Nitrogen-Doped Mesoporous Carbon Spheres

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Uniform Fe3O4/Nitrogen-Doped Mesoporous Carbon Spheres Derived from Ferric Citrate-Bonded Melamine Resin as an Efficient Synergistic Catalyst for Oxygen Reduction Haitao Wang, Wei Wang, Mengxi Gui, Muhammad Asif, Zhengyun Wang, Yang Yu, Junwu Xiao, and Hongfang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11608 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 16, 2016

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ACS Applied Materials & Interfaces

Uniform Fe3O4/Nitrogen-Doped Mesoporous Carbon Spheres Derived from Ferric Citrate-Bonded Melamine Resin as an Efficient Synergistic Catalyst for Oxygen Reduction Haitao Wang, Wei Wang, Mengxi Gui, Muhammad Asif, Zhengyun Wang, Yang Yu, Junwu Xiao* and Hongfang Liu*

Key Laboratory of Material Chemistry for Energy Conversion and Storage (Huazhong University of Science and Technology), Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, P.R. China.

KEYWORDS: ammonium ferric citrate, nitrogen-doped carbon matrices, iron oxides, mesoporous structure, oxygen reduction reaction

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ABSTRACT: Developing a facile strategy to synthesize an efficient and inexpensive catalyst for the oxygen reduction reaction (ORR) is critical to the commercialization of many sustainable energy storage and conversion techniques. Herein, a novel and convenient strategy was presented to prepare Fe3O4 embedded into nitrogen-doped mesoporous carbon spheres (Fe3O4/N-MCS) by the polycondensation between methylolmelamines and ammonium ferric citrate (AFC) and subsequent pyrolysis process. In particular, the polycondensation reaction was completely finished within a very short time (6.5 min), and the iron contents can be adjusted and had a great influence on the microstructure. Moreover, the Fe3O4/N-MCS can be used as a robust catalyst for the ORR in alkaline media, and the catalyst with the iron content of 3.35 wt% exhibited excellent electrochemical performance in terms of more positive onset potential (E0: 1.036 V vs RHE) and half-wave potential (E1/2: 0.861 V) and much better methanol tolerance and long-term durability, in comparison with that of 20% Pt/C. The remarkable performance was ascribed to the characteristics of large specific surface area, mesoporous structure, high contents of pyridinic N and graphitic N, as well as strong electronic interaction between Fe3O4 and protective N-doped graphitic layers.


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INTRODUCTION

Oxygen reduction reaction (ORR) was one of the most important reactions in clean energy conversion and storage techniques, such as, fuel cells and metal-air batteries.1-3 An efficient and cost-effective ORR electrocatalyst is extremely critical to achieve large-scale commercialization of these techniques.4-6 Unfortunately, so far, platinum (Pt)-based catalysts were still the most effective electrocatalyst for the sluggish kinetics of ORR.7-9 However, their low tolerance, poor durability, high cost and scarcity can’t meet the requirement of large-scale commercialization of these techniques. Thus, exploiting efficient and durable alternatives with low-cost for the ORR is critical for the practical applications of these techniques.10-13 In this respect, a wide range of alternative catalysts have been actively pursued, including non-precious metal14-18 and metal-free catalysts.19-21 Among non-precious metal catalysts, transition metal coordinated nitrogen nanocarbon materials (M/N-C, M = Fe, Co, etc.) have been found to be active and durable towards the ORR, where the catalytic activities are cooperatively contributed by surface nitrogen and metal species.3,

22, 23

Among them, the

Fe/N-C-based catalysts are generally considered to exhibit better catalytic activity than those containing other transition-metal (Co, Ni).24

Besides the chemical compositions, the structures of the catalysts also have an important influences on the catalytic activity due to the interfacial/surface reaction of ORR.25, 26 The high accessible surface area and hierarchal porous structures, which not only can maximize the exposed active sites, but can facilitate mass transfer for the reactants, intermediates and products.27-29 Therefore, the structural design is also of vital importance for enhancing the catalytic performance. Different structures based on M/N-C catalytic active sites have been ACS Paragon Plus 3 Environment


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successfully synthesized, including, nanotubes,30 nanosheets,31 nanoboxes32 and mesoporous spheres.33 Among them, mesoporous spheres can provide the efficient transport path for electrolyte and O2 during oxygen electrocatalysis because of the unique characteristics, such as, open-framework structures, high surface area, large porosity and good electric conductivity.

Several approaches have recently been reported for preparing (Fe/N-C)-based mesoporous carbon spheres (MCS). For example, Wei and Wang34 et al, synthesized the Fe-N/MCS via a hydrothermal reaction of phenol-formaldehyde resol and triblock copolymer pluronic F127 and subsequent the pyrolysis process. However, the synthesis method is quite tedious and time-consuming. The Fe/N-MCS also can be obtained via two-step pyrolysis of nitrogen-rich polymer spheres and iron-based salts. For instance, Xiao and Wang35 et al, reported the fabrication of Fe2N@MCS using dopamine (DA) and FeCl3 as the sources. Lu36 et al, reported the preparation of Fe@Fe3C in carbon spheres via the polymerization of DA and followed by high temperature pyrolysis with iron (II) acetate. However, the common problem is that the N and Fe species are separately incorporated using the aforementioned approaches. Moreover, the time-consuming prepared method, harsh treatments, expensive precursors in these approaches can’t meet the requirement of large-scale commercial applications, and their ORR catalytic activity can hardly match to that of Pt/C in terms of onset potential and diffusion-limiting current.

Based on the comprehensive consideration, we explored a novel and facile strategy to prepare Fe3O4/N-MCS with a uniform size via a two-step process including a simple polycondensation between methylolmelamines and ammonium ferric citrate (AFC) and ACS Paragon Plus 4 Environment

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subsequent pyrolysis process. Importantly, the polycondensation reaction can be completely finished within a very short time of 6.5 min, rather than several hours reported previously. The Fe3O4/N-MCS catalysts are with uniform distribution of Fe3O4 nanoparticle, high contents of pyridinic nitrogen and graphitic nitrogen, abundant mesoporous structure, high surface area and graphitization degree, as well as the synergistic effect of N-doped carbon and Fe3O4 nanoparticles, thus exhibit excellent electrochemical activity in terms of more positive half-wave potential, and much better methanol tolerance and durability in alkaline medium as the iron content is achieved to 3.35 wt%, in comparison with the commercial 20% Pt/C catalyst.

EXPERIMENTAL SECTION

Material Synthesis. The synthetic details of Fe3O4/N-MCS are described as follows: A typical process, different amounts (5 mg, 20 mg and 40 mg) of ammonium ferric citrate (C6H11FeNO7, AFC) and 0.54 g of polyvinyl alcohol (PVA) were completely dissolved in 90 mL of deionized water to form a homogeneous orange solution at 85 °C. 2.8 g of melamine was then added into 5.5 mL of formaldehyde with constant stirring for 20 min under 60 °C to get a clear solution. Subsequently, above solutions and 1.2 mL of acetic acid were mixed under continuous stirring at 60 °C for 6.5 min to form Fe containing nitrogen-rich carbon polymer spheres (Fe-NCPS). The products were collected by centrifugation, and dried up at 40 °C overnight, and were labeled by Fe-NCPS-1, Fe-NCPS-2 and Fe-NCPS-3, respectively, which were dependent on the amounts of AFC (5 mg, 20 mg, and 40 mg). Finally, the Fe-NCPS was pyrolyzed at 800 °C for 1 h under argon atmosphere with a heating rate of 1 °C min-1 to form Fe3O4 nanoparticles embedded into nitrogen doped mesoporous carbon spheres ACS Paragon Plus 5 Environment


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(Fe3O4/N-MCS). For comparison, iron-free nitrogen-rich carbon polymer spheres (NCPS) were synthesized in a similar way without the addition of AFC, and were decomposed into nitrogen doped mesoporous carbon spheres (NCPS and N-MCS).

Physical characterization. The surface morphology analysis was conducted on a field-emission scanning electron microscopy (FESEM, Nova Nano SEM 450). The microstructure was observed with a high resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-TWIN) with 300 kV. The crystallographic structure were recorded on Powder X-ray diffraction (XRD) using an X 'Pert PRO diffractometer (at a rate of 5 °C min-1). The surface properties analysis was evaluated by X-ray photoelectron spectroscopy (XPS) using an AXIS-ULTRA DLD-600W Instrument. Raman spectroscopy was performed on a LabRAM HR800 confocal Raman microscope. The specific surface areas and pore size distributions were obtained by adsorption-desorption measurements (Micromeritics ASAP2020) of nitrogen at 77 K.

Electrochemical measurements. The electrochemical tests were carried out on a CHI760E electrochemical workstation (CH Instruments, China) with the Pine electrochemical system (a Pine biopotentiostat and a rotation speed controller), using a conventional three-electrode electrochemical cell at 25 °C. A glassy carbon electrode (GCE) with an area of 0.196 cm2 was used as the substrate for the working electrode, a platinum foil and a saturated calomel electrode (SCE) were employed as reference and counter electrodes, respectively. All potentials in this work were referenced to a reversible hydrogen electrode (RHE), E(RHE) = E(SCE) + 0.059•pH + 0.243 V. The catalyst ink was fabricated as follows: 5 mg as-prepared sample was ultrasonically dispersed in a 1.0 mL solution (0.98 mL of isopropyl alcohol and ACS Paragon Plus 6 Environment

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0.02 mL of 5 wt% Nafion solution) for at least 30 min. Next, 12.5 µL of the ink was pipetted onto the GCE surface, then dry in air naturally. For comparison, the catalyst loading of Pt/C (20%, E-TEK) is 0.153 mg cm-2. The cyclic voltammetry (CV) measurements were conducted in O2 or N2-saturated 0.1M KOH electrolyte with a scan rate of 50 mV s-1. Linear sweep voltammetry (LSV) profiles were obtained in O2-saturated 0.1M KOH solution at the different electrode rotated speed with a scan rate of 5 mV s-1. For methanol poisoning tests, the chronoamperometric response were performed at 0.6 V (vs RHE) in O2-saturated 0.1 M KOH solution with a rotation rate of 1,600 rpm. The current-time durability measurement at 0.6 V was recorded by a rotating disk electrode (RDE) tests in O2-saturated 0.1 M KOH solution with a rotation rate of 1600 rpm. The ORR activity of the catalyst under the acid condition were also studied in 0.1 M HClO4 solution. The slopes of Koutecky-Levich plots (K-L plots) best linear fit lines was used to analyze the number of electrons transferred (n) during the ORR on the basis of the Koutecky-Levich equation (K-L equation). J-1 = JL-1 + JK-1 = B-1ω-1/2 + JK-1

(1)

B = 0.2nFC0D02/3υ-1/6

(2)

JK = nFkC0

(3)

Where J is the overall current density, JK and JL are the kinetic current density and O2 diffusion limiting current density, ω is the rotation speed, B is the Levich slope, F is the Faraday constant (96485 C mol-1), C0 is the bulk concentration of O2 (1.21×10-6 mol cm-3), D0 is the diffusion coefficient of O2 (1.9×10-5 cm2 s-1), υis the kinematic viscosity of the electrolyte (0.01 cm2 s-1), k is the electron-transfer rate constant. The constant 0.2 is adopted whenω ωis expressed in rpm. The electron transfer number (n) and JK can be calculated by



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linear fitting of J-1 vs.ω ω-1/2 at different electrode potentials and the inverse of the intercept.

RESULTS AND DISCUSSION

The Fe3O4/N-MCS was formed via a facile two-step method (polycondensation and high-temperature pyrolysis step), as schematically illustrated in Figure 1. The key of our synthesis was the addition of AFC rather than iron ions as the iron source in the reactants, since

iron species

can

be

uniformly

introduced

via

the

polymerization

between AFC and hydroxymethylated melamine. First of all, hydroxymethylation of the melamine and formaldehyde molecules resulted in the formation of methylolmelamines, in which hydrogen atoms in the melamine molecules were substituted by methylol groups (-CH2OH).37 Then methylolmelamines and AFC were polymerized under the catalysis of CH3COOH to form the Fe-NCPS. Finally, Fe-NCPS were pyrolyzed into the Fe3O4/N-MCS with the uniform size and abundant mesoporous structure. To gain an insight into the polycondensation reaction between methylolmelamines and AFC, the NCPS and Fe-NCPS were monitored by Fourier transformed infrared (FTIR) spectra (Figure S1). For highlighting the role of iron sources, Fe(NO3)3·9H2O and FeCl3·6H2O were used instead of AFC to prepared Fe-NCPS. The FTIR spectra of Fe-NCPS (Fe3+ ion as the iron source) exhibited similar absorption peaks with NCPS. However, the additional peaks at 1075 and 1793 cm-1 are appeared in the FTIR spectra of Fe-NCPS (AFC as the iron source), attributing to the stretching vibration of C-O-C and C=O in the ester groups, respectively, which confirms the polycondensation reaction between methylolmelamines and AFC. In addition, the bulk and surface mass percentages of iron in the Fe3O4/N-MCS were estimated on the basis of thermogravimetric analysis (TGA) and XPS measurement. The bulk mass percentages of Fe ACS Paragon Plus 8 Environment

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in the Fe3O4/N-MCS-1, Fe3O4/N-MCS-2 and Fe3O4/N-MCS-3 were found to be 1.72%, 3.35% and 6.84% according to TGA results, respectively (Figure S2), larger than that of 0.70%, 1.69% and 2.43% determined by XPS (Table S1), which can be ascribed to the limited depth of XPS analysis.

Figure 1. Schematic illustration of the fabrication process for Fe-NCPS and Fe3O4/N-MCS.



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Figure 2. (a), (b) FESEM and (c), (d) TEM images of Fe-NCPS-2.

The morphologies and microstructures were characterized via electron microscopy. The typical FESEM and TEM images show that Fe-NCPS-2 have well-defined solid spherical morphology with a diameters of ~ 650 nm (Figure 2a and 2c). Figure 2b and 2d further show that there are lots of the flakes at the surface of the spheres. Consistent with Fe-NCPS-2, NCPS and Fe-NCPS-1 both exhibited the similar morphology and microstructure (Figure S3 and S4), while the irregular spheres and the aggregates were appeared in the Fe-NCPS-3 (Figure S5a and S5d). After the pyrolysis process, the similar spherical morphology was retained (Figure 3a and 3b). However, the diameter obviously decreased from 650 nm to 300 nm due to the dehydration polymerization and shrinkage, and a fair amount of dark particles were embedded in the uniform spherical structure (Figure 3c). The XRD patterns were used


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for investigating the crystalline nature, and were shown in Figure 4a. The peaks at about 30.5, 35.9, 43.6, 57.6, and 63.2° were attributed to the crystal planes of (220), (311), (440), (511) and (440) of Fe3O4 (JCPDS No. 89-2355), revealing that the iron species were transformed into Fe3O4 after the pyrolysis process. Moreover, the characteristic peaks of Fe3O4 was gradually obvious with increasing the iron content. The diffraction peaks at ~ 26.2° is corresponding to the (002) reflection of graphite (JCPDS No. 75-1621). The diffraction peak ascribed to graphite gradually become sharp from N-MCS to Fe3O4/N-MCS, indicating that the introduction of iron species improved the graphitization degree of carbon matrices during the pyrolysis process, resulting in the increase of electric conductivity. The similar phenomenon was also seen from the transformation processes from NPCS to N-MCS (Figure S3b and 3d) and from Fe-NCPS-1 to Fe3O4/N-MCS-1 (Figure S6). For Fe-NCPS-3, only a small amount of the spheres were preserved (Figure S5b and 5c), and the additional large particles were appeared after the pyrolysis process (Figure S5e and S5f). Moreover, it’s found from XRD results (Figure 4a) that a faint diffraction peak at ~ 44.9° was appeared as increasing the iron content to 6.84 wt% (Fe3O4/N-MCS-3), corresponding to the crystalline facets of metallic Fe (JCPDS No. 87-0721).



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Figure 3. (a), (b) FESEM and (c), (d) TEM images of Fe3O4/N-MCS-2. (e), (f) HRTEM images of Fe3O4 nanoparticles in the Fe3O4/N-MCS-2 and corresponding lattice fringe images (Inset). (g) One typical TEM image of Fe3O4 nanoparticle encapsulated by graphitic carbon layer.

The microstructures of Fe3O4/N-MCS-2 were further investigated by HRTEM. In contrast to the dense spheres in the Fe-NCPS-2, the Fe3O4/N-MCS-2 spheres show the porous


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structure, and Fe3O4 nanoparticles with the diameter of 10 ~ 20 nm were uniformly embedded in the spheres (Figure 3d). The lattice fringes with an inter-planar distance of 0.297 and 0.253 nm in Figure 3e and 3f were ascribed to the lattice distances of the (220) and (311) plane of Fe3O4, respectively, and meanwhile the selected area electron diffraction (SAED) pattern also illustrated the crystalline phase of Fe3O4 (Figure S7). As seen from Figure 3g, it’s found that the Fe3O4 nanoparticles were entirely encapsulated by graphitic carbon layers, in which the lattice spacing at 0.340 nm was corresponding to the (002) plane of graphitic carbon. The confinement structure not only can effectively activate the surrounding graphitic carbon layer to enhance the reactivity of the sites, but can suppress the dissolution and agglomeration of Fe3O4 nanoparticles during the catalysis process,38, 39 thereby exhibiting a promising catalytic activity and long-term durability for ORR.

The structural defects and disordered graphitic structures of carbon in the N-MCS and Fe3O4/N-MCS were analyzed by Raman spectra, as shown in Figure 4b. The N-MCS and Fe3O4/N-MCS exhibited two sharp characteristic peaks at around 1358 cm-1 (D band) and 1592 cm-1 (G band), corresponding to the disordered carbon and graphitic sp2 hybridized carbon, respectively.40 The intensity ratio of the D band to G band (ID/IG) was cited frequently to measure the structural disorder of carbon materials.32, 41 The ID/IG values were calculated to be 1.04 for Fe3O4/N-MCS-1, 1.07 for Fe3O4/N-MCS-2 and 1.22 for Fe3O4/N-MCS-3, which were all higher than that of N-MCS (1.02), since the introduction of iron species increased the defect number of nitrogen doped carbon framework, in accordance with the XPS results as below.



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Figure 4. (a) XRD pattern of (1) N-MCS, (2) Fe3O4/N-MCS-1, (3) Fe3O4/N-MCS-2 and (4) Fe3O4/N-MCS-3. (b) Raman spectra with corresponding ID/IG ratios of N-MCS, Fe3O4/N-MCS-1, Fe3O4/N-MCS-2 and Fe3O4/N-MCS-3. (c) Nitrogen adsorption-desorption isotherms

of

Fe-NCPS-2

(inset),

N-MCS,

Fe3O4/N-MCS-1,

Fe3O4/N-MCS-2

and

Fe3O4/N-MCS-3. (d), (e) High resolution XPS spectra of Fe 2p and N 1s for Fe3O4/N-MCS-2. (f) The percentages of different nitrogen types in the N-MCS, Fe3O4/N-MCS-1, Fe3O4/N-MCS-2 and Fe3O4/N-MCS-3.

The specific surface area and pore size distribution of the Fe3O4/N-MCS were assessed based on Brunauer-Emmett-Teller (BET) method. Table S2 summaries the BET surface area, ACS Paragon Plus 14 Environment

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average pore size and total pore volume of the N-MCS and Fe3O4/N-MCS catalysts. The N-MCS and Fe3O4/N-MCS exhibited the type-IV isotherm (Figure 4c) with a pronounced hysteresis loop at relative pressure of around 0.4, suggesting the presence of mesopore, which also can be seen from the pore size distributions of N-MCS and Fe3O4/N-MCS (Figure S8a-d). The pore diameters are centered mainly at around 3.2 and 5.3 nm. However, the additional pores with the diameters of 9.9 and 16.8 nm are appeared in the Fe3O4/N-MCS-3, which may be due to the breakage of the spherical structure, as confirmed by FESEM and TEM images (Figure S5b, c, e and f). The specific surface areas were calculated to be 895.8, 795.1, 769.6 and 638.5 m2 g-1 for N-MCS, Fe3O4/N-MCS-1, Fe3O4/N-MCS-2 and Fe3O4/N-MCS-3, respectively. Evidently, the surface area decreased gradually with increasing the amounts of AFC, may since the pores are partially occupied by Fe3O4 nanoparticles, which also can be seen from the change in the average pore size and total pore volume of Fe3O4/N-MCS.

In order to probe the elemental compositions and chemical state, XPS measurement were performed to reveal the chemical bonding information. The full XPS spectra of the catalysts showed a clear N band, confirming that N atoms were successfully doped into carbon matrices (Figure S9a). Compared with N-MCS, the peaks centered at 710 eV in the Fe/N-MCS was ascribed to Fe 2p. The mass percentages of C, N, O and Fe in N-MCS, Fe3O4/N-MCS-1, Fe3O4/N-MCS-2 and Fe3O4/N-MCS-3 were summarized in Table S1. The chemical state of C, Fe and N elements in the Fe3O4/N-MCS-2 were further investigated. The high-resolution C 1s spectrum (Figure S9b) could be deconvoluted into four peaks: C-C (284.5 eV), C-N (285.2 eV), C=N (285.9 eV) and C=O (290.2 eV).42 The high-resolution Fe 2p spectrum (Figure 4d) reveal the presence of two prominent bands at around 710 eV and 724 eV, corresponding to



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Fe 2p2/3 and Fe 2p2/1 for Fe3O4,43 respectively. Moreover, the appearance of the satellite peak at 718.7 eV indicated the co-existence of Fe2+ and Fe3+.44

Figure 4e and Figure S9c-e display high-resolution N 1s spectrum of the N-MCS and Fe3O4/N-MCS, which can be fitted into three peaks at the binding energy of 398.6 eV, 400.7 eV, and 401.2 eV, corresponding to pyridinic N, pyrrolic N, and graphitic N, respectively. The relative percentages and contents of the different N species calculated according to the integrated area were listed in Figure 4f and Table S1. It’s found that the total nitrogen content in the Fe3O4/N-MCS gradually increased, in accompany with the increase of iron content, suggesting that the introduction of iron species was helpful to form strong Fe-N interaction,45 and hence nitrogen atoms were retained during the pyrolysis process. The binding energy of the N 1s peaks shifted positively from N-MCS to Fe3O4/N-MCS-2 (Figure S10), further confirming that Fe3O4 had a strong electronic effect on the neighbor nitrogen atoms.38, 46 Moreover, besides the total nitrogen content, the percentages of pyridinic N and graphitic N also increased with increasing the iron content, which have been reported to exhibit higher catalytic activity towards the ORR than pyrrolic N. Thus, the iron species were introduced into N-MCS to form highly efficient active sites, resulting in enhancing the catalytic activity.


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Figure 5. (a) LSVs of Fe3O4/N-MCS-2 obtained at different pyrolysis temperatures in O2-saturated 0.1 M KOH solution. (b) CVs of Fe3O4/N-MCS-2 and Pt/C in O2 or N2-saturated 0.1 M KOH. (c), (d) LSVs of N-MCS, Fe3O4/N-MCS-1, Fe3O4/N-MCS-2, Fe3O4/N-MCS-3, Pt/C and the corresponding kinetic current density JK at the potential of 0.75 V in O2-saturated 0.1 M KOH solution. (e) LSVs and the corresponding K-L plots of Fe3O4/N-MCS-2 at the potential range of 0.4 V to 0.7 V (inset). (f) Transferred electron number of (1) N-MCS, (2) Fe3O4/N-MCS-1, (3) Fe3O4/N-MCS-2, (4) Fe3O4/N-MCS-3 and (5) Pt/C.

The catalytic activities of the as-prepared catalysts toward the ORR was recorded by a RDE set-up in O2 or N2-saturated 0.1 M KOH. Cyclic voltammograms (CVs) and linear sweeping ACS Paragon Plus 17 Environment


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voltammograms (LSVs) for Fe3O4/N-MCS-2 obtained at different pyrolysis temperatures were firstly investigated, as shown in Figure S11 and Figure 5a. Noted that Fe3O4/N-MCS-2 obtained at the pyrolysis temperature of 800 °C exhibited the optimized catalytic activity with the oxygen reduction peak potential (EP) of 0.845 V and half-wave potential (E1/2) of 0.861 V, suggesting that the optimal pyrolysis temperature was 800 °C. Thus, the catalysts discussed as below were pyrolyzed at 800 °C. For comparison, N-MCS, Fe3O4/N-MCS-1, Fe3O4/N-MCS-2, Fe3O4/N-MCS-3 and commercial Pt/C were also investigated. As shown in Figure 5b and Figure S12, in the N2-saturated electrolyte, a quasi-rectangular voltammogram without obvious redox peak was observed. In contrast, a pronounced cathodic peak was clearly observed in O2-saturated electrolyte, which can be ascribed to the oxygen reduction. It’s found that the reduction peak of Fe3O4/N-MCS-2 (0.845 V vs. RHE) was more positive than that of N-MCS (0.702 V), Fe3O4/N-MCS-1 (0.801 V) and Fe3O4/N-MCS-3 (0.824 V), even surpassed that of Pt/C (0.831 V).

Such an effective performance was further supported by linear sweep voltammetrys (LSVs) tested in O2 saturated 0.1 M KOH solution using a rotating disk electrode (RDE) technique, and the results are shown in Figure 5c. A clear legibility of the comparison of all samples in the onset potential E0 (defined as the corresponding potential when reaching an ORR current density of 0.1 mA cm-2), E1/2 and diffusion-limiting current density Ji, which can be derived from Figure 5c, and were listed in Table S3. The N-MCS showed the E0 of 0.869 V and the Ji of 4.52 mA cm-2 at 0.2 V. Compared with N-MCS, the Fe3O4/N-MCS catalysts exhibited better catalytic activities. The E0 and Ji increased to 0.945 V and 4.74 mA cm-2 as the bulk content of Fe was 1.72 wt% (Fe3O4/N-MCS-1). The E0 and Ji further increased to 1.036 V and


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5.73 mA cm-2 when the bulk Fe content increased to 3.35 wt% (Fe3O4/N-MCS-2). However, the catalytic activity of Fe3O4/N-MCS-3 with the Fe content of 6.84 wt% was worsen with the E0 of 0.984 V and the Ji of 5.51 mA cm-2. The same tendency was also demonstrated in E1/2 (Table S3). The Fe3O4/N-MCS-2 was also benchmarked with commercial Pt/C catalyst for the ORR. Remarkably, the Fe3O4/N-MCS-2 exhibited more positive E0 and E1/2 than that of Pt/C catalyst (E0: 1.036 V versus 0.992 V, E1/2: 0.861 versus 0.837 V). Furthermore, the Ji of Fe3O4/N-MCS-2 (5.73 mA cm-2 at 0.2 V) was considerably larger than that of Pt/C (5.57 mA cm-2 at 0.2 V). To the best of our knowledge, the electrochemical activity of Fe3O4/N-MCS-2 for ORR was superior to most of the as-reported transition-metal catalysts in terms of EP and E0, as well as E1/2 (Table S4). The outstanding ORR catalytic activity of Fe3O4/N-MCS-2 was further confirmed by the kinetic current density (JK). Fe3O4/N-MCS-2 exhibited the JK of 283.85 mA cm-2 at the potential of 0.75 V (Figure 5d), which was three times as much as Pt/C (89.65 mA cm-2), and higher than that of N-MCS (2.43 mA cm-2), Fe3O4/N-MCS-1 (24.45 mA cm-2), and Fe3O4/N-MCS-3 (71.94 mA cm-2), indicating that O2 can be easily reduced under the catalysis of Fe3O4/N-MCS-2.

As we known, Fe3O4 and nitrogen-doped carbon matrix were co-existed in the Fe3O4/N-MCS, and hence there is a question what roles these two components played in determining the catalytic activity. Noted that although the Fe-free N-MCS was with larger surface area (895.8 m2 g−1) than the Fe3O4/N-MCS, it exhibited poorer ORR performance, and the content of Fe3O4 in the Fe3O4/N-MCS also influenced the catalytic activity. Moreover, the ORR activity was deteriorated as Fe3O4 nanoparticles in the Fe3O4/N-MCS were partially removed in the hot concentrated H2SO4 solution (0.5 M, denote as Fe3O4-leaching, Figure



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S13). It’s speculated that the remarkable ORR catalytic activity of Fe3O4/N-MCS-2 could be due to the strong electronic interactions between Fe3O4 nanoparticles and N-doped graphitic layers, which was confirmed that the binding energy ascribed to N 1s XPS spectrum in the Fe3O4/N-MCS-2 shifted positively,47-49 in comparison with that in the N-MCS (Figure S10). It’s in accordance with that metallic nanoparticles were encapsulated by graphitic carbon layer to enhance the ORR activity.3, 5, 39

Among the Fe3O4/N-MCS catalysts, the Fe3O4/N-MCS-2 exhibited the best ORR activity, which can be ascribed to several factors discussed as follows: Firstly, pyridinic N and graphitic N coordinated with C and Fe to form Fe-N-C active sites and alter the electron distribution due to the different electronegativity of carbon and nitrogen atoms, and hence increasing the reactivity of the sites.3, 50, 51 In the case, Fe3O4/N-MCS-2 contained higher content of pyridinic N and graphitic N than that of Fe3O4/N-MCS-1, thereby demonstrating better ORR performance; Secondly, the microstructures of nitrogen-doped carbon support also influenced the ORR activity. The uniform mesoporous spherical framework was observed in the Fe3O4/N-MCS-1 and Fe3O4/N-MCS-2. However, besides a small amount of mesoporous spheres, carbon matrices and large Fe3O4 and/or Fe particles were appeared in Fe3O4/N-MCS-3, resulting in the decrease of the specific surface area and porous volume. Hence, although Fe3O4/N-MCS-3 was with higher amounts of graphitic N and pyridinic N, in comparison with the Fe3O4/N-MCS-2, the ORR activity was deteriorated due to low specific surface area and pore volume, which were closely related to the number of the exposed active sites. In a word, the outstanding electrocatalytic performance of Fe3O4/N-MCS-2 was mainly ascribed to the characteristics of large specific surface area, mesoporous spherical framework


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structure, high content of pyridinic N and graphitic N, as well as strong electronic interactions between Fe3O4 and N-doped graphitic layers.

To further elucidate the pathway and kinetics of ORR, LSVs of all samples and Pt/C on RDE were recorded at different rotation rates (625 rpm to 2500 rpm) in O2-saturated 0.1 M KOH solution (Figure 5e and Figure S14 to S17). The kinetic parameters and the K-L plots at different potentials (0.4 V to 0.7 V) were obtained, and are shown in Inset of Figure 5e and Figure S14 to S17. Unlike the K-L plots of N-MCS, the Fe3O4/N-MCS exhibited good linearity with a similar slop, indicating that the first-order kinetics is related to the concentration of dissolved O2 at the potentials of 0.4 ~ 0.7 V.52 Figure 5f shows the transferred electron number (n) at the potential range from 0.4 to 0.7 V calculated according to the K-L equation. The average number of electrons transferred was 3.81 for Fe3O4/N-MCS-1, 3.99 for Fe3O4/N-MCS-2, 3.97 for Fe3O4/N-MCS-3 and 3.99 for Pt/C, indicating the Fe3O4/N-MCS catalyzed the ORR mainly through a dominant four-electron pathway in alkaline medium,53 just like Pt/C. In contrast, N-MCS exhibited low electron transfer number (~3.41), manifesting that the ORR under the catalysis of N-MCS is via a mixed 2e- and 4e- pathway. Hence, the introduction of Fe3O4 into N-MCS also can affect the pathway and kinetics of the ORR.

In addition, the catalytic performance of Fe3O4/N-MCS in acid media were evaluated by the RDE technique in O2-saturated 0.1 M HClO4, and the results are shown in Figure S18. Noted that the tendency of the ORR catalytic activity in acid media was similar with that in alkaline media. The Fe3O4/N-MCS-2 exhibited better catalytic activity with more positive E1/2 of 0.702 V and larger Ji of 5.16 mA cm-2, in comparison with N-MCS, Fe3O4/N-MCS-1 and ACS Paragon Plus 21 Environment


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Fe3O4/N-MCS-3. However, the catalytic activity of the Fe3O4/N-MCS-2 was slightly inferior to that of commercial Pt/C catalysts with the E1/2 of 0.830 V and Ji of 5.47 mA cm-2. The K-L plots were obtained according to the LSVs at the rotation rates of 625 ~ 2500 rpm, as shown in Figure S18b and S18c. It’s found that the ORR process under the catalysis of Fe3O4/N-MCS-2 was a first-order reaction at the potentials of 0.3 ~ 0.6 V in acid media. The corresponding electron transfer number was calculated according to the K-L plots using Equation 1-3 in experimental section, and the results are shown in Figure S18d. The electron transfer number of Fe3O4/N-MCS-2 was in the range of 3.79-3.87, obviously higher than that of Fe3O4/N-MCS-1, and Fe3O4/N-MCS-3, and was comparable to that of Pt/C catalyst, revealing that the ORR process under the catalysis of Fe3O4/N-MCS-2 was through a dominant four-electron pathway. Thus, above results demonstrate that the Fe3O4/N-MCS-2 catalyst exhibited good electrocatalytic activity towards the ORR in alkaline and acid media.

Figure 6. (a) LSVs of Fe3O4/N-MCS-2 and Pt/C in O2-saturated 0.1 M KOH or in the


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presence of 2 M methanol O2-saturated 0.1 M KOH. (b) Chronoamperometric responses of Fe3O4/N-MCS-2 and Pt/C at 0.6 V in O2-saturated 0.1 M KOH solution with 2 M methanol added. (c) LSVs of Fe3O4/N-MCS-2 and Pt/C before and after 3000 potential cycles at the potential range of 0 V to 1.2 V in O2-saturated 0.1 M KOH. (d) The current- time durability measurement of Fe3O4/N-MCS-2 and Pt/C at 0.6 V in O2-saturated 0.1 M KOH solution with the rotation speed of 1600 rpm.

Besides high catalytic activity, the crossover of methanol was important aspect of the catalysts in fuel cells, especially for direct methanol fuel cells. The tolerance of Fe3O4/N-MCS-2 and Pt/C to methanol was examined by CVs and LSVs in O2 saturated 0.1 KOH electrolyte containing 3 M methanol, and the results are shown in Figure S19 and Figure 6a. No activity specific to methanol on the Fe3O4/N-MCS-2 catalyst was observed when the methanol was added into the electrolyte. In contrast, for Pt/C, the cathodic ORR peak vanished and the methanol oxidation peak was observed obviously. The vastly different response to methanol was also found by investigating the chronoamperometric response at 0.6 V in O2-saturated 0.1 M KOH solution containing 2 M methanol (Figure 6b). After adding methanol, the current density of Pt/C shows an appreciable decrease, whereas no noticeable current response of Fe3O4/N-MCS-2 catalyst is observed. Above results demonstrated that the as-prepared Fe3O4/N-MCS-2 possessed considerably better tolerance to methanol crossover than Pt/C.

The durability was another key parameter of fuel cell catalysts. The stability of Fe3O4/N-MCS-2 and Pt/C were assessed by cycling the catalysts between 0 and 1.2 V at 100 mV/s in O2-saturated 0.1 M KOH. As shown in Figure 6c, the E1/2 for Pt/C shifts negatively ACS Paragon Plus 23 Environment


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by 13 mV, whereas the E1/2 for Fe3O4/N-MCS-2 show negligible loss after 3000 continuous cycles, suggesting the excellent operation stability of Fe3O4/N-MCS-2. In addition, the short-term durability of Fe3O4/N-MCS-2 and Pt/C toward ORR were further investigated through the chronoamperometric method at 0.6 V in O2-saturated 0.1 M KOH solution for 80,000 s (Figure 6d). Impressively, about 91.6 % of the original current density was retained for Fe3O4/N-MCS-2 catalyst, while the commercial Pt/C catalysts exhibited a much higher current loss of 26.8 %. These results unambiguously disclose that the Fe3O4/N-MCS-2 has a much better stability as a cathode catalyst for the future application of the direct methanol fuel cells than commercial 20% Pt/C catalysts.


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CONCLUSIONS

In summary, we proposed a fast polycondensation reaction of methylolmelamines and AFC to successfully introduce iron species into NCPS, and used it as the precursor to synthesize highly efficient Fe3O4/N-MCS catalysts toward the ORR. The presence of iron species not only can tempt the formation of hierarchical porous carbon supports for improving the mass transfer, but also can cooperate with N-MCS matrices to enhance the reactivity of active sites. Hence, owing to the favorable features of mesoporous structure, large specific surface area, high content of pyridinic N and graphitic N, as well as cooperative effect between Fe3O4 and protective N-doped graphitic layers, the synthesized Fe3O4/N-MCS-2 exhibited outstanding catalytic activity and stability for ORR, and even outperformed commercial Pt/C catalyst. In consideration of the simple synthetic preparation, low cost, large-scale industrial production, remarkable catalytic activity and long-term durability, the as-prepared electrocatalyst may serve as a promising alternative non-precious metal catalysts for energy conversion and storage technologies.



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ASSOCIATED CONTENT

Supporting information. This Supporting Information is available free of charge on the Internet at http://pubs.acs.org. FTIR spectra of NCPS and Fe-NCPS; TGA curves of Fe3O4/N-MCS; SEM and TEM of NCPS, Fe-NCPS and Fe3O4/N-MCS; XPS spectrum and pore size distribution curve of N-MCS and Fe3O4/N-MCS; CVs of Fe3O4/N-MCS-2 obtained at different pyrolysis temperatures CVs of N-MCS and Fe3O4/N-MCS; LSVs at different rotation rates and the corresponding K-L plots of N-MCS, Fe3O4/N-MCS and Pt/C; CVs of Pt/C and Fe3O4/N-MCS-2 in KOH containing 3 M methanol; Elemental composition and surface area of N-MCS and Fe3O4/N-MCS; and ORR catalytic activity comparison among N-MCS, Fe3O4/N-MCS and other transition-metal catalysts.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] (H. Liu).

*E-mail: [email protected] (J. Xiao).

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACS Paragon Plus 26 Environment

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ACKNOWLEDGMENT This research was supported by the Innovation Foundation of Huazhong University of Science and Technology Innovation Institute (No.2015ZZGH010 and 2015TS150), National Natural Science Foundation of China (Project No. 21401060) and Research Fund for the Doctoral Program of Higher Education of China (20130142120024). We acknowledge the support of the Analytical and Testing Center of the Huazhong University of Science and Technology for SEM and XPS measurements. REFERENCE (1) Steele, B. C.; Heinzel, A., Materials for Fuel-Cell Technologies. Nature 2001, 414, 345-352. (2) Chen, Z.; Higgins, D.; Tao, H.; Hsu, R. S.; Chen, Z., Highly Active Nitrogen-Doped Carbon Nanotubes for Oxygen Reduction Reaction in Fuel Cell Applications. J. Phys. Chem. C 2009, 113, 21008-21013. (3) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science 2011, 332, 443-447. (4) Debe, M. K., Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. (5) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P., Iron-Based Catalysts with Improved Oxygen Reduction Activity in Polymer Electrolyte Fuel Cells. science 2009, 324, 71-74. (6) Chen, C.; Yang, X.-D.; Zhou, Z.-Y.; Lai, Y.-J.; Rauf, M.; Wang, Y.; Pan, J.; Zhuang, L.; Wang, Q.; Wang, Y.-C., Aminothiazole-Derived N, S, Fe-Doped Graphene Nanosheets as High Performance Electrocatalysts for Oxygen Reduction. Chem. Commun. 2015, 51, 17092-17095. (7) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D., Structurally Ordered Intermetallic Platinum-Cobalt Core-Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. mater. 2013, 12, 81-87. (8) Greeley, J.; Stephens, I.; Bondarenko, A.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K., Alloys of Platinum and Early Transition Metals as Oxygen Reduction Electrocatalysts. Nat. chem. 2009, 1, 552-556. (9) Zhang, M.; Dai, L., Carbon Nanomaterials as Metal-Free Catalysts in Next Generation ACS Paragon Plus 27 Environment


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Nitrogen-Doped Mesoporous Carbon Spheres

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