Xiaobo Chen*a,
Jingguo Dinga,
Jing Jianga,
Guoce Zhuanga,
Zhihai Zhanga and
Peizhi Yang*b
aSchool of New Energy and Electronic Engineering, Yancheng Teachers University, Yancheng, 224051, P. R. China. E-mail: chenxbok@126.com
bKey Laboratory of Education Ministry for Advance Technique and Preparation of Renewable Energy Materials, Institute of Solar Energy, Yunnan Normal University, Kunming, 650500, P. R. China. E-mail: pzhyang@hotmail.com
First published on 20th August 2018
MoS2 and MoS2/carbon allotrope (MoS2/C) composites for use as anodes in supercapacitors were prepared via a facile hydrothermal method. In this study, we report the effects of various carbon-based materials (2D graphene nanosheet (GNS), 1D carbon nanotube (CNT), and 0D nano carbon (NC)) on the electrochemical performances. Among all nanocomposites studied, MoS2/CNT exhibited the best electrochemical performance. Specifically, the MoS2/CNT composite exhibits remarkable performances with a high specific capacitance of 402 F g−1 at a current density of 1 A g−1 and an outstanding cycling stability with 81.9% capacitance retention after 10000 continuous charge–discharge cycles at a high current density of 1 A g−1, making it adaptive for high-performance supercapacitors. The superiority of MoS2/CNT was investigated by field emission scanning electron microscopy and transmission electron microscopy, which showed that MoS2 nanosheets were uniformly loaded into the three-dimensional interconnected network of nanotubes, providing an excellent three dimensional charge transfer network and electrolyte diffusion channels while effectively buffering the collapse and aggregation of active materials during charge–discharge processes. Overall, the MoS2/CNT nanocomposite synthesized by a simple hydrothermal process presents a new and promising candidate for high-performance anodes for supercapacitors.
Two-dimensional (2D) layered MoS2 nanosheets may perform best for applications in electrochemical energy storage because of their large surface area and exposed active sites.3,4 Several recent reports have been focused on the supercapacitor performance of pure MoS2 with diverse morphologies. For examples, Huang et al. reported that MoS2 nanosheets as electrode materials exhibited a specific capacitance up to 129.2 F g−1 at 1 A g−1 with capacitance retention of 85.1% after 500 cycles;5 Wang et al. prepared flower-like MoS2 which showed a specific capacitance of 168 F g−1 at a current density of 1 A g−1 when applied as electrode materials for electrochemical capacitors.6 These results indicate that MoS2 is a promising electrode material for supercapacitors owing to its two-dimensional sheets-like morphology, which can provide large surface area for double-layer charge storage. In addition, the easy diffusion of electrolyte into the inner region (interlayers) of the MoS2 electrode at lower scan rates can provide faradaic capacitance, which acts a good part in improved charge storage capability.7 These studies on the supercapacitive properties of pure MoS2 showed only moderate performance as an electrode material due to the relatively poor electronic conductivity and stability. Therefore, the electrochemical properties of MoS2 can be improved via making composites with other electroactive materials, such as carbon allotrope materials and conductive polymer to overcome the above problem.4,7–10 Various kinds of carbon materials have been suggested as conductivity supporters (or additives) for MoS2, such as nano carbon black,10–12 graphene,3,4,13 and carbon nanotube.8,14,15 However, up to present, the comparative investigation about the effect of carbon supporters on the electrochemical performance for supercapacitor application is still rarely reported.
Herein, to combine all these merits and enhance the electrochemical performance of MoS2, a straightforward hydrothermal method has been developed in this work to synthesize MoS2/NC, MoS2/G and MoS2/CNT composites through hydrothermal method. The morphology, surface areas, porous structures and electrochemical properties of as-prepared MoS2/NC, MoS2/G and MoS2/CNT composites had been studied comparatively.
MoS2/C composites (MoS2/NC, MoS2/G and MoS2/CNT) were prepared through hydrothermal method. Firstly, 30 mg carbon based nanomaterials (NC, G, or CNT) were firstly blended with 20 mL ethanol and ultrasonicated for 1 h to obtain a solution A (carbon based nanomaterials dispersion). Secondly, 1 mmol Na2MoO4·2H2O and 5 mmol thiourea were dissolved in 60 mL of DI water and added to the above dispersion and stirred for 1 h. The reaction mixture was then transferred into a Teflon-lined autoclave, heated up to 210 °C, and maintained at this temperature for 24 h. The resultant black precipitate was centrifuged, washed three times with DI water, followed by ethanol washing, and dried under vacuum. For comparison, the pure MoS2 was also prepared via the same method without carbon.
The electrodes were obtained by mixing active material (85 wt%), acetylene black (10 wt%) and polytetrafluoroethylene (PTFE, 60 wt% dispersion in water) binder (5 wt%). Then, it was dried in hot air oven at 80 °C for 0.5 h, and pressed under the pressure of 10 MPa and finally dried at 100 °C for 12 h. Each electrode contained about 5 mg of active material.
As shown in Fig. 3(a), the TEM image of MoS2 forms two-dimensional nanosheets. The high-resolution TEM (HRTEM) image in the inset of Fig. 3(a) indicates the lattice figure with interlayer spacing of 0.67 nm which corresponds to the (002) plane of MoS2.16 Also, the lattice spacing of 0.27 nm which belongs to the (100) plane of MoS2 can be observed in the same image.17 SEM image of MoS2/NC (Fig. 3(b)) displays that the MoS2 shell coats nano carbon tightly. It can be also seen that GS are decorated randomly by MoS2 nanosheets to generate MoS2/G composite in Fig. 3(c). Fig. 4(d) shows more evidence of the extreme porous nature of the nanohybrid composed of ultrathin MoS2 nanosheets supported/separated by CNTs.
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Fig. 4 The N2 adsorption–desorption isotherms of MoS2 (a), MoS2/NC (b), MoS2/G (c) and MoS2/CNT composites (d). The insets show corresponding pore size distributions. |
Furthermore, the synthesized MoS2/CNT was also characterized by EDS. From Fig. S1(a),† the signals of C, O, S and Mo elements can be seen. From the observation of the four element mappings of Mo, S, C and O (Fig. S1(b)†), the elements are uniformly distributed throughout the MoS2/CNT composite. Therefore, the HRTEM and EDS results indicated that the MoS2/carbon hybrid has been successfully synthesized.
Fig. 4 shows the N2 adsorption–desorption isotherms and corresponding pore size distribution curves of samples. The isotherms of as-prepared materials all exhibit the type IV hysteresis loop as defined by IUPAC. The BET surface areas of the MoS2, MoS2/NC, MoS2/G and MoS2/CNT are 116, 219, 236 and 275 m2 g−1, respectively. Obviously, the surface area of MoS2/CNT is 2.4 times as much as pure MoS2, attributing to the compact interconnected network structure, which not only provides large surface area for charge storage but also facilitates electrolyte penetration through the mesopores. The pore size distributions of these samples are shown in the insets, confirming that the samples have mesoporous characteristics. The well-developed 3D interconnected porous network with multilevel pores can provide not only favorable transport channels for electrolyte but also strong mechanical strength. Based on the above mentioned merits, the MoS2/CNT composite can be employed as an excellent electrode material for SCs and the detailed electrochemical measurements are performed as follows.
MoS2 + Na+ + e− ⇌ MoS − SNa | (1) |
The output current of MoS2/CNT is larger than those of MoS2 and MoS2/NC and MoS2/G. One major weakness of pure MoS2 based SCs is its inherent resistivity.3,4 The results indicate that the conductivity of MoS2 can be improved by combining MoS2 and CNT together to form composites, meanwhile, the specific capacitance also increases. Fig. 5(b) shows the GCD curves of MoS2 and MoS2/NC, MoS2/G and MoS2/CNT at a current density of 1 A g−1 with voltage between −0.9 and −0.4 V. It is clear that the discharge time of MoS2/CNT is much longer compared to MoS2, MoS2/NC and MoS2/G. The specific capacitances can be calculated using the eqn (2):
Cs = I × Δt/(m × ΔV) | (2) |
CV measurement is carried out to further investigate the charge storage mechanism of MoS2/CNT in the three electrode system. Fig. 5(d) shows CV curve area of MoS2/CNT increases with increasing scan rates (from 2 to 100 mV s−1). The CV loop of MoS2/CNT electrode at 100 mV s−1 is still close to rectangular, suggesting that the MoS2/CNT composite can be employed as an excellent electrode material. It should be emphasized that the curves for the initial cycles at lower scan rates exhibit faradaic capacitance in addition to electric double layer capacitance, due to diffusion of the ions into the MoS2 interlayers at low scan rates,19 improving charge storage capabilities. The specific capacitances of the MoS2/CNT calculated from the GCD curves (Fig. 5(e)) are 402, 292, 215 and 172 F g−1 at current densities of 1, 2, 5 and 10 A g−1, respectively. Since such a decrease of specific capacitances is common with increasing scan rates, and caused by the difficulty of ion diffusion and the insufficient faradaic redox reaction at high scan rates.4
The cycle stability of MoS2/CNT composite is important for its practical applications. The cycling performance of MoS2/CNT at a current density of 1 A g−1 was shown in Fig. 5(f). Importantly, the specific capacitance still maintains 81.9% level after 10000 consecutive cycles. Particularly, there is no clear capacitance drop occurred over 1000 cycles (99.6% retention) due to the support between CNT and MoS2 nanosheets, presenting excellent cycling stability of MoS2/CNT. More interestingly, compared with other MoS2/carbon composites with electrolyte of 1 M Na2SO4, MoS2/CNT showed higher specific capacitance and more excellent electrochemical stability than those of most reported MoS2/carbon composite electrode (Table 1). The electrochemical reversibility can be further explained by EIS measurements (Fig. S3†). Noticeably, the EIS spectra are similar in terms of the curve shape except that a moderate change of equivalent series resistance. Before and after 10
000 cycles, the RESR values of MoS2/CNT electrode are 1.19 Ω and 1.62 Ω, respectively, indicating that the MoS2/CNT electrode has a relatively good stability. We further capture the SEM image of the hierarchical MoS2/CNT composite after 10
000 cycles in Fig. S4.† The possible reason can be found in Fig. S4,† the composite after 10
000 cycles with somewhat aggregation (highlighted by red circles), thus leading to decrease of the capacity.
Electrode materials | Preparation method | Maximum specific capacitance | Electrolyte | Cyclic ability | Ref. |
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a Abbreviations: MWCNT – multi-walled carbon nanotube; C – carbon; 3DG – three-dimensional grapheme; GS – graphene sheets; CNT – carbon nanotube. | |||||
MoS2/graphene | Solution phase exfoliation | 11 mF cm−2 at 5 mV s−1 | 1 M Na2SO4 | 250% at 1 mA cm−2 after 10![]() |
3 |
MoS2/graphene aerogel | Lithium intercalation and hydrothermal assembly | 268 F g−1 at 0.5 A g−1 | 1 M Na2SO4 | 93% at 1 A g−1 after 1000 cycles | 20 |
MoS2/graphene hybrid films | Layer-by-layer assembly | 282 F g−1 at 20 mV s−1 | 1 M Na2SO4 | 93% at 2 A g−1 after 1000 cycles | 21 |
MoS2/MWCNT | Hydrothermal method | 452.7 F g−1 at 1 A g−1 | 1 M Na2SO4 | 95.8% at 1 A g−1 after 1000 cycles | 8 |
MoS2/C | Hydrothermal method | 201.4 F g−1 at 0.2 A g−1 | 1 M Na2SO4 | 94.1% at 1.6 A g−1 after 1000 cycles | 10 |
MoS2/carbon aerogel | Hydrothermal method | 260 F g−1 at 1 A g−1 | 1 M Na2SO4 | 92.4% at 1 A g−1 after 1500 cycles | 22 |
MoS2 nanoflowers/3DG | Hydrothermal method | 410 F g−1 at 1 A g−1 | 1 M Na2SO4 | 113.6% at 2 A g−1 after 1000 cycles | 4 |
MoS2/CNT | Hydrothermal method | 402 F g−1 at 1 A g−1 | 1 M Na2SO4 | 99.6% at 1 A g−1 after 1000 cycles, 81.9% at 1 A g−1 after 10![]() |
This work |
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra05158e |
This journal is © The Royal Society of Chemistry 2018 |