Fe-Nx Sites Enriched Carbon Micropolyhedrons Derived from Fe

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Fe-Nx Sites Enriched Carbon Micropolyhedrons Derived from Fe-Doped Zeolitic Imidazolate Frameworks with Reinforced Fe-N Coordination for Efficient Oxygen Reduction Reaction Guanying Ye, Kuangmin Zhao, Zhen He, Rongjiao Huang, Yuchi Liu, and Suqin Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04105 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 7, 2018

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ACS Sustainable Chemistry & Engineering

Fe-Nx Sites Enriched Carbon Micropolyhedrons Derived

from

Fe-Doped

Zeolitic

Imidazolate

Frameworks with Reinforced Fe-N Coordination for Efficient Oxygen Reduction Reaction Guanying Ye,† Kuangmin Zhao,† Zhen He*,†,‡,§, Rongjiao Huang,† Yuchi Liu,† and Suqin Liu*,†,‡,§ †

College of Chemistry and Chemical Engineering, ‡Hunan Provincial Key Laboratory of

Chemical Power Sources, and §Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha, Hunan 410083, P. R. China. Corresponding Authors *E-mail: [email protected] (Zhen He); Fax: +86-731-88879616. *E-mail: [email protected] (Suqin Liu); Fax: +86-731-88879616. KEYWORDS: oxygen reduction reaction, Fe-N codoped carbon, metal-air battery, electrocatalysis, zeolitic imidazolate framework-8

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ABSTRACT: Rational design and facile synthesis of highly active and stable electrocatalysts for the oxygen reduction reaction (ORR) are crucial in the field of metal-air batteries. Here, we present a facile two-stage thermal synthesis of Fe-N codoped porous carbon (Fe-N/C) with abundant Fe-Nx active sites and mesopores from Fe-doped ZIF-8 precursors. The first-stage preheating treatment of the Fe-doped ZIF-8 precursors before the second-stage carbonization is the key to boost the coordination between the doped Fe and N-containing ligands, which contributes to a higher N content and more Fe-Nx sites in the final carbonized product. Besides, the preheating and Fe-doping both affect the morphology, porous structure, and catalytic performance of the fabricated Fe-N/C. The optimized Fe-N/C catalyst exhibits an outstanding ORR catalytic performance with a half-wave potential of 0.88 V and limiting current density of 6.0 mA cm-2 in 0.1 M KOH. A Mg-air battery assembled with a neutral electrolyte using the optimized Fe-N/C catalyst as the cathode exhibits an excellent power density of 72 mW cm-2 at 0.72 V. This developed two-stage synthesis strategy is facile and the preheating stage could be integrated into any carbonization process as an intermediate step for the fabrication of various metal, N codoped carbon materials with enhanced electrocatalytic performance.


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INTRODUCTION As promising new-generation energy conversion devices, metal-air batteries have gained much attention because of their high energy densities and environmental friendliness.1,2 However, the low coulombic efficiency of metal-air batteries, caused by the sluggish kinetics of the oxygen reduction reaction (ORR) at the air cathodes, is one of the key issues that need to be solved urgently.3 To overcome the poor reactivity of the air cathodes, many researchers have dedicated to developing high-performance ORR catalysts. The precious-metal-group materials represented by the Pt-group metal (PGM) catalysts exhibit efficient catalytic performance but account for unaffordable costs, i.e., nearly a half of the total cost of a metal-air battery.4 The resource scarcity and inferior stability have impeded the large-scale commercialization of the precious metal-based catalysts in metal-air batteries. Therefore, developing ORR catalysts with a high activity, long-term durability, and low cost is of great significance for the practical application of metal-air batteries.5 Transition-metal-based and carbon-based catalysts, such as metal carbides, metal phosphides, metal oxides, and carbon composites, have emerged as the precious metal-free ORR catalysts.6-8 Among them, the metal-nitrogen codoped carbon (M-N/C, M = Fe,9-12 Co,13 Cu,14,15 etc.) is considered as one of the most promising substitutes for the PGM catalysts for the ORR due to its abundant pore structure, high surface area, excellent electrical conductivity, and low cost.16-18 Particularly, much attention has been drawn to Fe-N/C due to its outstanding catalytic performance.19-21 Previous experimental and theoretical studies have both demonstrated that the Fe-Nx sites embedded in the carbon matrix could facilitate the adsorption of O2 and the subsequent breaking of the double bond of O2.22,23 However, the activity and long-term stability of Fe-N/C toward the ORR still need to be improved. Conventionally, the Fe-N/C catalysts were


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obtained through multi-step high-temperature pyrolysis of iron macrocycle complexes (e.g., iron phthalocyanines) or the physically mixed composites of iron salts and nitrogen-carbon containing precursors.24,25 However, the Fe-N/C catalysts synthesized using these approaches usually could not fully expose the catalytically active sites (i.e., the Fe-Nx sites) toward the ORR.26 Therefore, making full use of the catalytic activity of the doped metal atoms is essential for enhancing the activities and lowering the costs of the M-N/C ORR catalysts. Recently, zeolitic imidazolate frameworks (ZIFs), a class of metal-organic frameworks (MOFs) with a high nitrogen content, flexible adjustability, and three-dimensional structure, have been studied as precursors of carbon materials for energy-related applications.27,28 Zinc-based ZIF-8 has been widely used as a carbon precursor to prepare Fe-N/C by introducing Fe ions into the ZIF-8 followed by high-temperature pyrolysis.29 Different strategies have been developed to urge the formation of Fe-Nx sites rather than agglomerated Fe species (e.g., Fe, Fe3C, and Fe3N) in Fe-N/C for an enhanced ORR catalytic activity. Wang et al. fabricated a highly active and stable Fe/N-doped porous carbon by trapping Fe(acac)3 into ZIF-8 followed by high-temperature carbonization.30 Ye et al. prepared a Fe-N/C electrocatalyst by carbonizing ammonium ferric citrate-modified ZIF-8 nanoparticles to promote the exposure of Fe-Nx sites on the surface.31 Such macromolecular Fe sources could facilitate the dispersion of Fe, yet limited the content of the Fe-Nx sites in the final Fe/N-doped carbon, and the preparation processes were relatively complicate. Therefore, the development of a simple synthetic strategy for Fe-N/C with abundant Fe-Nx sites is still highly desired. Herein, we develop a facile two-stage thermal synthesis of Fe, N codoped carbon from an Fedoped ZIF-8 precursor, in which the preheating stage enhances the coordination between Fe ions and N-containing ligands in the Fe-doped ZIF-8 precursor and the followed carbonization stage


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forms abundant Fe-Nx sites in the final product, Fe-N/C. The effects of Fe-doping level and preheating temperature on the fabricated Fe-N/C have been investigated. The optimized Fe-N/C with the highest content of Fe-Nx active sites exhibits a remarkable electrocatalytic activity toward the ORR in both alkaline and neutral solutions. It shows a half-wave potential (E1/2) of 0.88 V (vs. RHE) and a limiting current density (jL) of 6.0 mA cm-2 in 0.1 M KOH solution, which is superior to the commercial Pt/C catalyst. Moreover, a Mg-air battery using the fabricated Fe-N/C catalyst as the air cathode and 1 M NaCl solution as the electrolyte shows an excellent electrochemical performance with a power density of 72 mW cm-2 at 0.72 V.

EXPERIMENTAL SECTION Synthesis of Fe-doped ZIF-8. Zn(NO3)2·6H2O (>99.0%), Fe(SO4)2·7H2O (> 99.0%), 2methylimidazole (> 99.0%), and methanol (> 99.5%) were of AR grade and purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received without further purification. The Fe-doped ZIF-8 (Fe-ZIF-8) was synthesized by a simple precipitation combined with chemical adsorption. In detail, 4.75 mmol of Zn(NO3)2·6H2O and 0.25 mmol of Fe(SO4)2·7H2O were dissolved in 100 ml of methanol. Another 100 ml of methanol containing 20 mmol of 2-methylimidazole was added into the pre-prepared Zn/Fe-containing solution under vigorous stirring for 2 h and the well-mixed solution was then aged for 24 h at room temperature to form Fe-ZIF-8. The Fe-ZIF-8 was collected by filtration, washed with methanol for three times, and dried at 60 °C in an oven overnight. The dried Fe-ZIF-8 was light-yellow powder. The Fe-ZIF-8 with different contents of Fe (denoted as x%Fe-ZIF-8, where x% represents the atomic ratio of Fe2+/(Fe2++Zn2+) in the used reagents during the synthesis of Fe-ZIF-8) were synthesized following the same procedures by adjusting the molar ratio of the added zinc and iron salts


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during the synthesis. For comparison, ZIF-8 was synthesized using the same approach without adding Fe(SO4)2·7H2O during the synthesis. Synthesis of Fe-N/C. The prepared x%Fe-ZIF-8 was carbonized under an Ar/H2 (90/10, v/v) flow in a tube furnace. During the pyrolysis process, two constant-temperature stages (i.e., the preheating treatment and final carbonization) were applied. First, the preheating stage was reached by raising the temperature from room temperature to 200 °C at a rate of 5 °C min-1 and then maintaining the temperature at 200 °C for 2 h. The material after this treatment was named as x%Fe-ZIF-8-200. After that, the temperature was further increased from 200 to 800 °C at 5 °C min-1 and kept at 800 °C for 2 h. Finally, the pyrolyzed product was treated in a 0.5 M H2SO4 solution for 12 h to remove the inactive components (e.g., Fe, Fe3C, and Fe3N), and then collected by filtration, washed with deionized water, and dried overnight at 60 °C in an oven. The as-prepared carbonized products were denoted as x%Fe-N/C. For comparison, x%Fe-N/C800 was synthesized via direct pyrolysis of x%Fe-ZIF-8 at 800 °C (with a heating rate of 5 °C min-1) for 2 h without undergoing the preheating stage. Physicochemical characterizations. The pyrolysis process of the Fe-ZIF-8 precursors was characterized by thermogravimetric/differential thermal analysis (TG/DTA) from room temperature to 900 °C in an Ar atmosphere with a heating rate of 10 °C min−1 on a thermal analyzer (DTA-50). The chemical bonding states and compositions of the precursors were investigated by Fourier transform infrared spectroscopy (FT-IR) in the range of 4000 to 400 cm−1 on a Nicolet-5700 spectrometer. The UV-vis spectra of the precursors were recorded by a UV1780 spectrometer from 800 to 200 nm. X-ray diffraction (XRD) was used to characterize the crystalline structure of the x%Fe-ZIF-8 precursors and x%Fe-N/C samples by using Cu Kα radiation (λ = 0.15418 nm) at a scan rate of 2°min-1 from the 2θ angle of 5 to 80 °on a Rigaku


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D/max 2500 diffractometer. The morphologies of the x%Fe-ZIF-8 precursors and x%Fe-N/C samples were investigated by using a scanning electron microscope (SEM, FEI NovaTM NanoSEM230) at an accelerating voltage of 15.0 kV and the detailed structure was investigated by high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F) operated at 300.0 kV. The X-ray photoelectron spectroscopy (XPS) study was carried out on a ThermoFisher ESCALAB 250Xi spectrometer using an Al Kα source (1486.6 eV). The low-pressure N2 adsorption-desorption isotherms for BET measurements were recorded on an Autosorb-iQ (Quantachrome) analyzer at 77K. Inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 5300DV) was used to determine the Fe content in the catalyst samples. Electrochemical measurements. The catalytic performance of the synthesized x%Fe-N/C catalysts was evaluated by the rotating-disk electrode (RDE) measurements on an Autolab PGSTAT204 electrochemical workstation in a three-electrode configuration at room temperature with a piece of Pt foil (1 cm2) as the counter electrode, an Ag/AgCl (in 3.5 M KCl) electrode as the reference electrode, and O2-saturated 0.1 M KOH (or N2-saturated 0.1 M KOH) or O2saturated phosphate buffer solution (PBS, pH ≈ 7, 100 ml aqueous solution containing 5.55 g of Na2HPO4·12H2O, 0.52 g of NaH2PO4·2H2O, and 8.50 g of NaCl) as the electrolyte. The working electrode was prepared as follows. About 6 mg of the as-prepared x%Fe-N/C catalyst was added into 1 ml of the solution composed of 950 µl of anhydrous ethanol and 50 µl of 5% Nafion solution under ultrasonication for at least 30 min to form homogeneous ink. Then, 8 µl of the ink was dropped on the surface of a glassy carbon RDE (with a diameter of 5 mm) and dried naturally at room temperature. The loading of the catalyst was about 0.245 mg cm-2. The linear sweep voltammetry (LSV) study was implemented by scanning the potential from 0.2 to -0.9 V (vs. Ag/AgCl) at 10 mV s-1 with different rotating rates (i.e., 400, 625, 900, 1225, and 1600 rpm).


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The cyclic voltammetry (CV) test was executed at a scan rate of 50 mV s-1 without electrode rotating. For comparison, the commercial 20 wt% Pt/C (named as Pt/C for short) was tested under the same conditions. The assembly and characterizations of the Mg-air battery using the optimized Fe-N/C catalyst as the air cathode are described in the Supporting Information (SI).

RESULTS AND DISCUSSION TG/DTA was first used to study the two-stage pyrolysis of the x%Fe-ZIF-8 precursors (Figure 1a, S1). The DTA curve of 5%Fe-ZIF-8 shows a shallow valley at the preheating temperature (i.e., 200 °C), indicating that an endothermic reaction occurs (see details in SI). To further explore the effect of the preheating treatment on the x%Fe-ZIF-8 precursors, the as-prepared ZIF-8, 5%Fe-ZIF-8, and 5%Fe-ZIF-8 treated at 200 °C (5%Fe-ZIF-8-200) were characterized by XRD and SEM. The XRD patterns (Figure 1b) of these three samples are almost identical, suggesting that neither Fe doping nor the preheating treatment at 200 °C has a significant influence on the crystalline structure of the synthesized MOF precursors. However, the Fedoping and preheating treatment significantly affect the morphology of the synthesized MOF precursors (Figure 1c-d, S2). That is, the Fe-doping increases the apparent particle size of the synthesized MOF crystals and the preheating treatment at 200 °C facilitates the formation of MOF crystals with a well-defined polyhedral morphology (Figure 1d). FT-IR spectroscopy was used to characterize the chemical structure of the synthesized 5%Fe-ZIF-8 samples treated with different preheating temperatures. A vibration band around 421 cm-1，which could be attributed to the Zn-Nx bonds,32 appears in each of the FT-IR spectra of the 5%Fe-ZIF-8 preheated at different temperatures (i.e., 200, 300, and 400 °C), as shown in Figure 1e. As the preheating temperature increases to 300 °C and 400 °C, the relative peak intensity of the Zn-Nx bonds


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Figure 1. (a) TG-DTA of 5%Fe-ZIF-8 under an Ar atmosphere from room temperature to 900 °C at 5 °C min-1. (b) XRD patterns of ZIF-8, 5%Fe-ZIF-8, and 5%Fe-ZIF-8-200. SEM images of (c) 5%Fe-ZIF-8 and (d) 5%Fe-ZIF-8-200. (e) FT-IR spectra of 5%Fe-ZIF-8 treated at 200, 300, and 400 °C for 2 h. decreases. This implies the decomposition of the Fe-ZIF-8 at such temperatures, which agrees well with the TGA result. Since the Fe content in 5%Fe-ZIF-8 is not high enough for an unambiguous measurement, the bonding state of Fe in the synthesized Fe-ZIF-8 was analyzed by using the 20%Fe-ZIF-8 sample with the same crystalline structure as 5%Fe-ZIF-8 (Figure S3). Compared with the FT-IR spectrum of ZIF-8, the FT-IR spectrum of 20%Fe-ZIF-8 shows characteristic peaks of Fe-2-methylimidazole (see details in Figure S4).33 It is worth mentioning that these Fe-2-methylimidazole peaks are more pronounced in the FT-IR spectrum of 20%FeZIF-8-200, suggesting that the preheating treatment at 200 °C could facilitate the coordination


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between Fe and 2-methylimidazole. In addition, the UV-vis spectrum of 20%Fe-ZIF-8-200 (Figure S5) exhibits a stronger absorption band at 385 nm (corresponding to the bonds between Fe and 2-methylimidazole) compared to that of 20%Fe-ZIF-8,34 which further indicates the enhanced coordination between Fe and 2-methylimidazole. These results all demonstrate that the preheating treatment integrated in the carbonization process as an intermediate step could enhance the bonding between Fe and 2-methylimidazoles. The as-synthesized x%Fe-ZIF-8 precursors were converted to x%Fe-N/C catalysts by a two-stage thermal treatment, i.e., the preheating treatment at 200 °C for 2 h and final carbonization at 800 °C for 2 h. The XRD patterns of all the x%Fe-N/C catalysts (Figure 2a) derived from the as-synthesized Fe-ZIF-8 precursors with different iron contents all exhibit two broad diffraction peaks at the 2θ angles of ~25 and ~44°, which could be assigned to the graphitic carbon (002) planes and disordered amorphous carbon (101) planes, respectively. No obvious characteristic diffraction peaks of Fe-based compounds could be found in these

Figure 2. (a) XRD patterns of x%Fe-N/C derived from x%Fe-ZIF-8 with different Fe contents. SEM images of (b) N/C, (c) 1%Fe-N/C, (d) 3%Fe-N/C, (e) 5%Fe-N/C-800, (f) 5%Fe-N/C, and (g) 7%Fe-N/C.


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x%Fe-ZIF-8 derived samples, suggesting that most of the possible products after carbonization such as metallic Fe and iron carbides have been removed by the acid etching.35 Since the Fedoping level has a profound effect on the morphology of the synthesized x%Fe-ZIF-8 (Figure S2), the morphologies of the x%Fe-N/C catalysts derived from the x%Fe-ZIF-8 with different Fe contents are also quite different (Figure 2b-g). The SEM study shows that the x%Fe-N/C after carbonization could largely inherit the morphology of the corresponding x%Fe-ZIF-8 precursor and the x%Fe-N/C derived from the x%Fe-ZIF-8 precursor with a higher Fe content has a larger apparent particle size. It is worth noting that severe cementation and agglomeration occur as the Fe content in the x%Fe-ZIF-8 precursor increases beyond 5% (e.g., 7%, 10%, and 15%, as shown in Figure 2g, S6). TEM and EDS were used to characterize the detailed structure and element distribution of the carbonized products (Figure 3). Compared with 5%Fe-N/C-800, the 5%Fe-N/C crystals derived from the 5%Fe-ZIF-8 precursor through the preheating treatment at 200 °C retain the precursor’s polyhedral morphology. After the high-temperature pyrolysis process, the surface of 5%Fe-N/C becomes rougher compared to that of its precursor, probably due to the dehydration and organic skeleton contraction during the pyrolysis process. These changes could also result in abundant pores, which actually favor the accommodation of a high density of active sites, and will be conducive to the diffusion of oxygen-containing species (like O2, HO2-, OH-, O2-) and the permeation of electrolyte.36 The high-resolution TEM study shows that 5%Fe-N/C is amorphous (Figure 3f), which is of benefit to obtain abundant defects and high efficiency ORR performance.37 In addition, the STEM image of 5%Fe-N/C shows no Fe particles and the EDS mapping images (Figure 3g-j) show that Fe, C, and N are uniformly distributed in 5%Fe-N/C, indicating that Fe is highly dispersed in the N-doped carbon matrix without noticeable


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aggregation.38 However, the agglomerated Fe species was found in the HRTEM images (Figure S7) of 7%Fe-N/C. This infers that the excessive increase of Fe in the synthesis process may lead to the aggregation of Fe and/or its compounds in the final carbon materials, which is disadvantageous to form highly dispersed Fe-Nx sites.

Figure 3. TEM images of (a-c) 5%Fe-N/C-800 and (d-f) 5%Fe-N/C at various magnifications. (g) STEM image of 5%Fe-N/C and the corresponding elemental mapping analysis of (h) carbon (red), (i) nitrogen (green), and (j) iron (yellow). Raman and XPS studies were performed to further analyze the presence of carbon and clarify the contents and chemical states of Fe and N in the fabricated x%Fe-N/C. The Raman


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spectra of 5%Fe-N/C and 5%Fe-N/C-800 (Figure 4a) both show two dominant peaks at 1350 (D band) and 1600 cm-1 (G band), corresponding to the disordered and graphitic carbon,

Figure 4. (a) Raman spectra of 5%Fe-N/C and 5%Fe-N/C-800. (b) The contents of N and Fe in N/C, 5%Fe-N/C-800, and 1%/3%/5%7%Fe-N/C based on the XPS results. (c) The content of three types of nitrogen in N/C, 5%Fe-N/C-800, and 1%/3%/5%7%Fe-N/C catalysts. (d) Highresolution N 1s XPS spectra of N/C, 5%Fe-N/C-800, and 1%/3%/5%/7%Fe-N/C.


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respectively.39 The intensity ratio of the D band and G band, ID/IG, for the 5%Fe-N/C-800 and 5%Fe-N/C is 0.88 and 1.01, respectively, suggesting that the preheating treatment at 200 °C contributes to the formation of more disordered carbon.40 Based on the results from the XPS survey spectra (Table S1), the atomic ratio of N in 5%Fe-N/C-800 (5.35 at%) is much lower than that in 5%Fe-N/C (7.88 at%), whereas the atomic contents of Fe in both samples are similar. This result along with the result of Raman spectroscopy demonstrate that the preheating treatment to the Fe-ZIF-8 precursors at an appropriate temperature prior to the final carbonization can promote the doping of N and formation of defect carbon in the MOF-derived carbon materials. The effect of Fe-doping level on the content and type of N in the fabricated x%Fe-N/C was further investigated by XPS (Figure 4b-d). The surface Fe contents based on the XPS analysis are 0.39 at%, 0.41 at%, 0.63 at%, and 0.42 at%, whereas the overall Fe contents based on the ICP-OES measurements are 0.75 wt%, 1.54 wt%, 2.93 wt%, and 3.87 wt% for 1%, 3%, 5%, and 7%Fe-N/C, respectively (Figure 4b). The unusually low surface Fe content in the 7%Fe-N/C could be attributed to that the excessive Fe doping in the x%Fe-ZIF-8 precursor leads to aggregation of metallic Fe and/or the formation of inorganic Fe-containing species, such as iron carbides after carbonization (as discussed above), which are removed by the acid etching process. Unlike the Fe content, the N content in the prepared Fe-N/C samples decreases from 9.93 at% in 1%Fe-N/C to 5.55 at% in 7%Fe-N/C. That is, as the Fe-doping increases, the N-doping level declines in the fabricated Fe-N/C. This is probably due to the removal of more iron nitrides forming at higher Fe-doping levels in the fabricated Fe-N/C via the acid-etching process.41 The high-resolution N 1s XPS spectrum (Figure 4c, 4d) can be deconvoluted into four peaks corresponding to four types of N-containing groups, including the pyridinic-N (Py-N) peak at


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398.3 eV (±0.2 eV), pyrrolic-N (Pyr-N) peak at 399.7 eV (±0.1 eV), graphitic-N (G-N) peak at 400.8 eV (±0.1 eV), and the Fe-Nx peak at 399.0 eV (±0.1 eV),36,42,43 indicating that the catalytically active Fe-Nx sites are successfully formed. In addition, the proportions of Py-N and Fe-Nx in the total N content of 5%Fe-N/C are significantly higher compared to those in 5%FeN/C-800 (Figure 4c). Since the Py-N and Fe-Nx have been demonstrated to be active toward the ORR,44,45 it can be presumed that the preheating process could enhance the catalytic activity of the Fe, N codoped carbon materials toward the ORR. As listed in Table S1, a higher Fe content in the prepared Fe-N/C catalyst corresponds to a higher proportion of Fe-Nx and G-N, but a lower proportion of Py-N. As the Fe-Nx is considered as a major active site toward the ORR, the result implies that 5%Fe-ZIF-8 has an optimized Fe-doping level, which results in a Fe-N/C material with the highest Fe-Nx content and a promising ORR activity. The number of effective Fe-Nx sites is not only related to the total Fe content, but also the surface area and pore structure of the fabricated x%Fe-N/C. N2 isothermal adsorption-desorption measurements were carried out to analyze the effect of Fe-doping level on the pore structure of the x%Fe-N/C (Figure 5a). The Brunauer-Emmett-Teller (BET) specific surface area of N/C is 819 m2 g-1, and the BET surface areas of x%Fe-N/C are 828, 667, 691, and 515 m2 g-1 for x% equaling to 1%, 3%, 5%, and 7%, respectively (Table S2). Generally, the BET specific surface area of the prepared x%Fe-N/C shows a decreasing trend as the Fe content increases (e.g., 10% and 15%Fe-N/C show a continuous decline of BET surface area in Figure S8a). This phenomenon is probably due to the Fe content-dependent morphology change of the x%Fe-N/C as evidenced by the SEM studies (Figure 2, S2, and S6). A higher Fe content results in a larger apparent particle size and therefore a lower specific surface area of the prepared x%Fe-N/C. It is noteworthy that the BET surface area of 1%Fe-N/C is slightly higher than that of the Fe-free N/C.


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Figure 5. (a) N2 adsorption-desorption isotherms of NC and x%Fe-N/C derived from x%Fe-ZIF8 with different Fe contents and (b) the corresponding BJH pore size distribution curves. Considering they have a similar particle size (Figure 2b, c), this difference in BET surface area could be attributed to the Fe-doping induced formation of mesopores in 1%Fe-N/C (Figure 5b, Table S2). With an appropriate Fe-doping, the prepared x%Fe-N/C clearly develops mesopores. However, with a too high Fe-doping level (e.g., in 7%, 10% and 15%Fe-N/C), the mesoporous structure almost disappears (Figure 5b, S8b). Therefore, an appropriate Fe-doping level in the MOF precursor is of great importance for the preparation of Fe-N/C with a suitable morphology, optimized pore structure, and abundant Fe-Nx active sites since Fe catalyzes the decomposition or dehydrogenation of the precursor frameworks and causes the formation of mesoporous structures.46,47 The electrochemically active surface area (ECSA) of the prepared Fe-N/C sample as a comprehensive variable was estimated by calculating the double layer capacitance (Cdl) based on the CV measurements.48 Since these samples are basically the same material, a larger Cdl corresponds to a higher ECSA (Figure S9). Unlike the trend presented by the BET measurement, the 5% Fe-N/C has the largest Cdl, meaning that it has the highest ECSA, which could provide more active sites to catalyze the ORR (see details in SI).


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The electrocatalytic performance of the as-prepared x%Fe-N/C catalysts toward the ORR was investigated by CV and LSV. Compared with the featureless CV curves measured in a N2saturated 0.1 M KOH electrolyte, the CV curves of the N/C, Pt/C, and 5%Fe-N/C catalysts measured in an O2-saturated 0.1 M KOH solution show ORR peaks at 0.72, 0.80, and 0.88 V (vs. RHE), respectively (Figure 6a). As Fe is incorporated, the ORR peak position on the 5%Fe-N/C shows an obviously positive shift, suggesting that an appropriate Fe-doping could enhance the catalytic performance of Fe-N/C toward the ORR. A RDE was used to further evaluate the catalytic performance of the prepared x%Fe-N/C catalysts. The LSV curves on various catalysts at a rotating rate of 1600 rpm are compared in Figure 6b, c. The 5%Fe-N/C shows a more positive half-wave potential (E1/2, 0.88 V) and a higher limiting current density (jL, 6.0 mA cm-2) than both the 5%Fe-N/C-800 (0.81 V, 5.5 mA cm-2) and commercial Pt/C (0.83 V, 5.7 mA cm-2). The catalytic performance of the fabricated 5%Fe-N/C toward the ORR is also comparable or even superior to the state-of-the-art Fe-containing ORR catalysts reported in literature (Table S3). The excellent ORR catalytic activity of the 5%Fe-N/C could be attributed to its high ECSA and abundant Fe-Nx sites arising from the appropriate Fe-doping level and the preheating treatment prior to the final carbonization. The relationship between the Fe content and ORR activity of the fabricated x%Fe-N/C was also investigated as shown in Figure 6c. Compared to N/C, all of the prepared x%Fe-N/C catalysts show a dramatic improvement in the electrocatalytic performance (more positive E1/2) toward the ORR. Among them, the 5%Fe-N/C is the best. The catalytic activity of the prepared x%Fe-N/C catalysts is related to the ECSA (represented by Cdl) of the catalysts as demonstrated by the Cdl-j’ plot (j’ represents the current density at 0.90 V vs. RHE on the LSV curve) and


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Figure 6. (a) CV curves on N/C, 5%Fe-N/C, and Pt/C in N2- or O2-saturated 0.1 M KOH solution at room temperature. (b) LSV curves of 5%Fe-N/C-800 and 5%Fe-N/C in O2-saturated 0.1 M KOH solution at room temperature. (c) LSV curves of N/C and 1%/3%/5%/7%Fe-N/C at 1600 rpm in O2-saturated 0.1 M KOH solution at room temperature. (d) Correlation between E1/2, j at 0.90V (vs. RHE) and Cdl. (e) The electron transferred numbers of N/C, Pt/C and 5%Fe-N/C. (f) Stability evaluation of Pt/C and 5%Fe-N/C tested at 0.60 V (vs. RHE) for 15000 s at 1600 rpm in O2-saturated 0.1 M KOH solution at room temperature.


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Cdl-E1/2 plots in Figure 6d. A larger ECSA could expose more active sites (e.g., the Fe-Nx sites) and therefore delivers an enhanced catalytic performance toward the ORR. The Tafel curves of 5%Fe-N/C and Pt/C (Figure S10) show similar Tafel slopes (i.e., 63 and 68 mV dec-1, respectively), which are smaller than that of N/C (108 mV dec-1). The kinetics of the ORR on the prepared x%Fe-N/C are further analyzed by using the Koutecky-Levich (KL)equation (Figure S11). The K-L plots on 5%Fe-N/C exhibit an excellent linearity in the potential range between 0.80 and 0.60 V and the average electron transferred number (n) calculated based on the slopes of the K-L plots is 3.97, which is close to the theoretical n of the ORR on the commercial Pt/C catalysts. The results show that 5%Fe-N/C ideally conducts a nearly 4e- reduction pathway as the Pt/C catalyst (Figure 6c). The stability measurements of 5%Fe-N/C and Pt/C were carried out at 0.60 V (vs. RHE) in an O2-saturated 0.1 M KOH electrolyte for 15000 s (Figure 6d). After the test, the 5%Fe-N/C shows a 7.6% current loss, which is much smaller than that of Pt/C (19.7%). This result demonstrates that the 5%Fe-N/C has a better stability under the ORR conditions in alkaline medium compared to Pt/C. Thus, the as-prepared 5%Fe-N/C exhibits a remarkable catalytic activity and long-term stability toward the ORR, which could be a promising alternative ORR electrocatalyst for Pt/C and applied in metalair batteries. The ORR electrocatalytic performance of the prepared x%Fe-N/C catalysts under pH-neutral condition was also investigated in order to meet the practical application requirements in neutral electrolytes. The LSV curves (Figure 7a) show that all of the prepared catalysts display a negative shift both for the onset potential and half-wave potential of the ORR compared to the results in alkaline solution, and the 5%Fe-N/C exhibits a more positive E1/2 (0.75 V) than that of Pt/C (0.68 V) and 5%Fe-N/C-800 (0.72 V). Obviously, the prepared 5%Fe-N/C catalysts in


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Figure 7. (a) LSV curves of Pt/C, N/C, 5%Fe-N/C-800, and 5%Fe-N/C in 1 M O2-saturated PBS buffer solution. (b) The discharge performance of the assembled Mg-air batteries with Pt/C or 5%Fe-N/C as the cathode. (c) Photograph of two assembled Mg-air batteries in series with an open circuit voltage of 3.73 V. (d) Photograph of strings of LED lights powered by the assembled two Mg-air batteries in series. neutral electrolyte also show an obvious reduction of jL, which might be due to the lower rate of charge transfer in neutral electrolyte (since the migration rates of Na+ and Cl- are much slower than those of H+ and OH-).5 Above results indicate that the prepared 5%Fe-N/C can be used as an excellent metal-air battery air cathode for pH-neutral electrolyte. To further verify the catalytic activity of the prepared Fe-N/C catalysts in neutral electrolyte, Mg-air batteries with 5%Fe-N/C or Pt/C as the cathode catalytic material, a Mg plate as the anode, and 1 M NaCl solution as the electrolyte were assembled. The plots of voltage and power density as functions of current density of the assembled Mg-air batteries are presented in Figure 7b. The Mg-air battery with the 5%Fe-N/C catalyst as the cathode exhibits a promising energy storage performance, achieving a promising open-circuit voltage of 1.78 V and a peak power density of 72 mW cm-2 at 0.72 V, which are better than those of the Mg-air battery with Pt/C as the cathode (60 mW cm-2 at 0.75 V). Besides, the assembled Mg-air battery also shows a good long-term stability (Figure S12). Furthermore, two assembled Mg-air batteries in series can achieve an open circuit voltage of up


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to 3.73 V, which is sufficient to illuminate the strings of LED lights (Figure 7c and 7d). The result demonstrates that the as-prepared 5%Fe-N/C catalyst is a promising substitute of the commercial Pt/C catalysts for the application in Mg-air batteries.

CONCLUSIONS Fe-N co-doped porous carbon with remarkable catalytic performance toward the ORR has been successfully fabricated from Fe-doped ZIF precursors using a facile two-stage thermal synthesis protocol. The preheating treatment at 200 °C (i.e., the first stage) before the final carbonization at 800 °C (i.e., the second stage) has been demonstrated to be beneficial to the coordination between Fe and N-containing ligands in the Fe-doped ZIF-8 precursors, which in turn facilitates the formation of Fe-Nx active sites with a higher N content in the final Fe-N/C catalysts. Besides, it also favors the growth of ZIF nanocrystals with a well-defined polyhedral morphology and the Fe-N/C nanoparticles with a suitable porous structure. The Fe-doping level also has significant effects on the morphology, pore structure, and N content of the fabricated Fe-N/C. With an appropriate doping of Fe and the preheating treatment during the synthesis, the optimized 5%FeN/C exhibits a limiting current density of 6.0 mA cm-2 (at 1600 rpm on RDE) and a half-wave potential of 0.88 V for the ORR in alkaline media with excellent ORR durability, which are better than those of Pt/C catalyst. Furthermore, the Mg-air battery using the fabricated 5%FeN/C as the cathode and 1 M NaCl as the neutral electrolyte shows an outstanding battery property with power energy of 72 mW cm-2 at 0.72 V, superior to that of the Mg-air battery using the commercial Pt/C catalyst. Such preheating treatment is effective for boosting the formation of Fe-Nx active sites and could be easily integrated into the carbonization process as an


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intermediate step. This strategy should also be applicable for the fabrication of a wide variety of metal-N/C catalysts (e.g., Co-N/C and Ni-N/C) for energy storage and conversion applications.

ASSOCIATED CONTENT Supporting Information. Fabrication of the air cathodes for Mg-air batteries; additional TG/DTA, SEM, TEM, XRD, XPS, FT-IR, UV-vis, and N2 adsorption-desorption characterizations; electrochemically active surface area measurement; Tafel measurement; stability test of the assembled Mg-air battery; calculation of the electron transfer number of the prepared catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Zhen He); Fax: +86-731-88879616. * E-mail: [email protected] (Suqin Liu); Fax: +86-731-88879616. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (grant nos. 51772332, 51372278, and U1507106), the Hunan Provincial Science and Technology Plan Project (grant nos. 2017TP1001 and 2016TP1007), the Natural Science Foundation of Hunan


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Province (grant no. 2018JJ2485), and Innovation-Driven Project of Central South University (grant no. 2016CXS031).

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Synopsis: MOF-derived Fe-Nx enriched porous carbon micropolyhedrons fabricated by twostage thermal synthesis exhibit extraordinary catalytic performance toward the oxygen reduction reaction. TOC Graphic:


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Fe-Nx Sites Enriched Carbon Micropolyhedrons Derived from Fe

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