+ All Categories
Home > Documents > Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment...

Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment...

Date post: 09-Jul-2020
Category:
Upload: others
View: 0 times
Download: 0 times
Share this document with a friend
127
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
Transcript
Page 1: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 2: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

© Copyright 2016

Daniel W. Lawrence. All Rights Reserved

Page 3: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

i

Dedications

To my parents, Christopher and Marie Lawrence

AND

To my younger brother, Ross Lawrence

Page 4: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 5: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 6: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 7: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 8: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 9: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 10: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 11: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 12: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 13: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 14: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 15: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell
Page 16: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 17: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 18: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 19: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 20: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 21: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 22: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 23: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 24: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 25: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 26: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 27: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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,

Page 28: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 29: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 30: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 31: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 32: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 33: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 34: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 35: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 36: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 37: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 38: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 39: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 40: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 41: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

26

Figure 7 – Absorption and desorption isotherms for PCNF and A-PCNF.

Figure 8 – Pore size distribution for PCNF and A-PCNF.

Page 42: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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,

Page 43: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 44: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 45: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 46: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 47: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 48: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 49: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 50: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 51: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 52: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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).

Page 53: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 54: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 55: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 56: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 57: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 58: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 59: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 60: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 61: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 62: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 63: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 64: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 65: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 66: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 67: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 68: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 69: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 70: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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:

Page 71: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 72: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 73: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 74: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 75: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 76: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 77: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 78: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 79: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 80: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 81: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 82: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 83: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 84: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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).

Page 85: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 86: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 87: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 88: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 89: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 90: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 91: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 92: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 93: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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)

Page 94: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 95: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 96: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 97: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 98: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 99: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 100: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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;

Page 101: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 102: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 103: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 104: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 105: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 106: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 107: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 108: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 109: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 110: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 111: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 112: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 113: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 114: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 115: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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.

Page 116: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 117: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

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

Page 118: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

103

directions is expected to drastically improve asymmetric supercapacitor energy

density and overall performance.

Page 119: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

104

List of References

1 B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and

Technological Applications, Kluwer Academic/Plenum Publishers, 223

Spring Street, New York, N.Y. 10013, 1999.

2 H. I. Becker, Low volage electrolytic capacitor, Google Patents, 1957.

3 T. C. Murphy. and R. B. Wright, Symposium on Electrochemical Capacitors II,

1997.

4 X. X. Guihua Yu, Lijia Pan, Zhenan Bao, Yi Cui, Nano Energy, 2013, 2, 213.

5 L. L. Zhang and X. S. Zhao, Chem. Soc. Rev., 2009, 38, 2520.

6 P. Simon and Y. Gogotsi, Nat. Mater., 2008, 7, 845.

7 E. Frackowiak and F. Beguin, Carbon, 2001, 39, 937.

8 M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An and R. S. Ruoff, Nano Lett.,

2008, 8, 3498.

9 V. Presser, M. Heon and Y. Gogotsi, Adv. Funct. Mater., 2011, 21, 810.

10 Z. S. Wu, D. W. Wang, W. Ren, J. Zhao, G. Zhou, F. Li and H. M. Cheng,

Adv. Funct. Mater., 2010, 20, 3595.

11 L. B. Hu, W. Chen, X. Xie, N. A. Liu, Y. Yang, H. Wu, Y. Yao, M. Pasta, H.

N. Alshareef and Y. Cui, ACS Nano, 2011, 5, 8904.

12 G. A. Snook, P. Kao and A. S. Best, J. Power Sources, 2011, 196, 1.

13 Y. L. Xu, J. Wang, W. Sun and S. H. Wang, J. Power Sources, 2006, 159, 370.

14 Y. Wang, J. Guo, T. F. Wang, J. F. Shao, D. Wang and Y. W. Yang,

Nanomaterials, 2015, 5, 1667.

Page 120: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

105

15 R. Kotz and M. Carlen, Electrochim. Acta, 2000, 45, 2483.

16 C. Zhong, Y. D. Deng, W. B. Hu, J. L. Qiao, L. Zhang and J. J. Zhang, Chem.

Soc. Rev., 2015, 44, 7484.

17 G. P. Wang, L. Zhang and J. J. Zhang, Chem. Soc. Rev., 2012, 41, 797.

18 H. Jiang, P. S. Lee and C. Z. Li, Energy Environ. Sci., 2013, 6, 41.

19 C. Tran and V. Kalra, J. Power Sources, 2013, 235, 289.

20 C. Tran, D. Lawrence, F. W. Richey, C. Dillard, Y. A. Elabd and V. Kalra,

Chem. Commun., 2015, 51, 13760.

21 J. Zeleny, Physical Review, 1914, 3, 69.

22 C. J. Angammana and S. H. Jayaram, Particulate Science and Technology,

2016, 34, 72.

23 G. Taylor, Proceedings of the Royal Society of London A: Mathematical, Physical

and Engineering Sciences, 1964, 280, 383.

24 W. E. Teo and S. Ramakrishna, Nanotechnology, 2006, 17, R89.

25 C. Huang, H. T. Niu, J. L. Wu, Q. F. Ke, X. M. Mo and T. Lin, J. Nanomater.,

2012, DOI: 10.1155/2012/473872, 7.

26 J. Zeng, Int. J. Mol. Sci., 2011, 12, 1633.

27 E. Fitzer, W. Frohs and M. Heine, Carbon, 1986, 24, 387.

28 J. C. Wang and S. Kaskel, J. Mater. Chem., 2012, 22, 23710.

29 S. S. A. Hashmi, J. Solid State Electrochem., 2014, 18, 465.

30 Sellam and S. A. Hashmi, Appl. Mater. Interfaces, 2013, 5, 3875.

31 M. Armand, F. Endres, D. R. MacFarlane, H. Ohno and B. Scrosati, Nat.

Mater., 2009, 8, 621.

Page 121: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

106

32 R. Y. Lin, P. L. Taberna, S. Fantini, V. Presser, C. R. Perez, F. Malbosc, N.

L. Rupesinghe, K. B. K. Teo, Y. Gogotsi and P. Simon, J. Phys. Chem. Lett.,

2011, 2, 2396.

33 E. Frackowiak, K. Metenier, V. Bertagna and F. Beguin, Appl. Phys. Lett.,

2000, 77, 2421.

34 C. Kim, Y. I. Jeong, B. T. N. Ngoc, K. S. Yang, M. Kojima, Y. A. Kim, M.

Endo and J. W. Lee, Small, 2007, 3, 91.

35 D. Carriazo, F. Pico, M. C. Gutierrez, F. Rubio, J. M. Rojo and F. del Monte,

J. Mater. Chem., 2010, 20, 773.

36 Y. W. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. W. Cai, P. J. Ferreira,

A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach

and R. S. Ruoff, Science, 2011, 332, 1537.

37 H. J. Fei, C. Y. Yang, H. Bao and G. C. Wang, J. Power Sources, 2014, 266,

488.

38 X. H. Liu, Z. B. Wen, D. B. Wu, H. L. Wang, J. H. Yang and Q. G. Wang, J.

Mater. Chem. A, 2014, 2, 11569.

39 Y. X. Xu, Z. Y. Lin, X. Q. Huang, Y. Liu, Y. Huang and X. F. Duan, ACS

Nano, 2013, 7, 4042.

40 J. Lee, W. Kim and W. Kim, Appl. Mater. Interfaces, 2014, 6, 13578.

41 Y. J. Kang, S. J. Chun, S. S. Lee, B. Y. Kim, J. H. Kim, H. Chung, S. Y. Lee

and W. Kim, ACS Nano, 2012, 6, 6400.

42 Y. J. Kang, H. Chung, C. H. Han and W. Kim, Nanotechnol., 2012, 23.

43 X. Yang, L. Zhang, F. Zhang, T. F. Zhang, Y. Huang and Y. S. Chen,

Carbon, 2014, 72, 381.

44 M. D. Stoller and R. S. Ruoff, Energy Environ. Sci., 2010, 3, 1294.

45 J. J. Zhou, J. S. Cai, S. R. Cai, X. Y. Zhou and A. N. Mansour, J. Power

Sources, 2011, 196, 10479.

Page 122: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

107

46 G. P. Pandey, A. C. Rastogi and C. R. Westgate, J. Power Sources, 2014, 245,

857.

47 R. Singhal and V. Kalra, J. Mater. Chem. A, 2015, 3, 377.

48 C. Tran, R. Singhal, D. Lawrence and V. Kalra, J. Power Sources, 2015, 293,

373.

49 F. W. Richey, C. Tran, V. Kalra and Y. A. Elabd, J. Phys. Chem. C, 2014, 118,

21846.

50 D. W. Lawrence, C. Tran, A. T. Mallajoysula, S. K. Doorn, A. Mohite, G.

Gupta and V. Kalra, J. Mater. Chem. A, 2016, 4, 160.

51 C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of

Sol-Gel Processing, Elsevier Science, 2013.

52 S. Lowell and J. E. Shields, Powder Surface Area and Porosity, Springer

Netherlands, 2013.

53 H. M. Lee, H. G. Kim, S. J. Kang, S. J. Park, K. H. An and B. J. Kim, J. Ind.

Eng. Chem., 2015, 21, 736.

54 G. Gupta, S. B. Rathod, K. W. Staggs, L. K. Ista, K. A. Oucherif, P. B.

Atanassov, M. S. Tartis, G. A. Montano and G. P. Lopez, Langmuir, 2009,

25, 13322.

55 J. E. B. Randles, Discussions of the Faraday Society, 1947, 1, 11.

56 L. S. Garca-Coln, L. F. del Castillo and P. Goldstein, Physical Review B,

1989, 40, 7040.

57 J. D. Kozlowski, C. S. Byington, A. K. Garga, M. J. Watson and T. A. Hay,

2001.

58 S. J. Zhang, N. Sun, X. Z. He, X. M. Lu and X. P. Zhang, J. Phys. Chem. Ref.

Data, 2006, 35, 1475.

59 Z. Tang, C. H. Tang and H. Gong, Adv. Funct. Mater., 2012, 22, 1272.

Page 123: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

108

60 H. L. Wang, Y. Y. Liang, T. Mirfakhrai, Z. Chen, H. S. Casalongue and H.

J. Dai, Nano Res., 2011, 4, 729.

61 T. W. Lin, C. S. Dai and K. C. Hung, Sci Rep, 2014, 4, 10.

62 F. X. Wang, S. Y. Xiao, Y. Y. Hou, C. L. Hu, L. L. Liu and Y. P. Wu, RSC

Adv., 2013, 3, 13059.

63 Z. J. Fan, J. Yan, T. Wei, L. J. Zhi, G. Q. Ning, T. Y. Li and F. Wei, Adv.

Funct. Mater., 2011, 21, 2366.

64 K. C. Ng, S. W. Zhang, C. Peng and G. Z. Chen, J. Electrochem. Soc., 2009,

156, A846.

65 M. Mastragostino, C. Arbizzani and F. Soavi, Solid State Ion., 2002, 148,

493.

66 A. Laforgue, P. Simon, J. F. Fauvarque, J. F. Sarrau and P. Lailler, J.

Electrochem. Soc., 2001, 148, A1130.

67 A. Laforgue, P. Simon, J. F. Fauvarque, M. Mastragostino, F. Soavi, J. F.

Sarrau, P. Lailler, M. Conte, E. Rossi and S. Saguatti, J. Electrochem. Soc.,

2003, 150, A645.

68 C. Arbizzani, M. Mastragostino and F. Soavi, J. Power Sources, 2001, 100,

164.

69 M. Selvakumar and S. Pitchumani, Korean J. Chem. Eng., 2010, 27, 977.

70 C. Peng, S. W. Zhang, X. H. Zhou and G. Z. Chen, Energy Environ. Sci.,

2010, 3, 1499.

71 S. Vaquero, J. Palma, M. Anderson and R. Marcilla, Int. J. Electrochem. Sci.,

2013, 8, 10293.

72 Z. X. Dai, C. Peng, J. H. Chae, K. C. Ng and G. Z. Chen, Sci Rep, 2015, 5, 8.

73 K. L. Van Aken, M. Beidaghi and Y. Gogotsi, Angew. Chem.-Int. Edit., 2015,

54, 4806.

Page 124: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

109

74 K. S. Ryu, K. M. Kim, N. G. Park, Y. J. Park and S. H. Chang, J. Power

Sources, 2002, 103, 305.

75 J. Stejskal and R. G. Gilbert, Pure Appl. Chem., 2002, 74, 857.

76 L. J. Pan, H. Qiu, C. M. Dou, Y. Li, L. Pu, J. B. Xu and Y. Shi, Int. J. Mol.

Sci., 2010, 11, 2636.

77 P. Xu, X. J. Han, B. Zhang, Y. C. Du and H. L. Wang, Chem. Soc. Rev., 2014,

43, 1349.

78 S. R. Sivakkumar and R. Saraswathi, J. Power Sources, 2004, 137, 322.

79 P. Novak, K. Muller, K. S. V. Santhanam and O. Haas, Chem. Rev., 1997, 97,

207.

80 K. M. Molapo, P. M. Ndangili, R. F. Ajayi, G. Mbambisa, S. M. Mailu, N.

Njomo, M. Masikini, P. Baker and E. I. Iwuoha, Int. J. Electrochem. Sci.,

2012, 7, 11859.

81 J. J. Cai, L. B. Kong, J. Zhang, Y. C. Luo and L. Kang, Chin. Chem. Lett.,

2010, 21, 1509.

82 Q. Cheng, J. Tang, N. Shinya and L. C. Qin, J. Power Sources, 2013, 241, 423.

83 C. M. Carlin, L. J. Kepley and A. J. Bard, J. Electrochem. Soc., 1985, 132, 353.

84 P. C. Innis, J. Mazurkiewicz, T. Nguyen, G. G. Wallace and D. R.

MacFarlane, Curr. Appl. Phys., 2004, 4, 389.

85 E. Song and J. W. Choi, Nanomaterials, 2013, 3, 498.

86 P. J. Hung, K. H. Chang, Y. F. Lee, C. C. Hu and K. M. Lin, Electrochim.

Acta, 2010, 55, 6015.

87 Z. B. Lei, Z. W. Chen and X. S. Zhao, J. Phys. Chem. C, 2010, 114, 19867.

88 D. Salinas-Torres, J. M. Sieben, D. Lozano-Castello, D. Cazorla-Amoros

and E. Morallon, Electrochim. Acta, 2013, 89, 326.

Page 125: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

110

89 J. L. Shen, C. Y. Yang, X. W. Li and G. C. Wang, ACS Appl. Mater.

Interfaces, 2013, 5, 8467.

90 F. J. Miao, C. L. Shao, X. H. Li, N. Lu, K. X. Wang, X. Zhang and Y. C. Liu,

Electrochim. Acta, 2015, 176, 293.

91 M. Dirican, M. Yanilmaz and X. W. Zhang, RSC Adv., 2014, 4, 59427.

92 H. Yang and A. J. Bard, Journal of Electroanalytical Chemistry, 1992, 339, 423.

93 L. Duić, Z. Mandić and S. Kovač, Electrochim. Acta, 1995, 40, 1681.

94 W. W. Focke, G. E. Wnek and Y. Wei, J. Phys. Chem., 1987, 91, 5813.

95 T. Brousse, P. L. Taberna, O. Crosnier, R. Dugas, P. Guillemet, Y.

Scudeller, Y. Zhou, F. Favier, D. Belanger and P. Simon, J. Power Sources,

2007, 173, 633.

96 L. Demarconnay, E. Raymundo-Piñero and F. Béguin, Electrochemistry

Communications, 2010, 12, 1275.

97 Y. Li, C. X. Lu, S. C. Zhang, F. Y. Su, W. Z. Shen, P. C. Zhou and C. L. Ma,

J. Mater. Chem. A, 2015, 3, 14817.

98 G. M. Kim, S. Baik and J. W. Lee, RSC Adv., 2015, 5, 87971.

99 M. M. Goldin, V. A. Kolesnikov, M. S. Khubutiya, A. G. Volkov, G. J.

Blanchard, A. K. Evseev and M. M. Goldin, J. Appl. Electrochem., 2008, 38,

1369.

100 P. L. Walker, Carbon, 1986, 24, 379.

101 J. Ding, H. Wang, Z. Li, A. Kohandehghan, K. Cui, Z. Xu, B. Zahiri, X. Tan,

E. M. Lotfabad, B. C. Olsen and D. Mitlin, ACS Nano, 2013, 7, 11004.

102 N. Petrov, T. Budinova and M. Razvigorova, 2nd Olle Indstorm Symposium

on renewable Energy-Bioenergy, 1999.

Page 126: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

111

103 J. T. Zhang, J. W. Jiang, H. L. Li and X. S. Zhao, Energy Environ. Sci., 2011,

4, 4009.

104 L. Demarconnay, E. Raymundo-Piñero and F. Béguin, J. Power Sources,

2011, 196, 580.

105 H. Peng, G. F. Ma, J. J. Mu, K. J. Sun and Z. Q. Lei, J. Mater. Chem. A, 2014,

2, 10384.

Page 127: Freestanding Nanofiber Electrodes for Supercapacitors...Daniel Wayne Lawrence in partial fulfillment of the requirements for the degree of ... Figure 32 – Photo of Swagelok T-cell

Recommended