A novel core–double shell heterostructure derived from a metal–organic framework for efficient HER, OER and ORR electrocatalysis

Abstract

The hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the cornerstone reactions of renewable energy technologies, like electrochemical water splitting and metal–air batteries. To promote these reactions, robust and efficient catalysts are highly desired. However, developing multifunctional electrocatalysts for integrating the HER, OER and ORR into one electrode still remains a huge challenge. Constructing intriguing nanostructures with multiple components and hierarchical interfaces could be an efficient method to develop multifunctional electrocatalysts. Herein, we report a strategy, derived from ZIF-67, to synthesize a novel core–double shell heterostructure Co₉S₈@Co₉S₈@MoS₂-0.5 as a trifunctional pre-catalyst for the HER, OER and ORR. Firstly, a core–shell Co@Co₉S₈ precursor was prepared through sulfurization and pyrolysis of ZIF-67. Then, a hydrothermal method was adopted to grow MoS₂ on the Co@Co₉S₈ precursor. Such a synthetic strategy endows the heterostructure with a strong interface effect that affects the electronic structure of materials and further boosts their electrocatalytic performance. The obtained Co₉S₈@Co₉S₈@MoS₂-0.5 heterostructure shows outstanding electrochemical performance. The overpotentials required for the HER and OER in an alkaline solution are 173 mV and 340 mV at 10 mA cm⁻², respectively. Moreover, Co₉S₈@Co₉S₈@MoS₂-0.5 also exhibits superior ORR performance with a half-wave potential of 0.77 V. The strategy described here can be extended to a number of integrated multifunctional electrocatalysts for water splitting and metal–air batteries.

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Introduction

Alternative energy storage and conversion technologies, such as metal–air batteries, water splitting devices, and fuel-cells, have attracted wide attention to meet the increasing demand of global energy and the crisis of fossil fuels during recent years^1–3 These renewable energy technologies usually involve three crucial reaction processes i.e., the ORR, OER and HER.^4,5 However, these reactions require a high overpotential to drive because of their sluggish kinetics. Although noble metal-based catalysts could improve reaction kinetics effectively, their high cost and vulnerability are not conducive to practical application.⁶ It is urgent to develop nonprecious metal-based electrocatalysts that possess high activity toward the HER, OER and ORR.

Construction of efficient tri-functional electrocatalysts for the HER, OER and ORR is of vital importance, yet challenging. Heterostructure materials, especially by introducing alien active components into the well-defined structure, show an activity superior to the single-component ones as electrocatalysts for the HER, OER and ORR.^7–10 For instance, Zhang et al. found that a 3D mesoporous heterostructure of N-MoS₂ grown on graphene exhibited excellent activity towards the OER, HER and ORR. The enhanced catalytic performance is ascribed to the integrated interfacial coupling and modified electronic structure of both N-MoS₂ and graphene, in which MoS₂ plays an important role in the HER.⁷ MoS₂ has been demonstrated to be a promising electrocatalyst for the HER because the undercoordinated Mo–S sites can be its active sites.¹¹ However, due to its serious aggregation and poor conductivity, MoS₂ requires a large overpotential to drive the HER. Enormous efforts have been devoted to improving the electrical conductivity of the MoS₂ nanostructure. Among them, integrating MoS₂ with other conductive materials is one of the most effective strategies.¹² Co₉S₈, a conductive transition-metal sulfide, has attracted wide attention recently for its relatively high electrical conductivity and advanced electrocatalytic performance.¹³ It is well recognized that the chemical environments of cobalt ions dominate the catalytic activity of Co₉S₈. Many methods, such as heteroatom doping, decorating with nanoparticles and developing a hierarchical nanoarchitecture, can be used to modify the chemical environments of cobalt ions.^14–16 Constructing a hierarchical nanoarchitecture has been proved to be an effective strategy and has been attracting more and more attention.^17–19 Possessing higher surface areas and more interfaces, hierarchical nanoarchitectures not only combine the individual advantages of each component, but also shorten the diffusion path of ions and decrease the interfacial contact resistance to synergistically improve their electrocatalytic activity.^16,17 Inspired by the above advances, integrating the unique advantages of MoS₂ and Co₉S₈ into one hierarchical nanostructure could be challenging but of great importance to develop electrocatalysts with multiple functions.

In this contribution, we report a hierarchical nanoarchitecture of Co₉S₈@Co₉S₈@MoS₂-0.5 derived from ZIF-67 as a tri-functional pre-catalyst for the HER, OER and ORR. The nanoarchitectures were prepared by following two procedures: firstly, a precursor Co@Co₉S₈ polyhedron was obtained by sulfurization of ZIF-67 followed by pyrolysis. Then, the MoS₂ layer was spread onto the surface of the precursor through a solvent-thermal method. In the first procedure, the Co core in Co@Co₉S₈ was converted into Co₉S₈ because of the sulfurization of L-cysteine. As we expected, an intriguing core–double shell hierarchical Co₉S₈@Co₉S₈@MoS₂-0.5 nanoarchitecture was finally obtained. The special synthetic strategy of this heterostructure can effectively regulate the electronic structure of each component, retain the integrity of morphology and generate abundant interfaces between phases, which facilitate electrocatalysis. This novel Co₉S₈@Co₉S₈@MoS₂-0.5 hierarchical nanostructure exhibits HER and OER activities, and also possesses advanced ORR electrocatalytic activity. In particular, Co₉S₈@Co₉S₈@MoS₂-0.5 requires an overpotential of 173 mV and 340 mV only for the HER and OER, respectively, at a current density of 10 mA cm⁻² in 1 mol L⁻¹ KOH. When Co₉S₈@Co₉S₈@MoS₂-0.5 is applied to the ORR, the half-wave potential (E_1/2) is 0.77 V in 0.1 mol L⁻¹ KOH and the diffusion-limiting current density is 4.76 mA cm⁻² at 0.6 V (vs. RHE), showing excellent activity for water splitting and application potential in metal–air batteries. The design and synthetic strategy of our components prospectively pave the new path for energy electrocatalysis and the design of heterostructures.

Results and discussion

The construction details are provided in Fig. 1. Firstly, ZIF-67 polyhedra were prepared by precipitating cobalt nitrate hexahydrate and 2-methylimidazole in methanol solution at room temperature for 12 h (step i). Then ZIF-67 was refluxed with thioacetamide (TAA) in ethanol at 90 °C for 18 min to give a core–shell nanostructure of ZIF-67@CoS (step ii). Afterwards, the intermediate product Co@Co₉S₈ was obtained through the pyrolysis of ZIF-67@CoS at 900 °C under an inert atmosphere for 100 min. During the progress of pyrolysis, the organic ligand was carbonized and Co²⁺ in ZIF-67 was reduced to a Co core (step iii). Subsequently, a series of Co₉S₈@Co₉S₈@MoS₂-x (x = 0.5, 2.0 and 4.0) were obtained by adjusting the mass ratio x of the Co@Co₉S₈ precursor to sodium molybdate dihydrate under hydrothermal conditions. During this process, MoS₂ grown onto the Co@Co₉S₈ surface and the Co core of Co@Co₉S₈ was sulfurized to Co₉S₈ simultaneously. Finally, the core–double shell Co₉S₈@Co₉S₈@MoS₂-x heterostructure was obtained (step iv).

Fig. 1 Schematic illustration of the synthesis of Co₉S₈@Co₉S₈@MoS₂-x (x = 0.5, 2.0, 4.0).

X-ray diffraction (XRD) was carried out to characterize the crystal structures of the as-prepared samples. As shown in Fig. S1,† ZIF-67 were identified as typical diffractions of the (011), (002) and (112) planes.²⁰ After sulfurization for 18 min, the outer parts of ZIF-67 turned into CoS while the inner parts still remained ZIF-67 due to the presence of (011), (002) and (112) planes. EDS mappings of ZIF-67@CoS showed that Co and S elements are uniformly dispersed on the ZIF-67@CoS surface (Fig. S2†), indicating the surface sulfurization of ZIF-67. Furthermore, diffraction peaks of the Co@Co₉S₈ sample at around 2θ = 44.2, 51.5 and 75.6°, shown in Fig. 2a, mainly originated from the Co (JCPDS No. 15-0806) core, and other peaks are attributed to the characteristic crystal planes of Co₉S₈. Owing to the overlap of diffraction peaks between Co₉S₈ and the Co core, we calcined ZIF-67 to obtain the metal Co phase by the same calcination process as that for the preparation of Co@Co₉S₈ (1.7 part of the ESI†). From the XRD patterns shown in Fig. S3,† the diffraction peaks of the Co phase are clearly observed. The XRD pattern of Co₉S₈@Co₉S₈@MoS₂-0.5 shows three main peaks at 29.8, 31.2 and 52.1°, corresponding to the (311), (222) and (440) planes of Co₉S₈ (JCPDS No. 73-1442). According to the results of XRF (X-ray fluorescence) (Table S1†), the weight ratio of m(MoS₂)/[m(MoS₂) + m(Co₉S₈)] in Co₉S₈@Co₉S₈@MoS₂-0.5 is calculated to be 12%. Even though the content of MoS₂ is higher than the detection limit of XRD (5%), the characteristic peaks of MoS₂ are obscure, which may be related to the low crystallinity of MoS₂. Upon increasing the addition of sodium molybdate dihydrate in step iv, the peaks of MoS₂ were gradually observed at 14.5, 33.0 and 58.3° in Co₉S₈@Co₉S₈@MoS₂-2.0 and Co₉S₈@Co₉S₈@MoS₂-4.0, corresponding to the (003), (006) and (110) planes of MoS₂ (JCPDS No. 17-0744) (Fig. S3†). This observation indicates that MoS₂ layers could grow restrictively on the precursor surface during the preparation. The carbon contents of Co₉S₈@Co₉S₈@MoS₂-x (x = 0.5, 2.0 and 4.0) were measured by CHNS elemental analysis and the results are listed in Table S2.† The content of residual carbon species in Co₉S₈@Co₉S₈@MoS₂-0.5 is about 6.8% and gradually reduced in Co₉S₈@Co₉S₈@MoS₂-2.0 and Co₉S₈@Co₉S₈@MoS₂-4.0. X-ray photoelectron spectroscopy (XPS) is applied to evaluate the surface composition and chemical states of the ions in the samples of Co₉S₈@Co₉S₈@MoS₂-0.5, Co@Co₉S₈ and MoS₂. As shown in Fig. S4,† the XPS survey spectrum of MoS₂ only shows the signals of Mo and S elements, while that of Co@Co₉S₈ exhibits the photoelectron peaks of Co and S elements. Comparatively, all Co, Mo and S photoelectron signals are detected for the sample of Co₉S₈@Co₉S₈@MoS₂-0.5. The presence of relatively intense shakeup satellite peaks in the Co 2p spectrum of Co₉S₈@Co₉S₈@MoS₂-0.5 (Fig. 2b) demonstrates that Co ions in this sample exhibit two different oxidation states. As shown in Fig. 2b, the peaks at about 785.4 and 801.4 eV observed for Co₉S₈@Co₉S₈@MoS₂-0.5 are assigned to the shakeup satellite peaks (noted as “Sat.”). The peaks at 779.16 and 794.29 eV are ascribed to 2p_3/2 and 2p_1/2 of Co³⁺, while those at 781.32 and 796.7 eV are assigned to 2p_3/2 and 2p_1/2 of Co²⁺, respectively.²¹ It should be noted that the Co 2p peaks of Co₉S₈@Co₉S₈@MoS₂-0.5 shift slightly toward higher binding energies compared with those of Co@Co₉S₈, demonstrating that the electronic density of Co changes with the strong coupling interaction between Co₉S₈ and MoS₂. For the Mo 3d spectra of Co₉S₈@Co₉S₈@MoS₂-0.5, the peaks at 232.5 and 226.5 eV correspond to Mo 3d_3/2 and Mo 3d_1/2, respectively (Fig. 2c).²² The peak at about 236.0 eV is attributed to mild oxidation of the surface Mo ions.²³ Compared with MoS₂, the Mo 3d peak of Co₉S₈@Co₉S₈@MoS₂-0.5 shifts slightly toward low binding energy. Fig. 2d compares the S 2p spectra of Co₉S₈@Co₉S₈@MoS₂-0.5, MoS₂ and Co@Co₉S₈. The peaks at 162.4 and 163.5 eV observed for Co@Co₉S₈ can be ascribed to S 2p_3/2 and 2p_1/2.²⁴ For Co₉S₈@Co₉S₈@MoS₂-0.5, double S 2p peaks at around 162.1 and 162.3 eV also shift toward low binding energy compared with those of MoS₂ and Co@Co₉S₈. X-ray adsorption near edge structure (XANES) spectroscopy, a sensitive technology for the determination of changes in the chemical bond and surface electronic structure, was performed to further investigate the electronic structure of surface sulfur. The S L-edge XANES spectra normalized from 160 to 185 eV are shown in Fig. S5.† The peaks at 162.8, 164.0 and 164.8 eV are ascribed to three dominant transitions originating from spin–orbit splitting.^25–28 Compared with MoS₂, the slightly decreased white line intensity of Co₉S₈@Co₉S₈@MoS₂-0.5 indicates a lower electron density at the S site, which is coincident with the shift of the S 2p XPS spectra.²⁹ Notably, obvious shifts in the XPS spectra and a decreased intensity in S L-edge XANES confirm the presence of a coupled interface, caused by the strong interaction between Co₉S₈ and MoS₂ and the successful synthesis of the heterostructure. The nature of MoS₂ was further proved by Raman spectroscopy recorded with a 532 nm line (Fig. S6†). The characteristic Raman peaks at around 379.28 and 401.86 cm⁻¹ correspond to the in-plane E¹_2g and out-of-plane A_1g vibration modes of the 2H MoS₂ phase, respectively.⁷

Fig. 2 (a) XRD patterns of Co₉S₈@Co₉S₈@MoS₂-0.5, Co@Co₉S₈, and MoS₂, and the standard patterns of Co (JCPDS No. 15-0806) (green line), Co₉S₈ (JCPDS No. 73-1442) (purple line), and MoS₂ (JCPDS No. 17-0744) (red line); XPS spectra of (b) Co 2p for Co₉S₈@Co₉S₈@ MoS₂-0.5 and Co@Co₉S₈, (c) Mo 3d for Co₉S₈@Co₉S₈@MoS₂-0.5, and MoS₂ and (d) S 2p for Co₉S₈@Co₉S₈@MoS₂-0.5, Co@Co₉S₈ and MoS₂.

A typical scanning electron microscopy (SEM) image combined with transmission electron microscopy (TEM) reveals that the size and polyhedral morphology of Co₉S₈@Co₉S₈@MoS₂-0.5 are well retained after the hydrothermal and pyrolysis processes (Fig. 3a, b). As further elucidated by the TEM image (Fig. 3b), MoS₂ grows on the surface of Co₉S₈ and forms a Co₉S₈@Co₉S₈@MoS₂-0.5 heterostructure. Furthermore, a high-resolution TEM (HRTEM) image of Co₉S₈@Co₉S₈@MoS₂-0.5 is shown in Fig. 3c, in which the lattice fringes with inter-planar distances of 0.64 and 0.50 nm correspond to the (002) plane of MoS₂ and the (200) plane of Co₉S₈, respectively. This observation is consistent with that of the XRD analysis above (Fig. 2a). The high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) image of Co₉S₈@Co₉S₈@MoS₂-0.5, shown in Fig. 3d, clearly proves the core–double shell structure. Energy-dispersive X-ray spectroscopy (EDS) analysis also indicates that Co is concentrated at the core and Mo are uniformly distributed in the shell of these materials. The polyhedral morphology of Co₉S₈@Co₉S₈@MoS₂-2.0 and Co₉S₈@Co₉S₈@MoS₂-4.0 also remains stable (Fig. S7†), i.e. Co₉S₈@Co₉S₈@MoS₂-x inherits the core–shell morphology well from the precursor Co@Co₉S₈ (Fig. S8†). After being encapsulated with MoS₂, the BET (Brunauer–Emmett–Teller) surface area of Co₉S₈@Co₉S₈@MoS₂-0.5 reduces greatly to 42.14 m² g⁻¹ from 115.84 m² g⁻¹ (Co@Co₉S₈), but it is still larger than that of MoS₂ (29.38 m² g⁻¹) (Fig. S9†).

Fig. 3 (a) SEM, (b) TEM and (c) HRTEM images of Co₉S₈@Co₉S₈@MoS₂-0.5; and (d) EDX mappings of elemental Co, Mo and S of Co₉S₈@Co₉S₈@MoS₂-0.5.

The above results clearly demonstrate that we successfully synthesized a polyhedral core–double shell Co₉S₈@Co₉S₈@MoS₂-0.5 nanostructure with strong interaction and a tunable electronic structure between the component phases. The polyhedral precursor Co@Co₉S₈ provides basic space to confine the subsequent growth of MoS₂, prevents the aggregation of MoS₂, and facilitates the exposure of more active sites. Eventually, the features of unique morphology, synergistic effect of component phases, interfacial interaction between Co₉S₈ and MoS₂, and abundant exposed sites enable the as-prepared heterostructure to show great potential as a gas-involving heterogeneous pre-catalyst.

We assessed the OER and HER electrocatalytic activities of the as-prepared samples firstly in 1 mol L⁻¹ KOH and also compared them with commercial Pt/C and IrO₂. As shown in Fig. 4a and Fig. S10,† Co₉S₈@Co₉S₈@MoS₂-0.5 exhibits an excellent catalytic performance for the OER. More specifically, the current density reaches 10 mA cm⁻² only at 1.57 V versus RHE, while Co₉S₈@Co₉S₈@MoS₂-2.0 and Co₉S₈@Co₉S₈@MoS₂-4.0 require overpotentials of 380 and 404 mV, respectively. As shown in Fig. S11a,† the OER performances of Co₉S₈@Co₉S₈, MoS₂ and a physical mixture of Co₉S₈@Co₉S₈ and MoS₂ were also examined. The plot of overpotential vs. the mole ratio of MoS₂ for the OER clearly confirms the presence of a synergistic effect in the physical mixture (Fig. S11b†). Therefore, the presence of a strong interface interaction and synergistic effect between Co₉S₈@Co₉S₈ and MoS₂ in Co₉S₈@Co₉S₈@MoS₂-0.5 leads to the lowest overpotential in comparison to those of the single components and physical mixture Co₉S₈@Co₉S₈@MoS₂-x-mix (x = 0.5, 2.0 and 4.0) (denoted as CCM-x-mix (x = 0.5, 2.0 and 4.0)). Usually, specific activity is vital to evaluate theintrinsic activity of a catalyst.^30,31 Co₉S₈@Co₉S₈@MoS₂-0.5 gives a high specific current density of 121 μA cm⁻² at 1.57 V (Fig. 4b), much better than 9 μA cm⁻² of MoS₂ and 31 μA cm⁻² of Co@Co₉S₈. This should be closely related with the presence of a remarkable synergistic effect and interface interaction between Co₉S₈@Co₉S₈ and MoS₂ after introducing MoS₂ to form a core–double shell nanostructure. Co₉S₈@Co₉S₈@MoS₂-0.5 also possesses a much lower Tafel slope of 82.7 mV dec⁻¹, implying its fast reaction rate. In addition to the OER electrochemical activity, Co₉S₈@Co₉S₈@MoS₂-0.5 exhibits impressive performance for the HER, too. The HER properties of Co@Co₉S₈, Co₉S₈@Co₉S₈@MoS₂-2.0, Co₉S₈@Co₉S₈@MoS₂-4.0 and Pt/C are also determined in 1 mol L⁻¹ KOH. As illustrated in Fig. 4c, Co₉S₈@Co₉S₈@MoS₂-0.5 requires an overpotential of 173 mV at 10 mA cm⁻², close to that of Pt/C (134 mV). Co₉S₈@Co₉S₈@MoS₂-2.0, Co₉S₈@Co₉S₈@MoS₂-4.0, Co@Co₉S₈ and MoS₂ require overpotentials of 184, 192, 255 and 366 mV, respectively. The change of the steady-state current density with the increase of overpotential, as described by the Tafel slope, is significant for evaluating the activity of the HER. The Tafel slope for Co₉S₈@Co₉S₈@MoS₂-0.5 is 71.5 mV dec⁻¹, smaller than 92.7 mV dec⁻¹ for MoS₂, suggesting a much faster reaction rate (Fig. 4d).³² MoS₂ has a relatively limited HER activity due to its low surface area and poor conductivity. Once MoS₂ is combined with Co₉S₈ to form a double core–shell nano-structure, the HER activities of the obtained samples are improved significantly, indicating that the unique core–double shell structure of Co₉S₈@Co₉S₈@MoS₂-0.5 plays an important role in improving the HER electrocatalytic performance. Among the three samples of Co₉S₈@Co₉S₈@MoS₂-x (x = 0.5, 2.0 and 4.0), Co₉S₈@Co₉S₈@MoS₂-0.5 achieves the highest improvement for HER activity. Charge transfer resistance (R_ct) can be evaluated by electrochemical impedance spectroscopy (EIS). Co₉S₈@Co₉S₈@MoS₂-x (x = 0.5, 2.0 and 4.0) samples have smaller R_ct values compared with MoS₂ and Co@Co₉S₈ tested at 1.57 V (Fig. S12†). Besides, the long-term stability of the electrocatalyst was also examined. The current–time (i–t) test of Co₉S₈@Co₉S₈@MoS₂-0.5 was carried out at a fixed overpotential of 200 mV for 12 hours. Apparently, the current density of Co₉S₈@Co₉S₈@MoS₂-0.5 remains stable over a long-time stability test (Fig. S12†). Moreover, the morphology and components of Co₉S₈@Co₉S₈@MoS₂-0.5 are nearly the same as that in the initial state (Fig. S13†). These results confirm the outstanding stability of Co₉S₈@Co₉S₈@MoS₂-0.5. The C_dl value of the as-prepared samples was also estimated by a cyclic voltammetry test. Co₉S₈@Co₉S₈@MoS₂-0.5 possesses a relatively large C_dl value, demonstrating its higher active area (Fig. S14 and S15†). Furthermore, Co₉S₈@Co₉S₈@MoS₂-0.5 also exhibits great HER performance in acidic solution containing 0.5 mol L⁻¹ H₂SO₄. As shown in Fig. S16,† when the current density reaches 10 mA cm⁻², the overpotential of Co₉S₈@Co₉S₈@MoS₂-0.5 is 268 mV, which is much lower than 412 mV for MoS₂. The Tafel slope of Co₉S₈@Co₉S₈@MoS₂-0.5 is 71.9 mV dec⁻¹, indicating a Volmer–Heyrovsky mechanism for the rate limiting step. Comparatively, the slope of MoS₂ is 162.8 mV dec⁻¹, suggesting the Volmer mechanism (Fig. S16†).²⁰ Co₉S₈@Co₉S₈@MoS₂-0.5 possesses a lower intrinsic charge transfer resistance than pure MoS₂ (Fig. S16†). The high HER and OER activity of Co₉S₈@Co₉S₈@MoS₂-0.5 is attributed to the increased active sites, enhanced conductivity, favorable reaction kinetics and interface interaction between Co₉S₈ and MoS₂.

Fig. 4 (a) LSV curves of samples for OER in 1 mol L⁻¹ O₂-saturated KOH; (b) relationship between Tafel slope, overpotential for 10 mA cm⁻² and specific activity for OER; (c) LSV curves of samples for HER in 1 mol L⁻¹ N₂-saturated KOH and (d) relationship between Tafel slope, overpotential for 10 mA cm⁻² and specific activity for HER. (The Loading mass of samples were all 0.20 mg cm⁻².)

Encouraged by the outstanding activity of the HER and OER, we further investigated the ORR activity of Co₉S₈@Co₉S₈@MoS₂-0.5 and other contrast samples in 0.1 mol L⁻¹ KOH solution. As shown in Fig. S17,† the behavior of Co₉S₈@Co₉S₈@MoS₂-0.5 in cyclic voltammetry (CV) was assessed by using a three-electrode system in an O₂- or N₂-saturated 0.1 mol L⁻¹ KOH solution. Co₉S₈@Co₉S₈@MoS₂-0.5 exhibits a distinct peak in the O₂ saturated electrolyte, showing a high activity for the ORR.³³ Linear sweep voltammetry (LSV) polarization curves of the samples were obtained using the rotating disk electrode (RDE) technique at 1600 rpm with a sweep rate of 10 mV s⁻¹. Fig. 5a shows the polarization curves. Co₉S₈@Co₉S₈@MoS₂-0.5 exhibits a relatively high diffusion-limiting current density (4.76 mA cm⁻² at 0.6 V vs. RHE) and half-potential (0.776 V), which are close to those of commercial Pt/C (diffusion-limiting current density of 6.37 mA cm⁻² and a half-potential of 0.835 V). As shown in Fig. S18 and S19,† the ORR performance of Co₉S₈@Co₉S₈@MoS₂-0.5 is also better than Co₉S₈@Co₉S₈, MoS₂, Co₉S₈@Co₉S₈@MoS₂-2.0, Co₉S₈@Co₉S₈@MoS₂-4.0 and CCM-x-mix (x = 0.5, 2.0 and 4.0). The LSV polarization curves of Co₉S₈@Co₉S₈@MoS₂-0.5, tested at different rotation speeds from 400 to 1600 rpm, demonstrate its rotation-dependent current density (Fig. 5b). Fig. S20† shows the Koutecky–Levich (K–L) plots (J⁻¹vs. ω^−1/2) calculated from the above-obtained LSV curves in Fig. 5b, which present good linearity and parallelism, suggesting the first-order reaction kinetics with respect to the dissolved oxygen concentration.^34,35 To identify the ORR mechanism of Co₉S₈@Co₉S₈@MoS₂-0.5, we subsequently performed the rotating ring-disk electrode (RRDE) test at 1600 rpm. The two-electron pathway of the ORR reduces oxygen to peroxide and then to water, whereas the peroxide produced from the two-electron pathway is detrimental to fuel cells. Based on the RRDE measurements, the number of electrons transferred in Co₉S₈@Co₉S₈@MoS₂-0.5 is nearly 4 (Fig. 5c), suggesting that the pre-catalyst proceeds via a four-electron pathway.³¹ Furthermore, the yield of HO₂⁻ is less than 6% during the ORR. Moreover, the chronoamperometric response confirms the great durability of Co₉S₈@Co₉S₈@MoS₂-0.5 because it retains a cathodic current of 89% after 12 h. Under the same test conditions, Pt/C retains only 42% (Fig. S17†). A methanol crossover test was carried out during the chronoamperometric response at 0.6 V vs. RHE. As shown in Fig. 5d, Co₉S₈@Co₉S₈@MoS₂-0.5 is not sensitive to methanol. By contrast, Pt/C is greatly affected by methanol and shows a significant decrease in the current density after the introduction of methanol. The above results indicate that Co₉S₈@Co₉S₈@MoS₂-0.5 is also a promising ORR pre-catalyst candidate in direct methanol fuel cells.

Fig. 5 (a) RDE polarization curves of Co₉S₈@MoS₂-0.5 and Pt/C tested in O₂-saturated 0.1 mol L⁻¹ KOH with a sweep rate of 10 mV s⁻¹ at 1600 rpm; (b) RDE polarization curves of Co₉S₈@Co₉S₈@MoS₂-0.5 with rotating rates from 400 to 1600 rpm of 10 mV s⁻¹; (c) electron transfer number (n) at the top and H₂O₂ yield at the bottom vs. potential; (d) methanol crossover effect test of Co₉S₈@Co₉S₈@MoS₂-0.5 and Pt/C. (The loading mass was 0.20 mg cm⁻² of Pt/C and 0.40 mg cm⁻² for other samples.)

Conclusions

In summary, a novel core–double shell heterostructure Co₉S₈@Co₉S₈@MoS₂-0.5 with tri-functional electrocatalytic activity for the HER, OER and ORR was constructed by two main steps. Firstly, the core–shell structure of the precursor Co@Co₉S₈ was prepared by sulfurization followed by the pyrolysis of ZIF-67. Then the MoS₂ grew on the Co@Co₉S₈ precursor surface under hydrothermal conditions. The Co@Co₉S₈ precursor provides abundant basic space to confine the growth of MoS₂, which greatly prevents the aggregation of MoS₂ and improves the conductivity of the nanostructure. Benefiting from the intriguing synthesis strategy and well-controlled structure, the core–double shell Co₉S₈@Co₉S₈@MoS₂-0.5 heterostructure exhibits synergistic interaction between Co₉S₈ and MoS₂ and a modified electronic structure. These features enable Co₉S₈@Co₉S₈@MoS₂-0.5 to show outstanding electrocatalytic performance, i.e., Co₉S₈@Co₉S₈@MoS₂-0.5 requires an overpotential of 173 mV and 340 mV at 10 mA cm⁻² for the HER and OER in alkaline solution, respectively. Besides, the ORR performance with a half-wave potential of 0.77 V and a diffusion-limiting current density of 4.76 mA cm⁻² at 0.6 V (vs. RHE) is also obtained. We believe that this work will not only provide creative views for facile synthesis of novel heterostructure materials, but also pave the way for exploring multifunctional electrocatalysts.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the NSFC (No. 21671077, 21771075, 21571176, and 21871106). We thank Dr Xiyang Wang, Jilin University, for XANES spectra analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qi01080g

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