REVIEW
National Science Review
4: 453–489, 2017
doi: 10.1093/nsr/nwx009
Advance access publication 2 March 2017
MATERIALS SCIENCE
Carbon-based supercapacitors for efficient
energy storage
Xuli Chen, Rajib Paul and Liming Dai
∗
Center of Advanced
Science and
Engineering for
Carbon (Case
4Carbon), Department
of Macromolecular
Science and
Engineering, Case
School of Engineering,
Case Western
Reserve University,
Cleveland, Ohio
44106, USA
∗
Corresponding
author. E-mail:
liming.dai@case.edu
Received 24 July
2016; Revised 6
September 2016;
Accepted 13
September 2016
ABSTRACT
e advancement of modern electronic devices depends strongly on the highly ecient energy sources
possessing high energy density and power density. In this regard, supercapacitors show great promise. Due
to the unique hierarchical structure, excellent electrical and mechanical properties, and high specic surface
area, carbon nanomaterials (particularly, carbon nanotubes, graphene, mesoporous carbon and their
hybrids) have been widely investigated as ecient electrode materials in supercapacitors. is review article
summarizes progress in high-performance supercapacitors based on carbon nanomaterials with an emphasis
on the design and fabrication of electrode structures and elucidation of charge-storage mechanisms. Recent
developments on carbon-based exible and stretchable supercapacitors for various potential applications,
including integrated energy sources, self-powered sensors and wearable electronics, are also discussed.
Keywords: electric double-layer supercapacitors, pseudocapacitors, hybrid supercapacitors, carbon
nanotube (CNT), graphene, exible and wearable electronics
INTRODUCTION
e ever increasing consumption of fossil fuels
and their soaring price have caused serious con-
cerns about the fast depletion of existing fossil-fuel
reserves and the associated alarming greenhouse-
gas emissions and pollutions in air and on soil.
erefore, it is important to develop environment
friendly energy-generation and storage technolo-
gies. In particular, there has recently been intensive
aention on the advancement of energy-storage de-
vices, includingelectrochemical supercapacitors and
baeries [
1–7]. Compared to baeries, electro-
chemical supercapacitors (ESCs) are capable of pro-
viding 100–1000 times higher power density, but
with 3–30 times lower energy density [
8]. As a
consequence, ESCs are particularly useful for high
power bursts, for example for accelerating/breaking
high-speed transportation systems. Moreover, ESCs
can sustain up to millions of charge/discharge cycles
via the electric double-layer charge storage free from
chemical reactions. In contrast, baeries suer from
volumetric modulation and swelling of active mate-
rials in the electrodes due to the excessive redox re-
actions during charge/discharge cycles [
8]. As far
as the safety issues are concerned, therefore, super-
capacitors are much more reliable than baeries. In
order to minimize/avoid possible decomposition of
the electrolyte, however, the operating voltage for
ESCs must be low as compared to baeries. Never-
theless, a high operatingvoltage is desirable for ESCs
with a high energy density, and hence an optimized
operating voltage is essential for high-performance
ESCs.
In an electrochemical supercapacitor, two elec-
trodes are kept apart by a separator between them
(Fig.
1). ese two electrodes are identical for a
symmetric supercapacitor (Fig. 1a), but dierent for
an asymmetric supercapacitor (Fig. 1b and c). e
separator is generally ion-permeable, but also elec-
trically insulating, soaked with electrolytes to allow
ionic charge transfer between the electrodes. Poly-
mer or paper separators are oen used with or-
ganic electrolytes while ceramic or glass-ber sepa-
rators are preferred for aqueous electrolytes [
8,9].
Depending on the ways in which energy is stored,
ESCs can be divided into electric double-layer ca-
pacitors (EDLCs), in which charge storage occurs at
the interfaces between the electrolyte and electrodes
(Fig.
1a), and pseudocapacitors (PCs), involving re-
versible and fast Faradaic redox reactions for charge
C
e Author(s) 2017. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, please e-mail:
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454 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Electrolyte
Separator
Electrolyte
Cathode
Cathode
Cathode
Separator
Anode
Anode
Anode
Anion
Cation
Solvent molecule
Metal oxide or redox active molecule
Electrolyte Separator
Li-based salt
Li
+
ions
(a) (b)
(c)
Li
+
Figure 1. Schematic representation of (a) electrical double-layer capacitor (EDLC),
(b) pseudocapacitor (PC) and (c) hybrid supercapacitor (HSC).
storage (Fig.
1b). When a supercapacitor stores
charges by matching the capacitive carbon electrode
with either a peseudocapacitive or lithium-insertion
electrode (Fig.
1c), it is then called a hybrid super-
capacitor (HSC). Owing to their availability in large
quantities at a relatively low cost, unique hierarchal
structures with a large surface/interface area and ex-
cellentelectrical/electrochemical/mechanical prop-
erties, nanoporous and/or mesoporous carbon ma-
terials are useful as the electrode materials in all types
of ESCs.
Along with the recent rapid development of ex-
ible/wearable electronics, there is an urgent need
for integrated power sources based on exible and
even stretchable electrodes. Consequently, exible
and stretchable ber-shaped or very thin superca-
pacitors (SCs) have recently aracted a great deal
of interest [
10]. In this context, carbon nanotubes
(CNTs) and graphene with a high mechanical sta-
bility and excellent bending strength have been
reported to be ideal electrode materials for exi-
ble and stretchable ESCs. us, carbon nanoma-
terials have been widely investigated for develop-
ing new electrode materials in various ESCs for
ecient energy storage. A huge amount of liter-
ature on carbon-based ESCs has been produced,
with the number of publications still rapidly increas-
ing every year. A timely review on such a rapidly
growing eld of such signicance is highly desir-
able. e aim of this article is to provide a timely,
concise and critical review by summarizing recent
important progress on the topic and presenting
critical issues related to the material/electrode
design and the elucidation of energy-storage mech-
anisms. rough such a critical review, our under-
standing of carbon-based electrode materials for
energy storage will signicantly increase, as will in-
sights for the future development.
CARBON NANOMATERIALS
Conventional carbon materials are divided into
three forms: diamond, graphite and amorphous car-
bon [
1]. eir properties vary depending on the ar-
rangement of carbon atoms. For example, diamond
is hard and rigid due to its special diamond cu-
bic crystal structure with sigma bonding between
sp
3
hybridized carbon molecules. Having a layered
structure with strong covalent bonding between sp
2
hybridized carbon atoms in the plane of individ-
ual layers and weak van der Waals interactions be-
tween adjacent layers, graphite is so. e recent de-
velopment of nanoscience and nanotechnology has
opened up a new frontier in carbon materials re-
search by creating new graphitic carbon nanomateri-
als with multi-dimensions, including dimension-less
(0D) fullerene, one-dimensional (1D) carbon nan-
otubes (CNTs) [
11–19] and two-dimensional (2D)
graphene [20–31]. Fullerene C
60
has a soccer-ball-
like structure containing 20 carbon hexagons with
12 carbon pentagons formed into a cage of trun-
cated icosahedrons. Fullerene C
60
is a perfect elec-
tron acceptor, which has been widely used in solar
cells for charge separation. Due to its intractability,
low electrical conductivity and small surface area,
fullerene has been rarely used for energystorage with
respect to other carbon nanomaterials. So far, CNTs
[
2,32–42], graphene [29,43–73], mesoporous car-
bon [
74–80] and their hybrids [81–94] have been
widely studied as supercapacitor electrodes because
of their excellent electrical conductivity, high spe-
cic surface area, outstanding electrochemical activ-
ity and the ease with which they can be function-
alized into multidimensional and multifunctional
structures with excellent electrical and mechanical
properties.
APPLICATION OF CARBON
NANOMATERIALS IN
SUPERCAPACITORS
Current research and development on energy-
storage devices have been mainly focused on super-
capacitors, lithium-ion baeries and other related
baeries. Compared with baeries, supercapacitors
possess higher power density, longer cyclic stability,
higher Coulombic eciency and shorter period for
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REVIEW Chen et al. 455
full charge–discharge cycles. us, supercapacitors,
particularly those based on carbon CNTs, graphene
and mesoporous carbon electrodes, have gained in-
creasing popularity as one of the most important
energy-storage devices.
EDLCs
Similarly to traditional capacitors, EDLCs also store
energy through charge separation, which leads to
double-layer capacitance. Unlike a traditional ca-
pacitor, however, an EDLC contains two separated
charge layers at the interfaces of electrolyte with pos-
itive electrode and negative electrode, respectively.
e separation between electrical double layers in
an EDLC is much smaller than that in a conven-
tional capacitor, leading to a several orders of m ag-
nitude higher specic capacitance for the EDLC.
Since there is no chemical reaction involved and
the transport of ions in the electrolyte solution or
electrons through the electrodes is responsible for
charge storage, EDLCs can be fully charged or dis-
charged within a short time with a high power den-
sity. Ideally, EDLCs require electrode materials with
a high specic surface area and excellent electri-
cal conductivity, which can be fullled especially by
CNTs and graphene.
CNTs in EDLCs
CNTs, with and without compositing with other
electrode materials, are highly suitable for super-
capacitor electrodes. e reported specic surface
area of pure CNTs is in between 120 and 500 m
2
/g
with the specic capacitance ranging from 2 F/g to
200 F/g [
2,32–34]. Using single-walled carbon nan-
otubes (SWNTs) as the electrode materials, a spe-
cic capacitance, power density and energy density
up to 180 F/g, 20 kW/kg and 7 Wh/kg, respec-
tively, have been reported [
35,36]. e specic sur-
face area can be enhanced by activating the CNT
walls and/or tips. For example, Pan et al.haveim-
proved the specic surface area of SWNTs from
46.8 m
2
/g to 109.4 m
2
/g through electrochemi-
cal activation, leading to a three-times increase in
the specic capacitance [
37]. Hata and coworkers
have reported a specic surface area of 1300 m
2
/g
for highly pure SWNTs [
38]. Using organic elec-
trolyte (1 M E t
4
NBF
4
/propylene carbonate) to en-
sure a high voltage of 4 V, these authors have re-
ported an energy density as high as 94 Wh/kg (or
47 Wh/L) and a power density up to 210 kW/kg (or
105 kW/L).
CNT diameters play a key role in controlling
the intrinsic surface area. It was reported that the
specic surface area of multiwall carbon nanotubes
(MWNTs) with outer diameter of 10∼20 nm
and inner diameter of 2∼5 nm varied from 128 to
411 m
2
/g with increasing diameters, and the
MWNTs exhibited the highest specic capacitance
of 80 F/g in 6 M KOH electrolyte [
39]. So far,
many of the reported EDLCs based on pure CNTs
showed high-rate capabilities and cyclic stabilities,
together with rectangular cyclic voltammograms
and symmetric triangular galvanostatic charge–
discharge proles, indicating high performance for
charge storage.
Apart from improving the specic surface area,
much eort has been made to improve the elec-
trical conductivity and increase the active sites on
CNTs. Heteroatom doping has been demonstrated
to be an important and ecient technique for these
purposes. For instance, nitrogen-doped (N-doped)
CNTs were synthesized by in-situ polymerization of
aniline monomers on CNTs, followed by carboniza-
tion of polyaniline (PANI)-coated CNTs [
40]. In
this study, the N-doping level was controlled by ad-
justing the amount of aniline used, leading to a high-
est specic capacitance of 205 F/g in 6 M KOH
electrolyte—a much higher value than 10 F/g for
the pristine CNTs, at 8.64% (by mass) nitrogen dop-
ing. Moreover, 97.1% of the initial capacitance was
maintained aer 1000 cycles. Recently, Gueon and
Moon prepared N-doped CNT-based spherical par-
ticles by emulsion-assisted evaporation of hexade-
cane, followed by N-doping using melamine [
41]. A
specic capacitance of 215 F/g was achievedat a cur-
rent density of 0.2 A/g—3.1 times the enhancement
as compared to that of the pristine CNTs. e ob-
served performance improvement was aributed to
the combination of more active sites with a higher
electrical conductivity induced by N-doping. Inter-
estingly, N-doped aligned CNT arrays have also
been synthesized and systematically characterized
for their application in supercapacitors [
42]. It was
found that the supercapacitor performance at a low
scan rate was highly dependent on the pyridinic ni-
trogen content in N-doped CNTs due to the net
charges induced onto the neighboring carbon atoms
through protonation of the pyridinic nitrogen.
Graphene in EDLCs
Having the basic carbon laice structure similar to
CNTs with all carbon atoms exposed at the sur-
face, the single-atom-thick 2D graphene sheets show
similar electrical and other properties to CNTs, but
with an even larger specic surface area [
1,2]. Like
CNTs, therefore, graphene sheets have also been
extensively studied as electrode materials in ESCs.
e availability of graphene oxide (GO) by acid
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456 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Compress
(b)(a)
(d)(c)
(f)(e)
Compress
Figure 2. Schematic illustration of the graphene and holey graphene foams. (a, b) Initial
3D macroporous (a) graphene foam and (b) holey graphene foam. (c, d) Compressed
lms of the (c) graphene foam and (d) holey graphene foam. (e, f) A c losed-up view
of (e) graphene and (f) holey graphene lms. The arrows highlighted the ion transport
pathway. Reproduced with permission from ref. [
49]. Copyright of Macmillan Publishers
Ltd (2014).
oxidation of graphite [
43–45], followed by chemi-
cal reduction [
29,43–45], provides an eective ap-
proach for low-cost mass production of reduced
graphene oxide (RGO), which can directly be used
as EDLC electrode materials. In this regard, Stoller
et al . used hydrazine hydrate as the reducing reagent
to produce RGO from GO [
29]. e resultant RGO
exhibited a specic capacitance of 135 F/g and spe-
cic surface area of 705 m
2
/g [29], which is much
lower than the theoretic value of 2630 m
2
/g, pre-
sumably due to RGO aggregation. To minimize the
RGO aggregation, Chen and coworkers synthesized
graphene with mesoporous structure through ther-
mal exfoliation of RGO at 1050
◦
Ctoproduceaspe-
ciccapacitance upto150 F/g in30% KOH aqueous
solution [
46]. Microwave irradiation in vacuum can
reduce the reduction temperature required for ther-
mal exfoliation, as demonstrated by Lv et al.[
47].
ese authors decreased the exfoliation temperature
down to 200
◦
C with a concomitant increase in the
specic capacitance up to 264 F/g [47]. By using mi-
crowave radiation to assist the exfoliation process,
Zhu et al. also eectively deducted the exfoliation
time to as short as 1 min and the produced graphene
could still exhibit specic capacitance of 191 F/gin
5 M KOH [48].
For conventional graphene and RGO electrodes,
electrolyte ions can only transfer charges between
graphene sheets, which inevitably leads to a much
longer ion-transport path with respect to ions
transferring through the graphene sheets (Fig. 2).
To address this issue, Xu et al. synthesized holy
graphene sheets, which allow ions to pass through
the holes with a minimized transport path while still
maintaining the electron-transport eciency [
49].
As a result, their hierarchically structured three-
dimensional (3D) holy graphene electrode exhib-
ited both high gravimetric and volumetric specic
capacitances of 298 F/g and 212 F/cm
3
, respec-
tively. Moreover, the energy density for a corre-
sponding fully packaged supercapacitor is as high as
35 Wh/kg (49 Wh/L), which is sucient to bridge
the gap between supercapacitors and baeries.
Similarly to CNTs, surface activation can also be
used to improve the specic capacitance of graphene
electrodes without a detrimental eect o n the elec-
trical conductivity. O f particular interest, Ruo and
coworkers obtained a dramatically improved spe-
cic surface area up to 3100 m
2
/g by activat-
ing exfoliated GO with KOH [
50], which is even
higher than the theoretically predicted specic sur-
face area of monolayer graphene (2630 m
2
/g) and
aributable to the presence of a 3Dnetwork contain-
ing pores with sizes of 1∼10 nm. In another study,
the same group activated RGO lms to produce
graphene lms of a specic capacitance of 120 F/g
at high current density of 10 A/g with correspond-
ing energy density and power density of 26 Wh/kg
and 500 kW/kg, respectively [
51]. Later, they fur-
ther improved the specic surface area up to 3290
m
2
/g by designing a mesoporous structure inte-
grated with macroporous scaolds [
52]. As a result,
specic capacitance of 174 F/g (100 F/cm
3
)was
achieved with energy density and power density of
74 Wh/kg and 338 kW/kg, respectively.
Doping graphene with hetero atoms can also im-
prove its electrical/electrochemical properties for
energy storage and many other applications [
53].
Indeed, Jeong et al. synthesized N-doped graphene
through a simple plasma process [
54], and the N-
doped graphene thus produced was found to exhibit
a specic capacitance of 280 F/g, which is four times
higher than that of the corresponding undoped pris-
tine graphene. is is because N-doping can in-
troduce charge-transferring sites through doping-
induced charge modulation and improve electrical
conductivity of graphene, and hence the improved
specic capacitance, along with an enhanced power
density of 8 × 10
5
W/kg and energy density of 48
Wh/kg. N-doped graphene can also be synthesized
through hydrothermal reduction of GO with nitro-
gen containing chemicals [55]. e resultant 3D
N-doped graphene framework has a very low den-
sity of 2.1 mg/cm
3
with a high specic capacitance
of 484 F/g in 1 M LiClO
4
electrolyte and main-
tains 415 F/g capacitance aer 1000 cycles at a high
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REVIEW Chen et al. 457
current density of 100 A/g [55]. Similarly, doping
graphene with other elements, such as boron, phos-
phorous and/or co-doping with N and P or B and N,
has also been demonstrated to signicantly improve
their energy-storage performance [
56,57].
Graphene-based self-assembled 3D structures
(such as hydrogels and aerogels) have recently
emerged as electrode materials for supercapacitors
due to their high porosity, low density and excel-
lent adsorption capacity [
58–60]. e detailed syn-
thetic processes and properties of graphene hydro-
gels and aerogels have been reviewed in references
[
58–60]. Briey, in an aqueous solution of GO, the
van der Waals aractions from the basal planes of
GO sheets and the electrostatic repulsions from the
functional groups of GO sheets are balanced each
other to maintain the well-dispersed state of GO
sheets. While this balance is lost, gelation of the
GO dispersion takes place, leading to the forma-
tion of 3D GO hydrogels that can be further re-
duced or functionalized to produce 3D graphene-
based architectures [
61–63]. Dierent techniques
have been used to produce graphene hydrogels, such
as hydrothermal reduction [
61], chemical reduction
[
63], cross-linking agent (including metal ions [64],
biomolecules [
65], polymers [66] etc.), sol-gel reac-
tion [
67], freeze-drying [68] and so on. Similarly to
hydrogels, graphene aerogels are made through re-
placing the solution part with a gas [
69–70]. ere
have been numerous studies on graphene aerogels
for supercapacitor applications. Such successful ef-
forts have been summarized in Table
1. For assem-
blies of graphene hydrogels, aerogels o r organogels
[
71], their overall conductivities are generally poor.
As the graphene-based nanostructured carbon
materials oen oer low density, in most cases,
the volumetric energy densities of carbon-based su-
percapacitors are low, which hinders their practical
application. Yang et al. and Yoon et al.havedemon-
strated graphene-based highly packed supercapaci-
tors with volumetric energy density of 59.9 Wh/L
and specic capacitance of 171 F/cm
3
, respectively
[
72,73]. However, much more eort must be made
in improving the volumetric energy density.
Mesoporous carbon in EDLCs
Activated carbon has been widely used as electrodes
in energy-storage devices because of their easy syn-
thesis, low cost and acceptable electrical conductiv-
ity. However, these advantages are hindered by its
low eective specic surface area due tothe presence
of randomly connected micropores with size less
than 2 nm that are hardly accessible by electrolyte
ions [
2]. To address this issue, mesoporous carbon
of a larger pore diameter (2–50 nm) was explored as
a supercapacitor electrode with a high specic sur-
face area, fast ion-transport pathway and high power
density. As an example, mesoporous carbon synthe-
sized through carbonization of poly(vinyl alcohol)
and inorganic salt mixture exhibited a specic ca-
pacitance of 180 F/g in aqueous H
2
SO
4
electrolyte
[74]. However, the volumetric specic capacitance,
energy density and power density of mesoporous
carbon electrodes couldbe inuenceddirectly by the
mesoporous size and content. A balanced popula-
tion of mesopores and micropores is desirable for ef-
cient electrochemical energy storage [
75,76].
As discussed above, the size and shape of the
pores in mesoporous carbon can be well controlled
through various synthetic techniques [
77]. When
mesoporous carbon is produced as an ordered meso-
porous carbon (OMC) with homogeneously dis-
tributed pores of regular size, it can facilitate charge
storage and transport, and hence both the capaci-
tance and rate capability can be improved. Highly
OMCs with pore sizes of 2.8 nm (C-1) and 8 nm (C-
2) have been synthesized using SBA-16 silica with
mesostructured templates and polyfurfuryl alcohol
asthe carbonsource[
77].e resultantOMCs,both
C-1and C-2 with a specic surface area of 1880 and
1510 m
2
/g, respectively, were tested as supercapaci-
tor electrodes in dierent electrolytes. It was evident
that the highest specic capacitance reached up to
205 F/g by the C-1 with a pore diameter of 2.8 nm
whereas the C-2 with a pore diameter of 8 nm exhib-
ited beer stability while increasing the rate.
For mesoporous carbons, activation can also be
performed to introduce micropores. For instance,
Xia et al. activated mesoporous carbon with CO
2
at
950
◦
C, which introduced micropores into the meso-
porous carbon to improve the specic capacitance
up to 223 F/g from 115 F/gin 6 M KOH [
78]. e
observed enhancement in specic capacitance can
be aributable to the formation of hierarchical pores
with a high specicsurface area (2749 m
2
/g) and the
well-balanced populations of micropores and meso-
pores. Recently, production of mesoporous carbon
through carbonization of non-conventional mate-
rials, like biomass, are becoming more and more
popular. For example, N-doped mesoporous car-
bon has been prepared by a one-step method of py-
rolysing the mixture of milk powder and potassium
hydroxide without using any template. e N-doped
mesoporous carbon (NMPC) showed a specic sur-
face area of 2145.5 m
2
/g and a pore volume of
1.25 cm
3
/g. As a supercapacitor electrode material,
the NMPC, with 2.5% N dopant, exhibited a specic
capacitance of 396.5 F/g at 0.2 A/g in 6 M H
2
SO
4
and stable capacitance retention of 95.9% aer 2000
cycles at 50 mV/s [79]. Furthermore, the shape and
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