Alcohol-assisted room temperature synthesis of different nanostructured manganese oxides and their pseudocapacitance properties in neutral electrolyte

Article abstract of DOI:10.1016/j.cplett.2008.01.042

Different nanostructured manganese oxides have been synthesized by a simple precipitation technique using KMnO₄ and different alcohols. The synthesized manganese oxides were extensively studied using TEM, XRD, XPS, surface area measurements and electrochemical studies. TEM observations showed a range of nanostructures from nanowiskers to nanoparticles. This synthesis method promises the tuning of electronic and structural properties of the nanostructured manganese oxides by simply varying the alcohols used in the reactions. MnO₂ shows more whisker-like morphology while the Mn₂O₃ shows particle morphology. The nanostructured manganese oxides showed excellent performance as a pseudocapacitor electrode in a neutral electrolyte.

Full text of DOI:10.1016/j.cplett.2008.01.042

Available online at www.sciencedirect.com
Chemical Physics Letters 453 (2008) 242–249
www.elsevier.com/locate/cplett
Alcohol-assisted room temperature synthesis of different
nanostructured manganese oxides and their
pseudocapacitance properties in neutral electrolyte
V. Subramanian, Hongwei Zhu, Bingqing Wei *
Department of Mechanical Engineering, University of Delaware, Newark, DE 19716, USA
Received 29 November 2007; in final form 16 January 2008
Available online 21 January 2008
Abstract
Different nanostructured manganese oxides have been synthesized by a simple precipitation technique using KMnO4 and different
alcohols. The synthesized manganese oxides were extensively studied using TEM, XRD, XPS, surface area measurements and electro-
chemical studies. TEM observations showed a range of nanostructures from nanowiskers to nanoparticles. This synthesis method prom-
ises the tuning of electronic and structural properties of the nanostructured manganese oxides by simply varying the alcohols used in the
reactions. MnO2 shows more whisker-like morphology while the Mn2O3 shows particle morphology. The nanostructured manganese
oxides showed excellent performance as a pseudocapacitor electrode in a neutral electrolyte.
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
Nanocrystalline manganese oxides have been reported
to be synthesized under various conditions by different
solution-assisted methods, such as sol–gel, hydrothermal,
reduction reactions, etc. [4–7]. Since manganese oxides
have variety of advantages such as low cost, eco-friendly
and abundantly available apart from their excellent electro-
chemical properties, various groups around the world are
working on this material. Of the various groups, Suib
and his co-workers have extensively synthesized a variety
of manganese oxides in various nanoarchitectures under
different conditions [8–11]. They have synthesized mixed
valent mesoporous manganese oxides using surfactants,
which showed very good structural stability even at high
temperatures [12].
In the case of solution-assisted methods using carboxylic
acids as chelating agents requires a post-formation heat
treatment for the removal of the carboxylic acids. The
exothermic decomposition of carboxylic acid has to be
carefully handled in large scale synthesis, which affects
the cost effectiveness of the synthesis procedure. Although
hydrothermal methods provide interesting nanostructures,
the longer reaction time is a major disadvantage and the
scaling up requires precise engineering of reaction vessel [7].
Supercapacitors or electrochemical capacitors have
gained a great attention in recent years because of their
potential applications ranging from cellular phones to elec-
tric vehicles [1]. Although RuO2 is the most promising sup-
ercapacitor electrode material, its cost and the use of strong
acidic electrolyte prevent the material from any large scale
applications such as in electric vehicles, thereby forcing the
research community to find an alternative. Manganese
oxide, one of the potential alternatives with promising
pseudocapacitor performance in neutral electrolytes [2,3],
has attracted intensive investigations. The synthesis of
manganese oxides with nanoscale features is therefore
among the top priorities as the nanostructured materials
are believed to have larger surface area and a higher reac-
tivity which are very critical parameters for capacitance
enhancement in electrochemical capacitor applications.
*
Corresponding author. Fax: +1 302 831 3619.
E-mail address: weib@udel.edu (B. Wei).
0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2008.01.042

V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
243
Here, in this Letter we report for the first time a simple
but novel approach for the precipitation of nanoscale man-
ganese oxide particles from KMnO4 using different alco-
hols at room temperature. All the earlier reported
methods involve aqueous solutions of potassium perman-
ganate and manganese (II) salt [13]. Compared to the other
room temperature precipitation techniques for the prepara-
tion of manganese oxide reported hither to, our approach
requires only one starting manganese precursor, KMnO4,
simplifying the post-synthesis treatment as well. This is
one of major advantages when considered for the scaling
up of the synthesis process for commercial purposes.
Cyclic voltammetry (CV) studies were performed using a
potentiostat/galvanostat (PGSTAT20, Autolab, EchoChe-
mie, The Netherlands) in a three electrode configuration
with the Ni mesh coated with MnxOy as the working elec-
trode, Pt-wire as the counter and saturated calomel elec-
trode (SCE) as the reference. CV was done between À0.2
and 0.8 V in a 1 M Na2SO4 neutral electrolyte at different
scan rates. The specific capacitance was evaluated from
the area of the charge and discharge curves of the CV plots.
Galvanostatic charge–discharge experiments were in a two
electrode configuration in a HS-Test cell (Hohsen Corpora-
tion, Japan). The electrolyte was soaked in a glass fiber fil-
ter paper which also acted as a separator. The cycling study
was performed at 200 mA/g and between À0.2 and 0.8 V
using a multi-channel battery tester (BT-4, Arbin, USA).
2. Experimental
Manganese oxides were prepared at room temperature
by the addition of different alcohols such as ethanol, meth-
anol, pentanol, isopropanol, glycerol and ethylene glycol
individually to the aqueous solution of KMnO4. Typically,
0.5 g of KMnO4 was dissolved in 30 ml of de-ionized water.
To this KMnO4 solution, 10 ml of ethanol (for instance)
was added drop-wise which led to the formation of brown-
ish precipitate of MnO2. The precipitate was filtered and
washed extensively with de-ionized water until the pH of
the washed water is 7. Then the precipitate was dried at
room temperature. Similar procedure was followed for
the synthesis of manganese oxides with other alcohols as
well.
The synthesized manganese oxides were analyzed for
structural, surface and electrochemical/electronic proper-
ties by X-ray diffraction (XRD), high-resolution transmis-
sion electron microscopy (HRTEM), X-ray photoelectron
spectroscopy (XPS) and electrochemical measurements.
The phase purity of synthesized materials was studied using
XRD (Siemens X-ray Diffractometer, Germany). The par-
ticle morphology and structural properties of the prepared
manganese oxides were further elucidated by TEM/SAED
(JEOL 2010F, Japan, at an accelerating voltage of 200 kV)
studies. Energy dispersive X-ray (EDX) was used to con-
firm the composition of the formed oxides and the presence
of other metal ions such as potassium. The oxidation states
of Mn in the synthesized oxides were studied in detail by
XPS (Kratos AXIS 165 XPS/SAM, USA). The surface
area of the synthesized materials was studied using BET
measurements (Quantachrome Instruments, Model NOVA
2000 Series, USA).
3. Results and discussions
3.1. Microstructural studies
TEM measurements were performed to study the mor-
phology and microstructure of the prepared manganese
oxides. There has been a variety of structures depending
on the alcohol used. As can be seen from Fig. 1, the man-
ganese oxides synthesized using ethanol and methanol,
show largely agglomerated whiskers in nanoscale. While
that from ethylene glycol, isopropanol and glycerol show
particle-like morphology in nanoscale. Alcohol oxidation
by KMnO4 is known from chemistry textbook, but no
much attention has been paid hither to on the nanoarchi-
tecture of the formed manganese oxide. This is very critical
in terms of chosen application, as manganese oxides can be
used as electrodes in primary and secondary batteries, sup-
ercapacitors and magnetic materials.
For example, the micrograph of manganese oxides
prepared using ethanol showed nanoscale architectures
resembling fine whiskers with size being in the range of
10–20 nm. Although the whiskers are in the nanometer
scale, there has been a profound agglomeration (Fig. 1c).
Electron diffraction studies revealed the formation of
crystalline MnO2, the lattice fringes can be clearly observed
from the HRTEM studies. The d-spacing of 0.70 nm
corresponds to (110) plane of MnO2 structure. Even after
copious washing of the formed manganese oxide precipi-
tate, there has been a trace amount of potassium as
evidenced from the EDX measurements.
The electrode for evaluating the electrochemical proper-
ties of the synthesized manganese oxides were fabricated by
mixing the prepared manganese oxides with 20 wt.% car-
bon black (Black Pearl 2000, Cabot Corp., USA) and
5 wt.% PVdF-HFP binder. A slurry of the above mixture
was made using N-methyl-2-pyrrolidone (NMP) as a sol-
vent which was subsequently brush-coated onto a Ni mesh.
The mesh was dried at 110 °C in air for 1 h to remove the
solvent. After drying the coated mesh was uniaxially
pressed to better adhere the electrode material with the cur-
rent collector.
In the case of MnO2 formed from methanol, the mate-
rial exhibits more amorphous nature than that prepared
using ethanol. There has been an earlier study on the prep-
aration of Ru nanoparticles using alcohols with different
carbon chain length. The larger chain length corresponds
to smaller nanoparticle [14]. In the present case, there also
have a difference in carbon chain length between ethanol
and methanol, resulting in difference of the formed particle
morphology and microstructure, which was confirmed by
the BET surface area analysis. MnO2 nanostructures syn-
thesized using ethanol showed a surface area of 250 m2/g.

244
V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
Fig. 1. SEM and TEM micrographs of manganese oxides synthesized using different alcohols: (a) a typical SEM image of MnO2 synthesized using
ethanol; (b) a typical SEM image of Mn2O3 prepared using ethylene glycol; (c) ethanol; (d) methanol; (e) ethylene glycol; (f) glycerol; (g) isopropanol;
(h) pentanol. It is found the samples synthesized using ethylene glycol glycerol and isopropanol contained a relatively high amount of amorphous phase
which was further identified as Mn2O3 by XPS measurements. The TEM results show that all the crystalline structures are MnO2. There is no connection
between the fringe distance shown in the images and the type of alcohols we used. The lattices displayed in TEM images depend on the applied zone axis
when doing TEM.
However, the surface area for the methanol-assisted MnO2
is 124 m2/g. This can be directly related to the whisker size
and the porosity of the formed MnO2. This clearly indi-
cates that the materials are not only in nanostructured
phase but also adequately porous. In general, as a superca-
pacitor electrode material, the porosity and surface area of
the material determine to a greater extent the electrochem-
ical performance of the system.
For the manganese oxides formed using ethylene glycol,
isopropanol and glycerol, there has been a totally different
morphology when compared with that from ethanol and
methanol. All of the manganese oxides from these alcohols
have predominantly amorphous phase which has been
identified as Mn2O3 (Mn in 3+) though there still are a
small amount of crystallites which show features corre-
sponding to MnO2 (Fig. 1e–g), confirmed with XRD mea-
surements (not shown here). This will be explained in detail
in the XPS discussions below. Similar to the ethanol/meth-
anol, there has been a considerable difference in the poros-
ity of the formed materials from ethylene glycol,
isopropanol and glycerol. In the case of ethylene glycol,
the particle size is about 10–20 nm, while that from isopro-
panol and glycerol show larger particles and highly porous
particles, respectively. This observation can be well supple-
mented from the surface area measurements. The manga-
nese oxide derived from glycerol showed the largest
surface area of 245 m2/g followed by the ethylene glycol
and isopropanol with 195 and 146 m2/g, respectively.
Another interesting observation from the surface area
study is the average pore diameter of the glycerol-assisted
˚
manganese oxide, which is the smallest, 18.82 A in the pres-
ent study. For all the other manganese oxides, the average
˚
pore diameter was ꢀ40 A. This can be addressed with
respect to the polarity of the alcohols. Further work to
understand the relation between the alcohol used and the
porosity of the formed manganese oxides using SAXS is
in underway.
The time for the precipitation reaction varies with the
alcohol used. Except for pentanol, all the alcohols show
a reaction time for complete precipitation less that 1 h.
In other words, KMnO4 was completely decolorized
within 1 h for all the alcohols studied except pentanol.

V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
245
Fig. 1 (continued )
Only in the case of pentanol we have an extended reaction
3.2. Reaction mechanism
time of up to 12 h. Also, there has been a pronounced
color difference among the precipitates formed depending
upon the alcohol used. The materials synthesized with
ethanol, methanol and pentanol shown a brown color
precipitate, while that for ethylene glycol, isopropanol
and glycerol it was a blackish brown precipitate. This is
mainly due to the valence state of the formed manganese
oxide.
KMnO4 used here acts as an oxidizing agent in a neutral
medium, which is water in the present case. As a typical
example, the ethanol is oxidized into acetic acid and per-
manganate is reduced into MnO2 with the formation of
KOH. In other words, the formation of KOH must be neu-
tralized by the formation of acetic acid which in turn forms
salt and water.
Table 1
Surface area, oxidation state and structural information for the manganese oxides synthesized using different alcohols
Alcohol used
Mn oxidation
statea
Sp. Capacitance
(F/g)b
Formed
compounda
Surface area
(m2/g)
Morphologyc
Ethanol
4+
4+
4+
ꢀ3+
200
160
170
133
MnO2
MnO2
MnO2
Mn2O3
240
126
Agglomerated nanowiskers
Agglomerated nanosheets
Agglomerated nanosheets
Amorphous and nanoparticles
(highly porous)
Methanol
Pentanol
Glycerol
245
Isoproponal
Ethylene glycol
ꢀ3+
ꢀ3+
152
113
Mn2O3
Mn2O3
146
196
Amorphous and nanoparticles
Amorphous and nanoparticles
a
Based on XPS studies.
b
Based on CV studied at 2 mV/s between À0.2 and 0.8 V in a 1 M Na2SO4 aqueous electrolyte.
c
Based on TEM studies and further confirmed by XRD studies.

246
V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
800
600
400
200
0
800
Mn3s - Ethanol
Mn3s - Methanol
600
400
200
0
4.70 eV
4.68 eV
-200
-200
70
75
80
85
90
95
100
105
70
75
80
85
90
95
100
105
Binding Energy (eV)
Binding Energy (eV)
1000
800
600
400
200
0
1000
800
600
400
200
0
Mn3s - Ethyene Glycol
Mn3s - Glycerol
-200
5.27 eV
5.38 eV
85
-200
65
70
75
80
90
95
100
105
65
70
75
80
85
90
95
100
105
Binding Energy (eV)
Binding Energy (eV)
600
Mn3s - Isopropanol
500
400
300
200
100
0
5.3 eV
-100
70
75
80
85
90
95
100
105
Binding Energy (eV)
Fig. 2. XPS spectra of manganese oxides synthesized using different alcohols.
However, the reaction mechanism is different when alco-
In general, these alcohols are oxidized by KMnO4 in an
aqueous media to ketones with the reduction of Mn7+ to
Mn3+. Since the tertiary alcohols can never be oxidized
by potassium permanganate using butyl alcohol resulted
hols such as ethylene glycol, glycerol and isopropyl alcohol
were used. KMnO4 oxidizes the respective alcohol to a
ketone with the formation of Mn2O3 in the aqueous media.

V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
247
0.002
0.001
0.000
-0.001
-0.002
0.0010
Ethanol
Methanol
0.0005
0.0000
-0.0005
-0.0010
-0.0015
-0.2
0.0
0.2
0.4
0.6
0.6
0.6
0.8
0.8
0.8
1.0
1.0
1.0
-0.2
0.0
0.2
0.4
0.6
0.6
0.6
0.8
0.8
0.8
1.0
Voltage (V)
Voltage (V)
0.0015
0.0010
0.0005
0.0000
-0.0005
-0.0010
-0.0015
-0.0020
-0.0025
0.0025
0.0020
0.0015
0.0010
0.0005
0.0000
-0.0005
-0.0010
-0.0015
-0.0020
Pentanol
Ethylene Glycol
-0.2
0.0
0.2
0.4
-0.2
0.0
0.2
0.4
1.0
Voltage (V)
Voltage (V)
0.003
0.002
0.001
0.000
-0.001
-0.002
-0.003
-0.004
0.0015
0.0010
0.0005
0.0000
-0.0005
-0.0010
-0.0015
-0.0020
Iso propanol
Glycerol
-0.2
0.0
0.2
0.4
-0.2
0.0
0.2
0.4
1.0
Voltage (V)
Voltage (V)
Fig. 3. Cyclic voltammograms of manganese oxides synthesized using different alcohols.
in no reaction. Since the oxidation state of formed manga-
nese oxide is very critical for the electrochemical perfor-
mance, XPS studies have been performed to ascertain the
exact Mn valence state.
X-ray photoelectron spectroscopic studies were per-
formed to evaluate the oxidation state of Mn in the prepared
materials and the results are summarized in Table 1. The
Mn3s spectra recorded for various manganese oxides pre-
pared using different alcohols are shown in Fig. 2. It has
been reported earlier [13,15], due to the parallel spin cou-
pling between the 3s electron and the 3d electron during
the photoelectron ejection the Mn3s spectra in manganese
oxides will exhibit a splitting. The difference in the binding
energy (BE) gives important information on the oxidation

248
V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
state of Mn in the prepared manganese oxide. The BE differ-
ence values close to 4.8 eV signifies the oxidation of Mn in
4+ state [13]. In the present case the difference in BE differ-
ence for ethanol and methanol was found to be 4.70 and
4.68 eV, respectively, which corresponds to Mn4+, confirm-
ing the reaction mechanism discussed above. When other
alcohols such as ethylene glycol, glycerol or isopropyl alco-
hol were used, the BE difference were about 5.3 eV. Accord-
ing to Chigane et al. [15], the BE difference for Mn2O3 in the
Mn3s spectra will be close 5.4 eV. This clearly indicates that
most of the formed oxides are having Mn in 3+ state. In the
present case, the manganese oxides prepared using ethylene
glycol, glycerol and isopropyl alchohol showed BE differ-
ence P5.3, which confirms the existence of Mn in 3+ state.
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
0
1000
2000
3000
4000
5000
6000
3.3. Electrochemical studies
Time (s)
Fig. 4. Typical charge–discharge curves for MnO2 synthesized using
ethanol.
Typical cyclic voltammograms recorded at a scan rate of
2 mV/s for the manganese oxides prepared using different
alcohols are shown in Fig. 3. The specific capacitance is
calculated by integrating the area of the CV curves. The
rectangular profiles of the CV curves show an ideal capaci-
tive behavior which is very important for a system to be
highly reversible. The specific capacitance for the ethanol-
assisted synthesis is 202 F/g, which is the highest in the
present study. For the methanol-assisted MnO2, the specific
capacitance was found to be 160 F/g. The main difference
in the specific capacitance may be attributed to the differ-
ence in the surface area of the synthesized materials. The
specific surface area of MnO2 prepared using ethanol was
found to be almost double of that of methanol. Pseudoca-
pacitance is more related to the actual redox reactions
occurring in the system, larger surface area always has a
more facile diffusion of electrolyte ions and hence a higher
reactivity. Although the profile of the CV curves obtained
for manganese oxides prepared using other alcohols such
as ethylene glycol, glycerol and isopropanol are similar to
that of MnO2 from ethanol and methanol, the actual spe-
cific capacitance values are much lower. The main reason
for the lower specific capacitance is the 3+ oxidation state
of Mn. The presence of Mn3+ has resulted in a reduction in
the specific capacitance. Table 1 summarizes the specific
capacitance values for the different manganese oxides pre-
pared using different alcohols.
tance of BP2000 has been estimated to be 70 F/g in a
1 M Na2SO4 electrolyte [16]. The specific capacitance of
MnO2 was 171 F/g after 200 cycles with a cycling efficiency
of 85%. Further work on long cycle performance and
impedance spectroscopy is in progress (see Fig. 5).
It is important to mention that the specific capacitance
obtained for the manganese oxide synthesized using this
simple technique is much better when compared to other
synthesis techniques which involve cumbersome reaction
procedures such as the hydrothermal and the sol–gel pro-
cesses [5,7]. If a post-treatment at 500 °C for 1 h is applied
for the synthesized MnO2 a decrease in specific capacitance
from 200 F/g to 40 F/g was observed. Similar results have
been observed for MnO2 synthesized by other techniques
as the specific capacitance depends on several structural
240
220
200
180
160
140
The charge–discharge experiments were performed in
order to understand the cycling capability of the synthe-
sized MnO2. The galvanostatic cycling is one of the reliable
experiments to evaluate the specific capacitance. The
cycling experiment was performed using ethanol-assisted
MnO2 at a high constant current of 200 mA/g and the typ-
ical charge–discharge curves are shown in Fig. 4. The
specific capacitance was found to be 205 F/g, which is very
much comparable to that calculated from the CV studies. It
should be mentioned that the capacitance values reported
in this Letter (both CV and charge–discharge) are after
the elimination of the contribution from the carbon black
(BP2000) added to the composite electrode. The capaci-
120
Charge
100
80
60
40
20
0
Discharge
0
50
100
150
200
250
Cycle Number
Fig. 5. Cycling performance of MnO2 synthesized using ethanol cycled
between À0.2 and 0.8 V at 200 mA/g in
a 1 M Na2SO4 aqueous
electrolyte.

V. Subramanian et al. / Chemical Physics Letters 453 (2008) 242–249
249
and surface properties such as surface area, crystallinity,
pore structure, etc. [2,6,7,13].
The synthesized manganese oxides showed excellent
pseudocapacitance properties in a neutral electrolyte with
specific capacitance ꢀ200 F/g for MnO2 prepared using
ethanol. This method for the preparation of MnO2 is sim-
ple and can be easily scaled up to industrial quantities.
Moreover, the absence of post-synthesis treatment is an
added advantage when focused in commercialization point
of view. By varying the alcohol used in the reaction, oxides
with different morphologies and with Mn in different
oxidation states can be achieved, which extends the possi-
bility of preparing materials for different applications.
The pseudocapacitance in MnO2 has been explained by
two reaction mechanisms. The first one is based on the con-
cept of intercalation of H+ or alkali metal cations such as
Na+ in the electrode during reduction and deintercalation
upon oxidation [17]. The second one is the adsorption of
cations in the electrolyte on the MnO2 electrode [3]. In
the present study the redox process is mainly governed by
the insertion and deinsertion of Na+ from the electrolyte
into the porous MnO2 matrix. The presence of additional
metal ion such as K+ is expected to play a similar role in
the electrochemical performance. However, the amount
of K+ is very negligible, as evidenced from XPS and
EDX studies (not shown). In our earlier study, there has
been an increase in the K+ content with respect to increase
in the hydrothermal treatment time [7]. Since the precipita-
tion was instantaneous, absence of any post-formation
treatment led to the lower K+ content, i.e., washing the
precipitate copiously with distilled water removes all the
unreacted potassium which may however can get impreg-
nated on the surface of MnO2 under the hydrothermal con-
ditions [7]. This simple precipitation reaction method is not
only faster but also avoids the coexistence of other metal
ions.
Acknowledgements
We thank Cheng Luo for his assistance in some experi-
ments. Also, the authors thank Dr. Prabhu Ganesan for
fruitful discussions. The authors gratefully acknowledge
financial support from National Science Foundation under
the NSF award number CMMI-0753462.
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4. Concluding remarks
In summary, different nanostructured manganese oxides
have been synthesized by a simple precipitation technique
using different alcohols and potassium permanganate.
The oxidation state of Mn in the synthesized manganese
oxides can be controlled by selecting different alcohols.
The surface of area of the synthesized manganese oxides
ranges from 125 to 250 m2/g, depending upon the alcohol
used. The interesting aspect of the present synthesis tech-
nique is not limited to the formation of Mn in different oxi-
dation states, but the morphological differences among the
formed manganese oxides. Different structures such as
whiskers to particles depend upon which kinds of alcohols
used. MnO2 shows more whisker-like morphology while
the Mn2O3 shows particle morphology, both in nanoscale
region.