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Boosting Oxygen Reduction Catalysis with Fe-N4 Sites Decorated Porous Carbons towards Fuel Cells Zhengkun Yang, Yu Wang, Mengzhao Zhu, Zhijun Li, Wenxing Chen, Weichen Wei, Tongwei Yuan, Yuteng Qu, Qian Xu, Changming Zhao, Xin Wang, Peng Li, Yafei Li, Yuen Wu, and Yadong Li ACS Catal., Just Accepted Manuscript • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019
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Boosting Oxygen Reduction Catalysis with Fe-N4 Sites Decorated Porous Carbons towards Fuel Cells Zhengkun Yang,†,∇ Yu Wang,§,∇ Mengzhao Zhu,† Zhijun Li,† Wenxing Chen,∥ Weichen Wei,† Tongwei Yuan,# Yunteng Qu,† Qian Xu,⊥ Changming Zhao,† Xin Wang,† Peng Li,† Yafei Li,*,§ Yuen Wu*,† and Yadong Li‡ †School
of Chemistry and Materials Science, Hefei National Laboratory for Physical
Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China ‡Department §Jiangsu
of Chemistry, Tsinghua University, Beijing 100084, China
Collaborative Innovation Centre of Biomedical Functional Materials, School of
Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, China ∥Beijing
Key Laboratory of Construction Tailorable Advanced Functional Materials and
Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China ⊥National #NEST
Synchrotron Radiation Laboratory (NSRL) in Hefei 230026, China
Lab, Department of Chemistry, College of Science, Shanghai University,
Shanghai 200444, PR China;
ABSTRACT It is highly desired but remains a great challenge to develop non-precious metal single-atom catalysts to supersede the Pt-based material for oxygen reduction reaction (ORR). Herein, we report a porous nitrogen-doped carbon matrix catalyst with 3.5 wt% single Fe atoms (Fe SAs/N-C) through a versatile molecules-confined pyrolysis strategy. In 0.1 M KOH condition, the Fe SAs/N-C catalyst possesses a half-wave potential of 0.91 V vs. RHE. In more challenge acidic solution, Fe SAs/N-C catalyst also offers good ORR activity, comparable to the commercial Pt/C. Impressively, Fe SAs/N-C shows extremely high stability both in alkaline and acidic media. In addition, this Fe SAs/N-C-derived Zn-air battery and proton exchange membrane fuel cells (PEMFCs)
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exhibit high performance. This work opens an avenue for designing and preparing single-atom catalysts.
KEYWORDS: single-atom Fe, porous carbons, electrocatalysis, oxygen reduction reaction, Zn-air battery, fuel cells
Due to high theoretical energy density, low-cost, and zero-emission of greenhouse gases, proton exchange membrane fuel cells (PEMFCs) and zinc-air (Zn-air) battery are promising energy storage appliance for next-generation portable electronics.1-3 However, the inferior energy conversion efficiency and low-lifetime are the main bottlenecks, impeding their widespread applications. These shortcomings predominantly derive from the intrinsically sluggish kinetics of the oxygen reduction reaction (ORR),4 as well as its poor stability in corrosive electrolytes. Therefore, great endeavors have been devoted to develop active and durable ORR catalysts. Although platinum (Pt)-based catalysts have been identified as the best performing ORR catalysts,5-9 their extensive commercial application is still limited by their high cost and severe scarcity. Therefore, the development of noble metal-free ORR catalysts is significantly important for Zn-air battery and PEMFCs. To date, atomically dispersed M−N−C catalysts have been regarded as the most promising alternatives to commercial Pt/C due to their outstanding performance in activation of oxygen.10-12 To meet the industrial requirements, achieving high power density is not only related to the intrinsic activity of reactive sites, but also the geometric and mass activity of as-prepared catalysts.13 The latter is highly determined by the density of active sites within limited surface of substrate. However, most current synthetic methods to the M−N−C catalysts include a general pyrolysis process. Simply increasing Fe concentration to increase the density of Fe SAs sites usually results in significant amount of inactive Fe clusters and nanoparticles (NPs). Recently, Sang Hoon Joo et al
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and Yu et al reported a silica-protected strategy that can preferentially fabricate Fe–N4 sites,14,15 but post silica remove is commonly necessary. Therefore, the development of a practical and simple strategy for the large-scale preparation of atomically dispersed M−N−C catalysts with high metal loadings, high catalytic activity and good chemical stability is still challenging. Herein, we report a straightforward and scalable molecules-confined pyrolysis strategy that can preferentially produce a highly stable single Fe sites on N-doped porous carbon nanosheets (Fe SAs/N−C) catalyst. As shown in Figure 1a, the polymer of polyetherimide (PEI) was selected as carbon precursor and 1,10-phenanthroline (Phen) ligand was employed as space isolation agent of Fe ions to promote their full transformation to single Fe atoms without forming Fe NPs. As a control, Fe nanoparticles were observed without addition of Phen molecules (Figure S1). Upon the high-temperature pyrolysis, the C3N4 was thermally removed (Figure S2),16 The confined Fe ions were directly reduced by carbonization of PEI to yield the isolated Fe atoms on the N-doped carbon nanosheets (Fe SAs/N-C) catalyst without post-acid treatment. As displayed in Figure 1b and 1c, the catalyst exhibits a markedly crumpled sheets structure with visible pores. By contrast, the transmission electron microscopy (TEM) image of the as-prepared sample without addition of Phen confinement agent (Fe NPs/N-C) shows a few Fe-based nanoparticles (Figure S3 and S4). The HRTEM image (Figure S5) reveals some irregular fringes. The selected-area electron diffraction (SAED) pattern demonstrated its poor crystallinity (inset in Figure S5). The Brunauer–Emmett–Teller (BET) surface area of Fe SAs/N-C catalyst was calculated to be 1097 m2 g−1 (Figure 1d) and the Barrett-Joyner-Halenda (BJH) pore size distribution curve indicates that Fe SAs/N-C catalyst contains mesopores appeared at about 2.4 and 19.2 nm in diameter, and pore volume is 5.3 cm3/g. In order to gain atomic insight into the structure of Fe SAs/N-C catalyst, the Aberration-corrected high-angle annular dark-field scanning TEM (AC HAADF-STEM) analysis was performed. As shown in Figure 1e and Figure S6, the brighter spots can be assigned to
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the atomic dispersed Fe atoms. Some isolated Fe atoms were circled for better observation (Figure 1f). The HADDF-STEM (Figure 1g) and energy-dispersive X-ray spectroscopy (EDS) mapping images show uniform dispersion of C, N and Fe over the whole architecture.
Figure 1. (a) Scheme of the fabrication Fe SAs/N-C. (b) SEM image, (c) TEM image, (d) N2 adsorption/desorption isotherms of Fe SAs/N-C catalysts and corresponding pore distribution. (e, f) magnified AC HAADF-STEM images of the Fe SAs/N-C. (g) HAADF-STEM image and corresponding EDS element maps of Fe SAs/N-C catalyst. The crystalline phase of the Fe SAs/N-C catalyst was firstly analyzed by X-ray diffraction (XRD). A clear diffraction peak centered at 26° is observed (Figure S7a), which can be ascribed to the (002) plane of graphitic carbon.17 No metal nitrides and carbides phases were detected. The Fe content was measured to be 3.5 wt% based on the coupled plasma atomic emission spectrometry (ICP-AES) analysis. Two predominant bands at 1591 and 1352 cm−1 are presented in the Raman spectrum (Figure S7b).18 As provided in Figure S8a, the survey X-ray photoelectron spectroscopy (XPS) spectrum 4
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shows peaks of C, N, O and Fe. The high-resolution N1s spectrum (Figure S8b) shows the coexistence of four types of N species: pyridinic-N, pyrrolic-N, graphitic-N and oxidized pyridinic-N.31 The peak of the Fe 2p3/2 is at 711.5 eV (Figure S9), which situated in-between Fe0 (710.7 eV) and Fe2+ (712.7 eV), suggesting that the nature of Fe in Fe SAs/N-C is ionic Feδ+ (0 < δ < 2). This is in good agreement with the previously reported results that the valence of Fe species in Fe SAs/N-C is between Fe (0) and Fe (II).19 To determine the chemical state and coordination environment of Fe SAs/N-C at atomic level, we performed X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The Fe K-edge XANES profile for Fe SAs/N-C in Figure 2a suggests the Fe single atoms in Fe SAs/N-C might be a low-valent state (0
