NiMoS3 Nanorods as pH-Tolerant Electrocatalyst for Efficient

Research Article pubs.acs.org/journal/ascecg

NiMoS3 Nanorods as pH-Tolerant Electrocatalyst for Efficient Hydrogen Evolution Jinxue Guo,† Xinqun Zhang,† Yanfang Sun,‡ Lin Tang,† and Xiao Zhang*,† †

Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Key Laboratory of Sensor Analysis of Tumor Marker (Ministry of Education), College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, No. 53, Zhengzhou Road, Qingdao 266042, China ‡ College of Science and Technology, Agricultural University of Hebei, Cangzhou 061100, China S Supporting Information *

ABSTRACT: To meet the increasing demands for sustainable and clean hydrogen energy sources, development of pH-tolerant electrocatalysts with high-performance and low-cost toward hydrogen evolution reaction (HER) is an important but challenging task. MoS2 is postulated as a promising candidate for HER in acidic solution, however, showing poor activity in alkaline media. Herein, to widen its application in various media, we first report the synthesis of NiMoS3 nanorods using a hydrothermal method that starts from NiMoO4 nanorods. The incorporation of Ni atoms in Mo−S could arouse the synergism of ternary Ni−Mo−S and create abundant defect sites, thus substantially improving the inherent catalytic activity and catalytic sites. More importantly, Ni endows Mo−S with excellent catalytic activity in alkaline solution. As a result, NiMoS3 exhibits large cathodic current, low overpotetnial, and stable durability for HER in H2SO4 and especially in KOH. The overpotetnial at current density of 10 mA cm−2 is as low as 126 mV in KOH, making it a promising candidate for HER electrocatalyst. KEYWORDS: NiMoS3, Self-template synthesis, Electrochemical hydrogen production, pH-Tolerant

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(Volmer step of HER in alkaline media).17 Considering that most of the catalysts toward oxygen evolution reaction work well in alkaline solution, development of MoS2 based catalysts with active HER in alkaline media is desired. Aiming at this target, Pu and co-workers have prepared 3D macroporous MoS2 film on Mo foil toward HER in alkaline media, which showed relatively good activity (184 mV at current density of 10 mA cm−2).18 However, efforts have rarely been made on this task, and the corresponding research urgently needs to be promoted. Very recently, ternary metal sulfides of M−Mo−Sx (M = Fe, Co, Ni) have been prepared and exhibited enhanced catalytic activity due to the synergistic effect between M and Mo.19−23 For instance, Shao et al. have prepared CoMoS4/NiMoS4 based electrocatalysts for HER, which showed efficient catalytic performance over a wide pH range. Among them, Ni is postulated to act as active sites for H−OH cleavage (the Volmer step) through the formation of Ni−OH bond, and thus facilitates the HER reaction kinetics in alkaline media.24,25 Therefore, Ni−Mo−S ternary sulfides that couple different functional species will lead to improved HER activity in both

INTRODUCTION Aiming at the sustainable improvement and environmental conservation of human society, there are ever growing demands to look for renewable and clean energy sources to replace fossil fuels. The electrochemical hydrogen evolution reaction (HER) has been well identified as a promising option, despite being not economically viable due to the high cost of Pt-based catalysts. Among the cost-effective alternatives, MoS2 has received specific research interest.1−16 To achieve the application of MoS2 based catalysts, intensive studies have been dedicated to producing more active sites and/or improving electrical conductivity to obtained improved HER activity.2−9 For instance, we have doped graphene quantum dots into MoS2 to create abundant active sites for enhanced catalytic activity.7 Lou’s group has used N-doped carbon nanoboxes as the active and conductive support to boost the HER activity of MoS2 nanosheets.9 On the other hand, doping heteroatoms into MoS2 has been adopted as an effective solution to achieve improved HER properties, because it could introduce additional edge sites, improve conductivity, and/or boost inherent unit catalytic activity.10−16 Unfortunately, these MoS2 based catalysts generally only display their good HER performance in acidic solution.2−16 It is reported that the poor HER activity of MoS2 in alkaline media is due to the strong Mo−OH bond that could prevent the water dissociation © 2017 American Chemical Society

Received: June 5, 2017 Revised: August 18, 2017 Published: September 6, 2017 9006

DOI: 10.1021/acssuschemeng.7b01802 ACS Sustainable Chem. Eng. 2017, 5, 9006−9013

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Low and (b) high magnification SEM images of NiMoS3 nanorods.

acidic and alkaline media.22,23 For instance, Miao and coworkers have prepared hierarchal Ni−Mo−S nanosheets on carbon fiber cloth, which exhibits good catalytic HER activity and stability in neutral electrolyte.22 However, there are few reports on the synthesis and application of advanced NiMoSx catalysts, particularly with specific nanostructures. And the substantial enhancement in activity and durability for NiMoSx catalysts is greatly desirable. Herein, we first develop a self-template method for the synthesis of NiMoS3 nanorods via a hydrothermal process using NiMoO4 nanorods as precursors. The as-obtained NiMoS3 nanorods are cost-effective and exhibit excellent activity and stability for HER performance, including a low overpotential (η10) of 212 mV generating a current density of 10 mA cm−2 in acidic media and an impressively small η10 of 126 mV in alkaline solution, making it a promising candidate as a pHtolerant electrocatalyst for HER.

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voltammetry (LSV) tests in 0.5 M H2SO4 and 1.0 M KOH at a scan rate of 2 mV s−1, respectively. The potentials are calibrated by a reversible hydrogen electrode (RHE). Commercial Pt/C is tested as the benchmark electrocatalyst. The potential required to afford a current density of 1 mA cm−2 is defined as the onset potential. The electrochemical impedance spectroscopy (EIS) tests are performed at the frequency range of 0.1 Hz to 100 kHz. The electrochemical double-layer capacitance (Cdl) is collected between the potential range where no faradic processes at varied scan rates. The electrochemically active surface area (ECSA) can be calculated based on the obtained specific capacitance.

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RESULTANTS AND DISCUSSION SEM images (Figure 1a,b) show a high yield of uniform NiMoS3 nanorods (∼100 nm in diameter and over 1 μm in length), and the 1D structure of which is well inherited from the NiMoO4 precursors (Figure S1). Interestingly, NiMoS3 nanorods show increased diameter and more rough texture in comparison with their precursors of NiMoO4 nanorods, suggesting the successful transformation. To further reveal the structure of NiMoS3, TEM characterization is used. Figure 2a confirms the successful synthesis of 1D rod structure, and the selected-area electron diffraction (SAED) pattern in the

EXPERIMENTAL SECTION

The NiMoO4 nanorods are obtained via a hydrothermal method. Ni(NO3)2·6H2O (2.5 mmol) are dissolved in 50 mL of water, which is then added with 2.5 mmol of Na2MoO4·2H2O under vigorous stirring. The resultant solution is transferred into an autoclave for heating at 180 °C for 12 h. The precipitate is collected by centrifugation and vacuum-dried at 80 °C. NiMoO4 nanorods (100 mg) and thioacetamide (300 mg) are dispersed in 30 mL of ethanol with ultrasonic treatment of 30 min. The dispersion is transferred into autoclave for hydrothermal treatment at 150 °C for 24 h. The resultant powders are sintered in a tube furnace at 400 °C for 3 h in Ar atmosphere with a heating rate of 5 °C every minute to obtain the NiMoS3 nanorods. Commercial Pt/C (10% in mass) is purchased from Aladdin Ltd. (Shanghai, China) and used without further purification. Pristine MoS2 is synthesized via a similar hydrothermal process: 350 mg of Na2MoO4·2H2O and 300 mg of thioacetamide in ethanol (30 mL) are heated at 150 °C for 24 h in an autoclave. The NiMoS3 nanorods are characterized by powder X-ray diffraction (XRD, Philips X’-pert X-ray diffactometer) and scanning electron microscopy (SEM, a JEOL JSM-7500F) techniques. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) are collected from an FEI Tecnai G2 F30. Raman spectroscopy is performed on a confocal microprobe Raman system (LabRam-e010, 632 nm as the excitation source). The X-ray photoelectron spectrum (XPS) is recorded with a RBD upgraded PHI-5000c ESCA system (PerkinElmer). The electrochemical HER measurements are conducted on a CHI760D (CH Instruments, Shanghai, China) with a three-electrode cell. A glassy carbon (GC) electrode modified with catalyst serves as the working electrode. Graphite rod and saturated calomel electrodes (SCE) are used as the counter electrode and reference electrode, respectively. The catalyst ink is a mixture containing 4 mg of NiMoS3 nanorods, 50 uL of Nafion solution (5 wt %), and 1.25 mL of water, which is drop-casted onto a GC electrode. The catalyst loading is about 0.5 mg cm−2. The HER activity is measured by linear sweep

Figure 2. (a) TEM image, (b) TEM image and EDX elemental mapping images of Ni, Co, and S, (C) HRTEM image, and (d) the corresponding EDX spectrum of NiMoS3 nanorods. The inset in panel a shows the SAED pattern of NiMoS3 nanorods. 9007


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Figure 3. (a) XRD and (b) Raman spectrum of NiMoS3 nanorods.

Figure 4. (a) Survey XPS spectrum of NiMoS3 nanorods. High-resolution XPS spectra of (b) Mo 3d and S 2s, (c) Ni 2p, and (d) S 2p of NiMoS3 nanorods.

similar to the interlayer spacing of 0.62 nm for MoS2. This proves that Ni atoms successfully dope into the crystal lattice of MoS2 by most probably locating at the Mo sites,19 namely the formation of Ni−Mo−S. Figure 3a displays the powder XRD pattern of NiMoS3 nanorods. The sharp diffraction peaks indicate the high crystalline. The peak located at 2θ = 14.2° corresponds to the characteristic (002) plane of crystalline MoS2, and all the XRD peaks agree well with 2H-MoS2 (JCPDS 37-1492) or hexagonal NiS (JCPDS 77-1624). The Raman spectrum of NiMoS3 nanorods is also collected (Figure 3b). Two main Raman peaks located at 373 and 405 cm−1 are clearly detected, which are assigned to the in-plane E12g mode and out-of-plane A1g modes of MoS2, respectively.10,22,26 The wide peak width of E12g, as well as the much lower peak intensity of E12g than that of A1g, show abundant defect sites, which could supply more catalytic sites for HER.22,26 The Raman peaks (labeled with #) related to NiS are observed at 165, 191, 241, 279, 516, and 572 cm−1,39 in accordance with the XRD results.

inset shows the polycrystalline texture of NiMoS3 with continuous diffraction rings.21 In the corresponding EDX elemental mapping (Figure 2b), the component elements of Ni, Mo, and S are homogeneously distributed over the nanorods. The EDX spectrum (Figure 2d) further shows that the obtained catalyst is composed of Ni, Mo, and S elements with the atomic ratio of 18:19:63, which is close to the theoretical value of 1:1:3. NiMoS3 nanorods are further characterized with inductively coupled plasma (ICP) opticalemission spectroscopy. The Ni:Mo:S atomic ratio of 1:1.1:3.3 is obtained, which is in consistent with the EDX result. For NiMoO4 precursor, Ni, Mo, and O elements are detected (Figure S1e), indicating the successful sulfuraction. In the highresolution TEM (HRTEM) image of Figure 2c, the layer structure with edge-terminated texture can be seen from the profile of NiMoS3 nanorods. Interestingly, defects (labeled with white arrows) can be observed on the exposed edges, assuring abundant exposed catalytic sites for HER. A distance of 0.64 nm is determined for the interlayer spacing of NiMoS3, which is 9008


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Figure 5. HER performance of NiMoS3 nanorods in (a,c,e) 0.5 M H2SO4 and (b,d,f) 1 M KOH: (a,b) polarization curves, (c,d), Tafel plots, and (e,f) measured capacitive currents plotted as a function of scan rate. The insets in panels e, f are the cyclic voltammograms of NiMoS3 nanorods obtained at a potential range where no faradic processes in (e) 0.5 M H2SO4 and (f) 1 M KOH.

and S 2p1/2 orbitals of S2−.16,22,26,27 The weak XPS peak centered at 169.0 eV is associated with S4+, which may be due to the partial oxidation during the hydrothermal process.23 The existence of oxygen peak in the XPS spectra of NiMoS3 should be ascribed to two possible contribution, in terms of the incomplete sulfuration of MoO42− species and partial oxidation of NiMoS3 during the hydrothermal process. The electrocatalytic HER activity of NiMoS3 nanorods is performed in 0.5 M H2SO4 and 1 M KOH, respectively. Pristine MoS2 and commercial Pt/C electrocatalysts are also measured as references. Figure 5a shows the polarization curves of different catalysts obtained with LSV in acidic solution. Pt/C delivers the best catalytic activity with the lowest onset overpotential. Pristine MoS2 shows poor catalytic activity toward HER. As expected, NiMoS 3 nanorods exhibit remarkably improved HER activity than pristine MoS2. The catalytic current density of NiMoS3 increases rapidly with the potentials negatively shifting, delivering a low η10 (the overpotential needed to generate a current density of 10 mA cm−2) of 212 mV. In alkaline media (Figure 5b), the similar

To further confirm the elemental composition and valence states of NiMoS3 nanorods, the XPS measurements are performed and shown in Figure 4. In the survey spectrum (Figure 4a), XPS peaks of Ni, Mo, S, C, and O elements are detected. The high-resolution XPS spectra of Mo 3d and S 2s are shown in Figure 4b. The XPS peak at 226.3 eV is associated with S 2s.16,22,27 Two sharp peaks located at 229.3 and 232.5 eV can be clearly observed, which are attributed to Mo4+ 3d5/2 and Mo4+ 3d3/2 in MoS2, revealing the dominance of Mo4+ in NiMoS3.16,22,27 The weak peak located at 235.8 eV that is associated with Mo6+ oxidation state is also detected, which may be due to the incomplete sulfuration of MoO42− species during the hydrothermal process.22,26 The high-resolution spectrum of Ni 2p (Figure 4c) displays a strong peak centered at 854.3 and 871.6 eV, which are assigned to Ni 2p2/3 and Ni 2p1/2, respectively.22,23,26,28 Interestingly, a satellite peak for Ni 2p2/3 is observed at 861.7 eV, indicating that Ni2+ is the major valence state for Ni species.23,28 Observed from the S 2p spectrum (Figure 4d), two peaks with binding energies of 162.1 and 163.2 eV are observed, which are assigned to the S 2p3/2 9009


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where b is the Tafel slope). In Figure 5c, a Tafel slope of 71 mV dec−1 is obtained in H2SO4, corresponding to the Volmer− Heyrovsky mechanism.10,21 In KOH, a Tafel slope of 82 mV dec−1 is observed. Based on this, the inherent HER activity of NiMoS3 nanorods is evaluated by the exchange current density (j0), which is determined based on the Tafel equation. In H2SO4, the j0 value of 0.019 mA cm−2 is obtained, which is slightly higher than that (0.011 mA cm−2) for the advanced electrocatalyst of CoMoS 3 hollow prisms in H 2SO 4.21 Strikingly, the present NiMoS3 nanorods exhibit much increased j0 of 0.28 mA cm−2, indicating the outstanding HER kinetics. This value is higher than most of the reported catalysts in alkaline media, for instance, the state-of-the-art porous carbon supported Ni/MoC (0.20 mA cm−2).32 In alkaline media, the HER process involves three steps: the first Volmer step including water cleavage and formation of intermediate Had (M + H2O + e− = M−Had + OH−), the followed Heyrovsky step (M−Had + H2O + e− = M + H2 + OH−) or Tafel recombination reaction (2M−Had = 2 M + H2). Beneficial from the optimum strength of Ni−OH bond in alkaline media, Ni2+ in Ni−Mo−S serves dual functions of facilitating O−OH cleavage (Volmer step) and the adsorption of OH− during HER.24,25 This should be responsible for the remarkably improved HER kinetics of NiMoS3 nanorods in KOH. To further understand the advantages of the present NiMoS3 nanorods, the double-layer capacitances are measured in both H2SO4 and KOH, and the corresponding Cdl values (Figure 5e,f) are calculated to evaluate the effective electrode surface area. For comparison, the Cdl (Figure S3a) of pristine MoS2 in 0.5 M H2OS4 is also obtained. Clearly, NiMoS3 nanorods possess a remarkably higher Cdl of 8030 μF cm−2, which is about 10.4-fold that (775 μF cm−2) for MoS2. This should be ascribed to two contributions of the Ni doping that could introduce rich defects and the edge-terminated texture assured by the rational synthesis strategy. In Figure 5e,f, NiMoS3 nanorods possess high Cdl values of 8030 and 26 170 μF cm−2 in H2SO4 and KOH, respectively, due to the synergistic effects of Ni, Mo, and S. It shows that NiMoS3 nanorods deliver more effective active sites, which is responsible for the better HER activity. The higher Cdl in alkaline media further reveals that Ni species possess more active feature in alkaline media. EIS measurements of NiMoS3 nanorods are also carried out to evaluate the electrode kinetics and interface reactions. In Figure S4, NiMoS3 nanorods show low charge transfer resistances (Rct) of about 65 Ω in H2SO4 and 108 Ω in KOH, indicating the fast reaction rate. In contrast, a very high Rct of ∼540 Ω is obtained for MoS2 in H2SO4 (Figure S3b). Durability, another important criterion to assess the NiMoS3 nanorod catalyst, is performed for over 10 h under a static overpotential of 210 mV in H2SO4, and 125 mV in KOH, respectively. Figure 6a displays the time-dependent curve in acid, in which the current density shows a negligible decline for over 10 h, showing the superior stability. Moreover, the LSV curve (Figure 6c) after 10 h is almost completely overlapped with the initial cycle, further confirming the stable catalytic performance of NiMoS3 in H2SO4. Good stability is also observed in KOH. The current density slightly fades during a long-term test (Figure 6b). As shown in Figure 6d, a small potential change of 9 mV is detected at a current density of 60 mA cm−2. It is indicated that NiMoS3 nanorods exhibit stable activity for HER in both H2SO4 and KOH. To probe the root for the good catalytic stability, the SEM images and XRD

catalytic performances are observed, in terms of Pt/C > NiMoS3 nanorods > pristine MoS2. Interestingly, NiMoS3 nanorods exhibit outstanding HER activity of low overpotential and high current density in KOH, which is even better than that in H2SO4. As listed in Table 1, an onset potential of 52 mV is Table 1. Electrochemical HER Parameters of NiMoS3 Nanorods Electrolyte

Onset potential (mV)

η10 (mV)

Tafel slope (mV dec−1)

Exchange current density (j0) (mA cm−2)

H2SO4 KOH

146 52

212 126

71 82

0.019 0.28

achieved in KOH, which is much lower than that of 146 mV in acid. And the η10 is obtained as low as 126 mV, which is also much lower than the value of 212 mV in H2SO4 electrolyte. To highlight the excellent catalytic activity of the present sample in alkaline solution, detailed comparison of overpotential at 10 mA cm−2 is conducted between NiMoS3 nanorods with other state-of-the-art non-noble-metal electrocatalysts in alkaline electrolyte. In Table 2, the present NiMoS3 nanorods deliver Table 2. Overpotentials at Current Density of 10 mA cm−2 Obtained from the Present NiMoS3 Nanorods and Other State-of-the-Art Non-Noble-Metal Electrocatalysts in Alkaline Media Samples

η10 (mV)

ref.

3D macroporous MoS2 on Mo foil Ni−Mo−S nanosheets on carbon fiber cloth NiMoS4 particles NiSe nanowire arrays on Ni foam Ni2P on Ni Foam Ni8P3 on Ni Foam Ni/Mo2C on porous carbon 3D Ni3S2 Ni3S2 nanorod array foam Ni2P CoP nanowire array NiFe layered double hydroxide on Ni foam Ni(OH)2 on Ni Foam NiCo2S4 nanowire arrays on Ni foam NiMoS3 nanorods

184 200 152 96 98 130 179 182 200 200 209 210 250 210 126

18 22 23 29 30 31 32 33 34 35 36 37 37 38 Present work

the lowest η10 in comparison with previously reported Ni− Mo−S catalysts, such as Ni−Mo−S nanosheets on carbon fiber cloth (200 mV)22 and NiMoS4 particles (152 mV).23 This value of 126 mV is also lower than most of the reported metal sulfides catalysts, including 3D macroporous MoS2 on Mo foil (184 mV),18 Ni3S2 nanorod array foam, (200 mV),34 and NiCo2S4 nanowire (NW) arrays on Ni Foam (NF) (210 mV).38 Moreover, our sample shows lower η10 than most of other kinds of earth-abundant electrocatalysts, such as Ni/ Mo2C on porous carbon (179 mV),32 Ni2P (200 mV),35 CoP NW array (209 mV),36 NiFe layered double hydroxide (LDH) on NF (210 mV),37 and Ni(OH)2 on NF (250 mV).37 It is among the best of the documented samples of NiSe NW on NF (96 mV),29 Ni2P on NF (98 mV),30 and Ni8P3 on NF (130 mV),31 showing great promise as advanced HER electrocatalyst in alkaline solution. The HER kinetics is assessed by the corresponding Tafel plot, which is fitted into the Tafel equation (η = b log j + a, 9010


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Figure 6. Durability tests of NiMoS3 nanorods in (a,c,e) 0.5 M H2SO4 and (b,d,f) 1 M KOH: (a,b) Time-dependent current density curves, (c,d) polarization curves after continuous test of 10 h, and (e,f) comparison of the theoretically calculated (black line) and experimentally measured (red dots) amount of evolved hydrogen versus time at −250 mV for 120 min.

its application in alkaline media, Ni is rationally selected to tune the HER activity of MoS2, because Ni can form a Ni−OH bond in alkaline and facilitate the reaction kinetics of HER. As a result, the as-obtained NiMoS3 nanorods exhibit excellent HER activity in alkaline, indicating that the addition of Ni endows NiMoS3 with intrinsic unit activity toward HER. The higher exchange current density of NiMoS3 in H2SO4 than in KOH reveals that Ni is more active in KOH. And the measurements on double-layer capacitances show that NiMoS3 possesses a higher Cdl in KOH than H2SO4, further revealing the active Ni species in alkaline solution. In addition, the structural benefits supply a great contribution for the outstanding HER performance of NiMoS3 nanorods. The introduction of Ni and rationally selected 1D nanostructure endow the NiMoS3 catalyst with great structural advantages of rich defects and edge-terminated texture, assuring abundant active sites. Therefore, NiMoS3 shows a high Cdl value in both H2SO4 and KOH. In comparison with the reported NiMoS4 particles in ref 23, our NiMoS3 nanorods deliver a lower η10 of 126 mV than that of 152 mV, which should be ascribed to the structural advantages

pattern of NiMoS3 nanorods after durable test are obtained. Observed from the SEM images (Figure S5a,b), the morphologies of NiMoS3 nanorods are well maintained after the 10 h test in both acid and alkaline solution, due to the good stability of 1D nanostructure. Figure S5c displays the XRD pattern obtained in H2SO4. The intensities of NiS peaks (labeled with #) decrease remarkably after the durability test, which should be ascribed to the dissolution of Ni in acidic media. In KOH, no detectable changes are observed in the XRD pattern (Figure S5d), indicating the excellent stability of NiMoS3 crystal in alkaline solution. Additionally, the Faradaic efficiency (FE) is also obtained. As shown in Figure 6e,f, NiMoS3 nanorods exhibit a Faradaic efficiency close to 100% for hydrogen evolution in both H2SO4 and KOH. It is generally postulated that two factors play key roles in determining the HER activity of MoS2 based electrocatalysts: the intrinsic unit activity on each catalytic site, and the extrinsic factor of exposed catalytic site numbers and electric conductivity. In KOH, pristine MoS2 shows no HER activity because it is inactive in alkaline solution (Figure 5b). To extend 9011


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hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. J. Am. Chem. Soc. 2005, 127, 5308−5309. (2) Zhang, X.; Lai, Z.; Tan, C.; Zhang, H. Solution-processed twodimensional MoS2 nanosheets: Preparation, hybridization, and applications. Angew. Chem., Int. Ed. 2016, 55, 8816−8838. (3) Karunadasa, H. I.; Montalvo, E.; Sun, Y.; Majda, M.; Long, J. R.; Chang, C. J. A molecular MoS2 edge site mimic for catalytic hydrogen generation. Science 2012, 335, 698−702. (4) Choi, W. I.; Wood, B. C.; Schwegler, E.; Ogitsu, T. Sitedependent free energy barrier for proton reduction on MoS2 edges. J. Phys. Chem. C 2013, 117, 21772−21777. (5) Zhang, X.; Zhang, Q.; Sun, Y.; Zhang, P.; Gao, X.; Zhang, W.; Guo, J. MoS2-graphene hybrid nanosheets constructed 3D architectures with improved electrochemical performance for lithium-ion batteries and hydrogen evolution. Electrochim. Acta 2016, 189, 224− 230. (6) Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; Zhang, P.; Cao, X.; Song, B.; Jin, S. Contributions of phase, sulfur vacancies, and edges to the hydrogen evolution reaction catalytic activity of porous molybdenum disulfide nanosheets. J. Am. Chem. Soc. 2016, 138, 7965−7972. (7) Guo, J.; Zhu, H.; Sun, Y.; Tang, L.; Zhang, X. Doping MoS2 with graphene quantum dots: Structural and electrical engineering towards enhanced electrochemical hydrogen evolution. Electrochim. Acta 2016, 211, 603−610. (8) Qi, K.; Yu, S.; Wang, Q.; Zhang, W.; Fan, J.; Zheng, W.; Cui, X. Decoration of the inert basal plane of defect-rich MoS2 with Pd atoms for achieving Pt-similar HER activity. J. Mater. Chem. A 2016, 4, 4025− 4031. (9) Yu, X. Y.; Hu, H.; Wang, Y.; Chen, H.; Lou, X. W. Ultrathin MoS2 nanosheets supported on N-doped carbon nanoboxes with enhanced lithium storage and electrocatalytic properties. Angew. Chem., Int. Ed. 2015, 54, 7395−7398. (10) Guo, J.; Li, F.; Sun, Y.; Zhang, X.; Tang, L. Oxygen-incorporated MoS2 ultrathin nanosheets grown on graphene for efficient electrochemical hydrogen evolution. J. Power Sources 2015, 291, 195−200. (11) Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y. Controllable disorder engineering in Oxygen-Incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution. J. Am. Chem. Soc. 2013, 135, 17881−17888. (12) Wang, H.; Tsai, C.; Kong, D.; Chan, K.; Abild-Pedersen, F.; Nørskov, J. K.; Cui, Y. Transition-metal doped edge sites in vertically aligned MoS2 catalysts for enhanced hydrogen evolution. Nano Res. 2015, 8, 566−575. (13) Zhang, X.; Zhang, Q.; Sun, Y.; Guo, J. Hybrid catalyst of MoS2CoMo2S4 on graphene for robust electrochemical hydrogen evolution. Fuel 2016, 184, 559−564. (14) Zhang, H.; Li, Y.; Xu, T.; Wang, J.; Huo, Z.; Wan, P.; Sun, X. Amorphous Co-doped MoS 2 nanosheet coated metallic CoS 2 nanocubes as an excellent electrocatalyst for hydrogen evolution. J. Mater. Chem. A 2015, 3, 15020−15023. (15) Merki, D.; Vrubel, H.; Rovelli, L.; Fierro, S.; Hu, X. Fe, Co, and Ni ions promote the catalytic activity of amorphous molybdenum sulfide films for hydrogen evolution. Chem. Sci. 2012, 3, 2515−2525. (16) Zhang, X.; Ding, P.; Sun, Y.; Guo, J. Layered FeMo4S6 nanosheets with robust lithium storage and electrochemical hydrogen evolution. Mater. Lett. 2016, 183, 1−4. (17) Staszak-Jirkovsky, J.; Malliakas, C. D.; Lopes, P. P.; Danilovic, N.; Kota, S. S.; Chang, K.-C.; Genorio, B.; Strmcnik, D.; Stamenkovic, V. R.; Kanatzidis, M. G.; Markovic, N. M. Design of active and stable Co-Mo-Sx chalcogels as pH-universal catalysts for the hydrogen evolution reaction. Nat. Mater. 2015, 15, 197−204. (18) Pu, Z.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X.; He, Y. 3D macroporous MoS2 thin film: in situ hydrothermal preparation and application as a highly active hydrogen evolution electrocatalyst at all pH values. Electrochim. Acta 2015, 168, 133−138. (19) Dai, X.; Du, K.; Li, Z.; Liu, M.; Ma, Y.; Sun, H.; Zhang, X.; Yang, Y. Co-doped MoS2 nanosheets with the dominant CoMoS phase

of rich defects and edge-terminated texture. Moreover, the 1D nanostructure with superior structural stability should be responsible for the good durable stability of NiMoS3 nanorods, which is evidenced by SEM characterizations in Figure S5. Therefore, it can be concluded that the excellent HER performances of NiMoS3 nanorods, in terms of low overpotential, high catalytic current, and excellent stability, are rooted in Ni addition and combined structural benefits.

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CONCLUSIONS In summary, to explore a pH-tolerant MoS2 based catalytst toward HER, we first demonstrate the synthesis of NiMoS3 nanorods via a hydrothermal method with NiMoO4 nanorods as a self-template. Guaranteed by the synergistic effect of Ni− Mo−S and the structural advantages, NiMoS3 nanorods are endowed with advantages to function as the efficient and stable catalysts for electrochemical HER in acidic solution and alkaline media. Specifically, a high j0 of 0.28 mA cm−2 and large Cdl of 26 170 μF cm−2 are achieved in KOH, showing that the incorporation of Ni atoms in Mo−S could not only improve the inherent catalytic activity but also increase active sites to achieve enhanced HER performance of low overpotentials of 126 mV at a current density of 10 mA cm−2 in KOH. We postulate that our present work would inspire the development of various advanced electrocatalysts toward HER application in alkaline electrolyte with the boosting effect of Ni, such as alloys, metal carbides, metal nitrides, and metal chalcogenides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01802. SEM image, XRD pattern, TEM image, HRTEM image, and the corresponding EDX of NiMoO4 nanorods, TEM image of pristine MoS2, the measured capacitive currents plotted as a function of scan rate for pristine MoS2, EIS spectrum of MoS2 in 0.5 M H2SO4, EIS spectrum of NiMoS3 nanorods in 0.5 M H2SO4 and 1 M KOH, SEM images and XRD pattern of NiMoS3 nanorods after i-t test in 0.5 M H2SO4 and 1 M KOH, polarization curves of NiMoO4 nanorods in 0.5 M H2SO4 and 1 M KOH (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 532 84022681; Fax: +86 532 84023927. E-mail address: [email protected] (X. Zhang). ORCID

Xiao Zhang: 0000-0003-0165-1561 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank for the financial support from Natural Science Foundation (2016GGX104019, ZR2014JL015, ZR2014EMM004) of Shandong Province.

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REFERENCES

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NiMoS3 Nanorods as pH-Tolerant Electrocatalyst for Efficient

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