Hollow microstructural regulation of single-atom catalysts for optimized electrocatalytic performance

Single-atom catalysts (SACs) and hollow microstructured materials have recently undergone significant advancements in the field of catalysis. The combination of SACs and hollow microstructured materials can further endow them with extraordinary characteristics, such as high loadings, uniform active sites and unique metal-support interactions and electronic structures. In this review, we focus on the design and construction of SACs supported on hollow microstructured materials for enhancing electrocatalytic reactions. Due to these unique hollow microstructures, most of the active sites can be fully exposed in catalysis. Therefore, the atomic utilization rate is greatly improved. Furthermore, the synergistic effect of SACs and hollow microstructured supports can bring about unpredictable characteristics. The hollow microstructures not only adjust the geometry and electronic structure of the SACs to improve their activity but the SACs can also serve as an auxiliary stimulus to the intrinsic activity of the support to achieve better performance. Compared with the reported traditional dual atomic catalysts, hollow microstructures are conducive for the isolation of atomically dispersed binary atom active sites on both sides of the particle shell. Combined with the current development status, we summarize the challenges and prospects in this area. This review is conducive to the reasonable design and manufacture of advanced SACs supported on hollow materials and the promotion of their future industrial usage in energy applications.

Hollow structures are types of functional nanomaterials with cavities or void spaces inside a defined shell [32] . Due to their unique structural characteristics and physical and chemical properties, advanced hollow structures exhibit some functions that simple NPs cannot obtain, especially in heterogeneous catalysis. Compared with their solid counterparts, hollow structures have several advantages. They effectively isolate the catalytic material and stabilize the NPs to prevent sintering. They can achieve cascade reactions by regulating the spatial position of catalytically active sites. By precisely controlling the pore size in the shell, the hollow structures improve the selectivity of the catalytic reaction through the difference in molecular sieve or nanopore diffusivity. In addition, hollow structures are generally suitable for improving mass transfer [33][34][35] .
The design concept of highly dispersed active species as heterogeneous catalysts be traced back to the research of Maschmeyer et al. [55] in 1995. They found that Ti highly dispersed on mesoporous MCM-41 could be used for the epoxidation of cyclic olefins by grafting organometallic complexes into mesoporous silica [55] . In 2003, Fu et al. [56] reported an ionic Au-cerium oxide catalyst for the water-gas shift reaction without Au particles. They demonstrated that the metal NPs were not active for the water-gas shift reaction. In 2007, Hackett et al. [57] reported the use of atomically dispersed Pd II supported on mesoporous Al 2 O 3 for the selective aerobic oxidation of allylic alcohols. In 2011, Qiao et al. [39] proposed the concept of SACs for the first time, which has attracted significant attention in the field of catalysis. They reported single Pt atoms dispersed on FeO x with high CO oxidation activity. In 2016, Yin et al. [58] made a major breakthrough in the preparation of high-loaded SACs (4 wt.%), which opened up new avenues for the general synthesis of various SACs.
Compared with conventional catalysts, SACs fully expose every atom as a catalytic site, which maximizes the atomic efficiency of metals and minimizes the amount of precious metals. SACs have the advantages of heterogeneous and homogeneous catalysts and bridge the gap between heterogeneous and homogeneous catalysts [59] . SACs can realize the rational use of metal resources and promote atomic economy by maximizing the efficiency of atom utilization [60,61] . Due to their quantum size and structural effects and strong metal-support interactions, SACs have additional advantages. In addition, the unique electronic structure and unsaturated coordination environment of the active sites in SAC can significantly improve the catalytic performance [62][63][64][65][66] . Moreover, SACs with uniform metal active sites are beneficial to the study of electrocatalytic mechanisms [67][68][69][70] .
A large number of reviews have focused on the influence of geometric and electronic structure regulations on the catalytic performances of SACs [38,[71][72][73][74][75][76][77] . In contrast, relatively few reviews have discussed the relationship between SACs supported by hollow microstructures and catalytic activity. The catalytic properties of SACs rely on the characteristics of the active sites and their accessibility to reactants, which are greatly affected by the hollow microstructures of the supports. Hollow structured materials as SAC supports have several unique features, including high surface area, low density and high loading capacity, for catalysis [33,[78][79][80][81] . Compared with bulk materials supporting SACs, confined SACs can be directly exposed as active sites on hollow microstructured supports, thus ensuring 100% exposure of single atoms to reactants. Cascade reactions can be achieved by placing SACs on hollow microstructures with controllable spatial localization. Hollow microstructures with thin shells provide more expedited mass-transfer processes. Finally, the thin shells in hollow structures enhance catalytic selectivity.
This review focuses on the design and construction of SACs supported on hollow microstructured materials for enhancing electrocatalytic reactions. Recent configurations of hollow microstructures supporting SACs can be divided into three types, namely, single-shell hollow, core-shell and yolk-shell structures [ Figure 1]. The accessibility of SACs in hollow microstructures and their performance in mass diffusion are highlighted. In addition, the research progress of SACs supported on hollow microstructures in electrocatalysis and their advantages/disadvantages in electrocatalysis are discussed. Moreover, we emphasize the importance and challenges in the rational design of hollow microstructured catalysts in electrocatalytic reactions, which have excellent potential for applications in energy conversion.

A HOLLOW MICROSTRUCTURAL MODULATION STRATEGY FOR SACS
The accessibility and nature of single atomic sites are significantly affected by the microstructures of the catalysts, including their morphology, size, shape and surface structure [35,82,83] . As a special type of microstructure, hollow microstructures have attracted particular attention [81,[84][85][86] . Compared with solid materials, hollow microstructures not only greatly increase the specific surface area of catalysts to expose the active sites but also facilitate the diffusion of the substrate [82,[87][88][89] . Therefore, the regulation of hollow microstructures can remarkably affect and even manipulate the catalytic performance of SACs [80,[90][91][92][93] . Herein, we focus on studying the methods, intrinsic properties and regulation of hollow microstructures to improve the electrocatalytic performance.

SACs supported on single-shell hollow materials
Single-shell hollow microstructures have empty spaces inside a single shell. The physicochemical properties of hollow microstructures can be modulated via a series of parameters, including external shape, internal configuration and shell architecture [81] . The shell composition of hollow microstructures has changed from carbon species (or metal oxides) to multiple compounds, such as metal-organic frameworks (MOFs), in recent years. Many strategies for the successful synthesis of hollow microstructures have been reported. Hollow microstructures with internal voids can effectively improve the weight fraction and mass activity of SACs. In addition, hollow structures provide more accessible areas to expose the active sites of SACs in catalysis [84][85][86][87][88][89] .
Recently, MOFs composed of metal ions/clusters and functionalized organic linkers have been investigated as ideal supports for anchoring SACs [25,28,31,47,[94][95][96][97][98][99] . For instance, Liang et al. [100] demonstrated a strategy for preparing isolated Fe atomic sites anchored in hollow carbon polyhedra (Fe-SAs/NPS-HC) derived from MOF@polymer composites via the Kirkendall effect [ Figure 2A]. A series of controlled experiments proved that sulfur plays a significant role in the construction of single-shell hollow microstructures. The uniform single-shell hollow morphology of Fe-SAs/NPS-HC was identified by transmission electron microscopy (TEM) and high-angle annular dark field scanning TEM (HAADF-STEM), with a shell thickness of ~20 nm [ Figure 2B]. In addition, energy-dispersive spectroscopy mappings proved the uniform spatial distribution of iron, carbon, nitrogen, phosphorus and sulfur throughout the hollow shell. Furthermore, aberrationcorrected HAADF-STEM (AC HAADF-STEM) and XANES analysis directly confirmed the atomic dispersion of Fe [ Figure 2C-E]. Due to the structure function and electronic control of Fe SACs, the catalysts exhibit significant oxygen reduction reaction (ORR) performance in both alkaline and acid media [ Figure 2F and G].
Similarly, Chen et al. [101] also constructed unique Co SACs supported on hollow carbon polyhedra (Co 1 -N 3 PS/HC). The synergistic coordination of N, P and S atoms was introduced into the single-shell hollow carbon polyhedra to adjust and optimize the electronic density of Co SAC active centers and construct novel optimal Co-N 3 PS active configurations. Co 1 -N 3 PS/HC exhibits an efficient ORR reactivity (176 mW cm -2 at a current density of 280 mA cm -2 ) better than commercial Pt/C (117 mW cm -2 ), significant ORR kinetics and excellent stability in both alkaline and acid media. Furthermore, a Zn-air battery based on Co 1 -N 3 PS/HC showed outstanding battery performance (786 mAh g -1 at 10 mA cm -2 ) and long-term charge/discharge durability. Moreover, they used the same method to synthesize a single-atom Ir catalyst on a single-shell hollow carbon substrate [78] . It was shown that the designed hollow structure plays an important role in accelerating the kinetics and improving performance. He et al. [102] synthesized a series of noble-metal (Ir, Pt, Ru, Au and Pd) single atoms immobilized on hollow nanotubes derived from a zirconiumporphyrinic MOF, which are equipped with square-planar four coordinate porphyrin units to anchor a single atom. This unique single-shell hollow structure was beneficial for fast mass diffusion.
Yang et al. [103] reported a solid phase thermal diffusion approach to prepare Mn isolated single-atom site catalysts on N-doped carbon nanotubes by the pyrolysis of a MnO 2 nanowires@ZIF-8 core-shell structure. After NH 3 treatment, the Mn SAs-N 2 coordination sites were controllably transformed into Mn SAs-N 4 coordination sites. This method generally uses acids and bases to etch the template, which may damage the composition and structure of the nanomaterial. Wang et al. [104] constructed single Fe atom catalysts supported on N-doped hollow carbon nanocages by the pyrolysis of ZIF-8 and pyrrole. As more single Fe atom active sites are exposed in the hollow carbon structure, the obtained Fe-ISAs/H-CN catalysts show high-efficiency activity in alkaline solutions. The typical ZIF-8 has been widely used to construct M-NC catalysts. Unfortunately, the resulting dense carbon framework hinders mass transfer and blocks many  [100] . TEM: Transmission electron microscopy; HAADF-STEM: high angle annular dark field scanning TEM; ORR: oxygen reduction reaction. metal sites. Li et al. [105] proposed a dual-linker zeolitic tetrazolate framework-engaged strategy to prepare single Ni atom catalysts supported on hollow plates. 5-Aminotetrazole was used as a self-sacrificing template and 2-methylimidazole was mainly used as a carbon and nitrogen source to form a hollow N-rich carbon matrix. The hollow structure of the obtained catalysts had more mesoporosity and available surface area, which promote mass transfer and provide abundant single Ni sites that are beneficial to enhancing the catalytic performance for electrochemical CO 2 reduction with high selectivity (~100%) to CO.
The synergistic effect based on the interface of binary atom sites provides a unique perspective for understanding the reaction mechanism in the electrocatalytic process [106] . Han et al. [107] reported atomically dispersed binary Co-Ni sites supported in N-doped single-shell hollow carbon cubes (CoNi-SAs/NC) by the pyrolysis of CoNi-MOFs [ Figure 3]. Such single-shell hollow carbon generates high surface areas and abundant mesopores, which shorten the paths for ionic/electronic diffusion and provide effective mass transfer channels. Benefiting from the abundant single-atom active sites and the synergistic effect of Co-Ni sites, the resultant CoNi-SAs/NC catalysts provide a low overpotential, high electron transfer number and good reversibility in alkaline electrolytes. Density functional theory results revealed that the ratedetermining step (RDS) on single-shell hollow NC and Ni-N is the hydrogenation of molecular O 2 , while the RDS on Co-Ni-N becomes the protonation of OH*, thereby highlighting the critical role of bimetallic Co/Ni sites in energetically facilitating the ORR process. Furthermore, they emphasize the accessibility of SACs with high activity and the synergistic interaction of Co-Ni-N in reducing the energy barrier and improving the reaction kinetics. Deng et al. [108] used a hollow ZIF-8 precursor to prepare N-doped hollow carbon polyhedra implanted with single Fe atoms (C-FeHZ 8 @g-C 3 N 4 ). The obtained FeHZ 8 @g-C 3 N 4 catalysts possessed an exclusively high density of Fe-N 4 sites and exhibited excellent ORR activity with a half-wave potential of 0.78 V in an acidic medium and 0.845 V in an alkaline medium. In addition, the catalysts display remarkable performance in H 2 /O 2 proton exchange membrane fuel cells.
Due to their large-scale preparation and high uniformity, SiO 2 materials as physical scaffolds are often used as hard templates. In short, the hard template synthesis includes the following steps: (1) preparing a hard template with a specific shape; (2) coating (or depositing) the target material on the template; and (3) removing the internal template. The hard template method is the common and effective method to fabricate  [107] . SEM: Scanning electron microscope; HRTEM: high resolution TEM; XANES: X-ray absorption near edge structure; OER: oxygen evolution reaction.
hollow nanostructures loaded with single atoms. Su et al. [109] used silica as a template to construct Fe-N/C and Fe-ND/C on hollow carbon spheres. The Fe-ND/C catalysts with the proposed coordination structure facilitate the desorption of OH*. Kuang et al. [110] prepared N-doped mesoporous hollow carbon spheres loaded with Pt single atoms (Pt 1 /NMHCS) via a SiO 2 template strategy. Due to the strong electronic metalsupport interaction between the single-shell hollow carbon support and Pt SACs, the obtained Pt 1 /NMHCS possessed more electron depletion and a higher oxidation state than single Pt atoms. This greatly promoted the outstanding hydrogen evolution reaction activity. Pt 1 /NMHCS exhibits a higher mass activity (2.07 A/mg Pt at a 50 mV overpotential) than that of Pt NP /MHCS (0.89 A/mg Pt ) and 20 wt.% Pt/C (0.58 A/mg Pt ).
The reasonable design of cost-effective non-precious metal-based catalysts to substitute Pt in the ORR is ideal for the development of sustainable energy conversion equipment. Recently, transition-metal SACs have become new cutting-edge catalysts with high activity, stability and selectivity, illustrating their remarkable potential for various electrocatalytic applications. For instance, Chen et al. [111] developed a modified hard-templating strategy to prepare SACs supported on functional Janus hollow graphene. As shown in Figure 4A [111] . TEM: Transmission electron microscopy; AC-HAADF-STEM: aberration-corrected high angle annular dark field scanning transmission electron microscopy; FT: fourier transforms; EXAFS: extended X-ray absorption fine structure; XANES: X-ray absorption near edge structure; ORR: oxygen reduction reaction; OER: oxygen evolution reaction.
Chen et al. [112] also reported a SiO 2 template strategy to synthesize single Fe atoms on single-shell hollow Ndoped carbon nanospheres (Fe-N-C HNSs). Benefiting from Fe-N 4 parts and a unique hollow structure with large surface area and high conductivity, the obtained Fe-N-C HNSs have significant ORR performance and high stability in alkaline solutions, as well as excellent methanol resistance. Compared with N-doped carbon hollow nanospheres without single Fe atoms and Fe-N-C samples without hollow nanospheres, the Fe-N-C HNSs emphasized the high activity of SACs and the structural advantages of single-shell hollow structures.
They revealed that the hollow carbon nanospheres with a thin shell and open framework could effectively expose active sites, thereby making fully use of the active Fe-N-C components.
Zhang et al. [113] constructed a single-shell hollow N-doped carbon matrix (HCM) by the SiO 2 template method to anchor isolated Ni atoms. The isolated Ni atoms coordinate with the surrounding N atoms and the synergistic effect of Ni-N coordination can reduce E F and the adsorption energy of intermediates, which is essential for promoting the OER kinetics in alkaline conditions [113] . In order to study the correlation between Ni-N x geometry and CO 2 RR performance, Xiong et al. [114] used SiO 2 as a hard template to construct a series of Ni-N 4 catalysts on carbon spheres with different geometric structures. They conducted an indepth study of the geometric parameters of the carbon support for the first time. The ultra-thin carbon shell changes the electronic structure of the carbon skeleton. By optimizing the carbon geometry, they obtained an ultra-thin carbon shell Ni/HMCS-3-800 with a higher pyrrole-N content and larger structural defects, which are conducive to CO 2 activation. Simultaneously, mesopores (7-10 nm) have also been proved to be beneficial to CO 2 transfer and adsorption. Thus, the geometry plays a key role in the RDS [114] .
Qiu et al. [115] discovered the influence of the dimensionality of the graphene matrix on the formation of Fe SACs [ Figure 5]. They revealed that single-shell hollow graphene spheres play a critical role in the preparation of single Fe atom catalysts. Due to the accumulation and aggregation of two-dimensional (2D) GO during the pyrolysis process, its surface area is reduced, single atomic sites are buried and mass transfer is blocked. Meanwhile, the FePc-immobilized 2D GO usually causes sintering and the aggregation of particles during pyrolysis, which has a negative impact on catalysis. In order to overcome these difficulties, single-shell hollow three-dimensional graphene spheres are used as scaffolds to immobilize Fe SACs (Fe ISAs/GHSs). The combination of single-atom Fe active sites and highly stable hollow structures endows Fe ISAs/GHSs with excellent ORR performance and improved activity and stability that are better than for most advanced commercial Pt/C catalysts. Li et al. [116] used silica spheres as hard templates to fabricate Co SACs anchored in single-shell hollow carbon spheres (CoSAs-NHCS). The Co atoms were uniformly dispersed on the NHCS and coordinated with N to construct Co-N-C active sites. The high surface area of CoSAs-NHCS can expose more active sites and promote mass and electron transport. The single-shell structure of CoSAs-NHCS not only lowers the reaction barrier but also contains more discharge products.
The hard template method often complicates the process of making hollow structures. According to the self-template method reported by Liang et al. [117] , single-shell hollow spheres with a single Fe atom were fabricated. The single-shell hollow structures increase the exposure of Fe-N 4 active sites, which show excellent ORR performance. Theoretical calculations combined with experiments revealed that Fe/NC(PS) has outstanding ORR catalytic performance due to the low reaction barrier, adjustable electronic structure and suitable adsorption energy in Fe-N 4 active sites. In addition, Zhang et al. [118] presented an efficient strategy to fabricate various SACs on single-shell hollow structured carbon. In this strategy, metal hydroxides or oxides [such as Co(OH) 2 nanoplates, MnO 2 nanowires and NiFe 2 O 4 NPs] are first wrapped with polymers, pyrolyzed and acid leached at high temperature and then SACs are immobilized on the internal wall of the single-shell hollow pipe. Zhao et al. [119] reported a urea-bridging strategy to prepare Ndoped hollow carbon tubes to anchor SACs (EA-SA). The results show that the atomically dispersed SACs on hollow carbon tubes are beneficial for increasing the exposed active sites and the transport of relevant species. These structural features endow EA-SAs with excellent ORR and OER activity. It is significant to fabricate heteroatom-doped metal tannic acid coordination materials with controllable morphology and stable chemical structures. Wei et al. [120] proposed a template-free strategy to prepare cross-linked polyphosphazene hollow nanospheres. After pyrolysis, they constructed single metal atoms on N/P-doped mesoporous carbon nanospheres with a high surface area (411.60 m 2 g -1 ). They showed the importance of mesoporous hollow structures and well-dispersed single Co atoms (Co-N 2 P 2 ) through controlled experiments. The Co-N 2 P 2 catalysts exhibited excellent electrocatalytic ORR activity, durability and methanol tolerance.

SACs supported on core-shell materials
Core-shell structures are composite materials where a solid core is directly coated with a different composition (shell). Therefore, core-shell structures can combine the characteristics of the core and the shell, rather than the single function of an individual NP. For instance, Hai et al. [121] reported a synthetic route to prepare a core-shell structured Co-SAC by pyrolysis of core-shell structured zeolitic imidazolate frameworks. The Co-N 3 C 1 @GC possessed a high density of Co-N 3 C 1 active sites in N-doped microporous carbon, which were coated by a mesoporous carbon (shell) [ Figure 6]. The catalysts showed excellent ORR performance and high stability (5000 cycles). Due to the reasonable design of the core-shell Co-N 3 C 1 @GC catalysts, the thermodynamic and kinetic constraints can be overcome to achieve high-efficiency  [121] . EDS: Energy-dispersive spectroscopy. EDS: Energy-dispersive spectroscopy; STEM: scanning transmission electron microscopy; AC STEM: aberration-corrected scanning transmission electron microscopy; FT-EXAFS: fourier transforms extended X-ray absorption fine structure; XANES: X-ray absorption near edge structure; LSV: linear sweep voltammetry.
performance. Compared with Co-N 2 C 2 and Co-N 4 , the Co-N 3 C 1 promotes both the electronic hybridization with O 2 and subsequent protonation of adsorbed O 2 * to form OOH*. Co-N 3 C 1 has a unique near-Fermi electronic structure before and after O 2 adsorption, resulting in outstanding ORR performance. The Co-N 3 C 1 -SAC core-shell structures significantly improve the mass and electron transfer, thereby further enhancing the performance of the ORR and Zn-air batteries.
Song et al. [86] reported that single Fe atoms supported on a 2D carbon nanoplate (Fe/CNP) derived from 2D ZnO@Zn/Fe-ZIF show excellent ORR performance. The core-shell NiFe-LDH@Fe/CNP present stable ORR and OER performance. He et al. [122] proposed a low-temperature CVD strategy to synthesize sheet-like open structures in which Ni NPs are coated by a Ni SAC supported carbon layer (Ni-NC@Ni) [ Figure 7]. The obtained Ni-NC@Ni catalysts present high Faradaic efficiency (~87%) for CO 2 electroreduction and significant current density (14.8 mA cm -2 at 670 mV), as well as stability (150 h). In addition, the core-shell structured Ni-NC@Ni promotes electron transport and mass transfer and the exposure of active site Ni-N species. Harzandi et al. [21] proposed an efficient method for constructing the core-shell OER catalysts, in which metallic Ru (core) is coated with RuO x (shell) supported Ni SACs. The obtained catalysts present outstanding activity (10 mA cm -2 at a low overpotential of 184 mV) and stability (~200 h) simultaneously in strong acidic media.
Pt SACs and bimetallic Pt 3 Co are considered the best ORR catalysts. Therefore, Lai et al. [123] constructed sustainable quasi-Pt-allotrope catalysts, which consist of a Pt 3 Co alloy (cores) and Pt 1 N-C in N-doped carbon (shells). The core-shell structure makes the internal and external spaces easy to access, thereby exposing a high active surface area and active sites. In addition, the novel Pt 1 N-C shells not only effectively protect the H-PtCo cores from agglomeration but also improve the efficiency of the ORR with the help of Pt SACs. Thus, the core-shell H-PtCo@Pt 1 N-C catalysts exhibit high stability (10000 cycles) in a HClO 4 solution. They thoroughly studied the synergistic effect between the hollow structure of PtCo and single Pt atoms. Compared with H-PtCo@Pt 1 N-C catalysts, the ORR performance of ZIF-67-50-750 is reduced due to the deformation of the hollow structure and the agglomeration of single Pt atoms after high-temperature treatment. They revealed that the ultra-fine Pt 3 Co alloy (cores), N-doped carbon (shells) and single Pt atoms synergistically endow H-PtCo@Pt 1 N-C with enhanced ORR performance. First-principles calculations showed that the smaller the number of N atoms adjacent to Pt, the smaller the corresponding overpotential, with PtNC 3 being the most favorable atomic local environment. Cheng et al. [124] demonstrated the synthesis of Co NPs (core) surrounded by N co-doped carbon nanofibers (shell) with Co SAC (Co@SACo-N-C). The core-shell Co@SACo-N-C presents outstanding ORR activity with an onset potential of 0.925 V and high stability in a 0.1 M HClO 4 solution. The results strongly reveal that the introduction of Co NPs (core) makes the free energy diagrams on Co-N-C shell closer to the ideal case, thus improving the ORR performance.

SACs supported on yolk-shell materials
Due to the limited storage capacity provided by the core-shell structure, researchers are seeking superior designs and microstructural modifications for greater flexibility. Yolk-shell structures combine the high storage capacity of hollow structures and the advantages of core-shell structures and create voids between the external shell and internal core.
For example, Zhao et al. [79] designed and constructed a yolk-shell biomimetic composite, in which Fe 1 sites were atomically dispersed in a N-doped carbon shell and Pd SACs were anchored in a MOF-derived yolk. HAADF-STEM images show that the obtained yolk-shell Pd 1 @Fe 1 has an ultrathin carbon shell (~10 nm) and an octahedral yolk derived from the MOF. This unique yolk-shell configuration may facilitate the exposure of catalytic sites and facilitate enhanced mass transfer during reactions. The Fe SACs strongly coordinate with four N atoms in the graphitic carbon shell. In addition, the corresponding fitting results show that both Pd and Fe have the same coordination number. Compared with Pd 1 -N 4 , the larger charge difference between the Fe and adjacent N atoms indicates that Fe 1 -N 4 is more stable. This unique yolk-shell structure effectively regulates the spatial distribution of Pd and Fe SACs in one configuration, which can simultaneously activate molecular oxygen and hydrogen in the catalytic cycles without influence. By directly using the O 2 and H 2 produced by the electrocatalytic overall water splitting, the Fe SACs in the carbon shell and the Pd SACs in the yolk can simultaneously catalyze the hydrogenation of nitroaromatic hydrocarbons  [122] . AC HAADF-STEM: Aberration-corrected high angle annular dark field scanning transmission electron microscopy. and the epoxidation of olefins, which leads to the cascade formation of amino alcohols. The void-free coreshell Pd 1 @Fe 1 (21%), yolk-shell Fe 1 @Pd 1 (4.9%) and hollow Pd 1 @Fe 1 (28%) exhibit weak chemoselectivity for 1-phenyl-2-(phenylamino) ethanol, which shows that the void and distribution order of the Pd and Fe SACs can synergistically accelerate the subsequent epoxide ring-opening amination reaction of the intermediate product.
Cai et al. [125] proposed an effective strategy where Au yolks can diffuse into and even be atomically dispersed in the Ni 2 P shell by simple pyrolysis of the yolk-shell Au@Ni 2 P [ Figure 8]. In-situ STEM shows the atomic diffusion of Au yolks and single-atom formation in the Ni 2 P shell. The obtained Au-Ni 2 P samples showed significantly improved OER performance, which was 16 times higher than that of a commercial IrO 2 sample. They revealed that the active sites originate from the synergy between Au SACs and the Ni 2 P shell. In addition, the Au-Ni 2 P samples may cause changes in electronic structure, which also enhance OER performance.
There is a significant need for efficient dual-function catalysts to accelerate the ORR and OER. Furthermore, interfacial adjustment also improves the activity of catalysts. For example, Gao et al. [126] reported a synchronous reduction method to prepare yolk-shell Co 3 O 4 @Co 3 O 4 /Ag catalysts. Due to the synergistic interaction, the obtained Co 3 O 4 @Co 3 O 4 /Ag shows a high initial capacity (12000 mAh g -1 at 200 mA g -1 ), high rate capability (4700 mAh g -1 at 800 mA g -1 ), low overpotential and long cycle life. The atomically dispersed Ag on the Co 3 O 4 shell not only strengthens the Ag/Co 3 O 4 interfacial binding but also adjusts the valence electronic structure of Ag and Co species and improves the electronic conductivity. The yolk-shell structure offers more active sites for the ORR and OER and also improves the catalytic activity.

CONCLUSIONS
Due to their maximum utilization of metal atoms and unique electronic/coordination structures, SACs supported on microstructures have attracted significant attention in the field of energy conversion. The catalytic properties of SACs rely on the characteristics of the active sites and their accessibility to reactants, which are greatly affected by hollow microstructures. In this review, we have summarized recent process regarding SACs supported on hollow microstructures, including single-shell hollow, core-shell and yolkshell structures, and their application in energy conversion. First, we showed the inherent characteristics of SAC related to the hollow microstructures. Due to the unique hollow microstructures, most of the active  [125] . AC HAADF-STEM: Aberration-corrected high angle annular dark field scanning transmission electron microscopy.
sites can be exposed during the actual catalysis process. Compared with the bulk supports, the atomic utilization rate is truly improved. In addition, the interaction between SACs and hollow microstructured supports brings about unpredictable characteristics. This means that hollow microstructured supports cannot only adjust the geometry and electronic structure of SACs to improve activity but SACs can also serve as an auxiliary stimulus to the intrinsic activity of support to achieve better performance. The local and global structure of SAC-NC can profoundly affect the electrocatalytic performance. In addition, it has been proved that the geometric factors of the hollow carbon support (shell thickness, compactness and pore size) affect the electrocatalytic performance. How to design isolated atom pairs through effective strategies while avoiding the coexistence of a large number of atoms is a challenge. Compared with the reported traditional dual atomic catalysts, hollow microstructures are conducive to the isolation of atomically dispersed binary atom active sites on both sides of the shell.
However, despite the great progress made in the past few decades, there are still some challenges that need to be solved urgently. Herein, we have described some challenges of SACs supported on hollow structures to demonstrate the necessary via microstructure regulation. Hollow structured materials usually have a large specific surface area but how to increase the loading of a single atom to make full use of the surface area is still a great challenge. The hard template method often complicates the process of making hollow structures. In addition, the microenvironment of SACs is complex and flexible, and the electronic structure and coordination environment of the metal active sites are also dynamically evolving during the catalytic process, which makes the variability of the microstructure and the complexity of the catalytic reaction mechanism. Simplifying the synthesis steps of hollow structures is another critical challenge. From a synthesis perspective, precise control and manipulation of highly complex hollow structures remains challenging. It is generally accepted that more complex target structures usually require more complex synthesis procedures. The ultimate goal of SAC is to be used in industry. However, the particularity and complexity of hollow microstructures may make it difficult for SACs to achieve industrial applications. In addition, the cost and complexity of the catalyst is another factor that should be considered. These problems make the real application of SACs/2DM still have a long way to go, and more efforts are needed to solve these problems as soon as possible. Ensuring and identifying the uniformity of actual active sites are very important for the study of catalytic properties but are also challenging issues. Due to the harsh synthesis conditions and interference in the hollow microstructured supports, the coordination structure of the real active site may be diverse, which is not conducive to exploring the real structure-performance relationship. Furthermore, adjusting the thickness of the shell is a key method to adjust the catalytic performance.
We expect that the application of SACs supported on hollow microstructures will continue to expand in electrocatalysis. In the future, SACs will be able to effectively solve actual energy and environmental problems.