Atomically Dispersed Iron–Nitrogen Active Sites ... - ACS Publications

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Mar 15, 2018 - and Rong Cao*,†,‡. †. State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy ...

Atomically Dispersed Iron−Nitrogen Active Sites within Porphyrinic Triazine-Based Frameworks for Oxygen Reduction Reaction in Both Alkaline and Acidic Media Jun-Dong Yi,†,‡,# Rui Xu,†,‡,# Qiao Wu,†,# Teng Zhang,† Ke-Tao Zang,§ Jun Luo,§ Yu-Lin Liang,∥ Yuan-Biao Huang,*,† and Rong Cao*,†,‡ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ University of the Chinese Academy of Sciences, Beijing 100049, China § Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon Technologies, School of Materials, Tianjin University of Technology, Tianjin 300384, China ∥ Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China S Supporting Information *

ABSTRACT: The rational design of highly efficient, low-cost, and durable electrocatalysts to replace platinum-based electrodes for oxygen reduction reaction (ORR) is highly desirable. Although atomically dispersed supported metal catalysts often exhibit excellent catalytic performance with maximized atom efficiency, the fabrication of single-atom catalysts remains a great challenge because of their easy aggregation. Herein, a simple ionothermal method was developed to fabricate atomically dispersed Fe−Nx species on porous porphyrinic triazine-based frameworks (FeSAs/PTF) with high Fe loading up to 8.3 wt %, resulting in highly reactive and stable single-atom ORR catalysts for the first time. Owing to the high density of single-atom Fe−N 4 active sites, highly hierarchical porosity, and good conductivity, the as-prepared catalyst FeSAs/PTF-600 exhibited highly efficient activity, methanol tolerance, and superstability for oxygen reduction reaction (ORR) under both alkaline and acidic conditions. This work will bring new inspiration to the design of highly efficient noble-metal-free catalysts at the atomic scale for energy conversion.

T

based electrocatalysts toward ORR have been extensively investigated.10−13 In particular, Fe-based catalysts supported in carbons have been demonstrated to be one of the most promising candidates to replace Pt-based catalysts. It has been proven that Fe−Nx species are the active sites for ORR.14−19 Nevertheless, most of the Fe-based materials often contain various Fe-based species (such as FeNx, Fe, Fe3C), thereby hindering the discrimination of the active sites. The design and fabrication of atomically dispersed active Fe− Nx species is a promising method to identify the active sites at the molecular level because single-atom catalysts (SACs) could

he rapid consumption of fossil fuels and the accompanying environment pollution have forced us to develop sustainable energy conversion technologies.1 Fuel cells and metal−air batteries, in which the cathodes are driven by oxygen reduction reaction (ORR), are considered as promising clean energy candidates for electric vehicles.2 However, the high overpotential for ORR with sluggish kinetic processes is one of the main challenges that impede the largescale commercialization of these clean energy devices. Although the precious Pt-based electrocatalysts have shown high activity for ORR, the drawbacks, including high cost, scarcity, and poor durability, as well as MeOH crossover have limited its largescale practical applications.3−6 Therefore, it is urgent to develop highly efficient non-noble metal electrocatalysts with excellent durability and resistance to MeOH to replace Pt-based electrodes.7−9 Over the past decade, earth-abundant-metal© XXXX American Chemical Society

Received: February 11, 2018 Accepted: March 15, 2018

883

DOI: 10.1021/acsenergylett.8b00245 ACS Energy Lett. 2018, 3, 883−889

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Cite This: ACS Energy Lett. 2018, 3, 883−889

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ACS Energy Letters Scheme 1. Schematic Illustration of the Formation of FeSAs/PTF

for rational design of highly effective catalysts at the atomic level. As shown in Scheme 1, FeSAs/PTFs were easily obtained by the trimerization reaction of 5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato]-Fe(III) chloride (Fe-TPPCN) catalyzed by molten zinc chloride in an evacuated pyrex ampule at 400− 600 °C for 40 h, followed by removal of ZnCl2 using diluted hydrochloric acid. The resulting samples are labeled as FeSAs/ PTF-x (x = temperature) based on the ionothermal temperature. Herein, the molten zinc chloride not only acted as Lewis acid to promote the trimerization reaction but also served as porogenic agent to produce mesopores. As shown in the FT-IR spectra in Figure S1, the disappearance of the characteristic band of carbonitrile at 2227 cm−1 suggested an almost complete conversion, while the presence of a strong absorption band at 1564 cm−1 assigned to triazine rings indicated the successful trimerization reaction.38−43 After polymerization, the formed FeSAs/PTFs samples showed significantly enhanced thermal stability (Figure S2). Interestingly, the samples FeSAs/ PTF-400, FeSAs/PTF-500, and FeSAs/PTF-600 have very high Fe contents of 8.3, 7.7, and 2.6 wt %, respectively, based on the inductively coupled plasma atomic emission spectroscopy (ICP-AES) results (Table S1). It is worth mentioning that the partial pyrolysis of Fe porphyrin moieties at higher temperature resulted in a sharp decrease of the Fe loading in FeSAs/PTF-600 (Figure S2). Nevertheless, the Fe loading of 2.6 wt % is higher than those of most reported metal singleatom catalysts.25−28 Such abundant Fe active sites could promote the activity of ORR. As illustrated in the powder X-ray diffraction (PXRD) patterns (Figure 1a), FeSAs/PTF-400 and FeSAs/PTF-500 show similar patterns and possess only a broad peak centered at 25.6°, which may be assigned to the amorphous (002) reflection of graphitic carbon or (001) of aromatic sheets.38 While in FeSAs/PTF-600, an additional weak peak at 43.0° was observed and corresponded to the (100)/(101) reflections of graphitic carbon, suggesting that a higher graphitic degree was produced at 600 °C. Interestingly, no diffraction peak assigned to Fe-based particles or carbides appeared, indicating that atomically dispersed Fe species might be confined in the PTFs. The Raman spectra (Figure 1b) also confirmed that any Febased particles were absent in the FeSAs/PTFs samples. Interestingly, high ratios of the G-band and D-band intensity (IG/ID) and a broad 2D peak at ca. 2800 cm−1 were observed, which indicated some layered graphene-like architectures were presented in all the three FeSAs/PTFs. Meanwhile, the X-ray photoelectron spectroscopy (XPS) measurements manifested that the ratio of graphitic N increases along with the increment

bridge the gap between heterogeneous and homogeneous catalysis. Furthermore, the low-coordinated SACs possess exposed active sites, thus maximizing the metal atom efficiency and usually achieving high catalytic performance.20−24 Although much effort has been devoted to fabricating SACs, it is still a great challenge to prepare atomically dispersed metal species because of their easy migration and agglomeration.25−28 To this end, it is highly desirable to develop suitable supports to provide strong interaction with Fe single atoms for achieving highly efficient ORR.29−34 However, to date, most SACs are stabilized by the defects or voids of metal oxides,35−37 which cannot be employed as electrocatalysts because of their poor electrical conductivity. Therefore, the judicious choice of a suitable support containing anchoring points or vacancies with good electrical conductivity is very important for preparation of single-atom catalysts (SACs) with high ORR efficiency. Recently, porous covalent triazine frameworks (CTFs) obtained from the trimerization of aromatic nitriles have emerged as promising multifunctional materials for separation and catalysis owning to their large surface areas and superstability.38−41 Therefore, the rich N-sites, defects in porous CTFs, could serve as anchoring points to stabilize SAs.42,43 Moreover, the CTFs synthesized by the ionothermal method possess good electrical conductivity because partial graphitization will inevitably occur at high temperature, which is beneficial for electrocatalysis. However, only a few examples of single atoms anchored on CTFs have been reported to date,42,43 and their applications in electrocatalysis have not been extensively explored. More importantly, porphyrin architectures containing four pyrrolic nitrogen sites could act as anchoring points to stabilize single-atom metals (e.g., Fe and Co) for effective promotion of ORR.44−46 Thus, the implantation of functional metalloporphyrin-like units into porous CTFs could result in atomically dispersed Fe−Nx species that can behave as open active sites for electrocatalysis. Herein, we fabricate highly stable atomically dispersed Fe− N4 species on porous porphyrinic triazine-based frameworks (FeSAs/PTFs) with excellent ORR activity via a direct de nova synthesis strategy under ionothermal conditions. Because of the highly porous structure, the high density of the atomically dispersed Fe−N4 active sites could be exposed to the highly diffused reactive species. Consequently, the optimal catalyst FeSAs/PTF-600 obtained at 600 °C with good enhanced electrical conductivity exhibited excellent activity, stability, and methanol tolerance for ORR in both alkaline and acidic electrolytes, which surpasses or is comparable to the commercial Pt/C catalyst. Our work will provide a new avenue 884

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Figure 1. (a) PXRD patterns and (b) Raman spectra of FeSAs/PTF400, -500, and -600. (c) Nyquist plots of different samples over the frequency range from 100 kHz to 10 mHz. (d) N2 adsorption− desorption isotherms of FeSAs/PTFs. Figure 2. (a) TEM image of FeSAs/PTF-600. (b) Corresponding EDS mapping reveals the homogeneous distribution of Fe, C, and N elements. (c and d) HAADF-STEM image and enlarged image of FeSAs/PTF-600. Some single Fe atoms are highlighted by red circles.

of synthesis temperature (Figure S3 and Table S2). These results implied that partial graphitization for the FeSAs/PTFs, particularly FeSAs/PTF-600, occurred. The graphitic degree of FeSAs/PTF-600 can be estimated as 64.3%, and the details are presented in the Supporting Information. This was further confirmed by the electrochemical impedance spectroscopy (EIS) measurement. The Nyquist plots (Figure 1c and Table S3) demonstrate that FeSAs/PTF-600 shows the minimum semicircle, implying that this material has the best conductivity among the three FeSAs/PTFs materials. This partial graphitization phenomenon could facilitate accelerating the electron transfer, thereby improving the activity of electrocatalysis. Although their amorphous feature, all the FeSAs/PTFs materials show large N2 adsorption uptakes and have high Brunauer−Emmett−Teller (BET) surface areas of up to 1067 cm3 g−1 (Figure 1d and Table S4). Moreover, obvious hysteresis loops were observed at the P/Po range of 0.45−1.0, suggesting meso- and/or macropores were produced in all the FeSAs/PTFs, which was attributed to the porogenic agent ZnCl2. The pore size distributions determined by the nonlocal density functional theory (NL-DFT) method (Figure S4) for FeSAs/PTFs demonstrated that their pores are in the range of 1−6 nm. Such highly porous structure could expose more active sites to the accessible reactive species, thus facilitating mass transportation and maximizing atom utilization efficiency. Transmission electron microscopy (TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and aberration-corrected HAADF-STEM with sub-angstrom resolution measurements were further conducted to investigate the atomic level of Fe sites in FeSAs/PTFs. As shown in Figures 2a and S5−S7, no visible Fe particles were observed in the TEM and HAADF-STEM images for all three FeSAs/PTFs samples, which is consistent with the PXRD (Figure 1a) and Raman measurement results (Figure 1b). Interestingly, the element mapping images (Figure 2b) reveal that Fe element was homogeneously distributed over the entire architecture, suggesting Fe species in an atomically dispersed form. The aberration-corrected HAADF-STEM measurements further proved the formation of the Fe SAs. A number of bright dots with atomic dispersion in Figure 2c,d for FeSAs/PTF-600 could be ascribed to the heavier Fe SAs.29−34

The atomically dispersed Fe sites with high density in porous FeSAs/PTFs could make them promising earth-abundant electrocatalysts for oxygen reduction reaction (ORR). The FeSAs/PTFs catalysts were investigated on a rotating disk electrode (RDE) in O2-saturated 0.1 M KOH solution. As shown in Figure 3a, among the three single-atom catalysts, FeSAs/PTF-600 exhibits the highest activity with the most positive onset potential Eonset (1.01 V vs RHE) and half-wave potential E1/2 (0.87 V vs RHE), which are much more positive than those of the commercial 20 wt % Pt/C (Eonset = 0.95 V and E1/2 = 0.81 V, respectively). Notably, it is one of the best values among all the reported nonprecious metal catalysts (Table S5). Meanwhile, FeSAs/PTF-600 also possesses the largest diffusion-limiting current density of 5.51 mA cm−2 (0.2 V vs RHE), which surpassed that of the Pt/C catalyst (5.14 mA cm−2). The Koutecky−Levich (K-L) plots for FeSAs/PTF-600 calculated from the LSV polarization curves at different rotation speeds (Figure 3b) displayed nearly parallel fitting lines, indicating first-order reaction kinetics associated with O2 concentration and a potential-independent electron transfer rate. Furthermore, the smaller Tafel slope of 62 mV dec−1 for FeSAs/PTF-600 (Figure 3c), compared with that of Pt/C (76 mV dec−1), further verified its superior activity. Very low yields of the two-electron product H2O2 were produced based on the RRDE measurements, indicating an efficient four-electron transfer process (Figure S8). The electron transfer number (n) for FeSAs/PTF-600 calculated according to rotating ring disk electrode (RRDE) tests is 3.88 (E = 0.2 V vs RHE) (Figure 3d), which is very close to that of Pt/C (n = 3.95) with a fourelectron ORR pathway. To examine the methanol crossover effects and durability, chronoamperometric tests were carried out. As shown in Figure 3e, there was no obvious change in the current density on FeSAs/PTF-600 electrode after injecting 1.0 M methanol into the electrolyte, indicating that FeSAs/PTF-600 was nearly free 885

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Figure 3. Electrochemical evaluation of catalysts in alkaline media. (a) Linear sweep voltammograms (LSVs) of FeSAs/PTF-400, -500, and -600 and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH (scan rate: 10 mV s−1). (b) LSVs of FeSAs/PTF-600 at different rotation speeds (inset: K-L plots and electron transfer numbers). (c) Corresponding Tafel plots obtained from the RDE polarization curves. (d) Electron transfer number of different samples obtained from the RRDE curves. (e) Methanol-crossover effects test of FeSAs/PTF-600 and Pt/C. (f) Current−time chronoamperometry for FeSAs/PTF-600 and Pt/C in an O2-saturated 0.1 M KOH solution. (g) LSVs of FeSAs/PTF-600, FeNPs/PTF-600, and PTF-600 without metal load. (h) LSVs of FeSAs/PTF-600 before and after the addition of 0.1 M NaSCN in 0.1 M KOH. (i) LSVs of poisoned FeSAs/PTF-600 in 0.1 M KOH.

negative Eonset (0.95 V) and E1/2 (0.85 V). The diffusionlimiting current density also decreased obviously from 5.51 to 4.42 mA cm−2 (0.2 V vs RHE). Interestingly, after the sample was rinsed several times with water until pH 7 and remeasured in 0.1 M O2-saturated KOH electrolyte, the activity of the recovered poisoned FeSAs/PTF-600 electrode could reach the level of the fresh catalyst. As shown in Figure 3i, the ORR polarization curve positively shifts after the first cycle test in the fresh electrolyte. After 15 cycles, the catalytic activity of the poisoned electrode finally recovered to the original level because of the sufficient dissociation of the SCN−. The poisoning and recovery experiments clearly elucidated that isolated Fe sites were the origin of the high ORR activity of FeSAs/PTF-600. Furthermore, the ORR performance of FeSAs/PTFs in O2saturated 0.1 M HClO4 was also investigated. As shown in Figure 4a, as in alkaline media, the FeSAs/PTF-600 electrode exhibited the highest activity among the three materials with the most positive onset potential of 0.89 V, which is close to that of the commercial Pt/C (ca. 0.96 V). Moreover, FeSAs/ PTF-600 showed a larger diffusion-limiting current density of 5.42 mA cm−2 (0.2 V vs RHE) and a smaller Tafel slope of 81 mV dec−1 (Figure 4b), compared with those of the Pt/C catalyst (4.92 mA cm−2 and 91 mV dec−1, respectively). A negligible H2O2 yield (less than 0.1%) for FeSAs/PTF-600

from the methanol crossover effect. In contrast, a dramatic decrease in the current density for Pt/C was observed under the same conditions. Moreover, FeSAs/PTF-600 exhibited a stability superior to that of the commercial Pt/C catalyst (Figure 3f). After 8 h of continuous potential cycling test, only a slight peak current decrease occurred for FeSAs/PTF-600, while 31% current decrease was observed for Pt/C. Such longterm durability can be ascribed to the fact that the single-atom Fe species were stabilized by the unique porphyrin-like structure in the CTF. To confirm the active site, a series of control experiments were conducted. For comparison, the Fe-free catalysts were also prepared by direct synthesis from 5,10,15,20-tetraki(4-cyanophenyl)porphyrin (TPPCN) at 600 °C (denoted as PTF-600) or removal of Fe species from FeSAs/PTF-600 in the presence of trifluoromethanesulfonic acid, which was labeled as (Fe)PTF-600. As shown in Figure 3g, compared with FeSAs/PTF600, both PTF-600 and (Fe)PTF-600 showed very poor activity for the ORR, which indicated that the activity of FeSAs/PTF-600 catalyst was largely attributed to Fe sites. In order to further prove the result, the atomic dispersed Fe species in FeSAs/PTF-600 were poisoned by SCN− ion, and then their ORR performances were compared.29 As shown in Figure 3h, the catalytic activity of FeSAs/PTF-600 decreased distinctly in the presence of SCN− ion, as evidenced by more 886

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Figure 4. Electrochemical evaluation of catalysts in acidic media. (a) LSVs of FeSAs/PTF-400, -500, and -600 and Pt/C at 1600 rpm in O2-saturated 0.1 M HClO4 (scan rate: 10 mV s−1). (b) Corresponding Tafel plots obtained from the RDE polarization curves. (c) Electron transfer number of different samples obtained by RRDE. (d) Current−time chronoamperometry for FeSAs/PTF600 and Pt/C in an O2-saturated 0.1 M HClO4 solution.

electrode (Figure S9) was observed, and the electron transfer number calculated by the RRDE curves was 3.99 in the range of 0.2−0.7 V (Figure 4c), suggesting an efficient four-electron ORR pathway in the acid media. In addition, the FeSAs/PTF600 catalyst in the acid media also showed a much better stability than the Pt/C electrode (Figure 4d) and was free from the methanol crossover effects (Figure S10). Previously reported DFT calculations suggested that the atomically dispersed Fe−N4 species are active sites for ORR.29−31 To further prove the atomic dispersion of Fe species and determine the highly efficient activity of FeSAs/ PTF-600, X-ray absorption near-edge structure (XANES) and X-ray absorption fine structure (EXAFS) were recorded for FeSAs/PTFs synthesized at various temperatures. As shown by the XANES results in Figure 5a, a weak pre-edge peak at about 7113 eV appeared in all the FeSAs/PTFs and the precursor FeTPPCN, which was ascribed to the 1s → 4pz transition with simultaneous ligand-to-metal charge transfer, further confirming that the FeSAs/PTFs samples contain square-planar Fe−N4 porphyrin-like structure with D4h symmetry.47 The highresolution N 1s peak at 400.1 eV in XPS results also demonstrated the presence of Fe−N bonds (Figure S3). Meanwhile, FeSAs/PTF-400 has an E0 value (the first inflection point) of 7132.1 eV, which is very close to that of the precursor Fe-TPPCN reference, indicating the predominance of Fe3+ in this sample. In comparison, FeSAs/PTF-600 has an E0 value of 7130.6 eV, very close to the Fe foil sample, suggesting that the main Fe species was Fe2+.29−34 This was also verified by the Xray photoelectron spectroscopy (XPS) measurements (Figure 5b). For FeSAs/PTF-400, the peaks at the binding energies of 711.4 eV (Fe 2p3/2) and 724.5 eV (Fe 2p1/2) are attributed to Fe3+ species, which is consistent with the XANES results. In comparison, the higher ratio of the areas of 709.6 eV in FeSAs/ PTF-600 indicated that Fe2+ species predominated in this sample.30 As shown in the Fe K-edge EXAFS curves (Figure 5c), no obvious peak at the position of Fe−Fe coordination was observed in all three FeSAs/PTFs samples and the monomer Fe-TPPCN, which was different from that of Fe foil. These

Figure 5. (a) Normalized Fe K-edge XANES spectra of the FeSAs/ PTFs, Fe-TPPCN, and Fe foil. (b) XPS spectra of the Fe 2p region of FeSAs/PTF-400, -500, and -600. (c) Fourier transform EXAFS spectra of different samples. The corresponding EXAFS fitting curves of (d) Fe-TPPCN, (e) FeSAs/PTF-400, and (f) FeSAs/PTF600.

results suggested that no Fe-based particle was formed and all the Fe sites are in an atomically dispersed form, which was consistent with the above HAADF-STEM results. Furthermore, the FeSAs/PTF-400 sample shows an obvious signal at 1.59 Å related to Fe−N coordination in the first shell,48 which was similar to that of the precursor Fe-TPPCN reference sample. This result suggested that the Fe center in FeSAs/PTF-400 may be coordinated with four nitrogen atoms and one chlorine atom in the axial direction perpendicular to the Fe−N4 plane, which was also confirmed by the presence of chlorine element (Figure S11) and ferric material based on XPS (Figure 5b). Notably, the corresponding R values in FeSAs/PTF-500 and FeSAs/PTF-600 shift to 1.47 and 1.44 Å, respectively, which was consistent with other reported Fe−N species’ scattering paths.29−34 The Fe−N coordination peak shifts to a low Rposition with the increase of ionothermal temperature, which may result from their different coordination models. The coordination model varies because some Fe3+ or almost all Fe3+ atoms were reduced to Fe2+ (Figure 5b) at high temperature by the in situ produced graphitized carbon (Figure 1a and 1b), thus forming a Fe−N4 coordination configuration in FeSAs/ PTF-x (x = 500, 600) samples, which is beneficial for the coordination and activation of O2.29−31 In order to prove our hypothesis, these two kinds of coordination models (Fe−N4Cl and Fe−N4) were used for EXAFS fitting of Fe-TPPCN and FeSAs/PTF, respectively. As shown in Figure 5d−f, the fitting curves were almost identical with the measured data, and the fitting parameters given in Table S6 were quite good. In addition, the EXAFS fitting results revealed that the Fe−N coordination numbers of FeSAs/PTF-400 and FeSAs/PTF-600 887

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Chinese Academy of Sciences (XDB20000000), NSFC (21671188, 21521061, and 21331006), Key Research Program of Frontier Science, CAS (QYZDJ-SSW-SLH045), Youth Innovation Promotion Association, and CAS (2014265). We thank the beamline BL14W1 station for XAFS measurements at the Shanghai Synchrotron Radiation Facility, China.

are 4.17 and 3.98, respectively (Table S6). This suggested that the Fe porphyrin-like structure was retained even when synthesized at 600 °C. Thus, the prominent ORR performance of FeSAs/PTF-600 is suggested to come from the unique porphyrin-like architectures; the compositional features of the catalyst, including high density and uniform distribution of the Fe−N4 active sites; the highly hierarchical porosity; high conductivity; and the superstable triazine-based framework. In conclusion, highly stable atomically dispersed Fe−N4 species embedded in porous porphyrinic triazine-based frameworks (FeSAs/PTF-600) were rationally designed and fabricated by using a one-step ionothermal synthesis approach. The unique iron single atoms with Fe−N4 configuration were unambiguously identified by spherical aberration-corrected transmission electron microscopy observation and X-ray absorption fine structure analyses. The resulting FeSAs/PTF600 exhibits excellent ORR activity, long-term durability, and good methanol tolerance in both alkaline and acidic media, which was attributed to the high porosity, abundant atomically dispersed Fe−N4 species, high electrical conductivity, and the superstable triazine-based network. The control experiments associated with poisoning tests confirmed that the main active site originated from Fe−N4 with metalporphyrin-like structure. The current study highlights the great advantages in the fabrication of single metal atom catalysts based on porous covalent triazine-based frameworks systems. The work presented here provides an avenue to design and preparation of stable nonprecious metal single atoms stabilized by CTFs toward diverse electrocatalytic applications, such as ORR, hydrogen evolution reaction, and CO2 reduction.





DEDICATION Dedicated to Professor Xin-Tao Wu on the occasion of his 80th birthday



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00245. Experimental details; electrocatalytic measurement details; FT-IR spectra, TGA, ICP, and EA results of catalysts; Rct values of catalysts; pore size distribution of catalysts; TEM and HAAD-STEM images of catalysts; H2O2 yield of catalysts in 0.1 M HClO4; methanolcrossover effect test of FeSAs/PTF-600 and Pt/C in 0.1 M HClO4; table of comparison of ORR performance between FeSAs/PTF-600 and other reported catalysts; EXAFS data-fitting results of samples (PDF)



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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected] ORCID

Yuan-Biao Huang: 0000-0003-4680-2976 Rong Cao: 0000-0003-2384-791X Author Contributions #

J.-D.Y., R.X., and Q.W. contributed equally.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS We acknowledge the financial support from the 973 Program (2014CB845605), Strategic Priority Research Program of the 888

DOI: 10.1021/acsenergylett.8b00245 ACS Energy Lett. 2018, 3, 883−889

Letter

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DOI: 10.1021/acsenergylett.8b00245 ACS Energy Lett. 2018, 3, 883−889

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