Fabrication of MoS2 loaded on expanded graphite matrix for high-efficiency pH-universal hydrogen evolution reaction

Journal Pre-proof Fabrication of MoS2 loaded on expanded graphite matrix for high-efficiency pHUniversal hydrogen evolution reaction Jun He, Siqi Chen, Shuqin Yang, Wenchao Song, Changtian Yu, Laizhou Song PII:

S0925-8388(20)30733-7

DOI:

https://doi.org/10.1016/j.jallcom.2020.154370

Reference:

JALCOM 154370

To appear in:

Journal of Alloys and Compounds

Received Date: 9 December 2019 Revised Date:

5 February 2020

Accepted Date: 13 February 2020

Please cite this article as: J. He, S. Chen, S. Yang, W. Song, C. Yu, L. Song, Fabrication of MoS2 loaded on expanded graphite matrix for high-efficiency pH-Universal hydrogen evolution reaction, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154370. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Fabrication of MoS2 Loaded on Expanded Graphite Matrix For

2

High-Efficiency pH-Universal Hydrogen Evolution Reaction

3

Jun Hea,*, Siqi Chena, Shuqin Yanga, Wenchao Songa, Changtian Yua, Laizhou Songb,*

4

a School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao 066004, China

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b Hebei Key Laboratory of heavy metal deep-remediation in water and resource reuse，Qinhuangdao, China

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* Correspondence: [email protected], [email protected]; Tel.: +86-335-8387741; Fax: +86-335-8061569

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ABSTRACT: As a promising electrocatalyst for hydrogen evolution reaction (HER), molybdenum disulfide

9

(MoS2) owns inadequate edge active sites and inferior conductivity which restricts its effective application.

10

In this study, the expanded graphite matrix (EGM) cathode was fabricated by tableting worm-like expanded

11

graphite (EG), acetylene black and poly tetra fluoroethylene (PTFE) emulsion, and MoS2 grew on EGM via

12

a hydrothermal synthesis process, subsequently employed for the HER process. Benefiting from the superior

13

conductivity and sufficient exposure of edge active sites on the rough structure of EGM, MoS2/EGM

14

exhibits a small overpotential of 230 mV (j = 10 mA·cm-2), a low Tafel slope of 77 mV·dec-1 and a lower

15

value of charge transfer resistance (Rct, 0.919 Ω·cm2). The improvement of HER performance of MoS2/EGM

16

could be due to the participation of the EGM as a highly conductive substrate. Excellent electrochemical

17

durability of MoS2/EGM was attested by the cyclic voltammetry and the amperometric (i−t) measurements.

18

This work may have the potential to develop a promising path to design advanced electrode materials for

19

HER.

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Keywords: Hydrogen evolution reaction, Electrocatalyst, Expanded graphite matrix, Molybdenum disulfide.

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1. Introduction

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Hydrogen energy as a kind of ideal, non-polluting and effective renewable energy, can solve the

24

environmental pollution problems caused by fossil fuel combustion [1-3]. And hydrogen producing by

25

electrochemical decomposition of water is a simple, ripe technology widely concerned with engineering

26

application prospects. An appropriate and efficient electrocatalyst is critical to realize the engineering

27

application of this technology for hydrogen evolution reaction (HER) [4-6]. Noble metals-based catalysts

28

were considered to be the most effective electrocatalysts for HER, like as Pt, Pd, which cannot be widely

29

applied for its scarcity [7-9]. Therefore, the development of novel cathode electrocatalysts with high

30

efficiency and lower price for HER has become a hot research topic. Molybdenum disulfide (MoS2) is deemed

31

to be one of the most promising catalysts for HER because of its low cost, high efficiency, abundance, and

32

hydrogen binding energy calculated by density functional theory closing to the binding energy of Pt group

33

metals [10-11]. However, theoretical and experimental evidences indicate that two main challenges hinder it

34

to become an ideal electrocatalyst for HER. Firstly, the active sites of MoS2 are situated at the edges where a

35

lot of unsaturated sulfur atoms are present instead of the basal planes. Secondly, the inferior conductivity of

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MoS2 restricts the transfer of electrons and impedes the property of HER [12-14].

37

In the past few years, many researchers have employed carbon materials loaded MoS2 to synthesize

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MoS2/carbon materials to overcome the two drawbacks mentioned above, such as carbon cloth, carbon fibers,

39

graphene, amorphous carbon, and carbon nanotubes. However, these composite materials are mostly

40

powders dropped on the glassy carbon electrode when testing electrochemical properties. Generally, a

41

polymer binder was used to effectively adjust the dissolved matter of the catalysts on the glassy carbon

42

electrode, which may not only increase the series resistance, hide the catalytic active sites and reduce the

43

electrocatalytic property, but also limit its application in engineering [15-20]. Therefore, the combination

44

with a conductive substrate to form a monolithic catalytic electrode for HER has aroused great interests in

45

developing efficient substrates with charge transportation. Expanded graphite (EG) is a kind of non-toxic and

46

easily produced carbon material, which has been applied in different electrochemistry fields such as

47

electrochemical sensors and active electrode material of fuel cell, etc. on account of its low cost, excellent

48

conductivity and good physical strength [21-25]. In addition, to our best knowledge so far, EG has not been

49

reported in terms of cathode matrix materials for HER.

50

In this paper, EGM prepared with EG and other mixtures was selected as the substrate material to load

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MoS2 to prepare a cathode electrode for improving HER performance. And the effect of MoS2 loading content

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on the surface of EGM for the structure and HER property of composites under acidic conditions was studied

53

by changing the concentration of MoS2 precursor. At the same time, the HER property of the prepared

54

materials in neutral and alkaline eletrolytes were explored.

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2 Experimental Section

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2.1 Chemicals

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The natural flake graphite (80 mesh) was purchased from Qingdao Tianhe Da Graphite Co., Ltd.

60

(Shandong, China), and the carbon content is 99.9 wt %., Ammonium molybdate ((NH4)6Mo7O24·4H2O),

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potassium permanganate (KMnO4) and thiourea (CH4N2S) were purchased from Kemiou Chemical Reagent

62

Co., Ltd. (Tianjin, China). Perchloric acid (HClO4 70%~72 wt %) was purchased from Oriental Chemical

63

Factory (Tianjin, China). Ammonium nitrate (NH4NO3) was supplied by Chemical reagent factory (Tianjin,

64

China). All of the experimental reagents mentioned above were analytical grade.

65

2.2 Preparation of MoS2/EGM

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2.2.1 Preparation of EG

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The chemical oxidation method was used for preparing sulfur-free expandable graphite (GIC). Firstly, 5

68

g of nature flake graphite was added into a beaker, and 2.25 g KMnO4 and 0.6 g NH4NO3 were added and

69

mixed fully, then 40 mL HClO4 was poured slowly into the above mixture. Secondly, the beaker containing the

70

mixture was sealed with plastic wrap and then placed in water bath stirring continuously at a constant

71

temperature of 30℃ for 1 h. Thirdly, after completion of the reaction, the products were washed with distilled

72

water and filtered, and then dried for 12 h at 65℃ to obtain GIC samples. Finally, the quartz beaker

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containing the GIC samples was placed in a muffle furnace heated to 950℃ in advance and puffed for 3-5 s.

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Afterwards, the EG samples were obtained.

75

2.2.2 Preparation of EGM

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Firstly, 150 mL absolute ethanol was slowly poured into a beaker containing 1 g EG and 0.1 g acetylene

77

black, and then the beaker was placed on a magnetic stirrer and stirred at room temperature for 30 min. 5 mL

78

of poly tetra fluoroethylene (PTFE) (60 wt %) solution was dropwise added into the mixture and stirred for 1 h

79

to mix completely. Next, the mixture was placed in a constant temperature water bath previously heated to 90℃

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to volatilize ethanol, and then the mixture was dried at 65℃ for 24 h. At last, the dried mixture powders were

81

taken out and placed in a mold of a powder tablet press for compression. The diameter of the mold was 50 mm,

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and the pressing pressure and time were set to 8 ton and 15 min, respectively. After the pressing was

83

completed, a pressed sample with a diameter of 50 mm and a thickness of 1 mm was taken out from the mold,

84

and sintered at 375℃ for 2 h in a muffle furnace. Finally, a sintered piece was cut up into a square of 20 mm ×

85

20 mm as an EGM sample.

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2.2.3 Preparation of MoS2/EGM

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The EGM sample was cleaned by sonication successively in absolute ethanol and distilled water for 15

88

min each. 35 mL distilled water was added into a beaker containing 1.236 g (NH4)6Mo7O24·4H2O and 2.28 g

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CH4N2S, the mixture was dissolved completely by a magnetic stirrer, then 20 mL of mixed solution and EGM

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(20 mm ×20 mm) were transferred into a 25 mL Teflon-lined stainless autoclave and placed in a 200℃ muffle

91

for 24 h. After stainless autoclave was naturally cooled to room temperature, the sample of MoS2/EGM-2 was

92

removed and washed successively with distilled water and absolute ethanol until no black solid particles were

93

present in the washing liquid, then dried at 60℃ for 24 h. The other two MoS2/EGM samples (MoS2/EGM-1,

94

MoS2/EGM-3) were also fabricated by controlling the additions of (NH4)6Mo7O24·4H2O and CH4N2S: 0.618 g

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(NH4)6Mo7O24·4H2O and 1.14 g CH4N2S for MoS2/EGM-1, and 2.472 g (NH4)6Mo7O24·4H2O and 4.56 g

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CH4N2S for MoS2/EGM-3. The flowchart of MoS2/EGM samples preparation was shown in Fig.1.

97 98 99

Fig.1. The flowchart of MoS2/EGM samples preparation

2.2.4 Preparation of MoS2-EGM and Pt/EGM

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MoS2 and Pt ink were dropped onto EGM (MoS2-EGM, Pt/EGM) as working electrodes compared with

101

MoS2/EGM for HER performance. Pt ink was prepared by adding 3.125 mg Pt powder into the solution

102

containing 0.75 mL distilled water , 0.25 mL absolute ethanol and 0.05 mL Nafion solution and the mixture

103

solution was sonicated until dispersed evenly. Then a Pt/EGM electrode was prepared via 0.1 mL catalyst ink

104

uniformly was loaded on the effective working area of EGM (5 mm × 5 mm) and dried at 65℃ for 24 h. The

105

same method was used to fabricate MoS2 ink with as-prepared pure MoS2 powder (where the dosage of MoS2

106

was 25 mg) and the preparation of a MoS2-EGM electrode was similar to the Pt/EGM electrode, but the

107

effective working area of EGM is 10 mm × 20 mm. The amount of Pt and MoS2 powder dripping on the EGM

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was all 1.25 mg/cm2 in comparative experiments, which was consistent with the load of MoS2 on the EGM

109

(MoS2/EGM-2).

110

2.3 Characterization of MoS2/EGM.

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The structures of blank EGM, pure MoS2 and MoS2/EGM samples were investigated by the X-ray

112

diffractometer (XRD, Smartlab, Rigaku, Japan) with Cu Ka radiation (λ=1.5418Å). A scanning electron

113

microscope (SEM, S–4800, Hitachi, Tokyo, Japan) was used to observe the morphologies of the prepared

114

composites and the compositions of the three MoS2/EGM samples were measured by an energy-dispersive

115

spectrometry (EDS) connected to the SEM mentioned previously. The X-ray photoelectron spectra (XPS,

116

ESCALAB MK II, Thermo Fisher Scientific, Waltham, MA, USA) was used to characterize the fabricated

117

MoS2/EGM-2 with Mg Kα being the excitation source, and the C 1s peak of graphite carbon at 284.6 eV was

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utilized to revise the binding energy.

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2.4 Electrochemical measurements.

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The HER property of all the prepared materials was measured under 0.5 M H2SO4 with a standard

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three-electrode system which was made up of blank EGM (MoS2-EGM, MoS2/EGM) with an effective

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working area of 10 mm × 20 mm as the working electrode, a saturated Ag/AgCl as the reference electrode, and

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a Pt wire as the counter electrode. Additionally, the HER property of the MoS2/EGM-2 electrode was also

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measured in 1 M phosphate buffer solution (PBS) and 1 M NaOH eletrolytes. Linear sweep voltammetry

125

(LSV) was measured from -0.5 to 0.2 V(vs. RHE) with the scan rate of 2 mV/s and the Tafel slope (b) can be

126

calculated from the Tafel equation (η = blog j + a, where, η is the overpotential and j is the current density).

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The frequencies ranging from 100 kHz to 100 mHz were selected to measure the electrochemical impedance

128

spectroscopy (EIS). During the investigation of stability, cyclic voltammetry (CV) was performed for 1000

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cycles with a sweep rate of 100 mV/s in the potential ranging from 0.1 to 0.3 V(vs. RHE). The amperometric

130

i−t curve was obtained at a static initial voltage of 250 mV. When electrical double-layer capacitor (Cdl) was

131

investigated, CV measurement was measured in the potential ranging from 0.1 to 0.3 V(vs. RHE) with scan

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rates of 20, 40, 60, 80 and 100 mV/s, respectively. All the above measurement methods were carried out by a

133

CHI 650C electrochemical analyzer (Chenhua Co. Ltd., Shanghai, China). Before the test, pure nitrogen was

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bubbled through the electrolyte for 30 min to deaerate the dissolved oxygen [27].

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3 Results and Discussion

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3.1 Characterization of MoS2/EGM.

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Fig. 2 shows the XRD patterns of the blank EGM, pure MoS2 and three MoS2/EGM composites. Three

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distinct diffraction peaks are at 2θ =13.8°, 32.26° and 57.18°. Compared with the standard diffraction pattern

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of MoS2, it is found that which is corresponding to the (002), (100) and (110) crystal planes of MoS2,

141

respectively. In addition, three distinct diffraction peaks also appear in the spectrum of pure MoS2 composites,

142

indicating that MoS2 was synthesized successfully via the hydrothermal method [28]. For blank EGM, two

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main characteristic diffraction peaks are at 2θ = 26.66° and 54.78° with higher diffraction intensities and

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sharp shapes, which indicates that the crystallinity of EGM is high and the internal particles are well

145

arranged. Furthermore, in the XRD spectrum of three MoS2/EGM composites, we can also observe that the

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partial diffraction peak of MoS2 also appears, demonstrating that EGM is loaded with MoS2 nanoparticles by

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hydrothermal synthesis reaction. Compared with the pure MoS2 pattern, the characteristic peaks of MoS2 in

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the three composites are weak, indicating that the addition of EGM reduces the crystallization properties of the

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MoS2 nanoparticles. In addition, as the content of MoS2 increases, the diffraction peak of MoS2 in the three

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composite materials becomes more and more obvious, and for MoS2/EGM-2, the diffraction peaks of the three

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crystal lanes of the MoS2 mentioned above are easily identified.

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Fig.2. X-ray diffraction patterns of EGM, MoS2 and various MoS2/EGM composites.

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Fig.3 displays the morphologies of blank EGM, pure MoS2, MoS2-EGM and MoS2/EGM-2. It can be

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seen from the SEM image of the blank EGM (Fig. 3(a) and the inset of Fig. 3(a)) that the surface of the EGM

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is rough, which could be favorable for the loading of MoS2 and also reduces the accumulation of MoS2 in

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favor of more edge sulfur active sites exposure. There was a distinct nanoflower-like material [20, 27] can be

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observed from Fig. 3(b), which reveals that MoS2 is successfully obtained by hydrothermal synthesis.

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Compared with MoS2 grown on EGM in Fig. 3(d), MoS2 dropped onto EGM in Fig. 3(c) aggregates seriously,

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resulting in a large number of active edges are covered [29]. Furthermore, Fig. 4(b) five times magnification

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than Fig. 3(d) shows that MoS2 was not only successfully loaded on the EGM, but also exposed to lots of

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edge active sites. With the content of MoS2 increasing as displayed in Fig. 4(a)-(c), the nanoflower-like

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structure in the three composites becomes more and more obvious, and the degree of accumulation becomes

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more and more serious. For MoS2/EGM-2, it may be the relatively evenly growth of MoS2 on the EGM

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surface, exposing the maximum active edges.

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The contents of C, S and Mo in the MoS2/EGM samples were tested by EDS when the samples were

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observed by SEM. The peak positions of the C element, the S element, and the Mo element can be seen from

169

the Fig. 4, which reveals that these elements do exist in the MoS2/EGM electrode materials. Moreover, the

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content of Mo and S elements in the three composite materials follows the following rules: MoS2/EGM-1<

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MoS2/EGM-2< MoS2/EGM-3, and the rule of the elemental test results is consistent with the loading of MoS2

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in the composite materials.

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Fig.3. Scanning Electron Microscope (SEM) images of the blank EGM, pure MoS2, MoS2-EGM and MoS2/EGM-2: (a) blank EGM (inset: 10000

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× image of blank EGM), (b) pure MoS2, (c) MoS2-EGM (inset: 10000× image of MoS2-EGM), (d) MoS2/EGM-2.

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Fig.4. Scanning Electron Microscope (SEM) images and Energy Dispersive Spectrometry (EDS) spectra of the three MoS2/EGM composites: (a)

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MoS2/EGM-1, (b) MoS2/EGM-2, (c) MoS2/EGM-3.

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XPS characterization measurement was performed to further explore the chemical valence of

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as-prepared MoS2/EGM-2 electrode material. Two characteristic peaks (Fig. 5(a)) at 228.9 eV and 231.9 eV

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are corresponding to Mo 3d5/2 and Mo 3d3/2 orbitals, suggesting the dominance of Mo4+ in the sample [20].

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Another weaker peak appears at the lower binding energy of 226 eV, representing S 2s of MoS2. Two

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obvious peaks can be observed at 161.7 eV and 162.8 eV in the S 2p spectrum shown in Fig. 5(b) attributed

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to S 2p3/2 and S 2p1/2 orbits, which originates from S2- of MoS2 [28]. Moreover, the peak at 169.2 eV is

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identified to the S4+ state locating at the edge of MoS2 structure in SO32- [27], which may be caused by the

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oxidation of sulfur in the hydrothermal reaction.

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Fig.5. X-ray photoelectron spectra of MoS2/EGM-2 composite: (a) Mo 3d and (b) S 2p, respectively.

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3.2 HER Performance of MoS2/EGM cathode

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In this study, the cathodic LSV polarization curves of blank EGM, MoS2-EGM, MoS2/EGM-1,

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MoS2/EGM-2, MoS2/EGM-3 and Pt/EGM were measured in 0.5 M H2SO4. Fig. 6(a) indicates that Pt/EGM

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has the best HER property among other tested materials in this study with a negligible initial overpotential,

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nevertheless, the blank EGM hardly shows any HER property within the measured voltage range. When the

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current density reaches 10 mA·cm-2, the corresponding overpotential of MoS2/EGM-2 electrode is 230 mV,

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exhibiting high catalytic activity during the HER process compared with the overpotential of MoS2-EGM of

200

370 mV, MoS2/EGM-1 of 280 mV and MoS2/EGM-3 of 270 mV. The reason is that MoS2/EGM-2 exposed

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more active sites owing to the relatively uniform growth of MoS2 on the EGM. In addition, this result is

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better than many MoS2/ carbon materials for hydrogen evolution as displayed in Table 1 [30-34].

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Table 1. Comparsion of HER performance of MoS2/EGM-2 with other MoS2/ carbon materials. Catalyst

Oneset Overpotential (mV)

Tafel slope (mV·dec-1)

Overpotential in 10

Ref.

mA·cm-2 (mV)

MoS2-CNT

_

47

290

[30]

1T MoS2/P-rGO

_

75

240

[31]

MoS2/g-C3N4

_

63

260

[32]

MoS2/C

122

64

_

[33]

MoS2 NSs/rCMWCNTs

190

66

_

[34]

MoS2/EGM-2

90

77

230

This work

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It is well known that the intrinsic reaction mechanism of the HER process can be explored by the Tafel

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slope. Generally, it is believed that there are three basic reactions in the HER process under acidic conditions,

209

as listed in equations (1)−(3) below, with the Tafel slopes of 120, 40, and 30 mV·decade-1. [35-39]. The Tafel

210

slope of the MoS2/EGM-2 electrode (77 mV·decade-1) is shown in Fig. 6(b), which is lower than MoS2-EGM

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electrode (108 mV·decade-1) and the other two samples (91 and 94 mV·decade-1), demonstrating that the

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Volmer−Heyrovsky mechanism of the MoS2/EGM composites occurred by a rapid adsorption reaction (eq. 1)

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firstly, and then an electrochemical desorption reaction (eq. 2) used to be the rate-determining reaction. In

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summary, the LSV polarization curve and the Tafel curve show an improvement of HER activity of

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MoS2/EGM-2. Compared with that of MoS2-EGM, this smaller Tafel slopes of the three MoS2/EGM

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samples may be originated for two reasons: (1) The close contact of MoS2 with the EGM promotes electrons

217

to transfer from the EGM to MoS2 during the cathodic reaction; (2) The finite layer along the (002) crystal

218

plane and the abundant exposed edge active sites of MoS2 loaded on EGM play the role of a catalytic center,

219

which is beneficial to the HER process.

220

Volmer reaction: H3O++ e- → Hads+ H2O

(1)

221

Heyrovsky reaction: Hads+ H++ e- → H2

(2)

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Tafel reaction: Hads+ Hads → H2

(3)

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Effective electron transfer is also an indispensable evaluation element for HER property of the electrodes.

224

It can be revealed from the Nyquist plots (Fig. 6(c)) that the charge transfer resistance (Rct) of MoS2/EGM-2

225

(0.919 Ω·cm2) is much lower than that of MoS2-EGM (236.2 Ω·cm2). The lower value means that the

226

conductivity of the composites is increased, thereby improving the charge transfer rate at the interface

227

between the electrocatalyst and the electrolyte owing to the participation of the EGM as a highly conductive

228

matrix material. It may be one of the reasons why the electrocatalytic activity of MoS2/EGM-2 has been

229

enhanced in HER process. Additionally, it is found that as the load increases, the value of Rct increases by

230

comparing the Rct value of the two materials (MoS2/EGM-1 and MoS2/EGM-3) with different loadings of

231

MoS2 on the surface of EGM (the inset of Fig. 6(c)), which possibly due to the excessive accumulation of

232

MoS2 to result in decrease of exposure of the sulfur active edge sites. Moreover, the MoS2/EGM-2 shows the

233

minimum Rct value among the three materials, indicating that MoS2/EGM-2 has better catalytic property of

234

HER, which is consistent with the results of the LSV polarization curve mentioned above.

235

The durability of catalysts is another critical factor considered in practical applications. The initial LSV

236

polarization curve of the MoS2/EGM-2 and the LSV polarization curve after 1000 cycles of the CV test are

237

shown in Fig. 6(d), which shows that the change of the LSV polarization curve before and after the CV test

238

can be ignored. Furthermore, the time-dependent current density curve of MoS2/EGM-2 (the inset of Fig. 6(d))

239

was measured to further explore the durability of catalysts, which illustrates that during the 12-hour

240

hydrogen evolution reaction, the curve is almost always level and there is no serious attenuation. In short, the

241

above analysis indicates that the MoS2/EGM-2 electrode has the long-time durability in acid media.

242 243

Fig.6. (a) Polarization curves and (b) corresponding Tafel slopes of as-prepared blank EGM, MoS2-EGM, Pt/EGM and MoS2/EGM composites in

244

0.5 M H2SO4 solution. (c) Nyquist plots of MoS2-EGM and MoS2/EGM-2 at 300 mV overpotential, (inset: Nyquist plots of the three MoS2/EGM

245

composites at 300 mV overpotential). (d) Polarization curves for MoS2/EGM-2 initially, after 1000 cycles were displayed (inset: time dependence

246

of cathodic current density curve for this sample).

247

The effective electrochemically active region of MoS2-EGM and MoS2/EGM composites was assessed

248

by calculating the value of the Cdl from the CV curves which were measured in a potential range without

249

Faraday current of 0.1 to 0.3 V (vs. RHE) because of the current response of this region was in charge of the

250

electrical double layer. The difference between the positive and negative current densities at the center of the

251

sweep potential range is plotted against the voltage sweep rate, where the half of slope is Cdl (Fig. 7(a)-(e))

252

[20]. The Cdl of MoS2-EGM, MoS2/EGM-1, MoS2/EGM-2 and MoS2/EGM-3 are 16.2, 60.83, 75.4 and 65.55

253

mF·cm-2 respectively, where the Cdl of MoS2/EGM-2 is more than nearly 5 times than MoS2-EGM and higher

254

than the other two composite materials.

255 256

Fig.7. (a-d) Cyclic voltammograms of (a) MoS2-EGM, (b) MoS2/EGM-1, (c) MoS2/EGM-2 and (d) MoS2/EGM-3 composites in 0.5 M H2SO4

257

solution at different scan rates. (e) Electrical double-layer capacitance (Cdl) of MoS2-EGM and the three MoS2/EGM catalysts.

258

259

The electrocatalytic performance of MoS2/EGM-2 was also tested under 1.0 M PBS and 1.0 M NaOH

260

for HER. Fig. 8(a)-(b) show the results of LSV and Tafel. At current density of 10 mA cm−2, the

261

overpotential of MoS2/EGM-2 in Fig. 8(a) is 250 mV and 360 mV, respectively, under 1.0 M NaOH and 1.0

262

M PBS conditions. Moreover, the result of Fig. 8(b) illustrates that MoS2/EGM-2 has different Tafel slopes

263

of 92 and 165 mV·decade-1 under 1.0 M NaOH and 1.0 M PBS medias, suggesting the Volmer–Heyrovsky

264

and Volmer mechanism, respectively [33]. Thus, synergistic effect of MoS2 and EGM for hydrogen

265

production at full pH values makes MoS2/EGM electrode as a promising candidate for practical water

266

decomposition.

267 268

Fig.8. (a) Polarization curves and (b) corresponding Tafel slopes of as-prepared MoS2/EGM-2 composite in 1.0 M NaOH and 1.0 M PBS solution.

269 270

4 Conclusions

271

In summary, the MoS2/EGM composites were successfully prepared by hydrothermal synthesis reaction.

272

The loading contents of MoS2 have a great effect upon the morphology, structure and HER property of the

273

materials. The lower overpotential, resistance, smaller Tafel slope and high electric double layer capacitance

274

confirm the synergistic effect between EGM and MoS2. The results show that MoS2/EGM-2 has the small

275

overpotential of 230 mV (j = 10 mA·cm-2), the Tafel slope as low as 77 mV·dec-1, and the large Cdl (75.4

276

mF·cm-2). The improvement of HER activity of MoS2/EGM may be due to the participation of the EGM as a

277

highly conductive substrate and the appropriate amount of MoS2 loaded, which results in MoS2/EGM having

278

more electrochemical regions and additional exposed active sites. The unique preparation methods of matrix

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material and synergistic optimization strategies between EGM and MoS2 are reported in this paper, which

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can be used not only to improve the performance of various electrocatalysts for HER, but also to provide

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opportunities for exploring new cathodic hydrogen evolution catalysts in the future.

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Acknowledgements This work was supported by National Natural Science Foundation of China(51608468).

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References

287 288

[1] Z.C. Li, J.J. Ma, Y. Zhou, Z.G. Yin, Y.B. Tang, Y.X. Ma, D.B. Wang, Synthesis of sulfur-rich MoS2

289

nanoflowers for enhanced hydrogen evolution reaction performance, Electrochim. Acta 283 (2018)

290

306-312.

291

[2] T.M. Su, Z.D. Hood, M. Naguib, L. Bai, S. Luo, C.M. Rouleau, L.N. Lvanov, H.B. Ji, Z.Z. Qin, Z.L. Wu,

292

Monolayer Ti3C2Tx as an Effective Co-catalyst for Enhanced Photocatalytic Hydrogen Production over

293

TiO2, ACS Applied Energy Materials 2(7) (2019) 4640-4651.

294

[3] Z. Liu, J. Yu, X. Li, L. Zhang, D. Luo, X. Liu, X. Liu, S. Liu, H. Feng, G. Wu, P. Guo, H. Li, Z. Wang,

295

X. Zhao, Facile synthesis of N-doped carbon layer encapsulated Fe2N as an efficient catalyst for oxygen

296

reduction reaction, Carbon 127 (2018) 636-642.

297 298 299 300

[4] G.Q. Li, D. Zhang, Q. Qiao, Y.F. Yu, S.Y. Huang, W.T. Yang, L.Y. Cao, Activating MoS2 for pH-universal hydrogen evolution catalysis, J. Am. Chem. Soc. 139 (2017) 16194-16200. [5] C-L. Wu, P-C. Huang, S. Brahma, J-L. Huang, S-C. Wang, MoS2-MoO2 composite electrocatalysts by hot-injection method for hydrogen evolution reaction, Ceram. Int. 43 (2017) s621-s627.

301

[6] T.M. Su, Z.D. Hood, M. Naguib, L. Bai, S. Luo, C.M. Rouleau, L.N. Lvanov, H.B. Ji, Z.Z. Qin, Z.L. Wu,

302

2D/2D heterojunction of Ti3C2/g-C3N4 nanosheets for enhanced photocatalytic hydrogen evolution, Nanoscale

303

11 (2019) 8138-8149.

304

305

[7] G.Q. Li, D. Zhang, Q. Qiao, Y.F. Yu, D. Peterson, A. Zafar, All the catalytic cctive sites of MoS2 for hydrogen evolution, J. Am. Chem. Soc. 138 (2016) 16632-16638.

306

[8] P. Salarizadeh, M.B. Askari, M. Seifi, S. M. Rozati, MoS2 coating on different carbonaceous materials:

307

comparison of electrochemical properties and hydrogen evolution reaction performance, J. Electroanal.

308

Chem. 847 (2019) 113198.

309

[9] T. Aditya, A.K. Nayak, D. Pradhan, A. Pal, T. Pal, Fabrication of MoS2 decorated reduced graphene

310

oxide sheets from solid Mo-precursor for electrocatalytic hydrogen evolution reaction, Electrochim.

311

Acta 313 (2019) 341-351.

312

[10] Z.Y. Zhang, W.Y. Li, M.F.Yuen, T-W. Ng, Y.B. Tang, C-S. Lee, X.F. Chen, W.J. Zhang, Hierarchical

313

composite structure of few-layers MoS2 nanosheets supported by vertical graphene on carbon cloth for

314

high-performance hydrogen evolution reaction, Nano Energy 18 (2018) 196-204.

315

[11] M. Zhu, Q. Ma, S.Y. Ding, Y.Z. Zhao, W.Q. Song, H.P. Ren, X.Z. Song, Z.C. Miao, A molybdenum

316

disulfide and 2D metal-organic framework nanocomposite for improved electrocatalytic hydrogen

317

evolution reaction, Mater. Lett. 239 (2019) 155-158.

318

[12] A.K. Kunhiraman, S. Ramanathan, B. Pullithadathil, Enlarged interlayer spaced molybdenum disulfide

319

supported on nanocarbon hybrid network for efficient hydrogen evolution reaction, Electrochim. Acta

320

264 (2018) 329-340.

321

[13] Q.H. Kong, X. Wang, A.W. Tang, W.B. Duan, B. Liu, Three-dimensional hierarchical MoS2 nanosheet

322

arrays/carbon cloth as flexible electrodes for high-performance hydrogen evolution reaction, Mater. Lett.

323

177 (2016) 139-142.

324

[14] J.Y. Huang, M.G. Chen, X.J. Li, X.W. Zhang, L.Y. Lin, W.P. Liu, Y.J. Liu, A facile layer-by-layer

325

fabrication of three dimensional MoS2-rGO-CNTs with high performance for hydrogen evolution

326

reaction, Electrochim. Acta 300 (2019) 235-241.

327

[15] L. Ma, L.M. Xu, X.Y. Xu, X.P. Zhou, J. Luo, L.L. Zhang, Cobalt-doped edge-rich MoS2/nitrogenated

328

graphene composite as an electrocatalyst for hydrogen evolution reaction, Mater. Sci. Eng.:B 212 (2016)

329

30-38.

330

[16] L. Yang, W. Zhou, J. Lu, D. Hou, Y. Ke, G. Li, Z. Tang, X. Kang, S. Chen, Hierarchical spheres

331

constructed by defect-rich MoS2/carbon nanosheets for efficient electrocatalytic hydrogen evolution,

332

Nanomater. Energy 22 (2016) 490-498.

333

[17] X. Li, G. Zhu, Q. Kang, Z. Huang, X. Feng, Y. Li, R. Liu, Y. Ma, Towards free-standing MoS2

334

nanosheet electrocatalysts supported and enhanced by N-doped CNT–graphene foam for hydrogen

335

evolution reaction, RSC Adv. 5 (2015) 55396-55400.

336

[18] Y.R. Liu, W.H. Hu, X. Li, B. Dong, X. Shang, G.Q, Y.M. Han, Y.Q. Chai, Liu, C.G. Liu, Facile

337

one-pot synthesis of CoS2-MoS2/CNTs as efficient electrocatalyst for hydrogen evolution reaction, Appl.

338

Surf. Sci. 384 (2016) 51-57.

339 340

[19] S.S. Tong, X.J. Wang, Q.C. Li, X.J. Han, Progress on electrocatalysts of hydrogen evolution reaction based on carbon fiber materials, Chinese J. Anal. Chem. 44 (2016) 1447-1457.

341

[20] N. Zhang, S.Y. Gan, T.S. Wu, W.G. Ma, D.X. Han, L. Niu, Growth control of MoS2 nanosheets on

342

carbon cloth for maximum active edges exposed: an excellent hydrogen evolution 3D cathode, ACS

343

Appl. Mater. Interfaces 7 (2015) 12193-12202.

344 345 346

[21] S.J. Zhao, P. Li, J. Adkins, L.L. Zhu, F.H. Du, Q. Zhou, J.W. Zheng, Carboxyl grafted sulfur-expanded graphite composites as cathodes for lithium-sulfur batteries, J. Electroanal. Chem. 823 (2018) 422-428. [22] Y.N. Wu, Q.C. Pan, F.H. Zheng, X. Ou, C.G. Yang, X.H. Xiong, M.L. Liu, D.L. Hu, C.L. Huang,

347

Sb@C/expanded graphite as high-performance anode material for lithium ion batteries, J. Alloys Compd.

348

744 (2018) 481-486.

349

[23] Y.L. An, H.F. Fei, G.F. Zeng, L.J. Ci, B.J. Xi, S.L. Xiong, J.K. Feng, Commercial expanded graphite as

350

a low-cost, long-cycling life anode for potassium-ion batteries with conventional carbonate electrolyte, J.

351

Power Sources 37 (2018) 66-72.

352 353

[24] R.J. Qu, S.H. Tang, X.L. Qin, J.W. Yuan, Y.X. Deng, L.S. Wu, Expanded graphite supported Ni(OH)2 composites for high performance supercapacitors, J. Alloys Compd. 728 (2017) 222-230.

354

[25] Y.G. Huang, X.L. Lin, X.H. Zhang, Q.C. Pan, Z.X. Yan, H.Q. Wang, J.J. Chen, Q.Y. Li, Fe3C@carbon

355

nanocapsules/expanded graphite as anode materials for lithium ion batteries, Electrochim. Acta 178

356

(2015) 468-475.

357

[26] T.F. Peng, B. Liu, X.C. Gao, L.Q. Luo, H.J. Sun, Preparation, quantitative surface analysis,

358

intercalation characteristics and industrial implications of low temperature expandable graphite, Appl.

359

Surf. Sci. 444 (2018) 800-810.

360

[27] J. He, L.Z. Song, J.Y. Yan, N. Kang, Y.L. Zhang, W. Wang, Hydrogen evolution reaction property in

361

alkaline solution of molybdenum disulfide modified by surface anchor of nickel–phosphorus coating,

362

Metals 7 (2017) 211.

363

[28] J.M. Zhang, L. Zhao, A.P. Liu, X.Y. Li, H.P. Wu, C.D. Lu, Three-dimensional MoS2/rGO hydrogel

364

with extremely high double-layer capacitance as active catalyst for hydrogen evolution reaction,

365

Electrochim. Acta 182 (2015) 652-658.

366 367

[29] T. Niyitanga, H. K. Jeong, Graphite oxide and molybdenum disulfide composite for hydrogen evolution reaction, Chem. Phys. Lett. 685 (2017) 451-456.

368

[30] H. Huang, W. Huang, Z. Yang, J. Huang, J. Lin, W. Liu, Y. Liu, Strongly coupled MoS2 nanoflake–

369

carbon nanotube nanocomposite as an excellent electrocatalyst for hydrogen evolution reaction, J. Mater.

370

Chem. A 5 (2017) 1558-1566.

371

[31] Y.Z. Liu, J.P. Liu, Z. Li, X.B. Fan, Y. Li, F.B. Zhang, G.L. Zhang, W.C. Peng, S.B. Wang, Exfoliated

372

MoS2 with porous graphene nanosheets for enhanced electrochemical hydrogen evolution, Int. J.

373

Hydrogen Energy 43 (2018) 13946-13952.

374 375

[32] P. Fageria, S. K. Sudharshan, R. Nazir, M. Basu, S. Pande, Decoration of MoS2 on g-C3N4 surface for efficient hydrogen evolution reaction, Electrochim. Acta 258 (2017) 1273-1283.

376

[33] L. Ma, X.P. Zhou, X.Y. Xu, L.M. Xu, L.L. Zhang, W.X. Chen, One-step hydrothermal synthesis of

377

few-layered and edge-abundant MoS2/C nanocomposites with enhanced electrocatalytic performance for

378

hydrogen evolution reaction, Adv. Powder Technol. 26 (2015) 1273-1280.

379

[34] J. Zhou, J.M. Cao, Y.Z. Zou, Y.F. Zhang, X.W. Liu, MoS2 nanosheets supported on reduced carboxylic

380

multi-walled carbon nanotubes: an advanced catalyst for the hydrogen evolution reaction, Microelectron.

381

Eng. 176 (2017) 89-93.

382

[35] P.C. Wang, L. Wan, Y.Q. Lin, B.G. Wang, MoS2 supported CoS2 on carbon cloth as a

383

high-performance electrode for hydrogen evolution reaction, Int. J. Hydrogen Energy 44 (2019)

384

16566-16574.

385

[36] X.Y. Meng, L. Yu, C. Ma, B. Nan, R. Si, Y.C. Tu, J. Deng, D.H. Deng, X.H. Bao, Three-dimensionally

386

hierarchical MoS2/graphene architecture for high-performance hydrogen evolution reaction, Nano

387

Energy 61 (2019) 611-616.

388

[37] X.L. Chen, K.G. Zhang, Z.Z. An, L.N. Wang, Y. Wang, S. Sun, T. Guo, D.X. Zhang, Z.H. Xue, X.B.

389

Zhou, X.Q. Lu, Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient

390

electrocatalyst for hydrogen evolution reaction, Int. J. Hydrogen Energy 43 (2018) 7326-7337.

Highlights MoS2/EGM cathode composites were easily synthesized by a hydrothermal method. The MoS2/EGM catalyst shows an excellent HER performance. MoS2 and EGM have a synergistic effect in HER at the full pH environments. The superior HER property of MoS2/EGM is due to conductivity of EGM and uniform growth of MoS2.

Credit Author Statement Jun He: Conceptualization, Methodology, Writing- Reviewing and Editing; Siqi Chen: Data curation, Writing- Original draft preparation; Shuqin Yang: Investigation; Wenchao Song: Formal analysis; Changtian Yu: Supervision; Laizhou Song: Validation, Funding acquisition

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: