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Supercapacitive behaviors of activated mesocarbonmicrobeads coated with polyaniline

Chun Wu, Xianyou Wang*, Bowei Ju, Xiaoyan Zhang, Lanlan Jiang, Hao Wu

Key Laboratory of Environmentally Friendly Chemistry and Applications of Minister of Education, School of Chemistry,

Xiangtan University, Hunan, Xiangtan 411105, China

a r t i c l e i n f o

Article history:

Received 21 April 2012

Received in revised form

23 June 2012

Accepted 20 July 2012

Available online 9 August 2012

Keywords:

Activated mesocarbon microbeads

Polyaniline

Composites

Electrochemical performance

Supercapacitor

* Corresponding author. Tel.: þ86 731 582920E-mail address: wxianyou@yahoo.com (X

0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2012.07.0

a b s t r a c t

The polyaniline/activated mesocarbon microbeads (PANI/ACMB) composites are prepared

by in situ chemical oxidation polymerization. Fourier infrared spectroscopy (FTIR), scan-

ning electron microscope (SEM) and transmission electron microscope (TEM) have been

utilized to characterize the structure and morphology of PANI/ACMB composites. It has

been found that PANI is uniformly deposited on the surface of the ACMB to form the

leechee-like morphology. The supercapacitive behaviors of the PANI/ACMB composites are

investigated with cyclic voltammetry (CV), galvanostatic charge/discharge and cycle life

measurements. The results obtained from cyclic voltammograms show that the compos-

ites have a maximum specific capacitance of 433.75 F g�1. Moreover, the electrochemical

performance of the coin supercapacitor used PANI/ACMB composites as electrode active

material represents both high specific capacitance and excellent cycle stability, indicating

that the PANI/ACMB composites will be a kind of potential electrode active materials with

excellent specific capacitance and enhanced cycle life for application in high performance

supercapacitors.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction storage mechanism, there are two types of supercapacitors,

The energy and pollution crisis currently becomes the great

problem around the world, and as a consequence novel

renewable and clean energy power sources must be consid-

ered. One of the prevalent alternative sources of electric

power is considered to be fuel cell, however, one of the main

weak points of fuel cell is its slow dynamics. At present, the

new high current supercapacitor technology has been devel-

oped to supply or to absorb high transient energy. This kind of

new supercapacitor is usually known as electrochemical

capacitors, which is electrochemical energy storage device

with unique properties, such as high power, long cyclic life,

and fast charge/discharge rates [1,2]. According to energy

60; fax: þ86 732 58292061. Wang).2012, Hydrogen Energy P87

viz., electrochemical double layer capacitors (EDLCs) and

redox supercapacitors. In the former case, energy storage

arises mainly from the ionic charge separation at the elec-

trode electrolyte interface [3]. In the latter case, a faradic

process, due to redox reactions, takes place at the electrode

materials (such as conducting polymers and metal oxides) at

characteristic potentials [4]. To develop an advanced super-

capacitor device, an active electrode material with high

capacity performance is indispensable [5e9].

Carbon materials have been widely used in the super-

capacitor electrodes because of their high surface area, good

electronic conductivity, and excellent stability [10e12].

Among various carbon materials, activated mesocarbon

.

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 4 3 6 5e1 4 3 7 214366

microbeads have attracted great attention because of their

higher energy densities per both weight and volume than

those of the conventional activated carbon with large power

densities [13,14]. In our group, Bai et al. have successfully

prepared the highly activated carbon microbeads (ACMB) by

glucose hydrothermal route and found that the ACMB elec-

trode exhibits a specific capacitance of 291.9 F g�1 at a scan-

ning rate of 1 mV s�1 in 6 M KOH electrolyte, and the loss of

specific capacitance was neglectable after 5000 cycles [15].

Usually, the carbon materials only possess double layer

capacitance, while metal oxides or conducting polymers

possess faradaic pseudocapacitance, and the faradaic pseu-

docapacitance is almost 10e100 times higher than double

layer capacitance [16]. Conducting polymers have been

extensively studied as promising materials for electro-

chemical capacitors due to their redox chemistry and high

conductivity in the doped state. In conducting polymers,

electric energy can be stored and delivered as delocalized-

electrons are accepted and released during electrochemical

doping/undoping, respectively. There are two types of doping

mechanism as followed [17]:

p� doping : ðpolymerÞ þ yA�/�ðpolymerÞyþyA��þ ye� (1)

n� doping : ðpolymerÞ þ yCþ þ ye�/�ðpolymerÞy�yCþ� (2)

In the series of the conducting polymers, PANI has been

considered as one of the most promising electrode materials

because of its unique advantages include easy synthesis, good

environmental stability in air, simplicity in doping, moder-

ately high conductivity in the doped form and lower cost than

many other conducting polymers [18]. All these have made it

an exceptionally versatile electrode material for super-

capacitors. An improvement in the capacitance of carbon

materials can be realized by preparing carbon-conducting

polymer composites. Such a modification induces faradaic

pseudocapacitance effects apart from electrostatically

charges accumulation. To take advantage of the admirable

properties of activated carbons and PANI, some researchers

used activated carbons as substrate materials to prepare

composites, our group has prepared the polyaniline (PANI)/

carbon aerogel (CA) composites by chemical oxidation poly-

merization and obtained a high specific capacitance of

710.7 F g�1 [19]. However, until recently, there is some lack of

knowledge about the PANI/ACMB composites and their

application in supercapacitor. In this paper, the PANI/ACMB

composite material has been successfully prepared by in situ

chemical oxidation polymerization based on our previous

ACMB studies [15]. The physical and electrochemical proper-

ties of PANI/ACMB are studied in detail.

2. Experimental

2.1. Material synthesis

2.1.1. Synthesis of ACMBTo prepare ACMB, the typical process was performed as

follows: (1) 27 g of glucose was fully dissolved in 100 mL

deionized water, diverting 75 mL of the solution into the

100 mL agitated reactor and setting it into the electric vacuum

drying oven after screwed, then the resulting solution was

aged at 180 �C for 18 h (2) The resultant precipitatewas filtered,

washed with alcohol and deionized water until pH ¼ 7, after

that dried at 80 �C to obtain the brown solid powder. The raw

material mesocarbon microbeads (CMB) could be gained by

carbonization at 750 �C for 3 h under Ar atmosphere. (3) The

as-prepared CMBwas activated in 16 M HNO3 at 70 �C for 24 h,

then ACMB could be acquired after washed to be neutral and

dried in oven at 100 �C.

2.1.2. Preparation of PANI/ACMB compositesThe PANI/ACMB composites were prepared using sodium

dodecyl sulfate (SDBS) as the surfactant through chemical

oxidation polymerization [20,21]. Fig. 1 shows a possible

formation mechanism of PANI/ACMB composites in the

presence of SDBS. There were oxygen-enriched and nitrogen-

enriched functionalities on the surface of the ACMB due to

16 M HNO3 activation. These functional groups acted as

anchor sites and enabled the subsequent in situ polymeriza-

tion of PANI attaching on the surfaces of ACMB.Meanwhile, by

introducing SDBS to the reaction system, which was hydro-

philic and made the aniline monomers easily absorbed onto

the surface of the ACMB. Besides, the pep electron stacking

interaction between the ACMB and the aniline was also

beneficial to in situ polymerization occurring on the surface of

ACMB. Therefore, the PANI would gradually grow along the

initial nuclei of PANI and resulted in the extending growth of

PANI [22]. To prepare the PANI/ACMB composites, the proce-

dure was divided into two steps: (1) ACMB (0.1 g) and SDBS

(0.2 g) were immersed in 1 M H2SO4 (15 mL) and kept for

ultrasound about 10 min, 0.2 mL aniline was added to the

above mentioned solution quickly with intensive stirring,

then being stirred under 0 �C for 1 h; (2) Ammoniumpersulfate

solution (the mass ratio of aniline/ammonium persulfate is

1:2.3) was added drop by drop to the solution mentioned in

step 1, then the mixture was stirred at 0 �C for 6 h. The black-

green product of the reaction was filtered and washed

repeatedly with distilled water and alcohol. The resulting

polymer was dried under vacuum at 70 �C for 12 h.

2.2. Preparation of electrodes

The working electrode containing 80 wt.% active material,

10 wt.% carbon black, and 10 wt.% polyvinylidene fluoride

(PVDF) were well mixed in N-methyl-2-pyrrolidone (NMP) to

form the slurry with proper viscosity. The slurry of the

mixture was painted onto a stainless steel mesh with the area

of 1 cm2 and dried at 120 �C for 12 h, then pressed under

a pressure of 1.6 � 107 Pa. The electrodes were well prepared.

2.3. Measurements

To confirm the polymerization of PANI deposited on ACMB,

the FTIR (Perkin Elicer Spectrum One) could be utilized. To

examine the surfacemorphologies andmicrostructures of the

PANI/ACMB composites and ACMB material, the scanning

electron microscopy (SEM) (JSE-6360LV) and transmission

electron microscope (TEM) (JEM-2100F, JEOL) were used. The

N2 adsorption/desorption isotherm at 77 K on

Fig. 1 e A possible formation mechanism of PANI/ACMB composites.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 4 3 6 5e1 4 3 7 2 14367

a Quantachrome autosorb automated gas sorption system

was used to determine the surface area and pore-size.

The performances of the PANI/ACMB electrode were tested

by cyclic voltammetry (CV), galvanostatic charge/discharge

test on a CHI660 (CH Instruments, USA) electrochemical

workstation, the voltage range for CV and galvanostatic

charge/discharge varied from �0.2e0.8 V. The current density

for thegalvanostaticmeasurementvaried from1to5Ag�1. The

usedelectrolytewas1MH2SO4 solution.Theexperimentswere

carried out using a three-electrode system, in which steel and

the saturated calomel electrode (SCE, 0.242 V vs. the normal

hydrogen electrode (NHE)) were used as counter and reference

electrodes, respectively. Moreover, cycle life measurements

were carried out by potentiostat/galvanostat (BTS 6.0, Neware,

Guangdong, China) on button supercapacitor.

Fig. 2 e FTIR spectra of ACMB and PANI/ACMB composites.

3. Results and discussion

3.1. Structural characterization

The FTIR is used to characterize the PANI deposited on the

ACMB. Fig. 2 is the FTIR spectra of ACMB and PANI/ACMB. In

Fig. 2, the peaks positioned at 1574.4 and 1713.4 cm�1 in the IR

spectrum of ACMB are clearly observed, which are attributed

to the characteristic stretching vibrations of C]N and C]O

group due to the activation of HNO3.While for the IR spectrum

of PANI/ACMB composites, the key characteristic peaks cor-

responding to the quinoid ring and the benzene ring are

observed at 1574.6 and 1492 cm�1, respectively. The other

peaks at 1281.1 and 1121.7 cm�1 can be assigned to the C-N

stretching of the secondary aromatic amine and aromatic

CeH in-plane bending, respectively [23]. The CeH out of plane

bending mode has been used as a key to identify the type of

substituted benzene. Thismode is observed in PANI as a single

band at 823.3 cm�1, which means the 1,4-substituted benzene

ring of PANI.

Themorphologies andmicrostructures of ACMB and PANI/

ACMB composites can be characterized by SEM and TEM. As

shown in Fig. 3a and c, the morphology of ACMB is smooth-

faced with a diameter about 1 mm. The SEM observation pre-

sented in Fig. 3b displays the microstructure of PANI/ACMB

composites. Comparing Fig. 3a with b, the external surface of

ACMB becomes rough, which is due to homogeneously

growing PANI on the external surface of ACMB. It has been

found that the morphologies of the PANI/ACMB composites

Fig. 3 e (a) SEM image of ACMB (b) SEM image of PANI/ACMB composites (c) TEM image of ACMB (d) TEM image of PANI/

ACMB composites.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 4 3 6 5e1 4 3 7 214368

are closely related to the amount of PANI deposited on the

surface of ACMB. During preparation process of PANI/ACMB,

when 0.2 mL aniline is added, the composites with 53 wt.%

PANI will be obtained, which forms the novel leechee-like

morphology. The TEM image presented in Fig. 3d further

illustrates that PANI has been coated on the surface of the

ACMB. It is obviously observed that a uniform layer of PANI

with a thickness of about 100 nm covers completely the

external surface of the ACMB, leading to a rough external

surface for PANI/ACMB. This kind of structure can signifi-

cantly offer the chance contacting between electrode mate-

rials and electrolyte, which could induce better

electrochemical performance. These properties will be proved

later by electrochemical measurements.

Nitrogen sorption isotherms are measured for calculating

SSA, pore volumes and pore size distribution (PSD) in terms of

the Bruauer-Emmet-Teller (BET) equation and Barrett-Joiner-

Halenda (BJH) method. The N2 adsorption/desorption

isotherms and pore size distributions are shown in Fig. 4. The

pore characteristics of the ACMB and PANI/ACMB composites

are shown in Table 1. For PANI/ACMB composites in Fig. 4a,

a hysteresis loop is observed, thus indicating that the

composite consists of micropore and mesopore, which are

beneficial to electrolyte ion movement. After coated by PANI,

the BET specific surface area of the PANI/ACMB decreased

remarkably, which can be ascribed to PANI blockage for the

micropore in the ACMB.

3.2. Electrochemical characterization

To evaluate the electrochemical characteristics of the PANI/

ACMB samples, the CV curves in 1 M H2SO4 electrolyte at

different scan rates are performed at the potential window

from�0.2e0.8 V versus SCE (Fig. 5). Notably, it can be seen that

due to the existence of the polarization, a positive shift of

oxidation peaks and a negative shift of reduction peaks are

observedwith the increase of the scan rate. The two couples of

apparent redox peaks are attributed to the redox transition of

PANI between a semiconducting state (leucoemeraldine form)

and a conducting state (polaronic emeraldine form) and the

emeraldine-pernigraniline transformation [24]. Themaximum

specific capacitance of the PANI/ACMBelectrode is 433.75 F g�1

at 1 mV s�1 from the CV curve according to Eq. (3) [20]:

C ¼ QV

¼Z

idtDV

(3)

where i is a sampled current (A), dt is a sampling time span (s),

and DV is a total potential deviation of the voltage window (V).

And the value retains 335 F g�1 at 5 mV s�1, which is higher

than the result reported by Yoon et al. [25] in which the PANI/

MWNT nanocomposites show specific capacitance values of

217 F g�1, 328 F g�1 and 139 F g�1 for leucoemeraldine base,

emeraldine salt and pernigraniline base, respectively.

In order to compare the performance of the PANI/ACMB

composites electrode and ACMB electrode, cyclic voltammo-

grams at 1 mV s�1 are carried out in Fig. 6. It can be seen that

three pairs of redox peaks labeled as A/A1, B/B1, C/C1, corre-

sponding to the different processes of the PANI redox transi-

tions of the PANI/ACMB composites. The two pairs of peaks

(A/A1 and C/C1) are associated with the redox of PANI mole-

cules (leucoemeraldine and perigraldine species) [26]. The

weak peaks B/B1 are attributed to the double-electron redox

transition between p-benzoquinone and the p-hydroquinone

through hydrolysis reaction of PANI [27e29]. Owning to PANI’s

Fig. 4 eNitrogen adsorption/desorption isotherm at 77 K (a)

and pore size distribution (b) of ACMB and PANI/ACMB

composites.

Fig. 5 e Cyclic voltammograms for PANI/ACMB electrode at

different scan rates.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 4 3 6 5e1 4 3 7 2 14369

homogeneously growth on the external surface of ACMB, the

composites can fully contact with electrolyte, so the chance

for storing energy could be largely increased. Meanwhile, the

area surrounded by CV curve is apparently larger than that of

the ACMB electrode, indicating that the PANI/ACMB electrode

has much more specific capacitance (433.75 F g�1) compared

with the value of the ACMB electrode (156.25 F g�1).

Fig. 7 shows the relationship between specific capacitances

and different scan rates. It is noted that the specific capaci-

tance of PANI/ACMB is much higher than that of ACMB at the

same scan rate. What is more, with the increase of the scan

rate, the ACMB material keeps excellent stability. The

maximum specific capacitance of 433.75 F g�1 is obtained at

a scan rate of 1 mV s�1 for PANI/ACMB composites compared

Table 1 e Pore characteristics of ACMB and PANI/ACMBcomposites.

BET-SSA(m2 g�1)

Averageaperture (nm)

Pore volume(cm3 g�1)

ACMB 333.8 1.186 0.049

PANI/ACMB 18.7 1.647 0.076

to 156.25 F g�1 for ACMB. The enhanced specific capacitance is

due to the synergistic effect between PANI and ACMB [18]. On

the one hand, ACMB undertakes some mechanical deforma-

tion in the redox process of PANI/ACMB composites, which

avoids destroying the electrode material and is benefited to

a better stability. On the other hand, the pseudocapacitance of

PANI in the composites is enhanced by its highly conductive

ACMB component, which favors the redox reaction of PANI

component [30]. In addition, the small nanometer-sized PANI

of the PANI/ACMB composites can exhibit the enhanced

electrode/electrolyte interface areas, providing high electro-

active regions and short diffusion lengths [31], which can

ensure the high utilization of PANI. Meanwhile, ACMB in the

composite reduces the internal resistance of the electrode.

The charge/discharge curves of PANI/ACMB electrode

measure at different current densities within a potential

window (�0.2e0.8 V vs. SCE) are showed in Fig. 8. It can be

noted that the discharge time increases distinctlywith current

density decreasing, suggesting an increase of the diffusion

resistance of electrolyte ions in the electrode material. And at

Fig. 6 e Cyclic voltammograms for the comparison of ACMB

and PANI/ACMB composites electrode at 1 mV sL1.

Fig. 7 e Specific capacitance as a function of potential

sweep rates for ACMB and PANI/ACMB composites

electrode.

Fig. 9 e Charge/discharge curves for the comparison of

ACMB and PANI/ACMB composites at 1 A gL1.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 4 3 6 5e1 4 3 7 214370

the beginning of discharge there are a few sudden potential

drops (IR drop) which are attributed to the resistance of elec-

trolytes and the inner resistance of ion diffusion [32]. Usually,

IR drop is a direct measure of equivalent series resistance

(ESR) which influences the overall power performance of

a capacitor. Low resistance leads to less energy being wasted

to produce unwanted heat during charging/discharging

processes. The specific capacitance decreases mainly because

charge and discharge rates are too fast, which leads to less

access of electrolyte ions to the active sites of PANI layers and

a reduced contribution of pseudocapacitance from the redox

reaction of PANI at the interface of the electrolyte/electrode.

The specific capacitance of PANI/ACMB composites reaches

351.7 F g�1 at 100 mA g�1, which can be calculated from Eq. (4)

[23,33]:

Cm ¼ Cm

¼ I� tDV �m

(4)

where Cm is the specific capacitance (F g�1), I is the charge/

discharge current (A), t is the discharge time (s), DV is the

Fig. 8 e Charge/discharge curves for PANI/ACMB electrode

at different current densities.

range of the discharge (V), and m is the mass of the active

material (g).

To gain a further understanding on the electrochemical

performance of the electrode material, the comparison of

galvanostatic charge/discharge curves for the PANI/ACMB

composites and ACMB electrode at 1 A g�1 is given. In Fig. 9,

the curve of ACMB electrode is deviated from typical

symmetrical triangle shape, because there are oxygen-

enriched and nitrogen-enriched functionalities on the ACMB

surface due to 16 M HNO3 activation. Also, the curve of PANI/

ACMB is not ideal triangle shape, which attributes to the

existence of double layer capacitance of ACMB and faradaic

capacitance of PANI. As being seen from Fig. 9, the discharging

curve of the PANI/ACMB composites electrode shows two

voltage stages in the ranges of 0.8 to 0.53 V and 0.53 to �0.2 V,

respectively. The former stage with a relatively short dis-

charging time is ascribed to double layer capacitance. Never-

theless, the latter stagewith amuch longer discharging time is

associated with the combination of the pseudocapacitance of

PANI/ACMB composites and partially from double layer

capacitance of ACMB [23]. Due to the deposition of PANI on the

surface of ACMB, the leechee-like morphology can enable the

electrochemical accessibility of electrolyte through the PANI

phase, which is fundamental for materials showing the

character of supercapacitors. Furthermore, the nanometer-

size reduces the distance within the PANI phase over which

the electrolyte must transport ions [18]. Therefore, an

enhanced charge storage capacity can be obtained.

The cyclic life of electrode material is one of the most

important parameters for supercapacitors. The cycle life

curves of the coin supercapacitors at the current density of

250mA g�1 are illustrated in Fig. 10. From the testing results of

the first 50 cycles, the specific capacitance for PANI/ACMB

composites material has an enormous loss, because the

doping/undoping of Hþ into or from the PANI chains results in

the swelling and shrinkage of the nanostructured conducting

polymer, which makes the structure of the PANI/ACMB

material changeable and leads to the mechanical degradation

of the polymer [34]. However, in the subsequent cycles the

Fig. 10 e Cycle life of ACMB and PANI/ACMB composites

supercapacitors under current density of 250 mA gL1.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 4 3 6 5e1 4 3 7 2 14371

specific capacitance stabilizes nearly at a fixed value

(110.21 F g�1), which is more than two times of pure ACMB

material (52.81 F g�1). And the decrease in specific capacitance

of the composites is smaller than that of the m-Csa/PANI-45

composites reported by Tan et al. [35]. Such a low decrease in

specific capacitance after the long charge/discharge cycle

indicates the high stability of the composite and its potential

prospect as an electrode active material for long-term super-

capacitors applications.

4. Conclusions

The obtained ACMBmaterials serve as an excellent matrix for

PANI growth. The PANI has been homogeneously grown on

the external surface of ACMB to form a uniform leechee-like

PANI/ACMB morphology with an amount of 53 wt.% PANI. A

drastically enhanced gravimetric capacitance of the PANI/

ACMB electrode has been presented, relative to pure ACMB in

H2SO4 aqueous solution, since the capacitance of the PANI/

ACMB is combination of double layer capacitance (ACMB) and

faradaic pseudocapacitance (PANI). The maximum specific

capacitance value of 433.75 F g�1 is achieved at a scan rate of

1mV s�1, which ismuch higher than that of ACMB at the same

scan rate (156.25 F g�1). In comparison with ACMB electrode

materials, the electrochemical performance of coin super-

capacitors used PANI/ACMB composites possesses both high

specific capacitance and excellent cycle stability. Hence, it is

notable that preparing PANI/ACMB composites will be an

effective way to offer the potential electrode active material

with excellent specific capacitance and an enhanced cycle life

for the application of high performance supercapacitors.

Acknowledgments

This work was financially supported by the National Natural

Science Foundation of China (Grant No. 51072173), Specialized

Research Fund for the Doctoral Program of Higher Education

(Grant No. 20094301110005).

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