Nanosheets/Mesopore Structured Co3O4@CMK-3 Composite as an Electrocatalyst for the Oxygen Reduction Reaction

Article abstract of DOI:10.1002/cctc.201701822

The exploration of highly effective and low-cost non-noble-metal electrocatalysts for the oxygen reduction reaction (ORR) will have a significant impact on the area of fuel cells and metal–air batteries. Here, we report a facile, efficient, and environmentally friendly method to deposit transition metal oxide nanosheets (NSs) on the backbone of ordered mesoporous carbon (CMK-3) by the pyrolysis of the metal-precursor@CMK-3. The optimized CMK-3 catalyst with a coating of Co₃O₄ NSs possesses a high surface area of 2396 m² g^?1 and demonstrates an excellent electrocatalytic activity towards the ORR in alkaline media, which is much better than that of other transition metal oxide@CMK-3 catalysts and other carbon-based catalysts (such as carbon nanotubes). In addition, the as-prepared catalyst possesses a comparable electrochemical performance and a superior durability and higher tolerance to methanol compared with commercially available Pt/C, which can be ascribed to the distinctive structure of the nanosheets and mesopores, indicative of potential applications in metal–air batteries.

Full text of DOI:10.1002/cctc.201701822

Accepted Article
Title: Nanosheets/Mesopore Structured Co3O4@CMK-3 Composite as
Electrocatalyst for Oxygen Reduction Reaction
Authors: Lang Gan, Mengran Wang, Langtao Hu, Jing Fang, Yanqing
Lai, and Jie Li
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To be cited as: ChemCatChem 10.1002/cctc.201701822
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Nanosheets/Mesopore Structured Co O @CMK-3 Composite as
3
4
Electrocatalyst for Oxygen Reduction Reaction
a
a
a
[a]
a
a
Lang Gan , Mengran Wang , Langtao Hu , Jing Fang * , Yanqing Lai , Jie Li
Abstract: Exploring highly effective and low-cost non-noble metal
electrocatalysts for oxygen reduction reactions (ORR) will have a
significance impact on the area of fuel cells and metal-air batteries.
In this manuscript, we report a facile, efficient and environmental
friendly method to deposit transition metal oxide nanosheets (NSs)
on the backbone of ordered mesoporous carbon (CMK-3) via
pyrolysis of the metal-precursor@CMK-3. The optimized CMK-3
activity and durability of catalysts and maximize their
electrochemical active sites. Self-ordered mesoporous carbon
material, as a new kind of nanostructured material, possesses
regular pore structure, large specific surface area and pore
volume, good thermal stability and chemical stability, which has
been widely explored as non-metal catalysts or as
a
[
8]
characteristic support for metal catalysts. This interesting
material was employed as a support by a mass of research
groups for making catalysts with inorganic metal oxide and
exhibiting a very good catalytic performance as electrode
3 4
catalyst with a coating of Co O NS possesses a high surface area
2
-1
of 2396 m g and demonstrates an excellent electrocatalytic activity
towards ORR in alkaline media, which is much better than other
transition metal oxide@CMK-3 catalysts and other carbon material-
based catalysts (such as carbon nanotubes). In addition, the as-
[
9]
material.
Particularly, the bulk of CMK-3 possesses plenty of ordered
interconnected pores, as an air diffusion electrode material, can
provide valid gas diffusion channels which has been considered
prepared
catalyst
possesses
comparable
electrochemical
performances, and even superior durability and higher tolerance to
methanol compared with commercially available Pt/C, which can be
ascribed to the distinctive structure of nanosheets/mesopore
[
10]
as potential electrocatalyst for ORR.
Accordingly, the metal
oxide loaded on the surface of CMK-3 can remain the gas
diffusion path of CMK-3 and get amply effective activity area of
three-phase interface (gas-catalyst-electrolyte) that are
(NSs/MP), indicative of great potential in the application of metal-air
batteries.
[
11]
[10]
indispensable for fuel cell or metal-air battery.
reported the high electrocatalytic performance of a mesoporous
carbon Mn @CMK-3 nanocomposite for oxygen reduction in
Wang et al.
3 4
O
Introduction
alkaline solution. Nevertheless, the gas diffusion channels of
CMK-3 may be blocked gradually with the loading increase of
metal oxide nanoparticles (NPs), resulting in enhanced ohmic
resistance and charge transfer resistance of ORR. Despite the
tremendous progress in CMK-3-based catalyst NPs, little effects
were done to the controllable assembly of non-precious metal
oxide with other micromorphology supported on this three-
Developing new non-noble metal ORR electrocatalysts is of
great importance for electrochemical devices directly in
connection with the efficient energy conversion and storage
technologies such as fuel cells and rechargeable metal-air
[
1]
batteries. It is well known that Pt and its alloys are the most
efficient ORR catalysts until now, unfortunately, the high cost
and low reserves of Pt prevent it from further development for
[12]
dimensional carbon networks as ORR catalysts.
[
2]
commercialization. As a consequence, the alternative catalysts
based on non-precious metals and metal oxide semiconductors
have been investigated as potential ORR eletrocatalysts which
[
3]
has drawn widespread concern by research teams.
Metal or metal oxide catalysts constantly suffer from
dissolution, disintegration, and agglomeration in the fuel cell
applications, which is not conducive to the development of
[
4]
electrocatalyst. To overcome this obstacle, carbon matrices
[
2a]
[5]
[6]
such as active carbon,
carbon nanotubes, graphene and
[7]
porous carbon used to support metal oxides have been
developed to ideal catalysts due to their high electro-conductivity,
popular prices and ultrastability, which can improve the catalytic
[
a]
L. Gan, M. Wang, L. Hu, Dr. J. Fang, Prof. Y Lai, Prof. J. Li
School of Metallurgy and Environment
Central South University
Changsha, Hunan, 410083 (China)
E-mail: fangjing526@csu.edu.cn
3 4
Scheme 1. The schematic display for the synthesis process of Co O @CMK-3
composite.
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Here, we demonstrate a NSs/MP structured Co
3
4
O @CMK-3
observed, respectively. After precipitation and the subsequent
composite which is successfully synthesized by using CMK-3 as
carbon support and Co(NO as a cobalt-precursor for exterior
wall decoration, and the synthetic routine is illustrated in scheme
. At first, the intermediate of CoOH or CoOOH nanosheets was
fabricated by a solvothermal method and then calcinations the
intermediates to graft ultrathin Co nanosheets onto highly-
ordered mesoporous carbon CMK- 3. The unique structure of
the resulting metal oxide NSs@CMK-3 (Co @CMK-3), an
interconnected multihole structure of carbon nanorods with
homogeneity deposition of Co NSs, leads to high electrical
conductivity, good stability, excellent catalytic activity and anti-
crossing effects for ORR. These electrochemical properties were
practically comparable or even superior in some ways to those
observed for a commercial Pt/C (20 wt%) with the same mass,
thus leading to a novel electrocatalyst for ORR. What’s more,
3 4
calcination treatment, the homogeneous Co O covered on the
)
3 2
surface of CMK-3 matrix in the form of standing NSs as shown
in Figure 1C, more importantly, the pore structure of CMK-3
doesn’t collapse during the low-temperature calcining process.
As we can seen from the TEM image in Figure 1D, the surface
of CMK-3 is covered with burrs and the NSs/MP structure of the
1
3 4
O
Co
Additionally, as shown in Figure S1A, B, the Co
uniformly aggregated in formation of the sphere-like structure. A
comparison of Co 4@CMK-3 composites derived from various
CMK-3 mass was also prepared, in Figure S1C–E, the well-
distributed Co NSs were epitaxial grown on the surface of
CMK-3. But the Co NSs are hardly observed on the surface
of CMK-3, when the usage of CMK-3 increased to 20 mg.
What’s more, Co supported on carbon nanotubes
(Co @CNTs) and other metal oxide@CMK-3 hybrid was also
successful prepared (Figure S2).
3 4
O @CMK-3 composite can also be clearly observed.
O
3 4
3 4
O
NSs were
O
3 4
3
O
3 4
O
3 4
O
3 4
O
Co
of CMK-3 and Co
Co @CNTs), other metal oxide@CMK-3 hybrid were also
prepared as referential samples, which verifies that the high
activity of Co NSs deposit on the outer wall of CMK-3 with
mesoporous architecture accounts for the excellent
3
O4@CMK-3 composites derived from different feed dosages
3 4
O
3 4
O
supported on carbon nanotubes
(
3 4
O
3 4
O
3
electrochemical catalytic performance of Co O4@CMK-3.
Results and Discussion
Figure 2. TEM images of Co
HRTEM image (C) and SAED pattern (D) of the corresponding Co
nanosheets; High angle annular dark field scanning transmission electron
microscopy image of Co @CMK-3 composites (E) and the corresponding
elemental mapping images of carbon (F) oxygen (G) and cobalt (H).
3 4
O @CMK-3 (A, B) hybrid nanostructures;
3 4
O
3 4
O
Figure 1. Typical SEM and TEM images of the bare CMK-3 template (A, B)
3 4
and the calcined Co O @CMK-3 composites with the best component ratio (C,
D).
3 4
The morphology and microstructure view of Co O @CMK-3
The morphology structure of the pristine CMK-3 template
and the Co @CMK-3 with the well-defined morphology after
were further conducted by a transmission electron microscopy
analysis. A typical TEM image, as presented in Figure 2A,
3 4
O
calcination are shown in Figure 1. The ordered three-
dimensional CMK-3 matrix consisted of plenty of rod-like
aggregates in the micrometer scale (Figure 1A) and the
sufficient ordered mesoporous channels of CMK-3 derived from
SBA-15 with a diameter of ~1.9 um (Figure 1B) can be clearly
3 4
indicating that the Co O @CMK-3 possess a NSs/MP structure
without any structure collapses after heat treatment process. In
order to observe the NSs in more details, a magnifying TEM
image of the Co
The stand upright Co
3
O
4
@CMK-3 composites is shown in Figure 2B.
NSs can be clearly observed on the
3 4
O
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ChemCatChem
10.1002/cctc.201701822
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backbone of CMK-3. Moreover, the thickness of a single
ultrathin Co O NSs is about 3.2 nm which is calibrated by
3 4
as shown in Figure 2E. Energy-dispersive X-ray (EDX)
elemental mapping confirmed that C, O, Co are principal
Digital Micrograph software, which could offer very convenient
transportation path for oxygen and the diffusion of the generated
3 4
elemental components of the Co O @CMK-3 which were also
performed on the single nanorod (Figure 2F-H). In the carbon
mapping, inferior carbon signal was detected out of the CMK-3
due to the nanorod lies on the carbon film. What’s more, the Co
and O are both homogeneously distributed around the template.
More importantly, the difference between the cross sections
H
2
O. In the HR-TEM images shown in Figure 2C, the inter-
planar distance of Co NSs is 2.46 Å, corresponding to the
311) crystal plane of Co . Additionally, three main diffraction
3
O
4
O
3 4
(
rings were obtained in the SAED patterns (Figure 2D), which are
readily assigned to the (111), (220) and (311) planes of the
diameters of the
demonstrate that the Co
3 and form a “core-shell” structure.
C
mapping and the Co,
O mappings
Co
results. Atomic-resolution HAADF image of the Co
reveal that it display morphology of NSs and nanorods structure,
O
3 4
phase, in good agreement with X-ray diffraction (XRD)
3 4
O
NSs deposit on the surface of CMK-
3
O @CMK-3
4
3 4
Figure 3. (A) X-ray diffraction patterns (B) Thermal gravity analysis in air (C) Nitrogen adsorption−desorption isotherm of the bare CMK-3 and Co O @CMK-3
composites with different component ratios.
Figure 3A shows the diffraction pattern of the as-
synthesized CMK-3 and the Co NSs@CMK-3 with different
component ratios. It can be clearly seen that two broad
Co
with the increasing addition of CMK-3. In the case of the
Co @CMK-3 composites with 15 mg CMK-3 templates, a
3 4
O @CMK-3 composites, the weight loss increased regularly
3
O
4
3 4
O
o
o
diffraction peaks at 22.9 and 43.3 exist in the XRD pattern of
CMK-3 which matching well with the (002) and (101) planes of
the graphitic structure, respectively, indicating that the CMK-3
have good electro-conductivity due to the graphite phase.
Besides, under the same calcination temperature, the amount of
sharply declining (56% by weight) appeared at 500 ℃, which
can be assigned to the oxidation of the CMK-3. Thus the high
interfacial area contact can accelerate the oxidation rate of the C
that directly combines with the interfacial oxygen from the oxide,
which is between the NSs oxide and the MP CMK-3. After the
Co
3
O
4
in the composites had little effect on the crystallinity of the
temperature increased to 800 ℃, the Co
total weight loss of 65.8% thus 34.2% of the Co
3
O
4
@CMK-3 shows a
remained in
final products and all of them presented the three main
3
O
4
o
o
o
characteristic peaks centered at 31.2 , 36.8 and 59.3
corresponding to the (220), (311) and (511) lattice planes of the
standard Co (JCPDS card No. 43-1003), respectively.
Meanwhile, with the increasing amount of CMK-3 in the
composites, the characteristic broad peak of CMK-3 in the range
the composite, indicating that it is essential to keep the heat
treatment temperature below 500 ℃ during the preparation
process, which could prevent CMK-3 from oxidizing by the
oxygen from the oxide. This is also why we hold the calcinations
temperature at 400 ℃ in the manuscript, in order to consume
the chemically labile species but not destroy the pore structure
of CMK-3.
3 4
O
o
of 20-25 can be observed, especially the composites with 15
and 20 mg CMK-3.
The thermal behavior of the as-synthesized CMK-3 and the
As depicted in Figure 3C, the nitrogen adsorption
Co
investigated through thermogravimetric (TG) analysis (Figure
B). For the pure CMK-3, very limited and andante
3
O
4
@CMK-3 composites with different component ratios was
−desorption isotherm of CMK-3 and other four Co
NSs@CMK-3 composites are both exhibited a Type IV isotherm,
which unequivocally confirmed that all of Co @CMK-3
3 4
O
3
3 4
O
decomposition occurred before 500 ℃ followed by a dramatic
decrease in the weight loss between 500-700 ℃, which is
attributed to the oxidation of the amorphous carbon consist in
the CMK-3. Finally the decomposition nearly finished and the
curve kept stabilized after 700 ℃ with little residual component
composites are successfully prepared and keeping mesoporous
structure of CMK-3. What’s more, a pronounced hysteresis loop
was presented at both raw CMK-3 and Co O @CMK-3
3 4
composites in the low relative pressure range of 0.4-0.6, which
resulted from their mesoporous structure. After the modification
that might come from the remaining SiO
CMK-3. The existence of Si in the component was verified by
our subsequent XPS measurement. In the other four
2
during the synthesis of
of the Co
from 1086 m
composite derived from 5 mg CMK-3), 1805 m g (10 mg CMK-
3 4
O NSs, the calculated BET surface area increased
2
-1
2
-1
g
(CMK-3) to 1470 m
g
3 4
(Co O @CMK-3
2
-1
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2
-1
2
-1
3
(
) and 2396 m g (15 mg CMK-3), but decreased to 946 m g
20 mg CMK-3), which clearly demonstrated that a suitable
amount of CMK-3 can effectively prevent the Co from
aggregating which working as a rigid support for depositing
Co NSs and lead to high surface area for the composite.
Furthermore, the NSs/MP structure of Co @CMK-3 achieves
larger specific surface area and exhibits excellent effective
interfacial contacts area among the three-phases, which is
considered to be very beneficial to promote charge transfer and
provide more active sites during the ORR. While the mass of
3 4
O
O
3 4
CMK-3 rise to 20 mg, the Co
3 4
O layer was self-aggregated and
3 4
O
blocked a part of the specific surface areas of CMK-3.
3 4
Figure 4. XPS spectra of (A) overall, (B) Co 2p and (C) O 1s of Co O @CMK-3 composites.
The detailed surface element chemical states of
@CMK-3 composites are further characterized by X-ray
which is more positive than the pure CMK-3 (Eonset of 0.84 V and
peak potential of 0.75 V), suggesting the Co NSs can
Co
O
3 4
3 4
O
photoelectron spectroscopy (XPS) spectroscopy as shown in
Figure 4, and all of the XPS spectroscopy was corrected by
enhance the catalytic activity for ORR. It is worth mentioning that
a slight but well-defined backward peak was obtained at CMK-3
electrode around 1.02 V, but was absented at Co O @CMK-3
3 4
electrode. This is powerful evidence that CMK-3 has a gas
channel, which does not allow the electrolyte to penetrate and
oxygen cannot be consumed due to the sluggish ORR kinetics
[
13]
setting C 1s peak at 284.6 eV.
Co @CMK-3 composites (Figure 4A) indicates the presence
of O, Co, C, and Si, where the Si (2p) peak comes from the
remnant of the SiO during the hard template synthesis process
of CMK-3. In Figure 4B, the peaks at binding energies of 779.9
The full survey spectrum of
O
3 4
2
on the bare CMK-3. However, the Co
surface of CMK-3 preserves the gas diffusion channels, so as
the Co NSs not only fabricates a unique three-phase
interface, but also provides ORR with highly active sites. That’s
to say, one side of the Co NSs exposes to the air transport
within the CMK-3, while the other side can fully contact the
electrolyte. More importantly, the performance of Co @CMK-
3 4
O NSs deposited on the
3+
eV and 795.9 eV should be ascribed to the Co in 2p3/2 and
1/2 states, respectively. Meanwhile, the other two major peaks
2
p
3 4
O
2
+
at 785.6 eV and 802.4 eV can also be attributed to the Co in
3/2 and 2p1/2 states, respectively, suggesting the co-presence
2
p
3 4
O
3+
2+
3+
2+
of Co and Co in the sample, and the ratio of Co : Co is
[
14]
about 2 : 1. This result was further confirmed by our previous
XRD analysis. In Figure 4C, the O 1s XPS spectrum was
deconvoluted into three photoelectron peaks of 529.5 (O1),
3 4
O
3 is comparable to that of Pt/C with the same mass (Eonset of
1.01 V and peak potential of 0.90 V).
5
30.6 (O2) and 531.6 (O3) eV that can be corresponded to the
[
15]
lattice oxygen defect in the spinel Co
3 4
O ,
the hydroxyl groups
and a small amount of adsorbed moisture on the surface,
respectively.
The ORR electrocatalytic properties of electrocatalysts were
first tested by cyclic voltammetry (CV). As illustrated in Figure 5A,
the composite with 15 mg CMK-3 reveals the best ORR
performances compared to the other three control samples, due
to the big BET surface area. In addition, the higher activity of the
electrocatalysts can also be concluded from the larger onset
potential (Eonset) and peak current (Ipeak). Lower values of Eonset
and Ipeak for the catalysts with 5 mg and 10 mg CMK-3 are due to
the fact that a comparatively more Co
blocked the active sites of CMK-3, while the less average
amount of Co NSs were obtained by using 20 mg templates,
3 4
O NSs have partially
3 4
O
which can vastly reduce the catalytic activity. So that the other
compared samples were prepared by using 15 mg templates in
3 4
Figure 5. (A) CVs of Co O @CMK-3 electrodes synthesized by using different
this paper. As depicted in Figure 5B, the Co
3 4
O @CMK-3 shows
3 4 3 4
amount of CMK-3 template. (B) CVs of Co O , CMK-3, Co O @CMK-3 and
an ORR Eonset of 1.03 V (vs RHE) and a peak potential of 0.90 V,
Pt/C electrodes.
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3 4
as-prepared Co O @CMK-3 in terms of Eonset, E1/2, and limiting
current density, proving the superiority of the surface method for
highly active Co depositing on mesoporous carbons (CMK-3)
3 4
O
over other transition metal oxide@CMK-3.
Table 1. Comparison of the ORR performance of various well-developed
electrocatalysts in the literature
Catalysts
E
onset vs.
E
1/2 vs.
Limiting
density (mA cm )
current
Ref
-
2
RHE (V)
RHE(V)
Co
3
O
4
@CMK-3
0.98
0.80
5.52
4.50
This work
[16]
Co@Co
1
3
4
O /NC-
0.92
0.80
Fe/Fe
2
3
O @Fe-
0.88
0.75
6.41
[17]
N-C-1000
N-Co
9
S
8
/G
0.94
0.94
0.92
-
6.00
3.36
5.30
[18]
[19]
[20]
3 4
Figure 6. (A) RDEs of Co O , CMK-3, NiO@CMK-3, ZnO@CMK-3,
Co
rotating rates and Koutecky-Levich plots (inset) of Co
represents the corrected current attributable to O ). (C) RRDE test of Co
CMK-3, Co @CMK-3 and Pt/C electrodes. (D) Comparison of electron-
3
O
4
@CMK-3 and Pt/C electrodes (from left to right). (B) RDEs at various
Co
3
O
4
/N-rmGO
-
3
4
O @CMK-3 electrode (j
2
3 4
O ,
NC@Co-NGC
DSNC
0.82
3
O
4
-1
transfer number on different electrodes at 0.6 V. Scan rate: 10 mV s .
ZnCo
2
O
4
/N-CNT
0.95
0.89
0.74
0.80
0.87
3.00
4.30
2.79
2.70
[21]
[22]
[23]
[24]
N,S-CN
0.74
To further evaluate the activity and dynamics property, the
as-obtained samples were prepared on rotating disk electrode
Mo-Doped MCG
-
-
(
0
1
RDE) for linear sweep voltammetry (LSV) in an O
2
-saturated
.1 M KOH solution, and the working electrode was rotated at
600rpm in order to remove the emerging bubbles during the
WN/N-carbon
black hybrids
electrochemical measurements. As demonstrated by Figure 6A,
both Co @CMK-3 and Pt/C show a sharply increasing LSV
curve. The Co @CMK-3 exhibits an Eonset of 0.98 V, a E1/2 of
.80 V and limiting current density of 5.52 mA cm , which is in
close proximity to that of Pt/C (Eonset of 0.99 V, E1/2 of 0.81 V and
limiting current density of 5.12 mA cm ) and other catalysts in
the literature (Table 1). In contrast, the LSV curve from bare
CMK-3 shows a relatively slow current increase and negative
GNPC
0.92
-
5.20
[25]
3
O
4
3 4
O
-
2
0
3 4
The LSV curve of Co O @CMK-3 at various rotating rates is
-
2
obtained, as shown in Figure 6B. The current density increases
with the acceleration of rotation rates, which can be ascribed to
the thinning diffusion layer, indicating that the NSs/MP structure
is expected to increase charge transfer. In addition, the
Koutecky–Levich (K–L) equations could be used to calculate the
kinetic parameter of electron-transfer numbers (n). As seen from
the inset picture in Figure 6B, the good linearity of the K–L plots
indicates the first-order reaction kinetics toward the consumption
E
1/2 of 0.74 V, implying that the pristine CMK-3 mainly occurs in
a two-electron-transfer during the ORR process. We also ran a
comparison with Co @CNTs, Co NSs deposited on the
surface of CNTs, which is prepared by the same method shown
in this work. As shown, the performance of Co @CNTs
including Eonset, E1/2, and limiting diffusion current is still far
behind the as-prepared Co @CMK-3. The superior
electrocatalytic performance of Co @CMK-3 comes from the
O
3 4
3 4
O
3 4
O
[
26]
of O
.91 by analyzing the slopes of the K–L plots at the potential
range of 0.55-0.70 V, indicating that the Co @CMK- 3
2
.
Dramatically, similar n value was calculated to be about
3
3
O
4
3 4
O
3
O
4
composite favours a four-electron-transfer reduction during the
ORR process, which is remarkably similar to that catalyzed by
the commercial Pt/C catalyst (n= 4.0, LSV polarization curves
seen in Figure S3), while n in the other cases is 1.2, 2.5, 2and
NSs/MP structure which could expand the reaction area. More
importantly, due to the hollow nature, CNTs possess gas
diffusion channels, but the oxygen would be unable to get pass
the carbon hollow tubes, resulting in slowly ORR electron
transfer process. But in CMK-3, this situation is completely
different, oxygen could be lightly passed through gas channels
because all pores inside the CMK-3 are connected and at the
same time contact with highly active Co O . The other metal
3 4
oxide deposited on CMK-3 (ZnO@CMK-3, NiO@CMK-3) was
also tested as another control sample. Though having the same
2.9, 3.1 for Co
3 4
O , CMK-3, ZnO@CMK-3, NiO@CMK-3, and
Co @CNTs, respectively (Figure 6D, LSV polarization curves
3 4
O
seen in Figure S4,S5).
This was further confirmed by RRDE measurement which
-
2
monitors the intermediate of peroxide species (HO
) produced
during the ORR process. The ring and disk currents were
recorded in Figure 6C which shows a more direct reaction
pathway of Co O @CMK-3 compared to other catalysts. The
3 4
formation NSs/MP structure as the Co
3 4
O @CMK-3, the ORR
catalytic properties of metal oxide@CMK-3 are still behind the
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result shows that both Co
3
O
4
@CMK-3 and Pt/C exhibit
not only fully contact the electrolyte solution, but also at same
time get improved oxygenation because of the interconnected
pores in the CMK-3 (see scheme 2), indicating a brilliant ORR
catalytic performance.
-
negligible HO
2
yields. This result is in close agreement with the
analysis of the K–L plots, verifying a mainly 4e reduction of
Co @CMK-3 for ORR. We also compare the bare CMK-3
catalyst, in which the average HO
than that of Co @CMK-3 during the same potential range.
This strongly suggests that the ORR active sites of our catalyst
originate from the Co . The advantages of Co @CMK-3 for
ORR could be concluded as following: 1. The Co @CMK-3
possess the NSs/MP structure which could provide more three-
phase interfacial areas for ORR. 2. The composite of Co NSs
3 4
O
-
2
yield value is much higher
3 4
O
O
3 4
3 4
O
3
O
4
3 4
O
and heat-treatment CMK-3 provide lager BET surface area,
sufficient active sites and good conductivity, reflecting in better
ORR catalysis performance.
3 4
Scheme 2. Schematic representation of the Co O @CMK-3 structure for
catalysis of ORR.
Conclusions
Figure 7. (A) Long-term stability test; (B) Methanol tolerance test of
Co
3
O
4
@CMK-3 and Pt/C by chronoamperometry. All chronoamperometric
-saturated 0.1 M KOH. The
tests were conducted at -0.35 V (vs Ag/AgCl) in O
2
In summary, we have designed catalyst structure of highly
active Co NSs couple with MP carbon support (CMK-3),
resulting in comparability electrocatalytic activity in comparison
to commercially Pt/C (20%) catalyst toward ORR. The Co O
4
NSs were vertically grown on the CMK-3 outer surface, rather
than inner ordered pores because of the extreme hydrophobicity
stability test was operated at a rotation rate of 1600 rpm.
3 4
O
3
Finally, long-term durability is used to evaluate the oxygen
reduction performance which is also a significant factor for their
practical application in metal-air batteries or fuel cells. The
of CMK-3. This framework includes a highly active Co
layer and interconnected oxygen pervasion channels from the
CMK-3, ensuring the Co NSs obtain abundant three phase
interfacial areas. The electrochemical testing results verify that
this unique NSs/MP Co @CMK-3 composite possesses
3 4
O NSs
chronoamperometry curves of Co
constant potential of 0.35 V (vs Ag/AgCl) were shown in Figure
A, obviously, the Co @CMK-3 possesses superior stability
3 4
O @CMK-3 and Pt/C at a
3 4
O
7
3 4
O
with a more slight current attenuation of 10% after 10 h of
continuous electrolysis, compared to the Pt/C that manifested a
current decay of 36%, this current declining was attributed to the
active species peeled off during the oxygen reduction reaction.
3 4
O
excellent electrocatalytic performance for ORR, which is
superior to pure CMK-3 and other carbon material-based
catalysts. We expect that this simple synthesis methodology for
More importantly, like other Pt-free catalyst, Co
shows stronger tolerant to methanol, in contrast to Pt/C (Figure
B), which is a crucial property for fuel cells which employ
methanol. This indicated that the long-term stability of
Co @CMK-3 is better than that of Pt/C.
The above data shows the mechanism for the enhanced
electrochemical performance of the Co @CMK-3 composite.
We believe that the high active and unusual Co NSs
3 4
O @CMK-3
deposition of Co
similar areas.
3 4
O NSs on CMK-3 can be extended to other
7
3 4
O
Experimental Section
Synthesis of materials
3 4
O
3 4
O
deposition on the outer wall of the pristine CMK-3 play a
significant role in its enhanced catalytic performance for ORR,
Furthermore, the unique NSs/MP structure could alleviate the
volume expansion and promote the electrons transport from
electrode to catalyst during ORR, thus reducing the resistance in
the ORR process which could improve the electrochemical
3 4
The fabrication process for 3D Co O @CMK-3 is demonstrated in
Scheme 1. First, the ordered mesoporous carbon (CMK-3) was obtained
by using a soft template method to synthesis the ordered silica template
SBA-15, followed by a hard template casting and carbonization process
[
27]
according to the previous literatures.
Co @CMK-3, 15 mg CMK-3 templates were first dispersed into 40 mL
of deionized water under ultrasound for 5min at room temperature, and
followed by addition of 145 mg Co(NO •6H O, 35 mg
hexamethylenetetramine (HMT) and 6.5 mg citric acid trisodium salt
In a typical synthesis of
3
O
4
3 4
performance. More importantly, the direct coverage of Co O
3
)
2
2
NSs on CMK-3 enables a strong bonding between Co
CMK-3, leading to excellent ORR stability. And the Co
3
O
4
and
NSs
3
O
4
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ChemCatChem
10.1002/cctc.201701822
FULL PAPER
2
-1 [29]
dehydrate, respectively. After forming homogeneous solution, the
resultant solution was transferred into a 100 mL round bottom flask, and
keeping at 90 °C for 6 h with magnetically stirring in an oil bath. After that,
the flask was cooled down to room temperature naturally and then the
mixtures were washed via centrifugation at 8000 rpm for 5min with
methanol for several times, and the obtained product was dried at 60℃
overnight. Finally, the as-prepared product was further pyrolysis
cm s ).
For rotating-ring-disk electrode (RRDE) measurements, the Pt ring is
employed to collect the intermediate products while the glassy carbon
disk is the main oxygen reaction place. The ring current and disk current
was recorded respectively, while a LSV program was operated on disk
electrode at a rate of 5 mV s and a constant potential (0.4 V vs.
Ag/AgCl) test was applied on Pt ring. The n and the %HO
-
1
-
2
field were
-1
[30]
treatment at 400℃ for 2 h at a temperature increase rate of 1℃ min in
Ar atmosphere. For comparison, the aggregated Co NSs,
ZnO@CMK-3, NiO@CMK-3 and Co @CNTs were synthesized via a
similar procedure except for without addition of CMK-3, using
Zn(NO ·6H O, Ni(NO ·6H O replace of Co(NO ·6H O and using
CNTs replace of CMK-3. Moreover, series of Co @CMK-3
calculated from the equations below:
3 4
O
3 4
O
3
)
2
2
3
)
2
2
3
)
2
2
a
3 4
O
composites were also prepared by adjusting the amounts of CMK-3
templates for comparison.
In the equation, I
d r
and I are corresponding to the ring current and disk
current, respectively. N is 0.37 which stands for the current collection
[31]
efficiency of the Pt ring.
In the measurements, the reference electrode was calibrated with
respect to the reversible hydrogen electrode (RHE) with high purity H
saturated 0.1 KOH electrolyte. The scan rate of linear sweep
voltammetry was 1 mV s , and the thermodynamic potential of the RHE
is calculated according to the average value of the two potentials at
which the current crossed zero. In 0.1 M KOH, ERHE = EAg/AgCl + 0.94 V.
Physicochemical Characterization
2
The XRD measurement was carried out by X-ray difraction (Rigaku-
TTRIII, Cu Ka, l =0.154056 nm). The morphology of products was
obtained by field emission scanning electron microscopy (SEM,TESCAN
MIRA3 LMU) and microstructure was observed through transmission
electron microscopy (TEM, Titan G2 6020) which is equipped with energy
dispersive X-ray spectrometry (EDS) to examine the homogeneity of the
C, N, O, S, Fe and Co in the product. X-ray photoelectron spectra (XPS)
were acquired on an ESCSLAB MK II X-ray photoelectron spectrometer
with Mg Kα as the exciation source. The Brunauer-Emmett-Teller (BET)
surface area and pore volume were measured by using an automatic
volumetric adsorption equipment of Mircomeritics ASAP2010. Thermo
gravimetric analysis (Perkin-Elmer TGA 7) was carried out under a flow
of air with a temperature ramp of 10 °C/min from room temperature to
M
-
1
Acknowledgements
The authors thank the National Natural Science Foundation of
China (Grant no. 51674299) and the financial support from the
China Postdoctoral Science Foundation funded project
(2015M572266). The authors also thank the Fundamental
Research Funds for the Central Universities of Central South
University (2015cx001), the Research Innovation Program for
College Graduates of Central South University (Grant No.
8
00 °C.
Electrochemical measurements
The ORR performances were studied on a standard three electrode
system working in Solartron1470E electrochemical workstation. A Pt wire
and an Ag/AgCl (3 M KCl) electrode served as the counter electrode and
the reference electrode, respectively. The working electrodes were
2
016zzts035) and other support from the Engineering Research
Centre of Advanced Battery Materials, the Ministry of Education,
China.
-1
prepared as follows: All catalysts inks with the concentration of 4 mg mL
were prepared by dispersing 4 mg catalyst in 750 L ethanol, 215 L
water, 35 L Nafion solution (5 wt.%, Alfa Aesar), followed by ultrasonic
treatment for 0.5 h. Then, the resulting 10L homogeneous suspensions
were pipetted onto glassy carbon rotating disk electrode (RDE, PINE, 5
Keywords: non-noble metal eletrocatalyst •oxygen reduction
reaction • metal oxide nanosheets • nanosheets/mosepore
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FULL PAPER
In this work, we uniformly deposition
L. Gan, M. Wang, L. Hu, J. Fang*, Y.
Lai, J. Li
of Co
O
3 4
nanosheets on the backbone
nanosheets not
of CMK-3. The Co
3 4
O
only fully contact the electrolyte
solution, but also at same time get
improved oxygenation because of the
interconnected pores in the CMK-3.
And the other metal oxides can also
be deposited on the surface of carbon
supports.
Page No. – Page No.
Nanosheets/Mesopore Structured
3 4
Co O @CMK-3 Composite as
Electrocatalyst for Oxygen Reduction
Reaction
Layout 2:
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Author(s), Corresponding Author(s)*
((Insert TOC Graphic here; max. width: 11.5 cm; max. height: 2.5 cm))
Page No. – Page No.
Title
Text for Table of Contents
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