Electrochemical capacitances of a nanowire-structured MnO2 in polyacrylate-based gel electrolytes

Article abstract of DOI:10.1149/2.112206jes

Nanowire-structured MnO₂ was synthesized by a sonochemical method and its electrochemical capacitances were investigated in three kinds of electrolytes: liquid-type KCl solution, potassium polyacrylate (PAAK), and potassium polyacrylate-co-polyacrylamide (PAAK-PAAM) gel polymer electrolytes (GPEs). The prepared MnO₂ with a nanowire structure had a high specific surface area and narrow pore distribution, which was very useful for gel-type electrolyte in supercapacitor applications. The specific capacitance of nanowire-structured MnO₂ with PAAK electrolyte exceeded 484 F g ^-1 at the conventional loading weight of MnO₂ (1.02 mg cm^-2) in the electrode. The nanowire-structured MnO₂ with PAAK electrolytes was a very promising electrode material for high-performance supercapacitors.

Full text of DOI:10.1149/2.112206jes

Journal of The Electrochemical Society, 159 (6) A899-A903 (2012)
A899
0013-4651/2012/159(6)/A899/5/$28.00 © The Electrochemical Society
Electrochemical Capacitances of a Nanowire-Structured MnO2 in
Polyacrylate-Based Gel Electrolytes
Ho-Seong Nam,a Nae-Lih Wu,b,∗ Kuang-Tsin Lee,b Kwang Man Kim,c,∗ Chul Gi Yeom,d
Lovely Rose Hepowit,d Jang Myoun Ko,d,z and Jong-Duk Kima
aCenter for Ultramicrochemical Process System, Department of Chemical and Biomolecular Engineering, Korea
Advanced Institute of Science and Technology (KAIST), Daejon 305-701, Korea
bDepartment of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
cResearch Team of Power Control Devices, Electronics and Telecommunications Research Institute (ETRI), Daejon
305-700, Korea
dDepartment of Applied Chemistry and Biotechnology, Hanbat National University, Daejon 305-719, Korea
Nanowire-structured MnO2 was synthesized by a sonochemical method and its electrochemical capacitances were investigated in
three kinds of electrolytes: liquid-type KCl solution, potassium polyacrylate (PAAK), and potassium polyacrylate-co-polyacrylamide
(PAAK-PAAM) gel polymer electrolytes (GPEs). The prepared MnO2 with a nanowire structure had a high specific surface
area and narrow pore distribution, which was very useful for gel-type electrolyte in supercapacitor applications. The specific
capacitance of nanowire-structured MnO2 with PAAK electrolyte exceeded 484 F g−1 at the conventional loading weight of MnO2
(1.02 mg cm−2) in the electrode. The nanowire-structured MnO2 with PAAK electrolytes was a very promising electrode material
for high-performance supercapacitors.
© 2012 The Electrochemical Society. [DOI: 10.1149/2.112206jes] All rights reserved.
Manuscript submitted August 11, 2011; revised manuscript received April 2, 2012. Published April 30, 2012.
Electrochemical capacitors are expected to be useful for high
power applications due to their high-power capability; low sensitivity
to temperature and easily monitored electrical behavior because they
can fully discharge in a few seconds; longer cycles of several hundred
thousand times; and high energy efficiency of charge and discharge.1
Among the metal oxide active materials, amorphous MnO2 has been
drawing tremendous attention because of its low cost, natural abun-
dance, and environmental safety.2,3 However, the specific capacitance
of MnO2 depends on its particle size, morphology, and crystal structure
because the MnO2 usually has low intrinsically electronic conductiv-
ity and clustered morphology.4 For a conventional MnO2 electrode in
an alkaline chloride electrolyte, the usual specific capacitance ranges
from 100 to 200 F g−1. At a low loading weight of MnO2 in the elec-
trode, however, the specific capacitance achieves a value greater than
300 F g−1; for instance, 350 F g−1 for 0.05 mg cm−2 and 678 F g−1
Experimental
Amorphous MnO2 was synthesized by the method of Lee and
Goodenough.3 First, 0.55 g of manganese acetate (Aldrich) was dis-
solved in 40 mL of distilled water; 0.237 g of potassium permanganate
(Aldrich) was dissolved in 15 mL of distilled water. The as-prepared
potassium permanganate solution was then added dropwise to the
manganese acetate solution using a syringe pump; ammonium hy-
droxide was added in the same manner under ultrasound irradiation
with a 37 kHz ultrasonic wave at 600 W output power from a bath-
type sonicator (Sonics & Materials, Danbury). During the sonication,
the glass vessel was cooled with flowing water. After sonication for
30 min, the resulting brown precipitate was separated by decanting and
washing several times with distilled water. It was subsequently dried
at 40◦C in a vacuum for 12 h to yield the nanowire-structured MnO2.
The as-prepared MnO2 was characterized by a crystalline property
analysis using an X-ray diffractometer (Siemens), morphology ob-
servations using a field emission scanning electron microscopy (FE-
SEM, Nova 230, FEI Comp.), a transmission electron microscopy
(FE-TEM) and selected electron diffraction patterns (JEM–2100F at
200 kV, JEOL). The specific surface area and the pore size distri-
bution of the as-prepared MnO2 were also measured by Brunauer-
Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) techniques,
respectively, using a Smartsorb-92/93 (Micromeritics) after degassing
at 100◦C for 2 h.
for 0.00105 mg cm−2 5,6
Such a difference is likely due to the elec-
.
trolyte accessibility to the active materials because amorphous MnO2
is difficult to be completely wetted by the electrolyte. In this stage,
the nanostructured MnO2 employed in the supercapacitor electrode
can be a critical issue in increasing its performance.7,8 In particu-
lar, one-dimensional nanowire-structured MnO2 has been proven to
provide a high specific area, short diffusion path in the electrode, and
good porous structures for electrolyte penetration. Subramanian et al.9
hydrothermally prepared manganese dioxide (MnO2) nanostructures
that had a specific capacitance of 168 F g−1. Jiang et al.10 synthesized
nanostrucutred MnO2 by co-precipitation in the presence of Pluronic
P123 and recorded a specific capacitance of 176 F g−1. In addition,
liquid-type electrolytes can be replaced with solid or gel-type elec-
trolytes to prohibit the leakage and corrosion of liquid electrolytes and
also to enhance the safety and reliability of the cell.11
In the present work, the nanowire-structured MnO2 electrode is
prepared by a sonochemical method to enhance the wettability of
gel electrolytes and maintain a conventional MnO2 loading weight
(1.02 mg cm−2).12 The supercapacitive properties of the nanowire-
structured MnO2 electrode are investigated by cyclic voltammetry in
different electrolyte systems: liquid-type KCl electrolyte, potassium
polyacrylate (PAAK)-based gel polymer electrolyte (GPE), and potas-
sium polyacrylate-co-polyacrylamide (PAAK-PAAM) GPE. The re-
sults are also compared with the previous reports6,11 using the hydrous
MnO2 and similar electrolyte systems.
A viscous slurry containing the as-prepared MnO2 powder
(59.5 wt%) as an active material, carbon black (Vulcan XC72)
(25.5 wt%) as a conductive agent, poly(vinylidene difluoride)
(Aldrich) (15 wt%) as a binder and N-methyl-2-pyrrolidone (Mit-
subishi Chemical) as a solvent was pasted onto a Ti foil and dried at
120◦C for 6 h in a vacuum oven. The dried electrode after the pressing
process was cut to 1 cm × 1 cm dimension with a thickness of 116 μm,
which contained 1.02 ( 0.03) mg of the MnO2. The theoretical den-
sity of the electrode was calculated to about 2.995 g cm−3 from the
theoretical densities of all the components in the electrode, i.e., 5.026,
1.95, and 1.77 g cm−3, corresponding to the MnO2, carbon black,
and binder, respectively. The packing density was also estimated by
considering the loading weight and film thickness to 1.094 g cm−3
.
The porosity of the nanowire-structured MnO2 was then calculated as
40% from the electrode and packing densities. However, the electrode
containing the conventional particles of MnO2, having the thickness
of 95 μm at the same composition and loading weight, showed the
packing density of 2.285 g cm−3 and the porosity of 23.7%. The
base polymers used to make GPE systems were PAAK (Aldrich) and
∗Electrochemical Society Active Member.
zE-mail: jmko@hanbat.ac.kr.
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A900
Journal of The Electrochemical Society, 159 (6) A899-A903 (2012)
Figure 1. X-ray diffraction pattern of the nanowire-structured MnO2.
PAAK-PAAM (Aldrich). For electrochemical characterizations, the
dried electrode was first immersed in a 10 cm3 solution of 1 M KCl,
and 1 g of polymer was subsequently added into the solution. The
concentration of KCl decreased slightly by the addition of polymer
to become the GPE. The resulting self-supported GPE consisted of 9
wt% polymer, 6.7 wt% KCl, and 84.3 wt% H2O.
Figure 2. (a) FE-SEM, (b) FE-TEM images of the conventional MnO2 without
the sonochemical method, (c) FE-SEM, and (d) FE-TEM images of nanowire-
structured MnO2. The inset shows the selected electron diffraction pattern.
The porous properties of the MnO2 electrode can be significant
from the viewpoint of allowing the electrolyte into the pores within
the electrode. As a porous property, Fig. 3 shows the N2 adsorp-
tion/desorption isotherm of the nanowire-structured MnO2. The N2
sorption isotherm of the nanowire-structured MnO2 shows an in-
crease/decrease in an adsorption/desorption curve, which represents a
uniform pore size or a narrow pore-size distribution13 by the condensa-
tion of the porous architecture produced by the sonochemical method.
To further understand the relationship between the microstructure of
porous membrane and N2 sorption isotherms, a decent textbook14 can
be referred. For the other porous properties, the BET surface area of
the nanowire-structured MnO2 is estimated to 250 m2 g−1. The BJH
pore size distribution is also shown in the inset of Fig. 3. The pore
is confirmed as a single modal type centered at around an average
diameter of 3.6 nm, implying that the porous nanowire structure is
facilitating the solvated ion transfer into the electrode up to a conven-
tional loading weight (1.02 mg cm2) of the active material.
Cyclic voltammetry was conducted with an electrochemical an-
alyzer (Eco Chemie P/Gstat 30, Autolab), carried out in a single
compartment with a three-electrode cell, and the distance between the
working and the counter electrodes was about 1 cm and the reference
electrode was Ag/AgCl saturated KCl (EG&G, 197 mV vs SHE at
25◦C). The cyclic voltammetry was conducted in a voltage range from
0.1 to 0.9 V to the reduction and oxidation of active materials, but
not of the electrolyte. The electrochemical window of ∼0.8 V is elec-
trochemically stable voltage range for all electrolytes adopted in this
study. The specific capacitance (Csp) was calculated from the cyclic
voltammogram according to the equation of Csp = (qa-qc)/2mꢀV,
where the qa, qc, m, and ꢀV denote the anodic and cathodic charges,
the mass of the active material, and the potential window of cyclic
voltammetry, respectively. A chronopotential charge-discharge test
was also performed between 0.1–0.9 V at the current densities of
1–3 A g−1. The electrochemical impedance spectra were obtained
from a frequency response analyzer (Eco Chemie P/Gstat 30, Auto-
lab) with a frequency range of 10–1-105 Hz and a bias voltage of 10 mV.
The room-temperature ionic conductivity (σ) of the electrolyte was
calculated from the impedance spectrum according to the equation of
σ = l/(Rs · A), where l was the distance (3 mm) between the reference
and working electrodes and A the surface area (1 cm2) of the electrode.
The supercapacitive properties of nanowire-structured MnO2 are
investigated by cyclic voltammetry between 0.1 and 0.9 V (vs.
Ag/AgCl) in different electrolytes, i.e., liquid electrolyte, PAAK, and
PAAK-PAAM based GPEs (see Fig. 4). Using the liquid-type KCl
Results and Discussion
Fig. 1 shows the X-ray diffraction pattern of the as-prepared MnO2.
The diffraction peaks of 2θ = 37.1, 42.4, 56.0, 68.8, and 78.9◦ cor-
respond to the crystalline planes of (100), (101), (102), (110), and
(200), respectively, in the amorphous α-MnO2 structure (JCPDS no.
44-0141). The shapes and distribution of the MnO2 particles can be
confirmed by FE-SEM and FE-TEM images, as shown in Fig. 2.
Conventional MnO2 prepared without the sonochemical method has
a spherical structure with a diameter of about 40 nm (see Figs. 2a and
2b).10 The as-prepared MnO2, however, has a nanowire structure com-
posed of one-dimensional nanoarrarys with a length of 300–700 nm
a diameter of about 10 nm (L/D = 30–70). Meanwhile, there are
some defects among the nanowires, which increased specific surface
area. The corresponding selected area electron diffraction pattern also
shows a faint ring image reflecting the amorphous structure of MnO2
(see Figs. 2c and 2d). Thus, the nanowire-structured MnO2 behaves
as one-dimensional nanoarrays interconnected in a disorderly way,
facilitating the electrolyte to penetrate the electrode more easily.
Figure 3. N2 adsorption-desorption isotherms of the nanowire-structured
MnO2. The inset indicates the pore size distribution of the nanowire-structured
MnO2.
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Journal of The Electrochemical Society, 159 (6) A899-A903 (2012)
A901
Figure 5. Specific capacitance of the nanowire-structured MnO2 cells at var-
ious scan rates.
electrolyte cell, 92% for the PAAK cell, and 62% for the PAA-PAAM
cells, compared with the hydrous MnO2 particles. In addition, Fig. 5
shows that the capacitance decreasing rates of GPEs with increasing
scan rate are higher than that of liquid electrolyte.
Fig. 6 shows the charge–discharge behavior of the nanowire-
structured MnO2 in the different electrolytes at the higher current
Figure 4. Cyclic voltammograms of the nanowire-structured MnO2 cells with
(a) the liquid-type KCl electrolyte, (b) the PAAK GPE, and (C) the PAAK-
PAAM GPE. The salt concentration in the liquid electrolyte is 1 M KCl.
electrolyte, the cyclic voltammogram shows nearly rectangular pro-
files, indicating nearly ideal capacitor behavior.15 Using GPEs, how-
ever, somewhat deviated cyclic voltammograms from the rectangular
shape with high specific capacitances are obtained, especially in the
low scan rate region. This is because of the nanowire-structured MnO2
in which the GPE components containing the carboxylate groups eas-
ily penetrate the porous architecture of the MnO2 which make it pos-
sible to further increase the interfacial contact between the electrode
and the electrolyte. Thus, it is likely that the specific capacitance can
be greatly enhanced by the adoption of polyacrylate-based GPE sys-
tems, as shown in Fig. 5. The cells composed of nanowire-structured
MnO2 and GPEs show higher specific capacitance than the liquid-
electrolyte cell. Of these, the electrode with the PAAK electrolyte
records the highest values of specific capacitance. For instance, at
a scan rate of 4 mV s−1, the nanowire-structured MnO2 achieves
250 F g−1 in the liquid-type KCl, 485 F g−1 in the PAAK, and 380 F g−1
in the PAAK-PAAM electrolytes. There is a 94% and 52% increases
in specific capacitance for the PAAK and PAAK-PAAM GPE cells,
respectively, compared with the liquid electrolyte cell. In contrast, hy-
drous MnO2 particles with the same electrolyte systems6 shows that
the maximum specific capacitances of 154, 253, and 235 F g−1, were
obtained at 4 mV s−1 for liquid-type KCl, PAAK, and PAAK-PAAM
electrolytes, respectively. Thus, the nanowire-structured MnO2 in the
present study enhances the specific capacitance by 62% for the liquid
Figure 6. Chronopotentialcharge-discharge curvesofthenanowire-structured
MnO2 under different current densities (1–3 A g−1) in (a) the liquid-type KCl
electrolyte, (b) the PAAK GPE, and (c) the PAAK-PAAM GPE. The salt
concentration in the liquid electrolyte is 1 M KCl.
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A902
Journal of The Electrochemical Society, 159 (6) A899-A903 (2012)
Figure 8. Schematic illustration of electrode layer between electrode and
gel electrolyte in (a) the conventional MnO2 and (b) the nanowire-structured
MnO2.
Figure 7. Nyquist plots of the nanowire-structured MnO2 cells in the range
of 10−1–105 Hz at the bias potential of 10 mV.
between the surface of MnO2 and the gel electrolyte is not electro-
static in nature but probably has specific interactions, e.g., hydrogen
bond and complexes between Mn atoms and carboxyl groups.17,18
Also, Lee at al.6 proved by analyzing X-ray absorption near edge
spectroscopy that the PAAK adsorbed on the MnO2 surface. Thus,
the gel electrolyte strongly adsorbed on the surface of the nanowire-
structured MnO2 is expected to play an important role of capacitance
enhancement. There are two mechanisms proposed for the charge
storage in MnO2. The first is intercalation or deintercalation of alkali
cations (K+) in the electrolyte according to the reaction of MnO2 + K+
+ e− ↔ MnOOK. The second is the adsorption or desorption of alkali
cations in the electrolyte on the MnO2 according to the reaction of
(MnO2)surface + K+ + e− ↔ (MnO2-K+)surface. Thus, the cation trans-
fer between the electrolyte and the MnO2 surface is a very important
factor in obtaining a high capacitance.9,19
Comprehensively considering the results above, a mechanistic ge-
ometry for the nanowire-structured MnO2 adopting the PAAK-based
GPEs can be suggested, as shown in Fig. 8b. In the electrode con-
taining conventional particles of MnO2, the gel electrolyte hardly
penetrates into the electrode layer because it is obviously more vis-
cous and less mobile than that of liquid electrolyte. Accordingly, the
presence of the cationic functional groups is expected not to be ef-
ficiently adsorbed at the surface of MnO2 in the electrode with the
conventional loading weight of MnO2, i.e., 1.02 mg cm−2. Thus, in
the gel type electrode, the permeability of the electrolyte into the
electrode does play an important role in the adsorption or desorption
of cations in the electrolyte on the surface of MnO2. However, the
nanowire-structured MnO2 electrode has plenty of space for efficient
penetration of gel electrolytes into the electrode (see Fig. 8b). Thus,
it could be that PAAK-based gel electrolytes are efficiently adsorbed
on the electrode layer composed of the nanowire-structured MnO2,
which stores the charges by means of adsorption/desorption than in-
tercalation/deintercalation to increase the capacitance.
density range of 1–3 A g−1. The charge-discharge curves are very
symmetric but their slopes maintain the constant value, independent
on the potential. These results indicate that the nanowire-structured
MnO2 electrodes in different electrolytes possess ideal capacitive be-
haviors in the higher current density range.
The electrochemical impedance spectra (see Fig. 7) of the
nanowire-structured MnO2 electrodes in the different electrolytes con-
sist of partially overlapped semicircles followed by straight lines with
a slope, which can be fitted by an equivalent circuit shown in the
inset of Fig. 7. The plot consists of the Warburg impedance (W), solu-
tion resistance (Rs), charge transfer resistance (Rct), and double-layer
capacitance (Cdl).16 The Rs is generally indicated by the first inter-
section of the semi-circle with the real Zꢀ-axis. The W corresponds to
the ion-diffusion effect on the interface between the active material
and electrolyte, typically revealed by a straight line with a slope at the
low frequency region. The Rct indicates the faradaic interfacial charge
transfer resistance, corresponding to the diameter of the semicircle
at the high frequency region. Table I summarizes those impedance
property values such as σ, Rs and Rct of sample cells. The σ of a GPE
system depends on the mobility of polymer chain in the electrolytes.
Accordingly, the trend of the σ values is in the order of PAA-PAAM
< PAAK < liquid electrolyte. PAAK-PAAM is more rigid, less flexi-
ble, and has a higher molecular weight than those of PAAK. Thus, the
electrolyte containing more rigid and less flexible molecules has lower
σ value than the less rigid and more flexible molecules. This agrees
well with the results of Lee et al.9 using hydrous MnO2 particles syn-
thesized by a conventional precipitation method. The σ of electrolytes
mainly affects the electrochemical performance, especially Rs. If the
σ of electrolytes is too low, it deteriorates the efficiency of the charge
and discharge at high scan rates. Thus, the specific capacitances of
PAAK and PAAK-PAAM, having a lower σ than liquid electrolyte, are
rapidly decreased at higher scan rates than that of liquid electrolyte.
In the present study, the Rct of the PAAK and PAAK-PAAM are
higher than that of the liquid electrolyte because of the strong ad-
sorption of the carboxylate group in the gel electrolytes on the MnO2
surfaces over a wide range of pH. Within the pH range 5.2–7.2, the
surface of MnO2 is negatively charged (isoelectric point of pH∼4.0),
while the carboxylic groups in PAAK are dissociated. The interaction
Conclusions
In summary, the supercapacitive properties of nanowire-structured
MnO2 in the PAAK and PAAK-PAAM GPEs are characterized. The
nanowire-structured MnO2 electrode with PAAK GPE shows a max-
imum specific capacitance of 485 F g−1 at a low scan rate, which is
much higher than that of the conventional MnO2 electrode. Thus, this
study confirms that the cell composed of nanowire-structured MnO2
is even more suitable for the PAAK-based GPE for supercapacitor
applications.
Table I. pH values and impedance properties of the
nanowire-structured MnO2 in various electrolytes.
Acknowledgments
Electrolyte system
pH
Rs (ꢁ)
Rct (ꢁ)
σ (S cm−1
)
This research was financially supported by the Ministry of Educa-
tion, Science Technology (MEST) and National Research Foundation
of Korea (NRF) through the Human Resource Training Project for Re-
gional Innovation and by a grant from the cooperative R&D Program
Liquid-type KCl
PAAK GPE
PAAK-PAAM GPE
6.2
5.2
7.2
2.16
2.30
2.63
2.13
3.86
2.02
0.139
0.130
0.114
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Journal of The Electrochemical Society, 159 (6) A899-A903 (2012)
A903
(B551179-10-01-00) funded by the Korea Research Council Indus-
trial Science and Technology. In addition, this research was financially
supported by National Science Council, Taiwan (NSC 98*3114-E-
007-011).
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(2010).
13. J. Y. Song, C. L. Cheng, Y. Y. Wang, and C. C. Wan, J. Electrochem. Soc., 149,
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