Article pubs.acs.org/Langmuir
Polyaniline Nanowire Arrays Aligned on Nitrogen-Doped Carbon Fabric for High-Performance Flexible Supercapacitors Pingping Yu, Yingzhi Li, Xinyi Yu, Xin Zhao, Lihao Wu, and Qinghua Zhang* State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, People’s Republic of China S Supporting Information *
ABSTRACT: A combination of vertical polyaniline (PANI) nanowire arrays and nitrogen plasma etched carbon fiber cloths (eCFC) was fabricated to create 3D nanostructured PANI/eCFC composites. The small size of the highly ordered PANI nanowires can greatly reduce the scale of the diffusion length, allowing for the improved utilization of electrode materials. A two-electrode flexible supercapacitor based on PANI/eCFC demonstrates a high specific capacitance (1035 F g−1 at a current density of 1 A g−1), good rate capability (88% capacity retention at 8 A g−1), and long-term cycle life (10% capacity loss after 5000 cycles). The lightweight, low-cost, flexible composites are promising candidates for use in energy storage device applications.
supercapacitors.18 To fabricate flexible supercapacitors with higher specific capacitance and longer cycle life, researchers are developing flexible synergistic composite materials. A flexible electrode of PANI/SWCNT/cloth synthesized by Wang et al. exhibited improved performance compared to SWCNT/cloth and PANI/cloth electrodes.19 Yan et al. demonstrated that composites of amorphous polyaniline nanoparticles coated on CNF paper retain the pristine flexibility of CNF paper and display remarkably improved electrochemical performance.20 He et al. reported the fabrication of a flexible PANI/CNF composite with long, ordered and needle-like PANI nanowires grown on CNF paper which exhibited a high specific capacitance of 497 F g−1 and a high Coulombic efficiency of 95% at a high current density of 50 A g−1.12 These results demonstrate that flexible composites combining the advantages of PANI (large capacitance) and CNF (high conductivity and flexibility) exhibit good electrochemical performance. However, the smooth surfaces of CNF restrict the absorption of aniline monomers, leading to unsatisfactory PANI morphologies in composites. Thus, creating nanostructured composites that simultaneously exhibit excellent electrochemical performance and high conductivity is crucial for the successful application of these materials in flexible supercapacitors. In this work, we describe the performance of a flexible supercapacitor based on a composite electrode of ordered conducting PANI nanowire arrays coated on nitrogen plasma etched carbon fiber cloth (eCFC), as shown in Scheme 1. Due to its high conductivity, chemical stability, highly porous 3D structure, and cost effectiveness, CFC was selected as the current collector and mechanical supporter. CFC, with its 3D macroporous structure that facilitates ion transport by
1. INTRODUCTION The development of low-cost, lightweight, high power, flexible electrochemical energy storage devices has attracted great attention due to the rapidly increasing energy consumption of modern society.1−3 Among energy storage devices, flexible supercapacitors have been intensively used because of their long cycle life, moderate energy density, and high power density performance.4,5 To date, flexible supercapacitors (SCs) have aroused broad interest due to their potential applications in portable electronic devices, hybrid electric vehicles, and medical devices.6,7 The vital component of a flexible supercapacitor is a binder-free electrode with a favorable mechanical strength and large capacitance. Therefore, the majority of recent research on supercapacitors has focused on the development of redox-active materials with high specific capacitance and good stability for pseudocapacitors.8−10 Flexible supercapacitors based on carbon materials mainly use free-standing carbon nanotube (CNT) films,11 carbon fiber (CNF),12 graphene oxide, and graphene nanosheet papers.13,14 However, the use of these individual carbon materials in highperformance flexible supercapacitors is limited by several factors, including the high cost of CNTs, the unavoidable aggregation of graphene layers, and the unsatisfactory performance of carbon fibers. Carbon materials store energy through charge accumulation at the electrode/electrolyte interfaces, which lead to a long cycle life (>105 cycles) and relatively low specific capacitance. 15,16 In contrast, metal oxide and conductive polymers store energy through their fast and reversible redox reactions at the electrode surface.17 Pseudocapacitors have higher specific capacitance but suffer from poor electronic conductivity as well as limited cycle life. Conducting polymers such as polyaniline (PANI) are among the most promising materials for flexible electronics due to their low cost, facile synthesis, and high pseudocapacitance. However, PANI exhibits poor stability, limiting its practical utility in © XXXX American Chemical Society
Received: July 1, 2013 Revised: August 24, 2013
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2.4. Electrical and Electrochemical Measurements. The flexible supercapacitors were assembled by sandwiching a separator between two electrodes with identical surface areas. The assembly was then encapsulated by a flexible polyethylene terephthalate (PET) film to produce a symmetric supercapacitor structure without using conductive additives or binders. Two Pt foils were used as the current collectors to form good contact with an electrochemical measurement instrument. The flexible electrodes were immersed in the electrolyte (1 M H2SO4 aqueous solution) for 30 min before assembly. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge measurements were conducted by the CHI660D electrochemical working station with the two electrode system. The PANI/eCFC5 and PANI/CFC5 electrodes were all assembled and measured by this method. The PANI nanowire electrodes were prepared by mixing PANI nanowire powder, carbon black (CB), and poly (tetrafluoroethylene) emulsion with a mass ratio of 85:10:5. A small amount of ethanol was added to obtain the slurry, which was subsequently pressed on a titanium sheet and dried at 95 °C for 12 h. The two pieces of titanium sheet were encapsulated in accordance with composite supercapacitors.
Scheme 1. Procedure to Fabricate the 3D Nanostructure Flexible Compositesa
a
(a) Unetched-CFC, (b) eCFC prepared by the nitrogen plasma etching for 2 min, and (c) PANI/eCFC composite prepared by the in situ polymerization of PANI nanowire arrays.
providing a smaller resistance and shorter diffusion pathways, is strongly recommended for the fabrication of advanced supercapacitors.21 The nitrogen plasma etching process can improve the surface activity and interactions between aniline monomers and eCFC. Moreover, a uniform distribution of PANI nanowire arrays reduces the ion diffusion length. The PANI/eCFC composite electrodes are binder-free electrodes that reduce interfacial resistance and enhance the electrochemical reaction rate. The supercapacitor based on PANI/ eCFC composite electrode possesses a high specific capacitance, excellent rate capacity, and cycling stability.
3. RESULTS AND DISCUSSION 3.1. Microstructure and Morphology. Carbon fiber cloths have been used as unique support materials for the development of wearable electronics due to their mechanical strength and flexibility, low mass, and modifiable surface.22 They usually display a hierarchical structure with smooth surface morphology and high microscopic porosity. XPS spectra of CFC and eCFC are shown in Figure 1, the peak at 284.8,
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Aniline (AN, CP) was distilled under reduced pressure and stored in a dark room before use. Ammonium persulfate (APS, AR) and sulfuric acid (98%) were purchased from Shanghai Chemical Co. and used without further purification. All solutions were prepared using deionized water. 2.2. Preparation of PANI/eCFC Composites. The cold plasma treatment was performed in a plasma chemical vapor deposition apparatus. The CFC was exposed to nitrogen/oxygen gas generated by a pulsed power supply (160 W) for 2 min without destroying the mechanical properties. PANI/eCFC composites were prepared by the in situ polymerization of aniline. The polymerization was carried out in an ice-water bath at a temperature of −5 °C. Aniline was added to 40 mL of 1 M H2SO4 aqueous solution, and eCFC or pristine CFC (1 × 1 cm2) was immersed into the above solution and then stirred for 10 min to ensure complete dispersion of the aniline. Another 40 mL of 1 M H2SO4 cold aqueous solution containing APS was rapidly added and stirred for 30 s. The molar ratio of AN to APS was 4:1. After reacting for 24 h, the PANI/eCFC composite was removed and washed with deionized water several times. A dark green layer was formed on the surface of the eCFC or pristine CFC. The composites were dried under vacuum at 40 °C for 10 h. A series of PANI/eCFC electrodes were fabricated from solutions with different aniline concentrations (CAN) of 0.03 M, 0.04 M, 0.05 M, 0.06 M, and 0.07 M. The as-prepared PANI/eCFC and composites were termed as PANI/eCFC3, PANI/eCFC4, PANI/eCFC5, PANI/eCFC6, and PANI/eCFC7, respectively. The weight of the PANI adsorbed on the cloth was calculated by the weight difference before and after the polymerization process. 2.3. Structural and Morphological Characterization. Field emission scanning electron microscopy (FESEM, Hitachi S-8010) was employed to characterize the morphology of the samples. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR spectrometer. Spectra in the range of 4000−400 cm−1 were collected by averaging 32 scans at a resolution of 4 cm−1. Raman spectra were recorded using a LabRam-1B Ramen spectroscope with He−Ne laser excitation at 632.8 nm and scanning for 50 s.
Figure 1. (A) XPS spectra of CFC and eCFC and (B) O 1s core level XPS spectrum of CFC.
398.8, and 531.9 eV (Figure 1A) can be assigned to C 1s, N 1s, and O 1s, respectively.23 Nitrogen elements were introduced to the surface of CFC as indicated by the N 1s signal. The O content increased significantly, consistent with the FTIR result that more −COOH groups were attached on the substrate. For pristine CFC, the deconvoluted three peaks at 531.5, 532.8, and 533.7 eV, can be assigned to CO, C−O, and O−H. It indicated that a few amount of functional groups such as −COOH also exist on the surface of CFC.24 By the plasma doping process, nitrogen atoms are expected to replace carbon atoms in the original CFC and form three types of Nconfigurations: pyridine N (N6), pyrrole N (N5), and graphite type N (NQ) (Figure 2A,B).25,26 For N-doped CFC, the peak at 284.8 eV is still assigned to the graphite type sp2 C, but the peak at 285.3 and 286.7 eV reflect the different bonding structures of the C−N, corresponding to N-sp2 C and N-sp3 C bonds (Figure 2C).27 In addition, the O 1s peak arises from two parts; one is due to the oxygen atoms existing at the carbon fiber defects, and other is ascribed to water adsorbed on the carbon fiber surface. There are three main peaks corresponding to CO (531.5 eV), C−O (532.1 eV), and O−H (533.7 eV).28 B
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Figure 2. (A) N 1s, (C) C 1s, and (D) O 1s XPS spectra of eCFC. Three types of nitrogen species in nitrogen-doped eCFC (B).
Figure 3A shows the smooth surface morphology of the original carbon fiber, while the eCFC shows comparatively Figure 4. FESEM images of different concentrations of PANI nanowires coating on eCFC: (A) pure PANI nanowires, (B) PANI/ eCFC3, (C) PANI/eCFC4, (D) PANI/eCFC5, (E) PANI/eCFC6, and (F) PANI/eCFC7.
thickness of PANI coating. Lower concentrations of aniline (0.03 and 0.04 M) result in sparse and short nanowires, as shown in Figure 4B,C. In contrast, self-nucleation of PANI nanowires is unavoidable at higher concentrations of aniline (0.06 and 0.07 M), resulting in both random connected PANI nanowires and aligned PANI nanowires (Figure 4E,F). During the polymerization of PANI nanowires, aniline monomers are adsorbed on surfaces of the eCFC via the π−π conjugation and chemical bonding effects. The adsorbed aniline molecules act as nucleation sites and react with adjacent aniline molecules to form PANI nanowires. The diameter of the PANI nanowires does not increase notably due to a low polymerization rate. However, a large number of dissociated PANI nuclei are adsorbed onto the CFC, leading to randomly connected PANI nanowires. The sheet resistance of CFC and eCFC is 3.83 and 3 Ω sq−1, which is much lower than SWCNT/cloth (60 Ω sq−1).19 After the in situ deposition of PANI nanowires, the loading weight of PANI in PANI/eCFC5 and PANI/CFC5 is ∼0.58 and 0.36 mg cm−2, respectively. The sheet resistance (measured by four point probe instrument) of PANI/CFC5 and PANI/eCFC5 is 108 and 118 Ω sq−1, respectively. FTIR spectra in Figure 5A display typical conducting state PANI vibrations. The bands at 1560 and 1479 cm−1 are
Figure 3. SEM images of the carbon fiber cloth before (A) and after (B) plasma-etching.
rough surfaces (Figure 3B). As-prepared CFC with its 3D ordered macroporous structure (inset of Figure 3A) was exposed to nitrogen/oxygen gas for 2 min to improve its surface wetability and electronic conductivity.29 This can be introduced chemically active sites into the carbon network by nitrogen doping. Figure 4A shows a typical SEM image of pure PANI nanowires with a one-dimensional nanostructure. The PANI nanowires were collected in the aqueous phase of the PANI/eCFC composite solution. Nitrogen doping of CFC can promote faster nucleation and growth kinetics of catalyst nanoparticles leading to their small size and uniform dispersion. Therefore, a layer of aligned PANI nanowires grown on the surface of the eCFC substrate was produced by the in situ polymerization of aniline monomers in H2SO4 aqueous solution (Figure 4D). Compared to PANI/CFC5 with less ordered PANI nanowires (Figure S1 of the Supporting Information, SI), vertically aligned PANI nanowires in PANI/ eCFC5 with a 50 nm average diameter approximately 150 nm in length are clearly observed, as shown in the inset image of Figure 4D. Small diameter PANI nanowires can greatly reduce the diffusion lengths and ensure increased utilization of electrode materials. The morphology of the PANI nanowires deposited on eCFCs can be controlled by varying the concentrations of the aniline monomer. The changes in morphologies of the PANI/ eCFC composites for samples prepared with 0.03 to 0.07 M aniline appear in Figure 4(B−F). For a fixed area of CFC (1 × 1 cm2), the optimized concentration of aniline is 0.05 M because of the uniform PANI nanowire arrays and reasonable
Figure 5. FTIR (A) and Raman (B) spectra of (a) CFC, (b)eCFC, (c) PANI, (d) PANI/CFC, and (e) PANI/eCFC. C
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Figure 6. Electrochemical characteristics of (a) CFC, (b) nitrogen-doped eCFC, (c) PANI, (d) PANI/CFC5, and (e) PANI/eCFC5 electrodes for the flexible supercapacitor in 1 M H2SO4 aqueous solution. (A) Schematic structure of our flexible supercapacitors consisting of two symmetrical 3D flexible composite electrodes, a polymer separator, and two PET membranes. The digital photograph shows the flexible supercapacitor. (B) and (C) CV curves of (a, b) and (c−e) at a scan rate of 10 mV s−1, respectively. (D) Galvanostatic charge/discharge measurements at 1 A g−1. (E) Specific capacitance plots at different current densities from 1 A g−1 to 8 A g−1.
3.2. Electrochemical Performance. To explore the advantages of the PANI/eCFC composites as active supercapacitor electrodes compared to PANI and PANI/CFC, supercapacitors based on PANI/eCFC5, PANI/CFC5, and PANI were systematically analyzed. A flexible supercapacitor was assembled by two electrodes with a separator, as shown in Figure 6A. The measurements of CV, EIS, and galvanostatic charge/discharge were tested by the two electrode system. Figure 6B shows the CVs of CFC and eCFC devices collected at 10 mV s−1. It can be seen that there is no remarkable peaks in all of the curves, and the nearly rectangular shape indicate an ideal double layer capacitor behaviors. Obviously, the eCFC exhibits a much higher current density than that of CFC, indicating a greatly enhanced capacitance. Such electrochemical properties suggest that the etching process improves the surface activities and interactions between aniline monomers and eCFC. In contrast, the CVs in Figure 6C of PANI/eCFC5 and PANI/CFC5 devices show a quasi-rectangle shape with two couples of peaks, indicating a combination of electrical doublelayer capacitance (EDLC) and pseudocapacitance originated from the redox reactions of PANI. The two pairs of redox peaks, O1/R1 and O2/R2, are attributed to the redox transitions of PANI between a semiconductive state (leucoemeraldine form) and a conducting state (polaronic emeraldine form) and
assigned to CC stretching vibrations of the quinoid and benzenoid rings, respectively. Bands from aromatic C−N, C N, and C−H stretching vibrations at 1298, 1241, and 1142 cm−1 can be recognized clearly, respectively.30,31 After nitrogen/oxygen plasma treatment, several additional bands appear in the spectrum of eCFC. The bands at 1638 and 1045 cm−1 are attributed to N−H deformation and bending vibration of pyrrole N. The bands at 1369 and 1317 cm−1 are assigned to be the stretching vibration of C−N of N-sp2 C and N-sp3 C bonds. Figure 5B shows the Raman spectra of (a) CFC, (b) eCFC, (c) PANI, (d) PANI/CFC, and (e) PANI/eCFC. There are two prominent peaks at 1340 and 1600 cm−1 in (a) and (b) spectra, corresponding to the well-documented D and G bands, respectively.32 The increase in the intensity of D band in eCFC confirmed the nitrogen doping, creating more defects sites on the CFC.33,34 These sites are favorable as they serve as anchoring sites for aniline monomers on subsequent polymerization.35 By comparison, the Raman spectra of (c−e) composites presents five new representative peaks attributed to PANI: the C−H bending of the quinoid ring at 1171 cm−1, the C−H bending of the benzenoid ring at 1253 cm−1, C−N+ stretching at 1337 cm−1, and C−C stretching of the benzene ring at 1502 and 1589 cm−1. The spectra of the PANI/eCFC composites corresponding to PANI in the conducting state.36 D
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Figure 7. (A) Nyquist plots of (a) CFC, (b) nitrogen-doped eCFC, (c) PANI, (d) PANI/CFC5 and (e) PANI/eCFC5 SCs at a frequency range from 0.1 MkHz to 10 mHz. The inset shows the high frequency part of the Nyquist plots. (B) Ragone plots of (a)−(e) SCs. (C) Capacitance retention of symmetric supercapacitor based on (a)−(e) PANI/eCFC5 electrodes. (D) and (E) FESEM images of PANI/CFC5 and PANI/eCFC5 electrodes after 5000 cycles, respectively.
nanostructure and high conductivity facilitate the utilization of PANI in the PANI/eCFC5 electrode and result in a larger specific capacitance. Additionally, along with the increase of scan rate, the CV curves nearly retain the same shape even at high scan rate of 500 mV s−1 (Figure S2A of the SI), indicating a good rate capability of our flexible electrode, which also can be proven from the similar charge/discharge plots at different current densities (Figure S2B of the SI). The specific capacitance of PANI/eCFC is larger than that of PANI nanowires at all current densities. The PANI/eCFC5 electrode maintains a high capacitance of 915 F g−1 at a current density of 8 A g−1, with 88% retention of its initial specific capacitance, higher than that of PANI/CFC5 and PANI (78% and 62%), showing a high rate capability performance. The improved rate capability of PANI/eCFC5 indicate that nanoscale diameter of PANI grown on the surface of eCFC provide a fast and short ion diffusion pathway. To further understand the superior power performance of these PANI/eCFC5 SCs, the kinetic features of the ion diffusions in the flexible electrode were investigated using electrochemical impedance spectroscopy at an equilibrium open circuit potential. Figure 7A shows the Nyquist plot obtained at a frequency range from 0.01 to 105 Hz, with an expanded view of the high frequency region in the inset. The CFC and eCFC plot do not show a semicircle response at the high-frequency range, suggesting that the interfacial charge transfer resistance of both CFC and eCFC is due to the high conductivity. In contrast, the plots of PANI, PANI/CFC5 and PANI/eCFC5 SCs display a semicircle at high frequencies, followed by a transition to linearity at low frequency. From the magnified data in the high frequency range, the equivalent series resistance (ESR) of PANI/eCFC5 and PANI/CFC5 obtained from the intersection of the Nyquist plot at the x-axis
redox pairs such as p-benzo/hydroquinone and p-aminophenol/benzoquinoneimine, respectively.37,38 Moreover, the PANI/eCFC5 electrode shows a much larger current density response with a larger curve area than that of pure PANI, indicating that the introduction of eCFC improves their capacities efficiently. The aligned PANI nanowires conformably adhere to the rough surface of each etched carbon fiber, maximizing the surface contact areas between the PANI nanowires and the flexible cloth, leading to a better ion access. Therefore, PANI/eCFC5 has the largest capacitance compare to that of PANI/CFC and PANI. This higher capacitance is also confirmed by the increased discharge time from charge/ discharge curves in Figure 6D. The total gravimetric specific capacitance of various composites was calculated from the discharge curves (calculated equations are shown in the SI). The triangle-shaped curves of galvanostatic charge/discharge demonstrate a typical behavior of supercapacitors; nitrogendoped eCFC shows a longer discharging time compared with that of CFC. PANI/eCFC5 has a smaller IR drop than that of PANI/CFC5 and PANI, which ensures a low internal resistance. As shown in Figure 6E, the specific capacitance of PANI, PANI/CFC5 electrodes is 501, 800 F g−1 at 1 A g−1, respectively. The highest value for PANI/eCFC5 achieved is 1035 F g−1 at 1 A g−1, exceeding those previously reported systems based on PANI/CFC (408 − 800 F g−1).12,20,24,39 The improved capacitance is due to the pseudocapacitance of the materials and their defined structures. PANI nanowires with small diameters (∼ 50 nm) can further increase the exposed electrochemical surface areas in the electrolyte and assist in the effective use of the active materials. Moreover, the threedimensional networks of the eCFC can serve as current collectors in PANI/eCFC and form a conductive network under the PANI layers. The synergistic effects of the ordered E
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is 1 Ω and 2.3 Ω, respectively. This suggests that the PANI/ eCFC5 electrode has a very low resistance with a good ion response at high frequency ranges. The Warburg resistance of PANI/eCFC5 is much lower, indicating a good accessibility of the electrolyte ions to the PANI nanowires. The PANI/eCFC5 electrode exhibits a shorter ion diffusion path length in the capacitive part of the Nyquist plot. The advantages of the PANI/eCFC5 composite electrode over the PANI/CFC5 are clearly demonstrated. The energy and power density of our flexible SCs were then depicted by Ragone plots (described in the SI) in Figure 7B. The energy density of PANI/eCFC5 can approach 22.9 Wh kg−1 at a power of 9.8 kW kg−1. Even at a high power density of 36.5 kW kg−1, the energy is maintained at approximately 20.3 Wh kg−1. These values are higher than that of reported selfdoped polyaniline/functionalized carbon cloth (9.3 Wh kg−1 at a power of 1 kW kg−1),24 on flexible SCs using PANI/CNT composite (0.5 Wh kg−1 at a power of 0.3 kW kg−1)40 and PANI/graphene SC (15.5 Wh kg−1 at a power of 6 kW kg−1).41 At the same power densities, the PANI/eCFC5 SC shows a larger energy density than that of the PANI/CFC5 SC and PANI SC. In addition, Figure 7C shows the cyclic stability of our devices at a constant current density of 1 A g−1. Initially, there is a sharp decrease in the specific capacitance of the pure PANI during the first 1000 cycles, followed by stabilization of the capacitance at 68% of the initial value. However, the capacitance of the PANI/eCFC5 SC with highly ordered PANI nanowire arrays nearly completely stabilizes after 1500 cycles and retains 90% of its initial capacitance after 5000 cycles, indicating both high rate performance and long-term stability. The result can be attributed to the swelling, shrinking, and overoxidation of the polymer lead to mechanical degradation of PANI.17,42 When compare to conventional preparation methods for PANI based on composite electrodes that mix active materials with insulating binder materials such as poly (tetrafluoroethylene) (PTFE) or poly (vinylidene fluoride) (PVDF), and conductive additives such as carbon black,15,43−45 the 3D conductive flexible SC can improve the electrical conductivity and the stability of electrodes. In fact, we characterize the morphology and structure of our hybrid electrodes after 5000 cycles and find that the morphology of PANI nanowires and the structural integrity of PANI/eCFC5 are well-maintained, as shown in Figure 7E. However, PANI nanowires in PANI/CFC5 have been disordered and became flat particles, as shown in Figure 7D. The morphology difference between PANI/eCFC5 and PANI/CFC5 composites after running 5000 cycles is mainly due to the different structures of CFC substrate before and after etching, as shown in Figure 3. The nitrogen-doped eCFC shows a rough surface and more hydrophilic compared to that of CFC, leading to uniform PANI nanowires arrays deposited on its surface. Such unique features also can effectively restrict the large volume changes of PANI during the doping/dedoping processes. In addition, the smooth surface of original CFC has less limitation to it. Therefore, the PANI nanowire arrays grown on eCFC can maintain its 1D structure after cycling test and exhibit a better stability. To test the feasibility for application as a flexible energy storage device, a PANI/eCFC5 SC was placed under constant mechanical stress, and its performance was analyzed. As shown in the inset photographs of Figure 8, the device has excellent flexibility and good mechanical properties and can undergo blending at various angles. More importantly, the CV curves of
Figure 8. CV curves of PANI/eCFC5 SC at a scan rate of 10 mV s−1 in different bending states. The inset shows photographs of bending the device at 0°, 60°, and 90°.
the device have no significant changes under various blending angles, further confirming that it has remarkable mechanical flexibility. This performance durability can be attributed to the high mechanical flexibility of the electrodes and the strong connection between the nitrogen-doped eCFC and PANI arrays, indicating our SCs may be ideal for flexible electronics.
4. CONCLUSIONS In summary, high performance flexible supercapacitors are successfully designed and assembled based on novel PANI/ eCFC composite electrodes. The highly ordered PANI nanowires on nitrogen plasma etched CFCs are synthesized via in situ polymerization of aniline. The nitrogen-doped eCFC with functional groups such as carboxylic acids and pyrrole N on the surface can better adsorb the aniline monomer unit by electrostatic interactions via interface diffusion leading to uniform PANI nanowires, compared to CFC. These flexible SCs can simplify the electrode manufacturing process by eliminating the need for conducting additives and binders. The resultant 3D PANI/eCFC5 composite architecture facilitates the transport of both electrolyte ions and electrons to the electrode surface. The uniform size and ordering of the PANI nanowire arrays in PANI/eCFC5 enhances the utilization of the pseudocapacitive materials, endowing the composite with a high specific capacitance, good rate capability, and long-term life. Therefore, these flexible and low-cost nanostructured composite supercapacitor electrodes are a promising candidate for use in energy storage systems.
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ASSOCIATED CONTENT
S Supporting Information *
SEM image of PANI/CFC5. CV curves and Galvanostatic charge/discharge plots of the PANI/eCFC5 electrode. Calculation equations of PANI/eCFC composites. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: 0086-21-67792854; E-mail: qhzhang@dhu. edu.cn. Notes
The authors declare no competing financial interest. F
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ACKNOWLEDGMENTS We thank Prof. Yonggen Lv for offering carbon fiber cloths. Financial support of this work is provided by SRFDP (20110075110009), 111 Project (111-2-04) and Innovation Funds for PhD Students of Donghua University (12D10630).
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