Sustainable energy technology has received enormous attention in recent years. Specifically, electrochemical water splitting is considered to be the cleanest technique for the production of promising fuels, for example, hydrogen and oxygen, where transition metal (di)chalcogenides (TMCs) as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been a growing interest. In this chapter, the typical preparation methods of TMCs such as chemical vapor phase deposition (CVD) and solvothermal synthesis are introduced. Then, several TMC materials for catalyzing HER and OER are reviewed. Most importantly, this chapter also introduced some in situ approaches to realize the mechanism of electrocatalytic behavior toward HER and OER. Finally, the conclusion and futuristic prospects of TMCs in HER and OER are discussed.
- energy conversion
- hydrogen evolution reaction (HER)
- oxygen evolution reaction (OER)
- transition metal chalcogenides (TMCs)
Energy crisis and environmental pollution arising from the burning of carbon-based fossil fuel in the past decades facilitate people to reconsider the way we utilized the resource on earth. Hydrogen as an ideal energy source came up to the stage due to its high energy density and environmental benignity [1, 2]. Electrochemical water splitting is not only regarded as the cleanest technique for hydrogen generation but also suitable to perform on a large scale. The appropriate electrocatalysts are developed to boost the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) because of the sluggish kinetics of these two core reactions [overpotential (η), Figure 1a]. Although platinum-based and iridium/ruthenium-based catalysts, respectively, have shown very promising performance in HER and OER, the industrial application is restricted due to their high cost and limited availability [3, 4]. Thus, developing alternative electrocatalysts based on non-noble metals or earth-abundant elements is still highly demanded.
The realization of HER and OER mechanisms plays a crucial role to design the efficient electrocatalysts, so the brief discussions on their mechanisms were introduced.
Two kinds of HER mechanisms have been completely studied and widely accepted [5, 6]. The first step, a proton from the solution adsorbs onto the catalytic sites of the electrode with a reduction process, is called as Volmer step:
Then, either the recombination of two adsorbed hydrogen atoms on the electrode surface is called as Tafel step:
or the reaction of an adsorbed hydrogen atom with the hydrated proton, which proceeds with an electron transfer from the electrode surface, is called as Heyrovsky step:
Nørskov et al. proposed a Volcano curve showing that experimental exchange currents of materials are as a function of the Gibbs free energy of the adsorbed hydrogen (ΔGH) (Figure 1b) [7, 8]. Ideally, the interaction of hydrogen with the electrode surface should be thermoneutral (ΔGH ≅ 0), otherwise either Heyrovsky or Tafel step (strong bonding) or Volmer step would become the rate-determining step. In the case of the OER, many researchers have proposed possible mechanisms at the anode in acidic electrolyte (Eqs. (4)–(8)). However, there are some differences around the reaction of forming oxygen. One route of forming oxygen is through the direct recombination of two MO (M represents Mn, Fe, Co, and Ni metals) (Eq. (6)), while the other route of forming oxygen is through the decomposition of the MOOH intermediate (Eq. (8)).
2. Preparations of TMCs
2.1 CVD synthesis of TMCs
The preparation methods of TMCs layers can be categorized into two main approaches: top-down and bottom-up methods (Figure 2). The TMC layers prepared from their bulk material by mechanical exfoliation are called the top-down method, while the TMC layers were produced from the elemental precursors on the target substrate in the bottom-up method, such as chemical vapor deposition (CVD) approach. The CVD method for TMC synthesis has been widely used due to its unique advantages such as scalable size, high crystallinity, and controllable thickness of TMCs [10, 11, 12, 13]. One of the most classic TMCs, molybdenum sulfide (MoS2), has been developed via CVD synthesis to replace the zero bandgap graphene.  There are two routes to synthesize MoS2 layer by CVD (Figure 3a). One is a two-step growth route, where the Mo-based precursors are first deposited and then conducted the sulfurization or decomposition process to form MoS2 (route 1, Figure 3a). The other growth way of MoS2 layer is an one-step growth, where the gaseous Mo- and S-based precursors are simultaneously introduced and react to form MoS2 on a substrate (route 2, Figure 3a). Recent reports have also demonstrated to grow MoS2 on other kinds of insulating substrates such as quartz, mica, and sapphire (Figure 3b) [15, 16]. Figure 3c shows a typical experimental setup for the low-pressure CVD synthesis of MoS2. The formation of entire MoS2 would be prevented according to the ternary Mo▬O▬S phase diagram (Figure 3d) if the reducing atmosphere is too weak . Figure 3e displays two possible mechanisms for the growth of MoS2. The well-established CVD synthesis of MoS2 is regarded as a prototype for the synthesis of other TMCs such as MoSe2 and WS2 [18, 19]. Except from the VI B group metals such as molybdenum and tungsten, the research works of TMCs based on VIII B group metals such as cobalt  and nickel  are also widely studied in recent years.
2.2 Solvothermal synthesis of TMCs
Wet chemical synthesis, a bottom-up process, is broadly used to synthesize TMC nanosheets with tunable thickness and size [24, 25, 26]. The desired nanosheets form directly in solution or onto substrate in high yield. The synthesis of TMCs via wet chemical method usually relies on the chemical reaction of metal salts and sulfur/selenide-based materials as precursors. The solvothermal method is one of the most typical routes of wet chemical method, in which the reaction is conducted in a sealed autoclave at suitable temperature (Figure 4) . As a representative example, Xie et al.  synthesized oxygen incorporated MoS2 nanosheets from the precursors of (NH4)6Mo7O24·4H2O and thiourea through a solvothermal method. By tuning the synthetic temperatures, the disordered structure and intrinsic conductivity of MoS2 could be controlled to a moderate degree. Thus, the optimal catalyst for electrocatalytic hydrogen evolution reaction exhibits onset overpotential as low as 120 mV. It is well known that the materials with different crystal orientations have the anisotropic properties. This phenomenon appears in TMCs depending on stacking sequence of the chalcogen and transition-metal coordination. The commonly found polymorphs in TMC materials are the so-defined 1 T, 2H, and 3R, where the number and letter, respectively, indicate the layer number in the unit cell and the type of symmetry with T, H, and R representing for tetragonal, hexagonal, and rhombohedral, respectively (Figure 5). For example, Zhang et al. synthesized metallic CoS2 nanopyramid array on carbon fiber paper by an one-step solvothermal synthesis . In brief, the carbon fiber papers were transferred to the precursor solution containing CoCl2·6H2O and CS(NH2)2, which was maintained at 180°C for 12 hours. The unique 3D nanostructure and intrinsic metallic properties of the CoS2 by this method contribute to the ultrahigh activity toward hydrogen evolution reaction.
3. Hydrogen evolution reaction (HER) based on TMC electrodes
In order to improve HER performance, three main factors including the number of active sites, intrinsic catalytic activity, and the conductivity of TMCs play crucial roles. In 2005, Hinnemann et al. suggested the active sites for HER only exist in the edges of TMCs by density functional theory (DFT) calculation . Thus, many research works are dedicated to enrich the defect sites and/or active sites by nanostructural engineering [31, 32, 33]. Zhang et al. synthesized the edge-rich 3D MoS2 coupling with conductive polymer polyaniline (PANI) as catalyst (Figure 6) . The MoS2 grown on 3D PANI substrate tends to grow vertically and expose abundant edge sites for HER. Consequently, excellent HER performance can be achieved with a low onset potential of 100 mV and a small Tafel slop of 45 mV dec−1. Additionally, MoS2/PANI achieved superior stability for HER electrocatalysis. Although the promising catalytic activity of MoS2 for HER was achieved by creating abundant edge sites and/or active sites, the performance was still limited by its intrinsic properties such as poor electrical transport and inefficient electrical contact to the catalyst . Lukowski et al. reported that the metallic nanosheets of 1T-MoS2, which were chemically exfoliated by lithium intercalation of semiconducting 2H-MoS2 nanostructures, dramatically enhanced HER performance (Figure 7) . The current density of 10 mA cm−1 can be reached at a low overpotential of −187 mV with a Tafel slope of 43 mV dec−1. The excellent performance can be attributed to the favorable kinetics, metallic conductivity, and increasing number of active sites in the metallic 1T-MoS2 nanosheets, which was proven by the dramatically decrease of charge-transfer resistance from 232 Ω of 2H-MoS2 nanostructures to 4 Ω of metallic 1T-MoS2 nanosheets. This finding proves that the metallic 1T polymorph of TMCs is competitive to earth-abundant catalysts in heterogeneous catalysis.
Lee et al. developed earth-abundant nanostructuring beaded stream-like cobalt diselenide (CoSe2) nanoneedles (CoSe2-BSND) as electrocatalyst for HER . The CoSe2 nanoneedles derived from the cobalt oxide (Co3O4) nanoneedle array directly formed on flexible titanium foils after selenization treatment (Figure 8). The CoSe2-BSND can drive the HER at a current density of 20 mA cm−2 with a small overpotential of 125 mV. Also, it possesses a small Tafel slope of 48.5 mV dec−1 suggesting that the HER follows the Volmer-Heyrovsky mechanism where a fast discharge of protons is followed by rate-determining electrochemical desorption. Moreover, the CoSe2-BSND electrode achieved great stability in an acidic electrolyte for 3000 cycles. The enhanced electrochemical activity is attributed to the highly accessible surface active sites, the improved charge transfer kinetics, and the super hydrophilic surface of CoSe2-BSND electrode.
4. Oxygen evolution reaction (OER) based on TMC electrodes
The TMC materials have shown promising performance toward HER as we mentioned above. Researchers are searching for the possibility of bifunctional electrocatalysts for both HER and OER to perform the overall water-splitting reaction. Moreover, TMC electrocatalysts have attracted tremendous attentions since Alonso-vante and coworker discovered that Mo4Ru2Se8 had a ORR activity comparable to platinum . As compared to ruthenium (Ru) and Rhenium (Rh), the low cost and earth abundant transition metals such as iron-, nickel- and cobalt-based TMCs have much attention for OER [39, 40, 41]. Liu and coworkers discovered that electrodeposited CoS nanosheet films on Ti mesh show high activity toward OER (Figure 9) . The CoS nanosheets tend to drive a current density of 10 mA cm−2 with an overpotential of 361 mV. In addition, this electrode maintains highly catalytic activity for at least 20 hours. The superior catalytic activity along with excellent stability of CoS nanosheets offers the good opportunity to become a cost-effective and industry-feasible electrode toward OER. On the other hand, Swesi et al. first reported that the catalytic activity of OER was observed by the nickel selenide (Ni3Se2) in alkaline condition (Figure 10) . The low overpotential required to reach 10 mA cm−2 was 290 mV, suggesting that this catalyst exhibits its competitivity among the oxide-based electrocatalysts. The catalytic ability of Ni3Se2 can be further improved through the modification of Se-deficient phase in Ni3Se2. Moreover, electrodeposited Ni3Se2 catalysts exhibited exceptional stability under OER for 42 hours. The effect of the underlying substrates such as glassy carbon, ITO-coated glass, and Ni foam on OER was also investigated. The results revealed that the glassy carbon substrate exhibited the lowest onset potential and the highest current density, suggesting that the interaction between the underlying substrate and the Ni3Se2 may play a role in O2 evolution reaction.
Although the electrocatalytic performance of TMCs toward OER was significantly enhanced, the reaction mechanism and actual active sites responsible for the reaction were in dispute. Recently, operando or
Electrocatalysts for HER and OER play an important role for sustainable energy, which require converting renewable energy to storable chemical fuels or employing clean energy. In this article, we have partially reviewed the promising candidates, transition metal (di)chalcogenides (TMCs), from preparation methods to electrochemical measurements toward HER and OER. It can be concluded that a good electrocatalyst should possess good conductivity and moderate adsorption energy to reactive species and/or intermediates. In addition, the large number of active/defect sites would be favorable for catalytic ability. The future challenges for TMCs toward HER and OER are summarized as follows. The realization of the mechanisms toward the HER and OER plays a key role to design a perfect electrocatalyst. The development of
This work was supported by the Ministry of Science and Technology (MOST) of Taiwan, under grant numbers 107-2113-M-845-001-MY3.
Bockris JOM. The origin of ideas on a hydrogen economy and its solution to the decay of the environment. International Journal of Hydrogen Energy. 2002; 27:731-740. DOI: 10.1016/S0360-3199(01)00154-9
Dresselhaus MS, Thomas IL. Alternative energy technologies. Nature. 2001; 414:332-337. DOI: 10.1038/35104599
Zhu Y, Chen HC, Hsu CS, Lin TS, Chang CJ, Chang SC, et al. Operando unraveling of the structural and chemical stability of P-substituted CoSe2 electrocatalysts toward hydrogen and oxygen evolution reactions in alkaline electrolyte. ACS Energy Letters. 2019; 4:987-994. DOI: 10.1021/acsenergylett.9b00382
Zhang J, Zhao Z, Xia Z, Dai L. A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nature Nanotechnology. 2015; 10:444-452. DOI: 10.1038/nnano.2015.48
Li Y, Wang H, Xie L, Liang Y, Hong G, Dai H. MoS2 nanoparticles grown on graphene: An advanced catalyst for the hydrogen evolution reaction. Journal of the American Chemical Society. 2011; 133:7296-7299. DOI: 10.1021/ja201269b
Conway BE, Tilak BV. Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H. Electrochimica Acta. 2002; 47:3571-3594. DOI: 10.1016/S0013-4686(02)00329-8
Zeng M, Li Y. Recent advances in heterogeneous electrocatalysts for the hydrogen evolution reaction. Journal of Materials Chemistry A. 2015; 3:14942-14962. DOI: 10.1039/c5ta02974k
Nørskov JK, Bligaard T, Logadottir A, Kitchin JR, Chen JG, Pandelov S, et al. Trends in the exchange current for hydrogen evolution. Journal of the Electrochemical Society. 2005; 152:J23. DOI: 10.1149/1.1856988
Suen NT, Hung SF, Quan Q, Zhang N, Xu YJ, Chen HM. Electrocatalysis for the oxygen evolution reaction: Recent development and future perspectives. Chemical Society Reviews. 2017; 46:337-365. DOI: 10.1039/c6cs00328a
Chia X, Eng AYS, Ambrosi A, Tan SM, Pumera M. Electrochemistry of nanostructured layered transition-metal dichalcogenides. Chemical Reviews. 2015; 115:11941-11966. DOI: 10.1021/acs.chemrev.5b00287
Huang JK, Pu J, Hsu CL, Chiu MH, Juang ZY, Chang YH, et al. Large-area synthesis of highly crystalline WSe2 monolayers and device applications. ACS Nano. 2014; 8:923-930. DOI: 10.1021/nn405719x
Liu G, Li Z, Hasan T, Chen X, Zheng W, Feng W, et al. Vertically aligned two-dimensional SnS2 nanosheets with a strong photon capturing capability for efficient photoelectrochemical water splitting. Journal of Materials Chemistry A. 2017; 5:1989-1995. DOI: 10.1039/c6ta08327g
Chang YH, Zhang W, Zhu Y, Han Y, Pu J, Chang JK, et al. Monolayer MoSe2 grown by chemical vapor deposition for fast photodetection. ACS Nano. 2014; 8:8582-8590. DOI: 10.1021/nn503287m
Lee YH, Zhang XQ, Zhang W, Chang MT, TeLin C, DiChang K, et al. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Advanced Materials. 2012; 24:2320-2325. DOI: 10.1002/adma.201104798
Ji Q, Zhang Y, Gao T, Zhang Y, Ma D, Liu M, et al. Epitaxial monolayer MoS2 on mica with novel photoluminescence. Nano Letters. 2013; 13:3870-3877. DOI: 10.1021/nl401938t
Yu Y, Li C, Liu Y, Su L, Zhang Y, Cao L. Controlled scalable synthesis of uniform, high-quality monolayer and few-layer MoS2 films. Scientific Reports. 2013; 3:1-6. DOI: 10.1038/srep01866
Feldman Y, Tenne R, Wasserman E, Srolovitz DJ. High-rate, gas-phase growth of MoS2 nested inorganic fullerenes and nanotubes. Science. 1995; 267:222
Zhao Y, Lee H, Choi W, Fei W, Lee CJ. Large-area synthesis of monolayer MoSe2 films on SiO2/Si substrates by atmospheric pressure chemical vapor deposition. RSC Advances. 2017; 7:27969-27973. DOI: 10.1039/c7ra03642f
Okada M, Sawazaki T, Watanabe K, Taniguch T, Hibino H, Shinohara H, et al. Direct chemical vapor deposition growth of WS2 atomic layers on hexagonal boron nitride. ACS Nano. 2014; 8:8273-8277. DOI: 10.1021/nn503093k
Wang B, Wang Z, Wang X, Zheng B, Zhang W, Chen Y. Scalable synthesis of porous hollow CoSe2-MoSe2/carbon microspheres for highly efficient hydrogen evolution reaction in acidic and alkaline media. Journal of Materials Chemistry A. 2018; 6:12701-12707. DOI: 10.1039/c8ta03523g
An T, Wang Y, Tang J, Wei W, Cui X, Alenizi AM, et al. Interlaced NiS2-MoS2 nanoflake-nanowires as efficient hydrogen evolution electrocatalysts in basic solutions. Journal of Materials Chemistry A. 2016; 4:13439-13443. DOI: 10.1039/c6ta05022k
Dong R, Kuljanishvili I. Review article: Progress in fabrication of transition metal dichalcogenides heterostructure systems. Journal of Vacuum Science and Technology B. 2017; 35:030803. DOI: 10.1116/1.4982736
Ji Q, Zheng Y, Zhang Y, Liu Z. Chemical vapour deposition of group-VIB metal dichalcogenide monolayers: Engineered substrates from amorphous to single crystalline. Chemical Society Reviews. 2015; 44:2587-2602. DOI: 10.1039/c4cs00258j
Wang S, Chen Y, Li X, Gao W, Zhang L, Liu J, et al. Injectable 2D MoS2-integrated drug delivering implant for highly efficient NIR-triggered synergistic tumor hyperthermia. Advanced Materials. 2015; 27:7117-7122. DOI: 10.1002/adma.201503869
Cheng L, Yuan C, Shen S, Yi X, Gong H, Yang K, et al. Bottom-up synthesis of metal-ion-doped WS2 nanoflakes for cancer theranostics. ACS Nano. 2015; 9:11090-11101. DOI: 10.1021/acsnano.5b04606
Shen S, Chao Y, Dong Z, Wang G, Yi X, Song G, et al. Bottom-up preparation of uniform ultrathin rhenium disulfide nanosheets for image-guided photothermal radiotherapy. Advanced Functional Materials. 2017; 27:1-9. DOI: 10.1002/adfm.201700250
Zhu S, Gong L, Xie J, Gu Z, Zhao Y. Design, synthesis, and surface modification of materials based on transition-metal dichalcogenides for biomedical applications. Small Methods. 2017; 1:1700220. DOI: 10.1002/smtd.201700220
Xie J, Zhang J, Li S, Grote F, Zhang X, Zhang H, et al. Controllable disorder engineering in oxygen-incorporated MoS2 ultrathin nanosheets for efficient hydrogen evolution. Journal of the American Chemical Society. 2013; 135:17881-17888. DOI: 10.1021/ja408329q
Zhang H, Li Y, Zhang G, Xu T, Wan P, Sun X. A metallic CoS2 nanopyramid array grown on 3D carbon fiber paper as an excellent electrocatalyst for hydrogen evolution. Journal of Materials Chemistry A. 2015; 3:6306-6310. DOI: 10.1039/c5ta00707k
Hinnemann B, Moses PG, Bonde J, Jørgensen KP, Nielsen JH, Horch S, et al. Biomimetic hydrogen evolution: MoS2 nanoparticles as catalyst for hydrogen evolution. Journal of the American Chemical Society. 2005; 127:5308-5309. DOI: 10.1021/ja0504690
Ren X, Pang L, Zhang Y, Ren X, Fan H, Liu S. One-step hydrothermal synthesis of monolayer MoS2 quantum dots for highly efficient electrocatalytic hydrogen evolution. Journal of Materials Chemistry A. 2015; 3:10693-10697. DOI: 10.1039/c5ta02198g
Du H, Gu S, Liu R, Li CM. Tungsten diphosphide nanorods as an efficient catalyst for electrochemical hydrogen evolution. Journal of Power Sources. 2015; 278:540-545. DOI: 10.1016/j.jpowsour.2014.12.095
Zhang X, Li L, Guo Y, Liu D, You T. Amorphous flower-like molybdenum-sulfide-at-nitrogen-doped-carbon-nanofiber film for use in the hydrogen-evolution reaction. Journal of Colloid and Interface Science. 2016; 472:69-75. DOI: 10.1016/j.jcis.2016.03.041
Zhang N, Ma W, Wu T, Wang H, Han D, Niu L. Edge-rich MoS2 naonosheets rooting into polyaniline nanofibers as effective catalyst for electrochemical hydrogen evolution. Electrochimica Acta. 2015; 180:155-163. DOI: 10.1016/j.electacta.2015.08.108
Laursen AB, Kegnæs S, Dahl S, Chorkendorff I. Molybdenum sulfides- efficient and viable materials for electro- and photoelectrocatalytic hydrogen evolution. Energy & Environmental Science. 2012; 5:5577-5591. DOI: 10.1039/c2ee02618j
Lukowski MA, Daniel AS, Meng F, Forticaux A, Li L, Jin S. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. Journal of the American Chemical Society. 2013; 135:10274-10277. DOI: 10.1021/ja404523s
Lee CP, Chen WF, Billo T, Lin YG, Fu FY, Samireddi S, et al. Beaded stream-like CoSe2 nanoneedle array for efficient hydrogen evolution electrocatalysis. Journal of Materials Chemistry A. 2016; 4:4553-4561. DOI: 10.1039/c6ta00464d
Vante NA, Tributsch H. Energy conversion catalysis using semiconducting transition metal cluster compounds. Nature. 1986; 323:431-432
Feng Y, He T, Alonso-vante N. In situ free-surfactant synthesis and ORR- electrochemistry of carbon-supported Co3S4 and CoSe2 nanoparticles. Chemistry of Materials. 2008; 20:26-28
Gao MR, Jiang J, Yu SH. Solution-based synthesis and design of late transition metal chalcogenide materials for oxygen reduction reaction (ORR). Small. 2012; 8:13-27. DOI: 10.1002/smll.201101573
Gao MR, Xu YF, Jiang J, Yu SH. Nanostructured metal chalcogenides: Synthesis, modification, and applications in energy conversion and storage devices. Chemical Society Reviews. 2013; 42:2986-3017. DOI: 10.1039/c2cs35310e
Liu T, Liang Y, Liu Q, Sun X, He Y, Asiri AM. Electrodeposition of cobalt-sulfide nanosheets film as an efficient electrocatalyst for oxygen evolution reaction. Electrochemistry Communications. 2015; 60:92-96. DOI: 10.1016/j.elecom.2015.08.011
Swesi AT, Masud J, Nath M. Nickel selenide as a high-efficiency catalyst for oxygen evolution reaction. Energy & Environmental Science. 2016; 9:1771-1782. DOI: 10.1039/C5EE02463C
Deng Y, Handoko AD, Du Y, Xi S, Yeo BS. In situ Raman spectroscopy of copper and copper oxide surfaces during electrochemical oxygen evolution reaction: Identification of CuIII oxides as catalytically active species. ACS Catalysis. 2016; 6:2473-2481. DOI: 10.1021/acscatal.6b00205
Zhang M, DeRespinis M, Frei H. Time-resolved observations of water oxidation intermediates on a cobalt oxide nanoparticle catalyst. Nature Chemistry. 2014; 6:362-367. DOI: 10.1038/nchem.1874