APPLIED PHYSICS LETTERS 92, 143108 共2008兲
Frequency response characteristic of single-walled carbon nanotubes
as supercapacitor electrode material
C. G. Liu,1,2 M. Liu,1 F. Li,1 and H. M. Cheng1,a兲
1
Shenyang National Laboratory for Materials Science, Institute of Metal Research,
Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, People’s Republic of China
2
State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams and School of Materials
Science and Engineering, Dalian University of Technology, Dalian 116024, People’s Republic of
China
共Received 7 December 2007; accepted 19 March 2008; published online 9 April 2008兲
Impedance analysis was employed to study the frequency response characteristics of
supercapacitors utilizing single-walled carbon nanotubes 共SWNTs兲 as electrode material. The knee
frequency is around 631 Hz for the electrochemically oxidized SWNT supercapacitor, much higher
than that of the as-prepared one 共around 316 Hz兲 and the common activated carbon supercapacitor
共⬍1 Hz兲. The good frequency response observed in this work is due to the introduction of nanosized
mesopores and oxygen-containing groups formed during the electrochemical oxidization of SWNTs.
The capacitance of electrochemically oxidized SWNT supercapacitor is 113 F / g, which is much
higher than the value of the as-prepared one 共20 F / g兲. © 2008 American Institute of Physics.
关DOI: 10.1063/1.2907501兴
Supercapacitors are energy storage devices with high
power density due to their fast charge propagation.1–10
Single-walled carbon nanotubes 共SWNTs兲 are predicted to be
a suitable candidate for the electrode of supercapacitors because of their high accessible surface area, low electric resistance, low mass density, high stability, and the network
structure of entangling with each other. Niu et al.3 reported
that the frequency “knee” of carbon nanotube supercapacitors was around 100 Hz, which suggests that most of their
stored energy is accessible at frequencies below 100 Hz.
This is unprecedented for this type of devices because the
highest reported knee frequency in other capacitors is 6 Hz
and the knee frequency for most commercially available supercapacitors, including those specially designed for high
power applications, is ⬍1 Hz.11 A better frequency response
means a better power performance. In particular, for low
frequency electrical filtering device application, the demand
of frequency response performance of supercapacitors is
high. Therefore, to find suitable materials which own good
frequency response is important. In this study, impedance
characterization of SWNTs was investigated, from which the
frequency response of the SWNT electrodes in supercapacitors was analyzed. The knee frequency around 631 Hz for
the electrochemically oxidized SWNT supercapacitor was
achieved due to the introduction of nanosized mesopores and
oxygen-containing groups.
The SWNTs used in this study were synthesized by a
hydrogen arc discharge method,12 in which graphite was the
carbon feedstock, hydrogen and argon gases were used as
buffer gas, a mixture of Ni, Co, and Fe powders as catalyst,
and FeS as growth promoter. The purity of the as-prepared
SWNTs was estimated to be over 40 wt % based on transmission electron microscopic observations and thermogravitric analysis results.
a兲
Author to whom correspondence should be addressed. Electronic mail:
cheng@imr.ac.cn.
Typically, 10– 80 mg of SWNTs was placed between
two sheets of nickel foam in a cylindrical steel mold and a
pellet with 8 mm diameter was formed by pressing the carbon material and nickel foam under 2 MPa. Supercapacitor
was assembled in a cell with two identical pellets separated
by a nylon cloth in 6M KOH aqueous solution at room temperature and atmospheric pressure. Some of the SWNT electrodes were at first electrochemically oxidized in 6M KOH
aqueous solution at the voltage of 1.5 V for 36 h. These will
be called as electrochemically oxidized SWNT electrode
below.
The impedance measurements were made at a dc bias of
0 V with a 10 mV amplitude sinusoidal signal over the frequency range of 100 kHz– 0.01 Hz using a Solartron model
1260 frequency response analyzer with a Solartron model
1287 potentiostat/galvanostat.
The complex-plane impedance plots of the as-prepared
SWNT and electrochemically oxidized SWNT supercapacitors are shown in Fig. 1. The impedance spectrum in the high
frequency range being fitted exhibits semicircles but straight
FIG. 1. 共Color online兲 Complex-plane impedance of a single cell supercapacitor fabricated with the as-prepared and electrochemically oxidized
SWNTs.
0003-6951/2008/92共14兲/143108/3/$23.00
92, 143108-1
© 2008 American Institute of Physics
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143108-2
Appl. Phys. Lett. 92, 143108 共2008兲
Liu et al.
TABLE I. Elemental composition of as-prepared and electrochemically oxidized SWNT surfaces detected by XPS.
SWNTs
Nonoxidized
Oxidized
FIG. 2. 共Color online兲 Bode angle plot of a single cell supercapacitor fabricated with the as-prepared and electrochemically oxidized SWNTs.
lines in the low frequency range, as shown in Fig. 1. An
electrode-solution system, especially those porous materials,
as in supercapacitors, has complex combinations of C and R
components. The interfacial impedance of a supercapacitor is
associated with a double-layer capacitance Cdl, a pseudocapacitance C p, a Faradaic charge-transfer resistance R f and the
sum of the electrolyte resistance, the electrode resistance,
and the contact resistance between the electrode and the current collector Rs. The presence of semicircles of SWNT supercapacitors is attributed to the Faradic reaction of oxygencontaining functional groups at the electrode surface.
From the value of Rs 共the point impedance curve intersects the Z⬘ axis at high frequency range兲, the equivalent
series resistance of the electrochemically oxidized SWNT
and the as-prepared SWNT supercapacitor can be obtained as
0.54 and 0.62 ⍀, respectively. The electrolyte resistance and
the contact resistance are identical in both SWNT supercapacitors. Therefore, a decrease of Rs indicates a decrease of
the SWNT electrode resistance in electrochemically oxidized
SWNT capacitor.
In the complex-plane impedance plots, SWNT supercapacitors differ somewhat from those of conventional
capacitors at the low frequency range. The ideal capacitor
exhibits a vertical line, while the plot of SWNT supercapacitors starts with a 45° impedance line and approaching an
almost vertical line at low frequencies. It is due to the diffusion process on the porous surface of SWNTs. It is consistent
with the porous nature of the electrodes when saturated with
electrolyte. The 45° region 共Warburg region兲 is a consequence of the distributed resistance/capacitance in a
porous electrode. At higher frequency, the resistance as well
as the capacitance of a porous electrode decreases because
only part of the active porous layer is accessible at high
frequency. The knee frequency for the electrochemically oxi-
C 1s, at. %
O 1s, at. %
O / C ration, %
87.2
58.5
12.8
41.5
14.7
70.9
dized SWNT supercapacitor was found around 631 Hz from
Fig. 1, much higher than those of the as-prepared one
共around 316 Hz兲. Figure 2 shows a Bode angle plot of the
as-prepared SWNT supercapacitor and electrochemically
oxidized SWNT supercapacitor. For frequency of up to 1 Hz,
the phase angle of the electrochemically oxidized SWNT
supercapacitor is close to −90°, while it is −50° for the asprepared one. This suggests that the electrochemically oxidized SWNT supercapacitor functions like an ideal capacitor
and better than the as-prepared one.
Such improved frequency response performance is
mainly achieved by the introduction of nanosized mesopores
and oxygen-containing groups, favoring the access of electrolyte to the surface of SWNT. After electrochemical oxidation treatment, parts of the tips and sidewall of SWNTs were
etched and the amount of small mesopores was increased.13
It is considered that the small mesopores serve as channels
for electrolytes and can be readily accessed in the charge/
discharge process. On the other hand, oxygen-containing
groups can form during the electrochemical oxidation process, which are confirmed by x-ray photoemission spectroscopy 共XPS兲 data, as shown in Fig. 3共a兲. A strong peak of O
1s
has
been
observed
for
electrochemically
oxidized SWNTs, indicating a high surface oxygen
concentration.14,15 Table I summarizes the elemental composition detected by XPS and the oxygen/carbon 共O / C兲 ration.
Obviously, electrochemically oxidized SWNTs show higher
O / C ration of ⬃70.9%, indicating that the surfaces were
severely oxidized during the electrochemical oxidation process. Figure 3共b兲 shows O 1s XPS analysis results of the
electrochemically oxidized SWNT. To identify the O 1s XPS
results, a curve-fitting procedure was employed. Dots shown
in Fig. 3共b兲 correspond to XPS experimental data and bold
solid lines are the fitting curves for the experimental data.
C u O and C v O groups on electrochemically oxidized
SWNT surfaces were identified. Moreover, the detection of
oxygen-containing functional groups in oxidized SWNTs is
in agreement with other reports.16,17 The increased oxygencontaining groups can enhance the wettability of carbon materials in aqueous electrolyte, which is important to maximize the access of the electrolyte to the surface of
carbon.10,18,19
FIG. 3. 共Color online兲 XPS survey
scan of as-prepared and electrochemically oxidized SWNTs 共a兲. O 1s peaks
of electrochemically oxidized SWNTs
共b兲.
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143108-3
Appl. Phys. Lett. 92, 143108 共2008兲
Liu et al.
FIG. 4. 共Color online兲 Galvanostatic
discharge curves 共a兲 and cycling
performance 共b兲 of a single cell supercapacitor fabricated with the asprepared and electrochemically oxidized SWNTs.
Not only own good frequency response of the electrochemically oxidized SWNT supercapacitors makes it suitable
for the application of strict frequency demand but also high
specific capacitance for the introduction of nanosized mesopores and oxygen-containing groups. The effect of electrochemical oxidation on the capacitance of SWNT capacitors
is shown in Fig. 4共a兲. The specific capacitance was measured
in galvanostatic experiments using an Arbin BT-2000 system
with the discharge current density of 50 mA/ g. The capacitance was obtained from the formula C = 共I ⫻ ⌬t兲 / ⌬v, where
I is the constant discharging current, ⌬t is the discharging
time measured from 0.6 to 0.4 V 共60%–40% of the peak
voltage兲, and ⌬v is the voltage change at a constant current
discharge. From the discharge curves in Fig. 4共a兲, the specific capacitance of 113 F / g was obtained for the electrochemically oxidized SWNT supercapacitor only compared to
20 F / g for the as-prepared SWNT supercapacitor. Figure
4共b兲 shows the cycling performance of the electrochemically
oxidized and the as-prepared SWNT supercapacitor. Both capacitors have good cycling performance and almost no capacitance decrease was observed after 300 cycles.
Impedance analysis was employed to study the frequency response characteristics of supercapacitors utilizing
SWNTs as an electrode material. It was found that the knee
frequency is around 631 Hz for the electrochemically oxidized SWNT supercapacitor, much higher than those of the
as-prepared one 共around 316 Hz兲 and the common activated
carbon supercapacitor 共⬍1 Hz兲. The good frequency response performance is due to the introduction of nanosized
mesopores and oxygen-containing groups. The former serves
as effective channels for electrolytes in the charge/discharge
process and the latter can improve the wettability of SWNT
electrode to maximize the access of the electrolyte to the
SWNT surface. Moreover, the electrochemically oxidized
SWNT supercapacitor has a specific capacitance of 113 F / g,
which is much higher than the specific capacitance 共20 F / g兲
of the as-prepared SWNT. From the results, we can expect
the electrochemically oxidized SWNTs to be good electrode
material in supercapacitor application.
This work was supported by National Science Foundation of China 共Grant Nos. 50328204 and 50632040兲.
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