Boosting the efficiency of electrocatalytic water splitting via in situ grown transition metal sulfides: a review

Abstract

The growing needs for sustainable and efficient energy sources have heightened interest in electrocatalytic water splitting (EWS), a promising method for hydrogen production as a clean and renewable energy carrier. EWS, which splits water molecules into hydrogen and oxygen, faces efficiency challenges due to the slow kinetics of the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode. Developing effective electrocatalysts is essential to overcome these limitations. Among the various electrocatalysts studied, transition metal sulfides (TMSs) have garnered significant attention due to their low cost and abundant active sites. Despite the superior electrochemical performance of TMSs compared to other materials, their inherent low conductivity and sluggish reaction kinetics remain major challenges. Recent advancements have focused on the in situ growth of TMSs on conductive substrates to enhance electron transfer and overall catalytic performance, eliminating the need for polymer binders and improving electrode stability. This review provides an in-depth analysis of the key aspects involved in the synthesis of in situ grown TMS electrodes, including the selection of TMS active materials, various substrates, and preparation strategies. The review then offers a comprehensive overview of different types of in situ grown TMS electrodes, with a focus on the most extensively researched materials: molybdenum sulfides, cobalt sulfides, nickel sulfides, and their composites. Finally, the limitations and future perspectives are discussed, highlighting potential directions for advancing the development of in situ grown TMS catalysts.

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1. Introduction

The pursuit of sustainable and efficient energy sources has significantly intensified the focus on electrocatalytic water splitting (EWS), a promising process for hydrogen production as a clean and renewable energy carrier. EWS entails the splitting of water molecules into hydrogen and oxygen by applying electrical energy. However, its efficiency is limited by the slow kinetics of the oxygen evolution reaction (OER) at the anode and the hydrogen evolution reaction (HER) at the cathode (Fig. 1a). Therefore, the development of effective electrocatalysts is crucial for improving the efficiency of EWS.

Fig. 1 (a) Schematic representation of the mechanisms of the HER and OER in both alkaline and acidic media; (b) schematic diagram comparing in situ grown and ex situ grown electrodes; (c) trends in scientific publications by category from 2014 to 2023.

To date, a wide range of electrocatalysts have been extensively studied to improve the EWS reaction efficiency, including noble metal-based electrodes such as platinum (Pt), iridium dioxide (IrO₂), ruthenium dioxide (RuO₂),^1,2 transition metal oxides (TMOs) such as molybdenum trioxide (MoO₃), cobalt oxide (CoO), manganese dioxide (MnO₂)^3–6 and TMSs such as molybdenum disulfide (MoS₂), vanadium sulfides (VS₂), nickel sulfides (Ni₃S₂).^7–9 Among these, TMSs have garnered particular attention due to their low cost and abundant active reaction sites.¹⁰ While noble metal-based materials are well-known for their high catalytic efficiency, their high cost and non-renewable nature considerably limit their broad practical application. Additionally, compared to TMOs, TMSs often exhibit superior electrochemical performance, which is mainly attributed to the following reasons: (1) oxygen is more electronegative than sulfur, meaning that when oxygen atoms are replaced by sulfur in TMSs, the bonding becomes less polar. Sulfur has a larger atomic radius and a lower electronegativity, which weakens the metal–sulfur bonds compared to metal–oxygen bonds. (2) The replacement of oxygen with sulfur tends to narrow the bandgap of the material. A narrower bandgap improves electrical conductivity, which is crucial for catalytic applications. (3) Sulfur atoms provide different surface electronic states compared to oxygen, which can create more favorable conditions for adsorption of reactants. This can increase the density and reactivity of active sites, leading to enhanced catalytic activity in various processes.^11–14 These features make TMS materials particularly well-suited for applications in electrocatalysis (Table 1).^15,16

Table 1 The conclusion summarizing the advantages and disadvantages of noble metal-based electrodes, TMO electrodes and TMS electrodes

	Advantages	Disadvantages
Noble metal-based electrodes	• Outstanding conductivity	• High cost
	• Superior catalytic activity	• Non-renewable
	• Excellent stability
TMO electrodes	• Good conductivity	• Low catalytic activity
	• Low cost	• Unstable structure
	• Simple preparation methods
TMS electrodes	• Low cost	• Low conductivity
	• Good catalytic activity	• Sluggish reaction kinetics
	• Adjustable structures

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However, TMS materials still face certain drawbacks, including inherent low conductivity and sluggish reaction kinetics.^17–19 To address these challenges and enhance the electrocatalytic efficiency of TMSs, conductive materials are often integrated during electrode preparation to improve the electrochemical performance of TMSs.^20,21 Nevertheless, the use of inert polymer binders can obstruct the active sites of TMS electrocatalysts, significantly diminishing their catalytic activity during EWS. In response to this issue, the in situ growth of TMSs on conductive substrates has attracted considerable attention. This method ensures close contact between the active materials and the substrate, thereby improving electron transfer and enhancing overall catalytic performance.^22,23

Recent research has increasingly focused on in situ grown electrodes due to several key advantages: Firstly, in situ grown electrodes eliminate the need for polymer binders and slurry ink, thereby reducing the cost of electrode preparation.²⁴ Secondly, improved contact between the active materials and substrates results in superior electron transfer and mass transport, which can facilitate the reaction kinetics.²⁵ Thirdly, the active materials achieve enhanced distribution across the substrate surfaces, which can expose more active sites, significantly improving the water-splitting performance.²⁶ Finally, circumventing the use of binders mitigates potential binder degradation, thereby enhancing electrode stability.²⁷ Over the years, there has been a noticeable upward trend in the number of in situ grown TMSs used as electrocatalysts. The increasing focus on electrocatalysts, as demonstrated in Fig. 1c, indicates a recognition within the scientific community of the potential of in situ grown TMSs to overcome the challenges inherent in energy conversion and storage. The schematic comparison between in situ grown and ex situ grown electrodes is illustrated in Fig. 1b.

In this review, we comprehensively examine recent advancements in the design, synthesis, and application of in situ grown TMSs for electrocatalytic water splitting. An overall introduction to electrodes is provided, including the active TMS materials, substrates and the preparation strategies. Furthermore, we also discuss the underlying mechanisms that contribute to their superior catalytic activities and highlight the role of structural and compositional modifications in optimizing their performance. By delving into the current challenges and potential solutions, this review aims to provide a holistic understanding of the advancements in TMSs as electrocatalysts. Our goal is to shed light on the future directions and opportunities in this rapidly evolving field, ultimately contributing to the realization of efficient and sustainable hydrogen production through EWS.

2. Overview of in situ grown TMS-based working electrodes for EWS

The selection of appropriate active materials is critical for optimizing the electrochemical performance of the electrodes, as these materials directly influence the efficiency and stability of the electrochemical reactions. Equally important is the choice of electrode substrates, which serve as the foundational support for the active materials, ensuring effective electron transfer and mechanical integrity during operation. Various fabrication techniques are employed to in situ grow these materials onto the substrates, with each method tailored to enhance the overall electrode architecture and improve its functional properties. This strategic combination of carefully selected materials and precise fabrication methods is essential for advancing the development of high-performance electrodes across various applications, including energy storage and catalytic processes. Therefore, in this section, a comprehensive overview of the key components is involved, including the active materials, electrode substrates, and fabrication methods.

2.1 TMS synopsis

TMSs are a class of compounds composed of transition metals and sulfur. These materials are characterized by their diverse crystal structures and chemical compositions, which result from the ability of transition metals to exhibit multiple oxidation states and coordinate with sulfur atoms in various ways. The history of TMS materials spans over a century. As early as 1923, Linus Pauling and colleagues reported the crystal structure of MoS₂.²⁸ In 1969, Wilson et al. provided a systematic introduction to transition metal dichalcogenides, detailing their optical, electrical, and structural properties.²⁹ The structures of TMS materials can be broadly classified into two categories: (1) layered structures and (2) non-layered structures. Transition metal dichalcogenides (TMDs), such as MoS₂ and tungsten disulfide (WS₂), belong to the layered structure category and are typically represented as MS₂ (where M stands for Mo, W, etc.). The remaining TMS materials, which include various transition metals like cobalt (Co), iron (Fe), and nickel (Ni), fall into the non-layered structure category and are generally denoted as M_xS_y.³⁰

In single layers of TMS materials, the transition metal (M) is sandwiched between two layers of chalcogens (S) to form an S–M–S trilayer structure. These single sheets are held together by van der Waals forces, forming bulk materials with layered structures.³¹ Within these layered structures, variations in surface orientation lead to basal and edge planes with anisotropic properties.³² Depending on the coordination geometry, layered TMS materials exhibit two primary phases, corresponding to octahedral and trigonal prismatic coordination. Each single layer in multilayered TMS materials can adopt either of these phases, resulting in structures with different coordination styles.³³ However, three of the most commonly observed structures are 1T, 2H, and 3R, as illustrated in Fig. 2a.³⁴ For instance, WS₂ and MoS₂ predominantly exhibit the 2H phase, which is the most thermodynamically stable.³⁸ Nonetheless, phase transitions, such as from 2H to 1T, can occur through ion intercalation, a process often used to modify electrochemical performance.

Fig. 2 (a) Unit cell structure of 1T MoS₂, 2H MoS₂ and 3R MoS₂; reproduced with permission from ref. 34. Copyright 2015 ACS. (b) Crystal structures of FeS₂; reproduced with permission from ref. 35. Copyright 2020 ACS. (c) Crystal structures of nickel sulfides; reproduced with permission from ref. 36. Copyright 2020 RSC. (d) Schematic diagram of three different types of substrates used for in situ grown TMS electrodes; reproduced with permission from ref. 37. Copyright 2023 MDPI. (e) Schematic diagram of the hydrothermal strategy for the preparation of in situ grown TMS electrodes. (f) Diagram of the electrodeposition strategy for the preparation of in situ grown TMS electrodes.

In contrast, non-layered TMS materials typically adopt either the pyrite or marcasite structure. In these structures, transition metals like Fe, Co, Ni, copper (Cu), and zinc (Zn) are coordinated with sulfur in an octahedral arrangement.³⁹ For instance, Fig. 2b illustrates the pyrite and marcasite structures of iron sulfides (FeS₂). In the pyrite structure, iron atoms, depicted as yellow spheres, occupy the corners and face centers of the cubic unit cell, while sulfur atoms, shown as orange spheres, form S₂ (disulfur) dimers aligned along the crystallographic axes. The pyrite structure is characterized by its high symmetry, with each iron atom octahedrally coordinated by six sulfur atoms. In contrast, the marcasite structure, while maintaining the octahedral coordination between iron and sulfur atoms, exhibits a more complex lattice arrangement. The FeS₂ units are connected differently compared to the pyrite structure, leading to a lower symmetry in the marcasite structure. The S₂ dimers in marcasite are more closely spaced and arranged in a manner that gives the structure its distinct, less symmetrical configuration.³⁵ Beyond pyrite and marcasite structures, TMSs can also exhibit other structural forms. For example, Fig. 2c depicts the crystal structures of various polymorphs of nickel sulfides, including NiS, NiS₂, Ni₃S₂, and Ni₉S₈. Each polymorph demonstrates distinct geometric arrangements and coordination environments of Ni and sulfur (S) atoms, as visualized through the depicted polyhedron and atomic networks. Each polymorph demonstrates distinct geometric arrangements and coordination environments of Ni and sulfur (S) atoms, as visualized through the depicted polyhedron and atomic networks.³⁶ The properties of non-layered TMSs vary depending on the d-orbitals of the transition metals. For instance, NiS₂ exhibits poor conductivity, whereas CoS₂ demonstrates metal-like conductivity.

2.2 The substrate materials for the preparation of in situ grown TMS electrodes

As illustrated in Fig. 2d, a wide range of substrates have been explored for the development of in situ grown catalysts. These substrates include carbon-based materials such as carbon fiber (CF), carbon cloth (CC), graphite, graphene, reduced graphene oxide (rGO), and carbon nanotubes (CNT);^40,41 metal-based materials like nickel foam (NF) and copper wire (CW);^42,43 and other conductive materials such as fluorine-doped tin oxide (FTO) and indium tin oxide (ITO). These materials not only address the conductivity challenges associated with traditional binders but also pave the way for new approaches in the design and application of TMSs to enhance the efficiency and durability of EWS technologies.

Carbon-based materials are particularly well-suited as substrates for TMS catalysts due to their high conductivity, outstanding stability, and cost-effectiveness. The in situ growth of TMSs on these carbon substrates substantially enhances catalytic activity by providing an efficient pathway for charge transfer.^44,45 CF and CC, in particular, are distinguished by their excellent mechanical and electrical properties, attributable to their unique preparation methods and high carbon content, making them highly effective in catalytic applications. Graphite, a naturally occurring crystalline form of carbon, is commonly used in lithium-ion batteries (LIBs) and is known for its unique structure and properties. It is one of the most stable forms of carbon under standard conditions, making it valuable in environments requiring stability under corrosive conditions. With its 2D-layered structure, graphite is an excellent conductor of electricity due to the mobility of free electrons within its layers, making it ideal for use in electrodes for batteries and catalysts.⁴⁶ Graphene, characterized by its single layer of carbon atoms arranged in a hexagonal lattice, is the thinnest material known and exhibits exceptional electrical properties.⁴⁷ Similarly, carbon nanotubes (CNTs), which are composed of carbon atoms arranged in a hexagonal pattern and rolled into tubular structures, offer a very high surface area due to their nanoscale dimensions, making them particularly beneficial for catalytic applications. rGO, produced by reducing graphene oxide, retains a combination of conductive graphene-like regions and residual oxygen functionalities. This hybrid structure, resulting from the partial restoration of the sp² carbon network, enhances its effectiveness as a substrate for various applications.⁴⁸

Metal-based substrates also play a crucial role in electrocatalysis. These substrates are chosen for their excellent conductivity, chemical stability, and ability to support active catalytic materials. NF, with its unique 3D porous structure, outstanding conductivity, and high surface area, not only serves as an excellent support for TMSs but also possesses inherent catalytic activity. Its interconnected network of nickel strands provides ample sites for catalyst loading.^49,50 Cu substrates are commonly used in the form of copper foil or CW. Cu's high conductivity and ease of fabrication make it a popular choice for electrocatalytic applications.⁵¹ Other metal-based substrates, such as stainless steel and titanium, are also utilized; however, their relatively poor surface activity and higher cost often limit their use compared to Ni and Cu-based substrates.⁵² The combination of conductivity, chemical stability, and structural support makes metal-based substrates essential for enhancing the performance and longevity of catalysts in water splitting and other electrochemical processes. These substrates not only facilitate the necessary electrochemical reactions but also provide a foundation for the development of advanced, cost-effective energy conversion technologies.

FTO and ITO are two widely used transparent conducting oxides that serve as effective substrates for catalysts in various electrochemical and photoelectrochemical applications. FTO is composed of tin oxide (SnO₂) doped with fluorine atoms, which enhances its electrical conductivity while maintaining high transparency in the visible spectrum. The doping process introduces free electrons into the tin oxide matrix, improving its conductive properties without significantly compromising its transparency.^53,54 ITO is a ceramic material composed of indium oxide (In₂O₃) and SnO₂, typically containing 90% In₂O₃ and 10% SnO₂. This combination results in a material with high electrical conductivity and excellent transparency across the visible light spectrum. ITO is also chemically stable and can withstand harsh conditions, making it a robust substrate in many catalytic applications.^55,56

2.3 Preparation strategies for in situ grown TMS materials

In the preparation of in situ grown electrodes, the active materials are directly grown on the surface of substrates without the use of binders and other conductive additives. Various methods, including hydrothermal, solvothermal,^57–59 electrodeposition,^60,61 and chemical deposition,⁶² have been widely employed to synthesize in situ grown catalysts. Among these, hydrothermal and electrodeposition strategies are particularly noteworthy for their ease of modification, straightforward operational procedures, and robust stability.

Hydrothermal is the predominant method for preparing in situ grown samples. This process utilizes water as the medium, where nucleation and conformal growth of TMS materials occur on the defects of substrates. The diagram of in situ grown TMS electrodes prepared via the hydrothermal method is shown in Fig. 2e. By adjusting preparation parameters such as temperature and pH value, various phases and morphologies of samples can be synthesized.⁶³ For instance, in 2019, Qu et al. synthesized a series of phosphorus-doped cobalt sulfides on carbon cloth by merely adjusting the hydrothermal temperature (160 °C, 180 °C, and 200 °C). The XRD results indicated that three different phases of cobalt sulfides were synthesized (P-CoS₂/Co_1−xS, P-Co_1−xS, and P-Co₉S₈).⁶⁴ Additionally, the high temperature and pressure during the preparation process provide substantial energy, facilitating the doping or insertion of other elements. Despite these advantages, the hydrothermal method sometimes requires combination with other techniques, such as thermal treatment, sulfurization, or an additional hydrothermal step, to achieve effective sample preparation.^65,66

Compared to the hydrothermal method, electrodeposition involves simpler processes and requires less preparation time and energy. This method is typically conducted in a three-electrode system, where the selected substrate serves as the working electrode, and the target materials are directly deposited onto its surface, accompanied by electron transfer (Fig. 2f). By adjusting the ratio of raw materials and deposition durations, electrodes with varying loading masses, morphologies and structures can be synthesized.⁶⁷ For example, Zeng et al. utilized a simple potentiostatic electrodeposition method to prepare a NiS_x catalyst on copper wire (NiS_x/CW), with the morphology varying according to the ratio of Ni to S.⁶⁸ Although doping is generally challenging to achieve via electrodeposition due to the lower energy input, ion intercalation can be readily facilitated by modifying the potential or current during the process. This technique is commonly employed to design novel electrodes with unique structures and outstanding electrochemical performance.

Hydrothermal and electrodeposition methods are the primary strategies for the preparation of in situ grown TMS electrodes and have been extensively discussed. However, other methods, such as thermal sulfidation, solvothermal, and template-assisted methods also play crucial roles in the preparation of in situ grown TMS catalysts. These alternative strategies offer unique advantages, including precise control over nanostructures and compositions, as well as the ability to produce catalysts with high surface areas, uniform pore distributions, and complex functional geometries. The advancement of these methodologies represents a significant evolution from traditional synthesis approaches, enabling the development of catalysts that are more active, selective, and durable.

3. Boosting the performance of electrocatalysis via in situ grown TMSs

In the preparation of in situ grown TMS catalysts, the active samples are directly grown on substrates, primarily serving as the key catalysts for overall water splitting.⁶⁹ As a result, the properties of these active materials have a significant impact on catalytic performance. Compared to TMOs, TMSs exhibit better electrical conductivity due to their narrow band gap. Moreover, the abundant active sites in TMSs can efficiently adsorb hydrogen and hydroxide ions from electrolytes, making them ideal materials for EWS.^70–72 For instance, in Zhang et al.’s research, a TMO-based material, MnCo₂O₄ was initially selected as a catalyst for the OER process. In LSV tests, the MnCo₂O₄/TM electrode required a high overpotential of 693 mV to reach a current density of 50 mA cm⁻², indicating substantial energy consumption. Subsequently, they used a sulfurization method to prepare a MnCo₂S₄/TM electrode, which showed a significant improvement, reducing the overpotential from 693 mV to 325 mV. Additionally, the Tafel slope improved from 228 mV dec⁻¹ to 115 mV dec⁻¹.⁷³ Similarly, in 2020, Joyner compared the HER performance of MoO₂/MoO₃ and MoS₂, both of which were prepared via ex situ growth strategies. The onset potential of MoS₂ for the HER was around −200 mV, representing a significant improvement compared to MoO₂/MoO₃ (−598 mV).⁷⁴ However, the −200 mV onset potential of MoS₂ still cannot meet the requirement for practical applications. In 2021, Dong and his team employed an in situ growth strategy to prepare a 1T MoS₂/rGO composite. This in situ grown composite demonstrated an exceptionally low Tafel slope of 56.7 mV dec⁻¹ and around 100 mV onset potential.⁷⁵

These studies highlight that TMS-based materials possess superior catalytic activities compared to TMO-based materials. Moreover, the adoption of in situ growth techniques can significantly enhance EWS performance. Therefore, the development of in situ grown TMS catalysts holds great promise for advancing electrochemical and catalytic technologies.

3.1 In situ grown molybdenum sulfides for enhancing the electrocatalytic HER

MoS₂ is generally considered to be more effective for the HER rather than for the OER. The electronic structure of MoS₂ is well-suited for facilitating the adsorption and subsequent reduction of protons, which is crucial for the HER. During the OER, MoS₂ can undergo oxidation, which compromises its structural integrity and reduces its catalytic performance over time. This makes it less stable under the harsh oxidative conditions of the OER compared to the HER, where such degradation is less of a concern. Despite this, researchers continue to investigate ways to improve its OER performance through doping, phase engineering, and other modifications. For instance, Zhao et al. composited MoS₂ with amorphous CoFeO_x(OH)_y and grew it in situ on carbon paper by the hydrothermal method for the preparation of high efficiency OER catalysts.⁷⁶ While these modifications can improve MoS₂ for OER applications, its inherent properties suggest that selecting alternative materials may currently be the most effective approach for OER catalysis.

3.1.1 In situ grown 2H-MoS₂ electrodes. MoS₂ can exist in three distinct phases, 2H, 1T and 3R, depending on its coordination structure. The 2H phase is the most stable structure for MoS₂; however, its poor conductivity and inert catalytic sites significantly hinder its application in EWS. To overcome these limitations, in situ growth of 2H-MoS₂ on conductive substrates has been explored as an effective strategy. In 2016, Tang et al. successfully synthesized a 2H-MoS₂/N-rGO nanocomposite through a two-step process. Initially, 2H-phase MoS₂ was prepared and then combined with N-rGO via a chemical deposition method. This approach ensured a uniform distribution of MoS₂ on the N-rGO substrate, effectively creating a robust, interconnected network that promotes efficient electron transport and catalytic activity. The resulting MoS₂/N-rGO-180 nanocomposite demonstrated excellent HER activity with an onset potential of just −5 mV and a small Tafel slope of 41.3 mV dec⁻¹. Additionally, cycle stability tests confirmed that the nanocomposite maintained its structural integrity and catalytic performance over 5000 CV cycles between −50 and −150 mV, indicating its suitability for long-term electrocatalytic applications. This research also intentionally increased the interlayer spacing within the MoS₂ structure (Fig. 3a), facilitating greater accessibility of catalytic sites and improving HER kinetics.⁷⁷ A similar enhancement was also observed by Huang et al. in 2020, who synthesized 2H-MoS₂ on graphite felt (GF) using a unique in situ growth approach. GF served a dual purpose as both the growth substrate for MoS₂ nanoflowers and a component of the composite catalyst. The as-prepared MoS₂/GF composite exhibited remarkable catalytic activity, maintaining stable HER performance even after 3000 cyclic voltammetry scans, indicating both high activity and durability (Fig. 3b). The promoted electron transfer at the MoS₂/GF electrodes, clearly displayed in Fig. 3d was a key factor contributing to this excellent catalytic performance.⁴⁵

Fig. 3 (a) The schematic of enlarged interlayer space of MoS₂; reproduced with permission from ref. 77. Copyright 2016 Wiley. (b) Polarization curves of MoS₂/GF activated at different voltages (−0.8 V, −1.2 V and −1.0 V), and polarization curves of MoS₂/GF before and after 3000th cycles; reproduced with permission from ref. 45. Copyright 2020 Elsevier. (c) Schematic diagram of the synthesis process of both the MoS₂/graphene superlattice and the control superlattice; reproduced with permission from ref. 78. Copyright 2018 ACS. (d) Schematic of the electron transfer route at the catalysts during the HER process; reproduced with permission from ref. 45. Copyright 2020 Elsevier. (e) Schematic illustration of the preparation of MoS_x/carbon cloth and the contact measurement result before and after H₂ plasma treatment; reproduced with permission from ref. 79. Copyright 2016 Wiley. (f) The schematic illustration of the mechanism of the HER process for the as-prepared 5.2% Rh–MoS₂ catalyst; reproduced with permission from ref. 80. Copyright 2017 Wiley. (g) The Gibbs free energy of hydrogen evolution for 2H-MoS₂, 1T-MoS₂ and V/1T-MoS₂; reproduced with permission from ref. 81. Copyright 2021 Elsevier.

As previously mentioned, MoS₂ has two types of catalytic sites, located on the basal plane and at the edge. The basal plane sites exhibit inert catalytic performance due to their low surface energy, while the edge sites are highly active. Therefore, strategies such as surface engineering or creating defects to expose more edge sites can further enhance the catalytic activity of in situ grown 2H-MoS₂. Lu et al. presented an innovative approach to enhance the catalytic efficiency of amorphous MoS_x on carbon cloth for the HER by creating high-density sulfur vacancies using remote H₂ plasma. This modification led to an increased density of active sites and improved surface hydrophilicity, significantly boosting catalytic activity and durability under high current densities. The preparation schematic and hydrophilicity test results are shown in Fig. 3e. The overpotential of a-MoS_x decreased by about 63 mV after plasma treatment, attributed to the increased active site density.⁷⁹

Other strategies have also been implemented to modify the electrochemical performance of in situ grown 2H-MoS₂ electrodes. For example, doping is widely used to improve the catalytic, electrical, and structural characteristics of materials, particularly in catalysts for energy conversion and storage. In 2017, Cheng et al. employed a solvothermal process to synthesize Rh-doped 2H MoS₂ catalysts to enhance HER performance. 2H-MoS₂ nanosheets, prepared via a lithium intercalation method, serve as the base onto which Rh nanoparticles are deposited using ethylene glycol at 90 °C. This synthesis approach resulted in a Rh–MoS₂ composite with exceptional HER performance, achieving a Tafel slope of only 24 mV dec⁻¹, attributed to the hydrogen spillover effect and the synergistic interaction between Rh and MoS₂; the mechanism underlying these effects is illustrated in Fig. 3f.⁸⁰ Another study by Qian et al. in 2021 demonstrated the enhancement of MoS₂'s electrochemical performance through zinc doping and the in situ growth of MoS₂ on reduced graphene oxides. The synthesized Zn-doped 2H-MoS₂ nanosheets on rGO (Zn–MoS₂-rGO) showcased superior HER performance, highlighted by a low Tafel slope of 69 mV dec⁻¹, with the improvement primarily attributed to enhanced conductivity.⁴⁴ In addition to surface engineering and elemental doping, intercalation can also efficiently improve the electrochemical performance by modifying the interlayer space. In 2022, our group introduced a facile electrodeposition method to produce in situ grown Ni-modified MoS₂. This innovative approach resulted in 2H-MoS₂ with increase interlayer spacing due to Ni intercalation, showcasing excellent electrocatalytic activity with an onset potential of 139 mV and a Tafel slope of 62 mV dec⁻¹, attributed to the synergistic effect of the Ni–Mo–S composition.⁸²

3.1.2 In situ grown 1T-MoS₂ electrodes. The 1T phase of MoS₂ offers several advantages over the 2H phase, making it particularly attractive for catalytic applications. Firstly, 1T-phase MoS₂ is metallic, providing significantly higher electrical conductivity than the semiconducting 2H phase. Secondly, the 1T phase features a distorted octahedral structure, which disrupts symmetry and creates more edge sites, thereby increasing the number of active sites. Thirdly, the 2D structure of 1T-MoS₂ typically results in a higher surface area compared to 2H-MoS₂, further enhancing the availability of active sites for chemical reactions and improving catalytic efficiency. Consequently, the selection of 1T-MoS₂ as the active material for the preparation of in situ grown electrodes has garnered significant attention in recent years.

In 2018, Xiong et al. introduced a single-layered MoS₂/graphene superlattice fabricated through a facile solution-phase restacking method. This novel preparation method, detailed in Fig. 3c, employed chemical exfoliation via Li intercalation to induce metallic properties in MoS₂ nanosheets, thereby improving charge transfer kinetics and catalytic activity. The resulting structure exhibited outstanding electrochemical properties as an HER catalyst, with an 88 mV onset potential and a Tafel slope of 48.7 mV dec⁻¹, highlighting the potential of two-dimensional nanosheets in advanced energy applications.⁷⁸ Further modifications have also been explored to improve the catalytic activity of 1T-MoS₂. In 2021, our group utilized vanadium (V) to fabricate in situ grown V-doped 1T MoS₂ catalysts on carbon paper via a straightforward one-step hydrothermal technique. The resulting V-doped 1T-MoS₂ exhibited exceptional HER performance, with a Tafel slope of just 54 mV dec⁻¹ and an onset potential of 102 mV. Additionally, during a 12-hour constant current test, the V-doped 1T-MoS₂ demonstrated remarkable stability with only a minimal increase in overpotential. The Gibbs free energy of hydrogen adsorption (ΔG_H), calculated using density functional theory (DFT) and shown in Fig. 3g, indicated that V-doped 1T-MoS₂ had the lowest ΔG_H (0.03 eV), underscoring its superior HER performance.⁸¹

However, the metallic 1T phase, despite its advantages of high conductivity and increased active sites, is thermodynamically less stable than the semiconducting 2H phase. This instability can result in a spontaneous phase transition, particularly under operational conditions, which diminishes the material's catalytic efficiency over time. Furthermore, the synthesis of 1T-MoS₂ typically requires complex procedures, such as chemical exfoliation or intercalation methods, making it challenging to scale up for industrial applications. Maintaining the 1T phase during the synthesis and operational stages is thus a significant obstacle. Researchers are actively exploring methods to stabilize the 1T phase, such as doping or using specific substrates, but achieving long-term stability remains a critical challenge that must be addressed before 1T-MoS₂ can be widely adopted in practical catalytic applications. The 3R phase of MoS₂ possesses a rhombohedral structure, similar to the 2H phase, but with a distinct stacking sequence. While it also exhibits semiconducting properties, the 3R phase is less commonly studied compared to the 2H phase. Its rhombohedral stacking and semiconducting nature result in lower catalytic activity compared to the metallic 1T phase, which provides higher conductivity and more abundant active sites. Consequently, the 3R phase has received less attention in research for applications that demand high conductivity and efficient catalysis.⁸³ The comprehensive comparison of the catalytic activities of the in situ grown molybdenum sulfide catalysts discussed in this study is shown in Table 2.

Table 2 Comparison of EWS performance of in situ grown molybdenum sulfide catalysts reported in the literature

Catalyst	Substrate	Preparation	Overpotential η (mV) at 10 mA cm^−2	Tafel slope (mV dec^−1)	Electrolyte	Ref.
			HER	HER
Defective-MoS2/rGO	rGO	Hydrothermal	154.77	56.17	0.5 M H2SO4	75
Annealed-MoS2/rGO	rGO	Hydrothermal	278.04	128.9	0.5 M H2SO4	75
Zn–MoS2-rGO	rGO	Hydrothermal	300 (169 mA cm^−2)	69	0.5 M H2SO4	44
MoS2/N-RGO-180	rGO	Hydrothermal	56	41.3	0.5 M H2SO4	77
a-MoSx/CC	Carbon cloth	Hydrogen plasma	143	39.5	0.5 M H2SO4	79
V-doped 1T MoS2/CP	Carbon paper	Hydrothermal	102 (onset potential)	54	0.5 M H2SO4	81
MoS2/graphene	Graphite	Chemical exfoliation	137	48.7	0.5 M H2SO4	78
MoS2/GF(−1.0 V)	Graphite felt	Hydrothermal	82	48	0.5 M H2SO4	45
Ni0.05Mo0.95S2/NF	Nickel foam	Electrodeposition	215	62	0.5 M H2SO4	82

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3.2 In situ grown cobalt sulfides for enhancing EWS

Cobalt sulfides are a class of inorganic compounds composed of cobalt and sulfur, with the general formula Co_xS_y, where x and y vary depending on the specific phase or stoichiometry of the compound. They exist in several phases, including cobalt monosulfide (CoS), cobalt disulfide (CoS₂), cobalt pentlandite (Co₉S₈), and cobalt trisulfide (Co₃S₄). Among them, CoS₂ exhibits the best electrical conductivity and abundant active sites, making it particularly valuable in electrocatalysis. Co₉S₈, with its cubic crystal structure, is more complex, featuring cobalt atoms occupying multiple distinct sites within the lattice. While it is a conductor, its conductivity is generally lower than that of CoS₂; however, its unique structure supports various electronic applications where moderate conductivity is required. CoS and Co₃S₄ are semiconductors, but their conductivity can be tailored through doping or the creation of nanostructures to enhance their performance in electronic and catalytic applications.

The in situ growth of cobalt sulfides on substrates results in a well-integrated interface that facilitates efficient electron transfer. This close interaction between the cobalt sulfide catalyst and the conductive substrate minimizes resistance and improves the overall catalytic performance, particularly in reactions such as the HER and OER. Therefore, the following section will first focus on in situ grown CoS₂, highlighting its exceptional conductivity and abundant active sites. Subsequently, other in situ grown cobalt sulfides, including CoS, Co₉S₈, and Co₃S₄, will be discussed in relation to their various valence states and catalytic properties.

3.2.1 In situ grown CoS₂ electrodes. The metallic nature of CoS₂ provides excellent conductivity, while its unique crystal structure offers numerous active sites for hydrogen adsorption and reduction. These features make in situ grown CoS₂ a promising catalyst for the HER, with the potential to replace more expensive noble metal catalysts in practical applications.

In 2013, Kong et al. discussed first-row transition metal dichalcogenides as novel, non-precious catalysts for the HER in acidic electrolytes. Their preparation method involved conformally coating carbon black nanoparticles with a cobalt layer as the precursor, followed by sulfurization in a tube furnace. By utilizing this facile process for growing polycrystalline dichalcogenide films, the study opened new avenues for developing efficient catalysts for energy technologies, particularly in applications requiring acid stability, such as proton exchange membrane electrolysis units. The electrochemical measurements revealed that Fe_0.43Co_0.57S₂ core–shell nanoparticles excelled in HER efficiency, ranking among the top non-precious material-based catalysts. These nanoparticles notably outperform their Fe_0.43Co_0.57S₂ film counterparts, achieving higher current densities at substantially lower overpotentials. Furthermore, the similarity in Tafel slopes between the nanoparticles and the films suggested similar surface chemical behaviours during the HER process, as detailed in Fig. 4a.⁸⁴ In 2014, Faber et al. synthesized CoS₂ on graphite disk substrates using three different methods, resulting in various morphologies: CoS₂ films, CoS₂ microwires (CoS₂ MWs) and CoS₂ nanowires (CoS₂ NWs), as shown in Fig. 4b. SEM images indicate that these three cobalt sulfides were intimately grown on the graphite substrate, enhancing their mechanical stability. Among them, CoS₂ NWs exhibited the most impressive HER performance, attributed to the facilitated release of gas bubbles generated at the electrode surface, which underscores the advantages of micro- and nano-structural optimization in catalyst design. CoS₂ NWs required only 145 mV overpotential to achieve a current density of 10 mA cm⁻², which is lower than that of CoS₂ films and CoS₂ MWs (190 mV and 158 mV), and the Tafel slope of CoS₂ NW was just 51.6 mV dec⁻¹ (Fig. 4c).⁸⁵

Fig. 4 (a) Polarization curves of the as-prepared catalysts grown on glassy carbon electrodes for the HER; reproduced with permission from ref. 84. Copyright 2013 RSC. (b) Preparation schematic of Co_xS_y/ACF by a simple hydrothermal method; reproduced with permission from ref. 85. Copyright 2014 ACS. (c) Polarization curves of the as-prepared catalysts for the HER process; reproduced with permission from ref. 85. Copyright 2014 ACS. (d) Preparation diagram of in situ grown CoS₂@Co nanoparticles on the CNT substrate; reproduced with permission from ref. 86. Copyright 2018 Elsevier. (e) Detailed preparation schematic for FeS₂-MoS₂@CoS₂-MOF; reproduced with permission from ref. 87. Copyright 2022 Elsevier.

In situ grown CoS₂ has also demonstrated significant potential as an OER catalyst, largely due to its ability to provide a high density of active sites. The in situ growth process ensures a robust connection between CoS₂ and the substrate, which not only facilitates efficient electron transfer but also significantly enhances the overall durability of the catalyst. As a result, in situ grown CoS₂ emerges as a promising candidate for overall water splitting, effectively catalysing both the HER and OER. Its bifunctional nature, coupled with cost advantages and the potential for further optimization, positions it as an attractive material for sustainable hydrogen production. In 2018, Wang et al. employed a novel preparation strategy that involved carbonization followed by vulcanization to create S and N co-doped carbon nanotube-encapsulated core–shelled CoS₂@Co nanoparticles. This structure, coupled with the Mott–Schottky effect between metallic cobalt and semiconductive cobalt disulfide, significantly boosted catalytic efficiency for both the HER and OER, offering a promising low-cost, high-activity, and stable alternative for water splitting applications (Fig. 4d).⁸⁶ In 2022, Chhetri et al. developed a novel catalyst for water splitting, featuring bimetallic sulfide-coupled mesoporous CoS₂ nanoarray hybrids derived from a 2D MOF. As shown in Fig. 4e, the electrocatalyst was synthesized through a method involving the growth of a 2D MOF on NF, followed by annealing with sulfur to form CoS₂ nanoarrays. These were then coupled with FeS₂@MoS₂ layers via a hydrothermal process, resulting in a structure with enhanced electrocatalytic activity due to its abundant heterointerfaces, mesoporosity, and multimetallic active centres. This innovative synthesis method yielded a material that demonstrates superior activity in both the HER (92 mV overpotential at 10 mA cm⁻² current density) and OER (211 mV overpotential at 20 mA cm⁻² current density).⁸⁷

3.2.2 Other in situ grown cobalt sulfide-based electrodes. In addition to the well-studied CoS₂, other in situ grown cobalt sulfides have also attracted considerable attention in the field of electrocatalysis. By leveraging the versatility of cobalt sulfides, researchers have developed a range of catalysts that offer promising alternatives to more expensive or less abundant materials.

In situ grown CoS catalysts are particularly valued for their structural integrity and resistance to degradation under operating conditions, making them highly effective for sustainable water-splitting applications. In 2018, Kale et al. prepared a binder-free cobalt sulfide thin film material on stainless steel via a chemical bath deposition (CBD) approach. The as-prepared CoS/SS displayed satisfactory conductivity due to its binder-free construction and thin-film structure. Compared to other cobalt sulfides, CoS/SS required lower kinetic energy in the OER process, with a Tafel slope of only 55 mV dec⁻¹ in an alkaline medium.⁸⁸ Subsequently, in 2020, Wang et al. employed a two-step electrodeposition method (Fig. 5a) to synthesize a novel CoS@NiFe/Ni foam heterostructure catalyst, creating an amorphous NiFe layer coated with a CoS film on commercial NF. This structure exhibited exceptional OER performance, requiring a low overpotential of 175 mV to drive a current density of 10 mA cm⁻² and demonstrating high stability in 1 M KOH alkaline solutions. The outstanding performance was attributed to the strong electronic interactions between the CoS film and the NiFe layer, which regulated redox states and promoted electrocatalytic activity (Fig. 5b).⁸⁹

Fig. 5 (a) Schematic illustration of the preparation of CoS@NiFe/NF by the electrodeposition method; reproduced with permission from ref. 89. Copyright 2020 ACES. (b) Polarization curves of the as-prepared catalysts for the OER; reproduced with permission from ref. 89. Copyright 2020 ACES. (c) Preparation diagram of edge-MoS₂/Co₃S₄@NFs; reproduced with permission from ref. 90. Copyright 2020 Elsevier. (d) Polarization curves of the as-prepared catalysts on carbon fibre for the OER; reproduced with permission from ref. 91. Copyright 2020 Wiley. (e) Intermediate adsorption configurations of *H and *OH on the NiFe LDH and NiCo₂S₄@NiFe LDH heterostructure surfaces, respectively; and the electron transfers at the interface between NiCo₂S₄ and NiFe LDH; regions of charge accumulation and depletion are depicted in yellow and light blue, respectively; reproduced with permission from ref. 92. Copyright 2017 ACS. (f) LSV curves of the as-prepared catalysts; reproduced with permission from ref. 93. Copyright 2019 Elsevier. (g) Preparation schematic of FeCo₂S₄/NF and their corresponding morphologies; reproduced with permission from ref. 94. Copyright 2018 ACS. (h) Preparation schematic of Zn doped and un-doped Co₉S₈ catalysts; reproduced with permission from ref. 95. Copyright 2020 Elsevier.

Co₃S₄, a cobalt sulfide compound with a spinel-type crystal structure, features a cubic lattice where cobalt atoms occupy both tetrahedral and octahedral sites, while sulfur atoms form a close-packed framework. This unique arrangement contributes to the material's distinct electrochemical properties, making it an attractive catalyst for the HER. In 2020, Peng et al. combined Co₃S₄ nanorods with MoS₂ nanoflakes via a multi-step preparation process involving the hydrothermal growth of Co₃S₄ nanorods on nickel foam, followed by decoration with MoS₂ nanoflakes (Fig. 5c). This arrangement maximized the exposure of catalytically active edges, significantly enhancing HER performance (η₁₀ = 90.3 mV and η₁₀₀₀ = 502.0 mV). The enhanced performance was attributed to the effective distribution of active sites and the synergistic effect of the Co₃S₄ and MoS₂ components. The hierarchical heterostructure, combined with NF, ensured optimal electron transfer throughout the catalyst, contributing to its enhanced electrocatalytic efficiency. The as-prepared edge-MoS₂/Co₃S₄@NFs maintained its performance over 20 hours at 100 mV current density without significant loss in activity, highlighting its potential for long-term applications.⁹⁰

Co₃S₄ features cobalt in mixed valence states (Co²⁺ and Co³⁺), which enhances redox activity and facilitates efficient electron transfer during catalytic reactions, particularly beneficial for the OER, where multiple electron transfers are required. Thangasamy et al. doped the Fe element into cobalt sulfides and grew the active samples on carbon fibre using a simple hydrothermal method. The standard reference pattern of Co₃S₄ was observed in PXRD results. The resulting Fe-Co_xS_y/ACF demonstrated a remarkably low overpotential (304 mV) at a constant current density (10 mA cm⁻²) and a small Tafel slope of 54.2 mV dec⁻¹ in an alkaline medium for the OER (Fig. 5d). The enhanced catalytic performance was primarily attributed to the 3D ferrocene-cobalt sulfide nanoarchitectures, which featured a high concentration of vertical nanosheets uniformly distributed with nanoparticles.⁹¹ Consequently, Co₃S₄ has gained attention as a highly effective bifunctional catalyst for both the HER and OER. In 2015, Danni Liu et al. successfully prepared NiCo₂S₄ nanowires on carbon cloth (NiCo₂S₄ NA/CC) via a straightforward hydrothermal process, followed by sulfidation treatment. In the as-prepared NiCo₂S₄ NA/CC, Co exhibited the coexistence of Co²⁺ and Co³⁺. The homogeneous nanostructure of NiCo₂S₄ NA/CC displayed excellent electrochemical activity for both the HER and OER, characterized by low overpotential (305 and 340 mV) at a current density of 100 mA cm⁻².⁹⁶ Similarly, in 2017 Jia Liu et al. employed a three-step hydrothermal method to synthesize a novel hierarchical NiCo₂S₄@NiFe LDH/NF catalyst, which exhibited outstanding HER and OER performance. The Co elements in NiCo₂S₄@NiFe LDH/NF also showed the coexistence of +2 and +3 values. The as-prepared catalysts displayed a low overpotential of 200 mV to drive a current density of 10 mA cm⁻² and exceptional stability for the HER, while the OER performance was also excellent, with only 201 mV overpotential at a current density of 60 mA cm⁻². DFT calculations demonstrated a strong synergistic effect between the NiCo₂S₄ and NiFe LDH components, which is beneficial for adjusting the interfacial electronic structure, as shown in Fig. 5e. Moreover, the overall water-splitting voltage of NiCo₂S₄@NiFe LDH/NF was only 1.6 V at 10 mA cm⁻², making it a promising material for efficient and cost-effective electrocatalysis in water-splitting applications.⁹² In 2018, Hu et al. adopted a two step strategy to synthesize FeCo₂S₄ on a nickel foam substrate (FeCo₂S₄/NF). According to XRD results, the main phase of the as-prepared catalysts was Co₃S₄. Initially, FeCo₂O₄/NF, as the precursor material, was prepared by a hydrothermal method. This precursor was then annealed at 160 °C for 8 hours to convert it into FeCo₂S₄/NF. The detailed preparation process is depicted in Fig. 5g. FeCo₂S₄/NF demonstrated outstanding HER and OER properties in a strong alkaline electrolyte.⁹⁴

Co₉S₈, another cobalt sulfide compound, features a unique crystal structure where cobalt atoms are arranged in both octahedral and tetrahedral sites within a sulfur-rich lattice. This configuration provides a diverse range of active sites, enhancing the material's catalytic performance. Karikalan and his group first prepared CoMoO₄/SDC precursors through a standard chemical reaction, and then CoMoS₄/SDC was synthesized via a hydrothermal strategy. According to the characterization results, Co₉S₈ was the main phase in CoMoS₄/SDC, and the formation of Co₉S₈ on the sulfur-doped carbon (SDC) surface, known for its advantageous role in the HER from prior studies, was observed. Compared with individual SDC or Co₉S₈, CoMoS₄/SDC exhibited better HER performance (Fig. 5f), which is attributed to the enhancement of ion exchange due to the interaction between Co²⁺ and sulfur ions within SDC. According to the characterization results, Co₉S₈ was the main phase in CoMoS₄/SDC, and the formation of Co₉S₈ on the sulfur-doped carbon (SDC) surface, known for its advantageous role in the HER from prior studies, was observed. Compared with individual SDC or Co₉S₈, CoMoS₄/SDC exhibited better HER performance (Fig. 5f), which is attributed to the enhancement of ion exchange due to the interaction between Co²⁺ and sulfur ions within SDC. During long-term cycling in 0.5 M H₂SO₄ at a high current density (220 mV cm⁻²), CoMoS₄/SDC exhibited notable stability compared to unsupported CoMoS₄, showing only a 12 mV deviation in LSV curves after 1000 cycles.⁹³ In 2020, Dong et al. employed a simple hydrothermal strategy to synthesize Zn-doped Co₉S₈ catalysts on copper foam (CF). As shown in Fig. 5h, the morphology of Co₉S₈@CF without Zn doping exhibited a spherical hierarchical structure; after doping with Zn, the morphology transformed into a neuron-like network. This Zn-doped Co₉S₈@CF-(1-1) catalyst demonstrates remarkable HER activity in both acidic and alkaline solutions, with low overpotentials (278 mV in 1 M KOH and 273 mV in 0.5 M H₂SO₄) to reach 10 mA cm⁻² and low Tafel slopes (114.4 mV dec⁻¹ in 1 M KOH and 85.2 mV dec⁻¹ in 0.5 M H₂SO₄), outperforming other samples.⁹⁵ The comprehensive comparison of the catalytic activities of the in situ grown cobalt sulfides catalysts discussed in this study is shown in Table 3.

Table 3 Comparison of EWS performance of in situ grown cobalt sulfide catalysts reported in the literature

Catalyst	Substrate	Preparation	Overpotential η (mV) at 10 mA cm^−2		Tafel slope (mV dec^−1)		Electrolyte	Ref.
			HER	OER	HER	OER
Fe0.43Co0.57S2	Metal film	Thermal sulfidation	∼190 (4 mA cm^−2)		55.9		0.5 M H2SO4	84
CoS/SS	Stainless steel	Chemical bath deposition		300		57	1 M KOH	88
FeCo2S4/NF	Nickel foam	Hydrothermal		270 (50 mA cm^−2)		59	1 M KOH	94
CoS@NiFe/NF	Nickel foam	Electrodeposition		175		73.1	1 M KOH	89
Edge-MoS2/Co3S4@NFs	Nickel foam	Hydrothermal	90.3		61.69		1 M KOH	90
NiCo2S4@NiFe LDH	Nickel foam	Hydrothermal	200	201 (60 mA cm^−2)	101.1	46.3	1 M KOH	92
FeS2-MoS2@CoS2-MOF	Nickel foam	Thermal sulfidation + hydrothermal	92	211 (20 mA cm^−2)	70.4	64.5	1 M KOH	87
FeCo2S4/NF	Nickel foam	Hydrothermal	132		164		1 M KOH	94
CoS2 NW	Graphite	Thermal sulfidation	145		51.6		0.5 M H2SO4	85
CoMoS4/SDC	Sulfur-doped carbon	Hydrothermal	180		48		0.5 M H2SO4	93
Zn-Co9S8@CF-(1-1)	Carbon fibre	Hydrothermal	273		85.2		0.5 M H2SO4	95
Zn-Co9S8@CF-(1-1)	Carbon fibre	Hydrothermal	278		114.4		1 M KOH	95
P–Co1−xS	Carbon cloth	Hydrothermal	110		—		0.5 M H2SO4	64
Fc-CoxSy/CNT	CNT	Solvothermal		304		54.2	1 M KOH	91
S, N-CNTs/CoS2@Co	N-doped CNT	Thermal treatment	112	157	104.9	76.1	1 M KOH	86

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3.3 In situ grown nickel sulfides for enhancing EWS

Nickel sulfides have gained significant attention as a versatile class of materials for electrocatalysis. Among these, NiS is notable for its hexagonal structure and good catalytic efficiency, attributed to its ability to facilitate electron transfer and provide active sites for hydrogen adsorption. NiS₂, with its distinctive pyrite-like cubic structure, provides a high density of catalytic sites and excellent electrical conductivity, making it particularly effective for the HER. On the other hand, Ni₃S₂ stands out due to its mixed valence states (+2 and +3) and rhombohedral structure, which contribute to its superior catalytic activity in both the HER and OER by enhancing redox flexibility and stability.

3.3.1 In situ grown NiS and NiS₂ electrodes. NiS is recognized for its moderate electrocatalytic activity in the HER. However, according to Jiang's report, NiS exhibited the poorest electrocatalytic activity among nickel sulfides, with an overpotential of 474 mV to reach 10 mA cm⁻² current density and a Tafel slope of 124 mV dec⁻¹ in an alkaline medium during the HER process.⁹⁷ In 2015, Yang et al. improved the catalytic activity of NiS by doping it with tungsten. They synthesized amorphous tungsten-doped nickel sulfide (NiWS) alongside tungsten doped cobalt sulfide (CoWS) for comparison. Unlike crystalline materials, amorphous structures provide more active sites due to their disordered atomic arrangement, potentially leading to higher catalytic activity. The as-prepared NiWS on the FTO substrate demonstrated exceptional HER performance, attributed to the increased number of active sites and improved charge transfer efficiency facilitated by the amorphous structure and composite nature of the material.⁹⁸ Further enhancement of NiS performance has been achieved through doping strategies. Zhang et al. doped NiS with iron and cobalt, synthesizing a sheet-like FeCoNiS_x/NF configuration on nickel foam. This composite structure featured a microporous framework conducive to facilitating oxygen bubble release, which is a critical factor for the continuous OER process. The low crystallization of FeCoNiS_x/NF contributed to a high abundance of unsaturated edge sites, enhancing its OER efficiency. Electrochemical assessments confirmed its low Tafel slope of 55 mV dec⁻¹ and a modest overpotential of 231 mV at a current density of 10 mA cm⁻², underscoring the composite's efficacy as an OER catalyst.⁹⁹ Recent advancements have also focused on structure management. Chen et al. reported a one-step solvothermal strategy to prepare a nanoworm-like nickel sulfide structure on NF, which is shown in Fig. 6f (NiS-NW/NF). The as-prepared NiS-NW/NF exhibited significant HER performance with an overpotential of 224 mV at a current density of 20 mA cm⁻² and a Tafel slope of 116.24 mV dec⁻¹. Additionally, NiS-NW/NF showed outstanding OER performance, achieving an overpotential of 279 mV at a high current density of 100 mA cm⁻², outperforming many recent OER catalysts.¹⁰⁵ The good durability of NiS further enhances its potential for catalyzing water splitting. In 2018, Rahman et al. grew NiS on FTO to enhance its electrochemical performance in the HER process. The morphology of the NiS-coated FTO could be controlled by adjusting the annealing temperature. Compared to pristine NiS, NiS-coated FTO demonstrated improved performance, with a lower overpotential of 364 mV at 10 mA cm⁻² and a smaller Tafel slope of 90.8 mV dec⁻¹.¹⁰⁷

Fig. 6 (a) Synthesis diagram of V-incorporated Ni_xS_y nanowires; reproduced with permission from ref. 100. Copyright 2017 RSC. (b) HER performance of the as-prepared Co-MOF/CP, Ni₃S₂/CP, Ni₃S₂@2D Co-MOF/CP, and 20% Pt/C; reproduced with permission from ref. 101. Copyright 2022 Elsevier. (c) LSV curves of the as-prepared Fe-Ni₃S₂/FeNi, FeS/Fe, Ni₃S₂/Ni and IrO₂; reproduced with permission from ref. 102. Copyright 2017 Wiley. (d) Preparation schematic of Ni₃S₂/NF; reproduced with permission from ref. 103. Copyright 2018 RSC. (e) Polarization curves of the as-prepared catalysts, nickel foam and 20 wt% Pt/C/NF for the HER and OER, reproduced with permission from ref. 104. Copyright 2018 ACS. (f) Different morphologies of NiS nanoplates and nanoworms; reproduced with permission from ref. 105. Copyright 2020 Elsevier. (g) LSV results and morphologies of NiS₂ catalysts before and after HER activation; reproduced with permission from ref. 106. Copyright 2017 Elsevier.

NiS₂, with its pyrite-like cubic structure, outperforms NiS due to its superior catalytic properties, including better electrical conductivity and a greater number of active sites for both the HER and OER. In 2017, Ma et al. employed a combination of hydrothermal synthesis and thermal treatment to prepare nanosheet nickel sulfide on nickel foam (NF-NiS₂). The as-prepared NF-NiS₂ was further activated after a 5-hour HER process, resulting in an activated sample denoted as NF-NiS₂-A. Comparative analysis between pristine NiS₂ and NiS₂-NF nanosheets revealed enhanced electrochemical activity in the latter, with a lower overpotential of 122 mV at 10 mA cm⁻² and a smaller Tafel slope of 86 mV dec⁻¹. Notably, the HER activity of NiS₂-NF-A demonstrated a significant increase after the chronopotentiometry test at 20 mA cm⁻². The overpotential required to reach 10 mA cm⁻² was reduced to 55 mV, and a mere 161 mV overpotential was necessary to reach 100 mA cm⁻², closely rivalling the efficiency of the Pt/C catalyst (∼145 mV). Fig. 6g illustrates the morphological changes before and after the HER process, which are attributed to the enhanced catalytic activity.¹⁰⁶

3.3.2 In situ grown Ni₃S₂ electrodes. Ni₃S₂ typically exhibits the highest catalytic performance among nickel sulfides, owing to its mixed valence states of Ni²⁺ and Ni³⁺ combined with a rhombohedral structure that offers a high density of active sites and enhanced redox capabilities. Cathodic activation is an effective method for enhancing the catalytic performance of the HER by modifying the surface properties of catalysts, increasing the availability of active sites and optimizing the electronic structure for hydrogen adsorption. This activation often involves electrochemical treatment that induces structural changes or creates favorable active sites for hydrogen evolution. Shang et al. extensively discussed the in situ cathodic activation (ISCA) of V-incorporated Ni_xS_y nanowires for enhanced HER. The ISCA process enhances hydrogen evolution activity by promoting the formation of an amorphous VOOH film on the V-incorporated Ni_xS_y nanowires (Fig. 6a), which accelerates the HER process. Additionally, the NiOOH formed during the ISCA process helps protect the active sites, leading to superior activity and structural durability. The activated A-VS/Ni_xS_y/NF displayed outstanding HER performance with only 125 mV overpotential required to reach 10 mA cm⁻² and a Tafel slope of 113 mV dec⁻¹.¹⁰⁰

Depositing Ni₃S₂ on substrates with excellent conductivity and surface performance is also a good way to enhance its HER performance. In 2022, Cheng et al. employed a solution crystallization method to grow a 2D Co-MOF on treated carbon paper (2D Co-MOF/CP), followed by an in situ electrodeposition strategy to deposit Ni₃S₂ on the surface (Ni₃S₂@2D Co-MOF/CP). As shown in Fig. 6b, the as-prepared hierarchical hetero-Ni₃S₂@2D Co-MOF/CP nanosheets exhibited excellent HER activity with a small overpotential of only 140 mV to reach a current density of 10 mA cm⁻² and a Tafel slope of just 90.3 mV dec⁻¹. The 2D structured Co-MOF provided a tunnel for ion transport and sufficient active sites for Ni₃S₂. This synergy significantly enhanced the HER activity of the as-prepared catalysts, even surpassing that of Pt/C catalysts when the current density exceeded 130 mA cm⁻².¹⁰¹

During the OER process, the presence of Ni³⁺ enhances the material's ability to facilitate the formation and release of oxygen molecules. This redox flexibility is a key factor in lowering the overpotential and increasing the overall efficiency of the OER. In 2017, Yuan et al. explored a simple solvothermal method to grow Ni₃S₂ nanosheets on FeNi alloy foils. This method simplifies the production of high-efficiency OER electrodes by using FeNi alloy foil as both the Fe and Ni source and the current collector, leading to an in situ grown electrode with a low overpotential (282 mV at a current density of 10 mA cm⁻²) and high long-term stability in alkaline solutions (Fig. 6c).¹⁰² Li et al. utilized metal–organic frameworks (MOFs) as self-sacrificial templates to synthesize oxygen-incorporated Ni³⁺ self-doped Ni₃S₄ nanosheets on NF substrates (Ni₃S₄/NF) in 2020, achieving outstanding catalytic activity. This method resulted in the production of vertically aligned nanosheets with numerous open pores, which are anticipated to enhance the density of active sites for the EWS process. As expected, Ni₃S₄/NF demonstrated an OER overpotential of 266 mV at 10 mA cm⁻² and a Tafel slope of 77 mV dec⁻¹, significantly showing improvement over pristine Ni₃S₄/NF. The introduction of oxygen into the material matrix notably improved catalytic activity, particularly in the OER, while maintaining stability at higher current densities over extended durations.¹⁰⁸

The unique properties of Ni₃S₂, particularly when synthesized in situ on conductive substrates, contribute to its dual-functionality and high efficiency in both the HER and OER. Ren et al. leveraged the structural benefits of nickel foam as both the substrate and an active component in the catalytic process, facilitating enhanced electrochemical performance through a synergistic effect. Ni₃S₂/Ni foam is produced through a rapid sulfurization process, where nickel foam is treated to form a nickel sulfide film directly on its surface (Fig. 6d). This method is noteworthy for its simplicity and efficiency, allowing for the creation of a uniform and functional catalytic surface without the need for complex equipment or procedures. The as-prepared Ni₃S₂/Ni foam exhibited excellent catalytic activity towards water splitting, capable of driving both the HER and OER with smaller overpotentials and larger current densities compared to many conventional catalysts.¹⁰³ In 2023, Zhu et al. combined electrodeposition and hydrothermal methods to synthesize MoO₃–NiS_x films with enhanced HER and OER performance by adjusting the deposition duration. The as-prepared samples exhibited bifunctional catalytic performance, with an overpotential of 142 mV for the HER (at 10 mA cm⁻²) and 294 mV for the OER (at 50 mA cm⁻²).¹⁰⁹ Introducing other elements into catalysts can significantly enhance electrochemical performance and stability by optimizing adsorption energies and modifying the electronic structure.¹¹⁰ In 2018, Jian et al. employed a two-step preparation strategy to synthesize ultrathin Sn-doped Ni₃S₂ nanosheets on nickel foam. The synthesis method involves a hydrothermal process followed by vulcanization, leading to a catalyst that required notably small overpotentials for both the HER (0.17 V) and OER (0.27 V) to reach 100 mA cm⁻² current density. The outstanding catalytic performance is shown in Fig. 6e. The enhanced catalytic activity is attributed to Sn doping, which increases the intrinsic electronic performance of the catalyst. Furthermore, only 1.46 V was required for overall water splitting to achieve a current density of 10 mA cm⁻².¹⁰⁴ Liu et al. also introduced phosphorus into the iron–nickel sulfide matrix to enhance the electrochemical performance and stability of the catalysts in 2019. The formation of P-(Ni,Fe)₃S₂ nanosheet arrays directly on an NF substrate provided a large surface area and ensured efficient charge transport. The outstanding catalytic performance of P-(Ni,Fe)₃S₂/NF for both the HER and OER makes it a versatile electrode for overall water splitting, requiring only 1.54, 1.58, and 1.72 V cell potential to reach 10, 20 and 100 mA cm⁻², respectively.¹¹¹ The comprehensive comparison of the catalytic activities of the in situ grown nickel sulfide catalysts discussed in this study is shown in Table 4.

Table 4 Comparison of EWS performance of in situ grown nickel sulfide catalysts reported in the literature

Catalyst	Substrate	Preparation	Overpotential η (mV) at 10 mA cm^−2		Tafel slope (mV dec^−1)		Electrolyte	Ref.
			HER	OER	HER	OER
FeCoNiSx/NF	Nickel foam	Room-temperature sulfuration		231		55	1 M KOH	99
Ni3S4/NF	Nickel foam	Hydrothermal		266		77	1 M KOH	108
Cu/Ni3S2/NF	Nickel foam	Hydrothermal	130		84.19		1 M KOH	24
NF-NiS2-A	Nickel foam	Hydrothermal/thermal sulfurization	67		63		1 M KOH	106
A-VS/NixSy/NF	Nickel foam	Hydrothermal/electrochemical activation	125		113		1 M KOH	100
NiS-NW/NF	Nickel foam	Solvothermal	224 (20 mA cm^−2)	279 (100 mA cm^−2)	116.2	38.44	1 M KOH	105
Ni3S2/Ni foam	Nickel foam	Mercaptoethanol solution/annealing	131	312	96	111	1 M KOH	103
Sn-Ni3S2/NF	Nickel foam	Hydrothermal	170	270	55.6	52.7	1 M KOH	104
Fe-Ni3S2/FeNi	FeNi alloy foils	Solvothermal		282		54	1 M KOH	102
Amorphous NiWS	FTO	Thermolytic process	250 (8.6 mA cm^−2)		55		0.5 M H2SO4	98
NiS/FTO	FTO	Chemical bath deposition	290		143.4		1 M NaOH	107
MoO3-NiSx (9H5E)/CF	Co foam	Electrodeposition/hydrothermal	294 (50 mA cm^−2)		114		1 M KOH	109
MoO3-NiSx (9H5E)/CC	Carbon cloth	Electrodeposition/hydrothermal	142		79		1 M KOH	109

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3.4 In situ grown TMS composites for enhancing EWS

Having explored the individual contributions of molybdenum, cobalt, and nickel sulfides as active materials in enhancing the catalytic performance of in situ grown electrodes, the focus now shifts to composite TMSs. These composite materials, formed by combining different TMSs, offer a synergistic enhancement of catalytic properties, surpassing the performance of their single-component counterparts. In this section, we will explore various composite TMSs that have been developed to optimize the efficiency and stability of electrocatalytic processes, particularly for the HER and OER. By examining the synthesis methods, structural features, and catalytic mechanisms of these composites, we aim to highlight their potential in pushing the boundaries of water-splitting technologies and other related electrochemical applications.

In 2016, An et al. utilized Ti foil as a substrate to synthesize NiS₂–MoS₂ catalysts with a hetero-nanowire structure optimized for HER performance. The schematic illustration for preparing NiS₂–MoS₂ is shown in Fig. 7a. This hybrid structure was created by the thermal conversion of nickel molybdate (NiMoO₄) precursors, resulting in a composite with abundant active edge sites and defects. The as-prepared NiS₂–MoS₂ demonstrates excellent HER activity in basic solutions compared to individual MoS₂ or NiS₂ nanostructures.¹¹² In 2020, Xue et al. introduced a MoS₂/Ni₃S₂ coaxial heterostructure on nickel foam synthesized via a one-step hydrothermal method followed by annealing. The structure schematic of the as-prepared MoS₂/Ni₃S₂ NW-NF is shown in Fig. 7b. This catalyst exhibits remarkable HER performance at high current density, requiring overpotentials of 182 and 200 mV to reach current densities of 500 and 1000 mA cm⁻², respectively.¹¹³ The OER performance of these materials has also been investigated. Dong and colleagues synthesized a NiFeS/NF catalyst through a two-step method. Initially, nanosheets of NiFe hydroxide were electrodeposited onto NF, creating a precursor with an extensive surface area. This was followed by a hydrothermal sulfurization process, which led to the formation of NiFeS/NF, an effective electrocatalyst for the OER. This synthesis approach enhanced the material's catalytic properties by incorporating sulfur into the NiFe structure, thereby improving its activity and stability during the oxygen evolution reaction. XRD results revealed mixed phases of NiS and Ni₃S₂. As shown in Fig. 7c, the outstanding OER performance was characterized by a small overpotential of 65 mV at a current density of 10 mA cm⁻², coupled with a large electrochemical active surface area of 251.25 cm⁻².¹¹⁴

Fig. 7 (a) Schematic illustration for preparing NiS₂-MoS₂ using sulfurization and CVD strategies; reproduced with permission from ref. 112. Copyright 2016 RSC. (b) Structure schematic of the as-prepared MoS₂/Ni₃S₂ NW-NF; reproduced with permission from ref. 113. Copyright Elsevier 2020. (c) LSV curves and double-layer capacitance of the as-prepared catalysts for the HER; reproduced with permission from ref. 114. Copyright 2016 RSC. (d) Schematic illustration of the process of overall water splitting with FeCoNiP₀S₁ and FeCoNiP_0.5S_0.5 as the electrodes along with LSV curves of the as-prepared catalysts for the HER, reproduced with permission from ref. 115. Copyright 2018 ACS. (e) Diagram and polarization curves of NiFeS/NF-P for the OER; reproduced with permission from ref. 116. Copyright 2018 Elsevier. (f) LSV curves of the as-prepared catalysts for the HER; reproduced with permission from ref. 117. Copyright 2020 Elsevier. (g) Schematic illustration for the preparation of CoMoS_x on nickel foam with its unique structure; reproduced with permission from ref. 118. Copyright 2021 Elsevier.

The ultimate goal in electrocatalysis is to develop materials that can efficiently catalyse both the HER and OER within the same system. Wang et al. synthesized amorphous multi-element electrocatalysts on a Ti sheet in 2018, integrating Fe, Co, Ni, S, P, and O through a singular electrodeposition strategy for water splitting. The as-prepared catalysts, named FeCoNiP_xS_y, were designed to modulate the non-metal composition, thereby altering their electrochemical performance for both the HER and OER. The schematic diagram is shown in Fig. 7d. The catalytic activity results indicate that S and P play crucial roles in enhancing the catalysts' intrinsic and apparent activities, highlighting a pathway toward optimizing multi-element catalysts for efficient electrocatalysis.¹¹⁵ In the same year, Yan et al. employed an electrodeposition strategy to deposit ternary NiFeMoS anemone-like nanorods on nickel foam. They investigated the influence of pluronic P123 and MoO₄²⁻ on the nanostructure and electrocatalytic activity of NiFeMoS. The anemone-like nanorods were formed due to the presence of P123 and MoO₄²⁻, which resulted in a scaly surface on NiFeMoS. The NiFeMoS/NF-P catalyst exhibits outstanding electrocatalytic performance, achieving a significant current density of 150 mA cm⁻² at a relatively low overpotential for the OER (280 mV) in alkaline media. Moreover, the HER performance of this as-prepared catalyst was also excellent, with a 100 mV overpotential at a current density of 10 mA cm⁻². Consequently, the full cell assembled with the as-prepared NiFeS/NF-P catalyst displayed a very low water electrolysis voltage of 1.52 V to reach 100 mA cm⁻² (Fig. 7e).¹¹⁶ In 2019, Shit et al. successfully synthesized a unique CoS_x/Ni₃S₂ heterostructure, which was skillfully integrated with nickel foam (CoS_x/Ni₃S₂@NF) using a one-pot hydrothermal method. Unlike many catalysts that require complex preparation or additional support materials, in situ grown CoS_x/Ni₃S₂@NF ensured strong adhesion and excellent electrical connectivity. The as-prepared CoS_x/Ni₃S₂@NF required remarkably low overpotentials (204 and 280 mV) to achieve significant current densities (10 and 20 mA cm⁻²) for the HER and OER.¹¹⁹ In 2020, Huang et al. reported a two-step synthesis method involving the hydrothermal growth of NiMo precursor cuboids on nickel foam, followed by nitrogen doping via vulcanization. The as-prepared N-NiMoS/NF exhibited a low overpotential of 68 mV at 10 mA cm⁻² current density and an 86 mV dec⁻¹ Tafel slope for the HER, demonstrating performance comparable to that of the most advanced Pt/C catalysts. Furthermore, the as-prepared catalyst maintained outstanding HER performance at high current density (Fig. 7f).¹¹⁷ Zhao et al. presented an innovative approach for fabricating a hierarchical electrocatalyst composed of Ni₃S₂-CoMoS_x on NF in 2020. As shown in Fig. 7g, by employing nickel foam as a conductive substrate, the study leveraged its 3D porous structure to facilitate electron transport and provide a large surface area for the uniform growth of the catalytic materials. The electrocatalyst demonstrated exceptional bifunctional activity for overall water splitting.¹¹⁸

In the previous section, we thoroughly explored the roles of molybdenum, cobalt, and nickel sulfides, as well as their composites, as active materials in enhancing the catalytic performance of in situ grown electrodes. These materials have been shown to significantly boost the efficiency and stability of electrochemical reactions, particularly in the context of the HER and OER. By combining different TMSs, we have seen how their composite form can achieve even greater catalytic performance, leveraging the synergistic effects between various metal elements. The comprehensive comparison of the catalytic activities of the in situ grown composite TMS catalysts discussed in this study is shown in Table 5.

Table 5 Comparison of EWS performance of in situ grown composite TMS catalysts reported in the literature

Catalyst	Substrate	Preparation	Overpotential η (mV) at 10 mA cm^−2		Tafel slope (mV dec^−1)		Electrolyte	Ref.
			HER	OER	HER	OER
NiS2-MoS2/Ti foil	Ti foil	Solvothermal + chemical vapor deposition	200 (onset potential)		70		1 M KOH	112
FeCoNiP0S1	Ti foil	Electrodeposition	135 (100 mA cm^−2)		99		1 M KOH	115
FeCoNiP0.5S0.5	Ti foil	Electrodeposition		258 (100 mA cm^−2)		49	1 M KOH	115
FS-0.9-30	Nickel foam	Electrodeposition	157		112.5		1 M KOH	120
N-NiMoS/NF	Nickel foam	Hydrothermal	68		86		1 M KOH	117
Ni3S2-CoMoSx/NF	Nickel foam	Sulfidation	234	90	125	75	1 M KOH	118
P-(Ni,Fe)3S2/NF	Nickel foam	Phosphorization + sulfuration	98	196	88	30	1 M KOH	111
NiFeMoS/NF-P	Nickel foam	Electrodeposited + hydrothermal sulfuration	100	280 (150 mA cm^−2)	121	69	1 M KOH	116
Co9S8-Ni3S2 HNTs/Ni	Nickel foam	Hydrothermal	85	281 (50 mA cm^−2)	83.1	53.3	1 M KOH	121
NF-Ni3S2/MnO2	Nickel foam	Hydrothermal	102	260	69	61	1 M KOH	122
(Ni-Fe)Sx/NiFe(OH)y/NF	Nickel foam	Electrodeposition	124 (100 mA cm^−2)	290 (100 mA cm^−2)	97	58	1 M KOH	123
Ni–S–B/NM	Nickel mesh	Electrodeposition	240		121.2		30 wt% KOH	124
Co:WS2/Co:W18O4/FTO	FTO	Thermal sulfidation	210		49		0.5 M H2SO4	125
1T/2H-MoS2@CC	Carbon cloth	Hydrothermal	114		52.8		0.5 M H2SO4	126
NS-500	N/S co-doped graphene	Chemical vapor deposition	130 (onset potential)		80.5		0.5 M H2SO4	127
MoSx/CNT-rGO/VTP	CNT-rGO	Hydrothermal	190 (onset potential)		59		0.5 M H2SO4	128
Co(II)1−xCo(0)x/3Mn(III)2x/3S	B/N-codoped mesoporous nanocarbon	In situ pyrolysis + hydrothermal		260 (20 mA cm^−2)		50	1 M KOH	129

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The exploration of TMSs, particularly in the context of in situ grown catalysts, has demonstrated significant potential in advancing the field of electrocatalysis. Through the comprehensive analysis of molybdenum, cobalt, and nickel sulfides, this study highlights the substantial improvements in catalytic performance, particularly for the HER and OER, when these materials are synthesized directly on conductive substrates. The superior electrical conductivity, increased density of active sites, and enhanced redox flexibility offered by these TMSs underscore their potential as efficient, cost-effective alternatives to traditional noble metal catalysts.

4. Conclusion and prospects

In conclusion, the pursuit of sustainable and efficient energy solutions has underscored the pivotal importance of electrocatalytic water splitting (EWS) as a transformative technology for hydrogen production, a clean and renewable energy source. The in situ grown TMSs on conductive substrates have been identified as a compelling alternative to noble metal-based catalysts due to their cost-effectiveness, structural diversity, and abundant active sites, which not only enhance electron transfer and catalytic performance but also eliminate the need for polymer binders, further improving electrode stability and reducing preparation costs. In this work, we first reviewed the three basic elements of EWS using in situ grown TMS-based catalysts, i.e., the TMS materials, suitable substrates for the in situ growth process, and the in situ growth methods.

Because the TMS materials are directly grown on the substrate, the performance characteristics of the substrate material crucially affect the performance of the synthesized catalysts. Therefore, the selection of substrate materials for in situ grown catalysts is crucial for achieving high catalytic efficiency in applications. Herein, carbon-based and nickel foam substrates are discussed in detail. Carbon-based materials, including carbon nanotubes, graphene, and carbon cloth, are characterized by their exceptional electrical conductivity, extensive surface area, and strong mechanical stability. These properties make them ideal substrates for supporting active catalytic materials. Nickel foam is a porous metallic substrate known for its high electrical conductivity, mechanical strength, and three-dimensional (3D) structure. The unique 3D porous network of nickel foam offers a large active surface area and facilitates mass transport, which is essential for efficient catalysis.

Furthermore, the most commonly adopted in situ growth techniques, hydrothermal synthesis and electrodeposition, are comprehensively investigated, offering the following advantages: (1) Direct growth on conductive substrates: they enable the in situ growth of active materials on substrates, facilitating the fabrication of cohesive, binder-free electrodes. This direct integration enhances electron transport and reduces interfacial resistance, beneficial for electrochemical applications. (2) Controlled morphology and structure: both these techniques allow for meticulous control over the morphology and structure of the resulting materials. In hydrothermal synthesis, parameters such as solvent composition, temperature, and pressure can be adjusted to tailor the size, shape, and arrangement of nanostructures. (3) Versatility in material types and compositions: both methods support the synthesis of a wide range of materials, including sulfides, metals, metal oxides, and hybrid composites. This versatility allows for the exploration of novel material systems and compositions tailored for specific applications. (4) Energy and cost efficiency: compared to other synthetic techniques, hydrothermal and electrodeposition processes are relatively energy-efficient and cost-effective, making them suitable for large-scale production.

While in situ grown TMSs have demonstrated promising catalytic performance as electrocatalysts for water splitting, several challenges and developmental shortcomings remain to be addressed:

(1) The durability and long-term operational stability of in situ grown TMS catalysts, which are crucial for their practical application in electrocatalytic processes, can be significantly affected by various factors. One such factor is the dissolution of catalytic materials, which leads to a gradual loss of active sites and a consequent decrease in catalytic activity. Additionally, structural degradation characterized by morphological changes, surface corrosion, or phase transformation under harsh operational conditions, can adversely affect the physical integrity and electrochemical performance of the catalysts. Together, these factors pose significant challenges to the efficiency and longevity of in situ grown TMS catalysts in efficient water splitting.

(2) Maximizing the exposure of catalytically active sites and increasing the surface area of in situ grown TMS catalysts are crucial for improving their performance. Advanced synthesis techniques that can control the nanostructure and morphology of TMS catalysts on the surfaces of substrates are needed. Such techniques should enable the precise manipulation of catalyst features including particle size, shape, and distribution, as well as the engineering of porosity at the nanoscale, directly on the surface of conductive substrates. By fine-tuning these parameters, it is possible to significantly enhance the active surface area, thereby facilitating a higher rate of catalytic reactions. Moreover, the introduction of structural features such as hierarchical porosities and interconnected networks can further improve mass transfer and gas diffusion efficiency, optimizing the overall catalytic performance.

(3) The interface between the TMS catalyst and the substrate plays a critical role in the overall catalytic performance. Optimizing this interface through better material integration and interfacial engineering can enhance the efficiency and durability of the catalysts. Better integration of TMS catalysts onto substrates involves creating a seamless connection that facilitates efficient electron transfer and minimizes interfacial resistance. Interfacial engineering, on the other hand, focuses on modifying the surface properties at the interface to improve catalytic performance. This can include the introduction of functional groups, dopants, or co-catalysts that can modulate the electronic structure of the interface, thereby optimizing the adsorption and desorption energies of reactants and intermediates.

(4) Some other TMSs with potential catalytic capabilities have not been covered in detail due to their relatively unexplored nature. Although these materials may not yet have the extensive research backing of the more established sulfides, they represent promising avenues for future exploration and development in the field of electrocatalysis. For example, Shi et al. adopted a multi-step strategy involving flame vapor deposition, sol-flame doping, and sulfurization to prepare Co-doped WS₂ with excellent HER activity.¹²⁵ Continued research into these lesser-known TMSs could uncover new insights and further expand the toolkit available for designing high-performance catalytic materials.

(5) Electrodeposition, through the manipulation of deposition parameters like voltage, current density, and electrolyte composition, offers precise control over the thickness, composition, and microstructure of the deposited layers. However, this technique also presents challenges, such as ensuring uniform coverage over complex geometries. Additionally, the choice of electrolyte and deposition conditions can significantly influence the crystallinity and phase purity of the resulting material, which in turn affects its electrocatalytic performance.

Addressing the challenges associated with in situ grown TMS catalysts will require a multi-faceted approach that includes improving structural stability, optimizing synthesis techniques, enhancing interfacial properties, and expanding the exploration of lesser-known materials. In the future, doping and heterostructure formation will play pivotal roles in overcoming conductivity limitations and improving the kinetics of catalytic reactions. These strategies can significantly enhance the efficiency, durability, and practical applications of TMS materials in electrocatalysis.

In conclusion, in situ grown TMS catalysts hold great promise for advancing electrochemical energy conversion technologies. In the future, more efforts should be focused on the development of novel and scalable synthesis techniques that allow for precise control over the morphology, crystallinity, and composition of TMS catalysts. Meanwhile, the creation of hybrid materials by combining TMSs with conductive carbon materials, polymers, or other metal oxides offers great potential for improving both conductivity and stability. Continued exploration of lesser-known TMS materials with novel compositions, such as mixed-metal sulfides and high-entropy TMSs, may lead to the discovery of new active materials with superior catalytic properties. We hope this review can provide a comprehensive overview of the latest progress and remaining challenges, serving as a critical resource for researchers and engineers seeking to optimize these materials further.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Author contributions

H. W. Jia, P. Y. Guan, L. Zhou, Y. Z. Zhou and D. W. Chu devised and outlined the draft of the review paper. H. W. Jia contributed to the scientific writing of the manuscript. H. W. Jia, L. H. Meng, Z. K. Dong, C. Liu, and P. Y. Guan constructed the figures for illustrations. H. W. Jia, L. H. Meng, Y. L. Lu, T. Y. Liang, Y. Yu and Y. F. Hu constructed the tables for illustrations. H. W. Jia, Y. Z. Zhou, P. Y. Guan, M. Y. Li, T. Wan, B. J. Ni, Z. J. Han, and D. W. Chu contributed to the review and editing of the manuscript. All authors contributed to the final polishing of the manuscript.

Conflicts of interest

The authors declare that they have no conflict of interest.

Acknowledgements

Haowei Jia would like to acknowledge the financial support from the University International Postgraduate Award (UIPA) funded by the University of New South Wales, Australia.

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