Substrate Engineering of Single Atom Catalysts Enabled Next-Generation Electrocatalysis to Power a More Sustainable Future

Abstract

Single-atom catalysts (SACs) are presently recognized as cutting-edge heterogeneous catalysts for electrochemical applications because of their nearly 100% utilization of active metal atoms and having well-defined active sites. In this regard, SACs are considered renowned electrocatalysts for electrocatalytic O₂ reduction reaction (ORR), O₂ evolution reaction (OER), H₂ evolution reaction (HER), water splitting, CO₂ reduction reaction (CO₂RR), N₂ reduction reaction (NRR), and NO₃ reduction reaction (NO₃RR). Extensive research has been carried out to strategically design and produce affordable, efficient, and durable SACs for electrocatalysis. Meanwhile, persistent efforts have been conducted to acquire insights into the structural and electronic properties of SACs when stabilized on an adequate matrix for electrocatalytic reactions. We present a thorough and evaluative review that begins with a comprehensive analysis of the various substrates, such as carbon substrate, metal oxide substrate, alloy-based substrate, transition metal dichalcogenides (TMD)-based substrate, MXenes substrate, and MOF substrate, along with their metal-support interaction (MSI), stabilization, and coordination environment (CE), highlighting the notable contribution of support, which influences their electrocatalytic performance. We discuss a variety of synthetic methods, including bottom-up strategies like impregnation, pyrolysis, ion exchange, atomic layer deposition (ALD), and electrochemical deposition, as well as top-down strategies like host-guest, atom trapping, ball milling, chemical vapor deposition (CVD), and abrasion. We also discuss how diverse regulatory strategies, including morphology and vacancy engineering, heteroatom doping, facet engineering, and crystallinity management, affect various electrocatalytic reactions in these supports. Lastly, the pivotal obstacles and opportunities in using SACs for electrocatalytic processes, along with fundamental principles for developing fascinating SACs with outstanding reactivity, selectivity, and stability, have been highlighted.

1. Introduction

In the last decade, the use of fossil fuels has expanded to provide primary energy needs. However, fossil fuels such as coal and natural gas were generating major environmental difficulties around the world, including greenhouse gas and carbon dioxide (CO₂) emissions [1]. The quest for sustainable energy fosters the growth of innovative catalytic materials for different electrochemical reactions such as the O₂ reduction reaction (ORR), O₂ evolution reaction (OER), H₂ evolution reaction (HER), water splitting, CO₂ reduction reaction (CO₂RR), N₂ reduction reaction (NRR), and NO₃ reduction reaction (NO₃RR). Table 1 illustrates the key electrocatalytic reactions, such as ORR, OER, HER, CO₂RR, NRR, and NO₃RR, and their intermediates for both acid and alkaline electrolytes. These reactions are crucial chemical transformation reaction processes that maintain eco-friendly energy conversions, in which the electrocatalytic activity, selectivity, and stability of electrocatalysts play an integral role in optimizing energy efficiencies and system performance [2]. Thus, the development of efficient and cost-effective electrocatalysts is critical and is emerging as a lucrative research area.

Single-atom catalysts (SACs) have recently sparked widespread attention, owing primarily to their distinct structural attributes, which encompass maximized metal usage, optimized reactive active sites, and promising electrocatalytic capabilities [3]. SACs constitute an innovative category of catalyst that involves strong interactions among different atomic species and nearby single atoms (SAs) on a substrate. SACs are disseminated over a variety of support substrates/materials, including carbon, metal oxides, alloys, transition metal dichalcogenides (TMD), MXenes, and metal–organic framework (MOF)-based supports [4]. Accordingly, SA-support interactions can have a remarkable impact on the electrocatalytic activity and durability of dispersed SACs [5]. Comprehending the strong metal-support interactions (SMSI) and the influence of the supports on their characteristics is critical for modifying SACs’ catalytic performance, stability, and selectivity. Because of their major effects on electrocatalytic performance, support and SMSI have been thoroughly studied [6]. Furthermore, it has been proven that the supports in SACs can efficiently affect the electrical characteristics to improve the transfer of electrons during electrochemical reactions, allocating them with optimum adsorption and/or desorption energy of their intermediate species [7]. Furthermore, different support materials can stabilize different atomic species to avoid aggregation, significantly improving the electrochemical robustness of SACs [8]. Additionally, extensive surface imperfections in support can significantly alter the local CE and its electronic structure, contributing to the presence of numerous coordinatively unsaturated sites and vacancies [9].

However, there are no limits to the quest for improved electrocatalytic performance. Dual and triple atom catalysts (DACs and TACs) are a newer type of electrocatalysis that can overcome the limitations of SACs. They have characteristics in common with SACs, including the ability to maximize metal dispersion, circumvent the conventional scaling relations that restrict extended surfaces, provide adjacent sites that are required for specific reaction mechanisms, and increase the tunability of the electronic structure and binding energies [10].

This review is distinguished by its thorough and multifaceted discussion of SACs in electrocatalytic applications. It contrasts with traditional reviews that concentrate solely on the electrocatalytic activity of SACs or their synthesis methods. Instead, it provides a comprehensive overview of the interaction between substrates and SACs, highlighting the significance of MSI, stabilization mechanisms, and CEs that influence electrocatalytic performance. The review distinctly merges insights from both bottom-up and top-down synthetic methodologies. Furthermore, it elucidates the study of diverse substrates such as carbon-based materials, metal oxides, alloys, TMDs, MXenes, and MOF-based substrates and emphasizes how the unique features of these substrates and their interactions with SACs affect reactivity and stability. Moreover, it concurrently explores the influence of different regulatory methods such as morphology engineering, vacancy engineering, heteroatom doping, facet engineering, and crystallinity control on these substrates in different electrocatalytic reactions, including ORR, OER, HER, CO₂RR, NRR, and NO₃RR, offering a framework for the development of advanced SACs with remarkable performance. The review ends by summarizing key insights and presenting future directions to address challenges and broaden the use of SACs in real-world applications. In contrast to prior reviews, which frequently focus on specific regulation strategies for SACs or individual electrocatalytic reactions, this review distinguishes itself by systematically linking substrate selection and structural engineering strategies to electrocatalytic performance [11]. While other studies discuss SACs without thoroughly investigating the influence of substrates or addressing the synergistic effects of regulatory techniques across different substrates and electrocatalytic processes, this work presents a thorough approach [3]. It combines substrate-specific analysis, recent synthetic techniques, and regulatory approaches to provide a comprehensive explanation of electrocatalytic reactions. Furthermore, it addresses the role of diverse substrates in overall water-splitting reactions, emphasizing the effectiveness of various performance-enhancing techniques. Unlike others that just focus on the synthesis techniques of SACs and their use for specific electrochemical reactions [4], it analyzes the performance of DACs and DSACs in different electrocatalytic reactions, challenges, and prospects of each substrate in different performance regulatory methods, filling a significant void in the literature. This thorough discussion not only emphasizes the key features of SAC design but also gives insights into overcoming recurrent challenges in the substrates, distinguishing this review from others in the sector.

Table 1. Overview of different electrocatalytic reactions and their intermediates in acidic and alkaline electrolytes.

Reaction	Electrolyte	Half-Reaction (Equation)	Reaction Pathways	Ref.
ORR	Acidic Alkaline	O2 + 4H+ + 4e− → 2H2O O2 + 2H2O + 4e− → 4OH−	Direct 4-electron pathway or 2-electron pathway (H2O2 intermediate) Same as acidic, but OH− formation is dominant	[12]
OER	Acidic Alkaline	2H2O → O2 + 4H+ + 4e− 4OH− → O2 + 2H2O + 4e−	Oxygen evolution via water oxidation Oxygen evolution via hydroxide oxidation	[13]
HER	Acidic Alkaline	2H+ + 2e− → H2 2H2O + 2e− → H2 + 2OH−	Proton reduction Water reduction	[14]
CO2RR	Acidic Alkaline	CO2 + 2H+ + 2e− → CO + H2O CO2 + H2O + 2e− → CO + 2OH−	Multiple pathways: CO, CH4, HCOOH, etc., based on the catalyst Similar to hydroxide products	[15]
NRR	Acidic Alkaline	N2 + 6H+ + 6e− → 2NH3 N2 + 6H2O + 6e− → 2NH3 + 6OH−	Ammonia production via stepwise hydrogenation A similar pathway with water-splitting	[16]
NO3RR	Acidic Alkaline	NO3− + 10H+ + 8e− → NH4+ + 3H2O NO3− + 6H2O + 8e− → NH4++ 10OH−	Stepwise reduction to NH4+ or N2 Similar reduction with hydroxide products	[17]

Table 1. Overview of different electrocatalytic reactions and their intermediates in acidic and alkaline electrolytes.

Reaction	Electrolyte	Half-Reaction (Equation)	Reaction Pathways	Ref.
ORR	Acidic Alkaline	O2 + 4H+ + 4e− → 2H2O O2 + 2H2O + 4e− → 4OH−	Direct 4-electron pathway or 2-electron pathway (H2O2 intermediate) Same as acidic, but OH− formation is dominant	[12]
OER	Acidic Alkaline	2H2O → O2 + 4H+ + 4e− 4OH− → O2 + 2H2O + 4e−	Oxygen evolution via water oxidation Oxygen evolution via hydroxide oxidation	[13]
HER	Acidic Alkaline	2H+ + 2e− → H2 2H2O + 2e− → H2 + 2OH−	Proton reduction Water reduction	[14]
CO2RR	Acidic Alkaline	CO2 + 2H+ + 2e− → CO + H2O CO2 + H2O + 2e− → CO + 2OH−	Multiple pathways: CO, CH4, HCOOH, etc., based on the catalyst Similar to hydroxide products	[15]
NRR	Acidic Alkaline	N2 + 6H+ + 6e− → 2NH3 N2 + 6H2O + 6e− → 2NH3 + 6OH−	Ammonia production via stepwise hydrogenation A similar pathway with water-splitting	[16]
NO3RR	Acidic Alkaline	NO3− + 10H+ + 8e− → NH4+ + 3H2O NO3− + 6H2O + 8e− → NH4++ 10OH−	Stepwise reduction to NH4+ or N2 Similar reduction with hydroxide products	[17]

2. Scope of This Review

As a revolutionary class of materials with great potential for a broad range of electrocatalytic reactions, SACs have undoubtedly emerged as a new frontier in the area of catalysis. Numerous SACs have been effectively created, and they all have unique performance traits that tackle important issues in environmental and energy conversion applications. The majority of reviews tend to concentrate mostly on the synthesis, characterization, and application aspects of SACs, even if this topic is developing quickly and there are an increasing number of fascinating review articles published one after the other. Nevertheless, there is still a lack of comprehensive research on how various support materials affect SACs’ electrocatalytic efficiency.

We aim to fill this gap in this review by providing a thorough analysis of how diverse supports, including carbon-based materials (graphene, graphitic carbon nitride), metal-based oxides (FeO_x, γ-Al₂O₃, and Fe (OH)_x), alloy-based materials, TMDs, MXenes (carbides and carbonitrides), and MOFs, affect the performance of SACs in various electrocatalytic reactions (Figure 1). In addition to giving SAs stability, these support materials actively work to improve the electrocatalysts’ electroactivity, selectivity, and overall efficiency. Employing mechanisms such as electronic interactions, the creation of new alloy phases, the enhancement of charge transfer, the generation of new interface sites, the optimization of atomic use, and the support of nanoscale geometric topologies could modify the local catalytic environment of SAC supports.

To further improve the performance of SACs, we also explore the many approaches that may be used on these supports, such as heteroatom doping, vacancy formation, facet and morphology engineering, and crystallinity control. These tactics affect the supports’ electrical characteristics, improve their capacity to stabilize and attach individual atoms, maximize the local CE surrounding the active sites, and in certain situations, even serve as active catalytic sites. This mini-review clarifies the critical role that support engineering plays in pushing the limits of SAC performance in electrocatalysis by methodically examining the synergistic link between SACs and their supports.

3. Synthesis of SACs

The synthesis procedures for developing SACs are critical but difficult, as they need the dispersion of single metal sites onto solid supports while preventing metal aggregation. Various methods for fabricating SACs have been constructed, which are extensively classified into two main strategies: bottom-up (impregnation, pyrolysis, ion-exchange method, atomic layer deposition (ALD), and electrochemical deposition) and top-down (host-guest strategy, atom trapping, ball milling method, chemical vapor deposition (CVD), and abrasion method).

3.1. Bottom-Up Strategies

The bottom-up strategy is widely used for developing SACs. It entails the adsorption of metal precursors onto substrates, followed by various reduction procedures to confine isolated metal atoms on the defect sites of the support. Metal atoms are atomically scattered in the precursors, making it easier to generate an SA structure in the final SAC. The most popular bottom-up technique for producing carbon-based SACs is to carbonize predesigned metal precursors at high temperatures, such as MOFs, metal-porphyrin compounds, and tiny organic molecules. The impregnation method (Figure 2a) is a wet chemistry technique for immobilizing highly distributed active species on supports. In this approach, the metal precursor is combined with the supports, and metal atoms or ions are gradually impregnated onto the supports’ surfaces or the entire framework [25]. The pyrolysis process (Figure 2b) is commonly used to create atomically distributed materials. These materials can be generated by decomposing various precursors at high temperatures in a gas atmosphere (like Ar and NH₃), resulting in atomically dispersed catalysts from a variety of precursors, including metal complexes, polymers, MOFs, and so on [26]. Furthermore, to create SACs, the ion exchange method (Figure 2c) substitutes single metal atoms for metal ions that are bound within a host material. This is usually accomplished through a controlled exchange process where the precursor material is loaded with metal ions, which are then exchanged with SAs by treating the material with an appropriate agent [27]. Furthermore, a strong method for creating uniformly dispersed SA thick films that can be used to create noble metal-based SACs is the ALD method (Figure 2d). The metal precursors undergo vaporization, deposition, and confinement on the solid substrate during the synthesis process [28]. Recently, the strong electrostatic adsorption (SEA) method has developed as a viable way for attaching metal precursors onto metal oxide supports to synthesize SACs. By altering the pH at the support’s point of zero charge (PZC), the surface becomes positively or negatively charged, allowing for electrostatic adsorption of oppositely charged metal precursors. SEA stabilizes single metal atoms by strong electrostatic interactions, reducing aggregation during synthesis phases such as mixing, drying, and reduction. This approach assures monolayer coverage of metal ions on the support, which promotes the production of highly scattered SA sites. SEA has been used successfully to synthesize SACs such as Pt and Pd on substrates such as silica, titania, alumina, and carbon [29]. Last but not least is electrochemical deposition (Figure 2e), an electrochemical process that involves reducing the cations in the cathode and dissolving and re-depositing the reactive metal anodes to load onto the cathode surface [30].

3.2. Top-Down Strategies

The top-down approach is completely different since it uses metal nanoparticles or even larger-scale metal elements as the precursors and uses various techniques to transform them into isolated locations on the support. Top-down approaches simplify and lower the cost of the entire production process since they do not require atomically distributed metal atoms in precursors, unlike bottom-up solutions. Furthermore, when bulk metals are reduced to single atoms in the structure, many flaws and vacancies may be produced. Metal precursors are encapsulated within a porous host material using the host-guest strategy method (Figure 2f), which uses the spatial confinement and unique interaction between the guest and host to regulate the stabilization and dispersion of the metal atoms at the atomic level [31]. Furthermore, the many vacancies in reducible supports are usually exploited by the atom trapping technique (Figure 2g), which was initially created to manufacture platinum group metals (PGMs) SACs. PGMs move, release volatile metal oxides, and are then confined by the defective support after annealing [32]. Additionally, the synthesis of SACs can be performed via ball milling (Figure 2h), a scalable, environmentally benign, and reasonably priced mechanochemical process that uses mechanical energy to break and rebuild chemical bonds without the need for extra solvents, templates, or additives [33]. Vaporized precursors are introduced into a CVD reactor (Figure 2i), where they attach to a substrate that is kept at a high temperature. After that, a solid coating is formed on the substrate by the adsorbed molecules either thermally decomposing or reacting with other gases or vapors [34]. The acid leaching method is an essential step in the synthesis of SACs, as it removes bulk or aggregated metal particles while leaving isolated metal atoms on the support material. During this process, the catalyst is treated with acids such as hydrochloric acid (HCl) or nitric acid (HNO₃). These acids selectively dissolve bigger metal particles due to their increased reactivity, leaving single metal atoms tightly bonded to the support via powerful coordination bonds. This approach improves not just the dispersion of active sites but also the catalyst’s surface area and accessibility. Furthermore, acid leaching can eliminate contaminants and improve the structural integrity of the support material, which is critical to achieving improved electrocatalytic performance [36]. Last but not least, the abrasion technique (Figure 2j) uses metal balls as a source of the active metal as well as a means of directly atomizing bulk metals onto various supports without the need for a solvent or the production of waste or byproducts [35].

There are specific challenges for both top-down and bottom-up approaches to SAC synthesis. It can be difficult to achieve uniform atomic dispersion with large metal loading without compromising stability in bottom-up techniques such as impregnation, pyrolysis, ion exchange, ALD, and electrochemical deposition. These methods frequently include expensive, intricate synthesis and less control over the location of atoms. Due to their high energy requirements, top-down methods such as host-guest strategy, atom trapping, ball milling, CVD, and abrasion have scalability problems that restrict large-scale manufacturing. Both approaches struggle to maintain controlled dispersion and stability, which can lead to aggregation. Optimizing precursors and synthesis conditions for effective atom use and reduced energy costs is one of the upcoming advancements in bottom-up techniques. Energy-efficient electrochemical deposition may expand its use, while improvements in ion exchange and ALD substrates may allow for precise atomic control. Aggregation may be decreased for top-down approaches by including new host matrices and improving atom trapping. Ball milling and CVD modifications that are scalable and energy-efficient may improve the sustainability of large-scale SAC synthesis. For the development of SACs with improved stability and catalytic performance for energy applications, these developments are crucial.

4. Substrate’s Engineering of SACs

In SACs, the substrate is essential because it not only offers a strong and conductive foundation to anchor active metal sites but also develops a specific environment for a variety of electrocatalytic processes, including water splitting, ORR, OER, HER, CO₂RR, NRR, and NO₃RR. Materials like carbon supports, which are renowned for their superior conductivity, chemical stability in both acidic and alkaline conditions, and mechanical robustness, provide the perfect foundation for SAs with careful substrate design. Furthermore, metal oxides can effectively bind isolated atoms due to their abundance of defect sites and surface hydroxyl (-OH) groups. They can also improve catalytic performance by utilizing characteristics like surface acidity, basicity, and redox activity. Furthermore, in single-atom alloys (SAAs), distinct atomic and electronic structures that promote effective electrocatalysis are produced by strong metallic contacts between isolated metal atoms and support. The large surface areas, high conductivity, and chemical stability of TMDs render them optimal substrates for atomically dispersed metals, particularly when combined with isolated metal sites or introduced vacancies that raise the local electronic density for enhanced activity. Interestingly, monolayer TMD substrates optimize available surface area and provide a distinct structure for in-depth analysis of electrochemical phenomena. Furthermore, MXenes and microporous materials such as MOFs serve as dependable supports by taking advantage of the coordinative environment surrounding SACs to produce active centers with special characteristics that either resemble homogeneous catalysts or deviate from the behavior of bulk metals, as in the case of zero-valent metal centers. In the meantime, SAC substrates’ performance in electrocatalytic reactions was further improved by implementing several techniques, including morphology, vacancy engineering, heteroatom doping, facet engineering, and crystallinity management. High-performance electrocatalyst design is made possible by this customized substrate engineering in SACs. The different advantages and disadvantages of SACs on different supports by using some performance regulatory approaches are illustrated in

Table 2. Advantages and disadvantages of SACs of different approaches based on diverse supports.

		Supports
Approach	Advantages and Disadvantages	Carbon Support	Metal Oxide Support	Alloy Support	Transition Metal Dichalcogenides (TMDs) Support	MXenes Support	Metal–Organic Framework (MOF) Support
Morphology Engineering	Advantages	High surface area, tunable porosity, lightweight, enhanced mass transport	Diverse morphologies, robustness, wide bandgap tunability	Tunable surface, improved electron transport, alloy synergy effects	Layered structures, abundant active edges, high catalytic activity	High conductivity, layered structure, enhanced active site exposure	Tailorable structure, ultrahigh porosity, and versatility in design
	Disadvantages	Limited chemical functionality; weak interaction with metal atoms	Brittle nature, low conductivity	Complexity in precise morphology tuning, potential phase segregation	Instability under certain conditions, higher synthesis cost	Susceptibility to oxidation, challenging defect creation	Structural fragility in liquid-phase reactions, scalability issues
Vacancy Engineering	Advantages	Improves catalytic activity and conductivity; lightweight	Enhances active site density and binding energies	Provides electronic tunability; synergetic effects enhance catalytic performance	Boosts catalytic sites at edges and vacancies; enables fine-tuning of electronic states	Promotes metallic conductivity and excellent adsorption properties	Facilitates enhanced adsorption properties and active site creation
	Disadvantages	Limited vacancy stability under reaction conditions	Difficulty in maintaining vacancy stability over time	Formation of vacancies can weaken overall structural integrity	Complex fabrication methods, moderate thermal stability	High sensitivity to oxidative and acidic conditions	Hard to control vacancies precisely
Heteroatom Doping	Advantages	Enhances electronic properties, provides functional groups for improved binding	Tailors catalytic activity via bandgap engineering	Strengthens catalytic performance by introducing new alloy phases and interactions	Modifies electronic structure to improve catalytic selectivity and activity	Facilitates synergistic effects with intrinsic conductivity	Allows diverse doping combinations, enhancing versatility
	Disadvantages	Limited thermal stability; uneven dopant dispersion	High temperature is often required for dopant diffusion	Dopant clustering may reduce uniformity and effectiveness	Reduced mechanical stability due to strain effects	Possible degradation of intrinsic conductivity upon heavy doping	May alter intrinsic stability, reducing long-term durability
Facet Engineering	Advantages	Enhances site-specific catalytic activity; tunable electronic interactions	Enables site-selective reactions and improved stability	Offers higher selectivity and activity by controlling atomic arrangements	Improves edge-site activity with facet exposure	Specific facet control for electronic property optimization	Facilitates selective catalysis with exposed active facets
	Disadvantages	Facet exposure control can be challenging in porous carbon	Limited options for controlling facets in certain oxides	Surface diffusion may compromise facet selectivity	Facet engineering may reduce stability in some catalytic environments	Limited reproducibility in precise facet exposure	Achieving stable facet configurations can be difficult
Crystallinity Control	Advantages	Enhances electronic conductivity and stability	Improves charge transport and catalytic stability	Reduces grain boundaries, improving structural integrity and activity	Increases intrinsic activity by reducing defects while maintaining conductivity	Enables fine-tuning of electronic and catalytic properties	Allows structural and catalytic property optimization
	Disadvantages	Difficult to achieve uniform crystallinity in disordered carbons	Requires high temperatures, challenging scalability	Balancing crystallinity with alloy composition can be challenging	Limited options for achieving precise control	Susceptible to oxidation and degradation	Trade-off between crystallinity and porosity

.

4.1. Carbonaceous Substrate

Carbon-based materials can generally be used as supports in SAC-based electrocatalysis catalysts. The robust covalent bonding between carbon atoms creates the distinctive chemical and physical properties of carbon nanostructures that confer their unique physical and chemical characteristics. The uniform dispersed single metal atoms on the carbon matrix can modify the electrical configurations and geometric shapes of reactive catalyst sites. The impact of varying core atom coordination on a particular electrochemical reaction must be considered while designing a catalyst. The reaction’s trajectory can even be altered by varying electron transfer quantities. We will discuss how various techniques, like morphology, vacancy engineering, and heteroatom doping in various electrocatalytic activities, can be used to tune the carbon matrix of SACs.

4.1.1. Morphology Engineering

Through the optimization of surface area, porosity, and accessibility of active sites, morphological engineering encompasses several dimensions, such as three-dimensional (3D), two-dimensional (2D), and one-dimensional (1D) carbon supports for SACs that are integral in improving electrocatalytic performance. Three-dimensional-based catalysts encompass porous carbon scaffolds, foams, and networks that can be customized for particular electrocatalytic purposes. These materials facilitate efficient mass movement and support a substantial quantity of active sites. These dimension structures offer superior mechanical qualities and extensive accessible surface areas, essential for improving reaction rates, particularly in different electrocatalytic processes due to the porous architecture facilitating efficient electron transport [5]. For example, Gong et al. [37] developed a catalyst denoted as PdZn-N-CMK-3-D with a dual catalytic site and achieved high faradic efficiency (FE) of up to 97.14% and durability in reducing CO₂ to carbon monoxide (CO) due to the PdZn DAC’s synergistic effect in between the thermodynamic pathway, which reduces the activation energy of *COOH intermediate species while concurrently generating CO. Furthermore, Wei Yu and his team [38] synthesized a 3D PtSAC@CC supported on carbon cloth (CC) for HER and showed 83.0 at η = 50 mV. In addition, a 3D-based catalyst known as Fe-N-C/N-OMC, which was developed by Han and his team [39], is embedded in an N-doped ordered mesoporous carbon (OMC) framework exhibiting elevated mass activity and half-wave potential towards ORR, because high Fe and N loadings create accessible active sites with high density, graphitic N dopants, which increase the intrinsic activity of FeN₄ sites, and the OMC structure facilitates electron and mass transport in 3D interconnected pores (Figure 3a,a1). Additionally, robust electrocatalytic performance for N₂RR has been reported for an isolated SA involving an Fe atom site supported on the N-doped-C-framework (ISAS-Fe/NC), and the electrocatalyst achieved an excellent NH₃ yield rate of 62.9 ± 2.7 μg h⁻¹ mgcat⁻¹ and long-term stability lasting beyond 24 h and high FE of 18.6 ± 0.8% [40]. Furthermore, Li and his team [41] created a 3D-based electrocatalyst, Co@CNB-Nx, which stabilized on N-doped carbon nanoboxes (CNB) promoting high efficiency as a bifunctional electrocatalyst for overall water splitting (OWS), driving a current density of 10 mA cm⁻² at 1.59 V owing to the uniformly dispersed Co atoms, which could be effectively and stably riveted on the CNB because of the dependable metal-nonmetal bonds between the Co-SA and the carbon substrate. Similarly, a CoSAs@NC-920 stabilized by N-doped carbon (NC) was synthesized by Lv and colleagues [42] and because of its Co-N₃C₁ SA configuration, large specific surface area, abundant N-active species, and 3D interconnected structures, it demonstrated exceptional ORR/OER performance (Figure 3b,b1). Preserving the structural integrity of 3D materials under severe reaction conditions and achieving a homogeneous distribution of active sites might be difficult. To solve this problem, 2D carbon-based morphologies such as graphene, graphene oxide, and layered carbon structures demonstrate substantial surface areas and improved electron conductivity inside the plane of the structure and show promising performance in electrocatalysis, in contrast to 3D carbon supports for SACs. Two-dimensional catalysts significantly enhance charge distribution and promote efficient charge transfer. Their elevated surface-to-volume ratio provides a substantial density of available active sites, enhancing catalytic activity in different electrocatalytic reactions [5]. For example, CuNi/rGO showed excellent electrocatalytic performance in ORR and OER, which was synthesized by Musa and colleagues [43] and achieved a lower wave potential and overpotential because of the stronger interactions of Cu and Ni with rGO. The primary active sites for ORR/OER in FeNi-NC are Fe-N₄ and Ni-N₄, and their coexistence enhances bifunctional electrocatalytic activity. Additionally, FeNi-NC supported on N-doped flowering hollow carbon spheres demonstrated good OER and ORR activity [44]. In another study, 2D-Pt SAC demonstrated increased HER activity supported by the N-doped carbon support [45] (Figure 3c,c1). Furthermore, it was demonstrated that Ni-SA-based porous carbon nanosheets had N/S co-coordination for CO₂ reduction, achieving over 95% CO selectivity rate and −7.8 mA cm⁻² current density. These results were ascribed to the nanosheets’ sheet-like shape, long-distance conductivity, and the abundant Ni-N₃S SA sites with high activity [46] (Figure 3d,d1). The computational study was also carried out in 2D SAC supported on a carbon matrix in different electrocatalytic reactions and proved enhanced performance [47,48,49]. However, the tendency of 2D materials to agglomerate or stack without appropriate surface modifications may restrict their effective surface area and electrocatalytic performance. In contrast to 2D, 1D structures, including nanowires, nanotubes, and nanorods, possess a high surface area and exhibit significant structural anisotropy. These morphologies offer a high concentration of active sites along their length and can enhance electron transport along the structural axis. This renders them suitable for electrocatalytic processes necessitating great charge mobility for electrochemical reactions [50]. For instance, Co, Zn SAs@Co-CNTs were synthesized by Huang et al. [51] and stabilized on Co-embedded carbon nanotubes (CNTs). Owing to the synergetic interactions between Co and Zn DASs, they exhibit superior ORR (E_1/2 = 0.92 V) and HER (10 = 89 mV; ~10 = 142 mV) activities with long-term operational stability. They also lower the adsorption–desorption energy barrier of H* in the HER and accelerate the formation of OOH* and the dissociation of OH* in the ORR. Furthermore, Wu and colleagues [52] synthesized bimetallic Fe-Ru SAC dispersed on N-doped carbon nanorod spheres and exhibited a maximized NH₃ yield of 43.9 μg h⁻¹ mg⁻¹ with an optimized FE of 29.3% for the NRR owing to robust synergistic interactions between the electronic structure of Fe sites, which were effectively influenced by Ru atoms, resulting in stronger N₂ adsorption and a shift in the d-band center, thus improving NRR performance (Figure 3e,e1). Moreover, Liu and his team [53] created a CuO-based catalyst with Pd SAs dispersed on its interlayer unsaturated bonds (Pd-CuO), and the NO₃ to NH₃ conversion could be improved and promoted by the added shear stress and the dynamic effects of support formation to the Pd SAs situated on dislocations, reaching a maximum NH₃ production and optimized FE (4.2 mol gcat⁻¹ h⁻¹ and 90%) (Figure 3f,f1). Furthermore, Chen et al. [54] synthesized a catalyst named Fe-N_x-S supported by CNT with atomically dispersed Fe species, and the presence of S dopants and abundant Fe-N_x species modified the electronic structure of the Fe-SAs, resulting in remarkable OER activity with a low overpotential of 326 mV at 10 mA cm⁻² and stability.

The impact of various dimensions, 1D, 2D, and 3D, on electrocatalytic performance in carbonaceous supports is substantial since the catalyst’s geometry and structure can affect charge transfer, active site accessibility, and reaction kinetics. For example, 1D structures dominate in rapid charge transfer along their length, whereas 2D-based catalysts offer an expanded surface area for active sites and enhanced electron conductivity in the plane, which is beneficial for reactions necessitating numerous active sites [55]. Further, 2D dimensions typically exhibit superior electrical properties, whereas 3D morphologies excel in enhancing mass transfer, reaction kinetics, and long-term stability. Three-dimensional structures are more appropriate for processes requiring intricate mass movement. Although 1D materials facilitate superior charge transfer, 3D structures often exhibit enhanced catalytic effectiveness owing to improved mass transport and increased surface area. The stability and practical scalability of 3D structures render them advantageous for extensive applications [56].

4.1.2. Vacancy Engineering

Vacancies are regarded as a type of crystal defect that can modify the electron distribution, lattice structure, and elemental composition. As a result, voids are typically complicated and require more careful design control. There are three types of vacancies: cationic, anionic, and mixed vacancies, depending on the charge polarity of the atoms in the structure. One efficient way to maximize the local environment for electrocatalysis is to introduce vacancies in the SA carbon matrix. For instance, to improve the ORR activity and stability, Tian et al. [57] showed that the generation of carbon vacancy (Cv) on Fe-N₄ atomic sites prevents the metal center from dissolving and may lower the adsorption energy barrier of OH*. Because of the good performance in ORR achieved by utilizing a C_vac, Guo and his colleagues [58] also utilized a C_vac in Fe-N₄-VC SAC next to the metal center in a DFT study and showed that elevated symmetry of the Fe-N₄ atomic site tuned the d-band center, hampered the adsorption of advantageous intermediates, and resultantly increased the ORR electrocatalytic performance. Moreover, Fe-N-C catalysts with pyridinic N vacancies (FeNC-VN) were synthesized by Lyu and associates [59] (Figure 4a). Adjacent VN sites are predicted by first-principles calculations to greatly improve ORR kinetics and stability by modestly weakening *OH binding energies and effectively enhancing the covalency of Fe-Nx moieties (Figure 4a1). Additionally, a catalyst with numerous catalytic reactive sites, containing the fast transfer of charge and with enhanced performance in OER was synthesized and denoted as δ-MnO2/CoNP@CoSAs-NCNTs/CC which was supported on a 3D-based structure exhibiting O_vac requires only 165 mV overpotential to reach a current density of 10 mA cm⁻² [60]. Moreover, a trifunctional catalyst for OER/ORR/ZAB was synthesized by Yi and his group [61] named ZnIn₂S₄/PC, and due to the combined action of N and F atoms, doped Sb-SAs, and sulfur vacancies (Svac), it reduces the energy potential barrier of intermediate products, consequently achieving remarkable performance (Figure 4b,b1). Furthermore, the divacancy of graphene defects (DG) successfully trapped Ni atoms to generate a Ni-C₄ site, which enhanced HER activity [62]. In another study, a Pt-C₃ configuration was developed by Yao and colleagues [63], which contains C_vac in the DG matrix and traps a large number of Pt-SAs, lowers the adsorbed H⁺ reduction, has a stronger ability to capture electrons, and accelerates the desorption of H₂, simultaneously improving HER activity. Additionally, an effective dual-site HER catalyst with a core–shell flatform structure was synthesized using an O_vac-containing LaCeO_x material that combined with N-doped graphene along with the dispersion of Ru SAs denoted as LaCeO_x @NGr/Ru₁ (Figure 4c). The H₂ was produced by using LaCeO_x @NGr/Ru₁, which causes the adsorption and dissociation of water on the surface of O_vac-rich LaCeO_x, along with the diffusion of produced H* species towards Ce³⁺-N-Ru₁ bridges, and H₂ evolute on the Ru₁ sites (Figure 4c1) [64]. Moreover, the DFT calculation for the SACs supported on an Ovs-enriched graphene matrix showed improved performance in the CO₂ reduction to CO product [65]. Similarly, Rong et al. [66] developed a Ni-N₃-Vac SAC with three N atoms demonstrating exceptionally high CO₂ reduction to CO, exhibiting FE above 90% due to the presence of Ni-N₃ vacancy. In addition, PSB-CuN₃ with local planar symmetry was synthesized and showed exceptionally high electrocatalytic activity and stability for CO2RR to formic acid (HCOO) because of the local D4h symmetry, which was broken to produce planar-like CuN₃ and CuN₂ moieties with lower local C₂v symmetry, resulting in charge redistribution and improved performance [67] (Figure 4d,d1). Additionally, TMs used as SA materials with abundant vacancies anchor on carbon substrates that provide localized d-orbitals [68,69] that effectively transfer electrons between TM and N₂, weakening the N₂ triple bond and enhancing their performance in NRR. In contrast, Ik Kwon [70] reported that the integrated kinetic evaluations with electronic and thermodynamic analyses on various SAC candidates, using boron carbon nitride (BCN) as a substrate, exhibited a high turnover frequency (TOF) (3.1 × 10⁻⁶ s⁻¹) among various vacancy sites, with Cr anchored to BCN exhibiting the least amount of TM aggregation over time (Figure 4e,e1). Further, TM-SACs dispersed on diverse N-doped graphene (NDG) systems with different combinations of pyridinic, graphitic N-doped, and pyrrolic structures with single and double vacancies were reported, and among them, Ru- and Ir-SACs/PD₂cGn showed high NO₃ reduction and stability [71]. Superior NO₃RR performance was also achieved at low overpotentials by the atomically dispersed Fe- SAs coordinated with P and N stabilized on hollow carbon nanocages, and modulating the P enhanced the Fe center electron density, which promotes the transfer of protons and adsorption of NO₃ ions along with their related intermediates species on the surface of the electrocatalyst [72] (Figure 4f,f1). Chen and his team [73] synthesized a Cu-N-C catalyst featuring Cu-N₃ coordination that attains a remarkable CO FE of 98% and exhibits exceptional stability (FE remains above 90% for 20 h) due to the Cu-N₃ coordination that may be stabilized by an extended carbon plane with six nitrogen vacancies, whereas three empty N sites are spontaneously protonated during the CO₂ reduction reaction. Consequently, the hydrogen bonds established between the adsorbed *COOH and neighboring protons markedly diminish the energy barrier for *COOH production. Further, Guo [74] and colleagues developed Ni-SAC@NC that exhibits superior performance for CO₂RR with a maximum FECO of 95% due to the defect-Ni-N₃ configuration serving as the highly active site in Ni-SAC@NC for CO₂ reduction reactions. Moreover, Liu and co-workers [75] synthesized a highly durable and efficient Fe-N-C catalyst and showed good performance in ORR by transforming defect-laden pyrrolic N-coordinated FeN₄ sites into extremely stable pyridinic N-coordinated FeN₄ sites. Further, by employing first-principles DFT calculations, Mavrikakis and researchers [76] constructed a pH-dependent microkinetic model to assess the comparative efficiency of SA-TM-based catalysts integrated within fourfold N-substituted double carbon vacancies in graphene for the OER, and reaction routes that involve intermediates co-adsorbed on the metal site are favored across all TMs, and these routes result in improvements in catalytic activity.

4.1.3. Heteroatom Doping

Although carbon-based materials showed good support for SACs, SACs find it challenging to adhere to these materials because of the weak metal-support interaction. Heteroatom doping is the most widely used technique to modify the CE of SACs and is a straightforward and efficient way to accomplish successful anchoring of SACs. It offers metal atom coordination sites that can greatly increase the resilience of SACs. Additionally, the CE of SACs can be controlled by judiciously regulating the atomic size and heteroatom electronegativity. By regulating the concentrations of N, S, P, and B in carbon-based SACs, their catalytic performance can be markedly enhanced. N doping, the most extensively researched heteroatom, increases the electron density at active sites, hence promoting the adsorption of essential reactants like oxygen or N. It possesses high electronegativity, facilitating electron localization and generating highly polarized environments that stabilize SAs and offer several active sites owing to diverse nitrogen configurations (e.g., pyridinic-N, graphitic-N, and pyrrolic-N). Its capacity to stabilize SAs through coordination bonds renders it essential in electrochemical reactions. For example, Zhang et al. [77] produced Co-N/C-based catalysts, and due to the occurrence of N coordinated site, it reduced the binding energy of the d-band center of the Co-N₄ site, thereby reducing the binding energy between the intermediates and Co-N₄ catalytic site consequentially increased O₂ reduction catalytic activity. Outstanding electrocatalytic performance on SA electrocatalysts by using N dopants on the carbon matrix for OER has been reported, such as N-doped-holy-graphene matrix (M-NHGFs) [78], Co/Ru SAs-N-C [79], and N-Co-Mo-GF/CNT [80]. For an effective electrocatalytic H₂ evolution, Xue’s group [81] binds Mo-SAs atomic sites with co-coordinated N, C, and O atoms stabilized on N-doped carbon fibers (NCF) denoted as Mo@NMCNFs, and these fibers demonstrated exceptional stability and great HER activity with an overpotential of 66 mV. Furthermore, Kuang et al. [82] dispersed PtSACs on N-doped materials to synthesize mesoporous-hollow carbon spheres named Pt₁/NMHCS that demonstrated remarkable stability and attained an overpotential of 40 mV at 10 mA cm⁻² for the HER. Musa and colleagues [83] synthesized atomically dispersed Fe-N₄ embedded on CNTs denoted as FeSA@CNTs (Figure 5a). Utilizing the protective properties of the CNTs structure through anchoring Fe-N₄ and facilitating electrolyte access to the active areas, the synthesized catalyst demonstrated superior ORR catalytic performance and electrochemical stability due to the Fe-N₄, which serves as the primary active site for catalysis and contains square planar in shape (D4h), and the interaction between the frontier orbitals of Fe in Fe-N₄ (dz²) and the LUMO of O₂ (π-orbital), thereby promoting the catalysts’ effective catalytic activity (Figure 5a1,a2). In contrast to N, doping with S induces distinct electrical and geometric alterations owing to its greater atomic radius and softer Lewis base characteristics. These characteristics improve selectivity and activity in different electrocatalytic reactions as the S possesses lower electronegativity than nitrogen, creating electron-rich regions that facilitate charge transfer. S atoms can establish robust coordination with metals, resulting in a synergistic effect that enhances catalytic activity. The doping process frequently leads to structures abundant in defects owing to the greater atomic scale. For instance, for the OER/ORR application, theoretical research was conducted on the electrocatalytic mechanism of S-doping in TM-N-C SACs, and the electrons flow from S sites to the substrate, and the presence of S dopant enhanced the geometry configuration, stability, and charge redistribution in TM-N-C SACs (Figure 5b–b2) [84]. Additionally, a dual single atom (DSA) denoted as Fe, Co/DSA-NSC was doped with N and S dispersed on porous carbon nanosheets, and these unsymmetrically organized N and S coordinated Fe/Co atomic sites boosted the transfer of charge, lowered the adsorption energy of intermediates, and enhanced the oxygenated reaction kinetics [85]. Some studies have shown outstanding performance in diverse electrochemical reactions by using S and N as the dopants on different carbon substrates in SACs, DACs, and DSACs [86,87,88]. P doping is recognized for facilitating significant electronic redistribution, which frequently enhances the binding energy modulation of intermediates, especially in reactions such as HER and ORR. Its capacity to affect charge transfer kinetics is notable among dopants. P imparts electron-rich properties akin to S, although it exhibits more robust metal-phosphorus interactions. P doping augments surface basicity, hence facilitating intermediate adsorption and catalytic reaction mechanisms. An increased atomic radius induces lattice aberrations, enhancing the accessibility of catalytic sites. Additionally, the role of P doping in Fe and Co bimetallic SACs, known as P/FeCo-NC HER-catalysts, is reported by Tao and his team [89]. The Fe-P-Co site was demonstrated as the most effective HER activity by modifying the electrical interactions and orbital hybridization in the P bridge between the Fe and Co sites, increasing the H₂ adsorption energy. P and Co co-regulate the electrical characteristics of the Fe sites, resulting in an ideal H₂ binding energy (Figure 5c–c2). To investigate the impact of distinct CE of Ni-SACs on water-splitting catalytic activity, Wang et al. [90] prepared a variety of Ni-SACs doped with different heteroatoms into porous CNFs, and among these, the Ni-SAC with the Ni-N₃-P structure (Ni-N, P/CNFs) demonstrated the best OWS performance. Furthermore, P doping was also investigated by Zhou and his team [91] by using P/Fe-N-C in OER. In addition, the N, S, and P, B doping endows a distinctive ability to generate p-type semiconducting properties, rendering it exceptionally efficient in processes where charge transfer regulation is essential. B is an electron-deficient element that creates Lewis acidic sites, hence improving the adsorption of reactants and enhancing the dispersion and stabilization of SAs. The capacity to take electrons alters the electronic configuration of metal centers, enhancing catalytic efficiency. For example, by adding single TM atoms and different NM atoms onto GY substrates, Long Li and his group [92] developed different SACs for NRR, and, in particular, Os-B-GY demonstrated exceptionally effective NRR activity with a low overpotential of 0.13 V within the distal pathway (Figure 5d–d2). The co-doping of TMs and B can modify the d-orbital electronic structure of transition metals, promoting moderately positive charge concentrations via a localized boron coordination framework. Moreover, the robust d-2π* interaction between TM and N₂ promotes electronic back donation, thereby activating N₂ and preparing it for NRR. Some studies have promising performance by using B and N as dopants in different electrocatalytic reactions. Furthermore, it is reported that rationally controlling the CE of SACs can successfully enhance the FE of NRR. For example, coordination with a B dopant in Fe-B₂N₂ can significantly enhance the Fe metal center, exhibiting a low limiting potential of −0.65 V, indicating its high NRR catalytic activity [93]. Additionally, Xin and colleagues [94] synthesized Mn-SACs by anchoring the Mn-O₃N₁ moieties on porous carbon, and the Mo-B₂N₂ configuration lowered the energy needed for N≡N bond-breaking and boosted the adsorption of N₂ at the active site, resulting in enhanced N₂ reduction to NH₃ with a yield rate of 66.41 μg h⁻¹ mgcat⁻¹. Moreover, Ye and co-workers [95] synthesized B, N co-doped Mo-SAC denoted as Mo/BCN for electrocatalytic N₂ reduction. SACs have recently been shown to have favorable action on NO₃RR, and there is still a need for improved catalysts with increased stability and activity. To prepare Fe-BCN catalysts, Lu and his team [96] added B to the NC matrix, and consequently B improved the electronic structure around Fe SAs and Fe-N, lowering the reaction barrier and increasing NO₃RR activity, demonstrating excellent NO₃RR performance (NH₃ yield of 2.17 mg h⁻¹ cm⁻², FE of 97.48%). Furthermore, by mounting a Ni single site to BNG supports, Ajmal and colleagues [19] developed a boron-spanning method to modify the coordination structure of Ni SACs and developed a rational NiN₄B₂ structure and allowed for the incorporation of active sites and electric fields into a catalyst. It is found that the electric field distribution surrounding the B-atom next to N-atoms coordinated with the Ni-atom is responsible for the charge distribution over the Ni single site in NiSA@BNG. The adsorption of NO₃⁻on the NiSA@BNG catalyst is facilitated by the existence of locally polarized B-N bonds, which are found to be essential in producing electric fields and require only −0.42 eV energy for NiSA@BNG. Some non-metal (NM) dopant SACs have recently been produced to support the high electrocatalytic activity and high selectivity of CO₂RR. For instance, a computational screening method was developed by Lu et al. [97] in which different NM dopants adjusted the CE of various SAs, and among these TM@N₄, Zn@N₄-Cl₁(II) with 0.15 V, Ni@N₃-B₁ with 0.43 V, Fe@N₄-F₁(I) with 0.75 V, and Cu@N₄-O₁(III) with 0.49 V, exhibit the lowest overpotentials for CO₂ reduction to CO. Additionally, the axial Cl atom, co-coordinated with Fe and N-atoms, represented as FeN4Cl/NC, showed a partial current of 9.78 mA cm⁻² and a FE of 90.5% for CO due to the Cl atom, which caused a negative shift in the d-band center, resulting in a weakened binding strength between the intermediates and the Fe atom [98]. Furthermore, Ma et al. [99] conducted DFT calculations and examined the different axial ligands that affected the CO₂ performances, and among them, the M-N₄ configuration showed that the axial ligands could alter the metal center’s electrical and magnetic characteristics, changing the activity and selectivity for CO₂ to CO. Furthermore, an F-doped Ni-SA denoted as Ni-SAs@FNC with high surface Ni-N₄ loading and exhibiting an ultrathin nanosheet structure was synthesized by Guo Han and colleagues [100]. Owing to the high Ni atom utilization, unique morphology, modulated Ni-N₄ configuration, and superior F-tuning effect, it allowed it to effectively boost the electroreduction of CO₂ to CO, as evidenced by its high FECO of approximately 95%. Furthermore, a multi-heteroatom doping of N, P, and Cl was investigated by Sui and co-workers [101], denoted as Ag₁-h-NPClSC anchoring on a hollow carbon matrix, and the synergetic effect between different heteroatoms and Ag SAs dramatically optimized the ORR catalytic activity and durability.

In conclusion, the introduction of non-metal heteroatoms like N, S, P, and B markedly affects the efficacy of SACs in diverse electrocatalytic reactions by altering their electronic structure and catalytic characteristics. N doping generally increases the electron density at the metal center, facilitating charge transfer and enhancing activity in electrochemical processes. S and P with lower electronegativity than nitrogen generate electron-rich regions that stabilize intermediates in processes. Moreover, S can provide dual functions by improving both catalytic activity and hydrophobicity. B, by inducing electron shortage, can modulate the adsorption energy of intermediates, so enhancing selectivity and efficiency. The heteroatoms together affect the local coordination environment, active site accessibility, and reaction kinetics, resulting in improved catalytic activity, selectivity, and stability in various electrocatalytic applications.

Carbon-supported SACs exhibit various morphologies, vacancy types, and doping configurations, showing considerable potential for catalytic applications owing to their high specific surface areas, swift mass transfer rates, and strong structural stability. Nonetheless, other issues remain that must be addressed. A significant problem is that the interior spaces of 1D geometries are frequently underutilized, resulting in diminished atomic utilization efficiency due to inadequate exposure to active sites, which can negatively impact catalytic performance. Moreover, 2D carbon substrates often undergo restacking due to robust π-π interactions during thermal treatments and catalytic reactions, requiring the development of effective ways to alleviate this phenomenon. In 3D porous carbon frameworks, numerous individual metal atoms become encapsulated within the carbon layers during high-temperature pyrolysis, complicating their complete exposure to catalytic reactions. Moreover, in the realm of morphological engineering, attaining exact control over the nanoscale configurations and forms of carbon substrates is essential for enhancing atomic dispersion and accessibility, which directly affects catalytic performance. Likewise, vacancy engineering is limited by the stability of active sites under diverse electrochemical circumstances, potentially leading to structural failure or deterioration, thereby compromising long-term performance. Furthermore, heteroatom doping encounters similar difficulties; the integration of dopants like N, S, and P requires meticulous adjustment to improve electron dispersion while maintaining conductivity. Despite the exploration of several methodologies for heteroatom doping, the predominant methods for synthesizing carbon-based SACs depend on high-temperature pyrolysis. Future initiatives should concentrate on advancing synthesis processes to provide stable, uniformly dispersed vacancies and customized doping profiles, perhaps resulting in SACs with enhanced durability and reactivity. Examining the interplay among these three engineering strategies may improve the accessibility of catalytic sites and refine electronic properties, potentially advancing the scalability and applicability of SACs in various electrochemical reactions for energy conversion applications.

4.2. Metal-Oxide-Based Substrate

Engineered metal oxides have gained attention recently as a possible option for creating high-performance HER electrocatalysts. The distinct benefits of earth-abundant metal oxides include great chemical and electrochemical stability, strong metal-support interaction (MSI), flexible tunability, structural diversity, and ease of synthesis. They are also inexpensive. However, some techniques like morphological tuning, phase, creating oxygen vacancies, doping (metal and non-metal), and controlling crystallinity are frequently used to improve the properties of pristine metal oxides, which are inert to electrocatalytic reactions because of their inadequate electrical conductivity, restricted number of catalytic active sites, and weak H₂ adsorption capacity. It is notably recognized that the electrocatalytic efficiency of a catalyst is highly associated with the electronic properties of the active sites, suggesting that the electrocatalytic activity of SACs can be competently tuned by maximizing the metal-metal (M-M) robust interaction. Following this guideline, diverse efforts were focused on the development of advanced transition metal compounds (TMCs)-based SACs by judiciously modulating the M-M and M-TMCs interactions. Significant advancements in the synthesis of advanced SACs have been achieved during the past decade due to rigorous efforts.

4.2.1. Morphology Engineering

A key tactic for improving electrocatalytic performance in SAC-based metal oxides is morphology engineering, which maximizes surface area, electrical characteristics, and active site accessibility. The density of exposed SAs can be greatly increased by modifying morphology at nanoscale dimensions, such as via hollow structures, nanowires, or nanosheets, which facilitates effective reactant adsorption and electron transport. These morphological modifications frequently improve mass transport efficiency, speed up reaction kinetics, and lower diffusion resistance in electrocatalytic reactions. Catalytic activity and selectivity in electrocatalytic reactions depend on precise control over defect sites and electronic structures, which can be achieved by tailoring morphologies. For instance, Zhou’s group [102] recently synthesized 3D Pt SACs by dispersing Pt SAs on NiO/Ni heterostructure, which was developed on Ag nanowires and demonstrated exceptional performance and 30 h durability, exhibiting a lower ΔGH* of 0.07 eV for Pt SAs and dissociating energy barrier for water attributed to the presence of dual active reactive sites in the Ni and O-vacancies-rich NiO sites for HER. Additionally, a 3D-based cerium hydroxide-doped copper catalyst was synthesized by Sargent and his team [103] and yielded a 43% FE. Mechanistic studies revealed that H_ad on Cu prefers the ethanol pathway over the ethylene pathway by targeting the Cu-C link of the *HCCOH intermediate. Additionally, using polystyrene nanospheres known as SA-FeN₄O₁-C, Xu and associates [104] synthesized a 3D-frame structure with organized pore channels, and the catalyst’s higher overpotential in ORR and OER is attributed to the special CE of the SA active site and 3D framework. Furthermore, Zheng et al. [105] deposited Ru-SA onto an Ov-riched SnO₂ matrix and synthesized a 2D-based Ru/SnO_2−x catalyst (Figure 6a). Ru SAs near Ov and the SnO_2−x substrate undergo a minimal transfer of electrons from Ru to O atoms, leading to electron loss of Ru atoms (Figure 6a1), and the catalyst produced a remarkable 32.4 μg h⁻¹ mgcat⁻¹ of NH₃, along with outstanding electrochemical stability and a FE of 16.2%.

4.2.2. Vacancy Engineering

A great way to control the CE, electrical characteristics, and catalytic activity of SACs is by defect engineering. Surface imperfections, for instance, might influence the density of charge close to the Fermi level and encourage molecular orbital hybridization, leading to more unsaturated coordination sites and suspended bonds, even at very low concentrations. C, O, N, S, cationic vacancy, and anionic vacancy defects are among the defect types employed to control the CE of SACs. To utilize uniformly dispersed Pt SAs on titania nanosheets (Pt₁O₁/Ti_1−xO₂), Lu and colleagues [106] used Tivac-enriched nanosheets, and these negatively charged Tivac readily trapped positively charged Pt⁺² ions, and an ultrahigh activity and exceptional stability (300,000 cycles) were promoted by the generation of the Pt-O dual atomic site by intrinsic charge transfer. Moreover, Wu and his team [107] synthesized a catalyst denoted as Ni/M-CeO₂ SACs in which the OV concentration was controlled by a doping series of different metal cations over CeO₂ surfaces. Among them, the Ni/CeO₂ SACs presented high initial activity in CO₂ reduction. Yuan’s team [108] synthesized Ru SA onto Ov-rich cobalt oxides (Ru/Co₃O_4−x), and the Ov causes electrons to delocalize around the Co atoms. This delocalization is further improved after loading Ru atoms, because of the coexistence of Ovs and Ru atoms, and achieves a low potential (280 mV @10 mA cm⁻²) and long-term durability for OER (Figure 6b,b1). Additionally, by controlling the electronic MSI, Yao and colleagues [109] described an intrinsic oxide anchoring technique to secure ligand-free, uniform Ru SAs on the amorphous layered surface of monolithic Ti support (Figure 6c). The strong electronic MSI was demonstrated by a clear charge rearrangement across the interface between the Ru SA and TiO_x/Ti, which generated a charge transfer from Ru SA to TiO_x/Ti and achieved a good performance in NO₃RR (Figure 6c1). In addition, Wang et al. [110] synthesized an electrocatalyst with Cu-Sn dual atomic active sites utilized by double Ovacs on CeO_2−x and these Ovacs are crucial in controlling the electrocatalytic performance and stability, highlighting the significance of controlling the electronic MSI between Cu-Sn diatoms and CeO_2−x and achieving a 90.0% FE for CO₂ conversion to formate.

Figure 6. (a) Charge density evaluation of Ru/SnO_2−x; (a1) AC-STEM image of Ru/SnO_2−x. Reprinted from Ref. [105] with copyright permission from Elsevier. (b) Structure of Ru/Co₃O_4−x; (b1) Bader charge numbers of Ru/Co₃O_4−x. Modified from Ref. [108] with copyright permission from The American Chemical Society. (c) Structure of Ru₁-TiO_x/Ti; (c1) difference in charge density of Ru₁-TiO_x/Ti. Adapted from Ref. [109] with copyright permission from The American Chemical Society. (d,d1) Model of Cu-CeO₂. Reprinted from Ref. with copyright permission from The American Chemical Society. (e) Structure of CuSA-Sn-SnO₂; (e1) DFT model of CuSA-Sn-SnO₂. Modified from Ref. [111] with copyright permission from Elsevier. (f) Structure of Co SA MoO₃; (f1) charge density difference in Co SA MoO₃ and MoO₃. Modified from Ref. [112] with copyright permission from Elsevier.

Figure 6. (a) Charge density evaluation of Ru/SnO_2−x; (a1) AC-STEM image of Ru/SnO_2−x. Reprinted from Ref. [105] with copyright permission from Elsevier. (b) Structure of Ru/Co₃O_4−x; (b1) Bader charge numbers of Ru/Co₃O_4−x. Modified from Ref. [108] with copyright permission from The American Chemical Society. (c) Structure of Ru₁-TiO_x/Ti; (c1) difference in charge density of Ru₁-TiO_x/Ti. Adapted from Ref. [109] with copyright permission from The American Chemical Society. (d,d1) Model of Cu-CeO₂. Reprinted from Ref. with copyright permission from The American Chemical Society. (e) Structure of CuSA-Sn-SnO₂; (e1) DFT model of CuSA-Sn-SnO₂. Modified from Ref. [111] with copyright permission from Elsevier. (f) Structure of Co SA MoO₃; (f1) charge density difference in Co SA MoO₃ and MoO₃. Modified from Ref. [112] with copyright permission from Elsevier.

4.2.3. Heteroatom Doping

Through altering the electrical environment surrounding the active metal sites, heteroatom doping in SACs has become a calculated strategy to improve their electrocatalytic performance. The electronic structure of the catalyst can be adjusted to increase its conductivity, stability, and reactivity in electrocatalytic reactions by adding NM atoms, such as S, P, N, or any other metal or NM, into the metal oxide support material. For example, the Ir-Co₃O₄ exhibited superior electrocatalytic OER performance and stability exhibited by the 3d-TM-based oxides doped with single noble metal atoms reported by Wang and his team [113]. Additionally, due to the surface hydroxylation and doping of Mo and W SAs, Yuan Lu and colleagues [114] created a highly efficient bifunctional water electrolysis-based catalyst, and due to the synergistic interactions between Mo and W SAs with the co-doped surface, hydroxylated NiFe oxide denoted as FN-MoWact maintains its stability after 100 h of operation and achieves ultralow overpotentials for OER, HER, and water splitting (1.504 and 1.729 V to deliver current densities of 10 and 500 mA cm⁻²). In addition, Yang and his group [115] showed that Cu-doped, single-atomic Cu-substituted CeO₂ and multiple Ovac-bound CeO₂ may maximize CO₂ electrocatalytic reduction to CH₄ and produce a remarkably efficient catalytic center for CO₂ activation and adsorption with a FE as high as 58%, indicating tremendous potential for designing more highly efficient electrocatalyst active centers to increase activity and selectivity (Figure 6d). Further, Cu-SACs supported on Sn-SnO₂, for instance, were synthesized together with the Cu doping denoted as CuSA-Sn-SnO₂. Cu-SAs modulated the electronic structure, which effectively influences the adsorption and stabilization of *CO intermediates and resultantly enhanced the performance of CO₂ (Figure 6e,e1) [111].

4.2.4. Facet Engineering

Facet engineering in metal-oxide-based SACs is critical for tuning catalytic activity because it exposes specific crystallographic planes that optimize the interaction of single metal atoms with the support. Different facets of metal oxides have distinct electronic structures, surface energies, and atomic arrangements, which influence the CE and electronic properties of the anchored single metal sites. For instance, Zhang et al. [116] reported the CEs of Pt SAs distributed on anatase TiO₂ supports with controlled shapes, specifically focusing on the (001) and (101) crystal facets. Among them, Pt SAs located on the (001) surface show reduced reactivity for CO oxidation, primarily due to restricted accessibility to gas-phase species. Further, Hu and his team [117] demonstrate that the crystal facets of CeO₂ play a crucial role in determining the form of Pd species. On the CeO₂(100) surface, Pd is present as isolated single atoms, whereas on CeO₂ (111), Pd tends to form clusters. This variation influences the catalytic behavior in N-alkylation reactions, demonstrating that as the catalytic component size reduces to the single-atom level, the impact of crystal facets becomes more significant and can potentially dominate the catalytic performance. Moreover, Chen and colleagues [118] reported that the electronic structure of Pt SAs (Pt₁) is affected by the CeO₂ facets. Pt₁ on CeO₂ (110) exhibits a positive charge, whereas, on CeO₂ (100), it tends toward a metallic state. These variations arise from the differences in Pt-O-Ce microstructures and electron transfer behaviors. The positively charged Pt1 shows weak CO adsorption, which favors oxidation, while metallic Pt₁ enhances H₂ dissociation, aiding CO oxidation and lowering the reaction barrier. Lastly, Li and co-workers [119] reported the SA-Pt catalysts supported on the CeO₂ surface with facets such as (111), (110), and (100) by using DFT calculations, and on all three ceria surfaces, it is thermodynamically advantageous to replace a lattice Ce ion with a single Pt atom. Whereas the Pt atom produces a planar structure with a lowered oxidation state of +2 on the (110) surface due to the spontaneous creation of surface peroxide (O₂²⁻) species, it adopts a + 4 oxidation state on the (111) and (100) surfaces after replacing Ce⁴⁺ and resultantly enhanced performance.

4.2.5. Crystallinity Control

Because it directly affects the electrical structure, stability, and accessibility of the active site, crystallinity management in SACs is essential for improving electrocatalytic activity. A well-defined atomic arrangement is usually ensured by the high crystallinity in SACs, which results in consistent coordination environments around the single metal atoms. For the best catalytic activity, precise electronic tuning is necessary, and this homogeneity aids in this process. Furthermore, a highly crystalline structure can offer strong support, improving SA stability in challenging electrochemical environments and reducing aggregation. But it is also critical to strike the desired balance between defect engineering and crystallinity; flaws like vacancies or grain boundaries can provide special electronic states that improve adsorption and charge transfer. Therefore, improving SACs for effective and long-lasting electrocatalytic applications requires deliberate flaw engineering in addition to regulated crystallinity. For instance, Xu and colleagues [120] developed Pt SAs on an amorphous MoO_x substrate and, due to the robust Pt-O bonding contact, obtained a good performance in HER with an overpotential of 19 mV at 10 mA cm⁻². Furthermore, to synthesize a Co-SA catalyst, Hyuk Moon and associates [112] used an amorphous-to-crystalline phase transition in the MoO₃ matrix (Figure 6f). The difference in charge densities of Co SA MoO₃ shows that it has improved hydrogen covalent bonding characteristics, and these H⁺ adsorption results provide more evidence for Co SA MoO_3’s strong HER activity (Figure 6f1).

SACs anchored on metal oxides exhibit considerable potential yet encounter numerous obstacles. A problem exists in morphological engineering, which, although capable of augmenting active surface area and modifying electrical characteristics, faces difficulties in attaining uniform and accurate shapes across diverse scales. Another critical strategy is vacancy engineering, which is vital for enhancing charge transfer and generating several active sites. Controlling the distribution and ensuring the stability of vacancies under reaction conditions presents significant challenges. Further, heteroatom doping presents challenges, as variations in the incorporation and activation of dopants can adversely impact catalytic efficiency and longevity. Moreover, achieving a balance between crystallinity and the introduction of defects and dopants is a challenging endeavor; an excess of defects may undermine structural integrity, whilst insufficient crystallinity might result in diminished conductivity. Future endeavors should prioritize the synergistic implementation of these engineering methodologies, concentrating on synthesis techniques that provide meticulous control over morphology, vacancy distribution, and dopant incorporation. The progression of in situ characterization techniques may yield real-time insights into structural and functional alterations, hence aiding the creation of SAC-supported metal oxides with improved activity and stability for electrocatalytic applications.

4.3. Alloy-Based Substrate

A single-atom alloy (SAA) is an innovative advancement in SACs. It amalgamates the advantages of conventional alloy catalysts with SACs, wherein minimal quantities of reactive metal centers are uniformly scattered inside inert metal matrices. In 2012, Sykes and his team [121] effectively dispersed isolated Pd atoms on a pristine Cu (111) active surface and introduced the term SAA. Subsequently, other SAAs have been documented and extensively employed in the electrocatalytic domain due to their distinctive physicochemical characteristics. This distinctive electronic configuration presents a possibility to disrupt the linear scaling correlations between the activation and reaction energy barrier. Simultaneously, SAA catalysts produce an intrinsic synergistic effect as dopant host metal atoms, and SAs on the reaction surface function as dual-catalytic centers for the activation of various supports. By separating the dissociation and catalytic active sites, the SAA catalyst presents the intriguing potential to circumvent linear scaling connections between the reaction energy barrier and the stability of intermediates species, which constrain the catalytic reactivity and selectivity of numerous catalysts. Additionally, we will examine the SAA structure by employing several techniques, including morphological manipulation, vacancy engineering, heteroatom doping, facet engineering, and crystallinity regulation in diverse electrocatalytic activities.

4.3.1. Morphology Engineering

Through the strong electronic MSI, modifying the SAA’s shape and nanostructure may alter the microenvironment and, consequently, its intrinsic activity. Due to their distinct advantages in electrocatalytic applications, SAA comes in a variety of morphologies, including 1D, which includes nanofibers, nanotubes, nanorods, and nanowires; 2D, which are nanosheets; and 3D, which is a hierarchical porous structure. For instance, a 1D SAA comprising Pt-Co dual reactive sites enclosed in N-doped graphite-CNTs, synthesized by Cheng et al. [122], exhibited exceptional ORR performance, attaining an ORR mass activity of 0.81 A mg⁻¹Pt. Furthermore, the direct conversion of NO₃ to NH₃ was accomplished by utilizing a 2D-based SA Ni-alloyed Cu electrocatalyst, exhibiting a high FE of approximately 100% and a yield rate of 326.7 μmol h⁻¹ cm⁻². This performance is attributed to the robust interaction between Ni and the essential NOOH* reaction intermediates, which facilitated a reduction in reactions’ limiting potential, consequently enhancing the preference for NH₃ synthesis [123]. Further, Li and his team [124] synthesized a PdFe₁ SAA metallene, initially developed as a potent and stable electrocatalyst for NRR, achieving an NH₃ yield of 111.9 μg h⁻¹mg⁻¹, a FE of 37.8%, and sustained stability over 100 h of electrolysis. This performance is attributed to the Pd co-coordinated Fe SAs serving as active sites, facilitating optimized N₂ activation through N₂-to-Fe σ-donation, alongside excellent thermodynamic stability, which collectively contributes to the excellent performance and durability of PdFe₁ in NRR applications. Mao and colleagues [125] similarly synthesized Co-substituted Ru nanostructures, which serve as extremely efficient Pt-free catalysts for HER, necessitating a low overpotential of 13 mV to attain a current density of 10 mA cm⁻². Furthermore, a CO₂-reduction-induced surface restructuring technique was accomplished through the innovative design of a nanoporous AgCu SAA catalyst, which demonstrates a self-reinforcing selectivity for CO₂ reduction, coinciding with the transfer of Cu to the reaction surface, and exhibits exceptional performance, with a CO FE of 97.5% [126] (Figure 7a,a1). Lastly, Gao and colleagues [127] synthesized a novel catalytic motif denoted as core/shell diluted SAA and ordered SAA. The Pauli repulsion from the fully occupied metal d-states resulted in a weaker *N binding, while the *NO₃ binding was boosted by the d-states hybridization in the ordered CuAu alloy layers. The tuning of the adsorption of important NO₃RR intermediates was not significantly impacted by the surface strain on Cu/CuAu-ordered SAA. Consequently, the outstanding ammonia selectivity and yield from NO₃RR on these Cu/CuAu-ordered SAA catalysts mark a significant advancement in the creation of core/shell, ordered intermetallic, and SAA catalysts.

4.3.2. Vacancy Engineering

The incorporation of vacancy engineering in SAAs is an effective approach for optimizing the electrocatalytic performance of these materials by generating active sites that improve catalytic activity, stability, and selectivity. The introduction of various vacancies, usually by the elimination of specific atoms in the alloy matrix, modifies the electronic structure and CE surrounding the individual atom sites. This structural alteration can reduce the activation energy of many electrocatalytic reactions, including O₂ and H₂ evolution, as well as CO₂ reduction, by enhancing the adsorption energies of reaction intermediates. For instance, Yao et al. [128] proposed the incorporation of Ru SAs into a Pt-rich environment (PtCu_x/Pt_skin) to mitigate the oxidation of Ru. This process increased the d-band center of Pt-atom vacancy, attributed to electron transfer between Pt and Ru. Consequently, the Ru₁-Pt₃ Cu configuration exhibited a lowered 90 mV overpotential to achieve a current density of 10 mA cm⁻². Moreover, Zhao and his team [129] reported a catalyst with a hierarchical pore structure denoted as Cu₁Au SAA as an effective electrocatalyst for CO₂ reduction and demonstrate exceptional CO₂ reduction performance, achieving nearly 100% CO FE, attributed to its hierarchically structured porous morphology with abundant vacancies and 3D catalytic reactive active sites. Furthermore, Zhang and colleagues [130] synthesized Au₁Cu (111) SAA with surface Covac (VCu-Au₁Cu SAAs), demonstrating an exceptional NH₃ with a FE of 98.7% and a yield of 555 μg h⁻¹ cm⁻² (Figure 7b). The local electronic structure of VCu-Au₁Cu SAAs was synergistically influenced by individual gold sites and Covac, hence facilitating the generation of *H that can be efficiently employed by NO³⁻ and its intermediates (Figure 7b1).

4.3.3. Heteroatom Doping

The doping of different heteroatoms N, S, or B in SAAs incorporates foreign atoms into the alloy matrix, facilitating improved regulation of the electronic structure between the dopant and adjacent atoms, active sites, and surface characteristics for electrocatalytic processes, hence increasing electron density at specific places and optimizing binding energies for required reaction intermediates. For example, Matthew and colleagues [131] conducted a DFT-based investigation that clarifies the thermodynamics of the ORR on Ag- and Au-based SAAs. They examine SAAs composed of Ni, Rh, Pd, Co, Pt, and Co incorporated as SAs into the (111) surfaces, revealing that the PdAu SAA demonstrates a marginally reduced theoretical overpotential and improved selectivity for 4-electron ORR activity. Furthermore, a study by Cheng’s group [132] was conducted on the competitive pathways for CO₂ reduction by using SAAs which reveals that doping of Au and Ag with different TMs facilitates tandem catalysis, and among them, the Au or Ag dispersed on the substrate reduced CO₂ to CO*, which is subsequently reduced by H₂ at the dopant metal to yield C1 hydrocarbons. Furthermore, Zheng and his team [133] introduced a SA Pb-alloyed Cu catalyst denoted as Pb₁Cu that can selectively reduce CO₂ into HCOOH with a high FE of around 96% wherein the Pb₁Cu electrocatalyst facilitates the conversion process on the modulated Cu sites. Thus, alloying Cu with isolated heteroatoms facilitates the C protonation of CO₂ to HCOO* on activated Cu sites, leading to exclusive electrochemical conversion of CO₂ to HCOOH with very high activity. Moreover, Wu et al. [128] synthesized a series of single Ru-doped PtCu_x/Pt_skin SAAs and among them the Ru₁-Pt₃Cu catalyst exhibiting the optimal OER performance, characterized by an overpotential of 220 mV at 10 mA cm⁻². Recently, the advancement of bifunctional electrocatalysts that operate effectively for both HER and OER across a broad pH spectrum has garnered significant attention. Liang’s group [134] recently synthesized a Ru-doped Co SAA catalyst encased in an N-doped carbon sheath, utilized for OWS. This catalyst exhibited exceptional bifunctional electrocatalytic activity achieving a minimal cell voltage of 1.55 V at 10 mA cm⁻², attributed to the doped Ru SAs that effectively reduced the adsorption of H* and other oxygenated reaction intermediates species. Recent studies have indicated a markedly improved reactivity and selectivity of SACs for N₂ reduction. Chen’s team [135] developed a series of SAA model systems M/Au(111) and assessed their NRR performance using DFT, revealing that the incorporation of TM SAs on Au(111) substantially enhances the N₂ binding affinity and reduces the activation energy barrier of the initial hydrogenation step. Furthermore, Huang et al. [136] developed a series of NiCoRu_x/SP and, in particular, NiCoRu_0.2/SP exhibited optimized activity attaining a lowered overpotential of 59 mV at 10 mA cm⁻² due to the incorporation of Ru atoms into the NiCo support would create a robust interaction at the Ru-Ni/Co interface thereby enhancing the HER activity (Figure 7c,c1).

Figure 7. (a) Fabrication of np AgCu SAA, (a1) Structure of np AgCu SAA. Modified from Ref. [126] with copyright permission from Elsevier. (b) Synthesis of the VCu-Au₁Cu SAAs; (b1) mechanism on VCu-Au₁Cu SAAs for NO₃RR. Modified from Ref. [130] with copyright permission from Elsevier. (c) Synthesis of NiCoRu/SP; (c1) mechanism of water splitting on NiCoRu_0.2/SP. Modified from Ref. [136] with copyright permission from Elsevier. (d) Exposed steps on diverse facets for various catalysts; (d1) structure of PtCu₃ (111). Modified from Ref. [128] with copyright permission from Nature. (e) Models of Cu-based SAAs with four different Miller indices. Modified from Ref. [137] with copyright permission from Elsevier. (f) Grand canonical Monte Carlo (GCMC) simulation on the Ag₅₄Co₁ cluster; (f1) Co₁O_x(OH)_y complexes configuration. Modified from Ref. [138] with copyright permission from The American Chemical Society.

Figure 7. (a) Fabrication of np AgCu SAA, (a1) Structure of np AgCu SAA. Modified from Ref. [126] with copyright permission from Elsevier. (b) Synthesis of the VCu-Au₁Cu SAAs; (b1) mechanism on VCu-Au₁Cu SAAs for NO₃RR. Modified from Ref. [130] with copyright permission from Elsevier. (c) Synthesis of NiCoRu/SP; (c1) mechanism of water splitting on NiCoRu_0.2/SP. Modified from Ref. [136] with copyright permission from Elsevier. (d) Exposed steps on diverse facets for various catalysts; (d1) structure of PtCu₃ (111). Modified from Ref. [128] with copyright permission from Nature. (e) Models of Cu-based SAAs with four different Miller indices. Modified from Ref. [137] with copyright permission from Elsevier. (f) Grand canonical Monte Carlo (GCMC) simulation on the Ag₅₄Co₁ cluster; (f1) Co₁O_x(OH)_y complexes configuration. Modified from Ref. [138] with copyright permission from The American Chemical Society.

4.3.4. Facet Engineering

Facet engineering is considered a highly effective strategy, as the crystal facet constitutes the most fundamental component of a SAC-based alloy. The surface atomic arrangement and configuration directly influence the reactant’s and intermediates adsorption and activation energy, which fundamentally dictate catalytic activity. Facet engineering has demonstrated its efficacy in improving the performance of electrocatalysts for various electrocatalytic reactions. For example, Du and his team [139] discovered that a Cu(100) surface doped with Pd-SA exhibited enhanced catalytic activity and selectivity for NO₃RR. Moreover, Feng and his team [140] demonstrated that the Fe- and Ru-decorated Cu (211) step edges were the most efficient in facilitating the CO₂ reduction process to CH₄. In addition, Zheng’s group [141] reported the development of Pt/Pd SAA catalysts, successfully dispersing Pt atoms onto Pd NPs with (100) and (111) facet surfaces, resulting in significantly enhanced HER and ORR activity. Moreover, Yao and his team [128] synthesized a series of Pt-Cu alloys along with the dispersion of Ru SAs and a controlled amount of the step, kink, and corner defects exposed on the facet’s active surface demonstrated that these processes augmented the surface defects which further function as anchoring reactive sites for dispersed Ru₁, thereby enhancing OER performance (Figure 7d,d1). Furthermore, Gao and colleagues [137] synthesized a series of copper-based SAA, incorporating doped TMs and exposing surfaces with various Miller indices, utilizing DFT calculations and among them W₁-Cu(111)) with −0.42V, Re₁-Cu(111) with −0.38 V, Os₁-Cu(211) with −0.49 V, Os₁-Cu(110) with −0.48V, and Re₁-Cu(100) with −0.35V exhibiting low limiting potentials for the N₂ reduction reaction (Figure 7e).

4.3.5. Crystallinity Control

Regulating crystallinity in SAAs is essential for enhancing electrocatalytic activity, as it affects active site distribution, electrical characteristics, and stability. Regulating the crystalline phase and grain boundaries allows for the precise adjustment of the atomic dispersion of single atoms within the alloy matrix, which directly influences the local electronic environment of catalytic sites and, consequently, the reaction kinetics. For example, Pu et al. [138] conducted DFT calculations demonstrating that the Ag₅₄M₁O_x(OH)_y complex serves as the crucial catalytic active site for facilitating the ORR in Ag-based SAA cluster electrocatalysts. The GCMC simulation demonstrates that the atomic O* species have a limited capacity to adsorb onto the Ag₅₄Co₁ cluster owing to the endothermic Gibbs free adsorption energy (Figure 7f). The suitable orbital hybridization between dopant metal and OH in the MO_x(OH)y complexes suggests that the Ag₅₄Co₁, Ag₅₄Pd₁, and Ag₅₄Au₁ clusters are the most effective Ag SAA catalysts for the ORR (Figure 7f1).

Besides the enhanced catalytic performance of alloy-based substrates in electrocatalysis, they still encounter numerous obstacles such as: (A) Understanding how the morphology of alloy-based SACs varies throughout prolonged catalytic operations is crucial to addressing the problem of catalyst deactivation. (B) It is difficult to predict how vacancies would affect the overall catalytic performance of alloy-based SACs due to the lack of clarity surrounding their function and interactions with other catalytic sites. (C) When heteroatoms are added, the metal lattice may experience strain and deformation, affecting the structural stability of alloy-based SACs and reducing the catalyst’s activity. (D) Heteroatoms may cause strain and distortions in the metal lattice, which could reduce the durability and risk the structural stability of alloy-based SACs. (E) Surface reconstruction may cause alloy-based SACs’ crystallinity to deteriorate across several catalytic cycles, potentially leading to a decrease in active sites and catalytic activity. Future considerations may improve the efficiency of SAA-based substrates by (A) better control over the morphology could help alloy-based SACs continue to maintain over time, preventing deactivation and making them more appropriate for widespread use in the energy application. (B) The development of new catalysts could be accelerated by using machine learning techniques to optimize vacancy engineering and determine the best vacancy kinds and densities for different catalytic reactions. (C) To improve catalytic performance, multi-element heteroatom doping in alloy-based SACs can optimize electrical and geometric features. (D) Multi-element heteroatom doping in alloy-based SACs can tune the geometric and electrical properties for increased catalytic performance. (E) The logical design of optimal compositions and structures can be facilitated by using advanced computational techniques to predict the crystallinity and stability of alloy SACs, offering important insights before experimental synthesis.

4.4. TMD-Based Substrate

TMDs are an essential category of 2D materials, which exhibit a high surface area, robust interaction with light fields, and elevated charge carrier mobility, rendering them optimal support for the incorporation of SACs. TMD materials generally manifest as MX₂, with M denoting the core TM atom and X signifying chalcogen atoms (S, Se, or Te). However, the saturated coordination atoms on the TMD substrate fundamentally restrict the accessible active sites for electrocatalytic processes. Consequently, the technique of utilizing TMD-supported TM atoms to fabricate SACs demonstrates considerable potential [142]. In recent years, various sophisticated TMD-loaded SA electrocatalysts have been developed, employing diverse techniques as outlined below.

4.4.1. Morphology Engineering

The intrinsic anisotropic behavior of TMDs in SACs renders morphological factors crucial in optimizing their catalytic performance. To fully exploit the potential of active edge sites and optimize the available surface area, various TMD-based SAC topologies have been developed, including nanosheets and hollow nanoboxes. For instance, Ling and co-workers [143] investigated the potential of the MoS₂ monolayer in the 1T′ phase for accommodating single TM atoms and assessed the ORR catalytic activity of these dispersed TM atoms through DFT calculations. Among the studied configurations, Pd@1T′ and Cu@1T′ exhibited the most favorable ORR performance, with overpotentials of 0.32 V and 0.41 V. Further, Liu and co-workers [144] predict that a single 3D TM co-doped with O-atoms in MoS₂ (3d-TMO₆@MoS₂) undergoes a transformation from a hexa-coordinated to a tetra-coordinated structure as oxidative intermediates approach, and among these, TM = Co, Mn, and Fe atoms exhibit superior catalytic activity for the OER. Additionally, Pattengale’s group [145] utilized 1T-MoS₂ as a substrate to incorporate Ni SAs into its basal plane, substituting Mo atoms (Ni@1T-MoS₂) and the resulting catalyst demonstrates significantly improved HER performance. The advancement of MoS₂ edge-supported SACs is essential for water splitting. Xu and his team [146] reported various single TM SACs embedded on MoS₂ edges as bifunctional electrocatalysts for OWS through DFT calculations, and among them, Pt-SAs containing T1-vacancy exhibits the lowest theoretical overpotential for H₂/O₂ (−0.10/0.46 V). Furthermore, Ren and colleagues [147] examined a range of SAs immobilized on MoS₂ nanosheets for CO₂ conversion to methane through DFT calculations, with Ni@MoS₂ with −0.45 V, Fe@MoS₂ with −0.39 V, and Co@MoS₂ with −0.24 V exhibiting limiting potentials for methane production. Ultimately, it has been determined that SA supported on several chalcogenides (WS₂, WSe_2, and MoSe₂) exhibits exceptional activity for CO₂RR. Utilizing first-principles high-throughput computations, Yang et al. [148] examined the catalytic activity of several 2D TM-SAs supported on a MoS₂ monolayer denoted as MoS₂ (SA@MoS₂-B) and the optimized configuration of Mo@MoS₂-M for NRR (Figure 8a). The dispersed TM-SAs on the sulfur-sulfur bridge site of MoS₂ (SA@MoS₂-B) facilitate charge transfer from the Mo-SA to N₂, which is crucial for both activating the nitrogen triple bond by populating the π antibonding orbital and stabilizing the adsorbed N₂ through Coulombic interactions (Figure 8a,a1), and among these configurations, Mo@MoS₂-M exhibits good stability by using the distal pathway for catalyzing N₂ reduction, with a calculated overpotential of 0.28 V. Further, Li and his team [149] developed a 2D Fe-MoS₂ catalyst for the NO₃RR, demonstrating exceptional performance with a maximum FE of 98% for the conversion of NO₃ to NH₃ (Figure 8b,b1). The energy levels of the α-spin and β-spin d atomic orbitals of Fe-MoS₂ significantly overlap with the 2π* orbitals of NO, resulting in the splitting and rearrangement of orbitals that generates new d-π* antibonding and bonding orbitals (Figure 8b2). Besides the MoS₂, WS₂-based SA was also reported for NO₃ reduction reaction by using DFT calculations [150].

4.4.2. Vacancy Engineering

Generating defects, such as vacancies, is an effective method for altering the electrical structure of SACs anchored on TMDs. Different concerted efforts have been undertaken to develop regulated techniques for the introduction of atomic vacancies in TMDs with SACs, thereby facilitating various electrocatalytic activities. For instance, Wang et al. [151] synthesized SA Ru-doped MoS₂ catalysts with a high 2H phase content denoted as Ru@2H-MoS₂ and Ru0.10@2H-MoS₂ exhibits the lowest HER overpotentials of 137, 51, and 168 mV at 10 mA cm⁻² due to the incorporation of Ru which creates additional reactive active sites and vacancies, thereby enhancing the HER performance. Additionally, various single TM atoms immobilized on MoS₂ edges have been evaluated as bifunctional electrocatalysts for OWS by DFT calculations [146] (Figure 8c,c1). The diffusion barrier of Pt on T1-vacancy is lower at 1.82 eV (Figure 8c2) and exhibited the lowest theoretical overpotential for the H₂/O₂ at −0.10/0.46 V for OWS, attributed to the adjusted band center of the single metal atom. Furthermore, Zhang and colleagues [152] utilized spin-polarized DFT-D₃ calculations to systematically examine the reaction mechanism of electrocatalytic CO₂RR on M@WTe₂ with a Te vacancy, finding that Ni@WTe₂ exhibits remarkable selectivity for HCOOH with an ultra-low overpotential of −0.11 V. Moreover, Jiang’s team [153] indicated that a non-contacting TM atom adsorbed on the Svac of the MoS₂ basal plane functions as a spin-active probe by causing significant spin polarization on the exposed Mo atoms. The electronic spin moment of the Mo atom can be adjusted across a wide range by altering the type of TM atom and its adsorption site. Consequently, the TM’s spin alignment properties significantly enhance the adsorption and polarization of N₂, facilitating the reduction of N₂ to NH₃. Furthermore, Yang and his team [154] adjust the phase of substrate MoS₂, adorned with a B-SA referred to as B@2H-MoS₂ and B@1T-MoS₂, by introducing Svac. Among these, B@1T-MoS₂ exhibits superior NO₃RR activity and remarkable stability, characterized by a metastable foundation, as validated by migration energy barrier and ab initio molecular dynamics (AIMD) simulations.

4.4.3. Heteroatom Doping

A key strategy for improving the catalytic reactivity of SACs supported by TMDs is the careful regulation of their physicochemical properties via heteroatom doping in their inert basal planes. The incorporation of metallic or non-metallic heteroatoms into the lattice structure can enhance H₂ adsorption affinity by modifying the electronic band structures of catalytically inactive basal planes. Striking a precise balance is crucial, since excessive doping may compromise lattice stability, consequently lowering catalytic reactivity and shortening the overall longevity of TMD-supported SACs. Therefore, the heteroatom doping technique must be well-optimized for the designated target materials. For example, Zhang and his team [155] synthesized an SA Ru catalyst supported on MoS₂, referred to as SA-Ru-MoS₂, which demonstrates a low overpotential of 76 mV at 10 mA cm⁻², attributed to the doping of Ru-SA which induces a phase transition in MoS₂ and the formation of Svac thereby markedly boosting the performance of inert 2D MoS₂ for HER. Moreover, Zhu and colleagues [156] investigated the potential of utilizing SA-dispersed Cu or Co on a 2H-MoS₂ monolayer denoted as Co@2H-MoS₂ and Cu@2H-MoS₂ for the OWS and these catalysts modulated the electronic band structure and charge density distribution of 2H-MoS₂ and the dispersion of Co or Cu SAs can activate inert in-plane S-atoms thereby enhancing the catalytic activity for both H₂ and O₂ reactions. Additionally, Hao et al. [157] developed an innovative heterostructure of MoS₂-Mo₂C nanosheets with minimal Ru loading, encapsulated atop TiN nanorod arrays supported by a carbon composite matrix (Figure 8d,d1). The ∆GH* value of Ru-MoS₂-Mo₂C was nearly 0, indicating its potential to attain an optimum ∆GH* value for superior HER performance (Figure 8d2) due to a synergistic effect that enhanced reaction kinetics for both the H₂ and O₂ evolution reaction as well as for OWS. Furthermore, the electrocatalytic NRR was performed utilizing a series of TM-doped Haeckelite-MoS₂ nanosheets (TM@HL-MoS₂) through DFT calculations and among these, Sc@HL-MoS₂ emerges as the most effective NRR catalyst, as it facilitates the activation of adsorbed N₂ molecules, particularly through the enzymatic pathway of the side-on configuration, which is distinguished by the lowest free energy (merely 0.001 eV) and an exceptionally low initial potential (−0.28 V) [158]. Magnetic field-enhanced heterogeneous SA spin catalysts increase metal utilization and reaction efficiency. The requirement for a high density of scattered active sites with both long-range ferromagnetic ordering and short-range quantum spin interactions makes their design difficult. In that case, Sun and his team [159] synthesized several SA spin catalysts with extensively adjustable substitutional magnetic atoms (M₁) in a MoS₂ host. Among all the M₁/MoS2 species, global room-temperature ferromagnetism is the result of the distorted tetragonal shape that Ni₁/MoS₂ adopts and this structure causes ferromagnetic coupling to neighboring Ni₁ sites and nearby S atoms consequently facilitates spin-selective charge transfer, resulting in triplet O₂. Additionally, the OER magnetocurrent is enhanced by approximately 2880% over Ni₁/MoS₂ in both saltwater and pure water splitting cells by a modest magnetic field of around 0.5 T due to a field-induced spin alignment and spin density optimization over S active sites resulting from field-regulated S(p)-Ni(d) hybridization which is responsible for a significant magnetic-field-enhanced OER performance over Ni₁/MoS₂.

4.4.4. Facet Engineering

Facet engineering is an effective approach for regulating the proportion of targeted crystal planes on the surfaces of SACs. Surface energy and atomic coordination can be modified by selectively exposing particular crystal facets, resulting in enhanced reactivity and selectivity in diverse electrochemical reactions. This method enables the incorporation of SACs onto selectively chosen surfaces of TMDs, enhancing electron transport and charge separation. Furthermore, facet engineering can enhance the stability of single atoms, inhibit their aggregation, and preserve elevated catalytic performance. For example, Qi and his team [160] developed a SA array catalyst, SA Co-D 1T MoS₂, for HER utilizing the top-down assembly/leaching methodology (Figure 8e,e1). The adsorption of H₂ at the upright position of Co on MoS₂ (111) is less favorable with the preference for the tilted top site linked to the ensemble effect at the Co-MoS₂ (111) interface. The energy acquired by transitioning from the upright to the tilted position is linked to the electrostatic attraction between the negatively charged proton and the positively charged S adjacent to Co (Figure 8e2), and the elevated HER activity of this SAC primarily results from an ensemble effect, facilitated by the synergy between the cobalt adatom and the sulfur of the MoS₂ (111) support, achieved through the modulation of the hydrogen-binding mode at the interface.

4.4.5. Crystallinity Control

Regulating crystallinity in TMD with SACs is essential for enhancing their catalytic efficiency in diverse electrochemical processes. Optimizing temperature and reaction time parameters might improve the uniformity and stability of the SACs inside the TMD matrix, hence promoting optimal atomic dispersion and interaction between the active sites and the matrix. Pan et al. [161] indicated that Pt SAs were incorporated into the 2H-MoS₂ lattice without disrupting its crystalline structure. Theoretical studies indicate a universal process of “onsite electrostatic polarization,” wherein electrostatic forces substantially polarize distributions of charge at individual atomic sites and modify the reaction kinetics of rate-determining steps, resulting in enhanced reaction performance. The field-induced on-site polarization provides a distinctive approach for modeling catalytic activities in natural enzyme systems using dynamic, quantitative, precise external electric fields (Figure 8f,f1).

Figure 8. (a) Structure of Mo@MoS₂-M, (a1) Charge redistribution on Mo@MoS₂-M, (a2) Structural optimization of the Mo and W SA in Mo|W@MoS₂-B. Modified from Ref. [148] with copyright permission from Elsevier. (b) Structure of Fe-MoS₂; (b1) mechanism for the NO₃RR on M-MoS₂; (b2) interaction scheme in-between M-MoS2 and NO. Modified from Ref. [149] with copyright permission from John Willey and Sons. (c) Structure of Pt₄/1T-vacancy; (c1) top view of Pt (111) slab; (c2) diffusion barrier of Pt on T1-vacancy. Modified from Ref. [146] with copyright permission from The Royal Society of Chemistry. (d) Synthesis of Ru-MoS₂-Mo2C; (d1) Ru-MoS₂-Mo₂C structure; (d2) ∆GH* of different catalysts for HER. Reprinted from Ref. [157] with copyright permission from Elsevier. (e) Structure of SA Co-D 1T MoS₂; (e1) illustration of Co and 2H MoS₂; (e2) H₂ adsorption modes on the SA Co-MoS₂. Modified from Ref. [160] with copyright permission from Nature. (f) Synthesis of Pt SAs-MoS₂; (f1) charge analysis of Pt SAs-MoS₂. Modified from Ref. [161] with copyright permission from Nature.

Figure 8. (a) Structure of Mo@MoS₂-M, (a1) Charge redistribution on Mo@MoS₂-M, (a2) Structural optimization of the Mo and W SA in Mo|W@MoS₂-B. Modified from Ref. [148] with copyright permission from Elsevier. (b) Structure of Fe-MoS₂; (b1) mechanism for the NO₃RR on M-MoS₂; (b2) interaction scheme in-between M-MoS2 and NO. Modified from Ref. [149] with copyright permission from John Willey and Sons. (c) Structure of Pt₄/1T-vacancy; (c1) top view of Pt (111) slab; (c2) diffusion barrier of Pt on T1-vacancy. Modified from Ref. [146] with copyright permission from The Royal Society of Chemistry. (d) Synthesis of Ru-MoS₂-Mo2C; (d1) Ru-MoS₂-Mo₂C structure; (d2) ∆GH* of different catalysts for HER. Reprinted from Ref. [157] with copyright permission from Elsevier. (e) Structure of SA Co-D 1T MoS₂; (e1) illustration of Co and 2H MoS₂; (e2) H₂ adsorption modes on the SA Co-MoS₂. Modified from Ref. [160] with copyright permission from Nature. (f) Synthesis of Pt SAs-MoS₂; (f1) charge analysis of Pt SAs-MoS₂. Modified from Ref. [161] with copyright permission from Nature.

SACs-based TMDs encounter numerous obstacles in electrocatalysis such as a. the reproducibility of TMD-based SACs with defined morphologies poses a considerable difficulty since minor alterations in experimental conditions can lead to huge inconsistencies in the catalyst’s performance. b. The production of TMD-based SACs with regulated morphology necessitates intricate processes that could limit their scalability by requiring costly precursors or producing low yields. c. developing vacancies in SAC-based TMDs to enhance catalytic activity by introducing supplementary active sites is difficult. d. It is difficult to predict how heteroatom doping would impact the catalytic activity and selectivity of SACs since the precise electrical changes caused by doping in TMDs are yet unclear. e. Further, it is still very difficult to distribute heteroatoms uniformly within the TMD framework. f. It is still difficult to achieve selective exposure of particular facets during the synthesis of TMD-based SACs. g. Defects in TMD can increase catalytic activity, but if left uncontrolled, they can weaken SACs’ crystallinity, which lowers their catalytic activity. The efficiency of SACs based on TMDs in electrocatalysis can be improved by; (A) the incorporation of throughput automation and high-throughput screening techniques may facilitate the swift optimization of synthesis methods, resulting in the scalable production of TMD-based SACs with accurate morphological control. (B) by regulating vacancy formation via etching or chemical processes, the type and density of vacancies can be refined in TMD-based SACs. (C) by improving their electrical characteristics and increasing catalytic efficiency, dual doping with distinct heteroatoms may improve the performance of TMD-based SACs, especially for complex processes. (D) The type, concentration, and distribution of heteroatoms at the atomic scale can be precisely controlled by developing high-throughput screening techniques. (E) The catalytic activity of TMD-based SACs can be increased synergistically by combining facet engineering with other techniques like doping or vacancy engineering, which will boost performance in a variety of processes. (F) This can be lessened by combining TMDs with additional materials, including metal oxide or carbon-based supports, which can help maintain the crystalline structure, reduce undesired phase shifts, and improve the stability and efficiency of SACs.

4.5. MXene-Based Substrate

Two-dimensional TM carbides, nitrides, and carbonitrides characterized by the generic formula M_n+1X_nT_X (where n = 1–3; M denotes a TM; X represents C and/or N; T signifies surface functional groups such as oxygen, hydroxyl, or fluorine) have proven to be advanced support for electrocatalytic reactions due to their remarkable electrical, mechanical, catalytic, and thermal features, possessing a substantial surface area, elevated active site density, exceptional durability, and hydrophilic characteristics; hence, these distinctive traits render them suitable as supports for SACs [162]. We elaborate below on several ways employed to boost their performance in distinct electrocatalytic activities.

4.5.1. Morphology Engineering

Morphology engineering is crucial for improving the performance of MXene-based SACs in electrocatalysis by adjusting their surface characteristics, structure, and electrical interactions. MXenes, characterized by their distinctive 2D layered architecture and elevated conductivity, provide an outstanding substrate for the immobilization of single atoms, hence optimizing active site accessibility. Furthermore, controlling morphology can enhance the accessibility of reactants to active sites and promote charge transfer, leading to reduced overpotentials and improved electrocatalytic performance. Kuznetsov’s group [163] synthesized Co-substituted MXene catalysts designated as Mo₂CT_x: Co where the substitution of Mo with Co significantly alters the electronic configuration and affects the adsorption-free energies of H₂ on adjacent O₂ atoms, thereby enhancing the suitability of these active sites for HER electrocatalysis. Similarly, Kuznetsov et al. [164] prepared a multilayered Mo₂CT_x: Fe by substituting Mo sites in Mo₂CT_x, exhibiting good catalytic activity and selectivity in the reduction of O₂ to hydrogen peroxide (Figure 9a–a2). Furthermore, Huang and co-workers [165] employed innovative 2D Mo₂CO₂ and Ti₂CO₂ MXene monolayers as effective electrocatalysts for N₂ reduction reactions utilizing first-principles computational methods. The incorporation of Mo and Ru onto Mo₂CO₂ and Ti₂CO₂ resulted in improved electrochemical activity for N₂RR and due to the elevated conductivity of Mo₂CO₂, Ru@Mo₂CO₂ exhibited a reduced negative potential. Moreover, Zhao and his team [166] reported an SA-Cu immobilized MXene for the electrocatalytic reduction of CO₂ to methanol, achieving a high FE of 59.1% and demonstrating commendable stability due to the SA Cu’s unsaturated electronic coordination sites (Cuδ+, 0 < δ < 2), which facilitates a low activation energy for the RDS with the conversion of HCOOH* to the absorbed CHO* reaction intermediate species), thereby enabling efficient electrocatalytic CO₂ reduction to methanol (Figure 9b–b2).

4.5.2. Vacancy Engineering

Vacancy engineering is essential for improving the performance of MXenes-based SACs in electrocatalysis. The electrical structure and surface properties of MXenes can be considerably modified by the strategic introduction of vacancies, including C, O, or TM vacancies. These vacancies establish distinct coordination sites that secure individual metal atoms, inhibiting their aggregation and improving their catalytic stability. For instance, Zhang’s group [167] synthesized double TM-SA dispersed on MXene denoted as Mo₂TiC₂T_x, characterized by abundant Mo vacancies and exposed basal planes in the outer layers, and achieved an H₂ evolution with low overpotentials (30mV @10 mA cm⁻², and 77 mV @100 mA cm⁻²) owing to the interaction between surface functional groups and protons of Mo₂TiC₂T_x (Figure 9c–c2). Moreover, Chen and colleagues [168] prepared a Mxene monolayer containing Ti vacancy resulting in remarkable stability and exceptional CO2RR activity. Further, Wang and his team [169] examined the NRR performance of TMn embedded in defective V₃C₂O₂ through DFT calculations, and these defects modify the d-band center and generate electron-deficient sites, enhancing N₂ activation, and among the series, W₃@V₃C₂O₂ with −0.31 V, Mo₂@V₃C₂O₂ with −0.31, Mo₃@V₃C₂O₂ with −0.33 V, and W₂@V₃C₂O₂ with −0.34 V exhibit commendable activity and selectivity, with the lowest limiting potentials (Figure 9d–d2). Additionally, the NO₃RR mechanism of SACs is methodically investigated by employing DFT to model single TM atoms on MXene with Ov, and among them, Cu/Ov-MXene with −0.34V and Ag/Ov-MXene with −0.24V demonstrate remarkable efficiency for NO₃RR to NH₃ [170].

Figure 9. (a) Structure of Fe₂O₃.xH₂O/C, (a1) SEM of Mo₂CT_x:Fe structure; (a2) Mo₂CT_x:Fe structure. Reprinted from Ref. [164] with copyright permission from The American Chemical Society. (b) Structure of SA-Cu-MXene; (b1) AC HAADF-STEM image of SA-Cu-MXene; (b2) difference in charge density in SA-Cu-MXene. Modified from Ref. [166] with copyright permission from The American Chemical Society. (c) Mechanism of MXene with immobilized Pt SAs; (c1) top view Mo₂TiC₂O₂-PtSA; (c2) structure of the Mo₂TiC₂T_x-VMo. Reprinted from Ref. [167] with copyright permission from Nature. (d) Top view of V₃C₂O₂ monolayer with O vacancy; (d1) side views of optimized adsorption configurations on TMn@V₃C₂O₂; (d2) NRR mechanism on TMn@V₃C₂O₂. Reprinted from Ref. [169] with copyright permission from Elsevier. (e) Atomic structure of TM/Ov-MXene; (e1) difference in charge density on Ag/Ov-MXene; (e2) structure of Ag/Ov-MXene. Modified from Ref. [170] with copyright permission from John Willey and Sons. (f) Structure of M₂CT₂-VT-Pt; (f1) structure of Nb₂Ta₂C₃T₂-VT-Pt; (f2) reaction paths and energy barriers of O₂ hydrogeneration or dissociation on M₂CT₂-VT-Pt. Reprinted from Ref. [171] with copyright permission from The American Chemical Society.

Figure 9. (a) Structure of Fe₂O₃.xH₂O/C, (a1) SEM of Mo₂CT_x:Fe structure; (a2) Mo₂CT_x:Fe structure. Reprinted from Ref. [164] with copyright permission from The American Chemical Society. (b) Structure of SA-Cu-MXene; (b1) AC HAADF-STEM image of SA-Cu-MXene; (b2) difference in charge density in SA-Cu-MXene. Modified from Ref. [166] with copyright permission from The American Chemical Society. (c) Mechanism of MXene with immobilized Pt SAs; (c1) top view Mo₂TiC₂O₂-PtSA; (c2) structure of the Mo₂TiC₂T_x-VMo. Reprinted from Ref. [167] with copyright permission from Nature. (d) Top view of V₃C₂O₂ monolayer with O vacancy; (d1) side views of optimized adsorption configurations on TMn@V₃C₂O₂; (d2) NRR mechanism on TMn@V₃C₂O₂. Reprinted from Ref. [169] with copyright permission from Elsevier. (e) Atomic structure of TM/Ov-MXene; (e1) difference in charge density on Ag/Ov-MXene; (e2) structure of Ag/Ov-MXene. Modified from Ref. [170] with copyright permission from John Willey and Sons. (f) Structure of M₂CT₂-VT-Pt; (f1) structure of Nb₂Ta₂C₃T₂-VT-Pt; (f2) reaction paths and energy barriers of O₂ hydrogeneration or dissociation on M₂CT₂-VT-Pt. Reprinted from Ref. [171] with copyright permission from The American Chemical Society.

4.5.3. Heteroatom Doping

Doping heteroatoms in MXenes-based substrates with SACs is a promising strategy to improve the catalytic performance and selectivity of these materials. Recently, several atoms, including N, S, P, Cu, and Fe, have been included in SACs-based MXenes to enhance their performance in diverse electrocatalytic reactions by adjusting their electronic structure, altering the elemental composition, and optimizing surface chemistry. Additionally, in the MXenes lattice and CE, the SACs can stabilize individual metal atoms through robust covalent connections, serving as active sites for electrocatalytic processes. For instance, Gao et al. [170] documented the O vacancy TM-doped MXene for NO₃RR utilizing DFT, revealing that Cu/Ov-MXene with −0.34V and Ag/Ov-MXene with −0.24V demonstrate remarkably low limiting potentials for NO3RR along with the adsorption energy of NO₃⁻ on Cu/Ov-MXene and Ag/Ov-MXene that exceeds that of the proton, hence promoting NO₃RR (Figure 9e–e2). Further, to demonstrate the significance of N and S doping, Ramalingam’s group [172] synthesized RuSA-Ti₃C₂T_X and RuSA-N-Ti₃C₂T_X incorporating N and S into the MXene matrix. The supported MXene enhances the electrical interaction between Ru SAs and the MXene substrate, optimizing ΔGH* to practically zero. Nonetheless, investigations into MXene materials remain nascent relative to other 2D supports. Kan and his team [171] described the construction of Pt SACs utilizing standard M₂CT₂ MXenes to investigate their catalytic characteristics for the ORR and the OER. The Pt atom was identified as the active site in the process. The catalytic capabilities predominantly depend on the submetal (M) of the MXenes, with Nb₂CF₂-VF-Pt and Cr₂CF₂-VF-Pt identified as effective bifunctional catalysts for ORR/OER. Both are predicated on the F terminator. All O-terminated systems demonstrated inadequate ORR or OER performance owing to the reduced charge density surrounding the Pt atom and the C layer (Figure 9f–f2).

SACs utilizing MXenes by incorporating different strategies face some challenges that encompass (1) the correlation between the morphology of MXenes as a support and their efficiency as SACs-based catalysts is complex. (2) Another significant issue in optimizing morphology for MXene-based SACs is maintaining the structural integrity of MXenes under harsh catalytic conditions. (3) Further, incorporating vacancies into the MXene structure containing SACs may trigger phase changes, thereby reducing the catalyst’s efficiency. (4) Nonetheless, not all vacancies exert identical effects differences in the nature and magnitude of vacancies can result in varying influences on the electronic structure and reactivity of them. Examining the impact of vacancy dimensions, morphology, and positioning on catalytic efficiency continues to be a significant challenge. (5) The influence of heteroatoms on the catalytic characteristics of MXenes-based SACs is not well understood. (6) Furthermore, attaining a high concentration of heteroatoms in MXenes’s SACs structure without reducing the catalyst’s stability or causing dopant aggregation is still a challenge. The performance of SACs based on MXenes in electrocatalysis can be enhanced by: 1. enhanced comprehension of the structure-function relationship can improve the creation of MXenes with specialized morphologies tailored for distinct catalytic processes by increasing the quantity of MXene layers, introducing surface imperfections, or altering edge configurations. 2. Their morphology can be optimized by regulating the number of layers, and thickness of MXenes, which can augment surface area, improve electron conductivity, and elevate the accessibility of active sites. 3. By meticulously regulating the concentration and types of vacancies, one can mitigate the probability of unfavorable phase transitions. 4. Moreover, by meticulously controlling vacancy dimensions and configurations, SACs-based MXenes can be tailored to establish certain electronic environments that improve selectivity. 5. By utilizing characterization techniques, such as DFT can provide significant insights into the impact of heteroatoms on the catalytic properties of SACs based MXenes. 6. Enhanced doping methodologies, including the utilization of low-temperature treatments or the application of coordination chemistry for superior heteroatom binding, may provide elevated doping concentrations while preserving the structural integrity of MXenes utilizing SACs.

4.6. MOF-Based Substrate

MOFs are advanced, porous, crystalline-based materials, consisting of diverse metal atoms and organic/inorganic ligands. The multi-component and well-defined structure allows for precise regulation of electrochemical characteristics through fundamental principles of molecular chemistry. In the past few years, due to several beneficial attributes, such as diverse functional groups, structured pores, unsaturated metal active sites, and plentiful surface-active sites, MOF, and MOF-derived materials have been investigated and utilized as novel substrates for SACs in various electrochemical reactions [3].

4.6.1. Morphology Engineering

The tailoring of morphology in SACs that anchor MOF-based substrates is crucial for improving catalytic efficacy and stability in diverse electrocatalytic processes. The structural dimensions of SACs, including nanofibers, nanowires, nanosheets, and nano boxes, have been optimized to synergistically improve catalytic performance. This optimization facilitates fast contact efficiency between the catalyst and reactants, minimizes mass transport distances, and ensures continuous electron transport pathways, thereby maximizing active sites and accessibility. For instance, a series of Fe_x-PCN-222, have been prepared by using a novel mixed-ligand strategy for ORR as reported by Jiao et al. [173] (Figure 10a). The hierarchical porous structure of FeSA-N-C, characterized by high-density Fe-N₄ active sites and orientated open mesoporous channels enhance elevated mass transfer, would significantly expedite the ORR process (Figure 10a1,a2). Another investigation demonstrates that enveloping a trinuclear Fe^III2Fe^II complex within the channels of distinctive MOF results in the formation of iron nanoclusters stabilized on a carbon layer, synthesizing a DAC referred to as Fe^II by Zn^II/Co^II, which displays effective OER activity [174]. Moreover, Yuan and colleagues [175] synthesized a rhombic dodecahedral catalyst, termed Co-SAs/N-C, using high-temperature pyrolysis, which has exceptional catalytic performance owing to the robust interaction between Co-SAs/N-C and Ru clusters. Furthermore, Jiao and his team [176] constructed a 3D SA Fe₁-Ni₁-N-C catalyst with N-bridged Fe and Ni SAPs for CO₂RR by direct pyrolysis of MOFs (Figure 10b,b1). The Fe-N-N-Ni structure exhibits the most uniform CE and interatomic distance and owing to the synergistic effect of adjacent Fe and Ni SA pairs in Fe₁-Ni₁-N-C demonstrates markedly enhanced performance for the CO₂ reduction (Figure 10b2,b3). In addition, Tao and his team [177] documented that a 3D single Ru site dispersed on N-doped porous carbon significantly enhanced the electroreduction of aqueous N₂ to NH₃, yielding an NH₃ formation rate of 3.665 mg NH₃ h⁻¹ mg⁻¹ and attained an NH₃ FE of approximately 21% at a low overpotential of 0.17 V. Similarly, Zhang and his team [178] developed 2D Fe SACs featuring distinctive FeN₂O₂ coordination via direct pyrolysis of MOFs demonstrating a remarkable FE of approximately 92% and an elevated NH₃ yield rate of 46 mg h⁻¹ mg cat⁻¹, attributed to the presence of O₂-atoms in FeN₂O₂ that modulate the d-band center of Fe, thereby influencing the adsorption activation energies of the NO₃RR intermediates.

4.6.2. Vacancy Engineering

The advancement of SACs with a precisely defined microenvironment has demonstrated advantages from the deliberate creation of surface imperfections on supports. These surface defects function as “traps” for the accumulation of metal atoms, aiding in this endeavor. The modification of SA site microenvironments predominantly occurs at defects, C_vac, S_vac, O_vac, or metal cations depending on the type of supports utilized. Cationic vacancies have been extensively employed to fabricate SACs with unique local environments based on MOFs. For instance, Shankar and his team [179] described a straightforward MOF-assisted electrodeposition technique for synthesizing Fe SA catalysts on C-Ni heterostructure nanosheets containing Ovs, which exhibit effective electrocatalytic activity for OER and HER, with low overpotentials of approximately 246 and 164 mV at 10 mA cm⁻² attributed to the synergistic properties of Fe SA sites on the C-Ni matrix, high-valent Fe⁴⁺ centers, characterized by a substantial volume of active sites, electrophilicity, rapid transfer of charge, and a minimal energy barrier for the rate-limiting *OOH formation, thereby enhancing the water-splitting reaction. Further, Ni- SA with varying N coordination numbers (designated as Ni-N_x-C) dispersed on faulty carbon supports generated from MOFs with Zn vacancies. The Ni-N₃-C catalyst, characterized by a reduced nitrogen coordination number around nickel atoms, exhibits enhanced catalytic efficacy for CO₂ reduction, and a lower free energy change of 0.66 eV for the production of COOH* in Ni-N₃-C when utilized as an electrocatalyst [180] (Figure 10c–c3).

Figure 10. (a) Structure of FeSA-N-C, (a1) Mechanism of ORR in FeSA-N-C, (a2) AC HAADF-STEM image of FeSA-N-C. Reprinted from Ref. [173] with copyright permission from John Willey and Sons. (b) Structure of Fe₁-Ni₁-N-C, (b1) AC HAADF-STEM for Fe₁-Ni₁-N-C, (b2) Model of Fe₁-Ni₁-N-C, (b3) Model of distorted Fe-Ni-N. Modified from Ref. [176] with copyright permission from The American Chemical Society. (c) Structure of Ni-N₃-C, (c1) AC HAADF-STEM of Ni-N₃-C, (c2) Geometric configurations of Ni-N₃-C, (c3) Geometric configurations of Ni-N₄-C. Adapted from Ref. [180] with copyright permission from John Willey and Sons. (d) Structure of W-SAC, (d1) Model of the W-SAC, (d2) Gibbs free energy (ΔGH*) of H2 adsorption on the different catalysts, (d3) Differential charge density for W-SAC. Modified from Ref. [181] with copyright permission from John Willey and Sons. (e) Structure of CoNi-SAs/NC, (e1,e2) Diatomic Ni-Co-N₆ models, (e3) LSV curves of different catalysts. Modified from Ref. [182] with copyright permission from John Willey and Sons. (f) Structure of C-Zn_xNi_y ZIF-8, (f1) Configuration of ZnN₄, (f2) Electrocatalytic performances of the CO₂RR over the catalysts, (f3) Energy diagram of C-Zn_xNi_y ZIF-8. Modified from Ref. [183] with copyright permission from The Royal Society of Chemistry.

Figure 10. (a) Structure of FeSA-N-C, (a1) Mechanism of ORR in FeSA-N-C, (a2) AC HAADF-STEM image of FeSA-N-C. Reprinted from Ref. [173] with copyright permission from John Willey and Sons. (b) Structure of Fe₁-Ni₁-N-C, (b1) AC HAADF-STEM for Fe₁-Ni₁-N-C, (b2) Model of Fe₁-Ni₁-N-C, (b3) Model of distorted Fe-Ni-N. Modified from Ref. [176] with copyright permission from The American Chemical Society. (c) Structure of Ni-N₃-C, (c1) AC HAADF-STEM of Ni-N₃-C, (c2) Geometric configurations of Ni-N₃-C, (c3) Geometric configurations of Ni-N₄-C. Adapted from Ref. [180] with copyright permission from John Willey and Sons. (d) Structure of W-SAC, (d1) Model of the W-SAC, (d2) Gibbs free energy (ΔGH*) of H2 adsorption on the different catalysts, (d3) Differential charge density for W-SAC. Modified from Ref. [181] with copyright permission from John Willey and Sons. (e) Structure of CoNi-SAs/NC, (e1,e2) Diatomic Ni-Co-N₆ models, (e3) LSV curves of different catalysts. Modified from Ref. [182] with copyright permission from John Willey and Sons. (f) Structure of C-Zn_xNi_y ZIF-8, (f1) Configuration of ZnN₄, (f2) Electrocatalytic performances of the CO₂RR over the catalysts, (f3) Energy diagram of C-Zn_xNi_y ZIF-8. Modified from Ref. [183] with copyright permission from The Royal Society of Chemistry.

4.6.3. Heteroatom Doping

Incorporating heteroatoms such as S, O, and P into coordinated atoms for a singular metal center anchored on a MOF substrate can serve as an effective technique for engineering the CE. Controlling the quantity and types of these heteroatoms in the coordinating layer is crucial for improving the activity and selectivity of SACs. This approach has been extensively utilized in numerous electrocatalytic processes. For example, Sun and coworkers [184] illustrated the fabrication of Pt SACs using an ALD technique, and the metal F-NC substrate is expected to form a strong covalent bond with Pt SAs due to its high density of N doping sites, resulting in enhanced support contact that improves the electrocatalytic performance and stability of Pt SACs for the ORR. Moreover, Chen and his team [181] synthesized a W-SAC by using W as the dopant for effective electrochemical HER, demonstrating a low overpotential of 85 mV at a current density of 10 mA cm⁻² (Figure 10d,d1). The optimal active site of W-SAC is the coordinated C atom adjacent to the N atom, exhibiting a remarkably low ΔGH* value of merely 0.033 eV. The difference in charge density before and after W SA doping indicated that the stabilization of H is attributable to the augmented electron density on the C atoms coordinated to W, which can significantly enhance the HER activity. (Figure 10d2,d3).

Furthermore, Han and colleagues [182] deposit dopamine on Co-Ni dual atomic active sites immobilized on N-doped hollow carbon denoted as the CoNi-SAs/NC hybrid, which exhibits exceptional electrocatalytic performance for bifunctional O₂ reactions attributed to the uniformly dispersed SAs with an elevated reactivity and attributed to the dual atomic Co-Ni sites, which lowered the activation energy barrier and enhanced reaction rates, along with the pre-adsorption of OH* on the catalyst surface, significantly optimizing electrocatalytic activity (Figure 10e–e3). Additionally, N co-coordinated Co or Fe atoms, isolated inside carbon matrices (M-N-C), featuring both bulk and edge-hosted M-N₄ coordination, were synthesized, and among the different configurations, Fe is inherently active in M-N₄ for the CO₂ reduction to CO, achieving a CO FE of 93% [185]. Additionally, an Fe porphyrinic MOF was decorated on the SA Fe immobilized on N-doped carbon catalysts (Fe₁-N-C). Leveraging the atomically dispersed SA Fe sites, hierarchical pore structure, and excellent conductivity, Fe₁-N-C exhibits an FE of 4.51% and an NH₃ yield rate of 1.56 × 10⁻¹¹ mol cm⁻² s⁻¹ [186].

4.6.4. Crystallinity Control

Regulating crystallinity in MOFs that include SACs is essential for enhancing their efficacy in diverse catalytic applications. The crystallinity of MOFs profoundly impacts their structural integrity, porosity, and accessibility of active sites, hence influencing the overall catalytic activity and selectivity of the included SACs. Strategies for attaining regulated crystallinity involve adjusting synthesis parameters, including temperature, solvent selection, and precursor concentration, alongside post-synthetic changes. For example, Bao et al. [183] synthesized coordinatively unsaturated Ni-N sites embedded in porous carbon through pyrolysis, demonstrating that the CO current density increases with overpotential, achieving 71.5 ± 2.9 mA cm⁻² while sustaining a high CO FE of 92.0–98.0%. DFT calculations indicate that the CO₂ reduction reaction occurs readily over coordinatively unsaturated Ni-N sites, thereby overcoming the limitations between FE and current density in the CO₂ reduction reaction (Figure 10f–f3).

In addition, the promising performance of MOF-based SACs in electrocatalysis, they still encounter numerous obstacles that need to be addressed. 1. Achieving consistent distribution of metal sites within the MOF is challenging, frequently resulting in performance differences. 2. Precise control over morphology is another considerable challenge for them. 3. The introduction of vacancies, whether at metallic sites or linkers, might undermine the crystallinity and stability of the material, potentially resulting in structural alterations under reaction conditions. 4. Moreover, achieving a uniform distribution of vacancies and their efficient utilization for SA anchoring poses challenges, as the misplacement of vacancies may lead to inactive or inadequately distributed metal sites, hence diminishing the catalyst’s efficiency. 5. Enhancing the interaction between dopants and metal sites is essential to avert undesirable side effects or instability. 6. Another problem is ensuring the long-term stability of these doped sites under catalytic conditions, as dopants may migrate or leach during reaction cycles, leading to a decline in catalytic activity. 7. Attaining high crystallinity in MOFs may restrict the accessibility of active sites, hence impeding overall efficiency. Prospective developments in SACs-based MOFs for electrocatalysis appear: i. Improved precision in morphology control would facilitate the development of hybrid structures, wherein MOFs with precisely designed shapes and sizes might be combined with conductive materials or other catalytic supports. ii. Improvements in synthesis methods, such as selective etching or thermal processing, may facilitate the accurate formation of vacancies, resulting in predictable structural effects and maintaining crystallinity and stability. iii. Improvements in doping methods, including post-synthesis alteration and in situ doping, will provide enhanced regulation of dopant distribution and concentration. iv. The integration of highly crystalline MOFs with amorphous or semi-crystalline phases or incorporating specific, adjustable dopants in high-crystalline MOFs to establish localized sites with improved accessibility to active sites can enhance the crystallinity of MOF-based SACs.

5. Conclusions

This paper thoroughly details the progress in the domain of SACs, highlighting the essential function of substrate engineering in improving electrocatalytic efficiency across many reactions, including ORR, OER, HER, water splitting, CO₂RR, NRR, and NO₃RR. This review elucidates how substrate engineering is fundamental for attaining high catalytic efficiency and stability in SACs, emphasizing substrate properties like electronic structure, surface morphology, and chemical composition, which profoundly affect the interaction between SAs and their support matrix, thereby determining their catalytic behavior. We have entailed a comprehensive analysis of various substrates, including carbon, metal oxide, alloy-based, TMD-based, MXenes, and MOF substrates, each providing distinct properties that enhance the performance and functionality of SACs.

Additionally, we have discussed synthesis strategies, classified into bottom-up and top-down methodologies. Bottom-up strategies encompass techniques such as the impregnation method, pyrolysis, the ion exchange method, ALD, and electrochemical deposition. Top-down strategies encompass the host-guest strategy, atom trapping method, ball milling method, CVD, and abrasion technique, all aimed at developing SACs. Both approaches are evaluated for their capacity to attain uniform dispersion and robust anchoring of SAs on substrates, hence ensuring long-term stability and elevated activity. Furthermore, we have reviewed the engineering of these substrates through various strategies, including morphology and vacancy engineering, heteroatom doping, facet engineering, and crystallinity control in multiple electrocatalytic reactions. These approaches are utilized to improve interactions with SAs, prevent aggregation, and sustain atomic dispersion under reaction conditions while underscoring their critical role in optimizing SAC performance. Finally, we have outlined several problems and future perspectives in the advancement of SACs, connecting fundamental understanding with practical applications.

6. Challenges

Substantial progress has been achieved in the catalytic efficiency of SACs in recent years by the application of efficient synthesis techniques. Significant obstacles persist in attaining a more accurate and logical design of CE for SACs, potentially obstructing further advancement. These challenges are as follows:

1. It is still difficult to study the many reactions of intermediate species adsorbed on the surfaces of catalytic reaction sites, even with the introduction of several enhanced characterization techniques to look at the influence of matrix on electrocatalytic properties.

2. The intricacies of precisely regulating the support’s electrical structure, surface morphology, and interactions with metal atoms make it extremely difficult to tune the properties of supports for SACs (Figure 11).

3. One major issue with electrocatalytic processes is the leaching of metal atoms from the substrate, which leads to the loss of active sites and decreased catalyst stability because of insufficient bonds between the metal and the support (Figure 11).

4. The support material, which acts as a ligand and interacts directly with the metal atoms in atomically dispersed metal catalysis, has a major impact on the electrocatalytic efficiency. Because different supports provide different local CEs, this dependency adds complexity and causes variations in catalytic behavior even when the same metal is used. To achieve improved catalytic performance, it is essential but challenging to comprehend and control these local interactions, such as nearby redox-active species, binding components, and coordination patterns.

5. The efficient dispersion and utilization of metal centers in SACs are limited by the absence of suitable active locations on supports for anchoring SA. Since insufficient binding sites prevent the effective distribution of metal atoms on the support, this restriction may lead to subpar catalytic activity and performance (Figure 11).

6. The compatibility between the substrate and the reactive site is still unclear despite a great deal of research and the usage of a variety of materials for SACs.

7. A limited range of supports, including metal oxides, MXenes, carbon-based, and TMDs (particularly MoS₂)-based supports, is necessary for the stabilization of the majority of SACs (Figure 11).

8. Weak metal-support interactions in SACs might result in instability, which can cause metal atoms to migrate or aggregate during catalytic processes, decreasing catalytic efficiency and impeding SACs’ long-term vitality (Figure 11).

9. In real-world applications, anchoring two or more sites on various supports to simultaneously increase electrical and catalytic activity is a significant challenge.

10. To enable higher SAC loading, a variety of synthetic methods have been developed to create uniform and adaptable single metal sites with built-in dopants and external ligands acting as anchoring points. However, little is known about how these changes impact the electrical and geometrical structures and how they therefore affect catalytic performance and selectivity.

Figure 11. Challenges and future prospects of SACs in different supports.

Figure 11. Challenges and future prospects of SACs in different supports.

7. Future Perspectives

In addition to the challenges, we have identified the following key prospects for the development of this rapidly growing field:

1. More convergent probe approaches are needed to capture transitory conditions on SACs. A greater emphasis should be placed on creating in situ characterization methods and monitoring the dynamic structure of SAs during the operational process.

2. Hybrid support materials, such as TMDs, alloys, and MXenes, can be precisely modified to optimize their properties (Figure 11).

3. Increasing MSI through functionalization, lattice strain, or dopants can improve bonding and reduce metal leaching on various supports (Figure 11).

4. Advanced computer modeling and in situ characterization approaches can provide valuable insights into the local CEs of atomically dispersed metals. These strategies can help to optimize support parameters, ensuring consistent catalytic performance across different supports.

5. Changing the supports of SACs using external variables such as temperature, pressure, light, or electric fields can improve catalytic efficiency, selectivity, and stability in energy applications (Figure 11).

6. To fully profit from monatomic catalysts, it is important to understand the compatibility between support and atomic sites and choose appropriate supports for metal atom deposition.

7. New functional support materials with complex surface topologies, including hierarchical and porous designs, are urgently needed (Figure 11).

8. Increasing MSI through surface modifications, chemical interactions, or dopants can increase metal atom stability and dispersion in SACs (Figure 11).

9. Novel support modification strategies, such as enhanced roughness, optimized morphology, and tailored surface chemistry, are of great interest. These modifications can create highly active reactive sites, allowing for stable and accurate attachment of dual or multimetal centers.

10. Engineering approaches, such as adding flaws or specialized anchoring locations, can improve SAC efficiency, durability, and catalytic efficacy (Figure 11).

11. To develop effective catalysts, real-time investigation of reaction intermediates, catalytic active sites, and fundamental mechanisms, as well as in operando or situ characterization techniques, are crucial.