Textural and capacitive characteristics of MnO2 nanocrystals derived from a novel solid-reaction route

Article abstract of DOI:10.1016/j.electacta.2008.08.057

Nanostructured manganese dioxide (MnO₂) materials were synthesized via a novel room-temperature solid-reaction route starting with Mn(OAc)₂·4H₂O and (NH₄)₂C₂O₄·H₂O raw

Full text of DOI:10.1016/j.electacta.2008.08.057

Electrochimica Acta 54 (2009) 1021–1026
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Textural and capacitive characteristics of MnO2 nanocrystals derived
from a novel solid-reaction route
Anbao Yuan∗, Xiuling Wang, Yuqin Wang, Jie Hu
Department of Chemistry, College of Sciences, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 April 2008
Received in revised form 17 June 2008
Accepted 26 August 2008
Available online 5 September 2008
Nanostructured manganese dioxide (MnO2) materials were synthesized via a novel room-temperature
solid-reaction route starting with Mn(OAc)2·4H2O and (NH4)2C2O4·H2O raw materials. In brief, the vari-
ous MnO2 materials were obtained by air-calcination (oxidation decomposition) of the MnC2O4 precursor
at different temperatures followed by acid-treatment in 2 M H2SO4 solution. The influence of calcina-
tion temperature on the structural characteristics and capacitive properties in 1 M LiOH electrolyte of
the MnO2 materials were investigated by X-ray diffraction (XRD), infrared spectrum (IR), transmission
electron microscope (TEM) and Brunauer–Emmett–Teller (BET) surface area analysis, cyclic voltamme-
try, ac impedance and galvanostatic charge/discharge electrochemical methods. Experimental results
showed that calcination temperature has a significant influence on the textural and capacitive charac-
teristics of the products. The MnO2 material obtained at the calcination temperature of 300 ◦C followed
by acid-treatment belongs to nano-scale column-like (or needle-like) ␥,␣-type MnO2 mischcrystals.
While, the MnO2 materials obtained at the calcination temperatures of 400, 500, and 600 ◦C followed
by acid-treatment, respectively, belong to ␥-type MnO2 with the morphology of aggregates of crystallites.
The ␥,␣-MnO2 derived from calcination temperature of 300 ◦C exhibited a initial specific capacitance
lower than that of the ␥-MnO2 derived from the elevated temperatures, but presented a better high-rate
charge/discharge cyclability.
Keywords:
Nanostructured manganese dioxide
Calcination temperature
Textural characteristic
Pseudocapacitive behavior
Lithium hydroxide electrolyte
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
high specific capacitance, low cost and good environmental com-
patibility.
Differing from using mild aqueous electrolytes, we previ-
ously reported a novel nano-MnO2/activated carbon (AC) hybrid
supercapacitor using Li+ ions available 1 M LiOH alkaline aque-
ous electrolyte [29]. The pseudocapacitance mechanism of the
MnO2 positive electrode was suggested to be associated with the
reversible insertion/extraction of Li+ ions in MnO2 solid during the
reduction/oxidation process. In the prior work, the nanostructured
␥,␣-MnO2 was prepared by a solid-reaction route starting with
manganese acetate (Mn(OAc)2·4H2O) and citric acid (C6H8O7·H2O).
Later, we reported the supercapacitive behavior of a nanosized
␥-MnO2 in 1 M LiOH electrolyte [30]. The MnO2 material was syn-
thesized by a sol–gel method starting with Mn(OAc)2·4H2O and
C6H8O7·H2O, and its capacitive properties was compared with that
of the previously solid-reaction derived ␥,␣-MnO2. Most recently,
another nano-scale ␥,␣-MnO2 material was prepared using a solid-
reaction route starting with MnCl2·4H2O and NH4HCO3, and its
capacitive behavior for MnO2/AC capacitor with 1 M LiOH elec-
trolyte was reported [31]. We know from these studies that the
electrochemical performance of an MnO2 material could be well
correlated with its preparation method and crystal structure.
In the present work, a novel solid-reaction route starting with
Supercapacitors, as charge storage devices with the features of
high power and long cycle life, can be used in electrical equip-
ments and hybrid electric vehicles [1,2], and attracted increasing
attentions in recent years [3–7]. Compared to electric double layer
capacitors (EDLC), electrochemical capacitors with faradic reaction
can yield higher specific capacitance originated from the faradic
pseudocapacitance in addition to electric double layer contribution.
Hydrous ruthenium oxide (RuO2·nH2O) was reported to deliver a
high capacitance of 760 F g−1 in sulfuric acid electrolyte [8], but it is
considered too expensive to be commercially attractive. Therefore,
other metal oxides with lower cost were investigated as electrode
materials for electrochemical supercapacitor applications, such as
NiO, Co3O4, MnO2, etc. [9–11]. Recently, nanostructured MnO2
materials have been intensively studied as electrodes for super-
capacitors with mild aqueous electrolytes [12–28], owing to their
advantages of large specific area, wide operating potential range,
∗
Corresponding author. Tel.: +86 21 66134851; fax: +86 21 66132797.
E-mail address: abyuan@shu.edu.cn (A. Yuan).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.08.057

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A. Yuan et al. / Electrochimica Acta 54 (2009) 1021–1026
Mn(OAc)2·4H2O and (NH4)2C2O4·H2O raw materials was employed
to synthesize nanostructured MnO2 materials. The MnO2 mate-
rials were obtained from calcination of the MnC2O4 precursor at
different temperatures followed by acid-treatment in 2 M H2SO4
solution, and their textural and capacitive characteristics were
investigated as supercapacitor electrode materials in 1 M LiOH
electrolyte, especially, in respect to the influence of calcination
temperature.
and Hg/HgO (1 M LiOH) electrode as reference electrode. 1 M LiOH
aqueous solution was used as an electrolyte. Cyclic voltamme-
try and ac impedance tests were carried out using a Solartron
1287 Electrochemical Interface coupled with a 1255B Frequency
Response Analyzer. Charge/discharge tests were conducted using a
LAND auto-cycler (China).
3. Results and discussion
2. Experimental
3.1. TG–DSC analysis of MnC2O4 precursor
2.1. Synthesis of nanostructured manganese dioxide materials
Fig. 1 shows the TGA–DSC curves of the MnC2O4 precursor. For
the TGA curve, the weight loss occurred in the temperature range
of ca. 50–100 ◦C should be ascribed to the loss of surface absorbed
water. Because the sample material was exposed to the air for a
long time prior to TG–DSC analysis, some physically absorbed water
was likely introduced into the material. Corresponding to the loss
of absorbed water, an endothermic peak can be observed in the DSC
curve. In the temperature range of ca. 320–420 ◦C, the observed fast
loss of weight should be attributed to the oxidative decomposition
of the MnC2O4. Correspondingly, two intensive exothermic peaks
can be observed in the DSC curve at ca. 345 and 420 ◦C, respectively.
These are similar to the observation reported in the literature [32],
where two exothermic peaks were also observed in the DTA curve
for MnC2O4 sample. With respect to oxidative decomposition of
MnC2O4, the theoretical weight loss for the formation of differ-
ent manganese oxides, namely, MnO2, Mn2O3, Mn3O4 and MnO
are 39.18, 44.78, 46.64, and 50.37%, respectively. Based on the data
in Fig. 1, the calculated weight loss for MnC2O4 (in the tempera-
ture range of ca. 320–420 ◦C) is ca. 47%, which is very close to the
theoretical value of 46.64% (Mn3O4). In the temperature range of
420–719 ◦C, almost no weight change can be observed. Judging from
the thermal analysis results, the oxidative decomposition product
should be mainly attributed to Mn3O4. Hence, in order to increase
the degree of oxidation of the obtained manganese oxides, acid-
treatment method was applied. In the process of acid-treatment,
the Mn3O4 can be transformed to MnO2 via a disproportionation
reaction mechanism [29].
Mn(OAc)2·4H2O and (NH4)2C2O4·H2O with a mole ratio of 1:1.2
were mixed and well ground in a mortar. In the grinding process,
the solid reaction takes place. Ground for 40 min, the mixture was
transferred to a beaker and was placed in a water bath of 80 ◦C.
Several hours later, a dry mixture was obtained. The dry mixture
was washed with distilled water, filtered to remove the dissoluble
substances, dried in an oven at 110 ◦C for 10 h, and then the white
MnC2O4 precursor was obtained. The precursor was divided into
four parts and the individual parts were air-calcined at 300, 400,
500 and 600 ◦C, respectively, in a muffle furnace for 10 h for oxida-
tive decomposition. The calcined products (manganese oxides)
were subjected to acid-treatment in 2 M H2SO4 solution at 80 ◦C
for 2 h under magnetic stirring in order to increase their degree of
oxidation. After acid-treatment, the products were washed thor-
oughly with distilled water, filtered, and dried at 105 ◦C, and then
the final products (MnO2 materials) were obtained.
2.2. Thermal analysis of precursor and characterization of
products
Thermogravimetric–differential
scanning
calorimetric
(TG–DSC) analysis of the MnC2O4 precursor was performed on a
Netzsch STA-409 PG/PC thermogravimetric analyzer (Germany)
with a sample mass of 8.800 mg. The TG and DSC curves were
recorded in a static atmosphere at a heating rate of 10 ◦C min−1 in
the temperature range of 29–719 ◦C.
X-ray diffraction (XRD) analyses of the MnO2 products were
conducted on a Rigaku D/max-2000 X-ray powder diffractometer
with a radiation source of Cu K␣ (40 kV, 250 mA). The test sam-
ples were scanned in the 2Â range of 10–90◦ with a scan rate of
0.02◦ s−1. Infrared spectrum (IR) analyses of the MnO2 products
were performed on a Nicolet Avatar 370 FT-IR Fourier transfor-
mation infrared spectrometer (KBr pelleting) in the wave number
range of 4000–400 cm−1. The morphology observations of the
MnO2 products were carried out using a JEOL JEM–200CX transmis-
sion electron microscope (TEM). Brunauer–Emmett–Teller (BET)
surface areas analyses of the MnO2 materials were conducted on a
3H-2000III type full automation nitrogen-adsorption specific sur-
face instrument (China).
3.2. Structural characterization and morphology observation of
MnO2 materials
Fig.
2 shows the XRD patterns of the MnO2 materials
obtained from different calcination temperatures followed by
acid-treatment. For the three samples derived from the calci-
nation temperatures of 400, 500 and 600 ◦C, respectively, the
diffraction patterns are similar. The diffraction peaks occurred at
2.3. Preparation and electrochemical testing of nano-MnO2
electrodes
The MnO2 electrodes were prepared as follows: nano-MnO2
active material, acetylene black conductor and polytetrafluoroethy-
lene binder with a mass ratio of 80:15:5 were mixed completely to
form a slurry. The slurry was coated onto a foamed nickel with an
apparent area of 1 cm × 1 cm, and was dried at 70 ◦C for 12 h and
roll pressed to ca. 0.6 mm thick.
Electrochemical tests of the MnO2 electrodes were performed in
a three-electrode cell, with MnO2 electrode and activated carbon
electrode as working electrode and counter electrode, respectively,
Fig. 1. TGA and DSC curves of MnC2O4 precursor.

A. Yuan et al. / Electrochimica Acta 54 (2009) 1021–1026
1023
calcination temperature. In addition to the TEM observations, the
BET surface areas of the MnO2 materials derived from the calcina-
tion temperatures of 300, 400, 500 and 600 ◦C are 136.9, 78.1, 72.4
and 63.3 m2 g−1, respectively, which are consistent with the mor-
phological observations in Fig. 4. That is to say, the particle growth
and agglomeration should be responsible for the reduction of BET
area at the elevated calcination temperatures.
3.3. Electrochemical characteristics of nano-MnO2 electrodes
Fig. 5 shows the cyclic voltammetry (CV) profiles of the MnO2
electrodes (derived from different calcination temperatures) in 1 M
LiOH electrolyte at a scan rate of 1 mV s−1. The specific current val-
ues (in A g−1) are calculated based on the mass of the active MnO2.
It can be seen that all the CVs show an oxidation peak around 0.52 V
and a reduction peak around 0.22 V (vs. Hg/HgO), which are con-
sistent with the previous studies [29–31], and can be attributed to
the electrochemical Li+ ions intercalation/deintercalation in MnO2
electrodes. Judging from the current responses, the specific capac-
itance of the MnO2 derived from the calcination temperature of
500 ◦C is the highest, which is little higher than that of the MnO2
derived from the calcination temperatures of 400 or 600 ◦C. These
current responses are higher than the previously reported results at
the same scan rate [29–31], suggesting the higher specific capaci-
tances of the presently synthesized materials. However, the specific
capacitance of the MnO2 derived from the calcination temperature
of 300 ◦C is obviously lower than that of the others. Although the
BET surface area of the ␥,␣-MnO2 obtained at 300 ◦C is much higher
than that of the ␥-MnO2 obtained at the elevated temperatures, the
specific capacitance of the former is obviously lower than that of
the latter. This may be related to their different crystal structures
and water contents in the structures.
Fig. 2. XRD patterns of MnO2 materials derived from different calcination temper-
atures.
22.36◦, 37.06◦, 42.52◦, 56.14◦ and 67.12◦, respectively should be
attributed to orthorhombic ␥-MnO2 (PDF no. 14-0644). Except for
these characteristic peaks, no additional peaks could be observed,
indicating the pure phase of ␥-MnO2. However, the diffraction pat-
tern for the sample derived from the calcination temperature of
300 ◦C is quite different. The diffraction peaks occurred at 22.36◦,
37.42◦, 41.70◦ and 56.14◦, respectively should be attributed to
␥-MnO2, while, the diffraction peaks occurred at 12.56◦, 17.82◦,
28.56◦, 37.42◦ (overlapped with the ␥-MnO2 diffraction), 49.68◦,
59.96◦, 65.26◦, 69.08◦ and 72.72◦, respectively should be indexed to
tetragonal ␣-MnO2 hydrate (PDF no. 44-0140). Hence, this sample
should be ascribed to a mixture of ␥- and ␣-MnO2.
Fig. 3 shows the FT-IR spectra of the MnO2 materials derived
from different calcination temperatures. Several absorption bands
can be observed at 3420, 1630, 1400, 1110, 560 and 530 cm−1
,
Fig. 6 shows the ac impedance plots of the MnO2 electrodes
(derived from different calcination temperatures) in 1 M LiOH elec-
trolyte measured at discharged state. The impedance values (in
ꢀ g) are calculated based on the mass of the active MnO2. All the
impedance plots consist of an arc and a slope line. The arcs at higher
frequency region should be attributed to the charge transfer resis-
tance at electrode/electrolyte interface in parallel with the electric
double layer capacitance, and the slope lines at lower frequency
region should be ascribed to the diffusion impedance of Li+ ions
in MnO2 solid. It can be seen from Fig. 6 that the charge transfer
resistance decreases with calcination temperature increasing. The
overall impedance of the ␥,␣-MnO2 electrode (derived from the cal-
cination temperature of 300 ◦C) is obviously larger than that of the
others, suggesting a lower electrochemical activity of the material.
respectively. The 3420 cm−1 band should be attributed to the O–H
stretching vibration, and the 1630, 1400 and 1110 cm−1 bands are
usually attributed to the O–H bending vibrations combined with
Mn atoms. While the 560 and 530 cm−1 bands should be ascribed
to the Mn–O vibrations in MnO6 octahedra [33]. The IR result sug-
gests the presence of somewhat bound water in the MnO2 structure,
which is generally believed to be favorable to the electrochem-
ical activity of a MnO2 material. It can be seen from Fig. 3 that
the absorption peak intensity at 3420 band for the MnO2 material
derived from the calcination temperature of 300 ◦C (␥,␣-MnO2) is
obviously lower than that of the other samples derived from the
elevated calcination temperatures (␥-MnO2). These results suggest
that the free water content in the former is likely lower than that
in the latter.
The TEM patterns of the MnO2 materials derived from dif-
ferent calcination temperatures are displayed in Fig. 4. It can be
seen from Fig. 4(a) that the ␥,␣-MnO2 derived from the calcina-
tion temperature of 300 ◦C possesses the morphology of isolated
column-like or needle-like crystallites with a length around 100 nm
and a diameter less than 30 nm. From the above XRD results, we
know that when the calcination temperature increased from 300
to 400 ◦C or above, the microstructure of the MnO2 changed from
␥,␣-MnO2 to pure ␥-form. Accordingly, seen from Fig. 4 we know
that when the calcination temperature increased to 400 ◦C, the
obtained ␥-MnO2 presents the morphology of petal-like aggre-
gates consisting of small crystallites. By comparing Fig. 4(b)–(d),
it can be found that the degree of aggregation increases with the
calcination temperature increasing. The ␥-MnO2 derived from the
calcination temperatures of 500 and 600 ◦C are the closely spaced
or compact aggregation of thickset column-like crystallites with
coarse morphology. These results indicate that the degree of nano-
agglomeration in the calcination process increases with increase in
Fig. 3. FT-IR spectra of MnO2 materials derived from different calcination temper-
atures.

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A. Yuan et al. / Electrochimica Acta 54 (2009) 1021–1026
Fig. 4. TEM patterns of MnO2 materials derived from calcination temperatures of: (a) 300 ◦C, (b) 400 ◦C, (c) 500 ◦C, and (d) 600 ◦C.
The specific capacitances of the MnO2 electrodes in 1 M LiOH
electrolyte at different charge/discharge current rates are shown
in Fig. 7. The specific capacitance values (in F g−1) are calcu-
lated based on the mass of the active MnO2. The electrode with
the MnO2 material derived from the calcination temperature of
600 ◦C exhibits the best rate charge/discharge performance, with
a specific capacitance reduction by 27.9% as the current rate
increased from 100 to 1000 mA g−1. While, the electrode with
the MnO2 material obtained at 300 ◦C presents the worst rate
charge/discharge behavior, with a specific capacitance reduction by
43.4%. These results are consistent with the ac impedance results
in Fig. 6. However, the discharge specific capacitance of the MnO2
derived from the calcination temperature of 500 ◦C is much higher
than that of the MnO2 derived from the calcination temperature
of 300 ◦C, and is little higher than that of the MnO2 obtained
at 400 and 500 ◦C, which is consistent with the CV results in
Fig. 5.
The representative charge/discharge profiles of the MnO2 elec-
trodes at the current rates of 500 and 1000 mA g−1 are present
in Fig. 8(a) and (b), respectively. It can be seen from Fig. 8(a)
that the charge and discharge profiles of the electrodes are
not standard straight lines, but are somewhat curved, exhibit
a pseudocapacitive characteristic. This is in accord with the CV
result in Fig. 5, where a couple of redox current peaks can be
observed. The charge–discharge durations for the four electrodes
derived from the different calcination temperatures are as follows:
500 > 600 > 400 > 300 ◦C, which is consistent with the result in Fig. 7.
In Fig. 8(b), the electric potential drops (when changed from charge
to discharge) for the electrodes are increased compared with that in
Fig. 8(a), due to the increase in current rate. The potential drops fol-
low this order: 600 > 500 > 400 > 300 ◦C, which is in agreement with
the ac impedance results in Fig. 6. In addition, when the current rate
increased from 500 to 1000 mA g−1, the charge–discharge dura-
tions for the three electrodes derived from the elevated calcination
Fig. 5. CVs of different MnO2 electrodes in 1 M LiOH electrolyte at a scan rate of
1 mV s−1
.
Fig. 6. Nyquist plots of different MnO2 electrodes in 1 M LiOH electrolyte.

A. Yuan et al. / Electrochimica Acta 54 (2009) 1021–1026
1025
Fig. 9. Charge/discharge cycle-life profiles of different MnO2 electrodes at a current
rate of 1000 mA g−1
Fig. 7. Current rate dependence of specific capacitance of different MnO2 electrodes
in 1 M LiOH electrolyte.
.
only by 10% in the 1000 cycles, exhibiting a good cyclability. While,
the initial specific capacitances of the electrodes with the MnO2
materials obtained at 400, 500 and 600 ◦C, respectively are all
around 200 F g−1, which are much higher than that of the MnO2
obtained at 300 ◦C. However, they exhibit different cycle stabili-
ties. For the electrodes with the MnO2 obtained at 400 and 600 ◦C,
the specific capacitances undergo a fluctuant variation. In the end
of cycling, their specific capacitances approach that of the MnO2
obtained at 300 ◦C. The MnO2 electrode derived from the calcina-
tion temperature of 500 ◦C presents the worst cyclability, only a
capacitance of 89 F g−1 can be maintained in the end, decreased
by 56% in the 1000 cycles. In point of cycle stability, the MnO2
derived from the calcination temperature of 300 ◦C exhibits the
best charge/discharge stability, but its initial specific capacitance
is lower than that of the others.
temperatures approach uniform, while the charge–discharge dura-
tion for the 300 ◦C derived electrode is obviously short than that of
the others. This is consistent with the result in Fig. 7.
Fig. 9 shows the charge/discharge cycle-life profiles of the MnO2
electrodes (derived from different calcination temperatures) at a
current rate of 1000 mA g−1 in the operating potential range of
0–1 V (vs. Hg/HgO). The specific capacitance values (in F g−1) are
calculated based on the mass of the active MnO2. For the electrode
with the MnO2 material derived from the calcination temperature
of 300 ◦C, the specific capacitance increased gradually in the initial
200 cycles, from the initial value of ca. 140 F g−1 to a maximal value
near 150 F g−1. Thereafter, the specific capacitance decreased grad-
ually to ca. 126 F g−1 in the end. The specific capacitance decreased
4. Conclusions
The influence of calcination temperature on the structure and
electrochemical performance of the solid-reaction derived MnO2
materials were investigated in this work. XRD and TEM results
revealed that the nano-MnO2 material derived from the calcina-
tion temperature of 300 ◦C possesses the column-like mixture of
␥- and ␣-type crystal structure. While, the nano-MnO2 materials
obtained at the elevated calcination temperatures of 400, 500, and
600 ◦C possess the ␥-form structure with aggregates of crystallites.
BET surface area of the MnO2 decreases with calcination temper-
ature increasing, which is in agreement with the morphological
observations. However, the ␥,␣-MnO2 derived from the calcination
temperature of 300 ◦C showed a specific capacitance lower than
that of the ␥-MnO2 derived from the elevated calcination tempera-
tures, but exhibited a better high-rate charge/discharge cyclability.
These results demonstrated that calcination temperature has a sig-
nificant influence on the crystal structures and electrochemical
performance of the nanostructured manganese dioxide materials
in 1 M LiOH electrolyte.
Acknowledgment
This work was supported by Leading Academic Discipline
Project of Shanghai Municipal Education Commission (Project
Number: J50102).
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