Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for oxygen reduction reaction

Article abstract of DOI:10.1016/j.ultsonch.2016.09.021

Porous MnO₂ as a non-noble metal oxygen reduction reaction (ORR) electrocatalyst was prepared by a simple sonochemical route. The as-prepared porous MnO₂ exhibited higher electrocatalytic activity, superior stability and better methanol tolerance than commercial Pt/C catalyst in alkaline media. Furthermore, the ORR proceeded via a nearly four-electron pathway. Cyclic voltammetry (CV) and rotating-disk electrode (RDE) measurements verified that the ORR enhancement was attributed to the porous structure and good dispersity, which facilitated sufficient transport of ions, electrons, O₂ and other reactants in the process of ORR. The results indicated that a facile and feasible sonochemical route could be used to prepare highly active porous MnO₂ electrocatalyst for ORR, which might be promising for direct methanol fuel cells.

Full text of DOI:10.1016/j.ultsonch.2016.09.021

Ultrasonics Sonochemistry xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
Sonochemical preparation of stable porous MnO2 and its application as
an efficient electrocatalyst for oxygen reduction reaction
b
a,
Ling-Xia Zuo a, Li-Ping Jiang a, , E.S. Abdel-Halim , Jun-Jie Zhu
⇑
⇑
a State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
b Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous MnO2 as a non-noble metal oxygen reduction reaction (ORR) electrocatalyst was prepared by a
simple sonochemical route. The as-prepared porous MnO2 exhibited higher electrocatalytic activity,
superior stability and better methanol tolerance than commercial Pt/C catalyst in alkaline media.
Furthermore, the ORR proceeded via a nearly four-electron pathway. Cyclic voltammetry (CV) and
rotating-disk electrode (RDE) measurements verified that the ORR enhancement was attributed to the
porous structure and good dispersity, which facilitated sufficient transport of ions, electrons, O2 and other
reactants in the process of ORR. The results indicated that a facile and feasible sonochemical route could
be used to prepare highly active porous MnO2 electrocatalyst for ORR, which might be promising for
direct methanol fuel cells.
Received 30 June 2016
Received in revised form 10 September
2016
Accepted 25 September 2016
Available online xxxx
Keywords:
Porous MnO2
Sonochemical preparation
Oxygen reduction reaction
Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction
there is a strong dependence of the catalytic activities of MnO2
on its crystal phase [13–16], shapes [14,17] and textural properties
ORR is a critical reaction of all the fuel cells, however, its slug-
gish reaction kinetics limits the performance of fuel cells. There-
fore, in order to attain efficient energy transformation, a highly
active ORR catalyst is necessary. Traditionally, Pt-based catalysts
have been widely used as active electrocatalysts for ORR in fuel
cells, which mainly depend on Pt-based catalysts to improve the
slow kinetics. However, high cost and scarcity of Pt severely limit
their application to large-scale commercialization. Therefore, it is
necessary to develop non-Pt, non-noble metal-based or metal-
free ORR electrocatalysts as potential replacements for the Pt-
based catalysts.
Non-noble metal electrocatalysts for cathode ORR represent one
of the most prominent families of electrocatalysts in fuel cells,
which have attracted considerable attention due to the catalytic
power comparable to or even superior to that of Pt. Among them,
the nanosized MnO2 is generally accepted as a state-of-the-art
non-noble metal catalyst due to its high catalytic activity, high
specific capacitance, nontoxicity, low cost, and minimum environ-
mental impact. Recent studies on MnO2 as an electrocatalyst or a
supercapacitor electrode have shown good performance with dif-
ferent morphologies such as nanoparticles [1–3], nanowires [4–
6], nanorods [7,8], nanosheets [9–12]. It has been reported that
including morphology and porosity. Some literatures reported that
MnO2 could actively catalyze ORR in alkaline media [14,17–20],
and the porous architecture of MnO2 was proven to be favorable
for oxygen adsorption and mass transport during ORR. Thus, the
use of MnO2 with a porous structure as ORR catalyst could be more
favorable over other structures, such as nanoparticles, nanorods,
nanotubes, nanosheets [16], since its porous structure could pro-
vide not only a high specific surface area but also a fast ion trans-
port. Some strategies for the design and the preparation of
mesoporous MnO2 catalysts, including soft or hard template [21–
28], electrodeposition [29–31], a soft interface synthesis between
CCl4 and H2O [32], ultrasound-stirred method [33] and
ultrasound-microwave-assisted method [25] were reported. How-
ever, the previously obtained mesoporous MnO2 materials gener-
ally require complicated procedures, expensive chemical
reagents, and relatively high temperatures and long reaction times,
which make the synthetic route complicated and render the cata-
lysts too expensive for widespread commercial use. Thus, a simple
method for preparing porous MnO2 with high activity is necessary.
The sonochemical method offers remarkable advantages,
including enhanced reaction rates and product yields, and simpli-
fied experimental conditions compared with traditional methods.
High intensity ultrasound can provide the extreme, transient con-
ditions, leading to temperatures of about 5000 K, pressures of
about 1000 atm, and heating or cooling rates above 1010 Ksꢀ1 due
⇑
Corresponding authors.
E-mail addresses: Jianglp@nju.edu.cn (L.-P. Jiang), jjzhu@nju.edu.cn (J.-J. Zhu).
http://dx.doi.org/10.1016/j.ultsonch.2016.09.021
1350-4177/Ó 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021

2
L.-X. Zuo et al. / Ultrasonics Sonochemistry xxx (2016) xxx–xxx
to cavitation. Furthermore, high intensity ultrasound is not only
the ability to form special nanostructures in the liquid, but also
the possibility to enhance the dispersity and activity of the as-
prepared nanoscale products [34]. Meanwhile, high intensity ultra-
sound is powerful for the formation of porous nanostructures.
Besides breaking up of larger aggregation, the crucial influence of
the ultrasound also contains improving the transport of crystallites
in the solution, generating denser micrometer spheres rather than
allowing the growth of flakes first [35,36]. A series of nanomateri-
als with different morphologies, structures, and compositions have
been prepared using sonochemical method [37]. Flower-like and
nanowire structured MnO2 [38] and mesoporous MnO2
[25,27,31,33] have also been prepared by sonochemical method,
however, the preparation for porous MnO2 with high catalytic
activity for ORR is still urgently needed.
Herein, it is demonstrated that the porous MnO2 can be synthe-
sized using a facile sonochemical route based on a redox reaction
without using any other additives. As a result, the as-prepared por-
ous MnO2 with the high BET showed a high ORR activity, excellent
durability and full tolerance to methanol. The improved ORR activ-
ity can be attributed to that the porous structure leads to high sur-
face area, which provides the possibility of efficient transport of
electrons and ions.
(0.012 M) aqueous solution were added into a 25-mL beaker in
water bath equipment. The mixture was rapidly exposed to high
intensity ultrasonic irradiation (Sonics VCX-750 ultrasonic proces-
sor with flat head tip, 750 W at 30% amplitude, 20 kHz, the ratio of
ultrasonic time and intermittent time: 6:4) for the total time was
20 min using water-bath by changing the water frequently to keep
under ambient air. Once the reaction was ended, a deep brown
solution was obtained. The product was collected by centrifugation
and washed for several times with water for further use in charac-
terization and electrochemical analysis.
For comparison, synthesis of porous MnO2 by a stirring method
(denoted as ST-PMO) has a similar procedure to that described in
the SC-PMO except using a stirring method.
2.4. Preparation of layered MnO2 using sonochemical method (denoted
as SC-LMO)
In a typical experiment, an aqueous solution (10 mL) containing
PVP (22 mg) and NaH2PO2 (20 mg) were added into a 25-mL bea-
ker in water bath equipment, and then mixed with the desired
amount of KMnO4 (0.01 M) aqueous solution, the solution was
rapidly exposed to high intensity ultrasound irradiation (Sonics
VCX-750 ultrasonic processor with flat head tip, 750 W at 30%
amplitude, 20 kHz, the ratio of ultrasonic time and intermittent
time: 6:4) for the total time was 20 min using water-bath by
changing the water frequently to keep under ambient air. The
resulting precipitate was collected by centrifugation at 9000 rpm
for 10 min, and then fully washed with deionized water and recol-
lected by centrifugation, dried at 50 °C in vacuo for 24 h and sealed
after grinding.
2. Experimental section
2.1. Materials
Potassium permanganate (KMnO4), manganese (II) acetate
tetrahydrate (Mn(CH3COO)2ꢁ4H2O), polyvinylpyrrolidone (K-30)
(PVP), sodium hypophosphite (NaH2PO2), isopropanol and ethanol
were all purchased from Shanghai Aladdin Chemical Reagent Com-
pany (China, Shanghai). Commercial Pt/C (20 wt%) was bought
from Alfa Aesar Chemicals Company (China, Shanghai). High purity
oxygen was obtained from Nanjing Specialty Gases Co., Ltd. (China,
Nanjing). All chemical reagents were commercially available and
were used as-received without further purification.
2.5. Electrode preparation
The glassy carbon electrode was polished with 0.3
lm and
0.05 m alumina slurries and then was rinsed with double distilled
l
water and dried using N2 flow before the catalyst layer was applied
to the disk. The as-prepared catalyst ink was prepared as follows:
2 mg of catalyst and 2 mg of Ketjen-300 J carbon were added into a
mixed solution containing deionized water (3.17 mL), isopropanol
(0.79 mL) and Nafion (0.04 mL), and dispersed by ultrasound for
2.2. Characterization
30 min to obtain a homogeneous suspension, 10 lL aliquot of the
The sizes and morphologies of each sample were observed by a
transmission electron microscope (TEM, JEM-2100, JEOL, Tokyo,
Japan). Scanning electron microscopy (SEM) images and SEM-
energy-dispersive X-ray spectroscopy (SEM-EDS) elemental map-
ping images were obtained on a Hitachi S-4800 scanning electron
microscope. The phase structures were characterized with an X-
ray Powder Diffractometer (XRD), which were performed on a Ger-
man Bruker D8 ADVANCE at a scanning rate of 2° minꢀ1 in the 2h
range from 5° to 80°. UV–Vis absorption spectra of the samples
were performed on a Shimadzu UV-3600 spectrophotometer (Shi-
madzu, Japan). The chemical composition was investigated by X-
ray photoelectron spectroscopy (XPS) using a PHI 5000 Versa
Probe. Electrochemical experiments were performed with a CHI
840B workstation (Shanghai Chenhua, Shanghai, China) with a sat-
urated calomel electrode (SCE) as reference and a platinum wire as
counter electrode, respectively. A glassy carbon (GC, diameter:
5.61 mm, area: 0.2471 cm2) disk and Pt ring (0.84 mm in width)
were used as the working electrode fixed on the rotating apparatus
(PINE Company, USA).
prepared slurry was drop-casted on the freshly polished GC elec-
trode with the help of a micro syringe and was further dried in
the fume hood for 2 h. For commercial Pt/C catalyst, 10 mg of Pt/
C was added into a mixed solution containing deionized water
(3.17 mL), isopropanol (0.79 mL) and Nafion (0.04 mL). Next, under
the same condition with the as-prepared catalysts, 10 lL of the cat-
alyst ink was loaded onto a GCE for electrochemical analysis. As a
consequence, the loading amount of pure Pt on the disk electrode
for commercial Pt/C (20 wt%) catalyst and the as-prepared catalyst
with the same quantity (5 lg) was studied for comparison.
2.6. Electrochemical measurements
The ORR activity measurements were evaluated using a RDE in
a conventional three-electrode system in 0.1 M KOH solution. All
potentials were measured and reported vs the potential of SCE
electrode. The CV experiments were recorded by applying a linear
potential scan at a sweep rate of 50 mV sꢀ1 from +0.2 to ꢀ1.2 V
after purging O2 or N2 gas for 30 min. The linear sweep voltamme-
try (LSV) measurements were performed at different rotating
2.3. Preparation of porous MnO2 using sonochemical method (denoted
as SC-PMO)
speeds from 400 to 2000 rpm with the scan rate of 10 mV sꢀ1
.
The chronoamperometry was conducted in O2-saturated 0.1 M
KOH solution at a potential of ꢀ0.3 V, sweep rate of 10 mV sꢀ1
,
For the synthesis of the SC-PMO catalyst, 10 mL of KMnO4
(0.008 M) aqueous solution, 10 mL of Mn(CH3COO)2ꢁ4H2O
and rotation rate of 1000 rpm. Methanol tolerance performance
of the catalysts was performed in O2-saturated 0.1 M KOH
Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021

L.-X. Zuo et al. / Ultrasonics Sonochemistry xxx (2016) xxx–xxx
3
+ 0.5 M CH3OH solution at a rotating speed of 1600 rpm. Rotating
ring-disk electrode (RRDE) measurements were carried out at
1600 rpm and the potential of the ring was poised at 0.5 V. The cur-
rent collection efficiency (N) of Pt ring was calibrated in N2-
saturated 1 M KNO3 + 6 mM K3[Fe(CN)6] mixed electrolyte (rota-
tion speed: 500–2000 rpm, sweep speed: 10 mV sꢀ1, scan potential
from 0.6 V to ꢀ0.2 V).
3. Results and discussions
It was reported that ultrasound irradiation could be used to fab-
ricate various nanomaterials [37]. The SC-PMO catalyst was pre-
pared by
a simple sonochemical route without using any
additives. In the synthesis, redox reaction between Mn from
KMnO4 at high oxidation states and Mn from Mn(CH3COO)2ꢁ4H2O
at low oxidation states occured under the ultrasound irradiation.
Scheme 1 is the schematic diagram illustrating the whole fabrica-
tion process. Fig. 1A and B shows transmission electron microscopy
(TEM) and high-magnification TEM images of the resulting SC-PMO
with porous structure. Scanning electron microscopy (SEM) images
of SC-PMO are shown in Fig. 1C. The SEM images show that the
morphologies of the SC-PMO were different from previously
reported MnO2 [4,8,14,39].
In the synthesis of MnO2 under ultrasonic irradiation, the con-
centration of Mn(CH3COO)2ꢁ4H2O greatly impacts the morphology
and dispersion of porous MnO2. When the concentration of Mn
(CH3COO)2ꢁ4H2O was reduced to 0.006 M, part of nonuniform
and irregular MnO2 would be obtained (Fig. 2A). Meanwhile,
increasing the concentration of Mn(CH3COO)2ꢁ4H2O to 0.018 M
can lead to inhomogeneous and slightly gathered MnO2 (Fig. 2C).
Therefore, when the concentration of Mn(CH3COO)2ꢁ4H2O was
0.012 M, porous MnO2 with good dispersity could be obtained
(Fig. 2B). Above results demonstrated that the concentration of
Mn(CH3COO)2ꢁ4H2O could influence the final shape and dispersity
of the SC-PMO catalyst under sonication, because it could control
the rate of nucleation and crystal growth [40].
SEM-EDS were also conducted, indicating the presence of ele-
mental O and Mn (Fig. 3A). The elemental maps (Fig. 3B and C) of
the SC-PMO revealed that Mn and O were homogeneously dis-
tributed in each individual MnO2 nanoparticle.
The UV–Vis absorption of SC-PMO exhibited a broad peak cen-
tered around 368 nm (Fig. 4A), which was the characteristic peak
of MnO2 nanomaterials [41]. X-ray diffraction (XRD) pattern of
the as-prepared SC-PMO has been shown in Fig. 4B, where the
diffraction peaks centered at 2h = 23.64°, 36.66°, and 65.66° corre-
sponded to the pure phase of d-MnO2 (JCPDS No. 18-0802). The
prepared oxide showed a low level of crystalline due to the
absence of long-range order of the MnO6 octahedral framework
[42]. The chemical composition and the oxidation state analysis
of Mn in the SC-PMO were investigated by X-ray photoelectron
spectroscopy (XPS) analysis. The survey scan spectrum in Fig. 4C
revealed the existence of O and Mn atoms. The high resolution
Mn 2p spectrum was presented in Fig. S1, in which the peaks at
641.75 eV (Mn 2p3/2) and 653.375 eV (Mn 2p1/2) were clearly
observed. The oxidation states of Mn of the SC-PMO could be cal-
Fig. 1. (A) TEM, (B) high-magnification TEM and (C) SEM images of the SC-PMO
obtained from a typical sonochemical synthesis.
culated from the deconvoluted XPS O 1s spectra results (Fig. 4D)
on the basis of the intensities of Mn-O-Mn, Mn-OH components
[43], indicating that the manganese oxidation state was close to
+4. The textural properties of the SC-PMO was investigated by N2
adsorption-desorption isotherms measurement (Fig. 5) and the
nitrogen sorption isotherm revealed a type-IV adsorption curve
with an H3-type hysteresis [44]. The inset was the Barrett-
Joyner-Halenda (BJH) pore size distribution curve, which exhibited
three groups of peaks from microporous, mesoporous and macrop-
orous region, respectively. The measured Brunauer-Emmett-Teller
(BET) surface area of 269.32 m2 gꢀ1 might exhibit fast electrode
kinetics and good stability, making it promising as an electrode
material for electrochemical catalysis. The SC-PMO obtained in
the present study has the highest BET than those previously
obtained mesoporous MnO2, whose important parameters are pre-
Scheme 1. Fabrication of the SC-PMO by a sonochemical method.
Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021

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L.-X. Zuo et al. / Ultrasonics Sonochemistry xxx (2016) xxx–xxx
Fig. 2. TEM images of the SC-PMO prepared with different amounts of Mn(CH3COO)2ꢁ4H2O (A) (conditions: KMnO4: 0.008 M, 10 mL; Mn(CH3COO)2ꢁ4H2O: 0.006 M, 10 mL;
ultrasound time: 20 min); (B) (conditions: KMnO4: 0.008 M, 10 mL; Mn(CH3COO)2ꢁ4H2O: 0.012 M, 10 mL; ultrasound time: 20 min); (C) (conditions: KMnO4: 0.008 M, 10 mL;
Mn(CH3COO)2ꢁ4H2O: 0.018 M, 10 mL; ultrasound time: 20 min).
Apparently, the porous structure was a key factor for the high
active noble metal-free ORR catalysts [45,46].
To further investigate electrocatalytic ORR mechanisms of the
SC-PMO electrode, RDE measurements were performed from 400
to 2000 rpm in the O2-saturated 0.1 M KOH solution. As shown
in Fig. 6C, the enhancement of limiting current density may be
due to the shortened diffusion layer with the increase of the rota-
tion rate. Fig. 6D was the corresponding Koutecky–Levich (K–L)
plots of the SC-PMO electrode at different potential, displaying per-
fect linearity and near parallelism of each other. The above results
were considered as typical of first-order reaction kinetics with
respect to the concentration of dissolved O2, based on the K–L
equation [47]:
1
j
1
jk
1
x1=2
¼
þ
ð1Þ
B
B ¼ 0:2nFðDO
Þ
2=3tꢀ1=6CO
ð2Þ
2
2
where j was the measured current density at mixed diffusion-
kinetics control region from the ORR polarization curve, jk was the
kinetic current density,
x was the rotation rate (rpm), n was the
number of electrons transferred in oxygen reduction, F was the
Faraday constant (F = 96,485 C molꢀ1), C was the concentration of
dissolved O2 in the solution, D was the diffusion coefficient of dis-
solved O2 in the solution, and
m was the kinematic viscosity of the
solution. From the slope of the Levich–Koutecky plots and the
parameters, the electron transfer number (n) on the SC-PMO was
determined to be ꢂ4.0, demonstrating a 4eꢀ oxygen reduction pro-
cess [48] from the SC-PMO catalysts in an alkaline electrolyte,
which was similar to the commercial Pt/C catalyst that was known
to catalyze ORR in a direct four-electron pathway. The ORR catalytic
performance was further evaluated by using RRDE technique.
Fig. 7A showed the obtained RRDE LSV data, among which the disk
current (jd) representative of the reduction of oxygen and the ring
current (jr) representative of the H2O2 oxidation were presented.
The electron transfer number (n) and percentage of H2O2 were
obtained from the ring/disk currents (jr/jd) through the following
equations:
Fig. 3. (A) SEM-EDS spectrum of the SC-PMO catalyst; Element-mapping images of
the SC-PMO (B) Mn and (C) O.
sented in Table S1. As can be seen from the Table S1, the SC-PMO
has more advantages concerning energy saving, simple experimen-
tal conditions and high BET surface area than other methods.
To further investigate the electrocatalytic properties of the SC-
PMO toward ORR, CV and LSV were performed using a RDE in
0.1 M KOH solution at room temperature. Fig. 6A presented the
CV curves of ORR in O2 or N2-saturated 0.1 M KOH solution. In par-
ticular, no characteristic peaks were observed in the CV curve in
N2-saturated electrolyte because of the absence of O2. However, a
strong oxygen reduction peak at ꢀ0.37 V versus SCE in O2-
saturated electrolyte indicated that the SC-PMO catalyst had highly
electrocatalytic activity for ORR. As shown in Fig. 6B, the polariza-
tion curves display two distinguishable potential regions: the dif-
fusion–limiting current region below ꢀ0.5 V and the mixed
kinetic-diffusion control region between ꢀ0.5 and 0 V. The limiting
current density of SC-PMO was close to commercial Pt/C electrode
at 1600 rpm in the O2-saturated 0.1 M KOH solution. The SC-PMO
exhibited a pronounced electrocatalytic ORR activity associated
with a lower loading and a higher steady-state cathodic current
density than the reported literatures [13,14,16,17,38] (Table S2).
jjdj
n ¼ 4 ꢃ
ð3Þ
jjdj þ jr=N
jr=N
jjdj þ jr=N
H2O2% ¼ 200 ꢃ
ð4Þ
where N was the current collection efficiency of RRDE (37%), jr was
the ring current density, jd was the disk current density. Fig. 7B
showed the lower H2O2 production yield and an apparent 4eꢀ
reduction in ORR process, which was consistent with the results
from the K–L determination.
Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021

L.-X. Zuo et al. / Ultrasonics Sonochemistry xxx (2016) xxx–xxx
5
Fig. 4. (A) UV–Vis absorption spectra and (B) XRD pattern of the SC-PMO; (C) XPS survey spectra of the SC-PMO; (D) the deconvoluted XPS O1s spectrum. The raw data are
represented by the solid lines, and the fitted data are represented by the dash lines.
Fig. 5. Nitrogen adsorption-desorption isotherms and pore size distribution curve
(inset) of the SC-PMO.
Fig. 6. (A) CV curves of the SC-PMO in O2 or N2-saturated 0.1 M KOH at 50 mV sꢀ1
;
(B) LSV curves of the SC-PMO and commercial Pt/C at a rotation rate of 1600 rpm;
(C) and (D) LSV curves of the SC-PMO in O2-saturated 0.1 M KOH at different
rotating rates (400–2000 rpm) with a scan rate of 10 mV sꢀ1 and corresponding
Koutecky–Levich plots at ꢀ0.5 V, ꢀ0.6 V, ꢀ0.7 V, ꢀ0.8 V, ꢀ0.9 V and ꢀ1.0 V.
The ORR performance of SC-PMO and commercial Pt/C catalysts
in the presence of methanol was also tested. As shown in
Fig. 7C and D, obviously, the ORR performance of the SC-PMO cat-
alyst was not influenced in the mixture of KOH and CH3OH. In con-
trast, for the commercial Pt/C catalyst, under the same
experimental conditions, the ORR on commercial Pt/C electrode
was seriously restrained in the presence of methanol due to the
occurrence of methanol oxidation reaction on Pt. Obviously, the
SC-PMO catalyst showed high selectivity for ORR with strong
methanol tolerance. The results indicated that the SC-PMO was a
perfect cathode electrocatalyst for the direct methanol alkaline fuel
cells.
Durability was a prominent concern for those materials used in
electrochemical applications. The SC-PMO exhibited excellent sta-
bility with a slow attenuation using chronoamperometry at ꢀ0.3 V
in O2-saturated 0.1 M KOH solution. As shown in Fig. 8A, there was
18% relative loss current of the SC-PMO catalyst after 16,000 s. In
contrast, the commercial Pt/C electrode showed a high relative cur-
rent loss of 41%. These results confirmed that the SC-PMO was
much more stable than the commercial Pt/C catalyst.
To confirm the significance of the sonochemical preparation
method and the porous structure on the ORR performance of
MnO2 catalysts, we further prepared the ST-PMO catalyst using a
stirring method and SC-LMO catalyst by a sonochemical method
and investigated their ORR catalytic activities. TEM and SEM
images (Fig. S2) revealed the morphology and structure of the
ST-PMO catalyst, which showed poor dispersity and inhomogene-
ity. The characteristic of the porous structure was confirmed by N2
adsorption-desorption isotherms (Fig. S3). The ST-PMO catalyst
possessed a N2 BET surface area of 249.76 m2 gꢀ1, which indicated
that it also had a porous structure. TEM and SEM images (Fig. S4)
revealed that the SC-LMO catalyst had a typical layered structure.
The powder XRD pattern (Fig. S5) of as-synthesized ST-PMO and
Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021

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L.-X. Zuo et al. / Ultrasonics Sonochemistry xxx (2016) xxx–xxx
guaranteed high ionic conductivity as well as continuous and effi-
cient O2 diffusion, but also provided much more active sites for
ORR than layered structure of SC-LMO. The high specific surface
area and the abundant pore structure of SC-PMO provided an effi-
cient mass transport [49] in the cathodic electrode and consequent
performance in ORR and thus resulting in significantly improved
activities in fuel cells.
For the SC-PMO and ST-PMO catalysts, the sonochemical
method could facilitate the uniform porous structure of SC-PMO.
Due to acoustic cavitation, the ultrasound improved transport of
crystallites in the solution and provided a homogeneous environ-
ment [50,51], leading to porous MnO2 with good dispersity. In
the conventional stirring method, however, porous MnO2 with
poor dispersity was obtained. Due to the better dispersity,
enhanced electrochemical performance of SC-PMO for ORR could
be obtained. Therefore, sonochemical method was demonstrated
to be an effective tool for the preparation of porous nanocatalysts
with better dispersion for fuel cells.
Fig. 7. (A) Disk-ring current density data of SC-PMO catalyst obtained by using
RRDE in O2- saturated 0.1 M KOH at a sweep rate of 10 mV sꢀ1; (B) percentage of
H2O2 and electron transfer number; LSV on RDE for SC-PMO (C) and commercial Pt/
C (D) catalysts in O2-saturated 0.1 M KOH and with 0.5 M CH3OH at a rotation rate
of 1600 rpm, respectively.
4. Conclusions
To summarize, we have demonstrated a facile method to pre-
pare MnO2 with a porous structure under ultrasonic irradiation.
The obtained porous MnO2 displays highly electrocatalytic activity,
good stability, fully tolerance to methanol and four-electron selec-
tivity for cathodic oxygen reduction reaction. Our results revealed
that the enhanced performance of ORR of the SC-PMO catalyst was
attributed to the highly porous structure by sonochemical method.
We expect that the prepared SC-PMO catalyst will be useful for
other electrocatalytic applications. Furthermore, our study further
might supply a practical preparation method for achieving high
activity cathodic ORR catalysts in fuel cells.
Notes
The authors declare no competing financial interest.
Acknowledgements
Fig. 8. (A) Current-time (i-t) chronoamperometric responses of the SC-PMO and
commercial Pt/C catalysts modified GC electrode at ꢀ0.3 V versus SCE in O2-
saturated 0.1 M KOH solution at a rotation rate of 1000 rpm; (B) Cyclic voltammo-
grams of ORR on three MnO2 samples in O2-saturated 0.1 M KOH solution; (C) RDE
voltammograms of SC-PMO, SC-LMO and ST-PMO catalysts in O2-saturated 0.1 M
KOH at 1600 rpm with a scan rate of 10 mV/s; (D) The electron transfer number of
SC-PMO, SC-LMO and ST-PMO at ꢀ0.5 V, ꢀ0.6 V, ꢀ0.7 V, ꢀ0.8 V, ꢀ0.9 V and ꢀ1.0 V.
We greatly appreciate the support of the National Natural
Science Foundation of China (21335004, 21475057). This work
was also supported by the Program for New Century Excellent
Talents in University (NCET-12-0256). The authors extend their
sincere appreciation to the Deanship of Scientific Research at King
Saud University for funding this Prolific Research Group (PRG-
1437-32). We thank Prof. Xi-Zhang Wang from Nanjing University
for his help to BET measurements.
SC-LMO showed evidence of two typical reflections at 2h values
36.66° and 65.66° corresponding to the SC-PMO catalyst.
To further assess the electrocatalytic performance of all three
MnO2 samples, the experimental results were compared. As could
be observed, the SC-PMO catalyst had the highest electrocatalytic
activity among the three different MnO2 catalysts in terms of both
the onset potential and reduction current density through the CV
(Fig. 8B) and RDE (Fig. 8C) measurements. On the other hand, the
electron transfer number per oxygen molecule (n) (Fig. 8D) on
the ST-PMO, SC-LMO electrode were calculated by RDE measure-
ments on the basis of K–L plots (Fig. S6), suggesting that the oxy-
gen reduction for the SC-LMO and ST-PMO catalyst proceeded via
Appendix A. Supplementary data
XPS profile (Fig. S1) of the SC-PMO sample; (Table S1) Com-
pared parameters of porous MnO2 prepared by different methods;
(Table S2) Comparison of the ORR performance of the reported
MnO2 catalysts. TEM and SEM images of the ST-PMO (Fig. S2);
Nitrogen adsorption-desorption isotherms (Fig. S3) of the ST-
PMO; TEM and SEM images (Fig. S4) of the SC-LMO; XRD patterns
(Fig. S5) of the ST-PMO and SC-LMO samples; Polarization curves
and corresponding K–L plots (Fig. S6) of the ST-PMO and SC-LMO
samples. Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.ult-
sonch.2016.09.021.
a
quasi-two-electron and
a
quasi-four-electron process,
respectively.
For the SC-PMO and SC-LMO catalysts, structure-activity rela-
tionship was an important factor that determined catalyst perfor-
mance. We found that the porous structure of SC-PMO not only
Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021

L.-X. Zuo et al. / Ultrasonics Sonochemistry xxx (2016) xxx–xxx
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Please cite this article in press as: L.-X. Zuo et al., Sonochemical preparation of stable porous MnO2 and its application as an efficient electrocatalyst for
oxygen reduction reaction, Ultrason. Sonochem. (2016), http://dx.doi.org/10.1016/j.ultsonch.2016.09.021