EQCM Investigation of the Electrodeposition of MnO
2
and Its Capacitance Behavior
S. Devaraj
z
and N. Munichandraiah
*
,z
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India
Electro-oxidation of Mn
2+
to MnO
2
by cyclic voltammetry on gold in acidic 0.1 M H
2
SO
4
and neutral 0.1 M Na
2
SO
4
media
was studied using electrochemical quartz-crystal microbalance EQCM. The cyclic voltammetric behavior of Au is different in
these electrolytes. From EQCM data of mass variation during cycling, the rate of electrodeposition of MnO
2
is higher in the
neutral medium than in the acidic medium. Specific capacitance of MnO
2
deposited from the neutral medium is higher than that
deposited from the acidic medium owing to different crystallographic structures.
© 2009 The Electrochemical Society. DOI: 10.1149/1.3152327 All rights reserved.
Manuscript submitted January 19, 2009; revised manuscript received May 10, 2009. Published June 12, 2009.
Manganese dioxide is an important material that exhibits diverse
crystallographic structures
1
with physical and chemical properties
suitable for applications such as ionic and molecular sieves,
2
catalysts,
3
sensors,
4
batteries,
5
supercapacitors,
6-12
electrochromic
devices,
13
magnetic materials,
14
etc. MnO
2
can be prepared by
chemical as well as electrochemical methods. By a variation in ex-
perimental conditions of synthesis, the crystallographic nature of
MnO
2
can be controlled. For electrochemically prepared MnO
2
, oxi-
dation of Mn
2+
in acidic electrolytes produces -MnO
2
. This is
known as electrolytic manganese dioxide, which is widely used as
the cathode material in aqueous primary batteries. Electrosynthesis
in neutral electrolytes leads to the formation of -MnO
2
, which is
less widely studied than -MnO
2
. -MnO
2
is a more appropriate
material for electrochemical supercapacitors than -MnO
2
.
15
Elec-
trochemical quartz-crystal microbalance EQCM is a useful tech-
nique to distinguish the properties of and phases of MnO
2
.
Recently, the reduction of MnO
4
-
to MnO
2
has been studied by
EQCM.
16
In the present work, electrochemical preparation of MnO
2
by
oxidation of Mn
2+
ions on gold in acidic and neutral electrolytes is
studied using EQCM. MnO
2
exhibits faster growth in a neutral me-
dium than in an acidic medium. Galvanostatic charge–discharge cy-
cling in 0.1 M Na
2
SO
4
is accompanied by variation in mass of
MnO
2
deposited on Au crystal.
Experimental
Analytical grade reagents, MnSO
4
·H
2
O, H
2
SO
4
Merck, and
Na
2
SO
4
BDH, were used for the experiments. All solutions were
prepared using doubly distilled water. EQCM measurements were
carried out in a Teflon cell, consisting of a Au coated quartz crystal
as the working electrode, a Pt-wire counter electrode, and a Ag/
AgCl, 3 M KCl reference electrode. The quartz crystal of 13.7 mm
diameter CH Instruments, model 400 A was AT-cut and sand-
wiched between 5.11 mm diameter vapor-deposited gold disks. The
resonating frequency of the crystal in air was 8 MHz, and the sur-
face area of the Au electrode was 0.205 cm
2
. The shear modulus
of the crystal was 2.947 10
11
g cm
-1
s
-2
and the density
was 2.648 g cm
-3
. All potential values were reported against a
Ag/AgCl, 3 M KCl reference electrode.
Before the electrochemical experiments, the working electrode
was conditioned by repeatedly cycling 20 times between 0 and 1.5 V
at a sweep rate of 5 mV s
-1
in 0.1 M H
2
SO
4
. Subsequently, 0.1 M
H
2
SO
4
was replaced by the experimental electrolyte. Electrochemi-
cal studies were carried out using a computer-controlled CH Instru-
ments potentiostat/galvanostat model 400 A. The software program
provided electrochemical data in the form of voltammograms as
well as frequency of the crystal as a function of the potential. The
data of changes in frequency f of the crystal due to electrode-
posits were converted into plots of mass changes m using the
Sauerbrey relationship, and plots of m vs potential were provided.
Charge–discharge cycling was carried out in 0.1 M Na
2
SO
4
solution
using a Solartron galvanostat/potentiostat model SI 1287. All experi-
ments were carried out at 20 2°C.
Results and Discussion
Deposition of MnO
2
in acidic and neutral electrolytes.— Au
crystal used to study the kinetics of electrodeposition of MnO
2
was
subjected to cycling repeatedly in 0.1 M H
2
SO
4
between 0 and 1.5
V. A typical cyclic voltammogram and the corresponding mass
change m of the Au crystal in a cycle are presented in Fig. 1.
Two broad peaks at around 1.1 and 1.3 V P
a1
and P
a2
in Fig. 1A
are due to the adsorption of oxygen on different crystal planes of the
Au working electrode.
17
There is a gradual increase in mass to about
30 ng during the forward sweep from 0.0 to 1.1 V Fig. 1B, fol-
lowed by a sharp increase up to about 70 ng when the potential is
swept to 1.50 V. During the reverse sweep, a sharp cathodic peak at
* Electrochemical Society Active Member.
z
E-mail: devaraj@ipc.iisc.ernet.in; muni@ipc.iisc.ernet.in
-0.02
-0.01
0.00
0.01
0.02
0.03
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
p
c
p
a2
p
a1
(A)
Current / mA
(B)
∆m/ μg
Potential / V vs. Ag/AgCl, 3 M KCl
Figure 1. A Cyclic voltammogram and B mass variation m vs poten-
tial recorded during activation of the Au crystal in 0.1 M H
2
SO
4
at a
5 mV s
-1
scan rate. The area of the electrode is 0.205 cm
-2
.
Electrochemical and Solid-State Letters, 12 9 F21-F25 2009
1099-0062/2009/129/F21/5/$25.00 © The Electrochemical Society
F21
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