3D Graphene-Carbon Nanotube Hybrid Supported Coupled Co-MnO Nanoparticles as Highly Efficient Bifunctional Electrocatalyst for Rechargeable Zn-Air Batteries

Article abstract of DOI:10.1002/asia.202000913

The development of high-efficiency and low-cost catalysts is one of the core and important issues to improve the efficiency of electrochemical reactions on electrodes, and it is also the goal we ultimately pursue in the commercialization of large-scale clean energy technologies, such as metal-air batteries. Herein, a nitrogen-doped graphene oxide (GO)-carbon nanotube (CNT) hybrid network supported coupled Co/MnO nanoparticles (Co/MnO@N?C) catalyst was prepared with a hydrothermal-pyrolysis method. The unique three-dimensional network structure of substrate allowed for the uniform dispersion of Co?MnO nanoparticles in the carbon skeleton. These characters enable the Co/MnO@N?C to possess the excellent bifunctional electrocatalysts. In alkaline electrolyte, the Co/MnO@N?C presents the outstanding oxygen evolution reaction (OER) performance comparable to the commercial RuO₂ catalyst and the exceedingly good oxygen reduction reaction (ORR) activity with positive half-wave potential of 0.90 V vs. RHE outperforming commercial Pt/C (0.84 V vs. RHE) and the recently reported analogous electrocatalysts. When it is applied to homemade Zn-air batteries, such a non-noble metal electrocatalyst can deliver a better power density, specific capacity and cycling stability than mixed Pt/C and RuO₂ catalyst, and exhibits a wide application prospect and great practical value.

Full text of DOI:10.1002/asia.202000913

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: 3D Graphene-Carbon Nanotube Hybrid Supported the
Coupled Co-MnO Nanoparticles as Highly Efficient Bifunctional
Electrocatalyst for Rechargeable Zn-Air Battery
Authors: Chaoqi Zhang, Fantao Kong, Yu Qiao, Qingbiao Zhao, Aiguo
Kong, and Yongkui Shan
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To be cited as: Chem. Asian J. 10.1002/asia.202000913
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10.1002/asia.202000913
Chemistry - An Asian Journal
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3D Graphene-Carbon Nanotube Hybrid Supported the Coupled
Co-MnO Nanoparticles as Highly Efficient Bifunctional
Electrocatalyst for Rechargeable Zn-Air Battery
Chaoqi Zhang[a], Fantao Kong[a], Yu Qiao[a] Qingbiao Zhao*[b], Aiguo Kong*[a] and Yongkui Shan*[a]
[a]
C. Zhang, F. Kong, Y. Qiao, A. Kong, Y. Shan.
School of Chemistry and Molecular Engineering
East China Normal University
Shanghai 200241(China)
E-mail: agkong@chem.ecnu.edu.cn
ykshan@chem.ecnu.edu.cn
[b]
Q. Zhao.
Department of electronic Science
East China Normal University
Shanghai 200241(China)
E-mail: qbzhao@ee.ecnu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: The development of high-efficiency and low-cost catalysts
is one of the core and important issues to improve the efficiency of
electrochemical reactions on electrodes, and it is also the goal we
ultimately pursue in the commercialization of large-scale clean
energy technologies, such as metal-air batteries. Herein, nitrogen-
doped graphene oxide (GO)-carbon nanotubes (CNTs) hybrid
network supported the coupled Co/MnO nanoparticles (Co/MnO@N-
C) catalyst was prepared with a hydrothermal-pyrolysis method. The
unique three-dimensional network structure of substrate allowed for
the uniform dispersion of Co-MnO nanoparticles in the carbon
skeleton. These characters enable the Co/MnO@N-C to possess
the excellent bifunctional electrocatalysts. In alkaline electrolyte, the
Co/MnO@N-C presents the outstanding oxygen evolution reaction
(OER) performance comparable to the commercial RuO2 catalyst
and the exceedingly good oxygen reduction reaction (ORR) activity
with positive half-wave potential of 0.90 V vs. RHE outperforming
commercial Pt/C (0.84 V vs. RHE) and the recently reported
analogous electrocatalysts. When it is applied to the homemade Zn-
air battery, such a non-noble metal electrocatalyst can deliver the
better power density, specific capacity and cycling stability than
mixed Pt/C and RuO2 catalyst, and exhibits a wide application
prospect and great practical value.
attracted a lot of interest and become one of the most hopeful
candidates to substitute Pt-based catalysts.
The fruitful researches have been carried out to demonstrate the
high activities of the Co-based catalyst for catalyzing ORR and
OER. Generally, these reported Co-based catalysts could be
roughly divided into the following categories: metal cobalt
nanoparticles (NPs) supported on carbon substrate,[4] Co-N co-
doped materials,[5] cobalt compounds (Co3O4, Co9S8, CoxP),[6]
[7]
Co-based non-noble metal alloy (FeCo, CoMn...)
and the
single atom cobalt.[8] Almost of these catalysts exhibit the
favorable electrocatalytic performances for ORR. The half-wave
potential (E1/2) of cobalt-based catalyst Fe0.3Co0.7/NC with the
highest activity is 880 mV vs RHE and 20 mV higher than that of
commercial Pt/C catalyst (E1/2 = 860 mV vs RHE), which indicate
that Co-based catalyst is a kind of good electrode material with
great potentialities of development. However, durability and
activity of the cobalt-based electrocatalysts were well below the
required levels for the large-scale production and commercial
operation of the renewable energy techniques.
The carbon-based materials such as graphene and carbon
nanotube have great potential in the field of the electrocatalysis
because of their lower prices, excellent stability and
electrochemical properties.[9] In addition, it has been reported in
the literature that the three-dimensional structure formed by the
hybridization of the graphene (Gph) and carbon nanotubes
(CNTs) can overcome the spontaneous agglomerate of them.
On this basis, the hybrid materials of Gph with CNTs are
considered the superior supports for the active species in the
electrocatalysts. Here, the 2D graphene oxide and 1D carbon
nanotube used as the carbon sources and building-blocks, the
urea as the nitrogen source, the 3D N-doped graphene oxide
(GO)-carbon nanotubes (CNTs) hybrid materials (N-DG/C) with
the hierarchically porous structure are synthesized and act as
the supports to capture and scatter the coupled Co-MnO
nanoparticles. The obtained catalysts, N-DG/C supported the
coupled Co-MnO nanoparticles (Co/MnO@N-C), has an
excellent catalytic performance superior to Pt/C and other similar
materials in 0.1 M KOH solution and comparable to commercial
Pt/C in 0.1 M H2SO4 solution for ORR. Moreover, the prepared
catalysts also possess the higher catalytic activity for OER than
Introduction
Development and research of non-precious metal electrocatalyst
with the high performance and cost-effectiveness for oxygen or
hydrogen transformation hold great promise for the
commercialization of the renewable energy techniques and
responding to the energy and environment crisis.[1] Related
research efforts have become extensive and a good deal of the
non-precious metal electrocatalysts with high efficiency has
been prepared by a variety of the synthesis techniques using a
great diversity of precursors.[2] Among the reported non-precious
metal
electrocatalysts,
the
transition
metal-based
electrocatalysts show the remarkable catalytic performance for
oxygen reduction reaction, oxygen evolution reaction (OER),
hydrogen evolution reaction (HER) and all water splitting
reaction.[3] Particularly, the cobalt-based electrocatalysts have
1
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10.1002/asia.202000913
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the widely-used IrO2 catalyst. The Zn-air battery assembled with
the Co/MnO@N-C showcase exceedingly good performance
compared to those with the mixed Pt/C and RuO2 catalyst and
promising market perspective.[10]
high-resolution TEM image (Fig. 1d) presents the apparent
lattice fringes with the interplanar spacings of 0.221, 0.223 and
0.258 nm, which represent the (200) and (020) facets of MnO
and the (200) facet of Co.[2d,12] The TEM mapping (Fig. 1e-g and
Fig. S4) showing the elemental distribution of Co, C, N, O and
Mn illustrates that Co and Mn are located in the same
nanoparticle and the distribution of C, N, O elements is uniform.
These phenomena clearly certify the formation of the coupled
Co-MnO nanoparticles, which provide the material basis of the
high-efficient catalytic activity coming from the synergistic effect
of Co and MnO nanoparticles.
Results and Discussion
Scheme 1. Schematic representation for the synthesis of Co/MnO@N-C
electrocatalysts.
Scheme 1 schematically presents the experimental process for
synthesizing Co/MnO@N-C through the hydrothermal-pyrolytic
method.[2c,11] First, 3D N-doped graphene-carbon nanotube hybrid
(N-DG/C) with network structure is assembled through the
hydrothermal treatment using graphene oxide and carbon
nanotubes as the carbon source and urea as the nitrogen
source. Then CoCl2·6(H2O) and MnCl2·4(H2O) were supported
on the prepared N-DG/C by ultrasonic dispersion, and followed
by the pyrolysis at high temperature under N2 atmosphere to
obtain the N-DG/C supported the coupled Co/MnO nanoparticles
(Co/MnO@N-C).
Figure 2. a) The XRD patterns of MnO@N-C, Co@N-C and Co/MnO@N-C; b)
Raman spectra of Co/MnO@N-C-X (X=800, 900, 1000); c) N2 adsorption and
desorption isotherms of Co/MnO@N-C-X; The corresponding pore size
distribution curves of Co/MnO@N-C (inset); d) N1s e) Mn2p f) Co2p XPS spectra
of the Co/MnO@N-C-900.
X-ray powder diffraction (XRD) pattern was utilized to investigate
the crystal structure of the prepared materials Co@N-C,
MnO@N-C and Co/MnO@N-C. As shown in Fig. 2a, the
characteristic peaks of Co@N-C and MnO@N-C indicate the
existence of the cubic phase of metal Co (JCPDS card no.15-
0808) and MnO (JCPDS card no.07-0230). In the characteristic
patterns of Co/MnO@N-C, three diffraction peaks located at
44.2°, 51.4° and 75.7° correspond to the (111), (200) and (220)
lattice planes of the cubic Co and centered at 34.8°, 40.5°, 58.6°,
70.2° and 73.8° correspond to the (111), (200), (220), (311) and
(222) planes of the cubic MnO. The results testify that Co2+
transforms into Co metal and MnCl2 transform into MnO in the
pyrolysis process. Raman scattering technique was used to
investigate the degree of graphitization of the prepared materials.
Raman spectra of Co/MnO@N-C-X (X=800, 900, 1000) obtained
by pyrolysis at 800, 900 and 1000°C show two strong
Figure 1. a), b) SEM, c) TEM, d) HR-TEM images and e)-g) the corresponding
EDS mappings of Co and Mn elements of Co/MnO@N-C.
characteristic peaks of D and G bands at 1350cm-1 and 1590
[13]
cm-1, respectively (Fig. 2b)
Theoretically, the G band is
The scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were used to observe the
morphology and microstructure of the prepared materials. As
displayed in Fig. 1a, the obtained Co/MnO@N-C possesses the
loosened foam-like structures constructed by the graphene
oxides and nanotubes. Abundant channels and hierarchical pore
structures can be seen from Fig. 1b, which are beneficial for the
dispersion of the nanoparticles and the mass transfer in
electrocatalytic process. From the TEM image (Fig. 1c and Fig.
s5), it shows that the Co and MnO nanoparticles disperse
uniformly on the foam-like substrate and the root of individual
CNT is directly connected to the graphene surface, which is
consistent with the results reported previously by us.[11] The
attributed to the stretching mode of the C-C bond (sp2 carbon
atom) in the graphite structure (such as carbon nanotube,
graphene and graphite nanosheets). The D band is related to
the vibrational mode of sp3 carbon atom from the disordered
structure in the carbon materials.[13,14] The peak intensity ratio
between D and G bands (ID/IG) is used to estimate the content of
defect or the degree of graphitization of carbon materials.[15] The
ID/IG values of Co/MnO@N-C-800, -900 and -1000 are 1.54, 1.49
and 1.37, respectively (Fig. 2b), which indicated that the degree
of graphitization goes up and the content of defects (or disorder)
in the graphene and carbon tube go down in the prepared
materials with the pyrolytic temperature.
2
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Tafel slope curves of OER on Co@N-C, MnO@N-C and Co/MnO@N-C-X
samples; i) LSV polarization curves of OER on Co/MnO@N-C-900 samples
before and after 2000 CV cycles. 0.1 M KOH is used as the electrolyte during
all the measurement.
The N2 adsorption-desorption isotherms of the Co/MnO@N-C-X
(X=800, 900, 1000 °C) shown type-IV curve with well-defined
H3-type hysteresis loops at the relative pressure (P/P0) range of
0.45 to 1.0, which illustrates the existence of mesoporous (Fig.
2c). The pore size distribution located at 2.25 nm was calculated
from the adsorption branch based on the Barrett-Joyner-
Halenda (BJH) models (inset in Fig. 2c). Co/MnO@N-C-900
presented the higher specific surface area (443 m2 g-1) and pore
volume (1.54 cm2 g-1) than that of Co/MnO@N-C-800 (432 m2 g-
1, 1.27 cm2 g-1), Co/MnO@N-C-1000 (358 m2 g-1, 0.364 cm2 g-1)
(Table S1) and other similar materials.[16] When the pyrolysis
temperature increased to 1000°C, many stacked holes collapsed
and thus resulted into the decreasing of the surface area. The
larger specific area and appropriate pore structure would be
advantageous to improve the electrocatalytic performance of the
catalysts.
The elemental compositions and bonding character of
component elements in the surface of the prepared materials
are explored by XPS spectrum. Survey XPS spectrum in Fig. S1
show that the Co/MnO@N-C-900 is composed the Co (1.34%),
Mn (2.41%), N (1.49%), O (4.25%) and C (90.51%). The high-
resolution of N 1s spectrum (Fig. 2d) could be split into four
peaks corresponding to pyridinic N (398.4 eV), pyrrolic N (400.4
eV), graphitic N (401.1 eV) and oxidized N (402.3 eV).[17] These
data prove that nitrogen can be doped into carbon skeleton
during the hydrothermal procedure. The Mn 2p XPS spectrum
(Fig. 2e) displays two obvious peaks at binding energies of
641.2 eV and 653.1 eV, corresponding to Mn2+ 2p3/2 and Mn2+
2p1/2, which confirm the formation of MnO.[18] The Co 2p
spectrum (Fig. 2f) of Co/MnO@N-C-900 reveals that the binding
energy of Co 2p1/2 (795.4 eV) and Co 2p3/2 (779.5 eV) fall in the
metal Co 2p energy region, indicating existence of the elemental
Co in the catalysts. The peaks at 797.7 eV and 782.6 eV in Fig.
2f suggest the presence of a very small amount of Co2+ in the
Co/MnO@N-C-900.[19]
To assess the electrocatalytic performance of the prepared
materials for oxygen reduction reaction (ORR), the cyclic
voltammetry (CV) curves of the Co/MnO@N-C-900 as the
electrode catalysts was shown in Fig. 3a and present an obvious
peak at 890 mV vs. RHE in O2-saturated 0.1 M KOH solution,
but not in N2-saturated solution, which indicated the efficient
ORR activity of the Co/MnO@N-C. The measurement results of
LSV in Fig. 3b show the onset potential and the half-wave
potential of ORR on the Co/MnO@N-C-900 is 1020 mV and 900
mV vs. RHE and more positive than those on the Co@N-C, the
MnO@N-C, the Co/MnO@N-C-800 and Co/MnO@N-C-1000.
Meanwhile, more remarkable, the half-wave potential on
Co/MnO@N-C-900 was 60 mV more positive than that on the
commercial Pt/C catalyst (E1/2 = 840 mV vs. RHE) and higher
than those on other similar materials reported in the literature
recently (Table S2).[2e,8,16a,16c,20] The corresponding Tafel slope
transferred from LSV curves was shown in Fig. 3c, which
reflected the behavior of ORR kinetics. The Tafel slope of ORR
on Co/MnO@N-C-900 (49.2 dec-1) was smaller than those on
Pt/C (59.8 mV dec-1), Co@N-C (64.8 mV dec-1), MnO@N-C
(80.2 mV dec-1), Co/MnO@N-C-800 (64.2 mV dec-1) and
Co/MnO@N-C-1000 (65.9 mV dec-1), indicating the faster
kinetics of ORR on Co/MnO@N-C-900. The apparent number of
electrons transferred in the ORR was calculated with Koutecky-
Levich (K-L) equation.[17] Fig. 4d shown the LSV curves at
different rotation rate of the electrode (400 to 2500 rpm). And the
inset of Fig. 3d presented the relation of linear change of j-1 and
ω-1/2 in the potential range from 0.1 to 0.6V. According to the
slope of the Koutecky-Levich plots at different potentials, the
number of electron transferred was counted to be 3.9, illustrating
that ORR occur mainly by a 4e- reduction pathway from O2 to
OH- on the Co/MnO@N-C electrode in the alkaline medium.
The methanol resistance properties of Co/MnO@N-C-900 and
Pt/C were investigated with time-current testing technology in
O2-saturated alkaline media. As shown in Fig. 3e, after methanol
was added into the electrolyte, there was no obvious decline of
current density for Co/MnO@N-C-900 compared with Pt/C. This
result reflected the outstanding methanol resistance
performance of Co/MnO@N-C electrode. The stability of the
catalyst was assessed by repeatedly CV scanning for 5000
cycles. As shown in Fig. 3f, the initial CV curve on Co/MnO-N-C-
900 could coincide exactly with the final one after 5000 cycles,
indicating the good stability of the Co/MnO@N-C in 0.1 M KOH
solution and which was apparently higher than that of Pt/C (Fig.
S2).
Furthermore, the electrocatalytic performance of Co/MnO@N-C
for ORR in acidic medium also was investigated by the same
techniques as those used in the alkaline media mentioned
above. The experimental results demonstrate that the
Co/MnO@N-C-900 possess the obvious ORR activity (Fig. S3a
and 3b), the favorable stability (Fig. S3e) and the excellent
methanol resistance (Fig. S3f) and can efficiently catalyze the
ORR with a fast-kinetic rate (Fig. S3c) by the 4e- reduction
pathway from O2 to H2O (Fig. S3d, inset) in 0.5 M H2SO4
solution. The onset potential (Eonset) and the half-wave potential
(E1/2) of ORR on the Co/MnO@N-C-900 are 880 mV and 780
Figure 3. a) CV curves of Co/MnO@N-C in O2 and N2 saturated electrolyte; b)
LSV polarization curves of ORR on Pt/C, Co@N-C, MnO@N-C and
Co/MnO@N-C-X samples at 1600 rpm; c) Tafel slope curves of ORR on Pt/C,
Co@N-C, MnO@N-C and Co/MnO@N-C-X samples; d) LSV curves at
different rotation rates with the scan rate of 5 mV s-1; K-L plot and the
corresponding electron transfer numbers (inset) (n) at various potentials for
Co/MnO@N-C; e) Current-time (i-t) chronoamperometric response of
Co/MnO@N-C-900 and Pt/C with the addition of methanol (3 wt %); f) The
stability test results for Co/MnO@N-C-900 by CV curves; g) LSV polarization
curves of OER on Co@N-C, MnO@N-C and Co/MnO@N-C-X samples; h)
3
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mV vs. RHE in 0.5 M H2SO4 and was only 40 mV and 30 mV
lower than those on commercial Pt/C (Eonset = 920 mV; E1/2= 810
mV vs. RHE) (Fig. S3b). There result indicate that the
Co/MnO@N-C-900 present the ideal catalytic performance for
ORR and excellent stability in acidic condition.
Co/MnO@N-C-800, and Co/MnO@N-C-1000 (Rct = 29 Ω), N-
DG/C (Rct = 41 Ω) and Pt/C (Rct = 57 Ω), illustrating the low
charge-transfer resistance and the fast charge transfer rate on
the Co/MnO@N-C-900 catalyst.[24] The electrochemical active
areas (ECSA) for Co@N-C, MnO@N-C, Co/MnO@N-C were
calculated by Parsons-Zobel plot method.[25] Within the range of
the potential where no electrochemical reaction occurs, the
limiting current density (j) is measured at different scanning rate
(v) and the results (CV curves) were shown in Fig. 4b-d present
the CV curves at different scanning rates, displaying that the
limiting current density increases with the rising of the scanning
rate. According to the CV curves, the linear relation between j
and v can be written as:
The research results for the OER performances of the Co@N-C,
MnO@N-C and Co/MnO@N-C prepared by pyrolyzed at
different temperatures were manifested in Fig. 3g-i. From the
LSV curves in Fig. 3g, it was seen that the Co/MnO@N-C-900
possessed an ideal OER activity with the overpotential (Ej=10) of
1.57 V at the current density of 10 mA cm-2, which was lower
than the Co@N-C (1.71 V), MnO@N-C (1.67 V), Co/MnO@N-C-
800 (1.64 V) and Co/MnO@N-C-1000 (1.61 V) catalysts and
only 0.02 V higher than RuO2 catalyst (Ej=10=1.55 V). Moreover,
OER activity of the Co/MnO@N-C-900 outperforms the most
cobalt and manganese composite materials reported in the
literature[21]. The Tafel slope of OER on Co/MnO@N-C-900 is
84.5 mV dec-1 and lower than those on Co@N-C (118.9 mV dec-
1), MnO@N-C (120.3 mV dec-1), Co/MnO@N-C-800 (113.0 mV
dec-1) and Co/MnO@N-C-1000 (115.5 mV dec-1) (Fig. 3h),
showing an excellent kinetic capability of OER on the
Co/MnO@N-C-900 catalyst. After 2000 CV cycles, the
polarization curve of the Co/MnO@N-C-900 shows a slightly
degradation compared with the initial curve, as shown in Fig. 3i,
illustrating the outstanding stability of the Co/MnO@N-C-900 for
OER in the alkaline solutions, and which was higher than those
of many noble metal-based catalysts[10,22] as well as the reported
Co-based catalysts.[2a,21a,23]
J = v퐶푑
And the slopes (Cd) of the linear equation were represented in
Fig. 4e-g. ECSA was further given through the following
equation:
퐶푑
ECSA =
A × 퐶푠
where A is the amount of the material coating on the surface of
electrode (mg·cm-2), Cs is an empirical constant representing the
capacitance per unit area (40mF·cm-2)[17]. The corresponding
values are summarized in Table S3. ECSA of the Co/MnO@N-
C-900 is 1470.88 cm2·mg-1 and larger than those of Co@N-C
(1376.01 cm2·mg-1) and MnO@N-C (1183.80 cm2·mg-1). From
this, it could be found that the combination of Co and MnO in the
Co/MnO@N-C not only change the chemical component of the
catalyst, but also increase its ECSA.
Moreover, the electrochemical reaction current density (JK) on
the catalysts was calculated with equation below.[26]
퐽퐷 × J
퐽푘 =
퐽퐷 − J
where JD is the limiting current density and J is the measured
current density. The calculated results were shown in Fig. 4h.
The electrochemical reaction current density on the
Co/MnO@N-C-900 at 0.85 V is 21.3 mA cm-2, the higher than
those on other prepared catalysts and 3.2 times that on Pt/C
(6.58 mA cm-2). This phenomenon implies that the Co/MnO@N-
C-900 maybe possesses the active site with very higher activity
and the higher density of the active sites.
The Tafel curves for ORR on different catalysts (Fig. 4i) show
the same curve shape between Co@N-C and Co/MnO@N-C-
900, indicating the same kinetic processes of electrochemical
reaction on them. This fact demonstrates that the cobalt was
acted as the major catalytic center and the manganese oxide
was used as cocatalyst. On the other hand, some studies have
proved that MnO can efficiently promote adsorption of the
oxygen species on the surface of the catalysts for oxygen
transformation reaction.[23,27] For this reason, we reckon that the
MnO can facilitate the enrichment of oxygen and transferring
oxygen species into Co active center and Co catalyze oxygen
transformation reaction on the surface of the Co/MnO@N-C-900.
Such synergistic effect between Co and MnO dramatically
enhances the performance of catalysts for the oxygen
transformation reactions. From what we have mentioned above,
it can be clearly seen that the excellent catalytic performance of
Co/MnO@N-C-900 should originate in the higher efficient active
center resulting from the synergistic effect between Co and MnO,
fast charge transfer rate and the higher density of the active
sites on it.
Figure 4. a) Nyquist plots of N-DG/C, Pt/C and Co/MnO@N-C-X (X=800, 900,
1000); b)-d) CV curves for Co@N-C, MnO@N-C and Co/MnO@N-C-900 in
the potential range from 1.02 V to 1.16 V at scan rates of 10 mV/s, 20 mV/s,
40 mV/s, 60 mV/s, 80 mV/s and 100 mV/s; e)-g) The current densities of
anode and cathode on Co@N-C, MnO@N-C and Co/MnO@N-C-900
measured at different scan rates; h) Comparison of kinetic current density at
0.85 V vs. RHE of Pt/C, Co@N-C, MnO@N-C and Co/MnO@N-C-X; i) Tafel
curves for ORR on Co@N-C, MnO@N-C and Co/MnO@N-C-900. 0.1 M KOH
is used as the electrolyte during all the measurement.
For further understanding the electrocatalytic behavior of the
Co/MnO@N-C, the electrochemical impedance spectroscopy
(EIS), Tafel curves and electrochemical active areas (ECSA) of
the prepared catalysts were determined. From Nyquist plots as
shown in Fig. 4a, it can be observed that the Co/MnO@N-C-900
exhibited the smaller semicircle (Rct = 14 Ω) than those on the
To further explore the potential for practical application of the
prepared catalysts, the home-made rechargeable zinc-air
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battery was assembled using carbon-fiber-cloth supported
Co/MnO@N-C-900 as the air electrode. Zinc plate as the anode
the fast charge transfer rate, the larger specific area and
abundant channels and hierarchical pore structures in the
and 6.0 M KOH + 0.2 M Zn(CH3COO)2 as the electrolyte solution. support endows the prepared catalyst with the excellent ORR
As shown in Fig. 5a, the zinc-air battery impelled by
Co/MnO@N-C-900 presents the open circuit potential of 1.65 V,
which was higher than that by Pt/C (1.42 V). The polarization
and power density curves of zinc-air battery were showed in Fig.
5b. It can be found that the battery derived from Co/MnO@N-C-
performance in basic (half-wave potential of 900 mV vs. RHE )
and acidic electrolyte (half-wave potential of 740 mV vs. RHE)
and the higher OER activity (the overpotential of 1.57 V at the
current density of 10 mA cm-2), which are superior to the
commercial Pt/C and outperform the most Co based catalysts
reported in the literatures. The uniformly dispersion of coupled
Co-MnO nanoparticles within the porous carbon skeleton
distinctly enhance the durability of the catalysts. Moreover, the
Zn-air battery using Co/MnO@N-C as the cathode exhibited the
excellent peak power density, specific capacity and stability,
comparing with the benchmark mixed Pt/C and RuO2 catalyst.
This work provides an advanced strategy to prepare cheap and
highly efficient bifunctional electrocatalyst for ORR and OER and
illustrates that the prepared catalysts possess broad prospects
for application in energy storage and conversion technologies.
900 presents
a
peak power density of 165 mW cm-2,
outperforming that from the Pt/C catalyst (100 mW cm-2). The
calculated specific capacity of the zinc-air battery driven by
-1
Co/MnO@N-C based on the consumed zinc was 821 mAh gZn
which was corresponded to an energy density of 1012 Wh kgZn
,
,
-1
exceeding those of Pt/C (757 mAh gZn-1g, 849 Wh kgZn-1) (Fig.
5c). The experimental results obtained by the galvanostatic
recurrent pulse technology at the constant current density of 10
mA cm-2 in Fig. 5d demonstrate that the Co/MnO@N-C-900
based battery possesses a superior stability after continuously
circulating for 45 h, whereas the Pt/C-based battery suffered an
obvious decline in the discharge voltage after 30 h. Moreover,
the Co/MnO@N-C-900 based battery exhibits the excellent initial
round-trip efficiency of 50.4% (discharge end potential divided
by charge end potential) with a voltage gap of only 1.12 V, and
the energy efficiency decreases to 45.6% after circulating for 30
h. By contrast, the mixed Pt/C and RuO2 catalyst-based battery
shows an initial energy efficiency of 41.6% with a voltage gap of
1.46 V, and such efficiency decreased to 27.6% after circulating
for 23 h, reflecting the superior energy efficiency and stability of
the Co/MnO@N-C-900 based battery (Fig. 6e). A light-emitting
diode (LED) could be powered by two serial zinc-air batteries
(Fig. 5e), indicating its feasibility of practical application in the
field of the clean energy techniques.
Experimental Section
Materials
All reagents are obtained from Sinopharm Chemical Reagent Co. Ltd and
used without further purification. The commercially Pt/C catalyst was
used. The percentage of carbon in Pt/C is 80 wt.%.
Synthesis of graphene oxides (GO)
GO was synthesized by the modified Hummers method.[28] Firstly, 5 g
graphite were added into a three-necked flask which contained 100 mL
H2SO4 (98%), 2.5 g P2O5 and 2.5 g K2S2O8 and the obtained mixture was
stirred at 80 °C for 6 h to finish the pre-oxidation of graphite. Then, the
pre-oxidation graphite (5 g) was mixed with 98% H2SO4 (115 mL) in the
ice bath and 15 g KMnO4 was added into the reactor slowly with the rapid
stirring to make sure that the reaction temperature was below 10 °C.
After continuing to stir for 2 h at the room temperature, the graphite was
oxidized completely. Next, 200 mL deionized (DI) water was added into
the product of the precious step, stirring continuously until the system
cooled down to the room temperature. Then 700 mL DI water and 12.5
mL H2O2 (30%) were poured into the mixture to wash away the residual
metal oxides and their salts. The obtained product was washed with
dilute hydrochloric acid solution (3%) for three times and dialysis (dialysis
membrane retained molecular weight: 8000-14000 Da) for one week to
get the final product.
Figure 5. The Co/MnO@N-C-900- and Pt/C-based zinc-air batteries. a) Open
circuit plots; b) The polarization and power density curves; c) Discharging
curves at 10 mA cm-2; d) Galvanostatic discharge-charge cycling curve at 10
mA cm-2; e) Long-term cycling performance at a current density of 10 mA cm-2
with Co/MnO@N-C-900 and Pt/C as air cathodes, respectively; (F)
Photograph of the LED powered by two serial zinc-air batteries with the
Co/MnO@N-C-900 as the cathode. Zincate (Zn(OH)42-) solution is used as the
electrolyte
Synthesis of nitrogen-doped graphene/carbon nanotube hybrid
The commercially multi-walled carbon nanotubes (MWCNTs) were used
to synthesize nitrogen-doped graphene/carbon nanotube hybrid, whose
specification is 10-30 nm in diameter, 5-15 μm in length, purity 95 wt.%.
Firstly, 40 mL MWCNT suspension (1 mg/mL), 25mL GO solution (2
mg/mL) and 24 g urea were mixed and sonicated uniformly. Then the
mixture was transferred to the poly(tetrafluoroethylene)(Teflon)-lined
autoclave and heated at 180 °C for 12 h. The product was filtered and
washed with the DI water until the filtrate was neutral. The black powder
was freeze-dried in lyophilizer overnight to obtain the N-doped graphene-
carbon nanotube hybrid (N-DG/C).
Conclusion
A facile method was proposed for synthesizing the novel carbon-
based non-precious metal catalyst. The obtained catalyst
Co/MnO@N-C is composed of the support with loosened foam-
like structures constructed by the GO and CNTs and the coupled
Co-MnO nanoparticles dispersed uniformly on the foam-like
support. The synergistic effect of cobalt and manganese oxide,
Synthesis of N-DG/C supported Co/MnO coupled nanoparticles
(Co/MnO@N-C)
50 mg CoCl2·6(H2O) (2×10-4 mol) and 40 mg MnCl2·4(H2O) (2×10-4 mol)
were mixed with the N-DG/C materials and dispersed into 20 mL ethanol
5
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10.1002/asia.202000913
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by ultrasound. The mixture was stirred at room temperature until the
solvent volatilized completely. The residual solid was ground and
transferred into the silica tube to be pyrolyzed at 900 °C for 4 h with a
heating rate of 2 °C min-1 in high-purity N2 atmosphere, and then cooled
down to ambient temperature to obtain the final product and named N-
doped graphene/carbon nanotubes hybrid supported Co/MnO coupled
nanoparticles (Co/MnO@N-C-900). As a contrast, N-doped graphene
carbon nanotubes hybrid supported Co nanoparticles (Co@N-C), MnO
nanoparticles (MnO@N-C) and coupled Co/MnO nanoparticles obtained
by pyrolysis at 800 °C (Co/MnO@N-C-800) and 1000 °C (Co/MnO@N-C-
1000) were synthesized with the similar method.
The Zn-air battery was prepared according to the following process.[11a]
The cathode was composed of carbon cloth supported catalyst with a
loading of 5 mg cm-1 and the zinc plate was used as the anode. The
electrolyte was prepared by mixing 6 M KOH and 0.2 M Zn(AC)2 to form
zincate (Zn(OH)42-) solution. The power density diagram and discharge
polarization curve were recorded by the Galvano-dynamic method.
Galvanostatic discharge-charge cycling test were conducted at ambient
temperature, and a whole cycle was consisted of a discharge and a
charge procedure at a constant current (10 mA cm−2) and the same
duration (45 h with a 10 min cycling).
Electrode preparation and electrochemical measurement
Supporting information
Cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques
were conducted in a standard three-electrode system. A Pt wire served
as a counter electrode, an Ag/AgCl electrode (KCl, 3.5 M) as the
reference electrode and a catalyst-modified glassy carbon electrode (5.6
mm in diameter, Pine) as working electrodes. The ORR experiments
were carried out in O2-saturated 0.1 M KOH solution and 0.5 M H2SO4
solution at a scan rate of 5 mV s-1 at the ambient temperature. The ORR
current is obtained by deducting the current measured in N2-saturated
electrolyte from the current measured in the O2-saturated electrolyte. The
catalyst inks were prepared by dispersing 10 mg of catalysts in 1.28 mL
alcohol solution containing 30 μL Nafion solution (5 wt %) by sonication
to form a homogeneous suspension. The obtained suspension was
pipetted onto a polished glassy carbon electrode surface and dried at
room temperature. The mass loading on the working electrode is 0.6 mg
cm-2 in 0.1 M KOH and 0.80 mg cm-2 in 0.5 M H2SO4 solutions. The
commercially available Pt/C catalyst (Johnson Matthey, 20 wt % Pt) with
the general loading of 20 μg Pt cm-2 was used for comparison. The
potential at which the ORR current density reached to 3 μA cm-2 in RDE
polarization curves is taken to be the ORR onset potential. The OER
property was investigated in an O2-saturated 1.0 M KOH solution at room
temperature. The general loading of catalysts in the OER on the working
electrode was 0.6 mg cm−2. The benchmark IrO2 catalyst electrodes with
the same mass loading as Co/MnO@N-C working electrode was utilized
for investigation and comparison. Linear Sweep Voltammetry (LSV)
curves were recorded at a scan rate of 5 mV s−1. All the potentials were
calibrated with a reversible hydrogen electrode (RHE). The electron-
transfer number (n) involved in a typical ORR process was estimated by
using the Koutecky-Levich (K-L) equation.
Supporting information is available from the Wiley Online
Library or from the author.
Acknowledgements
This work was financially supported by the China National
Natural Science Foundation (No. 21303058), Shanghai
Municipal Natural Science Foundation (No. 13ZR1412400), and
the key project of Shanghai Science and Technology Committee
(No. 14231200300).
Conflict of interest
The authors declare no conflict of interest.
Keywords: Co/MnO coupling NPs • 3D carbon material •
bifunctional electrocatalyst • metal-air battery
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Entry for the Table of Contents
Three-dimensional graphene-carbon nanotube hybrid supported the coupled Co-MnO catalysts could be figuratively described as the
luxuriant tree with fruit, whose structure reveals the unique advantages for oxygen conversion reaction. The outstanding
electrocatalytic performance employ the material with a wide application prospect in the sustainable energy conversion systems.
8
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