Effects of electrochemical activation and multiwall carbon nanotubes on the capacitive characteristics of thick MnO2 deposits

Article abstract of DOI:10.1149/1.1815151

Thick composites composed of crystalline manganese dioxide (MnO ₂) and multiwall carbon nanotubes (MWCNTs) were successfully codeposited onto a graphite substrate from an Mn(AcO)₂ · 4H₂O + MWCNTs solution. The rate/nucleation mechanism of MnO ₂ deposition was significantly influenced by the introduction of MWCNTs in the plating baths. The specific capacitance of thick MnO ₂-MWCNT composites, measured from cyclic voltammetry (CV) or chronopotentiometry (CP) in a potential window of 1.0 V, is monotonously decreased from ca. 160 to 80 F/g with increasing the oxide loading from 1.5 to 4.5 mg/cm₂. The lower specific capacitance of thick MnO₂ and MnO₂-MWCNTs deposits is reasonably attributed to the relatively poorer utilization of electroactive species as well as the compact structure in comparison with a thin Mn oxide deposit (<1 μm). The capacitive performance of these thick MnO₂ deposits in 0.1 M Na ₂SO₄ is significantly improved by the application of electrochemical activation and the introduction of MWCNTs, revealing the promising improvement in the capacity of electrodes.

Full text of DOI:10.1149/1.1815151

A2060
Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
0013-4651/2004/151͑12͒/A2060/7/$7.00 © The Electrochemical Society, Inc.
Effects of Electrochemical Activation and Multiwall Carbon
Nanotubes on the Capacitive Characteristics of Thick
MnO2 Deposits
,z
*
Yung-Tai Wu and Chi-Chang Hu
Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan
Thick composites composed of crystalline manganese dioxide (MnO2) and multiwall carbon nanotubes ͑MWCNTs͒ were suc-
cessfully codeposited onto a graphite substrate from an Mn͑AcO͒2•4H2O ϩ MWCNTs solution. The rate/nucleation mechanism of
MnO2 deposition was significantly influenced by the introduction of MWCNTs in the plating baths. The specific capacitance of
thick MnO2-MWCNT composites, measured from cyclic voltammetry ͑CV͒ or chronopotentiometry ͑CP͒ in a potential window of
1.0 V, is monotonously decreased from ca. 160 to 80 F/g with increasing the oxide loading from 1.5 to 4.5 mg/cm2. The lower
specific capacitance of thick MnO2 and MnO2-MWCNTs deposits is reasonably attributed to the relatively poorer utilization of
electroactive species as well as the compact structure in comparison with a thin Mn oxide deposit ͑Ͻ1 ␮m͒. The capacitive
performance of these thick MnO2 deposits in 0.1 M Na2SO4 is significantly improved by the application of electrochemical
activation and the introduction of MWCNTs, revealing the promising improvement in the capacity of electrodes.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1815151͔ All rights reserved.
Manuscript submitted March 14, 2004; revised manuscript submitted April 16, 2004. Available electronically October 29, 2004.
Due to the increase in demands for the power systems with both
dition, this ultrathin-film electrode exhibited an unacceptable capac-
ity under a fair current density of discharge for a very short time
interval ͑only 1-4 s͒, limiting its application. In our previous
work,10,13 thin manganese oxide deposits ͑Ͻ1 ␮m͒ prepared by elec-
trochemical deposition in approximately neutral media showed the
ideally capacitive characteristics for supercapacitors while its capac-
ity is not very high. In addition, the conductivity of thick MnO2 was
found to be too low to be a suitable electrode material,17,21 which
resulted in a relatively poor performance of Mn oxide in the thick
nature.22 Thus, MnO2-carbon composites were proposed to be suit-
able materials for the application of supercapacitors.17,21 The pur-
pose of this paper aims to show a simple one-step method for the
anodic deposition of thick MnO2-MWCNT composites since the
cold-rolling method is relatively complicated.17,21 The significance
of the introduction of MWCNTs as well as the application of elec-
trochemical activation on the improvement in electrochemical char-
acteristics of thick MnO2 deposits in 0.1 M Na2SO4 is demon-
strated. In addition, the influences of MWCNTs on the anodic
deposition behavior of MnO2 are also discussed briefly.
properties of high-energy and high-power densities, an integration of
conventional primary energy storage units ͑e.g., batteries and fuel
cells͒ and the electric energy storage devices in the high-power or
pulse-power forms ͑e.g., capacitors͒ becomes prime concern in the
development of new power systems. On the other hand, the energy
density of conventional capacitors are usually too low to be accept-
able for several future applications; The development of capacitors
with high energy densities ͑i.e., supercapacitors͒ for these applica-
tions has become the interesting subject of much research.1-4 More-
over, it is acceptable and reasonable to separate the high-power and
high-energy delivering devices in an integrated power system to
enhance their respective performances. Hence, development of su-
percapacitors becomes an attractive topic in the research for electro-
chemical energy storage/conversion systems.1-8
The electrode materials employed in supercapacitors are gener-
ally highly porous activated carbon ͑denoted as AC͒ for double-
layer capacitors6,9 or hydrous transition metal oxides for pseudo-
capacitors.1-5,7,8,10 In addition, a relatively high-frequency response
is an intrinsic requirement for the supercapacitors in the high-/pulse-
power applications, i.e., fast charge/discharge characteristics through
a proper utilization of the electroactive materials.11-13 To meet the
above requirements, carbon nanotubes ͑CNTs͒ and composites com-
posed of CNTs and electroactive materials ͑e.g., conducting poly-
mers and hydrous ruthenium oxide͒ were shown to exhibit the prom-
ising applicability to supercapacitors.11,14,15 However, the specific
capacitance of CNTs is relatively low because of their relatively low
specific surface areas11,16 and the cost of single-wall CNTs is very
high while their capacitive performance is obviously better than that
of CNT composites.14,15 Moreover, the steps and processes of elec-
trode fabrication proposed for CNTs-coated electrodes and CNT
composites are relatively complicated. Thus, it is more desirable to
employ a simple method for the preparation of CNT composite elec-
trodes with excellent capacitive characteristics ͑i.e., high reversibil-
ity, high capacity, and high power density͒ for the supercapacitor
applications.
Experimental
The MnO2-MWCNT and MnO2 deposits were electroplated di-
rectly onto 10 ϫ 10 ϫ 3 mm graphite substrates ͑Nippon Carbon
EG-NPL, N.C.K., Japan͒. These substrates were first abraded with
ultrafine SiC paper, degreased with acetone and water, then etched
in a 0.1 M HCl solution at room temperature ͑ca. 26°C͒ for 10 min,
and finally degreased with water in an ultrasonic bath. The exposed
geometric area of these pretreated graphite supports is equal to 1
cm2 while the other surface areas were insulated with PTFE ͑poly-
tetrafluorene ethylene͒ coatings. The plating solutions, consisting of
0.1 M Mn͑CH3COO͒2•4H2O with or without 0.05 g/L MWCNTs
with pH of 6.4, were agitated at 200 rpm during the deposition
process. The deposition was performed at 0.7 V with the total of the
passed charges from 2 to 6 C/cm2. After deposition, the PTFE films
were removed from the electrode and the electrode was doubly de-
greased with water. The deposit loading is the weight difference of
the electrode without PTFE coating before and after the application
of anodic deposition through a microbalance with an accuracy of 10
␮g ͑Sartorius BP 211D, Germany͒. The electrode before and after
oxide growth was dried by a cool air flow.
Recently, MnO2 thin films prepared by sol-gel or electrochemical
deposition methods were found to exhibit excellent capacitive prop-
erties in neutral media,4,17-19 which generally have specific capaci-
tance of 50-250 F/g although a very thin MnO2 film prepared by a
sol-gel-derived method was found to show a very high specific ca-
pacitance of ca. 700 F/g ͑based on the loading of MnO2).20 In ad-
For the preparation of an MWCNT-coated electrode, 5 mg
MWCNTs with the diameter Ϸ20 nm ͑Naonotech Port, PRC͒ were
dispersed in an ethanol solution of 20 cm3 in an ultrasonic bath. This
MWCNT-dispersed solution was dropped onto a pretreated graphite
substrate and dried at 80°C until ethanol was vaporized completely.
This procedure was repeated five to eight times until the loading of
* Electrochemical Society Active Member.
z E-mail: chmhcc@ccu.edu.tw
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
A2061
Figure 2. The grazing incidence XRD patterns for ͑1-3͒ MnO2-MWCNT
deposits with the charge density of deposition ϭ (1) 3; ͑2͒ 4; ͑3͒ 5 C/cm2;
and ͑4͒ an MnO2 deposit with the charge density of deposition ϭ 4 C/cm2.
before measurements were taken, and this nitrogen flowed above the
solution during the measurements. The solution temperature was
maintained at 25°C by means of a water thermostat ͑HAAKE DC3
and K20͒.
Figure 1. ͑a͒ Linear sweep voltammograms measured at 2 mV/s and
͑b͒ log(i) Ϫ E plots obtained from ͑1͒ 0.1 M Mn͑CH3COO͒2•4H2O and ͑2͒
0.1 M Mn͑CH3COO͒2•4H2O and 0.05 g/L CNTs with pH of 6.4.
Results and Discussion
Deposition of MnO2 and MnO2-MWCNT.—Linear sweep volta-
mmetry ͑LSV͒ and log(i) Ϫ E ͑Tafel͒ plots are used to examine the
influences of MWCNTs on the anodic deposition behavior of
MnO2 . Typical LSV curves and log(i) Ϫ E curves measured in 0.1
M Mn͑CH3COO͒2•4H2O ͑pH 6.4͒ without and with the presence of
0.05 g/L MWCNTs are shown in Fig. 1a and b as curves 1 and 2,
respectively. In Fig. 1a, sensible currents on both curves correspond-
ing to the anodic deposition of MnO2 commence at approximately
the same potential ͑ca. 0.35 V͒ and the deposition of MnO2 is under
diffusion control when the electrode potential reaches about 0.75 V.
The former result suggests that the decomposition potential of
Mn͑CH3COO͒2•4H2O may not be significantly influenced by the
introduction of MWCNTs into the deposition solution. On the other
hand, the difference in the limiting current density of deposition
between these two curves is obvious. This phenomenon is attributed
to the codeposition of MWCNTs onto the substrate, providing the
extra surface area for the deposition of MnO2 since the electronic
conductivity of MWCNTs is very good. Accordingly, the limiting
current density of MnO2-MWCNT codeposition is higher than that
of MnO2 deposition.
MWCNTs on the graphite is equal to ca. 2 mg/cm2. The adhesion of
MWCNTs on graphite substrate was confirmed to be very good.
The grazing incidence X-ray diffraction ͑XRD͒ patterns were
measured by an X-ray diffractometer ͑Rigaku, X-ray diffractometer
using a Cu target͒. Surface morphologies of electrodes were exam-
ined by a field-emission scanning electron microscope ͑FE-SEM,
Hitachi S-4700͒. Thickness of MnO2-MWCNT and MnO2 ͑ca. 3-10
␮m͒ was estimated from the cross sections of deposits.
Electrochemical deposition and measurements for all electrodes
were performed by means of an electrochemical analyzer system,
CHI 633A ͑CH Instruments, USA͒. All experiments were carried out
in a three-compartment cell. An Ag/AgCl electrode ͑Argenthal, 3 M
KCl, 0.207 V vs. SHE at 25°C͒ was used as the reference and a
platinum wire with an exposed area equal to 4 cm2 was employed as
the counter electrode. A Luggin capillary, whose tip was set at a
distance of 1-2 mm from the surface of the working electrode, was
used to minimize errors due to IR drop in the electrolyte. The elec-
trochemical activation used in this work was performed by cyclic
voltammetry at 25 mV/s between 0 and 1.0 V in 0.1 M Na2SO4 for
two cycles when deposits were dipped in the electrolyte for 0, 15,
30, 45, 60, 80, and 120 min.
In Fig. 1b, the Tafel slope of curve 2 in the low current density
region is obviously lower than that of curve 1 while two Tafel plots
approximately overlap when the electrode potentials are more posi-
tive than 0.4 V. The former result indicates that the nucleation and
growth of MnO2 onto the substrate should be influenced by the
presence of MWCNTs when the current density of deposition is
lower than 0.1 mA/cm2. However, the mechanism corresponding to
the deposition of MnO2 in the absence of MWCNTs dominates the
All solutions used in this work were prepared with 18 M⍀ cm
water produced by a reagent water system ͑MILLI-Q SP, Japan͒, and
all reagents not otherwise specified in this work were Merck, GR. In
addition, the plating baths and the electrolyte used to study the ca-
pacitive behavior of MnO2 were degassed with purified nitrogen gas
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A2062
Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
Figure 3. FE-SEM photographs of ͑a,b͒ MnO2 and ͑c-e͒ MnO2-MWCNT
deposits with the charge density of deposition ϭ 4 C/cm2.
deposition process when the electrode potentials are more positive
than 0.4 V. Based on the above results and discussion, MWCNTs in
the Mn͑CH3COO͒2•4H2O solution should not only act as electron
conductors but also influence the deposition mechanism in the low
overpotential region. On the other hand, due to the fact that the
deposition rate reaches the maximum when the limiting current den-
sity is reached, the potentiostatic deposition of MnO2-MWCNT and
MnO2 deposit is set at 0.7 V in this work.
onstrate the poor crystalline structure of MnO2 because the intensity
of the broad diffraction peaks corresponding to MnO2 ,4 centered at
37.1° and 66.3°, is very low ͑marked by o͒. Since the above two
peaks show similar intensities on these patterns, the crystalline qual-
ity of MnO2 should not be significantly influenced by the introduc-
tion of MWCNTs and the thickness of deposits.
Typical scanning electron microscopic ͑SEM͒ photographs for
MnO2 and MnO2-MWCNT deposits ͑charge density of deposition
ϭ 4 C/cm2͒ are shown in Fig. 3a-b and 3c-e, respectively. Under a
low magnification ͑see Fig. 3a and c͒, many of cracks are clearly
found on the surface of both MnO2 and MnO2-MWCNT deposits,
probably due to the shrinkage stress during drying while these de-
posits generally show the compact morphologies. The crack mor-
Textural analysis of MnO2 and MnO2-MWCNT.—Typical graz-
ing incidence XRD patterns for MnO2-MWCNT deposits with the
charge density of deposition equal to 3, 4, 5 C/cm2 and an MnO2
deposit with the charge density of deposition equal to 4 C/cm2 are
shown in Fig. 2, curves 1-4, respectively. Note that all patterns dem-
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
A2063
on the surface of MnO2-MWCNT, causing a rougher surface in
comparison with that of MnO2 . Under a high magnification, the
MnO2 deposit ͑see Fig. 3b͒ is composed of small particulates with
an average size of ca. 30 nm. The MnO2-MWCNT deposit, how-
ever, consists of short rods that make a less dense morphology ͑Fig.
3d͒, probably favoring the diffusion of electrolytes into the oxide
matrix. In addition, MWCNTs can be clearly found on the top of
some tumor-like structures, which are surrounded by many fine Mn
oxide nanowires ͑see Fig. 3e͒. Based on the above observation, the
tumor-like structures are considered as a newly codeposited product
of MnO2 and MWCNTs since fine MnO2 nanowires are easily
formed during the initial deposition of MnO2 .
Effects of electrochemical activation and MWCNTs on the
capacitive performance of MnO2.—Typical cyclic voltammograms
͑CVs͒ measured in 0.1 M Na2SO4 at 25 mV/s for MnO2 ,
MnO2-MWCNTs, and MWCNTs are shown in Fig. 4 as curves 1-3,
respectively. On curve 1, the voltammetric responses of MnO2 on
the positive sweep are not symmetrical to those on the following
negative sweep, indicating that the electrochemical reversibility of
this thick MnO2 deposit ͑deposition charge density ϭ 4 C/cm2͒ is
poor, which was also found in our previous work.22 Based on this
poor capacitive performance, there exists a limit on the loading ͑i.e.,
capacity͒ of MnO2 electrodes for the application of supercapacitors.
The decay in the electrochemical reversibility of MnO2 with a
higher loading probably results from an increase in ESR ͑equivalent
Figure 4. Cyclic voltammograms measured in 0.1 M Na2SO4 at 25 mV sϪ1
for ͑1͒ MnO2 ; ͑2͒ MnO2-MWCNTs; and ͑3͒ MWCNTs. The deposition
charge density for MnO2 and MnO2-MWCNTs is 4.0 C/cm2.
phology was also found previously when MnO2 deposits were an-
odically deposited at different potentials,4 while their surfaces were
very rough. On the other hand, there are some tumor-like structures
Figure 5. Cyclic voltammograms of ͑a͒ MnO2 and ͑b͒ MnO2-MWCNTs
deposits measured in 0.1 M Na2SO4 at 25 mV sϪ1 at the dipping time of 0,
15, 30, 45, 60, 80, and 120 min. ͑c͒ Cyclic voltammograms of an
MnO2-MWCNTs composite ͑1͒ as-prepared; ͑2͒ newly prepared and dipped
in 0.1 M Na2SO4 for 2 h; and ͑3͒ then, cycled between 0 and 1.0 V at 25
mV sϪ1 for ten cycles.
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A2064
Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
time in this electrolyte. Typical results are shown in Fig. 5a and b,
respectively. Note that on both figures, voltammetric currents ͑and
charges͒ are gradually increased with prolonging the dipping time
during the initial 30 min. When the dipping time is equal to or above
1 h, i-E curves on both positive and negative sweeps approximately
follow the same trace in the whole potential region of investigation.
On the other hand, the degree of enhancement in voltammetric cur-
rents is very low when freshly prepared MnO2-MWCNT ͑and
MnO2) deposits are dipped into the same medium for 2 h without
the application of electrochemical activation ͑see curves 1 and 2 in
Fig. 5c͒. In addition, voltammetric currents of this MnO2-MWCNT
deposit did not increase when an additional ten CV cycles were
applied from a comparison of curves 2 and 3 in Fig. 5c. The above
results reveal that the utilization of electroactive species is only
promoted by the repeated applications of CV in Na2SO4 at different
dipping times. Accordingly, the repeated applications of CV with
varying the dipping time are considered as the electrochemical acti-
vation method for thick MnO2 . Based on the above comparisons,
the compact and dense nature of thick MnO2 deposits on both elec-
trodes should depress the diffusion rate of electrolytes into the oxide
matrix. This effect is negligible for the deposits with a lower charge
density of deposition ͑i.e., 0.3-0.5 C/cm2͒ since the thin-film depos-
its showed the mesoporous nature with nanostructures.13 From a
comparison of Fig. 5a and b, the electrochemical reversibility of
thick MnO2 deposit is poorer than that of an MnO2-MWCNT com-
posite. For example, the rate of voltammetric currents reaching the
plateau value is faster in Fig. 5b when the dipping time is the same.
In addition, the symmetry of the i-E curves between the positive
and negative sweeps on the CV curves shown in Fig. 5b is some-
what better than that on the curves in Fig. 5a although the CV curves
of thick MnO2 deposit in Fig. 5a are similar in both shape and
currents to that of an MnO2-MWCNT deposit. Accordingly, the ca-
pacitive performance of MnO2 should be significantly improved by
the introduction of MWCNTs. Since the electrochemical reversibil-
ity of MnO2-MWCNTs deposit after the electrochemical activation
is as good as that of a thin MnO2 deposit, this thick composite,
promoting the energy density of the electrode, should further extend
the applications of supercapacitors.
Typical chronopotentiograms of both MnO2 and MnO2-
MWCNTs deposits with electrochemical activation for 0, 30, and
120 min are shown in Fig. 6a and b, respectively. Note the sharp
drops in electrode potentials when the applied currents are changed
from anodic to cathodic on all curves in Fig. 6. In addition, the
potential drops, obeying the ohmic law, become invisible with de-
creasing the charge-discharge current density. These phenomena
indicate the significant ESR on both deposits. The average specific
capacitance (CCP) of these deposits can be estimated on the basis of
Eq. 110,13
Figure 6. Chronopotentiograms of ͑a͒ MnO2 and ͑b͒ MnO2-MWCNTs de-
posits measured in 0.1 M Na2SO4 at 5 mA/cm2 with the dipping time of ͑1͒
as-deposited; ͑2͒ 30 min; ͑3͒ 120 min.
series resistance͒ for a heavier deposit because of the poor conduc-
17,21
tivity of MnO2
and/or the significance of the proton diffusion
barrier on this deposit. Note that very similar voltammetric behavior
is also observed on curve 2, indicating that the ESR of the thick
MnO2-MWCNTs deposit is still high. However, the voltammetric
currents and charges on curve 2 are higher than those on curve 1,
suggesting the influences of MWCNTs on improving the electro-
chemical characteristics of MnO2 since the deposit loading of both
electrodes is approximately the same. On curve 3, the capacitive
currents of MWCNTs are much lower than that of MnO2 and
MnO2-MWCNTs deposits. The specific capacitance (CS) of
MWCNTs estimated from this CV curve is only 5 F/g, although the
electrochemical reversibility is perfect. On the other hand, the spe-
cific capacitance of MnO2 and MnO2-MWCNTs deposits is about
98 and 108 F/g, respectively, revealing that MnO2 is the main spe-
cies for energy storage and delivery. Although both CS data are
much lower than 175 F/g from a thin MnO2 deposit ͑charge density
of deposition ϭ 0.5 C/cm2͒, the capacity of this MnO2-MWCNT
composite ͑Ϸ320 mF͒ is much larger than that of a thin Mn oxide
deposit ͑Ϸ65 mF͒.
CCP Ϸ i/ ⌬E/⌬t͒ ϫ w
͓͑
͓1͔
͔
where (⌬E/⌬t) indicates the average slope of these CP curves. The
average specific capacitance of MnO2 and MnO2-MWCNTs with
electrochemical activation for 120 min is ca. 135 and 157 F/g, re-
spectively. Each CCP datum is somewhat larger than the CS datum
estimated from the CV curve measured at 25 mV/s ͑i.e., 120 and 135
F/g, respectively͒. Note that MnO2 and MnO2-MWCNTs with elec-
trochemical activation for 120 min show the symmetric E-t curves
in Fig. 6 and the capacity of the MnO2-MWCNT composite reaches
470 mF, which is much larger than that of a thin MnO2 deposit ͑Ϸ65
mF͒.
The effect of electrochemical activation at various dipping times
in 0.1 M Na2SO4 on the ESR ͑including charge-transfer resistance,
electronic resistance, and ionic resistance͒ of MnO2 and
MnO2-MWCNTs deposits can be estimated from the iR drops of
chronopotentiograms when the direction of applied currents is al-
tered. Typical iR drop data of MnO2 and MnO2-MWCNTs deposits
are shown in Fig. 7a as curves 1 and 2, respectively. In addition, the
In order to gain a further understanding on the electrochemical
reversibility of thick MnO2 deposits, CV curves of MnO2 and
MnO2-MWCNT deposits with the charge density of deposition ϭ 4
C/cm2 were measured in 0.1 M Na2SO4 with prolonging the dipping
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
A2065
Figure 8. Dependence of CS on the deposition charge density for ͑1,3͒
MnO2 and ͑2,4͒ MnO2-MWCNTs deposits with the dipping time of ͑1,2͒ 0
and ͑3,4͒ 120 min. The MnO2 and MnO2-MWCNTs deposits for curves 2
and 4 have been dipped in 0.1 M Na2SO4 and activated by CV at 0, 15, 30,
60, and 120 min.
the significance of MWCNTs in improving the electronic conductiv-
ity of MnO2-MWCNTs, which shows the more suitable characteris-
tics for the application of supercapacitors.
The dependence of CS ͑estimated from CV curves measured at
25 mV/s͒ on the charge density of deposition ͑i.e., loading͒ for
freshly prepared MnO2 and MnO2-MWCNT deposits is shown in
Fig. 8 as lines 1 and 2, respectively. In addition, the dependence of
iR drops on the charge density of deposition for the MnO2 and
MnO2-MWCNT deposits having been electrochemically activated
for 2 h by CV ͑as proposed in the Experimental section͒ is shown as
lines 3 and 4, respectively. From a comparison of lines 1 and 2, the
effect of introducing MWCNTs on the promotion of specific capaci-
tance of MnO2 is not obvious although CS of MnO2-MWCNT is
generally somewhat larger than that of MnO2 ͑except the deposits of
Figure 7. ͑a͒ Dependence of IR drops on the dipping time and ͑b͒ depen-
dence of IR drops and loading on the charge density of deposition for ͑1,3͒
MnO2 and ͑2,4͒ MnO2-MWCNTs deposits.
6
C/cm2͒. On the other hand, the specific capacitance of
dependence of the iR drops for MnO2 and MnO2-MWCNTs depos-
its on the charge density of deposition is shown in Fig. 7b ͑see lines
3 and 4͒. Since iR drops of deposits should be strongly dependent
upon the deposit loading, the dependence of deposit loading on the
charge density of deposition for MnO2 and MnO2-MWCNTs depos-
its is also shown in Fig. 7b ͑see lines 1 and 2͒. From the finding that
all data points on lines 1 and 2 in Fig. 7b are overlapped, the de-
pendence of the deposit loading on the charge density of deposition
is not influenced by the presence of MWCNTs. This implies that the
amount of MWCNTs within the MnO2-MWCNTs deposits is low,
which is very difficult to be determined quantitatively.
MnO2-MWCNT is obviously larger than that of MnO2 when the
electrochemical activation has been applied. In addition, the effect
of MWCNTs becomes more obvious with increasing the charge den-
sity of deposition. From all the above results and discussion in this
section, the capacitive performance of thick MnO2 deposits in 0.1 M
Na2SO4 is significantly improved by the application of electro-
chemical activation and the introduction of MWCNTs, revealing a
promising improvement in the capacity of electrodes, further ex-
tending the applications of supercapacitors.
Conclusions
From an examination of curves 1 and 2 in Fig. 7a, the potential
drops were decreased with the activation time in 0.1 M Na2SO4 .
However, the decay in iR drops on curve 1 is less than that on curve
2. In addition, iR drops of the MnO2-MWCNT composite are in-
trinsically lower than that of the MnO2 deposit when the activation
conditions are identical. All the above results support the proposal
that the ESR of MnO2 and MnO2-MWCNTs deposits can be re-
duced by the repeated applications of CV at different dipping times.
Remember that the electrochemical reversibility of electroactive
manganese oxides is significantly improved by the electrochemical
activation in 0.1 M Na2SO4 as well as the introduction of
MWCNTs. The electrochemical activation at different dipping times
is likely to promote the diffusion of electrolytes into the oxide ma-
trix, rendering a decrease in the charge-transfer resistance of redox
couples since electron transfer and proton exchange occur simulta-
neously during the redox transition.10 The latter effect demonstrates
Thick composites composed of crystalline MnO2 and MWCNTs
were successfully codeposited onto a graphite substrate from an
Mn͑AcO͒2•4H2O ϩ MWCNTs solution with pH of 6.4. The rate/
nucleation mechanisms of MnO2 deposition are significantly influ-
enced by the introduction of MWCNTs in the plating baths. The
specific capacitance of thick MnO2-MWCNT composites ͑3-10 ␮m͒
is gradually decreased from ca. 160 to 80 F/g with increasing the
loading of deposits. The lower specific capacitance of thick MnO2
and MnO2-MWCNTs deposits is attributable to the relatively poor
utilization of electroactive species as well as the compact structure
in comparison with a thin Mn oxide deposit ͑Ͻ1 ␮m͒. The high
capacity and the excellent capacitive performance of thick
MnO2-MWCNT composites can be achieved by the repeated appli-
cations of electrochemical activation in Na2SO4 and the introduction
of MWCNTs.
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

A2066
Journal of The Electrochemical Society, 151 ͑12͒ A2060-A2066 ͑2004͒
8. J. P. Zheng, P.-J. Cygon, and T. R. Jow, J. Electrochem. Soc., 142, 2699 ͑1995͒.
9. D. Qu and H. Shi, J. Power Sources, 74, 99 ͑1998͒.
Acknowledgments
The financial support of this work, by the National Science
Council of Taiwan under contract no. NSC 92-2120-M-194-001, is
gratefully acknowledged.
10. C.-C. Hu and T.-W. Tsou, Electrochem. Commun., 4, 105 ͑2002͒; C.-C. Hu and
T.-W. Tsou, Electrochim. Acta, 47, 3523 ͑2002͒; C.-C. Hu and T.-W. Tsou, J. Power
Sources, 115, 179 ͑2003͒.
11. C. Niu, E. K. Sichel, R. Hoch, D. Moy, and H. Tennent, Appl. Phys. Lett., 70, 1480
͑1997͒.
12. C.-C. Hu and C.-H. Chu, J. Electroanal. Chem., 503, 105 ͑2001͒.
13. C.-C. Hu and C.-C. Wang, J. Electrochem. Soc., 150, A1079 ͑2003͒.
14. K. H. An, K. K. Jeon, J. K. Heo, S. C. Lim, D. J. Bae, and Y. H. Lee, J. Electro-
chem. Soc., 149, A1058 ͑2002͒.
National Chung Cheng University assisted in meeting the publication
costs of this article.
References
1. B. E. Conway, Electrochemical Supercapacitors, p. 29, Kluwer-Plenum Pub. Co.,
New York ͑1999͒.
2. A. Burke, J. Power Sources, 91, 37 ͑2000͒.
3. ͑a͒ C.-C. Hu and Y.-H. Huang, J. Electrochem. Soc., 146, 2465 ͑1999͒; ͑b͒ C.-C.
Hu and K.-H. Chang, Electrochim. Acta, 45, 2685 ͑2000͒.
4. J.-K. Chang and W.-T. Tsai, J. Electrochem. Soc., 150, A1333 ͑2003͒.
5. C. Lin, J. A. Ritter, and B. N. Popov, J. Electrochem. Soc., 145, 4097 ͑1998͒.
6. Y.-R. Nian and H. Teng, J. Electrochem. Soc., 149, A1008 ͑2002͒.
7. D. A. McKeown, P. L. Hagans, L. P. L. Carette, A. E. Russell, K. E. Swider, and D.
R. Rolison, J. Phys. Chem. B, 103, 4825 ͑1999͒.
15. J. H. Park, J. M. Ko, and O. O. Park, J. Electrochem. Soc., 150, A864 ͑2003͒.
16. K. H. An, W. S. Kim, Y. S. Park, Y. C. Choi, S. M. Lee, D. C. Chung, D. J. Bae, S.
C. Lim, and Y. H. Lee, Adv. Mater. (Weinheim, Ger.), 13, 497 ͑2001͒.
17. H. Y. Lee and J. B. Goodenough, J. Solid State Chem., 144, 220 ͑1999͒.
18. J. Jiang and A. Kucernak, Electrochim. Acta, 47, 2381 ͑2002͒.
19. Y. U. Jeong and A. Manthiram, J. Electrochem. Soc., 149, A1419 ͑2002͒.
20. S.-C. Pang, M. A. Anderson, and T. W. Chapman, J. Electrochem. Soc., 147, 444
͑2000͒.
21. H. Kim and B. N. Popov, J. Electrochem. Soc., 150, D56 ͑2003͒.
22. Y.-S. Chen, C.-C. Hu, and Y.-T. Wu, J. Solid State Electrochem., 8, 467 ͑2004͒.
Downloaded on 2014-11-17 to IP 132.239.1.230 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).