Uniform Multilayer Graphene-Coated Iron and Iron-Carbide as Oxygen

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Multilayer Graphene Coated Iron and IronCarbide Uniformly as Oxygen Reduction Catalyst Ailing Song, Lei Cao, Wu Yang, Yao Li, Xiujuan Qin, and Guangjie Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04319 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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ACS Sustainable Chemistry & Engineering

Multilayer Graphene Coated Iron and Iron-Carbide Uniformly as Oxygen Reduction Catalyst Ailing Song,†,‡ Lei Cao,†,‡ Wu Yang,†,‡ Yao Li,†,‡ Xiujuan Qin,*,†,‡ Guangjie Shao,†,‡ †

State key Laboratory of Metastable Materials Science and Technology, Yanshan

University, 438 Hebei West Avenue, Qinhuangdao 066004, China ‡

Hebei Key Laboratory of Applied Chemistry, College of Environmental and

Chemical Engineering, Yanshan University, 438 Hebei West Avenue, Qinhuangdao 066004, China

ABSTRACT: The industrialization of fuel cell is coming. The non-noble metals with carbonaceous coatings, as cathodic catalysts for fuel cells expected to replace the use of platinum for catalyzing the oxygen reduction reaction (ORR), have attracted much attention of researchers. This is mainly due to their low price, easy availability, high stability and gratifying catalytic activity. However, the high density and uniform dispersion of coated particles are still facing a challenge. In view of this, we prepared Fe3C/Fe@G catalysts with structure of nitrogen-doped multilayer graphene uniformly coating on iron and iron carbide (Fe3C/Fe) in a very simple way. The half wave potential of Fe3C/Fe@G-800 is comparable to commercial Pt/C, and it also highlights the excellent stability, these largely benefit from the complete coating of graphene layers to internal particles; highly dispersed Fe3C/Fe nanoparticles with small size in 

Corresponding author. Hebei Key Laboratory of Applied Chemistry, College of Environmental and Chemical

Engineering, Yanshan University, 438 Hebei West Avenue, Qinhuangdao, 066004, China. E-mail address: [email protected] (X.Qin), [email protected] (G. Shao).

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the graphene layer; the equilibrium degree of graphitization and the defect density. KEYWORDS: Non-noble metal catalysts, Coated structure, Oxygen reduction reaction INTRODUCTION Fuel cells with high energy conversion efficiency and none carbon emissions, attract many attentions of researchers and industries.1-6 Their promising future is primarily restricted by the cost at present, one of the main items of these expenditures is due to the slow cathode kinetic process required for the use of noble metal-based catalysts.7 The key issues in solving this problem is to reduce or even out of use of high-grade noble metal-based catalysts.8 Although the research on noble metal catalysts has been done in the state of atomic dispersion,9-10 and even single layer11 to improve utilization and reduce costs, but comparing to the use of non-noble metal catalysts, the cost is still high. Therefore, it is considered valuable and necessary to study the inexpensive non-noble metal catalysts. So far, the research on non-noble metal based oxygen reduction catalysts mainly concentrates on metal oxides,12-13 nitrides,14-15 oxynitrides,16 and carbonitrides;17 metal chalcogenides;18 carbon-based non-noble metal and metal-free catalysts,19-22 they are all inexpensive and have high performance in space. Since the pure metal compounds, above oxides, nitrides, oxynitrides, chalcogenides, have poor conductivity and stability, so the combination of these with stable carbon materials gained the favor of researchers. Guided in this direction, there have been some good jobs. For example, Chen et al. successfully synthesized the N-Fe/Fe3C@C nanorods, 2 ACS Paragon Plus Environment

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which showed notably improved activities towards ORR in neutral phosphate buffer solution in comparison with commercial Pt/C,23 both nitrogen-doping and core–Fe3C are presumed to be important for the advanced kinetics. Nonetheless, the material has the tendency of reunion from the observation of TEM images, which would not be conducive to the exposure of active sites, the arrival of electrolyte and oxygen. Then, Yao et al. used the sol-gel method to load Fe2N/C onto the graphene layer. The obtained catalyst showed good catalytic performance in 1 M HClO4,24 but if the particle distribution was more homogeneous, higher-density, the active sites may be more efficient. Also many other studies on carbon-based non-noble metal and metal-free catalysts have shown good catalytic properties, structure and cycle stability.25-26 Based on the above, we used a simple and effective method to prepare a carbon-coated non-noble metal-based catalyst, multilayer graphene-coated iron and iron-carbide catalyst

(Fe3C/Fe@G). The half-wave potential of

optimized

Fe3C/Fe@G-800 was comparable to commercial Pt/C, which indicated more accessible catalytic sites on the exposed surface of the sample.27 And the stability of Fe3C/Fe@G-800 compared to Pt/C was also remarkable. All of these outstanding properties benefit from the structure with multilayer-graphene coatings, the introduction of nitrogen from cyanoge namine to the graphene at the same time as well as the highly dispersed Fe3C/Fe nanoparticles in the graphene layer lead to the synergistic effect between the N-doped graphitized carbon layers and the coated particles, which makes the catalytic activity of the material more efficient; the 3 ACS Paragon Plus Environment


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equilibrium degree of graphitization and the defect density, while ensuring the catalytic activity of materials, the 2-dimensional plane structure enables the materials to form a continuous electron conduction path on the surface layer, so that the material has both activity and conductivity. EXPERIMENTAL SECTION Preparation of Fe3C/Fe@G.

Fe(NO3)3·9H2O (Tianjin Damao Chemical Reagent

Factory) and melamine (Kemi Chemical Reagent Co., Ltd.) were weighed at a mass ratio of 1:10. And then the mixture was ball milled for 4 h with a planetary ball milling. After that, a certain amount of mixture was placed in a tube and heated to 700, 800, and 900 ℃ with a ramp rate of 5 ℃/min under inert atmosphere in tube furnace respectively (These contents and temperatures were chosen to allow the metal to be encapsulated in the graphene carbon layer in the form of particles instead of other reactions), and then kept at their respective temperature for 2 h. After cooling to room temperature naturally, the black bar sample was taken out and placed in a dilute acid solution to remove the impurity particles physically attached to the surface. After 24 hours, the product collected by filtration was washed with ethanol, distilled water for several times and dried to obtain the final products named as Fe3C/Fe@G-700, Fe3C/Fe@G-800, Fe3C/Fe@G-900. Characterization of Materials. The morphology and microstructure of the Fe3C/Fe@G were observed by Transmission electron microscopy (TEM, JEM2010), and High magnification transmission electron microscope (HRTEM, H-7500) and Field-emission scanning 4 ACS Paragon Plus Environment

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electron microscopy (FE-SEM, Hitachi Modle S-4800, KV), the elements on the surface of sample were identified by energy dispersive X-ray spectroscopy (EDS) and quantified by X-ray photoelectron spectroscopy (XPS) with a Kratos XSAM-800 spectrometer. The crystal types of Fe3C/Fe@G were characterized by X-ray diffraction (XRD) spectrum on a Rigakud/MAX-2500/pc X-ray diffractometer operated at 40 kV using Cu Kα radiation at a scan rate of 2 °min-1. Iron is quantified by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Thermo Scientific iCAP6300, U.S.A). Raman spectra were recorded by a Horiva (LabRam HR-800) spectrometer (532 nm, 50 mW excitation laser). The electric conductivity of the prepared catalysts was measured by ST2722-SZ Semiconductor powder resistivity instrument. Nitrogen adsorption/desorption isotherms were conducted at 77 K by N2 physisorption on a Micromeritics V-Sorb 2800P analyzer. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Pore size distribution used the Barrette -Joynere-Halenda (BJH) method. Electrochemical measurements Ink solution for preparing working electrode film was prepared by dispersing 5.0 mg catalyst in the intermixture of 50 μL Nafion (DuPont, 5 wt. %) and 2 ml ethanol, followed by sonication for 30 min to be homogeneous. Then, dropped amount ink onto the glassy carbon electrode (GCE) with 5 mm in diameter (PINE Instruments) and dried in the air, the loading of Fe3C/Fe@G was 600 μg cm-2 (Pt/C, 60 μgPt cm-2). Ag/AgCl (saturated KCl) electrode was used as the reference electrode, Pt wire as the counter electrode, and the GCE modified with catalyst ink was used as the 5 ACS Paragon Plus Environment


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working electrode, so that the typical three-electrode system was applied to all subsequent electrocatalytic properties testing. PARSTAT4000 Princeton Applied Research was used in all electrochemical measurements. The electrolyte of the test system is 0.1 M KOH, and all the tests shown in the text were tested in O2 saturated KOH solution except for special cases. For the measurements of rotating disk electrode (RDE), the working electrode was scanned at 5 mV s-1 with various rotating speed at 400, 600, 900, 1,200, 1,600, and 2,000 rpm, respectively. Koutecky−Levich (K-L) plots (j-1 vs ω-1/2) were analyzed at different electrode potentials. The slopes of the fit lines were employed to calculate the electron transfer number (n) on the basis of the K−L equation. The detailed were introduced in the Supporting Information. Chronoamperometry test was obtained at -0.3 V (vs. Ag/AgCl) with a rotation rate of 1600 rpm for 30,000 s. Cyclic voltammetry measurements (CVs) under N2-saturated system in a non-Faradaic region at different scan rates (20, 40, 60, 80, 100, 120, 140, 160, and 180 mV s-1) were conducted to determine the double-layer capacitance of different catalysts. The relative ECSA values were then compared based on the proportional relationship of Cdl to ECSA: ECSA=

Cdl C*

(1)

C * is the unit characteristic capacitance of the material.

All measurement results were presented against the reversible hydrogen electrode (RHE): E(vsRHE) = E(vsAg/AgCl)+EAg/AgCl (reference) + 0.0591 V×pH

(2) 6

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(EAg/AgCl (reference) = 0.197 V vs NHE at 25 ℃) E (vsRHE) = E (vsAg/AgCl) +0.9653

(3)

RESULTS AND DISCUSSION Material Characterization Firstly, the morphology of the prepared catalysts was observed by transmission electron microscopy (TEM). From the appearance point of view, calcination at different temperatures of the catalyst materials were roughly the same, the graphene layers coated on the uniformly distributed nanoparticles with a diameter of about 10 nm (Figure 1(a-d)). The folds of the graphene layer in all samples were mainly due to the different C-N and C-C bond length caused by the incorporation of nitrogen. And a large hole in the thin graphene layer of Fe3C/Fe@G-900 can be observed from Figure 1(d), which was probably on account of the burst of graphene layer in high temperature. Then, HRTEM characterization of typical Fe3C/Fe@G-800 was further conducted to confirm the microscopic coating structure. As illustrated in Figure 1(e), the carbon layers around and far from the nanoparticles were all shown as a multilayered lattice fringe structure. Further, FFT transform of the lattice spacing of particles and their surrounding carbon layers was performed to determine the interplanar spacing of 0.335nm, 0.101nm and 0.34nm, respectively, corresponding to the (002) crystal plane of Fe3C, the (220) crystal plane of metal Fe, and the (002) crystal plane of graphitic C (Figure 1(f)). This fully proves that the prepared catalyst Fe3C/Fe@G is the structure of uniformly distributed Fe3C/Fe nanoparticles coated with multilayer graphene. Figure 1(g) is the scanning electron microscopy (SEM) 7 ACS Paragon Plus Environment


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image of Fe3C/Fe@G-800. The result was in accordance with the image of TEM, and the small, evenly dispersed particles are coated in the graphene layer. Additionally, in the energy dispersive X-ray spectroscopy (EDS) pattern of Fe3C/Fe@G-800 (Figure S1), C, N, O and Fe elements were evenly dispersed in the 2D architecture. Fe element of Fe3C/Fe@G-800 is further quantified by inductively coupled plasma-optical emission spectroscopy (ICP-OES), the calculated proportion is 52.8 wt. %. This shows that iron possesses a larger ratio in the material, exists mainly in the form of Fe and Fe3C. Above all, as the previous literature has been confirmed,28-30 the influence of the internal particles on the electronic structure of surface carbon and nitrogen atoms can not be ignored, which is also one of the major sources of intrinsic activity properties of Fe3C/Fe@G. The high-density, uniformly dispersed particles will increase the density of the area influenced by them in the 2-dimensional graphene plane and enhance the activity of the material.


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Figure 1. (a-b), (c), (d) TEM images of Fe3C/Fe@G-700, Fe3C/Fe@G-800, Fe3C/Fe@G-900; Embedded map of (c) for the local particle enlargement; (e-f) HRTEM image of Fe3C/Fe@G-800; (g) SEM image of Fe3C/Fe@G-800; (h) XRD of Fe3C/Fe@G-700, Fe3C/Fe@G-800, Fe3C/Fe@G-900. Subsequently, the crystal structure of the catalyst was characterized by X-ray diffraction pattern (XRD). All samples in Figure 1(h) showed a multi-peak state. Peaks at 26.1°in all curves are C (002) characteristic peak, also the graphitic carbon, and the interplanar distance d=0.34 nm. Peaks appeared at 44.6°, 64.9°, and 82.3° correspond to the (110), (200), (211) crystal face of metallic iron with cubic structure (PDF#06-0696). Leaving less prominent marked crystal peaks are the characteristic 9 ACS Paragon Plus Environment


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peaks of Fe3C crystal faces (PDF#03-0989). This means that there are two forms of iron in the samples, one is metallic Fe, and one is Fe3C, which is in line with the results of HRTEM. And the crystallinity of the coated metal and metal-carbides became better with temperature increasing. In addition, it was found that when the temperature increased to 800 ℃, the C (002) peak was lower and wider than other temperatures, which was primarily due to the present of defects causing by the introduction of nitrogen in the material. At low temperature of 700 ℃, due to the poor crystallinity of the metal particles, the carbon peaks appear to be stronger. At 900 ℃, graphitization degree of the material increases, peak shape becomes stronger and narrower.


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Figure 2. (a-c) Raman spectrum of Fe3C/Fe@G-700, Fe3C/Fe@G-800, and Fe3C/Fe@G-900; (d-f), (g-i), (j-l) and (m-o) are the XPS survey spectra and high-resolution scans of C 1s, N 1s, and Fe 2p3/2 of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900, respectively. We also performed Raman spectroscopy to further verify the above XRD results on graphitic carbon peaks by calculating the ratio of the intensities of D and G bands (ID/IG). As displayed in Figure 2(a-c), the D band of Fe3C/Fe@G-700 is wider and 11 ACS Paragon Plus Environment


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more intense, with a calculated ID/IG ratio of 1.38 versus 0.96 and 0.92 for Fe3C/Fe@G-800 and Fe3C/Fe@G-900. The lower graphitization degree and excessive defects of Fe3C/Fe@G-700 cause the large value. When the temperature rose to 800 ℃ and 900 ℃, the ratio increased. The main reason is that with the increasing of temperature, the doped nitrogen is running away, part or most heterocyclic rings are reduced to six-sided carbocyclic rings. Then, X-ray photoelectron spectroscopy (XPS) was used to confirm the loss of nitrogen after pyrolysis. From Figure 2(d-f), we can see that the N 1s peak gradually weakens with the temperature increasing, and the content of nitrogen decreases from 9.05 at. % of Fe3C/Fe@G-700 to 2.75 at. % of Fe3C/Fe@G-800 and 1.47 at. % of Fe3C/Fe@G-900. The result echoes with that of Raman. In addition to N, the XPS results also showed the existence of O and Fe as well as C that increased with temperature increasing (the content of each element is listed in Table S1). High-resolution of C 1s, N 1s and Fe 2p3/2 spectroscopy were done to further explore the existing state of each element. The proportion of C=C bonds to total carbon content (19.0 %, 28.5 % and 30.4 % for Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900, respectively) increased with temperature, which also indicated that the graphitization degree of the prepared catalysts enhanced with temperature increasing (Figure 2(g-i)). All these mean that the conductivity of the material increased with temperature increasing. We have verified this by the conductivity tests, the results are shown in Table S2. As the temperature increasing, the conductivity of the prepared catalyst increased, that is, the electronic conductive capacity increased. Then, from the N 1s spectrum (Figure 2(j-l)), we can see the 12 ACS Paragon Plus Environment

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pyridinic-N, pyrrolic-N and quaternary-N (graphinic-N) around at 398.3 eV, 399.9 eV and 400.8 eV, of which pyridinic-N and graphitic-N, which are considered to be the major contributors to the catalysis31-32. Their ratios accounted for the total nitrogen of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900 were 66.6 %, 72.4 % and 65.4 %, respectively. Among them, the effective nitrogen content of Fe3C/Fe@G-800 was the highest. Finally, Fe0 and Fe3+ at 705.6eV, 710.4eV can be observed from the Fe 2p3/2 spectrum (Figure 2(m-o)). It is consistent with the characterization results of HRTEM and XRD, iron is coated in the graphene layer mainly in the form of single iron and Fe3C particles. From the analysis of the results of XRD, Raman and XPS, it can be concluded that the graphitization degree of Fe3C/Fe@G-700 was low, and excessive incorporation of nitrogen caused too many defects. The graphitization degree of Fe3C/Fe@G-800 and Fe3C/Fe@G-900 were both high. Fe3C/Fe@G-800 had much higher nitrogen content than Fe3C/Fe@G-900, that is, the density of C-N sites introduced by nitrogen incorporation of Fe3C/Fe@G-800 was higher than that of Fe3C/Fe@G-900.

Figure 3. N2 absorption/desorption isotherms (a) and pore diameter distributions (b) of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900. 13 ACS Paragon Plus Environment


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The specific surface area and pore size distribution of the prepared catalyst were characterized by N2 absorption/desorption measurement. The results were shown in Figure 3. The isothermal hysteresis curve of Figure 3 (a) shows a hysteresis loop of type H3. Combined with the pore size distribution of Figure 3 (b), the material is a hierarchically micro/mesoporous structure. And the mesopores are mainly slit holes formed by the stacking of sheet-like (graphene sheets) structures. The specific surface areas of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900 were 33.1 m2 g-1, 69.9 m2 g-1 and 89.1 m2 g-1, respectively. The mesoporous characteristics of the prepared catalysts enhance the accessibility of the active sites. Summarizing the above characterization process, the prepared catalysts hold a structure in which uniformly distribution Fe3C/Fe nanoparticles coated with multilayer graphene. The graphitization degree, the specific surface area and the electronic conductive capacity of Fe3C/Fe@G all increases with temperature increasing. Fe3C/Fe@G-800 possesses the highest content of pyridinic-N and graphitic-N. Material Electrocatalysis After a series of characterization of Fe3C/Fe@G, we found that the catalyst prepared at 800 ℃ possessed a high content of pyridinic-N and graphitic-N, and the physical properties of other materials such as specific surface area and electrical conductivity all increased with the temperature increasing. We then tested the electrocatalytic properties of the material to see their catalytic activity.


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Figure 4. (a) LSV curves of Fe3C/Fe@G-700, Fe3C/Fe@G-800, Fe3C/Fe@G-900, and 20% Pt/C; (b) LSV curves of Fe3C/Fe@G-800 at various rotation rates and the corresponding K-L plots (c), also the dependence of electron transfer number (d). The linear sweep voltammetry (LSV) curves are obtained from RDE tests in Figure 4(a). Both the onset potential and half wave potential of Fe3C/Fe@G-700 and Fe3C/Fe@G-900 are slightly inferior compared with Fe3C/Fe@G-800, the E1/2 of Fe3C/Fe@G-800 is 0.80 V, which is consistent with the commercial Pt/C, although the onset potential has a small gap between with Pt/C (0.94 V, 0.98 V for Pt/C). We have also tabulated a table that intuitively compares the catalytic performance of Fe3C/Fe@G-800 and the same type of materials with different performance parameters. By comparison, it can be concluded that the ORR performance of Fe3C/Fe@G-800 is slightly better than that of other similar catalysts. It indicates that 15 ACS Paragon Plus Environment


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the electronic conductivity of Fe3C/Fe@G-800 may be better, that is, strong capability for transmitting electron, which is mainly due to the covering with graphene thin layer. The graphene layer not only increases the specific surface area of material, but also forms a large electron transport network in the material surface, thus increases the electronic transmission capacity of the material, and further improves the catalytic properties of Fe3C/Fe@G. It also indicates that the density of active sites of Fe3C/Fe@G-800 may be more enough for ORR, which is mainly due to the incorporation of appropriate amount of nitrogen, the degree of graphitization and the density of defects in the most favorable state of catalytic action. Table 1. Comparison of catalytic performance for Fe3C/Fe@G with some representative homogeneous catalysts. L-Pt

Eonset

2 (μg/cm )

(V)

0.83

50

0.923

0.81

400

0.928

FeNC-850

380

BFNCNTs

Catalyst

L-Catalyst

Eonset

E1/2 (V)

System

Cit

1.05

0.83

0.1M KOH

33

20

0.96

0.82

0.1M KOH

34

0.785

40

0.90

-

0.1M HClO4

35

0.91

0.81

31

0.92

0.83

0.1M KOH

36

750

0.94

0.74

150

0.93

0.74

0.1 M NaOH

37

FeNC-800

900

1.10

0.88

30

1.05

0.83

0.1M KOH

38

Fe3C/Fe@G-700

600

0.91

0.78

60

0.98

0.80

0.1M KOH

Fe3C/Fe@G-800

600

0.94

0.80

60

0.98

0.80

0.1M KOH

2 (μg/cm )

(V)

Fe3C/C-800

600

1.05

Fe−N/C-800

100

Fe3C/NCNTs/OB P-900

E1/2(V)

This wok This wok

Then, in order to further investigate the ORR mechanism of the prepared catalyst, we conducted LSV tests at various rotation speeds and make detailed analysis as follows. The recorded LSV curves of Fe3C/Fe@G-800 in Figure 4(b) show a swiftly 16 ACS Paragon Plus Environment

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enhanced diffusion current density with therotation speed increasing. Stable limiting diffusion current at the platform can be observed intuitively, indicating that the process of transferring electrons was very stable during the ORR. Moreover, the corresponding K-L plots of Fe3C/Fe@G-800 at different potentials (Figure 4(c)) reveal significant linearity (Specific calculation process, see the Supporting information), which implies the first order reaction kinetics of the catalyst in the process of ORR.39 Then we ulteriorly calculated the electron transfer number n of Fe3C/Fe@G-800 at different potentials, and the results are shown in the bar graph of Figure 4(d). The average of n is 3.71, indicating an appropriate 4 electron pathway for the reduction of oxygen. Moreover, the electron transfer process of Fe3C/Fe@G-800 during ORR is also very stable at different potentials.

Figure 5. (a) Chronamperometry test for Fe3C/Fe@G-800 and 20% Pt/C, respectively; (b) Steady-state ORR polarization curves of as-prepared and hot acid-treatment Fe3C/Fe@G-800. Further, the chronoamperometry test was applied to proof the stability of the materials (Figure 5(a)). The decay of Fe3C/Fe@G-800 is only 9 % after 30,000 s, keeping the current rate of 91%, while the result of Pt/C is nearly twice than that of 17 ACS Paragon Plus Environment


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Fe3C/Fe@G-800 (15%, 85%). It can be seen that the material prepared by us has more absolute stability than commercial Pt/C, which is mainly due to the thin coating layer of graphene on the surface of the material, which makes the structure of the material more stable, and the test process will not occur agglomeration of interior nanoparticles, like Pt particles. Then, to verify the interactivity between the internal metal particles and the surface of the nitrogen-doped multilayer graphene, the catalyst Fe3C/Fe@G-800 was leached in hot acid for 12 h to remove or extensively diminish internal particles. After leaching, the catalytic performance of the catalyst was significantly deteriorated, both the onset potential and the half-wave potential all decreased (Figure 5(b)). It is concluded that the Fe3C/Fe nanoparticles in the material play a very important synergism between it with the surface N-doped graphene layer in the catalytic ORR process, they have no neglecting influence on the surface structure. This makes active sites exposed on the prepared catalyst surface more efficient.


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Figure 6. (a-c) Cyclic voltammograms (CVs) of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900 measured at different scan rates from 20 to 180 mV s−1; (d) Cdl of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900, taken as the slope of the linear fits to the current density at 0.375 V vs the scan rate. The electrochemically active surface area (ECSA) for Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900 were also estimated from the electrochemical double-layer capacitance (Cdl) of the catalytic surface from voltammetry. All current is considered due to the capacitive charging. The Cdl values of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900 are 6.82 mF cm-2, 19.4 mF cm-2, and 20.8 mF cm-2, respectively. Due to the positive proportional relationship between Cdl and ECSA, this also implies that the electrochemically active surface area of the prepared catalyst increased with temperature increasing, which is consistent with the results of the specific surface area calculated by N2 adsorption/desorption test. The ECSA of 19 ACS Paragon Plus Environment


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Fe3C/Fe@G-800 and Fe3C/Fe@G-900 was larger than that of Fe3C/Fe@G-700, that is, when the system conducts ORR, the latter two can utilize more surface area of material. Combined with the above electrochemical test results, it can be judged that the catalyst prepared at 800 ℃ has the best catalytic performance. Compared with other catalysts of the same type, the performance is also prominent. The process of ORR is close to 4 electron transfer pathway and the stability of Fe3C/Fe@G-800 is also better than that of Pt/C. The performance of Fe3C/Fe@G-800 after hot-acid treatment worsened, which proved that the synergistic effect of the internal coating particles on the surface nitrogen-doped graphene layers could not be neglected. The electrochemically active surface area deduced by double-layer capacitance of Fe3C/Fe@G-800 and Fe3C/Fe@G-900 were close to each other and much higher than that of Fe3C/Fe@G-700. CONCLUSIONS The prepared Fe3C/Fe@G-800 catalyst possesses a half-wave potential (0.80 V) consistent with commercial 20 % Pt/C and superior stability than Pt/C (current retention of 91 % after 40,000 s chronoamperometry). Combined with the results of electrochemical measurements and characterization, the good stability and catalytic performance of Fe3C/Fe@G-800 samples came from four points: (1) the coating structure of the catalyst. We prepared a catalyst with a structure of uniformly dispersed Fe3C/Fe nanoparticle coating with multilayer graphene by a simple method. And the high degree of graphitization of the surface graphene makes the material have 20 ACS Paragon Plus Environment

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good electron conductivity; (2) the strong interaction between the different structural components of the catalyst. The synergism between the internal Fe3C/Fe metal particles and the N-doped graphene layer makes the C-N active sites of the surface more efficient; (3) the 2-dimensional planar structure of the catalyst. Layered structure makes the active sites more easily exposed and the mesoporous structure stacked between the graphene layers make the exposed active sites more accessible; (4) the higher graphitic-N and pyridinic-N content of the catalyst. Graphitic-N and pyridinic-N are considered to be the predominantly catalytically active nitrogen species. The high content of graphitic-N and pyridinic-N allow the Fe3C/Fe@G-800 to perform its structural advantage better than Fe3C/Fe@G-700 and Fe3C/Fe@G-900, making the exposed active sites more efficient. Then, the catalytic performance of the material becomes more excellent. This also intimates a good prospect for this kind of coated non-precious metal-based material in the field of catalysis. ASSOCIATED CONTENT Supporting Information SEM image of Fe3C/Fe@G-800 and the corresponding EDS. The electric conductivity of the prepared catalysts. The content of each element of Fe3C/Fe@G-700, Fe3C/Fe@G-800 and Fe3C/Fe@G-900 from XPS. The specific calculation of electron transfer number n with Koutecky-Levich plots. AUTHOR INFORMATION Corresponding Authors *E-mail address: [email protected]. Tel.: 0086-335-8061569; fax: 21 ACS Paragon Plus Environment


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0086-335-8059878. *E-mail address: [email protected]. Tel.:0086-335-8061569; fax: 0086-335-8059878. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT

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For Table of Contents Use Only

Fe3C/Fe@G are synthesized for oxygen reduction catalyst, cathode catalyst for green energy of fuel cell, high activity, sustainable utilization.


Uniform Multilayer Graphene-Coated Iron and Iron-Carbide as Oxygen

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