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Angewandte
Eine Zeitschrift der Gesellschaft Deutscher Chemiker
Chemie
www.angewandte.de
Akzeptierter Artikel
Titel: Rational Design of Single Mo Atoms Anchored on N-doped
Carbon for Effective Hydrogen Evolution Reaction
Autoren: Yadong Li, Wenxing Chen, Jiajing Pei, Chunting He, Jiawei
Wan, Hanlin Ren, Youqi Zhu, Yu Wang, Juncai Dong, Shubo
Tian, Weng-Chon Cheong, Siqi Lu, Lirong Zheng, Xusheng
Zheng, Wensheng Yan, Zhongbin Zhuang, Chen Chen, Qing
Peng, and Dingsheng Wang
Dieser Beitrag wurde nach Begutachtung und Überarbeitung sofort als
"akzeptierter Artikel" (Accepted Article; AA) publiziert und kann unter
Angabe der unten stehenden Digitalobjekt-Identifizierungsnummer
(DOI) zitiert werden. Die deutsche Übersetzung wird gemeinsam mit der
endgültigen englischen Fassung erscheinen. Die endgültige englische
Fassung (Version of Record) wird ehestmöglich nach dem Redigieren
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Für die AA-Fassung trägt der Autor die alleinige Verantwortung.
Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.201710599
Angew. Chem. 10.1002/ange.201710599
Link zur VoR: http://dx.doi.org/10.1002/anie.201710599
http://dx.doi.org/10.1002/ange.201710599
10.1002/ange.201710599
Angewandte Chemie
COMMUNICATION
Rational Design of Single Mo Atoms Anchored on N-doped
Carbon for Effective Hydrogen Evolution Reaction
Abstract: Highly efficient electrochemical hydrogen evolution
reaction (HER) provides a promising pathway to resolve energy and
environment problems. Herein, we designed an electrocatalyst with
single Mo atoms (Mo-SAs) supported on N-doped carbon with
outstanding HER performance. The structure of the catalyst was
probed by aberration-corrected scanning transmission electron
microscopy (AC-STEM) and X-ray absorption fine structure (XAFS)
spectroscopy, indicating the formation of Mo-SAs anchored with one
nitrogen atom and two carbon atoms (Mo1N1C2). Importantly, the
Mo1N1C2 catalyst displayed much more excellent activity compared
with Mo2C and MoN, and better stability than commercial Pt/C.
Density functional theory (DFT) calculation revealed that the unique
structure of Mo1N1C2 moiety played a crucial effect to improve the
HER performance. This work opens up new opportunities for the
preparation and application of highly active and stable Mo-based
HER catalysts.
Hydrogen is an attractive substitute for traditional fossil fuels,
and electrochemical hydrogen evolution reaction (HER) is
thought to be a method to generate hydrogen effectively, in
which the catalysts play a dominant role. [1] For HER, Pt-based
nanomaterials are considered as the best and practical catalysts,
with low overpotential, small Tafel slope and high exchange
current density, but blocked in their rare source, fancy price and
poor electrochemical stability. Recently, a variety of earthabundant Pt-free catalysts have been intensively researched
[*]
Dr. W. Chen[+], Dr. J. Wan, H. Ren, Dr. Y. Zhu, S. Tian, W. Cheong,
Prof. C. Chen, Prof. Q. Peng, Prof. D. Wang, Prof. Y. Li
Department of Chemistry, Tsinghua University, Beijing 100084
China
E-mail: wangdingsheng@mail.tsinghua.edu.cn
ydli@mail.tsinghua.edu.cn
J. Pei[+], S. Lu, Prof. Z. Zhuang
State Key Lab of Organic-Inorganic Composites and Beijing
Advanced Innovation Center for Soft Matter Science and
Engineering, Beijing University of Chemical Technology, Beijing
100029, China
Dr. C. He[+]
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry,
School of Chemistry, Sun Yat-Sen University, Guangzhou 510275,
China
Dr. Y. Wang
Shanghai Synchrontron Radiation Facility, Shanghai Institute of
Applied Physics, Chinese Academy of Science, Shanghai 201800,
China
Dr. J. Dong, Dr. L. Zheng,
Beijing Synchrotron Radiation Facility, Institute of High Energy
Physics, Chinese Academy of Sciences, Beijing, China
Dr. X. Zheng, Prof. W. Yan
National Synchrotron Radiation Laboratory (NSRL), University of
Science and Technology of China, Hefei, Anhui 230029, China
[+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
and developed as HER catalysts under acidic or alkaline
conditions.[2] Among them, Mo-based catalysts (MoxC, MoxN,
etc.) have attracted extensive attention due to their excellent
catalytic performance.[3] Although a number of efforts have been
made, it is still extremely necessary and great challenge to
understand the nature of the catalytic activity at atomic level for
rational design of highly efficient Mo-based catalysts. Recently,
single metal atom catalysts (SACs) are of great interest in
heterogeneous catalysis, mainly due to their high atom utilization
and unique catalytic performance.[4] Furthermore, SACs open a
new door for us to explore catalytic process at atomic scale.
However, to the best of our knowledge, the preparation and
investigation of single Mo atoms (Mo-SAs) catalyst hasn’t been
implemented yet.
Herein, for the first time, we successfully achieved synthesis
of Mo-SAs catalyst, with the help of chitosan, which is one of the
most abundant biopolymers in nature. The local atomic structure
of the catalyst is confirmed as Mo1N1C2 moiety by employing
synchrotron-radiation-based X-ray absorption spectroscopy (SRXAS). The as-prepared catalyst exhibited highly efficient activity
for HER in alkaline condition, with a low overpotential (onset
overpotential ηonset = 13 mV, and overpotential at a current
density of -10 mA cm-2 η10 = 132 mV) and a small Tafel slope (90
mV dec-1) in 0.1 M KOH solution, which is much better than that
of Mo2C and MoN. The Mo-SAC is among the excellent non-Pt
HER catalysts in alkaline media (Table S1). Additionally, the
catalyst also displayed excellent stability, with negligible activity
degradation after 1000 CV cycles. DFT calculation reveals the
different structure-activity relationships between Mo1N1C2 and
Mo2C or MoN. Moreover, this synthetic strategy for Mo-SAs can
also be applied to other metals (Cu, Pd, Pt, etc.), which gives
new opportunities for the research of SACs.
The single Mo atom catalyst (Mo-SAC) was prepared via
combining templated and pyrolysis method, using sodium
molybdate and chitosan (hydrophilic and with fruitful
uncoordinated amine groups) as precursors (Figure S1). The
synthesis detail was described in the supplement materials. The
morphology of the catalyst was characterized by scanning
electron microscopy (SEM) and transmission electron
microscopy (TEM). The SEM images in Figure S2 showed that
the obtained Mo-SAC possessed amounts of spherical voids
after removal of SiO2 templates, leading to a porous N-doped
carbon framework. Brunauer-Emmett-Teller (BET) absorptiondesorption isotherms also demonstrated that the catalyst
contained a highly open porous structure, with specific surface
area of 583 m2/g. It was emphasized that 3D porous structure
was of advantage to the accessibility of active sites and the
mass transport during the electrochemical catalytic process. [5]
The TEM and HRTEM images in Figure 1a and 1c further
showed that the obtained foam was fabricated by ultrathin Ndoped carbon walls. Moreover, small particles or clusters of Mo
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Wenxing Chen+, Jiajing Pei+, Chunting He+, Jiawei Wan, Hanlin Ren, Youqi Zhu, Yu Wang, Juncai
Dong, Shubo Tian, Weng-Chon Cheong, Siqi Lu, Lirong Zheng, Xusheng Zheng, Wensheng Yan,
Zhongbin Zhuang, Chen Chen, Qing Peng, Dingsheng Wang, Yadong Li
10.1002/ange.201710599
Angewandte Chemie
COMMUNICATION
peak located at 1.3 Å, which was mainly attributed to the
scattering of Mo-N or Mo-C coordination. Moreover, the
scattering peaks derived from Mo-Mo coordination were not
observed, in comparison with Mo foil, Mo2C and MoN. These
demonstrated the atomic dispersion of Mo in the N-doped
carbon substrate.
Figure 2. (a) XANES and (b) FT-EXAFS curves of the Mo1N1C2 at Mo K-edge,
(c) WT-EXAFS of the Mo1N1C2, Mo foil, Mo2C and MoN, (d) FT-EXAFS fitting
curves of the Mo1N1C2 at Mo K-edge (FT range: 2.0-10.5 Å-1, fitting range: 0.52.5 Å), the insert is the k space fitting curve of the Mo1N1C2, (e) atomic
structure model of the Mo1N1C2.
Figure 1. (a) TEM image of the Mo1N1C2, the insert is SAED pattern, (b) EDS
maps revealing the homogeneous distribution of Mo and N on the carbon
support, (c) HRTEM image and (d) AC-STEM image of the Mo1N1C2.
Synchrotron-radiation-based hard X-ray absorption fine
structure (XAFS) measurements were performed to further
investigate the structure of Mo-SAC at atomic level. Generally
speaking, the average oxidation state of Mo species can be
described through the absorption threshold position of Mo Kedge.[6] In the XANES curves (Figure 2a as well as Figure S5),
the position of the sample located between Mo2C and MoN,
indicating the oxidation state of Mo was between the two
references. As we know, Fourier transform (FT) is a fundamental
step for the data extraction and interpretation of EXAFS
spectra.[7] The FT-EXAFS of the sample was illustrated in Figure
2b. We found that the sample exhibited only one obvious FT
Wavelet transform (WT) is thought to be a wonderful
supplement for FT.[8] And owing to the powerful resolution in
both k and R spaces, WT-EXAFS was recently employed to
probe the atomic dispersion of SACs.[9] The Mo K-edge WTEXAFS of the sample was illustrated in Figure 2c, as well as the
comparison of the q–space magnitudes for FEFF-calculated k3weighted EXAFS paths in Figure S6. The WT contour plots of
the sample displayed only one intensity maximum at 7.2 Å-1,
corresponding to the Mo-N/C coordination. Additionally, the WT
signals related to Mo-Mo contribution weren’t detected,
compared with the WT plots of Mo foil, Mo2C and MoN. These
further demonstrated the formation of the Mo-SAs. Quantitative
EXAFS fitting was performed to extract the structural parameters,
and the fitting results were exhibited in Figure 2d, Figure S7 and
Table S2. The first shell of the central atom Mo displayed a
coordination number of four. Based on the EXAFS fitting, it was
thought that the Mo-SAs were atomically anchored in the Ndoped porous carbon matrix, three-fold coordinated by one N
atom and two C atoms (Mo1N1C2). Additionally, one O2 molecule
This article is protected by copyright. All rights reserved.
Accepted Manuscript
species weren’t observed in the field of view. The selected area
electron diffraction (SAED) pattern (the insert in Figure 1a)
exhibited the poor crystallinity of the N-doped carbon frame, as
well as the Powder X-ray diffraction (PXRD) pattern in Figure
S3a. Energy-dispersive X-ray spectroscopy (EDS) in a scanning
transmission electron microscope (STEM) suggested the
uniform distributions of Mo and N on the carbon frame (Figure
1b). The atomic dispersion of Mo on the N-doped carbon
substrate could be monitored directly by STEM, equipped with a
probe spherical aberration corrector. The Mo-SAs were
confirmed by monodispersed bright dots marked with red circles
for better observation (Figure 1d). X-ray photoelectron
spectroscopy (XPS) and soft X-ray absorption near-edge
structure (XANES) spectroscopy were applied to probe the
valence state and electronic structure of N and C in the sample
(Figure S4). The Mo content in the catalyst was 1.32 wt %,
according to the coupled plasma optical emission spectrometry
(ICP-OES) analysis.
10.1002/ange.201710599
Angewandte Chemie
COMMUNICATION
Figure 3. Electrocatalytic HER performance of the Mo1N1C2 in alkaline
condition (0.1 M KOH). (a) HER polarization curves for the Mo1N1C2 in
comparison with Mo2C, MoN and 20% Pt/C. (b) Overpotential for Mo1N1C2
compared with Mo2C, MoN and 20% Pt/C. (c) Tafel plots and (d) Nyquist plots
of the Mo1N1C2, Mo2C, MoN and 20% Pt/C. (e) TOF value of the Mo1N1C2. (f)
The long-term durability measurements of the Mo1N1C2. The polarization
curves were recorded initially and after 1000 CV cycles between 0 and -0.25 V
(vs RHE) at 50 mV S-1.
We investigated the electrochemical HER activity of the
Mo1N1C2 at alkaline condition (in 0.1 M KOH solution). A typical
three-electrode setup was adopted to conduct the
electrocatalytic measurements (Figure S9). For comparison, the
HER performance of Mo2C, MoN and Pt/C (20 wt %) was also
recorded under the same condition. The polarization curves with
iR correction (the specific percentage of the correction is 100 %)
were exhibited in Figure 3a and 3b, respectively. The Mo 1N1C2
displayed an onset overpotential (ηonset) of 13 mV, which was
much lower than that of the counterparts (107 mV for Mo2C and
121 mV for MoN). Additionally, the Mo1N1C2 needed only an
overpotential (η10) of 132 mV to reach the current density of 10
mA cm-2. The polarization curves of the sample and references
at different loading weight were also displayed in Figure S10S12. Tafel slope is usually employed to investigate the reaction
kinetics during HER process, and a smaller Tafel slope is more
advantageous for HER catalysis.[10] As shown in Figure 3c, the
Tafel slope of the Mo1N1C2 was as low as 90 mV per decade,
much smaller than that of Mo2C (102 mV per decade) and MoN
(163 mV per decade). The Nyquist plots (Figure 3d) indicated
that the charge transfer resistance of the Mo1N1C2 was lower in
comparison with Mo2C or MoN, suggesting the Mo1N1C2 had a
faster charge-transfer capacity during HER. The turnover
frequency (TOF) was then investigated (Figure 3e). The TOF
value of the Mo1N1C2 at thermodynamic potential (0 V vs RHE)
was calculated to be 0.047 S-1 by using exchange current
density. The values of Mo1N1C2 at the overpotentials of 50, 100
and 150 mV were 0.148, 0.465 and 1.46 S-1, respectively.
Besides the HER activity, the stability is another critical
factor to evaluate an electrocatalyst. To survey the stability of
the Mo1N1C2 in alkaline environment, long-term cyclic
voltammetry (CV) and chronopotentiometry measurements were
conducted. In Figure 3f, the polarization curves measured before
and after 1000 CV cycles ranging from 0 V to -0.25 V (vs RHE)
showed negligible degradation for both the HER overpotential
and the cathodic current density, indicating that the Mo 1N1C2
was of superior stability against long-period electrocatalytic
processes, agreeing well with the results obtained from the
chronometry curves at the current density of 10 mA cm-2 (Figure
S13-S14). The structural stability of the Mo1N1C2 sample was
further confirmed by SEM, TEM, HAADF-STEM and XAFS
measurements (Figure S15-S18 and Table S2). The high
durability of the Mo-SAC is due to the unique structure of the
Mo1N1C2 moiety, leading to strong interaction between active Mo
atoms and the N-doped carbon matrix. When tested in acidic
media (in 0.5 M H2SO4 solution), the Mo1N1C2 catalyst also
exhibited improved activity (Figure S19-S25). The catalyst
displayed a low overpotential (η onset = 48 mV, and η10 = 154 mV)
and a small Tafel slope (86 mV dec -1). The performance is
among the good activity for electrocatalysts based on non-noble
metal materials (Table S3). Moreover, the catalyst also exhibited
excellent stability toward HER at acidic condition (Figure S26S28).
Figure 4. (a) Gibbs free energy for H* adsorption on different catalysts of
Mo1N1C2, Mo2C and MoN. (b) The calculated DOS of the Mo1N1C2. The black
dashed line denotes the position of the Fermi level.
The adsorption free energy of H* (ΔGH*) is a key descriptor
of the HER activity in both alkaline and acidic conditions, and
generally the closer to zero, the better.[1b, 10] ΔGH* largely
depends on the geometric and electronic structures of the
catalysts.[11] The ΔGH* values of the Mo catalysts were
investigated using the density functional theory (DFT) method,
as shown in Figure 4a. Six possible active sites of the Mo 1N1C2
This article is protected by copyright. All rights reserved.
Accepted Manuscript
was considered to adsorb on the Mo-SA (as illustrated in Figure
2e). The best-fit EXAFS results of Mo foil, Mo2C and MoN were
exhibited in Figure S8 and Table S2. Combining the
morphological information with the corresponding structure
characterizations, the presence of individual Mo atoms was well
confirmed.
10.1002/ange.201710599
Angewandte Chemie
COMMUNICATION
Additionally, the developed strategy was also effective in the
synthesis of other M-SA (M=Cu, Pd, Pt, etc.) materials. The
typical EDS maps, HAADF-STEM images, XANES and EXAFS
curves of the as prepared Cu-SA (Figure S34-S36), Pd-SA
(Figure S37-S39) and Pt-SA (Figure S40-S42) were exhibited,
and the EXAFS fitting results were also listed in Table S4. We
found that the above single metal atoms (Cu, Pd and Pt) in the
N-doped carbon frames were normally three-fold coordinated by
N and C atoms, ignoring the adsorbed O2 molecules on the
single metal atom. These demonstrated the generality of the
synthetic strategy.
In conclusion, we successfully prepared a catalyst with MoSAs dispersed on N-doped carbon, with high catalytic activity
and stability for hydrogen evolution reaction. The structure of the
catalyst was characterized by electronic microscopy and XAFS
measurements. The unique catalytic properties of Mo1N1C2 for
HER was further investigated by DFT calculations. This work
opens up new opportunities for the preparation and application
of high-efficiency and stable Mo-based HER catalysts.
measurement. Chun-Ting He is thankful to the National Postdoctoral
Program for Innovative Talents (BX201600195).
Keywords: single Mo atom • N-doped carbon • hydrogen
evolution reaction • electrocatalyst
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Acknowledgements
This work was supported by China Ministry of Science and
Technology under Contract of 2016YFA (0202801), and the National
Natural Science Foundation of China (21521091, 21390393,
U1463202, 21471089, 21671117, 11405252). We thank the
BL14W1 in SSRF, BL10B and BL12B in NSRL for XAS
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This article is protected by copyright. All rights reserved.
Accepted Manuscript
had been considered (Figure S29). For comparison, the ΔGH*
values of the Mo2C, MoN and N-doped graphene were also
involved (Figure S30-S32). The calculated ΔGH* values of the
most favorable active sites for Mo1N1C2, Mo2C, MoN and Ndoped graphene were 0.082, -0.260, -0.401 and 0.672 eV,
respectively. Obviously, the Mo1N1C2 possessed the lowest
absolute value of ΔGH*, being consistent with the best
experimental HER performance among the research objects.
These indicated that Mo-SA was more nicely beneficial to the
HER process than those Mo in the bulk phases. The density of
states (DOS) was calculated to further study the electronic
structure of the Mo1N1C2 (Figure 4b and Figure S33). We found
that the DOS of Mo1N1C2 near the Fermi level was much larger
than that of Mo2C and MoN, leading to a higher carrier density
for the advantage to charge transfer during HER process,[12]
which is consistent with our charge transfer resistance
measurements. Moreover, the DOS near the Fermi level in
Mo1N1C2 was majorly contributed by the Mo d-orbital, while the
contributions of p-orbital of N and C can be neglected (Figure
4b), indicating that the individual Mo dispersion as well as
special coordination environment can effectively improve the delectron domination near the Fermi level and optimize the
catalytic activity. In a word, combining the controlled
experimental tests and theoretical calculations, we reasonably
discovered the perfect HER performance of the Mo1N1C2
catalyst.
10.1002/ange.201710599
Angewandte Chemie
COMMUNICATION
COMMUNICATION
Wenxing Chen, Jiajing Pei,
Chunting He, Jiawei Wan, Hanlin
Ren, Youqi Zhu, Yu Wang,
Juncai Dong, Shubo Tian, WengChon Cheong, Siqi Lu, Lirong
Zheng, Xusheng Zheng,
Wensheng Yan, Zhongbin
Zhuang, Chen Chen, Qing Peng,
Dingsheng Wang*, Yadong Li*
Page No. – Page No.
Rational Design of Single Mo
Atoms Anchored on N-doped
Carbon for Effective Hydrogen
Evolution Reaction
This article is protected by copyright. All rights reserved.
Accepted Manuscript
A catalyst with Mo-SAs dispersed
on N-doped carbon was
prepared, with high catalytic
activity and stability for hydrogen
evolution reaction. The structure
of the catalyst was characterized
by electronic microscopy and
XAFS measurements. The
unique catalytic properties for
HER was further investigated by
DFT calculations. This work
opens up new opportunities for
the preparation and application of
high-efficiency and stable Mobased HER catalysts.
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