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Graphene, 2012, 1, 1-13
http://dx.doi.org/10.4236/graphene.2012.11001 Published Online July 2012 (http://www.SciRP.org/journal/graphene)
Review of Electrochemical Capacitors Based on Carbon
Nanotubes and Graphene
Jian Li*, Xiaoqian Cheng, Alexey Shashurin, Michael Keidar
Department of Mechanical and Aerospace Engineering, The George Washington University, Washington DC, USA
Email: {*lijian, keidar}@gwu.edu
Received May 20, 2012; revised June 15, 2012; accepted July 20, 2012
Electrochemical capacitors, which can store large amount of electrical energy with the capacitance of thousands of Far-
ads, have recently been attracting enormous interest and attention. Carbon nanostructures such as carbon nanotubes and
graphene are considered as the potentially revolutionary energy storage materials due to their excellent properties. This
paper is focused on the application of carbon nanostructures in electrochemical capacitors, giving an overview regard-
ing the basic mechanism, design, fabrication and achievement of latest research progresses for electrochemical capaci-
tors based on carbon nanotubes, graphene and their composites. Their current challenges and future prospects are also
discussed.
Keywords: Electrochemical Capacitors; Ultracapacitors; Supercapacitors; Carbon Nanotubes; Graphene
Electrochemical capacitors (EC) are rechargeable elec-
trochemical energy storage devices, which can store
much larger amount of electrical energy with the capaci-
tance of thousands of Farads in the interfaces between
electrodes and electrolyte. Compared with other energy
storage devices, such as batteries, EC can offer great ad-
vantages of high power capability, high rates of charge
and discharge, high cycle life, flexible packaging and
low weight. The invention of EC aims to bridge the
critical performance gap between higher energy density
of battery and high power density of conventional elec-
trolytic capacitor [1-6]. As such, EC can facilitate and
benefit various applications, for instance, portable elec-
tronics, digital communications, hybrid electric vehicles,
and so forth [7-10].
In principle, EC stores the electrochemical energy in
two different ways [11]. The first storage mechanism is
non-Faradic, that is, only ion adsorption takes place be-
tween the interfaces of electrodes and electrolyte, there-
fore the storage of electric energy is electrostatic. Ac-
cording to the storage function, this category of EC is
dubbed as electrical double layer capacitors (EDLC), or
ultracapacitors. Because the charging and discharging of
such ultracapacitors involve no chemical reactions, they
have an unlimited degree of cyclability in theory. The
other category of EC is based on the Faradic processes,
similar to what take place in a battery, i.e. the energy
storage is achieved by electron transfer that follows re-
duction-oxidation (redox) reaction in electro-active ma-
terials. This kind of EC is called pseudo-capacitors or
supercapacitors. The terms ultracapacitors and superca-
pacitors are not distinguished strictly at the beginning of
invention; they both have very high level of capacitance
and constitute the two categories of EC. However, in
order to alleviate the confusion over nomenclature in this
paper, it is better to emphasize that ultracapacitors are
referred to the symmetric, electrostatic EC entirely based
on carbon nanostructures while supercapacitors are used
for the class of asymmetric and Faradic EC, which con-
tains metal oxide or conducting polymer [12].
To emerge as the revolutionary energy storage devices,
EC should possess the capability of higher energy den-
sity, while maintaining lower cost for large-scale produc-
tion in industry. Essentially, the performance of EC is
depending on the electro-active materials served as elec-
trodes and collectors. So far, the various carbon-based
materials, including activated carbon, ordered mesopor-
ous carbon, carbon nanotubes (CNT), graphene and their
composites, are intensively utilized for the fabrication of
EC. Among these nanomaterials, CNT and graphene are
the rising nanomaterials for electronic and energy storage
devices due to their highly accessible surface area, nano-
scale structure and good electrical conductivity since
they were discovered [13-15]. Thus the primary focus in
this paper will be on the recently developed progresses of
EC based on CNT, graphene and their composite in
laboratory and industry. The paper is organized as follows:
opyright © 2012 SciRes. Graphene
J. LI ET AL.
principle and mechanism of charge storage are firstly
described in terms of different types of EC in Section 2.
The fabrication methods and properties of EC based on
CNT, graphene or their mixture are described in Section
3 and sequentially the comparison with commercial
product is presented. The challenges and future research
prospects are summarized in Section 4.
2. Principle and Mechanism of Charge
According to the energy storage mechanism and elec-
trode materials, the major EC can be classified in fol-
lowing three types: carbon-based ultracapacitors, metal-
oxides-based supercapacitors and conducting-polymers-
based supercapacitors.
2.1. Electrical Double Layer Capacitors
(Ultracapacitors)
The first category is non-faradic EDLC, namely ultraca-
pacitor, where the electrodes are only based on carbon
materials with large SSA, including activated carbon [16],
CNT [17], and graphene [18]. They store energy at the
interface of electrolyte and electrodes through reversible
ion adsorption onto surface of the electro-active carbon
materials, thus charging double-layer capacitance. This
mechanism was firstly defined by Helmholtz [19]. The
major difference compared with other EC is that the
charge is stored only on the surface of carbon where no
redox reaction is involved in the energy storage proc-
esses.
The capacitance can be described as:

where ε is the electrolyte dielectric constant, A is the sur-
face area accessible to ions and d is the distance from
ions to the pore surface of carbon electrodes on the order
of angstroms. According to the equation, two approaches
can be taken to enhance the charge storage of EDLC ef-
fectively: increasing the SSA and reducing distance be-
tween ions and the carbon surface. Surface area is gener-
ally increased by the development of porosity in the bulk
of carbon materials [20]. In industry, the widely used
approach to increase surface area is to carry out surface
treatment in carbon materials. On the other hand, the
pore size inside the electrodes can determine the distance
between ions and the carbon surface, and further influ-
ence the specific capacitance of EDLC [21]. Three pore
groups can be classified according to the size distribution:
micro, meso and macro pores with diameters less than 2
nm, between 2 and 50 nm, and more than 50 nm, respec-
tively [22]. The traditional view anticipated that meso
pores with a size close to twice that of the solvated ions
lead to the maximum specific capacitance. In addition,
from the meso pores to the pore size of 1 nm, the specific
capacitance decreases simultaneously with the decrease
of the pore size [23]. However, the current research
showed when the pore size is less than 1 nm, the specific
capacitance dramatically increases, contrary to what was
expected in traditional theory [24,25]. According to this
view, CNT and graphene are promising candidates for
electrodes of EC because of their novel nanoscale size
and controllable size distribution.
2.2. Metal-Oxides-Based Supercapacitors
The second type is redox-based supercapacitors, where
transition metal oxides are utilized for electrodes owing
to fast and reversible redox reactions at the surface of
electro-active materials. The widely used metal oxides
nanostructured materials involve MnO2 [26,27], RuO2
[28,29], MoO3 [30], Co3O4 [31], VOx [32], and so forth.
To explain the mechanism of metal oxides in EC sys-
tem, MnO2 is taken as an example. There are two mech-
anisms proposed for the charge storage in MnO2 elec-
trodes. The first one is based on intercalation of alkali
metal cations (C+) such as Li+, Na+, K+ or protons (H+)
during reduction, and as well extraction of cations upon
oxidation [33,34].
MnO eCMnOOC

 
MnO eHMnOOH
The second mechanism is a surface process, which
involves the adsorption and desorption of alkali cations
or protons [35]. Therefore, in order to obtain consider-
able capacitance for this type of pseudo-capacitor, it is
believed to combine the redox reaction of metal oxides
and the large surface area of electro-active carbon mate-
rials as well. Additionally, metal oxides usually have a -
high electrical resistance, which might result in a low
power density, and thus need CNT or graphene to offset
the drawback in electrical properties.
2.3. Conducting-Polymers-Based
The third category is also pseudo-capacitor based on
faradic reactions, but the electrode materials are elec-
tronically conducting polymers [36], or their composites
such as polypyrrole (PPy) [37,38], polyaniline (PANi)
[39,40], poly(3,4-ethylenedioxythiophene) (PEDOT) [41],
and so on. This type of conducting polymers has a rela-
tively high conductivity, and a relatively low cost com-
pared to carbon-based electrode materials [42]. Faradic
reactions of conducting polymer are electrochemical
doping processes, which are extraction of electrons from
Copyright © 2012 SciRes. Graphene
the polymer backbone through the external circuit and
intercalation of an anion from solution into the polymer
film to balance the positive electronic charge. In order to
have the greatest potential energy and power densities, the
supercapacitors contain one positively charged (p-doped)
and one negatively charged (n-doped) conducting poly-
mer electrode; however, it is believed that the mechanical
stress on conducting polymers limits the stability of the
pseudo-capacitors during cycling redox reactions [43,44].
This process reduces the cycling stability and influences
the development of conducting polymer pseudo-capaci-
tors.
3. Fabrication and Properties of EC
In this section, the fabrication methods and the important
parameters of EC will be presented based on different
carbon nanomaterials according to the recent publication.
Specific energy of EC is the amount of energy stored
per unit mass, and is dependent on SSA of the electro-
active electrode materials and the range of operating
voltage. Specific power is the rate at which the energy is
converted per unit mass, and is greatly affected by the
structure of the electro-active materials that governs the
transport rate of electrolyte ions to and from the materials
surface. Figure 1 shows the comparison of the perform-
ance of batteries, capacitors and EC reviewed in this sec-
tion in a Ragone plot where their specific power and en-
ergy are displayed. Regarding the measurement tech-
nique, there are two major configurations. The first one is
the measurement in a symmetric two-electrode cell con-
figuration. The other one is individual electrode meas-
urements in a three-electrode cell configuration. If the
redox peaks in the positive and negative electrodes of the
two electrode cell do not take place at the same value of
the applied voltage, the difference by the factor of 3 or
more between such measurements. For a comparison of
different electro-active materials, it is more reasonable to
compare the values of specific capacitance obtained in
two-electrode cells, because it provides the most accurate
measure of a material's performance for EC [45,46].
Hence, only reference results obtained in a two-electrode
system were compared in this paper.
3.1. CNT
CNT are promising active materials for flexible elec-
trodes of electrochemical energy storage devices, due to
the high surface area, good electrical conductivity and
mechanical strength, good corrosion resistance and chemi-
cal stability, low mass density. There are two type of
CNT according to the physical structures: single-walled
carbon nanotubes (SWCNT) and multi-walled carbon
nanotubes (MWCNT). Compared with MWCNT, SWCNT
Figure 1. Ragone plot for electrochemical energy storage devices: batteries, capacitors and electrochemical capacitors.
have larger SSA accessible to ions in the electrolyte. For
SWCNT-based electrodes, a maximum specific capacitance
of 180 F/g was found in literature, whereas MWCNT-based
electrodes only have the specific capacitance in the range
of 4 - 137 F/g.
In order to realize the promising features that CNT
possess, three major approaches are utilized to create a
CNT thin film served as electrodes, including vacuum
filtration [47], wire wound rod coating [48], and electro-
phoretic deposition [49].
3.1.1. V acc uum Filtrat i o n
The vacuum filtration method is used for a freestanding
CNT film which is proper to integrate current collector
and electrode in a EC system. In this method the CNT
suspension is filtered through the pore size in the mi-
crometer range, i.e., 0.45 μm or 0.2 μm [50-52]. The
vacuum or lower pressure is created to facilitate CNT
suspension through the filter. As the liquid suspension
passes through the filter pores, CNT are trapped on the
surface to form a random oriented network. Two differ-
ent procedures could be carried out for the next step. The
thin film of CNT conjunct with filters can directly serve
as the integration of electrode and separator in an elec-
trolyte. In the case that only CNT thin film is needed, the
nitrocellulose filter also can be easily removed by the
chemical treatment such as immersing in acetone [53,54].
Preparing the electrodes of ultracapacitor by vacuum
filtration method, Niu et al. [55] have reported that
MWCNT reached the specific capacitance of 113 F/g and
a specific power of 8 kW/kg at a specific energy of 0.56
Wh/kg in a H2SO4 electrolyte. According to the relation-
ship between distribution of pore diameter of SWCNT
thin film and the temperature in heat treatment, An et al.
[56] reported a maximum specific capacitance of 180 F/g
with the treatment temperature of 1000˚C. In a solution
of 7.5 N KOH, the specific power and specific energy of
the ultracapacitor based on SWCNT are 20 kW/kg and 7
Wh/kg, respectively.
3.1.2. Wi re Wound Rod C o a ti ng
The wire wound rod coating process is utilized to fabric-
cate uniform CNT thin films for high volume manufac-
turing shown in Figure 2(a). The mechanism of this
technique is that space gaps between wire diameters can
be used to control the volume of fluid on a surface of
paper. In addition, the surface tension of the deposited
fluid can flatten the surface of CNT deposit [57].
By this technique, Hu et al. [58] prepared flexible
SWCNT ultracapacitor, where electrodes and separators
are integrated into single sheets of commercial paper. A
specific capacitance of 33 F/g and a high specific power
of 250 kW/kg were achieved with an organic electrolyte.
In order to reduce the sheet resistance of electrodes, Hu
et al. [59] also combined silver nanowire and SWCNT
thin films together by the rod coating method, achieving
a specific capacitance of 200 F/g. The flexible paper coat-
ing with SWCNT thin film has great potential to replace
metallic current collector in lithium battery, due to the
advantage of light weight, high conductivity and low cost.
3.1.3. Electrophoretic Deposition
In electrophoretic deposition (EPD) setup, voltage poten-
tial was applied to two electrodes to move charged parti-
cles of the coating materials from a solution illustrated in
Figure 2(b). To make the process effective, the elec-
trodes and the deposition particles in suspension should
be electrically conductive. Hence functionalization of
CNT by acid or other solution is necessary to make them
electrically charged before the EPD processes. Among
these fabrication methods, EPD has been found to yield
the lowest surface roughness of CNT thin film, minimal
surface resistance of CNT electrode and excellent mate-
rials utilization [60,61].
(a) (b)
Figure 2. (a) Schematic diagram of wire wound rod coating method; (b) Schematic graph of electrophoretic deposition method.
J. LI ET AL. 5
By EPD technique, Du et al. [62] reported the fabrica-
tion of MWCNT thin film as electrodes for ultracapaci-
tors, which have a very small equivalent series resistance
of 830 milli-ohm and superior frequency response due to
improved adherence of EPD coatings. The specific ca-
pacitance and specific power of the ultracapacitor are 21
F/g and 20 kW/kg, respectively.
3.1.4. S up e r-Aligned C NT
Super-aligned CNT conventionally stand for the entire
CNT that have the same orientation perpendicular to the
substrate [63,64]. The most attractive approach to obtain
aligned CNT is direct growth by chemical vapor deposi-
tion [65] with an external force, e.g., an electric field, the
gas flow or interactions with the substrate surface such as
sapphire which can provide high density and perfect
alignment [66,67]. It shows that uniform nanotubes ar-
rays fabricated on substrates can be transferred directly
as a thin film for electronics applications [68]. There are
several advantages to fabricate electrodes of EDLC by
aligned CNT. Firstly, the CNT in uniform orientation can
facilitate ion transport and make great contribution to
high power capability. Secondly, well-controlled micro-
scale pores can be introduced due to the structural fea-
tures of super-aligned CNT [69]. Lastly, super-aligned
CNT have superior electrons transport and less electrical
resistance than the CNT thin film with random network,
thus making them benefical for the electrodes of EC.
Izadi et al. [70] used the super-aligned SWCNT as
electrodes of ultracapacitor to achieve the lowest possible
amount of impurities and a very high specific surface
area of 1300 m2/g. The shear transfer technique by glass
slides was utilized to make the aligned SWCNT parallel
to the platinum current collectors of ultracapacitor. The
combination of high surface area and super-aligned struc-
ture enabled a high specific capacitance of 160 F/g in
organic electrolyte. Kim et al. [71] tested ultracapaci-
tors made of aligned CNT in ionic liquids electrolyte and
nonaqueous organic electrolyte. The specific capacitance
of ultracapacitor in ionic liquid was 160 F/g and the spe-
cific power reached very high value of 990 KW/kg be-
cause of the fast ion transport of the super-aligned CNT.
3.1.5. C NT Composit e
Although EDLC can deliver high specific power, specific
energy is still low so that it limits the usage of ultraca-
pacitor to few seconds. Increasing the specific energy of
EC would lead to extend discharge time and widen the
application market. On the other hand, the redox-based
capacitors which only contain metal oxides can reach
high specific energy and superb specific capacitance, e.g.
1300 F/g of MnO2 capacitor in aqueous electrolytes [72],
however the low electrical conductivity, poor compati-
bility with organic electrolytes, and short cycle life have
limited their practical application. To take the advantages
of reversible ions adsorption of carbon nanostructures
and faradic reaction of metallic oxidation, the hybrid
supercapacitors constituted by the combination of carbon
nanostructures and metal oxides have been extensively
studied recently [73-75].
Chen et al. [76] reported the fabrication of flexible
asymmetric supercapacitors constituted by MnO2 nano-
wire/SWCNT hybrid films as the positive electrode and
In2O3 nanowire/SWCNT hybrid films as the negative
electrode illustrated in Figure 3. This asymmetric design
allowed the devices to work in the potential window of 2
volt. In addition, the optimized hybrid nanostructured
supercapacitors exhibited a superior device performance
with specific capacitance of 184 F/g and high specific
energy of 25.5 Wh/kg.
3.2. Graphene
Theoretically SWCNT have an limit SSA of 1300 m2/g
[70], and also the high cost for mass production of high-
quality CNT is a challenge for the commercialization of
CNT-based EC. Graphene flakes and sheets have shown
a high electrical conductivity, a high theoretical SSA of
2630 m2/g and a very high intrinsic electrical conductiv-
ity in plane as well as high mechanical strength and
chemical stability comparable with or even better than
CNT [77,78]. Generally speaking, there are four ap-
proaches to synthesize graphene for the application of
EC, that is, mechanical exfoliation from bulk graphite
[15], arc discharge [79], chemical vapor deposition and
epitaxial growth, [80,81] and reduction of graphite ox-
ides [82]. The synthesis of graphene by exfoliation of
graphite is not in large scale so that it is only suitable for
fundamental study so far. We will review arc discharge
method to synthesize graphene in Section 3.3. Here we
mainly discuss the application of graphene in EC by the
latter two approaches and also consider the composites of
graphene with metal oxides or conducing polymer for
supercapacitors.
3.2.1. Chemi cal Vap or Deposition
Chemical vapor deposition is a promising method to
synthesize carbon nanostructures including CNT and
graphene. The mechanism is to pyrolyze hydrocarbon
resources such as methane or acetylene at well-controlled
high temperature. Upon the annealing process, the solu-
bility of carbon in the catalyst metal decreases and the
carbon nanostructures can precipitate on the catalyst sur-
face. Most often, nickel and copper films are used as
growth substrate for graphene [83-85].
Yoo et al. [86] fabricated an ultrathin ultracapacitor
based on continuous single and few-layer graphene films
by chemical vapor deposition technique on polycrystal-
line Cu foils using liquid precursor hexane. The in-plane
design was straightforward to implement so that the sur-
face of each graphene layer was exploited efficiently for
energy storage exploits efficiently. The performance of
the device was examined in both PVA/H3PO4 polymer-
gel electrolyte and an aqueous 1 M H2SO4 electrolyte.
The specific capacitance reaches high value of 247 F/g.
3.2.2. Chemical Reduction of Graphite Oxides
Graphite oxide is a compound of carbon, oxygen and
hydrogen in variable ratios, which can be obtained by
oxidation of graphite in the presence of strong acids and
oxidants. The synthesis method is to utilize a mixture of
H2SO4, NaNO3, and KMnO4 as oxidizers, developed by
Hummers et al. [87] in 1957 and still widely used nowa-
days. The great advantage of this approach is scalable,
affording high-volume production, and versatile in chemi-
cal functionalization.
Zhu et al. [88] improved the graphene synthesis tech-
nique and reported a simple activation with KOH of mi-
crowave exfoliated graphite oxides and thermally exfoli-
ated graphite oxides to achieve SSA around 3100 m2/g.
The specific capacitance is 200 F/g in organic electrolyte,
which was the highest value of ultracapacitor only based
on graphene in the organic electrolyte in 2011. For the
packaged cell, the specific power and specific energy
were 75 KW/Kg and 20 Wh/Kg, respectively. Liu et al.
[89] proposed a method to obtain very high specific energy
Figure 3 (a) Schematic diagram of the supercapacitor composed with a MnO2 nanowire/SWCNT hybrid film as a cathode
electrode and In2O3 nanowire/SWCNT hybrid film as an anode electrode; (b) The morphology of MnO2 nanowire/SWCNT
hybrid film by scanning electron microscope (SEM); (c) The SEM image of as-fabricated In2O3 nanowire/SWCNT hybrid
film. Reprinted with permission from reference [76]. Copyright 2010 American Chemical Society.
by utilizing intrinsic surface capacitance and surface area
of single-layer curved graphene. The specific energy of
the graphene-based ultracapacitor was tested as 85.6
Wh/kg at room temperature and 136 Wh/kg at 80˚C in an
ionic liquid, comparable to specific energy of Ni metal
hydride battery.
3.2.3. Laser Reduction of Graphite Oxides
In order to avoid the restacking of graphene sheet during
the chemical treatment of graphite oxides, a strategy to
produce graphene electrodes through a solid-state ap-
proach was proposed [90]. Compared with chemical re-
duction, the method using infrared laser to produce gra-
phene-based materials is more facile, cheap, and envi-
ronment-friendly. Notably, it is applicable for the fine
patterning of graphene sheet and has the potential to
simplify the manufacturing process of devices
El-Kady et al. [91] carried out the processes of laser
reduction of graphite oxide films to graphene directly by
a standard LightScribe DVD drive shown in Figure 4.
Initially, a thin film of graphite oxides dispersed in water
was drop-cast onto a flexible substrate. After laser reduc-
tion, the graphene-based materials can be used directly as
electrodes of ultracapacitors without binders or current
collectors. The SSA of graphene by this approach was
measured as 1520 m2/g. In addition, the ultracapacitor
offered a specific capacitance of 265 F/g in an organic
electrolyte with a wider operating voltage window of 3 V.
3.2.4. Gr a p hene Compos ites
Graphene is also a good candidate combining with metal
oxides and conducting polymer to provide higher specific
energy and electrical conductivity for the electrodes of
Cheng et al. [92] utilized electrophoretic deposition
method to mix MnO2 nanostructures with graphene film
by a cyclic voltammetric technique (250 mV/s). The
graphene-based supercapacitors showed a higher specific
capacitance due to the fact that the graphene can physic-
cally adapt different electrolyte ions, leading to a higher
accessibility of electrolyte ions and also a more effective
use of the specific surface area. The specific capacitance
was measured as 245 F/g for graphene thin film and 328
F/g for MnO2-coated graphene in a 1 M KCl aqueous
electrolyte solution. Also, the specific energy of MnO2-
coated graphene was 11.4 Wh/kg and the specific power
was 25.8 kW/kg.
Wu et al. [93] reported the supercapacitors based on
composite films of graphene and PANI conducting poly-
mer. The composite film has a layered structure, and
PANI polymer was sandwiched between chemically-
reduced graphene sheets. After treatment with HCl solu-
tion, the PANI component in the film can be re-doped.
Then the specific capacitance was measured as 210 F/g in
a 1 M H2SO4 electrolyte and the composite film showed
a good stability so that after 800 charging/discharging
cycles the high capacitance was maintained due to the
synergic effect of both components.
3.3. CNT and Graphene Composite
Composite materials of CNT and graphene have been
described in recent years to establish synergistic effects
between these two different carbon nanostructures both
having unique electronic, thermal and mechanical prop-
erties [94]. Another advantage of this hybrid structures is
that CNT can work as the spacer between graphene
sheets preventing the re-stack of graphene, and thus en-
hance the performance of electrodes based on carbon
nanostructures.
Figure 4 Schematic diagram of the fabrication of laser-scribed graphene-based electrochemical capacitors. ((A) to (D)) A
graphite oxide film supported on a flexible substrate is placed on top of a LightScribe-enabled DVD media disc, and a
computer image is then laser-irradiated on the graphite oxide film in a computerized LightScribe DVD drive; (E) As shown
in the photograph, the graphite oxide film changes from golden brown color to black as it reduced to laser-scribed graphene.
The low-power infrared laser changes the stacked GO sheets immediately into well-exfoliated few-layered laser-scribed
graphene (LSG) film, as shown in the cross-sectional SEM images; (F) A symmetric EC is constructed from two identical
SG electrodes, ion-porous separator, and electrolyte. From reference [L
91], reprinted with permission from AAAS.
3.3.1. Wet Sol u ti on
Cheng et al. [95] fabricated the electrodes of SWCNT
and graphene in wet solution method. The SWCNT and
graphene were first dispersed separately in ethanol. Then
the suspensions passed through a microporous filter pa-
per by vacuum filtration. Sequentially, the graphene and
CNT composite film was prepared by mixing graphene
and CNT by sonication in ethanol. Due to the effect of
SWCNT, the specific capacitances of the composite
films were tested as 291 F/g in 1 M KCl aqueous elec-
trolyte and 201 F/g in an organic electrolyte.
3.3.2. One-Step Synthesis in Arc
Anodic arc discharge is one of the most practical and
efficient methods to synthesize various carbon nanos-
tructures [96]. A great progress of improving the con-
trollability and flexibility of carbon nanostructures growth
in arc discharge was made by introducing mag-netic field
according to the strong magnetic responses of arc plas-
mas [97]. It demonstrated that the magnetically-enha-
nced arc discharge can narrow the diameter distribution
of metallic catalyst particles and carbon nanotubes, in-
crease the length of SWCNT, as well as change the ratio of
metallic and semiconducting carbon nanotubes [98-100].
In light of the introduction of magnetic field in arc, our
research group presented an approach to synthesize the
composite production of carbon nanotubes and graphene
simultaneously in a non-uniformed magnetically-enhan-
ced arc discharge, for the purpose of utilization of excellent
properties of SWCNT and graphene together [79,101,102].
The paper-based electrodes of ultracapacitor were fab-
ricated by wire wound rod coating method with the ink
of the composites and exhibited advantage in contrast to
the thin electrodes only made from SWCNT synthesized
without magnetic field. The sheet resistance of composite
film of SWCNT and graphene was much lower than that
of film made from SWCNT. The specific capacitance of
ultracapacitor based on SWCNT and graphene composite
in a KOH electrolyte was tested as 110 F/g, which is lar-
ger than that of ultracapacitor based on SWCNT of 40
F/g in the same conditions [103].
In addition to looking at research values it can also be
useful to have an understanding of the best in class
commercial devices that are available on the market to-
day. Commercial devices by necessity have typically
undergone extensive durability, reliability and charac-
terization testing to ensure that the product has a useful
life and will meet or exceed the expectations of custom-
ers. Although lower than research values, the values of
commercial EC as shown in Table 1 below are also a
good benchmark for what is achievable in a repeatable
large-scale production environment.
4. Challenges and Future
In this paper, we reviewed the current technology of
charge storage, and highlighted the latest fabrication and
achievement of EC in terms of different carbon nanos-
tructures including CNT and graphene. The carbon
nanostructures represent very exciting materials for EC
as they possess the excellent properties of large SSA,
good conductivity as well as fast ion transportation for
sufficient electrochemical reactions at their interfaces.
Supercapacitors, where very fast faradic redox reactions
take place in electrode material, have higher energy den-
sity and specific capacitance than ultracapacitor. But the
great cyclic stability of ultracapacitors makes them ideal
for the applications which need stable and large power
output. Regarding the materials of electrode, su-
per-aligned CNT facilitate the ion transport, which can
Table 1. The list of current commercial electrochemical capacitors with high performance.
Company Device name Country Electrode Electrolyte Voltage (V)ESR (mΩ) Capacitance (F)
AVX BestCap USA carbon polymer 3.6 - 16 25 - 600 0.068 - 1
Cap XX Super capacitor Australia 2.3 - 5.5 14 - 200 0.075 - 2.4
Cooper PowerStor USA 2.5 - 5 25 (min) 0.22 - 50
ELNA DYNACAP USA activated charcoalorganic solvent 2.5 - 5.5 0.22 - 100
ESMA Capacitor modules Russia nickel hydroxide/ACalkaline 12 - 52 100 - 100k
Kold Ban Kapower USA nickel/carbon KOH 4 - 16
Maxwell UltracapacitorUSA 2.5 - 1250.29 - 700 1 - 3000
NEC Super capacitor Japan activated carbon diluted sulfuric acid24 (max)1000 5
Ness EDLC Korea carbon Propylene
carbonate/acetonitrile 2.5, 2.7 3 - 5000
Panasonic Gold cap Japan 2.1 - 5.5 0.022 - 70
ECOND Super capacitor Canada 14 - 300 11 - 160
improve the power density of the capacitor, but the en-
ergy density is significantly low. Hence, one of the great
challenges in the development of EC technology is how
to utilize the advantages of different types of carbon ma-
terials. Another challenge is the relatively high cost when
compared to other energy devices. The cost of these
novel nanostructures will be greatly cut down with the
rapid development of nanotechnology, which is of great
importance in their practical application.
The major direction for future research in this area lies
in elaborately designing, controlling and tailoring the
strategy to synthesize novel carbon-based composites,
and to realize and exploit their more desirable functions
for EC application. For example, all-solid-state EC is one
of the trends for future research since the usage of liquid
electrolyte has several drawbacks in the aspects of pack-
aging and environment. Some research groups have al-
ready tried to fabricate EC with solid-state electrolyte
through the assembly of CNT or graphene [104,105].
However, the performance of the all-solid-state EC can
be improved further.
In conclusion, there is still tremendous room left for
improvement in the design and fabrication of EC based
on carbon nanostructures with high-performance elec-
trochemical capacitance. It is believed that any break-
throughs in the area will facilitate the development of
promising EC with a larger energy density, power den-
sity and more stable performance to bridge the gap of
battery and conventional capacitors.
This work was supported by NSF/DOE Partnership in
Plasma Science and Technology (NSF Grant No. CBET-
0853777 and DOE Grant No. DE-SC0001169). Authors
would like to thank the PPPL Offsite Research Program
supported by the Office of Fusion Energy Sciences for
supporting arc experiments.
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