Advanced Review
Review of nanostructured carbon
materials for electrochemical
capacitor applications: advantages
and limitations of activated carbon,
carbide-derived carbon,
zeolite-templated carbon, carbon
aerogels, carbon nanotubes,
onion-like carbon, and graphene
Wentian Gu and Gleb Yushin∗
Electric double layer capacitors, also called supercapacitors, ultracapacitors, and
electrochemical capacitors, are gaining increasing popularity in high power energy
storage applications. Novel carbon materials with high surface area, high electrical
conductivity, as well as a range of shapes, sizes and pore size distributions are
being constantly developed and tested as potential supercapacitor electrodes.
This article provides an overview of the electrochemical studies on activated
carbon, carbide derived carbon, zeolite-templated carbon, carbon aerogel, carbon
nanotube, onion-like carbon, and graphene. We discuss the key performance
advantages and limitations of various nanostructured carbon materials and
provide an overview of the current understanding of the structure–property
relationships related to the transport and adsorption of electrolyte ions on their
surfaces, specific and volumetric capacitance, self-discharge, cycle life, electrolyte
stability, and others. We discuss the impact of microstructural defects, pore
size distribution, pore tortuosity, chemistry and functional groups on the carbon
surface, nanoscale curvature, and carbon-electrolyte interfacial energy. Finally, we
review state-of-the art commercial large scale applications of supercapacitors,
including their use in smart grids and distributed energy storage, hybrid
electric and electric vehicles, energy efficient industrial equipment, ships, wind
power stations, uninterruptible power supplies, power backup, and consumer
devices. © 2013 John Wiley & Sons, Ltd.
How to cite this article:
WIREs Energy Environ 2013. doi: 10.1002/wene.102
INTRODUCTION
∗
Correspondence to: yushin@gatech.edu
Georgia Institute of Technology, School of Materials Science and
Engineering, Atlanta, GA, USA
Conflict of interest: The authors have declared no conflicts of
interest for this article.
E
lectric double-layer capacitors (EDLCs) represent
a unique type of high-power electrochemical
energy storage devices, where the capacitance arises
from the charge separation at an electrode–electrolyte
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
developed. As a result, the range of commercial
applications of electrochemical capacitor technology
is expanding (Figure 2) and novel materials are
being constantly developed and tested as potential
EDLC electrodes. Maxwell Technologies (the largest
US manufacturer of EDLC cells and modules) has
reported nearly six times increase in the EDLC sales in
the last 5 years, although from a relatively small base.
100 000
Specific power (W kg–1)
(a)
10 000
1000
100
10
1
10
100
Specific energy (Wh kg–1)
100 000
Volumetric power density (W L–1)
(b)
10 000
1000
100
10
1
10
100
Volumetric energy density (Wh L–1)
FIGURE 1 | Schematic energy characteristics of various types of
commercially available electrochemical capacitors in comparison with
lead-acid, nickel metal hydride and Li-ion batteries as a simplified
Ragone plot: (a) specific (mass-normalized) power versus specific
energy and (b) volumetric power versus energy density.
interface. The energy storage in EDLCs is based
on the adsorption of electrolyte ions on the large
specific surface area of electrically conductive porous
electrodes, most commonly porous carbons. The key
advantages of EDLCs over batteries include much
higher power density (Figure 1), much lower internal
resistance, broader temperature window of a stable
operation, very rapid charging (in seconds or less),
higher round-trip efficiencies and significantly longer
cycle life possible (millions of cycles). The main
limitation of EDLCs is their lower energy density
than batteries (Figure 1) combined with a high cost
per unit energy.
The cost of EDLCs in the last decade has been
decreasing significantly faster than that of batteries.
The progress in the performance improvements of the
EDLCs has been noticeably more rapid as well. In
addition, other types of electrochemical capacitors
(such as asymmetric capacitors where only one
electrode is of an EDLC-type) with higher energy
density (Figure 1) and lower cost have recently been
GENERAL PRINCIPLES AND CELL
CONSTRUCTION
The simplest building block of an EDLC consists
of two high surface area porous conductive
electrodes (commonly based on carbon) immersed
into electrolyte and separated by an ion-conducting
and electron-insulating separator membrane. When
a voltage is applied across these electrodes the
electrolyte ions of the opposite sign accumulate on
the surface of each electrode. The areal concentration
of such ions is commonly proportional to the applied
voltage. The charge separation between an electrode
space charge and a layer of electro-adsorbed ions is
called an electric double layer.1–3 During charging
and discharging of a pure EDLC no charge transfer
takes place across the electrode/electrolyte interface.
However, many carbon electrode materials contain
functional groups on their surfaces, which may
exhibit chemical and electrochemical interactions with
selected electrolytes. In case when such interactions
involve fast and reversible charge transfer reactions
between the carbon surface and the electrolyte ions,
a battery-like Faradaic charge storage process on
the electrode surface (often called pseudocapacitance)
may complement previously described electrostatic
ion adsorption of an ideal EDLC.
Somewhat similar to a gas adsorption, the
adsorption of ions under the application of an electric
field could be described by an adsorption isotherm,
which describes the ion surface coverage at constant
temperature as a function of their concentration,
controlled by the applied voltage.4–6 In contrast to gas
sorption, however, the ion adsorption has a very weak
dependence on temperature. The balance between the
changes in entropy and enthalpy of a system controls
the ion concentration on the electrode surface,
which minimizes the total Gibbs free energy of the
system. When ion adsorption sites have a very broad
distribution of enthalpies, the concentration of electroadsorbed ions follows a linear dependence on the
applied voltage for a wide range of concentrations, as
typically observed in some pseudocapacitive materials
and in EDLCs.7,8 However, if the enthalpy of ionelectrode interactions deviates weakly from its average
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
(a)
(b)
(c)
(d)
(e)
(f)
FIGURE 2 | Examples of the large-volume applications of electrochemical capacitors: (a) an electric bus with a small distance range but fast
(1 min) charging (Yaxing, China) (Reproduced from China Buses.com), (b) a hybrid energy-efficient forklift (Still, Germany) (Reproduced with
permission from Still GmbH), (c) off-shore variable speed wind turbines with enhanced reliability and efficiency (Reproduced with permission,
Copyright: F. Schmidt/Fotolia), (d, e) hybrid energy-efficient automated stacking and harbor cranes (Gottwald, Germany) (Reproduced with
permission of Gottwald Port Technology GmbH), (f) an electric ferry with rapid charging and low vibration operation (STX Europe, South Korea).
(Reproduced with permission from STX Europe)
value, the pseudocapacitance will exhibit a sharp peak
at certain voltage values.8
Energy density of an EDLC is determined by
the capacitance of positive, C+ , and negative, C− ,
EDLC
electrodes and the maximum voltage, Vmax
, at which
the device can be operated:
2
C− · C+
EDLC
EDLC
=
.
(1)
E
· Vmax
C− + C+
When the capacitance of both electrodes is the
same, the EDLC energy is maximized:
EEDLC =
1 EDLC 2
.
C· Vmax
2
(2)
To achieve high energy density, one shall
maximize the volumetric capacitance of each electrode
and increase the maximum allowed operational
voltage of EDLCs.
The maximum voltage is generally determined
by the stability window (from the lowest stable
stable
stable
potential Vlow
to the highest stable potential Vhigh
)
of the selected electrolyte (Figure 3(a)). Impurities
and functional groups on carbon, however, may
stable
catalyze electrolyte decomposition and increase Vlow
stable
or decrease Vhigh . The potential of each electrode
in a fully discharged symmetric EDLC is the same
stable
stable
and Vhigh
(Figure 3(a)).
and located between Vlow
When an EDLC is galvanostatically charged (by a
constant current), the potential of each electrode
changes linearly with time. In an ideal case, the
stable
at exactly the
negative electrode lowers to Vlow
same time as the potential of the positive electrode
stable
raises to Vhigh
. In this case, the maximum EDLC
EDLC
voltage Vmax can approach the maximum electrolyte
electrolyte
stable
stable
= Vhigh
− Vlow
. In a nonideal
stability Vmax
case, one of the electrodes reaches the boundary of
the allowed potential window faster than the other
(Figure 3(b)). In this case the highest voltage applicable
electrolyte
EDLC
will be lower than Vmax
.
to the EDLC Vmax
By doping one of the electrodes, however, one may
change their corresponding potential in the EDLC
fully discharged state in such a way as to counterbalance the difference in their rates of potential change
(Fig. 3c). Alternatively, by using different thicknesses
of the electrodes the slopes of the potential change
could be adjusted.
Asymmetric electrochemical capacitors utilize
different types of materials for positive and negative
electrodes. In most cases one of the double
layer porous carbon electrodes is replaced with a
battery-type higher capacitance electrode capable of
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
(a)
EDLC
(b)
electrolyte
Ideal EDLC: Vmax ≈ ΔV max
Electrolyte
oxidation
EDLC
Non-ideal EDLC: Vmax
Electrolyte
oxidation
stable
V high
Potential of an
EDLC cathode
Potential of an
EDLC cathode
Electrolyte
stability window
ΔV
electrolyte
< ΔV max
Initial potential of EDLC anode and cathode
electrolyte
max
Electrolyte
stability window
Max voltage
of an EDLC
Initial potential of EDLC anode and cathode
Potential of an
EDLC anode
electrolyte
ΔV max
EDLC
V max
Potential of an
EDLC anode
Max voltage
of an EDLC
EDLC
V max
stable
Electrolyte
reduction
Energy
(c)
V low
Electrolyte
reduction
Charge
Hold
Energy
Discharge
EDLC
electrolyte
Non-ideal after doping one of the electrode: Vmax
≈ ΔV max
Charge
Hold
Discharge
(d)
5.0
Electrolyte
oxidation
Organic electrolyte oxidation
Electrolyte
stability window
Initial potential of an EDLC cathode
Max voltage
of an EDLC
electrolyte
ΔVmax
Initial potential of a doped EDLC anode
EDLC
V max
Potential of doped
EDLC anode
Potential vs. Li/Li+
4.0
Potential of an
EDLC cathode
3.0
2.0
1.0
Electrolyte
reduction
Potential of EDLC
electrodes
Potential of an ‘EDLC’-like
cathode of an asymmetric
capacitor
Potential of a graphite anode
of an asymmetric capacitor
Vmax of an EDLC ~ 2.7 V
Vmax of an asymmetric cell ~ 4 V
Organic electrolyte reduction
0.0
Energy
Charge
Hold
Charge
Discharge
Charge
Hold
Hold
Discharge
Discharge
FIGURE 3 | Schematic energy diagrams of (a and b) regular and (c) doped EDLC electrodes as well as (d) electrodes in an asymmetric capacitor
upon charging and discharging. In (b) doping of one of the electrodes compensates for the difference in the capacitance between positive and
electrodes, thereby increasing the maximum operational voltage of an EDLC. In (d) a battery-like graphitic negative electrode operates at the
potential below the decomposition limit of the organic electrolyte. However, the layer of decomposed electrolyte (called SEI) is electrically isolative
and is stable, which prevents further electrolyte decomposition. The area within the discharge curves (colored in slightly darker green or darker blue)
is proportional to the energy stored in each device.
Faradaic ion storage. Such designs increase maximum
device voltage and the total energy density. For
example, in case of electrochemical capacitors with
organic electrolytes, lowering a potential of the
negative electrode (anode) below the stability value
will trigger electrolyte reduction. In some cases,
though, reduced electrolyte may produce stable
electrically isolative solid products that uniformly
coat the electrode surface, forming a so-called solidelectrolyte interphase (SEI) layer, which remain
impermeable to electrolyte solvent and thus prevents
further electrolyte reduction. Unfortunately, the SEI
disrupts formation of a double-layer and induces
resistance for the ion transport. However, if a double
layer negative electrode is replaced with a battery-type
electrode material capable of Faradaic ion storage
higher maximum device voltage can be achieved in
such an asymmetric capacitor (Figure 3(d)). In one
example, a porous carbon EDLC negative electrode
can be replaced with an intercalation-type graphitic
negative electrode in a Li-ion containing electrolyte. In
this case one can gain over 1.2 V of additional device
voltage (Figure 3(d)). Meeting the SEI stability and
high current performance requirements with graphite,
however, is challenging. The bonus advantage of
hybrid devices is significantly higher volumetric
capacity of battery-type electrodes over porous carbon
EDLC electrodes, which further increases the overall
device energy density. We will briefly review several
types of asymmetric capacitors in the end of this
review.
ELECTROLYTES
Electrolytes used in EDLCs may be divided into three
classes: (1) aqueous (solutions of acids, bases and
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
salts), (2) organic (for pure EDLCs—most commonly
solutions of tetraethylammonium tetrafluoroborate
(TEATFB) salt either in anhydrous acetonitrile (AN)
or propylene carbonate (PC) solvents), and (3) ionic
liquids (IL). Several recent publications provide a
comprehensive review of the room temperature IL
properties and compositions.9–11
Aqueous electrolytes are typically stable to
0.6–1.4 V in symmetric EDLCs, organic electrolytes to
2.2–3.0 V, and ILs to 2.6–4.0 V. As the energy density
of an EDLC strongly depends on the maximum voltage
applied to the device (Eqs. (1) and (2)), the choice of
electrolyte may significantly affect the energy density
of EDLC. For example, one of the key advantages of
IL-based EDLCs is their very high energy density. In
addition, ILs are nonflammable, which is important
for many mobile applications, including their potential
use in hybrid vehicles. Serious shortcomings of
ILs are their very high (often prohibitively high)
current cost and relatively low ionic mobility at
room temperature and below, which limits the
charge/discharge rate of IL-based EDLCs. Novel
methods of IL synthesis may potentially make them
more affordable. The advantages of using aqueous
electrolytes include their very low cost, safety and high
ionic conductivity. Their disadvantages, however,
include their low maximum applied voltage and, in
some cases, corrosion of EDLC electrodes observed
at higher temperatures and voltages (particularly for
acid-based electrolytes, such as H2 SO4 solutions),
which limits the cycle life of the EDLCs and
contributes to self-discharge. Organic electrolytes
are somewhat in between aqueous ones and ILs
in terms of the price, voltage and charge–discharge
time. Organic electrolyte-based EDLCs offer cycle life
in excess of 500,000, are used in the majority of
commercial EDLCs. In addition, EDLCs with organic
electrolytes are much less flammable than Li-ion
batteries.
ELECTRODE MATERIALS
This review largely focuses only on pure carbon
electrodes with only a few examples describing the use
of other materials in asymmetric capacitors. Current
technologies allow for commercial production of
various types of carbon materials with high surface
area, high electrical conductivity, as well as a range
of shapes, sizes, and pore size distributions (see
selected examples in Figures 4 and 5). Our goal
is to provide an overview of the published studies
of the common types of such carbons and, more
importantly, to share our view about their potential
advantages and limitations in EDLC applications. We
further aim to provide an overview of the current
understanding of the structure–property relationships
related to the transport and adsorption of ions within
these carbon materials. We would like to emphasize
that the extensive list of references provided in this
overview is not meant to be comprehensive.
We shall note that while volumetric capacitance
of capacitor electrodes is significantly more important
for applications than specific capacitance, the
majority of publications do not report volumetric
capacitance of the studied materials. In addition,
because volumetric capacitance could be affected by
calendaring (post-fabrication electrode densification)
and because precise estimation of electrode thickness
is more difficult than precise estimation of electrode
mass, reporting volumetric capacitance could be
prone to errors. As such, we mostly report specific
capacitance in our review.
Activated Carbons
Activated carbon (AC) is a form of disordered carbon
with small pores and large specific surface area (SSA).
ACs are prepared by thermal decomposition and
partial oxidation (activation) of organic compounds,
including a very wide selection of natural and
synthetic precursors. AC is the oldest and the most
common type of porous carbons. The use of AC
in Egypt was described as early as in 1550 BC.20
Industrial production of ACs in the U.S. started
in 1913.21 Although the first industrial production
of activated carbons started nearly a century ago,
achieving precise control over the pore structure has
been very challenging despite extensive studies and
improvements in activation processes. Most of the
pores in activated carbons are in the 0.4–4 nm range,
and the pore size distribution is generally relatively
wide, which may limit their performance in some
EDLC applications.
The common natural precursors for activated
carbon synthesis include coal, petroleum coke, pitch,
wood, nutshells, peat, lignite, and more exotic ones
include starch, sucrose, corn grain, leaves, coffee
grounds, and straw.22–42 More advanced ACs with
better developed porosity, reproducible properties and
more uniform microstructure and pores are produced
from synthetic polymers, such as polyacrylonitrile
(PAN), polyvinylidene chloride (PVDC), polyfurfuryl
alcohol (PFA), polyvinyl chloride (PVC), polypyrrole (PPy), polyaniline (PANI), polydivinylbenzene
(PDVB),22,43–49 to mention a few. In fact, most
organic materials rich in carbon that do not fuse upon
thermal decomposition can be used as precursors.
The activation process is generally divided into
two categories: (1) thermal (also called physical) and
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
(a)
SEM
5 μm
(d)
CDC
SEM
500 nm
(b)
SEM
2 μm
(e)
SEM
1 μm
ZTC
CDC
Graphene
(c)
SEM
5 μm
AC
(f)
TEM
5 nm
Carbon onions
FIGURE 4 | Electron microscopy images of high surface area carbon materials: (a) scanning electron microscopy (SEM) of micron-scale
micro-/meso-/macro-porous CDC (Reproduced with permission from Ref 12. Copyright 2006, Elsevier), (b) SEM of mesoporous silica-templated CDC
(Reproduced with permission from Ref 13. Copyright 2011, John Wiley and Sons, Ltd), (c) SEM of AC particles, (d) SEM of ZTC (Reproduced with
permission from Ref 14. Copyright 2010, American Chemical Society), (e) SEM of multilayer graphene flakes (Reproduced from Ref 15. Copyright
2011, Wiley-VCH), (f) transmission electron microscopy (TEM) of carbon onions. (Reproduced with permission from Ref 16. Copyright 2007,
Elsevier)
500 μm
AC fabric
40 μm
CNT forest
2 μm
CNT fabric
300 nm
CNT paper
AC fibers
(e)
(d)
0.4 mm
(c)
(b)
(a)
(f)
500 nm
CNT forest
FIGURE 5 | SEM micrographs of high surface area carbon materials: (a) AC fabrics (Reproduced with permission from Ref 17. Copyright 2013,
John Wiley and Sons, Ltd), (b) AC fibers (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd), (c) CNT fabric, (d and e)
vertically aligned CNT forest (Reproduced with permission from Ref 18. Copyright 2013, John Wiley and Sons, Ltd), (f) randomly oriented CNTs within
CNT paper mats. (Reproduced with permission from Ref 19. Copyright 2012, Royal Society of Chemistry)
(2) chemical. Production of ACs by physical activation
involves carbonization of a precursor (removal of
noncarbon species by thermal decomposition in inert
atmosphere) and gasification (development of porosity
by partial etching of carbon during the annealing
with an oxidizing agent, such as CO2 , H2 O, or
a mixture of both).50,51 Production of ACs by
chemical activation generally involves the reaction
of a precursor with a chemical reagent (such as
KOH,52–56 H3 PO4, 57 ZnCl2, 58 H2 SO4, 59 among a
few) at elevated temperatures and generally results
in smaller pores, more uniform pore size distribution
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
and higher specific capacitance in both aqueous and
organic electrolytes.
Activated Carbon Powders
Nearly all commercial EDLCs utilize high surface
area AC powders due to their well-developed
manufacturing technologies, easy production in
large quantities, relatively low cost and great
cycle stability.3 During electrode formation, AC
powders are mixed with polymer binders (often
polytetrafluoroethylene, PTFE), casted on current
collector foil (for EDLCs with organic and IL
electrolytes, thin Al foil is commonly used as a current
collector) and dried. Commercial ACs commonly offer
SSA in the range of 700–2200 m2 /g and moderately
high specific capacitance in the range of 70–200 F/g in
aqueous and 50–120 F/g in organic electrolytes60–64
(Table 1). Furthermore, the recent developments in
the synthesis of ACs having greatly enhanced specific
capacitance (up to 250–300 F/g in aqueous, organic,
and IL-based electrolytes) demonstrate that for a
significant portion of EDLC applications ACs may
remain the material of choice27,43,65,75,77,83,84,89,95–97
(Table 1).
Novel methods of AC synthesis offer better
control of their chemistry and pore size distribution.
In one recent paper, for example, direct activation of
synthetic polymers (such as PPy) was demonstrated
to produce ACs exhibiting pore volume in
the range of 1.1–2.39 cm3 /g and surface area
in the range of 2100–3430 m2 /g, as estimated
by nitrogen sorption technique. When tested in
symmetric EDLC with 1-ethyl-3-methylimidazolium
tetrafluoroborate (EMImTFB) IL electrolyte, these
carbons demonstrated specific capacitance of up
to 300 F/g, which is a more than two-fold
improvement compared to state-of-the-art commercial
ACs (Figure 6(a)). Furthermore, charge–discharge
tests performed at 60 ◦ C showed no visible
degradation and even a small increase in the
specific capacitance after 10,000 galvanostatic
cycles.
As readers may have noticed, some of
the reported surface area values in these novel
ACs exceed theoretical surface area of a perfect
single-walled graphene. However, formation of
vacancies or agglomeration of vacancies within
individual graphene layers can allow for the
nitrogen (gas molecule) adsorption on the edges of
graphene segments, leading to such incredibly high
(>2700 m2 /g) measured specific surface areas. We
shall also warn the readers that there are no tools
capable of estimating the surface area of microporous
materials precisely. Gas sorption techniques rely on
theories that simplify a gas sorption process and shape
of the pores in carbon. As a result, the reported
surface areas should be considered for estimating
purposes only.
In order to achieve higher surface areas and
eliminate bottle-neck pores (which may exist in ACs
and slow down transport of ions) while uniformly
enlarging the smallest micropores produced in the
course of carbonization of organic precursors, several
promising routes were proposed. According to one
method an equilibrium content of oxygen-containing
functional groups are uniformly formed on the
porous carbon surface during room temperature
treatment in acids.46 These groups together with the
carbon atoms are later removed via heat treatment
at 900 ◦ C. Repetition of the process of uniform
formation of chemisorbed oxygen functional groups
and subsequently removing them, allows for the
uniform pore broadening needed to achieve the
optimum PSD.46 In another study, an environmentally
friendly low-temperature hydrothermal carbonization
was utilized in order to introduce a network of
uniformly distributed oxygen within the carbon
structure in one step.27 This material, produced from
natural precursors (such as wood dust and potato
starch), was then transformed into microporous
carbons with high SSA of 2100–2450 m2 /g via
simultaneous heat-treatment (and thus uniform
removal of the oxygen-containing functional groups
from the internal material surface) and opening
of closed and bottle-neck pores by activation.
A very high specific capacitance of 140–210 F/g
was demonstrated in a TEATFB-based organic
electrolyte.27
Activated Carbon Films and Monoliths
Formation of EDLC electrodes from films and
monoliths of ACs allows for a significant increase in
their electrical conductivity (due to the elimination of
both the nonconductive binder and the high resistance
particle-to-particle point contacts) and volumetric
capacitance (due to the elimination of the large
macropores between the particles).47,72,79,82,87,88 For
example, meso/microporous AC monoliths (1 mm
thickness) produced by chemical activation (KOH) of
mesophase pitch precursor and exhibiting SSA of up
to 2650 m2 /g and surface area of micropores of up to
1830 m2 /g showed an outstanding initial capacitance
of up to 334 F/g (307 F/cm3 ) in 1 M H2 SO4 ,79
which is one of the highest volumetric capacitance
reported for carbon materials. Nitrogen-doped
macro/meso/microporous AC monoliths (cylindrical
shape with up to 17 mm diameter) having a very
moderate SSA of 772 m2 /g were recently shown to
exhibit high specific capacitance of up to ∼200 F/g
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
TABLE 1 Electrochemical Performance of Activated Carbon in EDLCs
Reported
Activation
Precursor
BET-SSA
Capacitance
(m2 g−1 ) (F g−1 )/Cell Type
Method
—
Physical-H2 O
2240
240/2 symm
—
Physical-CO2
1050
Melamine mica
30% HNO3 then
ammonia treatment
Wood sawdust
Pitch CF
Poly(vinylidene chloride)
(PVDC)
Chemical-KOH
—
Chemical-KOH
2500
Polyvinyl alcohol
Chemical-KOH
2218
Polyvinyl alcohol
Chemical-KOH
2218
Phenol formaldehyde resin
Chemical-KOH/ZnCl2
2387
Electrolyte
Organic
electrolyte
Ref
1.2 M TEABF4 in
acetonitrile (AN)
50
52/2 symm
1M TEABF4 in
polycarbonate (PC)
51
3487
148/2 symm
1M TEABF4 in PC
65
Chemical-KOH
2967
236/2 symm
1M TEABF4 in AN
27
Chemical-KOH
770
46/2 symm
TEABF4 in PC
66
2050
38/2 symm
1M TEABF4 in PC
54
110/2 symm
1M TEABF4 in AN
64
115/2 symm
1M Et3 MeNBF4 in PC
49
147/2 symm
1M LiPF6 in EC-DEC
49
142/2 symm
1M Et3 MeNBF4 in PC
67
Polybenzimidazol
Chemical-N2
1220
23/2 symm
0.8M TEABF4 in PC
68
Lignocellulosic materials
—
2300
95/2 symm
1.5 M TEABF4 in AN
61
Lignocellulosic materials
—
2315
125/2 symm
1.7M N(C2 H5 )4 CH3 SO3
in AN
62
Polyacrylonitrile (PAN)
—
1340
66/2 symm
1 M LiPF6 in EC-DEC
69
PAN
—
1340
90/2 symm
1 M TEABF4 in PC
70
Coconut shell
—
1692
22/2 symm
1 M LiClO4 in PC
71
Pitch CF
—
1000
21/2 symm
1 M LiClO4 in PC
71
Pitch CF
—
1500
24/2 symm
1 M LiClO4 in PC
71
Phenol resin
—
1232
3/2 symm
1 M LiClO4 in PC
71
Phenol resin
—
1542
18/2 symm
1 M LiClO4 in PC
71
Pitch
—
1016
1/2 symm
1 M LiClO4 in PC
71
Pitch
—
1026
2/2 symm
Sulfonated
poly(divinylbenzene)
physical-CO2
2420
206/3
Rubber wood sawdust
physical-CO2
913
138/2 symm
Poly (amide imide)
Physical-CO2
1360
196/3
Aqueous
electrolyte
1 M LiClO4 in PC
71
2M H2 SO4 aq. sol.
47
1M H2 SO4 aq. sol.
72
6M KOH aq. sol.
73
Pitch fiber
Physical-H2 O
880
28/2 symm
1M KCl aq. sol.
74
Coconut shell
Chemical-Melamine and
urea
804
230/2 symm
1M H2 SO4 aq. sol.
75
Melamine mica
Chemical-30% HNO3
then ammonia
treatment
86
115/2 symm
1M H2 SO4 aq. sol.
65
Phenolic resin
Chemical- 2M HNO3
—
60/2 symm
6M KOH aq. sol.
76
graphite
Chemical- HNO3 /H2 SO4
(1:1)
—
1071/3
0.1M KOH aq. sol.
59
Rice husk
Chemical- H2 SO4
—
175/3
6M KOH aq. sol.
40
Wood sawdust
Chemical-KOH
2967
143/2 symm
6M KOH aq. sol.
27
Eggshell
Chemical-KOH
221
297/3
6M KOH aq. sol.
41
polystyrene
Chemical-KOH
2350
258/3
6M KOH aq. sol.
77
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
TABLE 1 Continued
Reported
Precursor
Activation
BET-SSA
Capacitance
Method
(m2 g−1 )
(F g−1 )/Cell Type
Electrolyte
Ref
Viscose fibers
Chemical-KOH
1304
270/3
7M KOH aq. sol.
78
Celtuce leaves
Chemical-KOH
3404
273/2 symm
2M KOH aq. sol.
36
Polyvinyl alcohol
Chemical-KOH
2218
218/2 symm
30 wt.% KOH aq. sol.
49
Petroleum residue
(ethylene tar)
Chemical-KOH
2652
334/2 symm
1M H2 SO4 aq. sol.
79
Phenol-formaldehyde resin
Chemical-KOH
1900
100/2 symm
1M H2 SO4 aq. sol.
52
Pitch
Chemical-KOH
1611
220/2 symm
2M H2 SO4 aq. sol.
55
Pitch
Chemical-KOH
3160
295/2 symm
2M H2 SO4 aq. sol.
56
Pitch
Chemical-KOH
2860
130/2 symm
1M H2 SO4 aq. sol.
53
Seed shell
Chemical-KOH
2100
355/3
1M H2 SO4 aq. sol.
39
Viscose fibers
Chemical-KOH
1304
340/3
4M H2 SO4 aq. sol.
78
Beer lees
Chemical-KOH
3557
188/3
0.1M H2 SO4 aq. sol.
37
glucose
Chemical-KOH
—
220/3
1M Na2 SO4 aq. sol.
42
Styrene-divinylbenzene (SC)
Chemical-H3 PO4
434
210/2 symm
1M H2 SO4 aq. sol.
57
PAN
Chemical- ZnCl2
824
174/2 symm
6M KOH aq. sol.
58
Camellia oleifera shell
Chemical- ZnCl2
2080
184/3
6M KOH aq. sol.
38
Camellia oleifera shell
Chemical- ZnCl2
2080
230/3
1M H2 SO4 aq. sol.
38
PAN
Chemical-O2
1290
150/2 symm
1M H2 SO4 aq. sol.
80
—
Cold-plasma treated
1270
142/3
0.5M H2 SO4 aq. sol.
81
Resorcinol and
formaldehyde (RF)
—
603
198/2 symm
2M H2 SO4 aq. sol.
82
Seaweed
—
746
264/2 symm
1M H2 SO4 aq. sol.
83
Seaweed
—
270
198/3
1M H2 SO4 aq. sol.
84
PAN
—
302
202/2 symm
1M H2 SO4 aq. sol.
85
Coconut shell
—
1692
36/2 symm
1M H2 SO4 aq. sol.
71
Pitch CF
—
1000
32/2 symm
1M H2 SO4 aq. sol.
71
Pitch CF
—
1500
36/2 symm
1M H2 SO4 aq. sol.
71
Phenol resin
—
1232
40/2 symm
1M H2 SO4 aq. sol.
71
Phenol resin
—
1542
33/2 symm
1M H2 SO4 aq. sol.
71
Pitch
—
1016
1/2 symm
1M H2 SO4 aq. sol.
71
Pitch
—
1026
28/2 symm
1M H2 SO4 aq. sol.
71
Poly(vinylidene chloride)
—
700
64/2 symm
1M H2 SO4 aq. sol.
86
PAN
—
302
126/2 symm
6M KOH aq. sol.
85
Poly-aniline (PANI)
—
400
125/3
6M KOH aq. sol.
48
RF
—
722
199/3
6M KOH aq. sol.
87
—
—
1151
150/2 symm
6M KOH aq. sol.
88
—
—
2500
275/3
6M KOH aq. sol.
89
6M KOH aq. sol.
90
30 wt.% KOH aq. sol.
91
Li2 SO4 aq. sol.
89
Phenolic resin/PVA
—
416
171/3
Dimethyl acetamide/
polybenzimidazol
—
1220
178/2 symm
—
—
2500
210/3
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
TABLE 1 Continued
Reported
Precursor
Activation
BET-SSA
Capacitance
Method
(m2 g−1 )
(F g−1 )/Cell Type
Pitch fiber
Chemical-KOH
2436
224/2 symm
Electrolyte
Ionic liquid
electrolyte
N,N-diethyl-N-methyl(2methoxyethyl)ammonium
tetrafluoroborate (DEME-BF4 )
Ref
92
PAN
Chemical-NaOH
3291
196/2 symm
LiN(SO2 CF3 )2 + C3 H5 NO2
93
—
Oxygen-plasma
treated
2103
204/3
N,N-diethyl-N-methyl(2-methoxyethyl)
ammonium bis(trifluoromethyl
sulfonyl)imide (DEME-TFSI)
94
—
—
1428
60/3
N-butyl-N-methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide
(PYR14-TFSI)
95
in 6 M KOH electrolyte.87 In another recent study,
S-containing macro/meso/microporous AC monoliths
produced by physical (CO2 ) activation of carbonized
PDVB also demonstrated specific capacitance of up
to ∼200 F/g in 2 M H2 SO4 electrolyte47 (volumetric
capacitance was not provided in the last two studies,
but it is not expected to exceed ∼40 F/cm3 due to the
presence of high content of macropores). A recent
publication on patterned thin (1–3 μm) AC films
reported specific capacitance over 300 F/g in 1 M
H2 SO4 electrolyte.101
Activated Carbon Fibers
Activated carbon fibers/fabrics (Figure 5(a)) similarly
do not commonly require polymer binder for the formation of EDLC electrodes and exhibit high electrical
conductivity.66,68–71,73,74,78,81,85,86,92–94,102,103 In contrast to the monolithic electrodes, however, AC fabric
electrodes could offer very high mechanical flexibility. Their often higher power characteristics originate
from the smaller electrode thickness and the presence
of high volume of macro/mesopores between the individual fibers. Depending on the fiber diameter and the
activation process utilized, the ion transport and the
overall specific power of AC fiber-based EDLC may
vary in a broad range.
Apart from catalyst-grown carbon fibers and
CNTs (to be discussed in a subsequent section),
the smallest diameter AC fibers are produced
by carbonization and activation of electrospun
polymer solutions.67,68,85,90,91,104–106 The produced
AC nanofiber electrodes exhibit outstanding rate
capability, but suffer from low density.68,85 In fact,
the density of high-power AC fiber electrodes is often
noticeably lower than that of AC powder electrodes,
which leads to their lower volumetric capacitance.
However, several studies have demonstrated very
promising performance of dense AC fiber-based
EDLCs. For example, pitch-derived carbon fiber
electrodes (individual fiber diameter in the range of
2–30 μm) physically activated in an H2 O stream to
moderately high SSA of 880 m2 /g while retaining high
density (up to 0.8 g/cc) exhibited specific capacitance
of up to 112 F/g (90 F/cm3 ) in 1 M KCl electrolyte.74
Chemical activation parameters can similarly be
used to control the density and porosity of carbon
fibers. For example, chemically (KOH) activated
mesophase pitch based carbon fibers showed SSA
increase from 510 to 2436 m2 /g upon increase in
the KOH-to-C ratio from 1.5 to 4.92 The highest
gravimetric capacitance in both ILs and TEATFBbased organic electrolytes (up to ∼180 F/g) was
achieved in the sample with the highest SSA, while the
highest volumetric capacitance (up to ∼88 F/cm3 ) was
achieved in moderately activated fibers with SSA of
1143 m2 /g.92
Oxygen-containing plasma treatment of
AC fibers was found to increase their SSA and
specific capacitance in aqueous (0.5 M H2 SO4 )
electrolyte.81,94 Interestingly, treatment in a pure
O2 atmosphere at moderate temperatures (∼250 ◦ C)
did not significantly change the SSA, but introduced
higher content of C=O functional groups, which
resulted in an increase of specific capacitance from
120 to 150 F/g in 1 M H2 SO4 electrolyte, presumably
due to improved wetting and a higher contribution
from pseudocapacitance produced by the introduced
functional groups.80
In spite of multiple recent improvements, the
key challenge with traditional AC technology is
how to independently control the SSA, pore volume,
pore size and shape in these materials. Thus, other
synthesis techniques were developed to address these
limitations.
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
Electrolyte: EMImTFB (IL)
(a)
(b)
Tests performed at +60 °C
Activation at 600 °C At 650 °C At 700 °C
250
200
150
100
50
10 µm
0
AC electrodes (250 μm)
1
10
100
Scan rate (mV/s)
Tests performed at −70 °C
100
0
Produced at 800 °C
−100
−200
At 900 °C At 700 °C
−300
−400
−2.0
1000
ZTC electrodes (200 μm)
−1.5
100
Specific capacitance (F/g)
Contact pads
Carbon onions
electrodes
10–1
3.5V/25 mF
supercapacitor
63V/220 μF
Electrolytic capacitor
Substrate
20
40
60
80
1.5
2.0
100
120
140
160
180
Activated graphene electrodes
(20 μm)
100 mV/s
200 mV/s
500 mV/s
100
0
−100
−200
10–4
0
1.0
Tests performed at + 20 °C
200
10–3
−0.5 0.0
0.5
Voltage (V)
Electrolyte: BMIMTFB in AN
Tests performed at +20 °C
Carbon onions
Microsupercapacitor
Activated carbon
microsupercapacitor
10–2
−1.0
500 nm
(d)
Electrolyte: TEATFB in AN
101
Stack capacitance (F/g)
Specific capacitance (F/g)
Specific capacitance (F/g)
300
(c)
Electrolyte: SBPTFB in mixed organic solvents
200
200
250 nm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Potential (V)
Scan rate (V/s)
FIGURE 6 | Performance of some of the advanced high surface area carbons in EDLC applications: (a) specific capacitance of polymer-derived
activated carbons in an ionic liquid electrolyte at elevated temperature (Reproduced with permission from Ref 43. Copyright 2011, John Wiley and
Sons, Ltd), (b) low temperature cyclic voltammetry of ZTC recorded at the rate of 1 mV/second in an organic electrolyte (Reproduced with permission
from Ref 98. Copyright 2012, John Wiley and Sons, Ltd), (c) volumetric capacitance of micro-EDLC with OLC electrodes in an organic electrolyte
compared to that of typical electrolytic capacitor, EDLC and micro-EDLC with AC electrode recorded at ultra-high scan rates (Reproduced with
permission from Ref 99. Copyright 2010, Nature), (d) room temperature cyclic voltammetry of activated graphene in an organic electrolyte.
(Reproduced with permission from Ref 100. Copyright 2011, The American Association for the Advancement of Science)
Carbide Derived Carbons
Carbon produced by extraction of metals from metal
carbides is commonly termed carbide derived carbon
(CDC). Treatments in supercritical water or halogens
as well as vacuum decomposition have been explored
to etch metal atoms from various carbides, producing
carbon coatings and bulk porous material as well
as porous carbon powders, fibers and membranes.
Several recent publications review the progress in CDC
materials.107,108
The first manufacturing process reporting mass
production of CDC was developed in 1918109 and
was used until the 1960s as a method for the
production of SiCl4 from SiC by treating SiC in a
dry Cl2 gas etchant at temperatures above 1000 ◦ C.
The produced SiCl4 was collected in a condenser,
while CDC was commonly discarded or fully oxidized
during the chloride manufacturing process. In 1959,
the CDC itself drew some attention because in contrast
to carbon produced from organic precursors, CDC
was found to contain a minimal content of hydrogen
and other contaminants. In the last decade, it was
further discovered that CDC may offer other unique
technological advantages and additionally serve as a
good model material for systematic studies of ion and
gas adsorption.108,110–114
For EDLC applications, CDC is generally
produced by treatment of carbides in a chlorine gas.
In the case of binary carbides a simplified reaction can
be written as: MeCx + 0.5Cl2 = MeCly + xC, where
Me could be Al,115 B,116–119 Ca,120 Cr,121 Mo,122,123
Si,13,114,124–128 Ti,114,129–132 V,133 W,134 Zr,111,135
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
or other carbide-forming metals. In addition, CDC
with enhanced pore volume could be produced using
ternary carbides, such as Ti2 AlC, Ti3 AlC2 , and
Ti3 SiC2. 136–138 For thin films of less than several
microns the reaction is typically not limited by
diffusion139,140 and thus large size metal carbide
powders or thick films could be transformed
into CDC.
While various carbon structures can be
found in CDC, including nanodiamond,141 carbon
onions,141 graphene,142 graphite,142 and very dense
vertically aligned carbon nanotubes (CNTs),142,143
simple disordered carbon was found to be the
most attractive and abundant material for EDLC
applications.13,110,111,119,122,126,127,132,133 By selecting
different carbide precursors and changing the
chlorination temperature one can tune the average
pore size in CDC and study the effect of the pore size
on the electroadsorption of electrolyte ions of different
size.110,111,113
Carbide Derived Carbon Powders
and Nanopowders
The specific capacity of CDC in aqueous electrolytes is
moderately high. Depending on the preparation conditions and the selection of the initial carbide precursor
it commonly ranges from 60 to 220 F/g111,119,132,144
(Table 2).
In a 1 M H2 SO4 electrolyte, CDC produced
from micro-size particles of SiC (often termed
SiC-CDC) demonstrated specific capacitance up to
153 F/g,132 B4 C-CDC up to 147 F/g,119 ZrC-CDC
up to 160 F/g,111 Ti2 AlC-CDC up to 175 F/g,119
TiC-CDC up to 196 F/g,111,132 and CDCs derived
from a TiC/TiO2 mixture—up to 217 F/g.132 In
basic electrolytes, CDC may demonstrate higher
capacitance. For example, in 6 M KOH the
specific capacitance of B4 C-CDC and SiC-CDC
could be increased up to 178 F/g and 200 F/g,
respectively.116
While some of these values appear relatively
large, with the exception of Al4 C3 -CDC in 6 M KOH,
CDC cannot match the gravimetric performance
of some of the activated carbons in comparable
electrolytes. The surface chemistry of CDC may
partially explain such a moderate performance. The
high area-normalized capacity and frequency response
of aqueous electrolytes often correlates with the
degree of wetting of electrolyte on the carbon
surface.149 Unfortunately, Cl-terminated surface is
highly hydrophobic. Removal of Cl from CDC by
treatment in H2 or NH4 112,114 does not allow one
to achieve the desired high hydrophilicity of carbon
surface.
Because of the significantly reduced contribution of the pseudocapacitance, the total specific
and volumetric capacitance of CDC in organic
electrolytes is slightly smaller than in aqueous
electrolytes. Yet, in these high-voltage EDLC
applications CDCs outperform most of the nanostructured carbon materials, significantly exceeding
the capacitance of some commercial activated
carbons, particularly if normalized by the electrode
volume.13,110,111,115,119,122,123,127,129,132,146,150,151
TiC-CDC demonstrates one of the highest specific
capacitance in TEATFB, in excess of 160 F/g
Al4 C3 -CDC
showed
specific
(110 F/cm3 ).110
capacitance of up to 114 F/g in a triethylmethylammonium tetrafluoroborate
(EMATFB)—based
organic electrolyte.115 VC-CDC133 and Mo2 CCDC122 showed capacitance up to 133 F/g and
140 F/g in EMATFB electrolyte, respectively.
Activation of CDC may allow one to enhance
its ability for adsorption of gas molecules and
ions.152 Indeed, after activation of CDCs, their
surface chemistry could be modified and their pore
volume and specific capacitance could be significantly
enhanced. For example, the specific capacitance
of TiC-CDC in organic electrolyte was observed
to increase by up to 30% with chemical (KOH)
activation and reach 180 F/g.148
Decreasing the particle size of CDC to submicron
range was also shown to increase its surface area and
specific capacitance. A systematic study on SiC CDC
with varying particle size and synthesis temperature
showed up to a 200% increase in pore volume and
up to a 30% increase in BET SSA by decreasing
the size of CDC particles to 20 nm.145 The specific
capacitance in TEATFB—based electrolyte was found
to increase with increasing synthesis temperature
and decreasing particle size with the highest value
of 135 F/g achieved for 20 nm SiC-CDC synthesized
at 800 ◦ C. The rate performance of the produced
electrode, however, was not significantly improved
in spite of the dramatically shorter ion diffusion
path within individual particles. The nanoparticle
agglomeration within the electrode may have reduced
its ion migration capability. In addition, significantly
higher porosity of the EDLC electrode resulted in
inferior volumetric capacitance compared to that of
conventional CDC.
Carbide Derived Carbon Films
Formation of dense CDC films maximizes the volume occupied by the active material within the
EDLC electrode and thus maximizes its volumetric
capacitance.150,151 Unfortunately, however, processing CDC films makes it difficult to efficiently remove
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
TABLE 2 Electrochemical Performance of Carbide Derived Carbons in EDLCs
BET-SSA
Type of Carbide
2 −1
(m g )
Reported Capacitance
(F g−1 )/Cell Type
Electrolyte
Organic electrolyte
Ref
Al4 C3
1470
114/3
1M (C2 H5 )3 CH3 NBF4 in PC
115
Mo2 C
1855
143/2 symm
1M (C2 H5 )3 CH3 NBF4 in AN
122
Mo2 C
2082
132/3
1M (C2 H5 )3 CH3 NBF4 in PC
123
SiC
—
170/2 symm
1M TEABF4 in AN
107
SiC
2250
170/2 symm
1M TEABF4 in AN
127
SiC
2400
170/2 symm
1M TEABF4 in AN
126
SiC
2400
1853/2 symm
1M TEABF4 in AN
126
SiC
1300
135/2 symm
1.5M TEABF4 in AN
145
SiC
1190
105/2 symm
1.2M triethylmethylammonium
tetrafluoroborate (TEMA) in AN
128
TiC
1450
130/3
1M (C2 H5 )3 CH3 NBF4 in AN
146
TiC
1260
160/2 symm
1.5M TEABF4 in AN
110
TiC
1700
130/2 symm
1.5M TEABF4 in AN
130
TiC
1390
65/2 symm
1.5M TEABF4 in AN
147
TiC
1588
120/2 symm
1.2M TEMA in AN
129
TiC, KOH activated
1700
180/2 symm
1.5M TEABF4 in AN
148
VC
—
133/2 symm
1M (C2 H5 )3 CH3 NBF4 in AN
107
VC
1305
133/2 symm
1M (C2 H5 )3 CH3 NBF4 in AN
133
VC
1305
150/2 symm
1M TEABF4 in AN
133
B4 C
1400
147
1M H2 SO4 aq. sol.
119
B4 C
1461
177/3
6M KOH aq. sol.
116
CaC2
658
196/3
6M KOH aq. sol.
120
Mo2 C
1855
120/2 symm
6M KOH aq. sol.
144
Aqueous electrolyte
SiC
2729
200/2 symm
1M H2 SO4 aq. sol.
13
SiC
2430
153/2 symm
1M H2 SO4 aq. sol.
132
SiC
2380
200/3
6M KOH aq. sol.
116
—
175
1M H2 SO4 aq. sol.
119
TiC
1500
150/2 symm
1M H2 SO4 aq. sol.
111
TiC
1400
196/2 symm
1M H2 SO4 aq. sol.
132
TiC
1390
110/2 symm
1M H2 SO4 aq. sol.
147
TiC
—
150/2 symm
H2 SO4 aq. sol.
107
Ti2 AlC
ZrC
—
190/2 symm
H2 SO4 aq. sol.
107
ZrC
1350
190/2 symm
1M H2 SO4 aq. sol.
111
TiC
1269
160/2 symm
Ethyl-methylimmidazoliumbis(trifluoro-methanesulfonyl)imide
(EMI-TFSI)
113
the chloride contaminants during or after the synthesis and retain high rate capability. A recent study
provided a proof of concept for the fabrication of
EDLC electrodes with volumetric capacity in excess
of 175 F/cm3 in TEATFB electrolyte (the highest volumetric capacitance of carbon in organic electrolyte
IL
reported to date) and 160 F/cm3 in 1 M H2 SO4 for
2 μm thick films.151 The decrease in capacitance by up
to six times produced by increasing the film thickness
to 200 μm could be explained by the collapse of the
internal porosity caused by interface stresses and by
electrolyte starvation.
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
Carbide Derived Carbon Fibers
Formation of carbide nanofibers or nanofiber mats
has opened the opportunity for the formation of CDC
(nano)fibers153 with enhanced power characteristics.
CDC nanofibers produced from SiC precursor fibers
demonstrated a BET SSA of 1050 m2 /g and a pore
volume of 0.53 cm3 /g.128 Their specific capacitance
in TEATFB electrolyte reached only 78 F/g, which
was 25% lower than that of micron-sized SiCCDC powder tested at identical conditions.128
TiC nanofibers prepared using a combination of
electrospinning and carbo-thermal reduction allowed
formation of TiC-CDC nanofibers with 100–200 nm
diameters, BET SSA of up to 1390 m2 /g and pore
volume of up to 1.5 cm3 /g.147 While their specific
capacitance was found not to exceed 65 and 100 F/g in
TEATFB and H2 SO4 -based electrolytes, respectively,
they showed improved capacity retention at faster
sweep rate and higher current densities than TiC-CDC
produced from conventional powders.147 A likely
reason for the small capacitance values observed is
the presence of amorphous carbon in the initial TiC
precursor fibers.
Template-Assisted Synthesis of Carbide Derived
Carbon
In EDLC electrodes composed of micron-size
particles, the ion transport within individual particles
may largely govern the EDLC’s overall power
storage and the charge–discharge time.1 While
regular CDC powder allows one to achieve high
gravimetric and volumetric capacitance in organic
electrolytes,110,115,122,133,148 the slow ion transport
in small curved micropores leads to a moderate
rate of charge and discharge. Decreasing the size
of the particles to 20–200 nm dimensions by
synthesizing CDC nanopowder or felt128,145,147 makes
the electrode preparation more challenging, reduces
the volumetric capacitance and electrical conductance
of the electrode and at the same time does not
provide a break-through improvement in power
characteristics of the EDLCs because small curved
mesopores between individual nanoparticles impede
the ion transport within the electrode.
A more promising approach for power
enhancement may include the formation of CDC
powder with a combination of straight mesopore
channels for fast ion transport and subnanometer
pores in the volume between the large channels for
high specific capacitance.13,126,127 So far, formation of
such CDC has been demonstrated from mesoporous
SiC precursor, prepared using mesoporous SiO2
template and polycarbosilane precursor13,126,127
(Figure 7). While CDC produced from nonporous
Infiltration with
polycarbosilane
and pyrolysis
SBA-15: mesoporous SiO2
SiO2 etching
in HF
Microporous
CDC nanorods
Chlorination
of SiC
CDC: inverse replica of SBA-15
Large mesopores between the nanorods
for rapid electrolyte transport
Mesoporous SiC:
inverse replica of SBA-15
FIGURE 7 | Schematic of the formation of CDC with dual pore size
distribution—ordered mesopores for rapid ion transport and rapid
charging of EDLCs and small disordered micropores for high volumetric
capacitance. (Reproduced with permission from Ref 127. Copyright
2010, American Chemical Society)
SiC micropowder require at least 900 ◦ C for complete
chlorination108,154 mesoporous SiC powder can be
completely transformed into CDC at temperatures
below 700 ◦ C, and exhibits BET SSA in the range
of 2250–2729 m2 /g and pore volume in the range
of 1.4–2.0 cm3 /g.13,126,127 The produced SiC-CDC
exhibits impressive capacitance of up to 170 F/g
(64 F/cm3 ) in TEATFB-based organic electrolyte,127
up to 180 F/g (and up to 51 F/cm3 ) in 1-ethyl-3methylimidazolium tetrafluoroborate (EMITFB) ionic
liquid electrolyte at room temperature and up
to 200 F/g (68 F/cm3 ) in 1 M H2 SO4. 13 More
importantly, the preparation and application of
templated CDC overcame the present limitations
of nonaccessible surface because of bottle necks
and poor control over the pore size distribution
in the carbons currently used, and showed a route
for further performance enhancement. The ordered
mesoporous channels in SiC-CDC allow for very fast
ionic transport into the bulk of the CDC particles,
thus leading to an excellent frequency response
and outstanding capacity retention at high current
densities. The enhanced transport led to 85 and
90% capacity retention at current densities increasing
from 0.1 to ∼20 A/g in organic127 and aqueous13
electrolytes, respectively. This is even higher than what
was observed in CDC nanofiber felts.
Zeolite-Templated Carbons
Microporous zeolite-templated carbons (ZTCs) are
produced by carbon deposition on zeolite templates
followed by the removal of zeolite. A first publication
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
TABLE 3 Electrochemical Performance of Zeolite-Templated Carbons in EDLCs
BET-SSA
Type of Zeolite
Activation Method
2 −1
(m g )
Reported Capacitance
(F g−1 )/Cell Type
Electrolyte
Organic electrolyte
Ref
13X
Chemical-KOH
2970
176/2 symm
1.5M TEABF4 in AN
159
13X
Chemical-KOH
2864
160/2 symm
1.5M TEABF4 in AN
160
Y
—
1941
146/2 symm
1.5M TEABF4 in AN
161
FAU (diamond-like
framework)
—
1693
190/3
1.5M TEABF4 in AN
162
X
—
3040
168/3
1M TEABF4 in PC
163
Y
—
1820
300/2 symm
1M H2 SO4 aq. sol.
14
Y
—
1680
340/2 symm
1M H2 SO4 aq. sol.
164
Chemical-KOH
2970
259/2 symm
6M KOH aq. sol.
159
13X
reporting formation of porous carbon using sacrificial
zeolite templates in 1997 describes some of the
unique advantages of this technology.155 Zeolites
have uniform three-dimensional pore channels. By
depositing graphene monolayers uniformly on the
internal surface area of zeolites and by etching
the zeolite using a combination of acids, one may
produce porous carbon replicas with an ordered array
of micropores. The initial studies using propylene,
polyacrylonitrile (PAN) or polyfurfuryl alcohol (PFA)
as carbon precursors reported BET SSA in the
range of 1320–2260 m2 /g, micropore volume in the
range of 0.54–1.11 cm3 /g and total pore volume in
the range of 0.96–1.87 cm3 /g.155 At the time of
publication such surface area and well developed
porosity was the highest among carbons produced
without any activation. In the follow up studies a
combination of PFA infiltration and carbonization
with CVD carbon deposition using propylene allowed
the ZTC to retain the pore ordering of the original
zeolite template.156,157 BET SSA up to 3600 m2 /g was
reported.157 Further studies showed that using lowpressure vapor deposition could be sufficient for the
formation of uniform ZTCs.14,98 It was also found
that HF dissolution of zeolite template may leave small
cryolite (Na3 AlF6 ) residues at the end of the process,
which should be further dissolved using concentrated
H2 SO4 or other acids.14
A recent model proposes that at the atomic
level ideal ZTCs are made up of single wall carbon
nanotube (SWCNT) segments interconnected into a
three-dimensional regular network with aligned pore
channels.158 The important characteristics of the ZTC
for EDLC include the very high uniformity of pores,
as in the zeolite template, high surface area, pore
alignment and the opportunity to dope carbon without
distortion of the pore size distribution. The large
variety of zeolites with more than 300 types available
Aqueous electrolyte
allows one to select an appropriate template for the
desired pore size in a ZTC.
Very high specific capacitance of ZTC was
reported in both aqueous and organic electrolytes
(Table 3). By using an acrylonitrile (AN) precursor,
nitrogen-doped ZTC with a moderate BET SSA of
1680 m2 /g demonstrated an initial specific capacitance
of 340 F/g in 1 M H2 SO4 due to the pseudocapacitance
effect of the nitrogen-containing functional groups.164
The produced EDLCs retained more than 75%
capacity after 10,000 cycles. By using a low pressure
CVD from acetylene, ZTC was produced with
a BET SSA of up to 1820 m2 /g and a specific
capacitance in excess of 300 F/g in 1 M H2 SO4. 14 The
broad peak in the cyclic voltammetry curve revealed
pseudocapacitance reactions, which originate from
surface oxides unintentionally introduced at room
temperature during zeolite etching and purification.
In contrast to nitrogen-related pseudocapacitance,
which shows noticeable fading after 10,000 cycles,164
these ZTCs showed excellent stability as well as a
5% increase in capacitance after 10,000 cycles. More
importantly, this study demonstrated that ZTCs with
aligned pores may offer excellent rate capabilities
to EDLCs with the highest operating frequency (the
frequency at which 50% of the capacitance can
be accessed) approaching 10 Hz for 250 μm thick
electrodes.14
In organic electrolytes the performance of ZTC
is still very good but noticeably slower, which could
be related to the undesirable interactions between the
functional groups on the ZTC surface and electrolyte
solvents and possibly salts as well as larger ion
size, which may slow down the ion transport. ZTC
produced using ethylene or nitrogen saturated with
acetonitrile as carbon precursors and exhibiting BET
SSA in the range of 853–1941 m2 /g and total pore
volume in the range of 0.6–1.08 cm3 /g showed specific
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
capacitance in the range of 87–146 F/g in a TEATFBbased electrolyte.161 No positive effect of nitrogen
doping on the specific capacitance was observed
and, in contrast to many activated carbons165 and
CDCs,110 the specific capacitance linearly increased
with SSA, as would be expected for materials having
the same pore size and similar microstructure. The
pores in the produced samples were not well-aligned
and the capacitance decreased to 50–75% when
the current density was increased from 0.01 to 0.2
A/g.161 The highest decrease was observed in nitrogendoped ZTC having the highest nitrogen content.110
ZTC produced using acetylene CVD in different
types of zeolites (zeolite Y, zeolite X, and zeolite β)
and having aligned pores with a BET SSA in the
range of 1950–3040 m2 /g and pore volume in the
range of 0.97–1.59 cm3 /g showed specific capacitance
of 131–164 F/g in a TEATFB-based electrolyte.163 The
specific capacitance was found to linearly depend on
the BET SSA.163 The 180 μm thick ZTC electrodes
showed impressive retention of specific capacitance
(up to 96%) when the current density was increasing
from 0.05 to 2 A/g.163 Further improved ZTC
synthesis by the same group resulted in achieving
very impressive specific capacitance of up to 190 F/g
(83 F/cm3 ) and up to 80% capacitance retention for
current density increasing to 20 A/g in a TEATFBbased electrolyte.162
In addition to a high rate capability at room temperature, ZTCs are promising for high-performance
electrodes in EDLCs operating at temperatures as
low as −60 ◦ C98 (Figure 6(b)). This ability to quickly
store and deliver a significant amount of electrical
energy at ultra-low temperatures is critical for the
energy-efficient operation of high altitude aircraft and
spacecraft, as well as the exploration of the natural
resources available in Polar Regions and mountains.
According to conventional wisdom, mesoporous
electrochemical capacitor electrodes with pores large
enough to accommodate fully solvated ions are needed
for sufficiently rapid ion transport at lower temperatures. However, strictly microporous ZTC produced
using low pressure CVD with acetylene precursor and
having a moderate BET SSA of 1405 m2 /g demonstrated 146 F/g (90 F/cm3 ) at room temperature and
123 F g−1 (76 F/cm3 ) at −70 ◦ C with a proper organic
electrolyte.98 At −60 ◦ C selected cells based on 200 μm
electrodes exhibited characteristic charge–discharge
time constants of less than 9 seconds, which is
comparable to that of commercial devices operating
at room temperature and having smaller electrode
thickness.98
Two studies reported the effect of activation
on the ZTC performance. ZTC produced using PFA
infiltration and carbonization followed by CVD using
nitrogen saturated with acetonitrile as a precursor
with an optional KOH activation showed BET
SSA of up to 2864 m2 /g.160 The activation was
found to increase BET SSA and improve both
the ZTC’s H2 gas storage capability and specific
capacitance.160 Selected samples showed virtually no
decrease in specific capacitance (up to 160 F/g in
TEATFB-based electrolyte) when the current density
was increased from 0.25 to 2 A/g.160 In another
study on ZTC produced using ethylenediamine
and carbon tetrachloride as carbon precursors, a
KOH activation was found to significantly improve
the capacitance in both aqueous and organic
electrolytes.159 Interestingly, physical activation with
CO2 did not significantly improve the performance of
ZTCs. The KOH-activated ZTC with BET SSA up to
2970 m2 /g and micropore volume up to 1.04 cm3 /g,
showed specific capacitance up to 259 F/g in 6 M
KOH electrolyte and 176 F/g in 1.5 M TEATFB-based
electrolyte at the current density of 0.25 A/g.159
EDLC performance in an organic electrolyte, however,
was not very stable—the sample demonstrated 20%
capacity fading after 900 cycles.159
Carbon Nanotubes
Carbon nanotubes (CNTs) are carbon allotropes with
cylindrical 1-D structure. CNTs are consisted of either
one rolled-up graphitic sheet (single-walled CNT)
or several coaxial ones (multiwalled CNT). CNTs
(Figure 5(c)–(f)) are commonly grown via catalystassisted chemical vapor deposition (CVD) techniques
using hydrocarbon-based gaseous precursors, such
as methane, acetylene, propylene, and others. In
contrast to advanced ACs, CDCs, and ZTCs, CNTs
have relatively low SSA, low density and are
typically difficult to process into thick electrodes
using conventional electrode preparation methods.
Novel fabrication techniques, however, allow for
formation of vertically (relative to the substrate or
current collector) (Figure 5(d) and (e)) or horizontally
(Figure 5(c)) aligned CNT-based electrodes which do
not require any polymeric binder. While the low
CNT density and SSA limit volumetric capacitance
and energy density of CNT-based EDLCs’, the
high electrical conductivity and open porosity
of CNTs may allow for a fast ion transport
and thus high power characteristics of CNTbased EDLCs. Table 4 shows the broad range of
capacitance values reported for CNT-based EDLC
electrodes.
One of the earliest studies showing the potential
of thin (25 μm) CNT-based electrodes to provide high
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
TABLE 4 Electrochemical Performance of Carbon Nanotubes in EDLCs
BET-SSA
Activation Method
2 −1
(m g )
Reported Capacitance
(F g−1 )/cell Type
Chemical-KOH
1050
65/2 symm
Chemical-KOH
510
50/2 symm
Chemical functionalization
then LbL
—
250/half Li cell
Organic electrolyte
Electrolyte
Ref
1.4M TEABF4 in AN
166
1M LiClO4 in EC/DEC (1:1)
167
1M LiPF6 in EC/DMC (3:7 vol.)
168
—
985
90/2 symm
1.5M TEABF4 in AN
169
—
1064
79/2 symm
1M TEABF4 in AN
170
—
1000
80/2 symm
1M TEABF4 in PC
171
—
—
83/3
1M TEABF4 in PC
172
—
—
24/3
1M TBAPF6 in AN
173
—
—
79/3
1M LiClO4 in PC
174
—
782
75/3
Chemical-HNO3 ;
Mixed with PF then thermal
150
91/2 symm
Chemical-HNO3 ;
Mixed with PF then thermal
& chemical
120
21/2 symm
Chemical-HNO3
430
104/2 symm
38 wt.% H2 SO4 aq. sol.
178
Chemical-HNO3
411
80/2 symm
6M KOH aq. sol.
179
Chemical-KOH
1050
90/2 symm
6M KOH aq. sol.
166
Ammonia plasma
86.5
207/2 symm
6M KOH aq. sol.
180
Chemical-HNO3
430
49/2 symm
38 wt% H2 SO4 aq. sol.
181
Chemical functionalization
then LbL
—
160/3
1M H2 SO4 aq. sol.
182
Aqueous electrolyte
1M LiClO4 in PC
175
38 wt.% H2 SO4 aq. sol.
176
38 wt.% H2 SO4 aq. sol.
177
—
—
153/3
1M H2 SO4 aq. sol.
174
—
357
140/2 symm
7.5N KOH aq. sol.
183
—
—
18/2 symm
6M KOH aq. sol.
184
—
—
20/2 symm
6N KOH aq. sol.
185
—
—
20/2 symm
6M KOH aq. sol.
186
—
410
68
6M KOH aq. sol.
187
—
—
22
6M KOH aq. sol.
188
—
—
22.5/3
1M NaCl aq. sol.
189
∼400
440/2 symm
1-ethyl-3-methylimidazoliumbis
(trifluoromethylsulfonyl)imide
[EMIM][Tf2 N]
178
70
27/3
1-ethyl-3-methylimidazolium
tetrafluoroborate (EMIMBF4 )
190
Oxygen plasma
—
power and high rate performance was reported in
1997.181 In this study multiwalled CNTs (MWCNTs)
with an average diameter of ∼8 nm were treated
in nitric acid, processed into electrodes and tested
in concentrated (38 wt. %) H2 SO4 -based aqueous
electrolyte. While the CNTs showed moderately high
specific capacitance of 102 F/g, it offered a very high
operating frequency of up to 100 Hz.181
Ionic liquid
Another study performed in 1999 showed
MWCNTs with a diameter of 15–45 nm and
BET SSA in the range of 128–410 m2 /g having a
specific capacitance of up to 68 F/g in 6 M KOH
electrolyte when measured in three-electrode cells.187
A follow-up study by the same group investigated
electroadsorption of ions from aqueous (6 M KOH)
electrolyte on MWCNTs with 20–30 nm outer
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
diameter, BET SSA in the range of 128–411 m2 /g and
large pore volume in the range of 269–634 cm3 /g.179
Depending on the type of MWCNTs and the acid
treatments used, capacitance in the range of 4–80 F/g
was detected, with the highest values obtained in
MWCNTs with a herringbone morphology (edge
graphene planes exposed to the surface), the highest
BET SSA and a significant (up to 10.8 wt.%) amount
oxygen.179 The additional treatment of the best
MWCNT sample in concentrated (69%) nitric acid
at 80 ◦ C for 1 h increased the amount of functional
groups on its surface as well as its BET SSA to
475 m2 /g, leading to an increase in its capacitance to
137 F/g.179 It was noticed, however, that the oxygencontaining functional groups responsible for high
pseudocapacitance values often cause self-discharge in
the capacitors and thus the highly oxidized MWCNTs
may have limited applications.179
Carbon-MWCNT composite electrodes were
studied in 1999. After treatment in nitric acid
MWCNTs of 20–30 nm in diameter were mixed
with a phenol-formaldehyde (PF) resin, processed
into thin films and carbonized at 850 ◦ C, resulting
in a porous C-MWCNT composite with a BET SSA
of 150 m2 /g and pore volume of 0.45 cm3 /g.176 In
spite of the low SSA, a large capacitance of 90.8 F/g
(95.3 F/cm3 ) was reported in concentrated (38 wt.
%) H2 SO4 electrolyte.176 However, another study
by the same group reported much more modest
capacitance values (15–25 F/cm3 ) using very similar
processing technique: nitric acid—purified MWCNTs
of 20–30 nm in diameter, mixed with PF were
processed into thin films, carbonized at 850 ◦ C and
boiled in a mixture of sulfuric and nitric acids.177
These smaller capacitance values are more consistent
with the research results later reported by others on
MWCNTs (1–20 F/cm3 ).
Single-walled CNTs (SWCNTs) offer higher
SSA than MWCNTs, but tend to bundle and
may contain semiconducting tubes, which reduces
SSA available for ion storage. A 2000 report was
one of the first to describe the performance of
SWCNT bundles (1.2–1.4 nm tube diameter, 10 nm
bundle diameter) in EDLCs.189 The SWCNT bundles
were processed into a paper with a density of
0.3–0.4 g/cm3 and thickness of 25–40 μm, where
tubes hold together by van der Waals interactions.
BET SSA of the samples was not provided. In
three-electrode cells with aqueous electrolytes (1 M
NaCl, 7 M H2 SO4 , and 5 M KOH) most SWCNT
paper samples showed capacitance ranging from
19 to 26 F/g for scan rates up to 500 mV/second,
with several samples exhibiting capacitance as high
as 30–41 F/g in 1 M aqueous solution of NaCl.189
A small capacitance decrease (less than 15%) was
observed when the sweep rate was increased from 5
to 500 mV/second.189 Broad redox/pseudocapacitive
peaks observed in most SWCNT electrodes due to
the presence of oxygen-containing functional groups
were eliminated by annealing the SWCNT electrode
at 900 ◦ C in Ar.189 A follow-up study reported the
performance of SWCNT electrodes in 0.1 M aqueous
solutions of NaCl, CsCl, benzyltriethylammonium
chloride (BTEAC), LiNO3 , NaNO3 , Ca(NO3 )2 , and
trimethylammonium nitrate (TMAN), as well as in
organic solvent based electrolytes, such as LiClO4
and TBATFB in acetonitrile (AN) solvent.191 While
no well-defined Faradaic peaks were observed with
TBATFB-based electrolyte, a LiClO4 -based electrolyte
clearly demonstrated the presence of Faradaic
oxidation–reduction processes.191 Capacitance values
similar to that of Ref 189 were achieved. This study
was also one of the first to utilize electrochemical
quartz crystal microbalance (EQCM), and which
detected electrode mass changes of up to ∼1% in
aqueous electrolytes and in a TEATFB-based organic
electrolyte and up to ∼4% in a 1 M LiClO4 organic
solvent based electrolyte upon charging.191 When
used in IL electrolytes with different anions [such
as bis(trifluoromethanesulphonyl) amide (BFSA),
dicyanamide (DCA), hexafluorophosphate (HFP) and
p-tolunesulphonate(PTS)] and cations (such as R,R0imidazolium and R,R0-pyrrolidinium) SWCNT paper
demonstrated capacitance ranging from 14 to 24 F/g,
but selected IL electrolytes offer a very high voltage
stability window of up to 5.5 V, tested with Pt
electrodes.173 Highly purified SWCNTs with BET
SSA 462–782 m2 /g were tested in a 1 M LiClO4 -based
organic electrolyte in another study and exhibited
specific capacitance of 45–75 F/g, demonstrating good
rate capability, with less than 10% capacitance
decrease for current density increasing from 0.01 to
0.16 A/g.175
Significantly higher specific capacitance (up to
180 F/g in 7 M KOH electrolyte) of SWCNTs bundles
(with a bundle diameter of 10–20 nm) was soon
reported by another group.183 The heat-treatment
of SWCNT samples at temperatures up to 1000 ◦ C
increased their BET SSA from 210 to 350 m2 /g and
decreased the average mesopore size from 6.5 to below
3 nm.183 The higher volume of smaller pores was
offered as a possible explanation for the unusually
high capacitance observed.183
Chemical activation was proposed in 2002 to
increase electrochemical capacitance of MWCNTs
while retaining high electrical conductivity.166
Depending on the starting MWCNTs, the burnoff ranged from 20% to 45%, but the activation
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
allowed BET SSA increase up to 1050 m2 /g. The
specific capacitance in 6 M KOH aqueous electrolyte
reached 90 F/g, and in 1.4 M TEATFB-based organic
electrolyte reached 65 F/g.166 A similarly positive
effect of KOH activation of MWCNTs was also
observed in another study, which reported an increase
in BET SSA from 194 to 510 m2 /g after chemical
activation and a corresponding increase in specific
capacitance from 25 to 50 F/g in an organic electrolyte
based on LiClO4 salt.167
Composite electrodes composed of high surface
area porous carbons and CNTs may offer a
combination of high energy and power densities,
unattainable in one-component electrodes. For
example, the addition of 15 wt.% of CNTs to
‘regular’ ELDC electrodes based on activated carbons
was found to improve their power characteristics in
organic electrolytes without significant decrease in
specific capacitance.169
The first study of MWCNTs grown directly
on conductive graphite foil substrates was reported
in 2002. Misaligned MWCNTs with a diameter of
50 nm showed an unusually high specific capacitance
of 116–153 F/g in 1 M aqueous solution of H2 SO4 ,
and up to 13–79 F/g in 0.1 M LiClO4 organic solvent
based electrolyte.174 The calculation of MWCNT
capacitance in this study assumed invariance in the
capacitance and mass of graphitic foil substrates
after the CNT growth, which may be incorrect and
explain the unusually high capacitance observed.
Development of methods to grow CNTs on bulk
metal substrates (such as an Inconel alloy) improved
the current collector/CNT contact resistance and rate
capability.184 In this study a more moderate specific
capacitance of only 18 F/g was achieved in 6 M KOH
electrolyte.184 In another study, however, vertically
aligned MWCNTs grown to 0.15 mm on Inconel
foils showed specific capacitance as high as 83 F/g
at 1 mV/second and 47 F/g at 1000 mV/second in
1 M TEATFB organic solvent based electrolyte.172
Various MWCNT electrodes (including those grown
directly on Al foil) with BET SSA ranging from 47 to
1064 m2 /g and a density ranging from 0.1 to 0.9 g/cm3 ,
were also investigated in 1 M TEATFB organic
solvent based electrolyte.170 As their capacitance
values were quite modest (8–16 F/cm3 , over three
times smaller than that of commercial activated
carbons) the authors concluded that this technology
would unlikely be acceptable for mainstream EDLC
applications.170
Aligned CNT forests (Figure 5(d) and (e)) may
provide higher rate capability than misaligned CNTs
having similar thickness and mass loading due to
the reduced electrical resistance (no high resistance
point contacts which increase electrode resistance if
the individual CNTs are connected in a series) of the
electrode combined with a smaller pore tortuosity
and thus their lower ionic resistance. However,
the as-grown CNT forests commonly exhibit low
density (0.08–0.3 g/cc, which corresponds up to 98%
porosity) and thus low volumetric capacitance. A
simple method to produce electrodes with greatly
enhanced density was proposed in 2006.171 This
method described the introduction of liquid into
0.1–1.5 mm long aligned SWCNT forest (2.8 nm
average tube diameter), followed by the evaporation of
the liquid, which causes the electrode densification by
over 20 times (to an average tube separation distance
of less than 1 nm) and preserves the very high degree
of tube alignment and high BET SSA of 1000 m2 /g.171
The specific capacitance of these high-density SWCNT
electrode reached 80 F/g in TEATFB-based organic
electrolyte.171
Plasma treatment induces defects and function
groups on the surface of CNTs and other carbon
materials, which greatly enhance their specific
capacitance, as shown in Ref 180. Aligned ∼50 nm
diameter MWCNT electrodes were grown on Ni
foil substrates and studied in 6 M KOH electrolyte
solution. The ammonia plasma etching increased the
BET SSA from 9.6 to 86.5 m2 /g, while increasing
the specific capacitance from 37 to 207 F/g.180 These
values are unusually high for the modest surface area
of these samples (up to 239 μF/cm2 ). We shall note
that even if carbons are treated in plasma of oxygenfree or inert gases, their subsequent exposure to air
(which contains O2 , H2 O, CO2 and other species)
shall result in the satisfaction of the plasma-induced
dangling bonds in carbon with oxygen-containing
surface functionalities. It is likely that the introduced
functional groups provide high pseudocapacitance,
but the exact origin of such high area-normalized
capacitance remains unclear. Similarly high values of
specific capacitance of plasma-activated MWCNTs
were reported in the studies by another research
group.178 The use of oxygen plasma on 0.1–0.2 mm
tall vertically aligned MWCNT electrodes (5–10 nm
in diameter) resulted in opening of the CNT tips,
increasing the SSA to 400 m2 /g, as well as the
number of defects and functional groups on the
tubes’ outer walls, which, in turn, lead to outstanding
specific capacitance (>400 F/g) in IL electrolytes.178
This is significantly higher than that of similar
vertically aligned MWCNTs that were not exposed
to plasma etching/activation (27 F/g in ILs,190 and
11–22 F/g in 6 M KOH188 ). Unfortunately, plasma
activated samples decorated with functional groups
may potentially have issues with low cycle stability
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
and high leakage current, but such studies were not
presented in either of the articles.178,180
In addition to growing vertically aligned
CNTs on metal foil substrates170,171,174,180,184 or
transferring vertically aligned CNTs from ceramic
substrates to metal foils,192,193 the simplest and
the most conventional method for the preparation
of CNT electrodes for EDLC includes oxidation
or functionalization and dispersion of CNTs in
aqueous solutions followed by filtering and drying.189
This method, however, is not compatible with
commercial EDLC preparation techniques, does
not permit formation of thick (0.1–0.3 mm) and
dense (0.5–1 g/cm3 ) electrodes and suffers from low
precision and poor control over the electrode thickness
and porosity. One may argue that due to their
low specific surface area, pure CNT-based electrodes
will likely only find use in ultra-high power/low
energy applications anyway, which may, in turn, only
require thin (0.001–0.01 mm) electrodes. However, a
recent progress in the large-scale fabrication of ultrastrong CNT fabrics may open some opportunities for
the fabrication of lightweight, multifunctional and
flexible EDLCs with energy storage, electromagnetic
shielding and load-bearing functionalities, as recently
demonstrated for Li-ion batteries.194
Two common (for research laboratories)
methods for the formation of well-controlled
dense ultra-thin CNT films include layer-bylayer (LBL) deposition168,182 and electrophoretic
deposition.185,195 The repeated, sequential immersion
of a substrate into different aqueous solutions of
functionalized CNTs, some having only a positive
charge (such as amine functional groups, CNT–NH2 )
and others having only a negative charge (such
as carboxylic functional groups, CNT–COOH) on
their surface produces CNT-based LBL thin film
assembly.168,182 Ultra-thin (<1 μm) and nonetheless
self-supporting films could be prepared by crosslinking CNT bilayers at temperature as low as
150 ◦ C.182 The specific capacitance of LBL CNT films
was found to approach 160 F/g in 1 M H2 SO4, 182
which could be due to the pseudocapacitive effect of
the functional groups on their surface. Indeed, heattreatment of LBL CNTs in H2 atmosphere at 300 ◦ C
for 3 h reduced both the amount of functional groups
and the estimated specific capacitance (to 60 F/g).182
The binder-free LBL CNT films (up to 3 μm in
thickness) were investigated as a positive electrode in
asymmetric capacitors.168 The LBL assembled films
were estimated to exhibit unusually high density
of 0.8 g/cm3 (which corresponds to the porosity
of around 50%, noticeably smaller than typical
values for misaligned CNTs) and contained nearly
11 at.% oxygen and 4 at.% nitrogen.168 The LBL CNT
electrodes demonstrated ultra-high capacitance of
250 F/g in organic electrolyte based on a 1 M LiPF6 salt
dissolved in a mixture of carbonates when tested in the
voltage range from 1.5 to 4.5 vs. Li/Li+ .168 However,
a recent study of porous carbons (having higher SSA
and comparable content of similar functional groups)
showed over three times lower specific capacitance,
when measured in the same electrolytes in the same
potential range.196 As such, the pure contribution of
functional groups’ induced pseudocapacitance cannot
explain the origin of the extraordinary high specific
capacitance reported in LBL CNT films.
One serious limitation of the LBL assembling
route is a very slow fabrication rate. In comparison
to LBL assembling, electrophoretic (EP) deposition is
a significantly faster thin film fabrication technique. It
involves formation of a stable suspension of charged
CNTs and application of a potential (commonly
10–50 V) to a metal current collector to attract
charged CNT particles.185,195 The EP CNT films
with moderate capacitance (20 F/g in 6 M KOH)
demonstrated an ultra-high operating frequency in
excess of 5000 Hz and power density in excess of
20 kW/kg.185 Interestingly, the deposition of thin
MWCNT films on Ni foil from the CNT colloidal
suspension in dimethylformamide (DMF) was shown
to produce locally aligned MWCNT layers and result
in comparably high power characteristics.186
Onion-Like Carbon
Onion-like carbon (OLC) (Figure 6(f)), also called
‘carbon onions’ and ‘multiwalled fullerenes’,197,198 is
consisted of concentric spherical graphitic sheets. It
can be mass produced either by arc-discharge199 or by
annealing diamond nanoparticles (or nanodimond)
in an inert atmosphere at temperatures above
1200 ◦ C.200–202 Nanodiamond soot, in turn, is
commonly produced on a large scale by a controlled
detonation of carbon-containing explosives201 and
can be purified to high sp3 -content.200,203 With
moderately high BET SSA of 400–600 m2 /g, low
cost, relatively high conductivity, and the ability
to disperse in both polar and nonpolar solvents
OLC offer attractive properties for EDLCs and often
provide higher specific capacitance than MWCNTs16
(Table 5). In contrast to multiple publications devoted
to CNT-based capacitors, there are only a few papers
that have reported electrochemical studies of ND and
OLC.16,99,218–224
Annealing
of
nanodiamond
soot
at
1200–1800 ◦ C leads to an increase in electrical
conductivity of the compressed powders from 0.025
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
TABLE 5 Electrochemical Performance Of Onion-Like Carbon in EDLCs
BET-SSA
Precursor
Activation Method
Reported Capacitance
2 −1
(F g−1 )/Cell Type
(m g )
Nanodiamond (ND) —
520
38/2 symm
ND
—
520
∼1 F cm−3 /patterned 2D fingers
ND
Chemical-HNO3 + H2 SO4
(1:3 v/v)
420
143/3
ND
—
550
40/3
−3
ND
—
520
<5 F cm /3
ND
—
—
30/3
ND
—
550
100/3
to 4 S/cm, a decrease in the concentration of defects
and functional groups on the surface and a decrease in
the SSA-normalized capacitance from 9 to 4.5 μF/cm2
in organic TEATFB-based electrolyte.16 The specific
capacitance peaked at 38 F/g for the sample annealed
at 1200 ◦ C, while the best capacitance retention at
high current density was observed for the sample
annealed at 1800 ◦ C and containing less defects on
the surface.16 The operating frequency for the OLC
sample produced by 1800 ◦ C annealing approached
1 Hz for ∼0.4 mm thick (15 mg/cm2 ) electrodes.16
In aqueous electrolytes, OLC produced by a
similar route of annealing nanodiamond powders
exhibited specific capacitance in the range from 20
to 50 F/g in 1 M H2 SO4 and 15–20 F/g in 6 M KOH
electrolytes.218 OLC was also investigated in 1 M
H2 SO4 by using a microelectrode technique and
demonstrated excellent power characteristics with the
capacitance of OLC annealed at 1800 ◦ C remaining
virtually unchanged when the sweep rate was
increased from 0.1 to 10 V/second due to the low concentration of defects, small electrode thickness and the
lack of micropores in the sample.219 Yet, the volumetric capacitance of OLC remains small (<5 F/cm3 ).219
The specific capacitance of OLC could be significantly
enhanced by decorating its surface with pseudocapacitive materials,220,221 but this would decrease the
charging rate and is outside the scope of this review.
Without a binder, OLC films deposited
by casting are typically not stable in aqueous
solutions. However, OLC can be immobilized in
thin layers of tetra(n-octyl)ammonium bromide
(TOABr), poly(diallyldimethylammonium chloride)
(PDDA) or chitosan, which are insoluble in aqueous
solutions.222,223 In aqueous electrolyte solutions of
NaCl, NaClO4 , LiClO4 , and tetraethyl ammonium
chloride (TEACl) the OLC films immobilized with
the help of TOABr (OLC/TOABr) exhibit specific
capacitance of 10 F/g, while similar films composed
Electrolyte
Organic
Aqueous
electrolyte
Ref
1.5M TEABF4 in AN
16
1M TEABF4 in PC
99
1M H2 SO4 aq. sol.
224
1M H2 SO4 aq. sol.
218
1M H2 SO4 aq. sol.
219
0.1M H2 SO4 aq. sol. 223
6M KOH aq. sol.
218
of oxidized OLC films (OLC–COOH) immobilized
by TOABr show capacitance reduced to 6 F/g.222
The OLC/PDDA films of ∼25 μm in thickness
and OLC/chitosan films of ∼90 μm in thickness
demonstrated a moderately high capacitance of
20–30 F/gOLC in 0.1 M H2 SO4 and very good rate
capability with a constant capacitance for sweep rate
increasing from 0.02 to 1 V/second.223
Electrophoretic deposition is an alternative route
to deposit binder-free OLC films. In an organic
electrolyte, EP-OLC films of ∼5 μm in thickness
demonstrate power characteristics comparable to that
of electrolytic capacitors, while energy per volume is
an order of magnitude higher99 (Figure 6(c)). Small
changes in the specific capacitance were recorded
for sweep rate increasing up to an ultra-high value
of ∼ 100 V/second.99 The operating frequency of EPOLC films approaches 40 Hz,99 comparable to that
observed in EP-CNT films.185
OLC was also utilized in the preparation of
OLC-graphene composite films from an aqueous suspension of graphene oxide (GO) and nanodiamond.224
The OLC was found to efficiently prevent agglomeration of the hydrophobic graphene sheets during
the reduction of graphene oxide and thus achieve a
relatively high BET SSA of up to 417 m2 /g and a welldeveloped porosity in the 3–10 nm range.224 Specific
capacitance of the GO-OLC composite was surprisingly high for moderate SSA and ranged from 50 to
70 F/g in 1 M H2 SO4. 224 Furthermore, when oxidized
in a mixture of nitric and sulfuric acids the capacitance
nearly doubled and ranged from 80 to 143 F/g for
current densities from 10 to 0.2 A/g, respectively.224
Graphene
Graphene, in an ideal case, is one-atom thick layer of
graphite, which corresponds to the basal (0002) plane.
Graphene segments can be considered to be building
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
TABLE 6 Electrochemical Performance of Graphene in EDLCs
Reducing Reagent/
Method
Activation
Method
BET-SSA Reported Capacitance
(m2 g−1 )
(F g−1 )/Cell Type
Ref
1M TEABF4 in PC
204
1M TEABF4 in PC
205
1M TEABF4 in PC
206
H2
—
—
220/2 symm
Hydrazine
—
705
99/2 symm
Electrochemical
—
—
220/3
Hydrazine
—
—
132/3
Thermal exfoliation
—
925
117/2 symm
HBr
—
—
—
—
—
Hydrazine
—
320
205/2 symm
30% KOH aq. sol.
211
—
280/2 symm
6M KOH aq. sol.
204
H2
Organic
Electrolyte
Aqueous electrolyte 2M H2 SO4 aq. sol.
207
1M H2 SO4 aq. sol.
208
129/3
1M H2 SO4 aq. sol.
209
276/3
1M H2 SO4 aq. sol.
210
p-phenylene diamine
—
—
164/3
6M KOH aq. sol.
212
Hydrazine
—
—
120/2 symm
6M KOH aq. sol.
213
Hydrazine
—
705
135/2 symm
Thermal exfoliation
—
737
233/3
Chemical-KOH
492
136/3
1M Na2 SO4 aq. sol.
215
—
—
165/3
0.1M Na2 SO4 aq. sol.
216
Chemical-KOH
3100
166/2 symm
1-butyl-3-methyl-imidazolium
tetrafluoroborate (BMIMBF4 )
100
Thermal exfoliation
—
925
75/2 symm
N-butyl-N-methylpyroolidinium
bis(trifluoromethanesulfonyl)
imide (PYR14 TFSI)
208
HBr
—
—
65/3
1-butyl-3-methylimidazolium
hexafluorophosphate
(BMIHFP)
209
EMIMBF4
217
[Et3 NH][TFSA]
97
—
Electrochemical
—
—
—
500
250/2 symm
—
—
2500
164/3
blocks of many regularly structured carbons (such as
graphite and open-ended CNTs) and various irregular
porous carbons with disordered structure (such as AC,
CDC, and ZTC). The specific capacitance of graphene
varies in a broad range (Table 5).
One of the first studies of graphene structures
in different electrolytes was reported in 2008.208
Graphene was prepared by thermal exfoliation of
graphite oxide (GO) at 1050 ◦ C. This processing
resulted in a well-developed porosity and high BET
SSA (925 m2 /g). The specific capacitance of graphene
electrode in 1 M H2 SO4 (100–117 F/g for sweep rates
ranging from 1 to 0.01 V/second for exfoliated GO)
compared favorably with that of OLC, MWCNT and
SWCNT samples (Tables 4 and 6), but was inferior
to that of many AC, ZTC and CDC (Tables 1–3).
In ionic liquid (N-butyl-N-methylpyrrolidinium
bis(trifluoromethanesulfonyl)imide or PYR14 TFSI),
exfoliated and reduced GO exhibited specific capacitance of ∼75 F/g.208
Ionic liquid
5.5M KOH aq. sol.
205
2M KOH aq. sol.
214
An alternative method for graphene synthesis
involving suspending GO sheets in water and then
reducing them by using hydrazine hydrate was soon
introduced and provided formation of graphene
structures agglomerated into 15–20 μm size particles
exhibiting moderately high BET SSA of 705 m2 /g.205
Surprisingly, graphene particles showed a high specific
capacitance of 100–135 F/g in 5.5 M KOH electrolyte
and up to 99 F/g in organic electrolyte based on
1 M TEATFB salt.205 While both the specific and
more importantly volumetric capacitance of graphene
was significantly smaller than those of activated
carbons, these two publications generated significant
interest and multiple follow-up publications in
2009–2013.
In an effort to reduce agglomeration of
individual graphene layers, a reduction of GO
by hydrazine in gaseous phase was proposed.211
This process allowed one to increase the specific
capacitance of graphene to a very high initial value of
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
205 F/g in 30% KOH solution (reduced to 170 F/g
after 1200 cycles), in spite of the low measured
BET SSA of 320 m2 /g,211 presumably due to the
pseudocapacitance contribution of the functional
groups positioned near the defects.
Another method to reduce agglomeration of
graphene involves reducing exfoliated GO with pphenylene diamine (PPD), which causes positive
surface charge on graphene fragments induced by
adsorbing the oxidation product of PPD.212 The
EP deposition of charged graphene on Ni from
such a stable suspension allows preparation of thin
EDLC electrodes with the desired thickness and
specific capacitance up to 164 F/g in 6 M KOH.212
A stable aqueous suspension of graphene can also
be prepared by using a noncovalent functionalization
of graphene with 1-pyrenecarboxylic acid (PCA) via
a nondestructive π –π stacking (aromatic interaction)
mechanism.213 The –COOH groups of PCA greatly
enhance suspension stability in water and allow
formation of EDLC electrodes with capacitance
approaching 120 F/g.213 Reduction of the size of
graphene platelets offers a complementary strategy to
reduce their tendency to agglomerate. Exfoliation of
oxidized carbon nanofibers followed by reduction in
hydrazine showed specific capacitance of 82–132 F/g
in 2 M H2 SO4 , depending on the size of graphene
segments.207
Curved graphene with pore size of ∼4 nm and
mesopore SSA of 500 m2 /g was claimed to show
unexpectedly high specific capacitance of 100–250 F/g
(depending on the sample) in IMIMTFB ionic liquid,
but the actual data for the highest capacitance material
was not presented and stability for no more than
500 cycles (and only in the case of 120 F/g material)
was demonstrated.217
In situ electrochemical reduction of GO was
proposed in 2009, and involved gradual in situ
transformation of GO electrodes into graphene
electrodes in 0.1 M Na2 SO4 solution.216 After
2000 cycles the electrode exhibited impressive specific
capacitance of 165 F/g.216
The electrochemical performance of graphene
electrodes, prepared using thermal exfoliation of GO
at low temperature (250–400 ◦ C) in air and followed
by optional carbonization at higher temperature
(700–900 ◦ C) in N2 yielded functionalized graphene
with a BET SSA ranging from 329 to 737 m2 /g.
The produced material exhibited specific capacitance
of 221–233 F/g in 2 M KOH electrolyte, which
reduced to 95–114 F/g for sample after annealing
in N2. 214
Partial reduction of GO suspension by hydrobromic acid resulted in the formation of functionalized
graphene, which exhibited specific capacitance of
up to 129 F/g at 10 mV/second in 1 M H2 SO4
electrolyte and up to 50% of that in 1-butyl-3methylimidazolium hexafluorophosphate (BMIHFP)
ionic liquid.209 When evaluated in a three-electrode
configuration the electrodes exhibited capacitance of
up to 198 F/g when evaluated using CV tests and
up to 348 F/g when evaluated using galvanostatic
charge–discharge tests in 1 M H2 SO4. 209
An alternative approach to partially reduce
GO suspension in dimethylformamide (DMF) may
simply involve heating the suspension to 150 ◦ C in
an oil bath for different durations.210 Electrodes
with small electrode density (<0.2 mg/cm2 ) showed
a specific capacitance of ∼ 180 F/g when evaluated
from CV tests and up to 276 F/g when evaluated
using galvanostatic charge–discharge tests in 1 M
H2 SO4 (both tests performed in three electrode
configuration).210
Nitrogen doping was proposed to enhance
the pseudocapacitance of functionalized graphene
electrodes.204 In the proposed process GO was
reduced by a combination of hydrogen plasmaenhanced CVD, nitrogen plasma-enhanced CVD and
thermal annealing at 300 ◦ C to remove residual functional groups.204 Initial capacitance approached very
high values of 282 F/g in 6 M KOH and a very
impressive 220 F/g in organic TEATFB-based electrolyte according to galvanostatic charge–discharge
tests in three electrode configuration.204 In contrast to
many other studies, very good stability (>95% after
10,000 cycles) was demonstrated.204
Chemical activation of graphene granules
was shown to increase the BET SSA up to
3100 m2 /g and produce material with dual pore size
distribution—micropores of 0.6 nm and mesopores
of 4 nm.100 The produced material exhibited specific
capacitance of up to 166 F/g (60 F/cm3 ) in an
organic electrolyte based on IL salt (1-butyl-3methyl-imidazolium tetrafluoroborate (BMIMTFB),
Figure 6(d)) and up to 150 F/g in TEATFB-based
organic electrolyte in symmetric two electrode
configuration.100 In a similar work performed by
another research group a much more moderate
BET SSA was achieved (492 m2 /g), and the specific
capacitance of 136 F/g in aqueous 1 M Na2 SO4
electrolyte.215
Vertically aligned graphene sheets produced
by using plasma-enhanced CVD on carbon cloth
were proposed for ultra-high power and ultra-high
frequency performance.211 This idea was further
developed in 2010 by growing submicron long
graphene nanosheets on Ni foil using a similar
approach.225 The capacitance of the nanosheet-based
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
EDLC decreased from 375 to 140 μF when the
frequency was increased from 0.01 to 10,000 Hz,
which allowed the produced EDLC to be efficiently
used for AC filtering at a characteristic frequency of
120 Hz.225 In an organic electrolyte the capacitance
was 50% higher than in aqueous one but the frequency
response was inferior.225 The produced material
showed frequency response comparable to that of
the Al electrolytic capacitors, while offering higher
volumetric capacitance and maximum operating
voltage.225
Thermal reduction of GO (having initial
interlayer spacing of 0.6 nm) in Ar at 200–1000 ◦ C
produced a graphitic material with a very small
BET SSA of 5 m2 /g and interlayer spacing of
0.33–0.46 nm.206 The produced partially reduced GO
material, having negligibly small initial capacitance
was electrochemically activated in a traditional
TEATFB electrolyte in the potential window of −2.1
to +1.6 versus carbon quasireference electrode, which
caused the subsequent intercalation of anions and
cations in between graphene layers and a dramatic
increase in the specific capacitance up to impressive
220 F/g.206 While neither the pore size nor the density
of the activated sample were reported, its volumetric
capacitance is expected to be very high, due to
the small pore size being just large enough for ion
insertion.
Similar to CNTs, the main limitations of
most mesoporous graphene samples decorated
with multiple functional groups exhibiting high
pseudocapacitance include low electrode density (low
volumetric capacitance) and, more importantly, the
expected high leakage current and thus poor charge
storage stability. Both of these very serious issues,
however, are mostly ignored in the vast majority of
publications. In addition, the high specific capacitance
values most commonly do not come from symmetric
two-electrode tests and thus over-estimate the real
performance. In the view of the authors, mesoporous
graphene electrodes are unlikely to replace traditional
activated carbon-based EDLC electrodes with the rare
exception of devices operating at ultra-high frequency
and competing with electrolytic capacitors.
Carbon Aerogels
Aerogel is a highly porous solid material derived from
a gel, in which a liquid component is replaced with a
gas. One of the first reports on using carbon aerogels
for EDLC applications dates to 1993.226 In spite of
the long study history and moderately high SSA, the
reported values of aerogels’ capacitance are rather
modest (Table 7).
Aerogels with BET SSA ranging from 100 to
1000 m2 /g and having a density in the range of
0.3–1 g/cm3 demonstrated specific capacitance of up
to 40 F/g in 4 M KOH electrolyte.226 Follow up studies
demonstrated the opportunity to further increase
the surface area to 1100 m2 /g and, due to rapid
TABLE 7 Electrochemical Performance of Carbon Aerogels in EDLCs
Activation Method
BET-SSA
Reported Capacitance
(m2 g−1 )
(F g−1 )/Cell Design
Electrolyte
Organic
Ref
Physical-CO2
1655
120/2 symm
1.5M TEMA in PC
227
Chemical-grafting nonpolar
functional group
1655
130/2 symm
1.5M TEMA in PC
227
—
800
10/2 symm
1M TEABF4 in PC
228
750
104/3
1M H2 SO4 aq. sol.
229
—
<600
40/2 symm
1M H2 SO4 aq. sol.
230
—
706
87/3
1M H2 SO4 aq. sol.
231
—
800
183/3
1M H2 SO4 aq. sol.
232
—
700
220/2 symm
6M H2 SO4 aq. sol.
233
—
<600
35/2 symm
6M KOH aq. sol.
230
—
—
110/3
6M KOH aq. sol.
234
—
1000
184/3
6M KOH aq. sol.
235
—
800
45/2 symm
5M KOH aq. sol.
228
—
1000
40/2 symm
4M KOH aq. sol.
226
—
705
52/3
4M KOH aq. sol.
236
—
706
70/3
1M (NH4 )2 SO4 aq. sol.
231
Physical-CO2
Aqueous electrolyte
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
ion transport within the high volume of mesopores
in aerogels, utilize these materials in capacitive
deionization to remove various ions from wastewater
and seawater, with the initial experiments performed
in aqueous solutions of KOH, NaCl, NaNO3 , and
NH4 ClO4 237–239 and later additionally investigated
in NaBr, Na2 SO4 , MgBr2 , MgSO4 , KBr, KCl, KNO3 ,
RbCl, RbBr, and other aqueous solutions.240,241 The
use of similar carbon aerogels with BET SSA up to
800 m2 /g and pores <100 nm in EDLC showed specific
capacitance of up to 45 F/g (27 F/cm3 ) in 5 M KOH
and up to 10 F/g (7 F/cm3 ) in organic electrolyte based
on 1 M TEATFB in PC.228 Similar capacitance values
(40 F/g in H2 SO4 and 35 F/g in KOH solutions) were
also found for carbon aerogels by other research
groups,230 which is noticeably smaller than what
was observed in activated carbons. Later studies,
however, proved that monolithic carbon aerogels
may exhibit higher capacitance values of 110 F/g
in 6 M KOH, 87 F/g in 1 M Ha2 SO4 , and 70 F/g
in (NH4 )2 SO4. 234,231 Selected carbon aerogels with
BET SSA of up to 1000 m2 /g approached values as
high as 184 F/g in 6 M KOH when measured in a
three electrode configuration.235 Boron-doped carbon
aerogel monoliths with BET SSA up to 800 m2 /g have
been produced using pyrocatechol as a carbon source
and boric acid as a catalyst, and showed impressive
specific capacitance 183 F/g (185 F/cm3 ) in 1 M H2 SO4
electrolyte.232
Activation was found to be an effective tool
for increasing the specific and volumetric capacitance
of carbon aerogels. A moderately high specific
capacitance of up to 104 F/g in 1 M H2 SO4 and
up to 52 F/g in 4 M KOH electrolytes was reported
for physically activated aerogels with BET SSA of
up to 705 m2 /g and containing significant surface
area (of up to 429 m2 /g) of micropores <1 nm, as
obtained from CO2 adsorption data.229,236,242 A
very high specific capacitance of up to 220 F/g
in 6 M H2 SO4 was found in carbon aerogels
produced by pyrolyzing resorcinol–formaldehyde (RF)
organic aerogels and activated in air.233 Particularly
attractive for commercial applications was the
substitution of conventional supercritical drying with
a lower cost ambient drying technique based on
using acetone exchange and controlled evaporation
and a relatively high aerogel density (0.5 g/cm3
for high capacitance samples), which permits the
achievement of high volumetric capacitance (up
to 110 F/cm3 ).233 Activation of carbon aerogels
in CO2 allowed increase in their BET SSA from
835 to 1655 m2 /g and in micropore volume from
0.03 to 0.71 cm3 /g, which, in turn, allowed specific
capacitance to reach an impressive 100–120 F/g
in an organic electrolyte based on 0.8–1.5 M
EMATFB solution in PC.227,243 Furthermore, by
attaching nonpolar organic functional groups (such
as CH3 (CH2 )7 CH=CH(CH2 )7 − ) to carbon surface,
the surface of the activated aerogel became more
hydrophobic, which improved the affinity of the
carbon surface for nonpolar organic solvents (such
as PC), and accordingly increased the maximum
capacitance to 110–130 F/g and, more importantly,
improved the capacitance retention at high current
density and at increasing frequency.227,243 Recent
studies on CO2 activation proved that the BET
SSA may reach impressive 3125 m2 /g,244 which could
be expected to result in high specific capacitance
combined with high rate capability.
Carbon Electrodes in Asymmetric
Capacitors
As previously described, a building block in a typical
EDLC consists of two identical high SSA porous
carbon electrodes (Figure 3(a)–(c)). The energy storage
in the majority of commercial electrochemical energy
storage devices, including EDLCs, is controlled by
the average device voltage as well as the capability
of its electrodes to repeatedly store high content
of ions per unit electrode mass and, even more
importantly, volume. In double-layer electrodes, the
ions are only stored at the surface, which results
in a much lower volumetric capacity as compared
to battery electrodes, the capacity of which is due
to ion insertion in the bulk of the electrodes.
The potentials of both negative electrode and
positive electrode in an EDLC swing linearly during
charge/discharge (Figure 3(a)–(c)). As a result, the
cell average voltage (for full charge/discharge cycle)
does not exceed half of the electrolyte stability range,
which is undesirably small for both aqueous and
organic electrolytes. The low volumetric ion storage
ability and low average voltage prevents EDLC
energy density to exceed 5–10% of that of Li-ion
batteries.245
In order to increase the operating voltage,
volumetric capacitance and energy density of electrochemical capacitors while maintaining their high
power density, asymmetric electrochemical capacitor
(often termed or asymmetric capacitors and occasionally called ultra-batteries and supercabatteries)
had been proposed and investigated (Table 8). At
least One of the double-layer porous carbon electrodes in such devices is replaced with a battery-like
electrode. One of the first designs for asymmetric
capacitors was proposed and produced by MP Pulsar
(now Elit Co, Kursk, Russia) in 1989. The original
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
TABLE 8 Electrochemical Performance of Selected Asymmetric Capacitors
Negative
Electrode
Positive Electrode
Cell
Specific Energy
Voltage (V)
(Wh kg−1 )/Cell Design
Electrolyte
Ref
AC
Ni(OH)2 /AC
0.9
25/3
6M KOH aq. sol.
246
AC
NiO
1.5
15/3
6M KOH aq. sol.
247
AC
Al-doped Ni(OH)2
1.2
42/3
2M KOH aq. sol.
248
AC
V2 O5 0.6H2 O
1.8
20/3
0.5M K2 SO4 aq. sol.
249
AC
V2 O5 /AC
1
33/3
2M NaNO3 aq. sol.
250
AC
Co(OH)2
1.6
93/3
2M KOH aq. sol.
251
AC
Co(OH)2 /zeolite
1.4
35/3
1M KOH aq. sol.
252
AC
MnO2
2.3
21/2
2M KNO3 aq. sol.
253
AC
MnO2
2
12/2
0.5M K2 SO4 aq. sol.
254
AC
MnO2
2
21/3
0.1M Mg(NO3 )2 aq. sol.
255
AC
RuO2
1.3
17/2
1M H2 SO4 aq. sol.
256
AC
PbO2
1
29/3
0.1 M CH3 SO3 H + 0.1 M Pb(NO3 )2 + 4 M NaNO3
257
AC
PbO2
1.3
12/3
H2 SO4 aq. sol.
258
AC
Polyfluorene
3.2
47/2
1M TEABF4 in PC
259
AC
PAN
0.8
18/2
6M KOH aq. sol.
260
AC
poly(3-methylthiophene)
3.4
31/3
1-buthyl-3-methyl-imidazolium
261
Li4 Ti5 O12
AC
2.3
20/3
1M LiPF6 in EC/DEC (2:1)
262
Li4 Ti5 O12
Poly(fluorophenylthiophene)
1.3
20/3
1M LiPF6 in EC/DEC (1:1)
263
TiO2
AC
2.8
80/3
1M LiPF6 in EC/DEC (1:2)
264
CNT
Co(OH)2 /CNT
1.8
21/3
1M KOH aq. sol.
265
CNT
MnO2 /CNT
2.7
33/3
1M LiClO4 in EC/DEC (1:1)
266
RuO2
2
41/2
1M Na2 SO4 aq. sol.
267
graphene
CNT
MnO2 /graphene
0.9
11/2
1M KCl aq. sol.
268
Graphite
AC
4.5
104/2
1M LiPF6 in EC/DEC (1:1)
269
graphite
AC
2.5
17/2
1M TEABF4 in AN
269
AC
AC
1.5
11/2
1M H2 SO4 aq. sol.
270
design incorporated Ni(OH)2 -based positive electrode
and AC negative electrode, and the idea was slowly
picked up and explored in various laboratories worldwide. Various types of Faradaic electrodes have been
studied, including the ones based on graphite, metal
oxides and hydroxides as well as conductive polymers
and other materials. Both battery-like negative electrodes and positive electrodes have been investigated.
In contrast to majority of batteries having cycle life of
less than 2000 and charge time of more than half an
hour, many asymmetric capacitors exhibit cycle life in
the range of 5000–200,000 and charging time within
seconds-to-minutes.
Since our discussion is mainly focused on carbon
materials, we would like to refer the reader to
the detailed review on pseudocapacitive materials.271
In this section, we provide a very brief review of
carbons used as double layer electrodes in asymmetric
capacitors and conductive support or additives in
battery electrodes.
Nearly all carbon materials discussed previously have been explored for double layer
electrodes of asymmetric capacitors. ACs have
been used most frequently. For example, in 2002
Park et al. utilized Ni(OH)2 /AC composite as
positive electrode for asymmetric capacitor and
applied AC as the counter electrode.246 Similar
to Elit’s design, by introducing Ni(OH)2 powders
in the positive electrode, a much higher specific
capacitance (530 F/g) was achieved in a KOH
electrolyte, compared to that of symmetric EDLC
made by ACs, which was 230 F/g. Other researches
explored both NiO/Ni(OH)2 series247,248 and various
other pseudocapacitive metal oxides/hydroxides,
such as V O 249,250 Co(OH) 251,252 MnO 253–255
2
5,
2,
2,
RuO2, 23,256 PbO2 /PbSO4, 257,258,272 to name a few.
© 2013 John Wiley & Sons, Ltd.
Nanostructured carbon materials for electrochemical capacitor applications
(c)
(a)
160
140
120
600
500
100 ALD cycles
400
300
200
300 ALD cycles
100
500 ALD cycles
0
Uncoated CNT
5
100
80
60
0
100
200
300
400
500
Number of ALD cycles
(b)
10
15
Current density (A/g)
20
(d)
Specific capacitance (F/g(VOx))
Average tube diameter (nm)
180
Specific capacitance (F/ g)
WIREs Energy and Environment
VOx-coated CNT
500 ALD cycles
1600
100 ALD cycles
1200
800
300 ALD cycles
400
500 ALD cycles
0
5
(e)
10
15
Current density (A/g)
IR
Voltage (V)
0.6
20
Current
density = 20 A/g
0.3
100 ALD cycles
0.0
300 ALD cycles
–0.3
500 ALD cycles
1 µm
–0.6
0
4
8
Time (s)
12
16
FIGURE 8 | ALD deposition of vanadium oxide coating on a CNT paper: (a) average tube diameter changes with increasing the number of ALD
cycles, showing linear growth dependence, (b) a typical micrograph of the VOx -coated CNT paper sample, showing smooth and uniform coatings, (c
and d) specific capacitance of the selected samples calculated from the charge–discharge tests measured as a function of current density, (e) an
example of the charge–discharge test showing very small resistance drop at a very high current density. (Reproduced from Ref 19. Copyright 2012,
Royal Society of Chemistry)
Since most pseudocapacitive materials exploited
in these works suffer from low electrical conductivity,
miscellaneous methods other than physical mixing
have been developed to deposit metal oxides
homogeneously on the conductive backbone (such as
electrodeposition, coprecipitation, vapor deposition,
and others) to improve the capacity retention and
reduce the resistance. Surface-controlled reactions
involved in atomic layer deposition (ALD) processes,
in particular, may offer a route to achieve wellcontrolled and extremely uniform deposition of
smooth metal oxide-based coatings on electrically
conductive porous materials19,273 and thus optimize
the coating thickness, maximize utilization of the
active material and total capacity of the composite,
while maintaining low electrical resistance (Figure 8).
Most of the battery-like electrodes achieve
300–800 F/g at 1–10 mV/second for more than
1000 cycles. The cell voltage in aqueous electrolyte
is usually 1–2 V, in organic electrolyte and ionic
liquid 2–4 V, compared to ∼0.8 V in symmetric
carbon-based EDLCs. Besides metal oxides, electric
conductive polymer is also a major choice of
pseudocapacitive materials for the battery electrodes,
with lower cost, higher conductivity and more
available material options.259–261,274,275 With similar
specific capacitance as that of AC, well-selected
conductive polymers as positive electrodes usually
perform higher potential. For example, Machida et al.
studied a capacitor based on polyfluorene (PF) positive
electrode and AC negative electrode.259 The tested cell
showed high cell voltage of 3.2 V due to high redox
potential of PF. A moderate specific capacitance of
190 F/g was achieved. The capacitance retention was
60% after 10,000 cycles. Studies of capacitors based
on negative battery electrodes and positive carbon
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
electrodes have been considered as well.262–264,276
Low potential pseudocapacitive materials such as
Lix Tiy Oz , TiO2 , LiMnSiO4 have been adopted for
negative electrodes. In addition to multiple studies on
ACs coupled with different types of battery electrodes,
other carbon materials have been tested in asymmetric
electrochemical capacitors, including graphite,269,277
CNTs,265–267 and graphene.268,278 The low cost of
graphite combined with its commercial success in
Li-ion battery negative electrodes, have triggered
particular interest in this material as a negative
electrode in asymmetric capacitors. In one of the first
published reports Beguin et al. reported a capacitor
based on graphite negative electrode and AC positive
electrode in 2007.269 Lithium salt organic solution
was adopted as the electrolyte, very much like the case
of lithium ion batteries. Thanks to the low potential
of graphite (close to ∼0 V vs. Li/Li+ , Figure 3(d)),
the average cell potential may exceed 3 V during
charge–discharge.
Asymmetric capacitors based on functionalized
(or doped) porous carbons have also been proposed.
For example, Beguin et al. reported an asymmetric
capacitor using AC on both electrodes.270 AC was
oxidized with HNO3 to introduce pseudocapacitance.
By coupling two kinds of oxidized AC, the cell
voltage reached 1.6 V, a specific capacitance of 321 F/g
was achieved. Nitrogen is often considered to be a
promising material for doping (or functionalization)
of carbon and can be introduced into the carbon
structure by ammoxidation279,280 or by using
nitrogen-rich carbon precursor.161,281,282 Increase in
the total capacitance by nitrogen enrichment of carbon
in aqueous and organic electrolytes have been reported
in most of these manuscripts with a few exceptions.161
Unfortunately, however, doping and functionalization
of carbon may lead to increased leakage current and
lower cycle life.
STRUCTURE–PROPERTY
RELATIONSHIPS
In the last decade significant effort was put into studies
that reveal critical properties of carbon electrodes
that may affect the energy and power density as well
as specific energy and power of EDLC electrodes
and assembled devices. Below we provide selected
examples of such studies.
Pore Size Effect
Traditionally, a Helmholtz electric double layer (EDL)
consisting of solvated ions was used to understand the
energy storage mechanism (Figure 9(a)). Solvated ions
(a)
–
–
(b)
–
–
–
δ–
–
–
–
–
δ–
δ–
δ–
δ–
–
δ–
δ–
δ–
–
–
FIGURE 9 | Comparison of EDLC charge storage in a large
cylindrical mesopore and a small cylindrical micropore: (a) traditional
view on Helmholtz double-layer formation within a negatively charged
mesopore with cations (green spheres) approaching the pore wall to
form an electric double-cylinder capacitor with radii b and a for the
outer and inner cylinders and (b) a negatively charged micropore with
cations lining up to form an electric wire-in-cylinder capacitor. Solvent
molecules are shown in (a) as small light-blue spheres. (Reproduced
from Ref 283. Copyright 2008, John Wiley and Sons, Ltd)
were often believed not to enter the pores if their
size exceeded the pore dimensions. Both the partial
removal of the solvation shells and ion intercalation
into the graphitic structure were commonly believed to
be negligible in EDLCs. Thus, the larger pore size was
initially thought to be beneficial due to the supposedly
more accessible surface area.
However, by studying over 30 different porous
carbons in KOH electrolytes in 1996 it was reported
that SSA (either BET or DFT) does not have a clear
correlation with the specific capacitance.284 Furthermore, increasing SSA with a simultaneous increase
in the average pore size was shown not to increase
the specific capacitance either.284 Another systematic study investigated the correlation of the pore
size and SSA accessible to N2 gas to that accessible by positive and negative ions in both aqueous
and organic electrolytes.285 The authors concluded
that a distortion of solvation shells is dependent on
both the ion and the solvent. In the investigated electrolytes the effective relative size of ions (with or
without full solvation shells) follows: size of N2 gas
molecule ≈ Na+ (aq) ≈ Cl− (aq) < BF4 − < TEA+ (PC)
< Li+ (PC) and that K+ (aq) < Na+ (aq) < Li+ (aq).285
This preliminary work was instrumental for developing better understanding of the electrosorption
of ions in porous carbons and demonstrated that
improvements in EDLC energy storage characteristics
depends critically on the relative dimensions of pores,
electrolyte ions and the solvent molecules as well as
on the solvation energy.
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
In 2006, systematic studies performed by two
research groups110,286 showed significant enhancement of the BET SSA-normalized specific capacitance in small micropores (Figure 10(a)), where the
ion solvation shell becomes highly distorted and
partially removed.110 The resulting smaller charge
separation distance between the ion centers and
the pore walls was proposed to greatly increased
capacitance.110,283,286,287 In this case ions do not create layers on the surface of both walls of the slit-shape
pores or the inner cylinder coating on the surface of
the cylinder-shape pores. Instead, more appropriate
models of ion electrosorption would constitute formation of an ion monolayer sandwiched between slit
pore walls or the formation of an ion wire inside a
cylindrical pore (Figure 9(b)).283,288
The proposed observations generated great
interest, but the universality of such behavior still
remains the subject of the heated debates in the scientific community, as the enhanced surface-normalized
capacitance was not always experimentally observed
in other carbons and electrolytes. In fact, designing
unambiguous, clean and easy to interpret systematic experiments with independently controlled
parameters is not an easy task.
First of all, carbon materials with different
pore size distribution inevitably exhibit also different
microstructure, pore shape, concentration of defects
and surface functional groups, all of which may
impact the specific capacitance. Therefore, separating
the impact of pore size from that of microstructure
and chemistry becomes challenging.
Second, the accessibility of pores by gas
molecules (such as Ar or N2 utilized for the pore
size distribution and the SSA measurements) and
by electrolyte ions could be different. For example,
Li ions may access the space between the graphene
layers in graphite, the area which is not accessible by
gas molecules. Many electrolyte ions may potentially
intercalate into the tiny pores or small graphitic
segments during EDLC tests. A reverse argument
can also be used—smaller and more mobile gas
molecules may potentially enter the semiclosed pores
(separated from the rest of the pore volume by
narrow bottle-neck pores), while electrolyte may
not have access to this portion of the surface area.
Similarly, the poor electrolyte wetting (high energy of
the carbon–electrolyte interface or small gas bubbles
trapped in some of the pores) may contribute to the
reduced ion-accessible SSA.
Third, there are no accurate models capable
to deduce the SSA and pore size distribution from
the commonly used gas sorption measurements in
porous carbons with realistic pore shape and size.
The BET model is very simple and commonly
used, but it does not accurately represent the gas
sorption in microporous solids.289 More advanced
models, including the ones based on density functional
theory (DFT) calculations,290 are based on simplified
pore shapes (such as infinitely long slit-shaped
pores or cylindrical pores between perfect graphite
pore walls), which are quite different from the
realistic structure of microporous carbons. Centeno
et al. performed multiple studies of various porous
carbons with different pore size in organic and
aqueous electrolytes.291–294 The group found that
the volumetric capacitance is indeed often inversely
proportional to the average micropore size,293 which
could potentially be explained solely by higher surfaceto-volume ratio in carbons having smaller pores.
Since high volumetric capacitance is most critical
for maximizing the energy density for the majority
of practical applications, such results emphasize a
clear advantage of carbons with smaller pores. At
the same time, selecting different models for the SSA
calculations may have a major impact on the SSAnormalized specific capacitance. For example, while
smaller pores appear to show higher normalized
capacitance if a BET model is used, the trend
may completely disappear if other models (or a
combination of such models) are used in the SSA
calculation (Figure 10(b)).294 The way of calculating
the ‘average’ pore size may further affect the obtained
results and, in addition, very different pore size
distributions may formally give the same ‘average
pore size’, and obscure a possible pore size effect.
Finally, excessive amount of a binder within electrodes
may potentially block some of the smallest pores and
further obscure dependencies.
On the other hand, the community seems to
agree that carbons having too small pores might not
be attractive for EDLC applications either. When the
pore size becomes smaller than the critical (possibly
unsolvated ion) size, the access of the ions into the
pores becomes limited110,111 and, additionally, the
transport of ions inside such pores may become very
slow. Slightly larger well-aligned pores are needed for
rapid charging and discharging.14 Therefore, a tight
control over the carbon pore size distribution in the
small micropore range (primarily within 0.4–1 nm) is
critically important for maximizing the energy storage
characteristics of EDLCs, while the coexistence of
straight interconnected pores (with low concentration
of defects) within individual particles (primarily in the
range of 1–2 nm) is needed for improvements in power
characteristics of EDLCs.
Computer modeling and calculations may
provide additional unique insights about the
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
III
II
(b) 15
I
New results
10
(CH3CH2)4 N+
BF4−
0.68 nm diameter 0.33 nm diameter
5
Normalization using
BET specific surface area
Electrolyte: TEATFB in AN
Tests performed at +20 °C
0
1
2
3
4
5
Area-normalized capacitance (μF / cm)
Area-normalized capacitance (μF / cm)
(a) 15
Normalization using
“average” specific surface area Sav
10
5
Normalization using
BET specific surface area
Electrolyte: TEATFB in AN
Tests performed at +20 °C
0.5
Pore width (nm)
1
1.5
2
Pore width (nm)
FIGURE 10 | Effect of the average pore size on the specific capacitance normalized by SSA in porous carbons TEATFB-based electrolyte: (a)
carbon capacitance normalized by a BET specific surface area S BET (DFT specific surface area reveals weaker, but similar trends)110 ; Reproduced with
permission from Ref. 110. Copyright 2006, The American Association for the Advancement of Science. (b) comparison of carbon capacitance
normalized by a BET SSA and S av = 0.33(S Dubinin–Radushkevich + S phenol adsorption + S subtracting pore effect ). (Reproduced with permission from Ref 294.
Copyright 2011, Elsevier)
ion adsorption phenomena and their relationship
with the experimentally recorded electrochemical
performance, but the realistic materials systems are
too complex for ab initio simulations. Multiple
computer simulations have been performed in
simplified model systems,18,19,273,295–298 with many
publications focusing on IL electrolytes,18,19,273,297
which do not contain solvent molecules and thus
simplify the calculations. However, the assumptions
and parameters used in each of the described models
may significantly affect the obtained results. In
addition, the impact of the realistic pore structure
and electrolyte solvent—carbon pore wall interactions
on the calculated results may be significant. A recent
study, for example, demonstrated an importance of
taking into consideration a more realistic structure of
porous carbons in order to achieve a better correlation
with experimental data.299
The discussed above limitations of the current
modeling efforts make it challenging to utilize them
as prognostic tools for the rational optimization of
carbon electrode structure, porosity and chemistry for
the desired (application-specific) electrochemical performance. Therefore, the development and adoption
of new experimental techniques capable to identify the
ion adsorption sites in a broad range of microporous
solids (with different microstructure and chemistry,
and as a function of the applied potential) was critically needed to verify the molecular mechanisms
previously proposed and provide guidance to future
modeling efforts.
To address this need, Boukhalfa et al. proposed
to use neutron scattering to directly visualize ion
adsorption in microporous carbons as a function
of pore size and applied potential. Indeed, the
majority of electrolytes contain hydrogen atoms in
either their solvent molecules or ions or both. The
researchers have shown that by monitoring changes
in the distribution of hydrogen in the nanoconfined
electrolyte upon the application of external potential
to porous carbon electrodes one can elucidate where
the ion adsorption takes place. Interestingly, they
showed that depending on the solvent properties and
the solvent–pore walls interactions, either enhanced
or diminished ion adsorption may take place in
sub nanometer pores. The reported tests have been
performed in electrolytes based on the solution of
H2 SO4 in H2 O and D2 O (heavy water). Figure 11
shows some of the obtained results. In case of an
H2 O solvent, under the application of −0.6 V the
scattering intensity increases by ∼6 % (Figure 11(a)),
which represents the H enrichment of 6% within
the relatively large pores. The smallest pores
(Q > 0.5 Å−1 ) exhibit even higher ion adsorption
capacity, as manifested by higher H concentration
in such pores at negative potentials and lower H
concentration at positive potentials. In Figure 11(c)
showing the relative changes in the normalized
scattering intensities, such an effect can be seem most
clearly. In case of a D2 O solvent (Figure 11(b) and
(d)), the negative polarization increases the neutron
scattering intensity more dramatically because the
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
scattering contrast between carbon and D2 O is much
smaller than between carbon and H2 O. Under the
application of −0.6 V the concentration of H increases
by ∼35% in large pores. However, in sharp contrast
to a H2 O solvent studies (Figure 11(c)), cation
adsorption is significantly reduced in smallest pores
(Q > 0.4 Å−1 ) in case of a D2 O solvent (Figure 11(d)).
By analyzing the adsorption of D2 O and H2 O vapors,
it was found that a significant portion of the smallest
pores (Q > 0.3 Å−1 ) is not completely filled with
D2 O and not accessibly by electrolyte. The higher
energy cost of maintaining a stronger deuterium bond
network (when compared to a protium bond network)
at the subnanometer proximity to the hydrophobic
carbon surface was proposed to be responsible for the
formation of D2 O-based electrolyte depletion regions
and for the dramatic difference between the electroadsorption of D2 O- and H2 O-based electrolyte ions
in the smallest carbon nanopores. In contrast to
prior studies, where the impact of pore size was
studied by analyzing completely different carbons,
neutron scattering experiments allow unambiguous
observation of the different ion adsorption in various
pores of the same material. The reported methodology
may become instrumental in clarifying the currently
existing controversies. It may also contribute to
the formulations of the predictive models of ion
adsorption.
Particle Size and Electrode Thickness Effect
The power characteristics of EDLCs are commonly
limited not by the electrical resistance of the
electrodes but by the resistance faced by the
ions traveling to/from the electroadsorption sites
and, to a less extent, by the resistance of the
interface between the current collector foils and
the electrodes. Once the interface resistance is
minimized,61,300,301 improving EDLC power can be
achieved by improving the rate of ion transport
within porous carbon,13,14,16,127,217,234,302 minimizing
the ion diffusion distance145 or by utilizing both
approaches.99,186,225
Commercial EDLC electrodes consist of microsized activated carbon powder with large mesopores
or macropores between the particles. Therefore,
while both the electrode thickness and size of
microporous carbon particles are important for
considering the overall ion diffusion time, increasing
the electrode thickness to 50–300 μm allows one
to maintain a reasonable charge–discharge time
(commonly 1–20 seconds) and power in the EDLCs.
Larger electrode thickness is highly desirable for
minimizing the relative weight, volume and cost
of the inactive device components, such as metal
foil current collectors and separator membrane.
Therefore, the majority of commercial EDLCs have
electrode thickness in the range of 50–300 μm. When
developing new materials to replace activated carbons
for EDLC applications the overall device performance
(particularly its volumetric characteristics) should be
carefully considered.
In order to minimize the ion diffusion distance
within individual carbon particles, the particle size can
be reduced to submicron and even 10–30 nm range.145
However, the use of porous nanoparticles reduces
both the electrode density (and thus the energy density
of the fabricated device) and the size of pores between
the individual particles, which may ultimately lead to
high ionic resistance in thick electrodes and reduced
power density. In a systematic study performed on
CDC particles having different particle size but similar
electrode mass per unit current collector area, small
20 nm size particles (also having small interparticle
pore size), in fact, demonstrated power performance
inferior to that of 600 nm particles.145 In addition,
with the exception of activated carbon aerogels having
interconnected nanoparticles, the major particle size
reduction leads to a higher electrical conductivity
of the electrodes (due to point contacts between
individual particles), which may become significant
enough to impact the EDLC’s power characteristics.
When electrodes are produced not from
microporous particles, but from the individual
nonporous nanoparticles of graphene, carbon onions
and CNTs, the electrodes do not normally contain
large mesopores or macropores, and most of the
electrode pores are in the range of 1–5 nm. In
this case the time for the ion diffusion through
the full electrode becomes roughly proportional to
the square of the electrode thickness. As such,
very high EDLC power could only be achieved
in relatively thin electrodes.100,225 Indeed, most
of the relevant studies either don’t report the
electrode thickness or minimize it to 25 μm or below,
which produced unfair comparisons to state-of-the
art EDLCs. Unfortunately, the high power density
characteristics of graphene, CNTs and carbon onion
electrodes is extremely challenging to exploit in
electrodes with 100–300 μm thickness or mass loading
on the order of 10–20 mg/cm2 , which is common for
current commercial devices. Furthermore, by forming
large interconnected mesopores and eliminating the
micropores within carbon electrodes to achieve
higher power characteristics, the EDLC’s volumetric
capacitance becomes greatly reduced. Finally, strictly
mesoporous electrodes may require a significantly
increased amount of electrolyte needed to fill the
enlarged electrode pore volume. As the weight of
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
(a)
(b)
1.05
0.0 V
1
0.95
Smaller pores
0.6 V
0.9
0.5M H2SO4 in D2O
–0.6 V
1.4
1.3
1.2
Smaller pores
1.1
1.0
0.0 V
0.2
0.4
Q (per A)
0.1
0.6 0.8
1
1M H2SO4 in H2O
–0.6 V
1.03
1.02
1.01
0.0 V
1
0.99
0.98
0.97
0.4
0.2
0.4
Q (per A)
0.1
(d)
1.05
1.04
Relative changes in
normalized intensity (a.u.)
Normalized intensity (a.u.)
1.1
0.85
(c)
1.5
–0.6 V
1M H2SO4 in H2O
Relative changes in
normalized intensity (a.u.)
Normalized intensity (a.u.)
1.15
0.6
0.7
Q (per A)
0.8
0.9
0.6 0.8 1
1.3
1.2
0.5M H2SO4 in D2O
0.6 V
1.1
1
0.9
0.0 V
0.8
0.7
0.6 V
0.5
0.6 V
–0.6 V
1
0.4
0.5
0.6
0.7
Q (per A)
0.8
0.9
1
FIGURE 11 | In situ neutron scattering experiments on AC electrodes immersed into H2 O (a, c) and D2 O (b, d)-based electrolytes under an
application of a potential between the working and counter electrodes: (a and b) SANS profiles normalized by the 0 V one, (b, d) relative changes in
the intensity of the normalized SANS profiles. (Reproduced with permission from Ref 17. Copyright 2013, John Wiley and Sons, Ltd)
electrolyte becomes higher than the weight of carbon,
not only volumetric but also gravimetric device energy
density may be reduced. As a result, for many
applications currently utilizing commercial EDLC
which do not require ultra-fast 0.1–0.001 second
charging time or on-chip integration with electronic
devices, the use of nonporous carbon nanoparticles is
unlikely to offer any advantages. Similar arguments
are also valid for many bulk or thin film electrodes
having no mesopores.147,150,151 Such technology may
likely target only specialized applications and likely
be considerably more expensive (on a cost per energy
basis) than commercial EDLC devices.
Pore Alignment and Tortuosity
A recent study revealed the significant effect of
pore tortuosity on slowing down the transport
of ions within carbon nanopores.14 In that work,
zeolite-templated carbons were synthesized using lowpressure chemical vapor deposition. Depending on the
conditions of synthesis or annealing after the carbon
coating and before the zeolite template removal,
identical micropore (<2 nm) size distributions with
varying levels of pore tortuosity and pore alignment
were achieved, as detected by using gas sorption
measurements combined with X-ray diffraction
studies14 (Figure 12(a)). Electrochemical impedance
spectroscopy measurements (Figure 12(b)) and cyclic
voltammetry studies showed up to three orders
of magnitude enhancements in the ion transport
and frequency response accompanying the micropore
alignment and a decrease in the concentration of
obstacles for ion diffusion. This finding not only
proves that mesopores are not required for rapid
ion diffusion but also provides guidance for the
optimal design of porous carbons with improved
power storage characteristics.
Curvature Effect
The effect of nanoscale curvature of carbon surface
was investigated in another recent publication.303
The authors proposed that various carbon materials
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
(a)
(b) 350
(101)
(002)
Annealed
4h
8h
6h
4h
10
20
30
Tests performed at +20 °C
Electrolyte: H2SO4in H2 O
4h, annealed
300
40
50
Specific capacitance (F/g)
Intensity (a.u.)
Pore wall alignment in ZTC
originating from ordering
in zeolite template
250
8h
200
6h
150
100
4h
50
ZTC electrodes (250 µm)
0
1E–3
0.01
0.1
2 θ (°)
1
10
Frequency (Hz)
FIGURE 12 | Effect of pore tortuosity on the rate of the ion transport in microporous carbons14 : (a) effect of the ZTC synthesis on the alignment of
micropores originating from pore ordering in a zeolite template; (b) measured capacitance retention in the produced ZTC electrodes with increasing
operating frequency. A major improvement in the frequency response of ZTC electrodes having more aligned and thus less tortuous pores is clearly
seen. (Reproduced with permission from Ref 14. Copyright 2010, American Chemical Society)
(such as carbon onions, carbon nanotubes and
templated carbons) with small but positive curvature
(exohedral carbon materials) exhibit significant
enhancement of the specific capacitance with the
decrease of the particle size (smaller tubes or spheres)
from 5 to below 1 nm and a corresponding increase
in the surface curvature. Furthermore, the capacitance
of spheres increases faster than that of tubes, because
a sphere has locally two positive principal curvatures
while a tube only has one (the other being zero, along
the axis). If the hypothesis is correct, it may suggest
a new strategy for improving energy densities of
EDLCs by employing exohedral carbon materials with
the smallest nanoscale dimensions and additionally
contribute to building a universal model describing the
effect of pore shape and size on the specific capacitance
of various electrolytes.
Electrolyte–Carbon Interactions’ Effect
An impact of the electrolyte solvents on the
specific capacitance have been noted in multiple
publications.304–306 Many of such studies focused on
the commonly used AN and PC organic solvents.
While AN offers higher ionic conductivity, it exhibits
nearly 45% lower dielectric constant than PC.
However, in different porous carbons the AN-based
electrolytes inconsistently showed either higher304,305
or lower capacitance306 than PC-based ones with the
same salt.
The positive effect of electrolyte wetting on the
carbon surface has been often discussed.230,233,307,308
In some of such studies researchers observed
higher surface area-normalized capacitance in porous
carbons after surface oxidation, which were proposed
to improve electrolyte wetting of the carbon surface
and increasing the accessible SSA.230 However, one
shall be careful with such conclusions because surface
treatments may also modify carbon surface chemistry,
increase the concentration of defects16,309 and even
open bottle-neck pores, which may similarly increase
the capacitance. Indeed, defects on surface of carbon
were proposed to interact with the solvation shells of
the ions, decreasing the average carbon surface–ion
separation distance, and thereby increasing specific
capacitance.16,309 Unfortunately, little is known how
electrowetting phenomenon may counter-balance the
insufficient carbon wetting by the solvent. To the
best of our knowledge, only neutron scattering may
estimate the critical pore size, below which the inner
surface area of carbons may not be accesible by
electrolyte ions due to the poor wetting properties
(Figure 11(b) and (d)).
The use of surfactants to enhance wetting
of electrolytes on porous carbon surface was
investigated as well.149,243,310,311 Because of relatively
large molecular dimensions of surfactants (commonly
0.5–2 nm), they cannot improve electrolyte wetting
inside the smallest pores. As a result, their impact
on the maximum accesible capacitance is commonly
small and, in case when surfactant molecules block the
smallest pores, even negative (Figure 13(a)). However,
in some cases surfactants may facilitate the ion
transport in larger mesopores and, therefore, improve
EDLC rate capability and capacitance retention at
high current densities (Figure 13(a)).311 Concentration
of the surfactant should be optimized for optimum
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
(b)
(a) 180
6 mol/L KOH + 0.001 mol/L TRITON × 100
200
140
150
120
100
100
C F/g
C F/g
6 mol/L KOH + 0.005 mol/L TRITON × 100
250
6 mol/L KOH
160
300
80
pure
SDS
TPAB
60
40
TRITON × 100
20
LDS
TPAI
50
0
–50
–100
–150
surfactants concentration: 0.005 mol/L
–200
0
0
5
10 15 20 25 30 35 40 45 50 55
i A /g
0.0
0.2
0.4
U, V
0.6
0.8
FIGURE 13 | Effect of surfactants on the performance of a symmetrical EDLC: (a) capacity retention without and with surfactants at different
current density, (b) cyclic voltammetry recorded at the rate of 10 mV/second with Triton X® surfactant added to 6 M KOH electrolyte in two different
concentrations: below (black line) and above (red line) critical micellar concentrations. (Reproduced with permission from Ref 311. Copyright 2012,
Elsevier)
performance. If its concentration exceed so-called
critical micellar concentration, surfactant molecules
form micelle structures, which block the pores and
reduce specific capacitance311 (Figure 13(b)).
The pH of aqueous electrolytes may impact
electrolyte stability window, pseudocapacitance and
self-discharge312–314 (Figure 14). Neutral electrolytes
offer better stability at a high potential than basic
electrolytes and better stability at a low potential than
acidic electrolytes (Figure 14(a)).313 The properties
of the cations may have a major influence on the
energy storage characteristics of neutral electrolytes
(Figure 14(b)).312 As shown in Figure 14(b), the
same EDLC electrodes exhibit higher capacitance in
Li2 SO4 than in Na2 SO4 and K2 SO4 electrolytes in
spite of larger size and higher solvation energy of the
solvated Li+ (vs. solvated Na+ and solvated K+ ) and
the resultant smaller mobility of Li+ in the bulk of
electrolyte. It could be suggested that smaller size of
the un-solvated Li+ (vs. Na+ and K+ ) may allow it to
access larger SSA and possibly intercalate between the
closely spaced graphene layers of the AC electrode.
Self-Discharge
EDLCs are prone to gradual decrease of voltage during long-term storage. This spontaneous
(thermodynamically favored) process is named as
‘self-discharge’64,315–318 and is known to depend on
the initial voltage, purity of carbon and electrolyte
as well as electrolyte acidity. Acknowledged by most
researchers, Conway et al. first proposed a model
to explain this process.315,316 According to their
theory, self-discharge may take place due to three key
mechanisms:
(1) Overcharging. When the electrode is
polarized to a potential exceeding the electrochemical
window (or stable potential window) of the electrolyte
(Figure 3), the decomposition of electrolyte happens
at the electrode/electrolyte interface. This Faradaic
process reduces the cell potential continuously until
the electrode potential falls into the electrochemical
window of the electrolyte or the electrolyte is totally
consumed. Besides, this process usually produces
gases, which may block the pores of the electrodes
and the separator and even induce a separation of
individual particles within the electrode. This, in
turn, results in the capacitance fading (because of
reduction of accessible SSA of the electrodes) and
in the reduction of power performance (because of
increased separator resistance). It also leads to increase
of the equivalent series resistance.319
(2) Side reactions. Within the electrochemical
window of the electrolytes, some redox-active
impurities on the surface of electrodes or in
the electrolytes (such as O2 , H2 O, H2 O2 , metal
ions, and others) may be involved in undesirable
(parasitic) Faradaic processes which may consume
the charge stored in an EDLC or lead to
electrolyte degradation. The electrochemical window
of organic electrolytes is generally wider than that
of aqueous solutions. However, even ppm level
of H2 O impurity in organic electrolytes is enough
to greatly depress its electrochemical window. The
surface functional groups on the surface of carbon
electrodes are also claimed to be responsible for
the capacitance fading.320,321 Although these surface
groups contribute pseudocapacitance, they are often
thermodynamically unstable within the potential
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
(a) 750
(b) 200
500
150
250
100
Capacitance, F/g
j mA/g
6 mol / L KOH
0
–250
–500
1 mol/L H2SO4
–750
–1000
–1.4
1 mol/L Li2SO4
50
1 mol/L Na2SO4
0
1 mol/L K2SO4
–50
–100
–150
0.5 mol/L Na2SO4
–1.0
–0.6
–200
–0.2
0.2
0.6
1.0
0.0
E/V vs. NHE
0.2
0.4
0.6
cell voltage, V
0.8
FIGURE 14 | Effect of pH of electrolyte on its electrochemical stability during the operation of a symmetrical EDLC: (a) cyclic voltammograms
recorded at the rate of 2 mV/second and showing the potential stability window of EDLC electrodes with electrolytes composed of 6 M KOH, 1 M
H2 SO4 and 0.5M Na2 SO4 . Reproduced with permission from Ref 313. Copyright 2010, Elsevier; (b) cyclic voltammograms of symmetric EDLC with
electrolytes composed of 1M alkali metal (Li, Na, K) sulfate solutions recorded at the rate of 1 mV/second. (Reproduced with permission from Ref 312.
Copyright 2012, Royal Society of Chemistry)
range and readily decomposed, producing various
gases (such as NO2 , SO2 , SO3 , and others).63
Some researchers speculate that selected functional groups on carbon may possibly be stable in
the useful potential range of an EDLC. Unfortunately selective functionalization of carbon surface
only with some particular functional groups is a
challenging task. Furthermore, commonly available
surface chemistry characterization tools (Fourier
transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), chemical titration, etc.)
have difficulties to unambiguously interpret the presence of particular functional groups because the peaks
corresponding to different groups are relatively broad
and their position may be sufficiently close. Currently,
no comprehensive understanding exists on the specific
contributions of various functional groups to both the
desirable and undesirable performance characteristics
of EDLCs in various organic and aqueous electrolytes.
So far, the feasibility of the concept to induce pseudocapacitance on carbon without penalties in leakage
and degradation is unclear.196 We notice in our studies that the degree of leakage current may depend
on both the presence of functional groups in carbon
and the electrolyte utilized (pH-neutral aqueous electrolytes commonly induce lower leakage).322 Figure 15
shows an impact of the leakage current on the shape
of the cyclic voltammograms (Figure 15(a)) and the
often observed correlation between the identified fraction of a pseudocapacitance and the leakage current
(Figure 15(b)).322
The successful formation of stable functional
groups on carbon that do not induce leakage
has not been achieved in commercial devices.
Carbon electrodes utilized in commercial EDLCs
are nonfunctionalized and highly purified. In fact,
improvements in the purification procedures allowed
commercial devices to increase their cycle life from
tens of thousands to several million cycles. In addition,
such improvements in the purifications of carbons and
electrolytes allowed EDLC manufacturers to reduce
the characteristic self-discharge time constant from a
few hours to months (at room temperature).
(3) Ohmic leakage from unintended interelectrode contacts or leaky bipolar electrodes. According
to Conway’s model, self-discharge processes that follow different mechanisms would perform different
potential fading rate.316 Other than this work, studies
on proper modeling and simulation of self-discharge
have been performed by many,69,318,324,325 including
some supplement to Conway’s model. For example,
Kaus et al.324 suggested that charge distribution at an
open circuit may contribute to self-discharge.
GROWING LARGE-SCALE
APPLICATIONS
Smart Grid—Distributed Energy Storage
Electrical energy storage is a critical component of
the smart grids, which are designed to improve
the reliability of electrical energy distribution and
facilitate the employment of renewable energy sources.
Too high energy cost of EDLCs prevents their use
in shifting the supply and demand peaks long-term
(hours). Yet, their reliability and high power could
be efficiently utilized to improve the power quality
and reduce short (1 min or less) disturbances in the
electrical lines. EDLCs can significantly improve the
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
I
(b)
With no leakage current
C
V
I
With significant leakage current
C
V
Rleak
Pseudocapacitance contribution (%)
(a)
35
30
25
20
15
10
5
0
0.01
Ileak = V/Rleak
0.1
R –1 (per
leak
1
kOhm cm2)
FIGURE 15 | Leakage current cause by carbon functionalization: (a) schematics showing the cyclic voltammetry diagrams of idealized (infinitely
fast with stable electrolyte) electrochemical capacitors with no leakage current and with significant leakage current and the equivalent circuits for
these two conditions; (b) pseudocapacitance of functionalized activated carbon versus a leakage conductance in different aqueous electrolytes.
(Reproduced from Ref 323. Copyright 2013, Elsevier)
value of the energy harvested from renewable sources,
which commonly suffer from constantly changing
output power. In addition, integration of EDLCs
into grid energy storage systems may improve the
cycle life of low-cost batteries, many of which tend
to degrade rapidly if charged or discharged at high
currents. In order to overcome the high energy cost of
EDLCs, several companies are developing asymmetric
capacitors for grid power applications. For example,
ELTON (Troitsk, Russia) is developing devices based
on lead oxide and AC electrodes in an acidic
H2 SO4 electrolyte, while Aquion energy (Pittsburg,
USA) is developing asymmetric capacitors based on
MnO2 and AC electrodes in a pH-neutral Na2 SO4
electrolyte.
Hybrid Electric and Electric Vehicles
The significant reductions in electrochemical capacitors’ cost combined with their excellent reliability in
various climate conditions made them viable competitors to high-power Li-ion batteries for use in hybrid
power train of buses, trucks, trains, trams and metros.
The combined benefits of fuel cost savings, reduced
emission, reducing brake maintenance expenses and
enhanced performance of hybrid systems achieved by
capturing breaking energy and providing enhanced
power (torque assistance) during acceleration have
been projected to lead to the steady growth of the
hybrid systems in transportation. The growing popularity of the automatic start-stop technology further
expands the need for electrochemical capacitors or
high-power batteries in energy-efficient automobiles.
EDLC technology could be particularly attractive for
high performance or heavy transportation vehicles,
which have the most stringent requirements on the
performance and reliability (long cycle life and low
maintenance operation in a broad temperature range)
of high-power energy storage devices. The return of
investments associated with the additional cost of
hybrid systems in such vehicles is also the fastest.
In spite of the significantly higher cost of EDLCs
compared to lead-acid batteries (LABs), their use in
bus and truck starter system is growing rapidly due
to EDLCs’ efficient operation at low temperatures,
their more compact size, their better efficiency in
load stabilization and, most importantly, due to the
relatively high cost of occasional ‘down time’, ‘idle
heat up’, and ‘jump starting’ often associated with
LAB limitations.
EDLC-powered public buses capable of charging
at the regular stops are slowly gaining popularity in
China and Europe (Figure 2(a)). In these applications,
the high cost of energy in EDLCs (∼20 times higher
than that of Li-ion batteries) is offset by the small
energy storage requirements since the driving range
needed in such buses is 20–40 times smaller than that
commonly desired in regular electric vehicles (such as
Tesla Model S, USA).
Energy Efficient Industrial Equipment
Two of the most common use of EDLCs in industrial
equipment include hybrid forklift (Figure 2(b))
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
and hybrid cranes (Figure 2(d) and (e)). The key
advantages of EDLCs (over more conventionally used
LABs) in forklift applications include significantly
higher power (Figure 1) and good low temperature
operation (which becomes particularly important for
forklift use in refrigerated warehouses, such as food
distribution centers).
In the harbor crane applications EDLCs with
their reliable operation at deep discharge and
broad temperature range offer improved durability,
reliability and productivity and additionally save up
to 20% on fuel consumption realized via regenerative
braking and drop energy recovery. At the same time,
the EDLCs’ applications greatly reduce maintenance
and increase the equipment operational life by over
20%. Furthermore, for the same total power output,
smaller diesel engines could be utilized when combined
with EDLCs in hybrid systems, which offset the cost
increase associated with hybrid technology and reduce
CO2 emission by over 30%.
Ships
There are three potential applications of EDLCs and
asymmetric capacitors in ships. First, high cost of fuel
makes ultracapacitor storage an economically viable
solution for all-electric ferries (Figure 2(f)) traveling
short distances (3–30 min) and having sufficient
time for re-charging while boarding and unboarding
passengers. The additional benefit of an electric motor
is a significantly more quiet and comfortable ride. The
choice of EDLCs and ultracapacitors over batteries
can be justified by higher power needed during
acceleration and high-speed operation combined with
the reliable operation in a variable climate conditions.
Second, incorporating high-power energy storage into ships (not only diesel or natural gas-powered
but also the all-electric and hybrid electric batterypowered ships) increases the overall system efficiency
and provides emergency power when needed. For
example, having EDLC energy storage on board
decreases the number of bulky redundant diesel generators utilized to power auxiliary equipment. This
application is particularly attractive for well-equipped
military ships which have strong needs for high mobility, space and reliability.
Finally, similar to batteries, EDLCs and
ultracapacitors may increase the efficiency of the
marine engines when built into a hybrid engine–motor
system. Colossal engines used in large ships consume
tons (and even tens of tons) of fuel each day.
The recent increase in oil prices made the cost of
fuel a very large fraction of the sea transportation
cost. In addition, because ships commonly use the
cheapest, high-sulfur fuel, the air pollution from
ships now constitutes a significantly larger threat to
nature and society than the pollution from ground
transportation. The amount of the sulfur oxide
emitted by ships is over hundred times higher than
by cars and buses. The majority of marine engines
exhibit the highest efficiency when operating at an
optimal (relatively high) power. Most of the time
this power is underutilized and the overall engine
efficiency is sacrificed. By providing power to assist an
engine during acceleration and emergency breaking,
the ultracapacitor-based electrical motors reduce the
need to employ over-built engines, significantly reduce
the fuel consumption and reduce air and sea pollution.
Wind Power
EDLCs provide backup power to wind turbine blade
control system needed to maximize the turbines’
energy harvesting efficiency, while insuring their safe
operation by controlling the maximum rotor speed.
The high power of EDLCs, their long cycle-life and
their very reliable operation allowed them to take over
this market from other energy storage technologies.
EDLCs have been found to be particularly attractive
for off-shore wind turbine systems (Figure 2(c)), which
are subject to very high maintenance cost if their
reliability is sacrificed.
In addition, EDLCs improve the quality
of the output power of wind turbines, which
commonly suffer from ∼10% variations in their
output voltage in repeated, irregular wind condition
variations. Such variations commonly last from
milliseconds to minutes. EDLCs absorb turbine energy
during short high-power (high-voltage) periods and
provide additional energy during low-power periods,
significantly smoothing out the voltage supplied to the
electrical grid.
Uninterruptible Power Supplies and Power
Backup
Uninterruptable power is critically needed in many
key applications, where the cost of interruptions is
very high or where such interruptions may lead to
life- or health-threatening situations. Examples of
such applications include hospitals (particularly lifesustaining equipment areas), data centers, networking
and telecommunication facilities, broadcast systems
and industrial buildings. Backup standby generators
(commonly powered by diesel fuel, natural gas or
liquid propane) require seconds to minutes to power
up. Uninterruptable power supplies (UPS) bridge the
gap between the initiation of the electrical outage
and the time when standby generator powers up to
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
the required power level. In addition, UPS manages
a system’s electrical faults, such as voltage sages and
spikes, power surges, frequency variations, waveform
distortions, noise and others.
Historically, low-cost lead acid batteries (LABs)
dominated the market of large-scale UPS systems.
However, more reliable (with long and very
predictable life and easy to detect the ‘state of health’)
and faster responding (more powerful, Figure 1)
ultracapacitors and EDLCs with directly measurable
state of charge may reduce the total system cost
and virtually eliminate site maintenance visits. Such
important advantages will likely allow these devices
to significantly expand their market share in these
applications.
Consumer Electronics and Other Consumer
Devices
The majority of portable electronic devices now
require as much energy as possible to be packaged
in as small as possible volume. Therefore, EDLCs
simply cannot compete with Li-ion battery technology
that offers high specific energy and high energy
density (Figure 1). On the other hand, many of
such applications occasionally require high power
bursts. Examples of such demands include moving
camera and camera lens’ motors, producing highintensity long-range flash light from small sources
(and doing it quickly multiple times in a row), sending
information over wireless network or sending radio
signals, recording information onto flashcards, to
name a few. Li-ion batteries with high energy storage
capacity commonly suffer from relatively low power
and the reduced practical cycle life when operated at
high current modes. Higher power Li-ion cells which
can satisfy the needed power requirements suffer from
significantly reduced energy density (sometimes over
2 or more times compared to high energy Li-ion
cells) and higher cost per unit energy. Therefore,
the use of EDLCs in combination with high-energy
density Li-ion batteries offers unique performance
advantages: batteries provide energy needed for longterm operation while EDLCs satisfy the occasional
power needs and extend the battery cycle life.
Other high-power consumer applications, such
as toys (including remotely controlled helicopters,
airplanes, cars, etc.), emergency kits, flashlights,
personal gadgets (for kitchen, bar, home repair, and
other applications), various cordless tools and other
devices, may not need high energy storage as soon
as they could be charged quickly and reliably. Such
applications benefit greatly from using EDLCs, which
offer simpler and lower cost solution than that of
commercial batteries.
SUMMARY
We have provided a quick overview of state-of-theart EDLC technology and fundamental principles
governing EDLC operation. We have compared
synthesis, microstructural features and various
performance characteristics of ACs, CDCs, ZTCs,
carbon aerogels, CNTs, OLCs, and graphene.
In the view of the authors AC powders will likely
continue to dominate the mainstream EDLC market,
while thin electrodes composed of CNT, OLC, or
graphene structures may serve specialized applications
focused on ultra-high rate performance and competing
with electrolytic capacitors. Processing of CNTs and
graphene into flexible, strong and highly electrically
conductive fabrics, may allow them to serve as
electrodes in multifunctional EDLCs, serving weightsensitive applications in aerospace or military. Carbon
aerogels, CNTs and graphene may also serve as high
surface area conductive substrates for the deposition
of conductive polymers or metal oxides for higher
energy density asymmetric capacitor applications. The
use of CNTs and graphene as conductive additives will
also likely expand. ZTC and CDC technologies may
potentially serve high-end markets, where customers
would be willing to pay premium for reasonable
performance improvements. However, the need to use
and neutralize toxic chemicals during CDC and ZTC
syntheses currently slows down commercialization
of both technologies. The steady improvements in
the performance characteristics of ACs may threaten
investments in CDC and ZTC manufacturing.
Multiple structural parameters of nanostructured carbons have been proposed to impact their
performance in EDLCs: structural defects, pore size
distribution, pore tortuosity, chemistry and functional
groups on the carbon surface, nanoscale curvature
and carbon-electrolyte interfacial energy. In spite of
the significant progress in the relevant experimental
and modeling studies, significant gaps in fundamental
understanding of ion transport and adsorption phenomena within nanoscale carbon pores remain. The
adoption of advanced characterization techniques,
such as in situ small angle neutron scattering, and
close collaborations between theoreticians and experimentalists may allow the eventual formulations of
predictive models capable to describe the performance
of various carbon–electrolyte systems. Improvements
in the models describing gas sorption in carbon materials, needed for the accurate pore size distribution
calculations, will also benefit R&D efforts on EDLCs.
Several critical parameters shall be considered
when developing or evaluating novel materials for
EDLC applications. We can emphasize a few of such
parameters, which are often ignored in publications.
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
In the majority of applications, volumetric capacitance
(and volumetric energy density) is significantly more
important than gravimetric one. Therefore, porous
carbon materials with excessive volume of large
mesopores or macropores simply cannot compete with
microporous carbons used in commercial EDLCs.
Formation of multiple functional groups on the
surface of nanostructured carbons may lead to
significant leakage currents (rapid self-discharge) and,
therefore, should be carefully analyzed. In organic
and IL electrolytes functional groups may additionally
lead to rapid (within <100,000 cycles) performance
degradation.
The presence of several important market
segments, where EDLCs provide significant value
proposition to complement or replace various battery
technologies, will stimulate future efforts in research,
development and production of nanostructured
carbon materials for advanced EDLCs and asymmetric
capacitors with improved performance characteristics.
ACKNOWLEDGEMENT
Different aspects of this work were supported by the Energy Efficiency & Resources program of the Korea
Institute of Energy Technology Evaluation and Planning (KETEP) funded by the Korea Government Ministry
of Knowledge Economy (Grant 20118510010030) and by the US Army Research Office (Grant W911NF-12-10259).
REFERENCES
1. Conway BE. Electrochemical Supercapacitors, vol.
1. New York: Kluwer Academic/Plenum Publishers;
1999.
11. Lee SG. Functionalized imidazolium salts for taskspecific ionic liquids and their applications. Chem
Commun 2006, 10:1049–1063.
2. Beguin F, Frackowiak E. Carbons for Electrochemical
Energy Storage and Conversion Systems. Boca Raton,
FL: CRC Press; 2010.
12. Yushin G, Hoffman EN, Barsoum MW, Gogotsi
Y, Howell CA, Sanderman SR, Phillips GJ, Lloyd
AW, Mikhalovsky SV. Mesoporous carbide-derived
carbon with porosity tuned for efficient adsorption of
cytokines. Biomaterials 2006, 27:5755–5762.
3. Beguin F, Frackowiak E. Supercapacitors: Materials,
Systems, and Applications. Berlin, Germany: WileyVCH; 2013.
4. Frumkin A, Reichstein S, Kulvarskaja R. Ion
adsorption in aqueous surfaces. Kolloid-Zeitschrift
1926, 40:9–11.
5. Frumkin A. Affectation of the adsorption of neutral
molecules by means of electrical field. Zeitschrift Phys
1926, 35:792–802.
6. Conway BE, Gileadi E, Dzieciuch M. Deterination of
real surface areas and Temkin isotherm parameters
from analysis of adsorption pseudocapacity curves.
J Electrochem Soc 1963, 110:C68–C68.
7. Zheng JP. Ruthenium oxide-carbon composite
electrodes for electrochemical capacitors. Electrochem
Solid State Lett 1999, 2:359–361.
8. Conway BE. Transition from supercapacitor to
battery behavior in electrochemical energy storage.
J Electrochem Soc 1991, 138:1539–1548.
9. Galinski M, Lewandowski A, Stepniak I. Ionic
liquids as electrolytes. Electrochim Acta 2006,
51:5567–5580.
10. Pandey S. Analytical applications of room-temperature
ionic liquids: A review of recent efforts. Anal Chim
Acta 2006, 556:38–45.
13. Rose M, Korenblit Y, Kockrick E, Borchardt L,
Oschatz M, Kaskel S, Yushin G. Hierarchical Microand Mesoporous Carbide-Derived Carbon as a HighPerformance Electrode Material in Supercapacitor.
Small 2011, 7:1108–1117.
14. Kajdos A, Kvit A, Jones F, Jagiello J, Yushin G.
Tailoring the pore alignment for rapid ion transport
in microporous carbons. J Am Chem Soc 2010,
132:3252.
15. Evanoff K, Magasinski A, Yang J. NanoSi-coated
graphene granules as anodes for Li-ion batteries. Adv.
Energy Mater. 2011, 1:495–498.
16. Portet C, Yushin G, Gogotsi Y. Electrochemical performance of carbon onions, nanodiamonds,
carbon black and multiwalled nanotubes in electrical double layer capacitors. Carbon 2007, 45:
2511–2518.
17. Boukhalfa S, He L, Melnichenko YB, Yushin G. Small
angle neutron scattering for the in-situ probing of ion
adsorption inside micropores. Angew Chem Int Ed
2013, 52:4618–4622.
18. Evanoff K, Khan J, Balandin AA, Magasinski A,
Ready WJ, Fuller TF, Yushin G. Towards ultrathick
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
battery electrodes: aligned carbon nanotube-enabled
architecture. Adv Mater 2012, 24:533.
19. Boukhalfa S, Evanoff K, Yushin G. Atomic layer
deposition of vanadium oxide on carbon nanotubes for
high-power supercapacitor electrodes. Energy Environ
Sci 2012, 5:6872–6879.
20. Cooney DO. Activated Charcoal: Antidotal and other
Medical Uses. New York: Dekker; 1980.
pore structure and surface chemistry on electric double
layer capacitance in non-aqueous electrolyte. Carbon
2003, 41:1765–1775.
33. Xu B, Chen YF, Wei G, Cao GP, Zhang H, Yang
YS. Activated carbon with high capacitance prepared
by NaOH activation for supercapacitors. Mater Chem
Phys 2010, 124:504–509.
21. Baker FS, Miller CE, Repik ED. Kirk-Othmer
Encyclopedia of Chemical Technology, vol. 4. 4 ed.
New York: John Wiley; 1992, 1015–1037.
34. Rufford TE, Hulicova-Jurcakova D, Zhu Z, Lu GQ.
Nanoporous carbon electrode from waste coffee beans
for high performance supercapacitors. Electrochem
Commun 2008, 10:1594–1597.
22. Marsh H. Activated Carbon Compendium: A
Collection of Papers from the Journal Carbon
1996–2000. Elsevier; 2001, Gulf Publishing, Texas,
USA.
35. Li X, Han C, Chen X, Shi C. Preparation
and performance of straw based activated carbon
for supercapacitor in non-aqueous electrolytes.
Microporous Mesoporous Mater 2010, 131:303–309.
23. Subramanian V, Luo C, Stephan AM, Nahm KS,
Thomas S, Wei BQ. Supercapacitors from activated
carbon derived from banana fibers. J Phys Chem C
2007, 111:7527–7531.
36. Wang R, Wang PY, Yan XB, Lang JW, Peng C, Xue
QJ. Promising porous carbon derived from celtuce
leaves with outstanding supercapacitance and CO(2)
capture performance. ACS Appl Mater Interf 2012,
4:5800–5806.
24. Wei L, Yushin G. Electrical double layer capacitors
with activated sucrose-derived carbon electrodes.
Carbon 2011, 49:4830–4838.
25. Wei L, Yushin G. Electrical double layer capacitors
with sucrose derived carbon electrodes in ionic liquid
electrolytes. J Power Sources 2011, 196:4072–4079.
37. Lee SG, Park KH, Shim WG, Balathanigaimani MS,
Moon H. Performance of electrochemical double
layer capacitors using highly porous activated carbons
prepared from beer lees. J Ind Eng Chem 2011,
17:450–454.
26. Li QY, Wang HQ, Dai QF, Yang JH, Zhong YL.
Novel activated carbons as electrode materials for
electrochemical capacitors from a series of starch.
Solid State Ionics 2008, 179:269–273.
38. Juntao Zhang LG, Kang S, Jianchun J, Xiaogang Z.
Preparation of activated carbon from waste Camellia
oleifera shell for supercapacitor application. J Solid
State Electrochem 2012, 16.
27. Wei L, Sevilla M, Fuertes AB, Mokaya R, Yushin G.
Hydrothermal carbonization of abundant renewable
natural organic chemicals for high-performance
supercapacitor electrodes. Adv Energy Mater 2011,
1:356–361.
39. Elmouwahidi A, Zapata-Benabithe Z, Carrasco-Marin
F, Moreno-Castilla C. Activated carbons from KOHactivation of argan (Argania spinosa) seed shells as
supercapacitor electrodes. Bioresour Technol 2012,
111:185–190.
28. Balathanigaimani MS, Shim WG, Lee M-J, Kim C,
Lee J-W, Moon H. Highly porous electrodes from
novel corn grains-based activated carbons for electrical
double layer capacitors. Electrochem Commun 2008,
10:868–871.
40. Ding L, Wang ZC, Li YN, Du YL, Liu HQ, Guo
YP. A novel hydrochar and nickel composite for
the electrochemical supercapacitor electrode material.
Mater Lett 2012, 74:111–114.
29. Kierzek K, Frackowiak E, Lota G, Gryglewicz G,
Machnikowski J. Electrochemical capacitors based on
highly porous carbons prepared by KOH activation
(vol 49, pg 515, 2004). Electrochim Acta 2004,
49:1169–1170.
41. Li Z, Zhang L, Amirkhiz BS, Tan XH, Xu
ZW, Wang HL, Olsen BC, Holt CM, Mitlin D.
Carbonized chicken eggshell membranes with 3d
architectures as high-performance electrode materials
for supercapacitors. Adv Energy Mater 2012,
2:431–437.
30. Zhai D, Li B, Du H, Wang G, Kang F. The
effect of pre-carbonization of mesophase pitchbased activated carbons on their electrochemical
performance for electric double-layer capacitors.
J Solid State Electrochem 2011, 15:787–794.
42. Chun S-E, Whitacre JF. The evolution of electrochemical functionality of carbons derived from glucose
during pyrolysis and activation. Electrochim Acta
2012, 60:392–400.
31. Rufford TE, Hulicova-Jurcakova D, Khosla K, Zhu
ZH, Lu GQ. Microstructure and electrochemical
double-layer capacitance of carbon electrodes prepared by zinc chloride activation of sugar cane bagasse.
J Power Sources 2010, 195:912–918.
43. Wei L, Sevilla M, Fuertes AB, Mokaya R, Yushin
G. Polypyrrole-derived activated carbons for highperformance electrical double-layer capacitors with
ionic liquid electrolyte. Adv Funct Mater 2011,
22:827–834.
32. Lozano-Castello D, Cazorla-Amoros D, LinaresSolano A, Shiraishi S, Kurihara H, Oya A. Influence of
44. Yan J, Wei T, Qiao WM, Fan ZJ, Zhang LJ, Li TY,
Zhao QK. A high-performance carbon derived from
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
polyaniline for supercapacitors. Electrochem Commun
2010, 12:1279–1282.
phosphorus-enriched carbons. J Am Chem Soc 2009,
131.
45. Xiang X, Liu EH, Li LM, Yang YJ, Shen HJ,
Huang ZZ, Tian YY. Activated carbon prepared
from polyaniline base by K(2)CO(3) activation for
application in supercapacitor electrodes. J Solid State
Electrochem 2011, 15:579–585.
58. Xiaoxia XEL, Huang Z, Shen H, Tian Y, Xiao C,
Yang J, Mao Z. Preparation of activited carbon
from polyaniline by zinc chloride activation as
supercapacitor electrodes. J Solid State Electrochem
2011, 15.
46. Eliad L, Pollak E, Levy N, Salitra G, Soffer A, Aurbach
D. Assessing optimal pore-to-ion size relations in the
design of porous poly(vinylidene chloride) carbons for
EDL capacitors. Appl Phys A 2006, 82:607–613.
59. Nandhini R, Mini PA, Avinash B, Nair SV,
Subramanian KRV. Supercapacitor electrodes using
nanoscale activated carbon from graphite by ball
milling. Mater Lett 2012, 87:165–168.
47. Hasegawa G. Monolithic electrode for electric doublelayer capacitors based on macro/meso/microporous
S-Containing activated carbon with high surface area.
J Mater Chem 2011, 21:2060–2063.
60. Simon P, Gogotsi Y. Materials for electrochemical
capacitors. Nat Mater 2008, 7:845–854.
48. Kim K-S, Park S-J. Easy synthesis of polyaniline-based
mesoporous carbons and their high electrochemical
performance. Microporous Mesoporous Mater 2012,
163:140–146.
49. Guo SL, Wang F, Chen H, Ren H, Wang R,
Pan X. Preparation and performance of polyvinyl
alcohol-based activated carbon as electrode material
in both aqueous and organic electrolytes. J Solid State
Electrochem 2012, 16:3355–3362.
50. Jänes A, Kurig H, Lust E. Characterisation of activated
nanoporous carbon for supercapacitor electrode
materials. Carbon 2007, 45:1226–1233.
51. Yang H, Yoshio M, Isono K, Kuramoto R.
Improvement of commercial activated carbon and
its application in electric double layer capacitors.
Electrochem Solid-State Lett 2002, 5:A141.
52. Teng H, Chien Y-JC. To Hsieh performance of electric
double-layer capacitors using carbons prepared from
phenol-formaldehyde resins by KOH etching. Carbon
2001, 39.
61. Taberna PL, Simon P, Fauvarque JF. Electrochemical
characteristics and impedance spectroscopy studies
of carbon-carbon supercapacitors. J Electrochem Soc
2003, 150:A292–A300.
62. Gamby J, Taberna PL, Simon P, Fauvarque JF,
Chesneau M. Studies and characterisations of
various activated carbons used for carbon/carbon
supercapacitors. J Power Sources 2001, 101:109–116.
63. Frackowiak E, Beguin F. Carbon materials for
the electrochemical storage of energy in capacitors.
Carbon 2001, 39:937–950.
64. Azaïs P, Duclaux L, Florian P, Maasiot D, LilloRodenas MA, L-Solano A, Peres J-P, Jehoulet C,
Beguin F. Causes of supercapacitors ageing in organic
electrolyte. J Power Sources 2007, 171:1046–1053.
65. Hulicova-Jurcakova D, Kodama M, Shiraishi S, Hatori
H, Zhu ZH, Lu GQ. Nitrogen-enriched nonporous
carbon electrodes with extraordinary supercapacitance. Adv Funct Mater 2009, 19:1800–1809.
66. Endo M, Kim YJ, Maeda T, Koshiba K, Katayam K.
Morphological effect on the electrochemical behavior
of electric double-layer capacitors. J Mater Res 2001,
16:3402–3410.
53. Weng T-C, Teng H. Characterization of high porosity
carbon electrodes derived from mesophase pitch for
electric double-layer capacitors. J Electrochem Soc
2001, 148:A368.
67. Chen H, Wang F, Tong S, Guo S, Pan X. Porous
carbon with tailored pore size for electric double
layer capacitors application. Appl Surf Sci 2012,
258:6097–6102.
54. Kim YJ, Horie Y, Ozaki S, Matsuzawa Y, Suezaki H,
Kim C, Miyashita N, Endo M. Correlation between
the pore and solvated ion size on capacitance uptake of
PVDC-based carbons. Carbon 2004, 42:1491–1500.
68. Kim C. Electrochemical characterization of electrospun activated carbon nanofibres as an electrode in
supercapacitors. J Power Sources 2005, 142:382–388.
55. Alonso A, Ruiz V, Blanco C, Santamaria R,
M, Menendez R, Jager SGE. Activated
produced from Sasol-Lurgi gasifier pitch
application as electrodes in supercapacitors.
2006, 44:441–446.
Granda
carbon
and its
Carbon
56. Huang C-W, Hsieh C-T, Kuo P-L, Teng H. Electric
double layer capacitors based on a composite electrode
of activated mesophase pitch and carbon nanotubes.
J Mater Chem 2012, 22:7314.
57. Denisa Hulicova-Jurcakova AMP, Poddubnaya OI,
Suarez-Garcia F, Juan MD T, Gao Qing L.
Highly stable performance of supercapacitors from
69. Zhang Q, Rong JP, Ma DS, Wei BQ. The
governing self-discharge processes in activated carbon
fabric-based supercapacitors with different organic
electrolytes. Energy Environ Sci 2011, 4:2152–2159.
70. Hung KS, Masarapu C, Ko TH, Wei BQ. Widetemperature range operation supercapacitors from
nanostructured activated carbon fabric. J Power
Sources 2009, 193:944–949.
71. Endo M, Maeda T, Takeda T, Kim YJ, Koshiba
K, Hara H, Dresselhaus MS. Capacitance and
pore-size distribution in aqueous and nonaqueous
electrolytes using various activated carbon electrodes.
J Electrochem Soc 2001, 148:A910.
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
72. Taer E, Deraman M, Talib IA, Awitdrus A, Hashmi
SA, Umar AA. Preparation of a highly porous
binderless activated carbon monolith from rubber
wood sawdust by a multi-step activation process for
application in supercapacitors. Int J Electrochem Sci
2011, 6:3301–3315.
73. Seo MK, Park SJ. Electrochemical characteristics of
activated carbon nanofiber electrodes for supercapacitors. Mater Sci Eng B 2009, 164:106–111.
74. Nakagawa H, Shudo A, Miura K. High-capacity
electric double-layer capacitor with high-densityactivated carbon fiber electrodes. J Electrochem Soc
2000, 147:38–42.
75. Hulicova-Jurcakova D, Seredych M, Lu GQ, Bandosz
TJ. Combined effect of nitrogen- and oxygencontaining functional groups of microporous activated
carbon on its electrochemical performance in
supercapacitors. Adv Funct Mater 2009, 19:438–447.
76. Zhang X, Wang XY, Jiang LL, Wu H, Wu C, Su JC.
Effect of aqueous electrolytes on the electrochemical
behaviors of supercapacitors based on hierarchically
porous carbons. J Power Sources 2012, 216:290–296.
77. Han S-J, Kim Y-H, Kim K-S, Park S-J. A study
on high electrochemical capacitance of ion exchange
resin-based activated carbons for supercapacitor. Curr
Appl Phys 2012, 12:1039–1044.
78. Babel K, Jurewicz K. KOH activated carbon fabrics
as supercapacitor material. J Phys Chem Solids 2004,
65:275–280.
79. Ruiz V, Santamaria R, Ramos-Fernandez JM,
Martinez-Escandell M, Sepulveda-Escribano A,
Rodriguez-Reinoso F. An activated carbon monolith
as an electrode material for supercapacitors. Carbon
2009, 47:195–200.
80. Hsieh CT, Teng H. Influence of oxygen treatment on
electric double-layer capacitance of activated carbon
fabrics. Carbon 2002, 40:667–674.
81. Ishikawa M, Sakamoto A, Morita M, Matsuda Y,
Ishida K. Effect of treatment of activated carbon fiber
cloth electrodes with cold plasma upon performance
of electric double-layer capacitors. J Power Sources
1996, 60:233–238.
82. Carriazo D, Pico F, Gutierrez MC, Rubio F,
Rojo JM, Monte F. Block-Copolymer assisted
synthesis of hierarchical carbon monoliths suitable
as supercapacitor electrodes. J Mater Chem 2010,
20:773.
83. Raymundo-Pinero E, Cadek M, Beguin F. Tuning
Carbon Materials for Supercapacitors by Direct
Pyrolysis of Seaweeds. Adv Funct Mater 2009,
19:1032–1039.
84. Raymundo-Piñero E, Leroux F, Béguin F. A highperformance carbon for supercapacitors obtained by
carbonization of a seaweed biopolymer. Adv Mater
2006, 18:1877–1882.
85. Kim BH, Yang KS, Woo HG, Oshida K. Supercapacitor performance of porous carbon nanofiber
composites prepared by electrospinning polymethylhydrosiloxane (PMHS)/polyacrylonitrile (PAN) blend
solutions. Synth Met 2011, 161:1211–1216.
86. Endo M, Kim YJ, Takeda T, Maeda T, Hayashi T,
Koshiba K, Hra H, Dresselhaus MS. Poly(vinylidene
chloride)-based carbon as an electrode material for
high power capacitors with an aqueous electrolyte. J
Electrochem Soc 2001, 148:A1135.
87. Hao GP, Mi J, Li D, Qu WH, Wu TJ, Li WC,
Lu AH. A comparative study of nitrogen-doped
hierarchical porous carbon monoliths as electrodes
for supercapacitors. New Carbon Mater 2011,
26:197–203.
88. Garcia-Gomez A, Miles P, Centeno TA, Rojo
JM. Uniaxially oriented carbon monoliths as
supercapacitor electrodes. Electrochim Acta 2010,
55:8539–8544.
89. Sun XZ, Zhang X, Zhang HT, Zhang DC, Ma
YW. A comparative study of activated carbon-based
symmetric supercapacitors in Li2SO4 and KOH
aqueous electrolytes. J Solid State Electrochem 2012,
16:2597–2603.
90. Ma C, Song Y, Shi JL, Zhang DQ, Zhong M, Guo QG,
Liu L. Phenolic-based carbon nanofiber webs prepared
by electrospinning for supercapacitors. Mater Lett
2012, 76:211–214.
91. Kim C, Park S-H, Lee W-J, Yang K-S. Characteristics of supercapaitor electrodes of PBI-based carbon
nanofiber web prepared by electrospinning. Electrochim Acta 2004, 50:877–881.
92. Kim YJ, Matsuzawa Y, Ozaki S, Park KC, Kim
C, Endo M, Yoshida H, Masuda G, Sato T,
Dresselhaus MS. High energy-density capacitor based
on ammonium salt type ionic liquids and their mixing
effect by propylene carbonate. J Electrochem Soc
2005, 152:A710–A715.
93. Xu B, Wu F, Chen RJ, Cao GP, Chen S, Yang
YS. Mesoporous activated carbon fiber as electrode
material for high-performance electrochemical double
layer capacitors with ionic liquid electrolyte. J Power
Sources 2010, 195:2118–2124.
94. Okajima K, Ohta K, Sudoh M. Capacitance behavior
of activated carbon fibers with oxygen-plasma
treatment. Electrochim Acta 2005, 50:2227–2231.
95. Balducci A, Dugas R, Taberna PL, Simon P, Plee
D, Mastragostino M, Passerini S. High temperature
carbon–carbon supercapacitor using ionic liquid as
electrolyte. J Power Sources 2007, 165:922–927.
96. Liang YY, Schwab MG, Zhi LJ, Mugnaioli E, Kolb U,
Feng XL, Mullen K. Direct access to metal or metal
oxide nanocrystals integrated with one-dimensional
nanoporous carbons for electrochemical energy
storage. J Am Chem Soc 2010, 132:15030–15037.
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
97. Timperman L, Skowron P, Boisset A, Galiano
H, Lemordant D, Frackowiak E, Beguin F,
Anouti M. Triethylammonium bis(tetrafluoro
methylsulfonyl)amide protic ionic liquid as an
electrolyte for electrical double-layer capacitors. Phys Chem Chem Phys 2012, 14:
8199–8207.
98. Korenblit Y, Kajdos A, West WC, Smart MC, Brandon
EJ, Kvit A, Jagiello J, Yushin G. In-situ studies of ion
transport in microporous supercapacitor electrodes
at ultra-low temperatures. Adv Funct Mater 2012,
22:1655–1662.
99. Pech D, Brunet M, Durou H, Huang PH, Mochalin
V, Gogotsi Y, Taberna P-L, Simon P. Ultrahigh-power
micrometre-sized supercapacitors based on onion-like
carbon. Nat Nanotechnol 2010, 5:651–654.
100. Zhu YW, Murali S, Stoller MD, Ganesh KJ, Cai
WW, Ferreira PJ, Pirkle A, Wallace RM, Cychosz KA,
Thommes M, Su D. Carbon-based supercapacitors
produced by activation of graphene. Science 2011,
332:1537–1541.
101. Lu W, Yushin G. Lithographically Patterned Thin
Activated Carbon Films as a New Technology
Platform for On-Chip Devices. ACS Nano 2013,
7:6498–6506.
102. Lewandowski A, Olejniczak A, Galinski M, Stepniak I.
Performance of carbon-carbon supercapacitors based
on organic, aqueous and ionic liquid electrolytes.
J Power Sources 2010, 195:5814–5819.
103. McDonough JR, Choi JW, Yang Y, Mantia FL,
Zhang YG, Cui Y. Carbon nanofiber supercapacitors
with large areal capacitances. Appl Phys Lett 2009,
95:243109.
104. Pratyush D, Satish Kumar JL, Thein K. Experimental
and theoretical investigations of porous structure
formation in electrospun fibers. Macromolecules 2007,
40:7689–7694.
105. Wang T, Kumar S. Electrospinning of polyacrylonitrile
nanofibers. J Appl Polym Sci 2006, 102:1023–1029.
106. Rahatekar SS, Rasheed A, Jain R, Zammarano M,
Koziol KK, Windle AH, Gilman JW, Kumar S. Solution
spinning of cellulose carbon nanotube composites
using room temperature ionic liquids. Polymer 2009,
50:4577–4583.
107. Presser V, Heon M, Gogotsi Y. Carbide-derived
carbons—from porous networks to nanotubes and
graphene. Adv Funct Mater 2011, 21:810–833.
108. Yushin G, Nikitin A, Gogotsi Y. Carbide derived
carbon. In: Gogotsi Y, ed. Nanomaterials Handbook.
Boca Raton, FL: CRC Press; 2006, 239–282.
109. Hutchins, O. (US; 1918).
carbons on specific capacitance. J Power Sources 2006,
158:765–772.
112. Yushin G, Dash RK, Gogotsi Y, Jagiello J, Fischer
JE. Carbide-derived carbons: effect of pore size on
hydrogen uptake and heat of adsorption. Adv Funct
Mater 2006, 16:2288–2293.
113. Largeot C, Portet C, Chmiola J, Taberna P-L, Gogotsi
Y, Simon P. Relation between the ion size and pore
size for an electric double-layer capacitor. JACS 2008,
130:2730.
114. Gogotsi Y, Dash RK, Yushin G, Yildirim T, Laudisio
G, Fischer JE. Tailoring of nanoscale porosity in
carbide-derived carbons for hydrogen storage. J Am
Chem Soc 2005, 127:16006–16007.
115. Latt M, Kaarik M, Permann L, Kuura H, Arulepp M,
Leis J. A structural influence on the electrical doublelayer characteristics of Al(4)C(3)-derived carbon. J
Solid State Electrochem 2010, 14:543–548.
116. Wang HL, Gao QM. Synthesis, characterization and
energy-related applications of carbide-derived carbons
obtained by the chlorination of boron carbide. Carbon
2009, 47:820–828.
117. Kravchik AE, Kukushkina JA, Sokolov VV,
Tereshchenko GF. Structure of nanoporous carbon produced from boron carbide. Carbon 2006,
44:3263–3268.
118. Dash RK, Nikitin A, Gogotsi Y. Microporous carbon
derived from boron carbide. Microporous Mesoporous
Mater 2004, 72:203–208.
119. Chmiola J, Yushin G, Dash RK, Hoffman EN,
Fischer JE, Barsoum MW, Gogotsi Y. Double-layer
capacitance of selected carbide derived carbons in
sulfuric acid. Electrochem Solid State Commun 2005,
8:A357–A360.
120. Zheng LP, Wang Y, Wang XY, An HF, Yi LH. The
effects of surface modification on the supercapacitive
behaviors of carbon derived from calcium carbide.
J Mater Sci 2010, 45:6030–6037.
121. Thomberg T, Kurig H, Janes A, Lust E. Mesoporous
carbide-derived carbons prepared from different
chromium carbides. Microporous Mesoporous Mater
2011, 141:88–93.
122. Thomberg T, Janes A, Lust E. Energy and
power performance of electrochemical double-layer
capacitors based on molybdenum carbide derived
carbon. Electrochim Acta 2010, 55:3138–3143.
123. Leis J, Arulepp M, Kaarik M, Perkson A. The effect
of Mo(2)C derived carbon pore size on the electrical
double-layer characteristics in propylene carbonatebased electrolyte. Carbon 2010, 48:4001–4008.
110. Chmiola J, Yushin G, Gogotsi Y, Portet C, Simon P.
Anomalous increase in carbon capacitance at pore size
below 1 nm. Science 2006, 313:1760–1763.
124. Cambaz ZG, Yushin GN, Gogotsi Y, Vyshnyakova
KL, Pereselentseva LN. Formation of carbide-derived
carbon on beta-silicon carbide whiskers. J Am Ceram
Soc 2006, 89:509–514.
111. Chmiola J, Yushin G, Dash R, Gogotsi Y. Effect
of pore size and surface area of carbide derived
125. Rufino B, Mazerat S, Couvrat M, Lorrette C, Maskrot
H, Pailler R. The effect of particle size on the formation
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
and structure of carbide-derived carbon on beta-sic
nanoparticles by reaction with chlorine. Carbon 2011,
49:3073–3083.
126. Oschatz M, Kockrick E, Rose M, Borchardt L, Klein
N, Senkovska I, Freudenberg T, Korenblit Y, Yushin
G, Kaskel S. A cubic ordered, mesoporous carbidederived carbon for gas and energy storage applications.
Carbon 2010, 48:3987–3992.
127. Korenblit Y, Rose M, Kockrick E, Borchardt L, Kvit A,
Kaskel S, Yushin G. High-rate electrochemical capacitors based on ordered mesoporous silicon carbidederived carbon. ACS Nano 2010, 4:1337–1344.
128. Huczko A, Osica M, Bystrzejewsk M, Lange H,
Cudzilo S, Leis J, Arulepp M. Characterization of
1-D nanoSiC-derived nanoporous carbon. Phys Status
Solidi B 2007, 244:3969–3972.
129. Arulepp M, Leis J, Latt M, Miller F, Rumma K,
Lust E, Burke AF. The advanced carbide-derived
carbon based supercapacitor. J Power Sources 2006,
162:1460–1466.
130. Dash RK, Chmiola J, Yushin G, Gogotsi Y, Laudisio
G, Singer J, FIscher J, Kucheyev S. Titanium Carbide
Derived Nanoporous Carbon for Energy-Related
Applications. Carbon 2006, 44:2489–2497.
131. Permann L, Latt M, Leis J, Arulepp M. Electrical
double layer characteristics of nanoporous carbon
derived from titanium carbide. Electrochim Acta 2006,
51:1274–1281.
132. Fernandez JA, Arulepp M, Leis J, Stoeckli F, Centeno
TA. EDLC performance of carbide-derived carbons in
aprotic and acidic electrolytes. Electrochim Acta 2008,
53:7111–7116.
133. Thomberg T, Janes A, Lust E. Energy and power
performance of vanadium carbide derived carbon
electrode materials for supercapacitors. J Electroanal
Chem 2009, 630:55–62.
134. Gonzalez-Garcia P, Urones-Garrote E, Avila-Brande
D, Gomez-Herrero A, Otero-Diaz LC. Structural study
of carbon nanomaterials prepared by chlorination
of tungsten carbide and bis(cyclopentadienyl)tungsten
dichloride. Carbon 2010, 48:3667–3675.
135. Dash RK, Yushin G, Gogotsi Y. Nanoporous Carbon
Derived from Zirconium Carbide. Microporous
Mesoporous Mater 2005, 86:50–57.
136. Hoffman E, Yushin GN, Barsoum BM, Gogotsi G.
Synthesis of nanoporous carbide-derived carbon by
chlorination of titanium aluminum carbide. Chem
Mater 2005, 17:2317–2322.
137. Yushin G, Hoffman EN, Nikitin A, Ye JJ, Barsoum
MW, Gogotsi Y. Synthesis of nanoporous carbidederived carbon by chlorination of titanium silicon
carbide. Carbon 2005, 44:2075–2082.
138. Yachamaneni S, Yushin G, Yeon SH, Gogotsi Y,
Howell C, Sanderman S, Phillips G, Mikhalovsky
S. Mesoporous carbide-derived carbon for cytokine
removal from blood plasma. Biomaterials 2010,
31:4789–4794.
139. Ersoy DA, McNallan MJ, Gogotsi Y. Carbon coatings
produced by high temperature chlorination of silicon
carbide ceramics. Mat Res Innovat 2001, 5:55–62.
140. Cambaz ZG, Yushin GN, Vyshnyakova KL, Pereselentseva LN, Gogotsi YG. Conservation of shape
during formation of carbide-derived carbon on silicon carbide nano-whiskers. J Am Ceram Soc 2005,
89:509–514.
141. Gogotsi Y, Weltz S, Ersoy DA, McNallan MJ.
Conversion of silicon carbide to crystalline diamondstructured carbon at ambient pressure. Nature 2001,
411:283–287.
142. Cambaz ZG, Yushin G, Osswald S, Mochalin V,
Gogotsi Y. Noncatalytic synthesis of carbon nanotubes
and graphene on SiC. Carbon 2007, 46:841–849.
143. Kusunoki M, Rokkaku M, Suzuki T. Epitaxial carbon
nanotube film self-organized by decomposition of
silicon carbide. Appl Phys Lett 1997, 71:2620–2622.
144. Eskusson J, Janes A, Kikas A, Matisen L, Lust
E. Physical and electrochemical characteristics of
supercapacitors based on carbide derived carbon
electrodes in aqueous electrolytes. J Power Sources
2011, 196:4109–4116.
145. Portet C, Yushin G, Gogotsi Y. Effect of carbon
particle size on electrochemical performance of EDLC.
J Electrochem Soc 2008, 155:A531–A536.
146. Leis J, Arulepp M, Kuura A, Latt M, Lust E. Electrical
double-layer characteristics of novel carbide-derived
carbon materials. Carbon 2006, 44:2122–2129.
147. Presser V, Zhang LF, Niu JJ, McDonough J, Perez C,
Fong H, Gogotsi Y. Flexible nano-felts of carbidederived carbon with ultra-high power handling
capability. Adv Energy Mater 2011, 1:423–430.
148. Portet C, Lillo-Rodenas MA, Linares-Solano A,
Gogotsi Y. Capacitance of KOH activated carbidederived carbons. Phys Chem Chem Phys 2009,
11:4943–4945.
149. Qu DY. Studies of the activated carbons used in
double-layer supercapacitors. J Power Sources 2002,
109:403–411.
150. Heon M, Lofland S, Applegate J, Notte R, Cortes
E, Hettinger JD, Taberna P-L, Simon P, Huang PH,
Brunet M, Gogotsi Y. Continuous carbide-derived
carbon films with high volumetric capacitance. Energy
Environ Sci 2011, 4:135–138.
151. Chmiola J, Largeot C, Taberna PL, Simon P, Gogotsi
Y. Monolithic carbide-derived carbon films for microsupercapacitors. Science 2010, 328:480–483.
152. Sevilla M, Mokaya R. Activation of carbide-derived
carbons: a route to materials with enhanced gas
and energy storage properties. J Mater Chem 2011,
21:4727–4732.
153. Rose M, Kockrick E, Senkovska I, Kaskel S. High
surface area carbide-derived carbon fibers produced by
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
electrospinning of polycarbosilane precursors. Carbon
2010, 48:403–407.
lithium batteries from functionalized carbon-nanotube
electrodes. Nat Nano 2010, 5:531–537.
154. Paxman JR, Richardson JC, Dettmar PW, Corfe BM.
Daily ingestion of alginate reduces energy intake in
free-living subjects. Appetite 2008, 51:713–719.
169. Portet C, Taberna PL, Simon P, Flahaut E. Influence
of carbon nanotubes addition on carbon-carbon
supercapacitor performances in organic electrolyte.
J Power Sources 2005, 139:371–378.
155. Kyotani T, Nagai T, Inoue S, Tomita A. Formation of
new type of porous carbon by carbonization in zeolite
nanochannels. Chem Mater 1997, 9:609–615.
156. Ma ZX, Kyotani T, Tomita A. Preparation of a high
surface area microporous carbon having the structural
regularity of Y zeolite. PCCP 2000, 2:2365–2366.
157. Ma ZX, Kyotani T, Liu Z, Terasaki O, Tomita A.
Very high surface area microporous carbon with a
three-dimensional nano-array structure: synthesis and
its molecular structure. Chem Mater 2001, 13:4413.
158. Nishihara H, et al. A possible buckybowl-like
structure of zeolite templated carbon. Carbon 2009,
47:1220–1230.
159. Wang HL, Gao QM, Hu J. Preparation of
porous doped carbons and the high performance in
electrochemical capacitors. Microporous Mesoporous
Mater 2010, 131:89–96.
160. Wang HL, Gao QM, Hu J, Chen Z. High
performance of nanoporous carbon in cryogenic
hydrogen storage and electrochemical capacitance.
Carbon 2009, 47:2259–2268.
161. Portet C, Yang Z, Korenblit Y, Gogotsi Y, Mokaya
R, Yushin G. Electrical double-layer capacitance of
zeolite-templated carbon in organic electrolyte. J
Electrochem Soc 2009, 156:A1–A6.
170. Emmenegger C, Mauron P, Sudan P, Wenger P,
Hermann V, Gally R, Zuttel A. Investigation of electrochemical double-layer (ECDL) capacitors electrodes
based on carbon nanotubes and activated carbon materials. J Power Sources 2003, 124:321–329.
171. Futaba DN, Hata K, Yamada T, Hiraoka T, Hayamizu
Y, Kakudate Y, Tanaike O, Hatori H, Yumura
M, Iijima S. Shape-engineerable and highly densely
packed single-walled carbon nanotubes and their
application as super-capacitor electrodes. Nat Mater
2006, 5:987–994.
172. Gao LJ, Peng AP, Wang ZY, Zhang H, Shi ZJ,
Gu ZN, Cao GP, Ding BZ. Growth of aligned
carbon nanotube arrays on metallic substrate and its
application to supercapacitors. Solid State Commun
2008, 146:380–383.
173. Barisci JN, Wallace GG, MacFarlane DR, Baughman
RH. Investigation of ionic liquids as electrolytes for
carbon nanotube electrodes. Electrochem Commun
2004, 6:22–27.
174. Chen JH, Li WZ, Wang DZ, Yang SX, Wen JG,
Ren ZF. Electrochemical characterization of carbon
nanotubes as electrode in electrochemical double-layer
capacitors. Carbon 2002, 40:1193–1197.
162. Itai H, Nishihara H, Kogure T, Kyotani T. Threedimensionally arrayed and mutually connected 1.2-nm
nanopores for high-performance electric double layer
capacitor. J Am Chem Soc 2011, 133:1165–1167.
175. Shiraishi S, Kurihara H, Okabe K, Hulicova D,
Oya A. Electric double layer capacitance of highly
pure single-walled carbon nanotubes (HiPco (TM)
Buckytubes (TM)) in propylene carbonate electrolytes.
Electrochem Commun 2002, 4:593–598.
163. Nishihara H, Itoi H, Kogure T, Hou PX, Touhara
H, Okino F, Kyotani T. Investigation of the ion
storage/transfer behavior in an electrical double-layer
capacitor by using ordered microporous carbons as
model materials. Chem 2009, 15:5355–5363.
176. Ma RZ, Liang J, Wei BQ, Zhang B, Xu CL, Wu
DH. Processing and performance of electric doublelayer capacitors with block-type carbon nanotube
electrodes. Bull Chem Soc Jpn 1999, 72:2563–2566.
164. Ania CO, Khomenko V, Raymundo-Pinero E, Parra
JB, Beguin F. The large electrochemical capacitance of
microporous doped carbon obtained by using a zeolite
template. Adv Funct Mater 2007, 17:1828–1836.
177. Ma RZ, Liang J, Wei BQ, Zhang B, Xu CL, Wu
DH. Study of electrochemical capacitors utilizing
carbon nanotube electrodes. J Power Sources 1999,
84:126–129.
165. Barbieri O, Hahn M, Herzog A, Kotz R. Capacitance
limits of high surface area activated carbons for double
layer capacitors. Carbon 2005, 43:1303–1310.
178. Lu W, Qu LT, Henry K, Dai LM. High performance
electrochemical capacitors from aligned carbon
nanotube electrodes and ionic liquid electrolytes.
J Power Sources 2009, 189:1270–1277.
166. Frackowiak E, et al. Enhanced capacitance of carbon
nanotubes through chemical activation. Chem Phys
Lett 2002, 361:35–41.
179. Frackowiak E, Metenier K, Bertagna V, Beguin F.
Supercapacitor electrodes from multiwalled carbon
nanotubes. Appl Phys Lett 2000, 77:2421–2423.
167. Jiang Q, Qu MZ, Zhou GM, Zhang BL, Yu ZL. A
study of activated carbon nanotubes as electrochemical
super capacitors electrode materials. Mater Lett 2002,
57:988–991.
180. Yoon BJ, Jeong S-H, Lee K-H, Kim HS, Park CG,
Han JH. Electrical properties of electrical double layer
capacitors with integrated carbon nanotube electrodes.
Chem Phys Lett 2004, 388:170–174.
168. Lee SW, Yabuuchi N, Gallant BM, Chen S,
Kim B-S, Hammon PT, Shao-Horn Y. High-power
181. Niu CM, Sichel EK, Hoch R, Moy D, Tennent
H. High power electrochemical capacitors based on
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
carbon nanotube electrodes. Appl Phys Lett 1997,
70:1480–1482.
182. Lee SW, Kim BS, Chen S, Shao-Horn Y, Hammond
PT. Layer-by-layer assembly of all carbon nanotube
ultrathin films for electrochemical applications. J Am
Chem Soc 2009, 131:671–679.
183. An KH, Kim WS, Park YS, Choi YC, Lee SM, Chung
DC, Bae DJ, Lim SC, Lee YH. Supercapacitors using
single-walled carbon nanotube electrodes. Adv Mater
2001, 13:497.
184. Talapatra S, Kar S, Pal SK, Vajtai R, Ci L, Cictor P,
Shaijumon MM, Kaur S, Nalamasu O, Ajayan PM.
Direct growth of aligned carbon nanotubes on bulk
metals. Nat Nanotechnol 2006, 1:112–116.
185. Du CS, Pan N. High power density supercapacitor
electrodes of carbon nanotube films by electrophoretic
deposition. Nanotechnology 2006, 17:5314–5318.
186. Du CS, Yeh J, Pan N. High power density supercapacitors using locally aligned carbon
nanotube electrodes. Nanotechnology 2005, 16:
350–353.
187. Frackowiak E, Metenier K, Pellenq R, Bonnamy S,
Beguin F. Capacitance properties of carbon nanotubes.
In: Kuzmany H, Fink J, Mehring M, Roth S, eds.
Electronic Properties of Novel Materials—Science and
Technology of Molecular Nanostructures, vol. 486.
1999, 429–432.
188. Shah R, Zhang XF, Talapatra S. Electrochemical
double layer capacitor electrodes using aligned carbon
nanotubes grown directly on metals. Nanotechnology
2009, 20:395202.
189. Barisci JN, Wallace GG, Baughman RH. Electrochemical studies of single-wall carbon nanotubes in aqueous
solutions. J Electroanal Chem 2000, 488:92–98.
190. Zhang H, Cao GP, Yang YS, Gu ZN. Capacitive
performance of an ultralong aligned carbon nanotube
electrode in an ionic liquid at 60 degrees C. Carbon
2008, 46:30–34.
196. Gu W, Peters N, Yushin G. Functionalized carbon
onions, detonation nanodiamond and mesoporous
carbon as cathodes in li-ion electrochemical energy
storage devices. Carbon 2013, 53:292–301.
197. Ugarte D. Onion-like graphitic particles. In: Endo M,
Iijima S, Dresselhaus MS, eds. Carbon Nanotubes.
Oxford: Pergamon; 1996, 163–167.
198. De Heer WA, Ugarte D. Formation mechanism
of quasi-spherical carbon particles induced by
electron bombardment. Chem Phys Lett 1993, 207:
473–479.
199. Sano N, Wang H, Alexandrou I, Chhowalla M,
Teo KBK, Amaratunga GAJ, Iimura K. Properties of
carbon onions produced by an arc discharge in water.
J Appl Phys 2002, 92:2783–2788.
200. Osswald S, Yushin G, Mochalin V, Kucheyev SO,
Gogotsi Y. Control of sp(2)/sp(3) carbon ratio
and surface chemistry of nanodiamond powders by
selective oxidation in air. J Am Chem Soc 2006,
128:11635–11642.
201. Shenderova O, Jones C, Borjanovic V, Hens S,
Cunning ham G, Moseenkov S, Kuznetsov V, Mcguire
G. Detonation nanodiamond and onion-like carbon:
applications in composites. Phys Status Solidi A 2008,
205:2245–2251.
202. Kuznetsov VL, Chuvilin AL, Butenko YV, Malkov
IY, Titov VM. Onion-like carbon from ultradisperse diamond. Chem Phys Lett 1994, 222:
343–348.
203. Yushin GN, Osswald S, Padalko VI, Bogatyreva GP,
Gogotsi Y. Effect of sintering on structure of nanodiamond. Diam Relat Mater 2005, 14:1721–1729.
204. Jeong HM, Lee JW, Shin WH, Choi YJ, Shin HJ,
Kang JK, Choi JW. Nitrogen-doped graphene for highperformance ultracapacitors and the importance of
nitrogen-doped sites at basal planes. Nano Lett 2011,
11:2472–2477.
191. Barisci JN, Wallace GG, Baughman RH. Electrochemical characterization of single-walled carbon nanotube
electrodes. J Electrochem Soc 2000, 147:4580–4583.
205. Stoller MD, Park SJ, Zhu YW, An JH, Ruoff RS.
Graphene-based ultracapacitors. Nano Lett 2008,
8:3498–3502.
192. Evanoff K, Khan J, Balandin AA, Magasinski A,
Ready WJ, Fuller TF, Yushin G. Toward ultra-thick
battery electrodes: aligned carbon nanotube—enabled
architecture. Adv Mater 2011, 24:533–537.
206. Hantel MM, Kaspar T, Nesper R, Wokaun A, Kotz
R. Partially reduced graphite oxide for supercapacitor
electrodes: effect of graphene layer spacing and huge
specific capacitance. Electrochem Commun 2011,
13:90–92.
193. Hertzberg B, Alexeev A, Yushin G. Deformations in
Si-Li anodes upon electrochemical alloying in nanoconfined space. J Am Chem Soc 2010, 132:8548–8549.
194. Evanoff K, Benson J, Schauer M, Kovalenko I,
Lahmore D, Ready J, Yushin G. Ultra strong siliconcoated carbon nanotube nonwoven fabric as a
multifunctional lithium-ion battery anode. ACS Nano
2012, 6:9837–9845.
195. Du CS, Pan N. Supercapacitors using carbon
nanotubes films by electrophoretic deposition. J Power
Sources 2006, 160:1487–1494.
207. Sato J, Takasu Y, Fukuda K, Sugimoto W. Graphene
nanoplatelets via exfoliation of platelet carbon
nanofibers and its electric double layer capacitance.
Chem Lett 2011, 40:44–45.
208. Vivekchand SRC, Rout CS, Subrahmanyam KS,
Govindaraj A, Rao CNR. Graphene-based electrochemical supercapacitors. J Chem Sci 2008, 120:
9–13.
209. Chen Y, Zhang XO, Zhang DC, Yu P, Ma
YW. High performance supercapacitors based on
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
reduced graphene oxide in aqueous and ionic liquid
electrolytes. Carbon 2011, 49:573–580.
210. Lin ZY, Liu Y, Yao YG, Hildreth OJ, Li Z, Moon
K, Wong CP. Superior capacitance of functionalized
graphene. J Phys Chem C 2011, 115:7120–7125.
211. Wang Y, Shi ZQ, Huang Y, Ma YF, Wang CY,
Chen MM, Chen YS. Supercapacitor devices based
on graphene materials. J Phys Chem C 2009,
113:13103–13107.
212. Chen Y, Zhang X, Yu P, Ma YW. Electrophoretic
deposition of graphene nanosheets on nickel foams
for electrochemical capacitors. J Power Sources 2010,
195:3031–3035.
213. An XH, Simmons T, Shah R, Wolfe C, Lewis KM,
Washington M, Nayak SK, Talapatra S, Kar S. Stable
aqueous dispersions of noncovalently functionalized
graphene from graphite and their multifunctional
high-performance applications. Nano Lett 2010,
10:4295–4301.
214. Du QL, Zheng MB, Zhang LF, Wang YW,
Chen JH, Xue LP, Dai WJ, Ji GB, Cao JM.
Preparation of functionalized graphene sheets by
a low-temperature thermal exfoliation approach
and their electrochemical supercapacitive behaviors.
Electrochim Acta 2010, 55:3897–3903.
215. Li YM, van Zijll M, Chiang S, Pan N. KOH modified
graphene nanosheets for supercapacitor electrodes.
J Power Sources 2011, 196:6003–6006.
216. Shao YY, Wang J, Engelhard M, Wang CM, Lin YH.
Facile and controllable electrochemical reduction of
graphene oxide and its applications. J Mater Chem
2010, 20:743–748.
217. Liu CG, Yu ZN, Neff D, Zhamu A, Jang BZ.
Graphene-based supercapacitor with an ultrahigh
energy density. Nano Lett 2010, 10:4863–4868.
218. Bushueva EG, Galkin PS, Okotrub AV, Bulusheva LG,
Gavrilov NN, Kuznetsov VL, Moiseekov SI. Double
layer supercapacitor properties of onion-like carbon
materials. Phys Status Solidi B 2008, 245:2296–2299.
219. Portet C, Chmiola J, Gogotsi Y, Park S, Lian
K. Electrochemical characterizations of carbon
nanomaterials by the cavity microelectrode technique.
Electrochim Acta 2008, 53:7675–7680.
220. Park S, Lian K, Gogotsi Y. Pseudocapacitive behavior
of carbon nanoparticles modified by phosphomolybdic
acid. J Electrochem Soc 2009, 156:A921–A926.
221. Kovalenko I, Bucknall D, Yushin G. Detonation
nanodiamond and onion-like carbon—embedded
polyaniline for supercapacitors. Adv Funct Mater
2010, 20:3979–3986.
222. Plonska-Brzezinska ME, Palkar A, Winkler K,
Echegoyen L. Electrochemical properties of small
carbon nano-onion films. Electrochem Solid State Lett
2010, 13:K35–K38.
223. Breczko J, Winkler K, Plonska-Brzezinska ME,
Villalta-Cerdas A, Echegoyen L. Electrochemical
properties of composites containing small carbon
nano-onions and solid polyelectrolytes. J Mater Chem
2010, 20:7761–7768.
224. Sun YQ, Wu Q, Xu YX, Bai H, Li C, Shi GG.
Highly conductive and flexible mesoporous graphitic
films prepared by graphitizing the composites of
graphene oxide and nanodiamond. J Mater Chem
2011, 21:7154–7160.
225. Miller JR, Outlaw RA, Holloway BC. Graphene
double-layer capacitor with ac line-filtering performance. Science 2010, 329:1637–1639.
226. Mayer ST, Pekala RW, Kaschmitter JL. The
aerocapacitor—an
electrochemical
double-layer
energy-storage device. J Electrochem Soc 1993, 140:
446–451.
227. Fang B, Wei YZ, Kumagai M. Modified carbon
materials for high-rate EDLCs application. J Power
Sources 2006, 155:487–491.
228. Pekala RW, Farmer JC, Alviso CT, Tran TD,
Mayer ST, Miller JM, Dunn B. Carbon aerogels for
electrochemical applications. J Non-Cryst Solids 1998,
225:74–80.
229. Probstle H, Wiener M, Fricke J. Carbon aerogels
for electrochemical double layer capacitors. J Porous
Mater 2003, 10:213–222.
230. Kim SJ, Hwang SW, Hyun SH. Preparation of
carbon aerogel electrodes for supercapacitor and their
electrochemical characteristics. J Mater Sci 2005,
40:725–731.
231. Lee YJ, Jung JC, Yi J, Baeck S-H, Yoon JR, Song IK.
Preparation of carbon aerogel in ambient conditions
for electrical double-layer capacitor. Curr Appl Phys
2010, 10:682–686.
232. Moreno-Castilla C, Dawidziuk MB, Carrasco-Marin
F, Zapata-Benabithe Z. Surface characteristics and
electrochemical capacitances of carbon aerogels
obtained from resorcinol and pyrocatechol using boric
and oxalic acids as polymerization catalysts. Carbon
2011, 49:3808–3819.
233. Hwang SW, Hyun SH. Capacitance control of
carbon aerogel electrodes. J Non-Cryst Solids 2004,
347:238–245.
234. Li J, Wang XY, Huang QH, Gamboa S, Sebastian
PJ. Studies on preparation and performances of
carbon aerogel electrodes for the application of
supercapacitor. J Power Sources 2006, 158:784–788.
235. Li J, Wang XY, Wang Y, Huang QH, Dai CL,
Gamboa S, Sebastian PJ. Structure and electrochemical
properties of carbon aerogels synthesized at ambient
temperatures as supercapacitors. J Non-Cryst Solids
2008, 354:19–24.
236. Li WC, Reichenauer G, Fricke J. Carbon aerogels
derived from cresol-resorcinol-formaldehyde for
supercapacitors. Carbon 2002, 40:2955–2959.
237. Farmer JC, Fix DV, Mack GV, Pekala RW, Poco JF.
Capacitive deionization of NH4ClO4 solutions with
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
carbon aerogel electrodes. J Appl Electrochem 1996,
26:1007–1018.
238. Farmer JC, Fix DV, Mack GV, Pekala RW,
Poco JF. Capacitive deionization of NaCl and
NaNO3 solutions with carbon aerogel electrodes. J
Electrochem Soc 1996, 143:159–169.
239. Kalpana D, Omkumar KS, Kumar SS, Renganathan
NG. A novel high power symmetric ZnO/carbon
aerogel composite electrode for electrochemical supercapacitor. Electrochim Acta 2006, 52:1309–1315.
240. Gabelich CJ, Tran TD, Suffet IH. Electrosorption of
inorganic salts from aqueous solution using carbon
aerogels. Environ Sci Technol 2002, 36:3010–3019.
241. Welgemoed TJ, Schutte CF. Capacitive delonization
technology (TM): an alternative desalination solution.
Desalination 2005, 183:327–340.
242. Saliger R, Fischer U, Herta C, Fricke J. High surface
area carbon aerogels for supercapacitors. J Non-Cryst
Solids 1998, 225:81–85.
243. Wei YZ, Fang B, Iwasa S, Kumagai M. A novel
electrode material for electric double-layer capacitors.
J Power Sources 2005, 141:386–391.
244. Baumann TF, Worsley MA, Han TY-J, Satcher
JH Jr. High surface area carbon aerogel monoliths
with hierarchical porosity. J Non-Cryst Solids 2008,
354:3513–3515.
245. Zheng JP. The limitations of energy density of
battery/double-layer capacitor asymmetric cells. J
Electrochem Soc 2003, 150:A484.
246. Park JH, Park OO, Shin KH, Jin CS, Kim JH. An
electrochemical capacitor based on a Ni(OH)[sub
2]/activated carbon composite electrode. Electrochem
Solid-State Lett 2002, 5:H7.
247. Wang D-W, Li F, Cheng H-M. Hierarchical porous
nickel oxide and carbon as electrode materials for
asymmetric supercapacitor. J Power Sources 2008,
185:1563–1568.
248. Lang JW, Kong LB, Liu M, Luo YC, Kang
L. Asymmetric supercapacitors based on stabilized
α-Ni(OH)2 and activated carbon. J Solid State
Electrochem 2010, 14:1533–1539.
249. Qu QT, et al. V2O5·0.6H2O nanoribbons as cathode
material for asymmetric supercapacitor in K2SO4
solution. Electrochem Commun 2009, 11:1325–1328.
250. Chen L-M, Lai Q-Y, Hao Y-J, Zhao Y, Ji XY. Investigations on capacitive properties of the
AC/V2O5 hybrid supercapacitor in various aqueous
electrolytes. J Alloys Compd 2009, 467:465–471.
251. Kong L-B, Liu M, Lang J-W, Luo Y-C, Kang L.
Asymmetric supercapacitor based on loose-packed
cobalt hydroxide nanoflake materials and activated
carbon. J Electrochem Soc 2009, 156:A1000.
252. Liang Y-Y, Li H-L, Zhang X-G. A novel asymmetric
capacitor based on Co(OH)2/USY composite and
activated carbon electrodes. Mater Sci Eng A 2008,
473:317–322.
253. Khomenko V, Raymundo-Piñero E, Béguin F. Optimisation of an asymmetric manganese oxide/activated
carbon capacitor working at 2 V in aqueous medium.
J Power Sources 2006, 153:183–190.
254. Brousse T, Taberna P-L, Crosnier O, Dugas R,
Guillemet P, Scudeller Y, Zhou YK, Favier F,
Belanger D, Simon P. Long-term cycling behavior
of asymmetric activated carbon/MnO2 aqueous
electrochemical supercapacitor. J Power Sources 2007,
173:633–641.
255. Xu C, Du H, Li B, Kang F, Zeng Y. Asymmetric
activated carbon-manganese dioxide capacitors in
mild aqueous electrolytes containing alkaline-earth
cations. J Electrochem Soc 2009, 156:A435.
256. Algharaibeh Z, Liu X, Pickup PG. An asymmetric anthraquinone-modified carbon/ruthenium oxide
supercapacitor. J Power Sources 2009, 187:640–643.
257. Perret P, Khani Z, Brousse T, Bélanger D,
Guay D. Carbon/PbO2 asymmetric electrochemical
capacitor based on methanesulfonic acid electrolyte.
Electrochim Acta 2011, 56:8122–8128.
258. Yu N, Gao L, Zhao S, Wang Z. Electrodeposited PbO2
thin film as positive electrode in PbO2/AC hybrid
capacitor. Electrochim Acta 2009, 54:3835–3841.
259. Machida K, Suematsu S, Ishimoto S, Tamamitsu K.
High-voltage asymmetric electrochemical capacitor
based on polyfluorene nanocomposite and activated
carbon. J Electrochem Soc 2008, 155:A970.
260. Park JH, Park OK. Hybrid electrochemical capacitors
based on polyaniline and activated carbon electrodes.
J Power Sources 2002, 111:185–190.
261. Balducci A, Bardi U, Caporali S, Mastragostino M,
Soavi F. Ionic liquids for hybrid supercapacitors.
Electrochem Commun 2004, 6:566–570.
262. Amatucci GG, Badway F, A DP, Zheng T. An
asymmetric hybrid nonaqueous energy storage cell.
J Electrochem Soc 2001, 148:A930.
263. Du Pasquier A, Laforgue A, Simon P, Amatucci
GG, Fauvarque J-F. A nonaqueous asymmetric hybrid Li[sub 4]Ti[sub 5]O[sub 12]/poly
(fluorophenylthiophene) energy storage device. J Electrochem Soc 2002, 149:A302.
264. Brousse T, Marchand R, Taberna P-L, Simon P. TiO2
(B)/activated carbon non-aqueous hybrid system for
energy storage. J Power Sources 2006, 158:571–577.
265. Chen C-H, Tsai D-S, Chung W-H, Lee K-Y, Chen
Y-M, Huang Y-S. Electrochemical capacitors of
miniature size with patterned carbon nanotubes
and cobalt hydroxide. J Power Sources 2012,
205:510–515.
266. Wang G-X, Zhang B-L, Yu Z-L, Qu M-Z. Manganese
oxide/MWNTs composite electrodes for supercapacitors. Solid State Ionics 2005, 176:1169–1174.
267. Chen CH, Tsai DS, Chung WH, Chiou YD, Lee KY,
Huang YS. Miniature asymmetric ultracapacitor of
© 2013 John Wiley & Sons, Ltd.
WIREs Energy and Environment
Nanostructured carbon materials for electrochemical capacitor applications
patterned carbon nanotubes and hydrous ruthenium
dioxide. Nanotechnology 2012, 23:485402.
268. Cheng Q, Tang J, Ma J, Zhang H, Shinya N,
Qin LC. Graphene and nanostructured MnO2
composite electrodes for supercapacitors. Carbon
2011, 49:2917–2925.
269. Khomenko V, Raymundo-Piñero E, Béguin F. Highenergy density graphite/AC capacitor in organic
electrolyte. J Power Sources 2008, 177:643–651.
282. Lota G, Grzyb B, Machnikowska H, Machnikowski J,
Frackowiak E. Effect of nitrogen in carbon electrode
on the supercapacitor performance. Chem Phys Lett
2005, 404:53–58.
283. Huang JS, Sumpter BG, Meunier V. Theoretical model
for nanoporous carbon supercapacitors. Angew Chem
Int Ed 2008, 47:520–524.
284. Shi H. Activated carbons and double layer capacitance.
Electrochim Acta 1996, 41:1633.
270. Khomenko V, Raymundo-Piñero E, Béguin F. A
new type of high energy asymmetric capacitor with
nanoporous carbon electrodes in aqueous electrolyte.
J Power Sources 2010, 195:4234–4241.
285. Salitra G, Soffer A, Eliad L, Cohen Y, Aurbach
D. Carbon electrodes for double-layer capacitors—I.
Relations between ion and pore dimensions. J
Electrochem Soc 2000, 147:2486–2493.
271. Cericola D, Kötz R. Hybridization of rechargeable
batteries and electrochemical capacitors: principles
and limits. Electrochim Acta 2012, 72:1–17.
286. Raymundo-Pinero E, Kierzek K, Machnikowski J,
Beguin F. Relationship between the nanoporous
texture of activated carbons and their capacitance
properties in different electrolytes. Carbon 2006,
44:2498–2507.
272. Kazaryan SA, Razumov SN, Litvinenko SV, Kharisov
GG, Kogan VI. Mathematical model of heterogeneous electrochemical capacitors and calculation
of their parameters. J Electrochem Soc 2006, 153:
A1655.
273. Benson J, Boukhalfa S, Magasinski A, Kvit A, Yushin
G. Chemical vapor deposition of aluminum nanowires
on metal substrates for electrical energy storage
applications. ACS Nano 2012, 6:118–125.
274. Snook GA, Wilson GJ, Pandolfo AG. Mathematical functions for optimisation of conducting polymer/activated carbon asymmetric supercapacitors.
J Power Sources 2009, 186:216–223.
275. Kovalenko I, Bucknall DG, Yushin G. Detonation nanodiamond and onion-like-carbon-embedded polyaniline for supercapacitors. Adv Funct Mater 2010,
20:3979–3986.
276. Choi HS, Kim T, Im JH, Park CR. Preparation
and electrochemical performance of hyper-networked
Li4Ti5O12/carbon hybrid nanofiber sheets for a
battery-supercapacitor hybrid system. Nanotechnology 2011, 22:405402.
287. Feng G, Qiao R, Huang JS, Sumpter BG, Meunier V.
Ion distribution in electrified micropores and its role
in the anomalous enhancement of capacitance. ACS
Nano 2010, 4:2382–2390.
288. Huang JS, Sumpter BG, Meunier V. A universal model
for nanoporous carbon supercapacitors applicable
to diverse pore regimes, carbon materials, and
electrolytes. Chem 2008, 14:6614–6626.
289. Brunauer S, Emmett P, Teller E. Adsorption of gases
in multimolecular layers. J Am Chem Soc 1938,
60:309–319.
290. Ravikovitch PI, Vishnyakov A, Russo R, Neimark
AV. Unified approach to pore size characterization
of microporous carbonaceous materials from N-2,
Ar, and CO2 adsorption isotherms. Langmuir 2000,
16:2311–2320.
291. Centeno TA, Sereda O, Stoeckli F. Capacitance in
carbon pores of 0.7 to 15 nm: a regular pattern. Phys
Chem Chem Phys 2011, 13:12403–12406.
277. Sivakkumar SR, Milev AS, Pandolfo AG. Effect of
ball-milling on the rate and cycle-life performance
of graphite as negative electrodes in lithium-ion
capacitors. Electrochim Acta 2011, 56:9700–9706.
292. Sanchez-Gonzalez J, Stoeckli F, Centeno TA. The
role of the electric conductivity of carbons in the
electrochemical capacitor performance. J Electroanal
Chem 2011, 657:176–180.
278. Huang Y, Liang J, Chen Y. An overview of
the applications of graphene-based materials in
supercapacitors. Small 2012, 8:1805–1834.
293. Centeno TA, Stoeckli F. The volumetric capacitance
of microporous carbons in organic electrolyte.
Electrochem Commun 2012, 16:34–36.
279. Jurewicz K, Babe K, Źiókowski A, Wachowska H.
Ammoxidation of active carbons for improvement of
supercapacitor characteristics. Electrochim Acta 2003,
48:1491–1498.
294. Centeno TA, Stoeckli F. Surface-related capacitance
of microporous carbons in aqueous and organic
electrolytes. Electrochim Acta 2011, 56:7334–7339.
280. Krzysztof Jurewicz KB, Ziolkowski A, Wachowska
H, Kozlowski M. Ammoxidation of brown coals
for supercapacitors. Fuel Process Technol 2002,
23:77–78.
295. Choi NS, Chen ZH, Freunberger SA, Ji XL, Sun
Y-K, Amine K, Yushin G, Nazar LF, Cho J, Bruce
PG. Challenges facing lithium batteries and electrical
double-layer capacitors. Angew Chem Int Ed 2012,
51:9994–10024.
281. Frackowiak E. Carbon materials for supercapacitor application. Phys Chem Chem Phys 2007,
9:1774–1785.
296. Korenblit Y, Kajdos A, West WC, Smart MC, Brandon
EJ, Alexander K, Jagiello J, Yushin G. In situ
studies of ion transport in microporous supercapacitor
© 2013 John Wiley & Sons, Ltd.
wires.wiley.com/wene
Advanced Review
electrodes at ultralow temperatures. Adv Funct Mater
2012, 22:1655–1662.
297. Miller
JR, Simon P. Materials
science—
electrochemical capacitors for energy management.
Science 2008, 321:651–652.
298. Yuan LX, Feng JK, Ai XP, Cao YL, Chen SL, Yang
HX. Improved dischargeability and reversibility of
sulfur cathode in a novel ionic liquid electrolyte.
Electrochem Commun 2006, 8:610–614.
299. Urbonaite S, Juarez-Galan JM, Leis J, RodriguezReinoso F, Svensson G. Porosity development
along the synthesis of carbons from metal carbides. Microporous Mesoporous Mater 2008, 113:
14–21.
300. Portet C, Taberna PL, Simon P, Flahaut E.
Modification of Al current collector/active material
interface for power improvement of electrochemical
capacitor electrodes. J Electrochem Soc 2006, 153:
A649–A653.
311. Fic K, Lota G, Frackowiak E. Effect of surfactants
on capacitance properties of carbon electrodes.
Electrochim Acta 2012, 60:206–212.
312. Fic K, Lota G, Meller M, Frackowiak E. Novel
insight into neutral medium as electrolyte for highvoltage supercapacitors. Energy Environ Sci 2012,
5:5842–5850.
313. Demarconnay L, Raymundo-Pinero E, Beguin F. A
symmetric carbon/carbon supercapacitor operating at
1.6 V by using a neutral aqueous solution. Electrochem
Commun 2010, 12:1275–1278.
314. Bichat MP, Raymundo-Pinero E, Beguin F. High
voltage supercapacitor built with seaweed carbons
in neutral aqueous electrolyte. Carbon 2010,
48:4351–4361.
315. Conway BE, Pell WG, Liu T-C. Diagnostic analyses
for mechanisms of self-discharge of electrochemical
capacitors and batteries. J Power Sources 1997, 65:
53–59.
301. Portet C, Taberna PL, Simon P, Laberty-Robert
C. Modification of Al current collector surface
by sol–gel deposit for carbon-carbon supercapacitor
applications. Electrochim Acta 2004, 49:905–912.
316. Liu T, Pell WG, Conway BE. Self-discharge and
potential recovery phenomena at thermally and
electrochemically prepared RuO2 supercapacitor
electrodes. Electrochim Acta 1997, 42:3541–3552.
302. Fang B, Binder L. A modified activated carbon
aerogel for high-energy storage in electric double layer
capacitors. J Power Sources 2006, 163:616–622.
317. Niu J, Conway BE, Pell WG. Comparative studies
of self-discharge by potential decay and float-current
measurements at C double-layer capacitor and battery
electrodes. J Power Sources 2004, 135:332–343.
303. Huang JS, Sumpter BG, Meunier V. Curvature effects
in carbon nanomaterials: exohedral versus endohedral
supercapacitors. J Mater Res 2010, 25:1525–1531.
304. Arulepp M, Permann L, Leis J, Perkson A, Rumma K,
Janes A, Lust E. Influence of the solvent properties on
the characteristics of a double layer capacitor. J Power
Sources 2004, 133:320–328.
305. Tamai H, Kunihiro M, Morita M, Yasuda H.
Mesoporous activated carbon as electrode for
electric double layer capacitor. J Mater Sci 2005,
40:3703–3707.
306. Kotz R, Hahn M, Gallay R. Temperature behavior and
impedance fundamentals of supercapacitors. J Power
Sources 2006, 154:550–555.
307. Pandolfo AG, Hollenkamp AF. Carbon properties and
their role in supercapacitors. J Power Sources 2006,
157:11–27.
308. Bispo-Fonseca I, Aggar J, Sarrazin C, Simon P,
Fauvarque JF. Possible improvements in making
carbon electrodes for organic supercapacitors. J Power
Sources 1999, 79:238–241.
309. Gallagher KG, Yushin G, Fuller TF. The role of
nanostructure in the electrochemical oxidation of
model-carbon materials in acidic environments. J
Electrochem Soc 2010, 157:B820–B830.
310. Zhang K, Mao L, Zhang LL, Chan HSO, Zhao XS,
Wu JS. Surfactant-intercalated, chemically reduced
graphene oxide for high performance supercapacitor
electrodes. J Mater Chem 2011, 21:7302–7307.
318. Diab Y, Venet P, Gualous H, Rojat G. Self-discharge
characterization and modeling of electrochemical
capacitor used for power electronics applications.
IEEE Trans Power Electron 2009, 24:510–517.
319. Jerabek EC, Mansfield SF. Google Patents, 2000.
320. Morimoto T, Hiratsuka K, Sanada Y, Kurihara
K. Electric double-layer capacitor using organic
electrolyte. J Power Sources 1996, 60:239–247.
321. Qiao W, Korai Y, Mochida I, Hori Y, Maeda
T. Preparation of an activated carbon artifact
oxidative modification of coconut shell-based carbon
to improved the strength. Carbon 2002, 40:351–358.
322. Gu W, Sevilla M, Magasinski A, Fuertes AF, Yushin
G. Sulfur-containing activated carbons with greatly
reduced content of bottle neck pores for doublelayer capacitors: a case study for pseudocapacitance
detection. Energy Environ Sci 2013, 6:2465–2476.
323. Gu W, Sevilla M, Yushin G. Sulfur-containing activated carbons without bottle-neck pores for doublelayer capacitors: a case study for pseudocapacitance
detection. J Power Sources 2013, 53:292–301.
324. Kaus M, Kowal J, Sauer DU. Modelling the effects of
charge redistribution during self-discharge of supercapacitors. Electrochim Acta 2010, 55:7516–7523.
325. Zhang X, et al. The effects of surfactant template
concentration on the supercapacitive behaviors of
hierarchically porous carbons. J Power Sources 2012,
199:402–408.
© 2013 John Wiley & Sons, Ltd.