Ultrathin MnO2 Nanorods on Conducting Polymer Nanofibers as a

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Ultrathin MnO2 Nanorods on Conducting Polymer Nanofibers as a New Class of Hierarchical Nanostructures for High-Performance Supercapacitors Jie Han, Liya Li, Ping Fang, and Rong Guo* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, 225002, Jiangsu, P.R. China S Supporting Information *

ABSTRACT: A facile and scalable one-step strategy has been proposed for patterning ultrathin MnO2 nanorods (3 nm in diameter) on surfaces of almost all major classes of electronic conducting polymer nanofibers merely using conducting polymer nanofibers and KMnO4 as raw materials. The loading amount and patterning of MnO2 nanorods on surfaces of conducting polymers of typical polyaniline (PANI) nanofibers can be controlled by simply altering the KMnO4 concentration. Mechanisms involved in the formation of PANI− MnO2 composites were further revealed according to experimental results. In comparison with PANI nanofibers, the specific capacitance of PANI−MnO2 composites has substantially increased by almost fourfold, with values as high as 417 F g−1 achieved. The combination of ultrathin MnO2 nanorods and PANI nanofibers into one-dimensional hierarchical nanostructures show excellent electrochemical properties for energy storage applications, which evidence their potential application as supercapacitors. surface area, is seldom seen.2a Therefore, searching for facile and inexpensive routes to high-performance conducting polymer−MnO2 supercapacitor materials through a rational design to maximize the electrochemically active sites has always been greatly attractive but very challenging. Herein, we report on a one-step, scalable and general strategy toward 1D conducting polymer−MnO2 hierarchical composite nanostructures with MnO2 nanorods grown on surfaces of conducting polymer nanofibers. It is interesting to disclose that almost all major classes of conducting polymers, including polyaniline (PANI), polypyrrole (PPY), and poly(3,4-ethylenedioxythiophene) (PEDOT), show redox activities toward KMnO4, endowing the possibility for making conducting polymer−MnO2 composites merely using conducting polymers and KMnO4 as raw materials. In addition, patterning of MnO2 nanorods on surfaces of typical PANI nanofibers has been realized by simply altering KMnO4 concentration. Mechanisms involved have been revealed according to experimental results. Furthermore, the combination of MnO2 nanorods and conducting polymer nanofibers into 1D hierarchical composites have showed excellent electrochemical properties for energy storage applications, which evidence their potential for application as supercapacitors.

1. INTRODUCTION Since the pioneering work on the pseudocapacitive behavior of MnO2 in 1999 by Lee and Goodenough,1 manganese oxides have attracted particular attention as promising active electrode materials for electrochemical capacitors because of their many advantages such as excellent electrochemical activity, cheap price, facile synthesis, and low toxicity.2 However, the specific capacitance and power characteristics of manganese oxide electrodes are ultimately limited by the high charge-transfer resistance resulting from poor electrical conductivity of manganese oxide.3 Incorporation of electronic conducting polymers into MnO2 is an attractive alternative to alleviate the poor electrical conductivity of manganese oxide; meanwhile, the mechanical stability and flexibility of electrode materials can be enhanced.4 Until now, many efforts have been devoted to incorporate electronic conducting polymers to get mixed conducting polymer−MnO2 composite electrodes.5 Among various desirable morphologies, one-dimensional (1D) nanostructured materials as building components in electrochemical energy storage are more attractive because they provide short diffusion path lengths to ions, leading to high charge/discharge rates.6 Most of the present strategies involve the formation of a 1D conducting polymer−MnO2 composite in confined nanochannels through electrochemical or chemical reduction of KMnO4.7 The introduction and removal of template make this strategy complex and costly.7a,c In addition, MnO2 always appears as a film on surfaces of conducting polymers while MnO2, with size limited to several nanometers with high © 2012 American Chemical Society

Received: April 6, 2012 Revised: July 7, 2012 Published: July 9, 2012 15900

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Figure 1. (a) Schematic illustration for the formation of 1D conducting polymer−MnO2 composites. TEM images of (b) PANI and (c, d) PANI− MnO2 composites. (e) SAED pattern, and EDS maps of (f) Mn and (g) O from a single composite nanofiber in Figure 1d. Scale bars: (b, c) 100 nm; (d, f, g) 20 nm.

2. MATERIALS AND METHODS 2.1. Chemicals. Aniline monomer (Shanghai Chemical Co.) was distilled under reduced pressure. All other reagents were purchased from Aldrich and used without further purification. The water used in this study was deionized by a Milli-Q Plus system (Millipore, France), having 18.2 MΩ electrical resistivity. 2.2. Preparation of Conducting Polymer Nanofibers. (a) The synthesis of PANI nanofibers was performed according to the literature with a rapid mixing strategy.8 In a typical procedure, 0.3 mL of aniline monomer and 0.18 g of ammonium peroxydisulfate were dissolved in two vials containing 10 mL of 1.0 mol L−1 HCl, respectively. The newly prepared solutions were then poured rapidly into a 30 mL glass vial and shaken vigorously for ∼30 s. The mixtures were left still for 2 h. The obtained mixtures were thoroughly purified by deionized water and then excess 0.1 mol L−1 NH 4 OH(aq) for dedoping. After that, products were redispersed in deionized water for characterization and further use. (b) The synthesis of PPY and PEDOT nanofibers was performed according to the literature with an oxidative template.9 In a typical experiment, 0.01 M cetyltrimethylammonium bromide ((C16H33)N(CH3)3Br) (CTAB) is dispersed in 60 mL of 1.0 mol L−1 HCl under an ice bath. After the solution was magnetically stirred for 10 min, 0.03 mol of ammonium peroxydisulfate (APS) was added into the above solution and stirred for 10 min, resulting in reactive template in the form of white precipitates. All solutions were cooled to 0−3 °C. Pyrrole (0.12 mol) or 3,4-ethylenedioxythiophene (EDOT) (0.08 mol) was added into the as-prepared reactive template solution, self-assembly was conducted at 0−3 °C for 24 h, and the resulting black precipitate of PPY or naval blue precipitate PEDOT was thoroughly purified by deionized water and then excess 0.1 mol L−1 NH4OH(aq) for dedoping. After that, products were redispersed in deionized water for characterization and further use. 2.3. Preparation of PANI−MnO2 Composites. In a typical synthesis, KMnO4 aqueous solution (0.01 mol L−1) was mixed with aqueous solution containing PANI nanofibers (2.0 g L−1). The total volume of the reaction system was maintained

at 10 mL, the concentration of PANI nanofibers was set at 0.4 g L−1, and the concentration of KMnO4 ranged from 3.6 to 21.5 mmol L−1. Then the reaction system was maintained under magnetic stirring at room temperature for 1 h. Finally, the products were washed with deionized water and then dried in a vacuum at 60 °C for 24 h. 2.4. Characterization. The morphologies of the products were examined by a scanning electron microscope (SEM, XL30E Philip Co., Holland) and a transmission electron microscope (TEM, Tecnai-12 Philip Apparatus Co., USA), respectively. Samples for SEM measurements were sputtered with gold, whereas those for TEM measurements were deposited on copper grids. The FTIR spectra (Bruker Tensor 27, Germany) were recorded in the range of 400−4000 cm−1. The samples were prepared in pellet form with spectroscopicgrade KBr. The UV−vis spectra (UV-2501, Shimadzu Corporation, Japan) of samples dissolved in dimethylformamide were measured in the range between 260 and 900 nm. Thermal studies were performed using a thermagravimetric analyzer (Pyris 1, PerkinElmer, United States) with approximately 10 mg of each sample that were analyzed in a N2 atmosphere from 30 to 550 °C, and then with the gas switched to oxygen from 550 to 800 °C at 20.0 mL min−1 with a heating rate of 10 °C min−1 to ensure the complete removal of PANI polymer. XPS data were recorded on a Thermo ESCALAB 250 using a nonmonochromatized Al Kα X-ray (1486.6 eV) as the excitation source and choosing C 1s as the reference line. N2 adsorption−desorption measurements were conducted using Thermo Sorptomatic 1990 by N2 physisorption at 77 K. The as-calcined samples were outgassed for 4 h at 250 °C under vacuum (p < 10−2 Pa) in the degas port of the sorption analyzer. The Brunauer-Emmett-Teller (BET) specific surface areas of samples were evaluated using adsorption data in a relative pressure range from 0.05 to 0.25. 2.5. Electrochemical Performance Measurement. The working electrode was prepared by mixing 80 wt % of the synthesized PANI−MnO2 composites and 15 wt % acetylene black and 5 wt % polytetrafluoroethylene (PTFE). Briefly, the resulting paste was pressed on a sheet of nickel foam at 10 MPa. All of the experiments were performed in a three-electrode cell 15901

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Figure 2. TEM images of (a) P−M(42), (b) P−M(47), (c) P−M(58), and (d) P−M(77). All scale bars are 100 nm.

in 0.5 mol L−1 NaNO3 at 25 °C under normal atmosphere. The PANI−MnO2 composites were used as the working electrode, Hg/HgO as the reference electrode, and the Pt sheet as a counter electrode. The cyclic voltammetry (CV) measurements and chronopotentiometry were carried out with an electrochemical workstation (CHI 660C, Chenhua, Shanghai, China). The value of the specific capacitance (CS) was obtained from the charge−discharge cycling measurements according to the following equation:

The schematic illustration for the formation of 1D conducting polymer−MnO2 composites is given in Figure 1a. As-synthesized 1D nanofibers of conducting polymers showed excellent dispersion ability in water,12 which was favorable for heterogeneous reaction. After the addition of KMnO4 to aqueous solution containing conducting polymer nanofibers, the redox reactions between conducting polymers and KMnO4 should occur around surfaces of conducting polymer nanofibers where oxidant and reductant encounter each other. In the initial stage, MnO2 nuclei were inclined to be deposited on surfaces of conducting polymer nanofibers possibly due to the large surface energy of the conducting polymer nanofibers and interactions between conducting polymer nanofibers and MnO2. With reaction ongoing, directional growth along the long axis of 1D conducting polymer nanofibers led to the formation of MnO2 nanorods supported on conducting polymer nanofibers. It is seen that the color of aqueous solution changes to gray within several minutes after the addition of KMnO4 to aqueous solution containing PANI nanofibers, indicating the formation of MnO2 nanoparticles. The typical TEM images of PANI nanofibers before and after loading of MnO2 nanoparticles are shown in parts (b) and (c), respectively, of Figure 1. It is seen that the surfaces of PANI nanofibers before reaction are relatively smooth (Figure 1b), whereas the apparent increase in roughness of PANI nanofibers after reaction (Figure 1c) clearly indicates deposition of MnO2 nanoparticles. It should be noted that almost all PANI nanofibers (∼100%) are evenly supported by MnO2 nanorods with uniform morphology and narrow size distribution. It is interesting to discover in Figure 1d that all MnO2 nanoparticles are nanorods in morphology with diameter of 3 nm and length of 10−20 nm. The corresponding selected area electron diffraction (SAED) pattern in Figure 1e reveals

CS = I /[(dV /dt )m] where I is the discharge current, dv/dt is the potential change rate determined from galvanostatic charge/discharge curve, and m is the mass of electrochemical active materials. The electrochemical impedance spectroscopy (EIS) was measured by an Autolab PGSTAT30 at a frequency ranging from 50 kHz to 10 mHz with a potential amplitude of 10 mV.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Morphology of 1D P−M Hierarchical Composites. It is well-established that conducting polymers possess several intrinsic oxidation states and assynthesized conducting polymer are typically in the intermediate oxidation state between fully oxidized and reduced forms.10 Recent reports including ours have revealed that conducting polymers show reduction ability toward noble metal ions for obtaining conducting polymer-supported noble metal nanoparticle catalysts.11 Inspired by those results, it is interesting to disclose that conducting polymers also show reduction ability toward KMnO4 in aqueous solution at room temperature for obtaining conducting polymer−MnO2 composites. 15902

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Figure 3. XPS spectra for (a) P−M(72) and (b) manganese. (c) FTIR and (d) UV−vis spectra of P−M(72) and PANI nanofibers.

the cystallization of MnO2 nanorods.6a The energy dispersive X-ray spectroscopic (EDS) elemental maps of Mn (Figure 1f) and O (Figure 1g) from a single composite nanofiber in Figure 1d further confirm the uniform deposition of MnO2 nanorods on surfaces of PANI nanofibers. The compositions of the composites are then determined by TG analysis, where the weight ratio of MnO2 can reach as high as 77% for PANI− MnO2 composites (Figure S1). For clarity, PANI−MnO2 composites are specified as P−M(x), where x indicates the weight percentage of MnO2 in composites. It is found that the density and patterning of MnO2 nanorods on surfaces of conducting polymer nanofibers can be tuned by simply changing the concentration of KMnO4. Take PANI− MnO2 composites as an example. As given in Figure 2, the density of MnO2 nanorods increases with KMnO4 concentration at a fixed concentration of PANI nanofibers. As for P− M(42) composites (Figure 2a), individual MnO2 nanorods can be clearly identified, showing the sparse surface decoration. Significant increase in density of MnO2 nanorods is evidenced for P−M(47) (Figure 2b) and P−M(58) composites (Figure 2c) as the white spots on fiber surfaces come from a decreased in contrast. Disappearance of the white spots on fiber surfaces for P−M(77) composites (Figure 2d) displays a dense surface pattern of MnO2 nanorods. In addition, growth of MnO2 nanorods in length and location aside from PANI nanofibers has been identified. However, the diameter of MnO2 nanorods was almost unchanged with KMnO4 concentration. 3.2. Structural Characterization and Formation Mechanism of 1D P−M Hierarchical Composites. The chemical structures of P−M composites were then further confirmed by X-ray photoelectron spectroscopy (XPS) as shown in Figure 3a. The signatures of C and N for PANI nanofibers and Mn and O for MnO2 nanorods are clearly seen. Figure 3b represents the

XPS signature of the Mn 2p3/2 and Mn 2p1/2, which are centered at 642 and 653.7 eV, respectively, with a spin energy separation of 11.7 eV. This is in good agreement with reported data of Mn 2p3/2 and Mn 2p1/2 in MnO2.7a,13 To understand the mechanism of P−M composites, FTIR and UV−vis spectra of PANI nanofibers and P−M(72) composites were conducted. As shown in Figure 3c, the characteristics for PANI, such as N−H stretching vibrations at 3200−3500 cm−1, and CC stretching deformation of quinonoid and benzenoid rings at 1581 and 1494 cm−1, respectively, are clearly seen. The ratio of the relative intensity of the two peaks is about 1.0, which indicates that similar nitrogen quinonoid and benzenoid ring structures exist in the PANI chains.11c,14 As for P−M(72) composites, it is revealed that the relative intensity of CC stretching vibrations of quinonoid to benzenoid rings increases significantly after loading MnO2 nanoparticles, indicating partial benzenoid rings are oxidized into quinonoid rings. Besides, the appearance of a characteristic Mn−O vibrational peak centered at 522 cm−1 and another pronounced peak centered at 1619 cm−1 that attributed to O−H stretching vibrations on a Mn atom4d,15 indicated the presence of MnO2 in composites. The appearance of a peak centered at 657 nm corresponding to the benzenoid to quinonoid excitotic transition indicates the dedoped state of PANI nanofibers. As for the UV−vis spectrum of P−M(72) composites (Figure 3d), an obvious blue shift of peak at 657 to 536 nm indicates the transformation of benzenoid rings into quinonoid rings.16 It is also confirmed that more benzenoid rings are oxidized into quinonoid rings with increasing KMnO4 concentration by examining corresponding FTIR and UV−vis spectra of different P−M(x) composites as given in Figure S2 of the Supporting Information. As a result, it is believed that PANI nanofibers show excellent ability for reducing Mn(VII) 15903

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Figure 4. TEM images of (a) PPY, (c) PEDOT nanofibers, and (b) PPY−MnO2, (d) PEDOT−MnO2 composites. All scale bars are 100 nm.

supercapacitor electrode materials.13a,17 Above all, the influence of MnO2 loading amount on the capacitance property was investigated by galvanostatic charge/discharge (0.2 A g−1). As shown in Figure 5, it is seen that the specific capacitance

into Mn(IV) under mild conditions. Therefore, PANI nanofibers can be regarded as an ideal reactive scaffold for obtaining P−M composites by simply mixing PANI nanofibers and KMnO4 together. The synthetic strategy was insensitive to the raw material concentration; thus, it was easy to reproduce. Because of the excellent dispersion ability of PANI nanofibers, the concentration of stable PANI nanofibers aqueous suspension can reach as high as 0.4 g L−1. As a result, all the reaction systems mentioned were set at a relatively high concentration of PANI nanofibers suspension (0.4 g L−1) for obtaining more products in a single reaction. More importantly, we were able to synthesize as much as 900 mg of uniform P−M(72) composites in a single reaction of 1.0 L with the same morphology as shown in Figure 1c and such 1D P−M(72) composites could easily be redispersed in water. In addition to PANI nanofibers, the strategy is also applicable to synthesizing other 1D conducting polymer−MnO2 composites with well-defined morphology. For example, when using PPY and PEDOT nanofibers instead of PANI nanofibers, MnO2 nanorods evenly supported on surfaces of PPY and PEDOT nanofibers also can be fabricated (Figure 4). As the formation of MnO2 nanoparticles is largely dependent on the intrinsic redox nature of conducting polymer; thus it is anticipated that this strategy is also applicable to conducting polymer−MnO2 composites with other morphologies. Therefore, it is reasonable to believe that this is a general method for conducting polymer−MnO2 composites. 3.3. 1D P−M Hierarchical Composites for Energy Storage. As for the ultrathin MnO2 nanorods of large surface area supported on conducting PANI nanofibers, P−M composites are expected to show excellent performance as

Figure 5. Influence of MnO2 loading amount on the specific capacitance of P−M composites.

increases from 93 to 417 F g−1 with increasing MnO2 weight percentage from 42% to 72%. This may be related to the thin surface wrapping of MnO2 nanorods on nanofiber surface (Figure 1c and Figure 2a−c). The BET results have confirmed a significant increase in surface areas after loading MnO2 nanorods on PANI surfaces. As for dedoped PANI nanofibers, the BET surface area is 58.1 m2/g, which is comparable to the value reported.12f However, the BET surface area increases as high as 256.4 m2/g for P−M(72) composites. The increased surface area coming from surface patterning will be helpful for 15904

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is a powerful tool for the mechanism analysis of an interfacial process and the evaluation of rate constant, ionic and electronic conductivity, and double-layer capacitance, etc. The Nyquist plots of PANI nanofibers and P−M(72) composites (Figure 7c) show an inconspicuous semicircle which is related to the charge-transfer resistance from the interface structure between the electrode surface and electrolyte. From the point intersecting with the x-axis in the range of high frequency, the internal resistance can be evaluated. The internal resistance includes the ionic resistance of electrolyte, the intrinsic resistance of the active material, and the contact resistance at the interface of active material and current collector.21 This value is comparable to that of normally obtained PANI, but higher than that of PANI when conducted in acidic solution.22 As for P−M(72) composites, a considerable increase in internal resistance is observed due to the incorporation of MnO2 in PANI. It is reported that the most conductive form for PAN should have similar quinoid and benzoid rings as the π electrons delocalized the most.11e It is revealed in Figure 3c that PANI had similar quinoid and benzoid rings whereas P−M(72) composites had more quinoid rings. As a result, the deviation from the most conductive state for PANI in P−M(72) composites should also contribute to the increase in internal resistance. The intrinsic state of PANI in P−M(72) composites may be altered through redox chemistry23 to optimize the structure for further improvement in conductivity, which will be our continuing interest. The Nyquist plot of P−M(72) composite electrode represents a well-defined frequencydependent semicircle impedance curve over high frequencies followed by a straight line, but the PANI electrode has no defined semicircle. From Nyquist plots, it is seen that P−M(72) composites show a higher charge-transfer resistance. However, a line almost vertical to the y-axis in low-frequency region is observed, further indicating ideal capacitive behavior P−M(72) composites.24 The cyclability is another important quality required for application in supercapacitors. Figure 7d shows the specific capacitance variation for P−M(72) composites as a function of cycle number at a current density of 1.0 A g−1 within a voltage range between 0 and 1.0 V. It is not surprising that the PANI nanofibers suffer from a limited long-term stability during cycling because the swelling and shrinking of the polymer may lead to degradation.19 However, P−M(72) composites exhibit a much higher stability than PANI nanofibers. As seen in Figure 7d, the capacitance shows about 8.6% loss after 1000 consecutive cycles. This demonstrates that the charge and discharge process do not induce any remarkable change in structure of P−M(72) composites.25

the P−M composites to display enhanced capacitances due to the facilitation of better charge transfer at the interface of electrode material and supporting electrolyte. However, further increase in MnO2 loading amount leads to decrease in the specific capacitance. It is observed that a thin layer of MnO2 nanorods is evenly supported on PANI nanofibers and the density of nanorods increases with an increasing loading amount of MnO2 ranging from 42% to 72%. Further increase in MnO2 weight percentage to 77% leads to interconnected MnO2 nanorods on PANI nanofiber surfaces with thicker surface coating (Figure 2d), which results in a decrease in the specific capacitance.2a,18 Furthermore, random MnO2 nanorods aside from PANI nanofiber also can be observed. Those random connected MnO2 nanorods were not fully used for energy storage and might act as artifacts to lower the capacitance property of P−M composite.19 To further confirm the merits of P−M composites as a supercapacitor electrode, the electrochemical properties of P− M(72) composites were characterized by CV, galvanostatic charge−discharge, EIS, and cycling life measurement. Both CV curves of P−M(72) composite and PANI nanofibers electrodes show the typical rectangular shape of pseudocapacitive behavior (Figure 6). However, the inclusion of MnO2 nanorods has resulted in the capacitive current increasing almost eightfold.

Figure 6. CV curves of PANI nanofibers and P−M(72) composites within the potential window 0−1.0 V vs SCE at a scan rate of 50 mV/ s.

Galvanostatic charge−discharge curves (at 1.0 A g−1) of P− M(72) composites in Figure 7a showed a capacitive behavior with almost symmetric charge−discharge curves.6a Moreover, the small deviation to linearity is typical of a pseudocapacitive contribution, which shows that the capacitances of P−M(72) composites mainly originate from pseudocapacitance. The curve of the supercapacitor with PANI nanofibers is similar to that of the supercapacitor with P−M(72) composites. However, its “IR drop” is much lower that of the P−M(72) supercapacitor. This confirms the lower contact resistance in the composite. Therefore, it is believed that P−M(72) composites are more suitable for fabrication of safe and powerful supercapacitor compared with PANI nanofibers.20 The specific capacitances of both P−M(72) composites and PANI nanofibers decrease with increasing discharge current densities, but the capacitance of P−M(72) composites is always higher than that of PANI nanofibers in all current densities (Figure 7b). The specific capacitance of P−M(72) composites is 417 F g−1 at a discharge current of 0.2 A g−1 and can remain at 119 F g−1 even at a discharge current density of 3 A g−1. EIS

4. SUMMARY In summary, ultrathin MnO2 nanorod (3 nm in diameter) arrays grown on PANI nanofibers have been fabricated by a facile, reproducible, and scalable strategy merely using PANI nanofibers and KMnO4 as raw materials. Loading amount and patterning of MnO2 nanorods on surfaces of PANI nanofibers can be altered by simply changing the concentration of KMnO4. Mechanistic studies revealed that PANI showed excellent reducing ability toward KMnO4 for making nanosized MnO2 supported on surfaces of PANI nanofibers. This synthetic route will be instructive for designed synthesis of other conducting polymer−manganese oxide nanocomposites. Moreover, the combination of MnO2 and PANI into 1D nanostructures show excellent electrochemical properties for 15905

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Figure 7. (a) Charge−discharging curves of PANI nanofibers and P−M(72) composites; (b) specific capacitance of PANI nanofibers and P−M(72) composites at different current densities; (c) Nyquist plots of PANI nanofibers and P−M(72) composites with a frequency range from 50 kHz to 10 mHz; (d) stability of PANI nanofibers and P−M(72) composites. Cheng, L.; He, P.; Wang, C. X.; Xia, Y. Y. Adv. Mater. 2008, 20, 2166− 2170. (c) Yuan, C.; Su, L.; Gao, B.; Zhang, X. Electrochim. Acta 2008, 53, 7039−7047. (d) Zhang, X.; Ji, L.; Zhang, S.; Yang, W. J. Power Sources 2007, 173, 1017−1023. (e) McEvoy, T. M.; Long, J. W.; Smith, T. J.; Stevenson, K. J. Langmuir 2006, 22, 4462−4466. (f) Long, J. W.; Rhodes, C. P.; Young, A. L.; Rolison, D. R. Nano Lett. 2003, 3, 1155−1161. (5) (a) Sharma, R. K.; Karakoti, A.; Seal, S.; Zhai, L. J. Power Sources 2010, 195, 1256−1262. (b) Sivakkumar, S. R.; Ko, J. M.; Kim, D. Y.; Kim, B. C.; Wallace, G. G. Electrochim. Acta 2007, 52, 7377−7385. (6) (a) Hu, C. C.; Chang, K. H.; Lin, M. C.; Wu, Y. T. Nano Lett. 2006, 6, 2690−2695. (b) Arico, A. S.; Bruce, P.; Scrosati, B.; Tarascon, J. M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366−377. (7) (a) Liu, R.; Duay, J.; Lee, S. B. ACS Nano 2010, 4, 4299−4307. (b) Sharma, R. K.; Zhai, L. Electrochim. Acta 2009, 54, 7148−7155. (c) Liu, R.; Lee, S. B. J. Am. Chem. Soc. 2008, 130, 2942−2943. (d) Rios, E. C.; Rosario, A. V.; Mello, R. M. Q.; Micaroni, L. J. Power Sources 2007, 163, 1137−1142. (8) Huang, J.; Kaner, R. B. Angew. Chem., Int. Ed. 2004, 43, 5817− 5821. (9) Liu, Z.; Zhang, X.; Poyraz, S.; Surwade, S. P.; Manohar, S. K. J. Am. Chem. Soc. 2010, 132, 13158−13159. (10) (a) Tran, H. D.; D’Arcy, J. M.; Wang, Y.; Beltramo, P. J.; Strong, V. A.; Kaner, R. B. J. Mater. Chem. 2011, 21, 3534−3550. (b) MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581−2590. (11) (a) Han, J.; Wang, L.; Guo, R. J. Mater. Chem. 2012, 22, 5932− 5935. (b) Han, J.; Wang, L.; Guo, R. Macromol. Rapid Commun. 2011, 32, 729−735. (c) Han, J.; Li, L.; Guo, R. Macromolecules 2010, 43, 10636−10644. (d) Han, J.; Liu, Y.; Guo, R. Adv. Funct. Mater. 2009, 19, 1112−1117. (e) Han, J.; Liu, Y.; Li, L.; Guo, R. Langmuir 2009, 25, 11054−11060. (f) Guo, S.; Dong, S.; Wang, E. Small 2009, 5, 1869− 1876. (g) Gallon, B. J.; Kojima, R. W.; Kaner, R. B.; Diaconescu, P. L. Angew. Chem., Int. Ed. 2007, 46, 7251−7254. (h) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 6, 1077−1080.

energy storage applications, which evidence their potential for application as supercapacitors.

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ASSOCIATED CONTENT

S Supporting Information *

TG, FTIR, and UV−vis spectra of PANI nanofibres and P−M composites. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding is acknowledged from the National Natural Scientific Foundation of China (No. 20903079 and 21073156) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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REFERENCES

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dx.doi.org/10.1021/jp303324x | J. Phys. Chem. C 2012, 116, 15900−15907