![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 films of the (c) graphene foam and (d) holey graphene foam. (e, f) A closed-up view of (e) graphene and (f) holey graphene films. The arrows highlighted the ion transport pathway. Reproduced with permission from ref. [49]. Copyright of Macmillan Publishers Ltd (2014).](https://www.researchgate.net/profile/R_Paul/publication/316859480/figure/fig11/AS:667088582619146@1536057720559/Schematic-illustration-of-the-graphene-and-holey-graphene-foams-a-b-Initial-3D_Q320.jpg)
![Schematic illustration and SEM image of PANI/SWNT composite for pseudocapacitor. Reproduced with permission from ref. [99]. Copyright of American Chemical Society (2010).](https://www.researchgate.net/profile/R_Paul/publication/316859480/figure/fig1/AS:492847397904384@1494515384166/Schematic-illustration-and-SEM-image-of-PANI-SWNT-composite-for-pseudocapacitor_Q320.jpg)
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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, including electrochemical 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 operating voltage 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/mechanicalprop-
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
sp3hybridized carbon molecules. Having a layered
structure with strong covalent bonding between sp2
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 C60 has a soccer-ball-
like structure containing 20 carbon hexagons with
12 carbon pentagons formed into a cage of trun-
cated icosahedrons. Fullerene C60 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 energy storage 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 mag-
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 m2/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 m2/g to 109.4 m2/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 m2/g
for highly pure SWNTs [38]. Using organic elec-
trolyte (1 M Et4NBF4/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 m2/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 achieved at 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 closed-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 m2/g [29], which is much
lower than the theoretic value of 2630 m2/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-
ciccapacitanceupto150 F/g in 30% 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/cm3, 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 on the elec-
trical conductivity. Of particular interest, Ruo and
coworkers obtained a dramatically improved spe-
cic surface area up to 3100 m2/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 m2/g) and
aributable to the presence of a 3D network 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
m2/g by designing a mesoporous structure inte-
grated with macroporous scaolds [52]. As a result,
specic capacitance of 174 F/g (100 F/cm3)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 ×105W/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/cm3with a high specic capacitance
of 484 F/g in 1 M LiClO4electrolyte 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 or 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/cm3, 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 to the 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 H2SO4electrolyte
[74]. However, the volumetric specic capacitance,
energy density and power density of mesoporous
carbon electrodes could be inuenced directly 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
asthecarbonsource [77]. e resultant OMCs, both
C-1and C-2 with a specic surface area of 1880 and
1510 m2/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 CO2at
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 specic surface area (2749 m2/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 m2/g and a pore volume of
1.25 cm3/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 H2SO4
and stable capacitance retention of 95.9% aer 2000
cycles at 50 mV/s [79]. Furthermore, the shape and
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458 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Table 1. Carbon nanomaterials in electrical double-layer capacitors (EDLCs).
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
(i) Carbon nanotube (CNT)
CNT (functionalized by
–OH and –COOH)
0.075 M Hydroquinone
(HQ) into 1 M H2SO4
aqueous
3199 F/g at
5mV/s
– – 70% aer
350 cycles
32
PEDOT/MWCNT 1 M LiClO479 F/g 5000 W/kg 11.3
Wh/kg
85% aer
1000 cycles
33
PTFE/PANI-CNT 30% KOH in H2O 163 F/g at
0.1 A/g
–––34
C tubes 0.5 mol/l H2SO4315 F/g at
0.35 V
–––37
SWNT 1 M Et4NBF4/ propylene
carbonate
160 F/g at
4V
24 kW/kg 17 Wh/kg 96.4 aer
1000 cycles
38
MWCNT 6mol/lKOH 80–135F/g–––39
N-doped (8.64 wt. %) carbon
shellandaCNT-core
6 mol/l KOH 205 F/g – – 97.1% aer
1000 cycles
40
Spherical particles of N-doped
CNT
1MH
2SO4215 F/g at
0.2 A/g
– – 99% aer
1500 cycles
41
(ii) Graphene
rGO 1-butyl-3-
methylimidazolium
hexauorophosphate
(BMIPF6)
348F/g at
0.2 A/g
current
– – 120% aer
3000 cycles
at 10 mV/s
43
Graphene hydrogel 5 M KOH 220 F/g at
1A/g
current
30 kW/kg 5.7 Wh/kg 92% aer
2000 cycles
44
rGO by Na2CO36 M KOH 228 F/g at
5 mA/cm2
–––45
Graphene sheets 30 wt. % KOH in H2O 150 F/g at
0.1 A/g
current
– – 100% up to
500 cycles
46
Exfoliated rGO 30 wt. % KOH in H2O 264 F/g at
0.1 A/g
current
– – 97% aer
100 cycles
47
MW assisted rGO 5 M KOH 191 F/g at
0.15 A/g
current
––9%
reduction
at 0.6 A/g
48
Hollow graphene 1-ethyl-3-
methylimidazolium
tetrauorobo-
rate/acetonitrile
(EMIMBF4/AN)
298 F/g at
1A/g
current
1000 35 Wh/kg 49
Exfoliated rGO 1-ethyl-3-
methylimidazolium
tetrauorobo-
rate/acetonitrile
(EMIMBF4/AN)
166 F/g at
5.7 A/g
current and
3.5 V
250 kW/kg 70 Wh/kg 97% aer
10 000
cycles
50
rGO (KOH activated) Tetraethylammonium
tetrauoroborate
(TEABF4) in acetonitrile
120 F/g 500 kW/kg 26 Wh/kg 95% aer
2000 cycles
51
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REVIEW Chen et al.459
Table 1. Continued.
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
Graphene made porous carbon 1-ethyl-3-
methylimidazolium
bis(triuoromethylsulfonyl)
imide, [EMIM][TFSI] and
Acetronitrile in 1:1 ratio
174 F/g at
4.1 A/g
current
338 kW/kg 74 Wh/kg 94% aer
1000 cycles
52
N-doped graphene 1 M
tetraethylammoniumtetra-
uoroborate (TEA
BF4)
280 F/g at
20 A/g
current in 6
MKOH
800 kW/kg 48 Wh/kg 99.8% aer
230 000
cycles
53
N-doped (5.86 at.%) graphene
hydrogel
6 M KOH 308 F/g at
3A/g
current
– – 92% aer
1200 cycles
54
3D N-doped graphene 1 M LiClO4484 F/g at
1A/gand
415 at 100
A/g current
– – 100% aer
1000 cycles
at 100 A/g
current
55
B-doped rGO 6 M KOH solution 200 F/g at
0.1 A/g
current
∼10
kW/kg
∼5.5
Wh/kg
95% aer
4500 cycles
56
N/P doped rGO 6 M KOH solution 165 F/g at
0.1 A/g
current
80% aer
2000 cycles
at 0.5 A/g
57
Graphene hydrogel
(hydrothermal)
– 175 F/g at
10 mV/s
–––61
4.38% (at.) N-graphene hydrogel
(hydrogel)
5 M KOH 131 F/g at
80 A/g
current
144 kW/kg 4.5 Wh/kg 95.2% aer
4000 cycles
at 100 A/g
62
Graphene aerogel (3D printed) 3 M KOH 4.76 F/g at
0.4 A/g
current
4.08 kW/kg 0.26
Wh/kg
∼95.5%
aer 10 000
cycles at
200 mV/s
69
Graphene aerogel 0.5 M H2SO4325 F/g at
1A/g
7 kW/kg 45 Wh/kg ∼98% aer
5000 cycles
70
2D microporous covalent
triazine-based framework
Solvent-free ionic liquid
EMIMBF4
151.3 F/g
at 0.1 A/g
current
10 kW/kg ∼42
Wh/kg
85% aer
10 000
cycles at 10
A/g
71
(iii) Mesoporous carbon
Porous carbon (Surface area
(SBET)=1300 m2/g, pore
diameter (Dp)=5–15 nm)
2MH
2SO41M
(C2H5)4NBF4 in
acetonitrile
0.14 F/m2
0.075
F/m2(at 1
mA/cm2)
2600 W/kg
4000 W/kg
1Wh/kg3
Wh/kg
85% at 100
mA/cm2
74
Porous carbon (SBET =1600
m2/g, Dp=0.6–1.1 nm)
1.5 M (C2H5)4NBF4 in
acetonitrile
100–140
F/g At
1 mA/cm2
– – 100% up to
100
mA/cm2
75
Porous Carbon (SBET =1880
m2/g, Dp=2.8 nm)
6 M KOH 205 F/g at
1mV/s
– – 86.8% aer
3000 cycles
at 500
mA/g
77
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460 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Table 1. Continued.
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
Porous carbon (SBET =2749
m2/g, Dp=3–4 nm)
6 M KOH 223 F/g at
2mV/s
– – 73% at 50
mV/s
78
N-doped (2.5 at.%) mesoporous
carbon (SBET =2145.5 m2/g,
Dp=0.8–2.5 nm)
6MH
2SO4396.5 F/g
at 0.2 A/g
current
– – 95.9% aer
2000 cycles
at 50 mV/s
79
(iv) Hybrid carbon nanomaterials
Graphene/carbon black 6 M KOH 175 F/g at
10 mV/s
– – 98.9% aer
6000 cycles
(200
mV/s)
81
Porous carbon/rGO (SBET =
1496 m2/g, Dp=1.4–3.5 nm)
6 M KOH 171 F/g at
10 mV/s
scan
4.2 kW/kg 3.3 Wh/kg 74% at 0.1
A/g
82
rGO/CNT 0.1 M sodium phosphate
buered saline (PBS)
140 F/g at
0.1 A/g
current
– – 64.3% at
100 A/g
83
Graphene/CNT 1 M H2SO4124 F/g at
0.1 V/s
– – 95% at 1
V/s
85
Graphene/CNT ber Poly(vinyl alcohol)
(PVA)/H3PO4(1:1, mass)
31.5 F/g at
0.04 A/g
– – 100% aer
5000 cycles
86
SWNT/N-doped rGO ber PVA/H3PO4, 1MH
2SO4300 F/g at
26.7
mA/cm3,
305 F/g at
73.5
mA/cm3
1085
mW/cm3
6.3
Wh/cm3
93% aer
10 000
Cycles at
250
mA/cm3
87
3D N-doped graphene/CNT 6 M KOH 180 F/g at
0.5 A/g
current
– – 96% aer
3000 cycles
88
Freestanding graphene
hydrogel/carbon ber composite
1MNa
2SO4150.2 F/g
at 1 A/g
current
– – 97.9% aer
2000 cycles
89
structure of pores of mesoporous carbon have also
been explored to improve the electrochemical per-
formance [80].
Hybrid carbon nanomaterials in EDLCs
Carbon nanomaterials with distinct structures can
be combined to exhibit synergetic eects for electro-
chemical performance. For example, carbon black
has been used to separate graphene sheets to pro-
duce 3D hybrid materials with minimized aggrega-
tion of graphene, leading to a high specic capaci-
tance of 175 F/g at 10 mV/s scan rate in 6 M KOH
electrolyte [81]. In other work, mesoporous carbon
spheres were sandwiched between graphene sheets
and the resulting 3D structure exhibited a specic
capacitance of 171 F/g at the same 10 mV/s scan
rate in 6 M KOH [82]. More interestingly, CNTs
were intercalated between graphene sheets to retain
the specic surface area of graphene by minimizing
its aggregation [83]. e π−πinteraction between
graphene and CNTs can also improve electrical con-
ductivity and mechanical strength [84]. Just like GO
to disperse CNTs in solvents [83], oxidized CNTs
have been used to form composites with graphene
[85]. In this context, Yu and Dai produced hybrid
lms of CNT and graphene interconnected network
with well-dened nanoporous [85], which exhibited
a specic capacitance of 120 F/g in 1 M H2SO4elec-
trolyte and an almost rectangular cyclic voltammo-
gram even at a 1-V/s scan rate. Sun et al. reported
other interesting work [86], in which graphene
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REVIEW Chen et al.461
sheets were intercalated between CNTs in aligned
CNT ber. Although the resultant hybrid ber ex-
hibited a specic capacitance of only 31.5 F/g, it was
much higher than that of the pure CNT ber (5.83
F/g). By using a modied hydrothermal micro-
reactor, Yu et al. produced a continued CNT and
graphene hybrid ber with well-dened mesoporous
structures [87], which showed a specic surface area
as high as 396 m2/g with an electrical conductiv-
ity of 102 S/cm. e corresponding ber-shaped su-
percapacitor showed a volumetric specic capaci-
tance of 305 F/cm3at 26.7 mA/cm3current density
and a volumetric energy density of 6.3 mWh/cm3,
which is comparable to the energy density of a
4 V–0.5 mAh thin-lm lithium-ion baery. Further-
more, a 3D N-doped CNT/graphene network was
also synthesized through hydrothermal treatment
and freeze-drying, followed by carbonizing GO and
the pristine CNT mixture in the presence of pyr-
role [88]. e resultant hybrid carbon ber showed
high electrochemical performance, especially capac-
itance retention of 96% aer 3000 cycles [88]. In
case of self-assembled carbon-composite material
[89–94], freestanding 3D graphene hydrogel and
carbon nanober composite material demonstrated
150.2 F/g specic capacitance at 1-A/g current
with 97.8% capacitance retention aer 2000 cycles
[89]. Carbon nanober and nanotube network was
also synthesized from conjugated polymer for elec-
trochemical energy storage [90]. Table 1summa-
rizes carbon-based electrical double-layer superca-
pacitors (EDLCs).
Pseudocapacitors (PCs)
Pseudocapacitors store energy through reversible
Faradaic charge transfer, which involves fast and re-
versible electrochemical redox reactions on the in-
terface between the electrodes and electrolyte. As
such, the specic capacitance of a pseudocapacitor
is oen higher than that of an EDLC, as is the energy
density. As the redox reactions occur on the elec-
trode surface, a high specic surface area and high
electrical conductivity are essential for electrodes in
a high-performance PC. erefore, carbon nanoma-
terials, including CNTs, graphene, mesoporous car-
bon and their hybrids, have also been used as the
substrate to load active materials and/or current col-
lector to ensure high capacitance and fast charge
transfer for electrodes in high-performance PCs.
CNTs in pseudocapacitors
CNTs have been used in pseudocapacitors in ei-
ther a functionalized form or composited with
other active components, such as conductive poly-
mers and metal oxides. CNTs can be function-
alized through chemical or electrochemical meth-
ods. e most common way to functionalize CNTs
is acid oxidation (e.g. a mixture of concentrated
sulfuric acid and nitric acid) to introduce surface
carboxyl groups [95]. rough acid oxidation, the
specic capacitance of CNTs can be increased by
3.2 times due to the increased hydrophilicity of the
electrodes in aqueous electrolytes and the introduc-
tion of pseuducapacitance. Treatment of CNTs with
NaOH solution at 80◦C, followed by ultrasonica-
tion in H2SO4/HNO3solution, can also improve
the specic capacitance from 28 F/g for the pris-
tine CNTs to 85 F/g for the functionalized CNTs
[32]. However, the oxidation treatments inevitably
induced defects to degrade the CNT structure and
reduce the electrical conductivity. erefore, a del-
icate balance between the electrode performance
and its structure integrity is important for high-
performance pseudocapacitors (i.e. PCs).
Conducting polymers possessing good
electrical conductivities and redox activities of-
ten exhibit high specic capacitances when they are
composited with CNTs. In this regard, Bai et al.
increased the energy density of a CNT-based PC
by four times, up to 11.3 Wh/kg, by compositing
poly (3,4-ethylenedioxythiophene) (PEDOT)
homogenously onto the CNT electrode through
in-situ polymerization [96]. Similarly, polypyrrole
(PPy)/CNT composite electrodes have been
also synthesized to yield a specic capacitance of
165 F/g in 1 M KCl solution [97]. Compared
with PEDOT and PPy, PANI possesses a higher
theoretical specic capacitance [98], which was
conrmed by a high specic capacitance of 501.8
F/g reported for exible PANI/SWNT composite
lms synthesized through in-situ electrochem-
ical polymerization (Fig. 3)[99]. Subsequent
electrodegradation further increased the specic
capacitance to 706.7 F/g by forming charge transfer
channels via selective dissolution of polycrystalline
and o-lying disordered PANIs. Because PANI
changes its color during the charge–discharge
process, PANI/CNT composites have been used
for high-performance (308.4 F/g in PVA/H3PO4)
smart supercapacitors with highly reversible chro-
matic transitions during charge–discharge processes
for monitoring the energy-storage status by the
PANI color changes [100].
Metal oxides and hydroxides are two other
important classes of active electrode materials
for pseudocapacitors. Compared to conductive
polymers, metal oxides and hydroxides oen exhibit
a beer electrochemical stability but lower electrical
conductivity. For metal oxides and hydroxides to
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462 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
o
o
o
o
OH
NH
NH
NH
N
+
H
n
OH
NH
N
+
NH
H
N
+
H
n
NH
NH
N
+
H
NH
+
n
N+
H
N+
H
NH
NH
n
Emeraldine salt (ES)
+Electrochemical
polymerization
‘O
‘O
‘O
O
O
O
O
700
600
500
400
300
200
0 10 20 30 40
Electro-degradation cycles
Specific capacitances (F/g)
De5
SWNT/PANi90
De10
De20 De40
SWNT/PANi composite
Surface resistance (ohm/sq)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
O
O
O
O
O
O
OO
O
Figure 3. Schematic illustration and SEM image of PANI/SWNT composite for pseudocapacitor. Reproduced with permission
from ref. [99]. Copyright of American Chemical Society (2010).
be used in high-performance pseudocapacitors, the
corresponding metals must possess two or more
oxidation states, which can coexist and inter-transfer
freely. Examples include RuO2,MnO
2,NiO,V
2O5,
Fe2O3,Co
3O4,TiO
2,SnO
2,Mn
3O4and Ni(OH)2
[101–108]. In the metal oxide and CNT composite
electrodes, CNTs can not only provide the high
electrical conductivity and large specic surface
area for eciently loading the active materials, but
also eectively restrict the volumetric change of
metal oxides or hydroxides caused by the cyclic
charge–discharge processes [106]. As RuO2has
three oxidation states and a wide operation potential
window, RuO2/CNTs have become a typical com-
posite electrode material for pseudocapacitors. e
specic capacitance of RuO2with a large surface
area can reach up to 1170 F/g in 0.5 M H2SO4
electrolyte [108]. Reddy and Ramaprabhu have
synthesized RuO2/CNT, TiO2/CNT, SnO2/CNT
composites by chemical reduction of corresponding
salts to functionalize CNTs and demonstrated the
highest capacitance of 160 F/g for the TiO2/CNT
[107]. On the other hand, MnO2possesses a high
theoretical specic capacitance of 1370 F/g [109]
and has been electrochemically deposited onto
chemical vapor deposition (CVD)-grown
CNT arrays to exhibit a specic capacitance of
642 F/g in 0.2 M Na2SO4electrolyte [110].
Very high charge/discharge stability of up to
10 000 cycles can be obtained using MnO2/CNT
composite electrodes [111]. Furthermore, ex-
ible and wearable supercapacitors based on
MnO2/CNT composite bers prepared by elec-
trochemical deposition of MnO2on aligned
CNT bers were demonstrated to show a specic
capacitance of 3.707 mF/cm2[112].
Graphene in pseudocapacitors
Just like metal oxide and CNT electrodes, graphene
with a high specic surface area and high elec-
trical conductivity has also been composited with
other active materials, including conductive poly-
mers, metal oxides and hydroxides, as electrodes for
pseudocapacitors [113–119]. Indeed, PANI/GO
composites have been synthesized through in-situ
polymerization of aniline into PANI on GO [113].
Depending on the mass ratio of PANI to GO, per-
formance of the PANI/GO electrode varied and the
highest specic capacitance of 746 F/g has been
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REVIEW Chen et al.463
Adsorption
Without GNS
APS
GNS
Polymerization
Polymerization
APS
PANI coated GNS
PANI nanobars
Aniline+HCI phase
Aniline
Aniline anion
GO sheet
(a)
(b)
Nucleation
Growth
Growth further
Figure 4. Schematic illustration to the heterogeneous nucleation on graphene oxide
nanosheets and in bulk solution for the growth of PANI nanowires. Reproduced with
permission from Ref. [114]. Copyright American Chemical Society (2010). (b) Schematic
illustration for the synthesis of GNS/PANI composite. Reproduced with permission from
Ref. [115]. Copyright of Elsevier (2010).
achieved at a PANI/GO mass ratio of 200/1. Like a
pure PANI electrode, however, the PANI/GO com-
posite electrode showed very poor electrochemical
cyclic stability, with a retention rate of only 20% af-
ter 500 cycles. Increased electrochemical cyclic sta-
bility could be achieved by increasing the portion
of GO in the PANI/GO composite and a 73% re-
tention rate was obtained at the PANI/GO ratio of
23/1. By growing vertically aligned PANI nanowires
on GO substrate (Fig. 4a) [114], the morphology
of PANI and the mass ratio of PANI to GO can
be well controlled to produce a specic capacitance
as high as 555 F/g in 1 M H2SO4at 0.2 A/g with
a 92% retention rate aer 2000 cycles at 1 A/g.
Yan et al. also synthesized a PANI/graphene com-
posite with PANI nanoparticles (2 nm) uniformly
decorated on the graphene akes (Fig. 4b) [115].
In this work, graphene not only acted as a support
material to oer active sites for the nucleation of
PANI, but also provided a beer electron-transfer
path, leading to a specic capacitance of 1046 F/g
in 6 M KOH at a 1-mV/s scan rate, energy density
of 39 Wh/kg and power density of 70 kW/kg. To
further improve the performance, graphene sheets
with more active nucleation sites were prepared
by unzipping CNTs to produce graphene nanorib-
bons with a specic capacitance of 340 F/g and a
retention rate of 90% aer 4200 charge–discharge
cycles [116]. More interestingly, an even higher spe-
cic capacitance of 989 F/g in 6 M KOH elec-
trolyte was obtained when cobalt was introduced
into PANI/graphene composites through polymer-
izing aniline in the presence of both cobalt and
graphene [117].
3D PANI/graphene composite hydrogel was
also synthesized as a freestanding lm to prepare
exible supercapacitors without binders [118,119],
which exhibited excellent energy-storage properties
as well as outstanding exibilities for portable
electronic devices. More recently, a simple and
cost-eective method was reported to produce
graphene/polystyrene sulfonic acid-gra-aniline
(Gr/S-g-A) nanocomposite through direct ex-
foliation of graphite using S-g-A as a surfactant
[120]. e Gr/S-g-A composite electrode was
demonstrated to exhibit a superior specic ca-
pacitance of 767 F/g at 0.5 A/g current density
in 0.1 M Bu4NPF6/acetonitrile electrolyte with
92% capacitance retention aer 5000 cycles. e
corresponding supercapacitor showed an energy
density of 208.8 Wh/kg and a power density of
347.8 W/kg [120].
In addition to PANI, polypyrrole (PPy) has also
been composited with graphene simply by directly
mixing PPy with RGO [121]. e PPy/graphene
composite thus produced showed a specic capac-
itance of 400 F/g in 2 M H2SO4atacurrent
density of 0.3 A/g. PPy/GO composite was also
synthesized through in-situ chemically polymeriz-
ing pyrrole monomers in the presence of FeCl3to
improve uniformity of the composite, and hence
a slightly improved specic capacitance of 421.42
F/g in 0.1 M KCl electrolyte [122]. By substitut-
ing FeCl3with ammonium persulfate dissolved in
citric acid, the specic capacitance was further im-
proved to 728 F/g at 0.5 A/g current density with
a retention rate of 93% aer 1000 cycles [123].
Similarly to PANI, PPy can also be assembled with
GO into layer-by-layer composites through the elec-
trostatic interaction [124]. However, the electro-
chemical performance of PPy/GO composite is not
promising due to the poor electrical property of GO.
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464 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
rough electrochemical polymerization of pyrrole
monomer and simultaneous electrostatic deposi-
tion, Zhao et al. also synthesized 3D PPy/graphene
composites [125]. e resulting 3D PPy/graphene
composite foam exhibited a specic capacitance of
350 F/g at a current density of 1.5 A/g with no
change in the initial capacitance even aer 1000 cy-
cles. In another aempt, PPy/graphene composite
lms were prepared through electrochemical poly-
merization of PPy on graphene pre-deposited elec-
trophoretically onto a Ti substrate [126], leading to
a specic capacitance of 1510 F/g (or 151 mF/cm2,
or 151 F/cm3)in0.1MLiClO
4electrolyte at the 10-
mV/s scan rate. is ultrahigh specic capacitance
was aributed to the porous structure, eective uti-
lization of the pores and the large specic surface
area for rapid redox reactions during the charge–
discharge process in the pseudocapacitor.
As is the case for CNTs, metal oxides or hy-
droxides can also be composited with graphene-
based nanomaterials to improve the pseudocapaci-
tance. In this context, MnO2/GO composite with
MnO2nanoneedles deposited on the GO was syn-
thesized through the chemical reaction of KMnO4
and MnCl2in water/isopropyl alcohol mixture in
the presence of GO [127]. e as-synthesized
MnO2/GO composite exhibited a specic capac-
itance of 216 F/g and a retention of 84.1% af-
ter 1000 cycles. By perpendicularly graing poly-
mer brushes, poly(sodium methacrylic acid), onto
the GO sheets prior to loading of MnO2nanopar-
ticles, MnO2/GO composite with uniformly dis-
tributed MnO2on the GO substrate was prepared,
which showed an improved specic capacitance of
372 F/g in 1 M Li2SO4at 0.5A/g current den-
sity and an increased retention rate of 92% aer
4000 cycles [128]. Alternatively, MnO2/GO com-
posite with porous structure has also been syn-
thesized with the aid of microwave irradiation
[129,130] to exhibit a specic capacitance of 310
F/g at a 2-mV/s scan rate and 228 F/g at a high
scan rate of 500 mV/s. Besides, Mn3O4/graphene
composite was also prepared to show a specic ca-
pacitance of 271.5 F/g at 0.1 A/g and 180 F/g at
even a high current density of 10 A/g with no ob-
vious decay of capacitance even aer 20 000 cy-
cles [131]. A ltration technique was also developed
for preparing freestanding Mn3O4/graphene com-
posite lm as the electrode [132]. With regard to
carbon-supported metal oxide PCs, Vanadium phos-
phates (VOPO4)/graphene composited with verti-
cally aligned porous 3D structure obtained through
an ice-templated self-assembly process showed a
527.9-F/g gravimetric capacitance at 0.5-A/g cur-
rent density with 85% capacitance retention af-
ter 5000 cycles at a 100-mV/s scan rate in 6 M
KOH electrolyte—promising supercapacitive per-
formance [133].
RuO2, a highly active metal oxide for redox re-
actions, has also been composited with graphene
as electrodes in pseudocapacitors. RuO2/graphene
composite was synthesized most commonly through
a sol-gel technique, followed by annealing, to exhibit
a specic capacitance of 570 F/g in 1 M H2SO4and a
retention rate of 97.9% aer 1000 cycles [134]. is
method could eectively reduce the aggregation of
both RuO2nanoparticles and graphene sheets by
separating graphene sheets with the graed RuO2
nanoparticles. An energy density up to 20.1 Wh/kg
at 0.1-A/g current density was achieved, which re-
mained at 4.3 Wh/kg when the power density ap-
proached 10 kW/kg.
Cobalt oxide is another kind of metal oxide that
can demonstrate rapid and reversible redox reac-
tions during the charge–discharge process in pseu-
docapacitors. For example, Co3O4/graphene com-
posite was synthesized via a chemical reaction of
cobalt nitrate with urea in GO suspension under mi-
crowave irradiation to exhibit a specic capacitance
of 243.2 F/g in 6 M KOH [135]. Co3O4/graphene
composite can also be synthesized by a hydrother-
mal method, followed by calcination to improve the
electrochemical performance [136]. Morphology of
the resultant Co3O4/graphene composite depends
strongly on the ratio of Co3O4to graphene. At a
7% mass ratio of graphene, Co3O4nanoplates ho-
mogeneously grew on the graphene sheets, lead-
ing to improved specic capacitances of 667.9 and
385.1 F/g at the current densities of 1.25 and 12.5
A/g, respectively. Besides,Co3O4and Co(OH)2
could also be composited with graphene to prepare
pseudocapacitor electrodes. In particular, a sheet-
on-sheet-structured Co(OH)2/graphene compos-
ite was synthesized through a one-step in-situ hy-
drothermal method to exhibit a specic capaci-
tance of 540 F/g at a high current density of
10 A/g [137]. Increased specic capacitances with
charge–discharge cycles were observed for both the
Co3O4/graphene and Co(OH)2/graphene com-
posites, aributable to a gradual activation of the ac-
tive materials wrapped up by graphene during the
initial charge–discharge cycling [136].
Apart from Mn, Ru and Co oxides, oxides of
other metals with multiple valences, such as Ni,
Fe, Ti and Zn, have also been under research fo-
cus as alternative electrode materials that can be
composited with graphene to further improve pseu-
docapacitive performance [138–142]. For instance,
hexagonal nanoplates of single-crystal Ni(OH)2
have been grown on lightly oxidized conductive
graphene sheets using an in-situ synthesis tech-
nique [138]. e resultant composite exhibited an
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REVIEW Chen et al.465
ultrahigh specic capacitance of 1335 F/g in 1 M
KOH at a current density of 2.8 A/g with a remark-
ably high-rate capability of 953 F/g at 45.7 A/g and
an excellent cycling stability (94.3% specic capaci-
tance retained aer 3000 cycles).
Self-assembled graphene-based structures, such
as hydrogels and aerogels, were also extensively
utilized for pseudocapacitor electrode applications
[143]. In particular, Zhang et al. recently developed
a plasma treatment approach to fabricate 3D
N-doped graphene aerogel/Fe3O4nanostructures,
which showed a specic capacitance of 386 F/g
in 6 M KOH electrolyte with a 97% retention
rate aer 1000 cycles [144]. In-situ growth of
active materials on a graphene substrate can achieve
high energy-storage properties because of the
intimate interactions and ecient charge transfer
between active nanomaterials and the conductive
graphene substrate. Among other self-assembled
graphene-based 3D structures, graphene hydrogels
modied with dierent oxygen-containing groups
using hydroquinones [145], MnO2/graphene
hydrogel composites [146,147], graphene hydro-
gels modied with 2-aminoanthraquinone [148],
RuO2/reduced graphene-oxide hydrogels [149],
freestanding polyaniline/reduced graphene-oxide
composite hydrogels [150] and single-crystalline
Fe2O3nanoparticles directly grown on graphene
hydrogels [151] have been studied. Details of their
supercapacitive performance have been summarized
in Table 2. Self-assembled porous graphene net-
works synthesized from various organic chemicals
have also been used for supercapacitor applications.
ese structures are generally called organogels
[152–155]. However, more studies are needed
for beer understanding the mechanism of charge
storage in such organogels for improving their
performance as ecient supercapacitor electrodes.
Similarly to EDLCs, it is highly desirable to
improve the volumetric capacitance of graphene-
based PCs for practical applications. In this context,
Xu et al. recently reported a PC fabricated using
a PANI and graphene composite monolith, which
demonstrated 802 F/cm3volumetric capacitance at
54% PANI loading and 66% of the capacitance was
maintained when the current density increased by
100 times [156].
Mesoporous carbon in pseudocapacitors
Mesoporous carbon with functional groups can act
as ecient pseudocapacitor electrodes. e use of
strong activation reagents, such as sulfuric acid,
nitric acid and ammonium persulfate, to activate
mesoporous carbons can not only introduce micro-
pores, but also introduce various functional groups
with additional pseudocapacitance. For example,
OMC activated by nitric acid possesses small-sized
mesopores as well as functional groups, includ-
ing −OH, −COOH and/or −C=O, and showed
improvement in specic capacitance from 117 to
295 F/g at a 10-mV/s scan rate in aqueous al-
kali electrolyte [157]. By pyrolysing the iron fu-
marate metal organic frameworks, Wang et al.re-
cently adopted a simple and scalable technique
to synthesize porous carbon nanorods (Fe3O4-
DCN) supported by 3D kenaf stem-derived macro-
porous carbon (KSPC) for high-performance su-
percapacitors [158]. e 3D-KSPC/Fe3O4-DCN
was employed as an ecient electrode in super-
capacitors to exhibit a high specic capacitance of
285.4 F/g in 2 M KOH at 1-A/g current density
with the capacitance remaining at 220.5 F/g even
aer 5000 cycles at 2 A/g. Mesoporous carbon has
also been composited with other active materials, in-
cluding conductive polymers and metal oxides, as
electrode materials in pseudocapacitors, as exem-
plied by PANI nanowires grown onto ordered bi-
modal mesoporous carbon via chemical polymeriza-
tion [159]. e resultant PANI/mesoporous carbon
composite exhibited a specic capacitance of 517
F/g in 1 M H2SO4and a retention rate of 91.5% aer
1000 cycles due to the combination of the outstand-
ing electrochemical properties of PANI and hierar-
chical porous structures of mesoporous carbon.
In the case of N-doped carbon (NC), previous
studies suggested that four dierent types of nitro-
gen (pyrrolic N, pyridinic N, quaternary N/graphitic
N and N oxides of pyridinic N) can be introduced
into NCs, depending on the heat-treatment temper-
ature and nitrogen sources [160,161]. Since con-
ventional synthetic methods cannot produce NC
with a single N component (e.g. pyrrolic N, pyri-
dinic N or quaternary N/graphitic N), it is very
dicult to determine the real role of dierent N-
functional groups. NCs are reported to demonstrate
not only double-layer capacitance, but also pseudo-
capacitance [51,162]. e mechanism of pseudoca-
pacitance in NCs has yet been conrmed, although
some preliminary studies have indicated that the
presence of nitrogen atoms on the edges of graphene
sheets (e.g. pyridinic N) played a crucial role [160].
Hybrid carbon nanomaterials
in pseudocapacitors
As mentioned earlier, dierent carbon nanoma-
terials can be combined together to exhibit a
synergetic eect. For pseudocapacitors, CNT and
graphene have also been composited with PANI
via in-situ polymerization [163]. e resultant
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466 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Table 2. Carbon nanomaterials in pseudocapacitors (PCs).
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
(i) Carbon nanotube (CNT)
Poly(3,4-
ethylenedioxythiophene) or
PEDOT/MWCNT
1 M LiClO479 F/g at 1
A/g current
5 kW/kg 11.3 Wh/kg 85% aer
1000 cycles
96
Polypyrrole-coated MWCNT 1 M KCl 165 F/g at
0.5
mA/cm2
current
– – 100% aer
1000 cycles
97
Polyaniline (PANI)/SWNT 0.5 M H2SO4706.7 F/g
at 5 mV/s
–––99
PANI/aligned CNT PVA/H3PO4(1:0.85,
mass)
308.4 F/g
at
– – 73.9% 100
RuO2/MWCNT,
TiO2/MWCNT,
SnO2/MWCNT
1MH
2SO4138 F/g,
160 F/g,
93 F/g at
2mV/s
500 W/kg 36.8, 40.2,
25 Wh/kg
– 107
RuO2/CNT 0.5 M H2SO41170 F/g at
10 mV/s
– – 82% at 400
mV/s
108
MnO2/CNT 0.2 M Na2SO4642 F/g at
10 mV/s
– – 100% up to
700 cycles
111
MnO2/CNT 1 M Na2SO4201 F/g at
1A/g
current
600 W/kg 13.3 Wh/kg 100% up to
10 000 cycles
at 1 A/g
112
(ii) Graphene
GO-doped polyaniline (PANI) 1 M H2SO4531 F/g at
0.2 A/g
current
–––113
PANI nanowire/GO 1 M H2SO4555 F/g at
0.2 A/g
current
– – 92% aer
2000 cycles
at 1 A/g
114
Graphene sheet/PANI 6 M KOH 1046 F/g at
1mV/s
70 kW/kg 39 Wh/kg – 115
PANI nanorods/graphene
nanoribbon
1MH
2SO4340 F/g 3.15 kW/kg,
9.47 kW/kg
7.56 Wh/kg,
4.01 Wh/kg
90% aer
4200 cycles
116
Co-PANI/graphene 6 M KOH 989 F/g at
2mV/s
1.581 kW/kg 352 Wh/kg 79% aer
1000 cycles
117
3D graphene/PANI hydrogel 6 M KOH 334 F/g at
3A/g
current
– – 57% aer
5000 cycles
118
PANI/graphene 1 M H2SO4375.2 F/g
at 0.5 A/g
current
1 kW/kg 30.34 Wh/kg 90.7% aer
500 cycles at
3A/g
119
Graphene/poly(styrenesulfonic
acid-gra-aniline)
0.1 M
Bu4NPF6/acetonitrile
767 F/g at
0.5 A/g
current
0.35 kW/kg 208.8 Wh/kg 92% aer
5000 cycles
120
Graphene/polypyrrole (PPy)
nananotubes
2MH
2SO4400 F/g at
0.3 A/g
current
– – 88% aer
200 cycles at
1.5 A/g
121
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REVIEW Chen et al.467
Table 2. Continued.
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
PPy/GO 0.1 M KCl 421.4 F/g
at 2 mA
current
–––122
Graphene/PPy nanowires 1 M KCl 728 F/g at
0.5 A/g
current
– – 93% aer
1000 cycles
123
Pillared GO/PPy 2 M H2SO4510 F/g at
0.3 A/g
current
– – 70% aer
1000 cycles
at 5A/g
124
PPy/3D graphene foam 3 M NaClO4350 F/g at
1.5 A/g
current
– – 100% aer
1000 cycles
125
PPy/Graphene 0.1 M LiClO41510 F/g at
10 mV/s
3 kW/kg 5.7 Wh/kg Stable up to
25 cycles
126
GO/MnO2needles 1 M Na2SO4216 F/g at
0.15 A/g
current
– – 84.1% aer
1000 cycles
at 0.2 A/g
127
Amorphous MnO2/GO 1 M Li2SO4372 F/g at
0.5 A/g
current
– – 92% aer
4000 cycles
at 0.5 A/g
128
MnO2/porous graphene 1 M H2SO4256 F/g at
0.25A/g
current
24.5 kW/kg 20.8 Wh/kg 87.7% aer
1000 cycles
129
MnO2/graphene 1 M Na2SO4310 F/g at
2mV/s
– – 88% at 100
mV/s
130
Mn3O4/graphene sheets 6 M KOH 271.5 F/g
at 0.1 A/g
current
– – 100% aer
20 000 cycles
131
Mn3O4/graphene paper Potassium polyacrylate
(PAAK)/KCl
321.5 F/g
at 0.5 A/g
current
–––132
3D VOPO4/graphene 6 M KOH 527.9 F/g
at 0.5 A/g
current
– – 85% aer
5000 cycles
at 100 mV/s
133
RuO2/Graphene sheets 1 M H2SO4570 F/g at
1mV/s
10 kW/kg 20.1 Wh/kg 97.9% aer
1000 cycles
at 1 A/g
134
Co3O4/graphene sheets 6 M KOH 243.2 F/g
at 10 mV/s
– – 95.6% aer
2000 cycles
at 200 mV/s
135
Co3O4nanoplates/graphene
sheets
2 M KOH 667.9 F/g
at 1.25 A/g
current
– – 81.3% aer
1000 cycles
136
Co(OH)2/graphene sheet layers 1 M KOH 622 F/g at
2A/g
current
15.8 kW/kg 86.6 Wh/kg 80% aer
10 000 cycles
at 10 A/g
139
Ni(OH)2/graphene 1 M KOH 1335 F/g at
2.8 A/g
current
10 kW/kg 37 Wh/kg 100% aer
2000 cycles
at 28.6 A/g.
140
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468 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Table 2. Continued.
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
α-Fe2O3nanotubes/rGO 0.1 M K2SO4181 F/g at
3A/g
current
– – 108% aer
2000 cycles
at 5A/g
141
TiO2/graphene 1 M KOH 84 F/g at
10 mV/s
– – 87.5% aer
1000 cycles
at 2 A/g
142
ZnO/graphene 1 M KOH 62.2 F/g 8.1 kW/kg – 94.9% aer
200 cycles
143
Fe3O4/N-doped graphene
aerogel
6 M KOH 386 F/g at
5mV/s
– – 153% aer
1000 cycles
144
Graphene hydrogel
functionalized using
hydroquinones
1MH
2SO4441 F/g at
1A/g
current
– – 86% aer
10 000 cycles
at 10 A/g
145
MnO2/graphene hydrogel 1 M KOH 445.7 F/g
at 0.5 A/g
current
6.4 kW/kg ∼18 Wh/kg 82.4% aer
5000 cycles
at 50 mV/s
146
MnO2/rGO hydrogel and aerogel 1 M Na2SO4242 and
131 F/g at
1A/g
current
0.82 kW/kg 212 Wh/kg 89.6% aer
1000 cycles
147
Graphene hydrogel modied by
2-aminoanthraquinone
1MH
2SO4258 F/g at
0.3 A/g
current
Slightly
increased
aer 2000
cycles at 10
A/g
148
15% RuO2/rGO hydrogel 1 M H2SO4345 F/g at
1A/g
current
– – 100% aer
2000 cycles
at 1 A/g
149
PANI/graphene hydrogel 1 M H2SO4and 1 M
H2SO4+0.4 M
Hydroquinone
223.82 and
580.52 F/g
at 0.4 A/g
current
2.637 kW/l 13.2 Wh/l 87.5 and 70%
aer 5000
cycles at 10
A/g
150
Single-crystal Fe3O4/graphene
hydrogel
1 M KOH 908 F/g at
2A/g
current
– – 69% at 50
A/g
151
Graphene organogel 1 M propylene carbonate in
tetraethylammonium
tetrauoroborate
140 F/g at
1A/g
16.3 kW/kg 15.4 Wh/kg 64.3% at 30
A/g
152
Polyethyleneimine-induced
coagulated graphene-oxide
nanosheets from the dispersion
6 M KOH 46.3
mF/cm2at
0.1 A/g
– – 98% aer
1000 cycles
at 10 A/g
153
(iii) Mesoporous carbon
Oxygen/porous carbon (SBET =
578 m2/g, Dp=2.2, 5.3 nm)
6 M KOH 295 F/g at
10 mV/s
– – 100% aer
500 cycles at
10 mV/s
157
Fe3O4/porous carbon nanorods
(SBET =25.7 m2/g, Dp=14.9
nm)
2 M KOH 285.4 F/g
at 1 A/g
current
– – 104% aer
5000 cycles
at 2 A/g
158
PANI nanowire/porous carbon
(SBET =599 m2/g, Dp=2.4, 5
nm)
1MH
2SO4517 F/g at
0.1 A/g
current
– – 91.5% aer
1000 cycles
at 0.5 A/g
159
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REVIEW Chen et al.469
Table 2. Continued.
Electrode Electrolyte
Specic
capacitance
Power
density
Energy
density
Retention
capability Ref.
(iv) Hybrid carbon nanomaterial
Graphene sheet/CNT/PANI 6 M KOH 1035 F/g at
1mV/s
scan
– – 94% aer
1000 cycles
at 200 mV/s
163
N-doped porous
carbon/MWCNT (SBET =1270
m2/g, Dp=2.8 nm)
1MH
2SO4262 F/g at
0.5 A/g
current
–––164
PANI-graphene/CNT 1 M KCl 271 F/g at
0.3 A/g
current
2.7 kW/kg 188.4 Wh/kg 82% aer
1000 cycles
at 2 A/g
165
Graphene-MnO2/CNT 1 M Na2SO4/
polyvinylpyrrolidone
(PVP)
486.6 F/g
at 1 A/g
current
∼1.7 kW/kg 15 Wh/kg 92.8% aer
800 cycles at
1A/g
166
Cobalt chloride carbonate
hydroxide nanowire/AC
1 M KOH 1737 F/g at
2.5
mA/cm2
current
0.1 kW/kg 29.03 Wh/kg 85.6% aer
1000 cycles
at 7.5
mA/cm2
168
Patronite (VS4)/SWNT/rGO 0.5 M K2SO4558.7 F/g
at 1 A/g
current
13.85 kW/kg 174.6 Wh/kg 97% aer
1000 cycles
169
PANI/GO/graphene 2 M H2SO4793.7 F/g
at 1A/g
current
2.14 kW/kg 50.2 Wh/kg 80% aer
1000 cycles
at 100 mV/s
170
Ni(OH)2-graphene/CNT
stacked layers
2 M KOH 1065 F/g at
22.1 A/g
current
8 kW/kg 35 Wh/kg 96% aer
20 000 cycles
at 21.5 A/g
171
PANI/CNT/graphene composite showed a specic
capacitance as high as 1035 F/g in 6 M KOH elec-
trolyte at a 1-mV/s scan rate, which is comparable
with that of PANI/graphene (1046 F/g) and higher
than that of PANI/CNT (780 F/g) or pure PANI
(780 F/g). Moreover, 94% of the initial capacitance
was maintained aer 1000 cycles, though the cor-
responding retention rates for the PANI/graphene
and PANI/CNT composites were only about 52%
and 67%, respectively. Similarly, various eorts have
also been made to explore other ternary composites
(e.g. metal oxide/CNT/graphene, conductive poly-
mer/metal oxide/graphene, N-doped microporous
carbon/CNT) [164–167]. Of particular interest,
cobalt chloride carbonate hydroxide nanowire
arrays (CCCH NWAs) with an average length of
8 mm were synthesized on a Ni foam surface via
a simple hydrothermal process to show a specic
capacitance as high as 1737 F/g in 1 M KOH at
2.5 mA/cm2, and angood cycling stability with a
capacitance retention of 87.3% aer 2000 cycles at
a current density of 7.5 mA/cm2. An asymmetric
supercapacitor based on CCCH NWAs as posi-
tive and activated carbon as negative electrodes
exhibited an energy density of 29.1 Wh/kg and a
power density of 100 W/kg with a good stability
over a wide voltage rang of 0–1.6 V [168]. Patronite
(VS4) has also been demonstrated to perform
well in combination with SWNT and rGO to
show a specic capacitance of 558.7 F/g in 0.5 M
K2SO4at 1-A/g current density in 0.5 M K2SO4
electrolyte and to deliver an energy density of
174.6 Wh/kg with a power density of 13.85 kW/kg
[169]. Recently, the combination of both GO and
pristine graphene with PANI has led to a specic
capacitance of 793.7 F/g in 2 M H2SO4at a 1-A/g
current density [170]. is symmetric supercapac-
itor has exhibited an energy density of 50.2 Wh/kg
at a power density of 2.14 kW/kg, aributable to
the synergistic eects of the individual ingredient.
A graphene and CNT stacked structure has also
been conceived for supercapacitor applications
[171,172]. Table 2summarizes carbon-based
pseudocapacitors.
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470 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
GO + FeCl3
GO + sucrose
Hydrothermal Activation
3D grapheneIntermediate product
Solvothermal
Intermediate product
Annealing
Fe3O4/G nanocomposite
600
800
180
Cu foil
Separator
Al foil
PF6
Li+
-
Figure 5. Schematic illustration of the synthesis of the negative electrode material Fe3O4/G nanocomposite and the positive electrode material 3D
graphene, together for the conguration of a Li-ion containing organic hybrid supercapacitor. Reproduced with permission from ref. [179]. Copyright of
The Royal Society of Chemistry (2013).
Carbon-based hybrid supercapacitors
Hybrid supercapacitors (HSCs) are mainly intro-
duced to bridge the gap between ESCs that have
high power but low energy and baeries that have
high energy but low power. Actually, in most
cases, HSC consists of a capacitive carbon elec-
trode matched with either a peseudocapacitive or a
lithium-insertion electrode (Fig. 1)[173–176]. In
HSCs, the combination of the Faradaic intercalation
on cathode and non-Faradaic surface reaction on an-
ode (Fig. 1c) provides an opportunity to achieve
both high energy and power densities even without
compromising the cycling stability and aordability.
e reported carbon-based electrodes so far
used for the cathode in HSCs are graphite, CNTs,
graphene, activated carbon (AC), 3D mesoporous
carbons and dierent metal oxide or polymer-based
carbon composites [177]. 3D graphene/MnO2
composite has a maximum specic capacitance of
1145 F/g, which is about 83% of the theoretical
capacitance at a mass loading of 13% of MnO2
[178]. On the other hand, HSCs were fabricated
using a Fe3O4nanoparticle/graphene compos-
ite by a simple solvothermal method (Fig. 5).
Fe3O4/graphene-based half-cell exhibited a high
reversible specic capacity exceeding 1000 mAh/g
at a current density of 90 mA/g with an excellent
rate of capability and cycle stability [179]. When this
composite was assembled in a Li-ion-based (LiPF6)
HSC, energy densities of 204–65 Wh/kg and power
densities from 55 to 4600 W/kg were achieved
[179]. Besides, Lim et al. reported HSCs based on
a mesoporous Nb2O5/carbon-composite anode
and AC (MSP-20) cathode, showing excellent
energy and power densities of 74 Wh/kg and 18 510
W/kg (at 15 Wh/kg), respectively, with a capacity
retention rate of 90% at 1000 mA/g aer 1000
cycles in the mixture electrolyte of LiPF6 (1.0 M)/
ethylene carbonate and dimethyl carbonate (1:1
volume ratio) [180]. Li et al. reported a hybrid-type
supercapacitor based on N-doped AC, which
showed high material-level energy densities of 230
Wh/kg with a power density of 1747 W/kg and a
capacity retention of 76.3% aer 8000 cycles [181].
Carbon-based bendable supercapacitors
(lm-/ber-shaped)
Along with the recent development of exible and
wearable electronics, exible and wearable SCs, in
either a thin lm or ber-shaped (coating, fab-
ric/cloth, paper, textile, etc.), have aracted in-
creasing aention as advanced power sources. Due
to their large surface area, excellent mechanical
and electrical properties, and high electrochemi-
cal stability, carbon nanomaterials are also promis-
ing as electrode materials for exible supercapaci-
tors (FSCs). In this context, Chen et al. produced
exible and transparent supercapacitors based on
In2O3nanowire/CNT heterogeneous lms, and ob-
served an increase in specic capacitance up to
64 F/g with increasing numbers of In2O3nanowires
(up to 0.007 mg) dispersed on the CNT lms
[182]. In another study, a 2-mm-thick lm-based
FSC made of MnO2nanosheet-decorated carbon
nanober electrodes was demonstrated to show a
gravimetric capacitance of 142 F/g at a slow scan
rate (∼10 mV/s) when the electrode was inter-
faced with PVA-H4SiW12O40 ·nH2O[183]. Other
materials used for the electrode in FSCs include
TiO2/MWNT/PEDOT composited carbon bers
[184] and various carbon papers made of bers,
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REVIEW Chen et al.471
Vac uum
Vacuum-filtrationUltrasonication
Filter membrane
CNT
MnO2 NT
Dry & Peel off
(a)
(b) (c) (d)
10-2
10-1
10-1
100
101
100
Energy density (mWh/cm3)
Power density (mW/cm3)
0.0 0.2 0.4 0.6 0.8
Voltage (V)
80
60
40
20
0
-20
-40
Current density (mA/cm3)
Bent TwistNormal
Figure 6. (a) Schematic illustration of the fabrication process of exible freestanding
CNT/MnO2NT hybrid lm. (b) Ragone plots of the exible solid-state SC device. Inset
shows a group of LEDs (consisting of 32 green LEDs) powered by four series-connected
SCs. (c) CV curves collected at the same scan rate of 5 mV/s under normal, bent and
twisted conditions; insets are the digital images under the corresponding test condi-
tions. (d) An electronic watch wrapped around a transparent glass tube demonstrating
the exibility of the SC-integrated watch band. Reproduced with permission from ref.
[187]. Copyright of The Royal Society of Chemistry (2014).
aerogel and nanotubes [185]. Liu et al. fabricated
highly exible porous lms of carbon nanobers (P-
CNFs) by an electro-spinning technique combined
with a Co ion-assistant acid corrosion process [186].
e resultant bers have high conductivity and out-
standing mechanical exibility, with lile change in
their resistance under repeated bending many times,
even upto 180◦. e P-CNF electrode showed a spe-
cic capacitance of 104.5 F/g in 0.5 M H2SO4at
0.2 A/g current, eectively improved cycling stabil-
ity and 94% retention of specic capacitance aer
2000 charging/discharging cycles, along with a re-
tention rate of 89.4% capacitance aer 500 bending
cycles [186]. ese remarkable performances are at-
tributable to the high graphitization degree and the
unique hierarchical pore structures of P-CNF [186].
A low processing cost for exible electrode man-
ufacturing is always desirable. Du et al. developed a
low-cost, exible and high-performance hybrid elec-
trode based on a MnO2nanotube (NT) and CNT
composite lm obtained through a vacuum-ltering
method [187]. Due to the ultra-long 1D nanotube
morphology, the synergetic eects between pseudo-
capacitive MnO2-NTs and conductive CNTs, the
hierarchical porous structure of the freestanding lm
and the high mass loading of MnO2(4 mg/cm2),
the resultant MnO2-NT/CNT electrodes showed
excellent mechanical and electrochemical perfor-
mance with a volumetric capacitance of 5.1 F/cm3
in the polyvinyl alcohol (PVA)/LiCl gel electrolyte,
a high energy density of 0.45 mWh/cm3for the en-
tire FSC volume and the retention of capacitance at
about 105% of the initial capacitance aer 6000 cy-
cles due to a self-activation eect. As shown in Fig. 6,
these SCs can be integrated in wearable electronic
devices as exible power that can drive watches and
light emiing diodes (LEDs).
Flexible supercapacitors have also been fabri-
cated from conducting polymers with and with-
out compositing with other electrode materials (e.g.
CNTs, graphene). PANI, PPy and PEDOT possess
high specic capacitances of 1284, 480 and 210 F/g,
respectively [188]. PPy is a popular polymer elec-
trode material for FSCs due to its high environmen-
tal stability, excellent redox activity and easy avail-
ability. Moreover, the electrochemical properties of
PPy can further be enhanced by compositing with
CNTs, graphene or their hybrid/composites. Re-
cently, Yesi et al. prepared lms of CNT-PPy core–
shell composite by growing CNTs directly on car-
bon cloth (CC) as a skeleton, followed by elec-
tropolymerization of PPy on the CNTs [189]. e
direct fabrication of CNT-PPy on the exible CC
electrode increased the interfacial conductance and
the ion transport between the electrode and elec-
trolyte. e PPy/CNT-CC electrode thus prepared
exhibited 1038-F/g gravimetric capacitance per ac-
tive mass of PPy and up to 486.1 F/g per active mass
of the PPy/CNT composite, with excellent mechan-
ical exibility and cycle stability up to 10 000 cycles
with 18% capacitance reduction. At the same time,
the corresponding asymmetric supercapacitor (PPy-
CNT-CC/CNT-CC) showed a maximum power
density of 10 962 W/kg and energy density of 3.9
Wh/kg at 1.4-V potential.
For most solid-state supercapacitors based
on freestanding graphene materials, the specic
capacitance ranges from 80 to 135 F/g, while the
corresponding theoretical value should be around
550 F/g. e observed dierence is most probably
due to the restacking of graphene sheets, and hence
reduced active surface area of the graphene-based
electrodes [187]. To overcome the π–πrestacking
of graphene sheets, numerous aempts have been
made to control the electrode structure into, for
example, porous 3D graphene hydrogels on Ni foam
by CVD or freeze-drying GO. Mitchell et al.have
further proposed a strategy to produce hierarchical
and exible nanosheets of NiCo2O4-graphene-
oxide composite on nickel foam by using elec-
trochemical deposition [190], which exhibited a
specic capacitance of 1078 F/g in 3 M KOH elec-
trolyte at 1-mA discharge current and a relatively
poor cyclic stability (almost 45% reduction over 500
cycles). In another aempt to reduce the restacking
of graphene layers in a 3D graphene electrode, Li et
al. fabricated a solid-state asymmetric supercapaci-
tor (ASC) based on exible electrodes [191]. In this
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472 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Oxidative
polymerization
Step 1 Step 2
Step 3
PANI doping
Solution casting
Flexible substrate
NG doping
Sonication
SEM image
: PAA : Aniline-HCI : PANI : NG nanosheet
Figure 7. Illustration of the process from synthesis to obtaining a NG-PAA/PANI composite coating on CC; PANI is supplied
by both the rst and second steps. NG doping in step 3 improves the conductivity further and reduces swelling. Reproduced
with permission from ref. [192]. Copyright of Macmillan Publishers Ltd (2016).
case, the positive electrode was made from densely
packed graphene sheets with intercalated Ni(OH)2
nanoplates to give a gravimetric capacitance of 573
F/g and volumetric capacitance of 655 F/cm3in
1 M KOH electrolyte at 0.2 A/g current density,
excellent rate capability and cycling stability. On the
other hand, the negative electrode was fabricated
with CNT layers stacked in between highly dense
graphene sheets. e asymmetric supercapacitor
exhibited an energy density of 18 Wh/kg and power
density of 850 W/kg at 1-A/g current density. When
the current density increased up to 20 A/g, the cor-
responding energy density remained at 6.4 Wh/kg
with a high power density of 17 kW/kg. Bending
the device up to 180◦caused no inuence on the
electrochemical performance [191]. With the aim to
improve the specic capacitance in lm-based FSCs,
complex and hybrid structures like CNT/graphene
and Mn3O4nanoparticles/graphene paper elec-
trodes with a polymer gel electrolyte of potassium
polyacrylate (PAAK)/KCl were conceived and
specic capacitance of 72.6 F/g at 0.5 A/g current
was obtained [132].
As is the case for conventional ESCs discussed
above, composites of CNT/graphene and con-
ducting polymers have also been widely used in
FSCs. In particular, Wang et al. recently synthesized
polyacrylic acid/PANI composites enhanced by
nitrogen-doped graphene (NG) (NG-PAA/PANI)
[192] and demonstrated that the CC electrodes con-
taining 32 wt.% PANI and 1.3 wt.% NG showed a
high capacitance of 521 F/g in 1 M H2SO4elec-
trolyte at 0.5 A/g (Fig. 7). A symmetric supercapaci-
tor fabricated from 20 wt.% PANI-CC electrodes ex-
hibited four times higher capacitance of 68 F/g at 1
A/g than the previously-reported SCs based on ex-
ible PANI-CNT composites. e NG-PAA/PANI
electrode retained the full capacitance over large
bending angles with an energy density of 5.8 Wh/kg,
a power density of 1.1 kW/kg, a superior rate ca-
pability of 81% at 10 A/g and long-term electro-
chemical stability (83.2% retention aer 2000 cy-
cles) [192]. By incorporating graphene and CNTs
with PPy, Aphale et al. fabricated freestanding hy-
brid electrodes to show a specic capacitance of 453
F/g in 1 M H2SO4at 5 mV/s scan rate (Fig. 8)
[193]. Furthermore, the hybrid electrode demon-
strated an ultrahigh energy density of 62.96 Wh/kg
at a power density of 566.66 W/kg. Four such
SCs assembled in a series successfully lit up a
2.2 V LED.
For the ber-based FSCs, a wearable, ber-
shaped and all-solid-state asymmetric FSC was
recently fabricated with a 1.5-V operating voltage
using ultrathin MnO2nanosheets on carbon bers
as the positive electrode and graphene on carbon
bers as the negative electrode [194], leading
to an energy density of 27.2 Wh/kg and power
density of 979.7 W/kg. ese values are higher than
the latest reported for MnO2-based asymmetric
or symmetric supercapacitors, including MnO2
nanotubes/activated graphene (22.5 Wh/kg at
146.2 W/kg) [195], MnO2nanoowers/Bi2O3
nanoowers (11.3 Wh/kg at 352.6 W/kg) [196],
graphene foam (GF)-CNT-MnO2/GF-CNT-
PPy (22.8 Wh/kg at 860 W/kg) [197], 3D
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REVIEW Chen et al.473
Gold/Graphite/Pt
substrate
Polypyrrole (PPy)
CNT
Graphene
(CE) Platinum (Pt)
Pyrrole (Py)
(RE) Ag/AgCl
V
A
Potentiostat (a) (b)
CNT
PPy film
Graphene
Ions
(c) (d)
(e)
(f)
Figure 8. Fabrication and characterization of hybrid electrode. (a) Schematic representing the fabrication process of the nanocomposite electrodes. (b)
Illustration of hybrid nanocomposite lm forming a unique interface where graphene and CNT are embedded in situ during polymerization of PPy. (c, d)
Optical image of the actual freestanding lm on the graphite substrate with ∼2cm×2 cm area. (e) Cross-sectional SEM micrograph showing layered
formation of the polypyrrole lm. (f) Layers of graphene-CNT coated with PPy during polymerization forming a nanocomposite PCG lm. Reproduced
with permission from ref. [193]. Copyright of Macmillan Publishers Ltd (2016).
graphene-MnO2/3D graphene-MnO2(6.8 Wh/Kg
at 62 W/Kg) [198], multilayer MnO2-GO/porous
carbon (energy density of 46.7 Wh/kg at power
density of 100 W/kg) [199], 3D MnO2/graphene
hydrogel (21.2 Wh/kg at 0.82 kW/kg) [200]
and 2D planar MnO2/graphene (17 Wh/kg at
2.52 kW/kg) [201]. Moreover, this ber-shaped
asymmetric FSC further displayed suciently
good bendability and mechanical stability for being
connected in parallel and woven into coon textiles
(Fig. 9). By integrating this asymmetric FSC with a
nanowire-based photodetector into a self-powered
nanodevice, the ber-shaped asymmetric FSC
could eciently power a photo detector without
any requirement for an external bias [194].
Using a silica capillary column as a hydrothermal
micro-reactor as depicted in Fig. 10 [202], Yu and
coworkers demonstrated a large-scale manufac-
turing method for continuously producing carbon
microbers with a unique hierarchical structure
composed of an interconnected network of CNTs
with interposed nitrogen-doped reduced graphene-
oxide sheets. e resultant carbon ber electrode
showed an electrical conductivity of 102 S/cm,
volumetric capacity of 305 F/cm3in sulfuric acid
(measured at 73.5 mA/cm3in a three-electrode cell)
and 300 F/cm3in polyvinyl alcohol (PVA)/H3PO4
electrolyte (measured at 26.7 mA/cm3in a two-
electrode cell). e full micro-supercapacitor with
PVA/H3PO4gel electrolyte, free from binder,
current collector and separator, has a volumetric
energy density of 6.3 mWh/cm3, which is com-
parable to that of 4 V–500 mAh thin-lm lithium
baery while maintaining a power density more
than two orders of magnitude higher than that of
baeries as well as a long cycle life. is ber-based,
all-solid-state micro-supercapacitor was further
successfully interfaced into miniaturized exible
devices to power a TiO2-based ultraviolet photode-
tector and a light-emiing diode (Fig. 10). Another
type of cable/wire-shaped exible SC, as presented
in Fig. 11, was fabricated on a stainless-steel wire us-
ing hydrothermal rGO nanosheets [203]. In redox
additive electrolyte (PVA/H3PO4/Na2MoO4), this
exible SC exhibited a maximum length capacitance
and energy density of 18.75 mF/cm (areal capac-
itance of 38.2 mF/cm2) and 2.6 mWh/cm (areal
energy density of 5.3 mWh/cm2), respectively. e
exibility and stability of the above FSC device have
also been investigated and three serially connected
devices could be used to light up the green and blue
LEDs (Fig. 11)[203]. A similar aempt has been
made to demonstrate an rGO-based wearable SC
on Cu wire [204]. While recent progress in FSCs is
summarized above, interested readers are referred
to some earlier review articles on exible SCs related
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474 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Figure 9. (a) SEM image of carbon bers bundles. (b, c) SEM image of MnO2nanosheet
arrays on single carbon ber at a low and high resolution. (d) TEM of MnO2nanosheets.
(e–g) Photos of two ber-shaped ASCs connected in parallel under different bending
state which woven into a cotton textile. (h) CV curves of two asymmetric ber super-
capacitors in at and the corresponding bending states with a scan rate of 100 mV/s.
Reproduced with permission from ref. [194]. Copyright of Wiley (2016).
to MnO2[205] and carbon materials [188,196],
respectively.
Carbon-based stretchable and twistable
supercapacitors (lm-/ber-shaped)
Along with the FSCs described above, stretchable
and twistable FSCs are needed for advanced elec-
tronics, including polymer-based self-powered sen-
sors, polymer light-emiing diodes, polymer so-
lar cells and active matrix displays, to name a few
[196]. As well as early reports on stretchable SCs,
buckled SWNT/polydimethylsiloxane (PDMS)
electrodes have drawn considerable aention, as
they could show a strain up to 140% without any
change in resistance [206,207]. e use of crumpled
graphene papers reduced the cost and complexity
for fabricating stretchable and high-performance
electrodes for SCs [208,209]. e crumpled-
graphene-paper-based electrode demonstrated high
stretchability up to 300% linear strain and 800%
aerial strain with a high specic capacitance of 196
F/g in H3PO4-PVA electrolyte and reliability up to
1000 stretch/relax cycles [208].
In addition, Kim et al. have reported a delamina-
tion-free stretchable supercapacitor, in which all
the component layers were prepared with a single
matrix composed of an ionic liquid, 1-ethyl-3-
methylimidazolium bis(triuoromethylsulfonyl)-
imide, and a polymer, poly(vinylidene uoride-
hexauoropropylene), as an electrolyte and a
supporting layer, respectively, in the stretchable
supercapacitor [210]. e electrode layer was
fabricated by incorporating CNTs in the common
(polymer) matrix with all the layers being seam-
lessly fused into one body by dissolving the surface
of the composite with acetone. e operational
cell voltage was as high as 3 V due to the use of
ionic liquid-based gel electrolytes. Specic elec-
trode capacitance and areal cell capacitance were
67.2 F/g and 12.7 F/cm2, respectively. e standard
deviations of the capacitance were only ±2.1% and
±1.4%, respectively, aer 500 cycles of the lateral
and radial stretches at 0.5 strain.
Polypyrrole (PPy)-coated MnO2nanoparticles
were deposited onto CNT-based textile supercapac-
itor electrodes, which increases by 38% the electro-
chemical energy storage of MnO2/CNT-based ex-
ible (13% bend) and stretchable (21% tensile strain)
supercapacitors (Fig. 12)[211]. A specic capac-
itance of 461 F/g in H3PO4-PVA electrolyte was
reported at 0.2-A/g current density, which was at-
tributed to the delamination prevention of MnO2
nanoparticles by PPy coating. Furthermore, the ca-
pacitance retention was 96.2% even aer 750 000
bending (13%) cycles [211].
A thin SWNT lm and a honeycomb PDMS
structure have been utilized as electrode materials
and stretchable substrate supports for fabricating
stretchable micro-supercapacitors (MSC) [212].
An array of 4 ×4 MSCs showed that the maximum
strain in the MSC regions was almost 5 orders of
magnitude lower than that the applied strain (of
about 150%), and the device capacity remained
same even at 150% stretch. Yun and coworkers con-
structed a stretchable MSC of practical signicance,
where a stretchable paerned graphene gas sensor
driven by integrated micro-supercapacitor array on
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REVIEW Chen et al.475
Wet fibre
Dry fibre
Inch
(a) (b)
(c)
Pump
N
2
Oven
220 °C 6 h
GO
EDA
Feeding
solution
(d) (e) (f)
SWNT
(g) (h)
+
−
Fibre
PVA/H3PO4
PET substrate
0 200 400 600 800 1,000
Capacitance retention (%)
100
120
80
60
40
20
0
Cycle number
Bending 90° (i)
UV light
FTO
UV photodetector
Fibre micro-SC
A
TiO2 nanorod
Figure 10. (a) Schematic of the ber synthesis process by injecting a homogeneous solution containing acid oxidized SWNTs,
GO and EDA through a pump into a exible silica capillary column, followed by in-situ thermal treatment in an oven at
220◦C for 6 h before a continuous ber was pushed into a water reservoir by a pressurized nitrogen ow. (b) Photograph
of the as-prepared bers collected in water. (c) A dry ber with diameter of ∼50 mm and length of ∼0.5 m (∼20 inches).
(d) Planar structures obtained by bending bers. (e) Compressed and stretched ber springs. (f) A knitted textile fabricated
from bers. All scale bars: 0.5 cm. (g) Schematic of a micro-SC constructed using two ber-3 electrodes on a polyester
(PET) substrate. (h) Capacitance retention after 1000 cycles up to 908 bending angle. Inset: Photograph of a bent micro-SC.
(i) Current response assembly of the ultraviolet photodetector based on the TiO2nanorod array powered by the micro-SC.
Reproduced with permission from ref. [202]. Copyright of Macmillan Publishers Ltd (2014).
the same deformable substrate, as demonstrated
in Fig. 13 [213]. e paerned MSCs, which
consisted of PANI-wrapped multiwalled carbon
nanotubes (MWNTs) and an ion-gel electrolyte
of poly(ethylene glycol)diacrylate and 1-ethyl-3-
methylimidazoliumbis (triuoro-methylsulfonyl)
imide, exhibited excellent electrochemical perfor-
mance under a uniaxial strain of 50% and a biaxial
strain of 40%; their initial performance (capacitance
of 6.1 F/cm3at 5-mA/cm3current) characteristics
were retained even aer1000 cycles of repetitive uni-
axial (50%) and biaxial (40%) stretching. Moreover,
the paerned graphene sensor successfully detected
NO2gas for longer than 50 min via integration
with MSCs using the serpentine interconnections
even under uniaxial stretching by 50%. Such a
hybrid combination of SCs with other practical
electronic devices is highly desirable for near-term
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476 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
(b) (c)
(d) (e)
(a)
0 300 600 900
Time (s)
Voltage (V)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
4 mA 0.5 mA
Normal Stage 1 Stage 2 Stage 3
Different bending conditions
Length capacitance (mF cm-1)
10
8
6
4
2
0
Figure 11. (a) Schematic illustration of fabrication of rGO-based cable supercapacitor using PVA/H3PO4/Na2MoO4electrolyte.
(b) Galvanostatic charge–discharge curves at the different current densities (0.5–4 mA) for the fabricated cable SC. (c) Capac-
itance variation at the different bending states. Inset of (c): digital images of the different bending states. (d, e) Demonstration
of three serially connected devices can power the green and blue LEDs, respectively. Reproduced with permission from ref.
[203]. Copyright of Elsevier (2016).
applications. However, much more aention must
be given to further improving capacitive storage
capability in exible and stretchable supercapacitors.
Buckled CNT lm was also investigated for
stretchable electrodes in SCs, as shown in Fig. 14a–
c[214]. A comparative study between PANI com-
posites of buckled CNT with and without nitric
acid treatment revealed that acid-treated buckled
CNT@PANI electrodes exhibited a higher specic
capacitance of 1147.12 mF/cm2in H3PO4-PVA
electrolyte at 10 mV/s [214]. is observation can
be correlated with the formation of beer interfa-
cial bonding between acid-treated CNTs and PANI.
e acid-treated electrode also showed an energy
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REVIEW Chen et al.477
(a)
PEO based
gel electrolyte
Conductive polymer polypyrrole
MnO2
NP
SWNT
cotton
Textile based supercapacitor
Mechanical
bending stress
Mechanical
tensile stress
Current density (mA/cm2)
(b)
(c)
0.0 0.3 0.6 0.9
Voltage (V)
10
5
0
-5
Remaining 96.2%
energy capacity under 750,000
bending cycles
(13% bending strain)
w/o bending strain
250,000 bending cycles
500,000 bending cycles
750,000 bending cycles
13 % bending strain
Plate motion
Textile based
supercapacitor
Gap
(2R)
C-V
measurement
B0t0 + B1(t1 + 2t0t1)
22
s =
2(B0t0 + 2B1t1)
Figure 12. (a) Schematic illustration of the fabrication of polpyrrole−MnO2-
coated textile supercapacitor. (b) Schematic for bending test was performed on
polypyrrole−MnO2-coated supercapacitor. (c) Cyclic voltammetry of supercapacitor un-
der 13% bending strain. Reproduced with permission from ref. [211]. Copyright of Amer-
ican Chemical Society (2015).
density from 31.56 to 50.98 μWh/cm2with power
density changing from 2.294 to 28.404 mW/cm2at
the scan rate of 10–200 mV/s. e corresponding
supercapacitor sustained an omnidirectional strain
of 200%.
In the case of ber-shaped stretchable and
twistable SCs, recently, yarn-based SCs have
been demonstrated, which consist of core–shell-
structured coiled electrodes with pseudocapacitive
CNT-cores and MnO2-shells, as shown in Fig. 14d
and e [215]. e linear and volumetric capaci-
tances of the coiled yarn were determined to be
2.72 mF/cm and 34.6 F/cm3, respectively. In-
terestingly, around 84% of its static capacitance
was retained aer being reversibly stretched by
37.5% strain, while 96.3% dynamic capacitance was
maintained during 20% strain deformation despite
the extremely high strain rate of 6%/s. e yarn
supercapacitors exhibited 95% or 98.8% capaci-
tance retentions aer many stretching/releasing or
charge/discharge cycles [215]. Stretchable coaxial
ber-shaped SCs have also been fabricated using
CNT sheets wrapped on an elastic ber using a
polymer gel sandwiched between the two coaxial
CNT layers as the electrolyte and separator as well
[216,217].
Transparent SCs have also become important for
various optoelectronic applications. Indeed, trans-
parent energy-storage devices are highly desirable
for automobile/building windows or personal elec-
tronics with high aesthetic appeal. In this regard,
a simple dry press transfer technique has been
used to transfer thin SWNT lms to transpar-
ent substrates to prepare transparent and exible
EDLCs with a transparency of 92% at 550 nm and
high transparency over visible light and near in-
frared (NIR) wavelengths [218]. e resultant lms
showed an extremely high mass specic capacitance
of 178 F/g (which is 482 F/g when calculated per
mass of carbon) in PVA/H3PO4electrolyte and area
specic capacitance of 552 μF/cm2compared to
other reported carbon-based exible and transpar-
ent EDLCs [219]. e lms were also highly stable
in terms of specic capacitance over 10 000 loading
cycles [218]. Chen et al. also fabricated graphene-
based transparent and exible SCs [209], which are
aractive for various portable electronic devices. Ta-
ble 3summarizes carbon-based exible, stretchable,
wearable as well as transparent supercapacitors.
e advantages of using carbon materials for su-
percapacitor electrodes lay in the facts that they are
highly conductive, binder-free and exible with a
large surface area and excellent properties intrin-
sically associated with dierent carbon allotropes.
Carbon materials are Earth-abundant and environ-
mentally friendly with respect to metal- or polymer-
based electrode materials. rough using exible
carbon materials in supercapacitors, it would be pos-
sible to reduce the unnecessary use of metal foils
(such as Al) as the current collector. It is also pos-
sible to eliminate polymer-binders or other conduc-
tive additives. erefore, the use of carbon materials
can propel the exible supercapacitors to be lighter,
portable and more easily manufactured. For many
practical applications, however, their eciencies still
need to be improved by combining exible carbon
materials with pseudocapacitive materials (such as,
metal oxides and polymers).
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478 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Figure 13. (a) Schematic denition of the biaxial strain εbiaxial. (b) Optical microscope image of the MSC array (left) and the
strain distribution estimated by FEM analysis (right) under a biaxial strain of 40%. (c) Optical images (left) and the results
obtained from FEM analysis (right) of a serpentine interconnection in various uniaxial stretching states. The cross-sectional
view shown is for a strain of 50%. Reproduced with permission from ref. [213]. Copyright of Elsevier (2016).
Carbon-based ultrafast supercapacitors
for ac-line ltering
Alternating current (ac)-line ltering by using ultra-
fast supercapacitors is essential for domestic power
usage in order to remove undesired high-frequency
noises. ac electricity has a frequency of either 50
or 60 Hz. e combination of various nonlinear
loads from dierent electronic devices in domes-
tic requirements, portable electronics, automobiles
and medical appliances oen induces higher-order
harmonics (>120 Hz) of the basic generating fre-
quency [220]. In order to protect electronic devices
from such voltage ripples, aluminum electrolyte ca-
pacitors (AECs) are used for ac-line ltering [221–
223]. However, AECs have low specic capacitance,
and hence occupy a large space and volume in elec-
tronic circuits. In this regard, supercapacitors, pos-
sessing specic capacitance of 2–5 orders higher
in magnitude than that of AECs, could be used
for eective ac-line ltering with very negligible oc-
cupied space or volume in capacitive components
[224,225]. A supercapacitor generally acts like a re-
sistor at 120 Hz aer being introduced into transmis-
sion lines [223]. e typical resistor–capacitor (RC)
time constant for a supercapacitor is around 1 s, as-
sociated with the high electrochemical series resis-
tance and microporous structure of supercapacitor
electrodes, which is far too long to be useful for the
common application of 120-Hz ltering (∼8.3 ms
period)—indicating the necessity for smoothing the
leover ac ripples in most line-powered electronics
[226]. is is mainly because of the fact that unsuit-
able pore structures of supercapacitor electrodes im-
pede high-rate ion diusions or their high resistances
restrict ecient charge transfer [227]. us, the de-
sign and fabrication of highly conductive electrodes
with optimized micro-/nano-architectures for facile
electron/ion transportations can improve the per-
formance for ac-line ltering. us, high-surface-
area materials with less inherent porosity have been
studied for such applications.
Among a variety of other electrode materi-
als, including onion-like carbon [228] and CNTs
[229,230], carbide-derived carbon [231], metal ox-
ides [232], polymers [233] and mesoporous car-
bons [225], utilized to improve related rate capa-
bility for ac-line ltering, graphene-based materials
and graphene/CNT hybrid structures have recently
emerged to be promising over conventional carbon
materials because of their superior electrical conduc-
tivity and high specic surface area [227,234,235].
Graphene and porous carbon composites have also
been demonstrated to be excellent ac-line lters,
though their energy-storage capabilities still need to
be improved [236].
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REVIEW Chen et al.479
MnO2
shell
Parallel symmetric
fiber electrodes
CNT core
PVA/LiCl
gel electrolyte
(d)
(a)
(b)
1200
1000
800
600
400
200
00 50 100 150 200
Scan rate (mV s-1)
CNT
Acid treated CNT
CNT@PANI
Acid treated CNT@PANI
Ca (mF cm-2)
(c)
0 5 10 15 20 25 30 35 40
Strain (%)
1.0
0.8
0.6
0.4
0.2
0.0
Capacitance retention (A A0)
-1
(e)
Figure 14. (a) SEM images of buckled structures formed in CNT lms at 50% omnidirectional prestrain. (b) Digital images of buckled CNT lm at
various stretching states. (c) Specic capacitance variation for CNT/PANI SCs with scan rate. Reproduced with permission from ref. [214]. Copyright
of American Chemical Society (2016). (d) Schematic illustration for the complete solid-state coiled supercapacitor, which comprises two symmetric
MnO2/CNT core-shell-coiled electrodes and gel electrolyte. (e) Stress loading/unloading curves of the hybrid MnO2/CNT coiled electrode with tensile
strains from 20% to 40%. Reproduced with permission from ref. [215]. Copyright of Wiley (2016).
Lim et al. recently demonstrated that the substi-
tutional pyridinic nitrogen dopant sites in carbon
nanotubes can selectively initiate the unzipping
of CNT side walls at a relatively low electro-
chemical potential (0.6 V) [235]. e resultant
nanostructures consisting of partially zipped and/or
unzipped graphene nanoribbons wrapping around
carbon nanotube cores maintain the intact 2D
crystallinity with well-dened atomic conguration
at the unzipped edges (Fig. 15). e synergistic
interaction between the large surface area and
robust electrical connectivity of the unique nanoar-
chitecture led to ultrahigh-power supercapacitor
performance, which can serve for ac ltering with
the record high-rate capability of 85◦(very close to
that of AECs, 83.9◦) of phase angle at 120 Hz. Lin
et al. also fabricated a 3D graphene/CNT carpet
(G/CNTC)-based micro-supercapacitor on nickel
electrodes [237]. e G/CNTC showed an 81.5◦
phase angle at a 120-Hz frequency. However, much
work still needs to be done in order to realize super-
capacitors having both ac-line-ltering capability
and excellent charge-storage capability for practical
applications.
SUMMARY AND PERSPECTIVES
Carbon nanomaterials, including 1D CNTs, 2D
graphene, 3D mesoporous carbon and their com-
posites with conductive polymers or metal oxides,
have been widely used as electrodes in superca-
pacitors, such as EDLCs, PCs and HSCs. Gener-
ally speaking, pure carbon nanomaterials without
any functional groups are useful as EDLC-electrodes
because of their high specic surface area and ex-
cellent electrical conductivity. e advantages of
EDLC include its high-rate capability and outstand-
ing cyclic stability (e.g. retention of 95%∼100%
aer 1000∼10 000 cycles). e specic capaci-
tance for pure carbon nanomaterials in EDLC has
been demonstrated to be in the range of 10–300
F/g. For carbon-composite nanomaterials, the spe-
cic capacitance can be increased by one order of
magnitude: generally 100∼1000 F/g. By composit-
ing carbon nanomaterials with other materials hav-
ing pseudo-capacitances (e.g. conducting polymers,
metal oxides or hydroxyls), therefore, the energy
density can be largely improved, but their rate capa-
bility and cyclic stability may decrease to 60–90% af-
ter 1000 cycles.
Numerous recent eorts have been made to im-
prove the electrochemical performance of the su-
percapacitors based on carbon nanomaterials by
improving their specic capacitance, energy den-
sity, power density, rate capability and/or cyclic
stability. e design and development of advanced
3D electrode structures and compositing carbon
nanomaterials with other active materials have been
demonstrated to be eective approaches to high-
performance carbon-based SCs. Hybrid supercapac-
itors can ll the gap between a supercapacitor and a
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480 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Table 3. Carbon nanomaterials in exible, stretchable, wearable and transparent supercapacitors.
Electrode Electrolyte
Specic
capacitance Power density
Energy
density
Retention
capability Ref.
(i) Flexible
CNT-graphene/Mn3O4-
graphene (exibility: bendable
and twistable)
Potassium
polyacrylate
(PAAK)/KCl
72.6 F/g at
0.5 A/g
current
9 kW/kg 22.9 Wh/kg 86% aer
10 000 cycles
at 10 mV/s
132
MnO2/carbon nanober PVA-KOH,
PVA-H4SiW12O40
·nH2O
100 F/g,
142 F/g at
5 mV/s scan
– – 60%, 28% at
100 mV/s
183
TiO2/MWCNT 1 M H2SO436.8 F/g at
20 mV/s scan
– – 184
Porous carbon nanobers
(exibility: no degradation aer
100 times complete bending)
0.5 M H2SO4104.5 F/g at
0.2 A/g
current
600 W/kg 3.22 Wh/kg 94% aer
2000 cycles at
1A/g
186
MnO2nanotune/CNT
(exibility: no degradation on
bend and twist)
Polyvinyl alcohol
(PVA)/LiCl
29.3 F/g at
0.5 mA/cm2
current
13.8 mW/cm30.45
mWh/cm3
105% aer
6000 cycles at
1.2 mA/cm2
187
CNT (core)/PPy(shell)
(exibility: no degradation aer
20 bending cycles)
0.5 M H2SO4486.1 F/g at
1.25 A/g
current
10.96 kW/kg 3.9 Wh/kg 82% aer
10 000 cycles
at 8 A/g
189
NiCo2O4-GO (exibility:
no degradation up to
180◦bending and complete
twisting)
3 M KOH 1078 F/g at
1 mA current
––∼58% aer
100 cycles at
3mA
190
Ni(OH)2/dense stack of
graphene sheets (exibility: no
degradation up to 180◦
bending)
1 M KOH 573 F/g at
0.2 A/g
current
8.5 kW/kg 9 Wh/kg 89% aer
2000 cycles
191
N-doped graphene- polyacrylic
acid (PAA)/PANI (exibility:
no degradation up to
135◦bending)
1MH
2SO4,
H2SO4-PVA
521 F/g at 0.5
A/g current
1.1 kW/kg 5.8 Wh/kg 83.2% aer
2000 cycles at
1A/g
192
PPy/CNT-graphene
(exibility: not demonstrated)
1MH
2SO4453 F/g at
5 mV/s scan
566.66 W/kg 62.96 Wh/kg – 193
rGO coated on stainless-steel
wire (exibility: no degradation
aer 180◦bending)
PVA/H3PO4/Na2
MoO4polymer gel
38.2 mF/cm2
at 0.5 mA
current
5.3
microWh/cm2
63.7
microW/cm2
100% aer
2500 cycles
203
(ii) Flexible and wearable
MnO2sheets/carbon ber
(exibility: highly exible wire
without any degradation on
bending)
PVA-LiCl 634.5 F/g at
2.5 A/g
current
979.7 W/kg 27.2 Wh/kg 95.2% aer
3000 cycles at
20 A/g
194
SWNT/N-doped rGO ber
(exibility: 97% of capacitance
retention aer 1000 times
bending at 90◦)
1MH
2SO4305 F/cm3at
26.7 mA/cm3
current
1.085 W/cm36.3 mWh/cm393% aer
10 000 cycles
at 250 mA
cm/3
202
Graphene aerogel on Cu wire
(exibility: 99% of capacitance
retention aer 1000 times
bending at 160◦)
Polyvinylpyrrolidone
(PVP)
12.5 F/cm at
5 mV/s scan
– – 95% aer
10 000 cycles
at 1 A/g
204
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REVIEW Chen et al.481
Table 3. Continued.
Electrode Electrolyte
Specic
capacitance Power density
Energy
density
Retention
capability Ref.
(iii) Flexible/wearable and stretchable
Buckled SWNT macrolm
(no degradation aer 30%
strain)
1MEt
4NBF4/
propylene carbonate
54 F/g at 1
A/g current
1 kW/kg ∼3.5 Wh/kg 96.3% aer
1000 cycles
with 30%
strain at 1 A/g
206
SWNT/polydimethylsiloxane
(PDMS) (can sustain 120%
strain)
H2SO4-polyvinyl
alcohol (PVA)
53 F/g at 10
A/g current
with 120%
strain
32 kW/kg – 100% aer
1000 cycles
under 120%
strain
207
Crumpled graphene hydrogel
(can sustain 300% linear and
800% areal strain)
H3PO4-PVA 166–196 F/g
at 1 A/g
current
–30Wh/kg
under 300%
axial strain
95% aer
1000
stretching
cycles with
200% strain
208
Double-walled
CNT/poly(vinylidene
uoride-hexauoropropylene
(can sustain 50% lateral strain)
1-ethyl-3-
methylimidazolium
bis(triuoromethyl-
sulfonyl)imide
67.2 F/g at
2.5 A/g
current
3.7 kW/kg 20.3 Wh/kg
with 50%
lateral strain
97.9% aer
500 cycles of
50% lateral
strain
210
PPy-MnO2/CNT textile (can
sustain 21% tensile and 13%
bending strains)
H3PO4-PVA 461 F/g at 0.2
A/g current
22.1 kW/kg 31.1 Wh/kg 96.2% aer
750 000
bending
(13%) cycles
211
SWNT/honeycomb PDMS
(can sustain 150% stretch)
H3PO4-PVA 3.3 F/cm3at
10 V/s scan
– – 100% aer
150% stretch
212
PANI/MWCNT (can sustain
50% uniaxial strain and 40%
biaxial strain)
poly(ethylene
glycol)diacrylate/
Et4NBF4
6.1 F/cm3at
5 mA/cm3
current
3mW/cm
33.2 mWh/cm3Within 95%
up to 1000
cycles of 50%
uniaxial strain
213
Buckled CNT/PANI (can
sustain 200% strain)
H3PO4-PVA 364.6 F/g at
10 mV/s
scan.
2.34 to 30.04
mW/cm2
30.2 to 54.6
μWh/cm2
96.9% aer 20
stretching
cycles
214
MnO2-CNT (shell)/nylon
(core) (can sustain 150%
strain)
PVA-LiCl 5.4 mF/cm
(linear), 40.9
mF/cm2
(areal)
66.9 μW/cm22.6 μWh/cm287.8% with
17.1%
strainsfor
0–120% strain
cycles
215
CNT sheets coated on elastic
ber (can sustain up to 75%
strain)
H3PO4-PVA 41.4 F/g at
0.1 A/g
current
421 W/kg 0.363 Wh/kg 95% aer 100
stretching
cycles at 75%
strain
216
PEDOT-PSS/CNT/Elastic
wires (can sustain up to 350%
strain)
H3PO4-PVA 122.8 F/g at
0.5 A/g
current
– – 106% aer
100 stretching
cycles with
200% strain
217
(iv) Flexible/stretchable and transparent
In2O3nanowire/CNT
(∼60% transparent at 600 nm
and exible)
1 MLiClO464 F/g at 0.5
A/g
7.48 kW/kg 1.29 Wh/kg 82.8% aer
500 cycles at
0.5 A/g
182
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482 Natl Sci Rev, 2017, Vol. 4, No. 3 REVIEW
Table 3. Continued.
Electrode Electrolyte
Specic
capacitance Power density
Energy
density
Retention
capability Ref.
Wrinkled graphene (57%
transparent at 550 nm, can
sustain bending and 40% strain)
PVA/H3PO4/H2O7.6F/gat1
V/s scan
– – 100% aer
100 cycles
with bending
or 40% strain
209
SWNT (92% transparent at 550
nm and exible up to 180◦)
PVA/H3PO4178 F/g at
0.53 A/g
current
– – 100% up to
10 000 cycles
218
baery by improving both energy and power density
in a single electrochemical device.
Flexible, stretchable and even transparent
supercapacitors are also very important for the
next generation of wearable electronics. Although
compositing graphene and CNT with appropriate
conducting polymers has been demonstrated to be
an eective approach towards SCs with excellent
exibility and strain resistance while retaining their
electrochemical performance, conducting polymers
oen lose their inherent electrical properties in
composites.
As can be seen from the above discussions,
great progress has been achieved in the devel-
opment of carbon-based supercapacitors, includ-
ing EDLCs, PCs and HSCs in either traditional
or exible, stretchable and even transparent forms.
Compared with baeries, SCs possess a high power
density, short charging time, good discharge/charge
cyclability and broad-temperature-range applicabil-
ity. ese advantages make SCs useful for vari-
ous potential applications, including hybrid elec-
tric vehicles, renewable-energy-storage gadgets and
portable electronics. However, there are still some
challenges that need to be addressed to further im-
prove the performance of carbon-based electrodes.
First, carbon is well known to have a lower specic
capacitance as compared to other pseudocapacitive
materials, such as metal oxide and conducting poly-
mers. erefore, the carbon content, nature of het-
eroatom dopant(s), if any, and its crystallinity or
connectivity of a carbon network within composite
electrodes must be well understood in order to ob-
tain high electrode performance. Second, it is also es-
sential to further improve electrolytes and separators
for ecient charge storage as well as high-rate capa-
bility and cycling stability. ird, SCs with excellent
ac-line-ltering capability still need to be developed
to bring advanced SC technologies to the market-
place for various practical applications, ranging from
self-powered wearable optoelectronics to electrical
vehicles. Finally, the energy and power densities of
h 61h 8h 2
(b)
–90
–80
–70
–60
–50
–40
–30
–20
–10
0
10
100101102103104105
Phase angle (degrees)
Frequency (Hz)
AC filter
Vin
AC
C
R
Vout
Unzipped structure- 1 μm
Unzipped structure- 5 μm
NCNT- 5 μm
AEC
N-dopant-specific unzipping
Electrochemical oxidation
Nitrogen sites
Counter
part
O3/O2
(a)
H2O
(c)
Figure 15. (a) Schematic illustration of N-dopant-specic unzipping of NCNT.
(b) Aberration-corrected TEM images of 2, 8 and 16 h unzipped nanostructures (in 1
MH
2SO4at 0.8 V) (scale bar: 2 nm). (c) AC impedance phase angle versus frequency;
vertical dotted line indicates 120-Hz frequency. Reproduced with permission from ref.
[235]. Copyright of Macmillan Publishers Ltd (2016).
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on 31 August 2017
REVIEW Chen et al.483
SCs need to be further improved while their weight,
volume and cost to be further reduced.
ACKNOWLEDGEMENTS
We would like to thank colleagues, collaborators and peers for
their work cited in this article.
FUNDING
is work is supported by the National Science Founda-
tion (NSF-CMMI-1400274), the Air Force Oce of Sci-
entic Research (AFOSR-AF9550–12–1–0069), the Depart-
ment of Defense—Multidisciplinary University Research Initia-
tive (DoD-MURI-FA9550–12–1–0037) and the Dayton Area
Graduate Studies Institute (DAGSI-RQ20-CWRU-13–4).
Conict of interest statement. None declared.
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