N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction

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Available online at www.sciencedirect.com

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Facile synthesis of MoS2/N-doped macromesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction Xiaoling Chen a, Kangning Zhang a, Zhenzhen An a, Lina Wang a, Yan Wang a, Sen Sun a, Tong Guo a, Dongxia Zhang a, Zhonghua Xue b, Xibin Zhou a,*, Xiaoquan Lu b,** a

Key Laboratory of Bioelectrochemistry & Environmental Analysis of Gansu Province, College of Geography and Environment Science, Northwest Normal University, Lanzhou 730070, China b College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

article info

abstract

Article history:

A novel three-dimensional (3D) hybrid consisting of molybdenum disulfide nanosheets

Received 25 November 2017

(MoS2) uniformly bound at N-doped macro-mesoporous carbon (N-MMC) surface was

Received in revised form

fabricated by the solvothermal method. The resulting MoS2/N-MMC hybrid possesses few-

21 February 2018

layer MoS2 nanosheets structure with abundant edges of MoS2 exposed as active sites for

Accepted 26 February 2018

hydrogen evolution reaction (HER), in sharp contrast to large aggregated MoS2 nanoflowers

Available online xxx

without N-MMC. The high electric conductivity of N-MMC and an abundance of exposed

Keywords:

with a low onset potential of 98 mV, a small Tafel slope of 52 mV/decade, and a current

Molybdenum disulfide

density of 10 mA cm
2 at the overpotential of 150 mV. Moreover, the MoS2/N-MMC hybrid

N-doped macro-mesoporous carbon

exhibits outstanding electrochemical stability and structural integrity owing to the strong

Solvothermal method

bonding between MoS2 nanosheets and N-MMC.

edges on the MoS2 nanosheets make the hybrid excellent electrocatalytic performance

Hydrogen evolution reaction

© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Nowadays, the rapid consumption of fossil fuels has brought about a sequence of serious environmental issues and energy crisis. Thus, developing green, economical, and renewable energy is a solution with great promise to solve this problem [1e4]. Hydrogen energy has been considered as a promising alternative energy source to substitute for traditional sources in the future, and drawn extensive attention because of the fact that it is an environmentally friendly [5]. Hydrogen can be

sustainably produced by electrochemical of water splitting, the essential step in water electrolysis is the hydrogen evolution reaction (HER, 2Hþ þ 2e
/ H2) [6e8]. In general, Ptgroup metals, such as Pt and Pd, are considered as the most efficient electrochemical catalysts for HER due to the exceptionally low overpotential [9]. However, large scale applications of noble metals have been further confined due to their expensive cost and low abundance [10]. Thus, it is desirable to develop inexpensive and earth-abundant catalysts with nonplatinum rare metals for efficient hydrogen evolution,

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (X. Zhou), [email protected] (X. Lu). https://doi.org/10.1016/j.ijhydene.2018.02.163 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

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preferably based upon economic, non-poisonous and stable materials. In recent years, many effective noble-metal-free electrocatalysts for HER including transitional metal sulfides [11e13], selenides [14], phosphides [15e17], nitrides [18,19], and carbides [20e22] have been explored. Among them, molybdenum disulfide (MoS2) is proved to be a promising candidate in HER [23e26]. Both computational and experimental results verify that the HER catalytic activity derives from the sulfur edges of MoS2 plates [27]. However, dispersed MoS2 nanostructures easily aggregate and stack by reason of their high surface energy and interlayer van der Waals interaction, which limits the number of exposed active sites [28]. In addition, as a typical semiconductor material, the extremely low electrical conductivity of MoS2 nanostructure, especially along the adjacent interlayers, hampers the electron transfer for highly efficient HER [29]. Taking these factors into account, the researchers mainly focus on following key strategies: (1) increasing the exposed active sites of sulfur edges, and preventing MoS2 from agglomerating, and (2) further improving closely electronic contact between catalysts and electrode to accelerate Hþ adsorption and conversion into H2 [30]. Thus developing MoS2-based catalysts with layers on highly conductive substrate have been put forward as a promising solution, due to the fact that good conductivity of the substrates can lower the resistance of the catalysts. Carbon materials are ideal substrates for loading MoS2 to enhance their electrocatalytic performance due to the remarkable conductivity and stability, including graphene [31], graphene oxide (GO) [32,33], graphene aerogels [34], graphene quantum dots [35], carbon cloth [36], carbon nanotubes (CNTs) [37], CNTs aerogel [38] and amorphous carbon [39]. Hierarchical macro-mesoporous carbons (MMC) with mesopores located at the walls of the macropores have been drawn extensively interests due to large surface area, tunable pore structure, uniform and adjustable pore size,

chemically inert nature, mechanical stability, good conductivity, and efficient mass transfer through macropores [40]. These outstanding features make them ideal candidates in the applications of electrochemical energy storage and conversion [41]. Later on, some researches indicate that the incorporation of heteroatoms, such as B, N and O, into the carbon lattice can significantly enhance the mechanical, semiconducting, field-emission, and electrical properties of carbon materials [42]. Particularly, N doping can enhance the surface polarity, electric conductivity, and electron-donor tendency of the porous carbons [43]. By far, N-MMC has been successfully used in many fields including ORR [44], gas adsorption [45], biosensing research [46], supercapacitors [47], and so on. Based on above viewpoints, we describe the preparation of 3D hierarchical frameworks by the self-assembly of MoS2 nanosheets on N-MMC via a simple one-step hydrothermal process. In this process, N-MMC acting as the 3D conductive substrate not only provides matrix for the nucleation and subsequent growth of MoS2, but also improves the conductivity of the MoS2/N-MMC, thus facilitating electron transfer during the HER electrocatalytic process. The fabrication procedure of MoS2/N-MMC hybrid electrocatalyst is shown in Scheme 1.

Experimental Materials All chemicals were reagent grade and used as received without further purification in this work. Thiourea (CH4N2S, 99％), sodium molybdate dihydrate (Na2MoO4$2H2O, 99％) were purchased from Aladdin, N,N-dimethylformamide (DMF, 99.5％), and anhydrous ethanol were obtained from Tianjin Chemical Reagent plant, hydrazine hydrate was purchased

Scheme 1 e Schematic diagram of the fabrication procedure of MoS2/N-MMC hybrid. Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 2

from Shanghai zhongqin chemical reagent Co.,Ltd. Nafion solution (5 wt%) was purchased from DUPONT, 20% Pt/C was purchased from shanghai Macklin Biochemical Co.,Ltd. Sulfuric acid (H2SO4, 98%) was purchased from Beijing Chemical Works. Double distilled water and nitrogen gas (99.99%) were used for all experiments.

Synthesis of MoS2/N-MMC N-MMC was prepared by the method reported by Liang et al. previously [44]. MoS2/N-MMC hybrid was synthesized via onestep solvothermal method according to the method [48]. Briefly, 0.03 g N-MMC was dispersed in 30 mL DMF, the mixture was then sonicated for approximately 20 min to obtain homogeneous solution. This was followed by addition of 0.037 g Na2MoO4$2H2O and 0.1 g CH4N2S, the reaction solution was further sonicated for 20 min, this was followed by the addition of 30 mL of hydrazine hydrate as a reducing agent, well-mixed by magnetic stirring for 30 min until dissolved completely at room temperature. The obtained dispersion was transferred to a 50 mL Teflon stainless-steel autoclave and reacted at the temperature of 200  C for 12 h and down to room temperature naturally. The precipitates were washed by centrifugation at 8000 rpm for 10 min with water and anhydrous ethanol (1:1), at least five times to insure that most DMF was removed. Finally, the obtained product was dried at 80 Cfor 12 h, and denoted as MoS2/N-MMC(1:1) (mass ratio of Na2MoO4$2H2O and N-MMC is 1:1). For comparison, MoS2/N-MMC(1:2), MoS2/N-MMC(2:1) and MoS2 without N-MMC were prepared under the same conditions.

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(GCE, CHI104, F ¼ 3 mm, Gaoss Union Co.,Ltd, Wuhan, China) was used as the working electrode. Experimentally, 4 mg of the respective catalyst powders were dispersed in 1 mL of 4:1 (v/v) water/ethanol mixed solvents with 30 mL 5% Nafion solution under ultrasonication for 1 h to form uniform ink. A 5 mL portion of dispersion was dropcasted onto the glassy carbon electrode and dried at room temperature. The catalyst loadings were all 0.285 mg cm
2. Linear sweep voltammetry was carried out at a scan rate of 5 mV s
1 in 0.5 M H2SO4 solution (purged in pure N2 for 30 min before test) by sweeping the potential from 0 to
0.5 V (vs. SCE) with iR-compensation. Double-layer capacitance (CdI) was calculated according to the methods reported previously [49]. The electrochemical stability of the catalyst was evaluated by cycling the electrode for 2000 times with each cycle started at þ0.10 V and ended at
0.5 V (vs. SCE) with a scan rate of 100 mV s
1. Moreover, time-dependence curve for stability measurement was obtained at a static overpotential of 0.15 V (vs. RHE). In our electrochemical tests, the potential transfer from SCE to RHE was obtained according to the following equation: E (RHE) ¼ E (SCE) þ 0.284 V (Fig. 1). Current density referring to the geometric surface area (0.07 cm2) of the GCE was normalized. AC electrochemical impedance measurements were performed with the frequency range from 105 Hz to 0.1 Hz with amplitude of 5 mV. The turnover frequencies were calculated from the exchange current densities using the following relation [16]:


TOFðs-1 Þ ¼ j0 ;A cm-2 = 1:5  1015 sites cm-2 1:602  10-19 C=e ð2e- =H Þ 2

Characterization The scanning electron microscopy (SEM) image was collected on a JSM-6701 F (Japan). The transmission electron microscopy (TEM) image was acquired by using FEI-Tecnai G2 TF20 electron microscope (America) operating at HT 200 kV. Nitrogen adsorption-desorption isotherms were performed on an ASAP 2010 instrument (Micromeritics, USA) at
195  C, prior to analysis, the samples were degassed for 6 h at 373 K. X-ray diffraction (XRD) was performed on a Rigaku D/max-2400 diffractometer under Cu Ka1 0.154056 nm radiation at 40 kV and 150 mA with 2q ranges from 8 to 80 . Energy-dispersive Xray spectroscopy (EDX) was obtained on an AztecX-80 scanning electron microscopy (Japan) equipped with an energy dispersive X-ray spectroscopy detector. Raman spectra were obtained by inVia Renishaw confocal spectroscopy (Britain) with an excitation wavelength of 633 nm. X-ray photoelectron spectroscopy (XPS) was made based on PHI-5702 (USA).

Electrochemical measurements All electrochemical measurements were conducted on a CHI660E electrochemical workstation (Shanghai Chenhua Instrument Co., China) in a conventional three-electrode system at ambient temperature. The hydrogen evolution performance tests were performed in N2 saturated 0.5 M H2SO4 electrolyte solution. A saturated calomel electrode (SCE) and a graphite rod were used as the reference electrode and counter electrode, respectively. A modified glassy carbon electrode

Results and discussion Morphology and structure of MoS2/N-MMC The morphology and microstructure of MoS2/N-MMC hybrid were characterized by scanning electron microscopy (SEM), and transmission electron microscopy (TEM) (Fig. 1a-c, S2(ad)). As shown in Fig. S2(a-b), the N-MMC contains ordered 3D-interconnected macroporous architecture and possesses high porosity, which could provide a high surface area for the loading of MoS2 nanosheets, and the size of the macropores in N-MMC is observed to be ~250 nm, the pores of these carbon blocks consist of both 3D ordered macropores and 2D mesopores located at the walls of the macropores. Fig. S2c shows that the MoS2 nanoflowers with obvious ripples and corrugations are assembled by lamellar nanosheets, corresponding to the TEM images in Fig. S2d, pure MoS2 prepared without adding N-MMC, which are seriously stacked together and aggregated. From the Fig. 1a-c, after introduction of N-MMC by hydrothermal method, the SEM image of MoS2/N-MMC(1:1) hybrid displays that the MoS2 nanosheets are perpendicularly grow on the surface of N-MMC, and obviously different from the pure MoS2, which is in good accordance with TEM observations. Fig. 1c shows the high resolution transmission electron microscopy (HRTEM) image of MoS2/N-MMC(1:1), it can be clearly observed that MoS2 nanosheets are composed of few layers, and the interlayer spacing of MoS2 nanosheets is about

Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

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Fig. 1 e SEM and TEM images of MoS2/N-MMC(1:1) hybrid. 0.86 nm, indicates that the introduction of N-MMC can effectively inhibits the corrugations of MoS2, the intimate contact between MoS2 and hierarchical porous structure can enhance electron transfer quickly for electrochemical HER. Energy dispersive X-ray (EDX) elemental mapping of MoS2/NMMC(1:1) under the SEM conditions proves the coexistence and homogeneous distribution of C, N, O, Mo, and S elements (Fig. S3) on the surface of the N-MMC nanostructure, further indicating that MoS2 nanosheets evenly anchored on N-MMC successfully. In addition, the typical Nitrogen adsorptiondesorption tests results are showed in Fig. S4. It can be clearly seen that MoS2 exhibits a small BET specific surface area of only 12.56 m2g
1, lower than that (35.34 m2g
1) of MoS2/N-MMC(1:1), the MoS2/N-MMC(1:1) exhibits the high BET surface area. The pore-size distribution curves of composites were analyzed by the Barrett Joyner Halenda (BJH) method. The pore size of the MoS2/N-MMC(1:1) was from 4 to 70 nm in Fig. S4b. This demonstrates that there are mesopores and macropores in MoS2/N-MMC(1:1) composites. Thus, the large surface area and particular pore distribution, which actually stem from the introduction of N-MMC, lead to high catalytic efficiency. XRD was carried out to investigate the structural information of MoS2/N-MMC(1:1). As shown in Fig. 2, for N-MMC sample, the broad diffraction peak centered at 2q ¼ 23.7 can be assigned to the (002) planes, revealing the low crystalline degree of N-MMC. For MoS2 and MoS2/N-MMC(1:1) hybrid, the diffraction peaks present similarly to each other, indicating that no additional crystallization behavior is introduced into the MoS2/N-MMC(1:1) hybrid. Interestingly, three diffraction peaks centered at 2q ¼ 10.2 , 32.8 , 56.7 , are clearly observed for MoS2/N-MMC(1:1), which may be assigned to the (002), (100), and (110) crystal planes of MoS2, respectively. Noted that the (002) diffraction peak shifts to 10.2 (Fig. 2), when

compared to the standard 2H-MoS2(Joint Committee on Powder Diffraction Standards, JCPDS card no. 73e1508) with the (002) diffraction peak located at 14.4 , signifying an enlarged interlayer spacing. The calculated d spacings (0.86 nm) of MoS2/N-MMC(1:1) are clearly greater than that of pristine 2HMoS2 (0.61 nm) (calculated by using Bragg's law and position of the 002 XRD peak), suggesting the formation of a new lamellar structure, which is consistent with the HR-TEM image (Fig. 1c). Moreover, two almost invariant diffractions at 32.8 and 56.7 for the MoS2 (100) and (110) crystal planes indicate consistent atomic arrangements along the basal planes of MoS2/NMMC(1:1) frameworks. In addition, all their diffraction density of MoS2/N-MMC(1:1) become weaker by comparison with pure

Fig. 2 e XRD patterns of the N-MMC, MoS2, and MoS2/NMMC(1:1) hybrid.

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MoS2. All of results indicate that the low crystalline structure and the expanded interlayer spacing. The Raman spectra of N-MMC, MoS2, and the MoS2/NMMC(1:1) were depicted in Fig. 3. As we can see, the MoS2 displays three characteristic peaks, the E12g peak at 377 cm
1 and the A1g peak at 420 cm
1, which is associated with the opposite vibration of two S atoms with one Mo atom and the out-plane vibration of only S atoms [50]. Another strong peak of MoS2 that appeared at 454 cm
1 arises from a second-order process involving the longitudinal acoustic phonons at M point (2LA(M)) [51]. Meanwhile, the weak peaks around 219, 626 cm
1 are observed, derived from the resonance Raman (RR) scattering peaks of MoS2. Compared to MoS2, the peaks of MoS2/N-MMC(1:1) at 375, 408 and 454 cm
1 correspond to the in-plane E12g, out-of-plane A1g and 2LA(M) mode peaks of MoS2, which are greatly suppressed for MoS2/N-MMC(1:1) due to their being anchored on N-MMC surface. Previous studies confirmed that the frequency difference between E12g and A1g presented the number variation of crystal-plane layers [52]. Our results show that the differences are 43 and 32 cm
1 for MoS2 and MoS2/N-MMC(1:1), respectively. This could be attributed to the fact that the N-MMC can effectively prevent MoS2 nanosheets from stacking during the hydrothermal process [53]. The result is also in accordance with the above XRD analysis. The characteristic peaks at 1332 and 1580 cm
1 for N-MMC, can be ascribed to D band (represents edges, disordered carbon and defects) and G band (corresponds to vibration of ordered sp2-hybrided carbon) of carbon materials [54]. In comparison, the two characteristic peaks intensity of carbon materials decreased significantly, confirms that the MoS2 nanosheets covered over the N-MMC during the hydrothermal synthesis. Besides, the intensity of A1g mode is higher than the E12g mode, indicating that MoS2/N-MMC(1:1) have abundant edge-terminated structures and decreased number of S-Mo-S layers [55]. X-ray photoelectron spectroscopic (XPS) measurements were performed to investigate the chemical composition and atomic valence states of the MoS2/N-MMC(1:1) nanocomposite frameworks. Fig. 4a shows the XPS survey scan spectrum of

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the MoS2/N-MMC(1:1) nanocomposite. The elements of C, N, O, Mo and S can be clearly identified, which is consistent with EDX mapping in Fig. S3, and the corresponding atomic percentages (e.g., C ¼ 64.24, N ¼ 3.72, O ¼ 2.79, Mo ¼ 17.98, and S ¼ 11.27 at. %). As shown in Fig. 4b, the C1s spectrum can be deconvoluted into four peaks, the higher peak at 284.7 eV is mainly close to the sp2C ¼ C bond in aromatic ring, another peak at 286.6 eV corresponds to C
OH, which is commonly observed on carbon material. The emergence of the peak at 288.9 eV corresponding to C
N
C coordination indicates the bonding formation of doped nitrogen atoms to be sp3
C atoms [56]. And the presentence of the peak at 291.8 eV corresponds to
COOH coordination [57]. The N1s peak can be also deconvoluted into four components (shown in Fig. 4c), to be specific, the fitted peaks at 398.2 eV, 399.6 eV and 401.5 eV are consistent with pyridinic-N, pyrrolic-N and graphitic-N [58]. Based on other report, the pyridinic N and graphitic N is beneficial to improve the HER performance of N-doped carbon materials, in addition, the peak at 396.5 eV is indexed as Mo
N bond [59], indicating the strong adhesion between N-MMC and MoS2, rather than the simple physical mixture of two materials. The reason might be that the strong interaction of dorbital of transition metal Mo and the p-orbital of N-dopants may come into being robust bonding, forming Mo-N via coordination bond [60]. The O1s spectrum (Fig. 4d) are observed at 530.3 eV, 531.4 eV, 532.4 eV, and 533.6 eV assigning to C¼O, C
O/O
C
N, Mo¼O, and C
OH bonds, respectively. Fig. 4e shows that the XPS spectrum at the Mo3d region can be deconvoluted into four peaks, and one of these at 226.1 eV corresponds to the S2s peak of MoS2. Two characteristic peaks arising from Mo3d5/2 and Mo3d3/2 orbitals are located at 228.9 eV and 232.2 eV, suggesting the dominance of Mo (IV) oxidation state [61]. Besides, the relatively weak peak at 235.6 eV is Mo3d3/2 binding energy for the Mo (VI) oxidation state. In addition, the S2p XPS spectrum (Fig. 4f) shows two main peaks at 163.8 eV and 162.0 eV, which are attribute to the S2p1/2 and S2p3/2 binding energies, respectively, indicating the existence of divalent sulfide ions (S2
). Interestingly, a peak related to the S4þ is observed at 168.1 eV, due to the partial oxidation of S2
in the ambient air.

Electrochemical performance of MoS2/N-MMC hybrids

Fig. 3 e Raman spectrum of the N-MMC, MoS2, and MoS2/NMMC(1:1) hybrid.

The HER electrocatalytic activity of N-MMC, MoS2, MoS2/NMMC hybrids, and the physical mixture of MoS2 and N-MMC (pristine MoS2-N-MMC) was evaluated in N2-saturated 0.5 M H2SO4 electrolyte with a scan rate of 5 mV s
1 at room temperature. As a reference point, we also performed measurements using a commercial Pt/C catalyst (20 wt% Pt on Vulcan carbon black) exhibiting the highest HER catalytic performance (with a near zero onset overpotential). Fig. 5a shows the linear sweep voltammetry (LSV) polarization curves of all samples. As can be seen, N-MMC shows almost catalytically inactive for HER and its effect on the electrochemical tests can be nearly neglected. The MoS2 displays weak HER electrocatalytic performance with the large onset overpotential (210 mV) and low current densities, the poor HER performance is associated with the poor conductivity and small number of the active sites of pure MoS2. Apparently, pristine MoS2-N-MMC presents a moderate onset potential of

Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

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Fig. 4 e High-resolution XPS spectrum showing the binding energies of (a) survey spectra, (b) C1s, (c) N1s, (d) O1s, (e) Mo3d, and (f) S2p of MoS2/N-MMC(1:1) hybrid.

Fig. 5 e (a) LSV polarization curves for GCE modified with various catalysts in N2 purged 0.5 M H2SO4 solution with a scan rate of 5 mV s¡1. (b) Corresponding Tafel plots for pure MoS2, 20% Pt/C, and MoS2/N-MMC hybrids modified GCE with a catalyst loading of 0.285 mg cm¡2.

Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

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Table 1 e Tafel slopes, onset potentials, exchange current densities j0, and turnover frequencies (TOFs) for various catalysts. Catalysta

Tafel slope (mV/dec)

Onset potential (mV)

j-200 (mA cm
2)

j 0 (mA cm
2)

TOF(s
1)

107 62 52 66

210 109 98 121


0.79
19.37
51.06
15.42

0.0057 0.0125 0.0201 0.0115

0.012 0.026 0.042 0.023

MoS2 MoS2/N-MMC(1:2) MoS2/N-MMC(1:1) MoS2/N-MMC(2:1) a

All of the values were measured under the same conditions, loading weight (0.285 mg cm
2) in a 0.5 M H2SO4 solution.

140 mV and a current density compared with pure MoS2 and N-MMC, demonstrating that there is a synergistic effect between MoS2 and N-MMC, making the pristine MoS2-N-MMC present HER activity superior to those of each individual. However, when N-MMC was introduced, and the MoS2/NMMC was optimized by controlling the mass ratio of Na2MoO4$2H2O and N-MMC in the solvothermal process for hydrogen evolution. In sharp contrast, the obtained all of MoS2/N-MMC hybrids display higher current density and lower onset overpotential than pristine MoS2-N-MMC. Among them, MoS2/N-MMC(1:1) hybrid reveals the smallest onset potential of 98 mV relative to MoS2/N-MMC(1:2) (109 mV) and MoS2/N-MMC(2:1) (121 mV). Significantly, H2 evolution (j ¼ 10 mA cm
2) can be observed at a potential as low as 150 mV for MoS2/N-MMC(1:1), which is lower than other catalysts of MoS2/N-MMC(1:2) (173 mV) and MoS2/N-MMC(2:1) (191 mV). Especially, the MoS2/N-MMC(1:1) presents the best HER activity with a current density of 51.06 mA cm
2 at 200 mV, which is 2.5 times larger than that of MoS2/NMMC(1:2) (i.e. 19.37 mA cm
2), and 3 times larger than that of MoS2/N-MMC(2:1) (i.e. 15.42 mA cm
2), respectively (seeing Table 1). The current densities at the same potential are found to be in the order MoS2/N-MMC(1:1) > MoS2/NMMC(1:2) > MoS2/N-MMC(2:1). The enhanced HER activity of MoS2/N-MMC(1:1) is superior to most MoS2-based electrocatalysts reported elsewhere (seeing Table S2). Therefore, the MoS2/N-MMC(1:1) is proved as optimal for hydrogen evolution and corresponding the following experiments. The Tafel plots are generally adopted to elucidate the electron transfer kinetics [62]. The Tafel plots can be acquired on the basis of fitting the linear portion of polarization curves, and the Tafel slope is the slope of the fitting line, which displays the intrinsic properties of the electrocatalyst materials [63,64]. The linear portion of Tafel plots is fitted to the Tafel equation h ¼ a þ b log(j), where h is the overpotential, b is the Tafel slope, and j is the current density. It is commonly accepted that, three possible reaction steps have been suggested for the HER in acidic media [65]. First, the primary discharge step (Volmer reaction): H3Oþ þ e
/ Hads þ H2O b¼

(1)

2:3RT z120mV aF

Where R is the ideal gas constant, T is the absolute temperature, a z 0.5 is the symmetry coefficient, and F is the Faraday constant. This step is followed by either an electrochemical desorption step (Heyrovsky reaction),

Hads þ H3Oþ þ e
/ H2[ þ H2O b¼

(2)

2:3RT z40mV ð1 þ aÞF

or a recombination step (Tafel reaction), Hads þ Hads / H2 b¼

(3)

2:3RT z30mV 2F

The calculation and analysis of the Tafel slope are important for interpretation of the elementary steps involved. As shown in Fig. 5b, the Tafel slopes of the pure MoS2, pristine MoS2-N-MMC, MoS2/N-MMC(1:2), MoS2/N-MMC(1:1), and MoS2/N-MMC(2:1), are 107,69, 62, 52, and 66 mV/dec, respectively. As can be seen, the observed smallest Tafel slope value is ~52 mV/dec for MoS2/N-MMC(1:1) hybrid, which is originated from the MoS2 nanosheets with abundant exposed active sites for HER. Implying that electrochemical desorption is the rate-limiting step, and the operation of MoS2/NMMC(1:1) hybrid in the HER process is Volmer-Heyrovsky step mechanism by Eqs. (1) and (3), which can make a comparison for different kinds of MoS2 based materials performing as HER electrocatalysts in Table 1. In comparison with the pure MoS2, the enhanced HER performance of the MoS2/N-MMC(1:1) frameworks is likely due to the fact that the highly porous NMMC provides 3D conductive templates and prevents the aggregation of MoS2 nanosheets. The exchange current density (j0) is also a kinetic parameter to evaluate the intrinsic catalytic performance for the HER of the electrocatalyst. An excellent electrocatalyst with outstanding catalytic activities for the HER should possess a high exchange current density. By applying extrapolation method to the Tafel plots (Fig. 5b), exchange current densities are obtained (Fig. S5 and Table S1). The exchange current density values of all hybrids are listed in Table 1. It is evident that MoS2/N-MMC(1:1) exhibits the highest exchange current density (j0) of 20.1 mA cm
2, which is about 4 times larger than the value of MoS2 (5.72 mA cm
2), indicating the faster electron-transfer rate and the excellent activity for HER. To get a direct site-to-site comparison, the rough estimation of TOFs is calculated to compare the activity of pure MoS2, and MoS2/ N-MMC hybrids following Jaramillo's method [29], as shown in Table 1, MoS2/N-MMC(1:1) hybrid catalyst possesses much higher TOF value of 0.042 s
1. The superior HER activity of MoS2/N-MMC nanocomposites stems from the huge electrochemical active surface area (ECSA), and that, effective active surface area of electrocatalyst

Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

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Fig. 6 e Cyclic voltammograms of the (a) MoS2/N-MMC(1:2), (b) MoS2/N-MMC(1:1), (c) MoS2/N-MMC(2:1) and (d) MoS2 at various scan rates (20e200 mV s¡1). (e) Relations of the difference between anodic and cathodic currents at 0.15 V (vs. RHE) (△j0.15 ¼ ja-jc), the slope is double CdI. is linearly proportional to the electrochemical double-layer capacitance (CdI) [66,67]. The electrochemical double-layer capacitance (CdI) measured by cyclic voltammetry method (CV) selects the potential range (0.1 Ve0.2 V vs. RHE) where no faradic current is observed for the catalysts. Current response at different scan rates (20e200 mV s
1) should be only attributed to the charging of the double-layer (Fig. 6a-c). The double layer capacitances (CdI) for each catalysts are extracted by plotting the difference between anodic and cathodic currents (△j ¼ ja-jc) at a given potential (0.15 V) against various scan rates, where the slope is CdI. Fig. 6d shows that the CdI of MoS2/ N-MMC(1:1) reaches 32 mF cm
2, which is much higher than

those of MoS2/N-MMC(1:2) (16 mF cm
2), MoS2/N-MMC(2:1) (9 mF cm
2) and MoS2 (4 mF cm
2). The larger value of CdI indicates that the MoS2/N-MMC(1:1) hybrid possesses the highest ECSA for electrochemical reaction, which would definitely contribute to its better effective electrocatalytic activity [68,69]. The electrochemical impedance spectroscopy (EIS) is conducted to investigate the electrode kinetics and fluent charge transport of the electrocatalysts for the HER. Corresponding Nyquist plots are given in Fig. 7a, in which charge-transfer resistance (Rct) caused by the interfacial faradic reaction in electrode interface [70]. By comparison, it is obviously revealed that MoS2/N-MMC(1:1) hybrid displays a remarkable

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 2

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Fig. 7 e (a) AC impedance spectra of MoS2, and different ratio of MoS2/N-MMC catalysts by the introduction of N-MMC in a 0.5 M H2SO4 solution with the frequency range from 105 to 0.1 Hz and an amplitude of 5 mV. The inset is the equivalent model of system. (b) HER polarization curves for the MoS2/N-MMC(1:1) framework electrode before and after 2000 cycles in the stability test. (c) Time dependence of current density of the MoS2/N-MMC(1:1) at the overpotential of 0.15 V over 30000s.

decrease of Rct with the smallest value of only 98.2 U in the Nyquist plots, this is significantly lower than that of MoS2/NMMC(1:2) of 202.56 U, MoS2/N-MMC(2:1) of 568.7 U, as well as MoS2 of 3580 U, implying faster charge transfer capability and improved catalysts performance of MoS2/N-MMC(1:1). It is known that lower of Rct means more rapid of the Volmer reaction for HER. Therefore, MoS2/N-MMC(1:1) shows the best HER activity, which is consistent with result obtained from polarization measurement and Tafel slope analysis. The higher HER activity of MoS2/N-MMC(1:1) may be associated with underlying 3D ordered macro-mesoporous architecture of N-MMC, which will favor fast electron transfer and charge transport at the electrode interface and is one of the key factors contributing to the superior kinetics toward HER. The results indicate that the combination of N-MMC with MoS2 can greatly improve the conductivity of MoS2, and thus speed up their HER activity. Electrochemical stability is another significant criterion to estimate electrocatalyst for the HER. To search into the cycling stability of the catalysts developed in this study intuitively, the stability test of the MoS2/N-MMC(1:1) catalyst is conducted by utilizing CV measurements between þ0.1 V and
0.5 V (vs. SCE) at a scan rate of 100 mV s
1 in 0.5 M H2SO4. Fig. 7b shows

the polarization plots at the first cycle and after 2000 cycles. By contrast, it can be observed that the MoS2/N-MMC(1:1) catalyst displays similar polarization curve with negligible decay of cathodic current density, which confirms the better stability of intentionally designed electrocatalyst. To further confirm the electrochemical stability of the MoS2/N-MMC(1:1), the practical operation of the catalyst is examined at fixed potential over long period. The chronoamperometry measurement is also performed under static overpotential (h) of 0.15 V (vs. RHE). Fig. 7c presents the timedependent current density curve of MoS2/N-MMC(1:1). After a long HER testing period (30000 s), the current densities only show slight degradation, which could be owing to the consumption of Hþ in the reaction system and the hindrance of reaction by hydrogen bubbles remaining on the surface of the catalyst that hindered the reaction [71]. Fig. S6 shows that the morphology of the nanostructure barely changes from the TEM analysis after 30000s. All these results demonstrate the excellent electrochemical stability of MoS2/N-MMC(1:1) due to MoS2 robustly bound at the surface of N-MMC substrate, and the NMMC supports suppress the aggregation of MoS2 nanosheets which enhanced more exposed active surface, rapid diffusion of ions and electrons on the MoS2/N-MMC surface.

Please cite this article in press as: Chen X, et al., Facile synthesis of MoS2/N-doped macro-mesoporous carbon hybrid as efficient electrocatalyst for hydrogen evolution reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/ j.ijhydene.2018.02.163

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Conclusions In summary, a 3D MoS2/N-MMC hybrid has been developed via facile solvothermal method. The direct growth of MoS2 on N-MMC can offer intimate contact and excellent electrical connection between them. The optimized catalyst MoS2/NMMC(1:1) exhibits excellent HER electrocatalytic performance, such as low onset potential of 98 mV, a small Tafel slope as low as 52 mV/dec, large exchange current density of 20.1 mA cm
2, high double layer capacitance (32 mF cm
2), and remarkable stability. The superior performance of MoS2/NMMC (1:1) is attributed to conductive channels for accelerating the electron transfer of N-MMC and the abundant exposed active sites of sulfur edges of MoS2. As a consequence, MoS2/ N-MMC hybrid is a promising candidate as cost-effective HER electrocatalyst for hydrogen production.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 21165016, 21175108, 21265018) and the Science and Technology Support Projects of Gansu Province (Nos. 1011GKCA025, 090GKCA036, 1208RJZM289).

Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.02.163.

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