Electrode material of core-shell hybrid MoS₂@C/CNTs with carbon intercalated few-layer MoS₂ nanosheets

Highlights

• Non-covalent modification of CNTs with high conductive PEDOT:PSS, which is simple but effective to get individual and high conductive CNTs.

• Amorphous carbon adhesion to surface of MoS₂ nanosheets protect volume change of MoS₂ during electrochemical processes. The intercalated carbon between lamellar spacing of few-layer MoS₂ nanosheets provides electron transmission path.

• The MoS₂@C/CNTs hybrid electrode exhibits large specific capacitance and cycle stability at current density of 1A/g.

Abstract

MoS₂@C/CNTs is synthesized and applied as electrode of supercapacitors. Few-layer MoS₂ nanosheets, with amorphous carbon located at both the surface and interlayers, grow on the CNTs surface and form coaxial nanocable structure MoS₂@C/CNTs. The intercalated amorphous carbon broadens interlayer spacing of MoS₂ (002) planes, which provides both electron and ions transmission path as well as protection to volume change of MoS₂ during electrochemical processes. The MoS₂@C/CNTs electrode exhibits excellent electrochemical properties, such as high specific capacitance and excellent cyclic stability. The specific capacitance is 335 F/g at current density of 1 A/g and capacitance retention ratio of 127% is achieved after 40000 cycles.

Introduction

Molybdenum sulfide (MoS₂) is a promising electrode material to supercapacitors, because of its low-cost, large specific surface area, rich redox chemistry and high theoretic specific capacitance (1200 F/g) [[1], [2], [3]]. MoS₂ nanosheets possess two-dimensional (2D) lamellar structure that S–Mo–S layers are held together by Van der Walls’ forces [4,5], which is similar to graphene. The layered structure results in co-existence of anisotropic electrical conduction and predominant hopping mechanism, which is benefit to accelerate the intercalation and de-intercalation of electrolyte ions in the charge storage applications [[6], [7], [8]]. Xu, L.M. et al. synthesized MoS₂ microflowers as electrode, which showed specific capacitance of 196.9 F/g at the current density of 0.5 A/g [9]. Wang, P.Y. et al. designed a supercapacitor electrode with yolk–shell MoS₂ microspheres, whose specific capacitance reached to 343.0 F/g at the current density of 0.25 A/g [10]. Although the specific capacitance of MoS₂ has been improved by optimizing the morphology structure, the conductivity and rate performance of MoS₂ electrode are still maintained at a poor level. Especially when the current density exceeds 1 A/g, the specific capacitance decreases dramatically [[9], [10], [11]].

In order to improve the electrochemical capacitance and cycle stability of MoS₂ electrode, many efforts have been devoted to combining MoS₂ with conductive materials, such as carbonaceous materials (e.g. graphene, CNTs) and conductive polymers [12]. Ding, S.J. et al. apply glucose assisting growth of MoS₂ nanosheets on CNT backbone. It is found that the carbon derived from glucose provides an excellent contact between the CNT and the MoS₂, which plays a critical role in enhancing lithium storage properties. However, 40% of the irreversible capacity was lost after cycling of dozens of times [13]. Ge, Y. et al. fabricated a free-standing MoS₂/poly (3,4ethylenedioxythiophene): poly (styrenesulfonate) (PEDOT:PSS) film with excellent capacitance retention rate (98.6% over 5000 cycles). It shows good electrochemical cycle stability because PEDOT:PSS enhance the electrode conductivity. Unfortunately, its specific capacitance is low of 141.4 F/cm³ at 1 A/g [14]. Many researchers have attempted to study ternary hybrids of MoS₂ for high performance electrode materials. Wang, S.Z. et al. synthetized 3D network of MoS₂@CNT/RGO composites. Although capacitance retention of the hybrid composites remained 94.7% after 10000 cycles, it shows a relatively low specific capacitance of 129 mF/cm² at 0.1 mA/cm² [15]. Li, Z.J. et al. demonstrated CNTs/MoS₂/Fe₃O₄ hybrid electrode, which exhibited high specific capacitance (522.7 F/g at the current density of 0.5 A/g). However, the cyclic stability of CNTs/MoS₂/Fe₃O₄ hybrid electrode was not reported. Obviously, it's a hard issue to achieve high specific capacitance and high stability of the MoS₂ electrode materials simultaneously.

In order to effectively manifest the properties of each component in MoS₂-based hybrid electrode materials, researchers have designed various microstructures with different bonding modes. Krishnamoorthy, K. et al. obtained 2D electrode material by growing MoS₂ nanosheets on Mo foil with stable interfacial bonding. The MoS₂/Mo electrodes exhibit 98% capacitance retention over 1000 cycles, and delivered a specific capacitance of 192.7 F/g at current density of 1 mA/cm² [16]. Hu, A. et al. synthesized 3D interconnected network architecture hybrid of MoS₂/CNTs [5]. It shows better energy storage value and stability compared to the directly used sample of CNT@MoS₂ for lithium batteries [13]. However, the stacking of MoS₂ is easy to occur in both 2D plane structure and 3D network structure, which results in insufficient contact area between electrolyte and active substance [5,8,17]. Nakanishi, H. et al. designed a coaxial nanowire electrode. It consists of highly conductive metal cores and pseudocapacitive organic shell, which exhibits low internal resistance, excellent energy capacity and rate capability [18]. Liu, J.P. et al. combined Co₃O₄ and MnO₂ in the form of coaxial nanowires, which effectively prevented the stacking of metal oxides and improved the conductivity and plasticity significantly [19]. Coaxial nanowire structure can effectively solve the stacking phenomenon of active substances in the formation process [20,21]. However, lacking effective connection between the core substrate and the shell is a big problem to coaxial hybrid materials, which leads to unstable behavior during the electrochemical processes.

Herein, a stable core-shell nanocable structure of MoS₂@C/CNTs is obtained with high specific capacitance, high stability and excellent conductivity. Firstly, CNTs are homogeneously wrapped by highly conductive PEDOT:PSS forming PEDOT:PSS/CNTs coaxial structure. Secondly, MoS₂ subunits are homogeneously grown on PEDOT:PSS/CNTs surface with stable interface bonding. At last, MoS₂@C/CNTs hybrids are obtained with amorphous carbon located at both the surface and interlayer of MoS₂ nanosheets. The special core-shell structure of MoS₂@C/CNTs not only satisfies the need for rapid electron transport but also protects the MoS₂@C shell during electrochemical processes. Owing to the synergistic effect of carbon intercalated MoS₂, amorphous carbon and CNTs, the MoS₂@C/CNTs hybrid electrode exhibits excellent specific capacitance and electrochemical stability.