The applications of single‐atom alloys in electrocatalysis: Progress and challenges

The development of cost‐effective and highly efficient electrocatalysts to accelerate distinct electrochemical reactions is essential to help the industry to achieve a low‐carbon footprint. Single‐atom alloys (SAAs) with the characteristics of unique electronic structures, well‐defined active sites, and maximum atom utilization demonstrate promising potential to replace traditional noble metal catalysts. SAAs are expected to tailor the adsorption properties of reaction species, thus promoting electrocatalytic behaviors. Herein, representative synthetic strategies including wet chemistry, galvanic replacement, dealloying, and atomic layer deposition are introduced, followed by a summary of applications of SAAs in hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, carbon dioxide reduction reaction, and ethanol electro‐oxidation to provide an in‐depth understanding of the structure–activity relationship. Moreover, the challenges and perspectives in this emerging field of SAAs are discussed.


| INTRODUCTION
There is an urgent need to develop advanced electrochemical energy conversion technology, like hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO 2 RR), and ethanol electro-oxidation (EOR), to mitigate the energy crisis and achieve sustainable development. 1,2 It is important to fabricate highly efficient and costeffective electrocatalysts to improve energy conversion efficiency and promote industrialization. To date, tremendous efforts have been devoted to this study domain, like hybridization, 3-6 alloying, 7-10 preparation of single-atom catalysts (SACs), [11][12][13][14] and so forth. 15,16 Due to the advantages of alloy effects (geometric effect, ligand effect, strain effect, and bifunctional mechanism), alloying is an effective and promising strategy to tune the electronic structure of metal catalysts, 17,18 while metal alloys still follow the scaling relationships due to the existing continuous sites of the individual constituent elements. 19 Thus, it is difficult to simultaneously lower the activation barriers of intermediates and weaken the binding energy of key intermediates for the bulk alloy. 20 Single-atom alloys (SAAs), with the advantages of decoupling the dissociation and reaction sites in electrocatalysis, are expected to break the linear scaling relationship. Normally, SAAs are composed of a metal matrix dispersed with active metal atoms on the surface, without forming metallic bonds between neighboring active sites, as shown in Figure 1. 21 Compared with traditional alloys, the single atoms in SAAs retain a free-atom-like electronic structure despite being alloyed in the surface of the host. 22 The possibility of two or more contiguous doping metal atoms on the SAAs' surface is extremely low. Therefore, finding a bridge or threefold-binding sites of the doping metal is impossible. 23 The content of active metal in SAAs is about 1%, so the dopant is isolated in the surface layer of the matrix. 24 Owing to the formation of metallic bonds between the doping atom and the matrix, the single atom in SAA is more stable against phase separation than other types of SACs. Different from their constituent metals, SAAs normally have unique electronic and geometric structures, providing active sites and altering the reaction pathways. 25 It is reported that the construction of SAAs represents a promising approach that can combine the superiority of both homogeneous and heterogeneous catalysts. 21 Compared with the monometallic component, SAAs show enhanced chemical heterogeneity on their surface layer. 24 In addition, all single atoms participate in the electrocatalytic process, and catalytic sites are identical. The charged metal center bound to surface atoms makes SAA systems similar to the metal-organic complexes for homogeneous catalysis. 26 Microscopic characterization demonstrates that the single atoms usually act as catalytic sites, where the reactant activation occurs, and then spill over onto the matrix surface. This effect leads to the efficient decoupling of adsorption and desorption sites, thus avoiding overbinding of the adsorbates. Specifically, the well-defined catalytic center facilitates substrate dissociation before intermediate binding at shared host-dopant sites, followed by entropically driven spillover onto a weak binding metal matrix. 27 SAAs are promising to break the restriction of the Brønsted-Evans-Polanyi (BEP) relationship in this manner (BEP, which refers to a linear dependence between the activation barrier and the reaction energy for a chemical conversion), thus showing improved catalytic performance. 21 To date, SAAs have shown remarkable catalytic performance in various chemical reactions with high atom utilization, including hydrogenation, 28,29 dehydrogenation, 22,30 HER, 31 OER, 32 CO 2 RR, 33 and so forth. For instance, Li et al. 34 synthesized PtNi alloy nanowires, followed by partial electrochemical dealloying, designing single Ni atom-modified Pt nanowires with the optimum integration of specific activity, and an electrochemical active surface area (ECSA) for hydrogen evolution, methanol oxidation, and ethanol oxidation reaction. Abundant activated Pt sites adjacent to Ni atoms and minimal blockage of the surface Pt sites are achieved. Different from the traditional SACs, single atoms usually act as the catalytic center; here, single Ni atoms were dispersed on the Pt surface to tailor the local atomic and electronic structures of the neighboring Pt atoms, which are essential to tailor the corresponding electrocatalytic HER activity. In another study, taking advantage of the similar lattice constants of Pt and Pd atoms, Zhang et al. 35 deposited Pt single atoms on the (100) and (111) surfaces of Pd nanoparticles by atomic layer deposition (ALD). In comparison with the commercial Pt/C catalysts, the asprepared Pt/Pd SAA shows greatly improved HER and ORR performances. The combination of characterization techniques and theoretical simulation demonstrates that the superior HER activity originates from the higher unoccupied density of states of the 5d character and the lower Pt-Pt coordination number of F I G U R E 1 Schematic illustration of SAAs and representative SAAs reported in the field of electrocatalysis (single atoms: turquoise; matrix: yellow). SAA, single-atom alloy.
Pt single atoms compared with the core-shellstructured catalyst. Moreover, the low binding energy of OH on Pt/Pd SAA is responsible for the ORR activity. Cao et al. 29 prepared a NiGa catalyst with isolated Ni sites for acetylene semi-hydrogenation, which shows superior performance than the Ni and Ni 5 Ga 3 intermetallic catalysts. The isolated sites play an important role in the acetylene reactant π-adsorption and the desorption of ethylene.
Although extensive investigations of SAAs have been conducted on photocatalysis, 36 surface science, 37,38 and traditional catalysis, 38,39 limited research has been conducted on the applications of SAA in the field of electrocatalysis ( Figure 1). With the advantages of maximum atomic utilization, defined active sites, and peculiar electronic and geometric structures, SAAs are expected to replace traditional catalysts. Although there are several excellent reviews focused on the catalysis of SAAs, 40,41 a comprehensive summary on the electrocatalytic applications and mechanism is rare. In this regard, we introduce the recent progress in this field, including common synthetic approaches and representative electrocatalytic applications (including HER, OER, ORR, CO 2 RR, EOR) of SAAs. The structure-activity relationship of SAAs is also presented for an in-depth understanding of the catalytic mechanism. In addition, the challenges and opportunities are addressed in the review to provide insight on the rational design and further development of SAAs.

| SYNTHETIC STRATEGIES OF SAA
To synthesize SAA catalysts, various approaches have been explored. Frequently used methods include wet chemistry, galvanic replacement, dealloying, and ALD. Moreover, other methods like the laser ablation in liquid technique, 42 melting, 21 the sacrificial template method, 43 and so forth have been applied to fabricate SAAs.

| Wet-chemistry method
Wet-chemistry methods generally include coprecipitation, chemical reduction, coreduction, and so forth. Coreduction is frequently applied in prepared SAAs. Generally, a reducing agent and a surfactant are used to reduce the metals from the precursor. The composition of a single atom can be tailored by changing the amount of precursors added. As shown in Figure 2A, AgNi SAA electrocatalysts were synthesized using Ni(acac) 2 and AgNO 3 as precursors, oleylamine (OAm) as a coreducing agent and a surfactant, tetrabutylammonium (TBAP) as a phase reagent, and 1,2-dodecanediol (DDD) as a reduction agent. 44 The composition of atomic Ag was tailored by changing the amount of AgNO 3 added. OAm is reported to trigger nucleation, while DDD and TBAP can help to stabilize the alloy structure. Using comparison tests, the function of these raw materials was analyzed. The addition of OAm and oleic acid can promote inhibition of the growth of grains and TBAP can prevent the phase separation of Ni and Ag. Similarly, Li dissolved Ru(CO) 12 , Ni(acac) 2 in a mixture of OAm, octylamine, and tetradecanediol to form a uniform solution, followed by a solvothermal method to prepare Ni atomically dispersed Ru SAA. 45 Besides, Wang et al. 46 proposed the use of NaBH 4 as a reducing agent, and H 2 PdCl 4 , Bi (NO 3 ) 3 , and 1-(3-aminopropyl)-3-methylimidazolium bromide as precursors at 60°C for 1 h to synthesize PdBi SAA aerogels. By changing the ratio of Bi to Pd precursors, PdBi SAA with different compositions can be obtained easily. However, this method is complicated and time-consuming, and the low yield is unsuitable for large-scale production.

| Galvanic replacement
The hollow structure can be designed with galvanic replacement, with the morphology resembling the starting template. For instance, if the original template is a nanotube, a nanowire, or a nanorod, the synthesized product will have the hollow-nanotube morphology. This technique utilizes the semireduction potential of the precursors. The standard electrochemical reduction potential difference between the precursors and the template provides the driving force for galvanic replacement. Theoretically, as long as the standard electrochemical potential of the precursor is slightly higher than that of the template metal, the chemical transition will occur spontaneously, and it has thermodynamic advantages at room temperature. 49,50 The larger the difference in the potential between them, the faster the reaction. For instance, as shown in Figure 2B, CuMgAl-mixed metal oxides (MMO) was obtained by calcination of CuMgAl-layer double hydroxides in air, followed by reduction in a H 2 /Ar atmosphere to prepare Cu/MMO supported by Cu nanoclusters. 25 Finally, galvanic replacement was performed to introduce Pt single atoms on the surface of Cu nanoparticles. However, by galvanic replacement, SAAs in which the incorporated metals have a higher reduction potential than the host can be fabricated, but it is unsuitable for guest metals with lower reduction potential compared with the host metals, thus limiting the preparation of SAAs with various components.

| Electrochemical dealloying
Electrochemical dealloying is a morphological or atomic reconstruction process to achieve the nanoporous structure by selective etching of transition metals from alloys via a corrosion process driven by electrochemical potential cycling. 49 By coarsening the surface, higher surface area can be achieved and compressive strain may be introduced into the structure, which can boost the electrocatalytic performance. By controlling the applied potential, SAAs can be fabricated using this technique. For instance, single Ni atom-modified Pt nanowires were synthesized by electrochemical dealloying with abundant activated Pt sites adjacent to Ni atoms and the minimum blockage of the surface Pt sites, showing outstanding multifunctional electrochemical performance. 34 Moreover, Yu et al. 47 synthesized Ir 3 Ni 97 alloy ribbons by arc melting and meltspinning, followed by cyclic voltammetry dealloying in HCl to remove unstable nickel species and obtain IrNi SAAs with a large ECSA ( Figure 2C). Additionally, Luo et al. 48 used galvanic replacement between Bi and RhCl 3 ·3H 2 O to prepare Rh-atomically dispersed PdBi nanoplates (PtBi@Rh 1 ), followed by electrochemical dealloying to remove Bi atoms in HClO 4 under a relatively high potential, and eventually obtained a tensile-strained Pt-Rh SAA (denoted PtBi@PtRh 1 ; Figure 2D).

| Atomic layer deposition
ALD is a cyclic process that depends on the ordered selfterminated surface reaction between the gas precursor molecules and the solid substrate surface. Actually, it provides accurate thickness control and aspect ratio for conformal deposition of atomic layers. 51 The desired SAA electrocatalysts can be produced by tailoring the depositing cycles of the ALD, which show excellent electrochemical performance. 35 For example, Zhang et al. 35 used ALD to deposit Pt on Pd nanocubes/octahedrons, with N-doped carbon nanotubes serving as the substrate to avoid aggregation of Pd nanoparticles. During the process, MeCpPtMe 3 was utilized as the precursor, and PtPd SAA may be formed after several ALD cycles. The process is simple, highly efficient, and controllable in comparison with other strategies.

| ELECTROCATALYTIC APPLICATIONS OF SAAs
As mentioned above, SAAs are synthesized by introducing reactive single-metal atoms into the inert metal hosts. SAAs are prospective to alternate traditional noble electrocatalysts owing to the maximum atomic utilization, unique electronic structure, and well-defined active sites, which is conducive to break the linear scaling of BEP relation, tailor the adsorption energy of reaction species and pathways, thus enhancing the electrocatalytic behaviors. Considering the potential of SAAs, we summarize the electrocatalytic applications of SAAs (including HER, OER, ORR, CO 2 RR, EOR) in Table 1, 52-54 elucidate the structure-activity relationship, and provide insight on the rational design of electrocatalysts with high activity, selectivity, and durability.

| Hydrogen evolution reaction
HER is regarded as a promising and sustainable approach to produce clean hydrogen energy, 55,56 which occurs through the Volmer-Heyrovsky or Volmer-Tafel mechanism. 57 Normally, the free energy of hydrogen adsorption (ΔG H* ) is used as a descriptor to evaluate electrocatalytic performance, and the approaching zero ΔG H* value suggests the superior HER activity in acid media owing to the balanced hydrogen adsorption and desorption behaviors. 58 Too large a ΔG H* value will lead to the overbinding of H* with the surface of catalysts, while too small a ΔG H* value will make it difficult for H* to interact with the surface. 59 In contrast, in an alkaline electrolyte, the kinetic barrier of water dissociation and hydroxyl adsorption determines the HER activity. 60 The development of highly efficient catalysts to promote the HER process is critical to improve the energy conversion efficiency and accelerate the commercialization. Pt and Pt-based materials are state-of-the-art catalysts for use in HER currently, due to their fast reaction kinetics for driving the HER process. 61,62 However, the high cost, low abundance, and poor rigidity limit the practical applications of Pt-based catalysts. In this regard, designing Pt-based alloys is meaningful to reduce the usage of noble metals and modify the electronic structure of catalysts, which is fundamental to optimize the adsorption hydrogen energy and enhance the catalytic activity. Different from traditional alloys, construction of SAAs is useful to activate the maximum number of Pt atoms and ensure the maximum atomic utilization of Pt atoms. 63 For instance, Li et al. 63 fabricated Co-substituted Ru nanosheets with atomically dispersed Co into a Ru lattice, which was confirmed by high-angle annular dark-field-scanning transmission electron microscopy (HAADF-STEM) images. In comparison to a Ru foil, the negative shifting of the Ru K-edge of Co-substituted Ru suggests electron transfers from Co to Ru ( Figure 3A), while the Co K-edge shows positively charged Co atoms in the sample ( Figure 3B). Besides, extended X-ray absorption fine structure (EX-AFS) spectra demonstrate the formation of Ru-Ru and Ru-Co bonds, further confirming the existence of Co single atoms in the Ru nanosheet ( Figure 3C,D). The Cosubstituted Ru nanosheet needs only 13 mV to deliver the current density of 10 mA/cm 2 in an alkaline electrolyte.
In situ X-ray absorption fine structure (XAS) results show the stability of the as-prepared catalyst. Although the Co-substituted Ru has stronger hydrogen binding ability resulting from the more negative ΔG H* value, the energy barrier (ΔG w ) of the O-H cleavage is critical to the whole reaction rate ( Figure 3E). The ΔG H* value of the Tafel step is the smallest, indicating that the water dissociation is the rate-determining step ( Figure 3F). Additionally, as shown in Figure 3G, the introduction of Co atoms significantly reduces the energy barrier of water dissociation, while more Co substitution (RuCo 2 , RuCo 3 ) leads to sluggish water dissociation. The increased OH binding energy with the number of substituted Co atoms is attributed to the increased electron density in the saddle points around Co atoms ( Figure 3H). This promotes the scission of the O-H bond and desorption of OH, and eventually prevents the subsequent reaction. Thus, the moderate OH binding energy of the single Co atom-substituted Ru nanosheets can create a balance between promoting water dissociation and preventing active sites from inhibition by highaffinity hydroxides and thus promote the OER process. It is known that the mass activity (MA) determined by an ECSA and specific activity is an important parameter to evaluate the electrochemical performance of the catalysts. SACs with high MA and maximum atomic utilization are promising to replace noble metals. Similar to the work mentioned above, 63 Li et al. 34 proposed the utilization of partial electrochemical dealloying to prepare single Ni atom-modified Pt nanowires (SANi-PtNWs) toward HER. Ni atoms are used to activate most of the Pt atoms, while blocking the smallest surface Pt atoms to deliver the highest MA. HAADF-STEM images show that abundant surface defects, concave cavities, and steps exist in SANi-PtNWs. Electron energy loss spectroscopy elemental mapping suggests that the Ni atoms are dispersed on the PtNWs. Electrochemical measurements indicate that SANi-PtNWs show a significantly increased MA at −70 mV in an alkaline electrolyte compared with traditional PtNi nanomultipods and PtNi-O octahedra. Different from previous research in which single atoms serve as the active sites, here, the Ni single atoms introduced decorate the surface of Pt, modifying the atomic and electronic structures of the surrounding Pt atoms, thus regulating the electrocatalytic performance. Density functional theory (DFT) calculations show that the hydrogen binding energy of Pt atoms surrounding Ni single atoms is reduced, which is beneficial to the HER process.
In contrast, Zhang et al. 42 proposed the preparation of RuAu SAAs using the laser ablation in liquid technique, owing to the possible geometric and electronic structures arising from the immiscibility between Au and Ru atoms and the opposite hydrogen adsorption ability of Au and Ru. HAADF-STEM images demonstrate that Au atoms replace partial Ru atoms, forming RuAu SAA. LSV curves show that only 24 mV is required to deliver a current density of 10 mA/cm 2 . Theoretical calculations show that the electron transfer from Ru to Au atoms contributes to the favorable H 2 O adsorption ability and hydrogen adsorption energy, thus promoting the HER process.
Tailoring the coordination environment of Pt atoms with secondary or tertiary metal atoms is favorable for the optimization of hydrogen adsorption energy. Herein, Chao et al. 31 reported a two-step approach to prepare atomically dispersed Pt-Cu dual sites on Pd nanorings, which is different from the methods requiring harsh synthetic conditions. HAADF-STEM corroborates the atomic dispersion of Pt and Cu on the substrate. EXAFS spectra for the Cu K-edge and the Pt L 3 -edge show the existence of Cu-Pd/Pt and Pt-Pd bonds, respectively. Further electrochemical tests revealed significantly enhanced HER activity of Pd/Cu-Pt in comparison with the single-site Pd/Cu and 20 wt% Pt/C. DFT calculations demonstrate that the optimal adsorption energy of H on the Pt site for Pd/Cu-Pt contributes to the remarkable electrocatalytic performance. Importantly, a Cu atom is critical to balance the interaction between Pt and H atoms, ensuring that the adsorption energy is not too strong. This study provides an avenue for the rational design of dual sites SAA.
According to the above-mentioned study, the introduction of a single atom in the metal matrix can enable tailoring of the adsorption free energy of intermediates, thus boosting HER activity. Although satisfactory performance has been achieved, noble metals are used as the matrix, which cannot be applied for large-scale commercialization. Accordingly, designing suitable SAAs with high activity and low cost to replace expensive materials is fundamental to promote the development of electrocatalysis.

| Oxygen evolution reaction
As a half-reaction of water splitting, OER dictates the efficiency of the whole process. The discovery of superior OER electrocatalysts is expected to accelerate the sluggish OER kinetics resulting from the complicated four-electron transfer process. 64 Most of the reported catalysts can catalyze the OER process in an alkaline electrolyte, whereas the poor stability restricts applications in acid. 65 However, compared with the alkaline electrolyte, the high proton concentration in acid can promote a faster HER process. Therefore, it is important to develop OER catalysts that can be operated in acid for water splitting. Ru-and Ir-based materials are considered to be state-of-the-art electrocatalysts in acid. [66][67][68] It is reported that OER normally proceeds through two mechanisms: an adsorbate evolution mechanism (AEM) and a lattice oxygen-mediated mechanism (LOM). 69 In addition to water molecules, the oxygen species of LOM also come from lattice oxygen of the catalysts. LOM commonly occurs with the severe dissolution of metal and the formation of oxygen vacancies, leading to the structural instability of the catalyst. Nevertheless, structure collapse of catalysts is uncommon in catalysts through the AEM mechanism. Therefore, switching from the LOM mechanism to the AEM mechanism is beneficial to inhibit the dissolution of Ru. By tailoring the coordination environment of Ru, the fast bulk diffusion rates and surface exchange reaction kinetics can be impeded. Considering the much lower dissolution rate of Pt resulting from the weaker oxygen bonding, Yao et al. 32 proposed embedding Ru single atoms into a Pt-rich environment (PtCu x /Pt skin ) through sequential acid etching and electrochemical leaching to suppress the overoxidation of Ru. Atomically resolved elemental mapping demonstrates the atomic dispersion of Ru atoms. EXAFS spectra indicate the existence of Ru-O, Ru-Pt, and Ru-Cu bonds, further corroborating the formation of the atomically Ru in Pt 3 Cu/Pt skin . Electrochemical measurements show that Ru 1 -Pt 3 Cu needs an overpotential of only 220 mV to deliver 10 mA/cm 2 current density. In addition, compared with the samples with a higher Cu content (Ru 1 -PtCu, Cu@Ru 1 -PtCu 3 ), the electrocatalytic activity of Ru 1 -Pt 3 Cu is superior, indicating that Ru single atoms serve as the active sites. In situ XAS was performed to investigate the oxidation of Ru during the OER process. In situ X-ray absorption near-edge structure (XANES) spectra show that the oxidation state and the coordination environment remain almost unchanged when the applied potential varies from 0 to 1.86 V. The d band vacancy for Pt atoms increased, which resulted from the electron transfer from Pt to Ru. Furthermore, in situ attenuated total reflection infrared spectroelectrochemical measurements were conducted to capture OOH and verify the AEM mechanism. Apparently, the peak located at 1212 cm −1 is attributed to the vibration adsorption of adsorbed OOH, confirming that AEM dominates the OER process on the surface of Ru 1 -Pt 3 Cu, which ensures the stability of the as-prepared catalysts. DFT calculations show that the induced compressive strain of the Pt skin shell regulates the electronic structure of Ru, resulting in the optimal binding strength of oxygen intermediates and improved resistance to dissolution and overoxidation. This study provides an avenue for the rational design of Ru-based SAAs in an acid electrolyte.

| Oxygen reduction reaction
ORR is an important electrochemical conversion process of metal-air batteries and proton exchange membrane fuel cells, and the development of cost-effective catalysts with improved electrocatalytic activity and stability is vital to promote practical applications. 70 Oxygen is reduced by either the 2e − pathway to H 2 O 2 (O 2 + 2H + + 2e − → H 2 O 2 ) or the 4e − pathway to produce H 2 O (O 2 + 4H + + 4e − → 2H 2 O). 71 The 4e − pathway is certainly preferred, with higher reaction kinetics and efficiency. Highly efficient catalysts are required to accelerate the kinetics of the ORR process resulting from the difficulty in activating oxygen, break the O-O bond, and remove oxides. 72 Pt-based materials are considered as excellent catalysts with the 4e − pathway toward ORR. 73 Nevertheless, considering the exorbitant cost and scarcity of Pt, there is an urgent need to discover alternative catalysts with low cost and excellent durability. Doping of Pt atoms into a cost-effective metal host can enable optimization of the electronic and geometric structures of the catalysts. 53 Pd is regarded as an ideal substrate to deposit Pt atoms due to their similar lattice constants. 74 Besides, it is reported that deposition of Pt on Pd with various exposed facets is favorable to tailor the coordination environment and electrocatalytic performance. 75,76 Zhang et al. 35 deposited Pt on Pd nanoparticles using the ALD method ( Figure 4A). 35 Owing to the different surface energies of octahedral (111) and cubic (100) facets of Pd particles, Pt atoms are atomically dispersed on the former (denoted Pt/Pd SAA), while 2-3 atomic layer Pt forms on the latter (denoted Pd@Pt). Polarization curves show that the ORR activity of octahedral Pt/ Pd SAA is greatly improved compared with the octahedral Pd. Notably, as shown in Figure 4B, the MA of octahedral Pt/Pd SAA is 4 times (ORR) and 54 times (HER) higher than that of commercial Pt/C. DFT calculations were performed to gain a more in-depth understanding of the origin of the enhanced catalytic performance. As shown in Figure 4C,D, two different configurations representing Pt/Pd SAA and Pd@Pt were constructed. Theoretical calculations demonstrated that the binding energy of OH on Pt/Pd SAA was smaller than that of Pd@Pt, causing the weakened interaction between the OH* intermediate and Pt atoms in Pt/Pd SAA, thus accelerating the ORR process. In another study, Cheng et al. 53 fabricated Pt 1Con SAA (n = 20, 50, 100, 150) encapsulated with N-doped graphite carbon nanotubes (N-GCNT) for ORR. The HAADF-STEM image shows that Pt single atoms are dispersed on Co nanoparticles. Pt L 3 -edge XANES spectra suggest that metallic Pt species exist in the samples. EXAFS spectra demonstrate the formation of a Pt-Co bond and the absence of a Pt-Pt bond in Pt 1Con SAA, corroborating the formation of Pt-Co SAAs. Besides, Co K-edge XANES spectra show a similar spectrum to Co/N-GCNT and Co foil, indicating that the electronic structure of metallic Co remains unchanged with the incorporation of Pt. The corresponding EXAFS spectra also confirm the result that only a Co-Co bond can be detected in Pt 1Con SAA. ORR polarization curves show the boosted electrocatalytic activity of Pt 1Con SAA compared with Co-N/GCNT and Pt/C in 0.1 mol/L HClO 4 . The MA of Pt 1 Co 100 -GCNT is also superior to that of Pt/C. Notably, after a 10,000cycle accelerated durability test, the activity can also be maintained, demonstrating the remarkable stability of Pt 1Con /N-GCNT. Morphological and spectroscopic characterizations confirm the stability of the catalyst. DFT calculations indicate that Pt-Co dual sites in Pt 1 Co 100 /N-GCNT can facilitate the immobilization of OOH* and dissociation of OH*, promoting the ORR process in a 4e − pathway. This study sheds light on the rational construction of dual-site SAAs.

| Carbon dioxide reduction
The markedly increased concentration of carbon dioxide in the atmosphere originating from burning of fossil fuels like oil, coal, and natural gas is exacerbating the greenhouse effect as well as increasing the average temperature of the earth, which has an adverse effect on the environment. Previous research has focused on the capture and storage of CO 2 to reduce the greenhouse effect, whereas now, the development of CO 2 utilization technologies such as electrocatalytic CO 2 reduction (CO 2 RR) is an urgently needed and sustainable strategy for conversion to distinctly value-added products. 77,78 Owing to the existence of the stable C═O bond in the CO 2 molecule, the dissociation of CO 2 is extremely tough. Therefore, development of efficient electrocatalysts to lower the activation energy is meaningful. The experience gained and classical theory derived from other electrocatalytic techniques like HER and ORR is valuable to guide the rational design of electrocatalysts for CO 2 RR. 79 Due to their high surface-to-volume ratio, monometallic nanoparticles have been widely studied as electrocatalysts for CO 2 RR. 79 According to previous investigations, metal-based materials can be classified into three categories briefly. For Sn-, Pb-, Hg-, In-, and Bi-based catalysts, the adsorption ability of *COOH is weak, making it easy to desorb from the catalyst surface and generating HCOOH due to higher O affinity and lower H affinity, 80 whereas other metals like Ag, Au, and Zn with enough binding energy with *COOH, yet weak binding capability with *CO, are expected to produce CO. Moreover, Cu is the only transition metal that has the potential to convert CO 2 into various hydrocarbons and oxygenates through a multielectron transfer process. The overpotential of hydrocarbon production is dependent on the protonation of CO*, and the Cu surface is reported to provide the best moderate binding energy for CO 2 RR intermediates that can minimize the energy barrier to this step. Nevertheless, the drawback is that Cu can produce a mixture of gases, making it difficult to separate them efficiently. Therefore, modifying the catalytic behavior of Cu is decisive to improve the selectivity and activity. 81 As mentioned above, it is generally acknowledged that the CO 2 RR process that occurs on the surface of catalysts involves diverse intermediates. Consequently, it is unavailable to tailor the adsorption energy of key radicals individually without disturbing the binding energy of other intermediates. 82 As shown in Figure 5, theoretical analysis demonstrates that the adsorption energy of two key intermediates (COOH* and CO*) is correlated linearly due to the same adsorption sites, making it impossible to control the adsorption energy independently. In addition, the adsorption energy of H* (intermediates for the competitive HER process) linearly scales with those of COOH* and CO*. This means that even if decreasing the adsorption energy of COOH* can lower the potential required for CO production, the H 2 yield resulting from the HER will still increase. Furthermore, adsorption energies of other intermediates like carbon-bound species (CO*, COOH*, CHO*, and CH 2 O*) and oxygen-bound (OH*, OCHO*, and O*) species can be scaled with linear equations, demonstrating that CO 2 reduction with a low overpotential is intrinsically restricted on pure metal surfaces. 83 This correlation between intermediates is known as the scaling relationship, which is particularly prominent for Cu-based catalysts, where a huge quantity of products can be generated through numerous different reaction pathways. 82 Under such limitations, for the catalytic surface of a single-metal element, achieving the optimal interaction intensity of all intermediates observed along the reaction path is difficult. To get rid of the scaling relationship and increase degrees of freedom toward the energetics of intermediates, it is necessary to introduce additional binding modes on the catalyst surface to stabilize or destabilize some intermediates. It is feasible to decouple the binding energy of multiple intermediates in this way, since some intermediates can potentially allow additional binding modes owing to their molecular configuration, like oxygen atoms approaching the catalyst surface. Thus, some strategies for catalyst modification including surface functionalization and metal alloying have been proposed to be efficient to facilitate specific interactions with certain intermediates. 84 The electronic structures of the atoms on the surface of electrocatalysts are largely influenced by their adjacent atoms. This effect is negligible in the monometallic catalysts due to the similar electronic structure of the surrounding atoms. On the contrary, for the metallic alloy, this effect is more obvious due to the interaction of different metals, which leads to the variation of the binding energy of intermediates and catalytic performance. In fact, the effect of alloying has been proven to be the synergistic effect of the geometric and ligand effects of individual metals, influencing chemisorption of intermediates during the electrochemical process. 82 Although there have been various applications of SAAs in formic acid oxidation, 85 CO poisoning, 86 HER, 34 and hydrogen oxidation reaction, 45 limited investigations have been conducted on CO 2 RR. It is generally acknowledged that Au shows a relatively weak CO binding energy, whereas that of Pd is strong. Accordingly, CO is easy to desorb from the surface of Au and CO will form, while Pd is likely to undergo CO poisoning. Here, Wang et al. 19 proposed dispersal of Pd atoms with a controlled amount on the surface of Au nanoparticles, thereby lowering the CO 2 activation energy barrier and mitigating *CO poisoning ( Figure 6A). As shown in Figure 6B, a sequence of Pd@Au electrocatalysts is prepared, resulting in the formation of bimetallic surfaces involving Pd ensembles with different sizes. The effect of Pd ensemble size on CO 2 RR electrocatalytic performance was explored. Electrochemical measurements suggest that Pd with low amounts decorated on Au nanoparticles can activate CO 2 molecules more easily, and the CO 2 RR catalytic activity increases with the Pd content ( Figure 6C). However, the alloy with a higher amount of Pd acts more like a pure Pd catalyst, producing more HCOOH ( Figure 6D). A DFT simulation was conducted to investigate the adsorption properties of key intermediates (*CO and *COOH). Figure 6E shows that the strength of CO binding decreases with the content of the Pd, while the stability of *COOH was confirmed to increase with the Pd ensemble size. The linear scaling relationship between the binding strength of *CO and *COOH for (111) and (100) shown in Figure 6F demonstrates that a Pd ensemble with the appropriate size (Pd dimer) can efficiently balance the binding strengths of *COOH and *CO, contributing to a relatively lower CO 2 activation energy barrier, yet without undergoing severe poisoning by a *CO intermediate. Therefore, the surface enrichment of the atomically dispersed Pd on the as-prepared Pd@Au alloy is responsible for the nonlinear behavior of compositiondependent electrocatalytic activity. This study highlights the importance of fine-tuning the atomic structure of catalytic centers for the design of high-efficiency catalysts.
In another research, Zhi et al. 87 utilized DFT calculations to predict the selectivity of 12 Cu-based SAAs (M@Cu, M = Co, Ni, Ru, Rh, Ir, Pt, Pd, Au, Ag, Zn, In, Sn) as electrocatalysts for CO 2 reduction by evaluating several adsorption configurations and energetics. Based on an extensive analysis, the selectivity of M@Cu can be tailored by alloying with a dopant with different metal-oxygen (M-O) affinities and metal-hydrogen (M-H) affinities, which is ascribed to different d-band centers of M atoms. The observed product distribution provides an effective theoretical explanation for the previous reports of the CO 2 RR selectivity trend of Cu-based alloy catalysts. In addition, it provides a further analysis of the mechanism of the selectivity of CO 2 RR products for Cu-based bimetallic materials. The intrinsic properties of the catalysts are correlated to the selectivity trend, which is vital to the rational design of highly effective and active catalysts. Besides, Cheng et al. 88 investigated the selective reduction of CO 2 to C 1 hydrocarbons for 28 SAAs with quantum mechanical screening. Au or Ag composed of the majority of the alloy with isolated M atoms (M = Cu, Ni, Pd, Pt, Co, Rh, and Ir) dispersed on the surface. The formation of CO during the CO 2 RR process preferentially occurs on the surface of Au or Ag, and then CO binds to M. Most SAAs containing M (Co, Rh, Ir) promote the formation of *CHO or *COH over *H, suggesting CO 2 reduction as the major reaction pathway instead of hydrogen reduction. This simulation indicates that the catalytic performance of SAAs is determined by M atoms. Moreover, the ability to bind only one CO molecule of M is helpful for the preferential selectivity to C 1 products rather than larger hydrocarbons.
However, due to the simplicity of single active sites in the structure, it is normally only applicable for catalysis of single-molecule reactions, while the activation step usually involves multiple molecules in the majority of catalytic reactions. For instance, although the CO 2 molecule is regarded as the only species to be activated, the synergistic adsorption of CO 2 and H 2 O is important. Thus, designing of "atomic interfaces" with higher complexity and more intricate functionality (normally the interface between two atoms with different charges or even distinct chemical identities) is essential to extend the related applications to electrocatalytic processes requiring different types of functional active sites. On the basis of the strong C-O bond-cleaving tendency and low C-C bond scission ability of Cu-based materials, as well as the proneness of breakage of the C-C bond of Pt-based catalysts, Zhang et al. 25 proposed designing a PtCu SAA by spreading Pt atoms on Cu to the maximum advantage to realize high activity and selectivity for glycerol hydrogenolysis, where the interface sites serve as intrinsic active sites. Similarly, Jiao et al. 54

| Ethanol electro-oxidation
Direct ethanol fuel cells with the characteristics of an environmentally friendly nature and high energy density are promising to replace traditional energy devices. 89 Development of efficient and durable electrocatalysts toward EOR is fundamental to accelerate the reaction kinetics. Previous research demonstrates that Pt-and Pdbased electrocatalysts are favorable to EOR, 90-92 while these materials normally suffer from the poor stability and insufficient activity owing to CO poisoning. 93 The partial oxidation of ethanol through the C2 pathway will lead to low fuel oxidation. Thus, development of catalysts that promote C-C cleavage is critical to produce carbon dioxide or carbonate through C1 pathways. Considering the advantages of SAAs, including nearly 100% atomic utilization, the unique electronic structure, and abundant active sites, fabrication of SAAs as EOR catalysts is important to optimize the adsorption ability of intermediates and boost the electrocatalytic performance. A previous study reveals that adding transition metals is efficient to remove the intermediates and enhance the electrocatalytic activity owing to the regulated electronic structure of catalytic sites. 92,94 In this regard, Wang et al. 46 reported ionic liquid (IL)functionalized PdBi SAA (IL/Pd 50 Bi 1 ) using the NaBH 4 reduction method. HAADF-STEM and EXAFS spectra confirmed the atomic dispersion of Bi and the existence of Bi-Pd and Bi-O bonds. Electrochemical tests show that the MA and stability of IL/Pd 50 Bi 1 are superior to those of IL/Pd. Besides, CO stripping experiments were performed to verify the tolerance of poisonous intermediates. The results show that the peak potential of CO oxidation on IL/Pd 50 Bi 1 is negative compared to that of IL/Pd, indicating that Bi single atoms can weaken the adsorption ability of CO. DFT calculations show that the introduction of Bi atoms is conducive to lowering the energy barrier of the rate-determining step, thus accelerating the EOR process. Moreover, controlling the favorable interactions and the synergistic effect in SAAs is rather difficult, limiting their application in the field of electrocatalysis. With long-range ordering and isolated atoms on the surface, intermetallics are an ideal SAA matrix. Luo et al. 48 designed tensile-strained Pt-Rh SAA (PtBi@PtRh 1 ) by electrochemical dealloying from PtBi-3.6% Rh 1 nanoplates. HAADF-STEM images reveal the formation of tensile-strained Pt shells in the presence of Rh atoms on intermetallic PtBi nanoplates ( Figure 7A). The MA of PtBi@PtRh 1 is 12 times higher than that of Pt/C. Besides, the electrocatalytic activity of CH 3 CHO electro-oxidation of PtBi@PtRh 1 is superior, compared with the negligible activity of Pt/C. Besides, PtBi@PtRh 1 has enhanced selectivity of the C1 pathway, confirming that the introduction of Rh single atoms into the tensilestrain Pt is capable of facilitating the cleavage of the C-C bond. Moreover, the antipoisoning ability of PtBi@PtRh 1 is also improved. DFT calculations were conducted to gain an in-depth understanding of the improved electrocatalytic performance. Figure 7B shows the partial density of states (PDOS) of Pt and Rh d-states. The change of the d-band center due to the tensile strain suggests the redistribution of d-states, among which the d-band center of the metal atoms in Rh/Pt(110) + 4% to the Fermi lever is the closest, which will lead to easier electron transfer from d-orbitals to ethanol, thus enhancing ethanol adsorption. Figure 7C shows the free energy of C2 pathways for Rh/Pt(110) + 4%. Compared with CH 3 CH 2 O and CH 2 CH 2 OH, it is apparent that CH 3 CHOH is preferred due to the more stable CH 3 CHOH* intermediate, and the dehydrogenation contributes to the formation of CH 3 COH. Clearly, CH 3 CO is produced due to the easily broken O-H bond. Further reaction with OH − could lead to the formation of CH 3 COOH; subsequently, deprotonation will produce CH 3 COO − . Additionally, free energies of C1 pathways for various samples were also calculated. It was found that the introduction of tensile strain and Rh doping decreased the activation barrier of C-C bond cleavage resulting from the strongest adsorption of CH* 2 and CO* ( Figure 7D), which is conducive to the C1 pathway. The synergistic effect of the tensile strain and single-atom doping in the metal matrix improves the EOR activity.
However, the above-mentioned catalysts did not achieve high selectivity for the electro-oxidation of  48 PDOS, partial density of states. ethanol to CO 2 ; thus, attempts should be made to design catalysts with higher activity and selectivity.

| CONCLUSION AND OUTLOOK
Owing to the unique electronic and geometric structures, well-defined active sites, and maximum atomic utilization efficiency, SAAs have the potential to replace traditional electrocatalysts. In this review, the synthetic approaches to synthesize SAAs are summarized, including wet chemistry, galvanic replacement, dealloying, and ALD. Representative electrocatalytic applications of SAAs like HER, OER, ORR, CO 2 RR, and EOR are also introduced, and the structure-activity relationship is elucidated.
However, although substantial progress has been made in the domain of SAAs, some issues need to be further addressed, including the following: 1. Most of the matrices of SAAs applied in the field in electrocatalysts are noble metals, like Pt, Pd, and Au. Therefore, to lower the cost and achieve commercialization of SAAs, designing of SAAs composed of transitionmetal matrices cannot be neglected. Fabrication of new SAAs catalysts with various compositions that are available in the reported bimetallic, trimetallic, or even high-entropy catalysts is necessary. Besides, development of SAAs with dual or triple metallic catalytic sites is fundamental to break the linear scaling relationship, tailor the adsorption properties of intermediates, and enhance the electrocatalytic performance. Increasing the density of single atoms in SAAs is a direct method to improve the catalytic activity of SAAs, as those single atoms commonly serve as the active sites under electrochemical conditions. In addition, instead of laboratory preparation, discovery of novel approaches to achieve the scalable production of SAAs with high yields is necessary. 2. HAADF-STEM can be used to distinguish atoms with a huge difference in atomic number, and XAS can identify the metal-metal bond. However, the identification of atoms with similar atomic numbers is difficult. Thus, development of advanced characterization techniques is critical to confirm the detailed atomic structure and coordination environment of the SAAs. Besides, considering the unclear and debatable structure-activity relationship of SAAs, the utilization of in situ or operando characterization techniques (microscopic, spectroscopic, and electrochemical methods) with high resolution in time and space is conducive to monitoring the changes in electronic and atomic structures of active sites, identifying the active sites, and capturing the reaction intermediates during the catalytic process 95,96 and deepen our understanding of the catalytic mechanism, 97,98 especially for complicated catalytic reactions like CO 2 RR with abundant intermediates. Additionally, the poor stability of SAAs resulting from the high surface energy may lead to the easy aggregation of single-atom active sites, and the reconstruction normally occurs during the long-term operation in the electrochemical process. Accordingly, in situ characterization techniques like spectroscopic, microscopic, and electrochemical methods at an atomic scale are available for the sensitive detection of the structural changes and to gain an in-depth understanding of the structure-activity relationship. 3. Compared with the cumbersome, complicated, and laborious experimental process, designing and screening proper catalysts toward specific electrochemical reactions with theoretical calculations is highly required, to replace the conventional trial-and-error approaches. Different from the time-consuming and computationally expensive DFT calculations, machine learning (ML) has been employed to screen the most active catalysts. 99 For instance, d-band is normally regarded as descriptor to evaluate the reactivity of catalysts. However, d-band theory is inapplicable to those coinage metals with completely filled d orbitals. Kumar et al. 99 predicted the adsorption energy of adsorbates on SAAs toward adsorbate-metal interaction using available periodic properties of the elements with the assistance of ML. Besides, to study the stability of single atoms on SAAs, calculating the surface segregation energy is necessary. Salem et al. 100 created a framework with ML technique, elemental properties, and bond-centric model to predict the segregation energy of metals to accelerate SAA design. Moreover, break the linear scaling relationship is critical to fabricate ideal catalysts. Nevertheless, owing to the limited experimental conditions and experiments, it remains challenging for researchers to screen the most active catalysts. Dasgupta et al. 101 used ML to predict the reaction and adsorption energy of 300 SAAs to identify possible active materials not following the scaling relationship. Despite great progress has been made in using ML to explore descriptor to screen the possible SAAs for various reactions, no rational model has been constructed for electrocatalytic process. Therefore, it is urgently needed to create suitable framework to provide theoretical guidance for the design of highefficiently electrocatalysts.