared

to the spectrum of PEO–EMIHSO4, indicating that fillers may have different functionalities

in the different frequency regions. At low frequencies, the chemically inert SiO2 and TiO2

nanofillers may exhibit a barrier effect by physically obstructing the motion of ions and

hence slowing down EP, resulting in a longer τEP. At high frequencies, the greater flexibility

and faster relaxation of the polymer chains due to the fillers may reduce τR and increase ionic

conductivity.

So far, it was established that nanofillers can lead to more amorphous PEO–EMIHSO4

electrolytes, thus enhancing the flexibility of the polymer chain and the motion of the charge

carriers. However, if the function of the fillers were solely to reduce the crystallinity, one

would expect that at temperatures above the melting point of PEO, where all the polymer

electrolytes are amorphous, the dielectric response and conductivity of both PEO–

EMIHSO4–SiO2 and PEO–EMIHSO4–TiO2 would be similar. According to the results shown

in Figure 6-3, this is not the case. Thus, dielectric derivative of PEO–EMIHSO4, PEO–

EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 was studied at different temperatures to identify

the impact of the dielectric constant of the nanofillers.

The dielectric derivative vs. frequency curves over the temperature range from –10 °C to

+80 °C for the three electrolytes are shown in Figure 6-14a-Figure 6-14c. As temperature

increases, the magnitude of der in the EP region increases for all three polymer electrolytes,

suggesting an increase in double-layer capacitance, as expected. The relaxation of the

polymer electrolytes shifted towards higher frequencies as the polymer structures became

amorphous. While the relaxation peaks of PEO–EMIHSO4 and PEO–EMIHSO4–SiO2 shifted

gradually (Figure 6-14a and Figure 6-14b), an abrupt shift occurred in PEO–EMIHSO4–TiO2

(Figure 6-14c).


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Figure 6-14 Dielectric derivative vs. frequency for (a) PEO–EMIHSO4, (b) PEO–EMIHSO4–SiO2, and (c) PEO–EMIHSO4–TiO2 at different temperatures: −10 °C ( ), 0 °C (), 10 °C ( ), 20 °C ( ), 30 °C ( ), 40 °C ( ), 50 °C ( ), 60 °C ( ), 70 °C ( ), 80 °C ( )


134

The higher der in EP region in Figure 6-14 is the results of increased ionic mobility or

dissociation as temperature increased [147]. The extent of EP for PEO–EMIHSO4–TiO2

(Figure 6-14c) was higher than that of PEO–EMIHSO4–SiO2 (Figure 6-14b) and PEO–

EMIHSO4 (Figure 6-14a). A possible reason lies in the difference between the intrinsic

dielectric constants of TiO2 and SiO2. Since TiO2 has a high dielectric constant of 86, it may

promote the degree of ionic dissociation, and hence increase the charge density accumulated

at the electrode-electrolyte interface. This would lead to a greater ′ and increased

double-layer capacitance. To verify this hypothesis, the capacitance was calculated at 0.1 Hz,

and is shown at different temperatures in Figure 6-15. The specific capacitance of PEO–

EMIHSO4 and PEO–EMIHSO4–SiO2 cells was similar, while PEO–EMIHSO4–TiO2 based

cells had a higher capacitance, especially at high temperatures, suggesting that the dielectric

constant affects capacitance. While the addition of inert nanofillers was expected to decrease

cell capacitance by reducing the effective surface area at the interface [148] (also seen in

Figure 6-12a for the SiO2-containing electrolyte), this effect can be compensated for by the

addition of TiO2 that enhances ionic dissociation due to its high dielectric constant.

Figure 6-15 Capacitance of cells leveraging PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2, respectively, at different temperatures (at 0.1 Hz)


135

The difference in dielectric response observed in Figure 6-14, especially above the

melting temperature, suggests that the fillers may also affect the relaxation differently, which

will directly affect the conductivity of the electrolyte and the cell resistance. To determine the

contribution of the fillers on the rate of both EP and relaxation of PEO–EMIHSO4, EP and R

of the polymer electrolytes were extracted from Figure 6-14a-Figure 6-14c and plotted as

Arrhenius plots (see Figure 6-16) to illustrate their trends with respect to temperature. The

overall temperature dependence of EP and relaxation can be divided into three regions: the

semi-crystalline phase (T < 30 °C), the phase transition (30 ºC ≤ T ≤ 50 ºC), and the

amorphous phase (T > 50 °C). The effect of fillers on EP and R are compared at low and

high frequency processes in the semi-crystalline and the amorphous states.

Figure 6-16 Electrode polarization time constant EP (empty symbols) and relaxation time constant R (filled symbols) for PEO–EMIHSO4 ( ), PEO–EMIHSO4–SiO2 ( ), and PEO–EMIHSO4–TiO2 ( )


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a. Effect of fillers in the low f region (electrode polarization)

Since the trends of EP (i.e., cell capacitance) have already been illustrated in Figure 6-15,

this section will focus on τEP. Examining Figure 6-16, τEP of the filler-containing electrolytes

showed a flat temperature dependence relative to that of PEO–EMIHSO4 in both

semi-crystalline and amorphous states. Their EP process occurred at lower frequencies than

that of PEO–EMIHSO4 over the entire temperature range. This agrees with the notion that

fillers can act as a physical barrier to the motion of ions and hence slow down the formation

of the double layer in the EP region. As the rate of formation of the double layer depends on

the number and the motion of ions [108], τEP is also an indicator of the number density of

conducting ions and their mobility, specifically following the single ion conducting model

[109,149]. EP of both filler-containing electrolytes in low temperatures (T < 30 °C) was

longer than for PEO–EMIHSO4. In the amorphous state (T > 50 °C), where ionic mobility is

less dependent on the polymer segmental motion, PEO–EMIHSO4–TiO2 had a shorter EP.

This provides additional evidence for the controlling role of the dielectric constant. A higher

dielectric constant leads to higher ionic dissociation and higher mobility, and thus higher

capacitance and shorter EP.

b. Effect of fillers in the high f region (relaxation)

The overall trend in Figure 6-16 shows that R for all polymer electrolytes decreased with the

increase in temperature. At low temperatures (T < 30 ºC), the relaxation time of the three

polymer electrolytes showed a similar trend: PEO–EMIHSO4–SiO2 exhibits the fastest

response (the smallest R), followed by PEO–EMIHSO4–TiO2 and PEO–EMIHSO4. This

overall trend is expected, as the major impact of the fillers at low temperatures is to hinder

crystallization and promote structural relaxation. This in turn facilitates polymer chain

mobility and ionic motion which is manifested by the shift of the relaxation peak to higher

frequencies shown in Figure 6-14. A comparison of ionic conductivity (Figure 6-3) and R


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(Figure 6-16) at low temperatures reveals that the conduction mechanism of PEO–EMIHSO4

mainly relies on the polymer segmental relaxation.

At temperatures above 50 ºC, the amorphous and flexible PEO chains allow ionic motion

and relatively fast relaxation. Although both filler-containing electrolytes responded faster

than the filler-free PEO–EMIHSO4, R of PEO–EMIHSO4–TiO2 was noticeably shorter than

for PEO–EMIHSO4–SiO2. While the structure and chain movements of all three polymer

electrolytes are identical in the amorphous phase, the difference in dielectric constant may

again play a leading role. As temperature increases, the dipoles in PEO and the ionic species

become more thermally activated and have more rotational freedom [81]. TiO2, having a

higher dielectric constant, can induce greater polarity in the PEO–EMIHSO4 system than

SiO2. The additional polarity significantly enhanced the relaxation rate of PEO–EMIHSO4–

TiO2 at temperatures above the phase transition, fostering a faster relaxation response (see

Figure 6-16). This also explains the abrupt shift of relaxation to higher frequencies seen in

Figure 6-14c. The relaxation followed an Arrhenius-type behavior similar to the ionic

conductivity shown in Figure 6-3, which reveals the close relationship between these two

parameters. Since the conductivity of the electrolytes was also extracted from the impedance

data at high frequencies (at 0° phase angle), R directly represents the electrolyte conductivity

and the equivalent series resistance of an electrochemical capacitor cell.

6.5 Effect of Fillers on Device Performance

In the previous section, the impact of the SiO2 and TiO2 nanofillers on τEP and cell

capacitance was demonstrated for metallic cells enabled with PEO–EMIHSO4 electrolytes. It

was shown that the high dielectric constant of TiO2 promoted the ionic dissociation and

hence increased the capacitance of the PEO–EMIHSO4-based cells. This section concerns the

performance of graphite EDLCs employing filler-containing PEO–EMIHSO4 in order to


138

confirm the effect of the filler on the performance of the electrolytes specifically at the

electrode-electrolyte interface.

The cyclic voltammograms of graphite EDLCs using PEO–EMIHSO4 and PEO–

EMIHSO4–SiO2 are shown in Figure 6-17a. The addition of a large amount of SiO2 (i.e., 10

wt%) decreased the capacitance of the device from 2 mF cm-2 to 1.5 mF cm-2 at 1 V s-1. The

performance of these graphite EDLCs was further analyzed by EIS to examine the influence

of the filler addition on the rate response of the cells. As shown in Figure 6-17b, the

capacitance of cells was consistent with that observed in CV, and it decreased with the

addition of 10 wt% SiO2. On the other hand, the time constant decreased from 4 s for PEO–

EMIHSO4-based cell to 0.5 s for PEO–EMIHSO4–SiO2-based cells.

The performance of graphite EDLCs using PEO–EMIHSO4 and PEO–EMIHSO4–TiO2

was also characterized by CV and EIS as illustrated in Figure 6-18a and Figure 6-18b. The

electrochemical performance of the cells with TiO2-containing electrolytes was similar to the

SiO2-containing PEO–EMIHSO4-based cells: the capacitance of the graphite EDLCs

decreased with the addition of TiO2, but the time constant became shorter.


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Figure 6-17 (a) Cyclic voltammograms of graphite EDLC devices with PEO–EMIHSO4, PEO–EMIHSO4–SiO2 at 1 V s-1; (b) variation of real part C′ and imaginary part C″ of capacitance with respect to frequency for the respective cells


140

Figure 6-18 (a) Cyclic voltammograms of graphite EDLC devices with PEO–EMIHSO4 and PEO–EMIHSO4–TiO2 at 1 V s-1; and (b) variation of real part C′ and imaginary part C″ of capacitance with respect to frequency for the respective cells


141

The decrease in the capacitance could be due to the high content of inert SiO2 and TiO2

nano-particles in the electrolyte which reduces the effective contact surface area between the

electrodes and the electrolyte. Since graphite electrodes had a higher surface area than that of

metallic electrodes, the effect of the reduced surface area is more pronounced and

predominates the attribution of the dielectric constant on increasing the capacitance of cells

(see Figure 6-15). While the addition of SiO2 and TiO2 nanofillers decreased the capacitance

of cells (C), they reduced the resistance of PEO–EMIHSO4 electrolytes and hence the cell

resistance (R), resulting in shorter time constants (τ = RC) or faster response.

To examine the cycle stability of the filler-containing PEO–EMIHSO4, as an example,

the EDLC with PEO–EMIHSO4–10% SiO2 was further subjected to cycle life tests. Figure 6-

19 shows the CV profiles obtained at 1 V s-1 for the 1st, 500th, and 5000th cycles. The almost

overlapping CVs after 500 and 5000 cycles demonstrate an excellent cycle life and high-rate

response of the solid EC device enabled by this polymer electrolyte.

Figure 6-19 Cycle life test of graphite EDLC device with PEO–EMIHSO4–10% SiO2 electrolyte at 1 V s-1


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This study demonstrated that incorporation of SiO2 or TiO2 fillers is a feasible approach

that is beneficial to ionic conductivity and structural stability of PEO–EMIHSO4 for

application in ECs. However, the amount of fillers should be optimized to ensure a balanced

conductivity and capacitance or rate performance. Leveraging the fillers, the ionic

conductivity of the environmentally friendly polymer–ILs are approaching the level of the

fluorinated baseline at room temperature, and outperform the latter at high temperatures. This

is illustrated in Figure 6-20 where a progressive increase in ionic conductivity of PEO–

EMIHSO4 with the addition of SiO2 and TiO2 can be seen. For high temperature applications,

PEO–EMIHSO4–TiO2 will be a promising candidate.

Figure 6-20 Comparison between the ionic conductivity as a function of temperature of the starting PEO–EMIHSO4, the optimized PEO–EMIHSO4–SiO2 and PEO–EMIHSO4–TiO2, and PVdF-HFP–EMIBF4


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6.6 Summary

Amorphous SiO2 and crystalline TiO2 nanofillers at 3 wt% and 10 wt% were added into

PEO–EMIHSO4 electrolyte to improve its ionic conductivity. Incorporation of 3 wt% SiO2

into PEO–EMIHSO4 electrolytes increased the conductivity, while addition of 3 wt% TiO2

was less influential and increased the crystallinity, suggesting the dominating effect of the

filler structure in small quantity. The addition of the SiO2 and TiO2 nanofillers at 10 wt%

increased the ionic conductivity over the entire temperature range which indicated that the

fillers were effective not only in promoting the amorphous phase at lower temperatures, but

also in reducing the interaction between HSO4− and PEO, and thus facilitating ionic

conduction.

Ionic mobility and ionic dissociation are both contributing factors to ionic conductivity

and cell capacitance. In addition to structural characterizations, the complex capacitance and

the dielectric analyses on PEO–EMIHSO4 electrolytes with and without SiO2 or TiO2

nanofillers, have identified three influential factors: (i) A structural effect minimizing the

crystalline phase, where the enhanced segmental motion of PEO–EMIHSO4–SiO2 and PEO–

EMIHSO4–TiO2 over that of PEO–EMIHSO4 resulted in higher ionic conductivity. (ii) An

intrinsic dielectric constant effect in the amorphous phase, where the higher dielectric

constant of TiO2 considerably increased ionic mobility and dissociation, leading to enhanced

conductivity of PEO–EMIHSO4–TiO2 over PEO–EMIHSO4–SiO2. (iii) A barrier effect

which delayed EP at low frequencies, resulting in a slower double layer formation. While

both SiO2 and TiO2 prolonged EP of PEO–EMIHSO4, the high dielectric constant of TiO2

compensated for the slow rate by promoting ionic dissociation, resulting in a greater ′ and a

higher capacitance of metallic cells.

The ionic conductivity of PEO–EMIHSO4 increased to 2.1 mS cm-1 with the addition of

SiO2, approaching the conductivity of the PVdF-HFP–EMIBF4 at room temperature (6.5

mS cm-1), and with the addition of TiO2, it exceeded the conductivity of the fluorinated


144

polymer–IL at high temperatures. While this improvement in the conductivity of an

environmental friendly polymer–IL is promising, the ILs based on 1-ethyl-3-methyl

imidazolium cation are aprotic and hence are inactive in pseudocapacitive reactions

involving proton-electron transfer. Developing protic ionic liquids as proton conductors for

polymer electrolytes will not only improve the double-layer capacitance, but also promote

pseudocapacitance. The next chapter focuses on the investigation and the development of

protic ILs through tweaking the structure of cations and their application in pseudocapacitive

ECs.

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CHAPTER 7

PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE

This chapter considers a different class of ILs: proton conducting ILs or protic ILs (PILs).

First, the proton conductivity of solutions of EMIHSO4, MIHSO4, ImHSO4 with different

cationic functional groups (previously shown in Chapter 4) is examined (section 7.2). Then,

thermal properties of pure binary mixtures of PILs and their resultant phase diagrams are

discussed. Eutectic compositions of EMIHSO4-ImHSO4 and MIHSO4-ImHSO4 mixtures are

identified, and their proton activity is evaluated through their contribution to

pseudocapacitive reactions (section 7.3). Finally, the performances of solid pseudocapacitors

enabled by the polymer–eutectic PILs are compared to that of a polymer–aprotic IL-based

cell (section 7.4).

7.1 Proton Activity and Melting point

As described in section 2.3.2, the physicochemical and electrochemical properties of ILs can

be tuned by changing the size of the cationic alkyl chains, or by introducing functional

groups to the cation. In Chapter 4, it was shown that the thermal and the electrochemical

properties of EMIHSO4 changed with substituting the alkyl groups of imidazolium cation

CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE

146

with shorter chains and/or protons. It was also demonstrated that the ionic conductivity and

electrochemical performance of ILs in a protic solvent (e.g., MeOH) was superior to that in

an aprotic solvent (e.g., PC). Among the three IL/MeOH solutions, the ionic conductivity of

MIHSO4 and ImHSO4 electrolytes were higher than that of EMIHSO4. It is unclear whether

the higher ionic conductivity is due to the higher ionic dissociation or to the additional

contribution of dissociated protons. The interest in proton conductivity of ILs is associated

with the role of protons on promoting pseudocapacitive reaction of electroactive materials.

The higher specific capacitance of pseudocapacitors over that of double-layer capacitors

makes them attractive for further development.

On the other hand, there is a trade-off between achieving ILs with proton conductivity

and their high melting temperature. This is again illustrated in Table 7-1: the melting point

increased from EMIHSO4 (24 °C) to MIHSO4 (47 °C) and ImHSO4 (85 °C) as the cations

have more symmetric structure and contain more active protons. Imidazole (Im), known to be

proton-conductive, is a self-dissociating compound with high proton conductivity in the

liquid state. It contains two nitrogen sites, and its protonated and unprotonated nitrogen

functional groups can act as proton donor and acceptor in proton transfer reactions [1,150].

However, the high melting point prevents the application of these ILs at room temperature.

Further investigation is necessary to reduce the melting point of these ILs and maintain

proton conductivity for pseudocapacitors. Currently, the majority of room temperature PILs

are fluorinated-based. Developing benign PILs is therefore important from an environmental

standpoint.


147

Table 7-1 Structure and melting temperature of PILs with different cations.

Ionic liquid Structure Melting point

(°C) 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4)

24

1-Methylimidazolium hydrogen sulfate (MIHSO4) 47

Imidazolium hydrogen sulfate (ImHSO4) 85

One way to decrease the melting point of ILs is to increase the length of alkyl chain

substitution on the cation to make it asymmetric. However, the increase of the van der Waals

interactions between the long hydrocarbon chains can lead to high viscosity and melting

points [30]. More important, this approach is not applicable for PILs as alkyl chains have no

contribution to proton conduction.

Another approach that has been used to lower the melting point of high temperature

molten salts is to form eutectic mixtures of ILs [39]. The eutectic mixture of ILs involves

mixing two of the single ILs to distort the ion packing and avoid crystallization. This is

simple and advantageous as it eliminates the need for chemical reactions to produce novel

PILs. It was sought to verify the relative proton activity of EMIHSO4, MIHSO4, and

ImHSO4, and to reduce their high melting point to develop PILs with proton conductivities

equal or comparable to common fluorinated PILs. Accordingly, the following two

approaches were undertaken:

(i) Exploring the proton conductivity of the ILs by analyzing the electrochemical

performance of pseudocapacitive electrodes using IL/MeOH electrolytes.

(ii) Determining the eutectic compositions of binary IL systems to identify liquidus

regions at or below room temperature, and construct phase diagrams of these systems.


148

7.2 Proton Conductivity of PIL Solutions

To demonstrate the proton conductivity of the electrolytes, two types of known

pseudocapacitive electrodes were employed in the test vehicle: RuO2 and

carbon/polyoxometalate (POM) composite electrodes (see section 3.3). RuO2 exhibits

pseudocapacitance via a series of fast coupled proton–electron transfer redox reactions in a

protic electrolyte according to Equation 2-2. POMs also involve reversible multielectron

transfer reactions in proton containing electrolytes which generate charge storage and

delivery. The redox reactions for one type of POM: phosphomolybdate or PMo12O403−

(PMo12) is shown in Equation 7-1.

3 312 40 x 12 40PMo O xH xe H PMo O , x = 2,4,6 (7-1)

The pseudocapacitive performance of these electrodes has been demonstrated in aqueous

electrolyte, reported in [122,151]. Cyclic voltammograms of POMs in protic electrolytes

exhibit several characteristic oxidation/reduction peaks which can be used as a fingerprint to

compare different IL electrolytes. Both electrodes were used to assess the extent of proton

dissociation and the proton conduction in the IL electrolytes. The following discussion will

focus on the performance of RuO2 electrodes with the PIL electrolytes then will be followed

by the performance of POM electrodes in similar electrolytes.

7.2.1 Performance of RuO2 with PIL electrolytes

The electrochemical performance of RuO2 cells was examined to evaluate and compare the

proton conductivity of the respective PILs. Figure 7-1 shows the voltammograms of the

RuO2 cells employing EMIHSO4/MeOH, MIHSO4/MeOH, and ImHSO4/MeOH as well as

EMIHSO4/PC electrolytes. The two main observations agree with the results in Chapter 4: (i)

protic solvent promoted the proton dissociation, and hence the capacitance of RuO2 cell with

EMIHSO4/MeOH was higher than that with EMIHSO4/PC (also see Figure 4-2 and Figure 4-


149

7); and (ii) the higher capacitance of ImHSO4 and MIHSO4-based cells than that of

EMIHSO4-based cell indicated the effect of cation on proton conductivity and the ability of

these PILs to contribute to the electrochemical oxidation and reduction reactions of RuO2.

Figure 7-1 Cyclic voltammograms of RuO2 cells in ImHSO4/MeOH, MIHSO4/MeOH, EMIHSO4/MeOH, and EMIHSO4/PC electrolytes at (a) 100 mV s-1 and (b) 1 V s-1 (Liquid–2 configuration)


150

Table 7-2 ESR and capacitance of RuO2 cells using PIL electrolytes and H2SO4.

Electrolytea ESR (ohm) Cell capacitance (mF cm-2)

Cʹ (EIS)b C (100 mV s-1) C (1 V s-1) EMIHSO4/PC 21.4 5.0 11.0 5.5 EMIHSO4/MeOH 22.9 9.4 27.0 11.0 MIHSO4/MeOH 5.3 16.8 35.0 16.0 ImHSO4/MeOH 5.6 18.8 35.0 18.0 0.5M H2SO4 0.74 98.0 110.0 91.0

a concentration of all PIL electrolytes were 40 wt% of PIL in solvent b capacitance was extracted at 0.3 Hz which corresponds to DC measurements at 1 V s-1

Accordingly, one would expect ImHSO4 electrolyte that exhibit two cationic hydrogen

atoms to promote reaction 2-2 (pg. 10) more strongly than MIHSO4 electrolyte. However,

this is not the case in Figure 7-1 and also shown in Table 7-2. An acid-base titration was

performed to evaluate the degree of proton dissociation of the PILs in MeOH. The proton

concentrations for the three PIL electrolytes are listed in Table 7-3.

Table 7-3 Proton concentration of PIL/MeOH electrolytes obtained from titration with 0.1 M NaOH.

Electrolytes [H+] (M)

EMIHSO4/MeOH 0.10 MIHSO4/MeOH 0.24 ImHSO4/MeOH 0.26

Although Im cations exhibit two nitrogen sites, the proton concentration in ImHSO4

electrolyte was only slightly higher than that in MIHSO4. The estimated proton

concentrations (Table 7-3) and the comparison of pseudocapacitance of RuO2 cell (Figure 7-

1) indicate that the secondary proton of ImHSO4 may be partially dissociated. These

comparisons lead to the following conclusions:

(i) The studied PILs demonstrated proton conductivity increasing in the order of

EMIHSO4 ˂ MIHSO4 ≤ ImHSO4 (in MeOH solution).

(ii) The ion–ion association and the hydrogen bonding in ImHSO4 may cause partial

dissociation of the secondary proton, resulting in a pseduocapacitive performance

similar to that of MIHSO4.


151

(iii) The sweep rate dependency of the capacitance of the RuO2 cell in PIL/MeOH is

indicative of the lower proton dissociation and higher viscosity of PIL/MeOH

electrolytes compared to highly dissociated H2SO4.

The extent of proton dissociation and the rate of proton transfer in the three PIL/MeOH

electrolytes were further studied by analyzing the performance of the carbon/POM

pseudocapacitive electrodes.

7.2.2 Performance of carbon/POM in PIL electrolytes

The composite electrodes used in this study were based on MWCNT which were chemically

modified by PMo12O403− (PMo12). The proton contribution of the three PIL electrolytes to the

electrochemical reactions of these electrodes, referred to as carbon/PMo12, were examined

and compared to the bare carbon double-layer electrodes. For comparison, the respective

carbon/PMo12 electrodes were also tested in 0.5M H2SO4.

Figure 7-2 shows the cyclic voltammograms of carbon/PMo12 and bare carbon electrodes

in aqueous electrolyte. The CV profile of carbon/PMo12 exhibited three pairs of characteristic

oxidation/reduction peaks corresponding to the redox reactions of PMo12 (reaction 7-1).

These peaks were relatively sharp and reversible at a sweep rate of 100 mV s-1. The peak

intensities and their reversibility were used as a baseline to evaluate the proton-electron

reaction of the PIL electrolytes.


152

Figure 7-2 Cyclic voltammograms of bare carbon (dashed line) and carbon/PMo12 (solid line) electrodes in 0.5M H2SO4 at 100 mVs-1

7.2.2.1 Electrode performance in PIL electrolytes

The carbon/PMo12 electrodes were characterized using EMIHSO4/MeOH, MIHSO4/MeOH,

and ImHSO4/MeOH. Their CVs together with those of the bare carbon electrodes are shown

in Figure 7-3a-Figure 7-3c. Figure 7-3d overlaid the CVs of carbon/PMo12 electrodes in the

three electrolytes. The similarities between the overall performance of the electrodes in the

PIL electrolytes and that in H2SO4 were the presence of the characteristic redox peaks. This

verified that redox reaction of PMo12 occurred in PIL electrolytes, further confirming the

proton conduction of these PILs. However, compared to H2SO4, the peaks were less

reversible and sharp, leading to a lower charge storage. While protons were available in the

PIL electrolytes, their concentration was lower and their diffusion was slower than that in the

aqueous electrolyte.


153

Figure 7-3 Cyclic voltammograms of bare carbon and carbon/PMo12 electrodes in (a) EMIHSO4/MeOH, (b) MIHSO4/MeOH, (c) ImHSO4/MeOH electrolytes, and (d) comparison of cyclic voltammograms of carbon/PMo12 electrodes in the three PIL electrolytes (sweep rate: 100 mV s-1)

Among the three PILs, the proton activity of electrolytes increased in the order of

EMIHSO4 < MIHSO4 ≤ ImHSO4 as demonstrated by more distinct and reversible peaks. The

observed trend agrees with that seen for RuO2-based cells (see Figure 7-1), in which

ImHSO4/MeOH and MIHSO4/MeOH electrolytes promoted pseudocapacitive reactions of

both RuO2 and carbon/PMo12. The comparison of the performances of both electrodes to

those in aqueous electrolytes suggest that proton conduction in PIL electrolytes could be a


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combination of diffusion of the protonated ions and the proton transfer rather than pure

proton transfer.

According to Figure 7-3a and Figure 7-1, proton conduction occurred to some extent in

EMIHSO4/MeOH. This was further supported by acid-base titration (see Table 7-3). The

broad and distorted redox reaction peak in Figure 7-3a suggests a small fraction of

dissociated protons with a lower mobility. Since there are no active protons in EMI cation,

the proton conduction could be attributed to the relatively small dissociation of HSO4−.

Considering the small dissociation constant of HSO4−, 2

4 4HSO H SO , the equilibrium

may shift to the right in the presence of a protic solvent such as MeOH. As such, the

available protons in EMIHSO4 are contributed by HSO4− anion, while MIHSO4 and ImHSO4

have protons contributed from both HSO4− and their respective cations.

7.2.2.2 Device performance in PIL electrolytes

In addition to the characterization of the PILs at the electrode level, they were also tested in

devices using 2-electrode configuration. Figure 7-4a shows the CV profiles of the bare

carbon cells with EMIHSO4/MeOH, MIHSO4/MeOH, ImHSO4/MeOH, and EMIHSO4/PC.

The double-layer capacitance of the bare carbon cells was slightly higher in MIHSO4/MeOH

followed by ImHSO4/MeOH and EMIHSO4/MeOH.

The performance of carbon/PMo12 cells in the three PIL/MeOH electrolytes and in

EMIHSO4/PC are shown in Figure 7-4b. The pseudocapacitive reactions of carbon/PMo12

electrodes in PIL/MeOH electrolytes increased the capacitances over the double-layer

capacitances of the corresponding bare carbon cells (Figure 7-4a). The capacitance of

carbon/PMo12 cell was the highest in ImHSO4 electrolyte (Figure 7-4b), suggesting a higher

concentration of mobile protons in this electrolyte.


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Figure 7-4 Cyclic voltammograms of (a) bare carbon cells and (b) carbon/PMo12 cells in EMIHSO4/MeOH, MIHSO4/MeOH, ImHSO4/MeOH, and EMIHSO4/PC electrolytes at 1 V s-1 (Liquid–2 configuration)


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To examine the extent of proton transfer in these PIL electrolytes with respect to aqueous

electrolyte, the performance of same carbon/PMo12 cells were also characterized in H2SO4.

The capacitances of each cell using both H2SO4 and PILs are summarized in Table 7-4. For

comparison, the capacitance of each cell in PIL/solvent was calculated with respect to that in

H2SO4 and their ratio is reported as percentage in Table 7-4. The percentage of cell

capacitance increased in the order of EMIHSO4/PC < EMIHSO4/MeOH < MIHSO4/MeOH <

ImHSO4/MeOH. This trend was consistent with the capacitance of carbon/PMo12 electrodes

(Figure 7-4) and RuO2 cells (Figure 7-1), confirming the higher proton conducting

characteristics of ImHSO4 and MIHSO4.

Table 7-4 Capacitance of carbon/PMo12 cells in aqueous and the corresponding cells in PIL/solvent electrolytes at 100 mV s-1.

Electrolyte Cell capacitance (mF cm-2)

PIL/solvent 0.5 M H2SO4 CIL/Caq.(%)EMIHSO4/PC 4.4 32.3 14 EMIHSO4/MeOH 7.1 28.3 25 MIHSO4/MeOH 6.5 24.2 27 ImHSO4/MeOH 9.1 27.6 33

These observations supported the initial idea on tailoring the cationic functional groups to

develop PILs. The proton conductivity of the PILs was established and compared via

characterizing the pseudocapacitive performance of both RuO2 and carbon/PMo12 electrodes

in PIL/MeOH solutions. However, MeOH is not applicable in electrolytes due to its

volatility. The approach was to investigate binary mixtures of PILs to develop pure PILs that

have low melting temperatures and proton conducting characteristics.

7.3 Binary Mixtures of PILs

Both ImHSO4 and MIHSO4 showed much greater proton activity than EMIHSO4. The issue

is that the melting temperatures of these two PILs are above room temperature, so they are

solid in ambient condition different from EMIHSO4.


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Figure 7-5 shows the heating and cooling thermograms of EMIHSO4, MIHSO4, and

ImHSO4 between −90 °C and 150 °C. The melting point of EMIHSO4 is at 24 °C, and there

was no sign of recrystallization during cooling. EMIHSO4 remains liquid at room

temperature. At low temperature, EMIHSO4 exhibits a glass transition at −61 °C which is

typically reported between −70 °C and −90 °C for 1-alkyl-3-methylimidazolium salts [30].

Figure 7-5 DSC thermograms of pure EMIHSO4, MIHSO4, and ImHSO4 at heating and cooling scans of 10 °C min-1

The melting point of MIHSO4 is increased in comparison to EMIHSO4, at 47 °C, and is

further increased for ImHSO4 at 85 °C. Both MIHSO4 and ImHSO4 recrystallized on cooling,

Heating

Cooling


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indicating the formation of crystal structure at room temperature. The lower melting point of

EMIHSO4 is evidence of inefficient packing of ions due to its alkyl groups. Packing is more

efficient where the alkyl chains are replaced by hydrogen atoms or shorter chains, as in

MIHSO4 and ImHSO4 (see Table 7-1).

To develop true room temperature PILs, while retaining proton conduction in the IL

system, efforts were made to find eutectic compositions of two binary mixtures:

MIHSO4-ImHSO4 and EMIHSO4-ImHSO4. The impact of cation substitution groups of

EMIHSO4 and MIHSO4 on melting temperature of the binary mixtures was also studied and

compared.

7.3.1 MIHSO4-ImHSO4 binary system

When mixing MIHSO4 and ImHSO4 at various ratios, the binary system showed different

physical and chemical properties. Figure 7-6 shows examples of thermograms of the

MIHSO4:ImHSO4 mixtures with weight percentage ratios at 65:35, 70:30, 75:25, and 80:20.

The binary mixture with 65 wt% MIHSO4 and 35 wt% ImHSO4 showed a melting peak at

0 °C, which is much lower than that of MIHSO4 (47 °C) and ImHSO4 (85 °C). The binary

systems with compositions between 65 wt% and 80 wt% MIHSO4 showed negligible melting

peaks—in other words, no endothermic transitions. Specifically, the 70:30 composition of

MIHSO4-ImHSO4 remained liquid throughout the temperature region between −72 °C to

150 °C. This suggests that the system has reached the eutectic composition, where the IL

mixture remained as liquid until reaching its glass transition at approximately −65 °C. When

further increase the MIHSO4:ImHSO4 ratio to 80:20 wt%, a melting transition occurred and

its temperature reached that of pure MIHSO4 (Figure 7-5).


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Figure 7-6 DSC thermograms of various compositions of MIHSO4-ImHSO4 binary mixtures at heating and cooling scans of 10 °C min-1

In MIHSO4-ImHSO4 binary mixtures, the crystal structure of ImHSO4 was likely

disrupted due to the addition of more asymmetric MIHSO4, resulting in binary mixtures with

melting transitions at low temperature or none at all. The increase in the disorder of the

binary systems is further evidenced by the appearance of glass transitions at low

temperatures. The thermal properties of MIHSO4-ImHSO4 system for other compositions

were characterized in a similar way and the transition temperatures are summarized in Table

7-5.

Table 7-5 Thermal properties of MIHSO4-ImHSO4 binary system at different compositions. MIHSO4:ImHSO4 (wt%) Tg (°C) Tm1 (°C) Tm2 (°C)

(0:100) - 57 85 (30:70) −61 −14 64 (50:50) −61 4 42 (65:35) −69 - 1 (70:30) −72 - - (75:25) −69 - - (80:20) −69 - 29 (100:0) - - 47


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Based on the DSC results in Figure 7-6 and Table 7-5, a quasi-equilibrium phase diagram

of the binary system was constructed and illustrated in Figure 7-7. There are three main

phases for the MIHSO4-ImHSO4 binary system: liquid phase at temperatures above the

liquidus line, solid phase below the glass transition, and a two-phase system in between. By

mixing the two solid PILs, a binary system containing eutectic composition was developed.

At compositions between 65 wt% and 80 wt% MIHSO4, an eutectic region was obtained

which has an extended liquid phase all the way to its glass transitions at approximately

−70°C. Leveraging the eutectic compositions, the proton conductivity can be analyzed for

PIL systems without any solvent.

Figure 7-7 Phase diagram for MIHSO4-ImHSO4 binary system: () melting point; () solid-solid transition; ( ) glass transition

7.3.2 EMIHSO4-ImHSO4 binary system

Similar approach was also applied to liquid EMIHSO4 by adding solid ImHSO4 to introduce

proton activity into EMIHSO4 system. Table 7-6 shows the thermal properties of


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EMIHSO4-ImHSO4 at different compositions which were used to develop the phase diagram.

Examples of thermograms of the EMIHSO4:ImHSO4 binary systems are shown in Figure B-

3.

Table 7-6 Thermal properties of EMIHSO4-ImHSO4 binary system at different compositions.

EMIHSO4:ImHSO4 (wt%) Tg (°C) Tm1 (°C) Tm2 (°C) (0:100) - 57 85 (30:70) - 23 58 (40:60) −61 23 49 (50:50) −60 - 0 (60:40) −60 - 27 (65:35) −66 - 23 (70:30) −61 - - (75:25) −64 - - (80:20) −63 - - (85:15) −66 - - (100:0) −62 - 24

The resultant phase diagram for EMIHSO4-ImHSO4 is shown in Figure 7-8. Overall, the

addition of EMIHSO4 reduced the melting point of ImHSO4. While the three main phases

also exist in this binary phase diagram, there are two minima in Figure 7-8. At 50 wt%

EMIHSO4, the melting point of the binary system noticeably decreased to 0 °C reaching the

first minimum, but it increased with further addition of EMIHSO4 to 65 wt%.

At compositions greater than 65 wt% EMIHSO4, the phase diagram exhibited a second

minimum with Tg of approximately −61 °C corresponding to a eutectic region. The eutectic

EMIHSO4-ImHSO4 mixture was obtained over a wider range of compositions compared to

MIHSO4-ImHSO4 (see Figure 7-7), most likely due to the bulkier EMI cations interfering

with orderly packing and reducing ionic attraction to a greater extent than MI. Utilizing the

phase diagrams developed in Figure 7-7 and Figure 7-8, binary PIL liquids with eutectic

compositions through a wide temperature region (i.e., +150 °C to −70 °C) can be obtained.


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Figure 7-8 Phase diagram for EMIHSO4-ImHSO4 binary system: () melting point; () solid-solid transition; ( ) glass transition

7.3.3 Performance of RuO2 in eutectic PILs

To determine the proton conduction in these liquid binary mixtures, their electrochemical

performance was investigated using RuO2 as pseudocapacitive electrodes. The

electrochemical properties of the eutectic MIHSO4-ImHSO4 and EMIHSO4-ImHSO4 were

characterized at 70:30 percentage ratio and compared to that of pure EMIHSO4 and are

shown in Table 7-7. The ionic conductivity of EMIHSO4-ImHSO4 was similar to that of

EMIHSO4, whereas MIHSO4-ImHSO4 exhibited slightly higher ionic conductivity. This

could be due to greater available protons contributed by both MIHSO4 and ImHSO4. To

verify this hypothesis, the performance of RuO2 cells with EMIHSO4, EMIHSO4-ImHSO4,

and MIHSO4-ImHSO4 electrolytes were analyzed, and their CV profiles are shown in Figure

7-9.


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Table 7-7 Conductivity of pure EMIHSO4, eutectic EMIHSO4-ImHSO4, and eutectic MIHSO4-ImHSO4 and the capacitance of RuO2 cells enabled with respective PILs.

Electrolytes Conductivity

(mS cm-1) Capacitance of RuO2 cell (mF cm-2)

Cʹ (EIS)a C (5 mV s-1) EMIHSO4 1.5 16.7 18.8 EMIHSO4-ImHSO4 (70:30) 1.4 26.0 27.9 MIHSO4-ImHSO4 (70:30) 1.9 39.4 43.4 a capacitance was extracted at 0.001 Hz which corresponds to DC measurements at 5 mV s-1

*capacitance of RuO2 cell in 0.5M H2SO4 was 91 mF cm-2 at 1 V s-1

The cell capacitances from both CV and EIS measurements are reported in Table 7-7. As

depicted in Figure 7-9 and also shown in Table 7-7, the amount of charge stored for RuO2

cell is the highest with MIHSO4-ImHSO4, followed by EMIHSO4-ImHSO4 and EMIHSO4

electrolytes. This trend implied that the amount of protons or protonated ions is higher in

MIHSO4-ImHSO4, in which the cations from both MIHSO4 and ImHSO4 can contribute to

the proton conduction. Consequently, the available proton species are reduced in

EMIHSO4-ImHSO4 as the cation from ImHSO4 would be the dominating proton conductor,

and is further decreased in pure EMIHSO4 (i.e., EMI cation with no active protons).

Figure 7-9 Cyclic voltammograms of RuO2 cells using pure EMIHSO4, eutectic EMIHSO4-ImHSO4 (70:30), and eutectic MIHSO4-ImHSO4 (70:30) at 5 mV s-1


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Although the capacitance of RuO2 cell with MIHSO4-ImHSO4 (43 mF cm-2) was lower

than that with H2SO4 (ca. 90 mF cm-2), the higher electrochemical stability of the developed

PILs allows an operating potential of 1.5 V which is higher than aqueous electrolytes. The

eutectic PILs are also benign and less corrosive than H2SO4 electrolyte. The performance of

RuO2 in the eutectic PILs was comparable to a few available studies where they have used

fluorinated PILs. The capacitance of RuO2 electrode using EMIHSO4-ImHSO4 (37 F g-1)1

and MIHSO4-ImHSO4 (58 F g-1) were in the same order of that reported by Rocherfort et al.

using 2-methylpyridinium trifluoroacetate (83 F g-1) [52] and by Mayrand-Provencher et al

using 3-methylpyridazinium trifluoroacetate (45 F g-1) [53]. This implies that non-fluorinated

PILs such as eutectic MIHSO4-ImHSO4 and EMIHSO4-ImHSO4 can have promising proton

conduction characteristics.

7.4 Performance of Solid RuO2 Cell with Polymer–eutectic PILs

To demonstrate the viability of eutectic PILs for proton conducting polymer electrolytes,

eutectic EMIHSO4-ImHSO4 (70:30) and eutectic MIHSO4-ImHSO4 (70:30) were

incorporated into PEO to form polymer electrolytes. The performance of solid RuO2 cells

enabled with these polymer electrolytes were compared to that with PVdF-HFP–EMIBF4,

and their CV profiles are shown in Figure 7-10. Since PVdF-HFP–EMIBF4 is not proton

conducting, it serves as a baseline for double-layer capacitance.

1 The capacitances of RuO2 electrodes in mF cm-2 were converted to F g-1 for comparison to the literature.


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Figure 7-10 Cyclic voltammograms of solid RuO2 cells enabled with PVdF-HFP–EMIBF4, PEO–EMIHSO4-ImHSO4 (eutectic 70:30), and PEO–MIHSO4-ImHSO4 (eutectic 70:30) at (a) 5 mV s-1 and (b) 50 mV s-1


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The capacitance of RuO2 cells increased in the order of PVdF-HFP–EMIBF4 ˂ PEO–

EMIHSO4-ImHSO4 ˂ PEO–MIHSO4-ImHSO4 at both 5 mV s-1 and 50 mV s-1 (see Table 7-

8). This comparison denotes that: (i) proton activity exists in PEO–EMIHSO4-ImHSO4 and

PEO–MIHSO4-ImHSO4 electrolytes, enabling pseudocapacitive reactions of RuO2, and (ii)

the higher proton contribution of eutectic MIHSO4-ImHSO4 than that of eutectic

EMIHSO4-ImHSO4 maintained in polymer state, signified by the higher capacitance of RuO2

cell leveraging the former.

Table 7-8 Capacitance of RuO2 cells enabled with PVdF-HFP–EMIBF4, PEO–EMIHSO4-ImHSO4 (eutectic 70:30), and PEO–MIHSO4-ImHSO4 (eutectic 70:30).

Electrolytes Conductivity

(mS cm-1) Capacitance of RuO2 cell (mF cm-2)

C (5 mV s-1) C (50 mV s-1) PVdF-HFP–EMIBF4 4.8 ± 0.7 16.8 7.9 PEO–EMIHSO4-ImHSO4 0.6 ± 0.1 35.3 17.8 PEO–MIHSO4-ImHSO4 0.7 ± 0.3 40.9 20.4

The performance of RuO2 enabled with both PEO–eutectic PILs was not only similar to

that with liquid counterparts at 5 mV s-1, but also outperformed the liquid cells allowing

pseudocapacitive behavior of solid RuO2 cell at 50 mV s-1 (see Figure 7-10b). These results

further demonstrated the advantage of employing thin-film polymer electrolytes on

minimizing the influence of the high viscosity of eutectic PILs, enhancing the rate

performance of the pseudocapacitive device.

The higher capacitance of pseudocapacitors is expected to increase the energy density

over that of EDLCs. A comparison of the solid devices with the different polymer

electrolytes is presented in the Ragone plot in Figure 7-11. The specific power and energy

densities are estimated based on the volume and the area of the devices as the intended

applications are for thin-film and small-scale devices. Pseudocapacitors enabled by the

proton conducting polymer–ILs possess much higher energy density than that of EDLCs,


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while higher power density is obtained for EDLCs with fluorinated polymer–ILs due to their

higher ionic conductivity and operating voltage. The area specific power and energy densities

also showed the same trends. The performances of a lithium battery and an electrolytic

capacitor are overlaid in the Ragone plot only as a guideline. These devices were tested for

micro-devices as reported in [11].

Figure 7-11 Comparison of the specific energy and power density (per cm3 of stack cell) of solid ECs enabled with the polymer–ILs (volumetric energy and power densities are for the stack comprising the current collectors, the active material, and the polymer electrolyte)

7.5 Summary

The impact of cationic substitution groups on the proton conductivity of EMIHSO4,

MIHSO4, and ImHSO4 was studied for MeOH solutions of the respective PILs. The extent of

available protons and proton conductivity were examined by analyzing the ability of the

PIL/MeOH electrolytes to promote pseudocapacitive reactions of RuO2 and carbon/PMo12


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electrodes. Proton conductivity increased in the order of EMIHSO4 ˂ MIHSO4 ≤ ImHSO4 in

a good agreement with the proton concentration estimated from an acid-base titration. The

proton contribution was dominated by the proton-containing cations: MI and Im. A

contribution from anion was possible in such conditions that HSO4− dissociation was

activated (i.e., in protic MeOH).

To develop PIL electrolytes without solvent and have low melting temperatures, thermal

properties of binary mixtures of EMIHSO4-ImHSO4 and MIHSO4-ImHSO4 were determined.

Constructing phase diagrams, binary PIL liquids with eutectic compositions over a wide

temperature range were obtained. The performance of RuO2 pseudocapacitors in

EMIHSO4-ImHSO4 and in MIHSO4-ImHSO4 confirmed the proton activity of these eutectic

binary PILs; higher in MIHSO4-ImHSO4 binary system constituted of both proton-containing

cations.

The proton conductivity of eutectic binary PILs maintained in polymer electrolytes. Solid

RuO2 pseudocapacitors enabled with PEO–EMIHSO4-ImHSO4 and PEO–MIHSO4-ImHSO4

demonstrated promising performances at sweep rates higher than that of liquid cells and

higher energy densities than that of EDLCs. The study of the binary IL systems offers a new

approach to develop environmentally safe PILs by selecting the right combination of ILs.

The properties of eutectic PILs (i.e., proton dissociation) could be further enhanced in

thin-film polymer electrolytes by optimizing the material system.

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CHAPTER 8

CONCLUSIONS AND FUTURE WORK

8.1 Contributions

The results of current study have both practical and fundamental implications:

(i) Polymer–IL electrolytes for both EDLCs and pseudocapacitors were developed, and

their performance was enhanced to replace fluorinated polymer–IL electrolytes. This

was achieved via optimizing the material system, incorporating inorganic nanofillers,

tweaking the structure of ILs, and developing eutectic binary PILs.

(ii) Using electrochemical capacitor as a platform, this work has revealed the specific

functions of the IL, polymer matrix, and fillers in PEO–EMIHSO4 based electrolytes.

The ion transport mechanism in the polymer–IL was studied and the role of the

constituent materials on ionic conduction was identified. This was accomplished by

combining the complex capacitance and dielectric analyses to correlate the intrinsic

properties of the electrolytes and the performance of electrochemical capacitor cells,

and to provide insights for further improvements.

In addition, the findings and the methodology of this study may be applied to other

applications such as biodevices. The eutectic PILs developed in this work exhibit the

hydrogen bonding ability which is important to the dissolution of cellulose and maintaining

the reactivity of enzymes after dissolution for long-life biodevices. Proton conducting

CHAPTER 8. CONCLUSIONS AND FUTURE WORK

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polymer–ILs may also be used in the electrochemical actuators which produce a mechanical

bending or axial motion in response to an electrical stimulus, leveraging their ionic

conductivity coupled with high thermal and electrochemical stability.

8.2 Conclusions

The following conclusions have been drawn from the studies of the liquid IL electrolytes, the

polymer–IL with and without fillers, and the proton conducting polymer–PILs electrolytes.

Liquid electrolytes: The ionic conductivity, potential window, and resultant capacitance of

pure EMIHSO4 and EMIBF4 were investigated. The effects of anion and cation on these

properties were studied.

(1) EMIHSO4 with non-fluorinated anion (HSO4−) exhibited strong ion-ion interactions,

resulting in higher viscosity and lower conductivity than that of EMIBF4 with less

polarizable anion (BF4−). The high viscosity of EMIHSO4 was less influential on the

performance of enabled EDLC at low sweep rate (100 mV s-1), while it was a limiting

factor for the device operating at higher rates (1 V s-1).

(2) Addition of the solvent (PC) increased the ionic dissociation of EMIHSO4.

Conductivity and potential window at the optimum concentration were in the level of

common organic electrolytes.

(3) The substitution of cationic alkyl groups of EMIHSO4 with shorter alkyl chains

and/or protons in MIHSO4 and ImHSO4 increased the melting point of the ILs, as a

result of increased cationic symmetry and hence efficient ion packing.

Electrochemical properties examined for the solutions of the three ILs in polar protic

solvent demonstrated a higher ionic conductivity and resultant double-layer


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capacitance of MIHSO4 and ImHSO4 solutions, suggesting a higher proton

dissociation than that of EMIHSO4 solution.

Polymer–IL electrolytes: Thin-film and flexible PEO–EMIHSO4 were developed as viable

electrolytes for solid ECs. The ionic conductivity, structural characteristic, and performance

of enabled devices were characterized and compared to both liquid EMIHSO4 and

PVdF-HFP–EMIBF4.

(4) The ionic conductivity of PEO–EMIHSO4 was 0.8 mS cm-1 at room temperature.

Despite the higher viscosity of EMIHSO4 than that of EMIBF4, the decrease of ionic

conductivity in polymer state was less noticeable than that of PVdF-HFP–EMIBF4.

The lower activation energy of ionic conduction in PEO–EMIHSO4 than that in pure

EMIHSO4 supported the notion of different ion transport mechanism in polymer

which is less affected by high viscosity.

(5) Addition of EMIHSO4 into PEO substantially decreased the crystallinity of the

polymer (by 48%) and lowered its melting point. A similar effect of EMIBF4 on the

structure of PVdF-HFP confirmed that the ILs act as ionic conductors and plasticizers

in polymer electrolytes.

(6) An interaction between HSO4− and ether oxygen in PEO was revealed, which

enhanced the dissociation of the IL into EMI+ and HSO4− in polymer state.

(7) The capacitance of EDLCs leveraging the environmental friendly PEO–EMIHSO4

electrolyte was comparable to that of PVdF-HFP–EMIBF4-enbaled devices at 1 V s-1

over an operating voltage of 1.5 V. Also, the capacitive response of devices enabled

by solid PEO–EMIHSO4 exceeded the performance of the liquid counterpart devices,

especially at high rates.


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Polymer–IL with fillers: Amorphous SiO2 and crystalline TiO2 nanofillers were incorporated

into PEO–EMIHSO4 to improve its ionic conductivity. Ion conduction mechanism was

deduced, and the impacts of the fillers on ion transport process at different operating

conditions (temperature and frequency) were identified.

(8) The incorporation of both SiO2 and TiO2 nanofillers into PEO–EMIHSO4 electrolyte

effectively decreased the crystalline phase, resulting in a 2-fold increase in ionic

conductivity at room temperature. Structural and thermal characterizations showed

that the fillers primarily acted as “plasticizers” by inhibiting the crystallization of the

polymer chains. The addition of the fillers facilitated polymer segmental relaxation

which resulted in higher ionic conductivity. The difference in crystal structure of the

fillers had negligible impact on crystallinity and conductivity of PEO–

EMIHSO4-based electrolytes in the semi-crystalline state.

(9) Using the complex capacitance and dielectric analyses, additional effects of the fillers

were revealed. The dielectric constant characteristic of the filler was the main

contributor to ion conduction in the amorphous phase. Fillers with a high dielectric

constant increased the polarity of the polymer electrolyte and hence promoted ionic

dissociation. TiO2, with a much larger than SiO2, significantly increased ionic

conductivity and capacitance of PEO–EMIHSO4-based metallic cells at high

temperatures, where polymer is amorphous.

(10) The ionic conductivity of PEO–EMIHSO4 increased from 0.8 to 2.1 mS cm-1 with

the addition of SiO2, approaching the conductivity of the PVdF-HFP–EMIBF4 at

room temperature. For high temperature applications, the ionic conductivity of PEO–

EMIHSO4–TiO2 exceeded the fluorinated polymer–IL.


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Protic ILs and proton conducting polymer–PILs: The proton conductivity of EMIHSO4,

MIHSO4, and ImHSO4 (i.e., ILs with different cationic functional groups) were examined in

their respective methanol solutions.

(11) The pseudocapacitive behavior of RuO2 and carbon/PMo12 electrodes employing the

three IL solution electrolytes confirmed their proton activity. The proton conductivity

increased in the order of EMIHSO4 ˂ MIHSO4 ≤ ImHSO4 as a result of the

dissociation of protons of imidazolium cation in MIHSO4 and ImHSO4.

(12) Binary PIL liquids with eutectic compositions down to −70 °C were developed. Both

EMIHSO4-ImHSO4 and MIHSO4-ImHSO4 binary systems exhibited proton activity,

and the proton conductivity was higher for MIHSO4-ImHSO4 binary PILs due to the

presence of both proton-containing MI and Im cations.

(13) Polymer–PILs were developed by incorporating EMIHSO4-ImHSO4 and

MIHSO4-ImHSO4 eutectic binary systems into PEO. Their activities were

demonstrated in solid RuO2 pseudocapacitors. Polymer–binary PILs were promising

enabling electrolytes for solid psuedocapacitors with high energy density and high

rate performance than that of liquid devices.

8.3 Future Work

The following approaches are recommended to extend the work beyond the thesis:

Investigation of alternative polymer matrices: As shown in this study, the ion

conduction mechanism of salt-in-polymer electrolytes is controlled by the

characteristics of polymer matrix. Although it was shown that ILs can act as

plasticizers and reduce the crystallinity of polymers, further improvement in ion

transport process may be achieved with amorphous polymer network. Among several


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types of polymers, poly(acrylonitrile) (PAN) and poly(methyl methacrylate) (PMMA)

have been investigated for polymer–ILs. The former has a low degree of crystallinity

and the latter is an amorphous polymer. These polymers have polar groups (nitrogen

and oxygen atoms) which may enhance the ionic dissociation and particularly the

proton dissociation of the eutectic binary PILs.

Investigation of alternative anions for ILs: In this work, the effects of cationic

functional groups on the properties (melting point and proton conductivity) of the ILs

were investigated. Comparison of anions with similar structure but different

functional groups will lead to further understanding of the effect of functional groups

on the properties of the resulting IL systems, including the strength of ion/proton

dissociation, melting point, and viscosity. An example is sulfamate (derivative of

sulfamic acid) with a similar structure to hydrogen sulfate (HSO4), where an OH

group is replaced by NH2. Also, the additional protons and proton sites on the amine

group of sulfamate may be active in proton conduction process of PILs.

Investigation of the effect of filler size and its dispersion: Nano-sized inorganic fillers

were studied in this work. It would be interesting to investigate the effect of particle

size on the properties of the polymer–IL electrolytes, and to compare to the similar

polymer electrolytes reported in the literature. To analyze the dispersion of the fillers

in the polymer–IL electrolytes, techniques such as low-voltage scanning electron

microscopy (SEM) or transmission electron microscopy (TEM) can be used that

apply a low voltage beam, preventing the damage of polymer samples. Also,

cryo-SEM is another technique that can provide high resolution that is useful to

observe the dispersion of the fillers.


175

Improvement in understanding the ion transport properties: There is little

understanding of the degree of ionic dissociation of the ILs and in particular the

dominating conducting species in the polymer–ILs. This work has provided insights

into the ion transport mechanism and the role of the fillers on conduction process.

Further investigations on the transport properties of cations and anions as well as

mobile protons in these systems allow to develop guidelines for future improvements.

Electrochemical methods, i.e., impedance spectroscopy can be combined with

spectroscopic techniques, i.e., pulsed field gradient nuclear magnetic resonance

(NMR) to determine diffusion coefficients of cations and anions and their

transference number. The degree of ionic dissociation can be quantified as the ratio of

molar conductivities obtained from the two techniques.

Improvement in the fabrication/design: Minimizing the electrode-electrolyte contact

resistance will significantly enhance the overall performance of ECs. This could be

achieved by directly casting the precursor solution of polymer–ILs onto the electrode.

The material systems should be optimized to ease the processing. Alternative solvents

that are compatible with the electrolyte system, and have low toxicity and moderate

boiling point can be used.

176

LIST OF REFERENCES

[1] H. Ohno, Electrochemical aspects of ionic liquids, J. Wiley, Hoboken, N.J., 2005.

[2] W. Lu, L. Qu, K. Henry, L. Dai, High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes, J. Power Sources 189 (2009) 1270-1277.

[3] H. Cheng, C. Zhu, B. Huang, M. Lu, Y. Yang, Synthesis and electrochemical characterization of PEO-based polymer electrolytes with room temperature ionic liquids, Electrochim. Acta 52 (2007) 5789-5794.

[4] S. Kim, S.-J. Park, Preparation and electrochemical properties of composite polymer electrolytes containing 1-ethyl-3-methylimidazolium tetrafluoroborate salts, Electrochim. Acta 54 (2009) 3775-3780.

[5] S.K. Chaurasia, R.K. Singh, S. Chandra, Dielectric Relaxation and Conductivity Studies on (PEO:LiClO4) Polymer Electrolyte with Added Ionic Liquid [BMIM][PF6]: Evidence of Ion-Ion Interaction, Journal of Polymer Science Part B-Polymer Physics 49 (2011) 291-300.

[6] P.A.R.D. Jayathilaka, M.A.K.L. Dissanayake, I. Albinsson, B.-E. Mellander, Effect of nano-porous Al2O3 on thermal, dielectric and transport properties of the (PEO)9LiTFSI polymer electrolyte system, Electrochim. Acta 47 (2002) 3257-3268.

[7] Y.-X. Jiang, J.-M. Xu, Q.-C. Zhuang, L.-Y. Jin, S.-G. Sun, A novel PEO-based composite solid-state polymer electrolyte with methyl group-functionalized SBA-15 filler for rechargeable lithium batteries, Journal of Solid State Electrochemistry 12 (2008) 353-361.

[8] J.-H. Shin, S.-S. Jeong, K.-W. Kim, S. Passerini, FT-Raman spectroscopy study on the effect of ceramic fillers in P(EO)20LiBETI, Solid State Ionics 176 (2005) 571-577.

[9] B. Scrosati, F. Croce, L. Persi, Impedance spectroscopy study of PEO-based nanocomposite polymer electrolytes, J. Electrochem. Soc. 147 (2000) 1718-1721.

[10] P. Mustarelli, E. Quartarone, C. Tomasi, A. Magistris, New materials for polymer electrolytes, Solid State Ionics 135 (2000) 81-86.

[11] D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, et al., Ultrahigh-power micrometre-sized supercapacitors based on onion-like carbon, Nature Nanotechnology 5 (2010) 651-654.

[12] Z. Wu, X. Feng, H. Cheng, Recent advances in graphene-based planar micro-supercapacitors for on-chip energy storage, National Science Review (2013).

177

[13] X. Lu, M. Yu, G. Wang, Y. Tong, Y. Li, Flexible solid-state supercapacitors: design, fabrication and applications, Energy & Environmental Science 7 (2014) 2160-2181.

[14] K. Jost, D. Stenger, C.R. Perez, J.K. McDonough, K. Lian, Y. Gogotsi, et al., Knitted and screen printed carbon-fiber supercapacitors for applications in wearable electronics, Energy & Environmental Science 6 (2013) 2698-2705.

[15] B.E. Conway, Electrochemical supercapacitors : scientific fundamentals and technological applications, Kluwer Academic/Plenum Pub., New York, 1999.

[16] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nature Materials 7 (2008) 845-854.

[17] B.E. Conway, Transition from Supercapacitor to Battery Behavior in Electrochemical Energy-Storage, J. Electrochem. Soc. 138 (1991) 1539-1548.

[18] B.E. Conway, W.G. Pell, Double-layer and pseudocapacitance types of electrochemical capacitors and their applications to the development of hybrid devices, Journal of Solid State Electrochemistry 7 (2003) 637-644.

[19] J.R. Miller, R.A. Outlaw, B.C. Holloway, Graphene Double-Layer Capacitor with ac Line-Filtering Performance, Science 329 (2010) 1637-1639.

[20] J.R. Miller, P. Simon, Materials Science - Electrochemical capacitors for energy management, Science 321 (2008) 651-652.

[21] L. Bao, X. Li, Towards Textile Energy Storage from Cotton T-Shirts, Adv Mater 24 (2012) 3246-3252.

[22] F.M. Gray, Solid polymer electrolytes : fundamentals and technological applications, VCH, New York, NY, 1991.

[23] Y. Gogotsi, P. Simon, True Performance Metrics in Electrochemical Energy Storage, Science 334 (2011) 917-918.

[24] P.J. Hall, M. Mirzaeian, S.I. Fletcher, F.B. Sillars, A.J.R. Rennie, G.O. Shitta-Bey, et al., Energy storage in electrochemical capacitors: designing functional materials to improve performance, Energy & Environmental Science 3 (2010) 1238-1251.

[25] M. Galinski, A. Lewandowski, I. Stepniak, Ionic liquids as electrolytes, Electrochim. Acta 51 (2006) 5567-5580.

[26] S. Sowmiah, V. Srinivasadesikan, M.C. Tseng, Y.H. Chu, On the Chemical Stabilities of Ionic Liquids, Molecules 14 (2009) 3780-3813.

[27] General Chemical, Retrieved July 25, 2014, from http://www.generalchemical.com/.

178

[28] Spirig, Material Safety Data Sheet (MSDS), Retrieved July 25, 2014, from http://www.spirig.com/.

[29] M. Ue, K. Ida, S. Mori, Electrochemical properties of organic liquid electrolytes based on quaternary onium salts for electrical double-layer capacitors, J. Electrochem. Soc. 141 (1994) 2989-2996.

[30] P. Wasserscheid, T. Welton, Ionic liquids in synthesis, Wiley-VCH, Weinheim, 2003.

[31] P. Walden, Über die Molekulargrösse und elektrische Leitfähigkeit einiger gesehmolzenen Salze, Bull. Acad. Impér. Sci. (St. Petersburg) (1914) 405.

[32] M. Freemantle, An Introduction to ionic liquids, Royal Society of Chemistry (RSC), Cambridge, 2010.

[33] A. Fernicola, B. Scrosati, H. Ohno, Potentialities of ionic liquids as new electrolyte media in advanced electrochemical devices, Ionics 12 (2006) 95-102.

[34] M. Anouti, M. Caillon-Caravanier, Y. Dridi, H. Galiano, D. Lemordant, Synthesis and Characterization of New Pyrrolidinium Based Protic Ionic Liquids. Good and Superionic Liquids, The journal of physical chemistry.B 112 (2008) 13335-13343.

[35] A. Noda, A. Susan, K. Kudo, S. Mitsushima, K. Hayamizu, M. Watanabe, Bronsted acid-base ionic liquids as proton-conducting nonaqueous electrolytes, The journal of physical chemistry.B 107 (2003) 4024-4033.

[36] J. Sun, M. Forsyth, D. MacFarlane, Room-temperature molten salts based on the quaternary ammonium ion, J Phys Chem B 102 (1998) 8858-8864.

[37] T.D.J. Dunstan, J. Caja, Development of Low Melting Ionic Liquids using Eutectic Mixtures of Imidazolium and Pyrazolium Ionic Liquids, ECS Transactions 3 (2007) 21-32.

[38] M. Kunze, S. Jeong, G.B. Appetecchi, M. Schoenhoff, M. Winter, S. Passerini, Mixtures of ionic liquids for low temperature electrolytes, Electrochim. Acta 82 (2012) 69-74.

[39] M. Kunze, S. Jeong, E. Paillard, M. Winter, S. Passerini, Melting Behavior of Pyrrolidinium-Based Ionic Liquids and Their Binary Mixtures, Journal of Physical Chemistry C 114 (2010) 12364-12369.

[40] R. Lin, P. Taberna, S. Fantini, V. Presser, C.R. Perez, F. Malbosc, et al., Capacitive Energy Storage from -50 to 100 degrees C Using an Ionic Liquid Electrolyte, Journal of Physical Chemistry Letters 2 (2011) 2396-2401.

[41] N. Winterton, Solubilization of polymers by ionic liquids, Journal of Materials Chemistry 16 (2006) 4281-4293.

179

[42] M.A.B.H. Susan, T. Kaneko, A. Noda, M. Watanabe, Ion gels prepared by in situ radical polymerization of vinyl monomers in an ionic liquid and their characterization as polymer electrolytes, J. Am. Chem. Soc. 127 (2005) 4976-4983.

[43] S. Zhang, X. Lu, Q. Zhou, X. Li, X. Zhang, Ionic liquids: physicochemical properties, 1st ed., Elsevier, London, 2009.

[44] E. Frackowiak, Supercapacitors based on carbon materials and ionic liquids, Journal of the Brazilian Chemical Society 17 (2006) 1074-1082.

[45] D. Wei, T.W. Ng, Application of novel room temperature ionic liquids in flexible supercapacitors, Electrochemistry Communications 11 (2009) 1996-1999.

[46] C. Liu, Z. Yu, D. Neff, A. Zhamu, B.Z. Jang, Graphene-Based Supercapacitor with an Ultrahigh Energy Density, Nano Letters 10 (2010) 4863-4868.

[47] A. Lewandowski, A. Olejniczak, M. Galinski, I. Stepniak, Performance of carbon-carbon supercapacitors based on organic, aqueous and ionic liquid electrolytes, J. Power Sources 195 (2010) 5814-5819.

[48] C. Arbizzani, M. Biso, D. Cericola, M. Lazzari, F. Soavi, M. Mastragostino, Safe, high-energy supercapacitors based on solvent-free ionic liquid electrolytes, J. Power Sources 185 (2008) 1575-1579.

[49] R. Lin, P. Huang, J. Segalini, C. Largeot, P.L. Taberna, J. Chmiola, et al., Solvent effect on the ion adsorption from ionic liquid electrolyte into sub-nanometer carbon pores, Electrochim. Acta 54 (2009) 7025-7032.

[50] T. Tsuda, C.L. Hussey, Electrochemical applications of room-temperature ionic liquids, Electrochemical Society Interface 16 (2007) 42-49.

[51] L. Japan Radio Co., Retrieved July 25, 2014, from http://www.jrc.co.jp/jp/whatsnew/ 20061018/index.html.

[52] D. Rochefort, A.L. Pont, Pseudocapacitive behaviour of RuO2 in a proton exchange ionic liquid, Electrochemistry Communications 8 (2006) 1539-1543.

[53] L. Mayrand-Provencher, D. Rochefort, Influence of the conductivity and viscosity of protic ionic liquids electrolytes on the pseudocapacitance of RuO2 electrodes, Journal of Physical Chemistry C 113 (2009) 1632-1639.

[54] M. Lee, W. Tsai, M. Deng, H. Cheng, I. Sun, J. Chang, Pseudocapacitance of MnO2 originates from reversible insertion/desertion of thiocyanate anions studied using in situ X-ray absorption spectroscopy in ionic liquid electrolyte, J. Power Sources 195 (2010) 919-922.

[55] J. Chang, M. Lee, W. Tsai, M. Deng, H. Cheng, I. Sun, Pseudocapacitive Mechanism of Manganese Oxide in 1-Ethyl-3-methytimidazolium Thiocyanate Ionic Liquid Electrolyte Studied Using X-ray Photoelectron Spectroscopy, Langmuir 25 (2009) 11955-11960.

180

[56] C.A.C. Ruiz, D. Belanger, D. Rochefort, Electrochemical and Spectroelectrochemical Evidence of Redox Transitions Involving Protons in Thin MnO2 Electrodes in Protic Ionic Liquids, Journal of Physical Chemistry C 117 (2013) 20397-20405.

[57] R. Mysyk, E. Raymundo-Piñero, M. Anouti, D. Lemordant, F. Béguin, Pseudo-capacitance of nanoporous carbons in pyrrolidinium-based protic ionic liquids, Electrochemistry Communications 12 (2010) 414-417.

[58] D. Fenton, J. Parker, P. Wright, Complexes of Alkali-Metal Ions with Poly(ethylene Oxide), Polymer 14 (1973) 589-589.

[59] M.B. Armand, J.M. Chabagno, M. Duclot, Poly(ethylene oxide) is a medium capable of dissolving salt to form a conductive polymer, Second International Meeting on Solid Electrolytes, St Andrews, Scotland (1978).

[60] R.C. Agrawal, G.P. Pandey, Solid polymer electrolytes: Materials designing and all-solid-state battery applications: An overview, J. Phys. D 41 (2008).

[61] F.M. Gray, Polymer electrolytes, Royal Society of Chemistry, Cambridge, 1997.

[62] V. Di Noto, S. Lavina, G.A. Giffin, E. Negro, B. Scrosati, Polymer electrolytes: Present, past and future, Electrochim. Acta 57 (2011) 4-13.

[63] C.A. Angell, C. Liu, E. Sanchez, Rubbery Solid Electrolytes with Dominant Cationic Transport and High Ambient Conductivity, Nature 362 (1993) 137-139.

[64] C. Tiyapiboonchaiya, D. MacFarlane, J. Sun, M. Forsyth, Polymer-in-ionic-liquid electrolytes, Macromolecular Chemistry and Physics 203 (2002) 1906-1911.

[65] H. Ohno, M. Yoshizawa, W. Ogihara, Development of new class of ion conductive polymers based on ionic liquids, Electrochim. Acta 50 (2004) 255-261.

[66] S. Ahmad, Polymer electrolytes: Characteristics and peculiarities, Ionics 15 (2009) 309-321.

[67] H. Ohno, Electrochemical aspects of ionic liquids, J. Wiley, Hoboken, N.J., 2005.

[68] M. Ulaganathan, S. Rajendran, Effect of different salts on PVAc/PVdF-co-HFP based polymer blend electrolytes, J Appl Polym Sci 118 (2010) 646-651.

[69] C. Sequeira, D. Santos, Polymer Electrolytes: Fundamentals and Applications, Woodhead, Cambridge, 2010.

[70] M. Marzantowicz, J.R. Dygas, F. Krok, Z. Florjanczyk, E. Zygadlo-Monikowska, Influence of crystallization on dielectric properties of PEO:LiTFSI polymer electrolyte, J. Non Cryst. Solids 352 (2006) 5216-5223.

181

[71] J.R. MacCallum, C.A. Vincent, Polymer electrolyte reviews, Volume 2, Elsevier applied science, London and New York, 1989.

[72] F. Croce, L.L. Persi, B. Scrosati, F. Serraino-Fiory, E. Plichta, M.A. Hendrickson, Role of the ceramic fillers in enhancing the transport properties of composite polymer electrolytes, Electrochim. Acta 46 (2001) 2457-2461.

[73] H.(H.). Zhang, P. Maitra, S.L. Wunder, Preparation and characterization of composite electrolytes based on PEO(375)-grafted fumed silica, Solid State Ionics 178 (2008) 1975-1983.

[74] B. Singh, S.S. Sekhon, Polymer electrolytes based on room temperature ionic liquid: 2,3-dimethyl-l-octylimidazolium triflate, J Phys Chem B 109 (2005) 16539-16543.

[75] M.M.E. Jacob, E. Hackett, E.P. Giannelis, From nanocomposite to nanogel polymer electrolytes, Journal of Materials Chemistry 13 (2003) 1-5.

[76] D. Saikia, A. Kumar, Ionic conduction studies in P(VDF-HFP)-LiAsF6-(PC + DEC)-fumed SiO2 composite gel polymer electrolyte system, Phys. Status Solidi A-Appl. Res. 202 (2005) 309-315.

[77] S.A. Agnihotry, S. Ahmad, D. Gupta, S. Ahmad, Composite gel electrolytes based on poly(methylmethacrylate) and hydrophilic fumed silica, Electrochim. Acta 49 (2004) 2343-2349.

[78] S. Ramesh, T. Winie, A.K. Arof, Investigation of mechanical properties of polyvinyl chloride-polyethylene oxide (PVC-PEO) based polymer electrolytes for lithium polymer cells, European Polymer Journal 43 (2007) 1963-1968.

[79] G.B. Appetecchi, F. Croce, L. Persi, F. Ronci, B. Scrosati, Transport and interfacial properties of composite polymer electrolytes, Electrochim. Acta 45 (2000) 1481-1490.

[80] S.H. Chung, Y. Wang, L. Persi, F. Croce, S.G. Greenbaum, B. Scrosati, et al., Enhancement of ion transport in polymer electrolytes by addition of nanoscale inorganic oxides, J. Power Sources 97-98 (2001) 644-648.

[81] S.L. Agrawal, M. Singh, M. Tripathi, M.M. Dwivedi, K. Pandey, Dielectric relaxation studies on [PEO-SiO2]:NH4SCN nanocomposite polymer electrolyte films, J. Mater. Sci. 44 (2009) 6060-6068.

[82] G.P. Pandey, S.A. Hashmi, R.C. Agrawal, Hot-press synthesized polyethylene oxide based proton conducting nanocomposite polymer electrolyte dispersed with SiO2 nanoparticles, Solid State Ionics 179 (2008) 543-549.

[83] B.-K. Choi, Y.-W. Kim, K.-H. Shin, Effects of ceramic fillers on the electrical properties of (PEO)16LiClO4 electrolytes, J. Power Sources 68 (1997) 357-360.

182

[84] Y.W. Kim, W. Lee, B.K. Choi, Relation between glass transition and melting of PEO-salt complexes, Electrochim. Acta 45 (2000) 1473-1477.

[85] A. Lewandowski, A. Świderska, Electrochemical capacitors with polymer electrolytes based on ionic liquids, Solid State Ionics 161 (2003) 243-249.

[86] A. Lewandowski, A. Świderska, Solvent-free double-layer capacitors with polymer electrolytes based on 1-ethyl-3-methylimidazolium triflate ionic liquid, Applied Physics A: Materials Science and Processing 82 (2006) 579-584.

[87] Sellam, S.A. Hashmi, High Rate Performance of Flexible Pseudocapacitors fabricated using Ionic-Liquid-Based Proton Conducting Polymer Electrolyte with Poly(3,4-ethylenedioxythiophene):Poly(styrene sulfonate) and Its Hydrous Ruthenium Oxide Composite Electrodes, Acs Applied Materials & Interfaces 5 (2013) 3875-3883.

[88] A. Fernicola, S. Panero, B. Scrosati, M. Tamada, H. Ohno, New types of brönsted acid-base ionic liquids-based membranes for applications in PEMFCs, ChemPhysChem 8 (2007) 1103-1107.

[89] S. Lee, T. Yasuda, M. Watanabe, Fabrication of protic ionic liquid/sulfonated polyimide composite membranes for non-humidified fuel cells, J. Power Sources 195 (2010) 5909-5914.

[90] B. Lin, S. Cheng, L. Qiu, F. Yan, S. Shang, J. Lu, Protic ionic liquid-based hybrid proton-conducting membranes for anhydrous proton exchange membrane application, Chemistry of Materials 22 (2010) 1807-1813.

[91] G. Scharfenberger, W.H. Meyer, G. Wegner, M. Schuster, K.-D. Kreuer, J. Maier, Anhydrous polymeric proton conductors based on imidazole functionalized polysiloxane, Fuel Cells 6 (2006) 237-250.

[92] A. Lewandowski, M. Galinski, M. Krzyanowski, New polymer electrolyte poly(acrylonitrile)-sulpholane-(C2H5)4NBF4 for chemical capacitors, Solid State Ionics 158 (2003) 367-373.

[93] T. Osaka, X. Liu, M. Nojima, T. Momma, An electrochemical double layer capacitor using an activated carbon electrode with gel electrolyte binder, J. Electrochem. Soc. 146 (1999) 1724-1729.

[94] G.P. Pandey, Y. Kumar, S.A. Hashmi, Ionic liquid incorporated polymer electrolytes for supercapacitor application, Indian Journal of Chemistry - Section A Inorganic, Physical, Theoretical and Analytical Chemistry 49 (2010) 743-751.

[95] W. Lu, K. Henry, C. Turchi, J. Pellegrino, Incorporating ionic liquid electrolytes into polymer gels for solid-state ultracapacitors, J. Electrochem. Soc. 155 (2008) A361-A367.

[96] A. Lewandowski, A. Świderska, New composite solid electrolytes based on a polymer and ionic liquids, Solid State Ionics 169 (2004) 21-24.

183

[97] C.A. Angell, W. Xu, M. Yoshizawa, J.-P. Belieres, Ionic liquids: Inorganic vs. organic, protic vs. aprotic, and Coulomb control vs. van der Waals control, MOLTEN SALTS N 79 (2003) 1.

[98] R.P. Swatloski, J.D. Holbrey, R.D. Rogers, Ionic liquids are not always green: Hydrolysis of 1-butyl-3- methylimidazolium hexafluorophosphate, Green Chem. 5 (2003) 361-363.

[99] W. Ogihara, H. Kosukegawa, H. Ohno, Proton-conducting ionic liquids based upon multivalent anions and alkylimidazolium cations, Chemical Communications (2006) 3637-3639.

[100] T.E. Sutto, T.T. Duncan, T.C. Wong, K. McGrady, Ionic liquid batteries: Chemistry to replace alkaline/acid energy storage devices, Electrochim. Acta 56 (2011) 3375-3379.

[101] G.L. Henriksen, K. Amine, J. Liu, P.A. Nelson, Materials Cost Evaluation Report for High-Power Li-Ion HEV Batteries, Argonne National Laboratory (2002), Retrieved July 25, 2014, from http://www.ipd.anl.gov/anlpubs/2003/01/45346.pdf.

[102] L. Chen, M. Sharifzadeh, N. Mac Dowell, T. Welton, N. Shah, J.P. Hallett, Inexpensive ionic liquids: [HSO4]

−-based solvent production at bulk scale, Green Chem. 16 (2014) 3098-3106.

[103] E. Frackowiak, F. Béguin, Carbon materials for the electrochemical storage of energy in capacitors, Carbon 39 (2001) 937-950.

[104] A.J. Bard, L.R. Faulkner, Electrochemical methods : fundamentals and applications, 2nd ed., John Wiley, New York, 2001.

[105] P.L. Taberna, P. Simon, J.F. Fauvarque, Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors, J. Electrochem. Soc. 150 (2003) A292-A300.

[106] J.R. Miller, Pulse Power Performance of Electrochemical Capacitors: Technical Status of Present Commercial Devices, 8th International Seminar on Double Layer Capacitors and Similar Energy Storage Devices (1998).

[107] K. Cole, R. Cole, Dispersion and absorption in dielectrics I. Alternating current characteristics, J. Chem. Phys. 9 (1941) 341-351.

[108] F. Kremer, A. Schönhals, Broadband dielectric spectroscopy, Springer, Berlin, 2003.

[109] D. Fragiadakis, S. Dou, R.H. Colby, J. Runt, Molecular mobility, ion mobility, and mobile ion concentration in poly(ethylene oxide)-based polyurethane ionomers, Macromolecules 41 (2008) 5723-5728.

184

[110] U.H. Choi, M. Lee, S. Wang, W. Liu, K.I. Winey, H.W. Gibson, et al., Ionic conduction and dielectric response of poly(imidazolium acrylate) ionomers, Macromolecules 45 (2012) 3974-3985.

[111] D. Fragiadakis, S. Dou, R.H. Colby, J. Runt, Molecular mobility and Li+ conduction in polyester copolymer ionomers based on poly(ethylene oxide), J. Chem. Phys. 130 (2009) 064907.

[112] J.-L. Halary, F. Laupretre, l. Monnerie, Polymer Materials: Macroscopic Properties and Molecular Interpretations, J. Whiley, Hoboken, N.J., 2011.

[113] T. Lee, F. Boey, K. Khor, X-ray diffraction analysis technique for determining the polymer crystallinity in a polyphenylene sulfide composite, Polymer Composites 16 (1995) 481-488.

[114] D. Prusinowska, W. Wieczorek, H. Wyciślik, M. Siekierski, J. Przyluski, J. Soltysiak, Conductivity and structural studies of PEO-NH4SCN electrolytes, Solid State Ionics 72 (1994) 152-159.

[115] C. Zhao, G. Burrell, A. Torriero, F. Separovic, N. Dunlop, D. MacFarlane, et al., Electrochemistry of room temperature protic ionic liquids, The journal of physical chemistry.B 112 (2008) 6923-6936.

[116] Kynar by Arkema, Retrieved July 25, 2014, from http://americas.kynar.com/.

[117] S.P. Singh, K. Pal, A. Tarafder, M. Das, K. Annapurna, B. Karmakar, Effects of SiO2 and TiO2 fillers on thermal and dielectric properties of eco-friendly bismuth glass microcomposites of plasma display panels, Bull. Mater. Sci. 33 (2010) 33-41.

[118] K.M. Reddy, S.V. Manorama, A.R. Reddy, Bandgap studies on anatase titanium dioxide nanoparticles, Mater. Chem. Phys. 78 (2003) 239-245.

[119] S. Ketabi, K. Lian, Thermal and structural characterizations of PEO-EMIHSO4 polymer electrolytes, Solid State Ionics 227 (2012) 86-90.

[120] S. Ketabi, Z. Le, K. Lian, EMIHSO4-based polymer ionic liquid electrolyte for electrochemical capacitors, Electrochemical and Solid-State Letters 15 (2012) A19-A22.

[121] K. Lian, C.M. Li, Solid polymer electrochemical capacitors using heteropoly acid electrolytes, Electrochemistry Communications 11 (2009) 22-24.

[122] G. Bajwa, M. Genovese, K. Lian, Multilayer Polyoxometalates-Carbon Nanotube Composites for Electrochemical Capacitors, Ecs Journal of Solid State Science and Technology 2 (2013) M3046-M3050.

[123] A. Noda, M. Watanabe, Highly conductive polymer electrolytes prepared by in situ polymerization of vinyl monomers in room temperature molten salts, Electrochim. Acta 45 (2000) 1265-1270.

185

[124] A. Lewandowski, A. Świderska-Mocek, Ionic liquids as electrolytes for Li-ion batteries—An overview of electrochemical studies, J. Power Sources 194 (2009) 601-609.

[125] T.E. Sutto, Hydrophobic and hydrophilic interactions of ionic liquids and polymers in solid polymer gel electrolytes, J. Electrochem. Soc. 154 (2007) P101-P107.

[126] S.A. Hashmi, A. Kumar, K.K. Maurya, S. Chandra, Proton-conducting polymer electrolyte I. The polyethylene oxide NH4ClO4 system, J. Phys. D 23 (1990) 1307-1314.

[127] H.P. Srivastava, G. Arthanareeswaran, N. Anantharaman, V.M. Starov, Performance and properties of modified poly (vinylidene fluoride) membranes using general purpose polystyrene (GPPS) by DIPS method, Desalination 283 (2011) 169-177.

[128] K. Kim, J. Ko, N. Park, K. Ryu, S. Chang, Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolyte filled with rutile TiO2 nanoparticles, Solid State Ionics 161 (2003) 121-131.

[129] K. Kim, K. Ryu, S. Kang, S. Chang, I. Chung, The effect of silica addition on the properties of poly((vinylidene fluoride)-co-hexafluoropropylene)-based polymer electrolytes, Macromolecular Chemistry and Physics 202 (2001) 866-872.

[130] B. Wunderlich, Thermal analysis, Academic Press, Boston, 1990.

[131] S.K. Chaurasia, R.K. Singh, S. Chandra, Structural and transport studies on polymeric membranes of PEO containing ionic liquid, EMIM-TY: Evidence of complexation, Solid State Ionics 183 (2011) 32-39.

[132] M. Gargouri, T. Mhiri, A. Daoud, J.M. Réau, Disorder and protonic conductivity in RbH(SO4)0.81(SeO4)0.19 mixed crystals, Solid State Ionics 125 (1999) 193-202.

[133] M. Belhouchet, M. Bahri, J.M. Savariault, T. Mhiri, Structural and vibrational study of a new organic hydrogen sulfate, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy 61 (2005) 387-393.

[134] H. Zhang, J. Wang, Vibrational spectroscopic study of ionic association in poly(ethylene oxide)-NH4SCN polymer electrolytes, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy 71 (2009) 1927-1931.

[135] Y. Kumar, S.A. Hashmi, G.P. Pandey, Ionic liquid mediated magnesium ion conduction in poly(ethylene oxide) based polymer electrolyte, Electrochim. Acta 56 (2011) 3864-3873.

[136] J. Kiefer, J. Fries, A. Leipertz, Experimental vibrational study of imidazolium-based ionic Liquids: Raman and infrared spectra of 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl) imide and 1-ethyl-3-methylimidazolium ethylsulfate, Appl. Spectrosc. 61 (2007) 1306-1311.

186

[137] N.R. Dhumal, H.J. Kim, J. Kiefer, Electronic structure and normal vibrations of the 1-ethyl-3- methylimidazolium ethyl sulfate ion pair, Journal of Physical Chemistry A 115 (2011) 3551-3558.

[138] Q. Che, R. He, J. Yang, L. Feng, R.F. Savinell, Phosphoric acid doped high temperature proton exchange membranes based on sulfonated polyetheretherketone incorporated with ionic liquids, Electrochem. Commun. 12 (2010) 647-649.

[139] C. Schmidt, T. Gluck, G. Schmidt-Naake, Modification of Nafion Membranes by Impregnation with Ionic Liquids, Chem. Eng. Technol. 31 (2008) 13-22.

[140] H. Gao, K. Lian, High rate all-solid electrochemical capacitors using proton conducting polymer electrolytes, J. Power Sources 196 (2011) 8855-8857.

[141] B. Kumar, L.G. Scanlon, Polymer-ceramic composite electrolytes, J. Power Sources 52 (1994) 261-268.

[142] C. Fanggao, G.A. Saunders, E.F. Lambson, R.N. Hampton, G. Carini, A. Bartolotta, et al., Frequency dependence of the complex dielectric constant of poly(ethylene oxide) under hydrostatic pressure, Il Nuovo Cimento D 16 (1994) 855-864.

[143] M. Kumar, S.S. Sekhon, Role of plasticizer's dielectric constant on conductivity modification of PEO-NH4F polymer electrolytes, European Polymer Journal 38 (2002) 1297-1304.

[144] J. Shi, L. Wang, Y. Liang, S. Peng, F. Cheng, J. Chen, All-Solid-State Dye-Sensitized Solar Cells with Alkyloxy-Imidazolium Iodide Ionic Polymer/SiO2 Nanocomposite Electrolyte and Triphenylamine-Based Organic Dyes, J. Phys. Chem. C 114 (2010) 6814-6821.

[145] N.K. Karan, D.K. Pradhan, R. Thomas, B. Natesan, R.S. Katiyar, Solid polymer electrolytes based on polyethylene oxide and lithium trifluoro-methane sulfonate (PEO-LiCF3SO3): Ionic conductivity and dielectric relaxation, Solid State Ionics 179 (2008) 689-696.

[146] B. Natesan, N.K. Karan, M.B. Rivera, F.M. Aliev, R.S. Katiyar, Segmental relaxation and ion transport in polymer electrolyte films by dielectric spectroscopy, J. Non Cryst. Solids 352 (2006) 5205-5209.

[147] S. Emmert, M. Wolf, R. Gulich, S. Krohns, S. Kastner, P. Lunkenheimer, et al., Electrode polarization effects in broadband dielectric spectroscopy, European Physical Journal B 83 (2011) 157-165.

[148] H. Gao, K. Lian, A Comparative Study of Nano-SiO2 and Nano-TiO2 Fillers on Proton Conductivity and Dielectric Response of a Silicotungstic Acid–H3PO4–Poly(vinyl alcohol) Polymer Electrolyte, ACS Appl. Mater. Interfaces 6 (2014) 464-472.

187

[149] U.H. Choi, M. Lee, S. Wang, W. Liu, K.I. Winey, H.W. Gibson, et al., Ionic conduction and dielectric response of poly(imidazolium acrylate) ionomers, Macromolecules 45 (2012) 3974-3985.

[150] K. Kreuer, A. Fuchs, M. Ise, M. Spaeth, J. Maier, Imidazole and pyrazole-based proton conducting polymers and liquids, Electrochim. Acta 43 (1998) 1281-1288.

[151] M. Genovese, Y.W. Foong, K. Lian, Germanomolybdate (GeMo12O404-) Modified

Carbon Nanotube Composites for Electrochemical Capacitors, Electrochim. Acta 117 (2014) 153-158.

[152] L. Mayrand-Provencher, S. Lin, D. Lazzerini, D. Rochefort, Pyridinium-based protic ionic liquids as electrolytes for RuO2 electrochemical capacitors, J. Power Sources 195 (2010) 5114-5121.

[153] M. Yoshizawa, W. Xu, C.A. Angell, Ionic Liquids by Proton Transfer: Vapor Pressure, Conductivity, and the Relevance of ΔpKa from Aqueous Solutions, J. Am. Chem. Soc. 125 (2003) 15411-15419.

[154] J. Jiang, D. Gao, Z. Li, G. Su, Gel polymer electrolytes prepared by in situ polymerization of vinyl monomers in room-temperature ionic liquids, React Funct Polym 66 (2006) 1141-1148.

[155] A.-L. Pont, R. Marcilla, I. De Meatza, H. Grande, D. Mecerreyes, Pyrrolidinium-based polymeric ionic liquids as mechanically and electrochemically stable polymer electrolytes, J. Power Sources 188 (2009) 558-563.

[156] F. Yan, S. Yu, X. Zhang, L. Qiu, F. Chu, J. You, et al., Enhanced Proton Conduction in Polymer Electrolyte Membranes as Synthesized by Polymerization of Protic Ionic Liquid-Based Microemulsions, CHEMISTRY OF MATERIALS 21 (2009) 1480-1484.

[157] T.E. Sutto, The electrochemical behavior of trialkylimidazolium imide based ionic liquids and their polymer gel electrolytes, J. Electrochem. Soc. 154 (2007) P130-P135.

[158] Z. Li, H. Liu, Y. Liu, P. He, J. Li, A room-temperature ionic-liquid-templated proton-conducting gelatinous electrolyte, The journal of physical chemistry.B 108 (2004) 17512-17518.

[159] H. Ye, J. Huang, J.J. Xu, N.K.A.C. Kodiweera, J.R.P. Jayakody, S.G. Greenbaum, New membranes based on ionic liquids for PEM fuel cells at elevated temperatures, J. Power Sources 178 (2008) 651-660.

[160] M. Hanna, J. Lepretre, J. Sanchez, Electrolytes Based on Primary Ammonium Salts as Ionic Liquids for PEMFC-membranes, CHEMICAL AND BIOCHEMICAL ENGINEERING QUARTERLY 23 (2009) 107-119.

188

[161] M. Conte, Ionic Liquid-based Hybrid Power Supercapacitors, Publishable Final Activity Report (2010), Retrieved July 25, 2014, from http://www.transport-research.info/web/projects/project_details.cfm?id=28120.

189

APPENDIX A: PIL ELECTROLYTES AND POLYMER–IL SYSTEMS

Table A-1 PILs reported in the literature, and their conductivity, viscosity, and electrochemical window (using different electrodes) [34,43,115,152,153].

PIL σ

(mScm-1) η (cP) E (V)

pyrrolidinium formate [Pyr][HCOO]

32.9

2.5

2.1 (vs. Fc/Fc+) 2.3 (vs. Ag/AgCl) 1.5 (vs. Ag/AgCl)

GC

pyrrolidinium trifluoroacetate [Pyr][TFA]

16.4

21

2.1 (vs. Ag/AgCl) GC

pyrrolidinium nitrate [Pyr][NO3]

50.1

5.2

1.5 (vs. Ag/AgCl) GC

2-methylpyridinium trifluoromethanesulfonate (triflate) [2-MePy][Tf] (1:2)

11.2 16.9

3.5-2.5 GC-Pt

(vs. Ag wire )

2-methylpyridinium formate [2-MePy][HCOO]

11

2.5

-

4-methylpyridinium trifluoroacetate [4-MePy][TFA] (1:2)

8.3

15.5

3.1-2.5 GC-Pt

(vs. Ag wire ) 4-ethylpyridinium trifluoroacetate [4-EtPy][TFA] (1:2)

7.6

13.6

3.2-2.5 GC-Pt

(vs. Ag wire)


9.1

13

3.0-2.4 GC-Pt

(vs. Ag wire )


9.3

14.9

2.9-2.5 GC-Pt

(vs. Ag wire)

3-ethylpyridinium trifluoroacetate [EtPy][TFA] (1:2)

8.3

14.6

2.6-2.4 GC-Pt

(vs. Ag wire)

2-ethylpyridinium trifluoroacetate [EtPy][TFA] (1:2)

8.2

13.8

2.3-2.6 GC-Pt

(vs. Ag wire)

190

Table A-1 (Continued)

PIL σ

(mScm-1) η (cP) E (V)

Diethanolammonium formate [DEA][HCOO]

5.8

28

3.6-1.2 GC-Au

(vs. Ag wire)

pyrrolidinum acetate [Pyrr][AC]

5.9

30.2

1.5 GC

(vs. Ag/AgCl)

N-ethylimidazolium bis(trifluoromethyl sulfonimide) [EtIm][TFSI]

10

54

-

Diethanolammonium sulfamate [DEA][OSA]

14.2

720

4.2-2.0 GC-Au

(vs. Ag wire)

2-pentylpyridinium trifluoroacetate [PentylPy][TFA] (1:2)

3.4

19.9

2.2-2.3 GC-Pt

(vs. Ag wire)

Pyrrolidinium hydrogen sulfate [Pyrr][HSO4]

6.8

190.1

3.0 GC

(vs. Ag/AgCl) GC: glassy carbon; Pt: platinum; Au: gold electrodes

191

Table A-2 Polymer–IL systems developed by polymerization in ILs, and their ionic conductivity and potential window [65,123,154-156].

Monomer IL Solvent plasticizer initiator Technique σ

(mS/cm) E(V)

Methyl methacrylate (MMA)

[BMI][PF6]

[EMI][TFSI]

THF AIBN

BPO

free radical polymerization free radical polymerization

0.2

6.3

2

2-hydroxyethyl methacrylate (HEMA)

[EMI][BF4]

[BPyr][BF4]

BPO

BPO

free radical polymerization free radical polymerization

0.2

1

- -

Poly-cation type, diallyldimethylammonium (pyrrolidinium backbone)

[MBPyrr][TFSI] acetone

anion exchange reaction (casting)

0.1 7

Polyanion-type, (based on vinyl monomer acids)

[EtIm][VS] ethanol alkyl group, polyether

AIBN free radical polymerization

0.1

-

polymerizable surfactant, 1-(2-methylacryloyloxyundecyl)- 3-methyl imadizolium bromide (MAUM-Br)

[MIm][Tf] [EIm][Tf]

[DMIm][Tf]

styrene, acrylonitrile

microelmulsion, UV-light irradiation (PIL/MAUM-Br/monomer)

5.5 1.3 1.0

- - -

AIBN: azobisisobutyronitrile; BPO: benzoyl peroxide; THF: tetrahydrofuran

192

Table A-3 Polymer-IL systems developed by the incorporation of ILs into the matrix [85,96,138,139,157-160].

Polymer IL Solvent Plasticizer Technique σ (mS/cm)

E (V)

poly acrylonitrile (PAN)

[BMI][PF6] [BMI][PF6] [EMI][BF4] [EMI][BF4]

DMF DMF DMF DMF

TMS

TMS

casting casting casting casting

0.02 6.3 6.6 15.0

3 3 3 3

poly ethyleneoxide (PEO)

[BMI][PF6] [EMI][BF4] [EMI][TFSI] [EMI][TFSI]

AN AN AN AN

casting casting

0.3 0.6 3.3 2.2

3 3 3 3

poly vinylalcohol (PVA)

[EMI][BF4] [EMI][Tf]

Water Water

casting 1.6 3.4

3 3

PVdF-HFP [EMI][BF4] [EMI][Tf] [BMI][PF6] [TEA][BF4] [MMPI][TFSI] [MMBI][TFSI]

PC MP MP

EC+PC AcetoneAcetone

sandwiching sandwiching

3.2 2.2 0.6 5

6.0 2.7

poly methyl methacrylate (PMMA)

[EMI][Tf] Water 2.5 4

DMF: N,N-dimethyl formamide; MP: Methyl-2-pentanone

193

APPENDIX B: XRD, DSC, AND DIELECTRIC ANALYSES

The XRD results are summarized for the main crystalline peaks of the polymer–ILs. The

ratio of the intensity of the crystalline peaks with respect to that of the amorphous profile is

also listed.

Table B-1 Intensity of the crystalline peaks of all samples and the ratio of crystalline peaks with respect to the amorphous baseline. Sample 2θ (°) Intensity (counts) Ic/Ia PEO powder 19.34 2852 - 23.34 3215 - PEO film 19.28 2550 - 23.38 1444 - PEO–EMIHSO4 (1:1) 19.20 1018 4.52 23.32 949 4.22 PEO–EMIHSO4 (1:2) 19.20 1303 4.46 23.32 1043 3.57 PEO–EMIHSO4 (1:3) 19.72 676 1.61 23.58 724 1.72

PEO–EMIHSO4–3% SiO2 19.28 660 1.67 23.30 571 1.44 PEO–EMIHSO4–10% SiO2 18.98 441 1.56 22.98 400 1.42 PEO–EMIHSO4–3% TiO2 19.06 686 3.05 23.20 571 2.54 PEO–EMIHSO4–10% TiO2 18.72 361 0.92 22.72 303 0.77

PVdF-HFP powder 17.90 543 - 19.54 520 - PVdF film 20.02 441 - PVdF-HFP–EMIBF4 20.18 666 1.57 PVdF-HFP–EMIBF4–3% SiO2 20.32 520 1.30 PVdF-HFP–EMIBF4–3% TiO2 20.22 412 1.55

194

Figure B-1 Heating and cooling DSC thermograms for PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3% SiO2, and PVdF-HFP–EMIBF4–3% TiO2 electrolytes

Figure B-1 shows DSC thermograms of PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–3%

SiO2, and PVdF-HFP–EMIBF4–3% TiO2. As summarized in Table B-2, the addition of

3 wt% filler to PVdF-HFP–EMIBF4 resulted in somewhat similar behavior to that of PEO–

EMIHSO4: the crystallinity of PVdF-HFP–EMIBF4 slightly decreased with the addition of

SiO2, while the crystallinity was unaffected in PVdF-HFP–EMIBF4–3% TiO2.

Table B-2 Melting temperature (Tm), recrystallization temperature (Trc), and crystallinity (Xc) of PVdF-HFP film, PVdF-HFP–EMIBF4, PVdF-HFP–EMIBF4–SiO2, and PVdF-HFP–EMIBF4–3% TiO2 electrolytes.

Samples Tm (°C) Trc (°C) Xc (%) PVdF-HFP film 132 79 49 PVdF-HFP–EMIBF4 108 46 41 PVdF-HFP–EMIBF4–3% SiO2 107 48 38 PVdF-HFP–EMIBF4–3% TiO2 108 45 41

195

Figure B-2 Variation of (a) dielectric permittivity ′ and (b) dielectric loss ″ with respect to frequency for PEO–EMIHSO4, PEO–EMIHSO4–SiO2, and PEO–EMIHSO4–TiO2 at 30 °C

Figure B-2 show ′ and ″ as a function of frequency for PEO–EMIHSO4, PEO–EMIHSO4–

SiO2, and PEO–EMIHSO4–TiO2 at 30 °C. At low frequencies (0.1 to 1 kHz), ′ is high for all

three polymer electrolytes (Figure B-2a) due to the EP process. The loss spectra (Figure B-

2b) show a plateau for EP at lower frequencies (0.1 to 1 kHz) and a broad peak for relaxation

at high frequencies (1 kHz to 10 kHz).

196

Figure B-3 DSC thermograms of EMIHSO4-ImHSO4 binary mixtures at heating and cooling scans of 10 °C min-1

Figure B-3 shows DSC thermograms of the EMIHSO4:ImHSO4 mixtures with weight

percentage ratios at 50:50, 65:35, 70:30, and 80:20. The binary mixtures with 50 wt%

EMIHSO4 and 50 wt% ImHSO4 showed a melting peak at 0 °C (first minimum), which is

lower than that of EMIHSO4 and ImHSO4. At 70 wt% EMIHSO4 and 30 wt% ImHSO4, and

above this composition, the binary mixture reached eutectic compositions (second

minimum).

197

APPENDIX C: MATERIALS WEIGHT DISTRIBUTION

Figure C-1 Materials weight distribution for solid cells enabled by PEO–EMIHSO4 (left) and PVdF-HFP–EMIBF4 (right) (1 cm2 laminated pouch-type cells)

Figure C-1 shows the weight distribution for EDLCs fabricated with PEO–EMIHSO4 and PVdF-

HFP–EMIHBF4. In thin-film devices, the weight of electrode materials is very small compared to

that of polymer electrolyte and packaging. An example of EC cells based on liquid electrolyte (i.e.,

mixture of IL and solvent) for hybrid systems and application in electric vehicles has the weight

distribution shown in Figure C-2 [161]. Such devices for power applications require bulk active

materials.

Figure C-2 Weight distribution of EC modules with soft-pack assembly for hybrid electric vehicles

198

APPENDIX D: REPRODUCIBILITY OF CV MEASUREMENTS

The potential window of electrolytes was obtained in a three-electrode cell by sweeping the

potential towards positive and then negative at intervals. As shown in Figure D-1, the

potential window of EMIHSO4 was selected between −1.2 V and +1.2 V. Beyond these

potentials the reduction and oxidation peaks were observed.

Figure D-1 Cyclic voltammograms of pure EMIHSO4 at different potential intervals at 5 mV s-1

The CV and EIS measurements of EC cells were repeated for a minimum of five cells.

Figure D-2 shows an example of the CV profiles of six cells enabled with PEO–EMIHSO4–

SiO2. Cell 5 with a more rectangular CV was selected as the representative of these samples.

199

Figure D-2 Cyclic voltammograms of EDLCs enabled by PEO–EMIHSO4–SiO2 at 1 V s-1

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