MoS2 as negative electrode materials for supercapacitors

    Design and preparation of MoO 2 /MoS2 as negative electrode materials for supercapacitors Tong Zhang, Ling-Bin Kong, Mao-Cheng Liu, Yan-Hua Dai, Kun Yan, Bing Hu, Yong-Chun Luo, Long Kang PII: DOI: Reference:

S0264-1275(16)31228-X doi: 10.1016/j.matdes.2016.09.054 JMADE 2304

To appear in: Received date: Revised date: Accepted date:

5 June 2016 9 September 2016 14 September 2016

Please cite this article as: Tong Zhang, Ling-Bin Kong, Mao-Cheng Liu, Yan-Hua Dai, Kun Yan, Bing Hu, Yong-Chun Luo, Long Kang, Design and preparation of MoO2 /MoS2 as negative electrode materials for supercapacitors, (2016), doi: 10.1016/j.matdes.2016.09.054

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ACCEPTED MANUSCRIPT Design and preparation of MoO2/MoS2 as negative electrode materials for

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supercapacitors

State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals,

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1

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Bing Hu 1, Yong–Chun Luo 2, Long Kang 2

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Tong Zhang 1, Ling-Bin Kong 1, 2,*, Mao-Cheng Liu 1, Yan-Hua Dai 1, Kun Yan 1,

School of Materials Science and Engineering, Lanzhou University of Technology,

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Lanzhou University of Technology, Lanzhou 730050, P. R. China

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Lanzhou 730050, P. R. China

*Corresponding author. Tel.: +86-931-2976579; Fax: +86-931-2976578; E-mail: [email protected]

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ACCEPTED MANUSCRIPT Abstract In this work, the MoO2/MoS2 composite material with unique flower-like

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microsphere structure is prepared via a facile hydrothermal synthesis. Furthermore,

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the composite material is served as negative electrode for supercapacitors. Ascribe to

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the synergistic effect, including the improved electron conductivity, the coexistence of two energy storage mechanisms, and a stable flower-like microsphere structure, the

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as-fabricated MoO2/MoS2 exhibits a high specific capacitance of 433.3 F g−1 at a scan

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rate of 5 mV s-1 and a superior cycling stability of 84.41% retention after 5000 cycles compared to that of pure MoO2 (33.33% retention after 2000 cycles) and pure MoS2

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(58.33% retention after 2000 cycles) at a current density of 1 A g-1 in 1 M Na2SO4

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electrolyte. Moreover, we also synthesize a series of MoO2/MoS2 electrode materials,

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which were used to further illustrate the synergistic effect in MoO2/MoS2 composite materials. This work signifies such materials as negative electrodes exhibit bright

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application prospects for supercapacitors.

Keywords: Molybdenum dioxide; Molybdenum disulfide; Negative electrode; Synergistic effect; Supercapacitors

1. Introduction 2

ACCEPTED MANUSCRIPT With fast-pace development of a wide variety of electric vehicles, the demand for electrode materials in supercapacitor (SCs) has increased dramatically [1-6].

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Nowadays, metal compounds have attracted considerable attention as electrode

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materials for SCs [7-11]. However, the applications of metal compounds have always been limited because of shortages of themselves physical or chemical properties. For example, they are mostly not capable of furnishing satisfactory electrochemical

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behaviors because of poor electrical conductivity and instability, although metal

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oxides can achieve superior specific capacitance and facile preparation method [8,12]. In order to improve the electrochemical property of metal oxides, much attention has

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been paid to design specific architecture by combining different characteristics of

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electrode materials electrode materials [13]. For instance, Zhu et al. synthesized

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Fe3O4-doped MnO2 microspheres with a hierarchical porous, which showed an excellent cycling performance [7]. Hu et al. reported Ni2P/Co3V2O8 nanocomposite

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by a facile chemical precipitation technique, which combines separately the advantages of Ni2P and Co3V2O8, showing an ideal pseudocapacitive electrode material [11]. Moreover, Zhang et al. fabricated a hierarchical MoO2/Au/MnO2 heterostructure was prepared by a two-step electrodeposition method, which also exhibited displays an excellent electrochemical performance[14]. Compared with individual compound, the reported composite materials show excellent specific capacitance without any exception, but the preparation of these materials is complex. More importantly, the composites include two metal elements at least. Therefore, it is hard to say which contributes to improving electrochemical performance, different 3

ACCEPTED MANUSCRIPT elements in composite or unique structure after recombination of these two compounds. To solve this problem, we develop a strategy to design a composite

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electrode material, which includes only one metal element. Insight into the metal

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elements, Mo may be an electroactive element because of multiple valence states of +2, +3, +4, +5, and +6 [8,15], which is beneficial to the charge storage response. Molybdenum dioxide (MoO2), as one important member of transitional-metal oxides,

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has drawn tremendous attention in lithium ion batteries and SCs due to its low cost,

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low toxicity, and natural abundance [16-19]. However, less attention has been given to its practical applications because the drastic volume change of MoO2 cannot afford

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satisfactory cycling stability and rate performance [20, 21]. In this regard, it is

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necessary for MoO2 to combine with other electroactive materials. Among these

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Mo-based compounds, MoS2 may be a suitable candidate due to rich intercalation chemistry, including intrinsic ionic conductivity (higher than oxides) and higher

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theoretical capacity (higher than graphite) [22-29]. Moreover, MoS2 is quite attractive in various fields due to unique layered structure [30, 31] and there has been extensive research on the application of MoS2 as an electrode material for SCs [32-34]. Therefore, we think that MoO2/MoS2 composite material is a favorable electrode for SCs. First, it is well-known that a good electrical conductivity is beneficial to charge-transfer reactions and internal resistance. MoO2 has a poor conductivity because of its band gap of 3.85 eV [35]. However, MoS2 has a smaller band gap of 1.8 eV [36, 37], which can provide a favorable electrical conductivity to make up the deficiency of MoO2. Moreover, owning to lower electronegativity of sulfur (2.58) 4

ACCEPTED MANUSCRIPT than that of oxygen (3.44), MoS2 is also supposed to a suitable candidate to improve the conductivity of MoO2. Second, MoO2/MoS2 can provide more abundant active

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sites, including the oxide edge of MoO2 and the sulfur edge of MoS2. Third, the

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mechanism of charge storage is different. MoS2 possess both EDLC [15] and pseudocapacitive properties [38-40]. Therefore, the composite configuration of MoO2/MoS2 will make the contributions of two energy storage mechanisms to

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excellent capacitance and high energy density. In addition, there is no literature using

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MoO2/MoS2 composite materials as electrode materials for SCs. Herein, a novel three-dimensional (3D) MoO2/MoS2 composite electrode with

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flower-like microsphere structure has been successfully prepared via one-step facile

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hydrothermal process for the first time. The flower-like microsphere structure

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accelerates ion diffusion kinetics and electron transport between the electrode surface and the electrolyte. As expected, the results of the electrochemical characterization

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exhibit excellent specific capacitance and long lifespan. This work signifies that MoO2/MoS2 is a promising negative electrode for SCs. It also indicates that MoO2/MoS2 has great potential in advanced energy storage systems. 2. Experimental In a typical synthesis, 1.0000 g of (NH4)6Mo7O24∙4H2O and 0.2000 g of NaHB4 were dissolved in 20 ml distilled water and stirred for 30min. Subsequently, 20 mL distilled water containing a certain amount of Na2S∙9H2O was added dropwise. Then, the mixed solution transferred into Teflon-lined stainless steel autoclave at 200 °C for 12 h. Finally, filtration with distilled water and vacuum drying at 60°C for 18 h to 5

ACCEPTED MANUSCRIPT obtain the active materials, terming as MOS. Moreover, a series of MOS had been prepared by gradually increasing the quantity of Na2S∙9H2O (0.3500 g, 0.7500 g,

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1.1500 g, and 1.5500 g), and the products were assigned as MOS-0.35, MOS-0.75,

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MOS-1.15 and MOS-1.55, which was fabricated to further investigate the performance of the composite materials.

In addition, the detailed preparation of MoO2 and MoS2, materials used, structure

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characterization and electrodes preparation in this paper are given in the Supporting

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Information.

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3. Results and discussion 3.1. Structural characterization of MOS

Fig. 1.

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Fig. 1 shows the XRD pattern of a series of MOS samples. The diffraction peaks of the as-prepared products can be indexed to both MoO2 (JCPDF card No. 32-0671)

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and MoS2 (JCPDF card No. 37-1492), respectively. The peaks of the products are constituted by MoO2 and MoS2, which verifies the existence of MoO2 phase and MoS2 phase. It signifies the successful preparation of MOS composite materials. Obviously, the peak intensity and the peak width of the four samples are different from each other. As the decreased the quantity of Na2S∙9H2O, the peak intensity gradually weaken and the peak width gradually widen at an angle of 14.37º for the (002) plane. Moreover, more strong peak and more narrow peak is found at 26.03º _

_

for the (111) plane. Otherwise, the peak for the (111) plane shifts to lower angles for MOS-1.55, which results from embedding more S atoms in the crystal lattice of 6

ACCEPTED MANUSCRIPT MoO2 compared with the samples of MOS-1.15, MOS-0.75, and MOS-0.35. The TEM-EDS spectra display the Mo, O, and S characteristic peaks in Fig. S1. The peak

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at <1.0 KeV is from the overlapping signals of Mo and O. The peak at ~2.2 KeV is

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from the overlapping signals of Mo and S. The relatively weak peak at 16.3~20.1 KeV should be assigned to elemental Mo. These spectra prove that all the samples of MOS were successfully prepared. Moreover, it is easily found that the content of S is

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gradually decreased from MOS-1.55 to MOS-0.35, while the content of O is

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gradually increased from MOS-1.55 to MOS-0.35, which is due to the much less

Fig.2.

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Na2S∙9H2O. The XRD pattern of pure MoO2 and pure MoS2 is shown in Fig.S2.

The morphology and microstructure of MOS samples were characterized by

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scanning electronic microscopic (SEM) were shown in Fig. 2 and the morphology and microstructure of MoO2 and MoS2 were shown in Fig. S3. The agglomeration of the

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particles leads to a packed structure that can be observed in Fig. 2a. With the decrease of Na2S∙9H2O, a loosely packed structure appears, as shown in Fig. 2b. The morphology of MOS-1.15 is coarser and looser than MOS-1.55, which is ascribed to the coexistence of many nanoparticles and some rough nanosheets structures. With the further decreased of Na2S∙9H2O, rough nanosheets structures further were formed, coexisting with many nanoparticles. Upon gradual assembly of nanoparticles, massive nanosheets can be obtained. Afterwards, a large amount of smooth nanosheets stretch out towards the outside, many perfect flowerlike microspheres are obtained, as shown in Fig. 2c and inset. This structure contributes to improving porosity and providing a 7

ACCEPTED MANUSCRIPT stable structure in the composite materials. However, with less addition of Na2S∙9H2O, a more densely packed structure appears and the particle size is inhomogeneous, as

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shown in Fig. 2d, which could result from the flower-like microspheres collapse and

Fig. 3.

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pulverize.

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Furthermore, in order to clarify the component of the flower-like microspheres, the multi-petalled part (Section A in Fig. 3a) and the central part (Section B in Fig. 3a)

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of the flower-like microspheres was further investigated by TEM with corresponding

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elemental mapping images. It is easily found that the multi-petalled part of the

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flower-like microspheres contains elemental Mo (Fig. 3b), O (Fig. 3c), and S (Fig. 3d). Moreover, the central part of the flower-like microspheres also contains elemental Mo

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(Fig. 3e), O (Fig. 3f), and S (Fig. 3g). The results of elemental mapping images can prove that the Mo, O, and S are uniformly dispersed, confirming the formation of

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MOS-0.75 nanocomposite.

Fig. 4. Table 1

Nitrogen adsorption-desorption isotherms of MOS-1.55, MOS-1.15, MOS-0.75, and

MOS-0.35

was

further

investigated

by

measuring

the

nitrogen

adsorption–desorption isotherm at -196 ºC. The corresponding pore size distribution (PSD) profiles (Fig. 4 inset) were calculated using the density function theory (DFT) model from the absorption branch of the isotherm. It can be seen that all samples exhibit type-IV curves with obvious hysteresis loop at high relative pressures, which 8

ACCEPTED MANUSCRIPT indicates the existence of a typical mesoporous structures. Simultaneously, the PSD curves also indicate the existence of mesoporous structures. The MOS-1.55

displays

hierarchical

porous

structures,

including

microporous,

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MOS-1.15

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nanoparticles possess mesoporous structures and macroporous structures, while the

mesoporous, and macroporous structures, which is mainly attributed to the coexistence of the loosely packed nanoparticles and some rough nanosheets structures.

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With the further decreased of Na2S∙9H2O, the microporous structures disappear in the

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sample of MOS-0.75, more mesoporous and macroporous structures have been forming due to the existence of the large amount of nanosheets and flowerlike

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microspheres. With the destruction of the flowerlike structure, a more densely packed

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structure emerges, which results in the decrease of specific surface area and pore

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volume. The relevant structural parameters derived from the isotherms are summarized in Table 1. It is easily found that specific surface area and pore volume of

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MOS-1.15 is smaller than the sample of MOS-0.75, which results from the existence of microporous structures in MOS-1.15. 3.2. Electrochemical measurements Fig. 4. Table 2 In order to obtain insight on the electrochemical performance of as-prepared MOS, we performed a series of electrochemical measurements, including the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectra (EIS) techniques. All of the tests were performed in 1 M Na2SO4 aqueous electrolyte under three-electrode system. Fig. 5a displays the CV curves for 9

ACCEPTED MANUSCRIPT MOS-1.55, MOS-1.15, MOS-0.75 and MOS-0.35 samples at a scan rate of 10 mV s-1. It is found that all of the CV curves exhibit roughly rectangular shapes without any

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redox peaks, suggesting the existence of EDLCs behavior [41]. The specific

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capacitance C (F g-1) of MOS samples can be calculated using the equation S(1). The calculated capacitances of MOS-1.55, MOS-1.15, MOS-0.75, and MOS-0.35 are 306.1 F g-1, 334.2 F g-1, 381.9 F g-1, and 216.2 F g-1 at a scan rate of 10 mV s-1,

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respectively. More addition, we find the specific capacitance is proportional to the

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average area of a CV curve according to the equation S(1). It is obviously observed that the CV loop area of MOS-0.35 is the smallest, corresponding to the poorest

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specific capacitance of 216.2 F g-1. On the contrary, MOS-0.75 with the largest CV

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loop area is the optimum specific capacitance with 381.9 F g-1. It will be found their

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detailed values of capacitances based on the CV curves in Table S1. The corresponding rate capabilities will be found in Table 2. It is easily found that the rate

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capability is 51.5%, 45.9%, 57.4%, 52.7%, 67.3%, and 48.8% for the samples of MOS-1.55, MOS-1.15, MOS-0.75, MOS-0.35, MoO2, and MoS2 from 5 to 50 mV s−1, respectively. Obviously, the rate capability of MOS-0.75 electrode materials is superior to the other MOS electrode materials. The corresponding rate capability from 1 to 8 A g−1 is also superior to the other MOS electrode materials, which indicates the MOS-0.75 is suitable for SCs. Moreover, the values of capacitances based on the charge-discharge tests curves have been shown in Table S2 according to equation S(2). The corresponding energy density is 14.5 Wh kg-1, 18.1 Wh kg-1, 22.5 Wh kg-1, 9.1 Wh kg-1, 10.1 Wh kg-1, and 2.5 Wh kg-1 at current density of 1 A g-1 for MOS-1.55, 10

ACCEPTED MANUSCRIPT MOS-1.15, MOS-0.75, MOS-0.35, MoO2 and MoS2 can be calculated according to equation S(3). Moreover, the MOS-1.55, MOS-1.15, MOS-0.75 electrode materials is

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about 325.1 kW kg-1 at current density of 1 A g-1 according to equation S(4) . The

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power density of MoO2 and MoS2 is about 138.4 kW kg-1 and 132.7 kW kg-1. It is easily found that the MOS electrode materials are be superior to the electrode materials of MoO2 and MoS2. Moreover, the MOS-0.75 owns optimal energy density

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in MOS materials, suggesting the MOS-0.75 is promising candidate for SCs.

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The Fig. 5b shows the plots of the specific capacitance on the different MOS samples in proportion to CV profile. The specific capacitance of MOS electrodes

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decreases gradually with the increase of the scan rate. This can be ascribed to

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insufficient ion transfer at the electrode/electrolyte interface at higher scan rate.

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Moreover, it is easily found that the specific capacitance is gradually improved from MOS-1.55 to MOS-0.75 with less Na2S∙9H2O. However, the specific capacitance of

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MOS-0.35 suddenly drops when the addition of Na2S∙9H2O reduces again. The addition of Na2S∙9H2O controls the content of MoS2. Therefore, we think the values of specific capacitance are attributed to an effective synergistic effect between MoO2 and MoS2. For one thing, a suitable lattice disturbance may be beneficial to electrochemical performance. From the XRD results, it is found the existence of _

shifting in the lattice of MoO2 at the peak of the (111) plane, because the crystal lattice of MoO2 in composites appears different levels of disturbance due to the embedded S atom from the MoS2. The shifting reflects different degree of disturbance in four composites. The shifting of MOS-1.55 is the maximum value. It suggests that 11

ACCEPTED MANUSCRIPT the crystal lattice of MoO2 in MOS-1.55 composite material suffers from much disturbance, but the existence of much disturbance in MOS-1.55 composite leads to a

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poor capacitance. For another, the capacitance may be related to the content of MoO2

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and MoS2 in composites. The content of MoO2 gradually increases when the addition of Na2S∙9H2O gradually decrease. Meanwhile, the corresponding capacitance is gradually improved from MOS-1.55, MOS-1.15, to MOS-0.75. The content of MoO2

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in MOS-0.35 is more abundant, but its poor capacitance may be ascribed to a high

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degree of crystallinity. Nevertheless, MOS-0.75 exhibits an excellent electrochemical performance, manifesting an optimum content of MoO2 with adding 0.75 g

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Na2S∙9H2O. According to the data of EDS, the O : S weight ratio is about 4 : 3 in

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MOS-0.75 composite. Therefore, it is benefit for electrochemical performance to

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adjust the O : S weight ratio of 4 : 3 in MOS composites. Moreover, the MOS-1.55, MOS-1.35, MOS-0.75, and MOS-0.35 samples maintain 51.5%, 45.9%, 57.4%, and

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52.7% capacitance retention as the scan rate increased from 5 to 50 mV s-1, respectively. The GCD curves of the MOS electrode materials are acquired at a current density of 1 A g-1 (Fig. 5c). All of the GCD curves are symmetric during the process of charge and discharge, resulting in a perfect SCs property. The Nyquist plots of the impedance spectra are illustrated in Fig. 5d. It is composed of semi-arcs component at high frequency zone and almost similar linear at the low frequency zone. The four electrodes have similar linearity, which signify the existence of equivalent diffusion ability in MOS composite materials. The inset of Fig. 5d shows the spectra at high-frequency. Generally, it is easily found an obvious semicircle in the ideal state. 12

ACCEPTED MANUSCRIPT However, there are only semi-arcs rather than obvious semicircle, suggesting a lower charge transfer resistance and an ideal capacitive behavior. Additionally, the

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appearance of the resistance below 0 Ω is probably due to the presence of partly

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inductive reactance, which is aligned to previous reports [42, 43]. The x-intercept of the plots on real axis is defined as internal resistance (Rs), including the ionic resistance of electrolyte, intrinsic resistance of electrode materials and contact

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resistance at the electrode/electrolyte interface. The Rb is 1.98 Ω, 2.15, 2.23 Ω, and

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2.21Ω corresponding to the sample of MOS-1.55, MOS-1.15, MOS-0.75 and

Fig. 6.

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MOS-0.35, respectively, exhibiting a better electrical conductivity.

The CV curves of the MOS-0.75 electrode materials at various scan rates are

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displayed in Fig. 6a. The nearly mirror image shapes reveal the dominant reversibility and a fast reaction on the two-dimensional or quasi two-dimensional surface along the current-potential axis from -1.1 V to -0.3 V. The roughly rectangular shape without any obvious redox peaks infers a faster charge propagation and easy ion transport in the MOS electrodes due to the existence of ELDCs behavior. Moreover, the tiny deviation from perfect rectangular shape indicates the presence of a weak pseudocapacitive behavior, which results from a reversible redox reaction. The results exhibit that two mechanisms are involved in the MOS electrodes as follow: (1) The EDLCs mechanism, the charge-storage during electrochemical process is 13

ACCEPTED MANUSCRIPT ascribed to the adsorption/desorption alkali metal cations (Na+), which can be

(1)

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(MoS2) surface + Na+ + e- → (MoS2–Na+) surface

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described as following reaction (1):

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(2) The pseudocapacitance mechanism, the charge-storage during electrochemical process is ascribed to a reversible redox reaction, which can be described with the following reaction (2):

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MoO2 + xNa+ + xe- →MoO2-x (ONa)x

(2)

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The coexistence of two mechanisms involves a synergistic contribution to the charge-storage process, resulting in superior electrochemistry properties. The

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corresponding specific capacitance values calculated by equation(1) are 433.3 F g-1,

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381.9 F g-1, 331.8 F g-1, 308.5 F g-1, 277.1 F g-1, and 248.9 F g-1 at scan rates of 5 mV

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s-1, 10 mV s-1, 20 mV s-1, 30 mV s-1, 40 mV s-1, and 50 mV s-1, respectively. To get further insight into the electrochemistry properties of MOS-0.75, the GCD profile is

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studied at diverse current densities (Fig. 6b). The near linear and triangular curves signify a superior SCs behavior with a rapid I-V response. Additionally, all the GCD curves with a slight voltage drop (IR drop) indicate the dominance of the EDLCs behavior over pseudocapacitance behavior in MOS composites. Moreover, the IR drop decreases with the decrease in discharge current values, suggesting the fast I-V response and low internal resistance of the electrode materials. To clearly illustrate the advantages of the MOS electrode materials, a series of comparisons can be further revealed. Fig. 6c shows three CV curves corresponding to the samples of MOS-0.75, MoO2, and MoS2, respectively. The CV loop area of the 14

ACCEPTED MANUSCRIPT MOS-0.75 is obviously larger, implying that MOS-0.75 has an improved specific capacitance (381.9 F g-1) as compared to that of pure MoO2 (218.2 F g-1) and pure

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MoS2 (78.5 F g-1) at a scan rate of 10 mV s-1. The detailed values of capacitance can

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be obtained in Table S1. The specific capacitance change depends on the addition of Na2S∙9H2O. It can be obviously found in Fig. 6d. The capacitances of MOS samples are superior to that of pure MoO2 and pure MoS2, which further verifies the existence

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of synergistic effect in MOS composites. The value of capacitance can achieve the

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maximum of 433.3 F g-1 at a scan rate of 5 mV s-1 when the addition of Na2S∙9H2O is increased to 0.75 g. A decline of capacitance appears when the addition of Na2S∙9H2O

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is over 0.75 g. Therefore, the synergistic effect in MOS composites can be controlled

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by adjusting the addition of Na2S∙9H2O. What is more, Fig. 6e demonstrates the

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corresponding GCD profiles, which exhibit symmetric charge and discharge curves. It is easily found that there is small potential drop in all the GCD curves, which is

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attributed to resistance, including internal resistance of materials, charge transfer resistance, and contact resistance between the electrolyte and electrodes [31]. The EIS curves are shown in Fig. 6f in order to further evaluate the electrochemical performance of MOS-0.75, MoS2, and MoO2 electrode. It is easily found that the Rb of MOS-0.75 (2.23 Ω) between MoS2 (1.83 Ω) and MoO2 (2.38 Ω), suggesting the sample of MOS-0.75 with rapid electron transfer and optimal conductivity. The satisfactory value of Rb exhibits the advantage of combining MoO2 and MoS2. Here, it is necessary to explain that the Rb of MOS-0.75 is not the minimum values in four MOS samples, but the sample of MOS-0.75 shows the most 15

ACCEPTED MANUSCRIPT excellent specific capacitance. A favorable electronic conductivity is beneficial to electrochemical properties. However, evaluating the performance not only is related

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to the single parameter, but also depends on morphology and structures. The superior

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specific capacitance of MOS-0.75 as compared to the other MOS samples is mainly attributed to its flower-like microspheres, which creates more channels for electrolyte ion movement at the interface of the electrode/electrolyte.

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Fig. 7

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Table 3

Moreover, the equivalent circuit of the electrodes has been shown in Fig. 7. It is

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easily found that the mechanism of charge storage contains two types: EDLCs

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mechanism and pseudocapacitance mechanism, which is in good agreement with the

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CV curves analysis. The details equivalent circuit parameters also obtained by using fitting program, which is shown in Table 3.

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Fig. 8

In addition, the excellent electrical conductivity of MOS-0.75 can also be testified by linear sweep voltammetry. Fig. 8 shows the current-voltage (I–V) curves of MOS-1.55, MOS-1.15, MOS-0.75, MOS-0.35, MoO2, and MoS2. The slope of the straight line reflects the electrical conductivity of materials. Obviously, the slope of MOS electrode materials is bigger than MoO2 electrode, which is attributed to the existence of MoS2 in MOS materials. Moreover, the MOS-0.75 indicates a larger slope than other MOS electrode materials but a smaller slope than MoS2. It exhibits 16

ACCEPTED MANUSCRIPT that the electrical conductivity of MOS-0.75 is visibly superior to MOS-1.55, MOS-1.15, MOS-0.35, and MoO2, which is attributed to combine MoO2 with MoS2

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and also suggests that the design and preparation of MoO2/MoS2 is effective.

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Fig. 9

Fig. 9 exhibits plot of specific capacitance as a function of cycle number. The

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samples of MOS-0.75, MoO2, and MoS2 show a specific capacitance of 382.66 F g-1,

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151.25 F g-1, and 46.34 F g-1 at a current density of 1 A g-1, respectively. The capacitance retention of MOS-0.75 is 84.41% after 5000 cycles and the capacitance

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fading only occurs at initial cycles. Besides, the nyquist plot after cyclic stability

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display inset of Fig. 9a, the Rs after 5000th cycles is bigger than 1th, which results

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from the tiny change of the structure. However, the similar values of Rct and linear component at the low-frequency region indicate that such an electrode offers stable

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performance and good charge propagation (Fig. 9a, inset). The cycling performance is superior to MoO2 (retention of 33.33% after 2000 cycles) and MoS2 (retention of 58.33% after 2000 cycles). Moreover, the GCD curve even after 5000 cycles still keeps a similar triangle with first cycle, suggesting that the MOS-0.75 electrode has a better cycle stability. Compared with pure MoO2 and pure MoS2, the better electrochemical performances of MOS-0.75 are ascribed to the synergistic effect. Firstly, the oxide edge of MoO2 acts as active sites and the sulfur edge of MoS2 the sulfur edge acts as active sites. However, the integration of MoO2 with MoS2 can supply more active sites due to the coexistence of oxide edge and sulfur edge in 17

ACCEPTED MANUSCRIPT composite, which can facilitate fast electrochemical reaction of active material and enhance rapid electron transportation from electrode material to the current collector.

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Secondly, the flower-like microspheres of MOS-0.75 contribute to improve

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electrochemical behavior, which provide more pathways for charge transfer in electrolyte. Thirdly, MOS-0.75 has an ideal conductivity, which suggests a fast electron transfer. This is beneficial to obtain a better electrochemical performance.

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Finally, the coexistence of EDLCs mechanism and pseudocapacitance mechanism in

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composite is favorable for the charge-storage process, resulting in superior electrochemistry properties. Ascribe to the above four contributions, the MOS-0.75

MOS-0.75

composite

is

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that

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composite boosts an idea electrochemical property. These results not only confirm an

exceptional

electroactive

material

for

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high-performance SCs, but also suggest that MOS-0.75 composite has significant

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potential for use in energy devices.

4. Conclusions

In summary, a series of MoO2/MoS2 composite materials were synthesized via a facile hydrothermal synthesis and such materials served as negative electrode in supercapacitors for the first time. Ascribe to the synergistic effect, including the improved electron conductivity, the coexistence of two energy storage mechanisms, and a stable flower-like microsphere structure, the MoO2/MoS2 electrode exhibits a high specific capacitance with 433.3 F g-1 at a scan rate of 5 mV s-1 and good cycle stability with 84.41% capacitance retention over 5000 cycles, which suggests such materials are probable candidates for advanced energy device in the future. 18

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Acknowledgments

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We gratefully acknowledge the National Natural Science Foundation of China

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(No. 51362018) and the Foundation for Innovation Groups of Basic Research in Gansu Province (No. 1606RJIA322).

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ACCEPTED MANUSCRIPT Captions of Figures Fig. 1. XRD patterns of MOS-1.55, MOS-1.15, MOS-0.75, and MOS-0.35composited

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Fig. 2. SEM image of (a) MOS-1.55; (b) MOS-1.15; (c) MOS-0.75; (d) MOS-0.35. Fig. 3. Elemental mapping by TEM over the image: TEM images of MOS-0.75 (a) with corresponding elemental mapping images of Mo (b), O (c), and S (d) in section

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A, and Mo (e), O (f), and S (g) in section B.

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Fig. 4.Nitrogen adsorption-desorption isotherms of (a) MOS-1.55; (b) MOS-1.15; (c) MOS-0.75; (d) MOS-0.35. And the corresponding PSD (inset) calculated from the

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adsorption branch of the isotherm at -196 °C using the density function method (DFT)

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Fig. 5. Electrochemical characterization of MOS electrodes in 1 mol L-1 Na2SO4 aqueous electrolyte: (a) Cyclic voltammogram curves of MOS electrodes at a scan

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rate of 10 mV s-1 with a potential window from -1.1 V to -0.3 V; (b) Plots of the specific capacitance vs scan rates ranging from 5 mV s-1 to 50 mV s-1; (c) Galvanostatic charge/discharge curves of MOS electrodes at a current density of 1A g−1; (d) Nyquist plots of the impedance spectra measured in the frequency range from 105 Hz to 10-2 Hz at the open circuit potential with an alternate current amplitude of 5 mV-1. Fig. 6. (a) Cyclic voltammogramat different scan rates of 5 mV s-1 to 50 mV s-1; (b) Galvanostatic charge/discharge curves of MOS-0.75 at different current densities of 1 A g-1 to 8 A g-1; (c) Comparative Cyclic voltammogram of MOS-0.75 at scan rates of 25

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Fig. 7. An equivalent circuit used to fit the Nyquist plot using the software Zsimpwin.

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(Rs: Cell internal resistance, RCT: Charge transfer resistance, CDL: Double layer capacitance, W: Warbug diffusion element, CF: Faradic capacitance.)

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Fig. 8. .I-V curves of MOS-1.55, MOS-1.15, MOS-0.75, MOS-0.35, MoO2, and

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Fig. 9. Plot of specific capacitance as a function of cycle number of (a) MoO2, MoS2,

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Table 1. Structural parameters of MOS-1.55, MOS-1.15, MOS-0.75, and MOS-0.35. Table 2. Specific capacitances and rate capabilities for MOS-1.55, MOS-1.15, MOS-0.75, MOS-0.35, MoO2 and MoS2 electrodes in 1 M Na2SO4 neutral electrolyte. Table 3. Equivalent circuit parameters obtained by using fitting program.

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Fig. 6

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pore volume (cm3 g-1)

MOS-1.55

2.9868

0.0149

MOS-1.15

26.1555

0.3382

MOS-0.75

10.9632

MOS-0.35

1.9508

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CS at 5 mV s−1 (F g-1)

Rate capability from 5 to 50 mV s−1 (%)

MOS-1.55

350.7

51.5

MOS-1.15

384.1

MOS-0.75

433.3

MOS-0.35

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MoO2

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49.4

45.9

307.6

42.6

57.4

383.5

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52.7

153.4

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67.3

170.8

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RCT (Ω)

W (Ω.^-1/2)

CF (F)

MOS-1.55

1.995

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0.3218

0.3752

MOS-1.15

2.013

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0.3103

MOS-0.75

2.159

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MOS-0.35

2.177

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0.2114

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Graphical abstract

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The MoO2/MoS2 flower-like microspheres was successfully designed and prepared. Moreover, it was also served as negative electrode with excellent electrochemical performance for supercapacitors.

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ACCEPTED MANUSCRIPT Highlights

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1. MoO2/MoS2 is successfully synthesized via economical and simple hydrothermal synthesis and is firstly served as negative electrodes for supercapacitors.

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2. The MoO2/MoS2 electrode displays a high specific capacitance of 433.3 F g-1 at a scan rate of 5 mV s-1.

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3. The MoO2/MoS2 electrode indicates higher specific capacitance (381.9 F g-1) than that of pure MoO2 (218.2 F g-1) and pure MoS2 (78.5 F g-1) at a scan rate of 10 mV s-1 in 1 M Na2SO4 electrolyte.

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4. The MoO2/MoS2 electrode exhibits a long lifespan with cycling stability of 84.41% retention after 5000 cycles, which is superior to pure MoO2 (33.33%

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retention after 2000 cycles) and pure MoS2 (58.33% retention after 2000 cycles) at a current density of 1 A g-1.

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