Alkaline Polymer Electrolytes for Electrochemical Capacitors

114 Pages·2015·3.64 MB·English

Alkaline Polymer Electrolytes for Electrochemical
Capacitors
by
Jak Li
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Material Science and Engineering
University of Toronto
© Copyright by Jak Li 2015
Alkaline Polymer Electrolytes for Electrochemical Capacitors
Jak Li
Master of Applied Science
Material Science and Engineering
University of Toronto
2015
Abstract
Polymer electrolytes for electrochemical capacitors in place of liquid electrolytes were
developed to prevent issues such as leakage and safety concerns. Currently, there is a
lack of available OH--ion conducting polymer electrolytes compared to Li+-ion and H+-ion
conducting ones. OH--ion conducting polymer electrolytes with high ionic conductivity
and environmental stability in ambient conditions are required. TEAOH-PVA polymer
electrolyte, a suitable replacement for KOH-PVA, demonstrated similar pristine ionic
conductivities and superior shelf-life in ambient conditions. TEAOH-PVA also revealed
the highest ionic conductivity compared to TEAOH-PEO and TEAOH-PAA polymer
electrolytes. Optimization of TEAOH-PVA via light cross-linking produced a polymer
electrolyte with a high ionic conductivity of 1 x 10-2 S cm-1 and stable shelf-life over a
period of 80 days. An EDLC device made with this polymer electrolyte yielded an
excellent capacitance of 110 μF at 120 Hz, appropriate for high frequency filtering
applications.
ii
Acknowledgements
First and foremost, I would like to extend my appreciation to my supervisor Professor
Keryn Lian for giving me an opportunity to be a part of the Flexible Energy and
Electronics Laboratory (FEEL). Her guidance, support and encouragements have
profoundly impacted my growth as a student, researcher and person.
Second, I would like to thank past and present students in the FEEL. Han Gao for his
mentorship, ingenuity and expertise in the development of polymer electrolytes; Sanaz
Ketabi, Matthew Genovese, Haoran (George) Wu, and Blair Decker for countless
discussions and thought provoking ideas that were generated during the past couple of
years; and Alvin Virya and Yee Wei Foong for their dedication and assistance in FEEL.
Third, I would like to thank Dan Grozea for his encouragement to my work and support
in FTIR and DSC experiments and George Kretschmann for assisting me with XRD
experiments.
I would like to recognize a deep gratitude towards my friends, family and faith
community that have continuously embraced and encouraged my research endeavours.
Finally, I thank my Lord, Jesus Christ, for giving me purpose and hope that has helped
me persevere in my work.
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Table of Contents
Abstract ............................................................................................................................ ii
Acknowledgements ......................................................................................................... iii
List of Tables................................................................................................................... vi
List of Figures ................................................................................................................ vii
List of Appendices ........................................................................................................... xi
Abbreviations ................................................................................................................. xii
Introduction of Energy Storage and Electrochemical Capacitors ................... 1
Literature Review ........................................................................................... 6
2.1 Electrochemical capacitors .................................................................................... 6
2.2 Types of Electrolytes ............................................................................................. 7
2.3 Solid electrolytes ................................................................................................. 13
2.4 Polymer matrices in polymer electrolytes ............................................................ 16
2.5 Modification of polymer electrolytes .................................................................... 27
Objectives .................................................................................................... 33
Experimental ................................................................................................ 35
4.1 Preparation of polymer electrolyte precursor solution ......................................... 35
4.2 Preparation of graphite electrodes ...................................................................... 37
4.3 Fabrication of EDLC devices using solution cast method ................................... 38
4.4 Material characterization of polymer electrolyte materials ................................... 39
4.5 Electrochemical characterizations of polymer electrolyte material in simple EC
cells ..................................................................................................................... 40
Results and Discussion ................................................................................ 45
5.1 Polymer electrolyte TEA-VA vs K-VA .................................................................. 45
iv
5.2 Effect of polymer matrices ................................................................................... 54
5.3 Crosslinking Alkaline TEAOH-PVA Polymer Electrolyte ...................................... 66
Conclusions .................................................................................................. 83
Future Work ................................................................................................. 86
References .................................................................................................................... 87
Appendices .................................................................................................................... 97
v
List of Tables
Table 2-1: Summary of comparisons between the different types of electrolytes. .......... 8
Table 2-2: Diffusivity, stoke ionic radius and hydrated ionic radius of cations of the
studied hydroxides .........................................................................................................12
Table 4-1: Chemicals used for the preparation of polymer electrolytes .........................35
Table 4-2: Polymer electrolyte composition ...................................................................37
Table 5-1: Area capacitance of K-VA and TEA-VA in metallic solid cells on Day 1 and
Day 40 ...........................................................................................................................51
Table 5-2: Variations of capacitance and time constants of a graphite/TEA-VA EC and a
graphite K-VA EC, obtained from EIS and CV over time ...............................................52
Table 5-3: Overview of potential polymer matrices with their respective functional
groups, glass transition temperature, melting temperature and crystallinity nature .......54
Table 5-4: Ionic conductivity of TEA-VA, TEA-EO and TEA-AA metallic solid cells on
Day 1, 4 and 43 of tracking in ambient conditions..........................................................63
Table 5-5: Areal capacitance of TEA-VA, TEA-EO and TEA-AA metallic cells on Day 1
and Day 43 of tracking ...................................................................................................65
Table 5-6: Summary of thermal events for TEAOH, PVA and TEA-VA ..........................71
Table 5-7: Summary of thermal events for TEA-VA, TEA-VA G2(0.5%) and TEA-VA
G2(5.0%) .......................................................................................................................72
Table 5-8: Summary of ionic conductivities of TEA-VA and TEA-VA G2(0.5%) at
pristine, 75 and 45% RH conditions. ..............................................................................77
Table 5-9: Summary of capacitances of TEA-VA and TEA-VA G2(0.5%) on Day 1 and
Day 79. ..........................................................................................................................79
Table 6-1: Summary of developed OH--ion conducting polymer electrolytes. ................85

vi
List of Figures
Figure 1-1: Ragone plot of various energy storage technologies [4]. .............................. 2
Figure 1-2: Electrochemical process differences between supercapacitor cathode (A),
anode (C) and battery cathode (B) and anode (D) [3]. .................................................... 3
Figure 2-1: Electrochemical cells made with traditional liquid electrolytes (left) and solid
electrolytes (right). Adapted from [14]. ............................................................................ 7
Figure 2-2: Schematic of proton conduction mechanism in liquid water via the inversion
between different proton complexes. Adapted from [1]. .................................................. 9
Figure 2-3: Schematic of OH--ion conduction in liquid water involving the formation and
breaking of hypercoordinated complexes between OH--ion and water. The process is
known as structural diffusion [11]. ..................................................................................10
Figure 2-4: (a) pH and (b) ionic conductivities are compared at 0.1, 0.5 and 1 M
concentrations in water. .................................................................................................13
Figure 2-5: Segmental motion of polymer electrolyte facilitating the movement of Li+-
ions [2]. ..........................................................................................................................15
Figure 2-6: The effect of plasticizing on polymer electrolyte structure illustrated via XRD
patterns (a) and the conductivity of the polymer electrolyte (b). Adapted from [13]. ......18
Figure 2-7: Comparison of EC cells with and without Na MoO in H SO /PVA gel
2 4 2 4
electrolyte: (a) CV curves at 10 mV s-1 (b) charge-discharge curves at 1.56 A g-1.
Adapted from [17]. .........................................................................................................21
Figure 2-8: Ionic conductivity as a function of KOH content of anhydrous KOH/PVA
polymer electrolyte [15]. .................................................................................................23
Figure 2-9: (A) CV of porous carbon electrode in 8 M KOH solution and (B) CV of EDLC
with PVA/KOH polymer electrolyte at various scan rates. Adapted from [10] ................24
Figure 2-10: Ionic conductivity of polymer blends between PVA and PAA at different
compositions of (a) 10:3, (b) 10:5 and (c) 10:7.5 after being immersed in KOH for 24 hrs
[9]. The 10:5 blend improved both mechanical strength and ionic conductivity. ............26
Figure 2-11: Schematic of polymer structure changes from linear to branched to cross-
linked [6]. .......................................................................................................................28
Figure 2-12: Formation of radicals in the PEO backbone to propagate chemical bond
formation between the benzophenone and other parts of the PEO backbone [12]. .......29
vii
Figure 2-13: Schematic of reaction between PVA and GA and the formation of (A) fully
cross-linked species and (B) partially cross-linked species. Adapted from [8]. ..............30
Figure 2-14: Design and cross-linking scheme of PAA-HEMA modified polymer, where
(a) shows the HEMA modified PAA and (b) shows the cross-linked network via
photopolymerization [16]. ...............................................................................................31
Figure 2-15: Ionic conductivity of SiWA-PVA-H PO polymer electrolytes with varying
3 4
degrees of cross-linking [7]. ...........................................................................................32
Figure 4-1: Preparation of graphite ink electrodes. ........................................................38
Figure 4-2: Schematic of preparation of a simple EC cell. The active area of the cell is 1
cm2 and the thickness is denoted by "l". ........................................................................39
Figure 4-3: Schematic diagraph of the potentiostat setup. WE: working electrode, CE:
counter electrode, RE: reference electrode, PE: polymer electrolyte. ............................41
Figure 4-4: Ideal capacitive responses for an EDLC device from (a) CV and (b) CCD. .42
Figure 4-5: Schematic of Nyquist plot for EDLC with ideal capacitive response. ...........43
Figure 4-6: Schematic of real and imaginary capacitance plotted against frequency. ....44
Figure 5-1: The ionic conductivity of different compositions of TEAOH-PVA [5] ............45
Figure 5-2: K-VA and TEA-VA polymer electrolyte physical appearances after 3 days. 46
Figure 5-3: X-ray powder diffraction of K-VA and TEA-VA at (a) 40% RH and (b) 30%
RH ..................................................................................................................................47
Figure 5-4: TGA of K-VA and TEA-VA from 30 to 120 °C. .............................................48
Figure 5-5: Ionic conductivity as a function of time for K-VA and TEA-VA polymer
electrolytes. ....................................................................................................................49
Figure 5-6: CVs of solid metallic K-VA EC and TEA-VA EC at (a) day 1, and (b) day 40
(sweep rate = 5000 V s-1) ...............................................................................................50
Figure 5-7: CVs of solid graphite K-VA EC and TEA-VA EC at (a) day 1, (b)
day 16, (c) day 32 and (d) day 67 (sweep rate = 1 V s-1) ...............................................51
Figure 5-8: Real and imaginary capacitances of graphite electrode ECs with K-VA (a, b)
and TEA-VA (c, d) plotted against frequency after day 1, 16, 32 and 67 .......................53
Figure 5-9: Ionic conductivity map of (a) TEA-VA, TEA-EO and TEA-AA polymer
electrolytes and (b) aqueous TEAOH. ...........................................................................56
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Figure 5-10: Optical observations of TEA-VA, TEA-EO and TEA-AA films. ...................57
Figure 5-11: XRD of TEA-VA, TEA-EO and TEA-AA polymer electrolyte films
conditioned at (a) 75% RH and (b) 45% RH. .................................................................58
Figure 5-12: IR spectra of pure polymers from (a) 4000 – 700 cm-1 and (b) 1700 – 700
cm-1. IR spectra of TEA-VA, TEA-EO and TEA-AA at 3000:1 molar ratios from (c) 4000
– 700 cm-1 and (d) 1700 – 700 cm-1. ..............................................................................59
Figure 5-13: Thermal analysis of (a) PVA, PEO, PAA and their respective polymer
electrolyte systems (b) TEA-VA, TEA-EO, TEA-AA. ......................................................62
Figure 5-14: Ionic conductivity of TEA-VA, TEA-EO and TEA-AA in metallic ECs tracked
over a period of 43 days in ambient conditions. .............................................................63
Figure 5-15: CVs of TEA-VA, TEA-EO and TEA-AA in metallic ECs on (a) day 1 and (b)
day 43. (sweep rate = 5000 V s-1) ..................................................................................65
Figure 5-16: Ionic conductivity mapping of TEA-VA as a function of degree of cross-
linking. Cells were conditioned in 45% RH for three weeks prior to testing. ...................67
Figure 5-17: Difference in film properties between (a) TEA-VA and (b) TEA-VA
G2(0.5%) after wetting the film with two drops of water. ................................................68
Figure 5-18: XRD spectra of TEA-VA and TEA-VA G2(0.5%) films at (a) 40% RH and
(b) 30% RH ....................................................................................................................69
Figure 5-19: Thermal analyzes of (a) TEAOH, (b) PVA and (c) TEA-VA (4220:1) molar
ratio via DSC (solid), TGA (dash-dot) and DTG (short dash). Samples were conditioned
at 30% RH......................................................................................................................70
Figure 5-20: Thermal analysis of TEA-VA and TEA-VA G2(0.5%) via DSC (solid), TGA
(dash-dot) and DTG (short dash). Samples were conditioned at 30% RH. ....................72
Figure 5-21: FTIR measurement of (a) TEA-VA vs TEA-VA G2(0.5%). Semi-quantitative
analysis of changes in (b) hydroxyl as a function of cross-linking. .................................74
Figure 5-22: Absorbance ratio of (a) aldehyde peak and (b) acetal peak with increase in
GA content, adapted from [8], and (c) aldehyde and (d) acetal peaks from this study. ..75
Figure 5-23: Ionic conductivity tracking over 80 days of TEA-VA and TEA-VA G2(0.5%)
being conditioned at 75 and 45% RH. ............................................................................77
Figure 5-24: CV of metallic EC cells made with TEA-VA and TEA-VA G2(0.5%) polymer
electrolytes on (a) day 1 and (b) day 79 after conditioning in both 75 and 45% RH.
(sweep rate = 5000 V s-1) ...............................................................................................78
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Figure 5-25: Cycle-life test of (a) TEA-VA and (b) TEA-VA G2(0.5%) after storage for
one year and conditioned at 45% RH. The cycling was performed in ambient conditions
(Sweep rate = 5000 V s-1) ..............................................................................................79
Figure 5-26: Sandwiched graphene EDLC using TEA-VA G2(0.5%) polymer electrolyte.
The sample was sandwiched with glass slides and clipped for intimate contact between
the electrodes and the electrolyte ..................................................................................80
Figure 5-27: Nyquist (left) and Bode (right) plots of EDLC devices prepared from
graphene electrodes and TEA-VA G2(0.5%) polymer electrolyte ..................................81
Figure 5-28: Electrochemical characterization of graphene/TEA-VA G2(0.5%) EDLC
devices through (a) CV at 1000 V s-1 and (b) charge-discharge at 5 mA. ......................81
Figure 5-29: Real capacitance plotted against frequency of EDLC device with graphene
electrode and TEA-VA G2(0.5%) polymer electrolyte ....................................................82

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