A Bonded Double-Doped Graphene Nanoribbon Framework for

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A Bonded Double-Doped Graphene Nanoribbon Framework for Advanced Electrocatalysis Liang Chen, Jingjing Xiao, Baohong Liu, and Tao Yi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02522 • Publication Date (Web): 14 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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ACS Applied Materials & Interfaces

A Bonded Double-Doped Graphene Nanoribbon Framework for Advanced Electrocatalysis Liang Chen, Jingjing Xiao, Baohong Liu and Tao Yi*

Department of Chemistry and Collaborative Innovation Center of Chemistry for Energy Materials, and State Key Lab of Molecular Engineering of Polymers, Fudan University, Shanghai 200433, P.R. China. E-mail: [email protected] Keywords: Iron Carbide, Graphene Nanoribbons, Nonprecious Metal Catalyst, Electrocatalysis, Oxygen Reduction Reaction, Fuel Cells.

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Abstract: The preparation of a low-cost, high-efficient and stable electrocatalyst as an alternative to platinum for the oxygen reduction reaction (ORR) is especially important to various energy storage components, such as fuel cells and metal-air batteries. Here, we report a new type of bonded double-doped graphene nanoribbon-based nonprecious metal catalysts in which Fe3C nanoparticles embedded in Fe-N-doped graphene nanoribbon (GNRs) frameworks through a simple pyrolysis. The as-obtained catalyst possesses several desirable merits for the ORR, such as diverse high-efficiency catalytic sites, a high specific surface area, an ideal hierarchical cellular structure and a highly conductive N-doped GNR network. Accordingly, the prepared catalyst shows a superior ORR activity (an onset potential of 0.02 V and a half-wave potential of -0.148 V versus an Ag/AgCl electrode) in alkaline media, close to the commercial Pt/C catalyst. Moreover, it also displays good ORR behavior in an acidic solution.


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Introduction Electrochemical energy storage has become a significant enabling technology for sustainable and clean alternative energy sources.1-4 Therefore, fuel cells and metal-air batteries with high energy and power densities are highly desirable.5-10 The cathodic oxygen reduction reaction (ORR) is a critical process in fuel cells and largely decides their ultimate performance.11-14 Although platinum (Pt) and platinum-based alloys are the best known ORR catalysts, the scarcity and declining activity of Pt-based catalysts significantly impede their practical use.15 Therefore, the development of electrocatalysts with low price, high-efficiency and high stability as an alternative to Pt for the ORR is urgently needed. With this in mind, great efforts have been made to search for substitutes for Pt-based catalysts, such as metal oxides,16,17 heteroatom-doped carbon materials,18-21 transition-metal-coordinating macrocyclic compounds22,23 and the hybrids based on transition-metal-coordinating nitrogen doped carbon (M-N/C, M = Fe, Co Ni, etc.).24-28 Among those alternatives, M-N/C catalysts outperform many of their counterparts because of the fact that the active sites located in this type of catalyst are believed to involve M-N coordination bond at defects in graphitic carbon, which have been confirmed to be the most active ORR catalytic sites.10 Despite the above mentioned tremendous progress, the performance of alternative catalysts is still not satisfactory. This is because these alternatives always suffer from two barriers: (1) the limited number of active sites and poor mass/electron transmission capability of ORR-relevant species (H+/OH-, e−, O2, H2O), which are controlled by the specific surface areas (SSAs) and electrical conductivity; (2) the intrinsic character of the active sites, which is largely manipulated by the chemical constitution and the strong coupling effect between different components.29,30 Hence, to achieve high-quality ORR catalysts, the aforementioned two features should be considered. On the basis of these


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facts, we previously proposed a strategy to assemble graphene nanoribbons (GNRs) and carbon nanotubes (CNTs) into 3D porous frameworks followed by doping with nitrogen atoms, which can create additional effective active sites and promote mass/electron transfer while retaining its good intrinsic conductivity.31-33 Recently, a new type of nonprecious metal encapsulated in graphitic carbon was reported as an efficient ORR catalyst.34,35 The unique core-shell structure was considered very important to enhancing the electrocatalytic performance whereby the encapsulated metal species can activate the surrounding graphitic layers. Meanwhile, the encapsulation of metal species within graphitic carbon layers can further strengthen the coupling effect between the metal particles and the carbon shell and restrain the aggregation and dissolution of the active metallic substance.36 In addition, the graphitic carbon shells have been shown to promote electron transfer from the internal metal species to the graphitic carbon surface, thus, endowing its outstanding electrocatalytic properties.34,37 Therefore, a 3D porous framework, appropriate heteroatom doping and a well-defined core-shell structure are all highly desirable for superior electrocatalysis. However, to the best of our knowledge, producing such a material on a large scale has rarely been achieved. Herein, we report a facile and scalable approach to fabricate a new type of nonprecious metal catalyst based on Fe3C nanoparticles embedded in an Fe-N-doped GNR framework (named FeN-GNFs) through a cost-effective method. The precursors involved in this method are both easily accessible and eco-friendly, making the material suitable for industrial production. The 3D continuous N-doped GNR frameworks can not only generate an abundance of active sites but also provide a smooth channel for mass and electron transportation. Moreover, the core-shell structure (Fe3C@graphitic carbon) can further activate the surface of the graphitic carbon, thereby creating multiple highly effective intrinsic active sites. As a result, the as-prepared Fe-N-


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GNFs catalyst possesses all of the above desired features for the ORR, including abundant catalytic sites, a high SSAs (542 m2 g-1), an ideal interconnected mesoporous structure and a highly conductive N-doped GNR network. These unique bonded double-doped structural features lead to a superior ORR activity (an onset potential (E0) of 0.02 V and a half-wave potential (E1/2) of -0.148 V versus a Ag/AgCl electrode), comparable to a commercial Pt/C catalyst in alkaline media. Moreover, the Fe-N-GNFs catalyst also shows good ORR behavior in acidic solutions. This study affords a promising strategy for the preparation of high-efficiency nonprecious metal ORR catalysts. Materials and methods Synthesis of graphene oxide nanoribbons (GONRs): The graphene oxide nanoribbon aqueous solutions were synthesized according to our previous reports.31,32 Synthesis of Fe-N-GNFs catalyst: In a typical procedure, 15 mL of a homogeneous GONR solution with a concentration of 5 mg mL-1 and an 8 vol % pyrrole aqueous dispersion (distilled before use) were mixed together by sonication for several minutes to form a uniform suspension. Sealed the suspension in a 20 mL Teflon-lined autoclave at 80 °C for 16 h to form a robust GNR hydrogel. Then, the as-prepared hydrogel sample was immersed into 50 mL 0.24 M FeCl3•6H2O (16.2 g/250 mL) for 4 h to complete the polymerization process. The product was then solventexchanged with water and ethanol for several hours to remove all of the impurities. After that, the wet sample was then re-dispersed into 100 mL of the 0.24 M FeCl3•6H2O solution for 4 h under mild stirring to adsorb the Fe3+ until saturation was reached. Next, the processed hydrogel was taken out of the Fe3+ aqueous solution and freeze-dried to obtain the corresponding aerogel product. After heating the freeze-dried aerogel sample at 800 °C for 2 h (heating speed: 5 oC


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/min-1) under an Ar atmosphere, the pyrolyzed product was preleached in a 6 M HCl solution at 80 oC for 10 h to remove the exposed and ORR-nonreactive iron species. Finally, the product was washed with deionized water thoroughly (pH≈7) and then freeze-dried again to afford the final Fe-N-GNFs catalyst. In the case of optimization experiments to determine the pyrolysis temperature and the amount of pyrrole, the heating temperature changed from 700 to 1000 °C and the added amount of pyrrole changed from 4 vol% to 12 vol% (with respect to the volume of the GNR solution). Other preparation procedures and experimental conditions were kept the same. Synthesis of reference electrocatalysts (GNRs, N-GNRs and Fe-GNRs): For comparison, three reference catalysts: pristine GNRs, N-doped GNR frameworks (N-GNRs) and Fe-doped GNR frameworks (Fe-GNRs) were also prepared. For the GNRs, a certain amount of GONR powder was obtained by freeze-drying the GONR solutions and then heating in an Ar atmosphere at 800 °C for 2 h. The remaining N-GNRs and Fe-GNRs were fabricated with almost the same procedure as was used for the Fe-N-GNFs mentioned above, except the N-GNRs were polymerized with 50 mL of a 0.24 M ammonium persulfate (APS) solution, and then the process of adsorbing Fe3+ was omitted. The Fe-GNRs were prepared without adding any pyrrole, while the other procedure steps were kept unchanged. Characterization: The structure of the resultant samples was measured by X-ray diffraction (XRD) using monochromatic Cu Kα1 radiation. An ASAP 2010 (USA) was used to characterize the Brunauer-Emmett-Teller (BET) specific area, pore size distribution and total pore volume data, and the samples were processed at 77 K. The X-ray photoelectron spectroscopy (XPS) analysis was conducted using an AXIS Ultra spectrometer with a high-performance Al monochromatic source operated at 15 kV. The peak processing for XPS was conducted using the


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XPSPEAK41 software. Raman measurements were conducted on an XploRA with an excitation wavelength of 532 nm. Field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM) were performed on an Ultra 55 and an FEI Tecnai 20 at 200 KV. The elemental mapping (EDS) was performed on a scanning transmission electron microscope (STEM) with a high-angle annular dark-field (HAADF) detector (HITACHI S-5500) operating at 200 kV. The aerogel samples were dispersed in ethanol, sonicated for 30 min and then dropped onto a copper grid. Electrochemical Measurements: Before the preparation of the electrodes, 5 mg of catalyst was first ultrasonically dispersed in 1 mL ethanol with 50 µL of a Nafion solution (5 wt%); then, the homogenous catalyst suspensions (~5 µL) were dipped on a glassy-carbon (GC) electrode as a working electrode. The mass loading of the electrode was 0.127 mg cm-2. A commercial Pt/C (20 wt% Pt on Vulcan XC-72) electrode with a similar amount (~25 µg) was prepared by the same procedure. Catalyst loadings for all samples, including Pt/C, were 127 µg cm-2. The cyclic voltammetry (CV) tests were carried out on an electrochemical workstation (CHI 760D) with a three-electrode system (Ag/AgCl and Pt electrodes were acted as reference and counter electrode, respectively). Before the measurement, the test solution (including 0.1 M KOH and 0.5 M H2SO4) was first saturated with O2 by bubbling for 30 min. For control experiments in N2-saturated KOH, N2 was used instead of O2 while the other conditions remained unchanged. The rotating disk electrode (RDE) measurements were carried out on a MSRX electrode rotator (Pine Instrument) and the CHI 760D potentiosta at various rotating speeds from 400 to 2025 rpm at a scan rate of 10 mV s-1. The transferred electron number (n) was calculated by the Koutechy-Levich (K-L) equation.38 For the Tafel plot, the kinetic current was calculated from the mass-transport correction of RDE.


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When ORR performance was tested in acidic medium, the electrolyte changed to 0.5 M H2SO4 solution while other test conditions kept the same. Supporting Information. Details of material characterizations, additional images, spectra and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

Results and discussion Synthesis of the Fe-N-GNFs catalyst: The whole synthetic route for the Fe-N-GNFs catalyst is declared in Scheme 1. Before the synthesis of the catalyst, highly homogeneous GONRs were prepared by longitudinally unzipping multi-walled carbon nanotubes, as based on our previous reports.31 Then, a 3D GNR hydrogel was produced by a promoter-assisted hydrothermal reaction. The resultant hydrogel was then immersed in an aqueous solution of FeCl3 to fully polymerize pyrrole and form a polypyrrole/GNR (PPy/GNR) composite hydrogel. The PPy/GNR hydrogel was then washed with water and ethanol to remove polymerized byproducts and unpolymerized pyrrole. The wet gel was then re-dispersed in fresh FeCl3 solution to adsorb the most of Fe3+. Subsequently, the wet samples were directly freeze-dried and then processed to pyrolysis under an Ar atmosphere. In this process, the N atoms derived from PPy were doped into the GNR backbones to create N-doped GNR frameworks.39 Mean-while, the coordination of PPy and Fe3+ drove the formation of the Fe3C@graphitic carbon core-shell structure supported on the N-doped GNRs networks. After etching in acidic solution to remove the exposed and ORR-nonreactive iron species, the final Fe-N-GNFs catalyst was achieved. For comparison, pristine GNRs, NGNRs and Fe-GNRs were also prepared under similar conditions (Details in Materials and methods).


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Scheme 1. Illustration of the synthetic route for Fe-N-GNFs catalyst.

Characterization of the Fe-N-GNFs: The morphology of the as-prepared Fe-N-GNFs catalyst was first examined by SEM, TEM, and HRTEM. The SEM image (Figure 1a) shows that the FeN-GNFs exhibit a 3D porous structure with hierarchical pores. Moreover, we can also see several bright dots with the size of 20-50 nm in it, which can be assigned to the heavy atoms (from Fe3C nanoparticles in this case) that are uniformly distributed both on the shell and inside of the GNR frameworks (Figure 1b).40 The magnified TEM image shows that Fe-N-GNFs possesses a porous feature, and some of the pores may stem from the melting of Fe3C nanoparticles. HAADF-STEM and the relevant EDS test further confirm the nanoparticle-encapsulation structure and the uniform distribution of C, O, and N elements throughout Fe-N-GNFs (Figure 1c). It should be


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noted that except for the individual nanoparticle section, atomic Fe signals were also dispersed along the carbon/nitrogen matrix, probably revealing that some of the Fe species are fixed by coordination with N atoms.24 These Fe species are believed to form highly active Fe-N sites, which have been shown to exist in previous reports.24-27 HRTEM images of the Fe-N-GNFs catalyst show that the Fe3C nanoparticles are encapsulated in a highly graphitic carbon shell to form a core-shell structure (Figure 1d, e and Figure S1). An interplanar spacing of 0.35 nm can be easily observed in the graphitic layers, which corresponds to the (002) plane of graphitic carbon, and the thickness of the graphitic shell is approximately 10 nm. For the individual nanoparticle, a distance of 0.45 nm can be found, which can be assigned to the (001) lattice planes of the Fe3C nanoparticles.24 To study the effect of the pyrolysis temperature on the final properties of the Fe-N-GNFs, different pyrolysis temperatures are employed in the present work, and the related SEM and TEM images are provided in Figure S2. Moreover, the reference samples including GNRs, N-GNRs and Fe-GNRs are also investigated by electron microscopy tests (Figure S3).


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Figure 1. (a) An SEM image, (b) a TEM image (the arrows are representative of GNR nanosheets) and (c) a HAADF-STEM image and related EDS mapping of C, O, N and Fe for the Fe-N-GNFs catalyst. (d, e) HRTEM images of porous sections and single nanoparticle parts in the Fe-N-GNFs. The insets in (d) and (e) are magnified TEM pictures of the graphitic layer and the interplanar spacing of Fe3C nanoparticle, respectively.


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The crystalline structure of the nanoparticles was further examined by XRD, and the result is shown in Figure 2a. The peak located at ca. 26.5° is indexed to the (002) plane of the graphitic carbon or GNRs.41 The remaining diffraction peaks match well with the Fe3C phases (JCPDS No. 89-2867),24 and show no obvious change with the pyrolysis temperature in the range of 700~1000°C (Figure S4). Raman spectra were also used to determine the degree of graphitization. The appearance of the D band in the Fe-N-GNFs was indicative of defective carbon, and ID/IG values were found to decrease with increasing pyrolysis temperatures (Figure S5), which was consistent with the previously reported result. 42 The porosity of the resultant Fe-N-GNFs was studied using the nitrogen adsorptiondesorption technique. BET analysis illustrated that the SSAs of the Fe-N-GNFs pyrolyized at 800 °C can reach as high as 542 m2 g-1, which is much larger than those of GNRs (89 m2 g-1), NGNRs (270 m2 g-1), Fe-GNRs (385 m2 g-1) (Figure S6) and previously reported Fe3C-based materials.24,43,44 The pore size distribution for the Fe-N-GNFs presented two peaks centered at 2 nm and 80 nm (Figure 2b, c). These meso/macropores are expected to accelerate the reactant diffusion in the ORR process because the mass transfer performance largely depends on the pore structure and distribution.24 The chemical composition and electronic structure were further investigated by XPS test. From the image of the XPS survey spectrum, we can conclude that FeN-GNFs catalyst contains C, O, N and Fe elements (Figure 2d and Table S1), consistent with the EDS results. The high resolution N 1s spectrum of the Fe-N-GNFs indicates the presence of oxidized N, graphitic N as well as pyridinic N/Fe-N (Figure 2e), suggesting that the N heteroatom was doped into the GNR backbone.31 The doped N species, especially for Fe-N bondings, have been confirmed to be the most important catalytic sites in the ORR catalytic


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process.10 In the present work, the pyridinic nitrogen mainly derived from the pyrolysis of polypyrrole (PPy). Among them, a part of PPy which coordinated with Fe3+ could generate the Fe3C@graphitic carbon core-shell structure to form highly effective Fe-N active sites, which ensured the high catalytic ability of the Fe-N-GNFs. Moreover, the remaining PPy could serve as nitrogen sources and dope into the backbones of GNR to form N-doped carbon material (especially pyridinic N). According to the recently report,45 the pyridinic N in N-doped graphitic carbons can create the active sites for ORR reaction, and the carbon atoms next to the pyridinic N are suggested to be the active sites with Lewis basicity at which O2 molecules are adsorbed as the initial step pf the ORR. Therefore, in our work, the pyridinic N will play an crucial role in improving the electrocatalytic activity of Fe-N-GNFs. The high resolution Fe 2p can be deconvoluted into several pairs of peaks (Figure 2f). The Fe signals which located at 710.7 and 714.2 eV (for Fe 2p3/2), and at 723.4 and 727.4 eV (for Fe 2p3/2), respectively, indicate the existence of iron carbide (or its nitride and metallic phase).24 In addition, a normal satellite peak which appeared at 719 eV can also be detected. Meanwhile, according to the previous report, the Fe 2p peak at 710.7 eV was due to N-coordinated iron.46 Moreover, the chemical structure of the comparative catalysts was also analyzed by XPS and the relevant data are provided in Figure S7 and Table S1.


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Adsorption Desorption

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Amount Absorbed (cm3g-1STP)


740

710.7 eV 723.4 eV Fe2+ 2P3/2 Fe2+ 2P1/2 714.2 eV Fe3+ 2P3/2

727.4 eV Fe3+ 2P1/2

730

719.0 eV Statellite peak

720

710

Binding energy (eV)

Figure 2. (a) The XRD pattern, (b-c) nitrogen sorption isotherm and pore size distribution curve for the Fe-N-GNFs. (d-f) An XPS survey spectrum of the Fe-N-GNFs and the corresponding high-resolution N 1s and Fe 2p spectra.

Electrocatalysis performance of the Fe-N-GNFs: Generally, the activity of catalysts can be influenced by the concentrations of dopants (here N and Fe) to a large extend, which can be controlled by manipulating the addition amount of pyrrole (the content of iron is always


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saturated) and changing the carbonization temperature. Previous reports showed that the pyrolysis temperature played a crucial role in the electrocatalytic activity of catalysts by changing their chemical properties.33 Therefore before the detailed ORR test, the ORR activity of the Fe-N-GNFs catalysts was first investigated when prepared at different pyrolysis temperatures (700-1000 °C) with RDE measurements performed in an O2-saturated 0.1 M KOH electrolyte. The chemical information of these samples was provided in Figures S8-S9. The electrochemical results showed that the highest ORR activity can be obtained at 800 °C (refers to the E0, E1/2 and diffusion-limiting current obtained from linear sweep voltammetry (LSV), Figure 3a-b, Figures S10 and Table S2), which can be ascribed to the balance of electrical conductivity, porosity, and optimized Fe/N doping content. Hence, the Fe-N-GNFs catalyst discussed in the main text was prepared at this temperature unless otherwise specified. In addition, we also studied the effect of the amount of pyrrole on the final ORR performance, and the results are provided in Figure 3c-d, Figures S11-S12 and Table S3. It can be clearly shown that the optimum volume of pyrrole was 8 vol % (with respect to the volume of GNR solution), and a greater or smaller amount of pyrrole leads to an inferior ORR activity; thus, we use this volume of pyrrole to synthesize optimal samples of the Fe-N-GNFs.


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-2 -3 -4

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Figure 3. (a) LSV curves and (b) half-wave potentials as a function of the pyrolysis temperature; (c) LSV curves and (d) half-wave potentials as a function of the volume of pyrrole.

To demonstrate the efficient electrocatalytic activity of the Fe-N-GNFs, CV measurement were then conducted on the Fe-N-GNFs electrode and then compared with other reference catalysts (GNRs, N-GNRs and Fe-GNRs) and a commercial available Pt/C catalyst (20 wt% Pt) in a N2- or O2-saturated 0.1 M aqueous KOH electrolyte solution. As shown in Figure 4a, a featureless cathodic current with a potential range of -1.0 to 0.2 V was detected for all samples in the N2-saturated medium (Figure S10, S12 and S13). In contrast, well-defined redox peaks can be observed in the O2-saturated condition, indicating the remarkable electrocatalytic ability of the Fe-N-GNFs and the other reference catalysts.31,32 It is worth noting that the CV curve showed


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that the peak potential of the Fe-N-GNFs was located at -0.12 V (versus an Ag/AgCl electrode), which is close to that of the Pt/C catalyst (ca. -0.10 V) but much more positive than that of the other reference catalysts (GNRs: -0.35 V, N-GNRs: -0.21 V, and Fe-GNRs: -0.33 V, Figure S13 and Figure 4a).

-2

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-0.5 -1.0

-0.60

(h) 4 Pt/C (n=4.0)

0.0

Fe -N -

(f)

4

GN Fs

0.0

0.2


j (mA cm-2)

Fe-N-GNFs (n=3.93)

E= -0.60 V

-0.1

0.0

Potential (V vs. Ag/AgCl)

(e)

Potential (V vs. Ag/AgCl)

(g) 40

-0.1

Fe -G

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-0.2

NGN

-5

-0.3

-0.2

GN Rs

-4

-0.4

-0.3

Rs

GNRs N-GNRs Fe-GNRs Fe-N-GNFs Pt/C

Onset potential (V)

-3

-0.5

-0.4

E (V vs.Ag/AgCl)

-1

0.40

-2

-0.6

Half-wave potential Onset potential

-0.5

j/j0 (%)

j (mA cm-2)

0

j -1(cm2 mA-1)

(d)

-1

j (mA cm-2)

0.0 -0.2 O2 -0.4 N2 -0.6 0.2 Pt/C 0.0 -0.2 O2 -0.4 N2 -0.6 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 Potential (V vs. Ag/AgCl) 1

(c)-0.6

(b)0

Fe-N-GNFs

Electron transfer number (n)

j (mA cm-2)

(a) 0.2

jk (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt/C

60

67.1%

40

Fe-N-GNFs

0

20 -1 0

5

10

15

20

Time (min)

0

0

5000

10000

15000

20000

Time (s)

Figure 4. (a) CV curves of the Fe-N-GNFs and Pt/C catalysts in a N2- and O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s-1. (b) LSV curves of various catalysts in a RDE (1600 rpm) in an O2-saturated 0.1 M KOH medium. (c) A comparison of the onset potential and the half-wave potential of various catalysts. (d) LSVs of the Fe-N-GNFs at different rotating speeds. The inset declares the related Kouteckey-Levich plots at different potentials. (e) The n values of various catalysts in a potential range of -0.8 to -0.60 V. (f) Tafel slopes of different catalysts. (g) The kinetic limiting current density of different catalysts at potential of -0.6V with corresponding


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n value. (h) The methanol crossover effect for the Fe-N-GNFs and Pt/C upon the addition of 3 M methanol after approximately 10 min in an O2-saturated 0.1 M KOH solution at -0.4 V. (i) Durability tests of the as-prepared Fe-N-GNFs and Pt/C catalysts at -0.4 V in an O2-saturated aqueous solution of 0.1 M KOH at a rotation rate of 1600 rpm.

The catalytic activity of the samples was further studied by using LSV polarization curves, as shown in Figure 4b-4c. It can be observed that the pristine GNRs showed a very poor ORR activity due to the low SSAs and lack of effective active sites (because of their undoped nature). The N-GNR catalyst (with a high N content of ca. 10.03%) showed an enhanced catalytic performance compared with that of GNRs since the C-N bonding generated a massive amount of catalytic sites for ORR catalysis;31,32 however, the ORR performance is still not satisfactory compared with Pt/C, which is probably because of low SSAs and few exposed active sites. Although the Fe-GNRs possessed considerable SSAs values, the electrocatalytic performance is still relatively poor because of the small number of effective intrinsic active sites (a lack of C-N and Fe-N bonding). Therefore, for a high quality ORR catalyst, heteroatomdoping, a high SSA, an ideal porous structure and abundant effective active sites are necessary. As expected, the Fe-N-GNFs catalyst showed a superior ORR activity with an E0 of 0.02 V and an E1/2 of -0.148 V versus an Ag/AgCl electrode (Table 1) in alkaline media, which is more comparable to the Pt/C catalyst (The E0 is 0.02V and the E1/2 is 0.13 V, Figure 4c).


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Table 1. ORR performance of various electrocatalysts, including Fe-N-GNFs, Pt/C and three reference catalysts.

Sample

Onset Potential (V. vs Ag/AgCl)

Half-wave Potential (V. vs Ag/AgCl)

Current Density at -0.3 V (mA cm-2)

Current Density at -0.8 V (mA cm-2)

Tafel (mV/decade)

JK (mA cm-2)

GNRs

-0.27

-0.462

-0.28

-1.82

143

4.17

N-GNRs

-0.10

-0.273

-1.86

-3.47

82

20

Fe-GNRs

-0.22

-0.401

-0.39

-2.24

110

2.37

Fe-N-GNFs

0.02

-0.148

-4.09

-4.73

61

33.33

Pt/C

0.02

-0.130

-4.34

-4.63

56

28.54

The selective four-electron catalytic mechanism of the Fe-N-GNFs and Pt/C was also studied by K-L plots at several potentials. The K-L plots of the Fe-N-GNFs (Figure 4d) showed good linearity at all potentials, and the electron transfer number (n) was calculated to be ca. 3.93 to 4.0 for a potential of -0.60 to -0.80 V (Figure 4e), which is close to that of Pt/C (ca. 4.0), revealing that the ORR catalytic mechanism mainly follows a one-step four-electron route for the Fe-N-GNFs. In contrast, the n values calculated for the reference catalysts were approximately 23 (2.17-3.04, 3.13-3.52 and 2.29-2.58 for the GNRs, N-GNRs and Fe-GNRs, respectively, Figure S13). The stepwise sloping shape and no limiting current observed in GNRs and Fe-GNRs possibly attributed to their poor electrocatalytic ability. The excellent ORR activity of the Fe-NGNFs was further verified by the small Tafel slope of 61 mV per decade at low overpotentials, which is extremely close to the Pt/C catalyst (56 mV per decade) and much lower than those of the reference samples (143, 80 and 110 mV per decade for the GNRs, N-GNRs and Fe-GNRs,


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Page 20 of 29

respectively, Figure 4f). Moreover, the kinetic limiting current (Jk) value for the Fe-N-GNFs showed a high current density (ca. 33.33 mA cm-2), which is even a little higher than that of the Pt/C catalyst (28.54 mA cm-2), but much higher than the other reference samples (4.17, 20 and 3.57 mA cm-2 for the GNRs, N-GNRs and Fe-GNRs, respectively, Figure 4g). The high ORR current density further demonstrated the superior ORR electroactivity of the Fe-N-GNFs (Table S4). With respect to the methanol crossover effect, the Pt/C catalyst rapidly showed a poisoned response after the injection of 3 M methanol, whereas no apparent ORR current change is observed for the Fe-N-GNFs catalyst. The above results illustrated that compared to Pt/C catalyst, Fe-N-GNFs is more tolerant toward methanol (Figure 4h). On the other hand, durability is another important criterion for high performance catalysts. Therefore, we also used chronoamperometric measurements to assess the durability and the result showed that the asprepared Fe-N-GNFs catalyst showed a better long-term durability than the Pt/C catalyst (Figure 4i). The electrocatalytic performance of the Fe-N-GNFs for the ORR was also assessed in an acidic solution (0.5 M H2SO4). The CV curves of the Fe-N-GNFs showed pronounced electrocatalytic characteristics when the electrolyte (0.5 M H2SO4) was saturated with O2 (Figure S14a). The LSV curves indicated that the Fe-N-GNFs possessed a good catalytic activity in acidic solutions with an E0 of 0.60 V and an E1/2 0.35 V (Figure S14b). The n values calculated for the Fe-N-GNFs catalyst was found to be 3.60-3.71 when the potential changed from 0 to 0.2 V, further revealing that a high-efficient 4e- dominated ORR process was proceeded on this material in acidic solutions (Figure S14c-d). The excellent electrocatalytic activity of the Fe-NGNFs probably originates from: 1) The synergistic and strong coupling of different active


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constituents (Fe3C and N-doped carbon) because of the core-shell structure of Fe3C and the Ndoped graphitic carbon (an increase in the close contact between Fe3C and the graphitic layer preventing the Fe3C nanoparticles from aggregating or dissolving). 2) an ideal interconnected mesoporous structure (favorable for mass transfer); and 3) a highly conductive N-doped GNR network (conducive to electron transfer). Conclusions To summarize, we demonstrated a facile and scalable strategy for synthesizing a bonded double-doped graphene nanoribbon-based nonprecious metal catalyst in which Fe3C nanoparticles encapsulated in Fe-N-doped GNR frameworks. The physical measurement demonstrated that the Fe3C nanoparticles were well enwrapped in the N-doped porous graphitic layers to form a unique core-shell structure and then supported on the N-doped GNR frameworks to form a permanent and robust column- bracing material. As a result, the as-prepared Fe-NGNFs catalyst possesses all of the desired features for the ORR, including abundant catalytic sites, a high SSA, an ideal interconnected mesoporous structure and a highly conductive Ndoped GNR network. The superior ORR activity and stability of our catalysts in both acidic and alkaline solutions can be attributed to the particular closed structure and the synergistic effect produced by the active chemical composition (Fe3C and N-doped carbon). It is our belief that the synthetic protocol presented here is capable of being extended to prepare other nonprecious metal catalysts with improved ORR performances. Furthermore, our following work will focus more on further improving the ORR activity of this material in acidic media and determining exact sites.


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ASSOCIATED CONTENT Corresponding Author *Tao Yi; E-mail: [email protected] Acknowledgements The authors thank for the National Natural Science Foundation of China (51373039) and Specialized Research Fund for the Doctoral Program of Higher Education (20120071130008).

Supporting Information. Details of material characterizations, additional images, spectra and tables. This material is available free of charge via the Internet at http://pubs.acs.org.


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Table of Contents graphic


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A Bonded Double-Doped Graphene Nanoribbon Framework for

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