Recent Advances of Single-atom Metal Supported at Two-dimensional MoS2 for Electrochemical CO2 Reduction and Water Splitting

Due to increasing concerns about global warming and energy crisis, intensive efforts have been made to explore renewable and clean energy sources. Single-atom metals and two-dimensional (2D) nanomaterials have attracted extensive attention in the fields of energy and environment because of their unique electronic structures and excellent properties. In this review, we sum-marize the state-of-art progress on the single-atom metal supported at 2D MoS2 (single-atom metal/2D MoS2) for electrochemical CO2 reduction and water splitting. First, we introduce the advantages of single-atom metal/2D MoS2 catalysts in the fields of electrocatalytic CO2 reduction and water splitting, followed by the strategies for improving electrocatalytic performances of single-atom metal/2D MoS2 hybrid nanomaterials and the typical preparation methods. Further, we discuss the important applications of the nanocomposites in electrocatalytic CO2 reduction and water splitting via some typical examples, particularly focusing on their synthesis routes, modification approaches, and physiochemical mechanisms for improving their electrocatalytic performances. Finally, our perspectives on the key challenges and future directions of exploring high-performance metal single-atom catalysts are presented based on recent achievements in the development of single-atom metal/2D MoS2 hybrid nanomaterials.

Keywords:

Subject: Environmental and Earth Sciences - Environmental Science

1. Introduction

Due to the contamination and global warming problems, it is necessary to search for alternative environmentally friendly energy sources and decrease the concentration of CO₂ in the atmosphere [1,2,3]. The utilization of nonprecious metal electrocatalysts for water splitting and CO₂ fixation for producing high value-added fuels or chemicals may be the ultimate solution for sustainable and clean hydrogen energy and tackling the challenges posed by rising CO₂ levels and realizing a closed carbon cycle [4]. However, electrocatalysts for CO₂ reduction reaction (CO₂RR) and water splitting face some problems, such as low product selectivity, poor faradaic efficiency (FE), and/or hard experimental conditions (in acidic media) [5,6]. In particular, the catalytic activity and selectivity of non-precious metal catalysts are generally much lower than those of noble metal catalysts [7]. The activity of catalysts is not only related to the composition and structure, but also to the dimension or size. The most direct effect of the decrease in dimension or size is the change in the number of active sites. Compared with conventional nanoparticles, single-atom catalysts (SACs) have the advantages of unique electronic structure, strong metal-support interactions (SMSIs), and plenty of accessible active sites, and thus can significantly improve catalytic activity and selectivity [8,9,10]. Therefore, SACs have shown prominent activity in various electrochemistry processes including CO₂RR [11,12], oxygen evolution reaction (OER) [13,14], and hydrogen evolution reaction (HER) [15,16]. However, since the surface energy increases with the decrease of particle size, the single atoms tend to aggregate into clusters or nanoparticles [17,18], which leads to the degraded functions. Hence, it is indispensable to anchor the isolated atoms onto the appropriate supports to build the stable configurations with atomic distribution [19,20]. On the one hand, these supports can serve for stabilizing functions [21]. The strong metal-support interactions can effectively tune the electronic structure of SACs to improve the electrocatalytic activity and selectivity [22,23,24]. 2D materials are recognized as ideal supports for SACs and the preferable alternatives for catalysts due to their unique electronic properties, high specific surface area, and substantial number of active sites [25,26,27,28]. For example, Mn supported on 2D VTe₂ exhibits excellent catalytic performance for both HER and OER [29]. Recent advances have demonstrated that 2D materials can improve the performance of SACs and the inhomogeneity of active sites [30,31,32].

2D molybdenum disulfide (MoS₂) as the representative of transition metal dichalcogenides (TMDCs), has attracted much attention for HER, OER and CO₂RR due to its unique structure and ability to be chemically modified [33,34]. Owing to the high earth abundance, low price and high HER catalytic activity, 2D MoS₂ is regarded as a promising catalyst, compared with noble metal, for water splitting [35]. In addition, 2D MoS₂ has a great potential electrocatalyst for CO₂RR because Mo-exposed edges can enhance the chemisorption of the reactants, and thus improve the electrochemical catalysis with a low overpotential of CO₂RR (about ~54 mV) and high selectivity (the reduction product is only CO) [34,36]. However, the electrocatalytic performance of pristine 2D MoS₂ is still not satisfactory. For example, MoS₂ edges show poor oxygen evolution reaction (OER) activity [37] and the pristine basal plane of MoS₂ is inert to electrochemical reduction of CO₂ [38]. In contrast, Therefore, the successful combination of 2D MoS₂ and single-atom metal may remedy their respective shortcomings and increase the active sites, surface area and electronic conductivity, ultimately enhancing electrocatalytic CO₂RR and overall water splitting.

Recently, single-atom metal/2D MoS₂ hybrid nanomaterials have been booming in heterogeneous catalysis, especially electrocatalysis, due to their well-defined, precisely located metal centers, unique metal–support interaction, and identical coordination environment. However, there have been only a few studies on the systematic summarization of the nanomaterials for electrocatalytic CO₂RR and overall water splitting. Thus, to fill this gap, a comprehensive review on such a topic is extremely necessary and significant. In this review, we summarize the state-of-art progress on single-atom metal nanomaterials modified 2D MoS₂ for electrocatalytic CO₂RR and water splitting. We start briefly from a discussion of the unique advantages of single-atom metal/2D MoS₂ hybrid nanomaterials for electrocatalytic CO₂RR and water splitting. Further, the synthetic methods for single-atom metal/2D MoS₂ hybrid nanomaterials are summarized, including one-pot chemical method, electrochemical process, and polyoxometalate template-based synthetic strategy. After that, we highlight the significant applications of single-atom metal/2D MoS₂ hybrid nanomaterials in electrocatalytic CO₂RR and water splitting, particularly focusing on physiochemical mechanisms for improving their electrocatalytic activity. Finally, on the basis of previous studies, we provide our perspectives on the key challenges and future directions in utilizing single-atom metal/2D MoS₂ hybrid nanomaterials for CO₂RR and water splitting. We hope that this review can provide new insights for the further development and practical application of single-atom catalysts and 2D materials.

2. Synthesis Methods of Hybrid Nanomaterials

The catalytic performances are determined by concrete improvements of synthetic methodologies [39]. The most widely employed approaches for SACs are pyrolysis, atomic layer deposition (ALD) method, physical vapor deposition (PVD), wet-chemistry strategy, and electronic deposition [40,41,42]. However, it is still hard to manipulate atoms in a highly accurate way for the synthesis of theoretically designed SACs due to ultrahigh surface free energy. The synthesis methodologies for 2D MoS₂ can be divided into two categories: top-down and bottom-up methods. The former mainly includes chemical vapor deposition (CVD) and solvothermal or hydrothermal methods [43,44,45,46,47]; the latter includes mechanical exfoliation, chemical or electrochemical exfoliation methods, and liquid-phase exfoliation [48]. However, the number of successful cases for single-atom metal/2D MoS₂ hybrid nanomaterials is still limited compared to the synthesis methods for single-atom metal modified 3D supports, SACs preparation, and/or 2D materials. The methods for single-atom metal/2D MoS₂ hybrid nanomaterials are derived from the approaches for SAC@3D supports such as pyrolysis and coprecipitation. Considering that the common synthetic methods of SACs or 2D materials have been discussed in depth in previous reviews, this review focuses on some novel methods for single-atom metal/2D MoS₂ hybrid nanomaterials including one-pot chemical method, electrochemical process, and polyoxometalate template-based synthetic strategy, for single-atom metal/2D MoS₂ hybrid nanomaterials (Figure 1).

2.1. One-pot Chemical Method

The principle of this method is directly to mix the precursors of SACs and 2D MoS₂ for the following reactions under inert gas (e.g., Ar). The typical processes are described as follows, firstly, all precursors (e.g., (NH₄)₆Mo₇O₂₄·₄H₂O, H₂PtCl₆, and CS₂) are dissolved in a certain amount of deionized water to form a homogeneous solution [15]. Then, the resulting solution is transferred to a Teflon-lined stainless autoclave under Ar and maintained at a high temperature for a certain reaction time.

One-pot method has the advantages of simple operation and saving synthetic costs. In addition, the catalysts prepared by this method can have sulfur vacancies and the doping features of metal atoms at the same time [49]. Until now, Cu@1T-MoS₂, Ni@1T-MoS₂, Fe@1T-MoS₂, and Co@1T-MoS₂ have been easily prepared through this method [50].

2.2. Electrochemical Process

Electrochemical process is used to synthesize SACs modified 2D MoS₂ via electrochemical etching of big-size metal precursor. Taking the single-atom cobalt (Co) array modified 2D MoS₂ as an example (Figure 2), firstly 2D MoS₂ and Co nanodisks (NDs) are synthesized using standard solvothermal procedures and standard air-free procedures, respectively. Then, the combination of Co NDs and 2D MoS₂ is realized via an assembly process. Finally, single-atom Co array covalently bound onto distorted 1T-MoS₂ nanosheets (denoted as SA Co-D 1T-MoS₂) via Co-S bonds can be synthesized through electrochemical cyclic voltammetry (CV) leaching of Co nanodisks (NDs)-MoS₂ nanosheet hybrids. In addition to electrochemical CV leaching, the electrochemical deposition can be used to synthesize the nanocomposites of SACs modified 2D MoS₂, such as Pt, Cu, Sn, and Pd anchored on the 2D MoS₂, because these single metal atoms (from metal ions in the electrolyte solution) can be introduced onto the MoS₂ monolayer driven by applying the bias potential [51].

2.3. Polyoxometalate Template-based Synthetic Strategy

Highly purified and stable metallic 1T-MoS₂ can be obtained by a hydrothermal method, which introduces organic sulfur sources into (NH₄)₆Mo₇O₂₄·4H₂O (denoted as Mo₇). Here, Mo₇ is a precursor that is a butterfly-shaped metal oxide cluster (Figure 3), and it belongs to the β-isomer of Anderson-type polyoxometalates (POMs). In addition, its unique structure makes it possible to tune the chemical environment of 1T-MoS₂ with various metal atoms. Using Anderson-type polyoxometalates ([XH₆Mo₆O₂₄]ⁿ⁻) as precursors, atomically designing metal doping sites onto metallic 1T-MoS₂ can be achieved. [XH₆Mo₆O₂₄]ⁿ⁻ is denoted as XMo₆ (X = Fe^III, Co^III, n = 3; X = Ni^II, n = 4) [53].

3. Single-atom Metal/2D MoS₂ Hybrid Nanomaterials for Electrochemical CO₂ Reduction

3.1. Noble Metal Modified 2D MoS₂

Electrocatalytic CO₂RR includes three steps, namely, the chemisorption of CO₂ on the surface of electrocatalysts, the transfer of high-energy electrons and protons between two elements to break C=O bonds, and the desorption of products from the surface of the electrocatalysts [54]. Noble metal single-atom catalysts (NMSACs) have the advantages of high intrinsic activity, selectivity, and durability for electrocatalytic CO₂RR due to their unique electronic structure [55]. On the other hand, in addition to acting as catalytic centers, monodispersed single noble metal atoms can also activate inert in-plane S-atoms, and lead to the improved electrochemical activity of 2D MoS₂ [56]. Compared with the bulk MoS₂, 2D MoS₂ doped with a single-atom noble metal has the advantage of tunable electronic structures and spatial versatilities, exhibiting excellent performance in electrocatalytic CO₂RR [57]. In addition, these nanocomposites in experimental studies are usually prepared by standard electrochemical methods, which can accurately adjust the concentration of anchored single atoms by deposition time and anode voltage [58], and limiting potentials are usually used to evaluate the electrocatalytic activity of CO₂RR [11,59]. Up to now, there are few experimental studies on single-atom metal/2D MoS₂ hybrid nanomaterials for electrocatalytic CO₂RR; in contrast, a large amount of studies are focused on density functional theory (DFT) calculations for evaluating the electroactivity of single-atom metal/2D MoS₂ hybrid catalysts (as shown in Table 1). In the hybrid electrocatalysts, these single atoms in DFT calculations are mainly concentrated in Pt, Pd, Au, Ag, and Ru and have an optimal loading amount for avoiding the adverse effects on their performance. For example, single Pt atoms can introduce deep gap states, which leads to the decrease of the Fermi level upshifting and strong binding with COOH* [58]. The Ru doping can improve the H₂ adsorption and dissociation, avoiding the competing HER [60]. Moreover, because key intermediate *HCOO plays an important role in reaction pathway and potential determining steps (PDS) [61], the binding energy (

Δ E_{B i n d i n g}

) of *HCOO on 2D MoS₂ or single-atom metal as a substrate is identified as the effective reactivity descriptor to in theory calculations. Binding energy (

Δ E_{B i n d i n g}

) has been calculated using the following expression,

Δ E_{B i n d i n g} = E_{T o t a l} - (E_{I n t e r m e d i a t e s} + E_{S u b s t r a t e})

where

E_{T o t a l}

is the total DFT energy for the complex of intermediates (e.g., *HCOO) absorbed on a substrate (e.g., 2D MoS₂ or single metal atom),

E_{I n t e r m e d i a t e s}

is the DFT energy for the intermediates, and

E_{S u b s t r a t e}

is the DFT energy for a substrate. The promising dopants should enhance the CO₂ and *HCOO adsorption and meanwhile weaken the CO adsorption. In addition, the binding strength can be described by the energy of the center of the d states relative to the Fermi level according to the d-band center model. In general, the high d-band center in energy relative to the Fermi level means the strong CO adsorption.

Chemical doping with heteroatoms or surface decorating is one of the promising approaches to modulate the electronic and CO₂RR of 2D MoS₂. Proper dopants can enhance the CO₂ and COOH adsorption and meanwhile weaken the CO adsorption. Mao et al. investigated 29 kinds of single-atom metals (including Ag, Au, and other transition metal (TM) atoms) doped 2D MoS₂ with different doping concentrations and positions for electrochemical CO₂RR via high-throughput density functional theory (DFT) calculations (Figure 4a) for understanding the relationship between the doping elements and the catalytic performance [62]. For single noble metal atom doping, the binding energies of CO₂ and COOH on the Pt- and Pd-doped MoS₂ edge are greatly larger than that of the TM-doped MoS₂ (Figure 4b and Figure 4c) or pristine MoS₂; meanwhile, the binding energies of CO on Pt- and Pd-doped MoS₂ is much smaller than other TM-doped MoS₂, except for Ni-doped MoS₂. The stronger adsorption of CO₂ and COOH with MoS₂ edge can activate the reduction reaction, while the weaker adsorption of CO with MoS₂ could accelerate the rate-limiting step of CO desorption.

Atomic level electroplating can accurately control the doping level and anchoring sites to maximize the activity and stability of the catalyst [58]. Xuan et. al developed an electroplating method to synthesize monodispersed noble metal atoms on 2D MoS₂ [58]. To be specific, authors demonstrated a voltage gauged electrochemical deposition method to deposit single-atom Pt, Au, and Pd on 2D MoS₂. The surface atomic doping level for Pt, Au, and Pd can reach 1.1, 7.0, and 14 at.%, respectively, and the doping positions were accurately located on Mo- and S-vacancies. With the doping of Pt atoms, Pt, Mo and S atoms are homogeneously distributed on the MoS₂ flake (Figure 5a-d). After doping single-atom metal on 2D MoS₂, the electrochemical reactivity of the catalyst is driven higher than that of the pristine MoS₂ flakes (Figure 5e). In addition, monodisperse precious metal atoms can exhibit improved saturated CO tolerance (Figure 5f) and provide an extra pathway for CO₂RR. The overpotential of Pt-2D MoS₂ at 10 mA cm² only changes 9 mV after CO poisoning (Figure 5f), indicating that Pt-2D MoS₂ is almost immune to CO poisoning. Thus, due to this CO tolerance, Pt-2D MoS₂ electrocatalyst has more potential to be applied in electrocatalytic CO₂RR than Au-2D MoS₂ and Ag-2D MoS₂. The enhanced electrochemical performance is attributed to stabilized Pt (II) atoms with fewer free electrons to coordinate with CO than Pt⁰ [63]. It is believed that these findings can help the non-noble single-atom doping, especially the transitional metal, and be potentially realized within fine-tuned electrochemical windows.

3.2. Non-noble Metal Modified 2D MoS₂

Exploring MoS₂-based non-noble metal SACs for electrocatalytic CO₂RR is expected to solve the problems of high overpotential, low Faraday efficiency, and unsatisfactory selectivity in CO₂RR [64]. The geometric, electronic, spin states of the active center and magnetic states of transition metal SACs can be fine-tuned to determine the catalytic behavior and activity, which can help SACs be precisely designed with 2D MoS₂ as a support to improve electronic conductivity and increase the exposed active sites for electrocatalytic CO₂RR [65]. Moreover, the TM atoms doping can significantly modify the binding energies of intermediates (e.g., *COOH) on 2D MoS₂ edges, and thus regulate the electrocatalytic performance for CO₂RR [66]. Remarkably, in addition to the ability of changing the binding energies of key intermediates, non-noble metal single-atom catalysts also have high CO formation turnover frequency (TOF), which makes it possible for their electrocatalytic performance to surpass that of NMSACs. For example, it has been proved that Nb-doped 2D MoS₂ shows 2 orders of magnitude higher CO formation TOF than Ag-doped 2D MoS₂ at an overpotential range of 100–650 mV [67].

Yu et al. used the first principles quantum theory to explore the stabilities of SACs with the non-noble 3d-series of metal single atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) supported on 2D MoS₂ [68]_. Among them, Ni@MoS₂ is the most stable SAC, which has been proved by the analysis results including electron localization function (ELF), density of states (DOS), and band structure. All these enhancements of electrocatalytic performance for CO₂RR are attributed to the changes of the spin densities and charge density difference by Ni doping, and thus can improve the electrocatalytic performance for CO₂RR. To supplement the theoretical and experimental gaps in the single-atom metal catalysts on 2D MoS₂ toward CO₂RR, Ren et al. explored single atom (Fe, Co, Ni, Cu) supported on 2D MoS₂ for their CO₂RR performance by the combination of DFT calculations and computational hydrogen electrode (CHE) model [69]. The limiting potentials of Fe@MoS₂, Co@MoS₂, and Ni@MoS₂ are determined as −0.39 V, −0.24 V, and −0.45 V, respectively, for CH₄ production. The binding energy of *HCOO was regarded as an effective descriptor to screen potential active single-atom catalysts for CO₂RR [61].

The endothermic and rate-limiting CO desorption step largely limited the electrocatalytic CO₂RR performance of 2D MoS₂ [70]. Nb@2D MoS₂ had a high CO formation TOF and an extremely low onset overpotential (31 mV) for electrocatalytic CO₂RR because low Nb doping concentrations (<∼5%) can reduce the binding energies between intermediates and MoS₂ edge atoms [67]. Energy dispersive spectroscopy (EDS) mapping (Figure 6a) demonstrates that the distribution of Nb is homogeneous in the VA (vertically aligned)-Mo_0.95Nb_0.05S₂ structure. The changes of doping percentage will affect the electrocatalytic performance of Nb@2D MoS₂. When the doping percentage is 5%, the current density of Nb@2D MoS₂ is the highest (Figure 6b) and the overpotential is the lowest (Figure 6c), indicating that VA-Mo_0.95Nb_0.05S₂ has the best electrocatalytic performance for CO₂RR, far exceeding that of the pristine MoS₂ or Ta@2D MoS₂, because Nb doping can reduce the bonding strength between Mo edge and CO, and Nb@2D MoS₂ can result in a faster turnover for CO desorption than pristine MoS₂. However, a higher Nb doping percentage will bring about poor electron-transfer properties for CO₂RR. VA-Mo_0.95Nb_0.05S₂ generates a tunable mixture of CO and H₂ ranging from 12% to 82% of CO formation at the range of studied potentials −0.16 to −0.8 V (Figure 6d). At the low overpotential range of 0–150 mV, the overall electrocatalytic performance of VA- Mo_0.95Nb_0.05S₂ is one order of magnitude higher than that of the pristine VA-MoS₂ [71]. The electrocatalytic activity of VA-Mo_0.95Nb_0.05S₂ catalyst is also 2 orders of magnitude better than Ag NPs catalyst over the entire range of overpotentials (Figure 6e). The enhancement of electrocatalytic performance is due to the change of electronic properties of edge atoms by Nb doping.

For the TM atoms as dopants in MoS₂, the introduction of V, Zr, and Hf into MoS₂ can significantly promote the desorption of CO from the MoS₂ edge, and thus achieving the optimal performance for electrocatalytic CO₂ reduction, because the TM doping can significantly modulate the binding energies of the CO₂ reduction species. For example, Mao et al. screened out three dopants (i.g., V, Zr, and Hf) with outstanding electrochemical performance (Figure 7a) [62]. The dopants can not only activate the O-C-O bond by enhancing the binding energy of CO₂, but also allow CO to desorb more easily by reducing the binding energies of CO. Compared with pristine 2D MoS₂, the binding energies of CO are significantly decreased by 0.59, 0.59 and 0.58 eV for V-, Zr-and Hf-doped MoS₂ edges, respectively. In addition, the dopants position is extremely important to the catalytic activity for CO₂RR, but relatively the doping concentration is insignificant. Such conclusion has been early proved via the research work of Nørskov et al under DFT calculations about the electrocatalytic CO₂ reduction on MoS₂ edge with Mo edge replaced by TM atoms. As shown in Figure 7b, the closer the doping position is located to the active sites of the Mo edge, the more obvious the doping effect is, because single-atom metals can affect the local electronic structures of 2D MoS₂. This work developed an effective method to improve the electrocatalytic activity of 2D MoS₂ by doping near the active molybdenum at low doping concentrations. In addition, other studies about single-atom metal modified 2D MoS₂ (as shown in Table 1) also prove the importance of the intermediate *HCOO in the reaction pathway and potential determining steps.

4. Single-atom Metal/2D MoS₂ Hybrid Nanomaterials for Water Splitting

4.1. Noble Metal Modified 2D MoS₂

In a conventional water electrolyzer, HER reaction occurs at the cathode, and H₂ is separated out, while OER reaction occurs at the anode, and O₂ is separated out (Figure 8a) [73,74,75]. Under standard conditions, a thermodynamic potential of 1.23 V is required to drive electrochemical water splitting (Figure 8b) for HER and OER. However, in real conditions, the input potential of water splitting in practical electrolyzers is much larger than 1.23 V. In general, high-performance electrocatalyst for water splitting is still focused on noble metal-based catalysts (e.g., Pt for HER and IrO₂ or RuO₂ for OER); however, it is necessary to develop noble metal-free electrocatalysts or decrease the loading amount of noble metals electrocatalysts because of the prohibitive cost and scarce reserve.

Single-atom noble metal catalysts (SACs) can not only decrease the loading amount, but also significantly improve the number of catalytic active sites via reducing the size of materials to the atomic-scale range, which can fully take advantage of reaction sites in electrocatalysts. For the hybrid system of single-atom noble metal modified 2D MoS₂, the noble metal doping can enhance the adsorption capacity of 2D MoS₂ for gas molecule and metal ions [76]. The introduced noble metals can not only function as active centers for facilitating water adsorption and dissociation, but also improve the electrical conductivity via modifying the electronic structure of 2D MoS₂ [77]. In addition, noble metal single atoms are able to change the chemical bond between H and S/Mo atoms, which can enhance the intrinsic activity of each active center for electrocatalytic water splitting [78]. The in-plane S position adjacent to the doped noble atoms can produce new active sites for HER [79]. At present, Pt@2D MoS₂ is still a hot spot in the field of single noble metal atoms/2D MoS₂ hybrid nanomaterials for electrocatalytic water splitting, and this catalyst is usually prepared by the one-pot method, which is beneficial to the uniform distribution of Pt atoms in the 2D MoS₂ plane. Pt doping can significantly reduce the activation energy of electrocatalytic water splitting. In addition to Pt atoms, noble metal atoms such as Ru, Pd and Au are also applied to enhance the electrocatalytic water splitting performance of 2D MoS₂. For example, the synergistic effect between S vacancies and Ru sites can improve the catalytic performance of active sites [80]. Doping Pd atoms can introduce abundant S vacancies and convert 2H MoS₂ into stable 1T-MoS₂, enhancing the electrical transport [81]. In addition, doping with Au atoms can modify the band structure of the 2D MoS₂, which is beneficial for the electronic transfer [82]. All the improvements in electrocatalytic performance indicate that doping noble metal atoms can improve the electrocatalytic water splitting performance of 2D MoS₂.

Compared with H₂ evolution, the release of diatomic O₂ is challenging because there are four electrons and four protons involved in the eventual formation of an O–O bond. In general, the elementary substances of Cu and Co are used for either HER or OER, but rarely for electrochemically overall water splitting. Xu et al. screened 28 metal single atoms supported on MoS₂ edges as bifunctional electrocatalysts for overall water splitting by using DFT calculations [78]. Authors proposed a simple equation including the chemical environment and local structure of the active center, which can be used as a new structure descriptor to forecast the oxygen evolution reaction activities for MoS₂-based SACs. Among these metal single atoms, the T1-vacancy (in which 37.5% of sulfur atoms are added to the initial Mo termination) modified using a Pt single atom exhibits the lowest theoretical overpotential for HER and OER, which is comparable to those of the noble-metal-group benchmark catalysts for overall water splitting, which. Compared with pure MoS₂ terminations, the Pt single atom can increase the intrinsic activity of each active center by changing the chemical bond between H and S/Mo atoms. The high OER performance is attributed to the moderate d band center of the single metal atom. By analyzing the electrical structure of S/Mo near the hydrogen, the improvement of intrinsic HER activity is attributed to the change of the bond between H and S/Mo.

Similarly, Bao et al. [15] and Zhang et al. [82] also reported that the electrocatalytic activity of Pt@MoS₂ in the HER process was greatly improved compared with pure MoS₂. With (NH₄)₆Mo₇O₂₄, H₂PtCl₆ and CS₂ as precursors, Pt@2D MoS₂ was synthesized through a one-pot chemical method, which realized the uniform distribution of Pt atoms in the 2D MoS₂ nanosheets. The transmission electron microscopy (TEM) image (Figure 9a) shows that the structure of Pt@2D MoS₂ is flower-like 2D nanosheets. As shown in Figure 9b, single Pt atoms uniformly disperse on the entire 2D nanosheets and substitute the Mo atoms. Compared with blank glassy carbon (GC) electrode and bulk MoS₂, the electrocatalytic activity of Pt@2D MoS₂ for HER is significantly improved after the Pt atom doping (Figure 9c). As shown in Tafel plots, different from Pt/C electrocatalyst (32 mV dec⁻¹), the value of Pt@MoS₂ (96 mV dec⁻¹) is closer to that of pure few-layer MoS₂ nanosheets (FL-MoS₂) (98 mV dec⁻¹), which indicates that the S atoms instead of the Pt atoms are the active sites of HER reaction (Figure 9d). Moreover, Pt@2D MoS₂ electrocatalysts exhibit show extremely stable performance (Figure 9e and Figure 9f) because the current intensity and the potential values are nearly unchanged even after suffering from long-term cyclic voltammetric (CV) sweeps (>1000 CVs). All these enhancements are attributed to the activation of the in-plane S position adjacent to the doped Pt atom and the change of H adsorption free energy (ΔG_H) based on the DFT calculations, because Pt doping leads to the decrease of band gap, the improved the electron transport properties, and the reduced activation energy of water splitting.

4.2. Non-noble Metal Modified 2D MoS₂

Although anchoring single-atom noble metal onto 2D MoS₂ can enhance the electrocatalytic activity for overall water splitting, developing non-precious metals with comparable electrocatalysis activity with that of noble metal is still the best strategy for saving costs [83]. Transition-metal doping can not only modify the electronic structure of 2D MoS₂, but also induce lattice distortion and effectively increase the exchange current density, which can enlarge the surface active areas and improve the electrical conductivity [37,84,85]. 2D MoS₂ nanosheets exhibit excellent electrocatalytic performance for hydrogen evolution, especially the 2D MoS₂ with 1T-metallic phase with superior HER catalytic activity to 2H semiconducting phase. Therefore, coupling of MoS₂ with TM atoms with excellent OER activities can realize the improvement the overall water splitting performance [86,87].

Wang et al. developed a direct one-pot hydrothermal method to synthesize pure 1T-MoS₂ with flower-like morphology and high stability at a low temperature of 200 °C, and TMs (Ni, Co, Fe) were further doped by one-pot method to improve the HER activity and OER activity of 1T-MoS₂ in alkaline media [50]. As shown in Figure 10a, Gibbs free energy changes in the electrocatalytic reaction due to single-atom metals doping. This is because doping Fe, Co, and Ni atoms can change the electronic structure of MoS₂, which can decrease its adsorption of H₂O and increase the possibility of H₂O dissociation. In addition, consistent with XPS results, the charge difference density of Ni@1T-MoS₂ indicates that Ni plays a role in losing electrons and Mo acquires electrons (Figure 10b). In addition, Mo 3d spectrum exhibits a significant increase in electron density. Moreover, Ni@2D MoS₂ has the highest charge density at the Fermi surface (Figure 10c), which can accelerate electron exchange. In Figure 10d, DFT-optimized geometries of alkaline HER on Ni@1T-MoS₂ are exhibited, which shows the mechanism and intermediate steps of alkaline HER reaction for Ni@1T-MoS₂. Furthermore, as the molar ratio of Mo/Ni reaches ~6.7, the hybrid electrocatalyst can achieve the best electrocatalytic performance of η₁₀ of 112 mV for HER and 224 mV for OER. The overall water splitting reactivity is attributed to single atomic Ni that changes the electronic structure of 2D 1T-MoS₂.

Similarly, Zheng et al. prepared several selenium-doped MoS₂ nanofoam (Se-MoS₂-NF) with different Se content based on a one-pot solvothermal method [88]. Se-doped MoS₂ nanofoam has abundant spherical cavities (Figure 11a), which is beneficial for the mass transportation of reactants and increasing the number of active sites. Figure 11b shows the homogeneous distribution of Mo, S, and Se elements in Se-doped MoS₂ nanofoam. In addition, Se-MoS₂-NF has richer edges than both MoS₂ nanofoam (MoS₂-NF) and few-layer MoS₂ (MoS₂-FL) (Figure 11c), which means that the electrocatalytic HER performance of Se-MoS₂-NF can be significantly improved. HER polarization curves show that Se-MoS₂-NF exhibits much better HER activity than those of bulk MoS₂, MoS₂-FL, and MoS₂-NF (Figure 11d). Moreover, 9.1% is the best doping content of Se atom, and HER activity is the highest at this time (Figure 11e).

Qiao et al. proposed a simple, inexpensive way for modifying MoS₂ nanosheets on Cu nanorods to design MoS₂/Cu structure as a multifunctional catalyst [72]. The electrocatalyst shows markedly improved performances for both HER and OER and great durability in alkaline media. The facial and scalable synthetic effect between Cu nanorods and MoS₂ nanosheets leads to great electrocatalytic performance. This study provides the possible strategy to develop electrocatalysts with desired catalytic performance and stability through simple and large-scale manufacturing techniques. Similarly, Zhu et al. explored the electrocatalytic water splitting activity of Cu@2H-MoS₂ and Co@2H-MoS₂ by DFT calculations [89]. It is found that anchoring single-atom Cu or Co onto 2D 2H-MoS₂ can change the surface charge distribution and electronic band structure of 2H-MoS₂. By doping single-atom Cu or Co, inert in-plane S-atoms of 2D 2H-MoS₂ can be activated, thus significantly improving the electrochemical activity of overall water splitting. Relatively, Co@2H-MoS₂ shows better HER performance with smaller Gibbs free energy of the hydrogen adsorption (ΔG_H = 0.08 eV), and Cu@2H-MoS₂ exhibits lower energy barrier and a reduced overpotential of 1.25 eV for OER.

Qi et al. explored a top-down assembly/leaching method to realize Co array covalently anchored onto distorted 1T-MoS₂ nanosheets (SA Co-D 1T-MoS₂) [52]. As shown in Figure 12a-d, the atomic dispersion of Co atoms and the phase transformation of MoS₂ are observed. Specifically, both the HAADF-STEM image (Figure 12b) and simulated pattern (Figure 12c, d) demonstrate the obvious interface between SA Co-D 1T-MoS₂ and 2H MoS₂. Single Co atoms are homogeneously distributed on the top of Mo atoms on the MoS₂ slab. The SA Co-D 1T-MoS₂ electrocatalyst containing 3.54% Co (wt.%) shows an incredibly small onset overpotential for HER, even comparable with that of 10% Pt/C (Figure 12e). In addition, the Tafel slope of SA Co-D 1T-MoS₂ (32 mV dec⁻¹) is also comparable to that of Pt/C (30 mV dec⁻¹), preceding those of non-Pt based electrocatalysts (Figure 12f). Moreover, the low Tafel slope exhibits that a Tafel rate-determining-step mechanism, rather than the common Volmer reaction, is responsible for electrocatalytic HER of SA Co-D 1T-MoS₂ [90].

Table 2. Summary of typical single-atom metal/2D MoS₂ hybrid nanomaterials for water splitting.

Catalyst	Electrolyte	η (mV)/best ratio (w.t.% or concentration)	Tafel slope (mV dec ⁻¹)	Stability test	Ref
Co/2D MoS₂	0.5 M H₂SO₄	42/3.5% Co/1T-MoS₂	32	10,000 CVs	[52]
Pt/2D MoS₂	0.1 M H₂SO₄	60/1.5% Pt /MoS₂	96	5000 CVs	[15]
Pd/2D MoS₂	0.5 M H₂SO₄	89/1% Pd/1T-MoS₂	62	5000 CVs	[81]
Ni/2D MoS₂	0.5 M H₂SO₄	98/Ni/MoS₂	103	2000 CVs	[91]
Ru/2D MoS₂	0.5 M H₂SO₄	114/46 μg cm⁻² Ru/MoS₂	-	10 h	[92]
Cu/2D MoS₂	0.5 M H₂SO₄	131/1% Cu/MoS₂	51	7 h	[49]
Fe, Co, Ni, Pd, Pt/2D MoS₂	0.5 M H₂SO₄	140/2.7% Pd/1T-MoS₂	57	1000 CVs	[93]
Ni/2D MoS₂	0.5 M H₂SO₄	174/1% Ni/MoS₂	69	1000 CVs	[94]
Au, Pt, Pd/2D MoS₂	0.5 M H₂SO₄	210/1.1% Pt/MoS₂	104	5 h	[58]
Ni/2D MoS₂	0.5 M H₂SO₄	263/2.7% Ni/MoS₂	81	1000 CVs	[95]
Ru/2D MoS₂	1.0 M PBS	125/46 μg cm⁻² Ru/MoS₂	-	10 h	[92]
Ru/2D MoS₂	1.0 M KOH	41/46 μg cm⁻² Ru/MoS₂	114	20 h	[92]
Ir/2D MoS₂	1.0 M KOH	44/Ir/1T-MoS₂	32	9000 CVs	[96]
Ni/2D MoS₂	1.0 M KOH	110/Ni/MoS₂	119	2000 CVs	[91]

Note: overpotential value (η) and stability test were performed at the current density of 10 mA cm⁻²; CVs, cycles.

5. Challenges and Prospects

In this review, main advantages and features, synthesis methods, and applications of single-atom metal/2D-MoS₂ hybrid nanomaterials for electrocatalytic CO₂RR and water splitting were comprehensively discussed. Our discussions highlighted strategies, typical examples, and possible mechanism of improving electrocatalytic activity. Although research has considerably progressed in single-atom metal/2D-MoS₂ hybrid for CO₂RR and water splitting [59,97,98,99,100], certain challenges remain in the development of for fabricating low-cost and high-performance electrocatalysts, solving the aggregation of single-atom catalysts as number of active sites and loading amounts increasing [101,102], and clarifying the reaction mechanism [103,104,105].

Therefore, from a long-term view, more efforts in the future are focused on control synthesis, structure-property characterization, and reaction mechanism. First, it is necessary to work hard on controllable preparation to ensure the dispersion of single metal atoms and the catalytic activity of hybrid catalysts [106], because the control, high-quality, and large-scale synthesis of single-atom catalysts is the precondition or bottleneck for deeply understanding structure-property relationship and reaction mechanism. For example, the non-noble metals of SACs such as Fe, Ni, and Cu nanoparticles are easily oxidized in air [12]. Second, the research efforts in future may be focused on the development and utilization of some in-situ characterization methods (e.g., in situ Fourier transform infrared spectroscopy) and theoretical predictions (e.g., DFT or machine learning) for better understanding of structure-property relationship [107,108,109,110,111], especially the relationship between the defects of these hybrid nanomaterials and their electrocatalytic activity. The DFT calculation can simulate the electrocatalytic reaction efficiency of single-atom metal/ 2D MoS₂ hybrid nanomaterials and clarify the electron transfer mechanism between 2D MoS₂ and single metal atoms [12]. Last but not least, expect for the electrocatalyst itself, to accurately identify the clear catalytic mechanisms, the fundamental research and optimized standard experimental systems of electrocatalytic water splitting and CO₂RR are still required [9].

Author Contributions

Conceptualization, X. G. (Xiaorong Gan); writing—original draft preparation, J. W. (Jiaohao Wang); writing—review and editing, X. G. (Xiaorong Gan), T. Z. (Tianhao Zhu), D. Z. (Dan Zhao), Y. A. (Yanhui Ao), P. Z. (Peifang Wang); All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 52000059). Also, this study was funded by the Key Lab of Modern Optical Technologies of Jiangsu Province, Soochow University.

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Figure 1. Several common synthesis methods of single-atom metal/2D MoS₂ hybrid nanomaterials.

Figure 3. Structure of POM precursors. (a) Polyhedral representation of the XMo₆ precursors. (b) Ball and stick representation of the XMo₆ precursors. Reproduced with permission from ref [53]. Copyright 2019 Nature communications.

Figure 4. (a) 29 kinds of metals doping elements in MoS₂ as the calculation models for electrochemical CO₂RR. (b) Binding energies of CO₂ vs CO on the metal-doped MoS₂ edge. The gray rectangular area indicates the obvious decrease of CO binding energies. (c) Binding energies of COOH vs CO on the metal-doped MoS₂ edge. Reproduced with permission from ref [62]. Copyright 2020 The Journal of Physical Chemistry C.

Figure 5. (a-d) HAADF-STEM image and corresponding EDX mapping images of Pt-MoS₂. (e) Single-atom electroplating process related polarization curves, from bulk MoS₂, exfoliated MoS₂/GP, and Pt-MoS₂ electrode. (f) CO tolerance test of Pt-2H-MoS₂ catalyst in CO saturated 0.5 M H₂SO₄. Reproduced with permission from ref [58]. Copyright 2018 Chemistry of Materials.

Figure 6. (a) EDS maps Mo–K series, Nb–K series, and S–K series measured from the same region of the MoS₂: Nb sample. (b) Current density as a function of dopant percentage for Nb-doped and Ta-doped MoS₂ samples. (c) CV curves for Ag NPs, VA-MoS₂, VA-Mo_0.97Ta_0.03S₂, and VA-Mo_0.95Nb_0.05S₂ in CO₂ environment. (d) CO and H₂ Faradaic efficiency (FE%) at different applied potentials for VA-Mo_0.95Nb_0.05S₂. (e) CO formation partial current density for Ag nanoparticles, VA-MoS₂, and VA-Mo_0.95Nb_0.05S₂. Reproduced with permission from ref [67]. Copyright 2017 ACS nano.

Figure 7. (a) TM-doped MoS₂ structures with different positions of dopants. The green, bule, and yellow balls represent Mo, TM, and S atoms, respectively. (b) Binding energies of CO₂ vs CO for the M-x (x = 1, 2, 3, 4) MoS₂. The x indicates the row next to the Mo edge for TM doping. Reproduced with permission from ref [62]. Copyright 2020 The Journal of Physical Chemistry C.

Figure 8. (a) Illustration of conventional water electrolyzers. (b) Water splitting reactions under acidic and alkaline conditions.

Figure 9. (a) TEM image and (b) HAADF-STEM images of Pt-MoS₂. (c) HER polarization curves for Pt–MoS₂ in comparison with blank GC electrode, bulk MoS₂, FL-MoS₂, and 40% Pt/C. (d) Tafel plots for FL-MoS₂, Pt–MoS₂, and 40% Pt/C, respectively. (e) Durability measurement of Pt–MoS₂. (f) Potential values recorded initially and after every 1000 CVs from the polarization curves of durability measurements for Pt–MoS₂ at 1 mA cm⁻², 5 mA cm⁻² and 10 mA cm⁻², respectively. Reproduced with permission from ref [15]. Copyright 2015 The Royal Society of Chemistry.

Figure 10. (a) Gibbs free energy diagrams for the H₂ production pathway on the 1T-MoS₂ and TM-doped 1T-MoS₂. (b) Charge difference density of Ni@1T-MoS₂. (c) DOS of Ni@1T-MoS₂. (d) DFT-optimized geometries of alkaline HER on Ni@1T-MoS₂. Reproduced with permission from ref [50]. Copyright 2020 Small.

Figure 11. (a) SEM image, (b) EDX mappings, and (c) HRTEM image of the Se-MoS₂-NF. (d) HER polarization curves for the Se-MoS₂-NF with different Se deposited contents. (e) Changes of overpotentials depending on the Se deposited content at different current densities. Reproduced with permission from ref [88]. Copyright 2020 Nature communications.

Figure 12. (a) Aberration-corrected HAADF-STEM image of SA Co-D 1T-MoS₂. (b) Enlarged HAADF-STEM image in the red square area of Figure 12a. (c) Theoretical model and (d) simulated STEM images. (e) Polarization curves and (f) Tafel plots of different catalysts. Reproduced with permission from ref [52]. Copyright 2019 Nature communications.

Table 1. Summary of various single-atom metal supported at 2D MoS₂ for electrochemical CO₂ reduction.

Catalyst	Potential determining steps	Limiting potentials（V）	Overpotential （V）	Production for catalysts	Ref.
Fe@MoS₂	HCOO → HCOOH	−0.39	0.56	CH₄	[68]
Co@MoS₂	HCOO → HCOOH	−0.24	0.41	CH₄	[68]
Ni@MoS₂	CO₂ → *HCOO	−0.45	0.62	CH₄	[68]
Cu@MoS₂	OCH₃ → CH₄+O	−1.05	1.22	CH₄	[68]
Ru@MoS₂	CO → CHO	−0.73	0.90	CH₄	[72]
Pd@MoS₂	CO₂ → *HCOO	−0.96	1.13	CH₄	[61]
Pt@MoS₂	CO₂ → *HCOO	−0.50	0.67	CH₄	[61]

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Recent Advances of Single-atom Metal Supported at Two-dimensional MoS2 for Electrochemical CO2 Reduction and Water Splitting

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