Nitrogen-Doped Carbon@MoS2

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Hierarchical Porous Co9S8/Nitrogen-Doped Carbon@MoS2 Polyhedrons as pH-Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction Hongmei Li, Xing Qian, Chong Xu, Shaowei Huang, Changli Zhu, Xiancai Jiang, Li Shao, and Linxi Hou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b06384 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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ACS Applied Materials & Interfaces

Hierarchical Porous Co9S8/Nitrogen-Doped Carbon@MoS2 Polyhedrons as pH-Universal Electrocatalysts for Highly Efficient Hydrogen Evolution Reaction Hongmei Li, Xing Qian, Chong Xu, Shaowei Huang, Changli Zhu, Xiancai Jiang, Li Shao, Linxi Hou* College of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China *E-mail: [email protected]. Fax: +86-0591-2286 6244; Tel: +86-0591-2286 5220.

ABSTRACT: The development of highly active and stable earth-abundant electrocatalysts to reduce or eliminate the reliance on noble-metal based ones for hydrogen evolution reaction (HER) over a broad pH range remains a great challenge. Herein, hierarchical porous Co9S8/N-doped carbon@MoS2 (Co9S8/NC@MoS2) polyhedrons have been synthesized by a facile hydrothermal approach using highly conductive Co/NC polyhedrons composed of cobalt nanoparticles embedded in N-doped carbon matrices as both the structural support and cobalt source. The Co/NC polyhedrons were prepared by direct carbonization of Co-based zeolitic imidazolate framework (ZIF-67) in Ar atmosphere. Benefitting from the prominent synergistic effect of N-doped carbon enhancing the conductivity of the hybrid, MoS2 and Co9S8 providing abundant catalytically active sites as well as the well-defined polyhedral structure promoting mechanical stability, the as-synthesized Co9S8/NC@MoS2 shows excellent HER activity and good stability over a broad pH range, with onset overpotentials of 4, 38, and 45 mV, Tafel slopes of 60.3, 68.8, and 126.1 mV dec−1, and overpotentials of 67, 117, and 261 mV at 10 mA cm−2 in 1.0 M KOH, 0.5 M H2SO4, and 1.0 M phosphate buffer solution (PBS), respectively. This work provides a general and promising approach for the design and synthesis of inexpensive and efficient pH-universal HER electrocatalysts.

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ACS Paragon Plus Environment


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KEYWORDS: hydrogen evolution

reaction, Co9S8/N-doped carbon@MoS2,

polyhedrons, zeolitic imidazolate framework, synergistic effect, electrocatalysts

INTRODUCTION Along with the increasing global consumption of fossil fuels and the resulted environmental pollutions, considerable efforts have been devoted to developing high-efficiency, clean, and renewable energy sources. Among various sustainable energy sources, hydrogen is one of the most promising candidate for replacing traditional fossil fuels in the future because of its impressive energy density and environmental friendliness.1–3 Water electrolysis is regarded as an ideal approach for sustainable hydrogen production from water that has attracted significant attention, which requires highly efficient electrocatalysts for hydrogen evolution reaction (HER) to decrease the overpotential and increase energy efficiency.4,5 Typically, Pt-based materials have been proved to be the most effective catalysts for the HER. Nevertheless, their high cost and low reserves make them difficult to achieve the large-scale industrial application.6–8 Therefore, developing effective and economical substitutes to replace Pt-based catalysts for the HER is very imperative and significant. Various earth-abundant electrocatalysts with high HER activity have been widely explored in the past few years, such as transition-metal based sulfides,9–12 selenides,13,14 phosphides,15,16 carbides,17,18 nitrides,19 and so on. However, most of them can work well in an acidic or alkaline medium. In view of the inevitable proton concentration change during the HER process, the ideal catalyst should perform equally well in different pH conditions so that the water splitting process can be more energy-efficient.20,21 Unfortunately, this still remains a great challenge. As a typical transition metal sulfide (TMS) material, MoS2 has been considered as a promising catalyst for electrochemical HER due to its unique characteristics, such as low cost, good chemical stability, and outstanding electrocatalytic properties.22–24 However, the poor conductivity and serious aggregation largely limit the HER activity of MoS2. 2


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Recently, it was demonstrated that the catalytic activity of MoS2 could be remarkably enhanced by incorporating MoS2 with carbon-based conductive materials to form MoSx-based hybrid materials, which can not only improve the conductivity, but also protect the MoS2 from aggregating.25–27 Relative to the MoS2, cobalt sulfide (Co9S8) is also a transition metal sulfide with relatively higher electrical conductivity but exhibits inferior electrochemical performance and stability for HER, which is possibly related to its sluggish ion transport kinetics.28,29 In this respect, the combination of Co9S8 and MoS2 into rationally designed hybrid architectures could provide synergistic advantages, which demonstrate overall structural merits over the individual component.30,31 Up to now, there are very few reports on the synergetic effects of the MoS2–Co9S8 system for electrocatalytic HER. Moreover, the synthesis of multicomponent nanomaterials with novel polyhedral structures show unique and fascinating properties seems still challenging. Nanostructured carbons are known to possess many unique characteristics, such as high conductivity, high chemical stability and large surface area, have been widely utilized as substrates to hybridize with other active materials (such as MoS2) for various applications.32,33 For instance, Dai et al. developed a simple solvothermal approach for the synthesis of MoS2 nanoparticles on reduced graphene oxide (RGO) sheets, which displayed excellent electrochemical performances as advanced electrode materials for the HER.9 Wang et al. reported that a networked MoS2/CNT nanocomposite was successfully fabricated via a facile solvothermal strategy, which showed high HER activity.34 However, most of MoS2-based electrocatalysts can only work in acidic conditions and lack intrinsic catalytic activity and stability in neutral or alkaline medium. Accordingly, developing an effective strategy to prepare pH-universal MoS2-based catalysts is highly desirable and significant. In recent years, metal–organic frameworks (MOFs) have emerged as an ideal precursor or sacrificial template to construct porous functional materials.35–38 In particular, the Co-based zeolitic imidazolate framework (ZIF-67) created by bridging cobalt ions and N-containing imidazolate linkers through coordination bonds, have been used for the 3



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synthesis of a variety of Co-containing carbon-based electrocatalysts for the HER, OER or ORR process,39–42 and their high catalytic activity may depend on the following aspects: First, the metallic nanoparticles cores and the carbon shells in these materials synergistically enhance the electrocatalytic processes involved in the reactions.43 Second, carbon nanomaterials act as shells doped with heteroatoms can endow their unique electron structures and lower local work function.44 Furthermore, the carbon shells could effectively avoid the aggregation and dissolution of the nanoparticles cores, which is conductive to improve the stability of the material during electrochemical processes. Notwithstanding these advantageous structural features, to the best of our knowledge, only few materials derived from ZIF-67 have been reported for catalyzing HER, such as PNC/Co,45 CoP,46 NiCoP,47 CoSe2@DC,48 and [email protected] In addition, the combination of MoS2 with well-defined polyhedral structure comprising the metal chalcogenides nanoparticles encapsulated within N-doped carbon matrices which is expected to further enhance the HER activity in a broad pH range is not studied yet. Herein, we report a facile hydrothermal method to synthesize the nanostructured Co9S8/NC@MoS2 polyhedrons using highly conductive Co/NC polyhedrons derived from ZIF-67 as both the substrate and cobalt source. In this system, the as-prepared Co9S8/NC@MoS2 hybrid well inherits the morphology of ZIF-67 and exhibits a regular polyhedral structure. On the one hand, Co/NC polyhedrons could greatly improve the conductivity of the hybrid and act as the precursor for the in situ formation of Co9S8 nanoparticles, thereby facilitating the charge transfer at the electrode/electrolyte interface. On the other hand, thanks to the highly porous and activity surface of Co/NC polyhedrons, the nucleation and growth of MoS2 nanocrystals on Co/NC polyhedrons can be easily realized without any treatment of Co/NC or assistance of surfactant, thus significantly increasing the number of active sites or exposed edges for the HER process. This novel Co9S8/NC@MoS2 hybrid not only acts as a pH-universal electrocatalyst for HER by bringing together the intrinsic performance of individual MoS2 and Co9S8, but also shows impressive enhanced HER 4


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activity in comparison with pure Co9S8/NC, Co/NC, MoS2, and NC. As expected, the resulting Co9S8/NC@MoS2 polyhedron shows outstanding catalytic activity and stability towards HER, with a very low onset overpotential of 4 mV, a small Tafel slope of 60.3 mV dec−1, and an overpotential of 67 mV at 10 mA cm−2 in 1.0 M KOH. Additionally, this electrode provides good catalytic performance and stability in acidic and neutral conditions.

EXPERIMENTAL SECTION Chemicals and Reagents. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, Aladdin Chemistry Co., Ltd, AR), 2-methylimidazole (2-mIM, Aladdin Chemistry Co., Ltd, 98%), methanol (CH3OH, Tianjin Zhiyuan Chemical Reagent Co., Ltd, ≥99.5%), absolute ethanol (CH3CH2OH, Tianjin Zhiyuan Chemical Reagent Co., Ltd, ≥99.7%), potassium hydroxide (KOH, Aladdin Chemistry Co., Ltd, GR), sodium molybdate dihydrate (Na2MoO4·2H2O, Aladdin Chemistry Co., Ltd, AR), L-cysteine (Aladdin Chemistry Co., Ltd, 99%), Commercial Pt/C (Shanghai Hesen Electric Co., Ltd, 20 wt%) and Nafion solution (Sigma-Aldrich, 5 wt%) were used directly without any further purification. The deionized water used throughout all the experiments was purified using a Millipore system. Synthesis of ZIF-67 Nanocrystals. Typically, 4 mmol (1.1642 g) of Co(NO3)2·6H2O and 16 mmol (1.3136 g) of 2-methylimidazole were dissolved in 100 mL of methanol, respectively. Then, the latter clear solution was quickly poured into the former pink solution under vigorous stirring. The well mixed solution was aged without stirring for 24 h at room temperature. Finally, the final purple precipitate was collected by centrifugation and washed with absolute ethanol for three times, and then dried at 50 °C under vacuum for 12 h. Synthesis of Co/NC Polyhedrons. Typically, the obtained ZIF-67 powder was put into a tube furnace and then was annealed in Ar at 800 °C for 2 h with a heating rate of 2 °C min–1. After the furnace was allowed to cool to ambient temperature, the 5



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product was taken out and its color was found to change from purple to black. Synthesis of NC Polyhedrons. Typically, the obtained Co/NC polyhedron was dispersed in 2.0 M H2SO4 and stirred for 24 h at 80 °C to remove Co nanoparticle. After being cooled naturally, the resulting product was collected by centrifugation, washed thoroughly with deionized water and ethanol, and dried under vacuum at 50 °C for 12 h. Synthesis of Co9S8/NC@MoS2 Polyhedrons. Typically, 60 mg of Co/NC polyhedron was first ultrasonically dispersed in 60 mL of deionized water. Then 121 mg (0.5 mmol) of sodium molybdate dihydrate (Na2MoO4·2H2O) was added into the above solution and stirred to form a suspension. After adding 606 mg (5 mmol) of L-cysteine, the mixture was then poured into Teflon-lined stainless steel autoclave of 100 mL volume and heated at 200 °C in an air oven for 24 h. After cooling naturally to ambient temperature, the resulting black product was centrifuged and washed respectively with deionized water and ethanol three times. Finally, after being dried overnight, the product was annealed in Ar at 600 °C for 2 h with a heating rate of 5 °C min–1 to improve the crystallinity of MoS2. For comparison, the Co9S8/NC polyhedrons and MoS2 nanoflowers were also synthesized under the same conditions without addition of sodium molybdate dihydrate and Co/NC polyhedrons, respectively. Materials Characterization. The crystal structures of the samples were characterized by X-ray diffraction (XRD, X’Pert PRO, Cu Kα, λ = 0.1542 nm). Raman spectra was performed on a Raman spectrometer (Renishaw, inVia Reflex) with a 532 nm excitation laser. The morphologies and energy dispersive X-ray spectroscopy (EDS) mapping of the samples were recorded by a Hitachi S-4800 scanning

electron

microscope

(SEM)

equipped

with

an energy-dispersive

spectrometer. Transmission electron microscope (TEM) and HRTEM images were acquired on FEI Tecnai G2 F20. The X-ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALAB 250 spectrophotometer with Mg-Kα radiation. Nitrogen adsorption–desorption isotherms and the Brunauer-Emmett-Teller (BET) 6


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surface area of samples were evaluated by using an ASAP 2020M analyzer. Electrochemical Measurements.

All electrochemical measurements

were

conducted using a CHI660E electrochemical workstation in a typical three-electrode setup, with a glass carbon electrode (GCE, 3 mm in diameter) coated with the catalyst ink as the working electrode, a graphite rod as the counter electrode and an Ag/AgCl (sat. KCl) as the reference electrode. All the potentials were calibrated with respect to a reversible hydrogen electrode (RHE). In 1.0 M KOH (pH = 14), E(RHE) = E(Ag/AgCl) + 1.059 V. In 0.5 M H2SO4 (pH = 0), E(RHE) = E(Ag/AgCl) + 0.232 V. In 1.0 M PBS (pH = 7), E(RHE) = E(Ag/AgCl) + 0.646 V. Linear sweep voltammetry (LSV) measurements were conducted in electrolyte with a scan rate of 5 mV s−1. Corresponding Tafel curves were calculated from the LSV data. Electrochemical impedance spectroscopy (EIS) measurements were performed at an overpotential of 150 mV with a frequency range from 0.1 Hz to 100 kHz and an AC amplitude of 5 mV. Cyclic voltammetry was used to estimate the electrochemical double layer capacitance (Cdl) in non-faradaic region from 0.1 to 0.2 V vs. RHE with different scan rates (10–140 mV s−1). The Cdl was estimated by plotting j = janodic – jcathodic at 0.15 V vs. RHE against the scan rate. The linear slope is twice the Cdl. Electrochemical stability was measured using cyclic voltammetry (CV) sweeps at 100 mV s−1 between −0.4 and −0.1 V vs. RHE for 2000 cycles. The current density−time (I−t) curves were carried out at overpotentials of 67, 117 and 261 mV in 1.0 M KOH, 0.5 M H2SO4 and 1.0 M PBS, respectively. All data were corrected using iR compensation. The catalyst ink was prepared by dispersing 2 mg of catalyst into 0.5 mL of solution containing 460 µL of water/ethanol (v/v = 4:1) and 40 µL of 5 wt% Nafion solution, followed by sonicated for at least 30 min to form a homogenous ink solution. Then 5 µL of the catalyst ink was dropped onto the glassy carbon electrode and dried at room temperature. (loading: ~0.283 mg cm–2).

RESULT AND DISCUSSION 7



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Synthesis of Co9S8/NC@MoS2. The synthetic process of the Co9S8/NC@MoS2 polyhedrons is schematically illustrated in Figure 1. Firstly, highly uniform ZIF-67 nanoparticles with polyhedral morphology can be easily obtained by a room-temperature precipitation method in the existence of a cobalt salt and 2-methylimidazole (2-mIM) in methanol, which acted as the sacrificial template. Then, the ZIF-67 nanoparticles were transformed into Co/NC hybrids through one-step calcination treatment in Ar atmosphere at 800 °C for 2 h. During this process, the Co2+ species in ZIF-67 were converted into metallic Co nanoparticles, while the organic ligand (2-mIM) was concurrently carbonized to N-doped carbon that wraps the Co nanoparticles. Finally, the Co9S8/NC@MoS2 polyhedrons were synthesized by a facile hydrothermal process. It should be noted that the Co/NC polyhedrons played a dual role in this hydrothermal reaction, which not only acted as the structural support to guide the growth of MoS2 nanocrystals, but also provided the cobalt source to produce Co9S8 nanoparticles. During this step, L-cysteine was decomposed in the aqueous solution to release H2S, which acted as both the sulfur source and the reducing agent for the formation of MoS2 nanocrystals. Also, multifunctional groups (–SH, –NH2 and –COOH) of L-cysteine are in favor of the conjugation of metallic ions and the formation of the crystal nucleation (eqn (1)). Meanwhile, H2S reacted with the Co nanoparticles to in situ form Co9S8 nanoparticles (eqn (2)).50 MoO42– + 3H2S → MoS2 + 3H2 + SO42–

(1)

9Co + 8H2S → Co9S8 + 8H2

(2)

8


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Figure 1. Schematic illustration of the formation process of Co9S8/NC@MoS2 polyhedrons.

Morphologies and Compositions. Figure 2, Figure S1 (Supporting Information, SI), and Figure S2 show the X-ray diffraction (XRD) patterns of the as-synthesized samples. It can be seen that all the diffraction peaks in the XRD pattern (Figure S1) match well with the phase-pure ZIF-67, which is in agreement with previous reports.36 The XRD pattern of the MoS2 is shown in Figure 2 (the olive curve), where the main diffraction peaks centered at around 14.3º, 33.5º, 39.8º, and 58.2º can be respectively assigned to the (002), (101), (103), and (110) planes of hexagonally structured MoS2 (PDF No. 09-0312). After the calcination treatment of ZIF-67 (the purple curve), the characteristic diffraction peak at 25.8º is attributed to the (002) plane of graphitic carbon (PDF No. 23-0064), while the other three peaks located at 44.3º, 51.6º, and 75.9º can be ascribed to characteristic (111), (200), and (220) reflections of metallic Co (PDF No. 89-4307). This result reflects the metallic Co nanoparticles were successfully embedded in the graphitic carbon matrix. In contrast, the NC exhibits two broad peaks at 25.8º and 43.1º, which correspond to the (002) and (101) planes of graphitic carbon (Figure S2). These observations suggest the successful conversion of Co/NC into NC after being etched with acid. After hydrothermal treatment in L-cysteine solution, the metallic Co nanoparticles are converted into Co9S8 nanoparticles, as evidenced from the associated XRD pattern (the blue curve), in which the main peaks at 30.1º, 31.4º, 36.4º, 39.7º, 47.6º, 52.2º, and 54.6º can be indexed to (311), (222), (400), (331), (511), (440), and (531) planes of cubic Co9S8 (PDF No. 03-0631). Besides, the two broad peaks located at 26.2º and 44.5º corresponding to the (002) and (101) planes of graphitic carbon. Note that the characteristic diffraction peaks of metallic Co are not detected in the XRD pattern of Co9S8/NC, indicating that metallic Co nanoparticles were completely converted into cubic Co9S8 nanoparticles after sulfurization. When sodium molybdate was further added during the synthesis process, the peaks in the XRD pattern (the red curve) confirm the coexistence of graphitic carbon, Co9S8, and MoS2 phases in the hybrid. 9


C (101)

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(531)

Co9S8/NC@MoS2

Co/NC (110)

(103)

(220)

Co9S8/NC

(200)

C (101) (511) (440)

(400)

(331) (111)

C (002) C (002) (311) (222) (101)

(002)

Intensity (a.u.)

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C (002)


MoS2

Co9S8 PDF No. 03-0631

Co PDF No. 89-4307 MoS2 PDF No. 09-0312

10

20

30

40 50 2θ (degree)

60

70

80

Figure 2. XRD patterns of Co9S8/NC@MoS2, Co9S8/NC, Co/NC, and MoS2.

Raman spectroscopy was further applied to investigate the composition and the graphitization degree of the as-synthesized samples. As shown in Figure S3 (SI), two characteristic peaks are observed in the Raman spectra of Co9S8/NC@MoS2 and MoS2 at 375 and 402 cm–1, corresponding to the E12g and A1g vibrational modes of the hexagonal MoS2, respectively. Clearly, it can be observed that E12g vibrational mode possesses a larger peak width and weaker peak intensity compared with A1g, suggesting that the crystal structure of MoS2 may contain substantial defect sites, which would be beneficial for achieving better HER activity.51 The characteristic peak at around 662 cm–1 in the Raman spectrum of Co9S8/NC@MoS2 and Co9S8/NC can be indexed to Co9S8.28,52 In addition, the Co9S8/NC@MoS2, Co9S8/NC, Co/NC, and NC exhibit two typical peaks at the bands of ~1340 and ~1572 cm−1. Generally, the former is associated with sp3-hybridized disordered carbon or defective carbon (D-band), and the latter can be ascribed to the sp2-hybridized graphitic carbon (G-band). The relative intensity of ID/IG reflects the ratio of disordered carbon to graphitic carbon.48 The ID/IG values of Co9S8/NC@MoS2, Co9S8/NC, Co/NC, and NC are calculated to be about 0.98, 0.99, 1.05, and 0.99, respectively, indicating that the content of disordered carbon are almost same to that of graphitic carbon in the four 10


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hybrids and the graphitization degree of the hybrid remains unchanged during the hydrothermal process.

The morphologies of the as-synthesized samples were investigated by field-emission scanning electron microscopy (FESEM). As shown in Figure 3a and 3b, the as-prepared ZIF-67 particle shows a regular polyhedral morphology with high uniformity, smooth surface and average size of about 800 nm. After the calcination treatment at 800 °C in Ar atmosphere for 2 h, FESEM observations exhibit that the sample (Co/NC) well inherits the polyhedral morphology and uniformity of ZIF-67 but with a rough surface constructed by some tiny nanowhiskers (Figure 3c and 3d), which could provide sufficient active surface for the nucleation and growth of MoS2 nanocrystals. Meanwhile, it can be seen that each rhombic face of the sample appears little dimensional shrinkage compared to ZIF-67. This reason might be associated with the slight loss of organic components in ZIF-67 during the annealing process. Correspondingly, the polyhedral shape of Co/NC was perfectly retained in the NC after being etched by acid (Figure S4a and S4b). After sulfurization in L-cysteine solution, the Co9S8/NC polyhedron was successfully synthesized with similar diameter and polyhedral morphology as that of Co/NC (Figure 3e). The FESEM image recorded at high magnification (Figure 3f) clearly exhibits that the surface of Co9S8/NC becomes relatively smooth. During this process, metallic Co nanoparticles are converted into Co9S8 nanoparticles, which can be ascribed to the fact that the outward diffusion of cobalt cations from the core and subordinate inward transport of sulfur anions from the shell phase.36 When sodium molybdate was further added in L-cysteine solution, the FESEM image in Figure 3g reveals that the polyhedral morphology is well maintained during the hydrothermal process. Furthermore, it can be clearly observed that many wrinkled textures are attached on the outer surface of the polyhedron and nanowhiskers on the surface become nanoparticles due to the formation of MoS2 nanocrystals (Figure 3h and 3i), resulting in the sample has a rougher surface 11



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than that of the precursor (Co/NC) particle, which can provide abundant exposed active edges on its surface then finally enhance activity of electrocatalysis. In addition, the FESEM image of the MoS2 is shown in Figure S4c (SI), it can be seen that the surface of the particle is composed of the thin sheet-like structure with particle size of 200–300 nm.

Figure 3. SEM images of the (a and b) ZIF-67, (c and d) Co/NC, (e and f) Co9S8/NC, (g, h and i) Co9S8/NC@MoS2.

The energy-dispersive X-ray spectroscopy (EDS) elemental mapping images (Figure S5) reveal the uniform distribution of C, N, S, Co, and Mo elements throughout all the polyhedrons. Compared with C, N, and Co elements, Mo and S elements tend to distribute on the outer surface of these polyhedrons and exhibit much higher signal intensities, indicating the polyhedral structure with Mo and S in the shell as well as C, N, and Co in the core. The EDS result (Figure S6) exhibits that the atomic ratio of the C, N, S, Co, and Mo is about 16.7:2.0:50.9:11.3:19.1. It has been reported that the appropriate synergistic effect between different components and the well-defined polyhedral structure can play a vital role in enhancing HER activity.48 Additionally, nitrogen doping has been proved effective in improving catalytic 12


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activity and electrical conductivity of carbon-based materials.44 As a consequence, we believe that the Co9S8/NC@MoS2 hybrid could be a promising and efficient electrocatalyst for the HER. Transmission electron microscopy (TEM) characterizations were used to further investigate the detailed structure of Co9S8/NC@MoS2. As shown in Figure 4a, it can be clearly seen that Co9S8/NC@MoS2 hybrid has a well-defined polyhedral morphology with the diameter of about 800 nm, well in agreement with the above FESEM observations. In addition, this polyhedral structure is highly porous and that a number of tiny nanoparticles (dark dots) are uniformly encapsulated within the carbon matrices (grey region) (Figure 4b and Figure S7). Further HRTEM image (Figure 4c) exhibits that individual Co9S8 nanoparticle with a small size of around 8 nm is wrapped in the graphitic carbon layers, and the lattice fringes of 0.34 nm and 0.35 nm are respectively assigned to the (002) crystal plane of graphitic carbon and the (220) crystal plane of cubic Co9S8. The HRTEM image in Figure 4d reveals that the MoS2 nanocrystals present a layered crystal lattice structure with an interlayer spacing of 0.64 nm, consistent with the (002) crystal plane of hexagonal MoS2. Besides, it is found that the MoS2 nanocrystals consist of 3–5 layers are distributed on the polyhedron surface, which can provide abundant active edge sites for the HER.

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Figure 4. (a) TEM image and (b, c, and d) HRTEM images of Co9S8/NC@MoS2. We further carried out the X-ray photoelectron spectroscopy (XPS) measurements to evaluate the chemical compositions and elemental valence states of the Co9S8/NC@MoS2 and Co9S8/NC. The survey spectra in Figure 5a confirm the existence of C, N, S, Co, and O elements in both Co9S8/NC@MoS2 and Co9S8/NC. The presence of O element in the two samples is probably due to exposure of the samples to air, while the existence of Mo element in Co9S8/NC@MoS2 can be attributed to the formation of the MoS2 on the surface of the Co9S8/NC@MoS2 polyhedron. The C 1s spectra in Figure 5b exhibits that four peaks with binding energies of 284.6, 284.9, 285.8, and 288.1 eV for Co9S8/NC@MoS2 and Co9S8/NC can be observed, which should be related to the C=C–C, C–S, C–N/O, and C=O bonds, respectively. The observed peaks associated with C–S and C–N moieties should be owing to the substitution of C atom with S and N atoms in the hybrid.52 Figure 5c shows the N 1s spectra, which can be fitted into four peaks corresponding to the Mo 3p (394.9 eV) and three different type of N species including pyridinic (398.7 eV), pyrrolic (400.1 eV), and graphitic (401.0 eV) N species for Co9S8/[email protected] The nitrogen contents of Co9S8/NC@MoS2 and Co9S8/NC are 2.28 and 2.31 %, respectively, according to XPS results (Table S1). This is due to the decomposition of 14


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organic ligands (2-mIM) with abundant nitrogen during the high temperature calcination. Meanwhile, it can be found that pyridinic N and graphitic N are the dominant species, which are beneficial to enhance the activity for the HER.41 In the S 2p spectra (Figure 5d), the peaks located at 161.7 and 163.3 eV are assigned to the S 2p3/2 and S 2p1/2 of the Co9S8, respectively. The other two peaks at 162.7 and 164.3 eV are attributed to the S 2p3/2 and S 2p1/2 orbitals of divalent sulfide ions (S2–) in MoS2. In addition, the peak at 168.7 eV corresponding to the SO42– species, and the presence of SO42− manifests the sulfur species on the material’s surface is partly oxidized by air.31 The negative shift of the S 2p3/2 (161.5 eV) and S 2p1/2 (162.6 eV) in Co9S8/NC compared to that of Co9S8/NC@MoS2 (161.7 eV in S 2p3/2 and 163.3 eV in S 2p1/2) could further prove the presence of Mo–S in Co9S8/NC@MoS2. Meanwhile, the peak at 163.7 eV in Co9S8/NC is associated with the C–S–C species, suggesting that a certain amount of sulfur is doped into the lattice of carbon matrix.28 Figure 5e presents the Co 2p XPS spectra, where two spin-oribit doublets and two shakeup satellites (identified as “Sat.”) can be observed. The binding energy located at 779.2 eV in Co 2p3/2 and 794.2 eV in Co 2p1/2 for Co9S8/NC@MoS2 corresponds to the spin-orbit characteristics of Co3+, while those at 781.1 eV in Co 2p3/2 and 796.8 eV in Co 2p1/2 are assigned to Co2+.53 Compared to Co9S8/NC, the Co 2p peaks of Co9S8/NC@MoS2 slightly shift toward the lower binding energies, indicating that a strong interaction between the Co9S8 and MoS2. For the Mo 3d (Figure 5f), two peaks at binding energies of 232.0 and 228.8 eV could be respectively ascribed to the Mo 3d3/2 and Mo 3d5/2 core levels of Mo4+, further confirming the formation of MoS2 in the hybrid. The nearby S 2s peaks can be fitted into two peaks at 224.7 and 226.2 eV, corresponding to the two chemical states of S species bonding with Mo and Co ions (Mo–S and Co–S). Meanwhile, the two weak peaks located at 233.1 and 235.6 eV are attributed to the MoO3 because of slight surface oxidation.54

15



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Figure 5. XPS spectra of the Co9S8/NC@MoS2 and Co9S8/NC: (a) survey spectra, (b) C 1s spectra, (c) N 1s spectra, (d) S 2p spectra, and (e) Co 2p spectra of the Co9S8/NC@MoS2 and Co9S8/NC. (f) Mo 3d spectra of the Co9S8/NC@MoS2.

Brunauer−Emmett−Teller Analysis. The specific surface area and porosity of the as-synthesized samples were investigated by measuring the N2 adsorption–desorption isotherm. As shown in Figure 6a and Figure S8a (SI), all the samples display a type-IV curve with a distinct hysteresis loop, indicating the presence of mesopores structures in these samples.55 The specific surface area of NC is 311.2 m2 g–1, which is larger than that of Co/NC (298.1 m2 g–1), Co9S8/NC (276.7 m2 g–1), Co9S8/NC@MoS2 (189.2 m2 g–1), and MoS2 (92.7 m2 g–1). Correspondingly, the pore size distribution calculated using the Barrett–Joyner–Halenda (BJH) method reveals that these samples 16


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are mesoporous composites (Figure 6b and Figure S8b). Notably, the NC shows relatively broad pore size distribution in the range of 2–10 nm in comparison with other samples (2–5 nm), which might be associated with the dissolution of Co nanoparticles under strong acid condition. Meanwhile, the BJH desorption pore volumes of NC, Co/NC, Co9S8/NC, Co9S8/NC@MoS2 and MoS2 are 0.50, 0.34, 0.50, 0.23 and 0.35 cm3 g–1, respectively (pore diameter from 1.7 nm to 300 nm). The decreased surface area and pore volume of the as-synthesized Co9S8/NC@MoS2 compared with the Co/NC may be due to the pore volume of polyhedrons is occupied or covered by MoS2 nanocrystals during the hydrothermal process. Such a high specific surface area of Co9S8/NC@MoS2 with massive mesopores is of great importance for catalytic application, which would be beneficial to the exposure of

3 -1

Quantity adsorbed (cm g STP)

active sites and rapid transportation of HER-relevant species.

300

(a)

250

Co9S8/NC@MoS2 Co9S8/NC

200

Co/NC MoS2

150 100 50 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

0.25

3 -1

-1

Pore volumn (cm g nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 0.20 Co9S8/NC@MoS2

0.15

Co9S8/NC Co/NC MoS2

0.10 0.05 0.00 1

10

100

Pore diameter (nm)

Figure 6. (a) N2 adsorption–desorption isotherms of Co9S8/NC@MoS2, Co9S8/NC, 17



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Co/NC, and MoS2. (b) Corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curves.

HER performance of Co9S8/NC@MoS2. Figure 7a exhibits the polarization curve of Co9S8/NC@MoS2 in 1.0 M KOH with a scan rate of 5 mV s–1. For comparison study, the Co9S8/NC, Co/NC, MoS2, NC, and commercial Pt/C were also tested for HER. The corresponding data are summarized in Table S2. As expected, the Pt/C exhibits outstanding HER performance with an onset potential (η0) nearly 0 mV (vs. RHE) at 1 mA cm−2. Remarkably, the Co9S8/NC@MoS2 displays a very low onset potential of 4 mV for the HER, and further negative potential causes a sharp increase of cathodic current, corresponding to catalytic H2 evolution. Comparatively, the Co9S8/NC, Co/NC, MoS2, and NC give inferior HER performance with larger η0 of 34, 92, 146, and 218 mV, respectively. To achieve a current density of 10 mA cm−2, the Co9S8/NC@MoS2 requires an overpotential (η10) of only 67 mV which is much smaller than those of Co9S8/NC (121 mV), Co/NC (194 mV), MoS2 (270 mV), and NC (406 mV). Notably, the lowest overpotential for Co9S8/NC@MoS2 indicates that it possesses superior catalytic activity for HER in alkaline medium, suggesting the unique synergic effects of Co9S8, MoS2 and N-doped carbon matrices in this well-defined polyhedral structure; it is also very close to that of commercial Pt/C and better than most of non-noble metal HER catalysts in alkaline medium (Table S5), such as NiCo2S4 NW/NF (η10 = 210 mV),56 CoP nanowires (η10 = 210 mV),16 CoS2/CoSe2 (η10 = 80 mV),57 Co@N-C (η10 = 210 mV),58 NiS2@MoS2/rGO (η10 = 110 mV),59 and so on, showing its superiority as a HER catalyst having large potential in alkaline conditions. Based on many previous reports and the above test results, MoS2 is only active for the HER in acidic media, and it is not stable and exhibits poor activity under strong alkaline media.54 Relative to the pure MoS2, the HER activity is remarkably improved on Co/NC, which may be associated with the synergistic effect between the N-doped carbon matrices and the metallic Co nanoparticles. By contrast, the HER activity of Co9S8/NC is superior to that of Co/NC due to the successful phase 18


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transformation from Co to Co9S8. It is worth noting that by combining with MoS2, the Co9S8/NC@MoS2 shows outstanding HER performance because of the remarkable increase of active sites for the HER. This may benefit from the sufficient coverage of MoS2 nanocrystals on the surface of Co9S8/NC. The HER kinetics of the above catalysts were verified by studying the corresponding Tafel slopes, and a lower Tafel slope would lead to a faster increment of HER rate with increasing overpotential. The linear portions of the Tafel plots were fitted to the Tafel equation (ƞ = b log j + a, where b is the Tafel slope and j is the current density). As shown in Figure 7b, the Co9S8/NC@MoS2 delivers a Tafel slope of ~60.3 mV dec−1, suggesting that the corresponding HER abides by a Volmer– Heyrovsky mechanism, and the electrochemical desorption process could be the rate-limiting step.60 It can be found that the Tafel slope of Co9S8/NC@MoS2 is larger than that of Pt/C (36.1 mV dec−1) but obviously smaller than those of Co9S8/NC (79.8 mV dec−1), Co/NC (94.4 mV dec−1), MoS2 (114.5 mV dec−1), and NC (183.3 mV dec−1). Owing to the Tafel slope is directly associated with the reaction kinetics of electrocatalysts, the lower Tafel slope for Co9S8/NC@MoS2 implies it has faster catalytic kinetics and higher catalytic activity toward HER as compared with other samples. Moreover, the exchange current density (j0) of the electrocatalysts was calculated by extrapolating the Tafel plots to 0 V vs. RHE (Figure S9), which reflects the intrinsic catalytic activity of the electrode material under equilibrium conditions.61 The j0 value of Co9S8/NC@MoS2 is calculated to be 0.73 mV cm−2, which is larger than those of Co9S8/NC (0.33 mV cm−2), Co/NC (0.08 mV cm−2), MoS2 (0.04 mV cm−2), and NC (0.02 mV cm−2). The

favorable

kinetics

was

investigated

by

electrochemical

impedance

spectroscopy (EIS) at an overpotential of 150 mV (Figure 7c). The charge transfer resistance

(Rct)

is

associated

with

the

electrocatalytic

kinetics

at

the

electrocatalyst/electrolyte interface, and a smaller value indicates a faster electron transfer capacity.48 The electrochemical impedance spectra shows that the Rct value of Co9S8/NC@MoS2 is 40.2 Ω, much smaller than the values of 75.7, 178.1, 309.2, 19



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438.4 Ω for Co9S8/NC, Co/NC, MoS2, and NC, respectively, confirming that Co9S8/NC@MoS2 possesses a fast electron transfer and favorable HER kinetics at the electrocatalyst/electrolyte interface. In addition, to better understand their HER performance trend and intrinsic activities, the electrochemically active surface area (ECSA) of each catalyst should be measured and compared. Typically, the double-layer capacitances (Cdl) is linearly proportional to the ECSA, which can be estimated by measuring cyclic voltammograms with different scan rates in a non-faradaic region from 0.1 to 0.2 V vs. RHE (Figure S10).43,62 The halves of the differences between positive and negative current density at the center of the scanning potential ranges are plotted versus the scan rates in Figure 7d.63 The calculated Cdl value for Co9S8/NC@MoS2 is 22.98 mF cm−2, much higher than those of Co9S8/NC (13.84 mF cm−2), Co/NC (6.38 mF cm−2), MoS2 (2.40 mF cm−2), and NC (0.98 mF cm−2). These prove that Co9S8/NC@MoS2 exposes more effective active sites at the solid–liquid interface, resulting in higher electrochemical HER activity. Durability is another vital criterion to assess the property of a catalyst. The stability of Co9S8/NC@MoS2 was first tested by conducting CV with a potential range from −0.4 to −0.1 V (vs. RHE) at a scan rate of 100 mV s−1 in 1.0 M KOH (Figure 7e). The Co9S8/NC@MoS2 electrode exhibits an overpotential increase of only 3 mV after 2000 CV cycles to afford a current density of 10 mA cm−2. In addition, the long-term durability of the Co9S8/NC@MoS2 for the HER was evaluated by electrolysis at a fixed overpotential of 67 mV. Apparently, the cathodic current density remains almost the same over 12 h of electrolysis (Figure 7f), reflecting the outstanding stability of Co9S8/NC@MoS2 during the HER process. The excellent stability is probably associated with the good compositional and structural stability of Co9S8/NC@MoS2. XRD pattern and FESEM image on the Co9S8/NC@MoS2 electrode after durability test of 12 h exhibited that the composition and polyhedral structure remain similar to that of the electrode before the test (Figure S11 and S12a). In addition, we also performed XPS measurements to evaluate the chemical stability of the Co9S8/NC@MoS2 after durability tests (Figure S13). The survey spectra exhibit that 20


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the chemical compositions of Co9S8/NC@MoS2 are no change after durability tests. Note that the peak intensity of Co2+ species is increased, suggesting that a part of Co3+ in Co9S8/NC@MoS2 were reduced to Co2+ species after durability tests, which is in accordance with the previous reports.45 It can be found that the XPS spectra of C, N, S, and Mo elements have no obvious change as compared with the original results, indicating that the C, N, S, and Mo elements have the same valence states after durability tests. These results indicate that the Co9S8/NC@MoS2 polyhedrons are cathodically stable in the harsh alkaline environment.

Figure 7. (a) Polarization curves and (b) corresponding Tafel plots for the Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, NC, and Pt/C in 1.0 M KOH solution at a scan rate of 5 mV s−1. (c) Nyquist plots of the Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC at an overpotential of 150 mV and the corresponding equivalent circuit 21



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(inset). (d) The Cdl of the different samples obtained at 0.15 V vs. RHE. (e) Polarization curves of the Co9S8/NC@MoS2 before and after 2000 CV cycles with a potential range from −0.4 V to −0.1 V vs. RHE in 1.0 M KOH solution. (f) The current density−time (I−t) curve of Co9S8/NC@MoS2 in 1.0 M KOH solution at an overpotential of 67 mV for 12 h.

In addition to performing well as an electrocatalyst in alkaline solution for the HER, the Co9S8/NC@MoS2 was also found to be an active and stable electrocatalyst in acidic and neutral solution. Similarly, the Co9S8/NC, Co/NC, MoS2, NC, and commercial Pt/C were also tested for comparison under same condition. Figure 8a presents the polarization curves of all these samples in 0.5 M H2SO4 solution. The corresponding parameters are listed in Table S3. Overpotentials of 38 mV and 117 mV are required for Co9S8/NC@MoS2 to reach current densities of 1 mA cm−2 (η0) and 10 mA cm−2 (η10), respectively, revealing good catalytic activity of Co9S8/NC@MoS2 for H2 evolution in acid medium. Under the same conditions, these overpotentials are much lower than those of the Co9S8/NC (η0 = 84 mV and η10 = 184 mV), Co/NC (η0 = 117 mV and η10 = 237 mV), MoS2 (η0 = 208 mV and η10 = 347 mV), and NC (η0 = 287 mV and η10 = 474 mV). According to the detailed comparison in Table S6, these overpotentials are also comparable to most of the reported values for non-noble metal HER catalysts under acidic conditions, including CoP/CNT (η10 = 122 mV),61 Mo2C@CNTs (η10 = 152 mV),64 CoMoS3 (η10 = 171 mV),10 and CoS2/RGO-CNT (η10 = 142 mV).65 Accordingly, the Tafel plots of the above samples are shown in Figure 8b. Here, the Tafel slope (29.9 mV dec−1) of Pt/C is in good agreement with the reported results.66 As compared with Co9S8/NC (84.3 mV dec−1), Co/NC (111.3 mV dec−1), MoS2 (125.6 mV dec−1), and NC (189.2 mV dec−1), the Co9S8/NC@MoS2 exhibits a smaller Tafel slope of 68.8 mV dec−1, indicating it has faster kinetics for HER. In addition, the Rct values of Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC are 83.2, 125.7, 204.7, 316.5, and 502.7 Ω, respectively (Figure S14). Remarkably, the Co9S8/NC@MoS2 still exhibits good stability and durability even in such acid 22


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medium. After 2000 continuous CV cycles in 0.5 M H2SO4, the polarization curves of Co9S8/NC@MoS2 show little loss, and the overpotential slightly changes from 117 mV to 127 mV at 10 mA cm−2 (Figure 8c). Furthermore, the time-dependent current density curve at a static overpotential of 117 mV vs. RHE indicates Co9S8/NC@MoS2 can retain its catalytic activity for at least 12 h (Figure 8d). In addition, a post-mortem study

exhibited

that

the

structure

and

polyhedral

morphologies

of

the

Co9S8/NC@MoS2 remain almost unaltered after lasting electrolysis in 0.5 M H2SO4 solution (Figure S11 and S12b). We finally investigated the HER performance of the Co9S8/NC@MoS2 in 1.0 M PBS. The corresponding data are summarized in Table S4. Impressively, the polarization curve (Figure 8e) of Co9S8/NC@MoS2 measured under neutral condition shows an onset potential of 45 mV, close to the value from the Pt/C and much smaller than the values from Co9S8/NC (91 mV), Co/NC (128 mV), MoS2 (181 mV), and NC (278 mV). Additionally, the Co9S8/NC@MoS2 shows a higher catalytic activity than some other non-noble-metal HER catalysts under neutral conditions (Table S7), such as Co9S8@C (η0 = 150 mV),28 Mo2C (η0 = 200 mV),67 NiS2/CC (η0 = 54 mV),68 and CuMoS4 (η0 = 135 mV).69 Undeniably, the Co9S8/NC@MoS2 demands a relatively high overpotential of 261 mV to achieve a current density of 10 mA cm−2, whereas other samples have overpotentials of > 400 mV at the same current density. It is worth noting that all the samples show the different polarization curves in neutral electrolyte compared to those in alkaline or acidic electrolyte. This might be associated with a higher solution resistance and less effective proton transport in neutral medium, and the exact reason is not completely understood at present time.70 The linear part of Tafel plots demonstrates Tafel slopes for Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC are 126.1, 175.5, 212.3, 243.6, and 334.6 mV dec−1, respectively (Figure 8f). Meanwhile, the Rct values are 115.5, 159.5, 262.1, 395.0, and 621.4 Ω for Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC, respectively (Figure S15). Furthermore, the accelerated stability test in Figure S16a (SI) revealed that the final polarization curve suffers little degradation compared with the initial one after taking 23



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2000 continuous CV cycles. Additionally, Figure S16b (SI) exhibited that the current density over the material (Co9S8/NC@MoS2) in HER barely decreased after a 12 h long continuous electrolysis. XRD pattern in Figure S11 (SI) and FESEM image in Figure S12c (SI) indicate there is almost no difference in crystallographic structure and morphology before and after electrolysis. All the above results demonstrate that the Co9S8/NC@MoS2 electrocatalyst can be used as a pH-universal electrocatalyst with high HER activity and good stability, and its property is comparable and even superior to those of other non-noble metal HER electrocatalysts that could be applied under alkaline, acidic, or neutral conditions. The all-pH applicability of the Co9S8/NC@MoS2 electrocatalyst permits it to be utilized in different water electrolysis technologies, such as water-alkali and chlor-alkali electrolyzers,71 the proton exchange membrane cells (acidic conditions),72 and microbial electrolysis cells (neutral conditions).73 On the basis of the unique structural and compositional characteristics as well as the intrinsic properties of Co9S8/NC@MoS2, the excellent HER performance of Co9S8/NC@MoS2 over a wide pH range can be mainly ascribed to the following factors: (1) the well-coupled interface between MoS2 and highly conductive carbon skeletons is conductive to accelerate the electron and reactant transport as well as provide abundant active catalytic sites for the HER;34 (2) the carbon layers coated on the Co9S8 nanoparticles can not only effectively protect Co9S8 from undergoing dissolution and aggregation, but also improve the conductivity of the material, thus significantly enhancing the catalytic stability of the material during the HER process;28 (3) the existence of N dopants can reduce the work function of the carbon layers and optimize the electron structure, thereby promoting the electron transfer between carbon layers and reactants;44 (4) MoS2 and Co9S8 are known to be very active electrocatalysts for the HER, and the combination of Co9S8 and MoS2 into rationally designed hybrid architectures could provide synergistic advantages, which may bring together the intrinsic properties of the two components of materials;9,29,31 (5) the well-defined polyhedral structure with high specific surface area can offer more 24


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electroactive sites and facilitate the mass transfer for the electrochemical reactions.40 All these factors synergistically endow the Co9S8/NC@MoS2 with excellent catalytic activity and stability for the HER.

Figure 8. (a) Polarization curves and (b) corresponding Tafel plots for the Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, NC, and Pt/C in 0.5 M H2SO4 solution at a scan rate of 5 mV s−1. (c) Polarization curves of the Co9S8/NC@MoS2 before and after 2000 CV cycles with a potential range from −0.4 V to −0.1 V vs. RHE in 0.5 M H2SO4 solution. (d) The current density−time (I−t) curves of Co9S8/NC@MoS2 at an overpotentials of 117 mV for 12 h in 0.5 M H2SO4 solution. (e) Polarization curves and (f) corresponding Tafel plots for the Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, NC, and Pt/C in 1.0 M PBS at a scan rate of 5 mV s−1.

25



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CONCLUSION In summary, hierarchical porous Co9S8/NC@MoS2 polyhedrons have been successfully synthesized through a facile hydrothermal approach using Co/NC polyhedrons as both the template and cobalt source. The novel hybrid material shows high catalytic activity and good stability for HER over a broad pH range, thanks to the good conductivity offered by the N-doped carbon matrices, abundant catalytically active sites induced by MoS2 nanocrystals and Co9S8 nanoparticles as well as mechanical stability promoted by the well-defined polyhedral structure. In particular, the Co9S8/NC@MoS2 electrode delivers the overpotentials of only 4 and 67 mV at current densities of 1 and 10 mA cm−2, respectively, under alkaline conditions with a small Tafel slope of 60.3 mV dec−1. The strategy presented here may be extended to synthesize other advanced multicomponent materials with novel polyhedral structures for energy conversion applications.

ASSOCIATED CONTENT Supporting Information XRD

pattern

of

ZIF-67

polyhedrons

and

NC;

Raman

spectrums

of

Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC; FESEM images of MoS2 and NC; EDS elemental mapping, EDS spectra, and TEM image of Co9S8/NC@MoS2; N2 adsorption–desorption isotherm of NC; exchange current density and cyclic voltammograms of Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC; XRD patterns of Co9S8/NC@MoS2 polyhedrons before and after 12 h durability tests; FESEM images of Co9S8/NC@MoS2 after 12 h durability tests in different electrolytes; XPS characterization of Co9S8/NC@MoS2 after 12 h durability tests in 1.0 M KOH; Nyquist plots of Co9S8/NC@MoS2, Co9S8/NC, Co/NC, MoS2, and NC in 0.5 M H2SO4 and 1.0 M PBS; polarization curves of the Co9S8/NC@MoS2 before and after 2000 CV cycles in 1.0 M PBS; the current density-time (I−t) curves of HER with Co9S8/NC@MoS2 in 1.0 M PBS; the performance summary of different catalysts in different electrolytes; comparison of the HER performance of the Co9S8/NC@MoS2 26


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with other non-noble metal catalysts in different electrolytes. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Fax: +86-0591-2286 6244; Tel: +86-0591-2286 5220. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We are grateful for the financial support from the National Natural Science Foundation of China (No: 21676057).

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