Effect of Thermally Induced Oxygen Vacancy of α-MnO2 Nanorods toward Oxygen Reduction Reaction

Article abstract of DOI:10.1021/acs.inorgchem.9b00492

MnO₂ has been explored for various applications in environmental and energy aspects. However, the thermal sensitivity of the MnO₂ crystal structure never been studied. As a potential cathode material for fuel cell, α-MnO₂ has a higher specific activity than Pt/C based on per metals cost. In this work, the physical and electrochemical properties of α-MnO₂ nanorods were explored for the first time under thermal treatment with different temperatures (300, 400, and 500 °C). Under thermal treatment, oxygen vacancies were induced. The high-angle annular dark-field (HAADF) images and electron energy loss spectroscopy (EELS) have been taken to explore oxygen vacancies of α-MnO₂ materials. From EELS and X-ray photoelectron spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO₂ nanorods were strengthened with the temperature increasing. The sample with 400 °C treatment exhibited the best performance toward ORR, excellent methanol tolerance and higher stability compared to commercial Pt/C in alkaline media due to its combination of preferable growth on (211) plane and moderate oxygen vacancies as well as coexistence of Mn (IV)/ Mn (III) species. It was also observed the α-MnO₂ nanorods tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO₂ nanorods are thermal sensitive materials and their performance for ORR can be boosted under certain temperatures.

Full text of DOI:10.1021/acs.inorgchem.9b00492

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Effect of Thermally Induced Oxygen Vacancy of α‑MnO Nanorods
2
toward Oxygen Reduction Reaction
X. Shi, H. Zheng,* A. M. Kannan,† K. Per
†
,‡
,‡
ez-Salcedo, and B. Escobar
§
§
́
†Fuel Cell Laboratory, The Polytechnic School, Ira A. Fulton Schools of Engineering, Arizona State University, Mesa, Arizona
8
5212, United States
‡
Energy Centre, Council for Scientific and Industrial Research (CSIR), Pretoria, Gauteng 0184, South Africa
§
Centro de Investigacion
7302, Yucatan, Mexico
Científica de Yucatan, Carretera Sierra Papacal − Chuburna Puerto, km 5. Sierra Papacal, Merida, C.P.
́ ́ ́ ́
9
́
́
*
S Supporting Information
ABSTRACT: MnO has been explored for various applications
2
in environmental and energy aspects. However, the thermal
sensitivity of the MnO crystal structure never been studied. As a
2
potential cathode material for fuel cell, α-MnO has a higher
2
specific activity than Pt/C based on per metals cost. In this work,
the physical and electrochemical properties of α-MnO nanorods
2
were explored for the first time under thermal treatment with
different temperatures (300, 400, and 500 °C). Under thermal
treatment, oxygen vacancies were induced. The high-angle
annular dark-field (HAADF) images and electron energy loss
spectroscopy (EELS) have been taken to explore oxygen vacancies of α-MnO materials. From EELS and X-ray photoelectron
2
spectroscopy (XPS) analysis, the oxygen vacancies on the α-MnO nanorods were strengthened with the temperature
2
increasing. The sample with 400 °C treatment exhibited the best performance toward ORR, excellent methanol tolerance and
higher stability compared to commercial Pt/C in alkaline media due to its combination of preferable growth on (211) plane and
moderate oxygen vacancies as well as coexistence of Mn (IV)/ Mn (III) species. It was also observed the α-MnO nanorods
2
tended to become longer and thinner with increasing temperature. This work suggests that the α-MnO nanorods are thermal
2
sensitive materials and their performance for ORR can be boosted under certain temperatures.
INTRODUCTION
crystal polymorphs lead to different electronic structures,
which influence the electrochemical performance. Lu and co-
workers reported Pt nanoparticle support on carbon and
■
The oxygen reduction reaction (ORR) has been widely studied
and reported in recent decades,
1
−3
due to its important role in
MnO in acidic media and showed same activity and better
2
energy conversion technologies, such as alkaline membrane
14
4
5,6
durability compared to that of commercial Pt/C. Li et.al
fuel cell (AMFC), microbial fuel cell, direct methanol fuel
15
7
2,3,8
9−11
studied MnO N-doped carbon hybrid shell, Hang et al.
2
16
cell (DMFC), and energy storage (Li−air
and Zn−air
reported MnO -supported g-C N , Zhang and co-workers
2
3
4
batteries). However, the low kinetic exchange current of ORR
has slowed down the development and commercialization of
those environmentally friendly technologies. Pt group metal
studied phosphate-promoted α-MnO in alkaline media toward
2
17
ORR. The published ORR data have been compared, and it
was concluded that the specific current per dollar of α-MnO is
2
(
PGM) based catalysts deliver the highest ORR kinetic
much higher than that of Pt/C due to the metal cost and high
exchange current, but high cost, poor long-term durability,
and easy poisoning are still big challenges to face. In particular,
in DMFC, the Nafion membrane has a high methanol
crossover rate; when methanol reaches the cathode, the
18
α-MnO electrochemical properties. In particular, α-MnO2
2
shows onset potential and limiting current values close to those
of the Pt/C benchmark in alkaline media toward ORR.
The ORR process can have a two or four electron transfer
path, depending on the active sites of the catalytic surface. The
two electron path can give a higher cell potential; however, the
7
ORR will be influenced by methanol oxidation. Another
approach is to develop non-PGM ORR catalysts. Among noble
metal free ORR catalysts, manganese dioxides have received
extensive attention due to the abundance of magnesium (10th
element in the earth crust ) and the low cost, nonpoisonous
nature, no impact on environment, and high stability.
Additionally, manganese dioxides have many polymorphs
1
9
generated peroxides will bring damage to the membrane.
20
12
Typically, it is favorable to synthesis peroxides. For fuel cell
and metal air batteries, a four electron transfer path is
21,22
desired.
The noble metal Pt mainly goes through directly
(
e.g., α-MnO , β-MnO , δ-MnO , γ-MnO , and λ-MnO ) in
2 2 2 2 2
which the basic building blocks, [MnO ] octahedron, are
assembled to different tunnel and layer structures. Various
Received: February 19, 2019
6
13
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00492
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Inorganic Chemistry
Article
Figure 1. (a) X-ray diffractrograms of MO, MO300, MO400, and MO500 samples. (b) Integrated diffraction intensity of each plane for all the
samples.
−
four electron transfer, reducing the O into OH in alkaline
Oxygen vacancy (OV) is a defect inside the manganese
oxide crystalline materials, which can change the geometry and
electronic structures and elongate the adsorbed oxygen O−O
2
2
3−25
media (eq 1).
The ORR mechanism for α-MnO has not
yet been understood clearly; the commonly accepted
mechanism was summarized in Cheng’s paper. First, oxygen
2
26
bond. Li et al. has studied the OV in β-MnO along with the
2
molecule undergoes a two-electron partial reduction, forming
HO2 (eq 2) and then further reducing into OH (eq 3) or
experiment and the density functional theory (DFT)
calculation; their results concluded that moderate OV can
lower the band gap, increase Fermi levels, and improve the
−
−
−
27,28
decomposing into OH and O (eq 4).
It has been
2
−
37
reported that the catalytic speed of HO2 reduction or
decomposition is infinitely fast compared to that ineq 2, so the
generated peroxide ions will be reduced or decomposed
ORR performance. Similar results have been found in the
8
,38
literature.
Therefore, α-MnO has a better affinity to oxygen molecules
2
2
9−35
39
immediately on the MnO surface.
The MnO reacts with
water forming the active sites Mn OOH (eq 5). The oxygen
molecule either adsorbed onto two (eq 6a) or one generated
Mn OH (eq 6b) and then further reduced into OH (eq 7a)
or HO2 (eq 7b), respectively. The route shown in eqs 5, 6a,
and a higher activity toward ORR than that of β-MnO2. To
2
2
III
the best of our knowledge, a thermal sensitivity and OV study
in α-MnO has not been reported yet. In this work, α-MnO
2
2
III
−
nanorods were synthesized by hydrothermal method and
postheat treatment at various temperatures. The physical and
electrochemical properties were studies on the original and
−
and 7a illustrated eq 1 for the four electron transfer process,
and the route shown in eqs 5, 6b, and 7b depicted eq 2 for the
first two electron transfer reaction. Cheng’s results showed that
thermal treated α-MnO nanorods.
2
26
both routes exist in the ORR process in alkaline media. Cao
and co-workers have proposed that the increase in the
concentration (above 3 M) will decrease the ORR perform-
ance, and the whole process rate should be controlled by the
EXPERIMENTAL SECTION
■
α-MnO Preparation. MnO samples were synthesized by the
2
2
40
hydrothermal method in an autoclave. In brief, 0.2 g of MnSO ·H O
4
2
and 0.5 g of KMnO (Sigma-Aldrich) were dissolved in 15 mL of DI
III
36
4
oxidizing of Mn OOH.
water (Thermo Scientific Barnstead MicroPure, 18.2 MΩ.cm),
transferred into the autoclave (PARR Instrument), and heated at
−
−
O + 2H O + 4e → 4OH
(1)
(2)
(3)
(4)
2
2
1
40 °C for 12 h. The precipitate was collected by centrifugation and
O + H O + 2e → HO + OH−
−
−
washed thoroughly with DI water. The dark brown precipitate was
dried at 80 °C and the sample was labeled as MO. The postheat
treatments were carried out at 300, 400, and 500 °C in air for 2 h, and
the samples were labeled MO300, MO400, and MO500, respectively.
Electrode Preparation and Testing. In order to increase the
2
2
2
−
−
−
HO2 + H O + 2e → 3OH
2
−
−
2
HO2 → 2OH + O
2
conductivity of the MnO samples, 10 wt % of XC72 carbon was
2
added before ink making. The CV and durability test was only
−
III
−
conducted with manganese oxide film due to different stabilities of
MnO + H O + e ← → Mn OOH + OH
(5)
(6a)
(6b)
(7a)
(7b)
2
2
41
different carbon supports under oxidizing potential. Typically,
catalysts were dispersed in DI, 2-propanol. and Nafion dispersion
III
III
2
Mn OOH + O ← → 2(Mn OOH···O)
2
(
H O:2-propanol:Nafion (5 wt % Nafion, 1100 EW, Sigma-Aldrich) =
2
−
1
3
:1:0.016 (V:V:V)) under ultrasonic treatment to form a 4 mg·mL
III
III
−
1
Mn OOH + O ← → Mn OOH···O
(Pt/C 1 mg·mL ) slurry. A thin catalyst film was deposited by
dropping the required volume of the catalyst ink onto the polished
glassy carbon disk (5 mm diameter, Pine Research AFE5T050GC).
2
2
III
−
IV
−
Mn OOH···O) + e ← → Mn O + OH
2
The loading of α-MnO and Pt on working electrodes were ∼500 and
2
−
2
∼
50 μg·cm , respectively. The CVs in saturated N and O
electrolytes used to show the redox profile, ∼ 125 μg·cm , were
III
−
IV
−
2 2
Mn OOH···O + e ← → Mn O + HO
−2
2
2
2
B
DOI: 10.1021/acs.inorgchem.9b00492
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 2. SEM of MO (a, b), MO300 (c, d), MO400 (e, f) and MO500 (g, h) at different magnifications.
employed on the working electrode. Uniform and well-adhered
tested at 0.8 V vs RHE for 14 h in O -saturated 1 M KOH solution at
2
catalyst films were obtained by drying at ∼50 °C for 15 min under
700 rpm.
400 rpm. The methanol tolerance tests were conducted in 1 M KOH
with 1 M methanol content.
Linear sweep voltammetry (LSV) experiments were carried out in
Materials Characterization. XRD analyses were recorded using
Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation (λ =
0.154 nm, 40 kV, and 30 mA). The morphology and structure of α-
an O -saturated 1 M KOH solution, and all data were corrected by
2
subtracting that with N -saturated solution. LSV experiments were
2
−1
conducted in the potential range of 1 to 0.2 vs RHE at 20 mV·s scan
rate with Pt rod as counter electrode and saturated calomel electrode
as reference electrode using a PAR Bistat potentiostat at room
temperature. In order to evaluate the stability of the catalyst, cyclic
voltammetry (CV) was conducted for the disk electrodes at 50 mV·
MnO nanorods were evaluated by FESEM (JEOL-JSM 7500F at 2
2
and 3 keV). The high angle annular dark field (HAADF) images and
electron energy loss spectroscopy (EELS) data was characterized by
JEOL ARM-200F Scanning transmission electron microscopy
(STEM) at 200 keV. XPS analysis were conducted in K-Alpha (Al,
12 kV, Thermo Scientific, East Grinstead, U.K.) with surface charging
correction at 284.8 eV.
−
1
s
from 0.6 to 1.2 V vs RHE with a 500 rpm for 500 cycles in O -
2
42
saturated 1 M KOH solution. The chronoamperometries were
C
DOI: 10.1021/acs.inorgchem.9b00492
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Inorganic Chemistry
Article
RESULTS AND DISCUSSION
■
The XRD patterns of the α-MnO materials with/without heat
2
treatment are given in Figure 1a. The sharp peaks were
observed on all of samples at 2θ values of 12.75, 18.02, 28.79,
and 37.45° correspond to (110), (200), (310), and (211)
planes (PDF# 44-0142), respectively, indicating the high
crystallinity of the α-MnO samples. However, after 500 °C
2
treatment, the α-Mn O (PDF# 41-1442) peaks occur slightly
2
3
at 23.13, 32.95 and 55.19° corresponding to (116), (222), and
(
440) planes, respectively, as α-MnO is partially converted
2
into α-Mn O . Under the same condition of XRD measure-
2
3
ments, if a peak has higher integration intensity, it means that
43
the crystal is preferred to form that specific plane. To
highlight the intensity of planes, Figure 1b summarized the
peak area of planes on all the synthesized samples. For MO,
MO300, and MO500 samples, the intensity of peaks decreases
in the following order (310) > (200) > (211) > (110). For
MO400 sample, the peak intensity drops as follows: (310) >
(
(
211) > (200) > (110). DFT calculation shows that (211) and
310) are the first and second most active planes toward ORR,
9
respectively. In this case, (211) was favorable growth on α-
MnO sample during 400 °C treatment.
In order to study the surface morphology of the synthesized
materials, SEM was carried out and displayed in Figure 2. Parts
2
Figure 3. High angle annular dark field images of MO (a), MO300
(b), MO400 (c), and MO500 (d).
a and b of Figure 2 show the α-MnO nanorods shaped with an
2
average ∼80 nm diameter, ∼1 μm length, and ∼12.5 aspect
be omitted. Figure 4 displays electron energy loss spectrum for
OK and MnL2,3 edges on the prepared materials. The oxygen
K edge starts at around 532 eV corresponding to the oxygen K
shell core-loss, and the later signals start from 640 eV
corresponding to the Mn L2,3 core-loss (Figure 4a). The shape
ranges from 50 eV after oxygen threshold, this will determine
ratio. Under 300 °C heating treatment, the surface of α-MnO
2
nanorods (Figure 2c,d) became smoother and thinner (∼70
nm) in diameter and longer (∼1.5 μm) in length with an
aspect ratio of ∼21.4. When the temperature was increased to
4
00 °C (Figure 2e,f), the MO400 nanorods tended to become
sharper and longer, maintaining the same diameter as MO300
45
the Mn oxidation state, pointed out by Rask. Since all four
with an aspect ratio of ∼28. When the temperature reached to
samples have relatively different O edges shapes (especially
K
5
00 °C, MO500 materials stretched to nanowires. Even at
MO500), it can be seen from Figure 4a, the oxygen contents
on MO300 and MO400 are slightly different from MO, and
there is a huge difference observed on MO500 from the others.
The magnified spectra of Mn loss on the samples were
displayed in Figure 4b. The Mn edges consist of two major
peaks: the first one starts at 640 eV, corresponding to the Mn
L3 core-loss, and the second one starts around 651 eV,
attributed to Mn L . With increasing temperature, MnL on α-
lower magnifications, the wire ends are hard to observe (Figure
g,h). In summary, as the temperature increases, the aspect
2
ratios are increased, amd the MO nanorods tend to become
thinner and longer.
EELS and HAADF were carried out by STEM for all four
samples. Parts a−d of Figure 3 show the HAADF nanorod
images for MO, MO300, MO400, and MO500, respectively.
According to HAADF analysis, the darker the picture is, the
lower atomic density will be. In this case, we can observe from
2
3
MnO materials was shifted to lower energy loss accordingly.
2
The order of MnL energy loss on samples is MO > MO300 >
3
HAADF images that the structure of α-MnO nanorods were
2
MO400 > MO500. The manganese valence is decreased
changed/collapsed with heating temperature. The untreated
MO (Figure 3a) sample shows smooth and uniform atomic
distributions with the least defects among all four samples. At
(
higher OV content) when the MnL peaks shift to lower
3
4
5−48
energy loss.
Therefore, the OV content in the manganese
oxides decreases in the following order, MO400 > MO300 >
MO, and MO400 achieved the highest OV content. Since the
MO500 has a different phase proved by the XRD, it is difficult
to compare it with other samples.
3
00 °C (Figure 3b), the appeared gray lines indicate the
hollow channel was forming on α-MnO nanorods during the
2
heating. The darker lines were appeared on MO400 image
(
Figure 3c), indicating decreased atomic densities and hollow
To analyze OV quantitates, the ratio of O and Mn intensity
was calculated using Gatan DM software. In general, the
hydrogenic model is accurate enough for K edges while the
channels. In MO500 sample (Figure 3d), the HAADF image
shows the nanorod was collapsed partly. It is evidence that the
49
defects on α-MnO nanorod were enhanced because of the
Hartree−Slater model is used for L edges. The oxygen cross
section is estimated based on the hydrogenic model and the
manganese cross section estimation is based on the Hartree−
Slater model. All the samples were calculated under same
background and signal selecting condition using the above two
models. And all the samples are based on a standard criterion
2
form of Mn O impurities from α-MnO nanorods at 500 °C
2
3
2
(
seen in Figure 1).
In term of EELS analysis, dynamic scattering (scattered
more than twice) will interfere the data interpretation. From
published report, the mean free path for a 200 keV energy
electron in manganese oxide is ∼100 nm. Based on the
HAADF image, all the samples have great chance to be single
scattered that indicate interference of dynamic scattering can
44
with a stoichiometric formula of α-MnO . The results of
2
quantitative analysis are listed in Table 1. It can be seen from
Table 1 that the formulas of MO300, MO400, and MO500 are
D
DOI: 10.1021/acs.inorgchem.9b00492
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Inorganic Chemistry
Article
Figure 4. Electron energy loss spectrum (a) for O and MnL2,3 edges of MO, MO300, MO400, and MO500, respectively, and (b) zoomed in
K
MnL2,3 edges of MO, MO300, MO400, and MO500, respectively.
Table 1. List of Intensity Ratios of O/Mn and Mn L3/L2 in All MnO Samples
2
MO
MO300
MO400
MO500
O:Mn ratio (Mn valence)
I(L3)/I(L2) (Mn valence)
2.0 (+4)
1.79 (+4.05)
1.86 (+3.72)
2.04 (+3.96)
1.77 (+3.54)
2.12 (+3.9)
1.10 (+2.2)
3.49 (+2.5)
MnO1.86, MnO1.77, and MnO1.10, respectively. From the above
results, the oxygen content trend decreases as the sintering
temperature increased.
spectra on all the samples are attributed to Mn 2p1/2 (654.04
eV) and Mn 2p3/2 (642.36 eV). The Mn 2p 3/2 peaks can be
deconvoluted into four sub peaks P1 (640.80 eV), P2 (642.18
3
+
Another approach to determine Mn oxidation state is from
the Mn intensity ratio of I(L3)/I(L2). From Table 1, the ratios
of integrated intensity (area under the curve) on MO, MO300,
MO400, and MO500 are 1.79, 2.04, 2.12 and 3.49, which by
Kurata’s method result in Mn formal valences of 4.05, 3.96, 3.9,
eV), P3 (643.54 eV), and P4 (645.08 eV). P1 is due to Mn
4
+
binding electrons, P2 and P3 are ascribed to Mn ion and P4
53
belongs to shakeup satellite peak. The integrated intensity of
P1 is increasing with heating temperature, with value of 5.05,
5.54, 6.24, and 9.80% for MO, MO300, MO400, and MO500,
respectively. From another viewpoint, the sum of P2 and P3 is
decreasing with the increasing of the heating temperature at
87.04, 86.68, 86.12, and 79.67% for MO, MO300, MO400, and
50
and 2.5, respectively. This analysis also gives evidence that,
with increasing temperature, the Mn formal valence decreases
therefore increasing OV. Oxygen vacancy is a defect inside the
manganese oxides crystalline materials, the OV presence can
change the geometry and electronics structures and elongate
the O−O bond of the adsorbed oxygen. Moderate OV can
improve the ORR performance found in β-MnO and λ-MnO ,
4
+
MO500, respectively. Those results indicated more Mn is
being reduced with temperature increased, which lead to less
oxygen content inside the structure of the prepared materials.
From the quantification of the XPS survey scan (see Figure 1S
in SI), the chemical formula of MO, MO300, MO400, and
2
2
if the OV content is too high, the material will change the
8
,38
structure and become a different phase.
MO500 can be assigned to MnO , MnO1.98, MnO1.91, and
2
Figure 5 shows the deconvoluted peaks of the O 1s and Mn
p spectra on MO, MO300, MO400 and MO500 materials.
MnO1.84, respectively. The oxygen content from XPS analysis
is decreasing with increase heating temperature, which are
corresponding with results of EELS. However, the XPS
quantification showed higher oxygen content compared to
the analysis of EELS, which caused from difference measure-
ments from EELS and XPS, as EELS signal from the bulk and
XPS signal from surface of the materials.
2
The three deconvolution peaks of O 1s (Figure 5a−d) are
associated with Mn−O−Mn (529.79 eV) as lattice oxygen
(
O ), Mn−OH (531.26 eV), and H O adsorption (533.8
latt
2
51
eV). The latter two are adsorption oxygen (O ). From the
ad
quantification analysis, the ratios of O /O are 1.20, 1.08,
latt
ad
1
.01, and 1.09 for MO, MO300, MO400, and MO500,
The CVs on MO, MO300, MO400, and MO500 electrodes
respectively. The lower of the ratios indicates the stronger of
the manganese oxides to adsorption; this could boost the
ORR. The high-resolution Mn 2p spectra of the prepared
materials are shown in Figure 5e,f. Two peaks in the XPS
in O - or N -saturated 1 M KOH are shown in Figure 6. The
2
2
CVs in N (Figure 6, black) on all the electrodes showed no
2
5
2
observable redox peaks. However, in the O -saturated 1 M
2
KOH, the CVs (Figure 6, red) showed a reduction current
E
DOI: 10.1021/acs.inorgchem.9b00492
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Inorganic Chemistry
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Figure 5. XPS spectra of O 1s in (a) MO, (b) MO300, and (c) MO400 and of Mn 2p in (e) MO, (f) MO300, (g) MO400, and (h) MO500.
which belongs to ORR. From the CV curves in O -saturated 1
M KOH, the visible two onset point is showing that there is a
two-step reaction mechanism involved in all the samples. The
MO300, and MO500 at 0.36, 0.37, and 0.34 V vs RHE, which
indicates that MO 400 has better performance for ORR.
The electrochemical properties and durability performance
2
first onset point could be attributed to eq 6b and eq 7b for a
on MnO and Pt/C electrodes are shown in Figure 7. The LSV
2
−
two-electron transfer process forming HO ; when the greater
values on all the prepared samples and commercial Pt/C are
shown in the Figure 7a. The half-wave potential and limiting
current in Figure 7a are at 0.60, 0.62, 0.65, and 0.59 V vs RHE
and −5.09, −5.52, −6.17, and −5.36 mA·cm− for MO,
2
−
reducing potential is introduced, the as formed HO is further
being reduced to OH . The second onset point of MO 400
2
−
26
2
starting at ∼0.45 V vs RHE, is more positive than that for MO,
F
DOI: 10.1021/acs.inorgchem.9b00492
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Inorganic Chemistry
Article
Figure 6. O (red) and N (black) CV in 1 M KOH electrolyte of (a) MO, (b) MO300, (c) MO400, and (d) MO500.
2
2
Figure 7. Electrochemical analysis of (a) LSV of MO (black), MO300 (red), MO400 (blue), MO500 (black-dash), and Pt/C (red-dash) with N2
correction at 1600 rpm, (b) specific current density at 0.3 V vs RHE, MO (black), MO300 (red), MO400 (blue), and MO500 (black-white stripe),
(
c) Tafel slope of MO (black-solid), MO300 (red-solid), MO400 (blue-solid), MO500 (black-dash), and Pt/C (red-dash), (d) methanol tolerance
in 1 M CH OH+1 M KOH on Pt/C (black) and MO400 (red), (e) the Koutecky−Levich (K-L) plot for MO (black-solid), MO300 (red-solid),
MO400 (blue-solid), MO500 (black-dash), and Pt/C (red-dash), (f) the charge transfer number estimated from K-L analysis for MO (black-
3
square), MO300 (red-square), MO400 (blue-square), MO500 (black-circle), and Pt/C (red-triangle), (g) CV data in O -saturated electrolyte at
2
5
00 rpm, MO400 (red) after 500 cycles (red-dash) and Pt/C (black) after 500 cycles (black-dash), and (h) chronoamperometry on Pt/C (black)
and MO400 (red) for 14 h.
MO300, MO400, and MO500, respectively. With heating
limiting current were improved. The specific current densities
treatment up to 400 °C, both of the half-wave potential and
(Figure 7b) of MO, MO300, MO400, and MO500 at 0.3 V vs
G
DOI: 10.1021/acs.inorgchem.9b00492
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Inorganic Chemistry
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Table 2. List of Recent Published Manganese Oxide Catalyst Results toward ORR
sample
Ic (mA·cm−2) at 1600 rpm
sweep rate (mV·s−1
)
electrolyte
n
5
7
α-MnO in CNT
−3.6
−4.6
−3.5
−3.3
3.8
5.1
5.52
6.1
5
5
1 M KOH
0.1 M KOH
1 M KOH
1 M KOH
1 M KOH
1 M KOH
1 M KOH
1 M KOH
1 M KOH
NA
NA
3.83
3.34
3.85
2.90
3.85
4.00
2.20
2
5
8
α-MnO out CNT
2
6
MN/NrGO
MN/rGO
MnO /rGO
10
10
10
20
20
20
20
6
5
9
2
MO/C
MO300/C
MO400/C
MO500/C
5.3
−
1
RHE are 25.5, 27.6, 30.9, and 27.1 mA·mg , respectively. The
specific current density increases with temperatures up to 400
ORR mechanism is 4-electron path and follows reaction routes
of eq 5, eq 6a, and eq 7a. When the temperature reaches 500
°C, impurities of α-Mn O formed. n is 2.2 on MO500,
°
7
C and decreased at 500 °C. In addition, as displayed in Figure
2
3
−
1
c, the Tafel slope of kinetic current of MO400 (65 mV·dec )
indicating the oxygen reduction toward the 2-electron path but
−
is the smallest among the samples, indicating the high ORR
with a small amount of HO
highest content of OV; when too much OV is introduced, the
MnO structure starts to collapse by forming Mn , which
2
yield. However, MO500 has the
54
activity of MO400 at low overpotential, and the value of the
Tafel slope of MO400 close to 2.303RT/F (59 mV·dec at 25
C), in which reaction order on MO400 is similar to that of
−
1
O
2 3
2
°
changesthe electronic structure and adsorption energy, which
is one possible reason to cause a 2-electron mechanism on
35
Mn O/C under O -saturated KOH solution and γ-MnO2
x
2
55
37
under air-saturated KOH solution is given. In the kinetic and
diffusion mixed control region, MO400 has the Tafel smallest
slope at 142 mV·dec− as well. It is also an indicator of MO400
fast reaction kinetics toward ORR. From XRD, XPS, and EELS
results, the MO400 has the highest (211) diffraction peak
intensity and moderate OV content among all four samples,
and these could be reasons for its best electrochemical
MO500 catalyst. The results demonstrated that thermal
treatment on α-MnO further directs the ORR toward the 4-
2
1
electron path. Thus, the treatment on MnO
is of benefit to the
2
kinetic of the ORR before 500 °C.
The stability comparison was carried out o MO400 and Pt/
C by the CV technique and shown in the Figure 7g. After 500
cycles, the overpotential on Pt/C electrode increased by 12
−
2
4
+
3+
mV at −1 mA.cm and current density decreased by 9.5%
performance. On the other hand, the species of Mn /Mn
coexisted and was believed to favor the ORR on MnO /C,
and more Mn on the surface of MnO have shown higher
ORR activity than that with more Mn species. In this case,
we believe the coexistence of Mn / Mn species with certain
ratios accelerates the charge transfer to oxygen and thus favors
the oxygen reduction. The most favorable ratio of Mn /Mn
was produced on MO400, which shows the best ORR activity.
The methanol tolerance tests were performed on MO400
and Pt/C electrodes in 1 M CH OH + 1 M KOH as shown in
Figure 7d. The commerical Pt/C showed big methanol
oxidation peaks at ∼0.7 V vs RHE. However, no peak of
methanol oxidation was observed on the MO400 electrode.
This indicates the MO400 material has a high methanol
tolerance property compared to the commercial Pt/C catalyst.
The linear relationships of K-L analysis at 0.3 V vs RHE
−
2
35
from −3.07 to −2.77 mA.cm at 0.6 V vs RHE on Pt/C
electrode. However, the MO400 reduction current is
increasing with cycles at the beginning, the MO400 is first
cycled until the current is stable then test consecutive 500
cycles. After 500 cycles, overpotential on MO400 electrode
x
4
+
x
3+
56
4+
3+
−
2
4
+
3+
increased by only 6 mV at −1 mA.cm and current density
−2
decreased by 1.95% from −2.84 to −2.78 mA.cm at 0.6 V vs
RHE. The chronoamperometry was also performed for
durability evaluation and shown as Figure 7h, for 14 h holding
3
potential at 0.8 V vs RHE at 400 rpm with O bubbling. The
2
MO400 shows increasing the reduction current compare to the
initial by 10%. In the meanwhile, the Pt/C decreased 5%
reduction current after chronoamperometry testing. Evidently,
the MO400 electrode shows higher stability compared to the
commercial Pt/C electrocatalyst. Table.2 shows the LSV
comparison with other manganese oxide catalysts in the
(
Figure 7e) indicated the fast kinetics for all the materials. The
literature. Our results on MnO show slightly lower activity
2
calculated charge transfer number from K-L analysis on
different potential was displayed in Figure 7f. The charger
transfers number (n) of ORR on MO, MO300, MO400,
MO500, and Pt was calculated as around 2.9, 3.8, 4, 2.2, and 4,
respectively. The oxygen reduction on a Pt/C catalyst mainly
proceed by a direct 4e pathway (eq 1) with a serial 2 × 2 e
pathway, which is in agreement with our result on
compared to the reported results. Because carbon was mixed
with the prepared materials, which could cause less
conductivity than those of MnO /C composites. However,
2
with simple heat treatment on the α-MnO materials, the ORR
2
−
−
performances on the treated materials were boosted.
2
6
CONCLUSION
commercial Pt/C. The oxygen reduction on MnO has been
■
x
investigated to proceed to the first partial reaction with two
electrons (eq 2) followed by either the 2e reduction (eq 3) or
the chemical disproportionation (eq 4) of hydrogen peroxide
Pristine α-MnO nanorods were synthesized. Thermal
2
sensitivity of α-MnO nanorods was studied under various
2
calcination temperatures (300, 400, and 500 °C), and a series
of α-MnOx with different Mn valences and OV content were
−
35
(
HO ). For the MO sample, n = 2.9 at E = 0.3 V vs RHE,
2
showing that the ORR mechanism falls between 2- and 4-
electron pathways. As a result, the HO2− yield is large. The
ORR on MO300 is toward the 4-electron pathway (but not
totally) as n is 3.8, indicating that the HO2− yield is small. The
number of electrons (n) on MO400 is 4, showing that the
generated from the pristine α-MnO nanorods. With increasing
2
temperature, the α-MnO nanorods tend to become longer
2
and thinner. At 500 °C, XRD showed the presence of Mn O
2
3
impurity in α-MnO . Quantification analysis of EELS spectrum
2
and I(L3)/I(L2) methods have analyzed that the Mn state is
H
DOI: 10.1021/acs.inorgchem.9b00492
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
decreased with increasing temperature. The XPS also showed
decreasing oxygen content with increasing heating temper-
ature. The ORR onset potential and limiting/specific current
were improved by increasing the temperature up to 400 °C,
which then decreased due to too high of an OV content and
Mn O impurities on the MO500 sample. The methanol
electrochemical tuning and oxygen Vacancy Generation. Angew.
Chem., Int. Ed. 2016, 55, 8599−8604.
(
9) Mainar, A. R.; Colmenares, L. C.; Leonet, O.; Alcaide, F.; Iruin,
J. J.; Weinberger, S.; Hacker, V.; Iruin, E.; Urdanpilleta, I.; Blazquez, J.
A. Manganese oxide catalysts for secondary zinc air batteries: from
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tolerance testing indicates the MO400 is highly ORR selective.
In stability analysis, MO400 indicated higher durability after
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00492.
■
A. 1999, 96, 3447−3454.
*
S
(13) Pargoletti, E.; Cappelletti, G.; Minguzzi, A.; Rondinini, S.;
Leoni, M.; et al. High-performance of bare and Ti-doped a -MnO2
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AUTHOR INFORMATION
Corresponding Author
(H.Z.) E-mail: hzheng@csir.co.za.
ORCID
H. Zheng: 0000-0003-4180-0197
A. M. Kannan: 0000-0002-8412-1680
Notes
■
*
(15) Li, Y.; Cao, S.; Fan, L.; Han, J.; Wang, M.; Guo, R. Science
hybrid shells of MnO nanosheets encapsulated by N-doped carbon
2
towards nonprecious oxygen reduction reaction catalysts. J. Colloid
Interface Sci. 2018, 527, 241−250.
(16) Hang, Y.; Zhang, C.; Luo, X.; Xie, Y.; Xin, S.; Li, Y.; Zhang, D.;
Goodenough, J. B. α -MnO nanorods supported on porous graphitic
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Power Sources 2018, 392, 15−22.
(17) Zhang, T.; Tham, N. N.; Liu, Z.; Fisher, A.; Lee, J. Y.
Promotion of the bifunctional electrocatalytic oxygen activity of
manganese oxides with dual-affinity phosphate. Electrochim. Acta
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
2
(
018, 277, 143−150.
The authors would like to thank CSIR, USAID (US-Pakistan
Centers for Advanced Studies), and USAID (Global Develop-
ment Research) for financial support. We acknowledge the use
of facilities within the Eyring Materials Center at Arizona State
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