Simple-Cubic Carbon Frameworks with Atomically ... - ACS Publications

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Simple-Cubic Carbon Frameworks with Atomically Dispersed Iron Dopants toward High-Efficiency Oxygen Reduction Biwei Wang, Xinxia Wang, Jinxiang Zou, Yancui Yan, Songhai Xie, Guangzhi Hu, Yanguang Li, and Angang Dong Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b00004 • Publication Date (Web): 27 Jan 2017 Downloaded from http://pubs.acs.org on January 28, 2017

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Simple-Cubic Carbon Frameworks with Atomically Dispersed Iron Dopants toward High-Efficiency Oxygen Reduction Biwei Wang,† Xinxia Wang,‡ Jinxiang Zou,† Yancui Yan,† Songhai Xie,† Guangzhi Hu§, Yanguang Li,*‡ and Angang Dong*† †

Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key

Laboratory of Molecular Catalysis and Innovative Materials, and Department of Chemistry, Fudan University, Shanghai 200433, China. ‡

Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory

for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China. §

The Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of

Sciences, Urumqi 830011, China.

*To

whom

correspondence

should

be

addressed:

[email protected] (A.D.)

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ACS Paragon Plus Environment

[email protected];

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ABSTRACT Iron and nitrogen co-doped carbons (Fe-N-C) have attracted increasingly greater attention as electrocatalysts for oxygen reduction reaction (ORR). Although challenging, the synthesis of Fe-N-C catalysts with highly dispersed and fully exposed active sites is of critical importance for improving the ORR activity. Here, we report a new type of graphitic Fe-N-C catalysts featuring numerous Fe single atoms anchored on a three-dimensional simple-cubic carbon framework. The Fe-N-C catalyst, derived from self-assembled Fe3O4 nanocube superlattices, was prepared by in situ ligand carbonization followed by acid etching and ammonia activation. Benefiting from its homogeneously dispersed and fully accessible active sites, highly graphitic nature, and enhanced mass transport, our Fe-N-C catalyst outperformed Pt/C and many previously reported Fe-N-C catalysts for ORR. Furthermore, when used for constructing the cathode for zinc-air batteries, our Fe-N-C catalyst exhibited current and power densities comparable to those of the state-of-the-art Pt/C catalyst.

Keywords: Self-assembly • Fe-N-C catalyst • single atom • oxygen reduction reaction • zinc-air battery

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The sluggish oxygen reduction reaction (ORR) occurring at the cathode of fuel cells and metal-air batteries is a bottleneck reaction for the commercialization of these energy techniques.1-3 To date, platinum supported on carbon (Pt/C) has been regarded as the best catalyst for ORR. However, the high cost and poor durability of Pt-based catalysts significantly restrict their widespread applications in ORR.4, 5 To address these issues, intensive effort has been devoted to the development of non-preciousmetal-based

ORR catalysts

over the

past few years.6

Heteroatom-doped

nanostructured carbons represent an important class of low-cost yet efficient catalysts for ORR.7 In particular, transition metal and nitrogen co-doped carbons (M-N-C), especially Fe-N-C and Co-N-C, have been emerging as one of the most promising candidates to replace Pt/C because of their high catalytic efficiency and long-term stability.2, 5, 8 The superior ORR activity of M-N-C catalysts is attributed to the charge redistribution induced by nitrogen and metal doping atoms, which improves the O2 adsorption and reduction efficiency.9, 10 Although controversy still remains regarding the chemistry nature and working mechanism of M-N-C catalysts in the field, the ability to deposit abundant atomically dispersed metal dopants on a three-dimensional (3D), well-defined carbon support is essential to achieve the high ORR activity.6 Previous M-N-C catalysts are generally prepared by pyrolysis of a composite precursor containing N, C, and Fe or Co. Early precursors include metal-containing porphyrin and phthalocyanine complexes.11-13 Recently, metal-organic frameworks (MOFs) have been widely used as the composite precursor, enabling microporous MN-C catalysts by direct pyrolysis.14-17 Despite the tremendous progress, the aggregation of metal species is commonly observed during the carbonization of MOFs and other composite precursors, leading to catalytically inactive metal nanoparticles.14,

15, 18, 19

Another significant disadvantage associated with pyrolysis

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approaches is the difficulty to precisely control the location of metal dopants. Owing to the drastic structural change occurring at high temperatures, a high fraction of metal atoms could be embedded into the as-formed carbon matrix. Such carbonembedded metal dopants are hardly accessible and consequently contribute negligibly to the catalytic performance. Moreover, the carbon support derived from such composite precursors generally possesses a low graphitization degree,15, 20 which may hinder electron transport and lead to an unsatisfactory long-term stability. To make M-N-C catalysts truly competitive with Pt/C, new strategies that enable graphitic carbon architectures featuring well-defined porosity and fully-exposed active sites are therefore urgently needed. Herein, we report a simple and reproducible method of preparing 3D simplecubic carbon frameworks (SCCFs) with the surface decorated homogeneously with atomically dispersed Fe dopants, which upon ammonia (NH3) activation, exhibit unexpectedly superior ORR activity. Our method is based upon an acid etching reaction of Fe3O4 nanocube superlattices, with Fe residues predominantly existing as single atoms as revealed by aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM). Remarkably, the well-dispersed Fe atoms remain stable without agglomeration when the Fe-doped SCCFs (denoted as Fe-SCCFs) are subjected to NH3 treatment at temperatures as high as 900 oC. The resulting Fe, N codoped, graphitic SCCFs (denoted as Fe-N-SCCFs) exhibit superior ORR performance in alkaline media with a half-wave potential of 0.883 V (vs. reversible hydrogen electrode (RHE)), higher than that of Pt/C and many Fe-N-C catalysts reported previously. Furthermore, when used to construct the air cathode for zinc-air batteries, Fe-N-SCCFs display comparable current and power densities to the state-of-the-art Pt/C catalyst.

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Fe-N-SCCFs, derived from self-assembled Fe3O4 nanocube superlattices, were prepared by in situ ligand carbonization followed by acid etching and NH3 activation, as schematically illustrated in Figure 1. The transformation of Fe3O4 nanoparticle superlattices has proven to be very effective for producing ordered mesoporous carbons, with ultrathin pore walls derived from the monolayer hydrocarbon ligands (i.e., oleic acid) originally stabilizing Fe3O4 nanoparticles.21,

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We choose Fe3O4

nanocubes instead of conventional spherical nanoparticles in the present work, mainly because the self-assembly of nanocubes into simple-cubic superlattices allows the densest packing of nanoparticles (100% vs. 74% for face-centered-cubic superlattices assembled from spherical nanoparticles, if the interparticle spacing is neglected), thereby enabling carbon frameworks with maximized interior surface areas.23 Notably, Fe3O4 nanocubes play a multifunctional role for the formation of Fe-N-SCCFs, not only acting as the template but also providing the source of Fe dopants. Figure 2a shows a typical transmission electron microscopy (TEM) image of 20 nm Fe3O4 nanocubes, which tended to form 3D simple-cubic superlattices by a drying-induced assembly process. The in situ carbonization of the surface-coating ligands was accomplished by heating the as-assembled superlattices at 500 oC in Ar for 2 h. Highresolution scanning electron microscopy (HRSEM) indicated that the long-range ordering of Fe3O4 nanocubes was well retained during ligand carbonization (Figure 2b). To obtain carbon frameworks, the carbon-coated Fe3O4 nanocube superlattices were simply subjected to repeated acid treatment with HCl (3 M) until no Fe species in the supernatant could be detected by inductive coupled plasma (ICP). TEM revealed the formation of 3D carbon frameworks with simple-cubic mesopores derived from the removed Fe3O4 nanocubes (Figure S1). Survey of low-magnification

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TEM images confirmed the nearly complete removal of particulate Fe3O4, while the persistently detectable Fe signals in energy-dispersive X-ray spectroscopy (EDS) suggested the existence of a small amount of Fe residues in the as-formed carbon frameworks (Figure S1c). Fe-N-SCCFs were obtained by annealing such Fe-doped carbon frameworks in the presence of NH3 at 900 oC, which introduced N dopants while simultaneously graphitizing the amorphous carbon frameworks. The typical TEM image of Fe-N-SCCFs viewed from the [100] zone axis was provided in Figure 2c. Combined TEM (Figure S2), fast Fourier transforms (FFTs), and small-angle Xray scattering (SAXS, Figure 2e) established the long-range ordered simple-cubic structure exhibited by Fe-N-SCCFs, while high-resolution transmission electron microscopy (HRTEM) revealed that the ultrathin pore walls of Fe-N-SCCFs were constructed from a few layers of stacking graphene with interlayer distance of ~ 0.36 nm (Figure 2d). The high graphitization degree of Fe-N-SCCFs was also confirmed by X-ray diffraction (XRD, Figure 2f), in which the two diffraction peaks at 25 and 43o were ascribed respectively to the (002) and (101) diffractions of graphitic carbons.24 Notably, the diffraction peaks corresponding to metallic iron and iron oxide (or carbide and nitride) were almost undetectable in XRD, implying that the residual Fe species remained stable without agglomeration and/or crystallization during NH3 activation. The well-resolved G band 2D bands in Raman spectrum (Figure 2f, inset) as well as the high G-to-D intensity ratio (IG/ID = 1.47) also corroborated the highly graphitic nature of Fe-N-SCCFs.25 The Brunauer-Emmett-Teller (BET) surface area of Fe-N-SCCFs was 1180 m2 g-1 with a high pore volume of 2.53 cm3 g-1. The typeIV nitrogen adsorption-desorption isotherms with a large hysteresis loop was indicative of uniform, cage-like mesoporosity of Fe-N-SCCFs (Figure S3a).26 As indicated by the pore size distribution curve (Figure S3b), Fe-N-SCCFs possessed

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large mesopores centering at ~ 18 nm, which corresponded to the removed Fe3O4 nanocubes, consistent with TEM and SAXS results. The composition of Fe-N-SCCFs was further characterized using X-ray photoelectron spectroscopy (XPS) and EDS elemental mapping. XPS analyses suggested the co-existence of Fe and N in Fe-N-SCCFs with a content of 0.8 and 3.9 at%, respectively (Figure S4). The N 1s spectra could be deconvoluted into two dominant peaks at ~ 398.6 and 400.7 eV (Figure 3a), which were ascribed respectively to pyridinic-N (43.7%) and pyrrolic-N (56.3%).27-29 The high-resolution Fe 2p spectrum was shown in Figure 3b, in which the two peaks at 711.3 and 725.0 eV could be assigned to the Fe2+ 2p3/2 and Fe3+ 2p1/2 band, respectively, consistent with the previously reported results.30, 31 To investigate the variation of Fe oxidation states at different stages during sample preparation, we also collected the XPS spectra of carbon-coated Fe3O4 nanocube superlattices and Fe-SCCFs before NH3 treatment. As expected, Fe3O4 nanocube superlattices exhibited three peaks at 710.6, 712.6, and 724.6 eV in the Fe 2p spectrum, corresponding to the Fe2+ 2p3/2, Fe3+ 2p3/2, and Fe3+ 2p1/2 band, respectively (Figure S5a). Likewise, these three peaks were also observable in the Fe 2p spectrum of Fe-SCCFs (Figure S5b), despite the low signalto-noise ratio arising from the much lower Fe content. Given that the Fe3+ 2p3/2 band at 712.6 eV was almost invisible in the Fe 2p spectrum of Fe-N-SCCFs (Figure 3b), we speculated that a certain amount of Fe3+ species in Fe-SCCFs were reduced to Fe2+ during NH3 treatment. EDS elemental mapping of Fe-N-SCCFs established the uniform distribution of N and Fe over the entire carbon framework (Figure 3c). To elucidate the state of Fe dopants, we carried out Cs-corrected high-angle annular darkfield STEM (HAADF-STEM) measurements with sub-angstrom resolution, which were performed on the edge of carbon frameworks to minimize the influence of the

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sample thickness variation on the image quality (Figure 3d). Owing to the contrast difference between Fe and C, bright spots corresponding to heavier Fe atoms were clearly observed (Figure 3e), which were distributed homogeneously all over the carbon framework, in agreement with elemental mapping results. Importantly, careful examination of HAADF-STEM images indicated that Fe dopants predominantly existed as isolated single atoms as indicated by circles in Figure 3e. To unravel the chemical environment of Fe atoms, we performed synchrotronbased X-ray absorption fine structure measurements. As indicated by the Fe K-edge X-ray absorption near-edge structure (XANES), both Fe-SCCFs and Fe-N-SCCFs exhibited a spectrum distinct from that of the reference Fe foil (Figure 3f), with the absorption edge position shifted to the higher energy. These results suggested the oxidation of Fe atoms in both Fe-SCCFs and Fe-N-SCCFs, consistent with the aforementioned XPS results. Closer observation revealed that the main absorption feature of Fe-N-SCCFs was different from that of Fe-SCCFs, indicative of the variation of Fe chemical environment during NH3 treatment.32 Specifically, Fe-NSCCFs displayed a pre-edge feature at ~ 7116 eV with an additional peak at ~ 7130 eV, which was in general consistent with the absorption characteristics ascribed to Fe with a square planar geometry.33 Further structural information of Fe atoms could be obtained from the extended X-ray absorption fine structure (EXAFS), as shown in Figure 3g. Unlike the reference Fe foil which showed a prominent peak at the position of Fe-Fe coordination (2.49 Å), Fe-SCCFs exhibited a strong peak at 1.87 Å, which we attributed to the Fe-C coordination. The absence of the Fe-Fe peak at 2.49 Å strongly suggested that Fe dopants distributed in carbon frameworks predominantly existed as isolated atoms. After NH3 treatment, the coordination peak of Fe-N-SCCFs shifted to a high R-position at 1.96 Å assignable to the Fe-N coordination (Figure 3g,

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red curve). Remarkably, similarly to Fe-SCCFs, the Fe-Fe peak was also invisible in the EXAFS spectrum of Fe-N-SCCFs, corroborating that Fe atoms remained stable without agglomeration during high-temperature NH3 treatment, consistent with Cscorrected HAADF-STEM observations (Figure 3e). It should be noted that this simple etching-based synthetic strategy is very effective and reproducible for generating highly dispersed Fe atoms within the asformed periodic carbon framework. Despite the difficulty to precisely control the amount, Fe single atoms with a loading of 1-2 wt% (measured from EDS), which has been demonstrated to be appropriate for efficient ORR,15 are routinely achievable by acid etching of Fe3O4 nanoparticle superlattices. It is also worth mentioning that this acid etching strategy is general for a variety of transition-metal-oxide nanoparticles such as MnO and CoFe2O4 (Figure S6), providing a great flexibility in tuning the composition of metal atoms. Moreover, unlike previous precursor pyrolysis approaches, the top-down etching of nanoparticles ensures metal atoms exclusively anchored on the carbon surface, which are expected to be fully accessible for ORR given the 3D periodic structure of carbon frameworks. The ORR activity of Fe-N-SCCFs was first accessed by cyclic voltammetry (CV). In N2-saturated 0.1 M KOH, Fe-N-SCCFs exhibited a featureless CV curve, whereas a prominent cathodic peak at 0.87 V was observed when the electrolyte solution was saturated with O2 (Figure S7), suggesting their effective activity toward ORR in the alkaline medium. To further evaluate the catalytic performance of Fe-NSCCFs in O2-saturated 0.1 M KOH, the rotating ring-disk electrode (RRDE) technique was employed at a scanning rate of 10 mV s-1. Figure 4a showed the polarization curves of Fe-N-SCCFs and the commercial Pt/C catalyst (20 wt%) obtained at a rotation speed of 1600 rpm. Similar to Pt/C, Fe-N-SCCFs displayed an

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onset potential of 1.03 V with a well-defined plateau of diffusion-limiting currents below 0.8 V, implying an efficient catalytic reaction with a four-electron reduction pathway.34 The half-wave potential of Fe-N-SCCFs was determined to be 0.883 V, which was higher than that of Pt/C and comparable to that of most highly efficient FeN-C catalysts reported to date.2, 10, 15, 35-38 The RRDE technique also allowed us to monitor the evolution of peroxide (H2O2) during ORR and to determine the electron transfer number (n). As shown in Figure 4b, the H2O2 yield of Fe-N-SCCFs was below 5% with n ranging from 3.90 to 4.0 over the potential range of 0.2-1.0 V, similar to that of Pt/C. These results corroborated the direct four-electron reduction of O2 taking place on Fe-N-SCCFs. Moreover, we also assessed the electrochemical stability of Fe-N-SCCFs following the accelerated durability test protocol by cycling the catalysts between 0.6 and 1.0 V at 50 mV s-1.39 No obvious change in half-wave potentials was observed after 5000 and 10000 cycles (Figure 4c), demonstrating the outstanding durability of Fe-N-SCCFs for ORR. With the aim of accessing the role of Fe and N dopants in improving the ORR activity of Fe-N-SCCFs, we carried out several control experiments under identical testing conditions. Without NH3 treatment, Fe-SCCFs exhibited very poor ORR activity (Figure 4a, green curve). This result indicated that N-doping was indispensable in obtaining high ORR activity, consistent with previous results.7 To determine the role of Fe dopants, we investigated the ORR activity of Fe-N-SCCFs in 0.1 M KOH containing 5 mM KCN. It is well-known that CN- ions coordinate strongly to Fe and consequently position the Fe-containing active sites.39 As shown in Figure 4d, the introduction of CN- ions negatively shifted the polarization curve by more than 100 mV in both onset and half-wave potentials. The dramatically reduced

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ORR activity clearly demonstrated that the atomically dispersed Fe dopants indeed played a pivotal role in promoting the catalytic performance of Fe-N-SCCFs. To further investigate the contributing factor of the framework structural ordering on the catalytic performance, we intentionally prepared Fe, N co-doped but disordered carbon frameworks (Fe-N-DCFs) by transformation of randomly packed Fe3O4 nanocube assemblies resulted from fast solvent evaporation (Figure S8). The BET surface area of Fe-N-DCFs was 765 m2 g-1, much lower than that of their ordered counterparts. Despite the poorly defined porous structure and reduced surface area, elemental mapping, HAADF-STEM, and XPS confirmed the uniform distribution of comparable amounts of Fe and N atoms in Fe-N-DCFs (Figure S8c-f). Moreover, Fe-N-DCFs possessed a comparable electrical conductivity as Fe-NSCCFs, in the range of 1.2-1.6 S m-1. Interestingly, when evaluated as ORR catalysts, Fe-N-DCFs exhibited an inferior performance relative to Fe-N-SCCFs, with the onset and half-wave potentials shifting negatively by more than 35 and 50 mV, respectively (Figure 4a, blue curve). We speculate that, although Fe-N-DCFs had similar Fe and N contents and electrical conductivity as Fe-N-SCCFs, the number of accessible active sites was less than that of Fe-N-SCCFs owing to the reduced surface area, which led to the negative shift of the onset potential.40 These results clearly demonstrate that the highly ordered, fully accessible mesoporous structure of Fe-N-SCCFs indeed contribute to the observed exceptionally high ORR activity. The high and sustainable ORR activity of Fe-N-SCCFs further encouraged us to explore their potential use for real energy devices. To demonstrate this, primary zincair batteries were constructed by using Fe-N-SCCFs as the air cathode and zinc foil as the anode in a customized electrochemical cell filled with 6 M KOH electrolyte.41 A photograph depicting the electrochemical cell setup was given in Figure 5a. For

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comparison, zinc-air batteries employing Pt/C as the cathode catalyst were also fabricated and tested under the same conditions. Before the construction of zinc-air batteries, the catalyst powder of Fe-N-SCCFs was first loaded on the carbon fiber paper electrode (loading density = 1 mg cm-2) and accessed in O2-saturated 6 M KOH in a three-electrode configuration. The high ORR activity of Fe-N-SCCFs suggested the catalyst could be used for zinc-air batteries (Figure S9). Figure 5b depicted the polarization curves and corresponding power density plots of the two types of zinc-air batteries. At a voltage of 1.0 V, zinc-air cells with Fe-N-SCCFs as the cathode catalyst exhibited a high current density of ~ 205 mA cm-2, with the maximum power density close to 300 mW cm-2. Notably, both current and power densities of our Fe-NSCCFs, which were comparable to those of Pt/C catalyst (~ 235 mA cm-2 and 305 mW cm-2), were significantly improved over earlier primary zinc-air batteries employing non-precious-metal-based ORR catalysts as the cathode (Table S1). Galvanostatic discharge measurements showed that the voltage of Fe-N-SCCF cathodes was 1.34 and 1.25 V at the current density of 10 and 50 mA cm-2, respectively (Figure 5c), which were similar to Pt/C cathodes and higher than most previous Fe-N-C cathodes.41,

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Remarkably, unlike Pt/C cathodes which showed

declined performance, especially at 50 mA cm-2, almost no significant voltage drop was observed for Fe-N-SCCF cathodes after 16 h at 10 mA cm-2 and 10 h at 50 mA cm-2, demonstrating the excellent long-term stability of zinc-air batteries. In summary, our studies have established a simple yet highly efficient methodology, based on the acid etching reaction of Fe3O4 nanocube superlattices, to introduce abundant Fe single atoms into a highly ordered cubic carbon framework. The well-dispersed Fe atoms, anchored on the carbon frameworks, remain stable during NH3 activation without aggregation. The resulting Fe, N co-doped, graphitic

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carbon frameworks prove to be highly efficient Fe-N-C catalysts for ORR in alkaline media, with the half-wave potential higher than that of Pt/C and many carbon-based catalysts reported to date. The superior ORR activity of our Fe-N-C catalysts is attributed to the highly dispersed and fully accessible active sites as well as the enhanced mass transport and electron transfer properties. The practical application of this new type of Fe-N-C catalysts has also been demonstrated by assembling zinc-air batteries, which exhibit current and power densities comparable to those of the stateof-the-art Pt/C catalyst. ASSOCIATED CONTENT Supporting Information. SEM images, TEM images, EDS spectrum, XPS spectra, N2

adsorption-desorption isotherms, SAXS pattern, HAADF-STEM image, CV and polarization curves. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (A.D.); [email protected] (Y.L.) Notes The authors declare no competing financial interests. Acknowledgements A.D. acknowledges the financial support from MOST (2014CB845602), Natural National Science Foundation of China (21373052), and Shanghai International Science and Technology Cooperation Project (15520720100). Y.L. acknowledges the support form the Priority Academic Program of Jiangsu Higher Education Institutions, and Collaborative Innovation Center of Suzhou Nano Science and Technology. We

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also thank Dr. Jingyuan Ma at the BL14W beamline at the Shanghai Synchrotron Radiation Facility for the assistance on XAFS measurements. References 1.

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18. Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M.; Jaouen, F.; Mukerjee, S. Nat. Commun. 2015, 6, 7343-7352. 19. Jiang, W.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.; Wang, J.; Hu, J.; Wei, Z.; Wan, L. J. Am. Chem. Soc. 2016, 138, 3570-3578. 20. Liu, X.; Dai, L. Nat. Rev. Mater. 2016, 1, 16064-16075. 21. Jiao, Y.; Han, D.; Ding, Y.; Zhang, X.; Guo, G.; Hu, J.; Yang, D.; Dong, A. Nat. Commun. 2015, 6, 6420-6427. 22. Jiao, Y.; Han, D.; Liu, L.; Ji, L.; Guo, G.; Hu, J.; Yang, D.; Dong, A. Angew. Chem. Int. Ed. 2015, 54, 5727-5731. 23. Yu, S.; Lee, D. J.; Park, M.; Kwon, S. G.; Lee, H. S.; Jin, A.; Lee, K.; Lee, J. E.; Oh, M. H.; Kang, K.; Sung, Y.; Hyeon, T. J. Am. Chem. Soc. 2015, 137, 1195411961. 24. Worsley, M. A.; Pham, T. T.; Yan, A.; Shin, S. J.; Lee, J. R. I.; Bagge-Hansen, M.; Mickelson, W.; Zettl, A. ACS Nano 2014, 8, 11013-11022. 25. Cui, C.; Qian, W.; Yu, Y.; Kong, C.; Yu, B.; Xiang, L.; Wei, F. J. Am. Chem. Soc. 2014, 136, 2256-2259. 26. Deng, Y.; Yu, T.; Wan, Y.; Shi, Y.; Meng, Y.; Gu, D.; Zhang, L.; Huang, Y.; Liu, C.; Wu, X.; Zhao, D. J. Am. Chem. Soc. 2007, 129, 1690-1697. 27. Liang, H.; Wu, Z.; Chen, L.; Li, C.; Yu, S. Nano Energy 2015, 11, 366-376. 28. Sheng, Z.; Shao, L.; Chen, J.; Bao, W.; Wang, F.; Xia, X. ACS Nano 2011, 5, 4350-4358. 29. Jansen, R. J. J.; Bekkum, H. V. Carbon 1995, 1021-1027. 30. Niu, W.; Li, L.; Liu X.; Wang, N.; Liu, J.; Zhou, W.; Tang,Z.; Chen, S. J. Am. Chem. Soc. 2015, 137, 5555-5562. 31. Lin, L.; Zhu, Q.; Xu, A. J. Am. Chem. Soc. 2014, 136, 11027-11033. 32. Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Nat. Mater. 2015, 14, 937-942. 33. Zhou, J.; Duchesne, P.; Hu, Y.; Wang, J.; Zhang, P.; Li, Y.; Regier, T.; Dai, H. Phys. Chem. Chem. Phys. 2014, 16, 15787-15791. 34. Zhou, M.; Wang, H.; Guo, S. Chem. Soc. Rev. 2016, 45, 1273-1307. 35. Liang, H.; Wei, W.; Wu, Z.; Feng, X.; Müllen, K. J. Am. Chem. Soc. 2013, 135, 16002-16005.

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Figure 1. Schematic illustration of the fabrication of Fe-N-SCCFs.

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Figure 2. (a) TEM image of 20 nm Fe3O4 nanocubes used for self-assembly. (b) Representative HRSEM image of carbon-coated Fe3O4 nanocube superlattices. (c) TEM image and the corresponding FFT (inset) of Fe-N-SCCFs viewed from the [100] zone axis. (d) False-color HRTEM image of Fe-N-SCCFs, showing the ultrathin pore walls comprising few-layer stacking graphene with an interlayer distance of ~ 0.36 nm. (e) Representative SAXS pattern of Fe-N-SCCFs. Fitted peaks were shown in blue. (f) XRD pattern and Raman spectrum (insert) of Fe-N-SCCFs.

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Figure 3. (a, b) High-resolution N 1s and Fe 2p XPS spectra of Fe-N-SCCFs, respectively. (c) STEM image and the corresponding EDS elemental mapping of FeN-SCCFs, showing the homogeneous distribution of Fe and N over the entire carbon framework. (d) Representative Cs-corrected HAADF-STEM image of Fe-N-SCCFs. (e) Enlarged HAADF-STEM image of the region indicated in (d). Bright spots corresponding to the isolated Fe single atoms were indicated by circles. (f) Normalized Fe K-edge XANES spectra and (g) Fourier-transformed k3-weighted EXAFS spectra of Fe-SCCFs, Fe-N-SCCFs, and reference Fe foil.

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Figure 4. (a) RRDE polarization curves of Fe-N-SCCFs and Pt/C at a scan rate of 10 mV s-1 and a rotation speed of 1600 rpm. The polarization curves of Fe-SCCFs and Fe-N-DCFs obtained in control experiments were also provided for comparison. (b) Hydrogen peroxide yield (top) and electron-transfer number (bottom) of Fe-N-SCCFs and Pt/C. (c) Polarization curves of Fe-N-SCCFs after 5000 and 10000 cycles. (d) Polarization curves of Fe-N-SCCFs in O2-saturated 0.1 M KOH with (black curve) or without (red curve) the addition of 5 mM KCN.

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Figure 5. (a) Photograph of the customized electrochemical cell used for zinc-air battery measurements. (b) Discharge polarization curves and corresponding power density of zinc-air batteries using Fe-N-SCCFs and Pt/C. (c) Galvanostatic discharge curves of Fe-N-SCCFs and Pt/C at a current density of 10 and 50 mA cm-2, respectively.

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