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Applied Surface Science 256 (2010) 4339–4343

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

reparation and electrochemistry of one-dimensional nanostructured MnO2/PPyomposite for electrochemical capacitor

uan Lia,b, Li Cuia, Xiaogang Zhangc,∗

School of Science, Xi’an Jiaotong University, Xi’an 710049, ChinaCollege of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, ChinaCollege of Material Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

r t i c l e i n f o

rticle history:eceived 8 January 2010eceived in revised form 8 February 2010ccepted 8 February 2010

a b s t r a c t

One-dimensional nanostructured manganese dioxide/polypyrrole (MnO2/PPy) composite was preparedby in situ chemical oxidation polymerization of pyrrole in the host of inorganic matrix of MnO2, usingcomplex of methyl orange (MO)/FeCl3 as a reactive self-degraded soft-template. The morphology andstructure of the composite were characterized by infrared spectroscopy (IR) X-ray diffraction (XRD),

vailable online 13 February 2010

eywords:oft-templatene-dimensional nanostructureanganese dioxide

scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show thatthe MnO2/PPy composite consists of �-MnO2 and PPy with nanotube-like structure. Electrochemicalproperties of the composite demonstrated the material showed good electrochemical reversibility after500 charge–discharge cycles in the potential range of −0.4 to 0.6 V, the tube-like nanocomposite has thepotential application in electrochemical capacitor.

olypyrrolelectrochemical capacitor

. Introduction

Recently, materials possessing a combination of a wide rangef desirable properties have been received tremendous attentions they can combine the advantages of both components and haveotential applications in many fields such as molecular electron-

cs, electrochemical display devices, catalysis, electro-magnetichields, microwave-absorbing materials, supercapacitor and bat-eries, etc. [1–3]. Electrically conducting polymers have beenttracted great interest for being excellent candidates in the fabri-ation of various electronic devices [4–7], because of their varioustructures and metal-like conductivity. The most widely studiedonducting polymers include polyaniline (PANI), polypyrrole (PPy),nd polythiophene (PTh). Among these polymers, PPy has attracteduch attention owing to its unique electrical conductivity, redox

roperty, and optoelectrical properties and excellent environmenttability. PPy can be easily prepared by both chemical and electro-hemical approaches in various organic solvents and in aqueousolution [8]. Multifunctionalized PPy nanostructures have been

ynthesized by blending PPy with electrical, optical and magneticnorganic nanoparticles to form nanocomposites. Among the inor-anic nanoparticles manganese dioxide (MnO2) appears to be aromising material due to its low cost, natural abundance, envi-

∗ Corresponding author. Tel.: +86 25 52112918; fax: +86 25 52112626.E-mail address: azhangxg@163.com (X. Zhang).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.02.028

© 2010 Elsevier B.V. All rights reserved.

ronmental friendship and interesting electrochemical performance[9–11]. Because the combination of two extremely different com-ponents, at the molecular level, can provide a method to designnew nanocomposite materials as well as the ability to improve theelectrochemical properties of both components [12–18].

The electrode materials were also more applicable and extensivefor the application of supercapacitor in neutral medium [19,20].So, it is especially essential to study the electrochemical proper-ties of electrode material in neutral medium. In this paper, wereported a simple and cheap technique that allows the synthe-sis of one-dimensional nanostructured MnO2/PPy composite forsupercapacitor in neutral medium at lower temperature, usingcomplex of methyl orange (MO)/FeCl3 as a reactive self-degradedsoft-template and oxidant. To our knowledge, MnO2 and PPy areboth stable and can exist in mild electrolyte with various potentialranges [7,14,17,21]. We have attempted to find the most suitablepotential range of this material for an electrochemical supercapac-itor, and further discuss its cyclability.

2. Experimental

2.1. Synthesis of MnO2 nanoparticles

Pyrrole was distilled under reduced pressure before use. TheMnO2 was synthesized under hydrothermal condition. Rest ofthe chemical reagents used were analytical grade without furtherpurification. In a typical procedure, KMnO4 and MnSO4·H2O (the

4340 J. Li et al. / Applied Surface Scien

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at 1302 and 1158 cm are ascribed to the C H in-plane defor-mation and the C N stretching vibrations, while other bands at1040 and 962 cm−1 reflected N H in-plane deformation vibrationand the C H out-of-plane vibration, respectively, which impliedthe doping state of PPy [25]. The peaks centered at 772 cm−1 are

Fig. 1. X-ray diffraction patterns of the MnO2, PPy and MnO2/PPy composite.

olar ratio is 2:3) were dissolved in 40 ml volume of distilled waternd stirred strongly for ten minutes until a homogeneous solutionas formed, then the mixture was transferred into a Teflon-lined

utoclave with a stainless steel shell. The autoclave was kept at40 ◦C for 15 h and allowed to cool down to room temperature.he precursors were collected and washed with distilled waternd anhydrous alcohol several times. The final products were driednder vacuum at 60 ◦C.

.2. Preparation of MnO2/PPy composite and PPy

The composite were synthesized under static condition in aower temperature, the template in our work was similar to theiterature [22,23]. In a typical process, an appropriate amountf FeCl3 (0.486 g) was dissolved in 5 mM methyl orange (MO)e-ionized water solution (30 ml) under stirring. A flocculent pre-ipitate appeared immediately. Amount of MnO2 (0.174 g) wasdded to the mixture. After stirring and sonicating for better disper-ion of MnO2, 0.21 ml pyrrole monomer (0.201 g) was slowly addednto the above mixture, then the reaction was kept under static con-ition for 24 h at the temperature of −5 to 0 ◦C. The molar ratio ofyrrole monomer to MnO2 was 3:2. The resulting precipitate wasltered and washed with de-ionized water and anhydrous alcoholespectively. The products were dried under vacuum at 50 ◦C for4 h. The weight of the final product is about 0.31 g, indicating thatart of MnO2 (about 0.067 g) had participated in the oxidation ofyrrole. The percentage of MnO2 in the composite is about 35.5%.his was demonstrated by the TGA analysis of MnO2/PPy composites displayed in Fig. 1.

The PPy was synthesized by following the similar procedurehat adopted in the above composite synthesis except using MnO2nd performing stirring and sonicating step for better dispersion ofnO2.

.3. Instruments and characterization

The grain size and morphology of the composite were observedy transmission electron microscopy (TEM, Model Hitachi H-600,00 kV) and scan electron microscopy (SEM, Germany, Leo1430VP).

nfrared spectra were recorded using FT-IR spectrophotometerFT-IR, Germany, BRUKER-EQUINOX-55) to identify the chemicaltructure of composite. The crystalline structure of the productsas identified by X-ray diffraction analysis (XRD, Japan Rigaku

/Max 2400) using Cu K� radiation (� = 1.5405 Å) in the 2� rangef 10◦ to 60◦.

Electrodes for supercapacitors were prepared by mixing activeaterials (3 mg) with 15% acetylene black and 5 wt.% polytetraflu-

roethylene (PTFE) to make more homogeneous slurry. The slurry

ce 256 (2010) 4339–4343

was pressed on graphite current collector. Electrochemical studieswere carried out in a three-electrode system, the freshly preparedMnO2/PPy composite on graphite, a platinum electrode, and a sat-urated calomel electrode were used as working electrode, counterelectrode and reference electrode, respectively. The electrolytewas 1 M KCl solution. Cyclic voltammograms and galvanostaticcharge–discharge in different potential ranges were performedusing CHI660 electrochemical working station system (Shanghai,China) at the room temperature. The electrochemical impedancespectroscopy (EIS) measurements were performed at open-circuitpotential by using an AUTOLAB PGSTAT30. Data were collected inthe frequency range of 105 to 10−2 Hz taking 10 points per decade.

3. Results and discussion

3.1. XRD analysis

X-ray powder diffraction measurements were used to iden-tify the crystalline phase of the as-synthesized MnO2, PPy andMnO2/PPy composite, as displayed in Fig. 1. The XRD pattern ofpure PPy shows a broad peak located in the range of 15–30◦ andcentered at 2� = 21◦, which was the characteristic peak of amor-phous PPy with somewhat of a change in intensity and positioncompared to the XRD pattern in the literature [24]. The XRD pat-tern exhibited that the MnO2 hydrothermally prepared at 140 ◦Cis typical for �-MnO2. Whereas it can be noted that XRD peaksfor MnO2/PPy composite combined the diffraction peaks from �-MnO2 at 2� = 12.9◦, 23.5◦, 32.5◦, 36.9◦, 55.2◦, etc., and the broad peakfrom the PPy located in the range of 15–30◦ despite some differ-ences in intensity. The attenuation of the diffraction peaks maybeindicated that a part distortion in crystal structure of �-MnO2 andtransformation into amorphous phase has been occurred duringthe polymerization reaction.

3.2. FT-IR spectra analysis

FT-IR spectra of the prepared PPy and MnO2/PPy composite areshown in Fig. 2. The main characteristic peaks of PPy are assigned asfollows: the bands at 1537 and 1448 cm−1 are attributed to the anti-symmetric and symmetric pyrrole ring vibration. The bands located

−1

Fig. 2. FT-IR spectra of the PPy and MnO2/PPy composite.

J. Li et al. / Applied Surface Science 256 (2010) 4339–4343 4341

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Fig. 3. SEM images of as p

ssigned to the hydroxyl group peaks. The results are in agreementith the values in the literature [26]. The as-prepared MnO2/PPy

omposite has identical characteristic vibrations for PPy but incor-oration of MnO2 led to some peaks of PPy slightly shifted to highavenumber. It also could be seen from the Figs. 1 and 2 that thenO2/PPy composite reflected the mutual influence of PPy andnO2 from the differences in peak intensity and position.

.3. Morphology of the composite

To investigate the morphology of the as-prepared sample onlarge scale, the obtained sample was characterized by scanning

lectron microscopy (SEM). Fig. 3 displayed the typical low- andigh-magnification SEM images of the sample. The images indicatehat there seemed to exist a great deal of uniform one-dimensionalanostructured PPy covered with a layer of particles and no visiblegglomeration, the diameter of the particles is very small.

Transmission electron microscopy (TEM) images were used toirectly observe the morphology of the MnO2 existing in the com-osite materials (shown in Fig. 4). It can be observed that the sample

s the nanotubes with the outer diameter of 70–150 nm, while interiameter is between 30 and 60 nm. TEM images also prove that theanotubes were embedded by some small black particles (Fig. 4a).e compose that these particles are MnO2 nanoparticles. In addi-

ion, we think that the form of the MnO2 existing in the compositeaterials possesses two possibilities. One is that MnO2 nanoparti-

les are embedded in the wall of PPy nanotubes. From the Fig. 4b, it

as observed that the composite was core-shell structure and PPy

cted as the core. The other possibility is that MnO2 particles exist inhe pore channels during the polymerization process seen from theark area in the Fig. 4c. Since the hollow nature of PPy, the mate-ial may have higher capacitance. Furthermore, MnO2 nanosized

Fig. 4. TEM images of Mn

ed MnO2/PPy composite.

particles on polymer may have also lead to more sites that are sus-ceptible to redox reaction, a greater active area, and consequentlyhigher capacitance.

3.4. Electrochemical performance analysis

Fig. 5 give the cyclic voltammograms (CV) behaviors ofMnO2/PPy composite electrode in three-electrode system and thepotential ranges were from −0.8 to 0.5 V (a), from −0.4 to 0.6 V(b) and from −0.2 to 0.8 V (c) versus saturated calomel electrode(SCE) with scanning rates of 5, 10 and 20 mV/s. The rectangular andmirror images were observed from −0.4 to 0.6 V in which the CVtest was conducted, indicating high electrochemical reversibilityat the potential range and sweep rate. As the potential range wasbelow to −0.8 V (shown in Fig. 5a), a pair of broad oxidation andreduction current peaks at −0.5 V is clearly found and the behav-ior is more marked with increasing sweep rate, which is the redoxprocess of PPy in the composite [7]. In addition, as the potential isfurther expanded to +0.8 V (shown in Fig. 5c), the polarization ofelectrode/electrolyte is occurred. As we know, the maximum andminimum values of polarization potential of MnO2 electrode arecontrolled by the reactions Mn(IV) to Mn(II) and Mn(IV) to Mn(VII),respectively, which are irreversible because of the solubility of bothMn(II) and Mn(IV) in water [27]. Therefore the potential must becontrolled in the certain region, in which the reversible pseudo-capacitive behavior occurs.

The mirror-like charge–discharge curves of MnO2, PPy and

MnO2/PPy composite electrodes presented in Fig. 6. The spe-cific capacitance values can be calculated by I�t/m�V from thecharge/discharge curves. Where I is the constant discharging cur-rent, �t is the discharging time, �V is the potential drop, and m isthe mass of the active materials within the electrodes. The specific

O2/PPy composite.

4342 J. Li et al. / Applied Surface Science 256 (2010) 4339–4343

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The electrochemical galvanostatic charge–discharge stability ofMnO2/PPy composite electrode in 1 M KCl electrolyte was exam-ined by chronopotentiometry at 3 mA cm−2 in the potential rangefrom −0.4 to 0.6 V. Fig. 8 shows the specific capacitance of the

Fig. 7. (a) Galvanostatic charge–discharge curves of MnO2/PPy nanocomposite elec-trode at different current densities. (b) Specific discharge capacitance as function ofdischarge current density.

ig. 5. Cyclic voltammograms (CV) behaviors of MnO2/PPy composite with the dif-erent cut-off potentials: −0.8 to 0.5 V (a); −0.4 to 0.6 V (b); and −0.2 to 0.8 V (c) atarious scan rates.

apacitances of the above three electrodes are around 90, 142 and28 F g−1, respectively. It gives a clear proof that a synergic effect ofPy and MnO2 allows the capacitance of the composite electrode

igher than that of pure PPy or pure MnO2.

Fig. 7a gives the galvanostatic charge–discharge curves ofnO2/PPy composite electrode examined in 1 M KCl electrolyte at

arious current densities of 3, 6, 9 and 15 mA cm−2. The resulted

Fig. 6. Galvanostatic charge–discharge curves of MnO2, PPy and MnO2/PPy com-posite electrodes in the potential range from −0.4 to 0.6 V.

discharge capacitance per unit mass of the MnO2/PPy compositeelectrode is shown in Fig. 7b as a function of current density. Thecapacitance decreases slightly with increasing current density from3 to 15 mA cm−2. It lost about 11% of the initial specific capacitanceeven in the discharge current density up to 15 mA cm−2.

Fig. 8. Cyclic life of MnO2/PPy composite electrode in 1 M KCl at a current densityof 3 mA cm−2.

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[[30] Y.L. Xu, J. Wang, W. Sun, S.H. Wang, J. Power Sources 159 (2006) 370.[31] W.B. Zhong, J.Y. Deng, Y.S. Yang, W.T. Yang, Macromol. Rapid Commun. 26

(2005) 395.[32] K.K. Liu, Z.L. Hu, R. Xue, J.R. Zhang, Z.J. Zhu, J. Power Sources 179 (2008)

858.

ig. 9. AC impedance spectra of the MnO2/PPy composite electrode (insert is theIS of high-frequency region).

nO2/PPy composite electrode based supercapacitor with respecto charge–discharge cycle number. The specific capacitance cantill be stable after 500 charge–discharge cycles. These studiesropose that the composite electrode has a fast redox process,

eading to good cyclic power storage ability. Although the spe-ific capacitance is not obviously higher than other PPy basednorganic–organic composite thinner films [28,29] prepared bylectrochemical method, the facileness, bulk synthesis combinedith the morphology make this composite as a potential applica-

ion in electrochemical capacitor.EIS has been used to study the redox processes of the MnO2/PPy

omposite and to evaluate its ionic and electronic conductivity asell as specific capacitance [30]. The electrochemical impedance

pectroscopy was carried out at the open-circuit potential in therequency range of 0.01–105 Hz with ac-voltage amplitude of 5 mVnd the typical Nyquist plot of MnO2/PPy composite electrode ishown in Fig. 9. Firstly, the plot contains a distorted semicircle in theigh-frequency region which is attributed to the charge-transferesistance from the interface structure between the electrode sur-ace and electrolyte [31,32]. In addition, from the high-frequencyntercept of the real axis, the internal resistance (Rs ≈ 1.8 �) cane observed, which includes the resistance of the electrolyte, the

ntrinsic resistance of the active material, and the contact resistancet the interface active material/current collector. In the interme-iate frequency region, the 45◦ line is the characteristic of thearburg diffusion region as shown in the up-right corner of Fig. 9,

ttributable to the semi-infinite diffusion of ions into the poroustructure of the composite–electrolyte interface. On the other hand,line almost vertical to the real axis in the imaginary part of the

mpedance at the low-frequency region, i.e., an ideal capacitiveehavior, is due to the Faradaic pseudo-capacitance of the com-osite electrode [33].

. Conclusions

One-dimensional nanostructured MnO2/PPy composite hadeen successfully synthesized by in situ polymerization by using

[

ce 256 (2010) 4339–4343 4343

complex of methyl orange (MO)/FeCl3 as a reactive self-degradedsoft-template and oxidant. �-MnO2 nanoparticles attached in orout of the wall of uniform PPy nanotubes. A synergic effect ofPPy and MnO2 allowed the capacitance of the composite electrodehigher than that of pure PPy or pure MnO2. CV behaviors andgalvanostatic charge–discharge results showed that the compos-ite had good electrochemical reversibility in the potential range of−0.4 to 0.6 V in 1 M KCl mild solution. Furthermore, it is also con-vinced that this simple and effective synthetic strategy reportedabove could be viable to extend to other PPy nanotubes containingcontent of inorganic nanoparticles systems to explore other novel1D nanocomposites.

Acknowledgements

We thank the National Natural Science Foundation of China (No.20663006) for support of this work.

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