1. Introduction

Due to the high energy conversion efficiency and environmental friendliness, hydrogen fuel cells have been regarded as one of the most important clean energy conversion devices to solve the energy crisis and environmental pollution. However, the main challenge in hydrogen fuel cells is to find a highly efficient catalyst for the oxygen reduction reaction (ORR) [1]. The complex multi-step electron transfer process and the slow mass diffusion of the reaction itself lead to slow ORR kinetics [2]. Hitherto, platinum (Pt)-based catalysts are still considered as the most efficient ORR catalysts, but the high cost and poor durability severely limit their wide applications [3]. Therefore, these disadvantages have triggered intense interest in non-Pt catalysts with both high ORR activity and stability. Currently, a variety of non-platinum metal-based ORR catalysts, such as defective carbon materials [4], heteroatom-doped metal-free carbon [5], transition metal/heteroatom-co-doped carbon [6,7], intermetallic compounds [8], and metal nitrides [9], have been synthesized. Among them, two-dimensional (2D) materials with atomic or molecular thickness and infinite planar length exhibit peculiar physicochemical properties, including high specific surface area, adjustable band structure, and electromagnetic properties, which make 2D materials become one of the most important non-Pt-based ORR catalysts. [10–13].

A variety of 2D materials have been found for electrocatalysis. Among heteroatom-doped carbon [14], layered metal hydroxide (LDH) [15], layered metal oxide [16], 2D metal-organic frameworks (2D-MOFs) [17], 2D covalent organic frameworks (2D-COFs) [18], graphite carbonitrides (g-C3N4) [19], hexagonal boron nitride (h-BN) [20], black phosphorous (BP) [21], MXenes [22], and 2D transition metal dichalcogenides (TMDs) [23–25], TMDs are a class of materials with great application prospects and basic research value. The chemical formula of TMDs is MX2, where M represents transition metal elements, including group IV (Ti, Zr, or Hf), group V (V, Nb, or Ta), group VI (Mo, W), group VII (Tc, Re), or group X (Pd, Pt), and X refers to a chalcogen atom (S, Se). In a TMD, X-M-X units stack on each other to form a sandwich structure, and the layers are combined by van der Waals forces [26].

As the thickness decreases, 2D TMDs exhibit a series of special properties. During the conversion process of TMDs from bulk to single-layered 2D materials, significant bandgap transitions occur, resulting in excellent photoelectric performance. Besides, due to the high degree of structural controllability and anisotropy of 2D materials, 2D TMDs can be controlled to form various shapes and sizes, doped with different heteroatoms, and their surfaces can be modified with other materials to obtain a unique crystal structure [27]. More importantly, there are unsaturated coordination and dangling bonds at the edges of TMDs nanosheets. These special properties of TMDs nanosheets provide many inspirations for basic research in many fields including catalysis [28–33], transistor [34], energy storage [35–39] and sensor [40–42]. Among all 2D TMDs, MoS₂ is one of the few with a natural layered structure, indicating that MoS₂ can be stripped to obtain high-quality 2D MoS₂ without complicated chemical synthesis [26]. Therefore, the cost of preparing 2D MoS₂ is much lower than other 2D TMDs due to the simple synthesis conditions. More importantly, 2D MoS₂ is a 2D semiconductor with a direct band-gap, which has the best electric performance among 2D TMDs [43]. These make 2D MoS₂ get more attention among 2D TMDs.

2. Structural Properties of 2D MoS₂

The properties of 2D MoS₂ are highly dependent on their crystal phase, structure size and chemical composition. In the MoS₂ structure, the coordination of S elements and the stacking state between layers can form different electronic characteristics. The most common crystal structures of MoS₂ are 1T and 2H phases, where the number represents the number of layers of the S-Mo-S layer in a unit cell, and T and H represent the tetragonal crystal system, and the hexagonal crystal system, respectively [44]. As shown in Figure 1A，in the 2H phase, each Mo atom exists in the center of the hexagonal prism coordination structure and covalently bonded to six S atoms, and exhibits semiconductor properties. In the 1T phase, one Mo atom and six S atoms form a twisted octahedral coordination structure, with metallicity [45]. In MoS₂, the S-Mo-S of each planar unit is regarded as a single layer, which is composed of two layers of S atoms and an intermediate layer of Mo atoms [46]. Due to the weak van der Waals force connection between the layers, the peeling between the layers is easy to occur, which makes it possible to synthesize single-layer MoS₂ [47].

Figure 1. (A) Metal coordination, top view, and side view of single-layer 2H-MoS₂ and 1T-MoS₂. Reproduced with permission from [44] Copyright © 2021 The Royal Society of Chemistry. (B) Energy dispersion (energy versus wavevector k) in (1) bulk, (2) quadrilayer, (3) bilayer, and (4) monolayer MoS₂ from left to right. Reproduced with permission from [48] Copyright © 2021 The American Chemical Society.

As shown in Figure 1B, the bulk material of MoS₂ is an indirect-bandgap semiconductor, in which the top of the valence band is located at point G and the bottom of the conduction band is located at the midpoint of the axis of symmetry of G–K, while in single-layer MoS₂, the top of the valence band and the bottom of the conduction band coincide with point K, indicating that single-layer MoS₂ is a direct-bandgap semiconductor [48]. This indirect-to-direct transition of the bandgap from bulk to the single-layered 2D material is caused by the quantum confinement effect. The electronic properties of 2D MoS₂ are closely related to the d electrons existing in the d orbital between the bonding band and the anti-bonding band of the Mo–S bond. In 2H-MoS₂, the non-bonded d orbitals are fully occupied, resulting in semiconductor properties. In 1T-MoS₂, the non-bonded d orbitals are partially filled, resulting in higher conductivity. As the d-orbital electron occupancy rate decreases, 2D MoS₂ gradually changes from a semiconductor to a conductor, which endows the material with different physical and chemical properties [27]. When the thickness of MoS₂ drops to the atomic level, a large number of exposed surfaces can undergo oxidation or reduction reactions with various reagents [49]. These surface atoms can escape in the reaction to form vacancy defects, simultaneously leading to the disorder of the nanostructure and a reduction in the coordination number of surface atoms. By adjusting the vacancy defects, we can further modify the electronic characteristics of MoS₂. At the same time, unsaturated edges and dangling bonds often appear on the edges of 2D MoS₂. These unique edge defects also provide possible active sites for catalytic reactions [2,50].

So far, many 2D MoS₂-based materials have been applied to ORRs. At present, it is generally believed that the active site of 2D MoS₂ in the ORR is the unsaturated Mo site exposed at the edge. Because the Mo site is positively charged due to the possible polarization effect of the surrounding negatively charged S atoms, it can, therefore, be easily combined with negatively charged O atoms [51]. Simultaneously, the vacancy defects on the basal plane also contribute to ORR performance. In general, an excellent ORR electrocatalyst must have favorable stability, small overpotentials, high current densities, and low manufacturing cost [52]. Although MoS₂ has a low cost and high chemical stability, its large inert basal plane and poor electron transport properties limit its catalytic activity. However, it is undeniable that after adjusting the structure of 2D MoS₂, it still possesses unique advantages and broad development prospects for ORRs. Here are some advantages of 2D MoS₂ in ORRs:

1) The uncoordinated metallic edge centers not only provide a certain intrinsic ORR activity but also give an opportunity for effective functionalization with different groups.

2) Compared with bulk MoS₂, nanostructured MoS₂ has a smaller particle size and a larger specific surface area. The small size 2D MoS₂ is more sensitive to the catalytic reactions compared with bulk MoS₂. At the same time, the large surface area can anchor more active sites [53]. Besides, 2D MoS₂ has a unique open structure, which makes ORR reactants easily access the active site.

3) 2D MoS₂ with a uniformly exposed lattice plane can supply a much simpler condition as an ideal platform for combing experiments and theoretical results, which is helpful to further understand the origin of ORR activity.

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This entry is adapted from the peer-reviewed paper 10.3390/mi12030240