Atomically Dispersed IronâNitrogen Active Sites within Porphyrinic

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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 ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00245 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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Atomically Dispersed Iron-Nitrogen Active Sites

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within Porphyrinic Triazine-based Frameworks

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for Oxygen Reduction Reaction in Both Alkaline

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and Acidic Media

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Jun-Dong Yi,†,‡,# Rui Xu,†,‡,# Qiao Wu,†,# Teng Zhang,† Ke-Tao Zang,§ Jun Luo,§ Yu-Lin Liang,Ⅱ

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Yuan-Biao Huang,*,† and Rong Cao*,†,‡

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

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†

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of

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Matter, Chinese Academy of Sciences, Fuzhou 350002, China

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‡

University of the Chinese Academy of Sciences, Beijing 100049, China

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§

Center for Electron Microscopy, Institute for New Energy Materials and Low-Carbon

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Technologies, School of Materials, Tianjin University of Technology, Tianjin 300384, China

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Ⅱ

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Academy of Sciences, Shanghai 201204, China

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#

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

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

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ABSTRACT: The rational design of highly efficient, low-cost and durable electrocatalysts to

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replace platinum-based electrodes for oxygen reduction reaction (ORR) is highly desirable.

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Although atomically dispersed supported metal catalysts often exhibit excellent catalytic

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performance with maximized atom efficiency, the fabrication of single atom catalyst remains

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great challenge due to their easy aggregation. Herein, a simple ionothermal method was

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developed to fabricate atomically dispersed Fe-Nx species on porous porphyrinic triazine-based

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frameworks (FeSAs/PTF) with high Fe loading up to 8.3 wt%, resulting in highly reactive and

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stable single-atom ORR catalysts for the first time. Owning to the high density of single-atom

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Fe-N4 active sites, highly hierarchical porosity and good conductivity, the as-prepared catalyst

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FeSAs/PTF-600 exhibited highly efficient activity, methanol-tolerance and superstability for

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oxygen reduction reaction (ORR) under both of alkaline and acidic conditions. This work will

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bring new inspiration to design highly efficient noble-metal-free catalysts at the atomic scale for

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energy conversion.

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The rapid consumption of fossil fuels and the accompanying environment pollution has forced us

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to develop sustainable energy conversion technologies.1 Fuel cells and metal–air batteries, in

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which the cathode are driven by oxygen reduction reaction (ORR), are considered as promising

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clean energy candidates for electrical vehicles.2 However, the high overpotential for ORR with

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sluggish kinetic processes is one of the main challenges that impede the large-scale

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commercialization of these clean energy devices. Although the precious Pt-based electrocatalysts

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have shown high activity for ORR, the drawbacks including high cost, scarcity and poor

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durability, as well as MeOH crossover have limited its large-scale practical applications.3-6

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Therefore, it is urgent to develop highly efficient non-noble metal electrocatalysts with excellent

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durability and resistance to MeOH to replace Pt-based electrodes.7-9 Over the past decade, earth-

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abundant-metal-based electrocatalysts toward ORR have been extensively inverstigated.10-13 In

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particular, Fe-based catalysts supported in carbons have been demonstrated to be one of the most

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promising candidates to replace Pt-based catalysts. It has been proved that Fe-Nx species are the

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active sites for ORR.14-19 Nevertheless, most of the Fe-based materials often contains various Fe-

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based species (such as FeNx, Fe, Fe3C), thereby hindering the discrimination of the active sites.

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Designing and fabricating atomically dispersed active Fe-Nx species is a promising method to

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identify the active sites at the molecular level because single-atom catalysts (SACs) could bridge

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the gap between heterogeneous and homogeneous catalysis. Furthermore, the low-coordinated

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SACs possess exposed active sites, thus maximizing the metal atom efficiency and usually

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achieving high catalytic performance.20-24 Despite much effort has been devoted to fabricate

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SACs, it is still a great challenge to prepare atomically dispersed metal species because of their

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easy migration and agglomeration.25-28 To this end, it is highly desirable to develop suitable

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supports to provide strong interaction with Fe single atoms for achieving highly efficient in


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ORR.29-34 However, so far, most SACs are stabilized by the defects or voids of metal oxides,35-37

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which cannot be employed as electrocatlysts because of their poor electrical conductivity.

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Therefore, the judicious choice of suitable support containing anchoring points or vacancies with

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good electrical conductivity is very important for preparation single-atom catalysts (SACs) with

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high ORR efficiency.

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Recently, porous covalent triazine frameworks (CTFs) that obtained from the trimerization of

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aromatic nitriles have emerged as promising multifunctional materials for separation and

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catalysis owning to their large surface areas and superstability.38-41 Therefore, the rich N-sites,

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defects in porous CTFs could serve as anchoring points to stabilize SAs.42,43 Moreover, the CTFs

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synthesized by ionothermal method possess good electrical conductivity because partial

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graphitization will be inevitably occurred at high temperature, which is beneficial for

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electrocatalysis. However, only a few examples of single atoms anchored on CTFs have been

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reported to date,42,43 and their applications in electrocatalysis have not been extensively explored.

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More importantly, porphyrin architectures containing four pyrrolic nitrogen sites could act as

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anchoring points to stabilize single-atom metals (e.g. Fe, Co) for effective promotion of ORR.44-

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46

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endow atomically dispersed Fe-Nx species that can behave as open active sites for

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electrocatalysis.

Thus, the implantation of functional metalloporphyrin-like units into porous CTFs could

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Herein, we fabricate highly stable atomically dispersed Fe-N4 species on porous porphyrinic

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triazine-based frameworks (FeSAs/PTFs) with excellent ORR activity via a direct de nova

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synthesis strategy under ionothermal conditions. Thanks to the highly porous structure, the

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highly density of the atomically dispersed Fe-N4 active sites could be exposed to the highly

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diffused reactive species. Consequently, the optimal catalyst FeSAs/PTF-600 obtained at 600 oC


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with good enhanced electrical conductivity exhibited excellent activity, stability and methanol-

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tolerance for ORR in both of alkaline and acidic electrolytes, which surpasses or is comparable

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to the commercial Pt/C catalyst. Our work will pave a new avenue for rational design of highly

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effective catalysts at atomic level.

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As shown in Scheme 1, FeSAs/PTFs were easily obtained by the trimerization reaction of

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Scheme 1. Schematic illustration of the formation of FeSAs/PTF.

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5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato]-Fe(III) chloride (Fe-TPPCN) catalyzed by

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molten zinc chloride in an evacuated pyrex ampoule at 400-600 oC for 40 h, followed by removal

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of ZnCl2 using diluted hydrochloric acid. The resultant samples are labeled as FeSAs/PTF-x (x =

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temperature) based on the ionothermal temperature. Herein, the molten zinc chloride not only

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acted as Lewis acid to promote the trimerization reaction, but also served as porogenic agent to

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produce mesopores. As shown in Figure S1 of the FT-IR spectra, the disappearance of the

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characteristic band of carbonitrile at 2227 cm-1 suggested an almost complete conversion, while

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the presence of a strong absorption band at 1564 cm-1 assigned to triazine rings indicated the

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successful trimerization reaction.38-43 After polymerization, the formed FeSAs/PTFs samples

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showed significantly enhanced thermal stability (Figure S2). Interestingly, the samples

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FeSAs/PTF-400, FeSAs/PTF-500, and FeSAs/PTF-600 have very high Fe contents of 8.3, 7.7,

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and 2.6 wt%, respectively, based on the ICP-AES (inductively coupled plasma atomic emission


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spectroscopy) results (Table S1). It is worth mentioning that the partial pyrolysis of Fe porphyrin

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moieties at higher temperature resulted in a sharp decrease of the Fe loading in FeSAs/PTF-600

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(Figure S2). Nevertheless, the Fe loading of 2.6 wt% is higher than those most of reported metal

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single-atom catalysts.25-28 Such abundant Fe active sites could promote the activity of ORR.

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As illustrated in the powder X-ray diffraction (PXRD) patterns (Figure 1a), FeSAs/PTF-400

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Figure 1. a) PXRD patterns and b) Raman spectra of FeSAs/PTF-400, 500, 600. c) Nyquist plots

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of different samples over the frequency range from 100 kHz to 10 mHz. d) N2 adsorption-

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desorption isotherms of FeSAs/PTFs.

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and FeSAs/PTF-500 show similar patterns and only possess a broad peak centered at 25.6o,

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which may be assigned to the amorphous (002) reflection of graphitic carbon or (001) of

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aromatic sheets.38 While in FeSAs/PTF-600, an additional weak peak at 43.0o was observed and

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corresponded to the (100)/(101) reflections of graphitic carbon, suggesting that a higher graphitic

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degree were produced at 600 oC. Interestingly, no diffraction peak assigned to Fe-based particles

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or carbides was appeared, indicating that atomically dispersed Fe species might be confined in


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the PTFs. The Raman spectra (Figure 1b) also confirmed that any Fe-based particles were absent

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in the FeSAs/PTFs samples. Interestingly, the high ratios of the G-band and D-band intensity

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(IG/ID) and a broad 2D peak at ca. 2800 cm-1 were observed, which indicated some layered

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graphene-like architectures were presented in all the three FeSAs/PTFs. Meanwhile, the X-ray

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photoelectron spectroscopy (XPS) measurements manifested that the ratio of graphitic N

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increases along with the increment of synthesis temperature (Figure S3, Table S2). These results

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implied that partial graphitization for the FeSAs/PTFs, particularly FeSAs/PTF-600, was

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occurred. The graphitic degree of FeSAs/PTF-600 can be estimated as 64.3% and the details are

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presented in the Supporting Information. This was further confirmed by the electrochemical

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impedance spectroscopy (EIS) measurement. The Nyquist plots (Figure 1c and Table S3)

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demonstrate that FeSAs/PTF-600 shows the minimum semi-circle, implying that this material

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has the best conductivity among the three FeSAs/PTFs materials. This partial graphitization

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phenomenon could facilitate accelerating the electron transfer, thereby improving the activity of

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electrocatalysis.

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Although their amorphous feature, all the FeSAs/PTFs materials show large N2 adsorption

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uptakes and have high BET (Brunauer–Emmett–Teller) surface areas of up to 1067 cm3g-1

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(Figure 1d and Table S4). Moreover, obvious hysteresis loops were observed at the P/Po range of

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0.45–1.0, suggesting meso-, and/or macropores were produced in all the FeSAs/PTFs, which was

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attributed to the porogenic agent ZnCl2. The pore size distributions by non-local density

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functional theory (NL-DFT) method (Figure S4) for FeSAs/PTFs demonstrated that their pores

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are in the range of 1-6 nm. Such highly porous structure could make more active sites expose to

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the accessible reactive species, thus facilitating mass transportation and maximizing atom

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utilization efficiency.


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Figure 2. a) TEM image of FeSAs/PTF-600. b) Corresponding EDS mapping reveals the

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homogeneous distribution of Fe, C, and N elements. c, d) HAADF-STEM images and enlarged

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images of FeSAs/PTF-600. Part of single Fe atoms highlighted by red circles.

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Transmission electron microscopy (TEM), high-angle annular dark-field scanning

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transmission electron microscope (HAADF-STEM) and aberration-corrected HAADF-STEM

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with subangstrom resolution measurements were further conducted to investigate the atomic

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level of Fe sites in FeSAs/PTFs. As shown in Figure 2a and Figure S5-7, no visible Fe particles

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were observed in the TEM and HAADF-STEM images for all the three FeSAs/PTFs samples,

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which is consistent with the PXRD (Figure 1a) and Raman measurement results (Figure 1b).

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Interestingly, the element mapping images (Figure 2b) reveal that Fe element was

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homogeneously distributed over the entire architecture, suggesting that Fe species in an

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atomically dispersed form. The aberration-corrected HAADF-STEM measurements further


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proved the formation of the Fe SAs. A number of bright dots with atomic dispersion in Figure

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2c-d for FeSAs/PTF-600 could be ascribed to the heavier Fe SAs.29-34

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Figure 3. Electrochemical evaluation of catalysts in alkaline media. a) Linear sweep

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voltammograms (LSVs) of FeSAs/PTF-400, 500, 600 and Pt/C at 1600 rpm in O2-saturated 0.1

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M KOH (scan rate: 10 mV s-1). b) LSVs of FeSAs/PTF-600 at different rotation speeds (inset: K-

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L plots and electron transfer numbers). c) Corresponding Tafel plots obtained from the RDE

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polarization curves. d) Electron transfer number of different samples obtained from the RRDE

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curves. e) Methanol-crossover effects test of FeSAs/PTF-600 and Pt/C. f) Current-time

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chronoamperometry for FeSAs/PTF-600 and Pt/C in an O2-saturated 0.1 M KOH solution. g)

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LSVs of FeSAs/PTF-600, FeNPs/PTF-600 and PTF-600 without metal loaded. h) LSVs of


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FeSAs/PTF-600 before and after the addition of 0.1M NaSCN in 0.1 M KOH. i) LSVs of

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poisoned FeSAs/PTF-600 in 0.1 M KOH.

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The atomically dispersed Fe sites with high density in porous FeSAs/PTFs could allow them as

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promising earth-abundant electrocatalysts for oxygen reduction reaction (ORR). The

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FeSAs/PTFs catalysts were investigated on a rotating disk electrode (RDE) in O2-saturated 0.1 M

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KOH solution. As shown in Figure 3a, among the three single-atom catalysts, FeSAs/PTF-600

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exhibits the highest activity with the most positive onset potential Eonset (1.01 V vs. RHE) and

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half-wave potential E1/2 (0.87 V vs. RHE), which are much more positive than those of the

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commercial 20 wt% Pt/C (Eonset = 0.95 V and E1/2 = 0.81 V, respectively). Notably, it is one of

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the best values among all the reported non-precious metal catalysts (Table S5). Meanwhile,

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FeSAs/PTF-600 also possesses the largest diffusion limiting current density of 5.51 mA cm-2

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(0.2 V vs. RHE), which surpassed that of the Pt/C catalyst (5.14 mA cm-2). The Koutecky-Levich

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(K-L) plots for FeSAs/PTF-600 calculated from the LSV polarization curves at different rotation

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speeds (Figure 3b) displayed nearly parallel fitting lines, indicating first-order reaction kinetics

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associated with O2 concentration and a potential-independent electron transfer rate. Furthermore,

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the smaller Tafel slope of 62 mV dec-1 for FeSAs/PTF-600 (Figure 3c), compared with that of

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Pt/C (76 mV dec-1), further verified its superior activity. Very low yields of the two-electron

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product H2O2 were produced based on the RRDE measurements, indicating an efficient four-

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electron transfer process (Figure S8). The electron transfer numbers (n) for FeSAs/PTF-600

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calculated according to rotating ring disk electrode (RRDE) tests are 3.88 (E = 0.2 V vs. RHE)

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(Figure 3d), which are very close to that of Pt/C (n = 3.95) with a four-electron ORR pathway.


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To examine the methanol crossover effects and durability, chronoamperometric tests were

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carried out. As shown in Figure 3e, there was no obvious change in the current density on

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FeSAs/PTF-600 electrode after injecting 1.0 M methanol into the electrolyte, indicating that

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FeSAs/PTF-600 was nearly free from the methanol crossover effect. In contrast, a dramatic

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decrease in the current density for Pt/C was observed under the same conditions. Moreover,

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FeSAs/PTF-600 exhibited a superior stability than the commercial Pt/C catalyst (Figure 3f).

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After 8 h continuous potential cycling test, only a slight peak current decrease was occurred for

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FeSAs/PTF-600, while 17% current decrease was observed for Pt/C. Such long-term durability

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can be ascribed to that the single-atom Fe species were stabilized by the unique porphyrin-like

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structure in the CTF.

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To confirm the active site, a series of control experiments were conducted. For comparison, the

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Fe-free catalysts were also prepared by direct synthesis from 5,10,15,20-tetraki(4-

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cyanophenyl)porphyrin (TPPCN) at 600 oC (denoted as PTF-600) or removal of Fe species from

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FeSAs/PTF-600 in the presence of trifluoromethanesulfonic acid, which was labeled as (Fe)PTF-

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600. As shown in Figure 3g, compared with FeSAs/PTF-600, both of PTF-600 and (Fe)PTF-600

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showed very poor activity for the ORR, which indicated that the activity of FeSAs/PTF-600

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catalyst was largely attributed to Fe sites. In order to further prove the result, the atomic

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dispersed Fe species in FeSAs/PTF-600 were poisoned by SCN- ion and then compared their

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ORR performance.29 As shown in Figure 3h, the catalytic activity of FeSAs/PTF-600 decreased

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distinctly in the presence of SCN- ion, as evidenced by more negative Eonset (0.95 V) and E1/2

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(0.85 V). The diffusion limiting current density also decreased obviously from 5.51 mA cm-2 to

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4.42 mA cm-2 (0.2 V vs. RHE). Interestingly, after rinsing several times with water until pH = 7

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and re-measured in 0.1 M O2-saturated KOH electrolyte, the activity of the recovered poisoned


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FeSAs/PTF-600 electrode could reach the level of the fresh catalyst. As shown in Figure 3i, the

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ORR polarization curve positively shifts after first cycle test in the fresh electrolyte. After 15

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cycles, the catalytic activity of the poisoned electrode finally recovered to the original level due

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to the sufficient dissociation of the SCN-. The poisoning and recovery experiments clearly

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elucidated that isolated Fe sites were the origin of high ORR activity of FeSAs/PTF-600.

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Furthermore, the ORR performance of FeSAs/PTFs in O2-saturated 0.1 M HClO4 was also investigated. As shown in Figure 4a, like in alkaline media, the FeSAs/PTF-600 electrode

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Figure 4. Electrochemical evaluation of catalysts in acidic media. a) LSVs of FeSAs/PTF-400,

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500, 600 and Pt/C at 1600 rpm in O2-saturated 0.1 M HClO4 (scan rate: 10 mV s-1). b)

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Corresponding Tafel plots obtained from the RDE polarization curves. c) Electron transfer

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number of different samples obtained by RRDE. d) Current-time chronoamperometry for

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FeSAs/PTF-600 and Pt/C in an O2-saturated 0.1 M HClO4 solution.

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exhibited the highest activity among the three materials with the most positive onset potential of

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0.89 V, which is close to that of the commercial Pt/C (ca. 0.96 V). Moreover, FeSAs/PTF-600

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showed a larger diffusion limiting current density of 5.42 mA cm-2 (0.2 V vs. RHE) and a

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smaller Tafel slope of 81 mV dec-1 (Figure 4b), compared with that of the Pt/C catalyst (4.92 mA

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cm-2 and 91 mV dec-1, respectively ). A negligible H2O2 yield (less than 0.1%) for FeSAs/PTF-

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600 electrode (Figure S9) was observed and the electron transfer number calculated by the

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RRDE curves was 3.99 at the range of 0.2 V to 0.7 V (Figure 4c), suggesting an efficient 4e

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ORR pathway in the acid media. In addition, the FeSAs/PTF-600 catalyst in the acid media also

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showed a much better stability than the Pt/C electrode (Figure 4d) and free from the methanol

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crossover effects (Figure S10).

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Figure 5. a) The normalized Fe K-edge XANES spectra of the FeSAs/PTFs, Fe-TPPCN and Fe

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foil. b) XPS spectra of the Fe 2p region of FeSAs/PTF-400, 500, 600. c) Fourier transform

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EXAFS spectra of different samples. The corresponding EXAFS fitting curves of d) Fe-TPPCN,

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e) FeSAs/PTF-400 and f) FeSAs/PTF-600.

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The previous reported DFT calculation suggested that the atomically dispersed Fe-N4 species

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are active sites for ORR.29-31 To further prove the atomic dispersion of Fe species and figure out

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the highly efficient activity of FeSAs/PTF-600, X-ray absorption near-edge structure (XANES)

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and X-ray absorption fine structure (EXAFS) were recorded for FeSAs/PTFs synthesized at

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various temperatures. As shown the XANES in Figure 5a, a weak pre-edge peak at about 7113

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eV was appeared in all the FeSAs/PTFs and the precursor Fe-TPPCN, which was ascribed to the

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1s→4pz transition with simultaneous ligand-to-metal charge transfer, further confirming that the

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FeSAs/PTFs samples contain square-planar Fe-N4 porphyrin-like structure with D4h symmetry.47

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The high-resolution N 1s peak at 400.1 eV in XPS result also demonstrated the presence of Fe-N

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bonds (Figure S3). Meanwhile, FeSAs/PTF-400 has an E0 value (the first inflection point) of

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7132.1 eV, which is very close to that of the precursor Fe-TPPCN reference, indicating the

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predominance of Fe3+ in this sample. In comparison, FeSAs/PTF-600 has an E0 value of 7130.6

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eV, very close to the Fe foil sample, suggesting that the mainly Fe species was Fe2+.29-34 This

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was also verified by the X-ray photoelectron spectroscopy (XPS) measurements (Figure 5b). For

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FeSAs/PTF-400, the peaks at the binding energies of 711.4 eV (Fe 2p3/2) and 724.5 eV (Fe 2p1/2)

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are attributed to Fe3+ species and are in consistent with the XANES results. In comparison, the

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higher ratio of the areas of 709.6 eV in FeSAs/PTF-600 indicated that Fe2+ species predominated

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in this sample.30 As shown in the Fe K-edge EXAFS curves (Figure 5c), no obvious peak at the


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position of Fe-Fe coordination was observed in all the three FeSAs/PTFs samples and the

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monomer Fe-TPPCN, which was different from that of Fe foil. These results suggested that no

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Fe-based particle was formed and all the Fe sites are in an atomically dispersed form, which was

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in consistent with the above HAADF-STEM results. Furthermore, FeSAs/PTF-400 sample

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shows an obvious signal at 1.59 Å related to Fe-N coordination in the first shell,48 which was

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similar to that of the precursor Fe-TPPCN reference sample. This result suggested that the Fe

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center in FeSAs/PTF-400 may be coordinated with four nitrogen atoms and one chlorine atom in

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the axial direction perpendicular to the Fe-N4 plane, which was also confirmed by the presence

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of chlorine element (Figure S11) and Ferric based on XPS (Figure 5b). Notably, the

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corresponding R values in FeSAs/PTF-500 and FeSAs/PTF-600 shift to 1.47 Å and 1.44 Å,

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respectively, which was consistent with other reported Fe-N species scattering paths.29-34 The Fe-

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N coordination peak shifts to a low R-position with the increase of ionothermal temperature,

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which may be resulted from their different coordination models. The coordination model varies

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because partial of Fe3+ or almost of Fe3+ atoms were reduce to Fe2+ (Figure 5b) at high

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temperature by the in situ produced graphitized carbon (Figure 1a and 1b), thus forming Fe-N4

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coordination configuration in FeSAs/PTF-x (x = 500, 600) samples, which is beneficial for the

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coordination and activation of O2.29-31 In order to prove our hypothesis, these two kinds of

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coordination models (Fe-N4Cl and Fe-N4) were used for EXAFS fitting of Fe-TPPCN and

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FeSAs/PTF, respectively. As shown in Figure 5d-f, the fitting curves were almost identical with

20

the measured data and the fitting parameters given in Table S6 were quite good. In addition, the

21

EXAFS fitting results revealed that the Fe-N coordination numbers of FeSAs/PTF-400 and

22

FeSAs/PTF-600 are 4.17 and 3.98, respectively (Table S6). It suggested that the Fe porphyrin-

23

like structure was still retained even synthesized at 600 oC. Thus, the prominent ORR


15


Page 16 of 29

1

performance of FeSAs/PTF-600 is suggested to be come from the unique porphyrin-like

2

architectures, the compositional features of the catalyst including high density and uniform

3

distribution of the Fe-N4 active sites, the highly hierarchical porosity, high conductivity and the

4

superstable triazine-based framework.

5

In conclusion, highly stable atomically dispersed Fe-N4 species embedded in porous

6

porphyrinic triazine-based frameworks (FeSAs/PTF-600) were rationally designed and fabricated

7

by using one-step ionothermal synthesis approach. The unique iron single-atoms with Fe-N4

8

configuration were unambiguously identified by spherical aberration-corrected transmission

9

electron microscope observation and X-ray absorption fine structure analyses. The resulting

10

FeSAs/PTF-600 exhibits excellent ORR activity, long-term durability and good methanol

11

tolerance in both alkaline and acidic media, which was attributed to the high porosity, abundant

12

atomically dispersed Fe-N4 species, high electrical conductivity, and the superstable triazine-

13

based network. The control experiments associated with poisoning tests confirmed that the main

14

active site originated from Fe-N4 with metalporphyrin-like structure. The current study highlights

15

the great advantages in the fabrication of single metal atom catalysts based on porous covalent

16

triazine-based frameworks systems. This work presented here paves an avenue to design and

17

preparation of stable non-precious metal single-atom stabilized by CTFs toward diverse

18

electrocatalytic applications, such as ORR, hydrogen evolution reaction (HER) and CO2

19

reduction.

20

ASSOCIATED CONTENT

21

Supporting Information.

22

Experimental details; electrocatalytic measurements details; FT-IR spectra, TGA, ICP, EA

23

results of catalysts; Rct values of catalysts; pore size distribution of catalysts; TEM and HAAD-


16

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ACS Energy Letters

1

STEM images of catalysts; H2O2 yield of catalysts in 0.1 M HClO4; methanol-crossover effect

2

test of FeSAs/PTF-600 and Pt/C in 0.1 M HClO4; table of comparison of ORR performance

3

between FeSAs/PTF-600 and other reported catalysts; EXAFS data fitting results of samples.

4

AUTHOR INFORMATION

5

*Email: [email protected]

6

*Email: [email protected]

7

Notes

8

The authors declare no competing financial interest.

9

ACKNOWLEDGMENT

10

We acknowledge the financial support from the 973 Program (2014CB845605), Strategic

11

Priority Research Program of the Chinese Academy of Sciences (XDB20000000), NSFC

12

(21671188, 21521061, and 21331006), Key Research Program of Frontier Science, CAS

13

(QYZDJ-SSW-SLH045), Youth Innovation Promotion Association, CAS (2014265). We thank

14

the beamline BL14W1 station for XAFS measurements at the Shanghai Synchrotron Radiation

15

Facility, China.

16

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Scheme 1. Schematic illustration of the formation of FeSAs/PTF. 47x15mm (300 x 300 DPI)


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ACS Energy Letters

Figure 1. a) PXRD patterns and b) Raman spectra of FeSAs/PTF-400, 500, 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. 64x48mm (300 x 300 DPI)



Figure 2. a) TEM image of FeSAs/PTF-600. b) Corresponding EDS mapping reveals the homogeneous distribution of Fe, C, and N elements. c, d) HAADF-STEM images and enlarged images of FeSAs/PTF-600. Part of single Fe atoms highlighted by red circles. 82x82mm (300 x 300 DPI)


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ACS Energy Letters

Figure 3. Electrochemical evaluation of catalysts in alkaline media. a) Linear sweep voltammograms (LSVs) of FeSAs/PTF-400, 500, 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 loaded. h) LSVs of FeSAs/PTF-600 before and after the addition of 0.1M NaSCN in 0.1 M KOH. i) LSVs of poisoned FeSAs/PTF600 in 0.1 M KOH. 116x87mm (300 x 300 DPI)



Figure 4. Electrochemical evaluation of catalysts in acidic media. a) LSVs of FeSAs/PTF-400, 500, 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/PTF-600 and Pt/C in an O2-saturated 0.1 M HClO4 solution. 61x46mm (300 x 300 DPI)


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ACS Energy Letters

Figure 5. a) The 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, 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/PTF-600.


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