Graphene-Wrapped Polyaniline Nanowire Arrays on Nitrogen-Doped

Apr 24, 2014 - 1. INTRODUCTION. Supercapacitors (SCs) are attractive energy storage devices in flexible and wearable electronics, owing to their high ...
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Graphene-Wrapped Polyaniline Nanowire Arrays on Nitrogen-Doped Carbon Fabric as Novel Flexible Hybrid Electrode Materials for HighPerformance Supercapacitor Pingping Yu, Yingzhi Li, 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: We report the synthesis of reduced graphene oxide (RGO) sheet wrapped polyaniline (PANI) nanowire arrays grown on nitrogen-doped carbon fiber cloth (eCFC). The RGO coating layer is important to accommodate volume change and mechanical deformation of the coated PANI nanowires arrays during the long-term charge/discharge processes. The resulting hierarchical symmetric supercapacitor based on RGO/PANI/eCFC composites shows an enhanced capacitive behavior with a maximum energy density of 25.4 Wh kg−1, a maximum power density of 92.2 kW kg−1 and a specific capacitance of 1145 F g−1, which is higher than that of PANI/eCFC (1050 F g−1) and GO/PANI/eCFC (940 F g−1). Moreover, the assembled supercapacitor exhibits excellent charge/discharge rates and a good cycling stability, retaining over 94% of its initial capacitance after 5000 cycles.

1. INTRODUCTION Supercapacitors (SCs) are attractive energy storage devices in flexible and wearable electronics, owing to their high power characteristics and long cycle lifetime.1−3 High performance electronic applications require lightweight, wearable power conversion and storage devices. Recent research focus on the constructions of SCs using carbon-based materials, such as carbon nanotube,4,5 carbon onions,6 activated carbon,7 carbon fiber,8 and graphene.9 The pseudocapacitor materials such as RuO2,10 MnO2,11,12 and conducting polymers13−16 deposited on the surface of carbon materials further enhance their electrochemical performances. The realization of high performance flexible devices strongly depends on the electrical properties and mechanical integrity of the constitutive materials, and their controlled assembly into functional devices.17−21 Intrinsically conductive polymers (ICPs) have been extensively studied and widely applied in various organic devices. Polyaniline (PANI) is one of the most promising ICPs for SCs due to its excellent capacity for energy storage, low cost, easy synthesis, and controllable electric conductivity.22,23 The specific capacitance of PANI is larger than that of carbon materials using double layer charge storage.24 To improve the performances or extend the functions of the assembled devices, nanostructured PANI are usually synthesized through various chemical approaches by interfacial or rapid-mixing polymerization, obtaining various morphologies, such as aligned nanowires,25 nanotubes,26 and nanorods.27 However, the nanostructured CPs are usually insulating in their dedoped states and have a poor stability.28 Thus, various flexible carbon © 2014 American Chemical Society

materials are usually used as a conductive scaffold for fabricating CPs-based electrodes. Cong et al. demonstrated that the flexible graphene−PANI paper subsequently exhibits excellent supercapacitor performance with an enhanced specific capacitance (763 F g−1) and good cycling stability by electropolymerization of PANI nanorods on graphene paper.29 Sawangphruk et al. produced a new hybrid material of graphene, PANI, and silver nanoparticles coated on flexible carbon fiber paper, exhibiting high specific capacitance and capacity retention.30 A flexible electrode of polyaniline-coated electro-etched carbon fiber cloth was synthesized exhibiting a mass-normalized specific capacitance of 673 F g−1 and an areanormalized specific capacitance of 3.5 F cm−2, as reported by Cheng et al.31 Bian et al. prepared the self-doped polyaniline on functionalized carbon cloth, which showed a specific capacitance of 408 F g−1 at a constant charge−discharge current density of 1 A g−1.16 These results demonstrate that the flexible composites combining the electrochemical double layer capacitive graphene, pseudocapacitive PANI, and highly conductive carbon fiber cloth show good electrochemical performance. However, most of the previous reports lack control over the morphology of PANI, separated graphene sheets, and reasonable order of every single active material in the composites. Therefore, creating the special structured composites still remains a challenge, which allows highly Received: December 12, 2013 Revised: April 17, 2014 Published: April 24, 2014 5306

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deposition apparatus. The CFC was exposed to the nitrogen/oxygen gas generated by pulsed power supply (160 W) for 2 min. Graphite oxide was prepared from natural graphite by a modified Hummers method.36 PANI/eCFC composites were prepared by in situ polymerization of aniline. The polymerization was carried out in an ice−water bath at a temperature of −5 °C. Aniline (45.65 μL) was added to 40 mL of 1 M H2SO4 aqueous solution, and eCFC (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 aqueous solution containing APS (28.53 mg) was rapidly added and stirred for 30 s. The molar ratio of aniline 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. Then, the PANI/eCFC was immersed into the homogeneous GO suspension (0.5 mg mL−1) for 15 min, followed by rinsing with water three times for 1 min each time, which was named GO/PANI/eCFC. Finally, the conversion of GO to the reduced form RGO in the flexible electrode was achieved using hydroiodic acid (HI). The obtained composites (RGO/PANI/eCFC) were washed by deionized water and dried under vacuum at 40 °C for 10 h. The weight of the PANI and graphene 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 SU8000) was employed to characterize the morphologies of samples. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR spectrometer. Raman spectra were recorded using a LabRam-1B Ramen spectroscope, with an He−Ne laser excitation at 632.8 nm with a scanning time of 50 s. 2.4. Electrical and Electrochemical Measurements. The flexible SCs were assembled by two pieces of the same area electrodes with a separator sandwiched in between. Then they were encapsulated by flexible polyethylene terephthalate (PET) film to produce a symmetric supercapacitor device without conductive additives and binders. Two platinum wires were used as the current collectors to form a good contact between device and the electrochemical measurement instrument. The flexible electrodes were immersed into 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 on the CHI660D electrochemical working station. The specific capacitance of a supercapacitor cell (Ct) was calculated from the equation of Ct = IΔt/mΔV (1). In symmetric supercapacitors, the specific capacitance (Csc) of the electrodes (RGO/ PANI/eCFC, GO/PANI/eCFC, PANI/eCFC, and PANI) was calculated according to Csc = 4Ct. The power density (P) and energy density (E) could be achieved by the following equations:

efficient utilization of PANI for charge storage with facilitated transport of ions and good cycle stability. In our previous work, a novel hybrid composite electrode of ordered conducting PANI nanowire arrays coated on nitrogen plasma etched carbon fiber cloth (eCFC) was synthesized by in situ polymerization.32 Nitrogen-doped CFC enhances the capacitance of carbon electrodes of electrical double layer capacitors, and promotes faster nucleation and growth kinetics of catalyst nanoparticles leading to their small size and uniform dispersion. The 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), while the PANI/eCFC shows low cycle stability, due to the mechanical degrading because of swelling and shrinkage during the doping/dedoping process. The graphene is an ideal candidate as a high power, high energy, and long cycle life material for supercapacitors because of its intrinsically superior electrical conductivity, excellent mechanical flexibility, remarkable thermal conductivity, and high surface area.33−35 Therefore, the incorporation of graphene into the PANI/eCFC composites can greatly improve the electrochemical performance, especially cycle stability. In this work, we design and fabricate novel hybrid architecture by easily coating reduced graphene oxide (RGO) sheets on polyaniline nanowire arrays grown on flexible nitrogen-doped carbon fiber cloth. The general electrode fabrication procedure is schematically illustrated in Scheme 1. Scheme 1. Procedure to Coating RGO Nanosheets on PANI Nanowire Arrays Deposited on Nitrogen-Doped eCFC

Conducting PANI nanowire arrays were deposited on the nitrogen-doped CFC electrode to provide high pseudocapacitance (defined as PANI/eCFC). PANI nanowires with narrow diameters (ca., 100 nm) can provide high electroactive region, short diffusion path, and enhanced electrode/electrolyte interface areas. Then, a layer of RGO sheets was wrapped on the PANI/eCFC composite (denoted as RGO/PANI/eCFC) to buffer the PANI nanowire array volume change, improving the long-term cycle stability. The combination of RGO with PANI deposited on the 3D flexible substrate brings some intriguing properties in view of their synergistic effects. The flexible SCs assembled from these electrodes showed high rate capability and specific capacitance and excellent cycling stability.

2 E = CVmax /8

(2)

P = (Vmax − Vdrop)2 /(4RESR m)

(3)

where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the total mass of two active electrodes, and ΔV (V) is the potential window during the discharge process. Equivalent series resistance (RESR) is estimated with the formula RESR=Vdrop/(2I), and here Vdrop (V) is the voltage drop at the beginning of the discharge and I (A) is the constant current. The PANI nanowire electrodes were prepared by mixing PANI nanowire powder, carbon black, 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.

2. EXPERIMENTAL SECTION 2.1. Material. Natural graphite powder with an average particle size of 30 μm and a purity of >99% were supplied from Shanghai Yifan Graphite Co., Ltd. Aniline (AN, CP) was distilled under reduced pressure and stored in dark room prior to use. Ammonium persulfate (APS, AR) and sulfuric acid (98%) were purchased from Shanghai Chemical Co. All solutions were prepared with deionized water. 2.2. Preparation of RGO/PANI/eCFC Composites. The cold plasma treatment was performed in a plasma chemical vapor

3. RESULTS AND DISCUSSION 3.1. Microstructure and Morphology. Nitrogen-doped eCFC with its 3D ordered macroporous structure can be used as a robust and high conductive scaffold for growing graphene 5307

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and PANI, owing to their mechanical strength, flexibility, and lightweight. The morphology of GO, PANI, PANI/eCFC, and RGO/PANI/eCFC are shown in Figure 1. The GO nanosheets

area, but not preventing contact between the PANI nanowire arrays and ions in the electrolyte, and maintaining the structural integrity. This sandwich structure would form conductive bridges between RGO layers and PANI nanowire arrays, providing effective and short diffusion channels for ion transport compared to the pristine PANI nanowires. After deposition of PANI and RGO layer, the loading weight of PANI/eCFC and RGO/PANI/eCFC is 0.51 mg cm−2 and 0.62 mg cm−2, respectively, with each sheet having a resistance of 105 Ω sq−1 and 93 Ω sq−1. When the mass loading of PANI nanowires increases to 5.45 mg cm−2, the PANI nanowires were randomly connected on the eCFC and were also clearly observed under the RGO layer (SI Figure S5). The FTIR spectra of RGO/PANI/eCFC, PANI/eCFC, PANI, RGO, and eCFC are shown in Figure 2A. Compared

Figure 1. TEM image of GO (A), and SEM images of PANI (B), PANI/eCFC (C and D), and RGO/PANI/eCFC (E and F).

are entirely exfoliated in H2SO4 solution, exhibiting unfolded and crumpled transparency nanosheets in Figure 1A. GO nanosheets deposited onto a mica substrate from an aqueous dispersion, exhibiting the sheets heights of ∼1 nm (AFM image in Supporting Information, SI, Figure S1). The fact reveals the characteristics of a fully exfoliated GO nanosheet. Figure 1B 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. The low magnification SEM image of PANI/eCFC (Figure 1C) demonstrates that a large-scale of dense and aligned PANI nanowires grow uniformly on the skeletons of eCFC without any macroscopical defects. Arrays of PANI nanowires maintain their structures to profit from the fast access of ions to electrolytes. The average length of the uniformly aligned nanowires is up to approximately 150 nm, as shown in Figure 1D. The surface morphology of RGO/PANI/ eCFC (Figure 1E) shows the crumpled RGO sheets coating on PANI/eCFC and linking the adjacent carbon fibers to fill the void space. The RGO sheets not only interact with PANI through the large basal plane area, but also connect with each other to form the RGO layers on top of PANI nanowires. The features of PANI nanowire arrays under the RGO layers containing 7 monolayers graphene sheet still can be observed obviously in Figure 1F. The morphologies and structures of the RGO/PANI/eCFC are investigated by SEM and AFM, as described in the ESI and illustrated in SI Figures S2 and S3. When the dipping cycle increases, the PANI nanowires were covered by the dense RGO sheets. It is noteworthy that the newly introduced RGO sheets can fulfill the following requirements: contributing cycle stability, enlarging the surface

Figure 2. FTIR (A) and Raman spectra (B) of the eCFC, RGO, PANI, PANI/eCFC, and RGO/PANI/eCFC composites.

to that of eCFC and RGO, several typical bands can be observed for PANI, PANI/eCFC, and RGO/PANI/eCFC, indicating that PANI has been successfully synthesized in the composites, as prepared. The bands at 1558 and 1431 cm−1 are assigned to CC stretching vibrations of quinoid and benzenoid rings, respectively. The bands of aromatic C−N, CN, and C−H stretching vibrations at 1298, 1240, and 1078 cm−1 can also be clearly recognized, respectively.23 In the Raman spectra (Figure 2B), D and G bands can be observed for eCFC and RGO, the ratio of D- to G-band intensity (ID/IG) from eCFC and RGO is 1.07 and 0.94, respectively. By comparison, the Raman spectrum of PANI/eCFC composite presents several representative peaks attributed to PANI: the C−H bending of quinoid ring at 1171 cm−1, the C−H bending of the benzenoid ring at 1253 cm−1, C−N+ stretching at 1340 cm−1, and C−C stretching of the benzene ring at 1493 and 1584 cm−1, respectively.15 The reduced intensities of two peaks in the spectra of RGO/PANI/eCFC at 1253 and 1493 cm−1 are probably due to the strong interactions between PANI and graphene, indicating the successful fabrication of hierarchical 5308

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hybrid composites. Besides, an obvious D band at ∼1620 cm−1 can be observed for eCFC, revealing the defects caused by plasma etching and fabricated process. 3.2. Electrochemical Performance. In order to demonstrate the electrochemical performance of various flexible composites, we assembled symmetric SCs using PET as a thin flexible substrate and an aqueous electrolyte of 1 M H2SO4 with a separator (Figure 3). The flexible device based on RGO/

attached to the electrodes, serving as conductive connections. To address the influence the thickness of the RGO layer has on the capacitance, the CV curves and specific capacitance of RGO/PANI/eCFC composites after different dipping cycles in GO suspensions and HI reduction are illustrated in SI Figure S4. The decreasing capacitance of composites was caused by an increase in RGO sheet thickness, due to the restacking and aggregation of RGO sheets. The results show that the RGO/ PANI/eCFC exhibits better electrochemical performances. Figure 4A compares the CV curves of pristine PANI, PANI/ eCFC, GO/PANI/eCFC, and RGO/PANI/eCFC collected at 10 mV s−1. Two pairs of redox peaks are found on the CV curves, which are attributed to the redox transitions of PANI between a semiconducting state (leucoemeraldine form) and a conducting state (polaronic emeraldine form), the benzoquinone and aminoquinone couple, respectively.37,38 The appearance in the CV curve of RGO/PANI/eCFC and GO/PANI/ eCFC indicates that the electronic or electrolyte ion is transported into the internal PANI nanowire arrays deposited on eCFC. The strong π−π interaction ensures a good electron transfer between the conjugated structure of PANI and the basal plane of RGO sheets. The current signal is immediately reversed upon the reversal of the potential sweep, indicating the low equivalent series resistance of the device.39 The CV area of the RGO/PANI/eCFC SC is much larger than GO/PANI/ eCFC and PANI/eCFC SC at the same scan rate. This result indicates that the RGO layer enhances specific capacitance, while the insulated GO layer decreases the capacitance performance. CVs of RGO/PANI/eCFC SC at various scan rates of 10−200 mV s−1 are shown in Figure 4B. All the transport of counterions can insert into and out of the polymer.40 It is notable that the device shows the excellent

Figure 3. Schematic structure of the flexible supercapacitor consisting of two symmetrical 3D flexible composite electrodes, a polymer separator, and two PET membranes. The three digital photographs show the flexible electrodes as bent and twisted.

PANI/eCFC composites (the total mass of active materials in two electrodes is 1.24 mg) can be bent and twisted as shown in the bottom of Figure 3. The electrochemical performances were analyzed through both cyclic voltammetry (CV) and galvanostatic charge/discharge experiments by the two electrode system. No metal current collector, binders, or electroactive additives were used. Two metal clips were directly

Figure 4. (A) CV curves of PANI, PANI/eCFC, GO/PANI/eCFC, and RGO/PANI/eCFC SCs at 10 mV s−1 and (B) CV curves of RGO/PANI/ eCFC SC at various scan rates. (C) Galvanostatic charge/discharge curves of the SCs at 1 A g−1 in the 1 M H2SO4 aqueous solution. (D) Galvanostatic charge/discharge curves of RGO/PANI/eCFC SC at various current densities. 5309

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Figure 5. (A) Specific capacitance plots of PANI, PANI/eCFC, GO/PANI/eCFC, and RGO/PANI/eCFC SCs at various current densities from 1 to 8 A g−1. (B) Impedance comparison curves of all the flexible devices at the frequency from 0.01 to 105 Hz at open circuit potential with an ac perturbation of 5 mV. (C) Ragone plots of symmetric supercapacitors based on PANI, PANI/eCFC, GO/PANI/eCFC, and RGO/PANI/eCFC electrodes. (D) Cycle stability of the flexible devices at 1 A g−1, and the inset shows CV curves of RGO/PANI/eCFC at 10 mV s−1 for the 1st and 5000th cycle.

graphene/PANI multilayer film (375.2−878 F g−1),44−46 and graphene-wrapped PANI nanofibers (250 F g−1 ).47 It demonstrates that our hybrid RGO/PANI structure-based eCFC greatly yield an improved capacitance performance with ∼2 time increase in specific capacitance compared to that of PANI. The greatly enhanced specific capacitance of RGO/ PANI/eCFC composite is probably due to the synergetic effect between the two components. The PANI nanowires with narrow diameters provide an enhanced electrode/electrolyte interface area and short diffusion lengths, which can ensure a high utilization of PANI. RGO in the hybrids can offer highly conductive pathways by bridging the adjacent individual PANI nanowires together, thus facilitating rapid transport of the electrolyte ions in the electrode during the rapid charge/ discharge processes. Additionally, at various scan rates, CV curves for RGO/PANI/eCFC SC keep the same shape (Figure 4B), indicating a good rate capability of our flexible electrodes, which can be proven by the similar charge/discharge plots (Figure 4D). The galvanostatic charge/discharge curves of the electrodes at various current densities are illustrated in Figure 5A. As the current density increases by a factor of 8 (from 1 to 8 A g−1), the specific capacitance for RGO/PANI/eCFC has an 80% retention of its initial value, higher than that of PANI/eCFC (64%), showing a high rate performance. The improved rate capability of RGO/PANI/eCFC SC indicates that the structure of RGO layer contacts with all PANI nanowire arrays, which not only facilitates the ion diffusion between the electrolyte and electrode, but is also beneficial to overcome the poor electrical conductivity of PANI, both of which enhance the electrochemical performance of the supercapacitor. We obtained

electrochemical behavior in the wide range and the redox current increased with increasing scan rate, indicating a good rate capability. Figure 4C shows the galvanostatic charge−discharge curves of the samples at a current density of 1 A g−1. The almost symmetric and linear charge−discharge curves indicate that the electrodes had the good capacitive behavior of a highperformance supercapacitor electrode, where the deviation to linearity is typical of a pseudocapacitive contribution.41 It is notable that the“IR drop” of the RGO/PANI/eCFC is much lower than that of GO/PANI/eCFC and PANI/eCFC composites, which shows that the materials can be optimized to decrease their internal resistances by the reduction of graphene oxide. The total gravimetric specific capacitance of various composites was calculated from the discharge curves (Experimental Section). The specific capacitance (Csc) of PANI/eCFC and PANI is 1050 F g−1 and 520 F g−1, respectively. After the wrapping of GO and RGO on the PANI/ eCFC composites, the specific capacitances (Csc) of GO/ PANI/eCFC and RGO/PANI/eCFC were obtained to be 940 F g−1 and 1145 F g−1, respectively, at the same current density. This is mainly due to the insulating nature of GO sheets in the hybrid, and the total capacitance is mainly dominated by the pseudocapacitance from the conducting PANI/eCFC. Impressively, compared with GO wrapping layer, the effects of RGO in the nanocomposites not only decrease the internal resistance of the electrode, but also enhance the specific capacitance of the whole device. The highest value for RGO/PANI/eCFC achieved at 1 A g−1 exceeds those previously reported systems based on PANI/CFC (408−1079 F g−1),16,31,32,42 carbon nanofiber/graphene oxide/PANI film (450.2 F g −1 ),43 5310

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specific capacitance of 915 F g−1 when the RGO/PANI coating was 5.4 mg cm−2 (SI Figure S6). The optimized loading was 0.62 mg cm−2 for RGO/PANI/eCFC, and the highest value is 1145 F g−1. The increase of mass loading may induce an increase in diameter and length of PANI nanowires, thus only a thin layer of RGO/PANI was charged and discharged. Furthermore, electrochemical impedance spectroscopy (EIS) further confirmed the fast ion transport within the RGO/ PANI/eCFC electrodes. The complex plane plots of the impedance of the frequency ranging from 0.01 to 105 Hz for the three SCs are shown in Figure 5B with an expanded view provided in the inset. The EIS results are fitted using Zview software, and the equivalent circuits are given in SI Figure S7. The impedance curves are all similar in form, with an arc in the high-frequency region and a spike at lower frequencies. At the low-frequency region, the RGO/PANI/eCFC and PANI/ eCFC SCs show a more vertical shape than pure PANI, indicating better capacitive behaviors and lower ion diffusion resistance because the more vertical the curve is, the more closely the supercapacitor behaves as an ideal capacitor.48 At the high-frequency region, the bulky solution resistance (the intersection point on the real axis) and the charge transfer resistance (Rct, a semicircle in the high- to midfrequency region) were measured to be 6.5 and 51.6 Ω for PANI SC, respectively. In contrast, decreased Rct value for PANI/eCFC (1.5 Ω) and RGO/PANI/eCFC (1.4 Ω) are obtained, probably due to the relatively good electronic conductivity of eCFC and RGO. The Rct of GO/PANI/eCFC (3.0 Ω) is higher than PANI/eCFC, indicating a poor charge transfer rate. The small resistances are also confirmed by observing the negligible voltage drop at the beginning of discharge curves in Figure 4C. The 45° sloped portion known as the Warburg resistance is attributed to the ion diffusion inside the electrode. The higher Warburg slope of the RGO/PANI/eCFC SC (6.4 Hz) demonstrates the faster ion movement for the formation of the electrical double layer, compared to that of GO/PANI/ eCFC (3.2 Hz) and PANI/eCFC SC (4.5 Hz). In order to demonstrate the overall performance of our flexible SCs using various composites, the Ragone plots of PANI, PANI/eCFC, GO/PANI/eCFC, and RGO/PANI/ eCFC SCs are shown in Figure 5C, respectively. The maximum energy density for RGO/PANI/eCFC is 25.4 Wh kg−1 (at a power density of 52.5 kW kg−1) and the highest power density is 92.2 kW kg−1 (at an energy density of 20.3 Wh kg−1), respectively. These values are higher than those of reported self-doped polyaniline/functionalized carbon cloth (9.3 Wh kg−1 at a power of 1 kW kg−1)16 and comparable to CNT/ RGO/PANI hybrid system.44,49,50 As shown in Figure 5D, our device performances are very stable after 5000 cycles at a current density of 1 A g−1: capacitance retentions of 94%, 84%, 90%, and 68% for RGO/PANI/eCFC, GO/PANI/eCFC, PANI/eCFC, and PANI are obtained, respectively. The special structure of RGO/PANI/eCFC is also helpful for the relaxation of the volume expansion during the doping/dedoping process.51 The RGO layer can help the electrode materials withstand the strain relaxation and mechanical deformation, preventing the PANI nanowire arrays from seriously swelling, shrinking, and overoxidizing of the polymer and from mechanical degradation during the long charge/discharge process. This can be confirmed by the SEM image of the RGO/PANI/eCFC electrode after 5000 cycles (SI Figure S8), which still remains the structural integrity. The aligned PANI nanowire arrays can be clearly observed, keeping the 1D

structure after cycling test. As shown in the inset of Figure 5D, the curves with two couples of evident redox peaks are observed at first cycle for the RGO/PANI/eCFC SC. After 5000 cycles, those peaks become weak, probably owing to the reduced pseudocapacitance of PANI. The unique characteristics of the RGO/PANI/eCFC nanostructured composite, make it a promising candidate for high-performance SC electrode materials. The higher electrochemical performance of the RGO/PANI/eCFC electrode is due to its structure, which has an improved bond−bond interaction (Figure 6).45 First, there

Figure 6. Interaction bonds among the N-doped eCFC, PANI, and RGO.

are oxygen-containing groups and three types of N-configurations: pyridine N (N6), pyrrole N (N5), and graphite type N (NQ) on the nitrogen-doped eCFC surface. Aniline monomers are adsorbed on surfaces of the eCFC via the π−π conjugation and hydrogen bonding effects during the polymerization process, leading to uniform PANI nanowire arrays deposited on its surface. Second, negatively charged surfaces of GO will predominantly adsorb on the positively charged PANI due to the electrostatic interaction and hydrogen bonding interaction, apart from the π−π stacking under hydrophobic benzene ring and hydrophilic −NH groups in the PANI chains. This covalent link can reduce the charge transfer resistance of RGO/PANI/eCFC. RGOs act as conductive wires for interconnecting every PANI/eCFC unit, resulting in maintaining electronic contact with each other, subsequently enhancing the electrochemical stability. Therefore, the sandwich structure of RGO/PANI/eCFC shows a better cycle stability and rate capacity than PANI (or PANI/ eCFC) as the supercapacitor electrodes. In order to further evaluate the potential of RGO/PANI/ eCFC SC for flexible energy storage under real conditions, a device was placed under constant mechanical stress, and its performance was analyzed. The CV performance of this device under different bending conditions is shown in Figure 7. The as-prepared SC represents a high flexibility, which can be bent, twisted, and even rolled up without destroying its construction. The bending has a slight influence on the capacitive behavior; it can be bent arbitrarily without degrading performance. This performance durability can be attributed to the high mechanical flexibility of the electrodes and the strong connections between the graphene layers and PANI arrays, indicating our SCs may be ideal for next-generation flexible, portable electronics. The reproducibility of the electrochemical capacitances of PANI, PANI/eCFC, GO/PANI/eCFC, and RGO/PANI/ eCFC SCs are also tested by repetitive recording of CVs in 1 M H2SO4 solution.52 The relative standard deviation (R.S.D.) of the specific capacitance for 10 replicate determinations is 5311

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densities (Figure S6); and the electrical equivalent circuit used for fitting impedance spectra of PANI, PANI/eCFC, and RGO/PANI/eCFC (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: 0086-21-67792854; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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 Ph.D. Students of Donghua University (12D10630).

Figure 7. CV curves of RGO/PANI/eCFC supercapacitor bent with various angles from 0° to 180° at a scan rate of 10 mV s−1, and the inset shows the schematic structure of the bending angles.

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5.3%, 3.8%, 4.1%, 3.6%, and 3.2%, respectively. Ten pieces of the proposed electrode are prepared, and the R.S.D. for the individual determination is 5.2%, 3.6%, 4.0%, 3.4%, and 2.9%, respectively. These results demonstrate that fabricated supercapacitor electrodes have excellent reproducibility.

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4. CONCLUSIONS We have successfully demonstrated that flexible SCs based on hierarchical hybrid composites with a sandwich structure composed of electrical double-layer RGO, high specific capacitive PANI nanowire arrays, and good conductive nitrogen-doped carbon fiber cloth (eCFC). The enhanced hydrophilicity of eCFC not only facilitates the access of the electrolyte ions onto the graphene surface, but also improve the interactions between PANI and graphene. The aligned PANI nanowires with small size facilitate the ion diffusion and improve the utilization of the electroactive PANI during the charge/discharge processes. The RGO layer benefits the quick transfer of electrons and improves mechanical stability to refine the structure of PANI. The RGO/PANI/eCFC SC shows the excellent electrochemical performance: a gravimetric capacitance of 1145 F g−1 for RGO/PANI/eCFC SC higher than GO/PANI/eCFC (940 F g−1) and PANI/eCFC (1050 F g−1) at 1 A g−1, the maximum energy density of 25.4 Wh kg−1 (with power density of 52.5 kW kg−1) and the maximum power density of 92.2 kW kg−1 (with energy density of 20.3 Wh kg−1). Moreover, 94% capacitance retention after 5000 cycles is achieved. The hierarchical structured hybrid composites have the potential to be a new class of light, flexible, and binder-free charge storage devices.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

AFM image of GO (Figure S1); SEM images of RGO/PANI/ eCFC (7.2−7.6 nm), RGO/PANI/eCFC1 (11.9−12.2 nm), RGO/PANI/eCFC2 (19.9−20.0 nm), and RGO/PANI/ eCFC3 (25.0−25.8 nm) (Figure S2); AFM images of RGO/ PANI/eCFC (7.3 nm), RGO/PANI/eCFC1 (12.0 nm), RGO/ PANI/eCFC2 (19.2 nm), and RGO/PANI/eCFC3 (25.4 nm) (Figure S3); CV curves of RGO/PANI/eCFC dipped in the GO suspensions and specific capacitance plots of RGO/PANI/ eCFC with different RGO dipped cycles at various densities (Figure S4); SEM images of PANI7/eCFC and RGO/PANI7/ eCFC (Figure S5); CV curves and galvanostatic charge/ discharge curves of RGO/PANI7/eCFC at different current 5312

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