Synthesis of highly crystalline polyaniline with the use of (Cyclohexylamino)-1-propanesulfonic acid for supercapacitor

Abstract

Polyaniline (PANI) salt was prepared with 3-(Cyclohexylamino)-1-propanesulfonic acid (CAPS) as a novel dopant by aqueous polymerization pathway. Effects of sodium lauryl sulfate surfactant, mineral acid (H2SO4), and a combination of surfactant with mineral acid during the polymerization reaction were also determined. PANI-CAPS showed semicrystalline with flake-like morphology. The use of the sodium lauryl sulfate along with CAPS resulted in the formation of highly crystalline nanospheres with flake-like morphology. In order to find out the effect of surfactant, sodium lauryl sulfate was used in the reaction. The combination of sodium lauryl sulfate, CAPS, and H2SO4 brings about an extended nanosphere morphology. These polyaniline salts were used as electrode materials in the supercapacitor application, in a symmetric two-electrode cell configuration. The values of specific capacitance, energy, and power densities of PANI-CAPS-DHS-H2SO4 material at 2 mA cm−2 were 495 F g−1, 90 kJ kg−1, and 120 J Kg−1 s−1, respectively. Moreover, 85 % of the original capacitance was retained after 3,000 galvanostatic charge–discharge cycles with a coulombic efficiency of 96–99 %. The value of phase angle is close to 90 at low frequencies, indicating a good capacitive behavior.

Full text

RESEARCH ARTICLE
Synthesis of highly crystalline polyaniline with the use
of (Cyclohexylamino)-1-propanesulfonic acid for supercapacitor
Ravi Bolagam •Rajender Boddula •
Palaniappan Srinivasan
Received: 3 July 2014 / Accepted: 22 September 2014 / Published online: 2 October 2014
ÓSpringer Science+Business Media Dordrecht 2014
Abstract Polyaniline (PANI) salt was prepared with
3-(Cyclohexylamino)-1-propanesulfonic acid (CAPS) as a
novel dopant by aqueous polymerization pathway. Effects
of sodium lauryl sulfate surfactant, mineral acid (H
2
SO
4
),
and a combination of surfactant with mineral acid during
the polymerization reaction were also determined. PANI-
CAPS showed semicrystalline with flake-like morphology.
The use of the sodium lauryl sulfate along with CAPS
resulted in the formation of highly crystalline nanospheres
with flake-like morphology. In order to find out the effect
of surfactant, sodium lauryl sulfate was used in the reac-
tion. The combination of sodium lauryl sulfate, CAPS, and
H
2
SO
4
brings about an extended nanosphere morphology.
These polyaniline salts were used as electrode materials in
the supercapacitor application, in a symmetric two-elec-
trode cell configuration. The values of specific capacitance,
energy, and power densities of PANI-CAPS-DHS-H
2
SO
4
material at 2 mA cm
-2
were 495 F g
-1
,90kJkg
-1
, and
120 J Kg
-1
s
-1
, respectively. Moreover, 85 % of the ori-
ginal capacitance was retained after 3,000 galvanostatic
charge–discharge cycles with a coulombic efficiency of
96–99 %. The value of phase angle is close to 90 at low
frequencies, indicating a good capacitive behavior.
Keywords Conducting polymers Novel dopant 
Polyaniline Supercapacitor Crystallinity
1 Introduction
Supercapacitors, which are also termed as electrochemi-
cal capacitors or ultracapacitors, have been studied for
application in digital communication devices, digital
cameras, mobile phone, power supplies, and hybrid
electric vehicles. Supercapacitors have higher power
density and longer cycle life compared to secondary
batteries and higher energy density compared to con-
ventional electrochemical double-layer capacitors [1–3].
The performances of supercapacitors are primarily
determined by the electrode materials [4]. Capacitance
performance of supercapacitors depends on active elec-
trode materials based on carbon materials, metal oxides,
and conducting polymers, which are having their own
advantages and disadvantages. Carbon-based materials
can provide high power density and long cycle life, but
its low specific capacitance limits its application for high
energy density devices [5,6]. Metal oxides/hydroxides
possess pseudocapacitance in addition to double layer
capacitance and have wide charge/discharge potential
range, higher energy density, and better cycling stability,
but they have a key weakness of poor conductivity and
high cost [7,8]. On the other hand, conducting polymers
have been intensively studied as electrodes in superca-
pacitors due to their high electrical conductivity, elec-
trochemical reversibility, larger pseudo-capacitance, and
faster doping/dedoping rate during charge/discharge pro-
cess, but they have low mechanical stability and cycle
life [9]. Among the conducting polymers, polyaniline
(PANI) has been regarded as one of the most promising
conductive polymers due to its low cost, easy synthesis,
controllable electrical conductivity, and good environ-
mental stability [10]. We have written a chapter on
‘‘Recent advances in the approach of polyaniline as
R. Bolagam P. Srinivasan
Academy of Scientific and Innovative Research, New Delhi,
India
R. Bolagam R. Boddula P. Srinivasan (&)
Polymers & Functional Materials Division, CSIR-Indian
Institute of Chemical Technology, Tarnaka, Hyderabad 500007,
India
e-mail: palani74@rediffmail.com; palaniappan@iict.res.in
123
J Appl Electrochem (2015) 45:51–56
DOI 10.1007/s10800-014-0753-4

electrode for supercapacitor,’’ wherein we have covered
the polyaniline materials for supercapacitor application
[11]. Very recently, inkjet-printed polyaniline was used as
electrode in supercapacitor application [12,13].
Herein, we report (i) a facile synthesis of highly crystal-
line nanostructured polyaniline via an aqueous polymeriza-
tion pathway using 3-(Cyclohexylamino)-1-propanesulfonic
acid (CAPS) as a novel dopant; (ii) effects of surfactant and
mineral acid in the preparation of polyaniline salt; and (iii)
the use of these polyaniline salts as electrodes in superca-
pacitor cell and their performances.
2 Experimental
2.1 Instruments and characterization
Powder of polyaniline was pressed into a disk of 13-mm
diameter and about 1.5-mm thickness under a pressure of
120 kg cm
-2
. The resistance of the pellet was measured by
four-probe method using Keithley constant source (Model-
6220) and nanovoltmeter (Model-2182A) (Keithley,
Cleveland, Ohio, USA). Pellet density was measured from
mass per unit volume of the pressed pellet. FT-IR spectra
of polymer samples were registered on a FT-IR spec-
trometer (Thermo Nicolet Nexus 670, USA) using KBr-
pressed pellet technique. X-ray diffraction profiles for
polymer powders were obtained on a Siemens/D-500 X-ray
diffractometer, USA, using Cu Karadiation and the scan
speed of 0.045°min
-1
. Morphology studies (microstruc-
tural and elemental analysis) of the polymer samples were
carried out using a Hitachi S-4300 FE-SEM (Tokyo,
Japan). The sample was mounted on a carbon disk with the
help of double-sided adhesive tape and sputter-coated with
a thin layer of gold to prevent sample-charging problems.
The electrode was made by pressing 5 mg of polyaniline
sample on stainless steel mesh by the application of
100 kg cm
-2
pressure. Supercapacitor cell (Swagelok-type
cell) was constructed using two polyaniline electrodes in
1 M aqueous H
2
SO
4
electrolyte solution without a refer-
ence electrode, and a cotton cloth was utilized as a sepa-
rator. Cyclic voltammetry and galvanostatic charge–
discharge experiments were carried on supercapacitor cell
using a WonATech multichannel potentiostat/galvanostat
(WMPG1000, GyeongGi-do, Korea) equipment. Cyclic
voltammograms were recorded from -0.2 to 0.6 V at
various sweep rates. Galvanostatic charge–discharge
experiments were carried out from 0 to 0.6 V at various
current densities. Electrochemical impedance spectroscopy
measurements were carried out using IM6ex zahner-Elek-
trik (Germany) equipment in the frequency range of
40 kHz–10 mHz at various voltages using three-electrode
configuration, i.e., polyaniline salt as working electrode,
platinum as counter electrode, saturated calomel electrode
(SCE) as reference electrode, and 1 M aqueous H
2
SO
4
electrolyte solution. All the electrochemical measurements
were performed at the ambient temperature.
2.2 Preparation of PANI-CAPS salt
Aniline (0.93 g, 0.1 M) and 3-(Cyclohexylamino)-1-pro-
panesulfonic acid (2.21 g, 0.1 M) were dissolved in 50 ml
of distilled water. To this solution, 50 mL distilled water
containing ammonium persulfate (2.28 g, 0.1 M) was
added as a whole. The mixture was stirred constantly for
4 h at the ambient temperature. The green precipitate was
filtered and washed several times with distilled water fol-
lowed by acetone. The powder sample was dried at 50 °C
in oven.
2.3 Preparation of PANI-CAPS-DHS salt
In the above reaction, sodium lauryl sulfate (1 g, 0.035 M)
was taken along with aniline and 3-(Cyclohexylamino)-1-
propanesulfonic acid in 50 ml water, and further process
was carried out by the above procedure.
2.4 Preparation of PANI-CAPS-H
2
SO
4
salt
In the above reaction (PANI-CAPS), aqueous sulfuric acid
(1 M) was taken along with aniline and 3-(Cyclohexyl-
amino)-1-propanesulfonic acid in 50 ml water, and further
process was carried out by the above procedure.
2.5 Preparation of PANI-CAPS-DHS-H
2
SO
4
salt
In the above reaction (PANI-CAPS), sodium lauryl sulfate
(1 g, 0.035 M) and aqueous sulfuric acid (1 M) were taken
along with aniline and 3-(Cyclohexylamino)-1-propane-
sulfonic acid in 50 ml water, and further process was car-
ried out by the above procedure.
3 Results and discussion
In this work, an organic acid, 3-(Cyclohexylamino)-1-
propanesulfonic acid (CAPS), was used as a novel dopant
for the preparation of polyaniline salt (PANI-CAPS) via an
aqueous polymerization process by oxidizing aniline using
ammonium persulfate (Scheme 1). In order to find out the
effect of surfactant, sodium lauryl sulfate was used in the
reaction. In the course of polymerization, sodium lauryl
sulfate got converted into dodecyl hydrogen sulfate (DHS)
under the acidic condition and incorporated into polyani-
line system along with CAPS (PANI-CAPS-DHS). Mineral
acid (H
2
SO
4
) was also tried out along with an organic acid,
52 J Appl Electrochem (2015) 45:51–56
123

wherein, polyaniline got doped with both CAPS and H
2
SO
4
(PANI-CAPS-H
2
SO
4
). The use of mixture of surfactant
and mineral acid along with CAPS led to the formation of
PANI-CAPS-DHS-H
2
SO
4
. The values of yield and con-
ductivity for the polyaniline salts are included in
Scheme 1.
The values of yield and conductivity of sample PANI-
CAPS were 0.5 g and 0.03 S cm
-1
, respectively. The
values of yield and conductivity increased with the use of
surfactant and then increased further with the use of min-
eral acid and still further increased with the use of sur-
factant mineral acid mixture. This result indicates that the
oxidizing and doping power is higher with the use of sur-
factant and mineral acid. However, pellet density
(1.33 g cm
-3
) calculated from mass per unit volume of the
pellet was found to be independent of the use of surfactant
and mineral acid.
3.1 Infrared spectra of polyaniline salts
The FT-IR spectra of PANI-CAPS, PANI-CAPS-DHS,
PANI-CAPS-H
2
SO
4
, and PANI-CAPS-DHS-H
2
SO
4
are
shown in Fig. 1, and their corresponding peak positions are
reported in Table 1along with the reported ‘‘conventional’’
polyaniline SALT and BASE [14]. IR spectra of PANI-
CAPS-H
2
SO
4,
PANI-CAPS-DHS, and PANI-CAPS-DHS-
H
2
SO
4
are very nearly the same, which result is in turn
similar to that of the reported PANI SALT. Moreover, IR
spectrum of PANI-CAPS-DHS-H
2
SO
4
shows two addi-
tional peaks 1,635 and 1,010 cm
-1
which could be attrib-
uted to SO
3
H groups of H
2
SO
4
and/or CAPS. However, IR
spectrum of PANI-CAPS shows three peaks at 1,585,
1,500, and 820 cm
-1
which are assigned to PANI BASE,
and the remaining peaks are assigned to PANI SALT. A
peak at 1,040 cm
-1
originates from SO
3
H group of CAPS.
The IR spectrum of PANI-CAPS signposts that the doping
efficiency is less with the use of only CAPS as dopant.
3.2 XRD patterns of polyaniline salts
The X-ray diffraction profile registered for PANI-CAPS-
H
2
SO
4
(Fig. 2c) shows four clear peaks around 2h=15,
20, 25, and 27°which corresponds to semicrystalline
Polyaniline [15]. However, the X-ray diffraction pattern of
PANI-CAPS (Fig. 2a) shows only two peaks at 25 and 20°,
which indicates less crystallinity than that of PANI-CAPS-
H
2
SO
4
. The X-ray diffraction pattern of PANI-CAPS-
DHS-H
2
SO
4
(Fig. 2d) shows five peaks at 6.4, 15, 20, 25,
Scheme 1 Synthesis of PANI-
CAPS salts, their corresponding
yields, and conductivities
Fig. 1 Infrared spectra of (a) PANI-CAPS, (b) PANI-CAPS-DHS,
(c) PANI-CAPS-H
2
SO
4
, and (d) PANI-CAPS-DHS-H
2
SO
4
J Appl Electrochem (2015) 45:51–56 53
123

and 27°; the last four peaks indicate the semicrystalline
nature of polyaniline, and the first peak at 6.4°can be
assigned to the long-range ordering of polyaniline chains
via the doping of the surfactant molecules [16]. The X-ray
diffraction pattern of PANI-CAPS-DHS (Fig. 2b) displays
many peaks around 2h=14–44°; the main peaks at 17, 20,
22–24, and 29°are due to polyaniline, and the remaining
higher angle peaks are owing to the aromatic chain–chain
interaction in the polyaniline chain.
3.3 FE-SEM images of polyaniline salts
FE-SEM images of PANI-CAPS samples were recorded at
20 kV 980 k 9500-nm resolution and are shown in
Fig. 3. In the oxidative polymerization of aniline to poly-
aniline salt, the use of a weak acid (CAPS) results in the
formation of flake-like morphology (Fig. 3a); the use of
sodium lauryl sulfate surfactant along with CAPS results in
nanospheres with flakes (Fig. 3b); strong mineral acid and
CAPS upshot the formation of flakes (Fig. 3c); and the
combined use of sodium lauryl sulfate, mineral acid and
CAPS lead to the formation of nanospheres (Fig. 3d).
These results indicate that surfactant induces the formation
of nanospheres in the aniline polymerization.
3.4 Charge–discharge study of polyaniline salts
CD experiments were performed for the PANI-CAPS
samples in cell configuration at 1 mA cm
-2
, and the values
of specific capacitance (CD-C
s
), energy (E
d
), and power
densities (P
d
) by considering the weight of one electrode
were calculated. The values are reported in Table 2. The
Fig. 2 X-ray diffraction patterns of (a) PANI-CAPS, (b) PANI-
CAPS-DHS, (c) PANI-CAPS-H
2
SO
4
, and (d) PANI-CAPS-DHS-
H
2
SO
4
Fig. 3 FE-SEM images of aPANI-CAPS, bPANI-CAPS-DHS,
cPANI-CAPS-H
2
SO
4
, and dPANI-CAPS-DHS-H
2
SO
4
Table 1 IR peaks of PANI-CAPS samples in comparison with reported peak positions of PANI BASE and PANI SALT
PANI
BASE
PANI
SALT
PANI-CAPS-
H
2
SO
4
PANI-
CAPS-DHS
PANI-CAPS-
DHS-H
2
SO
4
PANI-
CAPS
N–H str. 3,425–3,445 3,425
NH
?
- indicative of doping i.e. salt formation NIL 3,228–3,222 3,225 3,225
C–H str. NIL 2,917–2,923 2,925 2,925 2,925 2,925 w
C=C str., quinonoid ring 1,585–1,590 1,558–1,570 1,560 1,565 1,585 1,585
C=C str., benzenoid ring 1,490–1,500 1,470–1,485 1,475 1,485 1,470 1,500
1,375–1,380 NIL NIL NIL NIL
C–N str., quinonoid ring) 1,312 1,298–1,302 1,300 1,290 1,265 1,300
C–N str., benzenoid ring 1,217–1,213 1,230–1,245 1,245 1,245 1,220
N=Q=N vibration, where Q represents
the quionoid ring)
1,158–1,162 1,110–1,130 1,105 1,105 1,140 1,120
1,4-disubstituted benzene 825–832 790–800 800 795 820
SO
3
H group 1,010 1,040
1,635
54 J Appl Electrochem (2015) 45:51–56
123

result shows that the values of specific capacitance and
energy densities increase with the increasing value of
conductivity of PANI-CAPS salts. In order to find out the
stability of the electrode, galvanostatic CD measurements
were carried out for PANI-CAPS-DHS-H
2
SO
4
from lower
to higher scan rates (Fig. 4). Ragone plot of energy density
versus power density is shown as an inset in Fig. 4.
The CD-Cs values calculated from the CD tests with
respect to the mass of one electrode are 530, 495, 515, 400,
and 365 F g
-1
at current densities of 1, 2, 3, 5, and
10 mA cm
-2
, respectively. Capacitance retention over
prolonged charge–discharge cycles is essential for practical
supercapacitor materials. Hence, CD experiments were
performed up to 3,000 cycles at 2 mA cm
-2
for PANI-
CAPS-DHS-H
2
SO
4
sample. CD-Cs, coulombic efficiency
(CE), and equivalent series resistance (ESR) with cycles
are shown in Fig. 5. The retention in the value of specific
capacitance at 3,000 cycles is 85 % with its initial capac-
itance value of 495 F g
-1
. CE values are almost constant
with cycle numbers (96–99 %). ESR value increases from
4to40Xat the end of 3,000 cycles.
3.5 Electrochemical impedance spectroscopy study
of polyaniline salts
EIS is an important analytical technique used to gain
information about the characteristic frequency responses of
supercapacitors and the capacitive phenomena occurring at
the electrodes. EIS experiments were carried out for the
PANI-CAPS systems in the frequency range from 40 kHz
to 10 mHz at an applied voltage of 0.7 V (Fig. 6), and the
EIS parameters are reported in Table 3. Solution resistance
of the four samples is nearly the same (0.5–0.7 X), indi-
cating the good conductivity of the electrolyte and very low
internal resistance of the electrode. The time constant value
is in the range of 0.1–0.4 ms.
The value of charge-transfer resistance is the main part of
the resistance of the supercapacitor. If the materials have
low charge-transfer resistance, then it has high electrical
conductivity and fast response ability of the electrode. The
value of charge-transfer resistance is in the range of
Fig. 4 Galvanostatic Charging–discharging curves of the PANI-
CAPS-DHS-H
2
SO
4
electrode recorded at different current densities
(a)1,(b)2,(c)3,(d) 5, and (e)10mAcm
-2
Fig. 5 Capacitance, coulombic efficiencies, and ESR of PANI-
CAPS-DHS-H
2
SO
4
with charge–discharge cycles at 2 mA cm
-2
current density
Fig. 6 Impedance spectra of aPANI-CAPS bPANI-CAPS-DHS,
cPANI-CAPS-H
2
SO
4
, and dPANI-CAPS-DHS-H
2
SO
4
electrode in
the range of 40 kHz–10 mHz at 0.7 V
Table 2 Specific capacitance,
energy, and power densities of
PANI-CAPS salts at
1mAcm
-2
current density
Sample Conductivity
(S cm
-1
)
Density
(g cm
-3
)
C
d
(F g
-1
)
Energy density
(kJ kg
-1
)
Power density
(J Kg
-1
s
-1
)
PANI-CAPS 0.03 1.32 320 57.6 120
PANI-CAPS-DHS 0.15 1.32 390 70.2 120
PANI-CAPS-H
2
SO
4
0.44 1.33 420 75.6 120
PANI-CAPS-DHS-H
2
SO
4
0.70 1.35 530 95.4 120
J Appl Electrochem (2015) 45:51–56 55
123

0.4–1.1 X, which indicates high electrical conductivity and
the fast response ability of the electrode. The value of
capacitance obtained at 10-mHz frequency for an applied
voltage of 0.7 V follows the order: PANI-CAPS-DHS-H
2-
SO
4
[PANI-CAPS-H
2
SO
4
[PANI-CAPS-DHS [PANI-
CAPS (Table 3). Bode plots of frequency versus phase angle
for PANI-CAPS samples carried out at 0.7 V applied volt-
age are shown in Fig. 7. The values of phase angles at
10 mHz obtained for PANI-CAPS, PANI-CAPS-DHS,
PANI-CAPS-H
2
SO
4
, and PANI-CAPS-DHS-H
2
SO
4
are 82,
85, 72, and 78, respectively. Ideal supercapacitor gives a
phase angle value of 90, and its value less than 90 shows
deviation from the ideal capacitor behavior. The value of
phase angle is close to 90 at low frequencies, indicating a
good capacitive behavior. Low values of phase angle
obtained in the case of PANI-CAPS-H
2
SO
4
and PANI-
CAPS-DHS-H
2
SO
4
salts indicate that the use of mineral
acid decreases the capacitive behavior.
4 Conclusions
Polyaniline salts containing organic acid (PANI-CAPS),
organic acid-surfactant (PANI-CAPS-DHS), organic and
mineral acids (PANI-CAPS-H
2
SO
4
), or organic and min-
eral acids and surfactant (PANI-CAPS-DHS-H
2
SO
4
) were
prepared. These polyaniline salts were explored as elec-
trode material for electrochemical supercapacitor. Among
the PANI salts, PANI-CAPS-DHS-H
2
SO
4
material showed
higher capacitance (530 F g
-1
at 1 mA cm
-2
current
density). Highly crystalline form with nanosphere mor-
phology was obtained for PANI-CAPS-DHS salt.
Acknowledgments The authors thank the Department of Science
and Technology, New Delhi, India for funding under the project DST/
TSG/PT/2011/179-G. The authors thank Dr. Lakshmi Kantam,
Director, CSIR-IICT for her support and encouragement. The authors
also thank Dr. Vijayamohanan K. Pillai, Director, CSIR—CECRI,
Karaikudi for his valuable suggestion. Authors BR and BR are
thankful to UGC, India for providing research fellowship.
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Fig. 7 Phase angle versus frequency of (a) PANI-CAPS (b) PANI-
CAPS-DHS, (c) PANI-CAPS-H
2
SO
4
, and (d) PANI-CAPS-DHS-
H
2
SO
4
electrode in the range of 40 kHz–10 mHz at 0.7 V
Table 3 Solution resistance (R
s
), charge transfer resistance (R
ct
),
time constant (s), and specific capacitance (C
s
) of Polyaniline salts
Sample R
s
(X)
R
ct
(X)
s
(ms)
C
s
at 10 mHz
(F g
-1
)
PANI-CAPS 0.5 1.01 0.24 125
PANI-CAPS-DHS 0.59 0.4 0.1 170
PANI-CAPS-H
2
SO
4
0.55 1.1 0.4 285
PANI-CAPS-DHS-H
2
SO
4
0.7 0.7 0.12 305
56 J Appl Electrochem (2015) 45:51–56
123