Optimizing the conductance and
capacitance of carbon
aerogel-based supercapacitors
Listiantono Nugroho (NTU) and Erik Stassen (DTU s071717)
Carbon aerogel supported by a piece of straw. Reproduced from [1].
NTU-DTU Innovation Workshop 2013
Nanyang Technical University, Singapore
26 July 2013
Abstract
Supercapacitors have attracted much interest due to their ability to generate a much higher
power density compared to conventional capacitors. Their high capacitance is contributed
to two important factors, Electrical Double Layer Capacitance (EDLC) and pseudocapacitance. Careful selection of electrode material is needed to facilitate these mechanism.
The desirable microstructure properties are large surface area and good electrical conductivity. Furthermore, the desired structure is one where ion diffusion will be possible
and reasonably quick to facilitate fast energy release. Carbon-based aerogels are viewed
as a promising material for supercapacitors as they nicely support these needs. In addition, there exists a template free carbon aerogel fabrication method that would be easily
scalable for mass production purposes.
To further achieve high capacity performance, it is important to pay close attention to the
electrode’s microstructure. The crucial parameters are pore size and pore size distribution,
which will affect specific surface area (SSA), ion diffusion rate as well as electrical conductivity. These closely related parameters and their tradeoffs need to be assessed carefully to
achieve an optimum level of desired performance. Moreover, the utilization of pseudocapactive material could further increase the capacitive performance. However, due the lack
of such material with good electrical conductivity and chemical stability, the application
of psudocapacitive material needs to be studied more for performance optimization.
ii
Contents
1 Introduction
1.1
1.2
Motivation
1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
1.1.1
Supercapacitor Development . . . . . . . . . . . . . . . . . . . . . .
3
1.1.2
Carbon aerogels in supercapacitors . . . . . . . . . . . . . . . . . . .
3
Outline of the Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
2 Theory and Basic Principles
2.1
2.2
2.3
5
Conventional capacitors and supercapacitors . . . . . . . . . . . . . . . . . .
5
2.1.1
Conventional capacitors . . . . . . . . . . . . . . . . . . . . . . . . .
5
Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2.1
Working Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
2.2.2
Important parameters and tradeoffs in supercapacitor optimization .
7
Carbon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.3.1
Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.3.2
Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
2.3.3
Carbon Aerogels . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
2.3.4
Hierarchical structures . . . . . . . . . . . . . . . . . . . . . . . . . .
9
3 Experiment
12
3.1
Experimental procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2
Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
iii
iv
CONTENTS
4 Conclusion and outlook
17
4.1
Conclusion
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2
Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1
Introduction
In the realm of energy storage development, the two general albeit important parameters
are energy and power density of the device. Conventionally batteries have been the champions of high energy density performance, while capacitors are known for their high power
density performance. Conventional capacitors high power density performance is made
possible by its fast energy release mechanism, which is purely electrostatic in nature.
A specially engineered device, known as a supercapacitor or ultracapacitor, is capable
of storing energy worth thousands of Farads [2] while maintaining the high power density nature of conventional capacitors. Supercapacitors high performance is due to two
main energy storage mechanisms; Electric Double Layer Capacitance (EDLC) and pseudocapacitance. These mechanisms arise from special electroactive materials placed on
the electrodes and the use of electrolytes. Different configurations of these two parameters result in supercapacitors with various operating energy and power density range.
Comparison of energy and power density performance of an energy storage device can be
illustrated using a Ragone plot as shown in Fig. 1.1.
Electroactive material selection is a very important aspect affecting supercapacitor performance. Electroactive materials ideal for supercapacitor applications should have a very
large surface area, good electrical conductivity and a microstructure that facilitates good
ion transportation. Properly engineered carbon nanostructures could fulfill the aforementioned needs and is thus of interest to the development of supercapacitors, as illustrated
in Fig. 1.2.
In this project, carbon aerogel-based supercapacitors were assessed. Particularly, the
optimization of the electrical conductivity and capacitance were studied. Aerogel itself
refers to a porous solid structure which was obtained by removing the liquid component
of a hydrogel [4].
1.1
Motivation
In the following section, the properties and applications of supercapacitors and carbon
aerogels are elaborated upon to better understand the motivational force driving forth the
development of this technology.
1
2
1. INTRODUCTION
Figure 1.1: Ragone chart illustrating the different operating regimes of Li-ion batteries
and hybrid and symmetrical supercapacitors. Reproduced from [3]
Figure 1.2: Diagram showing factors contributing to good electrochemical capacitance
and how carbon nanostructures could fulfill these needs. Reproduced from [2]
1.1. MOTIVATION
1.1.1
3
Supercapacitor Development
Supercapacitors have received much attention lately due to their ability to potentially
bridge the gap between high power and high energy storage devices. In addition to its
high power density nature, supercapacitors have a long life span of 10 - 100 thousand
cycles [2] which is 1 - 2 orders of magnitude longer than that of batteries, (for example,
the maximum cycle count for a Li-ion battery in a Macbook is 1000 cycles [5]). Moreover,
supercapacitors possess a charge time that is in the order of seconds. These wonderful
properties have helped to prove supercapacitors a viable solution for several energy storage
related device issues.
Its high power output capability could be utilized in conjunction with batteries or other
types of power supplies. This would give the system a boost in peak performance and
could extend the battery lifetime [6]. This application is especially appealing for mobile
devices, for example for fueling the cameras flash function.
Another application could be for regenerative breaking in electric vehicles. The energy
produced from breaking can be quickly stored in a supercapacitor and subsequently used
for accelerating.
An example of an application taking advantage of the short charging time of a supercapacitor is a supercapacitor-powered cable car in Austria. It connects the city Zell am
See to Schmittenhohen and often has to run 24/7 leaving almost no time for charging.
Supercapacitors were used due to their short charging time, which enables charging in the
short time interval where passengers board the lift at the station [7].
One down side of supercapacitors high power nature however, is the leakage current which
can be quite significant [8]. This is in contrast to batteries which have a lower leakage
current thus providing a longer voltage retention period. Therefore the usual usage for
supercapacitors are in situations where energy only needs to be stored for short period of
time.
1.1.2
Carbon aerogels in supercapacitors
As illustrated by Fig. 1.2, the inherent properties of carbon nanostructures makes them
a good candidate for acting as electroactive material in a supercapacitor. In addition
to its conductive nature, the porous structure of carbon aerogels has made more surface
area available for ions to adhere to. Furthermore, the porous structure will enhance ion
diffusion which a crucial component in the energy storage mechanism of supercapacitors.
The effect of pores in the electroactive material is shown in Fig. 1.3.
4
1. INTRODUCTION
Figure 1.3: The ion diffusion model on electroactive material of a super capacitor. The
pore size and pore distribution size highly affect SSA, ion diffusion rate and conductivity
of the carbon material. Reproduced from [3]
1.2
Outline of the Report
Firstly, the basic theoretical principles relevant to supercapacitors as well as carbon nanostructures are described. Subsequently, the aerogel fabrication method which was carried
out in the lab will be described and measurements of its performance as electrode material
in a supercapacitor will be presented and discussed. Lastly, suggestions for improvement
of the design of the supercapacitor as well as possible new ideas will be discussed.
2
Theory and Basic Principles
2.1
Conventional capacitors and supercapacitors
Capacitors serve as a useful energy storage device which exploits the very basic attractive
and repulsive nature of electric charges. Energy stored in capacitors is manifested by
electric fields generated by the charge separation. Due to this simple physical mechanism,
energy storage and release in capacitors takes place in a short period of time giving capacitors its high power nature. The amount of energy that can be stored in a capacitor
depends on the amount of charge that it can store per volt. This measure is defined as
capacitance and is given by:
C=
q
V
(2.1)
where C is capacitance, q is charge and V is voltage difference.
2.1.1
Conventional capacitors
Conventional capacitor design usually involves two parallel plates where charge is depleted
on one plate and accumulated on the other, when a voltage difference is applied. Keeping
a fixed distance between the plates, an electric field is generated and energy is stored.
The capacitance of such a capacitor can be calculated by considering the area of the
electrode, separation distance, and permittivity of space and dielectric material used [9].
It is expressed as
C = εr ε0
A
d
(2.2)
where εr is the relative static permittivity of the dielectric material between the plate, ε0
is the vacuum permittivity, A is the area of the plate and d is the separation between the
plates.
5
6
2. THEORY AND BASIC PRINCIPLES
Energy stored and power delivered for a capacitor can be calculated by considering capacitance, potential difference and equivalent series resistance [9] according to the following
equations
1
2
E = CV 2
P =
V2
4Rs
(2.3)
(2.4)
where E is energy, C is capacitance, V is the voltage difference, P is power and Rs is the
equivalent series resistance.
2.2
Supercapacitor
Unlike conventional capacitors, the energy storage mechanism in supercapacitors involves
a battery-like chemical reaction in addition to the conventional charge separation. As
compared to a battery however, supercapacitors distinguish themselves by having a charge
and discharge cycle in the order of seconds, a symmetric sloping charge-discharge profile
and a longer lifespan. [2].
2.2.1
Working Principles
In supercapacitors, energy stored in the form of charge separation is refered to as Electric
Double Layer Capacitance (EDLC) while energy stored in the form of fast redox reaction is
called Pseudocapacitance. In contrast with conventional capacitors, supercapacitors needs
an electrolyte to function. Electrolytes which could be used for supercapacitors are either
aqueous electrolytes, organic electrolytes or ionic liquids [10]. The working principles of
supercapacitors are summarized in Fig. 2.1.
Electric Double Layer Capacitance (EDLC)
EDLC is a purely physical form of energy storage caused by charge separation. As illustrated in Eq. (2.3) the amount of energy stored is affected by the capacitance of the
material. The capacitance is in turn affected by the surface area and the separation distance between charges as seen in Eq. (2.2). EDLC results from having a material with a
porous structure which is submersed in an electrolyte. This configuration results in the
formation of an ionic double layer which is electrostatically attracted to the electrode surface when a potential difference is applied to the electrodes, as shown in Fig. 2.1 a). Since
the separation distance between the charges are only in the order of a few åand the surface
2.2. SUPERCAPACITOR
7
Figure 2.1: Supercapacitor energy storage mechanism. a) Electric double layer capacitance caused by charge separation. b) Pseudocapacitance, a fast redox reaction acting as
a mechanism for storing energy. Reproduced from [11]
area of porous carbon is huge, supercapacitors are able to produce massive capacitance
and large energy storage capability.
Pseudocapacitance
Pseudocapacitance relies on a faradaic fast redox reaction between electroactive species
and the electrolyte as an energy storage mechanism as illustrated in Fig. 2.1. The electroactive materials which are most commonly used are transition metal oxides(MnO2 ,
NiO, Co3 O4 ), nitrides (VN), sulfides (CoSx) and conducting polymers (polyaniline, popypyrrole, polythiophenes, etc) [2].
Pseudocapacitance can generate more capacitance as compared to the EDLC mechanism
however, transition metal oxides have poor conductivity and conducting polymers suffer
mechanical degradation on charging processes, preventing both in becoming ideal electroactive material. A potential solution is to have a hierarchical structure with a carbon
base, which will aid electric conductivity, and incorporating a pseudocapacitive material
as the redox agent.
2.2.2
Important parameters and tradeoffs in supercapacitor optimization
In order to achieve the good performance of a supercapacitor, there are three main parameters which need to be optimized: surface area, ion diffusion rate and electrical conductivity.
8
2. THEORY AND BASIC PRINCIPLES
Surface area is important for both the EDLC and the pseudocapacitance mechanism.
Larger surface area would provide more space for the charges to adhere to for increasing the
EDLC. Furthermore, larger surface area could potentially provide more pseudocapacitive
reaction sites which will be obtained after functionalization of the carbon nanomaterial.
Ion diffusion through the microstructure is also important as ions are an essential component for charge accumulation in EDLC and for redox reactions in pseudocapacitance.
Lastly, the electrical conductivity of the electroactive material needs to be maximized.
Electrodes with small conductivity and high internal resistance could highly reduce the
power delivered by the capacitor, as illustrated by Eq. (2.4). Large resistance electrode
materials will consume big amounts of power and lead to undesirable dissipation energy
as heat.
Control of pore size and pore size distribution holds the key to optimization of the microstructure. A small pore size will provide a larger SSA, however it may prevent ion
diffusion in the system. In contrast, large pore size will decrease the SSA but it will enhance the ion diffusion rate. In addition, large pore size translates to a smaller number of
carbon atoms per unit area, which might reduce the electric conduction. Therefore, it is
important to balance out the factors in order to achieve maximum performance.
2.3
Carbon Nanostructures
Carbon nanostructures include for example Carbon Nanotubes (CNT), graphene and
fullerenes. CNTs and graphene will be further investigated in this chapter
2.3.1
Graphene
Graphene is in a sense a single sheet of graphite, carbon atoms arranged in a hexagonal
lattice, covalently bonded to each other. Graphene sports a superior specific surface area,
has high electron conductivity [12].
2.3.2
Carbon Nanotubes
A Carbon NanoTube (CNT) is basically a rolled up sheet of graphene. Several properties
combine to make CNTs very interesting for use in supercapacitors. They possess an
inherent flexibility and tensile strength ([13]) which can add mechanical stability to an
aerogel. CNTs are also very light.
The sheet of graphene can be rolled in different ways, making the edges connect in different
ways and causing what is refered to as chirality. A nanotube’s chirality is described by two
unit vectors (n, m) and for a given (n, m) nanotube, if n = m, the nanotube is metallic.
These nanotubes are termed armchair nanotubes. If nâm is a multiple of 3, then the
2.3. CARBON NANOSTRUCTURES
9
nanotube is a semiconductor with a tiny band gap, otherwise the nanotube will be a
moderate semiconductor. CNTs can be viewed as a 1D conductor.
2.3.3
Carbon Aerogels
There are several different types of carbon nanostructures. In this report, emphasize is
placed on carbon aerogels, more specifically on an aerogel created via a hydrogel method.
Many aspects of this kind of carbon aerogel makes it an interesting material. It’s scalable manufacturing process, ultra-low weight, flexibility and mechanical strength, and it’s
ultrahigh oil-absorption capabilities [1].
But it also shows specific promise for supercapacitor applications, due to it’s high porosity
and the amazing electrical attributes of it’s fundamental building blocks, graphene and
CNTs.
In Fig. 2.2 a SEM image of such a carbon aerogel is shown, and the nanostructure of the
material, revealing it’s high porosity becomes apparent.
Figure 2.2: Microscopical architecture of graphene aerogel. Left: SEM image of carbon
aerogel Right: Illustration of the microstructure of the carbonaerogel, showing CNTs
dispersed on the graphene. Reproduced from [1].
2.3.4
Hierarchical structures
A hierarchical structure is a combination of different building blocks, arranged in a somewhat (It can be hard to obtain complete control over the processes on a nanoscale) controlled way. Making a hierarchical structure has several benefits; each component may
contribute individual merits to increase the overall performance of the electrode, for example high conductance, extra active sites or better better liquid diffusion. An increased
SSA might also be obtained.
In Fig. 2.3 a possible design for a hierarchical carbon nanostructure has been sketched.
It consists of CNTs grown on a template, consisting of for example platinum with nickel
nanoparticels dispersed on it as a catalyst for the growth. The CNTs are grown via
10
2. THEORY AND BASIC PRINCIPLES
Figure 2.3: An example of possible design for a heirarchical carbon nanostructure.
plasma-enhanced chemical vapour deposition. The field in the plasma sheath controls the
degree of alignment of the CNTs [14], and can thus be reduced to increase the disordering
of the nanotubes. The structure has (ordered/aligned) CNTs near the (platinum) electrode, an moving up the heirarchical structure the plasma field is gradually lessened, and
the disordering of the nanotubes increased, creating the tree-like structure. The aligned
nanotubes near the template allow for fast transport, but are very densely packed, whilst
the disordered nanotubes might have larger pores, allowing for faster diffusion of the electrolyte.
Another heirarchical structure with CNTs seamlessly grown from a single graphene layer
has been realized in [15]. This structure has an SSA of 2000m2 /g.
In theory, the proposed heirarchical carbon nanostructures should be able to perform
better than the amourphous carbon aerogel alone, due to a larger SSA.
2.3. CARBON NANOSTRUCTURES
Figure 2.4: SWCNTs grown from a single sheet of graphene. Reproduced from [15]
11
3
Experiment
As discussed in Section 2.3, graphene and CNT exhibit different properties which could
complement each other to produce good electroactive material. Specifically from the
aerogel construction point of view, CNT could provide better structural support [1] as
compared to graphene. It is interesting to see how the composite aerogel could perform as
the electroactive material in a supercapacitor , as the electrical properties of the aerogel
has not been thoroughly studied previously.
3.1
Experimental procedure
Graphene oxide (GO) used in the experiment was synthesized from acid treatment of
graphite oxide [16], thus acid and salt impurities were expected to be present in the
solution. To purify it, GO was firstly centrifuged at 4000 rpm for 15 minutes. After
centrifugation, GO was accumulated in the bottom of the tube and the remaining water
was poured out to remove impurities contained in it. The residual accumulated graphene
was then mixed with deionized (DI) water and aggitated with the help of a vortex to
disperse the GO. This process was repeated 5 times.
The purified GO solution was then tested with a spectrophotometer (Cary 5000 UVVis-NIR Spectrophotometer) to determine its concentration. The GO concentration was
determined by observatoin of a peak at a wavelength of 230 nm which corresponds to
the π − π ∗ plasmon peak of GO [17]. The solution was then diluted until a clear brown
solution was obtained. Dilution was continued until the max relative absorbtion peak
value obtained was less than 1, in accordance with the instruments optimal accuracy (01). The absorption value is measured relative to that of DI water. The GO concentration
was obtained from the following relationship
36.15718C − 0.02291 = Abs230
(3.1)
where C is GO concentration and Abs230 is the 230 nm relative absorbtion value. Eq. (3.1)
was previously obtained from calibrating known concentrations of GO with its absorption
values.
12
3.2. RESULTS AND DISCUSSION
13
The obtained GO suspension was sonicated for 20 minutes to ensure a fairly even dipersion,
which later would be mixed with a CNT suspension to form the aerogel.
The chirality of CNTs is not yet a fully controllable entity, and as such the single-wall
CNTs used in this experiment were assumed to be a mix of metallic and semiconductive
ones.
The CNTs were submersed in nitric acid (68 wt%) and was heated in an oil bath at 140◦ C
for 3 hours, to purify them and remove any functional groups [18]. Excessive water was
then added to the suspension and the mixture was vacuum filtered with a masksize of
2µm. Once the filtered cake was obtained, 40 ml DI water was poured through the system
3 times to wash out the remaining acid. The resulting CNT filter cake was ovendried
overnight at 60◦ C. The dried CNT filter cake was then sonicated for 30 minutes in DI
water to obtain a CNT suspension, which later would be mixed with the GO suspension.
Suspensions with a graphene:CNT mass fraction of 1.00:0.00, 0.75:0.25 and 0.50:0.50 were
prepared. The resulting mixtures were again sonicated to ensure dispersion of the nanocarbons. The concentration used in the experiment was 4.9 mg/ml for both the graphene
and CNT suspension with the afore-mentioned mass fractions.
The suspension was then autoclaved for 18 hours in a 180◦ C oven. The volume of the
obtained hydrogel sample was 0.65 cm3 while the sample initially had a volume of 18 cm3 ,
making for a 27-fold size reduction.
Freeze drying was then used to remove the water component of the hydrogel. The approach
was adopted to avoid slow and relativelly hot drying of the (wet) hydrogel, which would
have collapsed the gel structure due to the surface tension of water. First, the hydrogel
was lightly tapped with tissue paper to remove any unbound water. The sample was then
put into an ALPHR 1-2 LD instrument. The freezing took place in a cooled chamber
of -40◦ C and lasted for about 20 minutes. The drying took place by chamber pressure
reduction to 0.120 mbars which lasted for 24 hours.
The aerogel sample was obtained after the freezedrying process. The electrode was constructed from the aerogel sample by slicing it up with a razorblade into 8 mm diametered
and 1 mm thick coinshaped objects. A piece of filterpaper (with a masksize of 2µm)
was placed between the two carbon aerogel coins, and the whole system was wetted with
sulfuric acid (1M). Electrical connection was obtained by placing a platinum plate in
contact with each of the aerogel pieces, and glass separators were inserted, finishing the
supercapacitor design.
3.2
Results and discussion
The obtained carbon aerogel was very brittle and had big pores see Fig. 3.1. The pores
were barely visible to the naked eye suggesting a pore size of approximately 100µm. It
is debatable whether this is actually the optimum poresize for use of the material in a
14
3. EXPERIMENT
supercapacitor; more likely, it would be best to have a poresize in the mesoporous or
microporous range [2].
A supercapacitor was constructed with carbon aerogel that had a 1:1 mass ratio of
graphene and CNT. The performance of this device was tested using the 3-electrode measurement method with counter and reference electrode clamped to one of the platinum
plates while the working electrode was connected to the other platinum plate.
Figure 3.1: Obtained carbon aerogel sample to be used for the electrodes of a supercapacitor.
A picture of the finished carbon aerogel supercapacitor can be seen in Fig. 3.2
Figure 3.2: The finished device.
The ESR (Equivalent Series Resistance) of the system can be found from the Nyquist plot
Fig. 3.3 by extrapolating the straight line to find the crossing with the real axis. This
gives an approximate value of Rs = 4.4 Ω, indicated by the arrow in the figure.
From the Cyclic-Voltammetry (CV) plot in Fig. 3.4, the capacitance can be calculated. A
voltage sweep applied to an ideal capacitor will create a current [19]
I=
dU
I
dQ
=C
↔C=
dt
dt
dU/dt
(3.2)
and we simply take the average current, which can be seen on Fig. 3.4 to be 1.2 mA and
3.2. RESULTS AND DISCUSSION
15
Figure 3.3: Nyquist plot of the supercapacitor, with the ESR indicated by the arrow.
The applied voltage is 5 mV, and the frequency is scanned from 0.001 Hz to 1 Hz.
divide it by the scanrate which is 0.01 V/s. So the capacitance is 120 mF. This is within
the range of performance expected of a supercapacitor [20].
The capacitance can also be found from the charge-discharge plot in Fig. 3.5. Notice that
the device is quite linear, which makes it possible to find the scanrate as the slope of the
curve. The slope is via graphical inspection found to be 1/120 V/s. The appliced current
is 1 mA and so the same result is gained, a capacitance of 120 mF. Combined electrode
mass that were used in the supercapacitor was 3.5 mg, thus the specific capacitance of the
carbon aerogel sample was 34.28 F/g.
16
3. EXPERIMENT
Figure 3.4: Cyclic voltammogram for the supercapacitor. The current is 10 mA at a scan
rate of 0.01 V/s.
Figure 3.5: Galvanostatic charge-discharge curve. Applied current is constant 1 mA.
4
Conclusion and outlook
4.1
Conclusion
The internal workings of a supercapacitor was investigated thoroughly, and a description
of the two concepts EDLC and pseudocapacitance mainly responsible for providing the
capacitance of the system was provided.
Next, the benefits of including carbon nanostructures including graphene, CNTs and carbon aerogel in the electrode material of the supercapacitor was investigated. The main
boosts in performance was found to come from pores, increased SSA and high conductivity.
It was also shown how a combination of all these different carbon nanostructures in a
hierarchical structure could improve the output of the supercapacitor.
The recipe for creating this carbon aerogel was described in detail, to enable reproduction
of the creation process, and a 3-electrode measurement of its properties was carried out. A
carbon aerogel based supercapacitor was then fabricated in the lab and its capacitance was
measured to be 120 mF, which is in the range of performance expected of a supercapacitor.
The equivalent series resistance was also found to be Rs = 4.4 Ω.
4.2
Outlook
In addition to the carbon aerogel with 1:1 mass ratio of graphene to CNT, a sample with
75 wt% and 100 wt% of graphene were obtained in the experiment. Further study could be
done to investigate the effect of the composition of carbon aerogels on their performance
as electroactive material in a supercapacitor.
The aerogel sample obtained form the experiment was observed to have a pore size of
100µm. As this size is much larger than the ion diameter, this pore size could potentially
be further reduced to achieve a higher SSA without considerable loss in ion diffusion rate.
The large pore size could be a result of ice crystal growth in the nanocarbon pores having
pushed the walls of the pores and thus enlarged them. This issue could potentially be
alleviated with the establishment of a very high heat removal rate. This could be achieved
17
18
4. CONCLUSION AND OUTLOOK
by quickly dipping the sample in liquid nitrogen.
The electrolyte used in a supercapacitor could also be further studied. In the microporous
regime the size of the solvation shell (the encasing of an ion by for example water molecules)
may affect the ion diffusion rate. The effective ratio of the pores and the solvation shell
could be optimized by further study to achieve better performance. In addition, the pore
size distribution of the material could be an important parameter to study for optimizing
ion diffusion. A BET measurement could determine the size of the pores via gas adsorption
[21].
To achieve high performing capacitor further study could be done on hierachical structure
that aim to enchance conductance of poorly conducting pseudocapacitive material such as
transition metal oxides. This could result in material with high specific capacitance and
good electrical conductance.
Another factor that could be further explored is the magnitude of leakage current and possible ways to prevent them. Further study on the nature and mechanism of leakage current
could potentially make supercapacitor an even more versatile energy storage device.
References
[1] Haiyan Sun, Zhen Xu, and Chao Gao. Multifunctional, ultra-flyweight, synergistically
assembled carbon aerogels. Advanced Materials, 25(18):2554–2560, 2013.
[2] C. Z. Yuan, B. Gao, L. F. Shen, S. D. Yang, L. Hao, X. J. Lu, F. Zhang, L. J. Zhang,
and X. G. Zhang. Hierarchically structured carbon-based composites: Design, synthesis and their application in electrochemical capacitors. NANOSCALE, 3(2):529–545,
2011.
[3] Xin Li and Bingqing Wei. Supercapacitors based on nanostructured carbon. NANO
ENERGY, 2(2):159–173, MAR 2013.
[4] Alex Capecelatro. aerogel.org, welcome to open source aerogel, June 2013.
[5] Apple. Apple.com, June 2013.
[6] Graham Pitcher. If the cap fits...@ONLINE, March 2006.
[7] Martin Randelhoff. Londoner emirates air line: Teuerste seilbahn der welt mit
fraglicher verkehrlicher bedeutung@ONLINE, July 2012.
[8] Xiaofan Jiang, J. Polastre, and D. Culler. Perpetual environmentally powered sensor
networks. In Information Processing in Sensor Networks, 2005. IPSN 2005. Fourth
International Symposium on, pages 463–468, 2005.
[9] David J. Griffiths. Introduction to Electrodynamics (3rd Edition). Benjamin Cummings, 1998.
[10] Arunabha Ghosh and Young Hee Lee. Carbon-Based Electrochemical Capacitors.
CHEMSUSCHEM, 5(3):480–499, 2012.
[11] Guihua Yu, Xing Xie, Lijia Pan, Zhenan Bao, and Yi Cui. Hybrid nanostructured materials for high-performance electrochemical capacitors. NANO ENERGY, 2(2):213–
234, MAR 2013.
[12] Jian-Hao Chen, Chaun Jang, Shudong Xiao, Masa Ishigami, and Michael S. Fuhrer.
Intrinsic and extrinsic performance limits of graphene devices on SiO2. Nature Nanotechnology, 3(4):206–209, March 2008.
19
20
REFERENCES
[13] Min-Feng Yu, Oleg Lourie, Mark J. Dyer, Katerina Moloni, Thomas F. Kelly, and
Rodney S. Ruoff. Strength and breaking mechanism of multiwalled carbon nanotubes
under tensile load. Science, 287(5453):637–640, 2000.
[14] Martin S Bell, Kenneth BK Teo, Rodrigo G Lacerda, WI Milne, David B Hash,
M Meyyappan, et al. Carbon nanotubes by plasma-enhanced chemical vapor deposition. Pure and applied chemistry, 78(6):1117–1125, 2006.
[15] Yu Zhu, Lei Li, Chenguang Zhang, Gilberto Casillas, Zhengzong Sun, Zheng Yan,
Gedeng Ruan, Zhiwei Peng, Abdul-Rahman O. Raji, Carter Kittrell, Robert H.
Hauge, and James M. Tour. A seamless three-dimensional carbon nanotube graphene
hybrid material. Nature Communications, 3:1225, November 2012.
[16] Sasha Stankovich, Dmitriy A. Dikin, Richard D. Piner, Kevin A. Kohlhaas, Alfred
Kleinhammes, Yuanyuan Jia, Yue Wu, SonBinh T. Nguyen, and Rodney S. Ruoff.
Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite
oxide. CARBON, 45(7):1558–1565, JUN 2007.
[17] Qi Lai, Shifu Zhu, Xueping Luo, Min Zou, and Shuanghua Huang. Ultraviolet-visible
spectroscopy of graphene oxides. AIP Advances, 2(3):032146, 2012.
[18] S. Mirershadi, S. Z. Mortazavi, A. Reyhani, N. Moniri, and A. J. Novinrooz. Effective
condition for purification of multi-walled carbon nanotubes by nitric acid. Synthesis
and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 39(6):312–
316, 2009.
[19] Testing Electrochemical Capacitors. Gamry.
[20] J Gamby, P.L Taberna, P Simon, J.F Fauvarque, and M Chesneau. Studies and characterisations of various activated carbons used for carbon/carbon supercapacitors.
Journal of Power Sources, 101(1):109 – 116, 2001.
[21] Stephen Brunauer, P. H. Emmett, and Edward Teller. Adsorption of gases in multimolecular layers. Journal of the American Chemical Society, 60(2):309–319, 1938.