Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Article abstract of DOI:10.1016/j.electacta.2010.02.027

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na ₂SO₄ and fumed silica (SiO₂). The MnO ₂ electrode showed a maximum specific capacitance of 151 F g ^-1 after 1000 cycles at 100 mV s^-1 when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g^-1 obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.

Full text of DOI:10.1016/j.electacta.2010.02.027

Electrochimica Acta 55 (2010) 7443–7446
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Supercapacitive properties of a nanowire-structured MnO electrode in the gel
2
electrolyte containing silica
Ho-Seong Nama,b, Jae Sung Kwon , Kwang Man Kim , Jang Myoun Ko , Jong-Duk Kimb
c
d
c,∗
a
Hanwha Chemical R&D Center, Daejeon 305-804, South Korea
Department of Chemical & Biomolecular Engineering, Center for Ultramicrochemical Process System, Korea Advanced Institute of Science & Technology (KAIST),
b
Daejeon 305-701, South Korea
c
Department of Applied Chemistry & Biotechnology, Hanbat National University, San 16-1, Duckmyung-dong, Yusong-gu, Daejeon 305-719, South Korea
Research Team of Power Control Devices, Electronics & Telecommunications Research Institute (ETRI), Daejeon 305-700, South Korea
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanowire-structured MnO2 active materials were prepared by a chemical precipitation method and
their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means
of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na2SO4 and fumed silica (SiO2).
Received 15 September 2009
Received in revised form 9 February 2010
Accepted 9 February 2010
−1
−1
The MnO2 electrode showed a maximum specific capacitance of 151 F g after 1000 cycles at 100 mV s
Available online 18 February 2010
−
1
when using the gel electrolyte containing 3 wt.% of SiO2, which is higher than 121 F g obtained when
using the 1 M Na2SO4 liquid electrolyte alone.
Keywords:
MnO2
©
2010 Elsevier Ltd. All rights reserved.
Nanowire
SiO2
Gel electrolyte
Supercapacitor
1
. Introduction
for the supercapacitor electrode is a critical issue in overcoming the
intrinsically low electronic conductivity and clustered morphology
of MnO2 [14,19].
A supercapcitor is a significant energy storage device that has
attracted much attention due to its high-power capability and long
cycle life, compared to conventional batteries [1]. Supercapacitors
have been employed in many applications such as memory back-up
in electronic devices and hybrid electric vehicles that use a fuel cell
or battery [2]. Supercapacitors may be classified into carbon-based
electric double layer capacitors [3–6] and conducting polymer-
and metal oxide-based pseudocapacitors [7–12]. In a pseudoca-
pacitor, the charge can be concurrently stored on the electrode
surface through a double layer and in the bulk of active material
In the electrolyte system of a supercapacitor, there are three
kinds of electrolytes: the liquid-type, gel-type, and polymeric solid
electrolytes. The liquid electrolyte shows generally high ionic con-
ductivity but the electrolyte leakage remains problematic. The low
ionic conductivity of a solid polymeric electrolyte can reduce the
performance of supercapacitors. However, compared to liquid or
solid polymeric electrolytes [20], a gel electrolyte containing inor-
ganic fillers, e.g., alumina (Al O ) and silica (SiO ), is advantageous
in that there is no leakage and because it exhibits high ionic con-
ductivity due to its thixotropic properties. In the present work,
nanowire-structured MnO2 electrodes are prepared by a chemi-
cal precipitation method and their supercapacitive properties are
investigated by means of cyclic voltammetry in aqueous gel elec-
2
3
2
via a faradic reaction. Various metal oxides, including RuO [10,13],
2
MnO2 [14–17] and NiOx [18], have been studied to evaluate the
charge storage behaviors in supercapacitor electrodes. Among the
metal oxides, MnO appeared to be one of the most attractive elec-
2
trode materials for supercapacitors, due to its low cost, availability,
environmental friendliness, and higher capacitance. However, the
supercapacitive performance of a MnO2 electrode depends on the
particle size of the active material, electrode morphology, and its
crystal structure. In addition, the nanostructured active material
trolytes consisting of 1 M Na SO4 and specified amounts of SiO2.
2
2. Experimental
Sodium dodecyl sulfate (Fluka) was used as received, and its
.2 M aqueous solution was prepared by dissolving in 200 ml of
0
distilled water and 0.06 mol of MnCl ·4H O (Aldrich). Then, 5 mol
2
2
∗ Corresponding author. Tel.: +82 42 821 1545; fax: +82 42 822 1562.
E-mail address: jmko@hanbat.ac.kr (J.M. Ko).
of H O (Junsei, 35%) and 40 ml of ethanol were added to the as-
2 2
prepared solution. After vigorous stirring for 1 h, 0.04 mol of KMnO4
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.02.027

7
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H.-S. Nam et al. / Electrochimica Acta 55 (2010) 7443–7446
Fig. 1. (a) Nyquist plots and (b) ionic conductivities of gel electrolytes containing a
M Na2SO4 aqueous solution and different amounts of SiO2.
1
(
Aldrich) aqueous solution, which has been prepared by dissolving
in 50 ml of distilled water, was added to the as-prepared solution.
After vigorous stirring for 3 h, the resulting brown precipitate was
separated by decantation and washed several times with distilled
Fig. 2. (a) SEM and (b) HR-TEM images of the prepared nanowire-structured MnO2.
◦
water. Subsequently, it was dried at 40 C under a vacuum for 12 h. A
The ionic conductivity of the gel electrolytes of 1 M Na2SO4
aqueous solution containing 0–5 wt.% SiO2 was determined from
complex impedance spectra measured using a frequency response
analyzer (Solartron HF 1225) in a frequency range of 100 kHz to
0.01 Hz. A conductivity cell composed of two stainless steel elec-
trodes on each side of the gel electrolyte film was used. Ionic
conductivity (ꢀ) was calculated from the bulk resistance (R) using
the relation ꢀ = l/(R × A), where l and A represent the thickness of
the gel electrolyte film and the conductivity cell area, respectively.
scanning electron microscope (SEM, Hitachi) and a high resolution
transmission electron microscope (HR-TEM, Jeol) were employed
to characterize the nanostructured morphology.
The MnO2 electrode was fabricated by mixing the prepared
MnO2 (80 wt.%) as an active material with a vapor-grown car-
2
−1
,
bon fiber (10 wt.%) (Showa Denko, specific surface area 13 m g
aspect ratio 67) as a conductive agent and poly(vinylidene fluoride-
co-hexafluoropropylene) as a binder (10 wt.%). The electrode
composite was made using N-methyl-2-pyrrolidone as a solvent.
The resulting slurry was cast on a Pt foil (50 ␮m thick) as a cur-
rent collector using a doctor blade apparatus to yield an electrode
with a thickness of 60 ␮m. The electrodes were dried overnight and
3. Results and discussion
Fig. 1(a) shows the Nyquist plots for the 1 M Na SO4 electrolyte
2
◦
then dried under a vacuum at 60 C for 24 h. After drying, the elec-
containing various amounts of fumed SiO . The real components
2
trode was uniaxially pressed to enhance contact and adherence to
of the impedance spectra at high frequency, corresponding to the
solution resistance and electronic/ionic resistance of the electrode
film, appeared around 10 ꢁ with slight deviations for other samples
with different SiO2 contents. Fig. 1(b) shows the ionic conductivi-
ties as a function of SiO2 content in the 1 M Na SO , as calculated
2
the foil. The electrode was cut to 1 × 1 cm and used as a working
electrode. Cyclic voltammetry was carried out using a potentiostat
(
EG&G, Model 273A) equipped with a three-electrode cell system.
An Ag/AgCl electrode (Metrohm AG 9101 Herisau, 3 M KCl, 0.222 V
2
4
◦
vs. SHE at 25 C) was used as a reference electrode, a coated MnO
from the real components. All of the conductivity values are in a
2
2
−1
current collector as a working electrode, and a 2 × 2 cm Pt plate as a
very small range between 0.125 and 0.147 S cm . That is to say,
counter electrode. Cyclic voltammograms were obtained between
the gel electrolytes containing SiO demonstrate potentials in ionic
2
0
1
.1 and 0.9 V potential windows in both the liquid electrolyte of
M Na SO and the silica gel electrolytes. The gel electrolytes used
conductivity comparable to those of the liquid electrolytes.
Fig. 2 shows SEM and HR-TEM images portraying the morpho-
2
4
were prepared by adding the specified amounts (0–5 wt.%) of fumed
logical properties of nanowire-structured MnO . The nanowires
2
silica (SiO ) (Aerosil 200, Degussa) with a hydrophilic surface into
grow completely to a well-defined length of 300–700 nm and
a diameter of 10–50 nm. In the HR-TEM image, the aggregated
2
the 1 M Na SO aqueous electrolytes.
2
4

H.-S. Nam et al. / Electrochimica Acta 55 (2010) 7443–7446
7445
Fig. 4. Variations of specific capacitance as a function of cycle number (scan rate
−
1
1
1
00 mV s ) for the nanowire-structured MnO2 in 1 M Na2SO4 aqueous solution and
M Na2SO4 containing 0–5 wt.% SiO2.
1
21 and 151 F/g in the liquid electrolyte and the gel electrolyte
containing 3 wt.% SiO , respectively. The increase and saturation
2
in the specific capacitance with the increase in the cycle number
may have resulted from the sequential activation from the surfaces
of the nanowire and MnO2 aggregates into the bulk phase of the
active material. This effect can also be attributed to the increase of
wettability between electrolyte components caused by long-term
immersion [22] or gradual penetration of the electrolyte into the
bulk electrode [23].
In addition, better capacitive behavior of the MnO2 electrode in
the gel electrolyte can be achieved from the hydrophilic SiO2 par-
ticles that exist in the electrical double layer. Because the surface
Fig. 3. Cyclic voltammograms before and after 1000 cycles (scan rate 100 mV s−1)
for the nanowire-structured MnO2 in (a) 1 M Na2SO4 aqueous solution and (b) 1 M
Na2SO4 aqueous solution containing 3 wt.% SiO2.
+
+
of hydrophilic SiO2 may interact with H , a large amount of Na
ions can be adsorbed around the –OH groups in the water com-
ponent of an aqueous electrolyte to satisfy the electron neutrality
of the MnO2 during the Mn4+/Mn reversible redox process [24].
For this reason, until the SiO2 content increases up to 3 wt.%, the
specific capacitance of the nanowire-structured MnO2 electrode in
a gel electrolyte may be higher than that in a liquid electrolyte.
3+
MnO2 species are distributed in the crosslinking points among
the nanowires. The nanowire part surrounding the crosslinks has
an average length above 0.8 ␮m and an average diameter below
When exceeding 3 wt.%, the inclusion of SiO particles may produce
2
1
0 nm. The large particles of MnO2 consist of aggregates of MnO2
a highly viscous medium within the gel electrolyte that inhibits
ion mobility by surpassing the interactions between the SiO2 sur-
face and electrolyte species, and thereby the specific capacitance
decreases. At this stage of higher SiO2 content, other key parame-
ters such as electrode porosity and pore tortuosity should also be
emphasized to determine the capacitance more accurately.
primary particles, whereas the nanowire part forms numerous
defects on the nanowire walls to increase the specific surface area.
This nanowire-structured MnO active material can possess higher
2
potentials in supercapacitive properties due to their higher specific
surface area like the MnO /activated carbon nanotube structure
2
[
21].
In order to investigate the supercapacitive properties of the
4. Conclusions
nanowire-structured MnO2 in the gel electrolyte containing SiO2,
cyclic voltammograms were obtained during 1000 cycles at
−
1
In summary, nanowire-structured MnO2 electrodes have been
prepared by chemical precipitation and their supercapacitive prop-
erties have been investigated by means of cyclic voltammetry in
1
00 mV s as shown in Fig. 3. The absolute current density in the
cyclic voltammogram gradually increased in accordance with the
increase in cyclic number until 1000 cycles. This indicates that the
ionic pathways are sufficiently developed for ions to approach the
electroactive sites in the bulk electrode. After 1000 cycles, both the
liquid and gel electrolytes exhibit nearly rectangular shapes with
a mirror image of the current response, indicating ideal capaci-
aqueous gel electrolytes consisting of 1 M Na SO4 and 0–5 wt.% of
2
SiO . The MnO electrode showed a specific capacitance value of
2
2
− −1
1
1
51 F g after 1000 cycles at 100 mV s when using the gel elec-
trolyte filled with 3 wt.% of SiO , which is higher than 121 F g
−
1
2
when using only the 1 M Na SO4 aqueous solution. Thus, it is
tance behavior. However, the MnO electrode in the gel electrolyte
2
2
suggested that the nanowire-structured MnO2 electrode in a gel
electrolyte with SiO2 is more suitable for use in supercapacitors.
with SiO2 exhibits higher current density and a better rectangu-
lar shape than those in liquid electrolyte. The specific capacitance
was calculated from the cyclic voltammograms using the equa-
tion C = (Qa + Qc)/(2mꢂV), where C, Qa, Qc, m and ꢂV denote the
specific capacitance, anodic and cathodic charges on each scan,
mass of the electroactive material, and the potential window of
the cyclic voltammetry, respectively. The specific capacitance val-
ues are shown in Fig. 4 as a function of cycle number. After 1000
Acknowledgements
This study was partially supported by Hanwha R&D Center, BK21
Project and by the Korean Ministry of Health and Welfare. One of the
authors (HSN) would like to thank the chairman of the Hanwha R&D
Center (S.W. Lee) for the financial support. Another author (JMK)
cycles, the specific capacitance value for the MnO electrodes were
2

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H.-S. Nam et al. / Electrochimica Acta 55 (2010) 7443–7446
appreciates the financial support for this study from the World-
Class University Program (R33-2008-000-10147-0) of the Korean
Ministry of Education, Science and Technology.
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