The effect of nonionic surfactant triton X-100 during electrochemical deposition of MnO2 on its capacitance properties

Article abstract of DOI:10.1149/1.2759618

Manganese dioxide has been considered as a promising material for electrochemical supercapacitors. In order to obtain a high specific capacitance, MnO2 has been electrodeposited from an aqueous solution of MnSO4 consisting of a neutral surfactant, namely, Triton X-100. The electrodeposited films of MnO2 in the presence of the surfactant possess greater porosity and hence greater surface area in relation to the films prepared in the absence of the surfactant. Cyclic voltammetry and galvanostatic charge-discharge cycling experiments reveal that specific capacitance is higher by about 59% due to the effect of Triton X-100. As surfactants are known to form micelles at a critical concentration, studies have been conducted to arrive at an appropriate concentration of Triton X-100. Accordingly, it has been found that 10 mM Triton X-100 in 0.5 M MnSO4 solution is the optimum composition of the electrolyte for obtaining a maximum specific capacitance. Extended charge-discharge cycling studies indicate that superior performance of MnO2 due to Triton X-100 is seen throughout the life-cycle.

Full text of DOI:10.1149/1.2759618

The Effect of Nonionic Surfactant Triton X-100 during
Electrochemical Deposition of MnO 2 on Its Capacitance Properties
S. Devaraj and N. Munichandraiah
J. Electrochem. Soc. 2007, Volume 154, Issue 10, Pages A901-A909.
doi: 10.1149/1.2759618
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Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
A901
0013-4651/2007/154͑10͒/A901/9/$20.00 © The Electrochemical Society
The Effect of Nonionic Surfactant Triton X-100 during
Electrochemical Deposition of MnO2 on Its Capacitance
Properties
S. Devaraj and N. Munichandraiah
,z
*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560-012, India
Manganese dioxide has been considered as a promising material for electrochemical supercapacitors. In order to obtain a high
specific capacitance, MnO2 has been electrodeposited from an aqueous solution of MnSO4 consisting of a neutral surfactant,
namely, Triton X-100. The electrodeposited films of MnO2 in the presence of the surfactant possess greater porosity and hence
greater surface area in relation to the films prepared in the absence of the surfactant. Cyclic voltammetry and galvanostatic
charge-discharge cycling experiments reveal that specific capacitance is higher by about 59% due to the effect of Triton X-100. As
surfactants are known to form micelles at a critical concentration, studies have been conducted to arrive at an appropriate
concentration of Triton X-100. Accordingly, it has been found that 10 mM Triton X-100 in 0.5 M MnSO4 solution is the optimum
composition of the electrolyte for obtaining a maximum specific capacitance. Extended charge-discharge cycling studies indicate
that superior performance of MnO2 due to Triton X-100 is seen throughout the life-cycle.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2759618͔ All rights reserved.
Manuscript submitted March 22, 2007; revised manuscript received May 22, 2007. Available electronically July 31, 2007.
literature.29 For instance, high-surface-area PbO2 deposits have been
prepared by the addition of a commercial surfactant, namely, Teepol,
to the electrolyte.30 Electrolytic manganese dioxide ͑EMD͒ has been
prepared in an acidic electrolyte in the presence of several surfac-
tants and evaluated for rechargeable alkaline manganese dioxide-
zinc battery application.31 The EMD prepared in the presence of
Triton X-100 has been shown to provide higher discharge capacity
in relation to the EMD prepared otherwise.31 We have recently re-
ported a specific capacitance of 310 F g−1 for MnO2 at a loading
level of 0.4–0.5 mg cm−2 by electrodeposition of MnO2 in the pres-
ence of an anionic surfactant, namely, sodium dodecyl sulfate.32 The
effect of Triton X-100 on electrocrystallization and electrochemical
performance of PbO2 electrode for lead-acid batteries has also been
reported recently.33
The aim of the present study was to electrochemically deposit
MnO2 in the presence of Triton X-100, which is a neutral surface-
active agent, in the electrolyte and to evaluate the electrodes for
capacitor properties. MnO2 electrodeposited in the presence of Tri-
ton X-100 was found to provide a SC of 355 F g−1 against
221 F g−1 obtained for MnO2 deposited in the absence of Triton
X-100.
Growing environmental concerns and fast depletion of fossil fu-
els have created great interest in studying alternative energy sources.
Electrochemical capacitors ͑ECs͒, also known as supercapacitors or
ultracapacitors, have become attractive energy storage systems for
applications involving high power requirements for a short duration
such as camera flash equipment, pulsed light generators, fire/smoke
alarms, backup power source for computer memory, etc.1-4 A prom-
ising future application of ECs is in electric vehicles in combination
with batteries or fuel cells. In such hybrid energy systems, ECs
provide high power for functions such as change of gear, hill climb-
ing, etc., and therefore the battery or fuel cell can be optimized
primarily for high energy density and long cycle life.
High-surface carbons, conducting polymers and transition metal
oxides are important electrode materials for supercapacitors.3
Among the various metal oxides investigated, RuO2·xH2O is a
promising electrode material as it provides a high specific capaci-
tance ͑SC Ӎ 760 F g−1͒ and a long cycle life.5-7 However, ruthe-
nium is expensive and toxic, and therefore is not attractive for com-
mercial applications. MnO2 has become an alternate electrode
material8-27 as it is cheap due to abundant natural resources, envi-
ronmentally friendly, and well studied as a battery material. How-
ever,
a
maximum SC reported for MnO2 at
a
loading of
14
0.4–0.5 mg cm−2 is 240 F g−1
.
According to the single electron-
Experimental
transfer redox process, a SC of 1109 F g−1 is expected for MnO2
within a 1 V window. This value can be realized in practice only
when the mass of MnO2 is at a level of a few micrograms per cm2
area. At such a low thickness range, almost complete utilization of
the active material occurs. However, thin layers of MnO2 are uneco-
nomical for practical capacitors and hence, it would be beneficial if
high values of SC are obtained at higher loading levels. In the
present study, enhancement in SC of electrochemically deposited
MnO2 at a loading level of 0.4–0.5 mg cm−2 was investigated by
suitably modifying the conditions of its deposition.
In general, the physical and morphological properties of elec-
trodeposited materials depend on the experimental conditions used
for their preparation, which include the presence of foreign mol-
ecules in the electrolyte. Surface-active molecules ͑surfactants͒ pos-
sess a polar head group attached to a long chain aliphatic nonpolar
tail.28 The head group can get adsorbed on the electrode surface and,
therefore, can influence the double-layer properties, kinetics, and
mechanism of electrochemical processes.
Analytical-grade MnSO4·H2O, K2SO4 were purchased from
Merck, Na2SO4 from BDH, and Li2SO4 from Spectrochem.
Reagent-grade Triton X-100 from Loba Chemie was used as the
surface-active agent. A high-purity Ni foil ͑0.18 mm thickness͒ of
battery grade was used as the substrate for MnO2 deposits. All so-
lutions were prepared using doubly distilled water. A glass cell of
about 70 mL capacity with suitable ground-glass joints to introduce
the Ni working electrode, Pt foil auxiliary electrodes, and a saturated
calomel reference electrode ͑SCE͒ was used for electrochemical
deposition of MnO2 and also for other electrochemical studies. All
potential values are reported against SCE.
A Ni foil of 10 mm width and 8 cm length was sectioned out of
a sheet; 2 cm2 area at one of the ends was exposed to the electrolyte
and the rest of its length was used as tag for taking electrical contact.
The Ni-foil was polished with successive grades of emery, cleaned
with detergent, washed copiously with doubly distilled water, rinsed
with acetone, dried in air, and weighed before using it for elec-
trodeposition of MnO2. A solution of 0.5 M MnSO4·H2O was used
as the electrolyte for electrodeposition. For deposition of MnO2 in
the presence of the surface-active agent, the required quantity of
Triton X-100 was added, followed by a thorough stirring of the
electrolyte to obtain a clear solution. Experiments were carried out
using several concentrations of Triton X-100 up to 100 mM in the
electrolyte. As the appropriate concentration of Triton X-100 was
Despite interesting applications of surfactants in electrochemistry
and electrodeposition, only a few studies have been reported in the
*
Electrochemical Society Active Member.
z E-mail: muni@ipc.iisc.ernet.in
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A902
Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
found to be 10 mM, a majority of the studies were carried out at this
concentration. The deposition of MnO2 on Ni was carried out by
galvanostatic, potentiostatic, and potentiodynamic techniques. Sub-
sequent to the deposition of the required quantity of MnO2, the
electrode was separated from the cell, washed with doubly distilled
water, dried at 100°C in air, and weighed. Quasi-steady-state polar-
ization data for the deposition of MnO2 were recorded at a sweep
rate of 0.01 mV s−1 while stirring the electrolyte using a magnetic
paddle. Current-time transients during the initial stages of
electrocrystallization of MnO2 were recorded using a Pt foil ͑area ϭ
0.6 cm2͒ as the working electrode.
A Sartorius balance of model CP225D-OCE with 0.01 mg sen-
sitivity was used for weighing the electrodes. The powder X-ray
diffraction ͑XRD͒ patterns of the samples were recorded using a
Philips XRD X’PERT PRO diffractometer using Cu K␣ ͑␭
= 1.5418 Å͒ as the source. The field-emission scanning electron mi-
croscope ͑FESEM͒ images were recorded using FEI-company mod-
els SIRION and Quanta 200 scanning electron microscopes.
Brunauer–Emmett–Teller ͑BET͒ surface-area measurements were
carried out using a SMARTSORB–92/93 surface area analyzer uti-
lizing N2 gas. Cyclic voltammetry and galvanostatic charge/
discharge experiments of MnO2 electrodes were carried out using
potentiostats/galvanostats EG&G PARC model Versastat II and So-
lartron model SI 1287. Electrochemical impedance studies were car-
ried out using a Solartron galvanostat/potentiostat model SI 1287
coupled with a frequency response analyzer model 1255. The cell
was excited with 5 mV ac signal and impedance was measured at
open-circuit potential in the frequency range from 100 kHz to
10 mHz. The data were analyzed using a nonlinear least-squares
͑NLLS͒ fitting program of Scribner Associates and an electrical
equivalent circuit. All experiments were conducted at 22 1°C.
Results and Discussion
Electrodeposition of MnO
2.— Electrodeposition of MnO2 in-
volves oxidation of Mn2+ in an aqueous electrolyte of manganese
salt
Figure 1. ͑a͒ Cyclic voltammograms of Ni in 0.5 M MnSO4 ͑i͒ and
0.5 M MnSO4 + 10 mM Triton X-100 ͑ii͒ at 20 mV s−1 sweep rate, and ͑b͒
Tafel plots for electro-oxidation of Mn2+ on Ni to form MnO2 ͑i͒ and
MnO2͑s͒ ͑ii͒.
Mn2+ + 2H2O → MnO2 + 4H+ + 2e−
The reaction can be carried out by galvanostatic, potentiostatic, or
potentiodynamic electroxidation routes. A kinetic study by potentio-
dynamic method in an unstirred electrolyte suggested the following
important steps in the mechanism34
͓1͔
The surfactant used in the present study, namely, Triton X-100, be-
longs to the class of nonionic surfactants, which have either poly-
ether or polyhydroxyl as the polar group.36 Contrary to ionic
surfactants, the physicochemical properties of nonionic surfactants
are not affected by electrolytes. Hence, Triton X-100 with
the chemical name t-octyl phenoxy polyethoxyethanol
͓4-͑C8H17͒C6H4͑OCH2CH2͒10OH͔ was used in the present work.
The oxides deposited from a solution in the absence and presence of
Triton X-100 surfactant are hereafter referred to as MnO2 and
MnO2͑s͒, respectively. The following experimental results provide
evidence for the influence of the surfactant during the preparation of
MnO2͑s͒.
Cyclic voltammograms of Ni recorded for electrodeposition of
MnO2 and MnO2͑s͒ are shown in Fig. 1a. Although the voltammo-
grams in the absence ͑curve i͒ and in the presence of Triton X-100
͑curve ii͒ are similar in shape, the anodic peak current is higher in
the latter case. The anodic peak ͑pa͒ at about 1.1–1.15 V corre-
sponds to the oxidation of Mn2+ to MnO2 and cathodic peaks ͑pc1
and pc2͒ at 0.8–0.84 and 0.39–0.41 V ranges correspond to a partial
reduction of Mn4+ to Mn3+ and Mn3+ to Mn2+, respectively.34 The
effect of Triton X-100 in the electrolyte is clearly reflected in the
voltammograms. As the adsorption of Mn2+ ions at the electrode
surface precedes the electron-transfer process ͑Reaction 1͒, it is in-
ferred that Triton X-100 molecules favor this process, which results
in higher anodic peak current for the oxidation of Mn2+ to MnO2͑s͒.
Quasi-steady-state polarization plots recorded for the oxidation
2+
3+
Mn → Mn + e−
͓2͔
ads
ads
3+
2+
ads
4+
ads
Mn → Mn + Mn
͓3͔
ads
The first step ͑Reaction 2͒ corresponds to the oxidation of Mn2+
2+
ads
3+
ads
3+
adsorbed at the electrode surface ͑Mn ͒ to Mn . Mn is unstable
ads
2+
ads
4+
ads
and undergoes disproportionate decomposition to Mn and Mn
.
Mn precipitates as MnO2 at the anode in aqueous medium. Mna2d+s
4+
ads
3+
ads
either undergoes oxidation to Mn at the electrode surface or un-
dergoes chemical oxidation and combines with OHads, resulting in
the formation of MnO2 at the anode.34 Surface-active molecules
tend to undergo adsorption at the electrode/electrolyte interface. As
a result, interfacial tension usually decreases and kinetics of the
electrochemical reaction changes. Manganese dioxide was deposited
on Ni substrate from an aqueous solution of 0.5 M MnSO4 with
varying concentrations of Triton X-100 from 0 to 100 mM. Gener-
ally, EMD is deposited in acidic electrolytes consisting of a Mn2+
salt, using an inert electrode as anode.35 EMD is known to crystal-
lize in ␥ form of crystallographic structure. When an electrolyte of
0.5 M MnSO4 + 0.5 M H2SO4 was used to deposit EMD on Ni
substrate, it was found that Ni started undergoing oxidative dissolu-
tion instead of deposition of MnO2 taking place on its surface. A
common, inexpensive metal such as Ni was considered to be impor-
tant as the substrate for MnO2 in view of practical application of the
electrodes for supercapacitors. Therefore, the electrolyte was
changed to 0.5 M MnSO4 without an acidic supporting electrolyte.
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Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
of Mn2+ to MnO2 and MnO2͑s͒ on well-stirred electrolytes are
A903
shown in Fig. 1b. The plots are fairly linear in the potential region of
1.25–1.35 V, suggesting that the data follow the Tafel relationship
log i = log i0 + ͓͑1 − ␣͒nF͑E − Er͔͒/͑2.3RT͒
͓4͔
where i0 is exchange current density, ␣ is transfer coefficient, and
the other symbols have their usual definitions. It is seen that the
currents are higher in the presence of the surfactant ͑Fig. 1b͒, similar
to the cyclic voltammetric anodic peak current ͑Fig. 1a͒. The overall
reaction is controlled by diffusion in Fig. 1a, while it is controlled
by electron-transfer in Fig. 1b. It is inferred from these two experi-
ments that the surfactant molecules present at the interface favor the
electrodeposition of MnO2 in both diffusion as well as electron-
transfer controlled processes. From Fig. 1b, ␣ was evaluated from
the slope of the plot. The values of ␣ for MnO2 and MnO2͑s͒ are
found to be 0.052 and 0.073, respectively. These values are very
low, suggesting that the Mn2+/MnO2 reaction is complex in nature.
The influence of Triton X-100 on the electrocrystallization of
MnO2 and MnO2͑s͒ was studied by recording current-time tran-
sients. Nucleation and growth are the two important steps in the
process of electrocrystallization.37 Generally, an electrode can pos-
sess surface crystallographic defects such as edges, kinks, vacancies,
screw dislocations, etc., at which a crystal can grow without nucle-
ation during an electrodeposition process.38,39 However, on a per-
fectly smooth single crystal, the deposition of species onto the sub-
strate lattice requires initial nucleation for the growth of crystals.38
This is a current-limiting process and hence by recording current-
time transients, kinetics of nucleation and crystal growth can be
understood.37,40 Mathematical expressions for current-time tran-
sients for different cases of nucleation ͑instantaneous and progres-
sive͒ and crystal growth ͑two- or three-dimensional͒ are reported in
the literature.41-43 During an electrocrystallization process, the rela-
tive rates of nucleation and growth of crystals greatly influence the
surface morphology of the deposit.37 The adsorbed surfactant mol-
ecules can favor one of these two steps. If the rate of nucleation
exceeds the growth rate, a high surface area of the deposit is ex-
pected.
Figure 2. Current-time transients on Pt electrode by stepping the potential
from 0.5 V vs SCE to ͑1͒ 0.8, ͑2͒ 1.0, ͑3͒ 1.2, and ͑4͒ 1.5 V vs SCE in ͑a͒
0.5 M MnSO4 and ͑b͒ 0.5 M MnSO4 + 10 mM Triton X-100.
Current-time transients of Pt at different potential steps in an
aqueous solution of 0.5 M MnSO4 in the absence and in the pres-
ence of 10 mM Triton X-100 are shown in Fig. 2a and b. The tran-
sients consist of three characteristic regions. When the potential is
stepped up, there is an increase in current due to double-layer charg-
ing ͑region I͒. The current decreases with time in region II where the
electron transfer process starts occurring. The nature of current-time
variation in this region corresponds to nucleation. There is a steady
flow of current in region III due to the growth of MnO2 crystals.
tammetry, polarization studies, and SEM micrographs clearly reflect
the influence of the surfactant during the deposition.
XRD and SEM studies
.— It is known that MnO2 exhibits
polymorphism with ␣, ␤, ␥, and ␦ forms.44 MnO2 prepared by elec-
trochemical oxidation of Mn2+ in an acidic medium ͑EMD͒
has ␥ structure.34 MnO2 prepared by coprecipitation of Mn7+ and
The current density of region II was plotted as a function of t1/2
,
t, t3/2, and t2 at a step potential of 1.5 V for MnO2 and MnO2͑s͒ and
the graphs for t1/2 are shown in Fig. 3. Current varies linearly with
t
1/2, indicating that the electrodeposition processes of MnO2 and
MnO2͑s͒ are diffusion controlled, instantaneously nucleated, fol-
lowed by three-dimensional growth, according to Eq. 537,40
1/2
i = k␲zFNϱ͑M/␳͒ ͑2cD͒3/2t1/2
͓5͔
where i is current density, z is valency, F is Faraday’s constant, Nϱ
is the number of active sites per cm2, M is molar mass, ␳ is density,
c is solution concentration of the depositing material, D is diffusion
coefficient, and t is time.
The three-dimensional growth of MnO2 and MnO2͑s͒ is further
supported by scanning electron microscopic study. SEM images of
MnO2 and MnO2͑s͒ deposited on Pt substrate at the step potential of
1.5 V for 60 s during the initial stages of deposition are shown in
Fig. 4. Spherical shaped deposits are obtained, thus confirming
three-dimensional crystal growth. The size of spherical crystals of
MnO2͑s͒ is smaller and number of crystals is more as compared to
MnO2, suggesting a higher rate of nucleation when Triton X-100 is
present in the electrolyte. Although the nature of electrocrystalliza-
tion is not altered due to the presence of Triton X-100, cyclic vol-
1/2
Figure 3. Current density in region II of Fig. 2 vs time ͑t͒ for ͑i͒ MnO2
and ͑ii͒ MnO2͑s͒ at 1.5 V vs SCE.
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A904
Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
Figure 5. XRD patterns of ͑i͒ MnO2 and ͑ii͒ MnO2͑s͒. The ͑hkl͒ planes
corresponding to ␦ form of MnO2 are given on the XRD patterns. The data
were recorded at a sweep rate of 0.5° min−1 using Cu K␣ source.
surface area obtained for MnO2 and MnO2͑s͒ samples were 31.06
and 84.46 m2 g−1, respectively. These results support that MnO2͑s͒
possesses a greater surface area than MnO2.
Figure 4. SEM images of electrodeposited ͑a͒ MnO2 and ͑b͒ MnO2͑s͒ on Pt
electrode at the step potential of 1.5 V vs SCE for 60 s.
Mn2+ has ␣ structure,12,45 and if prepared by hydrothermal-
electrochemical method from MnSO4 at high temperature and rela-
tive high pH, it has ␤ structure.46 ␣, ␤, ␥, and ␦ forms of MnO2
single-crystal nanowires/nanorods have been prepared by hydrother-
mal method by varying the concentration of inorganic cations ͑NH4+
and K+͒.47 Recently, nanostructured MnO2 with ␣, ␤, and ␥ crystal-
lographic forms have been prepared by hydrothermal method and
investigated as cathode materials for primary and rechargeable
batteries.48 In order to examine the crystallographic form, thick lay-
ers of MnO2 and MnO2͑s͒ were deposited on large-surface-area Ni
substrate and the oxides were scraped out of the substrate. The pow-
der XRD patterns of MnO2 and MnO2͑s͒ were recorded and shown
in Fig. 5. The data suggest that both samples are crystalline with
identical structure and hence the effect of Triton X-100 on the crys-
tallographic nature of MnO2 is absent. Furthermore, it is inferred
from the diffraction patterns that MnO2 and MnO2͑s͒ are in ␦
form.44,32
SEM images of thick layers of MnO2 and MnO2͑s͒ deposited on
Ni substrate by cyclic voltammetry in the potential range from 1.1 to
1.5 V at a sweep rate of 20 mV s−1 are shown in Fig. 6. It is seen
that the surface of MnO2͑s͒ ͑Fig. 6b͒ appears more uniform, rough,
and porous, with smaller particles, suggesting a higher surface area
than the smoother surface of MnO2 ͑Fig. 6a͒. The values of BET
Figure 6. SEM images of ͑a͒ MnO2 and ͑b͒ MnO2͑s͒ electrodeposited on
Ni. Mass of MnO2 or MnO2͑s͒ is 0.5 mg cm−2
.
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Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
A905
Table I. Effect of current density or potential on SC.
SC of
Current
density or
potential
SC of
MnO2
MnO2͑s͒
Method of
deposition
͑F g−1
͒
͑F g−1
͒
Galvanostatic
2.5 mA cm−2
5 mA cm−2
164
183
214
211
229
258
7.5 mA cm−2
Potentiostatic
1.0 V
1.2 V
1.5 V
178
182
302
202
219
342
The effect of electrochemical technique.— It was intended to
study the influence of the technique employed for electrodeposition
on capacitance of MnO2 and MnO2͑s͒. Several electrodes were pre-
pared by varying current density ͓galvanostatic, current density
͑cd͒ϭ2.5–7.5 mA cm−2͔, potential ͑potentiostatic, potentialϭ
1.0–1.5 V͒, and sweep rate ͑potentiodynamic, sweep rateϭ
10–200 mV s−1
͒ with nearly identical masses of MnO2 ͓or
MnO2͑s͔͒ and their capacitances were evaluated as described later.
The data of SC are tabulated ͑Table I͒. There is an increase in SC
with increasing cd or potential for both MnO2 and MnO2͑s͒. The SC
of MnO2͑s͒ is greater than that of MnO2 in all experiments. When
the cd or potential used for deposition of MnO2 and MnO2͑s͒ is
high, oxygen evolution also occurs along with the deposition of
MnO2 and MnO2͑s͒, resulting in porous and high surface-area de-
posits. Hence, higher SC was obtained at high cds or potentials. The
coevolution of oxygen decreases faradaic efficiency of deposition of
MnO2. Further studies in this direction were not conducted, as it was
found that MnO2 and MnO2͑s͒ prepared by potentiodynamic
method in the potential range between 1.1 and 1.5 V at a sweep rate
of 20 mV s−1 provided a maximum specific capacitance ͑Tables II
and III͒ among all the electrodes. The reason for obtaining higher
SC from potentiodynamically prepared electrodes could be a partial
reduction of MnO2 ͓or MnO2͑s͔͒, which starts occurring at about
1.1 V during the cathodic half-cycle. During repeated cycling, the
partial reduction and then further oxidation or deposition of the fresh
oxide layer on the previous layer could have resulted in a high
porosity of the oxides. Accordingly, electrodes prepared by the po-
tentiodynamic method between 1.1 and 1.5 V at 20 mV s−1 were
employed for the rest of the studies.
Figure 7. ͑a͒ Cyclic voltammograms recorded at a scan rate of 5 mV s−1 and
͑b͒ charge-discharge cycles at 0.5 mA cm−2 ͑1.11 A g−1͒ for ͑i͒ MnO2 and
͑ii͒ MnO2͑s͒ in 0.1 M Na2SO4. Curve ͑iii͒ in ͑a͒ is the cyclic voltammogram
of a bare Ni electrode.
Capacitance studies
.— Cyclic voltammograms of MnO2 and
MnO2͑s͒ electrodes in 0.1 M Na2SO4 are shown in Fig. 7a.
MnO2͑s͒ was deposited with 10 mM Triton X-100 in
0.5 M MnSO4. For comparison, a cyclic voltammogram of bare Ni
substrate is also shown. There is no significant current density on the
bare Ni electrode, thus suggesting that the contribution of the sub-
strate to the measured capacitance of MnO2 and MnO2͑s͒ is negli-
gibly small. Cyclic voltammograms of MnO2 and MnO2͑s͒ are
nearly rectangular in shape. Rectangular shape of the voltammo-
gram is a fingerprint for capacitive behavior.1 In addition to exis-
tence of double-layer capacitance, MnO2 and MnO2͑s͒ possesses
pseudocapacitance due to Mn4+/Mn3+ reversible redox reaction.
This process is accompanied by reversible insertion/deinsertion of
alkali cation ͑Na+͒or protons ͑H3O+͒ present in the electrolyte.8,21,26
Table II. Effect of deposition potential range at 20 mV s−1 on SC.
Potential range of
potentiodynamic method
͑V͒
SC of MnO2͑s͒
SC of MnO2
͑F g−1
͒
͑F g−1
͒
0–1.5
0.6–1.5
1.1–1.5
152
218
222
261
303
355
MnO2 + M+ + e− ꢀ MnOOM
͓6͔
where M+ is a cation.
Table III. Effect of sweep rate of deposition at 1.1–1.5 V range on
SC.
The electrodes were subjected to galvanostatic charge-discharge
cycling between 0 and 1.0 V in aqueous 0.1 M Na2SO4 electrolyte
at several current densities. The variation of potential during the first
few cycles with time of cycling at 0.5 mA cm−2 are shown in Fig.
7b. There is a linear variation of potential during both charging and
discharging processes. The linear variation of potential during
charging and discharging processes is another criterion for capaci-
tance behavior of material in addition to exhibiting rectangular
voltammograms.1 The durations of charge and discharge are almost
SC of MnO2͑s͒
Sweep rate
SC of MnO2
͑mV s−1
͒
͑F g−1
͒
͑F g−1
͒
10
20
50
100
200
205
222
174
148
145
310
355
237
210
202
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A906
Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
equal, implying high reversibility, and also the columbic efficiency
of charge-discharge cycling is close to unity. SC values are calcu-
lated from charge-discharge cycling using the following equation
SC = It/͑⌬Em͒
͓7͔
where I is discharge ͑or charge͒ current, t is discharge ͑or charge͒
time, and ⌬E͑=1.0 V͒ is the potential window of cycling. Discharge
SC values obtained from the second cycle for MnO2 and MnO2͑s͒
are 221.46 and 355 F g−1, respectively. These values are in close
agreement with those obtained from cyclic voltammetric data ͑Fig.
7a͒.
Effect of surfactant concentrations.— The effect of concentra-
tion of Triton X-100 in 0.5 M MnSO4 was studied by depositing
MnO2͑s͒ in 0.5 M MnSO4 consisting of Triton X-100 in various
concentrations. The concentration of the surfactant is important be-
cause of the formation of aggregates or micelles at a critical con-
centration, which is known as critical micelle concentration ͑CMC͒.
When the concentration is lower than the CMC, surfactant mol-
ecules influence the interface due to adsorption. At concentrations
higher than CMC, the surfactant molecules tend to form micelles,
and concentration of nonaggregates at the interface decreases, which
results in their decreased influence at the interface. Furthermore,
Triton X-100 is known to act as a molecular spacer through the
steric effect of the phenyl ring.49 The structure functions as a path
for Mn2+ ions, facilitating uniform current distribution. A moiety of
few ethylene oxide ͑EO͒ units in the film may mediate between
manganese ions solvated in the electrolyte and oxide film.50 The
oxygen atoms in the polyoxyethylene group of Triton X-100 may
attract the positive charge of Mn2+, resulting in Mn2+–Triton X-100
complexes. The transfer process through these types of complexes
formed probably by van der Waals forces may be affected by a
change in surfactant concentrations.51 At low concentrations of Tri-
ton X-100, the activation energy required to initiate gas/vapor
bubble nucleation may be reduced,52 and hence a large number of
smaller gas bubbles may be generated which nucleate uniformly on
the substrate surface and grow rapidly. At higher surfactant concen-
trations, however, the formation of micelles takes place, viscosity of
the medium increases, and therefore a slow diffusion of Mn2+ con-
trols the electro-oxidation.51 In fact, viscosity measurements of
0.5 M MnSO4 electrolyte consisting of Triton X-100 at different
concentrations revealed that there is a linear increase in viscosity
from 1.36 cP in the absence of Triton X-100 to 3.12 cP at 100 mM
Triton X-100. Hence, experiments were conducted by varying the
concentration of Triton X-100 in 0.5 M MnSO4 electrolyte. The
electrodes prepared at different concentrations of Triton X-100 were
subjected to charge-discharge cycling in 0.1 M Na2SO4. Variation of
specific capacitance of MnO2͑s͒ with concentration of Triton X-100
in the electrolyte is shown in Fig. 8. There is an increase in the SC
from 220 to 355 F g−1 when the concentration of Triton X-100 is
increased from 0 to 10 mM. This is followed by a steady decrease to
194 F g−1 at 100 mM. The initial increase in the SC ͑Fig. 8͒ when
Triton X-100 concentration is increased to 10 mM is ascribed to a
higher nucleation rate and the decrease thereafter is due to an in-
crease in the interfacial tension and viscosity. It was inferred from
the above results that the optimum concentration of Triton X-100 to
get maximum specific capacitance is 10 mM. Hence, MnO2͑s͒ elec-
trodes prepared from 0.5 M MnSO4 + 10 mM Triton X-100 electro-
lyte were used for further characterization studies.
Figure 8. Dependence of specific capacitance of MnO2͑s͒ on the concentra-
tion of Triton X-100 present in 0.5 M MnSO4 solution.
solutions between 0 and 1.0 V. It was found that the voltammo-
grams were rectangular in shape when recorded in Na2SO4 solution.
In Li2SO4 and K2SO4 electrolytes, however, there was an increase in
current at potentials close to 0 V and a decrease in current at poten-
tials at about 1 V, thus deviating from the rectangular shape as
shown in Fig. 9. This observation is found to be similar for both
MnO2 and MnO2͑s͒ ͑Fig. 9a and b͒. Hence 0.1 M Na2SO4 was used
for capacitance measurements of MnO2 and MnO2͑s͒.
The effect of electrolyte used for capacitance measurements.—
The charge-storage mechanism in MnO2 has been reported21,26 as
due to a pseudo-faradaic reaction, which involves insertion of cat-
ions from the electrolyte into MnO2 lattice. This single-electron
transfer process corresponds to about 1162 C per gram of MnO2. As
the kinetics of reversible insertion/extraction of the cation varies
with the electrolyte, it was intended to select an appropriate electro-
lyte for capacitor studies. For this purpose, cyclic voltammograms
of a MnO2 electrode were recorded in Li2SO4, Na2SO4, and K2SO4
Figure 9. Cyclic voltammograms recorded at a scan rate of 5 mV s−1 of ͑a͒
MnO2 and ͑b͒ MnO2͑s͒ in ͑i͒ 0.1 M Li2SO4, ͑ii͒ 0.1 M Na2SO4, and ͑iii͒
0.1 M K2SO4.
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Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
A907
Figure 11. Cycle-life data at 0.5 mA cm−2 ͑1 A g−1͒ of ͑i͒ MnO2 and ͑ii͒
MnO2͑s͒ in 0.1 M Na2SO4.
required. At higher loading levels, due to the low porous nature of
deposit, the electrolyte cannot permeate into the coating; only the
outer layer of the electrode material is exposed to the electrolyte and
hence SC values are low. The electrodes were subjected to charge-
discharge cycling at different current densities, and the variation of
SC with current density is shown in Fig. 10b. There is a decrease in
SC with increase in current density, which is ascribed to decrease in
the efficiency of material utilization of the active material at high
cds. Similar trends were reported in the literature.53
Evaluation of supercapacitors for their specific power ͑SP͒ and
specific energy ͑SE͒ has been of interest in the literature,1 because a
high SP is expected from electrochemical supercapacitors. Although
SP and SE are device properties, these parameters have been evalu-
ated for active materials also similar to the calculation of specific
capacitance. For instance, SP and SE of RuO2·xH2O have been cal-
culated and reported as Ragone plots from single-electrode
studies.54 In the present study, the values of SP and SE were calcu-
lated using Eq. 8 and 9, respectively
SP = I⌬E/͑2m͒
͓8͔
SE = It⌬E/͑2m͒
͓9͔
where t is discharge time. Variation of SE with SP is shown as
Ragone plots in Fig. 10c. It is seen that the SP of MnO2͑s͒ is higher
than that of MnO2 at all SE values. At a SE of 10 Wh kg−1, the SP
obtained for MnO2͑s͒ is 8.5 kW kg−1 against 4.3 kW kg−1 for
MnO2.
Cycle-life test
.— Both MnO2 and MnO2͑s͒ electrodes were sub-
jected to an extended cycle-life test at a cd of 0.5 mA cm−2, and the
variation of specific capacitance with cycle number is shown in Fig.
11. It is observed that the SC of MnO2 is 220 F g−1 during the initial
stages of cycling, and it decreases to 159 F g−1 after 500 cycles.
Thus, there is a decrease of about 28% of SC after 500 charge-
discharge cycles. For MnO2͑s͒, the SC of 355 F g−1 is obtained
during initial cycles, and it decreases to 282 F g−1 after 500 cycles
͑Fig. 11͒. Thus, the decrease in SC after 500 cycles is about 20% for
MnO2͑s͒ against 28% for MnO2.
Figure 10. Dependence of SC ͑a͒ on mass and ͑b͒ on current density of
charge-discharge cycling, and ͑c͒ Ragone plots of ͑i͒ MnO2 and ͑ii͒ MnO2͑s͒
in 0.1 M Na2SO4. In ͑c͒, the current density in mA cm−2 ͑A g−1͒ used for
charge-discharge cycling indicated at each data point.
Electrochemical impedance.— Superior electrochemical behav-
ior of MnO2͑s͒ is also reflected in impedance studies. Nyquist plots
of ac impedance spectra of MnO2 and MnO2͑s͒ are presented in Fig.
12a and Bode plots in Fig. 12b. The Nyquist plot consists of a
high-frequency intercept on the real axis corresponding to the elec-
trolyte resistance, a semicircle corresponding to a parallel combina-
tion of charge-transfer resistance ͑Rct͒ and double-layer capacitance
͑Cdl͒, and finally a linear region at low-frequency range. In general,
Effects of thickness and current density.— The variation of SC
of the electrodes with the mass of material is shown in Fig. 10a. At
all loading levels, the SC of MnO2͑s͒ is higher than MnO2. At a
mass of 0.04 mg cm−2, the SC of MnO2͑s͒ is 861 F g−1; to obtain
this value for MnO2, it is anticipated that still thinner deposits are
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A908
Journal of The Electrochemical Society, 154 ͑10͒ A901-A909 ͑2007͒
semicircle is depressed, and Q2 was used because the low-frequency
behavior is not ideal capacitive. The nonideal nature of capacitances
arise due to the nonhomogeneous nature of the electrode.55 The
impedance data were subjected to NLLS fitting program and the
impedance parameters were obtained. The theoretical curves gener-
ated from the fit results ͑solid lines in Fig. 12͒ match with the
2
experimental data and the ␹ fitting parameter is always less than
1 ϫ 10−3. An enlarged view of the Nyquist plot at the high-
frequency regime is given as an inset in Fig. 12a. It was found that
Rct of 3.17 ⍀ cm−2 for MnO2͑s͒ is smaller than 15.23 ⍀ cm−2 for
MnO2, which implies that the cation insertion/extraction process
into/from MnO2͑s͒ lattice is more facile than in the case of MnO2.
In Bode plots of impedance spectra of MnO2 and MnO2͑s͒ ͑Fig.
12b͒, maximum phase-angle values of about 51 and 74° are ob-
served for MnO2 and MnO2͑s͒, respectively. It is inferred that
MnO2͑s͒ is a better capacitive material in comparison with MnO2.
Conclusion
In order to improve the specific capacitance, ␦ MnO2 has been
electrodeposited from a neutral Mn2+ aqueous solution consisting of
Triton X-100 as the surfactant. The electrodeposited films of MnO2
in the presence of the surfactant possess greater porosity, and hence
surface area, in relation to the films prepared in the absence of the
surfactant. Cyclic voltammetry and galvanostatic charge-discharge
cycling experiments reveal that the SC is higher by about 59% due
to the effect of Triton X-100. It has been found that 10 mM Triton
X-100 in 0.5 M MnSO4 solution is the optimum concentration for
obtaining maximum SC. Extended charge-discharge cycling studies
indicate that the superior performance of MnO2 due to Triton X-100
is present throughout the life-cycle.
Acknowledgments
The authors thank K.C. Suresh for surface-area measurements.
S.D. acknowledges a senior research fellowship from the Council of
Scientific and Industrial Research ͑CSIR͒, New Delhi, India.
Figure 12. Electrochemical impedance spectra as ͑a, top͒ Nyquist plot and
͑b, bottom͒ Bode plot of ͑i͒ MnO2 and ͑ii͒ MnO2͑s͒ recorded at open-circuit
potential ͑0.7 V vs SCE͒. The experimental data points are shown as circles
and theoretical curves as solid lines. In ͑a͒ expanded view of Nyquist plot at
high-frequency regime is shown as inset and frequency values of some data
points in Hz are also shown.
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