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Metal-Organic Framework-Derived Hollow CoSx@MoS2 Microcubes as Superior Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Evolution Reactions Lifan Yang, Li Zhang, Guancheng Xu, Xin Ma, Weiwei Wang, Huijun Song, and Dianzeng Jia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02428 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Sustainable Chemistry & Engineering

Metal-Organic

Framework-Derived

Hollow

CoSx@MoS2

Microcubes as Superior Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Evolution Reactions

Lifan Yanga,b,c, Li Zhanga,b,c,d,*, Guancheng Xua,b,c, Xin Maa,b,c, Weiwei Wanga,b,c, Huijun Songa,b,c, Dianzeng Jiaa,b,c,*

a

Key Laboratory of Energy Materials Chemistry, Ministry of Education, Xinjiang University, Shengli Road No. 666, Urumqi, 830046, China b Key Laboratory of Advanced Functional Materials, Autonomous Region, Xinjiang University, Shengli Road No. 666, Urumqi, 830046, China c Institute of Applied Chemistry, Xinjiang University, Shengli Road No. 666, Urumqi, 830046, China d Physics and Chemistry Detecting Center, Xinjiang University, Shengli Road No. 666, Urumqi, 830046, China

* Corresponding +86-991-8580586

author.

E-mail:

[email protected],

[email protected].

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ACS Sustainable Chemistry & Engineering 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

Abstract Electrocatalytic water splitting catalysts play important roles in clean energy conversion systems. Herein, metal-organic framework (MOF)-derived hollow CoSx@MoS2 microcubes were successfully synthesized by a novel method. Co-MOF [(CH3)2NH2][Co(HCOO)3] prepared by a simple liquid precipitation method at room temperature reacted with S2− released from TAA to generate Co9S8 under solvothermal conditions. Through hydrothermal treatment, numerous MoS2 nanosheets grew on the surface of CoSx vertically and uniformly after introduction of sulfur and molybdenum sources, finally generating CoSx@MoS2 heterostructures. As bifunctional electrocatalysts, the heterostructures exhibited remarkable performance for hydrogen evolution reaction with a low overpotential of 239 mV when the current density increased up to 10 mA cm−2 and a small Tafel slope of 103 mV dec−1 in 0.5 M H2SO4. They also worked effectively for oxygen evolution reaction with a low overpotential of 347 mV at 10 mA cm−2 in 1 M KOH. The enhanced electrocatalytic activities of CoSx@MoS2 can be ascribed to their unique heterostructures and the synergism between CoSx and MoS2. Keywords: Metal-organic framework, CoSx@MoS2 heterostructure, Electrocatalyst, Hydrogen evolution reaction, Oxygen evolution reaction Introduction With continuous development and progress of the society, the human consciousness of protecting the ecological environment has been rising. Therefore, the replacement of more polluted fossil energy with renewable clean energy has attracted much attention.1-3 As one of renewable energy sources, hydrogen energy is regarded the cleanest in the 21st century, because only water is produced by combustion, thereby zero emission of CO2.4,5 Electrocatalytic water splitting to hydrogen and oxygen is acknowledged as a promising technology.6,7 Noble metals and their alloys (e.g. Pt and RuO2/IrO2) are still the best catalysts for water splitting, with remarkable catalytic performance and high exchange current density.8,9 Nevertheless, they cannot be applied to large-scale industrial production due to high price and scarcity. Hence, it is of great significance to develop non-noble metal catalysts with low costs, abundant resources and comparable properties to those of noble metal ones.10,11 In recent years, MoS2 has been highlighted because of potential applicability in the field of energy. It has rich resource, high catalytic activity and stability, as well as low cost and Gibbs adsorption energy, thus having become a promising catalyst for HER to substitute platinum.12-15 However, the application of MoS2 is limited owing to inherent limitations (e.g. insufficient catalytically active sites and low conductivity), so it is necessary to modify it, probably through preparing composite material. Transition-metal-based materials (e.g. oxides,16,17 layered double hydroxides18,19 and sulfides20,21) are efficient catalysts for OER. Fabricating bifunctional electrocatalysts for both HER and OER by combining MoS2 with transition-metal-based materials not only overcomes the inherent limitations of MoS2, but also enhance the activity for water splitting. However, related bifunctional electrocatalysts have seldom been fabricated hitherto. For one thing, heterostructures can shorten the diffusion channels of ions and electrons in electrocatalytic reactions; for another, they depend on different counterparts to exert the synergistic effects, then promoting the electrocatalytic activities.22 Bai et al. prepared Co9S8@MoS2 core-shell heterostructures by solvothermal method and subsequent annealing


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treatment.23 The unique core-shell heterostructures had heterogeneous nanointerface, synergistic effects and abundant catalytically active sites, which boosted the electrocatalytic performance. Furthermore, the importance of heterostructures to electrocatalytic reactions has also been well-documented.24-26 Thus, it is feasible to fabricate heterogeneous bifunctional electrocatalysts by combining of HER catalyst of MoS2 with OER catalysts of transition-metal-based materials. In the past ten years, hollow structures have attracted widespread research interest because of controllable morphology, uniform size, large specific surface area, large internal space, low density and nano-scale surface shell.27 Notably, they have been widely used in electrochemistry, catalysis, adsorption, drug delivery and many other fields.28 Currently, hollow structures are synthesized by template-assisted strategies, including hard templates, soft templates and self-sacrificed templates.29 Hard and soft templates are similar in principle, i.e. target product first forms on a template which is then removed selectively. In many cases, however, it is difficult to select appropriate templates or to remove the templates efficiently.30 As to self-sacrificed templates, the morphology and size of products can be directly controlled, benefiting the synthesis of hollow structures. Given the adjustable compositions, large specific surface areas and modifiable pore structures of metal-organic frameworks (MOFs), they as self-sacrificed templates have been extensively studied in many fields.31-35 Regardless, their industrialization remains challenging, requiring the development of easily available MOFs and MOFs-derived materials. Thereby motivated, we herein proposed a facile strategy to fabricate MOF-derived hollow CoSx@MoS2 microcubes (Scheme 1). In the first step, Co-MOF [(CH3)2NH2][Co(HCOO)3] was prepared by a simple liquid precipitation method at room temperature. In the second step, Co9S8 products were fabricated through the reaction of Co-MOF with TAA under solvothermal conditions. Finally, MoS2 nanosheets were grown on the surface of cobalt sulfide through hydrothermal treatment. Particularly, the cubic morphology of the Co-MOF precursor was well inherited. The unique CoSx@MoS2 heterostructures worked as an efficient bifunctional electrocatalyst for both HER and OER depending on MoS2 and MOF-derived CoSx respectively. This method may also be applicable to the synthesis of other MOF-derived composites.

Scheme 1. Schematic illustration for synthesis process of hollow CoSx@MoS2 microcubes. Experimental Section Materials Cobalt nitrate hexahydrate (Co(NO3)2·6H2O), thioacetamide and absolute ethanol were purchased from Tianjin Yongsheng Fine Chemical Industry. Thiourea, aqueous dimethylamine solution ((CH3)2NH) and formic acid (HCOOH) were purchased from Tianjin Zhiyuan Chemical Reagent Plant. Nafion solution and sodium molybdate dihydrate (Na2MoO4·2H2O) were purchased from Alfa Aesar Chemicals Co., Ltd.. Polyvinylpyrrolidone K30 (PVP K30) was purchased from Tokyo Chemical Industry Co., Ltd.. All chemical reagents were commercially available and used as received without further purification.



Preparation of MOF precursor [(CH3)2NH2][Co(HCOO)3] In a typical synthesis, (CH3)2NH (8 mmol, 33% in water), HCOOH (8 mmol, 88% in water) and PVP K30 (0.5 g) were firstly dissolved in 25 mL of absolute ethanol. Afterwards, Co(NO3)2·6H2O (1 mmol) and PVP K30 (0.5 g) were dissolved in another 25 mL of absolute ethanol. Subsequently, a pink transparent solution was dropped into the former solution, and the mixture was stirred for 1 h and aged for 24 h at room temperature. The products were collected by centrifugation, and washed with absolute ethanol several times to remove any possible residual reactants before vacuum drying at 60 oC overnight. Preparation of Co9S8 hollow microcubes The as-synthesized [(CH3)2NH2][Co(HCOO)3] (30 mg) was completely dispersed in 30 mL of absolute ethanol by magnetic stirring, and then 25 mg thioacetamide was added under constant stirring for 10 min. The mixture was transferred into a 45 mL Teflon-lined autoclave and heated at 120 oC for 6 h. The black products were collected by centrifugation, and washed with deionized water and absolute ethanol several times to remove any possible residual reactants before vacuum drying at 60 oC overnight. Finally, the dried products were annealed under N2 atmosphere at 350 o C for 2 h with a slow heating rate of 1 oC min−1 to improve the crystallinity.36 Preparation of CoSx@MoS2 heterostructures Typically, the as-synthesized Co9S8 hollow microcubes (15 mg), Na2MoO4·2H2O (32 mg) and thiourea (64 mg) were completely dispersed in 16 mL of deionized water by magnetic stirring, and then the mixture was transferred into a 40 mL Teflon-lined autoclave and heated at 200 oC for 14 h. The subsequent process of CoSx@MoS2 heterostructures was similar to that of Co9S8 hollow microcubes. Preparation of MoS2 microspheres The preparation process of MoS2 microspheres was similar to that of CoSx@MoS2 heterostructures, but Co9S8 hollow microcubes were not used. Physical Characterizations Crystal phase was determined by powder X-ray diffraction (PXRD; Bruker D8 advance diffractometer, Cu Kα radiation, λ = 0.15405 nm). Microstructure was characterized by field emission scanning electron microscopy (FESEM; Hitachi S-4800 microscope) and transmission electron microscopy (TEM; Hitachi H600 microscope). Composition was analyzed by energy-dispersive X-ray spectroscope (EDX). Elemental mapping was recorded using EDX attached to FESEM. X-ray photoelectron spectroscopy (XPS) was measured on an Escalab 250 Xi system (Thermo Fisher Scientific). N2 adsorption-desorption isotherm was measured on Quantachrome Instrument (Autosorb-iQ2) at 77 K. Specific surface area and pore size distribution were calculated by the BET method and BJH method, respectively. Electrochemical measurements Except for electrochemical impedance measurements using a CHI 760E electrochemical workstation, other electrochemical measurements were conducted on a WaveDriver 20 Potentiostat/Galvanostat workstation coupled with a rotating disk electrode (Pine Research Instrumentation) at a rotation speed of 1600 rpm. A standard three-electrode configuration was used in all electrochemical measurements, employing polished glassy carbon electrode (5 mm in diameter) as the working electrode, graphite rod as the counter electrode and Ag/AgCl (saturated KCl) electrode as the reference electrode. The working electrode was prepared as follows. Typically, 2.8 mg of catalyst was dispersed in a 500 µL of mixed solution containing 240 µL of


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deionized water, 240 µL of absolute ethanol and 20 µL of 5 wt% Nafion solution through ultrasonication for 30 min to generate a homogeneous ink. Then, 10 µL of the catalyst ink was dropped on the glassy carbon electrode (loading: 0.285 mg cm−2) and dried at ambient temperature. All potentials were referenced to a reversible hydrogen electrode (RHE) with the following equation: E(RHE) = E(Ag/AgCl) + (0.1989 + 0.059 pH) V (1) All data were not corrected for iR losses. Linear sweep voltammetry (LSV) was performed from 1.2 to 2 V vs. RHE in 1 M KOH (pH = 13.3) and from 0 to −0.8 V vs. RHE in 0.5 M H2SO4 (pH = 0.36) at a sweep rate of 5 mV s−1 to obtain polarization curves. Tafel plots were achieved by replotting the polarization curves. Electrochemical impedance spectroscopy (EIS) was carried out from 0.01 to 100 000 Hz with an amplitude of 5 mV at 1.5 V vs. RHE in 1 M KOH and −0.15 V vs. RHE in 0.5 M H2SO4. Long-term stability tests were performed by continuous cyclic voltammetry (CV) with a scan rate of 100 mV s−1 for 500 cycles. The electrochemical double layer capacitance was obtained by CV from 0.25 to 0.35 V vs. RHE in 1 M KOH and from 0.15 to 0.25 V vs. RHE in 0.5 M H2SO4 from 20 to 100 mV s−1. Results and discussion

Figure 1. FESEM images of Co-MOF (a, d), Co9S8 (b, e) and CoSx@MoS2 (c, f). EDX mapping of CoSx@MoS2 (g-j). Co-MOF was prepared by a facile liquid precipitation method at room temperature. According to the PXRD pattern (Figure S1a), all diffraction peaks are in good agreement with the corresponding simulated crystal structure pattern, verifying the harvest of pure phase [(CH3)2NH2][Co(HCOO)3]. As shown in FESEM images (Figure 1a and d), the as-prepared Co-MOF shows a well-defined cubic morphology, with a smooth surface and an average size of about 2 µm. Additionally, TEM image (Figure S1b) indicates its solid feature.



Co9S8 was fabricated through a solvothermal reaction. According to FESEM images (Figure 1b and e), the cubic morphology is well retained after sulfidation. In addition, hollow Co9S8 consists of a large number of particles and has a rough surface. TEM image (Figure S2b) further confirms the hollow feature which formed due to the Kirkendall effect. In the presence of ethanol, TAA decomposed and released H2S, and [Co(HCOO)3]− anion was ionized simultaneously. Co9S8 was fabricated through the anion exchange reaction of [Co(HCOO)3]− with S2−. Given a much smaller radius, Co2+ diffused outward much faster than S2− did inward. As the reaction proceeded, Co9S8 hollow microcubes finally formed.36 As shown in Figure S2a, all diffraction peaks match well with the characteristic reflections of Co9S8 (JCPDS No. 65-6801), without any impurity, suggesting high purity of the product. Moreover, the as-prepared Co9S8 had high crystallinity, as evidenced by the sharp peaks. Collectively, Co-MOF completely transformed into Co9S8 while retaining the cubic morphology, so the sulfidation conditions were suitable. Without Co9S8 template, pure MoS2 had high crystallinity (Figure S3a), and nanosheets stacked to form interconnected microspheres with poor dispersion (Figure S3b and c). Also, TEM image (Figure S3d) shows the same structure and proves its solid feature. CoSx@MoS2 was finally obtained after hydrothermal reaction. The hollow cubic morphology was completely maintained and decorated by numerous nanosheets vertically and uniformly (Figure 1c and f). Elemental mapping (Figure 1g-j) demonstrated that Co, S, and Mo elements existed in the nanocomposite.

Figure 2. PXRD pattern (a), TEM (b, c) and HRTEM images (d) of CoSx@MoS2. The phase structure of CoSx@MoS2 was further detected by using PXRD. As shown in Figure 2a, the diffraction peaks at 2θ = 16.3°, 26.8°, 31.5° and 38.3° are indexed to the characteristic (111), (220), (311) and (400) planes of Co3S4 (JCPDS No. 47-1738) respectively. Moreover, the diffraction peaks at 2θ = 28.0°, 32.4°, 36.3° and 47.0° correspond to the characteristic (111), (200),


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(210) and (220) planes of CoS2 (JCPDS No. 41-1471) respectively. The diffraction peak at 2θ = 55.1° is the superposition of two characteristic peaks. Interestingly, after loading MoS2 nanosheets, the as-synthesized Co9S8 transformed into mixed phases of Co3S4 and CoS2. The percentage of S increased, because excess thiourea was added during MoS2 preparation. Excessive sulfur may lead to defect which was beneficial to the improvement of electrocatalytic performance.13 The phase transition can also be attributed to the low stability of cobalt sulfide itself. In addition, the characteristic peaks of MoS2 are unobvious, because the crystallinity of cobalt sulfide exceeded that of molybdenum disulfide. Moreover, the diffraction peak at 2θ = 40.1° may be the superposition of the two characteristic peaks of CoS2 and MoS2, and that at 2θ = 50.4° may be the superposition of the two characteristic peaks of Co3S4 and MoS2. It is worth mentioning that there are no characteristic peaks of MoO3 in PXRD pattern (Figure S4a), meanwhile, Raman (Figure S4b) and FTIR spectra (Figure S4c) show that there are no obvious bands at the typical stretching frequencies of MoO3 (993, 817, and 666 cm-1).37,38 Given all that, the sample we prepared does not contain MoO3. The EDX spectrum (Figure S5) indicates that the atomic ratio of Co to Mo is 11.4:1, so the content of molybdenum disulfide is much less than cobalt sulfide, which may be one of the reasons why the characteristic peaks of MoS2 are not obvious. TEM images (Figure 2b and c) of CoSx@MoS2 display the same structural information as that in SEM images. With an average size of about 2 µm, the hollow microcube was decorated by numerous nanosheets. Meanwhile, the HRTEM image (Figure 2d) exhibits visible lattice fringes, and the interplanar spacings of 0.614, 0.285 and 0.271 nm can be assigned to the (002), (311) and (200) planes of MoS2, Co3S4 and CoS2 respectively.39-41

Figure 3. XPS survey spectrum (a) and high-resolution XPS spectra of Co 2p (b), Mo 3d (c) and S 2p (d) for CoSx@MoS2.



The elemental states and chemical compositions of CoSx@MoS2 were further analyzed by using XPS. The survey spectrum (Figure 3a) demonstrates that CoSx@MoS2 contains Co, Mo, S, C and O elements. Probably, C and O elements existed due to prolonged exposure in air. There are two spin-orbit doublets in the high-resolution Co 2p spectrum (Figure 3b). The peaks at 778.9 eV (Co 2p3/2) and 794.1 eV (Co 2p1/2) represent Co3+, and those at 781.3 eV (Co 2p3/2) and 797.5 eV (Co 2p1/2) correspond to Co2+. Moreover, there are two satellite peaks (identified as Sat.) at 786.9 eV and 803.1 eV.42,43 For the Mo 3d spectrum (Figure 3c), the peaks at 232.6 eV and 229.4 eV can be ascribed to Mo 3d3/2 and Mo 3d5/2 respectively, being in accordance with the oxidation state of Mo4+ in MoS2.44 Meanwhile, the peaks at 226.5 eV and 235.7 eV can be assigned to the Mo-S bond and Mo6+ respectively, and Mo6+ may originate from sample’s oxidation in air.45 In the S 2p spectrum (Figure 3d), the peaks at 162.1 eV and 163.4 eV correspond to S 2p3/2 and S 2p1/2 of the Mo-S bonds in MoS2 respectively, and the peaks at 161.8 eV and 163.2 eV correspond to S 2p3/2 and S 2p1/2 of the Co-S bonds in CoSx respectively.23,25,46 According to N2 adsorption-desorption isotherm measurement (Figure S6), the specific surface area of CoSx@MoS2 (54.34 m2 g-1) was much larger than those of Co9S8 (15.25 m2 g-1) and MoS2 (1.767 m2 g-1). It is well known that high specific surface area is not only conducive to the exposure of active sites, but also to the rapid mass transfer of electrolytes in reaction processes, thereby augmenting the electrocatalytic activity. As shown in Figure S3, MoS2 nanosheets form microspheres by self-assembly. However, the dispersibility of the microspheres is poor and they are connected to each other, so the specific surface area of MoS2 is very small even if the nanosheets grow vertically. Hollow Co9S8 microcubes are composed of a large number of particles, which are tightly bound and there are no obvious pores between them. Therefore, the specific surface area of Co9S8 is also small. Different from MoS2 and Co9S8, hollow CoSx@MoS2 microcubes have unique structural advantages. Hollow CoSx, as a template, prevents the stacking of MoS2 nanosheets effectively, the uniform dispersion and vertical growth of nanosheets greatly increase the specific surface area of CoSx@MoS2. In addition, the insets in Figure S6 show that the pore size distributions of the three samples are in a similarly mesoporous range (2-10 nm), so the contribution of pores can not be ignored.


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Figure 4. Polarization curves of CoSx@MoS2, Co9S8 and MoS2 (a) and corresponding Tafel slopes (b) in 0.5 M H2SO4. Polarization curves of CoSx@MoS2, Co9S8 and MoS2 (c) and corresponding Tafel slopes (d) in 1 M KOH. The electrocatalytic performance for HER was investigated by LSV in N2-saturated 0.5 M H2SO4. As shown in Figure 4a, hollow CoSx@MoS2 microcubes exhibit the best catalytic performance for HER with the lowest onset overpotential. When the current density increased to 10 mA cm−2, the overpotential of CoSx@MoS2 (239 mV) was lower than those of Co9S8 (272 mV) and MoS2 (491 mV). Clearly, pure MoS2 barely catalyzed HER, because the interconnected microspheres had tight structures that inhibited the exposure of active sites. Contrarily, vertical MoS2 nanosheets growing on the surface of CoSx increased the exposure of active sites and the specific surface area, which, in combination with cobalt sulfide, remarkably boosted the catalytic performance for HER. The HER kinetics was further analyzed by Tafel plots. The slopes were obtained according to the Tafel equation: η = a + b log(j) (2) (η is the overpotential, a is the overpotential when the current density reaches 1 mA cm−2, b is the Tafel slope and j is the current density). As an intrinsic characteristic of electrocatalyst, Tafel slope represents the change of overpotential when the current density is increased or decreased 10-fold, which is determined by the limiting step of HER. Meanwhile, a lower slope means faster kinetics, and the mechanism of HER can also be predicted by the slope magnitude. Electrochemical HER in acidic electrolytes can be divided into two pathways (Volmer–Heyrovsky mechanism or Volmer–Tafel mechanism) and three principal steps.47,48 (1) Volmer reaction (electrochemical hydrogen adsorption) H3O+ + e− → Hads + H2O (~120 mV dec−1) (3)



(2) Heyrovsky reaction (electrochemical desorption) H3O+ + Hads + e− → H2 + H2O (~40 mV dec−1) (4) (3) Tafel reaction (chemical desorption) Hads +Hads → H2 (~30 mV dec−1) (5) As shown in Figure 4b, the Tafel slope of CoSx@MoS2 heterostructures is lowest (103 mV dec−1), and those of Co9S8 and MoS2 are 107 and 190 mV dec−1 respectively. The Tafel slope of CoSx@MoS2 heterostructures suggested the Volmer-Heyrovsky mechanism. CoSx@MoS2 heterostructures were further investigated as OER electrocatalysts in N2-saturated 1 M KOH. Figure 4c displays the polarization curves of different products. The overpotential of hollow CoSx@MoS2 microcubes was 347 mV when the current density rose up to 10 mA cm−2, which was lower than that of Co9S8 (378 mV). Moreover, the Tafel slope of CoSx@MoS2 (147 mV dec−1) was lower than that of Co9S8 (157 mV dec−1) (Figure 4d). Therefore, CoSx@MoS2 heterostructures showed outstanding catalytic performance and kinetics for OER. Obviously, pure MoS2 has little effect as an inactive OER catalyst, but CoSx@MoS2 shows significantly improved catalytic activity compared to MoS2 and Co9S8, indicating that the synergism between CoSx and MoS2 built by the unique heterostructures can obviously improve the OER activity of CoSx@MoS2. In addition, although both CoSx@MoS2 and Co9S8 are hollow microcubes, the overpotential of CoSx@MoS2 is smaller than that of Co9S8, which is due to the unique heterostructure of CoSx@MoS2. Firstly, CoSx@MoS2 has a larger specific surface area, which is conducive to providing richer active sites and further reducing the diffusion length of ions and electrons. Secondly, the external MoS2 improves the HER performance by exposing more active edge sites15 and stabilizes the product in acidic electrolyte because of its excellent acid resistance. Thirdly, the synergism between CoSx and MoS2 contributes a lot in enhancing the catalytic performances. To sum up, the heterostructures are potentially applicable as efficient bifunctional electrocatalysts. Electrochemical surface area (ECSA) and charge-transfer resistance (RCT) are closely related to catalytic performance. The ECSA is proportional to the electrochemical double-layer capacitance (Cdl).49 Since no redox reaction occurred in the selected voltage range, there was no charge transfer, and the current only originated from the charge and discharge of the double layer. The Cdl values of CoSx@MoS2, Co9S8 and MoS2 in 0.5 M H2SO4 were calculated to be 6.06, 2.56, and 0.54 mF cm−2 respectively (Figure S7), and those in 1 M KOH were 6.33, 3.34, and 0.66 mF cm−2 respectively (Figure S8). Given that CoSx@MoS2 had the highest Cdl values in both acidic and alkaline electrolytes, electrochemical surface area was largest and maximum effective active sites were exposed.50 Owing to promoted proton adsorption and desorption, the catalytic performance was improved significantly. In addition, the charge-transfer impedance of hollow CoSx@MoS2 microcubes was much smaller than those of Co9S8 and MoS2 at the same potential in both acidic and alkaline electrolytes (Figure S9). Hence, CoSx@MoS2 heterostructures effectively reduced the charge-transfer impedance and promoted the electron conduction between electrolyte and electrode, being crucial to improvement of the catalytic kinetics.51 Furthermore, the exchange current density (j0), which describes the ability of an electrode to gain and to lose electrons and reflects the difficulty of an electrode reaction, was obtained by extrapolating the Tafel plot. Generally, j0 is an activity parameter of HER but not OER. As shown in Figure S10, j0 of CoSx@MoS2 (6.28×10−2 mA cm−2) is higher than those of Co9S8 (2.85×10−2 mA cm−2) and MoS2 (2.70×10−2 mA cm−2), indicating that a lower driving force was required by CoSx@MoS2 to start


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the reaction.52 The long-term stabilities of electrocatalysts were tested by continuous CV. For HER, MoS2 showed excellent stability, barely losing current density (Figure S11a). On the contrary, the performance of Co9S8 severely deteriorated after 500 cycles (Figure S11b), indicating its low stability in acidic conditions. After 500 cycles, the current density of CoSx@MoS2 only decreased slightly (Figure S11c), probably because the stability was improved by external MoS2 that protected internal CoSx. For OER, the current density of CoSx@MoS2 reduced less than that of Co9S8 did after 500 cycles in alkaline conditions (Figure S11d and e) due to its unique heterostructures. The outstanding catalytic performances of MOF-derived hollow CoSx@MoS2 microcubes for HER and OER can be attributed to the following advantages. (i) The hollow structures had a large specific surface area which was conducive to the exposure of active sites and shortened the diffusion channels of ions and electrons. (ii) The vertical MoS2 nanosheets boosted the HER performance by exposing more active edge sites. (iii) The synergism between CoSx and MoS2 played an important role in enhancing the catalytic performances for both HER and OER. (iv) The unique heterostructures simultaneously had the advatanges of cobalt sulfide and molybdenum disulfide. Cobalt sulfide, as a template, prevented the stacking of MoS2 nanosheets effectively. Also, the inherent metallic conductivity of cobalt sulfide facilitated the electron transfer from active sites to electrodes. Besides, external MoS2 with excellent acid resistance protected internal CoSx, managing to stabilize the product in acidic electrolyte. The detailed comparisons with other reported electrocatalysts are shown in Table S1 and S2. Conclusions In summary, MOF-derived hollow CoSx@MoS2 microcubes were successfully synthesized by a three-step method and used as bifunctional electrocatalysts for both HER and OER. Compared with pure Co9S8 and MoS2, CoSx@MoS2 heterostructures exhibited better catalytic properties, including low overpotential, small Tafel slope, large specific surface area and electrochemical surface area, as well as high current density, long-term stability and electrical conductivity. The unique heterostructures and the synergism between CoSx and MoS2 predominantly augmented the catalytic activities. Thus, MOF-derived hollow CoSx@MoS2 microcubes are promising bifunctional electrocatalysts for electrochemical energy applications, and the novel strategy may be extended to synthesize other MOF-derived composites. Supporting Information PXRD pattern and TEM image of Co-MOF. PXRD pattern and TEM image of Co9S8. PXRD pattern, FESEM and TEM images of MoS2. Nitrogen adsorption–desorption isotherms of CoSx@MoS2, Co9S8, MoS2 and corresponding pore size distribution (inset). EDX spectrum of hollow CoSx@MoS2 microcubes. Cyclic voltammograms of CoSx@MoS2 , Co9S8 and MoS2 measured at various scan rates in 0.5 M H2SO4, corresponding linear slopes at 0.2 V vs RHE. Cyclic voltammograms of CoSx@MoS2, Co9S8 and MoS2 measured at various scan rates in 1 M KOH, corresponding linear slopes at 0.3 V vs RHE. Nyquist plots of CoSx@MoS2, Co9S8 and MoS2 for HER collected at -0.15 V vs RHE and for OER collected at 1.5 V vs RHE. Exchange current densities of MoS2, Co9S8 and CoSx@MoS2 for HER. Stability tests of MoS2, Co9S8 and CoSx@MoS2 for HER and OER. Comparison of HER and OER activities of CoSx@MoS2 with



other reported electrocatalysts.

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Abstract Graphical

Herein, we designed a novel method to prepare MOF-derived hollow CoSx@MoS2 microcubes. Particularly, the cubic morphology of the Co-MOF precursor was well inherited. The unique CoSx@MoS2 heterostructures worked as an efficient bifunctional electrocatalyst for both HER and OER depending on their unique heterostructures and the synergism between CoSx and MoS2.


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