Alkaline Polymer Electrolytes for Electrochemical Capacitors

Transcription

1 Alkaline Polymer Electrolytes for Electrochemical Capacitors by Jak Li A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Material Science and Engineering University of Toronto Copyright by Jak Li 2015

2 Alkaline Polymer Electrolytes for Electrochemical Capacitors Abstract Jak Li Master of Applied Science Material Science and Engineering University of Toronto 2015 Polymer electrolytes for electrochemical capacitors in place of liquid electrolytes were developed to prevent issues such as leakage and safety concerns. Currently, there is a lack of available OH - -ion conducting polymer electrolytes compared to Li + -ion and H + -ion conducting ones. OH - -ion conducting polymer electrolytes with high ionic conductivity and environmental stability in ambient conditions are required. TEAOH-PVA polymer electrolyte, a suitable replacement for KOH-PVA, demonstrated similar pristine ionic conductivities and superior shelf-life in ambient conditions. TEAOH-PVA also revealed the highest ionic conductivity compared to TEAOH-PEO and TEAOH-PAA polymer electrolytes. Optimization of TEAOH-PVA via light cross-linking produced a polymer electrolyte with a high ionic conductivity of 1 x 10-2 S cm -1 and stable shelf-life over a period of 80 days. An EDLC device made with this polymer electrolyte yielded an excellent capacitance of 110 μf at 120 Hz, appropriate for high frequency filtering applications. ii

3 Acknowledgements First and foremost, I would like to extend my appreciation to my supervisor Professor Keryn Lian for giving me an opportunity to be a part of the Flexible Energy and Electronics Laboratory (FEEL). Her guidance, support and encouragements have profoundly impacted my growth as a student, researcher and person. Second, I would like to thank past and present students in the FEEL. Han Gao for his mentorship, ingenuity and expertise in the development of polymer electrolytes; Sanaz Ketabi, Matthew Genovese, Haoran (George) Wu, and Blair Decker for countless discussions and thought provoking ideas that were generated during the past couple of years; and Alvin Virya and Yee Wei Foong for their dedication and assistance in FEEL. Third, I would like to thank Dan Grozea for his encouragement to my work and support in FTIR and DSC experiments and George Kretschmann for assisting me with XRD experiments. I would like to recognize a deep gratitude towards my friends, family and faith community that have continuously embraced and encouraged my research endeavours. Finally, I thank my Lord, Jesus Christ, for giving me purpose and hope that has helped me persevere in my work. iii

4 Table of Contents Abstract... ii Acknowledgements... iii List of Tables... vi List of Figures... vii List of Appendices... xi Abbreviations... xii Introduction of Energy Storage and Electrochemical Capacitors... 1 Literature Review Electrochemical capacitors Types of Electrolytes Solid electrolytes Polymer matrices in polymer electrolytes Modification of polymer electrolytes Objectives Experimental Preparation of polymer electrolyte precursor solution Preparation of graphite electrodes Fabrication of EDLC devices using solution cast method Material characterization of polymer electrolyte materials Electrochemical characterizations of polymer electrolyte material in simple EC cells Results and Discussion Polymer electrolyte TEA-VA vs K-VA iv

5 5.2 Effect of polymer matrices Crosslinking Alkaline TEAOH-PVA Polymer Electrolyte Conclusions Future Work References Appendices v

6 List of Tables Table 2-1: Summary of comparisons between the different types of electrolytes Table 2-2: Diffusivity, stoke ionic radius and hydrated ionic radius of cations of the studied hydroxides...12 Table 4-1: Chemicals used for the preparation of polymer electrolytes...35 Table 4-2: Polymer electrolyte composition...37 Table 5-1: Area capacitance of K-VA and TEA-VA in metallic solid cells on Day 1 and Day Table 5-2: Variations of capacitance and time constants of a graphite/tea-va EC and a graphite K-VA EC, obtained from EIS and CV over time...52 Table 5-3: Overview of potential polymer matrices with their respective functional groups, glass transition temperature, melting temperature and crystallinity nature...54 Table 5-4: Ionic conductivity of TEA-VA, TEA-EO and TEA-AA metallic solid cells on Day 1, 4 and 43 of tracking in ambient conditions...63 Table 5-5: Areal capacitance of TEA-VA, TEA-EO and TEA-AA metallic cells on Day 1 and Day 43 of tracking...65 Table 5-6: Summary of thermal events for TEAOH, PVA and TEA-VA...71 Table 5-7: Summary of thermal events for TEA-VA, TEA-VA G2(0.5%) and TEA-VA G2(5.0%)...72 Table 5-8: Summary of ionic conductivities of TEA-VA and TEA-VA G2(0.5%) at pristine, 75 and 45% RH conditions Table 5-9: Summary of capacitances of TEA-VA and TEA-VA G2(0.5%) on Day 1 and Day Table 6-1: Summary of developed OH - -ion conducting polymer electrolytes vi

7 List of Figures Figure 1-1: Ragone plot of various energy storage technologies [4] Figure 1-2: Electrochemical process differences between supercapacitor cathode (A), anode (C) and battery cathode (B) and anode (D) [3] Figure 2-1: Electrochemical cells made with traditional liquid electrolytes (left) and solid electrolytes (right). Adapted from [14] Figure 2-2: Schematic of proton conduction mechanism in liquid water via the inversion between different proton complexes. Adapted from [1] Figure 2-3: Schematic of OH - -ion conduction in liquid water involving the formation and breaking of hypercoordinated complexes between OH - -ion and water. The process is known as structural diffusion [11] Figure 2-4: (a) ph and (b) ionic conductivities are compared at 0.1, 0.5 and 1 M concentrations in water Figure 2-5: Segmental motion of polymer electrolyte facilitating the movement of Li+ions [2] Figure 2-6: The effect of plasticizing on polymer electrolyte structure illustrated via XRD patterns (a) and the conductivity of the polymer electrolyte (b). Adapted from [13] Figure 2-7: Comparison of EC cells with and without Na2MoO4 in H2SO4/PVA gel electrolyte: (a) CV curves at 10 mv s -1 (b) charge-discharge curves at 1.56 A g -1. Adapted from [17] Figure 2-8: Ionic conductivity as a function of KOH content of anhydrous KOH/PVA polymer electrolyte [15] Figure 2-9: (A) CV of porous carbon electrode in 8 M KOH solution and (B) CV of EDLC with PVA/KOH polymer electrolyte at various scan rates. Adapted from [10]...24 Figure 2-10: Ionic conductivity of polymer blends between PVA and PAA at different compositions of (a) 10:3, (b) 10:5 and (c) 10:7.5 after being immersed in KOH for 24 hrs [9]. The 10:5 blend improved both mechanical strength and ionic conductivity Figure 2-11: Schematic of polymer structure changes from linear to branched to crosslinked [6] Figure 2-12: Formation of radicals in the PEO backbone to propagate chemical bond formation between the benzophenone and other parts of the PEO backbone [12] vii

8 Figure 2-13: Schematic of reaction between PVA and GA and the formation of (A) fully cross-linked species and (B) partially cross-linked species. Adapted from [8] Figure 2-14: Design and cross-linking scheme of PAA-HEMA modified polymer, where (a) shows the HEMA modified PAA and (b) shows the cross-linked network via photopolymerization [16] Figure 2-15: Ionic conductivity of SiWA-PVA-H3PO4 polymer electrolytes with varying degrees of cross-linking [7] Figure 4-1: Preparation of graphite ink electrodes Figure 4-2: Schematic of preparation of a simple EC cell. The active area of the cell is 1 cm 2 and the thickness is denoted by "l" Figure 4-3: Schematic diagraph of the potentiostat setup. WE: working electrode, CE: counter electrode, RE: reference electrode, PE: polymer electrolyte Figure 4-4: Ideal capacitive responses for an EDLC device from (a) CV and (b) CCD..42 Figure 4-5: Schematic of Nyquist plot for EDLC with ideal capacitive response Figure 4-6: Schematic of real and imaginary capacitance plotted against frequency Figure 5-1: The ionic conductivity of different compositions of TEAOH-PVA [5]...45 Figure 5-2: K-VA and TEA-VA polymer electrolyte physical appearances after 3 days. 46 Figure 5-3: X-ray powder diffraction of K-VA and TEA-VA at (a) 40% RH and (b) 30% RH...47 Figure 5-4: TGA of K-VA and TEA-VA from 30 to 120 C Figure 5-5: Ionic conductivity as a function of time for K-VA and TEA-VA polymer electrolytes Figure 5-6: CVs of solid metallic K-VA EC and TEA-VA EC at (a) day 1, and (b) day 40 (sweep rate = 5000 V s -1 )...50 Figure 5-7: CVs of solid graphite K-VA EC and TEA-VA EC at (a) day 1, (b) day 16, (c) day 32 and (d) day 67 (sweep rate = 1 V s -1 )...51 Figure 5-8: Real and imaginary capacitances of graphite electrode ECs with K-VA (a, b) and TEA-VA (c, d) plotted against frequency after day 1, 16, 32 and Figure 5-9: Ionic conductivity map of (a) TEA-VA, TEA-EO and TEA-AA polymer electrolytes and (b) aqueous TEAOH viii

9 Figure 5-10: Optical observations of TEA-VA, TEA-EO and TEA-AA films Figure 5-11: XRD of TEA-VA, TEA-EO and TEA-AA polymer electrolyte films conditioned at (a) 75% RH and (b) 45% RH Figure 5-12: IR spectra of pure polymers from (a) cm -1 and (b) cm -1. IR spectra of TEA-VA, TEA-EO and TEA-AA at 3000:1 molar ratios from (c) cm -1 and (d) cm Figure 5-13: Thermal analysis of (a) PVA, PEO, PAA and their respective polymer electrolyte systems (b) TEA-VA, TEA-EO, TEA-AA Figure 5-14: Ionic conductivity of TEA-VA, TEA-EO and TEA-AA in metallic ECs tracked over a period of 43 days in ambient conditions Figure 5-15: CVs of TEA-VA, TEA-EO and TEA-AA in metallic ECs on (a) day 1 and (b) day 43. (sweep rate = 5000 V s -1 )...65 Figure 5-16: Ionic conductivity mapping of TEA-VA as a function of degree of crosslinking. Cells were conditioned in 45% RH for three weeks prior to testing Figure 5-17: Difference in film properties between (a) TEA-VA and (b) TEA-VA G2(0.5%) after wetting the film with two drops of water Figure 5-18: XRD spectra of TEA-VA and TEA-VA G2(0.5%) films at (a) 40% RH and (b) 30% RH...69 Figure 5-19: Thermal analyzes of (a) TEAOH, (b) PVA and (c) TEA-VA (4220:1) molar ratio via DSC (solid), TGA (dash-dot) and DTG (short dash). Samples were conditioned at 30% RH...70 Figure 5-20: Thermal analysis of TEA-VA and TEA-VA G2(0.5%) via DSC (solid), TGA (dash-dot) and DTG (short dash). Samples were conditioned at 30% RH Figure 5-21: FTIR measurement of (a) TEA-VA vs TEA-VA G2(0.5%). Semi-quantitative analysis of changes in (b) hydroxyl as a function of cross-linking Figure 5-22: Absorbance ratio of (a) aldehyde peak and (b) acetal peak with increase in GA content, adapted from [8], and (c) aldehyde and (d) acetal peaks from this study...75 Figure 5-23: Ionic conductivity tracking over 80 days of TEA-VA and TEA-VA G2(0.5%) being conditioned at 75 and 45% RH Figure 5-24: CV of metallic EC cells made with TEA-VA and TEA-VA G2(0.5%) polymer electrolytes on (a) day 1 and (b) day 79 after conditioning in both 75 and 45% RH. (sweep rate = 5000 V s -1 )...78 ix

10 Figure 5-25: Cycle-life test of (a) TEA-VA and (b) TEA-VA G2(0.5%) after storage for one year and conditioned at 45% RH. The cycling was performed in ambient conditions (Sweep rate = 5000 V s -1 )...79 Figure 5-26: Sandwiched graphene EDLC using TEA-VA G2(0.5%) polymer electrolyte. The sample was sandwiched with glass slides and clipped for intimate contact between the electrodes and the electrolyte...80 Figure 5-27: Nyquist (left) and Bode (right) plots of EDLC devices prepared from graphene electrodes and TEA-VA G2(0.5%) polymer electrolyte...81 Figure 5-28: Electrochemical characterization of graphene/tea-va G2(0.5%) EDLC devices through (a) CV at 1000 V s -1 and (b) charge-discharge at 5 ma Figure 5-29: Real capacitance plotted against frequency of EDLC device with graphene electrode and TEA-VA G2(0.5%) polymer electrolyte...82 x

11 List of Appendices Appendix A-1: Phase diagram of KOH and H2O Appendix A-2: XRD pattern of TEAOH hydrates Appendix B-1: FTIR wavenumber assignment for polymer electrolytes Appendix B-2: Water content of polymer electrolytes in different RH conditions Appendix B-3: DSC of TEAOH in different RH conditions Appendix B-4: FTIR of various cross-linked TEA-VA polymer electrolytes xi

12 Abbreviations CV: Cyclic Voltammogram DSC: Differential Scanning Calorimetry ECs: Electrochemical Capacitors EDLC: Electric Double Layer Capacitors EIS: Electrochemical Impedance Spectroscopy ESR: Equivalent Series Resistance FTIR: Fourier Transform Infrared Spectroscopy KOH: Potassium Hydroxide PAA: Poly(acrylic acid) PEO: Poly(ethylene oxide) PVA: Poly(vinyl alcohol) RE: Reference Electrode TEAOH: Tetraethylammnonium Hydroxide TGA: Thermogravimetric Analysis WE: Working Electrode XRD: X-ray Diffraction xii

13 1 Introduction of Energy Storage and Electrochemical Capacitors The science of electrochemistry has brought many life-changing technologies into the modern era. Since Volta s discovery of the battery powered by electrochemistry in the 1800s, the demand for energy storage has increased exponentially from handheld consumer electronics to electric vehicles to grid power management. Especially when it comes to reducing the alarming rate of carbon emission growth, energy storage is essential to transition from the existing infrastructure that heavily depends on fossil fuels to an environmentally sustainable one. Among the various energy storage technologies, such as batteries and capacitors, electrochemical capacitors (ECs) bridge the gap between the low power limitations (1000 W kg -1 ) of batteries and low energy limitations of capacitors (0.1 Wh kg -1 ) as indicated in Figure 1-1 [4]. This attribute of ECs makes them an attractive option to address current energy challenges. Presently, ECs are being used in ASEA Brown Boveri s TOSA (Trolleybus Optimisation Systeme Almentation) electric bus technology [18], Maxwell Technologies Engine Start Modules for hybrid electric vehicles [19], and memory protection and back-up power in portable electronic and communications applications [20]. Commercial ECs and fundamental research to understand and extend their energy and power capacities are occurring simultaneously. ECs, like batteries, are a collection of electrochemical cells with two electrodes that are electronically conductive, a separator between the electrodes and an electrolyte with conductive ions. The electrochemical process in electric double layer capacitors (EDLCs) involves the adsorption of electrolyte ions on to the electrode material as

14 2 Figure 1-1: Ragone plot of various energy storage technologies [4]. opposed to diffusion-limited redox reactions in the bulk of the electrode as shown in Figure 1-2 (a) and (b) [3]. In the 1980s, Conway and others distinguished a second capacitive charge storage process in ECs that involved reversible redox reactions between electrode materials and electrolyte ions at the electrode/electrolyte interface. This psuedocapacitance enhanced charge storage while maintaining fast chargedischarge and long-cycle performance. The capacitance of an EC is described by the amount of charge at the surface of an electrode when a potential is applied across the electrodes (Eq.1). It has units of Farads (F). Energy is the amount of work required to move charges from one electrode to the other against the applied potential (Eq. 2 and 3). The energy of a cell is proportional to the capacitance and square of the voltage (Eq. 3). Therefore, to maximize energy, the capacitance and the operating voltage window are two key parameters.

15 3 Figure 1-2: Electrochemical process differences between supercapacitor cathode (A), anode (C) and battery cathode (B) and anode (D). From [3]. Reprinted with permission from AAAS. C = q V (Eq. 1) dw = q dq (Eq. 2) C Q E = W = q C dq = Q 2 C = 1 2 QV = 1 2 CV2 (Eq. 3) Power of a cell is defined as the rate of energy change per unit time. It is commonly described by Eq. 4. Like energy, power is also proportional to the square of the voltage and inversely proportional to the total resistance of the cell. The total resistance of the cell is a combination of resistances in the current collector, electrode material and the electrolyte. Typically, the electrolyte is the most resistive component. P = V2 R (Eq. 4)

16 4 Tremendous efforts have gone into both electrode and electrolyte materials to increase capacitance, widen voltage window and lower resistance. The focus of this thesis is on the development of electrolytes. In essence, electrolytes have three major influences in ECs: (1) operation voltage affects both energy and power, (2) resistance or ionic conductivity of the electrolyte is responsible for high power applications, and (3) appropriate ions in the electrolyte is necessary to take advantage of psuedocapacitance phenomenon for charge storage. An ideal electrolyte for EC applications would consist of properties such as: 1. High ionic conductivity 2. Large voltage windows 3. Reasonable areal capacitance when used in EDLC devices 4. Long cycle-life 5. Long shelf-life and high stability in ambient conditions 6. Easy to fabricate 7. Cost effective Electrolytes have evolved rapidly in recent years. Traditionally, liquid electrolytes were soaked in a separator, sandwiched between electrode materials and fabricated in cylindrical devices to provide maximum surface area and safe packaging. They exhibited high ionic conductivities up to 10-1 S cm -1, but resulted in bulky cell designs and leakage issues. The discovery of solid electrolytes enabled compact packaging and eliminated leakage issues, but suffered from lower conductivities. Since EC applications relied heavily on surface phenomenon, a major disadvantage when solid electrolytes were used was the lack of intimate contact between the electrode/electrolyte interfaces.

17 5 Promising systems were developed with polymers to address these issues, but with the growth of flexible and wearable electronics, advancement and understanding of ECs, particularly electrolytes are required. This thesis aims to contribute to the study of electrolytes in EC applications and the various topics are outlined below: Chapter 1: This chapter gives a brief introduction to electrochemical capacitors and outline of the thesis Chapter 2: This chapter offers a comprehensive review of available literature regarding different electrolyte systems including advantages, disadvantages and the direction of current research Chapter 3: This chapter states the objectives of this research and a detailed approach to develop new hydroxide-ion conducting polymer electrolytes Chapter 4: This chapter describes the experimental procedures including fabrication and characterization methods employed in this research Chapter 5: This chapter discusses the results and analyzes the results obtained from electrochemical and material characterization on the developed material Chapter 6: This chapter concludes the findings of this thesis and consequences thereof Chapter 7: This chapter discusses recommendations and future work

18 6 Literature Review 2.1 Electrochemical capacitors Traditional electrolytic capacitors store energy via electrostatic interaction between the electrode material and the dielectric material. Limited to small quantities of charge storage, electrolytic capacitors were usually employed for circuit protection applications. The technology of electrochemical capacitors (ECs) was first patented in 1957 by General Electric Company [21]. The first commercially successful double layer capacitor under the name super capacitor was launched by Nippon Electric Company (NEC) [22]. Later, Conway et al. described the fundamental phenomena of electric double layer capacitance (EDLC) and psuedocapacitance responsible for large increases in charge storage capacity [23]. Since then, research towards electrochemically active electrode and electrolyte materials has grown greatly [24-26]. Electrode materials were widely studied to leverage the two fundamental phenomena mentioned above. Since high surface area electrodes were desired for EDLC, carbon materials such as activated carbons, biochars, carbon nanotubes and graphene demonstrated promising performances as electrode materials [27]. For pseudocapacitance, metal oxides such as hydrous ruthenium oxides were studied and demonstrated highly increased capacitances [28]. However, due to the high cost of ruthenium oxide, finding alternative cost effective redox active species has been an active field in EC electrode research. Although studies in EC electrolytes were much less extensive compared to electrodes, their development have been increasingly important, specifically advanced solid polymer electrolytes. Figure 2-1 shows a schematic of a liquid and solid

19 7 electrochemical cell. In the liquid cell, a container is required to assemble the cell, as opposed to the freestanding electrolyte in a solid cell. This feature provides flexibility in designing any electrochemical cell. The following review on advanced solid electrolytes involves topics of: (1) ionic conductors, (2) electrolytes containing polymers, and (3) various ways to improve the ionic conductivity of polymer electrolytes. 2.2 Types of Electrolytes There are three broad groups of electrolytes: (1) aqueous, (2) organic and (3) ionic liquids (ILs). A list of properties for the three different types of electrolytes can be found in Table 2-1. Although aqueous based electrolytes are the oldest and most studied, they are limited by the decomposition of water at 1.23 V. Organic solvents have decomposition potentials of V and the decomposition of the IL ions can be designed to exceed 3 V. Using these electrolytes lead to higher energy densities for ECs, but they do not have intrinsic compatibilities with psuedocapacitive materials and Figure 2-1: Electrochemical cells made with traditional liquid electrolytes (left) and solid electrolytes (right). Adapted from [14].

20 8 are more costly compared to aqueous based electrolytes. This thesis is focused on the review and contribution of aqueous electrolytes. Table 2-1: Summary of comparisons between the different types of electrolytes. Electrolyte Voltage window Ionic conductivity Cost Toxicity Psuedocapacitance Aqueous 1 High Low Low Yes Organic Low High High No Ionic Liquid 3-6 Very low Very high Low No Hydronium and Hydroxide-ion Conducting Electrolytes Hydronium or proton conducting electrolytes are well-studied. Protons or H + -ions are the smallest available ions that demonstrate fast diffusivity. The movement of an ion from one position to another is known as ionic conductivity. Electrolytes with high ionic conductivities are required for high power applications. Protons are also required for many redox reactions, which enables psuedocapacitance in ECs. Aqueous proton conducting electrolytes are usually acids such as aqueous sulfuric acid H2SO4 or aqueous hydrochloric acid HCl with maximum ionic conductivities of ~0.75 S cm -1 at 30 and 20 % weight/weight of water (w/w) respectively [29]. H + -ion conduction in water is important in understanding many applications and was extensively studied with well-known conduction mechanisms. In ice, an H + -ion hops from one oxygen site to another, known as the Grotthuss mechanism [30, 31]. In water, an H + -ion is in the H3O + hydrated state. Movement of H3O + involves the interconversion between the (H9O4) + complex and the (H5O2) + complex as shown in Figure 2-2. This process is driven by the fluctuation of the second solvation shell of H3O + that reduces the first solvation shell coordination of H3O + [32], which lowers the overall energy. These

21 9 specific interactions between the H + -ions and water are what accounts for its high ionic conductivity. Hydroxides or OH - -ion in an aqueous solvent corresponds to an OH - -ion conducting electrolyte. OH - -ions and H + -ions are the fastest conducting ionic species, but OH - -ions have a lower diffusivity, 5.30 x 10-5 cm 2 s -1, compared to H + -ions, 9.31 x 10-5 cm 2 s -1 [33]. Just like H + -ions, OH - -ions are required for certain redox reactions to occur, enabling psuedocapacitance for materials that are active in alkaline media. This makes them great complementary electrolytes for EC applications. Alkaline electrolytes such as NaOH and KOH exhibited maximum ionic conductivities of 0.37 and 0.54 S cm - 1 at 15 and 30% w/w respectively [29, 34]. For a very long time, it was suspected that the OH - -ion conduction mechanism was strictly the proton-hole analogue of H + -ion conduction as described by Danneel (1905) and Huckel (1928). It wasn t until 2002 that alternative theories coupled with experimental results revealed more conclusive involvement of presolvated and hypercoordinated complexes in the OH - -ion conduction mechanism [33] as shown in Figure 2-3. The left schematic shows a four-fold coordinated OH - -ion in the OH - (H2O)4, Figure 2-2: Schematic of proton conduction mechanism in liquid water via the inversion between different proton complexes. Adapted from [1].

22 10 state. This state was described as the hypercoordinated complex. An approaching solvent molecule activates the transfer process where the OH - (H2O)4 complex breaks into a three-fold coordinated OH - (H2O)3 complex, while the OH - -ion donates a hydrogen bond to the approaching water molecule as shown in the center schematic. When the proton from the bulk water is transferred to the original OH - -ion, a properly solvated water molecule and a new hypercoordinated complex carrying the negative charge are formed. This state remains inactive until another solvent molecule approaches as shown in the right schematic. The transport of OH - -ions is fast because the charge transfer occurs via the formation of hydrogen bonds unlike vehicle diffusion of solvated ions, but due to the hypercoordinated complex and involvement of bulk water, it is slower than the transport of H + -ions. This type of OH - -ion transport is herein referred to as OH - - hopping. Alkaline electrolytes are a premature technology compared to acid electrolytes. It is important to investigate them to expand chemistries in EC applications and understand OH - -ion conduction in advanced solid electrolyte systems. Figure 2-3: Schematic of OH - -ion conduction in liquid water involving the formation and breaking of hypercoordinated complexes between OH - -ion and water. The process is known as structural diffusion [11].

23 11 Quaternary ammonium hydroxide Quaternary ammonium cations are positively charged polyatomic ions with a NR4 + structure where the R group are either aryl or alkyl groups. Unlike primary, secondary, tertiary ammonium cations and ammonium ions, NH4 +, the positive charge of quaternary ammonium cations remains the same regardless of the environment s ph. In the salt form, quaternary ammonium compounds are used as disinfectants, phase transfer catalysts in organic synthesis, fabric softeners and osmolytes. The tetraalkylammonium hydroxide (TAAOH) family with the structure N(CnH2n)4 + OH - 1 is an organic derivative of quaternary ammonium compounds. They are OH - -ion conducting electrolytes. The three smallest TAAOHs have a central nitrogen atom surrounded by four alkyl groups. They are obtained via treatment of tetralkylammonium halides with silver oxide [35] and are commonly used as reactants to produce other quaternary ammonium salts via neutralization with acids [36]. TAAOHs have a hydrophilic anion and a lipophilic cation that make them valuable phase transfer catalysts and modified catalysts used in many synthesis techniques [37-40]. Gao et al. performed a comparative study between TMAOH, TEAOH, TBAOH and KOH [5]. Table 2-2 shows the ionic size and diffusivity differences between the CH 3 CH 3 CH 3 CH N + 3 CH 3 O - H CH N + 3 CH 3 O - H CH 3 N + CH 3 1 CH 3 tetramethylammonium hydroxide (TMAOH) CH 3 tetraethylammonium hydroxide (TEAOH) CH 3 O - H tetrabutylammonium hydroxide (TBAOH)

24 12 cations. Figure 2-4 shows the ph and ionic conductivities of TMAOH, TEAOH, TBAOH and KOH in water with various concentrations. At 1 M, all electrolytes had a ph close to 14 indicating a similar amount of OH - -ion dissociation. TMAOH, TEAOH and TBAOH had ionic conductivities of 0.22 S cm S cm -1 and 0.11 S cm -1 respectively compared to 0.23 S cm -1 from KOH. From these potential OH - -ion conducting electrolytes, TEAOH was selected as an alternative to KOH over TMAOH and TBAOH due to the factors discussed below. Kuriyama et al. investigated TMAOH hydrates as solid ionic conductors that demonstrated high ionic conductivities via the formation of 3D hydrogen-bonded frameworks called clathrate hydrates that improved OH - -ion conduction in the solid state [41-43]. However, TMAOH was found to cause nerve and muscle damage [44], limiting its use in commercial application and was removed from further study by Gao et al. Unlike TMAOH, TEAOH and TBAOH cannot form clathrate hydrate networks because the cations cannot stabilize the specific 3D framework [45], leading to lower ionic conductivities. At the same time, they do not carry the same harmful effects as TMAOH. Ultimately, between TEAOH and TBAOH, the latter was removed from the study by Gao et al. because it had an ionic conductivity half that of TEAOH. Table 2-2: Diffusivity, stoke ionic radius and hydrated ionic radius of cations of the studied hydroxides TMA + TEA + TBA + K + Diffusivity a (x 10-5 cm 2 s -1 ) Stoke ionic radius (Å) Hydrated ionic radius (Å) a At infinite dilution.

25 13 Figure 2-4: (a) ph and (b) ionic conductivities are compared at 0.1, 0.5 and 1 M concentrations in water. In the past twenty years, the use of tetraalkylammonium hydroxides grew for alkaline fuel cell membranes, primarily by chemically synthesizing the tetraalkylammonium cations onto various polymers to facilitate OH - -ion conduction [46, 47]. Consequently, studies to understand their degradation pathways [48, 49] also emerged. Most of the examinations focused on using TMA as the cation of choice and showed promising applications in fuel cell alkaline membranes. However, using tetraalkylammonium hydroxides in battery and EC application remain sparse. 2.3 Solid electrolytes The science of solid electrolytes dates back from 1837 when Michael Faraday observed a poorly conducting material transition into a conducting state [50] to 1853 when Gaugain demonstrated an iron/air galvanic solid electrolyte cell [51]. These electrolytes remained in the field of physics until Nernst patented the Nernst glower filament in 1897 for lighting purposes and was later acknowledged as the first solid electrolyte gas cells using oxide ion conductors and platinum air electrodes [52]. Carl

26 14 Wagner and Walter Schottky made significant progress towards understanding ionic conductivity in the theory of ordered mixed phases published in 1930 that opened up new fields for electrochemistry [53]. Since then material science evolved from point defect ionic conduction materials to ionic conduction in disordered materials involving systems such as disordered ionic crystals, ionic-conducting glasses and polymer electrolytes [54]. Polymer electrolytes The development of polymer electrolytes began in the 1970s along with the emergence of multi-disciplinary research teams [55]. In 1973, a system with poly(ethylene oxide) PEO and alkali salts was first reported [56]. Later, Armand et al. proposed to use PEO solid polymer electrolytes for lithium batteries, which spurred on efforts to study and advance polymer electrolytes [57]. Particularly, sodium and lithium salts were combined with PEO as polymer electrolytes. Numerous studies demonstrated that the ionic conductivity of Na + or Li + -ions improved when the polymer electrolyte was amorphous instead of crystalline because of increased polymeric segmental motion [58, 59]. As shown in Figure 2-5, the movement of polymer chains facilitates the breaking and making of complexes between the Li + -ion and the oxygen group in PEO, which propagates movement of the ion. The crystallization of polymer electrolytes was inhibited by adding plasticizers and elastomers to keep the preferred amorphous structure [60, 61]. Interpretations of experimental ionic conductivities in largely amorphous polymer electrolytes were based on the free volume theory with the following assumptions: (1) molecular transport may occur only when voids having a volume greater than some critical value form by the redistribution of free volume and (2)

27 15 Figure 2-5: Segmental motion of polymer electrolyte facilitating the movement of Li+ions [2]. no energy is required for free volume redistribution [62]. Empirically, it is describe by the Vogel-Tamman-Fulcher (VTF) equation. Due to potential problems with the second assumption, Ratner proposed a dynamic percolation model based on a probabilistic hopping model between mutually accessible sites [63]. Over the years, many debates on the validity of these and other models have occurred. For convenience, a simplified VTF equation for conductivity is commonly used. To avoid crystallization of polymers, novel polymer electrolytes that are saltrich or polymer-in-salt were explored using polymers as binders for highly ionically conductive solids [64]. Solvents were introduced into these systems to further plasticize the polymer electrolyte [65, 66], which were identified as gel polymer electrolytes. As crystallization was prevented, new methods to improve ionic conductivity by increasing the free volume were discovered, such as incorporating inorganic fillers like ZrO2 and Al2O3 with polymer electrolytes and blending different polymers. These polymer electrolytes were referred to as hybrid polymer electrolytes [67].

28 Polymer matrices in polymer electrolytes Selecting a polymer matrix for a polymer electrolyte is complicated due to variations of polymer properties. Large molecular weight ranges, compatibilities with solvents and ionic conductors, and composition can all affect the polymer electrolyte system drastically. Although the prevention of polymeric crystalline regions is well known, there lacks a unifying theory regarding the polymer effects on the electrolyte system. A few commonly used polymers for polymer electrolytes were reviewed and studied in this thesis. Poly(ethylene oxide) Poly(ethylene oxide) (PEO) is a synthetic polyether 2 commonly known as poly(ethylene glycol) (PEG) when it has a molecular weight less than 100,000. Higher molecular weights are classified as PEO. These polymers are amphiphilic and soluble in many solvents [68]. They physically appear as waxy, white solids and have a melting point of about 67 C [69]. PEOs are nontoxic and approved by the FDA for different pharmaceutical formulations, foods and cosmetics [70]. The linear nature of PEO allows the main chains to easily fold upon each other to create crystalline regions, making it a semi-crystalline polymer. It is very adaptable and safe for many applications. The low melting point of PEO can cause potential issues for higher temperature applications. As H O O H 2 n Poly(ethylene oxide) (PEO)

29 17 a result of PEO s benefits, it is now one of the most utilized polymer matrices for polymer electrolytes. It was the first polymer used for polymer electrolyte development and fundamental understanding of ionic conductivity in polymer electrolytes. Armand provided the initial insight on the limitations of PEO due to its semicrystalline properties and suggested research in amorphous polymer electrolyte systems [71]. Nevertheless, many investigations remained on PEO based polymer electrolytes since the interactions of ions with PEO were well known and plasticizing PEO based polymer electrolyte systems was effective in overcoming its semi-crystalline limitations. Studies on PEO based hybrid polymer electrolytes to improve ionic conductivity revealed the multifaceted nature of ion transport due to the complexity of these systems [72]. For example, the addition of SiO2 to PEO-sodium iodide increased ionic conductivity from 4.1 x 10-6 to 6.9 x 10-6 S cm -1. However, adding poly(methyl methacrylate) (PMMA) to PEO-sodium iodide increased the ionic conductivity to 2.1 x 10-5 S cm -1 [73]. Even though the ionic conductivity increase with addition of PMMA was much higher than adding SiO2, explanations were not available. Another example of high ionic conductivity improvement from 1 x 10-7 to 5 x 10-5 S cm -1 was reported with the addition of poly(ethylene glycol) dimethyl ether to a PEO-Zn(II) system [74]. Effects of structural change on ionic conductivity can be seen in Figure 2-6, where amorphous polymer electrolytes demonstrated enhanced ionic conductivities from 2.63 x 10-7 up to 4.85 x 10-4 S cm -1 [13]. In the study, a quaternary electrolyte system, PEO- Li1.3Al0.3Ti0.7(PO4)3-LiClO4-PC was used to show that complexation between the Li + -ion and plasticizer reduced the interaction between the Li + -ion and PEO, thus improving ion mobility.

30 18 Figure 2-6: The effect of plasticizing on polymer electrolyte structure illustrated via XRD patterns (a) and the conductivity of the polymer electrolyte (b). Adapted from [13]. The above findings revealed that the inherent ionic conductivity of a salt in PEO was ~ S cm -1, but appropriate fillers or plasticizers improved the ionic conductivity up to ~10-4 S cm -1. At this point, there is sufficient understanding that fillers and plasticizers are effective in improving the flexibility and mobility of the polymer chains, but there is no conclusive evidence of a universally appropriate filler or plasticizer for polymer electrolytes. This approach generally worked, but lacks fundamental knowledge to intentionally design the ionic conductivity of polymer electrolytes. The above studies were focused on battery applications, where PEO based polymer electrolytes for EC applications are limited. Pioneering work by Ketabi et al. demonstrated an ionic liquid, 1-ethyl-3methylimidazolium hydrogensulfate (EMIHSO4) based polymer electrolyte. PEO was used to form a film due to the hydrophilic compatibility between it and the ionic liquid and an ionic conductivity of 7.5 x 10-4 S cm -1 at room temperature for EC applications was reported [75].

31 19 PEO-based polymer electrolytes evolved tremendously throughout the decades and reached ionic conductivities close to ~10-3 S cm -1, a key figure for EC application purposes. Important lessons regarding specific techniques to improve ionic conductivity were discovered, but the intrinsic crystallinity of PEO remains to be a challenge for PEO based polymer electrolytes. Poly(vinyl alcohol) Poly(vinyl alcohol) (PVA) is another well-studied polymer with a hydroxyl functional group as a part of its structure 3. It has a melting point of 230 C and a glass transition temperature of 85 C for fully hydrolyzed grades [76]. PVA is also a semicrystalline polymer being able to form crystalline regions through hydrogen bonding between its hydroxyl groups. It is nontoxic, biodegradable and water soluble. Coupled with its excellent film forming capabilities, it also has a wide range of applications. Due to its hydrophilic nature, its physical properties change readily with relative humidity (RH) since the absorbed water acts as a plasticizer for the polymer. This change in physical properties can be a potential issue in terms of environmental stability. PVA was first introduced into the field of electrochemical energy storage as a separator material for batteries [77]. Parallel to the initial development of PEO based Li + -ion conducting polymer electrolytes, permeation, diffusion and ionic conductivity of 3 OH n poly(vinyl alcohol) (PVA)

32 20 ions in hydrophilic polymers such as PVA were extensively studied. Since PVA is hydrophilic, water content is highly influential in the mobility of ions. The forming and breaking of bonds between polymer and ions in dry polymers were contrasted from the vehicle diffusion of ions in wet polymers [78, 79]. It was found that increased ion hydration and water content were the main factors that influenced ion movement in PVA, as opposed to the complexation of ions in PEO based systems. PVA based polymer electrolytes were first developed as proton conductors with H3PO4 and PVA used for hydrogen sensor applications by Polak et al. [80] These polymer electrolytes suffered from poor mechanical properties and were susceptible to dissolution in water. One particular way that improved modulus and decreased solubility of this polymer electrolyte was through the inclusion of methacrylic acid and methylenebisacrylamide followed by electron beam treatment [81]. Due to the demand for energy storage, PVA based polymer electrolytes were explored for these applications. The PVA/H3PO4 polymer electrolyte, first introduced by Polak et al., was proposed to be used for zinc/halogen batteries due to its compatibility with the PVA blinder in zinc/halogen electrodes and its large temperature range tolerance compared to other proton conducting solid polymer electrolytes. An acceptable ionic conductivity at room temperature of 3 x 10-5 S cm -1 was reported, but exact composition was not mentioned in the study [82]. A more thorough stoichiometric study of PVA/H3PO4 polymer electrolytes based on the molar ratio of proton per PVA monomer unit revealed complexation between the acid and the polymer. An optimal ionic conductivity of ~10-3 S cm -1 at a molar ratio of 0.77 H + /VA was achieved and attributed to increased conducting

33 21 species and the plasticizing effect of H3PO4 on PVA [70]. H3PO4/PVA polymer electrolyte systems are frequently used in EC applications. Kaempgen et al. demonstrated printable thin film EDLCs with single-wall carbon nanotubes (SWCNT) as electrodes [83] and El-Kady et al. used the same polymer electrolyte with graphene [84]. Both showed improved capacitance compared to an aqueous electrolyte. H2SO4/PVA is another common polymer electrolyte system for ECs. It was used with 3D graphene hydrogel electrolytes and compared to 1M H2SO4 [85]. The solid device showed improved stability and lower leakage current compared to the liquid device. The addition of a redox active mediator (Na2MoO4) to H2SO4/PVA based polymer electrolytes improved specific capacitance due to pseudocapacitive behaviors [17], but is liable to limitations in rate performance of the solid state device as shown by Senthilkumar et al. in Figure 2-7. PVA is suitable polymer with acids and form complexes that facilitate proton conduction. Its importance in ECs is critical and continues to grow. Figure 2-7: Comparison of EC cells with and without Na2MoO4 in H2SO4/PVA gel electrolyte: (a) CV curves at 10 mv s -1 (b) charge-discharge curves at 1.56 A g -1. Adapted from [17].

34 22 Alkaline polymer electrolytes technology lag that of acid polymer electrolytes, but they are equally important for energy storage applications. Fauvarque et al. first proposed to use KOH with PEO in anhydrous and water containing conditions. An ionic conductivity of ~10-3 S cm -1 was obtained with 50 wt% KOH [86]. Since PVA was water soluble, it was natural to assume its use in alkaline electrolytes. The polymer of choice for alkaline electrolytes shifted to PVA when Lian et al. (Motorola Inc.) patented its use with various metal hydroxides for zinc based rechargeable electrochemical cells. This KOH/PVA polymer electrolyte showed a quarter of the capacitance compared to aqueous KOH, but a two magnitude increase in cycle life [87]. Ionic conductivity of the polymer electrolyte system was not specifically mentioned in the patent. Lewandowski et al. showed that hydrated KOH/PVA has an ionic conductivity of ~10-3 S cm -1 [88]. This value was confirmed by Mohamad et al. with an anhydrous PVA/KOH systems that presented an ionic conductivity of 8.5 x 10-4 S cm -1 [15]. As shown in Figure 2-8, the ionic conductivity plateaued after 40 wt% KOH. The study reported that above 40 wt% KOH, the polymer electrolyte crystallinity increased. It became mechanically unstable and difficult to measure the ionic conductivity above 60 wt%. In the alkaline PVA based polymer system, the ionic conductivity increases with the presence of water, which prevents crystallization. However, the mechanical strength of PVA decreases with increased water content and a balance is required for specific applications. A hybrid polymer electrolyte KOH-PEO-PVA-H2O was studied by Yang for Ni-MH battery systems and showed a remarkable ionic conductivity of 3.8 x 10-2 S cm -1 at room temperature [89]. Wu et al. also demonstrated the use of nanofillers such as TiO2 in KOH-PVA-H2O polymer electrolytes for Zn-Ni secondary battery applications.

35 23 Figure 2-8: Ionic conductivity as a function of KOH content of anhydrous KOH/PVA polymer electrolyte [15]. Ionic conductivities of to S cm -1 were measured, approaching the liquid electrolyte systems [90]. Yang also reported using KOH/PVA for EDLC applications that demonstrated ionic conductivities on the order of 10-2 S cm -1 [10]. Figure 2-9 shows the CVs of porous carbon in KOH solution and EDLC devices with the KOH/PVA polymer electrolyte. The solid EDLC device showed rectangular shaped CVs and exhibited capacitances close to the electrodes with liquid electrolyte around 100 F g -1 even after 1000 cycles. Although these alkaline polymer electrolytes demonstrated excellent ionic conductivities for battery and EC applications, all the cells required lamination and a container for assembly, which hinders scale up possibilities. In the last five years, many advanced polymer electrolyte systems based on PVA were developed. From lithium-air battery applications that used PVA-lithium bis(oxalate)borate with ionic conductivities of ~10-4 S cm -1 [91] to PVA-ionic liquid

36 24 Figure 2-9: (A) CV of porous carbon electrode in 8 M KOH solution and (B) CV of EDLC with PVA/KOH polymer electrolyte at various scan rates. Adapted from [10] polymer electrolytes with 7.31 x 10-3 S cm -1 ionic conductivities for supercapacitors to novel H + -ion and OH - -ion conducting PVA based polymer electrolytes, PVA-H3PO4- SiWA-TiO2 and PVA-TEAOH respectively with ionic conductivities of ~10-2 S cm -1 [5, 92], PVA is a promising polymer for polymer electrolytes. Poly(acrylic acid) Poly(acrylic acid) (PAA) is a polyelectroyte with a carboxylic acid functional group 4. It appears as a white powder and can absorb water many times its own weight. It has a glass transition temperature of 106 C and at temperatures above 200 C it O OH 4 n Poly(acrylic acid) (PAA)

37 25 becomes an insoluble polymer anhydride [93]. Its lack of a melting temperature is an amorphous polymer characteristic, mainly due to its bulky functional group hindering the chains from packing. Because PAA can absorb water readily, it was first used in diapers in 1982 [94]. The carboxylic acid functional group in PAA can present a charge under certain environments. For example, in an aqueous solution, the carboxylic groups can be hydrolyzed to carry a negative charge. Due to this, it is weakly acidic and a mild irritant when exposed to eyes or skin. Upon mixing with other ions, complexation between adjacent functional groups of PAA causes change in solution properties such as a reduction in the viscosity [95]. The swelling properties of PAA due to ionization were studied to understand its capabilities as a water absorbent [96]. In recent years, PAA structure in solution was determined through dynamic molecular simulations and correlated with experimental data that showed change from a helicoidal structure at low degrees of ionization to flexible rods at high degrees of ionization [97]. The changes in PAA structure due to its interaction with water made it difficult for application in solid electrolytes. However, its ability to form flexible polymeric chains is an attractive feature for ion transport. As a result, acrylic acid polymers were used as additives to decrease crystallinity and introduce cooperative polar functional groups that facilitated ion transport, specifically in PEO-LiClO4 polymer electrolytes [98]. Its flexible structure was leveraged to entrap imidazole in a proton conducting polymer electrolyte that demonstrated ionic conductivities of ~10-3 S cm -1 at 120 C [99]. Through polymer blending, PAA and poly(1-vinyl-imidazole) formed networks that showed high

38 26 temperature resistance and provided a highly stable environment for the entrapment of enzymes in bio-sensing applications [100]. Wu et al. synthesized PVA/PAA membranes soaked in KOH as an OH - -ion conducting polymer electrolyte. Due to the swelling property of PAA, the membrane took up 80 wt% KOH without being destroyed as opposed to a pure PVA membrane. A balance between high ionic conductivity of S cm -1 and good mechanical properties was reported from this study as shown in Figure 2-10 [9]. PVA and PAA have a unique property of being completely miscible with one and another. Accordingly, there is tremendous interest in taking advantage of blending the two polymers for various types of applications [101, 102]. Figure 2-10: Ionic conductivity of polymer blends between PVA and PAA at different compositions of (a) 10:3, (b) 10:5 and (c) 10:7.5 after being immersed in KOH for 24 hrs [9]. The 10:5 blend improved both mechanical strength and ionic conductivity.

39 27 Unlike PEO and PVA, use of PAA as a polymer electrolyte is at its infancy. There are concerns with using PAA as a standalone polymer for polymer electrolytes because of unpredictable changes in its mechanical properties. The functional group of PAA forms radicals upon absorption of UV radiation and Gestos et al. demonstrated UV cross-linked ultrathin films of PAA to enhance its mechanical properties without a photoinitiator [103]. If this can be used appropriately, PAA can also provide water retention and structural flexibility that PEO and PVA lacks. Many possibilities can be realized with novel uses of PAA in the field of polymer electrolytes for EC applications. 2.5 Modification of polymer electrolytes Besides using different polymer matrices and ionic conductors to modify polymer electrolytes, addition of other components is another option. As discussed above, a few commonly used strategies are the blending of two or more polymers, the dispersion of inorganic fillers and the addition of plasticizers. These techniques were specifically aimed to improve mechanical properties and ionic conductivity through the formation of ionic conduction pathways within the polymer electrolyte [104]. Another type of modification is forming polymer networks via cross-linking. Cross-linking polymer matrix Cross-linking polymers is traditionally used to improve the mechanical properties of a polymer. It is achieved through bonding different polymer branches to form networks as illustrated in Figure As a result, the macroscopic structure of the polymer exhibits improved resistance to applied stresses and strains. Cross-linking polymers is achieved through chemical or physical bonding.

40 28 Figure 2-11: Schematic of polymer structure changes from linear to branched to crosslinked [6]. During the cross-linking process, specific cross-linking agents are used with an initiator to: (1) activate functional group of the polymer, and (2) create bonds between the cross-linking agent and the activated functional groups of the polymer. Another way to chemically cross-link is through high energy irradiation such as electron beam, gamma or UV irradiation. The high energy sources are used to initiate the cross-linking by forming free radicals that create bonds in the polymer. Chemical cross-linking has drawbacks in that some cross-linking agents are harmful and can remain in the polymer system as impurities. Cross-linking can also occur physically through partial crystallization by subjecting the polymer solution to freezing and thawing [105]. Cycling this process induces further crystallization. Although the technique avoids any undesired residues, it suffers from long term stability as the crystallites eventually change back into their original structure over a period of months to years. Cross-linking PEO was initiated for biomedical applications. It was prepared by electron beam irradiating thin layers of aqueous solution [106]. PEO was also

41 29 successfully cross-linked in both aqueous solution and solid state by UV irradiation with benzophenone as the photoinitiator [12]. Figure 2-12 shows that the UV irradiation activates the photoinitiator, which then reacts with the PEO backbone to form radicals in the main chain. A network is formed when two radicals from the main chain recombines. Using cross-linking agents to chemically cross-link PEO is a new area of research and was achieved through reactions with difunctional peroxides [107]. PVA was cross-linked in many ways and reviewed elsewhere by Bolto et al. [108]. When PVA was cross-linked by hexamethylene diisocyanate in dimethyl sulfoxide/n,n-dimethylformamide solvent, its mechanical properties initially decreased due to the original hydrogen bonded network breaking, but increased after 20% crosslinking from the formation of networks [109]. One of the more effective ways to crosslink PVA is using glutaraldehyde (GA) in an acidic environment. This was used to make water swelling membranes [110]. However, using acid catalysts can be a hindrance to particular applications. Figueiredo et al. cross-linked PVA with GA in a neutral environment at room temperature where the carbonyl groups 5 of GA react with the hydroxyl functional groups of PVA to form networks as shown in Figure A soluble membrane showed swelling up to 44% with a GA/PVA mass ratio of Increased GA Figure 2-12: Formation of radicals in the PEO backbone to propagate chemical bond 5 C=O Carbonyl group formation between the benzophenone and other parts of the PEO backbone [12].

42 30 content led to branching and decreased solubility of the film in water [8]. The reaction in a basic media is unclear because GA was found susceptible to self-condensation via aldol condensation at high ph [111]. It is suspected that only a low degree of crosslinking can be achieved in this condition. PAA was cross-linked by electron beam irradiation up to 75 kgy that improved gel content and swelling behaviors for drug delivery applications [112]. More successful cross-linking of PAA was achieved through modifying PAA with 2-hydroxyethyl methacrylate (HEMA) followed by UV irradiation as shown in Figure 2-14 [16]. Since the functional group of PAA is sensitive to UV irradiation, it was UV cross-linked without a photoinitiator [103]. However, an ultrathin film was required for this processing condition. Cross-linking is frequently used for optimization of polymer electrolytes. It was first proposed for Li + -ion conducting polymer electrolytes to form a standing film [113]. Figure 2-13: Schematic of reaction between PVA and GA and the formation of (A) fully cross-linked species and (B) partially cross-linked species. Adapted from [8].

43 31 Figure 2-14: Design and cross-linking scheme of PAA-HEMA modified polymer, where (a) shows the HEMA modified PAA and (b) shows the cross-linked network via photopolymerization [16]. UV cross-linking using methyl, butyl, or octyl methacrylate as photoinitiators improved the polymer electrolyte s mechanical properties without compromising their ionic conductivity [114]. Free radical cross-linking using dibenzoyl peroxide or azo bis isobutyronitirle showed similar results [115]. Both reports did not discuss the degree of cross-linking. H + -ion conducting polymer electrolytes for EC applications were cross-linked and ionic conductivities of ~10-3 S cm -1 were reported [7, 116]. Choudhury et al. indicated that the amount of GA used in the acidic PAA/PVA blend hydrogel electrolyte was optimal, but failed to mention specific quantities, whereas Gao et al. showed that the optimized molar ratio between GA/PVA was 0.013:1. There is a contradicting point of view regarding the effect cross-linking has on the ionic conductivity of an H + -ion conducting polymer electrolyte. Choudhury et al. suggested that the acid electrolyte conducts H + -ions through the Grotthuss mechanism, which is not affected by the

44 32 bonding of the macromolecular structure of the polymer. However, Gao et al. suggested that cross-linking improved water retention, increasing the ionic conductivity, but over cross-linking reduced the intimate contact of the electrode/electrolyte interface, decreasing the ionic conductivity as shown in Figure Since one system involved a mixture of silicotungstic acid (SiWA)/H3PO4/PVA where PVA was merely a binder for the polymer electrolyte system and the other is a PAA/PVA blend membrane soaked in HClO4 where the composition was dominated by the polymer, the influences of crosslinking varied accordingly benefiting one, but not the other. Cross-linking the polymer is an effective way to modify the polymer electrolyte for specific properties. Stabilizing a high performing polymer electrolyte for EC applications is necessary to reduce maintenance costs and improve shelf-life and environmental resistance when devices are expected to be used in high performing electronics. Figure 2-15: Ionic conductivity of SiWA-PVA-H3PO4 polymer electrolytes with varying degrees of cross-linking [7].

45 33 Objectives The development of advanced polymer electrolytes was reviewed in Chapter 2. While the main focus of these polymer electrolytes is still in battery applications, there are more advancements toward EC. L + -ion and H + -ion conducting polymer electrolytes are relatively mature technologies, but the development of OH - -ion conducting polymer electrolytes is in its beginning stages. The objective of this thesis is to develop an OH - -ion conducting polymer electrolyte for EC and, potentially, for other electrochemical systems. The literature review revealed a few important gaps: (1) there is a lack of high performance OH - -ion conducting polymer electrolytes, (2) there is a limitation with KOH as the standard OH - - ion conductor in polymer electrolytes due to solidification at high concentrations (see Appendix A: Figure A-1) and (3) there are uncertainties and discrepancies in terms of the effects polymers have in different systems. To address these gaps the following approaches were taken in this thesis: 1) Explore alternative OH - -ion conductors for polymer electrolytes by exploiting TEAOH-PVA and comparing it with current state-of-art KOH-PVA in terms of ionic conductivity and shelf-life stability together with structural and thermal characterizations of these polymer electrolytes. 2) Investigate OH - -ion conductor interactions with various polymer matrices (PEO vs PVA vs PAA) by determining the optimal composition between the OH - -ion conductor and the different polymers, and comparing their respective ionic conductivities, shelf-lives, structural and thermal properties.

46 34 3) Optimize the most promising OH - -ion conducting polymer electrolyte through chemical cross-linking to further improve ionic conductivity and shelf-life stability in ambient conditions, while structurally and thermally characterizing the polymer electrolyte to understand reasons for improvement. 4) Demonstrate the developed TEAOH-PVA and cross-linked TEAOH-PVA in solid thin film EDLC devices with reasonable areal capacitances, fast response times and stable shelf-life in ambient conditions.

47 35 Experimental This section is focused on the preparation and synthesis of polymer electrolytes from the precursor solution to the fabrication of EDLC devices. The chemicals used to prepare the polymer electrolytes are shown in Table 4-1. Various characterization methods used this thesis are also discussed here. Table 4-1: Chemicals used for the preparation of polymer electrolytes Materials Molecular Weight (g/mol) Supplier Polymer Poly(vinyl alcohol) 145,000 Sigma-Aldrich Poly(acrylic acid) 100,000 Sigma-Aldrich Poly(ethylene oxide) 300,000 Sigma-Aldrich Ionic conductor Potassium Hydroxide 56 Merck Millipore Tetraethylammonium hydroxide 147 Alfa Aesar Cross-linking agent Glutaraldehyde 100 Sigma-Aldrich 4.1 Preparation of polymer electrolyte precursor solution Preparation of polymer solutions PVA was used as received to prepare a 5 wt% solution with DI water where the total weight was 100 g. A beaker was filled with 95 g of DI water and heated on a hotplate with a magnetic stirrer to 60 C. 5 g of PVA powder was slowly introduced into the solution over a period of 30 min. Markings were made at the original level of water and a glass lid was placed on top of the beaker to prevent complete evaporation. The apparatus was left to stir for 24 hrs. Afterwards, the beaker was filled with DI water up to

48 36 the original marking. The homogeneous solution was cooled to room temperature. PEO was prepare in the same way as the 5 wt% PVA solution, the mixture was stirred for 48 hrs instead of 24 hrs to fully dissolve the higher molecular weight polymer. PAA was received as a 35 wt% aqueous solution. To be consistent with PVA and PEO polymer solutions, the PAA was diluted to 5 wt%. First, 3.57 g of 35 wt% of PAA(aq) was measured out in a 50 ml beaker. DI water was then added until the total mixture was 25 g. Using a magnetic stirrer, the solution was mixed at room temperature for 24 hrs until the solution was homogeneous Preparation of ionic conductor solutions KOH pellets were used as received. To create a 30 wt% aqueous KOH solution, 7.5 g of KOH was slowly added into 22.5 g of DI water. The solution was mixed with a magnetic stir for 24 hrs at room temperature. TEAOH was purchased as a 35 wt% aqueous solution. It was stored at room temperature and used as received. 25, 15, and 5 wt% aqueous solutions of TEAOH were prepared by measuring out 17.9, 10.7 and 3.6 g of 35 wt% TEAOH solution respectively and adding DI water until all solutions were 25 g total in weight. Preparation of binary polymer electrolyte solutions For the polymer electrolyte systems that consisted of only an ionic conductor and a polymer, the total precursor solution was 5 g each time. Based on the desired molar ratio between TEAOH and the polymer, the amount of ionic conductor and polymer in solution form were calculated as shown in Table 4-2. The mixtures were combined at room temperature and stirred with a magnetic stirrer for 24 hrs before use.

49 37 Table 4-2: Polymer electrolyte composition Polymer electrolytes Molar ratio TEA-EO TEA-VA TEA-AA 35% TEAOH 5% PEO 35% TEAOH 5% PVA 35% TEAOH 5% PAA 1000: g g g g g g 3000: g g g g g g 5000: g g g g g g Preparation of cross-linked polymer electrolyte solutions TEA-VA was cross-linked using GA as the chemical cross-linking agent. GA was used as received. A 1 wt% aqueous GA solution was prepared by measuring out 0.6 g of 25 wt% aqueous GA and adding DI water until the total weight was 15 g. As shown in Figure 2-13, one unit of GA can chemically bond with four units of hydroxyl groups in PVA. The amount of GA required for a specific degree of cross-linking was calculated as shown below, where % is expressed in decimals. GA solution (g) = crosslink % Solid PVA (g) PVA Mw ( g mol ) PVA DoP 4 GA Mw ( g mol ) 1 GA solution % To prepare the polymer electrolyte, 5 wt% PVA, 35 wt% TEAOH and 1 wt% GA was combined in a glass vial. The solution was mixed with a magnetic stir for three days at ambient conditions for the reaction to occur. 4.2 Preparation of graphite electrodes A commercial graphite ink was coated onto a nickel substrate and cut into graphite electrodes as shown in Figure 4-1. First, tape was applied to mark out a 2 cm width where the graphite ink was doctor-bladed on to the nickel substrate. The total

50 38 width of the nickel substrate was 5 cm. The graphite ink coated sample was then heated in an oven at 50 C for 1 hr and at 140 C for 1 hr. The electrodes were soaked in DI water overnight and air dried. Electrodes for EDLC devices were prepared by cutting the graphite coated nickel into 2.5 cm x 1 cm strips with an active area of 1 cm x 1 cm. 4.3 Fabrication of EDLC devices using solution cast method 6 A schematic for fabricating EDLC devices is illustrated in Figure 4-2 and steps are as follows: The active area of the electrodes was rinsed with DI water and isopropyl alcohol to remove any inorganic or organic contaminants such as dust and fingerprints. The electrolyte solution was solution drop casted on the active surface area of the electrodes with a plastic dropper. Two drops were applied on each electrode and spread evenly across the electrode surface without scrapping the material. The polymer electrolytes were set to dry from 30 minutes to two hours depending on the polymer electrolyte: Figure 4-1: Preparation of graphite ink electrodes. 6 Both section 4.2 and 4.3 refer to graphite electrodes. However, metallic electrode EC cells are prepared the same way except without coating the graphite ink

51 39 Figure 4-2: Schematic of preparation of a simple EC cell. The active area of the cell is 1 cm 2 and the thickness is denoted by "l". o 30 mins for TEA-EO polymer electrolytes. o 45 mins for TEA-VA polymer electrolytes. o 2 hrs for TEA-AA polymer electrolytes. Once the electrolyte was dried to a gel, the two electrodes were sandwiched together and sealed with a chemically inert tape. 4.4 Material characterization of polymer electrolyte materials X-ray diffraction (XRD) analyses were conducted using a Philips XRD system, including a PW1830 HT generator, a PW 1050 goniometer and PW3710 control electronics. The samples were analyzed with monochromatized Cu-Kα anode source operating at 40 kv/40 ma. The diffraction patterns were recorded from 5 to 50 2θ with a step scan of θ. All XRD samples were prepared at 25 C in an ambient environment. Differential scanning calorimetry (DSC) experiments were carried out in a TA Instruments Q50 DSC in a Nitrogen environment from 30 to 250 C with a heating rate of 10 C/min. All DSC film samples were prepared at 25 C in an ambient environment.

52 40 Thermogravimetric analyses (TGA) were carried out in a TA Instruments Q50 TGA in an Argon environment from 30 to 250 C with a heating rate of 10 C/min. All TGA film samples were prepared at 25 C in an ambient environment. Infrared (IR) spectra were recorded at room temperature on a Thermo Scientific Nicolet is5 FT-IR spectrometer with id1 transmission module. A liquid or polymer electrolyte solution was casted on the center of an IR transparent Si wafer. The electrolyte was allowed to dry under ambient conditions for 30 minutes to form a thin film. The samples were analyzed from cm -1 with a resolution of 2 cm -1. Each spectra consisted of 32 sample scans. Desiccators were used to store all the samples in desired relative humidity. Saturated salt solutions were prepared and kept in the desiccator to keep the relative humidity constant. LiCl was used for an environment of 75% RH. KCl was used for an environment of 40 45% RH. MnO was used for an environment of 30% RH. All the desiccators are kept at 25 C. 4.5 Electrochemical characterizations of polymer electrolyte material in simple EC cells Electrochemical properties of the electrolytes were characterized with metallic and graphite symmetric two-electrode EC cells by cyclic voltammetry (CV), constant charge-discharge (CCD), and electrochemical impedance spectroscopy (EIS) utilizing a CHI 670 C bipotentiostat setup as shown in Figure 4-3 with the working electrode measured against the reference and the counter electrode. The thickness of the assembled electrochemical cell is ~0.05 cm with an electrolyte thickness of cm.

53 41 Figure 4-3: Schematic diagraph of the potentiostat setup. WE: working electrode, CE: counter electrode, RE: reference electrode, PE: polymer electrolyte. Cyclic Voltammetry Multiple CV scans were performed on the EC cells. The voltage window was assigned from V to a decomposition of water. The sweep rate is the time it takes for the voltage to be scanned from beginning to end. The different sweep rates used were 1, 100 and 5000 V s -1. As the voltage increased, the current response of the EC cell was recorded by the potentiostat and plotted against the voltage. The resulting plot is called a cyclic voltammogram (CV). An ideal capacitive CV is represented by a rectangle as shown in Figure 4-4(a). Deviation from the ideal rectangular shape of CV is attributed to the resistance of the electrolyte. The area of the rectangle is proportional to the capacitance of the device. The areal capacitance was calculated using Eq.5, where the charge (Q) is divided by the voltage window (U) and the geometric surface area of the electrode (A). C = Q U A (Eq. 5)

54 42 Figure 4-4: Ideal capacitive responses for an EDLC device from (a) CV and (b) CCD. Constant Charge-Discharge CCD was performed on EDLC devices. The current applied to the device was 5 ma and the voltage range measured was selected from 0 1 V. An ideal capacitor has a symmetrical voltage versus time response for its charging and discharging states as shown in Figure 4-4(b). Deviations from this symmetry is a result of damage through excess voltage. Electrochemical Impedance Spectroscopy EIS was also used to characterize the EC cells. A small AC signal, 5 mv amplitude was applied across the cell from 1 Hz to 10 6 Hz and the impedance of the device was plotted. A Nyquist plot is the negative of the imaginary impedance being plotted against the real impedance. An ideal capacitor has a straight line response throughout the frequency range as shown in Figure 4-5. A more resistive response would tilt the line. The intersection of the plot with the real impedance axis at high frequencies is associated with the resistance of the electrolyte or the equivalent series

55 43 Figure 4-5: Schematic of Nyquist plot for EDLC with ideal capacitive response. resistance (ESR). Using the dimensions of the cell, the electrolyte thickness was determined as shown in Figure 4-3. The ionic conductivity was calculated using Eq. 6, where the thickness of the electrolyte (l) is divided by the ESR and the geometric area (A) of the electrodes. Ionic conductivity (σ) has units of (S cm -1 ). σ = l (ESR) A (Eq. 6) From EIS, other plots were used to assess properties of EDLC devices. Bode plots were used to observe the change in capacitive behavior as a function of frequency and are formed with the impedance magnitude and the phase angle plotted against frequency. For ideal capacitors, the phase angle between voltage and current is -90. EDLCs transition from being resistive to capacitive, at a phase angle of -45. A time constant was calculated by Eq. 7 where fo is the frequency at which the devices reaches -45. The real (C (ω)) and complex (C (ω)) capacitance of the EDLC devices were calculated by Eq. 8 and Eq. 9, where (ω) is 2πf, Z (ω) is the real impedance, and Z (ω) is the imaginary impedance obtained from EIS measurements.

56 44 τ = 1 f o (Eq. 7) C (ω) = Z (ω) (Eq. 8) ω Z(ω) 2 C (ω) = Z (ω) (Eq. 9) ω Z(ω) 2 When the real capacitance was plotted against frequency, the accessible capacitance at low frequency was extracted. This capacitance was compared to the capacitance obtained from CVs. When the imaginary capacitance was plotted against frequency, the maximum that appears with decreasing frequency typically corresponded to the relaxation frequency, but is also commonly associated with the time constant of an EDLC device as per Eq. 7 (see Figure 4-6). Figure 4-6: Schematic of real and imaginary capacitance plotted against frequency.

57 45 Results and Discussion In chapter 3 the objective of developing an OH - -ion conducting polymer electrolyte was discussed. Based on the approach, experiments were performed and this chapter is focused on the analysis of the results. In section 5.1 a comparative study between KOH and an alternative OH - -ion conductor as polymer electrolytes was discussed. Section 5.2 looked at the effects of the OH - -ion conductor in various polymer matrices. Finally the most promising OH - -ion conducting polymer electrolyte was optimized through cross-linking and discussed in section Polymer electrolyte TEA-VA vs K-VA TEAOH was incorporated with PVA (TEA-VA) and an optimal composition of 81:19 weight ratio (or 4220:1 molar ratio) between TEAOH and PVA was selected from five different compositions. Figure 5-1 shows the ionic conductivity of TEA-VA over a period of two months. Even though the 4220:1 composition had the second highest Figure 5-1: The ionic conductivity of different compositions of TEAOH-PVA [5].

58 46 ionic conductivity, the polymer electrolyte with higher TEAOH content exhibited greater fluctuations. KOH was made with PVA (K-VA) at a molar ratio of 4220:1 and used as a baseline for comparison. The molar ratio was selected to keep the amount of conducting species the same for a fair comparison. Material characterizations When the polymer electrolytes were being made, there were noticeable differences between K-VA and TEA-VA. Figure 5-2 shows K-VA and TEA-VA films casted on a glass slide after 3 days at ambient conditions. While K-VA was initially a transparent film, it quickly turned opaque and powder-like in texture. At the same time, TEA-VA retained its transparency and gel-like properties. Further insight towards this change was obtained through XRD. The films were exposed to the ambient environment and tested at a relative humidity (RH) of 40% and 30%. The 10% change in RH is representative of the fluctuations in ambient conditions. Figure 5-3(a) shows that both films at 40% RH had broad peaks from θ, TEA-VA K-VA Figure 5-2: K-VA and TEA-VA polymer electrolyte physical appearances after 3 days.

59 47 indicating an amorphous structure in the material. However, when the films were conditioned in 30% RH as shown in Figure 5-3(b), the XRD pattern for TEA-VA still had an overall broad peak, while five distinct peaks appeared in the K-VA XRD pattern, corresponding to hydrated KOH peaks [117]. Sharp peaks in the XRD pattern indicate crystalline structures. The physical observations were in good agreement with the XRD results. Disordered amorphous structures randomly diffract light through the material resulting in transparency, whereas ordered crystalline structures directionally diffract light, rendering the material opaque. Within three days, K-VA changed from a material with an amorphous structure to one with a crystalline structure while TEA-VA maintained its amorphous structure. TGA results revealed the underlying phenomenon behind the structural change observed from the XRD patterns. Figure 5-4 shows the weight loss of the polymer electrolyte films as a function of temperature. Over this temperature range, TEA-VA (a) RH=40% K-VA (b) RH=30% K-VA Intensity (a.u.) TEA-VA Intensity (a.u.) TEA-VA Scattering Angle ( 2 ) Scattering Angle ( 2 ) Figure 5-3: X-ray powder diffraction of K-VA and TEA-VA at (a) 40% RH and (b) 30% RH.

60 TEA-VA Weight (%) K-VA Temperature ( C) Figure 5-4: TGA of K-VA and TEA-VA from 30 to 120 C. showed a lower weight loss compared to K-VA. Since TEAOH and KOH both have decomposition temperatures higher than 120 C [118], the higher weight loss of K-VA was attributed to more adsorbed water in K-VA that was easily removed. Therefore, the crystallization of K-VA was due to the loss of adsorbed water that initially acted as a plasticizer. Since TEAOH is a hygroscopic material, the water in TEA-VA was more tightly bound compared to K-VA resulting in improved structural stability. Electrochemical characterizations Metallic solid cell performance The ionic conductivities of TEA-VA and K-VA cells were tracked over 40 days in ambient conditions as shown in Figure 5-5. In pristine conditions, the ionic conductivity of K-VA was higher than TEA-VA, but decreased two orders of magnitudes after 40 days. TEA-VA cells showed relatively constant ionic conductivities during the same period. These results correlated well with the structural changes of K-VA and TEA-VA.

61 49 Since the structure of TEA-VA showed little change, the movement of the OH - -ion in the polymer electrolyte remained consistent. When the structure of K-VA changed from amorphous to crystalline, the conduction pathways for OH - -ion also changed. In this case, an amorphous structure corresponded to a higher ionic conductivity compared to a crystalline one, consistent with literature findings. The OH - -ion conduction mechanism in polymers is influenced by two factors [11]: (1) OH - -hopping through the formation of hydrogen bonded complexes with existing water molecules and (2) polymer backbone facilitated movement of OH - -ions through segmental motion as discussed in section When K-VA crystallized, both factors were negatively impacted. The loss of water reduced the water molecules required for OH - -hopping. At the same time, crystallization reduced the movement of the polymer that aided OH - -ion transport. These mobility changes of OH - -ions in K-VA led to the large decrease in ionic conductivity as shown in Figure Conductivity (Scm -1 ) TEA-VA K-VA Time (Day) Relative Humidity (%) Figure 5-5: Ionic conductivity as a function of time for K-VA and TEA-VA polymer electrolytes.

62 50 It should be noted that the change in ionic conductivity of K-VA was more gradual than the structural change depicted through the XRD patterns in Figure 5-3. This discrepancy was accounted for by the tape packaging of the cell, which resulted in a slower dehydration of the polymer electrolyte. However, it reasonable to assume equilibrium was reached after 40 days. Figure 5-6 shows CVs of K-VA and TEA-VA metallic electrode cells with an ultrahigh sweep rate of 5000 V s -1. A summary of the areal capacitances for the K-VA and TEA-VA cells can be found in Table 5-2. Both K-VA and TEA-VA cells showed ideal rectangular CV profiles in the pristine condition indicating highly capacitive behaviors. After 40 days, the capacitance of TEA-VA cells reduced, but still had ideal rectangular CV profiles. On the other hand, the capacitance of K-VA reduced and the CV profile was no longer rectangular. The changes in capacitance were consistent with changes in the ionic conductivity and structure. When the mobility of OH - -ion decreased, the amount of ions that reached the electrode surface was reduced, lowering the capacitance. Current Density (Acm -2 ) 4.0x x x x10-2 (a) TEA-VA K-VA Cell Voltage (V) Day 1 Current Density (Acm -2 ) 8.0x x x x10-3 (b) TEA-VA K-VA Day Cell Voltage (V) Figure 5-6: CVs of solid metallic K-VA EC and TEA-VA EC at (a) day 1, and (b) day 40 (sweep rate = 5000 V s -1 )

63 51 Dehydration of polymer electrolyte also negatively affected the electrolyte/electrolyte interface preventing the ions from accessing the surface of the electrode. Table 5-1: Area capacitance of K-VA and TEA-VA in metallic solid cells on Day 1 and Day 40 K-VA Capacitance (μf cm -2 ) TEA-VA Capacitance (μf cm -2 ) Day Day Graphite solid cell performance Simple EDLC devices were made by sandwiching K-VA and TEA-VA between graphite electrodes. Figure 5-7 shows the evolution of the CVs over a period of two Current Density (Acm -2 ) 2.0x x x x x x10-3 (a) 1 Day Current Density (Acm -2 ) 2.0x x x x x x10-3 (b) 16 Days Current Density (Acm -2 ) -1.5x x x x x x x Cell Voltage (V) (c) 32 Days Current Density (Acm -2 ) -1.5x x x x x x x Cell Voltage (V) (d) 67 Days -1.5x Cell Voltage (V) -1.5x Cell Voltage (V) Figure 5-7: CVs of solid graphite K-VA EC and TEA-VA EC at (a) day 1, (b) day 16, (c) day 32 and (d) day 67 (sweep rate = 1 V s -1 )

64 52 months. A summary of the device capacitances can be found in Table 5-2. Generally, the capacitance decreased similarly to the metallic electrode cells in Figure 5-6, where the capacitance of K-VA devices decreased much more than TEA-VA devices. However, the rate that the capacitance decreased was slower in a device configuration. This was due to the interaction between the graphite material and the polymer electrolytes. During fabrication, the electrolyte soaked into the porous graphite material that was absent with the metallic cells. This extra interaction slowed down the dehydration and crystallization of the polymer electrolytes. However, when the adsorbed water eventually evaporated after 2 months, the capacitance was similar to the metallic cells. Further investigations of the EDLC devices were performed through EIS measurements. The real and imaginary capacitances were plotted against frequency as shown in Figure 5-8. Comparisons between capacitances obtained from EIS and CV and their time constants can be found in Table 5-2. Table 5-2: Variations of capacitance and time constants of a graphite/tea-va EC and a graphite K-VA EC, obtained from EIS and CV over time Capacitance (mf cm -2 ) Day 1 Day 16 Day 32 Day 67 TEA-VA a TEA-VA b K-VA a K-VA b Time constant (ms) TEA-VA K-VA a Capacitance at 1 Hz b Discharge capacitance from CV as shown in Figure 5-7

65 53 The capacitance measured from EIS were in agreement with those calculated from CVs. Initially, the time constant for the K-VA device was lower than TEA-VA, representing a faster response time. However, measurements made at day 16, 32 and 67 revealed that while the TEA-VA device showed slight changes in time constant and capacitance, the K-VA device lost capacitance as shown in Figure 5-7and Figure 5-8, while the time constant increased. According to the RC relationship, the resistance of the K-VA device increased much more significantly than the capacitance decrease. TEA-VA exhibited high OH - -ion conduction and stability in ambient conditions in both metallic solid cell and simple EDLC device. Even though the initial performance of Real capacitance C' (Fcm -2 ) Real capacitance C' (Fcm -2 ) 1.2x x x x x x (a) K-VA Day 1 Day 16 Day 32 Day Frequency (Hz) 1.2x10-3 (c) TEA-VA Day 1 1.0x10-3 Day 16 Day x10-4 Day x x x Frequency (Hz) Imaginary capacitance C'' (Fcm -2 ) Imaginary capacitance C'' (Fcm -2 ) 4.0x10-4 (b) K-VA Day 1 3.5x10-4 Day x10-4 Day x10-4 Day x x x x Frequency (Hz) 4.0x10-4 (d) TEA-VA Day 1 3.5x10-4 Day x10-4 Day x10-4 Day x x x x Frequency (Hz) Figure 5-8: Real and imaginary capacitances of graphite electrode ECs with K-VA (a, b) and TEA-VA (c, d) plotted against frequency after day 1, 16, 32 and 67

66 54 K-VA based cells were excellent, the device deteriorated quickly. TEA-VA has demonstrated to be a promising polymer electrolyte for EC based applications. 5.2 Effect of polymer matrices TEA-VA was a superior OH - -ion conducting polymer electrolyte than the state-ofart K-VA. However, it is premature to conclude that PVA is the best polymer host without preliminary screening of other polymers. PEO and PAA were investigated as alternative polymer hosts. Table 5-3 shows a summary of the properties that were discussed in section 2.4. Since the crystallinity, water content and complexation of TEAOH were associated with OH - -ion conduction of the polymer electrolyte, changing the polymer alters these properties. Polymer electrolytes with TEAOH in PVA, PEO and PAA were examined and compared with each other to understand OH - -ion conduction in these polymer and to identify the most promising system. Table 5-3: Overview of potential polymer matrices with their respective functional groups, glass transition temperature, melting temperature and crystallinity nature Structure Tg ( C) Tm ( C) Crystallinity PVA Semi-crystalline PEO Semi-crystalline PAA 105 Amorphous

67 55 Selection of polymer electrolyte composition Three different molar ratios between TEAOH and the polymers TEAOH-PVA (TEA-VA), TEAOH-PEO (TEA-EO) and TEAOH-PAA (TEA-AA) were made in metallic solid cells and tested for ionic conductivities in the pristine condition as shown in Figure 5-9(a). The ionic conductivity was plotted against the molar ratio due to the different molecular weights of the polymers 7. The 1000:1 composition between TEAOH and any given polymer corresponded to the lowest ionic conductivity for all three systems. As TEAOH content increased, higher ionic conductivities were obtained, in agreement with Gao et al. [26]. With compositions of 3000:1 and 5000:1, the ionic conductivity plateaued for TEA-EO and TEA-AA cells and decreased for TEA-VA cells. At the pristine condition, water was a large component of the polymer electrolytes. As a result, OH - -ion transport was through hopping via the formation of hydrogen bonded complexes as discussed in section To confirm this, metallic liquid cells were tested in various concentrations of aqueous TEAOH solution as shown in Figure 5-9(b). The results revealed a similar trend in ionic conductivity to the polymer electrolytes. Since the only influence toward OH - -ion conduction was water, it was suggested that conduction of OH - -hopping was the principal mode of OH - -ion transport in polymer electrolytes at the pristine condition. With this mode of OH - -ion transport, there were two possible causes for the lower ionic conductivity at higher TEAOH concentrations: (1) involved the dissolution of 7 Molar ratio between systems with different polymer matrices is a more precise representation of the amount of hydroxide-ions that are available for conduction.

68 56 Conductivity (Scm -1 ) 1.2x10-2 (a) TEA-VA TEA-EO 1.0x10-2 TEA-AA 8.0x x x x :1 3000:1 5000:1 TEAOH-Polymer Molar Ratio Conductivity (Scm -1 ) 1.4x10-1 (b) TEAOH(aq) 1.2x x x x TEAOH wt. Percentage (%) Figure 5-9: Ionic conductivity map of (a) TEA-VA, TEA-EO and TEA-AA polymer electrolytes and (b) aqueous TEAOH. TEAOH, and (2) involved the auto-ionization of water as shown in Eq H 2 O H 3 O + + OH (Eq. 10) In the first case, the amount of dissociated hydroxides increased with more TEAOH. However, at a certain concentration, the hydrated species formed large aggregates that regressed the formation of hypercoordinated complexes necessary for OH - -hopping In the second case, the equilibrium in Eq. 10 favored the presence of OH - -ions when the concentration of TEAOH is low, but the equilibrium shifted towards the formation of water at higher TEAOH concentrations, reducing the available OH - -ions, thus decreasing the ionic conductivity. It was suspected that due to the low activity of water, case (1) was more likely. Yet, it is difficult to confirm the hypotheses and ongoing work is under way. Because of the existence of an optimal composition between high ionic conductivity and efficient transport of OH - -ions, the 3000:1 molar ratio composition for the different polymer electrolytes were selected for further investigations.

69 57 Material characterizations Like the TEA-VA and K-VA (4220:1) films, there were notable differences between TEA-VA, TEA-EO and TEA-AA (3000:1) films as shown in Figure TEA- VA and TEA-AA were translucent after 3 days in ambient conditions, whereas the TEA- EO film turned from a transparent to opaque film. The adhesive properties between the polymer electrolytes were also evident. TEA-VA adhered to the glass substrate, TEA- EO was easily separated from it, and TEA-AA did not form a film. To characterize the structures of these electrolytes, the films were solution cast and stored in 75 and 45% RH conditioned desiccators to simulate pristine and ambient environments respectively. Figure 5-11(a) shows the XRD patterns for the polymer electrolytes stored in 75% RH. In the pristine condition, all three XRD results showed broad diffraction patterns that indicated amorphous structures. In the ambient condition, sharp peaks appeared in the TEA-EO XRD pattern, indicating increased crystallinity. On Figure 5-10: Optical observations of TEA-VA, TEA-EO and TEA-AA films.

70 58 (a) RH=75% TEA-AA (b) RH=45% TEA-AA Intensity (a.u) TEA-EO TEA-VA Intensity (a.u) TEA-EO TEA-VA Scattering Angle ( 2 ) Scattering Angle ( 2 ) Figure 5-11: XRD of TEA-VA, TEA-EO and TEA-AA polymer electrolyte films conditioned at (a) 75% RH and (b) 45% RH. the other hand, the XRD results for TEA-VA and TEA-AA films still showed broad peaks as shown in Figure 5-11(b), indicating unchanged amorphous structures. The respective structure of the polymer electrolytes agreed well with the film appearances. The two amorphous polymer electrolytes, TEA-VA and TEA-AA were translucent and the crystalline polymer electrolyte TEA-EO turned opaque in ambient conditions. FTIR experiments were performed to probe for interactions between TEAOH and the polymer matrices. Figure 5-12(a) shows the full IR spectra for TEAOH and the polymers with Figure 5-12(b) showing a zoomed in spectra. Figure 5-12(c) shows the IR spectra the polymer electrolytes with Figure 5-12(d) showing a zoomed in spectra. Each material has unique characteristic fingerprints, depicted in Figure 5-12(b). TEAOH characteristic peaks are the 1186 and 1002 cm -1 C-C-N stretches of the TEA cation and the 789 cm -1 NC4 stretch of the cation. According to Peppas et al., 1141 and 1094 cm -1 stretches of PVA are related to the crystallinity of the polymer [119]. The presence of the

71 59 triplet C-O-C stretches at 1149, 1114, and 1061 cm -1 with a maximum at 1114 cm -1 is confirmation of PEO semi-crystalline phase [120]. The well-known 1710 cm -1 carbonyl 8 stretch is due to its presence as a functional group in PAA. The comprehensive band assignments are shown in Table B-1 in Appendix B. Absorbance (a.u.) (a) OH PAA PEO PVA Characteristic Fingerprints Absorbance (a.u.) (c) TEA-AA TEA-EO TEA-VA TEAOH TEAOH Absorbance (a.u.) (b) PAA PEO PVA Wavenumber (cm -1 ) C-O C-O-C TEAOH C-C-N Wavenumber (cm -1 ) Absorbance (a.u.) (d) TEA-AA TEA-EO TEA-VA Wavenumber (cm -1 ) C-C-N C-O C-O-C TEAOH Wavenumber (cm -1 ) Figure 5-12: IR spectra of pure polymers from (a) cm -1 and (b) cm -1. IR spectra of TEA-VA, TEA-EO and TEA-AA at 3000:1 molar ratios from (c) cm -1 and (d) cm C O Carbonyl group

72 60 Since water content was an important aspect of polymer electrolytes, the hydrogen bonding hydroxyl 9 peaks were monitored around cm -1 and 1650 cm -1. Even though TEAOH, PVA, PEO and PAA all exhibited the hydroxyl peaks from cm -1, the hydrogen bonding was different in all cases: (1) TEAOH showed a broad hydroxyl peak, with small alkyl 10 signatures at 2992 and 2952 cm -1, (2) the hydroxyl peak for PVA shifted to lower wavenumbers and was more symmetrical compared to TEAOH, indicating decreased hydrogen bonding compared to TEAOH, (3) PEO showed the least intense hydroxyl peaks with the most intense alkyl peaks and (4) PAA exhibited the most broad hydroxyl peak and the smallest alkyl peaks. Accordingly, the water content decreased from PAA to TEAOH to PVA to PEO. It is not surprising that the polymer electrolytes at 3000:1 molar ratios all exhibited hydroxyl peaks similar to TEAOH since the predominant composition was TEAOH. However, the hydroxyl signature related to higher water content from PAA to PVA to PEO provided explanations for the film integrity differences between the polymer electrolyte, where TEA-AA could not form a film and TEA-EO dried up readily (see Figure B-2 in Appendix B for the water content of the polymer electrolytes). At lower wavenumbers, the signatures were from characteristic fingerprints. The characteristic peaks of PVA in TEA-VA changed noticeably. The distinct 1141 and 1095 cm -1 peaks merged into a broad 1121 cm -1. This indicated a reduction in crystalline 9 OH Hydroxyl group: related to hydrogen bonding 10 CH 3 CH 2 CH 3 Alkyl stretches: related to the backbone of the polymer

73 61 regions and increased amorphous regions of PVA [118], in agreement with XRD patterns. The triplet semi-crystalline peaks of PEO in TEA-EO exhibited no shift in wavenumber. The intensity of these peaks were greater than the C-C-N peaks from TEAOH, unlike TEA-VA and TEA-AA. This suggested that the crystalline phase of PEO was prevalent, which was also consistent with XRD results. There was no signature that corresponded to the crystallinity of PAA because it is an amorphous polymer as indicated by XRD result. The carbonyl stretch at 1710 cm -1 decreased in the TEA-AA system and a new peak independent of TEAOH and PAA at 1557 cm -1 emerged. This peak was attributed to asymmetric COO - vibrations [118]. The OH - -ions hydrolyzed the carboxylic functional group of PAA, which resulted in a reduced carbonyl stretch intensity and increased COO - stretch intensity. The COO - functional group is highly hydrophilic and was responsible for the high water content in PAA. The TEAOH characteristic peaks, 1186 and 1002 cm -1, were present in all the polymer electrolytes and did not shift in wavenumber, indicating that the cation remained intact. However, these peaks were broader in the polymer electrolytes than in TEAOH as shown in Figure 5-12(d), indicating complexation with the polymer host. The complexation of the cations with the polymer host could be sites for OH - -ion transport, but this requires further evidence to be conclusive. IR results further supported the observation of the crystalline structure of TEA- EO, and the amorphous structure of TEA-AA and TEA-VA. Moreover, it demonstrated higher intensities of hydroxyl stretches that can be attributed to higher water content, which influenced the film integrity of the polymer electrolytes.

74 (a) (b) Heat Flow (W/g) PVA -3.0 PEO -3.5 PAA Temperature ( o C) Heat Flow (W/g) TEA-VA -3.0 TEA-EO -3.5 TEA-AA Temperature ( o C) Figure 5-13: Thermal analysis of (a) PVA, PEO, PAA and their respective polymer electrolyte systems (b) TEA-VA, TEA-EO, TEA-AA. Thermal analyses of the polymer electrolytes are shown in Figure As polymers, from temperatures C, PEO had a large crystallization peak at 67 C, while PVA and PAA showed little thermal activity in this temperature range. In the DSC for TEA-EO, the crystallization peak for PEO decreased, but still existed, in agreement with IR and XRD results. Both TEA-VA and TEA-AA showed increased thermal activity over their respective pure polymer. The increased thermal activity was attributed to the evaporation of water. As the temperature increased towards 100 C, the thermal activity increased from TEA-EO to TEA-VA to TEA-AA, indicating that TEA- EO had the least amount of water and TEA-AA had the most. These results agreed with both the increased hydroxyl intensities from IR spectra and physical observations of the polymer electrolyte films. Electrochemical characterizations of metallic solid cells The ionic conductivity of TEA-VA, TEA-EO and TEA-AA were tracked over a period of 43 days in ambient conditions as shown in Figure A summary of the ionic

75 63 Conductivity (Scm -1 ) 1.6x10-2 TEA-VA TEA-EO TEA-AA 1.2x x x Time (Day) Relative Humidity (%) Figure 5-14: Ionic conductivity of TEA-VA, TEA-EO and TEA-AA in metallic ECs tracked over a period of 43 days in ambient conditions. conductivities are listed in Table 5-4. On day 1, TEA-VA and TEA-EO had ionic conductivities of approximately 10-2 S cm -1 and TEA-AA had an ionic conductivity closer to 10-3 S cm -1. After a few days, the ionic conductivity of TEA-VA reduced by 50% and TEA-EO decreased by an order of a magnitude. On the other hand, the ionic conductivity of TEA-AA increased to values close to TEA-VA. These values remained as such even after 43 days. Table 5-4: Ionic conductivity of TEA-VA, TEA-EO and TEA-AA metallic solid cells on Day 1, 4 and 43 of tracking in ambient conditions. TEA-VA Ionic Conductivity (ms cm -1 ) TEA-EO Ionic Conductivity (ms cm -1 ) TEA-AA Ionic Conductivity (ms cm -1 ) Day Day Day

76 64 The change in ionic conductivity of TEA-EO was related to the change from an amorphous system to a crystalline one, much like the K-VA system discussed in section 5.1. The loss of water and reduced polymeric segmental motion both hindered OH - -ion mobility in the polymer electrolyte. The decrease in ionic conductivity of TEA-VA was less than that of TEA-EO because, while OH - -hopping reduced from the loss of water, TEA-VA still had an amorphous structure that provided polymeric segmental motion to facilitate OH - -ion movement. Accordingly, one would expect TEA-AA, being both amorphous and highly hydrophilic to have the highest ionic conductivity. However, this was not the case. A couple of causes could account for this: (1) the high amount of water that TEA-AA possessed diluted the OH - -ion content in the polymer electrolyte so there were less species being conducted and (2) the hydrolysis of the carbonyl group in PAA by OH - -ions reduced the amount of available OH - -ions for conduction. The first cause accounted for the improvement in ionic conductivity after a few days due to the evaporation of water that increased the OH - -ion concentrations. The second cause could account for the lower ionic conductivity of TEA-AA compared to TEA-VA after 43 days since there were less OH - -ions available in the TEA-AA system. Nevertheless, a more detailed investigation is required to confirm this. The CVs of TEA-VA, TEA-EO and TEA-AA metallic cells are shown in Figure A summary of their capacitances are listed in Table 5-5. The capacitances of all three cells decreased over this period of time. The areal capacitance of TEA-VA cells was the highest throughout the duration of the tracking. The stable capacitance was due to the film s adhesive properties, which allowed the OH - -ions to consistently access the electrode surface to maximize charge storage. As the TEA-EO electrolyte crystallized

77 65 Current Density (Acm -2 ) 6.0x10-2 (a) TEA-VA 4.0x10-2 TEA-EO TEA-AA 2.0x x x x Cell Voltage (V) Day 1 Current Density (Acm -2 ) 2.0x10-2 (b) 1.0x x x10-2 TEA-VA TEA-EO TEA-AA Cell Voltage (V) Day 43 Figure 5-15: CVs of TEA-VA, TEA-EO and TEA-AA in metallic ECs on (a) day 1 and (b) day 43. (sweep rate = 5000 V s -1 ) and dehydrated, the contact between the electrode and electrolyte was damaged, preventing ions from reaching the electrode surface, which led to a large decrease in capacitance. The decrease in capacitance of TEA-AA cells was unlikely due to electrode/electrolyte interface issues because of the hydration of the system. Fewer ions reached the surface of the electrode was likely due to less available OH - -ions. This could be further evidence of reduced OH - -ions in the system due to the hydrolysis of the carbonyl functional groups. Table 5-5: Areal capacitance of TEA-VA, TEA-EO and TEA-AA metallic cells on Day 1 and Day 43 of tracking TEA-VA Capacitance (μf cm -2 ) TEA-EO Capacitance (μf cm -2 ) TEA-AA Capacitance (μf cm -2 ) Day Day This study showed the polymer host has significant influences on the polymer electrolytes. Although some explanations were inconclusive such as the effects that

78 66 determined the optimal composition and electrochemical properties of TEA-AA, it was clear that an amorphous polymer host and an appropriate degree of hydration were important factors to improve ionic conductivity. Furthermore, physical properties such as filmability and adhesion were also key in making stable cells. From the studied polymer electrolyte systems TEA-VA still is the best candidate for further optimization. 5.3 Crosslinking Alkaline TEAOH-PVA Polymer Electrolyte Cross-linking was used in both EC and fuel cell electrolyte systems to prevent swelling that led to the dissolution of the polymer electrolyte [26, 121] as discussed in section The following study chemically cross-linked the TEA-VA system with glutaraldehyde (GA) and explored its effects on material and electrochemical properties. Determining the degree of cross-linking A suitable degree of cross-linking was desired for the best balance between ionic conductivity and improved mechanical integrity. Since a polymer electrolyte with fast OH - -ion conduction is desired for ECs, ionic conductivity was used as a measure of improvement and was mapped against its degree of cross-linking as shown in Figure 5-16 after being conditioned in a 45% RH environment. A small degree of cross-linking (0.5%), increased the ionic conductivity by over 100% compared to linear TEA-VA. As the degree of cross-linking increased, the ionic conductivity decreased and plateaued because of increased rigidity of the polymer chains that impeded OH - -ion transport. A low degree of cross-linking stabilized the structure of the polymer electrolyte and allowed more efficient OH - -ion transport, without

79 67 1.5x10-2 TEA-VA G2 Conductivity (Scm -1 ) 1.2x x x x Theoretical Degree of Crosslinking (%) Figure 5-16: Ionic conductivity mapping of TEA-VA as a function of degree of crosslinking. Cells were conditioned in 45% RH for three weeks prior to testing. compromising the mobility of polymer chains. The lightly cross-linked polymer electrolyte, TEA-VA G2(0.5%) was selected for further testing. Material characterizations Since the degree of cross-linking was in comparison to the bulk component of the system, one way to physically observe the effect of cross-linking was to wet the film with water. TEA-VA and TEA-VA G2(0.5%) were solution cast onto glass slides in ambient conditions and left overnight to dry. Two drops of water were placed on the dried polymer electrolytes and was absorbed by the film. Only a couple drops of water were used because the polymer electrolytes were still soluble in water indicating that the cross-linking was minimal. When the polymer electrolytes were peeled from the glass slides with tweezers as shown in Figure 5-17, the TEA-VA film was more prone to breaking compared to TEA-VA G2(0.5%). The linear polymer electrolyte easily ripped, whereas the cross-linked counterpart was removed as a complete film without tearing.

80 68 Figure 5-17: Difference in film properties between (a) TEA-VA and (b) TEA-VA G2(0.5%) after wetting the film with two drops of water. This result supported the idea that the cross-linked film was more stable for efficient OH - -ion transport. XRD, DSC-TGA and FTIR were used to further characterize the polymer electrolyte. XRD measurements were performed to examine the bulk structural properties of the linear and cross-linked TEA-VA. As shown in Figure 5-18(a), both XRD patterns showed a broad baseline with a peak at 26 2θ. When the films were conditioned at 30% RH to represent the fluctuations in ambient conditions, the broad baseline did not change and a peak at 15 2θ emerged as shown in Figure 5-18(b). The presence of the broad baseline was evidence of an amorphous structure in all conditions, demonstrating that segmental motion still influenced the movement of OH - -ions. The peaks at 26 and 15 2θ are representative of TEAOH 5H2O and TEAOH 4H2O respectively [122,

81 69 (a) RH=40% (b) RH=30% Intensity (a.u.) TEA-VA G2(0.5%) TEA-VA Intensity (a.u.) TEA-VA G2(0.5%) TEA-VA Scattering Angle ( 2 ) Scattering Angle ( 2 ) Figure 5-18: XRD spectra of TEA-VA and TEA-VA G2(0.5%) films at (a) 40% RH and (b) 30% RH 123] as shown in Figure A-2 in Appendix A. Since the polymer electrolytes had a high content of TEAOH, the shift in hydration state with change in RH was expected. The key difference between the diffraction patterns was the consistently higher peak intensities from the XRD pattern of TEA-VA G2(0.5%) compared to TEA-VA at 15 and 26 2θ, indicating an increased presence of TEAOH 5H2O and TEAOH 4H2O in the polymer electrolyte system. The amplified intensity of TEAOH hydrates was suspected to be a result of more stable complexes due to cross-linking. Thermal studies via TGA, DTG and DSC were performed on TEA-VA and TEA- VA G2(0.5%). To comprehensively discuss the thermal properties of the polymer electrolytes, the thermal analyses of TEAOH, PVA and linear TEA-VA were performed first followed by the comparison between TEA-VA with TEA-VA G2(0.5%). Figure 5-19 shows the thermal signature of (a) TEAOH, (b) PVA and (c) TEA- VA. The important thermal transitions were summarized in Table 5-6. The thermal

82 70 Heat Flow (W/g) Heat Flow (W/g) Weight (%) Deriv. Weight (%/ o C) TEAOH PVA TEA-VA Temperature ( C) Weight (%) Deriv. Weight (%/ o C) Figure 5-19: Thermal analyzes of (a) TEAOH, (b) PVA and (c) TEA-VA (4220:1) molar ratio via DSC (solid), TGA (dash-dot) and DTG (short dash). Samples were conditioned at 30% RH. signature for PVA were in agreement with literature [8, 124]. The pure PVA film had a crystallinity of 57% using the heat of melting for 100% crystalline PVA as J/g. In the polymer electrolyte system, the crystallization peak at 230 C disappeared, which indicated that the system did not have any crystalline regions, consistent with XRD results. TEAOH has a decomposition signature at 213 C [118], which was reproduced in this study. A more obscure thermal signature appeared at 154 C. It was suspected to be a form of crystallized water in the TEAOH system and have been observed to fluctuate with changes in RH as shown in Figure B-3 in Appendix B. This signature

83 71 corresponded to peak 2 of DTG around 130 C that was present in both TEAOH and TEA-VA, which was used as an indication of water content when comparing TEA-VA and TEA-VA G2(0.5%). However, it remains an ambiguous signature and is under investigation. Table 5-6: Summary of thermal events for TEAOH, PVA and TEA-VA. TGA DTG DSC Residue (%) Peak Temperature ( C) Tm ( C) Hm (J/g) Tg ( C) TEAOH PVA TEA-VA TEA-VA and TEA-VA G2(0.5%) were prepared, conditioned and then tested for thermal properties similar to the individual components. Figure 5-20 shows the overlap between the two signatures and Table 5-7 shows a summary of the thermal events. TEA-VA G2(5.0%) was included in the table to help understand the effects of crosslinking. The thermal signatures for TEA-VA and TEA-VA G2(0.5%) essentially overlapped. There were no PVA crystallization peaks for TEA-VA G2(0.5%), which showed that the cross-linked polymer electrolyte was still an amorphous structure. If the signature at 130 C was associated with the water content of the polymer electrolyte, there was no conclusive evidence that the cross-linked polymer electrolyte possessed more water. Evidence of cross-linking was seen in the residue for TEA-VA G2(0.5%). It was higher than the linear component, and the 5% cross-linked polymer electrolyte had an even higher residue. As shown in Table 5-7, there were differences in Tg and Hm of

84 72 the cross-linked polymers. This could be due to cross-linking, but remains uncertain and requires confirmation. Heat Flow (W/g) Heat Flow (W/g) Weight (%) Deriv. Weight (%/ o C) TEA-VA TEA-VA G2(0.5%) Temperature ( C) Weight (%) Deriv. Weight (%/ C) Figure 5-20: Thermal analysis of TEA-VA and TEA-VA G2(0.5%) via DSC (solid), TGA (dash-dot) and DTG (short dash). Samples were conditioned at 30% RH. Table 5-7: Summary of thermal events for TEA-VA, TEA-VA G2(0.5%) and TEA-VA G2(5.0%) TGA DTG DSC Residue (%) Peak Temperature ( C) Tg ( C) Tm ( C) Hm (J/g) TEA-VA TEA-VA G2(0.5%) TEA-VA G2(5.0%)

85 73 FTIR experiments were performed to characterize and compare TEA-VA with TEA-VA G2(0.5%) to explore any changes in chemical bonding from cross-linking. The four main chemical bonding stretches indicated in Figure 5-21(a) were involved in crosslinking. The hydroxyl peak 11 of PVA at 3303 cm -1, the well-known methylene 12 stretches from PVA at 2943 cm -1, the aldehyde 13 peak at 1867 cm -1 and the acetal 14 peak at 1104 cm -1. Since the IR spectra for all cross-linked polymer electrolytes were qualitatively identical, a semi-qualitative analysis was used based on the findings of Figueiredo et al. [8]. The hydroxyl peaks were measured against the methylene peak and plotted as a function of cross-linking in Figure 5-21(b). The results for TEA-VA G2(2.5%) and TEA- VA G2(5.0%) were included for a better comparison and their respective IR spectra are shown in Figure B-4 in Appendix B. In the study by Figueiredo et al., the hydroxyl stretch, from the functional group of PVA was expected to decrease when reacted with GA. All hydroxyl content remained nearly constant for the OH - -ion conducting polymer 11 OH Hydroxyl group 12 O CH 3 Methylene group C OR C H OR Aldehyde group Acetal group

86 74 Absorbance (a.u.) (a) OH CH 2 COH COC TEA-VA G2(0.5%) TEA-VA Wavenumber (cm -1 ) Absorbance ratio (A 3303 /A 2943 ) 1.02 (b) Degree of Cross-linking (%) Figure 5-21: FTIR measurement of (a) TEA-VA vs TEA-VA G2(0.5%). Semi-quantitative analysis of changes in (b) hydroxyl as a function of cross-linking. electrolytes. This was because the hydroxyl stretches in the polymer electrolytes were mainly from TEAOH, which was the same concentration in all cases. The aldehyde and acetal stretches were measured against the methylene peak plotted as a function of the degree of cross-linking and compared with the results from Figueiredo et al. The qualitative trends were remarkably similar as shown in Figure The y-axis differed by an order of a magnitude and was suspected to be from the difference in the GA/PVA mass ratio. In the current study, the GA/PVA mass ratios corresponded to 0, 0.003, and while the referenced study had GA/PVA mass ratios of 0, 0.01, 0.1 and 1. The improvement with a small amount of GA in the system was due to the forward reaction towards cross-linked PVA. As shown in Figure 5-22(a) and (b), a small amount of GA, corresponding to a 0.01 GA/PVA mass ratio, increased both the aldehyde stretch, from the introduction of GA into the system, and the acetal stretch,

87 75 from the cross-linking of PVA. At a 0.1 GA/PVA mass ratio, the aldehyde stretches decreased because the cross-linking increased. At a GA/PVA mass ratio of 1, the aldehyde stretches increased again due to branching of PVA becoming the prevalent reaction over cross-linking. The reduced acetal stretching at higher GA/PVA mass ratios was not explained by Figueiredo et al. The above was unlikely to have happened in TEA-VA because even at 5.0% degree cross-liking, the mass ratio of GA/PVA was only Figure 5-22: Absorbance ratio of (a) aldehyde peak and (b) acetal peak with increase in GA content, adapted from [8], and (c) aldehyde and (d) acetal peaks from this study.

88 The differences in these studies were due to the presence of TEAOH, making it difficult to detect cross-liking from IR studies. Characterization of cross-linked TEA-VA was a challenge. Physical observations were the best indication of improved stability and film forming properties from lightly cross-linking. XRD, TGA and FTIR all confirmed the amorphous structure of the crosslinked polymer electrolyte, but revealed marginal differences between TEA-VA G2(0.5%) and TEA-VA that could have improved ionic conductivity such as a higher water content. Therefore, the increase in ionic conductivity was suspected to be due to the stabilizing of conduction sites that resulted in improved OH - -ion conduction efficiency. Further quantitative studies are required to convincingly characterize the cross-linked polymer electrolyte. Electrochemical characterizations Metallic solid cell performances Metallic solid cells were made with TEA-VA and TEA-VA G2(0.5%) polymer electrolytes. The ionic conductivity of the cells were tracked over a period of 80 days as shown in Figure A summary of the ionic conductivities are listed in Table 5-8. Conditioning in a 75% RH environment represented maximum OH - -ion transport for the polymer electrolytes. TEA-VA and TEA-VA G2(0.5%) had ionic conductivities of 7 x 10-3 S cm -1 to 18 x 10-3 S cm -1 respectively. Conditioning at 45% RH environment was a more realistic representation of ambient conditions. In this case, ionic conductivities of and 3 x 10-3 S cm -1 and 12 x 10-3 S cm -1 were measured for TEA-VA and TEA-VA G2(0.5%) respectively.

89 77 Conductivity (Scm -1 ) 2.0x10-2 (a) 75%RH TEA-VA TEA-VA G2(0.5%) 1.5x %RH 1.0x x Time (Day) Figure 5-23: Ionic conductivity tracking over 80 days of TEA-VA and TEA-VA G2(0.5%) being conditioned at 75 and 45% RH. Table 5-8: Summary of ionic conductivities of TEA-VA and TEA-VA G2(0.5%) at pristine, 75 and 45% RH conditions. TEA-VA Ionic Conductivity (x 10-3 S cm -1 ) TEA-VA G2(0.5%) Ionic Conductivity (x 10-3 S cm -1 ) Day 1 (Pristine) Day 24 (75 % RH) Day 79 (45 % RH) There was an absolute improvement in the ionic conductivity of TEA-VA G2(0.5%) compared to TEA-VA. This was suspected to be due to two reasons: (1) retention of amorphous structure allowed segmental motion to facilitate OH - -ion conduction and (2) improved stability that was observed physically, promoted the conduction of more efficient OH - -hopping. TEA-VA G2(0.5%) demonstrated among the

90 78 highest OH - -ion conducting ionic conductivities in ambient conditions reported to date as a thin film metallic EC cell. CV experiments were also performed throughout the duration of the tracking. Cyclic voltammograms are shown in Figure A summary of its capacitances are shown in Table 5-9. TEA-VA G2(0.5%) had a consistently higher capacitance compared to TEA-VA. The improved charge storage was evidence of better accessibility that the OH - -ions had towards the surface of the electrode. After a year of storage in ambient conditions, cycle-life tests were performed on TEA-VA and TEA-VA G2(0.5%) metallic cells as shown in Figure 5-25(a) and (b) respectively. The capacitance of TEA-VA increased over this period whereas TEA-VA G2(0.5%) stayed the same. This was suspected to be due to TEA-VA being less stable, absorbing and losing water readily in ambient conditions, whereas the TEA-VA G2(0.5%) maintained its performance without fluctuations. After 100,000 cycles, the Current Density (Acm -2 ) 1.5x x x x x x10-2 (a) Day Cell Voltage (V) TEA-VA TEA-VA G2(0.5%) Current Density (Acm -2 ) 1.5x x x x x x10-2 (b) Day Cell Voltage (V) TEA-VA TEA-VA G2(0.5%) Figure 5-24: CV of metallic EC cells made with TEA-VA and TEA-VA G2(0.5%) polymer electrolytes on (a) day 1 and (b) day 79 after conditioning in both 75 and 45% RH. (sweep rate = 5000 V s -1 )

91 79 Current Density (Acm -2 ) 1.5x x x x x x10-2 Cycle 1000 Cycle Cycle Cycle (a)tea-va Cell Voltage (V) Current Density (Acm -2 ) 1.5x10-2 Cycle 1000 Cycle x10-2 Cycle Cycle x x x x10-2 (b)tea-va G2(0.5%) Cell Voltage (V) Figure 5-25: Cycle-life test of (a) TEA-VA and (b) TEA-VA G2(0.5%) after storage for one year and conditioned at 45% RH. The cycling was performed in ambient conditions (Sweep rate = 5000 V s -1 ) capacitance of TEA-VA G2(0.5%) and TEA-VA both reduced by 3%. Stable cycle-life demonstrated the consistency of the OH - -ion transport of the system in an electric field. These results showed that upon charging and discharging, the system stayed the same, which is ideal for energy storage devices. Table 5-9: Summary of capacitances of TEA-VA and TEA-VA G2(0.5%) on Day 1 and Day 79. TEA-VA Capacitance (μf cm -2 ) TEA-VA G2(0.5%) Capacitance (μf cm -2 ) Day Day

92 Electrochemical characterizations with graphene electrodes Since TEA-VA G2(0.5%) demonstrated outstanding ionic conductivity, it is of interest to leverage it for high rate applications. One such application is for 120 Hz power filtering proposed by Miller, J.R [125]. Vertically grown graphene electrodes were provided by Miller, J.R. at JME, Inc. [126]. The cell was prepared and tested by Gao, H. from the flexible electronics and energy lab. The results have yet to be published. The assembled EDLC is shown in Figure CV was performed on the EDLC devices from 0 1 V at 1000 V s -1 as shown in Figure 5-28(a). The device demonstrated excellent capacitive behavior that resembled an ideal rectangular shape at an ultra-high sweep rate. This indicated that the device had a very fast response time. Constant current charge-discharge at 5 ma showed outstanding charge and discharge capability as shown in Figure 5-28(b). Figure 5-26: Sandwiched graphene EDLC using TEA-VA G2(0.5%) polymer electrolyte. The sample was sandwiched with glass slides and clipped for intimate contact between the electrodes and the electrolyte.

93 81 Figure 5-28: Electrochemical characterization of graphene/tea-va G2(0.5%) EDLC devices through (a) CV at 1000 V s -1 and (b) charge-discharge at 5 ma. EIS was performed on the graphene/tea-va G2(0.5%) solid device. The Nyquist and bode plots from EIS measurements are shown in Figure The ESR of the device was determined to be 0.72 Ω. The frequency at which the device reached a phase angle of -45 was 1585 Hz or 0.63 ms and the phase angle at 1 Hz was Figure 5-27: Nyquist (left) and Bode (right) plots of EDLC devices prepared from graphene electrodes and TEA-VA G2(0.5%) polymer electrolyte.

94 82 From the EIS measurements, the AC capacitance was plotted against frequency and a RC response time at 120 Hz was determined to be ms, with a capacitance of 110 μf. After the device was cycled through GC testing, the AC capacitance did not change as shown in Figure The EDLC was stored in 50% RH conditions for 9 months and showed very little change in capacitance. The EDLC prepared using TEA-VA G2(0.5%) demonstrated excellent response time at high frequencies up to 120 Hz. For power filtering applications, this polymer electrolyte satisfied the requirements with a significant margin. Along with fast rate performances, the stability of this device tested through cycling as well as shelf-life storage shows that this can be a potential candidate for these applications complementary to graphene electrodes provided. Figure 5-29: Real capacitance plotted against frequency of EDLC device with graphene electrode and TEA-VA G2(0.5%) polymer electrolyte.

95 83 Conclusions The objective of this work was to develop an OH - -ion conducting polymer electrolytes for EC applications. The developed OH - -ion conducting polymer electrolyte had high ionic conductivity, allowed high area capacitance, long cycle-life, stable shelflife and was easy to fabricate. However, the voltage window was still limited by the decomposition of water. A summary of the properties can be found in Table 6-1. The following conclusions can be drawn from the work: 1) TEA-VA polymer electrolyte was developed and compared to K-VA with a molar ratio of (4220:1). TEA-VA demonstrated a similar ionic conductivity to K-VA in pristine conditions and retained its ionic conductivity over 40 days in ambient conditions, when the ionic conductivity of K-VA decreased two orders of magnitude. Graphite electrode EDLC devices were made with TEA-VA and K-VA that both showed good capacitance of 1 mf cm -2 in pristine conditions, but the K- VA EDLC devices showed decreased capacitance to 0.01 mf cm -2 after 67 days. It was discovered the high performance was due to the TEA-VA polymer electrolyte retaining its amorphous structure. 2) TEA-VA was compared with TEA-EO and TEA-AA polymer electrolytes with a molar ratio of (3000:1). The results demonstrated that:

96 84 TEA-EO s ionic conductivity decreased from 11 x 10-3 S cm -1 to 0.4 x 10-3 S cm -1 after 40 days in ambient conditions due to the crystallinity of PEO that prevented segmental motion and hydration of the polymer electrolyte. TEA-VA s ionic conductivity decreased from 9 x 10-3 S cm -1 to 5 x 10-3 S cm -1 after 40 days in ambient conditions because PVA had a balance between filmability and hydration. TEA-AA s ionic conductivity increased from 1 x 10-3 S cm -1 to 4 x 10-3 S cm -1 after 40 days in ambient conditions due to the hydrophilic properties of PAA that prevented the polymer electrolyte from having stable film integrity. It was found that the ionic conductivity of the polymer electrolytes were greatly influenced by the polymer host material. 3) TEA-VA (4220:1) was lightly cross-linked with glutaraldehyde to 0.5 % and TEA-VA G2(0.5%) demonstrated: Outstanding ionic conductivity of 12 x 10-3 S cm -1 and stable ionic conductivity over a period of 80 days in 75 and 45% RH. Excellent cycle-life up to 100,000 cycles at 5000 V s -1 with 3% reduction in capacitance. Outstanding capacitance of 110 μf at 120 Hz with and excellent response time of 0.63 ms at a phase angle of -45 with graphene electrode EDLC devices TEA-VA G2(0.5%) showed an overall improvement in performance. It was suspected that the cross-linking improved hydration of the system while maintaining the amorphous structure and improving filmability. However, more quantitative evidence is required to justify these claims.

97 85 Table 6-1: Summary of developed OH - -ion conducting polymer electrolytes. Ionic conductivity (x 10-3 S cm -1 ) Capacitance (μf cm -2 ) Cycle life Shelf-life Fabrication method TEA-VA ,000 Excellent Easy TEA-EO Poor Easy TEA-AA Moderate Easy TEA-VA G2(0.5%) ,000 Excellent Moderate

98 86 Future Work The different types of polymer electrolytes developed showed promising results for EC applications. Much of the current work were characterized qualitatively through various experimental techniques. A more quantitative approach would be beneficial for conclusive evidence regarding the developed OH - -ion conducting polymer electrolytes. Furthermore, there is still room for improvement regarding temperature and voltage window limitations. 1. TEA-VA G2(0.5%) showed excellent ionic conductivity, but is still limited to applications under 80 C. To maintain the high ionic conductivity and improving thermal resistance, fillers can be incorporated. 2. TEA-AA showed poor stability due to fluctuations in mechanical properties. Cross-linking of TEA-AA can improve these physical properties while taking advantage of the amorphous nature of TEA-AA. 3. The voltage of all developed polymer electrolytes are limited to 1 V. To improve the voltage window, solid OH - -ion conductors such as layered double hydroxides can be explored to reduce the use of solvents. 4. The OH - -ion mechanism still remains difficult to confirm with the polymer electrolytes developed. Methods such as in-situ Raman spectroscopy can shed light onto the change in bonding as a function of temperature. 5. Other techniques such as nuclear magnetic resonance spectroscopy can be used to characterize the polymer electrolytes to determine the type of hydroxides being formed in the system. 6. Other amorphous polymers can be explored to better complement TEAOH.

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109 97 Appendices Appendix A-1: Phase diagram of KOH and H2O Phase diagram of crystallization limits of KOH and H2O Figure A-1: Phase diagram of KOH-H2O indicating formation of solids

110 98 Appendix A-2: XRD pattern of TEAOH hydrates Wiebcke et al. performed structural characterizations on TEAOH hydrates and produced the diffraction patterns from the crystallographic structure of the hydrates. The most intense peak for each diffraction pattern was used to indicate the presence of that particular hydrate. Figure A-2: XRD patterns of (a) TEAOH 4H2O and (b) TEAOH 5H2O.