The direct growth of highly dispersed CoO nanoparticles on mesoporous carbon as a high-performance electrocatalyst for the oxygen reduction reaction

Article abstract of DOI:10.1039/c6ra14394f

Cobalt monoxide (CoO) nanoparticles (NPs) and mesoporous carbon (MC) with large specific surface area were combined as a novel nanocomposite (CoO/MC) using a hydrothermal method to reveal outstanding electrocatalytic activity in the oxygen reduction reaction (ORR). The addition of polyvinylpyrrolidone (PVP) as the surfactant during the hydrothermal process is beneficial for the high dispersion of CoO NPs on the surface of MC. Among the as-acquired products, the CoO/MC nanocomposite prepared with 1.5 g PVP as the surfactant (CoO/MC-1.5) exhibits much better catalytic activity for the ORR with a more positive onset potential, a highly efficient four-electron transfer pathway and a larger current density than the others. Furthermore, the CoO/MC-1.5 nanocomposite demonstrates outstanding durability based on current-time chronoamperometric tests, which significantly prevails over a commercial Pt/C catalyst. The eminent catalytic activities of the CoO/MC-1.5 nanocomposite should be a result of the synergistic effect of the highly dispersed CoO nanoparticles and the ordered mesostructures with large specific surface area, which are advantageous for increasing the exposure of the active sites and promoting fast transfer of the reactants and products.

Full text of DOI:10.1039/c6ra14394f

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The direct growth of highly dispersed CoO
nanoparticles on mesoporous carbon as a high-
performance electrocatalyst for the oxygen
reduction reaction†
Cite this: RSC Adv., 2016, 6, 70763
a
a
a
a
Pengxi Li,ab Ruguang Ma, Yao Zhou, Yongfang Chen, Qian Liu, Guihua Pengb
*
a
*
and Jiacheng Wang
Cobalt monoxide (CoO) nanoparticles (NPs) and mesoporous carbon (MC) with large specific surface area
were combined as a novel nanocomposite (CoO/MC) using a hydrothermal method to reveal outstanding
electrocatalytic activity in the oxygen reduction reaction (ORR). The addition of polyvinylpyrrolidone (PVP)
as the surfactant during the hydrothermal process is beneficial for the high dispersion of CoO NPs on the
surface of MC. Among the as-acquired products, the CoO/MC nanocomposite prepared with 1.5 g PVP as
the surfactant (CoO/MC-1.5) exhibits much better catalytic activity for the ORR with a more positive onset
potential, a highly efficient four-electron transfer pathway and a larger current density than the others.
Furthermore, the CoO/MC-1.5 nanocomposite demonstrates outstanding durability based on current–
time chronoamperometric tests, which significantly prevails over a commercial Pt/C catalyst. The
eminent catalytic activities of the CoO/MC-1.5 nanocomposite should be a result of the synergistic
effect of the highly dispersed CoO nanoparticles and the ordered mesostructures with large specific
surface area, which are advantageous for increasing the exposure of the active sites and promoting fast
transfer of the reactants and products.
Received 3rd June 2016
Accepted 12th July 2016
DOI: 10.1039/c6ra14394f
www.rsc.org/advances
develop more efficient electrocatalysts for the ORR by adopting
different strategies.
Introduction
To date, theoretical and experimental results revealed that
transition metal oxides are an important class with promising
catalytic activity and reliability for the ORR in the alkaline
solution.5 Furthermore, carbon-based nanomaterials are
promising matrix for manufacturing hybrid and composite
materials, in terms of their high electrical conductivity, large
surface area and structural exibility.6 For example, FeOx
nanoparticles (NPs) embedded in porous carbon synthesized
using the chemical vapour deposition method have been re-
ported as an ORR electrocatalyst.7 Yusuke Yamauchi and co-
workers have reported nanoporous nickel oxide akes coated
with graphene oxide sheets to improve the ORR performance.8
Ketjen black carbon supported amorphous manganese oxides
nanowires have been investigated as a highly efficient electro-
catalyst for the ORR.9 Moreover, Dai and his co-workers have
studied Co3O4 NPs attached on graphene as a synergistic cata-
lyst for the ORR,10 as well as the CoO nanocrystals coupled with
carbon nanotubes.11 In comparison with other carbon materials
(e.g., carbon nanotube, graphene, carbon aerogel, and carbon
black),12 mesoporous carbon (MC) synthesized via a nano-
casting method has drawn a lot of attention in the eld of
energy storage and conversion, in terms of its high specic
surface area, outstanding electrical conductivity and adjustable
Nowadays, the oxygen reduction reaction (ORR) has been one of
the concerns of electrochemistry due to the persistent chal-
lenges in a series of energy applications containing fuel cells
and metal–air batteries.1 Highly efficient electrocatalysts are
desirable due to the slow kinetics of the ORR.2 Pt-based metals
have been considered as the most active catalysts for the ORR,
but they are subject to high cost and poor stability.3 The ORR
catalysts with high activity need to lower the overpotential and
endure the erosive operating conditions. Nevertheless, the
catalysts that have been reported in the literature are still far
from satisfying the demands of high catalytic activity, supernal
stability and low cost.4 Therefore, there are various challenges to
aState Key Laboratory of High Performance Ceramics and Superne Microstructure,
Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road,
Shanghai 200050, P. R. China. E-mail: jiacheng.wang@mail.sic.ac.cn; qianliu@
sunm.shcnc.ac.cn
bState Key Laboratory Cultivation Base for the Chemistry and Molecular Engineering of
Medicinal Resources, Ministry of Science and Technology of China, School of
Chemistry
& Pharmaceutical of Guangxi Normal University, Guilin 541004,
Guangxi, P. R. China
† Electronic supplementary information (ESI) available: Additional Fig. S1 and S2
mentioned in the text. See DOI: 10.1039/c6ra14394f
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mesopore sizes.13 Considering its excellent properties, MC can microscopy (TEM, JEM-2100F). X-ray photoelectron spectros-
be used as the outstanding matrix for various transition metal copy (XPS, ESCALAB 250 X-ray photoelectron spectrometer, Al
oxides as electrocatalysts for the ORR. MC with a large surface Ka radiation) was employed to analyze the surface of the ob-
area is benecial for the high dispersion of active nanoparticles, tained samples. Nitrogen adsorption/desorption measure-
thus resulting in the enhanced exposure of accessible catalytic ments (Micromeritics ASAP 2010, ꢁ196 ꢀC) were utilized to
sites. Thus, MC-supported catalytic materials can show analyze the surface area and pore size of the catalysts. All the
a signicantly enhanced activity for the ORR. We have studied catalysts were dried at 200 ꢀC overnight in vacuum before
spinel CoFe2O4 nanoparticles grown on ordered mesoporous their measurement. We used the Brunauer–Emmett–Teller
carbon in the electrocatalytic ORR.14 To the best of our knowl- (BET) method and Barrett–Joyner–Halenda (BJH) method to
edge, no more related reports on MC as the support utilized in calculate the surface area and pore size distributions,
the ORR have been reported in the previous studies.
respectively.
In this study, we demonstrated the direct growth of CoO NPs
on the surface of MC (CoO/MC) via a facile hydrothermal
approach with different contents of polyvinylpyrrolidone (PVP)
as the surfactant. The MC utilized as the matrix was synthesized
through a traditional nanocasting method with ordered meso-
porous silica rod as a hard template.15 The CoO/MC nano-
Electrochemical measurements
The electrochemical activities of the catalysts were tested using
a PINE instrument with traditional three electrode equipment
in 0.1 M KOH solution employed as the electrolyte. The refer-
ence and counter electrodes correspond to the saturated
calomel electrode (SCE) and Pt ake electrode, respectively. The
working electrode corresponds to glassy carbon electrode (GCE)
with a geometric surface area of 0.196 cm2. The geometric
surface area of the GC disk and Pt ring of the rotating ring-disk
electrode (RRDE) were 0.2475 cm2 and 0.1866 cm2, respectively.
The catalyst inks were prepared by mixing the active sample (5
mg), Naon solution (5 wt%, 30 mL), distilled water (500 mL) and
EtOH (500 mL), followed by ultrasonication for about 60 min to
form a homogeneous ink. Subsequently, 20 mL of ink was
pipetted on a GCE and dried at 50 ꢀC, which was employed for
further electrochemical test. The ink of commercial 20 wt% Pt/C
(Johnson Matthey) was prepared via the same method for
comparison.
For the ORR test, the electrolyte (0.1 M KOH) was purged
with high-purity O2 gas to ensure O2 saturation. The cyclic vol-
tammetry (CV) measurements were carried out at a sweep rate of
50 mV sꢁ1. The working electrode was cycled at least y times
ahead of assembling the data. Linear sweep voltammetry (LSV)
tests were investigated at a sweep rate of 10 mV sꢁ1, with the
electrode rotating at 400, 625, 900, 1225, 1600, and 2025 rpm.
Based on the LSV data, we acquired the current densities and
normalized the results according to the geometric surface area.
We analyze the kinetic parameters of the ORR test using the
Koutecky–Levich equation (eqn. (1)):
composite prepared using 1.5
g PVP as the surfactant
demonstrates excellent electrocatalytic activity for the ORR,
which was studied using a conventional three electrode equip-
ment in alkaline solution. The CoO/MC-1.5 nanocomposite
mainly favours a direct four electron reaction path for the ORR
and owns a high current density contrasted to commercial Pt/C.
Furthermore, the CoO/MC-1.5 nanocomposite reveals the
exceeding long-term reliability for the ORR to commercial Pt/C.
Experimental
Materials synthesis
MC was prepared via a nanocasting method with ordered
mesoporous silica (MS) as a hard template, which was prepared
in advance according to our previously reported procedure.14,16
The nanocomposites were synthesized as follows. Typically, MC
(0.06 g) was immersed into a mixture of 40 mL distilled water
and 20 mL EtOH under ultrasonication for 120 min. Then,
Co(NO3)2$6H2O (0.004 mol) was dissolved in the above-
mentioned suspending solution under vigorous stirring, fol-
lowed by the addition of different contents of
polyvinylpyrrolidone (PVP) as a surfactant (0, 0.5, 1, or 1.5 g).
Subsequently, aqueous ammonia (NH4OH, 6 mL), utilized as
the precipitator, was added dropwise under vigorous stirring.
Aer stirring for 60 min, the suspension was transferred into
a Teon-lined stainless autoclave and heated at 180 ꢀC for 12 h.
The obtained sediment was collected, centrifuged, washed with
ethanol and distilled water and dried in vacuum. Then, the
obtained powders were annealed at 400 ꢀC under a N2 atmo-
sphere for 4 h. The nal products were labelled as CoO/MC-X (X
¼ 0, 0.5, 1, or 1.5), in which X stands for the content of PVP used
in grams.
1
1
1
1
1
¼
þ
¼ þ
jk Bu1=2
(1)
j
jk jd
where j, jk and jd represent the measured disk current density,
the kinetic and diffusion limiting current densities, respec-
tively; u represents the electrode rotation speed; B can be
analyzed using the following equation (eqn (2)):
B ¼ 0.2nFCO DO 2/3nꢁ1/6
(2)
2
2
Structural characterization
where n represents the transferred electrons number, F repre-
X-ray diffraction (XRD, D8 ADVANCE, Cu Ka radiation, 40 kV, sents Faraday constant (96 485 C molꢁ1), DO represents the
2
200 mA) was used to determine the phase and crystal structure diffusion coefficient of oxygen (DO ¼ 1.86 ꢂ 10ꢁ5 cm2 sꢁ1), n
2
of the catalysts. The morphology and microstructure of the represents the kinetic viscosity of solution (n ¼ 0.01 cmꢁ2 sꢁ1
)
acquired catalysts were observed by scanning electron and CO represents the bulk concentration of O2 dissolved in the
2
microscopy (SEM, JEOL JSM-6700) and transmission electron electrolyte (CO ¼ 1.21 ꢂ 10ꢁ6 mol cmꢁ3).
2
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For the (RRDE) test, we held the ring potential at 0.2 V (vs.
SCE) at a sweep rate of 10 mV sꢁ1. We calculated the percentage
of H2O2 and the transferred electrons number during the ORR
procedure using the following equations (eqn (3) and (4)):
Ir=N
ꢁ
HO2 ¼ 200
(3)
(4)
Id þ Ir=N
Id
n ¼ 4
Id þ Ir=N
where Id represents the disk current, Ir represents the ring
current, and N represents the current collection efficiency of the
Pt ring (N ¼ 0.37).
Fig. 2 The X-ray diffraction patterns of the CoO/MC nanocomposites.
Results and discussion
The morphology and microstructure of MS and its replica MC
were surveyed using scanning electron microscopy (SEM). The
SEM image of the parent MS indicates that it is a macroporous
skeleton, including interconnected mesoporous silica rods
(Fig. 1a). SEM indicated that MC possesses fully interconnected,
homogeneous and sponge-like macroporosity (Fig. 1b), which is
the positive replica of the parent MS at the micrometer level.
Both MS and MC have inerratic branched rods as the building
blocks concatenating end-to-end with each other, which fabri-
cate the rigid macro-porous skeletons.
The CoO NPs were formed on the surface of MC via a facile
hydrothermal treatment, followed by pyrolysis at 400 ꢀC. The
resulting CoO/MC nanocomposites were further characterized
by the X-ray diffraction (XRD) measurements shown in Fig. 2.
Based on the standard PDF card (PDF#43-1004), all the char-
acteristic peaks of the samples can be well indexed as the CoO
phase without any collateral peaks. In addition, the content of
PVP has no signicant effect on the formation of the CoO phase.
Furthermore, the content of MC in the CoO/MC-1.5 nano-
composite was about 6.8 wt%, determined by thermogravi-
metric analysis.
absence of PVP as the surfactant. In the presence of PVP as the
surfactant (Fig. 3c), the CoO nanoparticles in the CoO/MC-1.5
nanocomposite were well dispersed on MC, which still main-
tained the ordered mesoporous structures. The histogram of
the CoO particle size distribution in CoO/MC-1.5 nano-
composite was in the range of 8–18 nm with a medial particle
size of 12.2 nm (Fig. 3d). The MC matrix with large surface area
may supply a large number of active sites (e.g. oxygen-
containing groups) for the germinating CoO NPs, which
possess smaller sizes. The high dispersion and uniformity of
CoO NPs on the MC are benecial for increasing the density of
the ORR active sites. Furthermore, the diffusion kinetic of
reactants, ions, electrons, and products can be improved in the
CoO/MC nanocomposites ascribed to the ordered mesoporous
channels. Accordingly, the electrocatalytic ORR performance of
the resulting composite will be improved.
The transmission electron microscopy (TEM) image of MC
demonstrates the triumphant synthesis of MC (Fig. 3a) with the
safeguard of regular ordered hexagonal nanowires arrays via
a nanocasting method (inset of Fig. 3a).15,17 MC with a unique
microstructure of ordered mesochannels and large surface area
can be employed as the catalytic matrix, which is benecial for
not only dispersing active sites, but also promoting fast transfer
of the reactants and resultants. As shown in Fig. 3b, the CoO
NPs severely agglomerated in CoO/MC-0 nanocomposite in the
Fig. 3 The TEM images of (a) pure MC, (b) CoO/MC-0 and (c) CoO/
MC-1.5; the particle size distribution of CoO nanoparticles in the CoO/
MC-1.5 nanocomposite (d); the inset of (a) represents the high-
magnification TEM images of the selected area.
Fig. 1 The SEM images of (a) MS and (b) MC.
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The cation oxidation state and surface chemical composition
of the CoO/MC nanocomposites were inquired by XPS. As
demonstrated in Fig. 4a, the XPS survey spectra of both CoO/
MC-0 and CoO/MC-1.5 nanocomposites display the peaks for
Co 2p, O 1s and C 1s. The de-convolutions of the Co 2p band in
the CoO/MC-0 and CoO/MC-1.5 nanocomposites show four
peaks (Fig. 4b). The two peaks of 780.5 and 796.3 eV correspond
to Co 2p3/2 and Co 2p1/2, respectively.18 The binding energies at
around 786.8 and 803.5 eV are two satellite peaks, which are
shake-up type peaks of Co at high binding energies. The
primary peaks and satellite peaks for Co 2p3/2 and Co 2p1/2
reveal the presence of Co2+ cations in the CoO/MC nano-
composites. This result is consistent with those obtained from
the XRD patterns showing the existence of the CoO phase in the
nanocomposites. Moreover, there exhibits no shi in the Co 2p
peaks whether PVP was used as the surfactant or not through
a comparison of Co 2p spectra of the CoO/MC-0 and CoO/MC-
1.5 nanocomposites, which implies that both Co cations have
the same chemical surroundings. The usage of PVP only
increases the dispersion of CoO NPs on the MC, which does not
change the chemical surroundings of the Co cations in the nal
samples. The C 1s XPS spectra (Fig. 4c) were tted based on the
three carbon bonding conditions: 285.0 eV (C–C), 287.0 eV (C–
O–C and C–O–H), and 288.7 eV (O–C]O).19 The O 1s XPS
spectra (Fig. 4d) show a strong distinct M–O–M peak at 530.3 eV,
indicating most of the oxygen atoms are in the lattice. Another
peak for the O–C]O bond at 531.7 eV, corresponding to the
oxygen functional groups on the surface of the MC are due to an
incomplete reduction or the moisture adsorbed onto the MC
under ambient conditions.20 Furthermore, it can be suggested
the existence of strong coupling between the CoO nanoparticles
and carbon matrix in the CoO/MC nanocomposites based on
previous reports.14,21
Fig. 5 The N2 adsorption–desorption isotherms of (a) CoO/MC-0 and
(b) CoO/MC-1.5, and the pore size distribution curves of (c) CoO/MC-
0 (c) and (d) CoO/MC-1.5.
desorption isotherms obtained for both the CoO/MC-0 and
CoO/MC-1.5 nanocomposites reveal that they are mesoporous
materials, with BET specic surface areas of 250.9 m2 gꢁ1 and
290.5 m2 gꢁ1, respectively. When compared to that found for
pure MC, the specic surface area of the CoO/MC nano-
composites decreased signicantly.16 The N2 sorption isotherm
obtained for MC is a typical IV isotherm with a BET specic
surface area of 1103 m2 gꢁ1 (Fig. S1a†). The pore size distribu-
tion curve for MC exhibits a dened peak at 3.84 nm (Fig. S1b†).
The MC with high specic surface area as the support for CoO
nanoparticles can help increase the accessible surface area. As
we can see, there are two capillary condensation steps in the P/
P0 range of 0.4–0.6 and 0.8–1.0, which imply the existence of
dual mesoporosity in both the CoO/MC-0 and CoO/MC-1.5
nanocomposites, respectively. The former was due to the
ordered mesoporous channels of the MC support and the latter
was ascribed to the large mesopores derived from the accu-
mulation of CoO nanoparticles grown on the MC.22 Fig. 5c and
d display the pore size distribution curves obtained for CoO/
MC. For CoO/MC-0, the two well-dened peaks, located at
3.38 nm and 11.3 nm, are reected by two hysteresis loops in the
isotherm. However, the peaks for CoO/MC-1.5 are located at
3.38 nm and 9.25 nm and the peak at 9.25 nm is gentle. As
shown in the TEM image, the CoO NPs agglomerated without
using PVP as the surfactant, thus decreasing the accessible
surface area and affecting the pore size distribution of the
hybrid catalysts. With PVP as the surfactant, the CoO NPs were
symmetrically distributed on the MC, which still maintained
the ordered mesoporous channels in the CoO/MC-1.5 nano-
composite. Furthermore, the diffusion kinetic of reactants,
ions, electrons, and products can be signicantly improved for
the CoO/MC nanocomposites by the unique mesoporous
structure and large specic surface, and thus, the catalytic
activity of the nanocomposites can be increased.
To investigate the porosity of pure MC and the CoO/MC
nanocomposites, N2 sorption measurements were performed
at ꢁ196 ꢀC. As shown in Fig. 5a and b, the N2 adsorption–
The electrochemical characterization of the CoO/MC nano-
composites was conducted using traditional three-electrode
systems in 0.1 M KOH solution; pure CoO, MC and Pt/C were
Fig. 4 (a) Full XPS spectra, (b) high-resolution Co 2p XPS spectra, (c)
high-resolution C 1s XPS spectra, and (d) high-resolution O 1s XPS
spectra of the CoO/MC nanocomposites.
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also tested as a comparison under the same conditions. Fig. 6a NiO/CNT,24 MnO2/CNT,25 Co3O4/graphene,26 and quasi oxygen-
exhibits the LSVs in O2-saturated 0.1 M KOH solution at a rota- decient indium tin oxide nanoparticles.27 It can be
tion rate of 1600 rpm. The onset potentials for all the CoO/MC concluded that the CoO/MC-1.5 nanocomposite prepared using
nanocomposites (ꢁ0.13 to ꢁ0.16 V) signicantly precede those 1.5 g PVP as the surfactant during the hydrothermal process has
obtained for pure CoO (ꢁ0.23 V) and MC (ꢁ0.29 V). This optimal activity for the ORR among the as-acquired catalysts,
signicant increase in activity found for the nanocomposite can probably due to having the highest dispersion of CoO nano-
be ascribed to the synergistic effect of both constituents. particles in the nanocomposite. Fig. 6c demonstrates the CV
Furthermore, the onset potential of the CoO/MC-1.5 nano- curves for the CoO/MC-1.5 nanocomposite in N2 and O2-satu-
composite (ꢁ0.13 V) was superior to those obtained for CoO/ rated 0.1 M KOH solution. In N2-saturated solution, no redox
MC-0 (ꢁ0.16 V), CoO/MC-0.5 (ꢁ0.16 V) and CoO/MC-1 (ꢁ0.14 peak can be found for the CoO/MC-1.5 nanocomposite.
V), which is due to the high dispersion of nanoparticles in the However, it exhibits an obvious reduction peak at ꢁ0.26 V in O2-
nanocomposite that can provide more active sites for the ORR. saturated solution, which could hint that O2 is reduced on the
To analyze the kinetic properties of the ORR on the as-prepared surface of the CoO/MC-1.5 nanocomposite. Fig. 6d demon-
electrocatalysts, Tafel slopes were obtained from the linear plots strates the LSVs at different rotation rates for the ORR catalyzed
of the LSVs (Fig. S2†). The CoO/MC-1.5 nanocomposite exhibits by the CoO/MC-1.5 nanocomposite of which the diffusion
a lower Tafel slope (95 mV decꢁ1) than CoO/MC-0 (101 mV limiting currents increase gradually as the rotation rate was
decꢁ1), implying enhanced ORR kinetics due to the high increased. Moreover, the RDE data were collected by the LSVs
dispersion of CoO nanoparticles on the MC. The CoO/MC-1.5 utilizing the K–L equation. The corresponding K–L plots for the
nanocomposite shows a more supernal limit current density CoO/MC-1.5 nanocomposite are shown in Fig. 6e, which were
(5.96 mA cmꢁ2) than the other catalysts at ꢁ1.0 V (Fig. 6b), and obtained at ꢁ0.5, ꢁ0.6, ꢁ0.7 and ꢁ0.8 V. The parallel and linear
the commercial Pt/C catalyst (5.52 mA cmꢁ2). Moreover, the K–L plots can reveal the rst-order dependence of the kinetics of
limiting current density of the CoO/MC-1.5 nanocomposite was the ORR on the CoO/MC-1.5 nanocomposite. The calculated
also higher than those for CoO/N-CNT,11 Fe3O4/N-graphene,23 results indicate an electron transfer number of ꢃ4.0. Fig. 6f
summarizes the number of transferred electrons (n) for CoO/
MC-0, CoO/MC-1.5 and commercial Pt/C, which demonstrate
the favorable selectivity for the 4-electron reaction pathway of
the CoO/MC-1.5 nanocomposite during the ORR. Furthermore,
our catalyst shows preferable or considerable performance to
the previous related report.28
To further verify the electron transfer pathway during the
ORR, rotating ring-disk electrode (RRDE) equipment was
employed, with which we could accurately calculate the amount
of H2O2 generated at the disk electrode. The LSVs for CoO/MC-0,
CoO/MC-1.5 and Pt/C (Fig. 7a) were collected in O2-saturated 0.1
M KOH solution using a ring potential of 0.2 V (vs. SCE). The
CoO/MC-1.5 nanocomposite shows the positive onset potential
and supernal current density than those of the CoO/MC-
ꢁ
0 nanocomposite. The formation of peroxide species (HO2
)
and the number of transferred electrons (n) according to the
corresponding RRDE data during the ORR procedure are shown
ꢁ
in Fig. 7b. The calculated HO2 yield of the CoO/MC-1.5 nano-
composite and Pt/C were 1.47–1.69% and 0.91–2.51%, respec-
tively, during the potential range from ꢁ0.8 to ꢁ0.4 V (vs. SCE).
The calculated n values obtained for the CoO/MC-1.5 nano-
composite and Pt/C were 3.96–3.97 and 3.95–3.98. The ultra-low
HO2ꢁ production and n value very close to 4 found for the CoO/
MC-1.5 nanocomposite indicate its high activity for the ORR
and high selectivity for the 4-electron reaction pathway during
the ORR process.
Fig. 6 (a) The LSVs of the CoO/MC nanocomposites, CoO, MC and
commercial Pt/C in O2-saturated 0.1 M KOH solution at a rotation rate
of 1600 rpm; (b) the limit current density of the CoO/MC nano-
composites and commercial Pt/C in O2-saturated 0.1 M KOH solution
The stability of the catalysts is one of the important factors
for the development of fuel cells or metal–air batteries. The
at a rotation rate of 1600 rpm; (c) the CV curves of the CoO/MC-1.5 durability of the CoO/MC nanocomposites and Pt/C were tested
nanocomposite in N2 and O2-saturated solution; and (d) the LSVs for
the ORR using the CoO/MC-1.5 nanocomposite in O2-saturated 0.1 M
KOH solution at different rotation rates; (e) the K–L plots based on the
ORR curves of the CoO/MC-1.5 nanocomposite, and (f) the number of
using chronoamperometry in O2-saturated 0.1 M KOH solution,
respectively. As indicated in Fig. 8, the current density of the
ORR for CoO/MC-1.5 decreased by about 8.6% at a constant
potential of ꢁ0.6 V (vs. SCE) aer 20 000 s of sustained opera-
transferred electrons (n) of CoO/MC-0, CoO/MC-1.5 and commercial
Pt/C.
tion, while the current density of the ORR for CoO/MC-
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Conclusions
In summary, we have synthesized highly dispersed CoO NPs
directly grown on the surface of mesoporous carbon (MC) via
a facile hydrothermal method. With 1.5 g PVP as the surfactant,
the resulting CoO/MC-1.5 nanocomposite shows optimal elec-
trocatalytic activity for the ORR among the various CoO/MC
catalysts prepared with different contents of PVP. The CoO/
MC-1.5 nanocomposite displays a 4-electron transfer path in
the ORR process, in terms of the K–L plots and RRDE data.
Moreover, the CoO/MC-1.5 nanocomposite exhibits the
supernal durability for the ORR, which exceeds that found for
Pt/C. The excellent electrocatalytic activity of the CoO/MC-1.5
nanocomposite was primarily attributed to the synergistic
effect of the uniform dispersed CoO NPs and ordered meso-
porous carbon matrix with high specic surface areas.
Acknowledgements
The authors would like to thank the nancial support provided
by the National Natural Science Foundation of China (No.
51502327, 21307145), Key Project for Young Researcher of State
Key Laboratory of High Performance Ceramics and Superne
Microstructure, One Hundred Talent Plan of Chinese Academy
of Sciences, the Youth Science and Technology Talents “Sail”
Program (No. 15YF1413800) and International Cooperation
Program (15520720400) of Shanghai Municipal Science and
Technology Commission, and the Research Grant (No.
14DZ2261203) obtained from the Shanghai Government.
Fig. 7 (a) The LSVs on the RRDE for the CoO/MC nanocomposites and
commercial Pt/C in O2-saturated 0.1 M KOH solution at a rotation rate
of 1600 rpm. The ring potential was held at 0.2 V (vs. SCE); (b) the
calculated number of transferred electrons (n) and the determined
peroxide percentage over various potentials based on the corre-
sponding RRDE data in (a).
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