Synthesis and characterization of nano- mn O2 for electrochemical supercapacitor studies

Article abstract of DOI:10.1149/1.2800163

Nanostructured Mn O2 was synthesized at ambient condition by reduction of potassium permanganate with aniline. Powder X-ray diffraction, thermal analysis (thermogravimetric and differential thermal analysis), Brunauer-Emmett-Teller surface area, and infrared spectroscopy studies were carried out for physical and chemical characterization. The as-prepared Mn O2 was amorphous and contained particles of 5-10 nm diameter. Upon annealing at temperatures >400°C, the amorphous Mn O2 attained crystalline α -phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods is evident from scanning electron microscopy and transmission electron microscopy (TEM) studies. High-resolution TEM images suggested that nanoparticles and nanorods grow in different crystallographic planes. Capacitance behavior was studied by cyclic voltammetry and galvanostatic charge-discharge cycling in a potential range from -0.2 to 1.0 V vs SCE in 0.1 M sodium sulfate solution. Specific capacitance of about 250 F g-1 was obtained at a current density of 0.5 mA cm-2 (0.8 A g-1).

Full text of DOI:10.1149/1.2800163

Synthesis and Characterization of Nano- MnO2 for Electrochemical
Supercapacitor Studies
P. Ragupathy, H. N. Vasan and N. Munichandraiah
J. Electrochem. Soc. 2008, Volume 155, Issue 1, Pages A34-A40.
doi: 10.1149/1.2800163
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A34
Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
0
013-4651/2007/155͑1͒/A34/7/$23.00 © The Electrochemical Society
Synthesis and Characterization of Nano-MnO2
for Electrochemical Supercapacitor Studies
a
a,z
b,
P. Ragupathy, H. N. Vasan, and N. Munichandraiah *
a
b
Solid State and Structural Chemistry Unit, and Department of Inorganic and Physical Chemistry,
Indian Institute of Science, Bangalore 560012, India
Nanostructured MnO was synthesized at ambient condition by reduction of potassium permanganate with aniline. Powder X-ray
2
diffraction, thermal analysis ͑thermogravimetric and differential thermal analysis͒, Brunauer–Emmett–Teller surface area, and
infrared spectroscopy studies were carried out for physical and chemical characterization. The as-prepared MnO was amorphous
2
and contained particles of 5–10 nm diameter. Upon annealing at temperatures Ͼ400°C, the amorphous MnO attained crystalline
2
␣
-phase with a concomitant change in morphology. A gradual conversion of nanoparticles to nanorods is evident from scanning
electron microscopy and transmission electron microscopy ͑TEM͒ studies. High-resolution TEM images suggested that nanopar-
ticles and nanorods grow in different crystallographic planes. Capacitance behavior was studied by cyclic voltammetry and
galvanostatic charge–discharge cycling in a potential range from −0.2 to 1.0 V vs SCE in 0.1 M sodium sulfate solution. Specific
capacitance of about 250 F g− was obtained at a current density of 0.5 mA cm ͑0.8 A g ͒.
1
−2
−1
©
2007 The Electrochemical Society. ͓DOI: 10.1149/1.2800163͔ All rights reserved.
Manuscript submitted April 25, 2007; revised manuscript received September 19, 2007.
Available electronically November 5, 2007.
Electrochemical capacitors ͑ECs͒ or supercapacitors ͑SCs͒ are
charge storage devices. They have gained enormous attention due to
their potential applications ranging from mobile devices to electric
vehicles ͑EVs͒. SCs are broadly classified into two categories: elec-
trical double-layer capacitors ͑EDLCs͒ and faradaic pseudocapaci-
tors.
Experimental
Preparation of MnO2 powder.— Analytical-grade chemicals
such as KMnO4, aniline, and Na2SO4 were procured from Ranbaxy
Fine Chemical, Ltd., India, and were used as such without any fur-
1
ther purification. First, 3.16 g of KMnO was dissolved in 200 mL
4
of distilled water. To this, 1 mL of aniline was added upon stirring
for about 20 min. Thus, the mole ratio of KMnO to aniline was 2:1.
A brown color precipitate was obtained and centrifuged, and the
precipitate was washed copiously with distilled water several times
followed by acetone. It was dried in a vacuum oven at around 50°C
The synthesis and characterization of nanomaterials, e.g.,
2
,3
4,5
6,7
4
nanowires, nanotubes, or nanorods, have become important in
the field of nanoscience and technology because these advanced
materials have promising applications in many areas such as
2
,7
2,8
9
photoelectronics, electronics, magnetic recording devices, and
overnight. The synthesized MnO was annealed for 3 h at different
2
10
energy storage/conversion. Active materials, which are useful as
supercapacitors, include high-surface-area carbon, hydrated ruthe-
temperatures up to 600°C in air.
11
nium dioxide, electronically conducting polymers, etc. Nanostruc-
tured metal oxides, which exhibit pseudocapacitance behavior, are
considered to be excellent materials for achieving high specific ca-
Characterization.— Powder X-ray diffraction ͑XRD͒ patterns of
MnO2 powders were recorded using a Philips XRD ‘X’ PERT PRO
diffractometer using Cu K␣ ͑␭ = 1.5418 Å͒ as a source. Brunauer–
Emmett–Teller ͑BET͒ surface area measurements of the samples
were carried out using a Smartsorb-92/93 surface area analyzer after
degassing overnight at around 150°C. Thermogravimetric analysis
͑TGA͒ and differential thermal analysis ͑DTA͒ were carried out in
the temperature range from ambient to 600°C in air at a heating rate
of 10°C per min using a NETZSCH TG 209 F1 thermogravimetric
analyzer. The IR absorption spectra were recorded using Fourier
transform infrared spectroscopy ͑FTIR͒ Spectrum 1000 ͑Perkin-
Elmer͒, using KBr pellets. A thin KBr pellet containing less than
1
2-14
pacitance ͑SC͒.
Among the various materials investigated over
the years, hydrated ruthenium dioxide ͑RuO ·xH O͒ has been found
2
2
as a leading material for supercapacitors as it exhibits a high SC of
1
4-16
about 720 F/g.
However, ruthenium is expensive and toxic, thus
deprived of its commercial applications. To this end, various other
metal oxides which are less expensive and abundant in nature have
17-19
been investigated.
MnO has been found to be an alternative for
2
2
0-23
RuO ·xH O.
2
2
Several routes have been reported for the synthesis of MnO .
2
0
.1 mg of the sample was heated in vacuum around 120°C over-
night. This was then transferred to a desiccator before mounting the
sample. The morphology and structural properties of the as-
24,25
Apart from the electrochemical methods,
wet chemical methods
of synthesis have also been reported. For instance, Jeong and
2
6
Manthiram have synthesized MnO by reduction of KMnO using
2
4
synthesized and annealed MnO were investigated using scanning
2
various inorganic reducing agents such as KBH , etc. Lee and
4
electron microscopy ͑SEM; FEI Sirion͒ by sonicating the sample in
acetone for 10–15 min and placing a few drops of this onto carbon
tape followed by gold coating. High-resolution transmission electron
microscopy ͑HRTEM; Tecnai͒ studies were carried out by placing a
few drops of sonicated samples in acetone onto carbon-coated cop-
per grids. A Sartorius balance of model CP22D-OCE with 10 ␮g
sensitivity was used for weighing electrodes and materials.
2
0
Goodenough have synthesized by the coprecipitation method in-
volving KMnO and manganese acetate. Toupin et al. have pre-
2
7
4
pared MnO by simply mixing aqueous solutions of KMnO and
2
4
MnSO . This reaction has also been carried out in a microemulsion
4
2
8
medium. Amorphous MnO2 was synthesized by reduction of
29
KMnO with fumaric acid by Reddy and Reddy. As an alternative
4
route, here we describe the use of a simple organic reducing agent,
namely, aniline to reduce KMnO4 at room temperature to obtain
amorphous MnO , which converts to crystalline ␣-MnO at higher
temperatures. It is characterized by both physicochemical and elec-
trochemical methods. The capacitive behavior of this compound is
studied.
Electrode preparation and electrochemical characterization.—
Electrodes were fabricated on a high-purity stainless steel ͑SS͒ of
grade 304 foil as a current collector. The SS was polished with
successive grades of emery, cleaned with detergent, washed with
doubly distilled water, rinsed with acetone, and dried in air. Then
2
2
7
5 wt % of MnO , 20 wt % of acetylene black ͑AB͒, and 5 wt % of
2
poly͑vinylidene fluoride͒ ͑PVDF͒ were ground in a mortar, and a
few drops of n-methyl pyrrolidinone ͑NMP͒ were added to form a
slurry. It was coated onto the pretreated SS and dried at 100°C under
*
Electrochemical Society Active Member.
E-mail: vasan@sscu.iisc.ernet.in
z
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Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
A35
Figure 2. TGA/DTA curves of MnO as prepared and dried at 50°C.
2
Figure 1. XRD patterns ͑Cu K␣ = 1.5418 Å͒ of MnO samples as prepared
and dried at 50°C ͑a͒ in air and annealed at ͑b͒ 200, ͑c͒ 300, ͑d͒ 400, ͑e͒ 500,
to the intake of oxygen. Beyond this temperature we observe a con-
stant plateau. XRD of the sample obtained from TGA heated up to
600°C showed the ␣-MnO phase. It is known that MnO trans-
2
and ͑f͒ 600°C for 3 h in air. The JCPDS pattern of ␣-MnO is shown at the
2
2
2
bottom. The hkl planes are indicated at the top.
26,30
forms into Mn O upon heating at 600°C.
In the present study,
2
3
however, even when the sample is subjected to prolonged heating
͑
3 h͒ at 600°C, it still exhibited the ␣-MnO phase. Tentatively, we
2
vacuum for 12 h. The loading level of the active material on the
attribute the thermal stability of MnO2 synthesized in the present
study to a high concentration of potassium ͑K/Mn mole ratio
= 0.234 as estimated from ICP͒ present in it, similar to an observa-
tion made in an earlier study.
IR spectra for the as-synthesized and annealed samples are
shown in Fig. 3a-f. A broad band at around 400–600 cm is as-
signed to the metal–oxygen bending vibration. A broad band at
around 3400 cm− is attributed to stretching frequency of H–O–H
and a weak band at around 1630 cm to the bending mode. The
intensity of these two bands gradually decreases with an increase of
−
2
current collector is typically 0.6 mg cm . Electrochemical studies
were carried out using a potentiostat/galvanostat Solartron model
3
1
1
286 in a three-electrode configuration with MnO -coated SS as the
2
working electrode, Pt as the counter electrode, and saturated calomel
electrode ͑SCE͒ as the reference electrode. All potential values re-
ported are against SCE. Discharge specific capacitance ͑SC͒ of
−
1
1
MnO was calculated using formula
2
−
1
SC = It/͑m⌬E͒
͓1͔
where I is current in amps used for charge/discharge cycling, t is
time in seconds of discharge, m is mass in grams of the active
material, and ⌬E is operating potential window in volt of charge or
discharge.
Results and Discussion
Figure 1 shows XRD patterns of the as-synthesized sample as
well as samples annealed at various temperatures. Samples annealed
up to 300°C exhibit only one unsymmetrical broad peak at 37.5°
͑
2␪͒, revealing the poor crystalline nature of the sample ͑Fig. 1a-c͒.
This peak corresponds to the main peak of the ␣-MnO2 crypto-
melane. The increase in intensity of this peak with temperature
shows a gradual evaluation to crystalline phase. A highly crystalline
sample was obtained at 400°C ͑Fig. 1d͒. All the diffraction peaks in
Fig. 1d-f are indexed to tetragonal phase ͑space group I4/m͒ of
␣
-MnO with lattice constants of a = 9.8172 Å and c = 2.8582 Å,
2
which are in good agreement with the reported values ͑JCPDS 44-
141͒.
Thermal analysis ͑TGA/DTA͒ is an important experimental tech-
0
nique usually carried out to estimate water of crystallization in the
sample and to determine the stoichiometry of the compound in com-
bination with the chemical analysis. Figure 2 shows TGA/DTA of
the as-prepared sample of MnO . The TGA curve initially shows a
2
steep decrease with the weight loss of about 22% at 150°C. This is
accompanied by a strong endothermic peak at around 55°C, fol-
lowed by an exothermic peak ͑150°C͒, indicating the loss of physi-
cally adsorbed and crystalline water, respectively. Further weight
loss is smooth, with a slight gain in weight ͑Ͻ1%͒, but with a sharp
exothermic peak at around 425°C. This gain in weight we attribute
Figure 3. IR spectra of MnO samples as prepared and dried at 50°C ͑a͒ in
air and annealed at ͑b͒ 200, ͑c͒ 300, ͑d͒ 400, ͑e͒ 500, and ͑f͒ 600°C for 3 h
in air.
2
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A36
Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
Figure 4. ͑Color online͒ SEM images of MnO ͑a͒ as prepared and dried at 50°C in air ͑inset shows energy-dispersive X-ray͒ and annealed at ͑b͒ 200, ͑c͒ 300,
2
͑
d͒ 400, ͑e͒ 500, and ͑f͒ 600°C for 3 h in air. Arrows in ͑c͒ indicate initiation of nanorods.
annealing temperature ͑Fig. 3a-c͒, and the bands completely disap-
pear at around 300°C ͑Fig. 3d͒. This is in agreement with results
reported previously.
600°C, are shown in Fig. 5. Agglomerated fine particles of MnO2
are seen in the as-prepared sample ͑Fig. 5a͒, and the diffused se-
lected area electron diffraction ͑SAED͒ ͑inset͒ pattern indicates the
amorphous nature of the compound. The TEM micrograph of the
annealed sample shows both nanorods and well-dispersed spherical
particles of size 4–6 nm ͑Fig. 5b and c͒. The corresponding SAED
and HRTEM images are shown in Fig. 6a and b, respectively, for the
individual particles of a rod and a sphere depicted by the boundaries
in Fig. 5. The distance between the lattice fringes of the nanorods
and nanospherical particles was found to be 3.08 Å ͓310͔ ͑Fig. 6a͒
and 2.35 Å ͓211͔ ͑Fig. 6b͒, respectively. These different lattice
planes suggest that the rods and spherical particles have grown in
different directions.
32
Figure 4 depicts SEM pictures showing the morphology of
MnO annealed at different temperatures. Amorphous MnO exhib-
2
2
its highly clustered granules of varying size between 5 and 10 nm
Fig. 4a͒. As the annealing temperature is increased, it is understood
͑
that the granules fuse ͑Fig. 4b͒ and slow growth of nanorods begins
at around 300°C ͑Fig. 4c͒, with complete formation of nanorods at
around 400°C ͑Fig. 4d͒. Upon further heating to 500 and 600°C,
well-defined nanorods of length 500–750 nm and diameter
50–100 nm are seen ͑Fig. 4e and f͒.
TEM images of MnO samples, as-synthesized and annealed at
2
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Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
A37
Figure 5. TEM image of as-prepared
MnO2 with the inset showing SAED pat-
tern ͑a͒, TEM images of MnO2 annealed
at 600°C ͑b͒, and nanoparticles at high
magnification ͑c͒.
+
−
Cyclic voltammetry is considered to be an ideal experimental
MnO + M + e ꢀ MnOOM
͓3͔
2
1
,11
technique to study the capacitive behavior of any material.
An
ideal capacitor shows a large magnitude of current and a rectangular
type of voltammogram, which is symmetric in anodic and cathodic
directions. Figure 7a shows a typical voltammogram recorded for
the as-synthesized MnO in 0.1 M Na SO electrolyte at different
The second mechanism is expected to be predominant in crystalline
MnO2.
The amount of conducting additives such as carbon is critical in
obtaining an optimum performance of the active material. The ca-
2
2
4
scan rates within a potential range of −0.2 to 1.0 V. It is clear from
Fig. 7a that there are no sharp redox peaks in the potential range
studied. The rectangular voltammograms exhibit a mirror image of
the cathodic and anodic parts, thus suggesting an ideal capacitive
behavior of the material. Variation of voltammetric cathodic current
density at 0.5 V with sweep rate is shown in Fig. 7b. There is a
linear increase in current density with an increase in sweep rate,
confirming the capacitive behavior of nanoparticles of MnO2.
There are two mechanisms proposed for the charge storage in
pacitance of MnO was evaluated by varying the amount of AB. It
was found that 20 wt % of AB is the optimum composition for
2
providing a good conducting path among MnO particles prepared
2
33,34
in the present study, similar to the studies reported earlier.
Fur-
ther experiments were carried out with the electrodes fabricated with
2
0 wt % AB.
Cyclic voltammograms recorded between −0.2 and 1.0 V in
−1
0
.1 M Na SO aqueous electrolyte at a sweep rate of 20 mV s for
2
4
2
0,23,27
MnO2 annealed at different temperatures for 3 h are shown in Fig.
a-f. Regular rectangular shapes are obtained for the samples an-
MnO2.
One of these mechanisms involves adsorption/
8
+
+
desorption of protons ͑H ͒ or cations ͑M ͒ from the electrolyte
nealed up to 300°C ͑Fig. 8a-c͒. The anodic or cathodic currents at
+
−
−
2
+
the middle region of the voltammograms at these temperatures
͑
MnO2͒surface + M + e ꢀ ͑MnO M ͒
͓2͔
surface
͑
ഛ300°C͒ are nearly the same, indicating capacitive behavior of
This is essentially a surface process, which is likely to be predomi-
the material. For the sample annealed at higher temperatures
͑Ͼ300°C͒, distorted rectangular voltammograms with broad current
peaks are obtained ͑Fig. 8d-f͒.
nant in amorphous MnO . The other mechanism is based on
2
+
+
intercalation/deintercalation of H or M
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A38
Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
Figure 7. ͑a͒ Cyclic voltammograms of as-prepared MnO2 recorded in
0
.1 M Na SO at ͑i͒ 10, ͑ii͒ 20, ͑iii͒ 40, ͑iv͒ 80, and ͑v͒ 100 mV s− . ͑b͒
1
2 4
Figure 6. HRTEM images of the nanorods ͑a͒ and nanoparticle ͑b͒ from the
squared regions of ͑b͒ and ͑c͒, respectively. The corresponding SAED pat-
terns are shown as insets.
Dependence of voltammetric current density on scan rate.
electrodes fabricated using MnO2 annealed at different tempera-
tures. The variation of SC with the previously annealed samples at
different temperatures is shown as a function of surface area in Fig.
10a. The as-synthesized sample of MnO2 exhibits a maximum SC
Capacitor studies of MnO are generally performed in the poten-
2
26,28
tial range from 0.0 to 1.0 V vs SCE.
Preliminary experiments in
the present study indicated linear variation of potential of MnO2
electrodes in 0.1 M Na SO during galvanostatic charge–discharge
−
1
−2
͑229 F g ͒ at a current density of 1.0 mA cm . For the sample
2
4
−
1
cycling between −0.2 and 1.0 V vs SCE. Hence, electrochemical
experiments were performed in this potential range. The electrodes
annealed at 200°C for 3 h, SC is 198 F g , which is 12% less than
−
1
the SC of the as-synthesized sample. A SC of 49 F g is obtained
3
7
of the as-prepared and annealed MnO were subjected to galvano-
for the sample annealed at 600°C. The SCs mainly depend on the
16,34
2
static charge–discharge cycling at several current densities in the
water content,
which is very essential for transportation of active
−
2
range 0.5–15 mA cm . Typical data for five charge–discharge
cycles for the as-prepared MnO2 at current density of
.5 mA cm ͑0.8 A g ͒ are shown in Fig. 9a. The time of charg-
ionic species and also on the structure ͑see below͒. In DTA studies
͑Fig. 2͒, it was observed that complete loss of water occurs at about
300°C. The samples of MnO2 annealed at 400°C and above are left
with no water, which leads to decrease in SC. Further, there is a
decrease in surface area with an increase in temperature as seen in
Fig. 10a The SC values are recalculated in terms of capacitance
a
−
2
−1
0
ing and time of discharging are almost the same, which implies high
reversibility, and also the Coulombic efficiency of charge–discharge
cycling is close to unity. SC values are calculated from galvanostatic
charge-discharge cycling using Eq. 1. The value of SC obtained
−
2
density ͑␮F cm ͒ and are plotted vs temperature of annealing in
Fig. 10b. It is seen that there are two segments in this plot. For
MnO2 samples heated to 300°C, which are predominantly in the
−
1
from the data of Fig. 9a is about 253 F g at a current density of
−
2
0
.5 mA cm . Variation of SC of the electrodes with current density
−
2
for as-prepared and annealed MnO is shown in Fig. 9b, showing a
amorphous state, capacitance density is about 100 ␮F cm . For the
2
decrease in SC with increase in current density. This is due to a
decrease in efficiency of utilization of active material at high current
density.
Charge–discharge cycling experiments were carried out for the
crystalline samples annealed between 400 and 600°C, it is about
300 ␮F cm . This increase in capacitance density suggests that Re-
−
2
3
5,36
action 3, corresponding to bulk, governs the crystalline phase and
2
2,23
Reaction 2 ͑surface͒ in amorphous phase.
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Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
A39
Figure 8. Cyclic voltammograms of
MnO as prepared ͑a͒ and annealed at ͑b͒
2
2
6
2
00, ͑c͒ 300, ͑d͒ 400, ͑e͒ 500, and ͑f͒
00°C recorded in 0.1 M Na SO at
2
4
−
1
0 mV s
.
Figure 10. ͑a͒ Dependence of SC on surface area at current density of
−
2
Figure 9. ͑a͒ Charge–discharge curves of as-prepared MnO at current den-
1 mA cm . Annealing temperatures are indicated at each point. ͑b͒ Capaci-
2
sity of 0.5 mA cm−2 and ͑b͒ dependence of SC on current density of charge–
discharge cycling for ͑i͒ amorphous and ͑ii͒ 600°C-annealed samples.
tance density on annealing temperature at current density of 1 mA cm
Potential window: −0.2 to 1.0 V vs SCE; electrolyte 0.1 M Na SO .
−2
.
2
4
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A40
Journal of The Electrochemical Society, 155 ͑1͒ A34-A40 ͑2008͒
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Figure 11. Cyclic-life data of as-prepared MnO cycled at current density of
2
−
2
0
.5 mA cm in 0.1 M Na SO between −0.2 and 1.0 V vs SCE.
2 4
1
1
2
2
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Cycle life testing was performed at 0.5 mA cm−2 for the as-
prepared MnO and the data are shown in Fig. 11. The SC of MnO
after 500 cycles was found to be about 136 F g , which is 54% of
the initial capacitance. Only 44% of the initial capacitance is avail-
2
2
−
1
22. T. Brousse, M. Toupin, R. Dugas, L. Athouel, O. Crosnier, and D. Belanger, J.
Electrochem. Soc., 153, A2171 ͑2006͒.
2
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able after 1000 cycles. A similar decrease in SC of MnO is reported
2
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1235 ͑1998͒.
38,39
upon repeated charge–discharge cycling.
2
2
2
2
2
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Conclusion
Nanoparticles of MnO are prepared from reduction of KMnO
by aniline. The as-synthesized amorphous MnO converts into crys-
talline ␣-form upon annealing at temperatures Ͼ400°C. There is a
conversion of amorphous nanoparticles into nanorods. Interestingly,
the MnO remains in the ␣ phase without converting into Mn O at
higher temperatures. The nanoparticles of MnO exhibit good ca-
pacitive behavior and specific capacitance of about 250 F g is
obtained.
2
4
2
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2
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Acknowledgment
3
3
3
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8. R. N. Reddy and R. G. Reddy, J. Power Sources, 124, 330 ͑2003͒.
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Zhou, F. Favier, D. Belanger, and P. Simon, J. Power Sources, In press.
H.N.V. thanks the Department of Science and Technology ͑DST͒,
Government of India, for financial assistance.
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