Proton Conducting Ionic Liquid-Polymer Electrolytes for Solid
Electrochemical Capacitors
by
Blair Decker
A thesis submitted in conformity with the requirements
for the degree of Master of Applied Science
Graduate Department of Materials Science and Engineering
University of Toronto
© Copyright by Blair Decker 2016
ii
Abstract
Proton conducting ionic liquid-polymer electrolytes for solid electrochemical capacitors
Blair Decker
Master of Applied Science
Graduate Department of Materials Science and Engineering
University of Toronto
2016
Electrochemical capacitors (ECs) are energy storage devices with high power density and
high efficiency, but their liquid electrolytes can contain environmentally unfriendly compounds
and are vulnerable to leakage. Ionic liquids (ILs) provide non-volatile ionic conductivity in flexible
polymer electrolytes, which can alleviate these issues.
The ILs 1-ethyl-3-methylimidazolium hydrogen sulfate (EMIHSO4) and 1-
methylimidazolium hydrogen sulfate (MIHSO4) were combined into a binary mixture with a
deeply depressed melting point, and a phase diagram was constructed after thermal analysis. On
ruthenium dioxide electrodes, 70-30 wt% EMI-MIHSO4 showed an increase in capacitance of
about 75% over that of neat EMIHSO4, demonstrating its proton activity.
The binary IL mixture was incorporated into a polyvinyl pyrrolidone polymer matrix to
form polymer electrolytes that showed good electrochemical performance and rate capability
under electrochemical impedance spectroscopy and cyclic voltammetry. On ruthenium dioxide
electrodes, the PVP-EMI-MIHSO4 electrolyte showed a 30% improvement in capacitance over
PVP-EMIHSO4.
iii
Acknowledgements
Heartfelt thanks to Dr. Keryn Lian, who has supervised me throughout this project and
provided invaluable guidance, support, and wisdom.
Thank you to Dr. S. Ketabi, who was my mentor during my time as a summer student and
at the outset of this project, and for providing the basis and model for this project. Big thanks to
Dr. D. Grozea, who was always patient with me and provided access to the DSC and FTIR. I would
also like to thank the past and present members of the Flexible Energy and Electronics Laboratory
for their support and discussion, and especially those who contributed electrodes to this project,
including M. Genovese, Y. Foong, and J. N’diaye. Thank you to J. Li for help performing FTIR
and Raman spectroscopy.
I would like to acknowledge the financial assistance I received from both the Applied
Science and Engineering Graduate Student Endowment Fund and the Ontario Graduate
Scholarship Fund. Thank you also to the administrative staff in the Materials Science and
Engineering department, who provided solutions to many issues, including M. Fryman, J. Prentice,
and F. Strumas-Manousos.
Finally, thank you to my family, whose love and support has allowed me to pursue my
education and always continue on.
iv
Table of Contents
1 Introduction ............................................................................................................................. 1
2 Background and Literature Review ........................................................................................ 5
2.1 Electrochemical Capacitors .............................................................................................. 5
2.2 Liquid Electrolytes ........................................................................................................... 8
2.3 Ionic Liquids .................................................................................................................. 10
2.3.1 Properties of ILs ...................................................................................................... 11
2.3.2 Application of ILs in ECs ....................................................................................... 14
2.4 Polymer Electrolytes ...................................................................................................... 16
2.4.1 Classification of Polymer Electrolytes.................................................................... 16
2.4.2 ILs in Polymer Electrolytes .................................................................................... 17
2.4.3 Polymer Selection ................................................................................................... 18
2.4.4 Polymer Additives .................................................................................................. 21
2.4.5 IL-Incorporating Polymer Electrolytes in ECs ....................................................... 22
2.5 Gap Analysis and Materials Selection ........................................................................... 23
2.5.1 Objectives ............................................................................................................... 24
3 Experimental Design ............................................................................................................. 26
3.1 Materials ......................................................................................................................... 26
3.2 Polymer Electrolyte Fabrication..................................................................................... 28
3.2.1 Preparation of PVP-IL gels ..................................................................................... 28
3.2.2 Electrode Fabrication .............................................................................................. 29
3.2.3 Device Fabrication .................................................................................................. 30
3.3 Characterization ............................................................................................................. 31
3.3.1 Electrochemical Characterization ........................................................................... 31
3.3.2 Physical Characterization........................................................................................ 36
4 Results and Discussion ......................................................................................................... 39
4.1 Liquid Electrolytes ......................................................................................................... 39
4.1.1 Thermal Properties of ILs ....................................................................................... 39
4.1.2 Binary Mixtures of ILs............................................................................................ 41
4.1.3 Physical Characterization of Liquid Electrolytes ................................................... 44
v
4.1.4 Electrochemical Cells with Liquid Electrolytes ..................................................... 47
4.2 Polymer Electrolyte ........................................................................................................ 51
4.2.1 Polymer Selection ................................................................................................... 51
4.2.2 Solvent Selection .................................................................................................... 52
4.2.3 Methanol-EG Cells ................................................................................................. 58
4.2.4 Effect of Additives on Methanol-EG Cells ............................................................. 65
4.2.5 Methanol-EG Cells on Double-Layer and Pseudocapacitive Electrodes ............... 70
5 Conclusions and Future Work .............................................................................................. 75
6 List of References ................................................................................................................. 78
7 Appendix A ........................................................................................................................... 83
vi
List of Tables
Table 2-1 – Summary of issues in liquid electrolyte mitigated by proposed polymer electrolyte
ECs .................................................................................................................................................. 8
Table 2-2 – Comparison of typical values for aqueous, organic, and IL electrolytes .................... 9
Table 2-3 – Data for imidazolium-based non-fluorinated ILs ...................................................... 13
Table 2-4 – Candidate polymers for polymer electrolyte with thermal properties ....................... 20
Table 3-1 – Structures of the ILs used in this study ..................................................................... 26
Table 3-2 – Properties of the polymer used in this study ............................................................. 27
Table 3-3 – List of solvents with structures and physical properties ............................................ 27
Table 3-4 – Properties of the additives used in this study ............................................................ 28
Table 3-5 – Summary of electrochemical parameters measured by EIS and CV ......................... 36
Table 4-1 – Structure and melting point for ILs used in this study .............................................. 40
Table 4-2 – Summary of thermal data for EMIHSO4-MIHSO4 binaries at given weight percentage
mixtures. (Tm – melting point, Tg – glass transition) .................................................................... 42
Table 4-3 – Solubility of mixture of polymer and solvent (× - not soluble, √ - soluble).............. 52
Table 4-4 – Solubility of PIL in soluble polymer-solvent systems from Table 4-3 (× - not soluble,
√ - soluble) .................................................................................................................................... 52
Table 4-5 – First-day electrochemical properties of PVP-IL gels in acetic acid solvent on titanium
current collectors ........................................................................................................................... 56
Table 4-6 – First-day electrochemical properties of 1-3 PVP-IL gels with various wt% EG content
on titanium current collectors ....................................................................................................... 63
Table 4-7 – Structures for organic additives ................................................................................. 66
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Table 4-8 – First-day electrochemical properties of 1-3 PVP-EMIHSO4 gels with organic additive
content on titanium current collectors........................................................................................... 67
Table 4-9 – First-day electrochemical properties of 1-3 PVP-IL gels with 10 wt% inorganic
additive content on titanium current collectors ............................................................................. 68
Table 7-1 – FTIR Assignments for EMIHSO4, MIHSO4, and EMI-MIHSO4.............................. 83
Table 7-2 – Raman Assignments for EMIHSO4, MIHSO4, and EMI-MIHSO4 ........................... 84
Table 7-3 – FTIR assignments for PVP ........................................................................................ 85
Table 7-4 – Raman assignments for PVP ..................................................................................... 85
Table 7-5 – FTIR Assignments for PVP-IL-EG ........................................................................... 86
Table 7-6 – Raman Assignments for PVP-IL-EG ........................................................................ 87
viii
List of Figures
Figure 2-1 – a) Commercial EC with spiral configuration and liquid electrolyte; b) Proposed
flexible polymer electrolyte EC ...................................................................................................... 7
Figure 2-2 – Examples of common IL cations and anions ........................................................... 10
Figure 3-1 – a) Beaker cell setup; b) Filter paper sandwich cell setup; c) Polymer electrolyte
sandwich cell setup; d) Photograph of sandwich cell ................................................................... 31
Figure 3-2 – Equivalent circuit for an EC, consisting of a resistor (Rleakage) in parallel with a
capacitor (C), all in series with a resistor (ESR). .......................................................................... 32
Figure 3-3 – Schematic drawings of CV profiles: a) ideal capacitor, b) capacitor with ESR, c)
pseudocapacitor............................................................................................................................. 35
Figure 4-1 – DSC thermograms for EMIHSO4 and MIHSO4....................................................... 41
Figure 4-2 – DSC thermograms for various wt% mixtures of EMI-MIHSO4 .............................. 43
Figure 4-3 – Quasi-equilibrium phase diagram for the binary mixture of EMIHSO4 and MIHSO4
....................................................................................................................................................... 43
Figure 4-4 – FTIR spectra of EMIHSO4, EMI-MIHSO4, and MIHSO4 ....................................... 45
Figure 4-5 – Raman spectra of EMIHSO4, EMI-MIHSO4, and MIHSO4 .................................... 46
Figure 4-6 – Cyclic voltammograms of titanium ECs with neat EMIHSO4 and EMI-MIHSO4 liquid
electrolytes at 1 V/s ....................................................................................................................... 48
Figure 4-7 – Nyquist (a) and Bode (b) plots of EIS data for titanium ECs with EMIHSO4 and EMI-
MIHSO4 liquid electrolytes........................................................................................................... 48
Figure 4-8 – Cyclic voltammograms of RuO2 ECs with EMIHSO4 and EMI-MIHSO4 electrolytes
at 2.5 mV/s .................................................................................................................................... 49
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Figure 4-9 – Nyquist (a) and Bode (b) plots of EIS data for RuO2 ECs with EMIHSO4 and EMI-
MIHSO4 electrolytes ..................................................................................................................... 50
Figure 4-10 – Electrochemical properties (a, conductivity; b, capacitance) of 1-2 weight ratio PVP-
EMIHSO4 gels with various solvents on stainless steel, with first-day and one week data ......... 54
Figure 4-11 – Cyclic voltammograms for ECs containing 1-2 PVP-EMIHSO4 gels with different
solvents on stainless steel current collectors at 1 V/s ................................................................... 55
Figure 4-12 – Cyclic voltammograms for ECs containing PVP-IL gels with acetic acid on titanium
current collectors at 1 V/s ............................................................................................................. 56
Figure 4-13 – Change in average conductivity over time for PVP-IL ECs with acetic acid on
titanium current collectors ............................................................................................................ 57
Figure 4-14 – Change over time in Nyquist electrochemical impedance spectrum of a sample 1-3
PVP-MIHSO4 cell with acetic acid ............................................................................................... 58
Figure 4-15 – FTIR spectrum for PVP powder ............................................................................ 59
Figure 4-16 – FTIR spectra for 1-3 PVP-EMIHSO4, PVP-EMI-MIHSO4, and PVP-MIHSO4 ... 60
Figure 4-17 – Raman spectrum for PVP powder .......................................................................... 62
Figure 4-18 – Raman spectra for 1-3 PVP-EMIHSO4, PVP-EMI-MIHSO4, and PVP-MIHSO4 62
Figure 4-19 – Change in average conductivity over time for 1-3 PVP-IL ECs with various wt%
EG content (a: EMIHSO4; b: EMI-MIHSO4; c: MIHSO4) ........................................................... 64
Figure 4-20 – Change in average conductivity over time for 1-3 PVP-IL ECs with 30 wt% EG
content (second attempt, higher initial solvent evaporation) ........................................................ 65
Figure 4-21 – Change in average conductivity over time for 1-3 PVP- EMIHSO4 ECs with various
organic additives ........................................................................................................................... 67
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Figure 4-22 – Change in average conductivity over time for 1-3 PVP-IL-EG ECs with 10 wt%
TiO2 additive ................................................................................................................................. 69
Figure 4-23 – Change in average conductivity over time for 1-3 PVP-IL-EG ECs with 10 wt%
SiO2 additive ................................................................................................................................. 69
Figure 4-24 – Comparison of three-electrode cell CV profiles of bare MWCNT and SiMo12-coated
MWCNT electrodes in 0.5 M H2SO4 solution .............................................................................. 71
Figure 4-25 – Cyclic voltammograms for ECs containing PVP-IL-EG gels on bare MWCNT
electrodes at 2.5 mV/s ................................................................................................................... 72
Figure 4-26 – a) Cyclic voltammograms for ECs containing PVP-IL-EG gels on MWCNT/ POM
electrodes at 2.5 mV/s; b) comparison between cyclic voltammograms of PVP-EMI-MIHSO4-EG
gels on MWCNT versus MWCNT/POM electrodes .................................................................... 72
Figure 4-27 – a) Cyclic voltammograms for ECs containing PVP-IL-EG gels with RuO2 at 2.5
mV/s; b) Cyclic voltammograms for ECs containing PVP-IL-EG gels with RuO2 at 100 mV/s . 73
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Nomenclature
Acronyms
AN Acetonitrile
CMC Carboxymethyl cellulose
CNT Carbon nanotube
CV Cyclic voltammetry
DMP Dimethyl phthalate
DSC Differential scanning calorimetry
EC Electrochemical capacitor
EDLC Electrochemical double-layer capacitor
EG Ethylene glycol
EIS Electrochemical impedance spectroscopy
EMIHSO4 1-Ethyl-3-methylimidazolium hydrogen sulfate
EMI-MIHSO4
Mixture of 1-ethyl-3-methylimidazolium hydrogen sulfate and 1-
methylimidazolium hydrogen sulfate (of 70-30 wt% composition
unless otherwise specified)
ESR Equivalent series resistance
FTIR Fourier transform infrared spectroscopy
IL Ionic liquid
ImHSO4 Imidazolium hydrogen sulfate
MIHSO4 1-Methylimidazolium hydrogen sulfate
MWCNT Multi-walled carbon nanotubes
PC Propylene carbonate
PEO Polyethylene oxide
PIL Protic ionic liquid
POM Polyoxometalate
PVA Polyvinyl alcohol
PVP Polyvinyl pyrrolidone
PVP-EMIHSO4 Polymer electrolyte containing polyvinyl pyrrolidone and 1-ethyl-3-
methylimidazolium hydrogen sulfate
xii
PVP-EMI-MIHSO4
Polymer electrolyte containing polyvinyl pyrrolidone and 70-30 wt%
1-ethyl-3-methylimidazolium hydrogen sulfate and 1-
methylimidazolium hydrogen sulfate
PVP-MIHSO4 Polymer electrolyte containing polyvinyl pyrrolidone and 1-
methylimidazolium hydrogen sulfate
Latin Symbols
C Capacitance (F)
C’ Real component of capacitance (F)
C” Imaginary component of capacitance (F)
E Energy density (Wh/kg)
Ec electrostatic attraction (N)
P Power density (W/kg)
Q Charge storage (C/cm2)
Tg Glass transition temperature (°C)
Tm Melting temperature (°C)
Z Impedance (Ω)
Z’ Real component of impedance (Ω)
Z” Imaginary component of impedance (Ω)
Greek Symbols
Λ Molar conductivity (S·cm2/mol)
η Dynamic viscosity (cP)
σ Ionic conductivity (mS/cm)
θ Phase angle (°)
ε Dielectric constant
τ Time constant (s)
ΔV Potential window (V)
1
1 Introduction
As we transition into an era of greater use of renewable energy and portable electronics,
the need for safe and effective energy storage has become paramount. Electrochemical capacitors
(ECs) bridge the gap between batteries and conventional capacitors with high power density, long
cycle life, low heat generation, and rapid charging [1]. Currently, good energy density is achieved
by potentially toxic fluorinated electrolytes [2-4] and expensive pseudocapacitive precious metals
[5]. If low-cost alternatives can be developed, applications of next-generation ECs can be further
evolved in the industrial, automotive, and personal electronics industries.
Commercial ECs face various limitations related to their electrolytes; aqueous electrolytes
limit the potential window, while organic electrolytes are flammable and volatile [1, 6, 7].
Furthermore, these liquid electrolytes require rigid, bulky containers and can be harmful to
humans. Polymer electrolytes provide an alternative through the formation of a solid matrix that
reduces leakage and permits thin, flexible EC design [8].
One class of ionically conductive material, ionic liquids (ILs), has drawn significant
interest as liquid electrolytes in their neat form or when incorporated into the polymer electrolyte.
ILs are organic salts that are liquid at room temperature, have wide potential windows, and are
non-flammable and non-volatile; however, they are hampered by low conductivity [1, 6, 7].
Fluorination of ILs is used to effectively improve ionic conductivity, but this is considered
environmentally unfriendly due to toxic breakdown products [2-4], and significantly increases the
cost of the materials and processes [9-11]. Relatively few studies have been conducted on non-
fluorinated ILs, and our work has focused on them and methods to overcome their limited
conductivity.
2
Neat ILs or mixtures of ILs can serve as liquid electrolytes, but non-fluorinated ILs are
often solid at room temperature, making them incompatible with this application. However, it is
possible to form a eutectic mixture of ILs with a melting point lower than that of either of its
components [12, 13]. This type of mixture would allow the use of more non-fluorinated and proton
active ILs, which are desirable for pseudocapacitance.
Double-layer ECs store energy through fast, efficient charge separation at the interface
between the electrodes and the electrolyte. Pseudocapacitance is the storage of energy through fast
redox reactions, providing 10-100 times the energy density of a double-layer capacitor, while
retaining fast cycling and low heat generation. Proton active materials such as protic ILs (PILs)
are required to support these pseudocapacitive reactions; PILs are relatively understudied, with
especially few studies of non-fluorinated PILs.
In this work we developed non-fluorinated, proton-conducting, and pseudocapacitance-
enhancing polymer electrolytes. To accomplish this, we incorporated novel deep eutectic ionic
liquid solutions to provide ionic conductivity; used amorphous polymers to permit high ionic
conductivity in polymer electrolytes while optimizing their mechanical and thermal properties;
and optimized polymer electrolyte properties using ILs and other organic and inorganic additives.
Previously polyethylene oxide (PEO) was used to prepare polymer electrolytes due to its
high chemical and thermal stability, high mechanical strength, and film forming properties for
processability [14, 15]. Good results were obtained with PEO and the weakly protic IL 1-ethyl-3-
methylimidazolium hydrogen sulfate (EMIHSO4), but it was less compatible with 1-
methylimidazolium hydrogen sulfate (MIHSO4), a similar compound with greater proton activity
[16].
3
Therefore, this provided an opportunity to develop a new polymer matrix for the polymer
electrolytes that would be compatible with these proton-conducting ILs, increasing device
pseudocapacitance. One of the drawbacks of PEO is its high crystallinity, which decreases ionic
conductivity and device performance. Polyvinyl pyrrolidone (PVP) was selected for its polar side
groups and amorphous character. This polymer is benign and is used as a pharmaceutical additive;
its lactam moiety has been shown to be compatible with imidazolium-based ILs [17, 18].
The PVP-based polymer electrolyte showed good initial performance and several inorganic
and organic additives were employed to improve its longevity and cohesion. Ethylene glycol (EG)
was used as a plasticizer to improve initial conductivity and device longevity. The organic
additives carboxymethyl cellulose (CMC) and dimethyl phthalate (DMP) greatly improved
cohesion at the expense of initial conductivity, and inorganic nanopowders of SiO2 and TiO2
improved device longevity and slightly improved cohesion. The polymer electrolytes were
matched with electrodes incorporating pseudocapacitive materials such as polyoxometalates and
ruthenium dioxide to demonstrate their protic activity.
This work addressed a number of challenges, including the development of eutectic
mixtures of PILs, the design of polymer electrolytes avoiding the use of fluorinated materials, and
the optimization of the electrolyte to best function in concert with pseudocapacitive electrodes.
This research provided insight for further work into this system, and its proton activity can be of
benefit to related electrochemical technologies including electrochemical sensors and batteries.
This document is organized as follows:
4
Chapter 2 discusses background information and performs a literature review regarding
ECs, polymer electrolytes, and the use of ILs as liquid electrolytes. A gap analysis is carried out
and objectives are presented, identifying the opportunities for study that this work explores.
Chapter 3 details the materials and the process of device fabrication, and discusses the
electrochemical and physical characterization methods used in this study.
Chapter 4 presents and discusses the results, beginning with the properties of the liquid
electrolyte and its electrochemical performance. This is followed by the process of developing the
polymer electrolyte and its interaction with additives. Finally, the performance of the polymer
electrolyte on pseudocapacitive electrodes is described.
Chapter 5 concludes the thesis and makes recommendations in regards to future
investigation.
5
2 Background and Literature Review
2.1 Electrochemical Capacitors
Portable, wearable, and flexible electronics are a growing industry requiring power sources
with high energy and power density. The majority of these electronics are currently supported by
batteries, but they have low power density, a limited cycle life, and lithium-ion batteries in
particular have demonstrated safety concerns. Electrochemical capacitors (ECs) can occupy a
niche between batteries and conventional capacitors, having high power density and high cycle
life, but will require higher energy density in order to meet the needs of these electronic devices.
ECs are categorized depending on their charge storage mechanism. Electrochemical
double-layer capacitors (EDLCs) store energy in a double layer at the interface between the
electrolyte and the electrode, while pseudocapacitors store energy in highly reversible Faradaic
redox reactions [1, 6].
Double layer capacitance is described by the following equation:
𝐶 =𝜀𝑟𝜀0𝐴
𝑑 (eq. 1)
where εr is the electrolyte dielectric constant, ε0 is the vacuum dielectric constant, d is the thickness
of the double layer, and A is the electrode surface area. Double-layer capacitance on a smooth
electrode surface is limited to about 20-50 µF/cm2 depending on the ions and environment [6];
where d is 5-10 Å, ε0 is 8.85 x 10-12 F/m, and εr is about 10 for a concentrated aqueous electrolyte,
the smooth double-layer capacitance would be about 10-20 µF/cm2 [19]. High capacitance
(mF/cm2) requires the use of high surface area materials such as carbon nanotubes (CNTs) or
activated carbon.
6
Pseudocapacitance involves charge storage through fast, reversible redox reactions at the
electrode surface, and occurs in materials such as transition metal oxides and conducting polymers
[6]. Pseudocapacitors can have 10-100 times the capacitance of EDLCs. The classic
pseudocapacitive material, ruthenium dioxide (RuO2), provides pseudocapacitance in protic
solutions due to fast, reversible proton/electron transfer reactions according to the following
reaction [6]:
𝑅𝑢𝑂2 + 𝑥𝐻
+ + 𝑥𝑒− ↔ 𝑅𝑢𝑂2−𝑥(𝑂𝐻)𝑥 , 0 ≤ 𝑥 ≤ 2 (rx. 1)
This reaction has high reversibility and RuO2-containing devices have a cycle life of hundreds of
thousands of cycles or higher, but its high cost as a precious metal oxide has led to a search for
alternative materials.
ECs have high power density, described by the following equation:
𝑃𝑚𝑎𝑥 = 𝑉2
4𝑅⁄ (eq. 2)
where P is the power density of an EC, but are currently limited by their energy density, which
lags behind that of batteries. The corresponding equation is:
𝐸 = 𝐶𝑉2
2⁄ (eq. 3)
where E is the energy density of an EC. Accordingly, the energy density of ECs can be improved
by increasing the capacitance (C) of the device, or by increasing the potential window V. The
potential window and resistance (R) have a direct effect on the power density of the device, and
the resistance affects the rate performance, efficiency, and heat generation [20]. A major
consideration toward improving the potential window has been to exclude water from the EC
7
system: the hydrolysis of water is a competing reaction that can limit the potential window to about
1.2 V.
Commercial ECs often employ liquid electrolytes, which can leak and require extensive
packaging, and are often hazardous due to acidity or volatility (Figure 2-1a). Rigid, battery-like
ECs containing liquid electrolyte are also not compatible with possible flexible electronic
applications, or with wearable and portable electronic devices. However, it is possible to
incorporate conductive chemicals into a polymer electrolyte, which functions simultaneously as
an ionic conductor and a separator, and is flexible and less prone to leakage (Figure 2-1b).
Figure 2-1 – a) Commercial EC with spiral configuration and liquid electrolyte; b) Proposed
flexible polymer electrolyte EC
Electrode
Polymer
electrolyte
Current
collector Electrode
Liquid
electrolyte
Current
collector
Separator
a) b)
8
While polymer electrolytes tend to have lower ionic conductivity than liquid ones, they can
be much thinner, with high surface area and multi-cell configurations (Table 2-1). These devices
also require less packaging, resulting in less dead mass and giving better energy and power density
indirectly. In addition, environmentally benign and non-hazardous materials can be selected to
construct these polymer ECs, making them safer for use in personal electronic devices.
Table 2-1 – Summary of issues in liquid electrolyte mitigated by proposed polymer electrolyte
ECs
Liquid electrolyte ECs Polymer electrolyte ECs
Corrosive or flammable liquid leakage No leakage
Additional components: separator, packaging No separator required, less packaging
Bulky size and increased weight Thin, flexible, space-efficient
2.2 Liquid Electrolytes
While aqueous electrolytes have high conductivity, they are often corrosive, and the
hydrolysis of water limits the potential window of the device to about 1.2 V. This places a major
limitation on energy and power density (Equation 2 and 3) due to the V2 dependence. Organic
electrolytes can tolerate larger potential windows before experiencing side reactions, but tend to
be low conductivity, flammable, and volatile. Acetonitrile (AN) and tetrahydrofuran are common
solvents for organic salts such as tetraethylammonium tetrafluoroborate (TEABF4)a, but they have
a
tetraethylammonium tetrafluoroborate (TEABF4)
9
high vapor pressure and are flammable, necessitating extensive packaging to prevent leakage, and
they pose a danger in the event of cell rupture or ignition of vapor.
Ionic liquids (ILs) are a possible alternative electrolyte. ILs are a class of liquid electrolytes
composed of organic salts that are liquid near room temperature. Like organic electrolytes, they
have wide potential windows and somewhat low conductivity (Table 2-2), but they have the
advantage of very low vapor pressure and low flammability [7, 21]. Their main drawback is high
viscosity, which can be altered through mixtures with solvents or through other chemical means.
Table 2-2 – Comparison of typical values for aqueous, organic, and IL electrolytes
Parameter Aqueous Organic Ionic Liquid Ref.
Conductivity High
825 mS/cm
(H2SO4 30 wt%)
Low
11 mS/cm
50 mS/cm
(TEABF4 0.65
M in AN)
Low
2.5 mS/cm (PYR14TFSI)b
14 mS/cm (EMIBF4)c
[22-
24]
Potential
Window Small
~1.2 V Wide
>2.5 V
Wide
3 V (PYR14TFSI)
2.2 V (EMIBF4)
[22,
23]
Thermal
Stability Low Volatile
High
e.g. decomposes ~400°C
(EMIBF4)
Negligible vapor pressure
[23,
25]
Viscosity Low
2 cP
(H2SO4 30 wt%)
Low
0.3 cP
(TEABF4 0.65
M in AN)
High
100 cP (PYR14TFSI)
43 cP (EMIBF4)
[22,
23]
b
N-butyl-N-methylpyrrolidinium bis(trifluoromethane
sulfonyl)imide (PYR14TFSI)
c
1-ethyl-3-methylimidazolium tetrafluoroborate
(EMIBF4)
10
2.3 Ionic Liquids
ILs are organic salts with melting points below 100°C, a property owing to the combination
of bulky cations and anions, or cations and anions that differ in size, which promotes dissociation
at low temperatures compared to familiar inorganic salts [7]. Millions of possible combinations of
ILs exist, and can be designed and fine-tuned through selection of cation and anion chemical
properties (Figure 2-2). Their very low vapor pressure, moderate conductivity, and wide potential
windows make them attractive for electrochemical applications, and also to support a wide variety
of chemical processes such as synthetic reactions and electrodeposition [7, 26-28].
Aprotic ILs lack protonated ions or proton conduction and form the greater part of those
ILs so far investigated in double-layer ECs. Protic ILs (PILs) do provide proton conduction and
are therefore potentially active in pseudocapacitive reactions; they are comparatively less-studied.
While PILs have narrower potential windows due to reactions involving their active protons, their
proton activity is an opportunity for much greater energy storage in comparison to EDLCs.
Cations Anions
Figure 2-2 – Examples of common IL cations and anions
alkylimidazolium
alkylammonium
alkylpyridinium
hexafluorophosphate triflate
bis(trifluoromethylsulfonyl)imide
11
2.3.1 Properties of ILs
Thermal properties. ILs have extremely low vapor pressure (on the order of nPa, 1012
times lower than common solvents such as water or ethanol) and do not evaporate significantly
even under ultra-high vacuum conditions, which has drawn attention toward their use to substitute
for volatile organic solvents in a number of applications [25]. In particular, aqueous and organic
solvent-containing ECs require airtight sealing and packaging to prevent evaporation, but the use
of ILs would require less stringent barriers.
The thermal stability of ILs depends on the electrostatic attraction (Ec) between its
component ions:
𝐸𝑐 = 𝑀𝑍+𝑍−
4𝜋𝜀0𝑟 (eq. 4)
where M is the Madelung constant, Z+ and Z- are the charges of the cation and anion, and r is the
inter-ion distance. The Madelung constant is a function of the interactions of surrounding crystal
lattice ions with a given ion. Stronger ionic interaction leads to a requirement for higher energy to
melt the salt; the strength of this electrostatic attraction is governed by the charge density of the
ions and their separation. Room temperature ILs therefore come about due to low charge density,
and due to bulky ion size and crystal packing mismatch.
For example, 1-ethyl-3-methyl imidazolium chlorided has a melting point of 77°C with the
relatively small chloride anion, but pairing the EMI cation with a bulky anion such as triflatee
d
1-ethyl-3-methyl imidazolium chloride
e
1-ethyl-3-methyl imidazolium triflate
12
results in a melting point of -13°C [23]. Increasing the bulk of the cation by addition of alkyl
groups reduces symmetry and impedes crystal packing, making it another approach to decrease IL
melting points. However, long alkyl chains (>8 carbons) reverse this effect due to the nonpolar
compatibility between chains [23].
The melting points of ILs can also be decreased through mixing of two or more ILs at a
eutectic composition. These mixtures can produce deep eutectics with very deeply depressed
melting points compared to the component ILs, and show appreciable conductivity at low
temperature as well. For example, binary mixtures of N-butyl-N-methylpyrrolidinium
bis(trifluoromethyl sulfonyl)imideb and N-methyl-N-propylpyrrolidinium bis(trifluoromethyl
sulfonyl)imidef had an acceptable conductivity of 0.1 mS/cm at -40°C, which was well below the
melting points (-18°C and 0°C, respectively) of the component ILs [12].
A binary mixture containing N-methyl-N-propylpiperidinium bis(fluorosulfonyl)imideg
and N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide h has been used as a liquid
electrolyte in an EDLC, resulting in a device with a broad operating range (-50 to 100°C) and a
wide potential window (3.7V) [13]. This was again well below the melting points (6°C and -18°C,
respectively) of the component ILs.
f
N-methyl-N-propylpyridinium bis(trifluoromethyl
sulfonyl)imide
g
N-methyl-N-propylpiperidinium
bis(fluorosulfonyl)imide
h
N-butyl-N-methylpyrrolidinium
bis(fluorosulfonyl)imide
13
A PIL binary mixture was developed composed of 1-methyl imidazolium hydrogen sulfate
(MIHSO4) and imidazolium hydrogen sulfate (ImHSO4), which were crystalline solids at room
temperature with melting points of 47°C and 85°C respectively (Table 2-3) [16]. The binary
mixture of MIHSO4 and ImHSO4 had a deeply depressed melting point below -70°C and stable
formation of a proton-active liquid phase at room temperature. It also had higher conductivity than
EMIHSO4 (2.5 mS/cm vs. 1.7 mS/cm).
Table 2-3 – Data for imidazolium-based non-fluorinated ILs
Ionic Liquid Structure Melting Temperature
1-ethyl-3-methylimidazolium
hydrogen sulfate (EMIHSO4)
24oC
1-methylimidazolium
hydrogen sulfate (MIHSO4)
47oC
imidazolium hydrogen sulfate
(ImHSO4)
85oC
Electrochemical properties. The electrochemical potential windows of ILs depend on the
limiting potentials of their components toward oxidation or reduction. Perfluorinated anions are
highly stable, while PILs tend to have a narrower potential window due to hydrogen reduction [29,
30], though generally still higher than that of aqueous solutions.
The main weakness of ILs is their viscosity, which stems from ion-ion interaction and
results in lowered conductivity according to Walden’s rule:
14
Λ𝜂 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (eq. 5)
here η represents the viscosity of the IL and Λ represents the molar conductivity. Fluorination of
the anion or cation can reduce ion-ion interactions due to the low polarizability of fluorine, and
bulky anions or cations also tend to reduce viscosity through decreased ion-ion interactions.
Current commercially employed ILs tend to be fluorinated, providing lower viscosity and higher
conductivity. However, the presence of fluorine can result in toxic breakdown products such as
hydrogen fluoride at high temperature or due to hydrolysis [2-4, 31]. In addition, many fluorinated
ILs are expensive due to their perfluorinated components or extensive synthesis requirements [9-
11]. Non-fluorinated ILs are considered to be more environmentally benign, but tend to have lower
conductivity and higher viscosity, which will need to be mitigated or improved to see their use in
similar electrochemical applications.
2.3.2 Application of ILs in ECs
ILs have been used as electrolytes in EDLCs due to their wide potential windows (> 3 V)
and acceptable conductivity, 1-2 mS/cm on the low end, with some ILs approaching or exceeding
the conductivity of organic solvents (~10 mS/cm). Typically EDLCs employing ILs make use of
high surface area carbon material electrodes such as activated carbon [32] or CNTs [33], and
improved performance can be achieved by matching the IL with the relative pore size of the
material to permit maximum intercalation and use of the surface [1, 34, 35].
Solvent can be added to ILs to decrease viscosity while maintaining the wide potential
window of the IL, although this introduces issues with the volatility of the given solvent [36].
Frackowiak reported that an activated carbon supercapacitor had a potential window of 0.8 V in 1
M H2SO4, 2 V in 1 M TEABF4 in AN, and 3.5 V in a high-viscosity phosphonium IL with 25%
15
AN [37], which also demonstrated the increase in potential window as a benefit of using an IL
electrolyte.
To build on the capacitance achieved by aprotic ILs in EDLCs, PILs have been considered
to replace the acidic electrolytes used in pseudocapacitors, which can store 10-100 times as much
charge as EDLCs. This would avoid the narrow potential window (about 1.2 V) of the aqueous
electrolyte.
An early work by Rochefort et al. showed that the specific capacitance of an RuO2 electrode
in the PIL α-picoline-trifluoroacetic acidi was ten times that of the specific capacitance obtained
in the aprotic IL EMIBF4, and it was on the same order as that obtained in acidic aqueous
electrolyte [29]. More recently, Nguyen et al. investigated the capacitance of RuO2 electrodes in
the PIL diethymethylammonium-trifluoromethanesulfonate j, which had a specific capacitance
about 20% that of RuO2 electrodes in 0.5 M H2SO4, and was stable up to 120°C [38]. Timperman
et al. used a PIL mixture of pyrrolidinium nitratek and pyrrolidinium bis(trifluoromethanesulfonyl)
imide l with activated carbon electrodes and achieved pseudocapacitance through intercalation
[39].
i
α-picoline-trifluoroacetic acid
j
diethymethylammonium-trifluoromethanesulfonate
k
pyrrolidinium nitrate
l
pyrrolidinium bis(trifluoromethanesulfonyl) imide
16
The use of PIL electrolytes in pseudocapacitors is a relatively new area of investigation
[29, 40-42] and study to date has focused on potentially environmentally unfriendly fluorinated
PILs. There is opportunity to explore the use of non-fluorinated PILs in pseudocapacitive ECs.
2.4 Polymer Electrolytes
Polymer electrolytes consist of systems in which a polymer matrix serves as the electrically
insulating support for an ionic conductor such as a salt, with conductivity of about
10-4 mS/cm or higher [43]. Criteria for polymer electrolytes for flexible ECs include the following:
high ionic conductivity, good electrochemical and thermal stability, good mechanical strength, and
good processability [43]. Due to the high viscosity of ILs discussed above, polymer electrolytes
provide an opportunity to incorporate those ILs into a polymer matrix, and can allow greater
dissociation and new conduction mechanisms to mitigate the low conductivity of the neat IL [44].
2.4.1 Classification of Polymer Electrolytes
Based on their conduction mechanism, polymer electrolytes may be divided into two broad
categories: salt-in-polymer and polymer-in-salt.
The ionic conduction in salt-in-polymer electrolytes is dominated by effects arising from
the structure and crystallinity of the polymer, with the ionic dissociation of the salt also playing an
important role. Selection of the polymer should balance its physical stability and compatibility
with the salt with the need for effective ionic conduction. In particular, while crystalline polymers
are often thermally and electrochemically stable, the rigidity of the polymer chains can be a barrier
to ion transport, especially below the glass transition temperature. Small molecule plasticizers can
be added to enhance the segmental motion of polymer chains, thereby increasing ionic mobility.
17
However, if plasticizers are added in excess or used to solubilize the polymer, issues can arise with
leakage and volatility of the polymer electrolyte components.
Polymer-in-salt electrolytes contain a large proportion of the ionic conductor with a small
amount of polymer binder to provide dimensional stability. This requires that the ionic conductor
have high conductivity, and have its ionic transport enhanced by the addition of the polymer. ILs
are suitable for this purpose due to their high thermal and electrochemical stability and low vapor
pressure. The polymer electrolytes in this study may be considered polymer-in-salt due to the
relatively lower proportion by weight of polymer in comparison to IL.
2.4.2 ILs in Polymer Electrolytes
One way to reduce the negative effect of the viscosity of ILs is to incorporate them into a
polymer electrolyte, which can act as a plasticizer and promote dissociation of the cation and anion,
and introduce new polymer-dependent conduction mechanisms. While IL properties are highly
tunable through the alteration of cation and anion combinations, it is difficult to systematically
predict polymer-IL compatibility and various empirical approaches are used [45]. ILs incorporated
into polymer electrolyte should have low Tm and Tg to appropriately plasticize the polymer system
and allow it to remain thin and flexible [46, 47]. The polymer matrix allows reasonable ionic
conductivity while maintaining structure and eliminating the need for a separator. Ideally these
electrolytes are directly cast onto the electrode or current collector to minimize interfacial
resistance, but relatively spongy or yielding free-standing films can also have good contact.
Alternatives to this method of polymer electrolyte production include in-situ
polymerization of a mixture of monomer and IL, or of an IL that is itself polymerizable. In the
former case, vinyl monomers are typically mixed with the IL and polymerized through free radical
18
polymerization, and the resulting polymer electrolytes reported have good conductivity and
mechanical stability [48-50]. Polymerizable ILs require that the IL contain an appropriately
reactive group such as a vinyl moiety, and produce polymer chains with the IL taking the place of
a side group. This tends to result in a lowered ionic conductivity (< 1 mS/cm) due to restriction of
IL motion, and requires the addition of an additional conductive salt [51, 52].
In-situ polymerization is an elegant method for incorporating an IL into a polymer matrix,
but it has the same difficulties as IL-incorporating polymer electrolytes in terms of matching
chemically compatible ILs with polymer, with the additional complication of managing the
polymerization reaction steps and controlling side reactions and impurities. For this reason,
integrating an IL into a polymer matrix through soluble mixing is generally the preferred method.
2.4.3 Polymer Selection
There are several considerations for matching the IL with an appropriate polymer matrix.
Amorphous polymers are preferred since increasing crystallinity serves as a barrier to ionic
motion. The IL must be able to be incorporated into the polymer matrix through compatible
chemical interactions, which often must be elucidated empirically [28]. Ideally, a polymer
electrolyte would be able to support the stability and non-volatility of the IL by being
electrochemically and thermally stable; by having an amorphous structure that would promote
chain motion and thus ionic conductivity; and by having a chemical structure compatible with the
IL that promotes dissociation and conduction of the ions.
Polyethylene oxide (PEO) was used due to its compatibility with EMIHSO4 and good
electrochemical and thermal stability, and extensive body of knowledge stemming from work in
lithium-ion batteries [14, 15]. PEO is semi-crystalline, which is an impediment to ionic
conductivity, but this issue was mitigated through the addition of plasticizers and metal oxide
19
nanopowders that promoted the formation of amorphous phases in the polymer matrix [53, 54].
Organic plasticizers were also used to increase ionic conductivity. PEO is fairly nonpolar, and
while it was compatible with EMIHSO4, MIHSO4 had a greater tendency to crystallize out of the
matrix due to its more polar nature.
Possible candidates for a replacement for PEO in this system are listed in Table 2-4. These
polymers were chosen due to their polar side groups, which would ideally lead to compatibility
with the ILs EMIHSO4 and MIHSO4, but this will have to be determined empirically. PVA has
been used in polymer electrolytes with 1-butyl-3-methylimidazolium chloridem in aqueous phase
[55], while PAA has been used in organic phase with EMIBF4 [56]. PEOX has also been shown
to be compatible with imidazolium ILs [57].
Polyvinyl pyrrolidone (PVP) is a benign polymer used as a binder in pharmaceutical
applications, in personal care products, and in adhesives [58]. It has been used recently to prepare
gel polymer electrolytes due to its compatibility with imidazolium ILs. Mishra et al. used a blend
of poly(vinylidenefluoride-co-hexafluoropropylene) (PVdF-HFP)n and PVP to incorporate a PIL,
1-butyl-3-methylimidazolium hydrogen sulfateo, forming a gel polymer electrolyte with an ionic
conductivity of about 3.9 mS/cm at ambient conditions and a stability window of 2 V [17].
m
1-butyl-3-methylimidazolium chloride
n
poly(vinylidenefluoride-co-hexafluoropropylene)
(PVdF-HFP)
o
1-butyl-3-methylimidazolium hydrogen sulfate
20
Table 2-4 – Candidate polymers for polymer electrolyte with thermal properties
Polymer name Molecular
weight Structure Tg Tm* Ref.
polyethylene
oxide (PEO) 1,000,000
-64 65 [16]
polyvinyl
alcohol (PVA) 195,000
85 >200 [59]
polyacrylic
acid (PAA) 200,000
106 -- [60]
poly(4-
vinylpyridine)
(P4VP)
60,000
137 >260 [61]
poly(2-ethyl-2-
oxazoline)
(PEOX)
500,000
70 >300 [62]
polyvinyl
pyrrolidone
(PVP)
1,300,000
180 >300 [63]
*or of decomposition
21
Syahidah et al. used a similar blend of PVdF-HFP and PVP with the IL 1-butyl-2,3-dimethyl
imidazolium tetrafluoroboratep and magnesium triflate q salt to form an EDLC with an ionic
conductivity of about 2.9 mS/cm and a stability window of 2 V [18].
There exists an opportunity to use PVP to form a polymer electrolyte with non-fluorinated
components, an imidazolium PIL, and pseudocapacitive electrodes. PVP is compatible with protic
solvents such as alcohols as well, and there is some knowledge of potential additives to modulate
its physical and thermal properties [58].
2.4.4 Polymer Additives
In the absence of plasticizing additives, cast PVP will lose solvent and harden into a
transparent film sometimes used as an adhesive. In a polymer electrolyte, it can be expected that
the IL will have a plasticizing effect on the polymer, but additives may be useful for retaining
plasticizing solvent molecules or adding bulk to improve polymer cohesion. A number of additives
are compatible with PVP, including polar organic molecules such as dimethyl phthalate (DMP)r
and sorbitol, and organic polymers and resins such as carboxymethyl cellulose (CMC)s and shellac
[58]. These additives are typically used improve cohesion and plasticization of PVP at ambient
conditions and typical humidity.
p
1-butyl-2,3-dimethylimidazolium tetrafluoroborate
q magnesium triflate
r
dimethyl phthalate (DMP)
s
carboxymethyl cellulose (CMC)
22
Metal oxide nanopowders of TiO2 and SiO2 are often used in polymer electrolytes to
improve their performance. Their effect has been described as to prevent crystallization and
enhance ionic conductivity [16, 53], to enhance ionic dissociation due to the polar groups at the
nanopowder surface [64, 65], to enhance the formation of liquid electrolyte channels in the
polymer matrix [66], and to improve mechanical strength [67, 68].
Since PVP is highly amorphous, the ability of the nanopowders to decrease crystallinity is
less important, but their potential to enhance ionic dissociation is useful. In general, ILs are less
able to dissociate, which is one factor toward their relatively low conductivity in comparison to
other salts, and are often mixed with solvents or other agents to promote this dissociation to provide
more charged species. PVP is also more likely to form a gel rather than a free-standing film, and
so the addition of nanopowders may improve mechanical strength and increase the cohesiveness
of the gel.
2.4.5 IL-Incorporating Polymer Electrolytes in ECs
Hundreds of polymer-IL formulations have been described in the literature, commonly
with fluorinated ILs or polymer. Although there are environmental concerns regarding these
fluorinated compounds as well as the potential health effects of common solvents such as dimethyl
formamide and N-methyl-2-pyrrolidone, these components have been able to achieve good ionic
conductivity in the 10-3 S/cm range with potential windows of >3 V. These formulations are often
equal to or better than common organic-based polymer electrolytes without the flammability of
the latter and may come to replace them.
Polymer-IL EDLCs generally show a smaller operating window than the neat IL or the
polymer-IL alone due to possible additional reactions at the electrode surface (e.g. high surface
area carbon). The ionic conductivity of the polymer-IL can exceed that of the neat IL through the
23
use of plasticizers; in Lewandowski et al., ionic conductivity was doubled in a polymer electrolyte
composed of polyacrylonitrile and 1-ethyl-3-methyl imidazolium triflatee through the addition of
sulfolane plasticizer, although a slightly lower potential window was observed [69]. The polymer
and plasticizer can increase ionic conductivity through promotion of IL dissociation.
The use of PIL-incorporating polymer electrolytes to support pseudocapacitors is a study
still in its infancy. Only a few studies have been done in this vein, with one by Sellam et al. using
EMIHSO4 in a matrix of PVA and PVP in an aqueous-based system with a conducting polymer
and RuO2 electrode [70]. A non-aqueous, non-fluorinated PIL-polymer electrode for
pseudocapacitors was developed by Ketabi using EMIHSO4 and MIHSO4 in a PEO matrix [16].
Since RuO2 is overly expensive, alternative pseudocapacitive materials have been sought out
including other metal oxides such as MnO2 [40] and redox active substances such as
polyoxometalates [71-73]. There are considerable opportunities to leverage the high capacitance
of pseudocapacitive materials as well as the stability and good electrochemical properties of
polymer-PIL electrolytes.
2.5 Gap Analysis and Materials Selection
Millions of combinations of cations and anions can form ILs, providing exciting
opportunity for chemical systems whose properties are highly tuneable for specific applications,
but as yet these investigations have been empirical in nature. Further progress is possible in the
development of high performance polymer electrolytes incorporating ILs for use in ECs, as well
as to better understand the fundamentals of the interactions and synergistic effects between
polymer matrix and IL. Here potential gaps in study and performance are discussed.
24
Liquid Phase ILs. Study of ILs for use in EDLCs is dominated by aprotic, fluorinated ILs
due to their high conductivity, wide potential windows, and low viscosity. While ILs have been
hailed as potential “green” replacements for flammable and reactive organic solvents, their good
thermal stability is not absolute and care must be taken to elucidate potential problems with side
reactions and breakdown products [2, 10, 31]. To avoid this and other environmental impacts,
benign non-fluorinated ILs should be developed. There are relatively few published studies
regarding PILs and especially few with non-fluorinated PILs. Leveraging protic activity from PILs
in a non-fluorinated, non-aqueous system would be a novel contribution. However, ILs lacking
fluorinated components tend to have higher viscosity and lower ionic conductivity, and so to
achieve better electrochemical properties, the ILs will likely need to be combined with a polymer
matrix.
IL-Incorporating Polymer Electrolyte. Very few studies exist involving non-fluorinated
IL-incorporating polymer electrolytes for energy storage devices. Sellam et al. prepared a device
mentioned earlier [70] and Sutto et al. developed a PVA-EMIHSO4 system for batteries [74]. Both
of these systems were in aqueous phase, so there exists opportunity for the development of a non-
fluorinated, non-aqueous PIL-incorporating polymer electrolyte for pseudocapacitors, a niche with
very limited exploration to date [16]. PVP is also relatively understudied in the polymer electrolyte
field, and it would be useful to characterize its electrochemical contributions and interaction with
ILs in a polymer matrix.
2.5.1 Objectives
This work presents the development of environmentally benign proton-conducting
polymer-IL electrolytes for use in pseudocapacitors. The objectives were as follows:
25
Develop and characterize the eutectic mixture of the PILs EMIHSO4 and MIHSO4 through
vibrational spectroscopy, and thermal and electrochemical techniques
Develop an amorphous, polar polymer matrix compatible with PILs and streamline the
manufacturing process of the polymer system
Explore possible additives to improve ionic conductivity, longevity, and cohesion of the
polymer electrolyte
Demonstrate the performance of the proton-conducting polymer-IL electrolyte using
pseudocapacitive electrodes
26
3 Experimental Design
3.1 Materials
Two non-fluorinated room-temperature PILs were selected for this study, 1-ethyl-3-
methylimidazolium hydrogen sulfate (EMIHSO4) and 1-methylimidazolium hydrogen sulfate
(MIHSO4) (Table 3-1). EMIHSO4 was used as the baseline weakly protic IL, while MIHSO4 was
chosen for its increased protic activity, and both were used to produce the binary IL mixture.
Table 3-1 – Structures of the ILs used in this study
Ionic Liquid Structure
1-ethyl-3-methylimidazolium hydrogen sulfate
(EMIHSO4, 95% purity Sigma-Aldrich)
1-methylimidazolium hydrogen sulfate
(MIHSO4, 95% purity Sigma-Aldrich)
Several polymers were initially tested for their compatibility with the ILs EMIHSO4 and
MIHSO4, and with protic solvents. The polymer PVP was selected for the bulk of the study (Table
3-2), but the rejected polymers and their properties are listed in Table 2-4 in the previous section.
Similarly, a number of solvents were considered for use in processing the polymer
electrolyte and are listed in Table 3-3. A protic solvent was preferred, and propylene carbonate is
included as a non-protic baseline.
The effect on the polymer electrolyte of various additives was investigated, including
inorganic nanopowders of silica (SiO2) and titania (TiO2), as well as the organic additives
carboxymethyl cellulose (CMC), and dimethyl phthalate (DMP) (Table 3-4).
27
Table 3-2 – Properties of the polymer used in this study
Polymer name Molecular weight Structure
polyvinyl pyrrolidone (PVP)
(Alfa Aesar) 1,300,000
Table 3-3 – List of solvents with structures and physical properties
Solvent Structure Acidity (pKa) Vapor Pressure
(kPa)
Methanol (MeOH)
15.5 13.02
Ethanol (EtOH)
15.9 5.95
Acetic Acid (HAc)
4.75 3.3
Ethylene Glycol
(EG)
14.22 0.5
Propylene
Carbonate (PC)
n/a 0.02
Finally, multi-walled carbon nanotubes (MWCNTs) (Cheaptubes) and graphite (Alfa
Aesar) were used to form carbon electrodes. These electrodes were also coated with
silicomolybdate polyoxometalates (POMs) (Strem Chemicals).
28
Table 3-4 – Properties of the additives used in this study
Additive Structure
Silicon (IV) dioxide (amorphous, 10-20 nm
powder, 99.5%, Alfa Aesar) SiO2
Titanium (IV) dioxide (anatase, 15 nm
powder, 99.7%, Alfa Aesar) TiO2
dimethyl phthalate (DMP, 99%, Alfa Aesar)
carboxymethyl cellulose (CMC, Alfa Aesar)
3.2 Polymer Electrolyte Fabrication
The polymer electrolytes in this study were produced by preparation of precursor solution
and direct casting onto electrodes or current collectors. All preparation was performed in an
environmental chamber under nitrogen atmosphere (oxygen
29
3. This was repeated, and followed by a final drop of solution before the electrodes were
sandwiched and taped together with electroplating tape. After equilibrating overnight the
electrochemical cells would be ready for testing.
These electrolytes were viscous gels with a thickness of approximately 0.2-0.4 mm. Typically
a mixture of 0.3g PVP and 0.9g IL would be prepared, with addition of additives, including 14
wt%, 23 wt%, and 30 wt% of EG as plasticizer, 10 wt% TiO2 and SiO2 as inorganic additives, or
5 wt% or 10 wt% CMC and DMP as organic additives. Methanol (1.5-2 g, not included in weight
percentage calculations) was used as solvent to dissolve the PVP powder and form a homogeneous
solution, and would evaporate rapidly both from the open film as well as the taped ECs.
3.2.2 Electrode Fabrication
Metallic current collectors were used to evaluate the electrochemical performance of the
gel electrolyte, with titanium shim (0.13 mm thick, McMaster-Carr) being used for the bulk of the
experiments although some early work was done with stainless steel shim (0.05 mm thick,
McMaster-Carr). These current collectors were superficially roughened with silicon carbide paper
prior to casting.
Electrodes composed of 25% MWCNT and 75% graphite coated on titanium shim using
PVA as a binder (about 1 cm2 in area and 100 µm thickness), and were used as double-layer
electrodes. These electrodes are referred to as bare MWCNT electrodes.
Pseudocapacitive electrodes of dimensionally stable RuO2 on titanium foil (about 0.8-0.9
cm2 in area) were used, referred to in the results section as RuO2 electrodes. The process of
manufacture for these RuO2 electrodes is described in [75], where RuCl3 salt is coated on titanium
30
foil and thermally oxidized, with approximately 1.5 mg/cm2 loading and 150-170 mF/cm2
capacitance in 0.5 M H2SO4 for a single electrode.
Pseudocapacitive carbon electrodes were also prepared using 25% MWCNT and 75%
graphite coated with silicomolybdate POMs (about 1 cm2 in area). These electrodes are referred to
as MWCNT/POM electrodes, and were fabricated according to the procedure in [73], which
involved the following steps:
1. Soaking of bare MWCNT electrodes in 9 M HNO3 for 2 minutes, followed by rinsing
in deionized water for 2 minutes;
2. Soaking in 4 wt% aqueous polydiallyldimethylammonium chloride solution for 10
minutes followed by rinsing in deionized water for 2 minutes;
3. Soaking in 10 mM POM solution for 20 minutes followed by rinsing in deionized water
for 2 minutes.
3.2.3 Device Fabrication
Conductivity measurements using a potentiostat were performed with titanium shim
current collectors submerged in liquid IL at about 4 mm distance, referred to as beaker cells (Figure
3-1a). Liquid cells were also produced using bare MWCNT, MWCNT/POM, or RuO2 electrodes
sandwiching an IL-imbued piece of filter paper (Whatman cellulose filter paper, medium porosity,
150 um thickness) (Figure 3-1b).
Gel electrolyte was sandwiched between titanium shim current collectors, bare MWCNT,
MWCNT/POM, or RuO2 electrodes (Figure 3-1c), and sealed with electroplating tape (Ideal Tape
Co. #9182) (Figure 3-1d). Unless specified, all experiments were carried out at room temperature
(about 25°C) inside a nitrogen atmosphere environmental chamber.
31
3.3 Characterization
3.3.1 Electrochemical Characterization
Electrochemical impedance spectroscopy (EIS). EIS involves the application of a low-
amplitude alternating voltage, and the measurement of the phase and magnitude of the responding
AC current. Typically, the electrode and electrolyte in an EC can be represented by an equivalent
circuit of a resistor (Rleakage) in parallel with a capacitor (C), all in series with a resistor (ESR)
Current
collector
(Titanium)
Liquid
electrolyte
Electrode (MWCNT,
MWCNT/POM, RuO2)
a)
IL-imbued
separator
Current collector
(Titanium)
b)
Figure 3-1 – a) Beaker cell setup; b) Filter paper sandwich cell setup; c) Polymer electrolyte
sandwich cell setup; d) Photograph of sandwich cell
Electrode (MWCNT,
MWCNT/POM, RuO2)
Polymer
electrolyte
Current collector
(Titanium)
c) d)
32
(Figure 3-2), where the ESR represents the internal and interfacial resistances of the EC, C its ideal
capacitance, and Rleakage its tendency toward self-discharge, or leakage current [76].
Impedance is the opposition to AC current, and is composed of real (Z’) and imaginary
(Z”) components as a function of frequency:
𝑍(𝑓) = 𝑍′(𝑓) − 𝑗𝑍"(𝑓) (eq. 6)
At high frequency, the system behaves as a pure resistor, with little or no imaginary
component and a phase angle of 0°. At low frequency, the capacitive response dominates and the
phase angle shifts to -90° for a pure capacitor. At -45° the contributions from resistance and
capacitance are equal, and the corresponding characteristic frequency is representative of the rate
performance of the EC [77]. A plot of |Z| vs. frequency and phase angle vs. frequency is referred
to as a Bode plot, and can be used to compare characteristic frequencies and phase angle responses
between systems.
The ionic conductivity is determined by immersing metallic electrodes in a liquid
electrolyte, or by sandwiching the polymer electrolyte between them. EIS can be used to produce
a plot of Z” vs. Z’, which is referred to as a Nyquist plot, and can be used to compare ESR and
Figure 3-2 – Equivalent circuit for an EC, consisting of a resistor (Rleakage) in parallel with a
capacitor (C), all in series with a resistor (ESR).
ESR
Rleakage
Rleakage
C
Rleakage
33
capacitance between systems. The ESR can be determined where the Z” is zero (or at a minimum)
and Z’ = R, and the conductivity calculated as follows:
𝜎 =𝑑
𝑅𝐴 (eq. 7)
where d is the distance between electrodes and A is their area. R represents the electrolyte
resistance as well as any interfacial resistances present in the cell.
Capacitance can be calculated from the impedance according to the following equations:
𝐶′ =−𝑍"
(2𝜋𝑓)|𝑍|2 (eq. 8)
𝐶" =−𝑍′
(2𝜋𝑓)|𝑍|2 (eq. 9)
Capacitance also consists of real (C’) and imaginary (C”) components, which correspond to
deliverable capacitance and irreversible energy loss, respectively [77]. At low frequency, C’ will
come to a plateau equivalent to capacitance from DC measurements (see cyclic voltammetry). A
plot of C” versus frequency will reveal a local maximum representative of the time constant (τ =
1/f) of the device, representing the frequency at which the device shifts from resistive to capacitive
behavior.
EIS was performed to determine the electrochemical performance of the liquid ILs and gel
electrolytes using a Princeton Applied Research VersaSTAT 3 potentiostat. EIS had a voltage
amplitude of 10 mV and the frequency range was typically 10-1 – 106 Hz, with some measurements
performed between 10-3 and 106 Hz.
34
The conductivity of the liquid ILs was measured using the beaker cell setup (Figure 3-1a),
with the ESR determined from the impedance spectrum where the imaginary component of
impedance was zero. EIS was used to calculate device capacitance for liquid ILs and gel electrolyte
in the sandwich cell setup (Figure 3-1b and c) to compare to the CV results.
Cyclic voltammetry. Another method of electrochemical characterization of ECs is cyclic
voltammetry (CV). During CV testing, a potentiostat alters the voltage difference between two
electrodes at a particular rate in volts per second (V/s), and the resulting current (A/cm2) is
measured. The current vs. potential plot is known as a cyclic voltammogram (Figure 3-3) and can
be used to determine rate capability, capacitance, and potential window (ΔV).
The capacitance of the device, C, can be calculated by:
𝐶 =
𝑄
∆𝑉 (eq. 10)
where ΔV is the potential range of the CV cycle. Integrating the area under the curve gives the
charge storage, Q, in coulombs per cm2. Increasing the cycle rate will give information on device
efficiency and ability to respond to fast changes in voltage. The shape of the CV profile indicates
the presence of redox reactions or other competing processes, and its symmetry indicates the
reversibility of the charge and discharge operations.
An ideal double-layer capacitor would have a rectangular CV cycle (Figure 3-3a),
indicating high reversibility and a current response that is stable with potential. However, in
practice, ECs have an equivalent series resistance (ESR) that creates deviations from this behavior,
leading to changes in current response with potential and less reversible behavior (Figure 3-3b).
35
Pseudocapacitive materials introduce an additional complication: at certain redox potentials, the
current will spike due to the chemical reaction, leading to peaks in the voltammogram (Figure
3-3c). The reversibility here can be estimated based on the vertical alignment of the peaks on the
charge and discharge sides.
The electrochemical potential window of an EC is the voltage range within which
irreversible Faradaic reactions are avoided. At certain oxidation and reduction potentials, the CV
will reveal exponential increase in current density corresponding with irreversible chemical
reactions of the chemical components of the EC, such as the hydrolysis of water near 1.2 V. This
type of reaction is to be avoided during the normal functioning of an EC since it will promote
irreversible reactions, mass loss, heat generation, and component breakdown.
CV testing was performed to determine the electrochemical performance of the liquid ILs
and gel electrolytes using a Princeton Applied Research VersaSTAT 3 potentiostat. CVs were
typically collected between 0 and 1.5 V at 0.1 and 1 V/s for metallic electrodes and 0 and 1 V at
2.5 mV/s for RuO2 electrodes.
Figure 3-3 – Schematic drawings of CV profiles: a) ideal capacitor, b) capacitor with ESR, c)
pseudocapacitor
Curr
ent
Curr
ent
Curr
ent
Potential Potential Potential
a) b) c)
ΔV
36
CVs were collected for the beaker cell setup (Figure 3-1a) and sandwich cell setup (Figure
3-1b and c) to measure the capacitance of the liquid ILs and the gel electrolyte on titanium and
pseudocapacitive electrodes.
A summary of the parameters measured by EIS and CV, and the device properties thereby
derived, are listed in Table 3-5.
Table 3-5 – Summary of electrochemical parameters measured by EIS and CV
Technique Parameter Properties
EIS
Potential window Operating voltage
Capacitance Charge storage ability, rate capability
Cycle life Device longevity
CV
High frequency cell impedance Equivalent series resistance (ESR)
Low frequency cell capacitance Charge storage ability
RC time constant Rate performance
3.3.2 Physical Characterization
Infrared (IR) Spectroscopy. When irradiated with light in the IR range, molecules will
absorb characteristic wavelengths that cause chemical bonds to vibrate. This technique can be used
to identify chemicals through their unique chemical bonds, as well as to reveal interactions
between mixtures of chemicals based on changes in spectrum. Asymmetric bonds are IR active
since IR spectroscopy requires a change in the dipole moment of the bond.
IR can be used to characterize binary ILs and IL-incorporating polymer electrolytes by
revealing intermolecular interactions between ILs and between IL and polymer by examining the
shift and intensity of the peaks between spectra.
37
FTIR was performed using a Thermo Scientific Nicolet iD5 ATR spectrometer in ambient
atmosphere, and spectra were obtained in the range of 4000-650 cm-1 with 2 cm-1 resolution.
Attenuated total reflectance (ATR) was used with the IL samples placed directly on the diamond
ATR crystal without further preparation.
Raman spectroscopy. Raman spectroscopy is similar to IR spectroscopy but involves the
scattering of photons by chemical bonds instead of their absorption. It can collect information from
vibration of symmetrical bonds since they will experience a change in polarizability instead of a
change in dipole moment. Raman can also be used to characterize binary ILs and IL-incorporating
polymer electrolytes and their intermolecular interactions, providing complementary information
to that supplied by IR.
Raman was performed using a Horiba XploRA Plus Confocal Raman Microscope in
ambient atmosphere with spectra obtained in the range of 100-2000 cm-1 using a 700 nm laser.
Differential Scanning Calorimetry. DSC is a measure of the difference in heat flow
between a sample and a reference as the temperature is modified. Thermal transitions appear as
peaks in heat flow as additional energy is added or removed to keep the sample and reference at
the same temperature. Using this method, thermal transitions such as glass transition temperatures,
recrystallization, and melting or freezing can be elucidated.
DSC can be used to characterize ILs and IL binary mixtures by revealing their melting
temperature and glass transition temperature, which are important considerations for the use of ILs
in polymer electrolyte. High melting temperature is indicative of stronger intermolecular
association, which impacts dissociation of the IL and ionic conductivity in polymer electrolyte.
38
Differential scanning calorimetry was performed using a DSC Q20 (TA Instruments).
Samples were prepared inside the glove box and a small amount (5-15 mg) was sealed inside
hermetic aluminium pans. Thermograms were recorded at 10°C/min, with initial equilibration at -
90°C followed by heating to 150°C and cooling to -90°C. This provided the temperatures of
various thermal transitions in the ILs.
39
4 Results and Discussion
4.1 Liquid Electrolytes
The first part of this study involved liquid electrolytes, namely EMIHSO4, MIHSO4, and
their binary mixture. Their thermal and electrochemical properties and applicability for use in ECs
were studied and compared on double-layer and pseudocapacitive electrodes.
EMIHSO4 was most extensively studied due to its room-temperature melting point, and it
had weak proton activity from the hydrogen sulfate anion [16]. To pursue greater proton activity
in IL electrolyte, MIHSO4 and ImHSO4 were studied. However, it was found that the proton
activities of MIHSO4 and ImHSO4 were equivalent through titration; the second proton of the
imidazolium cation is only marginally active even in strong base [16]. In addition, ImHSO4 is not
commercially available, and had to be produced in-house.
For these reasons, the current study focused on EMIHSO4 and MIHSO4, with EMIHSO4
as the baseline weakly pseudocapacitive material and MIHSO4 as a greater contributor of proton
activity.
4.1.1 Thermal Properties of ILs
The PIL EMIHSO4 is liquid at room temperature, having a melting point of about 24°C
(Table 4-1). The thermal and electrical properties of EMIHSO4 can be altered by removal of the
ethyl group to form MIHSO4, which has an additional active proton on the cation that can
contribute to pseudocapacitive reactions. However, MIHSO4 is solid at room temperature with a
melting temperature of 47oC (Table 4-1). This indicates a greater affinity for the neutral
undissociated state, and suggests that additives are required promote ionic dissociation and to make
the active proton available for ionic transport.
40
Table 4-1 – Structure and melting point for ILs used in this study
Ionic Liquid Structure Melting Point (°C)
EMIHSO4
24
MIHSO4
47
At room temperature, EMIHSO4 has the appearance of a dark yellow liquid, while MIHSO4
is a pale orange crystalline solid. The DSC thermograms of EMIHSO4 and MIHSO4 can be seen
in Figure 4-1. The peaks on the thermogram indicate a melting point at 24oC and a glass transition
at -61oC, and no recrystallization during cooling. In comparison, MIHSO4 has a melting peak at
47oC and a recrystallization peak at -11oC. The more symmetrical structure of the MI cation likely
contributes to this property, indicating a trade-off between the availability of active protons versus
the disrupted packing caused by the longer alkyl groups in EMIHSO4. This high melting
temperature prevents its application at room temperature as a liquid electrolyte.
One method to reduce the melting point of an IL is to introduce a solvent or similar IL to
form a binary mixture; in the next section, mixtures of EMIHSO4 and MIHSO4 were prepared to
determine a eutectic composition with a reduced melting point.
41
Figure 4-1 – DSC thermograms for EMIHSO4 and MIHSO4
4.1.2 Binary Mixtures of ILs
The formation of IL eutectic mixtures can be used to lower the melting point. The mixture
of two or more ILs introduces greater asymmetry into the crystal structure, impeding
crystallization at a given temperature.
Several binary mixtures of EMIHSO4-MIHSO4 were subjected to DSC analysis, with the
thermal data summarized in Table 4-2. Unlike the ther
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