Freestanding Nanofiber Electrodes for Supercapacitors
A Thesis
Submitted to the Faculty
of
Drexel University
By
Daniel Wayne Lawrence
in partial fulfillment of the
requirements for the degree
of
Master of Science in Materials Science and Engineering
June 2016
© Copyright 2016
Daniel W. Lawrence. All Rights Reserved
i
Dedications
To my parents, Christopher and Marie Lawrence
AND
To my younger brother, Ross Lawrence
ii
Acknowledgements
I would like to express my gratitude to my thesis advisor, Dr. Vibha Kalra,
without whom this would have not been possible. Her insight and guidance was
invaluable for the completion of this work, and I am grateful for the discussions
we had and the advice she has provided for both my thesis and life in general.
I would also like to thank Silas Simotwo for being my graduate mentor for
this work, as well as Rahul Pai, Caitlin Dillard, and Richa Singhal for their
invaluable insight and friendship within our lab group over the past few years.
Further, I’d like to thank the staff, faculty, and students in the Department of
Materials Science and Engineering, as well as the Centralized Research Facility,
at Drexel University for their support and for the use of university equipment. I
would also like to express my appreciation for Dr. Guatam Gupta and his group
at Los Alamos National Lab for their collaboration and contributions for
characterization of the solid electrolyte in that project.
Finally, I’d like to give special thanks to Ross Lawrence for proofreading
this thesis, Katie Van Aken for advice and experimentation support, and Nick
Pescatore, Kevin Bazzel, and Andrew Cieri for their camaraderie over the past
two years.
iii
Table of Contents
LIST OF TABLES .............................................................................................................. v
LIST OF FIGURES ........................................................................................................... vi
ABSTRACT ...................................................................................................................... xi
1. INTRODUCTION .........................................................................................................1
1.1 Motivation ....................................................................................................................1
1.2 Goals .............................................................................................................................2
2. BACKGROUND ............................................................................................................3
2.1 Energy Storage Devices ..............................................................................................3
2.2 Supercapacitors ...........................................................................................................4
2.2.1 Charge Storage Mechanisms ..................................................................................5
2.2.2 Electrolytes ................................................................................................................8
2.3 Experimental Materials ............................................................................................11
3. SOLID STATE SUPERCAPACITOR ........................................................................16
3.1 Introduction ...............................................................................................................16
3.2 Experimental ..............................................................................................................19
3.2.1 Carbon Nanofiber Fabrication .............................................................................19
3.2.2 Solid Supercapacitor Device Fabrication ............................................................20
3.2.3 Structural and Electrochemical Characterization ..............................................22
3.3 Results and Discussion .............................................................................................24
3.3.1 Carbon Nanofiber Characterization ....................................................................24
3.3.2 Solid Electrolyte Characterization .......................................................................27
3.3.3 Solid-State Device ..................................................................................................36
3.4 Conclusions ................................................................................................................50
4. ASYMMETRIC HYBRID SUPERCAPACITOR ......................................................51
4.1 Introduction ...............................................................................................................51
iv
4.1.1 Asymmetric Device Considerations ....................................................................52
4.1.2 Polyaniline Pseudocapacitors...............................................................................58
4.1.3 Polyaniline-Carbon Composite Electrodes in Supercapacitors .......................61
4.2 Experimental ..............................................................................................................65
4.2.1 Carbon Nanofiber Fabrication .............................................................................65
4.2.2 Galvanostatic Deposition of PANI on PCNFs ...................................................66
4.2.3 Electrochemical and Structural Characterization ..............................................68
4.2.4 Asymmetric Device Characterization .................................................................70
4.3 Results and Discussion .............................................................................................71
4.3.1 Replication of PANI-PCNF Electrodes ...............................................................71
4.3.2 Asymmetric Device Performance ........................................................................76
4.3.3 Expanded Asymmetric Window .........................................................................90
4.3.4 Negative Potential Regime for the Asymmetric Device ...................................93
4.4 Conclusions ................................................................................................................99
5. CONCLUSIONS AND RECOMMENDATIONS .................................................101
LIST OF REFERENCES ................................................................................................104
v
List of Tables
Table 1 – BET analysis data of activated (A-PCNF) and non-activated (PCNF)
samples. ............................................................................................................................27
Table 2 – Dependence of the charge transfer resistance of solid-state EMIM TFSI
electrolyte with temperature. ........................................................................................33
Table 3 – Time dependence of the charge transfer resistance of the solid-state
electrolyte. ........................................................................................................................35
Table 4 – Areal and gravimetric specific capacitance with increasing electrode
mass at a fixed scan rate of 20 mV s-1. A constant 3/8” diameter electrode size
was used, with increasing mass coming from electrode thickness. ........................49
Table 5 – Measured charged and discharged potentials of electrodes in the
asymmetric supercapacitor. Potentials were recorded by hand using a
multimeter and Ag/AgCl reference electrode within the voltage windows. .........83
Table 6 – Potential limits of the PANI-PCNF and PCNF electrodes in the
expanded voltage window asymmetric device. .........................................................91
Table 7 – Measured electrode potentials at the positive (1.2, 1.4, and 1.6 V) and
negative (-0.8 V) potential limits. ..................................................................................97
vi
List of Figures
Figure 1 – Schematics of (a) Helmholtz, (b) Goury-Chapman, and (c) Stern
electrostatic adsorption models by Zhang et al.5 (Reprinted with permission) .......6
Figure 2 – Ragone plot comparing power density and energy density of
electrochemical storage devices from Wang et al.17, who cited from the
permission of Database ©2009 IEEE. (Reprinted from permission) ..........................11
Figure 3 - Formation of Taylor cone and establishment of stable electrospinning
jet with an increasing applied potential by Zeng et al.26 (Reprinted with
permission).........................................................................................................................13
Figure 4 – Schematic showing the step-by-step procedure for solid-state device
fabrication. .........................................................................................................................21
Figure 5 – (Top) Swagelok cell with stainless steel current collectors with
(Bottom) solid-state supercapacitor inserted. ...............................................................22
Figure 6 – SEM images of (a) PCNF and (b) A-PCNF. ................................................25
Figure 7 – Absorption and desorption isotherms for PCNF and A-PCNF...............26
Figure 8 – Pore size distribution for PCNF and A-PCNF. ..........................................26
Figure 9 – FTIR spectra of ionic liquid electrolyte before and after gelation. ..........28
Figure 10 – Picture of gelled ionic liquid in vials. ........................................................29
Figure 11 – Impedance spectra of electrolytes with 0.05 M iodine and 0.5 M
lithium-iodide for characterization, with circuit fitted lines. Insets: right –
standard Randles circuit used for fitting the impedance spectra; left – picture of
the ionic liquid electrolyte after gelation. ......................................................................31
Figure 12 – Charge transfer resistance of the solid-state electrolyte with
temperature. .......................................................................................................................32
vii
Figure 13 – Conductivity of solid-state electrolyte as a function of inverse
temperature. The data fits well to a first order exponential, indicating an
adherence to Vogel-Fulcher-Tammann (VFT) behavior.56 ..........................................33
Figure 14 – Time dependence of charge transfer resistance of the solid-state
electrolyte. ..........................................................................................................................34
Figure 15 – SEM image of PCNF electrode with solid-state electrolyte....................37
Figure 16 – Solid-electrolyte-filled PCNF surface EDS, showing uniform
distribution of TMOS (Si) and EMIM TFSI (S, F). ........................................................37
Figure 17 – Solid-electrolyte-filled PCNF cross-section EDS showing uniform
distribution of TMOS (Si) and EMIM TFSI (S, F). ........................................................38
Figure 18 – Nyquist plots of liquid (control) and solid electrolyte devices
assembled with PCNF electrodes. ..................................................................................40
Figure 19 – Cyclic voltammetry of the solid electrolyte PCNF device at various
scan rates. ...........................................................................................................................41
Figure 20 – Cyclic voltammetry of PCNF device with liquid EMIM TFSI
electrolyte. ..........................................................................................................................42
Figure 21 – Galvanostatic charge-discharge curve for 1 A g-1 current density for
PCNF with solid electrolyte. ...........................................................................................42
Figure 22 – Plot of specific capacitance against current density for PCNF solid
electrolyte device. ..............................................................................................................43
Figure 23 – EIS Nyquist plots for liquid (control) and solid electrolyte devices
with A-PCNF electrodes. .................................................................................................44
Figure 24 – Cyclic voltammetry of the solid electrolyte A-PCNF device at various
scan rates. ...........................................................................................................................46
Figure 25 – Cyclic voltammetry of A-PCNF device with liquid EMIM TFSI
electrolyte. ..........................................................................................................................47
viii
Figure 26 – Galvanostatic charge-discharge curves from 0.5 to 5 A g-1 current
density for A-PCNF solid-state device. .........................................................................47
Figure 27 – Specific capacitance calculated at each current density for the A-
PCNF solid-state device. ..................................................................................................48
Figure 28 - Cyclability test of 2500 cycles at 100 mV s-1 in a 3 V window. ................48
Figure 29 – Galvanostatic charge-discharge curves at 10 mA cm-2 of Pica-
electrode supercapacitors in 0.5 M K2SO4 electrolyte with voltage windows of 1.4,
1.6, and 1.8 V, with individual electrode potentials shown. (a) Symmetric
supercapacitor device, with the positive electrode potential limit exceeded at 1.6
and 1.8 V device windows (m+/m- = 1) and (b) Asymmetric device (m+/m- = 2.46)
where both electrodes stay within their respective potential limits, and obtaining
the full possible window at 1.8 V. Image by S. Vaquero et al.71 (Reprinted with
permission).........................................................................................................................57
Figure 30 – Reduction and oxidation states of polyaniline (PANI) by Innis et al.84
(Reprinted with permission) ...........................................................................................60
Figure 31 – Cyclic voltammetry curve of polyaniline in an aqueous HCl (pH 1)
electrolyte with labeled redox couples by Song et al.85 ...............................................62
Figure 32 – Photo of Swagelok T-cell used in three-electrode testing and
galvanostatic deposition. .................................................................................................68
Figure 33 – SEM image of PANI-PCNF electrode. .......................................................72
Figure 34 – SEM images of galvanostatic PANI-coated porous carbon nanofibers.
Scale bars are (a) 1 µm and (b) 500 nm.48 .......................................................................72
Figure 35 – Cycling of PANI-PCNF electrode in a -0.1 V and -0.3 V to 0.65 V
window to show the effect of full emeraldine reduction on performance. Specific
capacitances are 252, 406, and 318 F g-1 for before reduction, reduction, and after
reduction scans, respectively. ..........................................................................................74
ix
Figure 36 – Comparison of PANI-PCNFs to the previous work48; similar specific
capacitances are achieved (318 F g-1 and 320 F g-1, respectively), with more
pronounced redox peaks..................................................................................................75
Figure 37 – EIS Nyquist plot of the PANI-PCNF electrode in the three-electrode
setup before and after PANI reduction. ........................................................................75
Figure 38 – PCNF three-electrode CV at 100 mV s-1 showing the available
negative potential window available in the asymmetric device. ...............................78
Figure 39 – Individual electrode CVs of an asymmetric cell cycled to 1.2 V. The
shifting of the PZV causes increased production of degradation products that
limits long-term cyclability. .............................................................................................84
Figure 40 – Full asymmetric CV of the electrodes shown in Figure 39. The PZV
shifts from 0.15 to 0.28 V vs. Ag/AgCl, causing the PANI-PCNF upper potential
limit to shift from 0.65 V to 0.72 V vs. Ag/AgCl. This precludes the use of the
emeraldine-leucoemeraldine redox pair for capacitance contribution. ....................84
Figure 41 – Nyquist EIS of the PANI-PCNF/PCNF asymmetric supercapacitor
before and after a 1000 cycle stability test. ....................................................................87
Figure 42 – CV of asymmetric PANI-PCNF/PCNF device at 50, 100, and 200 mV
s-1; specific capacitance values are 59.0, 49.5, and 44.4 F g-1 respectively. .................88
Figure 43 – Cyclability plot of the asymmetric supercapacitor device at 5 A g-1 in
a 1.2 V window. .................................................................................................................88
Figure 44 – Charge-discharge test of the asymmetric supercapacitor device at 1, 2,
5, and 10 A g-1. ...................................................................................................................90
Figure 45 – Asymmetric device scanned at 100 mV s-1 to 1.2, 1.4, and 1.6 V. ...........91
Figure 46 – Cyclic charge discharge test measuring capacitance retention at 5 A g-
1 current density for a 1.4 V window, retaining 77% of original capacitance after
1000 cycles. .........................................................................................................................92
x
Figure 47 – Cyclic charge discharge test measuring capacitance retention under a
5 A g-1 current density for a 1.6 V window, retaining 70% of original capacitance
after 1000 cycles. ................................................................................................................93
Figure 48 – CV comparison of an expanded asymmetric window with the stable
1.2 V window. The expanded window is brought to a potential of -0.8 V and 1.2
V, for a 2 V total window. ................................................................................................95
Figure 49 – CV comparison of an expanded asymmetric window with the 1.4 V
window. The expanded window is brought to a potential of -0.8 V and 1.4 V, for
a 2.2 V total window. ........................................................................................................96
Figure 50 – CV comparison of an expanded asymmetric window with the 1.6 V
window. The expanded window is brought to a potential of -0.8 V and 1.6 V, for
a 2.4 V total window. ........................................................................................................96
Figure 51 – CVs of the asymmetric device taken from -0.8 V to 1.2, 1.4, and 1.6 V
at 100 mV s-1. ......................................................................................................................98
Figure 52 - Charge-discharge of the asymmetric device in a -0.8 to 1.2 V window
for 1000 cycles. ...................................................................................................................99
xi
Abstract
Freestanding Nanofiber Electrodes for Supercapacitors
Daniel W. Lawrence
Supercapacitors are new and promising electrochemical energy storage
devices that possess a much higher energy per unit mass than conventional
capacitors while maintaining very high power handling capabilities. However,
the current energy storage capability of conventional supercapacitors is still
much less than that of batteries. The energy storage potential of a supercapacitor
is dependent on the total charge it can store and the potential at which storage
occurs. Improvements are constantly being made through alteration to device
capacitance through implementing more capacitive materials and increasing the
surface area where charge storage can take place. Perhaps more significantly,
serious effort has been taken to increase the operational potential of
supercapacitor devices to greatly increasing energy storage. Recent efforts
towards increasing this potential have been made through implementing
electrolytes stable at higher potentials, most notably room temperature ionic
liquids, as well as designing around the positive and negative potential limits
based on electrode-electrolyte interaction, maximizing the available window
through an asymmetric device design.
Electrospun porous carbon nanofibers are used as electrode materials in
supercapacitor devices due to their high conductivity, wettability, and
freestanding structure. Further treatments of the fibers result in a high specific
surface area with a hierarchical porous structure that facilitates electrolyte
diffusion and ion adsorption for rapid, low-resistance charge storage. In this
xii
work two supercapacitor designs are explored using these carbon nanofiber
electrodes that aim to maximize the operational potential voltage of the
supercapacitor device. The first is a solid-state supercapacitor with silica-based
ionic liquid gel electrolyte is developed that achieves a specific capacitance of 144
F g-1, and energy density of 61 Wh kg-1 based on the mass of the electrode active
material. An emphasis is placed on understanding the novel electrolyte design
and its strengths and weakness compared to an unmodified ionic liquid
electrolyte. The second is a hybrid electrode consisting of a porous carbon
nanofiber substrate with a thin film surface deposition of the conductive polymer
polyaniline, which is employed in an asymmetric supercapacitor design in order
to maximize its available potential window in an aqueous electrolyte. The
fundamentals of the more complicated asymmetric design are explored in detail
to ensure the optimization of device performance.
Chapter 1: Introduction
1.1 Motivation
As the demand for energy increases with the growth and modernization
of global economies, and the subsequent depletion of and beginning of an
exodus from fossil fuels, developing clean and renewable forms of energy is
urgently needed. Along with this need is the necessity of storage devices with
greater storage capacity and retention. Energy storage devices such as batteries
and capacitors have become ubiquitous in our everyday lives due to the ease at
which electrical energy can be stored or transported. Sources of renewable
energies, such as wind a solar, are receiving greater attention, and the transition
away from fossil fuel-based devices require new means of storing large
quantities of usable energy. Systems large and small demand higher energy
output, prompting advances in storage technology.
Electrochemical capacitors, or “supercapacitors”, have become attractive
energy storage devices due to their ability to maintain high power capability and
higher energy density, or maximum storable energy per unit mass, compared to
more traditional dielectric capacitors. Current battery technologies have much
higher energy density than supercapacitors, but due to different charge storage
2
mechanisms cannot provide that energy as rapidly, leading to a lower power
density. Improving the energy density of supercapacitors while maintaining high
power handling capability can result in the supersession of batteries in many
applications and provide the energy storage capabilities for high energy, high
power devices.
1.2 Goals
The goal of this work is to develop means of increasing the energy density
of nanofiber-based supercapacitor devices. This is done through the fabrication
of a binder-free supercapacitor with a large electrochemical window and safer
solid-state electrolyte, and an asymmetric supercapacitor that combines
pseudocapacitive and electric-double layer capacitive properties with mass
balancing to expand the electrochemical operating window beyond that of
symmetric supercapacitors. A literature review on both solid state and
asymmetric supercapacitors will be presented for comparison, followed by the
studies and experiments conducted for both projects, and finishing with
conclusions and avenues for improvement for each work.
3
Chapter 2: Background
2.1 Energy Storage Devices
Energy storage and release in electrochemical devices occurs through two
separate mechanisms: Faradaic oxidation and reduction reactions between the
electrolyte and electrode materials, and non-Faradaic electrostatic interactions.
In Faradaic reactions, chemical potential energy is stored and released
through redox reactions of electrochemically active reagents. This traditionally
occurs in batteries, as chemical changes in the anode and cathode material are
required for charge storage and release. These processes involve a potential
difference across the electrodes to change oxidation states, yielding a high energy
density through electron transfer and bond formation, but limiting cyclability
through reaction irreversibility that accumulates with subsequent cycling.
In non-Faradaic charge storage, no charge transfer occurs, with electric
charge being stored on surfaces at an elevated electric potential to one another
and separated by a vacuum or dielectric. This is the mechanism employed in
dielectric capacitors. The cyclability of these devices is extremely high, upwards
of one million cycles, as the charge storage process does not form bonds and is
4
surface based. However, a comparatively small amount of charge is stored,
limiting the total energy these capacitors can contain.1
2.2 Supercapacitors
Capacitors with high surface area electrodes in an electrolyte solution,
called electrochemical capacitors or “supercapacitors”, have become prominent
due to their higher energy density compared to dielectric capacitors. The first
patents for supercapacitors date back to 1957, when a high surface area carbon-
based design was proposed by Howard Becker.2 However, it wasn’t until the
1990’s that electrochemical capacitors gained widespread interest as part of a
DOE development program.3 Supercapacitors consist of two electrodes that are
physically separated, typically with a separator film, an electrolyte that ionically
connects the electrodes in the device, and current collectors on each electrode.
The charge a capacitor can store is called capacitance, C, and is measured in
charge stored per volt, in units of Farads. Capacitance can be described by the
parallel-plate equation shown in Equation 1:
(1)
Where εr is the relative electrolyte dielectric constant, ε0 is the vacuum dielectric
constant, A is the accessible surface area of the electrode, and d is the separation
5
distance between the two electrodes. Capacitance thus increases as surface area
increases and electrode separation distance decreases. Because of the higher
surface area of supercapacitors in comparison to conventional capacitors (on the
order of 500 – 2000 m2 g-1), supercapacitors achieve a much higher specific
capacitance, or capacitance normalized with respect to mass (herein reported in
units of F g-1).4
2.2.1 Charge Storage Mechanisms
There are two main charge storage mechanisms that occur in
supercapacitor devices: electric double layer capacitance (EDLC) and
pseudocapacitance. Under an applied voltage, two layers of polarized ions form
at the electrode-electrolyte interface. One is in the electrode surface itself through
atomic polarization from the applied voltage, similar to conventional capacitors.
The other is in the oppositely charged electrolytic ions that adsorb onto the
polarized electrode surface, separated from the surface by solvation molecules,
as seen in the models in Figure 1. The two layers form an electric field in this
surface solvation layer that corresponds with the applied voltage. This results in
an electric double layer, occurring at both electrodes in the device, essentially
making a supercapacitor a connection of two individual capacitors in series.
6
Figure 1 displays the progression of capacitor charge storage models, from
Helmholtz’s parallel plate model with linearly decreasing potential away from
the electrode surface, to Goury-Chapman’s incorporation of a diffuse layer for
electrolytic capacitors, to the combination of the two by Stern into the model
most commonly used today. The total capacitance of the supercapacitor can be
calculated in Equation 2 as:
(2)
C1 and C2 are the specific capacitances of the individual electrodes, and combined
in series yield Ctotal for the specific capacitance of the supercapacitor itself.
Figure 1 – Schematics of (a) Helmholtz, (b) Goury-Chapman, and (c) Stern
electrostatic adsorption models by Zhang et al.5 (Reprinted with permission)
7
A good electrode material for EDLC charge storage should be chemically
inert in regards to the electrolyte to prevent unwanted electrolyte or electrode
reactions and degradation, electrically conductive to facilitate the fast charge
transport associated with capacitors, possess a wide temperature operating
range, and have a large, accessible surface area to maximize surface adsorption
for ionic charge storage to occur. High surface area carbon is the dominant
electrode material for EDLC-based supercapacitors through possession of all of
these characteristics, along with a low cost.6 Carbon aerogels, xerogels, and
nanotubes7, graphene8, templated- and carbide-derived carbons9, and carbon
onions5 have all seen use as EDLC supercapacitor electrodes.
Pseuodocapacitance manifests in charge storage similar to that of
batteries, i.e. through redox reactions. For example, contemporary lithium-ion
batteries utilize lithium-based compounds that contain a transition metal as the
positive electrode for reversibility (e.g. LiCoO2, in which cobalt transitions from
Co3+ to Co4+ during charging). Intercalation of lithium ions in a negative
electrode, typically graphite, occurs during charging. These types of chemical
reactions are reversible but do not occur as rapidly as EDLC ion adsorption and
desorption. The redox reactions impart higher energy density than the EDLC
mechanism due to the bond formation, but suffer from the reduced power
handling, or ability to effectively charge and discharge quickly, associated with
8
Faradaic storage. Transition metals, such as RuO210 and MnO211, and conducting
polymers, such as polyaniline and polypyrrole12, are primary pseudocapacitive
materials due to their high theoretical specific capacitances (700, 1100, 750, and
620 F g-1, respectively)13, 14 and are oftentimes used in composites with carbon to
improve conductivity and provide mechanical support. The carbon electrodes
used in EDLCs may have functional groups such as oxygen or nitrogen on their
surfaces that induce faradaic reactions, slightly increasing their capacitance.15
2.2.2 Electrolytes
The total energy density of supercapacitors scales linearly with the
specific capacitance and with the square of the device’s maximum potential, as
described in Equation 3:
(3)
where E is energy density, normally reported in literature in units of Wh kg-1, C
is specific capacitance, which is determined by electrode surface area and
electrolyte dielectric constants as previously described, and V is the maximum
potential the supercapacitor can operate within, which is governed primarily by
the limits of the type of electrolyte used and the electrode materials.
9
The most common electrolytes are aqueous, organic, and room
temperature ionic liquid (RTIL) electrolytes. Aqueous electrolytes are grouped
into acid, basic, and neutral solutions, depending on the ionic composition (e.g.
H2SO4 is acidic, KOH is basic, and Na2SO4 is neutral), and have conductivities an
order of magnitude higher than organic or RTIL electrolytes, lowering the
equivalent series resistance for better power handling capability and capacitance.
However, their operating window, or the potential to which the electrodes are
brought with respect to one another through charging, is limited by the
decomposition of the water solvent at a 1.23 V difference between the electrodes.
The result is a lower energy density than organic or RTIL electrolytes due to the
second order dependence of energy density on the maximum potential. The
organic electrolytes used are conducting salts (e.g. tetraethylammonium
tetrafluoroborate (TEABF4)) dissolved in acetonitrile (ACN) or propylene
carbonate (PC) organic solvent. Electrochemical windows for devices are
expanded to a 2.5 to 2.8 V range, increasing the energy density of the device.
However, compared to aqueous electrolytes, they are more expensive, have
lower conductivity and specific capacitance, and have flammability and toxicity
concerns. RTILs are conducting salts with melting temperatures, as the name
suggests, at or below room temperature. The asymmetric pairing of anions and
cations in room temperature ionic liquids result in their low melting
10
temperatures and allow for potential “tuning” of RTIL properties using different
cation and anion combinations. Common cations are based on imidazolium or
pyrrolidinium (e.g. 1-ethyl-3-methylimidazolium ([EMIM]+) and 1-butyl-1-
methylpyrrolidinium (PYR14), respectively), while common anions are
tetrafluoroborate (BF4-) or bis(trifluoromethansulfonyl)imide (TFSI-). While RTILs
enjoy a very high electrochemical window (above 3 V) and better thermal
stability than organic electrolytes as the result of an absence of solvent, the high
cost, viscosity, and lower ionic conductivity are appreciable drawbacks. The
“tuneability” of RTIL properties through different ion pairings, however, may
eventually help overcome these disadvantages.16
In Figure 2 a Ragone plot is shown comparing the energy and power
densities of supercapacitors and different battery types. An ideal energy storage
device would have both high gravimetric energy density (energy per unit mass)
to store a large amount of energy in a device and high power density (power per
unit mass) to be able to discharge that stored energy as quickly as required. As
such, the two projects presented in this work aim to increase the specific energy
density of supercapacitor devices while retaining the power handling they are
known for, in order to “bridge the gap” between capacitors and batteries.
11
Figure 2 – Ragone plot comparing power density and energy density of
electrochemical storage devices from Wang et al.17, who cited from the
permission of Database ©2009 IEEE. (Reprinted from permission)
2.3 Experimental Materials
In this work, electrospun porous carbon nanofibers were used for the
electrode material due to their high specific surface area, good electrical
conductivity, controllable pore size and thickness, and their freestanding
structure. With proper heat treatments and processing, the formation of a binder-
free hierarchical porous structure that facilitates electrolyte ion diffusion during
operation is produced. The resulting pore distribution contains macropores that
12
function as electrolyte reservoirs to decrease ion diffusion distances to the
electrode surface, mesopores that act as pathways that enable low-resistance ion
transfer, and micropores that increase specific surface area and help facilitate
double-layer formation.18 The lack of binders used to hold other carbon-based
electrode active materials together eliminates “dead weight” in the device that
does not contribute to charge storage, avoiding reductions in device specific
capacitance and thus energy density and power density.19, 20
First studied in detail in 1914 by Zeleny21 with respect to the electrospray
technique, electrospinning allows for the creation of high surface-area-to-volume
fibers22, ideal for the surface-dependent charge storage mechanisms employed in
supercapacitors. A polymer solution or melt is ejected from a surface, usually a
droplet tip from a spinneret, forming a stable jet within an applied electric field.
The surface of a polymer solution droplet at the spinneret tip maintains its form
through surface tension, but the application of a high voltage at the spinneret
electrostatically charges the solution. The repulsion forces between the similarly
charged molecules in the solution causes the formation of a Taylor cone23, seen in
Figure 3. The applied electric field caused by the potential difference between the
spinneret tip and a grounded collector plate causes instability in the droplet,
causing the Taylor cone to eject its solution as a jet that travels toward the
collector plate. The travelling jet is subjected to coulombic, electrical, viscoelastic,
13
gravitational, and drag forces, causing rapid bending and whipping of the jet,
stretching the jet into an increasingly smaller diameter and evaporating the
solution solvent. The fibers are collected in a stochastic mat, though a more
advanced apparatus involving a rotating disk and parallel plate collector, among
others, can be employed for more ordered fiber mats.24 Rotating drums and disks
dipped in polymer solution bath have been used to increase throughput over
needle-tip electrospinning.25
Figure 3 - Formation of Taylor cone and establishment of stable
electrospinning jet with an increasing applied potential by Zeng et al.26
(Reprinted with permission)
14
In this work, polyacrylonitrile (PAN), a carbon precursor, is mixed with
Nafion, acting as a sacrificial polymer, in N,N-dimethylformamide (DMF)
solvent under gentle heating. A syringe is loaded with the solution and is placed
in a syringe pump in a chamber controlled for low humidity. A potential is
applied at the syringe tip, and a grounded collector plate is placed several inches
away. A stable Taylor cone is formed at a balance of the solution pump rate of
0.2 mL hr-1 to supply the solution and the applied voltage (8 to 15 kV) to create
the Taylor cone and jet, resulting in steady state electrospinning. During the jet
flight of the solution towards the collector plate, the solvent evaporates rapidly,
leaving a fiber of PAN and Nafion phase separated at the nanoscale. These
collected fibers are heat treated in air at 280°C for dehydrogenation, cyclization,
and oxidation of the fibers to transform the thermoplastic into a non-meltable
ladder polymer. The fibers then undergo carbonization at 1000°C in an inert
environment (i.e. N2) to split off nitrogen within the cyclic structure to form
planar polyaromatics.27 During carbonization the sacrificial Nafion decomposes,
resulting in an interconnected porous structure.
Microporosity and surface area of these porous carbon nanofibers
(PCNFs) can be further enhanced with physical or chemical activation.
Potassium hydroxide (KOH) is a common chemical activation material for
carbon because it yields a well-defined micropore size distribution, with pore
15
widening proceeding from an increase in activation temperature or KOH-carbon
mass ratio. Most importantly, carbon nanofibers are able to retain their
freestanding structure and textural properties while increasing microporosity
and interconnected pore networks.28
16
Chapter 3: Solid State Supercapacitor
3.1 Introduction
The long cycle life and high power density of supercapacitors in
comparison to batteries make supercapacitors propitious energy storage devices,
ideal for applications that require high power handling capability, i.e.
acceleration in electric vehicles and high cycle lifetime in portable electronics.
Supercapacitors store energy through physical adsorption and desorption at the
electrode-electrolyte interface (EDLC) or through rapid faradaic redox reaction
(pseudocapacitance); the former provides higher power and lower energy
density compared to the latter.17, 29, 30 To enhance the energy density of EDLCs,
room temperature ionic liquids have attracted much attention due to their
expanded electrochemical operational window (above 3V).31-36 Properties
common to RTILs, such as thermal, electrochemical, and moisture stability, make
them attractive EDLC electrolytes.
All-solid state supercapacitors developed with ionic liquid-based
electrolytes have also gained a great deal of attention due to the potential
integrity problems that RTIL leakage can cause. Efforts are focused on achieving
solid-state while maintaining the high energy and power density of these EDLCs.
17
However, recent works with solid-state supercapacitors are limited in their
operational potential window (commonly only up to 1V) due to the choice of
electrolyte used, and thus attain a relatively low energy density (<10 Wh kg-1).37-40
Another common limitation arises from low areal capacitance, or capacitance per
unit area, due to extremely low mass (<0.2 mg cm-2) and thickness (10-15 µm) of
the electrodes used, limiting total cell capacitance.41-43 Mass loading of this scale is
more than an order of magnitude lower than recommended for practical device
applications and testing.44 Performance of solid-state supercapacitors is
sometimes improved by testing the device at high temperatures to enhance
electrolyte conductivity and ionic transport, but such operation conditions either
fails to meet the practical operating conditions of many supercapacitor
applications38 or can cause more resistive behavior.45 As such, preserving the key
properties of low charge transfer resistance and high operating voltage in solid-
state RTIL electrolytes are of utmost importance.
Electrodes used in solid-state supercapacitor devices are primarily carbon-
based, created through the use of binding agents43, spray deposition40, 46, or
through pre-purchased materials such as nanofoam carbon paper.45 Other
electrodes fabricated via deposition of carbon nanotubes (CNT) of office paper42
or bacterial nanocellulose paper41 have also been reported. This work uses
porous carbon nanofibers for EDLC electrodes due to their high specific surface
18
area, good electrical conductivity, controllable thickness and pore size, and their
free-standing structure, creating binder-free carbon electrodes with through-
connected macropores for fast ion transport.19, 20, 47-49
I demonstrate in this work50 a binder-free solid-state EDLC with high
energy density. Free standing binder-free porous carbon nanofiber electrodes,
created using electrospinning, are combined with a silica-based ionic liquid gel
electrolyte. The device was fabricated by uniformly filling the freestanding
electrodes with a blend of silica sol-gel precursor and ionic liquid electrolyte to
ensure close electrode/electrolyte contact upon gelation. Porous carbon
nanofibers (PCNF) and activated porous carbon nanofiber (A-PCNF) were
incorporated as electrodes due to their high surface areas of 1218 m2 g-1 and 2282
m2 g-1, respectively, and were demonstrated as having excellent performance in
the EDLC device.
19
3.2 Experimental
3.2.1 Carbon Nanofiber Fabrication
Carbon nanofiber mats were fabricated using electrospinning followed by
a heat treatment process. Polyacrylonitrile (PAN, Sigma-Aldrich) and Nafion
(Ion Power) in 3:7 wt/wt ratio were mixed under low heat in N,N-
dimethylformamide (DMF, Sigma Aldrich) with a total solution concentration of
21 wt%. The resulting solution was drawn into a syringe and placed in an
automatic pumping device located in a sealed chamber with a low-moisture
environment. The pumping rate was set to 0.2 mL h-1 at a distance of 6-7 inches
from an aluminum foil collector plate, with an applied voltage of 10-15 kV
forming a stable jet and Taylor cone.
The resultant nanofiber mat was separated from the aluminum foil and
cut into smaller sections for heat treatment in a tube furnace, wherein the
samples were first stabilized in air by heating to 280°C at 5°C min-1 and holding
for five hours. They were then pyrolyzed in nitrogen by heating to 1000°C at a
rate of 2°C min-1 and holding for one hour. The heat treatments convert PAN to
carbon (see Fitzer et al.27) while also prompting Nafion decomposition to form
interconnected pores, forming PCNF.19 A-PCNF attains elevated specific surface
20
area through chemical activation of PCNF samples in potassium hydroxide
(KOH).28 PCNF mats were soaked in KOH solution (3:7 KOH:DI water wt/wt)
overnight. They were then blotted with a sterile wipe to remove excess KOH
solution and then heated in a tube furnace to 800°C at a rate of 5°C min-1 and
held for 30 minutes. The retrieved fibers were then thoroughly washed in DI
water with a few drops of hydrochloric acid (HCl) to remove any activation
remnants or byproducts. The fibers were then placed in a convection oven at 50
°C until dry.
3.2.2 Solid Supercapacitor Device Fabrication
The sol-gel precursor/ionic liquid mixture was prepared by first mixing
0.25g of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM
TFSI, Io Li Tec) with 45 µL of tetramethyl orthosilicate (TMOS, Sigma Aldritch)
to ensure homogeneous concentration distributions. Then, 4.5 µL of 0.1 M HCl
was added to induce the hydrolysis process51 and was mixed for one minute to
form the final mixture. The silica-based solid electrolyte recipe was a repurposed
solid electrolyte formulation from Dr. Guatam Gupta’s group at Los Alamos
National Lab and collaborators on this project, optimized for the use of EMIM
TFSI electrolyte. PCNF and A-PCNF samples were punched into 3/8” (0.9525 cm)
21
diameter electrodes and placed on either side of a Celgard 3501 separator. A 15
µL drop of the sol-gel precursor/ionic liquid mixture was placed between each
layer (electrode/separator/electrode) during stacking and the complete assembly
was sandwiched between two smooth Teflon plates. The entire unit was clamped
shut and allowed to solidify in air at room temperature for 5 hours. The cells
were then opened and dried in a convection oven at 50°C for 15 hours for
complete solidification and removal of excess moisture. This process is shown in
Figure 4. The now-solid supercapacitor was then transferred into a glove box
under an inert argon environment and placed in a Swagelok cell with stainless
steel current collectors for electrochemical testing, as seen in Figure 5.
Figure 4 – Schematic showing the step-by-step procedure for solid-state device
fabrication.
22
Figure 5 – (Top) Swagelok cell with stainless steel current collectors with
(Bottom) solid-state supercapacitor inserted.
3.2.3 Structural and Electrochemical Characterization
The forthcoming characterization and performance data of the device
materials were discerned using the following techniques. Specific surface area
(SSA) of the carbon nanofiber electrodes was measured using nitrogen sorption
isotherms at 77 K (Autosorb-1, Quantachrome) and CO2 sorption isotherms at
273 K (ASAP 2020, Micromeritics). All samples were degassed at 200°C under
vacuum for 24 h to remove impurities prior to the adsorption-desorption
measurements. Pore size distribution (PSD) was calculated based on these
adsorption-desorption curves through the use of the quenched solid density
23
functional theory (QSDFT) with the assumption of slit-shaped pores. Scanning
electron microscopy (SEM, Zeiss Supra 50VP) was conducted for the PCNF
electrodes before and after solid electrolyte fill. Energy dispersive spectroscopy
(EDS) was conducted for solid electrolyte-filled electrodes for atomic mapping.
All cells were assembled in a glove box (MBraun) with water content
below 1 ppm and were connected to a potentiostat (Gamry Reference 3000)
through electrical feedthroughs for electrochemical testing. Cyclic voltammetry
(CV) was performed with various scan rates from 0 to 3.5 V; similarly,
galvanostatic charge/discharge was carried out at different current densities
within the same window. Electrochemical impedance spectroscopy (EIS) was
conducted in the frequency range from 100 kHz to 10 mHz with an alternating
amplitude of 10 mV at open circuit voltage.
Equation 4 and Equation 5 evaluate the specific capacitance, C, and energy
density, E, based on cyclic voltammetry data:
∫
(4)
(5)
where I is the current response, m is the mass of one electrode, and v is the scan
rate (in mV s-1). Note that C is calculated using the weight of one electrode and E
is calculated for the device using the weight of both electrodes. Equation 6 shows
24
the calculation of specific capacitance, C, from galvanostatic charge-discharge
data:
(6)
where I is the current, Δt is the discharge time, m is the total mass of the two
carbon electrodes, and ΔV is the voltage operation window.
3.3 Results and Discussion
3.3.1 Carbon Nanofiber Characterization
The PAN/Nafion (3/7 wt%) blend was dissolved in DMF (21:79 wt%) and
electrospun, stabilized, and pyrolyzed to produce the PCNF mat through
carbonization of PAN and decomposition of Nafion. Surface area was further
increased through KOH activation (soaked in 30:70 KOH:DI wt%) to form A-
PCNF. SEM images of PCNF and A-PCNF are shown in Figure 6.
25
Figure 6 – SEM images of (a) PCNF and (b) A-PCNF. The freestanding fiber
structure was retained both (a) after decomposition of Nafion and (b) further
chemical etching of the nanofiber surface during KOH activation (b).
Surface characterization is shown in Figure 7, Figure 8, and Table 1. Figure
7 shows the adsorption isotherms for both PCNF and A-PCNF. Both isotherms
follow Type I (or Langmuir) sorption, indicating microporosity. The slight
hysteresis can be attributed to a degree of mesoporosity from Nafion
decomposition.52 The pore size distribution in Figure 8 corroborates this
viewpoint, with the majority of pores being microporous (< 2 nm) with some
mesoporosity ( 2 – 50 nm) (IUPAC Standard). KOH activation retains the overall
pore size distribution with an increase in mesoporosity due to etching not only of
new micropores but of existing micropores into mesopores.53 The average pore
size remains relatively unchanged. Specific surface area is found to be 1218 m2 g-1
and 2282 m2 g-1 through BET for PCNF and A-PCNF, respectively as seen in
Table 1, revealing the drastic increase in SSA due to KOH activation.
26
Figure 7 – Absorption and desorption isotherms for PCNF and A-PCNF.
Figure 8 – Pore size distribution for PCNF and A-PCNF.
27
Table 1 – BET analysis data of activated (A-PCNF) and non-activated (PCNF)
samples.
Samples SSA (m2 g-1)
Subnanometer
Volume (cc g-1)
Cumulative Pore
Volume (cc g-1)
Average
pore size
(nm)
PCNF 1218 0.24 0.72 1.88
A-PCNF 2282 0.49 1.24 1.80
3.3.2 Solid Electrolyte Characterization
The RTIL electrolyte, EMIM TFSI, and sol-gel precursor TMOS were well
mixed before adding 0.1 M HCl to induce the reaction for formation of the solid
state electrolyte. The minimum aqueous content (EMIM TFSI:TMOS:0.1 M HCl
0.25 g:45 µl:4.5 µl) required to sustain the condensation reaction for the solid
state electrolyte is used, while also preserving the voltage window of EMIM
TFSI. TMOS is hydrolyzed efficiently in acidic conditions followed by
condensation over a period of time resulting in Si-O-Si bond formation.51, 54 FTIR
analysis was conducted on both the ionic liquid and the solid electrolyte mixture;
the spectrum can be seen in Figure 9. After gelation, the features at 1020 cm-1,
28
1100 cm-1, and 442 cm-1 are indicative of Si-O-Si bond stretching and bending
vibrations. The broad band between 3600 and 3200 cm-1 are attributed to O-H
stretching frequencies of silanol groups. FTIR analysis shows complete gelation
of the electrolyte; Figure 10 shows images of the gelled electrolyte.
Figure 9 – FTIR spectra of ionic liquid electrolyte before and after gelation.
29
Figure 10 – Picture of gelled ionic liquid in vials.
Electrochemical impedance spectroscopy (EIS) was used to analyze the
solid-state electrolyte properties before their use in the devices. Figure 11 shows
the Nyquist plots of ionic liquid with and without gelation when placed between
two platinum electrodes. Both devices (with and without TMOS gelation)
followed the typical Warburg-type diffusion and fit with the standard Randles
circuit.55 The circuit diagram is shown in the inset of Figure 11, where Rs is the
series resistance of the contacts, electrodes, and electrolyte, Rct is the charge
transfer resistance (corresponding to surface ion absorption/desorption), Cdl is
the double layer capacitance, and Zw is the Warburg impedance element (which
models diffusion to and from the electrode surface). To account for the non-ideal
behavior of capacitance, a constant phase element with the frequency exponent
close to unity is used in place of the pure capacitor in this equivalent circuit. The
30
Rs values are 23.0 and 22.3 Ω cm-2 with and without gelation respectively,
showing nearly no variation in series resistance. This is expected due to the
unchanged electrolyte thickness and electrode resistance. After gelation, Rct
values increase from the range of 1.5-4.0 Ω cm-2 to 3.0-8.3 Ω cm-2, indicating an
increase of resistance of electron injection with gelation at the electrode-
electrolyte interface. As the Cdl values remain close to ~20 µF cm-2 in both cases, it
follows that the thickness of the electrical double layer at the electrode-electrolyte
interface does not change with TMOS. The effective diffusion constant (Deff) of
the ions with and without gelation are 2.7 x 10-6 cm-2 s-1 and 1.6 x 10-6 cm-2 s-1
respectively, obtained from the fitted Warburg impedance. These results indicate
that diffusion in the solid-state electrolyte is comparable to its liquid-state
counterpart.
31
Figure 11 – Impedance spectra of electrolytes with 0.05 M iodine and 0.5 M
lithium-iodide for characterization, with circuit fitted lines. Insets: right –
standard Randles circuit used for fitting the impedance spectra; left – picture
of the ionic liquid electrolyte after gelation.
The dependence of conductivity on time and temperature of the
electrolyte was investigated to better characterize the solid electrolyte. Figure 12
shows the change in charge transfer resistance (Rct) with temperature. The charge
transfer resistance decreases with increasing temperature, indicating elevated
device performance at higher temperatures beyond room temperature.
Conversely, Figure 13 indicates that conductivity increases with temperature,
further indicating improved device performance at elevated temperature. The
32
good fit of a first order exponential to the conductivity with respect to inverse
temperature implies the solid-state electrolyte follows Vogel-Fulcher-Tamman
(VFT) behavior.56 Table 2 tabulates the data shown in Figure 12 and Figure 13.
Figure 12 – Charge transfer resistance of the solid-state electrolyte with
temperature.
33
Figure 13 – Conductivity of solid-state electrolyte as a function of inverse
temperature. The data fits well to a first order exponential, indicating an
adherence to Vogel-Fulcher-Tammann (VFT) behavior.56
Table 2 – Dependence of the charge transfer resistance of solid-state EMIM
TFSI electrolyte with temperature.
Temperature Rct (Ω) 1000/T σ (S/cm)
298.15 4.60 3.35 0.217
304.15 4.00 3.29 0.249
309.15 3.46 3.23 0.289
314.15 3.20 3.18 0.312
319.15 2.96 3.13 0.337
324.15 2.66 3.08 0.375
34
Charge transfer resistance was measured over a two month period
through conductivity measurements as shown in Figure 14.The charge transfer
resistance shows a steep drop after day 1, potentially due to wetting of the
electrode. Rct remains stable for ~20 days, before marginally increasing and
stabilizing at 30 days. The preservation of key ionic liquid properties upon
gelation opens a path to fabricate solid-state devices with superior performance.
Table 3 tabulates the results from Figure 14.
Figure 14 – Time dependence of charge transfer resistance of the solid-state
electrolyte.
35
Table 3 – Time dependence of the charge transfer resistance of the solid-state
electrolyte.
Day Rct (Ω)
1 6.305
5 1.715
7 2.079
9 1.8705
10 2.029
11 2.036
12 2.148
13 2.387
18 2.4615
34 4.616
49 5.25
36
3.3.3 Solid-State Device
The carbon electrodes were uniformly penetrated with solid electrolyte;
an SEM of the full gelation of a PCNF electrode can be seen in Figure 15, and
energy dispersive spectroscopy (EDS) of the solid-state electrode front-on and
side-on can be seen in Figure 16 and Figure 17. The SEM shows a fully
penetrating solid electrolyte network that maintains the carbon nanofiber
morphology seen in Figure 6. The EDS of both the electrode face and cross
section indicate uniform distribution of silicon, which corresponds to the silica
network formed from the TMOS, and sulfur and fluorine, which in this device
are only provided by the EMIM TFSI electrolyte.
37
Figure 15 – SEM image of PCNF electrode with solid-state electrolyte.
Figure 16 – Solid-electrolyte-filled PCNF surface EDS, showing uniform
distribution of TMOS (Si) and EMIM TFSI (S, F).
38
Figure 17 – Solid-electrolyte-filled PCNF cross-section EDS showing uniform
distribution of TMOS (Si) and EMIM TFSI (S, F).
Both PCNF and A-PCNF electrodes were tested for performance in both
liquid EMIM TFSI and solid-state electrolyte using the Swagelok cell seen in
Figure 5 (stainless steel current collectors, Gamry Potentiostat) in an inert
glovebox (MBraun). Electrochemical impedance spectroscopy (EIS) Nyquist plots
of the PCNF electrodes can be seen in Figure 18 for both the liquid and solid
electrolyte. The series resistance (the x-intercept) is seen to increase from 2.15 Ω
cm-2 for the control (liquid EMIM TFSI) to 10.09 Ω cm-2 for the solid device,
indicative of slightly larger electrolyte resistance in the solid-state. This is
expected due to the physical entrapment of the ionic liquid in the solid-state
devices, as well as the lower adhesion of the solid electrolyte with the electrode
39
surface when compared to the liquid counterpart. The resistance increase is
minimal, suggesting the presence of a continuous ionic pathway through the
solid-state electrolyte and to the electrode surfaces. Circuit fitting finds that the
Rct values for liquid and solid electrolytes are 0.13 Ω cm-2 and 0.92 Ω cm-2
respectively, which is consistent with the findings from the platinum electrode
test devices of the solid electrolyte described previously. The high frequency
negative impedance values are attributed to either a measuring cable delay from
the connection wiring into the glovebox, or the formation of a passive layer at the
electrodes. These inductance values are 1.36 µH cm-2 and 1.24 µH cm-2 for the
liquid and solid electrolyte, respectively, and can be accounted for in the
equivalent circuit by using a virtual inductance in series with the standard
Randles circuit.57
40
Figure 18 – Nyquist plots of liquid (control) and solid electrolyte devices
assembled with PCNF electrodes.
Cyclic voltammetry curves were obtained at different scan rates for the
PCNF solid electrolyte devices shown in Figure 19; CVs of PCNF with liquid
EMIM TFSI electrolyte are shown in Figure 20 for comparison. The PCNF solid
device exhibits near-rectangular CV behavior even at a high scan rate of 100 mV
s-1, suggesting fast kinetics within the system. At a scan rate of 20 mV s-1, the
device shows a specific capacitance of 57 F g-1, 75% of which is retained at 100
mV s-1, again indicating fast ion transport through the gelled electrolyte
embedded within the hierarchical pore structure. Comparatively, the CV of
41
PCNF with liquid electrolyte achieves 59.5 F g-1 at 20 mV s-1 and 54.5 F g-1 at 100
mV s-1, showing similar performance between the solid and liquid electrolyte. In
both solid and liquid electrolyte, a peak is observed near the upper 3.5 V limit,
which could be indicative of the onset of EMIM TFSI degradation. This is not as
readily observed at the higher scan rates likely due to the smaller timeframe for
any adverse reactions to occur. Galvanostatic charge-discharge tests were also
performed at various current densities for the solid electrolyte, and attained
similar specific capacitances as shown Figure 21 and Figure 22.
Figure 19 – Cyclic voltammetry of the solid electrolyte PCNF device at various
scan rates.
42
Figure 20 – Cyclic voltammetry of PCNF device with liquid EMIM TFSI
electrolyte.
Figure 21 – Galvanostatic charge-discharge curve for 1 A g-1 current density for
PCNF with solid electrolyte.
43
Figure 22 – Plot of specific capacitance against current density for PCNF solid
electrolyte device.
The A-PCNFs, with their higher porosity and specific surface area due to
KOH activation, were used as electrodes to further increase specific capacitance
and energy density of the solid-state device. Electrochemical tests were done
with the same parameters used in testing the PCNF samples. Figure 23 shows
EIS Nyquist plots again comparing A-PCNF solid and liquid electrolyte samples.
Circuit fitting for these plots was done similarly to the PCNF samples. The series
resistance increases from 7.41 Ω cm-2 for the liquid electrolyte to 10.11 Ω cm-2 for
the solid-state electrolyte. The charge transfer resistance showed a small increase
(in the range of 1-3 Ω cm-2) for both the control and solid samples of A-PCNF
44
compared to PCNF, revealing that the activation process does not adversely
impact the electron injection process at the electrode-electrolyte interface. The
inductance values, 1.36 µH cm-2 and 1.31 µH cm-2 for liquid and solid-state
electrolyte respectively, are nearly equal to those of the non-activated samples
tested under the same parameters.
Figure 23 – EIS Nyquist plots for liquid (control) and solid electrolyte devices
with A-PCNF electrodes.
45
Cyclic voltammetry curves for the A-PCNF solid-state device is shown in
Figure 24; for comparison, CVs for A-PCNF device with liquid EMIM TFSI
electrolyte are shown in Figure 25. They all exhibit the near-ideal rectangular
behavior indicative of fast transport. These scan rates provide specific
capacitances of 144 F g-1 (0.24 F cm-2), 123 F g-1, 108 F g-1, and 96 F g-1, at 5, 20, 50,
and 100 mV s-1 respectively, indicating large retention at high scan rates, which is
indicative of high power handling capability. Comparatively, the A-PCNF liquid
electrolyte device achieves 125 F g-1 and 101 F g-1 at 20 and 100 mV s-1
respectively, closely matching the performance in the solid electrolyte. Similar to
the PCNF devices, peaks near the 3.5 V upper limit are likely due to the approach
of electrolyte degradation at that high potential, and are more easily observed at
lower scan rates due to the larger timeframe for those reactions to occur.
Galvanostatic charge-discharge tests for the solid A-PCNF device were
conducted at various current densities, with a specific capacitance of 142 F g-1
achieved at 0.5 A g-1; see Figure 26 and Figure 27. The device retained a specific
capacitance of 97 F g-1 at an order of magnitude higher 5 A g-1, again showing
excellent power handling capability. Finally, the cyclability of the solid A-PCNF
device was determined using 2500 cycles of charging to 3 V at 100 mV s-1, as seen
in Figure 28, to avoid any potential electrolyte degradation. After conditioning
46
the device for 100 cycles, capacitance retention of 81% after 2500 cycles was
achieved, with 94% retention at 1000 cycles.
Figure 24 – Cyclic voltammetry of the solid electrolyte A-PCNF device at
various scan rates.
47
Figure 25 – Cyclic voltammetry of A-PCNF device with liquid EMIM TFSI
electrolyte.
Figure 26 – Galvanostatic charge-discharge curves from 0.5 to 5 A g-1 current
density for A-PCNF solid-state device.
48
Figure 27 – Specific capacitance calculated at each current density for the A-
PCNF solid-state device.
Figure 28 - Cyclability test of 2500 cycles at 100 mV s-1 in a 3 V window.
49
An advantage of this electrospun nanofiber electrode is that areal mass
loading can be facilely tuned by modifying the nanofiber mat thickness through
the amount of electrospun solution. Table 4 shows the areal capacitance
corresponding to a range of electrode masses with a constant diameter of 3/8”,
cycled at a 20 mV s-1 rate. Areal capacitance can increase more than two fold,
from a mass increase from 2.40 mg to 5.46 mg, with minimal variations between
the related specific capacitance values, showing a potential for scalability to even
higher areal capacitance values. The limit to which the areal capacitance can be
increased through higher mass loading without sacrificing gravimetric
capacitance has yet to be explored.
Table 4 – Areal and gravimetric specific capacitance with increasing electrode
mass at a fixed scan rate of 20 mV s-1. A constant 3/8” diameter electrode size
was used, with increasing mass coming from electrode thickness.
Total Electrode
Mass (mg)
Gravimetric
Capacitance (F g-1)
Areal Capacitance
(F cm-2)
2.40 122.6 0.207
3.16 105.8 0.235
4.51 123.8 0.392
4.57 105.7 0.339
5.46 115.6 0.443
50
3.4 Conclusions
Excellent performance of an all-solid-state supercapacitor based on
electrospun carbon nanofibers and a silica-based ionic liquid gel electrolyte was
achieved. The freestanding CNF electrodes eliminate the need for binders and
allow for active material mass loading of 3-5 mg cm-2. The device exhibits a
specific capacitance of up to 144 F g-1 and areal capacitance of 0.443 F cm-2 within
a 3.5V window. This corresponds to an energy density of 61 Wh kg-1. Near
rectangular cyclic voltammetry curves at 100 mV s-1 indicate the high power
handling capability of the device. There are a plethora of RTIL electrolytes58 that
can be investigated and potentially employed in this solid electrolyte technique;
additionally, optimization of the areal capacitance through mass loading could
further improve this solid-state supercapacitor performance.
51
Chapter 4: Asymmetric Hybrid Supercapacitor
4.1 Introduction
Supercapacitors are devices that bridge the gap between conventional
electrostatic capacitors and batteries, possessing higher energy density than
electrostatic capacitors and higher power density than batteries. They are
attractive when coupled with batteries and fuel cells for low emission
transportation in providing the necessary transient power required for start-up
and acceleration, and regenerative recovery of energy from braking. Additional
uses like emergency doors on airplanes or other critical power applications, as
well as storage for alternate energy sources like solar and wind power, desire the
power and energy properties of supercapacitors. The three main subgroups of
supercapacitors are electric double layer capacitors, pseudocapacitors, and
hybrid capacitors. As previously stated, EDLCs store charge electrostatically
through surface adsorption of ions from an electrolyte, while pseudocapacitors
invoke fast faradaic reactions to chemically store charge in the form of bond
formation. The dominant electrode material for EDLCs is high surface area
carbon, while metal oxides and conducting polymers are the main
pseudocapacitive materials. Hybrid supercapacitors, however, are a combination
52
of the two, typically involving a pseudocapacitive positive electrode and an
EDLC negative electrode. Both metal oxides59-64 and conducting polymers65-69
have been paired with various carbon electrodes in the formation of hybrid,
asymmetric supercapacitor devices.
4.1.1 Asymmetric Device Considerations
As previously described, the energy stored in a supercapacitor, E, is
defined by its capacitance, C, and operational window, V, as stated previously in
Equation 3:
(3)
Similarly, total charge storage in the supercapacitor, Q, is the derivative of its
energy with respect to its operational voltage, leading to Equation 7:
(7)
The amount of charge each electrode stores must be the same. In symmetric
supercapacitors, as both electrodes are made of the same material, they both
have the same working potential range, requiring an equalization of the
electrodes’ capacitance for charge balancing. As shown previously, Equation 2
shows that, for the two electrodes in series:
(2)
53
where C1 and C2 correspond to electrode capacitances and Ctotal is the total
capacitance of the device. Since C1 = C2 = C for identical electrodes, Equation 8
simplifies the expression to:
(8)
such that the individual electrodes are twice the total capacitance of the device.
For asymmetric supercapacitors, this simplification does not hold. Equation 9
shows that the charge storage of both positive and negative electrodes must be
the same, such that:
(9)
where p subscripts denote the positive electrode, and n subscripts the negative
electrode. As the specific capacitance of either electrode in an asymmetric cell is
different due to the different electrode material used, Equation 9 can be further
broken down as shown in Equation 10:
(10)
in which m denotes the mass of the respective electrode and the sp subscript
denotes specific (or gravimetric) capacitance; both Cp and Cn were decomposed
into mass (g) and specific capacitance (F g-1).
The working voltage of a supercapacitor is defined by irreversible
reactions, be it the onset of electrolyte solvent decomposition or redox reactions
between the electrolyte and electrode. For example, Peng et al. report a carbon
54
electrode made of Cabot Monarch 1300 pigment black that is stable until around
-0.65 V in aqueous HCl solution in a 3 electrode setup against an Ag/AgCl
reference electrode, upon which the device experiences an H+ ion reduction. On
the positive side, oxidation of H2O occurred at 0.72 V, before the 0.13 V less
positive Cl- oxidation reaction.70 The operational window of aqueous electrolyte
supercapacitors is typically below 1.2 V, while organic electrolytes can reach
around 2.7 V15 and ionic liquids reach even higher (> 3 V).
In the symmetric cell, since electrode masses and specific capacitances are
equal, the operational windows of each electrode are also equal. During charging
of symmetric cells, each electrode increases in potential and charge at the same
rate in opposite directions; because of this, either a positive or negative
irreversible reaction acts as limiting factor in the operational window of the cell.
For instance, Vaquero et al. find the potential limits of their materials, Pica
carbon in 0.5 M K2SO4 electrolyte, to be -1.1 V and 0.7 V against an Ag/AgCl
reference.71 In a symmetric cell, they achieved a 1.4 V window, but beyond 1.4 V
electrolyte decomposition occurs. From the upper and lower limits, a 1.8 V
window should be possible, meaning the symmetric cell did not fully utilize the
entire operational window available to their system. Charge-discharge curves of
this can be seen Figure 29a.
55
Another important consideration is the potential of zero voltage, or PZV.72
As electrodes operational windows are typically characterized by their range
against a reference electrode, the electrodes themselves exhibit some “resting”
potential with respect to the reference; in other words, the open circuit voltage
(OCV) of device configuration. For example, the electrodes used by Vaquero et
al. had an OCV of 0.18 V, which is positive with respect to the Ag/AgCl
reference. Therefore, their operational window for a symmetric cell was truly ΔE+
= 0.52 V and ΔE- = -1.28 V for the positive and negative electrodes, respectively.71
The OCV acts as the neutral “starting point” from which the electrodes depart
from with the application of bias. In this case, with the 0.7 V positive potential
limit, the positive operational window is only between 0.7 V (the limit) and 0.18
V (the PZV), leading to an accessible window of 0.52 V from the PZV. In a
symmetric device, with the OCV of each electrode being essentially equivalent,
the device PZV is the same as their OCV. At the PZV, the electrodes have no
potential with respect to one another.
Asymmetric devices use mass balancing to influence the operational
windows of their electrodes. Returning to Vaquero et al., by changing their mass
balance from mp/mn = 1 to mp/mn = 2.46, they were able to achieve the full
operational window of -1.1 V to 0.7 V with respect to Ag/AgCl.71 The calculation
for this new mass balance ratio is seen in Equation 11:
56
(11)
As the electrode materials are the same, Csp,p = Csp,n, the equation reduces to
Equation 12:
(12)
Knowing the operational windows, Vp = 0.52 V and Vn = -1.28 V, Equation 13
solves for the mass ratio:
( ) ( )
(13)
This allows for the full operational window of the electrode material to be
utilized, as seen in Figure 29b, and thus a higher energy density achieved
according to Equation 1. Changing the mass balance is the most common way of
achieving the full potential window. Van Aken et al. use a novel method of
changing the PZV of a symmetric supercapacitor by using different ionic liquid
electrolyte blends. Two ILs with the same cation (EMI+) but different anions
(TFSI-, of larger size than the cation, and BF4-, of smaller size than the cation)
have different PZVs individually with the reported onion-like carbon (OLC)
electrodes, and a blend of the two results in an entirely different PZV.73 The non-
microporous OLC electrodes eliminated the effects of pore size and
transportation limitations to isolate surface adsorption mechanisms of blended IL
electrolytes, and the authors were able to show improved capacitance retention
57
during cyclability tests and minimal electrolyte degradation through altering the
operational window around the new PZV to be within both electrode limits.
Figure 29 – Galvanostatic charge-discharge curves at 10 mA cm-2 of Pica-
electrode supercapacitors in 0.5 M K2SO4 electrolyte with voltage windows of
1.4, 1.6, and 1.8 V, with individual electrode potentials shown. (a) Symmetric
supercapacitor device, with the positive electrode potential limit exceeded at
1.6 and 1.8 V device windows (m+/m- = 1) and (b) Asymmetric device (m+/m- =
2.46) where both electrodes stay within their respective potential limits, and
obtaining the full possible window at 1.8 V. Image by S. Vaquero et al.71
(Reprinted with permission)
58
4.1.2 Polyaniline Pseudocapacitors
Polyaniline (PANI) is a conducting polymer with high electrical
conductivity (between 0.1 and 5 S cm-1 when doped with Li salt electrolyte)74, 75,
low cost, and a number of inexpensive synthesis processes.76 The different redox
states of PANI can be seen in Figure 30. The three main forms, depending on the
oxidation state of the PANI, are leucoemeraldine ((C6H4NH)n, fully reduced),
emeraldine (([C6H4NH]2[C6H4N]2)n, half oxidized) and pernigraniline ((C6H4N)n,
fully oxidized). When doped with an acid (as seen in Figure 30), the emeraldine
base becomes the highly-conducting emeraldine salt. Similar salts can also be
formed for the leucoemeraldine and pernigraniline base states. Protonation
occurs on both amine and imine nitrogen atoms in the chain.
Charge storage occurs through Faradaic charge transfer between
electrolytic protons and nitrogen groups within the PANI chain. As such, protic
electrolytes are typically used in devices hoping to capture the charge storage
properties of PANI. It is further reported polymerization of PANI should
generally be carried out in acidic media to accommodate the PANI salt redox
states.77 A theoretical gravimetric capacitance of 750 F g-1, assuming a 50% dopant
per polymer unit in a 0.7 V window, has been predicted.12 Capacities ranging
from 44 to 270 mAh g-1 have been reported, varying with the precursor, polymer
59
morphology, synthesis procedure, and sample thickness.78, 79 Two common
polymerization techniques are electrodeposition and chemical synthesis. For
electrochemical polymerization, monomer units are oxidized at potentials that
allow for deposition onto a desired substrate. Chemical synthesis uses oxidants
such as ammonium persulfate (APS) to precipitate PANI from the reaction
solution onto the substrate.80 However, due to volume changes of the material
through ion diffusion into and out of its structure, PANI on its own possesses
poor mechanical stability and thus has reduced cyclability lifetime from charging
and discharging in a supercapacitor device.12
To circumvent this issue, deposition of PANI onto more stable structures,
typically carbons, is frequently pursued for device electrodes. For example, Cai
et al. used chemical polymerization of PANI on templated carbon in APS and
mixed the electroactive material with acetylene black, graphite, and
polytetrafluoroethylene (PTFE) to form slurries that were cast into electrodes.81
However, chemical polymerization takes longer than electrodeposition (hours as
opposed to minutes), and slurry-cast electrodes typically contain more
processing steps and some electrode dead weight in the form of binders. An
electrodeposition example is Cheng et al., who electrodeposited polyaniline onto
graphene/CNT composites through a two-step process, involving nucleation at a
constant potential and growth at a constant current to form PANI nano-cones for
60
a novel graphene/CNT/PANI nanostructured electrode.82 Electrodeposition has
the benefit of high surface adhesion onto the desired substrate. However, thicker
films of PANI made through electrodeposition yield an inhomogeneous
morphology with poorer capacitance and material utilization.83 As such, thin
deposition films are desired for supercapacitor purposes for full utilization of the
deposited material.
Figure 30 – Reduction and oxidation states of polyaniline (PANI) by Innis et
al.84 (Reprinted with permission)
61
4.1.3 Polyaniline-Carbon Composite Electrodes in Supercapacitors
A typical operational window of PANI in aqueous electrolyte can be seen
in Figure 31. The notable redox pairs occur at a relatively positive potential
(Ag/AgCl is -0.205 V with respect to SHE). Against Ag/AgCl, oxidation from
leucoemeraldine to emeraldine occurs around 0.2 V and from emeraldine to
pernigraniline around 0.8 V. Conversely, reduction from pernigraniline to
emeraldine occurs around 0.6 V and from emeraldine to leucoemeraldine around
0 V. It should be noted that the potential of the first pair of redox peaks (0 V and
0.2 V) are more or less independent of pH, while the second pair (in this example
0.6 V and 0.8 V) are will vary with potential based on the electrolyte pH level.
Similarly, the behavior of PANI electrochemically depends on temperature,
electrolyte, and electrode surface area.85 Neither leucoemeraldine nor
pernigraniline is conductive compared to the emeraldine state. Leucoemeraldine
is oxidized easily, allowing the redox reactions to emeraldine to be utilized for
charge storage, while pernigraniline, once formed, is easily degraded80, and can
prohibit long cyclability and capacitance retention in supercapacitor devices.
Further, irreversible degradation reactions of PANI can occur at potentials before
the onset of the pernigraniline redox transition in an aqueous electrolyte system.
62
Figure 31 – Cyclic voltammetry curve of polyaniline in an aqueous HCl (pH 1)
electrolyte with labeled redox couples by Song et al.85 (Reprinted with
permission)
Due to the relatively positive voltage location of the redox peaks, PANI is
commonly used as a positive electrode in supercapacitors, typically paired with
an electrode such as carbon which has a wide negative window in order to
maximize the potential operating window of a PANI-carbon device and thus
maximize energy density. This combination of different electrode types in one
device confers high power from the EDLC electrode and high energy density
from the pseudocapacitive electrode to the full supercapacitor.4 Hung et al. used
a wet chemical mixing method to synthesize PANI nanofibers to drop-deposit on
graphite supports as a positive electrode, combined with reduced graphene
63
oxide as the anode material, to form an asymmetric supercapacitor with an
energy density of 4.86 Wh kg-1 and power density of 8.75 kW kg-1, showing the
high capacitance and power handling capability that comes with pairing a
pseudocapacitive electrode and EDLC electrode.86 The capacitances of the
electrodes were mass balanced such that both electrodes scanned a 0.5 V window
from a PZV of 0.2 V, for a 1 V operational window. The authors state that the
narrow 0.2 V to 0.7 V window is recommended for PANI operation. However,
this misses the reduction to leucoemeraldine state and the corresponding redox
reaction from that transition. The PANI operates completely in the emeraldine
state, and further mass balancing could also increase the total device capacitance
by having the graphene anode expand more into the negative range.
As stated previously, PANI is frequently deposited onto carbon substrates
to compensate for their poor mechanical stability. Substrates like hollow carbon
spheres87, activated carbon fibers88, and graphene nanosheet/multiwalled
nanotube composites89 have been used as capacitive materials that were further
enhanced with the deposition of PANI. Even electrospun polyacrylonitrile (PAN)
has been used as a freestanding flexible substrate for high PANI mass loading
(~60 wt%) in core-shell nanostructures.90 Freestanding electrode structures have
the distinct advantage of not needing binders or supporting materials that do not
contribute to the performance of the electrode (i.e. “dead weight”). The
64
electrospun porous carbon nanofiber (PCNF) electrodes provide a high specific
surface area substrate with through-connected hierarchical pore structure, ideal
for facilitating ion diffusion to and from the electrode surface. For these PCNFs,
galvanostatic deposition of PANI was shown to provide uniform, conformal
coatings that achieve much higher capacitance in this composite form than as-is
PCNFs, up to 366 F g-1 from 140 F g-1.48
Dirican et al. produced similar PANI-coated porous carbon nanofibers
through electrospinning PAN with polymethylmethacrylate (PMMA) sacrificial
polymer which, after similar heat treatments, formed interconnected porous
fibers.91 PANI was chemically polymerized through a 12 hours process, and the
fibers achieved a slightly lower specific capacitance of 296 F g-1 in a three-
electrode device. They report a high voltage window of -0.5 V to 1.6 V for
asymmetric supercapacitors with PANI-PCNF working and PCNF counter
electrodes, but no cyclability data for the asymmetric device. This is likely due to
the excessive electrode potential windows used that are beyond PANI stability.
Further, the CVs show very resistive behavior and no distinct redox peaks at the
expected potentials for PANI.
Herein a continuation of the previous work48 on PANI-electrodeposited
porous carbon nanofibers is reported, moving towards asymmetric
supercapacitor device implementation. A full asymmetric device that operates
65
within entire confines of the potential available to the PANI-PCNF/PCNF is
successfully created that shows remarkable cyclic stability. The issues with a
limited 1.2 V potential window and incomplete PANI-PCNF electrode utilization
are discussed in depth, noting the path towards a vastly improved device.
4.2 Experimental
4.2.1 Carbon Nanofiber Fabrication
Porous carbon nanofibers (PCNFs) were fabricated with the same method
mentioned previously (see section 3.2.1) with the only difference being the
electrospinning solution composition. PAN/Nafion 4:6 at 17 wt% in DMF was
used in lieu of the 3:7 21 wt% composition, for the sole reason of accurately
replicating the previous PANI-PCNF electrode this work builds upon.48
To briefly summarize, PAN/Nafion 4:6 at 17 wt% were electrospun into
non-woven nanofiber mats. Subsequent heat treatments carbonized the PAN and
decomposed out the Nafion in order to create hierarchical porous freestanding
CNFs with high specific surface area and good conductivity.
66
4.2.2 Galvanostatic Deposition of PANI on PCNFs
As stated in the previous work, galvanostatic deposition of aniline onto
the PCNFs was shown to be the superior technique for PANI performance and
surface adhesion48; a near identical synthesis methods will be described here. As-
received aniline monomer solution (Alfa Aesar) was dissolved in 1 M H2SO4 for a
0.5 M aniline solution to create the acidic medium desired for PANI deposition.77
PCNF mats were punched into 3/8” diameter electrodes, both to be used for
PANI deposition and for use as the counter electrode. Galvanostatic
electrochemical polymerization of PANI was carried out using a T-type
Swagelok cell in a three-electrode cell setup, with Ag/AgCl as the reference
electrode and a platinum mesh as the counter electrode; this is shown in Figure
32. A graphite rod of 1/2" diameter was used as the current collector. A 3/8”
electrode was placed between the graphite rod and a thin, porous polymer mesh
separator. The graphite rod-PCNF-mesh was compressed to improve electrode-
current collector contact to reduce contact resistance; additionally, the graphite
rods were leveled and polished before use to further reduce contact resistance.
The platinum mesh counter electrode was inserted at the top of the T-cell. The
reference electrode (Ag/AgCl) was inserted at the other end of the T-cell, in very
close proximity to the substrate to lower solution resistance. The solution
67
resistance was kept below 0.3 Ω in 1 M H2SO4 aqueous electrolyte prior to
galvanostatic deposition to keep the measured potentials accurate and overall
resistance very low. Once low solution resistance was confirmed, the cell was
emptied and refilled with 0.5 M aniline solution. The PCNF was allowed to soak
in the aniline solution in the T-cell for 10 minutes to ensure full electrode
wetting. Galvanostatic deposition was carried out at a constant current of 9 mA
using a potentiostat (Gamry). For our device setup, to replicate the previous
work of ~40 wt% PANI mass in the final electrode, Equation 14 was developed to
determine the desired deposition time to ensure the final electrode achieved ~40
wt% PANI:
(14)
where t is the deposition time and m is the electrode mass in mg. After
deposition, the fibers were washed thoroughly with deionized water to remove
any residual or adsorbed electrolyte. The fibers were then blotted dry with a
sterile wipe and dried at 55°C to remove any excess moisture. The weight of the
electrode was taken before and after deposition and drying in order to confirm
the weight fraction of PANI.
68
Figure 32 – Photo of Swagelok T-cell used in three-electrode testing and
galvanostatic deposition.
4.2.3 Electrochemical and Structural Characterization
The PANI-PCNF electrodes were characterized electrochemically in the T-
cell using electrochemical impedance spectroscopy (EIS) in the frequency range
of 100 kHz to 100 mHz and cyclic voltammetry (CV) at scan rates of 20 and 100
mV s-1 in the voltage window of -0.1 V to 0.65 V in 1 M H2SO4 electrolyte.
Aqueous acidic electrolytes have high ionic conductivity, low cost and
environmental load, and possess H+ ions which are necessary for PANI redox
69
reactions.64 EIS was done prior to CV testing to observe the solution resistance,
charge transfer, and diffusion properties of the system, thus ensuring the redox
observations yielded by the CVs were accurate. The PANI-PCNFs were scanned
at 20 and 100 mV s-1 before and after at 20 mV s-1 scan in a -0.3 V to 0.65 V
window; as PANI is deposited in an unknown ratio of leucoemeraldine,
emeraldine, and pernigraniline states, this larger scan ensured full conversion to
leucoemeraldine at -0.3 V, and brought out the full extent of the
leucoemeraldine-emeraldine redox pair. As expected, after fully reducing PANI,
the subsequent scans in the -0.1 V to 0.65 V window saw these pronounced
peaks. Equations 15 and 16 calculate the composite capacitance, CPANI/C (F g-1):
(15)
∫
(16)
where m, V, and ϑ represent the total mass of the composite, operational
window, and scan rate, respectively; Q is the charge storage. The integral was
calculated using the Quick Integrate function in the Echem Analyst software
(Gamry).
The external morphology of the PANI-PCNF electrodes was characterized
using scanning electron microscopy (SEM, Zeiss Supra 50VP).
70
4.2.4 Asymmetric Device Characterization
The PANI-PCNF electrodes were electrochemically tested in asymmetric
cells in 1 M H2SO4 with PCNF counter electrodes. Both CV and EIS were again
used in characterizing the device. Charge balancing and operational windows of
the device will be discussed more in depth in Section 4.3. Calculations for specific
capacitance (Csp), energy density (Esp), and power density (Psp) of the asymmetric
device are given in the following Equations 17, 18, and 19, respectively:
(17)
(18)
(19)
where Q is the charge (calculated using the quick integrate function in the Gamry
software), m is the mass of both electrodes, V is the total potential window of the
device, and Δt is the time taken for full discharge.
71
4.3 Results and Discussion
4.3.1 Replication of PANI-PCNF Electrodes
The previous work48 had a sufficiently well-defined synthesis and
performance information for PANI-PCNF electrodes to allow for replication and
improvement. The authors note that, in their galvanostatic deposition at 9 mA, a
charge of 5.4 C was generated, which corresponds to ~2.15 mg of deposited PANI
assuming 2.4 electron consumption by each aniline unit. However a higher
amount (~3.0 mg) was measured, indicating the number of electrons consumed
by the aniline units is likely less than 2.4, and thus the galvanostatic method may
possess higher PANI formation efficiency than the potentiostatic method. An
SEM image of the galvanostatic electrodeposited PANI-PCNF electrode is seen in
Figure 33, and the SEM images of the previous work are shown in Figure 34.
Both images show a thin, conformal coating of PANI, indicating uniform,
homogeneous deposition conditions throughout the electrode.
72
Figure 33 – SEM image of PANI-PCNF electrode.
Figure 34 – SEM images of galvanostatic PANI-coated porous carbon
nanofibers. Scale bars are (a) 1 µm and (b) 500 nm.48
73
The three-electrode CV was used to compare composite performance
before and after reduction. As seen in Figure 35, the PANI appears to be
deposited in a mixed redox state. Upon full reduction, the redox peaks
drastically increase, showing elevated charge storage. The specific capacitances
at 20 mV s-1 for initial, reduction, and final scans are 252, 406, and 318 F g-1,
respectively. The PANI-PCNF CV is plotted against the previous work in Figure
36. The reproduced PANI-PCNF electrode has a more pronounced emeraldine-
leucoemeraldine redox pair (A and A’), and a seemingly lower byproduct
generation48 (B and B’). Though these byproducts (likely p-
hydroxydiphenylamine and hydroquinone)92, 93 provide reversible redox reaction
contributions to the overall capacitance, they may have poorer electrode
adhesion and may eventually deteriorate into the electrolyte. Three-electrode EIS
is shown in Figure 37. In the high frequency region, both series resistance (Rs)
and charge transfer resistance (Rct) are less than 1 Ω cm-1 for pre- and post-
reduction samples, showing a Rs of 0.12 and 0.11 Ω and Rct of 0.30 and 0.19 Ω for
pre- and post-reduction, respectively. Series resistance stays essentially constant,
due to the fact that the setup is not altered between these tests. Charge transfer
resistance decreases for the post reduction sample due to the state of the PANI
upon EIS testing, as during galvanostatic deposition PANI is deposited in the 0.6
V to 0.65 V potential range v. Ag/AgCl, implying a possible mixture of PANI
74
states. After reduction the PANI is uniformly oxidized into the more conducting
emeraldine state, improving charge transfer performance.94 The near-vertical
lines along the imaginary axis (-Z”) indicate ideally capacitive behavior of the
PANI-PCNF electrode owing to the rapid redox reactions of PANI, as well as
better ion diffusion in the reduced electrode, likely due to the cycling creating
defined ion pathways and more uniform PANI structure.
Figure 35 – Cycling of PANI-PCNF electrode in a -0.1 V and -0.3 V to 0.65 V
window to show the effect of full emeraldine reduction on performance.
Specific capacitances are 252, 406, and 318 F g-1 for before reduction, reduction,
and after reduction scans, respectively.
75
Figure 36 – Comparison of PANI-PCNFs to the previous work48; similar
specific capacitances are achieved (318 F g-1 and 320 F g-1, respectively), with
more pronounced redox peaks.
Figure 37 – EIS Nyquist plot of the PANI-PCNF electrode in the three-
electrode setup before and after PANI reduction.
76
4.3.2 Asymmetric Device Performance
Asymmetric devices consisted of PANI-PCNF positive electrode and
PCNF counter electrode with a Celgard 3501 separator and 1 M H2SO4
electrolyte. As stated previously, the total device window is determined by the
upper limit of the positive electrode and the lower limit of the negative electrode,
based on the earliest electrolyte or electrode degradation potentials. The PANI-
PCNF window, being the positive electrode, was limited by the transition from
emeraldine to the more easily degraded pernigraniline around 0.7 V vs.
Ag/AgCl; as such, the upper limit of PANI was kept to 0.65 V. The PCNF counter
electrodes were evaluated in an identical three-electrode setup in the negative
window in which they were to operate order to properly balance the charges. A
typical PCNF electrode vs. Ag/AgCl in three-electrode setup is shown in Figure
38. The negative limit for the PAN-based PCNF electrodes is around - 0.6 V.
Beyond that hydrogen evolution occurs in an irreversible degradation reaction.
This onset potential of hydrogen evolution is more positive than other carbons.
Activated graphene nanosheets89 operate comfortably at -0.8V in 1 M H2SO4, and
PICACTIF activated carbon95 can go slightly beyond -1.0 V in 1 M K2SO4. More
negative electrochemical windows can be achieved with Na2SO4, K2SO4, and
KOH aqueous electrolytes due to the larger negative potential required to evolve
77
the cation96, but PANI needs a protonated electrolyte for sustained operation.
This is further inhibited due to the nitrogen and oxygen heteroatoms present
within the CNF structure that push the negative limit to be more positive and
thus reduces the overall accessible window in an asymmetric device. Most of the
nitrogen from the PAN monomer is burned off as nitrogen gas at the highest
carbonization temperatures, but the final carbon fibers still contain a few percent
nitrogen in their composition, along with some fewer oxygen groups.27 Though
these heteroatoms are electrochemically active in the form of contributing a small
pseudocapacitance, and have been shown to improve surface wettability97, they
can act as active sites for unwanted redox reactions. Researchers are looking at
PAN-based carbons (called “nitrogen-doped”) for use in fuel cells as they are
able to compete with Pt-based catalysts due to the nitrogen sites within their
structure.98 The specific capacitance at 100 mV s-1 is 167 F g-1 for the -0.65 V to 0.2
V window. At a negative limit of -0.6 V (excluding the peak from hydrogen
evolution), specific capacitance drops to 159 F g-1.
78
Figure 38 – PCNF three-electrode CV at 100 mV s-1 showing the available
negative potential window available in the asymmetric device.
Additionally, both as-deposited PANI-PCNF and PCNF electrodes are
positive with respect to Ag/AgCl. The open circuit potential (OCP) for the PCNF
electrodes are typically between 0.3 and 0.4 V. Fully reduced PANI-PCNF
electrodes reside at a slightly lower potential of 0.2 V; similar PANI-coated CNTs
in 1 M HCl are 0.484 V vs. Ag/AgCl.72 This is again likely due to the nitrogen
content in the fibers. Nitrogen is more electronegative than carbon, and carbon-
nitrogen bonds are polarized heavily towards the nitrogen, improving fiber
wettability but increasing the surface potential. Goldin et al.99 measured the OCP
of granulated carbons from different sources in various electrolytes against an
Ag/AgCl reference electrode to determine the effect electrolytes had on the OCP.
-600
-500
-400
-300
-200
-100
0
100
200
300
400
-0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3
Sp
ec
ific
Cap
ac
itan
ce
(F
g-1
)
Voltage (V vs. Ag/AgCl)
79
The carbon derived from black coal, fossil coal, and peat are known to possess a
high amount of nitrogen in their base form and ultimately retain some fraction
after processing (decreasing with increasing carbonization temperature).100, 101
Similar to the PCNF fibers, these carbons were shown to have an overall positive
potential vs. Ag/AgCl across the different electrolytes, averaged to 0.132, 0.305,
and 0.360 V for black coal, peat, and fossil coal, respectively. Contrastingly, the
carbons derived from apricot and cherry stones that originally contained lower
fractions of nitrogen (>1 wt%) in their carbonized form with much higher oxygen
content (up to 12.4 wt%)102 resulted in more neutral (cherry, 0.071 V) or negative
(apricot, -0.021 V) OCP values. To the author’s knowledge there are no
publications directly correlating the measured open circuit potential of carbon
electrodes to their surface composition. As such, the stated observations in
publications of hetero-atomic content in carbon electrodes and their effect on the
resultant OCP seem to support the hypothesis of nitrogen and oxygen content
affecting the OCP of the material.
The OCPs act as “neutral” points for both electrodes, and play a pivotal
role in determining the operational window of the asymmetric device. The PZV
is the potential at which both electrodes minimize their potentials with respect to
one another through ion adsorption/desorption. For example, for two electrodes
with identical mass and specific capacitance, if one electrode resides at 0.3 V and
80
the other at 0.4 V vs. Ag/AgCl, then the two electrodes have a 0.1 V potential
between them. When brought together into a supercapacitor device with a
shared electrolyte and discharged fully to 0 V, the two electrodes will
“discharge” with respect to one another. Both will adsorb/desorb ions to bring
their potentials to 0.35 V vs. Ag/AgCl, and thus 0 V with respect to one another.
This can also happen if the supercapacitor is left untouched for a prolonged
period of time. Dai et al.72 explain the role of PZV in both symmetric and
asymmetric supercapacitors, and note this “relaxation” of electrodes through
charge redistribution or interaction with the electrolyte to reach a lower OCP
with respect to one another, and by extension electrode potentials that move
closer to their PZV. The inclusion of both OCP and PZV values is absent in much
of the literature of asymmetric supercapacitor devices; though operational
windows showing good specific capacitance and in some cases capacitance
retention are found, there is little mention of these factors outside the potential
limits of certain electrodes.81, 82, 103
With the currently employed materials, this poses a problem for fulfilling
the prime motivation for employing an asymmetric supercapacitor; maximizing
the voltage window and thus the energy density. As the energy stored by the
capacitor is dependent upon the square of the maximum voltage, the asymmetric
design attempts to capture the full possible operational window based on the
81
upper and lower potential limits for the positive and negative electrodes,
respectively. With the reduced PANI-PCNF electrode at 0.2 V and the PCNF
electrode close to 0.4 V, the combination of the two produce a PZV of around 0.3
V. This limits the PANI-PCNF operational window (ΔE+) to just 0.3 V (~0.3 to 0.6
V vs. Ag/AgCl), and the PCNF window (ΔE-) to 0.9 V (- 0.6 to ~0.3 V vs.
Ag/AgCl). This is beyond the leucoemeraldine-emeraldine redox pair (0.2 V
oxidation, 0 V reduction vs. Ag/AgCl), a major contributor to the high specific
capacitance achieved by the PANI-PCNF electrode. This potential range (0.3 to
0.6 V vs. Ag/AgCl) has been noted to be a within the more stable potential range
for PANI (0.2 to 0.7 V vs. Ag/AgCl) as mentioned previously.86 Both Dai et al.
and Demarconney et al. use this finding of the PZV to maximize their respective
device performances.72, 104
Conditioning of the electrodes, that is, holding both the PANI-PCNF and
PCNF electrodes at a negative potential for a prolonged period of time in the 1 M
H2SO4 electrolyte, was attempted to lower the PZV. This would allow for an
increase in ΔE+, and as PANI-PCNF is the electrode contributing the larger
capacitance, the more of the device window it contributes, the higher the
capacitance of the asymmetric device as a whole. Both PANI-PCNF and PCNF
electrodes were held at -0.3 V vs. Ag/AgCl for over 15 minutes against another
PCNF counter electrode to enable full PANI reduction to leucoemeraldine before
82
being combined in the Swagelok cell for asymmetric testing. The lowered PZVs
can be seen in Table 5. Though the conditioning produces lower initial PZVs, the
value generally shifts back to a more positive potential (~0.3 V vs. Ag/AgCl)
during device operation due to the eventual “relaxation” from the forced
reduction by the conditioning. The cycling at higher potentials oxidizes any
latent leucoemeraldine leftover from the conditioning, resulting in a higher
specific capacitance early on that decays with charging and discharging,
resulting in the PZV shift. Recalling the charge balance equation, if the masses
are balanced based on the specific capacitances and the expected voltage
windows of each electrode, and one electrode’s capacitance decreases, the
voltage window it operates in must increase to keep the charges balanced.
Hence, attempting to maximize the operational window of each electrode around
a PZV that gradually increases can lead to the exceeding of the upper potential
limit and thus impact long-term stability. This can be observed in initial
experiments constructed around these lower PZV values such as those shown in
Figure 39 and Figure 40. The PZV shifts from 0.15 V to 0.28 V as seen in Figure
39, causing the working electrode window to shift from 0.16 - 0.65 V to 0.28 - 0.72
V, and the counter electrode window to shift from (-0.55) – 0.15 V to (-0.48) – 0.28
V. The peak seen at around 0.2 V in cycle 2 corresponds to the oxidation of latent
leucoemeraldine, as the PZV is low enough to allow this oxidation to occur; this
83
results in a very high 81.4 F g-1 specific capacitance for the device at 20 mV s-1. By
cycle 100, however, the PZV has shifted considerably to 0.28 V. The only
available redox reaction of the by-products at 0.5 V is seen, as the
leucoemeraldine-emeraldine redox reaction is now outside the PANI-PCNF
window. The device still achieves a 75.9 F g-1 specific capacitance, with 93%
capacitance retention from the second cycle. The effect on the asymmetric device
as a whole is seen in Figure 40, with the pronounced by-products peak after PZV
shifting.
Table 5 – Measured charged and discharged potentials of electrodes in the
asymmetric supercapacitor. Potentials were recorded by hand using a
multimeter and Ag/AgCl reference electrode within the voltage windows.
Working Electrode
(PANI-PCNF)
Counter Electrode
(PCNF)
Operating
Window
PZV
Charged
Potential
(V vs.
Ag/AgCl)
Discharged
Potential (V
vs.
Ag/AgCl)
Charged
Potential
(V vs.
Ag/AgCl)
Discharged
Potential (V
vs.
Ag/AgCl)
Difference
in charged
potentials
(V)
Average of
Discharged
Potentials
(V vs.
Ag/AgCl)
0.579 0.161 -0.505 0.172 1.1 0.167
0.649 0.234 -0.548 0.271 1.2 0.235
0.628 0.19 -0.562 0.2 1.2 0.195
0.641 0.119 -0.519 0.137 1.2 0.128
0.616 0.232 -0.575 0.205 1.2 0.219
0.58 0.22 -0.58 0.225 1.2 0.223
0.625 0.28 -0.55 0.28 1.2 0.28
0.64 0.22 -0.55 0.25 1.2 0.235
84
Figure 39 – Individual electrode CVs of an asymmetric cell cycled to 1.2 V. The
shifting of the PZV causes increased production of degradation products that
limits long-term cyclability.
Figure 40 – Full asymmetric CV of the electrodes shown in Figure 39. The PZV
shifts from 0.15 to 0.28 V vs. Ag/AgCl, causing the PANI-PCNF upper potential
limit to shift from 0.65 V to 0.72 V vs. Ag/AgCl. This precludes the use of the
emeraldine-leucoemeraldine redox pair for capacitance contribution.
85
Additionally, it should be noted that though the electrodes were held for
over 15 minutes at these lower potentials, in the time it takes to remove the
electrodes from their conditioning setups and place them in the asymmetric
device, their potential invariably rapidly rises to 0.1 – 0.2 V vs. Ag/AgCl, further
indicating the instability of the conditioned state.
Herein the complexities of the current system are laid bare; 1) the specific
capacitance of the PANI-PCNF electrode depends greatly upon the potential
window in which it operates within due to the dependence of redox pairs on the
experienced potential, and 2) the PZV of the device resides in a positive region
that precludes the emeraldine-leucoemeraldine redox pair during asymmetric
testing due to the positive potential of the materials used.
A 1.2 V window for the asymmetric device was chosen to fit the positive
0.6 V limit of PANI for degradation at high voltages and the -0.6 V limit of
carbon due to hydrogen evolution. From prior experiments a PZV of 0.3 V was
used, with ΔE+ = 0.3 V (for a PANI-PCNF window of 0.3 to 0.6 V vs. Ag/AgCl)
and ΔE- = 0.9 V (for a PCNF window of -0.6 to 0.3 V). Specific capacitances of
both electrode materials are determined through three-electrode testing in their
operational windows; PCNF typically achieved 140 – 160 F g-1 at 100 mV s-1, and
PANI-PCNF achieved 280 – 350 F g-1 at 100 mV s-1. The total mass of the PANI-
PCNF electrode was normally > 2.5 mg for the 3/8” diameter electrode;
86
galvanostatic deposition targeted ~40 wt% PANI for the hybrid electrode.
Knowing the specific capacitances (Csp), operational windows (V), and the mass
of the PANI-PCNF electrode (mp), the mass of the PCNF counter electrode (mn)
could be determined through Equation 20:
(20)
Total active material exceeds 4 mg for the device, with only the Celgard 3501
separator contributing non-active material weight.
EIS of the asymmetric device is shown in Figure 41, before and after a 1000
cycle stability test. In the high frequency region, series resistance (Rs) shows a
slight increase from 0.136 Ω to 0.155 Ω, indicating a minimal change in electrode
conductivity with cycling. Charge transfer resistance (Rct) is similarly very low,
with nearly identical Rs measured before and after cycling (0.031 Ω to 0.024 Ω)
based on equivalent circuit curve fitting. Diffusion is high within the low
frequency region, indicating good electrolyte transport of ions.
The CVs of the asymmetric cell are shown in Figure 42 for scan rates of 50,
100, and 200 mV s-1. The PANI-PCNF maximum voltage was 0.61 V, and the
PCNF minimum voltage was -0.59 V vs. Ag/AgCl, showing excellent adhesion to
the desired operating windows for the two electrodes. Specific capacitance
values, based on the mass of both electrodes, are 59.0, 49.5, and 44.4 F g-1,
respectively. Though not as high of a specific capacitance as the aforementioned
87
81.4 F g-1 (at 20 mV s-1), the device is very stable, as seen in the cyclability plot in
Figure 43, wherein a 90% retention of capacitance was maintained over 1000
cycles at 5 A g-1 in the 1.2 V window. Comparatively, the 81.4 F g-1 dropped to
75.9 F g-1, 93% of its original specific capacitance, after only 100 cycles, indicating
a tradeoff between long-term cyclability and high specific capacitance for this
asymmetric system. As will be shown later, even higher specific capacitance
values can be achieved.
Figure 41 – Nyquist EIS of the PANI-PCNF/PCNF asymmetric supercapacitor
before and after a 1000 cycle stability test.
88
Figure 42 – CV of asymmetric PANI-PCNF/PCNF device at 50, 100, and 200 mV
s-1; specific capacitance values are 59.0, 49.5, and 44.4 F g-1 respectively.
Figure 43 – Cyclability plot of the asymmetric supercapacitor device at 5 A g-1
in a 1.2 V window.
89
A 1.2 V charge-discharge test at different current densities is shown in
Figure 44. Specific capacitance values calculated at 1, 2, 5, and 10 A g-1 were 57,
47, 40, and 34 F g-1 respectively; the 1 A g-1 value corresponds to an energy
density of 11.4 Wh kg-1 and power density of 596 W kg-1, whereas the 10 A g-1
corresponds to 6.8 Wh kg-1 and 6.03 kW kg-1, based on the total mass of the
electrodes. The pronounced time to reach 1.2 V for the lower current density of 1
A g-1 could be indicative of an adverse reaction that is more pronounced near the
electrode potential limits, such as the onset of either PANI decomposition or
electrolyte degradation at PCNF electrode. As the mass balancing to fully reach
each electrode’s limits is very sensitive, small perturbations could cause the
behavior observed here, and are an additional difficulty in implementing such
asymmetric systems. As such, lower current density and scan rate operation may
further decrease the acceptable potential window of the supercapacitor to avoid
the reactions that occur at the limits and thus maintain high cyclability.
90
Figure 44 – Charge-discharge test of the asymmetric supercapacitor device at 1,
2, 5, and 10 A g-1.
4.3.3 Expanded Asymmetric Window
The effects of expanding beyond the 1.2 V operational window with the
asymmetric device were explored, specifically for 1.4 and 1.6 V windows. This
was done through increasing the PANI-PCNF operational window while
allowing the PCNF electrode to stay within its limit. As the degradation of the
electrolyte was known, this allowed the testing of the reversibility of PANI
pushed beyond its regular limits, and to see if a higher operational window can
be achieved for the device without sacrificing much in the way of cyclability. CV
curves of 1.2, 1.4, and 1.6 V windows at 100 mV s-1 are shown in Figure 45, with
91
the corresponding calculated specific capacitance values being 49.5, 54.8, and
72.8 F g-1. Table 6 shows the measured potentials of each electrode at the fully
charged state in the asymmetric device.
Figure 45 – Asymmetric device scanned at 100 mV s-1 to 1.2, 1.4, and 1.6 V.
Table 6 – Potential limits of the PANI-PCNF and PCNF electrodes in the
expanded voltage window asymmetric device.
Working Electrode
(PANI-PCNF) Upper
Potential (V vs. Ag/AgCl)
Counter Electrode
(PCNF) Lower Potential
(V vs. Ag/AgCl)
Operating
Window
(V)
0.61 -0.59 1.2
0.80 -0.63 1.4
0.93 -0.65 1.6
92
Figure 46 and Figure 47 show cyclability data for 1.4 and 1.6 V windows,
respectively, at a 5 A g-1 current density. Even at a high scan rate where
degradation mechanisms have little time to manifest, significant drops in specific
capacitance are observed, with the 1.4 V window retaining 77% of its original
capacitance after 1000 cycles, and the 1.6 V window retaining even less, only 70%
of its original capacitance. This is indicative of the significant irreversibility of
PANI brought on by exceeding its upper potential limit and inducing
degradation and pernigraniline formation.
Figure 46 – Cyclic charge discharge test measuring capacitance retention at 5 A
g-1 current density for a 1.4 V window, retaining 77% of original capacitance
after 1000 cycles.
93
Figure 47 – Cyclic charge discharge test measuring capacitance retention under
a 5 A g-1 current density for a 1.6 V window, retaining 70% of original
capacitance after 1000 cycles.
4.3.4 Negative Potential Regime for the Asymmetric Device
The limited potential window of the PANI-PCNF in this asymmetric
device does not allow for the leucoemeraldine-emeraldine redox reaction to
occur, robbing the electrode of contributing more capacitance to the system. As
stated previously, this is due to the high PZV of around 0.2 to 0.3 V vs. Ag/AgCl
of the asymmetric device. When operating the asymmetric device in a standard
positive window (i.e. 0 to 1.2 V), the PANI-PCNF electrode only transitions from
the PZV to ~0.65 V vs. Ag/AgCl when the electrodes are brought to 1.2 V with
respect to one another, while the PCNF counter electrode moves from the PZV to
94
-0.6 V vs. Ag/AgCl at the same 1.2 V. With the OCP of the PANI-PCNF electrode
at 0.2 V vs. Ag/AgCl, the use of a more negative counter electrode has the
potential to make the PZV stable at 0 V vs. Ag/AgCl or lower, allowing the
leucoemeraldine-emeraldine redox reaction to occur during normal asymmetric
device operation. Activated non-PAN-based porous carbons, while having a
larger negative electrochemical window (to -1.0 V in some systems) and thus
higher energy density in asymmetric devices, have OCP values are typically
positive, and thus would not shift the PZV in the negative direction.99
The behavior of such a low-PZV asymmetric device can be ascertained by
cycling the device into a negative potential window. Figure 48 shows the CVs of
an expanded -0.8 to 1.2 V window compared to the previously shown 1.2 V
window at a scan rate of 100 mV s-1; similar CVs for 1.4 V and 1.6 V windows are
shown in Figure 49 and Figure 50, respectively. Specific capacitance for the 1.2 V
potential increased from 49.5 F g-1 to 67.3 F g-1, a 36% increase; the 1.4 V potential
increased in specific capacitance from 54.5 F g-1 to 72.8 F g-1, a 33% increase; and
the 1.6 V potential increased in specific capacitance from 58.9 F g-1 to 76.1 F g-1, a
29% increase. In these expanded potentials the emeraldine-leucoemeraldine
oxidation and reduction peaks are clearly visible around -0.2 and -0.5 V,
respectively. The measured electrode potentials at the upper (1.2, 1.4, and 1.6 V)
and lower (-0.8 V) limits are shown in Table 7. Note that the drop to -0.8 V
95
pushes the PANI-PCNF working electrode down to -0.1 V vs. Ag/AgCl, low
enough for the emeraldine-leucoemeraldine reduction reaction (~0 V vs.
Ag/AgCl) to occur in earnest; similarly, the oxidation peak (~0.2 V vs. Ag/AgCl)
is also observed. The PCNF counter electrode, stable up to 0.8 V vs. Ag/AgCl in
the positive region, stays within this boundary across full charge and discharge.
Figure 48 – CV comparison of an expanded asymmetric window with the
stable 1.2 V window. The expanded window is brought to a potential of -0.8 V
and 1.2 V, for a 2 V total window.
96
Figure 49 – CV comparison of an expanded asymmetric window with the 1.4 V
window. The expanded window is brought to a potential of -0.8 V and 1.4 V,
for a 2.2 V total window.
Figure 50 – CV comparison of an expanded asymmetric window with the 1.6 V
window. The expanded window is brought to a potential of -0.8 V and 1.6 V,
for a 2.4 V total window.
97
Table 7 – Measured electrode potentials at the positive (1.2, 1.4, and 1.6 V) and
negative (-0.8 V) potential limits.
Working Electrode
(PANI-PCNF)
Counter Electrode
(PCNF)
Operating
Window
Upper
limit
potential
(V vs.
Ag/AgCl)
Lower limit
potential (V
vs.
Ag/AgCl)
Upper
limit
potential
(V vs.
Ag/AgCl)
Lower limit
potential (V
vs.
Ag/AgCl)
Upper and
lower limit
(V)
0.61 -0.10 -0.59 0.60 -0.8 to 1.2
0.76 -0.12 -0.63 0.65 -0.8 to 1.4
0.89 -0.14 -0.68 0.68 -0.8 to 1.6
Cycling to -0.8 V simply causes an overlap of the PCNF and PANI-PCNF
windows, and though the magnitude of the total potential difference increases,
the working potential used to calculate energy density and power density does
not reflect this higher potential; it simply serves as a means of showing the
promising performance that can be achieved. A comparison of all expanded
window devices is seen in Figure 51. Using the specific capacitance values of 67.3
F g-1, 72.8 F g-1, and 76.1 F g-1 for the 1.2, 1.4, and 1.6 V expanded windows,
respectively, energy densities of 13.46, 19.81, and 27.10 Wh kg-1 can be achieved
assuming those potentials, and can be further increased with a new negative
electrode material. The asymmetric device was cycled over the 2 V window (-0.8
to 1.2 V) for 1000 cycles at a 5 A g-1 scan rate and maintained 81% capacitance
retention, as shown in Figure 52, showing good retention even over the
98
expanded electrode operational windows. This decrease as compared to the 1.2 V
retention of 90% could be echoing the departure from the 0.2 to 0.7 V stable range
noted by Hung et al.86 The increased mechanical strain caused by the increased
proton-nitrogen bond formation from the emeraldine to leucoemeraldine
transition is the likely cause of this sharper drop in retained capacitance.
Figure 51 – CVs of the asymmetric device taken from -0.8 V to 1.2, 1.4, and 1.6
V at 100 mV s-1.
99
Figure 52 - Charge-discharge of the asymmetric device in a -0.8 to 1.2 V
window for 1000 cycles.
4.4 Conclusions
The PANI-PCNF/PCNF asymmetric supercapacitor presented
possesses a significant degree of potential for further improvement as an energy
storage device. Deposition on A-PCNF fibers remains an easy next step for
further increasing the capacitance of the hybrid electrode, and implementation as
a negative electrode should further improve specific capacitance of the device.
Issues with lowering of the PZV remain unresolved, but new counter electrode
materials such as MoO3 nanobelts can be used to both pull down the PZV to
allow access to the PANI-PCNF emeraldine-leucoemeraldine redox pair as well
100
as extend further negative than the current PCNF counter electrode (to -0.8 V
from -0.6 V vs. Ag/AgCl).105 A high specific capacitance of 81.4 F g-1 at a 20 mV s-1
scan rate (Esp = 16 Wh kg-1, Psp = 271 W kg-1) at a 1.2 V window has been
demonstrated, but higher energy densities (27.10 Wh kg-1 for 1.6 V at 100 mV s-1)
and higher power densities (1.39 kW kg-1 for 1.6 V at 100 mV s-1) have been
shown. With the understanding of asymmetric devices and the PANI-PCNF
electrode described in this work, future efforts in lowering the PZV and
expanding the negative electrode window can further improve device
performance. Other avenues of improvement, such as the use of protic ionic
liquids as the electrolyte, can drastically increase the operating potential (up to 4
V)82 of the device and greatly increase the energy density.
101
CHAPTER 5: Conclusions and Recommendations
The porous electrospun carbon nanofibers provided excellent
supercapacitor device performance in both the solid-state ionic liquid electrolyte
and the aqueous asymmetric device. The freestanding electrodes eliminated the
need for binders and other additives to allow for high active mass loading
through complete utilization of the nanofiber material. The high specific surface
area and hierarchical porous structure allowed for fast ion adsorption and
desorption kinetics and high specific capacitance in both RTIL and aqueous
electrolytes.
The new silica-based solid state electrolyte supercapacitor device achieved
a high 3.5 V window that, when combined with the high specific capacitance
carbon nanofiber, achieved a high energy density of 61 Wh kg-1. High active
material mass loading ensured low resistivity and excellent charge transfer
conductivity. The near rectangular cyclic voltammetry curves at 100 mV s-1
demonstrated the high power handling capability of the device. Areal
capacitance shows improved energy storage per unit electrode surface area with
thicker electrode mats, providing the device with a degree of scalability. Future
work can incorporate new ionic liquid electrolytes into the silica gel design for
102
more fine-tuned electrode-electrolyte matching, and could be used in asymmetric
pseudocapacitive devices as well.
The PANI-PCNF/PCNF asymmetric supercapacitor utilized the full
electrochemical window available to its electrode materials. A high 318 F g-1
specific capacitance of the electrodeposited polyaniline-porous carbon nanofiber
composite demonstrated excellent combinative performance of EDLC and
pseudocapacitance energy storage mechanisms, and when fully reduced
produced a very high 406 F g-1 specific capacitance. The asymmetric device was
designed to operate within a 1.2 V window, maintaining excellent capacitance
retention of 90% over 1000 cycles. The fundamental components of asymmetric
design were discussed in detail within the scope of the PANI-PCNF/PCNF
device, notably the balance between performance and cyclability. Potential
solutions to the positive PZV were explored, and expanding into a negative
window was used to show the expected improvement in performance that comes
with a lower PZV. A high specific capacitance of 81.4 F g-1 at a 20 mV s-1 scan rate
(Esp = 16 Wh kg-1, Psp = 271 W kg-1) at a 1.2 V window has been demonstrated, but
higher energy densities (27.10 Wh kg-1 for 1.6 V at 100 mV s-1) and higher power
densities (1.39 kW kg-1 for 1.6 V at 100 mV s-1) have been shown. Improvements
through the use of a new electrolyte, such as a protic ionic liquid, and an
alternate negative electrode material are proposed, and future work in these
103
directions is expected to drastically improve asymmetric supercapacitor energy
density and overall performance.
104
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