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

Abstract

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 summarize 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. Furthermore, 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.

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, tackling the challenges posed by rising CO₂ levels, and realizing a closed carbon cycle [4]. Proper electrocatalysts are extremely useful for improving the reaction rate of CO₂ conversion to its reduced oxidation states, CO and formate/formic acid. From the perspective of reactivity, CO₂ is chemically inert, and the initial bond energy of C=O is 806 kJ mol^–1, so the conversion of CO₂ into various products is a difficult problem in thermodynamics. Therefore, to make CO₂ conversion happen at a reasonable rate, the potential energy necessarily needs to be much greater than the thermodynamic values. Once the reaction is started, CO₂ can be converted into a mixture of products, mainly including carbon monoxide (CO), methane (CH₄), ethylene (CH₂CH₂), methanol (CH₃OH), ethanol (C₂H₅OH) formic acid (HCOOH), and acetate (CH₃COOH). In addition, hydrogen is a by-product produced at a potential close to the potential for CO₂ reduction [5,6,7,8,9]. However, the electrocatalysts for the 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) [10,11]. In particular, the catalytic activity and selectivity of non-precious metal catalysts are generally much lower than those of noble metal catalysts [12]. 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 to 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 [13,14,15]. Therefore, SACs have shown prominent activity in various electrochemistry processes, including CO₂RR [16,17], oxygen evolution reaction (OER) [18,19], and hydrogen evolution reaction (HER) [20,21]. However, since the surface energy increases with the decrease in particle size, the single atoms tend to aggregate into clusters or nanoparticles [22,23], which leads to degraded functions. Hence, it is indispensable to anchor the isolated atoms onto the appropriate supports to build stable configurations with atomic distribution [24,25]. On one hand, proper supports can serve as stabilizing functions via metal-support interactions [26]. The strong metal–support interactions can effectively tune the electronic structure of SACs to improve the electrocatalytic activity and selectivity [27,28,29]. Two-dimensional materials are recognized as ideal supports for SACs and as preferable alternatives for catalysts due to their unique electronic properties, high specific surface area, and substantial number of active sites [30,31,32,33]. For example, Mn supported on 2D VTe₂ exhibits excellent catalytic performance for both HER and OER [34]. Recent advances have demonstrated that 2D materials can improve the performance of SACs and the inhomogeneity of active sites [35,36,37].

Two-dimensional 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 easy functionalization [38,39]. In addition, owing to the high earth abundance, low price, and high HER catalytic activity, 2D MoS₂ is regarded as a promising alternative for noble metals for water splitting [40]. In addition to water splitting, 2D MoS₂ has a great potential application in 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) [39,41]. However, the electrocatalytic performance of pristine 2D MoS₂ is still not satisfactory mainly due to the lack of active sites at its basal plane and low conductivity. For example, MoS₂ edges show poor oxygen evolution reaction (OER) activity [42], and the pristine basal plane of MoS₂ is inert to the electrochemical reduction of CO₂ [43]. Therefore, the successful combination of 2D MoS₂ and single-atom metal can not only minimize the drawbacks and maximize the advantages of the individual components; more importantly, in addition to the desired performances, some novel functions may be generated for enhancing electrocatalytic CO₂RR and overall water splitting.

Recently, single-atom metal/2D MoS₂ hybrid nanomaterials have been booming in heterogeneous electrocatalysis, due to well-defined 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 focusing 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 with a discussion of the unique advantages of single-atom metal/2D MoS₂ hybrid nanomaterials for electrocatalytic CO₂RR and water splitting. Furthermore, the synthetic methods for single-atom metal/2D MoS₂ hybrid nanomaterials are summarized, including the 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 with the discussions about some typical examples, 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 Single-Atom Metal/2D MoS₂ Hybrid Nanomaterials

The catalytic performances are determined by concrete improvements of synthetic methodologies [44]. The most widely employed approaches for SACs are pyrolysis, atomic layer deposition (ALD) method, physical vapor deposition (PVD), wet-chemistry strategy, and electronic deposition [45,46,47]. However, it is still hard to manipulate atoms in a highly accurate way for the control 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 [48,49,50,51,52]; the latter includes mechanical exfoliation, chemical or electrochemical exfoliation methods, and liquid-phase exfoliation [53]. 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 the one-pot chemical method, the electrochemical process, and the polyoxometalate template-based synthetic strategy (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 [20]; 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.

The 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 [54]. Until now, Cu@1T-MoS₂, Ni@1T-MoS₂, Fe@1T-MoS₂, and Co@1T-MoS₂ have been easily prepared through this method [55].

2.2. Electrochemical Process

The electrochemical process is used to synthesize SAC-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). 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 [56].

2.3. Polyoxometalate Template-Based Synthetic Strategy

Highly purified and stable metallic 1T-MoS₂ can be obtained via 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. Applications of Single-Atom Metal/2D MoS₂ Hybrid Nanomaterials

3.1. Electrochemical CO₂ Reduction

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 [59]. For the hybrid system composed of single-atom metals and 2D MoS₂, CO₂RR more frequently occurred on metal, and the reaction paths or products are strongly dependent on the components of metal. These reaction products or paths include three types: (1) the reduction of CO₂ to CO (e.g., on Au or Ag), (2) the reduction of CO₂ to formic acid (e.g., on Sn and Pb), and (3) the reduction of CO or carbon–oxygen compounds to hydrocarbons or alcohols (e.g., on Cu, Fe, and Mn).

3.1.1. Noble Metal Modified 2D MoS₂

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 [60]. 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 improved electrochemical activity of 2D MoS₂ [61]. Compared to the bulk MoS₂, 2D MoS₂ doped with a single-atom noble metal has the advantage of tunable electronic structures and spatial versatilities, exhibiting excellent performances in electrocatalytic CO₂RR [62]. In addition, these nanocomposites in experimental studies are usually prepared via standard electrochemical methods, which can accurately adjust the concentration of anchored single atoms by deposition time and anode voltage [63], and limiting potentials are usually used to evaluate the electrocatalytic activity of CO₂RR [16,64]. Up to now, there are few experimental studies on single-atom metal/2D MoS₂ hybrid nanomaterials for electrocatalytic CO₂RR; in contrast, a large number of studies are focused on density functional theory (DFT) calculations for evaluating the electroactivity of single-atom metal/2D MoS₂ hybrid catalysts. 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 adverse effects on their performance. For example, single Pt atoms can introduce deep gap states, which leads to a decrease in Fermi level upshifting and strong binding with *HCOO [63]. The Ru doping can improve the H₂ adsorption and dissociation, avoiding the competing HER [65]. Moreover, because key intermediate *HCOO plays an important role in the reaction pathway and potential determining steps (PDS) [66], the binding energy ($\Delta E_{Binding}$) of *HCOO on 2D MoS₂ or single-atom metal is identified as the effective reactivity descriptor for theoretical calculations. Binding energy ($\Delta E_{Binding}$) has been calculated using the following expression,

$$\Delta E_{Binding}=E_{Total}-(E_{Intermediates}+E_{Substrate})$$

where $E_{Total}$ is the total DFT energy for the complex of intermediates (e.g., *HCOO) adsorbed on a catalyst surface (e.g., 2D MoS₂ or single metal atom), $E_{Intermediates}$ is the DFT energy for the intermediates, and $E_{Substrate}$ is the DFT energy for a substrate. The promising dopants should enhance the CO₂ and *HCOO adsorption and meanwhile can 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 indicates strong CO adsorption. Chemical doping with heteroatoms or surface decorating is one of the promising approaches to modulating the electronic structure of 2D MoS₂ for CO₂RR. Proper dopants can enhance CO₂ and COOH adsorption and simultaneously weaken 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 with the PBE functional by using the projector augmented wave (PAW) method (Figure 4a) to understand the relationship between the doping elements and the catalytic performance [67]. Of note, the calculated energies for the adsorption, desorption, or dissociation of the intermediates (such as COOH* and CHO*) on the active sites of doped 2D MoS₂ may be suffered from the “basis set superposition error” (BSSE). In principle, the use of moderately diffuse basis functions for the valence set and crude approximations for the core orbitals will lead to the severe BSSE. 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,c) or pristine MoS₂; meanwhile, the binding energies of CO on Pt- and Pd-doped MoS₂ are 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 can 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 [63]. Xuan et al. developed an electroplating method to synthesize monodispersed noble metal atoms on 2D MoS₂ [63]. 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 levels 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. The doping 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 applications 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 [68]. It is believed that these findings are beneficial to understanding the non-noble single-atom doping for fine-tuned electrochemical windows.

3.1.2. 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 [69]. 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 allow SACs be precisely designed with 2D MoS₂ as a support to improve electronic conductivity and increase the exposed active sites for electrocatalytic CO₂RR [70]. Moreover, the TM-atom doping can significantly modify the binding energies of intermediates (e.g., *COOH) on 2D MoS₂ edges, and thus regulate the electrocatalytic performance for CO₂RR [71]. Remarkably, in addition to the ability to change 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 proven that Nb-doped 2D MoS₂ shows CO formation TOF two orders of magnitude higher than Ag-doped 2D MoS₂ at an overpotential range of 100–650 mV [72].

Yu et al. used the first principles quantum theory to explore the stability of SACs with the non-noble 3d-series of metal single atoms (e.g., Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) supported on 2D MoS₂ [73]_. Among them, the Ni@MoS₂ catalyst exhibits the most stability, which has been proved by the analysis results including electron localization function (ELF), density of states (DOS), and band structure. Spin-polarized projected DOS (PDOS) shows that Ni 3d orbitals play a critical role in bonding with the MoS₂ surface; the orbital hybridization (obvious overlapping with DOS) is responsible for Ni–S bonding interaction. The electrons from Ni 4s/4p states transfer to S and Mo, and consequently, such process enhance the structural stability of SACs because of the ionic interaction between Ni^δ+ and S^δ−. The improvement in electrocatalytic performance for CO₂RR is 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 atoms (Fe, Co, Ni, Cu) supported on 2D MoS₂ for their CO₂RR performance via the combination of DFT calculations and computational hydrogen electrode (CHE) model [74]. 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 can be regarded as an effective descriptor for screening potential single-atom catalysts for CO₂RR [66].

The endothermic and rate-limiting CO desorption step largely limits the electrocatalytic CO₂RR performance of 2D MoS₂ [75]. 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 [72]. Energy-dispersive spectroscopy (EDS) mapping (Figure 6a) demonstrates that the distribution of Nb is homogeneous in the vertically aligned (VA)-Mo_0.95Nb_0.05S₂ structure. The changes in doping percentage will affect the electrocatalytic performance of Nb@2D-MoS₂ catalyst. When the doping percentage is 5%, Nb@2D MoS₂ catalyst (VA-Mo_0.95Nb_0.05S₂) exhibits the highest current density (Figure 6b) and the lowest overpotential (Figure 6c) among the samples. The VA-Mo_0.95Nb_0.05S₂ catalyst has the best electrocatalytic performance for CO₂RR, far exceeding that of the pristine MoS₂ or Ta@2D MoS₂ catalyst, because Nb doping can reduce the bonding strength between Mo edge and CO, and Nb@2D MoS₂ catalyst 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 catalyst 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 catalyst is one order of magnitude higher than that of the pristine VA-MoS₂ [76]. The electrocatalytic activity of VA-Mo_0.95Nb_0.05S₂ catalyst is also 2 orders of magnitude better than Ag-NP 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, 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, or Hf) in 2D MoS₂ via high-throughput DFT calculations for outstanding electrochemical performance with the PBE functional by using the projector-augmented wave (PAW) method (Figure 7a) [67]. 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 to 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. Moreover, PDOS shows that the higher the energy of the d-band center relative to the Fermi level, the stronger the CO adsorption, which is consistent with the d-band center theory. Specifically, by doping Zr and Hf into 2D MoS₂, the binding energies of CO are reduced by 0.59 and 0.58 eV, respectively, so the center of the d-band shifts to Fermi level by 0.22 eV relative to the pristine MoS₂. In addition, the dopant position is extremely important to the catalytic activity for CO₂RR, but relatively, the doping concentration is insignificant. Such a conclusion has already been proven by the research work of Nørskov et al. via DFT calculations regarding the electrocatalytic CO₂ reduction on the MoS₂ edge with the 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. Summary of various single-atom metals 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	[73]
Co@MoS2	*HCOO → *HCOOH	−0.24	0.41	CH4	[73]
Ni@MoS2	CO2 → *HCOO	−0.45	0.62	CH4	[73]
Cu@MoS2	*OCH3 → CH4 + *O	−1.05	1.22	CH4	[73]
Ru@MoS2	*CO → *CHO	−0.73	0.9	CH4	[77]
Pd@MoS2	CO2 → *HCOO	−0.96	1.13	CH4	[66]
Pt@MoS2	CO2 → *HCOO	−0.50	0.67	CH4	[66]

) also prove the importance of the intermediate *HCOO in the reaction pathway and potential determining steps.

3.2. Electrochemical Water Splitting

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) [78,79,80]. 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, a 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) (Table 2); 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. For the reaction mechanisms (e.g., the Volmer–Heyrovsky mechanism) or paths for HER used in calculation models, there are three elementary steps regarding interactions between the water dissociates and a reactive hydrogen intermediate (absorbed hydrogen on the catalyst surface, H_ad), including the Volmer step followed by either the Heyrovsky step (H₂O + H_ad + e^– ↔ H₂ + OH⁻) or the Tafel recombination step (2H_ad ↔ H₂). The detailed mechanism or reaction path is dependent on the components of metal.

3.2.1. Noble-Metal-Modified 2D MoS₂

Single-atom noble metal catalysts (SACs) can not only decrease the loading amount but also significantly improve the number of catalytic active sites by 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 molecules and metal ions [81]. The introduced noble metals can not only function as active centers for facilitating water adsorption and dissociation but also improve the electrical conductivity by modifying the electronic structure of 2D MoS₂ [82]. 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 [83]. The in-plane S position adjacent to the doped noble atoms can produce new active sites for HER [84]. At present, Pt@2D MoS₂ catalyst 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 using 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 [85]. Doping Pd atoms can introduce abundant S vacancies and convert 2H MoS₂ into stable 1T-MoS₂, enhancing electrical transport [86]. In addition, doping with Au atoms can modify the band structure of the 2D MoS₂, which is beneficial for electronic transfer [87]. All the improvements in electrocatalytic performance indicate that doping noble metal atoms can improve the electrocatalytic water splitting performance of 2D MoS₂.

Compared toH₂ 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 [83]. 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. 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 in the bond between H and S/Mo.

Similarly, Bao et al. [20] and Zhang et al. [87] also reported that the electrocatalytic activity of Pt@2D-MoS₂ catalyst in the HER process was greatly improved compared to pure MoS₂. With (NH₄)₆Mo₇O₂₄, H₂PtCl₆ and CS₂ as precursors, Pt@2D-MoS₂ catalyst 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 catalyst 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 to blank glassy carbon (GC) electrode and bulk MoS₂, the electrocatalytic activity of Pt@2D-MoS₂ catalyst for HER is significantly improved after the Pt atom doping (Figure 9c). As shown in Tafel plots, differently from Pt/C electrocatalyst (32 mV dec⁻¹), the value of Pt@2D-MoS₂ catalyst (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 electrocatalyst exhibits show extremely stable performance (Figure 9e,f) because the current intensity and the potential values are nearly unchanged even after suffering from long-term CV sweeps (>1000 cycles). 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 a decrease in band gap, improved electron transport properties, and reduced activation energy of water splitting.

3.2.2. Non-Noble Metal Modified 2D MoS₂

2D MoS₂ nanosheets exhibit excellent electrocatalytic performance for hydrogen evolution, especially the 2D MoS₂ with 1T-metallic phase with superior HER catalytic activity to the 2H semiconducting phase. 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 [88]. TM-doping can not only modify the electronic structure of 2D MoS₂ but also induce lattice distortion and effectively increase the exchange current density. In essence, the doping enlarges the surface active areas and improves electrical conductivity [42,89,90]. Therefore, coupling of 2D MoS₂ with TM atoms with excellent OER activities can realize the improvement of the overall water splitting performance [91,92,93,94].

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 via the one-pot method to improve the HER activity and OER activity of 1T-MoS₂ in alkaline media [55]. As shown in Figure 10a, the changes of ΔG_H as the descriptor were used to evaluate the HER activity of TMs (Ni, Co, Fe)-doped 1T-MoS₂ for water splitting. The ideal ΔG_H should be close to 0 eV because a large positive ΔG_H is not kinetically favored for the adsorption of hydrogen on active sites of the catalyst, and a large negative ΔG_H indicates difficult spontaneous release of adsorbed hydrogen. Based on this principle, pristine 1T-MoS₂ has a strong adsorption effect on H₂O (Figure 10a), which is adverse to the dissociation and electrocatalytic processes. In contrast, 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 particular, Ni-doped 1T-MoS₂ electrocatalyst exhibits the best performance for HER, proved by its the lowest overpotential of 1.19 V. 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, the Mo 3d spectrum exhibits a significant increase in electron density because the S-3p orbital can strongly hybridize with the Ni-3d orbital. Moreover, the doping of the Ni atom can enhance the metallic property of the 2D MoS₂ based on the analysis of its TDOS; therefore, Ni@2D-MoS₂ catalyst has the highest charge density at the Fermi surface (Figure 10c), which can accelerate electron exchange. As shown in Figure 10d, DFT-optimized geometries of alkaline HER on Ni@1T-MoS₂ catalyst show that the active site for water splitting is the Ni atom. 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, which changes the electronic structure of 2D 1T-MoS₂.

Similarly, Zheng et al. prepared several selenium-doped MoS₂ nanofoams (Se-MoS₂-NF) with different Se contents based on a one-pot solvothermal method [93]. 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 catalyst exhibits better HER activity than those of bulk MoS₂, MoS₂-FL, and MoS₂-NF (Figure 11d). Moreover, 9.1% Se-doped in 2D MoS₂ can obtain the highest HER activity with a large current density of 1000 mA cm⁻² and a much lower overpotential of 382 mV than that of commercial Pt/C catalyst (Figure 11e). The introduction of Se confined in the surface of 2D MoS₂ can enable activation of the basal plane, stabilization of the edges, and optimization of the hydrogen adsorption activity.

Qiao et al. proposed a simple, inexpensive way for modifying MoS₂ nanosheets on Cu nanorods to design MoS₂/Cu structure as a multifunctional catalyst [77]. 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 a possible strategy for developing 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 via DFT calculations [94]. 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. In addition, the imbalance of charge distribution will bring about electron transfer. To be specific, the charge distribution of pristine MoS₂ should be 0.15 e for the Mo atom and −0.08 e for the S atom, but the Mo atom in Cu@2H-MoS₂ acquires 0.05 e, while the Mo atom in Co@2H-MoS₂ decreases by 0.12 e. The electron injection is beneficial for destabilizing the 2H-MoS₂ and promoting the phase transition of 2H-MoS₂ → 1T-MoS₂. Moreover, 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, the Co@2H-MoS₂ catalyst shows better HER performance with smaller Gibbs free energy than hydrogen adsorption (ΔG_H = 0.08 eV), and the Cu@2H-MoS₂ catalyst exhibits a 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) [57]. 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 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 the electrocatalytic HER of SA Co-D 1T-MoS₂ [95].

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	[57]
Pt/2D MoS2	0.1 M H2SO4	60/1.5% Pt/MoS2	96	5000 CVs	[20]
Pd/2D MoS2	0.5 M H2SO4	89/1% Pd/1T-MoS2	62	5000 CVs	[86]
Ni/2D MoS2	0.5 M H2SO4	98/Ni/MoS2	103	2000 CVs	[96]
Ru/2D MoS2	0.5 M H2SO4	114/46 μg cm−2 Ru/MoS2	-	10 h	[97]
Cu/2D MoS2	0.5 M H2SO4	131/1% Cu/MoS2	51	7 h	[54]
Fe, Co, Ni, Pd, Pt/2D MoS2	0.5 M H2SO4	140/2.7% Pd/1T-MoS2	57	1000 CVs	[98]
Ni/2D MoS2	0.5 M H2SO4	174/1% Ni/MoS2	69	1000 CVs	[99]
Au, Pt, Pd/2D MoS2	0.5 M H2SO4	210/1.1% Pt/MoS2	104	5 h	[63]
Ni/2D MoS2	0.5 M H2SO4	263/2.7% Ni/MoS2	81	1000 CVs	[100]
Ru/2D MoS2	1.0 M PBS	125/46 μg cm−2 Ru/MoS2	-	10 h	[97]
Ru/2D MoS2	1.0 M KOH	41/46 μg cm−2 Ru/MoS2	114	20 h	[97]
Ir/2D MoS2	1.0 M KOH	44/Ir/1T-MoS2	32	9000 CVs	[101]
Ni/2D MoS2	1.0 M KOH	110/Ni/MoS2	119	2000 CVs	[96]

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

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	[57]
Pt/2D MoS2	0.1 M H2SO4	60/1.5% Pt/MoS2	96	5000 CVs	[20]
Pd/2D MoS2	0.5 M H2SO4	89/1% Pd/1T-MoS2	62	5000 CVs	[86]
Ni/2D MoS2	0.5 M H2SO4	98/Ni/MoS2	103	2000 CVs	[96]
Ru/2D MoS2	0.5 M H2SO4	114/46 μg cm−2 Ru/MoS2	-	10 h	[97]
Cu/2D MoS2	0.5 M H2SO4	131/1% Cu/MoS2	51	7 h	[54]
Fe, Co, Ni, Pd, Pt/2D MoS2	0.5 M H2SO4	140/2.7% Pd/1T-MoS2	57	1000 CVs	[98]
Ni/2D MoS2	0.5 M H2SO4	174/1% Ni/MoS2	69	1000 CVs	[99]
Au, Pt, Pd/2D MoS2	0.5 M H2SO4	210/1.1% Pt/MoS2	104	5 h	[63]
Ni/2D MoS2	0.5 M H2SO4	263/2.7% Ni/MoS2	81	1000 CVs	[100]
Ru/2D MoS2	1.0 M PBS	125/46 μg cm−2 Ru/MoS2	-	10 h	[97]
Ru/2D MoS2	1.0 M KOH	41/46 μg cm−2 Ru/MoS2	114	20 h	[97]
Ir/2D MoS2	1.0 M KOH	44/Ir/1T-MoS2	32	9000 CVs	[101]
Ni/2D MoS2	1.0 M KOH	110/Ni/MoS2	119	2000 CVs	[96]

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

4. Challenges and Prospects

In this review, we have summarized the main advantages and features, synthesis methods, and applications of single-atom metal/2D-MoS₂ hybrid nanomaterials for electrocatalytic CO₂RR and water splitting. We highlight their synthesis routes, modification approaches, and physiochemical mechanisms for improving their electrocatalytic performances of single-atom metal/2D MoS₂ hybrid nanomaterials in electrocatalytic CO₂RR and water splitting with some typical examples.

Although research has considerably progressed in single-atom metal/2D-MoS₂ hybrid catalysts for CO₂RR and water splitting [64,102,103,104,105,106,107,108,109,110], certain challenges remain in the development of control synthesis methods for obtaining low-cost and high-performance electrocatalysts, solving the aggregation and stability of single-atom catalysts due to high surface energy [111,112], clarifying the reaction mechanisms of water or CO₂ on the surfaces or interfaces of catalysts [113,114,115], understanding the influences of metal–support interactions and the number, type, and microenvironment of active sites on the catalysis reactions. These problems limit their practical applications in future.

Therefore, from a long-term view, more efforts in the future are focused on control synthesis, structure–property characterization, and reaction mechanisms. 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 [116] because the control, high-quality, and large-scale synthesis of single-atom catalysts is the precondition or bottleneck for deep understanding of the structure–property relationship and the reaction mechanism. For example, the non-noble metals of SACs such as Fe, Ni, and Cu nanoparticles are easily oxidized in the air [17]. Most previous research has focused more on the coordination environment or components of single atoms, but relatively, the structural control (e.g., crystal phases and structural defects on the basal planes or plane edges) of 2D materials is not paid enough attention. In addition to the electrode materials, the overall modulations from the perspective of the whole electrochemical rection devices, such as the mass and electron transfer of electrode/electrolyte interface, should be carefully considered. Second, research efforts in the future may be focused on the development and utilization of some in situ characterization methods (e.g., in situ Fourier transform infrared spectroscopy), analysis methods for the specific coordination environment of the single atoms (e.g., X-ray absorption near edge structure spectra), and theoretical predictions (e.g., DFT or machine learning) for a better understanding of the structure–property relationship [117,118,119,120,121], especially the relationship between the defects of these hybrid nanomaterials and their electrocatalytic activity. The DFT calculations 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-atom metals [17]. Therefore, DFT calculations can act as a powerful tool for the prediction or screening of new electrocatalysts. Last but not least, except for the electrocatalyst itself, to accurately identify the clear catalytic mechanisms, fundamental research and optimized standard experimental systems of electrocatalytic water splitting and CO₂RR are still required [14]. Combination of DFT and machine learning based on data-driven scientific research motivated by artificial intelligence has great potential in understanding electrocatalytic mechanisms at an atomic level. We hope this review can provide some valuable suggestions for the development of advanced electrocatalysts for the environment and energy fields.