Novel ternary nanocomposites of MWCNTs/PANI/MoS2: preparation, characterization and enhanced electrochemical capacitance

In this work, nanoflower-like MoS2 grown on the surface of multi-walled carbon nanotubes (MWCNTs)/polyaniline (PANI) nano-stem is synthesized via a facile in situ polymerization and hydrothermal method. Such a novel hierarchical structure commendably promotes the contact of PANI and electrolyte for faradaic energy storage. In the meanwhile, the double-layer capacitance of MoS2 is effectively used. The morphology and chemical composition of the as-prepared samples are characterized by scanning and transmission electron microscopies, X-ray diffraction and Fourier transform infrared spectra. The electrochemical performance of the samples is evaluated by cyclic voltammogram and galvanostatic charge–discharge measurements. It is found that the specific capacitance of the obtained MWCNTs/PANI/MoS2 hybrid is 542.56 F g−1 at a current density of 0.5 A g−1. Furthermore, the MWCNTs/PANI/MoS2 hybrid also exhibits good rate capability (62.5% capacity retention at 10 A g−1) and excellent cycling stability (73.71% capacitance retention) over 3000 cycles.


Introduction
In recent years, global warming and the energy crisis have accelerated the development of advanced energy storage devices. As one of the most promising candidates, supercapacitors are receiving extensive attention due to their high power/energy density, excellent cycling stability and fast charge/discharge capability [1][2][3]. Supercapacitors can be divided into two types depending on their energy storage mechanisms: electrical double-layer capacitors (EDLCs) and pseudo-capacitors [4][5][6]. For the EDLCs, various carbonaceous materials, such as carbon nanotubes (CNTs), graphene and activated carbon, have been widely employed as electrode materials [7,8]. However, the specific capacitance of carbon electrodes is relatively low. In contrast, for the pseudo-capacitor, transition metal oxides and conducting polymers (e.g. polyaniline, polypyrrole and polythiophene) are very popularly used as electrode materials with higher energy storage capacity [9,10]. Among these conducting polymers, polyaniline (PANI) has been widely used as an ideal electrode material in the construction of high-performance supercapacitors due to its high theoretical specific capacitance, good electrochemical activity, good biocompatibility, low cost and ease of fabrication [11,12]. However, the main drawback restricting the application of PANI electrodes in supercapacitors is the mechanical degradation and poor cycling stability during the charge/discharge process [13,14]. To overcome these problems, many research works have been conducted on the preparation of PANI-based composites with EDLC electrode materials for hybrid capacitors, which are conducive to enhancing the specific capacitance, mechanical stability and cycling stability [15][16][17][18][19].
The combination of CNTs with PANI is an effective way to enhance the specific capacitance and cycling stability of PANI. For example, Li et al. [20] reported three types of nanocomposites synthesized by in situ chemical polymerization of aniline onto double-walled carbon nanotubes (DWCNTs), single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), respectively. They found that the specific capacitances were 576, 390 and 344 F g −1 for composites of DWCNTs/PANI, SWCNTs/PANI and MWCNTs/PANI, which were much higher than that of pure PANI (226 F g −1 ). The cycling stability of the three nanocomposites was also higher than that of pure PANI.
On the other hand, layered transition-metal dichalcogenides (WS 2 , MoS 2 and VS 2 ) have been successfully established as a new paradigm in the chemistry of nanomaterials and have aroused wide attention [21][22][23]. Especially, molybdenum disulfide (MoS 2 ), a typical type of transitionmetal dichalcogenide with a layered structure like graphene, has attracted tremendous attention, because it could be used as electrode material for supercapacitors due to its higher theoretical specific capacitance [21] and higher intrinsic fast ionic conductivity [24]. Therefore, many studies have combined PANI with MoS 2 to yield improved electrochemical performance [25][26][27]. Lei et al. [28] reported a hierarchical core-sheath PANI@MoS 2 nanocomposite via a hydrothermal redox reaction for high-performance electrochemical capacitor applications. They found that the nanocomposite electrode displayed a high specific capacitance of 450 F g −1 and excellent cycling stability (retaining 80% after 2000 charge/discharge processes), while the specific capacitance of individual PANI was 338 F g −1 , and the PANI electrode only retained 47% of the value of the first cycle. Wang and co-workers [29] reported MoS 2 /PANI hybrid electrode material via direct intercalation of aniline monomer and doped with dodecylbenzenesulfonic acid. They found that when the loading amount of MoS 2 was 38%, the obtained hybrid electrode exhibited a high specific capacitance of 390 F g −1 at a current density of 0.8 A g −1 and excellent cycling stability (86% retention over 1000 cycles).
In this paper, we introduce both MWCNTs and MoS 2 as a composite with PANI to be used as supercapacitor electrode material. The MWCNTs not only act as the initial supporter for the polymerization of aniline monomer, but also enhance the electrical conductivity and electrochemical properties of hybrid materials. Nanoflower-like MoS 2 is grown on the surface of the MWCNTs/PANI nano-stem through a facile hydrothermal reaction, where MoS 2 plays an important role in enhancing the charge storage capabilities and the cycling stability during the charge/discharge process. The preparation procedure is illustrated in figure 1. The obtained ternary nanocomposite exhibits high specific capacitance and excellent cycling stability.

Material and reagents
Thiourea was supplied by Sigma-Aldrich. MWCNTs were purchased from Chengdu Organic Chemicals Co. Ltd, Chinese Academy of Sciences. Aniline, ammonium peroxydisulfate ((NH 4 ) 2 S 2 O 8 , APS) and ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O) were purchased from Wuhan Shenshi Chemical Instrument Network Co. Ltd (China). All other chemicals and solvents were used as received without further treatment.

Synthesis of MWCNTs/PANI nanocomposites
MWCNTs/PANI composites were synthesized by an in situ oxidative polymerization using the procedures in our previous work [35]. Primarily, the MWCNTs were purified and functionalized by nitric acid at 80°C for 6 h. Then, 70 mg of the purified MWCNTs was dispersed in 50 ml of 1 M HCl with ultrasonic treatment for 0.5 h. In the meantime, 150 ml of 1 M HCl solution with 0.3 g of aniline monomer was treated by stirring for 1 h at room temperature. Afterwards, the above two solutions were mixed together and sonicated for another 0.5 h. Next, 0.5 g of APS was added into the mixture and stirred for 5 h at 9°C. Finally, the mixture was filtered and washed with deionized water three times and then dried at 60°C for 12 h with vacuum. Pure PANI was prepared through the same procedure without MWCNTs.

Preparation of MWCNTs/PANI/MoS 2 ternary nanocomposites
To synthesize the ternary hybrid MWCNTs/PANI/MoS 2 nanocomposites, the as-prepared MWCNTs/ PANI (30 mg) was dispersed in 25 ml of deionized water with the help of ultrasonication for 40 min. Ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 ·4H 2 O, 0.1 g) and thiourea (0.086 g) were added into the MWCNTs/PANI suspension, and the mixture was sonicated for 10 min. Then, the obtained mixed suspension was transferred into a 50 ml Teflon-lined stainless steel autoclave and heated at 190°C for 25 h. After cooling down to room temperature naturally, the resulting black precipitates were collected by centrifugation with deionized water and then dried in vacuum at 60°C for 24 h to obtain the ternary hybrid of MWCNTs/PANI/MoS 2 , abbreviated as MPM-0. 1 MWCNTs/PANIMoS 2 nanocomposites with different amounts of ammonium molybdate tetrahydrate (0.06 g and 0.14 g) were prepared by using the same process, which were denoted as MPM-0.06 and MPM-0.14 (the mass ratio of ammonium molybdate tetrahydrate to thiourea was 1.16).

Preparation of the modified electrodes
The working electrode was prepared as follows: firstly, a glass carbon electrode (GCE) was prepared by polishing and ultrasonic cleaning. The obtained nanocomposites were dispersed in Nafion (1%) solution and sonicated for 1 h to form a homogeneous mixture. Then, the mixture (10 µl) was dropped onto the pretreated GCE and dried at room temperature.
The electrochemical tests (cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD)) were carried out by using an Autolab (µ3AUT71018) electrochemical workstation with a three-electrode system, in which the GCE coated with the obtained samples was used as the working electrode, a platinum foil as the counter electrode and an Ag/AgCl electrode as the reference electrode in 1 M H 2 SO 4 electrolyte. The CV curves were recorded at scan rates of 10-100 mV s −1 in the voltage range from −0.2 to 1 V. GCD curves were measured at different current densities of 0.5, 1, 2, 4 and 10 A g −1 (0-0.8 V). The specific capacitances of electrode materials were calculated according to the following equation: where I, t, V and m are the charge/discharge current (A), discharge time (s), potential change (V) and the weight of active materials (g), respectively.      represents the representative CV curves of the MPM-0.1 nanocomposite at different scan rates. The peak current density and the CV loop area of the hybrid increase clearly with the increase in scanning rates, demonstrating the excellent rate property of the hybrid [38].

Electrochemical properties
The representative GCD curves of MoS 2 , PANI, MWCNTs/PANI, MPM-0.06, MPM-0.1 and MPM-0.14 at a current density of 0.5 A g −1 are displayed in figure 5a. The specific capacitance values of the six electrodes are 128.75 F g −1 , 438.46 F g −1 , 480.8 F g −1 , 498.32 F g −1 , 542.56 F g −1 and 410.27 F g −1 , respectively, calculated by equation (2.1). It is obvious that the MPM-0.1 electrode possesses larger specific capacitances than other electrode materials. This can be due to the fact that the nanoflower-like MoS 2 formed on the surface of MWCNTs/PANI produces large double-layer capacitance and this kind of nanostructure does not obstruct the transmission of protons and electrons between the PANI surface and H 2 SO 4 electrolyte, which can maintain the faradaic pseudocapacitance property of PANI. Furthermore, the excellent electrochemical properties are attributed to the synergistic effect of MWCNTs, PANI and MoS 2 . By contrast, MPM-0.06 possesses a few sheet-like petal structures of MoS 2 , which can provide small double-layer capacitance. MPM-0.14 with a great number of MoS 2 nanosheets exhibits a smaller specific capacitance. This phenomenon can be due to the excessive MoS 2 nanosheets agglomerated into the bulk and wrapped on the surface of MWCNTs/PANI, which block electron transfer between PANI and the electrolyte. The evaluation of the cycling stability of the four electrodes is carried out by GCD cycling at a current density of 1 A g −1 for 2000 cycles (figure 5d). It is found that the specific capacitance retention of the MPM-0.1 hybrid electrode is 74.25%, which is higher than those of MoS 2 (38.6%), PANI (41.76%) and MWCNTs/PANI (61.82%). Moreover, the cycling performance of MPM-0.1 for 3000 cycles was also tested. Its specific capacitance retention is 73.71% (electronic supplementary material, figure S3), revealing good cycle performance and stability of the hybrid. The improved stability is mainly attributed to the architecture with synergistic effect of nanoflower-like MoS 2 , MWCNTs and PANI. Firstly, MWCNTs acts as a framework to make PANI effectively accommodate the mechanical deformation caused by the swelling and shrinking of the nanostructures during the long-term GCD process. Secondly, the nanoflower-like MoS 2 covering on the surface of MWCNTs/PANI can suppress the volume change of PANI from the outside, which avoids the destruction of the electrode material and leads to outstanding stability.

Conclusion
In summary, a novel structured hybrid with nanoflower-like MoS 2 suitably grown on the surface of MWCNTs/PANI is successfully prepared by a facile in situ polymerization and hydrothermal method. In the hybrid, nanoflower-like MoS 2 not only acts as an active material with double-layer capacitance but also as an outer barrier to suppress the volume change of PANI; MWCNTs serve as a support framework to make PANI effectively accommodate the mechanical deformation and to improve the electrochemical property of the whole material; PANI provides the faradaic contribution to the overall capacitance. Electrochemical measurements show that the MWCNTs/PANI/MoS 2 hybrid electrode displays an ideal specific capacitance of 542.56 F g −1 at a current density of 0.5 A g −1 and excellent cycling stability with a capacitance retention of 73.71% after 3000 cycles, indicating its potential for high-performance electrical energy storage.