Nano Energy 78 (2020) 105234

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Nano Energy

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Recent advance and prospectives of electrocatalysts based on transition metal selenides for efficient water splitting

Xiang Peng a, Yujiao Yan a, Xun Jin a, Chao Huang b, Weihong Jin b,c, Biao Gao b,d,**, Paul K. Chu b,* a Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan, 430205, China b Department of Physics, Department of Materials Science and Engineering, Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China c Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan University, Guangzhou, 510632, China d State Key Laboratory of Refractories and Metallurgy and Institute of Advanced Materials and Nanotechnology, Wuhan University of Science and Technology, Wuhan, 430081, China

ARTICLE INFO ABSTRACT

Keywords: Electrochemical water splitting comprising the hydrogen evolution reaction (HER) and oxygen evolution reac­ Transition metal selenide tion (OER) plays critical role in energy conversion technology that transfers renewable electricity to hydrogen Hydrogen evolution reaction fuel and the proper catalysts are crucial to efficient electrochemical water splitting. Transition metal selenides Oxygen evolution reaction (TMSes) are potential electrocatalysts for both HER and OER due to the special layered structure, relatively Water splitting narrow bandgap, unique morphology, and low cost. However, their electrocatalytic HER and OER properties are Electrocatalysis still far from satisfactory from the standpoint of commercial implementation, especially the catalytic activity and durability for large charge densities in alkaline media. These drawbacks arise from the sluggish water dissoci­ ation kinetics, surface oxidization, and structure degradation. In this review, recent advance of TMSes is reviewed comprehensively from the perspectives of HER, OER, and overall water splitting. The electrochemical characteristics of TMSes are discussed and organized according to the metal cation species in single-metal TMSes and multi-metal TMSes. The composition and structural engineering of TMSes are summarized. Finally, the challenges and opportunities confronting TMSes-based electrocatalysts in advanced HER, OER and other elec­ trocatalytic applications are discussed.

1. Introduction However, the natural scarcity and high cost of noble metals have hin­ dered wider industrial adoption [6,7] and therefore, there are extensive Hydrogen which causes zero environmental pollution and has a high efforts to identify active, earth-abundant, and cost-effective materials gravimetric energy density is the ideal substitute for traditional fossil composed of transition metals to substitute for precious-metal-based fuels [1,2]. To produce pure hydrogen, water electrolysis (2H2O(l)→ electrocatalysts for large-scale commercial electrochemical water o 2H2(g) + O2(g), ΔE ≈ 1.23 V vs. RHE) is a promising technique. It splitting. consists of the hydrogen evolution reaction (HER) and oxygen evolution Recently, transition metals [8,9] and their hydro/oxides [10–12], reaction (OER) [3,4]. In common electrolysis systems, both HER and nitrides [13,14], carbides [15–17], phosphides [18,19], sulfides [20, OER are impeded by the high reaction overpotentials. So far, nano­ 21], borides [22], etc. have been explored as promising electrocatalysts structures composed of Pt, Pd, Ir, Ru, and their alloys and compounds for both HER and OER. For example, our group has developed a com­ have been studied as HER and OER catalysts due to the high efficiency posite catalyst comprising highly conductive vanadium nitride nano­ and balanced adsorption energy of the reaction intermediates [5]. sheets dispersed with metallic cobalt particles for OER [8] and more

* Corresponding author. ** Corresponding author.Department of Physics, Department of Materials Science and Engineering, Department of Biomedical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China E-mail addresses: [email protected] (B. Gao), [email protected] (P.K. Chu). https://doi.org/10.1016/j.nanoen.2020.105234 Received 19 June 2020; Received in revised form 22 July 2020; Accepted 25 July 2020 Available online 5 August 2020 2211-2855/© 2020 Elsevier Ltd. All rights reserved. X. Peng et al. Nano Energy 78 (2020) 105234 recently, we have prepared a hierarchical structure consisting of accelerates the HER kinetics of MoSe2. Introduction of two foreign cat­ nickel-doped amorphous iron phosphide nanoparticles, porous titanium ions can accommodate the crystal lattice change and iron-doped binary nitride nanowire arrays, and carbon cloth (CC) for advanced HER [18]. metal selenides have more active sites with the metal-OH bond to Among the different types of materials, transition metal chalcogenides improve the OER activity [36,37]. Recently, TMSes including have outstanding catalytic characteristics, high robustness, and unique single-metal selenides, multi-metal selenides, and their composites have capabilities that can accelerate OER [23,24] and HER [25,26]. In fact, been investigated extensively for HER, OER, and overall water splitting transition metal selenides (TMSes) share a similar structure with the applications, but there is a lack of a comprehensive review and summary corresponding sulfides but have higher electrical conductivity. For of these recent developments which will be very useful to researchers instance, MoSe2 shows higher intrinsic electrical conductivity than MoS2 working on electrochemical water splitting. because selenium is more metallic [27]. Therefore, TMSes are projected In this review, the crystal structure, electronic structure, and prep­ to be desirable HER and OER electrocatalysts for efficient water splitting aration methods of TMSes are described and recent development of [25,28]. TMSes-based electrocatalysts is discussed from the perspective of HER, Experimental and theoretical studies have demonstrated that the OER and overall water splitting. Important TMSes-based electrocatalysts electrocatalytic properties of TMSes depend on the exposed edge sites such as single-metal TMSes, multi-metal TMSes, and TMSes composites because the basal surfaces are inert catalytically [29,30]. Different are described comprehensively (Fig. 1) and the challenges and prospects techniques have been proposed to expose more edge sites, for example, of TMSes-based electrocatalysts are discussed. by decreasing the size of TMSes, designing ultra-thin nanosheets of TMSes, fabricating TMSes nanostructures in situ on conductive sub­ 2. Physical properties of TMSes strates like Ni foam (NF), CC, and carbon fiber paper (CFP). Pu et al. [31] have prepared NiSe2 nanoparticles on conductive titanium plates as Advanced electrode materials are crucial to high-efficiency electro­ stable catalysts for HER and OER and Wu et al. [32] have fabricated chemical water splitting because the efficiency depends largely on the ultra-thin nickel selenide on NF with vertically stacked nanosheets crystal structure and surface electronic states [38]. Transition metal which show superior and steady overall water splitting characteristics. chalcogenides especially TMSes have the desirable characteristics, for Besides increasing the exposed active sites on TMSes, the intrinsic example, properties spanning insulators, semiconductors, semi-metals, electrocatalytic capability and electrical conductivity of TMSes are and true metals. First of all, the electrical conductivity of electro­ important parameters. It has been demonstrated that synergistic metal catalysts impacts the catalytic properties such as charge transfer, reac­ doping of TMSes and construction of hierarchical/hetero-structures of tion kinetics, and energy conversion efficiency and secondly, the TMSes can optimize the surface/interface electronic structure and pro­ electronic structure has great impact on ion/molecule adsorption and mote the intrinsic catalytic activity and electrical conductivity [33,34]. desorption during the catalytic reactions. The physical properties of Zhao et al. [35] have reported that heteroatom doping (Ni and Co) TMSes are primarily determined by the intrinsic composition and

Fig. 1. Typical TMSes based catalysts for water splitting. Copyright permission has been received for all the images.

2 X. Peng et al. Nano Energy 78 (2020) 105234 structure such as the metal type, metal coordination, and d-band elec­ variable electrical conductivity results from the presence of the d-band trons [38]. at the Fermi level and different electronic state densities [47]. With regard to non-layered TMSes such as nickel selenide, the cubic phase has 2.1. Crystal structure superior catalytic OER activity as exemplified by the small overpotential and Tafel slope in Fig. 3b due to the cubic pyrite-type crystal structure Many transition metals can form selenides as shown in Fig. 2. TMSes that favors surface oxidation to form more active sites [48,49]. For can be divided into layered structures (groups IVB-VIIB) and non- reference, the crystal structures and properties of representative TMSes layered structures (group VIII) with the former having strong anisot­ (M for metals of IVB-VIIB, VIII, IB and IIB) are listed in Table 1. ropy in the electrical, chemical, mechanical and thermal properties [39, 40]. MSe2 (M for transition metal) has the typical layered structure with 2.2. Electronic structure each mono-layer containing three layers of atoms as shown in Fig. 2. The mono-layer with a thickness of 6–7 Å consists of transition metal atoms The electronic structure of TMSes is closely related to the coordi­ sandwiched between two layers of Se atoms bound covalently. However, nation (covalent) environment and d electron numbers and the different in the vertical direction, atoms are bound by weak van der Waals force electronic properties of the TMSes are ascribed to gradual filling of the so that the crystal can be easily cleaved along the surface of the layer non-bonding d bands of groups IVB-VIIB and VIII, as shown in Fig. 4 [38, [39,41,42]. The crystal structure of MSe2 consists of three common 39,76]. For example, 2H–NbSe2 has metallic properties due to the phases, 1T (trigonal), 2H (hexagonal), and 3R (rhombohedral), where partially filled orbital, whereas MoSe2 is a semiconductor resulting from the number represents the number of Se-M-Se (M for transition metal) in the fully occupied orbital [76]. The preferred phase of TMSes depends the stacking sequence in a unit cell as shown in Fig. 2. The phases of on the number of transition metal d-electrons. For instance, group IVB 0 MSe2 are categorized by the filling state of the d orbital. The metallic TMSes (featuring d transition metal centers) have the octahedral phase 1T represents the partially filledd orbital and the semiconducting structure, group VB TMSes (d1) have the octahedral and trigonal pris­ phase 2H represents the fully d orbital. Because of the metallic behavior matic phases, group VIB TMSes (d2) have mainly the trigonal prismatic of the 1T phase, phase conversion from 2H to 1T can improve the cat­ geometry, group VIIB TMSes (d3) commonly have the distorted octa­ alytic activity [39,43]. The crystal structure influence the electro­ hedral structure, and group VIII TMSes (d6) possess the octahedral catalytic properties. As illustrated in Fig. 3a, the 1T phase delivers better structure [39]. Group VIB bulk TMSes have indirect bandgaps but the HER performance since both the overpotential and Tafel slope decrease mono-layered structure is a direct bandgap semiconductor [77]. The with increasing 1T phase ratio in 2H/1T composite catalysts. The basal bandgap transformation from indirect to direct (bulk to mono-layer) plane sites of the 1T phase act as active catalytic sites, not just the edge arises from the quantum confinement effects [39], indicating that the sites as for the 2H phase, and the higher electrical conductivity of the 1T mono-layered structure has higher electrical conductivity than the bulk phase gives rise to the superior HER characteristics [44–46]. The counterpart. Hence, it is important to develop TMSes with the

Fig. 2. Illustration of TMSes (chalcogen stands for Se) and the typical 1T, 2H and 3R structures (top and side view). Reproduced with permission [42] and copy­ righted 2018, the Royal Society of Chemistry.

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Fig. 3. (a) HER performance of layered MoSe2 electrocatalysts composed of different phase contents of 2H and 1T [46]; (b) OER performance of non-layered nickel selenides electrocatalysts with different crystal structures [48].

active sites giving rise to superior activity in HER [4]. As for TMSes in Table 1 OER, transportation of the dioxygen molecule can be made faster to Crystal structure of common TMSes. accelerate the OER kinetics resulting from the negative charges on the Groups Common Typical Properties Refs. selenium sites and accumulated 3d-2p repulsion of the d-band centers Species compounds between the transition metal and selenium [4]. IVB TiSe2 TiSe2: 1T semi-metal with [50,51] Several principles should be followed to design and construct high- overlapping valence performance electrocatalysts based on TMSes. Firstly, the morphology and conduction of the catalysts plays an important role in the electrochemical active bands, Eg≈0.15 eV 3 VB VxSey: VSe2, VSe2: 1T metallic, ρ≈10 Ω [52,53] surface area. Design of TMSes nano-architectures with a larger available V5Se4, V2Se9 cm surface is a promising route to optimize the electrochemical properties. ρ ≥ – VIB MoxSey: MoSe2:hexagonal semiconductor, [54 58] In fact, the larger the electrochemical active surface area (ECSA), the Ω ≈ – MoSe2, (2H), 1 cm, Eg 1.1 1.4 larger the quantities of active sites. Since the edge planes of the TMSes Mo9Se, rhombohedral eV MoSe, (3R) have higher catalytic activity than the basal planes, exposing more edge Mo6Se8, planes can enhance the overall catalytic performance. Secondly, a MoSe3 higher electrical conductivity facilitates charge transfer to improve the WxSey: WSe3, WSe2: 2H [59] kinetics of electrocatalysis and so designing TMSes with a metallic W Se , WSe 3 4 2 behavior is effective in enhancing the catalytic activity. In addition, VIIB MnxSey: MnSe: rocksalt quasi-ionic [60–62] MnSe, MnSe2 (RS), zinc blende semiconductor preparation of TMSes directly on conductive substrates decreases the (ZB), wurtzite contact resistance between the catalysts and current collector to pro­ (WZ) mote charge transfer. Thirdly, doping TMSes with anion/cation or VIII FexSey: FeSex Fe7Se8: c-direction semiconductors [63,64] producing TMSes-based composites can regulate the electronic structure (1 ≤ x ≤ (3C, 4C) of the bare TMSes, balance the Gibbs free energy of hydrogen adsorp­ 1.35), FeSe2 CoxSey: Co9Se8: 4C pyrite, marcasite, [65,66] tion, and improve the electrical conductivity to accomplish better cat­ CoSe, Co1- and layer-type alytic performance. xSe, Co9Se8, metallic Bifunctional catalysts which have catalytic activities in both HER CoSe 2 and OER at the same time provide a convenient and simple means to NixSey: Ni3Se2: semiconductor, [67,68] NiSe2, Ni1- rhombohedral Eg≈0.4 eV, ρ ≈1 Ω contrive high-performance electrodes. To design high-efficiency OER/ xSe (x = cm HER bifunctional catalysts, the catalytic HER and OER aspects should be 0–0.15), taken into consideration. Firstly, the catalytic materials should offer Ni3Se2 plenty of HER and OER sites and fast catalytic kinetics in a small full- IB CuxSey: Cu2- Cu2-xSe (0.15 ≤ x semiconductor, [69,70] water splitting potential window. The proper nanostructures, surface xSe (0 ≤ x ≤ ≤ 0.2): face- Eg≈1.1–1.29 eV 1), CuSe, centered cubic modification, and construction of heterojunctions with HER and OER Cu3Se2, (FCC) active centers are needed to produce the desirable bifunctional catalysts. CuSe2 Moreover, the adjustable electronic structure of TMSes-based catalysts IIB Zn Se : ZnSe, ZnSe: hexagonal semiconductor, [71–73] x y offers the opportunity to increase the HER and OER activity of the (ZnSe)n wurtzite (HWZ), Eg≈2.7 eV cubic zinc blende bifunctional catalysts because the Gibbs free energy of hydrogen (CZB) adsorption can be tuned and the metal-H and metal-OH bonds can be CdxSey: CdSe CdSe: zincblende semiconductor, [74,75] balanced. Secondly, the chemical and electrochemical stability of ≈ (ZB), wurtzite Eg 1.7 eV bifunctional catalysts in alkaline media is key to practical applications. (WZ) + At a constant oxidation potential in alkaline media, Mn (M for transi­ ρ , in-plane electrical resistivity; Eg, band gap energy. tion metals, n for chemical valence) evolves into MOx or even MO(OH) which have highly active catalytic sites. When the MOx or MO(OH) layer mono-layer or few-layer structures for electrocatalytic water splitting formed on the TMSes surface reaches a certain thickness, it will protect [78,79]. the inner TMSes from further oxidation. Therefore, the stability of The intermediate bond strength and electronic structure of individ­ TMSes can be improved by surface modification. Thirdly, low cost and ual elements in the catalysts affect the water splitting efficiency. In the earth abundance of TMSes render them suitable bifunctional catalysts as metal selenide structure, the negative charges localized on Se attract substitutes to the more expensive Pt, Ir, and Ru-based catalysts [81,82]. protons and facilitate the discharge process to accelerate HER [4,80]. In addition, TMSes have weaker Se–H bonds than transition metal phos­ phides and sulfides thereby accelerating hydrogen desorption from the

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Fig. 4. Qualitative schematic showing progressive filling of the d orbital located within the bandgap of the bonding (σ) and anti-bonding states (σ*) in group IVB- VIIB and VIII TMSes. Reproduced with permission [39] and copyrighted 2013, Springer.

3. Preparation of TMSes mono-layer or few-layer structures can retain the properties and new properties emerge due to confinement effects [83,84]. Fig. 5 shows the TMSes with different sizes, compositions, and morphologies have typical LPE process highlighting the importance of each step including been prepared by physical and chemical techniques such as liquid phase immersion, insertion, exfoliation, and stabilization as well as the role of exfoliation (LPE), vapor deposition, hydrothermal/solvothermal tech­ solvents in the fabrication of mono-layer or few-layer TMSes [85]. Large nique, electrochemical deposition, and so on. The advantages, disad­ quantities of two-dimensional nanostructures can be prepared by vantages and scope of applications of the different synthetic methods are blending TMSes with suitable solvents and polymers. For instance, listed in Table S1 (Supporting Information). Smith et al. [86] have exfoliated inorganic layered compounds (TaSe2, MoSe2, NbSe2, etc.) in aqueous surfactant solutions by probe sonication. This exfoliation route is common and can be extended to other types of 3.1. Liquid phase exfoliation layered compounds. However, although this method is quick and easy, the extent of detachment is far less than that of ion-exfoliated products. Liquid phase exfoliation is more suitable for layered structures in Coleman et al. [87] have produced layered TMSes compounds such as electrochemical energy storage and catalysis [39,42]. To get over the MoSe2, TaSe2, and NbSe2 in general solvents as mono-layer and van der Waals force between TMSes layers, exfoliation of TMSes into

Fig. 5. Schematic of the LPE process consisting of immersion, insertion, exfoliation, and stabilization. Reproduced with permission [85] and copyrighted 2016, Wiley-VCH.

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◦ ◦ few-layer nanosheets and the technique can be scaled up to produce at 940 C, and microparticles at 980 C. The substrate also has signifi­ exfoliated materials for TMSes on a large scale. Direct sonication tech­ cant impact on the crystal structure of the deposited films due to the niques used to mechanically exfoliate layered structures tend to break lattice mismatch between the substrate and TMSes film. Single-crystal the nanosheets consequently reducing the lateral size and electro­ substrates have better lattice matching and more effective interactions chemical exfoliation is thus a better choice to produce large nanosheets with the MSe2 layers, thus providing more atomized planes to produce with less damage. Hou et al. have produced ultra-thin ternary molyb­ films with higher uniformity, continuity and crystallinity. denum sulfoselenide (MoSexS2-x) nanosheets by cathodic electro­ chemical exfoliation and the exfoliated MoSexS2-x nanosheets have high 3.3. Hydrothermal/solvothermal method structural integrity with dimensions up to 1.5 μm and thickness of 3 nm [88]. Lithium intercalation which is one of the best methods for exfo­ The hydrothermal/solvothermal approach is commonly employed to liation has been optimized by Zhang et al. in the synthesis of few-layer produce products with a controlled morphology. In the process, water is NbSe2, WSe2, and Sb2Se3 nanosheets by using different cut-off volt­ generally used as a reaction medium to dissolve and recrystallize nor­ ages [89]. mally insoluble or insoluble substances at a moderate temperature ◦ (normally less than 200 C) and high pressure in an autoclave. The 3.2. Chemical vapor deposition technique is commonly employed to produce nanostructured TMSes such as MoSe2 [94,95], FeSe2 [96,97], CoSe2 [98,99], NiSe2 [100,101], In chemical vapor deposition (CVD) which is simple and straight­ CdSe [102,103], ZnSe [104], and CuSe [105]. TMSes can be produced forward, transition metals or transition metal oxides are employed as the by a one-step hydrothermal route. For example, Mi et al. have prepared metal precursors and selenium powder as the Se precursor. It is usually nickel selenide arrays with different morphologies by adjusting the hy­ carried out in a tube furnace in an inert atmosphere, where the metal drothermal reaction time as shown in Fig. 6a. TMSes can also be syn­ and Se precursors are plated at the proper location in the furnace thesized by a two-step protocol using the corresponding transition metal through the interface between the gas and solid [42,43,90]. At a tem­ oxides or hydroxides as the template. Ho et al. have synthesized ◦ perature above the boiling point of Se (684.8 C), Se vapor reacts with ultra-thin two-dimensional NiSe nanosheets using ultra-thin two-di­ the metal precursors to form TMSes [91]. Taking WSe2 as an example, mensional Ni(OH)2 nanosheets as the template (Fig. 6b) and the hy­ WSe2 is prepared by placing the selenium powder upstream at a distance drothermal reaction becomes the selenation process to convert the of about 20 cm from the tube center where WO3 is placed. When the transition metal oxides or hydroxides into selenides. ◦ furnace is heated to 1000 C using the proper ramping rate, selenium TMSes can be produced hydrothermally with Se powder as the pre­ vapor diffuses and reacts with the WO3 precursor on the surface and cursor but strong reducing agents such as hydrazine hydrate and NaBH4 WO3 is selenized forming WSe2 on the substrate [92]. The temperature are needed [32,106]. The key challenges are the low solubility and plays important roles in the morphology and properties of the deposited density of Se in the solution leading to poor contact with the reactants. films. Gao et al. [93] have synthesized TMSes with different structures at To increase the concentration of Se2 in the hydrothermal solution, ◦ different temperature, for example, nanoparticles at 880 C, nanosheets Nitsche et al. [97] have synthesized FeSe2 by hydrothermal

Fig. 6. (a) Schematic diagram of the morphology evolution of crystalline nickel selenide grown in situ on NF. Reproduced with permission [239] and copyrighted 2013, the Royal Society of Chemistry. (b) Structural evolution of ultra-thin Ni(OH)2 and NiSe nanosheets by combining acid etching and topotactic selenization. Reproduced with permission [32] and copyrighted 2018, Wiley-VCH.

6 X. Peng et al. Nano Energy 78 (2020) 105234 recrystallization with Li2Se as the precursor. Dai et al. have employed an splitting because of the outstanding electrical conductivity, controllable organic precursor of Se, seleniumcyanoacetic acid sodium (NCSeCH2­ M Se (M for transition metals) coordination, as well as electronic and COONa) [107], and selenium in the form of Se2 . Therefore, there is no chemical properties. TMSes-based catalysts mainly include single-metal need for a strong reducing agent and ethylene glycol can be used instead. selenides, multi-metal selenides, and their composites. The catalytic The solvothermal method is different from the hydrothermal method performance of TMSes-based catalysts is comparing with that of other in that the solvent is usually organic rather than water. Replacing water transition metal compounds such as carbides, nitrides, borides, etc. in with non-aqueous solvents not only expands the scope of hydrothermal Table 2. To enhance the catalytic performance of TMSes-based catalysts, techniques, but also enables reactions that are not possible under normal several measures have been taken. To enlarge the electrochemical active conditions. In this regard, Gao et al. [81] have investigated the effects of surface areas and active sites, nanostructures such as ultrathin nano­ solvents (diethylenetriamine, N2H4⋅H2O, and de-ionized water) in the sheets with large numbers of exposed edge sites have been fabricated. solvothermal synthesis on the morphology and structure of NiSe nano­ Designing TMSes with a metallic behavior by bandgap engineering and crystals. The morphology and phase formation depend on the solution production of TMSes on conductive substrates directly decrease the composition and the synergistic effects of the three solvents have sig­ contact resistance and facilitate charge transfer during electrocatalysis nificant impact on the formation of uniform NiSe nanostructures. Wang and improve the kinetics to enhance the catalytic activity. Doping of et al. [108] have reported that a high temperature is beneficial to Ost­ multi-metal selenides with anions/cations or interface engineering to wald ripening in which high-crystallinity CdSe hollow spheres with the produce TMSes-based composites can regulate the electronic structure wurtzite structure are formed. However, the high temperature hinders of TMSes, balance the Gibbs free energy of hydrogen adsorption, and the process and causes the hollow structure to collapse. enhance the electrical conductivity to improve the catalytic characteristics. 3.4. Electrochemical deposition 4.1. Single-metal selenide catalysts During electrochemical deposition, the precursors are dissolved and deposited on the surface of a coating by cathodic reduction at a certain Single-metal selenides such as molybdenum selenides, tungsten sel­ potential. Si wafer, FTO (fluorine-tin-oxide) glass, metal foil, metal enides, iron selenides, nickel selenides, cobalt selenides, and copper foam, and nanostructured templates are usually used as the cathode selenides are suitable electrocatalysts for both HER and OER in a wide substrates. By means of electrochemical deposition, a variety of TMSes pH range as shown in Table 2. have been prepared, for instance, MoSe2 [109], WSe2 [110], FeSe [111], NiSe2 [112], CdSe [113], ZnSe [114,115], CuSe [116,117], MnSe [118], 4.1.1. Molybdenum selenides PbSe [119,120], and Ag2Se [121,122]. Several factors including the Molybdenum selenide, which is an efficient electrocatalyst for HER, potential, temperature, pH of the electrolyte, deposition time, concen­ is promising in replacing noble metals in catalytic electrochemical tration of the precursor, and substrate affect the morphology and hydrogen production [43]. However, molybdenum selenide is inactive structure of the product. Demura et al. have prepared iron selenide and on account of the limited exposed edge sites and there have been at­ studied the impacts of the synthesis potential and pH on the structure tempts to expose more edge sites and defects as active sites to achieve and composition of the product [111]. Nath et al. have prepared Ni3Se2 better electrocatalytic performance [58]. Kong et al. [25] have prepared electrocatalysts by electrochemical deposition and the deposition con­ vertically aligned MoSe2 films for HER. The active sites on the edge are ditions (pH of the electrolyte, deposition potential, and substrate similar to those on MoS2 as shown in Fig. 7a–c and the exchange current composition) affect the OER properties [123]. The power supply in density correlates directly with the density of the exposed edge sites. The electrochemical deposition also influences the structure of the product. synthesis setup schematic is shown in Fig. 7d and the proposed synthesis Moysiadou et al. [117] have synthesized copper selenide by a single-step mechanism is illustrated in Fig. 7e. The predominant exposed edges electrochemical deposition process using constant and pulsed potentials have a metastable structure with a high surface energy consequently and compared to potentiostatic deposition, the pulsed potential leads to promoting the catalytic activity. MoSe2 nano-films prepared on CFP are formation of crystallized copper selenides. more efficient HER electrocatalysts compared to the flat substrate due to the larger surface area and vertically orientated layers which facilitate 3.5. Other methods the catalytic process. The MoSe2 nano-films are stable in the acidic medium as verified by 15000 continuous potential cycles without In addition to the aforementioned methods, pulsed laser deposition noticeable damping [156]. has been applied to prepare TMSes. Fominski and co-workers have To confirm the relationship between the edge of MoSe2 and catalytic fabricated MoSex (x=1.5–2.4) thin films under different vacuum con­ performance, Qu et al. [150] have prepared two-dimensional MoSe2 ditions by pulsed laser deposition [124]. Grigoriev et al. [125] have nanosheets with a small vertical thickness and abundant defects on the synthesized WSex (x=1.5–2.2) films by shadow masked pulsed laser surface of the carbon fiber cloth (CFC) by the solvothermal method. deposition and their results agree qualitatively with Monte Carlo After NH4F etching, pits are formed in the catalytically inert plane of the simulation. Other TMSes containing transition metals such as Cu and Zn MoSe2 nanosheets to increase the active edge sites. Therefore, MoSe2-­ have also been prepared by pulsed laser deposition [126,127]. Ueda based catalysts have low initial potential, large exchange current den­ et al. [128] have fabricated Fe based TMSes superconducting films by sity, and small Tafel slope due to high activity of edge sites and high molecular beam epitaxy which is employed to grow ZnSe layers on GaAs conductivity. Lei et al. [26] have prepared ultra-thin and porous mo­ (001) under light illumination with energy of about 1.8 eV [129]. Other lybdenum selenide nanosheets by the modified liquid stripping method preparation methods including chemical bath deposition [130,131], for HER. The massive nanocrystalline sheets are peeled off to form the cation exchange [132,133], one pot synthesis [134,135], photochemical ideal ultra-thin and porous structure by H2O2 as illustrated in Fig. 8a. method [136,137], hot injection [138], sol-gel method [139,140], The ultra-thin and porous MoSe2 nanosheets have a high edge/substrate vacuum evaporation [141], plasma assisted method [142,143], thermal ratio to increase active site exposure. The lamellar structure maintains evaporation [144,145], spray pyrolysis [146,147], and sputtering [148, effective electron transportation in the electrode to the catalytic sites 149] are also commonly used to synthesize TMSes controllably. leading to good HER activity such as a low overpotential of ~75 mV for a hydrogen evolution current density of 10 mA cm 2 and long-term 4. TMSes catalysts for electrochemical water splitting stability. The effects of surface defects of the catalysts on the catalytic activity TMSes have attracted enormous attention in electrocatalytic water have been investigated. Nguyen et al. [151] have prepared amorphous

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Table 2 Summary of recently reported single-metal selenide catalysts in electrochemical water splitting in comparison with typical transition metal carbides, borides and sulfides.

Catalysts Electrolytes η10 Tafel slope [mV Morphology Mass loadings Electrode structures Refs. [mV] dec 1]

HER 2 MoSe2 Films 0.5 M H2SO4 – 105–120 films ~13.5 μg cm on GCE [25] ultra-thin MoSe2 nanosheets/CFC 0.5 M H2SO4 182 69 ultrathin nanosheets on carbon fiber cloth [150] ultra-thin and porous MoSe2 0.5 M H2SO4 150 80 ultra-thin and porous on GCE [26] nanosheets nanosheets 2 amorphous MoSea H2SO4 (pH = 0) 270 60 nanospherical shape 0.212 mg cm on GCE [151] 2 ultra-thin S-doped MoSe2 nanosheets 0.5 M H2SO4 – 58 nanosheets 0.28 mg cm on GCE [27] 1T-MoSe2 nanosheets 0.5 M H2SO4 152 52 nanosheets 0.14 ± 0.01 mg on GCE [46] cm 2 2 MoSe2 nanosheets/NCNTs 0.5 M H2SO4 102 53 nanosheets 0.15 mg cm on GCE [152] MoSe2 nanosheets/PCF 0.5 M H2SO4 – 65 nanosheets on GCE [153] 2 few-layered MoSe2 nanosheets/CFA 0.5 M H2SO4 179 62 nanosheets 0.28 mg cm on GCE [154] net-like MoSe2-AB/NF 0.4 M H2SO4 – 55 ultrathin nanosheets on neat Ni foam [155] WSe2 nanofilms/CFP 0.5 M H2SO4 300 77.4 nanofilms on carbon fiber paper [156] WSex thin film 0.5 M H2SO4 245 98 amorphous films on carbon substrate [157] 2 mono-layer WSe2 nanosheets 0.5 M H2SO4 245 76 monolayer 0.85 mg cm on GCE [158] 316 103 nanosheets 1 M KOH FeSe2 film 0.5 M H2SO4 – 65.3 film on GCE [28] 2 NiSe2 nanosheets 0.5 M H2SO4 198 72.1 Nanosheets ~0.35 mg cm on Ni foam [159] 217 28.6 nanoflakes NiSe nanoflakes ultra-thin non-layered NiSe 1 M NaOH 177 58.2 ultrathin nanosheets 0.69 mg cm 2 on Ni foam [32] 2 sea urchin-like NiSe nanofiber 0.5 M H2SO4 – 64 nanofiber ~0.28 mg cm on GCE [81] 2 Ni0.75Se nanocrystal 1 M KOH 233 86 nanocrystals 0.6 mg cm on carbon paper [48] 2 D-Ni0.85Se 0.5 M H2SO4 – 49.3 nanocrystalline 0.57 mg cm on GCE [160] porous NiSe2 nanosheets 0.5 M H2SO4 135 37.3 porous nanosheets on carbon paper [161] 1 M KOH 184 77 2 NiSe2/Ti 1 M KOH 96 82 nanoparticles 2.5 mg cm on Ti plate [31] NiSe2/NF 1 M KOH 104 93 nanosheets on nickel foam [162] Ni0.85Se films/GS 1 M NaOH 200 – films on GS [163] selenium-enriched NiSe2 nanosheets H2SO4 (pH = 117 32 nanosheets on carbon fibers [164] 0.67) CoSe2 NW/CC 0.5 M H2SO4 130 32 nanowires on carbon cloth [165] three-dimensional CoSe2/CFF 0.5 M H2SO4 141 68 nanobelts on carbon fiber felt [166] MoB 1 M H2SO4 55 particles (1–3 μm) on carbon-paste [167] 59 electrode 1 M KOH Mo2C 1 M H2SO4 56 particles (1–3 μm) on carbon-paste [167] 1 M KOH 54 electrode α-MoB2 0.5 M H2SO4 149 74.2 microsized particles on copper sheet [168] 2 MoS2/RGO hybrid 0.5 M H2SO4 ~150 41 nanoparticles 0.28 mg cm on GCE [169]

OER 2 FeSe2/NF 1 M KOH 245 – – 51 μg cm on GCE [64] 2 FeSe2 nanoplatelets 1 M KOH 330 48.1 nanoplatelets 10 μg cm on nickel piece [23] ultra-thin non-layered NiSe 1 M NaOH 290 77.1 ultrathin nanosheets 0.69 mg cm 2 on Ni foam [32] 2 Ni0.5Se nanocrystal 1 M KOH 330 51 nanocrystals 0.6 mg cm on carbon paper [48] Ni3Se2–Au@Glass 0.3 M KOH 320 ± 97.1 films on GC disk electrode [123] 20 Ni3Se2–Au@Glass (Annealed) 290 ± 97.2 10 Ni3Se2–Au@Si 300 ± 122.0 10 Ni3Se2-GC 310 ± 79.5 20 Ni3Se2–NF 270 ± 142.8 20 2 Ni3Se2 0.1 M KOH – 46 nanoparticles 0.2 mg cm on GCE [49] 2 NiSe2/Ti 1 M KOH – 82 nanoparticles 2.5 mg cm on Ti plate [31] Ni3Se2/NF 1 M KOH – 40.2 porous on Ni foam [170] NiSe2/NF 1 M KOH – 97 nanosheets on nickel foam [162] Ni0.85Se films/GS 1 M NaOH 302 – films on GS [163] vacancy-rich ultra-thin CoSe2 0.1 M KOH 320 44 ultrathin nanosheets on GCE [24] nanosheets three-dimensional coral-like CoSe 1 M KOH 295 40 coral-like NPs ~0.28 mg cm 2 on GCE [171] 2 CoSe thin film/Ti 0.5 M H2SO4 135 62 film 3 mg cm on Ti substrate [172] Co7Se8/GC 1 M KOH 260 32.6 film on GCE [173] 2 Co3Se4 thin nanowires/CF 1 M KOH – 44 nanowire arrays 2.6 mg cm on cobalt foam [174]

Overall water splitting 2 FeSe2/NF 1 M KOH 1.73 V – – 51 μg cm on GCE [64] (continued on next page)

8 X. Peng et al. Nano Energy 78 (2020) 105234

Table 2 (continued )

Catalysts Electrolytes η10 Tafel slope [mV Morphology Mass loadings Electrode structures Refs. [mV] dec 1] ultra-thin non-layered NiSe 1 M NaOH 1.69 V – ultrathin nanosheets 0.69 mg cm 2 on Ni foam [32] 2 Ni0.5Se-OER//Ni0.75Se-HER 1 M KOH 1.73 V – nanocrystals 0.6 mg cm on carbon paper [48] 2 NiSe2/Ti 1 M KOH 1.66 V – nanoparticles 2.5 mg cm on Ti plate [31] NiSe/NF 1 M KOH 1.63 V – nanowire film 2.8 mg cm 2 on Ni foam [175] Ni3Se2/NF (anode)//NiCo2S4/NF 1 M KOH 1.58 V – porous on nickel foam [170] (cathode) Ni0.85Se films/GS 1 M NaOH 1.73 V – films on GS [163] Co7Se8 nanostructures 1 M KOH 1.6 V – film on GCE [173] 2 Co3Se4 thin nanowires/CF 1 M KOH 1.59 V – nanowire arrays 2.6 mg cm on cobalt foam [174]

Fig. 7. (a) Layered crystal structure of molybdenum chalcogenide with individual S–Mo–S (or Se–Mo–Se) layers stacked along the c-axis by weak van der Waals interaction; (b) Schematics of MoS2 nanoparticles with the platelet-like morphology distributed on the substrate (left) and nanotubes and fullerene-like nanotubes of MoS2 and MoSe2 (right); (c) Idealized structure of edge-terminated molybdenum chalcogenide films with the layers aligned perpendicular to the substrate and maximally exposing the edges of the layer . (d) Schematic of the synthesis/setup in a horizontal tube furnace. (e) Schematic of the proposed synthesis mechanism. Reproduced with permission [25] and copyrighted 2013, American Chemical Society.

Fig. 8. (a) Schematic showing the preparation of ultra-thin MoSe2 nanosheets. Reproduced with permission [26] and copyrighted 2016, Wiley-VCH; (b) Schematic illustration of phase- and disorder-controlled synthesis of MoSe2 nanosheets by the hydrothermal technique. Reproduced with permission [46] and copyrighted 2017, Wiley-VCH; (c) Preparation process of Se-vacancies-rich WSe2 MLNSs by the mechanical exfoliation method followed by annealing. Reproduced with permission [158] and copyrighted 2016, The Royal Society of Chemistry.

MoSea by electrochemical corrosion and the materials had excellent electrochemical similarities to the amorphous molybdenum sulfide HER properties in a wide pH range. The electrochemical corrosion analogue but is more stable in the alkaline medium. The catalyst is a process generates a lot of molybdenum oxygenselenide species which potential candidate for water splitting in acidic, neutral, or basic solu­ impact the HER performance either as an electron reservoir or proton tions. Constructing surface defects by doping MoSe2 with anions have relay or both. As a result, the catalyst shows some structural and been reported to enhance the electrocatalytic activity. Xu et al. [27]

9 X. Peng et al. Nano Energy 78 (2020) 105234 have synthesized ultra-thin sulfur doped MoSe2 nanosheets with the HER activity. improved HER catalytic activity such as a low onset overpotential of 90 mV and Tafel slope of 58 mV dec 1. Anion-doping/incorporation is thus 4.1.3. Iron selenides useful in increasing the catalytic active sites density and electrical Iron-based compounds not only are abundant and inexpensive, but conductivity of TMSes. also have intrinsic semiconducting/metallic properties as well as unique The phase of molybdenum selenides affects the catalytic properties electronic structures that enhance the electrical conductivity and since the 2H phase is semiconducting, whereas the 1T phase has metallic adsorption of H2O [23]. Kong et al. [28] have prepared polycrystalline properties thus facilitating transportation of electrons through the FeSe2 films as HER electrocatalysts in 0.5 M H2SO4 and an overpotential electrode to active centers [176–180]. Transforming molybdenum sel­ of 270 mV can generate a hydrogen evolution current density of 4 mA 2 1 enides into the 1T phase enhances the catalytic activity. Yin et al. [46] cm with a Tafel slope of 65.3 mV dec . Moreover, FeSe2 has a rela­ have used excessive NaBH4 to regulate the electronic structure so that tively small Tafel slope among Fe, Co and Ni based dichalcogenides the MoSe2 framework is rearranged from 2H to 1T and the MoSe2 (Fig. 9a) and offers an opportunity to promote the performance of nanosheets have the 1T phase for HER, as shown in Fig. 8b. The 1T phase nonmetallic catalysts by adjusting the structure. Generally, TMSes share is formed by NaBH4 reduction and as a result, the intrinsic catalytic a similar structure with the corresponding sulfides but have higher activity and electrical conductivity are improved dramatically. Addi­ electrical conductivity, resulting in superior charge transfer in selenides tionally, the disordered structure resulting from the low temperature such as Fe and Co-based selenides as shown in Fig. 9a. The electrical synthesis has more defects as active centers and synergistic regulation conductivity is only one of the factors affecting the Tafel slope and other produces excellent characteristics such as a current density of 10 mA factors such as the electrode structure, morphology, crystal structure, cm 2, overpotential of 152 mV, and Tafel slope of 52 mV dec 1. surface defects, and so on also play important roles in the charge transfer Coupling molybdenum selenides with highly-conductive agents such and catalytic performance. In particular, NiSe2 shows a larger Tafel as carbon materials and metallic foams can improve the electrical con­ slope than NiS2. Gao et al. [23] have synthesized two-dimensional FeSe2 ductivity of electrocatalysts and facilitate the catalytic kinetics. For nanoplatelets by a hydrothermal process for OER (Fig. 9b and c). The instance, Qu et al. [152] have used MoO3/PANI nano-hybrid materials current density of FeSe2 is 2.2 times bigger than that of commercial to produce MoSe2 nanosheets with a small size and (002) plane RuO2 at an overpotential of 500 mV together with a small Tafel slope of expanding space on the surface of porous nitrogen-doped carbon 48.1 mV dec 1 and long-term stability (constant for 70h). The excellent nanotubes (NCNTs). The composite composed of NCNTs has high elec­ catalytic activity can be ascribed to the high density of exposed active trical conductivity and large quantities of exposed active edge sites and sites on the (210) crystal surface and two-dimensional nanostructures shows HER activity superior to that of layered metal chalcogenides re­ that improve the kinetics of water oxidation. Density-functional theory ported previously. Other carbon species including porous carbon fiber, calculation shows that Fe tunes the electronic structure and reduces the carbon fiber aerogel, and so on have been employed as substrates for OER overpotential [23]. MoSe2 to enhance the electrocatalytic properties [153,154]. Metallic Iron selenides can serve as catalysts in both HER, OER, and overall foam is a good skeleton for molybdenum selenides for HER. Huang et al. water splitting. Panda et al. [64] have studied the influence of size, have prepared the net-like MoSe2-acetylene black (AB) composite on NF structure, morphology, and electronic properties on the water splitting hydrothermally [155] and the three-dimensional MoSe2-AB nano­ performance of FeSe2. As an OER catalyst, an overpotential of 245 mV 2 structures composed of MoSe2 ultra-thin nanosheets have high HER can generate a current density of 10 mA cm , suggesting excellent ability as exemplified by the onset potential of 0.08 Vvs . RHE and Tafel electrocatalytic activity because of the formation of Fe(OH)2/FeOOH 1 slope of ~55 mV dec . active sites on the FeSe2 surface. Since the medium bonding with in­ termediates and products in HER result in good catalytic activity, the 4.1.2. Tungsten selenides Tungsten selenide is a semiconductor with a bandgap of 1.7 eV for the bulk and 1.4 eV for the single-layer structures [181,182] and the layered materials tend to expose the basal planes to minimize the surface free energy [27]. Generally, the basal planes of the layered materials are electrochemically inert, while the edge planes are chemically and elec­ trochemically active [20,30,39,55]. Wang et al. [156] have demon­ strated that WSe2 nano-films with molecular layers perpendicular to the rough and curved surfaces preferentially expose the active edge sites for HER and improved HER activity is observed from WSe2 on CFP compared to the flat films. In the early days, there is a lack of compre­ hensive theoretical studies on the structure and activity of WSe2 and the active sites on tungsten selenides during electrochemical water splitting are not well understood. Tsai et al. [183] have used density-functional theory to identify the active centers of tungsten selenide in HER and the Se-edges of WSe2 are the dominant catalytic sites but the basal planes are inactive. Hence, maximizing the exposure of Se edge sites enhances the performance of tungsten selenides based catalysts. The theoretical and experimental results presented by Cao et al. [158] sug­ gest that selenium vacancies enhance the catalytic activity of the basal planes of WSe2 (Fig. 8c). By introducing Se vacancies, favorable hydrogen adsorption is possible and the WSe2 catalyst has outstanding HER activity. Besides HER in the acidic solution, WSe2 is efficient in a basic solution. Romanova et al. [157] have prepared a series of WSe x Fig. 9. (a) Summary of Tafel slopes of transition metal dichalcogenide films. thin films for HER and the activity of the Se-rich films is improved. The Reproduced with permission [28] and copyrighted 2013, The Royal Society of highest catalytic activity is achieved from the films with the ­nano Chemistry; Schematic illustration of (b) Formation of FeSe2 nanoplatelets and crystalline layered structure, large surface area, and edge sites and (c) Application to OER. Reproduced with permission [23] and copyrighted therefore, the structure and chemical composition of WSex films affect 2017, Elsevier.

10 X. Peng et al. Nano Energy 78 (2020) 105234

Se–H bond strength is lower than that of S–H, so that it can act as a base that of MoS2 particles [169] revealing higher HER activity. to trap protons and facilitate deprotonation to release hydrogen in HER Two-dimensional non-layered materials are dominated by dangling [4,184]. An overall water splitting system using the two-electrode bonds on the surface and the atomic bonds in unconventional di­ configuration shows a small cell voltage and high durability, conse­ mensions render chemically active surfaces possible [187,188]. Nickel quently laying the foundation for the implementation of iron selenide as selenides based nanosheets have garnered much attention. Wu et al. high-efficiency catalysts in overall water splitting. [32] describe a self-limiting controllable acid etching method to syn­ thesize hierarchical ultra-thin (~0.96 nm) two-dimensional layered 4.1.4. Nickel selenides nickel hydroxide nanosheets as well as artificial topotactic phase engi­ Nickel selenides are excellent catalysts to replace precious Pt cata­ neering to form ultra-thin (~1.25 nm) non-layered nickel selenide lysts due to the abundant reserve, low cost, and good conductivity [43]. nanosheets (Fig. 10a). The elaborate ultra-thin layered structure allows Because of the unique electronic configuration of Ni (3d8 4s2) and small non-destructive topotactic selenization and conversion into non-layered difference in electronegativity between Ni and Se, the two can form counterparts, which is different from the bulk structures which succumb different stoichiometric compounds [68]. As an OER catalyst func­ to extra lattice expansion resulting from the large dimensions. The tioning at a constant oxidation potential, Se2 in nickel selenides loses catalyst based on ultra-thin nickel selenide nanosheets shows highly + electrons to form soluble selenoxide species, whereas Ni2 evolves into efficient OER and HER activity with low onset potentials and small Tafel NiOx or even NiO(OH) which have highly active catalytic sites. After the slopes as well as high stability in basic electrolytes. Since the NiSe NiOx or NiO(OH) layer formed on the nickel selenides surface reaches a nanosheets have steady activity in OER and HER, the overall water certain thickness, it will protect the inner nickel selenide from further splitting behavior is evaluated on a two-electrode system using NiSe oxidation and therefore, catalysts based on nickel selenides exhibit high nanosheets as the bifunctional catalysts. The voltage required for the activity and stability in OER [185]. The composition, microstructure, ultra-thin nickel selenides nanosheets based electrolyzer is 1.69 V for a and morphology influence the catalytic properties [38]. Bhat and current density of 10 mA cm 2, which is less than those of layered nickel Nagaraja [159] have synthesized nickel selenide nanostructures with hydroxide based catalysts (1.81 and 1.92 V for two-tiered and one-tiered different morphologies and compositions, for example, NiSe2 nano­ nickel hydroxide, respectively). sheets and NiSe nanoflakes. The HER catalysts show the Tafel slope of The atomic ratio of Ni/Se in NixSey influences the electronic struc­ NiSe nanoflakes is three times less than that of NiSe2 nanosheets and ture and electrocatalytic properties [189]. Nickel selenides with a comparable to that of the commercial Pt/C catalyst because the verti­ non-stoichiometry such as Ni0.85Se have unsaturated atoms [43] and it is cally aligned NiSe nanoflakes on the conductive NF framework expose of great significance to explore the electrocatalytic performance in water lots of active edge sites in HER. Gao et al. [81] have synthesized NiSe splitting. To understand the effects of the Se/Ni ratio of nickel selenides particles which are made of tens to hundreds of spikes (uniform, flaw­ on the electrocatalytic properties, Zheng et al. [48] have synthesized less, highly crystalline) that look like sea urchins and the loose, spiny Ni1-xSe (0.5 ≤ x ≤ 1) including Ni0.5Se, Ni0.75Se, Ni0.85Se, and NiSe structure provides a more active solid surface to activate and catalyze nanocrystals controllably for OER and HER. In OER, the overpotential HER. Hydrogen evolution from the sea urchin-like NiSe catalyst occurs for cubic Ni0.5Se nanocrystals is lower than those of other nickel sele­ at about 0.2 V vs. RHE similar to MoS2 electrocatalysts [20,186]. The nides based catalysts and comparable to that of commercial RuO2. It is Tafel slope of from the sea urchin-like NiSe catalyst is much lower than because the unique cubic pyrite-type favors surface oxidation and

Fig. 10. (a) Schematic illustration of two-tiered nanosheet formation via in situ self-regulating acid etching. Reproduced with permission [32] and copyrighted 2018, Wiley-VCH; Schematic illustration of the crystal structures represented with the ball-and-stick unit cell and solution conversion processes from NiGa LDH nanoplates to porous β-Ni(OH)2 and porous NiSe2 nanosheets: (b) NiGa LDH, (c) β-Ni(OH)2, and (d) NiSe2. The schematics on top illustrate (b, c) OER and (d) HER. Reproduced with permission [161] and copyrighted 2015, American Chemical Society.

11 X. Peng et al. Nano Energy 78 (2020) 105234 selenium enrichment promotes the number of surface active sites. With in the nickel framework. The synergistic effects of the intrinsic metal regard to HER, the best catalytic capability is achieved from Ni0.75Se state and anions in the metal matrix render the surface easier to nanoparticles because of the larger electrochemical active area and recombine and at the same time, addition of selenium atoms causes facilitated electron transportation in the catalytic process. The electro­ atomic displacement. The low coordination number and long bond lyzer comprising Ni0.75Se as the cathode and Ni0.5Se as the anode re­ length weaken the Ni–Ni bond, promote surface oxidation, and improve quires a cell voltage of 1.73 V to generate a current density of 10 mA the catalytic performance and as a result, Ni3Se2 exhibits optimal OER cm 2 with visible hydrogen and oxygen bubbles on the corresponding catalytic activity. electrodes. Moreover, the NixSe based catalysts exhibit less activity decay than commercial Pt/C//RuO2 catalysts after 10 h demonstrating 4.1.5. Cobalt selenides the outstanding durability of Ni0.5Se//Ni0.75Se in overall water splitting Cobalt selenides have attracted s attention because of the highly in addition to being cost effective and have large natural abundance. electrical conductivity, natural abundance, unique structure and excel­ Compared to non-stoichiometric nickel selenides compounds, NiSe2 lent electrochemical and electrocatalytic features [43,191]. The CoSe has a pyrite structure with a zero bandgap. The dumbbell-shape Se2 phase is stable at room temperature for OER. Liao et al. [171] have group in the middle of two Ni atoms renders it an excellent electro­ prepared three-dimensional coral-like CoSe for OER with high electro­ catalyst in water splitting [190]. Liang et al. [161] have converted catalytic performance and durability. An amorphous CoSe thin film is layered double hydroxide nanoplate precursors containing amphoteric prepared on Ti by Carim and co-workers by electrodeposition and the metals into porous NiSe2 nanosheets by selective etching for HER as catalyst exhibits good HER activity [172]. Cobalt selenide is useful in shown in Fig. 10b–d. The porous nanostructures have a large surface both OER and HER boding well for overall water splitting. Masud et al. area, abundant edge sites, and excellent durability. The porous NiSe2 [173] have prepared Co7Se8 nanostructures with a flake-like nanosheets show superior HER activity and excellent long-term dura­ morphology which shows high stability (more than 12 h) and Faradaic bility in both acidic and basic media. To produce a current density of 1 efficiency (99.62%) as the bifunctional electrocatalyst in both OER and 2 mA cm by the overall electrochemical water splitting configuration, a HER. The Co7Se8 bifunctional electrocatalyst is applied to water elec­ cell voltage of 1.7 V is required and the voltage needed to generate a trolysis at a cell voltage of 1.6 V in a basic medium. 2 6 1 constant overall water splitting current density of 5 mA cm increases The cubic CoSe2 with the t2geg electronic arrangement near the mildly from 1.76 to 1.8 V after 2 h indicating excellent durability. optimal eg filling is considered the ideal catalyst in electrocatalytic water Nanostructured nickel selenides grown in situ on conductive sub­ splitting [24,192]. Theoretical calculation reveals that the metallic strates are effective in improving the electrocatalytic capability. behavior of CoSe2 facilitates fast transportation of electrons between the Conductive substrates such as metallic plates, metallic foams, and car­ electrode and catalyst surface for efficient OER [24]. However, the bon species provide the skeleton to support the nanostructured active active sites on pristine CoSe2 is limited and the catalytic activity is still materials and current collector to increase the active sites and charge unsatisfactory despite optimal filling of eg and therefore, it is necessary transportation. The influence of the substrate on the OER performance is to improve the intrinsic catalytic capability [24]. Liu et al. [24] have studied and the catalysts deposited on Au-coated glass, Si substrate, and reduced the thickness of the CoSe2 nanosheets to the atomic level and a glassy carbon exhibit the highest current densities and earliest onset of small Tafel slope of 44 mV dec 1 is obtained (Fig. 11). Lots of Co va­ OER compared with others, suggesting that the interactions between the cancies are also produced on the ultra-thin nanosheets serving as active underlying substrate and Ni3Se2 impact the OER performance [123]. centers to catalyze OER. Metallic foams especially NF have been used to fabricate nickel selenides To improve the kinetics of the electrocatalytic reactions and enhance for electrochemical water splitting. Recently, Zhu et al. [162] have the catalytic activity further, CoSe2 nanostructures are produced on prepared pure NiSe2, NiSe, and Ni3Se2 on NF and the nickel selenide conductive substrates. Zheng et al. [166] have prepared a layered nanostructures deliver remarkable phase-dependent HER and OER structure of three-dimensional CoSe2 nanoribbons in situ on carbon felt performance. Typically, NiSe2 shows the best HER and OER activity. for HER and the densely grafted CoSe2 nanoribbons expose more active With respect to HER, the activity order is NiSe2>NiSe > Ni3Se2 and with sites. The modified active sites can fully combine with adsorbed H* to respect to OER, the order is NiSe2>Ni3Se2>NiSe. The difference in the accelerate proton-electron transfer and the three-dimensional porous electrocatalytic characteristics is related to the synergistic effects of structure provides active sites for hydrolysis and facilitates release of electrical conductivity and ECSA of the nickel selenide based electrodes. generated gaseous H2 from the electrode surface. As a result, the catalyst Density-functional theory calculation is commonly implemented to shows outstanding catalytic activity including a high exchange current study the electronic structure and ion/molecule adsorption and density and low overpotential. The good corrosion resistance and strong desorption properties and identity the active centers on electrocatalysts. bonding between carbon fiber and CoSe2 foster the stability during In order to understand the catalytic mechanism of nickel selenides, cycling. Li et al. [174] have prepared Co3Se4 nanowires on metallic Co Wang et al. [164] have prepared textured Se-enriched NiSe2 nanometer foam (CF) with high efficiency and catalytic durability for OER and sheet arrays and the Se-rich pyrite type NiSe2 is an efficient and earth overall water splitting. CF can be used as either the current collector or abundant catalyst for steady HER. The extra Se atoms on the NiSe2 precursor for Co3Se4 nanowires without additional Co sources. The surface facilitate transportation during HER resulting from the opti­ electrolyzer containing Co3Se4/CF as both the anode and cathode shows 2 mized intrinsic electrocatalytic capability of the NiSe2 nanosheets high performance. The electrolyzer shows 10 mA cm for more than 2 serving as promoters for H2 production. As a result, the catalyst exhibits 3500 h and 100 mA cm over 2000 h without obvious decay indicating distinctive kinetics and stable durability and the experimental results good durability. are corroborated by density-functional theory calculation which shows To enhance the electrocatalytic activity and electrical conductivity of that the Se and Ni sites have free energies of 0.13 and 0.87 eV, respec­ intrinsic cobalt selenides, anion doping is a viable solution. Hou et al. tively, suggesting that the Se sites are responsible for the excellent [193] have prepared a ternary catalyst of phosphorus-doped cobalt electrocatalytic characteristics of NiSe2 instead of the Ni sites. As a selenide nanosheets by hydrogenation and phosphation. The common nickel selenide, few reports have been reported for Ni3Se2. Xu phosphorus-doped cobalt selenide nanosheets have outstanding elec­ et al. [49] have studied the influence of the electrical conductivity on the trocatalytic capability and stability for HER in an alkaline medium, electrocatalytic capability in OER and revealed the relationship between which exceed those of transition metal dichalcogenides reported before the structure and OER behavior of non-oxide-based catalysts such as owning to the improved electrical conductivity and enlarged surface NiO, NiSe, and Ni3Se2. Density-functional theory calculation shows that area. Using phosphorus-doped cobalt selenide nanosheets as both the NiSe has a narrower bandgap than NiO and nickel selenide (Ni3Se2) anode and cathode in an overall water splitting electrolyzer, a low exhibits a metallic behavior when the selenium concentration is reduced voltage of 1.64 V is achieved to generate a current density of 10 mA

12 X. Peng et al. Nano Energy 78 (2020) 105234

′′ Fig. 11. (a) Schematic showing the formation of ultra-thin CoSe2 nanosheets and VCo vacancies; First-principles study of surface H2O adsorption on different sites and performance of various materials: (b) Geometries and binding energies of H2O molecules on cobalt sites and vacancies and (c) Calculated adsorption energies for H2O molecules on different Co sites. Reproduced with permission [24] and copyrighted 2016, American Chemical Society.

cm 2 and the performance is better than that of the commercial current density of 10 mA cm 2. Ir/C//Pt/C catalyst for sufficiently high overpotentials. Furthermore, theoretical calculation indicates that the synergistic effects of selenium 4.2. Multi-metal selenide catalysts vacancies and phosphorus replacement of selenium atoms around the selenium vacancies regulate the electronic structure to ensure fast Single-metal selenides have excellent electrocatalytic properties in charge transportation and ideal energy barrier of hydrogen desorption water splitting [197,198] but the electrochemical performance of to facilitate the reaction kinetics. multi-metal-based materials can sometimes be superior by modulating the 3d electronic structures. The improved electrochemical activity is 4.1.6. Other single-metal selenides associated with enhanced charge transportation between the foreign Compared with the above materials, other metal selenium com­ elements and host and more active sites by tailoring the local electronic pounds such as copper selenide have been studied less extensively as structures of the host materials. Electrochemical studies in combination electrocatalysis [191]. Zhang et al. [194] have prepared hierarchical with density-functional theory calculation indicate that the adsorption Cu2-xse/Cu foam for HER with sufficient interfacial contact for partial energy of OER intermediates can be modulated by tuning the 3d energy anion exchange. The graded copper selenide architecture consists of levels to improve the catalytic activity [199,200]. In recent years, stacked and vertically arranged nanosheets with a large proportion of several multi-metal selenides such as Ni–Fe, Fe–Co, Ni–Co, Mo–Co, and surface active sites exposed to the electrodes to improve the electro­ so on have been investigated as catalysts for water splitting and the catalytic activity. The materials show high current densities and cata­ catalytic properties are presented in Table S2 (Supporting Information). lytic stability in an acidic electrolyte in the acceptable overpotential range. CuSe has also been deposited on NF for overall water splitting by 4.2.1. Fe–Ni/Co selenides Driess et al. and the materials exhibit a low cell voltage (1.68 V) and Although nickel selenides with controlled chemical compositions long-term stability in overall water splitting [195]. Kukunuri et al. [196] and morphologies have been developed as advanced electrocatalysts, have prepared three different palladium selenide phases of Pd17Se15, their catalytic performance are far from satisfactory [201]. Recent Pd7Se4, and Pd4Se by thermolysis of different palladium organo­ studies show that synergistic metal doping can optimize the surface selenolate complexes and Pd4Se shows good electrocatalytic activity electronic structure of the electrocatalysts and incorporation of metal such as onset potentials of 0.03 V vs. RHE and 94 mV to attain a current atoms into nickel selenides is a viable strategy to promote the electro­ 2 density of 10 mA cm . Single- and few-layer InSe flakes have been catalytic activity. For example, Fe doping in Ni-based selenides modifies obtained by the LPE method from β-InSe single crystals by Petroni et al. the electronic structure and promotes the intrinsic catalytic activity [33] [78]. The exfoliated InSe few-layer flakes as an HER electrocatalyst is and Ni–Fe based multi-metal selenides with different Ni:Fe ratios have assessed using hybrid single walled carbon nanotubes/InSe been studied. Wang et al. [202] have prepared nickel-iron multi-metal hetero-structures in acidic and alkaline solutions. The smallest InSe selenide nanosheets on CFC (Ni0.75Fe0.25Se2/CFC) for OER and the flakes show promising overpotentials of 549 mV and 471 mVfora catalyst provides large quantities of active sites to expedite charge

13 X. Peng et al. Nano Energy 78 (2020) 105234 transportation and enhance desorption of oxygen bubbles from the co-workers [206]. The HER activity of the Fe1-xCoxSe2/RGO composite surface (Fig. 12a and b). The Ni0.75Fe0.25Se2 catalyst has the largest electrocatalyst depends on the Co concentration. Owing to the high ECSA compared to the other materials (Fig. 12c), indicating that the conductivity and unique structure, the Fe0.7Co0.3Se2/RGO composite is a active sites for OER are more abundant after Fe incorporation and the promising non-noble catalyst for HER. outstanding catalytic activity can be ascribed to Fe doping and seleni­ Although single-cation doping of TMSes to form binary TMSes is zation. Xu et al. [203] have fabricated nickel-iron multi-metal selenide beneficial to the catalytic performance of single-metal selenides, the (NixFe1-xSe2) for efficient OER in a basic solution and studied the in­ improvement is not significant on account of the limited change in the fluence of the atomic ratio of Ni:Fe on the OER activity. The highest electronic structure. Introducing two or more foreign cations simulta­ electrocatalytic capability is achieved for x=0.8 and the catalyst pre­ neously can more effectively tailor the atomic arrangement and elec­ sents outstanding OER activity. Additionally, the ECSA of NixFe1-xSe2 is tronic structure and iron doping of binary metal selenides can produce 2.4 times that of the NiFe layered double hydroxide after Fe doping. more active centers on metal-OH for OER to promote the electrocatalytic Compared to NiSe2, Ni3Se4 has higher conductivity and been inves­ activity of intrinsic TMSes [36,37,207]. Chi et al. [207] have synthe­ tigated as multi-metal selenides [49,164,204]. Du et al. [204] have sized trimetallic Ni/Fe/Co selenide (NiFeCoSex) on CFC for OER synthesized hierarchical Fe-doped Ni3Se4 ultra-thin nanosheets for showing a lower overpotential, smaller Tafel slope, larger double-layer alkaline media catalytic reaction with a low overpotential, small Tafel capacitance, and higher stability in comparison with single or binary slope, and good durability. The (Ni,Fe)3Se4 catalyst shows promising metal selenides because of the synergistic effects of trimetallic Ni/Fe/Co bifunctional catalytic ability in overall water splitting and using (Ni, atoms and selenide-CFC interactions [208]. The work suggests that Fe)3Se4 as both the anode and cathode, a cell voltage of 1.60 V is needed transition multi-metallic selenides on conductive substrates are good to generate a current density of 10 mA cm 2. The outstanding perfor­ OER catalysts. mance stems from more active sites and improved charge transportation To extend the application of multi-selenides, Fe–Co–Ni ternary sel­ by Fe doping. Coupling Ni–Fe based selenides with conductive agents is enides have been investigated. For example, Sun et al. [36] have syn­ a useful route to enhance the catalytic performance further. For thesized dual cation (Fe, Co) doped NiSe2 nanosheets (Fe, Co–NiSe2) and example, Umapathi et al. [205] have prepared FeNi2Se4 nanoparticles investigated the catalytic properties electrochemical water splitting. The segregated on nitrogen-doped reduced graphene oxide (FeNi2Se4-NrGO) dual cations contort the crystal lattice and cause stronger electronic for OER with low overpotential, small Tafel slope, and high current interaction giving rise to more active sites and harmonized adsorption density due to the synergistic interactions between the catalyst and energy of the reaction intermediates in comparison with the single graphene. cation doped or intrinsic single-metal selenides. The Fe0.09Co0.13-NiSe2 Generally, the active Fe center in FeSe2 has a similar structure like porous nanosheet catalyst has enhanced electrocatalytic properties. the [Fe–Fe]-hydrogenase active center thus providing good hydrogen Using Fe0.09Co0.13-NiSe2 as both the anode and cathode in overall water adsorption energy conditions [28]. To promote the catalytic properties splitting, a cell voltage of 1.52 V produces a current density of 10 mA 2 of FeSe2 based catalysts, the chemical composition can be tailored. For cm . The work points out the significance of dual cation incorporation instance, active and stable Co-doped FeSe2 electrocatalysts on graphene to promote the activity of TMSes. substrate (Fe1-xCoxSe2/RGO) for HER have been produced by Xu and

Fig. 12. (a) Schematic illustration of the synthesis of porous (Ni0.75Fe0.25)Se2 nanosheets on CFC; (b) iR-corrected polarization curves; (c) Charging current density differences plotted vs. scan rates. The linear slope is twice of the Cdl. Reproduced with permission [202] and copyrighted 2016, American Chemical Society.

14 X. Peng et al. Nano Energy 78 (2020) 105234

4.2.2. Ni–Co selenides cobalt-doped vanadium diselenide (VSe2) nanosheets for HER and Cobalt selenides and nickel selenides have high activity in HER and theoretical calculation discloses that appropriate Co doping decreases OER. The Co–Ni–Se system contains active Co and Ni centers boding the Gibbs free energy of hydrogen adsorption ΔGH. well for HER, OER, and overall water splitting since spectroscopic and Compared with the binary and ternary compounds, introduction of theoretical studies show that incorporation of Co atoms into Ni based heteroatoms into quaternary influences the electrocatalytic perfor­ catalysts improves its electrochemical activity by modifying the elec­ mance. Transition metal doping increases the quantity of active sites and tronic structure [209,210]. Density-functional theory calculation shows improves the rate determining steps by regulating OH adsorption on the that nickel lowers the atomic hydrogen adsorption free energy on sele­ electrocatalyst surface and modulating the local electron density near nium edge sites of CoSe2 to boost the intrinsic HER activity of Ni–Co–Se the active sites [11,223]. Additionally, the highly occupied d-levels catalysts. Introduction of nickel cations to cobalt selenides increases the improve the electrical conductivity with the matrix to accelerate charge electrical conductivity and improves the electrocatalytic kinetics [34]. transfer on the electrocatalyst surface. Cao et al. investigate the OER The polymorphic (orthorhombic and cubic structures) nature of catalytic activity of Co–Ni–Cu [(Co0.21Ni0.25Cu0.54)3Se2] and Fe–Co–Cu Ni–Co–Se increases the active sites on edges for enhanced HER [24,79]. [(Fe0.48Co0.38Cu0.14)Se] quaternary selenides [224,225] and the excel­ Ni atoms with a similar atomic radius and electron configuration as the lent electrocatalytic activity of the quaternary selenides stems from CoSe2 crystal lattice cause heterogeneous spin states and generate small possible electron configuration delocalization in the transition metal distortion in the lattice. Liu et al. [211] demonstrate that Co0.13Ni0.87Se2 sites through the d-bands resulting in faster electron transfer at the nanoparticles films on Ti have high electrocatalytic capability and sta­ catalyst-electrolyte interface and improved electrical conductivity. bility for HER and OER in strongly alkaline electrolytes. The catalytic activity is associated with the amount of Co dopant and replacement of 4.3. TMSes/TMSes composites Ni with Co does not change the NiSe2 structure but improves the overall performance. Compared with the multi-metal selenides in which the heteroatoms Preparation of advanced three-dimensional electrodes and optimi­ are incorporated into TMSes to modify the atomic arrangement and zation of the morphology are important to enhancing the electro­ electronic state, TMSes-based composites are different in principle. Each catalytic activity of catalysts. Growing selenide directly on a conductive component and the interface between components in the composite can substrate improves the catalytic activity because of the reduced resis­ promote individual steps in the catalytic process by exploiting the syn­ tance between the catalyst and current collector. NF is a commonly ergistic effects rendered by different electrocatalytic active parts to substrate to fabricate Ni–Co–Se nanostructures by forming the skeleton improve the electrocatalytic capability. Wang et al. [226] have reported and serving as the Ni precursor simultaneously [209,210,212]. The that the CoSe2–MoSe2 composites with reduced graphene oxide and Ni0.89Co0.11Se2/NF mesoporous nanosheet networks designed by Liu amorphous carbon (CS-MS/rGO-C) catalyst possess outstanding HER et al. [212] have outstanding HER catalytic properties and excellent catalytic activity in both acidic and basic solutions due to the bi-metallic durability in a wide pH range on account of the unique morphology and composition, porous rGO-C microsphere structure, and conductive electronic structure. Theoretical calculation discloses that the high HER rGO-C substrate and the abundant sites and high electrical conductivity + activity stems from improved adsorption of H and H2O, caused by Co. accelerate charge transportation. Chemical and engineering strategies Xiao et al. [209] have prepared (Ni, Co)0.85Se NSAs/NF for super overall such as construction of hierarchical structures and hetero-structures as water splitting. The NF substrate has a hydrophobic surface but it well as phase engineering have been proposed to enhance the electro­ changes to a super hydrophilic one after modification with porous (Ni, catalytic capability, electrical conductivity, and durability of Co)0.85Se NSAs. The catalysts grown on the substrate in situ are more TMSes-based composite catalysts. TMSes/TMSes composite catalysts likely to adsorb droplets and promote traps of electrolyte ions and access reported recently for electrochemical water splitting are listed in to active sites. The (Ni, Co)0.85Se NSAs/NF catalyst shows excellent Table S3 (Supporting Information). capability and stability in both OER and HER in alkaline medium. In addition to metal substrates, carbon materials such as CC and CFP are 4.3.1. Hierarchical TMSes/TMSes composites attractive [200,213–216]. Wang et al. [215] have produced porous Hierarchical structures offer more active sites, improve charge Ni0.7Co0.3Se2 nanosheets on CC for water splitting and the transportation, and suppress aggregation of materials. Zhang et al. [227] three-dimensional structure promotes transfer of charge and electrons have prepared hierarchical MoSe2/NiSe2 composite nanowire arrays on and increases the accessible catalytic active sites. CFP with improved HER activity because the three-dimensional hier­ archical structure exposes more active sites and prevents the nanosheets 4.2.3. Other multi-metal selenides from aggregation or re-stacking. Wang et al. [95] observe excellent Much work has been done to optimize the electrocatalytic properties conductivity and abundant exposed active sites on the hierarchical of TMSes by doping with transition metal atoms to modify the electronic three-dimensional [email protected] nanowire network during HER in and spatial structures. Zhao et al. [217] have prepared Mn-regulated the basic medium. Wang et al. [228] have synthesized unique hierar­ cobalt selenide nanosheets with modulated electrical conductivity for chical MoSe2/CoSe2 nanotubes with excellent HER performance under OER. By regulating the atomic disorder, modifying the electronic acidic and basic conditions due to the unique hierarchical structure structure, and improving the electrical conductivity with Mn, more consisting of uniformly distributed few-layered MoSe2 nanosheets, and active sites and better OER kinetics can be accomplished. Fe, Co, and Ni CoSe2 nanoparticles, more active sites or edges for HER, and suppressed are common heteroatoms to enhance the electrocatalytic active sites, aggregation of MoSe2 nanosheets. The hierarchical MoSe2/CoSe2 improve the electronic conductivity, and modulate the hydrogen nanotubes on graphene nanosheets promote electron transportation adsorption energy of TMSes [218–220]. Density-functional theory from the electrode to catalyst during HER [229]. The hierarchical calculation reveals that Ni or Co doping enhances the water adsorption mesoporous MoSe2@CoSe/N-doped carbon (N–C) composite prepared capability and optimizes the hydrogen adsorption free energy of the by Chen et al. [230] has excellent HER characteristics attributed to the TMSes [35,98]. Ou et al. [221] have synthesized Co-doped MoSe2 high electronic conductivity of N–C, interactions between MoSe2 and nanosheets with few layers with improved HER activity resulting from CoSe, and large electrode-electrolyte contact area. the structural merits, highly electrical conductivity, and large quantities of active edge sites. Zhao et al. [201] have studied a series of 4.3.2. Hetero-structured TMSes/TMSes composites nickel-tungsten selenide compounds and found that Ni0.54W0.26Se Hetero-structures have improved HER and OER electrocatalytic nanocrystals have excellent HER catalytic activity due to strong syner­ properties compared to single-component catalysts due to the modu­ gistic effects between W, Ni, and Se. Zhu et al. [222] have prepared lated physicochemical properties, enhanced interfacial contact, more

15 X. Peng et al. Nano Energy 78 (2020) 105234 active interface, and accelerated electron transportation. Therefore, (Fig. 13). Zhang et al. [235] have developed a new bifunctional nickel construction of well-defined hetero-structures is an effective approach to selenide (NiSe/Ni3Se2 hybrid) electrocatalyst supported on NF for du­ design high-performance electrocatalysts and composites of Co/Ni and rable HER and OER as a result of the unique three-dimensional structure, Mo based selenides have aroused attention. Zhou et al. [231] have large active area, and synergistic effects between NiSe and Ni3Se2. produced MoSe2–NiSe hetero-structures with a well-defined heteroge­ Theoretical calculation indicates that the strong interactions and inter­ neous interface by electronic structure modulation. Owing to the higher face reconstruction between NiSe and Ni3Se2 by Ni–Se bonding produce Fermi energy level of NiSe compared to MoSe2, electrons are transferred the outstanding electrocatalytic capability to support the experimental from the metal nanocrystal to the MoSe2 matrix to realize nanoscale results. Zhao et al. [236] control the charge state and electrocatalytic electron regulation consequently accelerating electron transfer in HER. characteristics by modulating the mass ratio of Co and Se. The exposed CoSe2/MoSe2 hetero-structures in which water-adsorption-favorable electrocatalytic centers and fast charge transfer capabilities of CoSe/­ CoSe2 species are decorated on MoSe2 nanosheets have been designed by Co9Se8 prepared on a Co foil result in active and stable OER, HER, and Zhao et al. [232]. The CoSe2/MoSe2 hetero-structures have better HER overall water splitting activities. catalytic activity than the individual MoSe2 and CoSe2 due to the syn­ In order to promote the electrocatalytic activity of nickel selenides ergetic effects between CoSe2 and MoSe2. The water adsorption/disso­ composites, Zhong et al. [237] regulate the phase conversion from ciation process is accelerated by additional water adsorption sites Ni3Se2 to NiSe in the preparation of Ni3Se2/NiSe nanorod arrays with provided by CoSe2 and the subsequent processes take place rapidly on enhanced overall water splitting ability as shown in Fig. 14. The degree the abundant exposed MoSe2. To further improve the activity, surface of phase conversion determines the number of junctions, which in turn defects have been proposed. Chen et al. [233] have synthesized defec­ affects the quantity of active sites, electrical conductivity and chemi­ + tive MoSe2/CoMoSe lateral hetero-structures by introducing cobalt into sorption free energies of H and OH to promote separation of inter­ the MoSe2 layers. Because of physical distortion in the CoMoSe alloy mediate states at the different interfaces [238]. As a result of nano-layers, atomic level coarsening in the hetero-structure interfaces, rearrangement of atoms, partial phase conversion can promote the and formation of defects in the MoSe2 nanolayers caused by Co, the electrical conductivity and produce extra active sites to achieve excel­ catalyst exhibits promoted HER capability compared to intrinsic MoSe2. lent HER, OER and overall water splitting activities.

4.3.3. Phase engineered TMSes/TMSes composites 5. Conclusion and outlook Regulating the phase of single-metal selenide can generate active phase interface, modulate the electronic structure, improve conductiv­ In this review, recent advance of TMSes-based catalysts including ity, and take advantage of the synergistic effects of different phase single-metal selenides, multi-metal selenides, and TMSes-based com­ constitutions. Chen et al. [234] have prepared NiSe–Ni0.85Se/CP posites as non-noble metal and low-cost electrocatalysts for HER, OER, hetero-structured catalysts by in situ selenylation of a NiO nanoflake and overall water splitting are described. TMSes-based catalysts possess array and controlling the initial Ni:Se molar ratio of the precursors as excellent physical and chemical properties such as high conductivity and bifunctional electrocatalysts for HER and OER under alkaline condi­ unique electronic structures which are responsible for the better per­ tions. The NiSe–Ni0.85Se/CP catalyst delivers better electrocatalytic formance in electrochemical water splitting. Because of the regulated performance in HER and OER than single-phase NiSe/CP and electronic structure and electrical conductivity, heteroatom-doped sel­ Ni0.85Se/CP because of easier absorption of H and OH on NiSe–Ni0.85Se enides (multi-metal selenides) and TMSes-based composites deliver

Fig. 13. (a) IR-corrected polarization curves; (b) Differences in the current density (Δj = ja jc) at 0.25 V as a function of scanning rates; (c) Nyquist plots of the electrochemical impedance spectra; (d) DFT-calculated HER free energy change. Reproduced with permission [234] and copyrighted 2018, Wiley-VCH.

16 X. Peng et al. Nano Energy 78 (2020) 105234

Fig. 14. Schematic illustration of the preparation of NiSe/NF catalysts. Reproduced with permission [237] and copyrighted 2018, Wiley-VCH. better electrocatalytic performance than single-metal selenides. How­ HER and OER are normally conducted in strong acidic or basic media ever, although considerable development has been made on TMSes- and so the physical, chemical, and electrochemical stability of non- based electrocatalysts recently, there is still room for improvement noble-metal-based electrocatalysts under such harsh conditions have and possible strategies are described in the following. significant impact on their electrocatalytic performance. The evolution of surface composition, atomic arrangement, elemental chemical states, (a) Surface engineering surface charge transfer efficiency, and defects during the electrocatalysis must be monitored by in situ observation techniques. To maintain the Catalytic reactions always occur on the surface of electrocatalysts via high catalytic activity and long-term stability of TMSes-based catalysts, charge transfer through the catalyst. Hence, the atomic arrangement, it is important to protect the nanostructured TMSes, such as coating, electronic structure, defects, and associated effects in the near surface encapsulating, anchoring, three-dimensional substrate supporting, etc., dominate the catalytic activity. Moreover, the conductivity of TMSes- from corrosion and structural collapse in the electrolyte during water based catalysts plays important role in charge transfer and catalytic splitting. kinetics. Fortunately, TMSes are mostly semi-conducting and con­ ducting (metallic) so that charge transportation can be enhanced (d) Commercial adoption routinely. However, TMSes tends to be easily oxidized form surface native oxide during OER consequently lowering the conductivity and Some high-performance HER, OER, and overall water splitting cat­ catalytic activity. The surface composition and chemical state of the alysts based on TMSes have been prepared in recently years but there is elements can be observed by X-ray photoelectronic spectroscopy and X- much room for improvement since most of the work has been conducted ray absorption fine structure. Therefore, it is essential to investigate the in the laboratory environment. Economical and convenient mass pro­ physical (conductivity, electronic structure, etc.) and chemical states duction is vital to the development of commercial TMSes based catalysts (atomic arrangement, atomic concentration, etc.) of the TMSes surface but it is still a challenge to scale up to production level. Therefore, and evolution during long-term water splitting and determine the rela­ commercial processes, especially environmentally friendly and tionship between the surface states and electrochemical properties. economical ones, must be developed to bring the technology to fruition.

(b) Synergistic mechanisms (e) Commercial prospects

Synergetic effects should be studied systematically and in-depth in TMSes have shown potential as electrodes in energy storage and order to understand the enhancement mechanisms in catalysis. With water splitting devices but the application should continue to be regard to multi-metal selenides and TMSes-based composite catalysts, extended due to materials merits such as the controllable bandgap, the improvement can be ascribed to synergetic effects but such effects electronic structure, composition, and phase. For example, bandgap should be different in these two systems, since the Se is in the form of engineering of TMSes by heteroatoms doping, atomic layers regulation, M1-Se-M2 (M1 and M2 for different transition metals) in multi-metal and heterojunction constructing may bode well for photochemical and selenides while composite of M1-Se/M2-Se (M1 and M2 for transition photo-electrochemical applications (photocatalysis, photodetectors, metals) in TMSes-based composite catalysts. Different bonding of Se photo-electrochemical sensors) and proper modulation of the electronic with transition metals would result in various surface atomic arrange­ structure of TMSes may render them desirable for applications such as ment and electronic structure. It is therefore important to investigate the CO2 reduction, nitrogen reduction, sensors, and more environmental individual mechanisms and furthermore, optimal regulation of the control processes. Considering the catalytic capability and lithium synergetic effects should be attempted to maximize the electrocatalytic storage capacity of selenides, the lithiation/delithiation process of activity of non-noble-metal-based electrocatalysts. In situ characteriza­ TMSes based electrodes may be catalyzed by themselves and the mate­ tion techniques such as Fourier transform infrared spectroscopy, atomic rials may have a bright future in the energy storage industry such as force microscopy, transmission electron microscopy, and others are lithium-Se battery systems. necessary to monitor the electrocatalysis processes and theoretical calculation is also helpful in providing clues for the synergistic effects. Declaration of competing interest (c) Long-term stability The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence

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[215] X. Wang, Y. Zheng, J. Yuan, J. Shen, J. Hu, A.-j. Wang, L. Wu, L. Niu, Porous NiCo diselenide nanosheets arrayed on carbon cloth as promising advanced catalysts – used in water splitting, Electrochim. Acta 225 (2017) 503 513. Xiang Peng received his PhD in physics and materials science [216] Z. Zhang, Y. Liu, L. Ren, H. Zhang, Z. Huang, X. Qi, X. Wei, J. Zhong, Three- from City University of Hong Kong in 2017. He was a post­ dimensional-networked Ni-Co-Se nanosheet/nanowire arrays on carbon cloth: a doctoral fellow at City University of Hong Kong from 2017 to flexible electrode for efficient hydrogen evolution, Electrochim. Acta 200 (2016) 2018. He is now a professor of Materials Science and Engi­ – 142 151. neering at Wuhan Institute of Technology. He has authored [217] X. Zhao, X. Li, Y. Yan, Y. Xing, S. Lu, L. Zhao, S. Zhou, Z. Peng, J. 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A 6 (2018) structures for electrochemical energy storage and conversion. 12701–12707. [227] L. Zhang, T. Wang, L. Sun, Y. Sun, T. Hu, K. Xu, F. Ma, Hydrothermal synthesis of 3D hierarchical MoSe2/NiSe2 composite nanowires on carbon fiber paper and their enhanced electrocatalytic activity for the hydrogen evolution reaction, J. Mater. Chem. A 5 (2017) 19752–19759. [228] X. Wang, B. Zheng, B. Yu, B. Wang, W. Hou, W. Zhang, Y. Chen, In situ synthesis of hierarchical MoSe2-CoSe2 nanotubes as an efficient electrocatalyst for the hydrogen evolution reaction in both acidic and alkaline media, J. Mater. Chem. A 6 (2018) 7842–7850. [229] X. Wang, B. Zheng, B. Wang, H. Wang, B. Sun, J. He, W. Zhang, Y. Chen, Hierarchical MoSe2-CoSe2 nanotubes anchored on graphene nanosheets: a highly efficient and stable electrocatalyst for hydrogen evolution in alkaline medium, Electrochim. Acta 299 (2019) 197–205. Chao Huang is presently a PhD candidate under the supervi­ [230] J. Chen, A. Pan, Y. Wang, X. Cao, W. Zhang, X. Kong, Q. Su, J. Lin, G. Cao, sion of Prof. Paul K. Chu in the Department of Physics, City S. Liang, Hierarchical mesoporous MoSe2@CoSe/N-doped carbon nanocomposite University of Hong Kong. He received his MS and BS degrees for sodium ion batteries and hydrogen evolution reaction applications, Energy from Wuhan University of Science and Technology. His Storage Mater. 21 (2019) 97–106. research focuses on synthesis of functional nanomaterials and [231] X. Zhou, Y. Liu, H. Ju, B. Pan, J. Zhu, T. Ding, C. Wang, Q. Yang, Design and their applications in energy storage and conversion. epitaxial growth of MoSe2-NiSe vertical heteronanostructures with electronic modulation for enhanced hydrogen evolution reaction, Chem. Mater. 28 (2016) 1838–1846. [232] G. Zhao, P. Li, K. Rui, Y. Chen, S.X. Dou, W. Sun, CoSe2/MoSe2 heterostructures with enriched water adsorption/dissociation sites towards enhanced alkaline hydrogen evolution reaction, Chem. Eur. J. 24 (2018) 11158–11165. [233] X. Chen, Y. Qiu, G. Liu, W. Zheng, W. Feng, F. Gao, W. Cao, Y. Fu, W. Hu, P. Hu, Tuning electrochemical catalytic activity of defective 2D terrace MoSe2 heterogeneous catalyst via cobalt doping, J. Mater. Chem. A 5 (2017) 11357–11363. [234] Y. Chen, Z. Ren, H. Fu, X. Zhang, G. Tian, H. Fu, NiSe-Ni0.85Se heterostructure Weihong Jin received her PhD in physics and materials Science nanoflake arrays on carbon paper as efficient electrocatalysts for overall water from City University of Hong Kong in 2015. She was a post­ splitting, Small 14 (2018) 1800763. doctoral fellow at City University of Hong Kong from 2015 to [235] F. Zhang, Y. Pei, Y. Ge, H. Chu, S. Craig, P. Dong, J. Cao, P.M. Ajayan, M. Ye, 2017. She is now an associate professor at Jinan University. Her J. Shen, Controlled synthesis of eutectic NiSe/Ni3Se2 self-supported on Ni foam: research interests are biomaterials and surface engineering. an excellent bifunctional electrocatalyst for overall water splitting, Adv. Mater. Interfaces 5 (2018) 1701507. [236] Y. Zhao, B. Jin, Y. Zheng, H. Jin, Y. Jiao, S.Z. Qiao, Charge state manipulation of cobalt selenide catalyst for overall seawater electrolysis, Adv. Energy Mater. 8 (2018) 1801926. [237] Y. Zhong, B. Chang, Y. Shao, C. Xu, Y. Wu, X. Hao, Regulating phase conversion from Ni3Se2 into NiSe in a bifunctional electrocatalyst for overall water-splitting enhancement, ChemSusChem 12 (2019) 2008–2014. [238] Q. Li, D. Wang, C. Han, X. Ma, Q. Lu, Z. Xing, X. Yang, Construction of amorphous interface in an interwoven NiS/NiS2 structure for enhanced overall water splitting, J. Mater. Chem. A 6 (2018) 8233–8237. [239] L. Mi, Q. Ding, H. Sun, W. Chen, Y. Zhang, C. Liu, H. Hou, Z. Zheng, C. Shen, One- pot synthesis and the electrochemical properties of nano-structured nickel selenide materials with hierarchical structure, CrystEngComm 15 (2013) 2624.

22 X. Peng et al. Nano Energy 78 (2020) 105234

Biao Gao received his MS and PhD in materials science from Paul K. Chu received his BS in mathematics from The Ohio Wuhan University of Science and Technology in 2012 and State University and MS and PhD in Chemistry from Cornell 2016, respectively. He is an associate professor in the State Key University. He is Chair Professor of Materials Engineering in the Laboratory of Refractories and Metallurgy at Wuhan University Department of Physics, Department of Materials Science and of Science and Technology and a Hong Kong Scholar at City Engineering, and Department of Biomedical Engineering in City University of Hong Kong. His research activity involves nano­ University of Hong Kong. He is Fellow of the American Physical structures for electrochemical energy storage. Society (APS), American Vacuum Society (AVS), Institute of Electrical and Electronics Engineers (IEEE), Materials Research Society (MRS), and Hong Kong Institution of Engineers (HKIE). He is also Fellow and a member of the membership committee of the Hong Kong Academy of Engineering Sciences (HKAES). His research interests are quite diverse spanning plasma surface engineering, materials science and engineering, surface sci­ ence, and functional materials. He is a highly cited researcher in materials science according to Clarivate Analytics of the Web of Science.

23 Supporting information

Recent Advance and Prospectives of Electrocatalysts Based on

Transition Metal Selenides for Efficient Water Splitting

Xiang Penga, Yujiao Yana, Xun Jina, Chao Huangb, Weihong Jinb,c, Biao Gaob,d*,

b Paul K. Chu *

a Hubei Key Laboratory of Plasma Chemistry and Advanced Materials, School of

Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205,

China b Department of Physics, Department of Materials Science and Engineering, and

Department of Biomedical Engineering, City University of Hong Kong, Tat Chee

Avenue, Kowloon, Hong Kong, China c Institute of Advanced Wear & Corrosion Resistant and Functional Materials, Jinan

University, Guangzhou 510632, China d State Key Laboratory of Refractories and Metallurgy and Institute of Advanced

Materials and Nanotechnology, Wuhan University of Science and Technology,

Wuhan 430081, China

* Corresponding authors: [email protected] (B. Gao), [email protected] (P.

K. Chu) Table S1. Summary of common preparation methods of transition metal selenides in electrochemical water splitting.

Preparation methods Advantages Disadvantages Scope of application

Large quantities of 2D layered TaSe2, MoSe2, NbSe2, WSe2, and Liquid phase exfoliation Generally organic solvents nanostructures, quick and easy Sb2Se3, etc High efficiency for 2D nanostructures, Specific equipment, High Chemical vapor deposition MSe2 layers simple and straightforward temperature

Hydrothermal/solvothermal High selectivity and efficiency for producing Unclear reaction mechanism, MoSe2, FeSe2, CoSe2, NiSe2, method nanostructure, controlled morphology time consuming CdSe, ZnSe, CuSe, etc

MoSe2, WSe2, FeSe, NiSe2, CdSe, Flexible process, easy to realize and Several influencing factors, Electrochemical deposition ZnSe, CuSe, MnSe, PbSe, Ag2Se, transform heterogeneous distribution etc Slower average deposition Pulsed laser deposition Short cycle, uniform MoSex, WSex, etc rate

Excellent controllability, good uniformity MoSe2 thin film, Cu3Se2, CuSe Chemical bath deposition Slow chemical reaction and low cost thin Films, etc High selectivity and efficiency, faster Cation exchange Uncontrolled process Ag2Se reaction rate One pot synthesis Economically and environmentally friendly Complex purification process CdSe, ZnSe Photochemical method Controllable sample thickness CdSe, PbSe

Hot injection High crystallinity, controlled morphology Low yield, impure products Co0.85Se Low synthesis temperature, easy reaction Sol-gel method Long time PbSe, ZnSe process Simple equipment, easy operation, high Low adhesion, poor process Vacuum evaporation CdSe efficiency repeatability Thermal evaporation Simple operation CdSe, ZnSe

Spray pyrolysis High film forming speed and low cost FeSe2, CdSe

Sputtering Good adhesion Limited range of use Co-Se thin films, WSex films, etc

Table S2. Summary of recently reported multi-metal selenide catalysts in electrochemical water splitting.

η10 Tafel slope Electrode Catalyst Electrolyte Morphology Mass loading Ref. [mV] [mV dec-1] structure HER

Co-MoSe2/NG nanosheets 0.5 M H2SO4 -- 58 nanosheets on GCE [1] 1 M KOH 206 81 -2 Ni-doped MoSe2 nanosheets 0.204 mg cm on GCE [2] 0.5 M H2SO4 180 67

V0.86Co0.14Se2/CC 0.5 M H2SO4 230 63.4 nanosheets on carbon cloth [3] nanosheets

-2 Ni0.54W0.26Se nanosheets 1 M KOH 162 74 nanosheets 0.45 mg cm on GCE [4]

0.5 M H2SO4 174 37 -2 Co-WSe2/MWNTs nanosheets ∼0.25 mg cm on GCE [5] 1 M KOH 241 -- dodecahedral on carbon fiber Ni-Co-Se/CFP 1 M KOH 250 72 ∼0.65 mg cm-2 [6] nanocages paper

Ni1/3Co2/3Se2 nanowires 131 40.1 nanosheet/nanowire 0.5 M H2SO4 on carbon cloth [7] Ni1/3Co2/3Se2 nanosheets 145 46.3 arrays

NiCoSe2/CC 1 M KOH 112.7 65 nanosheets on carbon cloth [8] -2 Fe0.7Co0.3Se2/RGO 0.5 M H2SO4 166 36 nanoparticles 1.1 mg cm on GCE [9]

NiCo diselenide/CC 0.5 M H2SO4 108 31.6 porous nanosheets on GCE [10] 278@100 mA Ni0.5Co0.5Se2/CC 0.5 M H2SO4 56.5 nanorod array on carbon cloth [11] cm-2 1 M KOH 85 52 mesoporous -2 Ni0.89Co0.11Se2 MNSN/NF 0.5 M H2SO4 52 39 2.0 mg cm on Ni foam [12] nanosheet networks 1 M PBS 82 78

Zn-doped MoSe2 nanosheets 0.5 M H2SO4 231 58 nanosheets on GCE [13]

Cu-Ni-CoSex porous quaternary porous 1 M KOH 50.2 49.6 0.668 mg cm-2 on FTO glass [14] nanocubes nanocubes triple-shelled Co-VSex -2 0.5 M H2SO4 81 39.1 nanocages 0.280 mg cm on GCE [15] hollow nanocages

OER

Fe-doped Ni3Se4 ultra-thin 1 M KOH 225 41 nanosheets 0.673 mg cm-2 on GCE [16] nanosheets

NixFe1-xSe2-DO 1 M KOH 195 28 nanoplates on Ni foam [17] dodecahedral on carbon fiber Ni-Co-Se/CFP 1 M KOH 300 87 ∼0.65 mg cm-2 [6] nanocages paper

NiCoSe2/CC 255.8 71 nanosheets on CC [8] NiCo diselenide/CC 1 M KOH 258 42.3 porous nanosheets on GCE [10] hybrid on carbon fiber -2 FeNi2Se4-NrGO 1 M KOH 170 62.1 ∼0.55 mg cm [18] nanocomposite paper on Cu nanowire Ni0.35Co0.65Se2/CNW 1 M KOH 193 58 porous nanosheets [19] arrays on carbon fiber Ni0.5Fe0.5Se2/CFC 1 M KOH -- 63 nanostructures [20] cloth

(Fe0.48Co0.38Cu0.14)Se thin on Au-coated 1 M KOH 256 40.8 thin films [21] film glass substrates on rotating disk -2 (Co4Mn1)Se2 nanosheets 1 M KOH 274 39 nanosheets 200 μg cm [22] electrode

Table S3. Comparison of recently reported TMSes/TMSes composite catalysts in electrochemical water splitting.

η10 Tafel slope Electrode Catalyst Electrolyte Morphology Mass loading Ref. [mV] [mV dec-1] structure

HER

MoSe2/CoMoSe lateral LH and VH 0.5 M H2SO4 305 95.2 on GCE [23] hetero-structures nanolayers hierarchical and MoSe2@CoSe/N-doped carbon 1 M KOH 66 54 [24] mesoporous structure

-2 MoSe2-NiSe hetero-structures 0.5 M H2SO4 210 56 nanohybrids ~0.285 mg cm on GCE [25] hierarchical MoSe2-CoSe2 0.5 M H2SO4 206 45 nanotubes on GCE [26] nanotubes 1 M KOH 237 89

MoSe2-NiSe@carbon 0.5 M H2SO4 154 76.3 nanosheets [27] hetero-structures 1 M KOH 180 80.6 Mn-Co-Se 1 M KOH 60 93 yolk shell structures [28]

Co-doped NiSe2/Ni3Se4/C 1 M KOH 90 81 porous film on Ni foam [29] on carbon NiSe-Ni0.85Se hetero-structure 1 M KOH 101 74 nanoflake arrays [30] paper 200 @17.5 box-in-box hollow CoSe2/(NiCo)Se2 0.5 M H2SO4 39.8 on GCE [31] mA cm-2 nanocubes

[email protected] nanowire 1 M KOH 117 66 nanowire network 6.48 mg cm-2 on Ni foam [32] network

0.5 M H2SO4 195 51.3 CoSe2-MoSe2/carbon microspheres on GCE [33] microspheres 1 M KOH 215 83.2

MoSe2-CoSe2 nanotubes on 1 M KOH 198 79 nanotubes 0.57 mg cm-2 on GCE [34] graphene nanosheets hierarchical MoSe2/NiSe2 249 @ 100 on carbon 0.5 M H2SO4 46.9 nanowires [35] composite nanowires mA cm-2 fiber paper

-2 NiSe/Ni3Se2/Ni 1 M KOH 92 101.2 nanosheet 5.7 mg cm on Ni foam [36]

-2 CoSe2/MoSe2 hetero-structures 1 M KOH 218 76 nanosheet ~ 0.204 mg cm on GCE [37] 268 @ 100 CoSe/Co9Se8 1 M KOH 61.4 3D network on Co foil [38] mA cm-2 168 @ 100 nanorods Ni3Se2/NiSe 1 M KOH 72.1 on Ni foam [39] mA cm-2 nanoparticles

OER Mn-Co-Se 1 M KOH 243 62 yolk shell structures [28] 275 @ 30 Co-doped NiSe2/Ni3Se4/C 1 M KOH 63 porous film on Ni foam [29] mA cm-2 on carbon NiSe-Ni0.85Se hetero-structure 1 M KOH 300 98 nanoflake arrays [30] paper 260 @ 20 -2 NiSe/Ni3Se2/Ni 69.2 nanosheet 5.7 mg cm on Ni foam [36] mA cm-2 280 @ 100 CoSe/Co9Se8 1 M KOH 40.4 3D network on Co foil [38] mA cm-2 370 @ 100 nanorods Ni3Se2/NiSe 1 M KOH 95.3 on Ni foam [39] mA cm-2 nanoparticles

Overall water splitting

Mn-Co-Se 1 M KOH 1.66 V @ 50 mA cm-2 yolk shell structures [28]

-2 Co-doped NiSe2/Ni3Se4/C 1 M KOH 1.6 V @ 10 mA cm porous film on Ni foam [29] 1.71 V @ 10 mA cm-2 on carbon -2 NiSe-Ni0.85Se hetero-structure 1 M KOH 1.62 V @ 10 mA cm nanoflake arrays [30] paper

-2 -2 NiSe/Ni3Se2/Ni 1 M KOH 1.6 V @ 10 mA cm nanosheet 5.7 mg cm on Ni foam [36]

-2 CoSe/Co9Se8 seawater 1.8 V @ 10.3 mA cm 3D network on Co foil [38] nanorods -2 Ni3Se2/NiSe 1 M KOH 1.61 V @ 10 mA cm on Ni foam [39] nanoparticles References

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