Synthetic Metals 161 (2011) 1141–1144
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Synthetic Metals
journal homepage: www.elsevier.com/locate/synmet
Letter
Toward a high specific power and high stability polypyrrole supercapacitors
a r t i c l e
Keywords:
Polypyrrole
Supercapacitors
High stability
High specific power
i n f o
a b s t r a c t
Polypyrrole (PPy) films doped by p-toluenesulfonic (PTS) are prepared by pulse current polymerization
(PCP PPy) in aqueous solutions. Cations (H+ , Li+ , Na+ , and K+ ions) in working electrolyte solutions have
a great influence on the electrochemical properties of PCP PPy films for supercapacitors. In 3 M aqueous
chloride solutions, the smaller cations lead to the better electrochemical properties of PCP PPy films. PCP
PPy films show ideally capacitive characteristics with a very high specific power of 110.9 kW kg−1 in 3 M
HCl when its specific energy reaches 18.4 Wh kg−1 . PCP PPy films are very stable in 3 M HCl, showing
less than 4% decay over 160,000 and 20,000 charge/discharge cycles at current density of 40 A g−1 within
voltage range of 0–0.4 V and 0–0.8 V, respectively. The results of SEM show that many pits and grooves
appear on the surface of PPy anode in 3 M KCl after 30,000 cycles, which should be main reason of rapid
decay of PPy capacitance in 3 M KCl.
Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction
In recent years, much attention has been focused on supercapacitors due to their high specific power for complements to
batteries in both portable electronics and fixed large-scale energy
storage applications in electric vehicles [1]. Conducting polymers
are considered as one of the most potential electrode materials for
supercapacitors [2]. Among these conducting polymers, polypyrrole (PPy) is an especially practical material for supercapacitors
due to its high conductivity, long-term environmental stability,
environmental friendliness, low cost and facile synthesis [2–4].
However, there are two critical problems to be solved before
the commercialization. Firstly, the poor stability, caused by the
degradation at high potential and big change of volume in oxidation/reduction process, need to be improved further [5,6]. Secondly,
the high specific energy and high specific power are not obtained at
the same time because of narrow electrochemical window (0–0.6 V
in aqueous solutions) and low diffusion rate of ion in conducting
polymers matrix [7]. Composites of carbon nano-tubes/polymers
have been often prepared for improving the performance of supercapacitors, because carbon nano-tubes play the role of a perfect
backbone for a homogenous distribution of PPy in the composite [8–10]. Unfortunately, the relatively complicated preparation
processes and high cost limit the commercialization of carbon
nano-tubes/polymers composites. Some researchers prepared PPy
films by potentiodynamic and pulse current methods to improve
the specific energy and stability of PPy films [11,12]. Sharma et al.
reported a high specific energy supercapacitor prepared by pulse
current polymerization (PCP PPy), but the specific power and stability of PPy films need to be enhanced further. Our group prepared
the PPy films by pulse current polymerization (PCP PPy), which
shows high charge/discharge rate and high stability within narrow
electrochemical window of 0–0.4 V in 3 M KCl aqueous solution
[12]. Only 14.5% decay of specific capacitance for PCP PPy films
is observed after 50,000 cycles. However, the stability of PCP PPy
films needs to be improved further within a broad electrochemical
window of 0–0.8 V.
To improve the stability of PCP PPy films within broad electrochemical window, we investigated the effect of cations in
electrolyte solutions on electrochemical properties of pulse polymerized polypyrrole films for supercapacitors. It was found that the
PCP PPy films exhibit high specific power and high stability in 3 M
HCl.
2. Experimental
Pyrrole (Capchem, 99.8%) was distilled prior to use and stored at
−10 ◦ C in a nitrogen atmosphere. All other chemicals were reagent
grade and used as received.
The polymerization of pyrrole was carried out in aqueous solution containing 0.1 M pyrrole, 0.1 M p-toluenesulfonate acid (PTSA),
and 0.3 M sodium p-toluenesulfonate (PTSS). A three-electrode
electrochemical cell was used for polymerizations. The working
electrode was a polished tantalum sheet of 1 cm × 1 cm and the
counter electrode was platinum sheet. All electrochemical potentials were measured against a saturated calomel electrode (SCE)
reference electrode. All polymerizations were performed in an icewater bath. The pulse current polymerization was carried out with
pulse on time of 10 ms, pulse off time of 100 ms, total number
of pulses of 20,000 cycles, and current density of 2 mA cm−2 . The
theoretical mass of PCP PPy films on tantalum plate electrode is
0.226 mg, which is calculated according to Ref. [15].
Two-electrode supercapacitors cells were assembled with two
1 cm × 1 cm PPy electrodes, and the distance between two electrodes was 2 mm. The morphology of PPy films was investigated
by field emission scanning electron microscope (JEOL, JSM-6700F).
0379-6779/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.synthmet.2011.01.011
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Letter / Synthetic Metals 161 (2011) 1141–1144
Fig. 1. CV curves of PCP PPy films at different scan rates (a) 500 mV s−1 and (b) 2000 mV s−1 in different electrolyte solutions (3 M HCl, 3 M LiCl, 3 M NaCl and 3 M KCl).
The electrochemical performance of PCP PPy films was investigated by cyclic voltammetry, constant current charge/discharge
and electrochemical impedance spectroscopy techniques. All electrochemical tests were performed on the Versatile Multichannel
Potentiostat 2/Z (VMP2, Princeton applied research). EIS measurements were performed at 0 V (versus SCE) in the frequency range
from 105 Hz down to 10−2 Hz, using an ac amplitude of 10 mV.
3. Results and discussion
Fig. 1 shows CV curves of PCP PPy films in symmetric twoelectrode system. All CV scans were performed within a voltage
range of −0.8 to +0.8 V. Even at a high scan rate of 500 mV s−1 , all
CV curves of PCP PPy films show rectangular shape and characteristics for an ideal capacitor in 3 M KCl, NaCl, LiCl and HCl aqueous
solutions, respectively. It indicates that PCP PPy films have high
reversibility and high specific power in these electrolytes. When the
scan rate increases to 2000 mV s−1 , an approximate rectangularshaped CV curve is obtained in 3 M HCl, however, CV curves have
changed into about cone-shaped in other electrolyte solutions. The
anion in all electrolyte solutions was Cl− with the same concentration, so the cations in electrolyte should be responsible for different
electrochemical properties of PCP PPy films. Some authors detected
an appreciable increase of volume in the first stage of reduction,
which was attributed to the initial incorporation of cations from
the electrolyte [6,13,14]. This implies that cations can contribute
to the capacitance of PCP PPy films. It is obvious that small radius
of cations benefit for it entering into/ejecting from PPy matrix in
the first stage of reduction. When the scan rate increasing from
500 mV s−1 to 2000 mV s−1 in Fig. 1b, the cation radius has greater
effect on the electrochemical properties of PCP PPy films in different
electrolyte solutions, the gap of the specific capacitance between
different electrolytes enlarging furthermore.
Fig. 2 shows charge/discharge curves of PCP PPy films at
charge/discharge current density of 150 A g−1 within a voltage
range of 0–0.8 V in two-electrode system. Under same test conditions, Table 1 exhibits the specific capacitance, specific energy
and specific power of PCP PPy films. The theoretical mass of PCP
PPy films on tantalum plate electrode is calculated according to
Fig. 2. Constant current charge/discharge curves of PCP PPy films at
charge/discharge current density of 150 A g−1 in different electrolyte solutions (3 M
HCl, 3 M LiCl, 3 M NaCl and 3 M KCl).
Ref. [15]. The values of specific energy 11.8 Wh kg−1 , 12.8 Wh kg−1 ,
14.3 Wh kg−1 , and 18.4 Wh kg−1 are obtained at charge/discharge
current density of 150 A g−1 in 3 M KCl, 3 M NaCl, 3 M LiCl and 3 M
HCl, respectively. A very high specific power of 110.9 kW kg−1 is
obtained in 3 M HCl. This value is distinctly higher than the most
data reported in the literature for PPy and PPy composite materials. High specific power of PCP PPy film doped by PTS makes it very
suitable for electrode material of supercapacitors.
Electrochemical impedance spectroscopy is a powerful means
for investigating the rate of charge transfer and charge transport
processes occurring in conducting polymer films and membranes
[16]. In Fig. 3, the Nyquist plot of PCP PPy films shows a straight
Table 1
The specific capacitance, specific energy and specific power of PCP PPy films at
charge/discharge current density of 150 A g−1 in different electrolytes.
Electrolytes
Specific
capacitance
(F g−1 )
Specific
energy
(Wh kg−1 )
Specific
power
(kW kg−1 )
3 M HCl
3 M LiCl
3 M NaCl
3 M KCl
232
198
189
178
18.4
14.3
12.8
11.8
110.9
102.9
96.0
93.5
Fig. 3. Electrochemical impedance spectroscopy and keen frequency of PCP PPy
films in different electrolyte solutions (3 M HCl, 3 M LiCl, 3 M NaCl and 3 M KCl).
Letter / Synthetic Metals 161 (2011) 1141–1144
1143
Fig. 4. Discharge capacitance retention for PCP PPy films at a charge/discharge current of 40 A g−1 within a voltage range of (a) 0–0.8 V and (b) 0–0.4 V in different electrolyte
solutions (3 M HCl, 3 M LiCl, 3 M NaCl and 3 M KCl).
Fig. 5. Scanning electron micrographs of PPy anode working for 30,000 cycles (a) in 3 M KCl and (b) in 3 M HCl.
line in the low-frequency region and an arc in the high-frequency
region. The vertical shape at lower frequencies indicates an ideal
capacitive behavior related to the film charging mechanism. The
arc part at higher frequencies is related to interfacial processes and
the diameter of arc corresponds to the interfacial charge transfer
resistance (Rct ). The two behaviors can be distinguished above and
below the knee frequency, which is marked in Fig. 3. The highest
knee frequency of PPy films in 3 M HCl indicates the H+ ion have best
conduction in the PPy films matrix during oxidation/reduction process, which can result from the smallest size of H+ ion. The smaller
the radius of the semicircle at higher frequencies indicates the
lower the Faraday resistance and the faster electrochemical reactions [16]. Rct of PCP PPy films in 3 M HCl is the smallest and about
0.4 . The very small Rct of 0.4 indicates PCP PPy films have easy
ion exchange with 3 M HCl in oxidation/reduction process compared to other electrolyte solutions. The Rct values of 1.8 , 3 and 7.1 were obtained in 3 M LiCl, NaCl and KCl, respectively. The
cations in electrolyte solutions have obvious effect on the values of
Rct , i.e., Rct increasing with the cationic radius increasing.
Fig. 4 shows discharge capacitance retention for PCP PPy films
at a charge/discharge current of 40 A g−1 . PCP PPy films is very stable in 3 M HCl within voltage range of 0–0.8 V, showing less than
3% decay after 20,000 cycles, which has not been reported before.
Especially within voltage range of 0–0.4 V, the specific capacitance
shows less than 4% decay after 160,000 cycles. While the capacitance deteriorated rapidly during the discharge/charge cycles in
3 M LiCl, NaCl and KCl. The specific capacitance of PCP PPy films
decreases by 30% in 3 M LiCl after 20,000 cycles. As is shown in
Fig. 5a, there are many pits and grooves on the surface of PPy anode
which has worked in 3 M KCl for 30,000 cycles. While pits and
grooves are few on the surface of PPy anode which has worked
in 3 M HCl for 30,000 cycles. The many pits and grooves on the surface of PPy anode should be main reason of the rapid decay of PPy
capacitance in 3 M KCl. The capacitance deterioration and damage
of PCP PPy films also correlates with radius of cations. The smaller
cations in electrolyte solution can contribute to high stability of
PCP PPy films. The volume of PCP PPy films increases in oxidation
process due to entering of Cl− and decreases in reduction process
due to ejecting of Cl− . Some authors considered the volume of PPy
films continues to increase in the first stage of reduction due to
cations entering the PPy matrix from the electrolyte [6,13,14]. So
the smaller cation can result in smaller change of PPy volume, which
can contribute to the good stability and little damage of PCP PPy
films during the discharge/charge process. A little change of volume caused by cation entering can be critical to PCP PPy stability,
especially when the volume of PCP PPy films has increased more
due to entering of Cl− after oxidation process. On the other hand, in
3 M HCl, the high concentration of H+ should reduce substitution of
hydrogen by hydroxyl ion or chloride ion in the polymer backbone,
which may contribute to high stability of PCP PPy films [17,18]. The
high stability of PCP PPy in HCl aqueous solution can promote the
commercialization of supercapacitors based on conducting polymers.
4. Conclusion
PPy films doped by p-toluenesulfonic (PTS) are synthesized
by pulse current polymerization for supercapacitors. The electrochemical properties of PCP PPy films are influenced by cations in
testing electrolyte solutions, displaying the capacitance and stability increasing with the cation radius decreasing. An approximate
rectangular-shaped CV curve is obtained at a high scan rate of
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Letter / Synthetic Metals 161 (2011) 1141–1144
2000 mV s−1 within a voltage range of −0.8 to +0.8 V in 3 M HCl,
which reveals the high rate of electrochemical process that can be
achieved in PCP PPy matrix. A high specific power of 110.9 kW kg−1
was obtained in 3 M HCl when its specific energy of PCP PPy films
reaches 18.4 Wh kg−1 . High specific power of PCP PPy film makes
it very suitable for electrode material of supercapacitors. Rct of
PCP PPy films increases with the cation increasing in electrolyte
solutions. Rct of PCP PPy films in 3 M HCl is very small and about
0.4 . PCP PPy films demonstrate excellent charge–discharge stability with no appreciable decay after 20,000 cycles within voltage
range of 0–0.8 V, whereas the capacitance deteriorated rapidly in
3 M LiCl, NaCl and KCl. The results of SEM show that many pits and
grooves appear on the surface of PPy anode in 3 M KCl after 30,000
cycles, which can result from the repeated expansion and shrinkage in oxidation/reduction process. The good stability of PPy in 3 M
HCl can result from smaller change of volume and less degradation
caused by overoxidation in oxidation/reduction process.
Acknowledgements
The authors wish to thank for the financial supports by the
National High Technology Research and Development Program of
China (Grant No. 2007AA03Z249) and the National Natural Science
Foundation of China (Grant No. 20804030).
References
[1]
[2]
[3]
[4]
[5]
[6]
D.B. Robinson, J. Power Sources 195 (2010) 3748.
M. Mastragostino, C Arbizzani, F. Soavi, Solid State Ionics 148 (2002) 493.
J. Wang, Y.L. Xu, X. Chen, J. Power Sources 163 (2007) 1120.
T.V. Vernitskaya, O.N. Efimov, Russ. Chem. Rev. 66 (1997) 443.
S. Ghosh, G.A. Bowmaker, R.P. Cooney, J.M. Seakins, Synth. Met. 95 (1998) 63.
T.F. Otero, H.J. Grande, J. Rodriguez, J. Phys. Chem. B 101 (1997) 3688.
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
E. Frackowiak, Phys. Chem. Chem. Phys. 9 (2007) 1774.
E. Frackowiak, V. Khomenko, K. Jurewicz, J. Power Sources 153 (2006) 413.
J. Wang, Y.L. Xu, X. Chen, X.F. Sun, Compos. Sci. Technol. 67 (2007) 2981.
Y.W. Ju, G.R. Choi, H.R. Jung, W.J. Lee, Electrochim. Acta 53 (2008) 5796.
R.K. Sharma, A.C. Rastogi, S.B. Desu, Electrochem. Commun. 10 (2008) 268.
J.P. Wang, Y.L. Xu, J. Wang, X.F. Du, F. Xiao, J.B. Li, Synth. Met. 160
(2010) 1826.
Q. Pei, O. Inganas, J. Chem. Phys. 96 (1992) 10507.
Q. Pei, O. Inganas, J. Chem. Phys. 97 (1993) 6034.
C.C. Hu, X.X. Lin, J. Electrochem. Soc. 149 (2002) A1049.
C.M. Li, C.Q. Sun, W. Chen, L. Pan, Surf. Coat. Technol. 198 (2005) 474.
C. Debiemme-Chouvy, T.T.M. Tran, Electrochem. Commun. 10 (2008) 947.
T. Shoa, M. Cole, N.R. Munce, V. Yang, J.D. Madden, Electroact. Polym. Actuators
Devices 6524 (2007) 52421.
Jingping Wang a,b
Electronic Materials Research Laboratory, Key
Laboratory of the Ministry of Education, Xi’an
Jiaotong University, Xi’an 710049, China
b Shaanxi University of Science and Technology, Xi’an
710021, China
a
Youlong Xu ∗
Jie Wang
Xianfeng Du
Electronic Materials Research Laboratory, Key
Laboratory of the Ministry of Education, Xi’an
Jiaotong University, Xi’an 710049, China
∗ Corresponding
author. Tel.: +86 29 82665161;
fax: +86 29 82665161.
E-mail address: [email protected] (Y. Xu)
4 September 2010
Available online 25 February 2011