Electrocatalysts based on MoS2 and WS2 for hydrogen evolution reaction: An overview

Recently, hydrogen energy has been significantly investigated by numerous technologies. To date, noble platinum group metals have often been employed to fabricate working electrodes for hydrogen evolution reaction (HER). Therefore, the demand of highly active HER catalysts based on effective and lower‐cost materials is becoming more and more critical. Transition metal dichalcogenide (TMD) materials could be one of the most suitable materials for these requirements because they possess numerous unique mechanical, electronic, and chemical characteristics that are greatly beneficial for HER processes. Among many TMD materials, tungsten disulfide (WS2) and molybdenum disulfide (MoS2) are the most well‐known TMD materials which have been intensively studied for different applications, including HER, batteries, and supercapacitors. In this review, we tried to cover the HER mechanism of catalysts and their parameters. Besides that, the structures, properties, preparation, and HER performance of catalyst materials based on WS2 and MoS2 are comprehensively discussed. After that, the challenges and future trends of catalysts based on WS2 and MoS2 for HER are also considered.

plate structures, such as graphene, transition metal dichalcogenides (TMDs), nanolayers, and nanofilms. Three-dimensional (3D) nanomaterials are materials that are not constrained to the nanoscale in any dimension. Three-dimensional materials could exhibit their morphologies in various shapes, including bulk powders, dispersions of nanoparticles, bundles of nanowires, nanotubes, and multi-nanolayers. Among different materials, 2D materials exhibit numerous outstanding properties that are suitable for many technologies. [4][5][6][7][8] The first report concerning 2D materials was published on graphene in 2004, which are materials with thicknesses between 0.4 and 1.5 nm. [9][10][11] Besides that, the weak interaction between basal planes of two layers (van der Waals forces-vdW forces) could lead to various unique physical or chemical properties. The vdW force is a distance-dependent interaction between atoms and molecules. The vdW forces can be repulsive or attractive forces which are highly dependent on the molecular surface area as well as the distance between molecules. The distances between layers ranging from 0.4 to 0.6 nm are ideal for the vdW forces to create ideal layered materials. The biggest advantage of 2D materials is that different thin layers can be integrated or stacked together to modify or prepare new heterostructures. 12 Therefore, 2D heterostructures with numerous structures can be prepared in which synergistic effects between different layers are formed. The synergistic effects of the heterostructure catalysts are crucial factors for their application because they can significantly enhance the catalytic activities of the synthesized materials. 13,14 Because of the layered structure, 2D materials could exhibit many unique characteristics, including larger active surfaces, smooth heterostructure interfaces, ultrafast carrier transport, and highly gate-tunable bandgaps. [15][16][17][18] Electronic carriers such as electrons or phonons in 2D materials can be transported more easily in the basal plane compared with the movement of carriers between different layers. Consequently, the movement of phonons and electrons in 2D catalyst materials is strongly dependent on the 2D plane properties and nanosized thickness between layers, which could cause significant changes in the electronic and optical properties of 2D materials. This explains why various types of 2D materials have been prepared with different characteristics, including insulators-hexagonal boron phosphorus, semiconductor-TMD, and semimetalblack phosphorus. 19 Along with the development of advanced technologies, semiconductors based on TMD materials have been extensively investigated because TMD materials are low-cost, easy process, earthabundant materials with many unique properties. In the last decades, TMD materials have been applied for many technologies, such as medical treatment, CO 2 reduction, energy storage, and catalytic materials and sensors. [20][21][22][23][24][25][26][27][28] TMD materials can be expressed by the general stoichiometry MX 2 , where M is a metal in transition group, including Mo, W, Ti, or Nb, and so forth, and X is an element in chalcogen group, such as S, Se, or Te. [29][30][31] Figure 1 shows the typical structure of TMD materials from different perspectives. As can be seen from Figure 1A,B, TMD materials normally exist in two typical structures: 1T and 2H. In both 1T and 2H structures, TMD monolayers are formed with three layers of X-M-X in which there is a hexagonally packed plane of metal atoms sandwiched between two planes of chalcogen atoms that are covalently bonded to the transition metal atoms. Transition metal atoms can have either octahedral or trigonal prismatic coordination. Because of the partially filled d-subshell in the outer shell of metal atoms, the different electronic behavior of TMD arises from the progressive filling of the nonbonding d-bands by transition metal electrons. Therefore, TMD materials could change their electronic properties from semiconducting to superconducting depending on their chemical compositions. In particular, group-VI TMD monolayers, such as MoS 2 , WS 2 , MoSe 2 , and WSe 2 , exhibit semiconductor behavior, with a direct bandgap in the range of 1-2 eV. In contrast, TMD multilayers or bulk TMD materials own an indirect bandgap. [32][33][34] As shown in Figure 1C-F, the structure of TMDs is observed clearly on many sides of view. It is clear that the TMDs possess the similar crystal structure and different physical characteristics, such as bandgap, thermoelectric conversion, and light-matter interaction. However, by the stacked heterostructures as shown in Figure 1G, different layers of TMDs could form numerous materials which vary dramatically the characteristics of prepared materials, including photovoltaic properties, thermal transmission, elasticity, and catalysis. Due to the d-band of metal atoms, sandwiched structure, layered materials caused by vdW forces, TMD materials possess various advantages, such as nanometer thickness, direct bandgaps, strong spin-orbit coupling, and favorable electronic and mechanical properties. These properties are beneficial for many electrochemical fields, including oxygen evolution reaction, photoelectrochemistry, hydrogen evolution reactions (HERs), batteries, and supercapacitors. [35][36][37][38][39][40][41] In Figure 1H,I, the layered structure of two typical TMD-tungsten disulfides (WS 2 ) and molybdenum disulfide (MoS 2 ) have been illustrated.
In recent decades, the energy crisis has become increasingly critical. Hydrogen energy is a great solution because it is an earth-abundant, eco-friendly, and renewable energy source. Numerous efforts have been made to produce hydrogen gas for various technologies on an industrial scale. [47][48][49][50][51][52] Among the many great catalytic materials, WS 2 and MoS 2 have been considered as the two flagship catalysts for the HER process with superior catalytic activities. [53][54][55] To fully understand the HER mechanism, numerous efforts from scientists, as well as the development of advanced technologies to precisely control the process are required. Most scientists agree that the HER performance of catalysts significantly depends on the electroconductivity and the active sites of the materials. For TMD materials, the active sites are usually considered as the edges of the catalysts. Therefore, the more edges catalysts possess, the higher HER performance could be exhibited. Due to their layered structure, which presents numerous active sites, TMD materials have been extensively studied for the HER process. The mechanisms of the HER processes based on MoS 2 and WS 2 catalysts could be depicted in Figure 2A,B which are in good agreement with previous reports. 56,57 As can be seen, the mechanisms of MoS 2 and WS 2 are highly similar. The H + ions in the solution are reduced by electrons from the cathode electrode, which is made from the synthesized catalysts. For that reason, electrode catalysts with more active sites, high conductivity, and large surface areas are ideal materials for preparing cathode electrodes where hydrogen gases can be released. In fact, as with many TMD materials, the electronic conductivity of MoS 2 and WS 2 is still poor that is the huge challenge to apply MoS 2 and WS 2 as practical catalysts for large scale. In the last decades, considerable efforts and various strategies have been intensively applied to improve the HER performance of electrode catalysts based on both MoS 2 and WS 2 .
In this work, we focus on the fundamental mechanism of HER process with theoretical calculations related to the HER process. First, different advanced techniques have been applied to explore the catalytic mechanisms and the electrochemical HER from the viewpoint of computational quantum chemistry. Second, the structures and characteristics of MoS 2 and WS 2 catalysts are comprehensively summarized. In the subsequent sections, we mainly discuss the state-of-the-art advances in HER using electrocatalysts electrodes based on MoS 2 and WS 2 materials. Synthesis protocols and new approaches to prepare MoS 2 and WS 2 materials were encapsulated. Different techniques to improve the efficiency of catalysts have been summed up including the increase of intrinsic activity and the increase of active sites of catalysts based on MoS 2 and WS 2 materials. The relationship between the catalytic activity, morphology, structure, composition, and synthetic method was also deeply reviewed. Finally, the challenges, perspectives, and research trends in future HER-related catalysts for the development of sustainable and clean hydrogen energy are put on the table. On the basis of the valuable analysis and the significant developments of technologies, catalyst materials based on MoS 2 and WS 2 could be considered as great candidates which can make a breakthrough in HER catalyst development.

| HER MECHANISM OF CATALYSTS
To date, electrochemical water splitting or hydrolysis of water has been considered one of the most promising approaches to produce hydrogen gas, which can be employed to store renewable electricity in the form of hydrogen energy or apply for hydrogen energy technologies. Figure 3 shows the overall water-splitting mechanism using a potential. Under the applied potential, water molecules are hydrolyzed to free H 2 at the cathodic electrode and O 2 at the anodic electrode. However, it is challenging to produce H 2 and O 2 on a commercial scale because of the high overpotential. This overpotential is the excess energy required to overcome the selfionization of water and sluggish reaction kinetics of the HER. To overcome the self-ionization of pure water, the HER process is carried out in an electrolyte that uses acidic or alkaline media. The kinetic energy barrier was decreased by using electrode catalysts. The HER activity in acidic media was significantly improved compared with that in alkaline media. 60 Typically, noble and scarce catalysts, including platinum-based catalysts, have been employed to prepare cathodic electrodes for H 2 production. Therefore, the synthesis of electrode electrocatalysts that can be used in acidic media with low cost, high catalytic activity, and good durability for electrolytic water splitting is challenging. In the electrochemical water-splitting process, the overall mechanism as well as the reduction and oxidation at the cathodic and anodic electrodes can be described by the following equations 61 Water splitting requires an energy input of ΔG = 237.13 kJ mol −1 at standard pressure (P = 100 kPa) and temperature (T = 298 K) conditions. [64][65][66] The theoretical input energy of this reaction corresponds to a thermodynamic voltage requirement of 1.23 V. In fact, because of the kinetic barrier for the process, splitting water needs higher power than the theoretical input to overcome the kinetic barrier, which is usually higher than 1.8 V. 67 The excess potential is overpotential η, which is needed to compensate for intrinsic activation barriers present on the surface cathode. To overcome the challenge, many solutions have been investigated. Among them, using electrocatalysts is a prominent pathway in which electrocatalysts play a vital role in increasing reaction rates and control processes in HER. Effective catalysts can reduce the energy barriers at each step of the process. To date, various electrocatalysts that can accelerate the reaction on electrode surfaces have been investigated. Therefore, the overpotential is an important factor for evaluating the activity of electrocatalysts. To save energy consumption, a superior catalyst requires a smaller excess potential for the HER to occur. [68][69][70] In fact, the overpotential values of catalysts which are needed to generate a current density of 10 or 50 mA cm −2 are often used to compare the catalytic activities of different catalysts. 71 As previously mentioned, the HER is an electrochemical phenomenon which took place at cathodic reaction in electrochemical water splitting. The HER process involves intermediate hydrogen adsorption and desorption H 2 from the electrode surface, and the adsorption energy depends on the nature of the electrode materials used, as well as the kinetics of the HER. In the HER process, two intermediate hydrogens in electrolyte are reduced by two electrons from cathode surface under applied potential to release H 2 . There are several mechanisms which have been suggested to elucidate HER reaction mechanism. Among them, the most popular theory agrees that the reaction mechanism of the HER process in acidic and alkaline solutions is related with the adsorption and/or desorption of intermediate hydrogen (H*) which could be described by the Volmer-Heyrovsky or the Volmer-Tafel mechanism as follows 72 In alkaline media, the reaction formula for the HER is represented as follows: The different mechanisms of HER processes are illustrated in Figure 4. In which, step by step of HER processes takes place under acidic and alkaline conditions are shown. It is clear that the required energy for each process is not the same because of the different energy barriers of each step. Therefore, the optimized catalysts have been intensively investigated to accurately control the process. In fact, catalytic materials exhibit poor activity and lower efficiency in alkaline solutions much more than in acidic solutions. However, under acidic media, the used catalysts are less stable. For instance, the results indicate that the obtained reaction rate for the HER with Pt in an acidic environment is significantly higher than that under alkaline conditions. This is likely caused by the initial water dissociation process on the Pt surface is inefficient, which is shown in the Volmer step of the alkaline HER. This caused the poor HER performance of catalysts in alkaline media. Because of the high cost and poor stability of catalysts based on platinum metal groups, recently, numerous earth-abundant catalysts with good catalytic activities have been intensively studied. Among them, WS 2 and MoS 2 are excellent catalysts with high performance and superior stability in acidic environments. [75][76][77][78] Therefore, the structures and characteristics of WS 2 and MoS 2 , as well as the HER performance of electrocatalysts based on WS 2 and MoS 2 , are mainly discussed in this review.

| Overpotential
In the HER process, overpotential of catalysts is evaluated as one of the most crucial parameters for considering its catalytic performance. In addition to the current density and Tafel slope, the overpotential of each catalyst has been carefully investigated to determine the catalytic activity of the prepared materials. As mentioned above, for the HER process, an extra potential is required compared with the thermodynamic potential (E HER ). This value is considered to be the overpotential of the catalysts, as illustrated in Figure 5A. Therefore, the applied potential for the HER process can be calculated as follows: where E HER is determined by thermodynamics, iR is the Ohmic potential drop caused by the current flow in ionic electrolytes, and η is the overpotential of the catalysts.
Electrocatalysts are typically used to decrease the reaction overpotential for saving the supplied energy by activating intermediate chemical transformations. Therefore, a good catalytic material requires the lowest overpotential to achieve the highest efficiency of the process. In fact, η 10 or η 50 , which indicates the overpotential at a current density of 10 or 50 mA cm −2 of catalysts, is usually calculated to compare the HER performance of materials ( Figure 5B). A smaller η value suggests that the catalyst is more conductive and has higher HER activity. To calculate and compare the actual catalytic activity of the prepared materials, the synthesized catalysts must be loaded with a standard amount of catalyst on an electrode with a standard area electrode to evaluate the HER behavior in a standard process.

| Volcano plots
For the HER process in acidic media, the free-energy change between the cation (H + ) and the formed material (H 2 gas) was calculated thermodynamically. This energy value is constant for all catalysts, since it is an intrinsic property of H 2 . As mentioned above, the thermodynamically ideal catalyst exhibits the lowest overpotential for higher current densities. In electrocatalytic reactions, the chemical bonds of elements have been reconstructed to prepare new materials. Because of that, the HER process can be considered as bonded electrontransfer process. As per the statement in Sabatier's theory, which concludes that the binding energy between the catalysts and the reactants should be reasonable that means it is not too strong nor too weak. If the binding energy is too weak, the cation H + could be more difficult to attach to the cathode surface which causes the rate of reduced reaction to take place very slowly. In contrast, if the binding energy is too strong, the released H 2 product could be difficult to detach from the cathode surface. This could also form a bubble layer on the cathode surface which can stop the process. Therefore, the ideal catalyst is the one with zero binding energy between the catalyst and the intermediate H*. In the HER process, the two steps of H + reduction and adsorbed H* desorption on the cathode surface. The key HER parameter is hydrogen binding energy (HBE). As shown in Figure 6A, a catalyst surface with very weak hydrogen binding (H-binding) cannot efficiently initiate the reaction. On the other hand, a catalyst surface with very strong binding would prohibit the desorption of H* to generate H 2 . For an ideal HER electrocatalyst, H-binding should be moderate to balance the two elementary steps. To calculate the HBE, recently, density functional theory (DFT) has been intensively studied to calculate the hydrogen-binding-free energy of a catalyst (ΔG H* ). The ΔG H* of MoS 2 and WS 2 and many other catalysts are shown in Figure 6B. It is clear that the Volcano peak is located at ΔG H* = 0, which is the ideal ΔG H* value of the catalyst. In which, Pt has a ΔG H* value closest to the peak position of the Volcano curve. This indicates that Pt-based catalysts are the most effective for electrochemical applications. Nevertheless, catalysts based on Pt metal groups have significant drawbacks, including high cost and low stability. Therefore, numerous efforts have been devoted to prepare catalysts with zero ΔG H* , low cost, easy process, and stability.

| Tafel slope
Along with overpotential, the Tafel slope is another critical factor for evaluating the HER performance of catalysts. Microkinetic analysis and the rate-determining step (RDS) of the HER process could be explained by using the Tafel value. The Tafel slope value is often studied to compare the electrocatalytic activities of catalysts and explain the mechanism of the HER performance. A smaller Tafel slope means that a lower overpotential is required to deliver the same current density increment, which means that the electron-transfer kinetics of the reduction-oxidation process are faster. A high-performance electrocatalyst exhibits a high-exchange current density and a small Tafel slope. To the best of our knowledge, Pt is the most effective catalyst, which shows the smallest Tafel slope of approximately 33 mV dec −1 . The Tafel slope values of other synthesized catalysts have been usually compared with that of Pt. The Tafel slope is used to represent the correlation between the catalytic current density (j) and overpotential η, which can be inferred from the Butler-Volmer kinetic model using the following equation [85][86][87] : where η is the overpotential of the reaction process, j is the current density, j 0 is the exchange current density, α is the charge-transfer coefficient, n is the number exchanged electrons of process, F is the Faraday constant, R is the ideal gas constant, and T is the reaction temperature. If overpotential is small (η < 0.005 V), the Butler-Volmer equation could be calculated as At a higher overpotential (η > 0.05 V), the Butler-Volmer equation can be expressed as The linear relationship between the overpotential η and log j is calculated as Tafel slope value: The Tafel slope can be obtained from the linear part of the Tafel plot by plotting the overpotential using Equation (14). On the basis of the obtained Tafel slope, the mechanism of the HER process could be analyzed. Figure 7 shows different RDSs of the HER process. The RDS of the process could be Volmer reaction, Heyrovsky reaction, and Tafel reaction which is corresponded with a Tafel slope of~120,~40, and~30 mV dec −1 , respectively. On the basis of the Tafel slope, the mechanism of the HER process could be elucidated.

| Electrochemical impedance spectroscopy (EIS)
The conductivity of catalysts applied for the HER is also a critical parameter for evaluating the activity of materials.
As mentioned above, the HER involves the adsorption process of H* onto the electrode surface and the desorption process of H 2 from the electrode surface. The Langmuir adsorption modern is often used to explain the mechanism of the adsorption process of H* onto the electrode surface. Recently, EIS has been intensively applied to study the changes in capacitance and interfacial electron-transfer resistance at the electrode surface. Therefore, EIS techniques could be employed to evaluate the rates and kinetics of adsorption and desorption processes of H 2 from electrode surfaces. The EIS profiles of hybrid materials between graphene oxide (GO) with CH 3 NH 3 PbI 3 (PSK) are described in the F I G U R E 7 Tafel slope for hydrogen evolution reaction as a rate-determining step. Reproduced with permission from Shinagawa et al. 87 Copyright: 2015 Springer Nature.  Nyquist Plot in Figure 8. In which, the internal electrical processes of materials were discussed. It can be seen that the equivalent circuit is composed of constant-phase elements and charge-transfer resistances. The chargetransfer resistances include the wire connection resistance (R s ), the resistance between the active material and electrolyte (R 1 ), and resistance between the active material and glassy carbon electrode (R 2 ). For the HER, a smaller R 1 value suggests a faster reaction rate, leading to a smaller overpotential.

| Stability of catalysts
The prolonged working time of the catalyst materials is also a vital factor in catalyst studies. The more stable the catalysts, the lower the cost of hydrogen energy; and the stability of the material is considered to be a key factor in evaluating catalytic activity. Meanwhile, the cyclic voltammetry (CV) test revealed the stability of the catalytic material after repeated cycling. The chronoamperometry (CA) and chronopotentiometry (CP) tests show the dependence of the catalyst stability on testing time at a potential or constant current density, respectively. Normally, CV tests are conducted for at least 1000 cycles to observe the changes in the linear sweep voltammetry (LSV) curves. Smaller changes in LSV curves indicate better stability of the catalyst. In the CA tests, the current density changes under fixed potential with time were carefully studied. In contrast, in the CP tests, the changes of overpotential at a constant current density could be intensively investigated. Both the CA or CP tests for catalysts are usually surveyed from 10 to 60 h. There are smaller changes in the CA and CP curves which indicate better stability of catalysts. Figure 9A,B shows the stability of catalysts for HER application by CV and CP tests, respectively. The stability of the catalysts is investigated by CV tests for 1000 cycles and CP measurements for 10 h.

| Turnover frequency (TOF)
To evaluate the HER performance, the TOF, which relates to the generation of products, is also a key parameter to consider. TOF can be considered as an accurate criterion of the intrinsic activity of catalysts, which can be employed to evaluate how efficient a catalyst is for HER performance. In 1968, Michel Boudart first reported a novel method to evaluate the catalytic rate of a catalyst in terms of the TOF. In which, the TOF value is calculated by the amount of product formed or the reactant consumed per unit time by a standard area. To date, the TOF concept has been quite complicated, and it still requires common agreement from researchers. 90 Recently, the TOF value was commonly agreed by many researchers as the total number of molecules transformed into the desired product per catalytic site per unit time. There are several methods to calculate the TOF using the following equations 91 : In Equation (15), j, N A , F, n, and I represent the current density, Avogadro constant, Faraday constant, number of electrons transferred to generate one molecule of the product, and surface concentration or exact number of active sites catalyzing the reaction (m −2 ), respectively. i and A in Equation (16) represent the current and area of the electrode, respectively, and x in Equation (17) stands for the number of moles of active sites available for the catalysis. As for the HER process, n = 2, which is the number of electrons transferred when a molecule of H 2 gas is formed. Figure 10 shows the calculated TOF of the amorphous MoS 2 thin film (thickness 9.4 nm) deposited on Au film using the atomic layer deposition (ALD) technique.
In fact, it is difficult to precisely calculate the TOF of the catalyst, especially for some hybrids or heterostructure materials. The different properties of each catalyst and the complicated mechanism of electrocatalytic process are the main challenges to calculate exactly TOF value. Although the calculated TOF is relatively imprecise, however, it is still considered as a helpful parameter to compare the catalytic activities of materials.

| Surface area and pore structure
In the HER process, hydrogen gas is released from the surface of the cathode. Therefore, the larger the cathode surface area, the more hydrogen gas escapes during hydrolysis. Therefore, catalytic materials with rough and perforated morphologies are more suitable for water splitting. Moreover, the larger the pore size, the easier it is for the hydrogen gas to be liberated. To determine the surface area and pore size of the catalysts, the Brunauer-Emmett-Teller surface area using the nitrogen adsorption-desorption isotherms and Barrett-Joyner-Halenda for the pore size distribution of the materials are measured and calculated, as done in a previous study, and shown in Figure 11. The high specific area and large total volume provided more surface-active sites and pore channels, resulting in good adsorption performance.

| Electronic density of state (DOS)
Electronic DOS is a crucial parameter in modern electronic structure theories. In theory, DOS controls the distribution of energy levels that can be occupied by electrons in a quasiparticle picture. In solid state physics and condensed matter physics, DOS of a system describes the number of modes per unit frequency range. By the obtained DOS value, the electrical and optical properties of the materials could be elucidated by the bandgap and effective masses of carriers. In recent decades, WC and W 2 C have been considered a great catalytic material that can be substituted for the Pt group. 94 The catalytic activity of WC and W 2 C is considerable because of its modified d-band electron structure with a greater DOS near the Fermi level upon the incorporation of carbon interstitial atoms. To compare the catalytic properties of WC and W 2 C, the DOS was calculated. Figure 12 compares the DOS of the W and C atoms in prepared WC and W 2 C materials. As can be seen, the calculated DOS data confirmed the considerable W 5d contribution at the Fermi level. The electronic DOS of W 2 C near the Fermi level is larger than that of WC. This result can be interpreted by the higher W concentration in W 2 C, which could enhance the metallicity. Therefore, the carrier density of W 2 C is significantly improved compared with that of WC. This improvement could benefit the HER process because of the increase in charge transfer. It is also confirmed by dband theory, it states that a lower d-band center results in weaker bonding between the catalyst and adsorbate. The obtained DOS indicates that the catalytic activity of W 2 C far exceeds that of WC. The crystallinity and structure of WS 2 and MoS 2 are highly similar, as confirmed by X-ray powder diffraction (XRD) and Raman spectroscopy in Figure 13. The chemical bonding and chemical components of WS 2 and MoS 2 are also shown in Figure 13. The corresponding binding energy levels of atoms on the surface of the W-S and Mo-S bonds are presented in Figure 13A-D. As can be seen, in pure MoS 2 , the outer layer of the Mo atom is Mo 3d, which has two main energy levels, Mo 3d 5/2 and Mo 3d 3/2 , which have binding energies of 231.9 and 22.9.8 eV, respectively. Similarly, in the pure WS 2 catalyst, the outer layer of the W atom is W 4f, which has two main energy levels, W 4f 7/2 and W 4f 5/2 , which have corresponding binding energies of 34.2 and 32.1 eV. The crystallinity and the chemical bonding of WS 2 and MoS 2 have been confirmed by the XRD patterns and Raman spectra, as shown in Figure 13E,F. The synthesized WS 2 shows various peaks that can be indexed to the standard JCPDS card-08-0237. The synthesized MoS 2 also shows similar peaks located in a similar position, and is assigned to the JCPDS card-37-1492. A significant difference between WS 2 and MoS 2 is regarding the Raman peaks, presented in Figure 13F. The E g

| Structural characteristics of WS 2 and MoS 2
In addition to similar crystallinities, the structures of WS 2 and MoS 2 are also similar. The detailed structures of WS 2 and MoS 2 are presented in Figure 14A, and the calculated structural parameters, binding energies, and energy bandgaps of the WS 2 and MoS 2 catalysts are listed in Table 1, which were calculated and reported in previous studies. [99][100][101] Figure 14B shows different phases from 1T and 2H to 3R for MoS 2 and WS 2 , where the number counts the layer material per unit cell. Besides that, the character denotes for the type of symmetry, including T is tetragonal (D 3d group), H indicates hexagonal (D 3h group), and R stands for rhombohedral (C v 3 5 group). Among them, the 1T phase owns the metallic properties. In contrast, both the 2H and 3R phases present semiconducting characteristics. Compared with the 2H-phase, the 1T phase of catalysts based on MoS 2 and WS 2 exhibits the metallic properties with extraordinary electronic conductivity and larger interlayer spacing as well as more active sites along the basal plane. That means the catalytic activities of the 1T phase are more advanced compared with that of the 2H phase. In nature, MoS 2 and WS 2 are usually found in the 2H phase and the 2H phase also exhibits better stability compare with the 1T phase. A monolayer of MoS 2 or WS 2 possesses a lamellar S-Mo-S or S-W-S structure with a layer thickness of 0.65 nm. The monolayer of MoS 2 or WS 2 is formed by one layer of Mo or W atoms sandwiched by two layers of S atoms by covalent interactions. The different layers of MoS 2 and WS 2 could be held by weak vdW forces. If the structure of MoS 2 or WS 2 is trigonal prismatic, which will form a 2H phase with the semiconductor characteristics. In contrast, if the structure is octahedral so the 1T phase will be formed with metallic properties. The metallic phase 1T of MoS 2 and WS 2 was first discovered when MoS 2 or WS 2 was exfoliated by lithium intercalation into MoS 2 and WS 2 powder. Another rare phase of MoS 2 or WS 2 is 3R. The 3R phase can be described by three layers per unit cell stacked with rhombohedral symmetry and trigonal prismatic coordination. It is difficult to prepare 3R phase of catalysts based on MoS 2 or WS 2 . Therefore, very little work has been conducted on 3R phase of those materials. In Figure 14C,D, the bandgaps from the monolayer to the bulk WS 2 and MoS 2 are presented. In semiconductor materials, there are two separated energy  The catalytic activities of electrocatalysts based on WS 2 and MoS 2 are highly affected by their layered structures. Therefore, the synthesis of monolayer and event metallic phases of WS 2 and MoS 2 by controllable processes and scalable and uniform catalysts is the main challenge to prepare effective catalysts for hydrogen technologies. In recent decades, significant achievements have been made to improve the performance of catalysts based on WS 2 and/ or MoS 2 materials. However, the HER performance of WS 2 and/or MoS 2 and their derivatives still need to be enhanced for practical and commercial products. The following sections summarize the cutting edge of technologies as well as the efficiency strategies that have been applied to prepare WS 2 and/or MoS 2 and their derivatives.

| Atomic layer deposition
ALD is a cutting-edge technology for preparing catalyst materials. This is a new development from the chemical vapor deposition (CVD) technique. Using the ALD technique, a very thin film of catalysts can be deposited by the sequential use of a gas-phase chemical process. The principle of ALD reactions is to use two or more chemical elements as precursors. In the ALD process, on the functionalized substrate, one-by-one precursors will be deposited on and/ or reacted with the surface of other layers deposited before. With precision timing and numbered layer control, a thin film is gradually formed with thickness control at Angstrom size and accurate composition of reactants. In recent years, considerable efforts have been devoted to the preparation of monolayers of MoS 2 and WS 2 , which exhibit significant improvements in catalytic activity toward electrochemical processes. Therefore, with the high advantages, ALD has recently attracted considerable attention from researchers to fabricate different devices based on semiconductor materials, including thin films based on MoS 2 and WS 2 , which could be employed for the HER process. [105][106][107] The growth mechanism of the catalyst materials based on MoS 2 and WS 2 is shown in detail in Figure 15.

| Exfoliation from the bulk materials
In nature, MoS 2 and WS 2 are semiconducting materials normally found with 2H trigonal prismatic phases. The raw materials of MoS 2 and WS 2 exhibit poor conductivity and low catalytic active sites, which is challenging for the preparation of catalytic materials based on bulk MoS 2 and WS 2 . Numerous studies have been conducted to boost the performance efficiency of catalysts based on these materials. Exfoliation is the very first and most effective route frequently used to overcome this obstacle. Using the exfoliation technique, the bulk materials are separated into thinner layers with 2-3 layers which propose a larger surface area and abundant active sites. Two popular techniques are often employed to exfoliate MoS 2 and WS 2 into layered catalysts: mechanical exfoliation and chemical exfoliation. Figure 16 shows the exfoliation routes to prepare layered catalysts by using both mechanical and chemical exfoliation. Chemical exfoliation can be used to prepare monolayer or bilayer catalyst materials that exhibit high catalytic activities. 110,111 Using chemical exfoliation, lithium, sodium, and potassium compounds have often been used to separate the layers of bulk catalysts. 112,113 The insertion of cations into bulk catalysts could lead to some unique phenomena, such as the transformation of the 2H catalyst to 1T or 1T′ materials, which exhibit better catalytic properties than primitive materials. 114,115 Smallyield catalysts are sufficient for study but not for practical use or commercial materials. Recently, a second exfoliation technique, mechanical exfoliation, has been intensively investigated to prepare catalytic materials in larger quantities and with higher quality. 110,116,117 Mechanical exfoliation has been significantly developed with noticeable results. However, chemical exfoliation still has some advantages over mechanical exfoliation which have been intensively studied by previous reports. 114,115,118 In summary, high-quality and layered 2D MoS 2 and WS 2 catalysts can be obtained by exfoliation of bulk materials via both mechanical and chemical routes. However, because of the time-consuming and difficult process control, as well as the small yield, the exfoliation technique faces difficulties in preparing practical catalytic materials.

| Hydro/solvothermal method
Hydrothermal and solvothermal methods are low cost, easy, and convenient for the large-scale synthesis of TMDbased nanomaterials. By changing one or more reaction conditions, such as the temperature, reaction time, type of metal precursors, surfactants, and/or solution reaction, the synthesized products with different morphologies, phases, or crystallinity could be prepared. Because of all the above benefits, hydrothermal and solvothermal processes are considered as a convenient process to prepare nanostructured materials. 121,122 Recently, MoS 2 and WS 2 have been intensively prepared by hydrothermal reactions to investigate their HER performances. Various morphologies, structures, and new materials have been prepared by changing one or more reaction conditions of the synthesis process as shown in Figure 17. Heterostructures of MoS 2 and WS 2 with other catalysts or MoS 2 /WS 2 heterostructures could also be prepared using these methods. [123][124][125][126] Generally, because of the controllable and large-scale process with different catalysts, hydrothermal and solvothermal techniques can be used to prepare various catalyst materials based on MoS 2 and WS 2 for electrochemical applications, including the HER, batteries, and supercapacitors.

| Chemical vapor deposition
CVD is a vacuum deposition method that is typically employed to prepare catalyst electrodes for thin films. A high-quality thin-film catalyst could be formed by a chemical reaction on the surface of substrates such as silicone or p-type silicone using one or more gaseous resources such as N 2 or CH 4 and/or elemental substances containing thin-film elements. The CVD route has various advantages to prepare catalyst thin films such as highquality film, low-defect, and exactly controllable thickness of catalyst film on substrates. One of the advances in vaporphase deposition involves the adjustment of the precursor ratio. The exact time and temperature of the components can be easily controlled, and well-contacted interfaces between nanostructures and substrates are favorable for charge migration. Additionally, high-quality catalysts obtained via the CVD method also provide a simple but pure platform to investigate the HER mechanism. In a typical CVD synthesis process, Mo or W is placed in the furnace at the center of the heating zone, while S powder is placed upstream relative to the flow direction of the carrier gas (Ar or N 2 ). When the temperature increases, the S vapor flows downstream, converting the Mo or W precursor into MoS 2 or WS 2 in the high-temperature zone. Figure 18A,B shows the synthesis of MoS 2 and WS 2 , respectively, by the CVD technique. The preparation of high-quality catalysts based on MoS 2 or WS 2 via CVD requires careful modulation of important parameters. The temperature is the most important parameter that must be checked for each process. The distance between the S powder and the Mo or W source is also a critical factor to prepare high-quality thin films. Another important factor is the flow of the carrier gas, usually Ar or N 2 , which needs to be controlled precisely so that the S powder can be properly deposited on the substrates where the Mo or W source is placed. In addition, the reaction time for different states must be adjusted correctly to form the highest-quality catalyst, as shown in Figure 18C.

| Other synthetic processes
In addition to the methods mentioned above, other synthetic pathways have also been used to fabricate catalysts based on MoS 2 and WS 2 materials. Thermal annealing is a straightforward method for preparing MoS 2and WS 2 -based materials. However, it is difficult to obtain MoS 2 or WS 2 with a well-controlled morphology and a number of layers. Moreover, high temperatures may often lead to material aggregation, which limits the HER performance. Even so, research attention has been paid to this method for obtaining electrocatalysts with decent HER catalytic activity. Chemical vapor transport (CVT) is typically utilized to obtain high-quality bulk single crystals. In this technique, raw materials react with transport agents, usually pure halogen elements or their compounds, in the hot zone, and are then transported to the cold zone, where the growth of single crystals is observed. The growth temperature, transport agent, and reaction time are the main parameters that should be carefully tuned to obtain high-quality MoS 2 and WS 2 single crystals. Sputtering is another effective method for preparing high-quality catalysts based on MoS 2 and WS 2 . Owing to the high purity of the as-grown films, these samples are also suitable for investigating the origin of the catalytic activities. Figure 19A,B shows the synthesis of catalysts by the CVT and sputtering methods, respectively. In addition, electrodeposition is also an effective route to prepare highly pure MoS 2 and WS 2 catalysts for HER applications, as shown in Figure 19C, which were also reported in detail by previous reports. 133,134 6 | ENGINEERING STRUCTURE OF CATALYSTS BASED ON MoS 2 AND WS 2 TO ENHANCE HER ACTIVITY The electrocatalytic activities of MoS 2 and WS 2 are much more efficient than those of TMDs and other materials, except for the Pt material group. However, to prepare practical and commercial materials, particularly at the industrial scale, the catalytic characteristics of catalysts based on MoS 2 and WS 2 require further investigation and improvements. In numerous studies, the catalytic activities of catalysts based on MoS 2 and WS 2 have been significantly improved by different techniques and methods, as listed below.
1. Improving the active surface of prepared catalysts. 2. Doping with other elements to boost catalytic activity. 3. Modifying the structure or morphology of catalysts. 4. Preparing heterostructure materials with other materials.
A detailed explanation, interpretation of research, and examples are provided to clearly understand each strategy as below.

| Preparing catalysts with larger active surface
As typical 2D catalysts, MoS 2 and WS 2 have a layered structure in which the catalyst layers can be stacked together by vdW forces. Although 2D catalysts exhibit large specific surface area, however, the poor conductivity and inactive basal planes could greatly hinder the HER process. In theory, the catalytic activity of 2D catalysts is basically located at the edge sites and defects of material. For the electrocatalytic process, the catalysts active area is one of the most crucial factors which could highly affect the kinetic and rate of process. Therefore, many strategies have been usually employed to elevate the active sites of catalysts surface which could be categorized into two common classes including increase of active site on catalyst edge and modifying the basal planes of catalysts. The first one is engineering the catalyst structure to increase the active sites which are considered as the edges of MoS 2 and WS 2 layers. To achieve that goal, two approaches are often employed including synthesis catalysts with the increase of edge length or preparing monolayer catalysts. In fact, to increase the edge length of catalysts, that means the size of catalysts also have to be lengthened, it is a really big challenge in nanoscience. Along with that, to prepare monolayer catalysts, there are also many drawbacks such as small yield and time consumption as we discussed in the above section. To date, modifying the structure of catalysts based on MoS 2 and WS 2 has been intensively studied to increase their catalytic activity. Different techniques have been applied to prepare various effective catalysts with a larger active surface. Among them, hollow structure and smaller size of catalysts are the VAN  prominent materials to improve HER efficiency as shown in Figure 20.
The second strategy is also investigated to prepare effective catalysts in which the basal planes of MoS 2 and WS 2 will be reconstructed to prepare dangling bond on that. It is clear that, the dangling bond states closer to the Fermi level (E F ), which activates the basal plane for HER. In fact, the basal planes of MoS 2 and WS 2 exhibit very low catalytic activity because of poor electron transport properties between layers. Therefore, many studies have been investigated to improve the catalytic activity of large inert basal planes by creating the defects. With structural defects on basal planes, the local electron density could be induced and that could increase the HER process by modulating the electronic structures of prepared catalysts. For instance, DFT results indicate that increasing the S atom vacancies (S-vacancies) could strengthen hydrogen adsorption and strain the S-vacancy sites, which would lead to an optimal ΔG H* and make the gap states close to the E F . [140][141][142] However, defect density must be controlled to avoid excess defects which could harm the catalytic activity of catalysts. In the last decades, various techniques, such as plasma treatment or chemical functionalization, have been used to increase the defect density of both MoS 2 and WS 2 catalysts. As shown in Figure 21A, after treatment with O 2 and H 2 , the catalytic activity of MoS 2 is significantly improved. As shown in Figure 21B, annealing in a H 2 atmosphere could prepare a catalyst with defects based on WS 2 surface, which also shows a significant improvement in the HER performance.

| Heteroatom doping
By adding a small amount of other elements to the layered MoS 2 and WS 2 , their electrocatalytic performance dramatically increases. As mentioned in the Volcano plot, the material with appropriate HBE which is not too weak nor too strong is a desirable catalyst for HER application. In those materials, the adsorption free energy of H* (ΔG H* ) is closed to the Volcano peak like Pt group. With ΔG H* ≈ 0, the intermediate hydrogen elements could be easily absorbed and desorbed into/from electrode surface because of the rapid proton/electron transfer between electrolyte and cathode. Therefore, the HER performance could be dramatically improved. Many studies have indicated that doping with very small concentrations of noble metal into MoS 2 and WS 2 catalysts could be the effective route to boost the catalytic activity of doped materials. The electronic interactivity among different atoms plays the role of modifying the electron structure of MoS 2 and WS 2 , especially, in which Mo and W are the d-band materials. The doped strategy could be grouped into two types of doped catalysts which are metal-doped catalysts and nonmetaldoped catalysts. Both routes have their own advantages and drawbacks as well as we have to choose the suitable doping elements for each host catalyst due to their different effects on the HER activity.
In metal doping methods, several metals such as Pt, Au, Co, Ni, and Cu have already been usually utilized as great doping elements into MoS 2 and WS 2 materials. Figure 22 shows the results of HER performance of various elements doped in MoS 2 and WS 2 . In Figure 22A, Pd-doped WS 2 exhibits a significant increase toward HER application with a Tafel slope of 54 mV dec -1 and an overpotential of −175 mV at −10 mA cm −2 . On the other hand, Figure 22B shows the remarkable catalytic activity of Zn-doped MoS 2 compared with that of other doping metals with the onset potential of −0.13 V versus reversible hydrogen electrode and the Tafel slope of 51 mV dec −1 . The significant improvement is considered to be the energy band alignment near the H + / H 2 reduction potential which could reduce ΔG H* and increase the density of the active sites as well as the improving conductivity. VAN  For the nonmetal doping process, nitrogen and phosphorus elements have been intensively investigated using various techniques. The improvement of catalytic activity toward HER application of catalysts based on N or P doped into MoS 2 and WS 2 is illustrated in Figure 23. In the structure of catalysts based on nonmetal-doped MoS 2 or WS 2 , there are different effects of doped elements onto host elements. At first, they could create the disorder structure of catalysts which is caused to increase the number of active sites in HER catalysts. Therefore, more active sites could be generated in doped catalysts. Second, based on the disorder of synthesized materials, the doped catalysts with higher carrier concentration could be prepared as well as the electronic structure of doped catalyst could be tunable. Therefore, the intrinsic conductivity of catalysts could be controllably enhanced and significantly improve after the doping process. On the basis of this principle, Xie et al. conducted the experiment to incorporate oxygen with MoS 2 . 147 The performance of O-doped catalysts is highly impressive with a current density of 126.5 mA cm −2 at an overpotential of 300 mV, a low onset overpotential of 120 mV, a small Tafel slope of 55 mV dec -1 . The excellent long-term stability is also achieved. The results indicate that the catalyst based on optimized O-doped MoS 2 could be a great material for HER application which could be considered as an alternative for Pt metal.

| Phase engineering
MoS 2 and WS 2 exhibit multiple crystal phases, as mentioned above. Normally, MoS 2 and WS 2 exist in nature as the 2H phase, which corresponds to a semiconductor, and those materials are often employed to prepare different catalysts based on MoS 2 and WS 2 . In contrast, the 1T and 1T′ phases of MoS 2 and WS 2 have more catalytic advantages, including the larger density of the active sites, metallic conductivity, and better electrode kinetics. Therefore, the 1T and 1T′ phases of MoS 2 and WS 2 show higher catalytic activities than those of the 2H phases. Because of the superior catalytic activity, numerous studies have been devoted to transform to prepare 1T and/or 1T′ phase of MoS 2 and WS 2 from the nature phase of materials. In Figure 24A, the free energy for HER of Mo atoms of 2H and 1T phases was calculated. We can see a significant decrease in free energy of the 1T phase compared with that of the 2H phase. In Figure 24B, the HER performance of different phases of catalysts based on WS 2 was investigated. It is clear that the metallic 1T phase of WS 2 exhibits a much better HER performance than those of the performance of the 2H phase. This enhancement in catalytic activity is caused by the increased number of active sites on MoS 2 and WS 2 at the edges sites as well as the metallic properties of polymorph in the 1T phase. Chemical exfoliation route is often employed to obtain single-or few-layered 1T/1T′ MoS 2 and WS 2 . 152,153 In which, lithium intercalation is the main mechanism to prepare layers catalysts. Element doping, heat treatment, or electrochemical treatment could be conducted to engineer the 2H phase into the 1T/ 1T′ phase of MoS 2 and WS 2 . 154,155 For instance, if 2H MoS 2 is treated by an electrochemical process, the 2H phase will be partially transformed into 1T phase to form in-plane 1T-2H MoS 2 hybrid phase. The 2H/1T hybrid phase enhances structural disorder and electronic conductivity as well as it causes the decrease of hydrogen adsorption-free energy. Compared with the initial phase of 2H-MoS 2 , the hybrid-phase MoS 2 electrocatalyst exhibits remarkably improved HER activity with a lower onset overpotential and Tafel plot. Specifically, the overpotential and Tafel plot of the electrochemically treated MoS 2 nanosheets were decreased, respectively, by 174 mV and 25 mV dec -1 at a cathodic current density of 10 mA cm -2 compared with pristine 2H-MoS 2 nanosheets. This significant improvement is assigned to the electrochemical treatments which synergistically modulate both the structural and electronic properties of multilayer 2H-MoS 2 nanosheets. 156 In many previous studies, most experimental results confirmed that the 2H phase of MoS 2 and WS 2 cannot be totally changed into the 1T phase. They indicated that there is maybe around 20% of the initial phase leftover after the process. That is why the obtained materials are usually mixed 1T and 2H phases. Another drawback of this method is the poor stability of the 1T phases of MoS 2 and WS 2 . Along with the working time, catalysts based on the 1T phase of MoS 2 and WS 2 can be easily converted back into the 2H phase, causing a quick collapse in HER performance. In fact, synthesis of electrocatalysts for the HER process under high temperatures by hydrothermal/ solvothermal techniques or CVD, the 2H phase of MoS 2 and WS 2 catalysts is usually formed. However, along with the development of technologies and numerous efforts of scientists, the metallic phases 1T or 1T′ of MoS 2 and WS 2 have also been easily prepared by a conventional process. Nevertheless, it is difficult to control the process, uniformity, and purity of the catalysts phase. More importantly, the catalytic performance, including the lifetime of electrodes based on 1T or 1T′ phases of MoS 2 and WS 2 , still needs to be further improved.

| Synthesis hybrid/composites catalysts
In the HER process, the reaction on the surface of electrodes highly depends on the effective interaction between the catalysts and hydrogen elements in the electrolyte. In the last decades, many effective hybrid catalysts based on MoS 2 and WS 2 have been reported as great materials for the HER process. Hybrid catalysts could present many distinct improvements because they have various advantages, including synergistic effects as well as strain effects and electronic interactions. In fact, MoS 2 and WS 2 could be combined with various types of materials to create different catalysts that can boost the HER performance. The hybrid catalysts based on MoS 2 and WS 2 can be classified into two main types. First, MoS 2 and WS 2 can be integrated with graphene or reduced graphene to create conductivity platforms for catalyst materials, as indicated in Figure 25. In these synthesized catalysts, the conductivity and the active site of the hybrid catalyst could be significantly improved by the effect of graphene layer as a conductive platform. In this heterostructure material, the carrier migration efficiency of MoS 2 and WS 2 can be accelerated, and the intrinsic catalytic activity can also be significantly increased.
Another hybrid material of catalysts based on MoS 2 and WS 2 is the heterostructure of MoS 2 and WS 2 with other catalysts, such as NiS 2 , CoS 2 , or MoO 3 . In these hybrid materials, the synergistic effect and electronic interaction of different catalysts are the main reason for the improvements in catalytic activity. In semiconductor physics theory, one of the important roles of heterostructure is the boost of the directional migration of electrons with properly matched band structures in different catalysts. The synergistic effect between different material phases has also been investigated in previously reported. For example, in the incorporated catalysts of Co/MoS 2 , the 3D isosurface indicates that coupling of Co with MoS 2 produces the redistribution of electron density, and the electron transfer occurs from Co to MoS 2 in the interfacial region. 161 The electron depletion on Co moiety and electron accumulation on the MoS 2 side is believed to strengthen the activation of H 2 O molecule and expedite the electron-transfer process in the Volmer and Heyrovsky steps, thus leading to the significantly enhanced electrocatalytic activities. The Gibbs free energy of water adsorption ∆ ( ) is generally considered as a vital parameter of the rate-limiting Volmer step in alkaline conditions. The DFT calculation shows that the ∆ ( ) G H O 2 on Co/MoS 2 is calculated to be −1.69 eV, which is drastically lower than that of MoS 2 (−0.21 eV). Other hybrid catalysts with catalytic improvement results are shown in Figure 26. It can be seen that the HER performance of prepared heterostructure catalysts is significantly improved compared with that of pristine MoS 2 or WS 2 .

| CONCLUSION AND PERSPECTIVES
Over the last decades, MoS 2 and WS 2 have been intensively investigated and employed for energy storage and electrochemical applications because of their VAN NGUYEN ET AL. abundant natural resources, low prices, low toxicity, and superior catalytic activity. The properties of MoS 2 and WS 2 are greatly affected by the size and electronic charge of transition metals, which causes various unique characteristics and the improved performance of the catalysts. Numerous effective approaches have been carried out to improve the electrocatalytic performance of catalysts based on MoS 2 and WS 2 materials. In this review, we briefly summarized the HER mechanism and its crucial parameters. The structure and properties of MoS 2 and WS 2 are also compared in detail. Besides that, the recent advances of catalysts based on MoS 2 and WS 2 materials for HER have been addressed.
MoS 2 is considered the most promising catalyst for HER applications after Pt-based catalysts. The ideal structure and Pt-like performance toward the HER are the key advantages of MoS 2 compared with other catalytic materials. Besides that, WS 2 has also been intensively investigated and considered a great candidate for HER applications on a practical scale because its structure and catalytic activities are highly comparable to that of MoS 2 . The layered 2D structure of MoS 2 and WS 2 leads to different special chemical and physical characteristics which are greatly beneficial for the HER process. Therefore, both MoS 2 and WS 2 could be considered as the prominent materials for catalytic applications which could be replaced for Pt group metals to prepared working electrode in electrochemical process. Significant efforts have been devoted to developing the efficiency as well as economic aspects of electrocatalyst materials based on MoS 2 and WS 2 for hydrogen energy technologies. Along with the significant developments of high technologies, the HER performances of these catalysts have been dramatically improved. However, it still remains a big challenge to prepare large-scale, high-quality, and commercial electrodes based on MoS 2 and WS 2 to produce hydrogen gas. Besides that, the synthesis process reactions of catalysts based on MoS 2 and WS 2 , as well as the mechanism of the HER application using MoS 2 and WS 2 , are not fully understood. So far, most studies have reported a simple combination of MoS 2 and WS 2 with doping elements and heterostructures with other materials. These improvements have not been appropriate for practical requirements. It is clear that a large research gap needs to be filled to increase the catalytic activity of catalysts based on MoS 2 or WS 2 materials. In fact, considerable efforts have been devoted as well as different strategies, methods and technologies have been applied to improve the electrocatalytic activities of MoS 2 and WS 2 . With the significant development of high technologies, as well as the critical energy demand for modern life, the prospect of hydrogen energy prepared by catalysts based on MoS 2 and WS 2 could soon become a reality. For those reasons, we believe that catalyst materials based on MoS 2 and WS 2 could be great candidates for energy storage and catalytic applications, especially for HER, in the near future.
Sang Hyun Ahn received his Ph.D. degree in School of Chemical and Biological Engineering of Seoul National University in 2012. After that he joined Thomas Moffat's group as postdoctoral researcher in National Institute of Standard and Technology, the USA. Currently, he is an associate professor in School of Chemical Engineering and Material Science of Chung-Ang University. His research focuses on electrochemical materials and devices for energy conversion and storage, including water, carbon dioxide, and ammonia electrolysis.