Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional

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Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn–air Batteries Jinman Bai, Tao Meng, Donglei Guo, Shuguang Wang, Baoguang Mao, and Minhua Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14997 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Co9S8@MoS2

Core-Shell

Heterostructures

as

Trifunctional Electrocatalysts for Overall Water Splitting and Zn–air Batteries Jinman Bai, Tao Meng, Donglei Guo, Shuguang Wang, Baoguang Mao, Minhua Cao* Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China.

KEYWORDS: Nanostructures; interfaces; electrochemistry; water splitting; Zn–air Batteries.

ACS Paragon Plus Environment

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ABSTRACT: The development of efficient non-noble metal electrocatalysts is of critical importance for clean energy conversion systems such as fuel cells, metal–air batteries, and water electrolysis. Herein, uniform Co9S8@MoS2 core–shell heterostructures have been successfully prepared via a solvothermal approach followed by an annealing treatment. Transmission electron microscopy, X-ray absorption near-edge structure and X-ray photoelectron spectroscopy measurements reveal that the core–shell structure of Co9S8@MoS2 can introduce heterogenous nano-interface between Co9S8 and MoS2, which can deeply influence its charge state to boost the electro-catalysis performances. Besides, due to the core–shell structure that can promote the synergistic effect of Co9S8 and MoS2 and provide abundant catalytically active sites, Co9S8@MoS2 exhibits a superior hydrogen evolution reaction (HER) performance with a small overpotential of 143 mV at 10 mA cm−2 and a small Tafel slope value of 117 mV dec-1 under alkaline solution. Meanwhile, the activity of Co9S8@MoS2 towards oxygen evolution reaction (OER) is also impressive with a low operating potential (~1.57 V vs. RHE) at 10 mA cm−2. Using Co9S8@MoS2 catalyst for full water splitting, an alkaline electrolyzer affords a cell voltage as low as 1.67 V at the current density of 10 mA cm−2. Also, Co9S8@MoS2 reveals robust oxygen reduction reaction (ORR) performance, making it as an excellent catalyst for Zn– air batteries with a long lifetime (20 h). This work provides a new means for the development of multifunctional electrocatalysts of non-noble metals for the highly demanded electrochemical energy technologies.


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1. INTRODUCTION To solve the energy crisis and environmental pollution, researchers have contributed considerable attention to clean energy conversion systems, such as, water splitting devices, fuel cells, and rechargeable metal–air batteries, as promising alternatives to fossil fuels. Hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), which are three fundamental electrochemical reactions, are generally involved in these energy systems mentioned above and that these reactions all often demand high-performance electrocatalysts to satisfy their practical applications.1-3 It is worth noting that the development of tri-functional electrocatalysts that are efficient synchronously for HER, OER, and ORR is of critical importance for electrochemical devices, especially for water splitting devices4,5 and rechargeable metal–air batteries,6 which are very effective strategies to obtain clean energy. However, many catalysts show high activity only towards one single reaction, and therefore developing ideal tri-functional catalysts for HER, OER, and ORR is still highly challenging because active ORR catalysts usually exhibit poor OER performances and vice versa. Up to now, Pt is still the most efficient HER and ORR electrocatalyst in alkaline media, 7,8 while IrO2 and RuO2 hold the benchmark for OER.9 However, these precious metals are scarce, expensive and poorly stable, which hinder their large-scale industrial applications. Therefore, developing highperformance tri-functional catalysts are highly necessary. Molybdenum disulfide (MoS2) with a large number of active unsaturated sulfur atoms has recently been considered to be effective HER catalyst based on both computational and experimental studies.10-12 Nevertheless, the HER activity of MoS2 is largely limited by its poor conductivity and serious aggregation. Great efforts have thus been focused on improving the electrocatalytic performance of MoS2. Despite significant success, controllable fabrication of


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ultra-small MoS2 with abundant active edge sites and good crystallinity towards high HER activity and high stability still remains a great challenge. In recent years, besides MoS2, other transition metal sulfides have also received wide attention because of their good electrochemical properties, particularly earth-abundant 3d metal (Co, Ni and so on) chalcogenides.13-21 These transition-metal chalcogenides have been making strides as highly promising and stable electrocatalysts for OER, ORR, and HER. For instance, N-, O-, and S-tri-doped carbonencapsulated Co9S8 was reported to exhibit a good OER activity with a low overpotential of 170 mV in alkaline medium.22 In another example, bifunctional MoS2/Ni3S2 heterostructure electrocatalyst grown on nickel foam delivers a current density of 10 mA cm-2 at a very low cell voltage of ca. 1.56 V for full water splitting.23 To the best of our knowledge, among transitionmetal chalcogenides, cobalt sulfides have attracted particular interest because of their excellent electrical properties.24,25 Cobalt sulfides include several different phases such as CoS2, Co3S4, CoS, Co9S8, etc.,26-28 in which the catalytic performance of the Co9S8 phase towards the OER has been widely investigated in alkali aqueous solutions.29 As reported, the activities of cobalt-based catalysts partly rely on the micro-chemical environment of cobalt sites.30 The under-coordinated metal sites on the surface are very important in OER process because of their excellent adsorption capacity for OH- or oxygen-containing intermediates. The properties of cobalt active centers would be improved by introducing doping ions, substrates, or another functional modification.29-31 Recent studies also indicate that composite catalysts often exhibit enhanced catalytic activities due to their strong synergetic effects.29-32 Recently, Feng group23 reveals using the density functional theory (DFT) calculations combined with experimental studies that the constructed interfaces between MoS2 and Ni3S2 can synergistically improve the chemisorption of hydrogen and oxygen-containing intermediates,


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thus facilitating the overall water splitting. And, Ramos et al.33 proposed a model, which then was confronted to a DFT approach to determine its feasibility as a reasonable description of the interface region between the individual MoS2 and Co9S8 phases. The synergetic effect between the interface regions of the two phases can increase surface area of the sulfide phases and the electrical conductivity, hence leading to remarkably improved activity for hydrodesulfurization. The advantages of nano-interfaces on Co9S8@MoS2/CNFs obtained by electrostatic spinning can strengthen electron transfer between Co and Mo through the intermediate sulfur atoms bonded to both metals, thus leading to promoted electrocatalytic activity.34 Although great efforts on heterostructures have been made to date, creating a new class of interfacial materials is still a great challenge. Afterwards, the researchers devoted to developing various effective strategies to engineer a hetero-interface to utilize its synergetic effects. The core–shell structured nanomaterials have gained our great attention due to the fact that the core–shell structure can increase the contact interface region. Besides, the core-shell materials also are interesting systems that can be used for a variety of applications. Especially in catalysis, such a structure offers combined properties of the constituents.35 Therefore, it is highly necessary to explore the core-shell heterostructures as novel trifunctional catalysts for all of the OER, HER and ORR. Herein, we demonstrate a facile strategy for synthesizing Co9S8@MoS2 core-shell heterostructures by solvothermal method followed by thermal treatment. The as-synthesized Co9S8@MoS2 core-shell heterostructures consist of uniform nanospheres with an average diameter of about 80 nm. Besides, transmission electron microscopy (TEM), X-ray absorption near-edge structure (XANES) and X-ray photoelectron spectroscopy (XPS) measurements reveal that the core-shell structure of Co9S8@MoS2 can introduce the heterogenous nano-interfaces between Co9S8 and MoS2, which can deeply influence its charge state to boost the electro-


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catalysis performances. Notably, Co9S8@MoS2 core-shell catalyst affords a small onset potential and a small Tafel slope of 117 mV dec-1 as well as good stability, performing pretty well among the noble-metal-free HER electrocatalysts in alkaline electrolyte. Also, the as-prepared Co9S8@MoS2 catalyst shows excellent ORR performance and it exhibits an onset potential close to that of the Pt/C catalyst. Meanwhile, Co9S8@MoS2 also shows an outstanding OER activity with the overpotential of only 342 mV at the current density of 10 mA cm-2. With respect to the good activities of Co9S8@MoS2 for HER, OER and ORR, Co9S8@MoS2 exhibits excellent activities as trifunctional electrocatalysts for overall water splitting and rechargeable Zn-air batteries. The excellent catalytic performance of Co9S8@MoS2 hybrid is likely due to the electrocatalytic synergistic effects between Co9S8 and MoS2. This work suggests a facile strategy for designing non-noble metal catalysts with enhanced catalytic performances that are comparable to those of the Pt-based and IrO2 electrocatalysts. 2. EXPERIMENTAL SECTION Synthesis of Co9S8@MoS2 heterostructures: In a typical process, 2.5 mmol of cobalt acetate, 2.5 mmol of sodium molybdate, and 12 mmol of thiourea as well as polyvinyl pyrrolidone (PVP) were dissolved in 50 mL of ethylene glycol. After being vigorously stirred for about 1 h, the resulting solution was transferred into 75 mL of Teflon-lined stainless steel autoclave (with a filling volume ratio of 80%). After that, the autoclave was sealed, kept at 200 °C for 48 h, and then cooled naturally to room temperature. The obtained deposit was washed with deionized water and ethanol in turn and then dried at vacuum at 60 °C for 6 h, and thus a precursor was obtained. Then this precursor was annealed at different temperatures (i.e., 650, 750, and 850 °C) for 3 h at a heating rate 3 °C min-1, under a flow of N2 in a quartz reactor. After being cooled to


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room temperature, Co9S8@MoS2 hybrids were obtained. For comparison, Co9S8 and MoS2 were also prepared with the same procedure but in the absence of Co or Mo precursor, respectively. Synthesis of Co9S8: Co9S8 was prepared with the process similar to that of the typical sample (Co9S8@MoS2) only without the sodium molybdate. Synthesis of MoS2: MoS2 was prepared with the process similar to that of the typical sample (Co9S8@MoS2) only without the cobalt acetate. Physically mixed Co9S8 and MoS2: The as-obtained Co9S8 was physically mixed with MoS2 by grinding treatment and the resultant sample was denoted as Co9S8& MoS2. The commercial Pt/C was purchased from Johnson Matthey Co. Ltd. (20 wt% platinum in carbon, crystallite size 3.5 nm, and carbon black-XC72R). Materials characterizations: The X-ray diffraction (XRD) patterns were carried out on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). The tested current and voltage were 40 mA and 40 kV with a scanning step of 10° min-1 from 10 to 80° (2θ). Raman spectra were taken at a laser excitation of 633 nm (Invia Raman spectrometer). The morphology and structure of the samples were characterized by transmission electron microscopy (TEM), which was taken on H-8100 TEM. Field emission scanning electron microscopy (FESEM) and energy dispersive X-ray (EDX) elemental mapping was taken on Hitachi S-4800 SEM units. X-ray absorption near edge structure (XANES) measurements were performanced at the Beamlines 1W1B at Beijing Synchrotron Radiation Facility (BSRF) using transmission modes. The X-ray photoelectron spectroscopy (XPS) was tested on ESCALAB 250 spectrometer. Electrochemical measurements: 4 mg of catalyst and 100 µL Nafion solution (5 wt%) were dispersed in 900 µL ethanol solution to prepare the ink. Then 20 µL of the ink was dropped onto


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a glass carbon (GC) electrode (diameter: 5 mm). OER tests were performed in a three-electrode configuration using a CHI 760E electrochemical workstation (CH instrument, Chenhua). In this system, Pt foil (1*1 cm-2) and Ag/AgCl (KCl, 4 M) electrode were employed as the counter electrode and the reference electrode, respectively. Potential was referred to reversible hydrogen electrode (RHE) by the following equation: E(RHE) = E(Ag/AgCl) + (0.205 + 0.0592 pH) V. The polarization curves were taken at a scan rate of 2 mV s-1 in O2-saturated, 1.0 M KOH solution. The Tafel slopes were then obtained with following Tafel equation from the corresponding LSV curves: η = b•log j (where η is the overpotential, j is the cathodic current density, and b is the Tafel slope). Electrochemical impedance spectroscopy (EIS) was performed at a certain potential of 1.6 V vs. RHE in a frequency ranging from 500 KHz to 1 Hz with the amplitude of 5 mV. To evaluate the electrochemical double-layer capacitance (Cdl) of all the materials, which is used to determine the corresponding electrochemical surface area (ECSA), cyclic voltammograms (CVs) were tested in 1.0 M KOH at different scan rates under the potential from 0.80 to 0.83 V. The longlifetime test was carried out by i-t curve at a constant working potential of 1.57 V for 10 h. All the data presented were not corrected for iR losses. The linear sweep voltammetry (LSV) curves over the electrocatalysts in HER are similar to those of OER, which were obtained in N2-saturated 0.5 M H2SO4 and 1.0 M KOH solutions. Tafel slopes were calculated from the corresponding LSV curves, just as mentioned above. The Co9S8@MoS2 was used to collect chronoamperometry data in 1.0 M KOH and 0.5 M H2SO4 solutions, which was obtained at the applied potential of 143 mV for 10 h and 171 mV for 15 h versus RHE, respectively. The overall water splitting was carried out in a two-electrode system. The Co9S8@MoS2


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was dropped onto nickel foam substrate and used as both the anode and cathode. The stability of the system was determined in a 100 mL glass beaker with the gas products in a N2 atmosphere. The ORR tests are similar to OER. CVs were obtained in both N2-saturated and O2-saturated KOH solutions. The LSV curves for ORR were obtained at room temperature and rotating speeds ranging from 400 to 2025 rpm with a scan rate of 10 mV s-1. The potential cycling was tested in O2-saturated solution. The kinetic parameters can be studied by the Koutecky-Levich equations: 1/J = 1/Jk + 1/ (Bω1/2) B = 0.2nFC0D02/3υ-1/6 Jk = nFkC0 Where J and Jk is the measured and kinetic current densities, respectively, ω is the rotation speed, n is the transferred electron number, F is the Faraday constant (96485 C mol-1), C0 is the saturated concentration of O2 in the electrolyte (1.21 * 10-3 mol L-1), D0 is the diffusion coefficient of O2 (1.9 * 10-5 cm s-1), υ is the kinetic viscosity of the electrolyte (0.01 cm2 s-1), and k is the electron-transfer rate constant.36 A home-made Zn-air battery was assembled by using the Co9S8@MoS2 catalyst as the air cathode, a Zn plate as the anode, and an aqueous solution containing 6 M KOH and 0.2 M Zn(Ac)2 as the electrolyte. Battery testing and cycling experiments were performed on a LAND CT2001A instrument at room temperature with each cycle of 10 min. 3. RESULTS AND DISSCUSSION The crystalline phases of the as-prepared precursor and corresponding annealed samples at different temperatures were first investigated by X-ray diffraction (XRD), as presented in Figure 1. It can be clearly seen from Figure 1b (blue curve) that no discernable diffraction peaks are


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observed, indicating the amorphous nature of the precursor. After being annealed at different temperatures (650, 750, and 850 oC), the resultant samples show obvious diffraction peaks in their XRD patterns. For clarity, the XRD pattern of the sample obtained at 750 oC is put separated (Figure 1a). By comparison with standard Co9S8 and MoS2 XRD patterns, it can be found that the main diffraction peaks at 2θ of 15.4°, 25.4°, 29.9°, 31.2°, 39.4°, 47.4°, and 52.0° can be assigned to (111), (220), (311), (222), (331), (511), and (440) planes of cubic Co9S8 phase (JCPDS No.19-0364), respectively, whereas those peaks at 14.5°, 33.0°, 44.4°, 58.3°, and 60.4° can be indexed to (002), (101), (104), (110), and (113) planes of hexagonally structured MoS2 (JCPDS No.17-0744), respectively. Furthermore, no other diffraction peaks were detected and meanwhile no noticeable carbon peaks were observed, presumably because of their relatively weak intensities compared with those of the crystalline Co9S8 and MoS2, but its existence has been confirmed by corresponding Raman spectroscopy with two peaks at 1335 and 1568 cm−1 (Figure S1a). For the other two samples obtained at 650 and 850 oC, their XRD patterns show that these two samples are also composed of Co9S8 and MoS2, and no other phases are detected (Figure 1b and Figure S1b). Meanwhile, it can be clearly observed that when the annealing temperature was increased, the diffraction peaks gradually become stronger, indicating improved

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(b)

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MoS2 JCPDS No.17-0744

30

40 50 60 2θ θ / degree

o

650 C

Co 9S 8 JCPDS No.19-0364

20

Precursor 750 C

Intensity ( a.u.)

(220)

(002) (111)

(a)

(311) (101)(222) (331) (104) (511) (440) (110) (113)

crystallinity, which is consistent with most of reported literatures.

Intensity ( a.u.)

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

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70

80

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o

850 C

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40 50 60 2θ θ / degree

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Figure 1. (a) XRD pattern of the as-obtained Co9S8@MoS2 obtained at 750 oC. (b) XRD patterns of the precursor and Co9S8@MoS2 samples obtained at different annealing temperatures.


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Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were conducted to investigate the morphology and microstructure of the resultant product. Figure 2a clearly shows the morphological features of Co9S8@MoS2 obtained by annealing the precursor in N2 at 750 oC. The sample is composed of spherical-like aggregations with relatively rough surface, and these aggregations have diameters in the range of 50-100 nm (Figure S2). Transmission electron microscopy (TEM) images of Co9S8@MoS2 hybrid clearly discloses its core-shell structure (Figure 2b,c), and the shell is about 15 nm (Figure 2d). The corresponding selected area electron diffraction (SAED) pattern of Co9S8@MoS2 reveals clear diffraction spots and also faint diffraction rings, further confirming the polycrystalline nature of this sample (inset of Figure 2b). Furthermore, high resolution TEM (HRTEM) image recorded on the edge of a single Co9S8@MoS2 nanosphere (Figure 2e) shows a lattice distance of 0.614 nm, which corresponds to (002) plane of MoS2, while the HRTEM image recorded on the inner discloses an (a)

(b)

(c)

20 nm Co Mo S

(d)

(e) (e)

(f)

0

40

80 120 160 Distance (nm)

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Figure 2. (a) FESEM image of the Co9S8@MoS2; (b−d) TEM images, SAED pattern, and (e) HRTEM image of the Co9S8@MoS2; (f) FESEM image (scale bar: 100 nm) and corresponding cross-sectional compositional line-scan profiles of the Co9S8@MoS2 hybrid and elemental mapping images recorded on Co9S8@MoS2.


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interplanar spacing of 0.573 nm, which can be assigned to (111) plane of Co9S8. Meanwhile, the (002) plane of MoS2 and the neighboring (111) plane of Co9S8 constitute atomic-level nanointerfaces denoted by the red dash line in Figure 2e. An element mapping investigation with the line-scan profiles across one nanosphere (Figure 2f) also reveals that the shell is MoS2 phase, which is consistent with above HRTEM observation. In addition, energy-dispersive X-ray (EDX) measurement proves the co-existence of Mo, Co, S, C and O elements in Co9S8@MoS2 (Figure S3) and that the atomic ratio of the Co and Mo is about 2:1. The distributions of these elements are further analyzed through surface-scanning element mappings (the bottom and top left in Figure 2f). Notably, the Co signal is relatively weak and the Mo signal is strong, which is probably due to the shielding effect of the shell, agreeing well with the result from TEM observations. The electronic and structural nature of Co9S8@MoS2 was further investigated by X–ray adsorption near–edge structure (XANES) and extended X–ray absorption ﬁne structure (EXAFS) analysis. Figure 3a shows the pre–edge of Co K–edge XANES spectrum of Co9S8@MoS2. Clearly, compared to pristine Co9S8, Co9S8@MoS2 shifts towards the higher energy, indicating the strongly electronic interactions between Co9S8 and MoS2 as well as the establishment of coupled interfaces.22,37,38 Besides, the Co K-edge k3χ(k) oscillation curve of Co9S8@MoS2 in Figure 3b presents an obvious difference from that of the bare Co9S8, revealing their distinct local atomic configuration, which is further confirmed by their R-space results in Figure 3c. Clearly, compared to that of the bare Co9S8, the EXAFS spectrum for Co9S8@MoS2 exhibits a relatively weak peak in the range of 1.5-2.5 Å along with an obvious shift due to the formation of the Co9S8/MoS2 nano-interface (proofed by HRTEM image), again revealing the strong coupling effect between Co9S8 and MoS2. Similar results are observed for Mo K-edge XANES data. Both


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Co K edge

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Figure 3. (a) The XANES spectra of Co K-edge for Co9S8@MoS2, Co9S8 and Co foil. (b) Co k3x(k) oscillation curves and (c) the corresponding Fourier transform k3-weighted EXAFS spectra of Co9S8@MoS2 and Co9S8. (d) The XANES spectra of Mo K-edge for Co9S8@MoS2, MoS2 and Mo foil. (e) Mo k3x(k) oscillation curves and (f) the corresponding Fourier transform k3-weighted EXAFS spectra of Co9S8@MoS2 and MoS2.

the XANES (Figure 3d) and K-edge k3χ(k) oscillation (Figure 3e) of Mo confirm distinctly different local atomic arrangement between Co9S8@MoS2 and bare MoS2. And, two peaks are observed in the range of 1.0-3.5 Å (Figure 3f), in which the first one, ranging from 1.0 to 2.5 Å, corresponds to Mo-S bond, and the second one in the range of 2.5−3.5 Å is attributed to Mo-Mo bond.39 For Co9S8@MoS2, the intensities of the two peaks both are weakened but with the second peak position slightly change, again confirming that the local atomic arrangement of the Co9S8@MoS2 is different from that of the MoS2. The more disorder in structure for Co9S8@MoS2 is due to the strongly coupled interfaces between Co9S8 and MoS2. All of the above results reveal that the electronic and structural nature of Co9S8@MoS2 can be changed by the coupled interfaces, which further can enhance its electro-catalysis performance. To further understand the surface chemical states of Co, Mo, and S elements in Co9S8@MoS2, X-ray photoelectron spectroscopy (XPS) analysis was performed (Figure 4 and Figure S4). Figure 4a shows a survey XPS spectrum of Co9S8@MoS2, from which S, Mo, C, O, and Co


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signals at 162.2, 230.64, 285.36, 531.87, and 780.57 eV, respectively, are clearly observed. The high-resolution XPS spectrum (Figure 4b) of Co 2p can be divided into two pairs of spin-orbit doublets and two shakeup satellites (identified as Sat.). The two pairs of doublets located at (780.0, 796.4 eV) and (783.2, 797.8 eV) are observed, which belong to Co 2p3/2 and Co 2p1/2 of Co3+ and Co2+, respectively.40 For the Mo 3d spectrum (Figure 4c), the peaks of Mo via peak fitting indicate that the Mo 3d5/2 and 3d3/2 peaks are observed at 229.19 and 232.54 eV, respectively, which match with the oxidation state of Mo4+. In the meantime,

(a)

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O 1s

Intensity ( a.u.)

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C 1s Co 2p Mo 3d S 2p

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400 600 800 1000 1200 810 805 800 795 790 785 780 775 Binding energy (eV) Binding energy (eV)

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240 238 236 234 232 230 228 226 224 Binding energy (eV)

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SO42-

Co-S Mo-S

Co9S8@MoS2 Co9S8 MoS2

174 172 170 168 166 164 162 160 Binding energy (eV)

Figure 4. (a) Survey XPS spectrum of Co9S8@MoS2. High resolution XPS spectra of Co9S8@MoS2, Co9S8 and MoS2: (b) Co 2p, (c) Mo 3d, and (d) S 2p.

the peak at 226.4 eV can be assigned to the Mo-S bond.23 The high-resolution XPS spectrum of S 2p displays four distinct peaks at (162.1, 163.1 eV) and (161.8, 163.5 eV) (Figure 4d), that is, S 2p in MoS2 and S 2p in Co9S8 crystal structures. The binding energy centered at 168.6 eV is attributed to SO42-.41 Notablely, compared with those of single-phased Co9S8 and MoS2, the Co 2p and Mo 3d peaks in Co9S8@MoS2 both shift towards higher binding energy, indicating


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strong coupling interaction between Co9S8 and MoS2 by their nano-interface, in well agreement with the XANES results discussed above. Therefore, the XPS results not only further manifest surface composition of Co9S8@MoS2, but also suggest the establishment of coupling interfaces between the two phases (Co9S8 and MoS2). The electrocatalytic OER activity of Co9S8@MoS2 was first evaluated using a three-electrode system. Co9S8@MoS2 was used as a working electrode and the polarization curves were obtained in an O2-saturated 1.0 M KOH electrolyte. In order to determine the effect of the annealing temperature on the electrocatalytic performance, the electrocatalytic activities of Co9S8@MoS2 samples obtained at different annealing temperatures were investigated. Figure 5a shows that Co9S8@MoS2 sample obtained at 750 °C (denoted as Co9S8@MoS2-750) exhibits more negative onset potential compared to the other two samples prepared at lower or higher temperatures(Co9S8@MoS2-650 and Co9S8@MoS2-850) and the precursor. Also at the same

Precursor

40

650 750 850

20

10 mA cm-2

0 1.2 50

1.3 1.4 1.5 1.6 1.7 Potential (V vs. RHE)

1.8

(d)

Co9S8

60

MoS2

40

Co9S8&MoS2

20

△

J (mA cm-2)

20 10

1.3 1.4 1.5 1.6 1.7 Potential (V vs. RHE) Co9S8 MoS2

0.8 Co9S8@MoS2

.3 20

0.6

0.0 2

4 6 Time (h)

8

10

F 7m

1.8

-1

0.2

7

V 9m

128

d mV

ec

MoS2

cm

150

Co9S8@MoS2

20 30 40 50 Scan rate (mV s-1)

0.0

2.4

(f)

Mixed Co9S8&MoS2

120

60

c V de

-1

de c

180

cm 3.82 mF -2 1.75 mF cm

10

-1

94 m

210

(e)

(c)

IrO2/C

-2

-2

0

Co9S8@MoS2

-1

-1 dec mV dec 170 mV 144

0.8 1.6 Log [J (mA cm-2)] Co9S8

0.4 0.2

MoS2

Co9S8&MoS2

0.4

IrO2/C

0 1.2

Co9S8

0.6

Co9S8@MoS2

1.0

30

0

80

1.2

40

0.8

(b)

Overpotentional (V)

60

J (mA cm-2)

J (mA cm-2)

80

100

(a)

-Z'' (ohm)

100

J (mA cm-2)

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


90 60 30 0 0

30 60 90 120 150 180 210 Z' (ohm)

Figure 5. (a) Polarization curves of Co9S8@MoS2 prepared with different annealing temperatures for OER in 1.0 M KOH (scan rate: 2 mV s-1). (b) Polarization LSV curves and (c) Tafel plots of Co9S8@MoS2, Co9S8&MoS2,Co9S8, MoS2, and IrO2/C in 1.0 M KOH (scan rate: 2 mV s−1). (d) i-t curve at a certain potential (the potential at 10 mA cm-2). (e) Plots of the current density at 0.815 V. (f) EIS of Co9S8@MoS2, physical mixed Co9S8 and MoS2, Co9S8 and MoS2 measured at 1.60 V.


15


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current density, the overpotential needed for Co9S8@MoS2-750 towards OER is smaller than those needed for the other two materials under the same conditions. From the previous XRD patterns (Figure 1b), we can see that Co9S8@MoS2-650 has a poor crystallinity, which may do not favor the electro-catalysis; while, Co9S8@MoS2-850 holds a large size (Figure S5), which results in the decrease of the amount of the active sites, thus affording poor electro-catalysis performance. Therefore, in the following discuss, Co9S8@MoS2-750 was used as a typical sample to further investigate its OER catalytic process, and it is abbreviated as Co9S8@MoS2. For comparison, Co9S8, MoS2 and IrO2/C toward OER were also examined under the same conditions and XRD patterns and FESEM images of Co9S8 and MoS2 are presented in Figure S6. It can be clearly seen that Co9S8@MoS2 shows significantly improved catalytic activity compared to Co9S8 and MoS2 (Figure 5b), indicating that the nano-interface built by the coreshell structure can obviously improve the OER activity of Co9S8@MoS2. Surprisingly, it is worthwhile mentioning that the OER activity of Co9S8@MoS2 is almost comparable to that of IrO2/C. Besides, the overpotentials needed for reaching 10 mA cm−2 for Co9S8@MoS2, Co9S8, MoS2 and IrO2/C are 342, 384, 486 and 320 mV, respectively. Furthermore, this overpotential at 10 mA cm−2 of Co9S8@MoS2 is also better than those reported Co-based electrocatalysts, including CoSe2/N–graphene (366 mV),42 Mn3O4/CoSe2 (450 mV),43 CoS2/N,S-GO (380 mV),44 and some non-noble metal catalysts (Table 1), revealing the excellent OER activity of Co9S8@MoS2. Tafel plots were used to further evaluate the OER kinetics. Figure 5c shows that the Tafel slopes are 94 mV dec-1 for Co9S8@MoS2, 144 mV dec-1 for Co9S8, 170 mV dec-1 for MoS2 and 128 mV dec-1 for IrO2/C, respectively. Clearly, the Tafel slope value of Co9S8@MoS2 is even smaller than that of the IrO2/C, further indicating its faster OER kinetics. Besides, the OER


16

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reaction mechanism occurring on Co9S8@MoS2 was studied by employing rotating ring disk electrode (RRDE) strategy. It can be identified as a four electron-dominated reaction pathway with negligible formation of peroxide intermediates and a Faradaic efficiency as high as 97.5% (Figure S7). Additionally, long-term durability is another key parameter for estimating the OER performance of the electrocatalyst. Figure 5d shows that the current density over the catalyst decreases slightly after a 10 h long continuous operation, revealing its long lifetime during the OER process. Moreover, after the OER electrolysis test, the Co9S8@MoS2 still maintains original morphology (Figure S8a), and XPS test demonstrates the formation of Co3+ and Mo6+ (Figure S8b,c), in good agreement with previous studies.45 This result suggests the evolution of CoOOH and MoOx species, which serve as the active phases during the OER electrochemical process.46 The increased oxidation states of Co and Mo is beneficial for the multi-electron transportation involved in OER process. Table 1 Comparison of the electrocatalytic performance of Co9S8@MoS2 core-shell heterostructures with recently reported materials in alkaline solution.

Overpotential (V vs. RHE) at J = 10 mA cm-2 (1.0 M KOH) Full water HER OER splitting

CV peak (V) (0.1 M KOH)

Co9S8@MoS2

0.143

0.342

1.67

0.884 (vs. RHE)

CoNC@MoS2/CNF

0.143

0.350

1.62

a

N.A.

51

MoS2-Ni3S2 HNRs/NF

0.098

0.249

1.50

N.A.

52

MoS2/Ni3S2 particles CoS-Co(OH)2@MoS2+x/NF

0.110 0.143

0.218 0.380

1.56 1.58

N.A. N.A.

23 32

NiS-Ni(OH)2@MoS2+x/NF

0.226

0.417

1.65

N.A.

32

Mo-N/C@MoS2 MoS2/NG Au NP/MoS2 films Co@MoS2-500 Ni/graphene-MoS2

0.117 N.A. N.A. N.A. N.A.

N.A. N.A. N.A. 0.271 0.214

N.A. N.A. N.A. N.A. N.A.

0.84 (vs. RHE) -0.23 (vs. SCE) -0.24 (vs. SCE) N.A. N.A.

53 54 55 56 57

MoS2 NDs/NGr

N.A.

N.A.

N.A.

0.82 (vs. RHE)

58

Ni2P/MoO2@MoS2

0.159

0.28

0.49

N.A.

59

Catalysts

Ref.

ORR This work

a N.A. stands for not given.


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Page 18 of 35

The high OER activity, fast kinetics, and excellent durability of Co9S8@MoS2 may result from its core-shell structure, which affords the synergistic effects between the Co9S8 core and the MoS2 shell as well as the close surface contact between them. As the Figure 5e shown, electrochemical double-layer capacitance (Cdl) based on CVs (Figure S9) was obtained to evaluate the effective active surface area. The Co9S8@MoS2 has a Cdl value as high as 20.37 mF cm-2, which is larger than those of the single-phase Co9S8 and MoS2 (1.75 and 3.82 mF cm-2). This indicates that the nano-interface between the Co9S8 and MoS2 can highly expose the effective active sites for Co9S8@MoS2, which is responsible for its excellent electro-catalysis activity. Besides, the nano-interface between the Co9S8 core and the MoS2 shell can influence the charge distribution (as proofed by XANES and XPS), which can significantly improve the charge transfer capability of the Co9S8@MoS2, thus endowing it with the enhanced OER activity and stability. In contrast, the physically mixed Co9S8&MoS2 sample shows a fade OER activity (Figure 5b), again confirming that the core-shell structure of Co9S8@MoS2 with nano-interface can enhance the OER performance. Moreover, the EIS plots of Co9S8@MoS2, Co9S8, MoS2 and the physically mixed one (Figure 5f) reveal that Co9S8@MoS2 has a much smaller semicircular diameter than MoS2 and Co9S8, which indicates remarkably decreased charge transfer resistance (Rct) for Co9S8@MoS2, exhibiting more facile electrode kinetics for enhancing the catalytic activity. The significantly enhanced electrocatalytic activity of Co9S8@MoS2 is most likely due to the synergistic effects between the Co9S8 core and the MoS2 shell as well as the close surface contact between them. The small overpotential, high current density and small contact impedance as well as low Tafel slope all indicate that Co9S8@MoS2 possesses very good electrocatalytic activity toward OER.


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Besides the electrocatalytic properties for OER in alkaline media, the electrocatalytic HER of all the as-prepared Co9S8@MoS2 materials was also investigated in 0.5 M H2SO4 and 1 M KOH electrolytes purged with N2, respectively. The polarization LSV curves of the precursor and Co9S8@MoS2 materials obtained with different temperatures were measured in both the acid and alkaline media, as shown in Figure S10. Similar to the observations from the OER tests, among all the tested samples, Co9S8@MoS2-750 hybrid affords the smallest overpotential of 143 mV in alkaline and 171 mV in acidic solution at the current density of 10 mA cm−2. For comparison, similar measurements were carried out over bare Co9S8, MoS2, and commercial Pt/C under the identical conditions. The Co9S8@MoS2 shows high HER activity in 0.5 M H2SO4 (Figure 6a). Specifically, Co9S8@MoS2 delivers a current density of 10 mA cm-2 with an overpotential of 171 mV. In contrast, the overpotentials (J = 10 mA cm-2) for Co9S8, MoS2 and the physically mixed Co9S8&MoS2 are 376, 235 and 272 mV, which are far high than that of Co9S8@MoS2. Besides, Co9S8@MoS2 also has a small Tafel slope of 123 mV dec-1, and this value is comparable to 103

Figure 6. (a) Polarization curves and (b) Tafel plots of Co9S8@MoS2, Co9S8&MoS2, Co9S8, MoS2 and Pt/C for

HER (N2-saturated 0.5 M H2SO4 with a scan rate of 2 mV s-1). (c) EIS plots of Co9S8@MoS2, Co9S8, and MoS2 collected at -0.17 V. (d) i-t curve at the potential of -0.17 V.


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mV dec-1 of MoS2, but much smaller than those of Co9S8 (195 mV dec-1) and Co9S8&MoS2 (163 mV dec-1) (Figure 6b). The EIS plots obtained in 0.5 M H2SO4 (Figure 6c) reveal remarkably decreased Rct for Co9S8@MoS2, indicating fast HER kinetics for enhancing its catalytic activity. We further investigated its ability to continuously generate the H2 gas (Figure 6d). After 10 h of test, a negligible decay is observed at high current density, which may be due to the MoS2 shell’s ability to prevent the Co9S8 core from undergoing corrosion in acidic solution. In order to satisfy the ever-growing applications in different environments, the HER performance of Co9S8@MoS2 in 1.0 M KOH was also investigated. Figure 7a shows that the overpotential of 143 mV for Co9S8@MoS2 to reach 10 mA cm-2 is far smaller than those of pure Co9S8 (461 mV), MoS2 (202 mV) and physically mixed Co9S8&MoS2 (389 mV) in the same tested conditions. Figure 7b further displays that the Tafel slope of Co9S8@MoS2 is about 117 mV dec-1. Although this value is higher than 75 mV dec-1 of the Pt/C catalyst, it is close to that of MoS2 and that far smaller than those of Co9S8&MoS2 (163 mV dec-1), Co9S8 (196 mV dec-1), and some non-noble metal catalysts (Table 1). Furthermore, significantly enhanced charge transfer for Co9S8@MoS2 compared to Co9S8 and MoS2 has also been confirmed by corresponding EIS plots obtained in 1.0 M KOH (Figure 7d). These results suggest that Co9S8@MoS2 has a favorable HER reaction kinetics corresponding to its high catalytic activity. Furthermore, the long-term stability of Co9S8@MoS2 catalyst was also studied by i-t measurement at a constant potential of -143 mV. Figure 7c shows that the i-t curve exhibits slight current attenuation within 10 h, demonstrating a high stability. And, after the long-time test, the FESEM image of Co9S8@MoS2 also maintain the spherical-like aggregations (Figure S11). This also explains the fact that among the series of Co9S8@MoS2 materials, Co9S8@MoS2-750 gives the best electrocatalytic performance for HER with a small overpotential and Tafel slope.


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Figure 7. (a) Polarization curves and (b) Tafel plots of Co9S8@MoS2 as well as control samples for HER in

1.0 M KOH (scan rate: 2 mV s-1). (c) i-t curve at the potential of -0.143 V. (d) EIS plots tested at -0.143 V. (e) Polarization curves for Co9S8@MoS2//Co9S8@MoS2 and Ni foam//Ni foam electrode in a two-electrode alkaline electrolyzer in 1.0 M N2-saturated KOH. (f) Chronoamperometric response at a constant voltage of 1.67 V over the course of 16 h for Co9S8@MoS2.

Based on the above experimental results, the excellent HER performance of Co9S8@MoS2 electrocatalyst could be attributed to the following factors: (i) the inherently catalytically active of Co9S8 nanoparticles may have played a synergistic role in the electrochemical HER with the MoS2 shells, as core–shell structures. Such a combination not only brings together the properties of the two types of materials, but also produces synergistic effects that are useful for electrocatalysts; (ii) the nano-interfaces formed between the Co9S8 core and the MoS2 shell can afford good donor-acceptor electronic interaction, where the electrons can transfer from Co9S8 to MoS2. This, in turn, greatly alters the electronic properties of the MoS2 shell and optimizes the electron transfer between electrocatalyst surfaces and reaction intermediates, leading to increased activity toward electrocatalysis; These possible processes proposed above reasonably account for


21


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the higher catalytic activity and the durability of the nonprecious electrocatalysts reported here in HER electrocatalysis. To further demonstrate the practical application of Co9S8@MoS2 catalyst, a two–electrode water electrolyzerwas assembled. As shown in Figure 7e, this electrolyzer delivers watersplitting current density of 10 mA cm-2 at applied potential of approximately 1.67 V, which is obviously smaller than that of the Ni foam (ca. 1.85 V) and those of other reported electrocatalysts such as Co@Co3O4-NC (ca. 2.004 V).47 Furthermore, the stability test indicates that the electrolyzer exhibits negligible degradation after 16 h of electrolysis, demonstrating its excellent durability (Figure 7f). This result also suggests promising practical application of Co9S8@MoS2 for overall water splitting. Interestingly, except the excellent overall water splitting performance demonstrated above, the Co9S8@MoS2 also exhibits good ORR activity. The electrocatalytic activity of Co9S8@MoS2 towards the ORR is first evaluated by CV tests carried out in N2 and O2-saturated 0.1 M KOH solution using a three electrode system at a scan rate of 10 mV s-1. As clearly shown in Figure 8a, almost no activity is detected for Co9S8@MoS2 in N2-saturated electrolyte. In contrast, Co9S8@MoS2 shows a substantial oxygen reduction process in O2-saturated electrolyte at about 0.884 V, which is far more positive in comparison to those reported catalysts (e.g. 0.817 V vs. RHE for Co0.85Se@NC,36 0.704 V vs. RHE for N-Co9S8/G,48 and -0.21 V vs. SCE for Co3O4/ONCNW49), and some non-noble metal catalysts, indicating an easier ORR process on Co9S8@MoS2. Figure 8b shows that the polarization curve displays a sharp increase in current density with increasing the rotation speed, which is due to the increased diffusion rate at high speeds. The corresponding Koutecky- Levich (K-L) plots of Co9S8@MoS2 all show fairly good linearity (Figure 8c), suggesting first- order reaction kinetics for the ORR with respect to the


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Figure 8. (a) CVs of the as-synthesized Co9S8@MoS2 in an O2-saturated and N2-saturated 0.1 M KOH

solution (scan rate: 10 mV s-1). (b) Rotating-disk electrode polarization curves of Co9S8@MoS2 in O2-saturated 0.1 M KOH with rotating rates from 400 to 2025 rpm (scan rate of 10 mV s-1). (c) K-L plots of Co9S8@MoS2 calculated from the RDE data at 0.581, 0.531, 0.481, 0.431, 0.381, 0.331, 0.281 and 0.231 V, and (d) the corresponding electron–transfer number. (e) Rotating-disk electrode LSVs of Co9S8@MoS2, Co9S8, MoS2, and Pt/C at 1600 rpm. (f) The i-t curve of Co9S8@MoS2 obtained at 0.67 V in O2-saturated 0.1 M KOH and the inset is the chronoamperometric response after adding methanol.

dissolved oxygen concentration at different potentials.50 Forthermore, the electron transfer number was calculated based on the rotating ring–disk electrode (RRDE) test of Co9S8@MoS2. The final results are shown in Figure 8d. We can see that the n values of Co9S8@MoS2 are in the range of 3.25 to 3.54 at the potentials ranging from 0.231 to 0.581 V. This clearly reveals that the Co9S8@MoS2 follows a desirable one-step, four-electron pathway (4OH-↔O2 + 2H2O + 4e-) for reversible OER and ORR process. For comparison, Co9S8@MoS2, Co9S8, MoS2 and commercial Pt/C catalyst were examined in 0.1 M O2-saturated KOH by using RDE technology. As shown in Figure 8e and Figure S12, both the ORR and onset (half-wave) potentials of Co9S8@MoS2 are superior to those of the individual Co9S8 and MoS2 due to the synergetic effect between Co9S8 and MoS2. Moreover, the chronoamperometric response for the Co9S8@MoS2 electrode (Figure


23


8f) implies relatively good durability in electrochemical processes, which is important for practical application. Another important criterion for practical fuel cells is the catalytic selectivity. In the case of Co9S8@MoS2 electrode, no activity specific to methanol is observed, indicating the high selectivity towards the ORR. The OER and ORR play important roles in rechargeable Zn–air batteries, and therefore a home-made rechargeable Zn-air battery device was constructed by using Co9S8@MoS2 as the air electrode (Figure 9a). Figure 9a shows that the open-circuit potential (OCP) measured by the electrochemical workstation for the Zn-air battery is 1.384 V. To investigate the cycling performance, galvanostatic discharge/charge testing on the Co9S8@MoS2-battery was performed at 10 mA cm-2. As shown in Figure 9b, the battery exhibits high cycling stability without obvious loss of both the charge and discharge performance. In addition, two Zn-air batteries in series show an OCP of about 2.73 V, as shown in Figure 9c. The prospect application of this device is (a)

2.5 Cell voltage (V)

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

Page 24 of 35

(b) 2.0

1.5

1.0 0.0

(c)

(d)

0.5

5 Time (h)

10

15

20

(e)

Figure 9. (a) Optical photograph of the Zn-air battery using Co9S8@MoS2 as the cathode, showing an OCP of

about 1.38 V. (b) Galvanostatic charge-discharge cycling at a current density of 10 mA cm-2 with each cycle being 5 min. (c) Photograph of two Zn-air batteries in series, showing an OCP of about 2.73 V. (d) A photograph of device integrated with two cells in series to power a LED under real battery operating conditions. (e) Photograph of two Zn-air batteries using Co9S8@MoS2 as the cathode in series to carry out the overall electrochemical water-splitting.


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further demonstrated in Figure 9d. Two single Co9S8@MoS2 batteries connected in series can light red light emitting diode (LED) in series, which operates at a minimum voltage of 2.0 V (Figure 9d). Also, this device can also be used to carry out the overall electrochemical watersplitting activity (Figure 9e). 4. CONCLUSIONS In summary, we have synthesized Co9S8@MoS2 core-shell nanospheres via a facile approach and they can be used as trifunctional electrocatalysts for overall water splitting and Zn-air batteries for the first time. Specifically, the two-electrode electrolyzer assembled using the assynthesized Co9S8@MoS2 as both the anode and cathode could afford a low cell voltage of 1.67 V at the current density of 10 mA cm-2 and good stability, while the obtained Zn–air batteries based on Co9S8@MoS2 exhibit long cycle life (up to 120 cycles). The excellent electrocatalytic performances of Co9S8@MoS2 towards water splitting and Zn-air batteries can be attributed to the synergistic effect of the coupling between Co9S8 and MoS2, the improved conductivity and active surface area. We believe that this work will facilitate the development of newly efficient, stable, earth-abundant, noble metal-free tri-functional catalysts based on transitional metal chalcogenides. ASSOCIATED CONTENT Supporting Information. The FESEM images of Co9S8@MoS2 precursor, Co9S8 and MoS2; Raman and EDX spectra of Co9S8@MoS2; OER activities of Co9S8@MoS2 with different annealing temperatures; XPS survey spectrum and high-resolution Co 2p XPS spectrum for Co9S8@MoS2 after OER test; XRD patterns of Co9S8 and MoS2; CVs for Co9S8@MoS2, Co9S8 and MoS2 tested at different scan rates; HER LSVs of Co9S8@MoS2 with different annealing


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temperatures in both the acid and alkaline media; CVs and LSVs for Co9S8@MoS2, Co9S8, MoS2, and Pt/C in 0.1 M KOH; Corresponding Author * E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21471016 and 21271023) and the 111 Project (B07012). The authors would like to thank the Analysis & Testing Center of Beijing Institute of Technology for performing FESEM and TEM measurements REFERENCES (1) Jiao, Y.; Zheng, Y.; Jaroniecb, M.; Qiao, S. Z. Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (2) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G. Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-6502. (3) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473.


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(4) Xu, C. H.; Lu, M. H.; Zhan, Y.; Lee, J. Y. A bifunctional oxygen electrocatalyst from monodisperse MnCo2O4 nanoparticles on nitrogen enriched carbon nanofibers. RSC Adv. 2014, 4, 25089-25092. (5) Zou, X. X.; Zhang, Y. Noble metal-free hydrogen evolution catalysts for water splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (6) Wang, Z. L.; Xu, D.; Xu, J. J.; Zhang, X. B. Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes. Chem. Soc. Rev. 2014, 43, 7746-7786. (7) Xia, B. Y.; Wu, H. B.; Wang, X.; Lou, X. W. Highly Concave Platinum Nanoframes with High-Index Facets and Enhanced Electrocatalytic Properties. Angew. Chem. Int. Ed. 2013, 52, 12337-12340. (8) Nie, Y.; Li, L.; Wei, Z. D. Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. (9) Lee, W. H.; Kim, H. S. Oxidized iridium nanodendrites as catalysts for oxygen evolution reactions. Catal. Commun. 2011, 12, 408-411. (10) Hinnemann, B.; Moses, P. G.; Bonde, J.; Jørgensen, K. P.; Nielsen, J. H.; Horch, S.; Chorkendorff, I.; Nørskov, J. K. Biomimetic Hydrogen Evolution: MoS2 Nanoparticles as Catalyst for Hydrogen Evolution. J. Am. Chem. Soc. 2005, 127, 5308-5309. (11) Jaramillo, T. F.; Jørgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science. 2007, 317, 100-102. (12) Merki, D.; Fierro, S.; Vrubel, H.; Hu, X. L. Amorphous molybdenum sulfide films as catalysts for electrochemical hydrogen production in water. Chem. Sci. 2011, 2, 1262-1267.


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Table of contents entry

Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn–air Batteries

MoS2

Co9S8@MoS2

core-shell

heterostructures

are

successfully

constructed

and

structural

characterizations reveal that the core–shell structure can introduce heterogenous nano-interfaces between Co9S8 and MoS2, which can deeply influence the charge state of Co9S8@MoS2 to boost its electrocatalysis performances for overall water splitting and Zn-air batteries.


35

Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional

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