Ultrahigh-Loading Zinc Single-Atom Catalyst for Highly Efficient Oxygen Reduction in Both Acidic and Alkaline Media

Article abstract of DOI:10.1002/anie.201902109

Atomically dispersed Zn–N–C nanomaterials are promising platinum-free catalysts for the oxygen reduction reaction (ORR). However, the fabrication of Zn–N–C catalysts with a high Zn loading remains a formidable challenge owing to the high volatility of the Zn precursor during high-temperature annealing. Herein, we report that an atomically dispersed Zn–N–C catalyst with an ultrahigh Zn loading of 9.33 wt % could be successfully prepared by simply adopting a very low annealing rate of 1° min^?1. The Zn–N–C catalyst exhibited comparable ORR activity to that of Fe–N–C catalysts, and significantly better ORR stability than Fe–N–C catalysts in both acidic and alkaline media. Further experiments and DFT calculations demonstrated that the Zn–N–C catalyst was less susceptible to protonation than the corresponding Fe–N–C catalyst in an acidic medium. DFT calculations revealed that the Zn–N₄ structure is more electrochemically stable than the Fe–N₄ structure during the ORR process.

Full text of DOI:10.1002/anie.201902109

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Ultrahigh-loading Zn Single-atom Catalyst for Highly Efficient
Oxygen Reduction Reaction in both Acidic and Alkaline Media
Authors: Jia Li, Siguo Chen, Na Yang, Mingming Deng, Shumaila
Ibraheem, Jianghai Deng, Jing Li, Li Li, and Zidong Wei
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201902109
Angew. Chem. 10.1002/ange.201902109
Link to VoR: http://dx.doi.org/10.1002/anie.201902109
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Angewandte Chemie International Edition
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COMMUNICATION
Ultrahigh-loading Zn Single-atom Catalyst for Highly Efficient
Oxygen Reduction Reaction in both Acidic and Alkaline Media
Jia Li, Siguo Chen*, Na Yang, Mingming Deng, Shumaila Ibraheem, Jianghai Deng, Jing Li, Li Li*, and
Zidong Wei*
Abstract: Atomically dispersed Zn-N-C nanomaterials are promising
Pt-free catalysts for oxygen reduction reaction (ORR). However, the
fabrication of high Zn loading Zn-N-C catalyst remains a formidable
challenge due to the high volatility of Zn precursor during high-
temperature annealing. Here, we report that the atomically dispersed
Zn-N-C catalyst with an ultrahigh Zn loading of 9.33 wt% can be
successfully prepared by simply adopting a very low annealing rate of
proved to be difficult.[7] This is due to the fact that Zn precursor is
easily removed due to its high volatility during high-temperature
pyrolysis process.
Herein, we markedly increase the loading of atomically
dispersion of Zn in Zn-N-C catalysts by strictly controlling the
gasification rate of Zn precursor. This route indeed well converts
the precursors, ZnCl
2
and ortho-phenylenediamine (oPD) into
1º/min. The Zn-N-C catalyst exhibits comparable ORR activity with
stable Zn-N active sites before their gasification, and thus
X
Fe-N-C catalyst, and significantly better ORR stability than Fe-N-C
catalyst in both acidic and alkaline media. Further experiments and
DFT calculations demonstrate that Zn-N-C catalyst is less susceptible
to protonate than Fe-N-C catalyst in acidic medium. DFT calculations
achieves a loading of Zn single-atom as high as 9.33 wt% (2.06
atom%). The atomically dispersed Zn-N-C catalyst exhibits not
only comparable ORR activity with Fe-N-C catalyst in both acidic
and alkaline media, but also better durability than Fe-N-C catalyst.
XPS analysis demonstrates that the Zn-N-C-1 (the number “1”
represents the heating rate of 1º/min during annealing) catalyst is
less susceptible to protonate than Fe-N-C-1 catalyst in acidic
medium. Density functional theory (DFT) calculations reveal that
4
reveal that the Zn-N structure is more electrochemically stable than
the Fe-N during ORR process.
4
Oxygen reduction reaction (ORR) plays a paramount role in a
variety of energy conversion and storage systems such as fuel
cells and metal-air batteries.[1] Atomically dispersed transition
metal (M) and N co-doped carbon materials (M-N-C) are
emerging as very promising Pt-free ORR catalysts,[ where M and
N atoms could induce uneven charge distribution and thus
4
the Zn-N structure is more electrochemically stable than the Fe-
N
4
structure during ORR process.
An optimal atomically dispersed Zn-N-C catalyst was
prepared by pyrolysizing the mixed precursor system of poly o-
phenylenediamine (PoPD) and ZnCl at a heating rate of 1º/min
Please see supporting information). The Zn loading of the optimal
2]
2
(
improve the O
2
adsorption and reduction.[3] The best M-N-C
Zn-N-C-1 catalyst is 9.33 wt% (2.06 atom%) by EDX (energy-
dispersive X-ray spectroscopy) method, and 5.64 wt% by ICP-MS
catalyst developed so far has a high initial activity with turnover
frequencies matching those of Pt/C;[ However, their practical use
was impeded by the unsatisfactory physical and chemical
stability.[5] Moreover, the reported transition metal sources are
mainly limited to the Fe, Co, Ni and Mn.[6] It has been proven that
4]
(
inductively coupled plasma mass spectrometry) analysis, which
is much higher than that of Fe-N-C-1 catalyst obtained at the
same annealing conditions (Table S2). For comparison, the
samples pyrolyzed at different annealing rates are also prepared
and denoted as Zn−N−C-X (X=annealing rates). The ORR
activities of Zn-N-C catalysts were evaluated in both acidic and
alkaline solutions by the rotating disk electrode (RDE) and rotating
ring-disk electrode (RRDE). As shown in Figure 1a, the ORR
activity of Zn-N-C in acidic medium increases quickly with the
decrease of annealing rate and reaches the most positive half-
wave potential of 0.746 V at 1º/min (Zn-N-C-1), which is
comparable to the Fe-N-C-1 catalyst (0.743 V). The same trend
is also observed in alkaline solution (Figure 1b), in which the half-
wave potential (0.873V) of the optimal Zn-N-C-1 catalyst is more
positive than that of commercial Pt/C catalyst (0.858V), and
slightly lower than that of the Fe-N-C-1 catalyst (0.880V). The
2
+
3+
residues of these transition metals, i.e. Fe or Fe (intermediate
valence of transition metals), and some incompletely coordinated
ions, can deteriorate the stability of the electrode and electrolyte
membrane during ORR.
Compared with Fe, Co, Ni and Mn, the element Zn has a full-
filled d orbital (3d104s ), thus cannot form oxidative ions with a
2
high valence state. Therefore, it is expected that Zn-N-C catalyst
would be harmless to the electrode and electrolyte membrane.
Unfortunately, the performance of the reported Zn-N-C catalysts
in ORR is not as good as Fe-N-C catalysts, which probably can
be assigned to the low active site density as listed in Table S1.
Generally, the fabrication of atomically dispersed Zn-N-C catalyst,
while maintaining high density of atomic Zn sites, has been
2 2
H O yield of Zn-N-C-1 catalyst in both acidic and alkaline
solutions is less than 5% indicating a four-electron reduction
pathway, which is comparable to that of Fe-N-C-1 catalyst (Figure
S1).
In order to understand the origin of the observed high ORR
activity of Zn-N-C-1 catalyst, a series of physical characterizations
has been carried out. The dominant diffraction peaks of Zn-N-C-
[*]
Dr. J. Li, Prof. G. S. Chen, Prof. L. Li, Prof. D. Z. Wei
Chongqing Key Laboratory of Chemical Process for Clean Energy
and Resource Utilization, School of Chemistry and Engineering,
Chongqing University
Shazhengjie 174, Chongqing 400044 (China)
E-mail: csg810519@126.com
1
catalyst in the X-ray diffraction (XRD) pattern are consistent with
liliracial@cqu.edu.cn
zdwei@cqu.edu.cn
the partially graphitized carbon structures (Figure S2), which is
confirmed by high-resolution transition electron microscopy
Supporting information for this article is given via a link at the end of
the document.
[8]
(
HRTEM) image (Figure 1c). Both XRD patterns and HRTEM
image verify the absence of any visible Zn-based particles or
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Figure 2. (a) Zn K-edge XANES spectra of Zn-N-C-1, ZnO, ZnPc, and Zn foil.
2
(
b) Fourier transformed (FT) k -weighted χ(k)-function of the EXAFS spectra for
Figure 1. ORR polarization curves of Zn-N-C-X and Fe-N-C-X catalysts
Zn K-edge. (c, d) Corresponding EXAFS fitting curves at k and R space,
respectively, inset showing the schematic model.
(
X=annealing rates, 0.5º/min, 1º/min, 3º/min, 5º/min 10º/min) recorded in O
2
-
saturated 0.1 M HClO (a) and 0.1 M KOH solution (b) at room temperature with
4
-
1
-1
a sweep rate of 10 mV s and a rotation rate of 1600 rotations min . (c) The
HRTEM image of Zn-N-C-1 catalyst. (d) The aberration-corrected HAADF-
STEM image of Zn-N-C-1 catalyst (single-atom Zn are the bright dots).
peak at about 1.53 Å. Meanwhile, metallic signal peaks of Zn-Zn
interactions in Zn foil and ZnO were not detected in Zn-N-C-1,
manifesting that the Zn element is atomically dispersed in the Zn-
N-C-1 catalyst. The best fitting result of the obtained EXAFS data
clusters in Zn-N-C-1 catalyst. To illustrate the existence state of
the Zn species, aberration-corrected atomic-resolution HAADF-
reveals that Zn-N
is the dominant coordination mode of Zn atoms in Zn-N-C-1
Figure 2c, Figure 2d, Table S3). This result clearly indicates that
the Zn-N-C-1 catalyst is actually a Zn single-atom catalyst with
Zn-N structure. The formation of Zn-N structure is also
confirmed by the high-resolution XPS (Figure S7, S8, Table S4).
The pore-making ability of ZnCl was also investigated by
4 2
rather than Zn-Zn/Zn-N coordination structure
STEM measurement has been carried out,[9] which clearly shows
(
that Zn single-atoms are uniformly dispersed in Zn-N-C-1 sample
at a very high density because the atomic number of Zn is much
higher than the support elements (Figure 1d). By counting the
amount of Zn single-atoms in the special area in Figure S3, the
density of Zn single-atom can be roughly estimated to be 9.31
4
X
2
2
scanning electron microscopy (SEM), HRTEM, and N2
adsorption-desorption tests. As revealed by SEM images (Figure
S9a and S9b), Zn-N-C-1 catalyst shows typical interconnected 3D
macroporous networks, and no 3D macropore can be found in N-
atoms/nm , which is indeed among the highest ones in all
reported single-atom catalysts (Table S1). The corresponding Zn
content obtained by EDX is about 9.33 wt% (2.06 at%) (Figure
S4), which is very close to the value of 2.47 at% determined by
XPS (Table S2), and larger than 5.64 wt% determined by ICP-MS
2
C catalyst obtained with the absence of ZnCl . The average
diameter of the macropores is over 500 nm (Figure S9a). The
HRTEM image shows that a large quantity of micropores and
small size mesopores surround the graphitic layers in the edge
regions of Zn-N-C-1 (Figure 1c). The lattice fringe spacing of
around 0.34 nm (Figure S10) agrees well with that of graphite.
The corresponding Raman spectrum analysis (Figure S11)
shows that the ratio of the relative intensity of the D band at 1340
(
Table S2 and Figure S5). Moreover, we found that a lower
heating rate of 0.5º/min cannot accomplish the conversion of
ZnCl into Zn-N-C catalyst (Table S2 and Figure S6). This is
2
because at such a low heating rate, PoPD precursor, after being
held at low-temperatures for a long time, would preferentially
transform into amorphous carbon, which while cannot capture Zn
species to form atomic Zn doping. That is why the Zn-N-C-0.5
catalyst shows significantly reduced activity compared to the Zn-
N-C-1 catalyst.
The valence state and coordination environment of the
atomic Zn of Zn-N-C-1 catalyst are further determined by X-ray
absorption near-edge structure (XANES) spectroscopy and
extended X-ray absorption fine structure (EXAFS) spectroscopy
analysis.[ As shown in Zn K-edge XANES spectra (Figure 2a),
the position of absorption threshold for Zn-N-C-1 is close to ZnO
and ZnPc, indicating that the valence state of zinc species in Zn-
N-C-1 is probably situated +2. The Fourier transforms of EXAFS
spectra (FT-EXAFS, without phase correction) in Figure 2b
reveal that Zn-N-C-1 exhibits similar Zn-N coordination with a
cm−1 to the G band at 1590 cm−1 (I
/I ) for Zn-N-C-1 is lower than
D G
that of N-C-1 catalyst, suggesting that the presence of ZnCl not
2
only benefits the formation of micro-/meso-/macro-pores, but also
ensures a higher graphitization degree. The Brunauer-Emmett-
Teller (BET) specific surface area of Zn-N-C-1 was measured to
2
-1
be 1002 m g (Figure S12), which is much larger than that of N-
C-1 (81 m2 g-1), further confirming that the existence of ZnCl2
results in the formation of micro-/meso-/macro-pores. By
10]
2
comparing the N adsorption-desorption results, we found that
tuning heating rates will not significantly change BET specific
surface areas for the Zn-N-C-X samples. Obviously, such
multimodal porous structure with well-balanced macro-/meso-/
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Figure 3. The ORR polarization of Zn-N-C-1 and Fe-N-C-1 catalysts before, and after 1000 cycles between 0.6 and 1.1 V versus RHE in 0.1 M HClO
.1 M KOH (b, d) solutions. High-resolution of N1s spectra of Zn-N-C-1(e) and Fe-N-C-1(f) catalysts before, and after 1000 cycles between 0.6 and 1.1 V versus
RHE in 0.1 M HClO and 0.1 M KOH solutions. (g) Schematic of the protonation reaction on Zn-N-C-1 and Fe-N-C-1 in acidic solutions.
4
(a, c) and
0
4
micro-porosities, high graphitization degree and high surface area
could partially explain the high activity of Zn-N-C-1, because: 1)
the high specific surface area hosts enormous active-sties for
ORR; 2) the high graphitization degree of carbon allows smooth
electron transfer; 3) the interconnected 3D macroporous networks
guarantee rapid mass exchanges during ORR process.
Since catalyst durability is extremely significant for practical
use of Zn-N-C catalyst, we investigated the ORR durability of Zn-
N-C-1 and Fe-N-C-1 catalysts in accelerated stress tests (AST) in
both acidic and alkaline media. As shown in Figure 3a, the half-
wave potential of Zn-N-C-1 catalyst decays only 19.88 mV after
1000 CV cycles in acidic medium, a new peak associated with the
protonation of pyridinic-N arised at approximately 401.4 eV
(protonation-pyridinic-N) for both Zn-N-C-1 and Fe-N-C-1
catalysts. By comparing the atomic percentages of pyridinic-N
before and after AST, one can find that both Zn-N-C-1 and Fe-N-
C-1 catalysts experienced protonation in acidic medium. However,
the Zn-N-C-1 catalyst exhibits a relatively small decrease in
pyridinic-N content from 36.62% to 12.88% as compared with the
Fe-N-C-1 catalyst from 24.23% to 3.74%. Moreover, we found
that the Zn-N-C-1 catalyst largely maintains its Zn-N
by showing only a slight decrease in Zn-N content from 5.79% to
4.08% in acidic medium, while the Fe-N-C-1 catalyst shows a
rather heavy decrease in Fe-N content from 10.74% to 4.96%. In
comparison, both of Zn-N-C-1 and Fe-N-C-1 catalysts largely
remain their Zn-N and Fe-N active sites after 1000 CV cycles in
X
active sites
X
2
1000 CV cycles between 0.6 and 1.1 V in the O -saturated 0.1 M
HClO , which is much less than that of Fe-N-C-1 catalyst (31.03
4
X
mV) (Figure 3b). The Zn-N-C-1 catalyst also exhibits excellent
stability in alkaline media, as evidenced by no obvious change in
half-wave potential after 1000 CV cycles (Figure 3c). By contrast,
the Fe-N-C-1 catalyst shows 18.67 mV fading of half-wave
potentials in alkaline media (Figure 3d). These results suggest
that Zn-N-C-1 catalyst is more electrochemically stable than the
Fe-N-C-1 catalyst in both acidic and alkaline solutions.
To deeply understand the origin of the observed high
durability of Zn-N-C-1 catalyst, the XPS measurements were
adopted to monitor the changes of N configurations for Zn-N-C-1
and Fe-N-C-1 electrodes before and after AST tests (Figure S13,
Table S4). As shown in Figure 3e and 3f, the high-resolution N1s
spectra of pristine Zn-N-C-1 and Fe-N-C-1 electrodes can be
deconvoluted into five peaks associated with the pyridinic-N
X
X
alkaline medium, respectively. These results indicate that the Zn-
N-C-1 catalyst is less susceptible to protonate than Fe-N-C-1
catalyst in acidic medium, and at the same time, the Zn-N
X
is more
stable than the Fe-N in acidic solution.
X
The protonation effect on the C-N bond stability of the
pyridinic-N doped graphene (PNG) and oxidized PNG (PNG-O)
was explored by DFT calculations.[12] As seen from Figure 4a, the
bond distances of C1-N in PNG show gradual increase from 1.32
+
+
Å to 1.37 Å and 1.45 Å after protonating with one H and two H ,
respectively. In comparison, the C1-N bond with no protonation in
PNG-O (1.44 Å) is much larger than that in PNG (1.32 Å), and this
+
bond will increase to 1.50 Å with one H and then break after
+
(
398.4eV), Zn(or Fe)-N
X
(≈399.6eV), pyrrolic-N (400.2eV),
protonating with two H (dC1-N=1.44 Å - 1.50 Å - break). These
[11]
graphitic-N (401.2eV), and oxidized-N, (403-405eV).
After
results indicate that the protonation of pyridinic-N in PNG with the
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N-C-1 and Pt/C catalysts dramatically increase from 42.4 mV at 1
mA cm-2 and 1 mV at 10 mA cm-2 to 25.7 mV at 100 mA cm-2
(
Figure S15b), suggesting that the Zn-N-C-1 catalyst is more
conducive to the mass-transfer at higher current densities. The
specific capacity was measured according to the consumption of
−2
Zn (Figure S16). At 100 mA cm , the Zn-N-C-1 catalyst enabled
the Zn–O
battery with a specific capacity of 683.3 mAh gZn−1
about 83.3% utilization of the theoretical capacity 820 mAh gZn−1),
2
(
−
1
corresponding to a high energy density of 666 Wh kgZn (about
6
−
1
1.3% of the theoretical energy density 1086 Wh kgZn ). These
values also significantly outperform those of the battery prepared
−
1
with Pt/C (specific capacity of 601.4 mAh gZn and energy density
−
1
of 563 Wh kgZn ), and even comparable to the best results
recently reported for Zn-O batteries (Table S5).
2
In conclusion, we have successfully synthesized ultrahigh
loading Zn single-atom catalysts by controlling the annealing rate
of ZnCl
experiments reveal that Zn-N
2
precursor at 1º/min. The XANES and EXAFS
is the main active site in Zn-N-C
4
catalyst. Electrochemical experiment tests show that the
atomically dispersed Zn-N-C catalyst exhibits not only
comparable ORR activity with Fe-N-C catalyst in both acidic and
alkaline media, but also better durability than that of Fe-N-C
catalyst. XPS analysis demonstrates that the Zn-N-C-1 catalyst is
less susceptible to protonate than Fe-N-C-1 catalyst in acidic
medium. Meanwhile, DFT calculations reveal that the Zn-N
4
Figure 4. (a) DFT-optimized structures of PNG and PNG-O as non-protonated,
structure is more electrochemical stable than the Fe-N structure
during ORR process. This work may represent a new class of
high-efficiency and stable single-atom site catalysts for both
fundamental research and practical applications.
4
protonation-H and protonation-2H. (b) Free energy diagrams for the Zn(OH)
Fe(OH) and Fe(OH) during the metal corrosion process based on M-N
M=Zn/Fe) structure.
2
,
2
3
4
(
existence of O
2
will accelerate the breaking of C-N bond, and thus
decrease the number of active sites and reduce the ORR activity. Acknowledgements
The anti-corrosion ability of Zn-N
4
and Fe-N
4
structures is
also evaluated by DFT calculation.[13] The OH , which is the ORR
*
This work was financially supported by the National Key Research
intermediate, can adsorb on the metal sites and then oxidize them. and Development Program of China (2016YFB0101202), and by
Once two OH* bond to a metal site, the oxidized metal tends to
dissociate in the solution. As shown in Figure 4b, the free energy
transferring Zn site (*Zn) to *Zn(OH) is 1.42 eV, which is much
larger than the value of 1.07eV for the formation of *Fe(OH).
the National Natural Science Foundation of China (Grant Nos.
21761162015, 91534205, 21436003, 21576031, and 21776023).
Keywords: fuel cells • metal-air battery • oxygen reduction
Moreover, we found that the formation of *Zn(OH)
is almost impossible because the free energy is larger than 10 eV.
While, the formation of *Fe(OH) from *Fe(OH) is feasible due to
the low free energy (ΔG = 1.68 eV). These results indicate that
the corrosion of *Zn site in Zn-N structure is tougher than that of
Fe site in Fe-N during ORR process. This phenomenon can be
attributed to the distinction of valence electron configurations
2
from *Zn(OH)
reaction • electrocatalyst • Zn single-atom catalyst
2
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4s
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2
), in which the Fe site with
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Mater. Chem. A 2018, 6, 15504-15509.
*
more d empty orbital tends to adsorb OH and form Fe(OH)
species in the ORR process.
X
X. F. Yang, A. Wang, B. Qiao, J. Li, J. Liu, T. Zhang, Acc. Chem. Res.
Finally, we constructed a Zn-O
potential of our catalyst for real energy conversion devices
Figure S14).[14] Commercial Pt/C was also tested under the
same conditions for comparison. The maximum power density for
2
battery to evaluate the
2
013, 46, 1740-1748.
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−
2
Zn-N-C-1 catalyst is 179 mW cm , slightly higher than that of Pt/C
−2
(
173 mW cm ) (Figure S15a). The potential gaps between Zn-N-
[
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N-C-1 catalyst. It was found that the potential gaps between Zn-
[
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[
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COMMUNICATION
The atomically dispersed Zn-N-C
catalyst with an ultra-high loading of
Jia Li, Siguo Chen*, Na Yang, Mingming
Deng, Shumaila Ibraheem, Jianghai
Deng, Jing Li, Li Li*, and Zidong Wei*
9.33 wt% Zn single-atom is
successfully prepared by decreasing
the annealing rate to 1º/min. The Zn-
N-C catalyst exhibits comparable
ORR activity with Fe-N-C catalyst, and
better ORR stability than Fe-N-C
catalyst in both acidic and alkaline
media.
Page No. – Page No.
Ultrahigh-loading Zn Single-atom
Catalyst for Highly Efficient Oxygen
Reduction Reaction in both Acidic
and Alkaline Media
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