# Electrode Properties ofMn2O3 Nanospheres  Synthesized by Combined Sonochemical/Solvothermal Method for Use in Electrochemical Capacitors

Authors: Teressa Nathan; Michael Cloke; S. R. S. Prabaharan
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

We report here an efficient single step combined sonochemical and solvothermal synthesis process to obtain bulk quantities of nanospherical particles of cubicMn2O3 and characterized its pseudocapacitive characteristics in relevance to electrochemical capacitors for the first time. It has been found that quantitative determination of specific capacitance yielded a value of capacitance of ∼100 Fg−1 within 0–0.4 V (versus SCE) potential range in a 6 M KOH alkaline electrolyte. The as-prepared nanopowders after being subjected to heat treatment at 400C∘ were characterized by using XRD which shows a typical cubic single-phase structure (space group Ia-3), the broad crystalline peaks indicating the presence of explicit nanostructure. Electron microscopic studies (FE-SEM and TEM) revealed that the synthesized powders exhibit nanospherical morphology with uniform sphere-like grains of ∼10–15 nm range. Two heat-treated samples were studied in the context of crystallinity versus electrochemical capacitance using rate-dependent cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a three-electrode system. The excellent well-refined redox behavior corroborates with EIS measurements. The presence of near symmetric redox couple observed in CV has been attributed to pronounced one-electron-transfer process owing to the presence of facile Mn redox centers facilitating the reversible one-electron transfer that accounts for its pseudocapacitance.


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## Body

## 1. Introduction

Although the concept
of electrochemical capacitor was initialized and industrialized almost 40 years
ago, the demand for high power pulse requirements in today's modern power
hunger electrical and electronic devices such as telecommunication
devices (mobile phones, PDA, etc.) standby power systems, memory protection of
computer electronics, and hybrid electric vehicles in the form of storage
components rendered a renewed interest in electrochemical capacitors (ECs).
Depending on their charge storage mechanism, ECs
utilize both nonfaradic and faradic capacitance, namely, electrochemical double-layer capacitance (EDLC) and pseudocapacitance,
respectively. The origin of capacitance in the EDLC
is charge separation at the electrode-electrolyte interface, whereas
pseudocapacitance arises from fast, reversible faradic redox reactions taking place in the bulk
of electrodes. The capacitance in an EDLC is in the order of tens and in the pseudocapacitor,
it is in the order
of hundreds ofμF per cm2 of interfacial area [1]. The materials being studied for EC applications are mainly three types:
carbons [2, 3], metal oxides [4–6], and
electronically conducting polymers [7]. In the first type, high-surface-area
carbon materials (activated carbon black, carbon aerogel, and carbon nanotubes)
are used as symmetric electrodes to form electrochemical double-layer capacitors
(EDLCs) in which the charge storage is a result of electrostatic charge
separation at the interface between the electrode and the electrolyte. Specific
capacitance of 280 and 120 Fg−1 can be achieved in aqueous and
nonaqueous electrolytes with the maximum voltages of 1 and ~3 V, respectively [8].
On the other hand, the above type-II and type-III category of materials possess both Faradic charge transfer reaction
(battery-like) and nondiffusional charge reaction (capacitor-like)
characteristics, together with high bulk electronic conductivity. It
has been demonstrated that even higher capacitance can be achieved by using
these redox-active materials. These materials store electrochemical charge
using highly reversible surface redox reactions, commonly described as pseudocapacitance in the literature. The
introduction of these fast,
reversible surface processes has drastically increased the charge-storage
performance. Amongst type-II category, hydrous ruthenium oxide
was reported as the most promising material with specific capacitance values as
high as 863 Fg−1  [9]. Though noble metal oxides or hydrous oxides
yield remarkably large value of capacitance, the high cost of these materials precludes
their application. On the other hand, conducting polymer-based electrochemical
capacitors represents an interesting class, thanks to the combination of high
capacitive energy density and low material cost [10–12]. However,
conducting polymer-based materials also have some disadvantages that include lower cycle life
and slow kinetics of ion transport because the redox sites in the polymer
backbone are not sufficiently stable for many repeated redox processes [13, 14].
Hence, it is extremely important to develop alternative electrode materials
with a combination of low cost and improved performances.Amongst many transition metal oxides being
investigated for EC applications, manganese oxides emerge as potential candidates for many niche
applications (high power batteries, chemical sensing devices, catalysts electronics, etc.) in terms of cost and environmentally benign qualities. As far as the chemical valence
states of manganese oxide are concerned, it possesses different oxidation states
(2+, 3+, 4+, and 6+). Amongst them, MnO2 is one of the common manganese oxide phases
which are extensively investigated because of its applications as catalysts in ion
sieves, as electrode materials in primary Li/MnO2 batteries as well
as in electrochemical capacitors [15–21]. The preparation of
MnO2 requires stringent procedures owing to its complicated means of
process control. There have been many synthesis methods adapted for MnO2 such as chemical coprecipitation [22], thermal oxidation [23], sol-gel [24], and improved reduction process [25].
In the context of
electrochemical capacitors, manganese oxide phases occupied a place of pride in
the recent past. Over the years, researchers reported various specific capacitance
values for MnO2 in the open literature [26, 27].As far as manganese oxide phases are concerned, the most common phases
of manganese oxide in air at different temperatures are shown below. That is,
the following thermal oxidation reactions are possible in manganese oxides [28].
It is well known that MnO2 exhibits thermochemical changes as shown
below,(1)MnO2→500°CMn2O3→900°CMn3O4→1700°CMnO. Amongst
different phases of manganese oxide, Mn2O3 possesses poor
electronic conductivity. Nevertheless, its pronounced electrochemical redox
activity via Mn3+ to Mn4+ transition giving rise to one
electron transfer. The diminished electron transfer reaction, though
significant from the point of view of electrode reaction in ECs, is still an
unexplored material for pseudocapacitance applications until Nathan and Fauzi [29]
demonstrated recently that Mn2O3 could be regarded as a
capacitor material that possesses significant pseudocapacitive characteristics.
Even though, the quantum of energy associated with one electron transfer is limited;
it is believed that cubic phase of Mn2O3 makes it
possible to be used as a pseudocapacitance electrode in ECs.In the present
study, we report here for the first time the synthesis ofMn2O3 nanospheres employing a novel solvothermal/ultrasonication,
a modified single step synthesis protocol from the one described
by us previously [30]. Using the
rate-dependent cyclic voltammetry, a single electrode pseudocapacitance behavior of Mn2O3 nanosphere electrodes has been studied using a three-electrode
cell comprised of a platinum counter and SCE as reference electrodes in alkaline
medium (6 M KOH).


## 2. Experimental Details

### 2.1. Synthesis of Nanospherical Manganese Oxide

The nanosphericalMn2O3 is a direct product of the soft-combustion
of Mn(NO3)2·4H2O with citric acid and acetone/isopropanol
solvent mixture. A 0.1 M aqueous solution of Mn(NO3)2 was prepared using deionised water.
The solution was thoroughly mixed with varying amounts of citric acid and acetone/isopropanol and subjected to sonication (Ultrasonic
processor, UP200S, 200 watts, 24 kHz, Hielscher, Germany) at ambient conditions
with a high-density ultrasonic Ti probe immersed directly in the solution.
During the ultrasonic irradiation, the temperature of the reaction solution
rose to ~80°C and the ultrasonic irradiation was continued to be maintained at this
temperature for 20 minutes. It is a similar method (without
sonication) that was previously reported by us to synthesize nanostructured NiO
powders of 10–15 nm size [30]. In the present synthesis protocol, citric acid was introduced as
a chelating agent which induces molecular level mixing by dissociating the ionic species of the starting material, while
acetone/isopropanol mixture acts as a handle to promote nonhydrolytic
condensation of Mn(NO3)2 dissolute. The solution remained
clear throughout the process without forming any
turbid solution (no precipitation) as it was gently stirred at 70°C for 1 hour
in ambient conditions until all the solvents were evaporated, leaving a wet
gel-like substance, the so-called precursor. As the precursor is further heated
up to 350°C, the wet gel-like substance turns into a brownish paste, releasing
brown-colored fumes as a result of the ignition of organic moieties and finally
yielding the as-prepared final product. Postannealing
was carried out at 400°C for 1 hour and 5 hours to study the grain growth and structural
stability and to ensure the structural perfection so as to obtain the
electrode-active characteristics.As describedMn2O3 nanoparticles were obtained via
a combined solvothermal/ultrasonication technique in which the precursor
solution was obtained after the ultrasonic irradiation of the initial phase of
precursor preparation. The presence of citric acid (chelating agent)
facilitated the complexation by modifying ligands in the original precursor
solution slows down the pace of hydrolysis and condensation. Citric acid is the
widely being used as a complexing agent for solution-based solvothermal methods
for preparing electrode-active transition metal oxides [30]. Citric acid (H3L)
is a weak triprotic acid which dissociates in a stepwise manner in the solution
depending upon the solution pH; only when the pH value of the solution is >6, the species L becomes the dominant one [31]. The complexation reactions
between metal ions and citric acid are also highly dependent upon the pH of the
solution and cannot occur, in general, in a very strong acidic solution.
However, we did not alter the pH of the total solution as it is the mixture
solution that
quickly forms a clear solution (without adding ammonia water) upon ultrasonication. Sonochemical reactions are
often explained by means of cavitation collapse or microjet [32]. The chemical
effect of ultrasound arises from acoustic cavitation phenomena such as
formation, growth, and implosive collapse of bubbles in a liquid medium [33]. The
initial reduction
of the starting material was facilitated by both the temperature and
the sonication followed by the reaction of chelating agent. Thus, we suggest
that citric acid coordinated to Mn ions in Mn(NO3)2 methanolic solution, making the nucleation complete at the early stage of the
solvothermal process and inhibiting the crystal growth.


### 2.2.Mn2O3 Composite Electrode Preparation

TheMn2O3 composite electrode was
prepared by using a well-known procedure previously reported [30].
The composite electrodes were fabricated
by forming the slurry comprising of a mixture of Mn2O3/polyvinylidene
fluoride (PVdF)/acetylene black in 80:10:10 ratio, respectively, and by stirring it for 30
minutes. Acetylene black (AB) was added to increase the conductivity of the Mn2O3 powders. The thick slurry was then coated on a stainless steel expanded mesh (EXMET, CT, USA) and dried
in a vacuum oven at 70°C overnight to
eliminate any residual moieties. The dried
pellets were cut into a 4 cm2 area circular disk-like composite
electrodes and used as a working electrode in a three-electrode cell in which a
platinum foil was
used as a counter electrode employing an aqueous electrolyte (6 M KOH) against
standard calomel electrode (SCE). The rate-dependent cyclic voltammetry was
performed between the potential range of 0 up to 0.4 V
versus SCE in the above three-electrode cell.


### 2.3. Physical and Electrochemical Characterization

The crystallographic phase of the annealed products
ofMn2O3 (400°C/1 hour and 5 hours in air) was confirmed
by powder X-ray diffraction (XRD) using a Siemens diffractometer D5000 using CuKα
radiation (λ=0.154056 nm). Nanostructure morphology of the annealed product was
observed under field-emission scanning
electron microscopy (FE-SEM, JEOL JSM-6701F, Tokyo, Japan) incorporating a
cold cathode field emission gun, equipped
with a conical FE gun and a semi-in-lens electron
optics. TEM analysis was carried out
using a Philips CM 12(s) TEM to determine the particle size. No coating was
done prior to analysis and images were obtained on the as-synthesized powders,
prior to loading. The rate-dependent cyclic voltammetry and ac impedance
measurements were carried out using a PAR VersaSTAT3 Potentiostat/Galvanostat (Ametek,
Pa, USA) equipped with V3-Studio software for data
acquisition and analysis.


## 2.1. Synthesis of Nanospherical Manganese Oxide

The nanosphericalMn2O3 is a direct product of the soft-combustion
of Mn(NO3)2·4H2O with citric acid and acetone/isopropanol
solvent mixture. A 0.1 M aqueous solution of Mn(NO3)2 was prepared using deionised water.
The solution was thoroughly mixed with varying amounts of citric acid and acetone/isopropanol and subjected to sonication (Ultrasonic
processor, UP200S, 200 watts, 24 kHz, Hielscher, Germany) at ambient conditions
with a high-density ultrasonic Ti probe immersed directly in the solution.
During the ultrasonic irradiation, the temperature of the reaction solution
rose to ~80°C and the ultrasonic irradiation was continued to be maintained at this
temperature for 20 minutes. It is a similar method (without
sonication) that was previously reported by us to synthesize nanostructured NiO
powders of 10–15 nm size [30]. In the present synthesis protocol, citric acid was introduced as
a chelating agent which induces molecular level mixing by dissociating the ionic species of the starting material, while
acetone/isopropanol mixture acts as a handle to promote nonhydrolytic
condensation of Mn(NO3)2 dissolute. The solution remained
clear throughout the process without forming any
turbid solution (no precipitation) as it was gently stirred at 70°C for 1 hour
in ambient conditions until all the solvents were evaporated, leaving a wet
gel-like substance, the so-called precursor. As the precursor is further heated
up to 350°C, the wet gel-like substance turns into a brownish paste, releasing
brown-colored fumes as a result of the ignition of organic moieties and finally
yielding the as-prepared final product. Postannealing
was carried out at 400°C for 1 hour and 5 hours to study the grain growth and structural
stability and to ensure the structural perfection so as to obtain the
electrode-active characteristics.As describedMn2O3 nanoparticles were obtained via
a combined solvothermal/ultrasonication technique in which the precursor
solution was obtained after the ultrasonic irradiation of the initial phase of
precursor preparation. The presence of citric acid (chelating agent)
facilitated the complexation by modifying ligands in the original precursor
solution slows down the pace of hydrolysis and condensation. Citric acid is the
widely being used as a complexing agent for solution-based solvothermal methods
for preparing electrode-active transition metal oxides [30]. Citric acid (H3L)
is a weak triprotic acid which dissociates in a stepwise manner in the solution
depending upon the solution pH; only when the pH value of the solution is >6, the species L becomes the dominant one [31]. The complexation reactions
between metal ions and citric acid are also highly dependent upon the pH of the
solution and cannot occur, in general, in a very strong acidic solution.
However, we did not alter the pH of the total solution as it is the mixture
solution that
quickly forms a clear solution (without adding ammonia water) upon ultrasonication. Sonochemical reactions are
often explained by means of cavitation collapse or microjet [32]. The chemical
effect of ultrasound arises from acoustic cavitation phenomena such as
formation, growth, and implosive collapse of bubbles in a liquid medium [33]. The
initial reduction
of the starting material was facilitated by both the temperature and
the sonication followed by the reaction of chelating agent. Thus, we suggest
that citric acid coordinated to Mn ions in Mn(NO3)2 methanolic solution, making the nucleation complete at the early stage of the
solvothermal process and inhibiting the crystal growth.


## 2.2.Mn2O3 Composite Electrode Preparation

TheMn2O3 composite electrode was
prepared by using a well-known procedure previously reported [30].
The composite electrodes were fabricated
by forming the slurry comprising of a mixture of Mn2O3/polyvinylidene
fluoride (PVdF)/acetylene black in 80:10:10 ratio, respectively, and by stirring it for 30
minutes. Acetylene black (AB) was added to increase the conductivity of the Mn2O3 powders. The thick slurry was then coated on a stainless steel expanded mesh (EXMET, CT, USA) and dried
in a vacuum oven at 70°C overnight to
eliminate any residual moieties. The dried
pellets were cut into a 4 cm2 area circular disk-like composite
electrodes and used as a working electrode in a three-electrode cell in which a
platinum foil was
used as a counter electrode employing an aqueous electrolyte (6 M KOH) against
standard calomel electrode (SCE). The rate-dependent cyclic voltammetry was
performed between the potential range of 0 up to 0.4 V
versus SCE in the above three-electrode cell.


## 2.3. Physical and Electrochemical Characterization

The crystallographic phase of the annealed products
ofMn2O3 (400°C/1 hour and 5 hours in air) was confirmed
by powder X-ray diffraction (XRD) using a Siemens diffractometer D5000 using CuKα
radiation (λ=0.154056 nm). Nanostructure morphology of the annealed product was
observed under field-emission scanning
electron microscopy (FE-SEM, JEOL JSM-6701F, Tokyo, Japan) incorporating a
cold cathode field emission gun, equipped
with a conical FE gun and a semi-in-lens electron
optics. TEM analysis was carried out
using a Philips CM 12(s) TEM to determine the particle size. No coating was
done prior to analysis and images were obtained on the as-synthesized powders,
prior to loading. The rate-dependent cyclic voltammetry and ac impedance
measurements were carried out using a PAR VersaSTAT3 Potentiostat/Galvanostat (Ametek,
Pa, USA) equipped with V3-Studio software for data
acquisition and analysis.


## 3. Results and Discussion

### 3.1. Structural and Microstructural (Nanophase) Characterization of the Active Electrode Material,Mn2O3

A single-phase cubic structure ofMn2O3 was evident from XRD analysis
as shown in Figure 1. The hkl planes are well-indexed to a cubic phase with lattice parameter a=9.4080 Å  having broad crystalline peaks indicating the presence of explicit
nanostructure. Obviously, the
broad peaks illustrate the presence of small crystallites (domains) and by
knowing the FWHM peak information from the powder diffraction data, the
particle size of the crystallites can be estimated by using Scherrer's formula. The
structural results are in good agreement
with the JCPDS 41-1442 database. The effect of postheat treatment versus
time has been investigated in order to study the electrode-active Mn2O3 nanospheres.
Figure 1 shows the XRD pattern obtained on a sample
annealed at 400°C for 1 hour and 5 hours as holding time. As the
holding time is increased from 1 hour to 5 hours (sample A and sample B, resp.), while maintained at 400°C,
XRD peaks became relatively sharper yet maintaining the features corresponding
to its nanostructure (broad crystalline peaks).Figure 1
X-ray diffractograms ofMn2O3 showing broad crystalline peaks; sample A heated at 400°C for 1 hour and sample B heated at 400°C for 5 hours.TEM images on the
annealed samples are reproduced in Figure2.
Figure 2(a) reveals that the
manganese oxide powders synthesized at 400°C exhibit spherical-shaped particles (within 10–15 nm range) with
porous morphology. The spherical shape of the product was also ascertained by
FE-SEM analysis as well. It has been found that as the annealing time increased
to 5 hours, the particle size remains the same and possible grain growth is not
explicitly evident as shown in Figures 3(a) and 3(b).
The porous particle
morphology was evident from the FESEM analysis.TEM pictures ofMn2O3 heated at 400°C for (a) 1 hour and (b) 5 hours.
(a)(b)NanosphereMn2O3 heated at 400°C for 5 hours (a) FE-SEM image and (b) Back scattered image.
(a)(b)


### 3.2. Electrochemical Characteristics ofMn2O3 Nanospheres

#### 3.2.1. Electrochemical Impedance Spectroscopy Analysis: Single Electrode Characteristics

Figure4 shows the ac impedance plots of the Mn2O3 electrodes of samples A and B in 6 M KOH electrolyte measured at OCV of ca. 0.4 V versus SCE
with a frequency range of 105–10−2 Hz with an applied 20 mV peak to peak sinusoidal perturbation voltage. As seen in Figure 4, the frequency dispersion
over the complex impedance plot is composed of a small solution resistance
followed by a semicircle at the mid frequency region and ~45° spike
accompanied by a straight line. The semicircle at higher frequency region is
possibly attributed to the charge transfer process at electrode/electrolyte
interface, and 45° the straight line at lower frequency region
should be ascribed to the diffusion process in solid. It can be seen from Figure
4 that the semicircle for sample A is
a little smaller than that of sample B.
This suggests that the electrochemical reaction resistance in sample A is
smaller comparatively. In addition, the straight line at the lower frequency
region for sample A is less inclined than that of sample B, suggesting
that the former is less capacitive than the later. Alternatively, one can think
of the possible surface roughness factor causing this depression at the double-layer
frequency response regime. Nevertheless, our interpretation of the effect of
depression at low-frequency regime for both samples corroborates with our CV
results. From the impedance data at the lower frequency region, it can be seen that
at the same frequency regime, the values of the real and the imaginary
impedances and thus the overall impedance of sample A are smaller than that of sample
B, suggesting that the latter has a smaller solid-state diffusion impedance
at the interface with the electrolyte and a larger capacitance. Considering the
little difference in the charge transfer resistance (semicircle portion) at higher
frequency region, the EIS results are corroborated with the cyclic voltamograms
as shown in Figure 5.Figure 4
ac impedance plots of the Mn2O3 electrodes of sample A and B in 6 M KOH electrolyte measured at OCV (0.4 V versus SCE) within a frequency range of 105–10−2 Hz.Cyclic voltammograms of sample A (Mn2O3 heated at 400°C/1 h) and sample B (Mn2O3 heated at 400°C/5 h) at different scan rates. Arrow indicates increasing scan rate (100, 70, 50, 20, 10, and 5 mVs−1).
(a)(b)


#### 3.2.2. Rate-Dependent Cyclic Voltammetry (CV): Single Electrode Properties

The pseudocapacitive behavior of the synthesized manganese oxide was examined by a cyclic
voltammetry technique at different scan rates.Mn2O3 heated
at 400°C, which was coated on a
stainless steel expanded mesh, was later used as a working electrode in a three-electrode
electrochemical cell, and the electrode potential was scanned between 0.0 and
0.4 V versus SCE in both anodic and cathodic directions. The rate-dependent
current responses were measured at different scan
rates from 5 mVs−1 up to 100 mVs−1 as shown in
Figure 5. Figures 5(a)
and 5(b) depict the
voltammograms recorded for sample A (Mn2O3 heated at 400°C/1 h)
and sample B (Mn2O3 heated at 400°C/5 h) at
various scan rates, respectively.The
electrochemical reaction ofMn2O3 in alkaline solution
has not been discussed much in the literature according to our knowledge. Recently,
Lin et al. described a sol-gel manganese oxide with Mn2O3 and
Mn3O4 mixture, suggesting that the relatively low valence
states of Mn2.67+ (Mn3O4) and Mn3+ (Mn2O3) changed into high valence Mn4+ after cyclic
voltammetry [34]. That
is, its pseudocapacitance is known to arise from electron transfer at the Mn
sites, and the charge transfer was thought to be balanced either by
chemisorption/desorption of electrolyte cations [35] or by
insertion/deinsertion of protons [16] as shown in the equation (2)MnO2+H++e−→MnOOH. However, in a
recent report it was suggested that the reaction involves bulk
insertion/extraction of both oxonium (H3O+) and
electrolyte cation as confirmed by electrochemical quartz-crystal microbalance
(EQCM) and in-situ XAS analysis [36].As far asMn2O3 is concerned, Mn3+ in Mn2O3 is reasonably described
as going through an oxidation reaction giving rise to one electron transfer
attaining a valence state of Mn4+. Accordingly, the CV profiles of
composite Mn2O3 electrode versus SCE are not perfectly
rectangular but exhibit near symmetric redox humps between 0 and 0.4 V.
As shown in Figure 6, well-resolved oxidation and reduction peaks are
evident at about 0.27 V and 0.17 V, respectively, during the charge/discharge
reaction. However, sample A shows no
clear oxidation peak unlike sample B.
The latter showed well-resolved redox peaks presumably due to facile crystal
structure caused by prolonged annealing of the sample. The extended holding
time could probably help in attaining the facile cubic structure as confirmed
by the well-resolved diffraction peaks in Figure 1.Figure 6
Cyclic voltammograms of nanostructuredMn2O3 (annealed at 400°C for 1 h and 5 h) as working electrode against SCE in a three electrode cell at 5 mV/s.This facilitates
the easy reaction kinetics of OH− being chemisorbed and/or
intercalated into host Mn2O3 cubic lattice. Accordingly,
the following electrochemical reaction is expected to take place during the
charge/discharge process involving OH− chemisorption/intercalation
into Mn2O3 structure:(3) The average specific
capacitance versus voltage for Mn2O3 deduced using the formula (4)Cspecific=isw, where i is the instantaneous current, s is the
potential scan rate, and w is the mass of the active material in an
electrode. Accordingly, a specific
capacitance of 40–50 Fg−1 has been obtained for sample A at 5 mVs−1 scan
rate (see Figure 7).Specific capacitance versus potential plot of  nanosphericalMn2O3. Arrow indicates decreasing scan rate (100, 70, 50, 20, 10, and 5 mVs−1).
(a)(b)As expected,sample B rendered almost twice the capacitance 90–100 Fg−1 at
5 mVs−1. One plausible cause for the lower specific capacitance is
probably the existence of Mn3+ oxidation state of Mn in Mn2O3. This has probably resulted in
diminished specific capacitance. Also,
there is another possible reason related to the presence of two distinct Mn
atom sites in Mn2O3 [37]; Mn3+ partially
occupies octahedral site as well as distorted
octahedral site. Perhaps this may have caused a portion of Mn ion present in Mn2O3 to be partially not available in the electrochemical oxidation reaction. Obviously, such
hindrances might diminish the specific capacitance even further. The studies on
hybrid asymmetric capacitor employing nanosphere Mn2O3 as
positive electrode against a mesoporous carbon negative electrode will be
reported elsewhere.


## 3.1. Structural and Microstructural (Nanophase) Characterization of the Active Electrode Material,Mn2O3

A single-phase cubic structure ofMn2O3 was evident from XRD analysis
as shown in Figure 1. The hkl planes are well-indexed to a cubic phase with lattice parameter a=9.4080 Å  having broad crystalline peaks indicating the presence of explicit
nanostructure. Obviously, the
broad peaks illustrate the presence of small crystallites (domains) and by
knowing the FWHM peak information from the powder diffraction data, the
particle size of the crystallites can be estimated by using Scherrer's formula. The
structural results are in good agreement
with the JCPDS 41-1442 database. The effect of postheat treatment versus
time has been investigated in order to study the electrode-active Mn2O3 nanospheres.
Figure 1 shows the XRD pattern obtained on a sample
annealed at 400°C for 1 hour and 5 hours as holding time. As the
holding time is increased from 1 hour to 5 hours (sample A and sample B, resp.), while maintained at 400°C,
XRD peaks became relatively sharper yet maintaining the features corresponding
to its nanostructure (broad crystalline peaks).Figure 1
X-ray diffractograms ofMn2O3 showing broad crystalline peaks; sample A heated at 400°C for 1 hour and sample B heated at 400°C for 5 hours.TEM images on the
annealed samples are reproduced in Figure2.
Figure 2(a) reveals that the
manganese oxide powders synthesized at 400°C exhibit spherical-shaped particles (within 10–15 nm range) with
porous morphology. The spherical shape of the product was also ascertained by
FE-SEM analysis as well. It has been found that as the annealing time increased
to 5 hours, the particle size remains the same and possible grain growth is not
explicitly evident as shown in Figures 3(a) and 3(b).
The porous particle
morphology was evident from the FESEM analysis.TEM pictures ofMn2O3 heated at 400°C for (a) 1 hour and (b) 5 hours.
(a)(b)NanosphereMn2O3 heated at 400°C for 5 hours (a) FE-SEM image and (b) Back scattered image.
(a)(b)


## 3.2. Electrochemical Characteristics ofMn2O3 Nanospheres

### 3.2.1. Electrochemical Impedance Spectroscopy Analysis: Single Electrode Characteristics

Figure4 shows the ac impedance plots of the Mn2O3 electrodes of samples A and B in 6 M KOH electrolyte measured at OCV of ca. 0.4 V versus SCE
with a frequency range of 105–10−2 Hz with an applied 20 mV peak to peak sinusoidal perturbation voltage. As seen in Figure 4, the frequency dispersion
over the complex impedance plot is composed of a small solution resistance
followed by a semicircle at the mid frequency region and ~45° spike
accompanied by a straight line. The semicircle at higher frequency region is
possibly attributed to the charge transfer process at electrode/electrolyte
interface, and 45° the straight line at lower frequency region
should be ascribed to the diffusion process in solid. It can be seen from Figure
4 that the semicircle for sample A is
a little smaller than that of sample B.
This suggests that the electrochemical reaction resistance in sample A is
smaller comparatively. In addition, the straight line at the lower frequency
region for sample A is less inclined than that of sample B, suggesting
that the former is less capacitive than the later. Alternatively, one can think
of the possible surface roughness factor causing this depression at the double-layer
frequency response regime. Nevertheless, our interpretation of the effect of
depression at low-frequency regime for both samples corroborates with our CV
results. From the impedance data at the lower frequency region, it can be seen that
at the same frequency regime, the values of the real and the imaginary
impedances and thus the overall impedance of sample A are smaller than that of sample
B, suggesting that the latter has a smaller solid-state diffusion impedance
at the interface with the electrolyte and a larger capacitance. Considering the
little difference in the charge transfer resistance (semicircle portion) at higher
frequency region, the EIS results are corroborated with the cyclic voltamograms
as shown in Figure 5.Figure 4
ac impedance plots of the Mn2O3 electrodes of sample A and B in 6 M KOH electrolyte measured at OCV (0.4 V versus SCE) within a frequency range of 105–10−2 Hz.Cyclic voltammograms of sample A (Mn2O3 heated at 400°C/1 h) and sample B (Mn2O3 heated at 400°C/5 h) at different scan rates. Arrow indicates increasing scan rate (100, 70, 50, 20, 10, and 5 mVs−1).
(a)(b)


### 3.2.2. Rate-Dependent Cyclic Voltammetry (CV): Single Electrode Properties

The pseudocapacitive behavior of the synthesized manganese oxide was examined by a cyclic
voltammetry technique at different scan rates.Mn2O3 heated
at 400°C, which was coated on a
stainless steel expanded mesh, was later used as a working electrode in a three-electrode
electrochemical cell, and the electrode potential was scanned between 0.0 and
0.4 V versus SCE in both anodic and cathodic directions. The rate-dependent
current responses were measured at different scan
rates from 5 mVs−1 up to 100 mVs−1 as shown in
Figure 5. Figures 5(a)
and 5(b) depict the
voltammograms recorded for sample A (Mn2O3 heated at 400°C/1 h)
and sample B (Mn2O3 heated at 400°C/5 h) at
various scan rates, respectively.The
electrochemical reaction ofMn2O3 in alkaline solution
has not been discussed much in the literature according to our knowledge. Recently,
Lin et al. described a sol-gel manganese oxide with Mn2O3 and
Mn3O4 mixture, suggesting that the relatively low valence
states of Mn2.67+ (Mn3O4) and Mn3+ (Mn2O3) changed into high valence Mn4+ after cyclic
voltammetry [34]. That
is, its pseudocapacitance is known to arise from electron transfer at the Mn
sites, and the charge transfer was thought to be balanced either by
chemisorption/desorption of electrolyte cations [35] or by
insertion/deinsertion of protons [16] as shown in the equation (2)MnO2+H++e−→MnOOH. However, in a
recent report it was suggested that the reaction involves bulk
insertion/extraction of both oxonium (H3O+) and
electrolyte cation as confirmed by electrochemical quartz-crystal microbalance
(EQCM) and in-situ XAS analysis [36].As far asMn2O3 is concerned, Mn3+ in Mn2O3 is reasonably described
as going through an oxidation reaction giving rise to one electron transfer
attaining a valence state of Mn4+. Accordingly, the CV profiles of
composite Mn2O3 electrode versus SCE are not perfectly
rectangular but exhibit near symmetric redox humps between 0 and 0.4 V.
As shown in Figure 6, well-resolved oxidation and reduction peaks are
evident at about 0.27 V and 0.17 V, respectively, during the charge/discharge
reaction. However, sample A shows no
clear oxidation peak unlike sample B.
The latter showed well-resolved redox peaks presumably due to facile crystal
structure caused by prolonged annealing of the sample. The extended holding
time could probably help in attaining the facile cubic structure as confirmed
by the well-resolved diffraction peaks in Figure 1.Figure 6
Cyclic voltammograms of nanostructuredMn2O3 (annealed at 400°C for 1 h and 5 h) as working electrode against SCE in a three electrode cell at 5 mV/s.This facilitates
the easy reaction kinetics of OH− being chemisorbed and/or
intercalated into host Mn2O3 cubic lattice. Accordingly,
the following electrochemical reaction is expected to take place during the
charge/discharge process involving OH− chemisorption/intercalation
into Mn2O3 structure:(3) The average specific
capacitance versus voltage for Mn2O3 deduced using the formula (4)Cspecific=isw, where i is the instantaneous current, s is the
potential scan rate, and w is the mass of the active material in an
electrode. Accordingly, a specific
capacitance of 40–50 Fg−1 has been obtained for sample A at 5 mVs−1 scan
rate (see Figure 7).Specific capacitance versus potential plot of  nanosphericalMn2O3. Arrow indicates decreasing scan rate (100, 70, 50, 20, 10, and 5 mVs−1).
(a)(b)As expected,sample B rendered almost twice the capacitance 90–100 Fg−1 at
5 mVs−1. One plausible cause for the lower specific capacitance is
probably the existence of Mn3+ oxidation state of Mn in Mn2O3. This has probably resulted in
diminished specific capacitance. Also,
there is another possible reason related to the presence of two distinct Mn
atom sites in Mn2O3 [37]; Mn3+ partially
occupies octahedral site as well as distorted
octahedral site. Perhaps this may have caused a portion of Mn ion present in Mn2O3 to be partially not available in the electrochemical oxidation reaction. Obviously, such
hindrances might diminish the specific capacitance even further. The studies on
hybrid asymmetric capacitor employing nanosphere Mn2O3 as
positive electrode against a mesoporous carbon negative electrode will be
reported elsewhere.


## 3.2.1. Electrochemical Impedance Spectroscopy Analysis: Single Electrode Characteristics

Figure4 shows the ac impedance plots of the Mn2O3 electrodes of samples A and B in 6 M KOH electrolyte measured at OCV of ca. 0.4 V versus SCE
with a frequency range of 105–10−2 Hz with an applied 20 mV peak to peak sinusoidal perturbation voltage. As seen in Figure 4, the frequency dispersion
over the complex impedance plot is composed of a small solution resistance
followed by a semicircle at the mid frequency region and ~45° spike
accompanied by a straight line. The semicircle at higher frequency region is
possibly attributed to the charge transfer process at electrode/electrolyte
interface, and 45° the straight line at lower frequency region
should be ascribed to the diffusion process in solid. It can be seen from Figure
4 that the semicircle for sample A is
a little smaller than that of sample B.
This suggests that the electrochemical reaction resistance in sample A is
smaller comparatively. In addition, the straight line at the lower frequency
region for sample A is less inclined than that of sample B, suggesting
that the former is less capacitive than the later. Alternatively, one can think
of the possible surface roughness factor causing this depression at the double-layer
frequency response regime. Nevertheless, our interpretation of the effect of
depression at low-frequency regime for both samples corroborates with our CV
results. From the impedance data at the lower frequency region, it can be seen that
at the same frequency regime, the values of the real and the imaginary
impedances and thus the overall impedance of sample A are smaller than that of sample
B, suggesting that the latter has a smaller solid-state diffusion impedance
at the interface with the electrolyte and a larger capacitance. Considering the
little difference in the charge transfer resistance (semicircle portion) at higher
frequency region, the EIS results are corroborated with the cyclic voltamograms
as shown in Figure 5.Figure 4
ac impedance plots of the Mn2O3 electrodes of sample A and B in 6 M KOH electrolyte measured at OCV (0.4 V versus SCE) within a frequency range of 105–10−2 Hz.Cyclic voltammograms of sample A (Mn2O3 heated at 400°C/1 h) and sample B (Mn2O3 heated at 400°C/5 h) at different scan rates. Arrow indicates increasing scan rate (100, 70, 50, 20, 10, and 5 mVs−1).
(a)(b)


## 3.2.2. Rate-Dependent Cyclic Voltammetry (CV): Single Electrode Properties

The pseudocapacitive behavior of the synthesized manganese oxide was examined by a cyclic
voltammetry technique at different scan rates.Mn2O3 heated
at 400°C, which was coated on a
stainless steel expanded mesh, was later used as a working electrode in a three-electrode
electrochemical cell, and the electrode potential was scanned between 0.0 and
0.4 V versus SCE in both anodic and cathodic directions. The rate-dependent
current responses were measured at different scan
rates from 5 mVs−1 up to 100 mVs−1 as shown in
Figure 5. Figures 5(a)
and 5(b) depict the
voltammograms recorded for sample A (Mn2O3 heated at 400°C/1 h)
and sample B (Mn2O3 heated at 400°C/5 h) at
various scan rates, respectively.The
electrochemical reaction ofMn2O3 in alkaline solution
has not been discussed much in the literature according to our knowledge. Recently,
Lin et al. described a sol-gel manganese oxide with Mn2O3 and
Mn3O4 mixture, suggesting that the relatively low valence
states of Mn2.67+ (Mn3O4) and Mn3+ (Mn2O3) changed into high valence Mn4+ after cyclic
voltammetry [34]. That
is, its pseudocapacitance is known to arise from electron transfer at the Mn
sites, and the charge transfer was thought to be balanced either by
chemisorption/desorption of electrolyte cations [35] or by
insertion/deinsertion of protons [16] as shown in the equation (2)MnO2+H++e−→MnOOH. However, in a
recent report it was suggested that the reaction involves bulk
insertion/extraction of both oxonium (H3O+) and
electrolyte cation as confirmed by electrochemical quartz-crystal microbalance
(EQCM) and in-situ XAS analysis [36].As far asMn2O3 is concerned, Mn3+ in Mn2O3 is reasonably described
as going through an oxidation reaction giving rise to one electron transfer
attaining a valence state of Mn4+. Accordingly, the CV profiles of
composite Mn2O3 electrode versus SCE are not perfectly
rectangular but exhibit near symmetric redox humps between 0 and 0.4 V.
As shown in Figure 6, well-resolved oxidation and reduction peaks are
evident at about 0.27 V and 0.17 V, respectively, during the charge/discharge
reaction. However, sample A shows no
clear oxidation peak unlike sample B.
The latter showed well-resolved redox peaks presumably due to facile crystal
structure caused by prolonged annealing of the sample. The extended holding
time could probably help in attaining the facile cubic structure as confirmed
by the well-resolved diffraction peaks in Figure 1.Figure 6
Cyclic voltammograms of nanostructuredMn2O3 (annealed at 400°C for 1 h and 5 h) as working electrode against SCE in a three electrode cell at 5 mV/s.This facilitates
the easy reaction kinetics of OH− being chemisorbed and/or
intercalated into host Mn2O3 cubic lattice. Accordingly,
the following electrochemical reaction is expected to take place during the
charge/discharge process involving OH− chemisorption/intercalation
into Mn2O3 structure:(3) The average specific
capacitance versus voltage for Mn2O3 deduced using the formula (4)Cspecific=isw, where i is the instantaneous current, s is the
potential scan rate, and w is the mass of the active material in an
electrode. Accordingly, a specific
capacitance of 40–50 Fg−1 has been obtained for sample A at 5 mVs−1 scan
rate (see Figure 7).Specific capacitance versus potential plot of  nanosphericalMn2O3. Arrow indicates decreasing scan rate (100, 70, 50, 20, 10, and 5 mVs−1).
(a)(b)As expected,sample B rendered almost twice the capacitance 90–100 Fg−1 at
5 mVs−1. One plausible cause for the lower specific capacitance is
probably the existence of Mn3+ oxidation state of Mn in Mn2O3. This has probably resulted in
diminished specific capacitance. Also,
there is another possible reason related to the presence of two distinct Mn
atom sites in Mn2O3 [37]; Mn3+ partially
occupies octahedral site as well as distorted
octahedral site. Perhaps this may have caused a portion of Mn ion present in Mn2O3 to be partially not available in the electrochemical oxidation reaction. Obviously, such
hindrances might diminish the specific capacitance even further. The studies on
hybrid asymmetric capacitor employing nanosphere Mn2O3 as
positive electrode against a mesoporous carbon negative electrode will be
reported elsewhere.


## 4. Conclusion

An efficient simple synthesis protocol has been reported to
synthesize nanospherical manganese oxide,Mn2O3 (grain
size within 10–15 nm), by means
of a simple combined solvothermal/ultrasonication at low temperature. The nanostructure
features have been confirmed by both XRD and electron microscopic analysis (FE-SEM
and TEM). The rate-dependent cyclic voltammetry revealed that both Mn2O3 samples exhibit redox reaction via
Mn3+/Mn4+ couple
yielding one electron transfer which corresponds to the electrochemical
capacitance of ~100 Fg−1 in alkaline medium (6 M KOH). The modest specific capacitance has
been attributed to the presence of facile crystal structure facilitating
the easy insertion/removal of electrolyte ions (OH−) in Mn2O3 nanospheres. The
EIS measurements also corroborate the findings of CV results with respect to
heat-treated samples as a function of holding time (sample A and sample B).



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*Source: 948183-2009-03-08.xml*