Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design

Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design

Review Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design Xiaoqian Wang,1 Zhijun Li,1 Yunteng Qu,1 ...

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Review

Review of Metal Catalysts for Oxygen Reduction Reaction: From Nanoscale Engineering to Atomic Design Xiaoqian Wang,1 Zhijun Li,1 Yunteng Qu,1 Tongwei Yuan,1 Wenyu Wang,1 Yuen Wu,1,* and Yadong Li2,*

Platinum (Pt)-based catalysts have been unanimously considered the most efficient catalysts for the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). Unfortunately, the exorbitant cost of Pt hampers the widespread adoption and development of PEMFCs. Scientists have devoted tremendous efforts to achieving higher catalytic activity with less Pt usage by constructing delicate nanostructures. Substituting Pt with cheaper metals may be a feasible solution but suffers from a relatively low intrinsic activity. Recently, single-atom catalysts (SACs), which possess the highest metal utilization and excellent activity because of the minimum size of metal and unique coordination structure, are developing rapidly and have been regarded as a potential alternative to Pt-based materials. Here, we review the development of conventional Ptand nonprecious-metal-based ORR catalysts and summarize recent achievement in SACs for the ORR. A brief perspective on the remaining challenges and future directions of SACs is also presented.

INTRODUCTION Since the first industrial revolution, advanced inventions and technologies have enabled us to enjoy warmth in the winter, cool in the summer, brightness at night, and convenient transportation all over the world.1 Most of these technologies depend heavily on our ability to exploit fossil sources of energy, resulting in an increasing demand for fossil fuel and excessive emissions of CO2. As projected by the International Energy Agency, the global annual energy demand will increase to 18 billion tons of oil equivalents, and 43 gigatons of CO2 will be released per year by 2035, which will aggravate energy crisis, increase the global average temperature, and acidify the ocean.1,2 Severe situations have motivated a large number of researchers to pursue reliable and clean energy options. Proton-exchange membrane fuel cells (PEMFCs), especially refueled with hydrogen from renewable energy, are generally considered one of the most promising solutions because of their competitive advantages, such as zero emission, high efficiency, fast refueling, and low upfront cost.3 In a typical PEMFC, fuel molecules (e.g., hydrogen) are oxidized on the anode, and oxygen gas is reduced on the cathode, outputting electric energy with pure water and heat as the only by-products (Figure 1A). Unfortunately, the difficulty in O2 activation, O–O bond cleavage, and oxide removal causes sluggish kinetics of the oxygen reduction reaction (ORR) on the cathode, thus demanding stringent requirements to the catalysts.4 After a long period of experimental exploration, platinum (Pt) and Pt-based catalysts are generally considered to be the most efficient ORR catalysts. Low-temperature PEMFCs currently adopt Pt nanoparticles (NPs) supported on carbon (Pt/C) or other Pt-rich materials as the cathode catalyst.5

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The Bigger Picture Proton-exchange membrane fuel cells (PEMFCs) are now of great interest because of zero emission and high efficiency. Current PEMFCs require an unaffordable amount of Pt-based catalysts to overcome the sluggish kinetics of the oxygen reduction reation (ORR) on cathodes, hampering the widespread adoption of PEMFCs. Tremendous efforts have been devoted to achieving higher catalytic activity with less Pt usage by nanoscale engineering. Substituting Pt with cheaper metals may be also a feasible solution but suffers from low intrinsic activity. Recently, singleatom catalysts (SACs), which possess the highest metal utilization and excellent activity because of the minimum size of metal and unique coordination structure, have been regarded as potential alternatives. Here, we review the development of Pt- and nonprecious-metal-based ORR nanocatalysts and summarize recent achievements in SACs for the ORR. At last, a brief perspective on the remaining challenges and future directions of SACs for the ORR is presented.

Nevertheless, the high cost of Pt greatly hampers further large-scale adoption of PEMFCs. According to the strategic analysis report, catalyst layers in the PEMFC system amount to US $11.24 kW 1 or over 20% of the total cost, in which over US $10 kW 1 attributes to Pt usage. How to reduce the dosage of Pt or substitute nonprecious metal for Pt without loss of performance is now of the greatest concern.3,5–7 At present, improving atom utilization and boosting the intrinsically catalytic activity of Pt by reducing Pt nanostructure sizes, alloying, and constructing specific nanostructures with Pt-rich skin are common strategies for reducing the dosage of Pt.4,5,7 A variety of delicate Pt-based nanostructures have been reported to exhibit significantly enhanced ORR activity. For instance, Li et al. fabricated ultrafine Pt jagged nanowires with diameters less than 5 nm, delivering 33 times the specific activity (catalytic activity normalized by surface area) or 52 times the mass activity (catalytic activity per given mass of Pt) of the commercial Pt/C catalyst.8 The Adzic group deposited Pt monolayers on PdAu NP surfaces by the galvanic displacement method to optimize Pt utilization. The ultra-low Pt content was found to be enough to achieve high ORR catalytic performance.9 In another important work by Chen et al., Pt3Ni nanoframes with Pt-skin surfaces were constructed after the interior of polyhedral PtNi3 nanocrystals was dissolved, significantly outperforming the commercial Pt/C catalyst for ORR activity.10 However, these fine nanostructures typically have a high propensity to agglomerate or deform during the electrochemical process, resulting in an unfavorable deactivation and poor stability during long-term operation.11 Meanwhile, the complicated synthetic procedures cause the manufacture of catalysts to be costly. These disadvantages make Pt-based catalysts still doubtful in further wide adoption. In consideration of the much lower price of nonprecious metals, such as iron (Fe), cobalt (Co), and nickel (Ni), the cost of PEMFCs could be significantly reduced by substituting nonprecious-metal catalysts for Pt-based catalysts. However, the ORR activity of conventional nonprecious NPs is lower than that of Pt counterparts by almost one order of magnitude, preventing them from directly acting as eligible ORR catalysts.12,13 Similar to the case of Pt-based electrocatalysts, regulation of morphological and electronic structure of the nonprecious-metal catalysts is a general strategy for improving their ORR activity. Unfortunately, despite the tremendous efforts, few results achieve satisfactory catalytic activity and durability because of the flagrantly low intrinsic ORR catalytic activity of nonprecious metals.4,5 The size of metal particles is a key factor in determining their catalytic performance given that the specific activity per metal atom generally increases with decreasing size of the particles. Single-atom catalysts (SACs) represent the theoretically ultimate size limit for metal particles, in which metal atoms are dispersed on specific supports and isolated from each other without appreciable interaction between them. Therefore, SACs are supposed to possess relatively high catalytic activity and maximum atom-utilization efficiency.14–16 In 2000, Heiz et al. prepared a series of Pdn cluster supported on magnesium oxide with the help of mass-selected soft-landing techniques.17 The single palladium (Pd) atom is surprisingly found to exhibit enough catalytic activity in acetylene cyclotrimerization to benzene. In another research by the Zhang group, atomically dispersed Pt atoms supported on Fe oxide (Pt1/FeOx) were successfully synthesized, carefully characterized, and applied in efficient and durable CO oxidation.18 In 2016, Liu and co-workers developed a convenient photochemical strategy to fabricate a stable SAC with Pd atoms supported on ultrathin TiO2 nanosheets.19 Such a Pd SAC exhibited high catalytic activity in hydrogenation of C=C bonds, outperforming commercial Pd/C catalysts and homogeneous

1School

of Chemistry and Materials Science, Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China

2Department

of Chemistry, Tsinghua University, Beijing 100084, China *Correspondence: [email protected] (Y.W.), [email protected] (Y.L.) https://doi.org/10.1016/j.chempr.2019.03.002

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Figure 1. Schematic Diagram of PEMFC, ORR Mechanism, Typical ORR Polarization Curves, Pure Pt ORR Catalysts, and Their ORR Performance (A) Schematic diagram of a typical PEMFC with H 2 as fuel. H2 is oxidized on the anode and O 2 is reduced on the cathode, outputting electric energy with pure water and heat as the only by-products. (B) Schematic representation of various possible ORR intermediate and mechanisms. Reprinted with permission from Keith et al.26 Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Typical ORR polarization curves and the parameters used to qualitatively compare activities. Reprinted with permission from Guo et al. 27 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (D and E) Schematic diagram of evolutionary steps in the synthesis of Pt cubic nanocages (D) and mass activities of Pt/C, cubic, and octahedral Pt nanocages (E). Reprinted with permission from Zhang et al. 34 Copyright 2015 AAAS. (F–H) Schematic diagrams of (F, i) jagged Pt nanowires, (F, ii) colored atoms showing 5-fold index, and (F, iii) atomic stress (in atm$nm 3 ) as well as (G) polarization curves and (H) specific activities and mass activities of Pt/C, regular Pt nanowires (R-PtNWs), and jagged Pt nanowires (J-PtNWs). Reprinted with permission from Li et al. 8 Copyright 2018 AAAS.

H2PdCl4. In the same year, the Li and Wu groups initiated a new method to construct isolated metal atoms anchored on three-dimensional nanostructures, achieving a high ORR activity.20 Other than this significative research, a series of SACs have been reported and showed surprisingly excellent performance in various catalysis processes, such as catalytic reduction of CO2,21,22 electrochemical synthesis of ammonia,23 methane conversion,24 selective acetylene hydrogenation,25 and other important chemical reactions.14,15 Inspired by the powerful achievements by SACs, scientists have made fruitful researches in tuning SACs into active, reliable ORR

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catalysts as an alternative to expensive Pt-based materials.15,16 In addition, SACs with uniform catalytically active sites provide us a golden opportunity of exploring the relationship between ORR catalytic performance and catalyst structure in an atomic scale, which could spur further research on the atomically rational design of ORR catalysts.22 In this review, we first introduce the mechanism and electrochemical evaluation of ORR. Then, we concisely retrospect the development of Pt-based ORR catalysts and demonstrate how scientists optimize their catalytic performance by controlling their component, morphology, size, and facet exposure. After that, we describe the ORR performance of nonprecious-metal-based catalysts and the following three common strategies of improving their performance: (1) increasing the intrinsic activity by composition modulation, (2) confining metal species into carbon shells to preserve metal from corrosion, and (3) increasing the accessibility by constructing porous or other large-area structures. Further, we briefly review the developments of SACs, summarize recent advances in SACs for ORR catalysis, and demonstrate how the ORR performance on SACs is promoted by support construction and regulation of electronic structures. At last, we also present a brief perspective on the remaining challenges and future directions of SACs for ORR.

THE MECHANISMS AND ELECTROCHEMICAL EVALUATION FOR ORR Mechanisms for ORR It is generally accepted that the ORR undergoes either a ‘‘direct’’ four-electron pathway to generate O2-species (H2O in acidic solutions or OH in alkaline solutions) or a ‘‘series’’ two-electron pathway to generate hydrogen peroxide (H2O2).4,5 The ‘‘series’’ way has been considered one of the competitive strategies of producing H2O2, and it could replace the energy-intensive anthraquinone process. However, the ‘‘direct’’ four-electron oxygen reduction pathway is unanimously recognized as the favorable pathway since H2O2 reduces energy-conversion efficiency and accelerates the degradation of the proton-conducting polymer electrolyte in PEMFCs.13 Different intermediates, including oxygenated (O*), hydroxyl (OH*), and superhydroxyl (OOH*) species, could be generated during oxygen reaction under common ORR conditions. Several possible transformations between these intermediates, as schematically shown in Figure 1B, make the ORR process more complicated.26 Despite the tremendous efforts to find the rate-determining step in ORR, there is still no definitive conclusion because the reaction pathway depends, to a great extent, on the catalysts and environmental parameters such as solvent, temperature, and applied electrode potential. In the majority of cases, the overall ORR rate is determined by one of these three steps: (1) the first electron transfer to adsorbed O2 molecule, (2) the hydration of O2, and (3) the final desorption of H2O.4 In addition, several studies have supported that oxygen coverage plays a critical role in ORR mechanisms. A high oxygen coverage causes O–O cleavage posterior to OOH* formation (so-called associative mechanism), whereas a low oxygen coverage makes O–O cleavage anterior to OH* formation (dissociation mechanism).27 Electrochemical Evaluation for ORR Based on Rotating Disk Electrode For a rigorous evaluation, a new ORR catalyst is supposed to be employed in a PEMFC and compared with acknowledged benchmarks, such as commercial Pt/C. Nevertheless, the complicated and costly fabrication of a PEMFC makes this approach impractical. In science labs, benefiting from operability as well as inexpensiveness, the rotating disk electrode (RDE) method has been widely adopted to quickly screen the ORR catalytic performance of new materials.5,27 Typically, the

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catalyst sample is first dispersed into a homogenous ink with mixing water, alcohol (isopropyl in some cases), and Nafion by an optimized ratio, which is then deposited on a glassy carbon RDE. For Pt-based catalysts, especially for Pt NPs, the Pt loading is usually controlled below 50 mg/cm2 to avoid mass-transport loss caused by catalyst agglomeration. After electrode preparation, in a typical procedure, a cyclic voltammogram (CV) is first investigated by cyclic voltammetry in inert-gas (N2 or Ar)-saturated acidic solutions (0.1 M HClO4, 0.5 M H2SO4, or 0.1 M KOH). H+ would be reduced and adsorbed on catalytically active sites on the catalyst surface during the cathodic scanning, corresponding to the H adsorption region in current-potential (I-V) curve. In reverse, the regeneration of H+ occurs during anodic scanning and corresponds to the H desorption region in the I-V curve. After mathematic conversion, the electrochemical active surface area (ECSA) can be obtained from the integral of H desorption area in the I-V curve. The ORR polarization curve is measured in an O2-saturated solution (usually 0.1 M HClO4, 0.5 M H2SO4, or 0.1 M KOH) in a potential scanning window between 0.05 and 1.20 V versus reversible hydrogen electrode (RHE). As the mass transfer largely influences the ORR catalytic performance, the RDE is rotating (usually at a speed of 1,600 rpm) to mitigate the mass transfer loss during the ORR evaluation. To minimize the contribution from capacitive current, the scan rate is usually controlled below 20 mV s 1. One should note that current PEMFCs adopt cation-exchange membranes to separate the anode and cathode, making RDE tests in an acidic medium more practically significative. After polarization curve measurements, the kinetic current (jk, catalytic current without the loss caused by mass transfer) can be extracted according to the Levich-Koutechy equation: 1 1 1 1 1 = + = + j jk jl;c jk 0:62nFAC0 D02=3 n

1=6 u1=2

;

where j is the apparent current density (extracted from the polarization curve under different applied potentials directly) and jl,c is the diffusion-limited current density (usually the highest current density under relatively negative potentials). Thus, the jl,c is determined by the average number of electron transferred during ORR (n), the faradic constant (F), the geometric area of electrode (A), the concentration of dissolved O2 in catalysis solution (C0*), the diffusion coefficient of O2 (D0), the kinetic of viscosity of the solution (n), and the RDE rotation speed (u).5 Based on a series of polarization curves under different rotation speeds and measurements of jl,c, the Levich-Koutechy equation allows us to extract jk (under 0.9 V versus RHE or other specific potentials), which can be further conversed into specific activity and mass activity by normalizing with ECSA and Pt (or other metal) loading. The catalytic capability of a material is generally evaluated by these two parameters. In addition, the half-wave potential (E1/2), which is the required potential to achieve a current that is half that of jl,c, is also widely used to describe the catalytic performance (Figure 1C). A higher E1/2 signifies a lower required overpotential to achieve 1/2 jl,c and thus reflects a higher catalytic activity. It is also noteworthy that the mass activity, especially for Pt-based catalysts, may be more significant to evaluate the ORR activity in consideration of the expensiveness of Pt. Researchers usually benchmark obtained ORR performance against the targets set by the United States Department of Energy (DOE). In these targets, a mass activity of 0.44 A mgPGM–1 (PGM: precious group metal) at 0.900 V in a PEMFC should be achieved before 2020.28

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RECENT METAL CATALYSTS FOR ORR Pt-Based Catalysts for ORR Facet-Dependent and Size Effect of Pure Pt Nanostructure As a common strategy, the shape and size control has been widely used in numerous catalytic systems to improve the catalytic performance of nanocatalysts. The ORR catalytic performance of different Pt facets, especially low-index planes (planes with low Miller indices), has been extensively studied. In strongly adsorbed electrolyte (such as H2SO4 solution), in which the strong adsorption of anion deactivates the Pt (111) surface dramatically, the ORR activity follows an order of Pt (111) < (100). While in weakly adsorbed electrolyte (such as HClO4 solution), the ORR active increases in the order of Pt (100) << Pt (110) z Pt (111).27 Several high-index planes (planes with high Miller indices) have been demonstrated to exhibit higher ORR catalytic activity. For example, the Xia group prepared Pt concave nanocubes by a synthetic method with Br as a capping agent to hinder the growth of the <100> axis.29 With the help of high-resolution transmission electron microscopy (HRTEM), these nanocubes were confirmed to be enclosed mainly by {720} as well as {510} and {830} facets. A substantially enhanced ORR catalytic activity was observed on these Pt nanocubes. These concave Pt nanocubes exhibited approximately three and two times the specific activity at 0.90 V of the Pt cubes and cuboctahedra (bounded by low-index facets) in ORR, respectively. Unfortunately, despite the achieved high specific activity, the mass activity of Pt concave nanocubes is unsatisfying mainly caused by the relatively large size (>15 nm). In addition, Pt NPs exposing high-index facets with at least one Miller index being larger than 1 have also been found to show relatively high ORR activity; these include but are not limited to tetrahexahedron (hk0), trapexezohedron (hkk), and trisoctahedron (hhk). The enhanced ORR activity is generally ascribed to dense surface steps, edges, and kinks on high-index facets.5 The main obstacle still lies in the difficulty in stabilization of these thermodynamically unstable shapes during long-term ORR catalysis operation in a more practical and large-scale synthesis. Generally, the activity of heterogeneous catalysts largely depends on the size of the metal particles. Reducing the particle sizes may boost the catalytic performance for multiple reasons. Cutting bulk materials into NPs brings a considerable portion of formerly inner atoms to surfaces where the catalysis reaction occurs. The smaller the NPs are the larger surface ration is. Meanwhile, small particles are more likely to possess dense low-coordinated species such as steps, edges, and kinks, which are more capable of achieving high catalytic performance because of the high surface free energy. Additionally, size reduction enhances metal-support interactions, which may rearrange the electronic structure of metal species and further promote the catalytic process. Thus, size reduction is widely regarded as one of the most effective strategies for improving atomic utilization and catalytic activity. Size effects of Pt in ORR have been deeply explored over the last several decades.4,5 There is now substantial theoretical and experimental research showing that the Pt mass activity is optimized when the size of particles is reduced into the range of 2–5 nm.30 When the size is further reduced, the mass activity is found to decrease with the decreasing of particle size. The reason for this unexpected phenomenon, however, is still in dispute.5 Being blocked by surface atoms, the interior atoms in NPs can be hardly collided by reactants and hence rarely contribute to catalytic activity directly. Therefore, constructing hollow Pt NPs by removing the interior Pt atoms in solid NPs may retain the original catalytic activity and decrease the Pt usage simultaneously. Additionally, a hollow particle offers two sides, the inner surface and outer surface, which may

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further improve the atom utilization. By a template-removal method, the Adzic group prepared hollow NPs, which were further found to exhibit higher ORR catalytic activity compared with solid Pt NPs with similar sizes.31 The enhanced catalytic performance, after careful investigation with experiments and density functional theory (DFT) calculation, was ascribed to lattice contraction induced by the hollow structure. A compressive strain is usually regarded to shift the Pt d-band center downward, weaken the adsorption of strongly adsorbed oxygenated intermediates, and finally improve the ORR catalytic activity.32,33 Further, Zhang et al. prepared Pt nanocages with sub-nanometer thickness and investigated their ORR catalytic performance.34 They first prepared Pd nanocubes with an edge length of ca. 18 nm and then deposited four atomic layers of Pt on the surface of Pd cubes by reducing Pt salt at 200 C. Finally, the Pd cube templates were selectively removed, leaving Pt cubic nanocages covered by {100} facets (Figure 1D). This method also allows them to prepare Pt octahedral nanocages covered by {111} facets by using Pd octahedral templates. The specific activity and the mass activity on octahedral nanocages were found to be five and eight times higher than those of commercial Pt/C, respectively (Figure 1E). Unlike previous research, this synthetic method provides the possibility of synthesizing Pt hollow structure with specific facets. He et al. prepared icosahedral Pt nanocages via a similar synthetic method, which also achieved outstanding ORR performance.35 Benefiting from the large surface area and relative better stability than NPs and nanowire, one-dimensional nanocrystal is considered an ideal structure to exhibit outstanding performance in electrocatalysis. Liang et al. prepared a free-standing Pt nanowires membrane through a multistep templating route.36 As expected, the Pt nanowires exhibited both higher mass activity of ca. 0.016 A/mgPt and better durability than the bulk Pt catalyst and commercial Pt/C. The Duan group prepared ultrafine jagged Pt nanowires with an average diameter of ca. 2 nm via a synthetic method with thermal annealing and electrochemical dealloying (Figure 1F).8 Such Pt nanowires exhibited an extremely high ORR catalytic activity with E1/2 of 0.935 V versus RHE (more positive than that of Pt/C by ca. 75 mV), a mass activity as high as 13.6 A/mgPt, and an excellent long-term durability. The enhanced activity is ascribed to the atomic stress as well as ORR-favorable rhombus-configuration on the surface (Figures 1F–1H). Alloying and Doping Tremendous works have demonstrated that alloying two or more metals may empower catalysts with unique properties. As early as 1993, scientists had already realized that the kinetics of ORR could be easily enhanced by at least three times by simply alloying Pt with transition metals, such as Ni, Co, and Mn.37 The atomic and electronic structures of Pt, as generally believed, can be improved by alloying to boost the ORR performance. Further, Markovic et al. systematically investigated the function of alloying metals by DFT calculation.38 The specific activities of PtM alloys were found to display a volcano-type relationship with the d-band center (as we discussed before, the d-band center can sever as a descriptor in ORR), elucidating that very strong and very weak oxygen-intermediate adsorption will limit the reaction rate by the removal of surface oxides and electron and proton transfer to adsorbed O2, respectively (Figure 2A). It is also noteworthy that Pt3Co, Pt3Ni, and Pt3Fe alloys dominate the top of the volcano and thus are supposed to exhibit higher ORR activity than that of other PtM alloys or pure Pt materials. Inspired by the above conclusions, scientists have devoted further efforts to tuning diverse PtM alloys into satisfactory ORR catalysts by modulating the component, shape, and size of PtM nanostructures.5,10,39,40 For example, the Yang group prepared Pt3Ni truncated-octahedral

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Figure 2. Investigation of PtM Alloys and Their ORR Performances (A) Volcano-like relationships between the catalytic properties and electronic structure of Pt 3 M alloys. Reprinted with permission from Stamenkovic et al. 38 Copyright 2007 Springer Nature. (B) Mass and specific activities of the icosahedral and octahedral Pt 3 Ni nanocrystal and Pt reference catalysts at 0.9 V versus RHE. Reprinted with permission from Wu et al. 42 Copyright 2012 American Chemical Society. (C) Atomic structures of (i) icosahedral Pt cluster with 309 atoms and (ii) octahedral Pt cluster with 146 atoms. A different color means a different coordination number. Surface strain fields of Pt (iii) icosahedral and (iv) octahedral nanocrystals with a diameter of 10 nm. Color indicates strain labeled in the color map. Reprinted with permission from Wu et al. 42 Copyright 2012 American Chemical Society. (D) (i) SXS spectrum and metal concentration profile extracted from SXS data; (ii) CV measurements of Pt3 Ni(111) and Pt(111); (iii) surface coverage extracted from CV in (ii) and polarization curves from rotating ring disk electrode (RRDE) test; (iv) green section represents H 2 O 2 production and ORR polarization curves showing that Pt 3 Ni(111) can achieve a more positive half-wave potential than that of polycrystalline Pt by ca. 100 mV. Reprinted with permission from Stamenkovic et al. 43 Copyright 2007 AAAS. (E) Influence of the surface morphology and electronic surface properties on ORR kinetics. Reprinted with permission from Stamenkovic et al.43 Copyright 2007 AAAS.

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Figure 2. Continued (F) The average site occupancies of the second layer of (i) the Ni1175 Pt 3398 nanocrystal and (ii) the Mo 73 Ni 1143 Pt 3357 nanocrystal, indicated by (iii) color map. (iv) Binding energies for an oxygen atom on the Mo6 Ni41 Pt 178 (111) surface, relative to the lowest binding energy. Gray spheres represent Pt atoms, and colored ones represent oxygen sites. Binding energy on the fcc site of Pt(111) surface and Pt3Ni(111) surface and the peak of Sabatier volcano are labeled for comparison. (v) The change in binding energies when a Ni 47 Pt 178 nanocrystal is transformed into a Mo 6 Ni 41 Pt 178 nanocrystal. Reprinted with permission from Huang et al. 44 Copyright 2015 AAAS.

nanocatalysts that dominantly expose {111} facets.41 The particles were found to exhibit a 4- and 1.8-fold higher ORR mass activity than commercial Pt/C and normal octahedral Pt3Ni particles, respectively. They then raised a synthetic strategy of preparation of uniform icosahedral nanocrystals of other PtM (M = Au, Ni, and Pd) alloys. Their investigation showed that ORR catalytic activity can be further enhanced by using icosahedral Pt3Ni as a catalyst (Figure 2B).42 Given that both octahedral and icosahedral Pt3Ni nanocrystals are bound by {111} facets, they speculated that the enhanced ORR performance may be ascribed to elastic strain. By using molecular dynamics simulations, they found a tensile surface strain on icosahedral particles but a compressive surface strain on the octahedral ones. These surface-strain differences, as they claimed, serve a vital role in the regulation of electronic structure of surface atoms and thus explain the enhanced ORR performance (Figure 2C). Since catalytic performance is sensitive to the surface structure, an in-depth investigation of the component, atomic, and electronic structure on the surface of the catalyst is of great concern. The Markovic group performed surface-sensitive techniques, including lowenergy electron diffraction, low-energy ion scattering, Auger electron spectroscopy, surface X-ray scattering (SXS), and synchrotron-based high-resolution ultraviolet photoemission spectroscopy,43 to investigate the Pt3Ni alloy surface. An oscillating structure of Pt3Ni, as shown in Figure 2D, was then proposed: the outermost layer is composed exclusively of Pt; the second layer is Ni enriched (52% of Ni content is larger than 25% of Ni content the bulk), and the third layer possesses a Pt-rich feature (87%). They also found that such a ‘‘segregation’’ surface structure is stable under a potential range from 0.05 to 1.00 V versus RHE, which was confirmed by in situ SXS measurements. With an unambiguous surface structure in mind, the d-band center and specific activities of different Pt3Ni surface morphology were investigated and summarized (Figure 2E), indicating the superior ORR catalytic activity of Pt3Ni (111). On the basis of the above achievement, the Huang group made a successful attempt to optimize ORR activity of Pt3Ni (111) by a surface-doping strategy.44 They tried a series of transition metals and found that Mo could improve the ORR catalytic performance the best by acting as a doping metal. Their DFT calculation suggests that Mo-doping increases the oxygen-binding energies of the center sites on (111) facet, explaining the enhanced ORR activity (Figure 2F). Another instructive inspiration by the Markovic’s research is that PtM alloy particles may possess a compositional-segregation feature and thus exhibit a different property with a different configuration. Two representatives by the Strasser group have promoted our understanding of segregation on shaped alloy nanocatalysts. In 2013, they followed the morphological and compositional evolution of three octahedral PtxNi1 x alloy NP under electrochemical conditions by employing aberration-corrected scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy.45 A leach in their facet centers and concave octahedral structure were observed. This dealloying and morphological evolution implies the complicacy with shape-selective alloying catalysts under operating conditions. In the following year, they did an intensive study on the element-specific anisotropic growth and degradation of PtM alloy nano-octahedra.46 The results, summarized in Figure 3A, forebode that a further enhancement in ORR activity may be achieved by rational synthesis of Pt alloy ORR electrocatalysts.

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Figure 3. Evolutions of Pt-Ni Alloys and Preparation of Pt-Skin Catalysts (A and B) HRTEM image of Pd-Pt nanodendrite (A) and ORR polarization curves for Pd-Pt nanodendrites, commercial Pt/C catalysts, and Pt black at room temperature and 60  C in O2 -saturated 0.1 M HClO 4 solution (B). Reprinted with permission from Lim et al. 48 Copyright 2009 AAAS. (C) Kinetic current densities, before and after 10,000 cycle test, recorded at 0.9 V on a variety of polycrystalline Pt5M catalysts, versus lattice parameter. Reprinted with permission from Escudero-Escribano et al. 50 Copyright 2016 AAAS. (D) Models and TEM images of PtNi 3 polyhedra, PtNi intermediates, hollow Pt3 Ni nanoframes, and Annealed Pt 3 Ni nanoframes with Pt(111)-skin-like surfaces dispersed carbon. Reprinted with permission from Chen et al. 10 Copyright 2014 AAAS. (E) TEM images of Pd cubes coated by Pt skin with different thickness. Reprinted with permission from Xie et al. 52 Copyright 2014 American Chemical Society. (F–H) An in-plate-view HAADF-STEM image of PtPd/Pt core/shell nanoplates (F), schematic models showing the top interface [(110)Pt//(100)PtPb] and the side interface [(110)Pt//(001)PtPb] (G), and ORR polarization curves and CVs (inset) of PtPb nanoplates, PtPb NPs, and commercial Pt/C catalysts (H). Reprinted with permission from Bu et al.54 Copyright 2016 AAAS.

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Besides the Pt-Ni system, scientists have also investigated other PtM alloy or bimetallic systems. The Adzic group prepared a Pt/C catalyst stabilized by Au nanoclusters via an under-potential deposition method.47 With in situ X-ray absorption near-edge spectroscopy and voltammetry data, the authors claimed that Au clusters raise the Pt oxidation potential and thus confer the stability during ORR operation. By reducing Pt salt with Pd nanoseeds, the Xia group prepared a Pd-Pt bimetallic nanostructure with Pt branches on a Pd core, which was found to exhibit high ORR activity at both room temperature and 60 C (Figures 3A and 3B).48 The improved ORR performance was ascribed to the large surface areas and high-index facets exposure of Pt branches. Greeley et al. investigated ORR performance on alloys of Pt and early transition metals such as Y and Sc.49 Particularly, polycrystalline Pt3Y showed higher ORR activity than pure Pt by a factor of 6–10. The Chorkendorff group synthesized eight Pt-lanthanide and Pt-alkaline earth catalysts and investigated their performance in ORR.50 A volcano-like relationship between the ORR catalytic activity and the bulk lattice parameter was demonstrated (Figure 3C). Thus, the lanthanide contraction was indicated to be capable of modulating strain effects and enhancing the activity and durability of Pt-based catalysts in ORR. Bu et al. prepared hierarchical Pt-Co nanowires bounded by high-index and Pt-rich facets, achieving an ORR mass activity approximately 30 times higher than that of commercial Pt/C catalysts.40 Pt-Skin Structure Constructing Pt-skin structure is also regarded as one of the promising strategies for simultaneously increasing ORR activity and lowering the usage of Pt. The performance of the Pt-skin may be regulated by controlling the composition, size, and shape of intimal metal. Here, we review some representative examples of Pt-skin catalysts with superior ORR catalytic activity. As we discussed in the above section, the component and morphology of Pt-Ni alloys can be protean during growth and catalysis process. Chen et al. prepared PtNi3 polyhedra with uniform morphology and size in oleylamine.10 Then, they dispersed the oleylamine-capped PtNi3 polyhedra in nonpolar solvents (e.g., hexane and chloroform) and kept this dispersion at room temperature for 2 weeks. The PiNi3 polyhedra were found to gradually transform into Pt3Ni nanoframes (Figure 3D). The obtained Pt3Ni nanoframes were then dispersed onto a carbon support and heated to ca. 400 C in protective argon gas. Most Pt3Ni nanoframes evolved into smooth nanoframes with Pt-skin surface (denoted as Pt3Ni nanoframe/C). Compared with commercial Pt/C catalyst, the Pt3Ni nanoframes/C delivered a factor of 36 and 22 enhancement in mass activity and in specific activity, respectively. The open structure, the sufficient exposure of Pt(111), and the surface strain of Pt atoms were demonstrated to contribute to the enhanced ORR performance. Niu and co-workers synthesized Pt–Ni rhombic dodecahedra (RD) at a lower synthetic temperature than typically reported.51 This lower synthetic temperature allows them to track the growth process during the prolonged synthetic period of time. They found the synthetic solution would turn from green to yellow, brown, and black in about 60 min. They collected the products at 3, 10, and 30 min after the solution turned black (denoted as RD-3, RD-10, and RD-30, respectively) and further selectively removed the Ni-rich phase within the collected products by chemical corrosion (denoted as RD-3-cor, RD-10-cor, and RD-30-cor, respectively). Thus, the morphology and dispersion of the Pt-rich phase were tracked. The frame-like morphology of RD-30-cor indicates the Pt migration from internal to external during Pt-Ni alloy growth. The RD-30-cor was also found to exhibit higher ORR activity than RD-3-cor, RD-10-cor, and commercial Pt/C catalyst. The Xia group coated Pd nanocubes with conformal thickness-controllable Pt shell by regulating the injection rate of Pt precursor and synthetic temperature.52 The thickness of the Pt shell could be adjusted to 1, 2, 3, 4, or 6 atomic layers (Figure 3E). All four

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materials showed higher ORR activity than Pt/C, and as expected, the nanocube with one Pt atomic layer delivered the highest mass activity. Hunt et al. demonstrated a self-assembly synthetic approach that allows them to control the particle size, surface Pt layer coverage, and heterometallic composition of the final Pt-M core-shell architecture.53 This synthetic method can be expanded to prepare a variety of core-shell systems. Even though the authors did not investigate ORR performance of the final core-shell particles, this method provides us with a golden opportunity to prepare Pt-skin catalysts with a controllable internal component, Pt-skin thickness and coverage, and doping by other novel metals. Recently, the Huang group reported PtPb/Pt core/shell nanoplates that exhibited superb ORR activity (Figures 3F and 3G).54 At 0.9 V versus RHE, such a catalyst achieved a specific activity of 7.8 mA cm–2 and a mass activity of 4.3 A/gpt (Figure 3H). As revealed by DFT calculation, the edge-Pt and top Pt (110) facets were under large tensile strains, explaining the optimized Pt–O bond strength and the final enhanced ORR performance. Pd, iridium, ruthenium, and other novel metal-based materials have also been studied as ORR catalysts.4 Despite tremendous efforts to optimize their performance, the best performance by them is barely equivalent to that of commercial Pt/C catalysts. In consideration of their expensiveness, it makes little sense to substitute Pt with these materials. Nonprecious-Metal-Based Catalysts for ORR Despite the high ORR activity, the cost and durability issues surrounding the Pt-based catalysts severely hamper their widespread commercialization.2,5 The development of low-cost and reliable ORR catalysts is now of great concern. In living organisms, several specific enzymes (e.g., cytochrome C oxidase and laccase) activate oxygen molecules into an electron acceptor that captures the electrons from fuels and thus supplies the energy. Although these enzymes feature nonprecious metals as a catalytic active site, they have demonstrated remarkably reduced overpotential for the oxygen reduction compared with man-made catalysts. This proves that a considerable improvement in ORR activity by nonprecious-metal catalysts is not an illegitimate target.12 During the past decades, various nonprecious-metalbased catalysts have been investigated for substituting expensive Pt-based catalysts in PEMFCs. Among them, nonprecious-metal-nitrogen-carbon composite (M-N-C), nonprecious-metal oxides, chalcogenides, and oxynitrides have been found to be potential candidates.4,12 Nonprecious M-N-C Catalysts The premier employment of M-N-C composite as ORR catalysts can be traced back to the research by Jasinski in 1964, in which Co phthalocyanine was found to exhibit ORR catalytic activity.55 After that, tremendous efforts have been devoted to the development of M-N-C ORR catalysts. Generally, there are three common strategies: (1) increasing the intrinsic activity by composition modulation, (2) confining metal species into carbon shells to preserve metal from corrosion (from acidic medium and oxidizing potentials), and (3) increasing the accessibility by constructing porous or other large specific-surface-area structures. Wu et al. prepared nonprecious-metal catalysts with Fe and Co confined in multiple C–N shells by using polyaniline (PANI) as a carbon-nitrogen precursor.56 Their best catalyst showed further improved ORR activity that was closer to that of commercial Pt/C (Figure 4A) and an enhanced durability in both long-term RDE and PEMFC tests, which was most likely generated from the protection of C–N shells. In 2009, the Dodelet group reported Fe-based catalysts with enhanced ORR activity.57 A mixture of ferrous acetate, carbon black, and phenanthroline was pyrolyzed in argon

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Figure 4. Representative M-N-C Catalysts (A) H 2 -O2 PEMFC performance of (1) metal-free PANI-C, (2) PANI-Co-C, (3) PANI-FeCo-C (1), (4) PANI-FeCo-C (2), (5) PANI-Fe-C, and Pt-based catalyst (gray dash line). Reprinted with permission from Wu et al. 56 Copyright 2016 AAAS. (B) Power density curves of as-prepared catalyst (blue stars), a catalyst reported previously (red circles) and Pt/C reference (green squares). Reprinted with permission from Proietti et al. 58 Copyright 2011 Springer Nature. (C) Volumetric activity (hollow stars or circles) and extrapolated Tafel plots (dash line) of as-prepared catalyst (blue) and a catalyst reported previously (red). The solid gray circle and star represent US DOE volumetric activity targets for the years 2010 and 2015, respectively. Reprinted with permission from Proietti et al. 58 Copyright 2011 Springer Nature. (D–F) TEM images of encapsulated Fe nanoparticles (D), schematic illustration of the ORR at the surface of Fe4@SWNT (E), and projected density of state of the p orbitals of C atoms (F). The charge transfer in (F) insets II and III is represented by red for increasing and blue for deceasing. Reprinted with permission from Deng et al. 59 Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (G) TEM image model illustration of as-prepared porous catalyst with vitamin B12 as a cobalt precursor and ordered mesoporous silica SBA-15 as a template. Reprinted with permission from Liang et al. 62 Copyright 2013 American Chemical Society. (H) Schematic illustration of the assembled electrode and ORR pathways under different proton transfer rate. Reprinted with permission from Edmund et al. 63 Copyright 2016 Springer Nature.

and ammonia successively after ball-mill. The obtained material was applied in PEMFC as a cathode catalyst and was found to exhibit improved initial ORR activity, which is competitive in comparison with Pt-based catalysts but has unsatisfying stability. Later, this group prepared another Fe-based catalyst by using zeolitic imidazolate framework (ZIF)-8 as a microporous host for phenanthroline and ferrous acetate.58 Compared with the previous one, this catalyst achieved an obvious enhancement of ORR activity in H2-O2 PEMFC with a power density of 0.75 W cm 2 at 0.6 V (Figure 4B). They then converted their data into volumetric

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activity of cathode to compare the obtained activity with that of the US DOE’s targets. The extrapolation of the Tafel slope (at 0.8 V) demonstrated that the volumetric activity by their catalysts was close to the target proposed by the US DOE (300 A cm 3 for non-PGM-based catalysts; Figure 4C). Unfortunately, an apparent activity decay of 15% after 100 h of operation demonstrated the unqualified durability. The Bao group prepared pea-pod-like carbon nanotubes (CNTs) encapsulating Fe NPs (Figure 4D) and found that such a confinement structure can deliver an enhanced durability during long-term PEMFC test.59 They suggested that a charge transfer from the encapsulated Fe cluster to the carbon tube turn the carbon atoms near Fe cluster into ORR active species so that ORR can proceed without the immediate contact between O2 and Fe atoms (Figures 4E and 4F). Thus, the enhanced durability can be explained because the carbon shells may preserve the Fe clusters from acid corrosion. The ORR activity of the encapsulated Fe NPs, however, still leaves much to be desired. In a typical H2-O2 PEMFC, the encapsulated Fe NPs yielded a voltage of ca. 0.5 V at a current density of 0.10 A cm 2, which was only ca. 60% of that given by Pt/C. Hu et al. prepared Fe3C NPs encapsulated by graphitic layers through a high-pressure pyrolysis process. In both acidic and alkaline electrolyte, this catalyst showed remarkable stability.60 The outer graphitic layers were also believed to play a protective role in stabilizing inner particle under corrosive conditions. While in acidic electrolyte, the as-prepared catalysts only achieved E1/2 of ca. 0.73 V, largely underperforming Pt/C. Given that a larger specific surface area usually guarantees a greater accessibility, constructing porous structure is generally considered a remedy for the nonprecious-metal-based catalysts that possess relatively low intrinsic activity. The Qiao group developed CNTs with Fe–N decoration from hierarchically porous carbon.61 Such a material was believed to possess desired merits that included high activity by Fe–N species, facile transportation from large pores, and adequate active-site exposure from large surface area. As expected, an improved ORR performance comparable to that of Pt/C was achieved in alkaline solution. Liang and co-workers prepared a series of mesoporous nonprecious-metal catalysts. Among them, a Co-based mesoporous catalyst, which is fabricated with vitamin B12 as the Co precursor and ordered mesoporous silica SBA-15 as the template, possesses the largest surface area of 568 m2 g 1 (Figure 4G).62 This mesoporous catalyst showed an outstanding ORR performance in acidic solution with E1/2 of 0.79 V versus RHE, an electron-transfer number of ca.3.95, and as excellent durability. The large surface area and the homogeneous distribution of abundant Co–Nx were claimed to contribute to the ORR performance. Selectivity is another noteworthy issue existing in ORR on nonprecious-metal catalysts. Several works have demonstrated the non-negligible and undesired side products, such as H2O2 and O2 , in the ORR process on nonprecious-metal catalysts. Recently, the Gewirth group presented their idea that the kinetics of proton transport in the ORR catalysts, to some extent, determine the product distributions.63 They fabricated a hybrid bilayer membrane with Au electrode modified with a selfassembled monolayer of Cu-based ORR catalyst and a monolayer of lipid consisting of proton carrier. Such a hybrid bilayer membrane allowed them to quantitatively regulate kinetics of proton transport to the catalyst by modulating the amount of proton carrier in lipid. As a result, an insufficient proton carrier results in proton transport that is too slow, and thus O2 would be reduced by 1 e to O2 ; an excessive proton carrier results in proton transport that is too fast, and the ORR process would undergo a 2 e pathway with H2O2 as the products (Figure 4H). Thus, a mismatch between proton and electron transport causes unfavorable products

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(O2 or H2O2), and a commensuration between proton transport and O–O bond breaking rate ensures 4 e ORR pathways with H2O as the product. Nonprecious Metal Oxides, Chalcogenides, and Oxynitrides Several nonprecious-metal oxides have also been found to be catalytically active for ORR. Inspired by the biological catalyst in photosystem, the Jaramillo group synthesized Mn oxide thin film through an electro-deposition method.64 This Mn oxide exhibited E1/2 of 0.73 V, which is more negative than that of Pt/C by 130 mV in alkaline solution. Later, Cheng et al. found that the ORR activity on MnO2 can be improved by introducing oxygen deficiency generated by high-temperature treatment in air or argon.65 A modified surface-oxygen interaction and a reduced reaction barrier by oxygen deficiency, as suggested by DFT calculation, were claimed to contribute to ORR performance. Recently, the Dong group reported free-standing tubular monolayer superlattices of hollow Mn3O4 nanocrystal (h-Mn3O4-TMSLs).66 The obtained h-Mn3O4-TMSLs delivered outstanding ORR performance in alkaline solution with an onset potential of ca. 0.91 V and E1/2 of 0.84 V (versus RHE), which is about 10 mV more negative than that of Pt/C. The mesoscale tubular geometry was believed to enhance mass transport, and the monolayer superlattice structure may be beneficial for molecular accessibility. The Dai group grew Co3O4 nanocrystals on reduced graphene oxide (RGO) and found such a hybrid material can sever as an efficient ORR catalyst in alkaline solution.67 The E1/2 by this hybrid material was ca. 0.83 V versus RHE, similar to that of Pt/C. Because neither Co3O4 nor graphene could hardly catalyze oxygen reduction, the enhanced ORR performance was ascribed to the synergetic chemical coupling effects of the hybrid structure. They further prepared a cobalt-oxide-carbon-nanotube hybrid, which showed an ORR onset potential of 0.93 V in 1 M KOH solution.68 This hybrid material was also found to be active and stable in 10 M NaOH at 80 C. Several perovskite materials are also active for ORR. The Shao-Horn group investigated ORR activity of a series of perovskites and found that the ORR activity correlates to s*-orbital occupation and the extent of covalency between B-site metal and oxygen.69 The Dai group fabricated Co1–xS-RGO hybrid material through a solution-phase process followed by solid-state annealing treatment.70 This hybrid material showed an active ORR performance with onset potential of 0.87 V versus RHE. The small size of Co1–xS NPs and the strong electrochemical coupling between RGO and Co1–xS NPs were believed to promote the ORR performance. Recently, the Xu group constructed honeycomb-like porous carbons with nitrogen and sulfur dual doping and Co9S8 NPs immobilized inside.71 They investigated the ORR activity of a series of such materials after calcination under different temperatures. The best catalyst exhibited an ORR performance with an onset potential of 0.05 V and E1/2 of 0.17 V versus Ag/AgCl. The sufficient accessibility from a honeycomb-like structure and the synergetic interactions between Co9S8 particles and the support may explain the enhanced ORR performance. Cao and co-workers prepared CoxMo1–xOyNz supported on carbon, which exhibited onset potentials of 0.918 and 0.645 V in 0.1 M KOH and 0.1 M HClO4, respectively.72 Other nonprecious compounds such as MnOOH, TiO2, NbO2, ZrOxNy, TaOxNy, and CoSe2 have also been widely investigated.4,5 In addition, metal-free catalysts, especially N-doped carbon materials, have been reported active for ORR.73,74 Unfortunately, these current nonprecious-metal-based and metal-free catalysts still leave much to be desired.5 In particular, because acidic PEMFC is currently much more prevalent than alkaline counterpart, the susceptibility to acid makes

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nonprecious-metal-based catalysts, especially the oxides, suffer from poor durability in acid medium and limits their widespread adoption in PEMFCs.12,13

RECENT ADVANCES IN SACs FOR ORR With the development of highly advanced characterization techniques, there is growing awareness that the single metal sites with M–N coordination in nonprecious-nitrogen-carbon composite, especially the porphyrin-like FeN4C12 moieties, may be capable of catalyzing the 4 e– pathway reduction of oxygen to water. In one important work reported by Zitolo et al., Fe–N–C catalysts quasi-free of crystallographic Fe were synthesized by a thermal treatment in either Ar or NH3.75 These catalysts had the same Fe-centered moieties but a much higher activity and basicity for NH3-treated Fe–N–C. After a detailed XANES study, two modes of FeN4 porphyrinic different O2 adsorption were identified. The authors noticed that it was difficult to integrate the porphyrinic moieties into graphene sheets, in sharp contrast to Fe-centered species assumed for pyrolyzed Fe–N–C. Such findings not only enlighten bottom-up synthesis strategies of M–N–C SACs but also underline the crucial role of the interaction between single metal atoms and support in ORR catalysis. Meanwhile, SACs have been widely assumed to possess high catalytic activity because of the minimum size of metal species and unique coordination structure. A large number of studies have demonstrated that SACs can exhibit distinctive catalytic performance for a wide variety of catalytic systems. Especially given that SACs may achieve an atomic economy of 100% atom utilization, SACs are logically considered potential ORR catalysts. Though much effort has been devoted, there are still hurdles to overcome before SACs become qualified ORR catalysts in practice. As practical ORR in PEMFC requires high catalytic activity in given geometric area, one challenge is how to improve the metal loading in SACs (typically below 1 wt % for the majority of SACs) or how to promote the accessibility of metal sites to increasing the catalytic activity per given area. Certainly, another challenge is how to improve the intrinsic activity of individual active metal sites. Correspondingly, two strategies are generally used to optimize the ORR performance of SACs. One is to create appropriate supports with a larger specific area to anchor or expose more single-metal sites, with the aim to provide more catalytically active sites. The second one is to modulate the electronic structure of metal sites by tuning the coordination environment or doping heteroatoms with the aim of optimizing the intrinsic catalytic activities. Here, we summarize recent advances in SACs for ORR catalysis and demonstrate how scientists attempt to overcome the abovementioned issues by constructing rational supports, regulating local coordination environment over single metal sites, or doping heteroatoms. Support Construction As we discussed before, an ideal support ought to provide dense coordinative sites to anchor sufficient isolated metal atoms, as well as large specific surface areas or porous structures for superior accessibility. Porous ZIFs with intrinsically isolated metal nodes and N-contained organic ligands are thus considered a potential precursor for SACs. In 2016, Yin et al. originally reported a facile and effective approach to prepare Co SAC with an extremely high metal loading over 4 wt % via thermal treatment of bimetallic Zn/Co MOFs (Figure 5A).20 One significant breakthrough of this work is the elegant use of Zn-Co ZIF for isolating single-metal atoms. Another noteworthy contribution is the greatly enhanced metal loading of the SACs prepared by this approach, highlighting the future direction for practical applications. After

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Figure 5. Formation Scheme of Representative SACs with Different Supports (A) Schematic illustration of the formation of Co SAC by pyrolysis of Co/Zn bimetallic ZIF. Reprinted with permission from Yin et al. 20 Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (B) Schematic illustration of the formation of Mn SAC. Reprinted with permission from Li et al. 77 Copyright 2018 Springer Nature. (C) Schematic illustration of the formation of Co isolated atoms supported on N-doped spherical carbon. Reprinted with permission from Han et al. 82 Copyright 2017 American Chemical Society. (D) Schematic illustration of the formation of hollow and porous Fe SAC through Kirkendall effect process. Reprinted with permission from Chen et al.83 Copyright 2018 Springer Nature. (E) Schematic illustration of the formation of Co isolated atoms supported on N-doped graphitic carbons. Reprinted with permission from Yang et al. 85 Copyright 2018 National Academy of Sciences. (F) Schematic illustration of the formation Co SAC derived from cattle bone. Reprinted with permission from Edmund et al. 87 Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

this initial disclosure, a variety of protocols for the synthesis of SACs via ZIFs have been reported. For example, Wu and Shao reported a chemical replacement method for constructing Fe-doped ZIF precursors by using an ionic exchange method.76 In this ZIF system, Fe ions can partially substitute Zn ions and bond with imidazolate ligands in three-dimensional frameworks, forming FeN4 complexes. The as-prepared Fe catalyst showed an outstanding ORR activity in acidic media, with E1/2 of 0.85 V, as well as an enhanced stability with a decay of only 20 mV in E1/2 after 10,000-cycle operation. Recently, Li et al. reported a two-step synthetic method to prepare Mn-based SACs with highly dense MnN4 sites. In the first step, a partially graphitized carbon host was prepared by carbonizing Mndoped ZIF-8 precursors. In the second step, additional Mn and N sources were then adsorbed into the obtained microporous carbon host, followed by a thermal activation, to increase the density of Mn sites. Measured by inductively coupled plasma mass spectrometry (ICP-MS), the Mn content was found to be more than 3 wt %. In 0.5 M H2SO4 electrolytes, this catalyst delivered an ORR performance with E1/2 of 0.80 V and encouraging durability (Figure 5B).77 In 2017, Li et al.

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developed a cage-encapsulated-precursor pyrolysis strategy to generate a highly stable isolated Fe SAC to endow excellent ORR performance.78 In this case, ZIF-8 was used as a molecular-scale cage to isolate and encapsulate the metal precursor Fe(acac)3 because of its special pore size and cavity. After pyrolysis treatment, the ZIF-8 was converted into nitrogen-doped porous carbon, and the Fe(acac)3 was reduced, forming isolated Fe sites anchored on N species. This SAC had a high Fe loading of 2.16 wt % and exhibited a highly efficient activity toward ORR in alkaline media with an E1/2 of 0.900 V and an exceptional Jk of 37.83 mA cm o at 0.85 V, superior to that of commercial Pt/C. To improve the electron conductivity, the Lin group employed surface-functionalized multiwalled CNTs as a template during the synthesis of Fe-Zn bimetallic ZIFs. After the pyrolysis process, a network of Ndoped carbon with atomically dispersed Fe atoms was prepared. The as-prepared Fe SAC gave an ORR performance with E1/2 of 0.81 V in 0.1 M HClO4, and a power density of 620 mW cm 2 in H2-O2 PEMFC.79 The use of polymers for the synthesis of SACs has also been explored, as the isolated metal atoms can be effectively stabilized by a coordination effect with the nitrogen atoms of N-doped carbon supports In 2018, Li group has offered a polymer encapsulation strategy to prepare SACs supported by porous nitrogen-doped carbon nanospheres.80 In brief, metal precursors were initially encapsulated in polymers by simply mixing the metal acetylacetonate complexes with monomers during the polymerization process. Then, a pyrolysis process was performed at a high temperature to give the polymer-derived porous nitrogen-doped carbon nanospheres with single metal atoms dispersed uniformly. This approach was found to be applicable to both noble and nonprecious metals. They noticed that the Co SAC showed the best performance among other SACs prepared by this approach and delivered a comparable ORR activity (E1/2 = 0.838 V) and Jk at 0.83 V to Pt/C in alkaline media, a good methanol tolerance, and an exceptional cycling stability even after 5,000 cycles. Similarly, a metal-organic polymer supramolecule strategy was introduced by Li and Guo for the construction of a SAC by ‘‘self-locking’’ between metal ions and a natural polysaccharide, sodium alginate (SA).81 SA has a great number of hydrophilic groups (–COOH and –OH) in a-L-guluronic (G-block) and b-d-mannuronic (M-block) acid units. It can bond with Fe ions to form a hydrogel, which leads to the formation of atomically dispersed Fe-Nx sites in highly porous, sheet-like structures. The resulting Fe SAC exhibited excellent ORR performance in 0.5 M H2SO4 and 0.1 M KOH, along with an admirable durability. Hollow and two-dimensional materials, as a result of high accessibility, are also considered efficient supports for SACs. The Li group reported an interesting template-assisted pyrolysis treatment to access Co SAC dispersed on hollow N-doped carbon spheres (Figure 5C).82 The single Co sites and the hollow carbon spheres collectively contributed to the excellent ORR performance in acid media. Later, they fabricated N-, P-, and S-co-doped hollow polyhedron with embedded single Fe atoms through the Kirkendall effect process (Figure 5D).83 In 0.1 M KOH solution, this Fe SAC achieved an outstanding ORR performance with E1/2 of 0.912 V, Jk of 71.9 mV cm 2 at 0.85 V, and a record-level Tafel slope of 36 mV dec 1. In 2017, the Bao group described a simple method to prepare a highly dispersed single Fe catalyst by ball milling of iron phthalocyanine (FePc) and graphene nanosheets (GNs).84 The resultant FeN4/GN showed a high ORR activity in alkaline electrolyte, similar to that of the commercial Pt/C. Importantly, it had a higher stability and resistance to SOx, NOx, and methanol than Pt/C. DFT calculations demonstrated the excellent ORR performance and stability result from the unsaturated Fe centers confined in the graphene nanosheets via four N atoms. Cao et al. introduced a

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surfactant-assisted approach for the preparation of an Fe SAC (Figure 5E) by pyrolyzing the layered-like precursor formed by Fe-loaded water-soluble surfactant F127 and g-C3N4.85 They found that the use of surfactant F127 enabled the uniform dispersion of Fe atoms to form Fe–Nx sites on the support. Additionally, the Fe-doped F127 sheets could strongly anchor on the g-C3N4 so that the Fe clusters could be easily removed by acid treatment. They demonstrated that the ORR activity of Fe SAC stemmed from the Fe-pyrrolic-N4 active sites. Its E1/2 was only 30 mV less than 20% Pt/C in acidic medium. For the H2-O2 PEMFC testing, it produced a current density of 0.85 A cm 2 at 0.6 V and 3.34 A cm 2 at 0.2 V and achieved the maximum power density of 823 mW cm 2. They attribute the good performance of the PEMFC to the accessible and high-density active sites on N-doped carbon nanosheets. Recently, Baek and co-workers synthesized a Cu SAC (up to 20.9 wt %) with isolated Cu atoms distributed in ultrathin nitrogenated carbon nanosheets.86 In their study, the use of L-glutamic was important for high Cu content in SAC because it could introduce additional N species during the synthesis process and effectively trap the metal atoms on the support. Moreover, dicyandiamide could react with the carboxylic acid groups in a trimesic acid to yield a two-dimensional sheet. Because of the synergetic effect between ultrathin nanosheet and high content Cu atoms, a favorable adsorption of O2 and OOH could be obtained. For the ORR testing, it showed over 54 times higher mass activity than Cu NPs at 0.85 V. In addition, it also delivered a lower Tafel slope (37 mV dec 1), higher methanol and carbon monoxide tolerance, and a longer-term stability than commercial Pt/C. The free energy diagrams of ORR pathway on CuN2 and CuN4 were further studied. They showed that under 0.4 V, the reaction on the CuN2 proceeded through a thermodynamically downslope route, implying an excellent electrochemical ORR activity. For CuN4, there was an upslope of 0.75 eV, meaning that the rate-determining factor is the relatively weak adsorption of O2 for ORR on CuN4. Several biomaterials are instinctively potential supports for SACs as a result of porosity. Dai and co-workers demonstrated that thermal treatment of unsubstituted phthalocyanine-FePc complexes within micropores of cattle bones can give atomically dispersed Fe atoms on hierarchically structured porous carbon frameworks (Figure 5F).87 The Fe SAC catalyst showed ORR performance comparable to that of the Pt/C in 0.1 M HClO4 (E 1/2 = 0.81 V) and an improved long-term durability (7 mV negative shift after 3,000 cycles). Under alkaline conditions, it outperformed the Pt/C in terms of activity (E 1/2 = 0.89 V) and long-term durability (1 mV negative shift after 3,000 cycles). Regulation of Electronic Structures Modulation of the electronic properties of the metal center has also been demonstrated to be an effective route for improving catalytic performance of M–CN catalysts.88 Generally, two approaches can affect the electronic properties of the active metal sites: one is regulating the center metal element, the species, and/or the number of the coordination atoms; another is using long-range interactions between metal sites and doped atoms on the support materials to adjust the electronic structures. For example, doping with heteroatoms (e.g., N, S, and P) also helps to tune the electronic structures to substantially improve the catalytic activity.5 Fe-based SACs are most commonly employed for ORR, and the catalytic properties are strongly dependent on the types of metal N2/N4 conformation. However, the problem of the existing FeNx-based catalysts is that the FeN4 is relatively stable sites, which might not be the most active sites based on theoretical predictions due to a strong interaction with O2* and OH*.89 In this regard, Guo and co-workers

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reported a viable template casting method to access isolated FeN2 species on N-doped ordered mesoporous carbon.90 One significant improvement of this approach is that the Fe precursor can be anchored on the surface of the template (SBA-15). DFT calculations show that the FeN2 outperforms FeN4 because of its lower interaction with OH* and O2* intermediates, along with improved electron transport. Interestingly, another work by Wang and co-workers demonstrated that FeN4 species can also be remarkably active for ORR by mini-trim on the atomic configurations of Fe-N–C moieties.91 The authors constructed atomically isolated Fe atoms on three-dimensional hierarchically porous carbon. In spite of merely 0.20 wt % of Fe metal loading, this SAC is highly efficient for ORR with E1/2 of 0.915 V in 0.1 M KOH, exceeding those of Pt/C (E1/2 = 0.85 V) and most M–N–C catalysts. Importantly, the atom-utilization efficiency in this study was superior to most previous reports. The experimental and DFT results demonstrate that the hierarchical carbon pores can effectively tune the electronic structure of FeN4 by modulating the local coordination of pyridine N. This leads to the selective cleavage of C–N bond near Fe centers, giving edge-hosted FeN4 moieties to lower the ORR barriers to obtain exceptional catalytic activity and durability (Figure 6A). Through modulating the electronic properties of the central metal with chlorine ions, the Li group originally developed a FeCl1N4 site catalyst (Fe loading 1.5 wt %) by a thermalmigrating method in 2018.92 The FeCl1N4/CNS showed a superior E1/2 of 0.921 V in 0.1 M KOH, 79 mV more positive than that of Pt/C (Figures 6B and 6C). Moreover, it had an excellent Jk of 41.11 mA cm‒2 at 0.85 V and an admirable cycling stability for 10,000 cycles, superior to those of most of the reported nonpreciousmetal electrocatalysts. DFT calculations were employed to investigate the effect of chlorine coordination and sulfur doping on the ORR. The catalyst showed a much lower overpotential of 0.44 V than FeN4/CN but a higher binding energy of O2 (Eb = 0.64 eV). They demonstrated a volcano curve showing the relationship between ORR overpotentials and O2 binding energies. That is, a higher O2 binding energy would lead to the difficulty in desorbing OH species, and weaker O2 binding may make the hydrogenation of O2* more sluggish. This suggests that the FeCl1N4/CNS has a moderate charge state and thus shows the highest ORR performance. The results reveal the near-range interaction with chlorine and the longrange interaction with sulfur of the Fe active sites contribute to the modulation of the Fe electronic structure. This work demonstrated the importance of modulation of electronic structure in the design and synthesis of SACs on their catalytic performance, an important future direction for this field. In another case, the Qiao group introduced a variety of graphitic carbon nitride (g-C3N4) coordinated transition metals (M–C3N4) for ORR. The g-C3N4 was used as an efficient support to construct a series of M–C3N4 electrocatalysts. They studied the Co–C3N4 in ORR and oxygen evolution reaction (OER) in alkaline media and found excellent activity stems from CoN2 in the g-C3N4 support, along with an optimal d-band of the catalyst.93 In addition to Fe, Co, Ni, and Mn SACs, carbides of groups have also been investigated as alternatives for precious electrocatalysts in PEMFCs. However, the low density and stability of these active sites greatly hampered their catalytic performance. To address the problem, Chisholm and co-workers originally prepared a new class of single niobium atoms-based carbide catalyst using an arc-discharge approach (Figure 6D).94 Single niobium atoms and ultra-small clusters stabilized in graphitic layers are identified as active sites for catalyzing the cathodic ORR. They found this unique structure greatly enhanced the conductivity to accelerate the exchange rate of ions and electrons, and strongly suppressed the chemical and thermal coarsening of the active species. Importantly, the single niobium atoms stabilized within the graphitic layers

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Figure 6. Representative Research on SAC and Pt-Co Catalyst for ORR (A) Five possible configurations with different cracking degrees (i). Free energy diagrams of ORR pathway under U = 0.77 V (ii) and U = 0.13 V (iii). Reprinted with permission from Jiang et al. 91 Copyright 2018 American Chemical Society. (B and C) ORR polarization curves of different catalysts recorded in O 2 -saturated 0.1 M KOH (B) and kinetic current densities at 0.85 V and half-wave potentials of different catalysts (C). Reprinted with permission from Han et al. 92 Copyright 2018 Royal Society of Chemistry. (D) Geometrical arrangement of single niobium site with adsorbed O 2 (i) and charge transfer from niobium atom (blue) into the O atoms (yellow). Reprinted with permission from Zhang et al. 94 Copyright 2013 Springer Nature. (E and F) Scanning electron microscope (SEM) image showing the porous structure of (CM+PANI)-Fe-C (E) and atomic-resolution HAADF-STEM image showing the atomically dispersed Fe atom in (CM+PANI)-Fe-C (F). Reprinted with permission from Chung et al. 98 Copyright 2017 AAAS. (G) Schematic illustration of the formation of (Fe,Co)/N-C. Reprinted with permission from Wang et al. 99 Copyright 2017 American Chemical Society. (H) Schematic of the preparation of Cu SACs from Cu bulk. Reprinted with permission from Qu et al. 100 Copyright 2018 Springer Nature. (I) Schematics of Pt-Co catalysts showing the coexistence of Pt-Co core shell NPs, Co NPs confined in graphene and Co-Nx -C y sites. Reprinted with permission from Chong et al. 102 Copyright 2018 AAAA.

can redistribute d-band electrons and therefore significantly facilitate O2 adsorption and dissociation. Heteroatom-doped M–N–C has also shown great potential in improving the ORR performance to substitute precious-metal-based catalysts.95 The S element can be

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easily introduced to the carbon support by thermal treatment of S-containing species. The relatively large atomic radius of p-block S element may create defects on the support, and the electronegativity of S may tune the electronic properties of M–N sites. In 2018, the Li group described an interesting pyrrole-thiophene copolymer pyrolysis strategy for constructing Fe SAC on S and N-codoped carbon with a loading of 0.947 wt %.96 The S and N contents could be changed by controlling the precursors. Interestingly, the catalytic efficiency of the SAC displayed a volcano-type curve with increasing amounts of S employed. The E1/2 of Fe-ISA/SNC was shown to be 0.896 V, and Jk was 100.7 mA cm o at 0.85 V, superior to that of Pt/C. X-ray absorption fine structure analysis and DFT showed that the incorporated S could elegantly tune the charges surrounding the Fe active sites. This makes the rate-limiting reductive release of OH* more favorable to give the enhanced ORR activity in alkaline conditions. Overall, this work shows a detailed understanding of the effects of heteroatom doping on the activity of SACs. In another important case, Xie and co-workers achieved isolated Fe–Nx on N- and S-co-decorated hierarchical carbon layers to serve as a bifunctional OER and ORR SAC.97 In their study, vertically aligned CNTs were employed to stabilize catalytic active sites. The isolated single Fe sites on N- and S-co-decorated hierarchical carbon layers were obtained by coating CNTs with 2,2-bipyridine and Fe salt, followed by pyrolysis and acid-leaching steps. Because of the abundance of active Fe sites, three-dimensional conductive networks, and unique hierarchical structure of the support, the Fe SAC catalyst showed an exceptional electrocatalytic performance for ORR and OER in alkaline conditions. By employing this Fe SAC, the polarization curves of Zn-air exhibited an open circuit voltage of 1.35 V and a charge-discharge voltage gap a little lower than that of Pt/C. The maximum power density was as high as 102.7 mW cm 2, and the cycling stability was excellent. Among the recent research on SAC for ORR, there are three works worth emphasizing. In the first work by the Zelenay Group in 2017, FeCl3, a polymer (PANI), and a simple organic compound (cyanamide, CM) were deliberately employed as precursors to endow the final Fe SAC (denoted as (CM+PANI)–Fe–C) with pore structures and high activity (Figures 6E and 6F). At voltages higher than 0.75 V in the H2-air PEMFC test, this Fe SAC achieved almost the same current densities as those obtained with a Pt cathode with a loading of 0.1 mgpt cm 2, underlying the great potential of SAC in practical PEMFCs.98 Later, Wang et al. described a double-solvent approach to creating Fe–Co dual sites on an N-doped porous carbon support (Figure 6G).99 The Fe3+ species were reduced and bonded with the adjacent Co atoms. Aberration-corrected high-resolution TEM, X-ray absorption fine structure spectroscopy, and Mo¨ssbauer spectroscopic measurements were performed to confirm the Fe–Co dual sites coordination environment. The as-obtained (Fe,Co)/N–C catalyst possessed excellent ORR performance (2.842 mA cm 2 at 0.9 V) in 0.1 M HClO4 solution, along with comparable onset potential (1.06 V) and E1/2 (0.863 V) and a superior cycling stability of 50,000 cycles. Remarkably, this catalyst reached maximum power density values of 0.98 W cm . in H2/O2 PEMFC and 0.51 W cm P in H2/air PEMFC. Moreover, constant-current operation testing showed that the working voltage of this catalyst was maintained even after a 100 h of operation. This Fe–Co dual-site catalyst outperformed the previously reported Pt-free catalysts in an H2/air PEMFC. DFT calculation showed that the activation of O–O was favored on the (Fe,Co)/N–C dual sites; therefore, the dual sites could greatly decrease the cleavage barrier of O–O bond to give high ORR activity and selectivity to the 4 e reduction pathway. This work not only reports an outstanding PEMFC

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performance by Pt-free catalyst but also demonstrates that the introduction of metal-metal bonds may be a feasible strategy of upgrading SACs into more competitive alternatives to Pt-based catalysts in PEMFCs. The third one reported a convenient synthetic method to prepare SACs from bulk metals. In this approach, ammonia gas first trapped metal atoms in upstream bulk metals by the strong Lewis acid-base interaction. Then, the M(NH3)x species were captured by the defects on the downstream supports (e.g., pyrolyzed ZIF-8), leaving isolated metal sites (Figure 6H). The as-prepared CuN4 SACs exhibited superior ORR performance with an E1/2 of 0.895 V in 0.1 M KOH solution. This synthetic strategy may serve as guidance for efficient preparation of SACs directly from bulk metals and demonstrates the potential for scaling up SACs toward industrial applications.100

PERSPECTIVE Despite the above achievement we reached, our ultimate goal—a highly active, stable ORR catalyst in PEMFCs with affordable costs—has yet to be achieved. For Pt- and other precious-metal-based catalysts, the primary drawback is still the prohibitively high cost. Although in RDE testing, some advanced catalysts have already achieved high mass activities even outdistancing 0.44 A mgPGM 1 at 0.9 V,10,44,54 few of them can also deliver such extraordinary performance in a practical PEMFC, leaving the 2020 DOE targets yet to be realized.101 One recent work by the Liu group provides a silver lining. As reported, a Pt-Co catalyst with ultra-low Pt content reached an excellent performance in H2-O2 PEMFC test with a current of 1.77 A mgPt 1 at 0.9 V, exceeding the DOE 2020 target by approximately four times (Figure 6I).102 Even though the use of extremely expensive Pt precursor and complicated synthetic process lessen the significance, this work demonstrates the feasibility of Pt-based ORR catalysts. However, for nonprecious-metal-based ORR catalysts, it seems that SACs possess greater competitiveness than conventional metal-particle-based materials mainly because of the relatively high activity and, in some cases, the tolerance to acid (while the intrinsic reason has yet to be revealed). Even so, state-of-the-art ORR SACs can still not meet the DOE targets. Thus, to further promote the activity is still the primary research direction for SACs. To fully realize the potential for ORR in PEMFC applications of SACs, more detailed and in-depth research is essential. Detailed structural studies at the atomic level are crucial for us to understand the factors influencing the properties and performance of SACs. So far, the pivotal characterization techniques for SACs are typically based on aberration-corrected HAADF-STEM and synchrotron radiation facility. These techniques are expensive and not easily available for every researcher in the community. Even so, the information we are able to collect by these advanced techniques still leaves much to be desired. In particular, efficient in situ characterization techniques, which enable us to observe catalysts under practical operation, are now urgently needed. In situ TEM,103 XAS,104 and other characterization techniques105 have already been reported to survey the formation process or catalytic behavior under practical conditions.106 With the help of these techniques, we may understand the relationship between catalyst structures and performance on a more profound level and thus prepare improved catalysts accordingly.107 In addition, given that commercialization usually requires low synthetic cost, another future research direction for ORR SACs would be to develop efficient synthetic strategies that allow us facilely prepare SACs on a larger scale. Several works have demonstrated the preparation of SACs directly from metal bulks.100,108 Further research may focus on developing more facile methods that avoid using high temperature or other energy-intensive treatments.

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ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China 2017YFA (0208300 and 0700104) and the National Natural Science Foundation of China (21522107 and 21671180).

AUTHOR CONTRIBUTIONS All authors devised the concept and built the framework of the review. X.W. and Z.L. wrote the manuscript. X.W., Z.L., T.Y., and W.W. organized the figures. X.W., Z.L., Y.W., and Y.L. edited and reviewed the manuscript. All authors approved the final version of the manuscript. X.W. and Z.L. contributed equally.

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