Pn

CST

a

ARRAA

KPHNCS

1

tetphvTmmitwmwtemeot

0d

Electrochimica Acta 55 (2010) 6857–6864

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

reparation and characterization of coaxial halloysite/polypyrrole tubularanocomposites for electrochemical energy storage

hao Yang, Peng Liu ∗, Yongqing Zhaotate Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou University,ianshui South Road 222#, Lanzhou 730000, China

r t i c l e i n f o

rticle history:eceived 15 January 2010eceived in revised form 25 May 2010ccepted 28 May 2010vailable online 4 June 2010

a b s t r a c t

Halloysite nanotubes/polypyrrole (HNTs/PPy) nanocomposites with coaxial tubular morphology for useas electrode materials for supercapacitors were synthesized by the in situ chemical oxidative poly-merization method based on self-assembled monolayer amine-functionalized HNTs. The HNTs/PPycoaxial tubular nanocomposites were characterized with transmission electron microscope (TEM), X-ray

eywords:olypyrrolealloysiteanocompositesoaxial tubular morphology

diffraction (XRD), thermogravimetric analysis (TGA), electrical conductivity measurement at differenttemperatures, cyclic voltammetry (CV), and galvanostatic charge–discharge measurements. The coaxialtubular nanocomposites showed their greatest conductivity at room temperature and a weak tempera-ture dependence of the conductivity from 298 K to 423 K. A maximum discharge capacity of 522 F/g aftercorrecting for the weight percent of the PPy phase at a current density of 5 mA cm−2 in a 0.5 M Na2SO4

electrolyte could be achieved in a half-cell setup configuration for the HNTs/PPy composites electrode,pplica

upercapacitor suggesting its potential a

. Introduction

Growing demands for the generation of power sources withransient high-power density have stimulated great interest inlectrochemical capacitors in recent years. It has been acceptedhat supercapacitors are the best candidates to provide the highower and long durability needed for new energy devices, such asybrid peak-power sources in electric vehicles, backup sources forarious electrical devices, and uninterrupted power supplies [1,2].he materials studied for supercapacitor applications include threeain types: (1) carbon; (2) metal oxides; and (3) conducting poly-ers [3–5]. Conducting electroactive polymers remain a subject of

ntense investigation of many research groups worldwide. Amonghem, polypyrrole, first synthesized in 1916 [6], is one of the mostidely investigated conducting polymers because of its good ther-al and environmental stability, and good electrical conductivity,hich are favorable for various applications, such as metalliza-

ion of dielectrics [7], batteries [8], antistatic coatings, shielding oflectromagnetic interference [9], sensors [10], actuators [11], and

icroactuators [12]. To solve problems, such as brittleness, low

longation, and poor processibilities, strategies have been devel-ped, such as preparing composites and copolymers, reforminghe monomers of these conducting polymers, and blending with

∗ Corresponding author. Tel.: +86 931 8912516; fax: +86 931 8912582.E-mail address: pliu@lzu.edu.cn (P. Liu).

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.05.080

tion in electrode materials for electrochemical capacitors.© 2010 Elsevier Ltd. All rights reserved.

some commercially available polymer that offers better mechanicaland/or chemical properties. The conducting polymer compositesrelated to supercapacitors containing transition metal oxides suchas RuO2, MnO2, TiO2, In2O3 and carbon materials, and carbon mate-rials have been synthesized and widely studied [13–19]. In thesecomposites, the transition metal oxides or carbon materials usedare conductive or semi-conductive.

Halloysite, which was first described by Berthier in 1826 as adioctahedral 1:1 clay mineral of the kaolin group, can adopt a vari-ety of morphologies, the most common of which is the elongatedtubule [20]. Recently, halloysite has gained growing interest in thesynthesis of complex structures as an economically available nan-otubular raw material [21–26].

At the present time, few approaches for preparing HNTs/PPycoaxial tubular nanocomposite have been reported. In this paper,low-costing natural halloysite nanotubes were used to formHNTs/PPy nanocomposites with well-defined coaxial tubular mor-phology for the first time. Furthermore, the halloysite nanotubesare insulating and only used as the supports for the polypyrrolecoatings here.

2. Experimental

2.1. Materials

Pyrrole monomer (Acros Organics) dehydrated with calciumhydride for 24 h was distilled under reduced pressure before

6858 C. Yang et al. / Electrochimica A

Table 1The conditions of the polymerizations investigated in this work.

Samples Pyrrole (mL) SAM-HNTs (g) SAM-HNTs (wt%)

S-1 1 0.0098 1S-2 1 0.0510 5S-3 1 0.1077 10S-4 1 0.2423 20S-5 1 0.6461 40

uCfTapcTd

2

wputa

2

bdoTwmotTTeani

2

mbitaltT0tcrat

−1

S-6 1 1.4537 60S-7 1 2.2612 70S-8 1 3.8764 80

se. Ammonium peroxodisulfate (APS) (Tianjin Chemical Reagento., Tianjin, China) as an oxidant and sodium p-toluene sul-

onate (STS) (Tianjin Guangfu Fine Chemical Research Institute,ianjin, China) as a dopant were used as received. Toluene, �-minopropyltriethoxysilane (APTES) and absolute ethanol wereurchased from Aladdin Chemical Reagent Co. Ltd. All the chemi-als were of analytical grade and used without further purification.he raw halloysite clay was obtained from Hebei, China. Doublyeionized water was used in all the processes.

.2. Chemical modification of halloysite nanotubes

The HNTs were chemically modified during the self-assemblyith APTES. Typically, HNTs (1.0 g) and APTES (2 mL) were dis-ersed in toluene (10 mL). The suspension was refluxed for 10 hnder dry nitrogen. The resulting modified HNTs were filtered andhen washed with ethanol. Finally, the product was dried in vacuumt 60 ◦C for 24 h to obtain amine-functionalized SAM-HNTs.

.3. Synthesis of HNTs/PPy coaxial tubular nanocomposites

The HNTs/PPy coaxial tubular nanocomposites were preparedy an in situ surface polymerization method. In a typical proce-ure, sodium p-toluene sulfonate (4.16 g) was dissolved in 100 mLf deionized water, and then the SAM-HNTs particles were added.he mixture was then ultrasonically dispersed, and pyrrole (1 mL)as added into the mixture with vigorous stirring. Afterwards, theixture was mechanically stirred for 30 min at 0 ◦C. Then, an aque-

us solution (20 mL) of APS (0.90 g) was added drop by drop tohe above mixture instantly to start the oxidative polymerization.he reaction was performed under mechanical stirring for 10 h.he resulting precipitates were washed with deionized water andthanol several times. Finally, the product was dried in vacuumt 60 ◦C for 24 h to obtain the desired HNTs/PPy coaxial tubularanocomposite as a dark powder. The conditions of the polymer-

zations are given in Table 1.

.4. Electrochemical behavior

The HNTs/PPy composite electrodes were prepared as follows. Aixture containing 80 wt% active materials (3 mg), 15 wt% carbon

lack, and 5 wt% polytetrafluoroethylene (PTFE) was well mixedn N,N-dimethylformamide (DMF) until they formed a slurry withhe proper viscosity, and then the slurry was uniformly laid on

piece of Ni foam about 1 cm2 that was used as a current col-ector and then dried at 50 ◦C for 24 h. The Ni foam coated withhe HNTs/PPy composite was pressed for 1 min under 1.0 MPa.he mass load of every sample is 3 mg. The electrolyte used was.5 M Na2SO4. The electrochemical behavior of the composite elec-

rode was evaluated by cyclic voltammetry (CV) and galvanostaticharge–discharge. All the electrochemical experiments were car-ied out in a three-electrode glass cell, a platinum counter electrode,nd a standard calomel reference electrode (SCE). CV and galvanos-atic charge–discharge tests were performed in a potential window

cta 55 (2010) 6857–6864

ranging from −0.2 V to 0.8 V (with respect to the SCE) using aCHI660A electrochemical working station.

2.5. Measurements

The FTIR measurements (Impact 400, Nicolet, Waltham, MA)were carried out with the KBr pellet method. Thermogravimetricanalysis (TGA) results were obtained with a TA Instrument 2050thermogravimetric analyzer at a heating rate of 10 ◦C/min from25 ◦C to 750 ◦C under a nitrogen atmosphere. The X-ray diffrac-tion (XRD) patterns were recorded in the range of 2� = 5–60◦ bystep scanning with a Shimadzu XRD-6000 X-ray diffractometer.Nickel-filter Cu K� radiation (� = 0.15418 nm) was used with a gen-erator voltage of 40 kV and a current of 30 mA. The morphologiesof the HNTs/PPy coaxial tubular nanocomposites were observedusing a JEM-1200 EX/S transmission electron microscope (TEM).The electrical conductivities of the nanocomposites were measuredusing an SDY-4 four-point probe meter (Guangzhou Semiconduc-tor Material Academy) at ambient temperature employing themethod on a pressed pellet. The temperature dependence of theconductivity was determined by a WDJ-1 temperature changeresistance measuring instrument (Institute of Chemistry, the Chi-nese Academy of Sciences) at a heating rate of 10 ◦C/min from 25 ◦Cto 150 ◦C.

3. Results and discussion

3.1. Characterization of HNTs/PPy coaxial tubularnanocomposites

To characterize the size and shape of the presented HNTs/PPynanocomposite, TEM was conducted. Fig. 1(b–h) depicts the typi-cal TEM images of HNTs/PPy nanocomposites, indicating that theshell/core products are the coaxial structure with tubular morphol-ogy. It is seen from the micrographs that clusters and granularstructures of polypyrrole on the SAM-HNTs tubes’ surfaces aremaintained even after the addition of the SAM-HNTs in PPy. How-ever, coaxial tubular structures could not be observed in compositeswith high PPy/SAM-HNTs ratios. This is probably due to the excessPPy encapsulating the SAM-HNTs.

It can be clearly observed that many PPy particles are locatedon the SAM-HNTs’ surfaces. From the TEM images with higherresolutions (Fig. 1(h)), the coaxial tubular structure of the nanocom-posite could be observed distinctly. Through this surface-modifiedprocedure, the HNTs/PPy nanocomposites were obtained withwell-defined coaxial tubular morphology (Fig. 1(e–g)). This wasprobably due to favorable specific interactions of the Lewisacid–base type between the basic amino group (n donor) and theacidic N–H bonds (�* acceptor) and/or the positively charged PPybackbone (n acceptor). The probable polymerization mechanismcan be seen in Fig. 2, and is similar to the mechanism that is pro-posed by Akar [27,28].

Fig. 3 shows the FTIR spectrum of the KBr pellet HNTs/PPynanocomposite in the range of 4000–400 cm−1. For the HNTs/PPysample, the absorption bands at 1560 cm−1 is assigned to the pyr-role ring, i.e., the combination of C C and C–C stretching vibrations.The peak at 1468 cm−1 is associated with the C–N stretching vibra-tion [29]. The peak due to C C stretching at 1560 cm−1 shifted tolow wavenumbers as the with HNTs content increased, indicat-ing that the doping level of PPy in HNTs/PPy nanocomposite is

higher than that of PPy [30]. The 1037 cm band is assigned tothe overlap peaks of the Si–O–Si symmetric stretching mode andthe C–H stretching mode in-plane bending. The band at 817 cm−1 isassigned to the overlap peaks of Si–O and C–H out-of-plane bend-ing. The band associated with ı Si–O is located at 467 cm−1.

ica A

pciit

C. Yang et al. / Electrochim

For the XRD spectra of the HNTs/PPy nanocomposites, the maineaks are similar to the SAM-HNTs particles, which reveal that the

rystal structure of SAM-HNTs is well-maintained after the coat-ng process under polymerization reaction conditions, as shownn Fig. 4. Because of the relatively thin layer and amorphous crys-allinity of the PPy coating prepared under this polymerization

Fig. 1. TEM images of raw HNTs (a) and th

cta 55 (2010) 6857–6864 6859

method, no obvious diffraction peak for the PPy is observed. Carefulanalysis of X-ray diffractograms of the HNTs/PPy nanocomposites

suggests that they exhibit semi-crystalline behavior.

Fig. 5 illustrates the results of the thermogravimetric analysisof the HNTs/PPy coaxial tubular nanocomposites. A sharp loss inmass is observed at 300 ◦C and continues to 750 ◦C, possibly due to

e HNTs/PPy nanocomposites (b–h).

6860 C. Yang et al. / Electrochimica Acta 55 (2010) 6857–6864

(Conti

tdttiH

3n

oH

that of PPy, which results in a high conductivity, as reported byNie [33,34]. In addition, the increase in the SAM-HNTs contentincreased the compactness of the sample and decreased the con-ducting PPy content, which resulted in two contrary effects on theconductivity of the HNTs/PPy nanocomposites. We can speculate

Fig. 1.

he large scale thermal degradation of the PPy chains [31] and theehydroxylation of HNTs [32]. The onset decomposition tempera-ures of the composites are higher than those of PPy and are shiftedowards the higher temperature range as the content of SAM-HNTsncreased. This can be attributed to the retardation effect of SAM-NTs as barriers for the degradation of PPy [33].

.2. Electrical conductivity of HNTs/PPy coaxial tubularanocomposites

The effect of SAM-HNTs content on the electrical conductivityf the HNTs/PPy nanocomposites is plotted in Fig. 6. As the SAM-NTs content increased, the conductivity first increased and then

Fig. 2. Proposed schematic for the polymerization mechanism.

Fig. 3. FTIR spectra of PPy and the HNTs/PPy nanocomposites.

nued).

decreased. The doping level of PPy in composite is higher than

Fig. 4. XRD patterns of PPy, the SAM-HNTs and the HNTs/PPy coaxial tubularnanocomposites.

Fig. 5. TGA patterns of PPy and the HNTs/PPy coaxial tubular nanocomposites.

C. Yang et al. / Electrochimica Acta 55 (2010) 6857–6864 6861

Ft

tp

HaTtntPTmth

�

wadnmara1at

ig. 6. Conductivities of the HNTs/PPy coaxial tubular nanocomposites at roomemperature.

hat the SAM-HNTs particles are inclined to the coaxial tubulararticles as suggested in Fig. 7.

The temperature dependence of the conductivity of theNTs/PPy nanocomposites is represented in Fig. 8. As the temper-ture increased from 283 K to 423 K, the conductivity increased.he temperature dependence of the conductivity shows that allhe samples had a semiconducting property. However, it is worthoticing that samples with high SAM-HNTs content showed a weakemperature dependence of the conductivity. Fig. 8 implies that thePy contributes predominantly in the charge conduction process.he inherent disordered nature is found in semiconducting poly-ers like polypyrrole [35]. Thus the temperature dependence of

he conductivity � (T) can be described by Mott’s variable rangeopping (VRH) model [36]:

(T) = �0exp −(

T0

T

)�

(1)

here �0 is the high temperature limit of the conductivitynd T0 is Mott’s characteristic temperature associated with theegree of localization of the electronic wave function. The expo-ent � = 1/(1 + d) determines the dimensionality of the conductingedium (d is defined as dimensionality). The possible values of �

re 1/4, 1/3, and 1/2 for three-, two-, and one-dimensional systems,

espectively. The best fit value of � is obtained by linear regressionnalysis. The lowest standard deviations are found for � = 1/4 for S-, S-5, and S-7, respectively. The linear dependence of ln � on T−1/4,s shown in Fig. 8, indicates that three-dimensional (3D) chargeransport occurs in the HNTs/PPy nanocomposites. The values of

Fig. 7. Schematic illustration of the synthesis process of the HN

Fig. 8. Temperature dependence of the conductivity of the HNTs/PPy nanocompos-ites and temperature dependence of the conductivity based on the VRH model.

Mott’s characteristic temperature T0 and the pre-exponential fac-tor �0 are obtained from the slopes and intercepts of Fig. 8 and aregiven in Table 2. The average hopping distance Rhop between twosites and the activation energy Whop are:

Rhop =(

38

)(T0

T

)1/4L (2)

Whop =(

14

)kT

(T0

T

)1/4(3)

where L is the localization length for the PPy phase, and the local-ization length of the pyrrole monomer unit is assumed to be about3 Å [37].

At room temperature the average hopping distances for the sam-ples S-1, S-5, and S-7 were about 18.3 Å, 15.2 Å, and 12.5 Å, and canbe reduced to about 16.5 Å, 13.8 Å and 11.3 Å by varying the PPycontent. The estimated activation energies for hopping, as shownin Table 2, are in the range of 133.8–67.6 meV. It implied that thedecrease in the hopping distance and hopping energy enhanced thestability of the conductivity when the temperature changed.

3.3. Electrochemical studies

To further evaluate the applicability of HNTs/PPy coaxial tubularnanocomposites for supercapacitors, the mass specific capacitance,the cycling performance and electrochemical impedance spec-

Ts/PPy nanocomposites with coaxial tubular morphology.

6862 C. Yang et al. / Electrochimica Acta 55 (2010) 6857–6864

Table 2Weight percentage of polypyrrole (x, %), parameters �0 and T0 as defined in Eq. (2), density of states hopping length Rhop, activation energy Whop.

Samples x (%) �0 (S cm−1) R283 K (Å) W283 K (meV) R423 K (Å) W423 K (meV)

tst

C

w�m

pctbtcaiHittAtdc

HTmtnncctd

Fta

To obtain more information about the ability of HNTs/PPynanocomposites to function as electrodes in supercapacitors, an EISexperiment was carried out in a 0.5 M Na2SO4 solution at 0.4 V (vs.SCE) as shown in Fig. 11. There are two features for both curves:

S-1 99 161497464 18.3S-5 60 3921560 15.2S-7 30 57526 12.5

roscopy of composites was investigated in a 0.5 M Na2SO4 aqueousolution. The specific capacitance (Cm) can be calculated accordingo Eq. (4):

m = C

m=

(I�t

�Vm

)(4)

here I is the charge–discharge current, �t is the discharge time,V is the electrochemical window, and m is the mass of activeaterial within the electrode.Fig. 9 shows the charge–discharge curves of the HNTs/PPy com-

osite electrodes with different HNTs content. As the SAM-HNTsontent increassd, the specific capacitance decreased from 522 F/go 241 F/g at a current density of 5 mA cm−2. The relationshipetween the specific capacitance of the HNTs/PPy composite vs.he weight percent of HNTs is also plotted in Fig. 9. The specificapacitance of the composite increased as more PPy coating wasdded at the low loading densities in the curve and reaches a max-mum of 522 F/g. Further increase of the PPy loading content of theNTs/PPy composite beyond 90 wt%, resulted in a sharp decrease

n the curve. This result implies that the excess PPy does not favorhe formation of effectively dispersed PPy on the surface of HNTs inhe composites and, therefore, results in the low utilization of PPy.

possible explanation may be due to favorable specific interac-ions of the Lewis acid–base type between the basic amino group (nonor) and the acidic N–H bonds (�* acceptor) and/or the positivelyharged PPy backbone (n acceptor).

Fig. 10(a) presents the cyclic voltammetry behavior of theNTs/PPy composite electrodes in 0.5 M Na2SO4 aqueous solution.he typical rectangle-like shape of all the CV curves in Fig. 10(b)easured at various scan rates in a 0.5 M Na2SO4 solution reveals

he perfect electrochemical capacitive behavior of the HNTs/PPyanocomposite electrodes. The curves at different scan rates show

o peaks, indicating that the electrodes are charged and dis-harged at a pseudo-constant rate over the complete voltammetricycle. However, as the scan rate increased, the effective interac-ion between the ions and the electrode decreased greatly, and theeviation from rectangularity of the CV became obvious.

ig. 9. Galvanostatic charge–discharge curves of the HNT/PPy nanocomposite elec-rodes at a current density of 5 mA cm−2 in a 0.5 M Na2SO4 solution. The mass of thective material in every sample is 3 mg.

98.9 16.5 133.882.4 13.8 111.367.6 11.3 91.4

Fig. 10. (a) Cyclic voltammogram of the HNT/PPy nanocomposite electrodes at10 mV s−1 in 0.5 M Na2SO4; (b) scan rate dependence of HNT/PPy nanocompositeelectrodes in 0.5 M Na2SO4.

Fig. 11. Impedance Nyquist plots of the HNTs/PPy nanocomposites.

C. Yang et al. / Electrochimica A

Ft

fir10ptdet

pe

A

wcFitctdhtP

4

spatteeses

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

ig. 12. High-rate discharge ability of the HNTs/PPy nanocomposite and PPy elec-rodes.

rst, in the high frequency intercept of the real axis, a solutionesistance Rs can be observed. The resistances of composites S-, S-3, S-5, and S-7 were approximately 12 �, 8 �, 17 �, 32 � in.5 M Na2SO4 electrolyte, and this conclusion is not contrary to therevious conductivity measurements. Secondly, in the medium-o-low-frequency region, an unequal semicircular pattern can beiscovered in both curves. At low frequency, the impedance plotsxhibited a vertical line, indicating a limiting diffusion process inhe electrolyte [38,39].

To compare the power properties between the HNTs/PPy com-osite and the pure PPy, the high-rate discharge ability (A) of thelectrode was also employed. A can be obtained using Eq. (5) [40]:

(%) = Cd

C1× 100% (5)

here Cd and C1 are the discharge capacity of electrodes at theertain current density of interest and 1 mA cm−2, respectively.ig. 12 shows the relationship between the high-rate discharge abil-ty and the discharge current density. It was clear from Fig. 12 thathe HNTs/PPy composite electrode exhibited better high-rate dis-harge ability when compared with the pure PPy electrode. Fromhe improved power characteristic of composite electrodes can beeduced that the introduction of HNTs makes the composite haveigher conductivity, lower charge-transfer resistance, more rela-ive chemical durability, and better rate capability than the purePy electrode.

. Conclusions

In summary, the present work demonstrates a novel and facileynthetic route for the preparation of the HNTs/PPy nanocom-osites with coaxial tubular morphology with natural nanotubess supports. The results show that the nanocomposites hadhe largest conductivity, up to 40 S/cm, and exhibited a weakemperature dependence of electrical conductivity. Furthermore,lectrochemical tests showed that the HNTs/PPy nanocompositesxhibited typical electrochemical supercapacitor behavior withpecific capacitances of approximately 522 F/g in a 0.5 M Na2SO4lectrolyte which makes them ideal candidates for applicationsuch as antistatic coatings and electrode materials.

eferences

[1] B.E. Conway, V. Birss, J. Wojtowicz, The role and utilization of pseudocapaci-tance for energy storage by supercapacitors, J. Power Sources 66 (1997) 1.

[2] C. Arbizzani, M. Mastragostino, L. Meneghello, Polymer-based redox superca-pacitors: a comparative study, Electrochim. Acta 41 (1996) 21.

[

[

cta 55 (2010) 6857–6864 6863

[3] C.M. Yang, Y.J. Kim, M. Endo, H. Kanoh, M. Yudasaka, S. Iijima, K. Kaneko,Nanowindow-regulated specific capacitance of supercapacitor electrodes ofsingle-wall carbon nanohorns, J. Am. Chem. Soc. 129 (2007) 20.

[4] C.C. Hu, Y.H. Huang, K.H. Chang, Annealing effects on the physicochemicalcharacteristics of hydrous ruthenium and ruthenium–iridium oxides for elec-trochemical supercapacitors, J. Power Sources 108 (2002) 117.

[5] Z. Mandic, M.K. Rokovic, T. Pokupcic, Polyaniline as cathodic material for elec-trochemical energy sources. The role of morphology, Electrochim. Acta 54(2009) 2941.

[6] L.X. Wang, X.G. Li, Y.L. Yang, Preparation, properties and applications ofpolypyrroles, React. Funct. Polym. 47 (2001) 125.

[7] C.M. Intelmann, V. Syritski, D. Tsankov, K. Hinrichs, J. Rappich, Ultrathinpolypyrrole films on silicon substrates, Electrochim. Acta 53 (2008) 4046.

[8] H. Nishide, K. Oyaizu, Toward flexible batteries, Science 319 (2008) 737.[9] J. Reut, A. Öpik, K. Idla, Corrosion behavior of polypyrrole coated mild steel,

Synth. Met. 102 (1999) 1392.10] M.T. Nguyen, A.F. Diaz, Novel method for the preparation of magnetic nanopar-

ticles in a polypyrrole powder, Adv. Mater. 6 (1994) 858.11] B. Lakard, O. Segut, S. Lakard, G. Herlem, T. Gharbi, Potentiometric miniaturized

pH sensors based on polypyrrole films, Sens. Actuator B: Chem. 122 (2007)101.

12] E.W.H. Jager, E. Smela, O. Inganäs, I. Lundström, Polypyrrole microactuators,Synth. Met. 102 (1999) 1309.

13] C.C. Hu, K.H. Chang, Cyclic voltammetric deposition of hydrous ruthenium oxidefor electrochemical supercapacitors: effects of the chloride precursor transfor-mation, J. Power Sources 112 (2002) 401.

14] C.C. Wang, C.C. Hu, Electrochemical catalytic modification of activated carbonfabrics by ruthenium chloride for supercapacitors, Carbon 43 (2005) 1926.

15] Z. Algharaibeh, X. Liu, P.G. Pickup, An asymmetric anthraquinone-modifiedcarbon/ruthenium oxide supercapacitor, J. Power Sources 187 (2009) 640.

16] Y. Wang, I. Zhitomirsky, Electrophoretic deposition of manganesedioxide–multiwalled carbon nanotube composites for electrochemicalsupercapacitors, Langmuir 25 (2009) 9684.

17] Y.G. Wang, Z.D. Wang, Y.Y. Xia, An asymmetric supercapacitor using RuO2/TiO2

nanotube composite and activated carbon electrodes, Electrochim. Acta 50(2005) 5641.

18] K.R. Prasad, K. Koga, N. Miura, Electrochemical deposition of nanostructuredindium oxide: high-performance electrode material for redox supercapacitors,Chem. Mater. 16 (2004) 1845.

19] S. Mitra, K.S. Lokesh, S. Sampath, Exfoliated graphite–ruthenium oxide compos-ite electrodes for electrochemical supercapacitors, J. Power Sources 185 (2008)1544.

20] E. Joussein, S. Petit, J. Churchman, B. Theng, D. Righi, B. Delvaux, Halloysite clayminerals a review, Clay Min. 40 (2005) 383.

21] M. Du, B. Guo, Y. Lei, M. Liu, D. Jia, Carboxylated butadiene-styrenerubber/halloysite nanotube nanocomposites: interfacial interaction and per-formance, Polymer 49 (2008) 4871.

22] D. Fix, D.V. Andreeva, Y.M. Lvov, D.G. Shchukin, H. Möhwald, Application ofinhibitor-loaded halloysite nanotubes in active anti-corrosive coatings, Adv.Funct. Mater. 19 (2009) 1720.

23] D.G. Shchukin, S.V. Lamaka, K.A. Yasakau, M.L. Zheludkevich, M.G.S. Ferreira,H. Möhwald, Active anticorrosion coatings with halloysite nanocontainers, J.Phys. Chem. C 112 (2008) 958.

24] S. Deng, J. Zhang, L. Ye, J. Wu, Toughening epoxies with halloysite nanotubes,Polymer 49 (2008) 5119.

25] G.S. Machado, C. de Freitas, A.D. Kelly, F. Wypych, S. Nakagaki, Immobiliza-tion of metalloporphyrins into nanotubes of natural halloysite toward selectivecatalysts for oxidation reactions, J. Mol. Catal. A: Chem. 283 (2008) 99.

26] D.G. Shchukin, G.B. Sukhorukov, R.R. Price, Y.M. Lvov, Halloysite nanotubes asbiomimetic nanoreactors, Small 1 (2005) 510.

27] B. Ustamehmetoglu, A. Akar, Soluble polypyrrole copolymers, J. Appl. Polym.Sci. 82 (2001) 1098.

28] C. Perruchot, M.M. Chehimi, M. Delamar, J.A. Eccles, T.A. Steele, C.D. Mair,SIMS analysis of conducting polypyrrole–silica gel composites, Synth. Met. 113(2000) 53.

29] C. Yang, T. Wang, P. Liu, H. Shi, D. Xue, Preparation of well-definedblackberry-like polypyrrole/fly ash composite microspheres and their electri-cal conductivity and magnetic properties, Curr. Opin. Solid State Mater. Sci. 13(2009) 112.

30] G. Qiu, Q. Wang, M. Nie, Polypyrrole–Fe3O4 magnetic nanocomposite preparedby ultrasonic irradiation, Macromol. Mater. Eng. 291 (2006) 68.

31] J. Jang, H. Yoon, Novel fabrication of size-tunable silica nanotubes usinga reverse-microemulsion-mediated sol–gel method, Adv. Mater. 16 (2004)799.

32] K.P. Nicolini, C.R.B. Fukamachi, F. Wypych, A.S. Mangrich, Dehydrated halloysiteintercalated mechanochemically with urea: thermal behavior and structuralaspects, J. Colloid Interface Sci. 338 (2009) 474.

33] J.S. Choi, J.H. Sung, H.J. Choi, M.S. Jhon, Effect of organoclay content on phys-ical characteristics of poly(o-ethoxyaniline) nanocomposites, Synth. Met. 153(2005) 129.

34] G. Qiu, Q. Wang, M. Nie, Polyaniline/Fe3O4 magnetic nanocomposite preparedby ultrasonic irradiation, J. Appl. Polym. Sci. 102 (2006) 2107.

35] C. Pandis, E. Logakis, V. Peoglos, P. Pissis, M. Omastová, M. Mravcáková, A. Janke,J. Pionteck, Y. Peneva, L. Minkova, Morphology, microhardness, and electri-cal properties of composites based on polypropylene, montmorillonite, andpolypyrrole, J. Polym. Sci.: Part B: Polym. Phys. 47 (2009) 407.

6 ica A

[

[

[

[39] A. Kepas, M. Grzeszczuk, Ionic transport in polypyrrole doped with dianioniccounterion hexafluorosilicate, Electrochim. Acta 51 (2006) 4167.

864 C. Yang et al. / Electrochim

36] S. Yoshimoto, F. Ohashi, T. Kameyama, Insertion of polypyrrole chains intomontmorillonite galleries by a solvent-free mechanochemical route, Macro-

mol. Rapid Commun. 26 (2005) 461.

37] K. Dutta, S.K. De, Optical and diode like current–voltage characteristics ofSnO2–polypyrrole nanocomposites, J. Phys. D: Appl. Phys. 40 (2007) 734.

38] T. Brousse, M. Toupin, R. Dugas, L. Athouel, O. Crosnier, D. Belanger, CrystallineMnO2 as possible alternatives to amorphous compounds in electrochemicalsupercapacitors, J. Electrochem. Soc. 153 (2006) A2171.

[

cta 55 (2010) 6857–6864

40] X. Wei, S. Liu, H. Dong, P. Zhang, Y. Liu, J. Zhu, G. Yu, Microstruc-tures and electrochemical properties of Co-free AB5-type hydrogen storagealloys through substitution of Ni by Fe, Electrochim. Acta 52 (2007)2423.