Rational design and application of electrocatalysts based on transition metal selenides for water splitting

Abstract

Transition metal selenides (TMSes) have gained significant attention as viable non-noble metal catalysts for water splitting due to their low cost and potential electrocatalytic activity. However, the electrocatalytic activity of TMSes can be hindered by poor conductivity and limited active sites. To overcome these limitations, numerous TMSe-based electrocatalysts have been recently designed and reported, aiming to optimize their performance. This review provides a comprehensive summary of the recent progress in TMSe-based electrocatalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and overall water splitting. Additionally, we discuss the various preparation methods and types of TMSe-based electrocatalysts. Furthermore, we outline future perspectives for the advancement of TMSe-based electrocatalysts. Overall, this review aims to offer new insights into the rational design and application of TMSe-based electrocatalysts for efficient water splitting.

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Haiyan Xiang

Haiyan Xiang received her PhD degree under the supervision of Prof. Song Liu from Hunan University. She is now employed in the College of Materials and Advanced Manufacturing, Hunan University of Technology. Her interest is in nanomaterial synthesis and their electrocatalytic application in the energy fuel cell field.

Qizhi Dong

Qi-Zhi Dong is an associate professor of Hunan University, PR China. Her research focuses on nanostructured materials for electrocatalysis and energy storage devices. She received her PhD from Hunan University. She was a visiting scholar at the University of Leicester, UK (2008–2009).

Meiqing Yang

Dr Meiqing Yang obtained her PhD degree from the School of Life Sciences, Nankai University, and later worked as a postdoctoral fellow at the School of Chemistry and Chemical Engineering, Hunan University. Currently, she is working in Hunan University of Arts and Science, focusing on the preparation and electrochemical properties of low-dimensional nanomaterials.

Song Liu

Dr Song Liu received his bachelor's degree in 2006 from Nankai University and PhD in 2011 from Peking University, China. He is a postdoctoral fellow working with Prof. Liming Dai (2011–2013) in Case Western Reserve University. After 3 years’ research in the National University of Singapore (2013–2016), he is now a full professor in the Institute of Chemical Biology and Nanomedicine in Hunan University, China. His research interest is on controlled synthesis of low-dimensional materials, application research of functional devices and nanobiological research.

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1. Introduction

Over the past few decades, the increasing population growth and depletion of conventional fossil fuels have led to a growing demand for innovation in energy technology. As a result, various alternative energy sources have been explored, including solar energy, hydropower, biofuels, wind energy, and geothermal energy. Among these alternatives, hydrogen energy has emerged as a promising candidate for future energy conversion systems due to its non-polluting nature and high energy density.^1–4 To obtain hydrogen energy, researchers have focused on water electrolysis, which can be achieved through both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).^5,6 However, the high reaction overpotentials associated with common HER and OER have posed challenges. To address this issue, nanostructures composed of Pt, Pd, Ir, and Ru, as well as their alloys and compounds, have been developed as catalysts.⁷ For example, Xu et al. have engineered the covalency of the Ru–O bond by introducing tensile strain to RuO₆ octahedra in a binary Ru–Sn oxide matrix, prohibiting the participation of lattice oxygen and the dissolution of Ru, thereby significantly improving the adsorption energy and long-term stability of OER.⁸ These catalysts have demonstrated high efficiency and balanced adsorption energy of the reaction intermediates. Nevertheless, their widespread applications have been hindered by limited reserves and high costs. Therefore, the development of low-cost, high-performance Pt-free HER catalysts holds significant importance.^9,10

Nanostructured transition metal dichalcogenides (TMDs) have been proven to be exceptional catalysts for both OER and HER, demonstrating superior performance and high robustness.^11–13 The emerging transition metal selenides (TMSes) share a similar structure to their corresponding TMDs. Selenium (Se) atoms possess an electronic structure of 4s²4p⁴ and can readily gain two electrons from less electronegative elements to form Se²⁻ ions or share electrons with more electronegative elements to form TMSes. TMSes can be primarily classified into two typical types based on their morphology: graphite-like layered MSe₂ structures (M = Mo, W, etc.) and non-layered M_xSe_y structures (M = Co, Ni, Fe, etc.). Each monolayer of MSe₂ consists of three atomic layers that are bonded by strong covalent bonds and linked to another layer through weak van der Waals forces. MSe₂ exhibits three well-known phases: 1T with octahedral coordination, 2H with trigonal prismatic coordination, and 3R with trigonal prismatic coordination.^14–16

TMSes generally exhibit higher electrical conductivity. For example, MoSe₂ demonstrates higher intrinsic electrical conductivity compared to MoS₂ due to the more metallic nature of selenium. Therefore, TMSes are expected to serve as efficient electrocatalysts for water splitting, involving both HER and OER.^17,18 The first transition metal selenides (NiSe) were prepared in 2012,¹⁹ and were subsequently utilized as catalysts for HER in acidic media. In 2013, MoSe₂ and WSe₂ were synthesized and proved to be efficient catalysts for electrocatalytic HER.²⁰ Since then, efforts have been made to enhance the performance of TMSes by improving the density of active sites and increasing conductivity. Various strategies have been explored, including defect engineering, control of structure and morphology, heteroatom doping, utilization of electro-conductive carbon materials as substrates, and fabrication of dual-native vacancies.^21–27 Such synthetic study aims to further expand the activity for the TMSes catalysis.

In this review, we provide an overview of the recent progress in the development of metal selenides as electrocatalysts for water splitting. Firstly, we summarize the mechanism of water splitting and relevant synthetic methods, including liquid phase exfoliation (LPE), chemical vapor deposition (CVD), hydrothermal/solvothermal techniques, and others. Secondly, we discuss the strategies for improving the electrocatalytic performance of metal selenides. Finally, we address future challenges and provide perspectives on this research area. We hope that this review will offer valuable information and guidance for future studies in the field of electrocatalysis and its applications in other fields.

2. The basic principle of electrocatalytic water splitting

As we know, water splitting as a significant approach to produce pure hydrogen has broad application prospects in energy storage and conversion. As shown in Fig. 1(a),²⁸ water electrolysis (2H₂O(l) → 2H₂(g) + O₂(g), ΔE_o ≈ 1.23 V vs. RHE) consists of two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER).^29,30 In common electrolysis systems, reaction mechanisms and kinetics are critical for different reaction and electrolyte environments. Therefore, understanding complicated reaction processes is necessary for the design of catalysts with high performance.

Fig. 1 (a) Schematic diagram of an electrocatalytic water decomposition cell and HER and OER reactions.²⁸ Reproduced with permission from ref. 28. Copyright 2021, American Chemical Society. (b) HER volcano plot for metals and MoS₂. (c) OER volcano plot for metal oxides.³³ Reproduced with permission from ref. 33. Copyright 2017, The American Association for the Advancement of Science.

2.1. Hydrogen evolution reaction

Hydrogen evolution is a pH-dependent multi-step reaction, which generally includes two possible reaction mechanisms. In acid media, the first step is the migration of hydroxonium ions from the bulk solution to the electrode surface, capturing an electron and generating H ads (Volmer reaction). In the second step, there are two possible reaction mechanisms, one of which is the chemical desorption process. Two adjacent H ads would combine to generate hydrogen, which is called the Tafel reaction. Another mechanism is the electrochemical desorption process, in which one H ads combines with a proton from the solution and with an electron from the electrode to produce H₂, which is called the Heyrovsky reaction. The reaction can be expressed as below:

Volmer reaction: H₃O⁺ + M + e⁻ → M–H* + H₂O (1)

Heyrovsky reaction: M–H* + H₃O⁺ + e⁻ → H₂ + H₂O + M (2)

Tafel reaction: M–H* + M–H*→ H₂ + 2M (3)

In acidic and basic media, as shown in Fig. 2(a) and (b),³¹ a water molecule is absorbed and dissociated into H⁺ and OH⁻ at the active sites; next, as in acidic solution, the Tafel reaction and Heyrovsky reaction of H ads produce the H₂ molecule.³² The reaction can be expressed as below:

Volmer reaction: H₂O + M + e⁻ → M–H* + OH⁻ (4)

Heyrovsky reaction: M–H* + H₂O + e⁻ → H₂ + M + OH⁻ (5)

Tafel reaction: M–H* + M–H* → H₂ (6)

Fig. 2 Mechanism of HER electrocatalysis in two different media. (a) Acid media. (b) Basic media. (c) The OER mechanism under alkaline conditions. (d) The typical HER and OER polarization curves.³¹ Reproduced with permission from ref. 31. Copyright 2022, Elsevier B.V. All rights reserved.

The process of reaction kinetics of HER in an acid electrolyte is generally more likely to happen than that in basic and neutral environments, because of the weaker covalent bond of hydroxonium ions compared to the covalent bond of water.

Furthermore, the value of the Tafel slope can reflect the speed control step of HER. When the Tafel slope value is high, hydrogen adsorption is the speed control step. If the dynamic process of hydrogen adsorption is fast, the Tafel slope value is small, the chemical or electrochemical desorption step is the speed control step. According to the Sabatier principle, the binding energy of the catalyst and reaction intermediate should be moderate. If the hydrogen bond to the surface is too weak, adsorption (Volmer step) will limit the total reaction rate, while if the binding is too strong, the desorption (Heyrovsky/Tafel) step will limit the reaction rate. Therefore, a necessary and insufficient condition for an active catalyst is the value of ΔG_H* close to zero. How to control the adsorption energy of reaction intermediates on the surface is the key to designing materials with better performance. Until now, the HER performance has been evaluated by some technical terms, e.g., overpotential, exchange current density, Tafel slope, turnover frequency, Faraday efficiency and stability. In addition, charge transfer resistance (R_ct), specific surface areas and free energy differential of hydrogen adsorption (ΔG_H*) are frequently used to determine the HER performance (Fig. 1(b)).³³

2.2. Oxygen evolution reaction

Designing high-activity OER electrocatalysts relies on in-depth understanding of the reaction mechanism of the OER process. Compared to HER, OER is kinetically slower because OER is a four-electron transfer reaction. Therefore, OER is a key process that controls the overall efficiency of electrochemical water decomposition. As shown in Fig. 2(c), OER reaction mechanisms are also different in different pH environments. There are two different mechanisms to generate O₂: one mechanism involves the formation of adsorption intermediates, like OH*, O*, and OOH*, and subsequently the formation of O₂, which is named the adsorbate evolution mechanism (AEM). In the reaction mechanism, the adsorption between the catalytic active center M and the reaction intermediate affects the overall electrocatalytic performance. As shown in equations (7)–(16), the water molecule is adsorbed on the surface of the metal site via the one-electron oxidation process to form OH⁻ and then an adsorbed M–OH, which subsequently undergoes proton coupling and electron removal to form M–O species. The following O–O bond formation step allows M–O to react with another water molecule to form M–OOH. Finally, M–OOH is oxidized by the one-electron transfer process to release O₂ and recover the initial M active site.³¹

In acidic solution

H₂O + M → M–OH* + H⁺ + e⁻ (7)

M–OH* → M–O* + H⁺ + e⁻ (8)

2M–O* → 2M + O₂ (9)

or

M–O* + H₂O → M–OOH* + H⁺ + e⁻ (10)

M–OOH* → M + O₂ + H⁺ + e⁻ (11)

In basic solution

OH⁻ + M → M–OH* + e⁻ (12)

M–OH* + OH⁻ → M–O* + H₂O + e⁻ (13)

2M–O* → 2M + O₂ (14)

or

M–O* + OH⁻ → M–OOH* + e⁻ (15)

M–OOH* + OH⁻ → M + O₂ + H₂O + e⁻ (16)

The difference between the Gibbs free binding energy of O* and OH* (ΔG_O − ΔG_OH) is used in the volcano-type correlation diagram of the OER reaction, which can evaluate the catalyst performance. Like the HER, the best catalyst displaying the minimum overpotential should bind intermediates on its surface neither too strongly nor too weakly. Cause the low potential, O* adsorption energy on the left side of the volcano (Fig. 1(c)) strong enough, the speed determination step is usually O* to OOH*. Inversely, on the right side of the volcano, the adsorption energy of O* is weaker, which resulted difficult formation of O* intermediates and this formation step is generally defined as decisive step. Among these major classes of metal oxide materials, all surfaces obey an unsatisfactory linear relationship between OOH* and OH* (ΔG_OOH* = ΔG_OH* + 3.2 ± 0.2 eV).

Another mechanism is the direct combination of 2 M–O intermediates to produce O₂; after O₂ release, the two empty metal centers are occupied by OH- to continue the reaction, which is known as the lattice oxygen participation mechanism (LOM). Unlike the traditional AEM mechanism system, LOM reactions occur at two adjacent metal sites rather than at a single metal site. Because *OOH is not an intermediate in the LOM catalytic cycle, limitation of the scaling relationship between *OH and *OOH can be broken. In the OER catalytic system, whether AEM or LOM mechanism, the study of structure–activity relationship is important, and continuous research is needed to develop more efficient and stable catalysis.

2.3. Overall water splitting

In general, the half reaction activities of HER and OER are set up in a three-electrode system for separate electrochemical measurements involving the counter electrode, reference electrode and working electrode as shown in Fig. 2(d). However, overall water splitting means that HER and OER occur simultaneously at the cathode and anode, which are assembled into an integrated circuit. The high efficiency of overall water splitting is dependent on the applied voltage to achieve sufficient value, which aims to realize the two half reaction (HER and OER) and overcome their sum of overpotentials. The theoretical decomposition voltage (E₀) of water is only 1.23 V, but the actual decomposition voltage (E) is much higher than 1.23 V. Currently, bifunctional catalysts have received a lot of attention for overall water splitting. Bifunctional catalysts can catalyze both HER and OER at the same time, simplifying the preparation process and reducing the cost, thus becoming an important research direction of overall water decomposition in practical applications.

3. Synthetic methods of TMSes

Because of the unique properties and abundant reserves of selenium on Earth, TMSes and their corresponding derivatives have garnered significant attention for their potential applications in the fields of electronics and catalysis. Numerous studies have demonstrated that the electrical and catalytic performance of TMSes is highly dependent on their size, composition, and morphology. To further enhance the value of TMSes, various techniques have been developed to engineer their structure and components. In this section, we have compiled and summarized typical synthetic methods of TMSes, including liquid phase exfoliation (LPE), chemical vapor deposition (CVD), hydrothermal/solvothermal methods, and so on.^34–37

3.1. Liquid phase exfoliation (LPE)

Single or few-layered TMSe flakes exhibit unique electronic properties and high specific surface areas, which are important for catalyst related applications. Single or few-layered TMSes can be exfoliated from bulk TMSes by overcoming the van der Waals force among layers, via direct sonication techniques, electrochemical exfoliation, LPE, etc. These methods can realize the large-scale production and meet the practical catalytic applications.

The typical LPE process comprises several important steps, as shown in Fig. 3(a),³⁸ including immersion, insertion, exfoliation, and stabilization. By utilizing appropriate solvents and polymers, it is possible to prepare large quantities of monolayer or few-layer TMSes. For instance, commercially available TMSe powders such as MoSe₂, TeSe₂, and NbSe₂ can be sonicated in various organic solvents with different surface tensions.³⁹ After centrifuging the dispersion, the supernatant containing monolayer and few-layer TMSe nanosheets can be obtained. This method is insensitive to air and water, and has the potential for scalable production of exfoliated materials. Moreover, this procedure facilitates the formation of hybrid films with enhanced properties. Coleman reported a similar direct sonication method using a sodium cholate/water solution. However, direct sonication mechanically exfoliates layered structures, resulting in the breakage of nanosheets and reduction in lateral size. To address this issue, electrochemical exfoliation was developed to produce TMSe nanosheets with high yield and minimal damage. Furthermore, the bulk nanosheet and the insertion source during electrochemical exfoliation can serve as part of the electrode or electrolyte in a two-electrode system.

Fig. 3 (a) Scheme of the whole LPE process containing immersion, insertion, exfoliation, and stabilization.³⁸ Reproduced with permission from ref. 38. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA. (b) The electrochemical lithium interaction process.⁴⁰ Reproduced with permission from ref. 40. Copyright 2017, Tsinghua University Press and Springer-Verlag Berlin Heidelberg.

In electrochemical exfoliation, as shown in Fig. 3(b),⁴⁰ the cation-intercalated substance can be inserted into the space between the layers of TMSe bulk materials, thereby increasing the interlayer distance and overcoming the van der Waals forces. In a typical electrochemical lithium intercalation process, first, the layered bulk material is initially mixed with acetylene black and a poly(vinylidene fluoride) (PVDF) binder dispersed in N-methylpyrrolidone (NMP). The resulting slurry is then uniformly coated onto a copper foil and dried. Subsequently, the bulk material-coated copper foil and lithium foil are used as the cathode and anode, respectively, with a polypropylene (PP) film serving as the separator. These components are assembled into a lithium-ion battery within an Ar-filled glove box, using a 1 M LiPF₆ electrolyte in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1 : 1.⁴¹

The performance of electrochemical exfoliation can be evaluated based on several parameters, including the amount of insertion, the voltage, and the discharging time. Firstly, the insertion amount. In the exfoliation process, the large sized of insertion can be formed under external voltage, which further increases the interlayer distance. For example, ultrathin ternary molybdenum sulfoselenide (MoSe_xS_2−x) nanosheets can be prepared through cathodic electrochemical exfoliation in non-aqueous electrolytes. The exfoliated MoSe_xS_2−x nanosheets exhibited high structural integrity, with lateral dimensions of up to ∼1.5 μm and an average thickness of approximately 3 nm.³⁹ The cathodic electrochemical exfoliation of bulk MoSe_xS_2−x was conducted in a two-electrode system, with bulk MoSe_xS_2−x serving as the cathode, Pt foil as the anode, and TBAB solution in acetonitrile as the electrolyte. During exfoliation, the interaction of cations induced the deformation of bulk MoSe_xS_2−x, resulting in the separation of ternary MoSe_xS_2−x nanosheets from the parent MoSe_xS_2−x and their dispersion in the TBAB acetonitrile solvent, exhibiting the Tyndall phenomenon. Secondly, the discharging voltage and time also influence the products of electrochemical exfoliation. In contrast to previously reported transition-metal sulfide crystals, which exhibit distinct discharge plateaus during electrochemical lithium intercalation, metal selenides show continuously descending discharge curves without a plateau. This makes it challenging to determine the cut-off voltage, which is the voltage at which the discharge stops, in order to achieve optimal Li insertion.⁴² Insufficient Li insertion can lead to ineffective exfoliation, while excessive Li insertion can result in the chemical decomposition of the crystals. In light of this, Zhang et al. successfully prepared few-layer NbSe₂, WSe₂, and Sb₂Se₃ nanosheets through electrochemical lithium intercalation at different cut-off voltages.⁴¹

In summary, for electrochemical exfoliation, the insertion amount, the voltage and the time of discharging can be carefully adjusted to achieve a high yield and maintain structural integrity of the layered TMSes.

3.2 Chemical vapor deposition (CVD)

CVD is a versatile, convenient, straightforward, and cost-effective method that has been successfully utilized for the synthesis of TMSe-based electrocatalysts. CVD is typically performed using a tube furnace under an inert atmosphere. The metal precursors used are either transition metals or transition metal oxides, while selenium powder serves as the precursor for the chalcogen element. These precursors are individually placed at appropriate positions in the furnace, allowing for gas–solid interactions. Upon reaching the boiling point of 684.8 °C, the solid selenium evaporates and reacts with the metal precursors, resulting in the formation of TMSes. In addition to transition metals or transition metal oxides, derivatives such as transition metal salts can also be employed as precursors.

Taking WSe₂ as an example, the synthesis of WSe₂ involves placing selenium powder upstream at approximately 20 cm from the center of the tube furnace, where WO₃ is located. When the furnace is heated to 1000 °C with an appropriate ramping rate, the selenium vapor diffuses and reacts with the WO₃ precursor, ultimately leading to the formation of WSe₂ on the substrate.⁴³ It is worth noting that, apart from WO₃, ammonium salts containing tungsten can also serve as precursors. Du et al. have reported the successful preparation of WSe₂ and W(Se_xS_1−x)₂ nanoflakes on carbon nanofibers using this approach.⁴³ In their process, polyacrylonitrile nanofiber mats were placed on an alumina boat and heated to 280 °C for 6 h in the furnace tube to thermally decompose (NH₄)₆H₂W₁₂O₄₀, resulting in uniformly dispersed WO₃ nanoparticles. Subsequently, a boat loaded with Se and S powder was positioned 20 cm upstream from the pre-prepared WO₃ nanoparticles at the center of the furnace. As the furnace temperature reached 1000 °C, the selenium vapor diffused and reduced the WO₃ nanoparticles on the surface, forming volatile suboxides of WO_3−x. These suboxides were rapidly selenized into the WSe₂ product for lateral growth.

The properties and morphology of deposited TMSes can be effectively modified by precisely controlling the conditions during the CVD, including the choice of precursors and temperature. In the case of TMDs, their ultrathin sheet structure exposes prismatic edges and basal planes, while the termination atoms at the edges (metal or chalcogen) depend on the morphology, which is defined by the chemical potential of the growth environment. Therefore, by carefully controlling the synthesis and growth conditions during CVD, the edge termination of TMSes can be tuned, leading to alterations in their surface chemical properties. Traditional CVD methods always used transition metal oxides and got thin nanosheets; additionally, the morphology of TMSe products can be modified by using other precursors. For instance, yolk–shell structures of bimetallic MnCo selenide have been successfully prepared.⁴⁴ In this process, Se powder and as-prepared MnCo glycolate powder were placed at the upstream and downstream sides of a tube furnace, respectively. The furnace was then heated to 450 °C and maintained to obtain the final product. As shown in Fig. 4(a), MoSe₂ of different morphology can be fabricated through CVD under non-equilibrium conditions at different temperatures,⁴⁵e.g., nanoplates were obtained at 880 °C, nanosheets at 940 °C, and microparticles at 980 °C. In the selenization process, the Se vapor can react with the target substrate without altering its original morphology. As shown in Fig. 4(c), this is exemplified by the preparation of pyrite-type beaded stream-like cobalt diselenide (CoSe₂) nanoneedles directly on flexible titanium foils. This was achieved by treating a cobalt oxide (Co₃O₄) nanoneedle array template with selenium vapor.⁴⁶ The property of TMSes is always connected with their morphology, and the choice of precursors and temperature have a significant effect on the practicle application of TMSes.

Fig. 4 (a) Schematic diagram of the synthesis of MoSe₂ structures by CVD at different temperatures; SEM images of the MoSe₂ samples fabricated: nanoplates at 880 °C, nanosheets at 940 °C, and microparticles at 980 °C.⁴⁵ Reproduced with permission from ref. 45. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) Formation mechanism of macroporous CoSe₂–CNT composite microspheres by the spray pyrolysis process and subsequent one-step post-treatment.⁴⁹ Reproduced with permission from ref. 49. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Schematic depiction of the synthesis of CoSe₂ nanoneedle arrays through treating a cobalt oxide (Co₃O₄) nanoneedle array template with selenium vapor. The experimental apparatus for step 2 (selenization) is shown as the inset.⁴⁶ Reproduced with permission from ref. 46. Copyright 2013, The Royal Society of Chemistry.

Heteroatom doping and heterojunction structures can be introduced into TMSes through the CVD method. For example, B-doped MoSe₂ nanosheets can be synthesized using a two-zone furnace. Se powder was placed upstream of the B₂O₃ and MoO₂ mixture, and subsequently heated to 550 °C and 920 °C.⁴⁷ The NiSe₂/Ni hybrid foam can be prepared by placing bare Ni foam and Se powder at the downstream and upstream of the furnace, ramping up to the anticipated temperature, and maintaining for 1 h under Ar flow.⁴⁸ As shown in Fig. 4(b), Kim et al. synthesized CoSe₂–CNT composite microspheres by placing the as-prepared CoO–CNT composite microspheres into a small alumina boat, which was loaded into a larger, covered alumina boat in a quartz reactor. The mixture was then heated to 300 °C for 6 h under a 10% H₂–Ar reducing atmosphere.⁴⁹ Selenium-enriched NiSe₂ nanosheets can be converted from as-prepared ultrathin Ni(OH)₂ nanosheets in a quartz socket. In the CVD process, Se powder and Ni(OH)₂/CF were respectively placed upstream and downstream. After CVD growth, uniform and vertically textured NiSe nanosheets covering the entire CFs can be obtained.⁵⁰ The introduction of heteroatoms or heterojunction structures in TMSes is significant for modifying their properties.

In the above studies, the Se and transition metal sources were placed separately. However, CVD selenization can also be achieved by uniformly mixing the reactants. For instance, the as-prepared Ni@nitrogen-doped graphene (NG) and Se powder can be annealed at 300 °C under a flow of argon for 3 h, thus obtaining a core–shell structure of NiSe₂@NG.⁵¹ In addition to Se powder, didecyl selenide can also be used as a Se source. By heating the precursor mixture in a tubular furnace at 250 °C for 1 h under an N₂ atmosphere, palladium selenide can be prepared.⁵²

3.3 Hydrothermal/solvothermal method

The hydrothermal/solvothermal method enables the production of nanomaterials with various phases and morphologies in a high-temperature and high-pressure environment. The TMSes can be formed by this method through the reaction of metal sources, Se precursors, and reduction reagents in an autoclave. To date, the hydrothermal/solvothermal method has been employed to produce TMSes with diverse transition metals and morphologies.

Several single-metal TMSes have been prepared by using the hydrothermal/solvothermal method, as shown in Fig. 5(a) and (b),^53,54 such as MoSe₂,^53,55 FeSe₂,^56,57 and CoSe₂,^54,58 and in Fig. 3(a), such as NiSe₂,^59,60 CdSe,^61,62 ZnSe,⁶³ and CuSe.⁶⁴ As shown in Fig. 5d, the MoSe₂ nanosheets can be prepared by reacting Na₂MoO₄·2H₂O with NaHSe at 220 °C for 12 h.⁵⁵ Yin et al. have used excessive NaBH₄ to regulate the electronic structure so that the MoSe₂ framework was rearranged from 2H to 1T, and the MoSe₂ nanosheets have the 1T phase for HER, as shown in Fig. 3(b).⁶⁵ Fu et al. used NaHSe and ethanediamine to synthesize Ni_0.85Se nanoparticles in the range of about 10–27 nm at 180 °C for 10 h.⁶⁶ With Li₂Se as the precursor, FeSe₂ can be prepared by hydrothermal recrystallization.⁵⁷ High-temperature conditions facilitated the formation of high-crystallinity CdSe hollow spheres with a wurtzite structure.⁶⁴ The Co_0.85Se nanosheets with a thickness of ∼30 nm can be obtained at 140 °C for 24 h, with Na₂SeO₃ as the Se source and water as solvent.⁵⁸

Fig. 5 (a) Schematic diagram of the synthesis of MoSe₂/Ni_0.85Se.⁵³ Reproduced with permission from ref. 53. Copyright 2017, Elsevier Ltd. All rights reserved. (b) Solvothermal synthesis with CoSe₂/DETA nanobelts as the substrate for the preparation of the MoS₂/CoSe₂ hybrid.⁵⁴ (c) Schematic diagram of the morphology evolution of crystalline nickel selenide grown in situ on nickel foam.⁶⁷ Reproduced with permission from ref. 67. Copyright 2013, The Royal Society of Chemistry. (d) Schematic illustration of phase- and disorder-controlled synthesis of MoSe₂ nanosheets by the hydrothermal technique.⁵⁵ Reproduced with permission from ref. 55. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

It is important to highlight that the quality of the product obtained from this method is significantly influenced by external conditions, including the reaction time, solvent, concentration of introduced additives, and so on. By adjusting the hydrothermal reaction time, nickel selenide arrays with various morphology can be achieved as shown in Fig. 5(c).⁶⁷ Dai et al.⁶⁸ utilized an organic precursor of selenium, namely selenium cyanoacetic acid sodium (NCSeCH₂–COONa), where selenium is in the form of Se²⁻, without the use of a strong reducing agent or ethylene glycol.

Bimetallic selenides as well as ternary and multi-metallic selenides can be easily synthesized using hydrothermal/solvothermal methods. For instance, Zn-doped MoSe₂ nanosheets can be prepared at 180 °C, which have been utilized as high-performance catalysts for the HER in acidic media.⁶⁹ Mo–Fe selenide nanosheets with an ultrathin morphology (thickness about 20 nm) can be synthesized at 180 °C.⁷⁰ Furthermore, Co–Fe–Se nanosheets with a thickness of about 2 nm can be obtained at 180 °C for 40 h by employing a mixed solution of EDTA and PVP.⁷¹

Various TMSe nanostructures such as heterostructure composites also can be formed by using the hydrothermal/solvothermal method. CoNiSe₂ hetero-nanorods can be prepared at 160 °C.⁷² For example, CNT/Ni_0.85Se–SnO₂ networks can be prepared at 140 °C for 22 h.⁷³ With hydrothermally synthesized NiMoO₄ nanowires as the template, the MoSe₂@Ni_0.85Se nanowire network can be achieved at 160 °C for 24 h.⁵³ Similarly, Zhang et al. used NiMoO₄ nanowires to synthesize 3D hierarchical MoSe₂/NiSe₂ composite nanowires through a two-step hydrothermal approach.⁷⁴ The hydrothermally prepared precursor was selenized at 220 °C for 12 h. The bare NF can be used as a substrate to grow 1D Ni–Se/NF nanostructures in different solvents, including ethanol, DMF and deionized water. With NaHSe solution as the Se source, the solvothermal process can be completed at 140 °C for 14 h.⁷⁵ Li et al. grew the CoSe₂/DETA nanobelts on Ti mesh at 180 °C for 16 h. The nanobelts exhibited widths of 30-300 nm and lengths of several micrometers.⁷² Yu et al. synthesized a CoSe₂/DETA nanobelt substrate by adding Na₂SeO₃ and heating at 180 °C for 16 h.⁵⁴ By introducing functional groups into MoSe₂ decorated CBC nanofibers, Liu et al. synthesized a MoSe₂/CBC composite by reacting Se powder with hydrazine hydrate, mixing it with Na₂MoO₄, and then heating at 180 °C for 12 h.⁷⁶

In general, the hydrothermal/solvothermal method is a common method for preparing TMSes with different phases, components, and morphology, and the final product can be obtained on a large scale in a convenient and low-cost manner. Through rational design, various morphologies and structures can be achieved to enhance the catalytic performance.

3.4. Other methods

In addition to the aforementioned methods, the electrochemical selenization method, pulsed laser deposition, and plasma-assisted selenization method have also been reported for the synthesis of transition metal selenides.

In the electrochemical process, various substrates such as Si wafers, FTO (fluorine-tin-oxide) glass, metal foil, metal foam, and nanostructured templates are utilized as cathodes. The precursors are dissolved and electrodeposited onto the surface of the cathode. For instance, Co(C₂H₃O₂)₂, SeO₂, and LiCl were added to a sealed electrochemical cell equipped with a graphite-rod counter electrode and an Ag/AgCl reference electrode. Amorphous cobalt selenide films were then electrodeposited onto a Ti substrate at a potential of −0.45 V.⁷⁷ As shown in Fig. 6(a), Chia et al. prepared porous MoSe_x films on a glassy carbon electrode substrate by performing chronoamperometry at a certain potential for 10 min, using an aqueous electrolyte containing H₂MoO₄ and SeO₂ precursors dissolved in NH₄OH.⁷⁸ The power supply during electrochemical deposition significantly impacts the structure of the resulting product. Copper selenide was synthesized through a single-step electrochemical deposition process using constant and pulsed potentials.⁷⁹ Compared to potentiostatic deposition, the pulsed potential led to the formation of crystallized copper selenides. The electrochemical method has also been employed to prepare other TMSes, including MoSe₂,⁸⁰ WSe₂,⁸¹ FeSe,⁸² NiSe₂,⁸³ CdSe,⁸⁴ ZnSe,^85,86 CuSe,^79,87 MnSe,⁸⁸ PbSe,^89,90 and Ag₂Se (Fig. 6(c)).^91,92

Fig. 6 (a) Illustration of the electrochemical fabrication of the inverse opal porous MoSe₂ films.⁷⁸ Reproduced with permission from ref. 78. Copyright 2018, American Chemical Society. (b) Schematics of MoSe₂/Mo core–shell 3D hierarchical nanostructures fabricated by glancing angle deposition (GLAD) followed by the plasma-assisted selenization process.⁹⁸ Reproduced with permission from ref. 98. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (c) Schematic diagram of the CTL detection system.⁹² Reproduced with permission from ref. 92. Copyright 2010, Elsevier B.V. All rights reserved. (d) Schematic diagram of the experimental setup used for deposition of WSe_x films by SMPLD in a buffer gas.⁹⁴ Reproduced with permission from ref. 94. Copyright 2013, Elsevier B.V. All rights reserved.

Pulsed laser deposition has been utilized by Fominski and co-workers to fabricate MoSe_x (x = 1.5–2.4) thin films under various vacuum conditions.⁹³ Similarly, WSe_x (x = 1.5–2.2) films can be prepared through shadow masked pulsed laser deposition (Fig. 6(d)).⁹⁴ Fe-based TMSes superconducting films have been synthesized using molecular beam epitaxy, where ZnSe layers were grown on GaAs(001) under light illumination with the energy of approximately 1.8 eV.⁹⁵ Additionally, other TMSes containing transition metals such as Cu and Zn have been obtained through pulsed laser deposition.^96,97

The plasma-assisted selenization method has been reported to synthesize transition metal selenides. Diphase engineered 1T- and 2H-MoSe₂/Mo catalysts can be prepared at a lower reaction temperature of 400 °C by using plasma to generate Se radicals.⁹⁸Fig. 6(b) illustrates the selenization process, where Se pellets are positioned in the heater located at the top of the chamber and heated to 220 °C under a flow of N₂/H₂ mixture (N₂ : H₂ = 100 : 40, 2 Torr), and the plasma was turned on and maintained for 8 min once the substrate reached the target temperature.⁹⁸

Other preparation methods have also been reported to synthesize TMSes, including cation exchange,^35,99 photochemical method,^100,101 sol–gel method,^102,103 hot injection,¹⁰⁴ one-pot synthesis,^36,37 thermal evaporation,^105,106 vacuum evaporation,¹⁰⁷ spray pyrolysis,^108,109 microwave assisted synthesis,¹¹⁰ sputtering, etc.^111–113 These synthetic methods provide more means to acquire various morphology and enhanced performance, thereby maximizing the full potential of catalyst conditions. We have summarized these synthetic methods and related materials with morphology and phase, metal precursor source of selenides, method and reaction in Table 1. Nevertheless, there is still significant room to explore more facile methods and conditions to optimize TMSe-based materials.

Table 1 Methodologies and precursors used to form the transition metal selenides

No.	Material with morphology and phase	Metal precursor	Source of selenides	Method and reaction conditions	Ref.
1	Few-layer 2H MoSe2, TeSe2, and NbSe2	Commercial TMSe powders	—	Liquid phase exfoliation, centrifuging the dispersion	39
2	Few-layer-thick BN, NbSe2, WSe2, Sb2Se3, and Bi2Te3	Layered bulk precursors	—	Electrochemical lithium intercalation process	41
3	Ultrathin ternary molybdenum sulfoselenide (MoSexS2−x)	Bulk MoSexS2−x	—	Cathodic electrochemical exfoliation	39
4	2H WSe2	WO3 precursor	Selenium powder	CVD	43
5	2H WSe2	(NH4)6H2W12O40	Selenium vapor	CVD	43
6	Yolk–shell structures of bimetallic MnCo selenide	MnCo glycolate powder	Se powder	CVD	44
7	2H MoSe2 nanoplates, MoSe2 nanosheets and MoSe2 microparticles	MoO2 precursor	Se powder	CVD process: nanoplates at 880 °C, nanosheets at 940 °C and microparticles at 980 °C	45
8	Pyrite-type beaded stream-like CoSe2	Cobalt oxide (Co3O4) nanoneedle array	Se powder	CVD	46
9	B-doped MoSe2 nanosheets	B2O3 and MoO2 mixture	Se powder	CVD	47
10	NiSe2/Ni hybrid foam	Ni foam	Se powder	CVD	48
11	CoSe2–CNT composite microspheres	CoO–CNT composite microspheres	Se powder	CVD	49
12	Selenium-enriched NiSe2 nanosheets	Ni(OH)2/CF	Se powder	CVD	50
13	Core–shell structure of NiSe2@NG	Ni@nitrogen-doped graphene (NG)	Se powder	CVD	51
14	Palladium selenide	Palladium acetate	Didecyl selenide	CVD	52
15	MoSe2 nanosheets	Na2MoO4·2H2O	NaHSe	Hydrothermal	55
16	1T phase MoSe2 nanosheets	Na2MoO4·2H2O	Se	Hydrothermal	65
17	10–27 nm Ni0.85Se/NGO	Ni(NO3)2·6H2O	Se powder	Hydrothermal	66
18	Metastable stoichiometric FeSe2	FeI2	Li2Se	Hydrothermal	57
19	High-crystallinity CdSe hollow spheres with the wurtzite structure	Cd(NO3)2·4H2O	Se powder	Solvothermal	64
20	Co0.85Se nanosheets with a thickness of ∼30 nm	Na2SeO3	Se source	Hydrothermal	58
21	Nickel selenide arrays	Nickel foam	Se powder	Solvothermal	67
22	MoSe2 hierarchical microspheres	(NH4)6Mo7O24·4H2O	NCSeCH2–COONa	Solvothermal	68
23	Zn-Doped MoSe2 nanosheets	Na2MoO4	Se powder	Hydrothermal	69
24	Mo–Fe selenide nanosheets with an ultrathin morphology	FeCl3·6H2O, Na2MoO4	Se powder	Hydrothermal	70
25	Co–Fe–Se nanosheets with a thickness of about 2 nm	CoCl2, FeCl3	Selenium powder	Hydrothermal	71
26	CoNiSe2 hetero-nanorods	CoNi-precursor/NF	NaHSe solution	Hydrothermal	72
27	CNT/Ni0.85Se–SnO2 networks	CNT@NiSn(OH)6 nanocube precursor	Sodium selenite	Hydrothermal	73
28	MoSe2@Ni0.85Se nanowire	NiMoO4 nanowires	Se powder	Hydrothermal	53
29	3D hierarchical MoSe2/NiSe2	NiMoO4 nanowires	Se powder	Hydrothermal	74
30	1D Ni-Se/NF nanostructures	Ni–Se/NF samples	NaHSe	Solvothermal	75
31	CoSe2/DETA nanobelts on Ti	Co(AC)2·H2O	SeO2	Hydrothermal	72
32	CoSe2/DETA nanobelt substrate	Co(OAc)2·H2O	Na2SeO3	Hydrothermal	54
33	MoSe2 decorated CBC nanofibers	Na2MoO4	Se powder	Hydrothermal	76
34	Cobalt selenide films were electrodeposited on a Ti substrate	Co(C2H3O2)2	SeO2	Electrochemical	77
35	Porous MoSex films on a glassy carbon electrode substrate	H2MoO4	SeO2	Electrochemical	78
36	Copper selenide	Cu(NO3)2	SeO2	Electrochemical	79
37	MoSex (x = 1.5–2.4) thin films	MoSe2 target	—	Pulsed laser deposition	93
38	WSex (x = 1.5–2.2) films	Microcrystalline WSe2	—	Shadow masked pulsed laser deposition	94
39	ZnSe layers on GaAs(001)	Zn fluxes	Se fluxes	Photo-molecular beam epitaxy	95
40	Diphase engineered 1T- and 2H-MoSe2/Mo	Mo film	Selenium pellets	Plasma-assisted selenization method	98

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4. TMSes as catalysts for electrochemical water splitting

Overall water splitting means that HER and OER occur simultaneously at the cathode and anode, which are assembled into an integrated circuit.¹¹⁴ TMSes as catalysts for electrochemical water splitting can serve as HER, OER or bifunctional catalysts. Bifunctional catalysts can catalyze both HER and OER at the same time, and simplify the preparation process and reduce the cost (Table 2). Se atoms have an electronic structure of 4s²4p⁴, and they can easily share electrons with more electronegative elements to form TMSes.⁵³ TMSes can be generally divided into two typical types: graphite-like layered MSe₂ structures (M = Mo, W, etc.) and non-layered M_xSe_y structures (M = Co, Ni, Fe, etc.). Layered structures usually possess a sandwiched structure with three layers of atoms such as Se–M–Se, which are bonded by covalent bonds and possess abundant exposed Se atoms, which are different from nonlayered structures.¹¹⁵ Meanwhile, the individual sheet has a thickness of 0.6–0.7 nm, and the interaction between individual sheets is through weak van der Waals force.¹¹⁶ Therefore, the performance of TMSes with layered structures is easily affected by the number of layers and surface effects. Moreover, the electronic properties of TMSes are also influenced by the progressive filling of the non-bonded d-bands of the TM.^114,117 The semiconducting behavior is caused by complete occupation of the d-bands, as in MoSe₂, while partial filling of the d-bands results in metallic behavior, as in CoSe₂. Actually, the catalytic performance depends on the electronic structure and the bond strength between the intermediate and active sites on the catalyst. Single-metal selenides such as cobalt selenides, molybdenum selenides, tungsten selenides, iron selenides, nickel selenides, copper selenides and other selenides have been designed and reported. This section introduces some representative single metal selenide-based electrocatalysts, and describes the strategies for improving the electrocatalytic activity of these TMSes in detail.

Table 2 The electrocatalytic performance of reported TMSes

Category	Catalyst	Electrolyte	HER		OER		Overall water splitting cell voltage (mV) at 10 mA cm^−2	Ref.
			η (mV) (mA cm^−2) (HER)	Tafel slope (mV dec^−1) (HER)	η (mV) (mA cm^−2) (OER)	Tafel slope (mV dec^−1) (OER)
Single-metal selenides	c-CoSe2	1 M KOH	190(10)	85	—	—	—	124
	c-CoSe2	0.5 M H2SO4	120(10)	51	200(10)	83	—	126
	CoSe and Co9Se8	1.0 M KOH	268(100)	61.4	—	—	—	13
	Thin CoSe2	pH = 13	—	—	320(10)	44	—	122
	Coral like CoSe2	1 M KOH	—	—	295(10)	40	—	123
	Flake-like Co7Se8	1 M KOH	472(10)	59.1	290(10)	32.6	1.6	124
	d-WSe2/CFM	0.5 M H2SO4	300(100)	80	—	—	—	134
	1T-WSe2	0.5 M H2SO4	101	67				133
	Ni0.85Se	1 M NaOH	200(10)	81	302(10)	—	—	135
	Ni0.85Se	0.5 M H2SO4	170(10)	49.3	—	—	—	136
	Two-tiered NiSe	1.0 M KOH	177(10)	58.2	290	77.1	1.69	142
	Textured NiSe2	H2SO4 (pH ≈ 0.67)	117(10)	32	—	—	—	137
	NiSe–NiOx/NF	1 M KOH	—	—	274(16)	—	1.74	138
	Ni0.75Se	1 M KOH	233(10)	86	—	—	1.73	139
	NiSe2	1 M KOH	—	—	—	—	1.43	140
	MoSe2	0.5 M H2SO4	300(10)	—	—	—	—	127
	MoSe2 films with vertically aligned layers	0.5 M H2SO4	159(10)	61	—	—	—	128
	a-MoSe	pH ≈ 0	270(10)	60	—	—	—	129
		pH ≈ 11	860(10)	860	—	—	—
	MoSex	0.5 M H2SO4	570(30)	118	—	—	—	78
	MoSe2/Mo core–shell nanoscrews	0.5 M H2SO4	166	34.7	—	—	—	98
	Pd4Se	0.5 M H2SO4	94	50	—	—	—	52
	PtSe2	0.5 M H2SO4	60	41	—	—	—	144
	InSe	pH = 1	549	126	—	—	—	149
		pH = 14	451	143
	FeSe2	1 M KOH	330	48.1	—	—	—	145
	2Fe–2Se	1 M KOH	245(10)				1.73	146
Heterogeneous structure	WSe2/CoSe2	0.5 M H2SO4	157	79	330	76	—	150
	CoSe2|CoP	1.0 M KOH	—	—	240(10)	46.6	—	151
	NiSe2–Ni2P/NF	1.0 M KOH	220(50)	45	102(10)	68	1.5	152
	MoSe2/NiSe2	0.5 M H2SO4	249(100)	46.9	—	—	—	74
	MoSe2/Bi2Se3	0.5 M H2SO4	300(85)	44	—	—	—	22
	(Ni,Co)Se2/CoSe2/NF	1.0 M KOH	65(10)	140.2	255(10)	148.6	1.56	205
	MOF-CoSe2@MoSe2 core–shell structure	1.0 M KOH	109.87(10)	68.91	183.81(10)	96.61	1.53	153
	Co(OH)2@CoSe nanorods (NRs)	1.0 M KOH	208(20)	152	268(20)	165	—	154
	MoSe2 HDH	0.5 M H2SO4	191(10)	72	—	—	—	40
	CoSe2–CNT	0.5 M H2SO4	174(10)	37.8	—	—	—	49
Conductive substrate	CoSe2/CF	1.0 M KOH	95(10)	52	—	—	—	155
	Ni3Se2 on Au-coated Si	0.3 M KOH	—	—	290(10)	—	—	93
	RGO/WSe2	0.5 M H2SO	300(38.43)	57.6	—	—	—	127
	Ni3Se2/NF-0.4	1.0 M KOH	95(50)	67	—	—	1.62	158
	NiSe-Ni0.85Se/CP	1.0 M KOH	101(10)	74	1.53(10)	98	1.53	156
	Ni0.85Se/rGO	0.5 M H2SO4	104(10)	50.7	—	—	—	66
	MoSe2 onto gold foil	0.5 M H2SO4	107.2(10)	31.8	—	—	—	80
	MoSe2/RGO	0.5 M H2SO4	0.05(10)	69	—	—	—	159
	MoSe2–rGO	0.5 M H2SO4	0.21(10)	57	—	—	—	160
	MoSe2/CNTs	0.5 M H2SO4	−0.07 V(10)	58	—	—	—	161
	MoSe2–rGO–CNT	0.5 M H2SO4	0.24(10)	53	—	—	—	162
	N-doped RGO/MoSe2	0.5 M H2SO4	0.229(10)	78.45	—	—	—	55
	CBC/MoSe2	0.5 M H2SO4	0.091	55	—	—	—	76
	NiSe2/NF	1 M KOH	104(10)	—	279(20)	—	—	157
Defect engineering	Dual-native vacancies of Se and Mo in 2H-MoSe2	0.5 M H2SO4	126 mV(100)	38	—	—	—	45
	Defect modulation in WSe2 MLNSs	0.5 M H2SO4	245(10)	76	—	—	—	164
	1T phase-MoSe2	0.5 M H2SO4	152(10)	52	—	—	—	116
Ternary selenides	CoP2xSe 2(1 − x)	1 M KOH	98	—	—	—	—	166
		0.5 M H2SO4	70	—
	Meso-CoSSe	0.5 M H2SO4	160(100)	52	—	—	—	167
	W(SexS1−x)2	0.5 M H2SO4	110(10)	59	—	—	—	168
	N-NiSe2/NF	1.0 M KOH	86(10)	69	—	—	—	169
	Ni0.5Mo0.5Se	0.5 M H2SO 4	197(10)	107	340	—	—	170
	NiSe1.2S0.8	0.5 M H2SO 4	144(10)	59	—	—	—	171
	N-doped MoSe2/TiC-C shell/core arrays	0.5 M H2SO 4	137(100)	32	—	—	—	206
	B-doped MoSe2	0.5 M H2SO 4	84(10)	39	—	—	—	47
	MoSSe nanodots	0.5 M H2SO 4	140(10)	40	—	—	—	175
	Zn-doped MoSe2	0.5 M H2SO4	231(10)	58	—	—	—	69
	MoSe2–Pt	0.5 M H2SO4	250(10)	85	—	—	—	176
	(Ni,Co)0.85Se nanoarrays	1.0 M KOH			216	77	—	177
	NiCoSe2 nanosheet	1.0 M KOH			255.8(10)	71		182
	(Ni,Co)0.85Se NSAs/NF	1.0 M KOH	169(10)	115.6	287(20)	86.7	1.65	183
	Co0.13Ni0.87Se2/Ti	1.0 M KOH	64(10)	63	320(100)	94	1.62	183
	CoNiSe2 (NRs)	1.0 M KOH	170(100)	40	307(100)	79	1.59	185
	Co-doped NiSe2 and Ni3Se4/C	1.0 M KOH	90(10)	81	275(30)	63	1.6	178
	Yolk-shelled Ni–Co–Se	1.0 M KOH	250	72	300	87	—	179
	CoxNi0.85−xSe	0.5 M H2SO4	103(10)	43	—	—	—	180
	CoNiSe2@CoNi-LDHs/NF	1 M KOH	106(10)	74	—	—	—	72
	(Ni,Co)Se2-GA	1 M KOH	128(10)	79	—	—	—	181
	(Ni,Fe)3Se4	1 M KOH	225(10)	41	—	—	—	189
	((Ni0.75Fe0.25)Se2)/CFC	1 M KOH	255(35)	47.2	—	—	—	192
	FeNi2Se4–NrGO	1 M KOH	170(10)	62.1	—	—	—	193
	(CoMn)Se2	1 M KOH	270(10)	39	—	—	—	195
Multi-metal selenides	Fe0.37Ni0.17Co0.36Se	1 M KOH	179(10)	115.3	—	—	—	65
	NixFe1−xSe2-DO	1 M KOH	195(10)	28	—	—	—	186
	Ni0.75Fe0.25Se2	1.0 M KOH			255	56	—	187
	Fe-doped NiSe	1.0 M KOH	163(10)	71.4	231(50)	43	1.585	190
	NiFeSe@NiSe|O@CC	1.0 M KOH	62(10)	48.9	270(10)	63.2	1.56	191
	Ni-FeSe	1 M KOH			197(10)	56	—	188
	(Fe–Co)Se2	1 M KOH	251(10)	47.6				207
	Co1−xFexSe2	1 M KOH			217	41	—	194
	MoCoSex@NC	0.5 M H2SO4	60(10)	67	—	—	—	196
	(Co0.21Ni0.25Cu0.54)3Se2	1.0 M KOH			272(10)	53.3	—	201
	(Fe0.48Co0.38Cu0.14)Se	1.0 M KOH	256(10)	40.8	—	—	—	200
	ZnNi0.5Co0.5Se2/Cu1.8Se	1.0 M KOH	—	—	370(325)	72	—	202
	Ni–Co–Fe–Se@NiCo-LDH	1.0 M KOH	113(10)	44.87	286(100)	78.9	—	203
	Fe0.09Co0.13–NiSe2	1.0 M KOH	251(10)		92(10)			17
	NiFeCoSex/CFC	1.0 M KOH	150(10)	85	—	—	—	199

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4.1 Cobalt selenides

Cobalt selenides have received extensive attention in electrocatalytic water splitting due to their high electrical conductivity, unique structure, excellent electrocatalytic features and natural abundance.^118,119 The electronic configuration (t^2g₆e^g₁) of CoSe₂ is near to optimal e_g filling and CoSe₂ is considered as an ideal catalyst for electrocatalytic water splitting due to its intrinsic metallicity.¹²⁰ Furthermore, cobalt selenides are considered as more stable in strong acids than cobalt sulfides, which are studied in acidic water solutions.¹²¹ However, the electrocatalytic performance of cobalt selenides is limited by the lack of exposed active centers despite optimal filling of e_g, and therefore, it is necessary to improve the intrinsic catalytic capability. Therefore as OER catalysis Liu et al.¹²² have improved the intrinsic catalytic capability of bulk CoSe₂ by reducing its thickness to the atomic scale (OER overpotential as low as 0.32 V at a current density of 10 mA cm⁻², Tafel slope of 44 mV dec⁻¹). Experimental and theoretical calculations verified that the formation of Co vacancies in the ultra-thin nanosheets should mainly contribute to the significantly improved OER catalytic performance. Liao et al.¹²³ have prepared three-dimensional coral-like CoSe by two-step solvothermal reaction fabrication. The prepared coral-like CoSe electrode exhibits a low overpotential of 295 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 40 mV dec⁻¹ in 1 M KOH solution. The chronoamperometric response of the coral-like CoSe electrocatalyst can retain 98% of its initial current density after 10 000 s at the overpotential of 350 mV. With Co as the active site for the OER, CoOOH/CoO_x will form on the surface of some cobalt-based OER catalysts during the electrochemical test. This increase in the oxidation state of Co with higher valence is believed to promote the multi-electron transportation process which is involved in the oxidation of water.

Among various cobalt selenides for HER electrocatalysis, the most representative one is cobalt diselenide (CoSe₂), which exists in three distinct crystal structures: orthorhombic marcasite-type (o-CoSe₂), cubic pyrite-type (c-CoSe₂, electronic configuration t^2g₆e^g₁), and polymorphic CoSe₂ (p-CoSe₂) with mixed orthorhombic and cubic phases. The electrocatalytic activity of c-CoSe₂ for HER under alkaline conditions surpasses that of other CoSe₂ materials.¹²⁴ As shown in Fig. 7,¹²⁴ the performance of o-CoSe₂ and c-CoSe₂ was compared through both experimental and DFT calculations. The water adsorption energy of the c-CoSe₂ catalyst (−0.163 eV) was significantly lower than that of the o-CoSe₂ catalyst (−0.106 eV), indicating easier water absorption on the surface of c-CoSe₂ in alkaline media. Theoretical investigations proved the easier water-adsorption process on cobalt cations and faster transformation of H_ads into H₂ on the Se active sites. Additionally, the electrical conductivity of c-CoSe₂ was also better. Consequently, the polarization curve of c-CoSe₂ achieved a low overpotential of 190 mV at a current density of 10 mA cm⁻².

Fig. 7 (a) The XRD patterns of c-CoSe₂/CC and o-CoSe₂/CC and (b) Raman spectra of c-CoSe₂ and o-CoSe₂ products. (c) and (d) The SEM images of c-CoSe₂/CC and o-CoSe₂/CC 3D electrodes. Inset: Corresponding high-resolution SEM images. (e) and (f) The HRTEM images of c-CoSe₂ and o-CoSe₂ products. Inset: Corresponding selected area electron diffraction (SAED) patterns. (g) Calculated HER free-energy change and (h) water adsorption energy for the o-CoSe₂ and c-CoSe₂ products. (i) IR-corrected polarization curves and (j) corresponding Tafel slopes of blank CC, Co(OH)F/CC, c-CoSe₂/CC, and o-CoSe₂/CC.¹²⁴ Reproduced with permission from ref. 124. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

As bi-functional catalysis for water splitting, cobalt selenides are also considered as high performance catalysts for both HER and OER. Masud et al.¹²⁵ have prepared Co₇Se₈ nanostructures with a flake-like morphology which show bifunctional catalytic activity for OER and HER in alkaline medium with long-term durability (>12 h) and high faradaic efficiency (99.62%). Co₇Se₈ exhibited an excellent OER activity in 1.0 M KOH with a small overpotential of 290 mV at a current density of 10 mA cm⁻². Moreover, this catalyst was found to be effective for hydrogen evolution reaction in the same media. It can effectively split water at a cell voltage of 1.6 V under ambient conditions. The catalytic performance can be further enhanced by modifying the morphology. To synthesize c-CoSe₂ nanoparticles embedded in carbon cages, Co-based metal–organic frameworks (MOFs) were utilized as precursors.¹²⁶ By controlling the annealing conditions, the phase transformation of CoSe₂ from orthorhombic to cubic phase was achieved. Due to its metallic nature, cubic phase CoSe₂ exhibits significantly improved electrocatalytic activities for both HER (η = 43 mV, Tafel slope = 51 mV dec⁻¹) and oxygen evolution reaction (OER) (η = 200 mV, Tafel slope = 83 mV dec⁻¹). The porous structure of the MOF cages not only exposes the active sites of incorporated CoSe₂, but also prevents the catalysts from agglomerating during the catalysis. Manipulating the charge state of Co species and controlling the mass ratio of Co and Se also can enhance the catalytic performance. Zhao et al. synthesized 3D free-standing cobalt selenide electrodes containing CoSe and Co₉Se₈ phases through one-step calcination of Co foil with Se powder in a vacuum-sealed ampoule.¹³ The electrocatalytic behaviors of CoSe₂ can be significantly affected by the change of Co charge states. Synchrotron-based mechanism study has demonstrated that a high Co charge state is beneficial for OER, while a low Co charge state promotes the HER activity. The 3D network structure of cobalt selenide electrodes facilitates efficient charge transfer, leading to enhanced and stable electrocatalytic performance.

4.2. Molybdenum selenides

MoSe₂ exhibits a band gap of 1.55 eV, which is smaller than that of MoS₂ (1.90 eV). Therefore, MoSe₂ shows better conductivity and catalytic performance than MoS₂. However, similar to MoS₂, the stable 2H phase of MoSe₂ is also restricted by the poor conductivity. Theoretical calculations have demonstrated that the basal planes of MoSe₂ are inactive, while the edge sites have catalytic activity for HER.

The morphology engineering has been able to increase the active sites. Ravikumar et al. synthesized MoSe₂ nanoflowers, which exhibited a higher number of exposed chalcogen sites and superior conductivity, contributing to their high performance in acidic HER.¹²⁷ As shown in Fig. 9(a)–(c), MoSe₂ films with vertically aligned layers were prepared through CVD, resulting in active sites located at the edges of MoSe₂.¹²⁸ Based on an electrochemical corrosion process, the amorphous molybdenum selenide can be activated under H₂-evolving conditions, and produce a steady overpotential of 270 mV at 10 mA cm⁻².¹²⁹ Chia et al. fabricated inverse opal porous MoSe_x films through electrosynthesis, where 2 ≤ x ≤ 3. In this process, MoSe_x was electro-deposited via a solid template-assisted method.⁷⁸ Molybdic acid and selenium dioxide were simultaneously reduced as the respective metal and chalcogen precursors in an aqueous electrolyte. After removing the template, the electrosynthesized porous MoSe_x films contained pores with diameters of 0.1, 0.3, 0.6, or 1.0 μm, depending on the size of the template. The porous MoSe_x films with a pore size of 0.1 μm exhibited the lowest HER overpotential of 0.57 V at −30 mA cm⁻² and a Tafel slope of 118 mV dec⁻¹. Phase-engineered 1T- and 2H-MoSe₂/Mo 3D-hierarchical nanostructures with controlled shapes were prepared using a low-temperature plasma-assisted selenization process.⁹⁸ The metallic 1T-MoSe₂/Mo core–shell 3D-hierarchical nanostructures were utilized for HER measurements and exhibited the highest catalytic activities. The increased density of exposed edges and the metallic phase of the MoSe₂ shell contributed to the high performance of the 1T-MoSe₂/Mo core–shell 3D-hierarchical nanostructures. The design and assembly of multi-dimensional structures can serve as a promising approach for the development of cost-effective and efficient TMSe catalysts for HER applications.

4.3. Tungsten selenides

Tungsten selenides are a kind of unique semiconductor the bulk phase of which shows a bandgap energy of 1.7 eV, while the single monolayer has a bandgap transition of 1.4 eV. WSe₂ exhibits similar electronic behavior and electrocatalytic performance as MoSe₂, since W atoms belong to the same group as Mo atoms. Similar to MoSe₂, the basal planes of the layered materials are electrochemically inert, while the edge planes are chemically and electrochemically active.^14,130–132 To solve the problem of low amount of active sites, inert basal planes and poor conductivity, which affect the catalytic activity of WSe₂, many strategies were used. Li et al.¹³³ induced the conversion of the 2H phase to 1T phase through a simple electron irradiation method. When the irradiation intensity is 5 × 10¹⁴ e cm⁻², 1T WSe₂ with abundant Se vacancies on the base surface after phase transition possessed a low onset potential of 101 mV for HER. Moreover, WSe₂ with different morphology, supported on carbon nanofiber mats (CFM) as a conductive substrate, can be prepared through the CVD method.¹³⁴ At the upstream of the furnace, the Se vapor concentration was super-saturated, leading to the formation of dendritic crystal structures of WSe₂ nanostructures (d-WSe₂/CFM). At the downstream, the Se vapor concentration was much lower, resulting in the formation of rose-like WSe₂ nanostructures (r-WSe₂/CFM) on account of the reduced driving force. The generated structures were tailored by the diffusion-controlled concentration of Se vapor. Due to many edge sites, d-WSe₂/CFM exhibited an overpotential of −300 mV vs. RHE at 100 mA cm⁻² and a relatively small Tafel slope of 80 mV dec⁻¹, presenting better HER performance than that of r-WSe₂/CFM.

MoSe₂ and WSe₂ as typical 2D-layered transition metal dichalcogenides and promising electrocatalysts to replace noble metals have been widely studied. Currently, because of the relatively wide band gap and large basal plane of MoSe₂ and WSe₂, they are mostly designed and applied for HER. For OER, the catalysts should possess high conductivity to meet the complicated kinetics. The 1T phase has metallic behavior, but its stability is poor. It is very important to find a suitable method to improve its stability for water splitting. Most MoSe₂ (WSe₂)-based electrocatalysts show good HER activity. The HER/OER performance in alkaline media is still not excellent.

4.4. Nickel selenides

Nickel selenides show high performance in both HER and OER. For HER, there are three stable phases of nickel selenides: Ni_1−xSe_(0<x<0.15), Ni₃Se₂ and NiSe₂. The representative Ni_1−xSe used as the HER catalyst is Ni_0.85Se, which has special electronic configuration and plentiful unsaturated atoms. As shown in Fig. 8, Wu et al. directly synthesized a porous film of hexagonal phase Ni_0.85Se on a graphite substrate. The Ni_0.85Se film supported on a graphite substrate exhibited excellent activities for both HER and OER.¹³⁵ It achieved a current density of 10 mA cm⁻² for HER and OER in 1.0 M NaOH with the overpotentials at about 200 and 302 mV, respectively. When Ni_0.85Se was used as the cathode and anode in a two-electrode system for overall water splitting in alkaline solution, the potential of 1.73 V was required to reach 10 mA cm⁻², and it remained stable for over 48 h. Ni_0.85Se synthesized with facile DMF-solvothermal reaction (D-Ni_0.85Se) exhibited much better HER electrocatalytic performance than hydrothermally synthesized Ni_0.85Se.¹³⁶ D-Ni_0.85Se displayed a very low onset overpotential of 170 mV in 0.5 M H₂SO₄ and a Tafel slope of 49.3 mV dec⁻¹. In order to understand the electronic structure and ion/molecule adsorption and desorption properties and identity the active centers on electrocatalysts, Wang et al.¹³⁷ have prepared textured NiSe₂ nanosheet arrays on flexible electrodes through selenization. The Se-rich pyrite type NiSe₂ is an efficient catalyst for steady HER, because the excess selenium on the NiSe₂ surface accelerates the charge transfer and thus optimizes the intrinsic catalytic capacity of the NiSe₂ nanosheets to a certain extent and acts as a promoter for H₂ production. This electrode shows a small overpotential of 117 mV at 10 mA cm⁻², and a low Tafel slope of 32 mV per decade and excellent stability. In combination with DFT calculations, the free energy for atomic hydrogen adsorption is much lower on the Se sites (0.13 eV) than on the Ni sites (0.87 eV), suggesting that the Se sites are responsible for the excellent electrocatalytic characteristics of NiSe₂ instead of the Ni sites.

Fig. 8 (a) Schematic illustration of the synthesis of Ni_0.85Se on a graphite substrate. (b) XRD patterns of the as-synthesized Ni_0.85Se/GS sample calcined at 400 °C. (c) A typical SEM image of Ni_0.85Se grown on GS. (d) A low magnification TEM image of the Ni_0.85Se catalyst. (e) A high resolution TEM image of the Ni_0.85Se catalyst and the corresponding fast Fourier transform (FFT) pattern (inset). Electrocatalytic performance of Ni0.85Se samples calcined at 400 °C in 1 M NaOH. (f) LSV curves of Ni_0.85Se/GS and graphite for HER. Inset: Tafel plot of Ni_0.85Se/GS derived from the LSV curve for HER. (g) LSV curves of Ni_0.85Se/GS and graphite for OER. Inset: Tafel plot of Ni_0.85Se/GS derived from the LSV curve for OER. (h) and (i) 48 h long-term stability test of Ni_0.85Se under constant current densities of 10 mA cm⁻² for HER and OER. (j) LSV curve of water electrolysis for Ni_0.85Se/GSNi_0.85Se/GS and RuO₂Pt/C. (k) Chronopotentiometric curve of Ni_0.85Se/GS. Inset: A photograph of the system showing hydrogen and oxygen generated during the durability test.¹³⁵ Reproduced with permission from ref. 135. Copyright 2016, Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Nickel sulfide as a promising OER electrocatalyst, the long bond length can weaken the Ni–Ni bond, promote surface oxidation, and improve the catalytic performance. Therefore, nickel selenide was thought to exhibit high performance towards OER reactions. In view of the easily oxidized nature of selenides under strong alkaline conditions, the possible electrochemical surface re-construction of the selenides during electrocatalysis is often neglected. Gao et al.¹³⁸ conducted a thorough and meticulous study on the NiSe oxygen evolution nanoelectrocatalyst during the OER process. In parallel with the OER process, Se²⁻ in NiSe loses electrons to form the soluble selenoxide species, whereas Ni²⁺ evolves into NiO_x. Once the NiO_x layer formed on the NiSe surface reaches a certain thickness, it will protect the inner NiSe from further oxidation. These results further imply that the electrocatalytic activity of the so-called NiSe catalysts should originate from the amorphous NiO_x shell, with the NiSe core having some auxiliary effects on the catalytic activity. In addition, the NiSe–NiO_x/NF material can serve as an efficient electrocatalyst for HER. They tested the water-splitting activity in 1 M KOH, and the current density was achieved at 1.74 V (20 mA cm⁻²).

Nickel selenides with non-stoichiometry such as Ni_0.85Se have unsaturated atoms, which is of great significance to explore the electrocatalytic performance in water splitting. To understand the effects of the Se/Ni ratio of non-stoichiometric nickel selenides on the electrocatalytic properties (Fig. 9(d)–(g)), Zheng et al. developed a novel hot-injection process at ambient pressure to control the phase and composition of a series of Ni_xSe by simply adjusting the added molar ratio of the nickel resource.¹³⁹ Among the synthesized Ni_xSe series, the cubic nanocrystalline Ni_0.5Se exhibits superior OER activity comparable to RuO₂ (330 mV, 10 mA cm⁻²), which may benefit from the pyrite-type crystal structure and Se enrichment in Ni_0.5Se. Ni_0.75Se is proved to be most efficient for HER, with an overpotential of 233 mV at an HER current density of 10 mA cm⁻²; the remarkable catalytic activity of Ni_0.75Se benefits from the good electronic conductivity and enhanced electrochemically active area. Ni_0.75Se as the cathode and Ni_0.5Se as the anode require a cell voltage of 1.73 V (10 mA cm⁻²) for overall water splitting. The analysis demonstrates that the electronic conductivity of Se/Ni and Se content in the Ni_xSe compounds account for the electrocatalytic activity. In addition to non-stoichiometric nickel selenide compounds, stoichiometric NiSe₂ has a pyrite structure with a zero bandgap. Swesi et al. prepared NiSe₂ films through electrodeposition for use as an efficient bifunctional electrocatalyst for overall water splitting.¹⁴⁰ The final product could effectively split water at 1.43 V and achieve an electrolysis energy efficiency of 83% producing current density as high as 100 mA cm⁻². The high performance is attributed to lowering of the oxidation potential of Ni²⁺ to Ni³⁺ which in turn is a consequence of changing the oxide lattice to the more covalent selenide lattice, as well as the preferential growth direction of the film which exposes the Ni-rich surface as the terminating lattice plane. The electrocatalytic performance can be further improved by increasing the number of active reaction sites. To provide more active sites, the surface roughness of NiSe₂ was enhanced using an acetic acid-assisted method. Zhang et al. successfully prepared porous NiSe₂ nanowrinkles anchored on nickel foam (NF), which exhibited efficient electrochemical activity for both HER and OER.¹⁴¹ Wu et al. developed ultrathin hierarchical nanosheets (≈0.96 nm) by a self-limiting tunable acid etching and topotactic selenization process.¹⁴² Through rationally controlled hydrolyzation with the self-regulated pH value, Ni foam was etched into ultrathin layered Ni(OH)₂. This ultrathin structure prevented excessive lattice expansion in bulk materials during selenization. Consequently, Ni(OH)₂ was artificially transformed to ultrathin NiSe (≈1.25 nm) through a simple and nondestructive topotactic phase engineering approach. Thus, highly efficient and stable OER and HER electrodes with low Tafel slopes and onset potentials can be achieved under alkaline conditions.

Fig. 9 (a) Schematic illustration of the preparation of perpendicularly oriented TMD nanosheets/GN hybrids. (b) and (c) SEM images of MoSe₂ nanosheets grown on GN with a graphite disc as the substrate.¹²⁸ Reproduced with permission from ref. 128. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Crystal structures of (d) NiSe, (e) Ni_0.85Se, (f) Ni_0.75Se, and (g) Ni_0.5Se nanocrystals.¹³⁹ Reproduced with permission from ref. 139. Copyright 2018, American Chemical Society. (h) Schematic of the HER measurement setup. (i) Polarization curves for the HER obtained on the basal plane, the edge and the whole flake. Inset: Optical image of a typical PMMA masked electrochemical microcell. (j) Optical images of CVT-grown PtSe₂ flakes with one to five and ∼20 layers, insets: corresponding AFM images of the PtSe₂ flakes with height profiles. (k) Polarization curves and Tafel plots for the HER obtained on 1–5 L and ∼20 L PtSe₂ flakes and a commercial Pt/C catalyst.¹⁴⁴ Reproduced with permission from ref. 144. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

4.5. Other metallic selenides

Apart from the above TMSe-based electrocatalysts, other selenides including palladium selenides, platinum selenides, copper selenides, gallium selenides, and indium selenides have also been studied, exhibiting an outstanding electrocatalytic activity and durability for water splitting (Fig. 10).¹⁴³

Fig. 10 (a) The exfoliation energies E_exf as a function of interlayer spacing D. (b) The Gibbs free energy ΔG_H diagrams for the pristine 3d-TMSe. (c) The ΔG_H summary of 3d-TMSe, 3d-TMSe/V1 and 3d-TMSe/V2, wherein V1 and V2 stand for the single cationic-vacancy and dual cationic-vacancy pair. The red dashed line indicates the screening criterion of the outstanding performance for HER catalysis. (d) The partial density of states (PDOS) of CuSe, CuSe/V1 and CuSe/V2. (e) The vacancy formation energies E_f for 3d-TMSe/V1 and 3d-TMSe/V2.¹⁴³ Reproduced with permission from ref. 143. Copyright 2020, Elsevier B.V. All rights reserved.

Pt and Pd are common elements used in energy conversion. They can also be synthesized as TMSe-based electrocatalysts for water splitting. For example, various palladium selenides (Pd₄Se, Pd₁₇Se₁₅, Pd₇Se₄) can be prepared through thermal decomposition using the same palladium and selenium precursors with different mole ratios. Among them, Pd₄Se exhibited the best HER catalytic activity in acidic media, with an overpotential of 94 mV to drive a current density of 10 mA cm⁻², an exchange current density of 2.3 × 10⁻⁴ A·cm⁻², an apparent Tafel slope of 50 mV dec⁻¹, and a turnover frequency (TOF) value of 0.47 s⁻¹.⁵² The combination of intrinsic lower charge transfer, larger electrochemical surface area, and moderate proton binding energy contributed to the high performance of palladium selenides. Furthermore, the Pd₄Se phase with tetragonal symmetry which possessed a higher d-band DOS had a smaller positive charge on Pd compared to the other two cases. Therefore, the oxidation state of Pd in Pd₄Se is smaller than that of the other two phases. This will help in the effective adsorption of protons on Pd₄Se as compared to Pd₇Se₄ and Pd₁₇Se₁₅. PtSe₂ demonstrates high catalytic activity for HER and notable layer-dependent electrical properties.¹⁴⁴ However, it is challenging to accurately characterize the layer structure using traditional electrochemical methods. Jiao et al. synthesized single crystalline 2D PtSe₂, as shown in Fig. 9(h)–(k), with a controlled number of layers and examined the HER catalytic activity of individual flakes using micro electrochemical cells. The number of layers influenced the number of active sites, electronic structures, and electrical properties of the 2D PtSe₂ flakes, thereby affecting their HER performance. This study precisely investigated the relationship between the layer number of nanosheets and their catalytic performance using micro electrochemical cells, significantly enhancing our understanding of the structure–activity correlations for 2D nanosheets.

In addition to Pt and Pd, other non-noble metals like Cu can also constitute TMSe-based electrocatalysts. DFT calculations identified CuSe as a promising candidate toward HER.¹⁴³ The pristine CuSe monolayer, without any treatment, exhibits relatively good activity, as indicated by the small thermodynamic barrier of 0.3 eV. In contrast, the Se site on the pristine basal plane of the remaining 3d-TMSes (including Ti, V, Cr, Mn, Fe, Co, Ni, and Zn) is chemically inactive. FeSe₂ has a relatively small Tafel slope among Fe, Co and Ni based dichalcogenides and offers an opportunity to improve the performance of nonmetallic catalysts by adjusting the structure. Generally, TMSes share a similar structure with the corresponding sulfides but have higher electrical conductivity, resulting in superior charge transfer in selenides such as Fe- and Co-based selenides. Gao et al.¹⁴⁵ have synthesized 2D FeSe₂ nanoplatelets via a hydrothermal reduction route. The FeSe₂ nanoplatelets exhibit highly enhanced OER catalytic activity compared with commercial RuO₂ (e.g., overpotential: 2.2 times higher than that of commercial RuO₂ at 500 mV; Tafel slope: 48.1 mV dec⁻¹; steady-state current densities remain constant after 70 h). The excellent catalytic activity can be ascribed to the 2D nanostructure, which could facilitate improvement of kinetics of water oxidation, and the high density of exposed active sites on the (210) crystal surface. Density functional theory calculations provide detailed insight into the energy level and water adsorption at the surface of FeSe₂ nanoplatelets.

Therefore, experimental and theoretical studies prove that FeSe₂ with intrinsic semiconductor properties and tunable electronic structures effectively reduces the OER overpotential. Iron selenides can serve as catalysts in HER, OER, and overall water splitting. Panda et al.¹⁴⁶ have investigated whether FeSe₂ can be fine-tuned to achieve high performance not only for bifunctional OER and HER but also for overall water-splitting, through the control of size, structure, morphology, and electronic properties. They have synthesized a bioinspired molecular 2Fe–2Se cluster core supported by a β-diketiminato ligand via a versatile thermolytic approach. The as-synthesized FeSe₂ was electrophoretically deposited on nickel foam and possessed an overpotential of 245 mV at a current density of 10 mA cm⁻², representing outstanding catalytic activity and stability due to the formation of Fe(OH)₂/FeOOH active sites at the surface of FeSe₂. Since the medium bonding with intermediates and products in HER results in good catalytic activity, Se–H bond strength is lower than that of S–H, so that it can act as a base to trap protons and facilitate deprotonation to release hydrogen in HER. Moreover, an overall water splitting setup was fabricated using a two-electrode cell which showed a small cell voltage and high durability. Such bioinspired molecular 2Fe–2Se cluster was creatively synthesized and as foundation for the application of iron selenide in overall water splitting. The MnSe electrode prepared on nickel foam showed an overpotential of 311 mV at 10 mA cm⁻² in an alkaline electrolyte. With the incorporation of iron into MnSe, the MnFeSe electrode demonstrated significantly enhanced OER activity and improved reaction kinetics.¹⁴⁷ To gain insight into the high OER/PEC activities of the Mn-based catalysts, especially the intermediate phase of MnSe, density functional theory (DFT) calculations were performed on six surface models of MnSe, Fe–MnSe, MnOOH, Se, Fe–MnOOH, MnO₂, and Se, Fe–MnO₂. It was found that both the rate-determining steps on MnSe and Fe–MnSe are O* + OH → OOH* + e⁻. Additionally, it was inferred that the Fe dopant can stabilize the O species, accelerating the MnSe oxidation and Mn (oxy)hydroxide formation of MnOOH and δ-MnO₂ after the OER. GaSe and GeS also possess unique electrochemical properties.¹⁴⁸ GeS is isoelectronic to black phosphorus with the same structure. GaSe is a layered material composed of GaSe sheets bonded in the sequence of Se–Ga–Ga–Se. It was found that the encompassing surface oxide layers on GaSe and GeS greatly influenced their electrochemical properties, especially their electrocatalytic capabilities towards HER. These findings provide new insights into the electrochemical properties of these IIIA–VIA and IVA–VIA layered structures. InSe, with small flakes, exhibits predominant edge effects. Elisa Petroni et al. produced few-layered InSe flakes through liquid-phase exfoliation of β-InSe single crystals in 2-propanol with a low boiling point (82.6 °C). These flakes have maximum lateral sizes ranging from 30 nm to a few micrometers and thicknesses ranging from 1 to 20 nm. The HER performance of InSe was tested in hybrid single-walled carbon nanotubes/InSe heterostructures. InSe flakes with smaller surface areas and thicknesses exhibited the lowest reported values of η₁₀ among the MX compounds (η₁₀ = 549 mV at pH = 1 and η₁₀ = 451 mV at pH = 14).¹⁴⁹

5. Modification strategies of TMSes for electrochemical water splitting

As described above, metallic selenides have shown promise as efficient and durable electrocatalysts for HER and OER, offering potential alternatives to precious Pt-based catalysts. However, the catalytic efficiency of TMSe catalysts reported is still hindered by the low density of active sites, low conductivity, and surface impurities. Several strategies have been employed to enhance the conductivity and active site availability of TMSe-based electrocatalysts, such as the construction of a heterogeneous structure, use of conductive substrates, defect engineering, and designing multi-metallic selenides.

5.1. The construction of a heterogeneous structure

Constructing heterogeneous metallic selenides can offer a range of superior properties that cannot be achieved by a single semiconductor material alone. These properties include quantum effects, enhanced mobility, unique two-dimensional space, and adjustable structural engineering properties. A well-designed heterojunction structure not only allows for the adjustment of the energy band structure between different materials, but also facilitates the coupling and interaction of excitons at the interface, thereby enabling the modulation of optical properties.

CoSe₂ nanoparticles were effectively dispersed in WSe₂ nanosheets through a hot-injection colloidal synthesis process for refining their electronic functionalities, as shown in Fig. 11.¹⁵⁰ The resulting WSe₂/CoSe₂ heterostructured nanohybrids exhibited enhanced electrocatalytic performance for both HER and basic OER, with lower overpotential values (η₁₀), and Tafel slope values of 157 mV and 79 mV dec⁻¹ for HER, and 330 mV and 76 mV dec⁻¹ for OER, respectively. The improved electrocatalytic performance can be attributed to enhanced conductivity and increased surface area. Furthermore, the hybrid catalyst of CoSe₂|CoP was synthesized and applied for OER.¹⁵¹ It was found that the true OER-active species in alkaline medium were oxyhydroxides and hydroxides of CoSe₂ and CoP, rather than selenides and phosphides. Selenides got oxidized to oxyhydroxides/hydroxides amid of OER, whereas phosphides took an extended period to fully oxidize. As a result, the derived oxides from the hybrid catalyst exhibited a relatively low overpotential (Fig. 12).

Fig. 11 (a) Schematic representation showing the formation of WSe₂/CoSe₂ heterostructured nanohybrids. (b)–(d) HRTEM images, (e) SAED pattern, (f) HAADF-STEM image, and (g)–(i) related EDX mapping images for WSe₂/CoSe₂ nanohybrids. (j) HER polarization profiles and (k) the corresponding Tafel plots of the WSe₂/CoSe₂ nanohybrids. (l) OER polarization curves and (m) the corresponding Tafel plots of the WSe₂/CoSe₂ nanohybrids.¹⁵⁰ Reproduced with permission from ref. 150. Copyright 2021, American Chemical Society.

Fig. 12 (a) Schematic illustration of the preparation of CoSe₂|CoP by solvothermal and phosphidation processes. (b) XRD data of CoSe₂|CoP. (c) iR-corrected polarization curves of CoSe₂–DO, CoP–DO, and CoSe₂|CoP–DO electrodes, along with RuO₂ for comparison. (d) Overpotential (η) required for j = 10 and 100 mA cm⁻². (e) Tafel plots derived from the corresponding polarization curves of CoSe₂–DO, CoP–DO, and CoSe₂|CoP–DO. (f) Overpotential required at 10 mA cm⁻² (η₁₀) and Tafel slope comparison of the catalysts in this work with other recently reported high-performance OER electrocatalysts.¹⁵¹ Reproduced with permission from ref. 151. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Nanowrinkles of NiSe₂ and Ni₂P on Ni foam (NiSe₂–Ni₂P/NF) were synthesized through full selenization and partial phosphorization.¹⁵² In alkaline solution, NiSe₂–Ni₂P/NF exhibited favorable catalytic performances for both OER and HER. The heterostructure of NiSe₂ and Ni₂P demonstrated a synergistic effect on the water adsorption process and catalytic kinetics enhancement. Moreover, this catalyst functioned as a bifunctional electrode for overall water splitting, requiring a cell voltage of only 1.50 V to achieve a current density of 10 mA cm⁻². In a different approach, MoS₂/CoSe₂ structures were integrated to achieve a synergistic effect between MoS₂ and CoSe₂. The catalyst exhibited an increased number of catalytic sites and reduced free energy, resulting in a Tafel slope of 36 mV dec⁻¹.⁵⁴

3D hierarchical MoSe₂/NiSe₂ composite nanowires (NWs) were prepared on a carbon fiber paper (CFP) skeleton.⁷⁴ The nanosheets of MoSe₂ and NiSe₂ NWs were uniformly and tightly distributed on CFP. Consequently, the formed 3D hierarchical MoSe₂/NiSe₂ delivered a current density of 100 mA cm⁻² with an overpotential of 249 mV and a Tafel slope of 46.9 mV dec⁻¹. In the electrocatalytic process, the 3D hierarchical structure could effectively suppress the aggregation or re-stacking of nanosheets, exposing more active sites to participate in HER.

Additionally, the highly conductive NiSe₂ dispersed in the nanosheets facilitated electron transfer from the electrode to the active edges of MoSe₂, further enhancing the activity. Hetero nanostructures can also be prepared by evenly loading small ultrathin hierarchical MoSe₂ nanosheets on the whole surfaces of single-layer Bi₂Se₃ hexagonal nanoplates. The MoSe₂/Bi₂Se₃ hybrids showed an overpotential (η) of 300 mV at a current density of 85 mA cm⁻², and a Tafel slope of 44 mV dec⁻¹.²² Another heterostructure electrocatalyst of (Ni,Co)Se₂/CoSe₂/NF was fabricated on NF and applied as a superior bifunctional electrode for overall water splitting. The as-synthesized (Ni,Co)Se₂/CoSe₂/NF electrocatalyst exhibited excellent electrochemical performance in alkaline solutions with HER and OER overpotentials of 65 mV and 255 mV, respectively, at a current density of 10 mA cm⁻². The alkaline electrolyzer for overall water splitting required a cell voltage of only 1.56 V to drive 10 mA cm⁻² current. The hybrid core–shell structure was another kind of heterogeneous structure; MoSe₂ 2D nanosheets were uniformly grown on a conductive Co-metal–organic framework (Co-MOF), resulting in cobalt diselenide laminated with molybdenum diselenide.¹⁵³ Such core–shell structure exhibited low overpotentials and a low voltage for overall water splitting. In the developed nanostructure, Co-MOF was converted into CoSe₂, providing conductive unidirectional pathways between the MoSe₂ nanosheets and carbon cloth (CC). The decoration of the MoSe₂ nanoflakes on the CoSe₂ nanowalls improved the resultant electroactive sites for the electrochemical process, and the porous 2D nanosheets of MoSe₂ allowed intimate contact with the electrolyte, and so rich reaction sites can be used more efficiently in core–shell structures which minimizes the ion diffusion length.

Hydroxides and TMSes also can constitute a heterogeneous structure to further enhance the activity. For instance, Co(OH)₂@CoSe nanorods were evaluated as electrocatalysts for both HER (overpotential of 208 mV at 20 mA cm⁻²) and OER (268 mV at 20 mA cm⁻²) at high current density in a 1 M KOH solution.¹⁵⁴ The nanorods exhibited superior electrocatalytic activity, attributed to the chemical coupling of Co–OH active sites between Co(OH)₂ and CoSe, which regulated hydrogen adsorption and desorption energy, and facilitated fast electron transfer. Additionally, the hetero-dimensional layered MoSe₂ nanodots (NDs) anchored on few-layer MoSe₂ nanosheets (NSs) (MoSe₂ HDH) via a one-pot hydrothermal route were designed and prepared as efficient electrocatalysts for HER.⁴⁰ MoSe₂ NDs with edge-abundant features and the defect-rich structure could introduce more active sites for HER. Random stacking of the flake-like MoSe₂ NSs on the surface of the supporting electrode can facilitate electron transport efficiency.

5.2. The application of conductive substrates

Conductive substrates, such as metallic plates, metallic foam, and carbon species, serve as the framework to support nanostructured active materials and current collectors, thereby increasing the active sites and facilitating charge transportation.

Damien et al. have developed a facile approach for depositing monodisperse single/few layers of MoSe₂ onto gold foil through electrochemical exfoliation. The resulting catalyst exhibited exceptional electro-catalytic activity towards the HER with a Tafel slope of 31.8mV dec⁻¹, i.e., a Pt-like activity for HER.⁸⁰ Overall, the research highlights the potential of TMSe catalysts for HER and the importance of a conductive substrate in improving performance. The substrate choice also has a significant effect on the OER catalytic performance. Ni₃Se₂ which was electrodeposited on different conducting substrates, including Au-coated glass, Au-coated Si, glassy carbon (GC),⁹³ ITO-coated glass, and Ni foam, has been synthesized and investigated for the OER activity. It was observed that it can generate 10 mA cm⁻² at an overpotential as low as 290 mV (upon annealing or using Au-coated Si as the growth substrate), and verified that the OER activity was influenced by electrodeposition parameters including the deposition time and pH of the electrochemical bath, annealing, and the nature of the substrate. In addition to metallic substrates, carbon-based, Ni foam and bio-substrate materials have been utilized to enhance conductivity and prevent the aggregation of different TMSes.

For CoSe₂, Dong et al. have reported a facile method for synthesizing CoSe₂ nanoparticles anchored on flexible carbon fiber (CF) electrodes via pyrolysis and selenization of ZIF-67 directly grown on CFs (CoSe₂/CF).¹⁵⁵ This CoSe₂/CF composite also exhibited excellent catalytic activity (95 mV at 10 mA cm⁻², Tafel slope of 52 mV dec⁻¹) and stability in alkaline media. Specifically, Kang et al. have developed an orthorhombic CoSe₂–CNT composite with an optimized morphological structure through spray pyrolysis and selenization.⁴⁹ This composite exhibits excellent catalytic activity for HER in an acidic medium. However, the bare CoSe₂ powders show moderate HER activity with an overpotential of 226 mV at 10 mA cm⁻², and Tafel slopes of 58.9 mV dec⁻¹. The high performance of the CoSe₂–CNT composite is attributed to its macroporous structure resulting from the CNT backbone, which has effectively improved the contact area of CoSe₂ active sites with the liquid medium, enhanced the H₂ removal rate, and increased electrical conductivity.

For WSe₂, to further increase the exposed active sites and electrical conductivity, Zhang et al. constructed an RGO/WSe₂ hybrid network and anchored it on graphene nanosheets.¹²⁷ Compared with bare WSe₂ nanoflowers with relatively poor HER performance, the RGO/WSe₂ hybrid demonstrated highly effective and stable electrocatalytic performance.

For NiSe, as shown in Fig. 13(a)–(h), Fu et al. have reported hetero-phase junction catalysts containing a hexagonal phase on carbon paper (CP), namely, NiSe/CP, Ni_0.85Se/CP and NiSe–Ni_0.85Se/CP.¹⁵⁶ The optimized NiSe–Ni_0.85Se/CP exhibited a remarkably higher catalytic activity for both OER and HER compared to single-phase catalysts. DFT calculations confirm that H and OH⁻ can be more easily adsorbed on NiSe–Ni_0.85Se than on NiSe and Ni_0.85Se catalysts. Tian et al. have prepared well-dispersed Ni_0.85Se nanoparticles on nitrogen-doped graphene oxide (NGO), resulting in high HER performance.⁶⁶ Among all the substrates, Ni foam (NF) is a common choice to fabricate nickel selenides for electrochemical water splitting. Zhu et al.¹⁵⁷ have synthesized NiSe₂, NiSe, and Ni₃Se₂ on NF via a facile electrodeposition route at a deposition potential of −0.35, −0.46, and −0.60 V, respectively. Such three phases of nickel selenide nanostructures exhibited clear phase-dependent electrocatalytic activities for HER and OER. Typically, NiSe₂/NF required overpotentials of only 104 mV and 279 mV to obtain the current densities of 10 mA cm⁻² for HER and 20 mA cm⁻² for OER, respectively. For the HER, the activity order is NiSe₂ > NiSe > Ni₃Se₂ and for the OER, the order is NiSe₂ > Ni₃Se₂ > NiSe. The phase engineering and utilization of NF verified the importance of the synergistic effects of electrical conductivity and ECSA of the nickel selenide based electrodes. Shi et al. have developed a solvothermal route to synthesize Ni₃Se₂ rich-grain-boundary nanowire arrays on nickel foam (Ni₃Se₂/NF). The optimized Ni₃Se₂/NF-0.4 exhibited superior OER and HER activity. Furthermore, it has been demonstrated as an efficient water electrolyzer, achieving a water-splitting current density of 10 mA cm⁻² at a cell voltage of 1.62 V.¹⁵⁸

Fig. 13 SEM images of (a) and (b) NiO/CP and (c) and (d) the NiSe–Ni_0.85Se/CP hybrid. (e) Polarization curves of NiSe/CP//NiSe/CP, Ni_0.85Se/CP//Ni_0.85Se/CP, NiSeNi_0.85Se/CP//NiSe-Ni_0.85Se/CP, and RuO₂/CP(+)//Pt–C/CP(−) pairs in a two-electrode configuration. (f) Multistep chronopotentiometric (CP) curve of the NiSe–Ni_0.85Se/CP//NiSe–Ni_0.85Se/CP electrolyzer at varying current densities. (g) Chronopotentiometric curves of NiSeNi_0.85Se/CP//NiSe–Ni_0.85Se/CP. (h) Schematic of the overall water splitting setup system using NiSe–Ni_0.85Se/CP as both cathode and anode.¹⁵⁶ Reproduced with permission from ref. 156. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (i) Formation mechanism of the macroporous MoSe₂–rGO composite with empty nanovoids created by applying a pilot-scale spray-drying process. Morphologies of the MoSe₂–rGO–L composite powders: (j) and (k) SEM images and (l) elemental mapping images.¹⁶⁰ Reproduced with permission from ref. 160. Copyright 2017, Elsevier B.V. All rights reserved.

For MoSe₂, MoSe₂/graphene hybrids have gained significant attention due to their potential in improving electron transfer efficiency. Yang et al. successfully prepared MoSe₂/RGO hybrids, which exhibited a lower overpotential of 0.05 V vs. RHE compared to previously reported MoS₂/graphene hybrids. This improvement can be attributed to the enhanced 3-D electrical contact and the presence of abundant folded edges in the MoSe₂/RGO hybrids.¹⁵⁹ In addition, the macroporous MoSe₂–rGO composite with empty nanovoids was prepared through one-step post-treatment of spray-dried powders as shown in Fig. 13(i)–(l). The MoSe₂–rGO–M composite (20 wt% rGO) exhibited a current density of 10 mA cm⁻² at a small overpotential of 0.21 V, and a Tafel slope of 57 mV dec⁻¹.¹⁶⁰ The outstanding HER performance was due to the synergistic effects of the rGO sheets and MoSe₂ nanocrystals, as well as the crumpled structure with empty nanovoids, providing a large number of active sites and facilitating high accessibility for electrolyte ions. Furthermore, the presence of rGO effectively prevented the stacking and growth of few-layered MoSe₂. Carbon nanotubes (CNTs) are another commonly used matrix for catalysts. Liu et al. reported a unique hierarchical nanostructure, where few-layered MoSe₂ nanosheets are perpendicularly grown on CNTs.¹⁶¹ This hybrid structure exhibited excellent catalytic activity for HER, with a low onset potential of −0.07 V vs. RHE, and a small Tafel slope of 58 mV dec⁻¹. The one-dimensional CNTs effectively prevented the agglomeration and restacking of MoSe₂ nanosheets, maximizing the number of exposed active sites and promoting electron transfer efficiency. Two types of substrates are adopted to regulate the catalytic performance; Kang et al. combined MoSe₂-reduced graphene (rGO) with CNTs to prepare highly porous MoSe₂–rGO–CNT powders by a spray pyrolysis process.¹⁶² MoSe₂–rGO–CNT exhibited an overpotential of 0.24 V at a current density of 10 mA cm⁻² and a Tafel slope of 53 mV dec⁻¹. The high aspect ratio of CNTs provided a porous, spherical backbone that minimized the stacking of rGO nanosheets, while the synergistic effect of CNTs and rGO minimized the growth of MoSe₂ nanocrystals. Overall, the synergy of rGO sheets, CNTs, and MoSe₂ nanocrystals, along with the unique porous structure of the material, contributed to its exceptional HER properties. In a similar vein, Jia et al. synthesized a MoSe₂/reduced graphene oxide/polyimide powder composite, which exhibited improved HER properties compared to unmodified MoSe₂.¹⁶³ Nitrogen doping into the substrate can further enhance the catalytic performance of substrates. Zhang et al. synthesized MoSe₂/N-doped reduced graphene oxide (NG) composites.⁵⁵ The composites consisted of dispersed nanoclusters of MoSe₂ nanosheets on folded NG nanosheets. Electrochemical measurements indicated that increasing the N/C ratio initially improved the activity of MoSe₂/NG. However, at high N/C ratios, an energy barrier suppressed electron transfer from NG to MoSe₂, leading to reduced activity. At low N/C ratios, the impact of the interfacial energy barrier between MoSe₂ and NG could be negligible, allowing effective electron transfer. Consequently, N-doped RGO/MoSe₂ with an intermediate N/C ratio exhibited the highest activity, with an overpotential of −0.229 V (vs. RHE) at 10 mA cm⁻² and a Tafel slope of 78.45 mV dec⁻¹. Other bio-substrates, such as bacterial cellulose-derived carbon nanofibers (CBC), have also been utilized.⁷⁶ Liu et al. employed CBC nanofibers as a substrate to facilitate the uniform growth of few-layered MoSe₂ nanosheets. The carbonized bacterial cellulose provided a 3D network for electrolyte penetration and accelerated electron transfer, resulting in fast hydrogen evolution kinetics with an onset overpotential of 91 mV and a Tafel slope of 55 mV dec⁻¹ in acidic media. Therefore, incorporating biotemplate materials offered a novel approach to develop high-performance catalysts for HER, simultaneously increasing active sites and conductivity.

5.3. Defect engineering

The introduction of vacancies into electrocatalysts is an efficient way to enhance their activity, which can tune the electronic structure, regulate active sites, and improve electrical conductivity.

Dual-native vacancies of Se and Mo can be introduced in 2H-MoSe₂ nanosheets, which alter the catalytic sites and improve the electrochemical performance.⁴⁵ These vacancies can be generated in CVD grown TMDs by controlling precursor vapor, pressure, and growth temperatures during synthesis. First-principles calculations revealed that both Se and Mo vacancies alter the catalytic sites, optimizing the basal plane and edges with the optimal free energy for HER. Additionally, the vacancies increase the number of gap states and electrons near the Fermi level, improving electron transport efficiency. Experimental results demonstrated that MoSe₂ nanosheets with a significant amount of dual-native vacancies exhibited a low overpotential and good stability for 20 h I–t testing. These findings indicate the effectiveness of defect engineering in enhancing catalytic activity.

As shown in Fig. 14, the controllable defect modulation can be achieved by engineering selenium vacancies in WSe₂ monolayer nanosheets through annealing.¹⁶⁴ The introduction of Se-vacancies created more active sites on the basal plane, allowing for favorable hydrogen adsorption. First-principles calculations were conducted to confirm the HER activity of the Se-vacancy sites. Increasing the number of Se vacancies enhanced neutral hydrogen adsorption, reduced the energy barrier, and lowered the overpotential.

Fig. 14 (a) Schematic illustration of the top (upper panel) and side (lower panel) views of WSe₂ with Se-vacancies on the basal planes, where the Se-vacancies serve as the active sites for hydrogen evolution. (b) HER free energy in the Se-vacancy range of 0–25%.¹⁶⁴ Reproduced with permission from ref. 164. Copyright 2016, The Royal Society of Chemistry.

Yin et al. introduced a novel method to simultaneously modulate the phase structure and unsaturated defects in partially crystalline 1T-MoSe₂.¹¹⁶ The synthesis process of 1T-MoSe₂ involved the reductant NaBH₄, which played a key role in forming the 1T phase. By conducting the synthesis at lower reaction temperatures, a disordered structure with numerous active defects can be obtained. The combination of the high conductivity of the 1T phase and defective surface led to a remarkable catalytic activity improvement for HER.

Overall, this part has highlighted and summarized the significance of defect engineering in enhancing the catalytic performance of electrocatalysts. The potential mechanism may be the modification of the catalytic sites and improvement of electron transport efficiency.

5.4. Designing multi-metallic selenides

In addition to developing nanohybrid structures, introducing heteroatoms also has been explored to improve the catalytic performance, which can generate new active sites and synergize with host substances. Computational studies show that the OER activity is mainly driven by the energetics of the OER intermediates (*OH, *O, and *OOH) on the surfaces, with the O to OH adsorption energy difference being the main descriptor for the observed activity trends among these materials. The foreign element can tune the local electronic structures of the host materials, and thus the energetics of OER intermediates and catalytic activity can be modulated. DFT calculations prove that the modulation of the adsorption energy of OER intermediates is attributed to the tuning of the 3d electronic structure.¹⁶⁵ This section provides a summary of recent advances in multi-metallic selenides, including ternary selenides, such as NiCo and NiSe, as well as other bimetallic and multi-metal selenides.

5.4.1. Ternary selenides. For CoSe₂, ternary necklace-like CoP_2xSe_2(1−x) nanowire arrays were obtained by simultaneously phosphorizing and selenizing Co(OH)₂ nanowires.¹⁶⁶ The activity of the nanowires varied based on the ratios of P and Se, as the P atom substituted in the ternary system. The ternary CoP_2xSe_2(1−x) nanowires exhibited high electrochemical HER activity in both acidic and alkaline media, achieving a current density of 10 mA cm⁻² at overpotentials of 70 mV and 98 mV, respectively. As shown in Fig. 15(a) and (b), Dutta fabricated mesoporous CoSSe through partial surface selenization, which can attain a current density of 100 mA cm⁻² at an overpotential of 160 mV vs RHE in acidic media and a low Tafel slope of 52 mV dec⁻¹ and a high exchange current density (j₀) of 70 μA cm⁻².¹⁶⁷ This study highlighted the significant role of sulfur, selenium, and S–Se in the HER process. The introduction of the conjugate histidine inhibited cobalt species from participating in the HER process, while HAuCl₄·3H₂O was used to block sulfur. The results demonstrated that the cobalt-blocked electrode exhibited the best HER activity, while the sulfur-blocked electrode showed comparatively poor activity. Furthermore, the activity declined further for the sulfur and cobalt blocked electrode.

Fig. 15 (a) Schematic representation of the synthesis of the mesoporous CoSSe catalyst. (b) Schematic structural representation of H adsorption on different {001} surface elements: (I) on Co of CoS₂, (II) on S of CoS₂, (III) on Co of CoSSe, and (IV) on S of CoSSe.¹⁶⁷ Reproduced with permission from ref. 167. Copyright 2019, American Chemical Society. (c) Schematic illustration outlining the fabrication of the W(Se_xS_1−x)₂ nanoporous architecture. (d) and (e) TEM images of the W(Se_xS_1−x)₂–15 NPA. (f) HRTEM image of the region marked with a yellow dashed box in panel (e). The light blue boxes represent lattice mismatch and defects. Fast Fourier transform analysis performed on the regions marked by (g) orange and (h) light green dashed boxes.¹⁶⁸ Reproduced with permission from ref. 168. Copyright 2017, American Chemical Society.

For WSe₂, heteroatom doping could engineer the phase to metallic 1T and fabricate defects simultaneously. For example, as shown in Fig. 15(c)–(h), Liang et al. developed the W(Se_xS_1−x)₂ nanoporous architecture (NPA) by substituting Se with S.¹⁶⁸ This ternary W(Se_xS_1−x)₂ NPA exhibited significant advantages for high-efficiency HER. The nanoporous morphology provided more electrochemically active sites for water splitting. The lattice mismatch and disordering in the mix-phased W(Se_xS_1−x)₂ film introduced additional defects, enhancing the catalytic activity. The formation of a conducting 1T phase in the strained W(Se_xS_1−x)₂ facilitated electron transfer during catalysis. As a result, the as-prepared W(Se_xS_1−x)₂ NPA achieved an onset overpotential of 45 mV and a Tafel slope of 59 mV dec⁻¹.

For NiSe₂, the introduction of nitrogen (N) in conductive NiSe₂ can promote the redistribution of charge densities and enrich the active sites, effectively enhancing its HER activity. Wang and co-workers developed a self-supported 3D structured nitrogen-doped nickel selenide (N-NiSe₂) by in situ growing NiSe₂ on nickel foam under an ammonia atmosphere (N-NiSe₂/NF).¹⁶⁹ This N-NiSe₂/NF electrode exhibited excellent electrocatalytic activity for the HER across a wide pH range (pH = 0.3, 7, and 14). In 1.0 M KOH, the N-NiSe₂/NF electrode demonstrated a small overpotential of 86 mV to achieve a current density of 10 mA cm⁻². The electrocatalytic effects of Mo in pure NiSe for the HER and OER have been explored.¹⁷⁰ Coral-like structured Ni_1−xMo_xSe has been prepared. The distinct morphology of the Ni_0.5Mo_0.5Se material exhibited enhanced electrochemical activity, following the Volmer–Heyrovsky mechanism. This work investigated the influence of the electrocatalytic properties of Mo in pure NiSe towards the HER and OER. Sun et al. synthesized an atomically thin Ni–Se–S based hybrid nanosheet (NiSe_1.2S_0.8).¹⁷¹ Theoretical results revealed that the hybrid electrocatalyst with sulfur (S) incorporation exhibited optimal adsorption free energy of hydrogen (ΔG_H*).

For MoSe₂, various strategies and doping techniques have been developed to enhance the catalytic activity of MoSe₂ for the HER. These strategies include doping other elements (N, B, S) to narrow the bandgap of MoSe₂ and increase active sites on the basal planes.^{10,47,172,173} For instance, as shown in Fig. 10, Tu et al. synthesized high-quality N-doped MoSe₂/TiC-C shell/core arrays via a facile hydrothermal plus annealing process, combining the phase and morphology engineering strategies together.¹⁷⁴ N-doping not only decreased the band gap of MoSe₂, but also reduced the energy barrier promoting easier phase transition from 2H to 1T. A systematic computational study revealed that B dopants can well function in lowing the formation energy in MoSe₂, suggesting the feasibility of B doping in activating the catalytic behavior of MoSe₂. Experiments have demonstrated that the doping of B into MoSe₂ could lead to the narrowed band and increased electrical conductivity, largely facilitating electron movement and charge transfer. Especially with the doping of more B atoms into the MoSe₂ lattice, the bands of MoSe₂ moved closer to the Fermi level, activating the conductivity of the basal plane.⁴⁷ The incorporation of S into MoSe₂ has also been reported, such as high-percentage 1T-phase, single-layer ternary nanodots (MoSSe) with high-density active edge sites.¹⁷⁵ The synthesis of MoSSe nanodots with high-density active edge sites promoted fast electron/charge transfer and resulted in an alloying effect between S and Se atoms, as well as the presence of Se vacancies on MoSSe nanodots. These factors contributed to the enhancement of catalytic activity on the basal plane. The MoSSe ternary nanodots exhibited significantly improved electrochemical HER performance, characterized by a low overpotential of −140 mV at a current density of 10 mA cm⁻², a Tafel slope of 40 mV dec⁻¹, and excellent long-term durability. Additionally, doping with Zn⁶⁹ and introducing Pt nanoparticles¹⁷⁶ have been investigated as effective strategies to improve the catalytic activity of MoSe₂. DFT calculations showed that the ΔG_H* of the Zn-doped site was −0.02 eV, and the adjacent Se site changed from 1.93 eV in pure MoSe₂ to −0.15 eV. The final Zn-doped MoSe₂ possessed the smallest onset potential (182 mV) and the lowest Tafel slope (58 mV dec⁻¹). The final MoSe₂–Pt catalyst exhibited an onset overpotential of 100 mV and a Tafel slope of ∼85 mV dec⁻¹.

Nickel–cobalt selenides (Ni–Co–Se) or their compounds with other non-metal materials are commonly used as multi-metal catalysts. Cobalt selenides and nickel selenides both can be regarded as high performance catalysts for HER and OER. The combination of active Co and Ni can further improve electrochemical activity and kinetics by modifying the electronic structure. The combined contribution of Ni and Co elements to the electronic structure of bimetallic selenides reveals the strong interaction of bimetals toward enhanced electrical conductivity. The synergistic effect between cobalt and nickel selenides could further improve the conductivity and activity, which is believed to contributed to their electrocatalytic activity for the OER. This is because OER is described as a kinetically sluggish process involving a multistep proton-coupled electron transfer. The intrinsically metallic Co_0.85Se can be further promoted by Ni doping. The metallic nature of Co_0.85Se, which is in sharp contrast to the semiconducting nature of other cobalt selenides such as CoSe and Co_0.85Se, makes it useful as an electroactive material or support material to non-conductive OER compounds.

Metallic Co_0.85Se and (Ni,Co)_0.85Se nanotube arrays were synthesized on a carbon fabric collector (CFC) for the OER.¹⁷⁷ The (Ni,Co)_0.85Se nanoarrays exhibited superior performance in an alkaline medium. This improvement can be attributed to the better electrical conductivity and higher defect concentrations, which serve as active sites for catalysis. Nanostructured Ni-doped cobalt selenides, with their excellent conductivity, corrosion resistance, and promising OER activity, can be utilized as advanced OER catalysts or as backbone materials for integrating insulating OER active materials in hybrid catalysts. A Co-doped nickel selenide (a mixture of NiSe₂ and Ni₃Se₄)/C hybrid nanostructure supported on Ni foam (Co–Ni–Se/C/NF) was prepared using the organic framework of zeolitic imidazolate framework-67 (ZIF-67) as a precursor.¹⁷⁸ The as-obtained catalyst exhibits excellent catalytic activity for the OER at a potential of 275 mV at 30 mA cm⁻², as well as efficient catalysis for the HER at a potential of 90 mV at 10 mA cm⁻². Furthermore, this hybrid nanostructure demonstrated good durability and achieved current densities of 10 and 30 mA cm⁻² at potentials of 1.6 and 1.71 V, respectively, when used as both cathode and anode for overall water splitting in basic media.

Other catalysts based on Co-doped nickel selenides have been developed for the HER and OER in water splitting. Each study has presented a different synthesis method and highlighted the performance and characteristics of the catalysts. Ao et al. synthesized yolk-shelled Ni–Co–Se nanocages on carbon fiber paper (Y–S Ni–Co–Se/CFP), which exhibited remarkable electrocatalytic activity and long-term stability for both HER and OER. This work provides another way to design MOF-derived functional materials with interesting nanostructures for diversified applications.¹⁷⁹ Zhang et al. regulated the electronic property and surface structures of cobalt-doped Ni_0.85Se chalcogenides and found that Co_0.1Ni_0.75Se showed the highest catalytic activity for HER.¹⁸⁰ They further improved its performance by using r-GO as a support (Co_0.1Ni_0.75Se/rGO), which exhibited an overpotential of 103 mV at 10 mA cm⁻² and a Tafel slope of 43 mV dec⁻¹ and the current tended to be stable for 30 h i–t testing. Gao et al. designed CoNiSe₂ heteronanorods decorated with layered-double-hydroxide (LDH) nanosheets on nickel foam (CoNiSe₂@CoNi-LDHs/NF), presenting enhanced HER kinetics due to balanced water dissociation and H desorption at the selenide–LDH interfaces.⁷² DFT calculations show that after Co-doping, CoNiSe₂@Co-LDHs and CoNiSe₂@Ni-LDHs presented a visibly downshifting d-band center of −2.91 and −2.84 eV, respectively, compared to bare CoNiSe₂ (−2.74 eV), indicating the electron transfer through CoNiSe₂–LDH interfaces. In addition, the H₂O chemisorption energy on the interface was also reduced to −1.83 and −1.55 eV after being decorated by Co- and Ni-LDHs. The CoNiSe₂@CoNi-LDHs/NF nanocomposites showed the best activity, with an overpotential of 106 mV at 10 mA cm⁻², a Tafel slope of 74 mV dec⁻¹ and a high value of ECSA of 186.2 cm². Wang et al. used Prussian blue analogues (PBAs) as precursors to synthesize penroseite (Ni,Co)Se₂ nanocages anchored on 3D graphene aerogel ((Ni,Co)Se₂–GA). Consequently, (Ni,Co)Se₂–GA showed an overpotential of 128 mV at −10 mA cm⁻² and a Tafel slope of 79 mV dec⁻¹ for water splitting with a low voltage of 1.6 V at 10 mA cm⁻² and possessed outstanding durability.¹⁸¹ Yu et al.¹⁸² designed novel NiCoSe₂ nanosheet networks grown on carbon cloth (CC) with crimped nanosheet configuration via an electrodeposition technique. NiCoSe₂ exhibits superior electrocatalytic performance and robust durability toward overall water splitting, especially toward oxygen evolution reaction (OER) with a low overpotential of 255.8 mV to deliver 10 mA cm⁻² current density. Xiao et al. have prepared (Ni, Co)_0.85Se NSAs/NF for super overall water splitting.¹⁸³ The NF substrate transforms from a hydrophobic surface to a hydrophilic one after modification with porous structure of (Ni,Co)_0.85Se NSAs, which perpendicularly grown on the NF. The catalysts grown on the substrate in situ are more likely to adsorb droplets and promote traps of electrolyte ions and access to active sites. The as-obtained (Ni,Co)_0.85Se NSAs exhibit a low overpotential of 169 mV at a current density of 10 mA cm⁻² for the HER and an overpotential of 287 mV at a current density of 20 mA cm⁻² for the OER, and superior performance toward overall water splitting with a cell voltage as low as 1.65 V at a current density of 10 mA cm⁻². A Ti plate can be used as the conductive substrate; Liu et al.¹⁸⁴ have reported Co_0.13Ni_0.87Se₂/Ti on a conductive Ti plate for both HER (10 mA cm⁻², 64 mV) and OER (100 mA cm⁻², 320 mV) in strongly basic media (1.0 M KOH). A voltage of only 1.62 V is required to drive 10 mA cm⁻² for the two-electrode alkaline water electrolyzer. Chen et al.¹⁸⁵ have prepared multilayered CoNiSe₂ nanorods (NRs) that grow radially from the substrate to form hierarchical sea urchin-like microstructures. The overpotentials required to deliver a current density of 100 mA cm⁻² are as low as 307 and 170 mV for the OER and HER, respectively; for water splitting reaction, the overpotential required is 1.591 V to achieve a current density of 10 mA cm⁻². These studies demonstrated the synthesis of various catalysts with improved performance for HER and OER, highlighting the importance of composition, structure, and support materials in designing efficient catalysts for applications.

Nickel-based selenide materials are promising candidates for oxygen evolution reaction due to their low cost and excellent performance. The incorporation of Fe into the nickel-based selenide can facilitate charge transportation, expand electrochemically active surface and enhance desorption of oxygen. In recent years, NiFe hybrid selenides have been proposed as exciting OER catalysts owing to the sufficient collaboration of nickel and iron. Based on this, various types of NiFe hybrid selenides have been developed as efficient catalysts for the OER in water splitting. One study has reported a nanostructured nickel iron diselenide (Ni_xFe_1−xSe₂), which acted as a precursor and then in situ converted into Ni_xFe_1−xSe₂-derived oxide (Ni_xFe_1−xSe₂-DO) under a stable current density of 5 mA cm⁻². As shown in Fig. 16(a)–(e), Ni_xFe_1−xSe₂-DO gave an ultra-low overpotential (195 mV at 10 mA cm⁻²). This work emphasized the importance of identifying the active species of OER catalysts.¹⁸⁶ The polyvinyl-pyrrolidone (PVP)-decorated Ni–Fe diselenide hollow nanoparticles (P-NFSHPs) with strong hydrophilicity have been explored as an electrocatalyst for OER. The strong polarity of lactam groups in PVP introduced a highly hydrophilic surface to the selenide catalyst. The as-prepared P-NFSHSs with enhanced wettability displayed enhanced OER performance with a low overpotential of 255 mV (vs. RHE) at 10 mA cm⁻², a low Tafel slope of 56 mV dec⁻¹ and decent long-term stability (only 13 mV degradation after 12 h electrolysis).¹⁸⁷ 3D porous Ni-FeSe with a rose-like microsphere architecture directly grown on Ni foam has also been developed with a two-step pathway.¹⁸⁸ The unique 3D mesoporous rose-like morphology facilitated mass and electron transport as well as O₂ bubble release. The resulted Ni_0.76Fe_0.24Se₄₅ exhibited superior OER performance. Du et al. have fabricated hierarchical (Ni,Fe)₃Se₄ ultrathin nanosheets which possess a low overpotential of 225 mV and a small Tafel slope of about 41 mV dec⁻¹ for OER, relying on abundant and accessible catalytically active sites,¹⁸⁹ facile charge transfer and a high specific surface area.

Fig. 16 (a) Illustration of the synthetic route to Co–Ni–Se/C/NF. (b) Polarization curve of Co–Ni–Se/C/NF in a three-electrode system in a wide potential window (−0.28 to 1.65 V vs RHE). (c) Polarization curves of RuO₂‖Pt/C, Co–Ni–Se/C/NF and NF. (d) Chronopotentiometric curve for Co–Ni–Se/C/NF under a constant current density of 50 mA cm⁻² for 100 h. (e) Energy efficiency of the Co–Ni–Se/C/NF electrolyzer as a function of current density.¹⁸⁶ (f) Schematic illustration of the synthesis strategy of the heterostructures. SEM images of (g) NiCH@CC, (h) NiFe-PBA@NiCH@CC, and (i) NiFeSe@NiSe|O@CC. Typical low-magnification SEM images of (j) NiFe-PBA@NiCH@CC. Electrochemical HER and OER performances of NiFeSe@NiSe|O@CC along with the NiFeSe nanocubes, NiSe|O@CC, CC, Pt/C@CC, and RuO₂@CC for comparison. (k) HER polarization curves in 1 M KOH. (l) Corresponding Tafel plots of the electrocatalysts in (k). (m) Time dependence of current density under static overpotential showing the durability of the electrocatalysts over 50 h. (n) OER polarization curves in 1 M KOH. (o) Corresponding Tafel plots of the electrocatalysts in (n). (p) Time dependence of current density under static overpotential showing the durability of the electrocatalysts over 50 h.¹⁹¹ Reproduced with permission from ref. 191. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Fe-doped NiSe (a mixture of hexagonal NiSe and orthorhombic NiSe) nanorod/nanosheet hierarchical arrays have been formed, which exhibited remarkable OER activity and decent HER activity.¹⁹⁰ The self-templated pseudomorphic selenization method has been adopted for transforming nickel iron Prussian blue analogue (NiFe-PBA) and nickel carbonate hydroxide (NiCH) precursors into a porous and defect-enriched nickel iron selenide hybrid on carbon cloth (NiFeSe@NiSe|O@CC). The final product exhibited highly efficient and stable electrocatalytic activity toward HER and OER, as well as overall water splitting in alkaline media with a low overpotential of 1.56 V, as shown in Fig. 16(f)–(p).¹⁹¹ Wang et al.¹⁹² reported the porous nickel–iron bimetallic selenide nanosheets ((Ni_0.75Fe_0.25)Se₂) on carbon fiber cloth (CFC) by selenization of the ultrathin NiFe-based nanosheet precursor. The as-prepared three-dimensional oxygen evolution electrode exhibits a small overpotential of 255 mV at 35 mA cm⁻² and a low Tafel slope of 47.2 mV dec⁻¹ and maintains high stability during a 28 h measurement in alkaline solution. Umapathi et al.¹⁹³ have reported FeNi₂Se₄ nanoparticles supported on nitrogen doped reduced graphene oxide (FeNi₂Se₄–NrGO) as an efficient catalysis for oxygen evolution under alkaline conditions. While FeNi₂Se₄ nanoparticles themselves show good catalytic activity for water oxidation, the constructed hybrid nanocomposite with NrGO as the supporting matrix shows enhanced catalytic activity with a small overpotential of 170 mV @ 10 mA cm⁻², a small Tafel slope of 62.1 mV per decade, and high current density. This study verified the effect of anion coordination on the catalytic performance, as well as the synergistic role of nanoscale interactions between the catalyst particles and graphene matrix in enhancing catalytic activity.

Other bimetallic selenides have been reported for water electrolysis; for example, a Fe–Co-based PBA precursor was designed by self-assembly and subsequently selenized to (Fe–Co)Se₂. The product of (Fe–Co)Se₂ only required an overpotential of 251 mV to drive a current density of 10 mA cm⁻² in the OER system, and 90 mV was required at a current density of 10 mA cm⁻² toward the HER activity, along with a low Tafel slope of 47.6 and 58.7 mV. Furthermore, the catalytic performance of Se-enriched Co_1−xFe_xSe₂ catalysts on nickel foam can be adjusted according to the Co/Fe ratios in the catalyst. The Co_0.4Fe_0.6Se₂ nanosheets not only exhibited superior OER performance, at a current density of 10 mA cm⁻² with an overpotential of 217 mV, but also possessed ultrahigh durability with a dinky degeneration of 4.4% even after a 72 h fierce water oxidation test in alkaline solution.¹⁹⁴

Fe, Co, and Ni are common heteroatoms to enhance the electrocatalytic active sites, improve the electronic conductivity, and modulate the intermediates adsorption energy of TMSes. In addition to Fe, Co, and Ni, other transition metal atoms have also been explored to optimize the electrocatalytic properties of TMSes. Zhao et al.¹⁹⁵ have prepared Mn-regulated cobalt selenide nanosheets. With the modulation of Mn, the tailored atomic disorder, tuned electronic structure, together with optimized electrical conductivity could be simultaneously achieved in CoMn selenide. The engineered structural and electrical properties led to the effective generation of active species and promoted the reaction rate during the OER process, in accordance with the high catalytic activity. The overpotential of (CoMn)Se₂ catalysts was 0.27 V at a current density of 10 mA cm⁻², much lower than that of CoSe₂ catalysts and the state-of-the-art IrO₂. MoCoSe_x encapsulated in N-doped hollow carbon nanospheres (MoCoSe_x@NC) has been reported and it exhibited an excellent low overpotential of 60 mV at 10 mA cm⁻² and great stability for HER.¹⁹⁶ V_0.86Co_0.14Se₂ was synthesized by doping cobalt into vanadium diselenide (VSe₂) nanosheets on a carbon cloth (CC), presenting a low overpotential of 230 mV at 10 mA cm⁻², and a small Tafel slope of 63.4 mV dec⁻¹.¹⁹⁷ Ternary refractory metal selenides (MWSe_x; M = Fe, Co, Ni and Mn) have been formed by introducing transition metals into tungsten selenide. The catalytic activity differences of ternary metal selenides have been compared, and they were ranked in the order of Fe ≈ Mn, Co < Ni. The addition of Ni and Co into the tungsten selenide made them more active catalysts compared to pristine WSe_x.¹⁹⁸ All these catalysts have the potential for practical catalytic applications in water electrolysis.

5.4.2. Multi-metal selenides. Single-cation doping of TMSes to form binary TMSes can improve the catalytic performance compared to the undoped structure, and the improvement is not significant on account of the limited change in the electronic structure. The dual cation incorporation can distort the lattice and induce stronger electronic interaction, leading to increased active site exposure and optimized adsorption energy of reaction intermediates. For example, the obtained Fe_0.09Co_0.13–NiSe₂ porous nanosheet electrode shows an optimized catalytic activity with a low overpotential of 251 mV for OER and 92 mV for HER (both at 10 mA cm⁻² in 1 M KOH).¹⁷ When used as bifunctional electrodes for overall water splitting, the current density of 10 mA cm⁻² is achieved at a low cell voltage of 1.52 V. Chi et al.¹⁹⁹ have synthesized NieFeCo selenides (NiFeCoSe_x) anchored on carbon fiber cloth (CFC) as an efficient electrocatalyst for OER (overpotential: 150 mV at 10 mA cm⁻², Tafel slope: 85 mV dec⁻¹).

Multi-metal selenides generally exhibit superior catalytic performance compared to their monometallic counterparts. This can be attributed to their enhanced electrical conductivity, higher structural stability, and favorable Gibbs free energy of hydrogen absorption (ΔG_H*). The ternary selenides of Fe–Cu exhibit reduced OER activity compared to their pure parent compounds. However, with the addition of the Co dopant, all Fe–Co–Cu quaternary selenides exhibited enhanced catalytic activity, with lower overpotential and lower Tafel slopes. The relative amounts of Fe and Cu in the catalysts play a significant role in catalytic activity, with increased amounts of either Fe or Cu leading to improved activity. The optimal catalyst, (Fe_0.48Co_0.38Cu_0.14)Se, required a low overpotential of 256 mV to achieve 10 mA cm⁻² and showed a favorable Tafel slope of 40.8 mV dec⁻¹. The enhanced catalytic activity is attributed to electron cloud delocalization and the formation of d-bands among the transition metal sites, as shown in Fig. 17(a)–(d).²⁰⁰ Multi-metal iron–nickel–cobalt selenide (Fe_0.37Ni_0.17Co_0.36Se) has been proved to be superior to the ternary nickel cobalt selenide in terms of conductivity and electrocatalytic activity, with an overpotential of −179 mV at 10 mA cm⁻² and a Tafel slope of 115.3 mV dec⁻¹.⁶⁵ The OER catalytic activity of Co–Ni–Cu selenides in alkaline medium is affected by transition metal doping, which is sensitive to the concentration of Cu in the catalysts. An increase in Cu concentration leads to an increase in activity. However, beyond a certain limit, a higher Cu concentration results in a decrease in catalytic efficiency. The Cu₃Se₂ structure shows excellent OER catalytic activity, with a low overpotential of 326 mV at 10 mA cm⁻². The (Co_0.21Ni_0.25Cu_0.54)₃Se₂ thin film also exhibited excellent OER catalytic activity (272 mV, 10 mA cm⁻²). Compared to ternary and binary selenides, quaternary compositions exhibit higher catalytic activity, indicating a synergistic effect of transition metal doping in enhancing catalytic activity, as shown in Fig. 17(e)–(h).²⁰¹ Furthermore, the catalytic activity of mixed metal selenides comprising various compositions of Fe, Co, and Cu selenides has been systematically investigated.

Fig. 17 (a) Schematic of combinatorial electrodeposition. (a) Ternary phase diagram for exploring the compositions of the mixed-metal selenide films examined in this work. Crossing vertices represent compositions of the precursor electrolyte with respect to the relative ratio of the corresponding metals. The black circle shows a typical example. (b) Typical experimental set-up for electrodeposition. Contour plots of overpotential η (in units of V) (c) at the onset of OER activity and (d) with a current density of 10 mA cm⁻² for the entire Ni–Fe–Co trigonal phase space.²⁰⁰ Reproduced with permission from ref. 200. Copyright 2019, American Chemical Society. Contour plots of onset overpotential (in units of V) (e) and overpotential (in units of V) (f) at a current density of 10 mA cm⁻² for the entire Co–Ni–Cu trigonal phase space. The color gradient corresponds to the overpotential measured in volts. (g) Polarization curves of (Co_0.21Ni_0.25Cu_0.54)₃Se₂ in comparison to the binary selenide films. (h) SEM image of the (Co_0.21Ni_0.25Cu_0.54)₃Se₂ film and the inset is a higher magnification image.²⁰¹ Reproduced with permission from ref. 201. Copyright 2019, The Royal Society of Chemistry.

Self-supported porous crystalline Zn–Ni–Co/Cu selenide microball arrays have been constructed through a series of steps.²⁰² Tubular Cu(OH)₂ arrays were first fabricated on carbon cloth and then covered with ultrathin Zn–Co–Ni layered hydroxide to form a core–shell structure. The Zn–Co–Ni layered ternary hydroxides were then deposited around the tubular arrays. Through selenization, the morphology changed from amorphous tubes to highly crystalline microballs, resulting in nanoporous ZnNi_0.5Co_0.5Se₂/Cu_1.8Se microballs. These advanced electrode materials possessed high conductivity, large specific surface areas, as well as multiple redox and electrocatalytic active sites for the OER. They exhibited a remarkable catalytic current of 325 mA cm⁻² at 1.6 V vs. RHE for OER in alkaline solutions. The catalytic activity of these materials is attributed to the synergistic effect of Ni, Co, Zn, and Cu redox sites within the well-crystallized cubic structure of ZnNi_0.5Co_0.5Se₂. Furthermore, the materials exhibit chemical stability and good mechanical strength, enabling them to withstand undesirable etching and accommodate volume changes during cycling.

Hierarchical trimetallic hydroxides on Ni foam can be selenized to obtain Ni–Co–Fe–Se@NiCo-LDH, exhibiting excellent electrocatalytic performance for overall water splitting in alkaline medium; it required an overpotential of 286 mV to deliver a current density of 100 mA cm⁻² for OER and 113 mV at a current density of 10 mA cm⁻².²⁰³ The catalyst required low overpotentials for both OER and HER. Additionally, the material showed excellent overall water splitting performance with a low cell voltage of 1.55 V at 10 mA cm⁻². DFT calculations indicated that the presence of Co-NiSe₂ accelerated hydrogen production kinetics, while Fe₇Se enhanced material conductivity. The synergistic effect of these components in the Ni–Co–Fe–Se@NiCo-LDH catalyst led to enhanced hydrogen production activity.

The mixed metal selenides generally show improved catalytic activity compared to their pure parent compounds. The addition of transition metal dopants, such as Cu, Co, Fe, Ni, and Zn, enhanced the catalytic activity by promoting electron cloud delocalization, improving charge transport resistance, and increasing film conductivity. Proper transition metal doping can further increase the quantity of active sites and modulate the reaction kinetics by regulating OH adsorption on the electrocatalyst surface and modulating the local electron density near the active sites. Furthermore, the highly occupied d-levels improve the electrical conductivity within the matrix to accelerate charge transfer on the electrocatalyst surface. The synergistic effects arising from the combination of different metal selenides also contribute to the overall improved catalytic activity. These findings offer valuable insights into the development of catalytic properties, which hold significant practical implications for application in electrocatalytic water splitting.

6. Conclusion and outlook

In this review, the basic principle of electrocatalytic water splitting and synthetic methods of TMSe-based catalysts have been firstly demonstrated, including LPE, CVD, hydrothermal/solvothermal technique, plasma-assisted selenization method, electrochemical and other synthetic methods. TMSes possess metallic properties due to the presence of Se atoms, which contribute to their good conductivity as metal selenides. Several TMSe-based catalysts for water splitting have been systematically discussed, such as cobalt selenides, molybdenum selenides, tungsten selenides, iron selenides, nickel selenides, copper selenides and other selenides. Moreover, benefiting from the excellent electrical structure and rich active sites, many strategies such as the construction of a heterogeneous structure, use of conductive substrates, designing multi-metallic selenides, and defect engineering are applied in designing cost-effective TMSe-based electrocatalysts.

The scalable and cost-effective production of clean and sustainable fuel alternatives, specifically hydrogen through electrochemical water decomposition, is an urgent goal due to the increasing energy crisis and environmental pressure. TMSes have emerged as promising electrocatalysts for water splitting, as they are abundant and can replace noble metal compounds. However, the performance of TMSe-based electrocatalysts, especially in terms of high-current-density and long-term stability, requires further improvement to meet the demands of industrial applications. Therefore, there are many potential directions for future research in this field.

6.1. Designing high performance catalysts

High performance catalysts are crucial for achieving energy conservation and emission reduction. The development of a facile, low-toxicity, and energy-efficient synthetic approach is incredibly important to realize large-scale production and application. Although various factors such as composition, atomic arrangement, morphology, elemental chemical states, surface charge transfer efficiency, and defects have been demonstrated to enhance catalytic activity, there is still room for improvement in terms of high current density and industrial applications. Furthermore, HER and OER are normally conducted in strong acidic or basic media, so the physical, chemical, and electrochemical stability of TMSe-based catalysts under such harsh conditions have a significant effect on their electrocatalytic performance. To maintain the high catalytic activity and long-term stability of TMSe-based catalysts, it is also important to increase the stability of the nanostructured TMSes through methods such as coating, encapsulating, anchoring, 3D substrate supporting, etc. These approaches can effectively protect the catalysts from corrosion and structural collapse in the electrolyte during water splitting.

6.2. In-depth understanding of the reaction mechanism

In addition to designing innovative and efficient TMSe-based electrocatalysts, it is essential to investigate the physical (conductivity, electronic structure, etc.) and chemical states (atomic arrangement, atomic concentration, etc.) of the TMSe surface, as well as their evolution during long-term water splitting. It is imperative to determine the relationship between the surface states and electrochemical properties, in order to find the real active sites and gain comprehensive understanding of the reaction mechanism. In conclusion, the development of advanced technologies and various characterization techniques, such as DFT calculations, and in situ characterization methods such as synchrotron radiation, Raman X-ray absorption spectroscopy, Fourier-transform infrared spectroscopy, atomic force microscopy, and transmission electron microscopy will further guide mechanism studies and the synthesis of the latest electrocatalysts based on TMSes.

6.3. Commercial adoption and prospects

TMSe-based electrocatalysts with high current density are crucial for industrial applications. There is a need for the development of economically viable and environmentally friendly commercial processes for their mass production. The U.S. Department of Energy (DOE) also states that the current density requirement for the central proton exchange membrane (PEM) water electrolysis would reach 1.66 V at 1600 mA cm⁻² in 2040.²⁰⁴ The design and preparation of some high-performance HER, OER, and overall water splitting catalysts based on TMSes have been developed. However, there is still significant room for improvement since most of the work has been conducted in the laboratory. Economic and convenient mass production is vital to the development of commercial TMSe based catalysts, and it is still a challenge to scale up to the production level. Developing commercial processes that are both cost-effective and environmentally sustainable is significant to make this technology viable for industrial use.

TMSes have shown potential as electrodes in energy utilization and water splitting devices, but their applications should be continuously extended. Bandgap engineering of TMSes can be applied for photocatalysis, photodetectors, and photo-electrochemical sensors, which can be further regulated by heteroatom doping, atomic layer regulation, and heterojunction constructing. Appropriate modulation of the electronic structure of TMSes may render them desirable for applications such as CO₂ reduction, nitrogen reduction, environmental control processes, sensors, devices (ion batteries, supercapacitors), and dye-sensitized solar cells.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 22175060, 21975067) and Natural Science Foundation of Hunan Province of China (2021JJ10014 and 2021JJ30092).

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