Compressive Strain Modulation of Single Iron Sites on Helical Carbon Support Boosts Electrocatalytic Oxygen Reduction

Article abstract of DOI:10.1002/anie.202109058

Designing and modulating the local structure of metal sites is the key to gain the unique selectivity and high activity of single metal site catalysts. Herein, we report strain engineering of curved single atomic iron-nitrogen sites to boost electrocatalytic activity. First, a helical carbon structure with abundant high-curvature surface is realized by carbonization of helical polypyrrole that is templated from self-assembled chiral surfactants. The high-curvature surface introduces compressive strain on the supported Fe?N₄ sites. Consequently, the curved Fe?N₄ sites with 1.5 % compressed Fe?N bonds exhibit downshifted d-band center than the planar sites. Such a change can weaken the bonding strength between the oxygenated intermediates and metal sites, resulting a much smaller energy barrier for oxygen reduction. Catalytic tests further demonstrate that a kinetic current density of 7.922 mA cm^?2 at 0.9 V vs. RHE is obtained in alkaline media for curved Fe?N₄ sites, which is 31 times higher than that for planar ones. Our findings shed light on modulating the local three-dimensional structure of single metal sites and boosting the catalytic activity via strain engineering.

Full text of DOI:10.1002/anie.202109058

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Accepted Article
Title: Compressive Strain Modulation of Single Iron Sites on Helical
Carbon Support Boosts Electrocatalytic Oxygen Reduction
Authors: Yuen Wu, Jia Yang, Zhiyuan Wang, Chun-Xiang Huang, Yida
Zhang, Qinghua Zhang, Cai Chen, Junyi Du, Xiao Zhou, Ying
Zhang, Huang Zhou, Lingxiao Wang, Xusheng Zheng, Lin Gu,
and Li-Ming Yang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202109058
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Compressive Strain Modulation of Single Iron Sites on Helical
Carbon Support Boosts Electrocatalytic Oxygen Reduction
Jia Yang,[a,b,§] Zhiyuan Wang,[b,§] Chun-Xiang Huang,[c,§] Yida Zhang,[d] Qinghua Zhang,[e] Cai Chen,[b]
Junyi Du,[b] Xiao Zhou,[b] Ying Zhang,[b] Huang Zhou,[b] Lingxiao Wang,[b] Xusheng Zheng,*[d] Lin Gu,[e]
Li-Ming Yang,*[c] Yuen Wu*[b]
[a]
[b]
Dr. J. Yang
Institutes of Physical Science and Information Technology, Key Laboratory of Structure and Functional Regulation of Hybrid Materials of Ministry of
Education, Anhui Graphene Engineering Laboratory
Anhui University
Hefei, Anhui 230601, China
Dr. J. Yang, Dr. Z. Wang, C. Chen, Dr. J. Du, Dr. X. Zhou, Dr. Y. Zhang, Dr. H. Zhou, L. Wang, Prof. Y. Wu
Hefei National Laboratory for Physical Sciences at the Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), School of
Chemistry and Materials Science
University of Science and Technology of China
Hefei, Anhui 230026, China
E-mail: yuenwu@ustc.edu.cn
[c]
[d]
C.-X. Huang, Prof. L.-M. Yang
Hubei Key Laboratory of Bioinorganic Chemistry and Materia Medica, Key Laboratory of Material Chemistry for Energy Conversion and Storage of Ministry
of Education, Hubei Key Laboratory of Materials Chemistry and Service Failure, Hubei Engineering Research Center for Biomaterials and Medical
Protective Materials, School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
Wuhan, Hubei 430074, China
E-mail: Lmyang.uio@gmail.com, Lmyang@hust.edu.cn
Y. Zhang, Prof. X. Zheng
National Synchrotron Radiation Laboratory (NSRL)
University of Science and Technology of China
Hefei, Anhui 230029, China
E-mail: zxs@ustc.edu.cn
[e]
[§]
Dr. Q. Zhang, Prof. L. Gu
Beijing National Laboratory for Condensed Matter Physics
Institute of Physics, Chinese Academy of Sciences
Beijing 100190, China
J. Yang, Z. Wang, C. Huang contribute equally
Abstract: Designing and modulating the local structure of metal
sites is the key to gain the unique selectivity and high activity of
single metal site catalysts. Herein, we report strain engineering of
curved single atomic iron-nitrogen sites to boost electrocatalytic
activity. First, a helical carbon structure with abundant high-curvature
surface is realized by carbonization of helical polypyrrole that is
templated from self-assembled chiral surfactants. The high-curvature
surface introduces compressive strain on the supported Fe-N4 sites.
Consequently, the curved Fe-N4 sites with 1.5% compressed Fe-N
bonds exhibit downshifted d-band center than the planar sites. Such
a change can weaken the bonding strength between the oxygenated
intermediates and metal sites, resulting a much smaller energy
barrier for oxygen reduction. Catalytic tests further demonstrate that
a kinetic current density of 7.922 mA/cm2 at 0.9 V vs. RHE is
obtained in alkaline media for curved Fe-N4 sites, which is 31 times
higher than that for planar ones. Our findings shed light on
modulating the local three-dimensional structure of single metal sites
and boosting the catalytic activity via strain engineering.
To further improve or regulate the catalytic activity of carbon
supported single metal site catalysts (M-Nx/C), various strategies
have been developed.[14] Considering that the catalytic activity of
the metal sites is closely related with the coordination structure,
numerous efforts are devoted to engineering the coordination
numbers, coordinated atoms, environmental atoms, axial guest
groups and etc. to tune the intrinsic activity and selectivity of the
metal sites for desired applications.[15] However, the above
strategies are mainly focused on the in-plane structure of metal-
nitrogen sites. The local three-dimensionally structural
modulation on the metal sites, which could mediate the catalytic
activity by strain effect, still remains challenging and has rarely
been discussed.[16] The local three-dimensional structure of
metal sites is highly correlated with the surface morphology and
geometric structure of support.[17] Compared with the flat surface,
high-curvature surface tends to create nonplanar curved
structure and introduce strain effect, which may be benificial to
improving the catalytic activity.[18] Single metal sites could also
be immobilized stronger on high-curvature support, affecting the
catalytic activity.[19-22]
Single metal site catalysts, with isolated single metal sites
dispersed on proper supports, have been emerging as a new
frontier in material and catalysis science.[1,2] They integrate the
advantages of homogeneous and heterogeneous catalysts such
as maximum atomic utilization efficiency, quantum-size effect
and tunable electronic environment, thus exhibiting ultra high
activity and unique selectivity in various catalytic reactions.[3-13]
Among which, carbon support single metal site catalysts with
atomic metal-nitrogen sites are promising candidates for energy
related catalytic reactions.[4,10]
Herein, we report strain engineering of curved single atomic
iron-nitrogen sites to boost electrocatalytic activity for oxygen
reduction reaction (ORR). Helical polypyrrole is firstly obtained
by using self-assembled chiral surfactants as the polymerization
template. After pyrolysis, right-handed helical single atomic iron
catalyst (D-Fe SAC) is obtained, featuring with isolated Fe-N4
sites on helical high-curvature hollow carbon nanofibers. The
high-curvature surface introduces compressive strain effect on
the supported Fe-N4 sites. As a result, the D-Fe SAC delivers
1
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higher activity for ORR compared with the non-helical LD-Fe
SAC. Theoretical calculations reveal that the curved structure
and compressive strain could regulate the electronic state of Fe-
N4 sites, and further reduce the energy barrier for ORR.
Addtionally, various curved metal-nitrogen sites were also
obtained based on other metal elements (e.g., Co and Ni). Our
research provides insights on modulating the local three-
dimensional structure of single metal sites and boosting the
catalytic activity via strain engineering.
Figure 1a illustrates the synthesis process of the helical
single atomic iron catalyst. Stearol chloride was reacted with D-
glutamic acid to give chiral surfactant N-stearoyl-D-glutamic acid.
The chiral surfactants underwent a self-assembly process to
form helical micelles, which were further used as the templates
for pyrrole polymerization.[23] The obtained D-polypyrrole shows
a right-handed helical morphology, as demonstrated by the
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) images (Figure S1). Taking advantage of the
interaction between the empty d orbitals of metal ions and the
lone electron pairs of nitrogen atoms on polypyrrole, iron species
can be introduced into D-polypyrrole.[24] The iron containing D-
polypyrrole was then pyrolyzed under inert atmosphere to afford
D-Fe SAC. Metal free D-PPy-Carbon was obtained by pyrolyzing
D-polypyrrole. At the same time, the orientation of the helical
morphology can be modulated. L-Fe SAC with left-handed
helical morphology was obtained from N-stearoyl-L-glutamic
acid (Figure S2 and S3). LD-Fe SAC was prepared from non-
chiral surfactant N-stearoyl-L/D-glutamic acid. Hollow tubular
structures without helical morphology are observed for LD-Fe
SAC (Figure S4).
Figure 1. (a) Schematic showing the synthesis process of D-Fe SAC. (b) SEM
image, (c) TEM image, (d) HAADF-STEM image, (e) elemental mapping
images of D-Fe SAC. (f-i) Aberration-corrected HAADF-STEM images of D-Fe
SAC (f, g) and LD-Fe SAC (h, i).
distributed in the straight flat carbon lattices (Figure 1h, 1i, S9
and S10).
The iron content in D-Fe SAC is 1.6 wt% by inductively
coupled plasma-mass spectroscopy (ICP-MS) measurement. X-
ray photoelectron spectroscopy (XPS) spectra were taken to
investigate the surface states of the D-Fe SAC. Peaks assigning
to carbon, nitrogen and iron are observed in the XPS full scan
spectrum (Figure S11 and 2a). The C 1s spectrum and fitting
results reveal the existence of carbon-nitrogen species on the
support.[29] Pyridinic N, pyrrolic N, graphitic N and oxidized N are
distinguished in N 1s spectrum.[30,31] The fitted Fe 2p3/2 spectrum
reveals that iron exists in ionic state with an oxidation state
between 0 and +3.[32,33] The iron content in LD-Fe SAC is 1.67
wt%. The PXRD pattern, Raman spectrum, nitrogen sorption
isotherm and XPS spectra of LD-Fe SAC (Figure S5, S6, S7 and
S12) indicate that LD-Fe SAC is similar to D-Fe SAC in aspects
of the iron loading, crystalline degree, graphitic degree, surface
area, pore size distribution and surface chemical states.
No characteristic peaks ascribed to iron related crystalline
species can be identified from the powder X-ray diffraction
(PXRD) patterns (Figure S5), and the broad peaks locating at
c.a. 26 degree are ascribed to carbon. This indicates that the
polypyrrole precursor is successfully transformed into carbon-
based material without the formation of crystalline iron
species.[25] Representative defective (1353 cm-1) and graphitic
bands (1590 cm-1) of pyrolytic carbon can be seen in Raman
spectra (Figure S6).[26] Nitrogen sorption isotherms (Figure S7)
could be classified into the type IV isotherm.[27] The steep gas
uptake at low pressure, together with the hysteresis loop,
reveals the micro and meso-porous nature of the carbon
material, which is also confirmed by pore size distribution results.
The fine structure of Fe species was clarified by X-ray
absorption near edge spectra (XANES) and extended X-ray
absorption fine structure spectra (EXAFS). The absorption edge
in Fe K-edge XANES of D-Fe SAC locates between Fe foil and
Fe2O3 (Figure 2b), in accordance with the Fe 2p XPS analysis
(Figure S11). In the Fourier Transformed (FT) EXAFS of D-Fe
SAC, the peak at ~1.4 Å is ascribed to the Fe-N interaction, and
no peaks assigning to Fe-Fe interaction are observed (Figure
2c).[34,35] These results agree with the Wavelet Transform
analysis, in which only the intensity maximum due to Fe-N
interaction is observed for D-Fe SAC (Figure 2d).[36,37] The
above results reveal that Fe species are atomically dispersed.
Further EXAFS fitting was performed to clarify the coordination
structure. The EXAFS fitting using Fe-N4 in a curved
configuration matches well with the experimental result for D-Fe
SAC (Figure 2e and Table S1). In contrast, the iron sites in LD-
Fe SAC are atomically dispersed with configuration of planar Fe-
N4 (Figure 2b-2e and Table S1). Compared to LD-Fe SAC, the
Right-handed helical morphology is retained for D-Fe SAC
after pyrolysis, which is confirmed by SEM and TEM images
(Figure 1b and 1c). No metal particles are visible in the TEM
images (Figure 1c and 1d). Elemental mapping results (Figure
1e) reveal that iron and nitrogen are homogeneously dispersed
throughout the carbon matrix. The bright dots with approximately
0.14 nm size in the aberration-corrected high-angle annular
dark-field scanning transmission electron microscopy (HAADF-
STEM) images reveal that iron disperse as isolated single sites
on the carbon support (Figure 1f).[28] Moreover, it can be clearly
observed that the helical morphology results in the edge curving
of the hollow nanofibers, creating abundant high-curvature
carbon lattices (Figure 1g). Consequently, large quantities of
single Fe sites are located in the high-curvature carbon lattices
(Figure 1f, 1g and S8). In contrast, no high-curvature carbon
lattices are observed for LD-Fe SAC. Single Fe sites are
2
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Figure 2. (a) N 1s XPS spectra of D-Fe SAC and LD-Fe SAC. (b) Fe K-edge XANES spectra, (c) FT-EXAFS spectra, (d) Wavelet Transform results of D-Fe SAC,
LD-Fe SAC, Fe foil and Fe2O3. (e) EXAFS fitting results of D-Fe SAC and LD-Fe SAC.
Fe-N bonds in D-Fe SAC are shortened by about 1.5%,
suggesting the compressive strain on Fe-N bonds created by the
curved structure. The XANES and EXAFS results demonstrate
that isolated Fe-N4 sites with curved structure and compressive
strain are successfully realized in D-Fe SAC.
catalyzed by D-Fe SAC is estimated to be 3.96, as calculated
from the LSV curves at different rotating speeds and Koutechy-
Levich (K-L) plots (Figure S17). The Tafel slope of D-Fe SAC
(60 mV/dec) is lower than those of LD-Fe SAC (74 mV/dec) and
Pt/C (71 mV/dec), indicating faster kinetics (Figure 3b).[38] The
peroxide yields and electron transfer numbers were also
The electrocatalytic activity of D-Fe SAC toward ORR was
then investigated. ORR activity was first investigated in 0.1 M
KOH and the linear sweep voltammetric (LSV) curves are shown
in Figure 3a. The D-Fe SAC exhibits an outstanding activity with
an onset potential (Eonset) of 1.015 V vs. RHE and a half-wave
potential (E1/2) of 0.91 V vs. RHE. The Eonset and E1/2 of D-Fe
SAC are more positive than those of platinum/carbon (Pt/C, 20
wt%, Eonset: 1.013 V vs. RHE and E1/2: 0.87 V vs. RHE). L-Fe
SAC exhibits comparable activity to D-Fe SAC with E1/2 of 0.9 V
vs. RHE (Figure S13). The ORR activity of non-helical LD-Fe
SAC and metal-free D-PPy-Carbon (Figure S14) was also
examined. The Eonset and E1/2 of LD-Fe SAC are 0.921 V and
0.81 V vs. RHE, which are more negative than those of D-Fe
SAC. Electrochemical active surface area (ECSA) measurement
results indicate that the ECSA of D-Fe SAC and LD-Fe SAC are
nearly the same (Figure S15). The LSV curves normalized by Fe
loading reveal that D-Fe SAC exhibits higher mass activity than
LD-Fe SAC (Figure S16). These highlight the critical role of the
curved structure and the compressive strain in achieving the
ultra high activity of D-Fe SAC. For D-PPy-Carbon, the Eonset and
E1/2 are 0.86 V and 0.735 V vs. RHE, indicating that iron play a
vital role in ORR. The electron transfer number during ORR
investigated
by rotating
ring-disk
electrode (RRDE)
measurements. The peroxide yields catalyzed by D-Fe SAC
maintain at below 2% and the electron transfer numbers are
close to 4 (Figure 3c). These results indicate that D-Fe SAC can
efficiently catalyze four-electron ORR under alkaline condition.
The kinetic current densities of the catalysts at 0.9 V vs.
RHE are shown in Figure 3d. D-Fe SAC exhibits a kinetic
current density of 7.922 mA/cm2, which is vastly enhanced
compared with LD-Fe SAC (0.25 mA/cm2), D-PPy-Carbon
(0.029 mA/cm2) and Pt/C (2.8 mA/cm2). The kinetic current
density of D-Fe SAC at 0.9 V vs. RHE is 31 times higher than
that of LD-Fe SAC and ranks among the highest values in
literatures (Table S2). Methanol tolerance was investigated by
introducing methanol during ORR. Negligible current density
change occurs for D-Fe SAC, while a sharp current density drop
occurs for Pt/C (Figure 3e). The stability of D-Fe SAC for ORR
was investigated. No obvious shift of the LSV curve can be
observed after 10000 cyclic voltammetric scans (Figure 3f).
These results indicate that D-Fe SAC exhibit high activity and
stability for ORR in alkaline electrolyte.
3
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