Surfactant assisted electrodeposition of MnO2 thin films: Improved supercapacitive properties

Article abstract of DOI:10.1016/j.jallcom.2011.08.029

In order to obtain a high specific capacitance, MnO₂ thin films have been electrodeposited in the presence of a neutral surfactant (Triton X-100). These films were further characterized by means of X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FESEM) and contact angle measurement. The XRD studies revealed that the electrodeposited MnO₂ films are amorphous and addition of Triton X-100 does not change its amorphous nature. The electrodeposited films of MnO₂ in the presence of the Triton X-100 possess greater porosity and hence greater surface area in relation to the films prepared in the absence of the surfactant. Wettability test showed that the MnO₂ film becomes superhydrophilic from hydrophilic due to Triton X-100. Supercapacitance properties of MnO₂ thin films studied by cyclic voltammetry, galvanostatic charge-discharge cycling and impedance spectroscopy showed maximum supercapacitance for MnO₂ films deposited in presence of Triton X-100 is 345 F g^-1.

Full text of DOI:10.1016/j.jallcom.2011.08.029

Journal of Alloys and Compounds 509 (2011) 10050–10054
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Surfactant assisted electrodeposition of MnO2 thin films: Improved
supercapacitive properties
D.P. Dubala,b, W.B. Kimb, C.D. Lokhandea,∗
a Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416004 (M.S.), India
b School of Materials Science and Engineering, Gwangju Institute of Science and Technology, 261 Cheomdan-gwagiro, Buk-gu, Gwangju 500-712, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
In order to obtain a high specific capacitance, MnO2 thin films have been electrodeposited in the presence
of a neutral surfactant (Triton X-100). These films were further characterized by means of X-ray diffrac-
tion (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy
(FESEM) and contact angle measurement. The XRD studies revealed that the electrodeposited MnO2 films
are amorphous and addition of Triton X-100 does not change its amorphous nature. The electrodeposited
films of MnO2 in the presence of the Triton X-100 possess greater porosity and hence greater surface area
in relation to the films prepared in the absence of the surfactant. Wettability test showed that the MnO2
film becomes superhydrophilic from hydrophilic due to Triton X-100. Supercapacitance properties of
MnO2 thin films studied by cyclic voltammetry, galvanostatic charge–discharge cycling and impedance
spectroscopy showed maximum supercapacitance for MnO2 films deposited in presence of Triton X-100
Received 6 October 2010
Received in revised form 1 August 2011
Accepted 6 August 2011
Available online 12 August 2011
Keywords:
Amorphous materials
Thin films
Chemical synthesis
Energy storage
is 345 F g−1
.
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
at the electrode/solution interface appeared to be an attractive,
simple methodology to improve the electrochemical processes at
the substrate/electrolyte interface. Though researches have been
carried out on surface active agents in electrochemistry for more
than six to seven decades, just a few classes of materials have
been well studied so far for a modification process in the pres-
ence of surfactants [11]. Employment of surfactants in this work is
a natural outgrowth of numerous and valuable articles on surface
active agents. As proposed by Rusling, surfactant layers are formed
on the electrode surface with well-defined microstructures and
these could serve as templates for the electrochemical reactivity
[12]. The addition of organic surfactants in the precursor solution
is extremely important due to their influence on morphological
and structural properties of electrodeposits. Organic surfactants
are commonly used in the electrodeposition to control the crys-
tal shape and size, in order to produce high quality deposits. The
role of the organic surfactants is to change the morphology of the
deposit owing to their concentration-dependent specific activity
during electrodeposition. The modified materials are expected to
afford superior mass transport properties, through regular meso-
porous networks as a result of modified growth on the surface of
adsorbed organic surfactant layers [13].
Electrochemical capacitor, also known as supercapacitor, is
a promising energy storage device for meeting the high-power
electric market [1,2]. Several metal oxides and hydroxides for
example, those of Ni, Co, V, and Mn, have been studied exten-
sively for high power redox supercapacitor applications [3–5].
Among the metal oxides, manganese dioxide (MnO2) is one of the
promising materials due to its availability, cost-effectiveness and
non-toxicity. There are several oxidation states, including Mn(0),
Mn(II), Mn(III), Mn(IV), Mn(V), Mn(VI), and Mn(VII), for manganese
oxides [6] hence they are often employed as the cathode materi-
als in rechargeable batteries [7]. Recently, manganese oxides have
been considered as a potential candidate for the electrode mate-
rial of supercapacitors. MnO2 can show excellent pseudo-capacitive
behavior in aqueous solutions and conduct a specific capacitance
of 160 F g−1 [8,9]. However, this value is competitive with carbon
supercapacitors, but falls far short of the 760 F g−1 obtained with
hydrous RuO2 [10].
Beneficial effects of localizing heat through non-isothermal
anode heating [16] on electro-kinetics, have led us to use a rela-
tively new technique with more efficiency in energy conversion for
the evaluation of electrodeposited MnO2. Adsorption of surfactants
In order to improve the supercapacitive properties, the surface
of MnO2 thin film should be modified. The aim of the present study
is 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. The effect of
∗
Corresponding author. Tel.: +91 231 2609225; fax: +91 231 2692333.
E-mail address: l chandrakant@yahoo.com (C.D. Lokhande).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.08.029

D.P. Dubal et al. / Journal of Alloys and Compounds 509 (2011) 10050–10054
10051
Ι
Δ- Stainless steel
ΙΙ
2
0
Δ
Δ
MnO2
-2
-4
-6
TX:MnO2
(b)
(a)
ΙΙΙ
1500
1000
500
0
-500
-1000 -1500
10
20
30
40
50
60
70
80
90 100
Voltage (mV/SCE)
2θ (Degree)
Fig. 1. Cyclic voltammograms for MnO2 and TX:MnO2 in the range of +1.3 to
Fig. 2. The XRD patterns of (a) MnO2 and (b) TX:MnO2 thin films onto SS substrate.
−1.2 V/SCE at the scan rate of 20 mV s−1
.
peak (III) (+1.0 V/SCE) which corresponds to the oxidation and
dissolution of deposit into ions. The standard potentials are shown
in bracket which is nearly the same. The slight variation in standard
and observed potential is seen which may be due to substrate effect.
The higher potential and alkaline medium assist the formation of
MnO2. As the adsorption of Mn2+ ions at the electrode surface pre-
cedes the electron-transfer process, it is inferred that Triton X-100
molecules favor this process, which results in higher anodic peak
current for the oxidation of Mn2+ to MnO2.
surfactant (Triton X100) on structural, morphological, contact angle
and supercapacitive properties of MnO2 thin films are investigated.
2. Experimental details
Plating baths for MnO2 and Triton X-100 assisted MnO2 (TX:MnO2) thin films
were prepared with AR grade chemicals using double distilled water. The bath con-
sisted of an aqueous solution of 0.1 M manganese sulphate (MnSO4·4H2O) with 0.1 M
citric acid {C (OH) (COOH) (CH2COOH)2·H2O} as a complexing agent, maintained at
a pH of ∼10.5 through the addition of 1 M sodium hydroxide (NaOH) solution. The
concentration of organic surfactant (Triton X-100) was kept constant at 1 wt% in the
final solution. The MnO2 and TX-MnO2 films are deposited onto a commercially pure
stainless steel (SS) foil (SS 304) by potentiodynamic deposition. Before deposition, SS
foil was polished with zero grade polish paper and then ultrasonically cleaned with
double distilled water. Pure graphite was used as an anode. All deposition poten-
tials were measured with respect to saturated calomel electrode (SCE) as a reference
electrode. The depositions of MnO2 and TX:MnO2 were carried out potentiodynam-
ically between the potential limits of +1.3 and −1.2 V/SCE at 20 mV s−1 scan rate
using Potentiostat (EG and G-263A).
3.2. Structural characterization
Film crystallinity was analyzed using X-ray diffraction. The XRD
patterns of MnO2 and TX:MnO2 on to the SS substrate are shown
in Fig. 2. Both the XRD patterns do not show well-defined diffrac-
tion peaks other than SS substrate, indicating that MnO2 films are
amorphous. Thus, the data suggest that both samples are amor-
phous and Triton X-100 do not alter the amorphous nature of MnO2.
The obtained amorphous phase is feasible for supercapacitor appli-
cation, since the protons can easily permeate through the bulk of
the amorphous MnO2 electrode materials and whole amount of
electrode is utilized for energy storage [14].
The structural characterization of the MnO2 and TX:MnO2 films was carried out
using X-ray diffraction within the range 10–100◦ on computer controlled Philips
˚
PW-3710 using CrK␣ radiations (ꢀ = 2.2897 A). The surface morphological studies of
films were carried out using FESEM (field emission scanning electron microscopy,
Model: JSM-6701F, JEOL, Japan). The Fourier transform infrared (FTIR) spectra of
the samples were collected using a ‘Perkin Elmer, FTIR Spectrum one’ unit. In order
to study interaction between electrolyte and electrode surface contact angle mea-
surement was carried out by Rame-hart USA equipment with CCD camera. The
supercapacitor study was carried out using the 263A EG & G Princeton Applied
Research Potentiostat forming an electrochemical cell comprising MnO2/TX:MnO2
film as a working electrode, platinum as a counter electrode and saturated calomel
electrode (SCE) as a reference electrode in 1 M Na2SO4 electrolyte. Charge-discharge
and impedance study was carried out using CHI 660D electrochemical workstation.
3.3. Surface morphological studies
Many investigations have shown that rechargeability, current
efficiency and stability depend significantly on the electrode
morphology. Surfactants strongly affect the morphology and
crystal growth in association with a promotion of nucleation and
current distribution [13]. In this regard, the structure of products is
controlled through a balance of thermodynamic and kinetic forces
within different inorganic, organics as well as inorganic/organic
interactions [15]. The SEM images of MnO2 and TX:MnO2 are
shown in Fig. 3(a and b), respectively. The morphologies showed
that the substrate is well covered with MnO2 nanoparticles. From
the Fig. 3(a), one can see the spherical grain morphology of MnO2
spread over whole surface with porous structure. It is seen that
the surface of TX:MnO2 appears more uniform, rough, and porous,
with smaller particles, suggesting a higher surface area than the
smoother surface of MnO2. Devaraj and Munichandraiah [16]
prepared Triton X-100 assisted MnO2 thin films and reported
that due to addition of triton X-100 the surface of MnO2 becomes
porous and rough
3. Results and discussion
3.1. Electrochemical deposition
Fig. 1 shows the typical cyclic voltammograms (CV) for MnO2
and TX:MnO2 in the range of +1.3 to −1.2 V/SCE at the scan rate of
20 mV s−1. The cathodic peak (I) formed at −0.42 V/SCE may be due
to the reduction of manganese at the surface of SS substrate. Depo-
sition potential shifted toward negative region due to the strong
complex formation in the solution.
2M(OH)2+ + 2e− → 2MnOOH + H2↑ (Peak I)
(−0.48 V/SCE)
(1)
2MnOOH + 2e− → 2MnO2 + H2↑ (Peak II)
(−1.25 V/SCE) (2)
+
Since, the bath is alkaline and complexed, the Mn(OH)2 ions
are formed which get attracted toward cathode at higher poten-
tials than −1.1 V/SCE (Peak II) and reduction takes place by the
formation of MnO2. It is seen that, in reverse scan, there is a
3.4. FTIR studies
The FTIR absorption spectra of MnO2 and TX:MnO2 samples
in the range 4000–400 cm−1 are shown in Fig. 4(a and b). The

10052
D.P. Dubal et al. / Journal of Alloys and Compounds 509 (2011) 10050–10054
Fig. 3. Scanning electron micrographs of (a) MnO2 and (b) TX:MnO2 thin films at ×100k magnification.
surfactant layer, rendering the surface more hydrophobic. Thus
(b)
(a)
the triton X-100 contains hydroxyl groups due to which the sur-
face becomes more hydrophilic [13] since there is cohesive force
between the water droplet and hydroxyl groups. The contact angle
factor is known to affect the wettability of a solid surface greatly.
Generally, low water contact angle increases the electrochemical
performance, where interfacial contact at electrolyte–electrode is
important [21].
3.6. Supercapacitive study
3.6.1. Effect of Triton X-100 on supercapacitive properties of
MnO2 thin films
4000
3500
3000
2500
2000
1500
1000
500
Cyclic voltammograms curves of MnO2 and TX:MnO2 electrodes
in 1.0 M Na2SO4 are shown in Fig. 6(a and b). Cyclic voltammo-
grams of both are rectangular in shape which is a fingerprint for
capacitive behavior [22]. In addition to existence of double-layer
capacitance, MnO2 and TX:MnO2 possesses pseudocapacitance due
to Mn4+/Mn3+ reversible redox reaction. This process is accompa-
nied by reversible insertion/deinsertion of alkali cation (Na+) or
protons (H3O+) present in the electrolyte [23].
Wavenumber (cm-1)
Fig. 4. FTIR spectra of (a) MnO2 and (b) TX:MnO2 thin films.
strong bands around at 614 cm−1 and 506 cm−1 are associated
with the characteristic vibrational mode of Mn–O lattice [17]. The
absorption peaks at around 1634 and 1107 cm−1 are assigned to
–OH bending vibrations combined with Mn atoms [18]. The sharp
absorption band at 3422 cm−1 is attributed to the –OH stretch-
ing vibrations. Shoulder of the 3422 cm−1 bond attributed to the
hydrous nature of the MnO2 material is highly intense in case of Tri-
ton X-100 assisted MnO2. Thus results indicated that, as MnO2 film
contained hydroxide and Mn–O bonds, which indicates that forma-
tion of hydrous MnO2 that may play important role in capacitive
behavior [19]. From the FTIR study it is concluded that electrode-
posited MnO2 thin films are free from stray organic surfactant
molecules.
MnO2 + M+ + e− ⇔ MnOOH
(3)
where M+ is a cation. The supercapacitace increases from 210 to
278 F g−1 due to Triton X-100 surfactant at 100 mV s−1. The main
reason for the enhancement in the supercapacitance is that the
presence of TX-100 could strengthen the skeleton of the MnO2
and prevents the capacity loss. Secondly, large surface area of
the crystals decreases the solid-state diffusion path length of pro-
tons and electrons into and out of the bulk of MnO2 films [24].
Therefore, more facilitated volume pulsations of MnO2 during
charge/discharge cycling, and consequently an increase of both
discharge capacity and coulomb efficiency could be expected.
Such features appear particularly important from the viewpoint
of conductance, as these nanoparticles should allow a much bet-
ter connection of MnO2 with graphite, explaining the higher cycle
performance of the corresponding cells [13].
3.5. Surface wettability test
Fig. 5(a and b) shows the measurement of water contact angles
for MnO2 and TX:MnO2 thin films. From the images the contact
angles for MnO2 and TX:MnO2 are found to be 45◦ and 10◦, respec-
tively. Decrease in contact angle for MnO2 in presence of Triton
X-100 is due to the hydroxyl bonds [20]. Triton X-100 molecules
tend to be oriented with the phenyl ring toward the solution [15].
The presence of hydroxyl groups would strengthen the adsorba-
bility toward the MnO2 electrode surface, forming an adsorbed
3.6.2. Effect of scan rate
The voltammetric responses of TX:MnO2 electrode at different
scan rates is shown in Fig. 7. The effect of the scan rate from 5
Fig. 5. Contact angle measurements of (a) MnO2 and (b) TX:MnO2 thin films.

D.P. Dubal et al. / Journal of Alloys and Compounds 509 (2011) 10050–10054
10053
Table 1
12
8
Values of specific energy, specific power and coulomb efficiency of MnO2 and
TX:MnO2 thin films.
TX-100: MnO2
MnO2
Name
Specific energy
Specific power
(W kg−1
Coulomb
efficiency, ꢁ (%)
4
(Wh kg−1
)
)
MnO2
TX:MnO2
2.3
3.4
0.5 × 103
0.6 × 103
91
93
0
-4
-8
-12
-10
-8
-6
-4
-2
0
1000
800
600
400
200
0
-200
TX: MnO2
Voltage (mV/SCE)
Fig. 6. Cyclic voltammograms of (a) MnO2 and (b) TX:MnO2 thin films in 1 M Na2SO4
electrolyte at 100 mV s−1
.
MnO2
12
E
D
C
8
4
B
A
0
2
4
6
8
10
12
Zr' (ohm cm2)
0
Fig. 9. Nyquist plots obtained for MnO2 and TX:MnO2 electrode at 0.8 V/SCE in 1 M
Na2SO4 electrolyte.
-4
-8
-12
A-5 mV s-1
B-10 mV s-1
C-20 mV s-1
D-50 mV s-1
E-100 mV s-1
3.6.3. Galvanostatic charge–discharge studies
MnO2 and TX:MnO2 electrodes were subjected to galvanos-
tatic charge–discharge cycling between 0 and 0.8 V/SCE in aqueous
1.0 M Na2SO4 electrolyte at 2 mA cm−2 current density. The varia-
tion of potential during the cycle with time of cycling is shown in
Fig. 8. There is a linear variation of potential during both charg-
ing and discharging processes. The linear variation of potential
during charging and discharging processes is another criterion for
capacitance behavior of material in addition to exhibiting rectan-
gular voltammograms [22]. The durations of charge and discharge
are nearly equal, implying high reversibility. The value of spe-
cific energy, specific power and coulomb efficiency for MnO2 and
TX:MnO2 are listed in Table 1.
1000
800
600
400
200
0
-200
Voltage (mV/SCE)
Fig. 7. Cyclic voltammograms of TX:MnO2 thin film at different scanning rates in
1 M Na2SO4 electrolyte.
0.8
MnO2
TX: MnO2
0.6
0.4
0.2
0.0
3.6.4. Electrochemical impedance analysis
Nyquist plots of ac impedance spectra of MnO2 and TX:MnO2 are
presented in Fig. 9. The Nyquist plot consists of a high-frequency
intercept on the real axis corresponding to the electrolyte resis-
tance, a semicircle corresponding to a parallel combination of
charge-transfer resistance (Rct) and double-layer capacitance (Cdl),
and finally a linear region at low-frequency range. From the Nyquist
plot the electrolyte resistance is 0.6 ꢂ as seen from the intercept on
the real axis. It was found that Rct of 3.9 ꢂ cm−2 for MnO2 in pres-
ence of Triton X-100 is smaller than that of 7 ꢂ cm−2 for MnO2,
this implies that the cation insertion/extraction process into/from
TX:MnO2 lattice is more facile than in the case of MnO2. It is inferred
that TX:MnO2 is a better capacitive material in comparison with
MnO2. The results obtained here are in good agreement with earlier
reported results [16].
0
100
200
Time (sec)
300
400
500
Fig. 8. Galvanostatic charge–discharge curves of MnO2 and TX:MnO2 thin film at
2 mA cm−2 in 1 M Na2SO4 electrolyte.
to 100 mV s−1 on supercapacitance of MnO2 electrode is studied
in 1.0 M Na2SO4 in voltage range of −0.1 to 0.9 V/SCE. There is a
linear increase in voltammetric current density with an increase
in scan rate, suggesting capacitive behavior of the electrodes [25].
The decrease in capacitance with the scan rate is attributed to the
presence of inner active sites, which cannot precede the redox tran-
sitions completely at higher scan rate of CV, probably due to the
diffusion effect of proton within the electrode. Maximum capaci-
tance obtained for hydrous MnO2 at lower scan rate (5 mV s−1) is
4. Conclusions
Electrochemically deposited TX:MnO2 thin films yields supe-
rior capacitive properties than plain MnO2. The addition of triton
X-100 does not alter the amorphous nature of MnO2 thin films.
FTIR studies revealed the formation of hydrous MnO2 thin films.
Due to addition of triton X-100, the MnO2 film becomes super-
hydrophilic from hydrophilic. The supercapacitance of TX:MnO2
decreased with increase in scan rate. Smaller particle size, greater
345 F g−1
.

10054
D.P. Dubal et al. / Journal of Alloys and Compounds 509 (2011) 10050–10054
porosity, higher specific surface area, and higher efficiency of uti-
lization of TX:MnO2 in relation to MnO2 are the factors responsible
for obtaining higher specific capacitance. A specific capacitance of
345 F g−1 for TX:MnO2 is obtained in the present study. The specific
energy, specific power and coulomb efficiency are also increased
due to Triton X-100. Thus, it is inferred that TX:MnO2 is a better
capacitive material in comparison with MnO2.
[5] V. Srinivasan, J.W. Weidner, J. Power Sources 108 (2002) 15.
[6] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National
Association of Corrosion Engineers, Houston, TX, 1966, p. 286.
[7] M.M. Doeff, A. Anapolsky, L. Edman, T.J. Richardson, L.C. De Jonghe, J. Elec-
trochem. Soc. 148 (2001) A230.
[8] T. Brousse, M. Toupin, R. Dugas, L. Athouel, O. Crosnier, D. Belanger, J. Elec-
trochem. Soc. 153 (2006) A2171.
[9] R.N. Ready, R.G. Ready, J. Power Sources 132 (2004) 315.
[10] J.P. Zheng, Electrochem. Solid State Lett. 2 (1999) 359.
[11] R. Vittal, H. Gomathi, J. Phys. Chem. B 106 (2002) 10135.
[12] J.F. Rusling, Colloids Surf. 81 (1997) 123.
[13] M. Ghaemi, L. Khosravi-Fard, J. Neshati, J. Power Sources 141 (2005) 340.
[14] V.D. Patake, C.D. Lokhande, Appl. Surf. Sci. 254 (2008) 2820.
[15] Y.Z. Khimyak, J. Klinowski, J. Mater. Chem. 10 (2000) 1847.
[16] S. Devaraj, N. Munichandraiah, J. Electrochem. Soc. 154 (2007) A901.
[17] C.M. Julien, M. Massot, C. Poinsignon, Spectrochim. Acta 60 (2004) 689.
[18] M. Ocana, Colloid Polym. Sci. 278 (2000) 443.
[19] W. Sugimoto, H. Iwata, Y. Murakami, Y. Takasu, J. Electrochem. Soc. 151 (2004)
A1181.
[20] A.V. Rao, S.S. Latthe, D.Y. Nadargi, H. Hirashima, V. Ganesan, J. Colloid Interface
Sci. 332 (2009) 484.
Acknowledgements
One of the authors Prof. C. D. Lokhande is grateful to All India
Council for Technical Education, New Delhi for financial sup-
port through scheme no. F. No.: 8023/BOR/RID/RPS-165/2009-10.
Authors are also grateful to the Council for Scientific and Industrial
Research (CSIR), New Delhi (INDIA) for financial support through
the scheme no. 03(1165)/10/EMR-II.
[21] R.S. Mane, J. Chang, D. Ham, B.N. Pawar, T. Ganesh, B.W. Cho, J.K. Lee, S.H. Han,
Curr. Appl. Phys. 9 (2009) 87.
[22] Conway BE, Electrochemical Supercapacitors, Kluwer Academic Publish-
ers/Plenum Press, New York, 1999, p. 1.
References
[1] B.E. Conway, V. Briss, J. Wojtowicz, J. Power Sources 66 (1997) 1.
[2] R. Kotz, M. Carlen, Electrochim. Acta 45 (2000) 2483.
[3] T.C. Liu, W.G. Pell, B.E. Conway, Electrochim. Acta 44 (1999) 2829.
[4] H.Y. Lee, J.B. Goodenough, J. Solid State Chem. 148 (1999) 81.
[23] M. Toupin, T. Brousse, D. Belanger, Chem. Mater. 16 (2004) 3184.
[24] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2000)
444.
[25] J.N. Broughton, M.J. Brett, Electrochem. Solid State Lett. 5 (A2) (2002) 79.