Microflower-like Co₉S₈@MoS₂ heterostructure as an efficient bifunctional catalyst for overall water splitting

Abstract

The development of a distinguished and high-performance catalyst for H₂ and O₂ generation is a rational strategy for producing hydrogen fuel via electrochemical water splitting. Herein, a flower-like Co₉S₈@MoS₂ heterostructure with effective bifunctional activity was achieved using a one-pot approach via the hydrothermal treatment of metal-coordinated species followed by pyrolysis under an N₂ atmosphere. The heterostructures exhibited a 3D interconnected network with a large electrochemical active surface area and a junctional complex with hydrogen evolution reaction (HER) catalytic activity of MoS₂ and oxygen evolution reaction (OER) catalytic activity of Co₉S₈, exhibiting low overpotentials of 295 and 103 mV for OER and HER at 10 mA cm⁻² current density, respectively. Additionally, the catalyst-assembled electrolyser provided favourable catalytic activity and strong durability for overall water splitting in 1 M KOH electrolyte. The results of the study highlight the importance of structural engineering for the design and preparation of cost-effective and efficient bifunctional electrocatalysts.

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Introduction

Owing to increasing energy demands and the unreasonable use of fossil fuels, there is an urgent need for clean and sustainable energy.¹ The development of sustainable, inexpensive, eco-friendly, and highly efficient energy resources is one of the most effective approaches for resolving environmental and energy problems. Electrocatalytic water splitting to H₂ and O₂ is an attractive and highly potential technology for renewable energy production because water resources are abundant and no carbon is involved.^2,3 The catalytic performance of precious catalysts, such as platinum⁴ and RuO₂/IrO₂,^5–7 remains unmatched for electrocatalytic water splitting. Nevertheless, prohibitive cost, scarcity, and poor durability of these precious catalysts restrain their large-scale practical applications.^8–10 Consequently, it is tremendously desirable to prepare high-efficiency and low-cost bifunctional catalysts for overall water splitting.^11,12

Recently, various investigations have shown that MoS₂ is a promising candidate for the HER.^13–16 However, MoS₂ also has drawbacks such as covering of active sites, poor conductivity, and low OER performance.^17–19 In contrast, transition metal sulphides and oxides of Co such as Co₃S₄,^20,21 Co₉S₈,^22–24 Co₄O₄,²⁵ and LaCoO₃^26,27 exhibit higher conductivity and efficient electrocatalytic performance for the OER, but their HER performance is unsatisfactory. Some studies have reported that the electronic structure of MoS₂ can be modulated and optimised by doping with transition metals (e.g. Ni and Co) to achieve enhanced catalytic activity.^28,29 Consequently, the integration of OER-active Co₉S₈ and HER-active MoS₂ into the same material is a judicious strategy for fabricating suitable bifunctional electrocatalysts for water electrolysis.³⁰

Several studies are currently focusing on the fabrication of bifunctional electrocatalysts comprising MoS₂ and Co₉S₈ nanocomposites to achieve efficient overall water splitting.^23,31–35 Cao et al.¹⁷ prepared a uniform Co₉S₈@MoS₂ heterostructure using a simple process that combined solvothermal treatment with pyrolysis. In an alkaline solution, the proposed heterostructure showed a current density of 10 mA cm⁻² for the HER and OER under overpotentials of 143 and 340 mV, respectively. Meanwhile, for overall water splitting, only a voltage of 1.67 V was demanded at 10 mA cm⁻² in alkaline electrolyser. Du et al.³⁶ designed and prepared a multicomponent Co₉S₈@MoS₂ nanohybrid derived from cobalt-doped polyoxometalate using a one-pot calcination technique. This nanohybrid exhibited a smallish overpotential (230 mV) for the OER but a relatively high overpotential (239 mV) for the HER at 10 mA cm⁻². Despite these advances, it is necessary to develop a novel and simple route to obtain cost-efficient bifunctional electrocatalysts with unique structures for the sake of enrichment of material preparation database, remarkable performance, and robust stability.

In this study, a bifunctional hybrid electrocatalyst comprising Co₉S₈ and MoS₂ with the structural merits of a flower-like morphology was synthesised using a facile hydrothermal treatment with metal salts and the strongly coordinating ligand ethylenediamine and formaldehyde, followed by annealing. A three-dimensionally interconnected network structure with the Co₉S₈ microflower tightly attached to the MoS₂ nanoflakes was materialised. The obtained Co₉S₈@MoS₂ heterostructure was further explored as bifunctional electrocatalysts for HER and OER. Compared with the sole component of Co₉S₈ and MoS₂, the prepared Co₉S₈@MoS₂ microflowers displayed low overpotentials of 295 and 103 mV to achieve a current density of 10 mA cm⁻² in alkaline electrolyte for the HER and OER, respectively, as well as a low overpotential (1.64 V) for overall water splitting at a current density of 10 mA cm⁻² when employed as a two-electrode assembly device. The results in this work highlight the importance of structural engineering for the design and preparation of low-cost, efficient, and bifunctional electrocatalysts.

Results and discussion

The two-step synthesis method for Co₉S₈@MoS₂ is illustrated in Fig. 1a. First, a flower-like composite precursor was formed by hydrothermal treatment of the metal salts in the presence of the strongly coordinating ligands thiourea, ethylenediamine, and formaldehyde.³⁷ Subsequently, the sample was pyrolyzed in a tube furnace under a nitrogen ambiance at 500 °C to obtain the Co₉S₈@MoS₂ heterostructure. The SEM images in Fig. 1b reveal that Co₉S₈@MoS₂ comprised flower-like structures, which is uniform spherical with a regular size of ∼2.0 μm. In contrast, MoS₂ comprised aggregated nanosheets with no clear macroporous structures (Fig. 1c). Co₉S₈ exhibits a flower-like morphology similar to that of Co₉S₈@MoS₂, but unexpected spherical particles are also present on the surface of the Co₉S₈ microflowers, indicative that the addition of Mo salt allows the surface coverage and keep the structural integrity (Fig. 1d). These morphology differences demonstrate the success assemble of MoS₂ and Co₉S₈ in the one flower-like component. Notably, the intact flower-like morphology of the Co₉S₈@MoS₂ composite implies that the formation of spherical particles was hindered by the surface coverage of MoS₂ nanosheets during carbonisation. Furthermore, the porous texture of Co₉S₈@MoS₂ can provide abundant channels for the release of electrolyte and gaseous products.

Fig. 1 (a) Schematic illustration of the fabrication of a typical sample Co₉S₈@MoS₂ as a bifunctional electrocatalyst for water splitting. SEM images for Co₉S₈@MoS₂ (b), MoS₂ (c) and Co₉S₈ (d).

The Co₉S₈@MoS₂ has a porous flower-like morphology comprising interconnected flakes with a thickness of approximately 20 nm (Fig. 2a and b). Fig. 2c shows that the Co₉S₈@MoS₂ particles comprised a dense inner Co₉S₈ layer and outer MoS₂ nanosheets. Lattice fringes with spacings of 0.30 and 0.62 nm are clearly visible in the High-Resolution TEM (HRTEM) images, which correspond to the (311) plane of Co₉S₈ and the (002) plane of MoS₂. In addition, the lattice fringes between Co₉S₈ and MoS₂ are clearly observed (Fig. 2e and f). Elemental mapping (Fig. 2g) confirms that Mo, S, and Co are evenly distributed in the Co₉S₈@MoS₂ heterostructure.

Fig. 2 The characteristics of Co₉S₈@MoS₂: (a and b) SEM images with different magnification; (c) HADDF-STEM image; (d) TEM image; (e and f) HR-TEM images and (g) overlap of elemental maps of Mo, Co, and S.

The three-dimensional (3D) flower-like Co₉S₈@MoS₂ heterostructural spheres are assembled by the interconnections of heterostructural 2D nanosheets, which not only prevent the restack/aggregation of nanosheets but also provide hierarchical three-dimensional porous structures, allowing the well-exposed interfaces and favourable diffusion channels and manifesting excellent electrocatalytic performance for overall water.

To obtain in-depth understanding of the elemental states and phase structures of the Co₉S₈@MoS₂ composite, XPS and powder XRD investigations were performed. The XRD pattern of Co₉S₈@MoS₂ in Fig. 3a exhibits characteristic diffraction peaks at 14.2°, 32.8°, 33.8°, 39.6°, 49.5°, and 58.9°, which can be attributed to the (002), (100), (101), (102), (110), and (008) planes, respectively, of MoS₂ (PDF#37-1492). In addition, the peaks at 29.8°, 47.5°, and 52.1° were well-matched with the (311), (511), and (440) planes, respectively, of Co₉S₈ (PDF#19-0364).

Fig. 3 XRD pattern of the prepared Co₉S₈@MoS₂ (a). The blue and red vertical lines indicate reference patterns of MoS₂ (PDF card no. 37-1492) and Co₉S₈ (PDF card no. 19-0364), respectively. High-resolution XPS spectra of (b) Co 2p, (c) Mo 3d, and (d) S 2p of Co₉S₈@MoS₂. The contrast of (e) Co 2p spectra in Co₉S₈@MoS₂ and Co₉S₈, and (f) Mo 3d spectra in Co₉S₈@MoS₂ and MoS₂.

The XPS survey spectrum (Fig. S1†) of as-prepared Co₉S₈@MoS₂ sample reveals the presence of Mo, Co, and S, which is in agreement with the XRD and EDS mapping results. In Fig. 3b, it is obvious that the high-resolution spectrum of Co 2p is deconvoluted into two spin–orbit doublets. The first doublet at 778.7 eV (Co 2p_3/2) and 793.7 eV (Co 2p_1/2) is assigned to Co³⁺, whereas the second doublet at 781.1 eV (Co 2p_3/2) and 797.3 eV (Co 2p_1/2) is assigned to Co²⁺. Further, the signals displayed at 802.4 and 786.3 eV are typical satellite peaks.^38,39 In Fig. 3c, the peak at 228.8 eV is attributed to Mo 3d_5/2, and the peak at 232.5 eV is ascribed Mo 3d_3/2. The two weak peaks signal at 225.8 and 226.5 eV are assigned to the S 2s shoulder peak, revealing that the S species bonded to Mo and Co ions have two chemical phases.⁴⁰ In the S 2p spectrum (Fig. 3d), the significant peaks at 161.2 and 162.7 eV are ascribed to S 2p_3/2 and S 2p_1/2 of Co₉S₈@MoS₂, whereas the peaks at 161.5 and 163.8 eV are attributed to S 2p_3/2 and S 2p_1/2 of MoS₂.⁴¹ Notably, as shown in Fig. 3e and f, the binding energy of the Co 2p3/2 from Co₉S₈@MoS₂ exhibits positive shifts of ∼0.6 eV relative to that pure Co₉S₈ while the Mo 3d_5/2 and Mo 3d_3/2 peaks in Co₉S₈@MoS₂ display ∼0.6 eV down-shift compared to the pure MoS_2, further confirming the existence of Co₉S₈@MoS₂ heterostructure with strong electronic interactions between Co₉S₈ and MoS₂. This change in binding energies preliminarily proves that the electrons are transferred from Co to Mo, which results in more negative charged Mo species generation and enhanced catalytic activity. Additionally, the Co₉S₈@MoS₂ microflowers show a high specific surface area up to 157.0 m² g⁻¹ and hierarchical porous structure with meso- and microporosity (Fig. S2†).

In 1 M KOH, the electrocatalytic performance of Co₉S₈@MoS₂ for the OER was assessed. The individual Co₉S₈ and MoS₂ materials were also examined for comparison. In addition, as a well-known OER catalyst, IrO₂ was used as a benchmark. Fig. 4a illustrates representative polarisation curves for the various catalysts. The OER evaluation results of the catalyst decrease in the following order Co₉S₈@MoS₂-iR > Co₉S₈@MoS₂ > Co₉S₈ > IrO₂ > MoS₂. The potential required by Co₉S₈@MoS₂ (1.525 V) to achieve at 10 mA cm⁻² is found to be lower than those required by IrO₂ (1.558 V), Co₉S₈ (1.558 V), and MoS₂ (1.692 V). Based on the Tafel slopes derived from the linear sweep voltammetry curves (Fig. 4b), Co₉S₈@MoS₂ exhibits the fastest kinetics for oxygen production. The Tafel slope for Co₉S₈@MoS₂ (175 mV dec⁻¹) is lower than those of Co₉S₈ (177 mV dec⁻¹), MoS₂ (265 mV dec⁻¹), and even IrO₂ (194 mV dec⁻¹). In the Nyquist plots (Fig. 4c), a semicircle appears at high frequencies corresponding to the charge-transfer resistance. The diameter of this semicircle is smaller for Co₉S₈@MoS₂ than for Co₉S₈, MoS₂, and IrO₂, indicating a lower charge-transfer resistance of the composite material.⁴² It is worth mentioning that, the in situ generated CoOOH of cobalt-based catalyst, which certainly include Co₉S₈@MoS₂ and Co₉S₈,^17,43 is generally claimed to be the actual active center for OER electrocatalysis.^44–47 In our study, the exposable density of active sites and charge-transfer resistance of the transformed CoOOH-containing catalyst are mainly determining the OER electrocatalytic activity, not involving the dominated high intrinsic activity tuned by defect engineering and heteroatomic doping.^44,48 Consequently, due to the nanostructural virtues of 3D porous framework and heterojunctions, the CoOOH-containing catalyst transformed from Co₉S₈@MoS₂ shows the low charge-transfer resistance and high exposed active site, resulting in slightly enhanced OER activity. These results also founded by the previous reported literatures.^3,17,40

Fig. 4 (a) LSV curves and (b) the corresponding Tafel plots of Co₉S₈@MoS₂, Co₉S₈@MoS₂ with iR compensation (Co₉S₈@MoS₂-iR), MoS₂, Co₉S₈, and IrO₂ electrodes for OER in a 1 M KOH electrolyte. (c) Electrochemical impedance spectra of different electrodes at +1.56 V for OER with an AC amplitude of 5 mV within the frequency range of 100 kHz to 0.01 Hz. (d) Chronoamperometric responses (i–t) collected on the Co₉S₈@MoS₂ electrode and IrO₂ at the same current density of 10 mA cm⁻² in 1 M KOH aqueous solution.

Durability and stability are also key parameters for estimating the electrocatalytic capability of Co₉S₈@MoS₂. The chronoamperometric responses (i–t) of catalysts were recorded at an applied potential of 1.558 V (vs. RHE). As demonstrated in Fig. 4d, the decrease in the current density of Co₉S₈@MoS₂ over 15 h is insignificant than that of IrO₂, showing the robust durability of Co₉S₈@MoS₂. To further understand the stability of the Co₉S₈@MoS₂, the SEM technique was performed after the OER test. As shown in the Fig. S3†, after the above long-time OER test, Co₉S₈@MoS₂ can largely retained its original morphology, although a slight aggregation and surface collapse is observed, indicative of its excellent structural stability. The excellent stability of Co₉S₈@MoS₂ can be ascribed to its unique heterostructures: (i) the interfacial interaction between Co₉S₈ and MoS₂ enhances the density of electron transfer at the joint boundaries; (ii) a unique core–shell nanostructure is constructed by the highly active Co₉S₈ core, along with a more stable MoS₂ shell; (iii) the 3D flower-like porous framework enables the easy release of bubbles formed on the surface of the electrode during operation, thus preventing the electrocatalyst from being removed from the electrode.

Similarly, the HER performance of Co₉S₈, MoS₂, and Co₉S₈@MoS₂ was assessed in 1 M KOH; a 20% Pt/C was also tested for comparison. Fig. 5a exhibits the polarisation curves of Co₉S₈, MoS₂, Co₉S₈@MoS₂, iR-corrected Co₉S₈@MoS₂, and Pt/C obtained at a scan rate of 10 mV s⁻¹ in 1 M KOH. Notably, the Pt/C exhibits superior HER activity, which merely need an overpotential of 54 mV at 10 mA cm⁻². While the HER activity of Co₉S₈@MoS₂ is weaker than that of Pt/C, it exhibits a lower overpotential (η₁₀ = 103 mV) than MoS₂ (η₁₀ = 323 mV) and Co₉S₈ (η₁₀ = 228 mV). The Tafel slopes (Fig. 5b) are estimated to be 336, 129, 124, and 67 mV dec⁻¹ for MoS₂, Co₉S₈, Co₉S₈@MoS₂, and Pt/C. As shown by the Nyquist plots in Fig. 5c, the charge-transfer resistance of Co₉S₈@MoS₂ is evidently lower than those of MoS₂ and Co₉S₈, which is favourable for the HER. This remarkable enhanced HER activity for Co₉S₈@MoS₂ is mainly due to its porous three-dimensional flower-like heterostructure spheres and the interfacial areas triggering electron transfer that can significantly increase the exposal of active sites and activates HER electrocatalysis.^3,23,31 Moreover, the chronoamperometric curve in Fig. 5d exhibits that Co₉S₈@MoS₂ is stable for more than 16 h and its microflower-like morphology can unambiguously observed in the SEM images taken after continuously catalyzing HER (Fig. S4†). In addition, in an acidic medium, the Co₉S₈@MoS₂ electrode also exhibits remarkable HER activity, requiring a low potential of 114 mV at 10 mA cm⁻² (Fig. S5†).

Fig. 5 (a) LSV curves and (b) the corresponding Tafel plots of Co₉S₈@MoS₂, Co₉S₈@MoS₂ with iR compensation (Co₉S₈@MoS₂-iR), MoS₂, Co₉S₈, and Pt/C electrodes for HER in a 1 M KOH electrolyte. (c) Electrochemical impedance spectra of different electrodes at −0.28 V for HER with an AC amplitude of 5 mV within the frequency range of 100 kHz to 0.01 Hz. (d) Chronoamperometric plots for comparing the operation stability of Co₉S₈@MoS₂ and Pt/C electrodes at the applied potential of −103 mV and −54 mV (vs. RHE), respectively.

To determine the origin of the remarkable HER property of Co₉S₈@MoS₂ in an alkaline solution, the electrochemically active surface areas (ECSAs) of the catalysts are considered. The electrochemical double-layer capacitance (C_dl) is positively correlated with the electrochemical surface area,^14,49 therefore, the ECSA of each catalyst is estimated from the C_dl value, as calculated using the cyclic voltammetry curves obtained at a series of scan rates (20 to 100 mV s⁻¹) in the non-faradaic reaction range. As observed in Fig. S6†, the C_dl value of Co₉S₈ (27.4 mF cm⁻²) and MoS₂ (1.8 mF cm⁻²) is lower than that of Co₉S₈@MoS₂ (40.0 mF cm⁻²) in 1 M KOH. Thus, it is observed that Co₉S₈@MoS₂ exhibits outstanding electrocatalytic performance.

To further explicitly explain the effect of MoS₂ concentration toward electrocatalytic HER and OER performance, two counterparts Co₉S₈@MoS₂ with varying MoS₂ concentration were prepared by feeding the different amount of Molybdenum salt and denoted as Co₉S₈@MoS₂-low and Co₉S₈@MoS₂-high. The electrochemical activities of Co₉S₈@MoS₂ with different MoS₂ concentration were investigated in alkaline media for OER and HER. As shown in Fig. S7†, Co₉S₈@MoS₂-low exhibits the worst performances for both OER and HER may owing to its thin MoS₂ nanosheets or less active sites on the interfaces of Co₉S₈ and MoS₂. Although Co₉S₈@MoS₂-high demonstrates a favorable improved the activities for both OER and HER compared to those exhibited by Co₉S₈@MoS₂-low, it still displays the lower catalytic activities as compared with that of Co₉S₈@MoS₂. This can be due to excessive MoS₂ is coated on the surface of Co₉S₈, which obstruct the activity of the inner Co₉S₈ layer.³⁸ Thus, Co₉S₈@MoS₂ heterostructure with an optimal concentration of MoS₂ were studied systematically in present work.

The above findings indicate that Co₉S₈@MoS₂ is a satisfactory electrocatalyst for both the OER and the HER. In a two-electrode setup, Co₉S₈@MoS₂ was used as both the anode and the cathode, and its catalytic performance for water splitting was assessed in 1 M KOH. In Fig. 6a, the Co₉S₈@MoS₂ electrode exhibits excellent performance, attaining water splitting at 10 mA cm⁻² under a low voltage of 1.64 V. Moreover, the long-term stability over 12 h was measured using chronoamperometry (Fig. 6b). The current density degraded only slightly, indicating the noteworthy stability of the Co₉S₈@MoS₂ electrode for the water splitting reaction.

Fig. 6 (a) LSV curves of Co₉S₈@MoS₂ as a bifunctional catalyst in 1 M KOH for overall water splitting; inset to panel (a) is the illustration of the two-electrode setup for overall splitting. (b) The stability of the Co₉S₈@MoS₂ on carbon cloth at a voltage of 1.64 V.

Thus, the flower-like Co₉S₈@MoS₂ heterostructure obtained in this study possesses unique structural merits such as a hierarchical 3D porous framework and a 2D sheet-like heterojunction structure, which facilitate short diffusion channels and well-exposed active sites, showing a high electrochemical active area and low electrochemical impedance. These features afford remarkable bifunctional electrocatalytic performance and stability for the HER and OER, which compares favourably to contemporary nonprecious metal or leading cobalt- and molybdenum-based electrocatalysts (Table S1†).

Conclusions

A Co₉S₈@MoS₂ bifunctional catalyst was designed and synthesised using a two-step process that combined a hydrothermal reaction and thermal annealing. During this process, surface coverage by MoS₂ nanosheets hindered the formation of spherical Co₉S₈ particles, and the obtained porous structure was advantageous for electrolyte infiltration and the transportation of intermediates. The flower-like Co₉S₈@MoS₂ heterostructure incorporated MoS₂ as a stable HER catalyst of MoS₂ and Co₉S₈ as a notable OER catalyst. Thus, Co₉S₈@MoS₂ acted as a bifunctional catalyst with a low charge-transfer resistance and a high ECSA. Co₉S₈@MoS₂ exhibited low overpotentials of 103 and 114 mV for the HER at 10 mA cm⁻² under alkaline and acidic conditions, respectively. Moreover, the OER performance of Co₉S₈@MoS₂ was satisfactory, and an overpotential of only 295 mV was required at 10 mA cm⁻². Moreover, for water splitting, a low voltage of 1.64 V was achieved by employing the microflower-like Co₉S₈@MoS₂ electrocatalyst as a two-electrode setup. The findings of this study provide insights for fabricating high-performance bifunctional electrocatalysts with advantageous structures for overall water splitting.

Author contributions

Chaohai Pang: conceived and designed the experiments. Chaohai Pang and Xiaojing Zhu: Performed the experiments. Chaohai Pang and Xionghui Ma: Analyzed the data. Chaohai Pang: writing – original draft. Xiaojing Zhu, Yuwei Wu and Shuhuai Li: writing – review & editing. Zhi Xu and Mingyue Wang: supervision, funding acquisition and project administration. All authors have discussed and given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Central Public-interest Scientific Institution Basal Research Fund for the Chinese Academy of Tropical Agricultural Sciences (Grant no.1251632022005). China Agriculture Research System of MOF and MARA (CARS-31). Key Laboratory of Tropical Fruits and Vegetables Quality and Safety for State Market Regulation (Grant no. ZX-2022002). Additional support was provided by the Foundation from Chemistry and Chemical Engineering Guangdong Laboratory (Grant no. 20111016) and the Basic and Applied Basic Research Foundation of Guangdong Province (Grant no. 2021A1515110111).

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Footnotes

† Electronic supplementary information (ESI) available. See https://doi.org/10.1039/d2ra04086g

‡ C. P and X. M contributed equally to this work.

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