Polymer Ionic liquid Electrolytes for Electrochemical Capacitors

Transcription

1 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

2 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 (EMIHSO 4 ) 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 EMIHSO 4 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 SiO 2 and TiO 2 nanofillers on ionic conductivity, crystallinity, and dielectric properties of PEO EMIHSO 4 were studied over a temperature range from 10 C and 80 C. Using an electrochemical capacitor model, impedance (complex capacitance) and dielectric ii

3 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 SiO 2 and TiO 2 promoted an amorphous structure in PEO EMIHSO 4 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 EMIHSO 4 and PEO binary PILs demonstrated a comparable energy density to that with polymer fluorinated ILs. iii

4 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. iv

5 To Mom, Dad, and my grandmothers v

6 CONTENTS ABSTRACT... ii ACKNOWLEDGEMENTS...iv CONTENTS...vi LIST OF TABLES...ix LIST OF FIGURES... xii NOMENCLATURE... xviii INTRODUCTION Objectives Thesis Overview... 4 BACKGROUND Fundamentals of Electrochemical Capacitors Types of electrochemical capacitors Advances in electrochemical capacitors Types of Liquid Electrolytes Ionic Liquids Classes of ionic liquids Properties of ionic liquids Ionic conductivity and conduction mechanism Application in ECs Polymer Electrolytes Classification of polymer electrolytes IL-based polymer electrolytes Polymer network Conduction mechanism in polymer electrolytes Effect of nanofillers on PEO-based electrolytes Polymer IL electrolytes for ECs Application in EDLCs Polymer PILs for pseudocapacitors Gap Analysis and Selection of Materials Characterization Techniques Electrochemical characterization Cyclic voltammetry (CV) Electrochemical impedance spectroscopy (EIS) vi

7 2.6.2 Structural characterization X-ray diffraction Differential scanning calorimetry Infrared (IR) spectroscopy EXPERIMENTAL METHOD AND CHARACTERIZATION Materials Ionic conductors Polymers Additives Polymer Electrolytes Fabrication Preparation of PEO EMIHSO Preparation of PVdF-HFP EMIBF Preparation of polymer IL with filler Device Fabrication Electrodes Liquid cells Solid cells Characterization Structural characterization Electrochemical characterization IONIC LIQUID ELECTROLYTES Effect of Anion Ionic conductivity Potential window Electrode capacitance and device performance Effect of Cation Ionic conductivity of IL solutions Device performance using IL solutions Summary POLYMER IONIC LIQUID ELECTROLYTES PEO EMIHSO 4 and PVdF-HFP EMIBF 4 Electrolytes Ionic conductivity Crystallinity and thermal characterizations XRD analyses DSC analyses Interaction Between Polymer and IL Effect of crystallinity Effect of interaction between PEO and HSO vii

8 5.3 Device Performance Summary POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Effect of Fillers on Ionic Conductivity Effect of Fillers on Crystallinity XRD analyses DSC analyses Effect of Fillers on Interaction Between PEO and EMIHSO Impedance and Dielectric Analyses Complex capacitance and dielectric analyses Capacitance and dielectric response of polymer electrolytes Effect of fillers Effect of Fillers on Device Performance Summary PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Proton Activity and Melting point Proton Conductivity of PIL Solutions Performance of RuO 2 with PIL electrolytes Performance of carbon/pom in PIL electrolytes Electrode performance in PIL electrolytes Device performance in PIL electrolytes Binary Mixtures of PILs MIHSO 4 -ImHSO 4 binary system EMIHSO 4 -ImHSO 4 binary system Performance of RuO 2 in eutectic PILs Performance of Solid RuO 2 Cell with Polymer eutectic PILs Summary CONCLUSIONS AND FUTURE WORK Contributions Conclusions Future Work LIST OF REFERENCES APPENDIX A: PIL ELECTROLYTES AND POLYMER IL SYSTEMS APPENDIX B: XRD, DSC, AND DIELECTRIC ANALYSES APPENDIX C: MATERIALS WEIGHT DISTRIBUTION APPENDIX D: REPRODUCIBILITY OF CV MEASUREMENTS viii

9 LIST OF TABLES Table 2-1 Comparison between the characteristic properties of liquid and polymer electrolytes and the enabled devices Table 2-2 Comparison of properties of aqueous, organic, and ionic liquid electrolytes for ECs Table 2-3 Some basic properties of ionic liquids Table 2-4 Some common polymer hosts with their corresponding chemical formula and thermal properties [65,66] Table 2-5 Polymer electrolytes used for activated carbon EDLCs and their electrochemical properties Table 2-6 Cost of the materials for cells fabricated with the polymer IL electrolytes (1 cm 2 laminated pouch-type cells) Table 3-1 Structure of the studied ILs Table 3-2 Properties of the polymer matrices Table 3-3 Properties of the SiO 2 and TiO 2 fillers [117,118] Table 3-4 Material components of the polymer electrolytes Table 3-5 Parameters of interest and the relationship between capacitor performance properties and the related electrolyte/polymer electrolyte properties Table 4-1 Conductivity, potential window, and viscosity of studied electrolytes (at room temperature) Table 4-2 Structure and melting temperature of the ILs with different cations Table 5-1 Conductivity and activation energy of ionic conduction for ILs and polymer ILs (viscosity of pure ILs is also listed) Table 5-2 Melting and recrystallization temperatures, and degree of crystallinity of PEO powder, PEO film, and the polymer electrolytes Table 5-3 Melting and recrystallization temperatures, and degree of crystallinity of PVdF-HFP film and PVdF-HFP EMIBF Table 5-4 FTIR band positions and associated bonding modes for PEO EMIHSO 4 in (1:2) composition and its components ix

10 Table 6-1 Room temperature ionic conductivity of PEO EMIHSO 4 and PEO EMIHSO 4 electrolytes containing SiO 2 and TiO 2 nanofillers, and activation energy (E a ) of ionic conduction for the respective electrolytes at low and high temperatures Table 6-2 Room temperature ionic conductivity of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, PVdF-HFP EMIBF 4 3% TiO 2, and the activation energy (E a ) of ionic conduction for the respective electrolytes Table 6-3 Melting temperature (T m ), recrystallization temperature (T rc ), and crystallinity (X c ) of PEO film, PEO EMIHSO 4, and PEO EMIHSO 4 filler electrolytes Table 7-1 Structure and melting temperature of PILs with different cations Table 7-2 ESR and capacitance of RuO 2 cells using PIL electrolytes and H 2 SO Table 7-3 Proton concentration of PIL/MeOH electrolytes obtained from titration with 0.1 M NaOH Table 7-4 Capacitance of carbon/pmo 12 cells in aqueous and the corresponding cells in PIL/solvent electrolytes at 100 mv s Table 7-5 Thermal properties of MIHSO 4 -ImHSO 4 binary system at different compositions Table 7-6 Thermal properties of EMIHSO 4 -ImHSO 4 binary system at different compositions Table 7-7 Conductivity of pure EMIHSO 4, eutectic EMIHSO 4 -ImHSO 4, and eutectic MIHSO 4 -ImHSO 4 and the capacitance of RuO 2 cells enabled with respective PILs Table 7-8 Capacitance of RuO 2 cells enabled with PVdF-HFP EMIBF 4, PEO EMIHSO 4 -ImHSO 4 (eutectic 70:30), and PEO MIHSO 4 -ImHSO 4 (eutectic 70:30) Table A-1 PILs reported in the literature, and their conductivity, viscosity, and electrochemical window (using different electrodes) [34,43,115,152,153] Table A-2 Polymer IL systems developed by polymerization in ILs, and their ionic conductivity and potential window [65,123, ] Table A-3 Polymer-IL systems developed by the incorporation of ILs into the matrix [85,96,138,139, ] Table B-1 Intensity of the crystalline peaks of all samples and the ratio of crystalline peaks with respect to the amorphous baseline x

11 Table B-2 Melting temperature (T m ), recrystallization temperature (T rc ), and crystallinity (X c ) of PVdF-HFP film, PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 electrolytes xi

12 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 Figure 2-2 Generic structures of common cations and anions for ionic liquids Figure 2-3 Synthesis of [EMI][TFSI] Figure 2-4 Synthesis of [α-pic][tfa] Figure 2-5 Variation in melting point with alkyl chain length for ionic liquids containing 1-alkyl-3-methylimidazolium cations and different anions [30] Figure 2-6 Oxidation and reduction of [Pyr][HSO 4 ] (structure 11) Figure 2-7 Classification Walden plot constructed from the literature data of some ILs Figure 2-8 Schematic representation of proton transfer via (a) Grotthuss mechanism of dissociated imidazole or H 3 PO 4 and (b) vehicular mechanism of imidazolium [1] 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] Figure 2-10 (a) Cyclic voltammetry sweep, cyclic voltammogram profiles for (b) ideal and resistive double-layer capacitance, and (c) pseudocapacitance Figure 2-11 Phasor diagram showing the relationship between alternating current and voltage signals at angular frequency ω [104] 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 Figure 2-13 Schematic of the real (solid line) and imaginary (dash dotted line) parts of the capacitance 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] xii

13 Figure 2-15 Schematic illustration of XRD pattern of a semi-crystalline polymer displaying the broad amorphous peaks and the crystalline diffraction peaks Figure 2-16 Schematic illustration of heating and cooling DSC thermograms including the thermal transitions, heat of crystallization, ΔH c, and heat of melting, ΔH m Figure 2-17 IR spectra (transmittance) of polyethylene displaying the main CH 2 vibrations Figure 3-1 Preparation of imidazolium hydrogen sulfate (ImHSO 4 ) ionic liquid Figure 3-2 Preparation steps of polymer IL electrolytes Figure 3-3 Filler-free PEO EMIHSO 4, and PEO EMIHSO 4 containing SiO 2 and TiO 2 nanofillers Figure 3-4 Schematic representation of device configuration for the (a) Liquid 1, and (b) Liquid 2 cells Figure 3-5 (a) Schematic representation of device configuration for the solid cells; (b) the resulting laminated cells Figure 4-1 Structure of EMIHSO 4 and EMIBF Figure 4-2 Conductivity as a function of EMIHSO 4 concentration in PC 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 EMIHSO 4, measurements were performed at a low sweep rate: 5 mv s -1 ) Figure 4-4 Cyclic voltammograms of graphite cells tested with EMIHSO 4 and EMIBF 4 at a sweep rate of 100 mv s -1 (Liquid 1 beaker cells) Figure 4-5 Real C and imaginary C part of the capacitance vs. frequency for graphite EDLCs with EMIHSO 4 and EMIBF 4 (Liquid 2 filter paper cells) Figure 4-6 Double-layer capacitance of glassy carbon electrode and conductivity as a function of EMIHSO 4 concentration in PC Figure 4-7 Conductivity of solutions of EMIHSO 4 ( ), MIHSO 4 ( ), and ImHSO 4 ( ) in methanol (filled symbols) and acetic acid (empty symbols) Figure 4-8 Cyclic voltammograms of graphite cells using EMIHSO 4 /PC, EMIHSO 4 /MeOH, MIHSO 4 /MeOH, ImHSO 4 /MeOH electrolytes at (a) 100 mv s -1 and (b) 1 V s Figure 5-1 Temperature dependence of the ionic conductivity of EMIHSO 4 and PEO EMIHSO 4 in (1:2), (1:3), and (1:4) compositions xiii

14 Figure 5-2 Temperature dependence of the ionic conductivity of PEO EMIHSO 4 and PVdF-HFP EMIBF Figure 5-3 XRD patterns of (a) PEO powder, PEO film, and PEO EMIHSO 4 (1:2) electrolyte; (b) PEO EMIHSO 4 electrolytes in (1:1), (1:2), and (1:3) compositions Figure 5-4 XRD patterns of PVdF-HFP powder, PVdF-HFP film, and PVdF-HFP EMIBF 4 electrolyte Figure 5-5 Heating and cooling DSC thermograms of PEO film, PEO EMIHSO 4 electrolytes in (1:2) and (1:3) compositions Figure 5-6 Heating and cooling DSC thermograms for PVdF-HFP film and PVdF-HFP EMIBF 4 electrolyte Figure 5-7 FTIR spectra of pure PEO film, pure EMIHSO 4, and PEO EMIHSO 4 in (1:2) composition Figure 5-8 FTIR spectra of PEO EMIHSO 4 electrolytes in (1:1), (1:2), and (1:3) compositions in the range of cm Figure 5-9 Cyclic voltammograms of graphite ECs with EMIHSO 4 and PEO EMIHSO 4 electrolytes at sweep rates of (a) 100 mv s -1 and (b) 1 V s Figure 5-10 (a) Real part C and (b) imaginary part C of the capacitance and vs. frequency for graphite ECs with EMIHSO 4 and PEO EMIHSO 4 electrolytes Figure 5-11 Cyclic voltammograms of graphite EDLCs with PVdF-HFP EMIBF 4 and PEO EMIHSO 4 electrolytes at sweep rate of 1 V s Figure 6-1 Temperature dependence of the ionic conductivity of (a) PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 ; and (b) PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO Figure 6-2 Temperature dependence of the ionic conductivity of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO Figure 6-3 Temperature dependence of the ionic conductivity of PEO EMIHSO 4, PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO Figure 6-4 The variation of ionic conductivity of PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO 2 over time Figure 6-5 XRD patterns for (a) SiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 ; and (b) TiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO Figure 6-6 XRD patterns of SiO 2 and TiO 2 nanofillers, PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 electrolytes xiv

15 Figure 6-7 XRD patterns of PEO film, PEO EMIHSO 4, PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO Figure 6-8 Heating and cooling DSC thermograms of (a) PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 ; and (b) PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO Figure 6-9 DSC heating and cooling thermograms of PEO film, PEO EMIHSO 4, and PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO Figure 6-10 FTIR spectra of SiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO Figure 6-11 FTIR spectra of PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO Figure 6-12 Variation of (a) real part C and (b) imaginary part C of capacitance with respect to frequency for cells leveraging PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 at 30 C Figure 6-13 Dielectric derivative vs. frequency for PEO film, PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 at 30 C Figure 6-14 Dielectric derivative vs. frequency for (a) PEO EMIHSO 4, (b) PEO EMIHSO 4 SiO 2, and (c) PEO EMIHSO 4 TiO 2 at different temperatures: 10 C ( ), 0 C ( ), 10 C ( ), 20 C ( ), 30 C ( ), 40 C ( ), 50 C ( ), 60 C ( ), 70 C ( ), 80 C ( ) Figure 6-15 Capacitance of cells leveraging PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2, respectively, at different temperatures (at 0.1 Hz) Figure 6-16 Electrode polarization time constant EP (empty symbols) and relaxation time constant R (filled symbols) for PEO EMIHSO 4 ( ), PEO EMIHSO 4 SiO 2 (), and PEO EMIHSO 4 TiO 2 ( ) Figure 6-17 (a) Cyclic voltammograms of graphite EDLC devices with PEO EMIHSO 4, PEO EMIHSO 4 SiO 2 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 Figure 6-18 (a) Cyclic voltammograms of graphite EDLC devices with PEO EMIHSO 4 and PEO EMIHSO 4 TiO 2 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 Figure 6-19 Cycle life test of graphite EDLC device with PEO EMIHSO 4 10% SiO 2 electrolyte at 1 V s Figure 6-20 Comparison between the ionic conductivity as a function of temperature of the starting PEO EMIHSO 4, the optimized PEO EMIHSO 4 SiO 2 and PEO EMIHSO 4 TiO 2, and PVdF-HFP EMIBF xv

16 Figure 7-1 Cyclic voltammograms of RuO 2 cells in ImHSO 4 /MeOH, MIHSO 4 /MeOH, EMIHSO 4 /MeOH, and EMIHSO 4 /PC electrolytes at (a) 100 mv s -1 and (b) 1 V s -1 (Liquid 2 configuration) Figure 7-2 Cyclic voltammograms of bare carbon (dashed line) and carbon/pmo 12 (solid line) electrodes in 0.5M H 2 SO 4 at 100 mvs Figure 7-3 Cyclic voltammograms of bare carbon and carbon/pmo 12 electrodes in (a) EMIHSO 4 /MeOH, (b) MIHSO 4 /MeOH, (c) ImHSO 4 /MeOH electrolytes, and (d) comparison of cyclic voltammograms of carbon/pmo 12 electrodes in the three PIL electrolytes (sweep rate: 100 mv s -1 ) Figure 7-4 Cyclic voltammograms of (a) bare carbon cells and (b) carbon/pmo 12 cells in EMIHSO 4 /MeOH, MIHSO 4 /MeOH, ImHSO 4 /MeOH, and EMIHSO 4 /PC electrolytes at 1 V s -1 (Liquid 2 configuration) Figure 7-5 DSC thermograms of pure EMIHSO 4, MIHSO 4, and ImHSO 4 at heating and cooling scans of 10 C min Figure 7-6 DSC thermograms of various compositions of MIHSO 4 -ImHSO 4 binary mixtures at heating and cooling scans of 10 C min Figure 7-7 Phase diagram for MIHSO 4 -ImHSO 4 binary system: ( ) melting point; ( ) solid-solid transition; ( ) glass transition Figure 7-8 Phase diagram for EMIHSO 4 -ImHSO 4 binary system: ( ) melting point; ( ) solid-solid transition; ( ) glass transition Figure 7-9 Cyclic voltammograms of RuO 2 cells using pure EMIHSO 4, eutectic EMIHSO 4 -ImHSO 4 (70:30), and eutectic MIHSO 4 -ImHSO 4 (70:30) at 5 mv s Figure 7-10 Cyclic voltammograms of solid RuO 2 cells enabled with PVdF-HFP EMIBF 4, PEO EMIHSO 4 -ImHSO 4 (eutectic 70:30), and PEO MIHSO 4 -ImHSO 4 (eutectic 70:30) at (a) 5 mv s -1 and (b) 50 mv s Figure 7-11 Comparison of the specific energy and power density (per cm 3 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) Figure B-1 Heating and cooling DSC thermograms for PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 electrolytes Figure B-2 Variation of (a) dielectric permittivity and (b) dielectric loss with respect to frequency for PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 at 30 C Figure B-3 DSC thermograms of EMIHSO 4 -ImHSO 4 binary mixtures at heating and cooling scans of 10 C min xvi

17 Figure C-1 Materials weight distribution for solid cells enabled by PEO EMIHSO 4 (left) and PVdF-HFP EMIBF 4 (right) (1 cm 2 laminated pouch-type cells) Figure C-2 Weight distribution of EC modules with soft-pack assembly for hybrid electric vehicles Figure D-1 Cyclic voltammograms of pure EMIHSO 4 at different potential intervals at 5 mv s Figure D-2 Cyclic voltammograms of EDLCs enabled by PEO EMIHSO 4 SiO 2 at 1 V s xvii

18 NOMENCLATURE Acronyms AN AIBN BPO DMF EC EP ESR MP NMP PC PAN PEO PMMA PVA PVdF PVdF-HFP PVP THF TMS Acetonitrile Azobisisobutyronitrile Benzoyl peroxide N,N-dimethyl formamide Ethylene carbonate Electrode polarization Equivalent series resistance Methyl-2-pentanone N-methyl-2-pyrrolidone Propylene carbonate Poly(acrylonitrile) Poly(ethylene oxide) Poly(methyl methacrylate) Poly(vinyl alcohol) Poly(vinylidene fluoride) Poly(vinylidene fluoride-co-hexafluoropropylene) Poly(vinyl pyrrolidine) Tetrahydrofuran Sulpholane English symbols C capacitance (F) C real part of capacitance (F) C imaginary part of capacitance (F) xviii

19 E energy density (Wh kg -1 ) E a activation energy (kj mol -1 ) E c f 0 Coloumbic attraction (J) characteristic frequency at 45 phase angle (Hz) P power density (kw kg -1 ) T b T c T g T m T rc boiling temperature ( C) crystallization temperature ( C) glass transition temperature ( C) melting temperature ( C) recrystallization temperature ( C) X c degree of crystallinity (%) Z impedance (Ω) Z real part of impedance (Ω) Z imaginary part of impedance (Ω) Greek symbols Λ molar conductivity (S cm 2 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) τ 0 τ EP τ R RC time constant (s) time constant of electrode polarization (s) relaxation time constant (s) ΔH c heat of crystallization (J g -1 ) xix

20 ΔH m heat of melting (J g -1 ) ΔH rc heat of recrystallization (J g -1 ) xx

21 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 1

22 CHAPTER 1. INTRODUCTION 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 (EMIHSO 4 ) were developed, and their 2

23 CHAPTER 1. INTRODUCTION 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 SiO 2, TiO 2, or Al 2 O 3 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 SiO 2 and TiO 2 nanofillers on the electrochemical performance and structural stability of a PEO EMIHSO 4 electrolyte was studied. Through complex capacitance and dielectric analyses, the ion transport mechanism in the filler-containing PEO EMIHSO 4 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 HSO 4 -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. 3

24 CHAPTER 1. INTRODUCTION 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 4

25 CHAPTER 1. INTRODUCTION 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 EMIHSO 4, a fluorinated IL, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4 ), 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). 5

26 CHAPTER 1. INTRODUCTION Aprotic IL EMIBF 4 EMIHSO 4 EMIHSO 4 (no protic functionality) ionic liquid (IL) (ionic conductor) protic functional group MIHSO 4 (one protic functionality) Protic IL ImHSO 4 (two protic functionality) eutectic binary systems EMIHSO 4 -ImHSO 4 MIHSO 4 -ImHSO 4 propylene carbonate plasticizer Polymer aprotic IL inorganic fillers mixed solvent amorphous SiO 2 (low dielectric constant) PEO EMIHSO 4 polymer electrolyte crystalline TiO 2 (high dielectric constant) Polymer protic IL protic ILs EMIHSO 4 -ImHSO 4 (eutectic) MIHSO 4 -ImHSO 4 (eutectic) 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

27 CHAPTER 1. INTRODUCTION ECs were fabricated with the optimized polymer electrolytes using double-layer and pseudocapacitive electrode materials. This is schematically illustrated in Figure 1-2. Polymer electrolytes ionic conductor polymer matrix additives Symmetric solid electrochemical capacitors Electrode materials double-layer capacitive pseudocapacitive 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. 7

28 CHAPTER 1. INTRODUCTION 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. 8

29 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 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]. 9

30 CHAPTER 2. BACKGROUND 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: C A r 0, (2-1) where ε r is the electrolyte dielectric constant, ε 0 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 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 (RuO 2 ) and manganese oxide (MnO 2 )) and electrically conducting polymers (e.g., polyaniline, polypyrrole, and polythiophene) are examples of these electroactive materials. Capacitance of pseudocapacitors is typically times larger than that of EDLCs (per real surface area). RuO 2 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 RuO 2 leads to a capacitive behaviour [17] according to Equation 2-2: 2 2 x x d RuO 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. 10

31 CHAPTER 2. BACKGROUND 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 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 1 V 2 4 ESR, (2-3) 2 E = CV and P = 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 11

32 CHAPTER 2. BACKGROUND V 2. 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 12

33 CHAPTER 2. BACKGROUND 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 Design /Package Ionic conductor with a porous separator (dead mass) Protective case and packaging Ionic conductor and separator (active mass) 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 13

34 CHAPTER 2. BACKGROUND 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. 14

35 CHAPTER 2. BACKGROUND Table 2-2 Comparison of properties of aqueous, organic, and ionic liquid electrolytes for ECs. Property Aqueous Organic Ionic liquid a Ref. Conductivity High Low Moderate H 2 SO 4 (30 wt%) (730 ms cm -1 ) KOH (30 wt%) (540 ms cm -1 ) TEABF 4 b (0.65 M in PC) (10.6 ms cm -1 ) TEABF 4 (0.65 M in AN) (49.6 ms cm -1 ) ~10 ms cm -1 [EMI][TFSI] (8.4 ms cm -1 ) [EMI][BF 4 ] (14 ms cm -1 ) [2,25] Operating voltage 1 V V V [2,25] Thermal stability Low Volatile (132 C) c High ( C) d [26] Viscosity Low Low High e H 2 SO 4 (1.5-2 cp) KOH (3.7 cp) 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 TEABF 4 (0.65 M in PC) (2.5 cp) TEABF 4 (0.65 M in AN) (0.3 cp) [EMI][TFSI] (28 cp) [EMI][BF 4 ] (40 cp) [2,25,27-29] 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 15

36 CHAPTER 2. BACKGROUND requirements for specific applications. The generic structures of some common cations and anions are shown in Figure 2-2. Cations Anions [BF 4 ] [HSO 4 ] [TFSI] [NO 3 ] [Tf] [PF 6 ] 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]. 16

37 CHAPTER 2. BACKGROUND 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 Relatively high ion density (10 ms cm -1 ) Wide operating potential window High electrochemical stability Tunable/Designable Various kinds of salts 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 PF 6, BF 4, Tf, and TFSI ions (Figure 2-2). 17

38 CHAPTER 2. BACKGROUND 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. 18

39 CHAPTER 2. BACKGROUND 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 (T g ) or melting (T m ), is governed by the structure of the cations and anions and the Coulombic attraction (E c ) between the ions: Ec M 4 0 r ZZ, (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][BF 4 ] (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 19

40 CHAPTER 2. BACKGROUND 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 20

41 CHAPTER 2. BACKGROUND 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 21

42 CHAPTER 2. BACKGROUND example, the room temperature viscosities of [HMI][BF 4 ] (structure 9) and [HMI][NO 3 ] (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][HSO 4 ] 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][HSO 4 ] (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). 22

43 CHAPTER 2. BACKGROUND 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 T g to remain rubbery at room temperature [42]. ILs with low T m and T g 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 Ionic conductivity and conduction mechanism The room temperature conductivity of RTILs, within a broad range of 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: E Aexp a RT, (2-6) where A is a pre-exponential factor, R is the gas constant, and E a is the activation energy of ionic conduction. In some cases, as temperature approaches the glass transition temperatures (T g ) of the ILs (i.e., below room temperatures), the conductivity deviates from linear 23

44 CHAPTER 2. BACKGROUND behavior. Their conductivity trend is better described by the empirical Vogel Tammann Fulcher (VTF) relation [30]: 1 E 2 a exp, (2-7) R(T T 0 ) where T 0 is the temperature at which the conductivity reaches zero. T 0 is ascribed to T g 50 K. A lower E a 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][BF 4 ]). 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][HSO 4 ]). 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. 24

45 CHAPTER 2. BACKGROUND 3 log Λ [S cm 2 /mol] 2 1 Super ionic liquids 0 low vapor pressure -1 Poor ionic liquids -2-3 high vapor pressures -4 Non ionic liquids 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 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 H 3 PO 4 ). 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). 25

46 CHAPTER 2. BACKGROUND (a) (b) Figure 2-8 Schematic representation of proton transfer via (a) Grotthuss mechanism of dissociated imidazole or H 3 PO 4 and (b) vehicular mechanism of imidazolium [1] 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][BF 4 ], [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 26

47 CHAPTER 2. BACKGROUND 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 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 (TEABF 4 ) 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 RuO 2 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 MnO 2 (a cheaper alternative to RuO 2 ) in an aprotic IL, 1-ethyl-3-methylimidazolium thiocynate [EMI][SCN] (structure 12), and suggested that MnO 2 is not compatible with PILs [54,55]. Recently, Ruiz et al. suggested that charge storage of MnO 2 occurs in a PIL composed of 27

48 CHAPTER 2. BACKGROUND 2-methoxypyridinium trifluoroacetate (TFA) (structure 13). However, the operating potential of MnO 2 in this PIL was limited to more positive potentials, so an asymmetric EC was suggested leveraging this PIL with MnO 2 as positive and carbon as negative electrodes, respectively [56]. Also, carbon electrodes containing surface functionalities, with pyrrolidinium nitrate [Pyr][NO 3 ] (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 28

49 CHAPTER 2. BACKGROUND Good film forming properties for easy processing 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 29

50 CHAPTER 2. BACKGROUND 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 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 30

51 CHAPTER 2. BACKGROUND 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 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 31

52 CHAPTER 2. BACKGROUND 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 Melting point Mw. T g ( C) T m ( C) Poly(ethylene oxide) PEO (CH 2 CH 2 O) n ,000 Poly(vinylidene fluoride) (CH 2 CF 2 ) n ,000 PVdF b Poly(vinylidene fluorideco-hexafluoropropylene) PVdF-HFP b (CH 2 CF 2 ) x (CF 2 CF( CF 3 )) y ,000 Poly(acrylonitrile) (CH 2 CH( CN)) n ,000 PAN Poly(methyl methacrylate) (CH 2 C( CH 3 )( COOCH 3 )) n a 996,000 PMMA 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 fluorideco-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 32

53 CHAPTER 2. BACKGROUND 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 (T b = 242 C) [66]. Another promising approach is to disperse inorganic nanofillers (e.g., SiO 2, TiO 2, and Al 2 O 3 ) 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 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 33

54 CHAPTER 2. BACKGROUND 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 LiClO 4 electrolyte [69]. Nevertheless, more detailed studies are needed on chemical and electrochemical stabilities of these polymer electrolyte systems 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. 34

55 CHAPTER 2. BACKGROUND 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 35

56 CHAPTER 2. BACKGROUND 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 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 SiO 2, TiO 2, or Al 2 O 3 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], 36

57 CHAPTER 2. BACKGROUND (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 SiO 2, TiO 2, and Al 2 O 3 in PEO LiClO 4 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 NH 4 SCN and PEO NH 4 HSO 4 decreased with the addition of nano-sized SiO 2 [81,82]. In contrast, Zhang et al. reported that nano-sized fumed SiO 2 increased the glass transition temperature, but had no effect on either the crystallinity or the conductivity of PEO LiClO 4 [73]. In another study of PEO LiClO 4 with various fillers including micro-sized fumed SiO 2 and TiO 2, 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 Al 2 O 3 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. 37

58 CHAPTER 2. BACKGROUND Polymer IL electrolytes for ECs 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 TEABF 4, a common salt for organic electrolytes, is also reported. Overall, the ionic conductivities of polymer IL electrolytes are higher than that of TEABF 4 -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 EMIBF 4 and PAN EMITf 38

59 CHAPTER 2. BACKGROUND 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] 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/ruo 2 composite electrode and 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMI][HSO 4 ]) 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 39

60 CHAPTER 2. BACKGROUND 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. 40

61 CHAPTER 2. BACKGROUND Table 2-5 Polymer electrolytes used for activated carbon EDLCs and their electrochemical properties. Composition Intrinsic property EDLC performance Polymer IL/salt Solvent a Plasticizer a (ms cm -1 ) window voltage (F g -1 ) c (mv s -1 ) Conductivity Potential Operating Capacitance Scan rate Ref. (V) b (V) PVdF [TEA][BF 4 ] PC+EC [92] PVdF-HFP [TEA][BF 4 ] PC+EC [93] [EMI][Tf] NMP [86] [EMI][Tf] acetone Mg (Tf) d 2, [94] NMP PC+EC [EMI][TFSI] Zeolite e 20 [95] PAN [TEA][BF 4 ] DMF [92] [EMI][BF 4 ] DMF [96] [EMI][BF 4 ] DMF TMS 15 f [85] [BMI][PF 6 ] DMF TMS [85] [EMI][Tf] DMF [86] [EMI][Tf] DMF TMS 16.2 f [86] PEO [BMPyr][TFSI] AN [85] [EMI][Tf] AN [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][BF 4 ] and [EMI][Tf] were ca. 14 ms cm -1 41

62 CHAPTER 2. BACKGROUND 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 PF 6 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 42

63 CHAPTER 2. BACKGROUND electrodes, and therefore a lack of understanding of the proton conduction mechanism in such systems. As alternatives to fluorinated ions, anions such as SO 2 4, PO 3 4, NO 3, 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 (EMIHSO 4 ) 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 EMIHSO 4 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 EMIHSO 4 [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 ). 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 43

64 CHAPTER 2. BACKGROUND 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. 44

65 CHAPTER 2. BACKGROUND Table 2-6 Cost of the materials for cells fabricated with the polymer IL electrolytes (1 cm 2 laminated pouch-type cells). Type of material Unit cost Cost per Unit cost Cost per Type of material ($) cell ($) ($) cell ($) EDLC with PEO EMIHSO 4 EDLC with PVdF-HFP EMIBF 4 Polymer electrolyte Polymer electrolyte IL (EMIHSO 4 ) /kg IL (EMIBF 4 ) /kg Polymer /kg Polymer /kg (PEO) (PVdF-HFP) Solvent (PC) 28.00/kg Solvent (NMP) 88.00/kg Filler (SiO 2 ) /kg - - Electrode a Aluminum current 1.00/m 2 collector Graphite carbon, 28.18/kg PVdF binder, NMP solvent Packaging Sealing tape 72.38/m 2 Aluminum 4.07/m 2 lamination Total a costs of electrode materials were obtained from [101] * the estimated cost of EDLC with liquid organic electrolyte (1 M TEABF 4 in PC) was $0.38/cell The analysis in Table 2-6 shows that the cost of devices enabled by polymer IL (HSO 4 -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 HSO 4 -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 EMIHSO 4 can decrease by an additional 20% per cell. 45

66 CHAPTER 2. BACKGROUND 2.6 Characterization Techniques 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 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. 46

67 CHAPTER 2. BACKGROUND 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 47

68 CHAPTER 2. BACKGROUND i C = 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, R s, 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 q C= Δ V [F cm -2 ], (2-10) where Δq is the integrated stored charge in C cm -2 over the potential range ΔV (V) [15] 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 48

69 CHAPTER 2. BACKGROUND 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 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

70 CHAPTER 2. BACKGROUND The impedance as a function of frequency, f, is expressed in the complex notation as Z( f ) = Z ( f ) jz ( 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. 50

71 CHAPTER 2. BACKGROUND a. Ionic conductivity The resistance of the electrolytes, R s, 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 Z C 2 (2-13) (2π f )Z Z C (2π f )Z 2 (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 51

72 CHAPTER 2. BACKGROUND to energy dissipation by an irreversible process (e.g., dielectric loss). The rise in C (f ) passes through a maximum at a frequency f 0 with a characteristic time constant τ 0 = 1/f 0. 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 τ 0 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 )/C 0, where C 0 = 0 A/d is the vacuum capacitance ( 0 is the permittivity of free space, A is the geometric 52

73 CHAPTER 2. BACKGROUND 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: Z ε = 2 (2-15) (2π f )C Z 0 Z ε = 2. (2-16) (2π f )C Z 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 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. 0 53

74 CHAPTER 2. BACKGROUND 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. 54

75 CHAPTER 2. BACKGROUND 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 (τ EP and τ R, τ = 1/f ). The time constant of EP (τ EP ) reflects the rate of charge and discharge of the double layer. The relaxation time (τ R ) represents the response time of the polymer motion. Also seen in Figure 2-14 is a derivative form of dielectric constant ε der, 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, ], the derivative spectra often better reveals EP and relaxation peaks in situations where broad peaks extend over several frequency decades. 2 ε ( f ) f ε der = π ln (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 τ EP 55

76 CHAPTER 2. BACKGROUND suggests a faster capacitive response of the cells, while a shorter τ R 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 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 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 56

77 CHAPTER 2. BACKGROUND 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 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. 57

78 CHAPTER 2. BACKGROUND Figure 2-16 Schematic illustration of heating and cooling DSC thermograms including the thermal transitions, heat of crystallization, ΔH c, and heat of melting, ΔH m 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 (T g ) 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 (T m ). On cooling, an exothermal peak indicates the recrystallization of the melted material at a temperature (T rc ) lower than T m. An exothermal crystallization (T c ) may occur on heating at above T g 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 58

79 CHAPTER 2. BACKGROUND 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 T g, T m, and T rc, and (ii) quantify the degree of crystallinity. The percent crystallinity is determined using the following equation [112]: ΔH m % Crystallinity = 100 ΔH m ο (2-19) The heat of melting (or heat of fusion), ΔH m, in J g -1 is obtained by integrating the areas under the endothermic melting peaks (see Figure 2-16). The term ΔH m 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 Infrared (IR) spectroscopy The mid-infrared IR spectra is usually acquired in the 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 The CH 2 groups in the polymer can vibrate in different ways and the strong bands are assigned to asymmetric 59

80 CHAPTER 2. BACKGROUND 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 CH 2 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]. 60

81 CHAPTER 3 EXPERIMENTAL METHOD AND CHARACTERIZATION 3.1 Materials Ionic conductors A non-fluorinated RTIL, 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO 4 ), was selected for this study. 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4 ) 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 (MIHSO 4 ) and imidazolium hydrogen sulfate (ImHSO 4 ) 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. ImHSO 4 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: 61

82 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION Table 3-1 Structure of the studied ILs. Ionic liquid 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO 4, 95% Sigma-Aldrich) Structure 1-Methylimidazolium hydrogen sulfate (MIHSO 4, 95% Sigma-Aldrich) + H Imidazolium hydrogen sulfate (ImHSO 4, synthesized) (Imidazole, 99% Alfa Aesar) H + 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4, 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 (ImHSO 4 ) ionic liquid 62

83 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION Polymers The focus of this work was on polymer electrolytes constituted of PEO as the polymer matrix and EMIHSO 4 as the ionic conductor. The baseline polymer electrolyte was composed of PVdF-HFP polymer matrix and EMIBF 4. 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 T g ( C) Melting point T m ( C) Poly(ethylene oxide) PEO (Alfa Aesar) (CH 2 CH 2 O) n 1,000, [66] Ref. Poly(vinylidene fluoride-cohexafluoropropylene) PVdF-HFP (Kynar Flex 2801) (CH 2 CF 2 ) x (CF 2 CF( CF 3 )) y 470, [116] Additives The effect of two types of inorganic nanofillers was investigated. For comparison, selected SiO 2 and TiO 2 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 SiO 2 and TiO 2 fillers [117,118]. Filler Structure Particle size (nm) Dielectric constant (ε) SiO 2 (Alfa Aesar) amorphous TiO 2 (Alfa Aesar) crystalline (anatase) 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 63

84 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION 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 Preparation of PEO EMIHSO 4 PEO EMIHSO 4 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) EMIHSO 4 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 64

85 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION (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 μm. The polymer IL composition is reported as a weight ratio of polymer to IL. PEO EMIHSO 4 were prepared with different weight ratios of PEO:EMIHSO 4 : (1:1), (1:2), (1:3), and (1:4) Preparation of PVdF-HFP EMIBF 4 PVdF-HFP EMIBF 4 electrolytes were prepared by a procedure similar to that for PEO EMIHSO 4. 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 EMIHSO 4 PEO Propylene carbonate (PC, 99% Alfa Aesar) EMIHSO 4 PVdF-HFP EMIBF 4 PVdF-HFP N-Methyl-2-pyrrolidinone (NMP, 99% Alfa Aesar) EMIBF 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), SiO 2 and TiO 2 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 EMIHSO 4 10% SiO 2 or PEO EMIHSO 4 10% TiO 2. The resulting PEO EMIHSO 4 films without filler were translucent, and turned opaque with the addition of SiO 2 and TiO 2 fillers (see Figure 3-3). 65

86 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION Figure 3-3 Filler-free PEO EMIHSO 4, and PEO EMIHSO 4 containing SiO 2 and TiO 2 nanofillers 3.3 Device Fabrication 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 cm 2 ), and (3) pseuodocapacitive electrodes: RuO 2 on Ti foils (50 μm thick, ca. 0.5 cm 2 ), the process of manufacturing RuO 2 electrodes is described in [121]. The loading of RuO 2 was about 1.5 mg cm -2, resulting in a capacitance of 150 to 170 mf cm -2 in an H 2 SO 4 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, PMo 12 O 3 40 (PMo 12 ) (25-30 μm thick, ca. 0.5 cm 2 ). The modification method and the electrode fabrication are reported in [122], and the pseudocapacitive performance of this composite electrode, referred to as carbon/pmo 12, has been demonstrated for ECs in aqueous electrolyte. 66

87 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION 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 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 SelfSeal TM 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 67

88 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION 3.4 Characterization Structural characterization Structural studies were conducted by XRD on a Philips PW 1830 HT diffractometer with a Cu Kα radiation source (λ = 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 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. 68

89 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION 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 AgNO 3 /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 69

90 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION measured using each technique and the relationship between the performance of capacitor and the properties of interest of liquid and polymer electrolytes. 70

91 CHAPTER 3. EXPERIMENTAL METHOD AND CHARACTERIZATION 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 potential window (E) operating voltage stability of electrolyte (oxidation/reduction) - CV 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 - - high frequency cell impedance (Z ) equivalent series resistance (ESR) ionic conductivity (σ) 2-12 EIS low frequency cell capacitance (C ) capacitance corresponding to DC capacitance low frequency dielectric constant (i.e., extent of electrode polarization) (ε,τ EP ) 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,τ R ) 2-14,2-16 CV & EIS energy density (E) and power density (P)

92 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 (HSO 4 ) imidazolium salt to a common fluorinated (BF 4 ) 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 (TEABF 4 /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, 72

93 CHAPTER 4. IONIC LIQUID ELECTROLYTES potential window, and resultant capacitance are key parameters for IL electrolytes in ECs. EMIHSO 4 was investigated in parallel with EMIBF 4, 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 EMIHSO 4 and EMIBF Ionic conductivity The ionic conductivity of pure EMIHSO 4 and EMIBF 4 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 TEABF 4 /PC organic electrolyte with a concentration of 0.65 M (10.5 wt%) is also listed. The high viscosity of EMIHSO 4, shown in Table 4-1, is due to the more localized charge on HSO 4 compared to that on BF 4 anion and its tendency to form hydrogen bonding with the cation. This increases the interaction between EMI + and HSO 4, 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 EMIBF 4. 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. EMIHSO ± b EMIBF a c [32,43] EMIHSO 4 /PC solution TEABF 4 /PC solution 11.9 a c [29] a values are consistent with the literature b provided by the manufacturer; c obtained from the literature 73

94 CHAPTER 4. IONIC LIQUID ELECTROLYTES To improve the ionic dissociation of EMIHSO 4 and compare its conductivity to organic electrolytes, PC (as a solvent) was added to EMIHSO 4. The conductivity at different concentrations of EMIHSO 4 /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 EMIHSO 4 which was in the same order as that of TEABF 4 /PC solution (see Table 4-1). This indicates the presence of ion pairs in pure EMIHSO 4 which was reduced with the addition of solvent. Figure 4-2 Conductivity as a function of EMIHSO 4 concentration in PC Potential window Figure 4-3 illustrates the electrochemical stability of EMIHSO 4, EMIBF 4, EMIHSO 4 /PC, and TEABF 4 /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 EMIHSO 4 and EMIHSO 4 /PC exhibited a potential window of ca. 2 V, similar to that of organic electrolyte. Although the addition of PC slightly narrowed the 74

95 CHAPTER 4. IONIC LIQUID ELECTROLYTES stability window of EMIHSO 4, it significantly increased the ionic conductivity. Comparing EMIHSO 4 and EMIBF 4, the wider potential window of EMIBF 4 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 EMIHSO 4, measurements were performed at a low sweep rate: 5 mv s -1 ) Electrode capacitance and device performance The capacitance of a glassy carbon electrode in EMIHSO 4 and EMIBF 4 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 EMIHSO 4 and 42.2 ± 1.6 μf cm -2 in EMIBF 4 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 EMIHSO 4 and EMIBF 4 as respective electrolytes. The 75

96 CHAPTER 4. IONIC LIQUID ELECTROLYTES 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 EMIHSO 4 -based cell was resistive and the resulting capacitance was lower than that of fluorinated EMIBF 4 -based cell. This suggests that the high viscosity of EMIHSO 4 affected the motion of ions and hence the high rate performance of the device. While the operating voltage of graphite EDLC enabled with EMIHSO 4 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 EMIHSO 4 and EMIBF 4 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 EMIHSO 4 and EMIBF 4. The deliverable capacitance of cells was obtained from Cʹ at low frequencies, and τ (RC time constant) was derived from Cʺ plot. This time 76

97 CHAPTER 4. IONIC LIQUID ELECTROLYTES 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 EMIHSO 4 and EMIBF 4 (Liquid 2 filter paper cells) At low frequency (0.1 Hz), the capacitance of EDLC using EMIHSO 4 was 1.1 mf cm -2, higher than that for EMIBF 4 -enabled device (0.75 mf cm -2 ). On the other hand, the charge delivery in the device using EMIBF 4 was much faster than that with EMIHSO 4 : 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 EMIHSO 4 -enabled cell was slightly higher, whereas EMIBF 4 -enabled cell performed faster charge/discharge. While EMIHSO 4 shows promising electrochemical performance, its high viscosity is a limiting factor for high rate applications. 77

98 CHAPTER 4. IONIC LIQUID ELECTROLYTES The effect of viscosity on the capacitance was examined by analyzing the double-layer capacitance of a glassy carbon electrode in EMIHSO 4 /PC solutions. Figure 4-6 shows the electrode capacitance and the ionic conductivity vs. the concentration of EMIHSO 4 in PC. The capacitance of the electrode increased with the EMIHSO 4 concentration. The highest capacitance was obtained at 40 wt% EMIHSO 4 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% EMIHSO 4, resulting in a high ionic conductivity and double-layer capacitance. With further addition of EMIHSO 4, 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 EMIHSO 4 concentration in PC Although EMIHSO 4 has much lower ionic conductivity than EMIBF 4, EMIHSO 4 -based EDLC can store and deliver the same or even higher amount of charge as that of EMIBF 4 -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 EMIHSO 4 may 78

99 CHAPTER 4. IONIC LIQUID ELECTROLYTES be less influential in a polymer matrix than in the liquid electrolyte. Therefore, developing polymer electrolytes using non-fluorinated EMIHSO 4 could be a viable approach for high performance and environmental friendly solid ECs. 4.2 Effect of Cation The electrochemical properties of EMIHSO 4 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 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO 4 ) Structure Melting point ( C) 24 1-Methylimidazolium hydrogen sulfate (MIHSO 4 ) + H 47 Imidazolium hydrogen sulfate (ImHSO 4 ) H H

100 CHAPTER 4. IONIC LIQUID ELECTROLYTES 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 MIHSO 4 and further in ImHSO 4. On the other hand, the melting temperature of the ILs increased from 24 C for EMIHSO 4 to 47 C and 85 C for MIHSO 4 and ImHSO 4, 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 MIHSO 4 and ImHSO 4 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 Ionic conductivity of IL solutions Solid MIHSO 4 and ImHSO 4 were dissolved in solvents to prepare electrolytes for electrochemical characterizations. For comparison, solutions of EMIHSO 4 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, EMIHSO 4, MIHSO 4, and ImHSO 4 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: 80

101 CHAPTER 4. IONIC LIQUID ELECTROLYTES (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 EMIHSO 4 < ImHSO 4 < MIHSO 4. 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 EMIHSO 4 in PC (Figure 4-2), Ac acid, and MeOH. Conductivity increased in the order of EMIHSO 4 /PC EMIHSO 4 /Ac acid EMIHSO 4 /MeOH. The much higher conductivity of EMIHSO 4 /MeOH than that of EMIHSO 4 /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 EMIHSO 4 /MeOH may indicate the influence of protic solvent on promoting proton dissociation. Figure 4-7 Conductivity of solutions of EMIHSO 4 ( ), MIHSO 4 ( ), and ImHSO 4 ( ) in methanol (filled symbols) and acetic acid (empty symbols) 81

102 CHAPTER 4. IONIC LIQUID ELECTROLYTES The methanol solutions of EMIHSO 4, MIHSO 4, and ImHSO 4 at 40 wt% IL were used for further electrochemical analyses. The performances of these three IL solutions were first compared in enabled EDLCs 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 EMIHSO 4, MIHSO 4, and ImHSO 4 and graphite two-electrode cells were assembled according to Liquid 2 configuration (see Figure 3-4b). A cell with the solution of 40 wt% EMIHSO 4 /PC was also prepared as a baseline. Figure 4-8 shows the device performance using EMIHSO 4 /PC, EMIHSO 4 /MeOH, MIHSO 4 /MeOH, and ImHSO 4 /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 MIHSO 4 and ImHSO 4 as ionic conductors for EC applications. The capacitance of the cells with methanol solutions is slightly higher for MIHSO 4 -based electrolyte followed by ImHSO 4 and EMIHSO 4 -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 MIHSO 4 and ImHSO 4 electrolytes. The comparison between the CVs of cells with EMIHSO 4 /MeOH and EMIHSO 4 /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

103 CHAPTER 4. IONIC LIQUID ELECTROLYTES Figure 4-8 Cyclic voltammograms of graphite cells using EMIHSO 4 /PC, EMIHSO 4 /MeOH, MIHSO 4 /MeOH, ImHSO 4 /MeOH electrolytes at (a) 100 mv s -1 and (b) 1 V s -1 83

104 CHAPTER 4. IONIC LIQUID ELECTROLYTES 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, EMIHSO 4 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 EMIHSO 4 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 EMIHSO 4 ; 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 MIHSO 4 and ImHSO 4 to obtain pure liquid ILs at low temperatures. The latter approach was investigated which is presented in Chapter 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 EMIBF 4 compared to EMIHSO 4. At low sweep rate, EDLC using EMIHSO 4 could store the same amount of charge as the EMIBF 4 -based cells. Addition of 84

105 CHAPTER 4. IONIC LIQUID ELECTROLYTES solvent can be used to reduce the ion-pair formation in EMIHSO 4, 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 EMIHSO 4 increased with substituting the cationic alkyl groups by shorter alkyl chains and/or hydrogen in MIHSO 4 and ImHSO 4. The electrochemical properties were studied for solutions of MIHSO 4 and ImHSO 4 and compared to EMIHSO 4 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 MIHSO 4 /MeOH and ImHSO 4 /MeOH exhibited a higher capacitance than that of EMIHSO 4 /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. 85

106 CHAPTER 5 POLYMER IONIC LIQUID ELECTROLYTES The feasibility of PEO EMIHSO 4 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 EMIHSO 4 and PVdF-HFP EMIBF 4. 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 EMIHSO 4 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 EMIHSO 4 are evaluated and compared to its liquid counterpart and to that of PVdF-HFP EMIBF 4 (section 5.3). 5.1 PEO EMIHSO 4 and PVdF-HFP EMIBF 4 Electrolytes The electrochemical properties of EMIHSO 4 and EMIBF 4 were characterized and presented in Chapter 4. Although the non-fluorinated EMIHSO 4 had a high viscosity and hence a low conductivity compared to fluorinated EMIBF 4, the low-rate performance of EMIHSO 4 -based device was comparable to that based on EMIBF 4. To investigate the viability of incorporating 86

107 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES the IL into a polymer, PEO was integrated with EMIHSO 4 to develop PEO EMIHSO 4 electrolytes. PVdF-HFP EMIBF 4 electrolytes were also developed and analyzed to serve as a baseline for the structural and the electrochemical comparisons Ionic conductivity The ionic conductivities of PEO EMIHSO 4 with different compositions of EMIHSO 4 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 EMIHSO 4 compared to that of pure EMIHSO 4 (1.5 ms cm -1 ) was not as severe as those PEO IL electrolytes reported in [85,86,96]. The average conductivity of PEO EMIHSO 4 (1:2) was 0.7 ms cm -1 and increased to 0.8 ms cm -1 in PEO EMIHSO 4 (1:3). This is within the range reported for PEO-based electrolytes such as PEO BMPyrTFSI (0.3 ms cm -1 [85]) and PEO MMPIBF 4 (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 EMIBF 4, 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 EMIBF 4 decreased at higher EMIBF 4 content, and the resulting polymer films were too soft. Thus, PVdF-HFP EMIBF 4 (1:2) was selected as the fluorinated polymer IL baseline in this study. 87

108 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 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 ) E a (kj mol -1 ) Low T High T EMIHSO 4 (liquid) ± PEO EMIHSO 4 (1:2) ± PEO EMIHSO 4 (1:3) ± EMIBF 4 (liquid) PVdF-HFP EMIBF 4 (1:2) ± The ionic conductivity of EMIHSO 4 and PEO EMIHSO 4 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 EMIHSO 4 from 25 C to 80 C as well as PEO EMIHSO 4 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 EMIHSO 4 and PEO EMIHSO 4 in (1:2), (1:3), and (1:4) compositions 88

109 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Since EMIHSO 4 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 HSO 4 ) is controlled by the viscosity of EMIHSO 4 [53,99]. The polymer IL electrolytes had lower conductivities than the pure IL. The ionic conductivity of PEO EMIHSO 4 increased with the increase of EMIHSO 4 from 1:2 to 1:3 compositions, and remained relatively unchanged for 1:4 composition within the temperature range. All three PEO EMIHSO 4 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 EMIHSO 4 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 EMIHSO 4 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 EMIHSO 4. 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 EMIHSO 4 electrolytes and pure EMIHSO 4 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 EMIHSO 4 suggests that addition of PEO did not add an extra energy barrier to the ionic motion of EMIHSO 4. Ionic conductivity at low temperatures can be achieved in the polymer electrolytes, while application of pure EMIHSO 4 is limited to temperatures above its melting point (24 C). Although the ionic conductivity of PEO EMIHSO 4 is within the range reported for PEO ILs, the performance of PEO EMIHSO 4 requires improvement to a level comparable to 89

110 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 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 EMIHSO 4 and PVdF-HFP EMIBF 4 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 EMIHSO 4 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 EMIHSO 4 and PVdF-HFP EMIBF Crystallinity and thermal characterizations As discussed in the previous section, the ionic conductivity of PEO EMIHSO 4 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 EMIHSO 4 electrolytes was at 90

111 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 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 XRD analyses XRD studies were performed to investigate the changes to PEO structure after it has been impregnated with EMIHSO 4. Figure 5-3a illustrates the XRD patterns of PEO powder, PEO film, and PEO EMIHSO 4 (1:2) electrolyte. Two dominant peaks at 19.3 and θ related to the crystalline phase of PEO [7,126] were observed in both PEO film and PEO EMIHSO 4. The intensity of these peaks decreased from PEO powder to PEO film and PEO EMIHSO 4, suggesting a corresponding decrease in the crystalline structure of the electrolytes. The peaks observed at 15.1, 15.5, 26.4, and θ 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 EMIHSO 4 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 EMIHSO 4 (1:3). This indicates that the incorporation of EMIHSO 4 into PEO promoted the degree of disorder in the polymer chains and hence improved the amorphous phase. 91

112 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-3 XRD patterns of (a) PEO powder, PEO film, and PEO EMIHSO 4 (1:2) electrolyte; (b) PEO EMIHSO 4 electrolytes in (1:1), (1:2), and (1:3) compositions 92

113 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES The structure of PVdF-HFP EMIBF 4 electrolyte was also analyzed parallel to that of PEO EMIHSO 4. Figure 5-4 shows the XRD patterns of PVdF-HFP powder, PVdF-HFP film, and PVdF-HFP EMIBF 4. Comparing this graph to Figure 5-3a, the effect of EMIBF 4 on the crystalline structure of PVdF-HFP is analogous to PEO EMIHSO 4 electrolyte. The XRD pattern of PVdF-HFP shows the characteristic peaks of the crystalline structure of PVdF at 17.9, 26.3, and θ corresponding to the large α-phase spherulites [127,128]. The peak at θ 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 θ were not visible, and the intensity of the diffraction peak at θ decreased. This peak disappeared with the addition of EMIBF 4 into PVdF-HFP. Comparing to the XRD pattern of PVdF-HFP film, although the peak at θ became sharper, the amorphous baseline was more pronounced. This can be attributed to the incorporation of EMIBF 4 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 EMIHSO 4 and EMIBF 4 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. 93

114 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-4 XRD patterns of PVdF-HFP powder, PVdF-HFP film, and PVdF-HFP EMIBF 4 electrolyte DSC analyses Using DSC, the melting temperature and the crystallinity of PEO EMIHSO 4 electrolytes were determined at different IL content. A PEO film was used as a reference. DSC thermograms of PEO film and PEO EMIHSO 4 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 EMIHSO 4, both melting and recrystallization peaks shifted to lower temperatures at 50 C and 42 C for PEO EMIHSO 4 electrolytes in 1:2 and 1:3 compositions, respectively. This indicates that the incorporation of EMIHSO 4 into PEO changed the phase structure of the polymer. 94

115 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-5 Heating and cooling DSC thermograms of PEO film, PEO EMIHSO 4 electrolytes in (1:2) and (1:3) compositions The crystallinity (X c ) of the polymer electrolytes were calculated from the melting heat of PEO film and are listed in Table 5-2 together with the melting (T m ) and the recrystallization (T rc ) temperatures. Observing the significant decrease in melting heat, it can be concluded that the crystallinity of PEO EMIHSO 4 was considerably reduced by 48% with increasing IL content in 1:3 composition. These results clearly demonstrate that the addition of EMIHSO 4 into PEO promoted the disorder of the polymer chains and hence decreased the crystallinity [130]. The melting temperature of PEO EMIHSO 4 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 EMIHSO 4 electrolyte is advantageous as most applications are at ambient conditions. 95

116 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Table 5-2 Melting and recrystallization temperatures, and degree of crystallinity of PEO powder, PEO film, and the polymer electrolytes. Samples T m ( C) T rc ( C) X c (%) PEO powder a PEO film a PEO EMIHSO 4 (1:2) 50.0 ± ± ± 6.0 PEO EMIHSO 4 (1:3) 42.3 ± ± ± 1.3 a values are consistent with the literature In the XRD analyses of PVdF-HFP EMIBF 4, it was observed that with the addition of EMIBF 4, 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 EMIBF 4 on the crystallinity of PVdF-HFP was not as clear as that of PEO EMIHSO 4. Accordingly, DSC analyses were performed to characterize the crystallinity of PVdF-HFP EMIBF 4. 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 EMIBF 4 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 EMIBF 4 decreased both melting and recrystallization of PVdF-HFP film and reduced its crystallinity to 41%. 96

117 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-6 Heating and cooling DSC thermograms for PVdF-HFP film and PVdF-HFP EMIBF 4 electrolyte Table 5-3 Melting and recrystallization temperatures, and degree of crystallinity of PVdF-HFP film and PVdF-HFP EMIBF 4. Samples T m ( C) T rc ( C) X c (%) PVdF-HFP film PVdF-HFP EMIBF 4 (1:2) Both XRD and DSC analyses indicated that while the crystalline phase was still present in PEO EMIHSO 4 and PVdF-HFP EMIBF 4, 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 EMIHSO 4 and EMIBF 4. The impact of EMIHSO 4 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 EMIHSO 4 which may affect the ionic conduction. 97

118 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 5.2 Interaction Between Polymer and IL FTIR analyses were performed to study the possible interaction between EMIHSO 4 and PEO through structural, compositional, and bonding characterizations of the polymer electrolyte. PEO EMIHSO 4 was investigated along with pure EMIHSO 4 and PEO film as references. Figure 5-7 shows the FTIR spectra of the polymer electrolyte and its individual components in the cm -1 range. The wavenumbers of the significant bands with their associated bonding interactions for pure EMIHSO 4, PEO film, and the PEO EMIHSO 4 (1:2) electrolyte are summarized in Table 5-4. To study the structural changes in PEO EMIHSO 4, the polymer electrolytes were analyzed as a function of EMIHSO 4 composition. As shown in Table 5-4, most of the cationic bands are located in the cm -1 region. As expected, the intensity of these characteristic bands increased with the increase of EMIHSO 4 ; 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 EMIHSO 4 in the cm -1 region. Comparing the FTIR spectrum of PEO EMIHSO 4 with the spectra of its individual components, almost all bands in the PEO EMIHSO 4 spectrum can be accounted for (see Figure 5-7). Although no additional bands were observed in the PEO EMIHSO 4 spectrum, two main effects can be derived: crystallinity and interaction between the polymer and the anion. 98

119 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-7 FTIR spectra of pure PEO film, pure EMIHSO 4, and PEO EMIHSO 4 in (1:2) composition 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 CH 2 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 EMIHSO 4. These changes, together with a decrease in the intensity of the CH 2 peak at 1342 cm -1 and a broadening of the C O C peak 99

120 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES at 840 cm -1, suggest that the addition of EMIHSO 4 resulted in a decrease in the degree of crystallinity of the PEO structure Effect of interaction between PEO and HSO 4 Based on the overlap between the anion and polymer vibrational peaks, there was a stronger interaction between HSO 4 and PEO than between EMI + and HSO 4 in the pure IL. The fingerprint region of pure EMIHSO 4 in Figure 5-7 shows the bands attributable to HSO 4 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 EMIHSO 4 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 EMIHSO 4 spectrum, changes in the C O C and CH 2 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 HSO 4 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 HSO 4 and PEO. The most important interactions were observed in the cm -1 frequency range. Figure 5-8 shows that the relative intensity of the bands at 1240, 1040, and 840 cm -1 (in PEO EMIHSO 4 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 HSO 4 and the ether oxygen of PEO. This interaction is most likely due to the formation of hydrogen bonds between HSO 4 and the oxygen atom. The presence of hydrogen bonds generally has a higher impact on the donor group (HSO 4 ) 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 100

121 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES with the slight change in the position of cationic bands indicate that the structural changes are the result of the HSO 4 and PEO interaction. Figure 5-8 FTIR spectra of PEO EMIHSO 4 electrolytes in (1:1), (1:2), and (1:3) compositions in the range of cm

122 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Table 5-4 FTIR band positions and associated bonding modes for PEO EMIHSO 4 in (1:2) composition and its components. Wavenumbers (cm -1 ) Band assignments PEO film EMIHSO 4 PEO EMIHSO OC CO vibrations, S OH stretching Ref. [126,132] CH 2 rocking and C O C deformation, S OH stretching 944, ,962 CH 2 symmetric and asymmetric rocking 1058,1100, ,1100,1168 C O C symmetric and asymmetric stretching, O SO 3 symmetric stretching N CH 2 and N CH 3 stretching, S O vibrations of SO CH 2 asymmetric twisting, O SO 3 asymmetric stretching 1342, ,1354 CH 2 symmetric and asymmetric wagging [114,133] [114,134] [114,126, ] [114, ] [126,133,136,137] [114,126,134] C=N stretching [136,138] CH 2 stretching [126,135] C H stretching of methyl group , ,3152 C H stretching of imidazole ring [137,138] [ ] 102

123 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 5.3 Device Performance The electrochemical performance of the developed PEO EMIHSO 4 and PVdF-HFP EMIBF 4 electrolytes was demonstrated in EDLCs. To understand the effect of the polymer on the electrochemical behavior of the ILs, ECs with pure EMIHSO 4 and EMIBF 4 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 (EMIHSO 4, Figure 3-4a) with the solid EC cell (PEO EMIHSO 4, 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 (EMIHSO 4, 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 EMIHSO 4 electrolyte had a smaller capacitance than the solid EC cell with PEO EMIHSO 4 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 EMIBF 4 and pure EMIBF

124 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-9 Cyclic voltammograms of graphite ECs with EMIHSO 4 and PEO EMIHSO 4 electrolytes at sweep rates of (a) 100 mv s -1 and (b) 1 V s

125 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 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 EMIHSO 4 electrolyte, the solid cell with the polymer electrolyte shows comparable rate response, demonstrating its improved performance. For the liquid EMIHSO 4 electrolyte, its high viscosity hinders the mobility of ions. However, the viscosity of EMIHSO 4 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 EMIHSO 4 was similar to that with PVdF-HFP EMIBF 4, but the latter had a time constant of 0.2 s which was the result of the higher conductivity of PVdF-HFP EMIBF

126 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES Figure 5-10 (a) Real part C and (b) imaginary part C of the capacitance and vs. frequency for graphite ECs with EMIHSO 4 and PEO EMIHSO 4 electrolytes 106

127 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES The performance of EC device employing PVdF-HFP EMIBF 4 with operating potentials of 1.5 V and 2 V is shown in Figure For comparison, the CV profile of PEO EMIHSO 4 -based EC is superimposed on this graph. The more rectangular CV profile of PVdF-HFP EMIBF 4 -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 EMIHSO 4 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 EMIHSO 4 system is expected. Figure 5-11 Cyclic voltammograms of graphite EDLCs with PVdF-HFP EMIBF 4 and PEO EMIHSO 4 electrolytes at sweep rate of 1 V s -1 The PEO EMIHSO 4 film has shown promising performance as an electrolyte for solid ECs exceeding the performance of liquid EMIHSO 4 electrolytes. This result may appear counter-intuitive. But similar observations have also been reported in the literature for other 107

128 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES 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 EMIHSO 4, 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 EMIHSO 4 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 EMIHSO 4 polymer electrolyte is less affected by the high viscosity of EMIHSO 4. 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 EMIHSO 4 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 EMIHSO 4 and PVdF-HFP EMIBF 4. The thin and flexible PEO EMIHSO 4 films showed an ionic conductivity of 0.8 ms cm -1 at room temperature. Considering the much higher viscosity of EMIHSO 4 compared to that of EMIBF 4, the 108

129 CHAPTER 5. POLYMER IONIC LIQUID ELECTROLYTES conductivity of EMIHSO 4 was not significantly affected in polymer state. Thus, addition of PEO did not hinder the ionic motion of EMIHSO 4. Structural and thermal analyses revealed that impregnating PEO with EMIHSO 4 had a positive impact on the structure of the polymer electrolyte: Addition of EMIHSO 4 into PEO not only decreased the crystallinity of the polymer, but also lowered the melting point of PEO. Interactions between EMIHSO 4 and PEO promoted the dissociation of ions as well as reduced the crystalline state of polymer. Structural characterization of PVdF-HFP EMIBF 4 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 EMIHSO 4 electrolyte showed a capacitive behavior up to 1 V s -1. Within a similar operating voltage, the capacitance of the device with PEO EMIHSO 4 was comparable to that using PVdF-HFP EMIBF 4 which makes PEO EMIHSO 4 a promising environmentally friendly electrolyte enabling solid ECs. Nevertheless, further improvement in ionic conductivity of PEO EMIHSO 4 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 EMIHSO 4 electrolytes, and to correlate the key parameters to the performance of ECs. 109

130 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 EMIHSO 4 electrolytes showed a reasonable conductivity of 0.8 ms cm -1 and the enabled EDLCs demonstrated high rate performance [120]. The interaction between EMIHSO 4 and PEO lowered the crystallinity of the electrolyte and promoted ionic dissociation [119]. Since a highly amorphous structure of the polymer 110

131 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 SiO 2 and crystalline TiO 2 were incorporated into PEO EMIHSO 4 to improve its performance and to determine their dominating function in ionic conduction, structural crystallinity, and performance of device. The PEO EMIHSO 4 composition, optimized at 1:3, was used as a baseline for investigating the impact of the SiO 2 and TiO 2 nanofillers on the ionic conductivity and structural properties of the electrolyte. To understand the influence of SiO 2 and TiO 2 fillers on the ionic conductivity of PEO EMIHSO 4, the relationship between temperature and ionic conductivity for PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 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 EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 as a function of temperature. Within the temperature range of 10 C to +80 C, the conductivity of electrolytes containing SiO 2 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% SiO 2. At low temperatures, the highest conductivity was obtained for PEO EMIHSO 4 10% SiO 2, whereas above the melting point, PEO EMIHSO 4 10% SiO 2 showed a lower conductivity than that of PEO EMIHSO 4 3% SiO

132 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-1 Temperature dependence of the ionic conductivity of (a) PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 ; and (b) PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO 2 112

133 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS The ionic conductivity of PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO 2 as a function of temperature is shown in Figure 6-1b. The trend of ionic conductivities of PEO EMIHSO 4 TiO 2 electrolytes was somewhat different from that of SiO 2 -containing electrolytes. The average ionic conductivity of PEO EMIHSO 4 3% TiO 2 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% TiO 2. Table 6-1 Room temperature ionic conductivity of PEO EMIHSO 4 and PEO EMIHSO 4 electrolytes containing SiO 2 and TiO 2 nanofillers, and activation energy (E a ) of ionic conduction for the respective electrolytes at low and high temperatures. Polymer electrolytes Conductivity (ms cm -1 ) E a (kj mol -1 ) Low T High T PEO EMIHSO ± PEO EMIHSO 4 3% SiO ± PEO EMIHSO 4 10% SiO ± PEO EMIHSO 4 3% TiO ± PEO EMIHSO 4 10% TiO ± 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 EMIHSO 4 : increased in the case of SiO 2, but unaffected in the case of TiO 2. 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 EMIBF 4 electrolytes with 3 wt% SiO 2 and with 3 wt% TiO 2 was also studied. Figure 6-2 shows the ionic conductivity of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 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% SiO 2 or TiO 2 did not significantly change the conductivity of PVdF-HFP EMIBF 4. This is also seen in Figure 6-2 where the ionic conductivity of PVdF-HFP EMIBF 4 3% SiO 2 is only slightly higher than that of PVdF-HFP EMIBF 4 and PVdF-HFP EMIBF 4 3% TiO 2. This trend was consistent with that seen in PEO EMIHSO 4 3% SiO 2 and PEO EMIHSO 4 3% TiO

134 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-2 Temperature dependence of the ionic conductivity of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 Table 6-2 Room temperature ionic conductivity of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, PVdF-HFP EMIBF 4 3% TiO 2, and the activation energy (E a ) of ionic conduction for the respective electrolytes. Polymer electrolytes Conductivity (ms cm -1 ) E a (kj mol -1 ) PVdF-HFP EMIBF ± PVdF-HFP EMIBF 4 3% SiO ± PVdF-HFP EMIBF 4 3% TiO ± So far, the comparison between the trends of ionic conductivity of filler-containing PEO EMIHSO 4 and filler-containing PVdF-HFP EMIBF 4 leads to the following observations: (i) The addition of 10 wt% SiO 2 and TiO 2 improved the conductivity of PEO EMIHSO 4 over the entire temperature range. In the case of PEO EMIHSO 4 SiO 2, the 3 wt% SiO 2 at amorphous state is more effective than that of 10 wt% SiO 2. This implies that the higher SiO 2 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 EMIHSO 4 SiO 2 electrolytes at high temperature region (see 114

135 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Table 6-1). The trends in conductivity of PEO EMIHSO 4 TiO 2 at 3 wt% and 10 wt% filler was parallel at both low and high temperature ranges, at which the conductivity of 3 wt% TiO 2 was lower. (ii) The lower SiO 2 and TiO 2 content (i.e., 3 wt%) was insufficient to show a clear trend in the ionic conductivity. While the addition of 3 wt% SiO 2 increased the conductivity of PEO EMIHSO 4, 3 wt% TiO 2 had no significant impact. This difference could be attributed to the structural characteristics of the SiO 2 and TiO 2 fillers, which could affect the structure of PEO EMIHSO 4 and the interaction between PEO and EMIHSO 4. (iii) Overall, the effect of 3 wt% filler on ionic conductivity of PVdF-HFP EMIBF 4 was negligible compared to that of PEO EMIHSO 4. 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 EMIHSO 4 10% SiO 2 and PEO EMIHSO 4 10% TiO 2 are superimposed in Figure 6-3 together with that of filler-free PEO EMIHSO 4. The transitions in PEO EMIHSO 4 and in PEO EMIHSO 4 10% TiO 2 were much more pronounced than the transition in PEO EMIHSO 4 10% SiO 2. The ionic conductivity of PEO EMIHSO 4 10% TiO 2 was similar to that of PEO EMIHSO 4 10% SiO 2 at low temperatures, but significantly increased at temperatures above the melting point. 115

136 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-3 Temperature dependence of the ionic conductivity of PEO EMIHSO 4, PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO 2 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 EMIHSO 4. Since the increase of conductivity was only observed at 10 wt% TiO 2 addition, the shelf life of PEO EMIHSO 4 10% TiO 2 is shown. As summarized in Table 6-1, the average conductivities of PEO EMIHSO 4 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 TiO 2 and SiO 2 fillers may affect the ionic dissociation and the ion transport in PEO EMIHSO 4 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 116

137 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS properties of SiO 2 and TiO 2 nanofillers influence the performance of the PEO EMIHSO 4 electrolyte. Figure 6-4 The variation of ionic conductivity of PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO 2 over time 6.2 Effect of Fillers on Crystallinity In section 6.1, it was shown that ionic conductivity of filler-containing PEO EMIHSO 4 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 SiO 2 and TiO 2 fillers on the structure of the polymer electrolytes, filler-containing PEO EMIHSO 4 and PVdF-HFP EMIBF 4 were characterized by XRD and DSC. 117

138 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS XRD analyses Figure 6-5a shows the XRD patterns of amorphous SiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2. As shown in Chapter 5, the incorporation of EMIHSO 4 into PEO increased the amorphous phase. The addition of 3 wt% and 10 wt% SiO 2 further decreased the intensity of the crystalline peaks of PEO at 19 and 23 2θ, suggesting a progressive decrease in the crystallinity of PEO EMIHSO 4. Meanwhile, the amorphous peak in the baseline of the PEO EMIHSO 4 spectrum became broader for PEO EMIHSO 4 3% SiO 2 and further for PEO EMIHSO 4 10% SiO 2. The XRD patterns of crystalline TiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO 2 are illustrated in Figure 6-5b. The XRD spectrum of TiO 2 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 EMIHSO 4 3% TiO 2 and PEO EMIHSO 4 10% TiO 2. A similar trend to that of PEO EMIHSO 4 SiO 2 was observed: the intensity of crystalline peaks substantially decreased from 3 wt% to 10 wt% TiO 2 (see Table B-1). The broad amorphous peak also indicated that crystallinity was suppressed in PEO EMIHSO 4 10% TiO

139 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-5 XRD patterns for (a) SiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 ; and (b) TiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO 2 119

140 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS From the XRD results, it appears that a low quantity of fillers (i.e., 3 wt% TiO 2 or SiO 2 ) 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 EMIBF 4 containing fillers was characterized. Figure 6-6 shows the XRD spectra of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 as well as the XRD patterns of SiO 2 and TiO 2 nanofillers. The comparison of the spectra revealed a small effect of 3 wt% fillers on the structure of PVdF-HFP EMIBF 4, supporting the observations of PEO EMIHSO 4 (Figure 6-5). Figure 6-6 XRD patterns of SiO 2 and TiO 2 nanofillers, PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 electrolytes Figure 6-7 compares the XRD spectra of PEO EMIHSO 4 10% SiO 2 and PEO EMIHSO 4 10% TiO 2 to that of PEO EMIHSO 4 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% SiO 2 and TiO 2 reduced the crystallinity of PEO EMIHSO 4 electrolytes. 120

141 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-7 XRD patterns of PEO film, PEO EMIHSO 4, PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO DSC analyses The degree of crystallinity of PEO EMIHSO 4 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 EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 are depicted in Figure 6-8a. The degree of crystallinity (X c ) 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 EMIHSO 4 was reduced by 48% over that of the PEO film due to the presence of a relatively large amount of EMIHSO 4 which itself can act as a plasticizer. With the addition of 3 wt% SiO 2, the crystallinity of PEO EMIHSO 4 decreased from 18% to 16%, which further decreased to 12% with 10 wt% SiO 2. This shows the same trend observed in XRD analyses (Figure 6-1a). 121

142 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-8 Heating and cooling DSC thermograms of (a) PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 ; and (b) PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO 2 122

143 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS The DSC thermograms of TiO 2 -containing PEO EMIHSO 4 together with that of PEO EMIHSO 4 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 EMIHSO 4 SiO 2, the trend of crystallinity in PEO EMIHSO 4 TiO 2 increased from 18% to 27% with 3 wt% addition, and then decreased to 12% with 10 wt% TiO 2, agreeing with the XRD results in Figure 6-1b. Table 6-3 Melting temperature (T m ), recrystallization temperature (T rc ), and crystallinity (X c ) of PEO film, PEO EMIHSO 4, and PEO EMIHSO 4 filler electrolytes. Electrolytes T m (ºC) T rc (ºC) X c (%) PEO film a PEO EMIHSO ± ± ± 1.3 PEO EMIHSO 4 3% SiO ± ± ± 0.4 PEO EMIHSO 4 10% SiO ± ± ± 0.1 PEO EMIHSO 4 3% TiO ± ± ± 0.5 PEO EMIHSO 4 10% TiO ± ± ± 0.9 a values are consistent with the literature The effect of 3 wt% SiO 2 and TiO 2 on the crystalline structure of PVdF-HFP EMIBF 4 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 EMIHSO 4 3% filler, and substantiated the small impact of 3 wt% filler on the structure of PVdF-HFP EMIBF 4. Figure 6-9 shows the comparison between the DSC thermograms of PEO EMIHSO 4 10% SiO 2 and PEO EMIHSO 4 10% TiO 2 which clearly reveals that the addition of 10 wt% SiO 2 and TiO 2 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 EMIHSO 4. 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. 123

144 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-9 DSC heating and cooling thermograms of PEO film, PEO EMIHSO 4, and PEO EMIHSO 4 10% SiO 2, and PEO EMIHSO 4 10% TiO 2 Also shown in Figure 6-9 and Table 6-3, the filler-containing electrolytes showed a slight increase in T m over PEO EMIHSO 4. This small increase in T m 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 TiO 2 and amorphous SiO 2 at 10 wt% reduced the crystallinity of PEO EMIHSO 4 implies that the crystal structure of nanofillers plays little role in the overall structural change of the polymer electrolyte. However, TiO 2 and SiO 2 fillers have different dielectric constants (see Table 3-3) than PEO (ε = 5) [142,143] which may affect the interaction between polymer and IL. 124

145 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 6.3 Effect of Fillers on Interaction Between PEO and EMIHSO 4 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 EMIHSO 4 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 SiO 2 or TiO 2 fillers [7]. To test this hypothesis, the filler-containing PEO EMIHSO 4 were characterized using FTIR and compared to PEO EMIHSO 4 in order to investigate the filler-il and filler-polymer interactions. Figure 6-10 shows the spectra of SiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2. The characteristic peaks of PEO were CH 2 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 EMIHSO 4 spectrum was assigned to a combination of individual C O C and HSO 4 characteristic modes, and the peak shifts reflected H-bonding between oxygen atoms on PEO chain and HSO 4 [119]. 125

146 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-10 FTIR spectra of SiO 2 nanofiller, PEO EMIHSO 4, PEO EMIHSO 4 3% SiO 2, and PEO EMIHSO 4 10% SiO 2 The spectrum of PEO EMIHSO 4 3% SiO 2 was similar to that of PEO EMIHSO 4 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% SiO 2, while there was no frequency change of the CH 2 peaks, the relative intensity of the CH 2 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 SiO 2 may interact with the polar or the charged species in the PEO EMIHSO 4 electrolyte. This in turn will reduce the attraction between PEO and HSO 4, and lead to a higher ionic dissociation in PEO EMIHSO 4 10% SiO 2. The increase in CH 2 and C O C intensities of PEO EMIHSO 4 10% SiO 2 together with thermal analysis and XRD, all suggest an increased polymer chain movement and vibration in the PEO matrix. 126

147 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS The effect of TiO 2 on the interaction between the polymer and IL is not visible as the IR active vibrations of TiO 2 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 EMIHSO 4 10% TiO 2 and PEO EMIHSO 4 were similar and no significant change in the peaks were observed. In the spectrum of PEO EMIHSO 4 3% TiO 2, 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 EMIHSO 4 3% TiO 2 in line with the DSC analyses. Figure 6-11 FTIR spectra of PEO EMIHSO 4, PEO EMIHSO 4 3% TiO 2, and PEO EMIHSO 4 10% TiO 2 The structural characterization supported the effect of SiO 2 and TiO 2 fillers on the ionic conductivity of PEO EMIHSO 4, leading to the following conclusions: 127

148 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS (i) 10 wt% SiO 2 and TiO 2 additions suppressed the crystallinity of PEO EMIHSO 4, increasing the ionic conductivity over the entire temperature. Despite their different crystalline structures, both SiO 2 and TiO 2 had plasticizing effects. (ii) There was little or no change on the ionic conductivity of PEO EMIHSO 4 with 3 wt% fillers. The low quantity of fillers was insufficient to noticeably decrease the crystallinity. (iii) The interactions between PEO and EMIHSO 4 was affected with 10 wt% SiO 2, suggesting further ionic dissociation. However, it appears that the lowered crystallinity may not be the only effect, since the ionic conductivity of PEO EMIHSO 4 10% SiO 2 and PEO EMIHSO 4 10% TiO 2 also increased at high temperatures (see Figure 6-3), where the polymer electrolyte is supposedly amorphous. There could be additional functions of SiO 2 and TiO 2 in the electrolyte that affects the interaction between polymer and IL and facilitates further ionic dissociation. 6.4 Impedance and Dielectric Analyses 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 EMIHSO 4. 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 This is the first attempt to combine these two approaches to 128

149 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 Capacitance and dielectric response of polymer electrolytes The concentration of SiO 2 and TiO 2 was held at 10 wt% in the polymer electrolytes for impedance and dielectric characterizations. Three types of metallic cells were assembled using PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 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. 129

150 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-12 Variation of (a) real part C and (b) imaginary part C of capacitance with respect to frequency for cells leveraging PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 at 30 C The dielectric permittivity ( ) and dielectric loss ( ) of PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 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 τ 0 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 (f 0 ) is used to evaluate the dipolar relaxation. Although a small contribution of capacitance from EP is still 130

151 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 at 30 C 131

152 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Effect of fillers The filler-containing PEO EMIHSO 4 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 EMIHSO 4, indicating that fillers may have different functionalities in the different frequency regions. At low frequencies, the chemically inert SiO 2 and TiO 2 nanofillers may exhibit a barrier effect by physically obstructing the motion of ions and hence slowing down EP, resulting in a longer τ EP. At high frequencies, the greater flexibility and faster relaxation of the polymer chains due to the fillers may reduce τ R and increase ionic conductivity. So far, it was established that nanofillers can lead to more amorphous PEO EMIHSO 4 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 EMIHSO 4 SiO 2 and PEO EMIHSO 4 TiO 2 would be similar. According to the results shown in Figure 6-3, this is not the case. Thus, dielectric derivative of PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 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 EMIHSO 4 and PEO EMIHSO 4 SiO 2 shifted gradually (Figure 6-14a and Figure 6-14b), an abrupt shift occurred in PEO EMIHSO 4 TiO 2 (Figure 6-14c). 132

153 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-14 Dielectric derivative vs. frequency for (a) PEO EMIHSO 4, (b) PEO EMIHSO 4 SiO 2, and (c) PEO EMIHSO 4 TiO 2 at different temperatures: 10 C ( ), 0 C ( ), 10 C ( ), 20 C ( ), 30 C ( ), 40 C ( ), 50 C ( ), 60 C ( ), 70 C ( ), 80 C ( ) 133

154 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 EMIHSO 4 TiO 2 (Figure 6-14c) was higher than that of PEO EMIHSO 4 SiO 2 (Figure 6-14b) and PEO EMIHSO 4 (Figure 6-14a). A possible reason lies in the difference between the intrinsic dielectric constants of TiO 2 and SiO 2. Since TiO 2 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 The specific capacitance of PEO EMIHSO 4 and PEO EMIHSO 4 SiO 2 cells was similar, while PEO EMIHSO 4 TiO 2 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 SiO 2 -containing electrolyte), this effect can be compensated for by the addition of TiO 2 that enhances ionic dissociation due to its high dielectric constant. Figure 6-15 Capacitance of cells leveraging PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2, respectively, at different temperatures (at 0.1 Hz) 134

155 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 EMIHSO 4, 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 EMIHSO 4 ( ), PEO EMIHSO 4 SiO 2 ( ), and PEO EMIHSO 4 TiO 2 () 135

156 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 τ EP. Examining Figure 6-16, τ EP of the filler-containing electrolytes showed a flat temperature dependence relative to that of PEO EMIHSO 4 in both semi-crystalline and amorphous states. Their EP process occurred at lower frequencies than that of PEO EMIHSO 4 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], τ EP 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 EMIHSO 4. In the amorphous state (T > 50 C), where ionic mobility is less dependent on the polymer segmental motion, PEO EMIHSO 4 TiO 2 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 EMIHSO 4 SiO 2 exhibits the fastest response (the smallest R ), followed by PEO EMIHSO 4 TiO 2 and PEO EMIHSO 4. 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 A comparison of ionic conductivity (Figure 6-3) and R 136

157 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS (Figure 6-16) at low temperatures reveals that the conduction mechanism of PEO EMIHSO 4 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 EMIHSO 4, R of PEO EMIHSO 4 TiO 2 was noticeably shorter than for PEO EMIHSO 4 SiO 2. 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]. TiO 2, having a higher dielectric constant, can induce greater polarity in the PEO EMIHSO 4 system than SiO 2. The additional polarity significantly enhanced the relaxation rate of PEO EMIHSO 4 TiO 2 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 SiO 2 and TiO 2 nanofillers on τ EP and cell capacitance was demonstrated for metallic cells enabled with PEO EMIHSO 4 electrolytes. It was shown that the high dielectric constant of TiO 2 promoted the ionic dissociation and hence increased the capacitance of the PEO EMIHSO 4 -based cells. This section concerns the performance of graphite EDLCs employing filler-containing PEO EMIHSO 4 in order to 137

158 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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 EMIHSO 4 and PEO EMIHSO 4 SiO 2 are shown in Figure 6-17a. The addition of a large amount of SiO 2 (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% SiO 2. On the other hand, the time constant decreased from 4 s for PEO EMIHSO 4 -based cell to 0.5 s for PEO EMIHSO 4 SiO 2 -based cells. The performance of graphite EDLCs using PEO EMIHSO 4 and PEO EMIHSO 4 TiO 2 was also characterized by CV and EIS as illustrated in Figure 6-18a and Figure 6-18b. The electrochemical performance of the cells with TiO 2 -containing electrolytes was similar to the SiO 2 -containing PEO EMIHSO 4 -based cells: the capacitance of the graphite EDLCs decreased with the addition of TiO 2, but the time constant became shorter. 138

159 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-17 (a) Cyclic voltammograms of graphite EDLC devices with PEO EMIHSO 4, PEO EMIHSO 4 SiO 2 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

160 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS Figure 6-18 (a) Cyclic voltammograms of graphite EDLC devices with PEO EMIHSO 4 and PEO EMIHSO 4 TiO 2 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

161 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS The decrease in the capacitance could be due to the high content of inert SiO 2 and TiO 2 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 SiO 2 and TiO 2 nanofillers decreased the capacitance of cells (C), they reduced the resistance of PEO EMIHSO 4 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 EMIHSO 4, as an example, the EDLC with PEO EMIHSO 4 10% SiO 2 was further subjected to cycle life tests. Figure 6-19 shows the CV profiles obtained at 1 V s -1 for the 1 st, 500 th, and 5000 th 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 EMIHSO 4 10% SiO 2 electrolyte at 1 V s

162 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS This study demonstrated that incorporation of SiO 2 or TiO 2 fillers is a feasible approach that is beneficial to ionic conductivity and structural stability of PEO EMIHSO 4 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 EMIHSO 4 with the addition of SiO 2 and TiO 2 can be seen. For high temperature applications, PEO EMIHSO 4 TiO 2 will be a promising candidate. Figure 6-20 Comparison between the ionic conductivity as a function of temperature of the starting PEO EMIHSO 4, the optimized PEO EMIHSO 4 SiO 2 and PEO EMIHSO 4 TiO 2, and PVdF-HFP EMIBF 4 142

163 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 6.6 Summary Amorphous SiO 2 and crystalline TiO 2 nanofillers at 3 wt% and 10 wt% were added into PEO EMIHSO4 electrolyte to improve its ionic conductivity. Incorporation of 3 wt% SiO 2 into PEO EMIHSO4 electrolytes increased the conductivity, while addition of 3 wt% TiO 2 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 HSO 4 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 EMIHSO 4 electrolytes with and without SiO 2 or TiO 2 nanofillers, have identified three influential factors: (i) A structural effect minimizing the crystalline phase, where the enhanced segmental motion of PEO EMIHSO 4 SiO 2 and PEO EMIHSO 4 TiO 2 over that of PEO EMIHSO 4 resulted in higher ionic conductivity. (ii) An intrinsic dielectric constant effect in the amorphous phase, where the higher dielectric constant of TiO 2 considerably increased ionic mobility and dissociation, leading to enhanced conductivity of PEO EMIHSO 4 TiO 2 over PEO EMIHSO 4 SiO 2. (iii) A barrier effect which delayed EP at low frequencies, resulting in a slower double layer formation. While both SiO 2 and TiO 2 prolonged EP of PEO EMIHSO 4, the high dielectric constant of TiO 2 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 EMIHSO 4 increased to 2.1 ms cm -1 with the addition of SiO 2, approaching the conductivity of the PVdF-HFP EMIBF 4 at room temperature (6.5 ms cm -1 ), and with the addition of TiO 2, it exceeded the conductivity of the fluorinated 143

164 CHAPTER 6. POLYMER IONIC LIQUID ELECTROLYTES WITH FILLERS 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. 144

165 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 EMIHSO 4, MIHSO 4, ImHSO 4 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 EMIHSO 4 -ImHSO 4 and MIHSO 4 -ImHSO 4 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 EMIHSO 4 changed with substituting the alkyl groups of imidazolium cation 145

166 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE 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 MIHSO 4 and ImHSO 4 electrolytes were higher than that of EMIHSO 4. 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 EMIHSO 4 (24 C) to MIHSO 4 (47 C) and ImHSO 4 (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. 146

167 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Table 7-1 Structure and melting temperature of PILs with different cations. Ionic liquid 1-Ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO 4 ) Structure Melting point ( C) 24 1-Methylimidazolium hydrogen sulfate (MIHSO 4 ) 47 Imidazolium hydrogen sulfate (ImHSO 4 ) 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 EMIHSO 4, MIHSO 4, and ImHSO 4, 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. 147

168 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE 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: RuO 2 and carbon/polyoxometalate (POM) composite electrodes (see section 3.3). RuO 2 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 3 delivery. The redox reactions for one type of POM: phosphomolybdate or PMo 12 O 40 (PMo 12 ) is shown in Equation 7-1. PMo O xh xe H PMo O, x = 2,4,6 (7-1) x 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 RuO 2 electrodes with the PIL electrolytes then will be followed by the performance of POM electrodes in similar electrolytes Performance of RuO 2 with PIL electrolytes The electrochemical performance of RuO 2 cells was examined to evaluate and compare the proton conductivity of the respective PILs. Figure 7-1 shows the voltammograms of the RuO 2 cells employing EMIHSO 4 /MeOH, MIHSO 4 /MeOH, and ImHSO 4 /MeOH as well as EMIHSO 4 /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 RuO 2 cell with EMIHSO 4 /MeOH was higher than that with EMIHSO 4 /PC (also see Figure 4-2 and Figure 4-148

169 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE 7); and (ii) the higher capacitance of ImHSO 4 and MIHSO 4 -based cells than that of EMIHSO 4 -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 RuO 2. Figure 7-1 Cyclic voltammograms of RuO 2 cells in ImHSO 4 /MeOH, MIHSO 4 /MeOH, EMIHSO 4 /MeOH, and EMIHSO 4 /PC electrolytes at (a) 100 mv s -1 and (b) 1 V s -1 (Liquid 2 configuration) 149

170 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Table 7-2 ESR and capacitance of RuO 2 cells using PIL electrolytes and H 2 SO 4. Electrolyte a ESR (ohm) Cell capacitance (mf cm -2 ) Cʹ (EIS) b C (100 mv s -1 ) C (1 V s -1 ) EMIHSO 4 /PC EMIHSO 4 /MeOH MIHSO 4 /MeOH ImHSO 4 /MeOH M H 2 SO 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 ImHSO 4 electrolyte that exhibit two cationic hydrogen atoms to promote reaction 2-2 (pg. 10) more strongly than MIHSO 4 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) EMIHSO 4 /MeOH 0.10 MIHSO 4 /MeOH 0.24 ImHSO 4 /MeOH 0.26 Although Im cations exhibit two nitrogen sites, the proton concentration in ImHSO 4 electrolyte was only slightly higher than that in MIHSO 4. The estimated proton concentrations (Table 7-3) and the comparison of pseudocapacitance of RuO 2 cell (Figure 7-1) indicate that the secondary proton of ImHSO 4 may be partially dissociated. These comparisons lead to the following conclusions: (i) The studied PILs demonstrated proton conductivity increasing in the order of EMIHSO 4 MIHSO 4 ImHSO 4 (in MeOH solution). (ii) The ion ion association and the hydrogen bonding in ImHSO 4 may cause partial dissociation of the secondary proton, resulting in a pseduocapacitive performance similar to that of MIHSO

171 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE (iii) The sweep rate dependency of the capacitance of the RuO 2 cell in PIL/MeOH is indicative of the lower proton dissociation and higher viscosity of PIL/MeOH electrolytes compared to highly dissociated H 2 SO 4. 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 Performance of carbon/pom in PIL electrolytes The composite electrodes used in this study were based on MWCNT which were chemically modified by PMo 12 O 3 40 (PMo 12 ). The proton contribution of the three PIL electrolytes to the electrochemical reactions of these electrodes, referred to as carbon/pmo 12, were examined and compared to the bare carbon double-layer electrodes. For comparison, the respective carbon/pmo 12 electrodes were also tested in 0.5M H 2 SO 4. Figure 7-2 shows the cyclic voltammograms of carbon/pmo 12 and bare carbon electrodes in aqueous electrolyte. The CV profile of carbon/pmo 12 exhibited three pairs of characteristic oxidation/reduction peaks corresponding to the redox reactions of PMo 12 (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. 151

172 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-2 Cyclic voltammograms of bare carbon (dashed line) and carbon/pmo 12 (solid line) electrodes in 0.5M H 2 SO 4 at 100 mvs Electrode performance in PIL electrolytes The carbon/pmo 12 electrodes were characterized using EMIHSO 4 /MeOH, MIHSO 4 /MeOH, and ImHSO 4 /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/pmo 12 electrodes in the three electrolytes. The similarities between the overall performance of the electrodes in the PIL electrolytes and that in H 2 SO 4 were the presence of the characteristic redox peaks. This verified that redox reaction of PMo 12 occurred in PIL electrolytes, further confirming the proton conduction of these PILs. However, compared to H 2 SO 4, 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. 152

173 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-3 Cyclic voltammograms of bare carbon and carbon/pmo 12 electrodes in (a) EMIHSO 4 /MeOH, (b) MIHSO 4 /MeOH, (c) ImHSO 4 /MeOH electrolytes, and (d) comparison of cyclic voltammograms of carbon/pmo 12 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 EMIHSO 4 < MIHSO 4 ImHSO 4 as demonstrated by more distinct and reversible peaks. The observed trend agrees with that seen for RuO 2 -based cells (see Figure 7-1), in which ImHSO 4 /MeOH and MIHSO 4 /MeOH electrolytes promoted pseudocapacitive reactions of both RuO 2 and carbon/pmo 12. The comparison of the performances of both electrodes to those in aqueous electrolytes suggest that proton conduction in PIL electrolytes could be a 153

174 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE 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 EMIHSO 4 /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 HSO 4. Considering the small dissociation constant of HSO 4, HSO 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 EMIHSO 4 are contributed by HSO 4 anion, while MIHSO 4 and ImHSO 4 have protons contributed from both HSO 4 and their respective cations 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 EMIHSO 4 /MeOH, MIHSO 4 /MeOH, ImHSO 4 /MeOH, and EMIHSO 4 /PC. The double-layer capacitance of the bare carbon cells was slightly higher in MIHSO 4 /MeOH followed by ImHSO 4 /MeOH and EMIHSO 4 /MeOH. The performance of carbon/pmo 12 cells in the three PIL/MeOH electrolytes and in EMIHSO 4 /PC are shown in Figure 7-4b. The pseudocapacitive reactions of carbon/pmo 12 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/pmo 12 cell was the highest in ImHSO 4 electrolyte (Figure 7-4b), suggesting a higher concentration of mobile protons in this electrolyte. 154

175 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-4 Cyclic voltammograms of (a) bare carbon cells and (b) carbon/pmo 12 cells in EMIHSO 4 /MeOH, MIHSO 4 /MeOH, ImHSO 4 /MeOH, and EMIHSO 4 /PC electrolytes at 1 V s -1 (Liquid 2 configuration) 155

176 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE To examine the extent of proton transfer in these PIL electrolytes with respect to aqueous electrolyte, the performance of same carbon/pmo 12 cells were also characterized in H 2 SO 4. The capacitances of each cell using both H 2 SO 4 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 H 2 SO 4 and their ratio is reported as percentage in Table 7-4. The percentage of cell capacitance increased in the order of EMIHSO 4 /PC < EMIHSO 4 /MeOH < MIHSO 4 /MeOH < ImHSO 4 /MeOH. This trend was consistent with the capacitance of carbon/pmo 12 electrodes (Figure 7-4) and RuO 2 cells (Figure 7-1), confirming the higher proton conducting characteristics of ImHSO 4 and MIHSO 4. Table 7-4 Capacitance of carbon/pmo 12 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 H 2 SO 4 C IL /C aq. (%) EMIHSO 4 /PC EMIHSO 4 /MeOH MIHSO 4 /MeOH ImHSO 4 /MeOH 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 RuO 2 and carbon/pmo 12 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 ImHSO 4 and MIHSO 4 showed much greater proton activity than EMIHSO 4. The issue is that the melting temperatures of these two PILs are above room temperature, so they are solid in ambient condition different from EMIHSO

177 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-5 shows the heating and cooling thermograms of EMIHSO 4, MIHSO 4, and ImHSO 4 between 90 C and 150 C. The melting point of EMIHSO 4 is at 24 C, and there was no sign of recrystallization during cooling. EMIHSO 4 remains liquid at room temperature. At low temperature, EMIHSO 4 exhibits a glass transition at 61 C which is typically reported between 70 C and 90 C for 1-alkyl-3-methylimidazolium salts [30]. Cooling Heating Figure 7-5 DSC thermograms of pure EMIHSO 4, MIHSO 4, and ImHSO 4 at heating and cooling scans of 10 C min -1 The melting point of MIHSO 4 is increased in comparison to EMIHSO 4, at 47 C, and is further increased for ImHSO 4 at 85 C. Both MIHSO 4 and ImHSO 4 recrystallized on cooling, 157

178 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE indicating the formation of crystal structure at room temperature. The lower melting point of EMIHSO 4 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 MIHSO 4 and ImHSO 4 (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: MIHSO 4 -ImHSO 4 and EMIHSO 4 -ImHSO 4. The impact of cation substitution groups of EMIHSO 4 and MIHSO 4 on melting temperature of the binary mixtures was also studied and compared MIHSO 4 -ImHSO 4 binary system When mixing MIHSO 4 and ImHSO 4 at various ratios, the binary system showed different physical and chemical properties. Figure 7-6 shows examples of thermograms of the MIHSO 4 :ImHSO 4 mixtures with weight percentage ratios at 65:35, 70:30, 75:25, and 80:20. The binary mixture with 65 wt% MIHSO 4 and 35 wt% ImHSO 4 showed a melting peak at 0 C, which is much lower than that of MIHSO 4 (47 C) and ImHSO 4 (85 C). The binary systems with compositions between 65 wt% and 80 wt% MIHSO 4 showed negligible melting peaks in other words, no endothermic transitions. Specifically, the 70:30 composition of MIHSO 4 -ImHSO 4 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 MIHSO 4 :ImHSO 4 ratio to 80:20 wt%, a melting transition occurred and its temperature reached that of pure MIHSO 4 (Figure 7-5). 158

179 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-6 DSC thermograms of various compositions of MIHSO 4 -ImHSO 4 binary mixtures at heating and cooling scans of 10 C min -1 In MIHSO 4 -ImHSO 4 binary mixtures, the crystal structure of ImHSO 4 was likely disrupted due to the addition of more asymmetric MIHSO 4, 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 MIHSO 4 -ImHSO 4 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 MIHSO 4 -ImHSO 4 binary system at different compositions. MIHSO 4 :ImHSO 4 (wt%) T g ( C) T m1 ( C) T m2 ( C) (0:100) (30:70) (50:50) (65:35) 69-1 (70:30) (75:25) (80:20) (100:0)

180 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE 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 MIHSO 4 -ImHSO 4 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% MIHSO 4, 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 MIHSO 4 -ImHSO 4 binary system: ( ) melting point; ( ) solid-solid transition; ( ) glass transition EMIHSO 4 -ImHSO 4 binary system Similar approach was also applied to liquid EMIHSO 4 by adding solid ImHSO 4 to introduce proton activity into EMIHSO 4 system. Table 7-6 shows the thermal properties of 160

181 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE EMIHSO 4 -ImHSO 4 at different compositions which were used to develop the phase diagram. Examples of thermograms of the EMIHSO 4 :ImHSO 4 binary systems are shown in Figure B- 3. Table 7-6 Thermal properties of EMIHSO 4 -ImHSO 4 binary system at different compositions. EMIHSO 4 :ImHSO 4 (wt%) T g ( C) T m1 ( C) T m2 ( C) (0:100) (30:70) (40:60) (50:50) 60-0 (60:40) (65:35) (70:30) (75:25) (80:20) (85:15) (100:0) The resultant phase diagram for EMIHSO 4 -ImHSO 4 is shown in Figure 7-8. Overall, the addition of EMIHSO 4 reduced the melting point of ImHSO 4. While the three main phases also exist in this binary phase diagram, there are two minima in Figure 7-8. At 50 wt% EMIHSO 4, the melting point of the binary system noticeably decreased to 0 C reaching the first minimum, but it increased with further addition of EMIHSO 4 to 65 wt%. At compositions greater than 65 wt% EMIHSO 4, the phase diagram exhibited a second minimum with T g of approximately 61 C corresponding to a eutectic region. The eutectic EMIHSO 4 -ImHSO 4 mixture was obtained over a wider range of compositions compared to MIHSO 4 -ImHSO 4 (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. 161

182 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-8 Phase diagram for EMIHSO 4 -ImHSO 4 binary system: ( ) melting point; ( ) solid-solid transition; ( ) glass transition Performance of RuO 2 in eutectic PILs To determine the proton conduction in these liquid binary mixtures, their electrochemical performance was investigated using RuO 2 as pseudocapacitive electrodes. The electrochemical properties of the eutectic MIHSO 4 -ImHSO 4 and EMIHSO 4 -ImHSO 4 were characterized at 70:30 percentage ratio and compared to that of pure EMIHSO 4 and are shown in Table 7-7. The ionic conductivity of EMIHSO 4 -ImHSO 4 was similar to that of EMIHSO 4, whereas MIHSO 4 -ImHSO 4 exhibited slightly higher ionic conductivity. This could be due to greater available protons contributed by both MIHSO 4 and ImHSO 4. To verify this hypothesis, the performance of RuO 2 cells with EMIHSO 4, EMIHSO 4 -ImHSO 4, and MIHSO 4 -ImHSO 4 electrolytes were analyzed, and their CV profiles are shown in Figure

183 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Table 7-7 Conductivity of pure EMIHSO 4, eutectic EMIHSO 4 -ImHSO 4, and eutectic MIHSO 4 -ImHSO 4 and the capacitance of RuO 2 cells enabled with respective PILs. Electrolytes Conductivity (ms cm -1 ) Capacitance of RuO 2 cell (mf cm -2 ) Cʹ (EIS) a C (5 mv s -1 ) EMIHSO EMIHSO 4 -ImHSO 4 (70:30) MIHSO 4 -ImHSO 4 (70:30) a capacitance was extracted at Hz which corresponds to DC measurements at 5 mv s -1 * capacitance of RuO 2 cell in 0.5M H 2 SO 4 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 RuO 2 cell is the highest with MIHSO 4 -ImHSO 4, followed by EMIHSO 4 -ImHSO 4 and EMIHSO 4 electrolytes. This trend implied that the amount of protons or protonated ions is higher in MIHSO 4 -ImHSO 4, in which the cations from both MIHSO 4 and ImHSO 4 can contribute to the proton conduction. Consequently, the available proton species are reduced in EMIHSO 4 -ImHSO 4 as the cation from ImHSO 4 would be the dominating proton conductor, and is further decreased in pure EMIHSO 4 (i.e., EMI cation with no active protons). Figure 7-9 Cyclic voltammograms of RuO 2 cells using pure EMIHSO 4, eutectic EMIHSO 4 -ImHSO 4 (70:30), and eutectic MIHSO 4 -ImHSO 4 (70:30) at 5 mv s

184 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Although the capacitance of RuO 2 cell with MIHSO 4 -ImHSO 4 (43 mf cm -2 ) was lower than that with H 2 SO 4 (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 H 2 SO 4 electrolyte. The performance of RuO 2 in the eutectic PILs was comparable to a few available studies where they have used fluorinated PILs. The capacitance of RuO 2 electrode using EMIHSO 4 -ImHSO 4 (37 F g -1 ) 1 and MIHSO 4 -ImHSO 4 (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 MIHSO 4 -ImHSO 4 and EMIHSO 4 -ImHSO 4 can have promising proton conduction characteristics. 7.4 Performance of Solid RuO 2 Cell with Polymer eutectic PILs To demonstrate the viability of eutectic PILs for proton conducting polymer electrolytes, eutectic EMIHSO 4 -ImHSO 4 (70:30) and eutectic MIHSO 4 -ImHSO 4 (70:30) were incorporated into PEO to form polymer electrolytes. The performance of solid RuO 2 cells enabled with these polymer electrolytes were compared to that with PVdF-HFP EMIBF 4, and their CV profiles are shown in Figure Since PVdF-HFP EMIBF 4 is not proton conducting, it serves as a baseline for double-layer capacitance. 1 The capacitances of RuO 2 electrodes in mf cm -2 were converted to F g -1 for comparison to the literature. 164

185 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE Figure 7-10 Cyclic voltammograms of solid RuO 2 cells enabled with PVdF-HFP EMIBF 4, PEO EMIHSO 4 -ImHSO 4 (eutectic 70:30), and PEO MIHSO 4 -ImHSO 4 (eutectic 70:30) at (a) 5 mv s -1 and (b) 50 mv s

186 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE The capacitance of RuO 2 cells increased in the order of PVdF-HFP EMIBF 4 PEO EMIHSO 4 -ImHSO 4 PEO MIHSO 4 -ImHSO 4 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 EMIHSO 4 -ImHSO 4 and PEO MIHSO 4 -ImHSO 4 electrolytes, enabling pseudocapacitive reactions of RuO 2, and (ii) the higher proton contribution of eutectic MIHSO 4 -ImHSO 4 than that of eutectic EMIHSO 4 -ImHSO 4 maintained in polymer state, signified by the higher capacitance of RuO 2 cell leveraging the former. Table 7-8 Capacitance of RuO 2 cells enabled with PVdF-HFP EMIBF 4, PEO EMIHSO 4 -ImHSO 4 (eutectic 70:30), and PEO MIHSO 4 -ImHSO 4 (eutectic 70:30). Electrolytes Conductivity (ms cm -1 ) Capacitance of RuO 2 cell (mf cm -2 ) C (5 mv s -1 ) C (50 mv s -1 ) PVdF-HFP EMIBF ± PEO EMIHSO 4 -ImHSO ± PEO MIHSO 4 -ImHSO ± The performance of RuO 2 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 RuO 2 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 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, 166

187 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE 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 cm 3 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 EMIHSO 4, MIHSO 4, and ImHSO 4 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 RuO 2 and carbon/pmo

188 CHAPTER 7. PROTIC IONIC LIQUIDS: LIQUID AND POLYMER STATE electrodes. Proton conductivity increased in the order of EMIHSO 4 MIHSO 4 ImHSO 4 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 HSO 4 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 EMIHSO 4 -ImHSO 4 and MIHSO 4 -ImHSO 4 were determined. Constructing phase diagrams, binary PIL liquids with eutectic compositions over a wide temperature range were obtained. The performance of RuO 2 pseudocapacitors in EMIHSO 4 -ImHSO 4 and in MIHSO 4 -ImHSO 4 confirmed the proton activity of these eutectic binary PILs; higher in MIHSO 4 -ImHSO 4 binary system constituted of both proton-containing cations. The proton conductivity of eutectic binary PILs maintained in polymer electrolytes. Solid RuO 2 pseudocapacitors enabled with PEO EMIHSO 4 -ImHSO 4 and PEO MIHSO 4 -ImHSO 4 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. 168

189 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 EMIHSO 4 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 169

190 CHAPTER 8. CONCLUSIONS AND FUTURE WORK 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 EMIHSO 4 and EMIBF 4 were investigated. The effects of anion and cation on these properties were studied. (1) EMIHSO 4 with non-fluorinated anion (HSO 4 ) exhibited strong ion-ion interactions, resulting in higher viscosity and lower conductivity than that of EMIBF 4 with less polarizable anion (BF 4 ). The high viscosity of EMIHSO 4 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 EMIHSO 4. Conductivity and potential window at the optimum concentration were in the level of common organic electrolytes. (3) The substitution of cationic alkyl groups of EMIHSO 4 with shorter alkyl chains and/or protons in MIHSO 4 and ImHSO 4 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 170

191 CHAPTER 8. CONCLUSIONS AND FUTURE WORK capacitance of MIHSO 4 and ImHSO 4 solutions, suggesting a higher proton dissociation than that of EMIHSO 4 solution. Polymer IL electrolytes: Thin-film and flexible PEO EMIHSO 4 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 EMIHSO 4 and PVdF-HFP EMIBF 4. (4) The ionic conductivity of PEO EMIHSO 4 was 0.8 ms cm -1 at room temperature. Despite the higher viscosity of EMIHSO 4 than that of EMIBF 4, the decrease of ionic conductivity in polymer state was less noticeable than that of PVdF-HFP EMIBF 4. The lower activation energy of ionic conduction in PEO EMIHSO 4 than that in pure EMIHSO 4 supported the notion of different ion transport mechanism in polymer which is less affected by high viscosity. (5) Addition of EMIHSO 4 into PEO substantially decreased the crystallinity of the polymer (by 48%) and lowered its melting point. A similar effect of EMIBF 4 on the structure of PVdF-HFP confirmed that the ILs act as ionic conductors and plasticizers in polymer electrolytes. (6) An interaction between HSO 4 and ether oxygen in PEO was revealed, which enhanced the dissociation of the IL into EMI + and HSO 4 in polymer state. (7) The capacitance of EDLCs leveraging the environmental friendly PEO EMIHSO 4 electrolyte was comparable to that of PVdF-HFP EMIBF 4 -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 EMIHSO 4 exceeded the performance of the liquid counterpart devices, especially at high rates. 171

192 CHAPTER 8. CONCLUSIONS AND FUTURE WORK Polymer IL with fillers: Amorphous SiO 2 and crystalline TiO 2 nanofillers were incorporated into PEO EMIHSO 4 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 SiO 2 and TiO 2 nanofillers into PEO EMIHSO 4 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 EMIHSO 4 -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. TiO 2, with a much larger than SiO 2, significantly increased ionic conductivity and capacitance of PEO EMIHSO 4 -based metallic cells at high temperatures, where polymer is amorphous. (10) The ionic conductivity of PEO EMIHSO 4 increased from 0.8 to 2.1 ms cm -1 with the addition of SiO 2, approaching the conductivity of the PVdF-HFP EMIBF 4 at room temperature. For high temperature applications, the ionic conductivity of PEO EMIHSO 4 TiO 2 exceeded the fluorinated polymer IL. 172

193 CHAPTER 8. CONCLUSIONS AND FUTURE WORK Protic ILs and proton conducting polymer PILs: The proton conductivity of EMIHSO 4, MIHSO 4, and ImHSO 4 (i.e., ILs with different cationic functional groups) were examined in their respective methanol solutions. (11) The pseudocapacitive behavior of RuO 2 and carbon/pmo 12 electrodes employing the three IL solution electrolytes confirmed their proton activity. The proton conductivity increased in the order of EMIHSO 4 MIHSO 4 ImHSO 4 as a result of the dissociation of protons of imidazolium cation in MIHSO 4 and ImHSO 4. (12) Binary PIL liquids with eutectic compositions down to 70 C were developed. Both EMIHSO 4 -ImHSO 4 and MIHSO 4 -ImHSO 4 binary systems exhibited proton activity, and the proton conductivity was higher for MIHSO 4 -ImHSO 4 binary PILs due to the presence of both proton-containing MI and Im cations. (13) Polymer PILs were developed by incorporating EMIHSO 4 -ImHSO 4 and MIHSO 4 -ImHSO 4 eutectic binary systems into PEO. Their activities were demonstrated in solid RuO 2 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 173

194 CHAPTER 8. CONCLUSIONS AND FUTURE WORK 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 (HSO 4 ), where an OH group is replaced by NH 2. 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. 174

195 CHAPTER 8. CONCLUSIONS AND FUTURE WORK 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. 175

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209 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] (vs. Fc/Fc + ) 2.3 (vs. Ag/AgCl) 1.5 (vs. Ag/AgCl) GC pyrrolidinium trifluoroacetate [Pyr][TFA] pyrrolidinium nitrate [Pyr][NO 3 ] 2-methylpyridinium trifluoromethanesulfonate (triflate) [2-MePy][Tf] (1:2) 2-methylpyridinium formate [2-MePy][HCOO] 4-methylpyridinium trifluoroacetate [4-MePy][TFA] (1:2) 4-ethylpyridinium trifluoroacetate [4-EtPy][TFA] (1:2) (vs. Ag/AgCl) GC (vs. Ag/AgCl) GC GC-Pt (vs. Ag wire ) GC-Pt (vs. Ag wire ) GC-Pt (vs. Ag wire) 2-methylpyridinium trifluoroacetate [2-MePy][TFA] (1:2) 3-methylpyridinium trifluoroacetate [3-MePy][TFA] (1:2) 3-ethylpyridinium trifluoroacetate [EtPy][TFA] (1:2) 2-ethylpyridinium trifluoroacetate [EtPy][TFA] (1:2) GC-Pt (vs. Ag wire ) GC-Pt (vs. Ag wire) GC-Pt (vs. Ag wire) GC-Pt (vs. Ag wire) 189

210 Table A-1 (Continued) PIL Diethanolammonium formate [DEA][HCOO] pyrrolidinum acetate [Pyrr][AC] N-ethylimidazolium bis(trifluoromethyl sulfonimide) [EtIm][TFSI] Diethanolammonium sulfamate [DEA][OSA] 2-pentylpyridinium trifluoroacetate [PentylPy][TFA] (1:2) Pyrrolidinium hydrogen sulfate [Pyrr][HSO 4 ] GC: glassy carbon; Pt: platinum; Au: gold electrodes σ (mscm -1 ) η (cp) E (V) GC-Au (vs. Ag wire) GC (vs. Ag/AgCl) GC-Au (vs. Ag wire) GC-Pt (vs. Ag wire) GC (vs. Ag/AgCl) 190

211 Table A-2 Polymer IL systems developed by polymerization in ILs, and their ionic conductivity and potential window [65,123, ]. Monomer IL Solvent plasticizer initiator Technique σ (ms/cm) Methyl methacrylate [BMI][PF 6 ] THF AIBN free radical polymerization 0.2 (MMA) [EMI][TFSI] BPO free radical polymerization 6.3 E(V) 2 2-hydroxyethyl methacrylate (HEMA) [EMI][BF 4 ] [BPyr][BF 4 ] BPO BPO free radical polymerization free radical polymerization Poly-cation type, diallyldimethylammonium (pyrrolidinium backbone) [MBPyrr][TFSI] acetone anion exchange reaction (casting) Polyanion-type, (based on vinyl monomer acids) [EtIm][VS] ethanol alkyl group, polyether AIBN free radical polymerization 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) AIBN: azobisisobutyronitrile; BPO: benzoyl peroxide; THF: tetrahydrofuran 191

212 Table A-3 Polymer-IL systems developed by the incorporation of ILs into the matrix [85,96,138,139, ]. Polymer IL Solvent Plasticizer Technique σ poly acrylonitrile (PAN) [BMI][PF 6 ] [BMI][PF 6 ] [EMI][BF 4 ] [EMI][BF 4 ] DMF DMF DMF DMF TMS TMS casting casting casting casting (ms/cm) E (V) poly ethyleneoxide (PEO) [BMI][PF 6 ] [EMI][BF 4 ] [EMI][TFSI] [EMI][TFSI] AN AN AN AN casting casting poly vinylalcohol (PVA) [EMI][BF 4 ] [EMI][Tf] Water Water casting PVdF-HFP [EMI][BF 4 ] [EMI][Tf] [BMI][PF 6 ] [TEA][BF 4 ] [MMPI][TFSI] [MMBI][TFSI] PC MP MP EC+PC Acetone Acetone sandwiching sandwiching poly methyl methacrylate (PMMA) DMF: N,N-dimethyl formamide; MP: Methyl-2-pentanone [EMI][Tf] Water

213 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) I c /I a PEO powder PEO film PEO EMIHSO 4 (1:1) PEO EMIHSO 4 (1:2) PEO EMIHSO 4 (1:3) PEO EMIHSO 4 3% SiO PEO EMIHSO 4 10% SiO PEO EMIHSO 4 3% TiO PEO EMIHSO 4 10% TiO PVdF-HFP powder PVdF film PVdF-HFP EMIBF PVdF-HFP EMIBF 4 3% SiO PVdF-HFP EMIBF 4 3% TiO

214 Figure B-1 Heating and cooling DSC thermograms for PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 electrolytes Figure B-1 shows DSC thermograms of PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 3% SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2. As summarized in Table B-2, the addition of 3 wt% filler to PVdF-HFP EMIBF 4 resulted in somewhat similar behavior to that of PEO EMIHSO 4 : the crystallinity of PVdF-HFP EMIBF 4 slightly decreased with the addition of SiO 2, while the crystallinity was unaffected in PVdF-HFP EMIBF 4 3% TiO 2. Table B-2 Melting temperature (T m ), recrystallization temperature (T rc ), and crystallinity (X c ) of PVdF-HFP film, PVdF-HFP EMIBF 4, PVdF-HFP EMIBF 4 SiO 2, and PVdF-HFP EMIBF 4 3% TiO 2 electrolytes. Samples T m ( C) T rc ( C) X c (%) PVdF-HFP film PVdF-HFP EMIBF PVdF-HFP EMIBF 4 3% SiO PVdF-HFP EMIBF 4 3% TiO

215 Figure B-2 Variation of (a) dielectric permittivity and (b) dielectric loss with respect to frequency for PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 at 30 C Figure B-2 show and as a function of frequency for PEO EMIHSO 4, PEO EMIHSO 4 SiO 2, and PEO EMIHSO 4 TiO 2 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). 195

216 Figure B-3 DSC thermograms of EMIHSO 4 -ImHSO 4 binary mixtures at heating and cooling scans of 10 C min -1 Figure B-3 shows DSC thermograms of the EMIHSO 4 :ImHSO 4 mixtures with weight percentage ratios at 50:50, 65:35, 70:30, and 80:20. The binary mixtures with 50 wt% EMIHSO 4 and 50 wt% ImHSO 4 showed a melting peak at 0 C (first minimum), which is lower than that of EMIHSO 4 and ImHSO 4. At 70 wt% EMIHSO 4 and 30 wt% ImHSO 4, and above this composition, the binary mixture reached eutectic compositions (second minimum). 196

217 APPENDIX C: MATERIALS WEIGHT DISTRIBUTION Figure C-1 Materials weight distribution for solid cells enabled by PEO EMIHSO 4 (left) and PVdF-HFP EMIBF 4 (right) (1 cm 2 laminated pouch-type cells) Figure C-1 shows the weight distribution for EDLCs fabricated with PEO EMIHSO 4 and PVdF- HFP EMIHBF 4. 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 197