MOF-Derived Metal Sulfides and Their Composites: Synthesis and Their Electrochemical Water Splitting

Abstract

Owing to their tunable electronic structures, exceptional structural stability, and superior catalytic performance, metal–organic framework (MOF)-derived metal sulfides have emerged as promising candidates for use in energy conversion systems. This review first summarizes the various synthesis methods for MOF-derived metal sulfides. Subsequently, recent progress in electrochemical water splitting, including the hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting are discussed. Finally, the current challenges of MOF-derived metal sulfides for electrochemical water splitting are also highlighted. We hope that this review will serve as a valuable reference for the rational design of novel MOF-derived metal sulfides for use in electrochemical water splitting.

1. Introduction

As a class of hybrid porous crystalline materials composed of organic ligands and metal ions, metal–organic frameworks (MOFs) exhibit high porosity, large specific surface area, tunable pore structures, and an abundance of metal coordination sites [1,2]. Furthermore, their structural versatility enables the design and synthesis of MOFs with tailored architectures and properties by selecting the appropriate metal ions and organic linkers [3]. Benefiting from their unique physical and chemical properties, MOFs have shown significant potential in energy-related applications, including water splitting [4], batteries [5], fuel cells [6], and supercapacitors [7]. However, the practical application of pristine MOFs is greatly limited by their relatively low stability and poor electrical conductivity [8]. To address these limitations, numerous researchers have focused on the development of MOF-derived composites, including MOF-derived phosphides [9], MOF-derived carbides [10], and MOF-derived sulfides [11], etc. These derivatives have proven to be an effective strategy for significantly enhancing the stability and electrical conductivity of the original MOFs. During the preparation of MOF-derived composites, the materials can be functionalized and improved further through heteroatom doping with elements such as nitrogen, phosphorus, and sulfur. This approach has been shown to enhance the electrochemical performance of carbon-based metal-free and non-noble metal catalysts, broadens the range of available catalysts, and mitigates some of the drawbacks associated with using pure MOFs directly.

Among the various MOF-derived materials, MOF-derived metal sulfides retain the high specific surface area and high electronic conductivity to MOFs. The incorporation of sulfide species introduces new active sites that can more effectively interact with reactants, thereby improving the catalytic and adsorption properties of the material [12]. Compared with other MOF-derived materials such as MOF-derived phosphides or MOF-derived carbides, MOF-derived metal sulfides have garnered significant attention in recent years [13,14,15]. Numerous studies have demonstrated the advantages of MOF-derived metal sulfides including low cost, uniform heteroatom doping, and excellent structural stability [16,17]. Furthermore, the distinctive band structure of metal sulfides derived from MOFs leads to enhanced intrinsic conductivity, which has the potential to result in superior catalytic activity [18,19]. Due to their tunable electronic structure, favorable conductivity, and abundant electrocatalytic active centers, MOF-derived metal sulfides are widely used in the field of electrochemical energy conversion systems such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting (OWS), etc. [20,21].

Over the past several years, researchers have adopted various strategies such as the solvothermal method [22], hydrothermal method [23], in situ sulfidation method [24], and other methods [25] to prepare MOF-derived metal sulfides using MOFs as precursors. These materials have been widely applied in various electrochemical energy conversion systems due to their outstanding physical and chemical performances, such as OER [26], HER [27], and OWS [28]. While several reviews have summarized the applications of MOF-derived transition metal sulfides in energy conversion [13,29,30,31,32], a comprehensive summary of the primary synthesis methods and electrochemical water splitting (such as OER, HER, OWS) of MOF-derived metal sulfides over the past several years remains lacking. Given the growing interest in this class of materials within the electrochemical research community, a systematic review of their synthetic approaches and application progress (Figure 1) is highly desirable.

In this review, the latest advances in the synthesis of MOF-derived metal sulfides and their composites are first summarized. The electrochemical water splitting such as OER, HER, and OWS are then discussed. Finally, it outlines current challenges and potential future research directions. This report is expected to provide valuable insights and guidance for the future design and development of efficient MOF-derived sulfide materials for use in energy conversion systems.

2. The Synthesis Methods of MOF-Derived Metal Sulfides and Their Composites

MOF-derived metal sulfides are usually prepared using MOFs or MOF-based composites as precursors, followed by sulfidation treatments to achieve the desired transformation. This process enables materials to be fabricated with large specific surface areas and enhanced structural stability. To date, numerous strategies have been developed to synthesize MOF-derived metal sulfides with diverse morphologies, including hollow structures [33], polyhedral frameworks [34], porous microspheres [35], nanosheets [36], and nanorod arrays [37]. This section provides a systematic summary of commonly used synthesis techniques, including the solvothermal method, hydrothermal reaction method, in situ sulfidation method, and other methods.

2.1. Solvothermal Method

The solvothermal method involves the execution of chemical reactions within a sealed vessel under conditions of elevated temperature and pressure, with an organic solvent acting as the reaction medium. The method has become a widely adopted technique for synthesizing MOF-derived composite materials due to its advantages of low cost, operational simplicity, and high yield. In recent years, significant efforts have been devoted to the preparation of MOF-derived metal sulfides via the solvothermal route [38]. During the synthesis process, the morphology, size, and composition of the sample can be precisely controlled by adjusting parameters such as reaction temperature, time, solvent type, and reactant concentration.

For example, Zhang and co-workers synthesized 1T-MoS₂ nanoflowers with ultra-large interlayer spacing through a one-step solvothermal sulfidation process based on Mo-MOF (Figure 2a) [18]. In this report, solution A (containing Na₂MoO₄ and thioacetamide (TAA)) and solution B (consisting of terephthalic acid and N,N-dimethylformamide, (DMF)) were initially amalgamated and agitated at room temperature. The mixture was then transferred into a high-pressure reactor and heated to 120 °C in order to synthesize Mo-MOF. Subsequently, the reaction temperature was increased to 200 °C and maintained for 5 h, during which the Mo-based MOFs were converted into 1T-MoS₂. Their experimental results demonstrated that the prepared 1T-MoS₂ exhibited excellent HER performance, achieving an overpotential of 98 mV at 10 mA cm⁻² in 0.5 M H₂SO₄. Importantly, one key factor contributing to this superior electrochemical behavior is the use of DMF as the solvent, which not only prevents structural collapse and active site coverage during carbonization but also increases the interlayer spacing of MoS₂ and enhances electron transfer kinetics.

In another example, Ke et al. synthesized the Fe_0.75Ni_0.25S₂ nanorods via a two-step solvothermal approach (Figure 2b) [39]. In the first step, FeCl₃·6H₂O, Ni(NO₃)₂·6H₂O, and terephthalic acid were dissolved in DMF and subjected to solvothermal treatment at 100 °C for 15 h, yielding the FeNi-MOF precursor. In the second step, the FeNi-MOF precursor and TAA were dissolved in ethanol and reacted under solvothermal conditions at 120 °C for 12 h. After post-treatment, the Fe_0.75Ni_0.25S₂ nanorods were synthesized. Particularly, the prepared Fe_0.75Ni_0.25S₂ nanorods catalyst exhibited a low overpotential of 247 mV for oxygen evolution at 10 mA cm⁻² in alkaline media, along with noteworthy electrochemical stability for over 40 h.

In addition to the aforementioned examples, a plethora of distinct MOF-derived metal sulfide types have been synthesized via the utilization of the solvothermal method. For example, Hou et al. fabricated a hierarchical Fe-Ni₃S₂@NiFe-MOF on nickel foam (Fe-Ni₃S₂@NiFe-MOF/NF) using NiFe-MOF nanosheet as a precursor through the solvothermal method [40]. In this report, prior to synthesis, commercial NF was pretreated to remove surface oxides and impurities. Subsequently, FeCl₃·6H₂O, Ni(NO₃)₂·6H₂O, 2-aminoterephthalic acid (H₂BDC-NH₂), TAA, DMF, and ethanol were uniformly mixed, and the pre-treated NF was immersed in the solution. The system was placed in a stainless-steel autoclave and reacted at 150 °C for 15 h to prepare the final product. This one-step strategy enabled the integration of clusters and ordered nanosheet arrays, thereby significantly increasing the specific surface area and facilitating electrolyte ion diffusion and gas bubble release. The optimized Fe-Ni₃S₂@NiFe-MOF/NF electrocatalyst exhibited excellent OER (226 mV@100 mA cm⁻²) and HER (170 mV@50 mA cm⁻²) performance in 1 M KOH.

Yu et al. constructed the flower-like porous Fe@CoMo₂S₄/Ni₃S₂/NF composites via a two-step solvothermal process using CoMo-MOFs/NF as the precursor [41]. In this report, the CoMo-MOFs/NF composites were first prepared via a simple process. The Fe@CoMo₂S₄/Ni₃S₂/NF composites were then obtained using CoMo-MOFs/NF as the framework, TAA as the sulfur source, and FeCl₃·6H₂O as the Fe source; sulfidation was carried out via solvothermal treatment at 150 °C for 6 h. The resultant Fe@CoMo₂S₄/Ni₃S₂/NF composites exhibited an overpotential of 167 mV for OER at 10 mA cm⁻².

Baek et al. synthesized CoS_x electrocatalyst using a solvothermal method with ZIF-67 as the precursor [42]. Initially, ZIF-67 was synthesized by stirring Co(NO₃)₂·6H₂O and 2-methylimidazole mixture at room temperature for 6 h. Subsequently, CoS_x was produced by reacting ZIF-67 with TAA in ethanol at 120 °C for 6 h. Zhao et al. synthesized the NiFe-MS/MOF@NF composites via a one-step solvothermal method [43]. Under an Fe³⁺/Ni²⁺ molar ratio of 1:4, using terephthalic acid and TAA as precursors, the target product was obtained after a 5 h reaction at 150 °C and demonstrated efficient performance. Dong et al. employed MIL-88A(Fe) as a precursor to fabricate a Co-doped FeS/MoS₂ electrocatalyst (Co-FeS/MoS₂@NF) via a two-step solvothermal–sulfidation approach [44]. The catalyst showed low overpotentials of 63 mV for HER at 10 mA cm⁻² and 230 mV for OER at 100 mA cm⁻², respectively. Zhong et al. synthesized N-doped Ni-Co bimetallic sulfide catalysts (N-Ni_xCo_y-S) via a solvothermal route [45]. The hollow spherical morphology contributed to excellent OER activity, with an overpotential of 286.2 mV at 10 mA cm⁻² in 1 M KOH. Wang et al. synthesized a MOF-derived highly active OER catalyst (NiFe-MOF-S@CNT) via a two-step solvothermal method [38]. The catalyst exhibited an overpotential of 237 mV for OER at 10 mA cm⁻² in 1 M KOH. Xiao et al. fabricated CoS₂/NC@MoS₂ composites using Co-MOF as a precursor through a combined high-temperature sulfidation and solvothermal approach [46].

2.2. Hydrothermal Method

The hydrothermal method has been identified as a particularly effective technique for the synthesis and processing of oxide-based functional materials or compounds that are not water-sensitive. This technique offers several advantages, including mild reaction conditions, operational simplicity, and cost-effectiveness. The hydrothermal method has been extensively utilized for the fabrication of nanomaterials with well-defined morphologies and dimensions due to the malleability inherent in the adjustment of pivotal parameters such as solvent type, reaction temperature, duration, and pH. Sulfur, despite its high solubility in water and organic solvents such as ethanol, can still be effectively extracted from metal sulfide frameworks precursors through meticulously designed hydrothermal reactions [47]. In recent years, researchers have successfully synthesized various MOF-derived metal sulfides with diverse morphologies using the hydrothermal method.

For instance, Kang et al. synthesized hollow rod-shaped NiCoMn-S ternary metal sulfide nanosheets through a combination of hydrothermal treatment and ion exchange reaction (Figure 3a) [48]. In this work, the ZIF-67 precursors with a rod-like morphology were obtained by reacting cobalt nitrate and 2-methylimidazole under hydrothermal conditions. These precursors were then introduced into a mixed solution of DMF and anhydrous ethanol containing Ni(NO₃)₂ and MnCl₂, followed by a reaction at 90 °C for 2 h to form NiCoMn-OH nanosheets. Finally, the NiCoMn-S nanosheets were obtained by treating the NiCoMn-OH via a sulfidation process using TAA at 120 °C for 4 h. The resulting composite exhibited a uniform hollow rod-like structure, which significantly improved ion diffusion and electron transport properties.

Ma and co-workers fabricated MoS₂/CoFe@nitrogen-doped carbo (MoS₂/CoFe@NC) electrocatalysts via a hydrothermal synthesis process (Figure 3b) [20]. In this report, the Co-Fe prussian blue analog (PBA) precursor was first synthesized using a self-assembly method. Subsequently, the Co-Fe PBA underwent high-temperature annealing in an Ar atmosphere to convert it into NC-coated CoFe alloy nanoparticles (CoFe@NC). Finally, TAA and sodium molybdate were introduced, leading to the deposition of MoS₂ nanosheets on the surface of CoFe@NC through a hydrothermal process, ultimately resulting in the formation of the MoS₂/CoFe@NC composites. The optimized MoS₂/CoFe@NC composites exhibited excellent HER (172 mV) and OER (337 mV) performance at 10 mA cm⁻² in 1 M KOH.

Moreover, Lu et al. developed a core–shell-structured CoS₂-C@ReS₂/carbon fiber paper (CoS₂-C@ReS₂/CFP) catalyst by combining low-temperature sulfidation with a hydrothermal method [50]. Initially, a Co-MOF was synthesized, followed by low-temperature sulfidation to obtain the CoS₂-C composite. Subsequently, ReS₂ was grown in situ on the CoS₂-C surface via hydrothermal treatment, forming the core–shell CoS₂-C@ReS₂ on carbon fiber paper (CFP). The CoS₂-C@ReS₂/CFP catalyst demonstrates excellent HER (85 mV) and OER (257 mV) performance at 10 mA cm⁻² in 1.0 M KOH.

Qin et al. synthesized ternary metal sulfide MoCoNiS nanoparticles by combining a hydrothermal method with an ion-exchange process [51]. In this work, a two-dimensional (2D) Co-MOF precursor was grown on an NF substrate to fabricate Co-MOF/NF composites. This precursor was then subjected to ion exchange using Na₂MoO₄·2H₂O to produce MoCo-LDH/NF. Finally, a hydrothermal treatment at 140 °C for 12 h was applied to obtain MoCoNiS/NF. As an OER electrocatalyst, this material exhibited an overpotential of only 151 mV at 10 mA cm⁻².

Liu et al. synthesized an FeNiZnS electrocatalyst via a two-step hydrothermal method. Initially, a series of ternary coral-like FeNiM (M = Zn, Co, Cd) MOF templates were prepared using a one-pot hydrothermal method [52]. Subsequently, the FeNiZn MOF template was converted into sulfides through hydrothermal sulfidation at 150 °C. With extended sulfidation time, the material evolved into dispersed nanosulfides. The optimized FeNiZnS-1 delivers a minimum overpotential of 249 mV at 10 mA cm⁻² for OER.

Chhetri et al. developed a 2D mesoporous hybrid nanostructure FeS₂-MoS₂@CoS₂-MOF on NF through a multi-step hydrothermal strategy [53]. In this report, 2D MOFs were first grown on NF substrates, followed by thermal annealing to form mesoporous CoS₂ nanosheets. Subsequently, FeS₂@CoS₂ layers were deposited onto the CoS₂ surface via a hydrothermal reaction process. The fabricated FeS₂-MoS₂@CoS₂-MOF composites demonstrated a low overpotential of 92 mV at 10 mA cm⁻² for HER in 1 M KOH.

Until now, other studies have also documented that MOF-derived metal sulfides can be synthesized via the hydrothermal method. For example, Xu et al. developed Ni_0.15Co_0.85S₂@MoS₂ catalysts using NiCo-MOFs as precursors through a facile hydrothermal method [23]. The optimized Ni_0.15Co_0.85S₂@MoS₂ demonstrates superior HER activity, achieving a low overpotential of 79 mV at 10 mA cm⁻² in 1.0 M KOH. Lu et al. synthesized NC-CoNi₂S₄@ReS₂/CC composites utilizing a combination of hydrothermal and ion exchange techniques [54]. The resulting NC-CoNi₂S₄@ReS₂/CC catalyst exhibited exceptional OER (253 mV) and HER (87 mV) performance at 10 mA cm⁻² in an alkaline electrolyte. Fu et al. synthesized bimetallic sulfide nanomaterials (Sn-doped Ni₃S₂) exhibiting a popcorn-like morphology using Ni-Sn MOF as a precursor via the hydrothermal method [55]. This unique structural configuration endowed the material with exceptional electrochemical performance. Based on the above research examples, the hydrothermal method for synthesizing MOF-derived metal sulfides represents a promising strategy, enabling the controlled fabrication of nanomaterials with diverse microstructures by adjusting reaction parameters.

2.3. In Situ Sulfidation Method

The in situ sulfidation method is a process in which a sulfur source is introduced concurrently with the formation of the MOF structure. This results in the direct conversion of the MOF into metal sulfides. This technique facilitates the preservation of the original structure and morphology of the MOF while ensuring a uniform distribution of metal sulfides. In recent years, a significant number of studies have utilized the in situ sulfidation method for the preparation of MOF-derived metal sulfides.

For example, Rong et al. developed the rose-like CuSNC@MoS₂-Pt composites through in situ sulfidation and solvothermal methods (Figure 4a) [56]. In this work, a rose-like Cu-TCPP MOF was first synthesized via solvothermal reaction at 80 °C for 4 h. Subsequently, Mo ions were immobilized on the surface of Cu-TCPP/Mo by heating at 120 °C for 4 h to form the Cu-TCPP/Mo precursor. The CuSNC@Mo composites with a stable architecture were obtained through treating the Cu-TCPP/Mo precursor under a calcination process. Finally, the MoS₂ nanosheets were in situ grown on the substrate surface through sulfidation at 180 °C for 24 h, with simultaneous introduction of Pt to yield CuSNC@MoS₂-Pt. Importantly, such CuSNC@MoS₂-Pt composites showed the hydrogen evolution activity of 102.6 mV at 10 mA cm⁻² in alkaline electrolyte.

In another example, Sun and co-workers utilized ZIF-67 as a template to construct a CoS_x composite membrane derived from ZIF-67 through an in situ sulfidation strategy (Figure 4b) [57]. Initially, ZIF-67 membranes were synthesized using sol–gel and in situ growth methods with raw materials including cobalt nitrate hexahydrate, ethanolamine, and 2-methylimidazole. Subsequently, TAA was introduced to the ZIF-67 framework, leading to the formation of the CoS_x composite membrane via in situ sulfidation method.

Moreover, Zeb and co-workers synthesized a novel POM-MOF-derived needle-like Fe,Mo-NiS/Ni₉S₈/NF heterostructure by employing a two-step hydrothermal procedure combined with in situ sulfidation process [58]. In this study, the NiMo₆ was first synthesized using (NH₄)₆Mo₇O₂₄·4H₂O and Ni(NO₃)₂·6H₂O as the precursor. Further, the NiMo₆@MIL-100 composites were obtained by mixing the NiMo₆, FeCl₃·6H₂O, and H₃BTC for 72 h by a hydrothermal reaction process at 130 °C. Subsequently, NiMo₆@MIL-100, TAA, and NF were reacted at 180 °C for 6 h via an in situ sulfidation process to prepare Fe,Mo-NiS/Ni₉S₈/NF composites. Their experimental findings suggested that the in situ sulfidation process effectively prevented the agglomeration of the nickel sulfide active phase, facilitated dual-atom doping, and enhanced electrical conductivity. The catalyst demonstrated exceptional OER performance in alkaline media, attaining an overpotential of 47 mV at 10 mA cm⁻².

Zhang et al. synthesized the MoNiS/Mo₂TiC₂T_x electrocatalyst through a combination of hydrothermal and in situ sulfidation methods [16]. Initially, the Al layer of the Mo₂TiAlC₂ precursor was etched using 50 wt% hydrofluoric acid. The resulting Mo₂TiC₂T_x MXene powder was then dispersed in TBAOH for intercalation treatment, followed by exfoliation into single- or few-layer nanosheets. The Ni(NO₃)₂·6H₂O was reacted with 2-methylimidazole to synthesize Ni-MOF material. Subsequently, Mo₂TiC₂T_x MXene and Ni-MOF were converted into MOF/Mo₂TiC₂T_x composites via solvothermal processing. Finally, in situ sulfidation was carried out in a tube furnace to obtain the MoNiS/Mo₂TiC₂T_x catalyst. In comparison with the conventional method of introducing Mo/S precursors to form MoS₂ clusters on the surface of MXene, this approach directly sulfides Mo₂TiC₂T_x MXene to generate MoNiS/Mo₂TiC₂T_x nanomaterials without requiring additional reagents, effectively reducing MoS₂ misalignment defects caused by interfacial interactions. It is worthy to note that the fabricated MoNiS/Mo₂TiC₂T_x composites exhibited excellent HER catalytic activity of 153 mV at 10 mA cm⁻² in alkaline electrolyte.

Li et al. employed an in situ sulfidation method to synthesize hollow CoS₂-MoS₂ heterostructured nanosheet arrays (denoted as CoS₂-MoS₂ HNAs/Ti) [59]. Initially, the Co-MOF nanosheet arrays were grown directly on titanium foil. Subsequently, an ion exchange and etching reaction between Na₂MoO₄ and the Co-MOF arrays were carried out, resulting in the formation of hollow Co-Mo layered double hydroxide (CoMo-LDH) arrays. Finally, in situ sulfidation converted the CoMo-LDH into CoS₂-MoS₂ HNAs/Ti. The obtained CoS₂-MoS₂ HNAs/Ti composites exhibited low overpotentials of 82 mV for HER and 266 mV for OER at 10 mA cm⁻², respectively.

Sahu et al. fabricated a Co-T-BPY MOF precursor using a hydrothermal method, followed by pyrolysis at 700 °C under an argon atmosphere to generate Co₃S₄ nanoparticles embedded within a N-doped carbon matrix [60]. The study revealed that the pyrolysis duration significantly influenced the optimization of the specific surface area of the resulting sulfide material. The optimized Co₃S₄-3h sample exhibited remarkable OER performance of 285 mV at 10 mA cm⁻² under alkaline electrolytic conditions. Sui et al. designed the Ni₇S₆@NiCo₂S₄ composites using a straightforward in situ sulfidation method by employing Ni-MOF as the precursor [24]. The distinctive 3D architecture provides numerous active sites and facilitates efficient electron transfer. Benefiting from the enhanced catalytic active sites and improved conductivity, the fabricated Ni₇S₆@NiCo₂S₄ composites exhibited outstanding electrochemical performance. Mao et al. employed sublimed sulfur along with solvothermally synthesized Co_0.7Fe_0.3-MOF-74 as precursors for preparing FeCoS₂/Fe_0.95S_1.05 composite materials through an in situ sulfidation process [61].

In addition to the methods mentioned above, there are other approaches for the synthesis of MOF-derived metal sulfides, including the chemical vapor deposition method, electrodeposition method, template method, etc. [62,63,64]. Therefore, the following part will introduce more information about these methods.

2.4. Chemical Vapor Deposition Method

The chemical vapor deposition method is widely employed for the synthesis of MOF-derived metal sulfides due to its operational simplicity, thorough sulfidation efficacy, and excellent scalability for industrial production. For example, Wang and co-workers synthesized a microsphere-structured CoNiZnS/C composites through the chemical vapor deposition method by treating the Co-Ni-Zn MOFs precursor (Figure 5a) [62]. Initially, Co-Ni-Zn MOFs microspheres were synthesized using Co(NO₃)₂·6H₂O, Ni(NO₃)₂·6H₂O, and Zn(NO₃)₂·6H₂O as metal sources and H₂BDC as the organic ligand. These Co-Ni-Zn MOFs were then used as precursors and subjected to sulfidation with TAA at 350 °C for 2 h under an Ar atmosphere in a tube furnace, ultimately forming CoNiZnS/C composites in which trimetallic sulfides were embedded within a carbon matrix. The resulting composites exhibited microspheres assembled from nanorods with abundant heterogeneous interfaces, offering numerous active sites and structural defects. Guo et al. prepared a CoFe₁Ni₂S@NC/CC electrocatalyst through the high-temperature sulfidation method (Figure 5b) [65]. In this report, CC was first immersed in a methanol solution containing Co(NO₃)₂·6H₂O and 2-methylimidazole at room temperature for 12 h to obtain ZIF-67/CC. The ZIF-67/CC was subsequently soaked in a mixed solution of FeSO₄ and NiSO₄ (Ni²⁺/Fe²⁺ molar ratio of 2:1) to form the Fe₁Ni₂/ZIF-67/CC precursor. Finally, the CoFe₁Ni₂S@NC/CC composites were obtained by treating the FeNi/ZIF-67/CC precursor via the chemical vapor deposition process. The optimized CoFe₁Ni₂S@NC/CC composites exhibited excellent OER activity of 175 mV at 20 mA cm⁻² in 1 M KOH. Liu et al. synthesized octahedral N-doped partially graphitized C-encapsulated CuS nanoparticles (CuS@N-C) using Cu-MOF as a precursor through chemical vapor deposition method [66]. This strategy effectively prevented the aggregation of active nanoparticles, enhanced electrical conductivity, and provided rigid structural protection for CuS@N-C. Zhang et al. synthesized hollow microsphere CoS₂ particles through a solvothermal method integrated with a one-step chemical vapor deposition process [25]. Initially, Co-MOF was prepared via the solvothermal method. Subsequently, the sulfidation process was conducted in a tube furnace under an Ar atmosphere to yield CoS₂.

2.5. Electrodeposition Method

Electrodeposition is regarded as a highly effective technique for fabricating functional materials, primarily due to its low cost, broad applicability, and absence of binder requirements [67].

For example, Ao et al. developed the CoS@CoNi-LDH composites on CC through a multi-step synthetic approach [63]. In this study, a Co-MOF precursor was synthesized on the CC substrate, followed by calcination and hydrothermal sulfidation to produce CoS nanorods. Finally, CoNi LDH was electrodeposited onto the CoS surface to form the CoS@CoNi-LDH/CC heterostructure. This material exhibited an excellent hydrogen evolution catalytic performance of 124 mV at 10 mA cm⁻². Cao et al. successfully prepared a core–shell-structured ZnCoS/ZnCo-LDH composite through electrodeposition method [68]. Initially, ZnCo-MOF material was synthesized and loaded onto NF. Subsequently, hydrothermal sulfidation was employed to obtain ZnCoS nanosheets. Finally, ZnCo-LDH was grown on the surface of ZnCoS/NF by electrodeposition to form a uniform and dense outer layer, thereby generating a core–shell-structured ZnCoS/ZnCo-LDH composite.

2.6. Template Method

The template method has also been used to synthesize MOF-derived metal sulfides with controllable morphologies and sizes. For instance, Wan et al. synthesized Ni-CoS/NC composites on N-doped mesoporous C using CoNi-MOF as both a precursor and a template [64]. Initially, CoNi-MOF was prepared and then mixed with sulfur powder, followed by thermal treatment at 700 °C for 2 h under an Ar atmosphere. This process led to the partial substitution of cobalt by nickel in cobalt sulfides, resulting in Ni-CoS/NC. In a 1.0 M KOH electrolyte, Ni-CoS/NC exhibited OER activity of 270 mV at 10 mA cm⁻². Spectroscopic characterization and theoretical calculations revealed that Ni-substituted CoS undergoes surface reconstruction during OER to form amorphous Co_xNi_1−xOOH, which serves as the true active center for OER. Moreover, Ni substitution induced a more negative charge on cobalt atoms and a more positive charge on nickel atoms, thereby optimizing the adsorption energy of OOH intermediates, reducing the OER overpotential, and enhancing overall OER activity.

Chen et al. employed the template method to synthesize the CoNi-S/NC composites by using CoNi-MOF as template [69]. Subsequently, dodecahedral ZIF-67 was synthesized using Co(NO₃)₂·6H₂O and 2-methylimidazole as raw materials. Subsequently, Ni²⁺ was introduced via hydrothermal treatment to obtain CoNi-MOF with a stacked lamellar nanosheet structure. Using CoNi-MOF as a template and TAA as the sulfur source, hollow dodecahedral CoNi-S/NC with surface-stacked nanosheets was obtained through high-temperature annealing. The optimized CoNi-S/NC composites demonstrated excellent OER (327 mV) and HER (106 mV) catalytic activity at 10 mA cm⁻² in 1 M KOH.

Table 1. Various synthesis methods and their advantages/disadvantages for the synthesis of MOF-derived sulfides.

Methods	Advantages	Disadvantages	Typical Examples (Ref.)
Solvothermal method	High yield, Low pressure, Uniform morphology	Harsh conditions, High cost	[18,39,40,41]
Hydrothermal method	Easy operation, Low temperature, Uniform size	Limited applicability, Harsh conditions	[20,48,52,53]
In situ sulfidation method	One-step synthesis, Unique structure, Avoids structural collapse	Complex to control, Limited applicability	[56,57,58,59]
Chemical vapor deposition method	Versatile, Rapid synthesis, Formed stable structure	Prone to structural collapse, High cost Low yield	[62,65,66]
Electrodeposition method	Rapid, Uniform deposition, Low cost	Limited precursor availability, poor scalability	[63,68]
Template method	Controllable morphology, Large surface area	Multi-step process, high cost	[64,69]

summarizes the advantages and disadvantages of the above-mentioned preparation methods for synthesizing MOF-derived metal sulfides.

3. Electrochemical Applications of MOF-Derived Metal Sulfides and Their Composites

3.1. OER

Electrocatalytic oxygen evolution plays a critical role in numerous green and efficient energy conversion and storage technologies, including water splitting, fuel cells, and metal–air batteries [70]. As a four-electron transfer process, OER typically exhibits sluggish kinetics, necessitating high overpotentials to overcome its activation barrier [71,72,73]. Therefore, the development of efficient OER electrocatalysts capable of reducing the energy input and accelerating the reaction rate is of paramount importance [74,75]. Among many functional materials, MOF-derived metal sulfides and their composites have emerged as promising candidates for oxygen evolution due to their favorable stability and electrical conductivity [36]. Notably, sulfide ions contribute significantly to modulating the electronic structure and enhancing the intrinsic catalytic activity of transition metals through their electronegativity [76]. In this section, we will summarize the representative examples of MOF-derived metal sulfides as active materials for OER.

For example, Chu et al. prepared a CoPS/Ni-iron foam (CoPS/NFF) heterostructured with an ultrathin nanosheet morphology through a one-step phosphidation–sulfidation process (Figure 6a) [26]. In this report, ZIF-L/NFF composites were first fabricated by growing the ZIF-L on NFF through a simple process. Subsequently, the CoPS/NFF composites were obtained by treating the ZIF-L/NFF composites via a simultaneous phosphidation and sulfidation process. The obtained CoPS/NFF composites particularly exhibit an ultrathin nanosheet morphology (Figure 6b). This structure, combined with abundant open space, provides a large specific surface area and numerous active sites, thereby significantly enhancing OER efficiency. As presented in Figure 6c, the linear sweep voltammetry (LSV) curves indicate that CoPS/NFF composites exhibit the highest OER activity, requiring an overpotential of 190 mV at 20 mA cm⁻². Moreover, it has a lower Tafel slope (Figure 6d) than those of other catalysts, suggesting superior reaction kinetics. It also demonstrates exceptional stability, maintaining activity for 100 h at 500 mA cm⁻² (Figure 6e), confirming its high activity and long-term durability. The outstanding OER performance of the CoPS/NFF composites is due to their high density of active sites and enhanced electron transport properties.

In another example, Yu et al. developed the Fe@CoMo₂S₄/Ni₃S₂/NF electrocatalyst using a solvothermal method combined with a stepwise sulfidation process by treating the CoMo-MOF precursor [41]. This catalyst showed an overpotential of 167 mV at 10 mA cm⁻² for OER. The remarkable OER performance was primarily attributed to the effective modulation of the anion environment surrounding the metal centers in the metal sulfides, which altered the cations' electronic structure and enhanced the intrinsic catalytic properties. This study demonstrates that amorphous Co/Mo/Ni oxygen sulfides formed under a heterogeneous anion environment are the true active species for OER.

Zhang et al. synthesized the N,P-Co₉S₈/CoS₂/Co_1−xS catalyst through an atmosphere-controlled pyrolysis strategy using MOF as precursor [77]. This catalyst exhibited an excellent OER activity (285 mV at 10 mA cm⁻²) in alkaline media. It also maintained good stability after 1000 cycles and 10 h of continuous operation. The enhanced catalytic performance was mainly because of two factors: (1) the presence of multiple active components providing abundant catalytic active sites at the interface and (2) optimization of the catalyst's valence orbitals via interfacial coupling regulation.

Zeb et al. fabricated the POM-MOF-derived needle-like Fe,Mo-NiS/Ni₉S₈/NF heterostructure via a two-step hydrothermal approach followed by a sulfidation process [58]. The resulting Fe,Mo-NiS/Ni₉S₈/NF heterostructure exhibited exceptional OER performance (47 mV at 10 mA cm⁻²) in alkaline media. This enhanced performance was attributed to its reduced interfacial resistance, fully accessible active sites, superhydrophilic characteristics, and strong electronic interactions. Furthermore, the Fe,Mo-NiS/Ni₉S₈/NF heterostructure exhibited excellent stability for over 100 h under alkaline conditions, meeting the requirements for an effective oxygen evolution electrocatalyst.

Bao and co-workers employed a pre-assembled NH₂-MIL-125@ZIF-67 MOF-on-MOF hybrid as a precursor, which was subsequently transformed into a “Pac-Man”-like Ti-CoS_x hollow superstructure (Ti-CoS_x HSS) electrocatalyst via a two-step sulfidation process [78]. This Ti-CoS_x HSS, composed of hollow nanocage assemblies, possesses a high specific surface area that not only enhances active site exposure but also facilitates electron transfer. Moreover, titanium doping modulates the electronic structure of cobalt centers, thereby improving intrinsic catalytic activity. As a result, the catalyst exhibits excellent OER performance, achieving an overpotential of only 249 mV at 10 mA cm⁻².

Qin et al. developed a ternary metal sulfide MoCoNiS electrocatalyst using a Mo-doped Co-MOF as the precursor [51]. Electrochemical OER tests indicated that in alkaline media, MoCoNiS achieved overpotentials of 151 mV and 226 mV at 10 and 100 mA cm⁻², respectively. Combined with density functional theory (DFT) calculations, this superior performance was found to be due to three factors: (1) the synergistic interactions between the molybdenum, cobalt, and nickel atoms; (2) the reduced reaction energy barriers; and (3) the enhanced utilization of the active sites.

In other representative studies, Zhang et al. utilized a Co-MOF precursor to synthesize a carbon sphere-supported CoS₂ nanoparticle system through a calcination-assisted sulfidation approach [25]. The optimized CoS₂-600 sample demonstrated superior OER activity with an overpotential of 232 mV at 10 mA cm⁻². Zhong et al. utilized Ni-MOF as a precursor and adopted a solvothermal method to synthesize a series of N-doped Ni-Co bimetallic sulfides (N-Ni_xCo_y-S) [45]. The optimized N-Ni₁Co₄-S material exhibited a low overpotential of 286.2 mV at 10 mA cm⁻² in alkaline media. Farooq et al. constructed a CoS@C/MXene composite by sulfiding the ZIF67/MXene precursor in a hydrogen sulfide gas atmosphere at elevated temperatures [79]. This composite exhibits an overpotential of 257 mV at 10 mA cm⁻² for the OER.

Table 2. The different types of MOF-derived metal sulfides and their composites for oxygen evolution.

Catalysts	Overpotential at 10 mA cm−2 (mV)	Tafel Slope (mV dec−1)	Electrolytes	Ref.
MoS2/CoFe@NC	337	84.6	1 M KOH	[20]
CoS2-600	232	77	1 M KOH	[25]
CoPS/NFF	190 (20 mA cm−2)	42.6	1 M KOH	[26]
NiFe-MOF-S@CNT	237	42.3	1 M KOH	[38]
Fe0.75Ni0.25S2	247	47.6	1 M KOH	[39]
Fe-Ni3S2@NiFe-MOF/NF	226 (100 mA cm−2)	67.5	1 M KOH	[40]
Fe@CoMo2S4/Ni3S2/NF	167	13.77	1 M KOH	[41]
NiFe-MS/MOF@NF	230 (50 mA cm−2)	32	1 M KOH	[43]
Co-FeS/MoS2@NF	230 (100 mA cm−2)	42.1	1 M KOH	[44]
N-Ni1Co4-S	286.2	54.8	1 M KOH	[45]
CoS2-C@ReS2/CFP	257	63.8	1 M KOH	[50]
MoCoNiS/NF	151	44.7	1 M KOH	[51]
MoCoNiS/NF	226 (100 mA cm−2)	44.7	1 M KOH	[51]
FeNiZnS-1	249	41.45	1 M KOH	[52]
NC-CoNi2S4@ReS2/CC	253	54.7	1 M KOH	[54]
Fe,Mo-NiS/Ni9S8/NF	47	40.8	1 M KOH	[58]
CoS2-MoS2 HNAs/Ti	266	104	1 M KOH	[59]
Co3S4-3h	285	109	1 M KOH	[60]
N,P-Co9S8/CoS2/Co1−xS	285	70	1 M KOH	[77]
Ti-CoSx HSS	249	45.5	1 M KOH	[78]
CoS@C/MXene	257	93	1 M KOH	[79]

provides a comprehensive summary of various MOF-derived metal sulfides and their composites as active materials for oxygen evolution electrocatalysts.

3.2. HER

With the intensifying global energy crisis and environmental pollution, the development of efficient, clean, and sustainable energy conversion technologies has become increasingly critical [80]. Of the various alternative energy sources, hydrogen is a promising candidate for addressing energy shortages and associated environmental challenges due to its cleanliness, renewability, and high energy density [81]. Electrochemical water splitting is considered a highly efficient and sustainable method for hydrogen production. Up to now, Pt-based electrocatalysts have demonstrated exceptional HER activity, owing to their near-zero onset overpotential and favorable hydrogen adsorption free energy [82,83]. However, their high cost and limited availability hinder large-scale commercial deployment [84]. Therefore, the exploration of cost-effective and environmentally friendly non-noble metal catalysts to enhance HER performance is of paramount importance. Research has shown that the HER activity of electrocatalysts is closely related to the electronic structure of the materials [85]. During the past several years, studies indicate that MOF-derived metal sulfides can significantly improve material conductivity, thereby enhancing electrochemical activity for the HER [86]. This is mainly due to sulfur’s empty 3d orbitals and two lone pairs of electrons in the 3p orbitals, which facilitate surface charge modulation and alter local charge density [87]. Therefore, MOF-derived metal sulfides have been recognized as high-performance HER catalysts [13,88,89].

Yao and co-workers prepared MOF-derived CoS₂/WS₂ composites by the combination of calcination and hydrothermal methods (Figure 7a) [90]. The ZIF-67 was first synthesized using cobalt nitrate hexahydrate and 2-methylimidazole as precursors. The ZIF-67 was then pyrolyzed at high temperature to obtain a Co₃O₄ framework. Subsequently, the hydrothermal method was employed to simultaneously sulfurize Co₃O₄ into CoS₂ while growing a small amount of WS₂ on its surface, forming the CoS₂/WS₂ composites. The fabricated CoS₂/WS₂ composites exhibited significantly superior hydrogen evolution performance compared to the other control samples (Figure 7b,c). Moreover, it also exhibits remarkable long-term durability for HER (Figure 7d). The catalyst was subjected to 1000 cycles at a scan rate of 5 mV s⁻¹ in 0.5 M H₂SO₄. The post-cycling LSV curve nearly overlapped with the initial one, with only a 6 mV increase in overpotential at 10 mA cm⁻². Furthermore, after a 50 h HER stability test at a fixed overpotential of 100 mV, the CoS₂/WS₂ electrode retained 92.61% of its initial current density. The flower-like CoS₂/WS₂ composites exhibited outstanding electrocatalytic activity and stability in acidic media due to two factors: (1) its large specific surface area, which provides more active sites, and (2) enhanced electrical conductivity, which facilitates charge transfer.

Moreover, Lu et al. synthesized NiSe₂/NiS₂@NC heterostructure with abundant phase interfaces using Ni-MOF as a precursor through a two-step process [86]. Initially, MOF nanosheets were prepared via a solvothermal method. Subsequently, the MOF nanosheets were calcined at 600 °C for 2 h under an Ar atmosphere to obtain Ni@NC. Finally, Ni@NC was mixed with sulfur and selenium powders and further calcined at 450 °C for 2 h under argon protection to yield the NiSe₂/NiS₂@NC electrocatalyst. This catalyst exhibited overpotentials of 188 mV and 211 mV at 10 mA cm⁻² in acidic and alkaline media, respectively. The study revealed that the heterojunction structure significantly increased the number of catalytic active sites, accelerated ion and gas transport, and optimized the interfacial electronic structure. This ultimately led to highly efficient HER performance by modifying the Gibbs free energy of hydrogen adsorption (ΔG*H).

Zhang et al. developed a sulfide-based electrocatalyst MoNiS/Mo₂TiC₂T_x through in situ sulfidation combined with hydrothermal treatment [16]. The resulting MoNiS/Mo₂TiC₂T_x catalyst exhibited a low overpotential of 153 mV at 10 mA cm⁻² in 1.0 M KOH solution, with a corresponding Tafel slope of 92 mV dec⁻¹. This performance surpasses that of most transition metal dichalcogenides (TMDs)-based catalysts reported in the literature. The superior HER performance of this catalyst can be attributed to the following reasons: (1) the incorporation of Ni₂S enhances the number of catalytically active sites on MoS₂/Mo₂TiC₂T_x; (2) Ni₂S improves the intrinsic activity of the MoNiS/Mo₂TiC₂T_x heterostructure by optimizing its electronic structure; (3) the synergistic coupling between MXene, Ni₂S, and MoS₂ promotes the exposure of active sites and enhances overall catalytic efficiency.

Rong et al. fabricated the CuSNC@MoS₂-Pt electrocatalyst using Cu-TCPP MOF as a precursor through a solvothermal method followed by in situ sulfidation [56]. This catalyst demonstrated exceptional HER performance in 1.0 M KOH electrolyte, achieving overpotentials of 102.6 mV, 165.6 mV, and 199.0 mV at 10, 50, and 100 mA cm⁻², respectively, along with a Tafel slope of 55.7 mV dec⁻¹. The performance remained nearly unchanged after 1500 cycles. Moreover, no significant fluctuations in catalytic activity were observed at current densities of 10 mA cm⁻² and 30 mA cm⁻² over a 24-h period, further confirming the excellent long-term stability of CuSNC@MoS₂-Pt. The research indicated that the synergistic effect of Pt doping and sulfur/nitrogen dual-doping within the carbon matrix effectively reduced the kinetic barriers for water dissociation and hydrogen generation, thereby significantly enhancing the HER kinetics.

Di et al. successfully constructed a S-N co-doped structure on a carbon shell substrate (denoted as CoS₂@NHCs) by embedding nitrogen atoms into ZIF-67 ligands at high temperature, followed by post-sulfidation treatment [91]. The results showed that the resulting electrocatalyst exhibited excellent catalytic activity and cycling stability for HER when pyrolyzed at 800 °C. CoS₂@NHCs-800 demonstrated overpotentials of only 98 mV in 0.5 M H₂SO₄ and 118 mV in 1 M KOH at 10 mA cm⁻². The superior HER catalytic performance of CoS₂@NHCs-800 is primarily attributed to the abundant active sites of CoS₂, the unique porous core–shell structure, and the enhanced conductivity of the carbon support due to N and S co-doping.

Shen et al. fabricated a porous carbon-based self-supported electrocatalyst, Co-NSC@CBC84, containing Co₉S₈ active components through pyrolysis and subsequent sulfidation of in situ grown ZIF-67 [92]. This catalyst exhibited excellent electrocatalytic performance. In acidic media, it achieved an overpotential of 138 mV at 10 mA cm⁻², along with a Tafel slope of 123 mV dec⁻¹. The enhanced electrochemical activity is mainly due to the strong synergistic coupling between Co-NSC nanoparticles and the CBC substrate.

In addition, Liu et al. synthesized Ni₃S₂-MoS₂@CoMoO₄/NF composite materials through a two-step solvothermal reaction using Co-MOF/NF as a template [27]. This catalyst demonstrated outstanding HER performance, exhibiting an overpotential of 62.4 mV at 10 mA cm⁻², along with excellent electrochemical stability. He and co-workers developed a layered Zr-MOF/NiS₂ electrocatalyst on NF substrates, utilizing Zr-MOF as the precursor [93]. This catalyst demonstrates overpotentials of 110 mV in acidic electrolyte and 72 mV in alkaline electrolyte at 10 mA cm⁻² for HER. Shi et al. synthesized the Er-MOF/NiS electrocatalyst via a solvothermal method followed by sulfidation treatment [22]. The obtained Er-MOF/NiS catalyst showed the HER activity of 115 mV at 10 mA cm⁻². Furthermore,

Table 3. The different types of MOF-derived metal sulfides and their composites for hydrogen evolution.

Catalysts	Overpotential at 10 mA cm−2 (mV)	Tafel Slope (mV dec−1)	Electrolytes	Ref.
MoNiS/Mo2TiC2Tx	153	92	1 M KOH	[16]
1T-MoS2	98	52	0.5 M H2SO4	[18]
MoS2/CoFe@NC	172	122.4	1 M KOH	[20]
Er-MOF/NiS	115	83.48	1 M KOH	[22]
Ni0.15Co0.85S2@MoS2	79	52	1 M KOH	[23]
Ni3S2-MoS2@CoMoO4/NF	62.4	142	1 M KOH	[27]
Fe-Ni3S2@NiFe-MOF/NF	170 (50 mA cm−2)	96.3	1 M KOH	[40]
NiFe-MS/MOF@NF	156 (50 mA cm−2)	82	1 M KOH	[43]
Co-FeS/MoS2@NF	63	53.9	1 M KOH	[44]
CoS2-C@ReS2/CFP	85	144.0	1 M KOH	[50]
FeS2-MoS2@CoS2-MOF	92	70.4	1 M KOH	[53]
NC-CoNi2S4@ReS2/CC	87	83.7	1 M KOH	[54]
CuSNC@MoS2-Pt	102.6	55.7	1 M KOH	[56]
CuSNC@MoS2-Pt	165.6 (50 mA cm−2)	55.7	1 M KOH	[56]
CuSNC@MoS2-Pt	199.0 (100 mA cm−2)	55.7	1 M KOH	[56]
CoS2-MoS2 HNAs/Ti	82	59	1 M KOH	[59]
CoS@CoNi-LDH	124	89	1 M KOH	[63]
NiSe2/NiS2@NC	188	46	0.5 M H2SO4	[86]
NiSe2/NiS2@NC	211	93.2	1 M KOH	[86]
CoS2/WS2	79	52	0.5 M H2SO4	[90]
CoS2@NHCs-800	98	85	0.5 M H2SO4	[91]
CoS2@NHCs-800	118	157	1 M KOH	[91]
Co-NSC@CBC84	138	123	0.5 M H2SO4	[92]
Zr-MOF/NiS2	110	17.95	0.5 M H2SO4	[93]
Zr-MOF/NiS2	72	11.45	1 M KOH	[93]

summarizes various types of MOF-derived metal sulfides and their composites that have been reported as efficient electrocatalysts for HER in recent years.

3.3. OWS

Hydrogen is recognized as a clean and sustainable chemical energy carrier and serves as a promising alternative to conventional fossil fuels [94,95]. The predominant hydrogen production methods are methane steam reforming [96], hydrogen generation from fossil fuels [97], and electrocatalytic water splitting [98]. Among these, water electrolysis has attracted considerable attention due to its mild reaction conditions, the ready availability of water as a feedstock, and its capacity to produce hydrogen of high purity. This renders it a key strategy for advancing the green energy transition [53,99]. Researchers have identified electrocatalytic water splitting as a viable route for the large-scale production of high-purity hydrogen [100].

However, the overall efficiency of water electrolysis is significantly constrained by the sluggish kinetics of both the OER at the anode and the HER at the cathode [101]. Precious metal-based catalysts such as Pt, Ir, and Ru are widely regarded as leading candidates for high-performance electrocatalysts in water splitting due to their excellent electrical conductivity and catalytic activity [102]. Nevertheless, substantial barriers to widespread commercial adoption are posed by their high cost, limited natural abundance, and insufficient long-term stability [103]. Consequently, there has been a surge of interest in the development of efficient, cost-effective, and abundant non-precious metal bifunctional electrocatalysts [104,105,106,107]. Among these alternatives, MOF-derived metal sulfides have emerged as promising candidates capable of replacing noble metals in OWS applications.

For example, Su and co-workers synthesized transition metal-doped bimetallic sulfide ultrathin nanosheets M (M = Fe, Cu, Zn, Mo)-Ni₃S₂/Co₃S₄ on NF through a solvothermal method combined with a simple sulfidation process (Figure 8a) [108]. Initially, the Co-MOF was first synthesized, followed by the preparation of M (M = Fe, Cu, Zn, Mo)-Ni(CO₃)(OH)/Co(CO₃)(OH) nanosheets containing different metals on NF via solvothermal methods as precursors. Subsequently, these precursors were sulfurized using TAA as the sulfur source at 180 °C to obtain M (M = Fe, Cu, Zn, Mo)-Ni₃S₂/Co₃S₄/NF. As shown in Figure 8b, the Cu-Ni₃S₂/Co₃S₄/NF sample consists of ultrathin nanosheets with a thickness of less than 10 nm. Notably, nanoparticles with sizes of 8–10 nm are clearly observed on its surface. The ultrathin nanosheet morphology shortens the electron transport path, while the presence of nanoparticles increases the specific surface area, thereby exposing more active sites and facilitating the adsorption and dissociation of water molecules. Figure 8c presents the LSV curves of several catalysts for HER in a 1.0 M KOH electrolyte. These curves indicate that Cu-Ni₃S₂/Co₃S₄/NF exhibits significantly higher HER activity than that of other samples. Specifically, Cu-Ni₃S₂/Co₃S₄/NF exhibited the catalytic activity of 79 mV and 150 mV to achieve current densities of 10 mA cm⁻² and 20 mA cm⁻², respectively. Figure 8d displays the LSV curves of the same catalysts for OER in 1.0 M KOH. Among all samples, Cu-Ni₃S₂/Co₃S₄/NF demonstrates the best OER performance, with overpotentials of 160 mV and 250 mV at 50 mA cm⁻² and 100 mA cm⁻², respectively. When Cu-Ni₃S₂/Co₃S₄/NF was used as a bifunctional electrocatalyst for OWS in a 1.0 M KOH electrolyte (Figure 8e), it achieved the low voltages of 1.49 V and 1.65 V at 10 mA cm⁻² and 35 mA cm⁻², respectively. The enhanced performance of Cu-Ni₃S₂/Co₃S₄/NF can be ascribed to the partial substitution of Co by Cu, which modulates the active centers and reduces the adsorption energy of HER/OER intermediates on Ni₃S₂/Co₃S₄, thereby enhancing the kinetics of both reactions.

In addition, Kim et al. synthesized a Ni₃S₂/MoS₂ hollow sphere electrocatalyst via a one-pot solvothermal method using NiCo-MOF as the precursor [109]. This catalyst exhibits outstanding catalytic activity for both OER and HER, with overpotentials of 303 mV and 166 mV, respectively, at 10 mA cm⁻² in 1 M KOH solution. When used as both the cathode and anode in an alkaline water electrolyzer, the Ni₃S₂/MoS₂ bifunctional electrocatalyst achieves a cell voltage of only 1.62 V at 10 mA cm⁻². The exceptional catalytic performance and stability of Ni₃S₂/MoS₂ are primarily attributed to the synergistic effects of its bimetallic system, the porous hollow carbon structure with a large surface area derived from the MOF precursor, and the exposure of abundant active sites due to the heterointerface between Ni₃S₂ and MoS₂ nanostructures.

Nguyen et al. coated sulfurized ZIF-67 with polypyrrole (PPy) to develop the Co₉S₈@N-HC electrocatalyst [110]. This catalyst exhibited outstanding electrocatalytic activity for both HER (201 mV) and OER (285 mV) in alkaline media at 10 mA cm⁻². An electrolyzer utilizing Co₉S₈@N-HC-800 as both the anode and cathode achieved a cell voltage of 1.63 V at 10 mA cm⁻². The device demonstrated exceptional long-term stability, with only a 9.6% decay in current density after a 50 h test. The superior electrocatalytic performance of Co₉S₈@N-HC-800 can be attributed to the synergistic effect between ultrafine Co₉S₈ nanoparticles and a N-doped porous carbon matrix characterized by a high heteroatom doping ratio. This combination provides an extensive specific surface area, abundant active sites, and excellent electrical conductivity.

Li et al. successfully synthesized nanoflower-like ZnCoNiS heterostructures using ZIF-8 as a precursor via a hydrothermal method [111]. Benefiting from the unique nanoflower architecture and strong interfacial interactions between different phases, the catalyst exhibits excellent electrocatalytic performance and remarkable stability. Specifically, it achieves low overpotentials of 134 mV for OER and 146 mV for HER at 10 mA cm⁻². When applied to overall water splitting, the system requires a cell voltage of 1.52 V at 10 mA cm⁻² while maintaining outstanding catalytic stability for more than 100 h.

Srinivas and co-workers synthesized Ni-M@C-130 using Ni-MOF@CNT as a precursor [112]. Ni-M@C-130 achieves overpotentials of 123 mV for HER and 244 mV for OER at 10 mA cm⁻² in 1.0 M KOH. Moreover, an electrolyzer assembled with Ni-M@C-130 as both electrodes requires 1.565 V at 10 mA cm⁻² for OWS. The enhanced performance of Ni-M@C-130 is attributed to the combination of MOF nanosheets and synergistic interactions.

Chen et al. synthesized layered porous metal sulfide nanosheets (Ni-Co-S HPNA) by utilizing 2D CoNi-MOF nanosheet arrays as precursors on conductive CC [113]. The Ni-Co-S HPNA catalyst delivered exceptional electrocatalytic performance in alkaline media, requiring overpotentials of 110 mV (HER) and 270 mV (OER) at 10 mA cm⁻². Notably, it achieved a low OWS cell voltage of 1.62 V at 10 mA cm⁻².

Liu et al. synthesized the FeNiZnS electrocatalyst using a two-step hydrothermal method and evaluated its performance for OWS in 1 M KOH [52]. Consequently, the FeNiZnS-1//Pt/C two-electrode configuration achieved a cell voltage of 1.54 V at 10 mA cm⁻². Dong's group synthesized a cobalt-doped FeS/MoS₂ electrocatalyst on NF using MIL-88A(Fe) as the precursor, which exhibited outstanding OWS performance [44]. The assembled electrolyzer required a low cell voltage of 1.45 V at 10 mA cm⁻². Lu et al. employed a Co-MOF precursor to synthesize CoS₂-C@ReS₂/CFP catalyst through hydrothermal method [50]. The CoS₂-C@ReS₂/CFP||CoS₂-C@ReS₂/CFP electrolyzer achieves a low cell voltage of 1.57 V at 10 mA cm⁻² for OWS.

Table 4. The different types of MOF-derived metal sulfides and their composites for overall water splitting.

Catalysts	Electrolytes	Cell Voltage (V) (j10)	Stability (h)	Ref.
Fe-Ni3S2@NiFe-MOF/NF	1 M KOH	1.60	50 (j50)	[40]
Fe@CoMo2S4/Ni3S2/NF	1 M KOH	1.513	120	[41]
NiFe-MS/MOF@NF	1 M KOH	1.61	50 (j50)	[43]
Co-FeS/MoS2@NF	1 M KOH	1.45	24	[44]
CoS2-C@ReS2/CFP	1 M KOH	1.57	15	[50]
FeNiZnS-1	1 M KOH	1.54	50	[52]
FeS2-MoS2@CoS2-MOF	1 M KOH	1.51	30	[53]
NC-CoNi2S4@ReS2/CC	1 M KOH	1.57	60	[54]
CoS2-MoS2 HNAs/Ti	1 M KOH	1.56	24	[59]
H-CoSx/NiFe-LDH	1 M KOH	1.59	100	[67]
Cu-Ni3S2/Co3S4/NF	1 M KOH	1.49	10	[108]
Ni3S2/MoS2	1 M KOH	1.62	100	[109]
Co9S8@N-HC-800	1 M KOH	1.63	50	[110]
ZnCoNiS	1 M KOH	1.52	100	[111]
Ni-M@C-130	1 M KOH	1.565	25	[112]
Ni-Co-S HPNA	1 M KOH	1.62	24	[113]

summarizes the OWS electrocatalytic properties of recently reported MOF-derived metal sulfides and their composites.

4. Conclusions and Outlook

This review provides a systematic summary of the synthesis strategies for MOF-derived metal sulfides and their composite materials, as well as recent advancements in their electrochemical applications, including the OER, HER, and OWS. The materials in question have been shown to possess a number of advantageous features, including porous architecture, high catalytic activity, tunable electronic structures and morphologies, and excellent structural stability. This has led to their demonstrable potential in energy conversion technologies. Here, the role of MOF precursor in the synthesis and electrochemical properties are discussed as follows:

(1) The core role of metal ions: The metal ions form the architectural backbone of the MOF and serve as the metal source for the sulfide. The type of metal ion (such as Co, Ni, Cu, etc.) dictates the chemical composition of the resulting metal sulfide (such as FeNi-MOF yields a bimetallic Fe_0.75Ni_0.25S₂ composite [39]). Moreover, the strength of the bond between the metal ion and the organic linker influences the framework's collapse rate during pyrolysis/sulfidation [14,18]. Stronger bonds can favor a more ordered structural transformation, preserving porosity [41,53].

(2) The key influence of organic linkers: Organic linkers are not merely “bridges”; their behavior during thermal decomposition critically determines the carbon matrix, morphology, and electronic structure of the final material [36,54]. For instance, function as a carbon source, protection and dispersion, enhanced conductivity, and so on.

(3) Ultimate impact on catalytic performance: The control enabled by the MOF precursor directly translates to enhanced catalytic performance. For instance, the high surface area and porosity inherited from the MOF ensure a large number of exposed active sites [59,67]. Strategies like metal doping, heterojunction formation, and carbon-matrix doping optimize the electronic structure, making each active site more efficient. For improving reaction kinetics, high conductivity ensures fast electron transfer, while a hierarchical porous structure promotes diffusion. To boost stability, the protective carbon matrix and robust structure enhance the material's long-term durability under harsh electrochemical conditions.

Nevertheless, several challenges remain to be addressed in both synthetic approaches and electrochemical performance.

(1) With regard to material synthesis and structural control: The high-temperature sulfidation process frequently results in framework collapse of MOF-derived metal sulfides, leading to a reduction in specific surface area, compromised pore architecture, and an uneven distribution of active sites (such as single atoms and heterointerfaces). Furthermore, in multi-metal systems, metal aggregation is frequently observed [114], which diminishes the synergistic catalytic effects. Future research should focus on lowering the sulfidation temperature, improving the design of MOF precursors [115], and achieving atomic-level dispersion of sulfide species through uniform metal node distribution.

(2) Concerning the balance between conductivity and catalytic activity: The majority of metal sulfides manifest semiconducting behavior, accompanied by constrained electron transport capabilities, a phenomenon that impedes the efficiency of redox kinetics. Furthermore, it is important to note that highly active sites may undergo deactivation during operation. The following potential solutions are proposed: the construction of heterostructures to enhance the electrical conductivity of composite systems [116]; the implementation of phase engineering to reduce charge transfer resistance; and the application of nitrogen-doped carbon coatings on sulfide surfaces to mitigate sulfur leaching and preserve catalytic activity.

(3) In terms of scalability and cost-effectiveness: The present reliance on costly MOF precursors, in conjunction with complex and aggressive multi-step sulfidation procedures, impedes large-scale production of MOF-derived sulfides and their composites. It is recommended that future efforts place a high priority on the development of low-cost transition metal-based MOFs (such as Fe, Co, Ni). This should be achieved by leveraging DFT calculations to screen suitable precursors [117], employing computational algorithms to predict optimal sulfide source ratios and compositions, and advancing green synthesis methodologies to minimize energy consumption and environmental impact.

It is recommended that future studies on MOF-derived sulfide catalysts incorporate in situ characterization techniques such as X-ray absorption spectroscopy and X-ray diffraction to elucidate reaction mechanisms and dynamic structural changes during catalysis. In addition, the optimization of synthesis protocols and the development of more eco-friendly and simplified preparation methods will enhance the controllability and reproducibility of the materials. It is anticipated that the perspectives and strategies delineated in this review will furnish innovative insights into the design and development of MOF-derived sulfide catalysts, thereby facilitating further breakthroughs in energy conversion technologies.