REVIEW www.advancedscience.com MXene-Supported Single-Atom Electrocatalysts Jianan He, Joshua D. Butson,* Ruijia Gu, Adrian Chun Minh Loy, Qining Fan, Longbing Qu, Gang Kevin Li,* and Qinfen Gu* MXenes, a novel member of the 2D material family, shows promising potential in stabilizing isolated atoms and maximizing the atom utilization efficiency for catalytic applications. This review focuses on the role of MXenes as support for single-atom catalysts (SACs) for various electrochemical reactions, namely the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO2RR), and nitrogen reduction reaction (NRR). First, state-of-the-art characterization and synthesis methods of MXenes and MXene-supported SACs are discussed, highlighting how the unique structure and tunable functional groups enhance the catalytic performance of pristine MXenes and contribute to stabilizing SAs. Then, recent studies of MXene-supported SACs in different electrocatalytic areas are examined, including experimental and theoretical studies. Finally, this review discusses the challenges and outlook of the utilization of MXene-supported SACs in the field of electrocatalysis. 1. Introduction Fossil fuels are primary energy sources widely used in many as- pects of modern life, which, however, results in significant emis- sions of greenhouse gases.[1] The development of renewable en- ergy and CO2 transformation technologies plays a vital role in realizing a carbon-neutral landscape.[2] Among current methods, electrocatalysis offers a sustainable and efficient approach to pro- ducing green fuels and reducing CO2 into valuable products. [3] For instance, water electrolysis, involving splitting water into hy- drogen and oxygen, is crucial for green hydrogen production.[4] With promising and tunable catalysts, the electrolysis process can J. He, J. D. Butson, R. Gu, A. C. M. Loy, Q. Fan, L. Qu, G. K. Li, Q. Gu Department of Chemical Engineering The University of Melbourne Parkville, VIC 3010, Australia E-mail: joshua.butson@unimelb.edu.au; li.g@unimelb.edu.au Q.Gu Australian Synchrotron ANSTO 800BlackburnRd, Clayton, VIC 3168, Australia E-mail: qinfeng@ansto.gov.au The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/advs.202414674 © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/advs.202414674 be achieved at low overpotential, realizing high energy efficiency. Additionally, a suit- able catalyst can help to improve the in- trinsic activity and Faradaic efficiency to- ward producing NH3. [5] The electrocatalytic CO2 reduction process, transforming CO2 into gas or liquid value-added carbon-based products, promotes the recycling of CO2 as a carbon-neutral feedstock.[6] The elec- trochemical activity and selectivity of CO2 conversion to specific products are nor- mally realized by engineering the electro- catalysts and the microenvironment be- tween catalysts and electrolytes.[7] During these electrocatalysis processes, including the hydrogen evolution reaction (HER), oxy- gen evolution reaction (OER), oxygen re- duction reaction (ORR), and carbon dioxide reduction reaction (CO2RR), catalysts play important roles in reaction performance and product rates. It is highly desirable to pursue endeavors in catalyst design and investigate novel materials and structures to alleviate the kinetic limitations observed in electrocatalytic pro- cesses. The key goals in this field are to enhance the efficiency and performance of catalysts for these reactions across a range of applications. Accordingly, significant efforts have been devoted to developing novel catalysts, such as Pt group metals,[8] metal carbides,[9] metal nitrides,[10] metal oxides,[11] metal sulfides[12] and metal phosphides.[13] Among them, platinum (Pt) is well known as the most effective catalyst for the HER and ORR,[8a,b] while ruthenium (Ru) possesses outstanding activity as an OER catalyst.[8c–e] They also show outstanding stability under strong acidic conditions. However, the application of precious metal cat- alysts is hindered by limited abundance and high costs, hence the atom efficiency needs to be improved. Alternatively, efficient non-noble metal catalysts are being increasingly employed to provide cost-competitive performance due to the low cost and rapid transfer of electrons.[14] There are still some challenges in the development of non-precious catalysts. One is lower activity than noble metals, leading to limited efficiency and increased en- ergy consumption.[15] Another is the insufficient stability of these catalysts, and elements might leach and aggregate during the reaction.[15b,16] To address these issues, size reduction of active catalysts is proven to be an effective strategy to maximize atom efficiency and maintain good catalytic performance. Single-atom catalysts (SACs), a class of material withmononu- clear metal complexes anchored on supports, have provided novel insights into creating economical and high-performance catalysts. Downsizing nanoparticles (above 1 nm)[17] into Adv. Sci. 2025, 12, 2414674 2414674 (1 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedscience.com mailto:joshua.butson@unimelb.edu.au mailto:li.g@unimelb.edu.au mailto:qinfeng@ansto.gov.au https://doi.org/10.1002/advs.202414674 http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ www.advancedsciencenews.com www.advancedscience.com nanoclusters (several to hundreds of atoms),[18] and even single atoms (SAs) can improve the atomic dispersion and maximize atomic efficiency for both noble and non-noblemetal catalysts.[19] Zhang et al. studied the impact of size on Nb-in-C through theo- retical and experimental approaches. By density functional theory (DFT) calculation, they showed that the electrons on the niobium SAs incorporated into the graphitic layers, leading to increased density of states (DOS) near the Fermi surface. According to fur- ther analysis of DOS, it is demonstrated that niobium SAs had much higher values than larger niobium particles (a niobium- terminated {111} surface and bulk niobium). This means the niobium SAs has superior catalytic potential than niobium nanoparticles. Experimentally, the ORR property of niobium SAs-in-C and the niobium nanoparticle-in-C were tested in an O2-saturated 0.1MKOH solution. The catalyst with niobium SAs shows a higher kinetic-limiting current density (12.3 mA cm−2 at −0.5 V) and quicker reaction pathway (4e−) than the catalyst with niobium particles (0.912 mA cm−2 at −0.5 V, 2e−), which means a better catalytic activity.[20] Additionally, in comparison to nanoclusters and nanoparticles, SAs exhibit unique active sites, consistent activity at each catalytic center, tunable mor- phology, and flexible electronic structures, which leads to a lower energy barrier of process and adjustable selectivity for various reactions.[21] There are many approaches to designing and constructing SACs, including wet chemistry,[22] atomic layer deposition,[23] and electrochemical deposition.[24] Different types of materials are employed as supports for anchoring SAs, such as carbon materials,[25] metal oxides,[26] metal-organic frameworks,[27] and zeolites.[28] The catalytic activity, selectivity, and mass loading of SACs are strongly influenced by the inter- action and coordination between metal atoms and neighboring non-metallic atoms of the support.[29] Additionally, due to the high surface energy of SAs, they are prone to agglomeration during catalytic processes.[30] Therefore, developing SACs with outstanding catalytic stability is another crucial aspect of their industrial applications. So far, constructing strong metal-support interactions (SMSIs) has been the primary strategy for synthe- sizing stable SACs. The strong surface interactions effectively help prevent aggregation and modify the electronic structure of electroactive sites to further enhance the catalytic stability of SAs through electronic metal-support interactions. Overall, suitable support is critical for efficient and stable SACs.[31] Since the discovery of graphene, 2D materials have attracted great attention as a powerful platform to support SAs, also in- cluding graphitic carbon nitride (g-C3N4), metal carbides (e.g., Mo2C, WC, and MXenes), and transition metal dichalcogenides (TMDs).[32] Compared with 3D catalysts, 2D materials provide several advantages due to their uniquemorphology, large specific surface area, numerous coordination-unsaturated sites, abun- dant intrinsic defects, and tunable electronic structure.[33] As a new type of 2D material, the 2D configuration of MXenes, comprising diverse forms of transition metal carbides, nitrides, and carbonitrides, demonstrates the potential to serve as a sup- porting platform for SAs. Besides the general advantages of 2D materials, MXenes possess many unique properties includ- ing tunable surface chemistry, excellent electronic conductivity, high hydrophilicity, and good mechanical properties. Unlike 2D metal oxides and g-C3N4, graphene and MXenes can be highly conductive.[34] In particular, MXene-supported catalysts show su- perior charge transfer kinetics compared to those supported by reduced graphene oxide (rGO). This is due to the fact that the properties of MXenes can bemore easily regulated through func- tionalization, alloying, and adjusting the chemical composition, resulting in enhanced interfacial and electronic interactions be- tween SAs and MXenes.[35] Specifically, for anchoring SAs, MX- enes have intrinsically electronegative surface functional groups that can adsorb cations and provide active sites. The inherent de- fects created during the etching process can also be used as sites to introduce heteroatoms.[36] This review begins by examining the characteristics and syn- thesis techniques of MXenes to elucidate the benefits of MXene support materials. Next, methods for anchoring SAs ontoMXene surfaces are explored and the sophisticated characterization tech- niques used to study isolated atoms on MXenes are discussed. Subsequently, a comprehensive discussion is provided on the re- cent applications of MXene-supported SACs in electrocatalysis. Finally, the future challenges and prospects of MXene-supported SACs for electrocatalysis are outlined. 2. MXene-Supported Single-Atom Materials 2.1. Characteristics of MXenes as Supporting Substrates Throughout the synthesis of MXene-supported SACs, support materials play a vital role in offeringmechanical stability, enhanc- ing atomic-active site efficiency, and accelerating charge transfer kinetics.[37] Therefore, an ideal supportmaterialmust exhibit sev- eral key properties, including the ability to prevent atom aggre- gation, high conductivity to facilitate electron transfer between the doped atoms and the support, strongmechanical and electro- chemical stability during oxidation-reduction reactions, and good hydrophilicity.[38] MXenes, 2D transition metal carbides and ni- trides, are new additions to the family of 2Dmaterials. Generally, MXenes can be synthesized by selectively etching a precursor of nanolaminate materials (MAX phases).[39] The general formula of MAX phases can be written as Mn+1AXn (n = 1–4), where M represents a left-side transition metal (TM), A represents a right- side TM or p-block element, and X denotes carbon and/or ni- trogen. The A atoms are etched to produce a MXene (Mn+1XnTx) with T surface functional groups, where T depends on the etching method. Figure 1 illustrates the compositions of MAX, MXenes, andMXene-supported SAs. MXenes have drawn considerable at- tention as a promising support material with the unique proper- ties of high conductivity, abundant vacancies for dopant atoms, and high redox activity.[40] First, the electronic structure, surface functional groups, and other properties can be manipulated through various syn- thetic conditions. Compared to other support materials such as graphene, MXenes possess easily adjustable electronic energy levels, charge mobility, and conductivity with a unique sandwich structure (X as the central layer and M as the outer layers, sur- rounded by terminal groups).[41] The electronic properties ofMX- enes mainly depend on the outer metal layer and surface func- tionalized substituents. MXenes can be metalloids, semiconduc- tors, topological insulators, or high insulators depending on the M metal and terminal groups.[42] For instance, the SMSI can be tunable. When the A atoms are substituted with surface groups such as -F, -Cl, -O, or -OH, they receive one or two electrons Adv. Sci. 2025, 12, 2414674 2414674 (2 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 1. The periodic table shows elements involved in the formation of MAX phases and MXenes in previous publications. Light blue: M atoms; gray: A atoms; dark green: X atoms; purple: (T) elements; solid circles: SAs discussed in the present work with experiments; dotted circles: SAs discussed in the present work with theoretical calculations. from each M metal atom. This results in the formation of a new bonding orbital by hybridizing the p and d orbitals of the M atoms, which ultimately leads to a reduction in the Fermi level. This is more difficult to achieve for other 2D materials.[43] Typically, MXenes have higher electrical conductivity compared to other 2D materials (10–100 S cm−1),[44] where Ti-based MX- enes have demonstrated conductivities of up to 24 000 S cm−1.[45] Due to the direction-dependent nature of electrons and holes, the chargemobility ofMXenes is anisotropic, with greater conductiv- ity within the plane than perpendicular to it.[46] Second, the surface of MXene layers is usually chemically sta- ble, although carbide MXenes are generally more stable than ni- tride MXenes.[47] Also, the stability of MXenes is typically de- termined by the terminal groups. Generally, the -F group with weak chemical bonding is prone to substitution by other groups; the -OH group easily decomposes. MXenes with the -O groups are the most stable, which benefits catalyst application in water electrolysis.[48] The remarkable stability of MXenes helps them to withstand harsh environments during the synthesis of MXene- supported SACs and the challenging electrochemical reaction conditions. Third, MXenes exhibit excellent hydrophilicity because of the abundant surface groups such as -F, -Cl, -O, and -OH.[49] Gas products generated on the surface of electrocatalysts can obstruct further reactions and reduce the real active sites.[50] It is impor- tant to accelerate bubble separation from the electrode surface to boost catalytic activity. Water molecules are attracted to hy- drophilic surfaces, which help to release gas bubbles and improve performance. Therefore, hydrophilicity is considered a key factor in optimizing catalysts.[51] The distinctive characteristics of MXenes render them appro- priate as support materials for SACs. With its 2D structure, high electrical conductivity, large surface area, tunable surface chem- istry, and chemical resilience, MXenes are proven to be an adapt- ablematerial for such applications. Consequently, understanding the synthesis of MXenes contributes to the future development of MXene-supported materials tailored for specific applications. 2.2. Synthesis Routes Toward MXenes SAs are anchored on substrates by coordination bonds, so they can be designed with a specific coordination environment for the target process.[52] MXenes can be synthesized by removing the A atoms in the MAX phase, which generates suspended bonds on the surface of M atoms. The surface terminal groups and mor- phology of MXenes are controlled by various synthesis methods, impacting the ability to immobilize SAs. Therefore, selecting an appropriate etching method is crucial. To further elucidate the advancement synthesis method, chronological progress over the last two decades is discussed as follows. 2.2.1. Fluoride-Based Etching Methods In 2011, the first MXene, Ti3C2Tx (T = -OH, -F), was synthesized by Naguib et al. by removing Al from Ti3AlC2 in hydrofluoric acid (HF) at room temperature (Figure 2A).[53] MXenes etched with HF typically exhibit multi-layer morphology, which can be then delaminated into single- or few-layer flakes.[54] In 2014, Ghidiu Adv. Sci. 2025, 12, 2414674 2414674 (3 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 2. A) Schematic of the exfoliation process for Ti3AlC2 with HF. Reproduced with permission.[53] Copyright 2011, Wiley-VCH. B) The reaction between Ti3AlC2 and aqueous NaOH solution under different conditions. Reproduced with permission.[60] Copyright 2018, Wiley-VCH. C) Schematic of the etching process for Ti3AlC2 with molten salt. Reproduced with permission.[61] Copyright 2019, American Chemical Society. D) Schematic illustration of the etching of Ti2AlC toward vacancy-enriched Ti2CClx MXene. Reproduced with permission.[62] Copyright 2024, Wiley-VCH. E) Schematic diagram of the synthesis by bottom-up method. Reproduced with permission.[65] Copyright 2023, The American Association for the Advancement of Science. Adv. Sci. 2025, 12, 2414674 2414674 (4 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com et al. proposed a safer, less toxic pathway to synthesizeMXenes by generating HF in situ using fluoride salts (e.g., LiF) with HCl.[55] This process allows the simultaneous intercalation of Li+ cations, increasing c-lattice parameters and enabling exfoliation into few- layeredMXenes through sonication and centrifugation. Nonethe- less, these approaches with HF directly or indirectly face chal- lenges including health hazards fromHF, limited applicability to Al-containingMAX phases, and instability of -F for atom doping. Consequently, it is crucial to explore alternative etching methods that are F-free. 2.2.2. Fluoride-Free Etching Methods In addition to the environmental benefits and operational safety, fluoride-free approaches aim to expand the types of MXenes,[56] boost conductivity,[57] enhance energy storage capacity,[58] and optimize the electrochemical activity[59] for various applications. Each method has advantages and disadvantages, and some typi- cal fluoride-free approaches are discussed in detail here. Theoretically, alkalis are a good choice for etching Al- containing MAX phases due to their strong reactivity with Al. Li et al. prepared high-purity multilayer MXene via an alkali- assisted hydrothermal method. Ti3AlC2 was soaked in 27.5 M NaOH at 270 °C to produce high purity (92%) multilayer Ti3C2Tx (T=O, OH) (Figure 2B).[60] However, the dangers of using highly concentrated alkali at high temperatures hinder its application at large scale. Li et al. first used molten salt ZnCl2 to synthesize Zn- containingMAX phases and -Cl terminatedMXenes (Figure 2C). This opened the door to explore hard-to-obtain MAX-phases via common powder metallurgy and modify MXenes with -Cl sur- face groups.[61] Lewis acid etching techniques were extended by another group to other MAX phases, expanding the range of pre- cursors available (A = Al, Si, Ga) and enabling control of surface groups (-Cl, -Br, -I, -S, -Se, and -NH) for tailored surface chem- istry in specific applications.[59] Recently, Guo et al. even syn- thesized the MXene Ti2CClx with metal chloride (ZnCl2) vapor within 5 min (Figure 2D).[62] Overall, this approach expands the potential applications of molten salt etching techniques. Never- theless, the resulting MXenes predominantly exhibit accordion- like structures, requiring further investigation to better support doped atoms. 2.2.3. Bottom-Up Synthesis of MXenes In addition to chemical etching of MAX or non-MAX phases, some bottom-up synthetic routes bypassing the MAX phase can also be employed to produce MXenes. The bottom-up methods to synthesis large-area MXenes assembles small molecules into ordered 2D layered structures via crystal growth.[63] By building the material from atomic or molecular components, it allows for more precise control over thematerial properties. Recently,Wang et al. successfully grew MXene sheets oriented perpendicular to the substrate by direct chemical vapor deposition (CVD) synthe- sis. Metals (Ti or Zr foil) were exposed to metal halides (TiCl4, ZrCl4 or ZrBr4) and an “X” source (CH4 or N2) atmosphere to produceMXenes (Ti2CCl2, Ti2NCl2, Zr2CCl2, and Zr2CBr2) at the temperature above 640 °C. This method not only offers the op- portunity for newMXenes (Zr2CCl2 and Zr2CBr2), but also diver- sifies the morphology from carpet-like to flower-like structures. Special morphology exposes fresher surface and edge sites with high catalytic activities, improving accessibility for ion interca- lation and chemical transformations which are good for future modifications and energy storage applications. The same group also demonstrated the direct synthesis of MXenes via sealing a mixture of Ti, carbon source (graphite), and TiCl4 in a quartz am- poule and heating it at high temperatures (950 °C) for 2 h. This is more efficient than traditional etching methods (24 h for in situ HF etching),[64] while the preparedMXene showed a high degree of structural perfection with sufficient -Cl coverage of the surface (Figure 2E).[65] Besides the typical methods mentioned above, several other techniques are also used for MXene synthesis, including etching with fluoride-based salts,[66] mechanical milling,[67] and etching with halogens.[68] Despite numerousmethods forMXenes prepa- ration, several MXene variants remain largely theoretical. More- over, single-layer production predominantly depends on the LiF + HCl or organic base intercalation (e.g., cetyl trimethyl ammo- nium bromide cation,[69] tetramethylammonium hydroxide[70]). Hence, improving and innovating new synthesis techniques for MXenes holds significant promise and is crucial for applica- tions as a substrate. Table 1 compares surface terminal groups and the structure of MXenes generated via various etching methods. 2.3. Synthesis of MXene-Supported SAs Because of their high surface energy, SAs tend to aggregate dur- ing synthesis and electrochemical reactions.[19a] Synthesis strate- gies based on principles including size effect, SMSI, electronic structure effect, and coordination environment effect, are vi- tal in the development of SACs.[72] There are several strategies to synthesize SACs, including wet chemistry,[73] atom trapping methods,[74] physical and chemical deposition,[24] pyrolysis,[75] and solvothermalmethods.[76] These common preparationmeth- ods are often combined with specific strategies to achieve atomic dispersion of metals with high loading, including defect engi- neering, ligand modification, and spatial confinement.[77] Due to the advantages ofMXenes as support materials, these techniques are employed to introduce precursors onto the MXene surface and transform them into SAs to synthesize MXene-supported SACs (Figure 3). The approach for incorporating SAs into MX- enes significantly influences the performance of the catalysts for particular applications. 2.3.1. Defect Vacancy Anchoring Defect engineering is an effective method to enhance the interac- tion between SAs and the substrate, which benefits the synthesis of SACs with high metal content and good stability. It has been proven that individual atoms can be fixed and stabilized by mod- ified support via pinning at electronic or structural defects as- sociated with coordinatively unsaturated sites.[78] Therefore, the concentration and stability of metal SAs intrinsically depend on Adv. Sci. 2025, 12, 2414674 2414674 (5 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Table 1.MXenes generated via various synthesized methods. Methods MXene type Functional group Structure Refs. HF etching Ti3C2TxZr3C2Tx -O, -OH, -F multi-layer,single layer [54,71] In situ HF etching Ti3C2Tx -O, -OH, -F few-layer,single layer [55] Fluoride-based salts etching Ti3C2Tx -O, -F, -N multi-layer, [66] Alkali etching Ti3C2Tx -O, -OH multi-layer,single layer [60] Mechanical milling Ti3C2Tx -O, -OH porous layer [67] Etching with halogens Ti3C2Tx -I, -Br monolayer, [68] Molten salt etching Ti3C2TxTi2CTx -O, -Cl, -S, -Se, and -NH accordion-like structures [59,61] CVD Ti2CCl2Ti2NCl2Zr2NCl2Zr2CBr2 -Cl, -Br carpet-like structures, flower-like structures [65] Direct synthesis Ti2CCl2Ti2NCl2 -Cl stack-like structures [65] the density and stability of defects. The removal of A atoms from MAX phases results in atomic vacancies such as metal vacan- cies and carbon vacancies in MXenes. With the introduction of surface functional groups, there may also be some vacancies like O-vacancies due to non-uniform distribution. These intrinsic de- fects have high reducing activity and provide promising anchor- ing sites for SAs. Zhang et al. synthesizedMo2TiC2Tx-PtSA with Pt SAs immobi- lized by Pt-C bonds on Mo vacancies, which were introduced by electrochemical exfoliation. DFT calculations show that Pt SAs Figure 3. A) Three methods for anchoring single metal atoms on MXenes; B) The advantages of MXenes to support SAs. Adv. Sci. 2025, 12, 2414674 2414674 (6 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com with positive charge comprise the adsorption position for H+ to benefit the HER.[79] In addition to metal atom vacancies such as VMo and VTi, oxygen vacancy (VO) is also generated from the transformation of surface groups. Zhang et al. designed Pt SACs on monolayer Ti3C2Tx by rapid thermal shock under an H2 at- mosphere. The PtSA anchoring on the VO of Ti3C2Tx achieved a Gibbs free energy of 0.02 eV and showed outstanding catalytic activity.[80] With the defect vacancy strategy, Ni,[81] Fe,[82] Cu, Co, Mn, Zn, In, Sn, Pb, and Bi SAs[83] were also successfully an- chored on MXenes. 2.3.2. Strong Metal-Support Interaction Besides anchoring on vacant sites, SAs can also be immobilized on supports with specific sites by strong metal-support interac- tion. Ramalingam et al. fixed Ru SAs on Ti3C2Tx through ni- trogen and sulfur heteroatom dopants.[84] A mixture of Ti3C2Tx, RuCl3·xH2O, and thiourea was freeze-dried and annealed at high temperatures. Ru SAs first interact with or are adsorbed on Ti3C2Tx by -O and -OH groups and are then stabilized with N and S dopants. Strong Ru-N (O) and Ru-S interactions were ob- served with XAFS, demonstrating electronic coupling between RuSA and the MXene via N and S atoms. This strategy has been effectively applied to Pd SAs as well, which were fixed on Ti3C2Tx via -O groups.[85] 2.3.3. Selective Atomic Substitution Due to the different redox activity of different transition metals (TMs), SAs can be introduced onto MXenes by selectively replac- ing surface atoms. Kuznetsov et al. developed a synthetic method to introduce Co SAs into Mo2CTx by Co-substitution. Initially, Co was intercalated into Mo2Ga2C, followed by the elimination of Ga through an HF etching procedure. The introduction of the Co dopant onto the MXene not only generated an additional re- action site but also modified the surface adsorption energetics, thereby impacting catalytic reaction kinetics.[86] 3. Characterization of MXene-Supported SAs A critical challenge concerning MXene-supported SACs involves characterizing the presence, distribution, and local geometric and electronic structure of SAs, as well as changes to SAs dur- ing reactions. Currently, numerous advanced techniques have emerged to study SAs at the atomic level, including high- angle annular dark-field scanning transmission electron mi- croscopy (HAADF-STEM), synchrotron X-ray absorption spec- troscopy (XAS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, theoretical calculations, and many others. Further- more, these techniques are extended to in situ characterization for studying SACs during reactions and providing detailed in- sights into reaction kinetics, reaction mechanisms, and active sites. 3.1. High-Resolution Electron Microscopy The development of advanced electron microscopes provides di- rect evidence of nanoparticles or even SAs. By employing vari- ous electronmicroscopy techniques, such as aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM), the location of SAs can be ob- served through bright and dark images. It also provides infor- mation about the coordination states of SAs, serving as a start- ing model for theoretical calculations. For instance, Zhang et al. demonstrated the successful formation of PtSA on Mo2TiC2Tx with STEM. The bright spots observed in the HAADF-STEM rep- resent heavy atoms (Pt) (Figure 4A–C).[79] Guo et al. investigated the sites of SAs (Ti vacancies) and the introduction of ZnSA and PtSA in Ti2CClx with STEM (Figure 4D–I).[62] Figure 4J–O shows a HAADF-STEM image of the NiSA@N-Ti3C2Tx, in which the light dots are firmly distributed with a distance of ≈ 0.50 nm. Moreover, STEM energy dispersive X-ray spectroscopy mapping shows the uniform distribution of Ti, C, Ni, and N elements.[87] Although STEM is pivotal for identifying SACs, distinguishing SAs with similar atomic numbers can prove challenging. Hence, complementary characterization techniques are necessary to dif- ferentiate various SAs within materials. In addition, this tech- nique can be extended to in situ STEM to trace the diffusion of SAs directly at different temperatures and specific reaction conditions.[88] 3.2. Synchrotron X-Ray Absorption Spectroscopy XAS has outstanding advantages in studying the interactions be- tween atoms in gas, liquid, and solid phases. It is widely used to determine the local structure and electronic structure which is hard to realize by traditional tools. According to the photon energy of X-rays, XAS can be classified into soft XAS (<2 keV) and hard XAS (>4 keV).[89] XAS spectra can be divided into two regions: X-ray absorption near-edge structure (XANES) and ex- tended X-ray absorption fine structure (EXAFS). XANES, within 30–50 eV of the absorption edge, is sensitive to the oxidation state and coordination chemistry.[90] Thus, it can provide infor- mation on the atomic valance state with the position and shape of the absorption edge. EXAFS, in the range from 50 to 1000 eV or more, is caused by a single-scattering or multi-scattering pro- cess of photoelectrons with neighboring atoms. Therefore, it can provide information on bond length and the number of coordi- nation species. Usually, these two forms of characterization can be assessed in the same scan. A simple single scan provides comprehensive detailed information, including oxidation states, bond lengths, bond angles, and the coordination number of ac- tive atoms. For SAs on MXenes, the dispersed SAs can be iden- tified by the metal-metal bonds in EXAFS spectra. XANES can further reveal the electronic and geometric coordination environ- ment of SAs by comparisonwith reference samples. For example, Guo et al. employed EXAFS to analyze Pt SAs and Ti2CClx. The Pt L3-edge XANES spectra reveals that the absorption edge position of Ti2CClx–PtSA lies between those of Pt and PtO2, suggesting a positive charge on the Pt SAs in Ti2CClx–PtSA (Figure 5A). From the Fourier transform (FT) of Pt L3-edge EXAFS (Figure 5B), it showed the existence of Pt-C (1.9 Å) instead of Pt-Pt bonds (2.7 Å). The wavelet transform (WT) shows an intensity maxima of 3 Å−1 (Pt-C) and 5 Å−1 (Pt-Ti), implying that Pt is atomically dispersed in Ti2CClx (Figure 5C). [62] Zhu et al. verified the atomic struc- ture of Pd in Pd1-Ti3C2Tx via Pd K-edge EXAFS. The major peaks Adv. Sci. 2025, 12, 2414674 2414674 (7 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 4. A) HAADF–STEM image of Mo2TiC2Tx–PtSA. B) Magnified HAADF–STEM image of Mo2TiC2Tx–PtSA and its corresponding simulated image, and illustration of the structure of Mo2TiC2Tx–PtSA, showing the isolated Pt atoms (circled). C) STEM–EDS elemental mapping of Mo2TiC2Tx–PtSA nanosheets. Reproduced with permission.[79] Copyright 2018, Springer Nature. D) Atomic-resolution HAADF-STEM image of Ti2CClx MXene. E) Line intensity profiles obtained from the rectangular regions in (D), showed the existence of Ti vacancies. F) Atomic-resolution HAADF-STEM image of Zn–Ti2CClx. G) Line intensity profiles obtained from the rectangular regions in (F), showed the existence of single-atom Zn. H) Atomic-resolution HAADF-STEM image, and I) the experimental and simulated HAADF-STEM images of Ti2CClx–PtSA. Reproduced with permission.[62] Copyright 2024, Wiley-VCH. J) HAADF-STEM image of NiSA@N-Ti3C2Tx. K) Corresponding EDS analysis. L) and M) Atomically resolved HAADF-STEM images. N) 3D atom-overlapping Gaussian-function fitting mapping for the g area in (L) illustrating the atomic structure and arrangement of NiSA@N-Ti3C2Tx. O) Corresponding profiles of the h area in (M). Reproduced with permission.[87] Copyright 2024, The Royal Society of Chemistry. Adv. Sci. 2025, 12, 2414674 2414674 (8 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 5. A) Pt L3-edge XANES spectra, B) Fourier transforms of Pt L3, and C) WT Pt L3-edge EXAFS spectra for Pt foil, PtO2, and Ti2CClx–PtSA. Reproduced with permission.[62] copyright 2024, Wiley-VCH. D) Pd K-edge EXAFS spectra of Pd foil, PdO, and 0.5% Pd1-Ti3C2Tx. E) EXAFS fitting curves for 0.5% Pd1-Ti3C2Tx. Reproduced with permission.[85] Copyright 2024, Elsevier. The experimental Ni K-edge F) XANES and G) EXAFS spectra of Ni SA@N-Ti3C2Tx and counterparts. Reproduced with permission.[87] Copyright 2024, The Royal Society of Chemistry. H) Normalized operando Ru K-edge XANES spectra for SA Ru-Mo2CTX under various conditions (applied voltage versus RHE) in 0.5 M K2SO4 solution, insert is the magnified image. I) The corresponding FT-EXAFS spectra derived from (H). J) The oxidation state of Ru and radial distance of the main peak under various conditions. Reproduced with permission.[92] Copyright 2020, Wiley-VCH. Adv. Sci. 2025, 12, 2414674 2414674 (9 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com at 1.78 and 2.48 Å are attributed to Pd-O and Pd-Ti bonds, re- spectively, with no peaks for Pd-Pd bonds (Figure 5D). By fur- ther fitting the EXAFS results, the coordination numbers of the Pd-O and Pd-Ti bonds were calculated to be 3.5 and 1.5, respec- tively. These results confirmed the presence of Pd SAs in the Pd1- Ti3C2Tx catalyst (Figure 5E). [85] Yang et al. studied the local coor- dination environment of Ni atoms in NiSA@N-Ti3C2Tx. The ad- sorption edge position of Ni atoms, which is betweenNi andNiO, indicates that the electronic structure of Ni𝛿+ (0 < 𝛿 < 2) is due to Ni-support interactions (Figure 5F). With the FT-EXAFS anal- ysis, a peak at 1.45 Å proved that Ni atoms were connected to the substrate by Ni-C/N bonds (Figure 5G).[87] In addition to studying catalyst structure, operando XAS is also applied in the study of catalytic mechanisms, which provides the opportunity to monitor the behavior of active sites in MXene- supported SACs at the atomic scale.[91] For instance, Peng et al. studied the intrinsic catalytic activity of RuSA@Mo2CTx for the electrochemical nitrogen reduction reaction (NRR) with operando XAS. First, RuSA@Mo2CTx was tested in N2-saturated andAr-saturated electrolytes respectively at open-circuit potential (OCP). As shown in Figure 5H–J, this catalyst exhibited higher energy of the RuK-edge, increased Ru oxidation state, and shorter radial distance in N2-saturated electrolytes. These changes may be attributable to the delocalization of unpaired electrons in the Ru 3d orbital, charge transfer from the Ru atom to the N atom, and the presence of Ru-N bonds. Second, RuSA@Mo2CTx was tested at -0.3 V versus RHE in N2-saturated electrolytes. When a potential was applied, the K-edge shifted to a lower energy, the oxidation state decreased, and the radial distance (R) increased (Figure 5H–J). These results demonstrated the continuous reduction capability of RuSA centers for N2. The Ru SAs work highlights important roles in catalytic intermediate adsorption and electron back-donation centers during the NRR process.[92] While this technique shows many advantages, it is difficult to identifymetal ormetal oxide clusters in samples with SAs by XAS only, and other assisted characterization techniques are needed. 3.3. X-Ray Photoelectron Spectroscopy XPS, a highly surface-sensitive method, can provide the com- position and electronic states of elements on the surface of catalysts.[93] For SACs, the SAs are dispersed in an unsaturated state and interact with the substrate, which is different from pure metals or metal oxides. By introducing suitable reactants and re- action temperatures to the XPS chamber, it can be upgraded to near-ambient pressure XPS (NAP-XPS), realizing the characteri- zation of electrocatalysts under the reaction atmosphere. The el- ement composition and surface states of Ti2CClx were charac- terized by XPS (Figure 6A–D). Compared with the Ti2CCl2 pre- pared in molten salts (MS-Ti2CCl2), there was no signal for Zn in Ti2CCl2 synthesized by ZnCl2 vapor, indicating the pure MX- ene with no metal impurities. Additionally, the peak shift of Ti in as-prepared Ti2CCl2 revealed a higher oxidation state gener- ated from Ti vacancies.[62] Liu et al. characterized the successful preparation of CuSA@N-Ti3C2Tx with XPS (Figure 6E–H). The binding energy of C in CuSA@N-Ti3C2Tx was higher than that in CuSA@Ti3C2Tx, indicating the influence of N atoms doping. The Cu spectrum was deconvoluted into Cu2+ and Cu0 peaks, reveal- ing a positive charge on the Cu atoms in CuSA@N-Ti3C2Tx. [94] However, the information obtained from XPS can only be used as supporting evidence and other techniques are needed to draw a definitive conclusion regarding the presence of SACs. In addition, the in situ NAP-XPS technique plays a crucial role in studying the behavior and thermal stability of atoms in cata- lysts. This enables the exploration of dynamic modification at a single atom-support surface, guiding sophisticated defect design for future SACs. It can also be applied to Ti3C2Tx to detect the bonding environment and terminations.[95] 3.4. Raman Spectroscopy Raman spectroscopy (ex-situ and operando) is a popular tool to obtain the structure of MXenes, where the A1g and Eg vi- brational modes represent out-of-plane and in-plane vibrations, respectively.[96] It also shows the presence of SAs onMXenes and identifies their anchoring sites by imaging modes. Moreover, it reveals the structure of SAs onMXenes by peak shift and changes in peak area ratio.[97] In situ Raman spectroscopy is an especially powerful technique to trace the intermediates and the changes in the structure of MXene-supported SACs under reaction condi- tions and identify the electrocatalytic centers.[98] Zhang et al. con- firmed the volumetric changes from Mo2TiAlC2 to Mo2TiC2Tx with peak changes in Raman spectra. The typical peaks at 178, 209, 607, and 708 cm−1 for Mo2TiAlC2 vanished, while strong peaks for Mo2TiC2Tx at 300–400 and 650 cm−1 were observed. The Raman spectra of Mo2TiC2Tx-PtSA and Mo2TiC2Tx delami- nated by tetrabutylammonium hydroxide (TBAOH) showed the same peaks, demonstrating the successful exfoliation process (Figure 7A–C).[79] Zou et al. investigated time-resolved informa- tion of RuSA@Ti3C2Tx during the HER in alkaline electrolyte for mechanistic investigations with in situ Raman spectroscopy.[99] The A1g peak shift of RuSA@Ti3C2Tx was smaller than that of Ti3C2Tx, indicating a lower surface coverage of the -OH func- tional groups (Figure 7D–I). This showed that Ru SAs enhanced the adsorption of H, facilitating H2 generation at a lower poten- tial and further reducing the protonation of exposed -O to -OH. 3.5. Theoretical Calculations In addition to the above characterizations, theoretical calcula- tions also provide vital contributions to understanding SACs with atomistic precision, a feat sometimes challenging to at- tain through experimentation.[100] First-principles calculations, the most widely used method to simulate SACs, were employed to study the origin of outstanding HER performance for the Ti2CClx-PtSA catalyst. Based on first-principles calculations, the hydrogen adsorption-free energy of Ti2CClx-PtSA (0.016 eV) is closer to zero than that of 10% Pt/C (-0.137 eV), meaning a more favorable hydrogen adsorption-desorption process (Figure 8A). The density of states (DOS) results, reflecting the electronic structure of materials, show that Ti2CClx-PtSA has a higher oc- cupied state around the Fermi level than Ti2CClx, resulting in good electrical conductivity and electrocatalytic performance (Figure 8B). This is consistent with the electrochemical results, where Ti2CClx-PtSA (Eonset = 19 mV, Tafel slope = 31 mV dec−1) Adv. Sci. 2025, 12, 2414674 2414674 (10 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 6. A) XPS survey spectra of Ti2AlC, MS-Ti2CClx, and Ti2CClx. High-resolution B) Al 2p, C) Zn 2p, and D) Ti 2p XPS spectra of Ti2AlC, MS-Ti2CClx, and Ti2CClx. Reproduced with permission.[62] Copyright 2024, Wiley-VCH. E) XPS survey spectra of CuSA@N-Ti3C2Tx and CuSA@Ti3C2Tx, F) Ti 2p spectra, G) C 1s spectra, and H) Cu 2p spectra. Reproduced with permission.[94] Copyright 2024, Springer Nature. Adv. Sci. 2025, 12, 2414674 2414674 (11 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 7. Raman spectra of A) Mo2TiC2Tx-PtSA, B) delaminated Mo2TiC2Tx, C) Mo2TiAlC2 and Mo2TiC2Tx. Reproduced with permission.[79] Copyright 2018, Springer Nature. D,E) In situ Spectro-electrochemical Raman spectra of Ti3C2Tx. F,G) In situ Raman spectra of RuSA@Ti3C2Tx. H,I) Schematic diagram of RuSA@Ti3C2Tx surface under operando HER conditions. Reproduced under the terms of the CC-BY 4.0 license.[99] Copyright 2022, Wiley- VCH. shows superior HER activity than 10% Pt/C (Eonset = 29mV, Tafel slope = 32 mV dec−1).[62] Especially for MXene-supported SACs, which are at the development stage, first-principles calculations help to reveal their potential as electrocatalysts and give guid- ance for designing materials. Cao et al. investigated the coordi- nation effect of MXene-supported SACs for electrochemical car- bon dioxide reduction reaction (ECO2RR) applications with DFT calculations. Based on the charge density difference of TM SAs on NS/NN-Ti3C2O2 (Figure 8C), electrons are transferred from TMs to O atoms and N or S bonds, which can enhance the stabil- ity of TMs and optimize electronic structure to further improve ECO2RR performance. The reaction mechanism of four ideal ECO2RR catalysts toward HOOH was explored with reaction en- ergy (ΔG). Negative ΔG means energetically favorable; positive ΔG means energy-requiring; the step with the greatest ΔG is the potential determining step in the reaction process. Therefore, V/Fe-NN-Ti3C2O2, which consumes less energy, was predicted to exhibit superior catalytic activity with formation of *OCHOas the potential determining step (Figure 8D–G).[101] Classical Molec- ular dynamics (MD) based on potential functions is a popular simulation method to model the formation of SAs.[102] Cheng et al. examined the thermal stability of Zn/Mo2CO2-𝛿 with MD Adv. Sci. 2025, 12, 2414674 2414674 (12 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 8. A) DFT-calculated hydrogen adsorption Gibbs free energy of Ti2CClx–PtSA and 10% Pt/C for HER. B) DOS of Ti2CClx and Ti2CClx–PtSA. Reproduced with permission.[62] Copyright 2024, Wiley-VCH. C) The charge density differences of TMs anchored onto NS/NN-Ti3C2O2 supports. The charge depletion and accumulation are depicted by cyan and yellow, respectively. The isosurface value is 0.005 e/A3. The reaction mechanisms of ECO2RR to HCOOH under 0 V versus SHE and operando conditions on D) Cr-NS-Ti3C2O2, E) Ti-NN-Ti3C2O2, F) V-NN-Ti3C2O2, and G) Fe-NN- Ti3C2O2. Reproduced with permission.[101] Copyright 2024, Elsevier Ltd. Adv. Sci. 2025, 12, 2414674 2414674 (13 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com and provided insights into its dynamic behavior at different tem- peratures. This study highlights the potential of Zn/Mo2CO2-𝛿 for low-temperature CO oxidation catalysis.[103] However, precise predictions can only be made when the simulated model is close to real conditions. Instead of using typical first-principles calcu- lations based on the constant charge model, which cannot al- ways accurately describe the electrochemical interface, the con- stant potential model has been developed to simulate real elec- trochemical systems. Ji et al. understand optimal Ru SAs site and expand the theoretical exploration under the constant potential model.[104] In summary, various methods can be utilized to characterize the SAs within SACs, such as STEM, XAS, XPS, Raman, first- principal calculations, and MD, among others. However, as each method presents its strengths and weaknesses, integrating mul- tiple techniques is essential to arrive at a robust conclusion. Addi- tionally, the characterizations and theoretical studies of MXene- supported SACs based on these techniques are limited and need more effort to explore the accurate position of SAs, the relation- ship between the SAs and support materials, the behavior of SAs during electrochemical catalytic processes, and the possi- ble promisingMXene-supported SACs for different applications. More in-depth research could offer valuable guidance for future studies on MXene-supported SACs. 4. Catalytic Applications of MXene-Supported SACs As 2D materials, MXenes possess remarkable properties that make them highly suitable for electrocatalytic applications. Their inherentmetallic conductivity and hydrophilicity contribute to ef- ficient charge transfer and favorable interactions with aqueous environments, respectively. Moreover, the unique MXene struc- ture maximizes the exposure of active sites on the surface, fur- ther enhancing the catalytic performance. Thus, pristine MXene materials are promising alternatives for catalytic reactions. For instance, Ti3C2Ox shows a good HER performance compared with typical support materials with low overpotential (190 mV @ 10 mA cm−2) and low Tafel slope (60.7 mV dec−1).[105] Single- layer Ti3C2 exhibited a desirable ORR performance and stabil- ity in 0.1 M KOH by a four-electron pathway.[106] Parui et al. predicted Ti2C(OH)2 as a CO2RR catalyst with high reactivity and selectivity to yield formic acid.[107] By introducing SAs onto the MXene structure, the catalytic efficiency can be significantly boosted. SA doping not only provides additional active sites for catalytic reactions but also allows precise control over the cat- alytic activity and selectivity. This level of control and enhance- ment often surpasses that achieved with conventional commer- cial catalysts, making MXene-supported SACs highly promising for a wide range of electrocatalytic applications, including wa- ter splitting, CO2 reduction, and nitrogen reduction. Here, we delve into recent advancements in MXene-supported SACs for electrocatalysis, encompassing studies on the HER, OER, ORR, CO2RR, and NRR, spanning both theoretical and experimental perspectives. 4.1. MXene-Supported SACs for the HER Hydrogen is considered one of the most promising candidates for the next generation of energy since it has a high energy den- sity without carbon emissions.[108] When it comes to hydrogen production, the HER via water splitting stands out as a cost- effective and environmentally friendly approach.[109] During this process, electrocatalysts play an important role and Pt is a near- ideal HER electrocatalyst. However, scarcity and high cost limit its large-scale usage in industrial applications. There are two strategies to overcome the challenges of Pt catalysts for the HER. One approach is to develop active and non-noble alternatives to Pt such as transitionmetal sulfides, phosphates, carbides, and ni- trides. MXenes possess high carrier mobilities and intrinsic lay- ering, which benefit the HER process.[110] However, the intrin- sic electrocatalytic activity for reported MXenes is poor, where Mo2CTx with high activity shows an overpotential of 283 mV at 10 mA cm−2 in a 0.5 M H2SO4. [111] The other approach is to im- prove the active site utilization efficiency by limiting their size to clusters or even SAs. To maximize the benefits of these two strategies, many researchers try to disperse SAs on ideal sup- port materials for high-performance HER catalysts. The pres- ence of heteroatoms onMXenes can provide new active sites and change the electronic structure, which affects the free energy of hydrogen adsorption and improves the performance of the HER process.[112] To guide the design of MXene-supported SACs for the HER, many researchers employ theoretical analyses. According to DFT studies, many MXene-supported SACs exhibit potential for HER activities in acidic and alkaline electrolytes, such as Ir/Ru/Pt- v-V2CCl2, [113] Fe/Cr/Cu@Nb4C3O2, [114] and Cl/Br/I-Mo2CO2 [115] etc. Wang et al. tuned the coordination microenvironment of V2CTx-based SACs (T = O, F, S, and Cl) by DFT calculations. They also showed that different surface groups play important roles in catalytic activity. Especially, the smallΔGH of Ir-v-V2CCl2 (0.01 eV), Ru-v-V2CCl2-(0.04 eV), and Pt-v-V2CO2 (−0.08 eV) are even better than commercial Pt (−0.09 eV) (Figure 9A).[113] Šljivančanin calculated the free energy of Hadsorption to de- sign O-terminated SACs. By introducing TM and noble metal atoms, the free energy approached that of Pt. They also in- vestigated the relationship between charge transfer and lo- cal reactivity (Figure 9B).[114] Besides metal atoms, non-metal SAs are also used as active sites to improve the HER per- formance of MXene materials. Zhang et al. studied the im- pacts of single non-metal atom (Cl, Br, and I) doping on the HER activity of molybdenum carbide MXene using DFT cal- culations. With the help of Cl, Br, and I, the ΔGH is close to 0 eV, meaning superior HER performance (Figure 9C).[115] These theoretical results provide valuable insights into the con- struction and mechanism of MXene-supported SACs for the HER. Some MXene-supported SACs have been shown experimen- tally to possess superior HER performance. Various noble metal atoms have been successfully introduced intoMXenes. For exam- ple, PtSA was introduced into Mo2TiC2Tx, Ti3C2Tx/SWCNTs, and Ti3C2Tx, showing improved HER activities. In Mo2TiC2Tx–PtSA, the surface Mo vacancies work as anchoring sites for the SAs and the Pt atoms can be further stabilized by forming strong covalent bonds (Pt-C). A low overpotential of −30 mV at 10 mA cm−2 was achieved with outstanding stability for 100 h with active sites of Mo-O and Pt, resulting in a 39.5-times increase in mass activity compared to commercial Pt/C (Figure 9D–F).[79] A self-supported Ti3C2Tx@Pt/SWCNTs catalyst was also prepared, Adv. Sci. 2025, 12, 2414674 2414674 (14 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 9. A) The calculated free-energy diagrams for H adsorption on M−v−V2CT2 SACs. Reproduced with permission.[113] Copyright 2023, Elsevier Inc. B) The variation in ΔG (H) with the Group number of substitutional impurities in the Nb4C3O2 monolayer. The points corresponding to Nb (representing the pristine Nb4C3O2) and impurities giving rise to the eV are labeled. Reproduced with permission.[114] Copyright 2024, Springer Nature. C)The calculated ΔGH at the S1 and S2 sites on NM-Mo2CO2 structures. Reproduced with permission.[115] Copyright 2024, Elsevier B.V. D) Illustration of the synthesis mechanism for Mo2TiC2O2–PtSA during the HER process. E) HER polarization curves of carbon paper (CP), Mo2TiC2Tx, Mo2TiC2Tx– VMo, Mo2TiC2Tx–PtSA and Pt/C (40%), acquired using graphite rod as the counter electrode in 0.5MH2SO4 solution. F) Stability test of Mo2TiC2Tx–PtSA through potential cycling, before and after 10000 cycles. Inset: chronoamperometry curve of Mo2TiC2Tx–PtSA and Pt/C. Reproduced with permission.[79] Copyright 2018, Springer Nature. G) HER polarization curves of Ti2CTx, Ti2CClx, Ti2CClx–PtSA, and 10% Pt/C collected in 0.5 M H2SO4 electrolyte. H) Tafel plots of Ti2CTx, Ti2CClx, Ti2CClx–PtSA, and 10% Pt/C. I) The stability of Ti2CClx–PtSA. Reproduced with permission.[62] Copyright 2024, Wiley-VCH. J) Schematic illustration of the fabrication of the Co@MXene composites on an example of V2CTx; K) HER LSV curves of Co@V2CTx, Co@Nb2CTx, Co@Ti3C2Tx and Pt/C electrodes at 5 mV s−1; L) HER chronopotentiometry response of three listed electrodes at a current density of 10 mA cm−2. Reproduced under the terms of the CC-BY 4.0 license.[117] Copyright 2022, Wiley-VCH GmbH. Adv. Sci. 2025, 12, 2414674 2414674 (15 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Table 2. Summary of electrocatalytic performance of MXenes, MXene-supported SACs, and other 2D-supported SACs. HER catalysts Research type Substrate Electrolyte Overpotential 𝜂 at 10 mA cm−2 [mV] Tafel slope [mV dec−1] Stability [hour/cycle] Loading amount [wt.%] Refs. Mo2CTx(T = O, OH) E GC 0.5 M H2SO4 283 70.0 2 h / [111] E-Ti3C2Tx(F, OH, O) E GC 0.5 M H2SO4 266 109.8 / / [105] E-Ti3C2Ox E GC 0.5 M H2SO4 190 60.7 2000 / [105] E-Ti3C2(OH)x E GC 0.5 M H2SO4 217 88.5 / / [105] Ti2CClx-Pt SA E GC 0.5 M H2SO4 41 31.0 18 h/5000 0.94 [62] Mo2TiC2Tx–PtSA E CFP 0.5 M H2SO4 30 30.0 -/10 000 1.20 [79] RuSA-N-S-Ti3C2Tx E CFP 0.5 M H2SO4 0.5 M Na2SO4 0.5 M NaOH 76 275 99 90.0 / / 16 h/- 1000 4000 1.20 [84] Pd SA-Ti3C2Tx E NF 1 M KOH 154 70.0 40h 0.5 [85] Co SA@V2CTx E CC 1 M KOH 35 109.1 10h 1.08 [117] CoNi-Ti3C2Tx E CC 1 M KOH 31 33.0 100h 5.60 [118] CoSA-N-graphene E Graphene paper 0.5 M H2SO4 1 M NaOH 147 270 82.0 / 10 h/1000 10 h/1000 0.57 [136] Co/Se-MoS2-NF E NF 0.5 M H2SO4 104 29.0 360 h 10.40 [137] 20% Pt/C (Commercial) E CFP CFP 0.5 M H2SO4 1 M KOH 13 28 32.0 24.0 / / 20.00 20.00 [80,112] Ti3C2I2-Ir T / / |ΔGH|<0.09eV / stable / [40] Ti3C2Br2-Cu T / / |ΔGH|<0.09eV / stable / [40] Ti3C2Br2-Pt T / / |ΔGH|<0.09eV / stable / [40] Ti3C2Cl2-Cu T / / |ΔGH|<0.09eV / stable / [40] Ti3C2Cl2-Pt T / / |ΔGH|<0.09eV / stable / [40] Ti3C2Se2-Au T / / |ΔGH|<0.09eV stable / [40] Ti3C2Te2-Nb T / / |ΔGH|<0.09eV stable / [40] Ir-v-V2CCl2 T / / |ΔGH| = 0.01eV stable / [113] Ru-v-V2CCl2 T / / |ΔGH| = 0.04eV stable / [113] Pt-v-V2CCl2 T / / |ΔGH| = 0.08eV stable / [113] Pt(111) T / / |ΔGH| = 0.09eV stable / [40] OER catalysts Research Type Substrate Electrolyte Overpotential 𝜂 at 10 mA cm−2 [mV] Tafel slope [mV dec−1] Stability [hour/cycle] Loading amount [wt.%] Refs. Co SA@V2CTx E CC 1 M KOH 245 90.4 10 h 1.58 [117] CoNi-Ti3C2Tx E CC 1 M KOH 298 79.8 100 h 5.60 [118] Ni-O-G SACs E CC 1 M KOH 224 42.0 50 h 3.10 [138] Ni-graphene E GC 1 M KOH 301 63.0 2000cycle 0.23 [139] Pt SA-Fe-N-C E GC 0.1 M KOH 310 62.0 50 h 2.10 [140] Rh-v-V2CS2 T / / 300 / / / [113] Pt-v-V2CO2 T / / 360 / / / [113] Rh-v-V2CO2 T / / 370 / / / [113] Rh-v-V2CCl2 T / / 490 / / / [113] Pt-v-V2CCl2 T / / 490 / / / [113] IrO2 T / / 650 / / / [113] (Continued) showing an overpotential of −62 mV at 10 mA cm−2 and re- maining stable for 800 h.[116] Guo et al. also employed vacancies to design a Ti2CTx-PtSA catalyst that fixed Pt SAs on the Ti vacancies, achieving an overpotential of −41 mV at 10 mA cm−2 and good stability over 5000 LSV cycles (Figure 9G–I).[62] Ra- malingam et al. doped Ru SAs on N-S-Ti3C2Tx through the formation of Ru-N and Ru-S bonds. Due to the high electroneg- ativity and different atomic radii of N and S atoms, they served as binding sites for Ru SAs. The catalyst possessed a small ΔGH of 0.08 eV and showed good HER activity under various pH conditions (76 mV at 10 mA cm−2 in acidic electrolyte; 275 mV at 10 mA cm−2 in neutral electrolyte; 99 mV at 10 mA cm−2 Adv. Sci. 2025, 12, 2414674 2414674 (16 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Table 2. (Continued) ORR catalysts Research Type Substrate Electrolyte Overpotential 𝜂 at 10 mA cm−2 [mV] Tafel slope [mV dec−1] Stability [hour/cycle] Loading amount [wt.%] Refs. Pt-v-V2CO2 T / / 380 / / / [113] Rh-v-V2CS2 T / / 400 / / / [113] Pt-v-V2CCl2 T / / 550 / / / [113] Pd/Ti3C2S2 T / / <500 / / / [123] Pd/Ti3C2O2 T / / <500 / / / [123] CO2RR catalysts Research Type Substrate Electrolyte Product Selectivity Faradaic Efficiency Loading amount [wt.%] Refs. CuSA-Ti3C2Tx (T = O, OH,F) E CC 1 M KOH EtOH/C2H4 98 71.0% 0.20 [128] CuSA@N-Ti3C2Tx(T = O, OH,F) E GC 0.5 M KHCO3 CO >97% 97.4% / [94] CuSA@Ti3C2Tx E GC 0.5 M KHCO3 >66% / / [94] CuSA-Ti3C2Clx E GC 0.1 M KHCO3 CH3OH / 59.1% 1.00 [141] NiSA-graphene E GC 0.5 M KHCO3 CO 97% 95.0% 0.44 at. % [142] MnSA-C3N4/CNT E CC 0.5 M KHCO3 CO / 98.8% 0.17 [143] Vo-CuO(Sn) E CC 0.1 M KHCO3 CO 95% 99.9 3.15 at. % [144] Co-Mo2CS2 T / / CH4 −0.304 (UL) / / [145] NiSA-Mo2CS2 T / / CHOOH −0.269 (UL) / / [145] FeSA-Mo2CS2 T / / CO −0.245 (UL) / / [145] CrSA-Mo2CS2 T / / CH4 −0.296 (UL) / / [145] CrSA@Mo2CS2 T / / CH4 −0.450 (UL) / / [127] NiSA-Ov-Ti2CO2 T / / HCOOH −0.27 (UL) / / [146] Co-NS-Ti3C2O2 T / / HCOOH −0.23 (UL) / / [101] Ti-NN-Ti3C2O2 T / / HCOOH −0.19 (UL) / / [101] V-NN-Ti3C2O2 T / / HCOOH −0.32 (UL) / / [101] Fe-NN-Ti3C2O2 T / / HCOOH −0.08 (UL) / / [101] NRR catalysts Research Type Substrate Electrolyte Product Faradaic Efficiency Loading amount [wt.%] Refs. FeSA/Ti3C2Tx E / 0.1 M Na2SO4 NH3 82.90% 1.69 [135] RuSA-Mo2CT E CFP 0.5 M K2SO3 NH3 25.77% 1.41 [92] RuSA@rGO/NC E / HCl NH3 17.90% 0.95 [147] Fe-MoS 2 E CFP 0.5 M K2SO3 NH3 18.80% 5.3 at. % [148] Au1/C3N4 E CFP 0.05 M H2SO4 NH3 11.10% 0.15 [149] Ag/Ov-MXene T / / NH3 −0.24 (UL) / [132] Cu/Ov-MXene T / / NH3 −0.34 (UL) / [132] V1@Ti2CO2 T / / NH3 −0.17 (UL) / [150] Abbreviations: E, experiment; T, theoretical; GC, glass carbon; CFP, carbon fiber paper; NF, nickel foam, GP, graphene paper, UL, limiting potential. in alkaline electrolyte).[84] Pd SAs are also successfully doped into Ti3C2Tx with regulated reaction kinetics. The as-prepared SAC, 0.5% Pd1-Ti3C2Tx, possessed an overpotential of 154 mV at 10 mA cm−2 and a Tafel slope of 70 mV dec−1, much better than Ti3C2Tx and Pd nanoparticles in Ti3C2Tx. [85] In addition to noble metal atoms, non-noble metals have also been introduced into MXenes, achieving efficiencies comparable to noble metal-doped MXenes. Zhao et al. anchored Co atoms onto MXenes (Nb2CTx, V2CTx, and Ti3C2Tx) to enhance the HER properties. Co@V2CTx possessed an outstanding HER overpotential of −35 mV and good stability for 10 h in 1 M KOH (Figure 9J–L).[117] Zhao et al. constructed a dual-atom catalyst CoNi-Ti3C2Tx by a surface modification method, where the Co and Ni were fixed in Ti3C2Tx via metal-O or metal-N bonds. It displayed an HER overpotential of −31 mV at 10 mA cm−2 with only 6.3% degradation after 100 h at a current density of 500 mA cm−2.[118] Based on the data presented in Table 2, MXene-supported SACs exhibit a favorable HER performance when compared to other 2D-supported SACs. Adv. Sci. 2025, 12, 2414674 2414674 (17 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com 4.2. MXene-Supported SACs for the OER and ORR In addition to the HER, the OER and ORR are also indispens- able for developing clean H2. The OER, the anodic half-reaction in water splitting, affects the potential required to drive the over- all water-splitting reaction.[8c] The ORR plays an important role in proton exchange membrane (PEM) fuel cells which helps to generate electricity from clean H2. [119] The sluggish kinetics of the OER and ORR are further limitations to developing clean en- ergy, and it is essential to develop novel and efficient ORR/OER catalysts to reduce the high overpotentials.[120] Currently, noble metals (e.g., Ru and Ir), metal oxides, andmetal sulfides are com- mon OER catalysts.[121] Although these advanced electrocatalysts can achieve outstanding OER performances, they tend to experi- ence elemental leaching and particle agglomeration under high voltage conditions, which affects catalyst stability. As such, 2D MXenes are emerging as a new candidate for OER catalyst sup- port due to their unique properties. Ram et al. investigated the OER performance of 21 individ- ual TM atoms on Mo2CO2 by DFT calculations. TM atoms act as OER active sites and can exchange charge with the Mo2CO2 MXene.[122] Cao et al. designed a series of TM atoms sup- ported on Ti3C2S2, analyzed by DFT calculations. Pd/Ti3C2S2 and Ir/Ti3C2S2 were found to be promising ORR catalysts with an overpotential lower than 0.5 V. SAs fixed on Ti3C2S2 exhib- ited better catalytic activity than Ti3C2O2. [123] In addition to these widely used MXenes, Dai et al. constructed efficient bifunctional catalysts for the ORR and OER by theoretical calculations with V2CO2 and Nb2CO2. [124] All of these results provide theoretical support and guidance for designing ORR and OER electrocat- alysts with MXene materials. As shown in Figure 10A–F, the Gibbs free energy diagrams of catalysts are studied to explore the ORR/OER thermodynamics. The potential-determining step in the ORR process of Co-H1-V2CO2 and Ni-H1-V2CO2 is the sec- ond proton-coupled electron transfer step, while that of Fe-H1- V2CO2, Co-Vo-Nb2CO2, Ni-H2-Nb2CO2, and Co-H2-Ta2CO2 is the fourth proton-coupled electron transfer step. For the OER pro- cess, Fe-H1-V2CO2 and Co-Vo-Nb2CO2 are limited by the *O to *OOH step, while Co-H1-V2CO2 and Ni-H1-V2CO2 are limited by the *OH to *O step. Based on their model, Co-H2-Ta2CO2 shows the lowest potential gap value (0.53 V), meaning a good overpo- tential for bifunctional catalytic performance. Some of the MXene-supported SACs have experimentally demonstrated good ORR/OER performance. Zhao et al. fur- ther studied the electronic structure and electrocatalytic activ- ity by synthesis of Co SAs on MXenes (V2CTx, Nb2CTx, and Ti3C2Tx) via a photochemical reduction method. Interestingly, Co@V2CTx not only exhibited a promising OER performance (242 mV at 10 mA cm−2) but also possessed remarkable HER activity (35 mV at 10 mA cm−2). This is due to the high elec- tron transfer in Co@V2CTx, which redistributes the electronic structure of Co and lowers the energy barriers in the HER and OER processes.[117] They also extended it to dual-atom Co/Ni catalysts doped on MXenes (Figure 10G–K). The synthe- sized CoNi-Ti3C2Tx displayed an OER overpotential of 241 mV and high stability for 100 h at an industrially relevant current density (500 mA cm−2).[118] As indicated in Table 2, MXenes demonstrate a notable ability to accommodate a higher density of SAs, thereby, enhancing the OER performance in alkaline electrolytes. 4.3. MXene-Supported SACs for the CO2RR and CORR Significant CO2 emissions from burning fossil fuels have con- tributed greatly to global climate change. Besides considering al- ternative clean energy sources,many researchers focus on captur- ing and transforming CO2 into valuable resources to address this issue. Among them, the CO2 electrocatalytic reduction reaction can realize the direct conversion of CO2 into carbon fuels, such as CO, HCOOH, CH4, C2H4, C2H6, and C2H5OH. [125] However, several challenges need to be addressed to facilitate the CO2RR, such as high overpotential, low selectivity for the target product, poor CO2 solubility, and low efficiency due to theHER.[126] There- fore, a cost-effective and efficient catalyst is required to enhance CO2 conversion efficiency. MXenes have attracted attention for CO2RR application due to the improved chemical activity and se- lectivity, which show the potential to overcome the limitations of traditional CO2RR catalysts. DFT calculations are commonly used to design MXene- supported SACs for the ECO2RR. Li et al. analyzed SACs of TM@Ti2CTx (T = -O, -S) monolayers for the CO2RR by first- principles calculations. TM@Ti2CO2 could better activate CO2 molecules than TM@Ti2CS2, while the theoretical limiting po- tential (UL)of the potential determining step of TM@Ti2CS2 is smaller than TM@Ti2CO2. This study offers new insights into designing catalysts by combining the advantages of differ- ent functional groups.[127] Cao et al. employed DFT to explore the ECO2RR activity toward HCOOH on two types of MXene- supported SACs, TM-NS-Ti3C2O2 and TM-NN-Ti3C2O2, where TM was Sc, V, Ti, Mn, Cr, Cu, Fe, Co, and Ni.[101] For TM-NS- Ti3C2O2, TM was coordinated with two O atoms, one S atom, and one N atom, while it was coordinated with two O atoms and two S atoms in TM-NN-Ti3C2O2. A catalyst with good stability should have a formation energy below 0 and a dissolution poten- tial above 0, where all the TM-NN/NS-Ti3C2O2 (TMs in the first row) meet these stability criteria (Figure 11A). During the reac- tion process, the adsorbed CO2 on the catalysts’ surface led to the formation of *OCHO. The dangling O atoms were then at- tacked by protons to form *HCOOH, which would be finally des- orbed as HCOOH (Figure 11B). Based on the above mechanism, the reaction energies of reactions on all catalysts were calculated and employed UL as a descriptor. According to the calculated UL values, Cr-NS-Ti3C2O2 (−0.23 V versus SHE) and Ti/V/Fe-NN- Ti3C2O2 (−0.19, −0.32, and −0.08 V versus SHE) whose UL val- ues are more positive than that of a benchmark SnO2 catalyst (−0.37 V versus SHE), are good choices for HCOOH production (Figure 11C). The adsorption and hydrogenation of the *OCHO intermediate is strongly associated with the catalytic activity. Fur- ther electronic structure analyses of *OCHO adsorption on cata- lysts revealed that both the electronic interactionmechanism and spin polarization nature of SACs affect the *OCHO intermediate. More electron transfer in Ti/V-NN-Ti3C2O2 and large spin polar- ization of Cr and Fe in Cr-NS-Ti3C2O2 and Fe-NN-Ti3C2O2 are responsible for their outstanding ECO2RR performance toward HCOOH (Figure 11D,E). Adv. Sci. 2025, 12, 2414674 2414674 (18 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 10. Free energy diagrams of A) Fe-H1-V2CO2, B) Co-H1-V2CO2, C) Ni-H1-V2CO2, D) Co-VO-Nb2CO2, E) Ni-H2-Nb2CO2, and F) Co-H2-Ta2CO2. The steps marked in purple and yellow represent the PDS of the ORR and OER, respectively. Reproduced with permission.[124] Copyright 2024, Elsevier Inc. G) Schematic illustration of anchoring L-tryptophan at the surface of Ti3C2Tx MXene, followed by fabrication of dual-atom CoNi-Ti3C2Tx composites. H) OER LSV curves of CoNi-Ti3C2Tx, Ni-Ti3C2Tx, Co-Ti3C2Tx, and RuO2 electrodes at 5 mV s−1. I) Tafel slopes of CoNi-Ti3C2Tx, Ni-Ti3C2Tx, and Co- Ti3C2Tx electrodes. J) Cdl values of CoNi-Ti3C2Tx, Ni-Ti3C2Tx and Co-Ti3C2Tx. K) OER chronopotentiometry response of CoNi-Ti3C2Tx electrodes at current densities of 10 and 500 mA cm−2. Reproduced with permission.[118] Copyright 2024, American Chemical Society. Adv. Sci. 2025, 12, 2414674 2414674 (19 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 11. A) The computed formation energy and dissolution potential of TMs in TM-NS/NN-Ti3C2O2. B) Schematic illustration of the reaction mech- anism of ECO2RR toward HCOOH on TM-NS/NN-Ti3C2O2. C) Theoretical limiting potential UL of ECO2RR toward HCOOH on TM-NS/NN-Ti3C2O2. D) Adsorption configurations of *OCHO intermediate and charge density difference on Ti-NN-Ti3C2O2 and V-NN-Ti3C2O2. The isosurface values are 3 × 10−3 e Bohr−3. Yellow and cyan regions represent increasing and decreasing electron densities, respectively. E) Projected density of states of cat- Adv. Sci. 2025, 12, 2414674 2414674 (20 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Bao et al. synthesized CuSA/Ti3C2Tx where the Cu SAs corre- sponded to O-coordinated Cu sites on the MXene matrix as car- bon monoxide reduction reaction (CORR) catalysts.[128] In 1 M KOH saturated with CO, the catalyst achieved a total Faradaic ef- ficiency (FE) of 98% at −0.7 V versus RHE, with product selectiv- ity of over 79% toward C2 products (EtOH and C2H4) at −0.6 to −0.9 V versus RHE. Compared to CuNP/Ti3C2Tx, CuSA/Ti3C2Tx exhibited a 3.2-fold increase in current density at −1.0 V versus RHE (−52.2 mA cm−2 vs. −16.2 mA cm−2). It also showed a sig- nificant improvement over CuNP/Ti3C2Tx in the FE of C2H4 and EtOH (21% at −0.7 V versus RHE). According to theoretical cal- culations, Cu SAs coordinated with oxygen remain stable during the reduction reaction, promoting the formation of intermediates and decreasing the free energy barrier of the rate-determining step. The good reactivity, structural simplicity, and uniform cat- alytic sites of CuSA/Ti3C2Tx contribute to the good selectivity of C2 products. Liu et al. incorporated Cu SAs into N-doped Ti3C2Tx for the CO2RR. The HAADF-STEM images (Figure 11F) and XAS spectra (Figure 11G,H) demonstrate the existence of Cu SAs.[94] CuSA@N-Ti3C2Tx exhibited a lower overpotential than CuSA@Ti3C2Tx and more efficient CO2 conversion (Figure 11I). With the help of C-Cu-N bridge fragments, the CO FE reached 97.4% at−0.58 V (Figure 11J), improving the CO FE (Figure 11K) and JCO (Figure 11L) of CuSA@Ti3C2Tx remarkably. Hence, the catalytic performance of MXenes for the CO2RR is influenced by the functional groups, SAs, and the type of metal (M) present. The current MXene-support SACs for CO2RR are summarized in Table 2. However, there is limited experimental research in this area, necessitating further in-depth investigation in the future. 4.4. MXene-Supported SACs for the NRR and NO3RR NH3, a vital chemical feedstock, is essential in industrial and agricultural production, as well as energy storage and conver- sion which could transport and supply hydrogen due to its high hydrogen density and liquid state under high pressure at room temperature. Currently, large-scale industrial production of NH3 relies on the Haber-Bosch process at high temperatures and pressures.[129] The NRR has been proposed as a sustain- able and carbon-free method for NH3 production under mild conditions.[130] Additionally, the electrochemical nitrate reduc- tion reaction (NO3RR) has also attracted attention as a method to produce NH3 from nitrate, which simultaneously realizes the re- cycling of nitrogen-containing pollutants.[131] The key challenges with these reactions are the low reactivity and selectivity for NH3 due to the competition of theHER. Thus, highly efficient and eco- nomical electrocatalysts are essential. MXene-supported SACs have been explored in these fields due to their excellent electri- cal conductivity, higher atomic utilization, and higher selectivity to adsorb specific ions. Many MXene-supported materials are theoretically predicted to have a good NRR performance. For instance, Gao et al. ex- plored the feasibility of SACs supported on Ti3C2O2 with oxy- gen vacancies for NRR applications.[132] As shown in Figure 12A, one O atom is removed from TM/MXene to form an O vacancy, and then the exposed Ti atoms are replaced by different TMs. With the DFT results, Pt/Ov-MXene shows the lowest energy bar- rier to produce NH3, however, the HER outcompetes the NRR on this catalyst. Ag/Ov-MXene and Cu/Ov-MXene are the next best catalysts with a higher preference toward NH3 than hydro- gen (Figure 12B). The formation energy Eform was calculated to evaluate the thermodynamic stability of the catalysts. The lower Eform of Ag/Ov-MXene (+2.3 eV) and Cu/Ov-MXene (+2.41 eV) were relative to that of PtSA/Ov-MXene (+2.62 eV), meaning that these two catalysts should be thermodynamic stable (Figure 12C). Qu et al. reported a series of superior NRR catalysts by com- bining Ti2NO2 with 28 different SAs via first-principles calcula- tions. According to the formation energy calculation, most can- didates are thermodynamic stable, while the elements with full or almost full electron shells (Zn, Ag, Cd, Au, and Hg) exhibit small formation energies. Thus, the MXene is a promising sub- strate to immobilize SACs.[133] For the early transitional metals, the side-on pattern is preferred where N2 would be horizontally anchored with both sides interacting with the substrate. How- ever, as the number of d electrons increases to d5 or d6, the end- on pattern is preferred where only one side of nitrogen coordi- nates with catalysts. For SAs/Ti2NO2, both the side-on and end- on patterns work due to the synergistic effects of SAs and Ti.With the enzymatic-distal mechanism, the optimal performance was obtained in the d8 orbital system with NiSA/Ti2NO2 (−0.13 V). Huang et al. designed 18 different SAs with two S-functionalized MXenes (Ti2CS2 and Nb2CS2) and explored their activity toward the NRR.[134] Compared to O-functionalized MXenes (Nb2CO2 and Ti2CO2), the ΔG*H of Nb2CS2 is large (0.8 eV), which indi- cates poor HER activity and the potential to be used for the NRR. Among all the candidates, Mo@Nb2CS2 is highly promising for its activity and selectivity. In the catalytic structure, Nb2CS2 acted as an electron donor during hydrogenation meanwhile MoS2 served as a medium to transfer electrons between the interme- diate and the MXene. All these calculations and research pro- vide a valuable reference for the future construction of SACs on MXenes. To further explore MXene-supported SACs for NRR applica- tions, experimental investigations have also been conducted. Ren et al. reported an effective FeSA/Ti3C2Tx catalyst for the NO3RR, which takes advantage of elevated activity, low nitrite selectivity of Fe and tunable electronic structure. It shows a higher NH3 Faradaic efficiency and selectivity (82.9% and 99.2%) than those of FeNP/MXene (69.2% and 81.3%, Figure 12D–F). Here, the NO3 − is adsorbed onto Fe single atom sites to first produce *NO3, which then breaks down to *NO2 and *NO. The pro- alysts and *OCHO adsorption on catalysts of Cr-NS-Ti3C2O2 and Fe-NN-Ti3C2O2. Reproduced with permission.[101] Copyright 2024, Elsevier Ltd. F) HAADF-STEM image of CuSA@N-Ti3C2Tx. The experimental Cu K-edge G) XANES and H) EXAFS spectra of CuSA@N-Ti3C2Tx and counterparts. I) LSV curves of CuSA@N-Ti3C2Tx, performed in CO2-saturated 0.5 M KHCO3. J) Potential-dependent FEs of H2 and CO for CuSA@N-Ti3C2Tx, at different potentials. K) The CO FEs of CuSA@N-Ti3C2Tx and CuSA@Ti3C2Tx at different potentials. L) Jco of CuSA@N-Ti3C2Tx and CuSA@Ti3C2Tx. Reproduced with permission.[94] Copyright 2024, Springer Nature. Adv. Sci. 2025, 12, 2414674 2414674 (21 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Figure 12. A) Top and side views of the atomic structure of TM/Ov-MXene. Atom labels: C (white), Ti (blue), O (red), and TM (purple). The screened TM atoms (from Ti to Au) are listed. B) Summary of limiting potentials on TM/Ov-MXene for NO3RR. C) Calculated formation energy of TM/Ov-MXene (TM = 3d to 5d transition metals). Reproduced with permission.[132] Copyright 2023, Wiley-VCH. D) Potential-dependent Faradaic efficiency, NO3 − removal, and NH3 selectivity for FeSA/MXene filter. E) Comparison of the highest Faradaic efficiency and NH3 selectivity for the FeSA/MXene filter at different NO3 − concentrations. F) The NH3 yield rate of a FeSA/MXene filter in 0.1 M Na2SO4 electrolyte with different concentrations of NO3 −. G) Gibbs free energy diagrams of nitrate reduction to NH3 and H2 evolution reaction (the top right) over FeSA/MXene and FeNP/MXene. Reproduced with permission.[135] Copyright 2023, American Chemical Society. Adv. Sci. 2025, 12, 2414674 2414674 (22 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com tonation reaction of *NO results in the generation of *NHO, *NH2O, and *NH2OH. The N-O bonds in NH2OH are cleaved and protonated to form *NH3. Finally, the *NH3 is released from the surface of catalyst to produce NH3 (Figure 12G). The MXene acted as a support for Fe SAs with Fe-O bonds and prevented them from corroding and leaching into the solution during the reaction.[135] According to Table 2, MXene-supported SACs exhibit significant promise for NRR applications, surpassing the performance of GO and MoS2. 5. Summary and Outlook SACs are considered some of the most promising alternatives to traditional electrocatalysts due to their low cost and high effi- ciency. The simplified components and structures of SACs make it easier to identify the active sites and reveal themechanism dur- ing electrochemical reactions. In this review, MXenes are intro- duced as potential support for SACs due to their unique prop- erties, including large surface area, high electrical conductivity, good hydrophilicity, and tunable surface groups. We first discuss the impacts of synthesis and properties of MXenes for the intro- duction of SAs. The morphology and surface functional groups can be easily controlled by synthesis methods, benefiting the de- sign of catalysts for specific applications. Moreover, the synthe- sis approaches of MXene-supported SACs are involved which provide the anchoring sites in MXenes for SAs, including de- fect vacancy anchoring, metal-support interactions, and selective atomic substitution. Advanced characterization techniques are frequently employed to validate the efficacy of SA doping. These methods, including STEM, XAS, XPS, andDFT, are instrumental in confirming the existence and coordination of SAs. Addition- ally, the applications of MXene-supported SACs in electrocatal- ysis are discussed for the HER, OER, ORR, CO2RR, NRR, and NO3RR from experimental and theoretical perspectives.Many ex- perimental and theoretical studies of MXene-supported SACs in recent years have demonstrated their remarkable potential for electrocatalysis. In addition, the anchored SAs can be tailored according to the specific reaction from noble metal to transi- tion metal atoms. However, the final performance of MXene- supported SACs still needed to be improved. In order to fully realize the potential of MXene-supported SACs, it is impera- tive to undertake further exploration in the following areas to overcome current obstacles, a task that poses challenges but is essential. 1) Enhancing current synthesis techniques and exploring novel approaches to produce single-layer or few-layer MXenes are crucial for practical applications. The prevailing methods pri- marily yield Ti-based MXenes through in situ HF etching, re- sulting in single-layer or few-layer structures. However, MX- enes with alternative functional groups like -Cl, -I, and -S pose challenges in exfoliation, limiting the surface area and active sites for SAs. Additionally, MXene surface groups influence substrate-SA, thereby affecting catalytic performance. Hence, innovative approaches for generating single- or few-layer MX- enes with diverse terminal groups could enhance the poten- tial of MXene-supported SACs for high activity. To address this issue, we suggest that on the one hand, appropriate ex- foliation techniques for MXenes (T = Cl, I, S) can be devel- oped; and on the other hand, direct synthesis methods like CVD can be employed to precisely produce few-layer or even single-layer MXenes. 2) While various types of MXenes hold potential as substrates for SACs, research has predominantly focused on Ti-based MXenes. Yet, other MXene variants, including Mo-based, Nb- based and V-based, have demonstrated viability for MXene- supported electrocatalysis. A comprehensive understanding of catalyticmechanisms associatedwith theseMXene variants is pivotal for advancing MXene-supported SACs. Besides ex- ploring the catalytic performance of pristine MXene variants, the ability to anchor SAs and their impact on the activity of SAs can also be investigated by experimental and theoretical research. 3) The current MXene-supported SACs suffer from insufficient loading of SAs. This is because the weak interaction between SAs and the substrate is not enough to overcome the strong surface energy of a single atom to prevent aggregation, result- ing in low loading of SAs and limited catalytic performance. Hence, gaining deeper insights into modifying MXene sub- strates during the synthesis process to enhance SA loading is crucial for achieving highly catalytic MXene-supported SACs. To realize high loading of SAs on MXenes, increasing the number of coordination sites on MXenes, such as enhanc- ing surface areas or rich defects, can be effective. In addition, controlling the synthesis conditions such as reducing the tem- perature also helps to increase SA loading without agglomer- ation. 4) While theoretical investigations have highlighted the poten- tial of MXene-supported SACs for electrocatalytic applica- tions, numerous assertions remain untested in experimen- tal settings. Consequently, prioritizing experimental endeav- ors is imperative to fully actualize the industrial applications of MXene-supported SACs. For the large number of theoret- ically predicted catalysts, the screening of them can be real- ized with high throughput computation and machine learn- ing. Suitable experimental methods can then be employed to synthesize and analyze the target catalysts. 5) Despite some MXene-supported SACs having exhibited out- standing electrochemical activity, their stability is still an unig- norable issue that limits industrial applications. Currently, strategies to enhance the stability of MXene-supported SACs mainly focus on two aspects. The first approach is to im- prove the stability of MXenes, which can be achieved through the following methods: a) modifying the surface functional group to a more inert one (halide or hydrocarbons) instead of -F, -OH, or -O; b) improving the quality of precursor MAX phase and etching it with non-aqueous solvent; c) cov- ering the MXene surface with an oxidation-resistant layer. The second approach is to improve the stability of fixed SAs by enhancing the interaction between MXene and SAs or adjusting the coordination condition of SAs. However, achieving both high activity and good stability is challeng- ing. There is a need to balance them during catalyst design and develop highly efficient and robust MXene-supported SACs. Adv. Sci. 2025, 12, 2414674 2414674 (23 of 27) © 2025 The Author(s). Advanced Science published by Wiley-VCH GmbH http://www.advancedsciencenews.com http://www.advancedscience.com www.advancedsciencenews.com www.advancedscience.com Acknowledgements J.H. was grateful for the Melbourne Research Scholarship from the Uni- versity of Melbourne. Q.G. thanks for the research support from ANSTO. Conflict of Interest The authors declare no conflict of interest. Keywords catalytic applications, MXenes, single-atom catalysts, support, synthesis and characterization Received: November 11, 2024 Revised: February 19, 2025 Published online: March 27, 2025 [1] a) E. Hu, Y. Feng, J. Nai, D. Zhao, Y. Hu, X. W. Lou, Energ. Environ. Sci. 2018, 11, 872; b) B. You, Y. Sun, Acc. Chem. Res. 2018, 51, 1571. [2] Y. Luo, Z. Zhang, M. Chhowalla, B. Liu, Adv. Mater. 2022, 34, 2108133. [3] a) Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Norskov, T. F. Jaramillo, Science 2017, 355, 146; b) P. De Luna, C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo, E. H. 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