Capacitive properties of PANI/MnO2 synthesized via simultaneous-oxidation route

Article abstract of DOI:10.1016/j.jallcom.2012.04.006

Polyaniline (PANI) and manganese dioxide (MnO₂) composite (PANI/MnO₂) was synthesized via a simultaneous-oxidation route. In this route, all reactants were dispersed homogenously in precursor solution and existed as ions and molecules, and involved reactions of ions and molecules generating PANI and MnO₂ simultaneously. In this way, PANI molecule and MnO₂ molecule contact each other and arrange alternately in the composite. The inter-molecule contact improves the conductivity of the composite. The alternative arrangement of PANI molecules and MnO₂ molecules separating each other, and prevents the aggregation of PANI and cluster of MnO₂ so as to decrease the particle size of the composite. The morphology, structure, porous and capacitive properties are characterized by scanning electron microscopy, X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, Branauer-Emmett-Teller test, thermogravimetric analysis, Fourier transform infrared spectroscopy, cyclic voltammetry, charge-discharge test and the electrochemical impedance measurements. The results show that MnO₂ is predominant in the PANI/MnO₂ composite and the composite exhibits larger specific surface area than pure MnO₂. The maximum specific capacitance of the composite electrode reaches up to 320 F/g by charge-discharge test, 1.56 times higher than that of MnO₂ (125 F/g). The specific capacitance retains approximately 84% of the initial value after 10,000 cycles, indicating the good cycle stability.

Full text of DOI:10.1016/j.jallcom.2012.04.006

Journal of Alloys and Compounds 532 (2012) 1–9
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Capacitive properties of PANI/MnO2 synthesized via simultaneous-oxidation
route
Jie Zhanga, Dong Shua,b,c,d,∗, Tianren Zhangb, Hongyu Chena,c,d, Haimin Zhaob, Yongsheng Wangb,
Zhenjie Suna, Shaoqing Tanga, Xueming Fangb, Xiufang Caoa
a School of Chemistry and Environment, South China Normal University, Guangzhou 510006, PR China
b Tianneng Group, Changxing 313100, Zhejiang Province, PR China
c Base of Production, Education & Research on Energy Storage and Power Battery of Guangdong Higher Education Institutes, Guangzhou 510006, PR China
d Key Laboratory of Electrochemical Technology on Energy Storage and Power Generation of Guangdong Higher Education Institutes, South China Normal University, Guangzhou
510006, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Polyaniline (PANI) and manganese dioxide (MnO2) composite (PANI/MnO2) was synthesized via a
simultaneous-oxidation route. In this route, all reactants were dispersed homogenously in precursor
solution and existed as ions and molecules, and involved reactions of ions and molecules generating PANI
and MnO2 simultaneously. In this way, PANI molecule and MnO2 molecule contact each other and arrange
alternately in the composite. The inter-molecule contact improves the conductivity of the composite. The
alternative arrangement of PANI molecules and MnO2 molecules separating each other, and prevents the
aggregation of PANI and cluster of MnO2 so as to decrease the particle size of the composite. The mor-
phology, structure, porous and capacitive properties are characterized by scanning electron microscopy,
X-ray diffraction spectroscopy, X-ray photoelectron spectroscopy, Branauer–Emmett–Teller test, ther-
mogravimetric analysis, Fourier transform infrared spectroscopy, cyclic voltammetry, charge–discharge
test and the electrochemical impedance measurements. The results show that MnO2 is predominant in
the PANI/MnO2 composite and the composite exhibits larger specific surface area than pure MnO2. The
maximum specific capacitance of the composite electrode reaches up to 320 F/g by charge–discharge
test, 1.56 times higher than that of MnO2 (125 F/g). The specific capacitance retains approximately 84%
of the initial value after 10,000 cycles, indicating the good cycle stability.
Received 11 November 2011
Received in revised form 30 March 2012
Accepted 4 April 2012
Available online 10 April 2012
Keywords:
Simultaneous-oxidation route
PANI/MnO2 composite
Inter-molecule contact
Supercapacitor
Specific capacitance
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
the appropriate conditions. The capacitive performance of super-
capacitors strongly depends on the properties of the electrode
Electrochemical supercapacitors, also known as supercapaci-
tors or ultracapacitors, are considered as the ideal energy-storage
devices. They show greater power density and longer cycle life
than batteries, and possess higher energy density compared with
conventional capacitors [1]. Recently increasing interest has been
focused on the development of electrochemical supercapacitors
in communications, transportation, consumer electronics, aviation
and so on [2]. Based on charge-storage mechanism, supercapaci-
tors can be categorized as two types [3]: (1) electrical double-layer
capacitors (EDLCs), in which energy is stored due to the for-
mation of an electrical double layer at the electrode–electrolyte
interface [4]. (2) Pseudocapacitors, which store energy by electro-
chemical faradic reactions between electrode materials and ions in
materials such as their surface area, conductivity, and electro-
chemical stability [5,6]. Carbon materials with high surface-area
have been most widely used in EDLCs. However, their low average
specific-capacitance value is a drawback [7]. Materials depending
on pseudocapacitance mechanism have higher capacitive nature
than carbon materials with electrical double-layer mechanism [8].
Nowadays, great interests are shown by researches on pseudoca-
pacitance materials with excellent electrochemical characteristics
such as metal oxides and conducting polymers [9]. Among the metal
oxides electrode materials, hydrous RuO2 has been studied exten-
sively. It exhibits high specific capacitance value ranging from 720
to 760 F/g [10]. However, the high cost and toxicity of RuO2 have
limited its applications and motivated the research of other transi-
tion metal oxides [11].
With the advantages of low cost, natural abundance, environ-
mental safety, and high theoretical specific capacitance (1370 F/g)
[12] MnO2 has been applied extensively in supercapacitors [13–16].
However, the poor electrical conductivity of MnO2 limits its
∗
Corresponding author at: School of Chemistry and Environment, South China
Normal University, Guangzhou 510006, PR China. Tel.: +86 020 39310212.
E-mail address: dshu@scnu.edu.cn (D. Shu).
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.04.006

2
J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
capacitive response [17]. Conducting polymers including polyani-
line (PANI), polypyrrole (PPy), and polythiophene (PTh) can offer
a cost-effective and flexible alternative to conventional elec-
trode materials due to their fast doping/dedoping abilities [18].
The advantages of easy synthesis and good conductivity make
PANI become a unique and promising polymeric material with
great potential applications in supercapacitors [19]. But it suffers
from the poor electrochemical cyclability [20]. Currently many
approaches have been taken to overcome these disadvantages
through synthesizing composite material of PANI/MnO2 to com-
bine the good conductivity of PANI and excellent cyclability of
MnO2. [21,22] Such a hybrid material possessing both the advan-
tages of the two moieties have been frequently proposed in past
years as the key solution to various challenges in the energy supply,
storage and production.
2.2. Preparation of PANI/MnO2 composites
Firstly, certain amount of aniline monomer and MnCl2·4H2O were dissolved in
80 mL 0.5 mol/L HCl aqueous solution and sonicated for 15 min. Then aniline was
added into the MnCl2 solution and sonicated for 30 min. Finally, KMnO4 dissolved
in 80 mL deionized (DI) water was added dropwise into the above mixed solution
in ice bath for 30 min under stirring. The reaction was initiated immediately after
the addition of KMnO4. The mixture was stirred for 4 h at constant temperature
to generate PANI/MnO2 composite. Then the reaction mixture was suction-filtered.
The black precipitate was washed repeatedly with double distilled water and alcohol
until the washing solution was colorless. The PANI/MnO2 composite was obtained
through the following equations [13]:
KMnO4 + aniline → MnO2·nH2O + PANI
2KMnO4 + 3MnCl2 + 2H2O → 5MnO2 + 2KCl + 4HCl
The MnO2 received by the reaction of KMnO4 with MnCl2·4H2O in the same
Various methods have been reported for the synthesis of
PANI/MnO2 composite including traditional template chemical
methods, stepped chemical methods [23] and electrochemical
routes [24,25]. Many researchers tended to prepare the PANI/MnO2
composite without using any template since the template removal
is tedious particularly when hard templates were used. Kim and
Park [23] obtained PANI/MnO2-MWNTs hybrid material by stepped
chemical method in which MnO2-MWNTs was prepared firstly
by in situ direct coating method, and then PANI was coated
onto the MnO2-MWNTs to synthesize PANI/MnO2-MWNTs, in
this work the composite was prepared in two separate steps. It
was similar to mechanical mixture, and would result in much
higher contact resistance between MnO2 and PANI. In compar-
ison with chemical methods discussed above, electrochemical
deposition offers a better control of the thickness of the com-
posite film and ease in constructing modified electrodes. Zhang
et al. [25] synthesized PANI/MnO2 composite on SnO2/Ti substrate
via potentiodynamic electrodeposition. The composite showed
high specific capacitance of 601.48 F/g, which is 1.69 times high
as that of PANI electrodes. However, the complicated process
and high cost of the electrochemical method limited their mass
production.
experimental condition was also investigated for comparison experiment.
2.3. Preparation of the working electrode
The working electrode was prepared by firstly mixing 75 wt.% active mate-
rial, 15 wt.% acetylene black and 10 wt.% poly(vinylidene fluoride) (PVDF) in
N-methylpyrrolidinone (NMP), and then the slurry was spread onto stainless steel
net with 1 cm2 geometry area. The electrodes were dried under vacuum at 60 ◦C for
24 h to evaporate solvent.
2.4. Characterization of the PANI/MnO2 composite electrode
2.4.1. Physical characterization
The morphologies of the composite and MnO2 were observed by scanning elec-
tron microscope (SEM) (JSM-6380) with an accelerating voltage of 15 kV. X-ray
˚
diffraction (XRD) analysis was carried out by using CuK␣1 (1.5406 A) radiation
on a Y-2000 X-ray generator. The X-ray intensity was measured over a diffrac-
tion 2ꢀ angle ranging from 5◦ to 80◦ with a velocity of 0.03◦ step−1 and 2◦ min−1
.
The measurements of thermogravimetric analysis (TG) were carried out using a
STA409PC thermogravimetric analyzer (0–800 ◦C) at a heating rate of 10 ◦C min−1
in air atmosphere. The specific surface area and pore size distribution of the com-
posite material and MnO2 were analyzed using the Brunauer–Emmet–Teller (BET,
ASAP-2020, Micromeritics, America) method by the adsorption and desorption of
N2. The Fourier transform infrared (FTIR) spectrum (Spectrum One, PerkinElmer
instruments, America) was used to determine the functional groups of PANI and
PANI/MnO2. The oxidation states of Mn and N elemental analysis on the surface
of the PANI/MnO2 were analyzed using X-ray photoelectron spectroscopy (XPS,
ESCALAB 250, Thermo Fisher Scientific Co., America).
Herein, in this paper, we present
a novel and simple
simultaneous-oxidation strategy for preparing PANI/MnO2 com-
posites and investigate their electrochemical properties as
supercapacitor electrodes. In this strategy, potassium perman-
ganate (KMnO4) is used as the oxidizing agent to oxidize aniline
and MnCl2 to prepare PANI and MnO2, and meanwhile KMnO4
is reduced to give product of MnO2. Compared with those con-
ventional methods, this method can synthesize PANI and MnO2
simultaneously. All the reactants were converted into products
totally, avoiding the generation of impurities. An excellent con-
tact in the inter-molecule level between PANI and MnO2 will
be formed, and it is beneficial for electron transfer [24]. Fur-
thermore, PANI and MnO2 could be separated from each other.
The separation between PANI and MnO2 prevents the aggre-
gation of PANI and cluster of MnO2. Therefore, the composite
maintain nanostructure, providing large specific surface area so
as to enlarge the interaction area between the composite and
electrolyte. The as-synthesized PANI/MnO2 composite electrode
shows a high specific capacitance of 320 F/g and excellent cycle
stability. The present synthetic strategy will be a promising fab-
rication technique for the highly efficient electrode materials of
supercapacitors.
2.4.2. Electrochemical characterization
All electrochemical experiments, including cyclic voltammetry (CV),
charge–discharge (CD) test and electrochemical impedance spectrometry (EIS)
were conducted by using Autolab PGSTAT30 (Eco Echemie B.V. company) and CHI
660A (CH Instrument, Inc.) electrochemical working stations in a three-electrode
mode (working electrode, a graphite stick as counter electrode, and a saturated
calomel electrode (SCE) as reference electrode).
3. Results and discussion
3.1. SEM
The morphologies of the PANI/MnO2 composite and MnO2 were
investigated by SEM. The results were displayed in Fig. 1. Both
MnO2 and PANI/MnO2 composite display compact granular mor-
phology. PANI/MnO2 composite exhibits spherical particles with a
radius of about 200 nm, demonstrating a three-dimensional nano-
structure. Such a structure allows a shorter ion diffusion path
which can improve the rate capability and specific capacitance [15].
Meanwhile it is observed that the granular size of the PANI/MnO2
composite (200 nm) is smaller than that of MnO2 (500 nm). This
is because that PANI and MnO2 were synthesized simultaneously
and separated from each other. Such a separation between PANI
and MnO2 prevents the aggregation of PANI or cluster of MnO2.
Therefore, the composite existed in nanostructure, providing large
specific surface area so as to enlarge the interaction area between
the composite and electrolyte. Further explanation, as for pure
2. Experimental
2.1. Materials
All chemicals used were of analytical grade. All solutions were prepared from
double distilled water. Aniline, MnCl2·4H2O (Aldrich), hydrochloric acid (HCl)
(Aldrich) and potassium permanganate (KMnO4) (Aldrich) were used as received.

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
3
Fig. 1. SEM images of the as-prepared (a and c) PANI/MnO2 powder and (b and d) MnO2.
MnO2 prepared in the same experimental condition, direct con-
tact between MnO2 molecules results in the formation of big MnO2
particles. However, in the PANI/MnO2 composite obtained from
our work, some MnO2 molecules contact with PANI molecules.
The PANI molecules could block the epitaxial growth of MnO2. In
other words, the PANI separated from MnO2 particles. Similarly,
the MnO2 could also separate with PANI and prevent PANI from
aggregation. So the granular size of the PANI/MnO2 composite is
small.
and PANI is about 9:1, indicating that MnO2 is predominant in the
composite. Besides, the decomposition of MnO2 in the temperature
range of 700–800 ◦C does not appear in the PANI/MnO2 curve, indi-
cating the enhancement of the thermal stability of PANI/MnO2. It
is because good contact and intermolecular force formed between
the PANI and MnO2 during the simultaneous-oxidation route.
3.3. XRD
The crystal structures of the composite and MnO2 were stud-
ied using XRD. Fig. 3 is the XRD patterns of PANI/MnO2 (Fig. 3(b))
and MnO2 (Fig. 3(a)). In Fig. 3(a), the obviously observed peaks
at 2ꢀ = 37◦, 42.5◦, 56◦ and 65.6◦ are corresponding to the (1 3 1),
3.2. TG
The thermal stability of the PANI/MnO2 composite was inves-
tigated by TG. Fig. 2 depicts the thermograms of MnO2 and
PANI/MnO2 composite. According to the TG curve of MnO2
(Fig. 2(a)), 7% weight loss under 170 ◦C is due to the removal of
physically adsorbed water. 10% weight loss from 170 to 250 ◦C is
attributed to the loss of acid dopant anions and water in the inter-
layer molecule. A small weight loss (3%) between 500 and 800 ◦C is
ascribed to the reduction of manganese from tetravalent to triva-
lent form accompanied by the evolution of oxygen [26]. In the TG
curve of PANI/MnO2 (Fig. 2(b)), 10% weight loss under 180 ◦C is due
to the dissipation of the physically adsorbed water. A further weight
loss of 5% observed from 180 to 260 ◦C is attributed to the loss of
acid dopant anions compensating the positive charge of PANI chain.
While a small weight loss about 10% between 260 and 800 ◦C is
ascribed to the degradation of the polymer chain. Finally, when the
temperature is higher than 800 ◦C, the percentages of the remain-
ing MnO2 and PANI/MnO2 composite are 82% and 74% respectively.
The content difference between the composite and MnO2 is consid-
ered as the content of PANI in the composite. If we exclude the small
quantity of water and other impurities, the weight ratio of MnO2
100
MnO2
(a)
80
(b)
PANI/MnO2
60
40
0
100
200
300
400
500
600
700
800
900
Temperature/oC
Fig. 2. TG patterns of (a) MnO2 and (b) PANI/MnO2.

4
J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
240
220
200
180
160
140
120
100
80
200
180
160
140
120
(131)
(300)
(a)
(b)
MnO2
MnO2
(160)
(421)
100
(a)
80
PANI/MnO2
PANI/MnO2
(131)
40
60
60
40
40
(421)
20
(b)
20
0
0
1000
2000
3000
0
20
60
80
Wavelength/cm-1
(2θ)/o
Fig. 4. FTIR spectrum of (a) MnO2 and (b) PANI/MnO2.
Fig. 3. XRD patterns of (a) MnO2 and (b) PANI/MnO2.
the characteristics of Mn4+ [31]. In XPS spectra, the full width at
half maximum (FWHM) of an element reflects the multiplicity of
molecular adsorption states. The N 1s spectrum in Fig. 5 shows a
wide FWHM of 3.5 eV (wider than 3 eV), which indicates more than
a single type of nitrogen exist (Fig. 5(c)). The spectra of N 1s are
reasonably deconvoluted into four Gaussian peaks with the bind-
ing energy of 398.4, 399.5, 401.0 and 402.5 eV respectively, which
(3 0 0), (1 6 0) and (4 2 1) planes of ␥-type MnO2 (JCPDS no. 14-0644)
respectively, indicating the crystalline state of MnO2 is ␥-MnO2.
The XRD patterns of PANI/MnO2 in Fig. 3(b) are similar to that of
MnO2, indicating the similar crystalline state of ␥-MnO2. But the
broad and weak peak of the composite may be related to the low
crystallization degree and the small size of the crystalline grain,
verifying a distortion of the crystalline structure of ␥-MnO2 in
PANI/MnO2 during the polymerization reaction. [17] As is well-
known that MnO2 can form several kinds of polymorphs such as
␣-, ␤-, ␥- and ␦- types according to the different linkage pat-
terns of basic unit [MnO6] octahedral. Among all the structures,
␥-MnO2 which is considered to be an intergrowth of ramsdellite
(1 × 2 tunnels) and pyrolusite (1 × 1 tunnels) have wide applica-
tions in electrochemical supercapacitors [27], lithium ion battery
[28,29] and dry-cell batteries [30]. In previous studies, PANI in its
emeraldine salt form shows characteristic peak at 20.8◦. While in
our PANI/MnO2 composite, other two low peaks at 2ꢀ = 20◦, 25◦
are found, due to the periodicity parallel and perpendicular to the
polymer chain, respectively [21]. Here in our work, PANI shows only
an amorphous halo without any diffraction feature due to the low
content and weak crystalline of received PANI. Therefore the XRD
peaks of MnO2 are predominant [11].
can be ascribed to imine-like structure [
N ], amine-like nitro-
gen atom [ NH ], [ ph NH ph ], and protonated imine [ N+ ],
respectively [22]. The ratio of different elements or statement can
be calculated according to the peak area. And the ratio of
and H components could illustrate the intrinsic doping state of
PANI [33–35]. Therefore, the peak areas of the four peaks calculated
are 19.7%, 15.8%, 40.2%, and 21.3%, respectively. The /N H ratio
N
N
N
is 41:59, indicating PANI in the composite is half oxidation-half
reduction status. In this status, PANI displays the highest electrical
conductivity, which is beneficial to improve the electrochemical
property of the PANI/MnO2 composites.
3.6. BET
The specific surface area is an important parameter of nanoma-
terial, which greatly affects their characteristics such as specific
capacitance. The N2 adsorption–desorption isotherm curve and
pore size distribution of MnO2 and PANI/MnO2 are shown in Fig. 6.
Their isotherms are similar to the shape of type IV [36]. It can be seen
that most pores are distributed in a narrow range from 14 to 17 nm,
suggesting that both of the MnO2 and PANI/MnO2 exhibit porous
structures mainly composed of mesopores (according to IUPAC
classification). The measured BET surface area of the PANI/MnO2
is 65.8 m2/g, which is higher than that of MnO2 (59.4 m2/g). The
specific surface area is closely related to the particle size. Smaller
particle size would lead to larger specific surface area [14]. There-
fore, the result indicates that PANI/MnO2 composite has smaller
particles, which is consistent with the SEM results.
3.4. FTIR
To obtain more specific information about the PANI/MnO2, FTIR
spectra of the composite and MnO2 were obtained. As seen from
Fig. 4, the peaks near 1308, 1490 and 1580 cm−1 are attributed
to Ph N, benzenoid structure and quinoid structure, respectively
[21]. Bands at 1623 and 1506 cm−1 correspond to the C C stretch-
ing of quinoid rings and C C stretching of the benzenoid rings.
The bands in the range of 1390–1400 cm−1 are the C N stretching
band of the aromatic amine. The band at 3320 cm−1 is attributed to
N
H stretching. Furthermore, the sharp peak at 560 cm−1 should
be ascribed to the contribution of the Mn O vibrations [31]. FTIR
spectrum of the PANI/MnO2 composite exhibits both the character-
istic band of PANI and MnO2, which confirms the presence of both
components in the composite [32].
3.7. Electrochemical characterization
The super capacitive (SC) behavior of a material can be evaluated
from CV and CD. The specific capacitance based formulas of CV or
CD cycling can be calculated by the following formulas:
3.5. XPS
The surface chemical composition of PANI/MnO2 was investi-
gated by XPS. Mn 2p and N 1s signals can be observed in the survey
scan (Fig. 5(a)) of PANI/MnO2. The Mn 2p region (Fig. 5(b)) consists
of a spin-orbit doublet: Mn 2p1/2 with binding energy of 654.2 eV
and Mn 2p3/2 with binding energy of 643.7 and 642.3 eV. They are
Q(C)
ꢁE(V)m(g)
SC(CV)(F/g) =
SC(CD)(F/g) =
(1)
(2)
I(A) × ꢁt(s)
ꢁE(V)m(g)

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
5
0.10
(a)
(a)
Mn2p3/2
O1s
0.08
0.06
0.04
0.02
0.00
Mn2p1/2
C1s
PANI/MnO2
MnO2
N1s
0
20
40
60
80
100
120
140
160
1200
1000
800
600
400
200
0
-200
Pore size/nm
Binding Energy/eV
280
260
240
220
200
180
160
140
120
100
80
(b)
(b)
Mn2p3/2
Mn2p1/2
Mn2p3/2
PANI/MnO2
MnO2
60
40
20
0
-20
0.0
0.2
0.4
0.6
0.8
1.0
660
655
650
645
640
635
Relative Pressure/(p/p0)
Binding Energy/eV
Fig. 6. (a) Pore size distributions and (b) N2 adsorption–desorption isotherms of
MnO2 and PANI/MnO2.
(c)
molar ratio of PANI to MnO2. Low HCl concentration is beneficial to
the deposition of MnO2, but not beneficial for PANI to obtain good
electrical conductivity and electrochemical capacitance.
[-ph-NH-ph-]
[-NH-]
[=N–]
By adjusting the acid concentration, we can control the MnO2
content in the composites. Different concentrations of HCl (0.1, 0.5,
1.0 and 2.0 mol/L) were used to synthesize the composites PM-0.1,
PM-0.5 PM-1.0 and PM-2.0, respectively. The CV test was carried
out in 1.0 mol/L Na2SO4 solution. Their specific capacitance cal-
culated from the CV curves in Fig. 7(a) and the content of PANI
obtained from TG measurement was presented in Table 1 and
Fig. 7(b). It can be found that:
+
[=N –]
(i) The concentration of HCl shows great effect on the specific
capacitances of PM-0.1, PM-0.5, PM-1.0 and PM-2.0. PM-0.5
exhibits the largest specific capacitance.
404
402
400
398
396
394
392
Binding Energy/eV
(ii) The content of MnO2 in the PANI/MnO2 composites increases
largely with the decrease of the concentration of HCl.
Fig. 5. XPS spectrum of (a) survey scan (b) Mn 2p and (c) N 1s for PANI/MnO2
composite.
Table 1
Here, ‘Q’ is the cathodic charge in coulomb, ‘I’ the discharge current
in ampere, ‘ꢂt’ the discharge time in second, ‘ꢂE’ the potential
difference in volt and ‘m’ is the active material mass in gram [35].
The effect of acid concentration on specific capacitance and MnO2 content of com-
posites measured by CV in Na2SO4.
Sample
Concentration of
HCl (mol/L)
Mass percent of
MnO2 in the
composite
Specific
capacitance of the
composite (F/g)
3.7.1. Characterization of PANI/MnO2 materials synthesized with
different concentrations of HCl
The electrochemical performance of PANI/MnO2 composite is
affected greatly by the molar ratio of PANI to MnO2. The concen-
tration of HCl during the preparation is a key factor to affect the
PM-2.0
PM-1.0
PM-0.5
PM-0.1
2
1
0.5
0.1
38%
68%
74%
91%
141
172
301
213

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J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
3
2
(a)
(a)
4.0
3.5
0.5M
1.0M
3.0
20mV/s
0.1M
2.5
2.0
10mV/s
1mV/s
1.5
1
1.0
0.5
0
2.0M
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-1
-2
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential/V
Potential/V
350
300
250
200
150
100
50
(b)
(b)
300
250
200
150
100
50
0
0
0.0
0.5
1.0
1.5
2.0
0
5
10
Scan rate/mVs-1
15
20
-1
Acid concentration/mol L
Fig. 7. (a) CV curves and (b) specific capacitance of the composites synthesized with
0.1 M, 0.5 M, 1.0 M and 2.0 M HCl at different scan rates.
Fig. 8. (a) CV curves and (b) specific capacitance of PANI/MnO2 electrode in Na2SO4
solution at different scan rate.
Therefore, in this work, 0.5 mol/L HCl is selected for the prepa-
ration of the composite.
(ii) Large specific surface area and high electrochemical utiliza-
tion of the composite material is realized by preventing the
composite particles from agglomerating.
(iii) The contribution of PANI in the composite may be partially
involved in Faradaic pseudocapacitance reaction [17].
3.7.2. CV and electrochemical stability test
Fig. 8(a) shows the CV curves of PANI/MnO2 at different potential
scan rates and Fig. 8(b) displays the calculated specific capacitance.
The rectangular and mirror images are observed in the CV curve,
indicating high electrochemical reversibility. This capacitive nature
may be caused by a combination of the double-layer capacitance
of the active material and the pseudocapacitance of PANI/MnO2. It
can be observed that the specific capacitance decreases from 301 to
144 F/g with the increase of potential scan rate from 1 to 20 mV/s.
It is because the electrolyte ions can only reach the outer surface
of the electrode at high scan rates, and the active material that
is accessible only through the deep pores does not actively con-
tribute to the pseudocapacitance [10]. Fig. 9 shows the CV curves
of the PANI/MnO2 and MnO2 obtained at a scan rate of 1 mV/s
in 1.0 mol/L Na2SO4 solution in the potential range of −0.1 and
0.9 V (vs. SCE). The specific capacitances of MnO2 and PANI/MnO2
composite were calculated to be 125 and 301 F/g, respectively. The
larger specific capacitance of PANI/MnO2 composite may be caused
by the following reasons:
3.0
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
MnO2
PANI/MnO2
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential/V
(i) The good contact in the inter-molecule level between PANI and
MnO2 formed during the simultaneous-oxidation process.
Fig. 9. CV curves of PANI/MnO2 and MnO2 at 1 mV/s in Na2SO4 solution.

J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
7
140
120
100
80
1.0
0.8
0.6
0.4
0.2
0.0
60
40
20
PANI/MnO2
MnO2
-0.2
0
0
500
1000
1500
2000
0
2000
4000
6000
8000
10000
Time/s
Cycle number
Fig. 12. Charge–discharge curves of MnO2 and PANI/MnO2 electrode in Na2SO4
solution.
Fig. 10. Cyclic performance of PANI/MnO2 electrode in the voltage range of −0.1 to
0.9 V (vs. SCE) in Na2SO4 solution.
3.7.3. Galvanostatic CD analysis
The stability of PANI/MnO2 electrode was investigated by CV
scanning for many cycles. Fig. 10 shows the cyclic voltammograms
recorded for 10,000 cycles with a scan rate of 20 mV/s in 1.0 mol/L
Na2SO4 solution. The specific capacitance of the PANI/MnO2 elec-
trode material increases with the increase of the cycle number at
the first 750 cycles. Afterwards, the capacitance slightly decays and
finally retains 84% of the initial value after 10,000 cycles, indicat-
ing the excellent cycle stability of the electrode. The excellent cycle
stability may be attributed to the cohesive inter-molecular contact
between PANI and MnO2 particles which prevents the dissolution
of MnO2. And the capacitance attenuation can be ascribed to two
reasons: (i) low dissolution of MnO2 is inevitable. (ii) The redox sites
in the PANI are insufficiently stable for many repeated redox pro-
cesses, which is probably due to the swelling and shrinking of PANI
under aqueous environment leading to the degradation of PANI
during the long term cycling [22]. In conclusion, both the specific
capacitance (320 F/g) and cycle stability (SC retention of 84% over
10,000 continuous cycles) of our result composite displays many
advantages comparing to the reported literatures [11,17,21,32,36].
It is particularly worth mentioning that in the existing literature,
few PANI/MnO2 composites can have such good cycle stability.
Fig. 11 shows the galvanostatic CD curves of PANI/MnO2 elec-
trodes at different current densities. Fig. 12 shows the galvanostatic
CD curves of the MnO2 and PANI/MnO2 electrodes at a current
density of 0.35 A/g. The linear variation of the voltage with time
is observed. The responses during anodic charging and cathodic
discharging processes are nearly symmetric, demonstrating that
both the electrodes display ideal capacitive properties in the poten-
tial range. The area under the PANI/MnO2 CD curve is larger
than that under the MnO2 CD curve. The specific capacitances
of the MnO2 and PANI/MnO2 calculated from the CD curves are
94 and 320 F/g respectively, which are similar to the result of
CV. Besides, IR drop is the electrical potential difference between
the two ends of a conducting phase during a current flow. It is
the product of the current (I) and the resistance (R) of the con-
ductor [21]. In the CD curves IR drop can be observed. The CD
curve of PANI/MnO2 composite shows a less IR drop compared to
MnO2. This further confirms the lower contact resistance in the
composite.
3.7.4. EIS analysis
EIS is a powerful tool for the mechanism analysis of interfacial
process and the evaluation of rate constant, ionic and electronic
conductivity and double-layer capacitance, etc. [24] The typi-
cal Nyquist plot of EIS measurement is shown in Fig. 13. Both
PANI/MnO2 and MnO2 exhibit a distorted semicircle in the high-
frequency region. It is attributed to the charge-transfer resistance
(Rct) from the interface structure between the electrode surface and
electrolyte [37,38]. From the point intersecting with the x-axis in
the range of high frequency, a very small internal resistance (Rs) can
be evaluated. The internal resistance includes the ionic resistance
of electrolyte, the intrinsic resistance of the active material, and
the contact resistance at the interface of active material and cur-
rent collector. A line almost vertical to the y-axis in low-frequency
region is also observed, indicating ideal capacitive behavior [39,40].
Fig. 13(a) and (b) demonstrates the Nyquist plots of MnO2 and
PANI/MnO2 at different potential range, and the charge-transfer
resistance (Rct) of PANI and PANI/MnO2 at different potentials is
shown in Fig. 13(c). It can be seen that the Rct of MnO2 is much larger
than that of PANI/MnO2. It further proved that the PANI improves
the conductivity of the composite, facilitating the charge-transfer
of the composite [33].
1.0
0.8
0.6
0.4
0.2
0.0
0.70A g-1
0.35A g-1
1.30A g-1
-0.2
-0.4
0
500
1000
1500
2000
Time/s
Fig. 11. Charge–discharge curves of MnO2 electrode at different current densities
in Na2SO4 solution.

8
J. Zhang et al. / Journal of Alloys and Compounds 532 (2012) 1–9
concentration plays an important role in the preparation of the
(a)
120
100
80
60
40
20
0
composite, and the optimum HCl concentration is 0.5 mol/L. (ii)
MnO2 is predominant in PANI/MnO2 composite and its crystalline
state is ␥-MnO2. (iii) PANI/MnO2 composite exhibits larger specific
surface area than that of pure MnO2. (iv) The maximum specific
capacitance of the composite electrode reaches up to 320 F/g in CD
test, which is 156% higher than that of MnO2 (125 F/g). (v) The spe-
cific capacitance of the composite electrode retains approximately
84% of the initial value after 10,000 cycles, indicating good cycle
stability of the composite. (vi) The excellent capacitive behavior of
the composite is due to the good contact in the inter-molecule level
between PANI and MnO2 formed during the preparation, and the
high electrochemical utilization of the composite material by pre-
venting the composite particles from agglomerating. The present
synthetic strategy can be extended to other conductive polymers
and inorganic metal oxides such as polypyrrole, polythiophene
and RuO2. It would also be a promising preparation technique for
highly efficient electrode materials of supercapacitors.
100mv
300mv
MnO2
0
20
40
60
80
100
120
Z'/ohm
140
120
100
80
(b)
Acknowledgments
PANI/MnO2
100mv
300mv
500mv
We are grateful for the financial support from the Natu-
ral Science Foundation of Guangdong Province, China (grant no.
S2011010003416); China Guangdong Province Science and Tech-
nology Bureau (grant no. 2010B090400552); China High-Tech
Development 863 Program (grant no. 2009AA03Z340) and National
Natural Science Foundation of China (grant no. 20877025).
60
40
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