CARBON
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
One-step fabrication and capacitive behavior
of electrochemical double layer capacitor electrodes using
vertically-oriented graphene directly grown on metal
Zheng Bo
a,b
, Zhenhai Wen b, Haejune Kim b, Ganhua Lu b, Kehan Yu b, Junhong Chen
b,*
a
State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, 38 Zheda Road, Hangzhou,
Zhejiang 310027, China
b
Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, WI 53211, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
We report on a one-step binder-free fabrication method for electrochemical double layer
Received 27 March 2012
(EDL) capacitor electrodes consisting of vertically-oriented graphene uniformly grown on
Accepted 10 May 2012
a metallic current collector. The double-layer capacitive behavior of the resulting electrode
Available online 18 May 2012
is studied in both aqueous and organic electrolytes. Compared with conventional graphene-based EDL capacitor electrode fabrication methods, this method offers the following
advantages: (a) no need to use a binder, (b) open channels for better ion access, and (c)
exposed edge planes for improved material wettability. These unique features lead to
excellent capacitive behavior in organic electrolytes, including a specific capacitance
slightly higher than that in aqueous electrolytes at the same potential scan rate and a high
knee frequency (3174 Hz in the current work).
Ó 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon materials in various forms and with high surface areas
have been widely employed as the active material of electrochemical double layer (EDL) capacitors (sometimes referred
to as ‘supercapacitors’ or ‘ultracapacitors’). Among which
activated carbons and carbon fibers are the most widely used
[1,2], and graphene-based materials, e.g., graphene/graphene
oxide (GO), are latest but promising candidates for the next
generation EDL capacitors [3–5]. A single layer graphene is
basically a two-dimensional (2D) lattice of sp2 carbon atoms
densely packed into a hexagonal structure and covalently
bonded along two planar directions [3,6]. The widespread
interest of employing graphene-based materials as the capacitor electrode initially stems from graphene’s high electrical
conductivity, good chemical stability, superior mechanical
strength and durability [6,7], and most importantly, its exceptionally high specific area up to >2600 m2/g [3,8]. The physical
charge mechanism of EDL capacitors, i.e., capacitance comes
from the charge accumulated at the electrode/electrolyte
interface, makes the high specific area of active material an
essential requirement for a high capacitance value. For graphene, a specific capacitance as high as 550 F/g was predicted
based on the full utilization of its huge surface area [3].
Current practice of fabricating graphene-based EDL capacitor electrode usually consists of three steps: the synthesis of
GO by the modified Hummer’s method, the reduction of GO
using thermal or chemical methods, and finally the assembly
of reduced GO on current collectors using binders [3,8]. A
major drawback of this method limiting the capacitive performance is the rapid restacking of graphene sheets due to the
strong van der Waals interactions, which would strongly reduce the inter-sheet open channels and decrease the actual
electrochemically accessible electrode surface area [8,9]. Hierarchical architectures were thus proposed by adding nanospacers between graphene sheets, such as carbon nanotubes
* Corresponding author: Fax: +1 414 229 6958.
E-mail address: jhchen@uwm.edu (J. Chen).
0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbon.2012.05.014
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(CNTs), mesoporous carbon spheres, and polyaniline nanofibers [10–12].
As an alternative, plasma-enhanced chemical vapor deposition (PECVD) techniques can directly produce graphene
sheets vertically standing on the substrate (or the current collector for capacitors). The inherent non-stacking morphology
of the as-grown vertically-oriented graphene (VG) sheets
could favor the ion diffusion and storage. Moreover, the vertical direction of VG growth on a substrate could potentially favor the intrinsic wettability of VG in organic electrolytes.
Compared with the aqueous electrolyte, the organic electrolyte has a higher voltage window, and thus potentially could
provide a greater specific energy which is the main concern
of EDL capacitors compared with batteries (theoretically, the
specific energy of EDL capacitor is proportional to the square
of the voltage window) [13]. Another advantage of this alternative method to produce capacitor electrode is avoiding
the introduction of a binder and its negative effects on the
material itself and the resulting capacitive behavior.
The PECVD-grown VG sheets are maze-like two-dimensional (2D) or flower-like three-dimensional (3D) networks of
graphene nanosheets vertically standing on the substrate,
which could be adjusted by growth parameters such as gaseous precursor [14], gas/substrate temperature [15], electric
field [16], and growth time [17]. Our group has recently demonstrated that 2D VG sheets are excellent atmospheric corona
discharge electrodes featuring a lower inception voltage and
minimized detrimental ozone emission, due to the larger lateral size of individual sheets, the exposure of sharp graphene
edges and an adequate inter-sheet spacing [18]. As for VGbased EDL capacitor applications, Zhao et al. have reported
the capacitive behavior of VG sheets grown on carbon cloth
in an aqueous electrolyte [19], and very recently Miller et al.
reported a EDL capacitor using VG sheets grown on a metallic
substrate as the active material, where more attention was
paid to its alternative current (ac) line-filtering performance
rather than its detailed capacitive performance such as specific capacitance, specific energy and specific power, and cycle
durability [20]. According to [21] where VG sheets were grown
on the top of an as-oxidized VG layer, the highly-branched VG
sheets seem to have a higher specific surface area, and thus
could favor capacitance applications.
In this work, one-step atmospheric PECVD fabrication of
highly-branched VG based capacitor electrodes and their
EDL capacitance are demonstrated. Detailed two-terminal
electrical responses of the as-fabricated VG sheet based electrodes are reported, including gravimetric and volumetric
specific capacitances, specific energy and specific power, the
cycle durability, and the electrical impedance.
2.
Experimental
2.1.
Electrode fabrication
The one-step electrode fabrication was realized by directly
synthesizing VG sheets on a 0.025-mm thick stainless steel
foil (type 304, Fe:Cr:Ni = 70:19:11 wt.%, Alfa Aesar) using an
atmospheric direct current (dc) normal glow discharge PECVD
technique, without the deliberate introduction of binders or
catalysts. A pin-to-wire electrode arrangement housed in a
quartz tube was used to create negative normal glow discharge plasma. A conical profile tungsten pin (taper: 1:5,
tip radius: 0.01 mm) was connected to a negative direct current high voltage (dcHV, 0 to 10 kV adjustable, 4000 Series,
EMCO High Voltage Corporation) and used as the discharge
electrode (cathode). A grounded stainless steel foil of
0.025 mm in thickness was used as the passive electrode (anode) and the substrate for VG synthesis. The inter-electrode
gap was fixed at 8 mm. Prior to the growth, the substrate
was brought to 700 °C using a tube furnace (Lindberg/blue M
TF55035 A-1) and held at that temperature for 10 min in an
Ar/H2 flow (1% H2 by volume) of 1000 standard cubic centimeters per minute (sccm). And then the negative dcHV was applied to the cathode to produce and sustain the normal
glow discharge plasma with the feed gases of CH4/H2O/Ar
mixture (CH4: 150 sccm, Ar: 1350 sccm, relative humidity:
32.3%). The whole process was conducted at atmospheric
pressure, which is different with prior PECVD synthesis conducted in vacuum or low pressure applying other plasma
sources [16,22,23], holding great potential for mass-production with a relatively high growth rate. Detailed VG growth
mechanism with this dc normal glow PECVD technique has
been discussed in our previous work [24,25].
2.2.
Material characterization
Transmission electron microscopy (TEM) analysis was performed with a Hitachi H 9000 NAR TEM, which has a point resolution of 0.18 nm at 300 kV in the phase contrast highresolution TEM (HRTEM) imaging mode. Scanning electron
microscopy (SEM) analysis was performed with a Hitachi
S-4800 SEM, which has a resolution of 1.4 nm at 1 kV acceleration voltage.To determine the interlayer spacing of VG sheets,
ten samples at three randomly selected positions were used to
calculate the graphene interlayer spacing, as shown on Fig. 1g.
The average interlayer spacing d is calculated as:
n
X
di
¼ i¼1
d
n
where n is the sample number, i.e., 10 in the current work.
The precision error Dd is calculated according to the
Student t-Distribution:
d
Dd ¼ ta=2 pffiffiffi
n
where ta is 2.262 (degree of freedom is set as n1 = 9), assuming a 95% confidence level, i.e., a/2 = 0.025; the sample standard deviation d was calculated as:
Fig. 1 – Components of a capacitor coin cell applying VGcoated electrode.
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"
#12
n
1 X
2
d¼
ðIi IÞ :
n 1 i¼1
2.3.
Electrochemical measurements
The electrochemical performance of the working electrode,
i.e., VG sheets-coated stainless steel foil, was evaluated using
a two-electrode configuration. The electrolytes used in the
current work included an aqueous electrolyte (6 M KOH) and
an organic electrolyte (1 M tetraethylammonium tetrafluoroborate (TEABF4) in acetonitrile (AN) solvent). The capacitor
test cells were assembled with the commonly used equipment for battery coin cells (CR2032). As shown in Fig. 1, the
test cell consisted of a metal cap, a metal case with polymer
seal, a spring, two stainless steel spacers, two current collectors coated with active materials (i.e., VG), and a membrane
separator. The capacitor stack (electrodes and separator)
was positioned between two spacers. Two symmetric
electrodes electrically separated by a membrane (hydrophilic
Millipore PVDF membrane for KOH electrolyte and Celgard3501 for TEABF4/AN electrolyte). A coin-cell crimper (MSK110) was used to seal the coin cell with a pressure of 100 kg/
cm2. For the capacitor test cells employing organic electrolyte,
the above assembly procedure was conducted in the glovebox to avoid oxygen and moisture.
With the CV curves, the gravimetric specific capacitance of
a single electrode (Cm, unit: F/g) was calculated as:
Z
Cm ¼ 2 IdV=ðv DV mÞ
where I is the response current (unit: A), v is the potential
scan rate (unit: V/s), DV is the potential window (unit: V),
and m is the mass of active materials in a single electrode
(unit: g), respectively. With the galvanostatic charge/discharge
plots, the specific capacitance of a single electrode (Cm, unit:
F/g) was calculated as:
Cm ¼ 2iDt=ðDv mÞ
where i is the constant discharge current (unit: A), Dt is the
discharge time (unit: s, excluding the time for IR drop), and Dv
is the voltage drop upon discharging (unit: V, excluding the IR
drop), respectively. The volumetric capacitance (Cv, unit: F/
cm3) was calculated as:
Cv ¼ Cm m=vol:
where vol. is the volume of the active material (unit: cm3). The
specific energy E (unit: Wh/kg) of the cell was calculated using
the following equation:
E¼
1000 Cm DV2
4 2 3600
where DV is the potential window (unit: V). The maximum
specific power P (unit: kW/kg) was calculated from the low
frequency data of the impedance spectra according to the
equation:
Pmax ¼
DV2
4MRs
where M is the mass of two electrodes (including the stainless
steel current collectors, unit: g), and Rs (unit: X) is the equivalent series resistance (ESR) obtained from the x-intercept of
the Nyquist plot in Fig. 7a and b.
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The mass difference of the current collector before and
after the VG synthesis (defined as Mbef and Maft, respectively)
cannot be directly used as the mass of the active material (m),
because the stainless steel foil can be easily oxidized at a temperature of 700 °C for 45-min growth with the presence of
moisture in the feed gases. Consequently, to determine the
mass contribution from the current collector oxidation, we
first conducted ten sets of control experiments. At each
experiment, we conducted the ‘synthesis’ without the carbon
precursor (methane); all other parameters (current collector
size, growth temperature, inter-electrode spacing, etc.) were
kept the same as those used for the real synthesis process.
The mass change of the substrate during the control experiment was solely attributed to the oxidation of the substrate,
and a mass increase ratio Ro can be determined. Therefore,
the mass of the active material VG (m) can be calculated as:
m ¼ Maft Mbef Mbef Ro
The volume of the active material (vol., unit: cm3) was calculated as:
vol ¼ SHð1 lÞ
where S and H are the VG growth area and growth height,
respectively, and l is a dimensionless coefficient reflecting
the free space between VG clusters. For the current work, l
was estimated as 0.4 according to SEM images.
3.
Results and discussion
3.1.
Material morphology and structure
TEM imaging, HRTEM imaging and SEM imaging were used to
characterize the as-grown deposits for different synthesis
time. Taking advantage of the atmospheric synthesis, the initial nucleation [24] was quick and completed in about 30 s.
After 5-min synthesis, continuous 2D VG sheet networks with
an average graphene lateral size of 1 lm were grown on the
metal foil, as shown in Fig. 2a. The morphology of the asgrown 2D VG sheets is similar to that used in Miller et al. work
[20], while the growth rate of our VG sheets could be much
higher due to the atmospheric synthesis [24]. Then the VG
sheets coagulated with the increase of growth time, as shown
in Fig. 2b for the 10-min growth. Finally with 45-min growth,
highly branched VG networks were formed, as shown in
Fig. 2c.
As shown in Fig. 2d, after 45-min growth, a VG layer of
4 lm in height was synthesized on the metallic substrate
in a relatively uniform manner, and meanwhile, the one-step
fabrication of VG-based capacitor electrode was achieved
without the need of binders. The diameter of the quasi-circle
VG-coated area on the current collector was measured as
6.5 mm. HRTEM image shown in Fig. 2e reveals that the
as-grown VG networks consisted of few-layer (3–7 layers
mainly) graphene sheets. As shown in Fig. 2g, the interlayer
spacing was calculated as 0.376 ± 0.018 nm, larger than the
d-spacing of graphite (0.335 nm). More importantly, there
was no obvious graphene stacking observed. The non-stacking VG sheets is expected to favor both the effective surface
area and the ion diffusion [3], as schematically shown on
Fig. 2h. As shown in the HRTEM image of Fig. 2f, VG sheets
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Fig. 2 – Top view SEM images of VG sheets after (a) 5-min, (b) 10-min, and (c) 45-min growth; (d) side view SEM image of VG
sheets after 45-min growth on stainless steel (inset: higher magnification image); HRTEM images of (e) few-layer VG sheets
and (f) interface between VG and the current collector; (g) graphene interlayer spacing at three randomly selected positions;
(h) schematic of ion diffusion in horizontal and vertical graphene.
with an interlayer spacing of 0.355 nm were strongly connected to the current collector surface; the interlayer spacing
of 0.488 nm is likely from the substrate alloys and compos-
ites (i.e., Fe/Cr/Ni/C) and/or their oxides. Our previous work
has demonstrated the excellent binding stability between
VG sheets and metal substrate, especially when using
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Fig. 3 – Comparison of CV curves of VG-coated stainless steel and bare stainless steel at a scan rate of 1000 mV/s in (a) 6 M
KOH and (b) 1 M TEABF4/AN.
Fig. 4 – CV curves of VG-coated stainless steel at different scan rates in (a) 6 M KOH and (b) 1 M TEABF4/AN.
stainless steel as the substrate material [18]. The robust binding between the active material and the current collector, as
shown in Fig. 2f, could play an essential role in the current
binder-free capacitor electrode assembly process.
3.2.
Cyclic voltammetry measurements
Cyclic voltammetry (CV) measurements were conducted on
the assembled test cells. Fig. 3a and b compare CV curves of
a VG-coated stainless steel electrode and a bare stainless steel
electrode at a scan rate of 1000 mV/s. For all test cells using
either electrolyte, the resulting voltammogram for one direction of potential sweep (e.g., positive-moving) was almost the
mirror image of that generated with the opposite direction of
sweep (e.g., negative-moving). The capacitance of VG-coated
electrode was one order of magnitude higher than that of
the bare stainless steel counterpart, indicating the significantly enlarged surface area (through VG sheets) available
for charge storage.
Fig. 4a and b shows the CV curves of VG-coated stainless
steel electrode at scan rates varying from 10 to 1000 mV/s in
both electrolytes. The CV curve area was gradually enlarged
with the increase of the voltage scan rate, while the shape
was kept as quasi-rectangular shape even at a scan rate as
high as 1000 mV/s, which indicates the predominant EDL
capacitive behavior [8,26,27]. The slight deviation from the
standard rectangular shape, especially at a relatively high
scan rate, could be attributed to the distributed charge storage
which is a classical porous electrode behavior [28–30]. The
Ohmic resistance in the electrolyte along the direction from
the open channel mouth to the bottom will result in a
potential difference; for VG-based active materials, the porous structure is mainly from the VG interlayer and intersheet
channels. Furthermore, for the current work, the passive
oxide layer on the stainless steel current collector could also
work as a parallel RC circuit in series with the EDL
capacitance.
At the scan rate of 20 mV/s, the gravimetric specific capacitance of a VG-coated electrode in 6 M KOH and 1 M TEABF4/
AN was calculated as 129 and 112 F/g, respectively. The specific capacitance was also determined from the galvanostatic
charge/discharge tests, based on the calculation from the
slope of the discharge curves as shown in Fig. 5a and b for
capacitors using 6 M KOH and 1 M TEABF4/AN, respectively.
Under a constant discharge current density of 1 A/g, the gravimetric and volumetric specific capacitances of a VG-coated
electrode in 6 M KOH and 1 M TEABF4/AN were calculated as
140 F/g (53 F/cm3) and 132 F/g (50 F/cm3), respectively. With-
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Fig. 5 – Galvanostatic charge/discharge curves of VG-coated stainless steel in (a) 6 M KOH and (b) 1 M TEABF4/AN at a constant
current density of 1 A/g.
out any optimization on the VG interlayer and the intersheet
spacings, these capacitance values are already comparable
with those reported for horizontally oriented graphene and
traditional activated carbon or carbon powders, but higher
than many of those reported for CNTs [8,27,31–34]. An
overly-small graphene spacing will increase the surface tension and hinder the formation of electric double-layer capacitance, while an overly-large graphene spacing will decrease
the specific surface area. For the current work, the interlayer
and intersheet spacing of the VG layer after 45-min growth
are 0.376 and few (2) nm, respectively. The hydrated ion
sizes of aqueous cations are at the level of 0.5 nm, and for
organic electrolytes, this size could be several times larger.
Consequently, further improvement on the specific capacitance is likely through optimization of morphology and structure of VG and the capacitor assembly process [8,35–37].
Beyond the specific capacitance itself, an interesting finding is that, for a potential scan rate higher than 50 mV/s, the
specific capacitance of the VG-coated electrode in 1 M TEABF4/AN was very close to or even slightly higher than that in
6 M KOH (i.e., CTEABF4/AN/CKOH = 95.6% and 91.9% for 500 and
1000 mV/s; CTEABF4/AN/CKOH = 104.8%, 108.1%, and 101.1% for
50, 100, and 200 mV/s), which is opposite to the typical trend
observed for activated carbon and horizontal graphene sheets
in these two types of electrolytes [8,20,37]. The capacitance
ratio (capacitance in organic electrolyte to that in aqueous
electrolyte) for horizontal graphene based supercapacitors
usually decreases with an increasing scan rate [8,38], while
the capacitance ratio in the current work maintained at a value higher than 90% even for a scan rate up to 1000 mV/s. This
result is, to a certain extent, in agreement with Miller et al.
work on 2D VG-based supercapacitor, which suggested that
the organic electrolyte capacitance could be 50% higher
than that of aqueous electrolyte [20]. This observation could
be explained as that, compared with horizontal graphene
sheets, the vertical direction and the non-agglomerated morphology could favor better wettability of graphene in organic
electrolyte. The relatively high capacitance density of VG in
TEABF4/AN is advantageous in terms of specific energy performance, due to the higher potential window in 1 M TEABF4/AN
(2.2 V) than that in 6 M KOH (0.9 V). For the current work, the
as-calculated specific energy of VG-coated electrode in 1 M
TEABF4/AN (potential window: 2.2 V) was 5.18–6.46 times
higher than that in 6 M KOH (potential window: 0.9 V) for
the same potential scan rate.
The cycle durability was tested by continuous CV measurements at the scan rate of 100 mV/s. Fig. 6a and b shows
the capacitance retention in 6 M KOH and 1 M TEABF4/AN
after 2900 and 1150 cycles, respectively. Both the shape and
the area of the CV curves were kept nearly the same as shown
in the insets of Fig. 6a and b, further proving that the pseudocontributions from functional surface groups, impurities, or
electrolyte redox couples [39], are minimal. The capacitance
in 6 M KOH slightly decreased for the initial 500 cycles with
a maximum capacitance decay of only 2.37%, then followed
by an increase of 4.30% up to 2900 cycles, which could be
attributed to the improved wettability over time. The capacitance in 1 M TEABF4/AN slightly decreased by 4.98% within
the initial 400 cycles and then exhibited excellent stability
with only a 0.75% decay up to 1150 cycles.
3.3.
Electrical impedance spectroscopy test
Electrical impedance spectroscopy (EIS, amplitude 5 mV in
the frequency range of 100 kHz–5 mHz) employing alternating
current (ac) impedance measurement was conducted on the
test cells. EIS data were presented using Nyquist and Bode
plots as shown in Fig. 7. Fig. 7a and b shows the complexplane (Nyquist) impedance for both electrolytes, which is
indicative of the capacitive characteristics (imaginary component, Z00 ) versus the Ohmic impedance (real component, Z 0 ) of
a test cell. Fig. 7c and d shows the Bode plots on the capacitor’s frequency response. For ideal capacitors with a series
RC circuit, the Nyquist plot should be vertical, i.e., the capacitive impedance is totally dependent on 1/C [13], and the
phase angle in the Bode plot at low frequencies should be
90°. For the current work, the imaginary component Z00
presents a sharp increase with a near-vertical line at low frequencies (as shown in Fig. 7a and b), and the phase angle at
low frequencies was nearly 80° but could not reach 90°
(as shown in Fig. 7c and d). This observation indicates the predominant EDL capacitance but likely with additional contri-
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Fig. 6 – Cycle durability in (a) 6 M KOH and (b) 1 M TEABF4/AN with insets of CV curves corresponding to different number of
cycles. Insets: CV curves at different cycles.
Fig. 7 – Complex-plane (Nyquist) impedance plots for cells with different electrolytes: (a) 6 M KOH and (b) 1 M TEABF4/AN;
Bode plots for cells with different electrolytes: (c) 6 M KOH and (d) 1 M TEABF4/AN. Insets: magnified portion of the Nyquist
plots near the origin.
bution from distributed charge storage. As discussed above on
the CV curves, the distributed charge storage could result
from the porous structure. We further prepared VG sheets-
coated nickel foils as the working electrode, where the fabrication procedure and parameters were kept the same but the
oxidation of the substrate (current collector) was reduced.
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Moreover, carbon has a relatively high solubility in nickel and
it is relatively easy to make Ohmic contact to it, while capacitive contact is usually made with the stainless steel. The EIS
test results (Nyquist and Bold plots) of the capacitor in 6 M
KOH electrolyte are shown in Fig. S1a and b, respectively (Supplementary data). It can be seen that, with a nickel foil as the
current collector, the Nyquist plot at low frequencies is more
vertical and the phase angle at low frequencies is around 85°
to 87°.
The insets of Fig. 7a and b are the magnified portion of the
Nyquist plots near the origin, which can provide more detailed information on the electrode impedance behavior at
high frequencies. The first intercept with the real-axis provides the value of a series resistance Rs, which is the sum of
the electrolyte resistance, the intrinsic resistance of the active
electrode material, and the contact resistance at the interface
of the active material and the current collector [40]. For the
cell using KOH electrolyte, a small Rs value of 800 mX was
observed, indicating that the active material/current collector
contact resistance was very low due to the absence of binder
(the KOH electrolyte resistance could be several 100 mX [41]).
The series resistance of the cell using TEABF4/AN electrolyte
is higher (6.8 X, see the inset of Fig. 7b). Because all active
materials and electrodes used in the current work were
synthesized and assembled using the same procedure with
controlled parameters, it is anticipated that all cells have
the same intrinsic resistance for the as-grown VG and the
same contact resistance between VG and stainless steel,
and consequently, the higher series resistance in TEABF4/AN
could be mainly attributed to the increased solution resistance. The maximum specific power of the capacitors using
aqueous and organic electrolytes were calculated as 23.1
and 16.2 kW/kg, respectively, which makes the new EDL
capacitors well suited for surge-power delivery applications
[38]. These values are somehow lower than those reported
for capacitors using carbon onions and MWCNTs [42]. Further
work is warranted to optimize the electrode aiming at a lower
internal resistance.
The following depressed semicircle of the Nyquist curve,
as shown in the inset of Fig. 7a and b, correspond to the
passive oxide layer resistance. By using nickel as the current
collector, the resistance of this part was significantly reduced
from 10 X to less than 1 X (see the inset of Fig. S1a in Supplementary data), indicating that the resistance of the passive
oxide layer was 9 X. For Miller’s work using maze-like 2D
VG-based active material and a nickel current collector [20],
this part of the resistance was almost eliminated.
The significance of the so called knee frequency in the Nyquist plot is the critical frequency where all surface area is
accessed, i.e., saturated. With few exceptions [40,41,43], most
carbon-based EDL capacitors show a knee frequency below
300 Hz [44–47]. For the current work, the knee frequency of
the cell using KOH electrolyte was 259 Hz, and notably, a
high knee frequency of 3174 Hz was achieved when TEABF4/
AN was used as the electrolyte. These knee frequency values
were consistent with those obtained from the Bode plots, as
shown in Fig. 7c and d. The high knee frequencies induced
by the good accessibility of the ions into the vertically-oriented graphene sheets would strongly favor the capacitor
power capability.
4.
Summary
VG nanosheets can be simply (one-step) synthesized on
metallic substrates using an atmospheric negative normal
glow discharge PECVD process. Compared with the graphene
prepared by the chemical oxidation of graphite and subsequent reduction of graphene oxide, this method can produce
vertically-oriented few-layer graphene nanosheets without
obvious stacking. CV and EIS tests indicate the predominant
EDL capacitive behavior for this novel electrode material.
The vertical direction and exposed edge planes of graphene
sheets could favor the ion accessibility at the electrolytematerial interface, and the non-agglomerated sheets would
result in a decreased obstruction of ion movement and diffusion. Compared with horizontal graphene sheets, the vertical
orientation and the non-agglomerated morphology of VG
nanosheets could lead to better wettability of graphene in
organic electrolyte, which provides a specific capacitance very
similar to, or even slightly higher than that in aqueous
electrolyte at the same potential scan rate. Future work will
be focused on the optimization of the VG morphology, especially on the graphene interlayer and intersheet spacing, aiming at better ion diffusion and storage kinetics to fully utilize
actual electrochemically accessible graphene surface area.
Acknowledgements
We acknowledge the financial support from the National Science Foundation (CMMI-0900509), the Department of Energy
(DE-EE-0003208), and the University of Wisconsin System. Z.
Bo acknowledges the research fellow support from the
UWM Research Foundation and the financial support from
the Innovation Program of the State Key Laboratory of Clean
Energy Utilization (ZJUCEU2011009). Finally, we thank anonymous reviewers for their valuable comments.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/
j.carbon.2012.05.014.
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