February
物理化学学报(Wuli Huaxue Xuebao)
Acta Phys. -Chim. Sin. 2016, 32 (2), 493-502
[Article]
doi: 10.3866/PKU.WHXB201511131
493
www.whxb.pku.edu.cn
聚吡咯/硝酸活化碳气凝胶纳米复合材料的制备表征
及其在超级电容器中的应用
李亚捷
倪星元*
沈
军
刘
冬
刘念平
周小卫
(同济大学物理科学与工程学院，上海市特殊人工微结构材料与技术重点实验室，上海 200092)
摘要：通过化学氧化聚合法制备出不同比例的聚吡咯(PPY)/硝酸活化碳气凝胶(HCA)复合材料。采用傅里叶
变换红外光谱(FT-IR)和扫描电子显微镜(SEM)表征材料的成分和形貌，结果表明，通过硝酸活化及与聚吡咯
的复合，并未破坏碳气凝胶的多孔形貌，硝酸活化碳气凝胶及聚吡咯/硝酸活化碳气凝胶都仍然保持着原碳
气凝胶的三维纳米多孔结构。采用对照实验的方法，设计并合成五组不同配比的复合材料，聚吡咯与硝酸活
化碳气凝胶的质量比例分别为 3 : 1、2 : 1、1 : 1、1 : 2、1 : 3，通过循环伏安法，恒流充放电，交流阻抗及
循环性测试等考察材料的电化学性能。结果证明，当聚吡咯与硝酸活化碳气凝胶比例为 1 : 1 时，复合材料显
示出最优电化学性能：比电容高达 336 F∙g－1，是纯碳气凝胶(103 F∙g－1)的三倍有余，除此还显示出卓越的
导电性与循环稳定性，2000 次循环后仍保持初始电容的 91%，具备优良的超级电容器电极材料性能。因此
聚吡咯/硝酸活化碳气凝胶复合纳米材料是超级电容器的理想电极材料。
关键词：活化碳气凝胶；聚吡咯；超级电容器；化学氧化聚合法；复合电极材料；电化学性能
中图分类号：O646
Preparation and Performance of Polypyrrole/Nitric Acid Activated
Carbon Aerogel Nanocomposite Materials for Supercapacitors
LI Ya-Jie
NI Xing-Yuan*
SHEN Jun
LIU Dong
LIU Nian-Ping
ZHOU Xiao-Wei
(Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Institute of Physical
Science and Engineering, Tongji University, Shanghai 200092, P. R. China)
Abstract: Polypyrrole (PPY)/nitric acid (HNO3) activated carbon aerogel (HCA) composites are prepared
through chemical oxidative polymerization with different PPY/HCA mass ratios. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM) were employed to investigate the components and
morphology of the samples. The results demonstrate that the synthesized materials maintain the threedimensional nanoporous structure of the carbon aerogel (CA); the activation by nitric acid and composition with
PPY do not destroy the porous structure of the carbon aerogel and the complex still has the original threedimensional nanoporous structure. Composites with different mass ratios (3 : 1, 2 : 1, 1 : 1, 1 : 2, 1 : 3) of PPY/
HCA were prepared and the electrochemical properties were measured by cyclic voltammetry, galvanostatic
charge-discharge test, and electrochemical impedance spectroscopy. The results confirm that the PPY/HCA
composite with a ratio of 1 : 1 exhibits the best electrochemical performances; it has a high specific capacitance
of 336 F∙g － 1, which is more than two times higher than that of CA (103 F∙g － 1); it also exhibits outstanding
conductivity and cycling stability, retaining 91% of its initial capacitance after 2000 cycles. Therefore, this
Received: June 25, 2015; Revised: November 10, 2015; Published on Web: November 13, 2015.
*Corresponding author. Email: [email protected]; Tel/Fax: +86-21-65986071.
The project was supported by the National Natural Science Foundation of China (51072137, 50802064, 11074189), Key Projects in the National
Science & Technology Pillar Program, China (2009BAC62B02), and Shanghai Committee of Science and Technology, China (11nm0501600).
国家自然科学基金(51072137, 50802064, 11074189), 国家科技支撑计划重点项目(2009BAC62B02)及上海科学技术委员会项目(11nm0501600)
资助
© Editorial office of Acta Physico-Chimica Sinica
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Acta Phys. -Chim. Sin. 2016
Vol.32
composite is quite a promising electrode material for supercapacitors.
Key Words: Activated carbon aerogel; Polypyrrole; Supercapacitor; Chemical oxidative polymerization;
Composite electrode material; Capacitive property
1
Introduction
Increasing attention has been paid to supercapacitors due to
their high power and energy densities, long cycle life and wide
range of operating temperature. Besides, they are safe and environmentally friendly1. Thus, they are widely used in power and
energy applications such as hybrid electric vehicles (HEVs), burst
power generation, and backup sources2.
According to the energy storage mechanism, supercapacitors
can be classified into two categories: electrochemical double-layer
capacitors (EDLCs) and pseudo-capacitors3. In the EDLCs, energy
is stored by the accumulation of ionic and electronic charges at the
interface between electrolytes and electrode materials4. Carbonaceous materials, such as carbon fibers5, porous carbons6, activated carbons7, and carbon aerogels8, are promising electrode
materials for their high specific area, long cycle life, and relatively
low cost. In particular, carbon aerogel (CA) is a unique class of
three-dimensional nanoporous carbon materials that have high
surface area, good electrical conductivity, and high porosity1,9.
However, its specific capacitance is lower than expected due to
poor wettability. Chemical surface modification in nitric acid
solutions has been reported to improve the wettability of carbon
materials, which results in a higher usable surface area, smaller
internal resistance, and higher specific capacitance10. By comparison, pseudo-capacitors store energy through relatively fast and
reversible faradaic processes in a redox- active material at characteristic potentials11. They are able to store charge not only in the
electrical double layer, but also throughout the body of the electrodes by rapid faradaic charge transfer. The faradaic pseudocapacitance of pseudo-capacitors is almost 10－100 times higher
than EDLCs, but the improvement of capacitance by pseudofaradaic reactions is not stable and the capacitance decays with
cycling11,12. Transition metal oxides including ruthenium and
tantalum oxides are ideal electrode materials for pseudo-capacitors
because they have great specific capacitance. However, their high
cost has limited the practical application13. Electrically conducting
polymers (ECPs), such as polyaniline (PANI)14, poly(3-methylthiophene) (pMeT)15, and polypyrrole (PPY)16, are promising
electrode materials due to their high conductivity and relatively
low cost. Among these ECPs, PPY has good thermal and environmental stability, high storage stability, and relative ease of
synthesis16. However, during the cyclic electrochemical oxidation
and reduction process, continuously injection and rejection of
solvated ions will lead to the framework swelling and contraction
of the polymer chain, which influences the cycling performance
of PPY17.
Nowadays, supercapacitors which combine the advantages of
both EDLCs and pseudo- capacitors have become a promising
subject. For example, some researchers combined carbonaceous
materials with ECPs or with transition metal oxides to synthesize
composite electrodes such as CNTs (carbon nanotubes)- PANI
composite18, CNTs (carbon nanotubes)- PPY composite19, TiO2activated carbon composite20, and manganese oxide/MWNTs
(multiwalled carbon nanotubes) composite electrodes21. The results
show that after recombination, the electrochemical performances
of electrodes are improved.
In this work, PPY/nitric acid activated carbon aerogel (HCA)
composites were synthesized via chemical oxidation polymerization with different PPY contents and we explored the optimum
mass ratio of HCA to PPY. The method involved in this paper is
simple, convenient and beneficial for commercial applications.
The final results indicate that the PPY/HCA composite with the
mass ratio of 1 : 1 has high specific capacitance, excellent conductivity, and long term stability.
2
Experimental
2.1 Preparation of carbon aerogel
The carbon aerogel was prepared via a sol-gel process: resorcinol (R) (analytical reagent) and formaldehyde (F) (analytical
reagent) were mixed in a 1 : 2 molar ratio with alkaline sodium
carbonate (C) (analytical reagent) as catalyst and deionized water
as solvent. The mass percentage of reactions in solution was 30%,
the molar ratio of R to C was held at a constant value of 500.
Stirring the above solution at room temperature for 30 min, then
the hydrosols were sealed and heat-treated at 30 °C for one day,
50 °C for one day, and 90 °C for three days, respectively. The
resultant RF hydrogels were rinsed in an ethanol bath for a week,
then CO2 supercritical drying at 31.8 °C and 7.3 MPa was carried
out to get cylindrical organic aerogels. The obtained RF organic
aerogels were carbonized at 1050 ° C for 3 h with the rising
temperature rate of 5 °C ∙min－1 in a tubular furnace under N2 flow
of 100 mL∙min－1 22,23. The obtained RF carbon aerogel sample was
denoted as CA.
2.2 Nitric acid activation of carbon aerogel
CA was dispersed in concentrated nitric acid (65%) at 60 °C for
12 h, filtered and washed with deionized water until the pH was
about 6, then the product was dried at 90 °C for 24 h, denoted as
HCA.
2.3 Synthesis of polypyrrole/nitric acid activated
carbon aerogel
Firstly, 0.1 g HCA was dispersed in 200 mL of deionized water,
then mixed with 0.02 g sodium dodecyl sulfate (SDS) (analytical
reagent) as surfactant, stirred for 30 min and kept for ultrasound
for 2 h. Secondly, pyrrole monomer (analytical reagent) was added
into the above solution in five kinds of pyrrole : HCA mass ratios
(1 : 1, 2 : 1, 3 : 1, 1 : 2, 1 : 3), stirred for 10 min and irradiated by
ultrasonic wave for 30 min. Thirdly, 0.2 g FeCl3 (93%) was added
No.2
LI Ya-Jie et al.: Preparation and Performance of PPY/HCA Nanocomposite Materials for Supercapacitors
into the solution as oxidant to motivate polymerization reaction,
stirred for 30 min and kept for ultrasound for 2 h. The reaction was
carried out under static condition for 24 h and then the PPY/HCA
composite precipitate was filtered and washed with deionized
water and ethanol. Lastly, the product was dried at 60 °C for 24
h 24. The name of PPY/HCA composite was abbreviated as PPYHCA-x, where x stands for the mass ratio of PPY to HCA. For
example, PPY-HCA-21 means that the mass ratio of PPY to HCA
was 2 : 1.
2.4 Structural characterization
The components of samples were determined by Fourier
transform infrared (FT-IR) spectroscopy (Bruker-TENSOR27).
The field emission scanning electron microscopy (FESEM, Philips
XL30FEG) was used to examine the structures and morphology
of materials. N2 adsorption isotherms were recorded with an
AUTOSORB-1 Surface Area Analyzer (Quantachrome Instrument
Corporation) at － 196 °C. Prior to measurements, the samples
were degassed at 300 °C for 2 h. The specific surface areas were
determined on the basis of the Brunauer-Emmett-Teller (BET)
method. The pore size distribution was obtained by employing
density functional theory (DFT). The total pore volume was estimated from the amount of N2 adsorbed at the relative pressure of
p/p0 = 0.99. And t- plot method was used to determine the micropore surface area and micropore volume.
2.5 Electrochemical measurements
Working electrodes were prepared by the following method: a
mixture of active materials, conductive carbon black, and
polytetrafluoroethylene (PTFE) at a mass ratio of 8 : 1 : 1 was
pasted onto nickel foam, pressed at 8 MPa and dried at 80 °C for
48 h. Electrochemical tests were carried out at room temperature
using an electrochemical work station (CHI660C, Chenhua,
China). A three-electrode cell was set up using nickel foam as
counter electrode, Hg/HgO electrode as reference electrode, PPY/
HCA composite as working electrode, and KOH (6 mol ∙ L － 1)
aqueous solution as electrolyte.
Cyclic voltammetry (CV) measurements were conducted over
a potential window from － 0.8 to － 0.1 V at different scan rates
ranging from 5 to 100 mV∙s － 1. Galvanostatic charge-discharge
measurements were ranged from － 0.8 to － 0.1 V at different
current densities ranging from 0.5 to 5 A∙g － 1. Electrochemical
impedance spectroscopy (EIS) measurements were recorded from
0.01 Hz to 100 kHz with 5 mV amplitude of AC signal.
3
495
and ＝C＝O with hydrophilic property, which could improve the
wettability of CA12. The spectra of three composites are similar but
they are quite different from CA and HCA, there emerge a series
of new peaks: 794 cm－1 is attributed to the C＝C deformation of
PPY, the broad band at 1320 cm－1 demonstrates the C―H and C―
N in-plane deformation vibration, 966 and 1047 cm－1 are attributed
to the C―H deformation of PPY, the peak near 930 cm － 1 is attributed to the characteristic absorption of pyrrole ring, the peak
near 1213 cm－1 is attributed to the C―N stretching vibration of
PPY, the peaks near 1462 and 1546 cm－1 are attributed to C―N
and C―C asymmetric and symmetric pyrrole ring stretching,
respectively. Additionally, 2850 and 2920 cm－1 present the C―H
asymmetric and symmetric stretching vibration of SDS which
acted as surfactant25,26. The above results illustrate that PPY/HCA
composites are successfully synthesized.
Fig.1(b) is the standard infrared spectrum of PPY27, where we
can see characteristic peaks: 794, 930, 966, 1047, 1213, 1320,
1462, and 1546 cm － 1, they are all attributed to PPY and are all
conformed to former analysis. Therefore, PPY/HCA composites
are successfully prepared via chemical oxidative polymerization.
The SEM images of CA, HCA and PPY/HCA composites are
shown in Fig.2. From Fig.2 (a, b), it can be seen that CA and HCA
consist of interconnected sphere nanoparticles with diameters of
30 to 40 nm. Both of them have porous structure, which suggests
that nitric acid activation did not destroy the overall skeleton and
Results and discussion
3.1 Structural characterization
Fig.1(a) shows the FT-IR spectra of CA, HCA, PPY-HCA-11,
PPY-HCA-21, and PPY-HCA-31. As shown in this image, the
spectrum of HCA is similar to that of CA, but a few new peaks
such as 1690 and 1725 cm － 1 appear, which represent the deformation vibration of ―COOH and ＝C＝O, respectively. Because
of the strong oxidation of concentrated nitric acid, oxidationreduction reaction may take place on the surface of CA, engendering a great deal of oxygen containing groups such as ―COOH
Fig.1 (a) FT- IR spectra of CA, HCA, PPY- HCA- 11, PPY- HCA21, PPY-HCA-31 and (b) standard infrared spectrum of PPY
496
Acta Phys. -Chim. Sin. 2016
Fig.2
SEM images of CA (a), HCA (b), PPY-HCA-11 (c), PPY-HCA-21 (d), and PPY-HCA-31 (e)
maintains the three-dimensional nanoporous structure of carbon
aerogel. Fig.2(c, d, e) shows the SEM images of PPY-HCA-11,
PPY-HCA-21, and PPY-HCA-31, respectively. They are similar
to CA, and appear no obvious clusters, which indicate that PPY
is uniformly coated on the carbon aerogel. The composites still
maintain three-dimensional nanoporous structure, which facilitates
the ion transfer in charging and discharging processes.
Fig.3 shows the N2 adsorption-desorption isotherms of CA, HCA,
PPY-HCA-11, PPY-HCA-21, and PPY-HCA-31, respectively,
which indicates a similar structure of 5 samples. As can be seen
from the left of the curves, it reaches single layer absorption very
Fig.3
Vol.32
fast, which demonstrates rich pore structures with diameter under
2 nm. The hysteresis loops on the right indicate abundant number
of pores between 2 and 50 nm. All 5 samples show abundant
structure of mesopore and micropore, which also proves the result
of the FESEM that the pore structure remains after activation and
composition.
Fig.4 shows the comprehensive N2 adsorption-desorption isotherms of CA, HCA, PPY-HCA-11, PPY-HCA-21, and PPY-HCA31, which shows the pore volume of 5 samples. As can be seen
from the left of the curves, CA has the highest volume platform
on the left, following with HCA, PPY-HCA-11, PPY-HCA-21, and
N2 adsorption-desorption isotherms of CA, HCA, PPY-HCA-11, PPY-HCA-21, and PPY-HCA-31
No.2
Fig.4
LI Ya-Jie et al.: Preparation and Performance of PPY/HCA Nanocomposite Materials for Supercapacitors
497
Comprehensive N2 adsorption-desorption isotherms of CA,
HCA, PPY-HCA-11, PPY-HCA-21, and PPY-HCA-31
PPY-HCA-31. According to the adsorption theory, higher volume
indicates more micropores, makes CA with the most micropores.
And same order of curve height can be observed on the right.
Higher volume indicates more pore structure and huger pore
volume. Therefore, CA has the most quantity of pore with hugest
pore volume and followed with HCA, PPY-HCA-11, PPY-HCA21, and PPY-HCA-31.
Fig.5 shows the pore size distribution of 5 samples. It can be
seen that most of the pores are mesopore between 10 and 50 nm.
But there is a 4 nm peak on the curve of HCA, indicates the
present of the micropore which may be generated during the activation. After polymerization, micropores are filled by PPY while
has little effect on mesopore. This result demonstrates that all 5
samples have rich mesopore structure consistent with FESEM and
N2 adsorption-desorption isotherms results.
The schematic of PPY/HCA composite preparation process is
shown in Fig.6. CA and SDS dispersed in the deionized water, the
hydrophobic groups-long chain alkyl of SDS makes it attached to
Fig.5
Fig.6 Schematic diagrams of the PPY/HCA composite
preparation process
CA quickly. Simultaneously, the hydrophilic groups-sulfate anions
of SDS form a negative charge layer on the surface of CA. Then
pyrrole monomer, which was added into the above solution, will
be attracted by the negative charge layer due to electrostatic interaction. Upon adding FeCl3 as oxidant, pyrrole monomer on the
surface of carbon aerogel will polymerize to PPY via chemical
oxidative polymerization28. Finally, PPY/HCA composite materials
are prepared.
3.2 Electrochemical performance
The cyclic voltammogram (CV) curves of samples at the scan
rate of 50 mV∙s－1 are shown in Fig.7. It can be found that the CV
curves of CA and HCA are rectangular and symmetrical, indicating a typical electric double layer (EDL) behavior and good
stability. The energy storage mechanism of carbon aerogel in
EDLCs has been described in previous literature29. HCA exhibits
Pore size distribution of CA, HCA, PPY-HCA-11, PPY-HCA-21, and PPY-HCA-31
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Acta Phys. -Chim. Sin. 2016
Fig.7
Cyclic voltammogram curves of samples at a
scan rate of 50 mV∙s－1
larger CV area than that of CA, revealing that it has larger specific
capacitance than CA. The CV curves of PPY-HCA-11, PPY-HCA21, PPY-HCA-31, PPY-HCA-12, and PPY-HCA-13 are similar:
compared with the CV curves of HCA, the oxidation and reduction peaks can be obviously seen, which are owing to the redox
reaction of PPY18.
This redox reaction of PPY is related with sequential Lewis baseor Lewis acid-producing steps: in the discharging process, a re-
Vol.32
duction process with release of hydroxyl ions is involved. With the
involvement of ions of the electrolyte, this Lewis ionization process
quotes a quasi-linear, one-dimensional cylindrical Helmholtz-like
double layer developed; in the charging process, due to a Lewis
ionization process which involves oxidation with electron transfer,
positive charges are introduced on the PPY chain by p-doping30,31.
In order to estimate the detail electrochemical properties, cyclic
voltammetry was carried out for each electrode at various scan
rates of 5, 10, 50, and 100 mV∙s－1, as shown in Fig.8. The specific
capacitance of the electrode can be calculated by the following
equation:
C = I/mv
(1)
where I is the average current, m is the effective mass of electrode
materials, and v is the scan rate32. The effective masses of CA, HCA,
PPY-HCA-11, PPY-HCA-21, PPY-HCA-31, PPY-HCA-12, and
PPY- HCA- 13 are 0.012, 0.011, 0.008, 0.012, 0.012, 0.012, and
0.012 g, respectively. The results obtained are listed in Table 1.
By comparing the specific capacitance of CA with that of HCA,
we can find that after nitric acid activation, the capacitance is 50%
higher than CA, which is owing to the improvement of wettability.
Thus the utilization rate of electrode materials in aqueous electrolytes increases. PPY/HCA composite electrode materials have
Fig.8 Cyclic voltammogram curves of CA (a), HCA (b), PPY-HCA-11 (c), PPY-HCA-21 (d), PPY-HCA-31 (e),
PPY-HCA-12 (f), and PPY-HCA-13 (g) at different scan rates
No.2
Table 1
LI Ya-Jie et al.: Preparation and Performance of PPY/HCA Nanocomposite Materials for Supercapacitors
Specific capacitances of materials at different scan rates
Specific capacitance/(F∙g )
－1
Sample
5 mV∙s－1
10 mV∙s－1
50 mV∙s－1
100 mV∙s－1
CA
103
95
87
80
HCA
139
136
133
124
PPY-HCA-11
336
325
257
228
PPY-HCA-21
316
295
221
214
PPY-HCA-31
334
318
229
174
PPY-HCA-12
210
191
171
150
PPY-HCA-13
202
185
174
140
much higher specific capacitance, almost 1－2 times higher than
that of CA. On the whole, PPY-HCA-11 has the highest specific
capacitance, followed by PPY-HCA-31, PPY-HCA-21, PPY-HCA11 and then PPY-HCA-13, their specific capacitances are all much
higher than those of HCA and CA. The specific capacitance of
PPY-HCA-11 reaches 336 F∙g－1 at a scan rate of 5 mV∙s－1, while
the capacitance of CA electrode is only 103 F∙g－1. According to
the above results and related theories, we can come to the following conclusions: composite materials combine the double-layer
capacitances of HCA and Faradic capacitances of PPY. Therefore,
the composites have a substantial increase in specific capacitance18.
In general, the specific capacitance of each sample increases
with the decreasing of scan rates, the reason is that electrostatic
adsorption-desorption reaction and oxidation-reduction reaction
occur not only in the surface of electrode, but also inside the
materials33. Hence at lower scan rate, the electrolyte can penetrate
well into the electrode materials, increasing the utilization rate of
materials, so the specific capacitance is improved.
The charge-discharge curves of samples measured in 6 mol∙L－1
KOH aqueous electrolyte at a current density of 1 A∙g－1 are shown
in Fig.9. The curves of CA and HCA are almost linear and present
typical symmetrical triangle shape, indicating that they have
double-layer capacitive behavior, while the curves of composite
electrodes are not linear due to the existence of the Faradic reaction of PPY.
The specific capacitance of samples can be calculated according
to equation (2):
Cm = It/m∆V
(2)
Fig.9
Charge-discharge curves of samples at a
current density of 1 A∙g－1
499
where, t is the discharge time, and ΔV is the voltage34. From Fig.9
we can find that the discharge time sequence of samples is PPYHCA-11 > PPY-HCA-31 > PPY-HCA-21 > PPY-HCA-12 > PPYHCA-13 > HCA > CA, so does the order of specific capacitance.
This sequence is resistent with the results acquired by cyclic
voltammetry, illustrates that composite electrodes have larger
specific capacitance attributed to the existence of pseudo-capacitance and confirms that PPY-HCA-11 has the highest specific
capacitance.
At the beginning of the discharge, a sharp voltage change,
which can be used to estimate the resistance of materials, is observed in each curves, the larger the voltage dip, the bigger the
equivalent series resistance (ESR). The resistance of samples can
be calculated by the following equation:
R = ∆U/I
(3)
where ΔU is the voltage dips and I is the charge-discharge current32. The resistances of CA, HCA, PPY-HCA-11, PPY-HCA-21,
PPY-HCA-31, PPY-HCA-12, and PPY-HCA-13 are 0.026, 0.018,
0.014, 0.013, 0.011, 0.013, and 0.017 Ω, respectively. PPY-HCA
composites have lower resistance than CA and HCA, indicating
that the addition of PPY improve the conductivity of CA. For
supercapacitors that utilize pseudo-capacitance, there are three
types of electrochemical processes in the energy storage: (1)
surface adsorption of ions from the electrolyte; (2) redox reactions
involving ions from the electrolyte; (3) the doping and undoping
of ECPs in the electrodes. In the above processes, the electrodes
must have high electronic conductivity to distribute and collect the
electron current3. Hence the composites with higher conductivity
are ideal electrode materials for supercapacitors.
Furthermore, to understand more about the electrochemical
behavior of PPY-HCA-11, galvanostatic charge-discharge tests
were carried out at various current densities of 0.5, 1, 2, 3, 4, and
5 A∙g－1, as shown in Fig.10. With the increasing of current density,
the curves still present typical symmetrical triangle shape.
However, the specific capacitance decreases due to the relatively
low rate of ions diffusion within micropores at large current
density.
Electrochemical impedance spectroscopy (EIS) is a useful
technique to characterize the electrochemical properties of elec-
Fig.10 Charge-discharge curves of PPY-HCA-11 at
different current densities of 0.5, 1, 2, 3, 4, and 5 A∙g－1
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Acta Phys. -Chim. Sin. 2016
trodes. The Nyquist plots of electrodes are shown in Fig.11(a). All
of them exhibit three connected parts: a semicircle in the high
frequency region which corresponds to the charge transfer reaction
at the interface of electrode and electrolyte, a 45° line in the intermediate frequency region associated with Warburg impedance
of ion diffusion inside the electrode materials and a straight line
in the low frequency region responding to the capacitive performance35.
The specific capacitance can be derived from the imaginary part
of the impedance spectrum and frequency according to the following equation:
C = －1/(mfπZʺ)
(4)
where, f is the frequency, and Zʺ is the imaginary impedance36.
Fig.11(b) shows the specific capacitance of samples on frequency
derived from impedance spectroscopy. The specific capacitance
of electrodes increased with the decreasing of frequency. This is
because the electrolyte ions can reach the inner surface sites of the
electrode materials under lower frequency37. In the lower frequency region, under the same frequency, the order of the specific
capacitances of electrodes is PPY-HCA-11 > PPY-HCA-31 > PPYHCA-21 > PPY-HCA-12 > PPY-HCA-13 > HCA > CA, which is
in accordance with the results obtained from cyclic voltammetry
and galvanostatic charge-discharge test. It confirmed that PPYHCA- 11 has the best capacitance- frequency response and the
highest specific capacitance.
The cycle life is a significant factor for supercapacitors. As
shown in Fig.12, the specific capacitance of CA and HCA is nearly
100% after 2000 times. The specific capacitance of HCA is 50%
Fig.11 AC impedance spectra of materials
(a) Nyquist plots, (b) capacitance vs frequency plots
Fig.12
Vol.32
Cyclic life of materials CA (a), HCA (b), PPY-HCA-11 (c),
PPY-HCA-21 (d), PPY-HCA-31 (e), PPY-HCA-12 (f),
and PPY-HCA-13 (g)
higher than CA, indicating that nitric acid activation does not
influence the cyclic stability of carbon aerogel and improve the
specific capacitance. Composite electrode materials have higher
capacitance, but their stability is inferior to CA and HCA. After
2000 times, the capacity deteriorations of PPY-HCA-11, PPY-HCA21, PPY-HCA-31, PPY-HCA-12, and PPY-HCA-13 are 9%, 15%,
21%, 12%, and 20%, respectively. The capacity deterioration rises
with the changing of the content of PPY, PPY-HCA-11 showing the
best stability among composite materials: the loss of capacitance
mainly happens at first 500 cycles, after 1000 times the specific
capacitance stabilizes at a fixed high value; after 2000 times, the
specific capacitance still remains 91% of the initial value, which is
still much higher than other samples. The order of the specific capacitances of electrodes is PPY-HCA-11 > PPY-HCA-31 > PPYHCA-21 > PPY-HCA-12 > PPY-HCA-13 > HCA > CA, which is
consistent with the results obtained from cyclic voltammetry, galvanostatic charge-discharge test, and electrochemical impedance
spectroscopy. This result also corroborates that PPY-HCA-11 has
the highest specific capacitance.
In the composite materials, PPY can enhance the capacitance
remarkably; meanwhile, as a conductive framework, HCA can
increase the cycling and physical stability of PPY. The above
results confirmed that the optimum ratio of PPY to HCA is 1 : 1.
The reasons are as follows: SEM shows that the sample has rich
pore structure and appears no obvious clusters, which indicates
that PPY is uniformly coated on the carbon aerogel, and maintains
the three-dimensional nanoporous structure after activation and
composition. The pore structure facilitates the ion transfer in
charging and discharging processes, thus propitious to the improvement of electrochemical properties.
N2 adsorption-desorption isotherms also show abundant structure
of mesopore and micropore in samples. The composite contains
carbon aerogel and PPY, which combine the double- layer capacitances of HCA and Faradic capacitances of PPY. If PPY is
over 50% , the pseudo-capacitance will be too huge for cycle
performance as well as huge impedance of PPY, and poor cycling
stability. If PPY is less than 50%, the pseudo-capacitance will be
too low to improve the specific capacitance.
No.2
According to the above results and related theories, we can
come to the following conclusions: composite materials combine
the double-layer capacitances of HCA and Faradic capacitances
of PPY. Therefore, the composites have a substantial increase in
specific capacitance. The above results confirmed that the optimum ratio of PPY to HCA is 1 : 1, the composite material shows
the best electrochemical properties. It is a promising electrode
material for supercapacitors.
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(14)
4
501
LI Ya-Jie et al.: Preparation and Performance of PPY/HCA Nanocomposite Materials for Supercapacitors
Conclusions
Wettability of CA can be improved by surface modification with
nitric acid. After activation, the specific capacitance of CA increases by 50%. Besides, the modified CA maintains excellent
cycle stability. The PPY/HCA composites have three-dimensional
nanoporous structure as CA. Because they possess not only doublelayer capacitance but also pseudo-capacitance, their specific capacitances are 1 － 2 times higher than that of CA. With the increasing of PPY content in composites, the conductivity increases,
but the long term stability becomes worse due to the existence of
Faradic pseudo-capacitance.
PPY- HCA-11 has the highest specific capacitance among
samples, its capacitance reaches 336 F∙g－1 at a scan rate of 5 mV∙
s－1, while the capacitance of CA is only 103 F∙g－1, it also has good
cycling stability and retains 91% of initial capacitance over 2000
times, which is still much higher than that of CA. Besides, its
conductivity is excellent and it is more cost- saving than other
composites. Consequently, the PPY/HCA composite with the ratio
of 1 : 1 is an ideal electrode material for supercapacitors.
Xu, H.; Li, J. L.; Peng, Z. J.; Zhuang, J. X.; Zhang, J. L.
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