Porous nano-MnO2: Large scale synthesis via a facile quick-redox procedure and application in a supercapacitor

Article abstract of DOI:10.1039/c0nj00712a

A new type of porous nano-MnO₂ for supercapacitors has been synthesized for the first time by a facile sonochemistry route from a quick-redox reaction between KMnO₄ and d-glucose. The crystal structure, morphology and chemical composition of the MnO₂ product were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The results show that the porous MnO₂ nanoparticles in the range 20-50 nm are interestingly composed of nanorods with diameters of about 2 nm and lengths of 4-8 nm. A possible growth mechanism of this nanostructure has been identified based on the experimental results. The electrochemical properties of the porous MnO₂ were investigated in a symmetric capacitor cell using both 1 M Na₂SO₄ and 9 M KOH aqueous solutions. The results indicate that the MnO₂ electrodes have a good capacitive performance in both cases.

Full text of DOI:10.1039/c0nj00712a

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PAPER
Porous nano-MnO : large scale synthesis via a facile quick-redox
2
procedure and application in a supercapacitor
a
a
a
a
Hong-Qiang Wang, Gui-fen Yang, Qing-Yu Li,* Xin-Xian Zhong,
a
Fang-Ping Wang, Ze-Sheng Li* and Ya-hao Lic
ab
Received (in Montpellier, France) 17th September 2010, Accepted 28th October 2010
DOI: 10.1039/c0nj00712a
2
A new type of porous nano-MnO for supercapacitors has been synthesized for the first time
by a facile sonochemistry route from a quick-redox reaction between KMnO and D-glucose.
4
The crystal structure, morphology and chemical composition of the MnO product were
2
investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). The results show
2
that the porous MnO nanoparticles in the range 20–50 nm are interestingly composed of
nanorods with diameters of about 2 nm and lengths of 4–8 nm. A possible growth mechanism of
this nanostructure has been identified based on the experimental results. The electrochemical
2
properties of the porous MnO were investigated in a symmetric capacitor cell using both 1 M
Na SO and 9 M KOH aqueous solutions. The results indicate that the MnO electrodes have a
2
4
2
good capacitive performance in both cases.
mild pathways for the synthesis of nano-MnO on a large scale
2
1
. Introduction
remains a challenge to a large extent.
2
Manganese dioxide (MnO ), as one of important energy
In past decades, sonochemistry has developed into a powerful
storage materials, has attracted a great deal of attention in
1
many technological applications, such as supercapacitors,
technique for generating nanostructured materials with unusual
13,14
properties.
The essence of the sonochemical process is the
2
zinc manganese batteries, lithium manganese batteries and
3
phenomenon of cavitation: the creation, growth and collapse of
4
fuel cells. The advantages of this metal oxide are its low cost,
bubbles that form in liquids. The extremely high temperatures
abundance and good environmental compatibility. Recently,
increasing demand for light-weight energy storage devices with
7
>5000 K), pressures (>20 MP) and cooling rates (>10 K s )
ꢀ1
(
generated during acoustic cavitation within the collapsing bubble
15,16
lead to many unique properties in irradiated solutions.
high energies and power densities has promoted the synthesis
5
of nanoscale MnO
Actually, nano-MnO
superior to its bulk counterpart because of its higher specific
2
(at least one dimension less than 100 nm).
We have previously prepared MnO2 microspheres based on
nanowires by a sonication-assisted hydrothermal synthesis
17
2
exhibits an electrochemical performance
process, while this technology has also been used in the
6,7
surface area. A variety of synthesis techniques have therefore
18
preparation other oxide nanostructures elsewhere.
been exploited to obtain nano-MnO with dispersible shapes,
2
In the present investigation, we report the large scale
synthesis of novel porous MnO2 nanoparticles via a quick
and facile sonochemical procedure. In this process, nano-
8
including hydrothermal, electrodeposition, a sol–gel method,
9
10
1
1
12
template synthesis, thermal decomposition, etc. However,
complicated operations, time/energy-consuming and expensive
reagents always seem to be obstacles for the commercialization
MnO is produced by an inexpensive D-glucose–permanganate
2
redox reaction in the presence of polyethylene glycol (PEG) at
ambient pressure. To our knowledge, this is the first report on
2
of nano-MnO . Hence, the development of quick, simple and
MnO
2
nanostructures from a D-glucose–permanganate redox
a
Key Laboratory for the Chemistry and Molecular Engineering of
Medicinal Resources (Ministry of Education of China),
School of Chemistry & Chemical Engineering of Guangxi Normal
University, Guilin, Guangxi, 541004, P. R. China.
E-mail: 13975808173@126.com, lzs212@163.com;
Fax: +86 0773 5854077; Tel: +86 0773 3310900
The State Key Laboratory of Optoelectronic Materials and
Technologies, School of Physics and Engineering,
Sun Yat-Sen University, Guangzhou, Guangdong,
510275, P. R. China
School of Chemistry & Chemical Engineering of Chong Qing
University, Chongqing, 400030, P. R. China
reaction, although D-glucose was used previously for the
1
reduction of KMnO
9
4 2
to MnO used in chemical analysis,
and MnOx microstructures have previously been reported
20
from KMnO
of the porous nano-MnO2 is investigated. Furthermore,
as potential electrode material for supercapacitors,
4
and simple sugars. The formation mechanism
b
c
a
the electrochemical properties of the nano-MnO2 are also
investigated through a symmetric capacitor cell using both
1 M Na SO and 9 M KOH aqueous solutions.
2
4
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3. Results and discussion
3
.1 Crystal structure, morphology and chemical composition
of the MnO product
2
2
To discover the crystallinity of the as-prepared MnO product,
XRD analysis was undertaken, and the resultant XRD pattern
is shown in Fig. 2. The two characteristic peaks (311) and
2
Fig. 1 Schematic of synthesis process of nano-MnO .
(400), at 37 and 661 (marked by .) clearly presented, were
confirmed to be MnO2 (PDF# 42-1169). The weak peak
2
. Experimental section
signals observed in the XRD pattern suggest the sample is in
In this study, as a green, safe and inexpensive raw material,
a poorly crystalline state with only a short-range crystal
21
D-glucose was used to prepare the MnO sample by a quick
2
form. The low degree of crystallinity of the MnO sample
2
redox reaction with KMnO . The sonochemical synthesis
4
can be attributed to the rapid redox reaction of D-glucose–
permanganate during the ultrasound irradiation. Since the
reaction rate was high, causing a large number of nuclei to
form in a very short period of time and leaving the crystals to
grow inadequately, the obtained product was thus poorly
process was carried out as follows: First, 1.50 g KMnO and
4
0
5
.05 g PEG-600 were dispersed in 100 ml de-ionized water in a
00 ml beaker. At the same time, 0.50 g D-glucose and 5 ml
absolute alcohol were dispersed in 50 ml de-ionized water in
another beaker. Then, the D-glucose aqueous solution was
poured into the potassium permanganate aqueous solution
rapidly. The above mixture was put into an ultrasonic
water bath for 10 min in a high-intensity ultrasonic source
22
crystalline.
Furthermore, the diffraction peaks of the MnO
2
were
highlighted using the Peak Paint feature in the JADE
software, and the corresponding peak areas were calculated
(
100 W and 40 kHz).
(
see Fig. 3). It can be seen that the two broadened diffraction
During the ultrasonic irradiation, the temperature of the
peaks (45 803 and 14 880 area counts) are a strong indication
3,24
reaction solution rose to around 60 1C. When the reaction was
complete, the brown precipitation product was collected and
washed thoroughly with de-ionized water and ethanol
alternately, and finally dried at 60 1C in a vacuum. The
synthesis route schematic is shown in Fig. 1. The conversion
rate of this sonochemical synthesis was more than 99.8%.
2
of nanocrystals in the MnO2,
and the average particle size
can be estimated by using the Scherer equation, expressed as
follows:
0
:9l
D ¼
b cos y
The morphology and microstructure of the MnO
were investigated by scanning electron microscopy (SEM)
FEI Quanta 200 FEG, Holand) and TEM (FEI TECNAI
2
sample
where D is the average particle size (nm), l is the wavelength of
the X-rays (0.15406 nm), y is the angle at the peak maximum
and b is the width of the peak at half height (peak FWHM).
The calculated mean sizes according to the diffraction peaks of
(
2
G
12, Holand). X-Ray diffraction (XRD) was preformed
with a Rigaku D/max 2550 V X-ray diffractometer
using Cu-Ka radiation (l = 0.15406 nm). The MnO2
electrodes were prepared in the form of disks (110 ꢁ 10 mm
(
311) and (400) were found to be 2.8 and 4.4 nm, respectively.
Fig. 4(a) shows a low-magnification SEM image of the
in thickness and 10 mm in diameter) by mixing 80 wt% MnO
2
as-prepared MnO
product has a three-dimensional (3-D) network structure
consisted of a large quantity of MnO nanoparticles (almost
00% in yield). The SEM image with a high-magnification
Fig. 4(b)) reveals that these nanoparticles have a size of
B50 nm and analogous spherical shapes. The MnO nano-
2
product. It is demonstrated that the
(
18.5 ꢁ 1.5 mg per one) in ethanol with 10 wt% of acetylene
black and 10 wt% of binder (polytetrafluoroethylene, PTFE).
Electrochemical measurements were performed using
symmetric capacitor cells (two-electrode mode), which were
assembled with a pair of identical electrodes separated by a
piece of microporous separator.
2
1
(
2
particles synthesized in our case are a uniform dispersion,
having no obvious secondary particle agglomerate larger than
2
4–28
5
00 nm, as reported in the literature,
indicating a fine
nanostructure of MnO in the present study.
2
Fig. 2 The XRD pattern of the as-prepared MnO
2
.
2
Fig. 3 Peak signals from the MnO XRD pattern.
4
70 New J. Chem., 2011, 35, 469–475
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2
Fig. 4 SEM images (a and b) and TEM images (c and d) of the as-prepared MnO .
2
Table 1 Compared parameters of MnO nanorods prepared by different methods
29
30
31
32
MnO nanorods Present study
2
Study A
Study B
Study C
Study D
Dimension size/nm
Synthesis route
Synthesis time
2 ꢂ 8
30 ꢂ 200
40 ꢂ 200
Hydrothermal
12 h
10 ꢂ 100
Template
1 h
5 ꢂ 50
Sonochemistry
10 min
60
Hydrothermal
Microemulsion
36 h
180
12 h
70
Temperature/1C
160
90
1
6
TEM observations confirm an exact diameter in the range
0–50 nm for the MnO nanoparticles, as shown in Fig. 4(c).
The high-magnification TEM image (Fig. 4(d)) shows that the
MnO nanoparticles are composed of many nanorods with
temperatures is an issue in material applications.
2
2
Consequently, due to the simple process, short reaction time,
high yield and safe precursors, the synthesis procedure of
2
nano-MnO
2
in our case may be applicable in practical
diameters of about 2 nm and lengths of 4–8 nm. The MnO2
nanorods produced in the present study are smaller than those
applications.
Particle agglomerates can be classified into two types: soft
agglomerate, in which the particles are held together by weak
van der Waals forces, and hard agglomerate, in which the
2
9,30
31
template and
obtained previously by hydrothermal,
3
2
microemulsion techniques, whose specific parameters are
presented in Table 1. As can be seen from the table, the
sonochemical method has more advantages concerning time/
energy savings and high efficiency than the other methods.
A similar result was also proved recently in another
3
3
particles are chemically bonded together by strong bridges.
It is interesting to note that, as shown in Fig. 4(d), the soft
agglomerates (overlap and side by side, marked by arrow) of
2
the MnO nanorods resulted in a nearly 10–50-times increase
sonochemistry-derived MnO
2
preparation from potassium
in size to become 20–50 nm nanoparticles, while lots of
nanopores (marked by circles) were formed simultaneously
7
permanganate and manganese acetate. It is known that
obtaining nanomaterials with a controlled size or shape
under mild conditions and with safe precursors at lower
from the surface to the inside (namely porous MnO
2
nano-
particles). The pore size is not homogeneous and it varies from
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2
Fig. 6 The SEM-EDS result of the as-prepared MnO : (a) SEM
image used and (b) corresponding images of EDS element distribution
mapping.
2
Fig. 5 The EDS pattern of as-prepared MnO .
1
–3 nm. Nevertheless, some hard aggregation can be observed
3
.2 Reaction courses and the formation mechanism of the
(
marked by the rectangle), where MnO nanorods have
2
MnO product
2
integrated into a solid body by strong chemical bonds due to
differences in local reaction conditions (e.g. the distribution of
surfactant). In addition, one can also deduce from the TEM
Generally, it is very difficult to study the exact reaction
mechanism of permanganate oxidation to organic substrates
due to the multitude of possible oxidation states. The
acceptable reaction courses of the permanganate oxidation
observations that the MnO
2
nanoparticles interconnect
physically into 3-D a network structure in form of a soft
3
agglomerate.
3
7–40
4
of D-glucose into MnO are described as follows:
2
The chemical composition of the obtained MnO product
2
ð1Þ
ð2Þ
was determined using energy-dispersive X-ray spectroscopy
(
EDS) analysis attached to the SEM. On the basis of the
appearance of the Mn-Ka, O-Ka, C-Ka and K-Ka X-ray
peaks, one can conclude that the constitutive elements of the
sample are Mn, O, C and K, as shown in the EDS pattern (see
Fig. 5). Generally, there is always a possibility of potassium
ð3Þ
ions co-existing in the MnO
permanganate, one of the starting materials.
the C element should be derived from the other starting
material, D-glucose. The composition content of the MnO
2
matrix, arising from potassium
3
5,36
Naturally,
ð4Þ
ð5Þ
2
sample is listed in Table 2. It can be seen from the table that
the O and Mn content are in absolute superiority, and that the
amount of K and C are very negligible for detection
using XRD.
ꢀ
Eqn (1) represents the protonation of MnO4 to HMnO . This
4
is a critical step in the D-glucose–permanganate redox reaction
SEM-EDS element distribution mapping is
a useful
that indicates that the initial reaction kinetics are dependent
technique utilized to identify areas of specific chemical
composition, in which several elements on the surface of
sample tested can be mapped simultaneously. Fig. 6 shows
3
7
on the hydrogen ion concentration. Eqn (2) shows the
formation of the D-glucose–Mn(VII) complex, which can
decompose into Mn(V) and a glucose-based radical [eqn (3)].
Mn(V) is highly unstable to the glucose-based radical and
immediately gets converted into Mn(IV) [eqn (4)], and the
formation of manganese dioxide can eventually occur in
the SEM-EDS result of the as-prepared MnO
2
product where
(
a) is the SEM image used in the EDS mapping test and (b) is
the resulting image of EDS element distribution based on
the SEM image in (a). It can be seen from Fig. 6(a) that
well-dispersed, spherical MnO2 nanoparticles are inter-
connected into a porous network structure. The corresponding
EDS mapping images (Fig. 6(b)) of both the Mn and
O elements have visible and well-dispersed spot distributions
that are consistent with the SEM image, as shown in Fig. 6(a).
In addition, the SEM-EDS analysis also indicates that there is
little K and C present, in view of the weak spot distribution of
the corresponding mapping images.
4
0
eqn (5).
The mechanism of sonochemical processes are usually based
ꢃ
ꢃ
on the formation of the short-lived radical species H and OH
4
1
eqn (6)], generated during violent cavitation events. In
[
ꢃ
ꢃ
aqueous solution, H and OH are strong reducing radicals
1
8,42,43
ꢃ
that are able to reduce metal ions.
In our case, the H
radical formed in eqn (6) (‘‘)))))’’ symbolizes ultrasonic action)
+
may act as a reducing species, trigger the reduction of K to K
and produce a large quantity of hydrogen ions [eqn (7)].
A high concentration of hydrogen ions produced in this step
Table 2 Content of C, O, K and Mn present in the MnO
2
sample
Total
2
is essential for MnO rapid synthesis within 10 min by
sonochemistry. When D-glucose was used for the reduction
of KMnO4 to MnO2 without ultrasonic treatment, the
Element
C-Ka
O-Ka
K-Ka
Mn-Ka
Quality (%)
Atom (%)
1.32
2.85
45.64
71.46
0.78
0.52l
52.26
25.17
100.00
100.00
precipitation was found to be complete only after at least
19
2
h, even at 80 1C. In addition, the pH of the solution was
4
72 New J. Chem., 2011, 35, 469–475
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ꢀ
1
measured before and after the sonochemical reaction. An
increase in the pH from 5.74 before the reaction to 8.62 after
supercapacitor cells at a current density of 280 mA g are
shown in Fig. 8(a) and (b) for 1 M Na SO and 9 M KOH
2
4
ꢀ
the reaction had occurred indicates the generation of OH
electrolytes, respectively. The typical triangle distributions
of the curves prove a good capacitive behavior of the
supercapacitors in both electrolytes. At this current density,
during the sonochemical process. This is probably due to the
reaction between the resulting K and H O [eqn (8)].
2
the single electrode specific capacitance calculated from the
12
ꢃ
ꢃ
H
2
O ))))) OH + H
(6)
(7)
(8)
2 4
discharging plots for the supercapacitor cells in 1 M Na SO
+
K
ꢃ
+
ꢀ1
+ H - K + H
and 9 M KOH electrolyte were 198.1 and 401.4 F g
respectively.
,
2
K + 2H
2
O - 2KOH + H
2
The dependence of the capacitance on the current density of
the MnO electrodes in different electrolytes is shown in Fig. 9.
Up to now, the definite formation mechanisms of MnO
nanorod structures have not be well understood and are
2
2
It was observed that the specific capacitance decreased
4
4
with increasing current density (280, 420, 540, 680 and
thought to vary between the different synthesis methods.
Recently, studies have demonstrated that sonochemistry can
ꢀ ꢀ1
1
8
20 mA g ). At a current density of 820 mA g , a high
ꢀ1
1
7,45,46
value of 196.7 F g in 9 M KOH electrolyte was obtained, but
promote the synthesis of 1-D metal oxides.
known that there are three regions of sonochemical activity:
i) inside of the collapsing bubble (T > 5000 K and
It is well
ꢀ1
a lower value of 100.2 F g was measured in 1 M Na SO
2
4
electrolyte, which indicates that in alkaline electrolyte, the
MnO electrodes possess better electrochemical properties
(
F > 20 MP), (ii) at the interface between the bubble and the
2
than in a neutral electrolyte.
liquid (T E 1900 K and F E 8 MP) and (iii) in the bulk
1
8,47
It is well known that the pseudo-capacitive behavior of
solution under ambient conditions.
Therefore, we propose
MnO
2
is a surface reaction: only the surface or a very
that the formation of MnO nanorods in the present study is at
2
thin surface layer of the oxide can participate in this pseudo-
9
the interfacial region where high temperature and high
pressure are present. Moreover, the extremely rapid cooling
,49
capacitive reaction.
2
The pseudo-capacitance of MnO is
7
ꢀ1
based on the reversible redox process that takes place in the
4
rates (>10 K s ) during acoustic cavitation can effectively
prevent the over-growth of the MnO nanorods, this being one
+
3+
redox transition Mn
2 Mn . The generally accepted
reactions for MnO in Na SO neutral aqueous electrolyte
2
6
key factor in the formation of the poorly crystallized MnO2.
Based on the experimental results and discussion above, a
possible formation mechanism of the porous MnO2 nano-
particles is proposed, as presented in Fig. 7. At first under the
2
2
4
3
5,50–55
are as follows:
MnO
MnO ) surface + Na + e 2 (MnO2 Na ) surface
+
ꢀ
2
+ Na + e 2 MnOONa
sonochemical conditions, MnO
according to the series of reaction process described
in eqn (1)–(5). Then, the MnO nanorods tend to softly
agglomerate and further form MnO nanoparticles by van
der Waals forces. Finally, under the effect of the surfactant
PEG-600, porous MnO nanoparticles consisting of stacked
MnO nanorods are generated. During this period, the
2
nanorods grow rapidly
+
ꢀ
ꢀ
+
(
2
2
where the second mechanism is based on the surface
2
adsorption of electrolyte cations on MnO . In the present
2
study, the redox process is mainly governed by the insertion
+
and de-insertion of Na from the electrolyte into the porous
2
2
nano-MnO matrix.
2
surfactant PEG-600 acts as an effective dispersant to inhibit
The redox mechanism of MnO in neutral solution has been
2
the MnO nanorods from hard-agglomerating, otherwise the
2
well investigated; however, in an alkaline electrolyte such as
5
6,57
porous nanoparticles would agglomerate into solid ones later
in the reaction. Actually, it has been found that in ultrasound
irradiation processes, the surfactant in the reaction system
plays an important role in the formation of porous or hollow
KOH, it has been rarely reported.
In the present study, the
observed specific capacitance values for the MnO electrode in
2
9 M KOH is much higher than that in 1 M Na SO . In an
2
4
alkaline electrolyte, the charge storage process may be
completed by the combination of the surface adsorption/
desorption of ions from the electrolyte and proton insertion
from water. This could be an interesting extension of the
1
4,48
oxide nanostructures.
3
.3 Electrochemical properties of MnO electrode in
2
supercapacitor applications
pseudo-capacitance of MnO
2
in aqueous solutions, and the
1
0,56
The electrochemical properties of MnO2 electrodes in 1 M
Na SO and 9 M KOH aqueous solutions were compared. The
possible reactions are as follows:
2
4
+
ꢀ
2ꢀ
+
(
MnO
MnO
MnO (OH)
2
) surface + K + e 2 (MnO K ) surface
galvanostatic charge/discharge curves of the two-electrode
ꢀ ꢀ
2
O + e 2 MnOOH + OH
2
+ H
+ nH + ne 2 MnOaꢀn(OH)b+n
+
ꢀ
a
b
.
When used for electrode materials, nanostructured metal
oxides have been considered to be more beneficial to high
capacitance than bulky ones due to their sufficiently high
Fig. 7 The proposed growth mechanism of the porous MnO
nanoparticles: (1) sonochemical synthesis of MnO nanorods,
2) soft-agglomeration of MnO nanorods and (3) generation of
porous or solid MnO nanoparticles.
2
2
5
,7,12,58
(
2
electrode surface area.
This may indicate that
2
electrolyte ions can effectively make contact with the active
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ꢀ2
2 2 4
Fig. 8 Charge–discharge curves of the MnO electrodes in (a) 1 M Na SO and (b) 9 M KOH electrolyte at a current density of 280 mA g .
The electrochemical stability of supercapacitor cells were
examined by charge–discharge cycling at a constant current
ꢀ
2
density of 280 mA g for 900 cycles (see Fig. 10). It was noted
ꢀ2
that the electrodes had more than 120 F g of electrode
specific capacitance in both 1 M Na SO and 9 M KOH
electrolytes after 900 cycles. Considering the network structure
nature of porous MnO electrodes, the dissolution process
2
4
2
should be more sensitive to a high concentration of alkaline
solution, which leads to faster capacitance fading within the
early 200 cycles in the 9 M KOH electrolyte. Although not
perfect compared with the MnO thin films previously
2
1
0
reported, this performance is still encouraging, especially in
view of the ease of material synthesis. One should also take
into account the fact that all of these results were investigated
from micron-level electrodes in symmetric cells, which is very
close to the actual application environment of supercapacitors.
Fig. 9 The dependence of capacitance on the current density of the
MnO electrodes in different electrolytes.
2
electrode materials in nanostructures. We believe that the 3-D
porous structure of the nano-MnO electrode here can
significantly facilitate the penetration of protons and the
4. Conclusions
2
In summary, we have accomplished a large-scale synthesis of
porous nano-MnO2 via a facile sonochemistry procedure
from inexpensive raw materials. The MnO2 nanoparticles
synthesized in this case are highly dispersed and fine nano-
5
9,60
transportation of electrons,
thus resulting in a facile
redox reaction and a fast charge/discharge in supercapacitor
applications. In addition, it is reasonable to suppose that the
structures. The good electrochemical properties of MnO
electrodes have been demonstrated both in 1 M Na SO and
M KOH aqueous solution. Furthermore, the simple process,
high yield and safe precursors of the synthetic procedure
indicate that porous nano-MnO should be a promising
2
2
nanopore structure (1–3 nm) of the MnO electrode could
2
4
also provide a desirable electric double layer capacitance in
1
symmetric electrode cells with aqueous electrolytes.
9
2
electrode material for supercapacitor applications.
Acknowledgements
Financial support by the National Natural Science
Foundation of China (nos. 20663001, 51064004 and
2
0763002) is gratefully acknowledged.
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ꢀ2
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74 New J. Chem., 2011, 35, 469–475
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