N Codoped Carbon

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Template-free synthesis of two-dimensional Fe/N codoped carbon networks as efficient ORR electrocatalysts Jingli Feng, Meiling Dou, Zhengping Zhang, and Feng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13445 • Publication Date (Web): 04 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Applied Materials & Interfaces

Template-free synthesis of two-dimensional Fe/N co-doped carbon networks as efficient ORR electrocatalysts Jingli Feng1,2, Meiling Dou1,2, Zhengping Zhang1,2 *, Feng Wang1,2 * 1 State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing 100029, P R China 2 Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China

KEYWORDS: two-dimensional carbon materials, template-free methods, direct pyrolysis synthsis, graphitic-carbon nitride intermediates, oxygen reduction reaction

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ABSTRACT: A direct-pyrolysis and template-free synthesis strategy is demonstrated to synthesize the two-dimensional (2-D) Fe/N co-doped carbon networks by virtue of 2-D graphitic-carbon nitride (g-C3N4) intermediates derived from melamine. Due to the stabilization and steric hindrance of additional N ligands with bis-nitrogen-containing groups (phenanthroline, phthalonitrile and phenylenediamine), the thin graphitic layered Fe/N co-doped carbon materials have successfully inherited the 2-D morphology from the g-C3N4 intermediate after direct carbonization treatment. After the easy removal of inactive Fe particles, the resultant sample exhibits numerous well-dispersed Fe atoms embed in the carbon layers with hierarchically (meso- and micro-) porous structure. Owing to the high active site density and open porous structure, the thin graphitic layered Fe/N co-doped carbon electrocatalysts exhibit the superior ORR performance (half-wave potential of 0.88 V and kinetics current density of 3.8 mA cm-2), even better than the commercial Pt/C catalysts (0.85 V and 1.6 mA cm-2, respectively). The facile and effective synthesis strategy without template to build the graphene-like nanoarchitectures inherited from the 2-D intermediates will lead a great development of 2-D carbon materials in various electrochemical applications.


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1. INTRODUCTION Along with the rising energy consumptions and environmental crisis, numerous concerns have been raised over the sustainable and clean energy. To develop the electrochemical conversion processes is one of the most effective strategies to ensure the security of our energy future.1-3 Electrocatalysts could play a substantial role in increasing kinetic rate and reaction efficiency in these energy conversion technologies.4, 5 However, most of state-of-the-art electrocatalysts are based on precious metals (e.g., platinum) with extremely high price and low yield.6, 7 The great challenge is to develop non-precious metal electrocatalysts with highly catalytic performance to popularize the widespread utilization of sustainable and clean energy technologies.8-11 As one kind of the potential alternatives to Pt-based electrocatalysts, carbon-based materials possess abundant supply and distinct merits for designable electrocatalysis owing to their adjustable doping-elements, flexible molecular structure and diverse morphology.4, 12-17 In the past decades, after transition metals and heteroatoms co-doping, the nanocarbon materials (typically iron/nitrogen co-doped carbon, FeNC) have been widely studied for various electrocatalysis systems, especially for oxygen reduction reaction (ORR).18-20 The boosted activity of the FeNC electrocatalysts has been widely illustrated due to their highly active Fe-NxCy co-doped structure. Among numerous synthetic strategies for FeNC materials, the in-situ doping methodology via the direct pyrolysis of the iron, nitrogen and carbon sources without carbon supports has been demonstrated as one kind of effective approaches in simple operation, apparent cost-saving and large-scale application.21 However, during the pyrolysis treatment, the thermal reduction of Fe ions always leads to a large number of Fe aggregations before the carbonization of organic molecules, which most likely catalyzes the small organic molecules into core-shell carbon nanotubes.22 This demerit restricts tailoring the structure and surface properties


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to achieve the rational design of the FeNC materials as advanced electrocatalysts. In addition, the non-active Fe/Fe3C particles at the tip and inside of the pyrolyzed carbon nanotubes make it difficult to remove by post-treatment with acid-leaching, which also decreases the mass activity of the FeNC electrocatalysts.23, 24 To overcome these issues, numerous approaches have been explored using metal organic frameworks (MOFs) or covalent organic frameworks (COFs) as precursors usually involving multi-step careful synthesis of MOFs or COFs, which emerge tedious and high-cost for bulk production with poor control over the inevitable metal aggregation yet.25-29 Some different approaches have also been submitted to employ the various templates to construct hybrid intermediates of Fe, N and C sources with metal salts/oxides (e.g., NaCl and MgO)/templates (e.g., silicon nanopowder, block copolymers, etc.).30-36 Pyrolysis of the hybrid intermediates with these templates is a feasible process; however, the resultant products show a low production yield with high cost. Besides, the removal for some kind of templates (e.g., removing silicon template) usually is required to use the toxic or hazard reagents (e.g., hydrogen fluoride or concentrated alkali), causing an environmental pollution and a health risk. Therefore, it is still highly challenging to design an effective and facile approach to controlling the carbon structure with numerous exposed active sites. As one kind of the promising nitrogen/carbon-containing intermediates, graphitic-carbon nitride (g-C3N4) can provide high nitrogen content (50 at.%) with graphene-like feature.37,

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Besides, g-C3N4 can be synthesized from simple feedstocks (e.g., melamine, urea) via polycondensation reactions (550-600 °C) by the direct pyrolysis treatment, which is expected to simplify the preparation process of the FeNC materials.39,

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Furthermore, g-C3N4 can be

constructed into a two-dimensional (2-D) structure, which is favorable to accelerating the mass transport, being significantly important in electrochemical convertion devices.41-44 Despite its


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superiority, the iron-nitrogen co-doped-carbon materials derived from melamine or urea, have rarely inherited the 2-D morphology from the g-C3N4 intermediate, due to the catalytic carbonization of decomposed small organic molecules by Fe aggregations.39,

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From these

analogies, we surmised that minimizing the electrostatic interaction of Fe ions to alter the isolation distance of Fe ions through the coordinated construction by additional ligand could suppress the aggregation of Fe nanoparticles, and hence build the 2-D nanoarchitectures inherited from the g-C3N4 intermediates. Herein, we demonstrate an effective in-situ Fe-doping method by introducing several nitrogencontaining ligands to prepare a series of 2-D FeNC electrocatalysts derived from the g-C3N4 intermediates (associated to the pyrolysis of melamine). The additional nitrogen-containing ligands with bis-nitrogen-containing groups (bis-NCGs) on various benzene structures (phenanthroline, PT; phthalonitrile, PN; phenylenediamine, PD) could effectively coordinate the Fe ions (Figure S1) serving as crabs to clamp Fe ions for expanding a steric hindrance and inhibiting the heterogeneous one-dimensional growth by Fe aggregations (Scheme 1). All the resultant FeNC materials derived from the bis-NCGs ligands (i.e., PT, PN and PD) revealed the thin graphitic layered fabric (designed as: g-FeNC-PT, g-FeNC-PN and g-FeNC-PD, respectively) with numerous exposed Fe-Nx-Cy sites (see, Experimental Section and Supporting Information for details). However, the tubular carbon materials with irremovable Fe nanoparticles (leached by 1 M H2SO4) inside the tip of carbon tubes (designed as: t-FeNC) were observed for the samples prepared in the same synthetic condition just without the addition of bis-NCGs ligands (see, Experimental Section for synthesis) by the scanning electron microscopy (SEM), transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) pattern (Figure 1a, 1b and Figure S2). For the 2-D FeNC samples, the species of bis-NCGs ligands were


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optimized and it was found that g-FeNC-PT exhibited the best electrocatalytic performance (Supporting Information), and hence only g-FeNC-PT was used for subsequent analysis in detail.

2. EXPERIMENTAL SECTION Synthesis of g-FeNC-PT catalysts: The 0.11 g FeCl3 and 0.47 g (0.002 mol) phenanthroline hydrochloride were dispersed in 20 mL ethanol, and stirred for 5 minutes. Subsequently, 2.00 g of melamine was added in the mixed solution and stirred for 3 h at room temperature, and dried at 80 °C for 12 h. Afterward, the obtained powder was heated at a rate of 5 °C min-1 to 600 °C for 2 h (condensation process) and to 800 °C for 2 h (carbonization process) under ﬂowing highly pure argon gas. Moreover, the obtained black powder was treated in 1 M H2SO4 for 12 h to remove the unstable metal nanoparticles and the acid-leached sample was washed with water several times, dried at 80 °C for 12 h, and then directly pyrolyzed again at 800 °C for 1 h to obtain g-FeNC-PT catalyst. The sample before acid leaching treatment was recorded as g-FeNCPT-BL. The g-FeNC-PN and g-FeNC-PD catalysts were prepared with the same synthesis condition by using 0.002 mol (0.2566 g) phthalonitrile and 0.002 mol (0.2156 g) ophenylenediamine to replace phenanthroline hydrochloride, respectively. The t-FeNC and tFeNC-BL samples were obtained with the same condition just without the addition of phenanthroline hydrochloride.

3. RESULTS AND DISCUSSION As shown in Figure 1c, the resultant g-FeNC-PT sample was constituted of numerous crumpled layers without obvious carbon tube, and the layered morphology was further observed in the TEM image (Figure 1d). These wrinkled carbon nano-networks without obvious Fe


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aggregation could construct into hierarchically porous structure with large specific surface area to facilitate the rapid mass transport on the carbon surface. Similar 2-D morphologies were also observed for g-FeNC-PN and g-FeNC-PD samples (Figure S3 and S4). The aberration-corrected high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and Enfina electron energy loss spectrometer (EELS) elemental mapping measurements were conducted to further investigate the chemical composition and doping structure of the pyrolyzed carbon. The HAADF-STEM image (Figure 1e) of g-FeNC-PT exhibited numerous well-dispersed Fe atoms, without noticeable metal atom aggregation, embedded in the carbon layers; meanwhile, the elemental mapping images (Figure 1f) verified that Fe and N elements were uniformly distributed in the nano-network after leaching, which inferred the high stability of the Fe/ N codoped-structure within the carbon in an acidic environment. Moreover, the Raman spectra were employed to detect the different carbon structure of g-FeNC-PT and t-FeNC (Table S1 and Figure S9). Despite the similar content of graphene-plane defects for both two samples, g-FeNC-PT possessed more nitrogen-doped structure, originated from its higher content of the related peaks called I-line than t-FeNC, which inferred that g-FeNC-PT should process more Fe-Nx-Cy active sites.46 To obtain the further insight into the surface chemical states and electronic structure of the tFeNC and g-FeNC-PT sample, the X-ray spectroscopic analyses were conducted. The X-ray photoelectron spectroscopic (XPS) survey spectra (Table S2 and Figure S10) were first used to explore the content (atomic ratio, at.%) of C, N, O and Fe elements for g-FeNC-PT (83.7, 7.4, 7.8 and 1.1 at.%, respectively) an d t-FeNC (89.1, 4.7, 5.3 and 0.9 at.%, respectively). The corresponding high-resolution XPS spectra for C 1s (C=C, 284.8 eV; C-C, 285.6 eV; C-N/C-O, 286.6 eV) and N 1s (pyridinic N, 398.6 eV; pyrrolic N, 400.1 eV; graphitic N, 401.3 eV) were


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both deconvoluted into three responding component peaks, respectively (Figure 2a and 2b).47-49 Compared with t-FeNC, the g-FeNC-PT possessed enriched Fe and N codoping structure and the higher content of pyridinic N species (1.41 times), which could provide the lone pair electrons to coordinate ionic Fe, at least partly, to form the active and acid-resistant Fe-Nx-Cy sites.49,

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Furthermore, we performed the X-ray absorption fine structure (XAFS) measurements to explore the microstructure and state of Fe in the t-FeNC and g-FeNC-PT samples. As shown in Figure 2c, the X-ray absorption near-edge structure (XANES) spectra showed that the Fe K-pre-edge of t-FeNC was closely near the reference of Fe foil, but not for the g-FeNC-PT sample, demonstrating that the metallic Fe0 valence state was still withheld in t-FeNC. Additionally, the g-FeNC-PT sample exhibited much higher half-edge energy and broader white line than t-FeNC, demonstrating that more free electrons in the local Fe structure transferred to nitrogen.51 The bonding environment of Fe atoms was carefully explored by the extended X-ray absorption fine structure (EXAFS, Figure 2d). The Fourier transforms of R space for g-FeNC-PT indicated that the formation of Fe-Nx-Cy complexes was consisted with the Fe-N and Fe-C bonding (the peaks at 1.5 Å and 2.6 Å, respectively); however, the obvious appearance of a Fe-Fe bonding peak (2.2 Å) in t-FeNC reconfirmed the existence of the large amounts of metallic Fe once again.24 The large divergence in the morphologies and structures between the resultant g-FeNC-PT and t-FeNC samples drew our attention. Multiple characterizations were performed to reveal the formation of these two samples differing vastly of the influence of direct pyrolysis and acid leaching processes. As for both of the g-FeNC-PT and t-FeNC before leaching (g-FeNC-PT-BL and t-FeNC-BL, respectively), the corresponding precursors were measured with the thermal gravimetric-differential thermal analysis (TG-DTA) measurements in a N2 atmosphere (Figure 3a). Two obvious weight-loss waves and the corresponding exothermic peaks (at 325 and


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675 °C) were both observed for g-FeNC-PT-BL and t-FeNC-BL, attributed to the thermal polycondensation for g-C3N4 and the further carbonization of pyrolyzed carbon, respectively. It was worth noting that though the final yields and trends of mass-loss waves for g-FeNC-PT-BL and t-FeNC-BL were same, the weight losses of the two samples in the same thermal process were different. The g-FeNC-PT-BL exhibited less weight losses in the polycondensation and a more loss in the carbonization stage. The phenomenon was reconfirmed with the thermal gravimetric-infrared (TG-IR) spectra (Figure S11). As for g-FeNC-PT-BL, less NH3 gas emission at the early thermal stage, which inferred that more N-containing ligands could be preserved to stabilize Fe ions and delay the thermal reduction process of Fe. Besides, the increased alkane gas emission was generated for the possible steam activation during the carbonization process. To observe the different changes of the intermediates, multiple physical characterizations were conducted for the g-FeNC-PT-BL and t-FeNC-BL at 600 °C (the region between the polycondensation and carbonization process), including powder XRD patterns (Figure S12), TEM images and elemental mapping images (Figure S13, S14), all indicated that the Fe doped g-C3N4 intermediates were successfully generated. As for the intermediates of gFeNC-PT-BL, the Fe ions were coordinated to the g-C3N4 host with uniformly dispersion, instead of the small Fe aggregation in that of t-FeNC-BL. After acid leaching, the XRD patterns in Figure S16 signified that the reduced Fe particles (metallic Fe and Fe3C) of g-FeNC-PT were basically removed, but not for the t-FeNC simples. N2 adsorption−desorption isotherms were also employed for the g-FeNC-PT and t-FeNC samples before and after acid leaching (Figure 3b). The g-FeNC-PT sample exhibited a type-IV isotherm characteristic of mesopores materials with a largest Brunauer-Emmett-Teller surface area (SBET) of 549 m2 g-1. Compared with the resultant samples before leaching, SBET of the g-FeNC-PT and t-FeNC (101 m2 g-1) were


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increased 2.7 and 1.6 times. The enlarged SBET and increased enhancements of g-FeNC-PT mostly originated from the considerable gas emission during carbonization and minimum retention of Fe aggregation, along with the wrinkled 2-D structure. In addition, density functional theory (DFT) pore size distribution plots (Figure 3c) derived from the N2 isotherms revealed that the g-FeNC-PT presented the hierarchically meso-/microporous (centered at 3.5/1.4 nm, respectively) structure with mesopore surface area (Smeso, 424 m2 g−1) and micropore surface area (Smicro, 125 m2 g−1). The micropores could provide numerous accessible Fe-Nx-Cy active sites in accord with the XPS results. The signiﬁcantly high total pore volume (0.67 cm3 g-1) was mainly contributed to the designable mesopores (the mesopore volume of 0.61 cm3 g-1), which could facilitate the fast diffusion of both reactants and electrolytes. Based on the above results, the illustrations of the formation process of g-FeNC-PT and t-FeNC were summarized in Figure 3d. The ORR activity of the resultant Fe/N co-doped carbon electrocatalysts were first evaluated with cyclic voltammetric (CV) measurements. As shown in Figure 4a, all the electrocatalysts, including g-FeNC-PT, t-FeNC, g-FeNC-PT-BL and t-FeNC-BL, presented the oxygen-reduction peaks in O2-saturated 0.1 M KOH electrolyte, but not in the N2-saturated electrolyte. The gFeNC-PT showed the most positive peak potentials of 0.84 V (versus reversible hydrogen electrode, vs. RHE, the same below), demonstrating the highest catalytic activity among all the samples (0.82 V for g-FeNC-PT-BL, 0.69 V for t-FeNC and 0.76 V for t-FeNC-BL). To investigate the advanced activity of linear sweep voltammetry (LSV) curves of the above four electrodes with the rotating disk electrode (RDE) were further conducted in comparison with the commercial Pt/C catalyst (20 wt.%, Johnson Matthey, UK). As shown in Figure 4b, the g-FeNCPT electrode exhibited the superior ORR activity with the positive half-wave potential (E1/2) of 0.88 V and the high diffusion limited current density (Jd) of 5.7 mA cm-2, which indicates much


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higher activity than t-FeNC, and even better than the commercial Pt/C (E1/2 = 0.85 V, Jd = 5.7 mA cm-2). Notably, the increased activity of g-FeNC-PT was observed for the g-FeNC-PT-BL sample (E1/2 = 0.84 V, Jd = 5.6 mA cm-2) after acid leaching; however, the t-FeNC electrode performed the similar and even worse ORR activity compared with t-FeNC-BL after acid leaching. The Tafel slopes were measured to evaluate the kinetics of the resultant electrodes and the Pt/C catalyst for oxygen reduction process (Figure 4c). At the low over-potential region, the gFeNC-PT and g-FeNC-PT-BL exhibited small Tafel slope (59 and 61 mV decade−1, respectively), indicating that the g-FeNC-PT and g-FeNC-PT-BL electrocatalysts shared the similar rate-determining step with Pt/C (64 mV decade−1). To verify the ORR catalytic pathways of the above electrodes, the electron transfer number (n) and the percentage of peroxide (% HO2−) were calculated from the monitor of rotating ring-disk electrode (RRDE) measurements (Figure S18), suggesting that the g-FeNC-PT electrode is efficient to reduce the peroxide yield of 2e pathway (below 2%) even much better than Pt/C (below 5%), demonstrating a 4-electron transfer pathway for ORR (Figure 4d). The electron transfer number for g-FeNC-PT was also consistent with that calculated from Koutecky–Levich (K–L) plots at various potentials (0.2–0.6 V, Figure 4e). The t-FeNC and t-FeNC-BL showed inferior ORR activity with the relatively high peroxide yield of above 10%, suggesting a trending 2–electron oxygen reduction process, mostly due to the participation of irremovable and inactive Fe aggregation for ORR. We also investigated the ORR activity of g-FeNC-PT and t-FeNC, along with Pt/C as reference, in a 0.1 M HClO4 electrolyte. As seen in Figure 4f and S17, the g-FeNC-PT electrode exhibited a passable catalytic activity (E1/2 = 0.77 V, Jd = 5.6 mA cm-2, n = 3.9), which was inferior to the


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commercial Pt/C (E1/2 = 0.83 V, Jd = 5.6 mA cm-2) but still much better than t-FeNC (E1/2 = 0.70 V, Jd = 4.8 mA cm-2). To better elucidate the origin of the enhanced activity, we also evaluated the ORR performance of the other two graphitic layered samples derived from phthalonitrile (PN) and phenylenediamine (PD) as bis-NCGs ligands. As shown in Figure 5a, all the graphitic layered samples, including g-FeNC-PT, g-FeNC-PN and g-FeNC-PD, exhibited the similar defined shape of the LSV characteristics with much improved activity for ORR. The corresponding behaviors in terms of E1/2, Jd, onset potential (E0) and kinetic current density (Jk), along with the SBET of the above three samples (Table S5 and Figure S21), were illustrated in Figure 5b in comparison with t-FeNC and commercial Pt/C. The parameters for ORR activity for all the resultant samples increased, followed by the trend of SBET (i.e., t-FeNC (101 m2 g-1) < g-FeNCPN (318 m2 g-1) < g-FeNC-PD (423 m2 g-1) < g-FeNC-PT (549 m2 g-1)), especially for the E1/2 and Jk. According to the N2 adsorption-desorption isothermals, the graphitic layered samples processed the open pore system (H3 hysteresis), facilitating the reduction of diffusion resistance and the enhancement of the ORR performance.30, 52 In this case, the surface area might correlate with the electrochemical surface area (ECSA), and the related catalytic activity. It was worth noting that all the graphitic layered samples presented the much larger limiting currents than that of t-FeNC despite the similar E0 of them. It was attributed to the unique 2-D structure that could effectively avoid pores blocking and lead fast kinetics, allowing the full employment of surface area as ECSA for ORR. Another interesting point is the increased surface area of the graphitic layered samples corresponded to the decreased thickness of the carbon layers in the different samples (g-FeNC-PT: 3.4 nm; g-FeNC-PD: 5.5 nm; g-FeNC-PN: 7.4 nm), which were reflected by TEM and atomic force microscopy (AFM) measurements (Figure S3, S4 and S22),


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respectively. The decreased thickness of carbon layer would lead to the increased accessible site density. Compared with other carbon-based ORR electrocatalysts, especially the FeNC (Table S6), the g-FeNC-PT catalyst shows more positive E1/2 and better mass activity than most carbonbased materials. The obvious enhancement of ORR performance is also contributed to the effective utilization of such numerous single Fe active sites. The electrochemical stability and methanol tolerance of g-FeNC-PT and Pt/C were tested by the amperometric i−t response in 0.1 M KOH electrolyte. As shown in Figure 5c, the g-FeNC-PT catalyst (current drop of 5.7 %) exhibited higher electrocatalytic stability than the commercial Pt/C catalyst (current drop of 42.8 %) after 10,000 s. In addition, the g-FeNC-PT also showed the outstanding methanol tolerance with almost free from the methanol crossover effect, which promised it serving as a potential electrocatalysts for methanol fuel cells (Figure 5d).

4. CONCLUSION In conclusion, g-C3N4 intermediate derived from melamine was used as an origin planar construction for the direct-pyrolysis and template-free synthesis of a hierarchically porous FeNC electrocatalysts with the 2-D nanoarchitectures. It turned out that a series of bis-NCGs ligands could effectively stabilize the Fe ions serving as crabs to clamp Fe ions to expand a steric hindrance, and prevent the uncontrollable structural changes during carbonization treatment. The direct and template-free pyrolysis of melamine and Fe salts with the additional bis-NCGs improves the efficiency and designability of the pyrolysis approach due to the simple synthesis of precursors and the full use of architectures from intermediates. The very facile 2-D constructing strategy shows great advantages compared to the template method such as higher production, less synthetic steps and lower cost, along with avoidance of harsh reagent. Owing to


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the high site density and open porous structure, the resultant g-FeNC-PT electrocatalyst exhibited the superior ORR performance, even better than the commercial Pt/C catalyst. Overall, this work demonstrates the new concept that constructs the 2-D metal and nitrogen co-doped carbon materials with hierarchical pores, processing a great improvement of their electrocatalytic performance. We anticipate our strategy developed in this work will also be applied for other electrochemical applications.

ASSOCIATED CONTENT Supporting Information Detailed materials, preparation of g-FeNC-PT and other Fe-based catalysts, characterizations and electrochemical Measurements; FT-IR spectra, XRD patterns, SEM image, TEM images with corresponding elemental maps, Raman spectra, XPS survey spectra, TG-IR patterns, AFM images, electrochemical tests for the as-prepared samples are shown in Supporting Information, including the optimization of Fe-based catalysts for ORR. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (F. Wang); [email protected] (Z. Zhang) Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here.

ACKNOWLEDGMENT This work was supported by the National Natural Science Funds of China (51432003, 51802011, 51125007); National Key R&D Program of China (2018YFB0105500); the Start-Up Fund for Talent Introduction of Beijing University of Chemical Technology (buctrc201806); the Fund of High-Performance Carbon-Based Electrodes for Energy Storage; and the Fundamental Research Funds for the Central Universities (JD1802).

REFERENCES (1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (2) Zhang, H.; Shen, P. K. Recent Development of Polymer Electrolyte Membranes for Fuel Cells. Chem. Rev. 2012, 112, 2780-2832. (3) Steele, B. C.; Heinzel, A. Materials for Fuel-Cell Technologies. Nature 2011, 414, 345-352. (4) Zheng, Y.; Jiao, Y.; Qiao, S. Z. Engineering of Carbon-Based Electrocatalysts for Emerging Energy Conversion: From Fundamentality to Functionality. Adv. Mater. 2015, 27, 5372-5378.


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Figures

Scheme 1. Illustration of the coordination of bis-NCG with iron ions.


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Figure 1. Typical SEM and TEM images of (a, b) t-FeNC and (c, d) g-FeNC-PT. e) HAADFSTEM image and f) the corresponding element mapping images for C (red), N (green) and Fe (blue) of g-FeNC-PT.


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Figure 2. High-resolution XPS spectra of a) C 1s and b) N 1s for the g-FeNC-PT and t-FeNC samples. c) Fe K-edge XANES spectra and d) Fourier transforms of k3-weighted χ(k)-function of the EXAFS spectra for the t-FeNC and g-FeNC-PT samples, along with iron foil as reference.


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Figure 3. a) TG-DTA curves for precursors of t-FeNC-BL and g-FeNC-PT-BL in N2 atmosphere. b) Nitrogen adsorption–desorption isotherms and c) the corresponding DFT pore size distributions of g-FeNC-PT, t-FeNC, g-FeNC-PT-BL and t-FeNC-BL samples. d) The formation of t-FeNC (top) and g-FeNC-PT (bottom).


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Figure 4. a) CV curves of g-FeNC-PT, t-FeNC, g-FeNC-PT-BL and t-FeNC-BL electrodes in O2-saturated (solid line) or N2-saturated (dashed line) 0.1 M KOH at a sweep rate of 50 mV s−1. b) RDE polarization curves of the above four electrodes and the commercial Pt/C electrode in O2-saturated 0.1 M KOH at a sweep rate of 5 mV s−1 with 1600 rpm. c) Tafel plots of the above five electrodes derived by the mass-transport correction of corresponding RDE data in b). d) The calculated % HO2− (top) and n (bottom) at various potentials calculated from the corresponding RRDE data in Figure S18. The RDE polarization curves of the g-FeNC-PT electrode with a sweep rate of 5 mV s−1 at the different rotation rates (625-2025 rpm) in O2-saturated e) 0.1 M KOH and f) 0.1 M HClO4. The insets in e) and f) show the corresponding K–L plots (J−1 vs. ω−1/2) at different potentials, respectively.


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Figure 5. a) RDE voltammograms of the t-FeNC, g-FeNC-PD, g-FeNC-PN, g-FeNC-PT and commercial Pt/C electrodes in O2-saturated 0.1 M KOH at 1600 rpm. b) The response potentials (E0 and E1/2, top) and response current density (Jd and Jk at 0.9V, bottom) versus the SBET of the as-prepared samples, compared with Pt/C. c) Durability evaluation and d) methanol-crossover from chronoamperometric (i−t) responses of the g-FeNC-PT and the commercial Pt/C electrodes in O2-saturated 0.1 M KOH.


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Table of Content


29

N Codoped Carbon

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