Heterostructured binary Ni-W sulfides nanosheets as pH-universal electrocatalyst for hydrogen evolution

Accepted Manuscript Full Length Article Heterostructured binary Ni-W sulfides nanosheets as pH-universal electrocatalyst for hydrogen evolution Shan-Shan Lu, Xiao Shang, Li-Ming Zhang, Bin Dong, Wen-Kun Gao, FangNa Dai, Bin Liu, Yong-Ming Chai, Chen-Guang Liu PII: DOI: Reference:

S0169-4332(18)30862-6 https://doi.org/10.1016/j.apsusc.2018.03.177 APSUSC 38925

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

15 January 2018 18 March 2018 22 March 2018

Please cite this article as: S-S. Lu, X. Shang, L-M. Zhang, B. Dong, W-K. Gao, F-N. Dai, B. Liu, Y-M. Chai, C-G. Liu, Heterostructured binary Ni-W sulfides nanosheets as pH-universal electrocatalyst for hydrogen evolution, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.03.177

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

Heterostructured binary Ni-W sulfides nanosheets as pH-universal electrocatalyst for hydrogen evolution Shan-Shan Lu, Xiao Shang, Li-Ming Zhang, Bin Dong*, Wen-Kun Gao, Fang-Na Dai, Bin Liu, Yong-Ming Chai*, Chen-Guang Liu College of Science, State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266580, PR China

Abstract Developing effective and robust electrocatalysts that are applicable for different pH conditions is promising for variable industrial hydrogen evolution reaction (HER), whereas it remains challenging for designing proper materials and protocols. Herein, we have developed a two-step electrodeposition-hydrothermal strategy to construct heterostructured binary Ni-W sulfides nanosheets based on carbon fiber (NiWS/CF). The electrodeposited nickel oxides film on CF in the first step is sulfurized and concurrently incorporated with tungsten disulfide in the following hydrothermal process. Benefiting from synergistic advantages of bimetallic sulfides as well as interwoven nanosheets for efficient mass/charge transport, the NiWS/CF electrode shows excellent HER performances over a broad pH range from acidic (pH=0), neutral (pH=7) to alkaline (pH=14) media. The NiWS/CF electrode also presents stability in long-term electrolysis in wide PH range for at least 12 h, and its interlaced nanosheets structure are well maintained. Our work may provide general and

* Corresponding author. Email: [email protected] (B. Dong), [email protected] (Y.-M. Chai) Tel: +86-532-86981376, Fax: +86-532-86981787 1

promising strategies to obtain inexpensive and efficient electrocatalysts for pH-universal hydrogen production. Keywords: Wide-pH; tungsten disulfide; nickel sulfide; electrodeposition; hydrogen evolution reaction

1. Introduction The hydrogen evolution reaction (HER) from water not only provides hydrogen fuels as a clean energy carrier to resolve energy crisis caused by fossil fuels, but also appears as an efficient strategy for energy conversion and storage into a chemical fuel from solar/wind-derived electricity [1-3]. The key to drive highly efficient water electrolysis is electrocatalysts to lower overpotentials and facilitate the reaction rates. However, the scarce and expensive Pt-based groups as the best catalysts are impossible for industrial application [4, 5], which stimulates extensive researches to explore earth-abundant materials especially transition metal groups. Apart from cost-effective merits of electrocatalysts, the ability to function over a wide pH range in electrolytes is also highly desirable. For example, microbial electrolysis cells (MEC) requires operation in neutral solutions [6-8]; proton exchange membrane (PEM) electrolysis needs acidic conditions [9, 10] while alkaline electrolysis cell requires catalysts in basic conditions [7, 11]. Analogous to MoS2 as typical layered transition-metal dichalcogenides (LTMDs), WS2 has also exhibited promising electrocatalytic performances on edge sites [12-21]. However, the HER activity of WS2-based materials are limited to acidic solutions [22, 23], which is restricted in neutral or alkaline media [24, 25]. The decreased proton concentration in 2

the electrolyte reduces the formation of absorbed H atoms in the first stage of Volmer step, bringing about much higher kinetic energy barrier in dissociating water molecules especially in alkaline solution [26-28]. Therefore, incorporating active species containing H2O dissociation centers [29-32] is the key to fast HER kinetics. Nickel-based species, especially nickel sulfides and their compounds [33-36], have demonstrated its high activity and excellent electron transport ability in both alkaline [37-39] and neutral [40, 41] media. Previous studies showed that the nickel sites are highly effective to decrease energy barrier of water dissociation and facilitate the desorption of the OH- [27, 42]. Therefore, integrating nickel sulfides into WS 2 materials is supposed to engineer water dissociation sites in neutral or alkaline HER. In terms of acid HER, the introducing of nickel species not only generate new active sites in catalyzing hydrogen evolution, but also provide synergistic advantages that may outperforms the individual components. The Chorkendorff and coworkers have theoretically and experimentally proved that cobalt-doping can reduce the free energy of hydrogen adsorption on S-edge of WS2 and result in enhanced HER properties [43]. Likewise, as in the same first-row transition metals, the doping or incorporating nickel species into WS2 is expected to exhibit better catalytic performances, whereas the corresponding research remains unexplored. Herein, based on our previous two-step electrodeposition-hydrothermal strategy of incorporating foreign metals [44-46], we have successfully incorporated NiS2 into WS2 nanosheets supported by carbon fiber (NiWS/CF). Such heterostructured bimetallic sulfides not only contain rich electrocatalytic active sites, but also result in 3

synergistic effects to intrigue intrinsic catalytic performances. Additionally, thanks to the excellent water dissociation abilities of nickel sulfide, the HER performances in neutral and alkaline conditions are remarkably improved. The electrochemical measurements demonstrate high HER performances of NiWS/CF ranging from acidic (pH=0), neutral (pH=7) to alkaline (pH=14) media with structural stability in long-term electrolysis for at least 12 h. The NiWS/CF exhibits its promising variability over broad electrolyte conditions, which may provide general accesses to inexpensive and efficient electrocatalysts for pH-universal hydrogen production.

2. Experimental Prior to the typical experiment, carbon fiber (CF, thickness of 0.35 mm, surface density of 190 g m−2, geometric area of 1×2 cm2) was under sonication in acid, acetone, ethanol and deionized water consecutively for 30 min. 2.1 Preparation of NiO/CF The electrodeposition of NiO/CF was conducted on a three-electrode setup (Gamry Reference 600, USA), comprising CF as the working electrode, an Ag/AgCl (in 3.5 M KCl) as reference electrode and a carbon rod as counter electrode. The electrolyte is protected in N2 atmosphere. A potentiostatic deposition of -1.0 V (vs. Ag/AgCl) was conducted in 0.1 mM Ni(NO3)3·H2O for 300 s in room temperature. Afterwards, the as-prepared sample was rinsed with deionized water and dried in air. 2.2 Preparation of NiS2/CF Typically, thioacetamide (1.5 g) was dissolved in 30 mL of deionized water to form uniform solution. Then the mixture was transferred into Teflon-lined stainless steel 4

autoclave (100 mL) with four pieces of as-prepared NiO/CF and heated at 200 °C for 24 h. After cooling down to room temperature, the black NiS2/CF products were rinsed with deionized water and ethanol for several times, followed by vacuum-drying at 60 °C for 8 h. 2.3 Preparation of WS2/CF In a typical synthesis, thioacetamide (1.5 g), ammonium metatungstate (0.2 g) and oxalic acid (1.2 g) were added in deionized water (30 mL) to form uniform mixture. Then it was transferred to Teflon-lined stainless steel autoclave (100 mL) with four pieces of CF at 200 °C for 24 h. After cooling down to room temperature, the black WS2/CF products were washed with deionized water and dried in vacuum. 2.4 Preparation of NiWS/CF The preparation of NiWS/CF was similar with WS2/CF, except for NiO/CF as precursor. 2.5 Physical characterizations X-ray diffraction (XRD) data were obtained on X’Pert PRO MPD (Cu KR). Scanning electron microscopy (SEM) images were collected on Hitachi (S-4800). X-ray photoelectron spectrum (XPS) was measured on VG ESCALAB MK II equipped with Al Kα of 1486.6 eV. Transmission electron microscopy (TEM) was performed on FEI Tecnai G2. X-ray fluorescence elemental analysis (EDX) was carried out on representative areas of as-prepared samples. The metal elemental analysis via inductively coupled plasma optical emission spectrometry (ICP-OES) has been conducted for as-prepared samples on Shidmadzu ICPE-9000. 5

2.6 Electrochemical measurements Electrochemical performances were estimated using a typical three-electrode setup (Gamry Reference 600, USA). The acid, alkaline and neutral electrolytes were 0.5 M H2SO4, 1.0 M KOH and 0.2 M phosphate buffer solution (PBS), respectively, which were in N2 atmosphere during the test. All samples were utilized as working electrodes, a carbon rod as the counter electrode. A saturated calomel (SCE) electrode was used as reference electrode for alkaline solution, while an Ag/AgCl (3.5 M KCl) was applied in acid and PBS media. For comparison, the HER performance of commercial Pt/C (20 wt %, Sinopharm Chemical Reagent Co., Ltd) was also tested. Typically, 4 mg of Pt/C was sonicated in 1 mL of water-ethanol (1:1 in volume ratio) solution for 0.5 h to form uniform ink. Then 5 μL of the dispersion was loaded onto a glassy carbon electrode (GCE, geometric area of 0.1256 cm2) as the working electrode in the three-electrode configuration. Linear sweep voltammetry (LSV) data were obtained at a scan rate of 5 mV s-1. Electrochemical impedance spectroscopy (EIS) measurements were performed at -0.45 V (vs. Ag/AgCl) in 0.5 M H2SO4, -1.15 V (vs. SCE) in 1.0 M KOH and -0.7 V (vs. Ag/AgCl) in 0.2 M PBS with frequency from 105 Hz to 10-1 Hz and an AC voltage of 5 mV. The double-layer capacitance (Cdl) was tested through cyclic voltammogram (CV) in different scan rates of 40, 60, 80, 100 and 120 mV s-1. The stability of as-prepared samples was measured by chronoamperometry for 12 h. The electrochemical data were under iR (current time internal resistance) correction on Gamry Framework Data Acquisition Software 6.11. The potentials conversion from vs. Ag/AgCl or vs. SCE to vs. RHE was as follows: 6

E (vs. RHE) = E (vs. SCE) + 0.244 V + (0.059 V) pH E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + (0.059 V) pH

3. Results and discussion The crystal structures of as-prepared samples characterized by XRD in Fig. 1. The broad peaks at 20-30° are carbon species originated from carbon fiber (PDF no.96-210-3201). The as-prepared NiO/CF contains Ni(OH)2 (PDF no. 00-001-1047) derived from electrodeposited products. The nickel hydroxides was transferred into NiS2 (PDF no. 01-080-0375) for NiS2/CF through hydrothermal sulfurization. The peaks at 15-25° show little impurities of Ni51S54 (PDF no. 96-900-7660) in NiS2/CF sample. The WS2 (PDF no. 00-008-0237) structure can be observed in WS2/CF sample in broad peaks, suggesting its low crystalline degree. The NiWS/CF possesses both WS2 and NiS2 indicating the hybrid structure. The surficial structure and morphologies of all samples are shown in Fig. 2 and Fig. S2. Compared with blank CF in Fig. S1, the Ni(OH)2 deposits on NiO/CF (Fig. 2a and Fig. S2a) show interwoven nanostructure and aggregate into sphere partially. The WS2/CF contains larger cross-linked nanosheets dispersing on the whole surface of carbon fiber as displayed in Fig. 2b and Fig. S2b. The Ni(OH)2 deposits can be transferred to small nanoparticles (Fig. 2c and Fig. S2c) after hydrothermal sulfurization. The incorporation of nickel sulfide into tungsten disulfide to form hybrids in NiWS/CF still displays interwoven nanosheets on CF (Fig. 2d and Fig. S2d). The related nanosheets structures on NiWS/CF are further depicted in TEM images of Fig. 3. Fig. 3a shows typical stacking of two-dimensional WS2 nanosheets 7

similar with SEM image of Fig. 2d. The high resolution of Fig. 3b illustrates that such nanosheet comprises WS2 and NiS2 structures such as (002) and (220) planes, respectively. Accordingly, the selected area electron diffraction (SAED) in Fig. 3c shows the existences of (101) and (311) facets derived from WS2 and NiS2, respectively, which also confirms the hybrid structure of NiWS/CF. Furthermore, the Ni, W, S elements detected in the nanosheet (Fig. S2e) distribute uniformly in hybrid structure proved by TEM mapping in Fig. 3d. The detailed elemental contents of NiWS/CF have been shown in Table. S1. The chemical information of main elements in NiWS/CF has been studied through XPS in Fig. 4. The Ni, W, S elements can be detected in the In the XPS survey of Fig. 4a except for C and O caused by contamination or carbon fiber. In the high resolution of Fig. 4b, the W 4f can be divided into doublets of W (IV) at 33.2 eV and 34.8 eV, [47, 48], and W (VI) at 35.6 eV and 37.8 eV [49, 50]. Note that the lower shift of binding energies of can be observed for W (IV), suggesting the influence of NiS2 incorporation similar with previous study [18]. In Ni 2p region of Fig. 4c, the peaks at 854.1 and 873 eV can be attributed to 2p3/2 and 2p1/2 of Ni (II) of nickel sulfide, while Ni (III) can also be found at binding energies of 855.9 and 873.8 eV [51]. The detected Ni (III) may be assigned to air exposure and surface oxidation [51]. For S 2p shown in Fig. 3e, W-S bond can be observed at 161 and 162.5 eV [18, 48], while Ni-S bond locates at binding energies of 161.7 and 163.6 eV [52]. The higher oxidation state in SO42- at 169 eV suggests the oxidation of S element caused by air [53]. The O 1s high resolution spectra (Fig. S3) can be divided into Ni-O, W-O and absorbed H2O 8

molecule centered at 530.5, 531.5 and 532.5 eV, respectively [54-56]. The detection of Ni-O, W-O bond may be assigned to the surface oxidation of metal sulfides. The HER performances of all the as-prepared samples have been tested in different PH values such as acid (0.5 M H2SO4, Fig. 5), alkaline (1.0 M KOH, Fig. 6) and neutral media (0.2 M PBS, Fig. 7). In polarization curves of Fig. 5a, the commercial Pt/C shows the best HER property with zero overpotential and high current density under low potentials. The NiWS/CF displays a very low overpotential of 56 mV to generate 10 mA cm-2, which is much smaller than counterparts of NiS2/CF (244 mV), WS2/CF (251 mV) and NiO/CF (266 mV). In Fig. 5b, NiWS/CF delivers Tafel slopes of 123 mV dec-1 suggesting the Volmer−Heyrovsky pathway in acid media. In addition, NiWS/CF has the smallest Tafel slope compared with NiS2/CF (129 mV dec-1), WS2/CF (152 mV dec-1) and NiO/CF (159 mV dec-1), implying the fastest catalytic kinetics for HER. Moreover, the Nyquist plots (Fig. 5c) to reflect electrochemical impedance spectroscopy (EIS) reveal that the NiWS/CF has the smallest semicircle and thus the excellent electrochemical conductivity favorable for charge transport. In order to obtain more insight into activities, the electrochemically active surface area (ECSA) has been measured through positively-related Cdl value derived from CV scans (Fig. S4), as shown in Fig. 5d. The calculated Cdl value for NiWS/CF is 596.2 mF cm-2, which is remarkably larger than NiS2/CF (458.4 mF cm-2), WS2/CF (30.0 mF cm-2) and NiO/CF (209.3 mF cm-2). It confirms that NiWS/CF can expose more effective active sites and induce highly efficient HER performance. The HER properties of as-prepared samples in alkaline solution are investigated in 9

Fig. 6. As displayed in polarization curves of Fig. 6a, in addition to the ultrahigh HER activity of Pt/C catalyst, NiWS/CF sample exhibits an overpotential of 38 mV at 10 mA cm-2, which dramatically outperforms other samples of NiS2/CF (127 mV), WS2/CF (150 mV) and NiO/CF (208 mV). In Fig. 6b, the smaller Tafel slope of NiWS/CF (98 mV dec-1) than NiS2/CF (127 mV dec-1), WS2/CF (170 mV dec-1) and NiO/CF (160 mV dec-1) also confirms the superior HER activity in alkaline. Meanwhile, the smallest semicircle for NiWS/CF in the Nyquist plots (Fig. 6c) demonstrates its excellent electrochemical conductivity than other samples. The largest calculated Cdl value (257.5 mF cm-2) of NiWS/CF (Fig. 6d) derived from Fig. S5 shows that it exposes abundant effective active sites at the electrolyte/catalyst interface which results in high electrochemical HER performance. The HER property in neutral media of NiWS/CF electrode is investigated in 0.2 M PBS as presented in Fig. 7. In polarization curve (Fig. 7a), NiWS/CF still possesses the lowest overpotential of 120 mV to deliver 10 mA cm-2 compared with NiS2/CF (150 mV), WS2/CF (420 mV) and NiO/CF (550 mV). NiWS/CF also exhibits smallest Tafel slope and electrochemical resistance as displayed in Fig. 7b and Fig. 7c, respectively. On basis of CV measurements (Fig. S6), the Fig. 7d shows that NiWS/CF has relatively larger C dl value (37.6 mF cm-2) than counterparts, illustrating that the abundant active sites are available for catalyzing HER in neutral solution. Fig. 8 describes the time-dependent current density curve of NiWS/CF catalyzing hydrogen evolution in acid (Fig. 8a), alkaline (Fig. 8b) and neutral solution (Fig. 8c) for 12 h. The results demonstrate that the HER performance of NiWS/CF maintains 10

stable in all-PH electrocatalysis. Furthermore, the post-HER characterization have been conducted to investigate the structures and morphologies of NiWS/CF. As shown in XRD patterns of Fig. 9a, most of facets of WS2 and NiS2 are retained with decreased peak intensities after stability test. It implies that the activity loss may be due to the slight damaging of crystalline structures. In addition, the overall interwoven nanosheet structures are kept well shown in Fig. 9b-Fig. 9d and Fig. S6a-Fig. S6c, except that the surface are roughened after HER in alkaline (Fig. 9c) and neutral media (Fig. 9d). The chemical states after long-term electrolysis have also been studied in Fig. S7. All the main elements including Ni, W and S are remained in the XPS spectra of Fig. S8a, Fig. S9a and Fig. S10a. In the high-resolution regions, the W 4f spectra (Fig. S8b, Fig. S9b and Fig. S10b) are nearly unchangeable compared with initial one shown in Fig. 4b. However, the NiOOH species are formed in the Ni 2p spectra (Fig. S8c, Fig. S9c and Fig. S10c) after HER. For S 2p regions, the increased peak intensity of SO42- after HER in alkaline (Fig. S10c) and neutral media (Fig. S10d) implies more transformation of sulfur species to higher oxidation states.

4. Conclusion In summary, we have developed a two-step strategy to build binary Ni-W sulfides heterostructures on carbon fiber (NiWS/CF). The synergistic effected derived from bimetallic composites as well as interwoven nanosheets for fast mass/charge transport contributes to excellent HER performances over a broad pH range from acidic (pH=0), neutral (pH=7) to alkaline (pH=14) solution. More importantly, it retains high HER activity in long-term electrolysis in wide PH range for at least 12 h, and its interlaced 11

nanosheets structure is maintained confirming the structural stability. Our work may provide general accesses to cost-effective and robust electrocatalysts for pH-universal catalysis, which is promisingly applicable for variable requirements in industrial hydrogen production.

Acknowledgements This work is financially supported by Shandong Provincial Natural Science Foundation (ZR2017MB059 and ZR2016BL22) and the National Natural Science Foundation of China (21776314 and 21771191) and the Fundamental Research Funds for the Central Universities (18CX05016A) and National Training Program of Innovation and Entrepreneurship for Undergraduates (No. 201710425069).

12

Reference [1] G. Li, D. Zhang, Q. Qiao, Y. Yu, D. Peterson, A. Zafar, R. Kumar, S. Curtarolo, F. Hunte, S. Shannon, All the catalytic active sites of MoS2 for hydrogen evolution, J. Am. Chem. Soc. 138 (2016) 16632-16638. [2] X. Zou, Y. Zhang, Noble metal-free hydrogen evolution catalysts for water splitting, Chem. Soc. Rev. 44 (2015) 5148-5180. [3] G. Han, Y.-H. Jin, R. A. Burgess, N. E. Dickenson, X.-M. Cao, Y. Sun, Visible-Light-Driven Valorization of Biomass Intermediates Integrated with H 2 Production Catalyzed by Ultrathin Ni/CdS Nanosheets, J. Am. Chem. Soc. 139 (2017) 15584-15587. [4] L. Xu, Q. Jiang, Z. Xiao, X. Li, J. Huo, S. Wang, L. Dai, Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction, Angew. Chem. Int. Ed. 55 (2016) 5277-5281. [5] M. Li, T. Liu, X. Bo, M. Zhou, L. Guo, S. Guo, Hybrid carbon nanowire networks with Fe–P bond active site for efficient oxygen/hydrogen-based electrocatalysis, Nano Energy 33 (2017) 221-228. [6] A. Kundu, J. N. Sahu, G. Redzwan, M. Hashim, An overview of cathode material and catalysts suitable for generating hydrogen in microbial electrolysis cell, Int. J. Hydrogen Energy 30 (2012) 1-13. [7] J. W. D. Ng, M. García-Melchor, M. Bajdich, P. Chakthranont, C. Kirk, A. Vojvodic, T. F. Jaramillo, Gold-supported cerium-doped NiOx catalysts for water oxidation, Nat. Energy 1 (2016) 16053. 13

[8] J.Q. Chi, K.L. Yan, Z. Xiao, B. Dong, X. Shang, W.K. Gao, X. Li, Y.M. Chai, C.G. Liu, Trimetallic Ni-Fe-Co selenides nanoparticles supported on carbon fiber cloth as efficient electrocatalyst for oxygen evolution reaction, Int. J. Hydrogen Energy 42 (2017) 20599-20607. [9] Z. Pu, Q. Liu, A. M. Asiri, Y. Luo, X. Sun, Y. He, 3D macroporous MoS 2 thin film: in situ hydrothermal preparation and application as a highly active hydrogen evolution electrocatalyst at all pH values, Electrochim. Acta 168 (2015) 133-138. [10] M. Hambourger, M. Gervaldo, D. Svedruzic, P. W. King, D. Gust, M. Ghirardi, A. L. Moore, T. A. Moore, [FeFe]-hydrogenase-catalyzed H2 production in a photoelectrochemical biofuel cell, J. Am. Chem. Soc. 130 (2008) 2015-2022. [11] R. LeRoy, Industrial water electrolysis: present and future, Int. J. Hydrogen Energy 8 (1983) 401-417. [12] J. Zhang, Q. Wang, L. Wang, X. a. Li, W. Huang, Layer-controllable WS 2-reduced graphene oxide hybrid nanosheets with high electrocatalytic activity for hydrogen evolution, Nanoscale 7 (2015) 10391-10397. [13] X. Shang, W. H. Hu, X. Li, B. Dong, Y. R. Liu, G. Q. Han, Y. M. Chai, C. G. Liu, Oriented Stacking along Vertical (002) Planes of MoS2: A Novel Assembling Style to Enhance Activity for Hydrogen Evolution, Electrochim. Acta 224 (2017) 25-31. [14] Y. Yang, H. Fei, G. Ruan, Y. Li, J. M. Tour, Vertically Aligned WS 2 Nanosheets for Water Splitting, Adv. Funct. Mater. 25 (2015) 6199-6204. [15] Z. Wu, B. Fang, A. Bonakdarpour, A. Sun, D. P. Wilkinson, D. Wang, WS2 14

nanosheets as a highly efficient electrocatalyst for hydrogen evolution reaction, Appl. Catal. B 125 (2012) 59-66. [16] X. Shang, J.Q. Chi, S.S. Lu, B. Dong, X. Li, Y.R. Liu, K.L. Yan, W.K. Gao, Y.M. Chai, Y.Q. Liu, C.G. Liu, Novel Co xSy/WS2 nanosheets supported on carbon cloth as efficient electrocatalyst for hydrogen evolution reaction, Int. J. Hydrogen Energy 42 (2017) 4165-4173. [17] L. Cheng, W. Huang, Q. Gong, C. Liu, Z. Liu, Y. Li, H. Dai, Ultrathin WS 2 nanoflakes as a high-performance electrocatalyst for the hydrogen evolution reaction, Angew. Chem. Int. Ed. 53 (2014) 7860-7863. [18] X. Zhou, X. Yang, M. N. Hedhili, K.-W. Huang, L.-J. Li, W. Zhang, Symmetrical synergy of hybrid CoS2-WS2 electrocatalysts for hydrogen evolution reaction, J. Mater. Chem. A 5 (2017) 15552-15558. [19] A. Ambrosi, Z. Sofer, M. Pumera, 2H→1T phase transition and hydrogen evolution activity of MoS2, MoSe2, WS2 and WSe2 strongly depends on the MX2 composition, Chem. Commun. 51 (2015) 8450-8453. [20] A. Y. S. Eng, A. Ambrosi, Z. Sofer, P. Simek, M. Pumera, Electrochemistry of transition metal dichalcogenides: strong dependence on the metal-to-chalcogen composition and exfoliation method, ACS Nano 8 (2014) 12185-12198. [21] X. J. Chua, J. Luxa, A. Y. S. Eng, S. M. Tan, Z. k. Sofer, M. Pumera, Negative electrocatalytic effects of p-doping niobium and tantalum on MoS2 and WS2 for the hydrogen evolution reaction and oxygen reduction reaction, ACS Catal. 6 (2016) 5724-5734. 15

[22] D. Voiry, H. Yamaguchi, J. Li, R. Silva, D. C. Alves, T. Fujita, M. Chen, T. Asefa, V. B. Shenoy, G. Eda, Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution, Nat. Mater. 12 (2013) 850-855. [23] J. Yang, D. Voiry, S. J. Ahn, D. Kang, A. Y. Kim, M. Chhowalla, H. S. Shin, Two-Dimensional Hybrid Nanosheets of Tungsten Disulfide and Reduced Graphene Oxide as Catalysts for Enhanced Hydrogen Evolution, Angew. Chem. Int. Ed. 52 (2013) 13751-13754. [24] P. Kuang, T. Tong, K. Fan, J. Yu, In Situ Fabrication of Ni-Mo Bimetal Sulfide Hybrid as an Efficient Electrocatalyst for Hydrogen Evolution over a Wide pH Range, ACS Catal. 7 (2017) 6179-6187. [25] J. Staszak-Jirkovský, C. D. Malliakas, P. P. Lopes, N. Danilovic, S. S. Kota, K.-C. Chang, B. Genorio, D. Strmcnik, V. R. Stamenkovic, M. G. Kanatzidis, Design of active and stable Co-Mo-Sx chalcogels as pH-universal catalysts for the hydrogen evolution reaction, Nat. Mater. 15 (2016) 197-203. [26] Z. Zhou, L. Wei, Y. Wang, H. E. Karahan, Z. Chen, Y. Lei, X. Chen, S. Zhai, X. Liao, Y. Chen, Hydrogen evolution reaction activity of nickel phosphide is highly sensitive to electrolyte pH, J. Mater. Chem. A 5 (2017) 20390-20397. [27] R. Subbaraman, D. Tripkovic, D. Strmcnik, K.-C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic, N. M. Markovic, Enhancing hydrogen evolution activity in water splitting by tailoring Li+-Ni(OH)2-Pt interfaces, Science 334 (2011) 1256-1260. [28] R. Subbaraman, D. Tripkovic, K. C. Chang, D. Strmcnik, A. P. Paulikas, P. Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N. M. Markovic, Trends in activity 16

for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts, Nat. Mater. 11 (2012) 550-7. [29] P. Wang, K. Jiang, G. Wang, J. Yao, X. Huang, Phase and Interface Engineering of Platinum–Nickel Nanowires for Efficient Electrochemical Hydrogen Evolution, Angew. Chem. Int. Ed. 55 (2016) 12859-12863. [30] N. Danilovic, R. Subbaraman, D. Strmcnik, K. C. Chang, A. Paulikas, V. Stamenkovic, N. M. Markovic, Enhancing the alkaline hydrogen evolution reaction activity through the bifunctionality of Ni(OH) 2/metal catalysts, Angew. Chem. Int. Ed. 51 (2012) 12495-12498. [31] Y. F. Xu, M. R. Gao, Y. R. Zheng, J. Jiang, S. H. Yu, Nickel/nickel (II) oxide nanoparticles anchored onto cobalt (IV) diselenide nanobelts for the electrochemical production of hydrogen, Angew. Chem. Int. Ed. 52 (2013) 8546-8550. [32] H. Liao, C. Wei, J. Wang, A. Fisher, T. Sritharan, Z. Feng, Z. J. Xu, A Multisite Strategy for Enhancing the Hydrogen Evolution Reaction on a Nano-Pd Surface in Alkaline Media, Adv. Energy Mater. 7 (2017) 1701129-1701136. [33] R. Chen, Y. Song, Z. Wang, Y. Gao, Y. Sheng, Z. Shu, J. Zhang, X. a. Li, Porous nickel disulfide/reduced graphene oxide nanohybrids with improved electrocatalytic performance for hydrogen evolution, Catal. Commun. 85 (2016) 26-29. [34] Y. Cui, C. Zhou, X. Li, Y. Gao, J. Zhang, High performance electrocatalysis for hydrogen evolution reaction using nickel-doped CoS2 nanostructures: experimental and DFT insights, Electrochim. Acta 228 (2017) 428-435. [35] K.L. Yan, J.F. Qin, Z.Z. Liu, B. Dong, J.Q. Chi, W.K. Gao, J.H. Lin, Y.M. Chai, 17

C.G. Liu, Organic-inorganic hybrids-directed ternary NiFeMoS anemone-like nanorods with scaly surface supported on nickel foam for efficient overall water splitting, Chem. Eng. J. 334 (2018) 922-931. [36] Y. Cui, R. Zhang, J. Zhang, Z. Wang, H. Xue, W. Mao, W. Huang, Highly active and stable electrocatalytic hydrogen evolution catalyzed by nickel, iron doped cobalt disulfide@ reduced graphene oxide nanohybrid electrocatalysts, Mater. Today Energy 7 (2018) 44-50. [37] L.-L. Feng, G. Yu, Y. Wu, G.-D. Li, H. Li, Y. Sun, T. Asefa, W. Chen, X. Zou, High-index faceted Ni3S2 nanosheet arrays as highly active and ultrastable electrocatalysts for water splitting, J. Am. Chem. Soc. 137 (2015) 14023-14026. [38] J. Zhang, R. Cui, X. a. Li, X. Liu, W. Huang, A nanohybrid consisting of NiPS 3 nanoparticles coupled with defective graphene as a pH-universal electrocatalyst for efficient hydrogen evolution, J. Mater. Chem. A 5 (2017) 23536-23542. [39] X. Shang, K.L. Yan, Y. Rao, B. Dong, J.Q. Chi, Y.R. Liu, X. Li, Y.M. Chai, C.G. Liu, In-situ cathodic activation of V-incorporated NixSy nanowires for enhanced hydrogen evolution, Nanoscale 9 (2017) 12353-12363. [40] N. Jiang, L. Bogoev, M. Popova, S. Gul, J. Yano, Y. Sun, Electrodeposited nickel-sulfide films as competent hydrogen evolution catalysts in neutral water, J. Mater. Chem. A 2 (2014) 19407-19414. [41] C. Yang, M. Gao, Q. Zhang, J. Zeng, X. Li, A. Abbott, In-situ activation of self-supported 3D hierarchically porous Ni3S2 films grown on nanoporous copper as excellent pH-universal electrocatalysts for hydrogen evolution reaction, Nano Energy 18

36 (2017) 85-94. [42] J. Zhang, T. Wang, P. Liu, S. Liu, R. Dong, X. Zhuang, M. Chen, X. Feng, Engineering water dissociation sites in MoS2 nanosheets for accelerated electrocatalytic hydrogen production, Energy Environ. Sci. 9 (2016) 2789-2793. [43] J. Bonde, P. G. Moses, T. F. Jaramillo, J. K. Nørskov, I. Chorkendorff, Hydrogen evolution on nano-particulate transition metal sulfides, Faraday Discuss. 140 (2009) 219-231. [44] B. Dong, X. Zhao, G.-Q. Han, X. Li, X. Shang, Y.-R. Liu, W.-H. Hu, Y.-M. Chai, H. Zhao, C.-G. Liu, Two-step synthesis of binary Ni–Fe sulfides supported on nickel foam as highly efficient electrocatalysts for the oxygen evolution reaction, J. Mater. Chem. A 4 (2016) 13499-13508. [45] X. Shang, K.L. Yan, S.S. Lu, B. Dong, W.K. Gao, J.Q. Chi, Z.Z. Liu, Y.M. Chai, C.G. Liu, Controlling electrodeposited ultrathin amorphous Fe hydroxides film on V-doped nickel sulfide nanowires as efficient electrocatalyst for water oxidation, J. Power Sources 363 (2017) 44-53. [46] X. Zhao, X. Shang, Y. Quan, B. Dong, G.-Q. Han, X. Li, Y.-R. Liu, Q. Chen, Y.-M. Chai, C.-G. Liu, Electrodeposition-Solvothermal Access to Ternary Mixed Metal Ni-Co-Fe Sulfides for Highly Efficient Electrocatalytic Water Oxidation in Alkaline Media, Electrochim. Acta 230 (2017) 151-159. [47] W. Jaegermann, F. Ohuchi, B. Parkinson, Interaction of Cu, Ag and Au with van der Waals faces of WS, and SnS2, Surf. Sci. 201 (1988) 211-227. [48] X. Xie, R. E. McCarley, Synthesis, Structure, and Characterization of N-Ligated 19

Tungsten Selenide Cluster Complexes W (6) Se (8) L (6), Inorg. Chem. 34 (1995) 6124-6129. [49] B. Chowdari, K. Tan, W. Chia, Raman and X-ray photoelectron spectroscopic studies of lithium phosphotungstate glasses, Solid state ionics 53 (1992) 1172-1178. [50] A. Katrib, F. Hemming, P. Wehrer, L. Hilaire, G. Maire, The multi-surface structure and catalytic properties of partially reduced WO3, WO2 and WC+O2 or W+O2 as characterized by XPS, J. Electron Spectrosc. 76 (1995) 195-200. [51] T. An, Y. Wang, J. Tang, W. Wei, X. Cui, A. M. Alenizi, L. Zhang, G. Zheng, Interlaced

NiS2-MoS2

nanoflake-nanowires

as

efficient

hydrogen evolution

electrocatalysts in basic solutions, J. Mater. Chem. A 4 (2016) 13439-13443. [52] H. Zhu, J. Zhang, R. Yanzhang, M. Du, Q. Wang, G. Gao, J. Wu, G. Wu, M. Zhang, B. Liu, When cubic cobalt sulfide meets layered molybdenum disulfide: a core-shell system toward synergetic electrocatalytic water splitting, Adv. Mater. 27 (2015) 4752-4759. [53] J. Huang, D. Hou, Y. Zhou, W. Zhou, G. Li, Z. Tang, L. Li, S. Chen, MoS 2 nanosheet-coated CoS2 nanowire arrays on carbon cloth as three-dimensional electrodes for efficient electrocatalytic hydrogen evolution, J. Mater. Chem. A 3 (2015) 22886-22891. [54] S. Xu, X. Li, Z. Yang, T. Wang, W. Jiang, C. Yang, S. Wang, N. Hu, H. Wei, Y. Zhang, Nanofoaming to boost the electrochemical performance of Ni@Ni(OH)2 nanowires for ultrahigh volumetric supercapacitors, ACS Appl. Mater. Interfaces 8 (2016) 27868-27876. 20

[55] L. Yang, X. Zhu, S. Xiong, X. Wu, Y. Shan, P. K. Chu, Synergistic WO 3· 2H2O nanoplates/WS2 hybrid catalysts for high-efficiency hydrogen evolution, ACS Appl. Mater. Interfaces 8 (2016) 13966-13972. [56] Y. Lei, J. Li, Y. Wang, L. Gu, Y. Chang, H. Yuan, D. Xiao, Rapid microwave-assisted green synthesis of 3D hierarchical flower-shaped NiCo2O4 microsphere for high-performance supercapacitor, ACS Appl. Mater. Interfaces 6 (2014) 1773-1780.

21

Figure Captions Scheme

1

Schematically

synthesis

of

NiWS/CF

through

two-step

electrodeposition-hydrothermal process. Fig. 1 XRD patterns of all as-prepared samples. Fig. 2 SEM images of (a) NiO/CF, (b) WS2/CF (c) NiS2/CF and (d) NiWS/CF. Fig. 3 (a) TEM and (b) HRTEM (c) SAED and (d) mapping of NiWS/CF scraped off the sample. Fig. 4 XPS spectra of NiWS/CF: (a) XPS survey, (b) W 4f, (c) Ni 2p and (d) S 2p. Fig. 5 Electrochemical measurements for all samples in 0.5 M H2SO4. (a) Linear sweep voltammogram (LSV) curves; (b) Tafel plots; (c) Electrochemical impedance spectroscopy (EIS) results; (d) Determined double-layer capacitance (Cdl). Fig. 6 Electrochemical measurements for all samples in 1.0 M KOH: (a) LSV curves, (b) Tafel plots, (c) EIS results and (d) determined Cdl. Fig. 7 Electrochemical measurements for all samples in 0.2 M PBS: (a) LSV curves, (b) Tafel plots, (c) EIS results and (d) determined Cdl. Fig. 8 Stability tests for NiWS/CF through chronoamperometry for 12 h in (a) 0.5 M H2SO4, (b) 1.0 M KOH and (c) 0.2 M PBS. Fig. 9 (a) XRD patterns of NiWS/CF after stability test. SEM images of NiWS/CF after stability test in (b) 0.5 M H2SO4, (c) 1.0 M KOH and (d) 0.2 M PBS.

22

Scheme 1

23

Fig. 1

24

Fig. 2

25

Fig. 3

26

Fig. 4

27

Fig. 5

28

Fig. 6

29

Fig. 7

30

Fig. 8

31

Fig. 9

32

Graphical Abstract

33

Highlights 

Binary Ni-W sulfides nanosheets based on carbon fiber (NiWS/CF) have been prepared.



The electrodeposited nickel oxides film on CF provides the base for NiWS/CF.



The binary sulfides and interwoven nanosheets of NiWS/CF show better HER activity.



NiWS/CF shows excellent HER performances over a broad pH ranging from 0, 7 to 14.

34