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Hydrogen Production by Electrocatalysts Based on Transition Metal Dichalcogenides
Nam Khen Oh, Hyesung Park
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DOI: 10.22661/AAPPSBL.2020.30.2.03

Hydrogen Production by Electrocatalysts Based on Transition Metal Dichalcogenides

NAM KHEN OH, HYESUNG PARK
DEPARTMENT OF ENERGY ENGINEERING, SCHOOL OF ENERGY AND CHEMICAL ENGINEERING,
LOW DIMENSIONAL CARBON MATERIALS CENTER, PEROVTRONICS RESEARCH CENTER,
ULSAN NATIONAL INSTITUTE OF SCIENCE AND TECHNOLOGY (UNIST), ULSAN 44919, REPUBLIC OF KOREA

ABSTRACT

Transition metal dichalcogenides (TMDs) are a class of two-dimensional materials with intriguing physical and chemical properties. Consequently, they have attracted significant interest in various disciplines. Owing to their high catalytic activity, excellent chemical stability, and large specific surface area, TMDs are potential alternatives to precious metal-based electrocatalysts in the hydrogen evolution reaction. This review presents strategies for efficient hydrogen generation by TMD-based electrocatalysis, and discusses future challenges for these material systems.

 



Fig. 1: Periodic table highlighting the representative transition metals and chalcogen elements in TMDs [1].

INTRODUCTION

Hydrogen generation technologies are considered as promising alternatives to unsustainable and ecologically damaging fossil fuels. Many studies have focused on the development of hydrogen generating water electrolysis catalysts. Precious metal-based catalysts such as Pt and Ru are the most efficient catalysts for the hydrogen evolution reaction (HER), but their scarcity, high-cost, and low stability limit their widespread industrial applications. Therefore, earth-abundant and low cost electrocatalysts that deliver the desirable electrocatalytic performance and stability are in high demand. Recently, transition metal-based electrocatalysts have shown promising potential as efficient hydrogen generating electrocatalysts, with performances comparable to those of precious metal-based catalysts. Among these materials, transition metal dichalcogenides (TMDs) deliver the prerequisite high catalytic activity, price competitiveness, and chemical stability required for HER electrocatalysts. As shown in Fig. 1, layered TMDs generally have the chemical formula MX2 (where M is a transition metal and X is a chalcogen such as S, Se, or Te). The unique characteristics of TMDs originate from their polymorphism, which depends on the atomic bonding arrangements between the transition metal and chalcogen atoms, and on the stacking order between each layer. Representative crystalline structures of TMDs are the 2H (hexagonal), 1T (trigonal), and 3R (rhombohedral) phases. The different electronic structures within each crystalline phase confer distinct catalytic activities. Density functional theory (DFT) calculations have revealed higher HER activity at the edge sites of TMDs than in the basal plane [1].

The so-called "volcano plot," describing the exchange current density as a function of metal-hydrogen bond strength, also confirms that the edge of 2H-MoS2 is electrochemically active whereas the basal plane remains inactive [2]. Therefore, many studies on TMD-based HER electrocatalysts have been dedicated to modulating the edge sites or surface structures of the electroctalyst, or optimizing the physical, chemical, and electronic properties of TMDs (e.g., their conductivity, charge transfer kinetics, intermediate adsorption free energy, and reaction kinetics) [1-4]. This review briefly summarizes the representative HER processes in acidic and alkaline media, then discusses strategies for improving the performance of TMD-based HER electrocatalysts. Finally, it discusses the remaining challenges in this promising field.

Reaction mechanism of HER

The electrocatalytic HER mechanism has been investigated in numerous experimental and theoretical studies. The HER proceeds in three steps: electrochemical hydrogen adsorption (1), electrochemical desorption (2), and chemical desorption (3). The proton source is H3O+ in an acidic medium and H2O in an alkaline medium. When a minimum external potential is applied at the electrode surface, an electron combines with a proton via the Volmer reaction (Eqs. 1 and 2), producing an adsorbed hydrogen atom (H*) on the electrode surface. Subsequently, H2 is generated by the Heyrovsky reaction, the Tafel reaction, or both reactions. The Heyrovsky reaction (Eqs. 3 and 4) generates H2 by combining a proton on the other electrode with an already produced H*. In the Tafel reaction (Eq. 5), H2 is generated by combining the generated H* with a nearby H*.

(1) E lectrochemical hydrogen adsorption (Volmer reaction)
Acidic media: H3O+ + * + e- → H* + H2O      (1)
Alkaline media: H2O + * + e- → H* + OH-     (2)

(2) Electrochemical desorption (Heyrovsky reaction)
Acidic media: H3O+ +e- + H* → H2 + H2O      (3)
Alkaline media: H2O + e- + H* → H2 + OH-    (4)

(3) Chemical desorption (Tafel reaction)
Acidic and alkaline media: H* + H* → H2       (5)

Strategies for enhancing the HER performance of TMDs-based electrocatalysts

Phase engineering

Since the Chhowalla group reported that the exchange current density and active sites can be improved by increasing the 1T phase ratio in WS2 [5], phase transitions of TMDs in electrocatalysts (e.g., MoS2, WS2, MoSe2, and WSe2) have been widely investigated for enhancing the HER performance of TMD-based catalysts. Owing to their intrinsic electrical conductivity and electronic structure rearrangements, metallic 1T phase and disordered 1T' phase TMDs exhibit more favorable kinetics for HER than semiconducting 2H phase TMDs. Gao et al. theoretically determined the bandgap of 2H-MoS2 as 1.74 eV, versus 0.006 eV in 1T and 1T'-MoS2. This finding suggests that the HER performance in metallic phase MoS2 can be improved by increasing the charge transfer capability [6].

Putungan et al. computationally demonstrated that the H* adsorption ability and metallic phase stability are higher in the basal plane of 1T'-TMDs (M = Mo, W; X = S, Se, Te) than in 1T-TMDs [7]. Voiry et al. and Lukowski et al. experimentally determined that 1T-WS2 and 1T-MoS2 (synthesized by lithium intercalation) deliver higher HER performance than 2H-WS2 and MoS2 [5, 8], consistent with theoretical analysis. In the typical lithium intercalation-assisted synthesis process, the phase transition of TMDs occurs via electron transfer from lithium to the TMD layers, which involves quite complex processes requiring a long reaction time in an inert environment. Recently, the Park group devised novel in-situ local phase transition of TMDs by synthesizing a TMDs/perovskite oxide heterostructure, which significantly improved the HER performance of a heterostructured catalyst [9]. Electron transfer from Co to Mo in the MoSe2/La0.5Sr0.5CoO3- (LSC) heterostructure alters the electronic structure of Mo, inducing a phase transition in MoSe2 (Fig.2).

 



Fig. 2: Potential interaction between MoSe2 and La0.5Sr0.5CoO3-灌, resulting in in-situ local phase transition of MoSe2 [9].

Defect engineering

The basal plane of 2H-TMDs has a semiconducting nature with insufficient numbers of active sites. Introducing defects in the basal plane increases the number of edge sites and modulates the Gibbs free energy (ΔGH*) of TMDs, thereby boosting the HER performance. For example, Ye et al. created hole and crack defects in pristine MoS2 through O2 plasma and H2 annealing. Such defects on the treated MoS2 surface became catalytically active sites that significantly improved the HER performance parameters, namely, the onset potential, Tafel slope, and current density, over those of pristine MoS2 [10]. Similarly, Hong et al. modulated the S-vacancy concentration of 2H-MoS2 by an Ar plasma treatment [11]. The DFT results demonstrated that S-vacancies provide additional active sites in the basal plane with the optimal ΔGH* (ΔGH* = 0). In addition, the S-vacancies enable direct bonding between hydrogen and Mo exposed on the MoS2 surface, thereby optimizing the HER performance. Recently, Park's group generated in-situ chalcogen vacancies by chemical vapor deposition (CVD) in the synthesis of MoSe2.

The HER performance of MoSe2 was significantly improved by in-situ generated Se vacancies, with a Tafel slope value similar to that of platinum [12]. It is worth noting that their approach generates vacancy sites in TMDs without requiring post-treatments such as a plasma process. In DFT calculations, the in-situ generated chalcogen vacancy exhibited lower hydrogen adsorption energy for HER and a lower hydrogen diffusion barrier for the Tafel reaction than pristine MoSe2 (Fig.3).

 



Fig. 3: Synthesis mechanism and effect of Se vacancies on the HER activity of MoSe2 [12].

Gao et al. theoretically demonstrated that introducing dual-native vacancies (both Mo and Se) in the basal plane and edge of MoSe2 modulates the ΔGH* value, bringing it close to 0 eV. Their results were experimentally supported in CVD-grown MoSe2 containing Mo and Se vacancies [13].

Heteroatom doping engineering

Heteroatom doping in TMDs is typically adopted to optimize ΔGH* of TMD-based electrocatalysts by modulating the electronic structure. Examples of TMD doping studies include non-metal doping (C, N, O, B, and P) [14-16], noble metal doping (Au, Pt, and Ru) [17-19], and non-noble metal doping (Fe, Ni, Co, and Zn) [20-22]. Kuo et al. demonstrated that C and O doping on defected MoS2 leads to higher hydrogen coverage [23]. Similarly, Xie's group reported that O doping in MoS2 induces synergistic effects in structural and electronic properties, leading to improved HER performance compared to pristine MoS2 (onset potential: -120 mV, Tafel slope: 55 mV dec-1) [24]. Oxygen-incorporated MoS2 has twice the hydrogen coverage compared to pristine MoS2, which is attributed to optimized ΔGH* (Fig.4).

 

Fig. 4: Calculated densities of states and charge-density distributions of O-incorporated MoS2 (top). Schematic of the disordered structure in O-incorporated MoS2 (bottom) [24].

Sun et al. synthesized flower-like N-doped WS2 via the sol-gel method. In a partial density of states analysis, the N doping induced strong hybridization between the p orbitals of the N atoms and the d orbitals of the W atoms, thereby increasing the number of charge carriers and the intrinsic conductivity of N-doped WS2. The HER performance is additionally enhanced by the increased surface area after N doping [25]. Bao's group synthesized Pt-MoS2 by a one-pot chemical method. When applied as a HER electrocatalyst, Pt-MoS2 delivered higher performance than pristine MoS2 because it optimizes the adsorption kinetics of the hydrogen atoms at the S sites near the Pt-doped regions [26]. Bonde et al. reported that the ΔGH* of MoS2 and WS2 can be modulated via Co doping at the S-edge [27], but is unaffected by Co doping at the Mo-edge. That is, the Co doping forms catalytically active sites only at the S-edge. Later, Zhang et al. reported a similar doping strategy with Ni, Co, and Fe, which improves the sluggish HER kinetics in MoS2 [28]. Among the three modifications, Ni-MoS2 showed the best HER performance, with an overpotential of -98 mV at 10 mA cm-2. Ni atoms doped in MoS2 effectively reduce the kinetic energy barrier of the initial water dissociation step, and facilitate the desorption of -OH intermediates. In addition, the ΔGH* of Ni-MoS2 was nearly 0 eV (-0.06 eV) in a computational calculation.

Heterostructure engineering

Although the aforementioned strategies (defect generation, doping, phase transition) improve the HER performance of TMD-based catalysts, the electrochemical properties of single type TMDs are intrinsically limited for the HER process. To improve this situation, researchers have fabricated heterogenous catalyst structures of TMDs and catalytically active materials. Such heterostructured TMD-based catalysts have further improved the catalytic performance of single type TMDs by optimizing ΔGH*, the electron transfer pathway, the electrochemically active surface area, and the intrinsic exchange current density. Among the reported heterostructures are TMDs/sulfides [29], TMDs/selenides [30, 31], TMDs/phosphide [32], TMDs/nitrides [33], TMDs/metal oxides [34, 35], TMDs/rGO [36], TMDs/black phosphorus (BP) [37], and TMDs/perovskite oxides [9]. Rheem et al. devised a MoS2/MoO2 heterostructure with a metallic MoO2 core surrounded by MoS2 nanosheets [35]. The MoS2 nanosheets improve the HER performance by increasing the overall surface area and conductivity of the composite. He et al. synthesized a MoS2/BP heterostructure by depositing MoS2 flakes on the surface of BP nanosheets [37]. In this electrocatalyst, the HER performance is enhanced by charge transfer at the MoS2-BP interface. With a higher Fermi level than MoS2, BP donates electrons to MoS2; consequently, the intrinsic exchange current density is 22 times higher in MoS2/BP than in MoS2 alone. The MoS2/BP heterostructure also delivers an excellent overpotential of -85 mV at 10 mA cm-2 (Fig.5).

 

Fig. 5: Comparison of HER performances (overpotential, Tafel slope, and charge transfer resistance) of BP, MoS2/C, and MoS2-BP in 0.5 M H2SO4 [37].

A heterostructure comprising highly conductive non-oxide materials and TMDs has also been applied as a HER electrocatalyst [38]. CoS2 nanowires coated with MoS2 nanosheets were synthesized by the hydrothermal method. The MoS2/CoS2 heterostructure produced an excellent overpotential in the HER (-87 mV at 10 mA cm-2), by virtue of the co-doped Co and O elements in the MoS2 nanosheets, structural defects, and excellent conductivity of CoS2 [38]. Recently, the Park group developed a novel TMDs/perovskite oxide heterostructure for HER electrocatalysts. The MoSe2/LSC heterogeneous catalyst delivered an excellent HER onset potential (MoSe2: -420, LSC: -330, and MoSe2/LSC: -200 mV) and Tafel slope (MoSe2: 237, LSC: 95, and MoSe2/LSC: 34 mV dec-1). This high performance originated from local 2H to 1T phase transitions in MoSe2, increased surface area, and enhanced surface hydrogen adsorption capability [9].

Summary and future challenges for TMDs-based HER electrocatalysts

Although the study of TMD-based HER electrocatalysts has greatly advanced through rigorous research endeavors, the industrial viability of these materials is limited by several remaining problems. First, the HER mechanism and descriptors of TMDs in alkaline electrolytes require further investigation. A thorough understanding of the HER process in alkaline electrolytes is critical for designing efficient HER electrocatalysts and for optimizing the overall water electrolysis with TMD catalysts. Second, operational stability is as important as performance in HER electrocatalysis. However, stability analyses of TMDs-based electrocatalysts have been rarely reported, and the exact mechanisms of stability are poorly understood. A fundamental understanding of the operational stability of electrocatalysis is a potential stepping stone for the practical industrialization of overall water electrolysis. Finally, the actual mechanism of hydrogen generation might differ from that derived in theoretical computational approaches. Therefore, establishing a concrete hydrogen generation mechanism in both acidic and alkaline electrolytes by in-situ characterization is highly desired.

Acknowledgement: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2019R1A2C1009025 and 2019R1A4A1029237).

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Nam Khen Oh is currently Ph.D candidate under the supervision of Prof. Hyesung Park in the Department of Energy Engineering at Ulsan National Institute of Science and Technology (UNIST), South Korea. His research interests lie in the synthesis of transition metal dichalcogenides, layered perovskites, and transition metal based heterostructures, mainly focusing on their dispersibility and electrocatalytic characteristics.

Hyesung Park is currently an Associate Professor in School of Energy and Chemical Engineering at Ulsan National Institute of Science and Technology (UNIST), South Korea. He received his M.S. and Ph.D. degrees in Mechanical Engineering and Electrical Engineering and Computer Science from Massachusetts Institute of Technology (MIT) in 2007 and 2012. He was a postdoctoral associate at MIT from 2012 to 2013 and postdoctoral fellow at Northwestern University in 2014. His research interests focus on synthesis of low-dimensional materials such as graphene, transition metal dichalcogenides, and nanowires, and their applications into functional devices in nanoelectronics, nanophotonics, energy harvesting, and bioelectronics.

 
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