Bimetallic sulfide particles incorporated in Fe/Co-based metal–organic framework ultrathin nanosheets toward boosted electrocatalysis of the oxygen evolution reaction

Abstract

The development of inexpensive, high-performance, and long-lasting electrocatalysts toward the oxygen evolution reaction (OER) has been proved to be crucial to enhance the efficiency of water splitting to obtain clean and sustainable energy. Herein, Fe/Co-based metal–organic framework (MOF) ultrathin nanosheets doped with (Fe)-Co₃S₄ metal sulfide (MS) particles (CoFe-MS/MOF) are reported as superior electrocatalysts for the OER. CoFe-MS/MOFs exhibit an outstanding OER performance with a small overpotential of 261 mV and a low Tafel slope of 60.3 mV dec⁻¹ at a current density of 10 mA cm⁻² while showing high durability in an alkaline medium. The existence of metal sulfide particles plays a pivotal part in promoting the charge transfer capacity as well as the formation of ultrathin MOF nanosheets. More significantly, they can also modulate the adsorption energy of oxygenated intermediates, thereby significantly enhancing the inherent electrocatalytic activity. This work provides guidance for rationally designing and synthesizing inexpensive and highly active bimetallic sulfide doped MOF electrocatalysts in the electrochemical energy field.

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1. Introduction

Due to severe environmental pollution and energy depletion resulting from extensive consumption of fossil energy, numerous researches have been dedicated to developing sustainable and clean energy sources,^1–4 among which, water electrolysis for the production of hydrogen is an eco-friendly and economically viable strategy.^5–7 Nevertheless, the sluggish OER severely affects the efficiency of overall water splitting. Since OER is a thermodynamically unfavorable process involving the transfer of four proton-coupled electron, it is a bottleneck for water splitting.^8–10 Therefore, exploring efficient OER electrocatalysts is a key requirement for accelerating the reaction by reducing the activation energy, and thus achieving an improvement of energy conversion efficiency.¹¹ Although Ir/Ru-based precious metal materials remain the state-of-the-art electrocatalysts toward OER by far, the scarcity, inferior durability and high cost severely limit their large-scale industrial application.^12,13 As a consequence, development of efficient, cost-effective, and stable noble-metal-free electrocatalysts toward the OER is of considerable scientific and research significance.^14,15

In recent years, a large amount of research has been dedicated to exploiting highly efficient and cost-effective electrocatalysts based on 3d transition metals (Co, Ni, Fe, etc.) for the OER due to their elemental abundance in the Earth's crust and similar valence electron structures of 3d^6–84s².¹⁶ Among them, metal–organic frameworks (MOFs) have emerged as promising OER catalysts owing to their intrinsically large surface area and porosity.^15,17,18 In electrocatalysis, MOFs are usually utilized as precursors or sacrificial templates for the preparation of different electrocatalysts, such as metal sulfides,¹⁹ nitrides²⁰ and carbides²¹ by pyrolysis under various conditions. However, the attractive structural and compositional characteristics of precursors will be completely destroyed during pyrolysis, causing restricted active sites and blocked reactant transport.²² Still, it is reasonable for MOFs to be used directly as (pre-)electrocatalysts to make the best of their distinctive properties. Nevertheless, the catalytic activity of pristine MOF materials is unsatisfactory, primarily on account of their poor inherent charge/mass transport capability.^23–25

The inherently poor conductivity of MOFs limits their electrocatalytic applications. To address this issue, abundant studies have been devoted to enhancing the electrical conductivity of MOFs. The incorporation of metal chalcogenide and oxide nanoparticles into MOFs through a post-synthesis process and the creation of polymetallic MOFs with more open metal sites and an enriched electron environment could effectively enhance the electrical conductivity and electrocatalytic properties of MOFs.^26–30 Furthermore, MOFs with high electrical conductivity have also been developed by using ligands with a π-conjugated aromatic structure.^31,32 Mass transfer in MOFs is restricted by the mismatched porous size of MOFs for the diffusion of reactants. The large mass resistance results in low exposure and underutilization of the active sites. In this aspect, two-dimensional ultrathin nanosheet-like MOF materials are highly satisfactory and can be successfully developed by liquid–liquid interfacial growth, ultrasound-assisted exfoliation, sacrificial template growth, epitaxial growth, and other methods, which exhibit excellent electrocatalytic properties.^25,33,34

Another superior strategy to improve the electrocatalytic activity of MOFs is the introduction of heterogeneous species into pristine MOFs.³⁵ In previous reports, tiny Ni₃S₂ or CoFeO_x nanoparticles embedded in MOFs by post-synthetic plasma treatment were able to enhance the electrical conductivity and electrocatalytic performance of MOFs.^27,28 Such heterogeneous species can enhance the surface area and availability of active sites through modulating the nanostructure of MOFs. Moreover, they can also trigger probable synergistic effects between diverse constituents through the generation of interfacial electronic coupling, providing a huge opportunity to optimize the adsorption energy of key intermediates, thereby enhancing the inherent catalytic activity.^36–38

Inspired by the above work, we report the CoFe-MS/MOF electrocatalyst formed by embedding metal sulfide particles into ultrathin Fe/Co-based MOF nanosheets using a facile one-step solvothermal method. It exhibits more outstanding OER electrocatalytic activity (10 mA cm⁻² at 261 mV for the OER) than CoNi-MS/MOF, CoCu-MS/MOF, CoFe-MOF and (Fe)-Co₃S₄. It is found that the existence of iron/cobalt sulfide particles is essential to improving the electrocatalytic properties. Embedding sulfide particles in MOFs not only contributes to the formation of ultrathin nanosheets, but also effectively improves the electrical conductivity of the nanosheets and, more significantly, modulates the surface properties and electronic structure of the electrocatalysts, which together enhance the electrocatalytic activity.

2. Results and discussion

As illustrated in Fig. 1, the CoFe-MS/MOF nanosheets were synthesized in the presence of thioacetamide (TAA, a S source) and terephthalic acid (TPA, the MOF ligand) under solvothermal conditions with an input Fe³⁺/Co²⁺ molar ratio of 1 : 4. For comparison, we also synthesized CoNi-MS/MOF and CoCu-MS/MOF nanosheets by the same method. The X-ray diffraction (XRD) pattern of the as-synthesized CoFe-MS/MOF (Fig. 2a) displays some characteristic diffraction peaks similar to those of the MOF-71 structure (no. 265 092, Cambridge Crystallographic Data Centre).^22,39 Two sharp diffraction peaks at 19.75° and 22.19° correspond to singlet sulfur (S, PDF#42-1278), probably due to the oxidation of sulfide in air. Some diffraction peaks at 16.37°, 32.14°, 38.33°, 47.21°, 50.53°, and 55.51° correspond to cobalt disulphide (Co₃S₄, PDF#47-1738), respectively.²⁶ The other diffraction peaks correspond to Fe₃S₄ (PDF#16-0713). This XRD pattern reveals that bimetallic sulfides and MOF-71 type MOFs coexist in the as-prepared CoFe-MS/MOF. As presented in the scanning electron microscopy (SEM) images of Fig. 2b and c, the CoFe-MS/MOF consists of a large number of ultrathin shuttle-shaped nanosheets (≈15–25 nm in thickness), and these ultrathin nanosheets form a large number of empty spaces by interconnecting and intersecting with each other. This distinctive open and layered porous structure facilitates mass transfer and provides highly specific surface areas for accelerating electrochemical reactions.^40–42

Fig. 1 Schematic diagram of the synthesis procedure for the CoFe-MS/MOF.

Fig. 2 (a) XRD patterns and (b and c) SEM images of the CoFe-MS/MOF.

To further investigate the microstructural characteristics of the CoFe-MS/MOF, transmission electron microscopy (TEM) tests were performed. The CoFe-MS/MOF is composed of various ultrathin nanosheets, as shown in the TEM image in Fig. 3a, which is consistent with the results observed by SEM. Compared to the CoNi-MS/MOF and CoCu-MS/MOF (Fig. S1†), the CoFe-MS/MOF with similar morphologies is significantly thinner. There were some black nanoparticles embedded in the ultrathin nanosheets (Fig. 3b), but no visible cracks, which implies good mechanical and electrical touch between various assemblies. The corresponding high-resolution TEM (HRTEM) image of the CoFe-MS/MOF (Fig. 3c) demonstrates the lattice d-spacing of approximately 0.209 and 0.254 nm, corresponding to the (400) and (422) lattices of Co₃S₄, agreeing well with the XRD pattern in Fig. 2a. It is noteworthy that no lattice stripes were observed for MOF-71, probably due to the disruption of the crystal structure of the MOF under a high-energy electron flow.^28,43

Fig. 3 (a) TEM image, (b and c) HRTEM images, (d) HAADF-STEM image, (e) the corresponding elemental mapping images of Co, Fe, O, C, and S, and (f) the SEM-EDX spectrum of the CoFe-MS/MOF nanosheet.

The high-angle annular dark-field scanning TEM (HAADF-STEM) image (Fig. 3d) and corresponding energy-dispersive X-ray (EDX) mapping images (Fig. 3e) of a single CoFe-MS/MOF nanosheet display the even distribution of Co, Fe, O, and C elements and the discrete distribution of the S element throughout the entire nanosheet, indicating that metal sulfide particles are successfully embedded in the Fe/Co-based MOF nanosheet. The EDX spectrum (Fig. 3f) unveils an Fe/Co atomic ratio of approximately 1 : 6.9 in the nanosheet, which is marginally different from the Fe/Co ratio employed for the solvothermal reaction, probably due to the different coordination capabilities of Co²⁺ and Fe³⁺ during the MOF growth.⁴⁴ The EDX data of CoNi-MS/MOF and CoCu-MS/MOF are displayed in Fig. S2.†

To investigate the formation mechanism of this metal sulfide particle inlaid in MOF ultrathin nanosheets, a series of controlled experiments was conducted. In the absence of TAA or TPA, the products are referred to as CoFe-MOF and (Fe)-Co₃S₄, respectively. The control experiments revealed that the CoFe-MOF displays several characteristic peaks of the MOF-71 MOF without the peaks of metal sulfide, while pure metal sulfide (Fe)-Co₃S₄ is the opposite (Fig. S3†). In this situation, TAA releases S²⁻ ions as a vulcanization reagent to form bimetallic sulfides with the coexistence of Co₃S₄ and Fe₃S₄ under solvothermal conditions.⁴⁵ Additionally, the diffraction peak intensity of the metal sulfide is positively correlated with the TAA dosage at the same TPA dosage, indicating that the sulfide amount in the nanosheets can be tuned by the TAA content. Notably, both the shuttle-shaped CoFe-MOF and the hollow microspherical (Fe)-Co₃S₄ are thicker than the CoFe-MS/MOF (Fig. S4†), indicating that the inlaid sulfide in the Fe/Co-based MOF is conducive to the formation of ultrathin nanosheets. In the current situation, the diffraction peaks of the MOF are proved to be rather weak, whereas those of sulfide are proved to be intense for a shorter growth time. Consequently, we assume S²⁻ liberated by TAA first reacts with the metal (Co and Fe) ions to form sulfide nanomicrospheres, and is subsequently converted into metal sulfide particles inlaid in the MOF ultrathin nanosheets in the presence of TPA ligands.⁴⁶

Among a series of bimetallic and monometallic catalysts, the optimized CoFe-MS/MOF catalyst synthesized with a 1 : 4 Fe/Co ratio after five hours of the solvothermal reaction displayed optimal OER activity in 1 M KOH (Fig. S5b and S6–S8†). As shown by the polarization curves in Fig. 4a, the CoFe-MS/MOF exhibits attractive OER activity as well as the lowest onset potential, which is significantly superior to those of the CoFe-MOF, (Fe)-Co₃S₄, CoCu-MS/MOF, CoNi-MS/MOF, and RuO₂. As shown in Fig. 4b, the CoFe-MS/MOF only demands a low overpotential of 261 mV to deliver a current density of 10 mA cm⁻², which is far smaller than those of the CoFe-MOF (312 mV), (Fe)-Co₃S₄ (306 mV), the CoNi-MS/MOF (305 mV), the CoCu-MS/MOF (299 mV), and commercial RuO₂ (313 mV). Meanwhile, the CoFe-MS/MOF electrode also displays a smaller Tafel slope (60.3 mV dec⁻¹) than those of the CoFe-MOF (73.6 mV dec⁻¹), (Fe)-Co₃S₄ (67.1 mV dec⁻¹), the CoNi-MS/MOF (111.8 mV dec⁻¹), the CoCu-MS/MOF (130.2 mV dec⁻¹), and RuO₂ (86.5 mV dec⁻¹), which is shown in Fig. 4c, implying more beneficial OER kinetics. In addition, electrochemical impedance spectra (EIS) were measured to study the charge transport and conductivity of catalysts. As indicated in Fig. 4d, the Nyquist plots can be fitted using an equivalent circuit, and the corresponding fitted results are presented in Table S1.† The terms R_s, CPE, and R_ct represent the solution resistance, constant phase elements, and charge-transfer resistance, respectively. Obviously, the charge transfer resistance (R_ct) of the CoFe-MS/MOF is only 9.50 Ω, much smaller than those of the CoFe-MOF (51.80 Ω), (Fe)-Co₃S₄ (10.10 Ω), the CoNi-MS/MOF (72.92 Ω), and the CoCu-MS/MOF (14.60 Ω), suggesting faster OER kinetics of the CoFe-MS/MOF in comparison with that of the other four electrocatalysts. It primarily arises from the synergistic effect between the incorporated sulfide particles and the MOF, which diminishes the charge transfer resistance, and the ultrathin nanosheet structure, which shortens the electron transfer distance.^25,47 The above comparisons clearly disclose that embedding sulfide particles into MOF nanosheets can essentially improve the OER activity of the as-prepared CoFe-MS/MOF electrocatalyst. The CoFe-MS/MOF also exhibits excellent OER durability. As shown in the current density over time (i–t) curve in Fig. 4e, the current density exhibits a slow decline. Simultaneously, the chronoamperometric curve at a current density of 10 mA cm⁻² reveals that the CoFe-MS/MOF remains quite durable after at least 40 h of continuous electrolysis (Fig. 4f), demonstrating superior durability toward the OER.

Fig. 4 Electrocatalytic OER activities in 1 M KOH. (a) Polarization curves, (b) histogram of overpotentials, and (c) Tafel plots at a current density of 10 mA cm⁻². (d) Nyquist plots. (e) The i–t curve under an overpotential of 270 mV over 14 h and (f) the prolonged chronopotentiometric curve over 40 h for the CoFe-MS/MOF.

To investigate the possible origin of the considerably boosted water oxidation activity mentioned above, electrochemical surface areas (ECSAs) of the CoFe-MS/MOF, the CoFe-MOF, (Fe)-Co₃S₄, the CoNi-MS/MOF, and the CoCu-MS/MOF were compared through their double-layer capacitance (C_dl) (Fig. 5a and Fig. S8†).⁴⁸ As shown in Fig. 5b, the C_dl value of the CoFe-MS/MOF (7.71 mF cm⁻²) is 2.6-, 4.7-, 4.0-, and 6.3-fold that of the CoFe-MOF, (Fe)-Co₃S₄, the CoNi-MS/MOF, and the CoCu-MS/MOF, individually, indicating a larger ECSA of the CoFe-MS/MOF, in accordance with the SEM and TEM observations. The electrocatalytic OER performance of the CoFe-MS/MOF and other comparison samples is listed in Table S2.† Nevertheless, the OER activity of the CoFe-MS/MOF fails to increase by a corresponding multiple, suggesting that it is not only determined by the increased ECSAs, but is also closely associated with the synergistic effect between the MOF nanosheets and the incorporated sulfide particles.

Fig. 5 (a) Cyclic voltammograms of the CoFe-MS/MOF. (b) Double layer currents of different catalysts versus the scan rate.

In order to illuminate the probable mechanism underlying such a synergistic effect, the alterations of the CoFe-MS/MOF in the structure and composition after electroactivation were investigated. After electrochemical activation (usually 20 cycles between 1.0 and 1.7 V (vs. RHE) at 10 mV s⁻¹), the diffraction peaks of the metal sulfide show no remarkable difference, while those of the MOF entirely vanish, as displayed by the XRD pattern in Fig. 6a. This indicates that the crystal structure of the MOF nanosheets has undergone a transformation during the electroactivation process, whereas the sulfide particles survive, as evidenced by the Raman spectra (Fig. 6b).^30,49 The survival of the sulfide particles is possibly due to the fact that they are sheltered from the phase changes during electroactivation by the surrounding oxyhydroxide substrate. The nanosheet morphology of the CoFe-MS/MOF remains invariant, yet its thickness is significantly enlarged (Fig. 6c and d), indicating the formation of (oxy)hydroxides during electrochemical activation, which act as the actual active sites of the OER.^26,50,51

Fig. 6 (a) XRD patterns and (b) Raman spectra of the CoFe-MS/MOF before and after electrochemical activation. (c and d) TEM images of the CoFe-MS/MOF after electrochemical activation.

X-ray photoelectron spectroscopy (XPS) was performed to examine the electronic interactions in the CoFe-MS/MOF. The survey spectrum of the CoFe-MS/MOF reveals the existence of Co, Fe, S, O, and C elements, as shown in Fig. 7a. The O 1s spectrum of the CoFe-MS/MOF in Fig. 7b displays three sub-peaks assigned to a M–O bond, a carboxyl (O=C–O–) bond, as well as the absorbed water. After electrochemical activation, the proportion of carboxyl O apparently decreases, whereas the M–O bonds grow, indicating the transformation of the MOF to oxyhydroxide during the OER process. Fig. 7c shows the Co 2p spectra from the CoFe-MS/MOF before and after electroactivation. The binding energies of 781.80 (Co 2p_3/2) and 797.70 eV (Co 2p_1/2) correspond to the Co²⁺ species, and 778.73 (Co 2p_3/2) and 793.87 eV (Co 2p_1/2) are associated with surface-oxidized Co³⁺, respectively.⁵² It is notable that the binding energy of Co 2p_3/2 for the CoFe-MS/MOF decreases after electrochemical activation (ΔBE = 0.87 eV). The peaks at 711.90 (2p_3/2) and 724.83 eV (2p_1/2) are indexed to Fe³⁺ and a resembling binding energy shift is noted in the Fe 2p spectra as well (Fig. 7d).⁴⁹ This indicates that their electronic structure is apparently altered to an electron-rich state by the metal sites in the CoFe-MS/MOF after electrochemical activation. It is caused by metal sulfide particles conferring electrons from their electron-rich metal atoms to the surface Co and Fe centers.^51,53,54

Fig. 7 (a) XPS survey spectra and (b) O 1s, (c) Co 2p, and (d) Fe 2p spectra of the CoFe-MS/MOF before and after electrochemical activation.

Recent studies further indicate that the OER activity of Fe/Co-based catalysts can be boosted through enhancing the adsorption of oxygenated intermediates such as *OH and *O on the surface.^35,55,56 As shown in Fig. 4c and d, the Fe/Co centers at the electron-rich state favor the enhancement of oxygen-containing intermediates for expedited OER kinetics,^16,57 evidenced by the smaller R_ct and smaller Tafel slope of the CoFe-MS/MOF in comparison with the CoFe-MOF in this work. Thus, a greater inherent OER activity of the CoFe-MS/MOF stems from the larger electron density at its metal sites, which is caused by the embedded metal sulfide particles.

To estimate the application of the CoFe-MS/MOF as a remarkable OER catalyst in a water-splitting cell, the overall electrocatalytic water decomposition performance of the CoFe-MS/MOF was investigated herein. The CoFe-MS/MOF and the commercial Pt/C catalyst serve as the anodic and cathodic working electrodes (WE) of a double-electrode cell in 1 M KOH, respectively (Fig. 8a). From the polarization curve (Fig. 8b), it is unambiguously discovered that this CoFe-MS/MOF//Pt/C couple entails a smaller cell voltage of 1.54 V in comparison with the RuO₂//Pt/C couple to achieve a current density of 10 mA cm⁻², reflecting its promising application toward water splitting.

Fig. 8 (a) The schematic diagram of the overall water splitting based on CoFe-MS/MOF//Pt/C electrodes. (b) Polarization curve of the overall water splitting over the CoFe-MS/MOF//Pt/C two-electrode couple.

3. Conclusions

To sum up, a range of CoM (M = Fe, Ni, Cu)-MS/MOF, CoFe-MOF, and (Fe)-Co₃S₄ samples was successfully synthesized through a facile one-step solvothermal method. Among these, the as-prepared CoFe-MS/MOF consisting of interconnected MOF ultrathin nanosheets with metal sulfide nanoparticles exhibits a much lower overpotential, smaller Tafel slope, and excellent stability, which indicate its greatly improved OER activity in alkaline conditions. Furthermore, by coupling with Pt/C, the CoFe-MS/MOF is able to drive water splitting to deliver a current density of 10 mA cm⁻² with an ultralow cell voltage of 1.54 V as well, indicating the promising application of the CoFe-MS/MOF for overall water splitting. The boosting mechanism results from the metal sulfide particles embedded in the nanosheets, which facilitates the formation of ultrathin nanosheets to significantly expand the ECSAs, while providing high electrical conductivity but, more significantly, effectively regulates the electronic structure of the catalytically active atomic sites to an electron-rich state through strong electronic interactions, enhancing the adsorption of O-containing intermediates to promote rapid electrochemical reactions. This research offers prospects for the sensible design of high-performance and cost-effective bimetallic sulfide doped MOF materials for electrocatalytic applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China (Grant No. 52073199 and 51873136).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi00125j

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