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Integration of Ultrathin MoS2/PANI/CNT Composite Paper in Producing All-Solid-State Flexible Supercapacitors with Exceptional Volumetric Energy Density I-Wen Peter Chen,* Yu-Chen Chou, and Po-Yuan Wang Department of Applied Science, National Taitung University, 369, Sec. 2, University Road, Taitung City 95092, Taiwan

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ABSTRACT: Due to the expeditious expansion of wearable electronics, all-solid-state flexible supercapacitors are being contemplated as promising energy-storage devices. Through the successful preparation of large quantities of high-quality semiconducting-type thin molybdenum disulfide (MoS2) sheets suspended in water, the authors have developed an environmentally friendly and simple method to fabricate ternary flexible electrodes with MoS2, polyaniline (PANI), and carbon nanotubes (CNTs). The resulting MoS2/PANI/CNT all-solid-state supercapacitor can be easily integrated in series to power commercial light-emitting diodes without an external bias voltage. In addition, such a supercapacitor exhibits remarkable energy density (0.013 Wh cm−3) and power density (1.000 W cm−3), thus making it superior to commercially available lithium thinfilm batteries (4 V/500 μA h) and 2.75 V/44 mF activated carbon electrochemical capacitors. These results demonstrate that the exfoliated MoS2-based composite is a promising material for the development of high-performance and low-cost energystorage devices.

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developed to produce thin MoS2 sheets.13 The prerequisites of a high throughput and low cost make mechanical exfoliation, hydrothermal synthesis, and intercalation more suitable for fundamental research.13−15 In contrast, the method of liquidphase exfoliation is high in scalability and relatively low in cost. The pioneering work by Coleman et al. has demonstrated that liquid-phase exfoliation of bulk MoS2 in N-methylpyrrolidone is a possible route to prepare MoS2 suspensions.16 However, exfoliated monolayered thin MoS2 sheets are rare, and the available exfoliated thin MoS2 sheets are usually submicrometer in size. Therefore, the large-scale production of highquality and large, thin MoS2 sheets requires a facile, effective, and safer route for controlled scalable synthesis, and this has not yet been accomplished. In addition, the poor electrical conductivity of MoS2 results in an unsatisfactory capacitance delivery. Many studies use the reduced graphene oxide (rGO)/ MoS2 configuration to enhance the electrical conductivity for MoS2. However, many functional groups and defects remain on the surface of the rGO, which result in low electrical conductivity.17 Carbon nanotubes (CNTs) offer superior electrical conductivity, mechanical properties, and chemical

INTRODUCTION With the increasing demand for soft electronics, significant effort has been put into developing flexible energy-storage devices while maintaining a high degree of energy over shorttime pulses.1,2 Therefore, the development of various types of supercapacitors is gathering pace3 and there is a need for supercapacitors that provide more energy and power in restricted spaces. However, the current level of supercapacitor technology development has inferior volumetric energy density. Thus, making energy-storage devices light weight (which offers a much higher volumetric energy density with less device mass and smaller dimensions) and flexible (sustaining the original performance quality even while bent) is still a challenging task.4,5 Two-dimensional transition metal dichalcogenides, such as semiconducting-type molybdenum disulfide (MoS2), are newly discovered supercapacitor materials due to their superior electric double-layer (EDL) properties.6−11 Stacked MoS2 sheets can be exfoliated by introducing foreign invaders. Because of its intrinsic layered structure and high theoretical capacitance, MoS2 is viewed as a potential electrode material for supercapacitors.12 As a result, low-cost mass production of MoS2 is highly important for foreseeable demands. Several methods, such as mechanical exfoliation, hydrothermal synthesis, intercalation, and liquid-phase exfoliation, have been © XXXX American Chemical Society

Received: April 30, 2019 Revised: June 15, 2019 Published: June 19, 2019 A

DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C stability, but the charge storage capabilities are limited.18−20 Thanks to expeditious developments in the synthesis of highquality CNTs and scaling-up techniques, breakthroughs in developing thin, conductive, flexible composite films for practical applications have significantly accelerated over the past few years. Hence, an alternative way to overcome the problem of the low electrical conductivity of MoS2 is the addition of one-dimensional high-quality CNTs to the thin MoS2 sheets.21,22 When the surfaces of the sheets come in contact with CNTs, the sheets can possibly store much more energy. Pseudocapacitors are another category of promising energystorage devices owing to their rapid faradaic processes, which are described as redox processes on electrode surfaces.23,24 Polyaniline (PANI) has attracted much attention due to its redox-based charge−discharge behavior, high degree of electrical conductivity, low cost, and excellent degree of environmental stability.25−28 Typical methods of fabricating PANI-based materials usually use in situ electrochemical polymerization of aniline monomers on the working electrode or another conductive substrate.29,30 Many PANI-decorated composite materials have been synthesized via in situ electrochemical polymerization; unfortunately, there are significant drawbacks to this method.31,32 One of the major obstacles is that it is hard to effectively control the morphology of the PANI molecules during in situ electrochemical polymerization. To overcome this drawback, self-assembly in the solution is an effective way to control the composition of PANI-decorated materials.33,34 PANI possesses a good degree of solubility in some neutral surfactant-type compounds or protonic acid,35 but there are rarely wet methods for fabricating PANI-decorated electrodes. According to related literature, interfacial modification between the surface of MoS2 and polymer chains can improve the capacitive performance of the composite electrodes. Therefore, achieving a high volumetric energy density combined with a desirable cycle stability by integrating the behavior of EDL and pseudocapacitance is a possible route for all-solid-state supercapacitor applications. In recent years, the combination of MoS2, PANI, and carbon-based materials has been integrated with the merits of the Faradaic process and non-Faradaic process to discover ways to develop new synthesis routes. For example, Yang et al. showed that aniline monomers were polymerized on unstable thermodynamic MoS2 sheets. Then, carbon dots were introduced to the as-prepared MoS2/PANI surface via hydrothermal reaction.23 Chao et al. synthesized MoS2/PANI nanosheets vertically aligned on rGO via multiple synthesis steps.36 Gopalakrishnan et al. developed a two-step synthesis method to synthesize a multilayered MoS2/PANI nanocomposite.9 Thakur et al. synthesized PANI on unstable thermodynamic MoS2 sheets and CNTs, which showed that the capacitance retention of the composite material was less than 80% after 1000 charge−discharge cycles in an acidic electrolyte.37 Zhang et al. showed that multiwalled CNT/ PANI/MoS2 nanocomposites disperse in a Nafion solution and exhibit a capacitance retention of 73% in acidic electrolytes.38 Ansari et al. showed that mechanically exfoliated MoS2 sheets coupled with PANI are produced via in situ polymerization.13 However, these synthesis steps are too complicated and limited for large-scale production. As mentioned before, considering the scalability and all-solid-state supercapacitor applications, a simple and facile route to prepare MoS2, PANI, and carbon-

based ternary freestanding paper is still a great challenge. To the best of our knowledge, there has been no research to date on integrating CNTs, exfoliant-assisted exfoliated thin MoS2 sheets, and PANI for flexible all-solid-state supercapacitor applications. In this study, the authors demonstrate, for the first time, an environmentally friendly and simple method for producing a freestanding and flexible MoS2/PANI/CNT composite paper using the synergistic effect of mutual π−π interaction and hydrophobic−hydrophobic interaction, which is aimed at achieving superior volumetric energy density. The MoS2/PANI/CNT supercapacitor has excellent values for energy density (0.013 Wh cm−3), power density (1.000 W cm−3), charge−discharge cyclic stability (1000 cycles), and capacitance retention at a 120° bend angle. Our work demonstrates a scalable and practical strategy to improve the electrochemical performance of thin MoS2 sheet-based supercapacitors by the inventive introduction of CNTs and PANI. In addition, the application of MoS2 could optimize other types of conductive polymers with exceptional energy-storage performance.

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METHODS Materials. Molybdenum(IV) sulfide (MoS2; 99% metalbased; ∼325 mesh powder, Alfa Aesar), imidazole (99%, ACROS), Triton X-100 (Innovation Business Co., Ltd.), poly(vinyl alcohol) (PVA: MW 30 000−70 000; 87−90 mol % hydrolyzed, Sigma-Aldrich), ammonium peroxodisulfate (APS: 98%, SHOWA), ammonium hydroxide solution (1 M, SigmaAldrich), and aniline (99.5%, Sigma-Aldrich) were used as received. CNTs were purchased from SouthWest NanoTechnologies Inc. (SWeNT CG 200) and Golden Innovation Business Co, Ltd. (AC tube-100L). Ethanol (EtOH: 99.5%), acetone (HPLC grade), hydrochloric acid (HCl: 35%, Showa), and sulfuric acid (H2SO4: 95%, Fisher Scientific) were used without further purification. Preparation of the MoS2 Suspension. To a 1 L beaker, 4 g of bulk MoS2 powder, 40 g of imidazole, and 1000 mL of deionized water were added. After leaving the beaker for 1 h, an L5M high-shear mixer head (Silverson Machines Ltd.) was lowered into the beaker and the rotation speed was controlled at a suitable speed for 1 h. Then, the sample was further sonicated in pulse mode for 4 h. The concentration of the MoS2 suspension was ∼2 mg mL−1. The exfoliated MoS2 suspension was centrifuged at 500 rpm for 10 min to collect the micrometer-sized exfoliated thin MoS2 sheets. Synthesis of PANI. First, a 1 L beaker containing 11.46 mL of the aniline monomer (1.6 M), 150 mL of chloroform, and 550 mL of HCl (1 M) was prepared prior to polymerization. Second, a 250 mL beaker containing 18.24 g of APS and 200 mL of HCl (1 M) was prepared prior to polymerization. Then, the APS solution was added drop-bydrop to the aniline monomer solution, and it was continuously stirred for 2 h. Thereafter, the mixture was kept in an ice bath for 24 h. After filtering the mixture, the crude PANI product was repeatedly washed with 1 M HCl for 30 min and then with deionized water. Later, the crude PANI polymers were repeatedly washed with 1 M NH4OH for 30 min and then with deionized water. Afterward, they were dried in a vacuum oven. The characterization of the synthesized PANI is shown in Figure S1. Preparation of the MoS2/PANI/CNT Composite Paper. To produce a 1 L-CNT suspension, 50 mg of CNTs were sonicated with the assistance of a Triton X-100 for 1 h. Figure B

DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Characterization of an exfoliated MoS2 monolayer. (a) Photograph of an exfoliated MoS2 suspension. (b) TEM image of the large dimension of exfoliated MoS2 sheets. (c) HRTEM image of the edge of an exfoliated MoS2 sheet. (d) Fast Fourier transform image of exfoliated MoS2 sheets. (e) UV−vis spectrum of an exfoliated MoS2 suspension. (f) Raman spectrum of MoS2 sheets. (g) XPS spectrum with deconvoluted peaks of exfoliated MoS2 sheets.

(Unicam UV-300). The exfoliated thin MoS2 sheets were characterized using a JEOL JEM-2100F transmission electron microscope (TEM) and a Hitachi H-7100 high-resolution transmission electron microscope (HRTEM). Field-emission scanning electron microscopy (FESEM, JEOL JSM-7600F) and elemental mapping analysis were used to assess the distribution of carbon, molybdenum, and nitrogen in the MoS2/PANI/CNT composite papers. The supercapacitor behavior of a single electrode was measured using a threeelectrode system in a 0.5 M H2SO4 aqueous solution. Cyclic voltammetry (CV) tests on the composite papers were performed using a CHI7927E electrochemical system in the potential range of 0−1.0 V at scanning rates of 1, 3, 5, 10, 20, 100, and 200 mV s−1. Galvanostatic charge−discharge (GCD) was measured over the potential range of 0−1.0 V. The allsolid-state MoS2/PANI/CNT supercapacitors were tested in a two-electrode system. The capacitance of one electrode was calculated via a discharging curve according to the following equation:

S2a shows the characterization of this suspension. Then, 250 mL of the CNT suspension (or 12.5 mg of CNTs), 100 mg of the exfoliated thin MoS2 sheet suspension, and 20 mg of the PANI suspension were mixed. The pH of the mixture was adjusted to 3 by titrating with HCl, which kept the PANI in its protonated form.39,40 After that, the mixture was sonicated via tip sonication (500 W) for 30 min to enhance the mixing of materials. The solution was filtrated via a filtration membrane with a pore size of 0.2 μm (cellulose acetate, Advantec). The MoS2/PANI/CNT composite paper was named M100/P20. To understand the effect of MoS2 and PANI with 12.5 mg of CNTs on energy storage, we prepared a series of Mx/Py nanocomposites with varying amounts of MoS2 and PANI (x and y indicate the amounts of MoS2 and PANI, respectively). The MoS2/PANI/CNT composite papers were washed thoroughly with deionized water to remove residues. All of the papers were dried at 100 °C for 1 h to eliminate moisture. The thickness of the papers ranged from 10 to 20 μm. Fabrication of an All-Solid-State MoS2/PANI/CNT Supercapacitor. First, the PVA electrolyte was prepared as follows: 10 g of PVA and 10 mL of deionized water were used to prepare a 50 wt % PVA gel solution; then, 10 g of H2SO4 were added to the stirred PVA gel solution for 1 h to obtain a homogeneous 1:1 PVA−H2SO4 gel electrolyte. Second, a 0.25 cm × 0.25 cm coating of this gel electrolyte was applied to the top surface of an MoS2/PANI/CNT composite paper. Another MoS2/PANI/CNT composite paper was wetted with the gel electrolyte and then stacked onto the first paper, which together formed an all-solid-state supercapacitor. Characterization. The exfoliated thin MoS2 sheets were characterized by Raman spectroscopy. Raman spectra measurements with a 532 nm laser were used with an iHR550 spectrometer (Horiba Jobin Yvon), and the signal of silicon was used as a reference. A PerkinElmer Frontier Fouriertransform infrared (FTIR) spectrometer was used for all data collection on the materials. The UV−vis spectra of all of the materials were collected using a UV−vis spectrophotometer

C=

∫0

V /ν

|J | dt

V

where C is the volumetric capacitance (F cm−3), J is the current density (A cm−3), and V and ν are the discharge potential (V) and scan rate (V s−1), respectively. The energy densities (E) and the power densities (P) were calculated by means of GCD curves using the equations E = (1/2) × C (ΔV)2 × (1000/3600) and P = (E/Δt) × 3600.

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RESULTS AND DISCUSSION Figure 1a shows a 1 L bottle of the exfoliated thin MoS2 sheets in water. Figure S3 shows that the highly dispersed suspensions of the sheets remained dark green after standing for a long time, indicating an excellent degree of stability. Figure 1b shows a TEM image of the sheets after ultrasonicating for 4 h. Surprisingly, the sheets had an average lateral size of over 10 C

DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) Photograph of a freestanding flexible MoS2/PANI/CNT composite paper electrode. (b) Top-view FESEM image of an MoS2/PANI/ CNT composite paper electrode combined with energy-dispersive spectroscopy (EDS) elemental mapping in the same location and relative intensities of (c) Mo (red), (d) N (orange), and (e) C (yellow) elements. (f) Raman spectrum of M100/P20 composite papers. (g) Schematic diagram of the MoS2/PANI/CNT process for the fabrication of a freestanding composite paper.

a novel high-pressure filtration technique. The FESEM images show that the materials used for making the paper are highly entangled (Figure 2b), whereas the cross-sectional image exhibits dense packing and interlocking with a layered stacking of freestanding sheets (Figure S4a). Figure S4b,c show that the PANI appeared on not only the outer surface of the MoS2 but also the inner surface of both the MoS2 and the CNTs between the MoS2 sheets. To assess the uniformity of the sheets and PANI in the MoS2/PANI/CNT composite paper, the surface distributions of MoS2, PANI, and CNTs were assessed using energy-dispersive spectroscopy (EDS) mapping. Figure 2 shows that the Mo (Figure 2c), N (Figure 2d), and C (Figure 2e) are distributed uniformly throughout the entire region. The interaction of MoS2/PANI/CNT materials was characterized by the Raman spectrum, and the results are depicted in Figure 2f. Figure S2b shows that the Raman spectrum of pure CNTs displays two peaks at 1332 and 1588 cm−1, which correspond to the D and G bands, respectively. The G band represents the vibration of the sp2-hybridized carbon configuration. The phonon mode at 1332 cm−1, also known as the D band, represents the sp3-hybridized carbon configuration. For pure PANI, the C−C stretching of benzoid is situated at 1588 cm−1. The peaks at 1588 cm−1 indicate the PANI or CNT structures and that they shifted to 1583 cm−1 (Figure 2f) when the MoS2, PANI, and CNTs were mixed. For pure CNTs, the D band at 1332 cm−1 depicts the CNT structure and its shift to 1325 cm−1 (Figure 2f) when the MoS 2 , PANI, and CNTs were mixed. These results demonstrate that strong chemical interactions took place between the materials. In addition, the peak at 277 cm−1 indicates the radial breathing mode of the pure CNTs and their shifts to 270 cm−1 when the MoS2/PANI/CNT

μm, which is almost 100 times larger than that produced by the liquid-phase exfoliation method.16,41 Figure 1c shows an HRTEM image depicting the edge of a sheet, showing that the sheet is monolayered. Moreover, the lattice structure of the surface of the sheet was not damaged during sonication, as shown in Figure 1d. To further characterize the phase of the sheets, UV−vis, Raman, and X-ray photoelectron spectroscopy (XPS) analyses were carried out. Figure 1e shows the typical absorption spectrum of the sheets. Two peaks appear between 600 and 700 nm, indicating the existence of high-quality sheets.42 Figure 1f depicts two Raman peaks at around 387 and 410 cm−1, corresponding to the E2g1 and A1g modes, respectively. Note that the difference in the Raman shift between 1E2g and A1g is consistent with that of protein or mechanically induced exfoliated monolayer MoS2 sheets.43,44 Figure 1g shows the Mo 3d of the XPS spectrum. The Mo 3d spectrum has peaks at around 229.1 and 232.2 eV, indicating the Mo4+ 3d5/2 and Mo4+ 3d3/2 components of MoS2, respectively. Moreover, the spectrum has two unobservable peaks (blue line) at 228.2 and 231.2 eV, indicating the Mo4+ 3d 5/2 and Mo4+ 3d3/2 components of metallic MoS2, respectively. These results are consistent with those of other studies on MoS2 crystals, demonstrating that most of the sheets are in stable thermodynamic form. To make these sheets large enough for real-world applications, the supercapacitor electrode should be made as large as possible while maintaining its original performance capacity. The MoS2/ PANI/CNT composite papers were fabricated using the synergistic effect of mutual π−π interaction and hydrophobic−hydrophobic interaction as shown in Figure 2a. Figure 2a shows that a flexible and freestanding MoS2/ PANI/CNT composite paper without any cracks was made via D

DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. (a) CV curves of M20/P0.5, M20/P5, M20/P10, M20/P20, and M20/P60 at a scan rate of 5 mV s−1 in 0.5 M H2SO4. (b) CV curves of M0/P20, M5/P20, M20/P20, M100/P20, M200/P20, M1000/P20, and M2000/P20 at a scan rate of 5 mV s−1 in 0.5 M H2SO4.

Figure 4. Electrochemical behavior of the M100/P20 composite paper electrode measured in 0.5 M H2SO4. (a) CV curves at different scan rates. (b) GCD curves. (c) Volumetric capacitances.

by adjusting the amount of MoS2 in the composite papers, and the results were also compared with those of the M20/P20 electrodes. A high correlation between the initial MoS2 weight in the controlled amount of PANI and the measured capacitance was observed. Figure 3b shows that the M100/ P20 exhibited a 3-fold electrochemical capacitance when compared to that of the M20/P20 electrode at the same scan rate of 5 mV s−1. Therefore, the composition of the M100/P20 composite paper electrode performed best in terms of energy storage. This large storage capacity is mainly attributed to the synergistic interfacial interaction between MoS2, PANI, and CNTs. Figure 4a shows the typical CV behavior of the M100/P20 composite paper electrode at scan rates of 1−200 mV s−1 in a 0.5 M H2SO4 electrolyte solution. It was found that the current density of the cathodic site increased and that the current density of the anodic site shifted to lower values at a higher scan rate, indicating that the M100/P20 composite paper electrode processes are controlled by mass transfer.29 There were no significant changes in the shapes of the measured curves, which indicate a desirable performance due to efficient charge transportation at the composite paper interfaces. The GCD curves for the M100/P20 composite paper electrode with various current densities are shown in Figure 4b,c. The shapes of the charging and discharging curves of the M100/ P20 composite paper electrode are slightly distorted triangles owing to the pseudocapacitive behavior of PANI, which is consistent with the results obtained in other studies.26,50−54 Impressively, Figure 4c shows that the volumetric capacitance of the MoS2-based composite paper electrode estimated from the GCD curves was 245 F cm−3 in a three-electrode system at a current density of 0.3 A cm−3. Moreover, nearly 80% of the initial capacity at 0.3 A cm−3 was restored upon increasing the current density to 19.2 A cm−3, which improved the results for power and energy density.

composite paper was fabricated, indicating that the diameter of the CNTs changed.45 Moreover, the Mo XPS data of the MoS2/PANI/CNT composite paper in Figure S5 is identical to that in Figure 1g, indicating that the fabrication process of the composite paper did not change or damage the structure of the materials. The peaks of CC, C−N, −NH, N+, and Mo−N were also found in the composite paper, indicating that the materials were uniformly distributed within.46,47 Looking at the SEM characterization, the top and side views of the paper exhibit layered and porous structures. In addition, the materials combined very well, which may be attributed to the synergistic effect of mutual π−π interactions and the hydrophobic− hydrophobic interaction of the polymer chains of PANI with MoS2 and CNTs. The interaction between MoS2, PANI, and CNTs not only provides efficient interfacial contact but also enhances the electrochemical performance of MoS2/PANI/ CNT. As capacitance values are greatly influenced by the material composition, a systematic and detailed examination of the MoS2/PANI nanocomposite seems necessary to assess the interfacial relationship between MoS2 and PANI. The capacitances of the MoS2/PANI/CNT composite paper electrodes were measured using a three-electrode system in which Ag/AgCl and platinum wire were used as the reference and counter electrodes, respectively. As expected, the current densities measured from these samples increased with increasing amounts of PANI (Figure 3a). Furthermore, the area under the CV curve of the M20/P20 electrode is nearly 5 times greater than that of the M20/P0.5 electrode, which quantitatively indicates a higher capacitance for the former. The decline in capacitance beginning with a high initial concentration of PANI is probably due to the higher degree of electrode resistance, longer diffusion pathways, and increasing difficulty for ion penetration to the inner site.48,49 To examine the effect of MoS2 loading, a comparative study was carried out E

DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. Flexible all-solid-state M100/P20 supercapacitor. (a) Schematic diagram, (b) side view, and (c) top view of the supercapacitor. (d) CV curves at different scan rates. (e) Digital photograph of the bendable supercapacitor. (f) CV curves of the supercapacitor at different bending angles of 30, 60, 90, and 120°. (g) Stability test of the supercapacitor for charge−discharge cycles.

Figure 6. (a) Nyquist plot of the M100/P20 all-solid-state supercapacitor. (b) GCD curves of a single all-solid-state M100/P20 supercapacitor and a series of three all-solid-state M100/P20 supercapacitors at a current of 0.45 A. (c) LED powered by a series of two all-solid-state M100/P20 supercapacitors.

Given the high quality of the exfoliated single-layer thin MoS2 sheets (with a large surface area) and the excellent degree of flexibility of the prepared M100/P20 composite paper, the composite paper was used to fabricate an all-solidstate symmetric supercapacitor. With two slightly separated MoS2/PANI/CNT composite paper electrodes solidified in a PVA−H2SO4 gel electrolyte solution, an all-solid-state supercapacitor was fabricated, and it has tremendous potential for powering ultrathin electronics. More importantly, owing to the ultrathinness of the electrodes and their efficient compatibility with the PVA gel electrolyte solution, the supercapacitor exhibited a remarkable degree of flexibility (Figure 5a,b). Figure 5c shows the top view of the supercapacitor. The binder-free supercapacitor displayed a typical double-layer capacitive behavior at scan rates ranging from 1 to 200 mV s−1 (Figure 5d). Figure S6 shows the GCD curves of the supercapacitor. The minorly distorted GCD curves are attributed to the intrinsic properties of PANI.25,26,30,54−57 The volumetric capacitance of the M100/P20 all-solid-state symmetric supercapacitor is 91.1 F cm−3, as shown in Table S1, which is superior to that of all-solid-state supercapacitors made using exfoliated graphene, rGO, and electrochemically exfoliated graphene.58−61 The high quality of single-layer MoS2

sheets resulted in a sufficiently large surface area for energy storage. The flexibilities of all of the tested symmetric all-solid-state supercapacitors were assessed by measuring their electrochemical performances as they were bent into different angles. A photograph of the assembled supercapacitor is shown in Figure 5e. Figure 5f shows CV curves that remain almost unchanged as the supercapacitor was bent from 0 to 120°, which indicates that the supercapacitor exhibited an excellent degree of flexibility. The supercapacitor exhibits an excellent degree of stability for the charge−discharge cycles owing to the gel electrolyte solution with a retention value of 80% after 1000 cycles (Figure 5g). Figure S7 shows that the morphology of the supercapacitor remained smooth as no pits formed on the surface, indicating that the supercapacitor has superior mechanical properties. To further demonstrate the merits of the supercapacitor as a superior electrode material, electrochemical impedance spectroscopy characterization was an effective tool in assessing electronic conductivity. The Nyquist plot in Figure 6a is composed of a semicircle in the high-frequency region and an approximately straight line in the low-frequency region. The high-frequency intercept at the x-axis corresponds to the F

DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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CONCLUSIONS In summary, the flexible MoS2 composite paper fabricated by a facile method on the macroscale can serve as a reliable subject to systematically study and discuss the interaction between related materials. Moreover, a novel high-pressure filtration technique was used to produce a freestanding, bendable MoS2/ PANI/CNT supercapacitor electrode. The MoS2/PANI/CNT composite paper as an all-solid-state supercapacitor electrode directly exhibited superior gravimetric capacitance owing to the synergistic effect between MoS2, PANI, and CNTs. The assembled supercapacitor based on the M100/P20 electrode exhibited a high energy density of 0.013 Wh cm−3 and an ultrahigh power density of 1.000 W cm−3. The sandwich structure yields a competitive electrochemical performance, making it a potential candidate for portable and flexible energystorage device applications.

equivalent series resistance (Rs), which is the sum of the intrinsic resistance of electrode active materials, the electrolyte resistance, and the interfacial contact resistance between the electrode and electrolyte. CPE1 is the constant phase element (pseudocapacitance). The charge-transfer resistance (RCT) was calculated from the arc in the high-frequency region. Although the approximately straight line at lower frequencies indicates the diffusion behavior of the ions in the electrode, the line with the steep slope indicates an ideal capacitive behavior with rapid ion diffusion in the electrolyte solution. Figure 6a shows that the Rs and RCT values were 19.09 and 13.13 Ω, respectively. These results exhibit an excellent degree of electrical conductivity and rapid ion diffusion, which indicate a high level of performance. As shown in Figure 6b, the potential window was extended from 1.0 V for a single solid-state supercapacitor to 3.0 V for a series of three solid-state supercapacitors. Moreover, the series of three solid-state supercapacitors demonstrates nearly identical charge and discharge times when compared with the single supercapacitor at the same current, indicating that the performance of each of the supercapacitors in series is maintained well. To further demonstrate the practical usage of these supercapacitors, connecting two supercapacitors in series can efficiently power an light-emitting diode (LED), as shown in Figure 6c, demonstrating the great potential of such supercapacitors as a flexible power source. The Ragone plots (Figure 7) clearly demonstrate the overall performance of the supercapacitor, indicating a maximum

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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/acs.jpcc.9b04046. UV−vis, FTIR, and Raman spectra; photo of PANI; TEM, Raman, UV−vis spectra of CNT; stability of the MoS2 suspension; FESEM images, XPS spectra, and GCD curves of the MoS2/PANI/CNT composite paper; FESEM image of the flexible solid-state M100/P20 supercapacitor after bending (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

I-Wen Peter Chen: 0000-0003-2532-1380 Notes

The authors declare no competing financial interest.

■ Figure 7. Ragone plots of volumetric energy density versus volumetric power density results for a flexible all-solid-state M100/P20 supercapacitor in comparison with commercially available products.

ACKNOWLEDGMENTS The authors are grateful to the Ministry of Science and Technology (MOST) of Taiwan (105-2119-M-143-001-MY2; 107-2628-M-143-001-MY2) for financial support. They would also like to thank S. J. Ji and C. Y. Chien of the Precious Instrument Center at NTU for their assistance with SEM, EDS, and TEM experiments.

volumetric energy density of 0.013 Wh cm−3 at a power density of 0.063 W cm−3; an energy density of 0.007 Wh cm−3 was still maintained at a high power density of 1.000 W cm−3. These results are considerably higher than those of the electrodes composed of three-dimensional-printed active carbon, CNT, and reduced graphene oxide (0.001 Wh cm−3)58 and commercially available 2.75 V/44 mF activated carbon electrochemical capacitors (0.7 mWh cm−3),62 50-fold higher than those of a 3.5 V/25 mF electrochemical capacitor (∼0.2 mWh cm−3),63 even nearly 1.5 times higher than those of a 4 V/500 μA h thin-film lithium battery (∼8 mWh cm−3),63 etc.64 These results demonstrate that the flexible M100/P20 all-solid-state supercapacitor is a great candidate for electrochemical energy-storage applications.

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DOI: 10.1021/acs.jpcc.9b04046 J. Phys. Chem. C XXXX, XXX, XXX−XXX