Hybrid Porous Catalysts Derived from MetalâOrganic Framework for

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Hybrid Porous Catalysts Derived from Metal-Organic-Framework for Oxygen Reduction Reaction in Anion Exchange Membrane Fuel Cell Kai-Chin Wang, Hsin-Chih Huang, Sun-Tang Chang, ChengHao Wu, Ichiro Yamanaka, Jyh-Fu Lee, and Chen-Hao Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05993 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 26, 2019

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ACS Sustainable Chemistry & Engineering

Hybrid Porous Catalysts Derived from Metal– Organic Framework for Oxygen Reduction Reaction in Anion Exchange Membrane Fuel Cell Kai-Chin Wang,† Hsin-Chih Huang,† Sun-Tang Chang,† Cheng-Hao Wu,† Ichiro Yamanaka,‡ Jyh-Fu Lee§, Chen-Hao Wang*,† †

Department of Materials Science and Engineering, National Taiwan University of Science

and Technology, No.43, Keelung Rd., Sec.4, Da'an Dist., Taipei 10607, Taiwan. *E-mail: [email protected] (C. H. Wang) ‡

Department of Applied Chemistry, Tokyo Institute of Technology, 2 Chome-12-1 Ookayama,

Meguro, Tokyo 152-8552, Japan §

National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park,

Hsinchu 30076, Taiwan KEYWORDS: Metal–organic framework; zeolitic imidazolate framework-67; oxygen reduction reaction; catalyst

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ABSTRACT: This study synthesized the metal–organic framework-derived composite Fe−N−Co@C-800-acid-leaching (Fe−N−Co@C-800-AL) through co-doped iron and nitrogen atoms in a zeolitic imidazolate framework-67, followed by pyrolysis at 800°C and the AL process. Fe−N−Co@C-800-AL was highly active in an oxygen reduction reaction, during which the number of transferred electrons (3.986) was close to an ideal four-electron transfer. In addition, Fe−N−Co@C-800-AL showed no obvious degradation even after potential cycling of half-cell measurement (30,000 cycles). The prepared material exhibited a porous structure composed of nanoparticles (NPs) that were randomly distributed on poly−hydrocarbon structures with a Brunauer–Emmett–Teller surface area of 449.0 m2 g−1. X-ray photoelectron spectroscopy demonstrated that the synthesized Fe−N−Co@C-800-AL contained large amounts of pyridinic nitrogen and graphitic nitrogen, which could significantly enhance the activity of the oxygen reduction reaction. Furthermore, X-ray absorption spectroscopy revealed the existence of Co−Co and Fe−Fe and a lack of Co−Nx and Fe−Nx moieties, which means an oxygen reduction reaction may occur on the microstructures of N-doped carbon with wrapped metal NPs (Co or Fe). These findings revealed that Fe−N−Co@C-800-AL had a porous structure, high surface area, and the presence of functional nitrogen, thereby making it suitable for oxygen reduction reaction.


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INTRODUCTION

Fuel cells produce water and electrical energy from hydrogen and oxygen and may serve as alternative power sources for stationary and portable power supplies as well as zero-emission electric vehicles. However, the fuel cell catalyst Pt is expensive and rare. Polyelectrolyte fuel cells (PEFCs) have been developed in the past decades. Among them, the anion exchange membrane fuel cell (AEMFC) is a promising candidate because non-precious metal catalysts (NPMCs) are low cost and can be used as electrodes to enhance the activity of oxygen reduction reaction (ORR) as well as improve the stability of NPMCs in an alkaline medium.1-6 Recently, transition metal–nitrogen–carbon (M–N–C, M = Fe or Co) materials with superior electrocatalytic activity have been regarded as one of the most promising NPMCs in both alkaline and acidic mediums. According to relevant studies, M–N–C materials possess intrinsic ORR active sites (M–Nx or N-doped carbon) for improving ORR activity.7-13 However, the active sites (M–Nx or N-doped carbon) of catalysts remain a subject of debate. Dehui et al. reported an excellent catalyst using transition metal nanoparticles (NPs) embedded in carbon nanotubes (CNTs) without M−Nx bonds. They considered that the electron could be transferred from transition metal NPs to the carbon layer, which would lead to a reduced local work function on the carbon surface.7 Metal–organic frameworks (MOFs) are materials with high porosity that usually include diverse metal ions combined with organic ligands to form three-dimensional (3D) architectures.14-22 MOFs exhibit a variety of structures because various metal ions integrate with different organic ligands, and some may have a uniform porous structure and high surface area. MOFs have attracted substantial attention for their numerous potential applications, such as gas storage, sensing, drug delivery, separation, and catalyzation. Numerous studies have reported using MOFs as catalysts for various processing methods that can enhance ORR activity.15, 17, 19, 3 ACS Paragon Plus Environment


21, 23-39

Chaikittisilp et al. reported that carbon–cobalt–oxide hybrid materials derived from ZIF-

9 with nanoporous carbon networks possessed an outstanding ORR and oxygen evolution reaction (OER).17 Ge et al. suggested that the high ORR activity of unique nitrogen-doped carbon/carbon nanotubular structures could be attributed to the benefits of MOFs (i.e., tunable N contents and high surface areas) and CNTs.23 In addition, because MOFs possess abundant metal ions for coordinating with nitrogen and carbon, they can easily form M–N–C materials after pyrolysis and possess active sites (M–Nx or N-doped carbon). Ma et al. asserted that the superior ORR activity of Co@NPCM/CNF probably arose from the microstructures of nitrogen-doped carbon-wrapping Co NPs with a lack of Co–Nx.11 Guo et al. suggested that COPTPP(Fe)@MOF-900 possesses optimal ORR activity, in which the existence of Fe−Nx coordination was proven by performing X-ray absorption fine structure (EXAFS) and X-ray photoelectron spectroscopy (XPS) analysis.12 In this study, we successfully synthesized Fe−N−Co@C-800-AL by using nitrogen and iron precursors doped in ZIF-67, followed by pyrolysis at various temperatures and the acid leaching (AL) process. Under optimized conditions, the prepared catalyst Fe−N−Co@C-800-AL exhibited a structure with high porosity and a large surface area, both of which provide uniformly distributed active sites. These features provide much help to the excellent ORR activity and stability of the ZIF-67-derived sample.


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EXPERIMENTAL SECTION

Preparation of catalysts To prepare ZIF-67, 1660.5 mg of cobalt (II) acetate and 1094.7 mg of 2-methylimidazole (MeIM) were mixed and ball milled for 10 min. The mixture was placed in Teflon-lined autoclaves, and 20 mL of ethanol was added to each. The autoclaves were sealed and heated in a microwave digestion system at 120oC for 30 min. After this process, purple products were filtered with ethanol. Finally, ZIF-67 was dried in a freeze dryer overnight. Subsequently, ZIF-67 was pyrolyzed at specific temperatures for 10 min in an argon atmosphere, and the product was named Co@C−T (T = pyrolysis temperature). The AL was conducted in 1 M HNO3 at 80°C for 2 h to eliminate impurities, and the product was named Co@C−T-AL. Scheme 1 presents a schematic of the synthesis of Co@C-800-AL. To modify ZIF-67 by using iron and nitrogen precursors, untreated ZIF-67 was immersed in methanol and CH2Cl2 and then dried for 24 h. Subsequently, 200 mg of ZIF-67 was mixed with 5.8 mg of iron (II) acetate and 30 mg of 1,10-phenanthroline in 10 mL of ethanol; then, ethanol was evaporated using a rotary evaporator. The pretreated sample was pyrolyzed at 800°C for 10 min in an argon atmosphere. Next, it was washed using AL in 1 M HNO3 at 80°C for 2 h to eliminate impurities and uncoordinated Fe. The product was named Fe−N−Co@C-800-AL.



Scheme 1. Schematic of the synthesis of Co@C-800-AL and Fe−N−Co@C-800-AL. Structural characterization A Bruker D2 Phaser (source: Cu Kα radiation = 1.54060 Å) was used to record the X-ray diffraction (XRD) of samples. For samples that had not undergone pyrolysis, scanning was performed between 5° and 50° at a scan rate of 4.8° min−1 in increments of 0.04°, whereas for pyrolyzed samples, scanning was conducted between 20° and 80° at a scan rate of 4.86° min−1 in increments of 0.0081°. Field-emission transmission electron microscopy (FETEM) (FEI Tecnai™ G2 F-20 S-TWIN) with an energy-dispersive detector (EDS) was employed to analyze the microstructure of samples. The compositions of catalysts were analyzed through high-angle annular dark field scanning-TEM (HAADF-STEM) in the EDS mode. XPS (Beamline 24A1 at the National Synchrotron Radiation Research Center [NSRRC]) was conducted to identify the chemical constituents of catalysts and the oxidation states of elements. This Beamline 17C1 possesses a multipole wiggler source (magnitude of energy = 2.7 KeV),


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which can record the Fe K-edge and Co K-edge of X-ray absorption near-edge structures (XANES) and extended X-ray absorption fine structures (EXAFS). Electrochemical characterization A potentiostat/galvanostat instrument (Biologic VSP) was used to obtain electrochemical measurements with a three-compartment cell. The reference and counter electrodes were a saturated calomel electrode (Hg/HgCl2 in a saturated KCl solution) and a piece of Pt foil, respectively. A rotating ring-disk electrode (RRDE, PINE AFE6R2GCAU) with a glassy carbon (GC) disk and a ring of gold was used as the working electrode. In this study, a reversible hydrogen electrode (RHE) was used as the reference in all potentials. The catalyst ink was prepared by dissolving 1.5 mL of n-propanol, 3.5 mL of deionized water, and 11.88 mg of catalyst. Next, 20 μL of catalyst ink was dropped onto a GC disk, and 5 μL of 0.1% Nafion was coated onto the catalyst layer, which was subsequently air dried at ambient temperature. Cyclic voltammetry (CV) was conducted in N2-saturated 0.1 M KOH solution (scan rate = 50 mV s−1). Furthermore, linear scan voltammetry (LSV) was employed at a specified scan rate. ORR curves on the GC disk were set at a rotation speed of 1600 rpm and a low scan rate of 10 mV s−1; thus, the catalyst avoided generating a substantial non-Faradaic current. A voltage of 1.1 V versus the RHE was applied to the ring to oxidize hydrogen peroxide ions, which were generated on the GC disk during ORR measurements. The electrochemical abilities of catalysts were compared with that of a commercial 40% Pt/C catalyst (Johnson Matthey, London, United Kingdom). To evaluate the durability of the as-prepared catalysts, LSV was applied at a potential range of 1–0.6 V (vs. RHE) in N2-saturated 0.1 M KOH (scan rate = 50 mV s–1). The ORR was measured in O2-saturated 0.1 M KOH (scan rate = 10 mV s–1; rotational speed: 1600 rpm) after every 10,000 cycles. 7 ACS Paragon Plus Environment


Single cell test The membrane electrode assembly (MEA) (area = 5 cm−2) was adopted to evaluate the performance of AEMFC. The MEA included a cathode, anode, and Fumapem® FAA-3 membrane (OH-, FuMA-Tech GmbH). The as-prepared catalyst was scattered in the ionomer solution to prepare the cathode catalyst ink, whereas Pt/C was scattered in the ionomer solution to prepare the anode catalyst ink. The ionomer solution was Fumion® FAA-3 solution (10.1 wt.% in NMP). For the cathode and anode, the mass ratio of dry FAA-3 to catalyst was 1:3. The catalyst ink was hand painted onto a carbon cloth with a microporous layer (MPL) to obtain catalyst loadings of 0.8 mgPt cm–2 and 3 mg cm–2 for the anode and cathode, respectively. The two electrodes and membrane were immersed in 1 M KOH for 24 h. The residual solution was quickly cleaned from the electrodes and membrane before the MEA was assembled. A fuel cell test station (Tension Energy, Inc.) measured the polarization curve of the samples at 60°C without back pressure. The fuel cell current and voltage were measured immediately under a steady-state condition. Before hydrogen and oxygen fuel flowed out into the MEA, both gasses were passed through humidifiers at 65°C. RESULTS AND DISCUSSION

Figure 1a displays the XRD pattern of the as-prepared ZIF-67, for which the characteristic peaks match those of the XRD pattern observed in the study conducted by Wang et al..20 Figure 1b illustrates various XRD patterns of Co@C-800-AL and Fe–N–Co@C-800-AL, which had the same characteristic peaks. The carbon peak (002) was at 26.3°. The characteristic peaks of metallic cobalt (JCPDS 15-0806) corresponded with (111), (200), and (220) and were approximately 44.2°, 51.5°, and 75.8°, respectively. AL can remove impurities and excessive Co particles; however, in this study, metallic cobalt structures were found in Co@C-800-AL and Fe–N–Co@C-800-AL. Later, TEM images are provided to prove that the poly-aromatic 8 ACS Paragon Plus Environment

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carbon structure protects some Co particles from AL. Figure S1 displays the XRD patterns of Co@C-700-AL and Co@C-900-AL, both of which exhibited obvious peaks of metallic cobalt.

Figure 1. XRD pattern of (a) ZIF-67 and reference20, and (b) Co@C-800-AL and Fe–N– Co@C-800-AL. Figure 2 displays TEM images of Fe–N–Co@C-800-AL at various magnifications. As shown in Figure 2a, the Fe–N–Co@C-800-AL microstructure mainly comprised a polyaromatic carbon structure, in which some NPs were randomly distributed. The highmagnification TEM image in Figure 2b shows NPs wrapped in this poly-aromatic carbon structure. According to the clear lattice fringes (spacing = 0.205 nm) of the particle, it matches the (111) plane of metallic Co, which is consistent with XRD patterns. Figure 2c presents a HAADF-STEM image of Fe–N–Co@C-800-AL, and Figure 2d, 2e, and 2f present the corresponding EDS elemental mapping images of carbon, cobalt, and iron, respectively. These images clearly verify that cobalt and iron were randomly dispersed over skeletons.



Figure 2. TEM images of Fe–N–Co@C-800-AL: (a) at low magnification; (b) at high magnification; (c) in HAADF-STEM mode; (d) carbon mapping image; (e) cobalt mapping image; and (f) iron mapping image. In an alkaline environment, the ORR pathway is mainly related to the reactions as follows: 𝑂2 + 2𝐻2𝑂 + 4𝑒 ― →4𝑂𝐻 ― … 𝐸0 = 0.410 𝑉

(1)

𝑂2 + 𝐻2𝑂 + 2𝑒 ― →𝑂𝐻 ― + 𝐻𝑂2― …𝐸0 = ― 0.065 𝑉

(2)

The direct ORR (Reaction (1)) and the indirect reduction pathway (Reaction (2)) produce four-electron and two-electron transfers, respectively. To convert hydrogen peroxide ions into hydroxide ions, the following further reduction (Reaction (3)) or chemical decomposition (Reaction (4)) will occur. 𝐻𝑂2― + 𝐻2𝑂 + 2𝑒 ― →3𝑂𝐻 ― …𝐸0 = 0.867 𝑉

(3) 10

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2𝐻𝑂2― →𝑂2 + 2𝑂𝐻 ―

(4)

Reaction (1) (the direct ORR pathway) is preferable to Reaction (2) (the indirect ORR pathway) because the direct ORR pathway has a greater thermodynamically reversible potential. Figure 3a presents the ORR activities of Co@C-800-AL, Fe–N–Co@C-800-AL, and commercial Pt/C obtained using the RRDE technique. The current density of the disk (Id) and current of the ring (Ir) are plotted as functions of applied potential and presented in the lower and upper charts, respectively. The electron transfer number (n) and yield of % 𝐻𝑂2― for the ORR are evaluated using the following equations:

𝑛=

4𝐼𝑑

(5)

𝐼𝑟

𝐼𝑑 + 𝑁

% 𝐻𝑂2― =

2𝐼𝑟 𝑁 𝐼𝑟

× 100%

(6)

𝐼𝑑 + 𝑁

where N means the RRDE collection efficiency and is set to 0.368. The n values and % 𝐻𝑂2― yields are shown in Figure 3b and 3c, respectively. Figure S2 shows that among all pyrolysis temperatures, 800°C (Co@C-800-AL) resulted in the highest value of Id and the lowest value of Ir. In addition, the n value of Co@C-800-AL was found to be 3.966, which approached the ideal four-electron transfer. Compared with Co@C-800-AL, the n value of Fe–N–Co@C-800AL was 3.986, which is greater than those of optimal Co@C-800-AL and commercial Pt/C. At a low potential, the ring current of commercial Pt/C suddenly increased because adsorbed hydrogen hinders the partial active sites of the surface. Some reports have proposed that OHads species block the partial active sites of the surface at a low potential, which inhibits the oxygen reduction on Pt to trend the indirect reduction pathway.40-42 Table 1 lists the ORR activities in an alkaline solution for non-precious metal catalysts. The demonstrated ORR activity of Fe–N– Co@C-800-AL is competitive with those of other catalysts described in relevant studies. Here, 11 ACS Paragon Plus Environment


we also measured the ORR activities of Co@C-800-AL, Fe–N–Co@C-800-AL, and commercial Pt/C in 0.1 M HClO4, as shown in Figure S3a. The ORR activities of the three samples in acid solution exhibited different trends in alkaline solution, as shown in Figure S3a. The ORR activity of Pt/C was superior to those of Co@C-800-AL and Fe–N–Co@C-800-AL. The n value and %H2O2 yield are shown in Figure S3b and S3c, respectively. The n values of Co@C-800-AL and Fe–N–Co@C-800-AL were found to be 3.944 and 3.953, respectively, indicating that they possess the route for a four-electron transfer. According to these results, the as-prepared samples possessed excellent ORR activities in both acid and alkaline solutions.

Figure 3. (a) ORR curves; (b) n values; and (c) % 𝐻𝑂2― of Co@C-800-AL, Fe–N–Co@C800-AL, and commercial Pt/C as functions of disk potentials.


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Table 1. ORR activities in an alkaline solution for non-precious metal catalysts reported in relevant studies. Onset potential (V vs. RHE)

Electron transfer number

Reference

Sample

Electrolyte

Co@C-800-AL

0.1 M KOH

0.93

3.96

This work

Fe-N-Co@C-800-AL

0.1 M KOH

0.91

3.98

This work

Co0.5Fe0.5S@N-MC

0.1 M KOH

0.91

3.80-4.00

43

Co/N-HCOs

0.1 M KOH

0.92

3.60-4.00

44

PCN-FeCo/C

0.1 M KOH

1.00

>3.90

45

Co3S4-S/G-800

0.1 M KOH

0.92

3.96-4.00

46

Co3(PO4)2C-N/rGOA

0.1 M KOH

0.96

>3.98

47

Co-N-OMMC-0.6

0.1 M KOH

0.92

3.90

48

CoNC-800

0.1 M KOH

0.93

3.99

49

Co@NPCM/CNF-850

0.1 M KOH

1.03

3.80

11

Co9S8/NSC-900-1

0.1 M KOH

0.95

3.90

50

Figure 4 shows the nitrogen adsorption–desorption isotherms of Co@C-800-AL and Fe–N– Co@C-800-AL. All samples exhibited type IV isotherms, which present a capillary condensation step and hysteresis loop with a relative pressure (P/P0) range of 0.5–1.0, suggesting that in both samples, micropore and mesopore structures exist.16,

51

At high

temperatures, ZIF-67 decomposes and then metal species aggregate to form cobalt NPs. Subsequently, these cobalt NPs etch the poly-aromatic hydrocarbon structure of ZIF-67, resulting in the large surface areas and porosity of the catalyst.20 Table 2 lists the BET surface areas and pore volumes of Co@C-T-AL and Fe–N–Co@C-800-AL. Fe–N–Co@C-800-AL had a BET surface area of 449.0 m2 g–1, whereas that of Co@C-800-AL was 437.6 m2 g–1. Large surface areas and high pore volumes can supply a greater number of active sites for ORR and thus increase catalytic activity.15, 27, 29, 33, 52 Figure S4 demonstrates the nitrogen adsorption– 13 ACS Paragon Plus Environment


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desorption isotherms and BET surface areas of Co@C-700-AL and Co@C-900-AL, and Table 2 lists the corresponding data. The BET surface area of Co@C-800-AL is slightly higher than that of Co@C-900-AL and much higher than that of Co@C-700-AL. Thus, 800°C is the optimal temperature for pyrolysis.

Figure 4. N2 adsorption–desorption isotherms of Co@C-800-AL and Fe–N–Co@C-800-AL. Table 2. BET surface areas and pore volumes of Co@C-T-AL and Fe–N–Co@C-800-AL. Sample

BET surface area (m2 g–1)

Pore volume (cm3 g–1)

Fe-N-Co@C-800-AL

449.0

0.069

Co@C-700-AL

397.9

0.099

Co@C-800-AL

437.6

0.037

Co@C-900-AL

436.2

0.027

Figure S5 displays the wide-scan XPS spectrum of Fe–N–Co@C-800-AL, which verifies the presence of C, N, O, Fe, and Co in Fe–N–Co@C-800-AL. Figure 5a and 5b display the XPS N1s spectra of Co@C-800-AL and Fe–N–Co@C-800-AL, respectively. The XPS N1s patterns had four sub-peaks, namely the pyridinic N group, pyrrolic N group, graphitic N group, and oxidized N group, which corresponded to 398.9 eV, 400.2 eV, 401.7 eV, and 405.3 eV, 14 ACS Paragon Plus Environment

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respectively.16,

21, 33, 52-56

Table 3 lists the corresponding deconvolution results of the two

samples. Several studies have reported that except for oxidized N, the other N-containing groups (i.e., pyridinic N and graphitic N) play crucial roles in the ORR process.21,

24, 57-58

Pyridinic N exerts a conjugation effect on its lone pair of electrons and π system, facilitating O2 adsorption and therefore improving ORR activity.6,

59-62

Graphitic N makes neighboring

carbons display more positive spin and charge densities, which produce active sites for ORR.63 The total contents of pyridinic N and graphitic N in Fe–N–Co@C-800-AL are higher than those in Co@C-800-AL, which is consistent with the trends of ORR activity. Li et al. proposed that the vital contributions of pyridinic N and graphitic N are assured in the ORR process, but the contribution of pyrrolic N is unclear.57

Figure 5. XPS N1s analysis of (a) Co@C-800-AL and (b) Fe–N–Co@C-800-AL. Table 3. The content of nitrogen species: Co@C-800-AL and Fe–N–Co@C-800-AL.

Sample

Nitrogen species (%) Pyridinic

Pyrrolic

Graphitic

Oxidized

Co@C-800-AL

35.93

31.87

18.46

13.74

Fe–N–Co@C-800-AL

39.65

26.30

15.85

18.20



Figure 6a displays the Co K-edge XANES spectra of all samples. The prominent pre-edge peak (1s → 3d transition) at approximately 7709 eV in ZIF-67 represents the characteristic of tetrahedral coordination, whereas the main absorption edge (1s → 4p transition) of ZIF-67 at approximately 7725 eV represents that ZIF-67 contains divalent cobalt ions.11, 64-67 The energy position at approximately 7732 eV corresponds with the cobalt foil.65 The Co K-edge spectra of Co@C-800-AL and Fe–N–Co@C-800-AL show similar results to the Co foil, revealing the presence of metallic cobalt.11 The calculated Fourier transforms of k3-weighted Co K-edge EXAFS spectra (Figure 6b) show that the local environments of Co@C-800-AL and Fe–N– Co@C-800-AL can be described well using the scattering path of Co–Co, in which clear peaks were found at approximately 2.49 Å and 2.50 Å, respectively. Table S1 lists the structural parameters of the fitted curve. This Co K-edge EXAFS spectrum for ZIF-67 differs from the spectra of Co@C-800-AL and Fe–N–Co@C-800-AL, which clearly show a scattering path of Co–N with a peak located at approximately 2.02 Å (listed in Table S1). Figure S6a depicts the Fe K-edge XANES spectra of Fe–N–Co@C-800-AL, which is different from the standard Fe foil. The pre-edge peak of the Fe K-edge XANES spectrum of Fe–N–Co@C-800-AL was noted as being similar to the Fe foil, indicating that iron-based crystalline structures exist in Fe–N–Co@C-800-AL. The Fe K-edge XANES spectrum possesses a pre-edge at 7114 eV, which has been recognized as the Fe−N4 square-planar structure.68-69 Yuan et al. reported the Fe K-edge XANES spectrum of FeNC-800, which was determined as being more similar to the spectra of Fe3C and Fe foil, indicating that FeNC-800 possessed Fe3C and/or metallic iron structures.68 The calculated Fourier transforms of the k3weighted Fe K-edge EXAFS spectra (Figure S6b) show that the local environment of Fe–N– Co@C-800-AL can be described well using two scattering paths of Fe–N and Fe–Fe, which have peaks located at approximately 1.96 Å and 2.53 Å, respectively. Table S2 lists the structural parameters of the fitted curves. 16 ACS Paragon Plus Environment

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Some reports have made controversial statements regarding the ORR active sites of catalysts with Fe–Nx or Co–Nx. Relevant reports have considered that Fe–Nx or Co–Nx compounds can play crucial roles in improving ORR activity.47, 70-72 By contrast, several reports have proposed that the presence of N-doped carbon can effectively increase the ORR activity of NPMCs.8, 11, 73

Strickland et al. reported that FePhen@MOF–ArNH3 has prominent ORR activity, as well as

proved the existence of N-doped carbon wrapped Fe/FexC NPs and the absence of direct Fe– Nx coordination using Mössbauer and XAS spectroscopy.8 Ma et al. reported that NPCM/CNF900 possessed outstanding ORR activity and stability in alkaline solution, and its ORR activity originated from N-doped carbon with wrapped Co NPs but was lacking obvious Co–Nx.11 In this case, according to the aforementioned ORR test results, XNAES, EXAFS, and the relevant literature, the microstructure of N-doped carbon with wrapped metal NPs (Co or Fe) contributed to the major ORR activity of our catalysts. Notably, Fe–N–Co@C-800-AL possesses both sources of active sites, which can make it superior to Co@C-800-AL in terms of ORR activity.

Figure 6. Co K-edge (a) XANES spectra and (b) EXAFS spectra of Co foil, ZIF-67, Co@C800-AL, and Fe–N–Co@C-800-AL.



Figure 7a and 7b present the results of stability tests of Fe–N–Co@C-800-AL and commercial Pt/C in alkaline media, respectively. Fe–N–Co@C-800-AL exhibited no obvious change in ORR activity, with a half-wave potential decay of only approximately 6 mV after 30,000 cycles. Zhang et al. suggested that the durability of their catalyst constituted a synergistic result of its highly graphitized carbon frameworks and the active sites formed from nitrogen doping, which may reinforce corrosion resistance and prevent discrete active sites during the stability test.28 Partial research has reported that transition metal active species encapsulated in graphitic carbon affect the charge distribution of the external carbon layers and enhance ORR activity.74-75 The half-wave potential of Pt/C exhibited an obvious decay (41 mV) after 30,000 cycles, as shown in Figure 7b. Thus, the long-term stability of commercial Pt/C may be considered inferior to that of Fe–N–Co@C-800-AL in alkaline electrolytes. The superior ORR activity of Fe–N–Co@C-800-AL with long-term stability can be ascribed to the active sites encapsulated in graphitic carbon structures, which prevent the detachment of active sites during stability tests. Figure 8 depicts the AEMFC single cell performance of Co@C-800-AL, Fe–N–Co@C-800AL, and commercial Pt/C as cathode catalysts. In an AEMFC, the electrolytic anion exchange membrane majorly affects its performance.76-79 Here, the AEMFC with Pt/C had a maximum power density of 118.5 mW cm-2. The maximum power densities of Co@C-800-AL (125.8 mW cm-2) and Fe–N–Co@C-800-AL (137.1 mW cm-2) were approximately 5.8% and 13.6% more than that of Pt/C, respectively. Table 4 lists the AEMFC single cell performance for nonprecious metal catalysts. The demonstrated AEMFC single cell performance of Fe–N–Co@C800-AL was competitive with those of other catalysts described in relevant studies.


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Figure 7. Stability tests of the ORR curves of (a) Fe–N–Co@C-800-AL and (b) Pt/C using a potential-cycling method in O2-saturated 0.1 M KOH.

Figure 8. Polarization curves of H2–O2 AEMFCs using Co@C-800-AL, Fe–N–Co@C-800-AL, and commercial Pt/C.



Page 20 of 35

Table 4. AEMFC single cell performance for non-precious metal catalysts reported in relevant studies.

Cathode

Anode

Membrane

Maximum power density (mW cm-2)

Reference

Co@C-800-AL

Pt/C

Fumapem® FAA-3

125.8

This work

Fe-N-Co@C-800-AL

Pt/C

Fumapem® FAA-3

137.1

This work

Co-Fe3O4/C

Pt/C

Fumapem® FAA-3

115

80

MnO/NG-900 (10%)

Pt/C

Fumapem® FAA-3

13

81

N-S/Gr-1000

Pt/C

Fumapem® FAA-3

19.8

82

CoMn/pNGr(2:1)

Pt/C

Fumapem® FAA-3

35

83

Co9S8/G-500 (1:1)

Pt/C

Fumapem® FAA-3

31

84

CONCLUSION

Among the samples tested in this study, Fe–N–Co@C-800-AL possessed the largest specific surface area, highest amounts of functional nitrogen, and Co NPs wrapped in N-doped carbon, which contribute to its high ORR activity. When we conducted a stability test, the half-wave potential of Fe–N–Co@C-800-AL only decayed by 6 mV. This result can be ascribed to the active sites (Co or Fe NPs) embedded in graphitic carbon structures, which prevented the detachment of active sites during the stability test. Applied in single fuel cells, the maximum power density of Fe–N–Co@C-800-AL was higher than that of Co@C-800-AL.


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ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. XRD pattern, ORR results, N2 adsorption-desorption isotherms, XPS survey spectrums, XANES spectra and EXAFS spectra of Fe K-edge and Co K-edge and Fe K-edge EXAFS curve-fitting results. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. H. Wang) Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank financial support from the Ministry of Science and Technology of Taiwan (MOST 104-2628-E-011-003-MY3). The authors acknowledge the National Synchrotron Radiation Research Center (Beamline 17C1 and 24A), Hsinchu, Taiwan for X-ray absorption spectrum (XAS) analysis facility and X-ray photoelectron spectroscopy (XPS) analysis facility, respectively.



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Fe–N–Co@C-800-AL possesses the high surface area and the functional nitrogen can be favored for ORR and outstanding performance in AEMFC. 336x121mm (96 x 96 DPI)

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