PANI Hybrid Electrode for High

Dec 8, 2015 - According to the energy storage mechanism, SCs can be classified into the electrical double layer capacitors (EDLCs) and pseudocapacitor...
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Three-Dimensional Tubular MoS2/PANI Hybrid Electrode for High Rate Performance Supercapacitor Lijun Ren, Gaini Zhang, Zhe Yan, Liping Kang, Hua Xu, Feng Shi, Zhibin Lei, and Zong-Huai Liu* Key Laboratory of Applied Surface and Colloid Chemistry (Shaanxi Normal University), Ministry of Education, Xi’an 710062, P.R. China School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, P.R. China S Supporting Information *

ABSTRACT: By using three-dimensional (3D) tubular molybdenum disulfide (MoS2) as both an active material in electrochemical reaction and a framework to provide more paths for insertion and extraction of ions, PANI nanowire arrays with a diameter of 10−20 nm can be controllably grown on both the external and internal surface of 3D tubular MoS2 by in situ oxidative polymerization of aniline monomers and 3D tubular MoS2/PANI hybrid materials with different amounts of PANI are prepared. A controllable growth of PANI nanowire arrays on the tubular MoS2 surface provides an opportunity to optimize the capacitive performance of the obtained electrodes. When the loading amount of PANI is 60%, the obtained MoS2/PANI-60 hybrid electrode not only shows a high specific capacitance of 552 F/g at a current density of 0.5 A/g, but also gives excellent rate capability of 82% from 0.5 to 30 A/g. The remarkable rate performance can be mainly attributed to the architecture with synergistic effect between 3D tubular MoS2 and PANI nanowire arrays. Moreover, the MoS2/PANI-60 based symmetric supercapacitor also exhibits the excellent rate performance and good cycling stability. The specific capacitance based on the total mass of the two electrodes is 124 F/g at a current density of 1 A/g and 79% of its initial capacitance is remained after 6000 cycles. The 3D tubular structure provides a good and favorable method for improving the capacitance retention of PANI electrode. KEYWORDS: polyaniline, molybdenum disulfide, 3D tubular structure, capacitance, rate performance, cycling stability

1. INTRODUCTION Supercapacitors (SCs) have drawn considerable attention in recent years due to their ability for delivering higher power density than batteries/fuel cells and storing higher energy density than conventional electrostatic capacitors.1,2 The SCs with high performances are expected to be developed for meeting the great demand for practical applications. Research results indicate that the capacitive property of the SCs is influenced by electrode materials, electrolyte, and the assembled technology, with the most important factor being the electrode materials.3 Therefore, it is of great importance to develop new electrode materials with high specific capacitance, good rate capability, and high cycling stability.4 According to the energy storage mechanism, SCs can be classified into the electrical double layer capacitors (EDLCs) and pseudocapacitors.5 The EDLCs are usually composed of carbon materials or carbon-based hybrid materials with high surface areas and suitable pore sizes,6 while the pseudocapacitors mainly employ electro-active conducting polymers or transition metal oxides for electrode materials.7,8 It has been demonstrated that pseudocapacitors can yield much higher specific capacitance and energy density than EDLCs.9 Therefore, many research works focus on the design, preparation and their capacitance of the multicomponent electrode materials © 2015 American Chemical Society

combining energy storage of electrostatic attraction and faradic reaction, in which active materials such as metal oxides or conducting polymers can enhance the capacitance remarkably, while the carbon-based material as a support, not only increases the effective utilization of active materials, but also improves the electrical conductivity and mechanical strength of the composite electrodes.10−15 Up to now, the transition metal oxides and conducting polymers as pseudocapacitive materials have been given wide attention because they possess higher theoretical capacitance.16 Although the transition metal oxides have high specific capacitance and excellent reversibility, the relatively high cost and low electrical conductivity hinder their widespread applications.17 Therefore, conducting polymers are widely researched as promising pseudocapacitive materials because of their large capacitance, good electric conductivity, ease of synthesis and low cost.18 Over the past years, polyaniline (PANI) has been used as the commonly conducting polymer electrodes, and the devices with low weight and good flexibility can be assembled with PANI due to its better flexibility and Received: September 9, 2015 Accepted: December 8, 2015 Published: December 8, 2015 28294

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

Research Article

ACS Applied Materials & Interfaces lower density in comparison with metal oxides.16,19 However, the low practical capacitance and poor cycling stability seriously limit their applications as promising electrode materials.20,21 Wei group has reported that three main reasons would cause the poor cycling stability of conducting polymer electrodes. First is the poor mechanical stability due to volumetric changes in the doping/dedoping process, and second is active material loss due to conducting polymers peeling off from the current collector or dissolving into the electrolyte, and third is overoxidative degradation due to the limited working potential range.22 To improve the poor cycling stability of PANI, many studies have been conducted on design and preparation of the nanostructured PANI and PANI based hybrid materials with different morphologies and dimensions, such as PANI arrays on graphene oxide sheets,23 PANI/single walled carbon nanotubes,24 SnO2@PANI nanofibers,25 PANI-MnO2 coaxial nanofibers,26 PANI/MoS2 nanosheets,27 coaxial PANI/TiN/PANI nanotube arrays,28 for improving their capacitance and cycling stability. Compared with pure PANI, the rate performance and cycling stability of these PANI hybrid materials have been improved in a certain range, but low ionic diffusion inside the solid phase is still not solved. Molybdenum disulfide (MoS2) with a typical layered structure is very attractive electrode material for SCs.29,30 However, the freshly prepared MoS2 layers easily aggregate during practical application even in the drying process, resulting in the loss of active sites or other unusual properties of ultrathin 2D nanostructures.31 For the practical applications of 2D nanostructures, it must to be solved the preventing aggregation, while constructing hierarchical 3D architectures based on 2D nanosheets is an effective approach to solve this problem.32,33 It has been reported that nanosheets assembling 3D tubular MoS2 architecture could facilitate the ion transport and storage, as well as withstand its volume change on cycling.34 In our previous work, PANI with four different morphologies including nanofibers, nanoflowers, nanorods, and nanotubes have been prepared, and the electrochemical measurements show that PANI electrode with nanotube morphology delivers the highest specific capacitance and best rate performance because of its high conductivity and good ion transport.35 Inspired by the work of PANI electrode with nanotube morphology, can improve the rate performance and cycling stability of PANI electrode by controllable growing PANI nanowire arrays on 3D tubular MoS2, in which the PANI nanowire arrays with very small diameter provide large number of active sites for electrochemical reaction, and the gaps between oriented PANI nanowires can serve as ion channels for fast electrolyte transport as well as accommodate the large volume changes. 36 Meanwhile, the 3D tubular MoS 2 architecture provides large surface area and acts as a substrate to protect the PANI structural pulverization during charge− discharge processes. In the present work, PANI nanowire arrays were vertically aligned on the surface of 3D tubular MoS2 and 3D MoS2/PANI hybrid electrode materials were prepared by a simple chemical oxidative polymerization method. The novel fabrication process was schematically illustrated in Scheme 1. The obtained MoS2/ PANI-60 hybrid material delivers enhanced capacitance retention of 82% from 0.5 to 30 A/g. Furthermore, the MoS2/PANI-60 based symmetric SC exhibits excellent cycling performance of 79% capacitance retention after 6000 charge− discharge cycles.

Scheme 1. Formation Schematic Illustration of the 3D Tubular MoS2/PANI

2. EXPERIMENTAL SECTION 2.1. Materials Preparation. Aniline (analytical grade) was distilled until color-less under reduced pressure before use. Other chemicals (analytical grade) were used as received without further. Preparation of 3D tubular MoS2/PANI hybrid materials: An acidic 3D tubular MoS2 suspension was first prepared in a typical process. 38.5 mg of S powder and 92.7 mg of ammonium molybdate [(NH)6Mo7O24·4H2O] were added into a mixed solution of octylamine (14 mL) and ethanol (13 mL). After the mixture was stirred for 20 min, the obtained orange suspension was then transferred into a 40 mL Teflon stainless steel autoclave, sealed, and heated at 200 °C for 24 h in an oven. After it was cooled to room temperature naturally, the precipitates were collected by centrifugation, washed several times with ethanol and followed by ultrapure water, and dried in a vacuum at 60 °C for 10 h, the precursor MoS2 was finally obtained. Then the precursor MoS2 (24 mg) was dispersed in 1 M H2SO4 (40 mL), again, an acidic 3D tubular MoS2 suspension (0.6 mg/mL) was obtained. The 3D tubular MoS2/PANI hybrid materials were prepared by in situ oxidative polymerization of aniline monomers on 3D tubular MoS2 using ammonium persulfate ((NH4)2S2O8, APS) as oxidizer. Typically, 146 μL aniline was first added into 40 mL acidic 3D tubular MoS2 suspension, and the obtained suspension was stirred in an ice bath (∼5 °C) for 30 min. After it was stirred in an ice bath for 30 min, APS with a molar ratio of 1:1.5 (APS: aniline) was quickly added in the suspension, stirred continually for 12 h in an ice bath. The obtained precipitates were followed by washing with ultrapure water and ethanol and drying in a vacuum at 60 °C for 10 h, 3D tubular MoS2/PANI-60 hybrid material was finally obtained. By variation of the amount of aniline monomers during polymerization process, 3D tubular MoS2/PANI hybrid materials with different amounts of PANI and controllable morphologies were prepared by using the same process, which were denoted as MoS2/PANI-x with x representing the mass percentage of PANI in the hybrid materials (x = 35, 46, and 73). On the other hand, pure PANI was also prepared by using the same process except MoS2 suspension in the reaction system. 2.2. Characterization. X-ray diffraction (XRD) measurements were carried out using a D/Max-3c X-ray diffractometer with Cu Kα (λ = 0.154 nm), using an operation voltage and current of 40 kV and 30 mA, respectively. A SU8020 field-emission scanning electron microscopy (FESEM) and JEM-2100 transmission electron microscope (TEM) images were used to observe the morphology of the obtained materials. Specimens for TEM observation were prepared by dispersing the material powder into ethanol by ultrasonic treatment. Infrared spectra were obtained by KBr method on a Fourier Transform Infrared Spectrometer EQUINX55. The electrochemical performance of the electrode materials was characterized on a CHI 660E (Chenhua, Shanghai) electrochemical workstation. 28295

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

Research Article

ACS Applied Materials & Interfaces 2.3. Electrochemical Measurement. The electrochemical performance of the obtained electrode materials was characterized by cyclic voltammetry (CV), galvanostatic charge−discharge and electrochemical impedance spectroscopy (EIS). Electrodes were prepared by mixing electroactive materials, acetylene black and PTFE (polytetrafluoroethylene) in a mass ratio of 85:10:5 to form homogeneous slurry. Then the slurry was spread onto the stainless steel cloth (Mesh sizes 500) (1 cm2) and dried at 60 °C for 10 h. After drying, the coated mesh was pressed to form working electrodes. The loading mass of the active material was about 3 mg. The electrochemical tests of the individual electrode were performed using 1 M H2SO4 as electrolyte in a three-electrode cell, in which platinum foil and Ag/AgCl electrode were used as counter and reference electrodes, respectively. The specific capacitance was calculated from the galvanostatic discharge process according to the following equation: C = I × Δt/ (ΔV × m). Where ΔV is the voltage change during the discharge process, excluding IR drop in the discharge process, m is the mass of active material for the working electrode (g), I is the applied current density (A), and t is the time of discharge stage (s). In a two electrode system, the symmetrical SC was configured with MoS2/PANI-60 as the electrode material and Celgard 3501 membrane as the separator. The specific capacitance was calculated using C = 4 × I × Δt/(ΔV × m), where m is the total mass of the electrode. Electrochemical impedance spectroscopy (EIS) was performed with an amplitude of 5 mV at a frequency range of 0.01 to 100 kHz.

sample, three crystalline peaks at 2θ = 15.1°, 20.5°, and 25.5° corresponding to (011), (020) and (200) planes of PANI are observed, suggesting that PANI are existed in emeraldine salt form.40 For MoS 2/PANI-60 hybrid material, its X-ray diffraction peaks are similar to that of MoS2 sample (marked with black asterisks) and the diffraction peak intensity is lower than those of MoS2 and pure PANI, suggesting a homogeneous combination between MoS2 and PANI throughout the whole 3D tubular architecture. Moreover, the weak diffraction peak around 20.5° for PANI is observed, suggesting PANI is existed in a semicrystalline state in the hybrid material.41 The morphology of MoS2 and MoS2/PANI-60 hybrid material is investigated by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), and the results are shown in Figure 2. It can be seen that the precursor MoS2 shows tubular morphology with a uniform external diameter of about 190 nm (internal diameter is about 80 nm) and a length of 1 μm. Moreover, an uneven layer assembled surface is also found (Figure 2a). TEM image shows an obvious contrast between the dark rough edge and pale interior in the tubular MoS2, suggesting its hollow structure characteristic (Figure 2b), and the ultrathin single nanosheets in rim of tubular MoS2 can be identified as labeled in Figure 2c. Compared to precursor MoS2, not only the external diameter of MoS2/PANI-60 hybrid material increases to 290 nm from 190 nm, but also its internal diameter decreases to 40 nm from 80 nm after PANI is grown on the surface of the tubular MoS2, suggesting that PANI nanowire arrays are grown on both the external and internal surface of the tubular MoS2 (Figure 2d). Moreover, the outside surface of the tubular MoS2 is covered by PANI nanowire arrays with a diameter of 10−20 nm, resulting in an apparent porous surface (Figure 2e). The HRTEM image of MoS2/PANI-60 hybrid material depicts that an interplanar spacing of 0.62 nm can be attributable to the (002) plane of MoS2 nanosheets, while no obvious interplanar spacing of PANI is observed (Figure 2f), suggesting nearly amorphous character of PANI in the hybrid material. The structure and component of MoS2/PANI-60 hybrid material is further investigated by FT-IR spectrum (Figure 3). For MoS2, the bands above 600 cm−1 are attributed to sulfate groups. These features are commonly present in amorphous or well dispersed MoS2 samples as a result of sample surface oxidation upon contact with air.42 Although the characteristic bands of PANI and MoS2 can be detected, their position and intensity obviously change in the hybrid material. The characteristic bands around 1566 and 1489 cm−1 corresponding to CC stretching mode for the quinoid and benzenoid rings are transferred to 1570 and 1492 cm−1, indicating the oxidized state of the emeraldine salt of PANI.43 The bands at 1295 and 1237 cm−1 are caused by the C−N stretching mode for the benzenoid ring. Because the band at 1135 cm−1 attributed to the inplane bending vibration of C−H represents the characteristic of the conductivity and the degree of electron delocalization of PANI, the strong band suggests that the conductivity and the degree of electron delocalization of PANI increase in the hybrid material. Moreover, the C−H modes have also been used as a key to identify the type of benzene substituted or synergetic effect with inorganic compounds. The position of C−H bending vibration changes from 1132 of pure PANI to 1135 cm−1, which is caused by the synergetic effect between PANI and MoS2.44 By varying the amount of aniline monomers during polymerization process, 3D tubular MoS2/PANI hybrid

3. RESULTS AND DISCUSSION The XRD patterns of samples MoS2, PANI, and MoS2/PANI60 are shown in Figure 1. All the diffraction peaks could be

Figure 1. XRD patterns of PANI, MoS2, and MoS2/PANI-60.

indexed to MoS2 phase with the refined lattice parameters of a = 3.161 Å, b = 3.161 Å, and c = 12.299 Å, in good consistence with previously reported hexagonal MoS2 (JCPDS No. 371492).37,38 It can be seen that the MoS2 exhibits obvious diffraction peaks at 2θ = 16.3°, 32.6°, and 56.7°, corresponding to the (002), (100), and (110) diffraction planes, respectively. In addition, the (002) plane shows relative weak diffraction peak, suggesting that the obtained MoS2 is assembled from single or few layers of MoS2 nanosheets.34,39 For pure PANI 28296

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

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ACS Applied Materials & Interfaces

Figure 2. FESEM, TEM, and HRTEM images of MoS2 (a−c) and MoS2/PANI-60 (d−f).

Figure 4. FESEM images of MoS2/PANI hybrid materials with different amounts of PANI: (a) MoS2/PANI-35, (b) MoS2/PANI-46, (c) MoS2/PANI-60, and (d) MoS2/PANI-73. Figure 3. FT-IR spectra of PANI, MoS2, and MoS2/PANI-60.

To explore their potential application in energy storage devices, the electrochemical properties of the precursor MoS2, PANI and the as-prepared 3D tubular MoS2/PANI hybrid materials with different amounts of PANI are first investigated by cyclic voltammetry (CV) and galvanostatic charge− discharge tests in a three electrode cell in 1 M H2SO4 electrolyte, and the optimized MoS2/PANI hybrid material is expected to be selected. At a scan rate of 5 mV/s, the CV curves of the precursor MoS2, PANI and the as-prepared 3D tubular MoS2/PANI hybrid materials with different amounts of PANI are shown in Figure 5a. The CV profile for tubular MoS2 electrode within the potential window of −0.3−0.4 V vs Ag/ AgCl reveals weak curve, indicates that the redox process in this sample.45,46 For PANI electrode, a pair of distinct redox peaks around at 0.10 and 0.25 V corresponding to the typical pseudocapacitive characteristic of the PANI electrode are observed, which is ascribed to the transformation between the leucoemeraldine base (LB) and emeraldine salt (ES) states of PANI.44 On the other hand, although the CV profiles of MoS2/ PANI hybrid electrodes with different amounts of PANI still show a pair of evident redox peaks centered at about 0.10 and

materials with different amounts of PANI and controllable morphologies were obtained. Their morphologies are also monitored with FE-SEM analysis (Figure 4). It can be seen that the morphology of the obtained MoS2/PANI hybrid materials changes with the amounts of PANI, and its external diameter gradually increases while the internal diameter decreases, indicating that PANI nanowire arrays are grown on both the external and internal surface of the tubular MoS2. When the amounts of PANI are relatively low, the MoS2/PANI hybrid material with randomly dispersed and tubular morphology can be obtained (Figure 4a, 4b). Whereas the amount of PANI is relatively high, in addition to the growth PANI nanowire arrays on tubular MoS2, some other PANI nanowires are formed because of self-nucleation growth of PANI along the initial nuclei (Figure 4d). For the MoS2/PANI-60 hybrid material, the suitable amount of PANI make it grow on both the external and internal surface of the tubular MoS2, and forming PANI nanowire arrays doped 3D tubular MoS2 hybrid material (Figure 4c). 28297

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

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ACS Applied Materials & Interfaces

of PANI are higher that of 73%, which can be ascribed to some aggregation of PANI random nanowires, leading to the decrease of contact area with MoS2. Since MoS2/PANI-60 hybrid electrode shows the largest capacitance, its electrochemical property is further investigated. The variation of the specific capacitance at different current densities is plotted in Figure 6a. It can be seen that its specific

Figure 5. CV profiles at 5 mV/s (a) and galvanostatic charge− discharge curves at 0.5 A/g (b) of MoS2, PANI, and MoS2/PANI hybrid electrodes with different amounts of PANI in 1 M H2SO4 electrolyte.

0.25 V corresponding to that of Faradaic capacitance of PANI.44 An obvious change has been observed for the CV curves of these electrodes. In comparison with the CV curve of PANI electrode, the redox peaks shift more negatively with the increase of the PANI amounts except for MoS2/PANI-60 hybrid electrode, suggesting a significant contribution of pseudocapacitance from surface redox reaction between PANI nanowire arrays and H2SO4 electrolyte for suitable PANI loading. Moreover, the controllable growth of PANI nanowire arrays on the tubular MoS2 surface provides an opportunity to optimize the capacitive performance of the obtained electrodes. It is clear indicates that the integration areas of CV curves for MoS2/PANI hybrid electrodes with different amounts of PANI are larger than that of MoS2, indicating a desirable capacitance enhancement due to a synergistic effect between MoS2 and PANI. More importantly, MoS2/PANI-60 hybrid electrode shows drastically enhanced current response, indicating that only a moderate amount ratio of PANI to MoS2 can obtain the desirable capacitance because the appropriate loading of PANI nanowire arrays can offer more active sites for redox charge transfer. The same trend can be found by comparison of galvanostatic charge/discharge curves for various samples at a current density of 0.5 A/g (Figure 5b). The minor distorted galvanostatic charge−discharge curves can be observed for MoS2/PANI hybrid electrodes with different amounts of PANI, it is corresponding to the combination of the redox characteristics of MoS2 and PANI. Moreover, the specific capacitances gradually increase to 552 from 276 F/g in company with the increase of PANI amount. When the amount of PANI increases to 60% (MoS2/PANI-60 hybrid electrode), a maximum capacitance of 552 F/g is obtained. In contrast, the capacitance decreases slightly when the amounts

Figure 6. Capacitance retention (a), Nyquist plots measured at frequency range from 100 kHz to 0.01 Hz (inset, the close-up view of the high-frequency region) (b), and cycling performances at 50 mV/s (c) of MoS2, PANI, and MoS2/PANI-60 hybrid electrodes in 1 M H2SO4 electrolyte.

capacitance decreases from 552 to 455 F/g as the current density increase from 0.5 to 30 A/g, which is higher than those of tubular MoS2 electrode (from 235 to 122 F/g) and pure PANI electrode (from 490 to 335 F/g), respectively. Meanwhile, the capacitance retention of MoS2/PANI-60 hybrid electrode is 82%, it is also better than those of MoS2 (52%) and PANI (68%). These results indicate the prepared MoS2/PANI60 hybrid electrode possesses the excellent rate capability in contrast with pure MoS2 and PANI electrodes. Moreover, the rate performance of MoS2/PANI-60 hybrid material is also better than other PANI hybrid electrode materials, such as 28298

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

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Figure 7. Capacitive performances of MoS2/PANI-60 based symmetric supercapacitor in 1 M H2SO4 electrolyte: (a) CV profiles at different scan rates, (b) galvanostatic charge−discharge curves from 1 to 30 A/g, (c) cycling performance at 3 A/g, and (d) galvanostatic charge−discharge curves at 1st, 500th, and 6000th cycles.

SnO2@PANI nanowire (capacitance retention is 37%),25 PANI/MnO2 coaxial nanofiber (capacitance retention is 35%),26 PANI/MoS2 nanosheets (capacitance retention is 53%),27 layered PANI/graphene film (capacitance retention is 50%),47 and so on.48−51 The remarkable rate performance can be mainly attributed to the architecture with synergistic effect between 3D tubular MoS2 and PANI nanowire arrays, in which the 3D tubular MoS2 not only acts as an active electrode material in the electrochemical reaction, but also as a framework to provide more paths for insertion and extraction of ions within the PANI nanowire arrays. Figure 6b shows the Nyquist plots of MoS2, PANI, and MoS2/PANI-60 electrodes. The plots are composed of a straight line at the lower frequency region and a semicircle at the higher frequency region. Their charge transfer resistance (Rct), which is derived from the radius of the high frequency arc on the real axis, is different and follows the order: MoS2 (0.41 Ω) < MoS2/PANI-60 (1.09 Ω) < PANI (1.83 Ω). For MoS2/PANI-60 hybrid electrode, the IR drop values at different current densities lie between those of MoS2 and PANI electrodes (Figure S1), which are consistent with their corresponding Rct order. Moreover, its smaller Rct and more vertical line along the imaginary axis indicate the lower resistance and better capacitive behavior.52,53 It is of particular note that the pure 3D tubular MoS2 has the smallest Rct, while pure PANI possesses high pseudocapacitance. Therefore, MoS2/PANI-60 hybrid electrode with 3D tubular structure not only has the conductive pathway and the favorable porosity for ion diffusion from the electrolyte to the PANI nanowire arrays, but also can maximize the utilization of PANI, which endows the remarkable rate performance and high capacitance behavior for MoS2/PANI-60 hybrid electrode.

Figure 6c shows the cycle performance of MoS2/PANI-60 hybrid electrode over 1000 charge−discharge cycles at a scan rate of 50 mV/s compared with those of tubular MoS2 and pure PANI electrodes. It is found that the tubular MoS2 electrode maintains a good charge−discharge performance, and the capacitance retention is 96% after 1000 cycles, while poor capacitance retention of about 62% after 1000 cycles is observed for pure PANI electrode. On the contrary, the specific capacitance of MoS2/PANI-60 hybrid electrode exhibits improved stability, and the capacitance retention of 88% after 1000 cycles has been obtained in comparison with pure PANI. The improved stability is mainly ascribed to a gradual access of electrolyte ions to the active sites of electrode surface due to the high stability of 3D tubular structure, in which 3D tubular structure provides a space between the oriented PANI nanowire arrays and effectively accommodate the mechanical deformation caused by the swelling and shrinking of the nanostructures during the long-term charge−discharge processes.54 To further investigate the reliable capacitive performance of MoS2/PANI-60 hybrid electrode, it is employed to assemble a symmetric SC with 1 M H2SO4 as electrolyte over a voltage window of 0−0.7 V. As expected, the redox peaks of CV profiles are maintained over a wide range of scan rates (Figure 7a). Moreover, the charge−discharge curves keep the same shape, and their discharge counterparts show no obvious IR drops even at higher current density (Figure 7b), demonstrating the excellent rate performance of the device.55,56 The specific capacitance based on the total mass of the two electrodes is 124 F/g at a current density of 1 A/g, corresponding to 496 F/g for a single electrode when 28299

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

Research Article

ACS Applied Materials & Interfaces

Shaanxi Province (2012KCT-21), the 111 Project, and the Fundamental Research Funds for the Central Universities (GK201301002 and GK201501007).

multiplying by a coefficient of 4, which complies with the test method as described by Ruoff and his co-workers.57 Moreover, the cycle performance of MoS2/PANI-60 based symmetric SC is investigated by a consecutive charge−discharge technique at a current density of 3 A/g (Figure 7c). The test results show that the specific capacitance has an obvious decay at the first 500 cycles, while no obvious delay is observed and 79% of its initial capacitance is remained during the following 500−6000 cycles. It is recognized that the improved capacitance retention is intrinsically determined by the internal resistance of the SC, which can be manifested in the voltage drop (IR drop) at the beginning of discharge curves. The charge−discharge curves measured at different cycling stages show that there is no obvious IR drop during the different cycling stages (Figure 7d), implying a lower equivalent series resistance (ESR) for MoS2/ PANI-60 based symmetric SC due to the considerable advantages of the 3D tubular structure. This capacitance retention is better than those of reported PANI based symmetric SCs, such as SnO2@PANI nanowire (60% after 500 cycles),25 PANI/reduced graphene oxide film (74% after 500 cycles),58 single walled carbon nanotubes/PANI nanoribbon paper (79% after 1000 cycles), 59 PANI-coated polypyrrole (52% after 200 cycles),60 and so on. Therefore, the 3D tubular structure provides a good and favorable method for improving the capacitance retention of PANI electrode.

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4. CONCLUSION By using in situ oxidative polymerization of aniline monomers, PANI nanowire arrays with a diameter of 10−20 nm are grown on both the external and internal surface of 3D tubular MoS2, and MoS2/PANI hybrid materials with 3D tubular structure are controllably prepared. MoS2/PANI hybrid electrode with 3D tubular structure not only has the conductive pathway and the favorable porosity for ion diffusion from the electrolyte to the PANI nanowire arrays, but also can maximize the utilization of PANI. The obvious synergistic effect between 3D tubular MoS2 and PANI nanowire arrays plays an important role in ion transport and storage, as well as withstands the volume change on cycling, which makes this type of materials has remarkable rate performance and high capacitance behavior. This novel method can be used to improve the practical capacitance and cycling stability of other conducting polymers as promising supercapacitor electrode materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08474. IR drop with discharge current density for MoS2, PANI, and MoS2/PANI-60 electrodes (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel: ++86-29-8153 0706. Fax: ++86-29-8153 0702. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51172137, 21471093), the Program for Key Science & Technology Innovation Team of 28300

DOI: 10.1021/acsami.5b08474 ACS Appl. Mater. Interfaces 2015, 7, 28294−28302

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