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Two composites based on CoMoO4 nanorods and PPy nanoparticles: Fabrication, structure and electrochemical properties
Synthetic Metals 215 (2016) 50–55
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Two composites based on CoMoO4 nanorods and PPy nanoparticles: Fabrication, structure and electrochemical properties Yong Chena,* , Guiying Kanga , Hui Xua , Long Kangb a b
College of Perochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China Colloge of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 October 2015 Received in revised form 27 January 2016 Accepted 2 February 2016 Available online xxx
In this work, we fabricate two composites based on CoMoO4 nanorods and PPy nanoparticles. The results of Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) prove that two composites structure are very different, and the major component of composites play an apparently decisive role in affecting the structure of composites. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) are applied to characterize the electrochemical performance of the composites. The specific capacitance of CoMoO4/PPy is 232 F g1 in 0.5 M Na2SO4 solution and that of PPy/CoMoO4 is 230 F g1 in 1 M KOH solution, considerably higher than their constituents. In contrast to PPy (13%) and CoMoO4 (67%), the capacitance retention of CoMoO4/PPy and PPy/CoMoO4 are 20.7% and 74.6% after 1000 cycles at the current density of 2 A g1. The results infer that for two composites, there is a synergistic effect between PPy and CoMoO4, and, to some extent, the electrochemical performance of composites is determined by the major component. ã 2016 Elsevier B.V. All rights reserved.
Keywords: Polypyrrole Cobalt molybdate Composite Supercapacitor
1. Introduction In recent years, conducting polymers have attracted scientists due to their unique properties and wide range of application potential in different electronics and device fields. There are a lot of research concerned with the processing and fabrication of conductive polymers. A variety of polymers, including Polypyrrole (PPy) [1], Polyaniline (PANI) [2], Polythiophene (PTh) [3] and their derivatives [4] have been researched widely. Among these polymers, PPy become one of the most studied conducting polymers owing to its fast charge/discharge kinetics, high energy density, excellent chemical stability, easy synthetic procedure and cost effectiveness [5–9]. At present, PPy has been widely used for commercial applications, including secondary batteries, fuel cells, supercapacitors, sensors and corrosion protection [10,11]. It is used for the electrode materials for the redox supercapacitors via the electrochemical deposition on the substrates or chemically oxidative polymerization. However, PPy has a critical defect that its main chains prone to rupture in the process of charging and discharging, leading to premature attenuation and decreasing of
* Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.synthmet.2016.02.006 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
specific capacitance of the electrode material. Therefore, more and more scientists begin to study composites based on PPy to improve its specific capacity and cyclic stability when it is used as electrode materials. For instance, Sahoo et al. [12] investigated the capacitance of composites based on PPy nanofiber and grapheme, the results show that the electrochemical performances of PPy can be enhanced greatly; Li et al. [13] fabricated graphene oxide (GO)/ PPy composites, the PPy nanowires grown on the surface of GO nanosheets, the results show that the introduction of GO can greatly improved the specific capacitance and cycle stability of PPy; Han et al. [14] fabricated hierarchical Co3O4/PPy/MnO2 core– shell–shell nanowire arrays with a remarkable long-term cycling stability; hydrochloride doped PPy was prepared by chemical oxidation at low temperature (0 C) by Zhang et al. [15], greatly improved the specific capacity and cycle stability of the PPy; Lin et al. [16] fabricated doped polypyrrole/PVA composite, the results showed that PVA had a certain influence on improving the performance of electrical conductivity and electrochemical properties of PPy. Although the preparation of composites based on PPy have been extensively studied by many research groups, its composite with double metal oxide are not yet explored fully for the electrochemical applications. Double metal oxides have the advantages of two metal oxides, and have outstanding electrochemical performance
Y. Chen et al. / Synthetic Metals 215 (2016) 50–55
(high specific capacity, rate performance and cycle stability) than that of single metal oxide. Therefore, in this work, we explored a facile and effective method to prepare the CoMoO4/PPy and PPy/ CoMoO4 binary composites, aiming to achieved composites electrode with excellent electrochemical properties. After examined the structure of the composites, and studied the differences of PPy/CoMoO4 and CoMoO4/PPy composites in electrochemical performance, we proved that the electrochemical properties of two composites, especially the specific capacity and cyclic stability are improved in contrast to PPy and CoMoO4. 2. Experimental section 2.1. Materials Sodium molybdate, Cobalt dichloride, and Pyrrole monomer are purchased from factory of Tianjin chemical reagent. Ammonium peroxodisulfate (oxidizing agent) is purchased from Double chemical Co.LtD. (Yantai, China). Pyrrole monomer is purified by distillation under reduced pressure before used. All other chemicals are used as received, without any further purification. Distilled water is used throughout the whole preparation process.
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2.3. Preparation of CoMoO4 CoMoO4 was prepared by chemical precipitation [17] 20 mL 0.25 M Na2MoO42H2O solution was added slowly to the 20 mL 0.25 M CoCl26H2O solution under electric stirring and the reaction lasted for 4 h at 70 C. The resultant product was dried at 80 C for 12 h to obtain a purple powder of CoMoO4. 2.4. Preparation of CoMoO4/PPy composite CoMoO4 (0.6 g) was added into 50 mL deionized water containing 1 mL pyrrole monomer and stirred for an hour at ambient temperature. The following steps as well as the preparation of PPy. The black powder composite of CoMoO4/PPy can be obtained (marked as CoMoO4/PPy). 2.5. Preparation of PPy/CoMoO4 composite 20 mL 0.25 M Na2MoO42H2O solution was added slowly to the 20 mL 0.25 M CoCl26H2O solution containing PPy (0.01 g) under electric stirring and the reaction lasted for 4 h and the product was dried at 80 C for 12 h to obtain powder of PPy/CoMoO4 composite (marked as PPy/CoMoO4).
2.2. Preparation of PPy 2.6. Electrochemical tests PPy was prepared by chemical oxidation polymerization. 1 mL Pyrrole monomer was dispersed in 50 mL distilled water and it was stirred for an hour at ambient temperature. Then, 50 mL 0.3 M (NH4)2S2O8 solution was added by drop wise into the mixture and the reaction was allowed to continue for 6 h under the electric stirring and the reactive temperature is about 0 5 C.The resultant product was dried at 60 C for 24 h to obtain a black powder of PPy.
Electrode active material (8.5 mg), acetylene black (1 mg), PTFE (0.5 mg), and a moderate amount of alcohol were mixed evenly. Then, the mixture was besmeared to the surface of nickel foam. It was dried at 60 C for 24 h and then compressed at 10 M Pa for 3 min. All electrochemical tests were carried out in a threeelectrode system, platinum foils and silver chloride electrode were used as counter and reference electrodes. The electrochemical
Fig. 1. SEM images of PPy (a), CoMoO4/PPy (b), CoMoO4 (d) and PPy/CoMoO4 (e); TEM images of CoMoO4/PPy (c) and PPy/CoMoO4 (f); the EDS microanalysis of CoMoO4/PPy (g) and PPy/CoMoO4 (h).
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Y. Chen et al. / Synthetic Metals 215 (2016) 50–55
characterization of CoMoO4/PPy and PPy/CoMoO4 composites was performed in 0.5 M Na2SO4 and 1.0 M KOH electrolyte solution. Because the CoMoO4 is easy to gain or loss electrons in the alkaline electrolyte which can improve the Faraday capacitance, we choose KOH as electrolyte to test the electrochemical performances of PPy/CoMoO4 electrode, on the other hand the structure of PPy electrode can be destroyed in the alkaline electrolyte, the neutral electrolyte Na2SO4 is selected during the test of CoMoO4/PPy. CV, EIS, and GCD measurements were performed with a CHI660B workstation. The voltage windows of CV and GCD tests of CoMoO4/ PPy composite were set in the range from 0.6 to 0.6 V, and the voltage window of CV and GCD tests of PPy/CoMoO4 composite were set in the range from 0.2 to 0.6 V and 0.2 to 0.5 V. After cyclic voltammograms test we found the PPy/CoMoO4 composite has good capacitance performance in the voltage range of 0.2 to 0.5 V, so we choose it during the following test of GCD. All EIS measurements were performed in the frequency range from 105 to 102 Hz at an open circuit potential with an AC perturbation of 5 mV. 2.7. Structure characterization The molecular structure of materials was identified by FTIR (Nicolet, type210, America), using KBr pellets, which was recorded between 4000 and 500 cm1. The X-ray diffraction patterns, using D/MAX-2400X X-ray diffractometer with CuKa radiation (l = 0.154056 nm), were recorded in the range of 2u = 5 80. The morphology of samples was observed by SEM (JEOL JSM6701F). TEM measurement was carried out on JEM-1200EX microscope at a voltage of 200 kV. Elemental analyses were performed by EDS (JEOL JSM-5600LV). 3. Results and discussion 3.1. Structure characterization The morphology of PPy, CoMoO4, CoMoO4/PPy and PPy/CoMoO4 were characterized by SEM firstly. As shown in Fig. 1a and d, the morphology of PPy and CoMoO4 are particles and short nanorods, respectively. CoMoO4/PPy composite exhibits more small particles than PPy and displays good dispersion (Fig. 1b), which indicates that the morphology of PPy is changed with the introduction of CoMoO4, while the short nanorods of CoMoO4 do not appear in the CoMoO4/PPy composite, the reason may be that the CoMoO4 nanorods were breaked during the process of reaction or the amount of nanorods was too little. In order to prove the PPy and CoMoO4 were successfully combined, TEM of CoMoO4/PPy was shown in Fig. 1c. It can be seen that the rods of CoMoO4 are presence. However, the morphology of PPy/CoMoO4 composite is
short nanorods with a small amount of particles, and the rods became slightly thinner than that of CoMoO4 (Fig. 1e). This indicates that the morphology of CoMoO4 has a little change with the introduction of PPy. The TEM of PPy/CoMoO4 was depicted in Fig. 1f. It can be seen that the particles of PPy were embedded in the rods of CoMoO4. These suggest that the PPy and CoMoO4 have been successfully combined. The energy dispersive spectroscopy (EDS) microanalysis of composites was depicted in Fig. 1g and h, which shows the presence of C, N, Co, Mo and O elements in CoMoO4/PPy and PPy/CoMoO4. XRD spectra of the PPy, CoMoO4, CoMoO4/PPy, and PPy/CoMoO4 were presented in Fig. 2a. PPy displays a broad peak in 20–30 degrees and the center is about 22 . This broad peak corresponds to the plane chain spacing of PPy molecules, and shows that the partial order of PPy molecules [18]. Because the aggregation ability of a and b of Py are similar, the main formed key is a-a0 , but also will form a small number of a-b key during the process of chemical oxidation synthesis of PPy, which destroyed the order of molecular structure of PPy, leading to the formation of amorphous PPy [19]. The structure XRD pattern of CoMoO4 nanorods were matched well with JCPDS 00-021-0868 [17]. By comparison, in CoMoO4/PPy samples, the width of PPy characteristic peak is relatively narrow than that of PPy and the reason may be attribute to the directional selection in the process of polymerization of Py [16]. In the meantime, all diffraction peaks of the PPy/CoMoO4 composite were almost as same as those of CoMoO4, moreover, it is noted that a broad diffraction peak appears in the range from 21 to 25 , which may be attributed to the insertion of PPy. All the above observations indicate that the crystal structure of CoMoO4/PPy is similar to PPy, while that of PPy/CoMoO4 is similar to CoMoO4. These may be attributed to the different major component of two composites. To understand the chemical environment of the composites, the FTIR analysis of PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 was carried out in the region of 4000 500 cm1, and shown in Fig. 2b. In the FTIR spectrum of PPy, peaks at 1560, 1470 and 3420 cm1 (broad peak) corresponds to C¼C, CC, N H stretching vibration in the pyrrole ring respectively [20–24]. The peaks at 1300 and 1180 cm1 are attributed to the in-plain vibrations of CH. Furthermore, peak at 1040 cm1 is assigned to the plane vibration of C H and N H, peak at 910 cm1 is deformation vibration of out-plane CH in the pyrrole ring [25]. By comparing the spectrum of CoMoO4/PPy, we can find the characteristic peaks of PPy are presence in CoMoO4/PPy composite, and the position of peaks is not changed. However, the intensity of peaks is weak, manifesting that the relationship between PPy and CoMoO4 is just a simple physical cladding [26]. Similarly, comparing the CoMoO4 and PPy/CoMoO4 composite, we also find the characteristic vibrations of CoMoO4 are existence in PPy/CoMoO4 composite,
Fig. 2. (a) XRD patterns for PPy, CoMoO4, CoMoO4/PPy, and PPy/CoMoO4; (b) FTIR spectra of PPy, CoMoO4, CoMoO4/PPy, and PPy/CoMoO4.
Y. Chen et al. / Synthetic Metals 215 (2016) 50–55
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Fig. 3. (a, c) CV curves for PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 at a scan rate of 5 mV s1; (b, d) CV curves for CoMoO4/PPy and PPy/CoMoO4 at different scan rates.
Fig. 4. (a, c) GCD curves of PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 electrodes at the current density of 0.6 A g1; (b, d) GCD curves of CoMoO4/PPy and PPy/CoMoO4 electrodes at different current density.
the position of characteristic peaks is not changed, and the intensity of peaks is strengthened. According to the analysis, we can conclude that the interaction between the CoMoO4 and PPy is also just a simple physical cladding.
and 0.41 V, which corresponding to the redox reaction between Co (II) and Co (III). Equations are as follow [28]: 3[Co(OH)3] $ Co3O4 + 4H2O + OH + 2e
(2)
3.2. Electrochemical measurement (three-electrode system)
Co3O4 + H2O + OH $ 3CoOOH + e
(3)
CoOOH + OH $ CoO2 + H2O + e
(4)
The electrochemical performances of the electrode materials were carried out by cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS). CV curves are one kind of potentiodynamic electrochemical measurement and can be used for the measurement of specific capacitance of the electrode materials as well as current response of the composites with applied potential, at different scan rate [12]. The CV curves of PPy and CoMoO4/PPy composite are displayed in Fig. 3a at a scan rate of 5 mV s1 between 0.6 and 0.6 V in 0.5 M Na2SO4 electrolyte. It can be found that the CV curves of PPy and CoMoO4/PPy appear a pair of redox peaks between 0.21 and 0.48 V roughly, and, to a certain degree, the current of redox peaks of CoMoO4/PPy composite increase compare to that of PPy. This may be a result of the doping/de-doping of sodium ion in the electrolyte [27], and the reaction mechanism can be expressed as follows (1). PPyþ ðA Þ þ e þ Mþ $PPy0 ðMþ A Þ
The CV curve of PPy/CoMoO4 exhibits stronger redox peaks, and the area of the curve is obviously bigger than that of CoMoO4. This is because PPy, as a component of composite, may change the faradic pseudocapacitance of pure CoMoO4. It can be seen from Fig. 3d, that the redox peaks are still visible with the increase of scanning rate, showing that the composite has good
ð1Þ
In addition, the CV curve of CoMoO4/PPy close to a rectangle in shape, and the area of cyclic voltammogram of CoMoO4/PPy is bigger than that of PPy, which indicating that CoMoO4/PPy composite can improve the electrochemical activity of PPy in Na2SO4 electrolyte. Besides, Fig. 3b shows the CV curves of CoMoO4/PPy composite at different scan rates (5 mV s1 40 mV s1). It can be observed that the peak currents increase with the scan rates, indicating that the kinetics of Interfacial Faradic redox reactions, the rates of electronic and ionic transports are even rapid enough at the scan rate of 40 mV s1. Fig. 3c depicts the CV curves of CoMoO4 and PPy/CoMoO4 at a scan rate of 5 mV s1 between 0.2 to 0.6 V in 1 M KOH aqueous media. We can find that the CV curve of CoMoO4 presents a pair of strong redox peak between 0.28
Fig. 5. AC impedance plots of PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 electrodes.
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Fig. 6. Equivalent circuit for the simulation of EIS spectra of PPy (a); CoMoO4/PPy (b); CoMoO4 (c); PPy/CoMoO4 (d).
Table 1 The simulated data of the equivalent circuit.
Rs Rp W
PPy
CoMoO4/PPy
CoMoO4
PPy/CoMoO4
4.06 0.32 2.36
4.92 0.3 2.29
2.42 4.11 27.65
3.65 1.30 2.25
electrochemical reversibility and can be applied as the material of large current charge and discharge. The specific capacitance values of composites can be calculated by GCD curves (Fig. 4) using the following equation: C¼
I Dt mDV
ð5Þ
Where I (A) is the discharge current, Dt (s) is the discharge time, DV (V) is the potential window and m (g) is the mass of the active material [29–32]. The specific capacitance values of PPy, CoMoO4/ PPy, CoMoO4 and PPy/CoMoO4 are 182, 232, 219 and 230 F g1, respectively. It illustrates that two composites enhance the specific capacitance of PPy and CoMoO4, which may be attributed to the synergistic effect between CoMoO4 and PPy. As a complement to galvanostatic charge-discharge test, EIS can provide more information of electronic or ionic conductivity of the electrode materials. From Fig. 5, we obviously observed that the semicircular diameter of high frequency area of PPy is as same as CoMoO4/PPy, indicating that the charge transfer resistance of PPy and CoMoO4/PPy are similar. However, the slope of straight line in the low frequency area of CoMoO4/PPy is larger than that of PPy, which suggested that the diffusion impedance of CoMoO4/PPy is close to the ideal capacitor compared with PPy. On the other hand, from Fig. 5 we find that the charge transfer resistance and diffusion impedance of PPy/CoMoO4 are smaller than those of pure CoMoO4 because of its smaller semicircular diameter and larger slope of straight line. The reason is probably that the PPy was embedded in the matrix of CoMoO4, intensifying the Faradic reaction. In addition, it can be seen from the distance of high frequency area semicircle with real axis intersection to origin, that the ascending order of internal resistance of samples is CoMoO4, PPy/CoMoO4, PPy and CoMoO4/PPy [33]. Thus, it is clearly seen that, whether CoMoO4/PPy or PPy/CoMoO4 composites will reduce the electrochemical impendence and improve the electrochemical performance of the pure PPy and CoMoO4. In order to confirm the above conclusions, the equivalent circuit diagram is simulated, as shown in Fig. 6. In the circuit, Rs is on behalf of the solution resistance, Rp is the charge transfer resistance, W is ionic diffusion process resulting in Warburg behaviour in the electrode, C represent the capacitance component [34]. Meanwhile, simulated data are showed in Table 1.
Fig. 7. Charge–discharge cycling test of PPy, CoMoO4/PPy, CoMoO4 and PPy/ CoMoO4 electrodes at the current density of 2 A g1.
Fig. 7 shows the charge storage capacity and durability of cycle lifetime of samples. The PPy, CoMoO4/PPy, CoMoO4 and PPy/ CoMoO4 as an electrode have been subjected to 1000 cycles at the current density of 2 A g1. It can be found that the cycle stability of the composites better than that of PPy and CoMoO4. The retention of specific capacitance of CoMoO4/PPy and PPy/CoMoO4 are 20.7% and 74.6%, bigger than that of the PPy (13%) and CoMoO4 (67%). These are illustrations of synergistic effect between PPy and CoMoO4. According to the analysis of electrochemical performance, it can be revealed that the composites have almost the same capacitor behaviour with its major material, and this consistency links to the similarity in their morphology and structure. This may be that the major material contacts with the electrolyte preferentially and completely, which makes the Faraday pseudocapacitance and double layer capacitance occurring on the major component firstly, hence, the other material only has a weak effect on electrochemical properties of composites. 4. Conclusions In summary, the CoMoO4/PPy and PPy/CoMoO4 composites were prepared by facile and effective method. The morphology and structure of CoMoO4/PPy are similar to PPy, while those of PPy/ CoMoO4 are same as CoMoO4. Due to properties determined by structure, the electrochemical performance of composites resembles its major component and exhibits some features as follows: the specific capacities of CoMoO4/PPy and PPy/CoMoO4 are higher than pure PPy and CoMoO4; the electrochemical impedance of two composites is lower than pure PPy and CoMoO4; and after
Y. Chen et al. / Synthetic Metals 215 (2016) 50–55
1000 cycles the capacity retention of CoMoO4/PPy and PPy/ CoMoO4 is higher than pure PPy and CoMoO4. These traits imply that two composites in the present work may be used as promising candidates for supercapacitors. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51503092). References [1] D. Plausinaitis, V. Ratautaite, L. Mikoliunaite, L. Sinkevicius, A. Ramanaviciene, A. Ramanavicius, Langmuir 31 (2015) 3186–3193. [2] R. Gottam, P. Srinivasan, J. Appl. Polym. Sci. 132 (2015). [3] H. Zhang, L. Hu, J. Tu, S. Jiao, Electrochim. Acta 120 (2014) 122–127. [4] S. Angaiah, D.S. Lakshmi, Polym. Adv. Technol. 19 (2008) 725–727. [5] O.N. Efimov, Russ. Chem. Rev. 66 (1997) 443. [6] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89–107. [7] T.V. Vernitskaya, O.N. Efimov, Cheminform 28 (1997). [8] R.D. Mccullough, R.D. Mccullough, Adv. Mater. 10 (1999) 93–116. [9] J. Xu, D. Wang, Y. Yuan, W. Wei, S. Gu, R. Liu, X. Wang, L. Liu, W. Xu, Cellulose 22 (2015) 1355–1363. [10] D.P. Dubal, H.L. Sang, J.G. Kim, W.B. Kim, C.D. Lokhande, J. Mater. Chem. 22 (2012) 3044–3052. [11] A. Ramanavicius, Y. Oztekin, A. Ramanaviciene, Sens. Actuators B Chem. 197 (2014) 237–243.
55
[12] S. Sahoo, S. Dhibar, G. Hatui, P. Bhattacharya, C.K. Das, Polymer 54 (2013) 1033– 1042. [13] J. Li, H. Xie, Y. Li, J. Power Sources 241 (2013) 388–395. [14] J. Liu, Y. Xu, X. Ma, J. Feng, Y. Qian, S. Xiong, Nano Energy 7 (2014) 52–62. [15] A.Q. Zhang, L.Z. Wang, Y. Zhang, F.B. Shen, Battery Bimonthly 40 (2010) 10–12. [16] S. Lin, New Chem. Mater. 39 (2011) 31–34. [17] M.C. Liu, L.B. Kong, C. Lu, X.M. Li, Y.C. Luo, L. Kang, M.C. Liu, L.B. Kong, C. Lu, X.M. Li, Mater. Lett. 94 (2013) 197–200. [18] Y. Chen, C. Zhang, Y. Wang, S. Cheng, P. Chen, J. Appl. Polym. Sci. 90 (2003) 3609–3616. [19] M.R. Karim, C.J. Lee, M.S. Lee, Polym. Adv. Technol. 18 (2007) 916–920. [20] S. Sahoo, G. Karthikeyan, G.C. Nayak, C.K. Das, Synth. Metal 161 (2011) 1713– 1719. [21] S. Bose, T. Kuila, N.H. Kim, J.H. Lee, M.E. Uddin, A.K.T. Lau, Polymer 51 (2010) 5921–5928. [22] S. Sahoo, P. Bhattacharya, G. Hatui, D. Ghosh, C.K. Das, J. Appl. Polym. Sci. 128 (2013) 1476–1483 (1478). [23] M. Moniruzzaman, C. Das, Macromolecular Symposia, Wiley Online Library, 2010, pp. 34–42. [24] C. Bora, S.K. Dolui, Polymer 53 (2012) 923–932. [25] C. Zhang, Y. Chen, Polym. Int. 54 (2005) 1027–1033. [26] X.U. Jun, X.R. Zeng, L.I. Peng, H.P. Deng, S.U. Hai-Xia, Chem. Adhes. 27 (2005) 127–130. [27] L. Yongfang, Polym. Bull. 4 (2005) 51–57. [28] M.C. Liu, L.B. Kong, X.J. Ma, C. Lu, X.M. Li, Y.C. Luo, L. Kang, New J. Chem. 36 (2012) 1713–1716. [29] S.H. Cho, J.S. Joo, B.R. Jung, T.M. Ha, J.Y. Lee, Macromol. Res. 17 (2009) 746–749. [30] J. Zang, X. Li, J. Mater. Chem. 21 (2011) 10965–10969. [31] H. Huang, X. Wang, Nanoscale 3 (2011) 3185–3191. [32] P. Zhu, T. Cai, G.Y. Han, J.J. Xiong, J. Inorg. Mater. 30 (2015) 505–510. [33] J. Zhang, Nanoscale 5 (2013) 3052–3057. [34] H. Xu, J. Wu, C. Li, J. Zhang, J. Liu, Electrochim. Acta 165 (2015) 14–21.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Two composites based on CoMoO4 nanorods and PPy nanoparticles: Fabrication, structure and electrochemical properties Yong Chena,* , Guiying Kanga , Hui Xua , Long Kangb a b
College of Perochemical Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China Colloge of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 28 October 2015 Received in revised form 27 January 2016 Accepted 2 February 2016 Available online xxx
In this work, we fabricate two composites based on CoMoO4 nanorods and PPy nanoparticles. The results of Scanning electron microscopy (SEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) prove that two composites structure are very different, and the major component of composites play an apparently decisive role in affecting the structure of composites. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) are applied to characterize the electrochemical performance of the composites. The specific capacitance of CoMoO4/PPy is 232 F g1 in 0.5 M Na2SO4 solution and that of PPy/CoMoO4 is 230 F g1 in 1 M KOH solution, considerably higher than their constituents. In contrast to PPy (13%) and CoMoO4 (67%), the capacitance retention of CoMoO4/PPy and PPy/CoMoO4 are 20.7% and 74.6% after 1000 cycles at the current density of 2 A g1. The results infer that for two composites, there is a synergistic effect between PPy and CoMoO4, and, to some extent, the electrochemical performance of composites is determined by the major component. ã 2016 Elsevier B.V. All rights reserved.
Keywords: Polypyrrole Cobalt molybdate Composite Supercapacitor
1. Introduction In recent years, conducting polymers have attracted scientists due to their unique properties and wide range of application potential in different electronics and device fields. There are a lot of research concerned with the processing and fabrication of conductive polymers. A variety of polymers, including Polypyrrole (PPy) [1], Polyaniline (PANI) [2], Polythiophene (PTh) [3] and their derivatives [4] have been researched widely. Among these polymers, PPy become one of the most studied conducting polymers owing to its fast charge/discharge kinetics, high energy density, excellent chemical stability, easy synthetic procedure and cost effectiveness [5–9]. At present, PPy has been widely used for commercial applications, including secondary batteries, fuel cells, supercapacitors, sensors and corrosion protection [10,11]. It is used for the electrode materials for the redox supercapacitors via the electrochemical deposition on the substrates or chemically oxidative polymerization. However, PPy has a critical defect that its main chains prone to rupture in the process of charging and discharging, leading to premature attenuation and decreasing of
* Corresponding author. E-mail address: [email protected] (Y. Chen). http://dx.doi.org/10.1016/j.synthmet.2016.02.006 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
specific capacitance of the electrode material. Therefore, more and more scientists begin to study composites based on PPy to improve its specific capacity and cyclic stability when it is used as electrode materials. For instance, Sahoo et al. [12] investigated the capacitance of composites based on PPy nanofiber and grapheme, the results show that the electrochemical performances of PPy can be enhanced greatly; Li et al. [13] fabricated graphene oxide (GO)/ PPy composites, the PPy nanowires grown on the surface of GO nanosheets, the results show that the introduction of GO can greatly improved the specific capacitance and cycle stability of PPy; Han et al. [14] fabricated hierarchical Co3O4/PPy/MnO2 core– shell–shell nanowire arrays with a remarkable long-term cycling stability; hydrochloride doped PPy was prepared by chemical oxidation at low temperature (0 C) by Zhang et al. [15], greatly improved the specific capacity and cycle stability of the PPy; Lin et al. [16] fabricated doped polypyrrole/PVA composite, the results showed that PVA had a certain influence on improving the performance of electrical conductivity and electrochemical properties of PPy. Although the preparation of composites based on PPy have been extensively studied by many research groups, its composite with double metal oxide are not yet explored fully for the electrochemical applications. Double metal oxides have the advantages of two metal oxides, and have outstanding electrochemical performance
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(high specific capacity, rate performance and cycle stability) than that of single metal oxide. Therefore, in this work, we explored a facile and effective method to prepare the CoMoO4/PPy and PPy/ CoMoO4 binary composites, aiming to achieved composites electrode with excellent electrochemical properties. After examined the structure of the composites, and studied the differences of PPy/CoMoO4 and CoMoO4/PPy composites in electrochemical performance, we proved that the electrochemical properties of two composites, especially the specific capacity and cyclic stability are improved in contrast to PPy and CoMoO4. 2. Experimental section 2.1. Materials Sodium molybdate, Cobalt dichloride, and Pyrrole monomer are purchased from factory of Tianjin chemical reagent. Ammonium peroxodisulfate (oxidizing agent) is purchased from Double chemical Co.LtD. (Yantai, China). Pyrrole monomer is purified by distillation under reduced pressure before used. All other chemicals are used as received, without any further purification. Distilled water is used throughout the whole preparation process.
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2.3. Preparation of CoMoO4 CoMoO4 was prepared by chemical precipitation [17] 20 mL 0.25 M Na2MoO42H2O solution was added slowly to the 20 mL 0.25 M CoCl26H2O solution under electric stirring and the reaction lasted for 4 h at 70 C. The resultant product was dried at 80 C for 12 h to obtain a purple powder of CoMoO4. 2.4. Preparation of CoMoO4/PPy composite CoMoO4 (0.6 g) was added into 50 mL deionized water containing 1 mL pyrrole monomer and stirred for an hour at ambient temperature. The following steps as well as the preparation of PPy. The black powder composite of CoMoO4/PPy can be obtained (marked as CoMoO4/PPy). 2.5. Preparation of PPy/CoMoO4 composite 20 mL 0.25 M Na2MoO42H2O solution was added slowly to the 20 mL 0.25 M CoCl26H2O solution containing PPy (0.01 g) under electric stirring and the reaction lasted for 4 h and the product was dried at 80 C for 12 h to obtain powder of PPy/CoMoO4 composite (marked as PPy/CoMoO4).
2.2. Preparation of PPy 2.6. Electrochemical tests PPy was prepared by chemical oxidation polymerization. 1 mL Pyrrole monomer was dispersed in 50 mL distilled water and it was stirred for an hour at ambient temperature. Then, 50 mL 0.3 M (NH4)2S2O8 solution was added by drop wise into the mixture and the reaction was allowed to continue for 6 h under the electric stirring and the reactive temperature is about 0 5 C.The resultant product was dried at 60 C for 24 h to obtain a black powder of PPy.
Electrode active material (8.5 mg), acetylene black (1 mg), PTFE (0.5 mg), and a moderate amount of alcohol were mixed evenly. Then, the mixture was besmeared to the surface of nickel foam. It was dried at 60 C for 24 h and then compressed at 10 M Pa for 3 min. All electrochemical tests were carried out in a threeelectrode system, platinum foils and silver chloride electrode were used as counter and reference electrodes. The electrochemical
Fig. 1. SEM images of PPy (a), CoMoO4/PPy (b), CoMoO4 (d) and PPy/CoMoO4 (e); TEM images of CoMoO4/PPy (c) and PPy/CoMoO4 (f); the EDS microanalysis of CoMoO4/PPy (g) and PPy/CoMoO4 (h).
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characterization of CoMoO4/PPy and PPy/CoMoO4 composites was performed in 0.5 M Na2SO4 and 1.0 M KOH electrolyte solution. Because the CoMoO4 is easy to gain or loss electrons in the alkaline electrolyte which can improve the Faraday capacitance, we choose KOH as electrolyte to test the electrochemical performances of PPy/CoMoO4 electrode, on the other hand the structure of PPy electrode can be destroyed in the alkaline electrolyte, the neutral electrolyte Na2SO4 is selected during the test of CoMoO4/PPy. CV, EIS, and GCD measurements were performed with a CHI660B workstation. The voltage windows of CV and GCD tests of CoMoO4/ PPy composite were set in the range from 0.6 to 0.6 V, and the voltage window of CV and GCD tests of PPy/CoMoO4 composite were set in the range from 0.2 to 0.6 V and 0.2 to 0.5 V. After cyclic voltammograms test we found the PPy/CoMoO4 composite has good capacitance performance in the voltage range of 0.2 to 0.5 V, so we choose it during the following test of GCD. All EIS measurements were performed in the frequency range from 105 to 102 Hz at an open circuit potential with an AC perturbation of 5 mV. 2.7. Structure characterization The molecular structure of materials was identified by FTIR (Nicolet, type210, America), using KBr pellets, which was recorded between 4000 and 500 cm1. The X-ray diffraction patterns, using D/MAX-2400X X-ray diffractometer with CuKa radiation (l = 0.154056 nm), were recorded in the range of 2u = 5 80. The morphology of samples was observed by SEM (JEOL JSM6701F). TEM measurement was carried out on JEM-1200EX microscope at a voltage of 200 kV. Elemental analyses were performed by EDS (JEOL JSM-5600LV). 3. Results and discussion 3.1. Structure characterization The morphology of PPy, CoMoO4, CoMoO4/PPy and PPy/CoMoO4 were characterized by SEM firstly. As shown in Fig. 1a and d, the morphology of PPy and CoMoO4 are particles and short nanorods, respectively. CoMoO4/PPy composite exhibits more small particles than PPy and displays good dispersion (Fig. 1b), which indicates that the morphology of PPy is changed with the introduction of CoMoO4, while the short nanorods of CoMoO4 do not appear in the CoMoO4/PPy composite, the reason may be that the CoMoO4 nanorods were breaked during the process of reaction or the amount of nanorods was too little. In order to prove the PPy and CoMoO4 were successfully combined, TEM of CoMoO4/PPy was shown in Fig. 1c. It can be seen that the rods of CoMoO4 are presence. However, the morphology of PPy/CoMoO4 composite is
short nanorods with a small amount of particles, and the rods became slightly thinner than that of CoMoO4 (Fig. 1e). This indicates that the morphology of CoMoO4 has a little change with the introduction of PPy. The TEM of PPy/CoMoO4 was depicted in Fig. 1f. It can be seen that the particles of PPy were embedded in the rods of CoMoO4. These suggest that the PPy and CoMoO4 have been successfully combined. The energy dispersive spectroscopy (EDS) microanalysis of composites was depicted in Fig. 1g and h, which shows the presence of C, N, Co, Mo and O elements in CoMoO4/PPy and PPy/CoMoO4. XRD spectra of the PPy, CoMoO4, CoMoO4/PPy, and PPy/CoMoO4 were presented in Fig. 2a. PPy displays a broad peak in 20–30 degrees and the center is about 22 . This broad peak corresponds to the plane chain spacing of PPy molecules, and shows that the partial order of PPy molecules [18]. Because the aggregation ability of a and b of Py are similar, the main formed key is a-a0 , but also will form a small number of a-b key during the process of chemical oxidation synthesis of PPy, which destroyed the order of molecular structure of PPy, leading to the formation of amorphous PPy [19]. The structure XRD pattern of CoMoO4 nanorods were matched well with JCPDS 00-021-0868 [17]. By comparison, in CoMoO4/PPy samples, the width of PPy characteristic peak is relatively narrow than that of PPy and the reason may be attribute to the directional selection in the process of polymerization of Py [16]. In the meantime, all diffraction peaks of the PPy/CoMoO4 composite were almost as same as those of CoMoO4, moreover, it is noted that a broad diffraction peak appears in the range from 21 to 25 , which may be attributed to the insertion of PPy. All the above observations indicate that the crystal structure of CoMoO4/PPy is similar to PPy, while that of PPy/CoMoO4 is similar to CoMoO4. These may be attributed to the different major component of two composites. To understand the chemical environment of the composites, the FTIR analysis of PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 was carried out in the region of 4000 500 cm1, and shown in Fig. 2b. In the FTIR spectrum of PPy, peaks at 1560, 1470 and 3420 cm1 (broad peak) corresponds to C¼C, CC, N H stretching vibration in the pyrrole ring respectively [20–24]. The peaks at 1300 and 1180 cm1 are attributed to the in-plain vibrations of CH. Furthermore, peak at 1040 cm1 is assigned to the plane vibration of C H and N H, peak at 910 cm1 is deformation vibration of out-plane CH in the pyrrole ring [25]. By comparing the spectrum of CoMoO4/PPy, we can find the characteristic peaks of PPy are presence in CoMoO4/PPy composite, and the position of peaks is not changed. However, the intensity of peaks is weak, manifesting that the relationship between PPy and CoMoO4 is just a simple physical cladding [26]. Similarly, comparing the CoMoO4 and PPy/CoMoO4 composite, we also find the characteristic vibrations of CoMoO4 are existence in PPy/CoMoO4 composite,
Fig. 2. (a) XRD patterns for PPy, CoMoO4, CoMoO4/PPy, and PPy/CoMoO4; (b) FTIR spectra of PPy, CoMoO4, CoMoO4/PPy, and PPy/CoMoO4.
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Fig. 3. (a, c) CV curves for PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 at a scan rate of 5 mV s1; (b, d) CV curves for CoMoO4/PPy and PPy/CoMoO4 at different scan rates.
Fig. 4. (a, c) GCD curves of PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 electrodes at the current density of 0.6 A g1; (b, d) GCD curves of CoMoO4/PPy and PPy/CoMoO4 electrodes at different current density.
the position of characteristic peaks is not changed, and the intensity of peaks is strengthened. According to the analysis, we can conclude that the interaction between the CoMoO4 and PPy is also just a simple physical cladding.
and 0.41 V, which corresponding to the redox reaction between Co (II) and Co (III). Equations are as follow [28]: 3[Co(OH)3] $ Co3O4 + 4H2O + OH + 2e
(2)
3.2. Electrochemical measurement (three-electrode system)
Co3O4 + H2O + OH $ 3CoOOH + e
(3)
CoOOH + OH $ CoO2 + H2O + e
(4)
The electrochemical performances of the electrode materials were carried out by cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS). CV curves are one kind of potentiodynamic electrochemical measurement and can be used for the measurement of specific capacitance of the electrode materials as well as current response of the composites with applied potential, at different scan rate [12]. The CV curves of PPy and CoMoO4/PPy composite are displayed in Fig. 3a at a scan rate of 5 mV s1 between 0.6 and 0.6 V in 0.5 M Na2SO4 electrolyte. It can be found that the CV curves of PPy and CoMoO4/PPy appear a pair of redox peaks between 0.21 and 0.48 V roughly, and, to a certain degree, the current of redox peaks of CoMoO4/PPy composite increase compare to that of PPy. This may be a result of the doping/de-doping of sodium ion in the electrolyte [27], and the reaction mechanism can be expressed as follows (1). PPyþ ðA Þ þ e þ Mþ $PPy0 ðMþ A Þ
The CV curve of PPy/CoMoO4 exhibits stronger redox peaks, and the area of the curve is obviously bigger than that of CoMoO4. This is because PPy, as a component of composite, may change the faradic pseudocapacitance of pure CoMoO4. It can be seen from Fig. 3d, that the redox peaks are still visible with the increase of scanning rate, showing that the composite has good
ð1Þ
In addition, the CV curve of CoMoO4/PPy close to a rectangle in shape, and the area of cyclic voltammogram of CoMoO4/PPy is bigger than that of PPy, which indicating that CoMoO4/PPy composite can improve the electrochemical activity of PPy in Na2SO4 electrolyte. Besides, Fig. 3b shows the CV curves of CoMoO4/PPy composite at different scan rates (5 mV s1 40 mV s1). It can be observed that the peak currents increase with the scan rates, indicating that the kinetics of Interfacial Faradic redox reactions, the rates of electronic and ionic transports are even rapid enough at the scan rate of 40 mV s1. Fig. 3c depicts the CV curves of CoMoO4 and PPy/CoMoO4 at a scan rate of 5 mV s1 between 0.2 to 0.6 V in 1 M KOH aqueous media. We can find that the CV curve of CoMoO4 presents a pair of strong redox peak between 0.28
Fig. 5. AC impedance plots of PPy, CoMoO4/PPy, CoMoO4 and PPy/CoMoO4 electrodes.
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Fig. 6. Equivalent circuit for the simulation of EIS spectra of PPy (a); CoMoO4/PPy (b); CoMoO4 (c); PPy/CoMoO4 (d).
Table 1 The simulated data of the equivalent circuit.
Rs Rp W
PPy
CoMoO4/PPy
CoMoO4
PPy/CoMoO4
4.06 0.32 2.36
4.92 0.3 2.29
2.42 4.11 27.65
3.65 1.30 2.25
electrochemical reversibility and can be applied as the material of large current charge and discharge. The specific capacitance values of composites can be calculated by GCD curves (Fig. 4) using the following equation: C¼
I Dt mDV
ð5Þ
Where I (A) is the discharge current, Dt (s) is the discharge time, DV (V) is the potential window and m (g) is the mass of the active material [29–32]. The specific capacitance values of PPy, CoMoO4/ PPy, CoMoO4 and PPy/CoMoO4 are 182, 232, 219 and 230 F g1, respectively. It illustrates that two composites enhance the specific capacitance of PPy and CoMoO4, which may be attributed to the synergistic effect between CoMoO4 and PPy. As a complement to galvanostatic charge-discharge test, EIS can provide more information of electronic or ionic conductivity of the electrode materials. From Fig. 5, we obviously observed that the semicircular diameter of high frequency area of PPy is as same as CoMoO4/PPy, indicating that the charge transfer resistance of PPy and CoMoO4/PPy are similar. However, the slope of straight line in the low frequency area of CoMoO4/PPy is larger than that of PPy, which suggested that the diffusion impedance of CoMoO4/PPy is close to the ideal capacitor compared with PPy. On the other hand, from Fig. 5 we find that the charge transfer resistance and diffusion impedance of PPy/CoMoO4 are smaller than those of pure CoMoO4 because of its smaller semicircular diameter and larger slope of straight line. The reason is probably that the PPy was embedded in the matrix of CoMoO4, intensifying the Faradic reaction. In addition, it can be seen from the distance of high frequency area semicircle with real axis intersection to origin, that the ascending order of internal resistance of samples is CoMoO4, PPy/CoMoO4, PPy and CoMoO4/PPy [33]. Thus, it is clearly seen that, whether CoMoO4/PPy or PPy/CoMoO4 composites will reduce the electrochemical impendence and improve the electrochemical performance of the pure PPy and CoMoO4. In order to confirm the above conclusions, the equivalent circuit diagram is simulated, as shown in Fig. 6. In the circuit, Rs is on behalf of the solution resistance, Rp is the charge transfer resistance, W is ionic diffusion process resulting in Warburg behaviour in the electrode, C represent the capacitance component [34]. Meanwhile, simulated data are showed in Table 1.
Fig. 7. Charge–discharge cycling test of PPy, CoMoO4/PPy, CoMoO4 and PPy/ CoMoO4 electrodes at the current density of 2 A g1.
Fig. 7 shows the charge storage capacity and durability of cycle lifetime of samples. The PPy, CoMoO4/PPy, CoMoO4 and PPy/ CoMoO4 as an electrode have been subjected to 1000 cycles at the current density of 2 A g1. It can be found that the cycle stability of the composites better than that of PPy and CoMoO4. The retention of specific capacitance of CoMoO4/PPy and PPy/CoMoO4 are 20.7% and 74.6%, bigger than that of the PPy (13%) and CoMoO4 (67%). These are illustrations of synergistic effect between PPy and CoMoO4. According to the analysis of electrochemical performance, it can be revealed that the composites have almost the same capacitor behaviour with its major material, and this consistency links to the similarity in their morphology and structure. This may be that the major material contacts with the electrolyte preferentially and completely, which makes the Faraday pseudocapacitance and double layer capacitance occurring on the major component firstly, hence, the other material only has a weak effect on electrochemical properties of composites. 4. Conclusions In summary, the CoMoO4/PPy and PPy/CoMoO4 composites were prepared by facile and effective method. The morphology and structure of CoMoO4/PPy are similar to PPy, while those of PPy/ CoMoO4 are same as CoMoO4. Due to properties determined by structure, the electrochemical performance of composites resembles its major component and exhibits some features as follows: the specific capacities of CoMoO4/PPy and PPy/CoMoO4 are higher than pure PPy and CoMoO4; the electrochemical impedance of two composites is lower than pure PPy and CoMoO4; and after
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1000 cycles the capacity retention of CoMoO4/PPy and PPy/ CoMoO4 is higher than pure PPy and CoMoO4. These traits imply that two composites in the present work may be used as promising candidates for supercapacitors. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51503092). References [1] D. Plausinaitis, V. Ratautaite, L. Mikoliunaite, L. Sinkevicius, A. Ramanaviciene, A. Ramanavicius, Langmuir 31 (2015) 3186–3193. [2] R. Gottam, P. Srinivasan, J. Appl. Polym. Sci. 132 (2015). [3] H. Zhang, L. Hu, J. Tu, S. Jiao, Electrochim. Acta 120 (2014) 122–127. [4] S. Angaiah, D.S. Lakshmi, Polym. Adv. Technol. 19 (2008) 725–727. [5] O.N. Efimov, Russ. Chem. Rev. 66 (1997) 443. [6] A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J.P. Ferraris, J. Power Sources 47 (1994) 89–107. [7] T.V. Vernitskaya, O.N. Efimov, Cheminform 28 (1997). [8] R.D. Mccullough, R.D. Mccullough, Adv. Mater. 10 (1999) 93–116. [9] J. Xu, D. Wang, Y. Yuan, W. Wei, S. Gu, R. Liu, X. Wang, L. Liu, W. Xu, Cellulose 22 (2015) 1355–1363. [10] D.P. Dubal, H.L. Sang, J.G. Kim, W.B. Kim, C.D. Lokhande, J. Mater. Chem. 22 (2012) 3044–3052. [11] A. Ramanavicius, Y. Oztekin, A. Ramanaviciene, Sens. Actuators B Chem. 197 (2014) 237–243.
55
[12] S. Sahoo, S. Dhibar, G. Hatui, P. Bhattacharya, C.K. Das, Polymer 54 (2013) 1033– 1042. [13] J. Li, H. Xie, Y. Li, J. Power Sources 241 (2013) 388–395. [14] J. Liu, Y. Xu, X. Ma, J. Feng, Y. Qian, S. Xiong, Nano Energy 7 (2014) 52–62. [15] A.Q. Zhang, L.Z. Wang, Y. Zhang, F.B. Shen, Battery Bimonthly 40 (2010) 10–12. [16] S. Lin, New Chem. Mater. 39 (2011) 31–34. [17] M.C. Liu, L.B. Kong, C. Lu, X.M. Li, Y.C. Luo, L. Kang, M.C. Liu, L.B. Kong, C. Lu, X.M. Li, Mater. Lett. 94 (2013) 197–200. [18] Y. Chen, C. Zhang, Y. Wang, S. Cheng, P. Chen, J. Appl. Polym. Sci. 90 (2003) 3609–3616. [19] M.R. Karim, C.J. Lee, M.S. Lee, Polym. Adv. Technol. 18 (2007) 916–920. [20] S. Sahoo, G. Karthikeyan, G.C. Nayak, C.K. Das, Synth. Metal 161 (2011) 1713– 1719. [21] S. Bose, T. Kuila, N.H. Kim, J.H. Lee, M.E. Uddin, A.K.T. Lau, Polymer 51 (2010) 5921–5928. [22] S. Sahoo, P. Bhattacharya, G. Hatui, D. Ghosh, C.K. Das, J. Appl. Polym. Sci. 128 (2013) 1476–1483 (1478). [23] M. Moniruzzaman, C. Das, Macromolecular Symposia, Wiley Online Library, 2010, pp. 34–42. [24] C. Bora, S.K. Dolui, Polymer 53 (2012) 923–932. [25] C. Zhang, Y. Chen, Polym. Int. 54 (2005) 1027–1033. [26] X.U. Jun, X.R. Zeng, L.I. Peng, H.P. Deng, S.U. Hai-Xia, Chem. Adhes. 27 (2005) 127–130. [27] L. Yongfang, Polym. Bull. 4 (2005) 51–57. [28] M.C. Liu, L.B. Kong, X.J. Ma, C. Lu, X.M. Li, Y.C. Luo, L. Kang, New J. Chem. 36 (2012) 1713–1716. [29] S.H. Cho, J.S. Joo, B.R. Jung, T.M. Ha, J.Y. Lee, Macromol. Res. 17 (2009) 746–749. [30] J. Zang, X. Li, J. Mater. Chem. 21 (2011) 10965–10969. [31] H. Huang, X. Wang, Nanoscale 3 (2011) 3185–3191. [32] P. Zhu, T. Cai, G.Y. Han, J.J. Xiong, J. Inorg. Mater. 30 (2015) 505–510. [33] J. Zhang, Nanoscale 5 (2013) 3052–3057. [34] H. Xu, J. Wu, C. Li, J. Zhang, J. Liu, Electrochim. Acta 165 (2015) 14–21.