MnWO4 nanocomposites as electrodes for pseudocapacitors

Applied Surface Science 258 (2012) 4881–4887

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Synthesis and characterization of polyaniline/MnWO4 nanocomposites as electrodes for pseudocapacitors S. Saranya a , R. Kalai Selvan a,∗ , N. Priyadharsini a,b a b

Solid State Ionics and Energy Devices Laboratory, Department of Physics, Bharathiar University, Coimbatore – 641 046, Tamil Nadu, India Department of Physics, PSGR Krishnammal College for Women, Coimbatore – 641 004, Tamil Nadu, India

a r t i c l e

i n f o

Article history: Received 22 September 2011 Received in revised form 16 January 2012 Accepted 17 January 2012 Available online 24 January 2012 Keywords: Polyaniline MnWO4 Composites X-ray diffraction Electrochemical properties

a b s t r a c t Polyaniline (PAni)/MnWO4 nanocomposite was successfully synthesized by in situ polymerization method under ultrasonication and the MnWO4 was prepared by surfactant assisted ultrasonication method. The thermal stability of PAni was determined by TG/DTA (Thermo Gravimetric/ Differential thermal analysis). The structural and morphological features of PAni, MnWO4 and PAni/MnWO4 composite was analyzed using Fourier transform infrared spectrometry, X-ray diffraction (XRD), scanning electron microscope (SEM) and Transmission electron microscope (TEM) images. The electro-chemical properties of PAni, MnWO4 and its composites with different weight percentage of MnWO4 loading were studied through cyclic voltammetry (CV) for the application of supercapacitors as active electrode materials. From the cyclic voltammogram, 50% of MnWO4 impregnated PAni showed a high specific capacitance (SC) of 481 F/g than their individual counterparts of PAni (396 F/g) and MnWO4 (18 F/g). The galvanostatic charge–discharge studies indicate the in situ polymerized composite shows greater specific capacitance (475 F/g) than the physical mixture (346 F/g) at a constant discharge current of 1 mA/cm2 with reasonable cycling stability. The charge transfer resistance (Rct ) of PAni/MnWO4 composite (22 ohm) was calculated using electrochemical impedance spectroscopy (EIS) and compared with its physical mixture (58 ohm). © 2012 Elsevier B.V. All rights reserved.

1. Introduction In the past few decades conducting polymers have been extensively studied because of their wide potential applications in many fields like electro-chromic devices, optical storage devices and energy storage devices and electrochemical sensors [1–2]. In particular, polymer nanocomposites consisting of inorganic materials have gained special attention as these have the advantages of organic polymers such as flexibility, toughness and coatability and ceramics or glasses such as hardness, durability and high refractive index. They also possess some synergetic properties which are different from that of parent materials. Polyaniline (PAni) is one of the most important conducting polymers because of its ease of synthesis at low cost, good processability, environmental stability and easily tunable conducting properties. It has a wide range of applications including electrodes for supercapacitors [3], corrosion inhibitors [4], NH3 sensors [5], etc. Different morphological products of PAni with various characteristics were synthesized by a number of polymerization techniques [6]. PAni nanofibers with interconnected network-like

∗ Corresponding author. E-mail address: [email protected] (R.K. Selvan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.104

structures have been fabricated on stainless steel substrates under galvanostatic method [7]. Wang et al. [8] have synthesized PAni nanosheets, nanofibers and nanoparticles through chemical oxidation method. PAni nanofibers with diameter of 30–50 nm and length about 300–1000 nm [9] and with diameters of 50 nm and lengths of 200 nm to several micrometers have been obtained using ultrasonication [10]. Variety of PAni/metal oxide composites have been prepared and used for various applications. Especially, PAni/BaTiO3 has attracted due to its magnetic and conductive properties [11], PAni/SnO2 for conducting as well as dielectric properties [12], PAni/Co3 O4 has shown good humidity sensing properties [13], PAni/TiO2 was used in photovoltaic cells [14] and as electrodes in fuel cells [15], PAni/V2 O5 in electrochromic and photoelectrochemical devices [16], PAni/Al2 O3 showed high thermal stability [17], PAni/MnO2 has been studied as electrodes in supercapacitors [18–19], PAni/WO3 in humidity sensors [20] and catalysts [21] and PAni/PbTe for thermoelectric materials [22]. Considering the importance of PAni based composites, we intended to prepare a new type of hybrids like MnWO4 impregnated PAni for supercapacitor applications. Among the metal tungstates, MnWO4 described as wolframite structure has interesting electrical properties, attractive antiferromagnetic [23], excellent photoluminescent [24] and sensing properties [25]. Also

S. Saranya et al. / Applied Surface Science 258 (2012) 4881–4887

2. Experimental methods PAni can be easily synthesized from the aniline monomer using an oxidant in the presence of acidic medium. Acidic medium is preferred for the polymerization of aniline because the monomer will dissolve easily. Besides it also induces the formation of emaraldine state of PAni and it improves the electronic conductivity of the polymer. In a typical synthesis, 0.125 mol of K2 Cr2 O7 dissolved in 50 ml of distilled water was added drop by drop to 0.1 mol of aniline dissolved in 40 ml of 2 M H2 SO4 and stirred for 5 min. The mixture turned dark green and polymerization was obtained within 2 h. The dark green solution was then centrifuged and washed with water and ethanol to remove the residual monomer. Finally, PAni was collected and dried in air at room temperature. The effect of ultrasonication on the formation of PAni was prepared using the same procedure under sonication bath. MnWO4 impregnated PAni were prepared via in situ polymerization method. In this process, the same oxidative polymerization method was adopted with the addition of different weight percentages (25%, 50% and 75%) of MnWO4 under ultrasonication. For the synthesis of MnWO4 , the stoichiometric quantities of MnCl2 ·4H2 O and Na2 WO4 ·2H2 O were dissolved in desired amount of distilled water individually. Subsequently, MnCl2 ·4H2 O was added to CTAB solution (0.001 mol) and then Na2 WO4 ·2H2 O was added drop by drop with constant stirring under ultrasonication. The resulting mixture was centrifuged and washed several times with distilled water to remove the impurities. Further the precipitate was collected and dried at 800 ◦ C for 2 h for the better crystallinity and compound formation of MnWO4 . Thermal gravimetric analysis (TG/DTA) was carried out through SDT Q600 V8.3 Build 101 at a heating rate of 20 ◦ C/min in inert atmosphere. X-ray diffraction (XRD) data were studied using BRUKER diffractor with Cu K␣ radiation. Fourier transform infrared spectra were recorded using PerkinElmer (−35) FT-IR instrument in the spectral region of 400–2000 cm−1 . The structural morphology was examined in a JEOL JSM 840A scanning electron microscope (SEM) and JEOL JEM 2100 High Resolution Transmission Electron Microscope (HRTEM) with 200 kV acceleration voltage. Electrochemical properties of MnWO4 , PAni and MnWO4 impregnated PAni were studied using CH instrument model CHI1102A. For the electrode preparation, 90% of active material and 10% of PVDF (polyvinylidene fluoride) was blended well in the solvent Nmethyl-2-pyrrolidone (NMP) to obtain a homogeneous mixture. The slurry was coated on the carbon electrode for the electrochemical studies. 3. Results and discussion 3.1. Thermal analysis Thermal stability of PAni and PAni/MnWO4 (50 wt%) composite are analyzed using TG/DTA measurements. Fig. 1(a) and (b), shows the TG and DTA curves of PAni and PAni/MnWO4 (50 wt%) composite prepared under ultrasonication. The TG curve of PAni shows three step thermal degradation process. The initial step weight loss up to 150 ◦ C is due to the removal of water molecules. The second step weight loss between 150 to 370 ◦ C is attributed to loss

a

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Weight loss (%)

the individual constituent of MnWO4 is MnO and WO3 which plays a major role in variety of electrochemical applications especially for electrodes in supercapacitor applications. So we have chosen MnWO4 as the filler for making the composite with PAni. In addition, to the best of our knowledge, there was no literature available for the synthesis of MnWO4 impregnated PAni and its usage in supercapacitor applications.

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Temperature, C Fig. 1. (a) TGA and (b) DTA curves of PAni and PAni/MnWO4 composites.

of acid dopant. The third step weight loss between 360 and 430 ◦ C is mainly due to degradation of polymer chain. The TG curve of PAni/MnWO4 (50 wt%) composite almost follows the similar steps of weight loss as pure PAni. However it is observed that the initial degradation temperature (Ti ) of the composite (373 ◦ C) is higher than that of pure PAni (360 ◦ C). This may be due to the interaction of MnWO4 with PAni chain, which increases the thermal stability of PAni [26–27]. The total weight loss up to 800 ◦ C of PAni and PAni/MnWO4 (50 wt%) composite are observed to be 95.8% and 73.5% respectively. The thermal degradation temperature of PAni is observed from the exothermic peaks of DTA curves and is found to be at 420 ◦ C. However for the prepared PAni/MnWO4 (50 wt%) composite, the exothermic peaks are observed at the higher temperature of 448 ◦ C. This clearly shows that the thermal degradation temperature of the composite increased about 24 ◦ C than PAni and hence it can be a better alternate for the applications which demands high thermal stability over PAni. 3.2. FT-IR analysis FT-IR spectrum is used to analyze the functional groups, nature of bonding and the chemical structure of compounds. It is also

S. Saranya et al. / Applied Surface Science 258 (2012) 4881–4887

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intensity of quinoid ring is higher than benzoid ring which shows that PAni exists more in the reduced form than in the oxidized form. This enumerates that PAni obtained via ultrasonication method is in highly doped emaraldine state. Fig. 2(c) shows the FT-IR spectrum of MnWO4 . It can be seen that the characteristic bands of tungstate groups were observed at 873, 805, 719, 598 and 445 cm−1 . The bands at 873 and 805 cm−1 corresponds to symmetric and asymmetric stretching vibration of short (in terminal WO2 group) W O bond. Similarly, the bands observed at 719 and 598 cm−1 indicates the asymmetrical stretching vibrations (Bu and Au ) of the longer W O bond in the (W2 O4 )n chain. The in-plane deformation modes of Au and Bu of the longer W O bond vibrates at 445 and 512 cm−1 respectively, which further confirms the formation of MnWO4 structures [32]. The representative FT-IR spectrum of MnWO4 impregnated PAni are shown in Fig. 2(d). The observed bands at 1560 and 1480 cm−1 belongs to the C C stretching vibration of quinoid and benzenoid ring respectively. C N stretching mode of benzenoid unit and C H in-plane bending vibration of quinoid unit vibrates at 1303 and 1113 cm−1 respectively. The characteristic bands of MnWO4 observed at 801, 506 and 433 cm−1 . The bands were shifted compared to the bands of pure MnWO4 due to the interaction of PAni with MnWO4 .

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3.3. XRD analysis

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Wavenumber (cm ) Fig. 2. FT-IR spectra of (a) PAni – A, (b) PAni – B, (c) MnWO4 and (d) PAni/MnWO4 composite.(For interpretation of the references to color in the text, the reader is referred to the web version of the article.)

used as the probe technique to identify the oxidation state of PAni. Fig. 2(a, b) shows the FT-IR spectra of PAni prepared by conventional stirring and ultrasonication methods and the corresponding characteristic bands are given in Table 1. For the PAni obtained by mechanical stirring, the bands observed at 507 cm−1 and 800 cm−1 explains the presence of C H out-of-plane bending vibrations and the C H para – substituted aromatic rings respectively. The peak at 614 cm−1 is caused by the deformation of benzene ring [28]. The appearance of the band at 1128 cm−1 is due to the in-plane bending vibration mode of C H, which is associated with high electrical conductivity and high degree of electron delocalization of PAni. The C N stretching vibration of secondary amine of PAni backbone can be explained by the band at 1291 cm−1 . The characteristic peaks at 1485 and 1642 cm−1 are associated with C C and C N stretching mode for the benzenoid and quinonoid rings [29]. These peaks confirmed that PAni exists in emaraldine state rather than in solely oxidized or reduced form. PAni synthesized by ultrasonication method also exhibited the characteristic peaks like in conventional stirring method. C C and C N stretching mode for the benzenoid and quinonoid rings are revealed by the bands at 1492 and 1572 cm−1 . These observed values are well matched with the reported values [30,31]. The former peak is blue shifted and the latter one is red shifted. The relative

Fig. 3(a, b) shows the X-ray diffraction patterns of PAni prepared by conventional stirring and ultrasonication method respectively. The diffraction peaks are observed at 2 = 18.4◦ , 21.2◦ , 24.1◦ , 25.4◦ for stirring method and 2 = 14.9◦ , 21.7◦ , 25.6◦ for ultrasonication method. The peaks at 21◦ and 25◦ may be ascribed to the periodicity, parallel and perpendicular to the polymer chains of polyaniline. Both the XRD pattern enumerates that PAni has some degree of crystalline nature [33]. In ultrasonication, the peak at

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Table 1 FTIR bands of PAni.

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PAni, (stirring) (cm−1 )

PAni, (ultrasonication) (cm−1 )

C N quinoid stretching C C benzenoid stretching C N stretching C H in plane bending C H para substitution Deformation of benzene ring C H out-of-plane bending

1642 1485 1291 1128 – 614 –

1572 1492 1298 1127 800 616 507

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2θ (degree) Fig. 3. XRD patterns of (a) PAni – A, (b) PAni – B, (c) MnWO4 , (d) PAni/MnWO4 (1:0.25), (e) PAni/MnWO4 (1:0.5) and (f) PAni/MnWO4 (1:0.75) composite.

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S. Saranya et al. / Applied Surface Science 258 (2012) 4881–4887

Table 2 Structural parameters of MnWO4 and Pani/MnWO4 composites. Sample

MnWO4 PAni/MnWO4 (1:0.25) PAni/MnWO4 (1:0.50) PAni/MnWO4 (1:0.75)

Lattice parameters ˚ a (A)

˚ b (A)

˚ c (A)

ˇ (◦ )

4.7960 4.8169 4.8092 4.8159

5.7457 5.7537 5.7270 5.7502

4.9802 5.0008 4.9793 4.9901

91.14 91.42 91.10 91.24

25.6◦ is stronger than 21.7◦ which is similar to that of highly doped emaraldine state. However, for stirring, the peak at 25.4◦ is weaker than that of 21.2◦ which is similar to that of less doped emaraldine state. So, this reveals that sonication improves the doping degree of PAni which also substantiates the FTIR results. The XRD pattern of MnWO4 is shown in Fig. 3(c). It shows the sharp and well defined peaks that indicates the single phase formation of MnWO4 without any impure phase. The observed diffraction peaks at 2 = 15.70◦ , 18.66◦ , 23.88◦ , 24.34◦ , 30.10◦ , 30.57◦ , 31.35◦ , 36.25◦ , 37.50◦ , 41.15◦ , 48.46◦ , 49.50◦ , 51.47◦ , 52.70◦ , 60.85◦ , 64.65◦ and 67.97◦ corresponding to the planes of (0 1 0), (1 0 0), (0 1 1), (1 1 0), (−1 1 1), (1 1 1), (0 2 0), (1 2 0), (2 0 0), (1 0 2), (0 2 2), (2 2 0), (1 3 0), (0 3 2), (2 0 2) and (1 4 0) respectively. ˚ b = 5.7457 A, ˚ The calculated structural parameters, a = 4.796 A, c = 4.9802 A˚ and ˇ = 91.34◦ reveals the monoclinic structure of MnWO4 . Fig. 3(d) shows the XRD pattern of MnWO4 impregnated PAni and the calculated structural parameters are given in Table 2. It shows that the XRD patterns of MnWO4 impregnated PAni, well resembles that of pure MnWO4 . The calculated lattice parameters infer the composite also having monoclinic structure. This attributes that the crystalline structure of MnWO4 is well maintained even after the impregnation with PAni [13]. The broad peaks of PAni disappeared after the incorporation of MnWO4 , because of the semi-crystalline nature of PAni and also the MnWO4 behave like impurities to hamper the growth of PAni crystallites. This indicates that PAni exists as amorphous in the composites. No

Cell volume (A◦ )3

Lattice density (g/cm3 )

Grain size (nm)

137.20 138.55 137.11 138.15

389.22 378.11 390.11 381.26

75 66 77 70

peak shift is found in the composite, which implies the composite behavior of the materials. Thus we inferred that MnWO4 in PAni existed in monoclinic structure as pure crystalline MnWO4 . 3.4. SEM and TEM analysis The morphological features of PAni, MnWO4 and PAni/MnWO4 (50%) composite were analyzed through SEM and TEM techniques which are shown in Fig. 4. The SEM and TEM images of PAni (Fig. 4(a) and (b)) indicates that the particles are in irregular shape which mainly looks like a cluster of flakes. No crystalline phase was observed for PAni in the TEM images. The formation of MnWO4 nanocrystals was confirmed through both the images (Fig. 4(c) and (d)). The average length of the crystals is 200–300 nm and the width is ∼50 nm. The SAED (Selected Area Electron Diffraction) pattern infers the polycrystalline nature of MnWO4 due to the formation of both spot and ring patterns (Fig. 4d (inset)). The SEM image of PAni/MnWO4 composite (Fig. 4(e)) looks like a similar morphology of PAni. It indicates that the MnWO4 nanocrystals are impregnated within the PAni which means that the PAni is coated on the crystals during in situ polymerization. The similar type of SEM observations has been reported by Xu et al. for the BaFe12 O19 impregnated polypyrrole [34]. In order to substantiate the coating behavior, the TEM images were carried out and are given in Fig. 4(f). However in some places we could see the coating behavior of PAni on MnWO4 crystals, unfortunately no uniform coating was observed. But it

Fig. 4. SEM and TEM images of (a) and (b) PAni – B, (c) and (d) MnWO4 and (e) and (f) PAni/MnWO4 composite [1:0.5].

S. Saranya et al. / Applied Surface Science 258 (2012) 4881–4887

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Fig. 5. CV curves of (a) PAni, (b) PAni/MnWO4 (1:0.25), (c) PAni/MnWO4 (1:0.50) and (d) PAni/MnWO4 (1:0.75) composites.

indicates that the presence of both MnWO4 and amorphous PAni as composites, which are segregated as separate phases due to the sonochemical treatment for the TEM procedure.

3.5. CV analysis Cyclic voltammetry (CV) is a prominent tool to study the electrochemical properties of the material. The influence of MnWO4 with different weight percentage in the capacitance behavior of polyaniline is investigated with the help of CV curves in the potential window of −0.2 to 1.0 V using 1.0 M of H2 SO4 as electrolyte. The CV curves explain the electrochemical activity characterized by the reduction and oxidation responses. PAni shows two redox peaks (Fig. 5), the peak at 0.31 V corresponds to the leucoemeraldine to emeraldine transition and the peak at 0.93 V corresponds to the emeraldine to pernigraniline transition [35]. The electrode will show an ideal reversibility for ioxidation /ireduction = 1 [36]. Here it is in the range of 1.1709–1.2853 for PAni and 1.196–1.681 for PAni/MnWO4 composite with 50% MnWO4 loading. The shape of the curve which gets deformed at high scan rate may be due to the slow diffusion of electrolyte ions into the active material of the electrode. The current rate increases with the increase in scan rate, which indicates a good rate ability and electrical activity of PAni. (SC) of a material can be calculated from the Specific capacitance  relation, Cs = I dv/(2ESW), where ‘I’ is the voltametric current, ‘E’ is the potential window, ‘S’ is the scan rate, and ‘W’ is the weight percentage of the active material in the electrode. Depending on the area of the CV curve, Cs value is found at different scan rates. The specific capacitance of PAni is determined to be 396 F/g at 5 mV/s and 323 F/g at 10 mV/s. This observed specific capacitance is much higher than the reported value of 258 F/g for the emeraldine form of polyaniline [30].

Similarly, the cyclic voltammogram of MnWO4 are also studied in the same experimental conditions (figure not given). Oxidation/ reduction peaks were not observed and quasi-rectangular shape of the curve attributes to the non-faradic/electrical double layer charge storage process of MnWO4 . The calculated specific capacitance values of MnWO4 are 18, 18, 17, 15, 15, 14, 11 and 10 F/g corresponding to the various scan rates of 5, 10, 20, 30, 40, 50, 100 and 200 mV/s respectively. It can be seen that the SC values were decreased with increase in scan rate reveals that the minimum utilization of bulk of the active material. The compounds are more stable in acidic conditions. The electrochemical behavior of PAni/MnWO4 composite electrode materials with different loading of MnWO4 (25 wt%, 50 wt% and 75 wt%) are studied (Fig. 5(b) and (c)). The specific capacitance of the composite with 25 wt% of MnWO4 is 418 F/g and that with 50 wt% of MnWO4 is 481 F/g at 5 mV. The hybrids show an enhanced specific capacitance than pristine PAni. This may be due to the synergetic effects between PAni and MnWO4 which induce the electrochemical utilization of PAni [37]. In acidic medium, the surface of tungstate is positively charged and so it adsorbed SO4 − ions from the electrolyte of H2 SO4 for charge compensation. It is well known that the aniline monomers are transformed in to cationic anilinium ions in acidic medium [38]. Hence, the electrostatic interactions take place between anions (SO4 − ) adsorbed on the tungstate surface and anilinium cations. Then, the in situ polymerization results in the formation of PAni/MnWO4 nanocomposites. After polymerization, the MnWO4 nanoparticles enhanced the stability of the composite by restraining the gradual structural deterioration of PAni chains [39] as the interaction between MnWO4 and PAni is greatly increased in the composite [40]. On the other hand, when the concentration of MnWO4 increases further (75 wt%), ‘Cs ’ value reduces to 335 F/g. The conducting particles influence the electrochemical performance of the electrode material [41]. This

S. Saranya et al. / Applied Surface Science 258 (2012) 4881–4887

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PAni- B MnWO4 PAni/MnWO4 (1:0.25) PAni/MnWO4 (1:0.50) PAni/MnWO4 (1:0.75)

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Specific capacitance (F/g)

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Scan rate (mV/sec) Fig. 6. Specific capacitance of PAni, MnWO4 and its composites at various scan rates.

may be due to the increased number of MnWO4 nanoparticles in the composite, reduces the available active sites of PAni. So, an appropriate quantity of MnWO4 is required to achieve high capacitance [42]. The specific capacitance of PAni, MnWO4 and it composites at various scan rate in the range of 5 mV/s to 200 mV/s are shown in Fig. 6. The specific capacitance is decreased with increase in the scan rate. The scan rate has a direct impact on the diffusion of H2+ ions into the electrodes, i.e., the H2+ ions reach only the outer surface of the electrode at higher scan rate. So, the active material at the inner surface does not get fully involved in the electrochemical process. Due to the limited ion diffusion, the capacitance value was also reduced on high scan rate [43]. From the figure, it is easily seen that PAni/MnWO4 nanocomposites with 50 wt% of MnWO4 loading, has high specific capacitance compared to PAni, MnWO4 and other composites. 3.6. Charge–discharge and EIS analysis The stability of the electrode which is an important factor for the practical applications of supercapacitors was analyzed through charge–discharge analysis. Based on the CV analysis, 50 wt% of MnWO4 loaded PAni showed the higher specific capacitance than the other samples. Hence, the charge–discharge and electrochemical impedance spectroscopy (EIS) studies were carried out with this composite and its physical mixture only. The galvanostatic charge–discharge curve at different current densities (1, 3 and 5 mA/cm2 ) over a potential window of −0.2 to 0.8 V for in situ polymerized PAni/MnWO4 nanocomposites and its physical mixture (1 mA/cm2 ) are shown in Fig. 7. It can be seen that the small deviation in symmetric charge–discharge profile was observed which indicates both the behavior of electric double layer and pseudocapacitance of the electrodes. Also in situ polymerized composite exhibits larger discharge time when compared with its physical mixture. From the charge–discharge curve, the specific capacitance can be evaluated using the formula, C = it/Vm, where i is charge–discharge current, V is the potential range, and m is the mass of active material in the electrode. The in situ polymerized composite shows greater specific capacitance of 475 F/g than the physical mixture (50 wt% of MnWO4 loaded PAni) of 346 F/g at a constant discharge current of 1 mA/cm2 . This observed higher capacitance for the in situ polymerized composite is may be due to the homogenous distribution and the interaction between the

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Time (s) Fig. 7. Galvanostatic charge–discharge curves of in situ polymerized PAni/MnWO4 composite at different current densities of (a) 1 mA/cm2 , (b) 3 mA/cm2 , (c) 5 mA/cm2 and (d) Physical mixture of PAni/MnWO4 composite at 1 mA/cm2 .

PAni and MnWO4 , which improves the conductivity in turn high capacitance. In order to investigate the cycling performance, the charge–discharge tests were carried out up to 100 cycles at the constant current density of 5 mA/cm2 for both the samples and the calculated specific capacitance is given in Fig. 8. The first and last three charge–discharge cycles of composite are given in Fig. 8 (inset). At the first cycle, the specific capacitance of in situ polymerized composite is 340 F/g whereas for physical mixture is 252 F/g. After 100 cycles it decays to 278 and 195 F/g respectively. The in situ polymerized PAni/MnWO4 composite provides the 82% capacity retention than the physical mixture (78%). In order to identify the origin of the difference in specific capacitance of in situ and physical mixture composites, the electrochemical impedance spectra were recorded and are given in Fig. 9. The Nyquist plot displays the distorted semicircle at high frequency region and spike in the low frequency region. It is well known that the diameter of the semicircle is equals to

Fig. 8. Cycle life curve of (a) in situ polymerized and (b) physical mixture of PAni/MnWO4 composites. (inset) Charge–discharge curves for first and last three cycles of in situ polymerized PAni/MnWO4 composites at 5 mA/cm2 .

S. Saranya et al. / Applied Surface Science 258 (2012) 4881–4887

References

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50

Z' (Ohm) Fig. 9. EIS spectra of PAni/MnWO4 composites prepared by (a) in situ polymerization and (b) physical mixture.

the charge transfer resistance (Rct ) of the system. The calculated Rct of the in situ polymerized PAni/MnWO4 composite (22 ohm) is very less when compared with physical mixture (58 ohm). This may be due to that at in situ polymerization, the MnWO4 nanoparticles are well dispersed/interacted with polyaniline matrix, which enhances the specific capacitance than the physical mixture. 4. Conclusion PAni was prepared by simple oxidation method via mechanical stirring and ultrasonication method. PAni/MnWO4 (organic/inorganic) nanocomposites were successfully synthesized via in situ polymerization method through ultrasonication. The functional groups of PAni and its oxidation state (emaraldine base) were analyzed by FT-IR specrum. The thermal stability of PAni was revealed through TG/DTA. The morphology of PAni, MnWO4 and PAni/MnWO4 nanocomposites were revealed by SEM images. PAni/MnWO4 nanocomposites have shown good electrochemical properties. PAni/MnWO4 nanocomposites with 50% of MnWO4 loading provides a high specific capacitance of 481 F/g when compared with the pristine polyanilne and PAni/MnWO4 nanocomposites with 25% and 75% of MnWO4 . Similarly, the galvanostatic charge–discharge studies also confirmed that the in situ polymerized PAni/50% MnWO4 composite shows higher specific capacitance (475 F/g) than the physical mixture (346 F/g) of PAni and MnWO4 at a constant discharge current of 1 mA/cm2 . The EIS spectra further confirmed the interaction of PAni and MnWO4 via the low resistance than the physical mixture. Overall, PAni/MnWO4 nanocomposites are found to be suitable electrode material for supercapacitor applications.

[1] D. Kumar, R.C. Sharma, European Polymer Journal 34 (1998) 1053–1060. [2] J. Janata, M. Josowicz, Nature Materials 2 (2003) 19–24. [3] T.C. Girija, M.V. Sangaranarayanan, Journal of Power Sources 156 (2006) 705–711. [4] Y. Chen, X.H. Wang, J. Li, J.L. Lu, F.S. Wang, Electrochimica Acta 52 (2007) 5392–5399. [5] S. Xing, C. Zhao, S. Jing, Y. Wu, Z. Wang, European Polymer Journal 42 (2006) 2730–2735. [6] D. Li, J. Huang, R.B. Kane, J. Huang, R.B. Kaner, Accounts of Chemical Research 42 (2009) 135–145. [7] Y. Guo, Y. Zhou, European Polymer Journal 43 (2007) 2292–2297. [8] J. Wang, J. Wang, Z. Yang, Z. Wang, F. Zhang, S. Wang, Reactive and Functional Polymers 68 (2008) 1435–1440. [9] Y. Wang, X. Jing, J. Kong, Synthetic Metals 157 (2007) 269–275. [10] X. Jing, Y. Wang, D. Wu, J. Qiang, Ultrasonics Sonochemistry 14 (2007) 75–80. [11] S. Khasim, S.C. Raghavendra, M. Revanasiddappa, M.V.N. Ambika Prasad, Ferroelectrics 325 (2005) 111–119. [12] S. Manjunath, K.R. Anilkumar, M. Revanasiddappa, M.V.N. Ambika Prasad, Ferroelectric Letters 35 (2008) 36–46. [13] N. Parvatikar, S. Jain, C.M. Kanamadi, B.K. Chougule, S.V. Bhoraskar, M.V.N. Ambika Prasad, Journal of Applied Polymer Science 103 (2007) 653–658. [14] S.X. Tan, J. Zhai, M.X. Wan, L. Jiang, D.B. Zhu, Synthetic Metals 137 (2003) 1511–1512. [15] R. Ganesan, A. Gedanken, Nanotechnology 19 (2008) 1–5. [16] R. Schweiss, N. Zhang, W. Knoll, Journal of Sol–Gel Science and Technology 44 (2007) 1–5. [17] Y.N. Qi, F. Xu, H.J. Ma, L.X. Sun, J. Zhang, T. Jiang, Journal of Thermal Analysis and Calorimetry 91 (2008) 219–223. [18] L.-J. Sun, X.-X. Liu, European Polymer Journal 44 (2008) 219–224. [19] L. Chen, L.J. Sun, F. Luan, Y. Liang, Y. Li, X.X. Liu, Journal of Power Sources 195 (2010) 3742–3747. [20] N. Parvatikar, S. Jain, S. Khasima, M. Revansiddappa, S.V. Bhoraskar, M.V.N. Ambika Prasad, Sensors and Actuators B 114 (2006) 599–603. [21] B.X. Zou, X.X. Liu, D. Diamond, K.T. Lau, Electrochimica Acta 55 (2010) 3915–3920. [22] Y.Y. Wang, K.F. Cail, B.J. An, Y. Du, X. Yao, Journal of Nanoparticle Research 13 (2011) 533–539. [23] S. Thongtem, S. Wannapop, T. Thongtem, Transactions of Nonferrous Metallurgical Society of China 19 (2009) 100–104. [24] Y. Xing, S. Song, J. Feng, Y. Lei, M. Li, H. Zhang, Solid State Sciences 10 (2008) 1299–1304. [25] W. Qu, W. Wlodarski, J. Meyer, Sensors and Actuators B 64 (2000) 76–82. [26] Y.-N. Qi, F. Xu, H.-J. Ma, L.-X. Sun, J. Zhang, T. Jiang, Journal of Thermal Analysis and Calorimetry 91 (2008) 219–223. [27] R. Ansari, M.B. Keivani, Journal of Chemistry 3 (2006) 202–217. [28] Q. Tang, J. Wu, X. Sun, Q. Li, J. Lin, L. Fan, Polymer 50 (2009) 752–755. [29] H. Pang, C. Huang, J. Chen, B. Liu, Y. Kuang, X. Zhang, Journal of Solid State Electrochemistry 14 (2010) 169–174. [30] V.S. Jamadade, D.S. Dhawale, C.D. Lokhande, Synthetic Metals 160 (2010) 955–960. [31] R. Ganesan, S. Shanmugam, A. Gedanken, Synthetic Metals 158 (2008) 848–853. [32] W. Tong, L. Li, W. Hu, T. Yan, X. Guan, G. Li, Journal of Physical Chemistry C 114 (2010) 15298–15305. [33] X. Wang, M. Shao, G. Shao, Z. Wu, S. Wang, Journal of Colloid and Interface Science 332 (2009) 74–77. [34] P. Xu, X. Han, C. Wang, H. Zhao, J. Wang, X. Wang, B. Zhang, Journal of Physical Chemistry B 112 (2008) 2775–2781. [35] E.I. Santiago, C. Pereira, L.O.S. Bulhoes, Synthetic Metals 98 (1998) 87–93. [36] G. Xu, W. Wang, X. Qu, Y. Yin, L. Chu, B. He, H. Wu, J. Fang, Y. Bao, L. Liang, European Polymer Journal 45 (2009) 2701–2707. [37] Y. Yang, E. Liu, L. Li, Z. Huang, H. Shen, X. Xiang, Journal of Alloys and Compounds 487 (2009) 564–567. [38] J. Jiang, L. Li, F. Xu, Journal of Physics and Chemistry of Solids 68 (2007) 1656–1662. [39] W. Zou, W. Wang, B. He, M. Sun, Y. Yin, Journal of Power Sources 195 (2010) 7489–7493. [40] L. Chen, L. Sun, F. Luan, Y. Lianga, Y. Li, X. Liu, Journal of Power Sources 195 (2010) 3742–3747. [41] R. Jiang, T. Huang, Y. Tang, J. Liu, L. Xue, J. Zhuang, A. Yu, Electrochimica Acta 54 (2009) 7173–7179. [42] G. Lv, D. Wu, R. Fu, Journal of Non-crystalline Solids 355 (2009) 2461–2465. [43] R.K. Sharma, H. Oh, Y. Shul, H. Kim, Physica B 403 (2008) 1763–1769.