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Synthesis and Electrochemical Characterization of Polypyrrole/Multi-walled Carbon Nanotube Composite Electrodes for Supercapacitor Applications

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  • Sacred Heart College Chalakudy, Kerala, India

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The nanocomposites of polypyrrole (PPy) and multi-walled carbon nanotube (MWCNT) with different composition are synthesized by the chemical oxidative polymerization method. In these composites, the MWCNTs are uniformly coated by PPy with different thickness. The electrochemical properties of the composite electrodes are investigated by cyclic voltammetry, galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy. The full cells assembled with the PPy/MWCNT composite electrodes deliver initial specific capacitances ranging from 146.3 to 167.2 F/g at 0.5 mA/ and exhibit stable cycling characteristics. The effect of content of MWCNT in the composite on cycling performance of the cells is also investigated.
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1228 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 5 Santhosh Paul et al.
DOI 10.5012/bkcs.2010.31.5.1228
Synthesis and Electrochemical Characterization of Polypyrrole/Multi-walled Carbon
Nanotube Composite Electrodes for Supercapacitor Applications
Santhosh Paul, Yoon- Sung Lee, Ji -Ae Choi, Yun Chan Kang, and Dong-Won Kim*
Department of Chemical Engineering, Hanyang University, Seoul 133-791, Korea. *E-mail: dongwonkim@hanyang.ac.kr
Department of Chemical Engineering, Konkuk University, Seoul 143-701, Korea
Received January 29, 2010, Accepted March 4, 2010
The nanocomposites of polypyrrole (PPy) and multi-walled carbon nanotube (MWCNT) with different composition
are synthesized by the chemical oxidative polymerization method. In these composites, the MWCNTs are uniformly
coated by PPy with different thickness. The electrochemical properties of the composite electrodes are investigated
by cyclic voltammetry, galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy. The
full cells assembled with the PPy/MWCNT composite electrodes deliver initial specific capacitances ranging from
146.3 to 167.2 F/g at 0.5 mA/cm2 and exhibit stable cycling characteristics. The effect of content of MWCNT in the
composite on cycling performance of the cells is also investigated.
Key Words: Cycling performance, Multi-walled carbon nanotubes, Nanocomposite, Polypyrrole, Superca-
pacitor
Tabl e 1 . Summary of reactant composition for synthesizing PPy/
MWCNT composites.
PCNT-10 PCNT-20 PCNT-30
MWCNT (g) 0.100 0.210 0.325
FeCl3 (g) 3.880 3.880 3.880
Pyrrole (g) 0.967 0.967 0.967
Water (mL) 100 100 100
Introduction
The growing demand for portable electronic systems, digital
communication devices and electric vehicles has prompted con-
siderable interest in supercapacitors with high power density
and long cycle life.1-3 The major types of materials used in super-
capacitors are: (i) high surface area activated carbons, (ii) re-
dox metal oxides and (iii) conducting polymers.4-6 Among them,
conducting polymers attracted great deal of interest due to their
high specific capacitance, low material cost, easy synthesis and
many other inherent properties.7-14 However, the main drawback
of conducting polymer-based supercapacitors is poor cycle life
due to the volume change during doping and dedoping process.
Hence it is necessary to strengthen adequately the electroche-
mically active sites of conducting polymers by the addition of
large surface area carbon materials or carbon nanotubes (CNTs).
CNTs have been attractive materials for electrodes of super-
capacitors due to their high accessible surface area, high elec-
tronic conductivity, chemical and mechanical stability. In order
to improve the mechanical and electrochemical properties of
electrodes based on conducting polymers, the composites of
conducting polymers and CNTs have been synthesized for
supercapacitor applications.15-22 It has been shown that CNTs
play a role of a perfect backbone for a homogeneous distribu-
tion of conducting polymer in the composites. Recently, Kim
et al. reported that high-specific capacitance and good high-rate
capability of polypyrrole(PPy)/CNT composite electrodes could
be achieved by controlling pore size in a three-dimensional
entangled structure of a CNT film.22
In this work, we tried to improve the cycling stability of the
cell without significant drop in specific capacitance. With the
aim of improving the cycleability of the PPy-based electrode,
we synthesized PPy/multi-walled carbon nanotube (MWCNT)
composites with different composition by the chemical oxida-
tive polymerization method. In this study, the electrochemical
properties of the PPy/MWCNT nanocomposite electrodes were
investigated by cyclic voltammetry, galvanostatic charge-dis-
charge cycling and electrochemical impedance spectroscopy.
The effect of content of MWCNT in the composite on electro-
chemical performance of the cells was also investigated.
Experimental
Synthesis of PPy/MWCNT composites. Pyrrole, anhydrous
ferric chloride (FeCl3), poly (vinyl pyrrolidone) (PVP), poly-
tetrafluoroethylene (PTFE), isopropanol (IPA), N-methylpyrro-
lidone (NMP) and potassium chloride (KCl) were purchased
from Aldrich chemicals. Pyrrole was purified before reaction
and rest of the chemicals were used as received. MWCNTs
(Hanwha Nanotech) prepared by chemical vapor deposition
were used without further purification. The PPy/MWCNT com-
posites were synthesized by the conventional oxidative chemical
polymerization method using anhydrous FeCl3. The content
of reactants for synthesizing PPy/MWCNT composites with
different composition is given in Table 1. Different quantity of
MWCNTs (0.100, 0.210, 0.325 g) was stirred vigorously in
100 mL aqueous FeCl3 solution. This suspension was sonicated
for 3 h, to facilitate the good dispersion of MWCNTs. Pyrrole
(0.967 g) was added very slowly and drop wise into the stirring
solution. The polymerization was allowed to continue for ano-
ther 4 h. The precipitated composite material was then filtered,
washed with water and methanol. It was vacuum dried at 70 oC
overnight. From the measurements of weight of PPy/MWCNT
Polypyrrole/Multi-walled Carbon Nanotube Composite Electrode Bull. Korean Chem. Soc. 2010, Vol. 31, No. 5 1229
Figure 1. FESEM images of (a) pristine MWCNTs, (b) PCNT-10, (c) PCNT-20, (d) PCNT-30 and (e) composite electrode film cast on Ti foil.
composites, the content of MWCNT was calculated to be 9.6,
20.1, 30.5 by weight for three composite materials, respectively.
The PPy/MWCNT composites synthesized are designated
PCNT-10, PCNT-20 and PCNT-30 according to the content of
MWCNT in the composites.
Electrode preparation and cell assembly. Electrode was pre-
pared by coating a IPA/NMP-based slurry of composite mate-
rial, super-P, PTFE and PVP (75:15:7:3 by weight) on a Ti foil.
The excellent water wetting and adhesive properties of PVP
have been well known and thus a proper amount of PVP was
added along with PTFE as a binder. The binder materials
(PTFE/PVP) were first dissolved in IPA/NMP mixed solvent
and then PPy/MWCNT composite and super-P powder were
added into the polymer binder solution. The solution was soni-
cated for 1 h and ball-milled for another 15 h in order to make
a homogenous slurry. The resultant slurry was cast on to Ti foil
by using a doctor blade. The electrodes were dried in a vacuum
oven at 80 oC for 24 h, and roll-pressed to enhance particulate
contact and adhesion to the foil. Sandwich-type cells were
assembled with two symmetric PPy/MWCNT electrodes (area:
1.0 cm2). The electrolyte used in assembling the cell was 1.0 M
KCl aqueous electrolyte solution with paper separator (thick-
ness: 40 µm, Nippon Kodoshi Co.). Both electrodes and sepa-
rator were soaked in the electrolyte before the cell assembly.
The symmetrical cell was enclosed in a metallized plastic bag
and vacuum-sealed.
Characterization and electrochemical analysis. Morpholo-
gies of pristine MWCNTs and PPy/MWCNT composites were
examined using a field emission scanning electron microscopy
(FESEM, JEOL JSM-6701). Cyclic voltammograms (CVs)
were recorded from 0 to 0.6 V at different scan rates. AC
impedance measurements were performed on CH Instrument
in the frequency range of 0.1 Hz - 100 kHz with an amplitude
of 10 mV. Galvanostatic charge-discharge cycling of the cell
was conducted over voltage ranges of 0 - 0.6 V with battery-test
equipment at constant current densities from 0.5 to 10 mA/cm2
at room temperature.
Results and Discussion
The FESEM images of the pristine MWCNTs and various
PPy/MWCNT composites are shown in Figure 1. The average
diameter of the pristine nanotubes (Figure 1 (a)) was measured
to be about 40 nm. From the images of PPy/MWCNT compo-
sites in Figure 1 (b)-(d), it can be seen that all the composites
exhibit well dispersed carbon nanotubes enwrapped uniformly
with PPy. This suggests that the interaction between polymer
molecules and MWCNTs overcomes the Van der Waals inter-
action between MWCNTs, which generally otherwise would
result in separate growth or aggregates of PPy. As the content
of MWCNT increases in the composite, a gradual decrease of
thickness of the PPy layer over nanotubes is clearly observed.
As shown in Figure (b)-(d), the average diameter of MWCNT
in the composites increases to about 77, 58, 51 nm and thus the
thickness of the PPy coating is estimated to be 18.5, 9.0, 5.5 nm
for PCNT-10, PCNT-20, PCNT-30, respectively. In these com-
posites, MWCNTs can offer good mechanical support to PPy
and also ensure good electronic conduction in the electrode.
The electrode film cast on Ti foil exhibits highly interconnected
and porous morphology, as shown in Figure 1 (e), which is
highly desirable for the fast ion diffusion and migration in the
electrodes. The co-addition of PVP as a binder material could
also help in opening many channels for the aqueous electrolyte
to pass through and it gave an improved wetting of the electrode
material with aqueous electrolyte.
The electrochemical performance of the PPy/MWCNT com-
posite electrode was evaluated by the CVs of the two-electrode
cells assembled with the same electrode. The voltammograms
obtained for the PCNT-20 composite electrode at different scan
rates are presented in Figure 2. The CVs obtained for the other
two composite electrodes also followed the same behavior as
shown in Figure 2. The CVs of the composite electrode look
almost like rectangular shape with good symmetry at all scan
rates, showing highly efficient capacitive behavior with good
charge propagation. CVs shown in Figure 2 are very similar to
1230 Bull. Korean Chem. Soc. 2010, Vol. 31, No. 5 Santhosh Paul et al.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Voltage (V)
6.0
4.0
2.0
0.0
2.0
4.0
6.0
Current (mA)
50 mV/s
30 mV/s
20 mV/s
10 mV/s
5 mV/s
1 mV/s
Figure 2. Cyclic voltammograms obtained for the symmetric two-
electrode cell assembled with PCNT-20 composite. (electrolyte: 1.0 M
KCl).
0 2 4 6 8 10 12 14 16
Time (min)
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Voltage (V)
1st 100th 2 00th 300th 400th 500th
Figure 3. Charge and discharge curves of the symmetric cell assembled
with PCNT-20 composite electrode.
0 100 200 300 400 500
Cycle number
200
180
160
140
120
100
80
Specific capacitance (F/g)
PCNT-10
PCNT-20
PCNT-30
Figure 4. Specific discharge capacitance as a function of cycle num-
ber for the cells assembled with three different composite electrodes,
which are obtained at a constant current density of 0.5 mA/cm2.
0.0 2.0 4.0 6.0 8.0 10.0
Current density (mA/cm2)
200
150
100
50
0
Discharge capacitance (F/g)
PCNT-10
PCNT-20
PCNT-30
Figure 5. Specific capacitance as a function of current density for the
cells assembled with different PPy/MWCNT composite electrodes.
those observed in the other PPy/CNT composite electrodes.17,20-23
These composite electrodes exhibited no significant change in
CV shape even after 500 cycles, which indicates good cycle-
ability of the PPy/CNT electrodes.
The symmetric cell assembled with PCNT-20 composite elec-
trode was subjected to charge-discharge cycling in the voltage
range of 0 - 0.6 V at a constant current of 0.5 mA/cm2, and its
charge-discharge curves are shown in Figure 3. These curves
are observed to be perfectly linear and there is no significant IR
drop, which indicates prominent capacitive behavior of the cell.
The charging and discharging time for the cell is maintained to
be constant throughout the cycling, which demonstrates that
the PPy/MWCNT composite electrode has high stability. The
supercapacitors based on PPy composites experienced either
very fast or gradual decrease in specific capacitance during re-
peated cycling, as previously reported.15,17 The specific capa-
citance of the electrode was calculated using the formula of
C (F/g) = 2 I t / (m E) where, I is the current applied for the
charging and discharging, t is the time of discharge, E is the
voltage difference between the upper and lower potential limit
and m is the mass of active materials (PPy and MWCNT) in
one of the electrodes. The factor of “2” arises from the fact
that the total capacitance measured from the cell is the addition
of two equivalent single electrode capacitors in series.17 The
discharge capacitance obtained for all the cells assembled with
different composite electrodes are shown in Figure 4. The initial
specific capacitances obtained from the galvanostatic charge-
discharge measurements ranges from 146.3 to 167.2 F/g, and
are found to be increased with increasing PPy content. It should
be noted that pure MWCNT has very low specific capacitance
not exceeding 40 F/g. An increase in specific capacitance of
PPy/MWCNT composite with increasing PPy content can be
thus associated with pseudo capacitance contributing from PPy.
It is also found that the loss of capacitance is little during the
charge and discharge cycles. This result suggests that the PPy/
MWCNT composite electrodes are very stable under charge
and discharge cycles and the active electrode materials are keep-
ing good interfacial contacts in the electrode. The data given
Polypyrrole/Multi-walled Carbon Nanotube Composite Electrode Bull. Korean Chem. Soc. 2010, Vol. 31, No. 5 1231
0 10 20 30 40 50
ZRe ()
PCNT-10
PCNT-20
PCNT-30
50
40
30
20
10
0
0.0 0.5 1.0 1.5 2.0 2.5
2.5
2.0
1.5
1.0
0.5
0.0
Figure 6. AC impedance spectra of the cells assembled with different
PPy/MWCNT composite electrode, which are obtained after 500 cycles
in the voltage range of 0 - 0.6 V.
in Figure 4 show that the cell assembled with PCNT-30 has
the best cycleability among the cells considered in this work.
Rate capabilities of cells assembled with PPy/MWCNT elec-
trodes were evaluated with varying current densities from 0.5
to 10 mA/cm2, which are shown in Figure 5. As expected, a
decrease in capacitance is observed with increasing the current
densities. The cell assembled with PCNT-20 composite has the
highest capacitance at high current rates. As compared, the
capacitance drop is the most prominent in the cell of PCNT-10
composite electrode. The reduction of capacitance in the cell of
PCNT-10 at high current rate may be associated with PPy layer
with high thickness, which gives high internal resistance of the
cell, as explained later. In case of PCNT-30 electrode, the amount
of PPy is not enough for giving high capacitance as compared
that of the cell assembled with PCNT-20. These results suggest
that there should be an optimum content of MWCNT in the
composite in order to provide both high specific capacitance
and good rate capability of the cell.
In order to investigate the effect of content of MWCNT on
the impedance behavior of the cells, the ac impedance of the
cells was measured. Figure 6 presents ac impedance spectra of
the cells, which are obtained at discharged state after 500 cycles
in the voltage range of 0 - 0.6 V. In all the spectra, the cells dis-
played a semicircle followed by a capacitive spike. The inter-
cepts of the real axis at high frequency are 0.58, 0.54, 0.53
for the cells assembled with PCNT-10, PCNT-20 and PCNT-30,
respectively. This intercept is known to be related to the intrinsic
resistance of the active materials, the electrolyte resistance,
the electrical leads, and the contact resistance at the interface
of active materials/current collector.24 A slight decrease in high
frequency-resistance with increasing content of MWCNT may
be due to the decrease in resistance of the active materials. On
the other hand, the semicircle appeared at middle-to-low fre-
quency region is related to the charge transfer reaction occurring
in the cells. Charge transfer resistances calculated from these
spectra are 1.21, 0.90 and 0.85 for the cells assembled with
PCNT-10, PCNT-20 and PCNT-30, respectively. A thin coating
of PPy on MWCNT can facilitate an effective penetration of
electrolyte into PPy, leading to enhanced participation of active
materials in the charge transfer reaction, which results in de-
crease of charge transfer resistance with increasing MWCNT
content in the PPy/MWCNT electrode. It suggests that the
introduction of proper content of MWCNTs into the composite
may facilitate the charge transfer and reduce the internal resis-
tance of the electrode.
Conclusions
The homogeneous coating of PPy over MWCNTs could be
achieved by the simple chemical oxidative polymerization
method. Stable cycling could be achieved for the cells fabricated
with these composites during the galvanostatic charge-discharge
tests. The performance of the cells depended mainly on the con-
tent of MWCNT in the composite electrode. The cell assembled
with PCNT-10 showed the highest specific capacitance due to
the higher content of PPy. In consideration of cycling stability
and rate capability, the cells based on PCNT-20 and PCNT-30
exhibited better performance. From these results, the PPy/MW
CNT composites with optimum composition could be consi-
dered as promising electrode materials in the application of
supercapacitors with high capacitance and long cycle life.
Acknowledgments. This work was supported by the Natio-
nal Research Foundation of Korea (NRF) grant funded by the
Korea government (MEST)(No. 2009-0071851). This work is
also the outcome of a Manpower Development Program for
Energy & Resources supported by the Ministry of Knowledge
and Economy (MKE).
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