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Heterogeneous Nanostructure Flexible Hybrid Quasi Solid-State
Supercapacitor based on Vertical Aligned Carbon Nanotubes
and Carbon Nanocups
A Thesis Presented by
Fabrizio Martini1
to
The Department of Mechanical and Industrial Engineering
in partial fulfillment of the requirements
for the degree of
Master of Science
in
Mechanical Engineering
in the field of
Design and Prototyping
Northeastern University
Boston, Massachusetts
May 2013
1Northeastern University
360 Huntington Avenue
Boston, MA 02115, USA
1
Acknowledgment
First and foremost, I would like to acknowledge my adviser, Professor Yung Joon Jung. His
continuous and comprehensive support guided me during this interesting research path. As busy
as he is, he still always had time to offer me assistance and positive encouragement. I would also
like to thank him for offering me the possibility to continue my research as a PhD candidate with
his prestigious research team; I am so honored about this opportunity and am taking it into
serious consideration. I would never have reached this accomplishment if not for him.
I wish to also thank Dr. Riccardo Signorelli, CEO of FastCAP Systems Corporation, who
followed me and supported me during my research of this thesis.
Finally, I would like to thank my family for their extremely important, unconditional, and
continuous support. They have kept me focused on the end goal and always trusted in my
potential even when I was leaving my home country without speaking any word of English.
Last but not least, thank you to all of the people, friends, classmates, and colleagues that have
been supporting me during this long path to complete the Master of Science degree. Each of you
is extremely important in my personal success.
2
Table of Contents
ABSTRACT .................................................................................................................................. 8
1. INTRODUCTION ...................................................................................................................... 9
1.1 Introduction to Supercapacitors ...................................................................................... 10
1.2 Hybrid and Quasi Solid-State configuration .................................................................. 12
1.3 Industry Application and Commercialization ................................................................. 16
2. EXPERIMENTAL PROCEDURE ........................................................................................... 20
2.1 Components Synthesis and Characterization ................................................................. 21
2.2 List of Experiments ....................................................................................................... 26
3. RESULTS AND DISCUSSION ............................................................................................... 32
3.1 Results ............................................................................................................................ 33
4. CONCLUSION ......................................................................................................................... 56
4.1 Conclusion ..................................................................................................................... 57
4.2 Future Work Opportunity ............................................................................................... 58
REFERENCES ............................................................................................................................. 59
3
List of Figures
Figure 1 - Ragone chart used for performance comparison of various energy storing devices.... 10
Figure 2 - Schematic design of the Solid-State Hybrid Supercapacitor (not in scale) ................. 12
Figure 3 - Polymer electrolyte based on PVDF-HFP on a pc screen............................................ 22
Figure 4 - Polymer electrolyte based on PVDF-HFP on a pc screen 2......................................... 22
Figure 5 - Partially lifted polymer electrolyte based on PVDF-HFP on a pc screen .................... 22
Figure 6 - Raman spectroscopy spectrum on VA-SWNT ............................................................. 24
Figure 7 - RBM peaks zoom on the Raman spectroscopy spectrum of VA-SWNT .................... 25
Figure 8 - Gel electrolyte .............................................................................................................. 27
Figure 9 - PVA/H2SO4 electrolyte ............................................................................................... 27
Figure 10 - Activated Carbon electrodes ...................................................................................... 27
Figure 11 - VA-SWNT electrodes ................................................................................................ 27
Figure 12 - Polymer mesh separator ............................................................................................. 27
Figure 13 - CNC on PDMS substrate ........................................................................................... 27
4
Figure 14 - Testing set up ............................................................................................................. 28
Figure 15 - Current collector connection on CNC electrode with PDMS substrate ..................... 28
Figure 16 - VA-SWNT facing CNC with current collector .......................................................... 28
Figure 17 - Testing set up for CNC-CNT experiments ................................................................ 29
Figure 18 - CNC and VA-MWNT on current collector with external tabs .................................. 29
Figure 19 - CNC and VA-MWNT on current collector ................................................................ 29
Figure 20 - PVDF-HFP membrane over a CNC array .................................................................. 30
Figure 21 - Set up for CNC+CNT experiment .............................................................................. 30
Figure 22 - Testing instrument VersaSTAT 4 by Princeton Applied Research ........................... 31
Figure 23 - Nyquist plot at 0.1V of different supercapacitor configurations................................ 33
Figure 24 - Cyclic voltammetry up to 3V of different supercapacitor configurations ................. 34
Figure 25 - ESR comparison between Regular and Reverse polarity configurations ................... 35
Figure 26 - Capacitance of the QSSH supercapacitor with Regular and Reverse configuration .. 36
Figure 27 - Bode Plot of the QSSH supercapacitor with Regular and Reverse configuration .... 38
Figure 28 - Cyclic voltammetry of Regular and Reverse polarity ................................................ 40
Figure 29 - Reverse polarity configuration - Sizes comparison .................................................... 40
5
Figure 30 - Three cyclic voltammetry at three different maximum voltages ............................... 41
Figure 31 - Three cyclic voltammetry using three different scan rates ........................................ 42
Figure 32 - Three Charge/Discharge in series at three different voltages..................................... 43
Figure 33 - Three Charge/Discharge with displayed current ........................................................ 43
Figure 34 - Charge/Discharge with two different currents ........................................................... 44
Figure 35 - Fast charge/discharge - first ten cycles of 1000 cycles .............................................. 45
Figure 36 - Current relative to the last ten of 1000 fast charge/discharge cycles ......................... 45
Figure 37 - Fast charge/discharge - last ten cycles of 1000 cycles ............................................... 46
Figure 38 - Current relative to the relative last ten of 1000 fast charge/discharge cycles ............ 46
Figure 39 - Charge/Discharge at 3V with long interruption (Energy IN and OUT) ..................... 47
Figure 40 - Voltage Drop at 3V at 0.1A discharge current ........................................................... 48
Figure 41 - Capacitance loss in 10,000 cycles .............................................................................. 49
Figure 42 - Self-discharge rate from three different voltages (1V, 2V and 3V) ........................... 50
Figure 43 - Bode Plot of four different energy storage devices based on different materials ...... 51
Figure 44 - Self Discharge of different energy storage devices .................................................... 52
Figure 45 - Electrical circuit for LED pulse test ........................................................................... 53
6
Figure 46 - LED powered by QSSH supercapacitor ..................................................................... 54
Figure 47 - Pulse test..................................................................................................................... 54
Figure 48 - Time LED on with the energy stored in the QSSH supercapacitor ........................... 55
Figure 49 - Three pulses in a row to light up a LED..................................................................... 55
7
List of Tables
Table 1 - Regular polarity vs Reverse polarity configuration ...................................................... 36
Table 2 - Relation between elements and corresponding colors in the Figures 28-49 ................. 39
Table 3 - Percentage of the capacitance gain due to CNTs doping effect .................................... 41
Table 4 - Efficiency (Energy IN vs Energy OUT) ........................................................................ 47
8
Abstract
High performance heterogeneous hybrid structure quasi solid-state electric double-layer
capacitors (supercapacitors) have been developed by assembling two morphologically different
nano-engineered carbon electrodes. The perfect interaction between these two graphitic
materials, such as vertically aligned carbon nanotubes (VA-CNTs) and high porous carbon
nanocups (CNC), permit the realization of a very thin film supercapacitor with a high frequency
response, low internal resistance and high performance. Additionally, the traditional dielectric
layer (separator) has been removed and has been replaced with a combination of gel electrolyte
and an ionic liquid polymer membrane, provides an innovative design to the quality of the quasi
solid-state device. This work provides a unique design that explores new boundaries of
supercapacitor technology, exploring new configurations and new possibilities for future
researches in the field.
Keywords: Vertical Aligned Carbon Nanotubes, Carbon Nanocups, High performance
Supercapacitor, Quasi Solid-State Supercapacitor, Gel electrolyte, Polymer membrane
9
CHAPTER 1:
1. INTRODUCTION
10
1.1 Introduction to Supercapacitors
Supercapacitors are electrochemical energy storage systems that, instead using chemical
reactions to store energy as batteries do, use direct and physical charge. Supercapacitors are
intermediate systems that bridge the power/energy gap between traditional dielectric capacitors
(high power) and batteries (high energy) [1,2]
. Most of the available supercapacitors are based on
activated carbon technology, due to its high surface area. The latest research shows that the use
of Carbon Nanotube structures as electrodes increase the performance of this kind of device [3]
.
Fig 1. Ragone chart used for performance comparison of various energy storing devices.
Supercapacitors, or “electric double-layer capacitors,” consist of carbon electrodes separated by
a conductive dielectric material. This dielectric often consists of a physical barrier such as a
polymer membrane infused with an electrolyte. Unlike batteries which use chemical reactions to
store energy within a cell, supercapacitors use a direct and physical charge stored in an electric
field. The advantage of supercapacitors over standard capacitors is that high surface area contact
11
between the carbon electrodes and thin electrolytic material allow for higher capacitance
densities; this, combined with fast discharge rates and potential for extreme thermal stability,
make supercapacitors a promising technology with numerous application possibilities [4,5]
.
Current supercapacitor research focuses on increasing the stability of capacitors while
maintaining the performance. Research into various electrode materials points to carbon
nanotubes (ideally single walled) as an excellent choice both with regard to performance and to
stability. Various dielectric materials have been tested and researched as well. Aqueous and
organic electrolytes have good conductivities and performance but lack the chemical and thermal
stabilities needed for some applications. Ionic liquids are currently being heavily researched as a
solution but while they appear to have increased stability, results are not very conclusive.
New research into various solid-state electrolytes is promising in regard to thermal and
electrochemical stability [6]
. While conductivities in these systems appear low, with further
research these dielectrics have the potential to set new thermal milestones for supercapacitors.
An intermediate solution would be a quasi solid-state electrolyte that has the higher electro-
chemical stability than liquid electrolyte and higher conductivity compare to solid-state
electrolyte [7,8]
. State of art of the quasi solid-state supercapacitors has been reviewed to
contribute to the design decision for this thesis [9-15]
.
12
1.2 Hybrid and Quasi Solid-State configuration
The design idea is to combine two different nanostructures, both carbon nanoparticles based.
Specifically, the design uses Vertical Aligned Single Walled Carbon Nanotubes (VA-SWNT) as
negative electrodes and Carbon Nanocups (CNC) as positive electrodes [16-18]
. To achieve the
solid-state design, a gel electrolyte has been developed based on ionic liquid that enable this kind
of device to achieve higher voltage with respect to the version of gel electrolyte based on
PVA/H2SO4 [19-21]
.
By proposing meaningful changes in the approach to this brand new technology, improvements
in the performance of supercapacitors will enable more comprehensive and diverse applications
in the related fields.
Fig 2. Schematic design of the Quasi Solid-State Hybrid Supercapacitor (NOT IN SCALE).
13
The two main goals to achieve are:
A) Quasi SOLID-STATE DEVICE
Quasi solid-state architecture enables this kind of supercapacitor energy storage to operate at
higher frequencies than the conventional electrolyte-based supercapacitor design. The main
advantages of this solution are:
High automation potential for electrode preparation and cell assembly techniques;
Broad operation temperature range;
No use of separator between the two electrodes;
No filling procedure (since there is no liquid-based electrolyte);
Intrinsic safety of the device;
High electrochemical stability;
High tensile strength and abrasion resistance;
Extended life time in harsh environment;
Flexible design;
Extra thin configuration.
14
B) HYBRID DEVICE
Looking at the structures of two excellent materials, such as Vertical Aligned Carbon Nanotubes
(VA-CNTs) and Carbon Nanocups (CNC), it is possible to see compatibility between them.
The advantages to use Carbon Nanotubes are:
Extremely high power and energy performance;
High surface area of the electrodes;
High conductivity;
No presence of impurities or binders.
The positive aspects of the Carbon Nanocups structure are:
High power and energy;
Flexibility;
Extremely low weight;
Extremely low thickness.
The technical characterizations between the quasi solid-state and the hybrid configurations will
enable and involve the supercapacitor in many potential applications not yet realized; from
industrial power applications to hybrid and fully electric cars, from public transportations to light
rails, from aerospace and military applications to micro/nano electronic devices and so on.
Combining these two brand new configurations will allow the already high application range of
CNTs-based supercapacitors to broaden further.
15
Liquid Electrolyte based EDLC has been tested with:
- Activated Carbon electrodes;
- Multi Walled Carbon Nanotubes electrodes;
- Single Walled Carbon Nanotubes electrodes;
- Vertical Aligned Single Walled Carbon Nanotubes electrodes.
From the preliminary tests, it is possible to understand how the nanotechnology helps improve
the performance of an energy system. In particular, Single Walled Carbon Nanotubes show a
very good performance in terms of energy density and power density. The liquid based device
reduces the lifetime and the application range of liquid electrolyte based EDLC.
16
1.3 Industry Application and Commercialization
Quasi Solid-State Hybrid Supercapacitor (QSSH Supercapacitor) is a solid, light, flexible, and
powerful device with high performances. The QSSH Supercapacitor has a wide range of
applications, including industrial power applications, hybrid and fully electric cars, public
transportations, light rails, aerospace and military applications, micro/nano electronic devices,
and memory backup systems.
There are many clean energy industrial applications for the new QSSH technology:
Energy efficiency;
Solar Thermal;
Photovoltaic;
Biofuels;
Wind power;
Geothermal;
Hydro energy;
Smart grid.
The twenty-first century is characterized by an increase in the global population and by an
exponential expansion of industrialization. It follows that the demand for high efficiency energy
storage and energy production is growing as well. A predicted global energy crisis and a
technologically advanced modern society demands the implementation and design of a new low
17
cost, highly efficient, and multifunctional framework for energy storage devices. As a result,
energy storage systems, in particular supercapacitor technology, is being adapted and optimized
with nanostructure components.
Nanomaterials applied for energy storage applications are designed to enhance the energy
density and the power density, and therefore the overall performance of the device relative to the
cost. This results in cost-effective, smaller, and more powerful energy storage devices that appeal
to both stationary and to portable energy storage solutions.
Of all storage media, electrochemical energy storage systems have emerged as being the most
promising for the future energy challenge. Supercapacitors in particular, also called electric
double-layer capacitors (EDLCs), have been studied for several years. The objective of the
research is to optimize the conventional supercapacitor, EDLC devices, to achieve better
performance using the combination of Vertical Aligned Carbon Nanotubes (VA-CNT) and
Carbon Nanocups (CNC) with a quasi-solid-state electrolyte. Developing a solid-state device
permits a wider field of applications.
Most of the applications that require an extremely high power and good energy are candidates to
use quasi solid-state and hybrid supercapacitors. Reported is a list of possible applications:
Hybrid and fully electric cars: One of the most important applications of the quasi solid-state
hybrid device is the application in electric vehicles. The integration of supercapacitors
working in parallel with the batteries packs can provide the extra power that vehicles require,
in term of acceleration and performance, reducing the oversized battery packs that are
currently applied in hybrid and fully electric vehicles. This change has the possibility to cut
down overall weight, volume, and cost of the vehicle.
18
Industrial power applications: The QSSH supercapacitor device can be applied to all
industrial applications requiring a high peak power. This kind of application would be able to
use the advantages of the extremely brief time of charge of the QSSH supercapacitor to
supply to a pulse power request.
Public transportations: Similar to the applications for vehicles, QSSH can be applied to
hybrid and fully electric public transportation systems, such as buses and subway systems.
Regenerative breaking and utilities charging can be considered.
Light rail: The QSSH device has potential to cut down the weight of the supercapacitors pack
that are already applied to light rail systems while still providing the same amount of power
and energy.
Aerospace: The QSSH supercapacitor can be used in a higher temperature range than the
electrolyte based supercapacitors.
Military applications: instruments that require an extremely high peak power can use QSSH
supercapacitors to provide the peak power requirements;
Micro/nano electronic devices: the quasi solid-state supercapacitor can be easily applied
without any enclosure on micro/nano electronic devices, such us micro/nano processor,
mother boards, nanoactuator systems, and nanosensors;
Memory backup systems: The QSSH supercapacitors can provide power during a temporary
failure of the primary power sources.
19
The current industries are increasing the usage of supercapacitor in their products and processes.
The new prototype of quasi solid-state hybrid supercapacitor will be able to solve many of the
issues that the liquid-based supercapacitors have in terms of temperature range, small volume
applications, and etc.
20
CHAPTER 2:
2. EXPERIMENTAL PROCEDURE
21
2.1 Components Synthesis and Characterization
Ionic Liquid Infused Silica Gels
Combining fumed silica nanoparticles, with a particle size between 7nm and 14nm, with
different ionic liquids in varying compositions of 10%, 8%, 5%, and 3% by weight, followed by
heating and stirring, yielded a thick gel that can be used as quasi solid-state electrolyte. This gel
electrolyte shows performance over a range of voltages and temperatures. Different recipes to
make a gel electrolyte have been executed. Cyclic voltammetry and frequency response analysis
tests were performed for many different compositions. As expected, we have seen that the
gelation process of the ionic liquid exhibits a wider electrochemical stability respect the liquid
version of the same electrolyte (stable up to 3.5V).
Polymer Electrolyte based on PVDF-HFP Copolymer
A polymer membrane based on the copolymer PVDF-HFP and ionic liquid has been created [22-
27]. First the copolymer PVDF-HFP has been dissolved in a solvent, such as Acetone, N-Methyl-
2-pyrrolidone (NMP) or others. Then after a period of stirring, to complete the disaggregation of
the copolymer, an amount of ionic liquid has been added. The solution has been stirred at 80ºC
for 4 hours to create a homogenous solution. Finally this solution has been positioned on a glass
light in the preferred shape, and baked at 100ºC for 12 hours under vacuum. Several ionic liquids
have been tested. Using this procedure it is possible to control the concentration of ions in the
polymer membrane controlling the weight ratio between copolymer and ionic liquid and it is also
possible to control the thickness of the membrane during the casting step. This membrane acts
22
both as electrolyte and separator. Using a polymer electrolyte membrane, it is possible to avoid
the usage of the separator. In the following pages there are displayed thee pictures of the film on
a computer screen.
Fig.3 Polymer electrolyte based on PVDF-HFP on a computer screen
Fig.4 Polymer electrolyte based on PVDF-HFP on a computer screen 2
Fig.5 Partially lifted polymer electrolyte based on PVDF-HFP on a computer screen
23
Carbon Nanocups Electrodes
Engineered low aspect ratio carbon nanocups have been used for this thesis. To produce the
highly ordered arrays of nanopores, a two steps anodization process has been used. In particular
a high purity aluminum foil has been anodized at 40V-45V for 4 hours in 3-5% oxalic acid
(C2H4O2) solution at room temperature. Then the aluminum foil has been placed in a mixture of
5% phosphoric (H3PO4) and 5% chromic (H2CrO4) for 24 hours to remove the formed aluminum
oxide layer. This process results in the formation of highly organized cup shape on the aluminum
surface. A second anodization process has been performed for 20-40 seconds to create the highly
organized nanocups (80-200nm in length) giving 103-10
5 time smaller L/D aspect ratio. Then the
aluminum foil has been soaked in 5% phosphoric acid solution for 1 hour, which results in the
widening of nanopores. Once the metal array was ready carbon nanocups have been synthesized
by using a chemical vapor deposition (CVD) process. The AAO template have been placed in a
quartz tube and evacuated to 15 mTorr. During heat-up, high purity argon gas was supplied and
the pressure was maintained at 760 Torr. When the temperature of the inside quartz reached
660ºC, acetylene (5 sccm)-argon (45 sccm) mixture gas was supplied as a carbon source for the
deposition of a graphitic carbon layer.
Vertical Aligned Carbon Nanotube Electrode
Using a chemical vapor deposition process, vertical aligned single wall carbon nanotubes and
multi walled carbon nanotubes have been grown. In particular the VA-SWNT has been
transferred from the forming substrate to a current collector.1
1 Details of the growing process of carbon nanotubes and transferring steps are proprietary information of the
Company, FastCAP Systems, and therefore will not be discussed in this thesis.
24
Characterization of the electrodes used for the experiments has been made by Raman
Spectroscopy. In Figure 6 it is possible to see the entire spectrum of the Raman spectroscopy
showing a single carbon nanotubes behavior. In particular:
Fig.6 Raman spectroscopy spectrum on VA-SWNT
In the following graph, Figure 7, a zoom in the radial breathing mode (RBM) peaks is shown.
From this section of the Raman spectroscopy spectrum, it is possible to calculate the diameter of
the VA-SWNT.
= (224/(230-10)) = 1.051 nm
0
200
400
600
800
1000
1200
1400
0 1000 2000 3000 4000
Ram
an
In
ten
sity
[arb
itra
ry u
nit
]
Frequency Shift [cm-1]
Raman Spectroscopy Entire Spectrum
Vertical
Aligned
Single
Walled
Carbon
Nanotubes
electrode
25
Fig.7 RBM peaks zoom on the Raman spectroscopy spectrum of VA-SWNT
0
200
400
600
800
1000
100 150 200 250 300 350 400
Ram
an
In
ten
sity
[arb
itra
ry u
nit
]
Frequency Shift [cm-1]
Raman Spectroscopy RBM peaks
Vertical
Aligned
Single
Walled
Carbon
Nanotubes
electrode
Diameter of CNT =
1.032864 nm
26
2.2 List of Experiments
The preliminary experimental process focused on assembling different kinds of supercapacitor
designs. Using three different carbon nanomaterials and different electrolytes solution, the
performances were then compared. Then once the recipes of all the components have been
optimized, the actual innovative design has been tested. Below there is a complete list of the
tested configurations:
1. Activated Carbon Quasi Solid-State Electrolyte PVA/H2SO4
2. Activated Carbon + Quasi Solid-State Electrolyte PVA/H2SO4
3. Activated Carbon + Quasi Solid-State Electrolyte SiO2/Ionic Liquid/PVA/H2SO4
4. VA-SWNT + Quasi Solid-State Electrolyte 3% SiO2/Ionic Liquid
5. VA-SWNT + Quasi Solid-State Electrolyte 3% SiO2/Ionic Liquid + Polymer Mesh
6. VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/Ionic Liquid
7. VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/Ionic Liquid
8. CNC + VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/Ionic Liquid
9. CNC + VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/Ionic Liquid
10. VA-MWNT + Quasi Solid-State Electrolyte 10% SiO2/New Ionic Liquid 30h under vacuum
+ Polymer Mesh;
11. VA-MWNT + Quasi Solid-State Electrolyte 10% SiO2/ New Ionic Liquid 32h under vacuum
+ Polymer Mesh;
12. VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/ New Ionic Liquid 32h under vacuum
+ Polymer Mesh;
27
13. VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/ New Ionic Liquid + Polymer Mesh;
14. CNC + VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/ New Ionic Liquid + Polymer
Mesh;
15. CNC + VA-SWNT + Quasi Solid-State Electrolyte 10% SiO2/ New Ionic Liquid + Polymer
Mesh.
16. CNC + VA-MWNT + Quasi Solid-State Electrolyte 8% SiO2 New Ionic Liquid + Polymer
electrolyte
The components that have been used to run these experiments are reported in the pictures below.
Fig.8 Gel electrolyte Fig.9 PVA/H2SO4 electrolyte Fig.10 Activated Carbon electrodes
Fig.11 VA-SWNT electrodes Fig.12 Polymer mesh separator Fig.13 CNC on PDMS substrate
28
Fig.14 Testing set up
Fig.15 Current collector connection on CNC electrode with PDMS substrate
Fig.16 VA-SWNT facing CNC with current collector
29
Fig.17 Testing set up for CNC-CNT experiments
Fig.18 CNC and VA-MWNT on current collector with external tabs
Fig.19 CNC and VA-MWNT on current collector
30
Fig.20 PVDF-HFP membrane over a CNC array
Fig.21 Set up for CNC+CNT experiment
31
Fig.22 Testing instrument VersaSTAT 4 by Princeton Applied Research
All the experiments have been run inside a glove box, MBraun LabMaster 130, with controlled
values of moisture (<0.1ppm) and oxygen (<3.5ppm).
32
CHAPTER 3:
3. RESULTS AND DISCUSSION
33
3.1 Results
Quasi Solid-State Hybrid (QSSH) Supercapacitor – Regular and Reverse Polarity
A combination of two different nanostructures, in this case Vertical Aligned Carbon Nanotubes
and Carbon Nanocups, implies that one of the two layers is a positive electrode and the other
layer is a negative electrode. In this case, the layers are considered as follows:
- Regular polarity: VA-CNT layer is considered as NEGATIVE electrode
- Reverse polarity: CNC layer is considered as NEGATIVE electrode
Characterization tests, such as frequency response analysis and cyclic voltammetry have been
run for both configurations. The results are reported below.
The Equivalent Series Resistance (ESR) of the Reverse polarity configuration is 1.1% more in
respect to the Regular polarity configuration (4.595Ω and 4.646Ω, respectively) (Fig 23). These
values, obtained from the interception of the plot at 45º, considering the quasi solid-state phase
of the electrolyte, manifest a good ionic conductivity and low internal resistance of the device.
Fig.23 Equivalent Series Resistance (ESR) comparison between Regular and Reverse Polarity configurations
0
1
2
3
4
5
6
0 1 2 3 4 5 6
- Z
im [
Oh
ms]
Zre [Ohms]
Nyquist plot at 0.1V - Regular vs Reverse polarity
Full FRA at 0.1V
- VA-CNT as
Negative
electrode
Full FRA 0.1V -
CNC as Negative
electrode -
Reverse polarity
45 Degree
34
A higher ESR is mostly due to a larger capacitance reached by using the Reverse polarity
configuration. In the following graph, shows the capacitance of the device at 0.1V and at
different frequencies (Fig.24). The total capacitance of the Reverse polarity at low voltage of
0.25Hz is about 73.1µF and the total capacitance of the Regular polarity at low voltage of
0.25Hz is only 71.5µF, which is 2.18% lower than the Reverse configuration.
Fig.24 Capacitance of the QSSH supercapacitor with Regular and Reverse configuration
The phases of both the configurations have been plotted in Figure 25. Both of the curves are
similar for high frequency response (>10Hz), but the Reverse polarity has higher phase at a
lower frequency (<10Hz). In particular, at 0.079Hz, the phase of the Regular polarity
configuration is about 76.64 degrees and the phase of the Reverse polarity configuration is 4.7%
lower at 80.46 degrees.
0
10
20
30
40
50
60
70
80
0.1 1 10 100 1000 10000 100000 1000000
Cre
[µ
F]
Frequency [Hz]
Capacitance at 0.1V - Regular vs Reverse polarity
Full FRA
at 0.1V -
VA-CNT
as
Negative
electrode
Full FRA
0.1V -
CNC as
Negative
electrode
- Reverse
polarity
35
Fig.25 Bode Plot (Phase) of the QSSH supercapacitor with Regular and Reverse configuration
In the last graph of this section (Fig.26), a cyclic voltammetry of both of the devices is observed
up to 3V. From this test, it is possible to visualize and calculate the overall capacitance of a
Reverse configuration in respect to the other Regular configuration. When comparing the two
total areas, it is clear that the Reverse polarity configuration has the higher capacitance. The total
charge of the Regular configuration is 9.4% lower than the Reverse configuration, due to higher
capacitance (682.223µC and 746.352µC, respectively).
Looking the high voltage section of the graph (> 2V), it is possible to notice that the HQSS
supercapacitor is more stable when comparing the Reverse polarity configuration to the Regular
version. During the Regular configuration, as the cyclic voltammetry approach to 3V, the current
begins to increase dramatically after 2.5V, which is a symptom of instability.
0
10
20
30
40
50
60
70
80
90
0.01 0.1 1 10 100 1000 10000 100000
- P
hase
[d
eg]
Frequency [Hz]
Phase comparison - Regular vs Reverse polarity
Full FRA
at 0.1V -
VA-CNT
as
Negative
electrode
Full FRA
0.1V -
CNC as
Negative
electrode -
Reverse
polarity
36
Fig.26 Cyclic voltammetry of Regular and Reverse polarity
Table 1 summarized data relevant seen above to Regular and Reverse polarity configuration. It is
possible to see how the Reverse polarity configuration has better performance for four points out
of five.
Table.1 Regular polarity vs Reverse polarity configuration
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3
Cu
rren
t [µ
A]
Potential [V]
Cyclic Voltammetry - Different polarities
CV _3V
full cell -
0.1mV/s
VA-CNT
as Negative
electrode
CV_3V full
cell - CNC
as Negative
electrode -
Reverse
polarity
Parameter Regular polarity Reverse polarity Percentage
Reverse vs Regular
Equivalent Series
Resistance ESR 4.595Ω 4.646Ω +1.1%
Capacitance
71.5µF 73.1 µF +2.18%
Phase 76.64º 80.46º +4.7%
Maximum Voltage V 2.5V 3V +20%
Charge of 1 cycle µC 682.223 µC 746.352 µC +9.4%
CW
37
Based on these presented results, the Reverse polarity configuration, where the Carbon Nanocups
layer is considered as the negative electrode, has shown higher performances and higher stability
in respect to the Regular polarity configuration.
This could be attributed to two main causes:
Interactions between the different nanostructures involved in the QSSH supercapacitor and
the different sizes of the ions in the chosen electrolyte;
Electrochemical interaction between positive electrode and negative ions and doping effect
of the VA-CNT.
A schematic graph visualizing the different sizes of the nanostructure and the ions of the ionic
liquid is shown below in Fig.27. In particular, the ionic liquid that has been used to run these
experiments has two different sizes of ions. The positive ions have a radius about 0.6nm and the
negative ions are much smaller with a radius only about 0.2nm.2 The bigger size of the carbon
nanocups has better interaction with large ions and the small vertical aligned carbon nanotubes
have a better interaction with smaller ions. Another possible explanation to be considered, beside
the interactions with different sizes of nanomaterial and ions, is the doping of the carbon
nanotubes electrode. It is known that the CNTs, due to their 1-D structure, have a Doping P
already at higher level those other carbon nanomaterials. Therefore, CNTs are able to dope more
easily as positive electrodes with negative charge than with the opposite. This effect is beneficial
in order to increase the capacitance at higher voltage [28-30]
.
2 Details about the ionic liquid are Company property information
38
Fig.27 Reverse polarity configuration - Sizes comparison
After all above considerations, from now on only results with the Reverse polarity configuration
are reported.
39
Quasi Solid-State Hybrid Supercapacitor – Reverse polarity results
Once the preliminary results have been analyzed and the most promising positive-negative
configuration has been decided, several tests have been run to understand comprehensively the
possible behaviors and the overall performances of the Quasi Solid-State Hybrid supercapacitor.
From this point forward the colors in the graphs will indicate different materials as it is reported
in Table 2, below:
Table.2 Relation between elements and corresponding colors in the Figures 28-49
In the following three graphs, Fig.28-30, a cyclic voltammetry (CV) has been run with nine
different parameters. Figure 28 has shown a cyclic voltammetry at 1V, 2V and 3V and has
maximum voltage using a scan rate of 0.1V/s for each. From these results, it is possible to note
that the supercapacitor is properly working for the entire three configurations and, in particular,
functions at up to 3V. In fact, the required current to reach 3V of operation does not have a
significant change in slope at the high voltage section (>2V); this means that the electrochemical
reactions are going to disappear with more cycles. Additionally, the overall capacitance of the
device continues to increasing with the voltage. The total capacitance of the device operating at
3V is 4.63 times larger than the total capacitance of the same device operating at 1V. When
comparing the total charge, the values are: 161.244µC at 1V, 366.17µC at 2V, and 746.352µC at
3V. Figure 29 demonstrates how this device is able to work with different scan rates of 0.05V/s,
Element Graph color
QSSH supercapacitor with CNCs as Negative electrode RED,YELLOW,GREEN
QSSH supercapacitor with VA-CNTs as Negative electrode BLUE
Activated Carbon electrodes BLACK
Carbon Nanotubes electrodes PURPLE
Electrolytic capacitor LIGHT BLUE
Current GRAY
40
0.1V/s and 0.2V/s. Even at faster scan rates of operation, the supercapacitor is able to provide the
full capacitance.
Fig.28 Three cyclic voltammetry at three different maximum voltages
Fig.29 Three cyclic voltammetry using three different scan rates
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3
Cu
rren
t [µ
A]
Potential [V]
Cyclic Voltammetry - Different Maximum Voltage
CV _1V
full cell
0.1V/s
CV _2V
full cell
0.1V/s
CV _3V
full cell
0.1V/s
-50
-40
-30
-20
-10
0
10
20
30
40
50
0 0.5 1 1.5 2 2.5 3
Cu
rren
t [µ
A]
Potential [V]
Cyclic Voltammetry - Different Scan Rate
CV _3V
full cell
0.05V/s
CV _3V
full cell
0.1V/s
CV _3V
full cell
0.2V/s
41
In Fig.30, three cyclic voltammetry cycles run in three different situations. To note is the
behavior of the green curve, that shows the CV of the QSSH supercapacitor right after the
assembly of the supercapacitor itself; an increase of the required current is necessary to reach 3V
of operation. The yellow curve represents the CV after one hour at maximum voltage (also
known as seasoning). From this graph, it is possible to see how the capacitance is increasing and
the reactions at high voltage are decreasing (a more flat end is shown). Finally, the red curve
indicates the CV after a long seasoning time of twelve hours. This is an extremely interesting
behavior, mostly due to the doping effect of the VA-CNTs.
Table.3 Percentage of the capacitance gain due to CNTs doping effect while seasoning the supercapacitor
Fig.30 Three cyclic voltammetry with/without seasoning
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
0 0.5 1 1.5 2 2.5 3
Cu
rren
t [µ
A]
Potential [V]
Cyclic Voltammetry - Capacitance Comparison
CV _3V
0.1V/s
CV _3V
0.1V/s
After 1h
at Max
Voltage
CV _3V
0.1V/s
After 12h
at Max
Voltage
Cycle Color line Total charge Percentage
Initial Green 746.244 µC 0 %
After 1h seasoning Yellow 762.733 µC +2.19 %
After 12h seasoning Red 854.488 µC +14.48 %
42
In the following graphs, Fig.31-34, show several galvanostatic charge/discharge cycles of the
QSSH supercapacitor. In Fig.31, three different charge/discharge curves, having the same current
0.1mA and reaching three different maximum voltages (1V, 2V and 3V respectively), are
observed. All three of these curves show nearly ideal triangular charge/discharge shape. This
means that the supercapacitor is able to work properly from 0.1V and up to 3V.
Fig.31 Charge/Discharge at three different voltages
Figure 32 is showing three charge/discharge cycles in series using three different voltages (1V,
2V and 3V respectively). From this graph it is possible to notice how all the three
charge/discharge have a similar shape, meaning good cycling repeatability. From Fig.33 it is
possible to see on the same graph the given current and the correspondent voltage. It is clear to
see how the voltage is following the current without any delay. This means that the QSSH
supercapacitor has very good frequency response.
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8
Volt
age[
V]
Time [s]
Charge/Discharge one cycle comparison
Charge/Dischar
ge cycle at 1V
0.1mA 3 cycles
Charge/Dischar
ge cycle at 2V
0.1mA 3 cycles
Charge/Dischar
ge cycle at 3V
0.1mA 3 cycles
43
Fig.32 Three Charge/Discharge in series at three different voltages
Fig.33 Three Charge/Discharge with displayed current
0
0.5
1
1.5
2
2.5
3
3.5
0 5 10 15 20
Volt
age[
V]
Time [s]
Charge/Discharge three cycles comparison
Charge/Discha
rge cycle at 1V
0.1mA 3
cycles
Charge/Discha
rge cycle at 2V
0.1mA 3
cycles
Charge/Discha
rge cycle at 3V
0.1mA 3
cycles
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
-150
-100
-50
0
50
100
150
0 5 10 15 20
Cu
rren
t [µ
A]
Time [s]
Charge/Discharge Current vs Voltage Plot
Current Voltage
44
Extremely fast charge/discharge
Another positive characteristic of the QSSH supercapacitor is the ability to be charged and to be
discharged in an extreme fast time. In the following graphs (Fig.34-38) it is possible to
appreciate this performance. In particular, Figure 34 is showing two extremely fast cycles
(charge/discharge) at two different currents (0.5mA and 1mA respectively). The cycle at 0.5mA
has:
The cycle at 1mA has the following times:
Fig.34 Charge/Discharge with two different currents
0
0.5
1
1.5
2
2.5
3
3.5
0 0.2 0.4 0.6 0.8 1 1.2
Vo
lta
ge[
V]
Time [s]
Fast Charge/Discharge
Fast Charge
Discharge
0.5mA
Fast Charge
Discharge
1mA
45
The next four graphs 35-38, show the first and the last ten fast cycles from the one thousand
cycles that have been performed on the QSSH supercapacitor. From these graphs, it is possible to
notice excellent fast cycle stability considering the high current rate and the number of cycle in
series.
Fig.35 Fast charge/discharge - first ten cycles of 1000 cycles
Fig.36 Current relative to the first ten of 1000 fast charge/discharge cycles
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
0 1 2 3 4 5 6 7
Vo
lta
ge[
V]
Time [s]
Fast Charge/Discharge 1st Ten Cycles Plot
Fast Charge
Discharge
cycle 3V 1mA
1000 cycles
-1.5
-1
-0.5
0
0.5
1
1.5
0 1 2 3 4 5 6 7
Cu
rren
t [m
A]
Time [s]
Fast Charge/Discharge 1st Ten Cycles Plot
Fast Charge
Discharge
cycle 3V 1mA
1000 cycles
46
Fig.37 Fast charge/discharge - last ten cycles of 1000 cycles
Fig.38 Current relative to the relative last ten of 1000 fast charge/discharge cycles
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
622 623 624 625 626 627 628 629 630
Vo
lta
ge[
V]
Time [s]
Fast Charge/Discharge Last Ten Cycles Plot
Fast Charge
Discharge
cycle 3V 1mA
1000 cycles
-1.5
-1
-0.5
0
0.5
1
1.5
622 623 624 625 626 627 628 629 630
Cu
rren
t [m
A]
Time [s]
Fast Charge/Discharge Last Ten Cycles Plot
Fast Charge
Discharge
cycle 3V 1mA
1000 cycles
47
Efficiency
From a charge/discharge curve, it is also possible to calculate the efficiency of QSSH
supercapacitor itself. In the graph below (Fig.39), it is possible to see the total energy stored in
the supercapacitor during charging (green area on the left hand side of the graph) versus the
energy that it is possible to extrapolate from the charged device. The values of the stored energy
(Energy IN) and the usable energy (Energy OUT) are reported in Table 4. An efficiency of
86.8% has been calculated (13.2% of energy loss).
Fig.39 Charge/Discharge at 3V with long interruption (Energy IN and OUT)
Table.4 Efficiency (Energy IN vs Energy OUT)
Charge Discharge Percentage
Discharge vs Charge
5.29 V*s 4.59 V*s
-13.2% 0.529 J 0.459 J
0.0001471 Wh 0.0001276 Wh
48
ESR calculations
The electrical model of a double-layer capacitor can be simply modeled as an ideal capacitance
in series with a resistance. The resistance of the cell includes both the resistance of the active
layer (in this case, VA-CNTs and CNCs) and the resistance of the charge collector. The voltage
drop at the beginning of each discharge curve, also known as iR drop, is a measure of the overall
resistance of the device.
Fig.40 Voltage Drop at 3V at 0.1A discharge current
The equivalent series resistance (ESR) of the device from Fig.37 is:
Considering that the diameter of the circular electrodes (both positive and negative) that has been
used is 1.6cm, the total area of each electrode is:
( )
2.6
2.7
2.8
2.9
3
3.1
3.2
13.5 13.55 13.6 13.65 13.7 13.75 13.8
Vo
lta
ge[
V]
Time [s]
Voltage Drop
Voltage
0.124 V
V2
V1
49
The ESR is inversely proportional to the surface area of the electrode (positive or negative), so in
this case:
The specific is 2.48Ω/cm2. This is a very good value considering the quasi solid-state
phase of the electrolyte used. It manifested by the good ionic conductivity of the electrolyte and
the low internal resistance of the used electrodes (VA-CNTs and CNC).
Lifetime test
A lifetime test run to see the performance of the QSSH supercapacitor during cycling yielded
very interesting results (see Fig.41). The following graph indicates the capacitance of this device
remains unchanged after 10,000 cycles of operation (1 cycle = 1 charge/discharge). This result
expresses an excellent cycling stability of this kind of device.
Fig.41 Capacitance loss in 10,000 cycles
0
20
40
60
80
100
120
0 2000 4000 6000 8000 10000 12000
Cap
aci
tan
ce [
%]
Number of cycles
Lifetime test
Capacitance
50
Self-discharge rate
A self-discharge test has been run to continue the characterization of the device. In Fig.42, it is
possible to see the comparison between different self-discharge rates of the same device from
three different voltages (1V, 2V and 3V). These self-discharge have been obtained by charging
the QSSH supercapacitor at the maximum voltage (Vmax) and then having the device in open-
circuit for 1000 seconds.
Fig.42 Self-discharge rate from three different voltages (1V, 2V and 3V)
0
0.5
1
1.5
2
2.5
3
0 200 400 600 800 1000
Volt
age
[V]
Time [s]
Self Discharge rate from different voltages
Self
Discharge
from 1V
Self
Discharge
from 2V
Self
Discharge
from 3V
51
Different devices comparison
For comparison purposes, data (obtained under the same dynamic conditions for each test) from
a commercial electrolytic capacitor, a commercial supercapacitor based on activated carbon, a
prototype supercapacitor based on carbon nanotubes, and the QSSH supercapacitor is shown in
the following graphs (Fig.43-44). In particular, in the Bode plot (Fig.43), it is possible to see how
the frequency response of the QSSH supercapacitor behavior reacts between an electrolytic
capacitor and supercapacitor. It shows a superior frequency response of the QSSH device with an
extremely small relation time. This reaction is a point of strength of this hybrid configuration, in
fact it has almost the same extremely high frequency response of an electrolytic capacitor, but it
also has higher capacitance than an electrolytic capacitor.
Fig.43 Bode Plot of four different energy storage devices based on different materials
0
10
20
30
40
50
60
70
80
90
100
0.01 1 100 10000
Ph
ase
[d
eg]
Frequency [Hz]
Phase comparison
Electrolytic
capacitor
Quasi Solid
State Hybrid
Supercapacitor
Activated
carbon
supercapacitor
Carbon
Nanotubes
supercapacitor
52
In the following graph, Fig.44, it is possible to compare the self-discharge of three different
devices, in particular an electrolytic capacitor, a commercial supercapacitor based on activated
carbon and the QSSH supercapacitor. This test confirms that the QSSH supercapacitor has an
electrochemical behavior in between a commercial supercapacitor and an electrolytic capacitor.
Fig.44 Self Discharge of different energy storage devices
0
0.5
1
1.5
2
2.5
3
3.5
0 200 400 600 800 1000
Volt
age
[V]
Time [s]
Self Discharge of different devices
Quasi Solid State Supercapacitor Activated Carbon Electrolytic capacitor
53
LED Pulse Test
To show the performance of the QSSH supercapacitor, an LED rated as operative at 1.8V has
been connected in series to the device. Figure 45 represents a similar electrical circuit to what it
has been created for this test. A 3.3 kOhms, a resistor has been added in series to the LED in
order to have the light turned on for a longer period time. In fact, after adding a high resistance in
series, it is possible to reduce the pulse current passing through the LED, and therefore the
energy required to turn on the light is lower. To test this, first, the QSSH supercapacitor is been
charged to Vmax (in this case 3V) and kept at a maximum voltage for 30 seconds. Next, the LED
is connected in series and the light turn on for 1 second. Figure 46 is a digital picture of the
system created for this kind of test, showing the LED turned on. The results of this test are
showed in the Fig.47 and Fig.48, where it is possible to see the reactions of the current and the
voltage at the moment that the LED is connected. In particular, Fig.48 is a close up view of the
moment the connection is made and the period of time in which the light was on is clear. Finally,
as it is shown in Fig.49, a three pulses test is run to see the repetitively of the results.
Fig.45 Electrical circuit for LED pulse test
54
Fig.46 LED powered by QSSH supercapacitor
Fig.47 Pulse test
-200
-100
0
100
200
300
400
0
0.5
1
1.5
2
2.5
3
3.5
0 20 40 60 80 100
Cu
rren
t [µ
A]
Volt
age
[V]
Time [s]
Pulse Test - LED - 1 cycle
Voltage Current
LED connected
55
Fig.48 Time LED on with the energy stored in the QSSH supercapacitor
Fig.49 Three pulses in a row to light up a LED
-200
-100
0
100
200
300
400
0
0.5
1
1.5
2
2.5
3
3.5
30 30.5 31 31.5 32
Cu
rren
t [µ
A]
Volt
age
[V]
Time [s]
Pulse Test - TIME LED ON
Voltage Current
1 second - LED ON
-200
0
200
400
600
800
1000
1200
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10
Cu
rren
t [µ
A]
Volt
age
[V]
Time [s]
Pulse Test - LED - 3 cycles
Voltage Current
56
CHAPTER 4:
4. CONCLUSION
57
4.1 Conclusion
In conclusion, the thesis focused on an overview of the possible design of a supercapacitor using
different kinds of electrodes, electrolytes, and separators and comparing their performances. In
particular, I have explored a unique configuration of supercapacitors combining two different
nano-engineered structures: quasi solid-state structures and hybrid structures. The hybrid
configuration merged the positive aspects of both of the used nanostructures, in this case carbon
nanotubes and carbon nanocups. The solid-state phase supercapacitor has been reached
developing a quasi-solid-state electrolyte, changing the viscosity of ionic liquid and developing a
solid-state polymer membrane that acts as separator and electrolyte at the same time. The
obtained results showed very high performances, in particular in terms of frequency response,
power density and lifetime. This hybrid quasi solid-state supercapacitor could be applied for
several high frequency applications and could be considered a device with performances between
an electrolytic capacitor and a supercapacitor.
58
4.2 Future work Opportunity
The fabricated design and the complexity of the structure has many interesting aspects. An
optimization of each component of this kind of device can be performed. Some of the further
research opportunities that are available, on the basis of this thesis, are as follows:
Investigation of the interaction between Carbon Nanotubes and Carbon Nanocups at the
nanoscale level.
Study of the gel electrolyte with different percentage of Silica Nanopowder and varying Ionic
Liquids.
Characterization and optimization of the polymer membrane used as separator in this
particular supercapacitor design.
Testing of the mechanical properties of this hybrid and quasi solid-state structure.
59
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