Polyacrylamide Microspheres-Derived Fe3C@N-doped Carbon Nanospheres as Efficient Catalyst for Oxygen Reduction Reaction

High-performance non-precious metal catalysts exhibit high electrocatalytic activity for the oxygen-reduction reaction (ORR), which is indispensable for facilitating the development of multifarious renewable energy systems. In this work; N-doped carbon-encapsulated Fe3C nanosphere ORR catalysts were prepared through simple carbonization of iron precursors loaded with polyacrylamide microspheres. The effect of iron precursors loading on the electrocatalytic activity for ORR was investigated in detail. The electrochemical measurements revealed that the N-doped carbon-encapsulated Fe3C nanospheres exhibited outstanding electrocatalytic activity for ORR in alkaline solutions. The optimized catalyst possessed more positive onset potential (0.94 V vs. reversible hydrogen electrode (RHE)), higher diffusion limiting current (5.78 mA cm−2), better selectivity (the transferred electron number n > 3.98 at 0.19 V vs. RHE) and higher durability towards ORR than a commercial Pt/C catalyst. The efficient electrocatalytic performance towards ORR can be attributed to the synergistic effect between N-doped carbon and Fe3C as catalytic active sites; and the excellent stability results from the core-shell structure of the catalysts.


Introduction
With the limited supply of fossil fuels and growing concern about environmental problems, the development of renewable energy and efficient energy conversion systems has become more important than ever. Fuel cells are regarded as one of the most promising energy conversion devices accessible nowadays [1][2][3][4][5][6]. Among various fuel cells, proton exchange membrane fuel cells (PEMFCs) and polymer electrolyte fuel cells (PEFCs) are considered as advanced electrochemical energy conversion systems due to their advantages of fast electrode reaction kinetics, high conversion efficiency and environmental friendliness [7,8]. In fuel cells, small molecule fuels are oxidized at the anode and concurrently oxygen is reduced at the cathode [9]. Efficient catalysts are necessary due to the sluggish kinetics of the oxygen-reduction reaction (ORR) at the cathode which is of great limit of the energy stirring, the mixed solution was put into a 55 • C water bath and the N 2 was aerated continuously to remove the air for about 40 min. Then the mixture was kept in a 55 • C water bath with stirring at the speed of 300 r/min in nitrogen atmosphere for 3 h to obtain crosslink-polyacrylamide reverse-phase microemulsion. The resulting w/o microemulsion was poured into excess acetone for demulsification. After washing with deionized water and absolute ethyl alcohol, the collected solids were dried at 70 • C overnight to obtain polyacrylamide microspheres. In order to obtain carbonization yield of polyacrylamide microspheres, in preliminary experiment, 2 g of dried polyacrylamide microspheres was pyrolyzed under flowing N 2 for 1 h at 900 • C with a heating rate of 5 • C min −1 and 0.4 g of carbon was collected. This indicates that the carbonization yield of polyacrylamide microspheres was about 20%.
Fe 3 C@N-doped carbon nanospheres were prepared by direct carbonization of iron precursors-loaded polyacrylamide microspheres. In a typical preparation process, a certain amount of ferroporphyrin and 1.6 g of KOH solid were dissolved in 15 mL deionized water, the mixture solution was completely absorbed by 4 g dried polyacrylamide microspheres. The KOH was added to dissolve the Ferroporphyrin and then activate the carbon. The sample was dried at 70 • C for 24 h and calcinated at 900 • C for 1 h filled with N 2 atmosphere and the ramping rate was 5 • C min −1 . The resulting dark powder was washed with deionized water for several times to remove alkali metal K which was produced during the activation and calcination process, followed by drying at 70 • C overnight. The final as-synthesized product has been obtained and labeled as Fe 3 C@N/C-x, and x represents the mass ratio of ferroporphyrin and carbon according to the theoretical dosage. For comparison, a series of Fe 3 C@N/C-x composites were also prepared via the same procedure except for the difference of ferroporphyrin additive amounts. These as-synthesized catalysts with various ferroporphyrin additive amounts were labeled as Fe 3 C@N/C-0, Fe 3 C@N/C-0.5, Fe 3 C@N/C-1, Fe 3 C@N/C-2, respectively.

Characterizations
Transmission electron microscope (TEM) images were collected using a JEM-2100F TEM (Tokyo, Japan), XRD measurements were measured with a D8 Advance X-ray diffractometer from Bruker AXS Company (Karlsruhe, Germany). X-ray photoelectron spectroscopy (XPS) analysis was recorded on a VG Multilab 2000 X-ray photoelectron spectrometer (VG Scientific, Waltham, MA, USA). The pore structure analysis was performed through nitrogen adsorption and desorption isotherm measurements (BET, ASAP 2020, Micromeritics). The dynamic function theory (DFT) method was applied to calculate the distribution of the micropore size.

Electrochemical Measurements
To evaluate ORR activity, the electrochemical performance of Fe 3 C@N-doped microporous carbon nanospheres catalysts were performed in a standard three-electrode glass cell with N 2 or O 2 -saturated 0.1M KOH solution at room temperature on the electrochemical workstation (CHI660E, CHI instrument) using a glassy carbon electrode (GCE, of 5.00 mm in diameter) as working electrode, a platinum wire as counter electrode, and Hg/HgO electrode as reference electrode, respectively. The electrochemical properties of as-synthesized catalysts were referred to the Hg/HgO electrode. The Hg/HgO reference electrodes were calibrated with respect to the reversible hydrogen electrode (RHE) before measurement [2]. The calibration values were E(RHE) = E(Hg/HgO) + 0.89 V in 0.1M KOH. 5.0 mg of as-synthesized catalysts power, 0.1 mL of deionized water, 0.9 mL of isopropanol and 0.02 mL of 5 wt % nafion solution was ultrasonically mixed to form the ink of the catalysts. The GCE was modified with 20 µL of the ink to serve as the working electrode. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were carried out to evaluate the ORR performances. CV curves were obtained in N 2 or O 2 -saturated 0.1M KOH electrolyte solutions without any rotation. The CV measurement data were carried out in the potential range from −0.6 V to 0.4 V at a sweep rate of 50 mV s −1 , and LSV measurements were carried out at a scan rate of 5 mV s −1 in the potential range from -0.8 V to 0.2 V under various electrode rotation rates (400, 800, 1200, 1600, 2000 rpm) in O 2 -saturated 0.1M KOH electrolyte solutions. All samples were also tested in N 2 -saturated for comparison. Chronoamperometric measurements for each as-synthesized catalyst were investigated in O 2 -saturated 0.1M KOH (−0.3 V vs. Hg/HgO) at room temperature. The ORR process kinetics was analyzed using the Koutecky-Levich (K-L) equation.

Results and Discussion
The morphology of polyacrylamide microspheres and the N-doped carbon encapsulated Fe 3 C nanospheres catalysts were characterized by TEM. Figure 1a shows the surface morphology of polyacrylamide microspheres. The results showed that the polyacrylamide microspheres are aggregates of spherical particles with an average diameter of about 30 nm. After adsorbing iron precursors and carbonization, the spherical morphology of samples can be maintained. The TEM images of Fe 3 C@N/C-1 were shown in Figure 1b,c. It can be observed that the core-shell structured catalysts were successfully synthesized. The high-resolution TEM (HRTEM) results in Figure 1d showed that the spacing between adjacent lattice fringe in nanospheres core was 0.21 nm, corresponding to the (211) crystal planes of Fe 3 C. The lattice fringe in outer shell was 0.348 nm, corresponding to the graphite (002) plane. It indicated that the Fe 3 C nanospheres were obtained and wrapped within graphite carbon layer during the carbonization process. vs. Hg/HgO) at room temperature. The ORR process kinetics was analyzed using the Koutecky-Levich (K-L) equation.

Results and Discussion
The morphology of polyacrylamide microspheres and the N-doped carbon encapsulated Fe3C nanospheres catalysts were characterized by TEM. Figure 1a shows the surface morphology of polyacrylamide microspheres. The results showed that the polyacrylamide microspheres are aggregates of spherical particles with an average diameter of about 30 nm. After adsorbing iron precursors and carbonization, the spherical morphology of samples can be maintained. The TEM images of Fe3C@N/C-1 were shown in Figure 1b,c. It can be observed that the core-shell structured catalysts were successfully synthesized. The high-resolution TEM (HRTEM) results in Figure 1d showed that the spacing between adjacent lattice fringe in nanospheres core was 0.21 nm, corresponding to the (211) crystal planes of Fe3C. The lattice fringe in outer shell was 0.348 nm, corresponding to the graphite (002) plane. It indicated that the Fe3C nanospheres were obtained and wrapped within graphite carbon layer during the carbonization process. In order to further identify the crystal structure of samples, X-ray diffraction (XRD) patterns of these samples were recorded and shown in Figure 2. The major peaks located at 37.8°, 43 [35]. In addition, the peaks at 26.4° and 44.0° were assigned to the (002) and (004) reflection of graphitic carbon [26]. There are only broad graphitic carbon diffraction peaks in Fe3C@N/C-0 sample. With the introduction of ferroporphyrin, the graphitic carbon and Fe3C crystalline phases coexisted in the other samples. The In order to further identify the crystal structure of samples, X-ray diffraction (XRD) patterns of these samples were recorded and shown in Figure 2. The major peaks located at 37 [35]. In addition, the peaks at 26.4 • and 44.0 • were assigned to the (002) and (004) reflection of graphitic carbon [26]. There are only broad graphitic carbon diffraction peaks in Fe 3 C@N/C-0 sample. With the introduction of ferroporphyrin, the graphitic carbon and Fe 3 C crystalline phases coexisted in the other samples. The relative intensity of Fe 3 C diffraction peaks increased with the iron content further enhanced in the synthesis process, indicating an increased ratio of Fe 3 C. The Fe 3 C nanocrystalline should originate from the reduction of iron precursor by carbon during the pyrolysis process. In the synthesis process of catalysts, ferroporphyrin was firstly dissolved in the KOH solution and homogeneously adsorbed in the polyacrylamide microspheres. Iron compound could be reduced by carbon to form metallic iron at high temperature, and these metallic iron atoms contribute to the catalytic graphitization of carbon [36]. When the iron atoms concentration is high enough, it would aggregate to form crystalline irons which react with the carbon atom to form a graphite layer-encapsulated Fe 3 C crystal during the process of pyrolysis and cooling. With the growth of Fe 3 C crystal, it is difficult for the large-size Fe 3 C crystal to make the inside carbon atoms diffuse onto the surface. Therefore, the relative intensity of graphitic carbon diffraction peaks weakens with the increase of iron content in the catalysts [35][36][37][38][39].
Polymers 2017, 9, x FOR PEER REVIEW 5 of 13 relative intensity of Fe3C diffraction peaks increased with the iron content further enhanced in the synthesis process, indicating an increased ratio of Fe3C. The Fe3C nanocrystalline should originate from the reduction of iron precursor by carbon during the pyrolysis process. In the synthesis process of catalysts, ferroporphyrin was firstly dissolved in the KOH solution and homogeneously adsorbed in the polyacrylamide microspheres. Iron compound could be reduced by carbon to form metallic iron at high temperature, and these metallic iron atoms contribute to the catalytic graphitization of carbon [36]. When the iron atoms concentration is high enough, it would aggregate to form crystalline irons which react with the carbon atom to form a graphite layer-encapsulated Fe3C crystal during the process of pyrolysis and cooling. With the growth of Fe3C crystal, it is difficult for the large-size Fe3C crystal to make the inside carbon atoms diffuse onto the surface. Therefore, the relative intensity of graphitic carbon diffraction peaks weakens with the increase of iron content in the catalysts [35][36][37][38][39]. XPS was conducted to illustrate the surface elements of catalysts. XPS spectra of Fe3C@N/C catalysts are exhibited in Figure 3(a), which revealed the existence of C, O, Fe and N in catalysts. There is only a weak Fe peak was detected in catalysts, which could be resulted from the coverage of graphitic layers on the Fe3C surface [33,39]. The high-resolution C 1s spectrum of Fe3C@N/C-1 shown in Figure 3b can be deconvoluted into four individual peaks that are assigned to C-C (284.6 eV), C-O (285.2 eV), C=O (286.5 eV), C-N (288.2 eV), respectively. The N 1s spectrum of Fe3C@N/C-1 shown in Figure 3c can be divided into three peaks, assigned to pyridinic N (398.4 eV), pyrrolic N (400.1 eV), graphitic N (401.2 eV), suggesting that nitrogen has been doped into the carbon authentically. It is generally believed that pyridinic N and graphitic N can serve as the efficient active sites for ORR. Pyridinic N increases the spin density and the density of π states of the carbon atoms near the Fermi level which could enhance the reduction of O2, while graphitic N can increase the conductivity of the catalysts [35,43,44]. In the case of the high-resolution Fe 2p spectrum (Figure 3d), the peaks at 711.0 eV and 724.4 eV are consistent with Fe 2p3/2 and Fe 2p1/2 states, respectively [40][41][42]. Fe, C and N content, N/C weight ratios and the relative atomic amount of N species in all of Fe3C@N/C-x samples are shown in Table 1. The amount of Fe incorporated into the final catalyst increases with the enhancement of ferroporphyrin loading during the synthesis process, while the amount of N declines slightly at the same time. In all cases, graphitic N and pyridinic N are dominant species. In general, with the increase of the amount of Fe3C catalytic active sites, the amount of N-doped carbon catalytic active sites declines. XPS was conducted to illustrate the surface elements of catalysts. XPS spectra of Fe 3 C@N/C catalysts are exhibited in Figure 3a, which revealed the existence of C, O, Fe and N in catalysts. There is only a weak Fe peak was detected in catalysts, which could be resulted from the coverage of graphitic layers on the Fe 3 C surface [33,39]. The high-resolution C 1s spectrum of Fe 3 C@N/C-1 shown in Figure 3b can be deconvoluted into four individual peaks that are assigned to C-C (284.6 eV), C-O (285.2 eV), C=O (286.5 eV), C-N (288.2 eV), respectively. The N 1s spectrum of Fe 3 C@N/C-1 shown in Figure 3c can be divided into three peaks, assigned to pyridinic N (398.4 eV), pyrrolic N (400.1 eV), graphitic N (401.2 eV), suggesting that nitrogen has been doped into the carbon authentically. It is generally believed that pyridinic N and graphitic N can serve as the efficient active sites for ORR. Pyridinic N increases the spin density and the density of π states of the carbon atoms near the Fermi level which could enhance the reduction of O 2 , while graphitic N can increase the conductivity of the catalysts [35,40,41]. In the case of the high-resolution Fe 2p spectrum (Figure 3d), the peaks at 711.0 eV and 724.4 eV are consistent with Fe 2p 3/2 and Fe 2p 1/2 states, respectively [42][43][44]. Fe, C and N content, N/C weight ratios and the relative atomic amount of N species in all of Fe 3 C@N/C-x samples are shown in Table 1. The amount of Fe incorporated into the final catalyst increases with the enhancement of ferroporphyrin loading during the synthesis process, while the amount of N declines slightly at the same time. In all cases, graphitic N and pyridinic N are dominant species. In general, with the   The nitrogen adsorption and desorption isotherms were conducted to investigate the pore structure of these catalyst samples and the results are shown in Figure 4a. The nitrogen adsorption and desorption isotherm curves of all catalysts reveal a Type-Ⅰ sorption isotherm with no hysteresis, manifesting the microporous structure of the Fe3C@N/C-x samples. In the synthesis process, KOH was used not only as an alkaline medium to dissolve ferroporphyrin, but also as an activator to form micropores. The abundant micropores and high specific surface area can improve the accessibility of electrolyte and oxygen to the active sites. The specific surface area and pore volume of all samples are shown in Table 2. With the increase of iron content, the total surface area of Fe3C@N/C-x catalysts reduce from 2484 to 1687 m 2 g −1 . Figure 4b illustrates the pore size distribution (PSD) curves of Fe3C@N/C-x catalysts. The results of PSDs are greatly distributed in the microporous region which is less than 2.0 nm and the peaks are mostly 0.78 nm and 0.59 nm.   The nitrogen adsorption and desorption isotherms were conducted to investigate the pore structure of these catalyst samples and the results are shown in Figure 4a. The nitrogen adsorption and desorption isotherm curves of all catalysts reveal a Type-I sorption isotherm with no hysteresis, manifesting the microporous structure of the Fe 3 C@N/C-x samples. In the synthesis process, KOH was used not only as an alkaline medium to dissolve ferroporphyrin, but also as an activator to form micropores. The abundant micropores and high specific surface area can improve the accessibility of electrolyte and oxygen to the active sites. The specific surface area and pore volume of all samples are shown in Table 2. With the increase of iron content, the total surface area of Fe 3 C@N/C-x catalysts reduce from 2484 to 1687 m 2 g −1 . Figure 4b illustrates the pore size distribution (PSD) curves of Fe 3 C@N/C-x catalysts. The results of PSDs are greatly distributed in the microporous region which is less than 2.0 nm and the peaks are mostly 0.78 nm and 0.59 nm.   CV and LSV measurements were performed to study the electrochemical activity of these catalysts. Figure 5 shows the CV curves of Fe3C@N/C-1 catalyst and 20 wt% Pt/C in N2 and O2-saturated 0.1 M KOH solution. Well-defined oxygen reduction peak for Fe3C@N/C-1 catalyst and 20 wt % Pt/C could be seen in the CV curves when saturating the alkaline solution with O2, illustrating the pronounced ORR activity of Fe3C@N/C-1 catalyst. LSV curves were carried out on a rotating disk electrode (RDE) for the further evaluation of the electrocatalytic activity of catalyst samples for ORR. The LSV measurement was tested in O2-saturated 0.1M KOH solution at a rotation rate of 1600 rpm with a potential scan rate of 5 mV s −1 (the solid line). All samples were also tested in N2-saturated for comparison (the same colour scheme as the dotted line). As shown in Figure 6, the Fe3C@N/C-1 catalyst with onset potential of 0.94V (vs. RHE) and diffusion-limiting current of 5.78 mA cm -2 reveals the remarkable catalytic performance towards ORR compared to the benchmark Pt/C (0.91 V and 5.55 mA cm −2 correspondingly). By contrast, the Fe3C@N/C-0 catalyst shows the lowest catalytic activity to ORR. It is reported that the encapsulated metal compound's nanoparticles could produce host-guest electronic interaction and change the local work function of the carbon shell, creating additional active sites to ORR [33]. Doping of nitrogen into the carbon shell could further improve the catalytic activity of these core-shell structured catalysts by modifying the electronic properties of carbon surface. In addition, there is a general agreement that N-doped graphitic carbon is playing an important role in enhancing the conductivity of the catalysts which is essential for ORR. Therefore, the enhanced electrocatalytic performance of Fe3C@N/C-1 can be attributed to the synergistic effect between N-doped carbon and Fe3C as catalytic active sites. It should be noted that Fe3C@N/C-2 showed obviously lower electrocatalytic activity than that of Fe3C@N/C-1, due to the reduced specific surface area and depressed graphitization degree of the carbon shell in the Fe3C@N/C-2 catalyst. In Table 3 below, the performance of the Fe3C@N/C-1 catalyst is compared to some other Fe3C-based electrocatalysts reported in literature. Overall, the Fe3C@N/C-1 catalyst possessed comparative onset potential with other Fe3C based electrocatalysts. Moreover, the diffusion limiting current of the Fe3C@N/C-1 catalyst was higher than that of most catalysts, indicating the excellent electrochemical activity towards ORR.  CV and LSV measurements were performed to study the electrochemical activity of these catalysts. Figure 5 shows the CV curves of Fe 3 C@N/C-1 catalyst and 20 wt % Pt/C in N 2 and O 2 -saturated 0.1 M KOH solution. Well-defined oxygen reduction peak for Fe 3 C@N/C-1 catalyst and 20 wt % Pt/C could be seen in the CV curves when saturating the alkaline solution with O 2 , illustrating the pronounced ORR activity of Fe 3 C@N/C-1 catalyst. LSV curves were carried out on a rotating disk electrode (RDE) for the further evaluation of the electrocatalytic activity of catalyst samples for ORR. The LSV measurement was tested in O 2 -saturated 0.1M KOH solution at a rotation rate of 1600 rpm with a potential scan rate of 5 mV s −1 (the solid line). All samples were also tested in N 2 -saturated for comparison (the same colour scheme as the dotted line). As shown in Figure 6, the Fe 3 C@N/C-1 catalyst with onset potential of 0.94 V (vs. RHE) and diffusion-limiting current of 5.78 mA cm −2 reveals the remarkable catalytic performance towards ORR compared to the benchmark Pt/C (0.91 V and 5.55 mA cm −2 correspondingly). By contrast, the Fe 3 C@N/C-0 catalyst shows the lowest catalytic activity to ORR. It is reported that the encapsulated metal compound's nanoparticles could produce host-guest electronic interaction and change the local work function of the carbon shell, creating additional active sites to ORR [33]. Doping of nitrogen into the carbon shell could further improve the catalytic activity of these core-shell structured catalysts by modifying the electronic properties of carbon surface. In addition, there is a general agreement that N-doped graphitic carbon is playing an important role in enhancing the conductivity of the catalysts which is essential for ORR. Therefore, the enhanced electrocatalytic performance of Fe 3 C@N/C-1 can be attributed to the synergistic effect between N-doped carbon and Fe 3 C as catalytic active sites. It should be noted that Fe 3 C@N/C-2 showed obviously lower electrocatalytic activity than that of Fe 3 C@N/C-1, due to the reduced specific surface area and depressed graphitization degree of the carbon shell in the Fe 3 C@N/C-2 catalyst. In Table 3 below, the performance of the Fe 3 C@N/C-1 catalyst is compared to some other Fe 3 C-based electrocatalysts reported in literature. Overall, the Fe 3 C@N/C-1 catalyst possessed comparative onset potential with other Fe 3 C based electrocatalysts. Moreover, the diffusion limiting current of the Fe 3 C@N/C-1 catalyst was higher than that of most catalysts, indicating the excellent electrochemical activity towards ORR.        Figure 7a. The Koutecky-Levich plots (Figure 7b) were calculated by the recorded reaction currents at −0.3 V on the LSV curves of the Fe3C@N/C-1 catalyst under various rotating speeds to study the reaction kinetics of this catalyst. The   Figure 7a. The Koutecky-Levich plots (Figure 7b) were calculated by the recorded reaction currents at −0.3 V on the LSV curves of the Fe 3 C@N/C-1 catalyst under various rotating speeds to study the reaction kinetics of this catalyst. The electron-transfer numbers of the Fe 3 C@N/C-1 catalyst were calculated by the K-L equation [47]. The n value for Fe 3 C@N/C-1 catalyst count from the slope of K-L plots is 3.85-4.00, which is close to 4, suggesting the Fe 3 C@N/C-1 catalyze ORR process through a quasi-four-electron process. H 2 O 2 was released during 2e − process which degrades the membrane electrolyte; therefore, the 4e − process is desired for a fuel cell [48][49][50][51]. The calculated Tafel slope is shown in Figure 8a, exhibiting a similar Tafel slope for the Fe 3 C@N/C-1catalyst (68.62 mV per decade) and Pt/C (67.8 mV per decade), indicating the similar reaction kinetics of ORR on the Fe 3 C@N/C-1 catalyst surface and the first electron is probably the rate-determining step.
Polymers 2017, 9, x FOR PEER REVIEW 9 of 13 electron-transfer numbers of the Fe3C@N/C-1 catalyst were calculated by the K-L equation [47]. The n value for Fe3C@N/C-1 catalyst count from the slope of K-L plots is 3.85-4.00, which is close to 4, suggesting the Fe3C@N/C-1 catalyze ORR process through a quasi-four-electron process. H2O2 was released during 2e -process which degrades the membrane electrolyte; therefore, the 4e -process is desired for a fuel cell [48][49][50][51]. The calculated Tafel slope is shown in Figure 8a, exhibiting a similar Tafel slope for the Fe3C@N/C-1catalyst (68.62 mV per decade) and Pt/C (67.8 mV per decade), indicating the similar reaction kinetics of ORR on the Fe3C@N/C-1 catalyst surface and the first electron is probably the rate-determining step.
(a) (b) The stability of catalyst is necessary for the practical application of fuel cells. The chronoamperometric measurements were investigated to evaluate the durability of the Fe3C@N/C-1 catalyst. The glassy-carbon electrode modified with Fe3C@N/C-1 and commercial Pt/C were tested at constant voltage of −0.3 V in an O2-saturated 0.1M KOH aqueous solution with rotation rate of 1600 rpm for 20,000 s. The current-time response is shown in Figure 8b. As we can see, the current-time chronoamperometric response for commercial Pt/C exhibited a rapid current decrease and the current loss is about 25% after 20,000 s. In comparison, the current retention for the Fe3C@N/C-1 catalyst is 94% after 20,000 s, implying the excellent stability under working conditions. The excellent stability of Fe3C@N/C-1 catalyst may in virtue of the appearance of the sufficient outer graphene layer. Thus, it can be concluded that the Fe3C@N-doped microporous carbon nanosphere catalyst is a promising alternative for costly Pt-based electrocatalysts to apply in fuel cells.  The stability of catalyst is necessary for the practical application of fuel cells. The chronoamperometric measurements were investigated to evaluate the durability of the Fe 3 C@N/C-1 catalyst. The glassy-carbon electrode modified with Fe 3 C@N/C-1 and commercial Pt/C were tested at constant voltage of −0.3 V in an O 2 -saturated 0.1M KOH aqueous solution with rotation rate of 1600 rpm for 20,000 s. The current-time response is shown in Figure 8b. As we can see, the current-time chronoamperometric response for commercial Pt/C exhibited a rapid current decrease and the current loss is about 25% after 20,000 s. In comparison, the current retention for the Fe 3 C@N/C-1 catalyst is 94% after 20,000 s, implying the excellent stability under working conditions. The excellent stability of Fe 3 C@N/C-1 catalyst may in virtue of the appearance of the sufficient outer graphene layer. Thus, it can be concluded that the Fe 3 C@N-doped microporous carbon nanosphere catalyst is a promising alternative for costly Pt-based electrocatalysts to apply in fuel cells.
Polymers 2017, 9, x FOR PEER REVIEW 9 of 13 electron-transfer numbers of the Fe3C@N/C-1 catalyst were calculated by the K-L equation [47]. The n value for Fe3C@N/C-1 catalyst count from the slope of K-L plots is 3.85-4.00, which is close to 4, suggesting the Fe3C@N/C-1 catalyze ORR process through a quasi-four-electron process. H2O2 was released during 2e -process which degrades the membrane electrolyte; therefore, the 4e -process is desired for a fuel cell [48][49][50][51]. The calculated Tafel slope is shown in Figure 8a, exhibiting a similar Tafel slope for the Fe3C@N/C-1catalyst (68.62 mV per decade) and Pt/C (67.8 mV per decade), indicating the similar reaction kinetics of ORR on the Fe3C@N/C-1 catalyst surface and the first electron is probably the rate-determining step.
(a) (b) The stability of catalyst is necessary for the practical application of fuel cells. The chronoamperometric measurements were investigated to evaluate the durability of the Fe3C@N/C-1 catalyst. The glassy-carbon electrode modified with Fe3C@N/C-1 and commercial Pt/C were tested at constant voltage of −0.3 V in an O2-saturated 0.1M KOH aqueous solution with rotation rate of 1600 rpm for 20,000 s. The current-time response is shown in Figure 8b. As we can see, the current-time chronoamperometric response for commercial Pt/C exhibited a rapid current decrease and the current loss is about 25% after 20,000 s. In comparison, the current retention for the Fe3C@N/C-1 catalyst is 94% after 20,000 s, implying the excellent stability under working conditions. The excellent stability of Fe3C@N/C-1 catalyst may in virtue of the appearance of the sufficient outer graphene layer. Thus, it can be concluded that the Fe3C@N-doped microporous carbon nanosphere catalyst is a promising alternative for costly Pt-based electrocatalysts to apply in fuel cells.

Conclusions
In summary, Fe 3 C@N-doped microporous carbon nanosphere catalysts were facilely and successfully synthesized by using polyacrylamide microspheres as a carbon and nitrogen resource and ferroporphyrin as an iron resource. The composite and nanostructure of catalysts were characterized by TEM, XRD, XPS and BET analysis and the electrochemical activity of these catalysts were investigated by CV and LSV measurements. The Fe 3 C@N/C-1 catalyst showed excellent ORR activity and long-term stability in alkaline medium, with an onset potential and diffusion limiting current of 0.94 V vs. RHE and 5.78 mA cm −2 , which is superior to that of commercial Pt/C catalyst. The outstanding performance is attributed to the synergistic effect between N-doped carbon and Fe 3 C as catalytic active sites, and the abundant microporous structure can improve the accessibility of electrolyte and oxygen to active sites. In addition, the carbon shell prevents Fe 3 C active sites from leaching out. These results show that this kind of core-shell catalyst could be a promising alternative to Pt-based catalysts for the further development of durable and efficient ORR catalysts.
Author Contributions: X.Y. and H.T. conceived and designed the experiments; M.C. and Y.J performed the experiments; P.M., X.Z. and W.X. analyzed the data; Q.Y. contributed analysis tools; X.Y. and Y.Z. wrote the paper.

Conflicts of Interest:
The authors declare no conflict of interest.