Revealing Isolated M?N3C1 Active Sites for Efficient Collaborative Oxygen Reduction Catalysis

Article abstract of DOI:10.1002/anie.202008325

Single atom catalysts (SACs) are of great importance for oxygen reduction, a critical process in renewable energy technologies. The catalytic performance of SACs largely depends on the structure of their active sites, but explorations of highly active structures for SAC active sites are still limited. Herein, we demonstrate a combined experimental and theoretical study of oxygen reduction catalysis on SACs, which incorporate M?N₃C₁ site structure, composed of atomically dispersed transition metals (e.g., Fe, Co, and Cu) in nitrogenated carbon nanosheets. The resulting SACs with M?N₃C₁ sites exhibited prominent oxygen reduction catalytic activities in both acidic and alkaline media, following the trend Fe?N₃C₁ > Co?N₃C₁ > Cu?N₃C₁. Theoretical calculations suggest the C atoms in these structures behave as collaborative adsorption sites to M atoms, thanks to interactions between the d/p orbitals of the M/C atoms in the M?N₃C₁ sites, enabling dual site oxygen reduction.

Full text of DOI:10.1002/anie.202008325

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Accepted Article
Title: Revealing isolated M-N3C1 active sites for efficient collaborative
oxygen reduction catalysis
Authors: Feng Li, Gao-Feng Han, Yunfei Bu, Hyuk-Jun Noh, Jong-
Pil Jeon, Tae Joo Shin, Seok-Jin Kim, Yuen Wu, Hu Young
Jeong, Zhengping Fu, Yalin Lu, and Jong-Beom Baek
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202008325
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10.1002/anie.202008325
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COMMUNICATION
Revealing isolated M-N3C1 active sites for efficient collaborative
oxygen reduction catalysis
Feng Li, Gao-Feng Han,* Yunfei Bu,* Hyuk-Jun Noh, Jong-Pil Jeon, Tae Joo Shin, Seok-Jin Kim, Yuen
Wu, Hu Young Jeong,* Zhengping Fu,* Yalin Lu, and Jong-Beom Baek*
Abstract: Single atom catalysts (SACs) are of great importance for
oxygen reduction, a critical process in renewable energy technologies
such as fuel cells and metal-air batteries. The catalytic performance
of SACs largely depends on the structure of their active sites, but
explorations of highly active structures for SAC active sites are still
limited. Herein, we demonstrate a combined experimental and
theoretical study of oxygen reduction catalysis on SACs, which
incorporate M-N3C1 site structure, composed of atomically dispersed
transition metals (e.g., Fe, Co, and Cu) in nitrogen doped carbon
nanosheets. The resulting SACs with M-N3C1 sites exhibited
prominent oxygen reduction catalytic activities in both acidic and
alkaline media, following the trend Fe-N3C1 > Co-N3C1 > Cu-N3C1.
Theoretical calculations suggest the C atoms in these structures
behave as collaborative adsorption sites to M atoms, thanks to
interactions between the d/p orbitals of the M/C atoms in the M-N3C1
sites, enabling dual site oxygen reduction. Compared with the
common single site mechanism, the proposed dual site mechanism
was determined to be more favorable for oxygen reduction catalysis,
in good agreement with the experimental findings.
Practical applications require efficient, stable and low-cost ORR
catalysts. An efficient ORR catalyst should exhibit a minimal
overpotential and fast reaction kinetics. Platinum and its alloys are
presently believed to be the most efficient ORR catalysts.
However, Pt-based catalysts suffer from material scarcity, poor
methanol tolerance and stability, and high cost, which limits their
utility and use.[2] Recently, transition metal based single atom
catalysts (SACs) have been developed as potentially better
alternatives for efficient and stable electrochemical ORR. In one
approach, Fe single atoms were embedded in a carbon basal
plane with a Fe-N4 structure. The edge-hosted Fe-N4 sites were
the major contributor to the high activity towards oxygen
reduction.[3] A concave-shaped SAC with dense Fe-N4 sites was
also synthesized, and showed high ORR catalytic activity.[4] In
addition to these, SACs have been incorporated with Co-N4, Mn-
N4 and Zn-N4 sites, and have exhibited acceptably high oxygen
reduction catalysis activity.[5]
The catalytic performance of SACs mainly relies on the
orbital interactions of adsorbed intermediates and their active
sites. The electronic and geometric features of the active sites’
structures play an important role in such interactions. To date,
however, few active site architectures beyond the M-N4 structure
have been developed and studied.
The oxygen reduction reaction (ORR) is an important process
because of its crucial role in fuel cells and metal-air batteries.[1]
In the present study, we conducted a rational examination of
oxygen reduction catalysis on SACs incorporated with highly
active M-N3C1 sites, combining experiment and theory. The SACs
were prepared with a series of atomically dispersed transition
metals on nitrogen doped carbon nanosheets (MNCSs, M = Fe,
Co or Cu). With the M-N3C1 structured active sites, the MNCSs
exhibited good catalytic performances toward both acid and
alkaline oxygen reduction, with the trend Fe-N3C1 > Co-N3C1 >
Cu-N3C1. Moreover, theoretical calculations revealed the
presence of collaborative adsorption sites in the M-N3C1 sites for
oxygen activation. It was also determined that ORR on the M-
N3C1 sites prefers to proceed via a more efficient dual site
mechanism, rather than the common single site mechanism on
M-N4 sites. This work suggests new opportunities for exploring
active sites with highly active structures, and provides deep
insight into the catalytic mechanisms of various catalytic
applications.
[*]
Prof. F. Li, Prof. G.-F. Han, H.-J. Noh, J.P. Jeon, Dr. S.J. Kim, Prof.
J.-B. Baek
School of Energy and Chemical Engineering/Center for Dimension
Controllable Organic Frameworks
Ulsan National Institute of Science and Technology (South Korea)
Email: jbbaek@unist.ac.kr, gfhan@unist.ac.kr
Prof. Z.P. Fu, Prof. Y.L. Lu
CAS Key Laboratory of Materials for Energy Conversion
Hefei National Laboratory for Physical Sciences at Microscale
University of Science and Technology of China (P. R. China)
Email: fuzp@ustc.edu.cn
Prof. T.J. Shin, Prof. H.Y. Jeong
UNIST Central Research Facilities, Ulsan National Institute of
Science and Technology (South Korea)
Email: hulex@unist.ac.kr
Prof. Y.F. Bu
Jiangsu Collaborative Innovation Center of Atmospheric
Environment and Equipment Technology (CICAEET), Jiangsu Key
Laboratory of Atmospheric Environment Monitoring and Pollution
Control (AEMPC), School of Environmental Science and
Technology, Nanjing University of Information Science and
Technology (P. R. China)
Email: yunfei.bu@nuist.edu.cn
Prof. Y.E. Wu
Department of Chemistry, Hefei National Laboratory for Physical
Sciences at Microscale, Collaborative Innovation Center of
Chemistry for Energy Materials (iChEM), University of Science and
Technology of China (P. R. China)
Figure 1a shows the procedure for preparing catalysts with
M-N3C1 sites. The MNCSs were obtained using a simple thermal
treatment of a mixture of trimesic acid, dicyandiamide and metal
species in an argon atmosphere at 800 °C for 3 h, without further
post treatment. Nitrogen doped carbon (NC) nanosheets without
metal were also prepared using a similar process.
A field emission scanning electron microscope (FE-SEM),
transmission electron microscopy (TEM) and X-ray photoelectron
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. a) Schematic illustration of the synthesis of catalysts with M-N3C1 sites. b-d) HAADF-STEM images of FeNCS, CoNCS and CuNCS. Scale bars: 2 nm. e-
g) XANES spectra of FeNCS, CoNCS and CuNCS. h-j) Fourier transform (FT) of the EXAFS spectra of FeNCS, CoNCS and CuNCS.
spectroscopy (XPS) were used to study the morphology and
chemical composition of the MNCSs (Figure S1-4, Supporting
Information). MNCSs compose of wrinkled nanosheets, which
contain C, N and a series of metal elements, respectively. In the
X-ray diffraction (XRD) patterns, the broad peaks of MNCSs at
around 26° can be assigned to the (002) plane of graphitic carbon
(Figure S5a, Supporting Information). No peaks belonging to the
relative metal or metal oxide were detected. Meanwhile, there are
two identical peaks appeared at approximately 1360 and 1560
cm−1 in the Raman spectra of MNCSs, which can be assigned to
the D and G bands, respectively (Figure S5b, Supporting
Information).
Using inductively coupled plasma optical emission
spectrometry (ICP-OES) analysis, the MNCSs were determined
to have a similar metal concentration of around 5 wt% (Table S1,
Supporting Information). Meanwhile, the specific surface areas of
FeNCS, CoNCS and CuNCS were measured to be 119.2, 137.2
and 136.5 m2 g−1, by nitrogen adsorption/desorption using the
Brunauer–Emmett–Teller (BET) method (Figure S6, Supporting
Information).
Details of the structures of the as-synthesized samples were
characterized by atomic-resolution transmission electron
microscopy (AR-TEM). The high-resolution TEM images of
MNCSs showed the lattice fringes with a spacing of approximately
0.34 nm, (002) plane of graphitic carbon, agreeing well with the
XRD measurement (Figure S7, Supporting Information). Element
mapping was conducted with electron energy-loss spectroscopy
(EELS) to confirm composition and spatial distribution, and it
revealed that C, N, Fe, Co and Cu elements were uniformly
distributed in the FeNCS, CoNCS and CuNCS, respectively.
High-angle-annular dark field scanning transmission electron
microscopy (HAADF-STEM) equipped with an imaging-side
spherical aberration (Cs) corrector was employed to determine
the form of the Fe, Co and Cu atoms. Because of the difference
in contrast, the HAADF-STEM images in Figure 1b-d clearly show
single metal atoms, which appear as white dots. These results
confirm that the metal elements are homogeneously distributed in
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Figure 2. a) Polarization curves for various catalysts in oxygen saturated 0.1 M aq. KOH solution. b) Corresponding potential dependent kinetic current densities
for various catalysts. c) Mass activities for Pt/C, FeNCS, CoNCS and CuNCS at a potential of 0.90 V. d) Polarization curves for various catalysts in oxygen saturated
0.1 M aq. HClO4 solution. e) Corresponding potential dependent kinetic current densities for various catalysts. f) Mass activities for Pt/C, FeNCS, CoNCS and
CuNCS at a potential of 0.80 V. Rotation speed: 1600 r.p.m. Scan rate: 5 mV s–1
.
the MNCSs in the form of single atoms, rather than nanoparticles.
The surface density of these single Fe, Co and Cu atoms were
subsequently calculated to be 2.1, 1.8 and 1.8 atom nm−2,
respectively (Table S2, Supporting Information).
sphere, their signal shifted to about 2 Å in the EXAFS spectrum
of CuNCS (Figure 1j).
Meanwhile, a possible formation mechanism of M-N3C1 sites
was further proposed. During the synthesis, trimesic acid and
dicyandiamide were used to extremely dilute the concentration of
metal species, and thus to create abundant carbon/nitrogen steric
hindrances and nitrogen species traps. The ultra-diluted metal
atoms cannot break through the solitary confinement for further
growth into nanoparticles. On the other hand, the nitrogen species
vanished heavily with respect to increasing reaction temperature.
As a result, it would be much easier to form M-N3C1 rather than
M-N4 sites at the relatively higher reaction temperature. When the
concentration of metal species and reaction temperature were
further increased, the solitary confinement would be broken,
leading to the formation of metal clusters.
Because of M-N3C1 sites in MNCSs had similar specific
surface areas and single atom surface densities, their intrinsic
activities could be evaluated in parallel. The oxygen reduction
tests for MNCSs were first conducted in an oxygen saturated 0.1
M aq. KOH solution (Figure S9-11, Supporting Information). NC
and benchmark Pt/C were compared under the same conditions.
The ORR polarization curves of various samples are shown in
Figure 2a. The FeNCS showed a more positive onset potential
than CoNCS, CuNCS and NC, which indicates that oxygen can
be reduced on Fe-N3C1 sites with a smaller overpotential. The
half-wave potentials (E1/2) of FeNCS, CoNCS and CuNCS were
0.882, 0.875 and 0.847 V, respectively. These values are more
positive than that of Pt/C (0.839 V) and other SACs (Table S4,
Supporting Information), suggesting their superior catalytic
activities.[6]
The chemical and structural natures of the metal atoms in the
MNCSs were further investigated using highly sensitive
synchrotron irradiation-based X-ray absorption spectroscopy
(XAS). Figure 1e shows the Fe K-edge X-ray absorption near-
edge structure (XANES) spectra of the Fe foil, FeNCS and iron(II)
phthalocyanine (FePc). The absorption edge of the FeNCS is
located between those of the Fe foil and FePc, revealing the
oxidation state of the Fe atoms is between 0 and +2. The
extended X-ray absorption fine structure (EXAFS) of the FeNCS
was also studied, to investigate the Fe coordination environment
(Figure 1h). The Fourier-transformed (FT) k3-weighted EXAFS
spectra of the FeNCS exhibited a main peak similar to FePc,
which can be attributed to the Fe-N/C coordination. Unlike the Fe
foil, the typical Fe-Fe coordination peak at around 2.2 Å was not
detected.
To identify the structural configuration of the Fe atoms, both
the XANES and FT transformed EXAFS spectra of the FeNCS
were simulated (Figure S8a-b, Supporting Information).
Importantly, the highly structural sensitive XANES spectrum of
FeNCS was found to be identical to the theoretical simulation
based on the Fe-N3C1 structure. Meanwhile, the FT transformed
EXAFS spectrum of FeNCS was also well fitted to the proposed
Fe-N3C1 structure (Table S3, Supporting Information). The
coordination environment of Fe atoms in FeNCS was confirmed
by combined experimental and theoretical XANES/EXAFS
spectra to be the Fe-N3C1 structure. Similar M-N3C1 structured
coordination environments were confirmed to occur with the
Co/Cu atoms in CoNCS/CuNCS, respectively (Figure 1, Figure
S8, Supporting Information). On the other hand, due to the longer
distance between the four C atoms in the second coordination
The potential dependent kinetic current densities (Jk) of
various samples were calculated using the Koutecky–Levich
equation, and further normalized with the surface accessible
metal contents (Figure 2b). As shown in Figure 2c, the mass
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Figure 3. a-c) The optimized structures of the Fe-N3C1, Co-N3C1 and Cu-N3C1 sites, respectively. d-f) The isosurfaces of the electron density difference for Fe-N3C1,
Co-N3C1 and Cu-N3C1 sites, respectively. The cyan and yellow isosurfaces denote an increase and decrease of 0.1 eꢀÅ−3 in electronic density, respectively. g-i) The
projected density of states distribution of C, N, Fe, Co and Cu in Fe-N3C1, Co-N3C1 and Cu-N3C1 sites, respectively. Deep yellow, light purple, coral, grey and blue
ivory balls represent iron, cobalt, copper, carbon and nitrogen atoms, respectively.
activities of Pt/C, FeNCS, CoNCS and CuNCS at 0.90 V were
25.8 mA mgPt−1, 300.6 mA mgFe−1, 183.6 mA mgCo−1and 54.7 mA
mgCu−1, respectively. With higher metal atom utilization, MNCSs
can offer mass activity comparable to or even higher than
commercial Pt/C. Significantly, these values indicated that the
intrinsic catalytic activities of M-N3C1 sites follow the trend Fe-
N3C1 > Co-N3C1 > Cu-N3C1. Beside the high catalytic activity, the
results confirmed the high selectivity and good stability of the
MNCSs.
were further studied using electron density difference calculations
(Figure 3d-f). As shown, the electrons of the M atoms prefer to
transfer to the bridge sites between the M and coordinated N/C
atoms in the M-N3C1 structures. Compared with the bridge sites
between the M and the coordinated C atoms in Cu-N3C1, those in
the Fe-N3C1 and Co-N3C1 sites exhibited much higher electron
densities, indicating stronger interactions between the d orbitals
of Fe/Co and the p orbitals of the C atoms.
On the other hand, the much lower electron densities of the
C atoms in the Fe-N3C1 and Co-N3C1 sites suggest these C atoms
are potential collaborative adsorption sites for oxygen molecules,
and can behave as dual adsorption sites together with Fe/Co
atoms, respectively. This is quite unlike the common single site
adsorption sites, such as M-N4. Compared with single site
adsorption, the O-O bond lengths of the adsorbed oxygen
molecule are significantly enlarged on a dual site, leading to a
much higher degree of activation (Figure S13, Table S5,
Supporting Information).
The ORR performance of the MNCSs were further studied in
an oxygen saturated 0.1 M aq. HClO4 solution. Remarkably,
FeNCS and CoNCS still exhibited catalytic activity comparable to
Pt/C. The E1/2s of FeNCS and CoNCS were only 23 and 40 mV
more negative than that of Pt/C, surpassing other recently
reported SACs (Figure 2d).[6e, 6g, 7] At a potential of 0.80 V, the
mass activities of FeNCS, CoNCS and CuNCS were 166.8 mA
−1
mgFe−1, 55.9 mA mgCo and 0.6 mA mgCu−1, respectively, the
same trend in intrinsic catalytic activity observed for M-N3C1 sites
in acidic media (Figure 2e-f).
The projected density of state (DOS) distributions of M, N and
C in the M-N3C1 sites were also analyzed (Figure 3g-i). For Fe-
N3C1 and Co-N3C1 sites, the sharp DOS peak of Fe/Co and C
above the Fermi level (EF) reveal that these atoms have favored
empty orbitals to accommodate the electrons transferred from
oxygen, which is responsible for the potential oxygen adsorption
To obtain deeper insight into the reaction mechanisms of
oxygen reduction on M-N3C1 sites, first principles density-function
theory (DFT) calculations were conducted. Figure 3a-c show the
optimized structures of the Fe-N3C1, Co-N3C1 and Cu-N3C1 sites
(Figure S12, Supporting Information). Their electronic structures
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Figure 4. a,b) Proposed reaction pathway on Fe-N3C1 site via single site and dual site mechanisms, respectively. c-e) The calculated free energy diagrams of
oxygen reduction on Fe-N3C1, Co-N3C1 and Cu-N3C1 sites at the potential of 1.23 V, respectively. Deep yellow, grey, blue, red and white ivory balls represent iron,
carbon, nitrogen, oxygen and hydrogen atoms, respectively.
and activation. Due to the weak interaction between Cu and C
atoms, the situation was quite different in the Cu-N3C1 sites.
With the above in mind, the oxygen reduction occurring on
M-N3C1 sites was further explored for both single site and dual
site mechanisms. Figure 4a illustrates that the four-electron ORR
proceeds on a single Fe atom in the Fe-N3C1 site, through the
adsorption of oxygen (*O2), the reduction of *O2 into *OOH, *O
and *OH, as well as the desorption of OH. For the dual site
mechanism, the C atom in the Fe-N3C1 site is proposed as the
collaborative adsorption site to the Fe atom (Figure 4b).
The calculated free energy diagram of oxygen reduction on
the Fe-N3C1 site at an equilibrium potential of 1.23 V is shown in
Figure 4c. The endothermic reaction process with a dominated
free energy change (ΔG) is considered to be the rate determining
step (RDS) of the reaction. The RDSs for the single site and dual
site mechanisms are the formation of *OH and *O, which exhibit
ΔGs of 0.73 and 0.46 eV, respectively. Remarkably, because it
has a much smaller ΔG value for the RDS (ΔGRDS), the proposed
dual site mechanism is more thermodynamically favorable than
the common single site mechanism for oxygen reduction on the
Fe-N3C1 site. The free energy diagrams for ORR on Co-N3C1 and
Cu-N3C1 sites were also determined (Figure 4d-e). For the Co-
N3C1 and Cu-N3C1 sites, the RDSs for the single site mechanism
were the formation of *OOH (0.86 eV) and *O (0.92 eV), while the
RDSs for the dual site mechanism were the formation of *O (0.55
eV) and the adsorption of O2 (0.74 eV), respectively. These
values also suggest the dual site mechanism is the favored
reaction mechanism for ORR on both the Co-N3C1 and Cu-N3C1
sites, and this was confirmed by experimental results (Figure S14,
Supporting Information).
N3C1 > Co-N3C1 > Cu-N3C1. The proposed mechanism agreed
well with the experimental findings, revealing the origin of the high
catalytic activity of the M-N3C1 sites from an unusual viewpoint.
In summary, we report highly activated M-N3C1 structured
active sites embedded in nitrogen doped carbon nanosheets for
efficient oxygen reduction catalysis. The M-N3C1 coordination
environments were established using both experimental and
theoretical XANES/EXAFS analyses. The MNCSs, incorporated
with M-N3C1 sites, exhibited significantly enhanced oxygen
reduction performance, which followed the trend Fe-N3C1 > Co-
N3C1 > Cu-N3C1. Theoretical calculations suggest the C atoms in
the M-N3C1 sites can provide additional collaborative adsorption
sites for oxygen activation, promoting dual site catalysis.
Compared with the common single site mechanism, the dual site
mechanism is more thermodynamically favorable for oxygen
reduction. This work not only explored highly activated structures
for SACs active sites, but also provides deeper insights into
catalytic mechanisms for various catalytic applications.
Acknowledgements
The authors would like to thank Pohang Light Source (PLS-II,
Korea) for the synchrotron beam time. This work was supported
by the Creative Research Initiative (CRI, 2014R1A3A2069102),
the Science Research Center (SRC, 2016R1A5A1009405), and
the BK21 Plus (10Z20130011057) program through the National
Research Foundation (NRF) of Korea.
Keywords: Active sites • single atom catalyst • oxygen reduction
reaction • collaborative catalysis
Importantly, the ΔGRDS values of the Fe-N3C1, Co-N3C1 and
Cu-N3C1 sites were ranked in the order of Fe-N3C1 < Co-N3C1 <
Cu-N3C1, indicating their catalytic activities follow the trend of Fe-
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10.1002/anie.202008325
Angewandte Chemie International Edition
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Isolated M-N3C1 active sites were
engineered into the single atom
catalysts for efficient oxygen reduction
catalysis. Theoretical calculations
proposed that the M/C atoms in the
active sites served as dual adsorption
sites, enabling more efficient
Feng Li, Gao-Feng Han,* Yunfei Bu,*
Hyuk-Jun Noh, Jong-Pil Jeon, Tae Joo
Shin, Seok-Jin Kim, Yuen Wu, Hu
Young Jeong,* Zhengping Fu,* Yalin Lu,
and Jong-Beom Baek*
Page No. – Page No.
collaborative catalysis.
Revealing efficient M-N3C1 active
sites for efficient collaborative
oxygen reduction catalysis
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Title
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