Iron-based single-atom electrocatalysts: synthetic strategies and applications

The performance and cost of electrocatalysts play an important role in the development and application prospects of energy conversion technology. Single-atom catalysts (SACs) have constituted a new frontier in the field of catalytic science in recent years. As a non-precious metal, iron (Fe)-SACs show great potential in the field of electrocatalysis, which is comparable to or even better than the performance of precious metal catalysts. However, a robust, generic synthetic strategy toward atomically dispersed Fe catalysts is still lacking, which is still a formidable challenge to maintain the dispersion of Fe atoms at high temperatures and to obtain high catalytic activity. In this review, the latest progress in the synthesis of Fe-SACs is introduced and summarized, and the electrochemical applications of Fe-SACs are further summarized and discussed. Herein, the relationship between the structural characteristics and performance of Fe-SACs is further introduced and discussed. Finally, the existing problems and development prospects of Fe-SACs are discussed.


Introduction
Single-atom catalysts (SACs) combine the advantages of heterogeneous catalysts and homogeneous catalysts, and bridge the gap that exists between them with their unique performance. It is expected to become a bridge between homogeneous catalysis and heterogeneous catalysis, which has been a rising star in the eld of catalysis in recent years. 1,2 SACs have the advantages of maximum atomic utilization, uniform active sites, adjustable electronic environment, high catalytic activity and selectivity, better stability, and excellent recyclability. However, whether in the process of preparation or in the subsequent application process, the strong trend of migration and aggregation of active atoms and the controllable preparation of SAC is still a challenging task. [3][4][5][6][7][8][9][10][11] Transition noble metal monatomic catalysts such as ruthenium (Ru), silver (Ag), rhodium (Rh), palladium (Pd), iridium (Ir), platinum (Pt), and gold (Au) have been widely used in various electrochemical reactions such as oxygen reduction reactions (ORRs), hydrogen evolution reactions (HERs), carbon dioxide reduction reactions (CO 2 RRs), and oxygen evolution reactions (OERs). [12][13][14][15] However, the high cost and low natural abundance of precious metals seriously hinder the wide range of applications of these technologies. Therefore, the monatomic catalyst of non-precious metals has become the research focus of the majority of scholars. The latest research shows that iron (Fe)-SACs can be used in a variety of electrocatalytic reactions such as HERs, ORRs, nitrogen reduction reactions (NRRs), and CO 2 RRs (Fig. 1), exhibiting the same or even better catalytic performance than precious metal catalysts (the specic performance of the catalyst is described in detail in the section describing applications of Fe-SACs). 8,[16][17][18][19] These results provide a new possibility for the substitution of precious metal catalysts and show a great application prospect.
To date, the preparation of Fe-SAC and its application in the eld of electrocatalysis have not been reviewed. Therefore, it is necessary to comprehensively review the determinants of its preparation method, structure, and activity to provide a basis for the design of this type of catalysts and promote further research and development in this eld. In this article, we present an in-depth understanding of Fe-based SACs from the following aspects: (1) the relationship between structural characteristics and performance, (2) the application in the eld of electrocatalysis, and (3) the preparation strategy of Fe-based SACs. Finally, we look forward to the prospects and challenges of Fe-SACs.

Active sites of monoatomic iron catalysts
Several factors affect the activity of Fe monatomic catalysts; however, it is still a great challenge to clarify their structureproperty relationship. According to the current research progress of Fe monoatomic catalysts, the active sites of Fe monatomic catalysts are closely related to the local environment of Fe atoms, that is, the coordination number and electronegativity of adjacent atoms. Therefore, most of the research studies are mainly focused on Fe-N-C catalysts. Although the Fe-N-C catalyst shows excellent catalytic performance, it usually leads to the formation of Fe active sites with different coordination congurations. The heterogeneity of these structures and components makes the determination of the active sites and the relationship between the structure and catalytic performance very complicated. Fe atoms have a large number of empty d orbitals and different types of coordination structures, thus according to the number of Fe atoms and coordinated nitrogen atoms in the active sites, there are mainly four types of active sites of Fe monoatomic catalysts: Fe-N 4 , Fe-N 2 , Fe-N 6 , and Fe-N x , among which Fe-N 4 is the most reported active site. The structure of the carrier comprising the location of these active sites and the surface loading rate also has a signicant impact on the catalytic performance. Different types of Fe active sites show different catalytic properties for different electrocatalytic applications. In recent years, research efforts have been devoted to the theoretical simulation and experimental verication of Fe-SACs, mainly to establish the relationship between the local atomic structure and catalytic performance, in order to explore its potential application in electrocatalysis.
The monatomic Fe catalyst dispersed on nitrogen-doped graphene (Fe/NG) reported by Zhang et al. 20 consists of a pyridine-type Fe-N 4 structure, and nitrogen doping plays an important role in improving the conversion of CO. A small amount of Fe atoms loaded in NG could form a Fe-N 4 site capable of enhancing the adsorption of CO 2 and improving the activity of CO 2 . Fig. 2a and b demonstrate that the mechanism includes the following steps: (1) CO 2 + * + H + + e À / COOH*, (2) COOH* + H + + e À / CO* + H 2 O, and (3) CO* / CO + *, where * represents the active site on the surface of the catalyst. Moreover, the three free energy diagrams with different nitrogen doping congurations of graphite indicate that the nitrogen substitution on graphene improves the catalytic activity of the Fe-N 4 moieties by lowering the energy barrier of COOH* formation, as well as facilitating the CO* desorption step.
The Fe monoatomic catalyst Fe SAs/N-C reported by Yang et al. 21 consists of a single Fe-N 4 as the active site and shows outstanding performance toward ORRs. They used extended Xray absorption structure (EXAFS) and X-ray absorption nearedge structure (XANES) to study the coordination environment and chemical state of Fe-SAs/N-C. Fig. 3a shows that the single atom of Fe in Fe SAs/N-C may be in a low-valence state (0  < d < 2). Fig. 3b demonstrates that the main peak of the catalyst corresponds to Fe-N coordination, and its central position is about 1.5Å. The results indicated that Fe atoms were loaded onto N-doped carbon nanosheets in the form of single atoms. Fig. 3c illustrates that the wavelet transform (WT) diagram of Fe SAs/N-C shows only one intensity maximum at 5Å À1 , which can be specied as Fe-N coordination. The EXAFS measurement and tting curve ( Fig. 3d and Table 1) indicate that the Fe single atom has a good Fe-N 4 structure. Three peaks of pyridine N p*   (399.5 eV), graphitized N p* (402.4 eV) transition, and the formation of C-N s* bond (408.5 eV) were observed in the N-K edge spectrum shown in Fig. 3e. The activity of the ORR was enhanced by selectively breaking the C-N bond at the edge of the active site of Fe-N 4 . 22 Five Fe-N 4 /C models with different edge N atoms were proposed through density functional theory (DFT) (Fig. 4a). At the equilibrium potential U ¼ À0.77 V (vs. SHE) (Fig. 4b), the free energy change (DG) of the four-electron reduction path was negative, thus it was exothermic. However, Fig. 4c shows that the rst electron transfer sub-step (*O 2 + H 2 O + e À / *OOH + OH À ) has a maximum positive DG; therefore, this step is a step that determines the rate of the reaction process. FeN 4 -6r-c1 and FeN 4 -6r-c2 have lower values of DG, which are 0.31 and 0.32 eV, respectively. However, for step 5 (OH* + e À / O À ), FeN 4 -6r-c1 showed higher resistance (0.72 eV) and better overcoming ability for FeN 4 -6r-c2 with a higher cleavage degree (0.13 eV). In general, the defect FeN 4 -6r-c2 is located at the edge of the cavity and has the lowest total reaction free energy change (0.32 eV). These results conrm that the C-N structure changes the charge density distribution and electronic structure of the active site.
Serov et al. 23 believed that the active site of the catalyst not only comprised Fe-N, but also included some in-plane and edge defects. Fig. 4d shows the schematic of in-plane Fe-N x defects and edge defects in a hypothetical graphene sheet. Edge defects include nitrogen and oxygen, and they may also contain Fe-N x . All these defects may act as potential active sites in ORRs.
Pan et al. 24 further revealed the intrinsic electronic properties of the coordination effect through DFT calculations, as shown in Fig. 5. The coordination mode of Fe-N leads to a change in the structure and electronic properties of the catalyst, thus affecting the path of the catalytic reaction and the formation of intermediates. The increase in the Fe-N coordination number facilitates the generation and activation of the crucial intermediate O]Fe]O species, thereby enhancing the BOR activity. More interestingly, when the coordinated N atom is replaced with one or two C atoms, the BOR activity of the monatomic Fe site catalyst decreases gradually, showing obvious coordination sensitivity. Therefore, the monatomic Fe center xed by four nitrogen atoms showed the highest BOR performance, with 78.4% conversion and 100% phenol selectivity at 30 C, exceeding those of all reported BOR catalysts.
The ORR activity with typical Fe-N 4 active sites was improved by introducing suitable carriers. For example, the intrinsic ORR activity of the original Fe phthalocyanine (FePc) catalyst with the added two-dimensional (2D) material Ti 3 C 2 T x as the carrier was twice as high as that of the FePc catalyst. 25 FeN 2 and FeN 6 active sites are considered to have great potential in the development of new advanced Fe-based monatomic electrocatalysts. Yansong Zhu 26 conrmed that the active site of a sixcoordinated Fe(III) complex [Fe(III) (porphyrin)(pyridine) 2 ] was six-coordinated FeN 6 . The non-noble metal catalyst for ORRs showed high activity and good durability. Zheng et al. 27 designed and developed a range of molecule-level graphitic carbon nitride (g-C 3 N 4 )-coordinated transition metals catalysts for these oxygen electrode reactions. The correlation of experimental and computational results conrmed that this high activity originated from the precise M-N 2 coordination in the g-C 3 N 4 matrix.
Asnavandi et al. 28 rst studied the overpotential of the surface without oxygen vacancies (OVs). Oxygen vacancies refer to vacancies formed in metal oxides or other oxygen-containing compounds where oxygen atoms (oxygen ions) in the crystal lattice break away, resulting in oxygen deciency. Fig. 6a shows the geometric structure of the OER and the calculated DG. DG max ¼ 1.78 eV, which led to an overpotential of 0.55 V when the OER occurred at the Fe site. When oxygen (represented by the circle in Fig. 6b) was removed to produce OV and the low coordination metals (LCMs) were marked as Fe 4c and Ni 4c, the reactant species (O, OH, and OOH) got adsorbed and relaxed completely on it. Its geometric structure and calculated value of DG are shown in Fig. 6b; DG max is 1.41 eV and overpotential h ¼ 0.18 V. Compared to the case without OV, the overpotential decreased by 0.37 V, which indicated that the LCM activity associated with OV was higher, which is consistent with the understanding in the earlier study that the high catalytic performance of NiFe-OOH results from poorly coordinated metals such as defects, steps, and edges. 29 In order to further describe the active sites and catalytic performance of Fe monoatomic catalysts, we collected some information about Fe monoatomic catalysts, including reaction precursors, types of active sites, and loading, as presented in Table 2. These are the key factors affecting its activity. Therefore, the activity of the catalyst can be regulated by selecting precursors or loading to form Fe active sites. For example, doping heteroatoms (such as P, S, and N) on the substrate aids in the adjustment of the electronic structure of the Fe active site. It is also possible to control the density of Fe active points by changing the Fe loading rate, thereby adjusting the performance and application of the catalyst.
In short, the main factors affecting the catalytic activity of Fe-SACs and its potential application in electrocatalysis include the coordination structure and electronic structure of Fe, the morphology and electronic structure of the conductive matrix, and the total number of Fe atom active sites.

Synthetic strategies of Fe-SACs
Recently, various experimental techniques for the preparation of Fe single atoms have been reported. Moreover, according to the reported process, the preparation of Fe monoatomic catalysts can be summarized into the following three strategies.

Strategy of wrapping iron active sites in roasted metalorganic frameworks
In recent years, metal-organic frameworks (MOFs) have attracted signicant attention because of their good porous structure and highly ordered arrangement of organic connectors and metal nodes. Both MOFs and MOF-derived materials can still maintain their specic shape and pore structure aer pyrolysis at high temperatures. Notably, they provide a pathway for the effective transfer and diffusion of substrates and products, providing a high-quality platform for Fe active sites. The synergistic effect and pore limitation effect can be produced between the active site of Fe and the MOF shell. Metal nodes and aromatic connectors in MOFs can establish charge transfer interactions with Fe active sites by coordination or p-p interaction, thus performing collective functions compared to single-component materials. Moreover, the MOF shell effectively wraps the Fe active site and prevents the agglomeration and leaching of Fe atoms, thus improving the stability of the catalytic process. Therefore, MOF coating strategy is widely used in the preparation of Fe-SACs. However, although intensive efforts and great progress have been made in this area, the study on Fe active sites coated with porous MOFs is still in its infancy and many challenges still need to be overcome. The composition and spatial distribution of Fe active sites in MOFs are still difficult to control, and it is also very hard and challenging to effectively balance the relationship between heterogeneous nucleation and controllable growth of MOFs around Fe active sites and self-nucleation and homogeneous growth. Moreover, the electrical conductivity and ionic conductivity of MOFs are also important factors signicantly affecting its electrocatalytic performance. For example, Qi-Long Zhu and his colleagues 31 used MIL-101-NH 2 as the MOF host to accommodate dicyandiamide, the nitrogen source with high nitrogen content, and FeCl 3 , the most readily available Fe source. In order to avoid the deposition of precursors on the outer surface of a MIL-101-NH 2 crystal, dicyandiamide and FeCl 3 were quantitatively coated by a double-solvent method. Fig. 7a shows the monatomic catalyst, Fe/N-GPC, obtained by pyrolysis and acid etching of the synthesized MOF composite, exhibiting that the complete MOF skeleton of the catalyst was well maintained. Xiao et al. 33 pyrolyzed Fe triacetylacetonate Fedoped ZIF-8 as the metal source, nitrogen source, and carbon source to form monoatomic catalyst Fe-N-C-950 (Fig. 7b). Han et al. 18 synthesized Fe-Phen complexes at room temperature using FeSO 4 and Phen as non-precious metal and organic ligands, respectively. Then, during the growth of ZIF-8, the Fe-Phen species were wrapped in nano-cages. The monoatomic catalyst Fe-N x -C was obtained aer pyrolysis in an argon atmosphere at 900 C (Fig. 7c). Jiang et al. 22 used ZIF-8 as the host of MOFs to hold Fe(II)Pc. During the assembly of Zn(II) and 2-methylimidazole, Fe(II)Pc molecules were encapsulated in the cavity of a zeolite imidazole skeleton to form FePc-x@zif-8 nanocomposites. Monatomic catalyst Fe SAs-N/C-20 was obtained by pyrolysis and acid etching, and the complete MOF of the catalyst was well maintained (Fig. 7d). Li et al. 37 synthesized a aky MOF by the interaction between N-rich dicyanoimidazole and ferric acetate. By using the MOF as the host, Fe acetate was coated, and the monoatomic catalyst DCI-Fe was prepared by pyrolysis (Fig. 7e). Lü et al. 19 prepared a bimetallic Fe/Zn zeolite imidazole framework (ZIF-8) by a hydrothermal method and then carbonized and etched it to obtain monoatomic catalyst ISAS-Fe/NC (Fig. 7f). Zhang et al. 35 synthesized a Fe-containing rod-like MOF from ZrOCl 2 and tetra(p-carboxyphenyl)porphyrin and tetra(p-carboxyphenyl)porphyrin Fe in DMF and triuoroacetic acid. In this porphyrin MOF, they successfully constructed the Fe atom-dispersed catalyst Fe1-N-C by reasonably controlling the distance between the adjacent Fe atoms (Fig. 7g)  converted into carbon with high specic surface area (SSA) to form a specic framework of the Fe-based porous carbon monoatomic catalyst Fe-ZIF. 6,32,34,38

Strategy including modied carbon-rich carriers
Carbon-rich carriers including graphene (or graphene oxide (GO)) sheets, carbon nanotubes (CNTs), carbon nanobers, carbon nanospheres, nano-carbon derived from polycarbonate, and other polymer-derived nano-carbon materials have porous structures for rapid mass and energy transfer and an adjustable chemical coordination environment, thus they can be considered as ideal carriers for Fe monoatomic catalysts. The use of chitosan, cyanamide, dicyandiamide, melamine, and porphyrin also shows the potential to capture Fe atoms via surface Fig. 7 (a)-(g) Schematic of Fe-SACs with a metal-organic framework as a catalytic application platform. modication of carbon-rich carriers, in order to achieve efficient electrocatalytic performance. Although this strategy has been developed to some extent, it still faces multiple challenges to be addressed. The loading of this type of Fe monatomic catalyst is relatively low, which may limit its further development in large-scale applications. The carrier (graphene or GO) mainly depends on the structure of graphene or GO, which is fragile and easy to break, thus it is difficult to be characterized and controlled by a synthesis strategy. This method of preparing Fe active site by introducing doping and manufacturing defects to adjust the local electronic state on the surface of the carrier is difficult to control regularly. Chen et al. 16 achieved monoatomic dispersion of Fe(S,N-Fe/N/C-CNT) on the layered carbon modied with N and S by coating 2-bipyridine and Fe salt precursors onto the surface of CNTs, followed by pyrolysis and acid etching in a nitrogen atmosphere (Fig. 8a). Zhu et al. 39 prepared CNT precursors by a one-step hydrothermal method using tellurium nanowires, glucosamine hydrochloride, and ammonium ferric sulfate (catalyst) at 180 C for 15 h. The resulting hydrogel was soaked several times in distilled water to remove impurities and then freeze-dried. Finally, the monatomic catalyst Fe-N-CNTAS-5-900 was obtained by heat treatment in a nitrogen atmosphere for 2 h (Fig. 8b). Yang et al. 13 used Fe-loaded F127 [F127 is a watersoluble surfactant of polyoxyethylene-polyoxypropylene-polyoxyethylene (PEO-PPO-PEO)] and g-C 3 N 4 to form layered precursors, and then obtained monoatomic catalyst SA-Fe/NG by pyrolysis (Fig. 8c). Further, Mun et al. 40 used ferrous chloride hexahydrate and 1,10-phenanthroline (Phen) as precursors and mesoporous carbon foam as a support, and obtained monoatomic catalyst FeNC-S-MSUFC by pyrolysis and acid etching (Fig. 8d). Yang et al. 21 used polyetherimide (PEI) polymers as carbon precursors and Phen ligands as spatial isolating agents for Fe ions to promote their complete conversion into a single Fe atom without forming Fe nanoparticles (NPs), and Ndoped porous carbon nanosheets as carriers to form a highly stable monoatomic catalyst, Fe SAs/N-C, by high-temperature pyrolysis (Fig. 8e). Zhang et al. 20 used GO as the precursor, annealed the mixture of GO and FeCl 3 in an Ar/NH 3 atmosphere at 800 C and xed the Fe atom on graphene by a N bond, and obtained Fe/NG with monoatoms dispersed on NG (Fig. 8f). Zhao et al. 17 reported a cascade anchoring strategy for synthesizing Fe-NC structures. Fe ions were rst chelated with chelating agents (such as glucose), then xed on oxygen-rich porous carbon carriers with high SSA, and nally pyrolyzed at a certain temperature to form Fe-N x catalytic sites. The function of the chelating agent is to protect the Fe ion and combine it with the oxygen-rich carbon carrier (Fig. 8g). Sa et al. 41 mixed CNTs with porphyrin Fe precursors, heated the CNTs adsorbed by porphyrin to 400 C, and formed CNTs with a porphyrin layer via p-p interaction. Second, the surface of the composite was covered with a layer of silicon dioxide (SiO 2 ). Finally, the ternary composite was pyrolyzed at high temperatures, and then the SiO 2 layer was etched to obtain the Fe monoatomic catalyst CNT/PC (Fig. 8h). Varela et al. 42 rst carried out in situ polymerization using Fe chloride and aniline as precursors and ammonium persulfate as an oxidant, and then added Kaijin black carbon powder for continuous annealing and acid leaching cycles to synthesize the monatomic Fe-doped N porous carbon black catalyst Fe-N-C. Choi et al. 43 prepared monoatomic catalyst Fe-N-C by pyrolysis and acid etching using ferric acetate and Phen as precursors and ZIF-8 as a carrier. Jiang et al. 44 coated CNTs with glucose in the presence of Fe salt precursors, and then prepared monoatomic catalyst Fe@C-FeNC by pyrolysis in the presence of melamine.

Strategy for in situ preparation of small molecule precursors
The preparation process of Fe-SACs using an in situ strategy is relatively simple, can control the dispersion of Fe atoms, and easily used on a large scale. However, the small molecule precursors usually have a relatively low thermal decomposition temperature, which makes the carbonization of precursors difficult. Therefore, appropriate selection of small molecule precursors is very important. Thus far, most of the precursors are nitrogen-rich molecules. Moreover, the catalyst prepared by this method is also difficult to have a consistent geometric morphology, and the selectivity of the catalysts may also be poor. For example, Zhou et al. 45 easily synthesized novel low-cost ferrocene-based porous organic polymers (POPs) by the Schiff base reaction between melamine and ferrocene aldehyde without any catalyst. Through the direct carbonization of the synthesized ferrocene-based porous organic polymer, the transformation from amorphous ferrocene-based POP to the crystalline Fe 3 C/Fe carbon composite was further realized to prepare monoatomic catalyst N-FC-800 (Fig. 9). Gu et al. 36 mixed glucose, dicyandiamide, and ferrous chloride tetrahydrate in an aqueous solution and then carried out freeze-drying and pyrolysis to obtain monoatomic catalyst Fe-N-C. Serov et al. 23 prepared monoatomic catalyst Fe-CBDZ by pyrolysis and acid etching using ferric nitrate and carbendazim as precursors and SiO 2 powder as a template. Owing to the elimination of articial carbon in the material design, the unique process method, and the reasonable selection of precursors based on high carbon content, high N/C ratio and low volatile content, the catalyst showed a higher active site density and a higher mass activity.

Electrochemical applications of iron-based single-atom catalysts
The development of efficient, durable, and economical nonprecious metal monatomic electrocatalysts is of great signicance to the development and commercial application of electrochemical energy conversion technology; however, this is still a huge challenge. In recent years, Fe-SACs used in various reactions have been developed, which have excellent activity, selectivity, and stability. In the following chapters, the electrochemical applications of Fe-based monatomic catalysts in key energy conversion reactions such as ORRs and CO 2 RRs are introduced and summarized, in order to demonstrate the great potential of Fe-based monatomic catalysts in achieving efficient and selective electrochemical processes.

Oxygen reduction reaction
Environmentally friendly energy storage process/device/ technology that converts chemical energy into electricity through devices such as fuel cells and metal-air cells is essential to reduce pollution and the greenhouse effect because the world needs to shi from carbon and fossil fuel (coal, oil, and natural gas)-driven economy to more sustainable and green options. 46,47 However, one of the main bottlenecks in the commercialization of these devices is the requirement of ORRs in their operation, because of their high overpotential due to the slow cathodic kinetics. 48 Fe-based monatomic catalysts are considered to be one of the most important (ORR) catalysts for oxygen reduction. They are potential substitutes for Pt-based catalysts; therefore, they have a bright future in the development of non-noble metal-based catalysts. At present, according to the different products, the path of the ORR can be divided into the following two types: one is the four-electron (4e) reaction mechanism of complete reduction, under acidic (or alkaline) conditions, in which oxygen is reduced to water (or OH À ); and the other is a partial reduction of two-electron (2e) reaction mechanism. Under acidic (or alkaline) conditions, oxygen is reduced to hydrogen peroxide (or HO 2À ). ORRs of two paths in different electrolytes are shown in Table 3. 49 In the fuel cell process, the 4e path is preferred because it provides high current efficiency, while the 2e reduction pathway is used in the industrial production of H 2 O 2 . Therefore, 4e complete reduction of ORR catalyst is required when selecting the ORR catalyst. 50 Fig. 10 shows the construction of a "volcano map" based on theoretical calculation and experimental data. 51 Although Fe is distributed at the bottom of the volcanic map, the activity of ORR can be improved by regulating the metal-carrier interaction and promoting the adsorption of key intermediate systems to reduce the Gibbs free energy of ORRs. Some Fe-based monatomic catalysts reported in recent years have excellent ORR catalytic activity, even exceeding that of the commercial Pt/ C catalysts at the top of the "volcano map".
The atomically dispersed Fe-N x -C catalysts prepared by Han et al. 18 showed ultra-high ORR activity, stability, and methanol resistance, and exhibited better performance than most Pt-free catalysts and industrial Pt/C catalysts in an alkaline electrolyte. The half-wave potential of ORRs of the catalyst was 0.91 V, which is higher than that of commercial Pt/C (0.82 V). The primary zinc-air battery with the Pt/C cathode catalyst showed excellent electrochemical performance with open-circuit voltage (OCV) as high as 1.51 V and power density as high as 96.4 mW cm À2 . The rechargeable zinc-air battery with the Fe-N x -C catalyst and an alkaline electrolyte showed excellent cycle performance within 300 h, and the initial round-trip efficiency was 59.6%, as shown in Fig. 11a-d. Moreover, the rechargeable all-solid-state ZABS with Fe-N x -C catalyst has a high OCV of 1.49 V, a long cycle life of 120 h, and foldability as shown in Fig. 12a-d. Zhu et al. 31 prepared electrode using the Fe/N-GPC catalyst, which exhibited high catalytic activity with an initial potential of 0.85 V and a half-wave potential of 0.63 V. The electron transfer number of Fe/N-GPC was calculated to be 3.62-3.73 at 0.4-0.2 V, suggesting that the Fe/N-GPC electrode also mainly favored an efficient four-electron transfer process in acidic media, showing great application potential. The catalyst FeSAs/N-C electrode prepared by Yang et al. 21 showed high ORR catalytic activity. Under alkaline conditions, the initial potential was 1.02 V and the half-wave potential was 0.91 V (Fig. 13a), while under acidic conditions, the initial potential was 0.95 V and the half-wave potential was 0.798 V (Fig. 13b). The electron transfer number of the FeSAs/N-C electrode was found to be 3.98 (Fig. 13c), indicating that the mechanism reaction of the FeSAs/N-C electrode is mainly 4e transfer process in an acidic or alkaline medium. The FeSAs/N-C catalyst is used as the cathode of proton exchange membrane fuel cell (PEMFC). At 0.1 and 0.2 MPa back pressure, the maximum power densities of FeSAs/N-C-based H 2 /O 2 PEMFC were found to be 0.68 and 0.75 W cm À2 , respectively (Fig. 13d). When using this FeSAs/N-C catalyst for H 2 /air PEMFC, this single PEMFC exhibited a peak power density of 0.35 W cm À2 , and the maximum power density of the Zn-air battery using the FeSAs/N-C catalyst was as high as 225 mW cm À2 (Fig. 13e and f), indicating that FeSAs/N-C is an excellent Pt-free catalyst.
Zhou et al. 45 prepared the catalyst (N-FC-800), which showed high initial potential and half-wave potential of 0.96 and 0.82 V, respectively, exhibiting good activity. Based on this, a rechargeable zinc-air battery was assembled using N-FC-800 as the positive catalyst. Compared to commercial Pt/C, the N-FC-800 battery has a higher power density of 178 MW AE cm 2 and a smaller charge-discharge voltage gap of 0.94 V (Fig. 14a), good stability, and less activity attenuation (1.0%) in long chargedischarge cycles (Fig. 14b), and it successfully lit the LED lamp ( Fig. 14c and d).

Oxygen evolution reactions
The OER is the reverse reaction process of ORRs, that is, the oxidation of H 2 O to O 2 is the reaction of the air electrode in the metal-air battery during the charging process. In the past few decades, electrocatalytic OERs have received extensive attention globally, and a large number of catalysts have been developed to improve the kinetics and stability of OER electrodes in different electrolytic environments. Among them, rutile RuO 2 and IrO 2 show good OER performance, and they are used as reference catalysts for research on OER performance. 52,53 Nonetheless, they are prone to further oxidation at high potentials, resulting in their instability at high potentials. More importantly, they are all precious metal catalysts, and their high costs are not suitable for large-scale production applications. Owing to the rapid development of monatomic catalysts in    recent years, it has been found that Fe-based monatomic catalysts also have good OER activity. For example, Junxing Han 18 prepared the Fe-N x -C catalyst and compared it with the most advanced OER catalyst RuO 2 . The overpotential of the Fe-N x -C catalyst was found to be 600 mV when the current density was 10 mA cm À2 (marked as E j10 ), which is close to the overpotential of RuO 2 (530 mV). Potential difference DE (DE ¼ E j10 À E 1/2 ) is an important parameter to evaluate the redox catalytic performance of electrocatalysts. The DE value of the Fe-N x -C catalyst is about 0.92 V, which is better than that of RuO 2 for OERs (E j10 ) (DE ¼ 0.94 V). Guoqiang Shen 54 report a facile adsorptionoxidation strategy to anchor Fe(III) atomically on ultrathin TiO 2 nanobelt to synergistically lower the spin state (e g lling $1.08) to enhance the adsorption with oxygen-containing intermediates and improve the electro-conductibility for lower ohmic loss. The electronic structure of the catalyst is predicted by DFT calculations and perfectly conrmed by experimental results. The catalyst exhibits superior performance for OERs with an overpotential 270 mV @ 10 mA cm À2 and 376 mV @ 100 mA cm À2 in an alkaline medium, which is much better than that of IrO 2 /C and RuO 2 /C.

Carbon dioxide reduction reactions
The electrochemical reduction of CO 2 to CO is a 2e/proton reaction, which includes two basic steps. 55-57 CO 2 is rst adsorbed on the surface of the catalyst, and one electron is transferred to the adsorbed CO 2 to generate surface-adsorbed CO 2 À ; the subsequent combination of CO 2 À and a proton occurs to form *COOH, where * represents the adsorbed substance on the surface. Then, *COOH reacts with protons to remove H 2 O to form * CO intermediates. Finally, the free gaseous CO was formed by peeling off the surface of the catalyst. Based on this reaction path, an ideal catalyst needs to have a suitable adsorption energy for the main intermediates, which is benecial to promote the activation of CO 2 to form *COOH and the desorption step of *CO. The electrochemical reduction of CO 2 to CO provides a promising pathway to reduce CO 2 emissions and increase the value of chemical fuel to alleviate the energy crisis. Therefore, development of efficient and selective CO 2 RR electrocatalysts is of great signicance. Fe-based monatomic catalysts exhibit the highest atomic efficiency and low coordination metal sites and show excellent activity in various reactions. Compared with the nano-cluster or nano-particle catalyst, the monatomic catalyst has a relatively uniform active center and geometric conguration, which gives the reaction substrate a similar environment, which is very benecial to improve the catalytic selectivity. Therefore, Fe monatomic catalysts show great hope in achieving high efficiency and selectivity in the CO 2 RR process. Gu et al. 34 prepared a catalyst Fe 3+ -N-C consisting of dispersed monatomic Fe sites, which could produce CO at an overpotential as low as 80 mV. At an overpotential of 340 mV, the local current density reached 94 mA cm À3 . The active sites of the catalyst were discrete Fe 3+ ions, which coordinated with pyrrole nitrogen (N) atoms on N-doped carbon carriers and maintained +3 oxidation states in the process of electrocatalysis, which was possibly achieved by electronic coupling with conductive carbon carriers. Electrochemical data show that compared to the traditional Fe 2+ site, the Fe 3+ site undergoes faster CO 2 adsorption and weaker CO absorption, thus obtaining better activity. Huan et al. 58 prepared Fe 0.5 d material as a catalyst with the highest activity and the best selectivity, which could produce CO with high FY (up to 90%) and low overpotential (190 mV overpotential relative to the equilibrium potential of CO 2 /CO in 0.5 M NaHCO 3 aqueous solution) and could be electrolyzed for a long time.
Interestingly, the selectivity of these materials for CO 2 reduction depends on the ratio of isolated FeN 4 sites to Fe-based NPs, and the higher the ratio, the better the selectivity. These positions are reminiscent of soluble Fe porphyrins, and they have been proved to be excellent catalysts for the conversion of CO 2 to CO. Zhang et al. 16 synthesized dispersed Fe atoms on nitrogen-doped graphene (Fe/NG) as efficient electrocatalysts for the reduction of CO 2 to CO. The reduction overpotential of Fe/NG was low, and the Faraday efficiency was as high as 80%. The evidence of structural analysis and control experiments of the catalyst system exhibited that the isolated Fe-N 4 structure was the main reason for the reduction of CO 2 to CO. Some Fe-N-C catalysts composed of uniformly dispersed Fe-N x sites showed impressive high activity and remarkable selectivity for CO and hydrocarbons. In the future industrial CO 2 gas diffusion cathode (CCCS), these catalysts can replace carbon-supported Au catalysts for the reaction of CO 2 RR to CO/H 2 gas mixtures. 42,59

Nitrogen reduction reactions
The traditional Haber-Bosch nitrogen production process requires high temperatures, high pressures, etc., and the ammonia synthesis process usually requires about 485 kJ mol À1 energy input. 60 Clearly, this is large energy consumption, and thus, ways to reduce this requirement are also highly desirable. Moreover, subsequent discussion also conrms that this is actually a very challenging goal. Under this background, electrochemical reduction of nitrogen to ammonia is considered as a promising alternative to the energy-and capital-intensive Haber-Bosch process; thus, it has attracted signicant attention in the scientic community. The basic equation of the process can be expressed as follows: 61 Anode (acid condition) Cathode (acid condition) Cathode (alkaline condition) Norskov et al. studied the NRR performance of different metal catalysts by theoretical calculation (Fig. 15) to reveal the relationship between N 2 adsorption and catalytic ammonia production. If only the activity of NRR is considered, the metals Mo, Fe, Ru, and Rh have been found to be more appropriate. 62 In the past few years, Fe-based monatomic catalysts have been proposed as catalysts for NRRs, and they have shown very high performance in terms of FE, NH 3 yield, and stability. The simplicity and uniformity of its structure are helpful for the comprehensive understanding of the relationship between structure and catalytic performance, which is helpful to achieve a reasonable catalyst design on the atomic scale. Lü et al. 19

Others
In addition to their applications in the above-mentioned elds, iron monatomic catalysts have also attracted the attention of researchers in the elds of HERs, electrochemiluminescence (ECL) and BOR. For example, Yurui Xue 8 anchored Fe atoms on diacetylene graphite by a simple electrochemical reduction method to obtain monoatomic catalyst Fe/GD. It exhibited excellent performance toward HERs, and the overpotential was 66 mV and the Tafel slope was 37.8 mV dec À1 at 10 mA cm À2 . Aer 5000 cycles, the polarization curve of Fe/GD remained unchanged in current density and was much better than that of commercial Pt/C in terms of stability. Gu et al. 36 for the rst time used the Fe monatomic catalyst (Fe-N-C SACs) as an advanced co-reaction accelerator to directly reduce dissolved oxygen (O 2 ) to reactive oxygen species (ROS). Owing to its unique electronic structure and catalytic activity, it could efficiently produce a large number of ROS. These ROS could react with luminol anions and signicantly amplied the ECL emission of luminol. Pan et al. 24 fabricated a Fe-N 4 SAs/N-C catalyst, which exhibited the highest BOR performance by the monoatomic Fe center anchored by four coordinated nitrogen atoms, with a benzene conversion of 78.4% and a phenol selectivity close to 100%. When the N atom in the Fe-N 4 SAs/N-C catalyst was replaced with one or two C atoms, the activity decreased gradually, and the low activity could be improved by increasing the Fe-N coordination number. The results indicated that the catalytic performance could be changed conveniently and effectively by adjusting the coordination environment of SACs.

Challenges and opportunities
In recent years, owing to the development of high-resolution characterization technology and theoretical simulation, reasonable progress has been made in elucidating the real catalytic system of Fe monatomic materials. The potential of Fe monatomic catalysts for electrocatalysis is obvious; however, there are still many scientic and technical challenges that need to be overcome. The research on Fe monoatomic materials for electrocatalysis is in the initial stage, and there are few reports available in the literature. Although some progress has been made in the research on Fe monoatomic materials for electrocatalysis, most of the studies are focused on the new preparation methods, and the performance is not outstanding. Fe monatomic materials consist of fewer active sites, resulting in only low current density, which is not suitable for practical applications. However, with the increase in the load of Fe atoms, it is inevitable to face the problem of migration and aggregation of Fe atoms to form NPs. At present, the activity of most reported Fe monatomic materials is mainly due to their Fe-N 4 active sites; nonetheless, studies on Fe-N 2 , Fe-N 6 , and other active sites with the same potential have rarely been reported. For example, the ORR activity of veried Fe-N-C-based catalysts is mainly attributed to their Fe-N 4 active sites. However, according to theoretical prediction, this is not the most active site, but Fe-N 2 . 65,66 Furthermore, basic problems such as Fe-N coordination environment still exist. When exploring the preparation of high-performance Fe-SACs with specic functions, signicant attention should be paid to the following points: (1) In order to obtain efficient Fe-SACs, development of more effective preparation methods is required to accurately regulate the anchorage point and the surface of the support. Defect engineering strategies, such as vacancies, amorphous phases, and structural defects, are used to adjust the electronic structure of the catalyst to obtain better catalytic performance. (2) The main challenge of Fe-SACs is that the loading of Fe active sites is very low, and further addition of Fe inevitably leads to the formation of Fe NPs. It is thus necessary to nd or develop porous carriers such as porous 2D materials and 0D materials with abundant accessible surfaces or vacancies that can carry more monatomic active sites. (3) Enhancing the electrical conductivity of Fe-SACs is also an important approach to improve its electrocatalytic activity, for example, it can be achieved by improving the graphitization of carbon-containing materials. (4) It is also essential to use the method of theoretical calculation to study the catalytic performance of the active site of Fe-SACs. Thus far, little work has been done in this area. Therefore, the use of theoretical calculation method to select suitable materials will also be of great signicance. (5) For researchers who study Fe-SACs, the development of its application in new elds should be regarded as an important task.
Although several challenges are encountered, there is a reason to be hopeful for Fe monoatomic catalysts that can be used for electrocatalysis, because the progress of in situ and non-in situ characterization techniques and the development of theoretical simulation can provide support for the study of the catalytic mechanism of Fe monoatomic catalysts. With the continued research on Fe monatomic catalysts, its application in the eld of electrocatalysis will be more mature and become the star of the catalyst research.

Conflicts of interest
The authors declare no conict of interest.