Mass-Production of Mesoporous MnCo2O4 Spinels with Manganese(IV)- and Cobalt(II)-Rich Surfaces for Superior Bifunctional Oxygen Electrocatalysis

Article abstract of DOI:10.1002/anie.201708765

A mesoporous MnCo₂O₄ electrode material is made for bifunctional oxygen electrocatalysis. The MnCo₂O₄ exhibits both Co₃O₄-like activity for oxygen evolution reaction (OER) and Mn₂O₃-like performance for oxygen reduction reaction (ORR). The potential difference between the ORR and OER of MnCo₂O₄ is as low as 0.83 V. By XANES and XPS investigation, the notable activity results from the preferred Mn^IV- and Co^II-rich surface. The electrode material can be obtained on large-scale with the precise chemical control of the components at relatively low temperature. The surface state engineering may open a new avenue to optimize the electrocatalysis performance of electrode materials. The prominent bifunctional activity shows that MnCo₂O₄ could be used in metal–air batteries and/or other energy devices.

Full text of DOI:10.1002/anie.201708765

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Mass-production of Mesoporous MnCo2O4 Spinel with
MnIV- and CoII-rich Surface for Superior Bifunctional Oxygen
Electrocatalysis
Authors: Wenhai Wang, Long Kuai, Wei Cao, Marko Huttula, Sami
Ollikkala, Taru Ahopelto, Ari-Pekka Honkanen, Simo Huotari,
Mengkang Yu, and Baoyou Geng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708765
Angew. Chem. 10.1002/ange.201708765
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Angewandte Chemie International Edition
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COMMUNICATION
IV
Mass-production of Mesoporous MnCo
2
O
4
Spinel with Mn - and
II
Co -rich
Surface
for
Superior
Bifunctional
Oxygen
Electrocatalysis
Wenhai Wang, Long Kuai, Wei Cao, Marko Huttula, Sami Ollikkala, Taru Ahopelto, Ari-Pekka
Honkanen, Simo Huotari, Mengkang Yu, and Baoyou Geng*
o
often beyond 800 C but the precise doping is difficult to
Abstract: A mesoporous MnCo
is successfully fabricated for bifunctional oxygen
electrocatalysis. The MnCo exhibits both Co -like
activity for oxygen evolution reaction (OER) and Mn
approaching performance for oxygen reduction reaction
ORR). The potential difference between ORR metric and
OER metric of MnCo is as low as 0.83 V. By XANES
and XPS investigation, the notable activity is resulted
from the preferred Mn - and Co -rich surface. Valuably,
the products can be obtained in large-scale with the
precise chemical components at relatively low
temperature. The surface state engineering maybe open a
new avenue to optimize electrocatalysis performance of
electrode materials. The prominent bifunctional activity
2
O
4
electrode materials
achieve.[
5c,d]
Contrastly, metal oxides are easily
synthesized at lower temperatures. However, ordinary
pure metal oxides with ideal crystal structure often
exhibit single oxygen electrocatalysis activity of either
OER or ORR. Various fabrication engineering has been
developed to modulate the structure or component of
2
O
4
3
O
4
2
O -
3
(
2
O
4
metal oxides to motivate their bifunctional performance.[
The structure or component engineering often involve
some complicated processes and the yield is limited.
Fortunately, previous researches have also revealed that
the electrocatalytic activity has a great connection with
the surface chemical state. Just as reported, average Mn
6]
IV
II
III
IV
valence locates in the region between Mn and Mn can
greatly improve the ORR activity of manganese oxides,
in which the driving force comes from the e orbit
number change from zero to one that can reduce oxygen
adsorption and facilitate the exchange of hydroxyl by
oxygen. It is noticed that MnCo
oxides possess the structural flexibility and mixed
valence states. Most of the reported MnCo catalysts
shows that MnCo
2
O has the possibility of being used in
4
metal-air batteries and/or other energy devices.
g
C
onsiderable interest has been focused on oxygen
electrocatalysis for its important role in energy storage
[7]
2
O is one of spinel
4
[1]
and conversion. The Pt-based materials are regarded as
the best catalysts toward ORR, but they are suffered from
a poor OER activity.
2
O
4
[2]
There are a few catalysts
have a good activity toward OER, whereas the ORR
possessing OER performance which can be comparable
[8]
activity is clumsy. The main reason is that the average
[3]
with Ir- or Ru-based materials. Nevertheless, Ir- or Ru-
based materials show a normal activity toward ORR.
Although alloys of Pt, Ir and Ru have been used as
bifunctional catalysts, their scarcity and high cost also
II
Mn valence of these MnCo
2
O catalysts is between Mn
4
III
and Mn , preventing ORR being improved.
We are fortunate to have developed a facile spray-
pyrolysis approach for the fabrication of mesoporous
with Pt/C comparable ORR activity via
modulating Mn chemical state. Therefore, it is also
expectable to achieve the bifunctional performance of
[4]
hamper their applications.
2
Mn O
3
[5]
Among the developed bifunctional oxygen-catalysts ,
heteroatoms doped carbon materials have been paid
numerious attention because of heteroatoms doping can
change the asymmetry spin density and the local charge
[9]
MnCo
2
O
4
through surface chemical state engineering.
possess
Herein, we reported a mesoporous MnCo
2
O
4
[5c-e]
density of carbon lattice
. In order to perfom the
IV
III
II
dominant Mn in the surface and Mn in bulk while Co
both in the surface and bulk, which exhibit high activity
toward both ORR and OER with the potential difference
between ORR and OER metrics of 0.83 V. The study of
Zn-air batteries device shows that the initial discharge
synergistic effects of heteroatoms, the synthesized
temperatures of heteroatoms doped carbon materials
[*]
Mr. W. H. Wang, Dr. L. Kuai, Mr. M. K. Yu, Prof. & Dr. B. Y. Geng
College of Chemistry and Materials Science
and charge potential of MnCo
2
O are respectively 1.21 V
4
2
The Key Laboratory of Functional Molecular Solids, Ministry of
Education, Anhui Laboratory of Molecular-Based Materials, Center
for Nano Science and Technology, Anhui Normal University
No.1 Beijing East Road, Wuhu, 241000, P. R. China.
E-mail: bygeng@mail.ahnu.edu.cn
Dr. W. Cao, Dr. Prof. M. Huttula
Nano and Molecular Systems Research Unit, University of Oulu,
P.O. Box 3000, FIN-90014 Oulu, Finland
Mr. S. Ollikkala, Mr. T. Ahopelto, Mr. A.-P. Honkanen, Prof. Simo
Huotari
Department of Physics, University of Helsinki, PO Box 64, FI-00014
Helsinki, Finland
and 2.05 V at the power density of 10 mA/cm . After 180
cycles, there is only a small potential drop in discharge
(1.10 V) and up in charge (2.32 V), which is superior to
Pt/C in the same condition. Notably, the products can be
synthesized in large-scale at a relatively low temperature.
The fabrication of the products is performed on the
home-made equipment.
Figure 1a-b and Figure S1a-b, d-e) reveal that the
obtaine products are porous spheres with the size of 1-2
μm, which is similar to that of Mn and Co
[10]
The SEM and TEM images
(
2
O
3
O
3 4
Supporting information for this article is given via a link at the end of
the document. ((Please delete this text if not appropriate))
prepared under same condition. The lattice fringe in
HRTEM image (Figure 1c) is indexed to the (220) plane
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Angewandte Chemie International Edition
10.1002/anie.201708765
COMMUNICATION
of MnCo
2
O . The SEM element mapping (Figure 1d)
4
proves the elements of Mn, Co and O distributing
uniformly in the product. The powder X-ray diffraction
(
XRD) pattern (Figure 1e) confirms the phases of the
samples can be assigned to Mn (JCPDS 41-1442),
Co (JCPDS 74-1656) and MnCo (JCPDS 77-
471). Compared to peaks of Co , the peaks of
MnCo shift slightly to smaller angles. The
substitution of larger size Mn cations results in the
2
O
3
3
O
4
2
O
4
4
0
3
O
2
O
4
[
11]
improvement of d (lattice fringe).
Figure 2. (a) Mn and (b) Co XPS spectra of MnCo
Mn and (d) Co K-edge XANES spectra of various samples. The inset of Figure
d is the zoom map of XANES ranging from 7719 to 7735 eV.
2 4 2 3 3 4
O , Mn O and Co O , (c)
2
Figure 1. (a) SEM, (b) TEM, (c) HRTEM, (d) SEM element mapping (scale
bars: 500 nm) images of MnCo ; (e) XRD patterns of MnCo , Co and
Mn
To evaluate the property of MnCo
2
O for OER, it was
4
2
O
4
O
2 4
3 4
O
carried out in O -saturated 0.1 M KOH by polarization
2
O .
2 3
curves. As a comparison, a bare glassy carbon (GC)
electrode, carbon (Vulcan XC-72), Pt/C (20 wt.% Pt),
The N
reveal that the surface areas of MnCo
2
adsorption-desorption curves in Figure S2a Mn
2
O
3
, Co
3
O
4
and the physical mixture of Mn
2
O and
3
O
4
, Mn
2
O
3
and Co
3
O
4
were also performed under the same condition.
2
2
2
2
Co
respectively. The N
show a loop occurs in the range of 0.9-1.0 P/P
mesoporous porous structure can promote the formation which is comparable with Co
of active sites (Figure S2b), improving the activity of Co-based spinel oxides relies on the relative
3
O
4
are 23.57 m /g, 40.15 m /g and 19.28 m /g, From Figure 3a, the bare GC and carbon don’t have a
-2
adsorption-desorption curves also great effect on catalyzing OER. At 10 mA cm (Figure
. The 3b), MnCo displays a small overpotential (0.40 V),
(0.39 V). The OER
2
2
O
4
0
3
O
4
II
III
II
electrochemistry activity of catalysts.
ratio of Co /Co . Liu et al. found that Co is beneficial
The X-ray photoelectron spectroscopy (XPS) was for forming cobalt oxygydroxide (CoOOH), which is
shown in Figure 2. In the Mn spectra (Figure 2a), the responsible for enhanced OER.[ In contrast, Co tends
12]
III
peaks at 644.2 eV and 654.7 eV are characteristic peaks to improve the strength between catalysts’ surface and
IV [8a]
of Mn .
The peaks at 641.3 eV and 653.3 eV are hydroxides groups, which lowers the activity of OER. It
III [8b]
assigned to Mn . The peaks of 641.5 eV and 653.0 eV is fortunate that the as-synthesized MnCo
2
O possesses
4
II [8c]
II
are ascribed to Mn .
Mn2p3/2 and Mn2p2/1 for MnCo
energy, showing the slightly higher surface Mn valence the OER activity for the physical mixture of Mn
of MnCo than Mn
. Apparently, the ration of Co is less active than MnCo , which indicates the
Co /Co for MnCo is higher than Co
in Figure 2c. importance of the chemical synergic coupling of Mn and
The calculated surface Co and Mn oxidation states of Co to enhance the electrochemical activity. Taking
MnCo
from XPS were 2.47 and 3.19 (Table S1), account of surface area and catalyst loading, the specific
which display the preferred state with Mn - and Co -rich activity (j
surface. Meanwhile, the Mn (Figure 2b) and Co (Figure compare OER activity of catalysts (Table S2). It is shown
d) K-edge x-ray absorption near edge structures that the specific activity and mass activity of MnCo
According to Figure 2a, the dominant Co in the both surface and bulk, causing the
shift to higher binding highly-comparable OER activity to Co . In addition,
and
O
4
3
O
4
2
2
O
3
O
4
2
O
3
3
O
4
2
O
4
2
II
III
2
O
4
3
O
4
2
O
4
IV
II
s
) and mass activity (j ) were also used to
m
2
(
O
2 4
-1
-2
XANES) were further investigated its electron structure (j
in bulk due to much larger detection depth. It is obvious Co
that the Co’s binding energy in MnCo is less than
From Figure 3c, we can find that the Tafel slope of
Co
(insets of Figure 2d), which matches with the XPS MnCo
s
: 20.60 mA g , j
m
: 90 μA cm ) are as good as that of
-1
-2
3
O
4
s
(j : 22.90 mA g , j
m
: 110 μA cm ).
2
O
4
-1
3
O
4
2
O
4
is 90 mV decade , which is slightly higher
II
-1
results, suggesting that the Co is dominant type in the than Co
3
O
4
(71 mV decade ) and lower than the physical
both surface and bulk of MnCo
2
O
4
. To keep the spinel mixture of Mn
2
O
3
and Co
3
O . A lower Tafel slope means
4
III
structure, the Mn atoms occupy the Co ’s sites, so the the relevant material can favor the kinetic of OER, which
III
[13]
Mn presents dominant Mn in the bulk of MnCo
2
O
4
,
indicates that MnCo
2
O has a great OER activity.
4
which is consistent with the Mn XANES (Figure 2b) Charge transport is a crucial factor for the kinetics of
OER.[ Electrochemical impedance spectroscopy (EIS)
14]
compared to Mn
2
O
3
, Mn
3
O
4
and MnO .
2
was used to study the charge rate of catalysts (Figure S3).
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Angewandte Chemie International Edition
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COMMUNICATION
The charge transfer resistance (Rct) follows the order of RDE results. The oxygen strength on metal oxides’
Pt/C > Mn
Co > MnCo
their OER activities.
2
O
3
> the physical mixture of Mn
2
O and surface is a key step of ORR. The oxygen strength will be
3
IV
III
3
O
4
2
O
4
> Co
3 4
O , which is associated with decreased by the reaction of Mn /Mn , resulting to the
acceleration of the kinetics of ORR.[ The smaller
7a]
Cyclic voltammetry (CV) was used to assess the ORR amount of peroxide generated on MnCo
electrocatalystic properties of MnCo
d, the peak potential of MnCo
which provides a similar ORR activity with Mn
V) and Pt/C (0.87 V). The peak potential of MnCo
slightly negative to Mn . There are two reasons MnCo
2
O
4
’s surface
III
IV
2
O . From the Figure should be originated from the coexistence of Mn /Mn .
4
III
IV
3
2
O
4
appears at 0.83 V, The introduction of Mn and Mn in Co
3
O
4
forming
2
O
3
(0.85 MnCo
2
O
4
makes great contributions to increasing the
is electron number of ORR. The long-term stability of
2
O
4
2
O
3
2
O
4
was studied by chronoamperometric
III
causing this gap. Firstly, by comparison with Mn and measurements. After 10 h, the loss of current density for
IV
Mn , Co-based species don’t have a great effect on the MnCo
enhancement of ORR. Secondly, Mn has a larger stability of MnCo
surface area than MnCo . Compared with Co (peak
potential: 0.69 V) the high ORR activity of MnCo can
2
O
4
is only 15 % (Figure 4b), indicating high
2
O
3
2
O
4
than Pt/C.
2
O
4
3
O
2
4
O
4
III
IV
be attributed to the substitution of Mn. Mn and Mn
are more active ORR sites than Co-based species. On
one hand, the presence of Mn can promote the ORR
[15]
III
[16]
kinetics.
can be decomposed by Mn . The physical mixture of
Mn and Co (peak potential: 0.78 V) offers lower
ORR activity than MnCo , suggesting that chemical
On the other hand, the amount of peroxide
IV
2
O
3
3
O
4
2
O
4
synergistic effect also has a great advantage in the
enhancement of ORR.
Rotating disk electrode (RDE) was employed to gain
insight into the ORR kinetic of catalysts. The onset
potential of MnCo
can be comparable with Mn
V). In addition, the limiting current of MnCo
2
O
4
is 0.95 V (Figure 3e and S4), which
(0.98 V) and Pt/C (1.00
is close
2
O
3
Figure 3. (a) OER polarization curves of catalysts at 1600 rpm; (b)
-
2
2
O
4
Overpotentials derived from OER polarization cures at j=10 mA cm ; (c) Tafel
plots derived from OER polarization cures; (d) CV of catalysts; (e) ORR
polarization curves of catalysts at 1600 rpm; (f) K-L plots of catalysts。 All
to Pt/C. The electron transfer number (n) was calculated
from polarization curves according to the Koutecky-
Levich (K-L) equation (Figure 3f). It is well known that
2
measurements were performed in O -saturated 0.1 M KOH.
[17]
Pt/C follows a 4e transfer mechanism. According to K-
L equation, the electron transfer number of MnCo was
calculated to be 3.94, which approaches that of Mn
3.97). Considering the contribution of catalysts’ mass
2
O
4
To quantitatively evaluate the bifunctional oxygen
electrode activity of MnCo , a metric was adopted and
used to compare the difference between the ORR
O
2 3
2
O
4
(
-2
and specific surface area to ORR, mass activity and
specific activity of catalysts were measured. From Table
potential at -3 mA cm and the OER potential at 10 mA
-
2
cm . The smaller the potential difference, the better the
catalyst’s bifunctional activity is. The potential difference
S3, the mass activity and specific activity of MnCo
O
2 4
-
1
-2
(
0
j
s
: 0.83 mA g ; j
m
: 30 μA cm ) is larger than Co
3
O
4
s
(j :
between ORR metric and OER metric of MnCo
2
O is
4
-1
-2
.21 mA g ; j
Mn /Mn in Co
m
: 10 μA cm ). The incorporation of
only 0.83 V (Table S4), which can be comparable with
the most excellent reported N, P-GCNs (0.71 V). The
unique surface state of Mn and Co is the source to the
III
IV
3
O
4
forms MnCo
2
O , leading to the
4
decrement of peroxide and the increment of the electron
transfer number. Consequently, the sluggish kinetic of
ORR would be facilitated.
Rotating ring-disk electrode (RRDE) was used to
explore the electron transfer number and the peroxide
IV
high performance. The dominant surface Mn gives birth
II
to excellent ORR activity while superior surface Co
contributes efficient OER performance. To determine this
claiming, the bifunctional oxygen catalysis of H
MnCo samples was investigated. As shown in Figure
S6a and b, the performance decreased after H -treatement
2
-treated
yield of MnCo
disk current for the reduction of O
for the oxidation of peroxide. The peroxide yield of
MnCo fluctuates between 4 % and 6 % over the
potential from 0.4 V to 0.7 V (Figure 4a), which is
similar to that of Mn and less than the reported
MnCo /N-rmGO (nearly 10%).
value is lower than MnCo and the peroxide yield
reaches 35 %, indicating that oxygen was mainly reduced
to peroxide. RRDE confirms that MnCo shows a great
excellent activity toward ORR, which is aligned with
2
O
4
(Figure S5). MnCo
2
O
4
shows a high
2
O
4
2
and a low ring current
2
o
o
at both 200 C and 400 C, and ORR activity inhibition
appears much more dominant. Based on the XANES
2
O
4
spectra of H
2
-treated MnCo
d) at 200 C, we can find that the bulk structure maintains
well compared to that of fresh MnCo sample.
2
O samples (Figure S6c and
4
o
2
O
3
[11]
2
O
4
For Co
3
O , the n
4
2
O
4
2
O
4
However, the OER and ORR activity presents notable
difference, indicating that the structure change happens in
o
2
O
4
the surface. Furthermore, after H
2
-treatement at 400 C,
the bulk structure changes apparently, where the binding
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Angewandte Chemie International Edition
10.1002/anie.201708765
COMMUNICATION
energy of Co moves to CoO and the Mn shifts toward Acknowledgments
Mn
3
O . While, the activity toward both OER and ORR
4
differs very slightly from that of MnCo
2
O
4
-H200 sample, This work was supported by the National Natural Science
suggesting that the contribution of bulk structure is much Foundation of China (21471006, 21271009), the Recruitment
less dominant compared to the surface state.
Program for Leading Talent Team of Anhui Province, the
Program for Innovative Research Team of Anhui Education
Committee, and the Research Foundation for Science and
Technology Leaders and Candidates of Anhui Province.
2 4
Keywords: MnCo O • mesoporous • mass production •surface
state • bifunctional oxygen electrocatalysis
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Mn
Chronoamperometric measurements of MnCo
(V vs Ag/AgCl) in -saturated 0.1
Discharge−charge cycling curves at 10 mA/cm2 of rechargeable Zn−air
2 3
O
, MnCo
2
O
4
, Pt/C and the physical mixture of Mn
O4 and Pt/C measured at -0.3
KOH at 1600 rpm. (c)
2 3 3 4
O and Co O . (b)
2
V
O
2
M
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stability was performed by the discharge-charge cycling
in 0.2 M Zn(CH COO) and 6.0 M KOH. The discharge-
charge curves were examined at 10 mA cm (Figure 4c).
The initial discharge and charge potential of MnCo
are respectively 1.21 V and 2.05 V. After 180 cycles,
there is small potential change in discharge (1.10 V) and
charge (2.32 V). Although Pt/C has a high potential (1.27
V) during discharge process at first, the stable discharge
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2
O
4
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3
2
-2
O
2 4
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9
[
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[
[
the stability of MnCo
practical application.
2
O
4
is still excellent during the
[
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In summary, a mesoporous MnCo
oxygen electrocatalyst was synthesized by spray-
pyrolysis route. The product possesses a remarkably high
2
O
4
bifunctional
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IV
116, 3594.
activity toward both OER and ORR. The unique Mn
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6021.
II
and Co -rich surface state is the source to the high OER
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and ORR activity. It makes possible that MnCo
2
O has
4
the potential of being applied to promote the
development of renewable energy technologies and
devices. Additionally, the products produced through
spray-pyrolysis can be sustainably obtained in a large
scale with the precise components. The surface state
engineering through modulating the chemical state of
elements opens up attractive opportunities to synthesize a
large number of new materials with excellent
performance.
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Angewandte Chemie International Edition
10.1002/anie.201708765
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Mass-production of mesoporous
Wenhai Wang, Long Kuai, Wei Cao,
Marko Huttula, Sami Ollikkala, Taru
Ahopelto, Ari-Pekka Honkanen, Simo
Huotari, Mengkang Yu, Baoyou Geng*
MnCo
bifunctional O
performed simply. The obtained
MnCo possesses dominant
surface chemical state, which
stimulate the excellent
bifunctional performance.
2
O
4
spinel with superior
2
electrocatalysis is
2
O
4
Page No.1 – Page No.6
Mass-production
of
Spinel with Mn and Co -
Surface for Superior
Mesoporous
IV
II
2 4
MnCo O
rich
Bifunctional Oxygen Electrocatalysis
This article is protected by copyright. All rights reserved.