English

• INTRODUCTION

  Direct methanol fuel cells (DMFCs) with oxygen reduction reaction (ORR) as the cathodic reaction and methanol oxidation reaction (MOR) as the anodic reaction have attracted much attention on account of the low environmental pollution, high energy conversion efficiency and convenient operation.^[1-5] However, ORR is a complicated multi-electron transfer process and its sluggish kinetics always results in serious cathode polarization with an overpotential of 300-400 mV, which greatly decreases the efficiency of DMFCs.^{[6, 7]} Nowadays, carbon-supported Pt-based catalysts are regarded as the most efficient ORR catalysts due to the large surface area, superior electric conductivity and rapid oxygen reduction kinetics, ^[9] whereas their high cost, low methanol tolerance and poor stability are still big obstacles for the widespread application of Pt-based catalysts in DMFCs.^{[4, 10-12]} Therefore, searching for highly efficient and cost-effective non-noble metal electrocatalysts as alternative materials for ORR is highly beneficial to reduce the cost and promote the commercialization of DMFCs.^[13]

  In the past decades, non-noble metal ORR catalysts formed from the earth-abundant elements such as C, N, S and Fe have attracted much attention.^[14-23] Among the catalysts, carbon-based materials have been regarded as the most promising ORR catalysts to replace Pt in future. In 2009, Dai's group prepared a vertically aligned nitrogen-containing carbon nanotubes as a metal-free electrode.^[24] The doping of nitrogen atoms with electron-accepting properties changes the electronic structures of the neighbor carbon atoms, resulting in a relatively high positive charge density of carbon atoms and thus enhances its catalytic activity toward ORR. Recently, carbon-based transition metal-nitrogen-containing complexes (M-N-C, where M = Fe and Co) are considered as the highly efficient non-noble metal catalysts for ORR.^[25-27] It is reported that this kind of M-N-C material can be synthesized via heat treatment of transition metal macrocycles such as Fe-porphyrin and Fe-phthalocyanine, or the mixed precursors comprised of Fe salts and N-rich compounds.^{[1, 9, 28, 29]} These raw materials are widely sourced and the preparation method is simple. Furthermore, the activities of this kind of FeNC catalyst with abundant Fe-N-C active sites are comparable to the commercial Pt/C catalyst and the long-term stability of them even surpasses the latter, ^{[30, 31]} which makes the non-noble metal FeNC materials exceptionally promising electrocatalysts for ORR.

  In the present work, a series of FeNC catalysts named as Fe_xNC-T-t (x, T and t refer to the mass ratio of Fe(NO₃)₃·6H₂O to melamine, pyrolysis temperature and time, respectively) are prepared through a facile pyrolysis process. XC-72 carbon black was introduced as the carbon support due to the outstanding electrical conductivity, large surface area and high stability under relatively high potential. Melamine was used as the N source and Fe(NO₃)₃ as the Fe source. The preparation conditions such as the mass ratio of Fe(NO₃)₃·6H₂O to melamine, pyrolysis temperature and time were optimized. Results show that Fe_0.50NC-800-1h exhibits the highest catalytic performance among the as-prepared Fe_xNC-T-t catalysts. The catalytic activity of Fe_0.50NC-800-1h is slightly lower than the noble metal Pt/C catalyst, whereas the stability and methanol tolerance are much superior to the latter. Considering the extremely low cost, we think the FeNC materials would have broad application prospects in the field of DMFCs and other energy conversion devices.

  RESULTS AND DISCUSSION

  Characterization of the FeNC Samples. Figure 1 shows the morphology of the representative Fe_0.50NC-800-1h sample. Clearly, the carbon black support exhibits a highly overlapped flocculent structure. The pyrolyzed FeNC nanoparticles with different sizes and morphologies were randomly doped in the carbon black. The HRTEM image (Figure 1c) shows that the nanoparticle was coated with a graphene layer with the thickness of 2-3 nm. The lattice fringes with interplanar spacing of 0.366 nm correspond to the (002) crystal plane of carbon.^{[32, 33]} Under the carbon layer, those with the interplanar spacing of about 0.209 nm were also detected. The spacing data of 0.209 nm are slightly larger than the standard value, 0.204 nm, of the (110) crystal plane of metallic Fe, which might be attributed to the incorporation of Fe atoms into nitrogen lattice and thus enlarges the lattice constant.^[8] In addition, we also detected the lattice fringes with the interplanar spacing of 0.301 nm (Figure 1d) in the nanoparticles, which corresponds to the (220) crystal plane of Fe₂O₃.^[34] Figure 1e shows the XRD patterns of the as-prepared Fe_xNC-T-t samples. All of the XRD spectra reveal a typical graphite-like structure. A broad diffraction peak at ca. 25.9º was detected, which is related to the (002) crystal plane of graphite carbon.^[8] The pronounced peaks at 30.2°, 35.6°, 43.3°, 57.3° and 62.9° can be assigned to the (220), (311), (400), (511) and (440) crystal planes of Fe₂O₃ (PDF#39-1346). For the Fe_0.50NC-800-1h sample, a new peak at 44.5° emerges, which comes from the (200) crystal plane of Fe₃N (PDF#49-1664). It is interesting that no peaks originating from other iron species such as metal iron and iron carbides are detected for the samples pyrolyzed at 800 ºC, suggesting that the crystal structures of the as-prepared Fe_xNC-800-t samples are similar and the iron species mainly consists of Fe oxides and Fe nitrides. When the pyrolysis temperature was increased to 900 ºC, the diffraction peak corresponding to Fe₃N became weak and another new peak at 49.4º was detected, which corresponds to the (102) crystal plane of metal Fe formed from the partial pyrolysis of Fe₂O₃ at relatively high temperature.

  Figure 1

  

  Figure 1.  (A-D) TEM/HRTEM images of the representative Fe_0.50NC-800-1h sample, (E) XRD patterns of the as-prepared Fe_xNC-T-t samples.

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  XPS was used to characterize the chemical valence state and surface composition of the samples. Figure 2 shows the XPS spectra of the representative Fe_0.50NC-800-1h sample. The signals of Fe and N elements were detected, suggesting that Fe and N elements were successfully doped in the carbon material.^{[35, 36]} The C 1s spectrum (Figure 2b) of Fe_0.50NC-800-1h sample has been deconvoluted into three peaks. The signal at 284.6 eV corresponds to the C=C in aromatic structure and that at 286.1 eV is concerned with the C=N/C-O structure derived from the pyrolysis of the carbon black.^[37] The broad peak at relatively high binding energy of 289.4 eV is assigned to the C=O bond. The N 1s spectrum of the Fe_0.50NC-800-1h sample is shown in Figure 2c. The deconvoluted five peaks correspond to the graphitic-N at 401.1 eV, pyrrolic-N at 400.2 eV, pyridinic-N at 398.2, N-Fe at 399.4 and oxidized-N at 401.7 eV.^{[13, 18, 38-40]} It is reported that not all of the N types can promote the catalytic process, and graphitic-N, pyridinic-N and N-Fe are in favor of ORR while the oxidized-N does not show similar performance.^[40] A semi-quantitative analysis based on the integration areas of different N types shows that the percentage contents of graphitic-N, pyrrolic-N, pyridinic-N, N-Fe and oxidized-N are 20.6%, 11.6%, 8.9%, 39.3% and 19.6%, respectively. Obviously, the content of N-Fe is much higher than that of the other four types of N elements, which might facilitate the catalysis of the catalysts.^{[24, 40-42]} Figure 2d shows the Fe 2p XPS spectra of the Fe_0.50NC-800-1h sample (11.4 wt.% Fe). The signals at 710.9 and 722.9 eV are assigned to the 2p_3/2 and 2p_1/2 of Fe(II), and those at 713.6 and 725.2 eV are associated with the 2p_3/2 and 2p_1/2 of Fe(III), respectively.^{[13, 43, 44]} In addition, there is a pronounced peak at about 719.7 eV, which might be related to the signal of satellite peak. These results together with the above analysis of N 1s XPS spectra suggested that the Fe₃N structure was formed in the Fe_0.50NC-800-1h sample, which is in good agreement with the above XRD and TEM results. The XPS spectra of the other two Fe_0.50NC samples calcinated at 700 and 900 ºC are shown in Figures S1 and S2. All of the XPS spectra have been deconvoluted and the percentage contents of various N species in total N amount calculated from the integration area are shown in Table S1. It can be seen that the calcination temperature has a great influence on the content of various N species in the samples. The percentage content of N-Fe in Fe_0.50NC-800-1h sample is much higher than those in Fe_0.50NC-700-1h (28.1%) and Fe_0.50NC-900-1h (8.8%) samples, suggesting that the catalytic performance of Fe_0.50NC-800-1h catalyst might be different from the other two Fe_0.50NC catalysts.

  Figure 2

  

  Figure 2.  (A) Entire XPS spectra, (B) C 1s, (C) N 1s and (D) Fe 2p XPS spectra of Fe_0.50NC-800-1h sample.

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  Catalytic Activity of the FeNC Catalysts for ORR in KOH Electrolyte. The investigation on the ORR activity in alkaline electrolyte is very important since the alkaline fuel cells allowed some non-precious metal materials to be used as the electrocatalysts.^[45] Figure 3a shows the CV curves of the representative Fe_0.50NC-800-1h catalyst in N₂- and O₂-saturated 0.1 M KOH solution. For the catalyst measured in N₂-saturated KOH solution, there is no significant redox peak in the CV curve. While for that measured in O₂-saturated electrolyte, a pronounced peak coming from the reduction of oxygen dissolved in the electrolyte was found at ca. 0.81 V, suggesting that the FeNC catalysts have a significant catalytic activity for ORR. Prior to the comprehensive evaluation on the catalytic performances via LSV curves, we optimized the preparation conditions of the catalysts and selected the most active FeNC catalyst. The influence of the mass ratio of Fe(NO₃)₃·6H₂O to melamine on the catalytic activity of the Fe_xNC-800-1h catalysts was firstly investigated. Figure S3 shows the LSV curves of ORR on the Fe_xNC-800-1h catalyst. with increasing the mass ratio of Fe(NO₃)₃·6H₂O to melamine from 0.25 to 0.50 g, the onset potential (E_o) of ORR shifted to higher value, and with further increasing the amount of Fe to 0.75 g, the E_o of ORR does not continue to increase but decreased by ca. 50 mV. Both Fe_0.25NC-800-1h and Fe_0.75NC-800-1h catalysts show comparable E_o of ca. 0.90 V, which is much lower than that on Fe_0.50NC-800-1h catalyst (0.94 V), suggesting that Fe_0.50NC-800-1h shows the fastest ORR kinetics among the three catalysts.

  Figure 3

  

  Figure 3.  (A) CV curves of the Fe_0.50NC-800-1h catalyst in N₂- or O₂-satu-rated 0.1 M KOH electrolyte. (B) LSV curves of Fe_0.50NC-800-1h catalyst at the indicated rotation speeds (rpm). (C) Koutecky-Levich plots of Fe_0.50NC-800-1h catalyst at different potentials. (C) Number of transfer electrons per oxygen molecule (n) for ORR at different potentials.

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  Besides the input amount of Fe precursor, the pyrolysis temperature and time were also optimized for the series of Fe_0.50NC catalysts. Figure S4 gives the comparison of the catalysts pyrolyzed at different temperatures. Compared with Fe_0.50NC catalyst pyrolyzed at 700 and 900 ℃, Fe_0.50NC-800-1h exhibits much higher catalytic activity, in terms of the ORR limiting diffusion current and more positive onset and half-wave potentials (E_1/2). In addition, the influence of pyrolysis time (1 h and 2 h) on the catalytic activity of the catalysts was also investigated, as shown in Figure S5. Both Fe_0.50NC-800 catalysts display comparable limiting diffusion current density while the E_1/2 of the catalysts are quite different. Fe_0.50NC-800-1h shows more positive E_1/2 (0.81 V), which is ca. 60 mV higher than Fe_0.50NC-800-2h catalyst (0.75 V). Considering the above comparison results, we chose Fe_0.50NC-800-1h catalyst to comprehensively investigate the catalytic activity for ORR in the present work. In addition, it is noteworthy to say that although Fe_0.50NC-800-1h catalyst shows the highest catalytic activity for ORR among the series of FeNC catalysts, its activity is slightly inferior to commercial Pt/C catalyst (Figure S6). The E_1/2 is about 38 mV lower than the Pt/C catalyst (0.85 V) and the limiting diffusion current is also smaller than the latter. The heavily stacked structure with the uneven active sites can increase the mass transfer resistance of the catalyst, which might decrease the catalytic activity as compared with the Pt/C catalyst.^[46]

  Figure 3b gives the LSV curves of Fe_xNC-800-1h catalyst at different rotation speeds (from 400 to 2025 rpm). The ORR currents increased sensitively with increasing the rotation speed of RDEs. At the high potential side of LSVs (> 0.85 V), electrochemical reaction was the rate determining step (rds) of ORR. Thereby, ORR is not sensitive to the rotation speed. When decreasing the potential, the electrochemical reaction rate was accelerated and the ORR kinetics was dependent on both electrochemical reaction rate and oxygen diffusion rate.^[17] With further decreasing the potential, the reaction rate was accordingly accelerated and the diffusion rate of oxygen became the rds. Generally, high rotation speeds result in faster oxygen flux to the surface of the electrode and thus a larger current of ORR. Based on the above analysis, both electrochemical reaction rate and oxygen diffusion rate are important for the catalysis, and their relative contributions to the measured overall oxygen-reduction current can be evaluated from the well-known Koutecky-Levich equation: ^{[1, 47]}

  Where i, i_k and i_d are the measured overall, kinetic and diffusion-limiting current densities, respectively. n is the number of transferred electrons for the reduction of an oxygen molecule, F the Faraday constant (96500 C mol^-1), and A the geometric area of the disk electrode (0.196 cm²). D₀ and C₀ are the diffusion coefficient (1.73 × 10^-5 cm² s^-1) and initial concentration (1.14 × 10^-6 mol cm^-3) of oxygen in the electrolyte, respectively. ω is the rotation angular speed (ω = 2πN, where N is the linear rotation speed) and ν is the kinetic viscosity of 0.1 M KOH solution (0.01 cm² s^-1). The number of transferred electrons involved in ORR is a crucial indicator for evaluating the catalytic activity of the catalysts.^[5] The data of n can be obtained from the slope of a series of Koutecky-Levich curves by plotting the inverse current density (i^-1) versus the inverse square root of the rotation speed (ω^-1/2).^[47] Figure 3c and 3d show the Koutecky-Levich curves at different potentials and the calculated number of transferred electrons (n), respectively. The electron transfer numbers (n) of all of Fe_xNC-T-t catalysts are shown in Figure S7, where all of the curves at different potentials show a linear relationship and the average data of n are calculated to be nearly to four, suggesting that the oxygen reduction process on the Fe_xNC-T-t catalysts proceeded mainly through the widely accepted four-electron transfer mechanism (O₂ + 2H₂O + 4e^- → 4OH^-). It is reported that the abundant Fe₃N active sites in the series of FeNC catalysts could increase the spin density and π electron density of the neighboring carbon atoms and thus increase the ORR activity.^{[44, 48, 49]} In addition, other N species also have special functions to the catalytic performances. For example, the graphitic-N can improve the stability of the FeNC catalysts through strengthening the π bond in the carbon skeleton.^[45] These positive effects might be responsible for the high catalytic performances of FeNC catalysts.

  To comprehensively investigate the catalytic stability of the catalysts, we firstly make the catalyst subject 5000 sequential cycles from 0.207 to 1.107 V vs. RHE in KOH solution (0.1 M). Figure 4a and 4b give the comparisons of the CV curves of Fe_0.50NC-800-1h and commercial Pt/C catalysts before and after 5000 cycles. For Fe_0.50NC-800-1h catalyst, there is almost no change for the two curves. While for the Pt/C catalyst, the double layer current significantly decreases, which might be due to the aggregation of the carbon support during the long-term repeated cycles.^{[4, 50]} Similar phenomenon is also found in the LSV curves of the two catalysts (Figure 4c and 4d). For the Fe_0.50NC-800-1h catalyst, the LSV curves do not show any marked change after 5000 repeated cycles and the E_1/2 only exhibits a negative shift of 15 mV. For the Pt/C catalyst, a pronounced potential attenuation of ca. 83 mV is found. Even after 5000 repeated cycles in the 0.207~1.107 V range, the Fe_0.50NC-800-1h catalyst still exhibits an invariable activity for ORR, indicating a distinguished durability and a promising application prospect in DMFCs.

  Figure 4

  

  Figure 4.  Comparison of the CV (A, B) and LSV (C, D) curves of Fe_0.50NC-800-1h (A, C) and commercial Pt/C (B, D) catalysts before and after 5000 cycles.

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  For DMFCs, owing to the anode methanol penetration, oxygen reduction and methanol oxidation simultaneously occur on the cathode, forming "cathodic mixed potential" and decreasing the cell performance. The resistance to methanol poisoning of the cathodic catalysts is thus of particular importance for DMFCs.^{[16, 51]} In this work, the methanol tolerance of Fe_0.50NC-800-1h was investigated through evaluating the change in current when methanol was added in the electrolyte during the chronoamperometric (CA) test (Figure 5a). For comparison, similar test of commercial Pt/C catalyst was also carried out. In the first 200 s, the Fe_0.50NC-800-1h catalyst only lost ca. 3% of the initial current whereas Pt/C catalyst lost nearly 10%. After the addition of methanol at 200 s, only a slight disturbance in the ORR current density was found for Fe_0.50NC-800-1h and the current density lost is only 9.9% at the time of 1000 s, indicating a high resistance ability to methanol. On the contrary, a dramatic current oscillation was found for the commercial Pt/C catalyst when adding methanol. The current quickly decreased from 90.0% to 65.1% and further decreased to 54.4% at the time of 1000 s. By comparing the results of Fe_0.50NC-800-1h and Pt/C, we can conclude that Fe_0.50NC-800-1h is an excellent electrocatalyst with high methanol resistance and long-term stability for ORR.

  Figure 5

  

  Figure 5.  (A) The chronoamperometric responses (1600 rpm) at 0.60 V (vs. RHE) of Fe_0.50NC-800-1h and Pt/C catalysts in O₂-saturated 0.1 M KOH solution with adding 3.0 M methanol (2.0 mL) at approximately 200 s. The LSVs (1600 rpm) of Fe_0.50NC-800-1h (B) and Pt/C (C) catalysts in O₂-saturated 0.1 M KOH with and without 3.0 M methanol.

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  The methanol resistance of the Fe_0.50NC-800-1h catalyst was further investigated by the LSV curves. Figure 5b and 5c show the LSV curves of ORR on Fe_0.50NC-800-1h and Pt/C catalysts at the rotation speed of 1600 rpm in KOH electrolyte with and without methanol. For Fe_0.50NC-800-1h catalyst, the E_o of ORR slightly shifted to more negative potential and the E_1/2 was lowered by ca. 27 mV. While for the Pt/C catalyst, significant change of the curve shape was found. The methanol oxidation current was extremely predominant whereas the ORR process was severely depressed. These results imply that Fe_0.50NC-800-1h catalyst shows much higher methanol resistance than Pt/C catalyst.

  EXPERIMENTAL

  Synthesis and Characterization of the FeNC Samples. In this work, melamine was used as the N resource due to its high N content (N wt.% = 67%). Firstly, 1.0 g of melamine was dissolved in 30 mL of deionized water. After stirring for 30 min at 80 ºC, a certain amount (0.25, 0.50 and 0.75 g) of Fe(NO₃)₃·6H₂O and 0.50 g of XC-72 carbon black was added in the above solution with vigorous stirring under 120 ºC until the water was completely evaporated. After the as-obtained product was grinded, 500 mg of the sample was placed in the corundum crucible and pyrolyzed at different temperatures (700, 800 and 900 ºC) under N₂ atmosphere. The rate of temperature increasing was controlled at 10 ºC min^-1, and the pyrolysis time was set at 1 or 2 h. The final products were grinded and denoted as Fe_xNC-T-t (x, T and t refer to the mass ratio of Fe(NO₃)₃·6H₂O to melamine, pyrolysis temperature and time, respectively).

  Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on an FEI Tecnai-F20 Super-Twin transmission electron microscope operating at 200 kV. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements were performed on the X'Pert3 PANalytical B.V. X-ray diffractometer (Cu Ka radiation) and ESCALAB 250XI (Al Ka radiation source) spectrometer, respectively. The scanning range of XRD was from 20° to 80° and the scanning speed was set at 8 ° min^-1. The actual Fe loading of the samples was measured by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7700ce spectrometer).

  Preparation of Working Electrode. Prior to each measurement, glassy carbon rotating disk electrode (RDE, d = 5 mm) was successively polished with 0.5 and 0.05 μm of alumina suspensions to a mirror finish, followed by ultrasonically rinsing with diluted HNO₃, ethanol, acetone and deionized water. Then, 5.0 mg of the FeNC sample was added in 1.0 mL of isopropanol and sonicated for 30 min to form a homogenous suspension. The working electrode was prepared by dropping 10 µL suspension onto the glassy carbon disk electrode with a pipette and the catalyst loading was controlled at 254.7 μg cm^-2. After the evaporation of solvent, 10 μL Nafion solution (Dupont, 0.05 wt.%) was coated onto the catalyst layer and then dried in the air.

  Electrochemical Measurements. In the present work, we mainly used cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometric (CA) techniques to investigate the catalytic activity and stability. All the electrochemical measurements were performed in a three-electrode cell at room temperature. Graphite rod and Hg/HgO electrode were used as the counter and reference electrodes, respectively. The potentials reported in the present work are converted in the reversible hydrogen electrode (RHE) scale based on the formula (E (vs. RHE)) = E (vs. HgO) + 0.059 × pH + 0.098).^[52] CV curves with the scan rate of 50 mV s^-1 were tested in the potential range from 0.207 to 1.107 V vs. RHE in 0.1 M KOH electrolyte. The LSV curves were measured with the scan rate of 10 mV s^-1 and rotation rates of 400, 625, 900, 1225, 1600 and 2025 rpm. Prior to each electrochemical test, high purity N₂ or O₂ (99.999%) was pumped to saturate the electrolyte. During the measurement, a gentle N₂ or O₂ flow was maintained above the electrolyte to eliminate the disturbance of the ambient atmosphere.

  CONCLUSIONS

  In this work, a series of non-noble-metal FeNC electrocatalysts were prepared through a simple pyrolysis process. The preparation conditions such as the mass ratio of Fe(NO₃)₃·6H₂O to melamine, pyrolysis temperature and pyrolysis time were optimized. Results show that the Fe_0.50NC-800-1h catalyst exhibits the highest catalytic performances toward ORR among the catalysts. The half-wave potential of ORR (0.81 V vs. RHE) is only 38 mV lower than the commercial Pt/C catalyst and the ORR process on the catalyst proceeds mainly through the four-electron transfer mechanism. After 5000 sequential cycles, the half-wave potential of ORR on the Fe_0.50NC-800-1h catalyst shows a decrease of only 15 mV, indicating an excellent catalytic stability. Chronoamperometric measurement shows that only a slight disturbance in the ORR current density was found for Fe_0.50NC-800-1h catalyst after the addition of methanol, and the current density lost only 9.9% of the initial at the time of 1000 s. While for the Pt/C catalyst, the current quickly decreased to 65.1% of the initial current density after the methanol addition and to 54.4% at the time of 1000 s. These data suggest that the non-noble metal Fe_0.50NC-800-1h catalyst not only has a high catalytic activity for ORR, but also has a high catalytic stability and methanol tolerance. Our findings would be informative for the design and preparation of high performance non-noble metal ORR catalysts for fuel cells and other applications.

  
  ACKNOWLEDGEMENTS: This work is supported by the National Natural Science Foundation of China (22072070) and the Natural Science Foundation of Shandong Province (ZR2019MB036). The authors declare no competing interests.
  COMPETING INTERESTS
  ADDITIONAL INFORMATION
  Supplementary information is available for this paper at http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0053
  For submission: https://www.editorialmanager.com/cjschem
  
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  Figure 1  (A-D) TEM/HRTEM images of the representative Fe_0.50NC-800-1h sample, (E) XRD patterns of the as-prepared Fe_xNC-T-t samples.

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  Figure 2  (A) Entire XPS spectra, (B) C 1s, (C) N 1s and (D) Fe 2p XPS spectra of Fe_0.50NC-800-1h sample.

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  Figure 3  (A) CV curves of the Fe_0.50NC-800-1h catalyst in N₂- or O₂-satu-rated 0.1 M KOH electrolyte. (B) LSV curves of Fe_0.50NC-800-1h catalyst at the indicated rotation speeds (rpm). (C) Koutecky-Levich plots of Fe_0.50NC-800-1h catalyst at different potentials. (C) Number of transfer electrons per oxygen molecule (n) for ORR at different potentials.

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  Figure 4  Comparison of the CV (A, B) and LSV (C, D) curves of Fe_0.50NC-800-1h (A, C) and commercial Pt/C (B, D) catalysts before and after 5000 cycles.

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  Figure 5  (A) The chronoamperometric responses (1600 rpm) at 0.60 V (vs. RHE) of Fe_0.50NC-800-1h and Pt/C catalysts in O₂-saturated 0.1 M KOH solution with adding 3.0 M methanol (2.0 mL) at approximately 200 s. The LSVs (1600 rpm) of Fe_0.50NC-800-1h (B) and Pt/C (C) catalysts in O₂-saturated 0.1 M KOH with and without 3.0 M methanol.

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