Covalent Connection of Polyaniline with MoS2 Nanosheets toward

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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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Covalent Connection of Polyaniline with MoS2 Nanosheets toward Ultrahigh Rate Capability Supercapacitors Rong Zeng,† Zongcai Li,† Longbin Li,† Yizhe Li,† Jun Huang,† Yingbo Xiao,† Kai Yuan,*,† and Yiwang Chen† †

College of Chemistry/Institute of Polymers and Energy Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

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ABSTRACT: Two-dimensional molybdenum disulfide (MoS2) is a promising electrode material for supercapacitors, attributing to attractive physical properties, outstanding electrical properties, and ultrahigh exposed surface area. However, MoS2 bulk suffers from low capacity due to the overlaying of the layers and the poor electric conductivity. Covalent functionalization of MoS2 is a promising, yet challenging, approach to overcome the drawbacks and boost electrochemical performance. Here, we report a series of sandwich-like 4-aminophenyl functionalized MoS2/polyaniline (MoS2−NH2/PANI) nanosheets by in situ growth of PANI on MoS2−NH2 templates. The optimized MoS2−NH2/PANI nanosheets express a high capacitance of 326.4 F g−1 at 0.5 A g−1 and a superior rate retention of 63.1% when the current density increased from 0.5 A g−1 to 1000 A g−1 in a three-electrode system. Impressively, the corresponding symmetric supercapacitors deliver an electrochemical cycling stability with 96.5% retention after 10000 cycles at 5 A g−1. Our strategy of covalent linking PANI onto functional MoS2 provides a feasible approach to improve the electrochemical performance of MoS2-based materials for energy storage. KEYWORDS: Molybdenum disulfide, Polyaniline, Covalent functionalization, Supercapacitors, Ultrahigh rate capacitance

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INTRODUCTION Various energy storage applications are in swift growth to pursue the rapid expansion of electronic products.1,2 Supercapacitors are one class of promising high-performance energy storage devices, due to their outstanding merits, such as excellent power density and long cycle life.3−5 Two-dimensional (2D) molybdenum disulfide (MoS2) with superior physical and (electro)chemical properties is considered as a promising electrode material for supercapacitors.6−14 It posesses a large distance (0.615 nm) between layers and a large specific surface area. Therefore, the effective insertion and adsorption of charge storage generates in the microstructure of MoS2 layers without a significant volume expansion.15−17 The Mo centers with a range of oxidation states from +2 to +6 are favorable to promote Faradaic charge, as well.18 However, the intrinsically low electric conductivity and the restacking of the MoS2 layers lead to fast capacity deterioration and inferior capacity.19,20 Hence, a number of synthetic routes have been developed to improve the electrochemical performance of MoS2-based supercapacitors. These strategies can roughly be summarized as follows: (1) transforming 2H phase MoS2 into 1T phase MoS2,21−23 which owns a 107 times higher electric conductivity than 2H MoS2 and (2) decorating MoS2 with other good capacity materials, such as graphene, transitionmetal compounds, conducting polymers, etc.2,24−26 The © 2019 American Chemical Society

superiority of using a collaborative approach is evident. The modified MoS2-based nanocomposites can not only be endowed with a high capacity from the other additions but also perform with a good electrochemical stability benefiting from the strong interaction between MoS2 and the dopants. Impressively, 2D MoS2/conductive polymer nanocomposites have obtained advanced achievements in energy storage. Conductive polymers (polyaniline (PANI), polypyrrole) are regarded as good pseudocapacitor electrode materials. They own benefits of a high theoretical capacity, an easy synthesis process, and low cost.27−34 Growing ultrathin conductive polymer (PANI or polypyrrole) on a 2D MoS2 template can endow the nanocomposites with an enhanced specific surface area.7,35,36 The unique 2D architecture facilitates electrodes to expose more activate sites for redox reaction and ion diffusion and also avoids the over swelling of polymer during the electrochemistry reaction. However, the traditional combination manner of MoS2 with conductive polymers confronts the following defects. (1) The uneven morphology of 2D MoS2/ conductive polymers leads to the underutilization of 2D MoS2 and insufficient exposure of conducting polymers to the Received: March 13, 2019 Revised: May 11, 2019 Published: May 29, 2019 11540

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of the Preparation of MoS2−NH2/PANI-x

Figure 1. SEM images of (a) MoS2−NH2 and (b, c) MoS2−NH2/PANI-100. (d, e) TEM images of MoS2−NH2/PANI-100 with different magnifications. Atomic force microscopy (AFM) images of (f) MoS2−NH2 and (g) MoS2−NH2/PANI-100 nanosheets, and inset maps are the corresponding extracted line profiles that demonstrate their height and length. (h−l) TEM image of MoS2−NH2/PANI-100 and corresponding elemental mapping images for Mo, S, N, and C.

a result, the optimized MoS2−NH2/PANI presents a high capacitance of 326.4 F g−1 at 0.5 A g−1 and a superior rate capacitance of 164.3 F g−1 even at an ultrahigh current density of 1000 A g−1, i.e., with a high rate retention of 63.1%. Impressively, the corresponding symmetric supercapacitor delivers an excellent electrochemical cycling stability with 96.5% retention after 10000 cycles.

electrolyte. (2) The nanocomposites without any interaction cannot guarantee efficient charge transfer, especially at the high current density.37 Therefore, the development of 2D MoS2/ polymers with well-defined interfaces, especially covalent connecting conducting polymers with MoS2, is essential, yet challenging, to resolve the drawbacks mentioned above. In this work, we demonstrate the synthesis of sandwich-like polyaniline covalently modified functional MoS2 nanosheets (MoS2−NH2/PANI) by controlling the thickness of PANI films targeted growing on 4-aminophenyl functionalized MoS2 (MoS2−NH2). The 4-aminophenyl group as a linker makes PANI tightly and orderly anchor on MoS2 via the S−C covalent bond. The final MoS2−NH2/PANI hybrids combine the large specific surface area of MoS2 and the high capacitance of PANI, while possessing plentiful active sites, displaying a typical pseudocapacitance behavior and high cycle stability. As

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RESULTS AND DISCUSSION The synthetic process of sandwich-like polyaniline covalently modified MoS2 nanocomposites is illustrated in Scheme 1. First, MoS 2 bulk was chemically exfoliated into MoS 2 monolayers (CE-MoS2) through the Li intercalation approach (Scheme S1). Subsequently, CE-MoS2 was substituted with 4aminophenyl diazonium salt under aqueous condition (Schemes S2 and S3) wherein the phenyl group forms a S− 11541

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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ACS Sustainable Chemistry & Engineering

Figure 2. (a) FT-IR spectra and (b) Raman spectra comparison of MoS2−NH2, PANI, MoS2−NH2/PANI-100, MoS2−NH2/PANI-150, and MoS2−NH2/PANI-200. (c) Raman image of the C−H bond in MoS2−NH2/PANI-100 at 1175 cm−1. (d) Raman image of the intensity ratio of J1 (at 228 cm−1) to C−H (at 1167 cm−1) of MoS2−NH2/PANI-100. (e) Mo 3d XPS spectra of MoS2−NH2, MoS2−NH2/PANI-100, MoS2−NH2/ PANI-150, and MoS2−NH2/PANI-200. (f) S 2p XPS spectra of MoS2−NH2/PANI-100, MoS2−NH2/PANI-150, and MoS2−NH2/PANI-200.

elements of MoS2−NH2 suggests the uniform distribution of 4aminophenyl groups on MoS2−NH2 (Figure S5). The new emerging peak of the C−S (at 687 cm−1) and the visible presence of the aniline peaks N−H (at 1620 cm−1) stretching vibrations in the FT-IR spectra of MoS2−NH2 (Figure S6) prove the successful covalent functionalization of MoS2 to produce MoS2−NH2.40−42 The Raman spectrum of MoS2− NH2 obviously presented four peaks of C−H (1175 cm−1), C− C (1275 cm−1), C−N (1341 cm−1), and C−C of the benzenoid ring (1586 cm−1), giving sufficient evidence of the 4-aminophenyl group on MoS2−NH2 (Figure S7).43 The J1 and J2 vibration modes at 182 and 228 cm−1 confirmed that MoS2−NH2 remained both in the 1T and 2H phases.44 The uniform distribution of C−H bond at 1175 cm−1 in the microRaman image of MoS2−NH2 further reflected that the 4aminophenyl was evenly grafted on the CE-MoS2 monolayers (Figure S8), which is consistent with the elemental mapping images. The weight percentage of 4-aminophenyl groups on MoS2−NH2 was 6.9% as determined by the thermogravimetric analysis (TGA) test (Figure S9). After in situ polymerization, MoS2−NH2/PANI-x samples, taking MoS2−NH2/PANI-100 as a typical example, remain in a two-dimensional sheet shape but have a rough surface

C bond with MoS2 to produce 4-aminophenyl functionalized MoS2 (MoS2−NH2).38−40 Finally, the MoS2−NH2 nanosheets were covalently modified with different amounts of polyaniline by in situ polymerization, while the polyaniline forms a C−N bond with MoS2−NH2 (Scheme S4). The final compounds were denoted as MoS2−NH2/PANI-x, where x = 100, 150, 200, and 300 stands for the volume (microliter) of aniline monomer. The thickness of 2D MoS2−NH2/PANI-x nanosheets can be effectively controlled by adjusting the ratio of aniline monomers to MoS2−NH2 during the syntheses process. The MoS2/PANI (with CE-MoS2 instead of MoS2−NH2 template) and pure PANI were also synthesized for comparison by using the same procedure. Synthesis and Characterization. Compared with MoS2 bulk, CE-MoS2 displayed a thin layer structure with wrinkles, as evidenced by scanning electron microscopy (SEM) images (Figure S1). The transmission electron microscopy (TEM) images (Figure S2) and X-ray diffraction (XRD) patterns (Figure S3) further confirmed the successful preparation of CE-MoS2. After functionalization, MoS2−NH2 remained transparent with an ultrathin sheet-like morphology, with layer sizes in the range of 100 to 800 nm (Figure 1a and Figure S4). The homogeneous distribution of Mo, S, C, and N 11542

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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ACS Sustainable Chemistry & Engineering

Figure 3. (a) CV curves of MoS2−NH2, MoS2−NH2/PANI-100, MoS2−NH2/PANI-150, and MoS2−NH2/PANI-200 at the scan rate of 100 mV s−1 in 1 M H2SO4 electrolyte in a three-electrode system. (b) CV curves of MoS2−NH2/PANI-150 at different scan rates from 5 mV s−1 to 100 mV s−1. (c) CV curves of MoS2−NH2/PANI-150 at different scan rates from 0.2 V s−1 to 5 V s−1. (d) GCD profiles of MoS2−NH2, MoS2−NH2/ PANI-100, MoS2−NH2/PANI-150, and MoS2−NH2/PANI-200 at 0.5 A g−1. (e) GCD profiles of MoS2−NH2/PANI-150 at different current densities. (f) Specific capacitance for MoS2−NH2, MoS2−NH2/PANI-100, MoS2−NH2/PANI-150, and MoS2−NH2/PANI-200 at different current densities.

elemental mapping images (Figures 1h−l and S14), in which Mo, S, C, and N elements are found to be uniformly distributed throughout the nanosheets. FT-IR was further employed to make sure PANI was successfully grafted onto MoS2−NH2. As presented in Figure 2a, MoS2−NH2/PANI-x showed both characteristic peaks of MoS2−NH2 and PANI, indicating a hybrid structure. Notably, the vibration peaks of CH, CN, and CN in MoS2− NH2/PANI-x were red-shifted as compared to those of pure PANI. It is caused by the covalent connection and strong π−π interaction between MoS2−NH2 and PANI. The intensity of peaks assigned to the groups of PANI in MoS2−NH2/PANI-x showed a stepwise increase with the increasing amount of PANI. Raman spectroscopy of the nanocomposites proved the successful hybridization of MoS2−NH2 and PANI as well. Compared with MoS2−NH2, MoS2−NH2/PANI-x presented a blue shift in both peaks for the out-of-plane CH motions and CN bond, attributing to the interaction between PANI and MoS2−NH2 (Figure 2b).43,48 In reference to PANI, MoS2−NH2/PANI-x samples showed a uniform distribution of CH bonds (Figures 2c and S15b−d). As shown in the micro-Raman images of MoS2−NH2, PANI, and MoS2−NH2/ PANI-x, the change tendency of the intensity ratio of J1 (at 228 cm−1) vs CH (at 1167 cm−1) was consistent with the amount of PANI in the nanocomposites (Figures 2d and S15). All of the above results indicated the successful connection of PANI onto MoS2−NH2 nanosheets. XRD analysis was carried out to investigate the crystalline structure of MoS2−NH2/PANI-x. The main diffraction peaks of MoS2−NH2/PANI-x at 14.4° and 43.1° correspond to the

morphology (Figures 1b and S10). It is noted that numerous PANI nanocluster arrays around 20 nm in diameter are distinctly seen on both sides of the nanosheets from the highresolution SEM image (Figure 1c). The obtained MoS2−NH2/ PANI-300 cannot keep a good sheet-liked amorphous structure due to the high concentration of aniline monomer in the reaction system (Figure S11). As a comparison, pure PANI displayed an irregular and agglomerate arrangement (Figure S12a). The layer surface of MoS2/PANI-150 is less uniform than that of MoS2−NH2/PANI-150 (Figure S12b). TEM images further confirmed the sandwich-liked structure of MoS2−NH2/PANI-x (Figures 1d and S13). It is clearly observed that PANI (marked with a white dotted line) was coated on the MoS2−NH2 nanosheets in MoS2−NH2/PANI100 from the high-resolution TEM image (Figure 1e). Moreover, the sheet-like morphology became more and more apparent with increasing the amount of the aniline monomer. It suggests that the thickness of PANI on MoS2− NH2 monolayers can be accurately tuned by varying the ratio of aniline to MoS2 (Figure S13). Atomic force microscopy (AFM) images showed that MoS2−NH2 and MoS2−NH2/ PANI-100 kept the ultrathin 2D nanosheet architecture (Figure 1f, g), indicating the well-matched and defined interface between MoS2−NH2 and PANI. The average thickness of MoS2−NH2 is about 1.0 nm, consistent with the result of the MoS2 monolayer,7,10,45−47 further testifying that MoS2−NH2 remains a single-layered property (Figure 1f). MoS2−NH2/PANI-100 has a thickness of about 64 nm (Figure 1g), indicating that the thickness of each PANI layer is about 32 nm. The homogeneous growth of PANI on MoS2−NH2 sheets is verified by a morphology image and corresponding 11543

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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Figure 4. (a) CV curves of MoS2/PANI-150 and MoS2−NH2/PANI-150 at 5 mV s−1. (b) GCD profiles of MoS2/PANI-150 and MoS2−NH2/ PANI-150 at the current density of 0.5 A g−1. (c) Specific capacitance variation with different current densities of MoS2−NH2/PANI-150 and MoS2/PANI-150. (d) Illustration of the charge process in MoS2−NH2/PANI-150 and MoS2/PANI-150. (e) Electrochemical mechanism of the sandwich-like MoS2−NH2/PANI nanosheets electrode.

structure.50 The peaks corresponding to S 2p shift toward higher binding energies of 2p3/2 (161.9 eV) and S 2p1/2 (168.3 eV), as shown in Figure 2f. Moreover, with the increased thickness of PANI, the coordinated S 2p3/2 peaks gradually become weakened. These results concluded that the outer electrons of the S atom (mainly the S 2p3/2 state) of MoS2− NH2 interact with PANI during the polymerization, and MoS2−NH2 is homogeneously covered by PANI.41 Electrochemical Performance. The electrochemical performance of the nanocomposites was first evaluated in a three-electrode cell with 1 M H2SO4 solution as the electrolyte. CE-MoS2 electrodes present a similar electrochemical performance with MoS2−NH2 electrodes (Figure S21), which suggests that the covalent functionalization of MoS2 does not significantly change the electrochemical property of the CEMoS2. As shown in Figure 3a, there are two pairs of redox peaks in the cyclic voltammetry (CV) curves of MoS2−NH2/ PANI-x, and even in the 2D CE-MoS2 and MoS2−NH2 electrodes, attributed to the Faradaic process of the oxidation/reduction reaction of Mo atoms and PANI (Figures S21 and S22a).18,23 The MoS2−NH2/PANI-x electrodes

(001) and (003) atomic planes of the hexagonally structured MoS2 (JCPDF no. 89-5112) (Figure S17). Because of the low amount of PANI and the much stronger crystallization of MoS2 than PANI, the diffraction peaks for PANI were hardly detected in MoS2−NH2/PANI-x.49 According to TGA analysis (Figure S18), the weight percentages of PANI in MoS2−NH2/ PANI-x were 12.7%, 13.9%, and 15.8% for MoS2−NH2/PANI100, MoS2−NH2/PANI-150, and MoS2−NH2/PANI-200, respectively. XPS measurements were performed to investigate the elemental components of MoS2−NH2/PANI-x nanocomposites (Figures 2e, 2f, S19, and S20). The coexistence of Mo, S, C, and N elements were observed in the survey spectra of MoS2−NH2 and MoS2−NH2/PANI-x, verifying successful covalent modification of MoS2 with PANI (Figures S19 and S20). The deconvolution of Mo4+ 3d peaks suggested a combination of the 1T phase and 2H phase for all of the samples, which benefits the electronic conduction (Figure 2e). After the grafting of PANI on the MoS2−NH2 nanosheets, all peaks shifted toward higher binding energies, indicating that electron transfer occurs in the MoS2−NH2/PANI hybrid 11544

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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Figure 5. (a) CV curves for supercapacitors of MoS2−NH2/PANI-150-based and MoS2/PANI-150-based devices at 100 mV s−1. (b) GCD profiles of devices based on MoS2−NH2/PANI-150 and MoS2/PANI-150 at 2 A g−1. (c) GCD profiles of a MoS2−NH2/PANI-150-based supercapacitor from 0.5 A g−1 to 10 A g−1. (d) Specific capacitance at different scan rates and current densities. (e) Nyquist plots. (f) Bode phase plots. The solid line highlights the characteristic frequency f 0 (1/τ0) at the phase angle of −45°. (g) Ragone plots. (h) Cycling performance of MoS2−NH2/PANI150 and MoS2/PANI-150-based SSCs at 5 A g−1 with comparison of other polymers/MoS2-based electrode supercapacitors (MoS2/PANI-I@C,55 polyprrole/MoS27,PANI/MoS2,56MoS2/PANI-60,57 MoS2@PANI,35 MoS2/RGO@PANI,58PANI-MoS259). (i) Digital images of the MoS2−NH2/ PANI-150-based supercapacitor turning an electronic fan and lighting a red LED.

densities of all the electrodes are shown in Figure 3f and Figure S23. At a current density of 0.5 A g−1, the order of the specific capacitance values is 326.4 F g−1 (MoS2−NH2/PANI150) > 260.7 F g−1 (MoS2−NH2/PANI-200) > 182.9 F g−1 (MoS2−NH2/PANI-100). It is intriguingly mentioned that MoS2−NH2/PANI-x electrodes keep a rate capability that is distinctly superior than that of PANI (Figure S22c). For instance, the MoS2−NH2/PANI-150 electrode retained 63.1% of its original capacity (164.3 F g−1) even at a high current density of 1000 Ag1−, while PANI kept only 39.6% of the pristine capacitance (165.7 F g−1). The super electrochemical rate performance of MoS2−NH2/PANI-x is a benefit from the sandwich-like PANI/MoS2−NH2/PANI ultrathin nanosheet with its large electrochemically active surface area for charge transfer and reduced ion diffusion length during the charge/ discharge process. Moreover, both MoS2−NH2 and PANI possess the pseudocapacitance to guarantee an improved capacitive performance.

possess a superior electrochemical behavior compared to MoS2−NH2. Moreover, MoS2−NH2/PANI-150 owns the best capacitance among the MoS2−NH2/PANI-x. These results not only highlight the synergistic effect of covalent linking PANI onto functional MoS2 but also emphasize that the thickness of PANI is key for the capacitive performance. All of the CV curves of MoS2−NH2/PANI-150 present similar shapes even at the high scan rate of 2 V s−1. This desirable fast charge/ discharge property is a benefit from the unique structure (Figure 3b, c). From the galvanostatic charge/discharge (GCD) plots for all the electrodes tested at 0.5 A g−1, the MoS2−NH2/PANI-x electrodes possess distinctly higher specific capacitances than that of MoS2−NH2 but slightly lower than that of PANI (Figures 3d and S22b). The unideal straight lines at different current densities in the GCD curves of the MoS2−NH2/PANI150 electrode also support pseudocapacitance charge storage behavior (Figure 3e). The relationships between specific capacitances (calculated from GCD curves) and current 11545

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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to the transitions between different doping states of PANI (Figures 5a and S25). Significantly, the resultant current density for MoS2−NH2/PANI-150 nanocomposites is higher than that for MoS2/PANI-150 at the scan rate of 100 mV s−1. This highlights that the covalent connection between MoS2− NH2 and PANI in MoS2−NH2/PANI-150 can synergistic improve capacitance. This result is further proved by the GCD plots (Figure 5b). The specific capacitance of the MoS2−NH2/ PANI-150 supercapacitor at 0.5 A g−1 (58.6 F g−1) is higher than that of MoS 2 /PANI-150 (46.6 F g −1 ). Typical chronopotentiometry curves of a MoS2−NH2/PANI-150// MoS2−NH2/PANI-150 device at different current densities (Figure 5c) show nearly linear slopes with only with a little IR drop, demonstrating a low charge delivery resistance and competent electrolyte diffusion within the MoS2−NH2/PANI150 electrodes. Notably, the specific capacitance of all the supercapacitors decreased with the increase of the current density or scan rate (Figure 5d). Evidently, the MoS2−NH2/PANI-150-based supercapacitor displays the best rate capability. It keeps a capacity of 52.2 F g−1 at 35 A g−1, holding 89% of its capacitance at 0.5 A g−1. Unfortunately, only 24.3% and 32.2% of the initial specific capacitance remained for PANI//PANI and MoS2/PANI-150//MoS2/PANI-150 devices, respectively (Figure S26). The high rate capability for MoS2/PANI-150based devices strongly benefits from the strong covalent binding of ordered PANI array and MoS2−NH2 to generate effective ion diffusion paths and advanced electrical conductivities. To further investigate the capacitance behaviors and electrolyte ion transport, electrochemical impedance spectroscopy (EIS) analyses were analyzed by utilizing Nyquist plots. The order of the intrinsic ohmic resistance (Rs) values for the electrochemical devices is MoS2−NH2/PANI-150 (0.48 Ω) < MoS2/PANI-150 (0.55 Ω) < PANI (0.98 Ω), implying that MoS2−NH2/PANI-150 owns the best electrical conductivity (Figures 5e and S27). Among the various SSCs, the MoS2− NH2/PANI-150 device presented the smallest charge transfer resistance (Rct) at high-frequency sections. The steepest sloped line in the low-frequency region for the MoS2−NH2/PANI150-based supercapacitor indicated a perfect capacitive behavior with the fastest ion transport, due to the strong interaction between the ordered PANI array and the 2D MoS2 nanosheet. A phase angle close to −90° at 0.01 Hz in the Bode curve for the MoS2−NH2/PANI-150 device (Figures 5f and S28), further proving its nearly ideal capacitive behavior. Moreover, the characteristic time constants (τ0 = 1/f 0) that depend on the characteristic frequency (f 0) at the phase angle of −45° for MoS2−NH2/PANI-150, MoS2/PANI-150, and PANI supercapacitors are 0.49, 0.58, and 4.34 s, respectively. The τ0 value of 0.49 s for the MoS2−NH2/PANI-150 supercapacitor not only is smaller than those for MoS2/ PANI-150 and PANI but also is lower than the values of 0.53 s for the 3D graphene scaffold,51 0.7 s for multiwalled carbon nanotubes,52 and 10 s for activated carbon.53 This result reveals that the MoS2−NH2/PANI-150 device owns a fast frequency response in good agreement with its outstanding rate capability confirmed by the CV and GCD data.54 Attributing to the wonderful rate performance at high charge/discharge current densities, MoS2−NH2/PANI-150// MoS2−NH2/PANI-150 owns an energy density of 4.0 Wh kg−1 at a power density of 175 W kg−1 (Figure 5g). When the power density increases to 14 kW kg−1, the energy density still

In order to further prove the merits of the unique heterostructure of MoS2−NH2/PANI nanosheets with welldefined interfaces, MoS2/PANI-150, the material of PANI directly grown on the 2D CE-MoS2, was also synthesized and characterized. It is evident that the MoS2−NH2/PANI-150 electrode possesses an integrated CV curve area that is larger than that of MoS2/PANI-150 at the same scan rate (Figure 4a), suggesting that the capacitance of MoS2−NH2/PANI-150 is higher than that of MoS2/PANI-150. The CV curves for the two kinds of electrodes exhibited a similar shape with two pairs of redox peaks nearly at 0.2 and 0.5 V, due to their pseudocapacitance mainly from PANI. Interestingly, the reversible capacity of the MoS2−NH2/PANI-150 is as high as 260.7 F g−1 at 0.5 A g−1, which is higher than the value of 207 F g−1 for MoS2/PANI-150 (Figure 4b). Moreover, the MoS2−NH2/PANI-150 electrode presents a much more outstanding electrochemical rate property. At a high current density of 1000 A g−1, MoS2−NH2/PANI-150 even remained at 63.1% of the pristine capacity at 0.5 A g−1, but the MoS2/ PANI-150 electrode remained at only 54.98% (Figures 4c and S24). According to the electrochemical property of MoS2−NH2/ PANI-150 and MoS2/PANI-150, the electron diffusion condition of the electrodes at the low and high current densities was illustrated. As shown in Figure 4d, whether the electrodes were charged at low or high current density, more electrons transfer into the MoS2−NH2/PANI-150 rather than MoS2/PANI-150 electrodes, resulting in a superior capacity and better stability for MoS2−NH2/PANI-150. The superiority of MoS2−NH2/PANI-150 can be explained as follows. (1) The ordered arrangement of PANI on MoS2−NH2 nanosheets in MoS2−NH2/PANI-150 rather than the disarray of PANI stacked on CE-MoS2 in the MoS2/PANI-150 electrode could provide a shorter diffusion distance of ions across their conductive direction. The SEM images of the electrodes can further confirm this conclusion (Figures S10a and S12b). (2) Compared with MoS2/PANI-150, the much more homogeneous nanosheets of MoS2−NH2/PANI-150 could supply more dense electrochemical active sites and numerous pathways for charge storage and delivery. (3) The special linkers of PANI and MoS2−NH2 (S−C bond) in MoS2−NH2/ PANI-150 could reduce the interface resistance gap between the MoS2 substrate and PANI. They also play a role as an electron “superhighway” to improve the ion/electron delivery rate, especially at the high current density. This conclusion can be verified by the Nyquist data plots. The electrochemical mechanism of the MoS2−NH2/PANI nanosheet electrode is illustrated in Figure 4e. The unique structure of the sandwichlike MoS2−NH2/PANI nanosheets with ordered PANI covalently connected on MoS2−NH2 with special linkers provides a superhighway for the diffusion of the ions during charge/discharge. The fast ion diffusion is preferred for high rate ability. The unique sandwich-like structure of the MoS2− NH2/PANI nanosheets could provide a large electrochemical surface area for charge storage, resulting in improved capacitance. Symmetric supercapacitors (SSCs) based on MoS2−NH2/ PANI-150 electrodes were assembled with 1 M H2SO4 solution electrolyte to appraise the potential practical applications. For comparison, the SSCs by using pure PANI and MoS2/PANI-150 electrodes were also constructed. The CV curves of the SSCs present a pseudocapacitance behavior with two couples of redox waves, which are mainly attributed 11546

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

ACS Sustainable Chemistry & Engineering

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remains at 3.5 Wh kg−1. The cycling stability of the MoS2− NH2/PANI-150, MoS2/PANI-150, and PANI-based SSCs were evaluated via the GCD approach at 5 A g−1 (Figures 5h and S29). Impressively, the MoS2−NH2/PANI-150-based device lost only 3.5% capacitance after 10000 cycles, and the value is lower than for other MoS2/PANI-based supercapacitors.7,35,55−59 The superior cycling performance can be attribute to the advanced sandwich-like structures and the covalent connection between MoS2 and PANI. The slight capacity loss of MoS2-NH2/PANI-150 is mainly caused by the degradation of the PANI polymer chain and the disconnection of MoS2−NH2 and PANI during long time cycling.34 Moreover, our MoS2−NH2/PANI-150//MoS2−NH2/PANI150 supercapacitor successfully powered an electronic fan and lighted a red LED (Figure 5i), demonstrating that our sandwich-like material has potential for practical energy storage devices.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.Y.). ORCID

Kai Yuan: 0000-0002-4507-1510 Yiwang Chen: 0000-0003-4709-7623 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC-DFG Joint Research Project (51761135114), the National Science Fund for Distinguished Young Scholars (51425304), the Natural Science Foundation of Jiangxi Province (20171ACB21009, 2018ACB21021), the National Natural Science Foundation of China (21704038, 51763018), the National Postdoctoral Program for Innovative Talents (BX201700112), and the China Postdoctoral Science Foundation (2018M632599).

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CONCLUSIONS In summary, sandwich-like PANI covalent modified functional MoS2 (MoS2−NH2/PANI) nanosheets were successfully synthesized. 4-aminophenyl group, as the linker of CE-MoS2 and PANI, makes PANI tightly anchor on MoS2 by the S−C covalent bonds, which significantly strengthens the structural durability and boosts the transportation of ions. The MoS2− NH2/PANI hybrids combine the 2D morphology of MoS2 nanosheets with high electric conductivity and high capacitance of PANI, while creating plentiful active sites, playing a typical pseudocapacitance behavior and enabling the ultrafast ion diffusion. The thickness of the MoS2−NH2/PANI monolayers was regulated by the amount of the aniline monomer. As a result, the optimized MoS2−NH2/PANI-150 nanosheets presents a high capacitance of 326.4 F g−1 at 0.5 A g−1 and a superior rate capacity with a retained 63.1% (164.3 F g−1) of original capacity even at 1000 A g−1. Impressively, the corresponding symmetric supercapacitor delivers a high energy density of 3.5 Wh kg−1 at a power density of 14 kW kg−1 and superior electrochemical cycling stability with 96.5% retention after 10 000 cycles. The sandwich-like MoS2−NH2/PANI heterostructure is capable of ultrahigh rate capacity and excellent long cycle life, which indicates a new route to rationally design conducting polymers/MoS2-based hybrid materials for supercapacitors with good stability and performance.

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Research Article

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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/acssuschemeng.9b01442. Experimental section; scheme of synthesized CE-MoS2, 4-aminophenyl diazonium salt, and 4-aminophenyl functionalized MoS2; scheme depicting the preparation of MoS2−NH2/PANI and corresponding SEM, TEM, and HRTEM images and XRD patterns, FT-IR spectra, Raman spectra, Raman images, TGA curves, and XPS spectra; additional electrochemical performance including CV curves, GCD profiles, specific capacitance at different scan rates and current densities, Nyquist plots, Bode phase plot, and cycling performance (PDF) 11547

DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549

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DOI: 10.1021/acssuschemeng.9b01442 ACS Sustainable Chem. Eng. 2019, 7, 11540−11549