From Metal–Organic Frameworks to Single-Atom Fe Implanted N-doped Porous Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic Media

Article abstract of DOI:10.1002/anie.201803262

It remains highly desired but a great challenge to achieve atomically dispersed metals in high loadings for efficient catalysis. Now porphyrinic metal–organic frameworks (MOFs) have been synthesized based on a novel mixed-ligand strategy to afford high-content (1.76 wt %) single-atom (SA) iron-implanted N-doped porous carbon (Fe_SA-N-C) via pyrolysis. Thanks to the single-atom Fe sites, hierarchical pores, oriented mesochannels and high conductivity, the optimized Fe_SA-N-C exhibits excellent oxygen reduction activity and stability, surpassing almost all non-noble-metal catalysts and state-of-the-art Pt/C, in both alkaline and more challenging acidic media. More far-reaching, this MOF-based mixed-ligand strategy opens a novel avenue to the precise fabrication of efficient single-atom catalysts.

Full text of DOI:10.1002/anie.201803262

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Accepted Article
Title: From MOFs to Single Fe Atoms Implanted N-doped Porous
Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic
Media
Authors: Hai-Long Jiang, Long Jiao, Gang Wan, Rui Zhang, Hua Zhou,
and Shu-Hong Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803262
Angew. Chem. 10.1002/ange.201803262
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10.1002/anie.201803262
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COMMUNICATION
From MOFs to Single-Atom Fe Implanted N-doped Porous
Carbons: Efficient Oxygen Reduction in Both Alkaline and Acidic
Media
Long Jiao+, Gang Wan+, Rui Zhang, Hua Zhou, Shu-Hong Yu, and Hai-Long Jiang*
Dedication ((optional))
Abstract: It remains highly desired but a great challenge to achieve
atomically dispersed metals in high loadings for efficient catalysis.
Herein, for the first time, we have developed porphyrinic metal-
organic frameworks (MOFs) based on a novel mixed-ligand strategy
to afford high-content (1.76 wt%) single-atom (SA) Fe implanted N-
doped porous carbon (FeSA-N-C) via pyrolysis. Thanks to the single-
atom Fe sites, hierarchical pores, oriented mesochannels and high
conductivity, the optimized FeSA-N-C exhibits excellent oxygen
reduction activity and stability, surpassing almost all non-noble-metal
catalysts and state-of-the-art Pt/C, in both alkaline and more
challenging acidic media. More far-reaching, this MOF-based mixed-
ligand strategy opens a novel avenue to the precise fabrication of
efficient single-atom catalysts.
candidates. MOFs have been demonstrated to be great precur-
sors/templates to produce porous carbon-based materials via
pyrolysis.[7,8] More importantly, their periodic structures give rise
to spatial separation of building units and thus inhibit potential
agglomeration of metal sites during pyrolysis, enabling MOFs
ideal precursors to create SACs. Currently, the reports on MOF-
pyrolyzed materials are mostly focused on the regulation of their
pore features and compositions.[7d-g,8] The precise fabrication of
active sites at molecule level based on MOF precursors has
been rarely reported. Very recently, zeolitic imidazolate frame-
works (ZIFs) were reported as precursors to produce SACs
based on structural tailorability, high nitrogen content and pore
confinement effect.[4b,9] Unfortunately, the microporous feature of
ZIFs and their derivatives hamper mass transfer and the
accessibility of active site.[9b] Therefore, it is imperative to
develop SACs (providing efficient active sites) with hierarchical
pores (benefitting mass transfer and active site accessibility) via
rational design of MOFs for improved catalytic performance.
With the above in mind, a porphyrinic MOF, PCN-222 (also
called MOF-545 or MMPF-6),[10] featuring 1D mesochannels with
Single-atom catalysts (SACs), a new research frontier in hetero-
geneous catalysis, have been intensively investigated in diverse
reactions, including electrocatalysis, photocatalysis, organic ca-
talysis, etc.[1-4] Given that only the coordinatively unsaturated
metal atoms on the surface of solid catalysts contribute to the
catalytic process, SACs would realize the utmost utilization of
metal sites and thus greatly benefit the catalytic efficiency.[2]
Furthermore, SACs are able to overcome the complicated multi-
components in traditional supported metal catalysts and
regarded as ideal model catalysts to identify active sites and
understand reaction process at a molecular level.[2,3] Though
great progress has been achieved, the rational fabrication of
SACs with high metal loadings (>1 wt%) remains a grand
challenge.[4] In addition to the introduction of more active metals,
the optimized pore structure, large surface area and excellent
conductivity in SACs, which would facilitate mass/electron
transfer, are also desirable to improve the activity.[5]
a
diameter of 3.2 nm (Scheme S1), was chosen as a
representative precursor. The ratio modulation between Fe-
TCPP (TCPP = tetrakis (4-carboxyphenyl)porphyrin) and H2-
TCPP mixed ligands affords a series of isostructural MOFs,
denoted as Fex-PCN-222 (x%: molar percentage of Fe-TCPP in
both ligands) (Scheme S2). The assembly of Fe-TCPP into 3D
networks of Fex-PCN-222 effectively inhibits the molecular
stacking, and the mixed-ligand strategy further expands the
distance of adjacent Fe-TCPP ligands. Upon pyrolysis, the
optimized Fe20-PCN-222 can be converted into single-atom (SA)
Fe sites implanted porous N-doped carbon, denoted as FeSA-N-
C, with the unique mesopore character (Scheme 1). By
integrating single Fe atoms (highly active sites) and hierar-
chically porous structure with oriented mesopores (facilitating
active site access and mass transfer), the optimized FeSA-N-C
exhibits superb oxygen reduction activity and ultrahigh stability,
surpassing almost all reported non-noble-metal catalysts and the
To meet these challenges, metal-organic frameworks
(MOFs),[6] a class of crystalline porous materials constructed by
metal ions/clusters and versatile organic linkers, are promising
[*]
L. Jiao,[+] R. Zhang, Prof. Dr. S.-H. Yu, Prof. Dr. H.-L. Jiang
Hefei National Laboratory for Physical Sciences at the Microscale,
CAS Key Laboratory of Soft Matter Chemistry, Collaborative
Innovation Center of Suzhou Nano Science and Technology,
Department of Chemistry, University of Science and Technology of
China, Hefei, Anhui 230026 (P.R. China)
E-mail: jianglab@ustc.edu.cn
Homepage: http://staff.ustc.edu.cn/~jianglab/
G. Wan[+]
Materials Science Division, Advanced Photon Source, Argonne
National Laboratory, Lemont, Illinois 60439 (USA)
G. Wan,[+] Dr. H. Zhou
X-ray Science Division, Advanced Photon Source, Argonne National
Laboratory, Lemont, Illinois 60439 (USA)
These authors contributed equally to this work.
[+]
Scheme 1. Illustration of the rational fabrication of single Fe atoms-involved
FeSA-N-C catalyst via a mixed-ligand strategy.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201803262
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state-of-the-art Pt/C, under both alkaline and acidic conditions.
A series of Fex-PCN-222, including Fe0-PCN-222, Fe20-PCN-
222 and Fe40-PCN-222, have been prepared and all present
similarly high surface areas with uniform pore size distribution
(Figure S1, S2). Taking Fe20-PCN-222 as a representative, its
typical type-IV N2 isotherms suggest a Brunauer-Emmett-Teller
five characteristic peaks, pyridinic N (398.5 eV), Fe-Nx (399.2
eV), pyrrolic N (400.3 eV), graphitic N (401.2 eV) and oxidized N
(402.7 eV), in which the Fe-Nx peak suggests the good
inheritance of Fe-Nx units from Fe-TCPP after pyrolysis (Figure
2a). High-resolution Fe 2p spectrum with two relatively weak
peaks centered at 710.7 eV (Fe 2p3/2) and 723.1 eV (Fe2p1/2
)
(BET) surface area as high as 2062 m2/g and two types of pores, illustrates partially oxidized Fe species in FeSA-N-C, further
with sizes of 1.2 nm and 3.2 nm, corresponding to the triangular
microchannels and hexagonal mesochannels, respectively. SEM
and TEM images present the uniform rod-shaped morphology
with a diameter of ~200 nm (Figure 1a, S3a). The highly oriented
mesopores with a size of ~3.2 nm along the c-axis can be well
identified in the TEM image (Figure 1a, inset), in good
agreement with the size distribution analysis above (Figure S2).
Upon pyrolysis at 800 °C , FeSA-N-C/ZrO2 composite with two
ZrO2 phases has been obtained (Figure S4). Upon subsequent
ZrO2 removal, Fe20-PCN-222 can be finally converted to the
FeSA-N-C without any identifiable metallic particle, and the rod
shape and highly oriented mesopores in Fe20-PCN-222 are
supporting the existence of Fe-Nx species (Figure S8b).[8c] The
existence of abundant N (4.67 wt%) and a high loading of Fe (up
to 1.76 wt%), strikingly consistent with XPS results, have been
quantified by elemental analysis and inductively coupled plasma
atomic emission spectrometry (ICP-AES) (Table S1, S2).
Aberration-corrected HAADF-STEM observation unambiguously
identifies single Fe atoms from the bright spots in randomly
selected areas, but no Fe particle is found (Figure 1f, S9).
In addition to the localized microstructrual information above,
X-ray absorption spectroscopy (XAS) was further studied to
unveil the electronic and structural information of Fe-involved
species in FeSA-N-C. From the Fe K-edge XANES spectra, the
inherited to the resultant FeSA-N-C to a large extent (Figure 1b-e). energy absorption threshold of FeSA-N-C locates between Fe foil
N2 sorption for FeSA-N-C suggests its high BET surface area of
532 m2/g and the evident hysteresis loop (P/P0: 0.6-0.8)
indicates its mesoporous structure (Figure S5a), which is recog-
nizable in the cross-section SEM and TEM images (Figure 1c-e,
S3b). The pore size distribution analysis further manifests its
hierarchical pores in 1-2 nm and ~9 nm diameters (Figure S5b).
It is generally believed that micropores would increase the
density of active sites and mesopores are beneficial to the mass
transfer, thus improving catalytic activity.[5d,9b]
Powder X-ray diffraction (PXRD) pattern of FeSA-N-C presents
two broad diffraction peaks in the ranges of 20-30° and 40-45°,
respectively corresponding to the (002) and (101) reflections of
graphitized carbon, and no peak associated with iron-based
species can be detected (Figure S6). Raman scattering
spectrum for FeSA-N-C gives low intensity ratio (ID/IG = 0.95) of D
band (~1345 cm-1) and G band (~1590 cm-1), further indicating
its high graphitization degree, which favors the electron transfer
(Figure S7). X-ray photoelectron spectroscopy (XPS) results
reveal that C, N, O and Fe elements are included in FeSA-N-C
(Figure S8a). High-resolution N 1s spectrum can be fitted into
and Fe2O3 and is closer to that of FeTPPCl and FePc, implying
the positively charged Feδ+ stabilized by N atoms in FeSA-N-C
(Figure 2b). Furthermore, the absence of the edge peak at
~7117.1 eV, which is the fingerprint of D4h symmetry, suggests
the lower symmetry of Fe-Nx sites in FeSA-N-C.[3b] FT-EXAFS
spectrum of FeSA-N-C at Fe K-edge presents only a main peak
at ~1.44 Å, which is attributed to the Fe-N scattering path, and
no Fe-Fe bond at ~2.13 Å can be detected, manifesting that the
evolution of single-atom Fe sites in FeSA-N-C (Figure 2c). To
further verify the coordination structure around Fe centers,
EXAFS fitting has been performed (Figure 2d, S10, Table S3).
The best fitting result for the first shell shows that each Fe atom
is coordinated by ~4 N atoms in average, illustrating that all
atomic Fe sites are four-coordinated by nitrogen species (Figure
2d, inset). It is noteworthy that when the molar percentage of Fe-
TCPP increases to 40% in PCN-222, Fe-Fe peak is identifiable
in the resultant catalyst (denoted as FeNP-N-C), manifesting that
Fe atoms might aggregate to nanoparticles (NPs) upon pyrolysis
Figure 2. (a) High-resolution XPS spectrum of N 1s for FeSA-N-C. (b) Fe K-
edge X-ray absorption near-edge structure (XANES) and (c) Fourier
transform-extended X-ray absorption fine structure (FT-EXAFS) spectra of
FeSA-N-C, 5,10,15,20-tetraphenylporphineiron(III) chloride (FeTPPCl) and iron
phthalocyanine (FePc), Fe2O3 and Fe foil. (d) EXAFS fitting for FeSA-N-C
(inset: schematic model of FeSA-N-C. The red, blue and grey spheres
represent Fe, N and C atoms, respectively).
Figure 1. (a) Scanning electron microscopy (SEM) image of Fe20-PCN-222
(inset: transmission electron microscopy (TEM) image showing the
mesochannels in Fe20-PCN-222). (b) SEM and (c) TEM images of FeSA-N-C.
(d) Cross-section SEM, (e) enlarged TEM images of the mesoporous structure
and (f) aberration-corrected high-angle annular darkfield scanning
transmission electron microscope (HAADF-STEM) images of FeSA-N-C.
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10.1002/anie.201803262
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with a higher percentage of Fe-TCPP, in consistent with high-
resolution TEM observation results (Figure S11).
mA/cm2), signifying its much more favorable kinetics (Figure 3b).
The LSV curve of FeSA-N-C/ZrO2 was also obtained and its
inferior performance to FeSA-N-C illustrates the importance of HF
etching (Figure S13). In addition, FeSA-N-C also shows better
activity than mix-Fe-N-C which is derived from the physical
mixture of Fe-TCPP and H2-TCPP, further demonstrating the
superiority of MOFs as precursors (Figure S13). All above
results suggest that FeSA-N-C possesses the optimal activity.
To gain more insight into the electron-transfer mechanism of
FeSA-N-C, LSV curves at different rotating rates of rotating disk
electrode (RDE) were recorded (Figure 3c). The K-L plots ob-
tained from the LSV curves exhibit excellent linearity, presenting
the first order reaction kinetics for ORR with a potential-
independent electron transfer rate (Figure 3c, inset). Based on
the K-L equation, the electron transfer number is determined to
be ~4.0, manifesting an ideal 4e- ORR mechanism. Rotating ring
disk electrode (RRDE) measurement for FeSA-N-C was further
performed to show the very low (<5.5%) H2O2 yields with an
electron transfer number larger than 3.9 (Figure S14), similar to
the results from the K-L plots. In addition, the negligible decay of
activity after stability tests and the methanol addition
experiments for FeSA-N-C demonstrate its excellent durability
and strong tolerance against methanol-crossover effect, in stark
contrast to the significant decline of Pt/C (Figure 3d, S15). The
results suggest that FeSA-N-C possesses the best ORR
performance among all non-noble metal catalysts ever reported
under alkaline condition (Figure 3a-d, Table S4).
The excellent ORR performance of FeSA-N-C in alkaline media
prompted our further exploration under the more challenging
acidic condition. Strikingly, superior ORR activity with a E1/2 of
0.776 V in 0.1 M HClO4 can be achieved for FeSA-N-C, which is
much higher than that of Fe0-N-C, FeNP-N-C, FeSA-N-C/ZrO2,
mix-Fe-N-C and even comparable to (5 mV lower) that of
benchmark Pt/C (Figure 3e, S16 and S17). The high kinetic
current density (9.60 mA/cm2) at 0.75 V manifests the favorable
kinetics of FeSA-N-C (Figure S16b). Further RRDE test suggests
a H2O2 yield lower than 1.0% with an ideal 4e- transfer process,
in consistence with the result from K-L plots (Figure S18).
Moreover, FeSA-N-C exhibits excellent durability with only 6 mV
decay after 5000 cycles and 4% drop after i-t test, which is much
better than the Pt/C, possibly due to the strong affinity between
atomic Fe sites and coordination N within carbon (Figure
The above results prove that Fe20-PCN-222 might possess an
appropriate Fe-TCPP content (20 mol%) to prepare single Fe
atoms. The fabrication mechanism for single Fe atoms and Fe
NPs has been proposed (Scheme S3). Firstly, the mixed ligands
of H2-TCPP and Fe-TCPP (Fe is coordinated by N atoms) are
employed to construct PCN-222. Its 3D skeleton composed of
metal nodes and organic ligands creates the average distance
(d) of adjacent Fe-TCPP ligands by simply changing the ratio of
the mixed ligands. When the distance reaches to a critical value
(for example, in Fe20-PCN-222), the formation of Fe NPs is
effectively inhibited in pyrolysis process. Upon the ZrO2 removal,
single Fe atoms anchored N-doped carbon can be successfully
obtained (Scheme S3b). Conversely, when more Fe-TCPP is
loaded and Fe…Fe distance is shortened, Fe aggregation would
be inevitable during the MOF pyrolysis (Scheme S3c).
In view of the intrinsic activity of Fe-N4 sites for efficient oxy-
gen reduction reaction (ORR),[3,11] the performance of FeSA-N-C
was first investigated in 0.1 M KOH. The CV curve of Fe0-N-C
sample (obtained from Fe0-PCN-222) without Fe doping displays
a much lower oxygen reduction peak than that of FeSA-N-C and
FeNP-N-C, reflecting an indispensable role of Fe-N sites on
boosting ORR activity (Figure S12). Linear sweep voltammetry
(LSV) curves indicate the best performance of FeSA-N-C among
all related catalysts, featuring a higher half-wave potential (E1/2
=
0.891 V) than Fe0-N-C (0.795 V), FeNP-N-C (0.889 V), and the
commercial Pt/C (0.848 V) (Figure 3a,b). In addition, the superb
activity of FeSA-N-C is supported by the much higher kinetic
current density (Jk) at 0.85 V (23.27 mA/cm2) than that of Fe0-N-
C (0.98 mA/cm2), FeNP-N-C (18.43 mA/cm2) and Pt/C (5.61
S19a,c,e). Additionally, FeSA-N-C demonstrates
a superior
tolerance to the crossover effect of methanol compared with
Pt/C, making it a promising candidate for the direct application of
methanol fuel cells (Figure S19b,d,f). Furthermore, a preliminary
constant-current test for FeSA-N-C was conducted in real fuel cell
device (Figure S20). It is noteworthy that, although much
endeavor has been devoted, very limited catalysts were reported
simultaneously possessing comparable performance with Pt/C in
both alkaline and acidic media, further highlighting the
conspicuous advantage of the single Fe atom-involved FeSA-N-C
over other reported catalysts (Table S5).
To understand the origin of the excellent ORR performance of
FeSA-N-C, more investigations were carried out. The SCN- (0.01
M), with strong affinity to Fe ion, was employed as a probe to
poison Fe-N4 sites.[9c] As a result, the half-wave potential of
FeSA-N-C for ORR in 0.1 M HClO4 decreased significantly by 50
mV (Figure 3f). The SCN--poisoned electrode was subsequently
Figure 3. (a) LSV curves and (b) E1/2 and Jk at 0.85 V for various catalysts in
0.1 M KOH. (c) LSV curves of FeSA-N-C at different rotating rates in 0.1 M
KOH (inset: K-L plots and electron transfer number). (d) Stability test of FeSA
-
N-C (inset: methanol tolerance test) in 0.1 M KOH. (e) LSV curves of various
catalysts in 0.1 M HClO4. (f) LSV curves of FeSA-N-C in 0.1 M HClO4 before
and after the addition of SCN-. (g) SAXS pattern of FeSA-N-C.
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10.1002/anie.201803262
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rinsed with pure water and re-measured in 0.1 M KOH. The E1/2
of the first LSV curve negatively shifts by 10 mV as compared to
the original catalyst. With increasing cycle numbers, the LSV
curves gradually overlap with the original one obtained before
SCN- treatment, which should be ascribed to the desorption of
SCN- from Fe site in 0.1 M KOH (Figure S21). The activity
recovery accompanied by the release of SCN- from blocked Fe-
N4 sites clearly reflects that single Fe atoms are responsible for
the high ORR activity of FeSA-N-C.
Advanced Photon Source is supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences,
under Contract No. DE-AC02-06CH11357. We are grateful to
the reviewers for valuable suggestions, and Dr. Chenxi Xu at
Hefei Univ. Technol. for great help on the test of fuel cell device.
Keywords: metal-organic frameworks • single-atom catalysts •
porous carbon • oxygen reduction reaction
In addition, the hierarchically porous structure in FeSA-N-C is
generally believed to affect the accessibility of active sites and
mass transfer in catalytic process. Small angle X-ray scattering
(SAXS) for FeSA-C-N gives a broad hump between 0.28 to 0.84
nm-1, hinting the ordering of pore structure with size distribution
from 7 to 22 nm (Figure 3g), in good agreement with the above
microstructure observation and N2 sorption data (Figure 1d,e
and S5). It should be pointed out that, though reported MOF-
derived carbon materials exhibit high surface areas, their pore
structures are usually disordered and non-interconnected after
uncontrollable pyrolysis, which is not favorable to mass transfer
in catalysis.[7e,8e] Therefore, the development of porous carbon
with oriented channels, which remains to be a long-term target
yet a great challenge to date, has been achieved for the first
time. The hierarchically porous FeSA-N-C, possessing abundant
micropores (accommodating high-density Fe-N4 active sites) and
open mesoporous channels with particular orientation
(facilitating the high-flux mass transfer), would greatly accelerate
the ORR process (Figure S22). This is in sharp contrast to
sluggish mass transfer in traditional MOF-derived carbons with
non-interconnected pores. Taking jointly the above Fe-N4 active
sites and pore structure merits, we may come to the conclusion
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that single-atom Fe sites accessible by hierarchal pores in FeSA
-
N-C mainly contribute to the superb ORR performance.
In summary, we have developed
a novel mixed-ligand
strategy in MOF system to fabricate SACs. The ratio
optimization of the mixed porphyrin ligands with and without FeIII
centers gives rise to long spatial distance of FeIII ions in the MOF
skeleton, favoring the formation of single Fe atoms upon
pyrolysis. Benefiting from periodic and tailorable MOF structures,
the spatial distance control of FeIII in porphyrinic MOFs
effectively suppresses the Fe aggregation during pyrolysis,
leading to atomic dispersion of Fe with a high loading (1.76 wt%)
in porous carbon. Thanks to the single-atom Fe sites (superior
activity), hierarchically porous structure (accessible active sites)
with oriented mesopores (fast diffusion of O2 and electrolyte)
and high conductivity (fast electron transfer), FeSA-N-C
demonstrates excellent ORR performance in both alkaline and
acidic media, surpassing any other reported non-noble-metal
catalysts and even the Pt/C. In light of the tremendous diversity
and tailorability of MOFs, this work opens up an avenue to the
rational synthesis of efficient SACs via multiscale control.
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10.1002/anie.201803262
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COMMUNICATION
COMMUNICATION
Based on a novel mixed-ligand
strategy, a porphyrinic MOF was
pyrolyzed to afford high-content
single-atom (SA) Fe implanted N-
doped porous carbon (FeSA-N-C).
Thanks to the FeSA sites, hierarchical
pores, oriented mesochannels and
Long Jiao+, Gang Wan+, Rui Zhang, Hua
Zhou, Shu-Hong Yu, and Hai-Long
Jiang*
Page No. – Page No.
From MOFs to Single-Atom Fe
Implanted N-doped Porous Carbons:
Efficient Oxygen Reduction in Both
Alkaline and Acidic Media
high conductivity, the optimized FeSA
N-C exhibits excellent oxygen
reduction activity and stability,
-
surpassing almost all non-noble-metal
catalysts and the Pt/C, in both alkaline
and more challenging acidic media.
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