2D Mesoporous Carbon Doped with Fe-N Active Sites for Efficient

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2D Mesoporous Carbon Doped with FeN Active Sites for Efficient Oxygen Reduction Yifan Ye, Haobo Li, Fan Cai, Chengcheng Yan, Rui Si, Shu Miao, Yanshuo Li, Guoxiong Wang, and Xinhe Bao ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02101 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 25, 2017

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2D Mesoporous Carbon Doped with Fe-N Active Sites for Efficient Oxygen Reduction Yifan Ye, a,b Haobo Li,a,b Fan Cai, a,b Chengcheng Yan,a,b Rui Si,c Shu Miao,a Yanshuo Li,*,d Guoxiong Wang,*,a and Xinhe Baoa a

State Key Laboratory of Catalysis, CAS Center for Excellence in Nanoscience, Dalian National

Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China. b

University of Chinese Academy of Sciences, Beijing, 100039, China.

c

Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese

Academy of Sciences, Shanghai, 201204, China. d

School of Material Science and Chemical Engineering, Ningbo University, Ningbo, 315211,

China. *E-mail addresses: [email protected], [email protected].

ABSTRACT Iron-nitrogen (Fe-N) sites confined within carbon are highly active to catalyze the oxygen reduction reaction (ORR). The mesoporous structure of carbon facilitates the mass transport and availability to the Fe-N sites during the ORR, however, the construction of mesoporous carbon architecture with highly exposed and accessible Fe-N active sites remains challenging. Herein, 3D-to-2D transformation of zeolitic imidazolate framework-7 (ZIF-7) was successfully achieved via convenient impregnation in ammonium ferric citrate aqueous solution. After pyrolysis, isolated Fe-N sites are generated and confined within highly mesoporous carbon without using any additional modulator or template. The optimal Fe-N catalyst exhibits excellent

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performance toward ORR in alkaline medium, exceeding commercial 40% Pt/C catalyst in terms of activity, stability and methanol resistance.

KEYWORDS: zeolitic imidazolate framework-7, oxygen reduction reaction, self-assembly, 3D-to-2D transformation, iron-nitrogen sites.

1. INTRODUCTION Oxygen reduction reaction (ORR) is an important electrocatalytic process in fuel cells and metal-air batteries, determining the eventual power density and energy conversion efficiency.1-4 Carbon nanomaterials doped with Fe-N sites have been screened as a novel class of efficient nonprecious-metal ORR catalysts in recent years.5-7 Metal-organic frameworks (MOFs) containing iron species with pre-designed structure have been investigated as a convenient strategy to achieve highly dispersed Fe-N active sites doped within carbon nanomaterials.8-9 However, decomposition of the parent MOFs during the pyrolysis process leads to a low surface area of the resulting carbon nanomaterials.10-12 In addition, a pre-dominantly microporous nature of the MOFs-derived carbon nanomaterials is not beneficial for oxygen mass transport during the ORR, and some Fe-N active sites located inside the carbon matrix are hardly to catalyze the ORR due to inaccessibility to the reactants.13-14 Therefore, constructing mesoporous carbon architecture with highly exposed and accessible Fe-N active sites on the surface is highly desired in order to facilitate the mass transport, make the best of catalytic active sites, and consequently improve the ORR performance.15-16

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2D mesoporous carbon nanomaterials have attracted great interests due to their high surface area for full exposure of active sites, continuous electronic-conducting pathway and facilitated mass transport.17-20 It would be expected to be an efficient route to manufacture 2D carbon nanomaterials through the construction of 2D-shaped MOFs precursor.21-22 However, constructing 2D MOFs usually needs expensive modulator or template agents with complicated preparation procedures.23-25 In particular, few 2D MOFs containing iron species have been synthesized up to now.26-27 Herein, we propose a facile strategy to achieve surface functionalization of iron species and subsequent 3D-to-2D transformation of MOFs by impregnating 3D ZIF-7 [Zn(benzimidazole)2] in aqueous ammonium ferric citrate (AFC) solution without using any additional modulator or template agents.28-29 The obtained 2D ZIF nanosheets

self-assembled

to

form

disk-like

shape.

After

pyrolysis,

the

morphology-preserved thermal transformation process resulted in disk-shaped carbon, consisting of 2D mesoporous carbon doped with highly exposed Fe-N active sites. The catalysts derived from 2D ZIF show much higher ORR activities than those derived from 3D ZIF-7, also outperforming commercial 40% Pt/C catalyst in terms of ORR activity, stability and methanol tolerance.

2. RESULTS AND DISCUSSION Scheme 1 illustrates the preparation route of 2D mesoporous carbon doped with Fe-N active sites. 1.0 g 3D ZIF-7 nanoparticles were immersed in AFC aqueous solution (12 mg AFC in 20 mL deionized water ) for 24 h. H2O acts as the only 3


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structure-modulator to induce the 3D-to-2D transformation of ZIF-7 as a result of hydrophobic force of benzimidazolyl group.30 The obtained 2D ZIF is denoted as ZIF-(12, 24h). After pyrolysis and acid leaching, Fe-N sites confined within carbon were obtained, and the sample was denoted as Fe©N-C-12. A series of specimens were prepared by varying the impregnation duration from 4 h to 24 h, while the AFC concentration was kept the same. It is noteworthy here to emphasize that the actual Fe loadings of all the samples with different impregnation durations were nearly 0.20% (Table S1 in the Supporting Information). Scanning electron microscopy (SEM) images show that ZIF-(12, 4h) exhibits a morphology similar to 3D ZIF-7, indicating that the AFC molecule was first anchored onto the surface of 3D ZIF-7 before 3D-to-2D transformation (Figures 1a~1b). 3D ZIF-7 has a cage-type pore structure with a cavity size of 6.3 Å * 4.1 Å, connected by its small pore apertures of 2.9 Å.31 Therefore, the bulky size of AFC makes it impossible to diffuse into the small pore of ZIF-7, and AFC molecules only anchor onto the surface of ZIF-7.32 The ion-exchange reaction between Zn2+ and Fe3+ ions was evaluated by theoretical calculations.33 The doping energy of Fe3+ from AFC and ionized Fe3+ to the skeleton of 3D ZIF-7 is 1.32 eV and -2.43 eV, respectively (Figures S1 and S2 in the Supporting Information), suggesting that the metal-ion exchange between AFC and ZIF-7 could be excluded due to the strong interaction between citrate ions and Fe3+.32 Furthermore, the ligand exchange between citrate ion and benzimidazole is expected to occur because of its strong coordination with Zn2+.34 As a result, AFC molecules bind to the surface of 3D ZIF-7, leaving a partially ligand-substituted Fe-doped 3D ZIF-7. The interaction 4


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between water and surface-functionalized 3D ZIF-7 would be influenced to a certain degree due to the blocking effect of AFC molecules on the surface. When 3D ZIF-7 was immersed in water for 8 h or 16 h, both disk-like nanosheets and the originally rhombododecahedral nanoparticles were observed, suggesting that the 3D-to-2D transformation is a gradual process in the presence of water (Figures 1c and 1d).35 Finally, the 3D structure of ZIF-7 was completely transformed into 2D morphology after impregnation for 24 h (Figure 1e). The disk-piled architecture originated from the well-organization of sub-nanosized 2D nanosheets, as indicated by the cross-sectional SEM image of ZIF-(12, 24h) (Figure 1f). The well-ordered assembly of 2D nanostructure was also observed in both ZIF-(12, 8h) and ZIF-(12, 16h) (Figures S3 and S4 in the Supporting Information). Therefore, the 3D ZIF-7 was gradually transformed into 2D ZIF and then self-assembled to form the well-organized disk-shaped nanosheets in water after surface functionalization with AFC. The interaction between H2O and 3D ZIF-7 is a key driving force in 3D-to-2D transformation. After pyrolysis, Fe©N-C-12 exhibits mesoporous morphology as indicated in Figures 1g, 1h and S5 in the Supporting Information. High-resolution transmission electron microscopy (HRTEM) image of Fe©N-C-12 reveals the absence of metal nanoparticles (Figure 1i). The white dots in the high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image suggest that the metallic single atoms are highly dispersed within the carbon matrix (Figures 1j and S6 in the Supporting Information).5, 7 Therefore, a self-template methodology was successfully explored to synthesize the 2D mesoporous carbon doped with Fe-N sites 5


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without the use of any hard template such as SBA-15, silica spheres and montmorillonite.36-38 Figure 2a shows XRD patterns of the five precursors aforementioned, illustrating that ZIF-(12, 4h) maintains the crystalline structure of 3D ZIF-7, while ZIF-(12, 8h) and ZIF-(12, 16h) are the mixtures of 3D ZIF-7 and 2D ZIF.39 ZIF-(12, 24h) shows only layered 2D structure that is completely different from 3D ZIF-7, consistent with the SEM result in Figure 1f.40 XRD patterns of their pyrolysis products only show two broad peaks corresponding to carbon without any diffraction peaks of metal nanoparticles (Figure 2b). The N2 adsorption–desorption isotherms in Figure 2c reveals pronounced hysteresis loop for C-ZIF-(12, 8h), C-ZIF-(12, 16h) and Fe©N-C-12, indicative of the mesoporous structure. Figure 2d shows the relationships of external surface area and ∆V (total pore volume minus micropore volume) versus impregnation duration. The increase of impregnation duration facilitates the formation of mesopores in the initial 16 h, rising from 268.0 m2 g-1 (0.398 cc g-1) for C-ZIF-(12, 4h) to 489.3 m2 g-1 (0.735 cc g-1) for C-ZIF-(12, 16h), followed by a decrease to 402.0 m2 g-1 (0.602 cc g-1) for Fe©N-C-12 (Table S2 in the Supporting Information). To enhance the Fe content in the pyrolysis carbon nanomaterials, we varied the AFC dosage from 12 mg to 480 mg while maintaining the impregnation duration of 24 h. The 2D morphologies of ZIF-(30, 24h) and ZIF-(60, 24h) in Figures 3a and 3b indicate the great applicability and generality in achieving 3D-to-2D transformation of ZIF-7. However, when the usage of AFC was increased to 120 mg, the mixed phases appeared similar to the preparation of ZIF-(12, 8h) and ZIF-(12, 16h) (Figure 3c). The 6


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morphology of ZIF-(480, 24h) even remained unchanged with only 3D ZIF-7 left after impregnation (Figure 3d). As proposed above, AFC molecule was first anchored to the surface of 3D ZIF-7 before 3D-to-2D transformation. At high concentration of AFC in water, the surface-anchored AFC would block the interactions between 3D ZIF-7 and H2O, lessening the driving force for 3D-to-2D transformation of ZIF-7 and resulting in co-existence of 3D ZIF-7 and 2D ZIF in ZIF-(120, 24h). If too much AFC (480 mg) was used for the precursor preparation, the blocking effect of AFC would be dominant, resulting in an unchanged 3D morphology (Figure 3d). Therefore, a sufficiently strong interaction between 3D ZIF-7 and H2O is the prerequisite for complete 3D-to-2D transformation of ZIF-7, considering the influence of both impregnation time and dosage of blocking agent (AFC). After pyrolysis, the carbon nanomaterials all maintained their precursors’ respective topology as shown in Figures 3e~3h. HRTEM images of Fe©N-C-30, Fe©N-C-60 and Fe©N-C-120 in Figures 3i~3k show carbon morphology without the formation of iron nanoparticles, which is obviously different from the metal nanoparticles encased by several carbon layers in Fe©N-C-480 (Figure 3l). XRD patterns of 3D ZIF-7 derived precursors with different AFC dosages also agree well with the SEM images in Figures 1a, 1e, 3a~3d. As indicated in Figure 4a, ZIF-(12, 24h), ZIF-(30, 24h) and ZIF-(60, 24h) reveal only 2D ZIF structure, which is different from the spectra of 3D ZIF-7 and ZIF-(480, 24h) as well as the mixed phases (3D ZIF-7 and 2D ZIF) of ZIF-(120, 24h). Except for Fe3C and Fe shown in Fe©N-C-480, only two weak carbon peaks appear in the spectra of other pyrolysis 7


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products in Figure 4b.41 To further understand the local electronic and geometric structure of three 2D ZIF-derived carbon nanomaterials, we performed the Fe-K edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of Fe©N-C-12, Fe©N-C-30, Fe©N-C-60 and Fe©N-C-480 compared with standard Fe foil and FePc (Figures 4c and 4d).42 The absorption edge and main transition energies of Fe©N-C-12, Fe©N-C-30 and Fe©N-C-60 are similar to FePc, indicative of similar valence of iron species. Fe©N-C-480 is located between Fe foil and FePc due to the co-existence of iron nanoparticles and Fe-N sites in Fe©N-C-480. Fourier transformation of EXAFS analysis confirms the Fe-N and Fe-Fe bonds in the pyrolysis products. Only Fe-N bond at 1.9 Å was observed in Fe©N-C-12, Fe©N-C-30 and Fe©N-C-60 while Fe-Fe bond around 2.5 Å was not observed.6 For Fe©N-C-480, both Fe-N and Fe-Fe bonds are detected. After complete 3D-to-2D transformation, the well-ordered assembly of 2D ZIF creates the gallery between each 2D nanosheet (Figures S3 and S4 in the Supporting Information).30 After pyrolysis, the specific surface areas of 2D ZIF-derived carbon nanomaterials are 594.5 m2 g-1 for Fe©N-C-12, 644.5 m2 g-1 for Fe©N-C-30 and 885.0 m2 g-1 for Fe©N-C-60, respectively (Figure S7 and Table S3 in the Supporting Information).43 As highlighted in Scheme 1, the distance between two layers of 2D ZIF is 0.988 nm, which may account for the micropores (< 1 nm) after pyrolysis.30 Besides, the gallery of 2D ZIF accounts for the mesopore-dominated and interlinked pore structure that would facilitate mass transport after pyrolysis, allowing a

8


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self-template strategy to generate mesopores without any additional templates (Figure S5 in the Supporting Information). Figure 5a and Figure S8 show the linear sweeping voltammetry (LSV) curves, electron transfer number and HO2- yield of 3D ZIF-7 derived carbon nanomaterials with different impregnation durations in O2-saturated 0.1 M KOH solution. Their ORR performance and electron transfer number increase with the impregnation duration, and Fe©N-C-12 exhibits the optimal ORR activity. The Fe loadings before and after pyrolysis for each catalyst were all kept nearly 0.20% and 0.37%, respectively (Table S1 in the Supporting Information). Therefore, the quantity difference of Fe-N active sites among these catalysts can be excluded. As characterized by Raman spectroscopy, the intensity ratio of G band to D band (IG/ID) as calculated from the peak intensity is between 0.92 and 0.99 (Figure S9 in the Supporting Information), indicative of similar graphitization degree, excluding the effect of electrical conductivity of the carbon nanomaterials on the ORR activity. In Fe-doped 3D ZIF-7, AFC was anchored onto the surface of 3D ZIF-7 by ligand exchange. After pyrolysis, both edge-hosted and bulk-hosted Fe-N sites were formed in the sample.44 In order to clarify the distribution of AFC on the 2D ZIF, we synthesized 2D ZIF without Fe-doping by immersing 3D ZIF-7 in water at room temperature for 24 h.30 After purification, the obtained powder was named ZIF-L. Figure S10 (Supporting Information) shows XRD patterns of ZIF-L, ZIF-(12, 24h), ZIF-(30, 24h) and ZIF-(60, 24h). Taking the major peak (located at ca. 9.1o) of ZIF-L as a reference, there is no obvious peak shift for other ZIFs, suggesting that AFC does 9


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not intercalate into the layers of 2D ZIF and is anchored at the edges after impregnation.45 The morphology of parent Fe-doped 2D ZIF is preserved after pyrolysis, therefore, Fe-N active sites are considered to be located at the edges instead of in the 2D layers. Previous reports have widely investigated the bulk-hosted Fe-N active sites for ORR by DFT calculations.46-49 Recently, Zelenay et al. used HAADF-STEM to verify that most of the highly dispersed Fe atoms were predominantly positioned at exposed basal-plane edges and steps (edge-hosted) in single layer graphene region.44 They further validated the higher ORR activity of the edge-hosted Fe-N sites than the bulk-hosted sites via quantum chemistry calculations. Therefore, the performance enhancement in Fe©N-C-12 is probably attributed to the accessible and active edge-hosted Fe-N sites in the mesoporous 2D carbon nanomaterials.50 C-ZIF-(12, 4h) and C-ZIF-(12, 8h) have inferior ORR activities, but their electron transfer numbers are obviously higher than 2.51 During the RRDE measurement, HO2- produced inside the thick and porous electrode needs to diffuse throughout the electrode, and then releases into the bulk electrolyte and are subsequently detected by the outer Pt ring electrode.52 During the limited HO2diffusion inside the C-ZIF-(12, 4h) and C-ZIF-(12, 8h) electrodes, some HO2molecules probably further react on the catalytic sites as follows:53 O2 + H2O + 2e- → HO2- + OH-

(Eq. 1)

2HO2-→O2 + 2OH-

(Eq. 2)

HO2- + H2O + 2e- → 3OH-

(Eq. 3)

10


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The above processes could convert HO2- molecules produced in the bulk of the electrode to OH-, thus resulting in an apparent selectivity obviously higher than 2 over C-ZIF-(12, 4h) and C-ZIF-(12, 8h) catalysts. LSV curves of the catalysts with different AFC usages in Figure 5b reveal that 3D ZIF-7 derived catalysts show very mediocre ORR activity for C-ZIF-7 due to the absence of Fe-N active sites. The 2D ZIF-derived Fe©N-C-12, Fe©N-C-30 and Fe©N-C-60 exhibit superior ORR activities, being 20 mV, 18 mV and 14 mV more positive than commercial 40% Pt/C catalyst in terms of half-wave potential, respectively. The kinetic current density at 0.8 V (vs. RHE) over Fe©N-C-12 is 11 times larger than that over Fe©N-C-480 derived from 3D ZIF-7, even though Fe content in Fe©N-C-12 is only ca. 1/6 of that in Fe©N-C-480 (Table S4 in the Supporting Information), highlighting the importance of accessible edge-hosted Fe-N sites in 2D mesoporous carbon architecture for ORR. As indicated in Figure 5c, Fe©N-C-12 also exhibits the highest metal mass activity (denoted as current density @ 0.80 V per microgram metal). LSVs of Fe©N-C-12 at different rotation rates were also measured to better compare with the single-atom non-noble-metal catalysts in recently reported literatures under the same conditions (Figure S11 in the Supporting Information). Fe©N-C-12 shows the highest mass activity among these catalysts (Table S5 in the Supporting Information). The electron transfer number n of the optimal Fe©N-C-12, Fe©N-C-30 and Fe©N-C-60 in ORR test all surpasses 3.87, suggesting that the ORR over these catalysts follows a near-4 electron pathway in the potential window from 0.2~0.9 V (vs. RHE), and results in the production of a small 11


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amount of HO2- below 7% (Figure 5d).54 Fe©N-C-120 and Fe©N-C-480 have inferior ORR activities, but the electron transfer numbers are also obviously higher than 2 due to the limited HO2- diffusion to Pt ring and the side-reactions (Eqs. (1-3)) inside the 3D microporous carbon nanomaterials,53 which are similar to those in C-ZIF-(12, 8h) and C-ZIF-(12, 16h). Fe©N-C-12 demonstrates a higher stability than commercial 40% Pt/C catalyst (Figures 5e and 5f). The half-wave potential of Fe©N-C-12 merely decreases by 8 mV after 10,000 cycles, while that of 40% Pt/C catalyst decreases by 21 mV after only 2,000 cycles and 60 mV after 10,000 cycles. In-situ Fe-K edge XANES and EXAFS characterizations show that the Fe-N active site structure of Fe©N-C-12 was well maintained during the ORR test (Figure S12 in the Supporting Information).55 The methanol resistance of Fe©N-C-12 is also superior to that of 40% Pt/C catalyst in O2-saturated alkaline medium added with 3 M CH3OH (Figures 6a and 6b). There is only a 24 mV decay in half-wave potential for Fe©N-C-12, compared with a pronounced deterioration for 40% Pt/C due to the strong CO-like intermediates binding on the Pt surfaces56. Figures 6c and 6d show the single cell performance of alkaline direct methanol fuel cell (ADMFC) with Fe©N-C-12 and 40% Pt/C as the cathode. The open-circuit voltage (OCV) of the ADMFC with Fe©N-C-12 as cathode catalyst is slightly higher than that of the ADMFC with 40 wt% Pt/C (0.91 V vs. 0.87 V). The corresponding peak power density of the ADMFC with Fe©N-C-12 reaches 36.0 mW cm-2, which is ca. 1.7 times of that for the cell with 40% Pt/C (20.9 mW

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cm-2), demonstrating higher ORR activity and methanol resistance of Fe©N-C-12 in ADMFC. 3. CONCLUSIONS In summary, we have developed a novel route to fabricate 2D mesoporous carbon doped with Fe-N active sites by surface functionalization of 3D ZIF-7 with AFC and subsequent 3D-to-2D transformation of ZIF-7 followed by pyrolysis. The well-ordered assembly of 2D nanosheets enables efficient utilization of Fe-N active sites and facilitates the mass transport during the ORR, thanks to the interconnected and mesopore-dominated pore structure. This self-template strategy opens an environmentally-friendly and convenient route for designing high-performance ORR catalysts.

4. EXPERIMENTAL SECTION Materials N,N-dimethylformamide

(DMF,

99.0%),

zinc

nitrate

hexahydrate

[Zn(NO3)2 ·6H2O, ≥ 99.0%] , benzimidazole (C7H6N2, ≥ 98.0%), potassium hydroxide (KOH, ≥85.0%), methanol (CH3OH, ≥ 99.5%) and ethanol (C2H5OH, ≥ 99.7%) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Nafion (D520, 5%) solution was received from Dupont, USA. Ammonium ferric citrate (C6H11FeNO7, 16.9% Fe, AFC) was purchased from J&K Chemical Ltd. Commercial 40% Pt/C and 60% PtRu/C were obtained from Tanaka Kikinzoku Corp. and Johnson Matthey Corp., respectively.

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4.1 Synthesis of ZIF-7 ZIF-7 was prepared by a modified protocol according to the report by Li et al.57 0.01 mol of Zn(NO3)2•6H2O and 0.065 mol of benzimidazole were soaked in 1000 mL DMF at room temperature. After stirring for 1 h, the solution was kept statically for 168 h. The ZIF-7 was refined by centrifugation and thoroughly refluxed with ethanol, followed by drying under vacuum at room temperature for 12 h. The yield of ZIF-7 is about 48% based on zinc. 4.2 Syntheses of ZIF-(12, 4h), ZIF-(12, 8h), ZIF-(12, 16h) and ZIF-(12, 24h) 12 mg of AFC was dissolved in 20 mL distilled water, then 1.0 g of ZIF-7 was added to the solution under vigorous stirring for 4 h, 8 h, 16 h and 24 h, respectively. The remnant samples were thoroughly washed with water for three times before drying in oven at 60 oC overnight. 4.3 Syntheses of C-ZIF-7, C-ZIF-(12, 4h), C-ZIF-(12, 8h), C-ZIF-(12, 16h) and Fe©N-C-12. 0.6 g of ZIF-7, ZIF-(12, 4h), ZIF-(12, 8h), ZIF-(12, 16h) and ZIF-(12, 24h) were pyrolyzed in Ar with a flow rate of 100 mL min-1 at 900 oC for 2 h with a ramping rate of 3 oC min-1, respectively. Then the black products were acid-leached in 150 mL 0.5 M HClO4 solution to remove the unstable species, followed by washing and drying in oven at 60 oC overnight. The final samples were named as C-ZIF-7, C-ZIF-(12, 4h), C-ZIF-(12, 8h), C-ZIF-(12, 16h) and Fe©N-C-12, respectively. 4.4 Syntheses of ZIF-L, ZIF-(30, 24h), ZIF-(60, 24h), ZIF-(120, 24h) and ZIF-(480, 24h) 14


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A certain amount (0/30/60/120/480 mg) of AFC was dissolved in 20 mL deionized water, then 1.0 g ZIF-7 was added to the solution under vigorous stirring for 24 h. The remnant samples were thoroughly washed with deionized water for three times before drying in oven at 60 oC overnight. The final samples were named as ZIF-L, ZIF-(30, 24h), ZIF-(60, 24h), ZIF-(120, 24h) and ZIF-(480, 24h), respectively. 4.5 Syntheses of Fe©N-C-30, Fe©N-C-60, Fe©N-C-120, Fe©N-C-480 0.6 g of ZIF-(30, 24h), ZIF-(30, 24h), ZIF-(120, 24h) and ZIF-(480, 24h) were treated under the same pyrolysis process and acid-washing as ZIF-(12, 24h). The final samples were named as Fe©N-C-30, Fe©N-C-60, Fe©N-C-120 and Fe©N-C-480, respectively. 4.6 Physicochemical characterization The morphologies of the samples were observed by a field emission SEM (FE-SEM, JSM-7800F) with an accelerating voltage of 3 kV and a JEM-2100 HRTEM operated at an accelerating voltage of 200 kV. HAADF-STEM measurements

were

performed

with

JEM-ARM200F

(JEOL,

Japan)

for

high-resolution images. The actual loadings of Fe in all the samples were measured by inductively coupled plasma optical emission spectroscopy (ICP-OES). X-ray diffraction (XRD) was performed using a Rigaku D/Max-2500 diffractometer with a Cu Ka radiation source (λ=1.5418 Å) at 40 kV and 200 mA at a scan rate of 5˚ min-1. Nitrogen adsorption/desorption was investigated by a Quantachrome Autosorb iQ2 system at 77 K, and the specific surface areas of different samples were determined by Brunauer-Emmett-Teller (BET) equation. XANES and EXAFS were investigated at 15


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the BL14W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF). All the catalysts were characterized by Fe-K edge XANES and EXAFS under ambient conditions. In-situ Fe-K edge XAS was conducted in homemade Teflon cell filled with O2-saturated 0.1 M KOH solution. 4.7 Electrochemical performance evaluation The electrochemical performance of all catalysts were evaluated by an Autolab potentiostat/galvanostat (PGSTAT 302N). 5.0 mg of catalyst was pre-mixed with 0.3 mL deionized water, 1.7 mL ethanol and 50 µL 5% Nafion solution by ultrasonication for at least 1 h to form a homogeneous ink. 25 µL of the catalyst ink was dripped onto the glassy carbon (GC) disk electrode for performance evaluation, with catalyst loading of 311 µg cm-2. As comparison, the loading of 40% Pt/C is 25 µg cm-2 using the same catalyst ink preparation method. The electrolyte, 0.1 M KOH solution, was pumped with O2 vigorously to guarantee an O2-saturated medium under the ORR test. The GC disk electrode was subjected to potential range between -0.8 to 0.2 V (vs. Hg/HgO) at a scan rate of 10 mV s-1 with a rotating rate of 2500 rpm. The potential difference between the Hg/HgO reference electrode and reversible hydrogen electrode (RHE) is 0.925 V. The graphite rod electrode was employed as the counter electrode. The net ORR current of the tested sample was calculated by deduction of the background capacitive current recorded in the same condition aforementioned, but in an Ar-saturated electrolyte. The durability test was conducted by recording the LSV curves of the catalysts before and after 10 k potential cycles (0.6 V→1.0 V→0.6 V) in O2-saturated 0.1 M KOH solution. The methanol resistance was evaluated by 16


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comparing the LSVs before and after the addition of 3 M CH3OH in O2-saturated 0.1 M KOH solution. To detect the electron transfer during the ORR test, the rotating ring-disk electrode (RRDE) was employed to record the current fluctuations in the disk and ring. 25 µL of the mixed ink was dropped onto the disk of the RRDE and the rotation rate was kept at 2500 rpm. The Pt ring potential was held constant at 1.225 V (vs. RHE). The electron transfer was calculated by the following equation (Eq. 4):

n=

4* Id (4) Id + Ir / N 0

The HO2- selectivity was determined by the following equation (Eq. 5):

HO 2 - (%) =

200 * Ir (5) N 0 * I d + Ir

Where, Id and Ir mean the disk and ring currents, respectively；n denotes the electron transfer number in the ORR performance test; N0 is the collection efficiency (0.38). 4.8 ADMFC test The catalyst layer in the anode and cathode were brushed onto the gas diffusion layer based on Toray TGP-H-060, respectively. The commercial 60% PtRu/C with a catalyst loading of 3.1 mg cm-2 was employed as the anode catalyst layer. For the cathode, the catalyst loadings of Fe©N-C-12 and 40% Pt/C were 2.2 mg cm-2 and 3.4 mg cm-2, respectively. Then the anode and cathode were placed onto the two sides of anion exchange membrane (Tokuyama A201) to form the membrane & electrode assembly (MEA). The MEA was assembled into a single cell and installed on a home-made Fuel Cell Testing System. The pure dry oxygen was fed into the cathode 17


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with a flow rate of 300 mL min-1, and 2 M methanol in 2 M KOH solution was fed into the anode with a flow rate of 2 mL min-1. The single cell was operated at 60 oC, and the polarization curves were recorded by a KFM 2030 impedance meter (Kikusui Corp.).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

ACKNOWLEGMENT We gratefully acknowledge financial support from the Ministry of Science and Technology of China (Grants 2016YFB0600901 and 2013CB933100), the National Natural Science Foundation of China (Grants 21573222, 21622607, 91545202 and U1532117), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB17020200). We thank staff at the BL14W1 beamline of Shanghai Synchrotron Radiation Facility (SSRF) for the kind help during XAFS measurements. G.X. Wang thanks the financial support from CAS Youth Innovation Promotion (Grant No. 2015145). Y.S. Li thanks the financial support from the K. C. Wong Education Foundation in Ningbo University.

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Scheme 1. Illustration of synthesis of Fe©N-C-12.

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Figure 1. (a-e) SEM images of ZIF-7, ZIF-(12, 4h), ZIF-(12, 8h), ZIF-(12, 16h), ZIF-(12, 24h). (f) Cross-sectional SEM image of ZIF-(12, 24h). (g-j) SEM, TEM, HRTEM and high-resolution HADDF-STEM images of Fe©N-C-12.

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Figure 2. (a) XRD patterns of ZIF-7 derived precursors with different impregnation durations. (b) XRD patterns and (c) Nitrogen adsorption–desorption isotherms of C-ZIF-7, C-ZIF-(12, 4h), C-ZIF-(12, 8h), C-ZIF-(12, 16h) and Fe©N-C-12. (d) External surface area and △V of ZIF-7 derived carbon materials with different impregnation durations.

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Figure 3. (a-d) SEM images of ZIF-(30, 24h), ZIF-(60, 24h), ZIF-(120, 24h), ZIF-(480, 24h). (e-h) SEM images of Fe©N-C-30, Fe©N-C-60, Fe©N-C-120, Fe©N-C-480. (i-l) HRTEM images of Fe©N-C-30, Fe©N-C-60, Fe©N-C-120, Fe©N-C-480.

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Figure 4. (a) XRD patterns of ZIF-7 derived precursors with different AFC dosages. (b) XRD patterns of pyrolysis products from ZIF-7 derived precursors with different AFC dosages. (c) XANES spectra and (d) Fourier transformed EXAFS spectra of the Fe-K edge for Fe foil, FePc, Fe©N-C-12, Fe©N-C-30, Fe©N-C-60 and Fe©N-C-480. R(Å), distance in angstroms. k, wave number.

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Figure 5. (a) LSV curves of different catalysts: 1, C-ZIF-7; 2,C-ZIF-(12, 4h); 3, C-ZIF-(12, 8h); 4, C-ZIF-(12, 16h); 5, Fe©N-C-12. (b) LSVs of Fe©N-C catalysts with different AFC dosages and 40% Pt/C. (c) Metal mass activities of in Fe©N-C catalysts with different AFC dosages at 0.80 V (vs. RHE). (d) Electron transfer number and HO2- selectivity of Fe©N-C catalysts with different AFC dosages. (e-f) LSV curves of Fe©N-C-12 and 40% Pt/C before and after 10 k potential cylces (0.6 V→1.0 V→0.6 V). All the data was obtained in O2-saturated 0.1 M KOH solution. Rotation speed: 2500 rpm.

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Figure 6. (a-b) Methanol resistance of Fe©N-C-12 and 40% Pt/C during the ORR test. (c) Cell voltage and (d) Power density of ADMFC with Fe©N-C-12 and 40% Pt/C as the cathode catalysts.

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2D Mesoporous Carbon Doped with Fe-N Active Sites for Efficient

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