Optimizing the conductance andcapacitance 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 2013Nanyang Technical University, Singapore

26 July 2013

Abstract

Supercapacitors have attracted much interest due to their ability to generate a much higherpower density compared to conventional capacitors. Their high capacitance is contributedto two important factors, Electrical Double Layer Capacitance (EDLC) and pseudocapac-itance. Careful selection of electrode material is needed to facilitate these mechanism.The desirable microstructure properties are large surface area and good electrical con-ductivity. Furthermore, the desired structure is one where ion diffusion will be possibleand reasonably quick to facilitate fast energy release. Carbon-based aerogels are viewedas a promising material for supercapacitors as they nicely support these needs. In addi-tion, there exists a template free carbon aerogel fabrication method that would be easilyscalable for mass production purposes.

To further achieve high capacity performance, it is important to pay close attention to theelectrode’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 conduc-tivity. These closely related parameters and their tradeoffs need to be assessed carefully toachieve an optimum level of desired performance. Moreover, the utilization of pseudoca-pactive material could further increase the capacitive performance. However, due the lackof such material with good electrical conductivity and chemical stability, the applicationof psudocapacitive material needs to be studied more for performance optimization.

ii

Contents

1 Introduction 1

1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1.1 Supercapacitor Development . . . . . . . . . . . . . . . . . . . . . . 3

1.1.2 Carbon aerogels in supercapacitors . . . . . . . . . . . . . . . . . . . 3

1.2 Outline of the Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Theory and Basic Principles 5

2.1 Conventional capacitors and supercapacitors . . . . . . . . . . . . . . . . . . 5

2.1.1 Conventional capacitors . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Working Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.2 Important parameters and tradeoffs in supercapacitor optimization . 7

2.3 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 parametersare energy and power density of the device. Conventionally batteries have been the cham-pions of high energy density performance, while capacitors are known for their high powerdensity performance. Conventional capacitors high power density performance is madepossible by its fast energy release mechanism, which is purely electrostatic in nature.

A specially engineered device, known as a supercapacitor or ultracapacitor, is capableof storing energy worth thousands of Farads [2] while maintaining the high power den-sity nature of conventional capacitors. Supercapacitors high performance is due to twomain energy storage mechanisms; Electric Double Layer Capacitance (EDLC) and pseu-docapacitance. These mechanisms arise from special electroactive materials placed onthe electrodes and the use of electrolytes. Different configurations of these two param-eters result in supercapacitors with various operating energy and power density range.Comparison of energy and power density performance of an energy storage device can beillustrated using a Ragone plot as shown in Fig. 1.1.

Electroactive material selection is a very important aspect affecting supercapacitor per-formance. Electroactive materials ideal for supercapacitor applications should have a verylarge surface area, good electrical conductivity and a microstructure that facilitates goodion transportation. Properly engineered carbon nanostructures could fulfill the aforemen-tioned needs and is thus of interest to the development of supercapacitors, as illustratedin Fig. 1.2.

In this project, carbon aerogel-based supercapacitors were assessed. Particularly, theoptimization of the electrical conductivity and capacitance were studied. Aerogel itselfrefers to a porous solid structure which was obtained by removing the liquid componentof a hydrogel [4].

1.1 Motivation

In the following section, the properties and applications of supercapacitors and carbonaerogels are elaborated upon to better understand the motivational force driving forth thedevelopment of this technology.

1

2 1. INTRODUCTION

Figure 1.1: Ragone chart illustrating the different operating regimes of Li-ion batteriesand hybrid and symmetrical supercapacitors. Reproduced from [3]

Figure 1.2: Diagram showing factors contributing to good electrochemical capacitanceand how carbon nanostructures could fulfill these needs. Reproduced from [2]

1.1. MOTIVATION 3

1.1.1 Supercapacitor Development

Supercapacitors have received much attention lately due to their ability to potentiallybridge the gap between high power and high energy storage devices. In addition to itshigh power density nature, supercapacitors have a long life span of 10 - 100 thousandcycles [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 wonderfulproperties have helped to prove supercapacitors a viable solution for several energy storagerelated device issues.

Its high power output capability could be utilized in conjunction with batteries or othertypes of power supplies. This would give the system a boost in peak performance andcould extend the battery lifetime [6]. This application is especially appealing for mobiledevices, for example for fueling the cameras flash function.

Another application could be for regenerative breaking in electric vehicles. The energyproduced from breaking can be quickly stored in a supercapacitor and subsequently usedfor accelerating.

An example of an application taking advantage of the short charging time of a super-capacitor is a supercapacitor-powered cable car in Austria. It connects the city Zell amSee 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 theshort time interval where passengers board the lift at the station [7].

One down side of supercapacitors high power nature however, is the leakage current whichcan be quite significant [8]. This is in contrast to batteries which have a lower leakagecurrent thus providing a longer voltage retention period. Therefore the usual usage forsupercapacitors are in situations where energy only needs to be stored for short period oftime.

1.1.2 Carbon aerogels in supercapacitors

As illustrated by Fig. 1.2, the inherent properties of carbon nanostructures makes thema good candidate for acting as electroactive material in a supercapacitor. In additionto its conductive nature, the porous structure of carbon aerogels has made more surfacearea available for ions to adhere to. Furthermore, the porous structure will enhance iondiffusion 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. Thepore size and pore distribution size highly affect SSA, ion diffusion rate and conductivityof the carbon material. Reproduced from [3]

1.2 Outline of the Report

Firstly, the basic theoretical principles relevant to supercapacitors as well as carbon nanos-tructures are described. Subsequently, the aerogel fabrication method which was carriedout in the lab will be described and measurements of its performance as electrode materialin a supercapacitor will be presented and discussed. Lastly, suggestions for improvementof 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 attractiveand repulsive nature of electric charges. Energy stored in capacitors is manifested byelectric 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 ca-pacitors its high power nature. The amount of energy that can be stored in a capacitordepends on the amount of charge that it can store per volt. This measure is defined ascapacitance 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 depletedon one plate and accumulated on the other, when a voltage difference is applied. Keepinga 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 theelectrode, separation distance, and permittivity of space and dielectric material used [9].It is expressed as

C = εrε0A

d(2.2)

where εr is the relative static permittivity of the dielectric material between the plate, ε0is the vacuum permittivity, A is the area of the plate and d is the separation between theplates.

5

6 2. THEORY AND BASIC PRINCIPLES

Energy stored and power delivered for a capacitor can be calculated by considering capac-itance, potential difference and equivalent series resistance [9] according to the followingequations

E =1

2CV 2 (2.3)

P =V 2

4Rs(2.4)

where E is energy, C is capacitance, V is the voltage difference, P is power and Rs is theequivalent series resistance.

2.2 Supercapacitor

Unlike conventional capacitors, the energy storage mechanism in supercapacitors involvesa battery-like chemical reaction in addition to the conventional charge separation. Ascompared to a battery however, supercapacitors distinguish themselves by having a chargeand discharge cycle in the order of seconds, a symmetric sloping charge-discharge profileand a longer lifespan. [2].

2.2.1 Working Principles

In supercapacitors, energy stored in the form of charge separation is refered to as ElectricDouble Layer Capacitance (EDLC) while energy stored in the form of fast redox reaction iscalled Pseudocapacitance. In contrast with conventional capacitors, supercapacitors needsan electrolyte to function. Electrolytes which could be used for supercapacitors are eitheraqueous electrolytes, organic electrolytes or ionic liquids [10]. The working principles ofsupercapacitors 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 il-lustrated in Eq. (2.3) the amount of energy stored is affected by the capacitance of thematerial. The capacitance is in turn affected by the surface area and the separation dis-tance between charges as seen in Eq. (2.2). EDLC results from having a material with aporous structure which is submersed in an electrolyte. This configuration results in theformation of an ionic double layer which is electrostatically attracted to the electrode sur-face when a potential difference is applied to the electrodes, as shown in Fig. 2.1 a). Sincethe separation distance between the charges are only in the order of a few aand the surface

2.2. SUPERCAPACITOR 7

Figure 2.1: Supercapacitor energy storage mechanism. a) Electric double layer capaci-tance caused by charge separation. b) Pseudocapacitance, a fast redox reaction acting asa mechanism for storing energy. Reproduced from [11]

area of porous carbon is huge, supercapacitors are able to produce massive capacitanceand large energy storage capability.

Pseudocapacitance

Pseudocapacitance relies on a faradaic fast redox reaction between electroactive speciesand the electrolyte as an energy storage mechanism as illustrated in Fig. 2.1. The elec-troactive materials which are most commonly used are transition metal oxides(MnO2,NiO, Co3O4), nitrides (VN), sulfides (CoSx) and conducting polymers (polyaniline, popy-pyrrole, polythiophenes, etc) [2].

Pseudocapacitance can generate more capacitance as compared to the EDLC mechanismhowever, transition metal oxides have poor conductivity and conducting polymers suffermechanical degradation on charging processes, preventing both in becoming ideal elec-troactive material. A potential solution is to have a hierarchical structure with a carbonbase, which will aid electric conductivity, and incorporating a pseudocapacitive materialas the redox agent.

2.2.2 Important parameters and tradeoffs in supercapacitor optimiza-tion

In order to achieve the good performance of a supercapacitor, there are three main parame-ters 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 theEDLC. Furthermore, larger surface area could potentially provide more pseudocapacitivereaction sites which will be obtained after functionalization of the carbon nanomaterial.Ion diffusion through the microstructure is also important as ions are an essential com-ponent 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 thepower delivered by the capacitor, as illustrated by Eq. (2.4). Large resistance electrodematerials will consume big amounts of power and lead to undesirable dissipation energyas heat.

Control of pore size and pore size distribution holds the key to optimization of the mi-crostructure. A small pore size will provide a larger SSA, however it may prevent iondiffusion in the system. In contrast, large pore size will decrease the SSA but it will en-hance the ion diffusion rate. In addition, large pore size translates to a smaller number ofcarbon atoms per unit area, which might reduce the electric conduction. Therefore, it isimportant to balance out the factors in order to achieve maximum performance.

2.3 Carbon Nanostructures

Carbon nanostructures include for example Carbon Nanotubes (CNT), graphene andfullerenes. 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 hexagonallattice, 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 propertiescombine to make CNTs very interesting for use in supercapacitors. They possess aninherent flexibility and tensile strength ([13]) which can add mechanical stability to anaerogel. CNTs are also very light.

The sheet of graphene can be rolled in different ways, making the edges connect in differentways and causing what is refered to as chirality. A nanotube’s chirality is described by twounit vectors (n,m) and for a given (n,m) nanotube, if n = m, the nanotube is metallic.These nanotubes are termed armchair nanotubes. If nam 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 amoderate 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 isplaced 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 scal-able manufacturing process, ultra-low weight, flexibility and mechanical strength, and it’sultrahigh oil-absorption capabilities [1].

But it also shows specific promise for supercapacitor applications, due to it’s high porosityand the amazing electrical attributes of it’s fundamental building blocks, graphene andCNTs.

In Fig. 2.2 a SEM image of such a carbon aerogel is shown, and the nanostructure of thematerial, revealing it’s high porosity becomes apparent.

Figure 2.2: Microscopical architecture of graphene aerogel. Left: SEM image of carbonaerogel Right: Illustration of the microstructure of the carbonaerogel, showing CNTsdispersed on the graphene. Reproduced from [1].

2.3.4 Hierarchical structures

A hierarchical structure is a combination of different building blocks, arranged in a some-what (It can be hard to obtain complete control over the processes on a nanoscale) con-trolled way. Making a hierarchical structure has several benefits; each component maycontribute individual merits to increase the overall performance of the electrode, for ex-ample high conductance, extra active sites or better better liquid diffusion. An increasedSSA 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 nickelnanoparticels 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 thedegree of alignment of the CNTs [14], and can thus be reduced to increase the disorderingof the nanotubes. The structure has (ordered/aligned) CNTs near the (platinum) elec-trode, an moving up the heirarchical structure the plasma field is gradually lessened, andthe disordering of the nanotubes increased, creating the tree-like structure. The alignednanotubes near the template allow for fast transport, but are very densely packed, whilstthe disordered nanotubes might have larger pores, allowing for faster diffusion of the elec-trolyte.

Another heirarchical structure with CNTs seamlessly grown from a single graphene layerhas been realized in [15]. This structure has an SSA of 2000m2/g.

In theory, the proposed heirarchical carbon nanostructures should be able to performbetter than the amourphous carbon aerogel alone, due to a larger SSA.

2.3. CARBON NANOSTRUCTURES 11

Figure 2.4: SWCNTs grown from a single sheet of graphene. Reproduced from [15]

3 Experiment

As discussed in Section 2.3, graphene and CNT exhibit different properties which couldcomplement each other to produce good electroactive material. Specifically from theaerogel construction point of view, CNT could provide better structural support [1] ascompared to graphene. It is interesting to see how the composite aerogel could perform asthe electroactive material in a supercapacitor , as the electrical properties of the aerogelhas not been thoroughly studied previously.

3.1 Experimental procedure

Graphene oxide (GO) used in the experiment was synthesized from acid treatment ofgraphite oxide [16], thus acid and salt impurities were expected to be present in thesolution. To purify it, GO was firstly centrifuged at 4000 rpm for 15 minutes. Aftercentrifugation, GO was accumulated in the bottom of the tube and the remaining waterwas poured out to remove impurities contained in it. The residual accumulated graphenewas then mixed with deionized (DI) water and aggitated with the help of a vortex todisperse the GO. This process was repeated 5 times.

The purified GO solution was then tested with a spectrophotometer (Cary 5000 UV-Vis-NIR Spectrophotometer) to determine its concentration. The GO concentration wasdetermined by observatoin of a peak at a wavelength of 230 nm which corresponds tothe π − π∗ plasmon peak of GO [17]. The solution was then diluted until a clear brownsolution was obtained. Dilution was continued until the max relative absorbtion peakvalue obtained was less than 1, in accordance with the instruments optimal accuracy (0-1). The absorption value is measured relative to that of DI water. The GO concentrationwas 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 absorptionvalues.

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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-wallCNTs used in this experiment were assumed to be a mix of metallic and semiconductiveones.

The CNTs were submersed in nitric acid (68 wt%) and was heated in an oil bath at 140Cfor 3 hours, to purify them and remove any functional groups [18]. Excessive water wasthen added to the suspension and the mixture was vacuum filtered with a masksize of2µm. Once the filtered cake was obtained, 40 ml DI water was poured through the system3 times to wash out the remaining acid. The resulting CNT filter cake was ovendriedovernight at 60C. The dried CNT filter cake was then sonicated for 30 minutes in DIwater 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 wereprepared. The resulting mixtures were again sonicated to ensure dispersion of the nanocar-bons. The concentration used in the experiment was 4.9 mg/ml for both the grapheneand CNT suspension with the afore-mentioned mass fractions.

The suspension was then autoclaved for 18 hours in a 180C oven. The volume of theobtained 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 approachwas adopted to avoid slow and relativelly hot drying of the (wet) hydrogel, which wouldhave collapsed the gel structure due to the surface tension of water. First, the hydrogelwas lightly tapped with tissue paper to remove any unbound water. The sample was thenput into an ALPHR 1-2 LD instrument. The freezing took place in a cooled chamberof -40C and lasted for about 20 minutes. The drying took place by chamber pressurereduction to 0.120 mbars which lasted for 24 hours.

The aerogel sample was obtained after the freezedrying process. The electrode was con-structed from the aerogel sample by slicing it up with a razorblade into 8 mm diameteredand 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 withsulfuric acid (1M). Electrical connection was obtained by placing a platinum plate incontact with each of the aerogel pieces, and glass separators were inserted, finishing thesupercapacitor design.

3.2 Results and discussion

The obtained carbon aerogel was very brittle and had big pores see Fig. 3.1. The poreswere barely visible to the naked eye suggesting a pore size of approximately 100µm. Itis 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 ormicroporous range [2].

A supercapacitor was constructed with carbon aerogel that had a 1:1 mass ratio ofgraphene and CNT. The performance of this device was tested using the 3-electrode mea-surement method with counter and reference electrode clamped to one of the platinumplates 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 superca-pacitor.

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 plotFig. 3.3 by extrapolating the straight line to find the crossing with the real axis. Thisgives 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. Avoltage sweep applied to an ideal capacitor will create a current [19]

I =dQ

dt= C

dU

dt↔ C =

I

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 withinthe range of performance expected of a supercapacitor [20].

The capacitance can also be found from the charge-discharge plot in Fig. 3.5. Notice thatthe device is quite linear, which makes it possible to find the scanrate as the slope of thecurve. The slope is via graphical inspection found to be 1/120 V/s. The appliced currentis 1 mA and so the same result is gained, a capacitance of 120 mF. Combined electrodemass that were used in the supercapacitor was 3.5 mg, thus the specific capacitance of thecarbon 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 scanrate 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 descriptionof the two concepts EDLC and pseudocapacitance mainly responsible for providing thecapacitance of the system was provided.

Next, the benefits of including carbon nanostructures including graphene, CNTs and car-bon aerogel in the electrode material of the supercapacitor was investigated. The mainboosts 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 ahierarchical structure could improve the output of the supercapacitor.

The recipe for creating this carbon aerogel was described in detail, to enable reproductionof the creation process, and a 3-electrode measurement of its properties was carried out. Acarbon aerogel based supercapacitor was then fabricated in the lab and its capacitance wasmeasured 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 with75 wt% and 100 wt% of graphene were obtained in the experiment. Further study could bedone to investigate the effect of the composition of carbon aerogels on their performanceas electroactive material in a supercapacitor.

The aerogel sample obtained form the experiment was observed to have a pore size of100µm. As this size is much larger than the ion diameter, this pore size could potentiallybe 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 havingpushed the walls of the pores and thus enlarged them. This issue could potentially bealleviated 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 microporousregime 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 shellcould be optimized by further study to achieve better performance. In addition, the poresize distribution of the material could be an important parameter to study for optimizingion 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 structurethat aim to enchance conductance of poorly conducting pseudocapacitive material such astransition metal oxides. This could result in material with high specific capacitance andgood electrical conductance.

Another factor that could be further explored is the magnitude of leakage current and pos-sible ways to prevent them. Further study on the nature and mechanism of leakage currentcould potentially make supercapacitor an even more versatile energy storage device.

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