Cheng et al. Nanoscale Research Letters (2019) 14:66 https://doi.org/10.1186/s11671-019-2902-5 NANO EXPRESS Open Access Highly Efficient Quasi-Solid-State Asymmetric Supercapacitors Based on MoS2/MWCNT and PANI/MWCNT Composite Electrodes Bing Cheng1,2, Renzhi Cheng1,2, Furui Tan1,2, Xiansheng Liu1,2, Jinghao Huo3 and Gentian Yue1,2* Abstract Molybdenum disulfide (MoS2) and polyaniline (PANI) electrodes were decorated with multi-walled carbon nanotubes (MWCNTs) on the basis of a facial hydrothermal and in situ polymerization methods and served in the asymmetric supercapacitor (ASC). The MoS2 and MWCNTs with a mole ratio of 1:1 in MoS2|MWCNTs electrode exhibited better electrochemical properties through extensive electrochemical studies, in terms of the highest specific capacitance of 255.8 F/g at 1 A/g, low internal resistance, and notable electrochemical stability with retention of the initial specific capacitance at 91.6% after 1000 cycles. The as-prepared PANI|MWCNTs electrode also exhibited good specific capacitance of 267.5F/g at 1A/g and remained 97.9% capacitance retention after 1000cycles. Then, the ASC with MoS2|MWCNTs and PANI|MWCNTs composite electrodes were assembled with polyvinyl alcohol (PVA)- Na2SO4 gel electrolyte, which displayed good electrochemical performance with the specific capacitance of 138.1 F/g at 1 A/g, and remained the energy density of 15.09 Wh/kg at a high power density of 2217.95 W/kg. This result shows that this ASC device possesses excellent electrochemical properties of high energy density and power output and thus showing a potential application prospect. Keywords: Molybdenum disulfide, Multi-walled carbon nanotube, Polyaniline, Quasi-solid-state, Supercapacitor Background research focus [4–7]. In particular, pseudocapacitive mate- With the energy crisis and environmental pollution in- rials with remarkably high capacitance have attracted a creasing seriously, the efficient and stable energy storage large number of concerns [8]. equipment has attracted extensive research due to the Among these reports, carbon materials are identified intermittent energy sources (such as solar energy, wind as one of the most popular because of their excellent power, and frictional power) cannot continuously and sta- conductivity, large surface area, environmental friendly bly output electrical energy for daily application [1, 2]. protecting, and low cost. However, the carbon materials Supercapacitors (SCs) with fast charging/discharging and store charges by electric double-layer capacitor mechan- long cycling life are considered as an ideal choice for en- ism possess high electrical conductivity but low capaci- ergy storage devices, which overall performance is mainly tance. Thus, to improve the electrochemical determined by the most important components, i.e., elec- performance of carbon-based materials, researchers have trode materials [3]. Typically, a significant amount of elec- devoted great efforts to grow nano-structured active ma- trode materials, like graphene, carbon nanotubes (CNTs), terials on them [9–15]. Molybdenum disulfide (MoS2), conducting polymers, and oxide metals, have become a as a typical family member of transition-metal dichalco- genides (TMDs), has attracted lots of attention for its special structural and chemical character, which is exten- * Correspondence: [email protected]; [email protected] 1Henan Key Laboratory of Photovoltaic Materials and Laboratory of sively applied in many fields including lithium-ion bat- Low-Dimensional Materials Science, Henan University, Kaifeng 475004, China teries, catalysis, and dye-sensitized solar cells. MoS with 2 2 School of Physics and Electronics, Henan University, Kaifeng 475004, China nanoscale recently has been chosen to use in capacitors Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Cheng et al. Nanoscale Research Letters (2019) 14:66 Page 2 of 12 owing to its higher intrinsic fast ionic conductivity than obtained 255.8 F/g and 267.5 F/g at 1 A/g, respectively. oxides and higher theoretical capacity than graphite be- The retention rate of the MoS2|MWCNTs electrode sides high surface area. Wang et al. [16] prepared MoS2 retained 91.6% after 1000 cycles at the scan rate of 30 with hierarchical hollow nanospheres structure as nega- mV/s. Also, the PANI|MWCNTs electrode also exhib- tive electrode material providing a maximum specific ited 97.9% retention at the scan rate of 60 mV/s after capacitance of 142 F/g at 0.59 A/g. Yang et al. [17] de- 1000 cycles. A quasi-solid-state ASC based on signed MoS2/graphene nanosheet composites and ob- MoS2|MWCNTs and PANI|MWCNTs electrodes with tained a specific capacitance of 320 F/g at 2 A/g. Hu et PVA-sodium sulfate (Na2SO4) gel electrolyte showed en- al. [18] prepared porous C|MoS2 electrode which ob- ergy density of 38.9 Wh/kg at the power density of tained the specific capacity of 210 F/g at 1 A/g and good 382.61 W/kg. Two such super quasi-solid-state ASC in stability with more than 1000 cycles at 4 A/g. However, series can easily light a red light-emitting diode, indicat- the poor specific capacitance of MoS2 attributed to the ing potential application prospects. stacking and collapse in the charge-discharge process of its two-dimensional layered structures, which limits the Methods application in SCs. Therefore, how to construct hier- Synthesis of MoS2|MWCNTs archical three-dimensional (3D) architectures is an ef- The preparation of MoS2 is done by using a simple fective way to solve the problem of aggregation and hydrothermal method [33]. Firstly, 0.726 g of sodium prepare high-performance supercapacitors based on molybdate and 0.684 g of thiourea were mixed together MoS2 composites electrode. in 35 ml of deionized water by stirring and sonicating In addition, with reported good conductivity, ease of syn- for 30 min successively. Then, a certain amount of thesis, low-cost monomer, tunable properties, and remark- MWCNTs with different contents was added into the able specific capacitance (500–2200 F/g), polyaniline above mixture and sonicated for another 30 min. Subse- (PANI) has been extensively studied as a pseudo-supercapa- quently, the mixed solution was adjusted the PH to less citor electrode material [19–21]. Recently, many re- than 1 with 12 M hydrochloric acid. After that, the solu- searchers have combined PANI with carbon materials and tion was transferred into a 50-ml Teflon liner and transition metal compound to improve the conductivity of heated at 200 °C for 24 h without intentional control of the PANI-based electrodes for enhancing the cycling stabil- ramping or cooling rate. When the temperature cooled ity and rate capability of the PANI-based pseudo-super-ca- down to room temperature, the precipitation was col- pacitors [22–27]. For example, Li and his workers [28] lected by filter and washed with ethanol and distilled prepared 3D CNTs|PANI fibers and obtained a specific water for five times, and then dried in a vacuum oven at capacitance as high as 242.9 F/cm in 1 M H2SO4 electrolyte. 60 °C for 24 h. Furthermore, the mole ratios of MoS2 Das et al. [29] reported an asymmetric supercapacitor and MWCNTs with 2:1, 1:1, and 1:2 were also researched (ASC) assembled with a Prussian blue/MnO2-positive elec- and labeled as MS2MWCNT1, MS1MWCNT1, and trode and a PANI/graphene nanoplatelet composite as the MS1MWCNT2 respectively. For comparison, pure MoS2 negative electrode in KNO3 electrolyte and exhibited mani- and MWCNTs were labeled as MS1MWCNT0 and fested favorable specific capacitance of 98 F/g at 1 A/g. MS0MWCNT1, respectively. Ghosh et al. [30] have fabricated high-energy density all-solid-state flexible ASC by using a facile novel 3D hol- Synthesis of PANI|MWCNTs low urchin-shaped coaxial MnO2@PANI composite as The preparation of PANI|MWCNTs was based on the positive electrode and 3D graphene foam as negative elec- previous reported [34, 35]. Firstly, 18 mg of MWCNTs trode materials with polyvinyl alcohol (PVA)/KOH gel elec- were dispersed in 10 ml of deionized water and soni- trolyte. Conductive PANI not only works as the bridge cated for 0.5 h and labeled as solution A. Then, 0.3 ml of between MnO2 and graphene to enhance the electrical con- aniline monomer was dissolved in 10 ml of 1 M hydro- ductivity, but also improves the specific capacitance of the chloric acid and labeled as solution B. Subsequently, electrode. Moreover, recent studies also exhibited that gel 0.21 g of potassium persulfate was dissolved in 10 ml of electrolyte has revealed the most potential application pros- 1 M hydrochloric acid and labeled as solution C. After- pects for supercapacitors [31, 32]. wards, solution B was added to solution A under mag- In view of these, we prepared MoS2 bridged by netic stirring followed by sequential addition of solution multi-walled carbon nanotubes (MWCNTs) through a C drop by drop, until the mixed solution turned dark facile hydrothermal method and prepared the PANI green. At room temperature, this reaction continued for decorated with MWCNTs composite by using in situ more than 5 h. After that, the product was collected by chemical polymerization process. After extensive centrifugation and washed repeatedly with deionized electrochemical testing, the specific capacity of water and absolute ethanol. The as-prepared sample was MoS2|MWCNTs and PANI|MWCNTs electrodes labeled as PANI|MWCNTs. Cheng et al.