Atomic-Level Modulation of Electronic Density at Cobalt Single-Atom Sites Derived from Metal–Organic Frameworks: Enhanced Oxygen Reduction Performance

Article abstract of DOI:10.1002/anie.202012798

Demonstrated here is the correlation between atomic configuration induced electronic density of single-atom Co active sites and oxygen reduction reaction (ORR) performance by combining density-functional theory (DFT) calculations and electrochemical analysis. Guided by DFT calculations, a MOF-derived Co single-atom catalyst with the optimal Co₁-N₃PS active moiety incorporated in a hollow carbon polyhedron (Co₁-N₃PS/HC) was designed and synthesized. Co₁-N₃PS/HC exhibits outstanding alkaline ORR activity with a half-wave potential of 0.920 V and superior ORR kinetics with record-level kinetic current density and an ultralow Tafel slope of 31 mV dec^?1, exceeding that of Pt/C and almost all non-precious ORR electrocatalysts. In acidic media the ORR kinetics of Co₁-N₃PS/HC still surpasses that of Pt/C. This work offers atomic-level insight into the relationship between electronic density of the active site and catalytic properties, promoting rational design of efficient catalysts.

Full text of DOI:10.1002/anie.202012798

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: Atomic-level modulation of electronic density of metal-organic
frameworks-derived Co single-atom sites to enhance oxygen
reduction performance
Authors: Yuanjun Chen, Rui Gao, Shufang Ji, Haijing Li, Kun Tang,
Peng Jiang, Haibo Hu, Zedong Zhang, Haigang Hao, Qingyun
Qu, Xiao Liang, Wenxing Chen, Juncai Dong, Dingsheng
Wang, and Yadong Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202012798
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Angewandte Chemie International Edition
10.1002/anie.202012798
RESEARCH ARTICLE
Atomic-level modulation of electronic density of metal-organic
frameworks-derived Co single-atom sites to enhance oxygen
reduction performance
Yuanjun Chen,[a],‡ Rui Gao,[b],‡ Shufang Ji, Haijing Li, Kun Tang, Peng Jiang, Haibo Hu, Zedong
[a]
[c]
[d]
[a]
[d]
[
a]
[b]
[a]
[a]
[e]
[c]
Zhang, Haigang Hao, Qingyun Qu, Xiao Liang, Wenxing Chen, Juncai Dong, Dingsheng
Wang, [a],* Yadong Li[a]
[
a]
Dr. Y. Chen, Dr. S. Ji, Dr. P. Jiang, Z. Zhang, Q. Qu, X. Liang, Prof. D. Wang, Prof. Y. Li
Department of Chemistry, Tsinghua University, Beijing 100084, China
E-mail: wangdingsheng@mail.tsinghua.edu.cn
[
[
[
[
b]
c]
d]
e]
Dr. G. Rui, Dr. H. Hao
College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China
H. Li, Prof. J. Dong
Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
K. Tang, Prof. H. Hu
School of Physics and Materials Science, Anhui University, Hefei 230601, China
Dr. W. Chen
Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
[‡
]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the document.
Abstract: Tuning the electronic structure of single-atom active sites
via engineering atomic configuration is an effective strategy to boost
oxygen reduction reaction (ORR), but still remains challenging
to engineer MOFs-derived M-N-C materials, their ORR catalytic
performance are not good enough as the alternatives to Pt-
based electrocatalysts.[5] One primary reason is the
because of the lack of rational guidance and effective methods. Here, compositional and structural heterogeneity of M-N-C catalysts
we demonstrate the correlation between atomic configuration
induced electronic density of single-atom Co active sites and ORR
performance by combining density functional theory (DFT)
calculations and electrochemical analysis. Guided by DFT
calculations, we design and synthesize a MOFs-derived Co single-
hinder the identification of exact active center and further
enhancement on catalytic performance.[
6]
Recently, single-atom catalysts (SACs) are emerging as a
promising class of catalysts due to maximum atom-utilization
efficiency and unique properties,[7] which have delivered
atom catalyst with the optimal Co
in hollow carbon polyhedron (Co
PS/HC exhibits outstanding alkaline ORR activity with a half-wave
potential of 0.920 V and superior ORR kinetics with record-level
1 3
-N PS active moiety incorporated
excellent catalytic performance in
a variety of important
1
-N
3
PS/HC). As predicted, Co
1
-
reactions.[ Moreover, SACs exhibits great potential to serve as
ideal platforms to identify and modulate the structure of active
centers for enhanced catalytic performance.[9] Much progress
has been made to reveal the intrinsic ORR activity of the active
metal of SACs follows the order of Fe > Co > Cu > Mn > Ni.[
However, the deleterious Fenton effect of Fe-based materials
limits their practical applications.[11] Therefore, enhancing the
ORR performance of Co-based SACs has regarded as an
important target for the development of ORR electrocatalysts.
8]
N
3
-
1
kinetic current density and ultralow Tafel slope of 31 mV dec ,
exceeding the Pt/C and almost all non-precious ORR
electrocatalysts. Even in more challenging acidic media, the ORR
10]
1 3
kinetics of Co -N PS/HC still surpasses that of Pt/C. This work offers
an atomic-level insight into the relationship between electronic
density of active site and catalytic property, promoting rational
design of highly efficient catalysts.
Currently, the active centers of the common M-N-C single-
atom ORR electrocatalysts are the four-coordinated structure of
M-N
4
moiety.[ Some recent researches have demonstrated
12]
Introduction
that the symmetrical planar structure results in the symmetric
electronic distribution, which is not beneficial to the adsorption
and activation of the ORR-relevant species, thereby leading to
limited ORR kinetics and performance.[
difference on electronegativity of different coordination atoms (N,
P, S etc.), modulating the types of coordination atoms of active
metal centers is an effective and sensible strategy to break the
symmetry of electronic density.[
rarely reported, due to the lack of reasonable guidance and
effective methods to achieve the precise control over the types
of coordination atoms at the atomic scale. Therefore, it is highly
desirable to advance scientific understanding of atomic
structure-property relationship and achieve atomic-level
regulation of electronic structure of single-atom active sites for
The commercial applications of energy conversion devices
such as fuel cells and metal-air batteries are strongly dependent
on highly efficient and cost-effective catalysts to drive
13]
Owing to the
electrochemical oxygen reduction reaction (ORR).[1] As
a
consequence, the exploration and development of efficient and
durable non-precious metal catalysts as substitutes for the
expensive and scarce Pt-based materials have attracted
14]
However, this strategy is
extensive research attention.[2]
MOFs) are a class of crystalline porous materials, featured with
Metal-organic frameworks
(
abundant porous structure, great designability and tunable
functionality,[3] which have been considered as promising
templates of metal-nitrogen-carbon (M-N-C) based ORR
[
4]
electrocatalysts. Despite numerous efforts have been devoted
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Angewandte Chemie International Edition
10.1002/anie.202012798
RESEARCH ARTICLE
enhanced ORR performance, but it still remains a grand
challenge.
of P atoms to form the CoN
represented the P and S atoms double doped single-atom Co
catalyst, while the P and S atoms uncoordinated with Co atom.
4
@P
2
S
2
model (Figure S3), which
Here, we demonstrate the electronic density of single-atom
Co active sites can be tailored through regulating atomic
configuration by the modulation of the types of coordination
atoms. Furthermore, the correlation between electronic density
of active sites and ORR performance is revealed by density
functional theory (DFT) calculations and electrochemical
analysis. Directed by DFT calculations, we design the synthesis
of MOFs-derived single-atom Co catalyst that comprises an
4 2 2
Furthermore, one of the two S atoms in CoN @P S was
transferred to coordinate with Co atom, while the other S atom
was unchanged or changed to close to the transferred S atom to
form different CoN S@P S (Figure S4); for comparison, other
3 2
four models with different sites of P atoms were also constructed.
Interestingly, it was found that, via structure optimizing, there
were two CoN
CoN PS@PS, in which the Co atom coordinated with five atoms
3N, 1P and 1S), and the CoN PS@PS-1 was the most stable
structure.
3 2
S@P S models that were distorted to form
3
optimal Co
polyhedron (Co
1
-N
3
PS active moiety supported on hollow carbon
-N PS/HC). The as-prepared Co -N PS/HC
(
3
1
3
1
3
exhibits excellent alkaline ORR performance with outstanding
half-wave potential (E1/2) of 0.920 V, which is 71 mV more
positive than that of the commercial Pt/C. Moreover, the record-
Consequently, to study the effect of the introduced
heteroatom (S and P) on the ORR activity, we calculated the
associative ORR reaction mechanisms (Figure S5, the
corresponding most stable intermediate structures of O *, OOH*,
2
O* and OH* are shown in Figures S6-S10) at the electrode
potential of U = 1.23 V vs. reversible hydrogen electrode (RHE)
level kinetic current density (J
unexpectedly remarkable Tafel slope of 31 mV dec are
accomplished over the Co -N PS/HC catalyst, far exceeding
most known non-precious metal ORR catalysts. Furthermore,
Co -N PS/HC also shows excellent ORR performance and
k
) of 28.0 mA cm-2 at 0.900 V and
-
1
1
3
1
3
on the seven models of CoN
4
, CoN
4 2 4 2 3
@P , CoN @S , CoN P@P,
stability in more challenging acidic media, whose E1/2 of 0.790 V
CoN S@S, CoN @P and CoN
3
4
2
S
2
3
PS@PS, respectively. In view
is closed to that of the Pt/C (0.798 V). The acidic ORR kinetics
of the presence of positively charged heteroatoms such as P
and S near the Co active site, the dissociative ORR reaction
mechanisms (Figure S11, the corresponding most stable
of 25.8 mA cm- at 0.775 V and a Tafel
2
of Co
1
-N
3
PS/HC with a J
k
-
1
slope of 43 mV dec even outperform that of the Pt/C with a J
of 10.0 mA cm at 0.775 V and a Tafel slope of 87 mV dec .
Moreover, Co -N PS/HC based Zn-air battery demonstrates
k
-1
-2
2 h h h
intermediate structures of O *, 2O*, O *+OH*, O * and OH * are
1
3
shown in Figures S12-S15, the suffixs of “h” represents the
species is adsorbed on the top site of the heteroatom of P or S)
were also calculated at U = 1.23 V on the six models with doped
heteroatoms of P and/or S. For comparison, it has been clearly
found that the first and third electrochemical steps of OOH*
great battery performance and charge/discharge durability,
suggesting the promising application in Zn-air battery.
Experimental and DFT results reveal that the high reactivity and
outstanding ORR kinetics are ascribed to unique electronic
density of single-atom Co active sites.
+
−
h h
* formation (O
* + H+
formation (O
+
2
* + H + e → OOH*) and OH
h
*) for the associative and dissociative mechanisms
e− → OH
are the rate determining steps on almost all these single-atom
Co catalysts except for the dissociative mechanism of
Results and Discussion
CoN
step of OH
the CoN PS@PS model has the lowest overpotential (η) of ORR
3
P@P, its rate determining step is the last electrochemical
+
−
h 2
* desorption (OH * + H + e → H O + *). By contrast,
To investigate the relationship between coordination
structure induced electronic density and the four-electron ORR
reaction energetics, seven different single-atom Co structural
h
3
both for the associative and dissociative mechanisms (Figure S5
and S11), and the former is much more favorable than the latter
4
models of CoN , CoN
4 2 4 2 3 3
@P , CoN @S , CoN P@P, CoN S@S,
(0.58 V vs. 1.16 V). Besides, to get in-depth understanding, the
CoN @P CoN
4
2
S
2
,
3
PS@PS were firstly constructed and
barriers of O * dissociation (Figure S16, O * + * → 2O*) were
calculated on the six models with doped heteroatoms of P
and/or S. The calculation results show that, only on the three
2
2
optimized by the density functional theory (DFT) simulations in
Figure S1, in which one graphite N atom was introduced
randomly on every model to balance the high elementary ratio of
N in the real M-N-C based ORR electrocatalysts. For these
models of CoN
4
@P
2
, CoN
4
@P
2
S
2
and CoN
3
PS@PS-re, the O
2
*
h
might be able to be dissociated forming the O * with a lower
4
models, the common accepted CoN was initially chosen as the
barrier (< 0.5 eV) at the room temperature, and all the three
formed O atom are adsorbed on the top P atom uncoordinated
with Co atom. Therefore, it can be reasonable concluded that
single-atom Co catalyst un-doped with heteroatoms. Then, two
P or S atoms were introduced symmetrically into the two ortho-
positions of CoN to form the CoN @P and CoN @S model,
4 4 2 4 2
h
although there may be some O * adsorbed on the top of P atom,
the associative mechanism still is the major ORR reaction
pathway on these single-atom Co catalysts, and the model of
which represented the single-atom Co catalyst doped with near-
range P or S atom. After that, one of the atomic P or S in the
model of CoN
with Co atom to form the new models of CoN
CoN S@S, which represented the single-atom Co catalyst
directly coordinated with P or S atom. Subsequently, via
comparing the doping energy of P and S atom in CoN @P and
CoN @S (Figure S2), it was found that the P atom was more
stable than the S atom. Subsequently, on the basis of CoN @P
two S atoms were further introduced into the two ortho-positions
4
@P
2
and CoN
4
@S
2
was transferred to coordinate
CoN
3
PS@PS has the highest ORR activity.
3
P@P and
3
For the associative mechanism, there is
a
parabolic
relationship (Figure 1a) for the overpotential (η) of ORR as a
function of the O * adsorption energy (∆GO2), in which the single-
4
2
2
4
2
atom Co active center coordinated by 3N, P and S atoms has
the highest ORR activity (η = 0.58 eV) and lowest ∆GO2 (0.02
4
2
,
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
eV), while the Co active center only coordinated by 4N atoms
relationship with the Bader charge of active center Co atom, the
(
∆
CoN
O2 (-0.30 eV). For the other five models, the Co active center
coordinated either by 4N atoms (CoN @P , CoN @S and
CoN @P ), or by 3N atoms and only one heteroatom
P@P and CoN S@S), although the ∆GO2 can be reduced
-0.27 ~ -0.06 eV), the overpotentials are still similar with the
, indicating only the synergistic coordination of P and S
4
) has the lowest ORR activity (η = 1.02 eV) and highest
highest active model of CoN
of Co . In other words, the appropriate charge of Co in
CoN PS@PS model turned by synergistic coordination of P and
3
S atoms can display the excellent ORR performance. In Figure
1c, the charge density differences of the active center Co atom
show that there are obvious different charge distributions of
these models. Compare with the perfectly symmetric electron
3
PS@PS has the moderate charge
δ+
δ+
G
4
2
4
2
4
2
S
2
(CoN
3
3
(
CoN
4
atoms can promote the ORR activity of the single-atom Co
active center.
structure of CoN
CoN @P and CoN
moiety in these two models become narrowed slightly, and
meanwhile the charge of Co atom in CoN @S is reduced
4
, some red area appears in the carbon layer of
4
2
4
@S , the blue area of the center CoN
2
4
4
2
slightly (0.811 vs. 0.862), indicating the electron can transfer
from P and S atoms to the carbon layer and then to the Co atom.
However, because of the symmetry of the two P and S atoms in
CoN
in these two models. In CoN
one of heteroatom is transferred to coordinate with Co atom, the
blue area of the center CoN moiety becomes narrowed
obviously, the charge of Co atom is further reduced (0.703 and
.677), and meanwhile the symmetry of the charge distribution is
broken. Moreover, the continued introducing of P and S leads to
the asymmetrical charge distribution of CoN @P and
CoN PS@PS. Relatively, because of the direct coordination
between P, S and Co atom, the charge of Co atom in the model
of CoN PS@PS is decreased more than that in the CoN @P
0.768 vs. 0.838), and the charge distribution is more asymmetry.
In Figure 1d, the pDOS analysis shows that the d-band center of
Co atom can be reduced dramatically from 0.86 eV (CoN ) to
1.98 eV (CoN PS@PS) by the synergistic coordination of P
4
@P
2
and CoN
4
@S
2
, the charge distributions are not broken
3
P@P and CoN S@S model, when
3
x
0
4
2 2
S
3
3
4
2 2
S
(
4
−
3
and S atoms, and the effect of the S atom is more obvious (-0.07
vs. 0.36 eV, and -0.64 vs. -0.23 eV). Thus, it is inferred that the
introducing of heteroatom P and S for synergistic coordination
can modulate the electronic structure of active center Co atom,
evoke the distortion of electronic density, break the symmetry of
Figure 1. DFT study for the ORR activity on different single-atom Co catalysts.
(
a) The linear relationship for ORR activity (i.e. the overpotential) as a function
RHE
of the O
2
* adsorption energy (∆GO2) at U
= 1.23 V; in the inserted model
4
Co-N moiety, and reduce the d-band center of the active center
structures, the Co, N, P and S atoms are in cyan, blue, pink and yellow, the
carbon layer is given in the grey stick. (b) The inverse volcanoes of ORR
overpotential for the Bader charge of active center Co atom, the negative
value represents electron donation. (c) The charge density difference of Co
atom in the seven catalyst models, the red and blue represents electron
Co atom. And then, the overpotential of ORR is reduced on the
single-atom Co catalysts. In summary, a rational design strategy
of single-atom Co catalysts for ORR is developed, which
achieves the optimal electronic density distribution to enhance
ORR activity and kinetics through modulating slightly the
coordination and electronic structure surrounding the metal
−
3
increase and decrease, respectively, the units for the color scale bar is a.u.
d) The partial density of states (pDOS) of Co atom in the models, the d-band
.
(
center is inserted with the red font and line.
center Co-N
4
moiety by synergistic coordination of the
heteroatom of P and S.
With that, to evaluate the electronic control effects of all the
seven models of single-atom Co catalysts, we analyzed the
Bader charge (Figure 1b), charge density differences (Figure 1c
and Figure S17) and partial density of states (pDOS, Figure 1d)
of the active center Co atom, as well as the Bader charge of the
other elements in these models (Table S1). The electronic
property analysis reveals that almost all the charge of N atoms
In light of the excellent ORR performance of CoN PS@PS
3
model through DFT studies, we developed a MOFs-derived two-
step pyrolysis strategy to prepare a unique single-atom Co
catalyst with an optimal Co-N PS active configuration supported
3
on hollow carbon polyhedron (Co -N PS/HC). The synthetic
1
3
scheme of Co -N PS/HC catalyst is illustrated in Figure 2a.
1
3
δ−
are negative (N ), while the charge of P and S atoms in these
δ+
models are positive (P/S ) except for the S atom coordinated
3
with Co atom in CoN PS@PS, suggesting the N atoms in the
single-atom Co catalysts are the electron acceptor, but the P
and S atoms are the electron donor, which finally leads to the
δ+
positive charge of metal center Co (Co ). Interestingly, Figure
1b shows that the overpotential of ORR has the inverse volcanic
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RESEARCH ARTICLE
Co/NPS-HC-650 at 950 oC under an Ar atmosphere, the final
Co
1
-N
3
PS/HC catalyst was obtained.
The transmission electron microscopy (TEM) image
demonstrates that Co -N PS/HC displays a hollow polyhedral
1
3
morphology with uniform size (Figure 2b). The high angle
annular dark field scanning transmission electron microscopy
1 3
(HAADF-STEM) shows the shell thickness of Co -N PS/HC
hollow structure is about 20 nm. And no obvious nanoparticles
are observed (Figure 2c). Energy-dispersive spectroscopy
1 3
(EDS) mappings of Co -N PS/HC reveal the presence of Co, C,
N, P and S elements (Figure 2d and Figure S20). To investigate
the dispersion state of Co species at the atomic scale, the
aberration-corrected
HAADF-STEM
(AC
HAADF-STEM)
measurement was carried out. As shown in Figure 2e and 2f,
several isolated bright dots associated with single Co atoms
(
marked by white circles) are clearly distinguished from
heteroatoms-doped carbon matrix. The atomic dispersion of Co
species in Co -N PS/HC is further confirmed by the analysis of
1
3
intensity profiles (Figure 2g).
1 3
Powder X-ray diffraction (XRD) pattern of Co -N PS/HC
1 3
Figure 2. Synthesis and structural characterizations of Co -N PS/HC. (a)
exhibits only one broad peak at approximate 25o (2 theta)
indexed to the (002) plane of the graphitic carbon, which is
similar with that of NPS/HC that derived from Co-free PZS@ZIF-
Scheme of the synthesis of Co
images of Co -N PS/HC. (d) Enlarged HAADF-STEM image and the
corresponding EDS mappings of Co -N PS/HC. (e) AC HAADF-STEM image
and (f) enlarged intensity image of Co -N PS/HC. (g) Intensity profiles of Co
PS/HC obtained in the dotted rectangle region 1# and 2# in (f).
1 3
-N PS/HC. (b) TEM and (c) HAADF-STEM
1
3
1
3
1
3
1
-
8
(Figure S21). And no typical peaks ascribed to crystalline Co
N
3
species are observed, excluding the existence of Co-related
nanoparticles. Inductively coupled plasma optical emission
spectrometry (ICP-OES) was used to determine the Co content
The highly cross-linked poly(cyclotriphospazene-co-4,4’-
sulfonyldiphenol)(PZS) coated zeolitic imidazolate framework
in Co
chemical state of C, N, P and S elements in Co
photoelectron spectroscopy (XPS) analysis was performed. And
the element atomic contents of the Co -N PS/HC are shown in
1
-N
3
PS/HC as 0.39 wt%. In order to investigate the
1
-N PS/HC, X-ray
3
(
ZIF-8) core-shell composites (ZIF-8@PZS) was firstly pyrolysed
o
at 650 C under an Ar atmosphere to form nitrogen, phosphorus
and sulfur co-doped hollow carbon polyhedron (NPS-HC-650)
sample (Figure S18). The formation procedure of hollow
structure follows
between the S ion from PZS shell and Zn ion from ZIF-8 core
1
3
the Figure S22. As shown in Figure S23, the C 1s spectrum can
be fit well with four peaks at the binding energy of 284.8 eV,
288.1 eV, 285.8 eV and 284.6 eV, assigned to graphitic C=C, C-
N, C-P and C-S, respectively. The N 1s spectrum in Figure S24
exhibits five peaks, which are indexed to pyridinic N (398.6 eV),
pyrrolic N (400.2 eV), graphitic N (401.5 eV), oxidized N (403.8
eV) and Co-N (399.1 eV). The P 2p spectrum demonstrates the
co-existence of four peaks at the binding energy of 132.7 eV,
Kirkendall effect.[15] The interdiffusion
a
2-
2+
2+
occurs during this process. Because the ionic radius of Zn ion
is smaller than that of S ion, the outward transport of Zn ion is
2-
2+
2-
faster compared with the inward transport of S ion. The
continuous unequal interdiffusion leads to the appearance of
Kirkendall voids and then generates hollow structure. Similar to
the S species, the P species from PZS shell also diffuses into
and uniformly disperses over the entire hollow NPS-HC-650
architecture (Figure S19). Subsequently, the cobalt precursor
was added, which can absorb on hierarchical pore on the NPS-
HC-650 (Co/NPS-HC-650). Followed by the pyrolysis of
133.9 eV, 129.5 eV and 130.4 eV, corresponding to P-C, P-O
and 2p3/2, 2p1/2 splitting of Co-P (Figure S25). The S 2p
spectrum can be deconvoluted into five peaks at 164.0 eV,
1
65.3 eV, 168.3 eV, 161.9 eV and 163.1 eV, associated with the
3/2, 2p1/2 splitting of the S 2p spin orbital (-C-S-C-), oxidized S
2p
and the 2p3/2, 2p1/2 splitting of Co-S, respectively (Figure S26).
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RESEARCH ARTICLE
Figure 3. Atomic structural analysis of Co
contour plots of Co -N PS/HC and reference samples. (c) Co K-edge EXAFS fitting analysis of Co
between the experimental FT-EXAFS spectra (purple line) and theoretically simulated FT-EXAFS spectra (red line) of Co
inset. (f) Co K-edge XANES spectra and (g) first derivative curves of Co -N PS/HC and reference samples. (h) Comparison between the experimental XANES
-N PS/HC based on DFT model of the inset.
1
-N
3
PS/HC. (a) Co K-edge FT-EXAFS spectra of Co
1
-N
3
PS/HC and reference samples. (b) Co K-edge WT-EXAFS
1
3
1
-N
3
PS/HC in R space and (d) k space. (e) Comparison
-N PS/HC based on DFT model of the
1
3
1
3
spectra (purple line) and theoretically simulated XANES spectra (red line) of Co
1
3
-
1
Synchrotron-radiation-based X-ray absorption fine structure
maximum at approximately 3.4 Å , which is close to the intensity
(
XAFS) measurements were conducted to confirm the local
structure of Co -N PS/HC. As shown in Figure 3a, the Fourier
transformed (FT) extended X-ray absorption fine structure (FT-
EXAFS) of Co -N PS/HC exhibits one major peak at about 1.4 Å
maximum of CoPc and the first-shell intensity maximum of
Co O , suggesting it can be attributed to the contribution from
3 4
1
3
the backscattering between Co and light atoms. Additionally,
compared with Co foil, the intensity maximum at approximately
1
3
and one shoulder peak at about 1.8 Å. In comparison with the
FT-EXAFS spectra of cobalt phthalocyanine (CoPc) and cobalt
6.8 Å-1 ascribed to Co-Co coordination is not observed in Co
1
-
N
3
PS/HC. The above results well confirm that Co species are
oxide (Co
3 4
O ), which are assigned to the backscattering
atomically dispersed in Co -N PS/HC without the existence of
1
3
between Co and light atoms (N, P, S etc.). The FT-EXAFS peak
at 2.2 Å from Co foil is associated with Co-Co coordination. The
metal-derived crystalline structures.
Quantitative least-squares EXAFS curve-fitting analysis was
carried out to investigate the coordination configuration. The
best-fitting result of Co -N PS/HC clearly demonstrates the
1 3
major peak at 1.4 Å and one shoulder peak at 1.8 Å are
attributed to superimposition of Co-N, Co-P and Co-S first-shell
coordination (Figure 3c, 3d and Figure S27). The structural
parameters were further extracted. The corresponding
absence of Co-Co scattering path in Co
demonstrates that Co species exist as the isolated single atoms.
To further confirm the atomic dispersion of Co species in Co
PS/HC, the wavelet transform (WT) analysis was performed,
1 3
-N PS/HC
1
-
N
3
which can discriminate the backscattering atoms and offer the
powerful resolution in both R and k spaces. As shown in Figure
1 3
3b, the WT contour plot of Co -N PS/HC exhibits one intensity
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
coordination numbers of N, P and S are estimated to be 3.1, 0.9
and 1.1 with the average bond lengths of 1.97 Å, 2.32 Å and
samples. The atomically dispersed Co species in Co
carry partially positive charges, which is determined by
comparison of the first derivative XANES curves of Co
PS/HC with reference samples (Figure 3g). Moreover,
XANES spectroscopy possesses a much high sensitivity to the
three-dimensional arrangement of atoms around center metal,
which is a powerful tool to identify the atomic configuration.
Therefore, we carried out XANES theoretical calculation and
1 3
-N PS/HC
2
.34 Å, respectively, which suggests a single-atom Co
moiety. The first-shell EXAFS curve-fitting results of CoPc,
Co and Co foil are also shown in Figure S28-S30 and Table
S2. The average bond lengths and coordination numbers of the
synthesized Co -N PS/HC are close to those of the DFT model
of CoN PS@PS (Table S3), indicating the optimal Co -N PS
1
-N
3
PS
1
-
N
3
O
3 4
1
3
3
1
3
active moiety is prepared. To further confirm it, we performed
theoretical simulation to reproduce the corresponding EXAFS
found that the XANES profile of Co
1 3
-N PS/HC can be well
reproduced by the structure of CoN PS@PS moiety from the
3
spectra of the Co
1
-N
3
PS/HC catalyst based on the structural
above DFT calculations (Figure 3h). The above XAFS analysis
results as well as the element atomic content analysis by XPS
and EDS mappings well confirm that the Co single-atom catalyst
models of CoN PS@PS that optimized by DFT calculations. As
3
shown in Figure 3e, the theoretically simulated EXAFS spectra
are in good agreement with the experimental EXAFS spectra.
1 3
with the optimal Co -N PS moiety is synthesized, which has
great potential to achieve highly efficient ORR performance, as
predicted by the DFT calculations.
X-ray absorption near-edge structure (XANES) spectroscopy
can reflect the electronic structure of center metal. Figure 3f
1 3
displays the XANES curves of Co -N PS/HC and reference
Figure 4. Electrocatalytic alkaline ORR performance of Co
Comparison of J at 0.90 V and E1/2 of Co -N PS/HC and reference catalysts. (c) Tafel plots of Co
-N PS/HC and the previously reported Co based catalysts and other non-precious metal catalysts in Table S4. (e) Electron transfer numbers (top) and
yield of Co -N PS/HC. (f) Electrochemical double-layer capacitance (Cdl) of Co -N PS/HC and reference catalysts. (g) Tolerance to methanol of Co
PS/HC compared with 20% Pt/C at 0.70 V. Methanol is injected into the 0.1 M KOH solution at the time of 200 s. (h) ORR polarization curves of Co -N PS/HC
-N PS/HC after durability test.
1
-N
3
1 3
PS/HC. (a) ORR polarization curves of Co -N PS/HC and reference catalysts in 0.1 M KOH. (b)
k
1
3
1
3
-N PS/HC and reference catalysts. (d) Comparison of Eonset and
E
1/2 of Co
1
3
H
N
2
O
2
1
3
1
3
1
-
3
1
3
before and after 10000 potential cycles. Inset is AC HAADF-STEM image of Co
1
3
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
The rotating disk electrode (RDE) technique was employed
which is 44 mV more negative than that of Co
indicating lower ORR activity of Co -N /HC. And, the kinetic
current density of Co -N PS/HC at 0.90 V outperforms than that
of Co -N /HC by a factor of 21. In comparison to Co -N /HC, the
obviously higher kinetic current density and smaller Tafel value
of Co -N PS/HC reveal that the better ORR kinetics is achieved
over Co -N PS/HC. Raman spectra demonstrate that Co
PS/HC has higher degree of graphitization compared with
NPS-HC, which is beneficial to the electrical conductivity for
enhanced electrochemical performance (Figure S41).
Furthermore, Co
1 3
-N PS/HC,
to assess the ORR activity of Co
M KOH. As shown in Figure 4a, the linear sweep voltammetry
LSV) measurements demonstrate that Co -N PS/HC exhibits
the most positive onset potential of 1.00 V. And Co -N PS/HC
1
-N
3 2
PS/HC in O -saturated 0.1
1
4
1
3
(
1
3
1
4
1
4
1
3
also shows excellent ORR activity in term of outstanding half-
wave potential (E1/2) of 0.920 V, which is 71 mV more positive
than that of the state-of-art commercial Pt/C (E1/2 = 0.849 V).
1
3
1
3
1
-
N
3
Moreover, Co
density (J
1
-N
3
PS/HC exhibits the record-value kinetic current
) of 28.0 mA cm- at 0.90 V, which is 30 times higher
2
k
than that of Pt/C (Figure 4b). As shown in Figure 4c, the ultralow
Tafel value of 31 mV dec-1 is observed over Co
-N PS/HC. The
record-value kinetic current density and unexpectedly
outstanding Tafel value of Co -N PS/HC suggests that the ORR
kinetics achieves a remarkable enhancement over Co -N PS/HC
1
-N
3
PS/HC exhibits highest electrochemical
-2
1
3
double-layer capacitance (Cdl) of 98 mF cm than that of NPS-
-
2
-2
1 4
HC (61 mF cm ) and Co -N /HC (63 mF cm ), indicating the
1
3
existence of N, P, S synergistic coordinated Co atomic sites
facilitates to increase the electrochemically active surface area
(ESCA) (Figure 4f and Figures S42-S44). Moreover, we
1
3
compared with the commercial Pt/C. Figure 4d and Table S4
show the comparison of alkaline ORR performance between
analyzed and compared the compositions of the Co
1 3
-N PS/HC,
Co
electrocatalysts, demonstrating Co
alkaline ORR electrocatalysts in term of onset potential, half-
1
-N
3
PS/HC and previously reported non-precious ORR
Co -N /HC and NPS-HC catalysts by combining ICP-OES, XPS
and EDS map sum spectra (Tables S5-S7), suggesting the
element contents as well as the ratio of N, P and S of these
1
4
1
-N PS/HC is one of the best
3
wave potential, kinetic current density and Tafel value. Moreover, three catalysts are similar with each other. The above results
the ORR pathway of Co -N PS/HC was evaluated by the RDE demonstrate that the outstanding ORR performance of Co
PS/HC is attributed to atomic dispersion of Co species and
1
3
1
-
measurements at various rotation rates (Figure S31a). As shown
in Figure S31b, the corresponding Koutecky-Levich (K-L) plot is
linear, suggesting Co -N PS/HC demonstrates the first-order
1 3
N
3
unique structure of active center with synergistic N, P and S
coordination, as further supported by the remarkable
reaction kinetics toward the concentration of dissolved oxygen.
Based on the K-L equation, the value of electron-transfer
number (n) is calculated to be in the range from 3.93 to 3.99
over the potential range from 0.3 to 0.7 V. This result is close to
the theoretical value of 4 for the four-electron ORR pathway.
Furthermore, rotating ring disk electrode (RRDE) measurement
was performed, which demonstrates H O yield of Co -N PS/HC
2 2 1 3
maintains below 3.5 wt% over the potential range from 0.3 to 0.9
V with the corresponding high electron-transfer number of 3.93-
1 3
enhancement of Co -N PS/HC on mass activity (MA) and
turnover frequency (TOF) in comparison to other reported
single-atom ORR catalysts (Table S8).
The tolerance to methanol crossover and durability of
electrocatalysts are of great importance for practical applications.
The methanol tolerance of Co
injecting methanol solution during the chronoamperometric
measurement. As shown in Figure 4g, Co -N PS/HC still
1 3
-N PS/HC was evaluated by
1
3
maintains a stable current density after methanol injection. In
contrast, the commercial Pt/C exhibits a sharp jump of current
density ascribed to the occurrence of methanol oxidation. These
1 3
3.98 (Figure 4e), indicating Co -N PS/HC catalyzes ORR via a
highly efficient and four-electron process.
To investigate the effect of single Co atoms on ORR
performance, the cobalt-free N, P, S co-doped hollow carbon
polyhedron (NPS/HC) material was prepared and characterized
1 3
results reveal that Co -N PS/HC possesses excellent tolerance
to methanol crossover. Moreover, we conducted accelerated
durability test (ADT) to investigate the long-term durability of
(
Figures S32-S34). The NPS/HC catalyst shows an obviously
Co
wave potential of Co
continuous cycles, revealing that Co
superior long-term durability. After durability tests, the hollow
polyhedral morphology of the used Co -N PS/HC still remains
(Figure S45), and the XRD pattern of the used Co -N PS/HC is
similar with that of the fresh Co -N PS/HC (Figure S46),
1
-N
3
PS/HC. As shown in Figure 4h, no obvious decay in half-
-N PS/HC is observed even after 10000
-N PS/HC exhibits
lower ORR activity with a negatively shifted E1/2 of 0.825 mV,
and more sluggish ORR kinetics with extremely low kinetic
current density of 0.287 mA cm at 0.90 V and large Tafel value
of 70 mV dec . The above results reveal the crucial role of
single Co atoms in enhancing ORR performance. To reveal the
effect of synergistic P and S coordination of Co
ORR performance, we synthesized the single Co atoms with
Co coordination active center supported on N, P, S co-doped
hollow carbon polyhedron (Co -N /HC) catalyst (Figures S35-
S38). The content of Co in the Co -N /HC catalyst is determined
as 0.42 wt% by the ICP-OES measurement. The FT-EXAFS
curve of Co -N /HC displays only one main peak at about 1.5 Å,
which is similar with that of CoPc, indicating that the main peak
is assigned to Co-N coordination. And no peaks associated with
Co-P and Co-S coordinations as well as Co-Co coordination are
1
3
1
3
a
-
2
-
1
1
3
1
3
1
-N
3
PS/HC on
1
3
demonstrating outstanding structural robustness. Further
observations in EDS mapping and AC HAADF-STEM image
1 4
N
1
4
reveal that the atomic dispersion of Co species in Co
is well preserved (Figure S47 and Figure 4h). These results
disclose that Co -N PS/HC has an excellent stability.
1 3
-N PS/HC
1
4
1
3
1
4
We also evaluated acidic ORR performance of Co
1 3
-N PS/HC
in 0.5 M H SO . As shown in Figure 5a, Co -N PS/HC exhibits
2
4
1
3
excellent acidic ORR performance in term of outstanding an E1/2
of 0.790 V, which is comparable to that of the commercial Pt/C
detected, suggesting the Co species in Co
atomically dispersed and coordinated by N species (Figure S39
and S40). Co -N /HC exhibits a half-wave potential of 0.876 mV,
1 4
-N /HC are
(
0.798 V). Furthermore, Co
1 3
-N PS/HC shows better acidic ORR
kinetics with a larger J
k
of 25.8 mA cm-2 at 0.775 V and a smaller
1
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Tafel slope of 43 mV dec-1 compared with Pt/C with a J
of 10.0
typical PANI-derived Fe-N-C with a E1/2 loss of 80 mV after 5000
k
mA cm-2 at 0.775 V and a Tafel slope of 87 mV dec-1 (Figure 5b
cycles.[16] The above results well reveal that Co
-N PS/HC
1 3
and 5c). It can be seen that Co
obviously poor acidic ORR performance, suggesting the
excellent acidic ORR performance of Co -N PS/HC is attributed
1
-N
4
/HC and NPS/HC show
delivers excellent acidic ORR performance. After potential
cycles in acidic media, the structure and atomic dispersion of Co
1
3
species of Co
1 3
-N PS/HC catalyst well maintain (Figures S53-
to atomically dispersed Co active sites and unique atomic
configuration with N, P and S synergistic coordination. As
S55), suggesting the excellent stability in acidic media.
displayed in Table S9, the acidic ORR performance of Co
1
-
N
3
PS/HC outperforms those of most of the previously reported
non-precious acidic ORR electrocatalysts.
Figure 6. Zn-air battery performance of Co
of Zn-air battery. (b) Discharge polarization curves and power density plots of
Co -N PS/HC and Pt/C based Zn-air battery. (c) Specific discharging
3
capacities of Co -N PS/HC and Pt/C based Zn-air battery at 10 mA cm . (d)
1 3
-N PS/HC. (a) Schematic diagram
1
3
-2
1
1 3
Galvanostatic discharge-charge cycling curves of Co -N PS/HC based Zn-air
battery.
Figure 5. Electrocatalytic acidic ORR performance of Co
1
-N
3
PS/HC. (a) ORR
SO
PS/HC and reference
PS/HC and reference catalysts. (d) ORR
3
PS/HC before and after 10000 potential cycles in
polarization curves of Co
b) Comparison of J at 0.775 V and E1/2 of Co
catalysts. (c) Tafel plots of Co -N
polarization curves of Co -N
.5 M H SO
-N
1 3
PS/HC and reference catalysts in 0.5 M H
2
4
.
(
k
1
-N
3
1
3
To demonstrate the potential application of Co
catalyst in energy conversion devices, we employed Co
PS/HC as the catalyst of air cathode and Zn foil as the anode
with 6 M KOH electrolyte to assemble the Zn-air battery (Figure
a). Co -N PS/HC based Zn-air battery shows an open-circuit
voltage of 1.47 V, which is higher than that of Pt/C+RuO based
1 3
-N PS/HC
1
1
-
0
2
4
.
N
3
6
1
3
The LSV curves of Co
were recorded and the corresponding K-L plots were obtained
Figure S48 and S49). The electron transfer number of Co
PS/HC in acidic media was estimated as 3.93-4.02 based on
K-L equation. Moreover, we carried out the RRDE tests, which
demonstrates the H yield of Co -N PS/HC maintains below
.7 % in the potential range from 0.1 to 0.6 V associated with the
high electron-transfer number of 3.945-3.955 (Figure S50). In
comparison, the H yield of Pt/C remains below 2.2 % and the
corresponding electron transfer number is calculated as 3.955-
1 3
-N PS/HC at different rotation rates
2
Zn-air battery (Figure S56). And a higher discharge current and
an obvious shrunken discharge-charge voltage gap are
(
N
1
-
3
accomplished in the Co
compared to the values of Pt/C+RuO
indicating the higher activity of the Co
enabling better rechargeability (Figure S57). As shown in Figure
b, Co -N PS/HC based Zn-air battery also exhibits an excellent
maximum powder density of 176 mW cm at a current density of
1
-N
3
PS/HC based Zn-air battery
based Zn-air battery,
-N PS/HC catalyst
2
2
O
2
1
3
1
3
2
6
1
3
2
O
2
-
2
-
2
-2
2
80 mA cm , which are both higher than those (117 mW cm ,
3
.970. The above results well suggest Co
1
-N
3
PS/HC undergoes
-
2
-
183 mA cm ) of Pt/C based Zn-air battery. The specific capacity
a high-efficient 4e catalytic process. In addition to excellent
of the Co
7
1
-N
3
PS/HC based Zn-air battery achieves as high as
acidic ORR activity, Co
to methanol. As shown in Figure S51, there is no obvious
change in CV curves of Co -N PS/HC after the injection of
1 3
-N PS/HC also exhibits great tolerance
86 mA h g-1 at the discharge current density of 10 mA cm ,
-2
-
1
which is larger than that (744 mA h g ) of Pt/C based Zn-air
battery (Figure 6c). As listed in Table S10, the Zn-air
1
3
methanol. For comparison, the typical oxygen reduction peak
becomes weaker and an inverse peak associated with methanol
oxidation appears in the CV curves of the Pt/C when methanol is
added into acidic electrolyte (Figure S52). The great stability for
ORR electrocatalysts in acidic media is urgently needed. As
performance of Co
surpasses most of reported catalysts. As shown in the inset of
Figure 6c, two Co -N PS/HC based Zn-air batteries in series can
easily power a blue light-emitting diode (LED) panel displaying
Zn air”. Moreover, we carried out galvanostatic charge and
discharge measurements to evaluate the rechargeability and
cyclic stability of Co -N PS/HC based Zn-air battery. As shown
in Figure 6d, no obvious increase in the charge/discharge
1 3
-N PS/HC as the catalyst of air cathode
1
3
“
shown in Figure 5d, Co
with a small E1/2 loss of only 12 mV in half-wave potential after
0000 potential cycles test in 0.5 M H SO , compared with the
1 3
-N PS/HC exhibits an enhanced stability
1
3
1
2
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
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In summary, we demonstrate that ORR performance of Co
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control of atomic configuration of active sites based on DFT
calculations and electrochemical analysis. Guided by DFT
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Keywords: metal-organic frameworks (MOFs) • single-atom Co
•
oxygen reduction • electronic density modulation
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.202012798
RESEARCH ARTICLE
RESEARCH ARTICLE
Yuanjun Chen,[ Rui Gao,
a],‡
[b],‡
Shufang
Ji,[ Haijing Li, Kun Tang, Peng
a]
[c]
[d]
Jiang,[ Haibo Hu, Zedong Zhang,
a]
[d]
[a]
[
b]
[a]
Haigang Hao, Qingyun Qu, Xiao
Liang,[ Wenxing Chen,[e] Juncai Dong,
a]
[c]
Dingsheng Wang, [a],* Yadong Li[a]
The correlation between atomic configuration induced electronic density of single-
atom Co active sites and oxygen reduction reaction (ORR) performance has been
established by combing density functional theory calculations and electrochemical
analysis. Directed by this correlation, we design the synthesis of MOFs-derived
Page No. – Page No.
Atomic-level modulation of electronic
density of metal-organic frameworks-
derived Co single-atom sites to enhance
oxygen reduction performance
single-atom Co catalyst that comprises an optimal Co
1
-N
3
PS active moiety
supported on hollow carbon polyhedron (Co
alkaline and acidic ORR performance.
1
-N
3
PS/HC), which exhibits superior
This article is protected by copyright. All rights reserved.