Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on Single Iron Atoms Dispersed on a Hierarchically Structured Porous Carbon Framework

Article abstract of DOI:10.1002/anie.201804958

Single Fe atoms dispersed on hierarchically structured porous carbon (SA-Fe-HPC) frameworks are prepared by pyrolysis of unsubstituted phthalocyanine/iron phthalocyanine complexes confined within micropores of the porous carbon support. The single-atom Fe catalysts have a well-defined atomic dispersion of Fe atoms coordinated by N ligands on the 3D hierarchically porous carbon support. These SA-Fe-HPC catalysts are comparable to the commercial Pt/C electrode even in acidic electrolytes for oxygen reduction reaction (ORR) in terms of the ORR activity (E_1/2=0.81 V), but have better long-term electrochemical stability (7 mV negative shift after 3000 potential cycles) and fuel selectivity. In alkaline media, the SA-Fe-HPC catalysts outperform the commercial Pt/C electrode in ORR activity (E_1/2=0.89 V), fuel selectivity, and long-term stability (1 mV negative shift after 3000 potential cycles). Thus, these nSA-Fe-HPCs are promising non-platinum-group metal ORR catalysts for fuel-cell technologies.

Full text of DOI:10.1002/anie.201804958

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International Edition: DOI: 10.1002/anie.201804958
German Edition: DOI: 10.1002/ange.201804958
Oxygen Reduction Reaction
Hot Paper
Efficient Oxygen Reduction Reaction (ORR) Catalysts Based on
Single Iron Atoms Dispersed on a Hierarchically Structured Porous
Carbon Framework
Zhengping Zhang, Junting Sun, Feng Wang,* and Liming Dai*
Abstract: Single Fe atoms dispersed on hierarchically struc-
tured porous carbon (SA-Fe-HPC) frameworks are prepared
by pyrolysis of unsubstituted phthalocyanine/iron phthalocya-
nine complexes confined within micropores of the porous
carbon support. The single-atom Fe catalysts have a well-
defined atomic dispersion of Fe atoms coordinated by
N ligands on the 3D hierarchically porous carbon support.
These SA-Fe-HPC catalysts are comparable to the commercial
Pt/C electrode even in acidic electrolytes for oxygen reduction
reaction (ORR) in terms of the ORR activity (E1/2 = 0.81 V),
but have better long-term electrochemical stability (7 mV
negative shift after 3000 potential cycles) and fuel selectivity.
In alkaline media, the SA-Fe-HPC catalysts outperform the
commercial Pt/C electrode in ORR activity (E1/2 = 0.89 V), fuel
selectivity, and long-term stability (1 mV negative shift after
3000 potential cycles). Thus, these nSA-Fe-HPCs are promis-
ing non-platinum-group metal ORR catalysts for fuel-cell
technologies.
stability and unsatisfactory catalytic activity in acidic electro-
lyte.[5]
Along with the development of metal (e.g., Pt, Fe)-based
nanoparticle catalysts, certain efficient Fe-based ORR cata-
lysts with single-molecule or single-atom (SA) active sites
have been studied.[3d,6] Although iron phthalocyanine (FePc),
one of the most popular Fe-based molecular catalysts,[7] could
facilitate the ORR process via a 4 electron pathway, deme-
talation of the catalytically active Fe atoms in FePc molecules
was demonstrated to lead a rapidly declining activity.[5b,8]
Besides, the inevitable Fenton reaction of FeII ions in FeIIPc
caused an instability in acidic electrolyte.[1,9] To eliminate the
demetalation and/or degradation of molecular catalysts, it is
highly desirable to develop SA-Fe catalysts, in which the
coordinated Fe atoms are directly stabilized on carbon
supports,[10] which can effectively prevent deactivation and
facilitate electron transport. Recent studies demonstrated
a few SA metal catalysts that are promising for energy-related
electrocatalysis.[11,12] To date, the pyrolysis of precursors
containing Fe, N, and C to generate unsaturated Fe atoms
with N and C coordination (Fe-N/C) serves as one of the most
important routes to Fe-N/C complex ORR catalysts.[13] To
avoid the aggregation of Fe atoms and the loss of N-
containing groups at high temperatures, the pyrolysis is
performed in harsh atmospheres (e.g., NH3), making it
difficult to tailor the active sites and optimize the ORR
performance by manipulating synthetic conditions.[12a,14]
Therefore, direct pyrolysis of N-rich Fe precursors pre-
supported on carbon substrates has recently attracted con-
siderable attention to allow adequate control of the active-
side structure.[14,15] However, it remains a challenge to
develop desirable coordination environments for the Fe
atoms in the carbon-supported Fe-containing precursors so
as to avoid aggregation during pyrolysis.[5b,12a,15]
Herein, we choose hierarchically structured porous
carbon (HPC) derived from biomass as the carbon support
and iron phthalocyanine/unsubstituted phthalocyanine (FePc/
Pc) complexes as the Fe precursors to fabricate the SA-Fe
catalyst for ORR (Scheme S1 in the Supporting Information).
To minimize the electrostatic interaction of Fe ions,[16] and
hence isolation of Fe atoms, we intentionally add unsubsti-
tuted phthalocyanine (Pc) molecules to assemble with FePc
molecules,[15b] through the p–p interaction of similar aromatic
macrocyclic compounds, to provide a steric hindrance for Fe
demetalation (Figure S1). Furthermore, the N-rich coordina-
tion structure in FePc/Pc complexes also facilitates the tuning
coordination environment of Fe atoms during pyroly-
sis.[5b,15b,17] As we previously demonstrated, the HPC derived
from cattle bones can provide various functional groups (e.g.,
T
o promote the large-scale application of fuel-cell technol-
ogies, cost-effective, high-performance non-platinum group
metal (non-PGM) catalysts for oxygen reduction reaction
(ORR) in both alkaline and acidic electrolytes have to be
developed as alternatives to the precious and scarce Pt-based
catalysts.[1–4] As one kind of non-PGM catalyst, Fe-based
catalysts have been widely considered as a promising candi-
date to replace the Pt-based ORR catalysts. However, Fe-
based catalysts still suffer from the poor electrochemical
[*] Dr. Z. Zhang, Dr. J. Sun, Prof. F. Wang
State Key Laboratory of Chemical Resource Engineering, Beijing Key
Laboratory of Electrochemical Process and Technology for Materials
Beijing University of Chemical Technology
Beijing 100029 (China)
E-mail: wangf@mail.buct.edu.cn
Dr. Z. Zhang, Prof. L. Dai
Center of Advanced Science and Engineering for Carbon (Case4-
carbon), Department of Macromolecular Science and Engineering
Case Western Reserve University
Cleveland, OH 44106 (USA)
E-mail: liming.dai@case.edu
Dr. Z. Zhang, Prof. F. Wang, Prof. L. Dai
Beijing Advanced Innovation Center for Soft Matter Science and
Engineering, College of Energy
Beijing University of Chemical Technology
Beijing 100029 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201804958.
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Figure 1. a) TEM and b) High-resolution HAADF-STEM images of the SA-Fe-HPC sample. c) BET characterization, nitrogen adsorption–desorption
isotherms (top) and the corresponding DFT pore size distributions (bottom) of HPC, Fe-N-C, NP-Fe-HPC and SA-Fe-HPC. d) XRD patterns for Fe-
N-C, NP-Fe-HPC and SA-Fe-HPC. High-resolution XPS spectra of e) Fe 2p and f) N 1s for NP-Fe-HPC and SA-Fe-HPC.
oxygen- and nitrogen-containing groups, Figure S2) and
numerous micropores[18] to effectively confine and stabilize
Fe atoms even during the optimized pyrolysis temperature of
8008C (see, Figures S3, S4 and the synthetic optimization in
Supporting Information) while its large surface area
(2537.9 m2 gÀ1, Table S1) can increase the concentration/
exposure of active sites. The same condition was used for
the preparation of all other samples investigated in this study,
including the HPC supported nanoparticle Fe (NP-Fe-HPC)
and the conventional iron–nitrogen codoped carbon (Fe-N-C)
electrocatalysts (see, Supporting Information).
Scanning electron microscopy (SEM) images of the SA-
Fe-HPC show a 3D honeycomb porous structure similar to
the open macropores (a diameter of ca. 500 nm of HPC
(Figures S5a, S5d). The corresponding transmission electron
microscopic (TEM) imaging revealed a channel mesoporous
structure (ca. 30–50 nm) for the resultant SA-Fe-HPC with-
out noticeable metal aggregation (Figure 1a). To gain insights
into the distribution of Fe atoms, we conducted the aberra-
tion-corrected high-resolution high-angle annular dark-field
scanning transmission electron microscopic (HAADF-
STEM) measurements on the SA-Fe-HPC sample. As
shown in Figure 1b, the single Fe atoms (the atomic scale of
ca. 1 ꢀ) are well-dispersed on the HPC support. Numerous
micropores (< 1 nm) consisting of disordered carbon layers
acted as anchor sites to stabilize the Fe atoms (Figures S6, S7).
However, the control sample derived from the pure FePc-
adsorbed HPC exhibited Fe aggregates on the HPC (desig-
nated as: NP-Fe-HPC), possibly formed during the high-
temperature pyrolysis (Figures S5b, S8). Similarly, the con-
ventional iron-nitrogen codoped carbon (Fe-N-C) produced
by pyrolysis of commercial carbon black and pure FePc)[19]
showed in homogeneously distributed Fe aggregates on the
carbon substrate (Figure S5c).
The N2 adsorption–desorption isotherm curves for all the
as-prepared samples derived from HPC (i.e., HPC, SA-Fe-
HPC and NP-Fe-HPC) displayed a similar I/IV-type isotherm
characteristic of micro/mesoporous materials (Figure 1c, top)
Owing to the large surface area and high pore volume
characteristics of HPC support (2537.9 m2 gÀ1 and
1.64 cm3 gÀ1),
the
SA-Fe-HPC
(1436.3 m2 gÀ1
and
0.96 cm3 gÀ1) and NP-Fe-HPC (936.8 m2 gÀ1 and 0.66 cm3 gÀ1)
exhibited a much larger Brunauer-Emmett-Teller (BET)
surface area and higher total pore volume than those of the
conventional Fe-N-C (68.5 m2 gÀ1 and 0.11 cm3 gÀ1). Com-
pared to the HPC support, the SA-Fe-HPC and NP-Fe-HPC
exhibited a decreased BET surface area attributable to the
adsorption of FePc/Pc complex and pure FePc, respectively.
The corresponding decrease in the nitrogen uptake (p/p0 <
0.1) for the SA-Fe-HPC and NP-Fe-HPC indicates that the
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adsorption occurred mainly into the micropores.[17a] In view of
N 1s spectra (Figure 1 f) into five component peaks at 397.7,
the similar adsorption capacity and molecular size of iron
phthalocyanine and unsubstituted phthalocyanine, we attrib-
uted the relatively small surface area of the NP-Fe-HPC with
respect to that of the SA-Fe-HPC to the blockage of the pores
and channels within the HPC support by Fe aggregates.[20]
In addition, we further conducted the density functional
theory (DFT) calculations to predicate the pore size distri-
bution (Figure 1c, bottom). It was found that all the samples
derived from HPC possessed the hierarchical porous struc-
ture with micropores and small mesopores centered at 0.8 and
3.5 nm, respectively. In consistence with the decreased nitro-
398.7, 399.8, 401.2 eV, and 402.4 eV corresponding to pyr-
À
idinic N, Fe N bonding, pyrrolic N, graphitic N and oxide N
(Figure 1 f), respectively.[22] Compared to the deconvoluted
high-resolution XPS N 1s spectrum of HPC (Figure S10b), it
is believed that the graphitic N and oxide N species in the SA-
Fe-HPC and NP-Fe-HPC are mainly originated from the N-
doped carbon skeletons in HPC (Table S4). Compared to NP-
À
Fe-HPC, the relatively high contents of pyridinic N, Fe N
bonding, and pyrrolic N observed for SA-Fe-HPC indicate
the presence of more N ligands.
To confirm the presence of single dispersed Fe atoms, we
further performed the X-ray absorption near-edge structure
(XANES) and extended X-ray absorption fine structure
(EXAFS) measurements. As seen in Figure 2a, the XANES
spectrum of SA-Fe-HPC shows the Fe K-pre-edge close to
that of the unpyrolyzed FeIIPc. However, the absence of the
gen uptake (p/p0 < 0.1), the microporous surface area (Smicro
)
of the HPC decreased from 2076.2 m2 gÀ1 to 1089.9 m2 gÀ1 for
SA-Fe-HPC and 752.6 m2 gÀ1 for NP-Fe-HPC by Fe adsorp-
tion. Compared with the mesoporous surface area (Smeso) of
HPC (461.7 m2 gÀ1), the SA-Fe-HPC showed a similar Smeso
(346.4 m2 gÀ1) whilst an obviously decreased Smeso
(184.2 m2 gÀ1) was observed for NP-Fe-HPC due,
most probably, to the blockage associated with the
Fe aggregates.[21]
To measure the iron and carbon crystalline struc-
tures, we further performed powder X-ray diffraction
(XRD). The SA-Fe-HPC and NP-Fe-HPC showed
a broad diffraction peak around 23.18 (Figure 1d)
corresponding to C(002) with a low-angle-shift in
respect to the commercial graphitic carbon (26.48).[22]
Apart from the broad C(002) peak, the SA-Fe-HPC
did not show any characteristic peak for metallic iron,
iron nitrate or iron carbide crystals. In contrast, the Fe-
N-C exhibited a set of sharp peaks (37.78, 43.78, 44.98,
and 48.68) characteristic of iron carbide (JCPDS card:
no. 35–0772). It is worth to note that the NP-Fe-HPC
sample also exhibited a few small, but noticeable,
characteristic peaks at 43.78 and 44.98, though its
microporous structure could stabilize Fe atoms and
confine Fe aggregates. Furthermore, Raman spectrum
of the NP-Fe-HPC showed a higher graphitization
Figure 2. a) Fe K-edge XANES spectra and b) Fourier transforms of k3-weighted
c(k)-function of the EXAFS spectra for the SA-Fe-HPC and NP-Fe-HPC samples
with iron foil and FePc as references. The 57Fe Mçssbauer transmission spectra
measured at 293 K for c) SA-Fe-HPC and d) NP-Fe-HPC.
degree with a higher G band to D band ratio (IG/ID, ca.
0.91) than those of the HPC and SA-Fe-HPC (ca. 0.84)
due probably to additional graphitization induced by
Fe nanoparticles during the high temperature pyrol-
ysis (Figure S9).
We have further carried out X-ray photoelectron
spectroscopic (XPS) measurements on HPC, SA-Fe-HPC and
NP-Fe-HPC. The XPS survey spectra (Figure S10a, Table S2)
indicate that all the samples are dominated by carbon, along
with some nitrogen and oxygen-containing moieties, while the
SA-Fe-HPC and NP-Fe-HPC samples show the presence of
Fe (0.80 and 0.45 at%, respectively). The corresponding high-
resolution XPS spectra of Fe 2p3/2 (Figure 1e, Table S3) were
deconvoluted into three component peaks at 707.1, 709.0 and
714.0 eV,[12b,13b] arising from the metallic iron, ferrous state,
and ferric state, respectively. FeII and FeIII species were
observed for both SA-Fe-HPC and NP-Fe-HPC. However,
Fe0 was observed only for the NP-Fe-HPC sample, indicating,
once again, the presence of the Fe aggregates. In view of the
different FeIII/FeII ratios in SA-Fe-HPC (0.71) and NP-Fe-
HPC (0.49), we further deconvoluted the high-resolution XPS
pre-edge peak at 7118 eV from SA-Fe-HPC revealed that the
square-planar D4h local symmetry of FePc was broken after
the pyrolysis treatment.[23] In contrast, the XANES spectrum
of NP-Fe-HPC is more similar to that of Fe foil, indicating the
presence of metallic Fe0 nanoparticles in the former. Fur-
thermore, the SA-Fe-HPC exhibited higher half-edge energy
than that of NP-Fe-HPC due to a negative charge transfer
from iron to nitrogen.[24] These results are consistent with the
XPS data.
To gain further structural information about Fe atom, we
carried out Fourier transformed EXAFS spectroscopic meas-
urements (R space) on the SA-Fe-HPC and NP-Fe-HPC
samples. As can be seen in Figure 2b, the SA-Fe-HPC
exhibited a slight shift in the Fe-N (1.5 ꢀ) coordination
peaks compared to Fe atom with four-coordinated nitrogen in
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Figure 3. a) CV curves of HPC, Fe-N-C, NP-Fe-HPC, and SA-Fe-HPC in O2-saturated (solid line) or N2-saturated (dashed line) in 0.1m KOH at
a sweep rate of 50 mVsÀ1. b) The corresponding RRDE voltammograms recorded in O2-saturated 0.1m KOH at 1600 rpm. The disk potential was
scanned at 5 mVsÀ1, and the ring potential was constant at 1.5 V. c) Tafel plots derived by the mass-transport correction from the corresponding
LSV data. d) LSV voltammograms recorded in O2-saturated 0.1m H2SO4 at 1600 rpm with a sweep rate of 5 mVsÀ1. e) LSV polarization curves of
the SA-Fe-HPC electrode in O2-saturated 0.1m H2SO4 at a sweep rate of 5 mVsÀ1 with the different rotation rates (400–2025 rpm). The inset in (e)
shows corresponding K-L plots (JÀ1 versus wÀ1/2) at different potentials. f) ORR polarization curves of SA-Fe-HPC and Pt/C before and after
3000 potential cycles over 0.6–1.1 V with the sweep rate of 100 mVsÀ1 in N2-saturated 0.1m H2SO4.
FePc (1.6 ꢀ), arising from the strong interaction between Fe
and N atoms associated with the axial-coordinated N-
ligands.[5b,11b,25] In contrast to the pronounce Fe-Fe bonding
peak observed in Figure 2b for the NP-Fe-HPC. The absence
of Fe–Fe paths (2.2 ꢀ) in the SA-Fe-HPC indicates unambig-
uously that Fe atoms are in the single-atomic dispersion
without the metallic Fe0 nanoparticle. Furthermore, we
performed 57Fe Mçssbauer transmission spectroscopy to
identify the different chemical states of iron in the SA-Fe-
HPC and NP-Fe-HPC samples (Table S5). In Figure 2c, the
Mçssbauer spectrum of SA-Fe-HPC was deconvoluted into
two doublets (D1 and D2), attributable to Fe-Nx complexes
with Fe atoms in a low- and medium-spin state, respectively.
However, the Mçssbauer spectrum of NP-Fe-HPC exhibits
a singlet, a doublet and two sextets corresponding to g-Fe, e-
FexN (x ꢀ 2.1), and a-iron/iron carbide, respectively.[12a,26]
Thus, the absence of the singlet and sextet components for
SA-Fe-HPC indicates that the SA-Fe-HPC is free from
inorganic Fe- phases (i.e., a-/g-Fe,). These experimental
data are further supported by density functional theory
(DFT) calculations (see, Figures S11 and S12 in the Support-
ing Information for details).
potentials between 0.75 and 0.90 V (vs. reversible hydrogen
electrode (RHE), the same below) in the O2-saturated 0.1m
KOH solution, but not in the N2-saturated solution. Among
them, the SA-Fe-HPC electrode showed the highest catalytic
activity with the most positive ORR peak potential and the
strongest cathodic current. Also, the linear sweep voltamme-
try (LSV) curves recorded on the rotating ring-disk electrode
(RRDE) in O2-saturated 0.1m KOH (Figure 3b, bottom and
Figure S14) showed that the SA-Fe-HPC electrode exhibited
the best ORR performance with the most positive half-wave
potential (E1/2) of 0.89 V and the highest diffusion limited
current density (Jd) of 5.4 mAcmÀ2 among all the electrodes
investigated in this study (HPC,
E1/2 = 0.63 V, Jd =
2.8 mAcmÀ2; NP-Fe-HPC, E1/2 = 0.82 V, Jd = 5.2 mAcmÀ2
;
Fe-N-C, E1/2 = 0.80 V, Jd = 5.0 mAcmÀ2), and even much
better than that of the commercial Pt/C (E1/2 = 0.84 V, Jd =
5.0 mAcmÀ2). As expected, the SA-Fe-HPC electrode also
displayed the lowest ring current (Figure 3b, top). Figure 3c
shows the corresponding Tafel curves and kinetic currents (Jk)
at 0.90 V with the mass-transport correction. Once again, the
SA-Fe-HPC
electrode
exhibited
the
highest
Jk
(3.72 mAcmÀ2), which is 5.4-fold higher than that of the
To evaluate the ORR catalytic activity of SA-Fe-HPC, we
first conducted cyclic voltammetric (CV) measurements for
SA-Fe-HPC in N2- and O2-saturated alkaline electrolytes
(0.1m KOH, Figure 3a). As can be seen, all the Fe-based
electrodes displayed a well-defined catholic ORR peak at
commercial Pt/C, and a smaller Tafel slope (49 mVdecÀ1
)
even than that of the Pt/C electrode (61 mVdecÀ1) in the low
over-potential region, indicating that the SA-Fe-HPC is an
efficient electrocatalyst—even better than the commercial Pt/
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C catalyst. As shown in Figure S15 and S16a, The SA-Fe-HPC
Keywords: electrocatalysis · hierarchical porous carbon · iron ·
showed a direct 4-electron transfer pathway for ORR.
It was further found that the acidic ORR performance
followed the same trend as in the alkaline electrolyte (vide
supra) with the catalytic activity increased in the following
order: Fe-N-C < NP-Fe-HPC < SA-Fe-HPC (Figure S17).
Figure 3d shows the corresponding LSV curves recorded in
O2-satured 0.1m H2SO4, which still shows a significantly
higher ORR activity (E1/2 = 0.81 V, Jd = 5.5 mAcmÀ2) for SA-
Fe-HPC than those of HPC, NP-Fe-HPC (E1/2 = 0.75 V, Jd =
4.6 mAcmÀ2) and Fe-N-C (E1/2 = 0.39 V), and even compara-
ble to the commercial Pt/C catalyst (E1/2 = 0.84 V, Jd =
5.5 mAcmÀ2). This could be attributed to a synergetic effect
associated with the high concentration of active sites and the
efficient mass transfer through the 3D porous structure.[9,14]
The electron transfer number of SA-Fe-HPC in acidic
electrolyte was calculated from the K-L plots at various
potentials (Figure 3e) to be 4, indicating a direct 4-electron
transfer pathway as for the commercial Pt/C (Figure S16b).
Finally, the SA-Fe-HPC electrode was found to be
remarkably stable in the harsh oxygen reducing environment
(Figures 3 f, S18, and S19) and almost free from the methanol-
crossover effect in both alkaline and acidic electrolytes
(Figure S20), having a much better fuel selectivity than that
of the commercial Pt/C. Unlike the easy demetalation and/or
degradation of FePc molecules, the enhanced interaction
between Fe atoms and the carbon support in SA-Fe-HPC
stabilized the Fe atom at its active site, especially in the acidic
environment.[9,17] Therefore, the SA-Fe-HPC catalyst is of
great potential as a Pt substituent for practical applications.
In conclusion, we have developed a general strategy to
prepare a Fe-based catalyst with atomic Fe dispersion on
a porous carbon support, which has been demonstrated to
show excellent performance for ORR in both alkaline and
acidic electrolytes, making it a promising candidate to replace
Pt-based ORR catalysts even for PEM fuel cells. The
remarkable ORR performance of the SA-Fe-HPC is attrib-
utable to the well-defined dispersion of single Fe atomic
active sites with an optimized coordination environment of Fe
atoms on a 3D hierarchical porous carbon support with a high
concentration/exposure of active sites. The methodology
developed in this study could be used for producing various
high-performance single-atom catalysts from earth abundant
biomasses and metal complexes.
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Acknowledgements
This work was supported by the National Natural Science
Funds of China (51432003, 51125007, 51732002) and the
International Joint Graduate-Training Program of BUCT. We
are thankful to the beamline 1W1B station of Beijing
Synchrotron Radiation Facility for the XAFS tests.
Conflict of interest
The authors declare no conflict of interest.
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Manuscript received: April 27, 2018
Version of record online: && &&, &&&&
6
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These are not the final page numbers!

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Communications
Oxygen Reduction Reaction
Stay single: Single-atom catalysts with Fe
atoms dispersed on hierarchical porous
carbons were prepared by pyrolysis of
iron phthalocyanine and unsubstituted
phthalocyanine complexes confined
within micropores of the porous carbon
supports. The resulting catalysts outper-
formed the commercial Pt/C electrode in
alkaline electrolytes and showed an elec-
trocatalytic activity comparable to the
commercial Pt/C electrode in acidic
media with a better long-term stability.
Z. Zhang, J. Sun, F. Wang,*
L. Dai*
&&&& — &&&&
Efficient Oxygen Reduction Reaction
(ORR) Catalysts Based on Single Iron
Atoms Dispersed on a Hierarchically
Structured Porous Carbon Framework
Angew. Chem. Int. Ed. 2018, 57, 1 – 7
ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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7
These are not the final page numbers!