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Metal-Organic-Framework-derived Fe-N/C Electrocatalyst with Fivecoordinated Fe-Nx Sites for Advanced Oxygen Reduction in Acid Media Qingxue Lai, Li Rong Zheng, Yan Yu Liang, Jianping He, Jingxiang Zhao, and Junhong Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02966 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017
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Metal-Organic-Framework-Derived Fe-N/C Electrocatalyst with Five-Coordinated Fe-Nx Sites for Advanced Oxygen Reduction in Acid Media Qingxue Lai,1,5 Lirong Zheng,2 Yanyu Liang,1,3,* Jianping He,1 Jingxiang Zhao,4 and Junhong Chen5,* 1
Jiangsu Key Laboratory of Materials and Technology for Energy Conversion,
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China 2
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese
Academy of Sciences, Beijing 100049, P. R. China 3
Jiangsu Collaborative Innovation Center for Advanced Inorganic Function
Composites, Nanjing 211816, P. R. China 4
College of Chemistry and Chemical Engineering, Harbin Normal University, Harbin
150025, P. R. China 5
Department of Mechanical Engineering, University of Wisconsin-Milwaukee,
Milwaukee, Wisconsin 53211, USA
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ABSTRACT: Even though Fe-N/C electrocatalysts with abundant Fe-Nx active sites have been developed as one of the most promising alternatives to precious metal materials for oxygen reduction reaction (ORR), further improvement of their performance requires precise control over Fe-Nx sites at the molecular level and deep understanding of the catalytic mechanism associated with each particular structure. Herein, we report a host-guest chemistry strategy to construct Fe-mIm nanoclusters (NCs) (guest)@zeolite imidazole framework-8 (ZIF-8) (host) precursors that can be transformed into Fe-N/C electrocatalysts with controllable structures. The ZIF-8 host network exhibits significant host-guest relationship dependent confinement effect for the Fe-mIm NCs during the pyrolysis process, resulting in different types of Fe-Nx sites with two- to five- coordinated configurations on the porous carbon matrix confirmed by X-ray absorption near edge structure (XANES) and Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) spectra. Electrochemical tests reveal that the five-coordinated Fe-Nx sites can significant promote the reaction rate in acid media, due to the small ORR energy barrier and the low adsorption energy of intermediate OH on these sites suggested by density functional theory (DFT) calculations. Such a synthesis strategy provides an effective route to realize the controllable construction of highly active sites for ORR at the molecular level.
KEYWORDS:
host-guest
chemistry;
confinement
effect;
Fe-Nx
sites;
five-coordinated configuration; oxygen reduction; electrocatalytic mechanism
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INTRODUCTION Clean and sustainable electric vehicles (EVs) have attracted growing attention due to widespread concerns on the increasing energy and environmental issues in the 21st century. Extensive application of EVs requires a stable, durable, and highly efficient dynamic power source system. Proton exchange membrane fuel cell (PEMFC) technology has been considered as one of the most promising power sources for EVs due to its great advantages over traditional fossil fuel-based energy sources, including huge energy and power density, high efficiency and no harmful emissions.1,
2
However, large-scale application of PEMFC in EVs has been enormously hindered by the unacceptable production cost mostly attributed to the use of massive Pt-group metal (PGM) catalysts, especially for the sluggish kinetically-controlled oxygen reduction reaction (ORR) on the cathode.3-5 In order to overcome this issue, development of cheap but highly efficient non-PGM ORR electrocatalysts in acidic media has been recognized as a potential strategy to reduce the production cost and to promote the commercialization of PEMFC technology in EVs.6-8 Among different types of non-PGM ORR electrocatalysts recently developed, Fe-N/C catalysts with iron and nitrogen co-doped into the carbon matrix have been recognized as one of the most promising alternatives to PGM materials for ORR due to their low cost as well as facile preparation.9-11 Extensive experimental results have demonstrated that the electrocatalytic activity of Fe-N/C in acidic solution mostly comes from the nitrogen-coordinated iron sites (Fe-Nx, x represents the coordination number) embedded in basal planes of carbon or bridging two graphene planes at their 3 ACS Paragon Plus Environment
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edges.12,
13
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Density functional theory (DFT) calculations further show that the
configuration of Fe-Nx (for example, coordination states) significantly affects electronic structures of the Fe center, which further affects the binding energy of reactants (O2), products (H2O), as well as intermediates (e.g., H2O2, OOH*, and OH*) with the Fe center, thereby leading to variations in electrocatalytic activity.14, 15 Conventional methods to construct highly active Fe-Nx sites are through rational pyrolysis of expensive iron macrocycle complexes (e.g., iron phthalocyanines, FePc) with definite Fe-N4 centers16,
17
which, however, turn to be uncertain new
configurations after the pyrolysis process and are almost impossible to control.18 This conspicuous drawback makes it hard to uncover the real active sites and thus understand the electrocatalytic mechanism, which are by now much desired to guide precise design and facile fabrication of highly efficient ORR electrocatalysts. Cao et al. reported a novel FePc electrocatalyst with an axial ligand anchored on single-walled carbon nanotubes fabricated via a chemical route instead of conventional pyrolysis paths.19 Experimental results and DFT calculations demonstrated that the rehybridization of Fe 3d orbitals with the axial ligand resulted in a significant change in electronic and geometric structure, leading to a greatly improved rate of oxygen reduction reaction in alkaline media. These interesting results suggest that controllable construction of definite Fe-Nx configuration in a carbon matrix is an effective way to improve the performance of Fe-N/C catalysts and promote the deep understanding of catalytic mechanism on particular Fe-Nx configuration. However, harsh conditions in the cathode of acid PEMFC require the 4 ACS Paragon Plus Environment
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definite Fe-Nx configuration strongly stabilized by the graphitic carbon plane, which cannot be achieved by a simple chemical reaction route. Thus, to develop a feasible high-temperature pyrolysis strategy to achieve controllable construction of Fe-Nx configurations embedded in a carbon matrix is much desired, which nevertheless remains a major challenge.20, 21 As
a
new
type
of
highly
porous
material,
metal-organic-frameworks
(MOFs)-derived nanostructures are recently widely investigated and applied in the field of electrochemical storage and conversion.22 Especially, their huge advantages of high surface areas with exceptionally abundant micropores, inherent presence of coordinated metal and heteroatoms, and tunable structure/composition at the molecular level make
them
ideal
precursors
for high-performance
ORR
electrocatalysts.23, 24 Herein, we proposed a host-guest chemistry strategy to construct Fe-mIm nanoclusters (NCs) (guest)@zeolite imidazole framework-8 (ZIF-8) (host) precursors that can be transformed into Fe-N/C electrocatalysts with controllable structures. The ZIF-8 host network exhibits significant host-guest relationship dependent confinement effect for the Fe-mIm NCs during the pyrolysis process, resulting in different types of Fe-Nx sites with two- to five-coordinated configuration on the porous nitrogen-doped carbon matrix. The obtained 5% Fe-N/C catalyst with abundant five-coordinated Fe-Nx sites displays a comparable current density of 5.12 mA/cm2 at 0.4 V vs. RHE with the commercial 30 wt.% Pt/C (5.81 mA/cm2) and a half potential (0.735 V) of only ~39 mV less than 30 wt.% Pt/C (0.774 V) in acid media. 5 ACS Paragon Plus Environment
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RESULTS AND DISCUSSION
Scheme 1. Illustration of the host-guest chemistry strategy to fabricate MOF-derived Fe-N/C catalysts. Conventional synthesis methods for MOF-derived Fe-N/C ORR electrocatalysts generally employed an iron macrocycle complex as an extra additive of effective precursor for Fe-Nx sites and/or a second heat treatment under NH3 to enhance the density of Fe-Nx sites.25, 26 Here we propose a host-guest chemistry strategy that can achieve high-performance Fe-N/C ORR electrocatalysts without using any expensive macrocyle complex or NH3 treatment. As shown in Scheme 1, during the construction of the ZIF-8 framework, Fe2+ ions were in situ introduced to the methanolic solution of Zn2+ and 2-mIm to form uniform Fe-mIm NCs that were well encapsulated by the simultaneously generated ZIF-8 framework [denoted as R% Fe-ZIF-8, where R represents the molar ratio of Fe2+/(Fe2++Zn2+)]. During the high temperature pyrolysis process (900 oC), the ZIF-8 host network exhibits significant confinement effect for the Fe-mIm NCs, which were more effectively transformed into highly active Fe-Nx sites instead of inactive inorganic phases like metallic Fe or its carbides. And such a 6 ACS Paragon Plus Environment
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confinement effect can be tuned by changing the ratio of the host and the guest components, leading to Fe-Nx sites with different states on the porous nitrogen-doped carbon matrix. After acid etching and second heat treatment, the final Fe-N/C catalysts were obtained. In contrast, the pure Fe-mIm synthesized without Zn2+ has a carbon yield of nearly zero after pyrolysis at 900 oC, probably due to the consumption of carbon by the intermediate product of iron-containing oxides.
Figure 1. (A) XRD pattern of pure ZIF-8, Fe-ZIF-8 with different densities of Fe-mIm NCs, and pure Fe-mIm; (B) XRD pattern of N/C and Fe-N/C from Fe-ZIF-8 with different densities of Fe-mIm NCs; (C-J) TEM images of (C, D) 5% Fe-N/C; (E, F) 10% Fe-N/C; (G, H) 15% Fe-N/C and (I, J) 25% Fe-N/C.
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We first constructed a series of MOF precursors with different densities of Fe-mIm NCs in the ZIF-8 framework through tuning the molar ratio of Fe2+/(Fe2++Zn2+) (See Experimental section in Supporting Information). X-ray diffraction (XRD) patterns in
Figure 1A show that pure ZIF-8 has a well-defined crystal structure completely consistent with previously reported results27, while pure Fe-mIm has an amorphous structure with a broad peak at around 26.0o. The 5% Fe-ZIF-8 sample shows the same diffraction peaks as those for pure ZIF-8 but with a significantly enhanced intensity, suggesting that the introduction of low concentration of Fe2+ doesn’t alter the structure of the ZIF-8 framework but enhances its crystallinity. As the R increases from 5% to 15%, these samples still maintain the typical structure of ZIF-8 while the diffraction intensity of these peaks gradually reduces, which should be attributed to the improved density of amorphous Fe-mIm NCs. When the R reaches 25%, typical peaks of ZIF-8 in obtained samples disappear and turn into a new broad peak at around 26.0o that is well consistent with the pattern of pure Fe-mIm, indicating that the MOF host has transformed from ZIF-8 to Fe-mIm. Moreover, transmission electron microscopy (TEM) images of these MOF precursors in Figure S1 demonstrate that the import of Fe2+ has a remarkable effect on their morphology. Under the low Fe2+ concentration of 5%, the ZIF-8 framework still keeps the typical polyhedral structure while its size expands to about 400 nm (vs. ~40 nm of pure ZIF-8). With the concentration of Fe2+ increasing to 10% and 15%, a yolk-shell structure with a further enlarged size is generated, probably originating from the thermodynamically controlled dissolution-recrystallization process due to the 8 ACS Paragon Plus Environment
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instability of the different building units derived from the different coordination numbers and geometries.28, 29 Ultimately a solid spherical structure with a size of about 300~400 nm is formed when the concentration of Fe2+ is high enough to transform the MOF host from ZIF-8 to Fe-mIm. These MOF precursors were facilely transformed into Fe-N/C ORR catalysts through pyrolysis at 900 oC under N2, followed by acid etching and second heat treatment under the same condition. XRD patterns of the as-prepared electrocatalysts in Figire 1B show that 5% Fe-N/C has two broad diffraction peaks centered at 25.3o and 43.7o corresponding to the partially graphitic carbon, indicating no obvious inorganic iron-containing phase is present. Nevertheless, non-ignorable metallic Fe and FeCx phases are observed in 10% Fe-N/C, 15% Fe-N/C and 25% Fe-N/C.30 The scanning electron microscopy (SEM) images (Figure S2A, B) of 5% Fe-N/C show a spherical structure with a rough surface, on which abundant mesoporous structure can be observed in the TEM images shown in Figure 1C, D. Distinctively, a nanosheet-assembled hierarchical shell with a hollow core is observed in the SEM and TEM images (Figure S2C, D and Figure 1 E, F) of 10% Fe-N/C, similar to its precursor. High-resolution TEM analysis (Figure S3) further reveals that the residual inorganic metal nanoparticles are well encapsulated with graphitic carbon shells. The inside metal nanoparticle has lattice fringes with a typical lattice spacing of 0.192 nm corresponding to the (112) plane of Fe3C (PDF#85-1317), suggesting some of the iron atoms exist as the Fe3C phase. For 15% Fe-N/C (Figure S2E, F and Figure 1G, H), its surface turns into relatively smooth, accompanied with a higher loading of inorganic 9 ACS Paragon Plus Environment
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metal nanoparticles than that of 10% Fe-N/C. Different from the mesoporous or hollow structure mentioned above, 25% Fe-N/C (Figure S2G, H and Figure 1I, J) shows a similar solid spherical structure with its precursor.
Figure 2. (A) N2 sorption isotherms at 77 K and (B) NLDFT pore size distribution of N/C, 5%/10%/15%/25% Fe-N/C; (C, D) high-resolution N1s XPS spectra of (C) 5% Fe-ZIF-8, N/C and 5% Fe-N/C, and (D) 5%/10%/15%/25% Fe-N/C; (E) high-resolution Fe2p XPS spectra of 5% Fe-ZIF-8 and 5%/10%/15%/25% Fe-N/C.
Significant differences were observed in the surface pore characteristics of the as-prepared electrocatalysts, measured by N2 adsorption/desorption measurements. As shown in Figure 2A and Table 1, the Brunauer–Emmett–Teller (BET) surface area of N/C is as high as 1,217.5 m2/g while the BET surface area of Fe-N/C gradually reduces from 1,036.4, 774.2, 736.3 to 551.9 m2/g with R increasing from 5%, 10%,
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15% to 25%. Furthermore, the obvious variation for the hysteresis loops in the N2 sorption isotherms indicates the significant difference of the channel distribution for these electrocatalysts, which is further confirmed by the nonlocal density functional theory (NLDFT) pore size distribution curves in Figure 2B. The predominant micropores for the as-prepared catalysts are centered at around 0.54 nm except for 5% Fe-N/C whose micropores are centered at around 0.75 nm and 1.20~1.50 nm, in which the highly active Fe-Nx sites are easier to be formed.31 Furthermore, these catalysts have typical mesopores centered at around 2.4~2.8 nm, with extra mesopores at 3.5~8.0 nm and 30~50 nm observed for 5% Fe-N/C and 10% Fe-N/C, respectively, which are beneficial to improve the mass transfer during ORR. As a result, the micropores occupy 69.4%, 61.3%, 50.9% and 24.9% of total surface area for 5% Fe-N/C, 10% Fe-N/C, 15% Fe-N/C and 25% Fe-N/C, respectively. To further analyze the states of Fe and N on the surface, X-ray photoelectron spectroscopy (XPS) spectra were then carried out. As shown in Figure 2C, the high-resolution N1s spectrum of 5% Fe-N/C can be well deconvoluted into five bonding types centered at 398.2, 399.0, 399.8, 400.8, and 402.7 eV. The peak centered at 398.2, 399.8, 400.8, and 402.7 eV can be assigned to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively, in agreement with the N states in N/C. But an extra peak centered at 399.0 eV in 5% Fe-N/C is observed which is similar to the peak of 5% Fe-ZIF-8, suggesting that part of Fe-Nx sites in the precursor survives after the high temperature pyrolysis.
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A comparison of N 1s spectra for Fe-N/C catalysts from the precursors with different content of Fe-mIm is shown in Figure 2D. The XPS results suggest that the content of Fe in the precursors plays an important role on the doping level of nitrogen. Increasing the Fe content significantly weakens the doping level from 11.46 at.% for N/C to 4.49 at.% for 5%Fe-N/C and continuously to 1.96 at.% for 25% Fe-N/C (Table 1).
Table 1. Summary of surface properties of as-prepared N/C and Fe-N/C catalysts obtained from BET and XPS. SBET Smicro 2 (m /g) (m2/g)
Smicro/ Pyridinic SBET N (at.%)
N/C
1217
854
70%
5% Fe-N/C
1036
720
10% Fe-N/C
774
15% Fe-N/C 25% Fe-N/C
Samples
Fe-Nx (at.%)
Pyrrolic N (at.%)
Graphitic N (at.%)
5.57
---
1.24
3.43
69%
1.37
0.55
0.53
1.43
475
61%
0.98
0.28
0.47
0.82
736
375
51%
0.72
0.17
0.31
0.78
552
137
25%
0.49
0.08
0.27
0.72
To understand the influence of Fe on the nitrogen doping level, phase changes of Fe in 25% Fe-ZIF-8 under different pyrolysis temperatures are investigated by XRD (Figure S4). It can be found that a distinct Fe3N phase is firstly formed under 500 oC, implying that abundant nitrogen atoms are extracted from the ZIF framework by iron to form inorganic metal nitride. This phase is stable under 600 oC but completely decomposed above 800 oC; at the same time, nitrogen atoms are released quickly from the sample in the form of N2 and/or NH3. Besides the nitrogen doping level, the part of nitrogen coordinated with Fe (Fe-Nx) on the surface of Fe-N/C is also reduced from 12.21 at.% (vs. total nitrogen content) 12 ACS Paragon Plus Environment
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for 5% Fe-N/C to 3.96 at.% for 25% Fe-N/C. As a result, the surface concentration of Fe-Nx is 0.55, 0.28, 0.17 and 0.08 at.% (vs. total atoms) for 5% Fe-N/C, 10% Fe-N/C, 15% Fe-N/C and 25% Fe-N/C, respectively. This significant reduction of Fe-Nx density is probably caused by the shrinking relative ratio of micropores with the increasing content of Fe. Notably, the similar trend is also observed for pyridinic N sites, manifesting that the Fe prefers to show a remarkable effect upon the two pyridinic N and Fe-Nx sites. Moreover, the Fe2p XPS spectra in Figure 2E show that the surface Fe content is 0.39 at.%, 0.47 at.%, 0.83 at.%, and 0.25 at.% for 5% Fe-N/C, 10% Fe-N/C, 15% Fe-N/C, and 25% Fe-N/C, respectively. It can be found that the surface Fe content increases with the increasing Fe content in the precursors from 5% to 15%. A smallest Fe content was found in 25% Fe-N/C, which should be attributed to the Fe-mIm as the host framework in the 25% Fe-ZIF-8, leading to large-sized inorganic Fe-containing nanoparticles that can be easily etched by acid. Because the large-sized Fe-mIm in ZIF-8 can be easily transformed into agglomerated inorganic Fe-containing nanoparticles, the surface content of Fe ions (FeII at 710.0±0.2 eV and FeIII at 711.9±0.2 eV), most of which are coordinated with N atoms, decreases with the increasing Fe content in the precursors from 5% to 25%. On the contrary, the percentage of Fe0 originated from metallic Fe and its carbides increases. As a result, the surface concentration of highly active Fe-Nx sites gradually reduces with increasing Fe content in the precursors, consistent with N1s XPS results.
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Figure 3. STEM images with corresponding elemental maps of N and Fe for 5% Fe-N/C (A), 10% Fe-N/C (B), 15% Fe-N/C (C), and 25% Fe-N/C (D). The
Scanning
Transmission
Electron
Microscopy
(STEM)
images
with
corresponding N and Fe mapping images in Figure 3 further demonstrate the space distribution states of N and Fe atoms in as-prepared Fe-N/C catalysts. The doped N atoms are distributed evenly in the carbon matrix while the distribution states of Fe atoms are obviously different for different Fe-N/C catalysts. For 5% Fe-N/C, a low-content but small-sized and evenly-distributed dots, most of which can be assigned to Fe atoms that are coordinated with N atoms, was observed. For 10% Fe-N/C, besides the small-sized dots, several large-sized and unevenly-distributed dots corresponding to aggregated inorganic Fe/FeCx nanoparticles appear. And the percentage of such large-sized and unevenly-distributed inorganic nanoparticles
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increases in 15% Fe-N/C. As for 25% Fe-N/C, the signal intensity from Fe element was weakened, and most of the Fe atoms were aggregated into several large Fe/FeCx nanoparticles with different sizes. The electrocatalytic performance of as-prepared catalysts was evaluated in 0.5 M H2SO4 with commercial 30 wt.% Pt/C catalyst (tested in 0.1 M HClO4) as reference. Figure 4A shows the Cyclic Voltammetry (CV) curves of N/C, 5% Fe-N/C and 30 wt.% Pt/C catalysts tested in O2 (solid line)- and N2 (dash line)-saturated solution. To visually observe the current response from ORR, the reduction segment of CV curves was corrected by subtracting the current in N2-saturated solution from that in O2-saturated one, as shown in Figure 4B. It can be found that N/C has a typical current response for electric double layer capacity instead of ORR whereas the 5% Fe-N/C catalyst exhibits a pronounced cathodic ORR peak at 0.720 V vs. Reversible Hydrogen Electrode (RHE) when the electrolyte is saturated with O2, closely approaching that of 30 wt.% Pt/C (0.806 V). The recorded linear sweep voltammetry (LSV) curvesof 5% Fe-N/C at various rotation rates in Figure 4C show a rapidly increased diffusion current density as the rotation rate increases. Furthermore, the corresponding K-L plots at different electrode potentials display good linearity, suggesting the first order reaction kinetics of ORR on this catalyst.
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Figure 4. (A) CV curves (50 mV/s, solid/dash line recorded in saturated O2/N2) of N/C, 5% Fe-N/C, and 30 wt.% Pt/C catalysts, and (B) their differential curves obtained by subtracting the current tested in N2-saturated solution from that in O2-saturated one; (C) LSV curves (10 mV/s) of 5% Fe-N/C at various rotation rates with the corresponding K-L plots; (D) LSV curves (10 mV/s) (bottom) and corresponding H2O2 yield (top) of N/C, 5% Fe-N/C, and 30 wt.% Pt/C; (E) LSV curves (10 mV/s, 1,600 rpm) (bottom) and corresponding H2O2 yield (top) of Fe-N/C with different Fe content in precursors; (F) Tafel plots of Fe-N/C with different Fe content in precursors obtained from (E) (The stars correspond to the Jk for
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as-prepared Fe-N/C catalysts at 0.7 V vs. RHE); (G) The relationship between E1/2 and different types of nitrogen content. Rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) LSV tests were then carried out for N/C, Fe-N/C, and 30 wt.% Pt/c catalysts to further illuminate their ORR activity and four-electron selectivity. The results are shown in Figure 4D, E and S5. The 5% Fe-N/C catalyst displays an onset potential of 0.861 V with a current density of 5.12 mA/cm2 at 0.40 V, which is much more positive than that of N/C (0.705 V, 1.95 mA/cm2) and close to that of the commercial 30 wt.% Pt/C (1.018 V, 5.82 mA/cm2). Meanwhile, the half-wave potential (E1/2) for 5% Fe-N/C is as high as 0.735 V, only 39 mV less than that of 30 wt.% Pt/C (0.774 V), further suggesting the high electrocatalytic activity for the former. Moreover, the H2O2 yield for 5% Fe-N/C is less than 1.0% at the potential region from 0.70 to 0.10 V, much lower than those of N/C and 30 wt.% Pt/C. As a result, the electron transfer number at 0.4 V vs RHE is 3.68, 3.98, and 3.89 for N/C, 5% Fe-N/C, and 30% Pt/C, respectively. These results strongly suggest that ORR on 5% Fe-N/C occurs through a high-efficiency nearly four-electron path. Further tests demonstrated that the content of Fe-mIm NCs in the ZIF-8 framework plays an important role on the electrocatalytic activity and four-electron selectivity of the obtained catalysts. The CV curves (Figure S6A) for as-prepared Fe-N/C catalysts tested in O2-saturated solution show a significant negative shift for the ORR peak position with the increasing Fe-mIm content. Meanwhile, an obvious pair of redox peak at around 0.64 V is observed in the CV curves (Figure S6B) tested in 17 ACS Paragon Plus Environment
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N2-saturated solution, probably corresponding to the Fe2+/Fe3+reduction/oxidation reaction, an important step for the ORR. It also can be found that such a reaction is gradually weakened with the increasing Fe-mIm content, caused by the decreased Fe-Nx site density. The same tendency is also observed for the E1/2 of them (Figure 4E bottom), shifting from 0.735 V for 5% Fe-N/C to 0.605 V for 25% Fe-N/C. On the other hand, the H2O2 yield is slightly elevated when the Fe2+ concentration is above 10% (Figure 3E top). In return, the electron transfer number is reduced slightly, making a small fraction of ORR through low-efficiency two-electron path. Moreover, the Tafel slope at the low overpotential region in Figure 4F also increases gradually from 55.6 mV/dec for 5% Fe-N/C to 68.7 mV/dec for 25% Fe-N/C,. Meanwhile, the kinetic current density at 0.7 V vs. RHE is 12.03, 7.57, 3.36, and 0.98 mA/cm2 for 5% Fe-N/C, 10% Fe-N/C, 15% Fe-N/C, and 25% Fe-N/C, respectively. These results obviously indicate the highest reaction rate of ORR on 5% Fe-N/C catalyst. As a result, the 5% Fe-N/C catalyst has the best electrocatalytic activity as well as four-electron selectivity. To understand the major factor that influences the as-prepared catalyst performance, the relationship between E1/2 and the surface content of different types of nitrogen was investigated, as shown in Figure 4G. It can be found that the surface content of Fe-Nx sites decreases faster than other types of nitrogen sites with the continuous negative shift of E1/2. This result suggests that Fe-Nx sites play a significant role in the electrocatalytic performance of as-prepared Fe-N/C catalysts in acid media. Of course,
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the reduced content of graphitic and pyridinic N also indicates that they play a non-negligible role in promoting the oxygen reduction process. Due to different catalytic activities for various Fe-Nx structures, it is necessary to resolve the structure of the Fe-Nx active sites in the as-prepared Fe-N/C catalysts by X-ray absorption near edge structure (XANES) and Fourier transform (FT) extended X-ray absorption fine structure (EXAFS) spectra. As shown in Figure 5A and S7A, it can be observed that the overall spectrum of 5% Fe-ZIF-8 is obviously different from that of the Fe foil and FePc but similar to that of Fe2O3, indicating that the Fe in Fe-mIm NCs has the similar oxidation state and coordination environment with that in Fe2O3. That’s probably caused by the oxidation of Fe2+ to Fe3+ during the synthesis of Fe-ZIF-8 under an air atmosphere. Distinctively, a clear shift of the absorption edge toward the lower energy region, close to that of FePc, is observed for 5% Fe-N/C in comparison with 5% Fe-ZIF-8, suggesting the similar structure states of Fe in 5% Fe-N/C and FePC. Furthermore, the intensity of the shoulder peak for 5% Fe-N/C at 7112.8 eV is obviously weaker than that for FePc, typical characteristics of square-planar D4h local symmetry of Fe ion surrounded by four-coordinated nitrogen ligands,32 suggesting a lower symmetrical and more complex coordination environment of Fe-Nx bonds in 5% Fe-N/C than in FePc. The Fe K-edge FT EXAFS spectra of the 5% Fe-ZIF-8 in Figure 5B reveals a major peak at approximately 1.50 Å, which is associated with the Fe-Nx bonding.33 Interestingly, the typical Fe-Nx peak still remains as the major peak in the FT EXAFS spectra of the 5% Fe-N/C, without an obvious signal at ~2.20 Å which corresponds to 19 ACS Paragon Plus Environment
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inorganic Fe-Fe bonds. These results suggest that most of the Fe-Nx sites in 5% Fe-ZIF-8 maintain their coordination structures instead of being transformed into inorganic phases after the high-temperature pyrolysis. A comparison of XANES spectra in Figure 5C and S7B shows a gradual shift toward the lower energy region for absorption edge from 5% Fe-N/C to 25% Fe-N/C, indicating that the oxidation state of Fe in Fe-N/C catalysts is reduced significantly with the increasing concentration of Fe-mIm NCs in the precursors. Meanwhile, the signal peak intensity of Fe-Fe bonds in Figure 5D is enhanced progressively with the same changes, suggesting the transformation of partial Fe-N bonds in the Fe-ZIF-8 to inorganic phases. These results reveal that such host-guest chemistry strategy can successfully protect the Fe-Nx bonds from transformation into inorganic phases under a low concentration of Fe-mIm NCs while a high concentration of them exceeds the confinement capacity of the ZIF-8 framework, leading to the formation of inorganic Fe-contained phases (Figure 5E).
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.
Figure 5. Fe K-edge XANES spectra (A, C) and FT EXAFS spectra (B, D) for (A, B) 5% Fe-N/C, 5% Fe-ZIF-8, FePc, Fe2O3 and Fe foil and (C, D) 5% Fe-N/C, 10% Fe-N/C, 15% Fe-N/C, 25% Fe-N/C and Fe foil; (E) illustration of the host-guest roles on the states of Fe in the resulting Fe-N/C catalysts. The coordination number and bond length of Fe-Nx can be obtained via curve-fitting the FT EXAFS spectra using FEFFIT (Figure S8). The 5% Fe-ZIF-8 shows typical six-coordinated Fe-Nx bonds with a bond length of 2.015 Å while five-coordinated Fe-Nx bonds with a bond length of 1.988 Å are observed in the 5% Fe-N/C. Furthermore, the coordination number is reduced to 2.4 for 10% Fe-N/C and 21 ACS Paragon Plus Environment
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1.3 for 15% Fe-N/C, accompanied with a shortened bond length of 1.969 and 1.966 Å, respectively. However, a high coordination number (2.2) and long bond length (2.057 Å) of Fe-Nx bonds are observed in 25% Fe-N/C. Considering the low content of Fe-N bonds, the obtained coordination number and bond length for 25% Fe-N/C are not exact and higher than that in reality due to the influence of high-content Fe-Fe bonds with high coordination number and bond length. These results manifest that, with the increasing Fe-mIm NCs concentration, the protection capacity for Fe-Nx configuration from ZIF-8 framework during the pyrolysis process is weakened, which leads to a reduced coordination number gradually for the survived Fe-Nx bonds. A consensus has been reached that different coordination structure of Fe-Nx configuration will result in distinct electrocatalytic performance for ORR.34 Here, combined with the electrocatalytic performance of our prepared Fe-N/C catalysts, it can be believed that the five-coordinated Fe-Nx configuration can promote the ORR more effectively than that with low coordination number in acid media. To prove this hypothesis, three typical Fe-Nx models with different coordination number, N-Fe-N4, Fe-N4 and Fe-N2 were built to conduct the spin-polarized DFT calculation. Based on the steady-state optimization, side-on adsorption conformation of O2 consistently binds more strongly than the end-on for N-Fe-N4 and Fe-N4 structures, with an opposite result for Fe-N2 structure. Figure 6A-C show the optimized structure of (i) O2, (ii) OOH, (iii) O, and (iv) OH adsorbed on (A) N-Fe-N4, (B) Fe-N4, and (C) Fe-N2, respectively. Figure 6D summarizes the trends of the calculated free energy for each
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intermediate reaction step relative to the initial state at 0 V and balance state at 1.23 V, respectively.
Figure 6. (A-C) Optimized structure of (i) O2, (ii) OOH, (iii) O, and (iv) OH adsorbed on (A) N-Fe-N4, (B) Fe-N4, and (C) Fe-N2; (D) Free-energy diagrams of the reduction of O2 to H2O on the N-Fe-N4, Fe-N4, and Fe-N2 structures in acid media. A decreasing free energy for all successive intermediate steps up to OH* was observed on the N-Fe-N4 and Fe-N4 structures at 0 V, suggesting that the ORR is easier to occur on these two structures than on the Fe-N2 structure. Furthermore, when a potential of 1.23 V was applied, an uphill process corresponding to an adsorption reaction was observed on the three structures, resulting in an obvious reaction energy barrier. Importantly, the N-Fe-N4 structure shows a lower energy barrier (0.67 eV) than Fe-N4 (0.75 eV) or Fe-N2 (1.99 eV), making O2 molecules easier to be 23 ACS Paragon Plus Environment
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completely reduced to H2O. On the other hand, the electrocatalytic activity of Fe-Nx configuration can be also measured by the adsorption energy of OH. As shown in Figure S9, the N-Fe-N4 structure exhibits an adsorption energy of OH of 2.88 eV, which is very close to that of Pt (111) surface (2.23 eV) and much lower than that of Fe-N4 (3.07 eV) or Fe-N2 (4.38 eV). Such a low adsorption energy of OH can protect the active sites from deactivation via permanent adsorption of OH and/or oxidization of the metal center by the OH or generated radicals.
CONCLUSION We demonstrated an effective host-guest chemistry strategy to fabricate high-performance MOF-derived Fe-N/C catalysts for ORR in acid media. With the host-guest relationship dependent confinement effect, the Fe-mIm NCs can be effectively transformed into Fe-Nx active sites with controllable coordination states instead of inactive inorganic phases. Experimental results and DFT calculations show that the definite Fe-Nx sites with five-coordinated configuration can significantly promote the ORR rate in acid media via narrowing the reaction energy barrier and reducing the adsorption energy of intermediate OH, resulting in excellent electrocatalytic activity and selectivity. Such a synthesis strategy provides a possibility to construct structure-definite Fe-Nx active sites for deeply revealing their structure-electrocatalytic performance relationship. ASSOCIATED CONTENT
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Supporting Information. Details of synthesis, physical and electrochemical characterizations of Fe-N/C catalysts, DFT calculation process, TEM images of Fe-ZIF-8, SEM images of Fe-N/C, XRD patterns of 25% Fe-ZIF-8 heated at different temperatures and additional electrochemical, XAFS, EXAFS, and DFT data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected] (Y. Liang)
[email protected] (J. Chen).
Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21273114), Natural Science Foundation of Jiangsu Province (Grant No. BK20161484), the Fundamental Research Funds for the Central Universities (Grant No. NE2015003) and the “Six Talent Peaks Program” of Jiangsu Province (Grant No. 2013-XNY-010), a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD).
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Table of Contents
An efficient host-guest chemistry strategy was developed to fabricate metal-organic-framework-derived Fe-N/C electrocatalysts with abundant five-coordinated Fe-Nx sites that show excellent electrocatalytic oxygen reduction performance in acid media.
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