Nano Energy 78 (2020) 105234 Contents lists available at ScienceDirect Nano Energy journal homepage: http://www.elsevier.com/locate/nanoen 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