**4. Applications of 2D TMDs for electrocatalytic energy conversion**

### **4.1. Hydrogen evolution reaction**

H2 is considered as one of the most promising energy carriers owing to its high energy density and environmentally benign character. Nevertheless, it is still a great challenge to produce H2 efficiently [7, 9]. Amongst various H2 production pathways, the electrocatalytic hydrogen evolution reaction is attracting tremendous attention due to its high energy‐converting efficiency and the abundant raw material, namely, water. As is well known, hydrogen molecules can be generated from a process named water splitting, which can be divided into two independent half‐reactions, HER to generate hydrogen in the cathode and OER to produce oxygen in the anode. However, the large electrolytic window of water means that appropriate electrocatalysts are required to lower the overpotential for water splitting.

**Figure 3.** TMDs as electrocatalyst for HER. (a) Plot of exchange current density as a function of DFT‐calculated Gibbs free energy of absorbed atomic hydrogen for MoS<sup>2</sup> and pure metals. Reproduced with permission from Jaramillo et al. [34]; copyright 2007, American Association for the Advancement of Science, (b) nitrogenase (left) and hydrogenase (middle) inorganic compounds designed to mimic the edge sites of MoS2 (right). Reproduced with permission from Hinnemann et al. [29]; copyright 2005 American Chemical Society, (c) HRTEM image and the Fourier transform pat‐ tern (inset) of the defect‐rich MoS<sup>2</sup> ultrathin nanosheets, (d) polarization curves of various MoS2‐based samples as indi‐ cated. Reproduced with permission from Xie et al. [87]; copyright 2013 Wiley, (e) schematics of MoO3‐MoS<sup>2</sup> core‐shell nanowires as catalysts for HER. Reproduced with permission from Chen et al. [90]; copyright 2011 American Chemical Society, (f) scheme for the vertically aligned MoS<sup>2</sup> layers. Reproduced with permission from Kong et al. [43]; copyright 2013 American Chemical Society, (g) exfoliation of 2H MoS2 to 1T MoS<sup>2</sup> by lithium intercalation. Reproduced with per‐ mission from Lukowski et al. [91]; copyright 2013 American Chemical Society, and (h) scheme for the MoS*x*/NCNT for‐ est hybrid catalyst. Reproduced with permission from Li et al. [94]; copyright 2014 American Chemical Society.

Pt is currently the most efficient electroactive and electrochemically stable catalyst for HER, but its high cost and rare existence limit its wide application. Hence, exploring other earth abundant materials with high catalytic activities has attracted intensive interest. The experi‐ mental explorations of TMDs as the electrocatalyst for HER were motivated by theoretical calculations on MoS2 materials, which demonstrated that the hydrogen‐binding energy of MoS2 was close to that of metals such as Pt, Rh, Re, and Ir (**Figure 3a**) [29, 34, 86]. The density functional theory (DFT) calculations showed that metallic edges of trigonal prismatic (2H) MoS2 clusters were highly active compared to the basal plane of the chalcogenide, where it remained inert from the electrochemical point of view [14]. The surface‐active sites of MoS<sup>2</sup> was also probed by biomimicry of Mo(IV)‐disulphide inorganic and organic (**Figure 3b**) [29, 35].

as H2S or CS2) to a transition metal halide precursor in solution was controlled to be slow

Although the wet‐chemical approaches may unavoidably alter the lattice structure of thin TMDs and introduce extrinsic defects during exfoliation process, these defects may be helpful

H2 is considered as one of the most promising energy carriers owing to its high energy density and environmentally benign character. Nevertheless, it is still a great challenge to produce H2 efficiently [7, 9]. Amongst various H2 production pathways, the electrocatalytic hydrogen evolution reaction is attracting tremendous attention due to its high energy‐converting efficiency and the abundant raw material, namely, water. As is well known, hydrogen molecules can be generated from a process named water splitting, which can be divided into two independent half‐reactions, HER to generate hydrogen in the cathode and OER to produce oxygen in the anode. However, the large electrolytic window of water means that appropriate

**Figure 3.** TMDs as electrocatalyst for HER. (a) Plot of exchange current density as a function of DFT‐calculated Gibbs free energy of absorbed atomic hydrogen for MoS<sup>2</sup> and pure metals. Reproduced with permission from Jaramillo et al. [34]; copyright 2007, American Association for the Advancement of Science, (b) nitrogenase (left) and hydrogenase (middle) inorganic compounds designed to mimic the edge sites of MoS2 (right). Reproduced with permission from Hinnemann et al. [29]; copyright 2005 American Chemical Society, (c) HRTEM image and the Fourier transform pat‐ tern (inset) of the defect‐rich MoS<sup>2</sup> ultrathin nanosheets, (d) polarization curves of various MoS2‐based samples as indi‐ cated. Reproduced with permission from Xie et al. [87]; copyright 2013 Wiley, (e) schematics of MoO3‐MoS<sup>2</sup> core‐shell nanowires as catalysts for HER. Reproduced with permission from Chen et al. [90]; copyright 2011 American Chemical Society, (f) scheme for the vertically aligned MoS<sup>2</sup> layers. Reproduced with permission from Kong et al. [43]; copyright 2013 American Chemical Society, (g) exfoliation of 2H MoS2 to 1T MoS<sup>2</sup> by lithium intercalation. Reproduced with per‐ mission from Lukowski et al. [91]; copyright 2013 American Chemical Society, and (h) scheme for the MoS*x*/NCNT for‐ est hybrid catalyst. Reproduced with permission from Li et al. [94]; copyright 2014 American Chemical Society.

**4. Applications of 2D TMDs for electrocatalytic energy conversion**

electrocatalysts are required to lower the overpotential for water splitting.

enough to favour the lateral (2D) growth over 3D growth (**Figure 2d**).

70 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

in electrocatalytic reactions [14, 16, 22].

**4.1. Hydrogen evolution reaction**

The activities of TMDs are usually limited by the proportion of active edge sites [16]. To tackle this problem, Lou and co‐workers fabricated the defect‐rich MoS<sup>2</sup> ultrathin nanosheets by adding excess thiourea in the precursors (**Figure 3c** and **d**) [87]. The excess thiourea played a key role in the formation of defect‐rich MoS2, which not only worked as a reductant to reduce VIMo to IVMo but also worked as a capping agent to stabilize the morphology of MoS2 nanosheets. The resultant defect‐rich MoS2 showed outstanding electrocatalytic activity towards HER. It held a low overpotential of 120 mV, a large current density, and a small Tafel slope of 50 mV decade-1. They attributed the superior performance to the additional active edge sites exposed on defect‐rich MoS2 ultrathin nanosheets. To increase the active surface of MoS2, Kibsgaard et al. fabricated a 3D MoS2 porous network [88]. Chen et al. synthesized vertically oriented MoO3‐MoS2 core‐shell nanowires, in which the MoS2 shell contributes to the out‐ standing catalytic response as well as to protection against corrosion (**Figure 3e**) [89, 90]. Cui's group synthesized MoS2 films with vertically aligned layers [43]. The structure predominantly exposes the edges on the film surface maximally (**Figure 3f**). The edge‐terminated surface is obtained by overcoming the free energy barrier kinetically through rapid sulphurization.

Besides the active sites, the electric conductivity of MoS2 is another crucial factor to affect its electrocatalytic activity. Jin and co‐workers reported metallic 1T‐MoS<sup>2</sup> nanosheets, which were prepared by chemical exfoliation via lithium intercalation to from semiconducting 2H‐MoS<sup>2</sup> nanostructures (**Figure 3g**) [91]. This catalyst exhibited metallic conductivity and achieved a current density of 10 mA cm-2 at an overpotential of -187 mV vs. RHE. Additionally, a small Tafel slope of 43 mV decade-1 was reported for this catalyst. Xie's group studied the influence of active sites and conductivity of MoS2 on the electrocatalytic activity and achieved the balance between them by controlling disorder engineering and oxygen incorporation in MoS2 ultrathin nanosheets. This oxygen‐doped MoS2 with synergistically structural and electronic modula‐ tions achieved high‐efficient HER activity [84].

In order to further improve the catalytic efficiency and stability of TMD‐based electrocatalysts, enormous research efforts have been devoted to the incorporation of TMDs with other materials, such as noble metals and carbon materials (**Figure 3h**) [92–96]. Zhang and co‐ workers demonstrated the wet‐chemical synthesis of noble metal nanostructures epitaxially grown on TMD nanosheets. The noble metal‐TMD composites exhibit good electrocatalytic activity in hydrogen evolution reaction [97, 98].

#### **4.2. CO2 reduction reaction**

Despite the tremendous efforts being made to implement renewable energy sources, there remains a need in the longer term to be able to sustainably generate liquid fuels for applications including aviation and mining [9, 11]. Electrochemical CO2 reduction, recycling CO2 back to fuels, and commodity chemicals utilizing renewable energy as a power source could poten‐ tially provide a solution to this problem [99]. However, CO2 is very stable under environmental conditions and HER often prevails over CO2 reduction in aqueous electrolytes under cathodic polarization [100, 101], making it essential to find a suitable catalyst to achieve cost‐effective CO2 reduction with high efficiency and selectivity. Metals and especially nanostructured metals derived from metal oxide have been widely studied as electrocatalysts for CO2 reduction [100, 102–106]; however, these systems generally show low activities and/or selectivity for a solo product (such as CO, formate, methanol, methane, ethylene, and ethanol) or need nonaqueous solvents which may limit practical application.

**Figure 4.** TMDs as electrocatalysts for electrochemical reduction of CO2. (a) Binding energies Eb(COOH) vs. Eb(CO) for transition metals and Mo and S edges of MoS2. Reproduced with permission from Shi et al. [108]; copyright 2014 Royal Society of Chemistry, (b) raw greyscale HAADF and false‐colour low‐angle annular dark‐field (LAADF) image (inset) of MoS2 edges (scale bar, 5 nm), (c) cyclic voltammetric (CV) curves for bulk MoS2, Ag nanoparticles (Ag NPs), and bulk Ag in CO2 environment. The electrolyte is a mixture of 96 mol% water and 4 mol% EMIM‐BF4, (d) CO and H2 Faradaic efficiency (FE) at different applied potentials. Reproduced with permission from Asadi et al. [109]; copyright 2014 Nature Publishing Group, (e) CVs of rGO‐PEI‐MoS2‐modified GCE in N2‐saturated (black curve) and CO2‐saturat‐ ed (red curve) 0.5 M aqueous NaHCO3 solution. Scan rate was 50 mV s-1, (f) Faradaic efficiency for CO (red bars) and H2 (blue bars) as a function of potential, (g) amount and Faradaic efficiency of H2 (circles) and CO (squares). Potentio‐ static electrolysis at -0.4 V in CO2‐saturated 0.5 M aqueous NaHCO3 solution and (h) Tafel plot of CO production parti‐ al current density vs. overpotential on rGO‐PEI‐MoS2. Reproduced with permission from Li et al. [110]; copyright 2016 Royal Society of Chemistry.

Recently, Nørskov et al. demonstrated theoretically that MoS2 or MoSe2 could possibly be electrocatalysts for CO2 reduction by DFT calculation [107, 108]. Their results indicate the edge site of MoS2 or MoSe2 is active for electrochemical CO2 reduction due to the different scaling relationships of adsorption energies between key reaction intermediates (\*CO and \*COOH) on the edges of MoS2 or MoSe2 compared to transition metals (**Figure 4a**). Experimental results of MoS2 as electrocatalyst for CO2 reduction were firstly reported by Asadi et al. [109] (**Figure 4b**–**d**). They uncovered that MoS2 showed superior CO2 reduction performance compared with the noble metals with a high current density and low overpotential (54 mV) in an ionic liquid. They also utilized DFT calculations to reveal the catalytic activity mainly arises from the molybdenum‐terminated edges of MoS<sup>2</sup> due to their metallic character and a high *d*‐ electron density. The experimental result that vertically aligned MoS2 showed an enhanced performance compared to bulk MoS2 crystal supported their calculations.

Li and co‐workers reported amorphous MoS2 on a polyethylenimine‐modified reduced graphene oxide substrate as an effective catalyst for electrocatalytic CO2 reduction (**Fig‐ ure 4e**–**h**) [110]. The catalyst is capable of producing CO at an overpotential as low as 140 mV and reaches a maximum Faradaic efficiency (FE) of 85.1% at an overpotential of 540 mV. Another interesting point is that at an overpotential of 290 mV with respect to the formation of CO, it catalyses the formation of syngas with high stability, which could be readily utilized in the current Fischer‐Tropsch process and produce liquid fuels, such as ethanol and methanol. Their detailed mechanism investigation indicated that the efficiency and selectivity towards CO2 reduction rather than hydrogen evolution at the optimal applied potential were attributed to the synergetic effect of MoS2 and PEI: (1) the intrinsic properties of MoS2 that it can selectively bind the intermediate during the CO2 reduction reaction path is the principal factor contribu‐ ting the CO2 reduction and (2) PEI, an amine containing polymer with outstanding CO2 adsorption capacity, can stabilize the intermediate and thus lower the energy barrier by hydrogen bond interaction.
