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Borotungstic Acid (BWA)-Polyacrylamide (PAM) Solid Polymer Electrolytes for Electrochemical Capacitors
by
Yee Wei Foong
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science
Department of Materials Science and Engineering University of Toronto
© Copyright by Yee Wei Foong 2017
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Borotungstic Acid (BWA)-Polyacrylamide (PAM)
Solid Polymer Electrolyte for Electrochemical Capacitors
Yee Wei Foong
Master of Applied Science
Department of Materials Science and Engineering University of Toronto
2017
Abstract
Solid polymer electrolytes (SPEs) are key enablers for thin and flexible electrochemical
capacitors in wearable technologies. Polyacrylamide (PAM) is one such promising hygroscopic
polymer host, but its performance had not been optimized. This thesis enhanced PAM with
borotungstic acid (BWA) as the heteropolyacid conductors. The BWA-PAM electrolyte achieved
a high initial conductivity of ca. 27 mS cm-1, but suffered from a short service life (< 40%
conductivity retention after 28 days) due to dehydration.
BWA-PAM modified with acidic (H3PO4) and neutral (glycerol) plasticizers showed improved
conductivity of ca. 30 mS cm-1 and service life (≥ 70% conductivity retention after 28 days). The
high BWA and H3PO4 content accelerated the hydrolysis of PAM to polyacrylic acid, resulting
in the undesirable precipitation of NH4+-substituted BWA; whereas, glycerol was found to
suppress this reaction. The solid CNT-graphite cells with the optimized electrolytes
demonstrated a capacitance of ca. 19.5 mF cm-2; a high rate capability (≥ 75% capacitance
retention) at 1Vs-1; excellent cycle life (≥ 90% retention of its initial capacitance); and
maintained ca. -85o phase angle over 10,000 charging-discharging cycles.
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Acknowledgments
First and foremost, I would like to express my utmost gratitude to my thesis supervisors,
Professor Keryn Lian, Professor Steven Thorpe, and Professor Donald Kirk for their continuous
support and mentorship throughout my MASc program. They have helped me to grow in critical
analysis and research planning. I would also like to thank my committee members, Professor
Tom Coyle and Professor Charles Jia, for their valuable advice and suggestions.
I am grateful for the financial support from NSERC, NSERC CREATE, Ms. Vijayalakshmi
Graduate Scholarship in Sustainable Energy, Haultain scholarship, and Department of Materials
Science and Engineering. I would not have made it this far without their generous support.
Many thanks to Flexible Energy and Electronic lab and my colleagues, including Han Gao,
Matthew Genovese, Alvin Virya, Jak Li, Haoran Wu, Blair Decker, and Jeanne N’Diaye. Both
Han and Matthew were my mentors who introduced me to various aspects of academic research.
Alvin and Jak provided guidance on various material characterizations. Haoran helped me to
optimize the cell fabrication. Blair and Jeanne motivated me throughout my research.
I would also like to extend my appreciation to Maria Fryman and Fanny Strumas-Manousos from
the Department of Materials Science and Engineering for always being there to assist me
whenever I have concerns about the MASc program. Jessica Barnes-Burley, Krista Haldenby,
and Sabrin Mohamed are also very helpful in my preparations for the seminar presentation.
I would also like to recognize Sal Boccia for his useful advice in scanning electron microscopy
and Dan Grozea for his support in fourier transformed infrared (FTIR) spectroscopy and
differential scanning calorimetry (DSC). George Kretchmann from the Department of Earth
Sciences has also taught me about powder and film x-ray diffraction (XRD).
I am especially grateful to Professor Reeve and Albert Lee for being excellent mentors, inspiring
me to achieve lifelong learning and critical thinking to become a professional engineer leader.
Many thanks to my other friends, professionals, and professors, especially Amelia Susanto and
William Lee Yu Lin for being very supportive throughout my degree. I would like to express my
gratitude to my mother, father, and sister. I would not be able to study abroad in University of
Toronto without their support. Most importantly, I would like to acknowledge my church family
who are always there to support me through encouragement and persistent prayers.
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Table of Contents
Acknowledgments.......................................................................................................................... iii
Table of Contents ........................................................................................................................... iv
List of Tables ................................................................................................................................. ix
List of Figures ................................................................................................................................ xi
List of Abbreviations .................................................................................................................. xvii
List of Symbols and Units .......................................................................................................... xviii
Chapter 1 Introduction .....................................................................................................................1
1.1 Demand for Energy Storage.................................................................................................1
1.2 Energy Storage Devices .......................................................................................................1
1.3 Electrochemical Capacitors (ECs) .......................................................................................3
Chapter 2 Literature Review ............................................................................................................6
2.1 Solid Polymer Electrolytes (SPEs) ......................................................................................6
2.2 Charge Storage Mechanisms in ECs ..................................................................................10
2.2.1 Electric Double Layer Capacitance (EDLC) .........................................................10
2.2.2 Pseudocapacitance .................................................................................................11
2.3 Proton conduction mechanisms in SPEs ............................................................................13
2.4 Polymer hosts .....................................................................................................................16
2.4.1 Polyvinyl alcohol ...................................................................................................20
2.4.2 Polyacrylamide ......................................................................................................21
2.5 Keggin-type heteropolyacids .............................................................................................25
2.6 Polymer-in-salt electrolytes ...............................................................................................27
2.6.1 Heteropolyacid-based membranes .........................................................................29
2.7 Additives for polymer-in-salt electrolytes .........................................................................31
2.7.1 Acidic plasticizers ..................................................................................................32
2.7.2 Neutral plasticizers .................................................................................................33
v
Chapter 3 Research Plan ................................................................................................................34
3.1 Motivation ..........................................................................................................................34
3.2 Experimental Design ..........................................................................................................35
3.2.1 Phase 1: Optimization of BWA-PAM Binary System ...........................................35
3.2.2 Phase 2: Enhancing of BWA-PAM with H3PO4 Plasticizer ..................................36
3.2.3 Phase 3: Enhancing BWA-PAM with Glycerol Plasticizer ...................................36
3.3 Thesis Outline ....................................................................................................................36
Chapter 4 Experimental Method ....................................................................................................38
4.1 Materials ............................................................................................................................38
4.1.1 Heteropolyacids (BWA) ........................................................................................38
4.1.2 Polymer host (PAM) ..............................................................................................38
4.1.3 Additives ................................................................................................................39
4.2 Preparation of polymer electrolytes ...................................................................................39
4.2.1 BWA-PAM ............................................................................................................39
4.2.2 BWA-PAM + H3PO4 .............................................................................................39
4.2.3 BWA-PAM + Glycerol ..........................................................................................40
4.3 Working Electrodes ...........................................................................................................40
4.3.1 Metallic Electrodes ................................................................................................40
4.3.2 Carbon Electrodes ..................................................................................................41
4.4 Construction of liquid cells ................................................................................................42
4.4.1 Liquid 3-electrode cells ..........................................................................................42
4.4.2 Liquid 2-electrode cells ..........................................................................................42
4.5 Construction of solid cells .................................................................................................43
4.5.1 Solid capacitor cells ...............................................................................................43
4.6 Electrochemical characterization methods ........................................................................44
4.6.1 Cyclic voltammetry (CV) ......................................................................................44
vi
4.6.2 Electrochemical impedance spectroscopy (EIS) ....................................................46
4.7 Material and structural characterizations ...........................................................................48
4.7.1 Simulated cells .......................................................................................................48
4.7.2 Optical microscopy ................................................................................................49
4.7.3 Raman spectroscopy ..............................................................................................49
4.7.4 Fourier transform infrared (FTIR) spectroscopy ...................................................49
4.7.5 X-ray diffraction (XRD) ........................................................................................49
Chapter 5 Development of BWA-PAM binary system .................................................................50
5.1 BWA characterizations ......................................................................................................50
5.1.1 Ionic conductivity of BWA (Two-electrode Beaker Cell) .....................................50
5.1.2 Electrochemical window of BWA (Cyclic Voltammetry) .....................................51
5.1.3 Chemical bonding in BWA (Raman Spectroscopy) ..............................................52
5.1.4 Structure of BWA (XRD Analysis) .......................................................................54
5.2 PAM characterizations .......................................................................................................56
5.2.1 Water retention in PAM (Raman and FTIR Analysis) ..........................................56
5.2.2 Structure of PAM (XRD Analysis) ........................................................................57
5.3 Proton conductivity ............................................................................................................58
5.4 Material characterization ...................................................................................................60
5.4.1 Chemical bonding in BWA-PAM binary electrolytes (Raman Analysis) .............60
5.4.2 Structure of BWA-PAM binary electrolytes (XRD Analysis) ..............................63
5.5 Summary ............................................................................................................................65
Chapter 6 Effects of plasticizers in BWA-PAM ............................................................................66
6.1 Acidic plasticizer ...............................................................................................................66
6.1.1 Proton conductivity ................................................................................................66
6.1.2 Crystallization at high content of BWA and H3PO4 (Visual inspection)...............68
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6.1.3 Chemical bonding analyses of BWA-PAM + H3PO4 Ternary Electrolytes (Raman analysis) ....................................................................................................70
6.1.4 Structural and chemical analyses of the precipitates (XRD and Raman analysis) .................................................................................................................72
6.2 Neutral plasticizer ..............................................................................................................75
6.2.1 Proton conductivity ................................................................................................75
6.2.2 Chemical stability of BWA-PAM + Gly Ternary Electrolytes (Raman analysis) .................................................................................................................77
6.2.3 Water retention of BWA-PAM + Gly Ternary Electrolytes (XRD analysis) ........81
6.3 Summary ............................................................................................................................82
Chapter 7 Device performance of the optimized ternary electrolytes ...........................................83
7.1 Acidic vs. Neutral Plasticizers ...........................................................................................83
7.2 Solid Cells with 75BWA-PAM + 10% H3PO4 Electrolyte ................................................84
7.2.1 Liquid Cells versus Solid Cells ..............................................................................84
7.2.2 Cycle Life Analysis................................................................................................86
7.3 Solid Cells with 85BWA-PAM + 10% Gly Electrolyte ....................................................87
7.3.1 Liquid Cells versus Solid Cells ..............................................................................87
7.3.2 Cycle Life Analysis................................................................................................88
7.4 Summary ............................................................................................................................90
Chapter 8 Summary and outlook ...................................................................................................91
8.1 Conclusions ........................................................................................................................91
8.2 Future work ........................................................................................................................93
References ......................................................................................................................................94
Appendix A – Raman and FTIR Peak Assignments ....................................................................114
Appendix B – FTIR Analysis for SiWA and BWA .....................................................................118
Appendix C – Acid attack on PAM .............................................................................................119
Appendix D – Change in electrolyte thickness due to dehydration .............................................120
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Appendix E – Electrochemical impedance spectroscopy (EIS) analyses for the liquid cells and the solid cells ....................................................................................................................121
ix
List of Tables
Table 1: Comparison of the energy storage performance for capacitor, electrochemical capacitor
(EC), and battery [11]. .................................................................................................................... 2
Table 2: Summary of different polymer hosts for EC applications. ............................................. 17
Table 3: Compositions of BWA-PAM polymer electrolytes. ....................................................... 39
Table 4: Compositions of BWA-PAM + H3PO4 polymer electrolytes. ........................................ 40
Table 5: Compositions of BWA-PAM + Glycerol polymer electrolytes. .................................... 40
Table 6: The comparison of conductivity as a function of acid content for BWA-PAM and
H3PO4-PAM electrolyte systems [163]. ........................................................................................ 59
Table 7: Peak intensity ratios of the υ(C-N), δ(NH2), υ(C=O), and υ(COO-) peaks in PAM,
50BWA-PAM, 75BWA-PAM, and 85BWA-PAM normalized against the δ(CH2) at 1458.4 cm-1
from the polymer backbone as the reference. ............................................................................... 63
Table 8: Peak intensity ratios of the υ(C-N), δ(NH2), υ(C=O), and υ(COO-) peaks in PAM,
75BWA-PAM, 75BWA-PAM + 10% H3PO4, 85BWA-PAM, 85BWA-PAM + 10% H3PO4, and
PAA normalized against the δ(CH2) at 1458.4 cm-1 from the polymer backbone as the reference.
....................................................................................................................................................... 71
Table 9: Peak assignments for the Raman spectra of SiWA and BWA powders dried at 60oC for
24 hours prior to the characterization [220-222]. ....................................................................... 114
Table 10: FTIR peak assignments for SiWA and BWA powders dried at 60oC for 24 hours prior
to the characterization [115, 173, 237-241]. ............................................................................... 114
Table 11: Raman peak assignments for PAM film conditioned at 45% RH for 7 days [225]. ... 115
Table 12: FTIR peak assignments for PAM film conditioned at 45% RH for 7 days [226]. ..... 115
Table 13: Peak assignments for the Raman spectra of BWA-PAM electrolyte systems [175, 176,
220-222, 225, 242]. ..................................................................................................................... 116
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Table 14: Raman peak assignments for BWA-PAM + H3PO4 electrolytes [175, 220-222, 225,
242-246]. ..................................................................................................................................... 116
Table 15: Raman peak assignments for BWA-PAM + glycerol electrolytes [175, 176, 220-222,
225, 242, 245, 247, 248]. ............................................................................................................ 117
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List of Figures
Figure 1: Ragone chart for various energy storage devices [10]. ................................................... 2
Figure 2: Schematic of ECs based on SPEs in (a) flexible sandwiched cell configuration, (b)
interdigitated finger cell configuration, and (c) coaxial fiber cell configuration [37]. ................... 7
Figure 3: The different types of SPEs and the conducting species in aqueous-based SPEs. .......... 8
Figure 4: Charge separation in EDLC during a charging process. ............................................... 10
Figure 5: Charge separation in a pseudocapacitive material through oxidation (Ox) and reduction
(Red) reactions. .............................................................................................................................. 12
Figure 6: Pseudocapacitive profile of ruthenium dioxide, depicting the evenly distributed redox
potentials when subjected to incremental voltage scan [68]. ........................................................ 12
Figure 7: Ionic conduction mechanisms in polymer electrolytes: (a) ion hopping, (b) vehicular
diffusion, and (c) segmental motion [37]. ..................................................................................... 14
Figure 8: Structure of a PVA. ....................................................................................................... 20
Figure 9: Structure of a PAM. ....................................................................................................... 21
Figure 10: Room temperature conductivity as a function of acid/AM molar ratio for PAM gel
electrolyte (40 vol% water) embedded with H3PO4 and H2SO4 [165]. ........................................ 22
Figure 11: Room temperature conductivity of 3D crosslinked PAM embedded with H3PO4 (a) as
a function of H3PO4 loading in anhydrous state; and (b) as a function of swelling volume ratio
from H2O uptake [161]. ................................................................................................................ 23
Figure 12: Room temperature conductivity of (a) H3PO4/H2O system and (b) H3PO4/PAM
system as a function of wt% H3PO4 [163]. ................................................................................... 24
Figure 13: (a) Ball-and-stick view of heteropolyacids with the Keggin-type structure, and (b)
polyhedral representation of the Keggin structure showing the central tetrahedron (MO4); the
terminal oxygen atoms of each X3O13 group (X-Od); the bridging bonds between edge-sharing
oxygen atoms (X-Oc-X); and corner-sharing octahedra (X-Ob-X). [177, 178]. ........................... 25
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Figure 14: The trend of reduction potential is shifted in a predictable manner with respect to the
different anion charge for various heteroatom in HPAs [188]. .................................................... 27
Figure 15: Morphology model for a polymer electrolyte as a function of salt content [189]. ...... 28
Figure 16: Three electrode CVs at a scan rate of 100 mV s-1 for (a) SiWA–XLPVA and (b)
BWA–XLPVA using graphite electrodes; CVs at a scan rate of 100 mV s-1 for symmetric solid
graphite cell with (c) a SiWA–XLPVA and (d) a BWA–XLPVA electrolytes at incremental cell
voltage; electrode potential limits of both cathode and anode versus cell voltage for the solid
cells with (e) SiWA–XLPVA and (f) BWA–XLPVA electrolyte [202]. ..................................... 30
Figure 17: Conductivity against temperature of BWA-XLPVA and SiWA-XLPVA at various
humidity conditions [202]. ............................................................................................................ 31
Figure 18: Research Outline ......................................................................................................... 35
Figure 19: The steps for doctor-blading of carbon ink onto the titanium substrate. ..................... 41
Figure 20: A liquid 2-electrode cell. ............................................................................................. 42
Figure 21: The assembly of an electrochemical capacitor cell. .................................................... 43
Figure 22: Rectangular CV shape of an ideal electrolyte material [216]. .................................... 44
Figure 23: CV comparisons of an ideal capacitor vs capacitor with resistivity. ........................... 45
Figure 24: (a) AC perturbation input signal with various frequencies for EIS, and (b) an
equivalent circuit for capacitor with resistance. ........................................................................... 46
Figure 25: (a) Nyquist plot and (b) Bode plot obtained from EIS of an electrolyte test cell. ....... 46
Figure 26: Room temperature conductivity of SiWA and BWA solutions as a function of
concentration. ................................................................................................................................ 51
Figure 27: Third scan for the cyclic voltammograms in a 3-electrode cell with a glassy carbon
working electrode and platinum mesh counter electrode, showing the electrochemical stability
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windows for 0.01 M SiWA and 0.01 M of the synthesized BWA at a scan rate of 50 mV s-1
against a saturated Ag/AgCl reference electrode. ......................................................................... 52
Figure 28: Raman spectra of (a) SiWA powder and (b) the synthesized BWA powder dried at
60oC for 24 hours prior to characterization. The Keggin-type structures of SiWA and BWA are
shown as inset figures. The υ(Si-O) and υ(B-O) peaks were highlighted in blue color, while the
δ(W-O-W) peak was highlighted in red color. ............................................................................. 54
Figure 29: X-ray diffraction patterns of (a) SiWA powder and (b) the synthesized BWA powder
dried at 80oC for 24 hours prior to the characterization. The structure of Keggin-type HPA was
included as an inset. The hydrated HPA peak was highlighted in red color, while the dehydrated
HPA peaks were highlighted in blue color. .................................................................................. 55
Figure 30: (a) Raman and (b) FTIR spectra of pure PAM film conditioned at 45% RH for 7 days.
Inset shows the structure of PAM. The acrylamide functional group peaks were highlighted in
blue, while the polymer backbone peak used as the reference in this thesis was highlighted in red.
....................................................................................................................................................... 57
Figure 31: XRD pattern for PAM film conditioned at 45% RH for 7 days prior to the
characterization. ............................................................................................................................ 58
Figure 32: Room temperature conductivity versus time of electrolytes with PAM, 50BWA-PAM,
75BWA-PAM, and 85BWA-PAM conditioned at 45%RH over a period of 28 days. ................. 59
Figure 33: (a) Photo of electrolyte delamination from the electrode surface on day 28; and (b)
optical micrograph of the delaminated area of the electrolyte, showing precipitation due to
electrolyte dehydration. ................................................................................................................. 60
Figure 34: Raman spectra for dried films of PAM, 50BWA-PAM, 75BWA-PAM, and 85BWA-
PAM conditioned for 7 days at 45% RH. The υ(B-O) peak was highlighted in red color, while
the δ(O-H) was highlighted in blue color. .................................................................................... 61
Figure 35: Raman spectra for dried films of PAM, 50BWA-PAM, 75BWA-PAM, and 85BWA-
PAM conditioned for 7 days at 45% RH (focus on the range of 1000-1800 cm-1). The acrylamide
xiv
functional group peaks with changing intensity were highlighted in blue color, while the
additional peak was highlighted in red color. ............................................................................... 62
Figure 36: X-ray diffraction patterns for dried films of PAM, 50BWA-PAM, 75BWA-PAM, and
85BWA-PAM conditioned for 7 days at 45% RH........................................................................ 65
Figure 37: Room temperature conductivity versus time of electrolytes with 75BWA-PAM and
85BWA-PAM, with 5 wt% and 10 wt% H3PO4, conditioned at 45%RH over 28 days. .............. 67
Figure 38: Day 1 and day 28 observations against a University of Toronto logo as the
background of the electrolyte in 1cm x 1cm simulated cells for (a) 75BWA-PAM + 5% H3PO4,
(b) 85BWA-PAM + 5% H3PO4, (c) 75BWA-PAM + 10% H3PO4 and (d) 85BWA-PAM + 10%
H3PO4; and (e) optical micrographs of the 85BWA-PAM + 10% H3PO4 electrolyte. ................. 69
Figure 39: Raman spectra of PAM, 75BWA-PAM, 75BWA-PAM + 10% H3PO4, 85BWA-PAM,
85BWA-PAM + 10% H3PO4, and PAA. The characteristic peaks for PAM were highlighted in
blue color, while the PAA peak was highlighted in red color. ..................................................... 70
Figure 40: X-ray diffraction patterns of dried films for (a) 75BWA-PAM; (b) 85BWA-PAM; (c)
75BWA-PAM + 10% H3PO4; and (d) 85BWA-PAM + 10% H3PO4. .......................................... 72
Figure 41: (a) X-ray diffraction patterns of dried samples for BWA powder, 85BWA-PAM +
10% H3PO4 electrolyte, and NH4+-substituted BWA powder; and (b) Raman spectra of non-
crystal and crystal regions in 85BWA-PAM + 10% H3PO4 electrolyte. The shifted Raman peaks
were highlighted in blue color. ..................................................................................................... 74
Figure 42: Room temperature conductivity versus time of electrolytes with 75BWA-PAM,
85BWA-PAM, 75BWA-PAM + 10% Gly, and 85BWA-PAM + 10% Gly conditioned at 45%RH
over a period of 28 days. ............................................................................................................... 76
Figure 43: Raman spectra of PAM, BWA, and glycerol compared to Gly-PAM, 75BWA-PAM,
75BWA-PAM, and 85BWA-PAM modified with (a) 5 wt% Gly and (b) 10 wt% Gly. The
characteristic peaks for BWA were highlighted in blue color, glycerol characteristic peak was
highlighted in red color, and the PAM characteristic peaks were highlighted in green color. ..... 78
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Figure 44: Raman spectra of PAM, 85BWA-PAM, 85BWA-PAM + 10% H3PO4, 85BWA-PAM
+ 10% Gly, and PAA. The glycerol peak was highlighted in blue color, the PAM functional
group peaks were highlighted in red color, and the PAA peak was highlighted in green color. .. 80
Figure 45: X-ray diffraction patterns of dried films for (a) PAM, 5% Gly-PAM, 75BWA-PAM,
75BWA-PAM + 5% Gly, and 85BWA-PAM + 5% Gly; and (b) PAM, 10% Gly-PAM, 85BWA-
PAM, 85BWA-PAM + 10% Gly, and 85BWA-PAM + 10% Gly. .............................................. 82
Figure 46: Room temperature conductivity versus time of 75BWA-PAM + 10% H3PO4, and
85BWA-PAM + 10% Gly conditioned at 45%RH over a period of 28 days. .............................. 84
Figure 47: Third scan of cyclic voltammograms (a) at 50 mV s-1 and (b) at 1 V s-1 for 75BWA-
PAM + 10% H3PO4 on CNT-graphite solid cells against the liquid electrolyte cells; and (c) the
discharging capacitance for 75BWA-PAM + 10% H3PO4 at various scan rates. Inset showed the
solid capacitor cells. ...................................................................................................................... 85
Figure 48: (a) The capacitance retention rate of 75BWA-PAM + 10% H3PO4 solid cells
throughout the 10,000 charging-discharging cycles (CDC) at 6 A g-1 with the 1-10 cycles and
9,991-10,000 cycles shown in the inset; (b) Nyquist plots and (c) third scan of CVs at 50 mV s-1
of 75BWA-PAM + 10% H3PO4 solid cells before and after the CDC. ........................................ 86
Figure 49: Third scan of cyclic voltammograms (a) at 50 mV s-1 and (b) at 1 V s-1 for 85BWA-
PAM + 10% Gly on CNT-graphite solid cells against the liquid electrolyte cells; and (c) the
discharging capacitance for 85BWA-PAM + 10% Gly at various scan rates. Inset showed the
solid capacitor cells. ...................................................................................................................... 88
Figure 50: (a) The capacitance retention rate of 85BWA-PAM + 10% Gly solid cells throughout
the 10,000 charging-discharging cycles (CDC) at 6 A g-1 with the 1-10 cycles and 9,991-10,000
cycles shown in the inset; (b) Nyquist plots and (c) third scan of CVs at 50 mV s-1 of 85BWA-
PAM + 10% Gly solid cells before and after the CDC. ................................................................ 89
Figure 51: FTIR spectra of (a) SiWA powder and (b) the synthesized BWA powder dried at 60oC
for 24 hours prior to the characterization. ................................................................................... 118
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Figure 52: The observations on the electrolytes for (a) 75BWA-PAM, (b) 85BWA-PAM, and (c)
90BWA-PAM after 6 months. .................................................................................................... 119
Figure 53: The change in electrolyte thickness over a tracking period of 21 days conditioned at
45% RH. ...................................................................................................................................... 120
Figure 54: The Nyquist (left) and Bode (right) plots for (a) 75BWA-PAM + 10% H3PO4 and (b)
85BWA-PAM + 10%Gly solid cells against their liquid cell counterparts. ............................... 121
xvii
List of Abbreviations
Abbreviations Name AC Alternating current BWA Borotungstic acid CNT Carbon nanotube CV Cyclic voltammetry / cyclic voltammogram EDLC Electrochemical double-layer capacitor EIS Electrochemical impedance spectroscopy ESR Equivalent series resistance FTIR Fourier transform infrared spectroscopy HPA Heteropolyacid IPA Isopropanol MWCNT Multi-wall carbon nanotube PAA Poly(acrylic acid) PAM Poly(acrylamide) PANI polyaniline PEO Poly(ethylene oxide) PFSA Perfluorosulphonic acid PTFE Polytetrafuorethylene PVDF Poly(vinylidene fluoride) PWA Phosphotungstic acid RC Resistor-capacitor RH Relative humidity SiWA Silicotungstic acid SPE Solid polymer electrolyte SPEEK Sulfonated poly(ether ether ketone) SPEEKK Sulfonated poly(ether ether ketone ketone) SWCNT Single-wall carbon nanotube XRD X-ray diffraction
xviii
List of Symbols and Units
Symbol Name (unit) A Area (m2) C Capacitance (F) C(ω) Complex capacitance (F) C’ Real part of complex capacitance (F) C” Imaginary part of complex capacitance (F) d Thickness (m) E Energy (J) f Frequency (Hz) j Imaginary unit (-) P Power (W) Q Charge (C) R Resistance (Ω) or gas constant (JK-1mol-1) T Temperature (K or oC) t Time (s) U Cell voltage (V) W Weight of electrolyte films (g) Z(ω) Complex impedance (Ω) Z’ Real part of complex impedance (Ω) Z” Imaginary part of complex impedance (Ω)
Permittivity of vacuum (Fm-1) Dielectric constant (-)
Impedance phase shift (deg.) Pi (-) Conductivity (Sm-1) Angular frequency (rads-1)
1
Chapter 1 Introduction
1.1 Demand for Energy Storage
Thin, flexible, and lightweight electronics have gained interests because of recent technological
advancements [1-6]. This flexible form factor opens up a whole new dimension of design
possibilities that cannot be achieved with conventional rigid electronic components. Some
examples of flexible electronics are wearable devices, textile electronics, and flexible displays.
These flexible and wearable electronics have potentially high market values when incorporating
energy-harvesting and energy-storing capabilities.
Large scale commercialization of such devices remains challenging. One of the main limitations
is the rigidity and bulkiness of the existing energy storage devices. The bulky packaging of the
conventional energy storage devices not only adds to the deadweight of the electronic devices,
but also precludes the flexible design aspect of wearable technologies.
Thin and flexible energy storage technologies become the key enabler for the future
commercialization of most flexible electronics [7]. By replacing the conventional energy storage
devices that contain liquid electrolytes, flexible energy storage devices can also eliminate
potential electrolyte leakage, alleviate safety concerns, provide a thin and flexible form factor,
and a higher energy density to power the wearable technologies.
1.2 Energy Storage Devices
Current commercial energy storage devices are dominated by fuel cells, batteries, flywheels,
conventional capacitors, and electrochemical capacitors (ECs) [8, 9]. These devices can store
different amounts of energy (energy density) and perform at different charging-discharging rates
(power density) as represented in the Ragone chart in Figure 1 [10]. Their energy storage
performance is summarized in Table 1 [11].
Batteries and fuel cells have high energy density and are frequently used in applications that
require high energy storage capacity such as hybrid vehicles and grid energy storage [12-14].
However, they suffer from slow charging-discharging rates and a low specific power of ≤ 1 kW
kg-1 as demonstrated in Table 1. In contrast, the conventional capacitor displays a fast charging-
2
discharging rate, but has a limited energy density [15-17]. Therefore, conventional capacitors are
often used in applications that require fast energy response such as smoothing power supplies
and backup circuits for electronic devices. The goal of this thesis is to engineer an energy storage
device that can achieve both a high energy density and high power density.
Figure 1: Ragone chart for various energy storage devices [10].
Table 1: Comparison of the energy storage performance for capacitor, electrochemical
capacitor (EC), and battery [11].
Characteristics Capacitor EC Battery Specific energy (W h kg-1) < 0.1 1-10 10-100 Specific power (W kg-1) >> 10,000 500-10,000 < 1,000 Discharge time 10-6 to 10-3 1 s to 2 min 0.3-3 h Charge time 10-6 to 10-3 1 s to 2 min 1-5 h Columbic efficiency (%) ca. 100 85-98 70-85 Cycle life Almost infinite >500,000 ca. 1,000
3
1.3 Electrochemical Capacitors (ECs)
Among all energy storage devices, only the electrochemical capacitor (EC) demonstrates hybrid
properties by bridging the gap between batteries and conventional capacitors [18-21]. As shown
in Figure 1, the EC exhibits 10-100 times higher specific energy than conventional capacitors
[22, 23]. They also have longer cycle life (> 500,000 cycles) and fast charging-discharging
capability (1-120s) compared to batteries [22, 24]. Due to their excellent properties, ECs can
operate in either a stand-alone configuration, replace batteries, or form the battery-EC hybrid
systems to provide peak assist and regenerative power in vehicles, as well as to reach high
energy densities for fast-charging consumer electronics.
However, ECs have a lower energy density than batteries and require bulky packaging which
adds to their deadweight. The energy density in an EC is defined by the voltage and capacitance
in the following equation [25]:
12
(1)
where , , and .
This equation is derived by integrating the voltage change in the discharge process. Therefore, to
maximize energy, the capacitance and the operating voltage window are two key parameters.
The power of an EC is given by:
(2)
where , , and .
The current can then be represented by the equation:
(3)
where , , and .
4
By substituting equation (3) into (2) yields:
(4)
The maximum power from equation (4) can be found by taking the first differential with respect
to R and setting it to zero:
2
0 (5)
Solving equation (5) yields:
(6)
Substituting the result in (6) back into equation (4) yields the maximum power of an EC defined
by the voltage and resistance as given by equation (7) [25]:
4
(7)
where , , and .
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 in a
metallic cell. The power is also calculated by the energy released or restored during a charging-
discharging period [25]:
(8)
where , , and .
5
From the aforementioned equations, increasing the voltage improves the power density and
energy density of the ECs, while reducing the resistance prevents losing the inherently high
power density of ECs. Both the voltage and resistance are closely related to the ion movement in
the electrolyte.
A conventional EC is constructed using an electrically insulating but ionically conducting
separator film impregnated with a liquid electrolyte and sandwiched between two electrodes. The
liquid electrolyte enables fast ion movement, thus the low resistance in liquid electrolyte helps to
achieve high power density in EC devices. This thesis aims to engineer solid polymer
electrolytes for ECs to replace the conventional liquid electrolytes and improve the energy
density without losing the high power density of ECs.
6
Chapter 2 Literature Review
2.1 Solid Polymer Electrolytes (SPEs)
A typical solid polymer electrolyte (SPE) consists of a polymer host matrix, an ionic conductor,
and additives. The polymer host functions as the structural support. Due to the low intrinsic
conductivity of the polymer host, usually ionic conductors and additives are added to improve
the electrolyte performance. SPEs not only act as an ionic conductor, but also function as the
separator between the electrodes in cells or batteries. This enables SPEs to replace conventional
liquid electrolytes to eliminate the occurrence of hazardous gas or corrosive solvent leakage, as
well as remove the requirement for inert porous spacers to achieve thin, lightweight, and flexible
designs [26-29].
SPEs also offer advantages such as high mechanical strength to withstand physical deformation
to the cell. These advantages enable SPEs to have long service life and wide operating
temperature range [30-33]. SPEs are easy to process and fabrication can be full automated, and
configured into various geometries [33-35]. Various flexible solid ECs can be made using SPEs
as shown in Figure 2. Figure 2 (a) depicts a multi-cell configuration assembled by sandwiching
the polymer electrolyte between the bi-polar plates and terminal electrodes, which is favorable
for compact energy storage designs. Interdigitated finger cells in Figure 2 (b) are optimal for 2D
planar EC applications such as thin and flexible displays. Coaxial fiber ECs in Figure 2 (c) are
highly flexible to be woven within textiles, cotton, or fabric for wearable electronics and medical
monitoring devices [36].
7
Figure 2: Schematic of ECs based on SPEs in (a) flexible sandwiched cell configuration, (b)
interdigitated finger cell configuration, and (c) coaxial fiber cell configuration [37].
The SPEs can be categorized into three types: organic-based, aqueous-based, and ionic liquid
(IL)-based electrolytes as shown in Figure 3 [38]. Depending on the types of solvent, the
electrochemical stability window can vary, leading to different energy densities for the ECs.
Among the three categories, aqueous-based electrolytes have the narrowest electrochemical
stability window limited by the water decomposition at 1.23 V [39-42]. Organic solvents have a
wider decomposition potentials of 2.5 – 2.7 V [42-46], whereas the decomposition of the IL can
exceed 3 V [47-51]. Using organic and IL-based electrolytes may lead to higher energy densities
for ECs, but they are more expensive compared to aqueous-based electrolytes. The ionic
conductivity of aqueous-based electrolytes is also the highest as compared to the organic and IL-
based electrolytes.
Aqueous solvents are non-flammable, which is a valuable safety factor over organic solvents
[52-54]. Aqueous solvents are also stable in air as compared to the volatile organic solvents and
the ionic liquids which require inert atmosphere due to their sensitivity to moisture [55, 56].
Aqueous solvents are also relatively environmentally safe as compared to ionic liquids and
organic solvents. These advantages favor aqueous-based SPEs compared to organic or IL-based
SPEs for printable and flexible EC applications. Therefore, this thesis will focus only on
engineering high-performing aqueous-based SPEs.
8
Figure 3: The different types of SPEs and the conducting species in aqueous-based SPEs.
Electrolytes with high ionic conductivities are required for high power applications. Polymers
are generally non-conductive and require the addition of conducting species to increase their
conductivity. The three different types of conducting species commonly added are hydroxyl ions
(OH-), proton (H+), or a neutral salt as depicted in Figure 3.
Hydroxyl-conducting electrolyte utilizes the OH- ion as the conducting species. OH- ions (5.30 x
10-5 cm2 s-1) have a lower diffusivity than H+ ions (9.31 x 10-5 cm2 s-1) due to their larger ion size
[57]. The most commonly studied OH- conductor is potassium hydroxide (KOH) due to its wide
applications in polyvinyl alcohol (PVA)-based electrolytes [58]. The KOH-PVA electrolytes can
achieve high room temperature conductivities of 8-10 mS cm-1 as reported by Yang et al. [59]
and Ma et al. [60].
Proton-conducting electrolytes are widely studied for EC applications due to their high ionic
conductivity and compatibility to pseudocapacitive electrodes. Since protons or H+ ions are the
smallest available ions, they demonstrate fast ion mobility for conduction. Aqueous proton-
conducting electrolytes are usually polymers embedded with acids such as sulfuric acid (H2SO4),
hydrochloric acid (HCl), or phosphoric acid (H3PO4). These aqueous acids can achieve high
room temperature conductivity of 0.8 S cm-1 [61].
9
Neutral salts have the potential advantages of being less corrosive and a wider electrochemical
stability window when compared with hydroxide and proton conducting electrolytes. This wider
stability window is attributed to the local change in the pH of the neutral electrolyte near the
electrode surfaces; thus achieving a higher potential window of 1.5 V as compared to the limited
window of 1.2 V for proton and hydroxide conducting electrolytes [62]. However, the
conductivity of neutral salts is lower than that of the proton-conducting electrolytes.
Before going forward with optimizing an SPE, it is crucial to understand the desired properties of
an SPE. An ideal SPE should have the following properties:
Electrically insulating: An SPE that is not electrically insulating will allow electrons to
pass through the electrolyte, thus resulting in short-circuiting of the two electrode
terminals, which would lead to the failure of the ECs.
High ionic conductivity: An SPE with high electrolyte resistance will hinder the
movement of ions for energy storage and thus impede the power performance of the ECs.
Large electrochemical stability window: A small electrochemical stability window
leads to limited cell voltage (U), resulting in lower energy storage (E) and maximum
power (Pmax) in the ECs according to equations (1) and (2). If the SPEs are pushed
beyond its stability window, they may undergo O2 or H2 evolution. The electric charges
will participate in these irreversible reactions, resulting in leakage current and
degradation of the SPE after multiple charging-discharging cycles.
Good contact/compatibility with electrode surface: A major disadvantage of switching
from liquid to solid electrolytes was the lack of intimate contact at the
electrode/electrolyte interface, resulting in high interfacial resistance. An SPE that is not
compatible with the electrode can also lead to corrosion and degradation of the
pseudocapacitive materials at the electrode surface.
The properties listed above are the fundamental characteristics of an ideal SPE electrolyte. For
feasible commercial deployment of the SPEs, the cost, ease of fabrication, and stability should
also be considered.
10
2.2 Charge Storage Mechanisms in ECs
To improve the performance of an EC, it is important to understand the charge storage
mechanism. ECs store energy by the separation of charges in the electrolyte through electric
double layer capacitance (EDLC) or pseudocapacitance.
2.2.1 Electric Double Layer Capacitance (EDLC)
Electric double layer capacitance (EDLC) utilizes reversible adsorption and desorption of
solvated electrolytic ions onto the electrode surface electrostatically as shown in Figure 4 [63,
64]. The adsorbed ions will form a Helmholtz double layer at the electrode-electrolyte interface,
resulting in non-faradaic energy storage.
Figure 4: Charge separation in EDLC during a charging process.
The charge stored at the electrode/electrolyte interface is proportional to the capacitance as
expressed in:
(9)
where , , and .
11
The capacitance of an EC (unit: Farad, F) is described by the amount of charge (unit: Coulomb,
C) at the surface of an electrode when a potential (unit: voltage, V) is applied across the
electrodes. A typical smooth metallic electrode in a liquid electrolyte solution can achieve a
double-layer capacitance of ca. 15-30 μF cm-2 [65]. The double layer capacitance can be
calculated by:
(10)
where , ,
, , and
.
Since EDLCs rely only on electrostatic charge without charge transfer between the electrode and
electrolyte, they can achieve high power performance with excellent conductivity and cycle life
in the order of 105 to 106 cycles [66]. However, the transition from a conventional liquid
electrolyte to SPEs results in a two to three orders of magnitude reduction in conductivity. This
problem can be compensated by the ability of SPEs to form thin film configurations to reduce the
effective electrolyte thickness.
2.2.2 Pseudocapacitance
Pseudocapacitance is another charge storage mechanism that relies on fast and reversible redox
reactions on both of the electrodes as depicted in Figure 5. The faradaic redox reactions lead to
charge separation of the ions in the electrode materials, resulting in multiple electron transfer
process that is similar to the effect of an EDLC [67].The overlapping redox potentials due to the
electron transfer process can contribute to the pseudocapacitance, resulting in a capacitance 100
times larger than that of an EDLC.
12
Figure 5: Charge separation in a pseudocapacitive material through oxidation (Ox) and
reduction (Red) reactions.
The mechanism of the pseudocapacitance depends on the potentials of the faradaic fast redox
reaction of the 1-2 monolayers of surface materials on the electrodes. This implies that the redox
properties of the electrode materials have large influence on the pseudocapacitive behavior of the
EC. Ideally, the redox reactions of the electrode materials should have multiple overlapping
oxidation states that have evenly distributed potentials to allow for a stable current flow during
the charging and discharging of the EC. For instance, ruthenium dioxide (RuO2) is a near ideal
pseudocapacitive material which has evenly distributed redox potentials that results in an almost
rectangular capacitance profile as shown in Figure 6 [68, 69].
Figure 6: Pseudocapacitive profile of ruthenium dioxide, depicting the evenly distributed
redox potentials when subjected to incremental voltage scan [68].
13
Previous work demonstrated that this ideal rectangular profile can be also achieved by using
conducting polymers such as polypyrrole (PPy) and polyaniline (Pani) which store charges in the
bulk through redox reactions [70, 71]. Conducting polymers suffer from low power because of
their slow bulk ionic diffusion [72]. Oxides, such as manganese oxide (MnO2), are also widely
used as pseudocapacitive materials because of its low cost and low toxicity [73, 74].
Adsorption of protons from the electrolyte is required for these pseudocapacitive reactions [75].
Proton-conducting SPEs are especially promising as they tend to have high ionic conductivity
and contribute many protons during their self-dissociation to the faradaic redox reactions on the
pseudocapacitive electrode surfaces [76-78]:
RuO2 + xH+ + xe- ↔ RuO2-x(OH)x (11)
HxMnO2 ↔ MnO2 + xH+ + xe- (12)
Panired + nA- + nH+ ↔ Paniox + ne- (13)
where .
Therefore, proton conducting SPEs are versatile materials that can be used in applications for
both EDLC and pseudocapacitive ECs to store more energy. Other than energy density, another
property of interest as mentioned in Chapter 1 is the power density of the ECs, which is heavily
depended on the ion conduction in the SPEs.
2.3 Proton conduction mechanisms in SPEs
Ionic conductivity is a crucial characteristic for high-power applications. Therefore, it is
important to understand the mechanism in which ion conduction occurs within an SPE system.
The three distinct mechanisms for ion conduction in SPEs based on an aqueous system are
displayed in Figure 7.
14
Figure 7: Ionic conduction mechanisms in polymer electrolytes: (a) ion hopping, (b)
vehicular diffusion, and (c) segmental motion [37].
The specific description for each proton conduction mechanism is as follows:
a) Ions hopping/Grotthuss mechanism
The kinetics of hydrogen bond formation or bond breaking between a proton and a water
molecule or other hydrogen-bonded liquids determine the ion mobility [79, 80]. Due to
hydrogen-bonding, protons can exist at various states of H5O2+, H7O3
+, or H9O4+ which
allows it to jump across different water molecules in the polymer matrix through the
hydrolyzed ionic site as seen in Figure 7 (a) [81]. Site-to-site hopping requires local
rearrangement and reorientation of molecules involving two potential wells separated by
a barrier of only a few kJ mol-1: the proton donor and the proton acceptor, respectively
[82, 83]. Therefore, the Grotthuss mechanism is prominent in a system with many
hydrogen bonding because of the low activation energy (typically under 15 kJ mol-1) for
conduction and the high proton mobility in this mechanism [84-86].
15
b) Vehicular diffusion mechanism
A proton can bond with the solvent molecules (e.g. water) to form a solvated complex
that diffuses across the polymer matrix as in Figure 7 (b) [87, 88]. The diffusion process
is commonly observed in a system with weak hydrogen bonding. Diffusion of the
hydrogen-water ions may decrease because of hydrogen bonding with other solvent
molecules. This diffusion process is much slower than proton hopping and is
characterized by a higher activation energy (typically above 20 kJ mol-1) and lower
proton mobility [85, 86, 89].
c) Segmental motion mechanism
The ion moves together with the polymer side chains as shown in Figure 7 (c) [90, 91].
During the polymer chains movement, the polymer chain conformation changes and
alters the bonding distance between the cation and the side chain. When a neighboring
side chain becomes sufficiently close to the cation, it possesses sufficient attraction such
that the cation can move from one side chain to the neighboring side chain. This
mechanism is highly dependent on the flexibility for polymer matrix motion or vibration
to reduce or eliminate the distance for proton conduction. Thus, this type of proton
transportation is restricted to the amorphous phase of the solvating polymers, where the
polymer molecules are free to move and as such become more prominent at temperature
above the polymer glass transition temperature (Tg) of the polymer.
The understanding of the proton conduction mechanism enables researchers to better engineer
different SPE systems. The previous work on different proton-conducting SPEs are discussed in
the next section.
16
2.4 Polymer hosts
Proton conduction in SPEs relies heavily on the properties of the polymer hosts to remain
amorphous, flexible, and have sufficient water retention capability to facilitate ion migration.
Therefore, the selection of suitable polymer hosts for proton-conducting SPEs is essential for
solid EC applications.
Wright et al. were the first group to attempt research in SPEs in 1975 [92, 93]. Armand et al.
proposed the technological application of SPEs in batteries and electrochemical devices [92, 94].
The “dry solid” polymer electrolyte was later introduced as a solvent-free system without using
the organic liquid [95]. This SPE demonstrated poor conductivity at room temperature.
Thereafter, researches had been carried out to investigate various types of polymers for solid EC
applications. The molecular structure, properties, and applications of previously studied polymer
hosts are summarized in Table 2.
The benchmark materials for SPEs in energy storage application is perfluorosulfonic acid
(PFSA) ionomers [96], which is often referred to as Nafion®. Due to the hydrophobic
polytetrafluoroethylene (PTFE) backbone, PFSA exhibits high chemical resistance and
compatibility with various pseudocapacitive electrode materials. The hydrophilic property of the
terminal sulfonic acid (-SO3H) group enables the dissociation of protons and facilitates hydrated
proton mobility through Grotthuss and vehicle mechanism [97-99]. The -SO3H groups can only
bond to water molecules loosely, causing the film to lose water rapidly at low RH conditions
[100]. Thus, proton conduction of Nafion relies heavily on the hydration state of the electrolyte.
Although PFSA solid electrolyte is highly conductive at high temperatures and RH conditions,
its proton conductivity drops significantly at room temperature [101, 102]. Yongheng et al. has
impregnated phosphoric acid organic framework into the Nafion and achieved higher
conductivity (ca. 60 mS cm-1 at 51% RH and 80oC) than the unmodified Nafion [103].
Similarly, Rafael et al. pretreated Nafion with hydrochloric acid and achieved 23.8% higher
conductivity than untreated Nafion at 40% RH [99]. Other modifications to Nafion include
hygroscopic materials such as inorganic oxides particles [104-107], clays [108-110], conductive
polymers [111-113], and solid acids [114-116]. The preparation of Nafion® involves
environmentally hazardous fluorine-based technologies [117-119]. Therefore, the commercial
deployment of Nafion-based ECs is unfavorable due to environmental concerns.
17
Tab
le 2
: S
um
mar
y of
dif
fere
nt
pol
ymer
hos
ts f
or E
C a
pp
lica
tion
s.
Pol
ymer
typ
e M
olec
ula
r st
ruct
ure
M
onom
er
Mw (
gmol
-1)
Tg
(o C)
Pro
per
ties
A
dva
nta
ges
Ap
pli
cati
ons
Per
fluo
rosu
lfon
ic
acid
[P
FS
A]
544.
14
100
Am
orph
ous
H
igh
chem
ical
st
abil
ity
H
igh
ther
mal
st
abil
ity
H
ighl
y pe
rmea
ble
to w
ater
P
olym
er e
lect
roly
te f
or
fuel
cel
l
Ion-
sele
ctiv
e se
nsor
s
sulf
onat
ed
poly
(eth
er e
ther
ke
tone
) [S
PE
EK
]
288.
30
214
Am
orph
ous
H
igh
mec
hani
cal
stre
ngth
Hig
h th
erm
al
stab
ilit
y
Low
cos
t
F
uel c
ell p
roto
n ex
chan
ge m
embr
ane
H
igh
tem
pera
ture
sha
pe
mem
ory
appl
icat
ion
Pol
y(et
hyle
ne
oxid
e)/
Pol
y(et
hyle
ne
glyc
ol)
[PE
O/P
EG
]
44.0
5 -6
7 S
emi-
crys
tall
ine
S
truc
tura
l int
egri
ty
B
ioco
mpa
tibl
e
Low
Gla
ss
tran
siti
on
D
rug
deli
very
Sol
id e
lect
roly
te f
or
elec
troc
hem
ical
dev
ices
Pol
y(vi
nyl
pyrr
olid
one)
[P
VP
]
111.
14
180
Am
orph
ous
W
ater
sol
uble
Goo
d ad
hesi
on
E
xcel
lent
fil
m
form
ing
B
inde
r in
ph
arm
aceu
tica
l tab
lets
Foo
d ad
diti
ves
Pol
y(ac
ryli
c ac
id)
[PA
A]
72.0
6 10
6 A
mor
phou
s
Wat
er s
olub
le
H
ygro
scop
ic
T
hick
enin
g ag
ent f
or
adhe
sive
s
Mic
ella
r dr
ug d
eliv
ery
Pol
yvin
yl a
lcoh
ol
[PV
A]
44.0
5 85
S
emi-
crys
tall
ine
W
ater
sol
uble
Exc
elle
nt f
ilm
fo
rmin
g
F
ilm
for
wat
er tr
ansf
er
prin
ting
pro
cess
Wat
er-s
olub
le f
ilm
for
pa
ckag
ing
Pol
y(ac
ryl a
mid
e)
[PA
M]
71.0
8 16
5 A
mor
phou
s
W
ater
sol
uble
Hyg
rosc
opic
Exc
elle
nt f
ilm
fo
rmin
g
M
ediu
m f
or
elec
trop
hore
sis
A
bsor
bent
in w
ater
tr
eatm
ent
Pol
y(ac
ryla
mid
e-
co-a
cryl
ic a
cid)
[P
(AM
-co-
AA
)]
143.
14
177
Am
orph
ous
W
ater
sol
uble
Hyg
rosc
opic
Bio
com
pati
ble
P
olym
er e
lect
roly
te
D
rug
deli
very
for
in
suli
n re
leas
e
18
Non-perfluorinated polymers were also good candidates for SPE applications. They have
hydrocarbon backbones with various combinations of benzene rings or aromatic heterocyclic
rings. These combinations enable different functionalization (i.e. introduction of polar sites)
through doping, chemical grafting, or direct sulfonation to increase proton conductivity and
water uptake [120]. Recently, sulfonation of non-perfluorinated polymers were widely
investigated for energy storage applications, especially sulfonated poly(ether ether ketone)
(SPEEK) [121-124].
Although sulfonation slightly decreases the chemical resistance of SPEEK, the SPEEK materials
still exhibit excellent chemical resistance due to the higher C-H bond strength in the benzene
ring compared with aliphatic C-H bond strengths [122, 125]. Other advantages of these materials
include low cost, excellent chemical resistance, and good mechanical properties [122], but their
overall performance is still inferior to Nafion. Application of the SPEEK family to ECs is still
limited to proton exchange membrane in fuel cells or direct methanol fuel cells because of their
higher temperature stability. Currently, only SPEEKK and SPEEK have been used for ECs
applications due to their higher conductivity at low temperature.
Poly(ethylene oxide) (PEO) is a synthetic polyether commonly known as poly(ethylene glycol)
(PEG) at a molecular weight less than 100,000 g mol-1. The PEO polymers are amphiphilic and
soluble in many solvents. They are also nontoxic for different pharmaceutical, food, and
cosmetic formulations [126]. Due to these benefits, it is one of the earliest polymer matrix
developed and most utilized for polymer electrolytes development and fundamental
understanding of conductivity [94, 127]. In 1975, Wright et al. developed PEO-Na+ complex
with conductivity of 10-7 S cm-1 [93]. Thereafter, PEO has been mixed with various type of
materials such as ionic liquids, salts, plasticizers, and fillers to enhance its conductivity close to
10-3 S cm-1 [127]. These promising characteristics enabled PEO for high power ECs applications.
The linear nature of PEO also leads to semi-crystalline properties for PEO because the linear
main chains can easily fold upon each other to create crystalline regions. The intrinsic
crystallinity of PEO remains to be a challenge for PEO based polymer electrolytes. Therefore,
the limitations of PEO due to its semi-crystalline properties were documented by Armand, which
redirected the research attention into other amorphous polymer electrolyte systems [128].
19
Poly(vinyl pyrrolidone) (PVP) is an inexpensive amorphous polymer with a high glass transition
temperature due to the pyrrolidone group [129, 130]. PVP is also non-toxic, biocompatible, and
is readily soluble in polar solvents, which is ideal for fast printable SPEs fabrication methods
such as ink jet printing. Due to the extremely low intrinsic conductivity of PVP at 10-8 mS cm-1
at room temperature, sodium chlorite and sodium nitrite were added as the ionic conductors to
enhance the conductivity to 10-4 mS cm-1 [131, 132]. Vijaya et al. also developed PVP doped
with ammonium iodide and achieved ca. 10-2 mS cm-1 at ambient conditions [133].
Ravi et al. modified a PVP electrolyte with nano-TiO2 fillers and successfully demonstrated
conductivity of ca. 1 mS cm-1 at room conditions [130]. However, PVP is very sensitive to the
humidity because it was reported as the best candidate material for use in the application of a
humidity sensor combined with iodine and cobalt [134]. This implies that the water retention
capability of PVP is poor. Therefore, there is an interest to explore other hygroscopic polymer
electrolytes for ECs applications.
Poly(acrylic acid) (PAA) is a polyelectrolyte that can readily absorb water as it was first used in
diapers in 1982 [135-138]. Some other frequent applications of PAA include superabsorbent,
coagulating agents, and hydrophilic adhesives [19]. Its bulky carboxylic acid functional group
hinders the chains from packing, thus resulting in amorphous properties. The proton on the side
chain also enables PAA to become hydrophilic and adhesive [139].
The weakly acidic PAA carboxylic groups can be hydrolyzed to release proton, thus, promoting
proton conduction in polymer electrolytes. A gel electrolyte consisting of PAA and 1M H2SO4
exhibited high conductivity [37]. Complexation can occur upon mixing PAA with other ions that
changes the physical properties of the PAA solution such as a reduction in viscosity [140].
Through polymer blending, PAA and poly(1-vinyl-imidazole) formed networks that showed high
temperature resistance [141]. PVA/PAA blend also demonstrated that PAA can retain more
water and increase the conductivity up to five times compared to PAA alone [142].
The research of PAA-based SPEs is still in its infancy. The PAA carboxylic groups can form
radicals for crosslinking through UV radiation without a photo initiator [143]. This can enhance
the mechanical stability [144], as well as improving its water retention and structural flexibility
that PEO lacks. Many possibilities are yet to be discovered with uses of PAA as SPEs for EC
applications.
20
2.4.1 Polyvinyl alcohol
Polyvinyl alcohol (PVA) is a very promising candidate for SPE applications. PVA is
hygroscopic due to the hydroxyl functional group in its structure (see Figure 8). The small
hydroxyl functional group also allows PVA to pack efficiently in zig-zag confirmation and form
crystalline regions through hydrogen bonding between its hydroxyl groups; thus, exhibiting
semi-crystalline properties [136].
Figure 8: Structure of a PVA.
PVA also has advantages such as high chemical resistance, non-toxic, water soluble, and
biodegradable properties which enabled its frequent use in various applications, including
adhesives, textiles, pharmaceutical, and biomedical industries [145]. PVA has also been widely
studied for both Li-ion batteries and ECs owing to its good film formability and ease of forming
gel or hydrogel for the electrolytes.
PVA was first investigated as a separator material for batteries in the field of electrochemical
energy storage [146]. PVA were first developed as proton conductors with H3PO4 for hydrogen
sensor applications by Polak et al. [147]. The PVA-H3PO4 polymer electrolyte was reported to
have a conductivity of 3 x 10-2 mS cm-1 at room temperature [147]. The complexation between
the acid and the polymer led to an increase in conducting species and the plasticizing effect of
H3PO4 on PVA can enhance the conductivity to 1 mS cm-1 [148]. Thus, PVA is suitable polymer
with acids and form complexes that facilitate proton conduction.
Due to the demand for energy storage, PVA based polymer electrolytes were explored for EC
applications and first patented by Lian et al. [149]. Kaempgen et al. demonstrated printable thin
film EDLCs with single-wall carbon nanotubes (SWCNT) as electrodes [150] and El-Kady et al.
used the same polymer electrolyte with graphene [151]. Both work demonstrated improved
capacitance compared to an aqueous electrolyte.
21
Other than H3PO4, strong acid such as H2SO4 are also frequently used in PVA polymer
electrolytes. It was demonstrated in a solid device with 3D graphene hydrogel electrolytes and
exhibited comparable stability and lower leakage current compared to the 1M H2SO4 liquid
device [85]. Fei et al. also demonstrated PVA-H2SO4 flexible devices with graphene-carbon
black electrodes [152]. Recent efforts have involved the research into PVA-ionic liquid (IL)
blends which show improved performance [153], as well as crosslinked PVA with sulfosuccinic
acid to improve the conductivity to the range of 10-3 to 10-2 S cm-1 [154].
Since PVA is hydrophilic, the water content is highly influential in the mobility of ions. At the
same time, this hydrophilic nature caused rapid changes in physical properties of PVA with RH
because the absorbed water acts as a plasticizer for the polymer. This change in physical
properties can be a potential issue in terms of the environmental stability of PVA electrolytes.
These properties caused PVA solid electrolyte to suffer from poor mechanical properties and
were susceptible to hydrolysis in water [155].
2.4.2 Polyacrylamide
In recent years, polyacrylamide (PAM) caught the attention of researchers for applications in
ECs. This is due to the beneficial properties of PAM such as the higher water retention capability
of PAM compared to PVA [156]. PAM has an amide functional group (see Figure 9) which
helps in forming hydrogen bonding with water; thus, improving its water retention capability.
Due to the bulky functional group, the chain alignment results in an amorphous structure. PAM
also has an excellent film-forming capability. These features enabled its deployment in fuel cell
electrolytes, absorbents for waste treatment, and moisture storage for soils [157-159].
Figure 9: Structure of a PAM.
22
In the field of energy storage, PAM-based electrolytes are widely used in fuel cell systems such
as borohydride fuel cells and direct methanol fuel cell applications. Sumathi et al. demonstrated
that hydrogel electrolytes consisting of PAM mixed with paratoluenesulfonic monohydrate acid
and methane sulfonic acid showed a good conductivity around 10-2 S cm-1 to 10-1 S cm-1 at
ambient conditions in batteries [160]. The increased conductivity was attributed to the higher
amount of mobile H+ ions from the acids.
Most of the reported work on proton-conducting SPEs consisted of PAM with either sulfuric acid
(H2SO4) or phosphoric acid (H3PO4) additions for enhancing conductivity [161-165]. Wieczorek
et al. compared the proton transport for PAM doped with H3PO4 and H2SO4. Doping PAM with
these acids exhibit similar room temperature conductivity that increased to ca. 20 mS cm-1 at
1.6:1 acid/acryl amide monomer ratio and reached a plateau at higher acid concentration as
shown in Figure 10 [165].
Figure 10: Room temperature conductivity as a function of acid/AM molar ratio for PAM
gel electrolyte (40 vol% water) embedded with H3PO4 and H2SO4 [165].
23
Tang et al. embedded H3PO4 into 3D crosslinked PAM network for proton exchange membrane
[161]. The hydrophilic nature of PAM and the 3D framework enabled H3PO4 to be absorbed and
retained in the membrane. At anhydrous state, the conductivity of H3PO4-PAM membrane
increased with higher H3PO4 loading as shown in Figure 11 (a). A sudden increase in
conductivity at ca. 60 wt% H3PO4 was attributed to the formation of percolation proton-
conducting pathways. This H3PO4-PAM membrane can achieve 0.65 mS cm-1 at 75.9 wt%
H3PO4 loading in anhydrous state. Tang et al. also discovered that this conductivity can be
further improved with H2O absorption in the membrane, reaching a maximum of 73.3 mS cm-1 in
fully hydrated state (329% swelling volume ratio) as shown in Figure 11 (b). The conductivity of
PAM membrane was highly dependent of the relative humidity of the electrolyte [161].
Figure 11: Room temperature conductivity of 3D crosslinked PAM embedded with H3PO4
(a) as a function of H3PO4 loading in anhydrous state; and (b) as a function of swelling
volume ratio from H2O uptake [161].
Rodriguez et al. also investigated the anhydrous PAM blended with H3PO4, focusing on the acid-
PAM interaction and their compatibility [163]. H3PO4 is not strong enough to protonate PAM,
but interact with the acrylamide functional groups via hydrogen bonding as follows [163]:
24
This resulted in anhydrous H3PO4-PAM electrolyte to be relatively more stable from acid
hydrolysis as compared to stronger acids such as HCl and H2SO4 [163]. Figure 12 (a) and (b)
revealed the composition dependence of the conductivity for the H3PO4-PAM electrolyte.
Rodriguez et al. reported in Figure 12 (a) that at low H3PO4 content, the active protons were
isolated and cannot form connection. Further addition of H3PO4 result in percolation to form the
conduction pathway, reaching a maximum of 260 mS cm-1 at ca. 45 wt% H3PO4 due to extensive
self-ionization in water. Figure 12 (b) showed that adding PAM resulted in reduction in the
conductivity, but the slope of this variation was lower than any other polymer investigated [163].
This implies that acid-embedded PAM can be a good candidate for proton-conducting SPEs.
Figure 12: Room temperature conductivity of (a) H3PO4/H2O system and (b) H3PO4/PAM
system as a function of wt% H3PO4 [163].
Apart from the linear configuration of PAM, PAM copolymers with PAA (PAM-co-PAA) are
also known for good water absorption due to the two hydrophilic COOH and NH2 groups. The
carboxylic group of PAA may link with the acryl amide group of PAM through hydrogen
bonding [166, 167]. This property enables PAM-co-PAA to retain water efficiently for its
applications in superabsorbent as well as drug delivery system [168, 169]. However, the
applications of PAM-co-PAA in ECs have not been studied.
This thesis will focus on PAM as the polymer host for the SPE system. Since PAM is
intrinsically not very conductive, proton conductors will be added into the polymer host to form
an electrolyte matrix to improve its conductivity. The next section will discuss the selection of
the proton conductors to be used with PAM to formulate the electrolyte system.
25
2.5 Keggin-type heteropolyacids
The utilization of liquid acids can achieve high conductivity, but the resulting electrolyte system
may suffer from issues of leakage over time. Recently, the utilization of solid proton conductor
was demonstrated to have high ionic conductivity. The most promising solid proton conductors
are Keggin-type heteropolyacids (HPAs) [170-172]. These hydrous acids are metal-oxide
clusters of early transition metals formulated as Hz[MX12O40], where z is the charge of the anion,
M is the heteroatom, and X is the addenda atom as shown in Figure 13 (a) [173, 174].
The heteroatom M forms an internal tetrahedron unit (MO4x-) with four oxygen atoms (Oa) [175,
176]. This tetrahedron unit is surrounded by a cage of twelve XO6 octahedral units. Three XO6
octahedral units can link together to form X3O13 structural unit through sharing edges. Four of
these X3O13 structural units will share corner oxygen to form the cage that encapsulates the
internal tetrahedron unit through oxygen bonds to form the Keggin-type spherical structure.
Figure 13 (b) indicates the four different types of oxygen atoms in the Keggin-type HPAs:
tetrahedral oxygen (MO4), edge-sharing oxygen (Ob), corner-sharing oxygen (Oc), and terminal
oxygen (Od) [86].
Figure 13: (a) Ball-and-stick view of heteropolyacids with the Keggin-type structure, and
(b) polyhedral representation of the Keggin structure showing the central tetrahedron
(MO4); the terminal oxygen atoms of each X3O13 group (X-Od); the bridging bonds between
edge-sharing oxygen atoms (X-Oc-X); and corner-sharing octahedra (X-Ob-X). [177, 178].
26
The heteroatom can be P5+, Si4+, Ge4+, and B3+; while the most common addenda atoms include
W6+, Mo6+, and V5+ [179, 180]. The two commonly used commercial Keggin-type HPAs are
silicotungstic acid (SiWA, H4SiW12O40) and phosphotungstic acid (PWA, H3PW12O40).
The Keggin-type HPAs exhibit high proton conductivity at room temperature as inorganic solid
proton conductors due to the large number of crystallized water molecules in the crystal hydrate.
Its hygroscopic nature also allows high water absorption and leads to a “quasi-liquid” state [181-
183]. Two major factors contribute to their proton conductivity: (a) highly hydrogen-bonded
conduction pathways in the crystal lattice, and (b) the dynamic dissociation of crystallized water
molecules in the crystal hydrate via interactions with oxygen atoms of the Keggin anion.
The pathways result from many crystallized water molecules in the crystal hydrate (e.g.
SiWA•nH2O), leading to “quasi-liquid” states that facilitate fast proton transport [184-186]. The
protons (i.e. both intrinsic and dissociated protons), in the forms of H+-nH2O clusters (e.g. H3O+
or H5O2+), are transferred by hopping from H+-nH2O donor sites to nH2O acceptors in the HPA,
yielding a high proton conductivity in the solid-state (27 mS cm-1 for SiWA•28H2O) [187].
HPAs are strong acids and all intrinsic protons of Keggin-type HPAs dissociate equally in
aqueous solution. Their high ionic conductivity in aqueous solutions results from (a) the high
ionic strength of the electrolyte solutions due to the complete dissociation of ions and (b) the
high mobility of the Keggin anions because of their un-solvated nature, in which its
hydrodynamic radius is similar to its crystallographic radius.
The reduction potential of HPAs are dependent on the chemistry of molecule. These potentials
are linearly related to the charge carried by the anions as shown in Figure 14 [179, 180]. When
the heteroatom is replaced with another element, the reduction potential decreases with the
valence of the central heteroatom, indicating their strong dependence on the Keggin structure.
The trend in Figure 14 is only an indication as there are other reactions (e.g. hydrogen evolution
reaction) that can complicate the situation. Nevertheless, the redox reactions of W-based HPAs
occur at more negative potentials compared to the Mo- or V-based HPAs; thus, the
electrochemical inactive window is wider. The wider electrochemical inactive window ensures
that no current leakage occurs within that range due to the reduction of the HPAs, which is an
important characteristic of an ideal electrolyte.
27
Figure 14: The trend of reduction potential is shifted in a predictable manner with respect
to the different anion charge for various heteroatom in HPAs [188].
2.6 Polymer-in-salt electrolytes
The polymer hosts are usually added with inorganic proton conductors to improve its
conductivity for SPE applications. Depending on the proton-conducting species content in the
polymer, this inorganic/polymer proton-conducting electrolytes can be further divided into two
subgroups: (a) salt-in-polymer or (b) polymer-in-salt, as shown in Figure 15 [189].
28
Figure 15: Morphology model for a polymer electrolyte as a function of salt content [189].
Salt-in-polymer electrolytes are prepared by dispersing small amount of acids, bases, or salts in a
polymer. Most salt-in-polymer electrolytes are aqueous-based polymers, such as poly(ethylene
oxide) (PEO), poly(acrylonitrile) (PAN), or poly(methylmethacrylate) (PMMA), forming
acid/polymer blends or acidic hydrogels with liquid acids (such as H2SO4 or H3PO4) because of
their self-ionization and self-dehydration reactions in their pure states [190].
6H2SO4 ↔ H3SO4+ + H3O+ + HSO4
- + HS2O7- + H2S2O7 + H2O (14)
5H3PO4 ↔ 2H4PO4+ + H3O+ + H2PO4
+ + H2P2O72- (15)
Polymer-in-salt electrolytes are prepared by using only a small amount of polymer material as a
matrix to hold the solid proton conductors together during film formation. At high conductor
concentration, the individual conductor clusters will interconnect to form a percolation pathway
for ion conduction as shown in Figure 15. Most of the proton conductors have water molecules
embedded in the structure (e.g. crystallized water in hydrous acids), which enable polymer-in-
salt electrolytes to have much higher water retention capability compared to the loosely bound or
free water in salt-in-polymer electrolytes. The crystallized water on the solid proton conductors
can facilitate high conductivity for polymer-in-salt configurations.
29
2.6.1 Heteropolyacid-based membranes
The development of HPAs in SPEs for EC applications gained momentum in recent years. Li et
al., Lei et al., Lian et al., and Xu et al. separately developed PVA-phosphotungstic acid (PWA)
electrolytes which demonstrated high conductivity values [191-194]. PWA-based solid
electrolytes for EC applications were patented by Li et al. [195, 196]. The PWA is reported to
have high leakage current, thus research has focused on other HPA chemistries for EC
applications [197]. Other HPA-based electrolytes have also been widely studied in PVA-based
systems because of the high water retention ability of PVA which facilitates high ionic
conductivity and better environmental stability.
Silicotungstic acid (H4SiW12O40, SiWA) has been studied intensively by Gao et al. for both
aqueous and polymer electrolytes [173, 198-200]. SiWA is one of many Keggin-type
heteropolyacids, acid with huge cluster of metal oxides as its anions. The anion consists of one
central heteroatom (Si) surrounded with 12 addenda atoms (W), the anion is illustrated in Figure
13. It has previously been demonstrated to be a highly promising electrolyte both as aqueous and
polymer electrolytes. Performance optimization of SiWA-PVA polymer electrolytes by
crosslinking and use of fillers has raised the conductivity up to the range of 10 mS cm-1 to 20 mS
cm-1 with stable performance at different humidity conditions. The SiWA-PVA solid electrolytes
were also demonstrated to be compatible with pseudocapacitive electrodes [201].
Recently, Gao et al. impregnated a crosslinked polyvinyl alcohol (XLPVA) polymer electrolyte
with borotungstic acid (BWA, H5BW12O40) to form BWA-XLPVA solid electrolyte [202]. In a
three electrode setup, BWA demonstrated 0.2 V larger electrochemical stability window than
SiWA as shown in Figure 16 (a) and (b), agreeing with the trend in Figure 14. The capacitor cell
with BWA-XLPVA can achieve a larger maximum cell voltage of 1.3 V as shown in Figure 16
(c) than the 1.1 V of SiWA (Figure 16 (d)). Figure 16 (e) and (f) reveals that the BWA cell
voltage is limited by oxygen evolution at the positive voltage of 0.7 V and tungsten reduction in
BWA at the negative potential of -0.6 V. The larger cell voltage (U) allowed BWA-XLPVA
capacitor cell to have higher energy and power performance according to according to equations
(1) and (2).
30
Figure 16: Three electrode CVs at a scan rate of 100 mV s-1 for (a) SiWA–XLPVA and (b)
BWA–XLPVA using graphite electrodes; CVs at a scan rate of 100 mV s-1 for symmetric
solid graphite cell with (c) a SiWA–XLPVA and (d) a BWA–XLPVA electrolytes at
incremental cell voltage; electrode potential limits of both cathode and anode versus cell
voltage for the solid cells with (e) SiWA–XLPVA and (f) BWA–XLPVA electrolyte [202].
The capacitor cells fabricated from BWA-XLPVA electrolyte also achieved a high conductivity
of 13 mS cm-1 at 45% RH and 20 oC as shown in Figure 17 [202], similar to SiWA-XLPVA.
The high conductivity values were attributed to the large number of crystallized water molecules
in BWA and SiWA. The performance of these HPA-based capacitor cells still suffered from
dehydration. Thus, it is necessary to leverage a highly hygroscopic polymer such as PAM
combined with a suitable HPA to further improve these solid polymer electrolytes.
31
Figure 17: Conductivity against temperature of BWA-XLPVA and SiWA-XLPVA at
various humidity conditions [202].
Even though Figure 14 shows that HPAs with cobalt (CoWA) and copper (CuWA) heteroatoms
can theoretically achieve even wider voltage window than BWA for better energy storage
performance, the copper and cobalt heteroatoms have their own redox potential Cu+/Cu2+ and
Co+/Co2+ which limits the voltage window. Thus, this thesis will focus on BWA as the proton
conductor in the SPEs.
2.7 Additives for polymer-in-salt electrolytes
Additives are sometimes used in polymer electrolytes to further enhance its electrochemical
properties (e.g. improving ionic conduction) as well as improving the environment stability of
the SPEs. Previous work on various plasticizers such as tetraglyme, ethylene glycol (EG),
dimethyl formamide (DMF), and propylene carbonate (PC) demonstrated that plasticized
32
electrolytes such as urethane crosslinked polyethers, polymethylmethacrylate, and poly(2-
hydroxyethylacrylate) can achieve 1-2 orders of magnitude higher conductivity while a decrease
of Tg by 40 oC [203, 204].
A common additive in polymer electrolytes are plasticizers which are small molecules that
increase polymer flexibility to ease segmental motion, as well as acting as the solvent that ease
ion diffusion – both of which can improve conductivity of the electrolyte. The plasticizers can be
categorized into acidic plasticizers and neutral plasticizers. The acidic plasticizers and neutral
plasticizers will be discussed in the following section.
2.7.1 Acidic plasticizers
Acidic plasticizers are commonly used in proton-conducting polymer electrolytes because of
their intrinsic conductivity. The proton dissociation from the plasticizers can help in providing
more conducting species for higher conductivity in the electrolyte system. Some common
examples of acidic plasticizers are strong acids such as phosphoric acid and sulfuric acid [205].
In the past, phosphoric acid (H3PO4) has been widely used as a plasticizer in proton-conducting
SPEs [148, 206]. In these studies, the H3PO4-doped electrolyte system employed basic polymers
such as PEO, polyvinyl acetate (PVAc), polyacrylamide (PAM), and polyethyleneimine (PEI) as
the polymer hosts [207-210]. The extensive self-ionization of H3PO4 can result in 53 mS cm-1
conductivity near room temperature.
The conductivity can be increased to a maximum of 0.27 S cm-1 with 45 wt% acid due to the
decrease in viscosity and water facilitation in the acid dissociation. Further increase in water will
dilute the acid and decreases the number of charge carriers more rapidly [205]. High H3PO4
plasticizer concentrations will result in higher conductivity at the expense of sacrificing the
mechanical stability of the polymer electrolytes due to the acid attack on some of the polymers,
especially at high temperatures.
Sulfuric acid (H2SO4) modified PAM has comparable room conductivity (ca. 20 mS cm-1) to
H3PO4, except that H2SO4 is generally more acidic and has higher oxidizing properties which
result in faster degradation of PAM polymer by H2SO4 [163, 165]. A better chemical stability can
be achieved by utilizing highly thermostable polymers including polyimides, poly(ether ketones)
(PEK) or poly(ether sulfones) (PES).
33
2.7.2 Neutral plasticizers
Neutral plasticizers are another alternative to avoid acid degradation of polymers. Similar to
acidic plasticizers, neutral plasticizers can act as the solvent for the ionic conductors and soften
the polymer chains. The neutral plasticizers also have functional groups that facilitate hydrogen
bonding with water molecules; thus, enhancing the hygroscopic properties of the polymer
electrolytes. Typical neutral plasticizers include glycerol and ethylene glycol.
Glycerol is a simple polyol compound that is non-toxic, which enabled its widespread
deployment in food and pharmaceutical applications. It has three hydroxyl functional groups that
help in its solubility in water and hygroscopic nature. Due to its hygroscopic nature, glycerol is
often used both in sample preparation and gel formation for polyacrylamide gel electrophoresis.
Alves et al. demonstrated that glycerol promotes higher conductivity in chitosan-based polymer
electrolytes because glycerol contributes to inefficient polymer chain packing and allow more
chain flexibility that results in an improvement of the ionic transport [211]. Adding too much
glycerol will result in adverse effects such that the solutions become too viscous to be handled
properly due to the hydrophilic functional groups that bind with the water molecules [212].
Further addition of glycerol can lead to phase separation of the polymer and glycerol [211].
Similar to glycerol, ethylene glycol (EG) can also improve the polymer adhesion as well as the
polymer water retention from its hydrophilic characteristics. EG is a smaller molecule than
glycerol, which may require more EG quantity to achieve the same plasticizing effect as glycerol
[213]. Therefore, this thesis will be focusing on using glycerol as the neutral plasticizers for the
proton-conducting SPEs.
34
Chapter 3 Research Plan
3.1 Motivation
The literature review reported PAM as a promising candidate for SPEs due to its film-forming
capability, as well as its hygroscopic nature and amorphous structure that enhance conductivity
for fast-charging EC applications. The low intrinsic conductivity of PAM can be improved by
adding proton conductors, especially Keggin-type HPAs such as BWA due to their high solid-
state conductivity. To date, HPA-based SPEs are only demonstrated in PVA system, and the
compatibility of HPAs in PAM have not been studied. Due to the lack in the research on the
HPA-based PAM solid electrolyte, some gaps in previous research work were identified and
listed below. This project aims to address those gaps and develop high-performing SPEs for
flexible solid EC devices modified with plasticizers which exhibit the following properties:
a) Wide electrochemical stability window for maximum cell voltage for higher energy;
This project will continue to use BWA as the ionic conductor due to their wide stability
window. Focus will be placed on addressing the lack of characterization of the
compatibility of PAM and BWA as SPEs.
b) High ionic conductivity for high power;
The electrochemical performance of BWA-PAM electrolyte has not been studied. Focus
will be placed on optimizing the BWA composition in PAM to achieve high ionic
conductivity. The effects of acidic and neutral plasticizers on the conductivity of BWA-
PAM electrolyte will be addressed.
c) High environmental stability for long service life.
The stability of BWA-PAM + plasticizer electrolytes in EC applications have not been
investigated. Focus will be placed on analyzing the cycle life of the capacitor cell
fabricated with BWA-PAM + plasticizer electrolytes.
35
3.2 Experimental Design
The research objective can be achieved systematically by focusing on the aforementioned
attributes to develop BWA-PAM based polymer electrolytes and to understand the mechanisms
leading to their performance and properties. The experimental design developed was based on
these attributes as shown in Figure 18. The following sub-sections describe the specific strategy
in each phase of this research.
Figure 18: Research Outline
3.2.1 Phase 1: Optimization of BWA-PAM Binary System
Previous work reported that the cell voltage can be improved with wider stability window by
using custom HPAs. Recent work demonstrated BWA in PVA system with 1.3 V cell voltage
[202]. This phase of the research focused on developing the BWA-PAM SPE system. The
chemical and electrochemical compatibility of BWA and PAM were analyzed along with the
electrolyte conductivity and stability. The root cause of failure in this BWA-PAM electrolyte
system was also investigated.
36
3.2.2 Phase 2: Enhancing of BWA-PAM with H3PO4 Plasticizer
Additives such as acidic plasticizers can enhance the film water retention for better environment
stability. However, the nature of additive‐polymer interactions are not well studied. This phase of
the research focused on enhancing the ion conduction and environmental stability of BWA-PAM
electrolyte system with acidic plasticizers. Chemical characterizations using Raman spectroscopy
was performed to develop a fundamental understanding of the interactions between the BWA,
PAM, and H3PO4. The nature of these interactions were then correlated with the electrolyte
conductivity and performance. The plasticizer and electrolyte composition were then optimized
and demonstrated in capacitor cells with CNT-graphite electrodes.
3.2.3 Phase 3: Enhancing BWA-PAM with Glycerol Plasticizer
The use of different plasticizers may result in different electrochemical performance in the
BWA-PAM electrolyte system. This phase of the research focused on understanding the
interactions between the BWA, PAM, and glycerol, as well as on comparing its properties with
the H3PO4-modified BWA-PAM electrolytes. The plasticizer and electrolyte composition were
then optimized and demonstrated in capacitor cells with CNT-graphite electrodes.
3.3 Thesis Outline
This thesis will describe a systematic approach to enhancing the performance of a solid ECs
through engineering of proton-conducting solid polymer electrolytes (SPEs). The literature
reviews and research plan have been described in Chapter 1-3. The rest of the thesis is organized
as described below:
Chapter 4: This chapter describes the experimental design and techniques to engineer and
characterize the proton-conducting SPEs. A detailed description is given for each
characterization technique in order to establish how they provide a complementary
characterization of the materials linked mechanistically to cell performance.
37
Chapters 5: This chapter describes the development of SPE with borotungstic acid (BWA) as the
proton conductor and polyacrylamide (PAM) as the polymer host. The electrochemical and
material characterizations on BWA-PAM electrolytes are analyzed to understand the
compatibility of the materials and their performance. The compositions for the binary BWA-
PAM electrolyte with highest conductivity and shelf life are chosen to proceed to the next stage
of the research.
Chapter 6: This chapter describes the modification of BWA-PAM with both acidic plasticizer
(H3PO4) and neutral plasticizer (glycerol). The effects of these plasticizers on the BWA-PAM
material properties are discussed and correlated back to their electrochemical performance. The
optimized electrolyte compositions with each plasticizer are chosen to proceed to the next stage.
Chapter 7: This chapter describes the SPE optimizations and solid cell demonstrations. The best
electrolyte with acidic plasticizer and neutral plasticizer are demonstrated in solid cells with
carbon electrodes and compared against the liquid cells.
Chapter 8: This chapter summarizes the highlights of this thesis and the findings from each stage
of the research. Plans and experiments are suggested for the possible future continuation of this
research.
38
Chapter 4 Experimental Method
4.1 Materials
This thesis analyzed the performance of BWA-PAM electrolyte system with electrochemical
characterization and material characterization techniques. The experimental method from
electrolyte preparation to SPE characterizations are described in this chapter.
The BWA-PAM SPE consists of polyacrylamide (PAM) as the polymer host and borotungstic
acid (BWA) as the proton conductors. PAM, acidic plasticizers, and neutral plasticizers were
commercially available, but BWA had to be synthesized as it is a novel HPA. The synthesis
method of BWA is described below.
4.1.1 Heteropolyacids (BWA)
H5BW12O40 ∙ xH2O (BWA) was synthesized by a modified Copaux's method [202, 214]. Briefly,
Na2WO4 ∙ 2H2O (5 g) was mixed with H3BO3 (4 g) in hot water (25 mL). The solution was
adjusted to pH 5.5 using 6 N HCl and heated at 60 oC for 2 hours under stirring. Subsequently,
the pH was further adjusted to 3.8 using 6 N HCl and the solution was heated at 80 oC for 6
hours. Finally, the solution was allowed to cool to room temperature for 6 hours. Solid
precipitates (primarily tungsten oxides) were filtered out and the filtrate solution was extracted
with diethyl ether. The extracted solid was washed with DI H2O and dried, yielding ca. 2 g of a
white crystalline product.
4.1.2 Polymer host (PAM)
The PAM was purchased from Scientific Polymer and was used as-received. The PAM has
average molecular weight Mw of 5,500,000 to 6,000,000 g/mol.
39
4.1.3 Additives
Phosphoric acid (H3PO4) was used as the acidic plasticizer, while glycerol (Gly) was used as the
neutral plasticizer. The H3PO4 comes at 85 wt% concentration in H2O, while Gly has above
99.5% purity. Both of these plasticizers were purchased from Sigma Aldrich and were used as-
received.
4.2 Preparation of polymer electrolytes
This thesis explored three systems of the proton-conducting SPEs: BWA-PAM binary system,
BWA-PAM + H3PO4 ternary system, and BWA-PAM + Gly ternary system. The preparation
method and composition of each electrolyte system are listed below. The precursor solutions
were cast in ambient conditions to form freestanding films. The film thickness was measured
with a Mitutoyo Model 293-831 digital micrometer; the average film thickness was ca. 150 μm.
4.2.1 BWA-PAM
The BWA-PAM electrolyte was prepared by mixing a 3% PAM solution (Scientific Polymer,
Mw: 5-6 000 000 g mol-1) with BWA powder to form precursor solutions at 50 wt%, 75 wt%,
and 85 wt% BWA composition.
Table 3: Compositions of BWA-PAM polymer electrolytes.
Sample ID BWA:PAM molar ratio
BWA wt% PAM wt%
PAM 0 0 100 50BWA-PAM 2,000 50 50 75BWA-PAM 5,000 75 25 85BWA-PAM 10,000 85 15
4.2.2 BWA-PAM + H3PO4
The BWA-PAM + H3PO4 electrolytes was prepared through adding phosphoric acid (H3PO4,
Sigma Aldrich) as a plasticizer to the BWA-PAM electrolyte precursor solutions with the weight
ratio controlled at ca. 5 wt% and ca. 10 wt%.
40
Table 4: Compositions of BWA-PAM + H3PO4 polymer electrolytes.
Sample ID H3PO4:PAM molar ratio
BWA wt% PAM wt% H3PO4 wt%
75BWA-PAM + 0% H3PO4 0 75 25 0 75BWA-PAM + 5% H3PO4 10,000 70 25 5 75BWA-PAM + 10% H3PO4 20,000 67 24 9 85BWA-PAM + 0% H3PO4 0 85 15 0 85BWA-PAM + 5% H3PO4 20,000 81 14 5 85BWA-PAM + 10% H3PO4 40,000 76 14 10
4.2.3 BWA-PAM + Glycerol
The BWA-PAM + Glycerol electrolytes was prepared through adding glycerol (Sigma Aldrich)
as a plasticizer to the BWA-PAM electrolyte precursor solutions with the weight ratio controlled
at ca. 5 wt% and ca. 10 wt%.
Table 5: Compositions of BWA-PAM + Glycerol polymer electrolytes.
Sample ID Glycerol:PAM molar ratio
BWA wt% PAM wt% Gly wt%
75BWA-PAM + 0% Gly 0 75 25 0 75BWA-PAM + 5% Gly 10,000 71 25 4 75BWA-PAM + 10% Gly 25,000 66 24 10 85BWA-PAM + 0% Gly 0 85 15 0 85BWA-PAM + 5% Gly 20,000 81 14 5 85BWA-PAM + 10% Gly 40,000 76 14 9
4.3 Working Electrodes
All working electrodes have a geometric area of 1 cm2 unless otherwise specified. Two types of
working electrode materials (metallic electrodes and carbon electrodes) were used to characterize
the properties of the electrolyte and demonstrate the performance in solid-state ECs.
4.3.1 Metallic Electrodes
The metallic electrodes were cut from Ti foil (127 μm thick, McMaster-Carr). The electrode
surface was rinsed with isopropanol (IPA) and DI water before use. Electrodes were cut into two
sizes, 3 x 1 and 3 x 1.2 cm. The slightly larger size of one electrode can prevent short-circuiting
when the edge of two electrodes meet. A polyester tape (0.1 mm thick, ABI Tape, Series 9180)
was used to limit the active area of the electrodes of 1 cm2.
41
4.3.2 Carbon Electrodes
The CNT-graphite electrodes were prepared using a modified method from a previous report
[215]. The 75% CNT ink synthesis is as follows: 0.1050g (75 wt%) CNT and 0.0350g (25 wt%)
graphite powders were mixed homogeneously. 0.7 g of 5 wt% PVA binder solution was diluted
to 2 mL with DI water. The solution was stirred in a 20 mL disposable vial with a magnetic
stirrer while slowly adding the carbon powders. The ink was stirred for 8-9 hours to ensure
homogeneous mixing before adding 0.0035g glutaric acid as the crosslinker. The ink was stirred
for another 5 days before coating onto the Ti current collector. The resultant 75% CNT ink has a
composition of 20 wt% crosslinked PVA binder with 60 wt% CNT and 20 wt% graphite.
The procedure for doctor-blading the carbon ink is outlined in Figure 19. The Ti current collector
was secured flatly on the bench with double sided tapes. Two strips of 60 μm thick tape were
aligned along the ground area to control the thickness of the applied carbon ink. The smooth
edge of a metallic plate was used as the doctor blade to coat the ink. The carbon coated electrode
was dried for 1 hour in an oven pre-heated to 50 °C. The temperature was raised to 120 °C for
the next hour to crosslink the PVA binder. The electrode was cooled in DI water overnight.
The electrode was cut with a paper trimmer into individual electrode with 1cm x 1cm carbon ink
area. First, the two carbon ink electrodes were taped such that only the carbon ink area was
exposed. The electrodes were then soaked in DI water for 1-2 hours to remove the loose carbon
particles. The carbon electrodes had ca. 1.5-2 mg cm-2 carbon loading with 60 wt% CNT, 20
wt% graphite, and 20 wt% crosslinked PVA binder. The geometric area of the electrode is 1 cm2.
Figure 19: The steps for doctor-blading of carbon ink onto the titanium substrate.
42
4.4 Construction of liquid cells
4.4.1 Liquid 3-electrode cells
To understand the electrochemical behavior of the electrolyte, a three-electrode cell was
employed. The setup consists of a working electrode (WE), counter electrode (CE), and
reference electrode (RE) connected to the electrolytes. In three-electrode setup, the potential
changes in the working electrode were measured against a saturated Ag/AgCl reference
electrode. This isolation allows an accurate study of specific reaction that might occurs at the
electrode-electrolyte interface, evaluates the performance of the individual ink electrodes, and
matches the electrodes with similar capacitance for the symmetric cell construction. The carbon
ink electrode served as the working electrode with the ink exposed in the 0.5 M BWA
electrolyte, while a platinum (Pt) mesh as the counter electrode which has at least 10x the surface
area of the working electrode. The setup was connected to the potentiostat for cyclic
voltammetry test.
4.4.2 Liquid 2-electrode cells
The two-electrode liquid cells were fabricated to assess the electrochemical performance of the
electrolyte. The setup consists of a Whatman glass microfiber (1 mm thick, 1.2 μm pore size,
Sigma Aldrich) impregnated with 0.1 M BWA electrolyte solution sandwiched in between two
electrodes. The electrodes were taped with polyester tape (0.1 mm thick, ABI Tape, Series 9180)
to limit the 1 cm2 geometric area exposed to the electrolyte. The cell was stabilized by clipping
with two glass slides (see Figure 20). Ten test cells were fabricated and connected to the
potentiostat for cyclic voltammetry test.
Figure 20: A liquid 2-electrode cell.
43
4.5 Construction of solid cells
4.5.1 Solid capacitor cells
The test cells were constructed with titanium foil (127 μm thick, McMaster-Carr) as electrode in
the electrolyte performance study, while CNT-graphite electrodes coated on a titanium foil
current collector were used for the solid EDLC cell. Ten test cells were fabricated for each
electrolyte compositions. The CNT-graphite electrodes will be characterized with liquid 3-
electrode setup prior to symmetric cell construction. The solid cells were assembled by:
1. Electrode Fabrication: Electrodes were cut into two sizes, 3 x 1 and 3 x 1.2 cm. The
slightly larger size of one electrode prevent short-circuiting when the edge of two
electrodes meet. A tape was used to limit the active area of the electrodes of 1 cm2.
2. Electrolyte Application: 2-3 drops of precursor solution containing combinations of
polymer, salt, and plasticizer were applied onto the electrode active surface cleaned
with IPA and DI water. The electrolyte was left at ambient until it formed a film.
3. Cell Construction: Two electrodes/electrolyte assemblies were placed together as
shown in Figure 21 and taped with polyester tape (0.1 mm thick, ABI Tape, Series
9180). Constant pressure was applied with 20-30 kPa pressure for 20 min on the cells
to ensure good contact between electrode and electrolyte.
Figure 21 visualizes the assembly of the cells. After assembly, all cells were stored in a 45%
humidity desiccator and were only taken out just before testing. The test cells’ thickness was
measured with a Mitutoyo Model 293-831 digital micrometer; the average cell thickness was 0.6
mm with ca. 0.2 mm thick electrolyte. All solid cells were stored at room temperature in a 45%
RH desiccator.
Figure 21: The assembly of an electrochemical capacitor cell.
44
4.6 Electrochemical characterization methods
Electrochemical characterization was intensively utilized in this thesis. There are 2 techniques to
characterize the electrochemical performance of the cells: cyclic voltammetry (CV) and
electrochemical impedance (EIS).
4.6.1 Cyclic voltammetry (CV)
Cyclic voltammetry (CV) was used to characterize the capacitance and voltage window by
measuring the current response with respect to change in voltage. CV was performed on
Potentiostat/Galvanostat Model 263A from the EG&G Princeton Applied Research at ambient
condition. This potentiostat was operated with the CorrWare software and the data analysis is
performed with the CView software version 3.3d.
This technique starts at a pre-set initial voltage and scans to a pre-set maximum voltage with a
fixed linear sweep rate. Upon reaching the final voltage, it reverses the scan to the pre-set
minimum voltage. The combinations of forward and reverse scan produce a cyclic
voltammogram as shown in Figure 22. The ideal CV shape is represented by the blue line in
Figure 22. The electrolyte should have a near rectangular CV shape (black line) when operated
within its stable voltage window. As the electrolyte is pushed beyond its stability window, the
irreversible peaks (grey line) indicate faradaic losses that can occur due to water breakdown.
Figure 23 illustrates the rectangular voltammogram shape of an ideal capacitor versus the
parallelogram voltammogram shape of a capacitor with resistivity.
Figure 22: Rectangular CV shape of an ideal electrolyte material [216].
45
Figure 23: CV comparisons of an ideal capacitor vs capacitor with resistivity.
There are two important parameters that can be obtained from CV:
1. Charging/discharging capacitance
For areal capacitance analysis, dc capacitance (unit: Farad) can be calculated by the
following equations: C=Q/U where Q is charge stored/discharged and U is the voltage
window in which the CV is performed; both Q (unit: Coulomb) and U (unit: Volt) can be
extracted from the voltammogram data. In this project, CV on most cells was performed
from 0 to 0.8 V with scan rate of 1 V s-1. It should also be noted that the capacitance is
directly proportional to the area under the voltammogram curve.
2. Voltage window
Voltage window of an electrochemical cell can be measured using incremental CV where
the voltage positive and negative limits are expanded incrementally until certain reactions
(such as O2 or H2 evolutions and heteropolyacids reduction) occur on either cathode or
anode. These reactions can be identified with slow scan rate of 50 mV s-1 by the peaks in
CV similar to the grey line in Figure 22. Potential window can be investigated through
two-electrode test setup and/or three-electrode setup.
46
4.6.2 Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) is a common technique to extract frequency
dependent responses from the cell. EIS subjected the tested cell to an alternating current (AC)
perturbation with a range of frequencies as shown in Figure 24 (a). This allows the use of
software to model the tested cell with an equivalent circuit as shown in Figure 24 (b) and
calculate the relevant electrochemical parameters of the cell.
Figure 24: (a) AC perturbation input signal with various frequencies for EIS, and (b) an
equivalent circuit for capacitor with resistance.
EIS was performed with 1255 HF Frequency Response Analyzer from Solartron Schlumberger.
The EIS data was analyzed with ZView software version 2.9b. In this project, the initial potential
in the EIS was set at 0V bias and the amplitude was set to be 0.005V with frequency ranging
from 0.1 Hz to 100 kHz. From the EIS technique, a Nyquist plot or Bode plot can be generated
as shown in Figure 25.
Figure 25: (a) Nyquist plot and (b) Bode plot obtained from EIS of an electrolyte test cell.
47
From the Nyquist plot in Figure 25 (a), the Zreal intercept corresponds to the ionic resistance of
the cell. This information can be extracted by using an equivalent circuit for a capacitor as shown
in Figure 24 (b) to model the electrolyte test cell. Ionic conductivity of the cell was calculated by
the following equations:
(16)
where d is thickness of electrolyte, R is the ionic resistance, and A is the active area (in this
project: 1 cm2).
In the Bode plot of Figure 25 (b), the impedance angle and magnitude are plotted against the
frequency. An ideal capacitor should have an impedance angle of -90° and therefore it is
desirable for the impedance angle of EC to reach -90° as fast as possible in the Bode plot.
For ac capacitance, the real part of the capacitance (C’) and its corresponding imaginary part
(C”) were separated [217]. In this method, the EC frequency behavior is considered. Then its
impedance is
1
(17)
Since the impedance, Z(ω), can be represented in its complex form
′ " (18)
By rearranging equations (17) and (18),
1
′ "" ′| |
(19)
is also be represented in its complex form:
′ " (20)
48
Combination of equations (19) and (20) leads to ′ equation:
′"
| | (21)
and the " equation:
"′
| | (22)
These two values ( ′ and " ) can be plotted as a function of frequency. ′ represents
the accessible capacitance of the cell at the corresponding frequency, which approaches the
capacitance obtained from dc methods at low frequencies. " corresponds to energy
dissipation by an irreversible process. The maximum of the " vs. frequency curve represents
a time constant, at which a transition from resistive-dominated to capacitive-dominated behavior
occurs (i.e. capacitor phase angle reaches -45°). This time constant was used as a “factor of
merit” to compare the rate capability of ECs [218].
4.7 Material and structural characterizations
The electrochemical characterization discussed in the previous section should be coupled with
material characterization so that the material properties can be correlated to the performance and
establish deeper understanding of the underlying mechanisms governing the performance. The
electrolytes were cast in ambient conditions and dried into thin free-standing films for materials
characterizations.
4.7.1 Simulated cells
The simulated cells were assembled by casting the precursor solution onto each microglass slides
followed by sandwiching the electrolyte between two microglass slides and stabilizing it with
tape. The test cells’ thickness was measured with a Mitutoyo Model 293-831 digital micrometer;
the average cell thickness was 0.6 mm with ca. 0.2 mm thick electrolyte. The simulated cells
were used for optical microscopy and Raman spectroscopy.
49
4.7.2 Optical microscopy
The optical microscopy was performed with Jenaphot 2000 Reflected-light Photomicroscope.
Images were collected with Camera Control Pro 2 software at various magnifications in bright
field and dark field modes.
4.7.3 Raman spectroscopy
Raman spectroscopy measurements were recorded on a Horiba XploRATM PLUS Raman
microscope system with a 532 nm laser and diffraction grating of 1800 g/mm. The spectra were
recorded between 15 cm-1 and 4000 cm-1 wavenumbers. Raman film samples were cast and dried
into films under ambient conditions and re-conditioned at 45% RH for 7 days before being
characterized.
4.7.4 Fourier transform infrared (FTIR) spectroscopy
Fourier transformed infrared (FTIR) spectroscopy was performed with Bruker ALPHA FTIR
Spectrometer and the data was collected with OPUS 7.2 software. The spectra were recorded at a
resolution of 4 cm-1 within the range from 4000 cm-1 to 400 cm-1. FTIR film samples were cast
and dried into films under ambient conditions and re-conditioned at 45% RH for 7 days before
being characterized.
4.7.5 X-ray diffraction (XRD)
X-ray diffraction (XRD) measurements were carried out using a Philips XRD system, including
a PW 1830 HT generator, a PW 1050 goniometer, and PW 3710 control electronics. The samples
were analyzed with a Cu-Kα source of wavelength 1.5406 Å operating at 40 kV/30 mA. The
diffraction patterns were recorded from 5o to 50o 2θ with a step size of 0.02o 2θ and a scan step
time of 2 seconds. All XRD film samples were cast and dried into films under ambient
conditions and re-conditioned at 45% RH for 7 days before being characterized. The resulting
pattern can be used to analyze the crystallinity of the electrolyte.
50
Chapter 5 Development of BWA-PAM binary system
5.1 BWA characterizations
This chapter discusses the development of a binary electrolyte system with borotungstic acid
(H5BW12O40, BWA) as the heteropolyacid (HPA) proton conductor and polyacrylamide (PAM)
as the polymer host. Before incorporating the proton conductor with the polymer host, the
synthesized BWA was characterized with cyclic voltammetry (CV), Raman spectroscopy,
Fourier Transformed Infrared (FTIR), and x-ray diffraction (XRD). The characterization results
were compared against the commercial Keggin HPA, silicotungstic acid (H4SiW12O40, SiWA)
that had a well-established database to examine the purity of the synthesized BWA.
5.1.1 Ionic conductivity of BWA (Two-electrode Beaker Cell)
The ionic conductivity of BWA solution was analyzed in a 2-electrode beaker cell. Figure 26
depicts the conductivity of SiWA and BWA solutions at various concentrations. The
conductivity of both SiWA and BWA increased linearly with concentration due to the higher
amount of protons available for conduction. This conductivity trend agreed with the work by
Gao et al. [214]. As BWA had a more negative ionic charge than SiWA, BWA can provide a
higher amount of protons for conduction. This led to the consistently higher conductivity of
BWA compared to that of SiWA at all concentrations studied. This suggested that BWA is a
better HPA candidate for ionic conduction than SiWA for SPEs.
51
Figure 26: Room temperature conductivity of SiWA and BWA solutions as a function of
concentration.
5.1.2 Electrochemical window of BWA (Cyclic Voltammetry)
BWA solutions were also characterized with cyclic voltammetry to investigate the window of
their electrochemical stability. Figure 27 shows the cyclic voltammograms (CVs) of 0.01 M
SiWA and 0.01 M of the synthesized BWA at 50 mV s-1 in a 3-electrode cell with glassy carbon
working electrode against a saturated Ag/AgCl reference electrode. Similar to SiWA, the CV of
BWA reached an upper limit of oxygen evolution at 1.5 V. BWA demonstrated a wider
electrochemical stability window at lower potential (-0.3V) than SiWA (-0.1 V). This lower limit
was attributed to the redox activity of HPAs at negative potentials [214].
This 0.2 V increase in the electrochemical stability window from SiWA to BWA was in
agreement with the trend reported by Song et al. [179, 180]. As the boron heteroatom had a
larger anionic charge than silicon, the less electronegative boron had a lower tendency to attract
electrons from the W12O36 cage, leading to a higher electron density on the cage. Thus, a greater
overpotential was required to reduce the W12O36 cage, resulting in a linear negative shift of the
redox potential [179, 219]. Due to the lower reduction potential, BWA can achieve a 1.8 V
window as compared to the 1.6V for SiWA, in agreement with the reported values in literature
[214]. This wider electrochemical window is one of the critical design concepts to achieve high
energy and power ECs according to equations (1) and (2).
52
Figure 27: Third scan for the cyclic voltammograms in a 3-electrode cell with a glassy
carbon working electrode and platinum mesh counter electrode, showing the
electrochemical stability windows for 0.01 M SiWA and 0.01 M of the synthesized BWA at
a scan rate of 50 mV s-1 against a saturated Ag/AgCl reference electrode.
5.1.3 Chemical bonding in BWA (Raman Spectroscopy)
The heteroatom and the chemical bonding of BWA were analyzed with Raman spectroscopy.
Figure 28 depicts the Raman spectra of BWA in comparison with SiWA and the peak
assignments are listed in Table 9 of Appendix A. The Raman peaks for SiWA were assigned
according to the well-established literature reports [220-222]. Since the Raman analysis on BWA
was not previously reported, the peaks were assigned from a cross comparison of known Keggin-
type structures [220-222].
The Raman spectrum of SiWA can be divided into three major regions. The low Raman shift
area revealed the W12O36 cage vibration signals below 400 cm-1 [222]. The high-intensity peak at
1006.1 cm-1 was characteristic of the υ(Si-O) peak for the SiWA heteroatom [220-222]. The
broad peak at ca. 3500 cm-1 belonged to the δ(O-H) peak of the crystallized water surrounding
the SiWA [220-222]. Thus, the spectrum of SiWA is typical of the spectrum for Keggin-type
HPAs.
1.6 V window
1.8 V window
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-0.4
-0.2
0.0
0.2
-1.0 -0.5 0.0 0.5 1.0 1.5 2.0-0.4
-0.2
0.0
0.2
0.4
0.01M SiWA
Cu
rre
nt (
mA
cm
-2)
Potential vs. Ag/AgCl (V)
0.01M BWA
53
The Raman spectrum of BWA powder had similar vibration signals to that of the SiWA powder
below 400 cm-1 for the W12O36 cage. This suggested that the synthesized BWA had the W-based
Keggin cage structure. BWA also showed a high-intensity peak around 989.3 cm-1 which can be
assigned to the υ(B-O) signals. Figure 28 showed that BWA also had a broad δ(O-H) peak at ca.
3500 cm-1, suggesting that it also contained crystallized water.
As compared to SiWA, the peaks below 400 cm-1 in BWA were shifted to slightly lower
frequencies (see Table 9 in Appendix A) likely due to the larger size of the BWA structure. The
higher peak intensity of δ(W-O-W) at 216.8 cm-1 in the BWA spectrum may indicate minor
residual tungsten oxides from the synthesis process in the BWA powder. The shift of the υ(B-O)
peak (989.3 cm-1) to a lower frequency than υ(Si-O) peak (1006.1 cm-1) in SiWA agrees with the
trend reported by Bridgeman et al. from density functional theory (DFT) calculations [222]. As
BWA and SiWA had similar Keggin metal oxide cage sizes, the larger anionic charge and
smaller ionic radius of boron led to a longer B-O than Si-O bond in the Keggin structure [219,
222]. The longer B-O bond caused the shift in the υ(B-O) peak to a lower frequency. This
Raman characterization confirmed that the BWA had a Keggin-type structure with a boron
heteroatom. The BWA also showed a higher peak intensity at 3500 cm-1, suggesting that it
retained more crystallized water as compared to SiWA.
FTIR analysis was also performed to characterize the BWA powder (refer to Appendix B). Both
Raman and FTIR analyses confirmed that the BWA powder was successfully synthesized with a
Keggin-type structure and a boron heteroatom. Since the Raman spectra was more sensitive and
had sharper peaks than FTIR, it was used as the primary tool to characterize the chemical
bonding for the rest of this thesis.
54
Figure 28: Raman spectra of (a) SiWA powder and (b) the synthesized BWA powder dried
at 60oC for 24 hours prior to characterization. The Keggin-type structures of SiWA and
BWA are shown as inset figures. The υ(Si-O) and υ(B-O) peaks were highlighted in blue
color, while the δ(W-O-W) peak was highlighted in red color.
5.1.4 Structure of BWA (XRD Analysis)
The structure of the synthesized BWA was also investigated using XRD analysis. Figure 29
shows the XRD patterns of BWA in comparison with SiWA, a commercial HPA with known
Keggin-type structure. The XRD pattern of SiWA revealed the typical signature peaks for
Keggin-type structure [214]. The three strongest Keggin signature peaks were at 10.0o, 25.2o, and
34.4o. In the XRD pattern, the peak at 10.0o and ca. 25.2o were associated with the dehydrated
SiWA [223, 224].
55
The synthesized BWA powder had a similar XRD pattern as SiWA with the three strongest
peaks at 10.2o, 25.3o, and 34.6o, confirming that BWA retained a Keggin structure. One
additional peak was observed at ca. 7.7o, which was the hydrated structure for Keggin-type
HPAs [223, 224]. This implied that although both SiWA and BWA were dried under the same
condition prior to the XRD characterization, BWA showed a slightly higher tendency to retain its
hydrated state. Thus, the XRD analysis confirmed that the synthesized BWA retained the same
Keggin-type structure with a slightly higher level of hydration than SiWA, which agrees with the
Raman analysis in section 5.1.3. Thus, this custom synthesized BWA was further investigated
for solid electrolytes in this project.
Figure 29: X-ray diffraction patterns of (a) SiWA powder and (b) the synthesized BWA
powder dried at 80oC for 24 hours prior to the characterization. The structure of Keggin-
type HPA was included as an inset. The hydrated HPA peak was highlighted in red color,
while the dehydrated HPA peaks were highlighted in blue color.
56
5.2 PAM characterizations
5.2.1 Water retention in PAM (Raman and FTIR Analysis)
The PAM was characterized with Raman, FTIR, and XRD to investigate its water retention
capability and structure. The chemical bonding within the PAM film was characterized with
Raman spectroscopy as shown in Figure 30 (a). The signature peaks of PAM were also tabulated
in Table 11 of Appendix A. The Raman spectrum for PAM agreed with the peaks reported by
Gupta et al. [225]. This PAM spectrum was used as the baseline to compare against the polymer
electrolytes developed in this thesis. The broad δ(O-H) peak at 3355.3 cm-1 corresponded to a
large amount of water retention in the PAM film, suggesting the highly hygroscopic property of
PAM which is desirable for polymer electrolyte applications.
The PAM had a strong peak with strong intensity at 1458.4 cm-1, correspond to the δ(CH2)
backbone in PAM. This signature peak was used as the baseline for peak intensity analysis in the
characterization of the electrolytes because the CH2 polymer backbone is usually inert to
chemical reactions as compared to the functional group represented by υ(C-N), δ(NH2), and
υ(C=O) peaks at 1426.9 cm-1, 1610.0 cm-1, and 1666.3 cm-1, respectively.
Figure 30 (b) revealed the FTIR characterization of the PAM film. The FTIR peaks were
assigned according to Lu et al. [226] and tabulated in Table 12 in Appendix A. Similar to the
Raman analysis, a large broad peak was observed at 3324.3 cm-1 that matched the δ(O-H) of
water, suggesting that the PAM film can retain water within its structure. This hygroscopic
nature of PAM is desirable for SPE applications.
57
Figure 30: (a) Raman and (b) FTIR spectra of pure PAM film conditioned at 45% RH for 7
days. Inset shows the structure of PAM. The acrylamide functional group peaks were
highlighted in blue, while the polymer backbone peak used as the reference in this thesis
was highlighted in red.
5.2.2 Structure of PAM (XRD Analysis)
Figure 31 shows the XRD pattern for PAM film. The broad peaks in the XRD pattern suggest
that the polymer film is amorphous. The peak at 11.9o was a silicone signature [227], which
likely originated from the silicone gel used to attach the film samples during XRD
characterization. The broad peak at ca. 15-25o was deduced to be the amorphous structure of
PAM, according to Murthy et al. [228-230]. This amorphous structure is beneficial to enable the
segmental motion of the polymer to enhance proton conductivity, as well as retain more water to
facilitate ion migrations as mentioned in Chapter 2.
58
Figure 31: XRD pattern for PAM film conditioned at 45% RH for 7 days prior to the
characterization.
5.3 Proton conductivity
After characterizing the individual components, the BWA and PAM are mixed together to form
the electrolyte precursor solutions. The BWA-PAM mixtures formed homogeneous precursor
solutions that dried into translucent films when cast in ambient conditions. The tested electrolyte
compositions were selected to be at least 50 wt% BWA to form the polymer-in-salt system as
discussed in Chapter 2. The stability of PAM was compromised in highly acidic solution beyond
a threshold of 85 wt% BWA as reported in Appendix C. Therefore, the precursor solutions of
50BWA-PAM, 75BWA-PAM, and 85BWA-PAM were selected to construct solid cells. Their
conductivity were characterized to optimize the electrolyte compositions.
Figure 32 showed the room temperature conductivity of BWA-PAM binary electrolytes as a
function of composition over 28 days while conditioned at 45% RH. The PAM electrolyte film
had a low intrinsic conductivity of ca. 1 mS cm-1. Adding BWA increased the conductivity of the
as-fabricated solid electrolytes from ca.7 mS cm-1 for 50BWA-PAM, ca. 17 mS cm-1 for
75BWA-PAM, to ca. 27 mS cm-1 for 85BWA-PAM due to the extra protons available for
conduction.
59
Figure 32: Room temperature conductivity versus time of electrolytes with PAM, 50BWA-
PAM, 75BWA-PAM, and 85BWA-PAM conditioned at 45%RH over a period of 28 days.
The conductivity of H3PO4-PAM reported by Rodriguez et al. [163] was extracted from Figure
12 (b) and compared against BWA-PAM in Table 6. As expected, BWA-PAM exhibited
significantly higher conductivity than the H3PO4-PAM because BWA can provide more protons
per molecule than H3PO4 to enhance the conductivity.
Table 6: The comparison of conductivity as a function of acid content for BWA-PAM and
H3PO4-PAM electrolyte systems [163].
Acid content (wt %) Conductivity (mS cm-1) BWA-PAM H3PO4-PAM
50 7.2 0.032 75 16.9 2.5 85 27.8 7.9
At the same condition of 45% RH under room temperature, the conductivity of as-fabricated
85BWA-PAM (ca. 27 mS cm-1) was twice that of 90BWA-XLPVA developed by Gao et al. (ca.
13 mS cm-1) [202] due to the more hygroscopic nature of PAM as compared to PVA [156]. This
result is very promising as they demonstrated the potential to use BWA-PAM binary system as
highly-conductive SPE for EC applications.
0 5 10 15 20 25 300
5
10
15
20
25
30
Co
nd
uct
ivity
(m
S c
m-1
)
Days
PAM 50BWA-PAM 75BWA-PAM 85BWA-PAM
60
The conductivity of 50BWA-PAM was comparatively lower, thus only 75BWA-PAM and
85BWA-PAM were selected for further analysis. The conductivity of these two compositions
decreased rapidly with time as shown in Figure 32, resulting in an average conductivity of ca. 7
mS cm-1 (≤ 40% conductivity retention) after 28 days of room temperature exposure to 45% RH.
The metallic cells were disassembled at this point to investigate the origin of the reduction in
conductivity. The electrolytes were found to be dried, brittle, and delaminated from the electrode
surface as shown in Figure 33 (a). Part of the electrolyte film turned an opaque white color
because of the precipitation of BWA due to dehydration (see Figure 33 (b)). The size of the
precipitation ranged from 10 µm to 40 µm. Thus, the reduction of conductivity was likely due to
electrolyte dehydration, resulting in the short shelf life of the binary electrolytes.
Figure 33: (a) Photo of electrolyte delamination from the electrode surface on day 28; and
(b) optical micrograph of the delaminated area of the electrolyte, showing precipitation due
to electrolyte dehydration.
5.4 Material characterization
5.4.1 Chemical bonding in BWA-PAM binary electrolytes (Raman Analysis)
Since dehydration was identified as the cause of failure, the BWA-PAM binary electrolytes were
analyzed with Raman spectroscopy and XRD to investigate the water bonding in the electrolyte.
Figure 34 showed the Raman spectra for BWA-PAM electrolytes as a function of composition.
The corresponding Raman peak assignments of the BWA and PAM in the binary electrolytes are
given in Table 13 of Appendix A.
61
The Raman spectra for BWA and PAM were previously discussed in sections 5.1 and 5.2
respectively. After adding 50 wt% BWA to PAM, the Raman spectrum of 50BWA-PAM
generally consisted of the signature peaks of both BWA and PAM components without any peak
shifting (see Table 13). The characteristic strong peaks for BWA-PAM below 1000 cm-1
matched that of the BWA powder, suggesting that it retained the Keggin structure in PAM. The
PAM peaks primarily in 1000-1800 cm-1 were overshadowed by the BWA signals, except the
strong υ(CH2) peak at 2937.4 cm-1. The broad peak at ca. 3500 cm-1 suggested that the 50BWA-
PAM binary electrolyte can retain water in the film.
The 75BWA-PAM and 85BWA-PAM showed similar spectra to that of the 50BWA-PAM
material without any peak shifting (see Table 13). The relative peak intensity ratio of BWA
(represented by υ(B-O) at 989.3 cm-1) to PAM (represented by υ(CH2) at 1458.5 cm-1) increased
with increasing BWA content in the electrolytes. As more BWA was added into the electrolytes,
the δ(O-H) peak at ca. 3300 cm-1 was reduced in intensity and shifted towards ca. 3500 cm-1,
suggesting that higher content of BWA led to a lower level of hydration in the film. This
explains the more rapid reduction in conductivity of 85BWA-PAM due to dehydration over 28
days as compared to 75BWA-PAM and 50BWA-PAM as shown in Figure 32.
Figure 34: Raman spectra for dried films of PAM, 50BWA-PAM, 75BWA-PAM, and
85BWA-PAM conditioned for 7 days at 45% RH. The υ(B-O) peak was highlighted in red
color, while the δ(O-H) was highlighted in blue color.
500 1000 1500 2000 2500 3000 3500
500 1000 1500 2000 2500 3000 3500
500 1000 1500 2000 2500 3000 3500
500 1000 1500 2000 2500 3000 3500
500 1000 1500 2000 2500 3000 3500
BWA Powder
85BWA-PAM
75BWA-PAM
50BWA-PAM
Co
un
ts (
a.u
.)
Raman Shift (cm-1)
PAM Film
62
Since the PAM peaks were overshadowed by the high-intensity BWA signals, a focused analysis
was performed over the range of 1000-1800 cm-1 in Figure 35. BWA powder did not have any
peak in this region. The PAM backbone peaks at 1116.2 cm-1, 1329.8 cm-1, and 1458.5 cm-1 were
identical for all binary electrolytes. They also showed similar acrylamide functional group peaks
at 1426.9 cm-1, 1610.0 cm-1, and 1666.3 cm-1 without significant peak shifting, but the peak
intensities at these three locations decreased with increased BWA content (see Table 7). Figure
35 also revealed that an additional peak appeared at 1712.8 cm-1 for all BWA-PAM electrolytes,
which did not correspond to either PAM or BWA. This additional peak became more
pronounced at higher BWA concentration (see Table 7), suggesting a possible reaction between
BWA and PAM.
Figure 35: Raman spectra for dried films of PAM, 50BWA-PAM, 75BWA-PAM, and
85BWA-PAM conditioned for 7 days at 45% RH (focus on the range of 1000-1800 cm-1).
The acrylamide functional group peaks with changing intensity were highlighted in blue
color, while the additional peak was highlighted in red color.
1000 1100 1200 1300 1400 1500 1600 1700 1800
1000 1100 1200 1300 1400 1500 1600 1700 1800
1000 1100 1200 1300 1400 1500 1600 1700 1800
1000 1100 1200 1300 1400 1500 1600 1700 1800
1000 1100 1200 1300 1400 1500 1600 1700 1800
BWA Powder
85BWA-PAM
75BWA-PAM
50BWA-PAM
Co
un
ts (
a.u
.)
Raman Shift (cm-1)
PAM Film
63
Table 7: Peak intensity ratios of the υ(C-N), δ(NH2), υ(C=O), and υ(COO-) peaks in PAM,
50BWA-PAM, 75BWA-PAM, and 85BWA-PAM normalized against the δ(CH2) at 1458.4
cm-1 from the polymer backbone as the reference.
Sample ID Peak intensity ratio
υ(C-N) at 1426.9 cm-1
δ(NH2) at 1610.0 cm-1
υ(C=O) at 1666.3 cm-1
υ(COO-) at 1720 cm-1
PAM 0.80 0.85 0.61 N/A 50BWA-PAM 0.67 0.65 0.62 0.27 75BWA-PAM 0.55 0.43 0.51 0.64 85BWA-PAM 0.49 0.41 0.47 0.80
A previous study on PAM by Pei et al. demonstrated that adding strong acids such as HCl to
PAM can initiate chemical reactions to partially hydrolyze the acrylamide group in PAM to form
PAM-co-polyacrylic acid (PAA) as shown below [231]:
This extra peak at 1712.8 cm-1 matched the υ(COO-) signature peak of PAA [232] suggests that
the highly acidic BWA may partially transform PAM to PAM-co-PAA. This was also supported
by the change in relative peak intensities of the acrylamide functional groups and PAA peaks as
a function of BWA content (see Table 7) because more BWA would initiate higher degree of
PAM-co-PAA transformation. This transformation may be helpful for the electrolyte hydration
capability because PAM-co-PAA is more hygroscopic than both PAM and PAA homopolymers
[138, 233]. The improved hydration capability can facilitate ion conduction to achieve high
initial conductivities for the binary electrolytes in Figure 32.
5.4.2 Structure of BWA-PAM binary electrolytes (XRD Analysis)
XRD analysis was performed to investigate the change in electrolyte structure due to the PAM to
PAM-co-PAA transformation. Figure 36 is the XRD patterns for BWA-PAM films at various
BWA concentrations conditioned at 45% RH for 7 days. The PAM film without BWA addition
had been previously described in Section 5.2.
After adding 50 wt% BWA, the broad profile without any sharp peaks indicated that the
50BWA-PAM film was amorphous and well-hydrated with the same broad peak at ca. 15-25o as
the PAM. The appearance of a peak at ca. 6o and a broad peak at ca. 30o can be associated with
64
the hydration level of the BWA [223, 224]. Similar to the trend observed in 50BWA-PAM,
adding 75 wt% and 85 wt% BWA to PAM also yielded a similar XRD pattern as PAM,
suggesting that all binary systems have an amorphous structure despite the high concentration of
crystalline BWA.
The peak at ca. 6o systematically shifted with higher BWA content, from 7o for 75BWA-PAM to
8o for 85BWA-PAM due to the slightly lower hydration level of BWA. This was also supported
by the increase in the relative peak intensity at ca. 30o that implied a lower hydration of BWA.
This led to a higher crystallinity of the film with increased BWA content as indicated by the
narrowing of the first peak at 8.0o.
Despite the slight difference in hydration level, all BWA-PAM electrolyte films were well-
hydrated and retained an amorphous structure. This amorphous structure was desirable for ion
conduction as discussed in Chapter 2. Thus, the amorphous structure of BWA-PAM binary
electrolytes enabled them to achieve high initial conductivity values as shown in Figure 32.
However, the dehydration at higher BWA contents led to the short shelf life of the electrolytes,
thus plasticizers were investigated to enhance the conductivity and shelf life of the binary
electrolytes.
65
Figure 36: X-ray diffraction patterns for dried films of PAM, 50BWA-PAM, 75BWA-
PAM, and 85BWA-PAM conditioned for 7 days at 45% RH.
5.5 Summary
A proton-conducting polymer electrolyte consisting of borotungstic acid (BWA) and
polyacrylamide (PAM) was developed for electrochemical capacitor applications. The
synthesized BWA powder was confirmed to have a Keggin structure. After mixing BWA with
PAM, the precursor solution can form homogeneous electrolyte films. The strong acidity of
BWA partially transformed PAM to PAM-co-PAA which also had a desirable water retention
capability to facilitate ion conduction. When being cast into a film, the PAM had an intrinsic
conductivity of only ca. 1 mS cm-1, thus requiring the addition of BWA as a proton conductor to
enhance its electrochemical performance. BWA-PAM electrolytes exhibited high conductivity of
ca. 27 mS cm-1 for as-fabricated 85BWA-PAM due to the high concentration of BWA. However,
the binary electrolytes suffered from low service life due to dehydration, leading to electrolyte
brittleness and delamination.
66
Chapter 6 Effects of plasticizers in BWA-PAM
6.1 Acidic plasticizer
The water retention capability of binary BWA-PAM electrolytes can be improved by adding
plasticizers to form a BWA-PAM + plasticizer ternary system. An acidic plasticizer was first
investigated as previous work reported that acidic plasticizers not only help to retain water in the
electrolyte system, but also provide extra protons for conduction [148, 205, 206].
In this work, phosphoric acid (H3PO4) was chosen to be the acidic plasticizer due to its higher
stability with PAM as compared to HCl and H2SO4 [163, 165]. The composition was selected to
be 5% and 10% H3PO4 because adding concentrations of H3PO4 beyond 10 wt% into the
electrolyte system will result in electrolyte leakage issue in the cell.
6.1.1 Proton conductivity
The BWA-PAM + H3PO4 ternary electrolytes were sandwiched between two Ti electrodes to
assemble the test cells and conditioned under room temperature at 45% RH. The ternary
electrolytes were characterized with EIS for conductivity analysis. Figure 37 shows the
conductivity of 75BWA-PAM and 85BWA-PAM with 5 wt% and 10 wt% H3PO4 additions over
a period of 28 days.
The addition of H3PO4 improved the initial conductivity in both 75BWA-PAM and 85BWA-
PAM, reaching an initial conductivity of ca. 25 mS cm-1 for 5 wt% H3PO4 and ca. 30 mS cm-1
for 10 wt% H3PO4. On the 28th day, all of the ternary electrolytes were found to have retained
most of the initial conductivity as compared to the binary counterparts, indicating an improved
shelf life with the addition of a plasticizer. No electrolyte delamination was observed for the
ternary systems, suggesting that their water retention capability was better than that of their
respective binary systems. Therefore, H3PO4 addition not only provided extra protons for
conduction but also prolonged the electrolyte shelf life by reducing the rate of dehydration.
67
After adding the H3PO4 plasticizer, the initial conductivity of 75BWA-PAM (ca. 17 mS cm-1)
improved by 1.5x (ca. 25 mS cm-1) with 5 wt% H3PO4 and the 75BWA-PAM + 10% H3PO4
achieved almost twice the conductivity (ca. 30 mS cm-1) of its binary counterpart. In contrast,
adding 5 wt% H3PO4 into 85BWA-PAM resulted in a similar conductivity as its binary
counterpart (ca. 25 mS cm-1), while 10 wt% H3PO4 only provided a minor enhancement in
conductivity (ca. 30 mS cm-1).
It was apparent that the H3PO4 affects the electrolyte differently in the 85BWA-PAM that
resulted in less improvement in conductivity compared to the 75BWA-PAM system. Visual
inspection of the electrolytes over time was performed to note any change in the electrolyte
material that could lead to the discrepancy in the observed performance of the two compositions.
Figure 37: Room temperature conductivity versus time of electrolytes with 75BWA-PAM
and 85BWA-PAM, with 5 wt% and 10 wt% H3PO4, conditioned at 45%RH over 28 days.
0 5 10 15 20 25 300
5
10
15
20
25
30
35
Con
duct
ivity
(m
S c
m-1
)
Days
75BWA-PAM 75BWA-PAM + 5% H3PO4
75BWA-PAM + 10% H3PO4
85BWA-PAM 85BWA-PAM + 5% H3PO4
85BWA-PAM + 10% H3PO4
68
6.1.2 Crystallization at high content of BWA and H3PO4 (Visual inspection)
Visual inspection of the ternary electrolytes over 28-days period against a dark background was
carried out by sandwiching the electrolyte films in between two microglass slides to create 1cm x
1cm simulated cells. The micrographs in Figure 38 (a) and (b) showed that both electrolytes
modified with 5 wt% H3PO4 remained translucent. The addition of 10 wt% H3PO4 to 75BWA-
PAM and 85BWA-PAM showed a significant difference in Figure 38 (c) and (d); where the
75BWA-PAM + 10% H3PO4 remained translucent, but the 85BWA-PAM + 10% H3PO4
exhibited phase separation. This suggested that the BWA-PAM electrolytes were compatible
with 5 wt% H3PO4 and stayed well-hydrated in a homogeneous condition, whereas the addition
of 10 wt% H3PO4 was compatible with 75BWA-PAM but initiated an undesirable reaction to
deteriorate the stability of the 85BWA-PAM electrolyte.
In Figure 38 (e), a photo from the optical microscope over the white phase in 85BWA-PAM +
10% H3PO4 electrolyte revealed the formation of crystals with ca. 10 μm radius. The crystals
agglomerated into a second phase different from that of the bulk electrolyte. The crystals were
likely BWA precipitated from the polymer matrix, suggesting that the presence of 10 wt%
H3PO4 may have accelerated the BWA to hydrolyze significantly more acrylamide groups and
this drastic change in the polymer structure may lead to the BWA precipitation [231]. Raman
spectroscopy was performed on the electrolytes with 10 wt% H3PO4 to test this hypothesis.
69
Figure 38: Day 1 and day 28 observations against a University of Toronto logo as the
background of the electrolyte in 1cm x 1cm simulated cells for (a) 75BWA-PAM + 5%
H3PO4, (b) 85BWA-PAM + 5% H3PO4, (c) 75BWA-PAM + 10% H3PO4 and (d) 85BWA-
PAM + 10% H3PO4; and (e) optical micrographs of the 85BWA-PAM + 10% H3PO4
electrolyte.
70
6.1.3 Chemical bonding analyses of BWA-PAM + H3PO4 Ternary Electrolytes (Raman analysis)
As observed in Section 5.3, the most noticeable change in the Raman spectra due to the
interactions between BWA and PAM were in the range of 1000-2000 cm-1. Since the 10 wt%
H3PO4 was hypothesized to accelerate this reaction, the Raman spectra in Figure 39 focused on
1000-2000 cm-1 to reveal the chemical bonding present in each of the binary and ternary
electrolytes with respect to PAM and PAA. The corresponding peak assignments according to
the individual electrolyte component are listed in Table 14 of Appendix A.
Figure 39: Raman spectra of PAM, 75BWA-PAM, 75BWA-PAM + 10% H3PO4, 85BWA-
PAM, 85BWA-PAM + 10% H3PO4, and PAA. The characteristic peaks for PAM were
highlighted in blue color, while the PAA peak was highlighted in red color.
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
PAM Film
75BWA-PAM
75BWA-PAM + 10% H3PO4
85BWA-PAM
85BWA-PAM + 10% H3PO4
Co
un
ts (
a.u
.)
Raman Shift (cm-1)
PAA
71
Figure 39 showed that both PAM and PAA had similar Raman spectra for the polymer
backbones, represented by skeletal υ(C-C), CH2 wagging, and δ(CH2) at 1116.2, 1329.8, and
1458.4 cm-1 respectively. The strong δ(CH2) peak at 1458.4 cm-1 was used as the reference for
peak intensity analysis. The PAM had signature peaks for υ(C-N), δ(NH2), and υ(C=O) at
1426.9, 1610.0, and 1666.3 cm-1 respectively [225], whereas PAA had a signature peak for
υ(COO-) at 1717.2 cm-1 [232]. These signature peaks were used for the Raman spectra analyses.
Section 5.3 concluded that the binary electrolytes exhibited both characteristic peaks of PAM
and PAA due to the formation of PAM-co-PAA. After adding 10 wt% H3PO4, all of the ternary
electrolytes displayed similar peaks for their polymer backbones at 1116.2 cm-1, 1329.8 cm-1,
and 1458.4 cm-1. This observation was expected because the PAM-co-PAA formation primarily
involved the functional groups.
The 75BWA-PAM + 10% H3PO4 spectrum in Figure 39 displayed slightly lower PAM and
slightly higher PAA relative peak intensities than that of its binary counterpart as shown in Table
8. In contrast, the addition of 10 wt% H3PO4 into 85BWA-PAM resulted in the spectrum very
different than that of the binary electrolytes due to the absence of PAM characteristic peaks at
1426.9 cm-1 and 1666.3 cm-1, as well as a significant reduction of the 1610.0 cm-1 peak intensity.
The 85BWA-PAM + 10% H3PO4 also displayed twice the intensity of the PAA peak at 1717.2
cm-1 compared to its binary counterpart.
Table 8: Peak intensity ratios of the υ(C-N), δ(NH2), υ(C=O), and υ(COO-) peaks in PAM,
75BWA-PAM, 75BWA-PAM + 10% H3PO4, 85BWA-PAM, 85BWA-PAM + 10% H3PO4,
and PAA normalized against the δ(CH2) at 1458.4 cm-1 from the polymer backbone as the
reference.
Sample ID Peak intensity ratio
υ(C-N) at 1426.9 cm-1
δ(NH2) at 1610.0 cm-1
υ(C=O) at 1666.3 cm-1
υ(COO-) at 1717.2 cm-1
PAM 0.80 0.85 0.61 - 75BWA-PAM 0.55 0.43 0.51 0.64 75BWA-PAM + 10% H3PO4 0.52 0.40 0.46 0.67 85BWA-PAM 0.49 0.41 0.47 0.80 85BWA-PAM + 10% H3PO4 0.00 0.28 0.00 1.64 PAA - - - 1.00
72
At a constant level of H3PO4, 75BWA-PAM + 10% H3PO4 showed a slightly higher extent of
PAM-co-PAA formation than its binary counterpart due to the acid addition; whereas 85BWA-
PAM + 10% H3PO4 showed a spectrum very similar to PAA with almost no characteristics of
PAM, implying a high degree of conversion from PAM to PAA. This affirmed that adding
H3PO4 can accelerate the transformation of PAM to PAM-co-PAA and result in a greater extent
of PAA conversion at very high BWA concentration due to the strong acidity in the electrolyte.
6.1.4 Structural and chemical analyses of the precipitates (XRD and Raman analysis)
The conversion process from PAM to PAA will release NH4+ ions as a by-product [231], which
can neutralize the BWA in the electrolyte system to form NH4+-substituted BWA. The different
solubility of salts and acids may be the reason behind the phase separation. X-ray diffraction
(XRD) was performed to further characterize the crystallinity of these electrolyte films. Figure
40 shows the XRD patterns for 75BWA-PAM, 85BWA-PAM, 75BWA-PAM + 10% H3PO4, and
85BWA-PAM + 10% H3PO4. The XRD analysis of the binary electrolytes were previously
discussed in section 5.4.2.
Figure 40: X-ray diffraction patterns of dried films for (a) 75BWA-PAM; (b) 85BWA-
PAM; (c) 75BWA-PAM + 10% H3PO4; and (d) 85BWA-PAM + 10% H3PO4.
73
After adding 10 wt% H3PO4, the 75BWA-PAM + 10% H3PO4 film in Figure 40 (c) retained the
same broad peak at ca. 15-25o as its binary counterpart (Figure 40 (a)). The significantly
decreased intensity of the peak at 7.5o after the 10 wt% H3PO4 addition was due to a lower
relative BWA content in the ternary electrolyte compared with that of the binary electrolytes (see
Table 3 and Table 4). The XRD pattern of 75BWA-PAM + 10wt% H3PO4 showed a flat profile
without any high-intensity peak, indicating a more amorphous structure to better facilitate ion
conduction as compared to the binary counterparts [33]. Thus, this ternary electrolyte
composition was preferable for high conductivity as shown in Figure 37.
A significant difference was observed in the ternary systems, where 75BWA-PAM + 10% H3PO4
film in Figure 40 (c) still showed an amorphous structure sufficiently hydrated to facilitate this
ion conduction [33], while the 85BWA-PAM + 10% H3PO4 film in Figure 40 (d) showed
crystalline peaks resulted from second phase precipitation in Figure 38 (b) and (c). These
crystalline peaks in XRD pattern correspond to (NH4)5BW12O40 crystals in Figure 41 [234-236].
Figure 41 (a) shows the signature peaks for Keggin-type BWA powder dried at 80-90oC
overnight prior to characterization. As discussed in Section 5.1, the BWA powder showed XRD
peaks characteristic of a Keggin-type structure. (NH4)5BW12O40 powder was synthesized by
drop-wise titration of ammonia carbonate, (NH4)2CO3 into a solution of 0.3M BWA according to
the stoichiometric ratio to substitute the proton with NH4+ ions. The powder was dried at 80-
90oC overnight and characterized with XRD as shown in Figure 41 (a).
Figure 41 (a) shows that the XRD pattern of (NH4)5BW12O40 powder was very similar to that of
BWA powder, implying that it retained a Keggin-type structure. The appearance of a few extra
peaks at 10o, 17.2o, and 20o were due to the different amounts and type of crystallized water in
the (NH4)5BW12O40 powder. The (NH4)5BW12O40 powder had its peaks shifted to higher 2θ
values compared to BWA powder due to the change in lattice spacing from NH4+ substitution.
The crystalline peak from 85BWA-PAM + 10% H3PO4 electrolyte film exhibited similar pattern
as the BWA powder XRD with the three strongest peaks at 10.7o, 26.0o, and 36.0o, implying that
the BWA in the electrolyte remained in Keggin structure. All of the peaks in 85BWA-PAM +
10% H3PO4 electrolyte film were shifted to higher values as compared to BWA powder. The
extent of crystalline peak shifting of the 85BWA-PAM + 10% H3PO4 electrolyte film to higher
74
angles matched the (NH4)5BW12O40 powder, thus confirming that the second phase precipitation
on the 85BWA-PAM + 10% H3PO4 electrolyte film originated from the NH4+-substituted BWA.
Raman spectroscopy was performed to differentiate the chemical bonding of the bulk electrolyte
(top spectrum) and the crystals (bottom spectrum) in Figure 41 (b). Both the bulk electrolyte and
crystal phase had the W12O36 cage peaks below 300 cm-1 and strong υ(B-O) peak near 1000 cm-1.
The BWA peak locations in the bulk electrolyte matched the BWA powder characterized in
Section 5.1. The precipitated crystal had shifted υ(W-Oc-W) peak from the original 539.9 cm-1 to
564.3 cm-1 and υ(B-O) from 989.3 cm-1 to 1007.2 cm-1, which implied a change in bond length
within the BWA due to the NH4+ substitution. This supported the notion in Figure 41 (a) that the
phase separation is indeed the precipitation of NH4+-substituted BWA from the bulk electrolyte.
Figure 41: (a) X-ray diffraction patterns of dried samples for BWA powder, 85BWA-PAM
+ 10% H3PO4 electrolyte, and NH4+-substituted BWA powder; and (b) Raman spectra of
non-crystal and crystal regions in 85BWA-PAM + 10% H3PO4 electrolyte. The shifted
Raman peaks were highlighted in blue color.
Although BWA-PAM electrolytes would form PAM-co-PAA due to the reaction of acids with
PAM, most of them remained amorphous and well-hydrated to maintain a high level of
conductivity. The only exception was the 85BWA-PAM + 10% H3PO4 which showed phase
separation due to an extensive conversion to PAA and formation of excess NH4+-based
crystalline side product. This explains the discrepancy in the optical, structural, and
electrochemical analyses of the ternary electrolytes. Thus, an overly high BWA and H3PO4
content is undesirable for BWA-PAM electrolytes. Since 75BWA-PAM + 10% H3PO4 remained
homogeneous and well-hydrated structure, it was selected for further investigation in Chapter 7.
75
6.2 Neutral plasticizer
Section 6.1 showed that a high concentration of BWA and H3PO4 can hydrolyze the PAM
functional group and induce crystallization of the electrolyte system. Hence, a neutral plasticizer
was investigated in the BWA-PAM system to evaluate the performance and compatibility of the
electrolyte components.
In this work, glycerol (Gly) was selected as the neutral plasticizer because of its highly
hygroscopic property. The BWA-PAM + Gly formed homogeneous precursor solutions. Higher
concentrations of glycerol significantly increased the viscosity of the precursor solution. The
precursor solutions with more than 10 wt% of glycerol were too viscous to be stirred well,
resulting in non-homogeneous electrolytes. Thus, two compositions of glycerol were
investigated, 5 wt% and 10 wt% Gly.
6.2.1 Proton conductivity
The BWA-PAM + Gly ternary electrolytes were sandwiched between two Ti electrodes to
assemble the test cells. The test cells were conditioned under room temperature at 45% RH. The
ternary electrolytes were characterized with EIS for conductivity analysis. Figure 42 shows the
conductivity tracking for BWA-PAM and BWA-PAM with glycerol plasticizers conditioned at
45% RH over the period of 28 days.
The conductivity of as-fabricated 75BWA-PAM improved with increased glycerol content from
an initial value of ca. 17 mS cm-1, to ca. 24 mS cm-1 with 5 wt% glycerol, and to ca. 27 mS cm-1
with 10 wt% glycerol. As for the 85BWA-PAM system, although the addition of 5 wt% glycerol
showed no improvement on the conductivity (ca. 27 mS cm-1), a higher concentration of 10 wt%
glycerol led to increased conductivity up to ca. 32 mS cm-1. The improvement in conductivity
was attributed to the hygroscopic property of glycerol that helped to retain water inside the solid
electrolyte to facilitate ion conduction [211, 212].
76
When conditioned at 45% RH under room temperature, the conductivity of the glycerol-modified
electrolytes decreased over time due to electrolyte dehydration, reaching ca. 19 mS cm-1 at day
28. They can retain ≥ 70% of the initial conductivity over 28 days compared to the binary
system, suggesting an improved shelf life due to the glycerol addition. When the failed cells
were dissembled, no delamination was observed because of their better water retention capability
compared to their corresponding glycerol free binary systems.
Figure 42: Room temperature conductivity versus time of electrolytes with 75BWA-PAM,
85BWA-PAM, 75BWA-PAM + 10% Gly, and 85BWA-PAM + 10% Gly conditioned at
45%RH over a period of 28 days.
0 5 10 15 20 25 300
5
10
15
20
25
30
35
Con
duct
ivity
(m
S c
m-1
)
Days
75BWA-PAM 75BWA-PAM + 5% Gly 75BWA-PAM + 10% Gly 85BWA-PAM 85BWA-PAM + 5% Gly 85BWA-PAM + 10% Gly
77
6.2.2 Chemical stability of BWA-PAM + Gly Ternary Electrolytes (Raman analysis)
The glycerol-modified BWA-PAM electrolytes were characterized with Raman, FTIR, and XRD
to investigate their chemical bonding and structure. The chemical stability of glycerol in BWA-
PAM electrolyte was analyzed with Raman spectroscopy. Figure 43 showed the Raman spectra
for BWA-PAM modified with glycerol and the peak assignments are tabulated in Table 15.
The Raman analyses of PAM and BWA were previously discussed in sections 5.1and 5.2. BWA
was represented by its characteristic peak at 989.3cm-1, while the PAM functional group was
represented by the peaks at 1610.0 cm-1 and 1666.3 cm-1. Glycerol has strong peaks at three
positions (859.1 cm-1, 1065.1 cm-1, and 1474.8 cm-1), but most of these are overshadowed by the
PAM peaks due to the relatively low concentration of glycerol in the BWA-PAM electrolytes.
The only exception was the glycerol peak at 1065.1 cm-1 which did not overlap with any peaks
from BWA and PAM, thus this peak was used to identify glycerol in the electrolytes.
When 5 wt% glycerol was added into PAM in the absence of BWA, the resultant spectrum in
Figure 43 (a) showed both PAM and glycerol peaks. The intensity of glycerol peaks were lower
due to the lower concentration of glycerol in the electrolyte. When 75 wt% BWA was added into
the PAM modified with 5% glycerol, the resultant spectra showed all characteristic peaks for
BWA, PAM, and glycerol. However, the glycerol peak was overshadowed by the relatively high-
intensity BWA peaks. Adding 85 wt% BWA also resulted in the spectrum displaying all peaks
from BWA, PAM and glycerol. The difference was the change in peak intensities, which were
the simple addition of the BWA, PAM, and glycerol peaks according to their respective
concentration.
Similar trend as the 5 wt% glycerol-modified electrolytes can be observed for the electrolytes
modified with 10 wt% glycerol in Figure 43 (b). The ternary electrolytes showed all
characteristic peaks from BWA, PAM, and glycerol. The only difference was the more
prominent glycerol peak due to the higher glycerol content in the electrolytes. Thus, the BWA-
PAM is compatible with glycerol because neither peak shifting nor additional peaks that did not
belong to BWA, PAM, and glycerol were observed.
78
Figure 43: Raman spectra of PAM, BWA, and glycerol compared to Gly-PAM, 75BWA-
PAM, 75BWA-PAM, and 85BWA-PAM modified with (a) 5 wt% Gly and (b) 10 wt% Gly.
The characteristic peaks for BWA were highlighted in blue color, glycerol characteristic
peak was highlighted in red color, and the PAM characteristic peaks were highlighted in
green color.
Figure 44 shows a closer observation and comparison of the Raman spectra of 85BWA-PAM
modified with glycerol in the range of 1000-2000 cm-1 in comparison with its binary counterpart
and 85BWA-PAM modified with 10 wt% H3PO4. As reported in Section 6.1, adding BWA into
PAM will cause a partial transformation of PAM to PAM-co-PAA, which can be accelerated by
H3PO4. This transformation was indicated by the decreased peak intensities for the PAM
functional group of υ(C-N), δ(NH2), and υ(C=O) at 1426.9, 1610.0, and 1666.3 cm-1
respectively, and an increased PAA peak intensity of υ(COO-) at 1717.2 cm-1.
400 600 800 1000 1200 1400 1600 1800 400 600 800 1000 1200 1400 1600 1800
PAM Film
BWA Powder
Glycerol
5% Gly-PAM
75BWA-PAM + 5% Gly
(b)C
ou
nts
(a
.u.)
Raman Shift (cm-1)
85BWA-PAM + 5% Gly
(a) PAM Film
BWA Powder
Glycerol
10% Gly-PAM
75BWA-PAM + 10% Gly
Co
un
ts (
a.u
.)
Raman Shift (cm-1)
85BWA-PAM + 10% Gly
79
The resultant spectrum of the 85BWA-PAM electrolyte modified with glycerol had a similar
peak distribution as that of the pure PAM film, except for the δ(CH2) peak at 1060 cm-1 from the
glycerol (see Table 15 in Appendix A). The slightly reduced υ(C-N) peak at 1426.9 cm-1 for
85BWA-PAM + 10% Gly as compared to the PAM and the appearance of small PAA υ(COO-)
peak at 1717.2 cm-1 suggested a slight conversion of PAM to PAM-co-PAA.
When comparing 85BWA-PAM + 10% Gly with its binary counterpart, the PAM functional
group peak intensities at 1426.9, 1610.0, and 1666.3 cm-1 where higher and the PAA peak at
1717.2 cm-1 had a smaller intensity. This suggested that the extent of PAM-co-PAA conversion
in 85BWA-PAM + 10% Gly was much lower than its binary counterpart. The 85BWA-PAM +
10% H3PO4 showed a spectrum very similar to PAA, indicating that almost all PAM was
converted to PAA. In contrast, glycerol as a neutral plasticizer showed a different interaction
with BWA-PAM since the Raman spectrum still showed high intensities for the PAM peaks at
1426.9, 1610.0, and 1666.3 cm-1 and just a minor PAA peak at 1717.2 cm-1.
This suggested that the glycerol suppressed the conversion of PAM to PAA, thus preventing the
crystallization and phase separation as observed in 85BWA-PAM + 10% H3PO4 as reported in
Section 6.1. Therefore, glycerol is effective at enhancing the chemical stability of the BWA-
PAM electrolytes.
80
Figure 44: Raman spectra of PAM, 85BWA-PAM, 85BWA-PAM + 10% H3PO4, 85BWA-
PAM + 10% Gly, and PAA. The glycerol peak was highlighted in blue color, the PAM
functional group peaks were highlighted in red color, and the PAA peak was highlighted in
green color.
1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
PAM Film
85BWA-PAM
85BWA-PAM + 10% H3PO4
85BWA-PAM + 10% Gly
Co
un
ts (
a.u
.)
Raman Shift (cm-1)
PAA
81
6.2.3 Water retention of BWA-PAM + Gly Ternary Electrolytes (XRD analysis)
Figure 45 is the XRD patterns of BWA-PAM electrolytes modified with glycerol. Figure 45 (a)
shows that when PAM was modified with 5 wt% glycerol in the absence of BWA, the peaks at
ca. 11.9o and ca. 15-25o merged into one as compared to that of PAM, suggesting a more
amorphous characteristic of the films due to the water retention capability of glycerol.
When 75 wt % BWA was added into the PAM modified with 5 wt% glycerol, the XRD pattern
in Figure 45 (a) showed a broad profile at ca. 15-25o, indicating that the film retained an
amorphous structure. The appearance of the strong peak at ca. 7.0o and a broad peak at ca. 30o
was due to the BWA hydrates. At the higher concentration of 85 wt% BWA, the 85BWA-PAM
+ 5% Gly also showed the same broad peak at ca. 15-25o, a strong peak at ca. 7.7o, and a broad
peak at ca. 30o as that of 75BWA-PAM + 5% Gly. The difference lies in the peak shifting from
ca. 7.0o for 75BWA-PAM + 5% Gly to ca. 7.7o for 85BWA-PAM + 5% Gly and a slightly
higher intensity of the peak at ca. 30o. These observations suggested that the BWA became less
hydrated at higher BWA concentration in the electrolyte.
Similar to the 5 wt% glycerol-modified electrolytes, the electrolytes modified with 10 wt%
glycerol in Figure 45 (b) showed a similar trend where the 10% Gly-PAM showed a broad peak
at 20o-25o and adding BWA resulted in an additional strong peak at 6.7-7.3o and a broad peak at
ca. 30o. At higher BWA content, 85BWA-PAM + 10% Gly showed a shift of the peak from ca.
6.7o for 75BWA-PAM + 5% Gly to ca. 7.3o and an increase of peak intensity at ca. 30o, again
implying a lower level of hydration of BWA with increased BWA content.
When comparing Figure 45 (a) and (b), adding more glycerol shifted the strong peak at 7.0o-7.7o
for 5 wt% glycerol down to 6.7o-7.3o range for 10 wt% glycerol, indicating a higher degree of
film hydration with more glycerol. These observations were strong evidence that the hygroscopic
glycerol helped to retain water in the films that resulted in a well-hydrated, amorphous structure.
The high conductivity achieved with the addition of glycerol seen in Figure 42 was due to the
enhanced water retention capability of the electrolytes to facilitate ion conduction. Since the
85BWA-PAM + 10% Gly showed both high conductivity and chemical stability, it was chosen
for further investigation in Chapter 7.
82
Figure 45: X-ray diffraction patterns of dried films for (a) PAM, 5% Gly-PAM, 75BWA-
PAM, 75BWA-PAM + 5% Gly, and 85BWA-PAM + 5% Gly; and (b) PAM, 10% Gly-
PAM, 85BWA-PAM, 85BWA-PAM + 10% Gly, and 85BWA-PAM + 10% Gly.
6.3 Summary
The conductivity of BWA-PAM electrolyte can be improved by adding either an acidic
plasticizer (H3PO4) or a neutral plasticizer (glycerol). Adding phosphoric acid (H3PO4)
plasticizer can achieve higher conductivity of ca. 30 mS cm-1 and longer service life by slowing
the electrolyte dehydration. The combination of two acids (85 wt% BWA and 10 wt% H3PO4) at
high concentration accelerated the hydrolysis reaction of PAM to PAM-co-polyacrylic acid
(PAA) with NH4+ as by-product. While partial conversion to PAA generally improves the
hygroscopic behavior of the polymer, the NH4+ by-product can react with BWA to form an
undesirable phase separation. The neutral glycerol can lessen the conversion of PAM to PAM-
co-PAA and mitigate this crystal precipitation issue. Thus, glycerol was proven to be better at
maintaining the chemical stability and compatibility of the electrolyte system. Glycerol addition
also demonstrated better water retention capability in the film to achieve a high conductivity of
ca. 30 mS cm-1 and longer service life than its binary counterpart.
83
Chapter 7 Device performance of the optimized ternary electrolytes
7.1 Acidic vs. Neutral Plasticizers
In Chapter 6, both 75BWA-PAM + 10% H3PO4 and 85BWA-PAM + 10% Gly solid polymer
electrolytes were determined to be the optimal compositions because of their high conductivity
and extended shelf life. Therefore, both electrolytes were tested in capacitor cells for further
investigation. Figure 46 showed the room temperature conductivity of 75BWA-PAM + 10%
H3PO4 and 85BWA-PAM + 10% Gly conditioned at 45% RH.
In general, both as-fabricated electrolytes modified with acidic and neutral plasticizers had
comparable conductivity at ca. 30 mS cm-1. After 28 days of tracking the cells, both electrolytes
demonstrated a similar decrease in conductivity over time, resulting in ca. 70% retention of their
initial conductivity at day 28. The cells dehydrated over the tracking period due to the simple
tape sealing of the cell setup (see Appendix D), which resulted in the reduction in the electrolyte
thickness and self-shorting of the cells after day 28.
The high conductivity (ca. 30 mS cm-1) achieved by both 75BWA-PAM + 10% H3PO4 and
85BWA-PAM + 10% Gly outperformed the SiWA-PVA electrolyte with 10 wt% H3PO4
modification (ca. 20 mS cm-1) developed by Gao et al. [117, 224]. Both of the BWA-PAM +
plasticizers were able to retain sufficient water to facilitate ion conduction at a high level
comparable to the room temperature conductivity of SiWA-XLPVA + H3PO4 electrolyte (ca. 33
mS cm-1) at 45% RH condition [224]. Thus, both 75BWA-PAM + 10% H3PO4 and 85BWA-
PAM + 10% Gly had comparable electrochemical performance with high conductivity and long
shelf life suitable for EC applications. Both optimized ternary electrolytes were then tested with
CNT-graphite electrodes in solid capacitor cells.
84
Figure 46: Room temperature conductivity versus time of 75BWA-PAM + 10% H3PO4,
and 85BWA-PAM + 10% Gly conditioned at 45%RH over a period of 28 days.
7.2 Solid Cells with 75BWA-PAM + 10% H3PO4 Electrolyte
A pair of CNT-graphite electrodes was first tested in liquid cells with 0.1 M BWA soaked in
glass microfiber as the electrolyte. The same pair of electrodes was then used to assemble the
75BWA-PAM + 10% H3PO4 solid cells to evaluate the performance difference of replacing the
liquid electrolyte with the solid polymer electrolytes.
7.2.1 Liquid Cells versus Solid Cells
Figure 47 shows the cyclic voltammograms (CVs) and the discharging capacitance as a function
of scan rates for 75BWA-PAM + 10% H3PO4 solid cells against their BWA liquid cell
counterparts. As shown in Figure 47 (a), the 0.1M BWA liquid cells demonstrated a near
rectangular CV with a 1.2 V window at 50 mVs-1. The 75BWA-PAM + 10% H3PO4 solid cells
showed a different CV profile than their liquid cell counterparts likely due to the shift in open
circuit potential.
At higher scan rate of 1 V s-1, the liquid cells in Figure 47 (b) a near rectangular CV profile but
slightly tilted as compared to that at 50 mVs-1 (Figure 47 (a)). The 75BWA-PAM + 10% H3PO4
solid cells also showed a near rectangular CV at 1 V s-1, but exhibited a slightly larger tilt than
the cells with 0.1M BWA liquid electrolyte due to the increased resistance from the polymer as
compared to the liquid. Nyquist and Bode plots for the solid cells are provided in Appendix E.
0 5 10 15 20 25 300
5
10
15
20
25
30
35
Co
nd
uct
ivity
(m
S c
m-1
)
Days
75BWA-PAM + 10% H3PO4
85BWA-PAM + 10% Gly
85
The rate capability of 75BWA-PAM + 10% H3PO4 solid cells was compared against the liquid
counterparts in Figure 47 (c) using the same electrodes. The cells with 0.1M BWA liquid
electrolyte exhibited a discharging capacitance of ca. 11.5 mF cm-2 at 50 mV s-1, and retained ≥
75% of this capacitance (ca. 9.6 mF cm-2) at high scan rate of 1 V s-1. The solid cells with
75BWA-PAM + 10% H3PO4 electrolyte showed ≥ 95% discharging capacitance (ca. 11.0 mF
cm-2) as compared to that of the liquid electrolyte (ca. 11.5 mF cm-2), suggesting that replacing
the liquid electrolyte with solid electrolyte did not significantly compromise the cell
performance. The 75BWA-PAM + 10% H3PO4 solid cells also retained ≥ 75% of their
discharging capacitance at 1 V s-1 as compared to that at 50 mV s-1 (see Figure 48 (c)), implying
that the solid cells were capable of high rate performance.
These results suggested that the modification of BWA-PAM with 10 wt% H3PO4 plasticizers
allowed the BWA-PAM-enabled capacitor cells to have high rate performance that is comparable
to the liquid cells. Therefore, the solid cells with 75BWA-PAM + 10% H3PO4 electrolytes are
very promising for solid ECs.
Figure 47: Third scan of cyclic voltammograms (a) at 50 mV s-1 and (b) at 1 V s-1 for
75BWA-PAM + 10% H3PO4 on CNT-graphite solid cells against the liquid electrolyte cells;
and (c) the discharging capacitance for 75BWA-PAM + 10% H3PO4 at various scan rates.
Inset showed the solid capacitor cells.
86
7.2.2 Cycle Life Analysis
A new set of CNT-graphite electrodes were used to assemble solid cells with 75BWA-PAM +
10% H3PO4 electrolyte for cycle life analysis in a galvanostatic cycling test over the voltage
range of 0-1.2 V at 6 A g-1. Figure 48 (a) depicts the charging-discharging cycle (CDC) curve for
cycles 1-10 and 9,991-10,000 of the 75BWA-PAM + 10% H3PO4 solid cells. The capacitance of
the solid cells dropped from ca. 9.88 mF cm-2 in the first cycle to ca. 8.97 mF cm-2 in the
10,000th cycle, an equivalent of ≥ 90 % capacitance retention that suggests a good cycle life.
Figure 48 (b) showed that the ESR of the 75BWA-PAM + 10% H3PO4 capacitor cells remained
in a similar range of 1.0 Ω-1.2 Ω with a phase angle of ca. -85o before and after the CDC,
suggesting that the electrolyte has a high stability under ambient condition to retain their highly
capacitive behavior. The impedance response after the galvanostatic test showed a slightly larger
hemispherical shape likely due to minor carbon ink delamination from the Ti substrate. The
overlapping CVs in Figure 48 (c) for 75BWA-PAM + 10% H3PO4 cells suggests good cycle life
to maintain their performance over the rigorous charging-discharging cycles.
Figure 48: (a) The capacitance retention rate of 75BWA-PAM + 10% H3PO4 solid cells
throughout the 10,000 charging-discharging cycles (CDC) at 6 A g-1 with the 1-10 cycles
and 9,991-10,000 cycles shown in the inset; (b) Nyquist plots and (c) third scan of CVs at 50
mV s-1 of 75BWA-PAM + 10% H3PO4 solid cells before and after the CDC.
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000
10
20
30
40
50
60
70
80
90
100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 2 4 6 8 10 12 14 16 18 200.0
0.2
0.4
0.6
0.8
1.0
1.2
0 200 400 600 800 1000 1200 14000
-200
-400
-600
-800
-1000
-1200
-1400
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.02.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Cap
acita
nce
Ret
entio
n (%
)
Cycle Number
Cycles 1-10
Vol
tage
(V
)
Time (s)
Cycles 9,991-10,000
Z''
(Ohm
)
Z' (Ohm)
Before Galvanic Test After Galvanic Test
Z''
(Ohm
)
Z' (Ohm)
(c)
(b)
Cur
rent
(m
A c
m-2
)
Voltage (V)
Before Galvanostatic Test After Galvanostatic Test
Scan rate: 50 mV s-1
(a)
87
7.3 Solid Cells with 85BWA-PAM + 10% Gly Electrolyte
A new pair of CNT-graphite electrodes was tested in 0.1 M BWA liquid cells and was used to
assemble the 85BWA-PAM + 10% Gly solid cells to evaluate the performance difference of
replacing the liquid electrolyte with the solid polymer electrolytes.
7.3.1 Liquid Cells versus Solid Cells
Figure 49 (a) revealed a near rectangular CV for the 85BWA-PAM + 10% Gly solid cells at 50
mVs-1, which is close to the liquid cell counterparts. Similar to the observation in 75BWA-PAM
+ 10% H3PO4 solid cells, the 85BWA-PAM + 10% Gly solid cells showed a different CV profile
than their liquid cell counterparts likely due to the shift in open circuit potential. The 85BWA-
PAM + 10% Gly solid cells in Figure 49 demonstrated a slightly larger capacitance (ca. 15.6 mF
cm-2) than that of 75BWA-PAM + 10% H3PO4 solid cells in Figure 47 (ca. 11.5 mF cm-2) due to
the difference in CNT-graphite loading on the electrodes.
Figure 49 (b) reveals that 85BWA-PAM + 10% Gly solid cells exhibited a near rectangular CV
profile at 1 V s-1, but with a slightly more resistive response than its liquid cell counterpart as
indicated by the tilting of the CV. Nyquist and Bode plots for the solid cells are provided in
Appendix E. Figure 49 (c) shows that replacing the liquid electrolyte (ca. 16.2 mF cm-2) with
85BWA-PAM + 10% Gly solid cells (ca. 15.6 mF cm-2) also resulted in ≥ 95% discharging
capacitance retention. The 85BWA-PAM + 10% Gly solid cells had high rate performance with
≥ 75% retention of their discharging capacitance at 1 V s-1 (ca. 10.8 mF cm-2) as compared to
that of 50 mV s-1 (ca. 15.6 mF cm-2). Therefore, the solid cells with 85BWA-PAM + 10% Gly
electrolyte are comparable to that of the liquid cells and are very promising for fast-charging
solid ECs.
88
Figure 49: Third scan of cyclic voltammograms (a) at 50 mV s-1 and (b) at 1 V s-1 for
85BWA-PAM + 10% Gly on CNT-graphite solid cells against the liquid electrolyte cells;
and (c) the discharging capacitance for 85BWA-PAM + 10% Gly at various scan rates.
Inset showed the solid capacitor cells.
7.3.2 Cycle Life Analysis
A new set of CNT-graphite electrodes were used to assemble solid cells with 85BWA-PAM +
10% Gly electrolyte for cycle life analysis in a galvanostatic cycling test over the voltage range
of 0-1.2 V at 6 A g-1. The 85BWA-PAM + 10% Gly cells exhibited a high initial capacitance of
ca. 19.5 mF cm-2 and retained ≥ 90% of its initial capacitance (ca. 18.2 mF cm-2) after 10,000
CDC as shown in Figure 50 (a). The symmetric triangular shape of the CDC curve after the
galvanostatic cycling suggests that the cells have very good cycle life.
Figure 50 (b) compared the ESR of the 85BWA-PAM + 10% Gly cell before (0.83 Ω) and after
(0.85 Ω) the 10,000 galvanostatic cycles which were very close to each other, implying that the
glycerol-modified cells also have a high stability after the CDC. The impedance response after
the CDC showed a slightly larger hemispherical shape likely due to minor carbon ink
delamination from the Ti substrate.
89
The cyclic voltammograms of 85BWA-PAM + 10% Gly cells at slow scan rate of 50 mV s-1 in
Figure 50 (c) showed a near rectangular shape before and after the CDC. The ca. -85o phase
angle and the near rectangular CV shapes for 85BWA-PAM + 10% Gly solid cells suggest a
great cycle life and highly capacitive behavior.
In general, the performance of both 75BWA-PAM + 10% H3PO4 and 85BWA-PAM + 10% Gly
solid capacitor cells were comparable to liquid cells and demonstrated long cycle life (≥ 90%
capacitance retention and ca. -85o phase angle over 10,000 charging-discharging cycles).
Therefore, the optimized ternary electrolytes have similar electrochemical performance suitable
for high-rate and long-lasting EC applications.
Figure 50: (a) The capacitance retention rate of 85BWA-PAM + 10% Gly solid cells
throughout the 10,000 charging-discharging cycles (CDC) at 6 A g-1 with the 1-10 cycles
and 9,991-10,000 cycles shown in the inset; (b) Nyquist plots and (c) third scan of CVs at 50
mV s-1 of 85BWA-PAM + 10% Gly solid cells before and after the CDC.
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 100000
10
20
30
40
50
60
70
80
90
100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 2 4 6 8 10 12 14 16 18 200.0
0.2
0.4
0.6
0.8
1.0
1.2
0 100 200 300 400 500 600 7000
-100
-200
-300
-400
-500
-600
-700
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.02.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Cap
acita
nce
Ret
entio
n (%
)
Cycle Number
Cycles 1-10
Vol
tage
(V
)
Time (s)
Cycles 9,991-10,000
Z''
(Ohm
)
Z' (Ohm)
Before Galvanostatic Test After Galvanostatic Test
Z''
(Ohm
)
Z' (Ohm)
Scan rate: 50 mV s-1
Cur
rent
(m
A c
m-2
)
Voltage (V)
Before Galvanostatic Test After Galvanostatic Test
(b)(a)
(c)
90
7.4 Summary
The optimized ternary electrolytes with composition of 75BWA-PAM + 10% H3PO4 and
85BWA-PAM + 10% Gly were demonstrated to have similar conductivity despite the different
plasticizers and BWA composition. These electrolytes can be made into solid capacitor cells that
not only demonstrated a near rectangular CV window of 1.2 V up to 1 V s-1 scan rate, but also
displayed excellent cycle life of ≥ 90% capacitance retention and remained at ca. -85o phase
angle over 10,000 charging-discharging cycles. The capacitance and rate performance of these
solid cells were not significantly compromised when replacing the liquid electrolyte with solid
polymer electrolytes. Therefore, both the 85BWA-PAM + 10% Gly and the 75BWA-PAM +
10% H3PO4 electrolytes are very promising candidates for SPE applications in fast-charging and
long-lasting ECs.
91
Chapter 8 Summary and outlook
8.1 Conclusions
In this thesis, multiple electrochemical and material characterization techniques were performed
to explore the structural and chemical bonding of the electrolyte system and the interactions
between polymer, proton conductors, and additives. A thorough analysis of the experimental
results helped to establish an understanding of the compatibility of the electrolyte materials; the
effects of additives on the electrolytes; and the effects of material properties on conductivity.
The key attributes affecting the electrolyte properties and compatibility have been identified:
water retention capability, concentration of ionic conductor, electrochemical stability window of
HPAs, and types of plasticizers. The following understandings can be summarized.
BWA-PAM Binary Electrolytes: The synthesized BWA had high purity. The BWA-
PAM binary electrolyte system had been successfully developed. High concentration of
BWA resulted in higher conductivity in PAM due to the availability of more ionic
conductors. The strong acidity of BWA can hydrolyze acrylamide groups to partially
transform PAM to PAM-co-PAA. This transformation may be beneficial as PAA is also
hygroscopic. However, the BWA-PAM electrolyte system still suffers from water
evaporation over time.
BWA-PAM + Plasticizers Ternary Electrolytes: The dehydration of BWA-PAM can
be improved by the addition of plasticizers. Two different plasticizers: acidic plasticizer
(H3PO4) and neutral plasticizer (glycerol) were investigated. The hygroscopic H3PO4 not
only helped to retain the water in the electrolyte, but also provide extra protons for
conduction. The H3PO4 can increase the conversion of PAM to PAA. At high
concentration of 85 wt% BWA, the by-product of this conversion reacted with BWA to
form NH4+-substituted BWA precipitation, which is undesirable for electrolyte
performance. In contrast, glycerol assisted the polymer electrolytes to stay well-hydrated.
The neutrality of glycerol helped to mitigate the conversion of PAM to PAA, thus
eliminating the issue of phase separation as observed in the case of H3PO4 plasticizer.
92
Solid CNT-Graphite EC Cells: The best compositions for acidic plasticizer- and neutral
plasticizer-modified BWA-PAM electrolytes were used to assemble solid cells with
CNT-graphite electrodes compared against the liquid cell counterparts. The electrolytes
modified with both plasticizers had comparable conductivity and long-term performance.
The solid cells demonstrated near rectangular CV profile of 1.2 V window and
maintained ≥ 75% discharging capacitance from 50 mV s-1 to 1 V s-1. Both cells with
H3PO4-modified and glycerol-modified electrolytes demonstrated excellent cycle life and
highly capacitive behavior by retaining ≥ 90% capacitance and ca. -85o phase angle over
the rigorous 10,000 charging-discharging cycles. Therefore, both the 85BWA-PAM +
10% Gly and the 75BWA-PAM + 10% H3PO4 ternary electrolytes are very promising
candidates for SPE applications in ECs.
93
8.2 Future work
This thesis had developed solid polymer electrolytes based on BWA-PAM system and
investigated the effects of different plasticizers on its performance. However, there are some
properties that can be examined to further enhance this electrolyte system. This research can be
carried forward by investigating the following aspects:
The cell failure is attributed to the water loss due to poor sealing technique. Better sealing
techniques such as hot press lamination can be explored to design ECs with longer
service life.
The characterizations presented in this thesis are limited to ambient temperature and 45%
relative humidity (RH). The cell performance at lower RH and various temperature
ranges can be investigated to analyze the versatility and feasibility of the cell under
different environmental conditions.
Other amorphous polymers can be explored to engineer a compatible electrolyte system
with HPAs.
Other structures and chemistries of HPAs can be investigated to further expand the
electrochemical stability window for the electrolyte systems
Crosslinking of PAM or adding nanoparticles such as nano-silica (nano-SiO2) or nano-
titanium oxide (nano-TiO2) as solid plasticizers to enhance the dimensional stability of
the electrolyte.
The compatibility of this electrolyte system with pseudocapacitive electrodes can be
investigated.
The conduction mechanism of the electrolyte systems can be characterized.
94
References
[1] L. Dong, C. Xu, Y. Li, Z. H. Huang, F. Kang, Q. H. Yang, et al., "Flexible electrodes and supercapacitors for wearable energy storage: a review by category," Journal of Materials Chemistry A, vol. 4, pp. 4659-4685, 2016.
[2] Z. Wang, W. Zhang, X. Li, and L. Gao, "Recent progress in flexible energy storage materials for lithium-ion batteries and electrochemical capacitors: A review," Journal of Materials Research, vol. 31, pp. 1648-1664, 2016.
[3] F. R. Fan, W. Tang, and Z. L. Wang, "Flexible Nanogenerators for Energy Harvesting and Self‐Powered Electronics," Advanced Materials, vol. 28, pp. 4283-4305, 2016.
[4] N. Yu, H. Yin, W. Zhang, Y. Liu, Z. Tang, and M. Q. Zhu, "High‐Performance Fiber‐Shaped All‐Solid‐State Asymmetric Supercapacitors Based on Ultrathin MnO2 Nanosheet/Carbon Fiber Cathodes for Wearable Electronics," Advanced Energy Materials, vol. 6, pp. 1-9, 2016.
[5] Y. Yang, Q. Huang, L. Niu, D. Wang, C. Yan, Y. She, et al., "Waterproof, Ultrahigh Areal‐Capacitance, Wearable Supercapacitor Fabrics," Advanced Materials, vol. 29, pp. 1-9, 2017.
[6] X. Pu, L. Li, M. Liu, C. Jiang, C. Du, Z. Zhao, et al., "Wearable self‐charging power textile based on flexible yarn supercapacitors and fabric nanogenerators," Advanced Materials, vol. 28, pp. 98-105, 2016.
[7] X. Lu, M. Yu, G. Wang, Y. Tong, and Y. Li, "Flexible solid-state supercapacitors: design, fabrication and applications," Energy & Environmental Science, vol. 7, pp. 2160-2181, 2014.
[8] M. Winter and R. J. Brodd, "What are batteries, fuel cells, and supercapacitors?," vol. 104, ed. Chem. Rev.: ACS Publications, 2004, pp. 4245-4270.
[9] R. Kötz and M. Carlen, "Principles and applications of electrochemical capacitors," Electrochimica Acta, vol. 45, pp. 2483-2498, 2000.
[10] Y. Zeng, M. Yu, Y. Meng, P. Fang, X. Lu, and Y. Tong, "Iron‐based supercapacitor electrodes: advances and challenges," Advanced Energy Materials, vol. 6, pp. 1-17, 2016.
[11] A. González, E. Goikolea, J. A. Barrena, and R. Mysyk, "Review on supercapacitors: technologies and materials," Renewable and Sustainable Energy Reviews, vol. 58, pp. 1189-1206, 2016.
[12] F. Schipper and D. Aurbach, "A brief review: Past, present and future of lithium ion batteries," Russian Journal of Electrochemistry, vol. 52, pp. 1095-1121, 2016.
95
[13] J. Jaguemont, L. Boulon, and Y. Dubé, "A comprehensive review of lithium-ion batteries used in hybrid and electric vehicles at cold temperatures," Applied Energy, vol. 164, pp. 99-114, 2016.
[14] A. Chu and P. Braatz, "Comparison of commercial supercapacitors and high-power lithium-ion batteries for power-assist applications in hybrid electric vehicles: I. Initial characterization," Journal of power sources, vol. 112, pp. 236-246, 2002.
[15] C. Emmenegger, P. Mauron, P. Sudan, P. Wenger, V. Hermann, R. Gallay, et al., "Investigation of electrochemical double-layer (ECDL) capacitors electrodes based on carbon nanotubes and activated carbon materials," Journal of Power Sources, vol. 124, pp. 321-329, 2003.
[16] H. Douglas and P. Pillay, "Sizing ultracapacitors for hybrid electric vehicles," in Industrial Electronics Society, 2005. IECON 2005. 31st Annual Conference of IEEE, 2005, p. 6.
[17] M. Jayalakshmi and K. Balasubramanian, "Simple capacitors to supercapacitors-an overview," Int. J. Electrochem. Sci, vol. 3, pp. 1196-1217, 2008.
[18] Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li, et al., "Anchoring hydrous RuO2 on graphene sheets for high‐performance electrochemical capacitors," Advanced Functional Materials, vol. 20, pp. 3595-3602, 2010.
[19] P. Sharma and T. Bhatti, "A review on electrochemical double-layer capacitors," Energy Conversion and Management, vol. 51, pp. 2901-2912, 2010.
[20] P. Simon and Y. Gogotsi, "Materials for electrochemical capacitors," Nature materials, vol. 7, pp. 845-854, 2008.
[21] I. Hadjipaschalis, A. Poullikkas, and V. Efthimiou, "Overview of current and future energy storage technologies for electric power applications," Renewable and sustainable energy reviews, vol. 13, pp. 1513-1522, 2009.
[22] Y. Zhang, H. Feng, X. Wu, L. Wang, A. Zhang, T. Xia, et al., "Progress of electrochemical capacitor electrode materials: A review," International journal of hydrogen energy, vol. 34, pp. 4889-4899, 2009.
[23] S. Sarangapani, B. Tilak, and C. P. Chen, "Materials for electrochemical capacitors theoretical and experimental constraints," Journal of the Electrochemical Society, vol. 143, pp. 3791-3799, 1996.
[24] Y. Wang, Y. Song, and Y. Xia, "Electrochemical capacitors: mechanism, materials, systems, characterization and applications," Chemical Society Reviews, vol. 45, pp. 5925-5950, 2016.
[25] A. Lewandowski and M. Galinski, "Practical and theoretical limits for electrochemical double-layer capacitors," Journal of Power Sources, vol. 173, pp. 822-828, 2007.
96
[26] S. Ramesh and L. C. Wen, "Investigation on the effects of addition of SiO2 nanoparticles on ionic conductivity, FTIR, and thermal properties of nanocomposite PMMA–LiCF3SO3–SiO2," Ionics, vol. 16, pp. 255-262, 2010.
[27] S. Ramesh, C. W. Liew, E. Morris, and R. Durairaj, "Effect of PVC on ionic conductivity, crystallographic structural, morphological and thermal characterizations in PMMA–PVC blend-based polymer electrolytes," Thermochimica Acta, vol. 511, pp. 140-146, 2010.
[28] C. W. Liew, R. Durairaj, and S. Ramesh, "Rheological Studies of PMMA–PVC Based Polymer Blend Electrolytes with LiTFSI as Doping Salt," PloS one, vol. 9, p. e102815, 2014.
[29] S. R. Raghavan, M. W. Riley, P. S. Fedkiw, and S. A. Khan, "Composite polymer electrolytes based on poly (ethylene glycol) and hydrophobic fumed silica: dynamic rheology and microstructure," Chemistry of materials, vol. 10, pp. 244-251, 1998.
[30] J. Adebahr, N. Byrne, M. Forsyth, D. MacFarlane, and P. Jacobsson, "Enhancement of ion dynamics in PMMA-based gels with addition of TiO2 nano-particles," Electrochimica acta, vol. 48, pp. 2099-2103, 2003.
[31] I. Nicotera, L. Coppola, C. Oliviero, M. Castriota, and E. Cazzanelli, "Investigation of ionic conduction and mechanical properties of PMMA–PVdF blend-based polymer electrolytes," Solid State Ionics, vol. 177, pp. 581-588, 2006.
[32] S. Rajendran and T. Uma, "Lithium ion conduction in PVC–LiBF4 electrolytes gelled with PMMA," Journal of power sources, vol. 88, pp. 282-285, 2000.
[33] K. S. Ngai, S. Ramesh, K. Ramesh, and J. C. Juan, "A review of polymer electrolytes: fundamental, approaches and applications," Ionics, vol. 22, pp. 1259-1279, 2016.
[34] S. Ramesh, C.-W. Liew, and K. Ramesh, "Evaluation and investigation on the effect of ionic liquid onto PMMA-PVC gel polymer blend electrolytes," Journal of Non-Crystalline Solids, vol. 357, pp. 2132-2138, 2011.
[35] S. Ramesh, T. Winie, and A. Arof, "Investigation of mechanical properties of polyvinyl chloride–polyethylene oxide (PVC–PEO) based polymer electrolytes for lithium polymer cells," European polymer journal, vol. 43, pp. 1963-1968, 2007.
[36] Q. Huang, D. Wang, and Z. Zheng, "Textile‐Based Electrochemical Energy Storage Devices," Advanced Energy Materials, 2016.
[37] H. Gao and K. Lian, "Proton-conducting polymer electrolytes and their applications in solid supercapacitors: a review," RSC Advances, vol. 4, pp. 33091-33113, 2014.
[38] A. Lewandowski, A. Olejniczak, M. Galinski, and I. Stepniak, "Performance of carbon–carbon supercapacitors based on organic, aqueous and ionic liquid electrolytes," Journal of Power Sources, vol. 195, pp. 5814-5819, 2010.
97
[39] C. Wessells, R. A. Huggins, and Y. Cui, "Recent results on aqueous electrolyte cells," Journal of Power Sources, vol. 196, pp. 2884-2888, 2011.
[40] L. Suo, O. Borodin, T. Gao, M. Olguin, J. Ho, X. Fan, et al., "“Water-in-salt” electrolyte enables high-voltage aqueous lithium-ion chemistries," Science, vol. 350, pp. 938-943, 2015.
[41] C. Wessells, R. Ruffο, R. A. Huggins, and Y. Cui, "Investigations of the electrochemical stability of aqueous electrolytes for lithium battery applications," Electrochemical and Solid-State Letters, vol. 13, pp. A59-A61, 2010.
[42] W. Li, D. Chen, Z. Li, Y. Shi, Y. Wan, G. Wang, et al., "Nitrogen-containing carbon spheres with very large uniform mesopores: the superior electrode materials for EDLC in organic electrolyte," Carbon, vol. 45, pp. 1757-1763, 2007.
[43] J. Read, "Ether-based electrolytes for the lithium/oxygen organic electrolyte battery," Journal of The Electrochemical Society, vol. 153, pp. A96-A100, 2006.
[44] P. Azaïs, L. Duclaux, P. Florian, D. Massiot, M.-A. Lillo-Rodenas, A. Linares-Solano, et al., "Causes of supercapacitors ageing in organic electrolyte," Journal of power sources, vol. 171, pp. 1046-1053, 2007.
[45] V. Khomenko, E. Raymundo-Piñero, and F. Béguin, "High-energy density graphite/AC capacitor in organic electrolyte," Journal of Power Sources, vol. 177, pp. 643-651, 2008.
[46] W. Y. Tsai, R. Lin, S. Murali, L. L. Zhang, J. K. McDonough, R. S. Ruoff, et al., "Outstanding performance of activated graphene based supercapacitors in ionic liquid electrolyte from −50 to 80oC," Nano Energy, vol. 2, pp. 403-411, 2013.
[47] A. B. McEwen, H. L. Ngo, K. LeCompte, and J. L. Goldman, "Electrochemical properties of imidazolium salt electrolytes for electrochemical capacitor applications," Journal of the Electrochemical Society, vol. 146, pp. 1687-1695, 1999.
[48] F. Wu, N. Zhu, Y. Bai, L. Liu, H. Zhou, and C. Wu, "Highly Safe Ionic Liquid Electrolytes for Sodium-Ion Battery: Wide Electrochemical Window and Good Thermal Stability," ACS Applied Materials & Interfaces, vol. 8, pp. 21381-21386, 2016.
[49] M. C. Buzzeo, C. Hardacre, and R. G. Compton, "Extended electrochemical windows made accessible by room temperature ionic liquid/organic solvent electrolyte systems," ChemPhysChem, vol. 7, pp. 176-180, 2006.
[50] M. Armand, F. Endres, D. R. MacFarlane, H. Ohno, and B. Scrosati, "Ionic-liquid materials for the electrochemical challenges of the future," Nature materials, vol. 8, pp. 621-629, 2009.
[51] Z. Lei, Z. Liu, H. Wang, X. Sun, L. Lu, and X. Zhao, "A high-energy-density supercapacitor with graphene–CMK-5 as the electrode and ionic liquid as the electrolyte," Journal of Materials Chemistry A, vol. 1, pp. 2313-2321, 2013.
98
[52] T. Abdallah, D. Lemordant, and B. Claude-Montigny, "Are room temperature ionic liquids able to improve the safety of supercapacitors organic electrolytes without degrading the performances?," Journal of power sources, vol. 201, pp. 353-359, 2012.
[53] W. J. Cho, C. G. Yeom, B. C. Kim, K. M. Kim, J. M. Ko, and K. H. Yu, "Supercapacitive properties of activated carbon electrode in organic electrolytes containing single-and double-cationic liquid salts," Electrochimica Acta, vol. 89, pp. 807-813, 2013.
[54] G. Pandey, Y. Kumar, and S. Hashmi, "Ionic liquid incorporated polymer electrolytes for supercapacitor application," 2010.
[55] M. Galiński, A. Lewandowski, and I. Stępniak, "Ionic liquids as electrolytes," Electrochimica Acta, vol. 51, pp. 5567-5580, 2006.
[56] P. Kurzweil and M. Chwistek, "Electrochemical stability of organic electrolytes in supercapacitors: Spectroscopy and gas analysis of decomposition products," Journal of Power Sources, vol. 176, pp. 555-567, 2008.
[57] M. E. Tuckerman, A. Chandra, and D. Marx, "Structure and dynamics of OH- (aq)," Accounts of chemical research, vol. 39, pp. 151-158, 2006.
[58] M. Mokhtar, E. Majlan, M. Talib, A. Ahmad, S. Tasirin, and W. Daud, "A short review on alkaline solid polymer electrolyte based on polyvinyl alcohol (PVA) as polymer electrolyte for electrochemical devices applications," International Journal of Applied Engineering Research, vol. 11, pp. 10009-10015, 2016.
[59] C. C. Yang, S. T. Hsu, and W. C. Chien, "All solid-state electric double-layer capacitors based on alkaline polyvinyl alcohol polymer electrolytes," Journal of power sources, vol. 152, pp. 303-310, 2005.
[60] G. Ma, J. Li, K. Sun, H. Peng, J. Mu, and Z. Lei, "High performance solid-state supercapacitor with PVA–KOH–K3[Fe(CN)6] gel polymer as electrolyte and separator," Journal of Power Sources, vol. 256, pp. 281-287, 2014.
[61] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, and J. Zhang, "A review of electrolyte materials and compositions for electrochemical supercapacitors," Chemical Society Reviews, vol. 44, pp. 7484-7539, 2015.
[62] S. T. Vindt and E. M. Skou, "The buffer effect in neutral electrolyte supercapacitors," Applied Physics A, vol. 122, pp. 1-6, Feb 2016.
[63] L. L. Zhang and X. Zhao, "Carbon-based materials as supercapacitor electrodes," Chemical Society Reviews, vol. 38, pp. 2520-2531, 2009.
[64] H. Ji, X. Zhao, Z. Qiao, J. Jung, Y. Zhu, Y. Lu, et al., "Capacitance of carbon-based electrical double-layer capacitors," Nature communications, vol. 5, 2014.
[65] B. E. Conway, Electrochemical supercapacitors: scientific fundamentals and technological applications: Springer Science & Business Media, 2013.
99
[66] R. Agrawal, M. Beidaghi, W. Chen, and C. Wang, "Carbon microelectromechanical systems (C-MEMS) based microsupercapacitors," in SPIE Sensing Technology+ Applications, 2015, pp. 94930C-94930C-7.
[67] G. Bajwa, "Nanocarbon/Polyoxometalate Composite Electrodes for Electrochemical Capacitors," MASc, Materials Science and Engineering, University of Toronto, 2012.
[68] J. P. Zheng, "Ruthenium Oxide‐Carbon Composite Electrodes for Electrochemical Capacitors," Electrochemical and Solid-State Letters, vol. 2, pp. 359-361, 1999.
[69] I. H. Kim and K. B. Kim, "Electrochemical characterization of hydrous ruthenium oxide thin-film electrodes for electrochemical capacitor applications," Journal of The Electrochemical Society, vol. 153, pp. A383-A389, 2006.
[70] V. Khomenko, E. Frackowiak, and F. Beguin, "Determination of the specific capacitance of conducting polymer/nanotubes composite electrodes using different cell configurations," Electrochimica Acta, vol. 50, pp. 2499-2506, 2005.
[71] Y. Huang, H. Li, Z. Wang, M. Zhu, Z. Pei, Q. Xue, et al., "Nanostructured Polypyrrole as a flexible electrode material of supercapacitor," Nano Energy, vol. 22, pp. 422-438, 2016.
[72] G. A. Snook, P. Kao, and A. S. Best, "Conducting-polymer-based supercapacitor devices and electrodes," Journal of Power Sources, vol. 196, pp. 1-12, 2011.
[73] S. W. Lee, J. Kim, S. Chen, P. T. Hammond, and Y. Shao-Horn, "Carbon nanotube/manganese oxide ultrathin film electrodes for electrochemical capacitors," ACS nano, vol. 4, pp. 3889-3896, 2010.
[74] G. X. Wang, B. L. Zhang, Z. L. Yu, and M. Z. Qu, "Manganese oxide/MWNTs composite electrodes for supercapacitors," Solid State Ionics, vol. 176, pp. 1169-1174, 2005.
[75] Q. Tian and K. Lian, "In situ characterization of heteropolyacid based electrochemical capacitors," Electrochemical and Solid-State Letters, vol. 13, pp. A4-A6, 2010.
[76] C. Zhan and D. E. Jiang, "Understanding the pseudocapacitance of RuO2 from joint density functional theory," Journal of Physics: Condensed Matter, vol. 28, p. 464004, 2016.
[77] N. Jabeen, Q. Xia, S. V. Savilov, S. M. Aldoshin, Y. Yu, and H. Xia, "Enhanced Pseudocapacitive Performance of α-MnO2 by Cation Preinsertion," ACS Applied Materials & Interfaces, vol. 8, pp. 33732-33740, 2016.
[78] D. Gu, C. Ding, Y. Qin, H. Jiang, L. Wang, and L. Shen, "Behavior of electrical charge storage/release in polyaniline electrodes of symmetric supercapacitor," Electrochimica Acta, 2017.
[79] N. Agmon, "The grotthuss mechanism," Chemical Physics Letters, vol. 244, pp. 456-462, 1995.
100
[80] D. T. Chin and H. H. Chang, "On the conductivity of phosphoric acid electrolyte," Journal of applied electrochemistry, vol. 19, pp. 95-99, 1989.
[81] L. Vilčiauskas, M. E. Tuckerman, G. Bester, S. J. Paddison, and K. D. Kreuer, "The mechanism of proton conduction in phosphoric acid," Nature chemistry, vol. 4, pp. 461-466, 2012.
[82] D. Marx, "Proton Transfer 200 Years after von Grotthuss: Insights from Ab Initio Simulations," ChemPhysChem, vol. 8, pp. 209-210, 2007.
[83] K. Kreuer, "Fast proton conductivity: A phenomenon between the solid and the liquid state?," Solid State Ionics, vol. 94, pp. 55-62, 1997.
[84] A. Chernyshev and S. Cukierman, "Thermodynamic View of Activation Energies of Proton Transfer in Various Gramicidin A Channels," Biophysical Journal, vol. 82, pp. 182-192, 2002/01/01/ 2002.
[85] X. Tong, X. Wu, Q. Wu, W. Zhu, F. Cao, and W. Yan, "Pentadecatungstotrivanadodiphosphoric heteropoly acid with Dawson structure: Synthesis, conductivity and conductive mechanism," Dalton Transactions, vol. 41, pp. 9893-9896, 2012.
[86] H. Cai, T. Huang, Q. Wu, and W. Yan, "Synthesis and conduction mechanism of high proton conductor H6SiW10V2O40⋅14H2O," Functional Materials Letters, vol. 9, p. 1650048, 2016.
[87] K. D. Kreuer, A. Rabenau, and W. Weppner, "Vehicle mechanism, a new model for the interpretation of the conductivity of fast proton conductors," Angewandte Chemie International Edition in English, vol. 21, pp. 208-209, 1982.
[88] K.-D. Kreuer, "Proton conductivity: materials and applications," Chemistry of Materials, vol. 8, pp. 610-641, 1996.
[89] H. Cai, X. Wu, Q. Wu, and W. Yan, "Synthesis and high proton conductive performance of a quaternary vanadomolybdotungstosilicic heteropoly acid," Dalton Transactions, vol. 45, pp. 14238-14242, 2016.
[90] M. Armand, "Polymers with ionic conductivity," Advanced Materials, vol. 2, pp. 278-286, 1990.
[91] M. A. Ratner and D. F. Shriver, "Ion transport in solvent-free polymers," Chemical Reviews, vol. 88, pp. 109-124, 1988.
[92] K. Murata, "An overview of the research and development of solid polymer electrolyte batteries," Electrochimica Acta, vol. 40, pp. 2177-2184, 1995.
[93] P. V. Wright, "Electrical conductivity in ionic complexes of poly (ethylene oxide)," Polymer International, vol. 7, pp. 319-327, 1975.
101
[94] P. V. Wright, "Polymer electrolytes—the early days," Electrochimica Acta, vol. 43, pp. 1137-1143, 1998.
[95] W. H. Meyer, "Polymer electrolytes for lithium‐ion batteries," Advanced materials, vol. 10, pp. 439-448, 1998.
[96] S. J. Osborn, M. K. Hassan, G. M. Divoux, D. W. Rhoades, K. A. Mauritz, and R. B. Moore, "Glass transition temperature of perfluorosulfonic acid ionomers," Macromolecules, vol. 40, pp. 3886-3890, 2007.
[97] J. T. Hinatsu, M. Mizuhata, and H. Takenaka, "Water uptake of perfluorosulfonic acid membranes from liquid water and water vapor," Journal of the Electrochemical Society, vol. 141, pp. 1493-1498, 1994.
[98] L. Liu, W. Chen, and Y. Li, "An overview of the proton conductivity of nafion membranes through a statistical analysis," Journal of Membrane Science, vol. 504, pp. 1-9, 2016.
[99] R. Kuwertz, C. Kirstein, T. Turek, and U. Kunz, "Influence of acid pretreatment on ionic conductivity of Nafion® membranes," Journal of Membrane Science, vol. 500, pp. 225-235, 2016.
[100] M. Schalenbach, W. Lueke, W. Lehnert, and D. Stolten, "The influence of water channel geometry and proton mobility on the conductivity of Nafion®," Electrochimica Acta, vol. 214, pp. 362-369, 2016.
[101] S. Slade, S. Campbell, T. Ralph, and F. Walsh, "Ionic conductivity of an extruded Nafion 1100 EW series of membranes," Journal of the Electrochemical Society, vol. 149, pp. A1556-A1564, 2002.
[102] M. Maréchal, J. L. Souquet, J. Guindet, and J. Y. Sanchez, "Solvation of sulphonic acid groups in Nafion® membranes from accurate conductivity measurements," Electrochemistry communications, vol. 9, pp. 1023-1028, 2007.
[103] Y. Yin, Z. Li, X. Yang, L. Cao, C. Wang, B. Zhang, et al., "Enhanced proton conductivity of Nafion composite membrane by incorporating phosphoric acid-loaded covalent organic framework," Journal of Power Sources, vol. 332, pp. 265-273, 2016.
[104] L. G. Boutsika, A. Enotiadis, I. Nicotera, C. Simari, G. Charalambopoulou, E. P. Giannelis, et al., "Nafion® nanocomposite membranes with enhanced properties at high temperature and low humidity environments," International Journal of Hydrogen Energy, vol. 41, pp. 22406-22414, 2016.
[105] F. A. Zakil, S. K. Kamarudin, and S. Basri, "Modified Nafion membranes for direct alcohol fuel cells: An overview," Renewable and Sustainable Energy Reviews, vol. 65, pp. 841-852, 2016.
102
[106] M. G. Hosseini and E. Shahryari, "Fabrication of novel solid-state supercapacitor using a Nafion polymer membrane with graphene oxide/multiwalled carbon nanotube/polyaniline," Journal of Solid State Electrochemistry, pp. 1-16, 2017.
[107] H. Yang, W. Lee, B. Choi, and W. Kim, "Preparation of Nafion/Pt-containing TiO2/graphene oxide composite membranes for self-humidifying proton exchange membrane fuel cell," Journal of Membrane Science, vol. 504, pp. 20-28, 2016.
[108] J. H. Chang, J. H. Park, G. G. Park, C. S. Kim, and O. O. Park, "Proton-conducting composite membranes derived from sulfonated hydrocarbon and inorganic materials," Journal of Power Sources, vol. 124, pp. 18-25, 2003.
[109] R. C. Greaves, S. P. Bond, and W. R. McWhinnie, "Conductivity studies on modified laponites," Polyhedron, vol. 14, pp. 3635-3639, 1995.
[110] C. Felice and D. Qu, "Optimization of the Synthesis of Nafion− Montmorillonite Nanocomposite Membranes for Fuel Cell Applications through Statistical Design-of-Experiment," Industrial & Engineering Chemistry Research, vol. 50, pp. 721-727, 2010.
[111] S. J. Lee, N. Muthuchamy, A. I. Gopalan, and K. P. Lee, "New Nafion/Conducting Polymer Composite for Membrane Application," 2016.
[112] C. O. Baker, X. Huang, W. Nelson, and R. B. Kaner, "Polyaniline nanofibers: broadening applications for conducting polymers," Chemical Society Reviews, vol. 46, pp. 1510-1525, 2017.
[113] J. Jalili, S. Borsacchi, and V. Tricoli, "Proton conducting membranes in fully anhydrous conditions at elevated temperature: Effect of Nitrilotris (methylenephosphonic acid) incorporation into Nafion-and poly (styrenesulfonic acid)," Journal of Membrane Science, vol. 469, pp. 162-173, 2014.
[114] H. S. Thiam, M. Y. Chia, Q. R. Cheah, C. C. H. Koo, S. O. Lai, and K. C. Chong, "Proton conductivity and methanol permeability of Nafion-SiO2/SiWA composite membranes," in AIP Conference Proceedings, 2017, p. 020007.
[115] H. Zhang, H. Huang, and P. K. Shen, "Methanol-blocking Nafion composite membranes fabricated by layer-by-layer self-assembly for direct methanol fuel cells," international journal of hydrogen energy, vol. 37, pp. 6875-6879, 2012.
[116] Y. Hu, X. Li, L. Yan, and B. Yue, "Improving the Overall Characteristics of Proton Exchange Membranes via Nanophase Separation Technologies: A Progress Review," Fuel Cells, 2017.
[117] H. Gao, H. Wu, and K. Lian, "A comparative study of polymer electrolytes for ultrahigh rate applications," Electrochemistry Communications, vol. 17, pp. 48-51, 2012.
[118] Y. Chang and C. Bae, "Acidity Effect on Proton Conductivity of Hydrocarbon-Based Ionomers," ECS Transactions, vol. 33, pp. 735-741, 2010.
103
[119] Z. Wei, S. He, X. Liu, J. Qiao, J. Lin, and L. Zhang, "A novel environment-friendly route to prepare proton exchange membranes for direct methanol fuel cells," Polymer, vol. 54, pp. 1243-1250, 2013.
[120] M. J. Parnian, S. Rowshanzamir, and F. Gashoul, "Comprehensive investigation of physicochemical and electrochemical properties of sulfonated poly (ether ether ketone) membranes with different degrees of sulfonation for proton exchange membrane fuel cell applications," Energy, vol. 125, pp. 614-628, 2017.
[121] S. Qu, Y. Sun, and J. Li, "Sulfonate poly (ether ether ketone) incorporated with ammonium ionic liquids for proton exchange membrane fuel cell," Ionics, vol. 23, pp. 1607-1611, 2017.
[122] M. J. Parnian, F. Gashoul, and S. Rowshanzamir, "Studies on the SPEEK membrane with low degree of sulfonation as a stable proton exchange membrane for fuel cell applications," Iranian Journal of Hydrogen & Fuel Cell, vol. 3, pp. 221-232, 2017.
[123] L. Zhao, F. Li, Y. Guo, Y. Dong, J. Liu, Y. Wang, et al., "SPEEK/PVDF binary membrane as an alternative proton-exchange membrane in vanadium redox flow battery application," High Performance Polymers, vol. 29, pp. 127-132, 2017.
[124] W. Xu, Y. Zhang, G. Yang, X. Yu, J. Liu, and Z. Jiang, "ZnPc-MWCNT/sulfonated poly (ether ether ketone) composites for high-k and electrical energy storage applications," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 24, pp. 720-726, 2017.
[125] S. Y. Lee, H. S. Woo, J. M. Song, J. Y. Sohn, and J. Shin, "Preparation of cross‐linked SPEEK‐zirconium phosphate hybrid membranes by electron beam irradiation," Polymer Composites, 2016.
[126] F. Fuertges and A. Abuchowski, "The clinical efficacy of poly (ethylene glycol)-modified proteins," Journal of controlled release, vol. 11, pp. 139-148, 1990.
[127] M. Armand, "The history of polymer electrolytes," Solid State Ionics, vol. 69, pp. 309-319, 1994.
[128] M. Armand, "Polymer solid electrolytes-an overview," Solid State Ionics, vol. 9, pp. 745-754, 1983.
[129] N. Vijaya, S. Selvasekarapandian, G. Hirankumar, S. Karthikeyan, H. Nithya, C. Ramya, et al., "Structural, vibrational, thermal, and conductivity studies on proton-conducting polymer electrolyte based on poly (N-vinylpyrrolidone)," Ionics, vol. 18, pp. 91-99, 2012.
[130] M. Ravi, K. K. Kumar, V. M. Mohan, and V. N. Rao, "Effect of nano TiO2 filler on the structural and electrical properties of PVP based polymer electrolyte films," Polymer Testing, vol. 33, pp. 152-160, 2014.
104
[131] K. N. Kumar, T. Sreekanth, M. J. Reddy, and U. S. Rao, "Study of transport and electrochemical cell characteristics of PVP: NaClO3 polymer electrolyte system," Journal of power sources, vol. 101, pp. 130-133, 2001.
[132] M. J. Reddy, T. Sreekanth, M. Chandrashekar, and U. S. Rao, "Ion transport and electrochemical cell characteristic studies of a new (PVP1NaNO3) polymer electrolyte system," Journal of materials science, vol. 35, pp. 2841-2845, 2000.
[133] N. Vijaya, S. Selvasekarapandian, H. Nithya, and C. Sanjeeviraja, "Proton Conducting Polymer Electrolyte based on Poly (N-vinyl pyrrolidone) Doped with Ammonium Iodide," Int. J. Electroactive Mater, vol. 3, pp. 20-27, 2015.
[134] A. A. De Queiroz, D. A. Soares, P. Trzesniak, and G. A. Abraham, "Resistive‐type humidity sensors based on PVP–Co and PVP–I2 complexes," Journal of Polymer Science Part B: Polymer Physics, vol. 39, pp. 459-469, 2001.
[135] D. Santhiya, G. Nandini, S. Subramanian, K. Natarajan, and S. Malghan, "Effect of polymer molecular weight on the absorption of polyacrylic acid at the alumina-water interface," Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 133, pp. 157-163, 1998.
[136] R. Hodge, G. H. Edward, and G. P. Simon, "Water absorption and states of water in semicrystalline poly (vinyl alcohol) films," Polymer, vol. 37, pp. 1371-1376, 1996.
[137] M. Silberberg‐Bouhnik, O. Ramon, I. Ladyzhinski, S. Mizrahi, and Y. Cohen, "Osmotic deswelling of weakly charged poly (acrylic acid) solutions and gels," Journal of Polymer Science Part B: Polymer Physics, vol. 33, pp. 2269-2279, 1995.
[138] C. W. Liew, H. Ng, A. Numan, and S. Ramesh, "Poly (Acrylic acid)–Based Hybrid Inorganic–Organic Electrolytes Membrane for Electrical Double Layer Capacitors Application," Polymers, vol. 8, p. 179, 2016.
[139] H. Park and J. R. Robinson, "Mechanisms of mucoadhesion of poly (acrylic acid) hydrogels," Pharmaceutical research, vol. 4, pp. 457-464, 1987.
[140] H. Nishide, N. Oki, and E. Tsuchida, "Complexation of poly (acrylic acid) s with uranyl ion," European Polymer Journal, vol. 18, pp. 799-802, 1982.
[141] A. Arslan, S. Kıralp, L. Toppare, and A. Bozkurt, "Novel conducting polymer electrolyte biosensor based on poly (1-vinyl imidazole) and poly (acrylic acid) networks," Langmuir, vol. 22, pp. 2912-2915, 2006.
[142] M. Genovese, J. Jiang, K. Lian, and N. Holm, "High capacitive performance of exfoliated biochar nanosheets from biomass waste corn cob," Journal of Materials Chemistry A, vol. 3, pp. 2903-2913, 2015.
[143] A. Gestos, P. G. Whitten, G. M. Spinks, and G. G. Wallace, "Crosslinking neat ultrathin films and nanofibres of pH-responsive poly (acrylic acid) by UV radiation," Soft Matter, vol. 6, pp. 1045-1052, 2010.
105
[144] Z. Adamczyk, A. Bratek, B. Jachimska, T. Jasiński, and P. Warszyński, "Structure of poly (acrylic acid) in electrolyte solutions determined from simulations and viscosity measurements," The Journal of Physical Chemistry B, vol. 110, pp. 22426-22435, 2006.
[145] M. I. Baker, S. P. Walsh, Z. Schwartz, and B. D. Boyan, "A review of polyvinyl alcohol and its uses in cartilage and orthopedic applications," Journal of Biomedical Materials Research Part B: Applied Biomaterials, vol. 100, pp. 1451-1457, 2012.
[146] M. E. Kagan and M. N. Yardney, "Interelectrode separator for rechargeable batteries," ed: Google Patents, 1953.
[147] A. Polak, S. Petty-Weeks, and A. Beuhler, "Applications of novel proton-conducting polymers to hydrogen sensing," Sensors and Actuators, vol. 9, pp. 1-7, 1986.
[148] A. Bozkurt, M. Ise, K. Kreuer, W. H. Meyer, and G. Wegner, "Proton-conducting polymer electrolytes based on phosphoric acid," Solid State Ionics, vol. 125, pp. 225-233, 1999.
[149] K. K. Lian, C. Li, R. H. Jung, and J. G. Kincs, "Electrochemical cell having symmetric inorganic electrodes," ed: Google Patents, 1996.
[150] M. Kaempgen, C. K. Chan, J. Ma, Y. Cui, and G. Gruner, "Printable thin film supercapacitors using single-walled carbon nanotubes," Nano letters, vol. 9, pp. 1872-1876, 2009.
[151] M. F. El-Kady, V. Strong, S. Dubin, and R. B. Kaner, "Laser scribing of high-performance and flexible graphene-based electrochemical capacitors," Science, vol. 335, pp. 1326-1330, 2012.
[152] H. Fei, C. Yang, H. Bao, and G. Wang, "Flexible all-solid-state supercapacitors based on graphene/carbon black nanoparticle film electrodes and cross-linked poly (vinyl alcohol)–H2SO4 porous gel electrolytes," Journal of Power Sources, vol. 266, pp. 488-495, 2014.
[153] C. W. Liew, S. Ramesh, and A. Arof, "Characterization of ionic liquid added poly (vinyl alcohol)-based proton conducting polymer electrolytes and electrochemical studies on the supercapacitors," international journal of hydrogen energy, vol. 40, pp. 852-862, 2015.
[154] J. W. Rhim, H. B. Park, C. S. Lee, J. H. Jun, D. S. Kim, and Y. M. Lee, "Crosslinked poly (vinyl alcohol) membranes containing sulfonic acid group: proton and methanol transport through membranes," Journal of Membrane Science, vol. 238, pp. 143-151, 2004.
[155] J. C. Park, T. Ito, K. O. Kim, K. W. Kim, B. S. Kim, M. S. Khil, et al., "Electrospun poly (vinyl alcohol) nanofibers: effects of degree of hydrolysis and enhanced water stability," Polymer journal, vol. 42, p. 273, 2010.
[156] J. Li, Z. Suo, and J. J. Vlassak, "Stiff, strong, and tough hydrogels with good chemical stability," Journal of Materials Chemistry B, vol. 2, pp. 6708-6713, 2014.
106
[157] S. Vicini, M. Castellano, M. C. Faria Soares Lima, P. Licinio, and G. Goulart Silva, "Polyacrylamide hydrogels for stone restoration: Effect of salt solutions on swelling/deswelling degree and dynamic correlation length," Journal of Applied Polymer Science, vol. 134, 2017.
[158] M. S. Johnson, "The effects of gel‐forming polyacrylamides on moisture storage in sandy soils," Journal of the Science of Food and Agriculture, vol. 35, pp. 1196-1200, 1984.
[159] A. Bhardwaj, I. Shainberg, D. Goldstein, D. Warrington, and G. J Levy, "Water retention and hydraulic conductivity of cross-linked polyacrylamides in sandy soils," Soil Science Society of America Journal, vol. 71, pp. 406-412, 2007.
[160] S. Sumathi, V. Sethuprakhash, and W. Basirun, "A Comparative Studies on Methanesulfonic and p-Touluene Sulfonic Acid Incorporated Polyacrylamide Gel Polymer Electrolyte for Tin-Air Battery," World Academy of Science, Engineering and Technology, International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering, vol. 7, pp. 948-953, 2013.
[161] Q. Tang, H. Cai, S. Yuan, X. Wang, and W. Yuan, "Enhanced proton conductivity from phosphoric acid-imbibed crosslinked 3D polyacrylamide frameworks for high-temperature proton exchange membranes," International Journal of Hydrogen Energy, vol. 38, pp. 1016-1026, 2013.
[162] H. J. Kim, K. Talukdar, Y. H. Kim, Y. Park, H. C. Lee, and S. J. Choi, "Study of Semi-Interpenetrating Networks in Nafion®/Polyacrylamide Proton Conducting Membranes," Journal of Nanoelectronics and Optoelectronics, vol. 10, pp. 569-573, 2015.
[163] D. Rodriguez, C. Jegat, O. Trinquet, J. Grondin, and J. Lassegues, "Proton conduction in poly (acrylamide)-acid blends," Solid State Ionics, vol. 61, pp. 195-202, 1993.
[164] W. Wieczorek, J. R. Stevens, and Z. Florjanczyk, "Proton conducting polymer gels based on a polyacrylamide matrix," Electrochimica Acta, vol. 40, pp. 2327-2330.
[165] W. Wieczorek and J. Stevens, "Proton transport in polyacrylamide based hydrogels doped with H3PO4 or H2SO4," Polymer, vol. 38, pp. 2057-2065, 1997.
[166] D. Eustace, D. Siano, and E. Drake, "Polymer compatibility and interpolymer association in the poly (acrylic acid)–polyacrylamide–water ternary system," Journal of applied polymer science, vol. 35, pp. 707-716, 1988.
[167] M. Moharram, L. Balloomal, and H. El‐Gendy, "Infrared study of the complexation of poly (acrylic acid) with poly (acrylamide)," Journal of applied polymer science, vol. 59, pp. 987-990, 1996.
[168] Y. Qiu and K. Park, "Environment-sensitive hydrogels for drug delivery," Advanced drug delivery reviews, vol. 53, pp. 321-339, 2001.
107
[169] Z. Liu and G. Rempel, "Preparation of superabsorbent polymers by crosslinking acrylic acid and acrylamide copolymers," Journal of Applied Polymer Science, vol. 64, pp. 1345-1353, 1997.
[170] T. Uma and M. Nogami, "Proton-conducting glass electrolyte," Analytical chemistry, vol. 80, pp. 506-508, 2008.
[171] A. B. Bourlinos, K. Raman, R. Herrera, Q. Zhang, L. A. Archer, and E. P. Giannelis, "A liquid derivative of 12-tungstophosphoric acid with unusually high conductivity," Journal of the American Chemical Society, vol. 126, pp. 15358-15359, 2004.
[172] M. Yamada and I. Honma, "Heteropolyacid-encapsulated self-assembled materials for anhydrous proton-conducting electrolytes," The Journal of Physical Chemistry B, vol. 110, pp. 20486-20490, 2006.
[173] H. Gao and K. Lian, "Characterizations of proton conducting polymer electrolytes for electrochemical capacitors," Electrochimica Acta, vol. 56, pp. 122-127, 2010.
[174] H. Gao, A. Virya, and K. Lian, "Monovalent silicotungstate salts as electrolytes for electrochemical supercapacitors," Electrochimica Acta, vol. 138, pp. 240-246, 2014.
[175] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, and R. Thouvenot, "Vibrational investigations of polyoxometalates. 2. Evidence for anion-anion interactions in molybdenum (VI) and tungsten (VI) compounds related to the Keggin structure," Inorganic Chemistry, vol. 22, pp. 207-216, 1983.
[176] R. Thouvenot, M. Fournier, R. Franck, and C. Rocchiccioli-Deltcheff, "Vibrational investigations of polyoxometalates. 3. Isomerism in molybdenum (VI) and tungsten (VI) compounds related to the Keggin structure," Inorganic Chemistry, vol. 23, pp. 598-605, 1984.
[177] L. Swenson, J. C. Orozco, J. A. Kaduk, and M. I. Khan, "Electrostatic self-assembly of composite materials containing Keggin structure polyoxoanions and functionalized Anderson structure polyoxometallic cations," Inorganica Chimica Acta, vol. 444, pp. 43-50, 2016.
[178] M. Genovese, Y. W. Foong, and K. Lian, "The unique properties of aqueous polyoxometalate (POM) mixtures and their role in the design of molecular coatings for electrochemical energy storage," Electrochimica Acta, 2016.
[179] I. K. Song and M. A. Barteau, "Redox properties of Keggin-type heteropolyacid (HPA) catalysts: effect of counter-cation, heteroatom, and polyatom substitution," Journal of molecular catalysis A: Chemical, vol. 212, pp. 229-236, 2004.
[180] I. K. Song, R. B. Shnitser, J. J. Cowan, C. L. Hill, and M. A. Barteau, "Nanoscale characterization of redox and acid properties of Keggin-type heteropolyacids by scanning tunneling microscopy and tunneling spectroscopy: Effect of heteroatom substitution," Inorganic chemistry, vol. 41, pp. 1292-1298, 2002.
108
[181] U. Mioč, P. Colomban, and A. Novak, "Infrared and Raman study of some heteropolyacid hydrates," Journal of Molecular Structure, vol. 218, pp. 123-128, 1990.
[182] Q. Wu, "Synthesis and conductivity of tritungstovanadoselenic heteropoly acid," Materials Letters, vol. 50, pp. 78-81, 2001.
[183] E. M. Serwicka and C. P. Grey, "ESR and solid state MAS NMR study of the silica-supported H3+nPVnMo12−nO40 (n= O, 1, 2, 3) heteropolyacids," Colloids and surfaces, vol. 45, pp. 69-82, 1990.
[184] U. Mioč, M. Todorović, M. Davidović, P. Colomban, and I. Holclajtner-Antunović, "Heteropoly compounds—From proton conductors to biomedical agents," Solid State Ionics, vol. 176, pp. 3005-3017, 2005.
[185] A. Micek-Ilnicka, "The role of water in the catalysis on solid heteropolyacids," Journal of Molecular Catalysis A: Chemical, vol. 308, pp. 1-14, 2009.
[186] O. Nakamura, I. Ogino, and T. Kodama, "Temperature and humidity ranges of some hydrates of high-proton-conductive dodecamolybdophosphoric acid and dodecatungstophosphoric acid crystals under an atmosphere of hydrogen or either oxygen or air," Solid State Ionics, vol. 3, pp. 347-351, 1981.
[187] K. Kreuer, M. Hampele, K. Dolde, and A. Rabenau, "Proton transport in some heteropolyacidhydrates a single crystal PFG-NMR and conductivity study," Solid State Ionics, vol. 28, pp. 589-593, 1988.
[188] J. J. Altenau, M. T. Pope, R. A. Prados, and H. So, "Models for heteropoly blues. Degrees of valence trapping in vanadium (IV)-and molybdenum (V)-substituted Keggin anions," Inorganic Chemistry, vol. 14, pp. 417-421, 1975.
[189] M. Forsyth, J. Sun, D. MacFarlane, and A. Hill, "Compositional dependence of free volume in PAN/LiCF3SO3 polymer‐in‐salt electrolytes and the effect on ionic conductivity," Journal of Polymer Science Part B: Polymer Physics, vol. 38, pp. 341-350, 2000.
[190] P. Colomban, Proton Conductors: Solids, membranes and gels-materials and devices vol. 2: Cambridge University Press, 1992.
[191] Y. Li, H. Wang, Q. Wu, X. Xu, S. Lu, and Y. Xiang, "A poly (vinyl alcohol)-based composite membrane with immobilized phosphotungstic acid molecules for direct methanol fuel cells," Electrochimica Acta, vol. 224, pp. 369-377, 2017.
[192] L. Li, L. Xu, and Y. Wang, "Novel proton conducting composite membranes for direct methanol fuel cell," Materials Letters, vol. 57, pp. 1406-1410, 2003.
[193] W. Xu, C. Liu, X. Xue, Y. Su, Y. Lv, W. Xing, et al., "New proton exchange membranes based on poly (vinyl alcohol) for DMFCs," Solid State Ionics, vol. 171, pp. 121-127, 2004.
109
[194] K. Lian and Q. Tian, "Solid asymmetric electrochemical capacitors using proton-conducting polymer electrolytes," Electrochemistry Communications, vol. 12, pp. 517-519, 2010.
[195] C. Li and R. H. Reuss, "Electrochemical capacitor with solid electrolyte," ed: Google Patents, 1999.
[196] C. Li, R. H. Reuss, and M. Chason, "Electrochemical capacitor with hybrid polymer polyacid electrolyte," ed: Google Patents, 1998.
[197] V. Goffman, V. Sleptsov, N. Kovyneva, N. Gorshkov, O. Telegina, and A. Gorokhovsky, "Effect of Nanosized Potassium Polytitanate on the Properties of Proton-Conducting Composite Based on Phosphotungstic Acid and Polyvinyl Alcohol," Theoretical and Experimental Chemistry, vol. 52, pp. 318-322, 2016.
[198] H. Gao and K. Lian, "High rate all-solid electrochemical capacitors using proton conducting polymer electrolytes," Journal of Power Sources, vol. 196, pp. 8855-8857, 2011.
[199] H. Gao, Q. Tian, and K. K. Lian, "Proton conducting polymer electrolytes for electrochemical capacitors," ECS Transactions, vol. 28, pp. 77-82, 2010.
[200] H. Gao, Q. Tian, and K. Lian, "Polyvinyl alcohol-heteropoly acid polymer electrolytes and their applications in electrochemical capacitors," Solid State Ionics, vol. 181, pp. 874-876, 2010.
[201] K. Lian and C. M. Li, "Solid polymer electrochemical capacitors using heteropoly acid electrolytes," Electrochemistry Communications, vol. 11, pp. 22-24, 2009.
[202] H. Gao and K. Lian, "A H5BW12O40-polyvinyl alcohol polymer electrolyte and its application in solid supercapacitors," Journal of Materials Chemistry A, vol. 4, pp. 9585-9592, 2016.
[203] M. Forsyth, P. Meakin, and D. MacFarlane, "A 13 C NMR study of the role of plasticizers in the conduction mechanism of solid polymer electrolytes," Electrochimica acta, vol. 40, pp. 2339-2342, 1995.
[204] D. MacFarlane, J. Sun, P. Meakin, P. Fasoulopoulos, J. Hey, and M. Forsyth, "Structure-property relationships in plasticized solid polymer electrolytes," Electrochimica acta, vol. 40, pp. 2131-2136, 1995.
[205] J. Lassegues, J. Grondin, M. Hernandez, and B. Maree, "Proton conducting polymer blends and hybrid organic inorganic materials," Solid State Ionics, vol. 145, pp. 37-45, 2001.
[206] S. Majid and A. Arof, "Electrical behavior of proton-conducting chitosan-phosphoric acid-based electrolytes," Physica B: Condensed Matter, vol. 390, pp. 209-215, 2007.
110
[207] P. Donoso, W. Gorecki, C. Berthier, F. Defendini, C. Poinsignon, and M. Armand, "NMR, conductivity and neutron scattering investigation of ionic dynamics in the anhydrous polymer protonic conductor PEO(H3PO4)x," Solid State Ionics, vol. 28, pp. 969-974, 1988.
[208] P. Gupta and K. Singh, "Characterization of H3PO4 based PVA complex system," Solid State Ionics, vol. 86, pp. 319-323, 1996.
[209] J. Lassegues, B. Desbat, O. Trinquet, F. Cruege, and C. Poinsignon, "From model solid-state protonic conductors to new polymer electrolytes," Solid State Ionics, vol. 35, pp. 17-25, 1989.
[210] A. Schechter and R. F. Savinell, "Imidazole and 1-methyl imidazole in phosphoric acid doped polybenzimidazole, electrolyte for fuel cells," Solid State Ionics, vol. 147, pp. 181-187, 2002.
[211] R. Alves, F. Sentanin, R. Sabadini, A. Pawlicka, and M. M. Silva, "Influence of cerium triflate and glycerol on electrochemical performance of chitosan electrolytes for electrochromic devices," Electrochimica Acta, vol. 217, pp. 108-116, 2016.
[212] R. F. Marcondes, P. S. D'Agostini, J. Ferreira, E. M. Girotto, A. Pawlicka, and D. C. Dragunski, "Amylopectin-rich starch plasticized with glycerol for polymer electrolyte application," Solid State Ionics, vol. 181, pp. 586-591, 2010.
[213] A. Pawlicka, M. Danczuk, W. Wieczorek, and E. Zygadło-Monikowska, "Influence of plasticizer type on the properties of polymer electrolytes based on chitosan," The Journal of Physical Chemistry A, vol. 112, pp. 8888-8895, 2008.
[214] H. Gao, A. Virya, and K. Lian, "Proton conducting H5BW12O40 electrolyte for solid supercapacitors," Journal of Materials Chemistry A, vol. 3, pp. 21511-21517, 2015.
[215] M. Genovese, Y. W. Foong, and K. Lian, "Germanomolybdate (GeMo12O404−) Modified
Carbon Nanotube Composites for Electrochemical Capacitors," Electrochimica Acta, vol. 117, pp. 153-158, 1/20/ 2014.
[216] J. Biener, M. Stadermann, M. Suss, M. A. Worsley, M. M. Biener, K. A. Rose, et al., "Advanced carbon aerogels for energy applications," Energy & Environmental Science, vol. 4, pp. 656-667, 2011.
[217] P. Taberna, P. Simon, and J.-F. Fauvarque, "Electrochemical characteristics and impedance spectroscopy studies of carbon-carbon supercapacitors," Journal of The Electrochemical Society, vol. 150, pp. A292-A300, 2003.
[218] F. Wolfart, D. P. Dubal, M. Vidotti, R. Holze, and P. Gómez-Romero, "Electrochemical supercapacitive properties of polypyrrole thin films: influence of the electropolymerization methods," Journal of Solid State Electrochemistry, vol. 20, pp. 901-910, 2016.
111
[219] K. Nakajima, K. Eda, and S. Himeno, "Effect of the Central Oxoanion Size on the Voltammetric Properties of Keggin-Type [XW12O40]n−(n= 2− 6) Complexes," Inorganic chemistry, vol. 49, pp. 5212-5215, 2010.
[220] U. Lavrencic Štangar, N. Grošelj, B. Orel, and P. Colomban, "Structure of and interactions between P/SiWA keggin nanocrystals dispersed in an organically modified electrolyte membrane," Chemistry of materials, vol. 12, pp. 3745-3753, 2000.
[221] G. Zukowska, J. Stevens, and K. Jeffrey, "Anhydrous gel electrolytes doped with silicotungstic acid," Electrochimica acta, vol. 48, pp. 2157-2164, 2003.
[222] A. J. Bridgeman, "Density functional study of the vibrational frequencies of α-Keggin heteropolyanions," Chemical physics, vol. 287, pp. 55-69, 2003.
[223] G. Brown, M. Noe-Spirlet, W. Busing, and H. Levy, "Dodecatungstophosphoric acid hexahydrate, (H5O2
+)3(PW12O403−). The true structure of Keggin'spentahydrate'from
single-crystal X-ray and neutron diffraction data," Acta Crystallographica Section B: Structural Crystallography and Crystal Chemistry, vol. 33, pp. 1038-1046, 1977.
[224] H. Gao and K. Lian, "Advanced proton conducting membrane for ultra-high rate solid flexible electrochemical capacitors," Journal of Materials Chemistry, vol. 22, pp. 21272-21278, 2012.
[225] M. K. Gupta and R. Bansil, "Laser Raman spectroscopy of polyacrylamide," Journal of Polymer Science: Polymer Physics Edition, vol. 19, pp. 353-360, 1981.
[226] X. Lu and Y. Mi, "Characterization of the interfacial interaction between polyacrylamide and silicon substrate by Fourier transform infrared spectroscopy," Macromolecules, vol. 38, pp. 839-843, 2005.
[227] A. Oyane, K. Nakanishi, H. M. Kim, F. Miyaji, T. Kokubo, N. Soga, et al., "Sol–gel modification of silicone to induce apatite-forming ability," Biomaterials, vol. 20, pp. 79-84, 1999.
[228] N. Murthy and H. Minor, "General procedure for evaluating amorphous scattering and crystallinity from X-ray diffraction scans of semicrystalline polymers," Polymer, vol. 31, pp. 996-1002, 1990.
[229] N. Murthy, H. Minor, C. Bednarczyk, and S. Krimm, "Structure of the amorphous phase in oriented polymers," Macromolecules, vol. 26, pp. 1712-1721, 1993.
[230] N. Murthy, S. Correale, and H. Minor, "Structure of the amorphous phase in crystallizable polymers: poly (ethylene terephthalate)," Macromolecules, vol. 24, pp. 1185-1189, 1991.
[231] Y. Pei, L. Zhao, G. Du, N. Li, K. Xu, and H. Yang, "Investigation of the degradation and stability of acrylamide-based polymers in acid solution: Functional monomer modified polyacrylamide," Petroleum, vol. 2, pp. 399-407, 2016.
112
[232] C. Murli and Y. Song, "Pressure-induced polymerization of acrylic acid: a Raman spectroscopic study," The Journal of Physical Chemistry B, vol. 114, pp. 9744-9750, 2010.
[233] D. E. Owens, Y. Jian, J. E. Fang, B. V. Slaughter, Y. H. Chen, and N. A. Peppas, "Thermally responsive swelling properties of polyacrylamide/poly (acrylic acid) interpenetrating polymer network nanoparticles," Macromolecules, vol. 40, pp. 7306-7310, 2007.
[234] T. Ito, K. Inumaru, and M. Misono, "Structure of porous aggregates of the ammonium salt of dodecatungstophosphoric acid,(NH4)3PW12O40: unidirectionally oriented self-assembly of nanocrystallites," The Journal of Physical Chemistry B, vol. 101, pp. 9958-9963, 1997.
[235] T. Ito, K. Inumaru, and M. Misono, "Epitaxially self-assembled aggregates of polyoxotungstate nanocrystallites, (NH4)3PW12O40: Synthesis by homogeneous precipitation using decomposition of urea," Chemistry of materials, vol. 13, pp. 824-831, 2001.
[236] J. S. Santos, J. A. Dias, S. C. Dias, F. A. Garcia, J. L. Macedo, F. S. Sousa, et al., "Mixed salts of cesium and ammonium derivatives of 12-tungstophosphoric acid: Synthesis and structural characterization," Applied Catalysis A: General, vol. 394, pp. 138-148, 2011.
[237] T. Rajkumar and G. R. Rao, "Synthesis and characterization of hybrid molecular material prepared by ionic liquid and silicotungstic acid," Materials Chemistry and Physics, vol. 112, pp. 853-857, 2008.
[238] K. Checkiewicz, G. Żukowska, and W. Wieczorek, "Synthesis and characterization of the proton-conducting gels based on PVdF and PMMA matrixes doped with heteropolyacids," Chemistry of materials, vol. 13, pp. 379-384, 2001.
[239] L. Adamczyk and K. Miecznikowski, "Solid-state electrochemical behavior of Keggin-type borotungstic acid single crystal," Journal of Solid State Electrochemistry, vol. 17, pp. 1167-1173, 2013.
[240] Z. Sun, X. Duan, J. Zhao, X. Wang, and Z. Jiang, "Homogeneous borotungstic acid and heterogeneous micellar borotungstic acid catalysts for biodiesel production by esterification of free fatty acid," Biomass and Bioenergy, vol. 76, pp. 31-42, 2015.
[241] J. Wang, J. Li, and J. Niu, "Synthesis, crystal structure and characterization of a 2-D network organic-inorganic hybrid polymer with [α-BW12O40]5− as building blocks," Science in China Series B: Chemistry, vol. 49, pp. 437-444, 2006.
[242] A. Martinelli, A. Matic, P. Jacobsson, L. Börjesson, M. A. Navarra, D. Munaò, et al., "A study on the state of PWA in PVDF-based proton conducting membranes by Raman Spectroscopy (Accepted-In Press)," Solid State Ionics, 2006.
[243] W. W. Rudolph, "Raman-and infrared-spectroscopic investigations of dilute aqueous phosphoric acid solutions," Dalton Transactions, vol. 39, pp. 9642-9653, 2010.
113
[244] J. M. Bregeault, M. Vennat, S. Laurent, J. Y. Piquemal, Y. Mahha, E. Briot, et al., "From polyoxometalates to polyoxoperoxometalates and back again; potential applications," Journal of Molecular Catalysis. A, Chemical, vol. 250, pp. 177-189, 2006.
[245] J. Dong, Y. Ozaki, and K. Nakashima, "Infrared, Raman, and near-infrared spectroscopic evidence for the coexistence of various hydrogen-bond forms in poly (acrylic acid)," Macromolecules, vol. 30, pp. 1111-1117, 1997.
[246] N. Tanaka, H. Kitano, and N. Ise, "Raman spectroscopic study of hydrogen bonding in aqueous carboxylic acid solutions. 3. Polyacrylic acid," Macromolecules, vol. 24, pp. 3017-3019, 1991.
[247] E. Mendelovici, R. L. Frost, and T. Kloprogge, "Cryogenic Raman spectroscopy of glycerol," Journal of Raman Spectroscopy, vol. 31, pp. 1121-1126, 2000.
[248] A. Mudalige and J. E. Pemberton, "Raman spectroscopy of glycerol/D2O solutions," Vibrational Spectroscopy, vol. 45, pp. 27-35, 2007.
114
Appendix A – Raman and FTIR Peak Assignments
This section presents the peak assignments for the Raman spectra for each of the samples
discussed in this thesis. Table 9 and Table 10 were the Raman and FTIR peak assignments for
SiWA and BWA. Table 11 and Table 12 were the Raman and FTIR peak assignments for PAM.
Table 13, Table 14, and Table 15 were the Raman peak assignments for BWA-PAM binary
electrolytes, BWA-PAM + H3PO4 ternary electrolytes, and BWA-PAM + Gly ternary
electrolytes, respectively.
Table 9: Peak assignments for the Raman spectra of SiWA and BWA powders dried at
60oC for 24 hours prior to the characterization [220-222].
Raman Shift (cm-1) Band Assignments SiWA BWA 101.0 98.9 δ(W-O-W) 160.9 156.2 δ(W-O-W) 235.9 216.8 δ(W-O-W)
- 244.2 δ(W-O-W) 547.8 539.9 δ(W-Oc-W) 926.2 919.1 υ(W-Ob-W) 980.5 - υ(W-Od)
- 989.3 υ(B-O) 1006.1 - υ(Si-O) 3504.3 3504.3 δ(O-H)
Table 10: FTIR peak assignments for SiWA and BWA powders dried at 60oC for 24 hours
prior to the characterization [115, 173, 237-241].
Wavenumber (cm-1) Band Assignments SiWA BWA 758.4 754.0 υ(W-Ob-W)
- 896.3 υ(B-O) 909.3 - υ(Si-O) 977.3 955.1 υ(W=Od)
- 1001.4 υ(B-O) 1016.3 - υ(Si-O) 1618.0 1615.6 δ(O-H) 3500.2 3499.7 δ(O-H)
115
Table 11: Raman peak assignments for PAM film conditioned at 45% RH for 7 days [225].
PAM Raman Shift (cm-1) Band Assignments 100.8 Lattice vibrations 485.7 skeletal δ(C-C) 638.6 ω(C=O) 776.2 (CH2) rocking 846.4 υ(C-C) side-chain 1116.2 skeletal υ(C-C) 1217.0 (NH2) wagging 1329.8 δ(C-H) 1426.9 υ(C-N) 1458.4 δ(CH2) 1610.0 δ(NH2) 1666.3 υ(C=O) 2877.9 υ(CH2) 2937.4 υ(CH2) 3199.9 υ(NH2) 3355.3 δ(O-H)
Table 12: FTIR peak assignments for PAM film conditioned at 45% RH for 7 days [226].
PAM Raman Shift (cm-1) Band Assignments 1117.6 Skeletal (C-C) vibration 1178.8 (NH2) wagging 1317.5 δ(C-H) 1344.0 (CH2) wagging 1411.3 υ(C-N) 1446.0 δ(CH2) 1596.9 δ(NH2) 1645.8 υ(C=O) 2930.7 υ(C-H) 3175.4 υ(NH2) 3324.3 δ(O-H)
116
Table 13: Peak assignments for the Raman spectra of BWA-PAM electrolyte systems [175,
176, 220-222, 225, 242].
Raman Shift (cm-1) Band Assignments BWA PAM 50BWA-
PAM 75BWA-
PAM 85BWA-
PAM 85.9 - 84.1 85.9 82.2 δ(W-O-W) 98.9 - 95.2 98.9 95.2 δ(W-O-W)
- 100.8 - - - Lattice vibrations 156.2 - 154.4 158.1 154.4 δ(W-O-W) 216.8 - 215.0 218.6 215.0 δ(W-O-W) 244.2 - 242.4 249.6 246.0 δ(W-O-W)
- 485.7 478.6 - - skeletal δ(C-C) 539.9 - 534.7 539.9 534.7 δ(W-Oc-W)
- 638.6 598.9 600.7 605.8 ω(C=O) - 776.2 774.5 774.5 777.9 (CH2) rocking - 846.4 843.1 846.4 846.4 υ(C-C) side-chain
919.1 - 910.9 915.8 914.2 υ(W-Ob-W) 989.3 - 981.2 986.1 984.5 υ(B-O)
- 1116.2 1111.5 1114.6 1111.5 skeletal υ(C-C) - 1217.0 1215.5 1228.6 1220.2 (NH2) wagging - 1329.8 1325.8 1328.3 1329.9 δ(C-H) - 1426.9 1425.4 1423.9 1426.9 υ(C-N) - 1458.4 1453.9 1459.9 1453.9 δ(CH2) - 1610.0 1605.7 1604.2 1604.2 δ(NH2) - 1666.3 1666.3 1667.7 1667.7 υ(C=O) - - 1714.3 1715.8 1712.8 υ(COO-) - 2877.9 2873.5 2872.1 2872.1 υ(CH2) - 2937.4 2928.5 2932.6 2937.4 υ(CH2) - 3199.9 3194.2 3202.2 3197.6 υ(NH2)
3355.3 3355.3 3347.5 3347.5 3353.1 δ(O-H)
Table 14: Raman peak assignments for BWA-PAM + H3PO4 electrolytes [175, 220-222,
225, 242-246].
Raman Shift (cm-1) Band Assignments BWA PAM H3PO4 PAA
- 1116.2 - 1116.2 skeletal υ(C-C) - 1217.0 - (NH2) wagging - 1329.8 - 1329.8 δ(C-H) - 1426.9 - υ(C-N) - 1458.4 - 1458.4 δ(CH2) - 1610.0 - δ(NH2) - 1666.3 - υ(C=O) - - - 1717.2 υ(COO-)
117
Table 15: Raman peak assignments for BWA-PAM + glycerol electrolytes [175, 176, 220-
222, 225, 242, 245, 247, 248].
Raman Shift (cm-1) Band Assignments BWA PAM Glycerol PAA
- - 82.2 - 85.9 - - - δ(W-O-W) 98.9 - - - δ(W-O-W) 156.2 - - - δ(W-O-W) 216.8 - - - δ(W-O-W) 244.2 - - - δ(W-O-W)
- - 425.6 - δ(C-C) - 485.7 485.7 - skeletal δ(C-C)
539.9 - - - δ(W-Oc-W) - 638.6 - - ω(C=O) - - 688.2 - δ(CCO) - 776.2 - - (CH2) rocking - - 828.1 - υ(C-C) - 846.4 - - υ(C-C) side-chain - - 859.1 - υ(C-C)
919.1 - - - υ(W-Ob-W) - - 935.9 - δ(CH2) - - 989.3 - δ(CH2)
989.3 - - - υ(B-O) - - 1065.1 - δ(CH2) - 1116.2 1122.6 1116.2 skeletal υ(C-C) - 1217.0 - - (NH2) wagging - 1329.8 - 1329.8 δ(C-H) - 1426.9 - - υ(C-N) - 1458.4 - 1458.4 δ(CH2) - 1474.8 - υ(CH2) - 1610.0 - - δ(NH2) - 1666.3 - - υ(C=O) - - - 1717.2 υ(COO-)
118
Appendix B – FTIR Analysis for SiWA and BWA
This section presents the FTIR for SiWA and BWA. Figure 51 showed the FTIR spectra of
BWA in comparison with SiWA and the peak assignments were tabulated in Table 10 of
Appendix A. The FTIR peaks for SiWA and BWA were assigned according to the well-
established literature reports [115, 173, 237-241].
Both BWA and SiWA had δ(O-H) peaks near 1615 cm-1 and 3500 cm-1 which corresponded to
the crystallized water in the HPAs. The FTIR spectrum for BWA was very similar to that of
SiWA below 1100 cm-1 region without any extra peak for the signature peaks of Keggin
structure (see Table 10 in Appendix A). This suggested that BWA also had W-based Keggin
structure. The slightly shift of BWA spectrum to a lower frequency as compared to that in SiWA
was due to the larger size of BWA Keggin structure. Similar to observations in Raman
spectroscopy, the BWA υ(B-O) peaks at 896.3 cm-1 and 1001.4 cm-1 were located at lower
frequency than υ(Si-O) peaks (909.3 cm-1 and 1016.3 cm-1) due to the longer B-O than Si-O
bond in the Keggin structure.
Thus, the FTIR results confirmed that the synthesized BWA had the required chemical bonding
similar to SiWA to maintain the Keggin structure. The higher peak intensity of υ(W-Ob-W) at
754.0 cm-1 as compared to SiWA agreed with the observation in Raman spectroscopy,
suggesting that minor residual tungsten oxides may remain in BWA powder.
Figure 51: FTIR spectra of (a) SiWA powder and (b) the synthesized BWA powder dried at 60oC for 24 hours prior to the characterization.
119
Appendix C – Acid attack on PAM
Figure 52: The observations on the electrolytes for (a) 75BWA-PAM, (b) 85BWA-PAM,
and (c) 90BWA-PAM after 6 months.
In this thesis, it was found out that the higher concentration of BWA can lead to higher
conductivity of the electrolyte. Figure 52 shows the observation of the BWA-PAM electrolyte
precursor after 6 months from their preparation. The 75BWA-PAM and 85BWA-PAM remained
at similar viscosity to its pristine conditions. It was obvious from Figure 52 that 75BWA-PAM
and 85BWA-PAM remained translucent after 6 months, suggesting the high stability of the
electrolytes.
However, the 90BWA-PAM electrolyte became less viscous and turned into orange color with
some solid precipitate at the bottom, which is an indication of the hydrolysis of PAM by the
strong acids [231]. From this observation, it was deduced that 85BWA-PAM should be the upper
limit for the electrolyte composition because the electrolyte is highly susceptible to chemical
instability beyond this threshold. Therefore, this thesis will only investigate BWA concentration
up to 85 wt% BWA.
120
Appendix D – Change in electrolyte thickness due to dehydration
Figure 53: The change in electrolyte thickness over a tracking period of 21 days
conditioned at 45% RH.
The BWA-PAM electrolyte undergoes dehydration over time, which may result in the decrease
in the electrolyte thickness from the water loss. Figure 53 shows the change in thickness of the
electrolyte over a period of 21 days. Even though the device is stored in 45% RH for the entire
tracking period, the electrolyte thickness still decreases over time for the cells packaged with
tapes and packaged with photo laminates.
The taped cells have thickness decreased by roughly 30 μm per week. This indicates that the
sealing of the cell with the polyester tape is not optimal for the flexible device. The electrolyte
will still dry out over time. When the cell is packaged with photo laminates, the sealed cells have
thickness decreased by roughly 20 μm per week. This indicates that a better sealing technique
can help to improve the service life of the cell.
0 5 10 15 20 2570
80
90
100
110
120
130
140
150
160
170
180
190
200T
hic
kne
ss (
μm
)
Days
75BWA-PAM + 10% H3PO4 (taped)
75BWA-PAM + 10% H3PO4 (sealed)
85BWA-PAM + 10%Gly (taped) 85BWA-PAM + 10%Gly (sealed)
121
Appendix E – Electrochemical impedance spectroscopy (EIS) analyses for the liquid cells and the solid cells
Figure 54: The Nyquist (left) and Bode (right) plots for (a) 75BWA-PAM + 10% H3PO4
and (b) 85BWA-PAM + 10%Gly solid cells against their liquid cell counterparts.
The Nyquist plots for both 75BWA-PAM + 10% H3PO4 and 85BWA-PAM + 10% Gly
solid cells showed reduced resistance as compared to their liquid cell counterparts. This
reduction in resistance is attributed to the lower cell thickness enabled by solid polymer
electrolytes. The Bode plots reveal that switching from liquid electrolytes to solid electrolytes in
the cells also maintained the near -85o phase angle at low frequency, indicating a very capacitive
behavior of the solid cells.
0 200 400 600 800 1000 1200 14000
-200
-400
-600
-800
-1000
-1200
-1400
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.02.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
10-2 10-1 100 101 102 103 104 105
10-1
100
101
102
103
104
10-2 10-1 100 101 102 103 104 105
40
20
0
-20
-40
-60
-80
Z''
(Oh
m)
Z' (Ohm)
0.1M BWA 75BWA-PAM + 10% H3PO4
Z''
(Oh
m)
Z' (Ohm)
(a)
Mag
nitu
de (
|Z|)
Frequency (Hz)
0.1M BWA 75BWA-PAM + 10% H3PO4
Th
eta
(deg
.)
Frequency (Hz)
0.1M BWA 75BWA-PAM + 10% H3PO4
0 100 200 300 400 500 600 700 8000
-100
-200
-300
-400
-500
-600
-700
-800
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.02.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
10-2 10-1 100 101 102 103 104 105
10-1
100
101
102
103
104
10-2 10-1 100 101 102 103 104 105
60
40
20
0
-20
-40
-60
-80
(b)
Z''
(Oh
m)
Z' (Ohm)
0.01M BWA 85BWA-PAM + 10% Gly
Z''
(Oh
m)
Z' (Ohm)
Mag
nitu
de (
|Z|)
Frequency (Hz)
0.01M BWA 85BWA-PAM + 10%Gly
The
ta (
deg
.)
Frequency (Hz)
0.01M BWA 85BWA-PAM + 10%Gly