Single‐atom catalyst application in distributed renewable energy conversion and storage

In recent years, owing to the depletion of fossil energy and the aggravation of environmental pollution, the conversion and storage of distributed renewable energy (such as solar energy, wind energy, and tidal energy) based on electrochemical technology have attracted extensive attention. Electrocatalytic processes with high efficiency and high selectivity play a key role in clean energy conversion and storage. With the nearly 100% atomic utilization rate and unique catalytic activity, single‐atom catalysts (SACs) have been rapidly developed and widely used in the field of energy conversion and storage. In this review, we first introduce the characteristics of SACs. Then, we focus on the application of SACs in energy conversion, including water electrolysis reaction, nitrogen reduction reaction, nitrate reduction reaction, oxygen reduction reaction, and carbon dioxide reduction reaction. In terms of energy storage, we focus on supercapacitors and Li–S batteries. Further, we enumerate some of the methods for the synthesis of SACs in high metal loading or large scale. Finally, the main challenges and opportunities for this emerging field in the future are discussed and prospected.


INTRODUCTION
energy, wind energy, and tidal energy) conversion and storage due to the sufficient active sites and ultrahigh reaction selectivity. [3][4][5][6][7][8] However, when the size of metal particles decreases to an SA level, the metal surface free energy is increased dramatically. So, neighboring SAs have the tendency to aggregate into clusters or nanoparticles. Therefore, how to prepare SACs with stable structures in high metal loading is still a huge challenge. 9,10 Moreover, in consideration of the industrial applications, it is necessary for SACs to be prepared on a large scale. 6 In this review, some recently reported literatures are introduced for obtaining SACs in high metal loading or on a large scale.
Electrocatalysis catalytic reactions with high efficiency and high selectivity play a key role in energy conversion and storage. With the nearly 100% atomic utilization rate and unique catalytic activity, SACs have been rapidly developed and widely used in the fields of energy conversion and storage. [11][12][13][14][15][16] The applications of SACs in electrochemical energy conversion, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of electrolytic water, the reduction reaction of nitrogen/nitrate (NRR/NO 3 RR), oxygen reduction reaction (ORR), carbon dioxide reduction reaction (CO 2 RR), and energy storage applications containing supercapacitors and batteries, are discussed in detail. Finally, we look forward to the prospect of SACs consisting of four aspects: (1) preparation of SACs with high loading synthesis and large-scale simultaneously; (2) develop novel in situ characterization techniques for characteristic the SACs; (3) improvement of the stability of SACs; (4) use machine learning to predict efficient SACs.

SACS FOR ELECTROCHEMICAL ENERGY CONVERSION AND STORAGE
The applications of recent advanced SACs to some typical electrocatalytic energy conversions and storage systems are detailly introduced in this part ( Figure 1).

2.1
SACs for electrochemical energy conversion 2.1. 1 Hydrogen evolution reaction (HER) Hydrogen (H 2 ) is one of the cleanest energies with no pollutant products during combustion. The development of electrocatalysts for efficient H 2 production is very important toward the sustainable economy. [17][18][19][20] Platinum (Pt)-based SACs have the advantages of remarkable catalytic activity and excellent stability in acidic HER. [21][22][23] Wu et al. 24 reported a thermal emitting synthesis strategy (Figure 2A) to obtain Pt SAs/DG from bulk Pt, which greatly reduced the synthesis cost. The prepared Pt SAs/DG showed a remarkable catalytic activity toward HER, and the activity of Pt SAs/DG is 31.5 times higher than that of commercial Pt/C ( Figure 2B). To further reduce costs, researchers have also developed non-Pt catalysts. Among them, ruthenium (Ru) has received widespread attention on account of its low cost compared to Pt. Because the price of Ru is only 42$ per ounce, which is much lower than that of Pt (992$ per ounce).
Li et al. 25 reported a simple impregnation method ( Figure 2C) to anchor Ru SAs on a phosphorus nitride (PN) carrier that utilized the strong coordination interaction between d orbital electrons of Ru SAs and lone pair electrons of nitrogen in PN. The carrier could effectively separate Ru metal sites and inhibit them from aggregating ( Figure 2D,E). The obtained Ru SAs@PN could be used as an outstanding electrocatalyst for HER in 0.5 M H 2 SO 4 , providing a small overpotential of 24 mV at 10 mA cm −2 . Similarly, Zhou et al. 26 reported an anti-Ostwald maturation strategy to realize Ru SAs separated on N-doped molybdenum carbide (Mo 2 C) nanosheets (Ru SAs/N-Mo 2 C NSs). The Ru SAs/N-Mo 2 C NSs showed excellent HER activity as an electrocatalyst, the overpotential of which was 43 mV at 10 mA cm −2 and apparent long-term constancy. The density functional theory (DFT) results revealed the high performance of Ru SAs/N-Mo 2 C NSs was derived from the synergistic effect between the Ru SAs and N-Mo 2 C NSs ( Figure 2F-H). Reproduced with permission from Ref. [24]. Copyright 2019, American Chemical Society. (C-E) Reproduced with permission from Ref. [25]. Copyright 2018, Wiley-VCH. (F-H) Reproduced with permission from Ref. [26]. Copyright 2020, Elsevier.

Oxygen evolution reaction (OER)
OER is one of the key half-reactions of water splitting, rechargeable metal-air cells, and other catalytic reactions, in which the efficiencies of these reactions were restricted by the slow kinetic process of OER. In recent years, the developments of OER catalysts with high activity and stability have become one of the research hotspots. Among the advanced catalysts toward OER, Ru-based SACs have attracted extensive attention owing to their high electrocatalytic activity for OER. Many studies have focused on improving the stability of Ru-based SACs. For example, Yao et al. 27 implanted Ru SAs in Pt-Cu compound as OER electrocatalysts to improve the stability ( Figure 3A,B).
In situ X-ray absorption fine structure (XAFS) analysis showed that the oxidation state of Ru 1 had almost no changes within the scope of OER catalytic potential ( Figure 3C). In contrast to Ru-based SACs, Ir-based SACs have enough stability toward OER. However, the performance is not good enough and needs to be further improved. For this purpose, Cao et al. 28 reported a synthetic strategy to anchor Ir SAs on the Ni (3−x) Fe x S 2 nanosheet array (Ir 1 /NFS) ( Figure 3D). The separation of matrix structure and the deposition of Ir SAs made Ir atoms only distributed on the surface of the support, causing the maximum utilization of precious metals. The experimental results showed that Ir 1 /NFS possessed an excellent performance of OER activity in 1.0 M KOH, only needing the overpotential of 170 mV to achieve the current density of 10 mA cm −2 ( Figure 3E). Moreover, the spin-polarized DFT calculations were employed to simulate the OER process based F I G U R E 3 (A) High-resolution image and relevant FFT graphics of Ru 1 -PtCu; (B) the steps exposed in diverse aspects of three representative samples were counted; (C) in situ X-ray absorption fine structure (XAFS) spectra transformation of the Ru K-edge; (D) the synthetic process of Ir 1 /NFS; (E) polarization curves of Ir 1 /NFS, Ir 1 /NFH, Ir/C (5 wt%), and IrO 2 in 1.0 M KOH; (F) the 4e − theory of oxygen evolution reaction (OER) on Ir 1 /NFS; (G) the stability tests of IrO 2 , Ir/C, Ir 1 /NFH and Ir 1 /NFS at 100 mA cm −2 . Source: (A and B) Reproduced with permission from Ref. [27]. Copyright 2019, Springer Nature. (C-G) Reproduced with permission from Ref. [28]. Copyright 2022, Springer Nature. on the 4e − mechanism proposed by Nørskov ( Figure 3F). The DFT calculation showed that Ir SAs created a suitable chemical environment through Ir-S-M bond (M represents Fe or Ni), which effectively reduced the dynamic barrier of *O generation of *OOH, thus speeding up the OER procedure. Finally, the stability of Ir 1 /NFS was excellent, with the voltage increased by 3.6% after 350 h testing ( Figure 3G).

2.1.3
Nitrogen reduction reaction (NRR) Ammonia (NH 3 ), as one of the most basic chemicals, is widely used in agriculture, plastics, and pharmaceutical industries. 29 In the past, the effective solution for sustainable NH 3 synthesis was electrocatalytic N 2 fixation using renewable electricity. 30 However, due to the high polarization barrier of inert N 2 molecules, the performance of NRR is severely limited, resulting in low NH 3 yield. 31 In addition, HER is a competitive reaction to NRR; therefore, designing unique catalysts that can effectively activate N 2 and inhibit HER is very important. The unique structure and electronic properties of SACs are not only beneficial to improve the efficiency of NRR but also improve the selectivity for a product of NH 3 . 32-34 Natural biological nitrogenases contain Fe and Mo elements, which are considered to be active metal elements for N 2 fixation. 35 The preparation of SACs mimicking natural nitrogenases is an effective strategy toward NRR. Inspired by this, a series of Fe-and Mo-based SACs have been designed for NRR. For example, Shi et al. 36 synthesized Mo SAs supported by B/N co-doped porous carbon nanotubes (Mo/BCN) ( Figure 4A). Mo/BCN could be used for electrocatalytic NRR at room temperature because of its unique tubular structure and abundant coordination of unsaturated metal atoms. The B and N atoms fixed the Mo site on the nanotube, which could activate the N 2 molecules. Therefore, Mo/BCN had a high catalytic activity for NRR, with an NH 3 yield of 37.67 μg h −1 mg −1 and an FE of 13.27% in 0.1 M KOH ( Figure 4B).
Liu et al. 37 established neutral aqueous electrolyte NRR processing on Fe-based SACs. The catalyst was synthesized by hydrothermal and annealing treatment of a ZIF mixture containing Fe/Zn. The Fe SA/NC showed high performance, with an NH 3 yield of 62.9 μg h −1 mg −1 at −0.4 V in 0.1 M phosphate buffer solution. The excellent NRR performance was attributed to the high density of atom-dispersed Fe-N 4 species. The Fe atom coordinated was able to provide electrons, which increased the length of the N≡N bond; thus, the efficiency of the subsequent hydrogenation reaction was improved. Furthermore, Chen et al. 38 developed a lattice-limited strategy to prepare S coordination Fe SACs on TiO 2 support (Fe 1 S x @TiO 2 ) for electrocatalytic NRR ( Figure 4C). The FeS 2 O 2 coordination configuration was the active site for NRR. In addition, the mesoporous structure was designed as a nanoreactor to provide a nano boundary effect for mass transfer in the catalytic reaction ( Figure 4D), improving the efficiency of NRR. At the same time, the introduction of S made rich defects in Fe 1 S x @TiO 2 ( Figure 4E). The results showed that the S-coordination process could effectively regulate the local electrical structure of a single Fe atom on a TiO 2 lattice, which could greatly adsorb and activate N 2 molecules to generate NH 3 . Therefore, the NH 3 yield reached 18.3 μg h −1 mg −1 , and the FE was 17.3% at −0.20 V. It was worth noting that this lattice-limited synthesis strategy was used to the coordination of Co, Mo, and Ni SAs supported by mesoporous TiO 2. It greatly promoted the design and generation of biomimetic NRR electrocatalysts.
In addition to the effect of the SACs, the low solubility of N 2 molecules in water also affects the yield of electrocatalytic NH 3 synthesis. Therefore, adjusting the electrochemical reaction pressure can also effectively promote the NRR process. Based on this, Zou et al. 39 reported that stereo-confinement induced compact SA (such as Rh, Ru, Co) on a graphite diacetylene substrate composed of M SA/GDY. In the pressurized reaction system, more N 2 molecules were transferred to the electrode surface, which could enhance the catalytic performance of NH 3 synthesis ( Figure 4F-H). Under 55 atm N 2 pressure, the yield of NH 3 was 74.15 μg h −1 mg −1 , the FE was 20.36%, and the partial current density of NH 3 was 0.35 mA cm −2 . Compared with the environmental conditions, the amounts of increase were 7.3, 4.9, and 9.2 times, respectively. The strategy could cooperate with an efficient catalyst and pressurized reaction systems and promote the development of electrocatalytic NH 3 synthesis reactions.

2.1.4
Nitrate reduction reaction (NO 3 RR) is also a widely available source of nitrogen and a green alternative to NH 3 synthesis with inert nitrogen. 40,41 The polar N=O bond energy has low energy and is also easily activated at low energies. 42 NO 3 − has the unique advantage of electrical NH 3 synthesis, which is expected to replace the Haber-Bosch process with a long history. 43 However, the formation of NO 3 − to NH 3 also requires the transfer of multiple electrons/protons, leading to the low reaction kinetics. Therefore, how to improve efficiency and selectivity is very important. 44,45 Fe-based SACs are considered a high-performance catalyst for NO 3 RR due to their low price, high electron-giving ability, and high activity. Wang et al. 46 synthesized Fe SAC with high selectivity for electrochemical NO 3 RR to NH 3 ( Figure 5A). In this catalyst, Fe existed on the support in the form of Fe-N 4 . Because it was short of adjacent metal sites, the Fe SAC could be capable of blocking the N−N coupling step, thus improving the selectivity of NH 3 products. 47,48 The maximum yield was 20 000 μg h −1 mg −1 , the FE was up to 75% at −0.66 V, and the local current den-sity of NH 3 reached ∼100 mA cm −2 at −0.85 V. Although the amount of Fe loading was about 1.51%, the NH 3 yield was significantly higher than that of Fe nanoparticle catalyst. Yu et al. 44 proposed a polymer-hydrogel strategy for preparing N coordination Fe sites with uniform atomic dispersion on carbon ( Figure 5B-D), which had the good performance of NH 3 production by NO 3 RR. The maximum NH 3 yield of the catalyst was 2.75 mg NH 3 h −1 cm −2 , and the FE was close to 100%. The occupancy mechanism of the Fe(II) site prevented competition for HER before Fe(0) formation that usually occurred in the monolithic catalyst due to the prohibition of water adsorption, making Fe SACs have good activity and selectivity.
To further improve the Fe site activity, Li et al. 49 synthetized a supported Fe SA-MoS 2 for NO 3 RR ( Figure 5E). The unique 3D porous structure was beneficial to the eminent NO 3 RR performance of the SACs. The FE of NO 3 RR for NH 3 was up to 98%. The selective enhancement of NH 3 generation of Fe SA-MoS 2 was due to the decrease of the deoxidation energy barrier between *NO and *N at 0.38 eV ( Figure 5F,G).

2.1.5
Oxygen reduction reaction (ORR) In general, there are two pathways of ORR reaction: two-electron pathways and four-electron pathways under acidic or alkaline conditions, which are shown in the following square equation 50 : Two-electron pathways: Four-electron pathways: Hydrogen peroxide (H 2 O 2 ), as the two-electron reduction product of ORR, has the functions of sterilization, disinfection, bleaching, and so on. H 2 O 2 is also widely used in industries, such as papermaking, electroplating process, and industrial wastewater treatment. 33,[51][52][53][54] At present, most of the industrial production of H 2 O 2 still adopts the traditional anthraquinone processes, which are complex, costly, and produce many organic by-products. The direct synthesis of H 2 O 2 by electrochemical method shows the advantages of energy saving, environmental protection, and safety. 55  Figure 6G). Chen et al. 57 prepared three types of Co-N SACs (Co-N SAC Dp , Co-N SAC Pc , and Co-N SAC Mm ) through a pyrolysis strategy. They found that Co-N SAC Dp (pyrrole type CoN 4 ) had the best HOO* adsorption ability and the highest ORR catalytic activity. generate H 2 O 2 . The incorporation of Fe atoms was theoretically proved to promote the catalytic activity for the generation of H 2 O 2 , specifically, it was able to effectively disinfect >99.9999% of bacteria at a 125 L h −1 m −2 electrode treatment rate ( Figure 6I).

2.1.6
Carbon dioxide reduction reaction (CO 2 RR) The use of fossil fuels has not only greatly improved the living standard of human beings but also promoted the development of industry. 59,60 However, fossil fuels release large amounts of CO 2 , causing atmospheric pollution and global warming. Therefore, it is important to develop efficient CO 2 conversion technologies. At present, the development of electrocatalytic technology to convert CO 2 into fuels and chemicals has a wide range of prospects. CO 2 molecular is a linear symmetric structure; therefore, the kinetics of CO 2 conversion reactions are slow. In addition, the presence of competitive responses (such as HER) reduces the selectivity of CO 2 conversion. [61][62][63] The developments of advanced catalysts are important to improve the activity and selectivity of CO 2 RR.
N-coordinated Ni SACs (Ni-NC) are effective CO 2 RR catalysts. 64 synthesis of SAC Ni-NC with high transition metal (TM)atom loadings ( Figure 7A). Two atomic catalysts with an Ni load of 7.5 or 15 wt% were obtained, and their electrochemical CO 2 RR performances were investigated. The results showed that the two catalysts with different Ni loads achieved high CO selectivity (>90%) ( Figure 7B). The CO partial current density of the catalyst with 15 wt% Ni load reached 122 mA cm −2 at the voltage of 2.55 V. The results showed that the CO 2 RR activity increased significantly with the increased loading of Ni.
Yan et al. 67 synthesized coordinated unsaturated Ni-N sites doped in porous carbon by pyrolyzing the Zn/Ni bimetallic zeolite imidazolate skeleton, with the loading of Ni up to 5.44% ( Figure 7C). At the coordination unsaturated Ni-N site, CO 2 RR was more likely to occur than HER, which greatly improved the selectivity of CO 2 RR. With the increase of the potential, the current density of CO boosted significantly. The current density of CO reached 71.5 ± 2.9 mA cm −2 at the voltage of 1.03 V, with the FE of 92.0%-98.0%. The high performance of CO 2 RR contributed to the Ni SAC proved by DFT ( Figure 7D,E). Similarly, Ren et al. 68 prepared a conciliatory direct solid-state pyrolysis using an Ni SA nanoparticle catalyst (NiSA/NP) ( Figure 7F,G). Electrons are delivered to Ni(I)-N-C sites by Ni nanoparticles via the carbon nanotube network, and the current density was up to 346 mA cm −2 at −0.5 V versus RHE under alkaline conditions. When combined with NiFe oxidation anode into a zero-gap membrane electrolyzer, it provides a current density of 310 mA cm −2 at −2.3 V, equivalent to 57% of the total energy efficiency. Due to the dense active center and the strong adsorption of the middle COOH* on the Ni SA, CO 2 had excellent reduction performance ( Figure 7H,I).
The conversion of CO 2 to high-value-added products (such as CH 3 OH and CH 4 ) is important. A large number of studies indicate that Cu-based materials have the ability to reduce CO 2 to CH 3 OH or CH 4 . Yang et al. 69 synthesized highly efficient single-atom copper on through-hole carbon nanofibers (CuSAs/TCNFs) for CO 2 RR ( Figure 8A-G). Because of the high binding energy of Cu to *CO intermediates, *CO was easily reduced to CH 3 OH. These CuSAs/TCNFs as the cathode of CO 2 RR in the liquid phase generated CH 3 OH with the FE of 44%. Moreover, the through-hole structure and self-support of CuSAs/TCNFs not only reduce the number of embedded metal atoms but also increase the number of effective Cu SAs, so that the atoms actually involved in CO 2 RR were greatly increased. In addition, Zheng et al. 70 synthesized a Cu-N-C by the pyrolysis of Cu-MOF. Importantly, the selectivity of the product could be changed by adjusting the Cu SAs concentration and the Cu-N x distance by pyrolysis in different temperatures ( Figure 8H-K). At a high concentration of Cu (4.9%), the Cu-N x species were very close to each other, and the electrocatalyst was favorable for C-C coupling to generate C 2 H 4 . In contrast, at low Cu concentrations (2.4%), adjacent Cu-N x species became more distant from each other, thus tending to produce C 1 product CH 4 . The CO 2 RR to different hydrocarbons by modulating the site of active Cu was an important breakthrough.

Supercapacitor
Supercapacitor is a new type of energy storage equipment between rechargeable battery and traditional electrochemical capacitor. It can charge quickly in a few minutes and then release energy according to the needs of the application equipment. 71 At the same time, it also has the energy storage characteristics of batteries, with charge and discharge times up to 500 000 times, whereas the life of common battery is generally only 1000 times. This characteristic makes supercapacitors widely used in solar energy collection systems, renewable energy systems, electric vehicle, and so on. Therefore, the development of supercapacitors is crucial to accelerate the revolution of energy structure. [72][73][74] Generally, the materials of positive and negative electrodes show significant impacts on the performance of supercapacitors. At present, N-doped carbon-based loading metal SAs have been proven as one of the most efficient materials for supercapacitors. 75,76 For example, Li et al. 77 proved that F I G U R E 9 (A) CV curves of Zn 1 NC and NC for capacitive (gray) and pseudo-C (purple/green), respectively; (B) the capacitance contribution of Zn 1 NC at different scanning rates was calculated by the Dunn method; (C) comparison of specific capacitance and specific current density before and after acid leaching; (D) Raman spectra of Zn 1 NC under electrode scanning; (E) the Raman frequencies of OH anchored Zn-N 4 groups in graphene support were simulated υ 1 , υ 2 , and υ 3 bands, the illustrations show the proposed atomic structure; (F) the Raman band intensity variation chart of Zn 1 NC centered on 285 and 829 cm −1 with electrode change; (G) the CV curves of Zn 1 NC and NC at 2.0 mV s −1 within a voltage span of −1.0 to 0.5 V; (H) the variation of single-atom Zn on OH adsorption through density functional theory (DFT) calculation, inserted illustration indicated the differential charge analysis of Zn-N 4 before (a-ZnN 4 ) and after (b-ZnN 4 -OH) OH adsorption. Source: Reproduced with permission from Ref. [77]. Copyright 2022, Springer Nature. the capacitive performance of N-doped carbon nanomaterials (CMS) could be improved thanks to Zn SAs dispersed on the support ( Figure 9A). The obtained Zn 1 NC material provided a capacitance of 621 F g −1 at 0.1 A g −1 , excellent magnification performance (about 65% at 100 A g −1 ), and outstanding stability ( Figure 9B,C). In situ Raman showed that the embedding of Zn atoms in CMS regulates the microenvironment of the whole material, and then, the adsorption behavior of OH − was affected ( Figure 9D-G).
Further DFT calculation revealed that the accumulated charge between Zn and N became discrete after the adsorption of OH − at the Zn-N 4 site, facilitating the rapid transfer of enriched electrons around the Zn atom to the adsorbed species ( Figure 9H). At the same time, the accumulation of more positive charges in the local region of HO-ZnN 4 indicates that there are additional C and N sites in addition to Zn atoms for anion storage, thus enhancing both the double-layer capacitance and the pseudocapacitance. Sodium ion hybrid capacitor (SIHC) has the characteristics of both sodium ion battery and capacitor. [78][79][80][81][82][83][84][85] In recent years, metal SAs embedded in C have been widely studied for their high conductivity and high adsorption energy in SIHC. 1,86 Hu et al. 87 reported that the atom-dispersed Mn-N 4 site was conducive to improve the reaction kinetics of the Na + storage anode and increase fake electric capacity, electrical conductivity, and the capacitive performance of the ClO 4 − storage cathode. In addition, SA Mn-N 4 implanted within N and F co-doped carbon nanoflakes (MnSAs/NF-CNs) as electrodes could validly offset the imbalance of slow dynamics and low capacity, thus producing significant energy, remarkable power density, and long-term cycle stable performance of the capacitor ( Figure 10A-C). The initial constant current charge-discharge (GCD) curve was 405 mA h g −1 at 0.2 A g −1 , demonstrating a notable reversible capacity of MnSAs/NF-CNs ( Figure 10D-F). MnSAs/NF-CNs exhibited the superior capacity in the discharge potential of 0.5 V, which showed that the implantation of MnSAs could increase the surface defect sites adsorbed by Na + . Meantime, MnSAs/NF-CNs with Janus characteristics made SIHCs have surprisingly significant energy, robust power density (maximum 197 W h kg −1 /9350 W kg −1 ), and an ultra-long cycle life of more than 10 000 times, after which capacity retention rate was 85.2% ( Figure 10G,H).

Li-S battery
Because the lithium-ion battery is widely used in business, the actual energy density increases from the original 150 to 300 W h kg −1 , which is not far from the 420 W h kg −1 (1400 W h L −1 ) theoretical limit, leading to the development into a bottleneck period. Therefore, new energy storage systems with longer cycle life and higher energy density are popular. As a cathode material, lithium metal can provide very low reduc- tion potential (−3.04 V), very high theoretical specific capacity (3860 mA h g −1 ), and minimum weight density (0.534 g cm −3 ), whereas the theoretical specific capacity of S cathode is 1672 mA h g −1 . Thus, Li-S batteries have a weight-energy density (2500 W h kg −1 ) and theoretical specific capacity (1675 mA h g −1 ). Moreover, S is very suitable for the commercial production of Li-S batteries due to its great abundance, low price, and environmental friendliness. Recently, SACs have been universally used in Li-S batteries. Because the synergistic interaction between metal SAs and the corresponding matrix can give the composite unique physical and chemical properties to meet various requirements. In addition, SACs not only promote the uniform deposition of Li on the negative electrode but also effectively absorb soluble LiPSs to promote the catalytic conversion of the positive electrode, which is a good material for Li-S batteries. 88 Zhou et al. 89 synthesized large-scale vanadium SAC (SAV@NG) on graphene to achieve Li-S batteries with high sulfur loading (80 wt%), fast kinetics (645 mAh g −1 at 3C ratio), and long life ( Figure 11). The best catalytic potential for the decomposition of Li 2 S was demonstrated by SAV@NG, which exhibited the shortest Li 2 S decomposition barrier (1.10 eV) and could sustain a low lithium diffusion barrier. In addition, S had good compatibility with graphene and catalyst, and good absorption capacity of S 6 2− in SAV@NG, which showed great potential for the application of SAC@NG in high-performance Li-S batteries. The initial specific capacity of S-SAV@NG electrode was 780 mAh g −1 , which was stable at 551 mAh g −1 after 400 cycles. Metal atoms could capture dissolved lithium polysulfide and catalyze the conversion of LiPSs/Li 2 S in the cycling process, which could improve S utilization, cycle life, and rate capacity.

Synthesis methods of SACs
At present, the common synthesis strategies of SACs include atomic layer deposition method, chemical vapor deposition (CVD), mass-selected soft-landing method, and wet chemical method. The following section mainly focuses on the recent advances in the preparation of SACs with high metal loading or mass production.

SACs with high metal loading
Pyrolysis, as one of the CVD methods, is a common method to prepare SACs, which refers to the hightemperature decomposition of metal-containing precursors. On account of diverse precursors, the pyrolysis method can be divided into two categories, pyrolysis of mixed metal and carbon sources, specific metal-containing complexes (mainly including metal-organic-frameworkbased precursors), irregular metal-containing complexes, and metal-containing organic polymers. Meantime, various precursors have different pyrolysis temperature requirements for the synthesis of SACs. Under certain pyrolysis processes, SACs with high metal loading can be obtained via rational design of precursor composition and structure. For instance, Xia et al. 65 reported a synthesis strategy of high metal loading SACs applying graphene quantum dots (GQDs). GQDs could stably and uniformly confine TM cations on their surfaces when selectively combined with amine groups (GQDs-NH 2 ) and form a mixture with TM salts in solutions, because of the robust chelation and complexation effect between metal cations and amine groups. Then, compact TM SACs could be obtained by pyrolysis in an atmosphere rich in ammonia. Among them, the SA loading amount of Ir, Pt, and Ni in samples reached up to 41.6, 32.3, and 15 wt%.

Large-scale preparation of SACs
Ball milling method, a top-down mechanochemical route, refers to putting one or mixed metal salts into a ball mill, adding ball milling beads for milling for a period of time to obtain atomic-scale metal balls, which interact with the carriers to form SACs. 90,91 The advantages of this method are no templates, solvents, or additives, and are easy to scale up and expand, which has the potential for largescale production. Ji et al. 92 successfully prepared 60, 200, and 1000 g noble metal SACs by ball milling with large ball mills and more feed quantities, and the catalyst structure remained unchanged ( Figure 12).

CHARACTERIZATION METHODS
The typical characterization methods include a special aberration-corrected transmission electron microscope (AC-TEM AC), an extended XAFS (EXAFS), and X-ray diffraction. 93 In this part, we will focus on the introduction of atom probe tomography (APT) technology. APT can confirm the atom type, reconstruct its spatial position intuitively, and display the 3D spatial distribution of different element atoms in the material relatively true. It has become a high spatial resolution analysis and test means. For example, Wang et al. 94 synthesized a TM atom in the graphene shell as the active center for CO 2 RR to CO, with the FE exceeding 90% at a current of 60 mA mg −1 . In order to clarify the essence of dispersed Ni sites in graphene layers to a better perspective of the catalytic active sites of NiN-GS, they used APT technology to judge the dispersion degree of Ni atoms and the coordination with N. Figure 13A-C shows the 2D atom picture of the catalyst. Each green dot stands for one single metal atom of Ni. The local coordination environment of Ni atoms in the graphene layer was observed as shown in Figure 13D.
There was a small amount of Ni SA coordinated with one N atom in the graphene vacancies, and nickel clusters were not observed, which proved that Ni atoms were uniformly dispersed. Statistical and quantitative analysis data showed that 83% of the Ni atoms in the amplified area are Ni SAs, and most of the Ni form coordination with C atoms ( Figure 13E).

OUTLOOK
In recent years, the performance and application of SACs have been widely studied, and SACs are also facing great opportunities and challenges. In this part, for promoting the industrialization of SACs, we outlook some important directions for developments of SACs in the future.

Practical applications require the preparation of SACs in high metal loads and large-scale simultaneously
The active sites of the SACs are related to the metal loading; therefore, increasing the metal contents can facilitate obtaining high catalytic activity. Large-scale preparation is the premise of practical application. To meet the needs of the practical application of electrocatalysis, increasing the load of active components in the SACs and achieving large-scale preparation of SACs are two major difficulties in the area of SACs synthesis. 95 However, in current synthesis strategies, 96,97 high-load and large-scale preparation are hard to meet simultaneously. The synthesis of highload SACs is a great obstacle, which seriously affects the practical application of SACs. Therefore, new synthetic techniques need to be developed to obtain the SACs with high-load and large-scale.

Accurate characterization of the active site in the catalytic process
With the accurate understanding of catalyst structure, the structure-activity relationship between catalyst structure and properties can be constructed more practically. Tradition of in situ characterization methods can indicate the current state before and after chemical reactions of catalyst. Steady state is only one condition for the catalytic reaction; however, more emphasis should be placed on the transient response, which includes the detection of catalyst atomic structure change, the adsorption of molecules into catalytic product intermediates, the chemical state of catalyst, the oxidation or reduction, and coordination atoms move. This series of intermediate reaction processes for the study of catalytic reaction mechanism, the choice of catalyst, has a crucial role. 98 At the same time, understanding exactly the structure of the catalyst and the determination of the coordination environment and electronic structure of the metal SA is also conducive to the construction of the theoretical model, so as to obtain a more realistic, more reasonable, and effective theoretical structure. Therefore, advanced in situ experimental methods need to be established.

Improvement in the stability of SACs
In the actual reaction process, the deactivation of SAC is an urgent problem to be solved. [99][100][101] To solve this problem, we need to increase the interaction between the carrier and metal atoms to improve the stability of SAC. We can do this by choosing the right metal and bracket. In general, carriers rich in unsaturated sites can be selected, which can not only improve their charge transfer ability in trapping individual atoms but also stabilize the charge on the carrier. The support doped with coordination atoms can enhance the interaction between metal and support, thus effectively restraining the migration and aggregation of SA. Strong chemical bonds between singlemetal atoms and graphene-containing oxygen-containing functional groups or metal oxides enhance stability and make it a good carrier choice. By selecting the appropriate support to promote the substrate activation, the catalytic activity of the catalyst can be improved. Therefore, it is very important to select suitable support for the synthesis of SAC.
In addition, in the process of SAC preparation, the aggregation of metal atoms is also an important factor leading to the deactivation of the catalyst. As the particle size decreases, the surface energy of the particle will also increase, accompanied by the decrease of stability. Therefore, individual atoms will migrate or aggregate to reduce the surface energy, thus forming larger particles and more stable clusters, which results in a significant reduction in the density of a single active site and a great decrease in the catalytic performance. If a strong interaction can be formed between the metal SA and the supporting coordination atoms around it, that is, a strong supporting assisted covalent bond, the chemical potential of the SASC system can be reduced, and the internal stability of the system can be improved. Therefore, space confinement, defect capture, ligand anchoring, and low temperature reduction of the thermal motion of the molecules can effectively inhibit the polymerization of metal precursors, achieve atomistic dispersion and isolation of metal precursors, and thus, limit their migration and aggregation on the support to prevent the deactivation of catalysts.

5.4
Accelerate screening efficiency by combining theoretical calculations with machine learning After decades of development, theoretical calculation and machine learning have been widely used in the research of electrocatalysts and have made great progress to predict the performance of electrocatalysts and help design efficient catalysts. In terms of research methods and means, we should pay attention to the combination of experimental research and theoretical calculation, as well as the combination of theoretical calculation and machine learning. On the one hand, with the help of theoretical calculation, more realistic catalytic reaction mechanisms can be simulated, and the gap between current theoretical calculation and experimental research can be narrowed or eliminated. On the other hand, machine learning can be used to quickly analyze the structure-activity relationship and catalytic mechanism of SACs, laying an important scientific foundation for rapid and accurate design of SACs for different electrocatalytic reactions.

A C K N O W L E D G M E N T S
This work was supported by Taishan Scholars Project Special Funds (tsqn201812083), Natural Science Foundation of Shandong Province (ZR2022QE076, ZR2021JQ15, ZR2019YQ20,) and the National Natural Science Foundation of China (52002145, 52202092, 51972147, 52022037). Li X., Xu W., and Fang Y. contributed equally to this work.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.