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Zheng Bo,* Weiguang Zhu, Wei Ma, Zhenhai Wen,* Xiaorui Shuai, Junhong Chen,
Jianhua Yan, Zhihua Wang, Kefa Cen, and Xinliang Feng*
Supercapacitors, that is to say, electrochemical double-layer
(EDL) capacitors, a fast-rising class of energy-capture and
-storage devices with advantages beyond rechargeable batteries
in terms of power density and cycling stability, are considered
very promising candidates for emerging renewable energy
applications.[1] Typically, a supercapacitor with high-rate capability can tolerate charge/discharge at an elevated current level,
and thus is particularly attractive for high-power applications
such as heavy transport and electric vehicles. Considering the
important roles of the morphology/structure of active materials and the interface of active-layer/electrolyte on the rate
performance of a supercapacitor, continuous endeavors have
been directed towards the development and optimization of
advanced active materials. For instance, the rate capability of
graphene-based supercapacitors can be significantly improved
by mitigating the self-restacking of graphene sheets by inserting
carbon black nanoparticles, carbon spheres, or water into intersheet spaces.[2–4] However, the role exerted by the interface of
the active layer/current collector in implementing fast transportation of electrons has not received the attention it deserves.
It is well known that one of the major roles of current collectors in a supercapacitor is to effectively collect/transport charge
carriers (e.g., electrons) from/to the active materials during the
charge and discharge processes. Thus, a high-quality interfacial
contact between the current collector and the active materials
is highly desirable to reduce the contact resistance and lead
to high power and rate capabilities.[1,5–10] At the microscale,
Prof. Z. Bo, W. Zhu, W. Ma, X. Shuai, Prof. J. Yan,
Prof. Z. Wang, Prof. K. Cen
State Key Laboratory of Clean Energy Utilization
Institute for Thermal Power Engineering
Department of Energy Engineering
Zhejiang University
38 Zheda Road, Hangzhou
Zhejiang Province, 310027, P. R. China
E-mail: bozh@zju.edu.cn
Dr. Z. Wen, Prof. J. Chen
Department of Mechanical Engineering
University of Wisconsin-Milwaukee
3200 North Cramer Street
Milwaukee, Wisconsin, 53211, USA
E-mail: wen@uwm.edu
Dr. Z. Wen, Prof. X. Feng
Max Planck Institute for Polymer Research
Ackermannweg 10, 55128, Mainz, Germany
E-mail: feng@mpip-mainz.mpg.de
DOI: 10.1002/adma.201301794
Adv. Mater. 2013, 25, 5799–5806
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Vertically Oriented Graphene Bridging ActiveLayer/Current-Collector Interface for Ultrahigh Rate
Supercapacitors
neither the current-collector surface nor the active material
surface is ideally flat due to the manufacturing process and the
nature of the materials.[11] Based on classical contact theory, a
constriction/spreading resistance will be inevitably formed at
two surfaces meeting at a limited number of contact spots for
charge transport.[11,12] Consequently, such a situation will lead
to a considerable contact resistance at the asperity interface of
the active layer and the current collector.
According to Holm’s theory, the constriction/spreading
resistance is highly dependent on the electrical resistivity of
the contact spots.[12–14] Graphene, due to its superior electrical
conductivity, is therefore attractive to use as ideal “bridges” to
offer high-quality electrical links at the interface between current collector and active material. Since graphene shows higher
in-plane than out-of-plane electrical conductivity,[15–17] the ideal
graphene bridges should be perpendicular to the current-collector surface so that electrons can be efficiently transferred
along the graphene plane. In addition, perpendicular graphene
bridges can provide a large number of exposed graphene edges
as contact points, and thus further decrease the constriction/
spreading resistance. In our previous work, we reported the
rapid growth of vertically oriented graphene (VG) nanosheets
on various substrates (e.g., planar or cylindrical metals, and
carbon nanotubes) through plasma-enhanced chemical vapor
deposition (PECVD) without the introduction of catalysts,
binders, or other additives.[18–20] Successful applications of VG
in a wide range of fields, including as the active material for
supercapacitors, demonstrate its good electrical conductivity,
robust binding with the substrate, and high chemical tolerance
in electrolyte.[18,21–24] These favorable features of VG should be
beneficial for serving as a bridging interface of active layer/current collector in energy-storage devices.
Inspired by the above facts, we demonstrate herein that the
critical issue of contact resistance in a supercapacitor can be
solved to a large extent by building bridges of VG nanosheets
between the current collector and active materials. With respect
to the counterpart devices applying traditional current collectors,
the VG-bridged supercapacitor brings a significantly improved
rate performance, which was evidenced by a capacitance retention
of ca. 90% when the cyclic voltammetry (CV) scan rate increased
from 20 to 1000 mV s−1 or the galvanostatic charge/discharge
current density increased from 1 to 100 A g−1. The as-proposed
VG bridged supercapacitor, though it employs conventional graphene films as the active material, outperforms almost all of the
previously reported high-rate counterparts that use optimized/
improved graphene as active materials (Table S1, Supporting
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Figure 1. a–c) Optical images of graphene films: the original and vertically oriented graphene (graphene-bridged) coated nickel foams, and the fabricated VG/Foam-CC working electrode. d) SEM image of graphene film. e) SEM images of the wrinkled graphene film surface. f) Cross-sectional SEM
image of graphene film. g) SEM image of vertically oriented graphene-coated nickel foams. h) High-magnification SEM image of the graphene bridges
standing vertically on the nickel-foam surface. i) SEM image of the contact region between VG and graphene film.
Information).[2–4,25,26] Moreover, a power capability of the VGbridged graphene-film supercapacitor of up to 112.6 kW kg−1 was
achieved even at a high current density of 600 A g−1.
To demonstrate the advantages of VG bridges, four types of
working electrodes were fabricated for comparison by applying
graphene films as the active materials in a symmetric twoelectrode supercapacitor: i) without current collector (No-CC);
ii) with nickel-foil current collector (Foil-CC); iii) with nickelfoam current collector (Foam-CC); and iv) with VG-bridged
coated nickel-foam current collector (VG/Foam-CC).
The active materials (i.e., graphene films) were prepared
by vacuum filtration of reduced graphene oxide (GO) dispersion and dried at room temperature.[27] The as-prepared freestanding graphene films (Figure 1a) are highly flexible and
mechanically robust. The overall thickness of the flat graphene
film is estimated to be 8.0 μm, according to the scanning electron microscopy (SEM) measurement (Figure 1d). A small
number of wrinkles and ripples with open structures were
observed both on the surface of the film and between the layered stacking of graphene sheets (Figure 1e,f); these open intersheet channels are beneficial for the capacitive behavior.[3,21,28,29]
The graphene films were further characterized by using Raman
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spectroscopy and X-ray diffraction (XRD; Figure S1, Supporting
Information). The Raman spectrum indicates the formation
of graphene films with a D-to-G peak intensity ratio of 1.38,
in which the high D peak can be attributed to the presence of
defects, wrinkles, or ripples on graphene. According to the XRD
pattern, the interlayer spacing of graphene films was calculated
to be ca. 0.37 nm, a value that is slightly larger than that of pristine graphite (0.34 nm).
To construct the “bridges”, VG nanosheets were directly
grown on the surface of nickel foam current collector, via a
facile one-step PECVD process. With three minute growth,
a light black layer was formed on the nickel foam surface, as
shown in the optical (Figure 1b) and SEM images (Figure 1g).
Figure 1h further shows the SEM image of a close view of
the as-grown VG, which confirms that highly dense graphene
nanosheets were standing vertically on the nickel-foam surface.
The lateral length and height of a single VG nanosheet were
estimated to be ca. 150 nm. Details of the characterization of the
as-employed direct current (dc) normal glow discharge plasma,
and the as-grown VG, as well as the connection between VG
and growth substrate, can be found in our previous work.[18–20]
It is worth noting that, by using a microwave PECVD reactor, a
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Adv. Mater. 2013, 25, 5799–5806
As shown in Figure 2d, the first intercepts of Nyquist plots with
the x axis were almost the same for VG/Foam-CC and FoamCC, which indicates that the VG bridges and the bridges/
current-collector connection did not increase the series bulk
resistance. This observation can be attributed to the ultralow
mass and the good electrical conductivity of VG bridges, as well
as to the high solubility of carbon in nickel.[1,30,33] For various
systems reported previously, the loop at high frequencies in a
Nyquist plot was normally interpreted as the charge-transfer
resistance, the charge-transport resistance in active materials,
and/or the contact resistance.[4–8,10,24,33] Herein, the predominant EDL capacitive behavior was demonstrated by the above
EIS tests and the following CV measurements; consequently,
the charge-transfer resistance at the electrode/electrolyte does
not explain the loops. Meanwhile, it is reasonable to ignore
the influence of charge-transport resistance in active materials
since the same active material (i.e., graphene film) was used in
all four types of working electrodes. Therefore, it is reasonable
to attribute the loops for No-CC, Foil-CC, and Foam-CC to the
interfacial contact resistance.[5–10] As schematically shown in
Figure 2e, due to the surface roughness of both the nickel foil
and the graphene film, there are only a very limited number
of contact points at the contact interface for Foil-CC, which
implies that the electrons have to travel a long way to reach,
go through, and subsequently spread out of the constriction
during the charge/discharge processes.[11,12] The considerable
constriction/spreading resistance will certainly affect the electrochemical performance as a result of such a detoured electron
transport, especially when running at high charge/discharge
rates. As shown in Figure 2f, with a 3D-structured nickel-foam
current collector, the number of contact points can be increased
to a certain extent; however, such macroscale bulk contact is
still uneven and insufficient. However, when VG nanosheets
are used to bridge the active materials with the current collector, as shown in Figure 2g, the dense exposed edges of graphene could provide numerous pathways for charge transport,
significantly facilitating the electron transport through highly
electrically conductive graphene channels.
Figure 3a and 3b show the CV curves of four types of
working electrodes at scan rates of 20 and 1000 mV s−1,
respectively. For all the working electrodes at a relatively low
scan rate of 20 mV s−1, the resulting voltammogram for one
direction of potential sweep was almost the mirror image of
that generated with the reverse sweep, and the shapes of all
the CV curves were nearly rectangular. However, with a closer
view of the transition parts connecting the reverse sweeps,
it was found that the slopes of ΔI/ΔV, which indicates the
charge/discharge-rate responses to the applied potential,
became larger in the order of No-CC < Foil-CC < Foam-CC <
VG/Foam-CC.[1] This trend became more significant as the scan
rate was increased to a relatively high level (i.e., 1000 mV s−1),
where the shapes of the CV curves for No-CC, Foil-CC, and
Foam-CC were evidently distorted. Actually, the distorted
shape of CV curves started at a scan rate of ca. 100 mV s−1 for
No-CC, Foil-CC, and Foam-CC (Figure S4). With an increasing
scan rate, the VG/Foam-CC also manifested an enhanced
area of CV curve but maintained the quasirectangular shape
very well, as shown in Figure 3c. The specific capacitance
was calculated based on the CV curves (Figure S6). When
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COMMUNICATION
much sparser VG network with larger lateral length and height
for individual VG nanosheets can be obtained (Figure S2). The
dense network of VG produced by the current dc PECVD process has a tiny mass of ca. 0.01 mg, which can be negligible
in comparison to that of the original current collectors (nickel
foil: ca. 150 mg; nickel foam: ca. 63 mg) and graphene-film
active materials (ca. 0.45 mg). Thus, it is reasonable to ignore
the capacitance values contributed by the VG bridges with
respect to the two orders of magnitude greater capacitance
from the graphene film (Figure S3). To fabricate the VG/FoamCC electrode, both the graphene film and the VG-coated nickel
foam were dipped in electrolyte until completely wet, and the
graphene film was then attached onto the VG-coated nickel
foam. One side of the VG nanosheets was naturally connected
to the current collector, while the other side (i.e., a large number
of exposed sharp VG edges) could provide numerous contact
points to bridge the active materials (Figure 1i). Figure 1c
exhibits a digital photograph of the binder-free VG/FoamCC working electrode, corroborating the good structural and
mechanical integrity of the as-prepared electrode.
Electrochemical impedance spectroscopy (EIS) was conducted on No-CC, Foil-CC, Foam-CC, and VG/Foam-CC
working electrodes to investigate their dielectric and transport
properties. Figure 2a shows the phase response of the frequency for different working electrodes. Compared with the
other three counterparts, the VG/Foam-CC electrode presented a significantly enlarged frequency range for the capacitive features with phase angles of nearly −90°, and a ca. 1–2
orders of magnitude higher knee frequency (the maximum
frequency at which the capacitive storage of electrical energy
is dominant).[30] As shown in Figure 2b, the operating frequency at half maximum capacitance of the VG/Foam-CC electrode was 30.45 Hz, which was much higher than those of the
No-CC (0.65 Hz), Foil-CC (1.06 Hz), and Foam-CC (2.76 Hz)
(see calculation method in Section S6, Supporting Information)) electrodes. The corresponding characteristic relaxation time constant (CRTC) of the VG/Foam-CC electrode was
32.8 ms, significantly lower than those of the No-CC (1538.5 ms),
Foil-CC (943.4 ms), and Foam-CC (362.3 ms) electrodes. This
result suggests a better charge/discharge rate performance for
VG/Foam-CC-based supercapacitor than for the other counterparts.[2,31] The CRTC for VG/Foam-CC is also much lower than
that of freeze-dried (434.8 ms) and dried/thermally annealed
graphene films (2941.2 ms) with Pt foil as the current collector.[2] The Nyquist plot of VG/Foam-CC was nearly vertical
to the axis of the real component of the impedance (Figure 2c),
which indicates that the capacitive impedance was highly
dependent on the reciprocal of capacitance, consistent with an
ideal EDL capacitive behavior.[1] As estimated from the Nyquist
plots, the internal resistances of four types of working electrodes were in the order of VG/Foam-CC < Foam-CC < FoilCC < No-CC (Figure 2c). More interestingly, the distinctive loop
at high frequencies almost disappeared for VG/Foam-CC; the
rest of the Nyquist plot at high frequencies with a 45° slope,
usually termed Warburg impedance, was mostly related to ion
diffusion and transport in the electrolyte.[32]
A detailed comparison between the Nyquist plots of VG/
Foam-CC and Foam-CC was carried out to achieve a better
understanding of the role of VG bridges on contact impedance.
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200
a
90
80
-1
Specific capacitance (F g )
60
50
40
30
No-CC
20
10
Foam-CC
Foil-CC
0
-10
-2
10
-1
10
0
10
1
10
2
10
b
180
VG/Foam-CC
70
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160
140
VG/Foam-CC
120
100
80
Foam-CC
60
40
Foil-CC
20
3
4
10
5
10
No-CC
0
-2
10
10
-1
10
100
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VG/Foam-CC
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Foam-CC
Foil-CC
40
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20
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0
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2
3
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10
4
10
5
10
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20
40
60
80
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100
Imaginary component of impedance (Ohms)
Imaginary component of impedance (Ohms)
Frequency (Hz)
60
0
10
10
d
9
8
7
VG/Foam-CC
6
5
4
3
2
Foam-CC
1
0
0
2
4
6
8
10
12
Real component of impedance (Ohms)
14
16
Figure 2. a) Phase response as a function of frequency, b) frequency response of the capacitance, and c) Nyquist plots for No-CC, Foil-CC, FoamCC, and VG/Foam-CC working electrodes. d) Magnified portion of the Nyquist plots near the origin for Foam-CC and VG/Foam-CC. e–g) Schematic
illustration of electron transport between active materials and Foil-CC, Foam-CC, and VG/Foam-CC, respectively.
the scan rate was increased from 20 to 1000 mV s−1, the
specific capacitance decreased by 85.7%, 77.7%, and 54.1%
for No-CC (14.5 F g−1), Foil-CC (30.2 F g−1), and FoamCC (76.6 F g−1), respectively. In contrast, the VG/Foam-CC
retained a specific capacitance of 160.7 F g−1 at a high scan rate
of 1000 mV s−1, which was decreased by only 10.7% in specific
capacitance compared with the value at a scan rate of 20 mV
s−1 (Figure 3d). Remarkably, the CV curve still presented a
nearly rectangular shape and the VG morphology was kept
well after 2000 CV cycles at a high scan rate of 1000 mV s−1
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(Figure S5), which implies that the as-grown VG nanosheets
possess excellent mechanical strength and chemical
resistance.
Figure 4a shows the magnified galvanostatic charge/discharge plots in the high-potential range (0.9–1.0 V) for different
working electrodes at a relatively low current density of 1 A g−1.
The voltage (IR) drops were less than 40 mV for all the working
electrodes due to the low current density. Upon increasing the
current density to 20 A g−1, the IR drops became serious (>50 mV)
for No-CC, Foil-CC, and Foam-CC, while VG/Foam-CC only
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0.0015
a
0.04
VG/Foam-CC
0.0010
VG/Foam-CC
0.03
0.02
Current (A)
Current (A)
b
0.05
0.0005
No-CC
0.0000
Foil-CC
Foam-CC
-0.0005
Foam-CC
No-CC
0.00
-0.01
-0.03
-1
20 mV s
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-1
-0.04
1000 mV s
-0.05
0.9
0.0
0.1
0.2
Potential (V)
0.05
0.04
250
c
-1
Specific capacitance (F g )
Current (A)
0.01
0.00
-1
20 mV s
-1
50 mV s
-1
100 mV s
-1
200 mV s
-1
500 mV s
-1
1000 mV s
-0.02
-0.03
-0.04
0.0
0.1
0.2
0.3
0.5
0.5
0.6
0.7
0.8
0.9
0.6
0.7
0.8
0.9
No-CC
Foil-CC
Foam-CC
VG/Foam-CC
200
150
100
50
0
0.4
0.4
d
VG/Foam-CC
0.02
-0.01
0.3
Potential (V)
0.03
-0.05
Foil-CC
0.01
-0.02
-0.0010
-0.0015
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0.0020
0
200
400
600
800
1000
-1
Scan rate (mV s )
Potential (V)
Figure 3. CVs of different working electrodes at a scan rate of a) 20 and b) 1000 mV s−1. c) CVs of VG/Foam-CC supercapacitor at different scan rates
between 20 and 1000 mV s−1. d) Specific capacitance dependence on potential sweep rates for supercapacitors using different working electrodes.
Electrolyte: 6.0 m KOH aqueous solution.
showed an IR drop as tiny as 8 mV (Figure 4b). This trend
was more obvious with a further increase of the current density up to 50 A g−1 (Figure 4c). Actually, the IR drop of VG/
Foam-CC at 100 A g−1 is even comparable to those of No-CC,
Foil-CC, and Foam-CC running at 1 A g−1. Such significantly
reduced IR drop at high current densities can be attributed
to the VG-bridged high-quality contact at the interface of
active layer/current collector, which leads to an internal resistance value of 0.93 Ω that is much lower than those of No-CC
(15.30 Ω), Foil-CC (31.11 Ω), and Foam-CC (73.66 Ω), as shown
in Figure 4d. This result is consistent with the Nyquist plots
shown in Figure 2c. As expected, the specific capacitance of VG/
Foam-CC was less affected by the increasing charge/discharge
current density, as shown in Figure 4f, in stark contrast to that
of the No-CC, Foil-CC, and Foam-CC counterparts (Figure S6).
The Ragone plots shown in Figure 4g further demonstrate
the excellent rate performance of VG/Foam-CC with high
energy and power densities. At the same energy density, the
power density of VG/Foam-CC was ca. 1–2 orders of magnitude higher than that of the No-CC, Foil-CC, and Foam-CC
Adv. Mater. 2013, 25, 5799–5806
counterparts. For instance, at a high current density of
100 A g−1, the specific capacitance, energy density, and power
density of VG/Foam-CC were 156 F g−1, 4.98 W h kg−1, and
24.1 kW kg−1, respectively. Significantly, VG/Foam-CC showed
an ultrahigh power density of 112.6 kW kg−1 (specific capacitance of 130 F g−1) at a current density of 600 A g−1, much
higher than those of previously reported Foil-CC supercapacitors employing graphene-based active materials (e.g., highly
conductive graphene hydrogels, 30 kW kg−1; graphene film pillared by carbon black nanoparticles, 5.1 kW kg−1).[3,25]
To further clarify the role of VG bridges, especially the vertical orientation, on the enhanced rate and power capabilities
of a supercapacitor, control experiments were performed by
inserting lamellar graphene films (LGF, 0.2 mg) at the activelayer/current-collector interface. According to CV and EIS
tests on the LGF-bridged Foam-CC samples (Figure S7), the
specific capacitance decreased by 55.6% when the scan rate
was increased from 20 to 1000 mV s−1, and the CRTC was
calculated to be 603.4 ms; both values are significantly higher
than those of VG/Foam-CC (10.7% and 32.8 ms, respectively).
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1.00
a
IR
-1
20 A g
IR
IR
IR
0.98
b
1.0
-1
1Ag
IR
0.8
Potential (V)
Potential (V)
IR
0.96
VG/Foam-CC
Foil-CC
0.94
Foam-CC
0.6
VG/Foam-CC
0.4
Foam-CC
Foil-CC
0.2
0.92
No-CC
No-CC
1.0
60
80
100
120
140
Time (s)
160
180
200
0
2
IR
80
0.6
0.4
VG/Foam-CC
Foam-CC
0.2
1.0
1.5
2.0
2.5
Time (s)
1.0
IR
IR IR
10
3.0
3.5
No-CC
Foil-CC
Foam-CC
VG/Foam-CC
60
40
Ave. 31.11
Ave. 15.30
20
Ave. 0.93
0
1
10
4.0
IR
IR
8
Ave. 73.66
0
0.5
6
d
-1
50 A g
IR
0.0
0.0
4
Time (s)
100
c
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0.0
Internal resistance (Ohm)
0.90
2
3
10
10
-1
Current density (A g )
10
e
VG/Foam-CC
IR
Potential (V)
0.8
0.6
0.4
-1
100 A g
-1
400 A g
0.2
200 A g
-1
300 A g
-1
500 A g
-1
0.0
0.0
-1
600 A g
0.2
0.4
0.6
0.8
Time (s)
1.4
1.6
10
f
160
g
140
-1
Energy density (Wh kg )
-1
1.2
1
180
Specific capacitance (F g )
1.0
120
100
80
60
No-CC
Foil-CC
Foam-CC
VG/Foam-CC
40
20
0
0
10
No-CC
Foil-CC
Foam-CC
VG/Foam-CC
-1
0
100
200
300
400
-1
Current density (A g )
500
600
10
2
10
3
4
10
5
10
10
6
10
-1
Power Density (W kg )
Figure 4. Magnified portion at the initiation of discharge in galvanostatic charge/discharge plots for different working electrodes at a current density of a) 1 and b) 20 A g−1 for different working electrodes. c) Galvanostatic charge/discharge plots at a current density of 50 A g−1 for Foam-CC
and VG/Foam-CC. d) Internal resistances of different working electrodes (a pair) obtained from galvanostatic charge/discharge plots with different
current densities. e) Galvanostatic charge/discharge plots at relatively high current densities of ca. 100–600 A g−1 for VG/Foam-CC. f) Dependence
of specific capacitance on current density for different working electrodes. g) Ragone plots for different working electrodes. Electrolyte: 6.0 m KOH
aqueous solution.
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Synthesis of Vertically Oriented Graphene (Bridges): Vertically oriented
graphene nanosheets were grown using an atmospheric dc normal
glow discharge PECVD system.[18,19] A needle-shaped working electrode
connected to negative high voltage was used to provide a locally
enhanced electric field. The grounded nickel foam was used as the
passive electrode as well as the growth substrate. The interelectrode
voltage drop was about 1000 V during the synthesis. Prior to the growth,
the substrate was brought to 700 °C and held at this temperature for the
entire growth procedure. The growth was conducted using CH4/Ar/H2O
mixture (CH4: 10 vol.%; relative humidity: ca. 40%) as the precursor.
After 3 min growth at atmospheric pressure, the dc power was shut
down and the sample was cooled down to room temperature under the
protection of a mixture of Ar/H2.
Material Characterization and Electrochemical Measurements: The
morphologies and structures of materials were inspected by scanning
electron microscopy (SEM, SU-70, HITACHI). For electrochemical
tests, graphene-film-based supercapacitors were assembled in a twoelectrode system with a layered structure and all the components were
sandwiched between two pieces of plastic sheet.[38,41] Two pieces of
graphene film of ca.15 mm in diameter were separated by a porous
polypropylene film in a 6.0 m KOH aqueous electrolyte solution.
For No-CC, the graphene film was attached to a platinum wire. For
Foil-CC, Foam-CC, and VG/Foam-CC, the graphene films were attached
to a nickel foil, a nickel foam, and a vertically oriented graphenecoated nickel foam, respectively, and a platinum wire clipped onto
the end of each current collector. Electrochemical performances of
the supercapacitors were tested by cyclic voltammetry, galvanostatic
charge/discharge, and electrochemical impedance spectroscopy on an
electrochemical workstation (PGSTAT302N, Metrohm Autolab B.V.) at
room temperature.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors acknowledge financial support from the National Basic
Research Program of China (973 Program, No. 2011CB201500,
2012CB933404), ERC grant on 2DMATER. Z.B. acknowledges financial
support from the Zhejiang Provincial Natural Science Foundation of
China (No. LY13E020004), the Foundation of National Excellent Doctoral
Dissertation of China (No. 201238), the Specialized Research Fund for
the Doctoral Program of Higher Education (No. 20120101120140), and
the Qianjiang Talent Project of Zhejiang Province (No. 2012R10028). Z.B.
also acknowledges helpful discussion with Profs. Gaoquan Shi (Tsinghua
Univ.) and Yanwu Zhu (USTC) on test supercapacitor assembly.
Received: April 22, 2013
Revised: July 14, 2013
Published online: August 14, 2013
Experimental Section
Preparation of Graphene Film (Active Materials): Graphite oxide (GO)
was synthesized from graphite powder (XF010, XF NANO) by using a
modified Hummer's method. GO power was obtained after a drying
process under vacuum at 35 °C. Then the as-prepared GO powder
(250 mg) was dispersed in deionized water (1 L) and ultrasonicated
(FB15150, 300 W, Fisher Scientific) for 2 h. The resulting dispersion was
mixed with ammonia solution (4 mL, ca. 25–28 wt% in water, Sinopharm
Chemical Reagent Co. Ltd) and hydrazine hydrate (206 μL, 85 wt% in
water, Sinopharm Chemical Reagent Co. Ltd). The mixture was then kept
in a 95 °C oil bath and stirred for 1.5 h. Graphene film was fabricated
by vacuum filtration of the resulting dispersion through a membrane
filter of 0.22 μm in pore size, followed by air drying at room temperature.
Finally the graphene film was peeled off the membrane for use.
Adv. Mater. 2013, 25, 5799–5806
COMMUNICATION
Notably, the CRTC of the control sample is even slightly higher
than that of Foam-CC (362.3 ms), which indicates that the
addition of lamellar graphene films at the active-layer/currentcollector interface may cause a negative effect (e.g., increase
of the internal resistance between planar graphene sheets)
on the rate capability of a supercapacitor. Based on the literature, it is believed that the surface modification of current collectors is a feasible and promising alternative to improve the
power and rate capabilities of supercapacitors. Possible routes
could include electrochemical etching, growth of metal nanostructures, coating, deposition, and decoration of carbon materials (e.g., fullerenes, carbon nanotubes, mesoporous carbon
capsules, and graphene/graphene-based derivatives). [8,34–37]
On the other hand, a variety of parameters, including the morphology of additives, structure, element composition, bulk
resistance, mechanical strength, and chemical stability, should
be carefully considered to lower internal contact resistance and
improve device performance.
In conclusion, we demonstrate that the introduction of VG
bridges on a nickel-foam current collector can greatly reduce the
constriction/spreading resistance caused by the limited contact
points at the active-layer/current-collector interface. By taking
advantage of the unique growth orientation and ultrahigh inplane electrical conductivity of graphene, dense exposed sharp
edges, robust binding with the substrate, and high chemical
tolerance, VG nanosheets can build up a short-cut and highspeed bridge between the current collector and active materials
to facilitate electron transport during the charge/discharge processes, implementing supercapacitors with outstanding rate
and power capabilities. As a result, the VG-bridged supercapacitor using conventional graphene films as the active materials exhibits a capacitance maintenance of about 90% with a
two-order increase of the charge/discharge current density or a
50-fold increase of the CV scan rate.[2–4] Further improvement
of the VG-bridged supercapacitor performance is envisaged by
optimization of the active materials.[2,3,38–40] Notably, the advantages of the VG/Foam-CC working electrode over its Foam-CC
counterpart were also demonstrated in an organic electrolyte
system (Figure S8). We expect that this work will open up new
opportunities for the application of VG bridges in a broad range
of emerging electrochemical energy storage and conversion
devices, for example, EDL-, pseudo-, and hybrid capacitors,
as well as secondary batteries, to advance the corresponding
performances.
[1] B. E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, Kluwer Academic/Plenum Publisher, New York 1999.
[2] X. Yang, J. Zhu, L. Qiu, D. Li, Adv. Mater. 2011, 23, 2833.
[3] G. Wang, X. Sun, F. Lu, H. Sun, M. Yu, W. Jiang, C. Liu, J. Lian,
Small 2011, 8, 452.
[4] Z. Lei, N. Christov, X. S. Zhao, Energy Environ. Sci. 2011, 4, 1866.
[5] S. Biswas, L. T. Drzal, Chem. Mater. 2010, 22, 5667.
[6] M. M. Shaijumon, F. S. Ou, L. Ci, P. M. Ajayan, Chem. Commun.
2008, 44, 2373.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
5805
www.advmat.de
COMMUNICATION
www.MaterialsViews.com
5806
[7] K. H. An, W. S. Kim, Y. S. Park, J. M. Moon, D. J. Bae, S. C. Lim,
Y. S. Lee, Y. H. Lee, Adv. Funct. Mater. 2001, 11, 387.
[8] C.-W. Huang, H. Teng, J. Electrochem. Soc. 2008, 155, A739.
[9] C. S. Du, J. Yeh, N. Pan, Nanotechnology 2005, 16, 350.
[10] K. H. An, W. S. Kim, Y. S. Park, Y. C. Choi, S. M. Lee, D. C. Chung,
D. J. Bae, S. C. Lim, Y. H. Lee, Adv. Mater. 2001, 13, 497.
[11] L. Li, J. E. Morris, IEEE Trans. Compon., Packag., Manuf. Technol.,
Part A 1997, 20, 3.
[12] R. Holm, Electric Contacts: Theory and Application, Springer, Berlin,
Germany 2000.
[13] S. V. Morozov, K. S. Novoselov, M. I. Katsnelson, F. Schedin,
D. C. Elias, J. A. Jaszczak, A. K. Geim, Phys. Rev. Lett. 2008, 100,
016602.
[14] X. Du, I. Skachko, A. Barker, E. Y. Andrei, Nat. Nanotechnol. 2008, 3,
491.
[15] V. Varshney, S. S. Patnaik, A. K. Roy, G. Froudakis, B. L. Farmer,
ACS Nano 2010, 4, 1153.
[16] A. G. Moghaddam, M. Zareyan, Phys. Rev. B 2009, 79, 073401.
[17] A. Bachtold, C. Strunk, J. P. Salvetat, J. M. Bonard, L. Forro,
T. Nussbaumer, C. Schonenberger, Nature 1999, 397, 673.
[18] Z. Bo, K. Yu, G. Lu, S. Cui, S. Mao, J. Chen, Energy Environ. Sci.
2011, 4, 2525.
[19] Z. Bo, K. H. Yu, G. H. Lu, P. X. Wang, S. Mao, J. H. Chen, Carbon
2011, 49, 1849.
[20] K. H. Yu, G. H. Lu, Z. Bo, S. Mao, J. H. Chen, J. Phys. Chem. Lett.
2011, 2, 1556.
[21] J. R. Miller, R. A. Outlaw, B. C. Holloway, Science 2010, 329, 1637.
[22] L. Jiang, T. Yang, F. Liu, J. Dong, Z. Yao, C. Shen, S. Deng, N. Xu,
Y. Liu, H.-J. Gao, Adv. Mater. 2012, 25, 250.
[23] Z. Bo, Y. Yang, J. Chen, K. Yu, J. Yan, K. Cen, Nanoscale 2013, 5,
5180.
[24] Z. Bo, Z. Wen, H. Kim, G. Lu, K. Yu, J. Chen, Carbon 2012, 50, 4379.
[25] L. Zhang, G. Shi, J. Phys. Chem. C 2011, 115, 17206.
wileyonlinelibrary.com
[26] C.-M. Chen, Q. Zhang, C.-H. Huang, X.-C. Zhao, B.-S. Zhang,
Q.-Q. Kong, M.-Z. Wang, Y.-G. Yang, R. Cai, D. S. Su, Chem.
Commun. 2012, 48, 7149.
[27] D. Li, M. B. Mueller, S. Gilje, R. B. Kaner, G. G. Wallace, Nat. Nanotechnol. 2008, 3, 101.
[28] H. Gwon, H.-S. Kim, K. U. Lee, D.-H. Seo, Y. C. Park, Y.-S. Lee,
B. T. Ahn, K. Kang, Energy Environ. Sci. 2011, 4, 1277.
[29] W. Lv, D. M. Tang, Y. B. He, C. H. You, Z. Q. Shi, X. C. Chen,
C. M. Chen, P. X. Hou, C. Liu, Q. H. Yang, ACS Nano 2009, 3, 3730.
[30] M. Hughes, M. S. P. Shaffer, A. C. Renouf, C. Singh, G. Z. Chen,
J. Fray, A. H. Windle, Adv. Mater. 2002, 14, 382.
[31] A. Kajdos, A. Kvit, F. Jones, J. Jagiello, G. Yushin, J. Am. Chem. Soc.
2010, 132, 3252.
[32] T. Y. Kim, H. W. Lee, M. Stoller, D. R. Dreyer, C. W. Bielawski,
R. S. Ruoff, K. S. Suh, ACS Nano 2011, 5, 436.
[33] X. Li, J. Rong, B. Wei, ACS Nano 2010, 4, 6039.
[34] C. Portet, P. L. Taberna, P. Simon, C. Laberty-Robert, Electrochim.
Acta 2004, 49, 905.
[35] P. L. Taberna, C. Portet, P. Simon, Appl. Phys. A: Mater. Sci. Process
2006, 82, 639.
[36] S. Murali, D. R. Dreyer, P. Valle-Vigon, M. D. Stoller, Y. W. Zhu,
C. Morales, A. B. Fuertes, C. W. Bielawski, R. S. Ruoff, Phys. Chem.
Chem. Phys. 2011, 13, 2652.
[37] J. R. Potts, D. R. Dreyer, C. W. Bielawski, R. S. Ruoff, Polymer 2011, 52, 5.
[38] Y. W. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. W. Cai,
P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes,
D. Su, E. A. Stach, R. S. Ruoff, Science 2011, 332, 1537.
[39] Z. Wen, X. Wang, S. Mao, Z. Bo, H. Kim, S. Cui, G. Lu, X. Feng,
J. Chen, Adv. Mater. 2012, 24, 5610.
[40] L. L. Zhang, X. Zhao, M. D. Stoller, Y. Zhu, H. Ji, S. Murali, Y. Wu,
S. Perales, B. Clevenger, R. S. Ruoff, Nano Lett. 2012, 12, 1806.
[41] M. D. Stoller, S. J. Park, Y. W. Zhu, J. H. An, R. S. Ruoff, Nano Lett.
2008, 8, 3498.
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2013, 25, 5799–5806