An Adjacent Atomic Platinum Site Enables Single-Atom Iron with High Oxygen Reduction Reaction Performance

Article abstract of DOI:10.1002/anie.202105186

The modulation effect has been widely investigated to tune the electronic state of single-atomic M-N-C catalysts to enhance the activity of oxygen reduction reaction (ORR). However, the in-depth study of modulation effect is rarely reported for the isolated dual-atomic metal sites. Now, the catalytic activities of Fe-N₄ moiety can be enhanced by the adjacent Pt-N₄ moiety through the modulation effect, in which the Pt-N₄ acts as the modulator to tune the 3d electronic orbitals of Fe-N₄ active site and optimize ORR activity. Inspired by this principle, we design and synthesize the electrocatalyst that comprises isolated Fe-N₄/Pt-N₄ moieties dispersed in the nitrogen-doped carbon matrix (Fe-N₄/Pt-N₄@NC) and exhibits a half-wave potential of 0.93 V vs. RHE and negligible activity degradation (ΔE_1/2=8 mV) after 10000 cycles in 0.1 M KOH. We also demonstrate that the modulation effect is not effective for optimizing the ORR performances of Co-N₄/Pt-N₄ and Mn-N₄/Pt-N₄ systems.

Full text of DOI:10.1002/anie.202105186

Angewandte
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How to cite:
International Edition: doi.org/10.1002/anie.202105186
German Edition: doi.org/10.1002/ange.202105186
Single-Atom Catalysts
An Adjacent Atomic Platinum Site Enables Single-Atom Iron with
High Oxygen Reduction Reaction Performance
+
+
Ali Han , Xijun Wang , Kun Tang, Zedong Zhang, Chenliang Ye, Kejian Kong, Haibo Hu,
Lirong Zheng, Peng Jiang, Changxin Zhao, Qiang Zhang, Dingsheng Wang,* and Yadong Li
[
3]
Abstract: The modulation effect has been widely investigated
to tune the electronic state of single-atomic M-N-C catalysts to
enhance the activity of oxygen reduction reaction (ORR).
However, the in-depth study of modulation effect is rarely
reported for the isolated dual-atomic metal sites. Now, the
H O electrochemical production. The exploration of ORR
2 2
electrocatalysts with high efficiency has garnered intensive
attention as the kinetic process could significantly determine
the conversion efficiency of energy-related devices. Unfortu-
nately, the corresponding ORR suffers from an extremely
ꢀ1
catalytic activities of Fe-N moiety can be enhanced by the
high O=O bond energy value of 498 KJmol and sluggish
4
adjacent Pt-N moiety through the modulation effect, in which
kinetics of multielectron transfer process. Precious Pt-group
metals (PGMs) related materials are widely known as
efficient catalysts to active the strong O=O bond electro-
4
the Pt-N acts as the modulator to tune the 3d electronic orbitals
4
of Fe-N active site and optimize ORR activity. Inspired by this
4
[
4]
principle, we design and synthesize the electrocatalyst that
comprises isolated Fe-N /Pt-N moieties dispersed in the
catalysts, however, the large-scale commercialization is
significantly hindered by the high-cost and low-stability.
Downsizing the electrocatalysts to sub-nanometer or even
single-atomic size is an efficient strategy to maximize the
4
4
nitrogen-doped carbon matrix (Fe-N /Pt-N @NC) and exhibits
4
4
a half-wave potential of 0.93 V vs. RHE and negligible activity
degradation (DE1/2 = 8 mV) after 10000 cycles in 0.1 M KOH.
We also demonstrate that the modulation effect is not effective
for optimizing the ORR performances of Co-N /Pt-N and
[5]
number of active metal sites and lower the cost of PGMs.
Recently, non-precious single-atom catalysts (SACs) com-
posed of Fe-N moieties have ranked among one of the most
4
4
4
Mn-N /Pt-N systems.
promising alternatives to manifest encouraging activity due to
their cost-minimization and ultrahigh metal utilization (ca.
4
4
[
2,6]
Introduction
100%).
However, the well-defined structure of Fe-N is
4
endowed with symmetric electron distribution, leading to the
discontented adsorption energies to oxygen intermediates
(for example, OOH*, O*, and OH*), as compared to the
The oxygen reduction reaction (ORR) is widely recog-
nized to be one of the most vital catalytic processes since it is
involved in vast advanced energy conversion and storage
[
6e,7]
benchmark Pt/C catalysts.
It is still challenging to sub-
[
1]
[2]
technologies, such as fuel cells, metal–air batteries, and
stitute PGM catalysts by Fe-N4 catalysts while exhibiting
superior performance and robust stability in practical appli-
cations.
[
+]
[
*] Dr. A. L. Han, Z. D. Zhang, Dr. C. L. Ye, K. J. Kong, Dr. P. Jiang,
Prof. D. S. Wang, Prof. Y. D. Li
Department of Chemistry, Tsinghua University
Beijing 100084 (China)
Weakening O=O bonds and optimizing the oxygen
adsorption energies to oxygen intermediates for the single
site of Fe-N are still very limited. Nature catalyzes the O via
4
2
E-mail: wangdingsheng@mail.tsinghua.edu.cn
an metalloenzyme called cytochrome c oxidase (CcO), which
[
+]
Dr. X. J. Wang
is composed of heme a /Cu (heme-copper oxidases, HCOs)
3
B
Hefei National Laboratory for Physical Sciences at the Microscale,
CAS Center for Excellence in Nanoscience, School of Chemistry and
Materials Science, University of Science and Technology of China
Hefei, Anhui 230026 (China)
[
8]
dual metal centers and surrounding organic ligands. In
special, the copper centers are in close proximity (5.0 ꢀ) to
iron heme metal centers (HMCs, similar to Fe-N ) and have
4
been suggested to be able to assist O=O double bonding
breakage and efficiently catalyze 4 e ORR without forma-
K. Tang, Prof. H. B. Hu
School of Physics and Materials Science, Anhui University
Hefei 230601 (China)
ꢀ
tion of incomplete reduction intermediates (for example,
[
8,9]
Prof. L. R. Zheng
H O ).
Inspired by nature, constructing adjacent metal
2
2
Beijing Synchrotron Radiation Facility Institute of High Energy
Physics, Chinese Academy of Sciences
No. 19 Yuquan Road, Beijing 100049 (China)
centers to regulate the electronic localization of Fe-N should
4
[10]
be a promising strategy to promote the ORR performance.
[2a,11]
[10b]
[12]
Although different Ni-N4,
Mn-N4
and Pt-N4 moi-
C. X. Zhao, Prof. Q. Zhang
eties have proved to promote the ORR activity of Fe-N4
moieties, the general principle that governs the structure-
Beijing Key Laboratory of Green Chemical Reaction Engineering and
Technology Department of Chemical Engineering
Tsinghua University, Beijing 100084 (China)
dependent catalytic properties of Fe-N /M-N4 have rarely
4
+
[
been elucidated. It was revealed that Pt-N moiety was
4
] These authors contributed equally to this work.
[
3e,13]
insufficiently active for ORR,
however, they are endow-
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.202105186.
ed with vacant 5d orbitals, providing the possibility of
&
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facilitating the adsorption of O and O-O breakage on the Fe- Results and Discussion
2
[
14]
N active site.
4
In this work, we demonstrate the modulation relationship
between the co-existence of Fe-N and Pt-N moieties and the
ORR performance using density functional theory (DFT)
calculations and electrochemical analysis. The theoretical
The Fe-N moiety was constructed with four pyridine N
4
atoms surrounding the metal-center based on those previous-
4
4
[
15]
ly investigations. Given that no FeꢀPt metallic bond was
found, four possible models of Fe and Pt co-embedding in
graphene were built (Figure S1). It was shown that the models
in Figure S1a–c are less thermodynamically stable than the
model in Figure S1d, which was selected for the subsequent
simulations. The distance between Fe and Pt (ca. 5.3 ꢀ) in the
selected model is close to the distance between two metal
studies predict that the adjacent atomic Pt-N modulator can
4
assist the O activation on the Fe-N active center, as well as
2
4
modulate the rehybridization of Fe d and O p orbitals to
optimize the adsorption of oxygen intermediates to accelerate
the whole ORR kinetic processes. Consequently, we design
and synthesize the optimal Fe-N /Pt-N @NC, which delivers
[8a]
centers (dFe-Cu, 5.0 ꢀ) in the HCOs systems and a bioin-
4
4
[
10a]
an ultrahigh half-wave potential of 0.93 V, robust activity
stability and high methanol tolerance, outperforming the
activity of commercial Pt/C and mostly reported metal-based
SACs in basic solution. Moreover, Fe-N /Pt-N @NC catalysts
spired HCOs system (dPt-Cu, ca. 5.05 ꢀ),
showing the
rationality of the selected model. In general, the O absorp-
2
tion and OꢀO bond stretching are particularly key parame-
[7d,16]
ters determining the ORR performances of Fe-N .
To
4
4
4
have proven excellent Zn-air battery performance and
charge/discharge durability, manifesting the promising appli-
cation in metal-air battery. Simultaneously, inspired by the
reveal the effect of adjacent Pt-N on the properties of Fe-N
4 4
sites, DFT calculations were carried out. As shown in
Figure 1a, the adsorption energies for O on Fe-N and Fe-
2
4
modulation effect on Fe-N , we also analyze its impact on the
N /Pt-N4 systems are 0.46 eV and 0.55 eV, respectively,
4
4
systems of Co-N /Pt-N and Mn-N /Pt-N . However, both the
suggesting a more favorable O adsorption process on Fe-
4
4
4
4
2
theoretical and experimental results demonstrate that the co-
N /Pt-N than that on Fe-N . Moreover, a higher stretching
4
4
4
existence of Pt-N moieties shows negligible enhancement for
degree of OꢀO bond (ca. 1.30 ꢀ) was observed on Fe-N /Pt-
4
4
the ORR performance of Co-N /Pt-N @NC. In contrast to the
N
system as compared to the Fe-N4 system (1.28 ꢀ),
4
4
4
[16]
positive modulation effect of Pt-N site on the Fe-N site, the
manifesting an easier brokage of O molecule. In addition,
4
4
2
Mn-N /Pt-N @NC system exhibits an inferior ORR activity
the adsorption energies and OꢀO bonds on the Co-N /Pt-N
4
4
4
4
due to the too strong adsorption energy of OH* after the Pt-
N modulation on the Mn-N site.
and Mn-N /Pt-N systems were also calculated as displayed in
4 4
Figure S2–S3, respectively. In contrast to the imperfect
impacts of Pt-N on the Co-N and Mn-N in O adsorption
4
4
4
4
4
2
and O-O activation, the adjacent Pt-N moiety could obvi-
4
ously enables O adsorption and O-O activation on the Fe-N
2
4
site.
Figure 1. a) O adsorption models of Fe-N and Fe-N /Pt-N . b) Gibbs free energy diagram for ORR on Fe-N and Fe-N /Pt-N . c),d) Computed
2
4
4
4
4
4
4
projected density of states of Fe-N (c) and Fe-N /Pt-N (d) after OOH* adsorption. e),f) Computed projected density of states of Fe-N (c) and
4
4
4
4
Fe-N /Pt-N (d) after OH* adsorption. s and p represent the bonding molecular orbitals between d orbital of Fe and p orbital of O. s* and p*
4
4
represent the antibonding molecular orbitals between d orbitals of Fe and p orbital of O.
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To investigate the effect of adjacent Pt-N moiety on the
increased by 0.18 and 0.20 eV, respectively, making the
potential-limiting step from OOH* adsorption to OH*
desorption and lowering down the overpotential from 1.18
to 1.03 V after co-embedding Pt-N with Fe-N . Such over-
4
ꢀ
ORR activity, the adsorption configurations of 4 e ORR
ꢀ
reaction intermediates (e.g., OOH*, O*, and OH*) and 2 e
ORR reaction intermediates (e.g., OOH* and H O ) for Fe-
2
2
4
4
N , Pt-N4 and Fe-N /Pt-N4 were shown in Figure S4–S6,
potential reduction caused by enhancing the adsorption of
intermediates has also been reported in many other graphene-
based systems, such as FeCoN5. Figure 1e and 1 f show the
4
4
respectively. Then, the Gibbs free energy (DG) diagrams of all
intermediates of the ORR were calculated as a function of the
cell potential, as plotted in Figure S7. Clearly, we can see that
[7a]
PDOS for the d orbitals of Fe and p orbital of O for Fe-N and
4
in the Fe-N /Pt-N4 system, the adsorptions of all the
Fe-N /Pt-N after OH* adsorption, respectively. Clearly, with
4
4
4
considered reaction intermediates are stronger on Fe-N sites
Pt co-embedding, the Fe dz
2
and O p orbitals can form s2
4
ꢀ
than those on Pt-N for the direct 4 e process, leading to
bond, the Fe d and O p orbitals can form p bond, and the Fe
4
xy
1
much lower overpotentials on Fe-N (1.03 V) than those on
d , d
x2ꢀy2
and O p orbitals can form p bond. These additional
4
xy
2
[25]
Pt-N (1.85 V). As the Pt-N metal center is not as oxophilic as
bonding orbitals thereby strengthen the Fe-OH* bonding.
4
4
the Fe-N site, it is more prone to OOH* protonation to
In addition to these orbital analyses, we also performed
4
produce H O due to the energetically unfavorable breaking
charge analyses on Fe-N4 and Fe-N /Pt-N4 as shown in
2
2
4
ꢀ
[3a,b,17]
of OꢀO bond (2 e process, Figure S8, orange lines),
Figure S9, it shows that Pt co-embedding can introduce more
electrons to the system and thus reshape the spatial charge
distribution at the Fe single-atom site, which slightly reduces
the net charge of Fe site from + 0.541 to + 0.537. However,
such a charge redistribution alone was unlikely to directly
determine the adsorption of reaction intermediates because
no clear linear correlation was found between the net charge
of metals and the adsorption free energy of OH* for different
whereas Fe-N site is inclined to break the OOH* intermedi-
4
ꢀ
ates to O*, leading to the formation of H O (4 e process,
2
Figure S8, green lines). Meanwhile, considering the much
lower competitiveness of Pt-N to snatch O than Fe-N active
4
2
4
[3e]
centers, O molecules bonded with Pt-N center was not
2
4
considered in the calculated reaction pathways. Based on
these reasons, Fe-N is demonstrated as the only active center,
4
[
25]
while Pt-N is considered as the modulator (not active center)
species (Figure S10).
Therefore, we conclude that the
4
to tune the electronic state of Fe-N for the reaction pathway,
increase of ORR activity of Fe-N /Pt-N mainly originate
4
4
4
which is completely different from the previously reported
from the electronic orbitals modulation of Fe-N by Pt-N .
4 4
ꢀ
“
synergetic effect” on the binary metal active centers, for
Besides, the Gibbs free energy barriers of the key steps of 4 e
ORR were also studied for Fe-N and Fe-N /Pt-N as shown in
[
18]
[19]
[20]
example, Co-Pt-NC,
NC and Cu-N /Zn-N , Fe-N /Mn-N ,
Fe-Pt-NC,
Fe-Mn-NC,
Fe-Ni-
4
4
4
[
21]
[22]
[10b]
[23]
Fe-N /Co-N4
Figure S11 and S12, respectively. Clearly, all these steps are
highly exothermic with relatively small reaction barriers. The
4
4
4
4
4
[
2a,11]
and Fe-N /Ni-N4
et al. The theoretical hORR of each
4
catalyst can be determined by examining the reaction Gibbs
rate-determining step on Fe-N is the last step (OH* + H*!
4
free energies of the different mechanistic steps for Fe-N and
H O*) with an energy barrier of 0.88 eV, which is reduced to
4
2
Fe-N /Pt-N . As revealed in Figure 1b and Figure S7, the Fe
0.24 eV after the co-embedding of Pt. In contrast, the rate-
4
4
site in the structure of Fe-N /Pt-N exhibits the lowest
determining step on Fe-N /Pt-N is the second step (OOH* +
4
4
4
4
overpotential of 1.03 V and the optimized limiting-potential
U ) of 0.2 V, indicating its superior ORR properties than the
H*!O* + H O) with an energy barrier of 0.51 eV, indicating
2
(
that Fe-N /Pt-N has higher overall reaction rate of ORR than
L
4
4
Fe-N system. From Figure 1b, the potential-determining step
that on Fe-N4.
4
of ORR on pristine Fe-N4 is the OOH* adsorption step
As guided by the predicated results of excellent ORR
performance of Fe-N /Pt-N @NC, the electrocatalyst was
designed and synthesized via a spatial confinement strategy,
as seen details in Experimental Section and Figure 2a. We pay
close attention to the zeolite imidazole frameworks (ZIF8s),
because they have been known as the most ideal N-rich
(
O *!OOH*) with the overpotential h = 1.18 V and the
2
4
4
limiting-potential U = 0.05 V. Encouragingly, the height of
L
the O *!OOH* step is decreased to 0.81 V for the config-
2
uration of Fe-N /Pt-N due to the enhancement of the OOH*
4
4
adsorption caused by the introduction of Pt-N4.
To further investigate the impact of Pt-N site on the Fe-
N site for OOH* adsorption, the projected density of states
carbon precursors to generate exclusive M-N active sites for
4
4
[
1b,6o,25,26]
enhanced ORR activity.
Firstly, Fe/Pt-doped ZIF8s
4
(
PDOS) calculations was carried out for the d orbitals of Fe
are synthesized by in situ encapsulating Fe(acac) (diameter,
3
and p orbital of O for Fe-N and Fe-N /Pt-N after OOH*
ca. 9.7 ꢀ) and Pt(acac) (diameter ca. 10.0 ꢀ) in the cages of
4
4
4
2
adsorption, respectively. As depicted in Figure 1c, for OOH
ZIF8s (nanocavity, 11.6 ꢀ), ensuring a uniform distribution of
Fe and Pt atomic sites in the subsequent pyrolysis. After
bonding on Fe site in pristine Fe-N , the frontier orbitals for
4
FeꢀO bonding are mainly p* anti-bonding orbitals, leading to
relatively weak Fe-OOH bonding. In contrast, as shown in
Figure 1d, the introduction of Pt greatly varies the orbital
energy levels and spatial distributions of Fe 3d orbitals,
carbonization at 10008C under N atmosphere for 2 h, the Fe-
2
N /Pt-N4 are homogeneously embedded into the three-
4
dimensional (3D) porous carbon matrix. Other different
metal SACs, including Fe-N @NC and Pt-N @NC were also
4
4
making the original p * anti-bonding orbitals become p
fabricated by the similar method. Powder X-ray diffraction
(PXRD) patterns of pure ZIF8s and metal-doped ZIF8s
showed the same zeolite-type structures, indicating the high
crystalline structures of different metal sources doped-ZIF8s
(Figure S13). The morphologies of different metal-based
SACs supported on the carbon matrix were characterized
3
bonding orbitals by the rehybridization of Fe d and O p
z2
[
24]
orbitals. This thus strengthens the FeꢀO bond and makes
the adsorption free energy of OOH* intermediate increases
by 0.37 eV. Intriguingly, the free energy diagrams in Figure 1b
also show that the adsorption free energy of O* and OH* are
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Figure 2. Structural characterizations of Fe-N /Pt-N @NC. a) Synthesis process of Fe-N /Pt-N @NC. b) Atomic-resolution HAADF-STEM image of
4
4
4
4
Fe-N /Pt-N @NC. c) Magnified atomic-resolution HAADF-STEM images of Fe-N /Pt-N @NC show the atomically dispersed Fe atoms and Pt
4
4
4
4
atoms, which are highlighted by orange dashed rectangles. d) The average intensity profiles of Fe/Pt atoms (indicated by orange dashed
rectangles) reveal the distance between Pt and Fe is ca. 5.2 ꢁ. e) EDS mapping images of C (green), N (blue), Fe (red) and Pt (orange),
respectively. f),g) Fe K-edge and Pt L -edge XANES spectra, respectively. h) Magnitude of Fourier transform of k2-weighted EXAFS spectra. Scale
3
bars: b) 2 nm; c) 1 nm.
ꢀ
1
by transmission electron microscopy (TEM), indicative of the
initial dodecahedral shapes without formation of large-size
particles (Figure S14). Moreover, the porous structures were
much pronounced with the removal of Zn elements (boiling
point: 9058C) upon the pyrolysis at high temperatures,
confirmed by the G-band peaks at ca. 1590 cm in the
Raman spectra (Figure S16).
To further confirm the co-existing Fe-N and Pt-N in the
4
4
NC supports, sub-angstrom-resolution aberration-corrected
high-angle annular dark-field scanning TEM (HAADF-
STEM) images were acquired. Atomic-resolution HAADF-
STEM image in Figures 2b,c show many isolated bright dots
distributed on the carbon matrix with two different contrasts.
Particularly, isolated Fe and Pt atoms were clearly identified
by relatively brighter Pt dots and darker Fe dots, as marked by
the dashed rectangles in Figure 2c. As shown in the intensity
profile in Figure 2d, the distances between two neighboring
Fe and Pt atoms are in the range of ca. 5.3 ꢀ, consistent with
[
1b,2b]
leading to much rougher surfaces.
No peaks for metal,
metal oxides or metal carbides were observed from the XRD
patterns of carbon supported M-N @NC or M-N @Pt-N @NC
4
4
4
(
Figure S15), suggesting no large-size crystalline particles,
which were in good agreement with the TEM results. The
main peaks at ca. 258 and ca. 438 in the XRD patterns could be
well ascribed to (002) and (101) facets of the pyrolyzed
[
1b,6h,19,27]
graphitic carbon,
respectively, which was further
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the distance between Pt and Fe atoms in the most stable
structure in Figure S1d. The Fe atoms where no Pt appeared
were also observed as marked by the white circles. The
corresponding energy-dispersive spectroscopy (EDS) map-
ping of Fe-N /Pt-N @NC showed uniform dispersions of C, N,
observed, strongly excluding the existence of FeꢀPt bond or
Fe-Pt clusters formation. From the comprehensive analysis by
XPS and EXAFS, we can conclude that the different isolated
Fe-N and Pt-N were formed on the NC substrate.
4
4
The ORR performance of Fe-N /Pt-N @NC was evaluat-
4
4
4
4
Fe and Pt throughout the entire carbon matrix, as displayed in
Figure 2e. In addition, the presence of Fe and Pt was further
confirmed by inductively coupled plasma optical emission
spectrometry (ICP-OES) with 1.12 wt% Fe and 0.45 wt% Pt
ed using a rotating ring-disk electrode (RRDE) in a O2
saturated 0.1 M KOH electrolyte. NC, Pt-N , Fe-N and Pt/
4
4
C catalysts were used for comparisons. The metal-free NC
derived by pure ZIF8s exhibited poor activity from the linear
sweep voltammetry (LSV) results in Figure 3a. The half-wave
potential (E ) of Fe-N or Pt-N showed a positive shift after
on the NC supports (See Table S1). Other Fe-N @NC and Pt-
4
N @NC were also successfully prepared, as confirmed by
4
1/2
4
4
EDS mapping in Figures S17–18.
dispersing in the NC substrates. In addition, the optimized
catalyst, Fe-N /Pt-N @NC showed the best ORR activity, with
To acquire the chemical states of C, N, Fe and Pt in the Fe-
N /Pt-N @NC, X-ray photoelectron spectroscopy (XPS) was
4
4
an onset potential (Eonset) of 1.03 V and a half-wave potential
(E ) of 0.93 V vs. RHE. Meanwhile, the catalysts containing
4
4
investigated, as shown in Figure S19. From Figure S19b,
pyridinic-N and graphitic-N were dominantly existed in the
N 1s spectrum. The prominent graphitic-N was also confirmed
by the Raman spectra in Figure S16. Except from the signals
of C 1s and N 1s, the Fe and Pt signals were also detected with
much weaker intensity, which should be caused by the low
atomic density of Fe and Pt. After deconvoluting the Fe 2p
1
/2
only Fe-N and Pt-N exhibited much lower E of 0.885 Vand
4
4
1/2
0.825 V, respectively, see details in Figure S23a. The gap at E1/
between Fe-N /Pt-N @NC and Fe-N @NC was 45 mV,
2
4
4
4
manifesting the adjacent Pt-N could remarkably enable the
4
ORR activity of Fe-N , which was consistent with the DFT
4
results. Although the N-dopant in the NC, Fe-N @NC and
4
2
+
signal, Fe 2p spectrum was resolved into doublet peaks of Fe
Fe-N /Pt-N @NC was similar (Figure S24 and Figure S19b),
4
4
3
+
peaks (709.8 eV), Fe peaks (710.8 eV) and a weak satellite
peak (717.5 eV), indicating the dominated oxidation of Fe-N4
between + 2 and + 3 with the formation of Fe-N coordination.
Meanwhile, the Pt 4f was deconvoluted into two peaks of Pt
however, Fe-N @NC and Fe-N /Pt-N @NC exhibited much
4 4 4
better ORR activity than the NC support, indicating the
importance of metal SACs for the enhanced ORR activity. In
addition, we have also investigated the effect of N-dopant on
the ORR performance, as shown in Figure S25–S26 and
Table S3, suggesting the significant N-dopant for the ORR
activity of Fe-N /Pt-N @NC. The Fe-N /Pt-N @NC exhibited
4
f7/2 at 72.3 eV and Pt 4f at 75.6 eV, which was attributed to
5/2
II
[28]
the specie of Pt after formation of PtꢀN bonds.
No
0
0
metallic Fe or Pt were observed, further excluding the Fe or
Pt nanoparticles in the synthesized samples.
4
4
4
4
ꢀ2
a high kinetic current density (J ) of 27.2 mAcm at 0.85 V
k
X-ray absorption near-edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS) are
carried out to detect the chemical state and coordination
environments of Fe and Pt species in the NC substrates. The
structural parameters of EXAFS are shown in Table S2 in
detail. As observed in Figure 2 f, the Fe K-edge XANES
(Vs. RHE), which was about three times higher than that of
ꢀ
2
Pt/C (7.5 mAcm ) from Figure S23b at a given potential of
0.85 V vs. RHE. Moreover, the measured Tafel slope of Fe-
ꢀ
1
N /Pt-N @NC was 78 mVdec , which was smaller than that
4
4
ꢀ
1
of Pt/C (139 mVdec ) (Figure S27), illustrating a much faster
kinetic process catalysis. It is worth noting that the value of E1/
observed in Fe-N /Pt-N @NC makes it one of Fe-based
profile (orange line) shows the edge energy (E ) value of Fe-
0
2
4
4
N /Pt-N located between Fe foil and Fe O , confirming that
the average oxidation state of Fe was positively charged
below + 3, consistent with the Fe 2p XPS result in Fig-
SACs or dual metal sites electrocatalysts ever reported, as
shown in Figure 3b and Table S4–S5. In addition to the Fe-N4/
Pt-N atomic sites, mononuclear Fe-N sites were probably
4
4
2
3
4
4
ure S19c. The Pt-L edge adsorption fine structure in Fig-
present, however, the dominated enhancement of ORR
3
ure 2g shows that the Pt in the Fe-N /Pt-N is in the same
activity was contributed from Fe-N /Pt-N4 with a more
4
4
4
oxidation state around + 2, matching well with the previously
positive E .
1
/2
[19,27b,28]
ꢀ
reported literatures
and Figure S19d. The coordina-
The HO2 monitor of Fe-N /Pt-N @NC was measured by
4 4
RRDE in Figure 3c, yielding a low HO2 production of less
than 5%, which was lower than that on the Fe-N sites (ca.
8%, Figure S28). The low production of HO2 by the Fe-N4/
ꢀ
tion configurations were determined by the Fourier trans-
formed (FT) EXAFS (Figure 2h). Different from Fe foil and
Fe O , only the peak at ca. 1.5 ꢀ (without phase shift
4
ꢀ
2
3
correction, orange line) was observed in the FT profile of
Pt-N @NC system further confirmed that the Pt-N was not
4
4
Fe K edge, indicating the dominant Fe-N coordinatio-
the active center and followed the “modulation effect”
principle to tune the electronic state of Fe-N4 for the
enhancement of 4 e pathway. Otherwise, much more HO
2
[
2a,6m,27b]
n.
The detailed Fe-N coordinate structure was con-
4
ꢀ
ꢀ
ꢀ
firmed based on the EXAFS experimental spectra and fitted
curves (Figure S20 and Figure S21). The FT profile of Pt-L3
edge in Figure 2h (green line) showed that no Pt-Pt bond in
Fe-N /Pt-N was observed, revealing that atomically dispersed
could be detected if Pt-N was acted as active center for a 2 e
4
ꢀ
reaction pathway (Figure S5). The HO2 yield of Pt-N @NC
was above 30%, revealing a 2 e reaction pathway was
4
ꢀ
4
4
Pt atoms without aggregation of Pt NPs or cluster. First
coordination shell at R = 1.6 ꢀ, which was ascribed to the Pt-
N4 moiety with FT curve-fitting analyses in Figure S22.
Interestingly, in both orange and green lines, the secondary
peak at ca. 2.3 ꢀ (the distance of FeꢀPt bond) was not
involved, which was consistent with the simulated result in
[3e]
Figure S5 and previously reported results. The correspond-
ꢀ
ing electron transfer number measurement indicates a 4 e
reduction pathway in Fe-N , Fe-N /Pt-N and Pt/C, respec-
4
4
4
tively. These efficient kinetics were further affirmed by the
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Figure 3. ORR performance of Fe-N /Pt-N @NC 0.1 M KOH. a) Linear sweep voltammetry of different catalysts in a O -saturated KOH electrolyte
4
4
2
ꢀ1
with a scan rate of 5 mVs . b) Comparisons of Eonset and E1/2 of Fe-N /Pt-N @NC (orange square) and previously reported Fe based SACs (dark
green squares) and low Pt catalysts (light green squares) in Tables S3,S4. c) Electron transfer numbers (n) and HO2 production of Fe-N /Pt-
4
4
ꢀ
4
N @NC and control samples. d) ORR polarization curves of Fe-N /Pt-N @NC before (solid line) and after cyclability (dash line). e) I-t curve for
4
4
4
Fe-N /Pt-N @NC and its comparison with Fe-N @NC and commercial Pt/C at 0.70 vs. RHE. f) Methanol tolerance tests of Fe-N /Pt-N @NC and
4
4
4
4
4
Pt/C.
Koutecky-Levich (K-L) plots shown in Figure S29–S31,
(Figure S35), approximating the theoretical value of 1.65 V.
The ZAB driven by Fe-N /Pt-N @NC delivered a peak power
respectively. However, the Pt-N showed an electron transfer
4
4
4
ꢀ
ꢀ
ꢀ2
number of 3.4 (Figure 3c), indicating a 2 e and 4 e reduction
pathway simultaneously involved, in accordance with the
density of ca. 200 mWcm , much higher than that of
ꢀ
2
commercial Pt/C (119 mWcm ) and other reported NC
supported metal-based SACs (Table S6). Besides, the ZAB
based on the Fe-N /Pt-N @NC exhibited a larger discharge
[
13]
previously reported work.
Furthermore, the Fe-N /Pt-
4
N @NC also exhibits robust stability with only a 8 mV
4
4
4
ꢀ
1
ꢀ2
negative shift on E1/2 after continuous 10000 cycles (Fig-
specific capacity of 749.8 mAhg at 10 mAcm , as high as
91.4% utilization of the theoretical capacity value of
820 mAhg (Figure 4c). Moreover, the two tandem ZABs
could lighted up light-emitting diodes for few days, strongly
demonstrating the promising practical application in powder
energy devices. Notably, negligible decay was observed at
ure 3d), while the Fe-N showed a much pronounced negative
4
ꢀ1
shift of 15 mV (Figure S32) after continuous 5000 cycles. The
excellent stability of Fe-N /Pt-N @NC was also demonstrated
4
4
by the chronoamperometry (CA) test, as shown in Figure 3e,
revealing a 6% decay after 12 h test in an O -sataurated
2
ꢀ
2
electrolyte. In contrast, the current density retentions of Fe-
a current density of 10 mAcm over a period of 40 h,
however, the Pt/C-based system exhibited an obvious degra-
dation within 20 h (Figure 4e), revealing the notable dura-
bility of Fe-N /Pt-N @NC than commercial Pt/C.
N @NC and Pt/C present 86% and 70% of its initial current
4
density value. Superior to Pt/C catalysts, no obvious variation
was observed for Fe-N /Pt-N @NC (Figure 3 f) in the meth-
4
4
4
4
anol tolerance test, while the current density value of Pt/C
was sharply decreased, suggesting the excellent methanol
tolerance merit of Fe-N /Pt-N @NC. The XAFS results of Fe-
Intriguingly, inspired by the significant enhancement by
modulation effect of Pt-N to boost the Fe-N ORR perfor-
4
4
mance, we also tested if this principle was effective on the
other generally reported Co-N4 and Mn-N4 systems. The
ORR oxygen intermediates absorbed on Co-N site or Pt-N
4
4
N /Pt-N @NC after ORR tests, indicating the robust structure
4
4
during the reaction (Figure S33). Additionally, the Fe and Pt
atoms were uniformly embedded in the carbon matrix after
the ORR test (Figure S34), demonstrating its superior
durability.
4
4
ꢀ
ꢀ
site for both the 2 e and 4 e reaction processes were all
considered, as seen in Figure S36–S37. The corresponding
ꢀ
overpotential for the 4 e pathway on Co site in Co-N , Co
4
Furthermore, a homemade Zn-air battery (ZAB) was
assembled using Fe-N /Pt-N @NC as the electrocatalysts on
and Pt sites in Co-N /Pt-N4 are 1.30, 1.27, and 2.13 V,
respectively, from the Gibbs free energy diagrams for ORR
4
4
4
air cathode, as depicted in Figure 4a. The open-circuit voltage
OCV) of the Fe-N /Pt-N @NC was as high as ca. 1.48 V
in Figure 5a. Meanwhile, it was revealed that the Co-N site
4
ꢀ
(
was the main active site for the 4 e pathway, as Pt-N site
4
4
4
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Figure 4. Zn-air battery performance of Fe-N /Pt-N @NC. a) The as-constructed Zn-air battery; b) Charge/discharge polarizations profiles and
4
4
power density curves of the Zn-air batteries based on Fe-N /Pt-N @NC (green) and Pt/C (orange) catalysts, respectively. c) The discharge profiles
4
4
ꢀ2
of the Zn-air batteries at 10 mAcm constant current. Inset: optical picture of a LED lighted by two Fe-N /Pt-N @NC based Zn-air battery in
4
4
series. d) Galvanostatic discharge–charge cycle profiles of the Zn-air batteries based on Fe-N /Pt-N @NC (green) and Pt/C (orange) catalysts at
4
4
ꢀ
2
1
0 mAcm , respectively.
required relatively much higher overpotential (2.13 V, Fig-
ure S38) to participate in a 2 e pathway. Importantly, the
In the case of Mn-N4 and Mn-N /Pt-N4 systems, the
4
ꢀ
related ORR oxygen intermediates absorbed on Mn-N site
4
negligible change of overpotentials in the Co-N (1.30 V) and
or Pt-N site were seen in Figure S40–S41. As displayed in
4
4
Co-N /Pt-N (1.27 V) systems suggested the Pt-N moieties
Figure 5b and Figure S42, although the intermediates adsorp-
4
4
4
showed much weaker modulation effect on the Co-N4
tions were strengthened in the Mn-N /Pt-N4 system, the
4
moieties than on the Fe-N moieties. The impact of Pt on
Co site was further examined with the orbital analyses in
Figure S39. Comparing the OOH bonding on Co site in Co-N4
overpotential became larger (3.02 V, Figure 5b) than Mn-N4
system (1.66 V, Figure 5b) due to the too strong adsorption of
OH*, leading to a disadvantage modulation effect of Pt-N4
4
[
1b]
(
Figure S39a) and Co-N /Pt-N (Figure S39b), it is clear that
on the Mn-N . The increase of MnꢀO bonding can be
4
4
4
the s * and s * antibonding orbitals can merge into s * due to
rationalized by the orbital analyses as shown in Figure S43.
1
2
3
the Pt-induced orbital reorganization between Co dz
2
and O p
For OOH bonding, it is clear that the s * antibonding orbital
1
orbitals. This thus strengthens the CoꢀO bonding. Unfortu-
can be converted into p bonding orbital due to the hybrid-
2
nately, such a strength is offset by the p * antibonding orbitals
ization of Mn d , d and O p orbitals caused by Pt
3
yz
xz
formed by the rehybridization of Fe dyz and O p. For the OH
bonding (Figure S39c,d), the introduction of Pt makes the
originally symmetric s and s bonding orbitals becomes p
introduction (Figure S43a,b). For OH bonding, the co-em-
bedding of Pt leads to intricate orbital rehybridizations, and
thus form new bonding orbitals like s and p (Figure S43c,d).
3
4
1
5
4
and p , and the originally symmetric p * and p * becomes p *.
These additional bonding orbitals thereby greatly increase the
bonding energies of Mn-OH and Mn-OOH.
2
4
5
6
Although Pt reshapes the electronic structure of Co, these
orbital reconstructions brought only minor changes to the
bonding energy (Figure 5a).
To experimentally confirm that the modulation effect was
not effective on the Co-N /Pt-N and Mn-N /Pt-N systems,
4
4
4
4
the Co-N /Pt-N @NC and Mn-N /Pt-N @NC were prepared
4
4
4
4
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Figure 5. a) Gibbs Free energy diagram for ORR on Co-N , Co-N /Pt-N at 0 and 1.23 V. The overpotential on Co-N , Co and Pt sites in Co-N /Pt-
4
4
4
4
4
N are 1.30, 1.27, and 2.13 V, respectively. b) Gibbs Free energy diagram for ORR on Mn-N , Mn-N /Pt-N at 0 and 1.23 V. The overpotential on
4
4
4
4
Mn-N , Mn and Pt sites in Mn-N /Pt-N are 1.66, 3.02, and 1.22 V, respectively. c) Magnified atomic-resolution HAADF-STEM images of Co-N /Pt-
4
4
4
4
N @NC show the atomically dispersed Co atoms and Pt atoms, which are highlighted by orange dashed rectangles. The white dashed circles
4
represent the Co SACs. The average intensity profiles of Co/Pt atoms (indicated by orange dashed rectangles) reveal the distance between Pt and
Co is ca. 5.2 ꢁ. d) Magnified atomic-resolution HAADF-STEM images of Mn-N /Pt-N @NC showing the atomically dispersed Mn atoms and Pt
4
4
atoms, which are highlighted by orange dashed rectangles. The white dashed circles represent the Mn SACs. The average intensity profiles of Mn/
Pt atoms (indicated by orange dashed rectangles) reveal the distance between Pt and Mn is ca. 5.2 ꢁ. e) Linear sweep voltammetry of Co-N @NC
4
(
orange) and Co-N /Pt-N @NC (green) in a O -saturated KOH electrolyte. f) Linear sweep voltammetry of Mn-N @NC (orange) and Mn-N /Pt-
4 4 2 4 4
N @NC (green) in a O -saturated KOH electrolyte. Scale bars: c),d) 1 nm.
4
2
by similar methods (see experimental section). The TEM
images in Figure S44 and XRD patterns in Figure S45
indicated that no agglomerate nanoparticles can be detected
on the carbon matrix in the both control samples and Co-N4/
Pt-N @NC and Mn-N /Pt-N @NC. Further EDS mapping
EXAFS spectra of the Co-N /Pt-N @NC and Mn-N /Pt-
4 4 4
N @NC were shown in Figure S50–S52, indicating the for-
4
[23,29]
[1b]
[30]
mations of Co-N4,
N-doped carbon matrix. Furthermore, the Co-N /Pt-N @NC
Mn-N4 and Pt-N4 structures in the
4
4
and Mn-N /Pt-N @NC were evaluated by the RRDE, as
4
4
4
4
4
(
Figure S46–49) and atomic-resolution HADDF-STEM im-
shown in Figure 5e,f. The negligible shift of E1/2 between Co-
ages in Figure 5c,d show the atomically dispersed Co/Pt and
Mn/Pt SACs on the NC supports. The XPS spectra and
N /Pt-N @NC and Co-N @NC was observed, suggesting the
4
4
4
addition of Pt-N making no difference for the enhancement
4
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of ORR performance for Co-N . Additionally, the values of
Keywords: dual atomic sites · modulation effect ·
oxygen reduction reaction · platinum · single-atom catalysts
4
ꢀ
n and HO2 production remained the same between Co-N4/
Pt-N @NC and Co-N @NC systems (Figure S53). These
4
4
results were matching well with the calculated Gibbs free
energy diagram in Figure 5a that Pt-N4 showed weak
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4
ment of Pt-N on Fe-N site, the Mn-N /Pt-N @NC showed
4
4
4
4
a negative shift of E1/2 after introduction of adjacent Pt-N site
4
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4
caused by the too strong adsorptions of OH* (Figure 5b),
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4
4
4
ꢀ
@
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ꢀ
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4
ꢀ
2
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4
2
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4
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4
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8
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4
4
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The authors declare no conflict of interest.
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[
[
[
Manuscript received: April 15, 2021
Revised manuscript received: June 17, 2021
Accepted manuscript online: June 22, 2021
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b) W.-H. Lai, B.-W. Zhang, Z. Hu, X.-M. Qu, Y.-X. Jiang, Y.-X.
Version of record online: && &&
,
&&&&
Angew. Chem. Int. Ed. 2021, 60, 2 – 12
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
&&&&
These are not the final page numbers!

Angewandte
Research Articles
Chemie
Research Articles
Single-Atom Catalysts
We have designed an efficient oxygen
reduction reaction (ORR) electrocatalyst
of Fe-N /Pt-N @NC with Fe-N and Pt-N
4
dual atomic sites. The enhancement for
the ORR activity is demonstrated by the
A. L. Han, X. J. Wang, K. Tang,
Z. D. Zhang, C. L. Ye, K. J. Kong, H. B. Hu,
L. R. Zheng, P. Jiang, C. X. Zhao,
Q. Zhang, D. S. Wang,*
4
4
4
modulation effect: Pt-N plays the role in
4
Y. D. Li
&&&& — &&&&
modulating the Fe-N active center to
4
optimize the ORR performance. The
modulation effect is not effective for the
enhancement of ORR performance in the
systems of Co-N /Pt-N @NC and Mn-N /
An Adjacent Atomic Platinum Site
Enables Single-Atom Iron with High
Oxygen Reduction Reaction Performance
4
4
4
Pt-N @NC.
4
&
&&&
www.angewandte.org
ꢀ 2021 Wiley-VCH GmbH
Angew. Chem. Int. Ed. 2021, 60, 2 – 12
These are not the final page numbers!