ISSN 1998-0124 CN 11-5974/O4

2019, 12(9): 2067–2080 https://doi.org/10.1007/s12274-019-2345-4

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Metal organic frameworks derived single atom catalysts for electrocatalyticenergy conversion Tingting Sun1, Lianbin Xu2, Dingsheng Wang1 (), and Yadong Li1

1 Department of Chemistry, Tsinghua University, Beijing 100084, China 2 State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Received: 1 January 2019 / Revised: 14 February 2019 / Accepted: 15 February 2019

ABSTRACT The development of efficient and cost-effective catalysts to catalyze a wide variety of electrochemical reactions is key to realize the large-scale application of renewable and clean energy technologies. Owing to the maximum atom-utilization efficiency and unique electronic and geometric structures, single atom catalysts (SACs) have exhibited superior performance in various catalytic systems. Recently, assembled from the functionalized organic linkers and metal nodes, metal-organic frameworks (MOFs) with ultrafine porosity have received tremendous attention as precursors or self-sacrificing templates for preparing porous SACs. Here, the recent advances toward the synthesis strategies for using MOF precursors/templates to construct SACs are systematically summarized with special emphasis on the types of central metal sites. The electrochemical applications of these recently emerged MOF-derived SACs for various energy-conversion processes, such as oxygen reduction/evolution reaction (ORR/OER), hydrogen evolution reaction (HER), and CO2 reduction reaction (CO2RR), are also discussed and reviewed. Finally, the current challenges and prospects regarding the development of MOF-derived SACs are proposed.

KEYWORDS single atom catalysts, metal organic frameworks, electrocatalytic, energy conversion

1 Introduction Regarding the dramatically increasing demand of energy and rapid depletion of fossil fuel, the development of energy-conversion technologies that are sustainable and renewable is a necessary requirement [1–6]. Among various types of next-generation energy sources, fuel cells, electrochemical water splitting, metal-air batteries, and CO2 reduction have arisen as the most promising devices owing to their high efficiencies and environmental benignancy [7–12]. The efficiency of these technologies is greatly limited by the sluggish kinetics of some key reactions, such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and CO2 reduction reaction (CO2RR) [13–16]. In this regard, developing highly active and durable electrocatalysts is imperative to expedite these reactions and thus promote the energy conversion efficiency. At present, noble-metal-based materials (e.g., Pt, IrO2, RuO2, and Au) are identified as state-of-the-art electrocatalysts for a variety of electrochemical reactions, but their scarcity and high cost as well as insufficient durability severely hamper their large-scale commercialization [17–23]. Therefore, the exploration of cost-effective earth-abundant electrocatalysts with high activity and durability is a key step for the practical use of clean energy technologies [24–26].

The catalytic performance of the electrocatalysts is generally dominated by two factors, including the number of catalytically active sites and the intrinsic activity of individual active site [27]. Recent studies have demonstrated that increased number of catalytically active sites and more exposed active surface areas can be engendered by diminishing the size of catalyst particles and enhanced intrinsic properties of the metal sites can be achieved through tuning their structural configuration and electronic structure

[28–31]. Downsizing the catalyst particles to atomic level offers an effective way to realize maximum utilization efficiency of metal atom and improve the intrinsic catalytic activity of the catalyst [32]. Compared with homogeneous catalysts, anchoring homogeneously distributed well-defined metal single sites on supports to achieve heterogeneous catalysts with the merits of outstanding recyclability and good stability provide ideal platforms to connect heterogeneous and homogeneous catalysis [33–37]. The tunable electronic structure and low-coordinated configuration of the single atom catalysts (SACs) have been demonstrated to boost the catalytic performance in various reactions [38–44]. In addition, the homogeneity in the well-defined isolated active sites of SACs is beneficial to provide precise mechanistic investigation of the catalytic reaction and identification of the active sites, thereby facilitating the construction of optimized activity through proper catalyst design [45, 46]. Because of these distinct features, SACs have attracted tremendous attention as superior electrocatalysts for electrochemical energy conversion [47–54].

To advance the electrocatalytic performance of SACs, the following crucial parameters ought to be acquired [55–60]: 1) homogeneously dispersed isolated active sites over the supports for catalytic reaction; 2) promoted mass/electrons transport and enhanced active sites accessibility; 3) high durability and activity; 4) strong bonding between metal single atoms and coordination atoms. The preparation strategies for the SACs are critical to meet these demands. Recently, the metal-organic frameworks (MOFs) have been generally used as novel self-sacrificed precursor/template for the fabrication of nanostructure carbon materials with improved catalytic performance [61–65]. Due to the metal sites are atomically distributed in MOFs, homogeneous single metal atoms supported on carbon materials

Address correspondence to wangdingsheng@mail.tsinghua.edu.cn

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can be readily produced from MOFs. The SACs derived from MOFs have many advantages for electrochemical energy conversion [66, 67]: 1) Tunable structure, size, and composition can be obtained based on the targeted performance through controlling the synthesis parameters; 2) the interconnected carbon framework derived from the organic linkers possesses high conductivity for allowing fast electron transport; 3) various heteroatoms in the organic linkers can be in situ doped within the carbon skeleton, creating a favourable microenvironment for the electronic regulation of the active metal center; 4) the MOF-derived SACs inherited many merits from the MOFs, such as high three-dimensional (3D) porosity and large surface area, which are beneficial to facile mass diffusion and sufficient exposure of active sites. Recently, MOF-derived SACs with various metal sites were synthesized and demonstrated as high-performance electrocatalysts for different electrochemical

reactions, which triggered a new hot topic in the area of MOF- derived carbon materials [68, 69].

In this review, recent progress on the design strategies of SACs derived from MOF-based materials for advanced electrocatalytic energy-conversion applications is summarized and classified based on the types of metal sites in MOF-derived SACs. The catalytic performances of MOF-derived SACs for various crucial reactions in electrochemical energy conversions and storages were demonstrated through recent case investigations. This review aims to afford the readers with a wider overview and deep understanding of the important role and contributions of the single atomic active sites in MOF-derived materials for the applications in the field of the electrochemical energy-conversion, and promote more significant breakthroughs in developing of MOF-derived SACs. Table 1 summarized typical MOF-derived SACs for different electrocatalytic

Table 1 Summary of MOF-derived SACs for energy conversiona

MOF precursor Catalyst and atomic structure Metal content Synthetic process Appli-

cation Electrolyte Performance Reference

Zn/Co bimetallic MOF

Co SAs/N- C(900) (Co-N2)

4.3 wt.% according to the ICP-AES

Pyrolysis of the Zn/Co bimetallic MOF at 900 °C under N2 flow for 3 h

ORR 0.1 M KOH Eonset: 0.982 V E1/2: 0.881 V

[78]

Co-ZIF-8 20Co-NC-1100 (CoN4)

95.7 at.% according to ICP-MS

Pyrolysis of Co-ZIF-8 from 600 to 1,100 °C under N2 flow for 1 h

ORR 0.5 M H2SO4

Eonset: 0.93 V E1/2: 0.80 V

[79]

Co-ZIF-8@F127 Co-N-C@F127 (CoN2+2)

0.77 wt.% according to the XRF

Pyrolysis of Co-ZIF-8@F127 nanocrystals at 900 °C under N2 flow for 3 h

ORR 0.5 M H2SO4

Eonset: 0.93 V E1/2: 0.84 V

[80]

ZIF-67 CUMSs-ZIF-67 (ZIF-67-3.44N)

13.45 at.% according to XPS

Dielectric barrier discharge (DBD) plasma etching of ZIF-67 under nitrogen atmosphere

OER 0.5 M KBi η10: 410 mV b: 185.1 mV·dec−1

[75]

Bimetallic Co/Zn ZIFs

Co-N2 0.25 wt.% according to the ICP-AES

Pyrolysis of bimetallic Co/Zn ZIFs at 800 °C for 1 h, 900 °C for 1 h and 1,000 °C for 1 h successively under flowing argon gas

CO2RR 0.5 M KHCO3

94% CO formation Faradaic efficiency at a overpotential of 520 mV

[77]

Fe(acac)3@ZIF-8 Fe-ISAs/CN (Fe-N4 with O2 adsorbed on the Fe center)

2.16 wt.% according to the ICP-OES

Pyrolysis of Fe(acac)3@ZIF-8 at 900 °C under flowing argon for 3 h

ORR 0.1 M KOH E1/2: 0.900 V [81]

FePc@ZIF-8 Fe SAs-N/C-20 (Fe-N4)

0.20 wt.% according to the ICP-MS

Pyrolysis of FePc-x@ZIF-8 at 900 °C under N2 flow for 3 h, followed by acid- leaching

ORR 0.1 M KOH E1/2: 0.915 V [82]

Fe-Zn-ZIF Fe-ZIF catalysts (Fe-N4)

— Pyrolysis of Fe-Zn-ZIF from 500 to 1,100 °C under N2 flow for 1 h

ORR 0.5 M H2SO4

E1/2: 0.85 V [83]

Fe20-PCN-222 FeSA-N-C (Fe-N4)

1.76 wt.% according to the ICP-AES

Pyrolysis of Fe20-PCN-222 at 800 °C under N2 flow for 2 h

ORR 0.1 M KOH E1/2: 0.891 V [84]

Ni2+@ZIF-8 Ni SAs/N-C (Ni-N3-C)

1.53 wt.% according to the ICP-OES

Pyrolysis of Ni2+@ZIF-8 at 1,000 °C under Ar flow for 2 h

CO2RR 0.5 M KHCO3

Faradaic efficiency for CO production is over 71.9% at −0.9 V

[85]

Ni-MOF A-Ni-C 1.5 wt.% according to the ICP-OES

Pyrolysis of the Ni-MOF at 700 °C under N2 flow for 5 h, followed by HCl leaching and electrochemical activation

HER 0.5 M H2SO4

η10: 34 mV b: 41 mV·dec−1

[86]

Cu foam and ZIF-8 Cu-SAs/N-C (Cu-N4)

0.54 wt.% according to ICP-AES

Pyrolysis of Cu foam and ZIF-8 at 1,173 K in a stream of argon for 1 h and then NH3 for 1 h

ORR 0.1 M KOH Eonset: 0.99 V E1/2: 0.895 V

[87]

Mn-ZIF-8 20Mn-NC-second (Mn-N4)

3.03 wt.% according to ICP-MS

Pyrolysis of Mn-ZIF-8 and subsequent a second adsorption step, followed by thermal activation at 1,100 °C under N2 flow for 1 h

ORR 0.5 M H2SO4

E1/2: 0.8 V [88]

WCl5/UiO-66-NH2 W-SAC (W1N1C3) 1.21 wt.% according to the ICP-OES

Pyrolysis of WCl5/UiO-66-NH2 at 950 °C under Ar flow for 3 h followed by leaching with HF solution

HER 0.1 M KOH η10: 85 mV b: 53 mV·dec−1

[89]

Fe and Co doped ZIF-8

(Fe,Co)/N-C ((Fe,Co)-N6）

0.93 wt.% (Fe) and 1.17 wt.% (Co) accor-ding to ICP-OES

Pyrolysis of the Fe, Co doped ZIF-8 at 900 °C under Ar flow for 2 h

ORR 0.1 M HClO4

Eonset: 1.06 V E1/2: 0.863 V

[90]

Pyrolyzed ZIF-8 with adsorbed Fe and Co

FeCo-ISAs/CN 0.964 wt.% (Fe) and 0.218 wt.% (Co) accor-ding to ICP-OES

Pyrolysis of pyrolyzed ZIF-8 with adsorbed Fe and Co at 900 °C under Ar flow for 2 h

ORR 0.1 M KOH Eonset: 0.995 V E1/2: 0.92 V

[91]

Co-MOF A-CoPt-NC (a(Co-Pt)@N8V4)

1.72 wt.% (Co) and 0.16 wt.% (Pt) accor-ding to ICP-AES

Electrochemical activation of pyrolyzed Co-MOF with a Pt wire as the counter electrode

ORR 0.1 M KOH E1/2: 0.96 V [92]

aEonset: onset potential (vs. RHE); E1/2: half-wave potential (vs. RHE); η10: overpotential at 10 mA·cm−2; b: Tafel slope; ICP-OES: inductively coupled plasma optical emission spectrometry; XRF: X-ray fluorescence; ICP-AES: inductively coupled plasma atomic emission spectrometry; ICP-MS: inductively coupled plasma-mass emission spectrometry; XPS: X-ray photoelectron spectroscopy.

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energy-conversion reactions. Finally, the main research challenges and perspectives for the development of MOF-derived SACs in electrochemical applications are addressed.

There are many advanced characterization techniques to elucidate the local atomic and electronic configuration of the atomically dispersed metal sites in SACs. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) with atomic resolution can be employed for the direct observation of the metal single atoms distributed on the supports based on the different Z-contrast between the isolated metal species and the supports [70]. Synchrotron-radiated X-ray absorption fine-structure (XAFS) test is a powerful technique for obtaining detailed information on SACs, such as the oxidation states, electronic structures, and coordination configurations of the atomic metal sites [71]. Electron energy loss-spectroscopy (EELS) attached to the microscope was used to confirm the elements distribution of the SACs. Density functional theory (DFT) calculations were conducted to probe the most stable atomic structure of SACs and provide insight into the catalytic mechanisms and verify the active sites.

2 MOF-derived single atom catalysts

2.1 Cobalt single atoms in MOF-derived materials

Cobalt-loaded nitrogen-doped porous carbon materials with Co-Nx-C active configuration have exhibited excellent electrocatalytic performance for various applications [72–74]. MOFs composed of Co nodes and N-containing organic linkers can serve as ideal precursors for the preparation of porous carbon electrocatalysts containing N-coordinated Co sites due to the flexible control of the structure, high porosity and surface area, as well as the uniformly

dispersed Co and N atoms [75–77]. These MOF precursors can be in situ converted into porous Co-N-C materials through high- temperature pyrolysis. Moreover, the original Co–N coordination bond could directly form Co-Nx active sites for electrochemical reactions. However, direct high-temperature pyrolysis of Co-based MOFs was found to be inclined to cause severe agglomeration of Co atoms to produce Co aggregates instead of forming atomic Co-Nx dispersion in the catalysts. The inactive Co aggregates in the obtained catalysts could decrease the accessibility of the Co-Nx active sites and reduce the activity of the catalyst. Thus, acid-leaching process becomes inevitable for removing the Co-containing particles. Additionally, a subsequent heat treatment is required after the acid treatments to repair the broken carbon structures for further improving the catalytic properties. Despite these tedious efforts, a large number of metallic Co species protected by graphitic carbon shells still retain and lead to significant loss in carbon surface and decline of the catalytic performance. It is thus extremely desirable to directly synthesize atomically dispersed Co-Nx on porous carbon materials after pyrolysis without forming metallic cobalt nanoparticles. As a subclass of MOF, the zinc-based zeolitic imidazolate framework-8 (ZIF-8) with high content of N dopants in the organic ligands and flexibility to dope various metals into the 3D frameworks has emerged as a new platform to prepare MOF-derived SACs via one-step pyrolysis strategy. During the pyrolysis process, Zn2+ in the ZIF-8 was reduced to Zn atoms and further evaporated away under high temperature, leading to the formation of isolated metal-Nx sites from the doped non-volatile metal species. For example, Yin et al. recently prepared single Co atoms anchored on N-doped porous carbon support based on one-step pyrolysis of predesigned bimetallic Zn/Co ZIF containing evenly distributed Zn2+ and Co2+ ions and the organic linkers of 2-methylimidazole (Figs. 1(a)–1(e)) [78]. In

 Figure 1 (a) Formation process of N-doped carbon with Co nanoparticles (top) and single Co atoms (bottom). (b) TEM and (c)–(e) HAADF-STEM images of thesingle Co atoms on N-doped carbon obtained from Zn/Co-ZIF. EXAFS fitting curves of the synthesized single Co atoms on N-doped carbon by pyrolysis ofZn/Co-ZIF at (f) 800 and (g) 900 °C. (h) ORR polarization curves of various catalysts under O2-saturated 0.1 M KOH. Reproduced with permission from Ref. [78],© Wiley-VCH 2016.

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the bimetallic Zn/Co ZIF (Zn/Co atomic ratio = 1:1), a certain proportion of Co2+ sites were substituted by the Zn2+ ions, giving rise to an expanded spatial interval between two adjacent Co atoms and thus avoiding the formation of Co–Co bonds at higher temperature. Upon the following high-temperature calcination, the Zn and Co cations can be reduced in situ to metallic Zn and Co by carbonization of the organic linker. Subsequently, Zn atoms were evaporated off due to their low boiling point, producing substantial free N anchoring sites to stabilize single Co atoms. Extended X-ray absorption fine structure (EXAFS) spectroscopies revealed that the dominant single Co atoms obtained at the pyrolytic temperature of 800 and 900 °C present as planar Co-N4 and Co-N2, respectively (Figs. 1(f) and 1(g)). The single Co-N2 sites catalyst obtained at 900 °C showed superior electrocatalytic performance for ORR in alkaline electrolyte with an onset potential (Eonset) of 0.982 V vs. RHE and a half-wave potential (E1/2) of 0.881 V vs. RHE compared with the single Co-N4 sites catalyst obtained at 800 °C and commercial Pt/C catalyst (Fig. 1(h)). Moreover, the Co-N2 single sites electrocatalyst exhibited remarkable thermal stability and chemical stability DFT calculations revealed that the Co-N2 sites interacted with peroxide more strongly than Co-N4 species and favored a four electrons (4e−) ORR pathway, which contributed to the higher catalytic activity of the Co-N2 sites catalysts. In another case, Wang et al. used one-step heat activation to convert the Co-doped ZIF precursors having different Co contents into N-coordinated atomically single Co sites catalyst [79]. The influences of Co doping contents and pyrolytic temperature on the catalytic activity toward ORR were also investigated. The supreme single Co sites catalyst with optimal chemical and

structural performances showed high ORR activity and long-term durability under acidic condition, giving an E1/2 of 0.80 V vs. RHE and retaining 83% of initial activity after 100 h of continuous operation in 0.5 M H2SO4.

To enhance the catalytic activity of the Co-Nx-C catalysts, increasing the density of atomically dispersed Co-Nx active sites while avoiding the formation of inactive Co aggregates is of significant importance. He et al. synthesized a type of core-shell structured Co-N-C electrocatalysts with single atomic Co-Nx sites via direct pyrolysis of surfactants coated Co doped ZIF-8 crystals (Figs. 2(a)– 2(d)) [80]. During the thermal treatment, the confinement effect produced by the interface interaction between the surfactant and Co-ZIF-8 inhibited the collapse of the carbon microporous structure of ZIF-8 and simultaneously prevented the aggregation of isolated Co atomic sites. The catalysts prepared by employing surfactant F127 (Co-N-C@F127) possessed higher density of atomic CoN4 active sites than the surfactant-free catalysts or the catalysts obtained using other surfactants and gave rise to the optimal performance. Based on DFT calculations, the Co-N-C@F127 catalyst contains abundant CoN2+2 active sites, which are capable of catalyzing ORR through a 4e− pathway (Fig. 2(e)). The Co-N-C@F127 catalyst showed outstanding ORR activity with an E1/2 of 0.84 V vs. RHE as well as long-term durability in acidic condition (Figs. 2(f)–2(h)). Furthermore, fuel cell tests confirmed that the Co-N-C@F127 can be used as an effective cathode catalyst in proton-exchange membrane fuel cells (PEMFCs).

2.2 Iron single atoms in MOF-derived materials

Apart from the Co-Nx-C sites catalysts, carbon materials with

 Figure 2 (a) Synthesis strategy of core–shell Co-N-C@surfactants materials (the grey, yellow, and blue spheres correspond to Zn, Co, and N atoms, respectively). (b) High-resolution transmission electron microscopy (HRTEM) image of Co-N-C@F127 catalyst. (c) Aberration-corrected HAADF-STEM image of Co-N-C@F127. (d) Fourier transform of the k2-weighted EXAFS fitting of the Co-N-C@F127 catalyst. (e) DFT calculations for ORR on CoN2+2 and Co-N4. (f) ORR linear sweep voltammogram (LSV) curves and (g) H2O2 yield plots for various Co-ZIF-8@surfactants derived samples in 0.5 M H2SO4. (h) Chronoamperometry test for 100 h at 0.7 V. Reproduced with permission from Ref. [80], © The Royal Society of Chemistry 2019.

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Fe-Nx-C configuration have also been extensively studied as highly efficient electrocatalysts for ORR. Chen et al. applied ZIF-8 with Fe(acac)3 encapsulated in the cages (Fe(acac)3@ZIF-8) as s precursor to fabricate single Fe atoms atomically dispersed on N-doped porous carbon matrix (ISA Fe/CN) (Figs. 3(a)–3(e)) [81]. Aberration- corrected high-angle annular dark-field SEM (AC HAADF-STEM) observation and EXAFS measurements confirmed the atomic dispersion of single Fe atoms across the carbon skeletons. EXAFS fitting revealed that the atomic structure of the single Fe site was Fe-N4 while vertically adsorbing one O2 molecule on the Fe atom (Fig. 3(f)). The resulting ISA Fe/CN catalyst exhibited superior ORR activity under alkaline solution with a E1/2 of 0.900 V, which is significantly better than that of the state-of-the-art Pt/C (Fig. 3(g)), as well as a high kinetic current density (Jk) of 37.83 mA/cm2 at 0.85 V. Moreover, it had a high tolerance of methanol and catalytic durability. Control experiments on Fe single atoms combined with nanoparticles (NPs) on CN (Fe-ISAs&NPs/CN) and SCN− poisoning tests uncovered that the isolated Fe-N4 sites contributed to the high ORR activity. DFT calculations further demonstrated that the promoted ORR reactivity is attributed to the high transfer efficiency of electrons from the atomic Fe sites to the adsorbed OH species (Fig. 3(h)).

It has been speculated that the N bonding structures adjacent to Fe-N4 sites can also affect the catalytic activity. Jiang et al. synthesized hierarchically micro-mesoporous Fe-N-C catalyst with well-dispersed atomic Fe-N4 sites through pyrolysis of ZIF-8 encapsulated by iron(II) phthalocyanine (FePc) and subsequent acid-leaching [82]. By changing the amount of FePc, the carbon pore size in the catalysts can be adjusted (Figs. 4(a)−4(d)). The Fe-N4 sites electrocatalyst with 0.20 wt.% Fe showed excellent ORR activity (E1/2 is 0.915 V vs. RHE) under alkaline condition, and the atom-utilization efficiency was improved nearly 10 times compared with previous reports. DFT calculations revealed that the selective C–N bond cleavage triggered by the enhanced porosity modulated the local coordination of pyridinic

N to generate Fe-N4 sites at the edge of holes, thus reducing the overall ORR free energy change. Controllably building a specific Fe-Nx configuration was considered to be an effective method to enhance the performance of the Fe-Nx-C catalyst. For instance, Lai et al. used Fe-2-methylimidazole (mIm) nanoclusters (guest)@ZIF-8 (host) as precursors, and prepared Fe-N/C electrocatalysts for ORR through pyrolysis and subsequent acid leaching (Fig. 4(e)) [93]. EXAFS fitting spectra indicated the formation of two- to five-coordinated Fe-Nx structures from the ZIF precursors with different Fe-mIm contents. Electrochemical tests demonstrated that five-coordinated Fe-Nx sites can catalyze ORR more efficiently than that with lower coordination number under acid medium. It showed an E1/2 of 0.735 V vs. RHE, which is only  39 mV less than 30 wt.% Pt/C (0.774 V vs. RHE). DFT calculations confirmed that the N-Fe-N4 configuration has lower energy barrier and adsorption energy of intermediate OH than Fe-N4 or Fe-N2, contributing to the enhanced ORR activity of five-coordinated Fe-Nx sites catalyst.

Chen et al. constructed a type of Fe catalyst with single atomic Fe sites anchored on hollow nitrogen, phosphorus and sulfur co-doped carbon polyhedron (denoted as Fe-SAs/NPS-HC) based on a ZIF-8@polymer composite using Kirkendall effect, meanwhile an electronic tuning of the Fe active center by coordination with neighboring N atoms and interaction with long-range S and P was obtained (Figs. 5(a)−5(f)) [94]. Atomically dispersed Fe sites were directly confirmed by AC HAADF-STEM analysis. The as-synthesized Fe-SAs/NPS-HC catalyst reveals a superior activity for ORR under alkaline condition with a positive E1/2 of 0.912 V vs. RHE and a large Jk of 71.9 mA/cm2 at 0.85 V. The Fe-SAs/NPS-HC catalyst also showed remarkable ORR catalytic activity in acidic electrolyte with an E1/2 of 0.791 V, approaching that of the Pt/C. Moreover, it has high tolerance against methanol and long-term durability. Control experiments demonstrated the significant role of the hollow structure for facilitating kinetics and improving catalytic activity. DFT calculations confirmed that the enhanced catalytic efficiency and

 Figure 3 (a) Schematic illustration of the synthesis of ISA Fe/CN. (b) TEM and (c) HAADF-STEM and element mappings of the distribution of Fe (yellow), C (red), and N (orange). (d) and (e) AC HAADF-STEM of the ISA Fe/CN. Single Fe atoms were marked by red circles. (f) EXAFS r space fitting curves and schematic model of ISA Fe/CN. (g) ORR polarization curves for various catalysts. (h) Free-energy paths of ORR on different catalysts. Reproduced with permission from Ref. [81], © Wiley-VCH 2017.

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 Figure 4 (a) Schematic illustration for the fabrication of atomic Fe-N4 sites catalyst from ZIF-8 encapsulated FePc. (b) TEM image of FePc@ZIF-8 composite. (c) HRTEM and (d) AC HAADF-STEM of the atomic Fe-N4 sites catalyst. Reproduced with permission from Ref. [82], © American Chemical Society 2018. (e) Schematic illustration of the host–guest strategy for the synthesis of Fe-N/C Catalysts (reproduced with permission from Ref. [93], © American Chemical Society 2017).

kinetics of Fe-SAs/NPS-HC benefited from the atomically distributed of Fe-N coordination sites and electronic modulation from the long-range S and P atoms by transferring electrons to the Fe centers, which is beneficial to achieve less positively charged Fe (Feδ+) and weaker binding of the adsorbed OH species. Furthermore, H2-air fuel cell and Zn-air battery measurements suggested the promising application of Fe-SAs/NPS-HC in clean energy conversion and storage systems.

It is expected that more exposed Fe-Nx active sites for improved ORR electrocatalytic efficiency can be engendered by decreasing the catalyst size. Zhang et al. systematically explored the effect of the particle size of the ZIF-derived atomic Fe-N4 sites catalysts on the ORR activity (Figs. 5(g)–5(k)) [83]. The atomically dispersed Fe sites catalysts with various particle sizes were prepared by first producing Fe-doped ZIF-8 precursors ranging from 20 to 1,000 nm and subsequent high-temperature pyrolysis step. They found that the ORR catalytic activity of the resultant atomic Fe sites catalysts was significantly dependent on the particle size. A continuous improvement in ORR activities was exhibited as the size of the catalyst particle decreased from 1,000 to 50 nm, which was confirmed by the gradual positive shift of E1/2 (Fig. 5(l)). The dependence of the activity on the catalyst size was attributed to the enhanced accessibility of atomic Fe active sites and increased catalytically surface areas for ORR as the catalyst size decreased. Further reduction in size to 20 nm resulted in a declined activity. This is because the severe particle agglomeration led to a decrease in accessible part of active sites (Fig. 5(m)). It was also elucidated that larger number of graphitic N and isolated Fe-N4 sites were generated as the heating temperature increased, thereby improving the activity toward ORR. The best atomic Fe catalyst (50 nm, 1,100 ºC) for ORR obtained an E1/2

of 0.85 V vs. RHE in acidic media, merely 30 mV less than Pt/C (60 μgPt/cm2). Additionally, this catalyst had a superior durability with a negative shift of merely 20 mV in E1/2 after 10,000 cycles.

Some other studies developed MOF-derived Fe-N-C catalysts with high density of accessible isolated Fe-Nx sites imbedded in carbon frameworks for enhanced ORR activities through rational design of the porous structures of the resultant carbon materials. For example, by pyrolysis of a MOF composite precursor composed of dicyandiamide and FeCl3 incorporated in the pores of MIL-101-NH2 and the following acid etching, Zhu et al. synthesized hierarchically porous carbon frameworks with atomic dispersion of Fe-Nx species [95]. The hierarchical pore structures of the as-synthesized Fe/N-doped carbon catalyst contain abundant macro-, meso-, and micropores, which can significantly enhance active site accessibility and accelerate mass transport. Thanks to its unique open character and sufficient exposure of accessible Fe-Nx active sites, the resulting catalyst afforded a superior electrocatalytic ORR property in terms of the E1/2 and durability under alkaline condition compared to the Pt/C catalyst. In another example, Jiao et al. prepared a type of hierarchically porous single Fe atom catalyst (denoted as FeSA-N-C) with rich micropores and unique oriented mesopores from a porphyrinic Fex-PCN-222 precursor through pyrolysis [84]. HAADF-STEM observation together with X-ray absorption spectroscopy (XAS) manifested the isolated Fe-N4 configuration in FeSA-N-C catalyst. Benefiting from the abundant atomic Fe-N species as highly catalytically active sites and hierarchical micro-mesopores for enhanced active sites accessibility and mass/charge diffusion, the optimal FeSA-N-C catalyst demonstrated superb ORR activity and durability, better than the benchmark Pt/C under both alkaline and acidic solutions.

2.3 Nickel single atoms in MOF-derived materials

Recent reports indicated that in addition to Fe and Co, porous carbon materials supported single atomic dispersion of Ni sites can also be yielded based on MOFs precursors. Zhao et al. adopted Ni-incorporated ZIF-8 to afford single atomic Ni sites dispersed on N-doped carbon matrix through the one-step high temperature pyrolysis (Figs. 6(a)−6(d)) [85]. A Ni(NO3)2 aqueous solution used as the Ni precursor was evenly implanted into the voids of the ZIF-8 based on a double-solvent strategy. Upon pyrolysis, the Zn nodes were evaporated away, forming a large number of free N defects. These N-doping defects are capable of serving as anchoring sites to stabilize and atomically disperse single Ni species. HAADF-STEM images of the resultant catalyst unveiled that individual Ni atoms were evenly dispersed on the carbon materials. XAFS characterizations confirmed the absence of metallic Ni nanoparticles and the evolution of isolated atomic Ni sites, and each central Ni atom was approximately coordinated by three N atoms (Figs. 6(e) and 6(f)). The resultant single Ni atom catalyst can serve as a robust electrocatalyst for selective reduction of CO2 to CO, showing a high turnover frequency (TOF) of 5,273 h−1 and a Faradaic efficiency of above 71.9% at an overpotential of 0.89 V (Fig. 6(g)). Control experiments and DFT calculations certified the critical role of single atomic Ni-N3-C active sites for the activation of CO2 and reduction of CO adsorption energy. Fan et al. produced isolated single atomic Ni sites embedded on graphitized carbon materials from a Ni-MOF precursor by using a unique method (Figs. 7(a)–7(d)) [86]. First, metallic Ni aggregates encapsulated in graphene layers (denoted as Ni@C) was obtained via pyrolysis of Ni-MOF precursor at 700 °C in N2. The Ni@C sample was then leached in HCl for removing most of Ni metal nanoparticles. An electrochemical activation process was conducted to completely dissolve the remaining metallic Ni particles that were protected by graphitic layers, thus affording atomically isolated single Ni sites (denoted as A-Ni-C). The presence of isolated Ni atoms was illustrated by X-ray diffraction (XRD), XPS, as well as atomic resolution HAADF characterizations. The Ni-C catalyst showed significantly

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improved electrocatalytic HER activity, with an overpotential of 34 mV at 10 mA/cm2, a Tafel slope of 41 mV/dec, and much promoted electron transfer after electrochemical activation (Figs. 7(e) and 7(f)). Chronoamperometric test further verified the high durability with negligible activity loss after continuous operation for 25 h.

2.4 Copper single atoms in MOF-derived materials

The previously reported routes for synthesizing SACs involve tedious fabrication procedures and are also difficult to achieve large scale production. The exploration of direct routes to mass production of highly stable SACs is significantly important in this area. Qu et al. proposed a NH3-assisted gas-transport strategy to straightly convert various bulk metals into individual atoms trapped in a support, satisfying the scalable production of SACs (Figs. 8(a)–8(c)) [87]. Specifically, Cu foam and ZIF-8 powder were located in a porcelain boat separately. Pyrolysis of ZIF-8 under Ar flowing led to the formation of porous N-doped carbon materials with rich defect sites. Under a stream of NH3, the superficial Cu atoms on the Cu foam

were dragged out by coordinating with NH3 to yield Cu(NH3)x species. Subsequently, the defective sites in the N-doped carbon trapped the volatile Cu(NH3)x species, producing the single atomic Cu sites catalyst (Cu-SAs/N-C). The atomic distribution of isolated Cu atoms in the Cu-SAs/N-C was confirmed by the AC HAADF- STEM observations and a Cu-N4 coordination configuration was revealed by EXAFS fitting. The as-produced Cu-SAs/N-C catalyst with a high surface area of 832 m2/g and abundant surface single Cu active sites can effectively catalyze ORR via a near 4e− pathway under alkaline media. It showed a more positive E1/2 of 0.895 V vs. RHE and a much higher TOF of 0.68 Hz than commercial Pt/C catalyst (0.87 V vs. RHE and 0.19 Hz). The single Cu atom catalyst showed negligible loss in activity and maintained stable Cu-N4 coordination structure after durability test for 5,000 potential cycles, confirming the outstanding electrochemical durability. Moreover, the Cu-SAs/N-C catalyst produced by this strategy afforded a robust thermal stability with stable single atomic Cu sites after heat treatment under 973 K for 1 h in Ar.

 Figure 5 (a) Scheme of the preparation of Fe-SAs/NPS-HC. (b) HAADF-STEM image and element mappings, Fe (yellow), C (blue), N (cyan), S (orange), and P (green).(c) and (d) AC HAADF-STEM images of Fe-SAs/NPS-HC catalyst. (e) EXAFS fitting at R space and the corresponding EXAFS k space fitting curves of Fe-SAs/NPS-HC. (f) ORR polarization curves for various catalysts. Reproduced with permission from Ref. [94], © Nature Publishing Group 2018. (g) Synthesis illustration of the Fe single atom catalysts derived from Fe-doped ZIF and size control of the Fe-doped ZIF catalysts from 20 to 1,000 nm. (h)–(k) TEM and HAADF-STEM images of the optimal Fe-doped ZIF catalyst with particle size of 50 nm and the inset shows the EELS analysis. (l) ORR polarization curves for Fe-doped ZIF catalysts in 0.5 M H2SO4 and Pt/C catalysts in 0.1 M HClO4 at 25 ºC and 900 rpm. (m) Correlation between the catalytic activity for ORR and electrochemically accessible surface area(Sa), revealing a clear size dependence. Reproduced with permission from Ref. [83], © American Chemical Society 2017.

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2.5 Manganese single atoms in MOF-derived materials

Although a large number of highly efficient Fe-Nx-C and Co-Nx-C catalysts have been developed toward ORR, they still suffer from

unsatisfying selectivity and durability in acidic media, which restricts their practical uses for PEMFCs. Li et al. synthesized single atomic Mn-N4 sites supported on partially graphitic carbon materials via a two-step approach containing doping and the following adsorption

 Figure 6 (a) Scheme of the preparation of single atomic Ni sites catalyst from the Ni-incorporated ZIF-8. (b) and (c) HAADF-STEM images and (d) EDS maps of thesingle atomic Ni sites catalyst. (e) XANES spectra at Ni K-edge and (f) EXAFS fitting curves. Inset is the proposed Ni-N3 configuration. (g) Partial CO current density (based on geometric surface area) curves and TOFs of Ni single atom and Ni particle catalysts at different potentials. Reproduced with permission from Ref. [85],© American Chemical Society 2017.

 Figure 7 (a) Scheme of synthesis and activation process of the A-Ni-C catalysts. (b) HRTEM image of the pyrolyzed Ni-MOF after acid leaching. (c) HRTEM image of the pyrolyzed Ni-MOF after HCl leaching and electrochemical activation. (d) XRD patterns of the pyrolyzed Ni-MOF (Ni@C), the pyrolyzed Ni-MOF after HCl leaching(HCl-Ni@C), and the pyrolyzed Ni-MOF after HCl leaching and electrochemical activation (A-Ni-C), respectively. (e) HER polarization curves of HCl-Ni@C, A-Ni-C,and Pt/C. (f) Tafel plots derived from (e). Reproduced with permission from Ref. [86], © Nature Publishing Group 2016.

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 Figure 8 (a) Schematic illustration of the synthesis of Cu-SAs/N-C. (b) electron energy loss spectroscopy (EELS) maps of copper (yellow), carbon (red), and nitrogen (green). (c) AC HAADF-STEM image of Cu-SAs/N-C. Reproduced with permission from Ref. [87], © Nature Publishing Group 2018.

processes (Figs. 9(a)–9(d)) [88]. First, Mn-doped ZIF-8 crystals were prepared as precursors by partial substitution of Zn2+ ions in ZIF-8 with Mn3+ ions. The Mn-ZIF-8 precursors were pyrolyzed in

N2 atmosphere followed by leaching in acidic solution to generate Mn and N co-doped porous carbon with abundant micropores. In the second adsorption step, the 3D porous carbon was used as host to adsorb extra Mn and N precursors and then subjected to thermal treatment, producing Mn-N-C catalyst with increased density of isolated atomic Mn-N4 sites. The two-step chemical doping and adsorption strategy led to a distinct increase in the density of single atomic Mn-N4 sites, which was verified by EELS and inductively coupled plasma-mass spectrometry (ICP-MS) results. Homogeneously dispersed atomic Mn-N4 coordination sites in the carbon structure were established by X-ray absorption spectroscopy (XAS) analysis and straightly observed in the aberration-corrected medium-angle annular dark field (MAADF) STEM images. The excellent electrocatalytic ORR performance of the Mn-N-C catalyst in acidic solution was verified by an E1/2 of 0.80 V vs. RHE, which approaches that of Fe-N-C catalysts, as well as a superior long-term stability. Control experiments demonstrated that the single atomic Mn-N4 sites with increased density and the carbon with enhanced corrosion resistance in the catalysts contributed to the robust performance. This Mn-N-C catalyst was also capable of acting as a high-performance platinum group metal (PGM)-free cathode based on the fuel cell tests. DFT results further confirmed the key role of the atomically isolated Mn-N4 sites for the activity for ORR going through a 4e− route in acidic condition.

2.6 Tungsten single atoms in MOF-derived materials

Electrocatalytic HER offers a most appealing way to produce hydrogen gas from water. As an attractive class of alternatives to Pt-based catalysts, tungsten-based catalysts have received considerable attention as effective catalysts for HER. Chen et al. achieved a catalyst with single atomic W sites embedded in N-doped carbon framework via pyrolysis of a MOF (UiO-66-NH2) precursor for HER applications (Figs. 10(a)–10(c)) [89]. The W precursor (WCl5) was encapsulated in the cavities of UiO-66-NH2 to form WCl5/UiO-66-NH2. After pyrolysis and the following treatment in acid solution to remove the unnecessary ZrO2, the single W atom catalyst was produced. HAADF- STEM and XAS analysis demonstrated the atomically dispersed isolated W sites on the support, as well as suggested that the local

 Figure 9 (a) Schematic of the atomic Mn-N4 sites catalyst synthesis. (b) AC MAADF-STEM image. (c) Fourier transforms of Mn K-edge EXAFS fitting curves of the Mn-N-C catalyst. (d) ORR polarization curves in 0.5 M H2SO4 to investigate the effect of different carbon hosts used for the second adsorption process on the activityof the resultant Mn-N-C catalyst. Reproduced with permission from Ref. [88], © Nature Publishing Group 2018.

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 Figure 10 (a) Schematic Illustration of the formation of the single W atom catalyst. (b) The AC HAADF-STEM image. (c) HER polarization curves for various catalysts at the scan rate of 5 mV/s. Reproduced with permission from Ref. [89], © WILEY-VCH 2018.

configuration of the W species was likely to be W1N1C3. The resultant W-N-C catalyst showed superb electrocatalytic properties toward HER under alkaline solution, which was verified by a low overpotential at 10 mA/cm2 (85 mV) and a small Tafel slope (53 mV/dec) as well as an outstanding durability. The HER activity of this W-N-C catalyst is close to that of commercial Pt/C catalyst. DFT calculation revealed that the unique W1N1C3 sites were the primary active sites for the high HER performance.

2.7 Mixed-metal atoms in MOF-derived materials

Two or more different metal site electrocatalysts have been shown to have superior catalytic activity compared to their corresponding single metal sites catalysts, likely due to the synergistic effects between different metal species [96–98]. Isolated mixed metal sites supported on carbon materials can be prepared by employing MOFs implanted by multiple metal species as precursors, and the application of Zn-based MOFs precursors can effectively avoid the agglomeration of metal sites. According to the above thinking, Wang et al. synthesized hollow carbon framework with isolated atomic Fe-Co dual sites via the one-step pyrolysis of a Zn/Co bimetallic MOF (host) with Fe salts (guest) encapsulated in the cavities (Fig. 11(a)) [90]. A double-solvent strategy was used to completely incorporate the Fe sources within the pores of the Zn/Co MOF. During the pyrolysis, the Fe species acted as the catalyst to force the formation of cavities inside the pyrolyzed MOF and the reduced Fe bonded with adjacent Co atoms to form Fe-Co dual sites (Fe,Co)/N-C). Aberration- corrected HAADF-STEM and XAFS experiments confirmed the atomic dispersion of the dual Fe-Co sites with porphyrin-like structure (Figs. 11(b)–11(d)). The as-prepared Fe-Co dual sites electrocatalyst afforded excellent ORR catalytic performance under acidic electrolyte, showing an onset potential of 1.06 and an E1/2 of 0.863 V vs. RHE, which are comparable to that of Pt/C, as well as superb durability and high methanol/CO tolerance. Importantly, a kinetic current of more than 550 mA/cm2 at 0.6 V and a peak power density of larger than 505 mW/cm2 at 0.42 V were achieved by the H2/air fuel cell using these dual sites incorporating carbon materials as catalyst.

This property outperformed most non-Pt electrocatalysts. DFT calculations revealed that the Fe-Co dual sites were favorable for adequately activating and dissociating the O–O bonds, which can reduce the energy barrier for O–O bond breakage, thereby affording enhanced ORR activity. Through pyrolysis of Fe and Co species adsorbed pyrolyzed ZIF-8, Zhang et al. synthesized isolated single Fe and Co atoms on N-doped carbon with superb electrocatalytic performance for ORR under alkaline condition [91]. In another case, Zhao et al. prepared isolated single Co and Fe atoms supported on carbon framework with hierarchical porosity at submillimeter-scale by using Zn-based triazole-rich energetic MOFs (EMOFs) doped by Co and Fe sources as precursors (Figs. 11(e) and 11(f)) [99]. During pyrolysis, Zn nodes were reduced and volatilized while the triazole ligands decomposed and produced lots of gases, resulting in the formation of the carbon networks with hierarchically porous structure. Simultaneously, the evenly distributed Fe and Co ions were in situ transformed into single atomic metal sites. The as-prepared hierarchically porous CoFe@C catalyst with great conductivity and abundant atomically dispersed Fe and Co active sites shows excellent performance for electrocatalytic ORR with a E1/2 of 0.886 V vs. RHE under 0.1 M KOH (Fig. 11(g)). In addition to Fe-Co dual sites catalysts, Zhang et al. prepared atomic Pt-Co sites on N-carbon-based materials through electrochemical activation of core–shell Co-NC produced from Co-MOF by using a Pt wire as the counter electrode (Figs. 12(a)–12(f)) [92]. The obtained Pt-Co dual sites catalyst (denoted as A-CoPt-NC) achieved an E1/2 of 0.96 V vs. RHE for ORR under alkaline medium, 90 mV positive than that of Pt/C. Until now, MOF-derived mixed-metal sites catalysts are limited to ORR active Fe-Co and Pt-Co dual sites. Due to the MOF crystal

 Figure 11 (a) Synthesis of the Fe-Co dual sites catalyst (Fe,Co)/N-C. (b) AC HAADF-STEM image shows the atomic dispersion of Fe-Co dual sites in (Fe,Co)/ N-C. (c) EXAFS fittings at Fe K-edge. (d) Proposed configuration of Fe-Co dual sites. Reproduced with permission from Ref. [90], © American Chemical Society 2017. (e) Preparation of submillimeter-scaled CoFe@C through the thermal treatment of CoFe doped MET-6 particles. (f) TEM image of CoFe@C. (g) ORR polarization curves of CoFe@C and Pt/C at 10 mV/s and 1,600 rpm in 0.1 M KOH. Reproduced with permission from Ref. [99], © Wiley-VCH 2018.

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can be incorporated by diverse metal species, it is expected that two or more isolated atomic sites supported on carbon materials for other energy technologies could be constructed based on MOFs.

3 Summary and perspective Benefiting from the advantageous characteristics inherited from heterogeneous and homogeneous catalysts, SACs hold great promise for bridging the heterogeneous and homogeneous catalysis. MOFs with atomically distributed metal sites and ultrafine porosity have recently emerged as significantly attractive platforms to design and construct catalysts having isolated single atomic metal sites. At increased temperature, the organic linkers in the MOFs can be converted into porous heteroatom-doped carbon, offering rich coordination sites to anchor single metal atoms. A variety of MOF- derived SACs have been successfully synthesized and shown fascinating performance in electrocatalytic application for renewable energy-conversion systems attributing to their enhanced electrical conductivity, high chemical/thermal stability, and hierarchical pore structure. In this review, we systematically summarized the design and preparation strategies of MOF-derived SACs containing various isolated metal sites, such as Fe, Co, Ni, Cu, Mn, W, and dual Fe-Co and Pt-Co sites, and their applications in electrocatalytic ORR, HER, OER, and CO2RR. Although great advances have been achieved in the area of MOF-derived single atom electrocatalysts, there are still some challenges for the development of more efficient SACs based on MOF in the future. (1) Among thousands of reported MOFs, only a few MOFs, such

as ZIF-8, ZIF-67, MIL-101-NH2, and UiO-66-NH2, have been well-studied as precursors/templates to produce SACs for enhanced electrocatalysis. Novel design strategies to broaden the availability of MOF-based precursors are highly desirable. In addition, up to now, MOF-derived SACs for electrocatalytic energy conversion are limited to Fe, Co, Ni, Cu, Mn, W sites, as well as Fe-Co and Pt-Co dual sites, developing other single atomic sites on MOFs derived materials is expected.

(2) Synthesizing SACs from MOFs usually involves pyrolysis at high-temperature. Thus, the limited metal content in the MOF precursor was used to avoid the agglomeration of the metal species. To fulfill the demand for practical energy-conversion applications, novel strategies to large-scale preparation of SACs with high density of isolated atomic active sites while preventing the generation of metal aggregates are needed. Moreover, the precise control of the coordination environment of isolated

metal sites that is very promising to achieve enhanced catalytic performance of SACs is desirable but less reported.

(3) The morphology of MOF-derived SACs has a close relationship with the catalytic efficiency. Nevertheless, most of the MOF- derived SACs are in the form of particles having high isotropic shape. The effective strategies for the synthesis of MOF-derived SACs with other morphologies, such as one-dimensional (1D) and two-dimensional (2D) structure, as well as 3D hierarchical architectures are still lacking.

(4) The coordination configuration of the atomically dispersed single atoms on MOF-derived carbon can serve as perfect model systems for the investigation of the catalytic mechanism at the atomic level. However, the characterization technologies and theoretical calculation on the catalytic mechanism are still limited. Theoretical modeling, in situ electron microscopy and in situ X-ray absorption spectroscopy measurements that are capable of unveiling the mechanism of the catalytic reactions on the single active sites are urgently needed. These can offer unprecedented insight and novel design rules to develop MOF-derived SACs with desirable performance for the targeted energy-conversion process.

Acknowledgements This work was supported by the National Key R&D Program of China (No. 2016YFA0202801), the National Natural Science Foundation of China (Nos. 21671117, 21871159, 21890383, and 21676018), and the China Postdoctoral Science Foundation (No. 2017M610864).

References [1] Wu, H. B.; Lou, X. W. Metal-organic frameworks and their derived

materials for electrochemical energy storage and conversion: Promises and challenges. Sci. Adv. 2017, 3, eaap9252.

[2] Wang, J. B.; Chen, W. L.; Wang, T.; Bate, N.; Wang, C. L.; Wang, E. B. A strategy for highly dispersed Mo2C/MoN hybrid nitrogen-doped graphene via ion-exchange resin synthesis for efficient electrocatalytic hydrogen reduction. Nano Res. 2018, 11, 4535–4548.

[3] Bu, L. Z.; Shao, Q.; E, B.; Guo, J.; Yao, J. L.; Huang, X. Q. PtPb/PtNi intermetallic core/atomic layer shell octahedra for efficient oxygen reduction electrocatalysis. J. Am. Chem. Soc. 2017, 139, 9576–9582.

[4] Chen, P. Z.; Zhou, T. P.; Xing, L. L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L. D.; Yan, W. S.; Chu, W. S.; Wu, C. Z. et al. Atomically dispersed iron- nitrogen species as electrocatalysts for bifunctional oxygen evolution and reduction reactions. Angew. Chem., Int. Ed. 2017, 56, 610–614.

 Figure 12 (a) Schematic illustration of the synthesis of A-CoPt-NC. (b) Optical photo of the Co-MOF nanocrystal. The inset image exhibits the Co-MOF in a DMF solution. TEM images of the (c) Co-NC composite and (d) A-CoPt-NC. (e) Top view of the charge densities of a(Co-Pt)@N8V4 (N8 refers to the number of nitrogen atoms and V4 represents the number of vacant carbon atoms). (f) ORR polarization curves of Co-NC, A-CoPt-NC, and Pt/C in 0.1 M KOH. Reproduced with permission from Ref. [92], © American Chemical Society 2018.

Nano Res. 2019, 12(9): 2067–2080

| www.editorialmanager.com/nare/default.asp

2078

[5] Yan, X. C.; Jia, Y.; Zhang, L. Z.; Soo, M. T.; Yao, X. D. Defective graphene anchored iron-cobalt nanoparticles for efficient electrocatalytic oxygen reduction. Chem. Commun. 2017, 53, 12140–12143.

[6] Gao, S. Y.; Geng, K. R.; Liu, H. Y.; Wei, X. J.; Zhang, M.; Wang, P.; Wang, J. J. Transforming organic-rich amaranthus waste into nitrogen-doped carbon with superior performance of the oxygen reduction reaction. Energy Environ. Sci. 2015, 8, 221–229.

[7] Zhang, L.; Liu, B. R.; Zhang, N.; Ma, M. M. Electrosynthesis of Co3O4 and Co(OH)2 ultrathin nanosheet arrays for efficient electrocatalytic water splitting in alkaline and neutral media. Nano Res. 2018, 11, 323–333.

[8] Sun, T. T.; Zhao, S.; Chen, W. X.; Zhai, D.; Dong, J. C.; Wang, Y.; Zhang, S. L.; Han, A. J.; Gu, L.; Yu, R. et al. Single-atomic cobalt sites embedded in hierarchically ordered porous nitrogen-doped carbon as a superior bifunctional electrocatalyst. Proc. Natl. Acad. Sci. USA 2018, 115, 12692–12697.

[9] Yan, D. F.; Guo, L.; Xie, C.; Wang, Y. Y.; Li, Y. X.; Li, H.; Wang, S. Y. N, P-dual doped carbon with trace Co and rich edge sites as highly efficient electrocatalyst for oxygen reduction reaction. Sci. China Mater. 2018, 61, 679–685.

[10] Kuang, M.; Wang, Q. H.; Ge, H. T.; Han, P.; Gu, Z. X.; Al-Enizi, A. M.; Zheng, G. F. CuCoOx/FeOOH core–shell nanowires as an efficient bifunctional oxygen evolution and reduction catalyst. ACS Energy Lett. 2017, 2, 2498–2505.

[11] Gupta, S.; Zhao, S.; Wang, X. X.; Hwang, S.; Karakalos, S.; Devaguptapu, S. V.; Mukherjee, S.; Su, D.; Xu, H.; Wu, G. Quaternary FeCoNiMn-based nanocarbon electrocatalysts for bifunctional oxygen reduction and evolution: Promotional role of Mn doping in stabilizing carbon. ACS Catal. 2017, 7, 8386–8393.

[12] Dai, Z. F.; Geng, H. B.; Wang, J.; Luo, Y. B.; Li, B.; Zong, Y.; Yang, J.; Guo, Y. Y.; Zheng, Y.; Wang, X. et al. Hexagonal-phase cobalt mono-phosphosulfide for highly efficient overall water splitting. ACS Nano 2017, 11, 11031–11040.

[13] Cao, J. M.; Feng, Y. Q.; Liu, B. Y.; Li, H. G. Carbon skeleton doped with Co, N, S and P as efficient electrocatalyst for oxygen evolution reaction. Sci. China Mater. 2018, 61, 686–696.

[14] Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R. H.; Liu, S. H.; Zhuang, X. D.; Feng, X. L. Interface engineering of MoS2/Ni3S2 heterostructures for highly enhanced electrochemical overall-water-splitting activity. Angew. Chem., Int. Ed. 2016, 55, 6702–6707.

[15] Jia, N.; Weng, Q.; Shi, Y. R.; Shi, X. Y.; Chen, X. B.; Chen, P.; An, Z. W.; Chen, Y. N-doped carbon nanocages: Bifunctional electrocatalysts for the oxygen reduction and evolution reactions. Nano Res. 2018, 11, 1905–1916.

[16] Zhang, M. D.; Dai, Q. B.; Zheng, H. G.; Chen, M. D.; Dai, L. M. Novel MOF-derived Co@N-C bifunctional catalysts for highly efficient Zn-Air batteries and water splitting. Adv. Mater. 2018, 30, 1705431.

[17] Wu, S. S.; Zhu, Y. G.; Hou, Y. F.; Luo, Y. C.; Zhang, L. H.; Wan, Y.; Nan, B.; Cao, L. J.; Wang, Z. Y.; Li, M. C. et al. Bimetallic organic frameworks derived CuNi/carbon nanocomposites as efficient electrocatalysts for oxygen reduction reaction. Sci. China Mater. 2017, 60, 654–663.

[18] Qiao, Y. Y.; Yuan, P. F.; Hu, Y. F.; Zhang, J. N.; Mu, S. C.; Zhou, J. H.; Li, H.; Xia, H. C.; He, J.; Xu, Q. Sulfuration of an Fe-N-C catalyst containing FexC/Fe species to enhance the catalysis of oxygen reduction in acidic media and for use in flexible Zn-Air batteries. Adv. Mater. 2018, 30, 1804504.

[19] Song, J. F.; Xiang, J. Y.; Mu, C. P.; Wang, B. C.; Wen, F. S.; Su, C.; Wang, C.; Liu, Z. Y. Facile synthesis and excellent electrochemical performance of CoP nanowire on carbon cloth as bifunctional electrode for hydrogen evolution reaction and supercapacitor. Sci. China Mater. 2017, 60, 1179–1186.

[20] Sun, K.; Cheng, T.; Wu, L. N.; Hu, Y. F.; Zhou, J. G.; Maclennan, A.; Jiang, Z. H.; Gao, Y. Z.; Goddard, W. A., III.; Wang, Z. J. Ultrahigh mass activity for carbon dioxide reduction enabled by gold-iron core–shell nanoparticles. J. Am. Chem. Soc. 2017, 139, 15608–15611.

[21] Yan, B.; Krishnamurthy, D.; Hendon, C. H.; Deshpande, S.; Surendranath, Y.; Viswanathan, V. Surface restructuring of nickel sulfide generates optimally coordinated active sites for oxygen reduction catalysis. Joule 2017, 1, 600–612.

[22] Zhang, H. B.; An, P. F.; Zhou, W.; Guan, B. Y.; Zhang, P.; Dong, J. C.; Lou, X. W. Dynamic traction of lattice-confined platinum atoms into mesoporous carbon matrix for hydrogen evolution reaction. Sci. Adv. 2018, 4, eaao6657.

[23] Stoerzinger, K. A.; Rao, R. R.; Wang, X. R.; Hong, W. T.; Rouleau, C. M.;

Shao-Horn, Y. The role of Ru redox in pH-dependent oxygen evolution on rutile ruthenium dioxide surfaces. Chem 2017, 2, 668–675.

[24] Lu, C. B.; Tranca, D.; Zhang, J.; Hernández, F. R.; Su, Y. Z.; Zhuang, X. D.; Zhang, F.; Seifert, G.; Feng, X. L. Molybdenum carbide-embedded nitrogen-doped porous carbon nanosheets as electrocatalysts for water splitting in alkaline media. ACS Nano 2017, 11, 3933–3942.

[25] Wu, Y. S.; Liu, X. J.; Han, D. D.; Song, X. Y.; Shi, L.; Song, Y.; Niu, S. W.; Xie, Y. F.; Cai, J. Y.; Wu, S. Y. et al. Electron density modulation of NiCo2S4 nanowires by nitrogen incorporation for highly efficient hydrogen evolution catalysis. Nat. Commun. 2018, 9, 1425.

[26] Sun, T. T.; Xu, L. B.; Li, S. Y.; Chai, W. X.; Huang, Y.; Yan, Y. S.; Chen, J. F. Cobalt-nitrogen-doped ordered macro-/mesoporous carbon for highly efficient oxygen reduction reaction. Appl. Catal. B: Environ. 2016, 193, 1–8.

[27] Su, J. W.; Ge, R. X.; Dong, Y.; Hao, F.; Chen, L. Recent progress in single- atom electrocatalysts: Concept, synthesis, and applications in clean energy conversion. J. Mater. Chem. A 2018, 6, 14025–14042.

[28] Liu, J.; Jiao, M. G.; Lu, L. L.; Barkholtz, H. M.; Li, Y. P.; Wang, Y.; Jiang, L. H.; Wu, Z. J.; Liu, D. J.; Zhuang, L. et al. High performance platinum single atom electrocatalyst for oxygen reduction reaction. Nat. Commun. 2017, 8, 15938.

[29] Sun, T. T.; Xu, L. B.; Yan, Y. S. Zakhidov, A. A.; Baughman, R. H.; Chen, J. F. Ordered mesoporous nickel sphere arrays for highly efficient electrocatalytic water oxidation. ACS Catal. 2016, 6, 1446–1450.

[30] Tang, T.; Jiang, W. J.; Niu, S.; Liu, N.; Luo, H.; Chen, Y. Y.; Jin, S. F.; Gao, F.; Wan, L. J.; Hu, J. S. Electronic and morphological dual modulation of cobalt carbonate hydroxides by Mn doping toward highly efficient and stable bifunctional electrocatalysts for overall water splitting. J. Am. Chem. Soc. 2017, 139, 8320–8328.

[31] Panda, C.; Menezes, P. W.; Walter, C.; Yao, S. L.; Miehlich, M. E.; Gutkin, V.; Meyer, K.; Driess, M. From a molecular 2Fe-2Se precursor to a highly efficient iron diselenide electrocatalyst for overall water splitting. Angew. Chem., Int. Ed. 2017, 56, 10506–10510.

[32] Long, B.; Tang, Y.; Li, J. New mechanistic pathways for CO oxidation catalyzed by single-atom catalysts: Supported and doped Au1/ThO2. Nano Res. 2016, 9, 3868–3880.

[33] Chen, Y. J.; Ji, S. F.; Chen, C.; Peng, Q.; Wang, D. S.; Li, Y. D. Single-atom catalysts: Synthetic strategies and electrochemical applications. Joule 2018, 2, 1242–1264.

[34] Wang, X.; Chen, W. X.; Zhang, L.; Yao, T.; Liu, W.; Lin, Y.; Ju, H. X.; Dong, J. C.; Zheng, L. R.; Yan, W. S. et al. Uncoordinated amine groups of metal-organic frameworks to anchor single Ru sites as chemoselective catalysts toward the hydrogenation of quinoline. J. Am. Chem. Soc. 2017, 139, 9419–9422.

[35] Zhang, J.; Wu, X.; Cheong, W. C.; Chen, W. X.; Lin, R.; Li, J.; Zheng, L. R.; Yan, W. S.; Gu, L.; Chen, C. et al. Cation vacancy stabilization of single-atomic-site Pt1/Ni(OH)x catalyst for diboration of alkynes and alkenes. Nat. Commun. 2018, 9, 1002.

[36] Gao, C.; Chen, S. M.; Wang, Y.; Wang, J. W.; Zheng, X. S.; Zhu, J. F.; Song, L.; Zhang, W. K.; Xiong, Y. J. Heterogeneous single-atom catalyst for visible-light-driven high-turnover CO2 reduction: The role of electron transfer. Adv. Mater. 2018, 30, 1704624.

[37] Lin, R. H.; Albani, D.; Fako, E.; Kaiser, S. K.; Safonova, O. V.; López, N.; Pérez-Ramírez, J. Design of single gold atoms on nitrogen-doped carbon for molecular recognition in alkyne semi-hydrogenation. Angew. Chem., Int. Ed. 2019, 58, 504–509.

[38] Qiao, B. T.; Wang, A. Q.; Yang, X. F.; Allard, L. F.; Jiang, Z.; Cui, Y. T.; Liu, J. Y.; Li, J.; Zhang, T. Single-atom catalysis of CO oxidation using Pt1/FeOx. Nat. Chem. 2011, 3, 634–641.

[39] Huang, X. H.; Xia, Y. J.; Cao, Y. J.; Zheng, X. S.; Pan, H. B.; Zhu, J. F.; Ma, C.; Wang, H. W.; Li, J. J.; You, R. et al. Enhancing both selectivity and coking-resistance of a single-atom Pd1/C3N4 catalyst for acetylene hydrogenation. Nano Res. 2017, 10, 1302–1312.

[40] Zhang, C. H.; Sha, J. W.; Fei, H. L.; Liu, M. J.; Yazdi, S.; Zhang, J. B.; Zhong, Q. F.; Zou, X. L.; Zhao, N. Q.; Yu, H. S. et al. Single-atomic ruthenium catalytic sites on nitrogen-doped graphene for oxygen reduction reaction in acidic medium. ACS Nano 2017, 11, 6930–6941.

[41] Tao, H. C.; Choi, C.; Ding, L.-X.; Jiang, Z.; Han, Z. S.; Jia, M. W.; Fan, Q.; Gao, Y. N.; Wang, H. H.; Robertson, A. W. et al. Nitrogen fixation by Ru single-atom electrocatalytic reduction. Chem 2019, 5, 204–214.

[42] Yang, J.; Qiu, Z. Y.; Zhao, C. M.; Wei, W. C.; Chen, W. X.; Li, Z. J.; Qu, Y. T.; Dong, J. C.; Luo, J.; Li, Z. Y. et al. In situ thermal atomization to

Nano Res. 2019, 12(9): 2067–2080

www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano Research

2079

convert supported nickel nanoparticles into surface-bound nickel single- atom catalysts. Angew. Chem., Int. Ed. 2018, 57, 14095–14100.

[43] Ahn, S. H.; Klein, M. J.; Manthiram, A. 1D Co- and N-doped hierarchically porous carbon nanotubes derived from bimetallic metal organic framework for efficient oxygen and tri-iodide reduction reactions. Adv. Energy Mater. 2017, 7, 1601979.

[44] Wei, S. J.; Li, A.; Liu, J. C.; Li, Z.; Chen, W. X.; Gong, Y.; Zhang, Q. H.; Cheong, W. C.; Wang, Y.; Zheng, L. R. et al. Direct observation of noble metal nanoparticles transforming to thermally stable single atoms. Nat. Nanotechnol. 2018, 13, 856–861.

[45] Fang, X. Z.; Shang, Q. C.; Wang, Y.; Jiao, L.; Yao, T.; Li, Y. F.; Zhang, Q.; Luo, Y.; Jiang, H. L. Single Pt atoms confined into a metal-organic framework for efficient photocatalysis. Adv. Mater. 2018, 30, 1705112.

[46] Pan, Y.; Lin, R.; Chen, Y. J.; Liu, S. J.; Zhu, W.; Cao, X.; Chen, W. X.; Wu, K. L.; Cheong, W. C.; Wang, Y. et al. Design of single-atom Co-N5 catalytic site: A robust electrocatalyst for CO2 reduction with nearly 100% CO selectivity and remarkable stability. J. Am. Chem. Soc. 2018, 140, 4218–4221.

[47] Han, A. J.; Chen, W. X.; Zhang, S. L.; Zhang, M. L.; Han, Y. H.; Zhang, J.; Ji, S. F.; Zheng, L. R.; Wang, Y.; Gu, L. et al. A polymer encapsulation strategy to synthesize porous nitrogen-doped carbon-nanosphere-supported metal isolated-single-atomic-site catalysts. Adv. Mater. 2018, 30, 1706508.

[48] Liang, Z. Z.; Fan, X.; Lei, H. T.; Qi, J.; Li, Y. Y.; Gao, J. P.; Huo, M. L.; Yuan, H. T.; Zhang, W.; Lin, H. P. et al. Cobalt-nitrogen-doped helical carbonaceous nanotubes as a class of efficient electrocatalysts for the oxygen reduction reaction. Angew. Chem., Int. Ed. 2018, 57, 13187–13191.

[49] Liang, H. W.; Brüller, S.; Dong, R. H.; Zhang, J.; Feng, X. L.; Müllen, K. Molecular metal-Nx centres in porous carbon for electrocatalytic hydrogen evolution. Nat. Commun. 2015, 6, 7992.

[50] Lu, Z. Y.; Wang, B.; Hu, Y. F.; Liu, W.; Zhao, Y. F.; Yang, R. O.; Li, Z. P.; Luo, J.; Chi, B.; Jiang, Z. et al. An isolated zinc-cobalt atomic pair for highly active and durable oxygen reduction. Angew. Chem., Int. Ed. 2019, 58, 2622–2626.

[51] Li, X. G.; Bi, W. T.; Chen, M. L.; Sun, Y. X.; Ju, H. X.; Yan, W. S.; Zhu, J. F.; Wu, X. J.; Chu, W. S.; Wu, C. Z. et al. Exclusive Ni-N4 sites realize near-unity CO selectivity for electrochemical CO2 reduction. J. Am. Chem. Soc. 2017, 139, 14889–14892.

[52] Zhang, Z. P.; Sun, J. T.; Wang, F.; Dai, L. M. Efficient oxygen reduction reaction (ORR) catalysts based on single iron atoms dispersed on a hierarchically structured porous carbon framework. Angew. Chem., Int. Ed. 2018, 57, 9038–9043.

[53] Zhang, J. F.; Liu, J. Y.; Xi, L. F.; Yu, Y. F.; Chen, N.; Sun, S. H.; Wang, W. C.; Lange, K. M.; Zhang, B. Single-atom Au/NiFe layered double hydroxide electrocatalyst: Probing the origin of activity for oxygen evolution reaction. J. Am. Chem. Soc. 2018, 140, 3876–3879.

[54] Chen, W. X.; Pei, J. J.; He, C. T.; Wan, J. W.; Ren, H. L.; Zhu, Y. Q.; Wang, Y.; Dong, J. C.; Tian, S. B.; Cheong, W. C. et al. Rational design of single molybdenum atoms anchored on N-doped carbon for effective hydrogen evolution reaction. Angew. Chem., Int. Ed. 2017, 56, 16086–16090.

[55] Fei, H. L.; Dong, J. C.; Wan, C. Z.; Zhao, Z. P.; Xu, X.; Lin, Z. Y.; Wang, Y. L.; Liu, H. T.; Zang, K. T.; Luo, J. et al. Microwave-assisted rapid synthesis of graphene-supported single atomic metals. Adv. Mater. 2018, 30, 1802146.

[56] Han, Y. H.; Wang, Y. G.; Chen, W. X.; Xu, R. R.; Zheng, L. R.; Zhang, J.; Luo, J.; Shen, R. A.; Zhu, Y. Q.; Cheong, W. C. et al. Hollow N-doped carbon spheres with isolated cobalt single atomic sites: Superior electrocatalysts for oxygen reduction. J. Am. Chem. Soc. 2017, 139, 17269–17272.

[57] Han, Y. H.; Wang, Y. G.; Xu, R. R.; Chen, W. X.; Zheng, L. R.; Han, A. J.; Zhu, Y. Q.; Zhang, J.; Zhang, H. B.; Luo, J. et al. Electronic structure engineering to boost oxygen reduction activity by controlling the coordination of the central metal. Energy Environ. Sci. 2018, 11, 2348–2352.

[58] Sun, T. T.; Zhang, S. L.; Xu, L. B.; Wang, D. S.; Li, Y. D. An efficient multifunctional hybrid electrocatalyst: Ni2P nanoparticles on MOF-derived Co, N-doped porous carbon polyhedrons for oxygen reduction and water splitting. Chem. Commun. 2018, 54, 12101–12104.

[59] Qiu, H. J.; Ito, Y.; Cong, W. T.; Tan, Y. W.; Liu, P.; Hirata, A.; Fujita, T.; Tang, Z.; Chen, M. W. Nanoporous graphene with single-atom nickel dopants: An efficient and stable catalyst for electrochemical hydrogen production. Angew. Chem., Int. Ed. 2015, 54, 14031–14035.

[60] Sun, T. T.; Shan, N. N.; Xu, L. B.; Wang, J. X.; Chen, J. F.; Zakhidov, A. A.; Baughman, R. H. General synthesis of 3D ordered macro-/mesoporous materials by templating mesoporous silica confined in opals. Chem. Mater.

2018, 30, 1617–1624. [61] Ahn, S. H.; Yu, X. W.; Manthiram, A. “Wiring” Fe-Nx-embedded porous

carbon framework onto 1D nanotubes for efficient oxygen reduction reaction in alkaline and acidic media. Adv. Mater. 2017, 29, 1606534.

[62] Indra, A.; Song, T.; Paik, U. Metal organic framework derived materials: Progress and prospects for the energy conversion and storage. Adv. Mater. 2018, 30, 1705146.

[63] Chong, L.; Wen, J. G.; Kubal, J.; Sen, F. G.; Zou, J. X.; Greeley, J.; Chan, M.; Barkholtz, H.; Ding, W. J.; Liu, D. J. Ultralow-loading platinum-cobalt fuel cell catalysts derived from imidazolate frameworks. Science 2018, 362, 1276–1281.

[64] Li, F.; Han, G. F.; Noh, H.-J.; Kim, S.-J.; Lu, Y. L.; Jeong, H. Y.; Fu, Z. P.; Baek, J.-B. Boosting oxygen reduction catalysis with abundant copper single atom active sites. Energy Environ. Sci. 2018, 11, 2263–2269.

[65] Ye, Y. F.; Cai, F.; Li, H. B.; Wu, H. H.; Wang, G. X.; Li, Y. S.; Miao, S.; Xie, S. H.; Si, R.; Wang, J. et al. Surface functionalization of ZIF-8 with ammonium ferric citrate toward high exposure of Fe-N active sites for efficient oxygen and carbon dioxide electroreduction. Nano Energy 2017, 38, 281–289.

[66] Yang, L.; Zeng, X. F.; Wang, W. C.; Cao, D. P. Recent progress in MOF- derived, heteroatom-doped porous carbons as highly efficient electrocatalysts for oxygen reduction reaction in fuel cells. Adv. Funct. Mater. 2018, 28, 1704537.

[67] Liu, J. L.; Zhu, D. D.; Guo, C. X.; Vasileff, A.; Qiao, S.-Z. Design strategies toward advanced MOF-derived electrocatalysts for energy-conversion reactions. Adv. Energy Mater. 2017, 7, 1700518.

[68] Peng, Y.; Lu, B. Z.; Chen, S. W. Carbon-supported single atom catalysts for electrochemical energy conversion and storage. Adv. Mater. 2018, 30, 1801995.

[69] Liang, Z. B.; Qu, C.; Xia, D. G.; Zou, R. Q.; Xu, Q. Atomically dispersed metal sites in MOF-based materials for electrocatalytic and photocatalytic energy conversion. Angew. Chem., Int. Ed. 2018, 57, 9604–9633.

[70] Krivanek, O. L.; Chisholm, M. F.; Nicolosi, V.; Pennycook, T. J.; Corbin, G. J.; Dellby, N.; Murfitt, M. F.; Own, C. S.; Szilagyi, Z. S.; Oxley, M. P. et al. Atom-by-atom structural and chemical analysis by annular dark-field electron microscopy. Nature 2010, 464, 571–574.

[71] Sun, Z. H.; Liu, Q. H.; Yao, T.; Yan, W. S.; Wei, S. Q. X-ray absorption fine structure spectroscopy in nanomaterials. Sci. China Mater. 2015, 58, 313–341.

[72] Zhu, C. Z.; Shi, Q. R.; Xu, B. Z.; Fu, S. F.; Wan, G.; Yang, C.; Yao, S. Y.; Song, J. H.; Zhou, H.; Du, D. et al. Hierarchically porous M-N-C (M = Co and Fe) single-atom electrocatalysts with robust MNx active moieties enable enhanced ORR performance. Adv. Energy Mater. 2018, 8, 1801956.

[73] Zheng, Y.; Jiao, Y.; Zhu, Y. H.; Cai, Q. R.; Vasileff, A.; Li, L. H.; Han, Y.; Chen, Y.; Qiao, S. Z. Molecule-level g-C3N4 coordinated transition metals as a new class of electrocatalysts for oxygen electrode reactions. J. Am. Chem. Soc. 2017, 139, 3336–3339.

[74] Fei, H. L.; Dong, J. C.; Arellano-Jiménez, M. J.; Ye, G. L.; Kim, N. D.; Samuel, E. L. G.; Peng, Z. W.; Zhu, Z.; Qin, F.; Bao, J. M. et al. Atomic cobalt on nitrogen-doped graphene for hydrogen generation. Nat. Commun. 2015, 6, 8668.

[75] Tao, L.; Lin, C.-Y.; Dou, S.; Feng, S.; Chen, D. W.; Liu, D. D.; Huo, J.; Xia, Z. H.; Wang, S. Y. Creating coordinatively unsaturated metal sites in metal-organic-frameworks as efficient electrocatalysts for the oxygen evolution reaction: Insights into the active centers. Nano Energy 2017, 41, 417–425.

[76] Zhao, W. P.; Wan, G.; Peng, C. L.; Sheng, H. P.; Wen, J. G.; Chen, H. R. Key single-atom electrocatalysis in metal-organic framework (MOF)-derived bifunctional catalysts. ChemSusChem 2018, 11, 3473–3479.

[77] Wang, X. Q.; Chen, Z.; Zhao, X. Y.; Yao, T.; Chen, W. X.; You, R.; Zhao, C. M.; Wu, G.; Wang, J.; Huang, W. X. et al. Regulation of coordination number over single Co sites: Triggering the efficient electroreduction of CO2. Angew. Chem., Int. Ed. 2018, 57, 1944–1948.

[78] Yin, P. Q.; Yao, T.; Wu, Y. E.; Zheng, L. R.; Lin, Y.; Liu, W.; Ju, H. X.; Zhu, J. F.; Hong, X.; Deng, Z. X. et al. Single cobalt atoms with precise N-coordination as superior oxygen reduction reaction catalysts. Angew. Chem., Int. Ed. 2016, 55, 10800–10805.

[79] Wang, X. X.; Cullen, D. A.; Pan, Y. T.; Hwang, S.; Wang, M. Y.; Feng, Z. X.; Wang, J. Y.; Engelhard, M. H.; Zhang, H. G.; He, Y. H. et al. Nitrogen-coordinated single cobalt atom catalysts for oxygen reduction in proton exchange membrane fuel cells. Adv. Mater. 2018, 30, 1706758.

[80] He, Y. H.; Hwang, S.; Cullen, D. A.; Uddin, M. A.; Langhorst, L.; Li, B. Y.;

Nano Res. 2019, 12(9): 2067–2080

| www.editorialmanager.com/nare/default.asp

2080

Karakalos, S.; Kropf, A. J.; Wegener, E. C.; Sokolowski, J. et al. Highly active atomically dispersed CoN4 fuel cell cathode catalysts derived from surfactant-assisted MOFs: Carbon-shell confinement strategy. Energy Environ. Sci. 2019, 12, 250–260.

[81] Chen, Y. J.; Ji, S. F.; Wang, Y. G.; Dong, J. C.; Chen, W. X.; Li, Z.; Shen, R. A.; Zheng, L. R.; Zhuang, Z. B.; Wang, D. S. et al. Isolated single iron atoms anchored on N-doped porous carbon as an efficient electrocatalyst for the oxygen reduction reaction. Angew. Chem., Int. Ed. 2017, 56, 6937–6941.

[82] Jiang, R.; Li, L.; Sheng, T.; Hu, G. F.; Chen, Y. G.; Wang, L. Y. Edge-site engineering of atomically dispersed Fe-N4 by selective C–N bond cleavage for enhanced oxygen reduction reaction activities. J. Am. Chem. Soc. 2018, 140, 11594–11598.

[83] Zhang, H. G.; Hwang, S.; Wang, M. Y.; Feng, Z. X.; Karakalos, S.; Luo, L. L.; Qiao, Z.; Xie, X. H.; Wang, C. M.; Su, D. et al. Single atomic iron catalysts for oxygen reduction in acidic media: Particle size control and thermal activation. J. Am. Chem. Soc. 2017, 139, 14143–14149.

[84] Jiao, L.; Wan, G.; Zhang, R.; Zhou, H.; Yu, S. H.; Jiang, H. L. From metal- organic frameworks to single-atom Fe implanted N-doped porous carbons: Efficient oxygen reduction in both alkaline and acidic media. Angew. Chem., Int. Ed. 2018, 57, 8525–8529.

[85] Zhao, C. M.; Dai, X. Y.; Yao, T.; Chen, W. X.; Wang, X. Q.; Wang, J.; Yang, J.; Wei, S. Q.; Wu, Y. E.; Li, Y. D. Ionic exchange of metal-organic frameworks to access single nickel sites for efficient electroreduction of CO2. J. Am. Chem. Soc. 2017, 139, 8078–8081.

[86] Fan, L. L.; Liu, P. F.; Yan, X. C.; Gu, L.; Yang, Z. Z.; Yang, H. G.; Qiu, S. L.; Yao, X. D. Atomically isolated nickel species anchored on graphitized carbon for efficient hydrogen evolution electrocatalysis. Nat. Commun. 2016, 7, 10667.

[87] Qu, Y. T.; Li, Z. J.; Chen, W. X.; Lin, Y.; Yuan, T. W.; Yang, Z. K.; Zhao, C. M.; Wang, J.; Zhao, C.; Wang, X. et al. Direct transformation of bulk copper into copper single sites via emitting and trapping of atoms. Nat. Catal. 2018, 1, 781–786.

[88] Li, J. Z.; Chen, M. J.; Cullen, D. A.; Hwang, S.; Wang, M. Y.; Li, B. Y.; Liu, K. X.; Karakalos, S.; Lucero, M.; Zhang, H. G. et al. Atomically dispersed manganese catalysts for oxygen reduction in proton-exchange membrane fuel cells. Nat. Catal. 2018, 1, 935–945.

[89] Chen, W. X.; Pei, J. J.; He, C. T.; Wan, J. W.; Ren, H. L.; Wang, Y.; Dong, J. C.; Wu, K. L.; Cheong, W. C.; Mao, J. J. et al. Single tungsten atoms supported on MOF-derived N-doped carbon for robust electrochemical hydrogen evolution. Adv. Mater. 2018, 30, 1800396.

[90] Wang, J.; Huang, Z. Q.; Liu, W.; Chang, C. R.; Tang, H. L.; Li, Z. J.; Chen, W. X.; Jia, C. J.; Yao, T.; Wei, S. Q. et al. Design of N-coordinated dual- metal sites: A stable and active Pt-free catalyst for acidic oxygen reduction reaction. J. Am. Chem. Soc. 2017, 139, 17281–17284.

[91] Zhang, D. Y.; Chen, W. X.; Li, Z.; Chen, Y. J.; Zheng, L. R.; Gong, Y.; Li, Q. H.; Shen, R. A.; Han, Y. H.; Cheong, W. C. et al. Isolated Fe and Co dual active sites on nitrogen-doped carbon for a highly efficient oxygen reduction reaction. Chem. Commun. 2018, 54, 4274–4277.

[92] Zhang, L. Z.; Fischer, J. M. T. A.; Jia, Y.; Yan, X. C.; Xu, W.; Wang, X. Y.; Chen, J.; Yang, D. J.; Liu, H. W.; Zhuang, L. Z. et al. Coordination of atomic Co-Pt coupling species at carbon defects as active sites for oxygen reduction reaction. J. Am. Chem. Soc. 2018, 140, 10757–10763.

[93] Lai, Q. X.; Zheng, L. R.; Liang, Y. Y.; He, J. P.; Zhao, J. X.; Chen, J. H. Metal–organic-framework-derived Fe-N/C electrocatalyst with five-coordinated Fe-Nx sites for advanced oxygen reduction in acid media. ACS Catal. 2017, 7, 1655–1663.

[94] Chen, Y. J.; Ji, S. F.; Zhao, S.; Chen, W. X.; Dong, J. C.; Cheong, W. C.; Shen, R. A.; Wen, X. D.; Zheng, L. R.; Rykov, A. I. et al. Enhanced oxygen reduction with single-atomic-site iron catalysts for a zinc-air battery and hydrogen-air fuel cell. Nat. Commun. 2018, 9, 5422.

[95] Zhu, Q.-L.; Xia, W.; Zheng, L.-R.; Zou, R. Q.; Liu, Z.; Xu, Q. Atomically dispersed Fe/N-doped hierarchical carbon architectures derived from a metal–organic framework composite for extremely efficient electrocatalysis. ACS Energy Lett. 2017, 2, 504–511.

[96] Wei, L.; Karahan, H. E.; Zhai, S. L.; Liu, H. W.; Chen, X. C.; Zhou, Z.; Lei, Y. J.; Liu, Z. W.; Chen, Y. Amorphous bimetallic oxide-graphene hybrids as bifunctional oxygen electrocatalysts for rechargeable Zn-Air batteries. Adv. Mater. 2017, 29, 1701410.

[97] Chao, T. T.; Luo, X.; Chen, W. X.; Jiang, B.; Ge, J. J.; Lin, Y.; Wu, G.; Wang, X. Q.; Hu, Y. M.; Zhuang, Z. B. et al. Atomically dispersed copper- platinum dual sites alloyed with palladium nanorings catalyze the hydrogen evolution reaction. Angew. Chem., Int. Ed. 2017, 56, 16047–16051.

[98] Jiang, K. Z.; Wang, P. T.; Guo, S. J.; Zhang, X.; Shen, X.; Lu, G.; Su, D.; Huang, X. Q. Ordered PdCu-based nanoparticles as bifunctional oxygen- reduction and ethanol-oxidation electrocatalysts. Angew. Chem., Int. Ed. 2016, 55, 9030–9035.

[99] Zhao, R.; Liang, Z. B.; Gao, S.; Yang, C.; Zhu, B. J.; Zhao, J. L.; Qu, C.; Zou, R. Q.; Xu, Q. Puffing up energetic metal-organic frameworks to large carbon networks with hierarchical porosity and atomically dispersed metal sites. Angew. Chem., Int. Ed. 2019, 58, 1975–1979.