Applications of MXene‐Based Single‐Atom Catalysts

Single‐atom catalysts (SACs) consist of isolated metal sites on the support through stable coordination bond, which usually have high catalytic activity and selectivity. With large surface area and electron tunability, metallic carbides, nitrides, or carbonic nitrides (MXenes) are suitable carriers for supporting isolated single‐metal atoms. The abundant surface‐functional groups and vacancy defects on MXenes are the ideal anchoring sites for isolated metal atoms. Herein, the advanced synthesis and characterization methods for MXene‐based SACs are first introduced. Three strategies (adsorption at functional groups, anchoring at metal vacancies, and anchoring at surface terminating group vacancies) appear to be feasible in ensuring the non‐aggregation of single‐metal atoms, which are attributed to the strong bonding between the single‐metal atoms and the carrier. The applications of MXene‐based SACs in electrocatalysis (including hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and nitrogen reduction reaction), energy storage (including Li‐ion batteries, metal–air batteries, and supercapacitors), and sensors (including gas sensors and biological sensors) are fundamentally reviewed. Finally, own insight on the current challenges and prospects of MXene‐based SACs is presented.

further reduction and anchoring. In addition, the preparation process of MXenes usually requires the addition of etching agents to selectively remove unwanted metal atoms, but some metal atoms inside the MXene lattice will inevitably be etched away, resulting in metal defect sites. The defect sites of unsaturated coordination have certain reducibility, where the adsorption and reduction of metal cations can be simultaneously achieved by one step method.
MXene-based SACs have been widely studied and applied in various fields. In this review, we first discuss the properties and advantages of MXenes as carriers to load single-metal atoms, and summarize the existing characterization methods of single-metal sites. Second, the theoretical basis, synthesis methods, and strategies for the stabilization of single-atomic sites on MXenes are systematically summarized. Adsorption on surface-functional groups and filling in M and T vacancies are the main anchoring strategies. Then, the applications of MXene-based SACs in the fields of electrocatalysis, energy storage, and sensor are introduced. Finally, the prospect of MXene-based SACs is proposed.

Synthesis of MXenes
In 2011, the first MXene Ti 3 C 2 was synthesized by Naguib et al. by selectively removing Al atoms from Ti 3 AlC 2 , which was defined as MAX (A is Al element). [16] Due to the weak binding energy of Al in MAX, it exhibited higher reactivity than Ti species. After treatment in 50% hydrogen fluoride (HF) solution, the Al atoms combined with F to form AlF 3 and volatilized from the precursor Ti 3 AlC 2 , retaining MXene Ti 3 C 2 . Subsequently, this top-down strategy was popularized and was the dominant method for MXene synthesis (Figure 1a). Ti-and Mo-based MAX are commonly used precursors that can be easily purchased, while other specific MAX are usually synthesized in the laboratory. By mixing a molar ratio of TM, Al, and graphite, and annealing at high temperature, MAX with target metal can be obtained, including bimetallic MAX. For example, Nb 4 AlC 3 , with multiple atomic layers, was synthesized by treating Nb, Al, and C mixtures under high temperature and pressure. [17] In addition, MXenes can be also obtained by the removal of the A element. The commonly used etching agent is HF and the concentration and temperature required for the synthesis of different MXenes are different. Jiang et al. synthesized Ti 3 C 2 and V 2 C quantum dots with an average size of 4.5 nm under different conditions, at 25°C with 40% HF for V 2 AlC and at 50°C with 48% HF for V 2 AlC. [18] The HCl/LiF solutions are also used in the etching process, which are more controlled, mild, and environmentally friendly than HF. In addition, Li þ can enter the MXene interlayer during the etching process to obtain MXene with a few layers or single layer. Moreover, MXenes can be synthesized by the stripping of multilayer MXenes. Multilayer MXene obtained by F À etching is washed with deionized water and ultrasonic until the pH is neutral. During this process, H 2 O molecules would enter the interlayer and reduce the tendency of MXene aggregation. To avoid the unsatisfactory small layer structure caused by prolonged ultrasound, additional intercalators, such as metal cations, organic molecules, and so on, are added to widen the layer spacing and accelerate stripping and stratification of MXenes. Due to the high corrosion and danger of fluorine-containing reagents, fluorine-free etching has been widely developed, including high-temperature alkali treatment, molten salts etching, electric etching, and so on. [19][20][21] In addition, MXene with chlorine-functional groups would be obtained through by etching MAX with molten chloride salts. [22] In addition to MXenes with monometal and bimetals, highentropy MXenes with more than three TMs have been designed. Leong et al. calculated the thermostatically stable configurations of TiVNbMoC 3 and TiVCrMoC 3 , and found that the metals showed obvious preferential occupation at high temperatures, with Cr most inclined to the outer layer, followed by Mo. [23] In addition, the metals in high-entropy MAX precursor were in a solid solution state above 1873 K, while Nb and V atoms showed significant separation in the inner layer below 928 K. Figure 1. a) Synthesis and structure of Ti 3 C 2 T x . b) High-angle annular dark-field (HAADF) and c) Annular bright-field images of Ti 3 C 2 T x observed along the a/b axis. Reproduced with permission. [120] Copyright 2015, American Chemical Society. d) Top view of 2D M 2 X (X = C or N). Side views of configuration of e) fcc, f ) hcp, g) fcc þ hcp, h) top, and i) bridge. Reproduced with permission. [121] Copyright 2018, Elsevier.

Abundant Sites for Anchoring Isolated Metal Atoms
MXene, with a hexagonal lattice structure, is composed of M metal layer and carbon/nitrogen atom layer arranged alternately, where carbon and nitrogen atoms are in the inner layer (Figure 1d-i). Due to the high surface energy, metals exposed in the outer layer will usually bond with functional groups such as F, O, and OH, while coordinating with three carbon or nitrogen in the inner layer to form a six-coordination structure. There are two main coordination configurations of surface-functional groups, face center cubic (fcc) and hexagonal close packed (hcp), and the optimal coordination configurations are different for different TMs. [24] The electronegative surface-functional groups are ideal sites for fixing isolated metal atoms, which can easily adsorb metal cations. Theoretical calculations showed that the stability of different single atoms at the terminal is different. [25] The stability of SACs is often associated with the intrinsic properties of metalsupport interaction, including kinetic stability and thermodynamic stability. [26] When the formation energy of an isolated metal atom on the carrier is lower than that of a nanoparticle, the formation of SA site is thermodynamic favorable. Under certain conditions, such as the addition of a small number of metal precursors, SA sites are preferentially generated, which can remain dispersed state due to the presence of a high aggregation barrier. Under the guidance of theoretical calculations, it is meaningful to design and synthesize the advantageous terminal. Furthermore, the type and proportion of surface terminals are related to the method of MXene synthesis, and the oxygen end is the more stable end group due to the strong M─O bond. [27] MXene obtained by strong alkali etching achieved 100%─O/ÀOH coverage. [28] However, high concentration of HF would increase the content of F on the surface, and high temperature can also remove part of the O-functional group, correspondingly increasing the ratio of F. [29] In addition, the unanticipated lattice defects of MXene are advantageous for the anchoring of single atoms. [30][31][32] In the process of MXene synthesis, in addition to Al atoms, some metal atoms will be taken away by strong acids, leaving highly active metal vacancies on the surface of MXene. Due to unsaturated coordination structure of defect sites, the adsorbed metal ions are reduced in situ without the need for additional high temperature or reducing atmosphere. According to different distribution of metals, bimetallic M 1 ,M 2 -MXene can be divided into ordered MXene and solid solution MXene, which both exhibit more favorable properties for loaded metal atoms. Selective etching of metals M 1 from M 1 ,M 2 -MXene results in uniformly dispersed vacancies for filling the target single atoms. In addition, by taking the target single atom as metal M 1 and adjusting the ratio of metal M 1 and M 2 , atomically dispersed M 1 single atom can be directly obtained.

Strategies of Anchoring Isolated Metal Atoms onto MXenes
The high tendency of polymerization of single atoms on the surface of the MXene is a major challenge in the synthesis of SACs. Therefore, uniform dispersion and stability of single atoms in the catalytic process is the key to improve the utilization rate of single-atom active sites. The T-functional groups of MXene have negative charge and can be designed as coordination sites to facilitate the adsorption and fixation of metal atoms, which is the preferred single-atom synthesis strategy. Another efficient strategy is immobilizing metal atoms using defect sites to limit atomic migration. The presence of natural M and T vacancies within the MXene structure leads to the formation of unsaturated sites, which are conducive to the formation of isolated atoms. Recent synthesis methods of MXene-based SACs are summarized in Table 1.

Theoretical Basis
Fixation of single atoms on the surface of MXene requires specific adsorption sites. For primitive MXene, the adsorption of single atoms mainly occurs at four possible sites: top site (T), two different hollow sites fcc and hcp, and bridge sites (B), where the sites of hcp and fcc are the preferred adsorption sites (Figure 2a,b). Theoretical calculations displayed that the TM atoms were more easily adsorbed on the surface of Ti 3 C 2 than graphene, with adsorption energy between À1.05 and À7.98 eV (Figure 2c). [33] Furthermore, the increase of atomic number of TM led to the d electron center away from the Fermi level, resulting in the weakened adsorption. Moreover, Oschinski et al. studied the aggregation trend of TMs on M 2 C by calculating diffusion energy. [34] The cohesive energy of Zn was greater than 0 on the surface of all M 2 C, suggesting that Zn single atom was stable on MXene surface. However, V, Cr, Mn, Fe and Co exhibited unfavorable aggregation trend on M 2 C. In addition to atomically dispersed atoms on MXene surface, the charge (partial oxidation or partial reduction) state can be controlled. Nevertheless, the adsorption model of TM on MXene with functional groups (O, OH, F) should be constructed because MXene is easy to be oxidized. The adsorption sites were basically consistent with those of MXene without functional groups, top, hcp, fcc, and bridge. With the low tendency to form clusters and high diffusion barrier, Sc and Ti were the viable to form single-atom sites on M 2 CO 2 . [35]

Synthesis Method
MXene is primarily synthesized by fluoride ion etching, and there are F-or O-functional groups on the surface rather than bare MXene. Negatively charged functional groups are the structural basis for the adsorption of individual metal atoms on MXene. Usually, single-metal atoms on the surface of MXene are synthesized by electrostatic adsorption and in situ reduction in two steps. First, the metal cations are adsorbed on the negatively charged functional groups on the surface of MXene to form ionic bonds driven by electrostatic attraction. Subsequently, the metal cations are reduced and fixed on the MXene to form covalent bonds such as M─O/F by adding reducing agents or annealing at high temperatures. In the presence of NaBH 4 , Co 2þ ions are bonded to O on MXene under the drive of electrostatic attraction ( Figure 3a). [36] The successful synthesis of Co single atoms, which are coordinated with O and C, making the accumulation degree of Ti 3 C 2 T x significantly reduced. The addition of NaBH 4 avoided the excessive oxidation of MXene and facilitated the Ru SA -N-S-Ti 3 C 2 T x 1.2 wt% Thermal-reaction method HER [53] Cu SA/Ti 3 C 2 T x 0.2 wt% Assisted reduction method CO reduction reaction [50] Figure 2. a) MXene structure of M 2 C and the sites for anchoring single-metal atoms. Reproduced with permission. [34] Copyright 2021, American Chemical Society. b) Adsorption model of transition metal (TM) on M 2 C. Reproduced with permission. [122] Copyright 2018, The Royal Society of Chemistry. c) The relationship between adsorption energy and the d-electron center of TM. Reproduced with permission. [33] Copyright 2018, Elsevier.
www.advancedsciencenews.com www.small-structures.com reduction of metal ions. The Ru 3þ ions were adsorbed on oxygen end groups and reduced to Ru atoms by adding reducing tannic acid. [37] The tannic acid was beneficial to the synthesis of Ru SAs with low load (0.43 wt%), while the corresponding Ru SAs synthesized with NaBH 4 contained 2.14 wt% Ru and Ru nanoparticles. High-temperature annealing facilitates the formation of individual atoms from bulk metals. Peng et al. annealed Rh 3þ /Ti 3 C 2 O x in an argon atmosphere to obtain Rh SA/Ti 3 C 2 O x , in which 0.17 wt% Rh was uniformly dispersed on the surface of MXene. [38] At high temperature, the structure of Ti 3 C 2 O x nanosheet was not damaged, and it has a thickness of 2 nm, a transverse size of hundreds of nanometers, and a high specific surface area of 441.81 m 2 g À1 . Annealing in hydrogen atmosphere can induce the morphologic transformation of Ir species on Mo-based MXene from nanoparticles to single atoms with the increase of annealing temperature (Figure 3b). [39] At 650°C, Ir species exhibited octahedral nanoparticles with an average diameter of 3.5 nm, and the Ir atomic layer would be formed when continuing to heat up to 750°C. Further, single-Ir-atoms-dominated Ir species were formed on the surface of MXene after annealing at 850°C. There are strong metal-carrier interactions between Ir and Mo. Owing to the easy sublimation of AlCl 3 , Zhao et al. selectively etched Al from the quaternary MAX phase Ti 3 (Al 1Àx Cu x )C 2 , which contained a mixed A layer, and obtained Cu single atoms on the surface of MXene (Figure 3c). [40] Only 1.0 wt% of Cu was uniformly distributed on the surface of MXene, where Cu coordinated with three O with coordination bonds of 1.55 Å.
In addition to the O and F atoms introduced during the preparation of MXene, it is feasible to adding additional other heteroatoms for coordination atoms. The introduction of heteroatoms can not only increase the specific anchorage sites, but also further regulate the electronic structure of metal atoms, which is conducive to the improvement of catalytic activity. High-temperature annealing can also reduce the adsorbed metal ions. After annealing in argon at 550°C for 120 min, Liu et al. doped Ru and N atoms into MXene at the same time, using RuCl 3 and melamine as precursors. [41] S and P heteroatoms are also doped for the synthesis of single atoms. Therefore, the strategy is efficient to achieve high-performance SACs due to the provision of an easily regulated coordination environment.

Theoretical Basis
Defect engineering refers to that the TM atoms are fixed to the carrier through strong interactions with unsaturatedcoordination vacancies. Metallic vacancies are natural sites on MXene that are highly reactive and can be used to anchor individual atoms. Li et al. calculated the forming energy of Zr, Hf, Ta, Figure 3. a) Synthetic process of Co-Ti 3 C 2 T x single-atom catalysts (SAC). Reproduced under the terms of CC-BY license. [36] Copyright 2021, The Author(s), published by Elsevier. b) The conversion scheme of Ir from nanoparticles to flattened layer and then to ultrathin layer on Mo 2 TiC 2 T x . Reproduced with permission. [39] Copyright 2022, Wiley-VCH GmbH. c) The fabrication of single atom (SA)-Cu-MXene via selective etching quaternary MAX. Reproduced with permission. [40] Copyright 2021, American Chemical Society.  [1] found that Ru and Os filling on the Ti 3 C 2 Ti showed the lowest forming energy. Therefore, metal M vacancy is a suitable monatomic fixation site, which is conducive to the formation of SACs.

Synthesis Method
In the synthesis process of MXene, the metal atoms on the surface will be unavoidably stripped to form metal vacancies, which have unsaturated coordination and certain reducibility. Through the self-reduction method, Zhao et al. fixed Pt onto the Ti vacancy of Ti-based MXene, and the loading amount of Pt was 0.2 wt %. [42] Without additional reductant, the adsorption and reduction of Pt 4þ occurred simultaneously ( Figure 4a). The Pt atom was anchored at the Ti site rather than the lattice gap, which was proven by the clear Pt spots located at the lattice plane walls of the Ti columns (Figure 4b,c). The M vacancy of MXene is related to the concentration of etching agent and etching time. Park et al. etched V 2 CT x with 49% HF for 5 days, and the obtained V vacancies can be used to anchor-isolated Pt atoms by self-reduction. [43] Also, Ni SACs/Ti 3 C 2 T x , with Ni content of 2.93 wt%, has been synthesized using the reactivity of Ti vacancies. [44] Assisted reduction method refers to the reduction of metal ions to single atoms by additional auxiliary processing after adsorption occurred. In CO 2 atmosphere, Zhao et al. calcined Se powder and Ti 3 C 2 at 350°C for 120 min, and obtained 4.99 wt% Se SA injected into Ti-vacant Se-Ti 3 C 2 ( Figure 4d). [45] Different from inert gases, CO 2 promoted the formation of Ti vacancy by reacting with Ti. In addition, the oxidation of CO 2 atmosphere was conducive to the generation of ÀO terminal and the approach of Se to Ti vacancy, because the adsorption energy of Se on the O─MXene is far less than that on the F─MXene. Moreover, Se─C and Se─O bonds were observed, demonstrating the filling of Se atoms in the Ti vacancies. However, under the condition of argon, Se species synthesized by the same method was not fixed into Ti vacancy, but aggregated to form nanoparticles. For V M -SACs prepared by reduction method, the number and distribution of M vacancies will affect the loads of TM atoms and further affect the catalytic activity. Simply adding M vacancies increases the theoretical load of TM, but damages the structural stability of MXene.
The construction of quaternary hybrid MAX is a simple and fast method to obtain isolated metal sites, in which the single atom is completely embedded in the lattice. When the metal layer of A is removed with an etching agent, two different metals, including the target single-metal atoms, will be retained in the MXene. Kuznetsov et al. synthesized β-Mo 2 C:Co with the mixture of (NH 4 ) 6 Mo 7 O 24 ·4H 2 O and Co(NO 3 ) 2 ·6H 2 O, and then successfully constructed a bimetallic MAX (Mo 2 Ga 2 C:Co) by adding metal Ga. [46] Subsequently, Mo 2 Ga 2 C:Co was treated with HF, etching the metal Ga and excess Co atoms away, and Figure 4. a) Self-reduction-stabilization process for the preparation of Pt 1 /Ti 3À x C 2 T y . b) Magnified HAADF image of Pt 1 /Ti 3 Àx C 2 T y . c) Corresponding intensity maps. Reproduced with permission. [42] Copyright 2019, American Chemical Society. d) Synthesis process of the SASe-Ti 3 C 2 . Reproduced with permission. [45] Copyright 2021, Wiley-VCH GmbH. e) Schematic representation of the Mo 2 CT x :Fe structure. Reproduced under the terms of CC-BY license. [47] Copyright 2021, The Authors, published by American Chemical Society. Mo 2 CT x :Co with 0.04 wt% Co single atom was finally obtained (Figure 4e). The MXene substrate synthesized by this method has a big transverse dimension of 1 À 2 μm. Further, this research group synthesized Fe SAs catalyst Mo 2 CT x :Fe by the similar method, in which Fe content was 0.42 wt%. [47] Both Fe and Co exhibited an oxidation state of þ2, and the coordination environment are similar to that of Mo, suggesting that the construction of bimetallic MAX is a general strategy for the synthesis of single-atom sites anchored in M vacancies. Under high temperature and high pressure, quaternary hybrid MAX with different metal proportions can be synthesized, and MXene-based SACs with stable structure can be obtained more controllably.

Theoretical Basis
The functional groups ÀO, ÀF, and ÀCl are easy to form defects or be replaced in the reaction process, which provides convenient conditions for the anchorage of metal single atoms in MXenes. Due to the high stability of O-terminated MXenes, more studies have focused on oxygen vacancies. The oxygen vacancy on the Mo 3 C 2 O 2 can be formed in reducing atmosphere (H 2 or CO). [48] Cheng et al. studied the adsorption of Pd at different sites on the surface of Mo 2 CO 2 , and found that Pd anchored onto oxygen vacancies showed high stability and had good CO oxidation activity. [25] With an energy barrier of 2 eV to diffuse to other nearby adsorption sites, Pd anchored on oxygen vacancies had a little tendency to migrate and form clusters. In the Mo 2 CO 2 system constructed by Cheng et al., the formation energy of oxygen vacancy was 3.64 eV. [49] Furthermore, the binding energy of Fe, Co, Ni, Cu, Zn, Ru, Rh, Ag, Ir, Pt, and Au atoms to the oxygen vacancies of Mo 2 CO 2 was calculated. Four initial sites for anchoring TMs were calculated, including B, H 1 , H 2 , and oxygen vacancy (Figure 5a,b). However, no matter where the initial adsorption site was, TMs (except Fe) would always move to more stable oxygen vacancy during structural optimization. Moreover, the formation of metal nanoparticles in oxygen vacancies is more difficult than that of individual metals. Thus, it is practicable to synthesize the vacancies at T sites on MXenes by experiment and to use them for anchoring isolated metal atoms.

Synthesis Method
TM atoms can be directly anchored to T vacancies or substituted for the T atoms to form SACs. Unlike the surface adsorption behavior, metal anchoring at the T-vacancy implies doping into Reproduced with permission. [49] Copyright 2019, Wiley-VCH GmbH. c) The synthesis of Cu-SA/Ti 3 C 2 T x . Reproduced under the terms of CC-BY license. [50] Copyright 2021, The Author(s), published by Springer Nature. d) The synthesis of Co@V 2 CO 2 . Reproduced under the terms of CC-BY license. [51] Copyright 2022, The Authors, published by MDPI.  Figure 5c). [50] The distance between copper atoms was 0.61 nm, which was much larger than the length of Cu─Cu bond in the Cu dimer, indicating that there was no aggregation of Cu. Due to the coordination structure of Cu-O 3 , the catalyst exhibits Faraday efficiency (FE) of 71% in the electrocatalytic reduction of CO to ethylene. Using freezing photochemical reduction method, Zhao et al. added CoCl 2 solution in the form of ice cube to V 2 CT x solution, and maintained the condition of 0°C to ensure the slow release of Co 2þ to avoid the formation of clusters ( Figure 5d). [51] The mixture was then exposed to ultraviolet light for 1 h to reduce and fix the Co onto the MXene. The Co─O and Co─V bonds were formed, demonstrating the successful doping of Co on V 2 CT x . The synthesis of Co SACs on Nb-and Ti-based MXene can also be achieved by this method. The oxygen-functional groups of Ti 3 C 2 T x would be removed in the form of H 2 O, leading to the form of oxygen vacancy during the process of high-temperature annealing. In Ar/H 2 , Zhang et al. reduced Pt 2þ to zero-valence Pt and anchored it on the generated oxygen vacancy at 400°C. [52] The oxidation state of Pt increased by bonding with Ti. Ramalingam et al. mixed sulfourea, Ru, and Ti 3 C 2 T x uniformly. After freeze-drying, it was annealed at 500°C to obtain N-, S-, and Ru-co-doped Ti 3 C 2 T x , where Ru was coordinated with N and S simultaneously. [53] The photocatalytic hydrogen evolution reaction (HER) performance of Ru SA -N-S-Ti 3 C 2 T x was higher than that reported on precious metal catalysts. Therefore, the performance of MXenebased SACs can be improved by the T-vacancy filling strategy. During the synthesis of SACs by this strategy, the construction of uniform T-vacancy anchoring sites needs to be optimized, and suitable metal precursors should be explicitly selected to form specific coordination structures to avoid the loss of defects.

Characterizations of Isolated Metal Atoms in MXenes
SACs have been widely used in various fields, but in the experimental process, the successful synthesis of single atoms needs accurate characterization to prove their dispersion. [32] Existing methods for the characterization of single atoms fall into two main categories: 1) microscopic characterization, including scanning transmission electron microscope (STEM) and scanning tunneling microscope (STM) and 2) spectral characterization, including X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDX), electron energy loss spectroscopy (EELS), and surfaceenhanced Raman spectroscopy (SERS).

STEM
The rapid development of STEM makes the characterization of atomic-scale metals simple and intuitive. High-angle annular dark-field STEM(HAADF-STEM) clearly shows the existence of existence of isolated single atoms, taking advantage of the difference of atomic number between metal atoms and the support. When electrons accelerated by high-voltage attack metal atoms with high atomic number, high-angle diffraction would be produced, showing brighter images, while low-atomic number atoms cause small-angle electron scattering, showing dark-field images. The metals with higher atomic numbers produce brighter images. For MXene, 2D material-containing layers of TM atoms, the different kinds of metal atoms anchored to its surface can be identified by brighter or darker imaging. Zou et al. obtained a Ru-based SAC by equipping Ru atoms on the surface of Ti 3 C 2 T x by impregnation method (Figure 6a). [54] The bright spots in the HAADF-STEM image corresponded to the highly dispersed Ru atoms and the enlarged HAADF-STEM image showed more pronounced Ru atoms anchored inside the lattice of Ti 3 C 2 T x (Figure 6b-d). The isolated Pt atoms on MXene aerogel were synthesized by Peng and coworkers (Figure 6e). [55] The atomic dispersion of Pt was proven by highly dispersed bright spots in the HAADF-STEM. In addition, the distance between atoms was significantly larger than the radius of Pt, and Pt atoms were uniformly distributed in EDX spectra, which all demonstrated that no Pt nanoparticles were formed. More, other different types of single atoms on MXene have been detected via HAADF-STEM, such as Ir, Sb, Cu, Co, Cr, Ni, and Se. [44,45,[56][57][58][59][60] The STM was invented in 1982 with a resolution of nanometers both horizontally and vertically, marking the dawn of a new era in which atoms could be observed directly. If a voltage is applied between the STM probe with less than 1 nm and the sample, there will be tunnel current due to tunnel effect. When the STM probe is scanned on the surface of the material, fluctuations of less than 0.1 nm will result in an exponentially varying tunnel current, achieving highly accurate detection of a single atom. However, the maximum resolution of STM can only be achieved on atomically flat samples. In addition, unlike STEM, STM can only detect the surface information of the sample.

XAS
The coordination environment of single atoms, including the electronic and geometric structure, can be studied through XAS spectra. In the characterization process, the electrons in the inner layer of a single atom are excited by high-energy electrons to the vacant orbitals in the outer layer that participate in bonding, while the excess energy that is not absorbed eventually causes oscillation of signal. Therefore, the absorption edge in the XAS spectrum has a characteristic, which is related to both the central atom and the coordination atom. Depending on the energy range, XAS can be divided into X-ray absorption nearedge structure (XANES) and extended X-ray absorption fine structure (EXAFS), which provide information on the oxidation states and coordination chemical bonds of the central atoms, respectively. For the XANES, it is more sensitive to chemical information such as valence states, unoccupied electron orbitals, and charge transfer. The higher the oxidation state of the atom is, the more the absorption moves toward the high-energy direction. XANES spectroscopy can also provide 3D structure information for the complement of EXAFS to a certain extent. EXAFS can provide the coordination number of single atoms, coordination atoms, the bond length, and the average ligand structure plane. Yang et al. synthesized different amounts of Pt on TiN and studied the coordination environment of Pt. [61] For 0.35 wt% Pt sample, the EXAFS spectra show that the strong peak at 2 Å corresponds to the presence of Pt-Cl, and its coordination number is 3.038, much larger than that of Pt-Pt (0.583). Zhao et al. synthesized Pt 1 /Ti 3Àx C 2 T y for CO functionalization. [42] In the EXAFS spectra, the peak at 1.5 Å was attributed to the first shell of Pt-C with coordination number of 3.1 AE 0.8 and bond length of 1.99 AE 0.02 Å, and the peak at 2.2 Å was attributed to the higher shell of Pt-Ti with coordination number of 3.8 AE 0.6 Å and bond length of 2.68 AE 0.02 Å. The incorporation of Pt atoms resulted in a slight increase in Figure 6. a) Fabrication procedure of RuSA@Ti 3 C 2 T x . b) HAADF scanning transmission electron microscope (HAADF-STEM) image, c) Magnified HAADF-STEM image and the contrast profile of the highlighted region, and d) energy-dispersive X-ray spectroscopy (EDX) images of RuSA@Ti 3 C 2 T x . Reproduced under the terms of CC-BY license. [54] Copyright 2022, The Authors, published by MDPI. e) Synthetic route of Pt SA-PNPM. f ) HAADF-STEM image of Pt SA-PNPM. g) The line-scanning intensity profile in the area marked by red rectangles in (e). h) STEM-EDX mapping of Pt SA-PNPM. Scale bars: f ) 1 nm, h) 100 nm. Reproduced with permission. [55] Copyright 2022, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com the oxidation state of Ti, but did not change the structure of MXene. Zhou et al. used XANES analysis to uncover that the oxidation number of Pt in Pt SAs/In 2 S 3 /Ti 3 C 2 was between 0 and 4. In the Fourier transform (FT-) EXAFS spectrum, the peak of Pt-S is located at 2.2 Å and the coordination number is 2.71 (Figure 7a,b). [62] Jin et al. characterized Fe single atoms on the surface of bimetallic MXene, and found that the absorption edge position was located between the absorption edge of bivalent and trivalent FeO x . [63] The peak at 1.42 Å is corresponded to the Fe─O bond, and the fitting results indicated that Fe─O 6 was the main structure (Figure 7c,d). Single Fe atoms synthesized by Kuznetsov et al. exhibited different oxidation states and coordination environments. [47] The edge absorption was close to that of FeO, indicating an oxidation state of þ2. In addition, two characteristic peaks in the XANES spectrogram, located at 7121.5 and 7127.9 eV, respectively, were simulated to prove the substitution of Fe to Co (Figure 7e-g).
In addition to structural analysis of single-atom sites, in situ XAS technique has been widely used to study catalytic mechanisms. Nitrogen reduction reaction (NRR) catalytic behavior of Ru atom on Mo vacancy in Mo 2 CT x has been studied by Peng et al. through operando XAS measurements. [64] The oxidation state of Ru in nitrogen was higher than that in argon, which was attributed to the electrons transfer from Ru to N 2 . In the Figure 7. a) X-ray absorption near-edge structure (XANES) spectra of Pt foil, PtO 2 , and Pt SAs/In 2 S 3 /Ti 3 C 2 at the Pt L 3 -edge. b) Fourier transform (FT) of the X-ray absorption fine structure (EXAFS) at the k3-weighted R space for Pt SAs/In 2 S 3 /Ti 3 C 2 and the Pt foil and PtO 2 as reference samples. Reproduced with permission. [62] Copyright 2022, Wiley-VCH GmbH. c) FT-EXAFS spectra of Fe-SA/Mo 2 TiC 2 T x , d) FT-EXAFS fitting spectrum. Reproduced with permission. [63] Copyright 2022, American Chemical Society. e) Normalized Fe K-edge XANES spectra of Mo 2 CT x :Fe and reference materials. f ) Comparison between the experimental XANES spectrum of Mo 2 CT x :Fe and the theoretical simulation. g) Phase-uncorrected FT of the Fe K-edge EXAFS function for Mo 2 CT x :Fe. Reproduced under the terms of CC-BY license. [47] Copyright 2021, The Authors, published by American Chemical Society.

XPS
The highly dispersed metal atoms usually exist in an unsaturated state and have electron states different from metals oxides, which can be analyzed by XPS spectra. Under X-ray irradiation, electrons with certain kinetic energy are excited from the surface atoms in SACs, and the resulting spectra are used for qualitative and quantitative analysis of the surface atoms. [66,67] For specific monochromatic excitation sources and specific atomic orbitals, the photoelectron energy is characteristic. When the excitation source energy is fixed, the photoelectron energy is only related to the type of element and the atomic orbital excited by ionization. In addition, the chemical environment of atoms is different as the shift of the spectral peak. Yi et al. reduced H 2 PtCl 6 to Pt and fixed it on the heterojunction constructed by N, B, and Fdoped ReS 2 (NBF-ReS 2 ) and Mo 2 CT x . [68] The binding energy of the fixed Pt was close to that of the zero-valent Pt, with a positive chemical shift of 0.9 eV, which proved that the Pt single atoms are successfully synthesized and strongly interacted with the carrier (Figure 8a). SA Pt catalysts synthesized by Cui et al., with different amounts of Pt, all exhibited zero-valent oxidation state and forward chemical shift (Figure 8b). [69] Nonmetallic B Figure 8. a) Pt 4f X-ray photoelectron spectroscopy (XPS) spectra of Pt/NBF-ReS 2 /Mo 2 CT x and Pt foil. Reproduced with permission. [68] Copyright 2021, Elsevier. b) Pt 4f peaks of Ti 3 C 2 T x @1Pt and Ti 3 C 2 T x @5Pt. Reproduced with permission. [69] Copyright 2020, Wiley-VCH GmbH. c) B 1s, d) C 1s, and e) V 2p XPS spectra of VC/CNFs and B 11 -VC/CNFs. f ) N 2 temperature programmed desorption curves of VC/CNFs and B 11 -VC/CNFs. Reproduced with permission. [70] Copyright 2021, Wiley-VCH GmbH. XPS spectra of g) V 2p in Co@V 2 CT x and V 2 CT x ; h) Nb 3d in Co@Nb 2 CT x and Nb 2 CT x ; i) Ti 2p in Co@Ti 3 C 2 T x and Ti 3 C 2 T x . Reproduced under the terms of CC-BY license. [51] Copyright 2022, The Authors, published by MDPI. atoms formed B─C─V bonds on the surface of vanadium carbide, in which electrons flowed from B and V to C. [70] The B doping caused V 2p binding energy to move forward, and the content of V in the high state increased, suggesting the electron contribution of V to C (Figure 8c-e). In addition, the B─C─V enhanced the adsorption of nitrogen on the catalyst, which was conducive to the nitrogen reduction reaction (Figure 8f ). Zhao et al. anchored Co single atoms on the surface of V-, Nb-, and Ti-based MXenes. [51] In the doping process, the oxidation of V, Nb, and Ti occurred to different degrees, which was attributed to the redistribution of charge (Figure 8g-i).

Hydrogen Evolution Reaction
As a clean energy, hydrogen produced by electrocatalytic water splitting can be used as an alternative to fossil fuels. As the key substance in the process of electrolytic hydrogen production, the electrocatalyst of HER plays an important role in the industrialization of electrolytic water. High dispersion of precious metals (Pt, Ir, Ru, Rh, etc.) can improve the atomic utilization while maintaining high HER activity. By electrochemical stripping method, Zhang et al. fixed single-atom Pt on the Mo vacancy on the surface of bimetallic MXene to form stable Pt─C bond. [71] Under acidic conditions, it achieved a current density of 100 mA cm À2 at an overpotential (η 100 ) of 77 mV, with a high mass activity (about 40 times than that of Pt/C), and could maintain stability for 100 h. Through a spontaneous reduction method, Cui et al. synthesized MXene@Pt, which kept stable for 800 h at the current density of 10 mA cm À2 . [69] Using the reducibility of V 2 CT x , Park et al. fixed successfully 0.88 wt% Pt atoms in the V vacancy of V 2 CT x , exhibiting high HER activity in both acidic and alkaline media. [43] Through the bonding of Pt with N and O, Kang et al. fixed Pt in nitrogen-doped Ti 3 C 2 , and the obtained catalyst showed high FE in a practical device. [72] By electrostatic adsorption and high-temperature treatment, Peng et al. synthesized Pt sites on porous N-and P-co-doped MXene (PNPM) to obtain 3D Pt SA-PNPM, where N/P-coordinated Pt increased the intrinsic activity of the electrocatalyst. [55] In addition, the Pt SA-PNPM catalyst displayed low overpotential and small Tafel slope comparable to Pt/C over a wide PH range, but had higher mass activity than Pt/C (Figure 9a- (Figure 9i-o). [52] Ru SAs-MXene synthesized by Zou et al. showed excellent alkaline HER properties, which could reach the current density of 1 A cm À2 at the overpotential of 435 mV, in which the loading capacity of Ru was only 0.44 wt%. [54] In addition, the mass activity of the Ru SAs-MXene increased with increasing potential and was two orders of magnitude higher than that of Pt/C at high current density. In addition, theoretical calculations showed that the adsorption of the intermediate H occurred on the three oxygen atoms adjacent to Ru atom rather than Ru atoms, where the ΔG was close to 0 eV. Nitrogen-and sulfur-co-doped Ir [58] or Ru [53] in the surface of MXene also exhibited enhanced HER activity, which was attributed to the redistribution of the charge of the active metal atom, improving the adsorption of the intermediate H. [39] By calculating the formation energy of the anchored TM at the Ti vacancy, Kong et al. found that TM tended to embed MXene as a single atom rather than a bulk metal, with a negative adsorption energy (Figure 10a,b). [73] In addition, Mo-Ti 2 N was a potential high-performance HER catalyst with a hydrogen adsorption-free energy close to that of Pt (Figure 10c). Kuznetsov et al. replaced part of Mo in Mo 2 C with Co to form an isolated Co-based catalyst Mo 2 CT x :Co, similar to bimetallic MXene solid solution, where Mo and Co had the same coordination structure. [46] Under acidic conditions, the HER activity of Mo 2 CT x :Co was significantly higher than that of Mo 2 CT x with the same load, and even better than that of Mo 2 CT x with ten times loading (Figure 10d-f ). The increased activity was attributed to the perturbation of the electronic structure on the surface of MXene by Co doping, which made the adsorption of H by oxygen atoms more moderate and conducive to the occurrence of HER (Figure 10g-i). The doping of Ni at the Mo vacancy also exhibited low overpotential and fast kinetics. [74] It is worth noting that most TM doping improves HER activity by promoting the reduction of Ti 4þ to Ti 3þ and inducing oxygen vacancy, while Cr doping is more inclined to the SAC-driven mechanism, in which the reduction of Cr 3þ to Cr 2þ occurs during HER process. [65]

Oxygen Electrode Reaction
Bifunctional oxygen electrode is an important part of rechargeable metal-air batteries and reversible fuel cells. [75,76] However, due to large overpotentials required to overcome the large reaction barriers of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), the energy efficiency of metal-air batteries and fuel cells is greatly reduced, which limits their industrialization. In recent years, MXene-based SA electrocatalysts have been widely studied for OER and ORR. Xu et al. loaded Pt nanoparticles onto MXene for the ORR, and meanwhile added CNT to suppress the distortion of Pt nanoparticles. [77] The reduction of the atomic scale and the MXene substrate increased the mass activity of the catalyst by 3.4 times compared with Pt/C, even if the catalyst did not reach the single-atomic scale. Moreover, SA Pt was synthesized via a static method, and only 0.35% Pt was loaded on TiN carrier. [61] The coordination number of Pt-Pt of 0.35 wt% Pt/TiN was 0.583, which was much smaller than that (8.310) of 5 wt% Pt/TiN, proving that there was no nanoparticles. During the ORR process, 0.35 wt% Pt/TiN mainly underwent two electron transformations, presenting high selectivity (65%) for H 2 O 2 . Liu et al. calculated the effect of surfacefunctional groups on single-Pt atoms, and the results showed that the ÀF was more conducive to the ORR catalysis than ÀO. [27] If a certain degree of strain is applied, the metal d-band center and work function of MXene would shift, improving the electrocatalytic performance. The overpotential of Pt-V F -Ti 2 CF 2 , with Pt loaded on the F vacancy, decreased significantly under 14% compressive strain and 4% tensile strain (0.28 V for ORR and 0.03 V for OER). [78] Furthermore, Ma et al. studied the effect of supports on bifunctional ORR/OER performance and discovered the η, η 10 , and ΔE (E η10 À E 1/2 ) of Pt-V S -Ti 2 CSBr, Pt-V I -Ti 2 COI, and Pt-V S -Ti 2 CSBr as ORR, OER, and ORR/ OER catalysts were 0.35, 0.34, and 0.79 V, respectively. [79] Kuznetsov et al. synthesized a highly dispersed singleiron-atom catalyst by injecting 1 mol% iron atoms into the Mo vacancy of Mo 2 CT x . [47] Under oxygen reduction conditions, Mo 2 CT x :Fe would degrade in situ to a carbon skeleton to form Fh/C, in which the ferric hydroxide species were the true active species to activate ORR (Figure 11a). Under alkaline conditions, the catalyst showed excellent ORR performance and its selectivity to H 2 O 2 was still up to 90% when the applied voltage was greater than 0.4 V (Figure 11b-d). To adjust the electronic structure of Fe atoms, Liu et al. constructed an axial Fe-O-Ti structure with Ti 3 C 2 T x as a substrate, which could change the spin state of Fe-N 3 O site and improve its adsorption of O 2 (Figure 11e). [80] In 0.1 M KOH, the catalyst had a half-wave potential (E 1/2 ) of 0.861 V and a mass activity of 182.1 mA mg À1 , which was 4.3 times better than that of the catalyst before optimization.
What's more, the catalyst that underwent the four-electrontransfer reaction provided greater power in the fuel cell.
The OER activity of SACs has been screened by theoretical calculations. Chen et al. studied the loading of a series of mid-to-late TM atoms on M 2 NO 2 (M = Ti, V, Cr) and proved that Cu-Ti 2 NO 2 was a hopeful electrocatalyst for the OER, with a low aggregation trend and a low theoretical overpotential of 0.24 V. [81] Via a controlled pyrolysis to remove Zn atoms, Kou et al. successfully synthesized Cu-, Co-, and Ni-doped Mo 2 C with imidazole salts as raw materials, in which Co-based Mo 2 C showed the best OER/HER-bifunctional activity. [82] For OER, the Co-based Mo 2 C had a low η 10 of 270 mV and high turnover frequency (1.37 times higher than that of Co SA without MXene), owing to the improved adsorption of OH* intermediate on the Co sites. Theoretical calculations revealed that the coordination structure of Co and three Mo broke the proportional relationship between adsorption energies of adjacent intermediates, increasing the intrinsic activity of the catalyst.  [55] Copyright 2022, American Chemical Society. Density functional theory (DFT) calculations of Ti 3 C 2 O 2 -based catalysts for the HER. i-n) Atomic structures for Ti 3 C 2 O 2 , Ti 3 C 2Àz O 2Ày -N-Pt SA , and Ti 3 C 2 O 2Ày -Pt SA , respectively. o) Calculated Gibbs-free energies of adsorbed hydrogen. Reproduced with permission. [52] Copyright 2022, American Chemical Society. Bifunctional oxygen electrodes with high OER and ORR activity are more practical applications in fuel cells and metal-air batteries. Kan et al. constructed Pt and Pd on MXene, and studied the influence of M (Nb, Ti, V, or Cr), X (C or N), and T (O or F) on the performance of SA electrocatalysts. [83,84] Theoretical calculations indicated that the combination of Nb-based carbides, F-functional groups, and Pt doping displayed the best OER/ ORR-bifunctional catalytic performance. Furthermore, Wei et al. constructed an dual-atom catalyst (DAC) model of double transition metal-doped MXene, and screened out Ni1/Ni2-and Fe1/Ni2-MXene with OER/ORR-bifunctional catalytic activities, which were even better than the Pt and IrO 2 ( Figure 12). [85] The electron induction between Fe, Ni, and oxygen atoms and moderate tensile strain changed the electronic structure of MXene surface, which provided the theoretical basis for the design of bimetallic atom electrocatalysts for OER/ORR.

Carbon Dioxide Reduction Reaction
The electrocatalytic reduction of CO 2 to hydrocarbons is a green and economy strategy to mitigate the greenhouse effect. [86] Copper-based catalysts are potential electrocatalysts for CO 2 reduction due to their ability to break C─O bonds and low HER competitiveness. By etching Al atoms from quaternary Ti 3 (Al 1 À x Cu x )C 2 , Zhao et al. synthesized SA-Cu-MXene with atomic copper immobilized on the surface of Ti 3 C 2 , where Cu was connected with end oxygen. [40] In 0.1 M KHCO 3 saturated Figure 10. a) Adsorption energy (E ads ) and b) differences between adsorption energy and cohesive energy (E ads À E coh ) of TM atoms. c) Correction of ΔG H and d-band center value on TM sites. Reproduced with permission. [73] Copyright 2022, American Chemical Society. d) Mo 2 CO 2 :Co model structure used for DFT calculations. e) Computed values of ΔG H on Mo 2 CO 2 and Mo 2 CO 2 :Co surfaces. f ) Reaction coordinate for the hydrogen evolution on Mo 2 CO 2 and Mo 2 CO 2 :Co. g) Mo 2 CO 2 :Co model structure used for DFT calculations. h) Computed values of the free energies of hydrogen adsorption (ΔG H ) on Mo 2 CO 2 and Mo 2 CO 2 :Co surfaces. i) Reaction coordinate for the hydrogen evolution on Mo 2 CO 2 and Mo 2 CO 2 :Co with average |ΔG H | values computed for the hydrogen adsorption on six oxygen atoms shown in red on panel (g). Reproduced with permission. [46] Copyright 2019, American Chemical Society. with CO 2 , SA-Cu-MXene reduced CO 2 to methanol, and when the applied voltage was 1.4 V, the FE of methanol production reached a maximum of 59.1%, much higher than that of Cu nanoparticles (Figure 13a-d). Theoretical calculations showed that the transformation from HCOOH* to CHO* was ratedetermining step for the SA-Cu-MXene (Figure 13e,f ). Furthermore, by simple standing m8ethod, 1.04 wt% of Cu was fixed to Ti 3 C 2 T x , which reduced CO 2 to formic acid with an FE of 58.1%, four times that of Ti 3 C 2 T x . [87] Theoretical calculations showed that, in addition to Cu species, other transition metals, even Ti 2 CO 2 also had a positive effect on the enhancement of CO 2 RR activity. [88] Oxygen-functional groups bonded with TM provided additional electrons into the 2π* u orbital of CO 2 , and split 2π* u into the 2π* u (unocc) orbital and d-2π* u (occ) orbital (Figure 13g). Such orbital rebuilding resulted in the transformation of CO 2 structure from linear to V-shaped, which is conducive to the first hydrogenation step. In addition, the maximum Gibbs-free energy (ΔG max ) on Cr-anchored Ti 2 CO 2 during CO 2 RR was smaller than that during HER, presenting high CO 2 RR selectivity. In addition, Co-Ti 3 C 2 T x was also synthesized for the CO 2 RR through a coordination process facilitated by electrostatic interaction (Figure 13h). [36] In addition, nonmetallic SA doping significantly improves the CO 2 RR performance of MXenes. By calcining in ammonia, Qu et al. introduced nitrogen atoms and Ti vacancies into MXene, which effectively reduced CO 2 to CO in seawater. [89] The maximum values of FE CO (92%) and j CO (À16.2 mA cm À2 ) could be reached at À0.7 and À0.8 V, respectively, which were equivalent to the best electrocatalysts including precious metals. The coordination of nitrogen atoms and Ti vacancies improved the electronic structure of active sites and regulated the adsorption and desorption of intermediates.

Nitrogen Reduction Reaction
The industrial production of ammonia mainly relies on Haber-Bosch method, which needs high temperature and high pressure. Electrocatalytic NRR is a promising green alternative to  [47] Copyright 2021, The Authors, published by American Chemical Society. e) The synthesis process of O-MQFe. Reproduced with permission. [80] Copyright 2022, Wiley-VCH GmbH. fix N 2 at room temperature and pressure. Boron catalyst has been widely applied in the field of NRR, and adding nonmetal boron into the MXene can effectively promote the reduction of N 2 . Wen et al. synthesized vanadium carbide containing a high concentration of B atoms, in which the redistribution of electrons on the B─C─V bond regulated the adsorption of N 2 and reduced the reaction energy barrier. [70] The ammonia yield of the catalyst was 0.443 mol h À1 cm À2 and the FE was 46.1% in 0.5 M K 2 SO 4 solution (Figure 14a,b). The electron-deficient property of B makes it have a high tendency to combine with electron-rich N, which is conducive to the first hydrogenation, but not favorable to the last hydrogenation. However, transition metals in MXene can adjust its electron-donating capacity to medium, which is advantageous to the release of NH 3 (Figure 14c-f ). [90] For NRR, the theoretical limit potentials of B-Mo 2 CO 2 and W 2 CO 2 were only À0.2 and À0.24 V, respectively. In addition, B@V 3 C 2 O 2Àx was also predicted to display ideal NRR catalytic performance. [91] Additionally, single-carbon atoms injected into the oxygen vacancy of MXene showed NRR activity. [92] As interacting with adjacent TM atoms, the electron filling in the P z orbitals of active carbon atoms changed, further promoting the activation of N 2 . With low limit potential (À0.14-À0.38 V), single-C atoms on Nb 2 CO 2 , Mo 2 CO 2 , and W 2 CO 2 were screened as active NRR catalysts with high selectivity. Peng et al. fixed single-ruthenium atoms into the Mo site of Mo 2 CT x for NRR under mild conditions, where Ru content was 1.41 wt% (Figure 15a). [64] At À0.3 V, Ru-Mo 2 CT x had an ammonia yield of 40.57 μg h À1 mg À1 , more than double that of commercial Ru/C, and an FE of 25.77%. Theoretical calculations indicated that the interaction between Ru 3d and C 2p orbitals was stronger than the hybridization between Mo 3d and C 2p, making the adsorption and activation of N 2 easier, while making the reaction barrier of the speed control step lower. Since Ru-O 4 has the lowest formation energy on the surface of MXene compared with the combination of Ru with other functional groups, Ru SAs/Ti 3 C 2 O catalyst was synthesized by Chen et al via an alkali treatment method. [37] At a low voltage of À0.2 V, the catalyst achieved high ammonia yield (27.56 μg h À1 mg À1 ) and FE (23.3%), which could remain stable for 20 h (Figure 15b-g). The Ru-O 4 site as the active center of NRR catalyzed the reaction by a more favorable distal/alternating hybrid path, with a significantly reduced ΔG max of 0.78 eV, where the hydrogenation from *NNH 2 to *NHNH 2 was the rate-limiting step.
In general, the adsorption strength of the intermediate on the catalyst can serve as a descriptor of the NRR theoretical overpotential, and the first protonation of N 2 and the last generation of NH 3 determine the overall NRR overpotential. [93] Gao et al. calculated the theoretical overpotential of N 2 activation and reduction when TM was anchored to Ti 3 C 2 O 2 , and found that the load of single-Fe, Co, Ru, or Rh atoms had NRR activity. [94] Furthermore, the NRR overpotential as low as 0.16 V was calculated on Mo@Mo 2 CO 2 , which catalyzed NRR through distal or Figure 12. a) Configuration of a rechargeable metal-air battery. b) Electrocatalytic mechanisms for the formation of Fe/Co/Ni-DACs based on Ti 2 CO 2 and for the oxygen evolution reaction/oxygen reduction reaction (OER/ORR). c) Theoretical polarization curves for Ni0-SAC, Ni1&Ni2-DAC, and Fe1&Ni2-DAC and experimental curves for reported bifunctional electrocatalysts in the full range of ORR/OER potentials. d) Calculated ORR and OER activities of bifunctional catalysts. Reproduced with permission. [85] Copyright 2021, Wiley-VCH GmbH. hybrid mechanism. [95] The covering of other nonmetallic functional groups (ÀOH, ÀF or ÀS) on MXene could further reduce the limiting potential compared to the covering of pure oxygenfunctional groups. [96] For example, the presence of ÀOH accelerated the adsorption of N 2 and the desorption of NH 3 . [97] On the surface of nitride Ti 2 NO 2 , single-Mo atom exhibited the best NRR performance (ΔG NH3 = 0.12 eV), with high inhibition for HER and high conductivity, accelerating electron transfer. [98] In addition, other 2D MXenes have been screened as suitable carriers of single-TM-atom catalysts for electrocatalytic NRR. [99][100][101][102][103]

Applications in Energy Storage
MXene modification can make it show better performance in the battery system. [104] Ion reserve and capacity retention are the indicators to evaluate the performance of ion battery. [105] The insertion of Co 2þ ions into V 2 C doubled the Li-ion capacity of the electrode material from 686.7-1117.3 mA h g À1 at 0.1 A g À1 , owing to the interlayer expansion induced by V-O-Co (Figure 16a-c). [56] Further, the V-O-Sn structure was synthesized in the Sn/V 2 C electrode by ion exchange, achieving a high lithium capacity (1284.6 mA h g À1 at 0.1 A g À1 ) and maintaining stability for 1600 cycles (Figure 16d-f ). [106] Compared with Li þ , large-radius K þ ion is easy to cause deformation of electrode material, which seriously affects the recycling of battery. Guo et al. synthesized an electrode material SQ@MA with high potassium storage by simultaneously embedding Sb SAs and Sb quantum dots (QDs) into 3D Ti-based MXene aerogel (Figure 16g,h). [59] Among them, the high load of Sb QDs ensured the storage of K þ , and Sb SAs accelerated the charge-transfer dynamics between Sb QDs and MXene. Meanwhile, the porous structure of aerogel provided a fast electron transport path and stable structure, prolonging the battery life. As the anode, SQ@MA provided a high reversible capacity (521 mAh g À1 at 0.1 A g À1 ) and high capacity retention (94% at 1 A g À1 even after 1000 cycles) (Figure 16i,j). Due to the dendrite growth of the Li anode and the slow kinetics of the S cathode, Li-S batteries face the problems of low coulomb efficiency and poor electrochemical reversibility even if they have high energy density. Gu et al. replaced part of Ti atoms Reproduced with permission. [40] Copyright 2021, American Chemical Society. g) Schematic diagram of CO 2 activation mechanism on TM/Ti 2 CT x . Reproduced with permission. [88] Copyright 2022, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. h) Mechanism of photocatalytic CO 2 reduction. Reproduced under the terms of CC-BY license. [36] Copyright 2021, The Author(s), published by Elsevier. in Ti 3 C 2 Cl x with 0.87 at% Zn, which effectively inhibited the dendrite of Li. [107] Since Li can form an alloying phase with Zn, Li tended to nucleate uniformly on the surface of the Zn-Ti 3 C 2 Cl x layer, and then grew further vertically at the nucleation sites due to a strong lightning rod effect at the edges, thus providing bowlshaped lithium rather than lithium dendrites (Figure 17a).
The electrode life of lithium anode with Zn-Ti 3 C 2 Cl x membrane was greatly extended (1200 h), which was attributed to controlled Li nucleus growth. The performance of S cathode can also be improved by the Zn single atom on MXene. Zn-MXene was used as the main body of sulfur cathode, showing high reversible capacity (1136 mAh g À1 at 0.2 C), high area capacity (5.3 mAh cm À2 at 6 C), and high rate capacity (640 mAh g À1 ). [108] Due to the electrondonating ability of Zn, the single-atom Zn had a strong affinity with polysulfide, which was conducive to the adsorption and nucleation of polysulfide. In addition, the injection of Zn increased the valence band energy level of MXene, and reduced the rate-limiting reaction barrier from 0.91 to 0.71 eV, accelerating the redox kinetics of polysulfides. In addition to serving as the main electrode, the diaphragm in battery play an important role in reducing the dendrite crystallization of the Li anode and avoiding the harm caused by the diffusion of polysulfide. [109] Zhou et al. anchored Pt in In 2 S 3 /Ti 3 C 2 compound by photochemical reduction method for the modification of polypropylene (PP) diaphragm of lithium-sulfur battery (Figure 17b,c). [62] When 0.43 mg cm À2 Pt-In 2 S 3 /Ti 3 C 2 was supported on the PP film, 8.94 μm film was formed, which completely covered the void on the PP film and inhibited the migration of hydrogen polysulfide, but did not affect the diffusion of Li þ . In addition, the modification of Pt SAC on PP film not only promoted the adsorption of polysulfide and the oxidation of Li 2 S on the sulfur cathode, but also regulated the uniform distribution of Li and inhibited the dendrite growth on the Li anode. The initial cell capacity of the cell equipped with modified PP film was improved, the coulomb efficiency was close to 100%, and the capacity attenuation per cycle was only 0.019%. Briefly, MXene modified by single atoms has a promising application prospect in battery system, including cathode, anode, and diaphragm.
Metal-air batteries, with air as the part of cathode, have an uninterrupted cathode active substance to provide energy in theory. The common air batteries are Li-O 2 battery and zinc air battery, in which oxygen participates in the reaction of cathode, and OER and ORR occur in the process of charging and discharging, respectively. Hence, rapid kinetics of OER/ORR and cycle stability are essential for metal-air batteries. Due to the unsaturated ligand of surface TM atoms, MXene QDs displayed good performance in Li-O 2 batteries, with high selectivity, significant charge/discharge potential (0.62 V), and good stability (240 cycles at 200 mA g À1 ). [110] Single-atom Se doped to Ti 3 C 2 via a thermal reaction strategy, which was a high-performance Li cathode in Li-O 2 battery. [45] Furthermore, the active center Se single atom was favorable to the adsorption and nucleation of LiO 2 , and then promoted the charge transfer between the discharge products of Li 2 O 2 , resulting in the accelerated redox reaction. Li-O 2 battery assembled with SeSA-Ti 3 C 2 as cathode had extremely high discharge capacity (17 260 mAh g À1 at 100 mA g À1 ). In addition, the single-atom Pt-doped heterojunction NBF-ReS 2 /MoC 2 T x was found to have bifunctional OER/ORR activity, stable charging, and discharging for 100 h as the anode of the zinc-air battery. [68] [111] Supercapacitors are another type of energy-storage devices, which store energy through charge accumulation and reversible redox reactions. Compared with batteries, supercapacitors have a faster charge and discharge rate but a faster decay of energy density. [112] Li et al. inserted Al ions into Ti 3 C 2 T x , providing a specific capacity of 175 F g À1 , and due to the increased layer spacing, more Li ions were stored for subsequent ion exchange. [113] V 3 CrC 3 T x was synthesized by replacing part of the V atoms with Cr atoms, which increased the adsorption capacity for Zn ion. [57] The synthesized Zn anode material based on V 3 CrC 3 T x presented increased capacitance and discharge potential, with high surface energy density (51.12 mWh cm À2 ) and foldability (80.2% capacitance retention after 10 000 cycles), indicating high competitiveness in the use of flexible portable power supplies.

Gas Sensors
Applications of MXenes in gas sensors have been reported including theoretical calculations and experiments. As a raw material for many industrial productions, NH 3 is one of the common industrial pollutions and is harmful for the environment and human body, thus there is an urgent need for fast and convenient NH 3 sensor. Single-layer Ti 3 C 2 was synthesized by NaF þ HCl etching without the addition of any single atom for the detection of NH 3 at room temperature. [114] Not only NH 3 , single-layer Ti 3 C 2 also responded to other seven gases (CH 4 , H 2 S, H 2 O, etc.), but had the highest selectivity for NH 3 , which was four times more responsive than the next highest. However, both the multilayer Ti 3 C 2 and the addition of Figure 15. a) The fabrication mechanism of SA Ru-Mo 2 CT x . Reproduced with permission. [ metallic SA sites exhibited higher response to NH 3 than singlelayer Ti 3 C 2 . Wu et al. fixed Ag onto the surface of oxygen-rich Ti 3 C 2 T x via a self-reduction method, and tested the NH 3 sensor preformation of in dry and wet nitrogen, respectively. [115] At the humidity of 40% relative humidity, the response sensitivity of Ag-Ti 3 C 2 to NH 3 was the highest, which was attributed to the affinity between intercalated water and NH 3 . However, excessive water would hinder the adsorption of NH 3 at the active site. With the conspicuously strong adsorption capacity for NH 3 , Ag nanoparticles first adsorbed NH 3 and then the adsorbed NH 3 diffused to Ti 3 C 2 T x , increasing the response resistance (Figure 18a-d).
Triethylamine (TEA) has a strong smell of ammonia and is toxic to human body, which is urgent to be detected with a high sensitivity sensor. Zong et al. injected Pt single atoms into Ti 3 C 2 T x , greatly improving the sensitivity and selectivity of Ti 3 C 2 T x for TEA detection (Figure 18e,f ). [116] Theoretical calculations indicated that the reduced detection limit (14 ppb) was due to the chemical sensitization of Pt atom and the strong adsorption between Pt and TEA. Through NiCl molten salt etching, a NO 2 sensor Ni 1 /TiC 0.5 N 0.5 was synthesized with a low detection limit (190 ppb). [117] Since MXene is easily oxidized in wet environments, more efforts are needed to obtain MXene-based SA sensors with better performance.

Biological Sensors
Photoelectrochemical (PEC) response is a new technique used for the detection and analysis of gases and organic compounds by amplifying photocurrent signals. Light capture ability and interface redox reaction are the key to signal amplification. SACs as excellent biosensing catalysts have the active sites with the same coordination structure, corresponding to the unique enzyme active sites, and unique SA electron cloud distribution, which is conducive to signal recognition and amplification. In addition, 2D MXenes in combination with SACs with large interface, excellent conductivity, and easily adjustable electronic structure, have been investigated for biosensing systems. Qin et al. added single-Fe atoms to heterojunction of MXene and cuprous oxide to accelerate the interfacial redox reaction of P-type semiconductors. [118] The introduction of Fe-N 4 site changed the adsorption of oxygen reduction intermediates on Cu 2 O. In addition, the insoluble enzyme precipitation induced by Fe SACs inhibited PEC signal, suggesting that peroxidase could be detected by Fe SACs (Figure 19a). Furthermore, the addition of acetylcholinesterase regulated the enzyme activity of Fe and amplified the photoelectric signal, resulting in a low detection limit (0.08 ng mL À1 ) and a wide linear range (0.5 % 600 ng mL À1 ) for organophosphorus pesticides. Moreover, SACs on MXenes show promising applications in cancer diagnosis. Through calcinations, single-gold atoms were embedded in the Ti vacant of MXene, redistributing the charge of MXene through metal-carrier interactions (Figure 19b,c). [119] At the SA Au sites, hydrogen peroxide obtained electrons and converted to hydroxyl radicals, resulting in the significant increase of electrochemical luminescence intensity, which was twice that of Au NPs -MXene. In addition, in the synthesis process, part of Ti would be oxidized into TiO 2 , and the heterojunction of MXene and TiO 2 was constructed, which accelerated the formation of Figure 16. Schematic of expansion and shrinkage for a) V2C@Co MXene d) and V2C@Sn MXene. b,e) Rate performance. c,f ) Long-term cycling performance. Reproduced with permission. [56] Copyright 2018, Wiley-VCH GmbH. (d-f ) Reproduced with permission. [106] Copyright 2018, Wiley-VCH GmbH. Selected area electron diffraction patterns of the Sb SQ@MA electrode at g) fully discharged and h) charged states. i) Rate performance of Sb SQ@MA electrode. j) Long-term cycling performance of the Sb SQ@MA. Reproduced with permission. [59] Copyright 2022, American Chemical Society. free radicals and facilitated the generation of electrochemical signals. For the clinical detection of miRNA-187 in triple negative breast cancer tumor tissue, the luminescence intensity was positively correlated with the concentration of miRNA-187.

Summary and Outlook
SACs combine the advantages of heterogeneous and homogeneous catalysts and show great potential in various fields.
MXenes show promising prospects as substrates for SACs. In this review, we discuss the favorable properties of MXenes as carriers of single-metal sites, including abundant anchorage sites, adjustable electronic structure, and so on. Additionally, the characterization techniques of single atoms are summarized, including direct electron microscope images and indirect spectral analysis. Moreover, three strategies for fixing isolated metal sites on MXenes are condensed, including adsorption on the surface by bonding with functional groups, filling in M vacancies and filling in T vacancies. We not only analyze the feasibility of the strategy by the stability and aggregation trend of single atoms at the anchorage sites, but also summarize the existing experimental studies on the successful synthesis of single-atom sites on MXenes. The synthesis of single atoms on MXenes by different methods involves the following three steps: 1) metal cations approach MXene under the drive of electrostatic attraction or high surface energy; 2) the metal cations are reduced to zero valence by the reducibility of MXene itself or additional reductants; and 3) the metal is oxidized to a high valence state through generating significant electron transfer with MXene. For surface adsorption strategies, single atoms have high surface energy and readily available heteroatomic modifications that allow for more possibilities in their electronic structures. For the vacancy filling strategy, isolated atoms enter the MXene lattice, which means better anchoring effect. However, to ensure the stability of the structure, the types and loads of TM atoms are limited. The catalytic activity of SACs is closely related to their coordination structure, and the advanced characterization techniques Figure 17. a) Synthesis process of Zn atoms immobilized on MXene layers (Zn-MXene) for the Li nucleation and growth. Reproduced with permission. [107] Copyright 2020, American Chemical Society. b) Schematic illustrations of polysulfides suppressing and Li anode deposition process on PP and heterostructure-modified PP separator. c) Cycling performance of the LiÀS battery with a Pt SAs/In 2 S 3 /Ti 3 C 2 @PP separator at different folding angles. Reproduced with permission. [62] Copyright 2022, Wiley-VCH GmbH. make them possible to understand the coordination number and coordination atom of single atoms on MXenes. Through in situ characterization, the catalytic mechanism of SA sites in the catalytic process can be further analyzed. Moreover, the combination of the theoretical calculation and experiment is efficient means for improving the performance of SACs, especially the known atomic structure of MXene and controllable coordination sites are advantageous to the construction of calculation model. However, even the structures of SA active sites with high catalytic activity have been successfully screened by theoretical calculations; it is still challenging to synthesize these sites with specified coordination structure on MXene by experimental means.
Therefore, to obtain excellent-performance SACs supported by MXenes, it is necessary to further understand the structureactivity relationship between coordination environment and catalytic performance by in situ characterization. In addition, combining experiments with theoretical calculations could facilitate in depth understanding of the reaction mechanisms.
The applications of MXene-based SACs in electrocatalysis, energy storage, and sensors are summarized. The tunability of M, X, and T elements in MXene is conducive to the regulation of electronic structure, which can improve the adsorption and desorption behavior of reaction intermediates at single-atom sites and reduce the electrocatalytic reaction barrier.  [115] Copyright 2022, Elsevier. e) Schematic diagram of the synthesis of Pt SA-Ti 3 C 2 T x . f ) High specificity of the sensors toward triethylamine (TEA) compared with other volatile organic compounds and reducing gases. Reproduced with permission. [116] Copyright 2022, American Chemical Society. Figure 19. a) Operation of Fe SACs/Cu 2 O/Ti 3 C 2 T x -based photoelectrochemical (PEC) analytical platform. Reproduced with permission. [118] Copyright 2022, American Chemical Society. b) Fabrication of the Au SA /MXene and c) Electrochemiluminescence sensing system based on the Au SA /MXene and fluid nanoislands for detecting miRNA-187. Reproduced with permission. [119] Copyright 2022, American Chemical Society. For energy-storage devices, MXene-based SACs play an active role in the cathode, anode, and electrolyte diaphragm in advanced battery systems, which accelerate the reaction kinetics of the cathode, inhibit the dendrite precipitation of the metal anode, while avoiding adverse diffusion from the cathode to the anode. However, improvements in battery performance, including ion storage and cyclic stability, are limited only by the single-atomic sites due to the instability. Constructing additional MXene-based heterojunctions is usually necessary, where the single-metal atoms are further stabilized. MXene-based SACs show high responsiveness and selectivity to specific gases or organic compounds, but due to their easily oxidized properties, more efforts are needed for their detection applications in humid environments. Additionally, the application prospects of MXenebased SACs can be extended to more other fields, which could be studied by the combination of theoretical calculation and experimental verification.