Electrochemical and capacitance properties of rod-shaped MnO2 for supercapacitor

Article abstract of DOI:10.1149/1.1904912

A novel kind of electrode material, rod-shaped MnO₂, was synthesized under high viscosity for the first time. The MnO₂ was proved to be the multicrystal with the mixture of α- and γ-MnO ₂ by X-ray diffraction. From transmission electron micoscopy, it could be seen that the sample has a length of 250 - 350 nm and a diameter of 10 - 15 nm. The ratio of length to diameter could reach more than 20:1. The electrochemical properties were examined by cyclic voltammograms (CV), galvanostatic charge/ discharge, and electrochemical impedance spectroscopy (EIS) in three electrode cell. According to CV results, 2 mol L^-1 (NH₄)₂SO₄ aqueous solution was a proper electrolyte for MnO₂ supercapacitors. A maximum specific capacitance (SC) of 398 F g^-1 was obtained from galvanostatic charge/discharge curve in 2 mol L^-1 (NH₄)₂SO₄ solution at constant current of 10 mA. The SC value was 328 F g^-1 at a high current of 50 mA which exhibit high power characteristics of the rod-shaped MnO₂. The electrolyte resistance was 0.21 Ω cm ² by researching on EIS. Long cycle-life and high coulombic efficiency of rod-shaped MnO₂ were also demonstrated by charging and discharging 1000 cycles.

Full text of DOI:10.1149/1.1904912

Electrochemical and Capacitance Properties of Rod-Shaped MnO2
for Supercapacitor
Chen Ye, Zhang Mi Lin and Shi Zhao Hui
J. Electrochem. Soc. 2005, Volume 152, Issue 6, Pages A1272-A1278.
doi: 10.1149/1.1904912
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A1272
Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
0013-4651/2005/152͑6͒/A1272/7/$7.00 © The Electrochemical Society, Inc.
Electrochemical and Capacitance Properties of Rod-Shaped
MnO2 for Supercapacitor
Chen Ye,z Zhang Mi Lin, and Shi Zhao Hui
School of Chemical Engineering, Harbin Engineering University, Harbin 150001, China
A novel kind of electrode material, rod-shaped MnO2, was synthesized under high viscosity for the first time. The MnO2 was
proved to be the multicrystal with the mixture of ␣- and ␥-MnO2 by X-ray diffraction. From transmission electron micoscopy, it
could be seen that the sample has a length of 250 ϳ 350 nm and a diameter of 10 ϳ 15 nm. The ratio of length to diameter could
reach more than 20:1. The electrochemical properties were examined by cyclic voltammograms ͑CV͒, galvanostatic charge/
discharge, and electrochemical impedance spectroscopy ͑EIS͒ in three electrode cell. According to CV results, 2 mol L−1
͑NH4͒2SO4 aqueous solution was a proper electrolyte for MnO2 supercapacitors. A maximum specific capacitance ͑SC͒ of
398 F g−1 was obtained from galvanostatic charge/discharge curve in 2 mol L−1 ͑NH4͒2SO4 solution at constant current of 10 mA.
The SC value was 328 F g−1 at a high current of 50 mA which exhibit high power characteristics of the rod-shaped MnO2. The
electrolyte resistance was 0.21 ⍀ cm2 by researching on EIS. Long cycle-life and high coulombic efficiency of rod-shaped MnO2
were also demonstrated by charging and discharging 1000 cycles.
© 2005 The Electrochemical Society. ͓DOI: 10.1149/1.1904912͔ All rights reserved.
Manuscript submitted September 20, 2004; revised manuscript received February 9, 2005. Available electronically May 13, 2005.
Recently, supercapacitors have drawn much attention for their
Experimental
higher power density compared to batteries and higher energy den-
sity compared to common capacitors.1,2 Moreover, as a new kind of
energy storage device, the supercapacitor has long cycle life, high
efficiency, and is environmentally friendly. Supercapacitors are
based on utilization of conducting materials having large specific
areas, such as carbon powder,3 aerogels,4 and nanotube,5 and also
conducting metal oxides ͑RuO2,MnO2,NiO͒,6-8 or conducting
polymers.9 At the carbon surface, the capacitance is mainly from
electrostatic double layer charging. Activated carbon capacitors are
In this work all reagents were obtained guaranteed as analysis
pure grade and used without further purification and treatment. The
molecular weight of polyethylene glycol ͑PG͒ was 6000 and poly-
acrylamide ͑PAM͒ was 14,000,000.
In a typical experiment, some starting
Preparation of MnO2.—
solutions, such as aqueous solution KMnO4 ͑0.5 mol L−1͒, MnCl2
͑2 mol L−1͒, PG ͑1%,w/w͒, and PAM ͑0.1%,w/w͒, were prepared
prior to use. At first, 150 mL KMnO4 and 10 mL PG solutions were
mixed and the pH value was adjusted to 6 ϳ 8 by NaOH solution
͑10%,w/w͒ in a flask. Then, PAM solution was introduced to con-
trol the viscosity of the mixture to 40 ϳ 50 MPa s at 65°C. At last
60 mL MnCl2 solution was added slowly into the system under
strong stirring. The obtained solution was aged for 12 h. After that,
the precipitation was filtered and washed carefully by hot deionized
water. The precipitation was filtered again and treated by steam for 3
h. Finally it was dried in vacuum at 80°C and the black powder
MnO2 was obtained.
typical double layer capacitors, and the specific capacitance ͑SC͒
10
value was about 125 F g−1
.
But at metal oxides, it arose from
redox pseudocapacitance couple with double layer capacitance. The
hydrous ruthenium oxides was reported as the most promising ma-
terials with SC value of 863 F g−1.6 So the energy density of elec-
trochemical supercapacitors based on a combination of faradaic
pseudocapacitance and double layer capacitance is higher than that
of double layer capacitors.11 But the application of noble metal ox-
ide capacitors are limited due to high cost and carbon-based capaci-
tors have large internal resistance in high working current.10 The
transition metal oxides with several oxidation states are considered
as the most promising materials for supercapacitor electrodes and
may show excellent pseudocapacitor characteristics. Currently, some
transition metal oxides, such as MnO2, NiO and Co3O4, are under
development.8,12,13
Among these oxides, tetravalent manganese oxide ͑MnO2͒ is one
of the most promising materials in many technological
applications.7,14 For its high positive electrode potential and relative
low molecular weight, MnO2 successfully acts as the electrode in
high energy density lithium batteries15 and supercapacitors.12 Some
researchers have reported on the preparation and application of
MnO2.16-18 However, electrochemical properties of nanostructured
MnO2 strongly depend on its dimensionality, powder morophology,
crystalline structure, and bulk density.13 In this paper, the rod-shaped
MnO2 was synthesized under high viscosity for the first time. The
sample was characterized by X-ray diffraction ͑XRD͒, transmission
electron microscopy ͑TEM͒, and high-resolution TEM ͑HRTEM͒.
The relevant electrochemical properties were then investigated by
means of cyclic voltammetry ͑CV͒, galvanostatic charge/discharge,
and electrochemical impedance spectroscopy ͑EIS͒.
Characterization.—XRD data was collected on Rigaku
D/Max-A diffractometer using Cu K␣ radiation at wavelength
1.54178 nm ͑30 KV, 20 mA͒. The diffractogram was obtained in the
2␪ range of 10-70° with divergence and scatter slits of 1°. The step
interval was 0.1° and the scans speed was 0.1°/s. HRTEM of MnO2
was studied by a JEM-2010 microscope, operating at an acceleration
voltage of 200 kV. The electron diffraction ͑ED͒ pattern was also
measured by the electron microscope. To prepare the
electrode, MnO2, graphite, acetylene black, and polytetrafluoroeth-
ylene ͑70:15:10:5, w/w͒ were mixed in absolute ethanol and the
resulting mixture was pressed on macroporous nickel current collec-
tor under 12 MPa. The electrode was tested by CV at various sweep
rate and galvanostatic charge/discharge at different currents in
2 mol·L−1 ͑NH4͒2SO4 solution using a conventional three-electrode
cell. MnO2 was used as the working electrode, platinum sheet was
used as the counter electrode, and Hg/HgO coupled with a luggin
capillary was used as the reference electrode. The impedance spec-
troscopy analyzer, IM6e ͑Zahner, Germany͒ with Thales software
was employed to measure and analyze the impedance spectra. The
potential amplitude was kept as 5 mV and a wide frequency range
from 50 mHz to 60 kHz was used.
Results and Discussion
Figure
1 shows the
Characteristics of rod-shaped MnO2.—
XRD pattern of rod-shaped MnO2. It was found that the sample was
composed of two phase, which are ␣-MnO2 ͑JCPDS 44-0141͒ and
␥-MnO2 ͑JCPDS 14-0644͒. The diffraction peaks at 12.4, 17.7, 28.4,
z E-mail: chenye@vip.0451.com
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Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
A1273
Figure 1. XRD pattern of rod-shaped MnO2.
37.2, 49.8, 59.8, and 69.3 are assigned to ␣-MnO2 and 41.8, 56.0,
and 65.1 to ␥-MnO2. All peaks in the pattern can be attributed to
various reflections.
Figure 2 is a TEM image of MnO2. From the TEM image, it can
be seen that the sample is rod shaped. The rod has the average
length of 250 ϳ 350 nm and a diameter of 10 ϳ 15 nm. The ratio
of length to diameter could reach more than 20:1.
The HRTEM micrograph and ED photo of MnO2 are shown in
Fig. 3. The ED photo accounts for a strong crystalline of the rod-
shaped MnO2. It also could be observed that the MnO2 is not a
single phase which is consistent with the result of XRD. The HR-
TEM result reveals that the rod-shaped structure of the sample is
Figure 3. ͑a͒ ED photo; ͑b͒ HRTEM morphology of MnO2.
Figure 2. TEM of rod-shaped MnO2.
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A1274
Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
Figure 4. CV curves of MnO2 in different aqueous electrolytes with 10 mV/s ͑a͒ 2.0 mol L−1 ͑NH4͒2SO4, ͑b͒ 2.0 mol L−1 Na2SO4, ͑c͒ 0.5 mol L−1 K2SO4, ͑d͒
2.0 mol L−1 NH4NO3, ͑e͒ 2.0 mol L−1 NaNO3, and ͑f͒ 2.0 mol L−1 KNO3.
indeed partially crystalline, obvious amorphous domain are also
shown. The d-spacing between the crystal fringes is measured in
corresponding magnified inset, determined values in the range of
7.0 ϳ 7.5 Å.
Electrochemical properties in aqueous electrolytes.—For su-
percapacitors, electrolyte is another important factor. In this work,
some neutral aqueous solutions, such as 2.0 mol L−1 Na2SO4,
0.5 mol L−1 K2SO4, 2.0 mol L−1 ͑NH4͒2SO4, 2.0 mol L−1 KNO3,
2.0 mol L−1 NaNO3, and 2.0 mol L−1 NH4NO3, were chosen as
electrolytes for supercapacitors. Figure 4 shows the CV curves of
MnO2 electrode in different aqueous electrolytes between 0
ϳ 0.9 V at 10 mV/s. All these CV curves show nearly rectangular
shape, which could be attributed to the good electrochemical prop-
erties of rod-shaped MnO2. The above results indicate that the su-
percapacitor may show good charge/discharge properties and rod-
shaped MnO2 is a promising electrode material.
In Fig. 4a-c, all measured curves show good capacitive behaviors
in different sulfate solutions. The same phenomenon can be ob-
served in nitrate solutions. Because the specific capacitance of sul-
fate solutions are larger than that of nitrate solutions, sulfate solution
is more suitable than nitrate solution. Furthermore, the CV proper-
ties are similar in electrolytes with same cations. In Fig. 4a and d, no
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Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
A1275
Figure 5. Galvanostatic charge/discharge curves of MnO2 in different electrolytes ͑a͒ 2.0 mol L−1 ͑NH4͒2SO4, ͑b͒ 0.5 mol L−1 K2SO4, ͑c͒ 2.0 mol L−1 Na2SO4,
͑d͒ 2.0 mol L−1 ͑NH4͒2NO3, ͑e͒ 2.0 mol L−1 KNO3, and ͑f͒ 2.0 mol L−1 NaNO3.
obvious redox peaks are shown in the range between 0 and 0.9 V
when the electrolytes with the same NH4 cations. In Fig. 4b and e,
haviors are not better than ͑NH4͒2SO4 solution, which may be
caused by their relatively low conductivity and solubility. Further
work is needed to clarify and explain the mechanism.
From CV results, ͑NH4͒2SO4 solution was the most promising
electrolyte for rod-shaped MnO2 supercapacitors.
Galvanostatic charge/discharge is a more reliable method for get-
ting the specific capacitance than CV. Figure 5 shows the charge/
discharge curves of MnO2 in different electrolytes at 10 mA. All
charge/discharge curves are almost linear. The charge/discharge spe-
cific capacitance was calculated from the liner part of charge/
discharge curves using
+
a reduction peak at 600 mV could be seen and the corresponding
oxidation peaks could be nearly observed. In Fig. 4c and f, the redox
peaks at 100 and 700 mV could be seen clearly. The charge/
discharge mechanism of MnO2 in aqueous solution is a complicate
process. During the charge/discharge procedure, some researchers
described the process as following reversible reaction19
charge
MnO2 + H+ + e
→ MnOOH
͓1͔
discharge
But the whole charge/discharge procedure might be also influ-
enced by cations presented in electrolytes.11,20 In this work, different
+
C = ͑it͒/͑mV͒
͓2͔
cations in electrolytes exhibited different results. In NH4 solutions,
+
the NH4 could provide more H+ than other solutions which make it
where i is the constant current, ⌬t is the time internal for charge/
discharge in voltage ⌬V,m is the weight of MnO2 and C is specific
capacitance.
possible for supercapacitor fast charge/discharge process. In other
CV results, although the CV is close to rectangle, capacitance be-
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A1276
Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
Table I. Specific capacitance and coulombic efficiency of rod-shaped MnO2 in different aqueous electrolytes at different constant currents.
Charge
Discharge
Coulombic
efficiency
͑%͒
capacitance
capacitance
͑F g−1
͒
͑F g−1
͒
2.0 mol L−1 ͑NH4͒2SO4
0.5 mol L−1 K2SO4
2.0 mol L−1 Na2SO4
2.0 mol L−1 ͑NH4͒2NO3
2.0 mol L−1 KNO3
405
357
345
355
358
363
398
338
323
339
327
325
98.3
94.7
93.6
95.5
91.3
89.3
10 mA
50 mA
2.0 mol L−1 NaNO3
2.0 mol L−1 ͑NH4͒2SO4
0.5 mol L−1 K2SO4
2.0 mol L−1 Na2SO4
2.0 mol L−1 ͑NH4͒2NO3
2.0 mol L−1 KNO3
352
332
311
325
308
313
328
302
276
298
268
275
93.2
91.0
88.7
91.7
87.0
87.9
2.0 mol L−1 NaNO3
The values of coulombic efficiency are calculated according to
i
i
C =
=
d␸
m
v
m
dx
td
␩ =
͓3͔
tc
where i is current, is scan speed, C is specific capacitance, and m
v
is mass of MnO2. From the equation, the value of specific capaci-
tance is 389, 351, 318, and 273 F g−1 at sweep rate from 10 to
80 mV/s. The results demonstrated the linear relationship of current
to sweep rate and indicated high power properties for rod-shaped
MnO2 supercapacitors.
Figure 7 shows CV curves with the variation of ͑NH4͒2SO4 con-
centration increased from 1 mol L−1 to saturated solution. The value
of specific capacitance is 367, 389, 392, and 397 F g−1. The higher
the concentration is, the larger the specific capacitance is. The
͑NH4͒2SO4 would be separated from the aqueous solution when the
concentration is higher than 2 mol L−1. So 2 mol L−1 ͑NH4͒2SO4
aqueous solution was a appropriate electrolyte.
The cyclic characteristics of MnO2 supercapacitor was also ex-
amined by galvanostatic charge/discharge at 10 mA over 1000
cycles. Figure 8 shows the relationship of cycle number to discharge
capacitance and coulombic efficiency. After 1000 cycles, it could be
seen that the charge loss does not exceed 10%, the specific capaci-
tance decreased form 398 F g−1 to 364 F g−1. The first cyclic cou-
lombic efficiency is only 97.8%, but it reached to 99.5% after 1000
cycles. The results reveal that supercapacitors have stable coulombic
where td and tc are the discharge and charge time.
The capacitance values are listed in Table I. The results of gal-
vanostatic charge/discharge are consistent with CV results. From
Table I, the specific capacitance could reach to 398 F g−1 and the
coulombic efficiency is 98.3% in 2 mol L−1 ͑NH4͒2SO4 solution at
10 mA. When the constant current increased from 10 to 50 mA, the
specific capacitance and coulmobic efficiency decreased simulta-
neously.
Figure
6
Electrochemical properties in (NH4)2SO4 solution.—
shows the CVs of MnO2 electrode with sweep rates from 10 to
80 mV/s in a potential range between 0 ϳ 0.9 V in 2 mol L−1
͑NH4͒2SO4. It can be seen clearly that there are no redox peaks in
the working potential. The CVs present a slight deviation from rect-
angular form and exhibit symmetrical shape. The large magnitudes
of current are not influenced by different sweep rates which are the
characteristics of the idea capacitor. The voltammograms reveal the
charge transfer at the interface between MnO2 and electrolyte and
show the typical behavior of supercapacitors.
From CV curves the specific capacitance could be calculated by
Figure 7. CV of MnO2 electrode with different concentration with 10mV/s.
Figure 6. CV of MnO2 electrode with different sweep rates.
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Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
A1277
Figure 8. Relationship of specific capacitance and coulombic efficiency with
cycle number for MnO2 electrode in 2 mol L−1 ͑NH4͒2SO4.
efficiency over 1000 cycles. The long cycle-life prove that the su-
percapacitor has good supercapacitor properties of high electro-
chemical stability and high reversibility.
Figure 10. Electrochemical impedance spectra of MnO2 in 2.0 mol L−1
͑NH4͒2SO4 after 1000 cycles ͑a͒ EIS from 50 mHz to 60 kHz and ͑b͒ EIS in
the high-frequency region.
The electrochemical behaviors of MnO2 are also investigated by
EIS. The Nyquist diagram for MnO2 in 2 mol L−1 ͑NH4͒2SO4 solu-
tion is exhibited in Fig. 9. A single semicircle in the high-frequency
region and a straight line in the low-frequency region could be ob-
served clearly. From the point intersecting with the real axis in high-
frequency resistance of electrolyte was about 0.21 ⍀ cm2 for
͑NH4͒2SO4 solution under this condition. In the low-frequency re-
gion, the Nyquist diagram shows a 45° slope relating close to that of
the Warbug impedance which corresponding to porous nature of
nano-MnO2.21
Figure 10 shows the EIS of MnO2 supercapacitor after 1000
cycle. Only a quarter circle in high-frequency region could be ob-
served and the slop line didn’t change in the low-frequency region.
The total resistance of the bulk electrolyte is 0.98 ⍀ cm2 after long
cycles.
Conclusion
The rod-shaped MnO2 was synthesized under high viscosity for
the first time. The MnO2 was characterized by XRD, TEM, and
HRTEM. It was proved to be the mixture of ␣- and ␥-MnO2. The
TEM image reveals that the sample has a length of 250 ϳ 350 nm,
a diameter of 10 ϳ 15 nm, and that the ratio of length to diameter
could reach more than 20:1. The electrochemical properties of rod-
shaped MnO2 were studied in a three-electrode cell. Six different
electrolytes were chosen for their electrochemical properties. Fi-
nally, the ͑NH4͒2SO4 solution was chosen as the electrolyte from its
Figure 9. Electrochemical impedance spectra of MnO2 in 2.0 mol L−1
͑NH4͒2SO4 ͑a͒ EIS from 50 mHz to 60 kHz and ͑b͒ EIS in the high-
frequency region.
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Journal of The Electrochemical Society, 152 ͑6͒ A1272-A1278 ͑2005͒
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CV characteristics and galvanostatic charge/discharge value. The
maximum specific capacitance was 398 F g−1 at 10 mA. The elec-
trochemical properties were also investigated carefully in
͑NH4͒2SO4 solution. The best concentration of ͑NH4͒2SO4 solution
is 2.0 mol L−1. The CV results demonstrate that the rod-shaped
MnO2 has high power density. In EIS spectra, the electrolyte resis-
tance value is 0.21 ⍀ cm2. After 1000 charge/discharge cycles, the
synthesized MnO2 shows high efficiency and stability. Although fur-
ther research should be undertaken to clarify the synthesis mecha-
nism and electrochemical properties, the rod-shaped MnO2 is un-
doubtedly a promising material for supercapacitor.
Acknowledgments
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The authors thank the Hei Long-jiang province key scientific and
technological projects for financial support ͑no. GB01A2050͒.
Harbin Engineering University assisted in meeting the publication costs
of this article.
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