Two-dimensional transition-metal dichalcogenides for electrochemical hydrogen evolution reaction

Journal Pre-proofs Two-Dimensional Transition-Metal Dichalcogenides for Electrochemical Hydrogen Evolution Reaction Kunlei Zhu, Chenyu Li, Zhihong Jing, Xicheng Liu, Yuanchun He, Xiaoxia Lv, Yan Wang, Kai Liu PII: DOI: Reference:

S2452-2627(19)30087-X https://doi.org/10.1016/j.flatc.2019.100140 FLATC 100140

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FlatChem

Received Date: Revised Date: Accepted Date:

30 May 2019 23 September 2019 11 October 2019

Please cite this article as: K. Zhu, C. Li, Z. Jing, X. Liu, Y. He, X. Lv, Y. Wang, K. Liu, Two-Dimensional TransitionMetal Dichalcogenides for Electrochemical Hydrogen Evolution Reaction, FlatChem (2019), doi: https://doi.org/ 10.1016/j.flatc.2019.100140

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Two-Dimensional Transition-Metal Dichalcogenides for Electrochemical Hydrogen Evolution Reaction Kunlei Zhu,a,b* Chenyu Li,b Zhihong Jing,a Xicheng Liu,a Yuanchun He,a Xiaoxia Lv,a Yan Wang a and Kai Liu b* aCollege

of Chemistry and Chemical Engineering, Qufu Normal University, Jingxuan West Road NO.57, 273165,

Qufu, Shandong, People’s Republic of China. E-mail: [email protected]. bState

Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering,

Tsinghua University, Beijing 100084, P. R. China. E-mail: [email protected].

Abstract In this review, we comprehensively review some recent progress of two-dimensional (2D) transition-metal dichalcogenides (TMDs) in the application of hydrogen evolution reaction (HER), for the purpose of offering a reference for related researchers, especially for new beginners in this research field. The mechanism of HER is first presented in detail, followed by an introduction of evaluation methods and some considerations in HER tests. Then, we make a brief description of composition, structure and crystal phases of TMDs. Next, we highlight the intrinsic factors determining HER performance and summarize the recent development of strategies for improving the HER performance of TMDs. At last, an overall summary is presented and important challenges are discussed as well.

1. Introduction With the development of economy and growth of population, it is highly needed to exploit renewable energy and develop advanced technologies preventing the environmental degradation and providing sufficient green energy for human [1-3]. As the cleanest fuel, hydrogen (H2) is regarded as one of the most promising sources. However, the industrial production of H2 involves catalytic stream methane reforming, partial oxidation and coal gasification, accompanied by the consumption of fossil fuels and the emission of carbon dioxide (CO2) [2,4]. Thus, it is urgent demand to develop a green, sustainable and efficient method for production of H2. The electrolysis of water is a promising and green strategy which is used for production of H2, with clean raw materials and without emission of polluted products. As we all know, the reaction equation is expressed as [5-7]:

H2O(l)

electrolysis

1

H2(g) + 2O (g) (∆𝐺θ = +237.2 kJ mol ―1, ∆𝐸θ = 1.23 V) 2

(1.1)

The potentials of the hydrogen and oxygen evolution are 0 and 1.23 V, respectively, versus reversible hydrogen electrode (RHE) at standard conditions (298.15K, 1 atm). The reaction 1.1 consists of two half reactions involving hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) processes [6-7], and experiences different processes in acid solution and alkaline/neutral solution: In acidic conditions: 2H3O + (aq) + 2e ― → H2(g) +2H2O(l)

(1.2)

1

1

3H2O(l)→2H3O + (aq) + 2O (g) +2e ― 2

(1.3)

In neutral/alkaline conditions: 2H2O(l) + 2e ― → H2(g) +2OH ― 1

2OH ― (aq)→H2O(l) + 2O (g) +2e ― 2

(1.4) (1.5)

As the heart of energy storage and conversation systems in the future of renewable energy, HER (ep 1.2 and 1.4) is attracting more and more attentions and a large number of studies have been performed. The traditional electrocatalyst materials for HER are platinum (Pt) and Pt-group noble metals which are efficient and work at nearly zero overpotential. However, these catalysts are rare and expensive, thus leading to the limitation of their applications in large scale. This dilemma motivates scientists to develop new catalysts and a great many of materials have been used for efficient hydrogen evolution reaction, such as two-dimensional (2D) materials, Ni-based alloys, and transition metal phophides et al [4]. Among them, 2D materials which have a thickness of only single- or few-atoms thick (typically less than 5 nm) represent one of the hottest catalysts for HRE. They present unique properties due to confinement effects compared to their bulk counterparts, thus attracting sharply increasing interest especially after the discovery of graphene in 2004. The 2D materials include a wide range of nanomaterials including graphene [8,9], transition metal dichalcogenides (TMDs) [10,11], hexagonal boron nitride (h-BN) [12-13], graphitic carbon nitride (g-C3N4) [14-15], black phosphorus (BP) [16-17], MXenes [18-19], layered double hydroxides (LDHs) [20], metal oxides [21], metal-organic frameworks [22] and covalent-organic frameworks [23]. In 2007, Thomas group reported remarkable HER performance of MoS2 with a hydrogen adsorption Gibbs free energy (ΔGH) of only 0.08 eV which is even lower than that of Pt with a value of 0.09 eV [24-25]. This highlights that 2D TMDs could be one of the most promising candidates for HER. Since then, numerous studies have focused on improving HER performance of 2D TMDs. On the one hand, the target is to increase the number of active sites. Generally, the sites along the edges are active, while the sites within the basal plane are inactive [25-27]. Besides the edge sites, Cao group revealed that sulfur vacancies and grain boundaries are both active as well [28]. One the other hand, the target is to improve the conductivity of 2D TMDs since the majority compounds are semiconductors. Many researchers prepared metallic phase such as metastable 1T MoS2 improving the performance of HER [29-31]. So far, it is still very challenging to both increase the number of active sites and improve the electrical conductivity to achieve optimal HER performance. In this timely review, we focus on a comprehensive overview of TMDs and their application on HER. Firstly, we introduce the HER mechanism involving multistep reaction processes, also including evaluation methods and some choices in the HER practice. Then, we make a brief introduction of categories, composition and crystal phases of TMDs. Next, we highlight the good activity descriptor ΔGH of HER and summarize the recent progress of feasible strategies for improving the HER performance of TMDs. Eventually, we make a brief summary and present some insights on the prospects and important challenges of TMDs nanomaterials involving with HER application. 2

2. Mechanism, Relevant Evaluation Methods and Considerations of HER 2.1 Mechanism of HER For HER, hydrogen is the only product, whereas the reaction is actually a multi-step process comprised of adsorption, reduction and desorption processes [4,32-35]. The overall reactions are different in acid and neutral/alkaline conditions, which are listed as follows (‘A’ denotes an active site on the surface of a catalyst)：

2H3O + + 2e ―

A

H2 +2H2O (in acidic solution)

A

2H2O + 2e ― H2 +2OH ― (in neutral/alkaline solution)

(2.1)

(2.2)

The overall reaction of HER involves three steps. In the step 1 of Volmer reaction, the reaction involves a process of hydrogen adsorption on the electrode surface by combining a hydronium ion from the acid solution and an electron transferred through electrode (eq 2.3). However, this reaction needs a previous step of water dissociation in alkaline solution (eq 2.4), which could bring in additional energy barrier probably limiting the whole reaction rate. Step 1, electrochemical hydrogen adsorption process (Volmer reaction):

H3O + + e ―

A

Hads + H2O (in acidic solution)

A

H2O + e ― Hads +OH ― (in neutral/alkaline solution)

(2.3)

(2.4)

In this step, the Tafel slope (b1) can be obtained by eq 2.5:

𝑏1 =

2.3𝑅𝑇 𝛼𝐹

(2.5)

where R is the ideal gas constant, T is the absolute temperature, α is the symmetry coefficient with a value of 0.5, and F is the Faraday constant. The value of b1 is calculated to be b1 = 118 mV dec-1 at 25 °C. The following step relates to a process of electrochemical desorption (Heyrovsky reaction) or chemical desorption (Tafel reaction). The Heyrovsky reaction involves a process on the electrode surface where an adsorbed hydrogen atom (Hads) combines a hydronium ion from the acid solution (eq 2.6) or a dissociated water molecule (eq 2.7) from the alkaline solution. Step 2, electrochemical desorption process (Heyrovsky reaction):

Hads + H3O + + e ―

A

H2 +H2O

(in acidic solution)

(2.6)

3

Hads + H2O + e ―

A

H2 +OH ― (in neutral/alkaline solution)

(2.7)

In this step, the Tafel slope (b2) can be calculated by eq 2.8: 2.3𝑅𝑇 + 1)𝐹

𝑏2 = (𝛼

(2.8)

The value of b2 is calculated to be b2 = 39 mV dec-1 at 25 °C. Or, the second step is Tafel reaction which is a process of chemical desorption on the electrode surface where two adsorbed hydrogen atoms combine to generate one hydrogen molecule (eq 2.9). Step 2’, chemical desorption process (Tafel reaction): A

2Hads H2

𝑏′2 =

2.3𝑅𝑇 2𝐹

(2.9)

(2.10)

The value of b’2 is calculated to be b’2 = 29 mV dec-1 at 25 °C in this step (eq 2.10). In the real electrocatalytic tests, these steps are strongly related to the inherent electronic and chemical properties of the electrode surface [34]. The Tafel slope from polarization curves can be used for determining which reaction is the rate-controlling step. 2.2 Evaluation Methods and Considerations in HER Tests HER is performed in a three-electrode electrolytic tank consisting of a working electrode (prepared by electrocatalysts), a counter electrode, a reference electrode and a gas entrance. The evaluation methods involving series of parameters and key points are discussed in detail here for beginners to reasonably and conveniently perform HER tests. 2.2.1 Overpotential Overpotential, which is an additional potential beyond the thermodynamic requirement is needed to drive a reaction at a certain rate. The onset overpotential which is activation overpotential for initiating HER is an important intrinsic parameter used for evaluating the performance of electrocatalyst. The value of onset overpotential for Pt electrode is ~ 0 V. The other overpotentials at fixed current densities are usually used as a quantitative activity parameter to evaluate the catalytic activity of electrocatalysts. The typical current density is 10 mA cm-2 which is a value corresponding to a 10% efficiency solar-to-fuel device under an illumination of AM1.5G [36]. In some cases, the overpotential at high current densities (e.g. 50, 100 and even 1000 mA cm-2) are also treated as alternate activity parameters for high-performance electrocatalysts [37-38]. The absolute value of overpotential is the difference between standard hydrogen electrode (SHE) and the onset potential initiating HER by an electrocatalyst. The SHE is the standard reference point for eq 1.2 or 1.4, with its potential, Eθ, assigned as 0.0000 V at all temperatures with 1 atm hydrogen pressure. In the realistic experiment, the hydrogen pressure influences the 4

overpotential as indicated by the Nernst equation (eq 2.11) [39]: 𝐸 = 𝐸θ +

𝑅𝑇 [H + ] 𝐹 ln 𝑃H

(2.11)

2

It is indicated that the partial pressure of H2 (PH2) can alter the overpotential (E) of HER. Therefore, to maintain the equilibrium potential at the standard value (0 V versus RHE) for HER, H2 gas should be introduced to keep the saturation of gas [40-41]. The overpotential can be mainly divided into activation overpotential, concentration overpotential and resistance overpotential according to the origins of polarization of electrodes [4]. The activation overpotential which is an intrinsic property of materials can be greatly reduced by employing modified electrocatalysts. For example, the overpotential at 10 mA cm-2 of MoS2 can be greatly reduced from -320 mV to -187 mV after the as-grown 2H-MoS2 changes into 1T-MoS2 nanosheets via lithium intercalation [29]. However, the concentration potential which derives from the concentration difference of involved ions caused by the slow ion diffusion rate, can only be lower to limited extent by some external force (e.g. stirring) due to the existence of a diffusion layer. But, the external force may disturb the related reactions, in turn. The resistance overpotential, as known as junction overpotential, which is the potential drop caused by Ohmic resistance (RΩ) including contact resistances and solution resistance. It needs to be corrected to obtain the real potential applied on the electrocatalysts. The method of iRΩ compensation is used to effectively eliminate this resistance overpotential, as expressed by eq. 2.12. 𝐸corrected = 𝐸measured ― 𝑖𝑅Ω

(2.12)

where, Ecorrected is the iRΩ-corrected potential, Ecorrected is the measured potential against RHE, and i is the measured current without background-correction. RΩ, also known as uncompensated resistance, can be obtained by electrochemical methods such as electrochemical impedance spectroscopy (EIS) or be directly measured by some advanced electrochemical workstations. In a typical EIS measurement, the AC impedance spectrum is acquired with frequencies ranging from 100 kHz to 0.1 Hz by applying a voltage perturbation of 5 or 10 mV at a fixed potential (usually open circuit potential). The value of RΩ which is the leftmost cross point at high frequency can be directly read out or be obtained by data fitting in some cases.

5

Fig. 1 (a) Pourbaix diagram of water which presents the potentials of OER and commonly used reference electrodes at various electrode versus SHE. (b) Calibration curve of a SCE reference electrode versus RHE by cyclic voltammetry (CV) method at a scan rate of 10 mV s-1 in a 0.1 M KOH solution saturated by H2, with Pt rotating disk electrode as the working electrode (rotation speed: 1600 rpm) and Pt as the counter electrode. Inset indicates there are two intercepts at 0 A in the CV curve [41]. Copyright© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

In the real tests, it is recommended that all the measured potentials are normalized to RHE for convenience. If the potentials are normalized to SHE, the values of SHE vary with the pH of solution as shown Pourbaix diagram (Fig. 1a). For example, the equilibrium potential of HER is 0 V against SHE at pH 0 in an acidic solution, whereas the value changes to -0.83 V against SHE at pH 14 in an alkaline solution [41]. The same problems are encountered, if the value of equilibrium potential are normalized to referencing electrodes (e.g. Ag/AgCl electrode, red curve in Fig. 1a). The potentials depend on the pH value of different solutions complexing the comparison of study results with different electrolytes, thus all the obtained potentials are suggested to normalize to RHE which has relation with SHE expressed by eq 2.13:

𝐸RHE = 𝐸SHE +pH ×

2.303𝑅𝑇 𝐹

(2.13)

where ERHE and ESHE are the potentials of the used reference electrode against RHE and SHE, respectively. It is obvious that the dependence of pH is considered when ERHE is set as reference point, thus leading to the disregard of influence from solution pH and the intuitive comparison of study results. 2.2.2 Election of Reference Electrode and Counter Electrode The absolute value of overpotential of catalysts (working electrode) is not directly measured and is a difference value versus a reference electrode. Thus, selection of reference electrode is 6

very important. The reference electrode should have a stable and well-defined potential for accurately measuring or regulating the potential applied on catalysts in the selected solution. Using an RHE as the reference electrode is the most reliable choice, and the value of measured potential is can be directly used. In practical test, other kind of commercially available reference electrodes are generally used for convenience and low-cost. Two kinds of reliable electrodes are most widely used as reference electrodes in acid solution. One is the saturated calomel electrode (SCE), which follows a reversible redox reaction (Hg2Cl2 + 2e- ⇆ 2Hg + 2Cl-) in KCl solution with saturated concentration. The other is the silver chloride electrode (Ag/AgCl), which follows a reversible redox reaction (AgCl + e- ⇆ Ag + Cl-) in KCl solution with a selected concentration. Generally, the reference is equipped with a double junction or a salt bridge separated from the work reference to ensure its stable and fixed potential and minimize junction resistance [42]. In alkaline solution, the Ag/AgCl electrode is not suitable for being used as reference electrode, due to the possible diffusion of OH- into its chamber triggers additional redox reactions (e.g. 2AgCl + 2OH- ⇆ Ag2O + H2O + 2e-). In alkaline condition, the Hg/HgO (HgO + 2e- + H2O ⇆ Hg + 2OH-) is a better choice. It is noted that all the measured potential should be changed into RHE scale, whatever reference electrode is employed, as already discussed in 2.2.1. It is strongly suggested that the calibration of reference electrode is executed before the HER test for obtaining a reliable overpotential value. A commonly used method is introduced here. The calibration is performed by CV method with the voltage range of HER in the same solution as the studied electrocatalysts used, where Pt rotating disk electrode, the calibrated electrode and Pt electrode are employed as the working electrode, the reference electrode and the counter electrode, respectively. The conversion relation follows by eq 2.14: 𝐸RHE = 𝐸measured ― 𝐸offset

(2.14)

where Emeasured is the measured potential of studied electrocatalysts against the calibrated reference electrode in HER tests. Eoffset which is an offset value can be obtained by CV curve. Fig. 1b presents an example for the calibration of a SCE electrode at a scan rate of 10 mV s-1 in a 0.1 M KOH solution saturated by H2 [41]. The value of Eoffset is the average value of two voltage intercepts at zero current in the HER CV curve (Fig. 1b, inset). The counter electrode, an auxiliary electrode, which accepts electrons from solution and forms current loop with the working electrode should sustain enough current and have high stability without disturbing the HER of working electrode. Thus, the selection of counter electrode is important as well. First, the counter electrode should have strong ability to acquire electrons from solution for the reduction of H3O+/H2O at the working electrode. The Pt electrodes such as Pt meshes, Pt wires and Pt plates are able to provide large current for HER. In addition, researchers suggested that choosing a counter electrode with higher projected area than that of working electrode, guaranteeing the rapid transport of electrons without limiting reaction rate of the working electrode is not influenced by the counter electrode. Second, the counter electrode should have high stability without disturbing the HER measurement. Although Pt electrode is rather robust, it may dissolve in acidic/alkaline media [41, 43-44], thus disturbing the activity of working electrode. Especially for non-metal electrocatalysts, their activity could even be largely enhanced by very limited amount of deposited Pt from Pt counter electrode, 7

leading to misleading activity results [44]. Thus, it should be noted that Pt is not suggested as the counter electrode in HER study. It is a feasible choice by using carbon electrodes (e.g. graphite rod) to replace the Pt electrode. It should be noted that the purity of carbon electrode is as high as possible, or impurities within carbon electrode might leach into solution and affect the activity of electrocatalysts. There is a wise choice to eliminate the interference from the counter electrode by separating the compartment of working electrode and counter electrode using a separator such as Nafion membrane. At last, it is also important to place the counter electrode relative to the position of working electrode. The reasonable placement of the counter electrode will provide homogenous electric field between the counter and working electrode, thus ensuring the stability of HER. Generally, a desired homogeneous electric field occurs between two plates or between a plate and a wire if they are completely separated. 2.2.3 Polarization curves, Tafel slope and Exchange current density

Fig. 2 (a) Polarization curves of the 2H and 1T MoS2 nanosheets. The curves plotted using dashed lines are the iRΩ-corrected polarization curves. (b) The corresponding Tafel plots extracted from these polarization curves. Reprinted with permission from [30]. Copyright© 2013 American Chemical Society.

The polarization curves of electrocatalysts which are generally collected by using linear sweep voltammogram (LSV) or chronoamperometry method can be presented by plotting current density versus potential. The lower potential is presented at same current density or the higher current density is obtained at the same potential in polarization curves, which indicates that the studied electrocatalysts have better catalytic activity. For example, the 1T MoS2 nanosheets (dashed line in red, Fig. 2a) exhibit much improved catalytic activity compared to that of the 2H MoS2 nanosheets (dashed line in blue, Fig. 2a), due to their lower onset potential (~100 mV vs >250 mV) and potentials at same current density (e.g. 10 mA/cm2) [30]. The Tafel plots are gained by replotting polarization curves using overpotential versus log|current density|. It should be noted that the influence of background current is caused by the contribution from the capacitive currents of electrocatalysts (especially the materials with high specific surface area). However, the execution method is lack for the corrections of background current in the reported literatures. It is recommended that the contribution from background current is minimized using low-scan rate LSV or chronoamperometry method to measure the catalytic activity [41]. In real data processing, the Tafel slopes (b) are usually estimated by fitting the linear region of these Tafel plots, which follows by eq. 2.15 𝑖

𝜂 = 𝑏log𝑖0

(2.15) 8

where, η is the overpotential, i is the current density and i0 is the exchange current density. An example of fitting Tafel plots is present as the dashed lines in Fig. 2b. The smaller value of the Tafel slope indicates a fast charge transfer kinetic, because the smaller potential need to be applied for obtaining the same current density. Thus, it is concluded that the catalytic activity of 1T MoS2 is substantially improved compared to that of the 2H MoS2 [30], due to the faster charge transfer kinetic as indicated by lower Tafel slope (40 mV dec-1 vs 75 mV dec-1). As discussed above, there two possible processes are Heyrovsky reaction (electrochemical desorption process) or Tafel reaction (chemical desorption process) in the second step of HER. The values of Tafel slope are 39 mV dec-1 and 29 mV dec-1 as calculated by eq. 2.8 and eq. 2.10, respectively. In this case of 1T MoS2, its value of Tafel slope is 40 mV dec-1, indicating that the Volmer-Heyrovsky mechanism dominates the HER where the electrochemical desorption reaction is the rate-limiting step. However, Tafel slopes which are extracted from polarization curves may contain the contribution of catalyst resistance. And the effect of catalyst resistance depends highly on the electrical conductivity and high mass loading of the electrocatalysts. Hu’s group has proposed another method to acquire the Tafel slopes by using the impedance data [45]. The Tafel slopes which are obtained by the plot of logRct vs. overpotential reflect purely the charge transfer kinetics, avoiding the effect from catalyst resistance. Here, Rct which is the charge transfer resistance can be obtain from Nyquist plots by using EIS method. Besides the Tafel slope, the exchange current density (i0) is an activity parameter of HER as well. Almost all the reported electrocatalysts have similar kinetics, it is because that the kinetics of HER is fast beyond the defined overpotential. Thus, the exchange current density is directly correlated to the onset overpotential and can be regarded as an activity parameter. It should be noted that the value of exchange current density may be overestimated due to the contribution from capacitive currents by using LSV method. The exchange current density is actually acquired by extrapolating the linear region, and it is the current density when the value of overpotential is zero, as expressed eq. 2.15. Even though performed at low-scan rates, the value exchange current density of may be higher that its true value, especially for those electrocatalysts with high specific area. 2.2.4 Surface Area The surface area of electrocatalysts is a key parameter for normalizing the value of current. Generally, there are three kinds of surface area for normalizing current to plot polarization curves and Tafel plots: geometric surface area, Brunauer-Emmett-Teller (BET) surface area, and electrochemically active surface area (ECSA). The geometric surface area is the area of electrodes, such as disk surface area of glassy carbon electrode. The unit of normalized current density is generally written as A cmgeo-2. This method is widely used in HER tests and it is easy to do fair comparison with previous reports. However, this kind current density could be an artificial effect and is not able to reflect the intrinsic catalytic properties of the electrocatalysts [46]. In addition, the mass loading of electrocatalysts which could have significant impact on applied potential is out of consideration. It is reasonable to use the geometrical area normalizing current density, if the loaded electrocatalysts are single layer with a coverage of 100%. Thus, the geometric surface area is fitted for planar electrodes (e.g. deposited thin films and foils). In the most cases, the mass 9

loading is either low where many substrate inactive sites are exposed or very high where only the surface layer of electrocatalysts actually involves the HER. In such cases, it is not fair to acquire the current density using geometrical area and large errors will be caused. Thus, the demand of electrode preparation is very rigorous and the optimum of mass loading is needed to be determined. The surface area which is obtained using gas adsorption-desorption curves by BET method is one of the most used method for normalizing current density. N2 gas is generally used as the probe gas. This BET method could be best suited to determination of the surface area of porous and power-type electrocatalysts. The absolute capacity can be easily determined by this independently BET surface area. But, this method is lack of experimental accuracy as well, due to not all gas adsorption sites are electrocatalytically active. Thus, this method is not able to reflect the intrinsic catalytic properties and does not provide anything to the validity of an internal comparison [47]. Moreover, the experimental incongruity gets larger when the electrocatalysts are composed of more than one element [38]. It is generally accepted that the most reasonable method is using ECSA to estimate specific activity of electrocatalysts. One method is to employ non-Faradic double-layer capacitance measured by CV method in a selected non-Faradaic potential window. For obtaining the value of ECSA, firstly the non-Faradic double-layer capacitance (CDL) can be derived from the following relational expression [47]: 𝑖 = 𝐶DL × 𝜈

(2.16)

where i is the response current density, ν is the scan rate of CV. Plotting i against ν obtains a linear relationship as expressed by eq. 2.16, where the slope is regarded as CDL. In addition, the value of CDL can be obtained by EIS as well [48-50]. It is noted that the applied potentials should fall in a potential region where no Faradaic processes occur [50]. Then the ECSA of a catalyst material is obtained by using CDL according to eq. 2.17:

ECSA =

𝐶DL 𝐶S

(2.17)

where Cs is the specific capacitance of the catalyst or the capacitance of an atomically smooth surface of the material per unit area under the same solution [50-51]. For example, the reported values of Cs are 0.13 mF cm-2 and 0.025 mF cm-2 for Cu and Ni electrode in 1 M NaOH solution [52-53]. The reported values of Cs are 0.050 mF cm-2 and 0.020 mF cm-2 for Cu and Ni electrode in 0.5 M H2SO4 solution [54-55]. However, as discussed by a previous literature, the determination of electrochemical surface area remains a big challenge, especially for nonmetal catalysts [41]. There could be large experimental inconformity between different evaluation methods, such as for CV method and EIS method. At present, ex situ methods such as the BET method may be more accurate than in situ electrochemical approaches such as non-Faradic double-layer capacitance measurement. 2.2.5 Turnover Frequency (TOF) The TOF is a quantitative parameter of HER employed to assess the catalytic activities of 10

electrocatalysts at designated overpotentials. It is defined as the number of H2 molecules produced per unit time with the unit of s-1. The TOF for HER can be calculated by eq. 2.17 [38]: 𝐼𝑁A

TOF = 𝐴𝐹𝑛Γ

(2.18)

where I is the current, Na is the Avogadro constant, A is the geometrical surface area, F is the Faraday constant, n is the number of transferred electrons, and Γ is the number of active sites. The key point is the determination of Γ and several methods are available. The quantity of active sites can be achieved by the Cu underpotential deposition method [56], which is well established for the noble metals [57]. The concentration of active sites (e.g. sulfur vacancy) can be estimated by scanning transmission electron microscopy (STEM) high-angle annular dark field (HAADF) images [28]. The number of active sites can be also calculated based on the average size of electrocatalyst by the Avogadro’s number method [58]. 2.2.6 Stability

Fig. 3 Stability measurement: (a) The polarization curves of ReSSe nanodots recorded before and after 20000 potential cycles in a 0.5 M H2SO4 with a potential range of 0 to -0.202 V (vs RHE) [59]. Copyright© 2018 American Chemical Society., Weinheim. (b) The I-t curve of the crumpled graphene/WS2/WO3 electrocatalyst at a fixed potential of -0.120 V (vs RHE) in an Ar-saturated 0.5 M H2SO4 solution [60]. Copyright © 2018 Elsevier Ltd.

Stability is another important parameter for assessing the HER catalytic performance of electrocatalysts. There are two mainstream methods for the stability. One is the CV method with a repeated potential cycles in a fixed potential range including onset potential, an example presented in Fig.3 a. After repeated CV cycles, the less increase of potential is needed to achieve the same current value compared to that of the initial cycle, indicating the better stability. The difference between the cycles can be directly observed in CV curves. After 20000th cycle, the polarization curve exactly overlaps with that of the initial cycle, demonstrating the superior stability of ReSSe nanodots [59]. The other method is potentiostatic or galvanostatic electrolysis with monitoring the value variation of current density or potential. The smaller value declines for current density or potential with increasing time at fixed potential or current, suggesting the better stability. An example of potentiostatic method is shown in Fig. 3b. The crumpled graphene/WS2/WO3 electrocatalyst exhibits hardly noticeable drop of current density (a 99.7 % retention of the catalytic current) at a fixed potential of -0.12 V after 15 hours, manifesting its 11

long-term stability [60]. 2.2.7 Faradic Efficiency For HER, the Faradic efficiency is the ratio of the electrons used for driving the HER to the electrons provided by external circuit, or the ratio of experimentally obtained H2 amount to the theoretical H2 amount. The Faradic loss could be caused by the by by-product produced during electrode reaction and heat loss. The experimental amount of H2 can be detected by gas chromatography or water-gas displacing method. The theoretical amount of H2 can be estimated by integrating the curves obtained by constant charge method.

3. Composition, Structure and Crystal Phases. The 2D TMDs nanomaterials deliver their HER catalytic activity varying with their intrinsic properties including composition, structure and crystal phases. Here, a briefly introduction is presented for readers to know 2D TMDs nanomaterials better. 3.1 Composition

Fig. 4 The layered TMDs compounds composed of highlighted and partially highlighted elements in the periodic table of elements [61]. Copyright © 2013 Macmillan Publishers Limited.

The general chemical formula of TMDs is MX2, where M denotes a transition metal element (e.g. Mo, W etc. elements highlighted in Fig.4) and X is a chalcogen element (S, Se or Te). The number of layered TMDs is about 40, accounting for two-thirds of all TMDs compounds which are about 60 in total number [61-62]. The TMDs compounds which are composed by the highlighted transition metals and chalcogen elements in Fig. 4, are mainly layered. For example, TiS2, WS2, MoS2, MoSe2 and MoTe2 are all layered TMDs compounds. However, only some TMDs compounds are layered if the transition metals are the ones partially highlighted in Fig. 4. For example, NiTe2 is a layered TMDs compound, whereas NiS2 has a structure of apyrite [61]. 3.2 Structure For these layered TMDs compounds, the transition metal atoms provide four electrons binding with chalcogen atoms to form MX2, leading to the valence states are +4 and -2 for transition metal and chalcogen atoms, respectively. These chalcogen atoms terminate the surface of layers in the absence of dangling bonds, thus guaranteeing the stability of those layers without reacting with ambient species [61]. These layered TMDs compounds have a graphite-like layered structure and consist of many individual layers which are coupled by weak van der Waals forces. The thickness of each 12

individual layer is typically 0.6-0.7 nm. The individual layer is actually a sandwich structure composed by a hexagonally packed layer of transition metal atoms sandwiched between two layers of chalcogen atoms. The intralayer interactions, i.e. the M-X bonds, are covalent in nature, and are much stronger than interlayer interactions. This means that stable single layer TMDs can be obtained by breaking these van der Waals forces among individual layers. These single layers are able to exist in a stabilized form of a ripple structure like as graphene [63], as demonstrated by ultrathin MoS2 [64-67]. After the exfoliation of these bulk TMDs, the single or few layer TMDs will exhibit various novel properties due to the quantum confinement effect in addition to the majority properties of their bulk counterparts [68-70]. 3.3 Crystal Phases

Fig. 5 Crystal phases of single layer TMDs and multilayer TMDs by stacking single layers [71]. Copyright © 2015 The Royal Society of Chemistry.

Fig. 5 presents the crystal phase of TMDs. There four different crystal structures: 2H (1H for single layer), 1T, distorted 1T (1T’) and 3R, where the digital represents for the number of X-M-X unit(s) in a unit cell (also the number of layer(s) in the stacking sequence) and the capital letters denote trigonal, hexagonal and rhombohedral, respectively. The single layer TMDs have only two polymorphs of trigonal prismatic and octahedral phases [71]. The trigonal prismatic phase belongs to the D3h point group and is conventionally referred as 1H phase (Fig. 5a), while the octahedral phase is assigned to be D3d group and is referred as 1T phase (Fig. 5b). In the 1H phase, the coordination of transition metal atoms is trigonal prismatic and the stacking sequence of atoms is AbA with a hexagonal closed packing symmetry [71-72], where the capital and lowercase letters denote chalcogen and transition metal atoms, respectively. In the 1T phase, the coordination of transition metal atoms is octahedral with a tetragonal symmetry and the stacking sequence of atoms is AbC in each layer [71-72]. The 1T’ phase which is similar to 1T phase has an octahedral coordination as well (Fig. 5c), whereas the superlattices are formed due to the shift of atoms from their equilibrium positions [72]. Taking MoS2 and WS2 for examples, and the unit cells arrange with 2a × a (or √3a × a), √3a × √3a, or 2√3a × 2√3a superlattice replacing the original ‘a × a’ unit cell [61, 73], where ‘a’ denotes the superlattice parameter. There are two different stacking ways for the 1H phase single layer constructing 2H crystal phase (Fig. 5d) [71]. The one way is a stacking sequence of AbA BaB, leading to the formation of 13

2H phase with hexagonal symmetry. The other way is the stacking sequence of AbA CaC BcB, resulting in the formation of 3R phase with rhombohedral symmetry. In addition, the AbC AbC sequence can be formed by stacking the 1T single layers.

4. Activity Descriptor and Optimization Methods of HER Performance. The intrinsic properties of TMDs include hydrogen adsorption Gibbs free energy ΔGH, active sites and electronic conductivity, which plays a fundamental role in determining the catalytic activity of HER. Especially for ΔGH, it is obtained by theoretical calculation and imply the HER catalytic activity of materials. Here, we highlight the vital role of ΔGH and the related optimization methods, meanwhile including the strategies for improving HER catalytic activity. 4.1 ΔGH

Fig. 6 (a) Schematic diagram of a HER volcano plot by correlating the experimentally measured j0 and theoretically computed ΔGH [78]. Copyright© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) The calculated ΔGH diagram of HER at 0 V vs. RHE, pH=0 [24]. Copyright© 2005 American Chemical Society. (c) Comparison of ΔGH of Td-WTe2 and 2H-MoS2 with different hydrogen adsorption sites [81]. Copyright© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (d) Comparison of ΔGH of 1T’-MoS2 basal plane, 2H-MoS2 edge on graphene and 2H-MoS2 on Au (111) at 25% and 50% H coverage [82]. Copyright© 2015 Macmillan Publishers Limited.

As a good activity descriptor, it is found that ΔGH largely determines the overall reaction kinetics of the HER [24, 74-77]. The ΔGH is always acquired through theoretical calculations, while the exchange current density j0 can be obtained by experimentally measurement. And the Sabatier Volcano of HER can be Plotted by j0 vs. ΔGH, shown in Fig. 6a [78], if the adsorption of hydrogen is too weak (ΔGH>0), it is difficulty for the combination of the proton and electrocatalyst, thus leading to a limited Volmer step. In turn, if the adsorption is too strong (ΔGH<0), the adsorbed hydrogen is hard to separate from surface of electrocatalysts, resulting in 14

a limited desorption process (via either a Heyrovsky step or Tafel step) due to the active sites mostly occupied. That means that the value of ΔGH closer to zero indicates the higher catalytic activities [4,24, 79-86]. In 2005, Hinnemann group revealed that the (1 0 -1 0) Mo-edge of MoS2 has a ΔGH of ~0.08 eV (approaching the optimum value of 0 eV) at H coverage of 50% (Fig. 6b) [24-25], demonstrating MoS2, as a typical TMDs material, a very promising candidate for HER. In further, it is found that the value of varies largely with different adsorption sites [87-88]. For example, a lower ΔGH of 0.08 eV on the Mo-edges versus a ΔGH of 0.18 eV on the S-edges, as shown in Fig. 7 [87]. The similar trend is observed for other TMDs reported by Cha group [88]: Td-WTe2 has a ΔGH with large value at the γ(W) site (the basal plane site), whereas the value is smallest for the α (W) site (the edge site) (Fig. 6c). Thus, ΔGH depends on different sites and it is necessary to find strategies to optimize ΔGH for improving HER catalytic activity. 4.2 Phase Engineering The optimization of ΔGH could be achieved by selecting favorable crystal phase of TMDs or performing phase engineering [29-31, 89-93]. Many theoretical calculations suggest that both the basal and edges sites of 1T-MoS2 are electrochemically active for HER [31, 90-92], with the value of ΔGH close to zero. After the investigation of 1T and 1T′-MX2 (M = Mo, W; X = S, Se, Te) TMDs as HER catalysts using density functional theory, the results indicate that 1T′-MoS2 and WS2 with optimal ΔGH could be the best HER electrocatalysts among these MX2 [93]. Phase engineering can be performed by lithium intercalation which leads to a transformation of MoS2 from 2H (trigonal prismatic) to 1T’ (clustered Mo). Brinker group found that the ΔGH of 2H MoS2 basal plane is > + 1.6 eV according to their calculation, and the value of ΔGH is reduced to the point approaching 0 eV after the basal plane changing into 1T’ phase [82]. As shown in Fig. 6d, the ΔGH of 1T’ MoS2 is as low as 0.13 eV at H coverage of 0.0625. Furthermore, the absolute value of ΔGH on the 1T’ MoS2 is superior, comparable to that of 2H Mo-edge supported on Au (111) and graphene [90]. Thus, the transformation of 1T’ improve the behavior of H adsorption/desorption, leading to the catalytic activation of the 2H MoS2 basal plane and the improvements of HER performance. Besides, it is noted that the value of ΔGH varies with different TMDs [94]. According to the theoretical calculations of ΔGH of basal planes, the ΔGH of 2H structure is stronger than that of the 1T structure for the Ti, Zr and Hf dichalcogenides, whereas the behavior is opposite for the Cr, Mo, and W dichalcogenides. 4.3 Heteroatom Doping

15

Fig. 7 (a) Model of Mo/WS2 exposing both Mo/W-edge and S-edge (left) and calculated ΔGH (right) [87]. Copyright© 2015 The Royal Society of Chemistry. (b) Model of 1T-MoS2 with metal doping. The circled atom denotes the H binding site and the purple sphere, cyan spheres and yellow spheres represent doping site, Mo atoms and S atoms, respectively (right). The diagram of calculated ΔGH of doped- and pristine 1T-MoS2 [31]. © 2016 American Chemical Society.

Introduction of exotic elements can optimize of ΔGH as well. Some TMDs, e.g. WS2, has no difference in value of ΔGH (both are 0.22 eV) at W edge and S edge, as presented in Fig. 7a [87]. Interestingly, the value of ΔGH can be lowered by the incorporation of cobalt. The promotion effects exhibit for both S edge of MoS2 and WS2: after the incorporation of cobalt into the S edge of MoS2 and WS2, the ΔGH of MoS2 and WS2 are optimized to 0.10 and 0.07 eV which both approach 0 eV, indicating a remarkable enhancement. Bao group has revealed that Pt-doped MoS2 nanosheets exhibit a significantly improved HER catalytic performance compared to the pristine MoS2, resulting from that the adsorption behavior of H atoms is optimized by the S atoms close to Pt atoms [95]. For 1T-MoS2, the HER performance can be improved by cation doping [31]. As shown in Fig. 7b, after density functional theory (DFT) calculations, it is found that the value of ΔGH of 1T-MoS2 is can be optimized by doping with Mn, Cr, Cu, Ni or Fe [31]. The anion dopants have been incorporated into TMDs to regulate their electronic conductivity and catalytic activity, as well. Kuo et al have revealed that C and O dopants are able to reduce the ΔGH and allow the defected MoS2 to operate at higher H coverages, according to the results of the first principles calculation [96]. This indicates that introduction of C and O 16

elements during the fabrication process could elevate the HER efficiency of TMDs. TMDs containing oxygen have been experimentally demonstrated to be as high-activity electrocatalysts for water splitting [40, 97]. For example, Xie group has prepared oxygen-incorporated MoS2 ultrathin nanosheets [40]. As presented by the calculated density of states (DOS), the bandgap of oxygen-incorporated MoS2 is only 1.30 eV, which is smaller than that of the pristine 2H-MoS2, indicating oxygen-incorporation could provide more charge carries and lead to improved electronic conductivity. Furthermore, according to the DFT calculations for ΔGH at edges of electrocatalysts, oxygen-incorporated MoS2 has an optimized value compared to the pristine MoS2 with an increasing H coverage from 25% to 50%, suggesting a lower energy barrier driving the HER. The experimental results demonstrate that the oxygen-incorporated MoS2 exhibits superior HER performance with lower onset overpotential, smaller Tafel slope and excellent long-term stability. Other anion-doped TMDs, e.g. N-doped WS2 nanosheets which possess a narrower bandgap of 1.5 eV compared to the pristine WS2 according to the partial density of states (PDOS) result, deliver enhanced electrocatalytic activity due to the increased charge carriers and improved electronic conductivity introduced by N doping [99]. 4.4 Strain Engineering

17

Fig. 8 STEM images of as-exfoliated 1T-WS2 (a) and annealing treated 2H-WS2 (b), Inset presents the strain tensor map obtained from the STEM-HAADF image, where yellow and black colour denote the strain tensed region and compressed region, respectively. Scale bars, 1 nm. (c) The catalytic activity versus the 1T phase concentration curve generated by annealing the as-exfoliated 1T-WS2 in an inert atmosphere. The error bars are from several measurements of multiple samples. (d) ΔGH of WS2 samples with different strain calculated by DFT with H coverage of 0.0625 S atoms and corrected for zero point energies and entropy [56]. Copyright© 2013 Macmillan Publishers Limited. Variation curves of on XT (e) and XC (e) sites under different biaxial tensile strain values. XT: the tensile-strained sulfur site. XC: the compressive-strained sulfur site [93]. Copyright © 2015 the Owner Societies.

The introduction of strain can optimize ΔGH and thus promote HER performance, as well [56, 93, 100-103]. Chhowalla group has demonstrated that the strained chemically exfoliated WS2 nanosheets can improve catalytic activity of HER. The as-exfoliated WS2 nanosheets are obtained by lithium intercalation of commercially available WS2 powder. The STEM image (Fig. 8a) shows that the structure of as-exfoliated WS2 is highly distorted 1T structure with a 2a0 × a0 superlattice [56, 104-105]. The superlattice structure is the strained 1T phase developed during Li intercalation [104], whereas the 1T-WS2 can relax to the stable 2H-WS2 (Fig. 8b) with annealing treatment. The exchange current density gradually decreases with decreasing 1T phase and strain caused by increasing annealed temperature (Fig. 8c), obviously indicating that strain plays an important role in regulating the catalytic activity of HER. As shown in Fig. 8d, the DFT calculation results confirm that the value of ΔGH equals to zero with a strain of 2.7%, agreeing well with the experimental results. Kuo group has investigated the effect of strain on the 1T’-MX2 (M = Mo, W; X = S, Se and Te) TMDs using DFT method [93]. As shown in Fig. 8e, the biaxial tensile strain lowers the ΔGH on the tensile-strained sulfur (XT) site of all 1T’-MX2, and thus optimizes significantly H adsorption behavior. Especially for MoS2, a strain of 2% make the ΔGH of MoS2 approach zero, while an optimal value of ΔGH can be obtained with a strain of ~3% for WS2. The same trend appears for the compressive-strained sulfur (XC) sites of all the 1T’-MX2 (Fig. 8f). Especially for MoTe2, if the strain value is higher than 6%, the ΔGH of XC sites is much lower than that of XT sites, indicating a remarkable enhancement of HER activity. In addition, Zheng group has firstly proved the optimization of ΔGH and activation of the basal plane of monolayer 2H-MoS2 for HER by employing strain and sulfur (S) vacancies. In the absence of S-vacancies, the ΔGH of basal plane of 2H-MoS2 is still large even a large elastic strain of 8% applied, suggesting that elastic strain is not enough to activate the basal plane. The optimal ΔGH of 0 eV can be achieved by multiple combinations of S- strain magnitudes and vacancy concentrations: greater strain combined with smaller S-vacancy concentrations (e.g. 8% strain combined with 3.12% S-vacancy) or smaller strain combined with larger S-vacancy concentrations (e.g. 1% strain combined with12.5% S-vacancy). Thus, the optimal ΔGH = 0 eV can be easily accomplished by proper combinations of strain and S-vacancy, eventually leading to the highest intrinsic HER activity among molybdenum-sulphide-based catalysts at that time [102]. 4.5 Defect and Vacancy Engineering

18

Fig. 9 (a) Structural models of defect-free and defect-rich MoS2. (b) The synthetic pathways obtaining the two structures in a. (c) nd annealing treated 2H-WS2 (b), Inset presents the strain tensor map obtained from the STEM-HAADF image, where yellow and black colour denote the strain tensed region and compressed region, respectively. Scale bars, 1 nm. (c) Polarization curves of various samples. (d) The corresponding Tafel plots. [106]. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Defect and vacancy engineering can optimize ΔGH and increase active sites, thus improving catalytic activity of HER [28, 106-109]. As well known, the basal plane of 2H-MoS2 with large ΔGH is inactive [24-25, 110]. Xie group has performed a pioneering work of defect-rich MoS2 ultrathin nanosheets [106]. Excess thiourea was added as both reductant and stabilizer to obtain the defect-rich ultrathin nanosheets [102, 111], whereas defect-free MoS2 nanosheets could be prepared with high concentration of precursors and less thiourea, as shown in Fig. 9a and 9b. Besides, defect and morphology modulations can be controllably adjusted by simply regulating the concentration of the precursors. The defect engineering causes the formation of cracks on the surface of MoS2 nanosheets, leading to more active edge sites exposed and thus dramatically enhancing the HER catalytic activity. As demonstrated in Fig. 9c, the defect-rich MoS2 nanosheets deliver a much smaller onset overpotential, compared to these highly crystalline samples. Moreover, the Tafel slope of defect-rich MoS2 nanosheets is as low as 50 mV dec-1, which is much smaller than that of the defect-free nanosheets, thicker nanosheets assemblies and calcined MoS2 nanosheets. Ye et al have demonstrated that more active sites can be created on the pristine monolayer MoS2 via the formation of defects within the monolayer after oxygen plasma exposure and hydrogen annealing [107]. As a result, the treated monolayer MoS2 possesses a high density of exposed edges and exhibits an improved HER catalytic activity. Han group has investigated on the HER at anion vacancy of TMDs by Ab initio computational screening [109]. ΔGH is used as the activity descriptor to study the HER catalytic activity at the anion vacancy of 40 TMDs. The calculation results show that ΔGH can be optimized after introducing anion vacancy. For example, it is found that ZrSe2 and ZrTe2 have similar ΔGH as Pt at low vacancy density, and an optimal ΔGH value is achieved for MoS2, MoSe2, MoTe2, ReSe2, ReTe2, WSe2, IrTe2, and HfTe2 at proper vacancy densities. Thus, vacancy engineering could activate the basal planes of many TMDs, and make them become promising candidates for HER. In addition, as discussed above in section 4.4, the ΔGH of basal plane of 2H-MoS2 can be well optimized to 0 eV using S-vacancy combined elastic strain and it is experimentally demonstrated that the 19

2H-MoS2 with S-vacancies deliver very impressive HER catalytic activity [102]. 4.6 Heterostructure

Fig. 10 (a) Schematic diagram of the flexible hybrid membrane of WS2/WO2.9/C sythesis. The photograph (b), SEM image (c) and HAADF-STEM image (d) of the hybrid membrane. Polarization curves (e) and corresponding Tafel plots (f) of samples. (g) ΔGH of various adsorption sites on the edge or basal plane of WS2 and WO2.9 (100) surface. Here, 3.13%-Vac denotes 3.13% of S vacancies in the basal plane of WS2, similar for 6.35%-Vac and 9.38%-Vac. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Heterostructured electrocatalysts can achieve the optimization of ΔGH [112-114], improvement of electronic conductivity [112, 115-118] and exposure of more active sites [115, 119-121], thus leading to the significant enhancement of HER catalytic activity. Lv group has recently reported a hybrid membrane of WS2/WO2.9/C which was prepared by using a precursor mixture of N, N-dimethylformamide solution of ammonium tetrathiotungstate and polyacrylonitrile spin-coated on graphite paper and thermal treatment in Ar/H2 flow in a chemical vapor system [112] (Fig. 10a). The flexible hybrid (Fig. 10b) consists of WS2 nanosheets with S vacancies, WO2.9 with oxygen vacancies, N-hybridized aromatics and graphite paper as support (Fig. 10b and 10c). The strain within WS2 could be the reason for creating S vacancies which could exist within the basal plane of WS2 and increase the number of catalytically active sites, thus enhancing the catalytic activity of HER. As a better conductor than WO3 due to oxygen vacancies, the WO2.9 nanowires can act as electron pathways across the WS2 nanosheets and serve as an effective electrocatalyst of HER as well. In addition, nitrogen-doped conjugated and 20

N-hybridized aromatics is produced by the pyrolysis of polyacrylonitrile and acts as connection of graphite paper with WS2 and WO2.9, which facilities the electron transport within the hybrid. Based on the above reasons, the WS2/WO2.9/C hybrid membrane exhibits the best HER catalytic activity among these WS2-based electrocatalysts, with the lowest onsetpotential (Fig. 10e) and smallest Tafel curve (Fig. 10f). This experimentally demonstrates that the S vacancies on the basal plane and the specific W sites of WO2.9 are both the active sites that play a vital role in improving the catalytic activity of HER. DFT calculations suggest that ΔGH is almost zero for the WS2 basal plane with 6.25% and 9.38% vacancies, whereas the adsorption of hydrogen is relatively strong for S-edge and W-edge with the ΔGH of -0.41 and -0.61 eV, respectively (Fig. 10g). Moreover, the ΔGH is approximately 0 eV for three adsorption sites (S-1W, S-2W and S-3W shown in Fig. 10g) of W terminal on WO2.9 (100), indicating very high catalytic activity. Thus, theoretical calculations confirm that both WS2 and WO2.9 play important roles in enhancing HER activity. He et al have constructed MoS2-black phosphorus (BP) heterostructures via the deposition of BP on MoS2 nanosheets [113]. Due to the higher Fermi level of BP than that of MoS2, electrons flow from NP to MoS2 in the heterostructures, thus resulting in remarkable HER performance of the heterostructures with an only 85 mV overpotential at 10 mA cm-2. Moreover, DFT calculations confirm that the ΔGH of MoS2-BP heterostructures is lower than that of MoS2, indicating that the injected electrons from BP improve the catalytic activity of original active sites in MoS2 rather than increases the number of active sites. In addition, it has recently predicted that 2D Janus TMD monolayers are promising electrocatalysts of HER using first-principles DFT calculations [114]. Herein, the WSSe Janus system has an optimized ΔGH close to thermos neutrality in the presence of S/Se-vacancies with intrinsic concentration and is expected to exhibit improved HER catalytic activity, resulting from the intrinsic strain and inter electric field brought by the Janus asymmetry. The electronic conductivity and number of active sites of TMDs can be improved and maximized by incorporation of proper conductive support forming heterostructures [115-117]. Dai group has developed a selective solvothermal preparation of MoS2 nanoparticles on the reduced graphene oxide (RGO) sheets [115]. These nanoscopic few-layer MoS2 structures are well dispersed on the RGO, leading to the forming an abundance of MoS2 exposed edges. And, the electronic conductivity of MoS2 is largely promoted due to the excellent electrical coupling to the underlying RGO network. As a result, the MoS2/RGO hybrid delivers superior electrocatalytic activity of HER, with a Tafel slope as low as ~41 mV/decade. 4.7 Nanostructure and morphological Engineering Nanostructure and morphological engineering are able to maximize the surface area/reaction interface, shorten the transport distances of ions and mass, and expose more active sites and thus enhances HER catalytic activity [27, 122-132].

21

Fig. 11 (a) Synthesis process of the mesoporous MoS2 with a DG morphology. (b, c) TEM images of the DG MoS2. (d) Comparison of iR-corrected CV curves of DG MoS2 versus core-shell MoO3-MoS2 nanowires at 5 mV s-1. (e) Ratios of surface area, active sites per surface area and total HER activity of the DG MoS2 films versus the MoO3-MoS2 nanowires [126-127]. Copyright © 2012 Macmillan Publishers Limited.

Jaramillo group has prepared a contiguous large-area thin film of a highly ordered double-gyroid (DG) with abundant nanopores to expose more edge sites for enhancing HER catalytic activity (Fig. 11) [127]. The silica DG template was firstly fabricated with EO19-PO43-EO19 as the surfactant [133] and was dip-coated onto the fluorine-doped tin oxide substrates. Then, the DG MoS2 was prepared via electrodeposition of Mo into the silica template with different deposition time to regulate the film thickness, followed by a sulphidization treatment with H2S. Finally, the electrodeposited DG MoS2 film which represents the negative of the silica template morphology was obtained after the removal of silica template using 2% HF (Fig. 11a). The TEM images present the ordered structure and high porosity of the mesoporous DG MoS2 thin films: the pore-to-pore distance is ~7 nm (Fig. 11c) and the thickness of a single MoS2 network is ~4nm with ~3 nm wide pores (Fig. 11c). When used as HER electrocatalyst, the DG MoS2 exhibits superior catalytic activity (iR-corrected) compared to that of core-shell MoO-MoS2 nanowires [127], as presented in Fig. 11d. Even for the thinnest MoS2 DG film (10s, ~50nm) with only half the surface area, it has high active sites density per surface area and outperforms the MoO-MoS2 nanowires by a factor of 1.6× (Fig. 11e). Moreover, the total HER activity of these MoS2 DG films is estimated using the exchange current densities and shows a remarkable increase relative to the core-shell MoO-MoS2 nanowires. Thus, this work demonstrates that morphological engineering of materials at the nanoscale is able to significantly influence the surface structure at the atomic scale, offering new chances for improving surface properties for HER catalysis. 22

As discussed above, there are two general types of surface sites on these TMDs: edge sites on the side surfaces and terrace sites on the basal planes. For HER of TMDs, edge sites are active sites, e.g. MoS2 and MoSe2 [24-25,27], whereas the sites on the basal planes are active e.g. 3R-NbS2 as recently reported by Zhang et al [134]. Thus, different strategies of structure engineering need to be executed for different TMDs. To maximize the expose of the edge sites, Cui group has developed a method to grow MoS2 and MoS2 thin films with vertically aligned layers on flat oxidized silicon substrate [27]. Furthermore, the MoS2 and MoS2 thin films have been proved that their layers with vertical orientation are able to grow on the curved and rough substrate such as nanowires and microfibers as well [130]. And, these films deliver outstanding HER activity with no degradation after 15000 catalytic cycles. In addition, hierarchically porous structures are also good choice due to the expose of more active sites. Moreover, the electrocatalysts with hierarchical structures at both the nano- and microscale not only generate a strong capillary force to pump solution, but also reduce interfacial interactions facilitating generated H2 gas bubble release, as indicated by the solid-liquid-gas interface theory [37]. For example, three-dimensionally hierarchical MoS2/graphene [133] and hierarchically porous nickel sulfide [132] both exhibit remarkable HER performance. For some other TMDs (e.g. 3R-NbS2) with active sites on the basal planes, synthesis of single crystal with large size may be an effective strategy maximally exposing their active sites. At last, we summarized typical and latest HER activities of various materials including TMDs [21,30,59,60,107,112,115,135-138], modified carbon materials [139-141], metal oxides [79,142], metal phophides [143,144] and metal carbides [37,145] (Table 1), as reference for readers. Table 1. Typical HER activities of various materials Catalyst

Mass loading (mg cm-2)

ηonset (mV)

MoS2/RGO Oxygen-Incorporated MoS2 1T phase MoS2 Defected MoS2 Zn-MoS2 1T-2H MoS2 1T WS2 WS2/WO2.9/C rGO/WS2/WO3 ReSSe 2H Nb1.35S2

0.28 0.285

~100 120

~0.05

~ 100 ~ 300 130 320 80-100 ~20 96 32 <100

~0.11 0.28 1-2×10-4 ~0.4 ~0.707 0.25

Cobalt-Doped FeS2

~7

~90

N-doped carbon

0.35

65

ηa10-5000 (mV)

300126.5

300139 32020 12010 11310 8410 12310 3701000 4205000 ~12020 ~170100 18010

Tafel slope (mV dec-1) ~41 55 ~40 147 51 65 55 ~36 37 50.1 ~38

Jb0 (mA cm-2)

0.0126

0.020 0.020

0.8

~46 56.7

Ref.

115 21 30 107 135 136 56 112 60 59 137

138 0.057

139 23

BA-TAP-Fe-800 Co@N-CNTs@rGO WO2−Carbon

0.283 ~0.5 0.35

65 35

TiO2 NDs/CoNSNTs-CFs

0.75

40

Carbon-Shell-Coated FeP NiCo2Px/CNTs MoS2/Mo2C

0.44

Mo2C

0.25

23 0.3 52

33010 10810 5810 7820 10810 14520 19550 235100 7110

200 56.7 46

4710 191500 2201000 11010

0.96 0.64

140 141 79

62

142

52

143

57.0 43

144 37

49.7

0.056

145

ηa20-1000：overpotential of the electrocatalyst at the current density of 10-5000 mA cm-2. Jb0: exchange current density.

5. Summary and Outlook In summary, HER definitely plays a vital role in the future of sustainable energy system and exploring low-cost and practicable electrocatalysts is highly imperative. The HER process processes through a Volmer-Heyrovsky or Volmer-Tafel mechanism, which can be determined by fitting Tafel plots. The parameters which contain onsetpotential, overpotential, Tafel slope, exchange current density, TOF, stability and Faradic efficiency are generally used to evaluate catalytic activity of HER. It should be careful in selecting suitable reference electrode and counter electrode for HER with different solutions, and the calibration of reference is needed. And the overpotential is recommended to perform the iRΩ compensation to reflect the inherent catalytic activity. The surface area which is used for normalizing the current density should be rationally selected depending on different electrocatalysts. As the possible alternatives for the ideal HER electrocatalyst of Pt, the 2D TMDs nanomaterials have demonstrated tremendous potential and been attracting more and more attentions. There are about 40 layered TMDs compounds comprised by many individual layers which are coupled by weak van der Waals forces. And, the individual layer is actually a sandwich structure composed by a hexagonally packed layer of transition metal atoms sandwiched between two layers of chalcogen atoms. The crystal phases consist of 2H (1H for single layer TMDs), 1T, distorted 1T and 3R phase. ΔGH which is a good activity descriptor determines the overall reaction kinetics of the HER based on the theoretical calculations. The methods which are used for optimizing ΔGH and enhancing HER catalytic activity usually involve phase engineering, heteroatom doping, strain engineering, defect and vacancy engineering, heterostructure building, and nanostructure and morphological engineering. Outlook combined with challenges are summarized as follows: 1) The electronic conductivity and crystallinity of TMDs could be largely lowered, when ΔGH and HER catalytic activity are optimized by creating more active sites, vacancies and defects. 24

Thus, the balance which involves the electronic conductivity and crystallinity of TMDs with the exposure of more active sites, vacancies and defects should be discreetly considered. 2) The catalytic activity and stability of TMDs should be investigated in the solutions with universal pH. In the reported cases, the HER tests are mainly performed in acidic solution. However, if these electrocatalysts are measured in alkaline solution. the HER catalytic activity may be 2-3 order of magnitude lower that in acidic solution. 3) At present, ΔGH can be successfully employed as the sole descriptor for the HER in acidic medium, whereas its availability remains to be further confirm in alkaline medium and controversies still exist among the experimentalists. 4) The effective and accurate method for correcting the background current is less well documented in the reported papers and literature. 5) Measuring the ECSA of TMDs remains a big challenge. It is urgent desirable to develop reliable methods for calculating ECSA and thus rationally and fairly evaluating the HER activity of TMDs. Acknowledgements The research was financially supported by the National Natural Science Foundation of China (51972193 and 51602173) and Laboratory Open Foundation of Qufu Normal University (sk201722). References: [1]

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Graphical abstract

31

Highlights: 1, We comprehensively review the recent progress of two-dimensional (2D) transition-metal dichalcogenides (TMDs) involving hydrogen evolution reaction (HER). 2, The mechanism of HER is first presented in detail, followed by an introduction of evaluation methods and some considerations in HER tests and a brief description of composition, structure and crystal phases of TMDs. 3, We highlight the intrinsic factors determining HER performance and summarize the recent development of strategies for improving the HER performance of TMDs. 4, An overall summary is presented and important challenges are discussed as well. 5, This review provides a reference for related researchers, especially for new beginners in this research field.

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