Influence of the solution pH on the nanostructural, and electrochemical performance of electrolytic manganese dioxide

Article abstract of DOI:10.1016/j.jallcom.2009.02.145

In this paper, during electrodeposition of electrolytic manganese dioxide (EMD) by anodic deposition from MnSO₄ solution, the changing of pH is studied. The pH shows a decreasing trend. Two samples are electrodeposited at fixed pH (2 and 5) and

Full text of DOI:10.1016/j.jallcom.2009.02.145

Journal of Alloys and Compounds 481 (2009) 446–449
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Influence of the solution pH on the nanostructural, and electrochemical
performance of electrolytic manganese dioxide
H. Adelkhani , M. Ghaemib
a,∗
a
Department of Optic & Spectroscopy, Lasers & Optics Research School, NSTRI, P.O. Box 11365-8486, Tehran, Iran
Department of Chemistry, Science Faculty, Golestan University, P.O. Box 49138-15739, Gorgan, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, during electrodeposition of electrolytic manganese dioxide (EMD) by anodic deposition
from MnSO4 solution, the changing of pH is studied. The pH shows a decreasing trend. Two samples are
electrodeposited at fixed pH (2 and 5) and their physico-chemical properties are characterized by X-ray
diffraction (XRD), scanning electron microscopy (SEM), the Barrett–Joyner–Halenda (BJH) method, and
rechargeable alkaline manganese dioxide (RAM) battery tests. The results have confirmed that the pH of
the solution has remarkable effects on the nanostructure, crystal structure, porosity, and the electrochem-
ical performances of the EMD. Whereas the electrodeposition at pH 2 creates irregular multi-branched
morphology with meso/microporosity, a large number of regular nanospheres with microporosity are
obtained at pH 5. The battery tests study has revealed that ␣/␥-MnO2 electrodeposited at pH 2 exhibits
good electrochemical performances, and the ␣-MnO2 displays very stable cycling behavior.
© 2009 Elsevier B.V. All rights reserved.
Received 27 January 2009
Received in revised form 25 February 2009
Accepted 26 February 2009
Available online 14 March 2009
Keywords:
MnO2
pH
Nanostructures
X-ray diffraction
Rechargeable alkaline batteries
1. Introduction
ogy, and the electrochemical performances are obtained for the
EMD deposits. From the viewpoint of structural properties then, ␥-
In the past decades, manganese dioxide (MnO ) has been one of
MnO consists of a Ramsdellite-like (2×1 channels) basic structure
2
2
the most intensively researched substances due to its applications
as electroactive materials for batteries (primary, secondary, and Li-
ion batteries) and supercapacitors [1–4].
and shows good electrochemical activity. Nevertheless, depend-
ing on the electrodeposition conditions, the mixtures of ␣- or ␤-
and ␥-MnO2 (␣/␥ or ␤/␥) can be obtained. Both ␣-MnO2 and
␤-MnO2 show low electrochemical activity when used as active
Electrodeposition is known to be a useful method for the prepa-
ration of MnO . In this method, electrolytic manganese dioxide
material. Therefore, the electrochemical performance of MnO , as
2
2
(
EMD) is prepared based on an anodic reaction from a MnSO4
active material in supercapacitors, Li-ion and rechargeable alka-
line manganese dioxide (RAM) batteries, is strongly affected, but in
electrolyte solution. The anodic and cathodic reactions can be sum-
marized as follows:
proportion to the contribution of ␣ or ␤ in MnO [4–12].
2
In the present work, we first consider the changing pH during
electrodeposition. By examining electrodeposition at fixed pH, we
subsequently study the solution pH effects on the physico-chemical
properties of the EMD samples. Finally, we investigated the effect of
the pH on the samples’ electrochemical performances when used
as cathode in the RAM batteries.
Mn2 + 2H O → MnO + 4H + 2e− anodic reaction
+
+
(1)
(2)
2
2
+
+ 2e− → H2 cathodic reaction
2
H
The anodic and cathodic reactions include the formation (reac-
tion (1)) and consumption (reaction (2)) of H . The unbalancing of
the H formation and its consumption may change the pH during
+
+
MnO2 electrodeposition and drastically influence the physico-
chemical properties of the EMD. Because of the dependence on
the acidity of the electrolyte solution (pH) and other electrodepo-
sition conditions (temperature and composition of the electrolyte
solution, current density, types of electric currents (direct or pulse
current), presence of surfactants), various structures, morphol-
2.
Experimental
The EMD samples were produced from
a
solution containing MnSO4
−
1
◦
−2
(
0.74 mol l ) at temperature 80 C and an anodic current density of 0.8 A dm . A
titanium plate was used as the anode between two Pb plates as the cathodes. The
pH of the electrolyte solution was monitored in situ by a pH meter. Two samples
were electrodeposited at fixed pH 2 and 5 by adding NH4OH (10%), naming EMD2
and EMD5, respectively.
Powder X-ray diffraction (XRD) analysis was carried out by using a Philips X-ray
diffractometer (model PW 1800) using Cu K␣ radiation source operating at 30 kV.
Each diffraction pattern was recorded in the 2ꢀ range 10–80 , with a step size of 0.05
∗ Corresponding author. Tel.: +98 21 88221085; fax: +98 21 88221083.
◦
E-mail address: hadelkhani@aeoi.org.ir (H. Adelkhani).
2ꢀ and a count time of 2.5 s per step. Scanning electron microscopy (SEM), Philips
0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.02.145

H. Adelkhani, M. Ghaemi / Journal of Alloys and Compounds 481 (2009) 446–449
447
model XL30, was used to study the morphology of the samples. The pore size distri-
bution was determined using desorption isotherm by the Barrett–Joyner–Halenda
BJH) method. Charge/discharge cycle performances of the EMD samples were mea-
(
sured as the main cathodic component of laboratory designed rechargeable alkaline
manganese dioxide (RAM) batteries. Detailed descriptions of experiments can be
found in our previous work [4].
3. Results and discussion
3.1. The pH changing
In the first step of research, an electrolyte solution was chosen
that contained only MnSO , without adding NH OH. The plot of
4
4
pH vs. time is shown in Fig. 1. The acidity of electrolyte solution
is showing an increasing trend during electrodeposition. The ini-
tial pH was approximately 6, which decreases to approximately
1
when electrodeposition is complete. As mentioned, there is an
unbalancing occurring between the formation and consumption of
+
+
H ; consequently, the excess production of H is expected during
Fig. 2. XRD pattern of the EMD2 and the EMD5. Peaks marked with an asterisk
correspond to ␣-MnO2 phases.
+
electrodeposition. In fact, the excess production of H due to the
electro-oxidation of Mn2 to MnO is responsible for the decrease
+
2
in pH.
[
11,12]. Mixing of ␤- and ␥-MnO resulted in an increase in the pH
2
or the temperature of the solution. Furthermore, lower pH or lower
temperatures were suitable for the formation of the mixtures of
3.2. Characterization of the EMD
␣
- and ␥-MnO . The formation of ␣-MnO2 at low pH levels was
2
+
Relative to the change in pH, two samples are electrodeposited at
attributed to a template or structure-directing effect due to H O .
3
fixed pH (2 and 5). The 10% ammonia solution was previously used
as a neutralized the EMD after electrodeposition [8], therefore, it
is used as a pH-stabilizing agent in this work. The XRD patterns
of the obtained powders are illustrated in Fig. 2. In the EMD elec-
Increasing the pH creates a region of stability for the formation of ␤-
MnO . In other words, the amount of pyrolusite intergrowth began
2
by increasing the pH of the solution.
The SEM of the EMD2 and the EMD5 samples are shown in
Fig. 3, and a notable difference appears between the micro/nano-
structures of the samples. The EMD5 demonstrates a uniform/
trodeposition of ␥-MnO , the product is strongly affected by De
2
Wolff (Pr) and microtwinning (Tw) defects [6]. The Pr represents the
intergrowth of ␤-type pyrolusite within the ramsdellite structure.
Generally, a shift to higher angle of the (1 1 0) line corresponds to
an increase in the Pr. The Tw, which represents the number of faults
generated by twinning, is shown to be associated with the Mn4+
vacancies generated by oxygen evolution during MnO2 electrode-
position. A narrowing of the distance between the (2 2 1) and (2 4 0)
lines corresponds to an increase in the Tw.
For the EMD5, it is observed that the (1 1 0) line is shifted to a
higher angle. In addition, the EMD5 shows a sharper peak for (2 2 1)
◦
and (2 4 0) lines at 2ꢀ = 56 . The reflections given by marked Miller
indices may be attributed to the presence of ␣-MnO in the EMD2.
2
Consequently, the EMD5 has a structure with high Pr and Tw, and it
consists of the mixture of ␤- and ␥-MnO . The EMD2 has a structure
2
with low Pr and Tw and it consists of the mixture of ␣- and ␥-MnO2.
According to previous reports that have studied the effects of the
pH on the structural characteristics of ␥-MnO and its compounds
2
containing a metal M (with M: Al, Co or Ni), the temperature and the
pH have the same effects on the crystal structures of MnO samples
2
Fig. 3. SEM of MnO2 obtained via the electrodeposition at (a) pH 5 (EMD5), and (b)
pH 2 (EMD2).
Fig. 1. The plot of pH vs. time.

4
48
H. Adelkhani, M. Ghaemi / Journal of Alloys and Compounds 481 (2009) 446–449
regular morphology consisting of nanosphere (Ø: 140 nm), while
electrodeposition at pH 2 creates an irregular thistle-like morphol-
ogy that has many branches.
In accordance to our previous studies, the formation of
oxygen containing intermediate Mn (III) species (e.g., MnOOH,
3
+
[
Mn(OH ) ] , Mn O ) causes the growth of passive layers on the
2 6 2 3
anode surface during electrodeposition [4]. The thickness of pas-
sive layers plays an important role in the morphology of the EMD
by the inhibition of prevailing grains growth mechanism on the
anode/electrolyte interface [4]. Furthermore, it is well known that
the stability of passive layer is low at the acidic pH.
At pH 5, the grain growth is prevented because of the higher
stability of the Mn (III) species and the increasing thickness of
the passive layers. These conditions lead to a large number of
nanospheres that have regular appearances as seen in Fig. 3a. At
pH 2, by decreasing the stability and thickness of passive layers,
the growth of crystal grains randomly occurs to produce multi-
branched thistle-like morphology (see Fig. 3b).
The EMD5 shows a uniform and dense appearance, but the EMD2
has an irregular morphology. Nevertheless, the EMD5 has higher
defects (higher Pr and Tw) than the EMD2 (as seen in XRD results).
In other words, it shows an independent relationship between the
morphology and the crystal structure of the samples. There are
some reports with similar results. According to Machefaux et al.
and Hill et al., the morphologies obtained for ␥-MnO can be inde-
2
pendent of the structural parameters. In addition, the higher pH can
cause the formation of denser products [11,12]. Consequently, the
morphology may be controlled by other parameters (the presence
of oxygen containing intermediate Mn (III) species, the stability, and
thickness of passive layers).
The nitrogen adsorption/desorption isotherms and the pore size
distribution of samples from the Barrett–Joyner–Halenda (BJH) test
are presented in Fig. 4. Both of the resulting isotherms could be
identified as type Langmuir I isotherms [13]. As is seen for Fig. 4, the
gap between the two curves at about 0.2–0.9 of P/P is wider for the
0
EMD2, implying an increased proportion of mesopores by decreas-
ing the pH, however the EMD5 does not differ greatly. Based on the
BJH results, the EMD5 is predominantly composed of micropores,
while the EMD2 composed of both micro- and mesopores.
3.3. The electrochemical performances of the EMD
Fig. 4. Nitrogen adsorption/desorption isotherms (a) and pore size distributions (b)
of the EMD2 and the EMD5.
The electrochemical performances of the EMD samples are
studied by the RAM batteries, which have been discharged galvano-
−1
statically at a current density of 30 mAh g MnO , and discharge
2
capacities are recorded to a cut-off voltage (COV) of 0.9 V. The bat-
teries charged by voltage limited taper current (VLTC) method to
1.72 V. The discharge curves of both samples are shown in Fig. 5. It
is seen that the EMD2 shows higher capacity than the EMD5 in first
discharging cycle. In continuation of charging/discharging process,
the cumulative capacity is considered. The cumulative capacity per
each cycle is calculated by adding the capacity for each cycle and
the sum of capacities for all previous cycles. Fig. 6 plots the rela-
tion between cumulative capacities and cycle numbers of samples.
It clearly demonstrates that the EMD2 displays better capacity and
cycle life compared to the EMD5. This could be explained via the
physico-chemical properties of samples.
The micro/mesoporosity can affect the charge transfer pro-
cess between the electrolyte and the crystal phase [4,14]. During
charge/discharge cycling, a decrease in the pore size of samples may
lead to the reduction of accessible electrochemical active sites. The
BJH results reveal that the porosity of the EMD5 is on the microscale.
In addition, the XRD results show that the main part of the crystal
structure of the EMD5 consists of ␤-MnO2 [6]. Consequently, the
lower capacity of the battery with the EMD5 cathode is related to
the rutile type lattice distortion and the narrower diffusion path of
Fig. 5. Voltage–time curves of the EMD2 and the EMD5 samples used as the main
cathodic compound of the RAM batteries for the first discharge cycle.

H. Adelkhani, M. Ghaemi / Journal of Alloys and Compounds 481 (2009) 446–449
449
This assumption is supported from the XRD results that revealed
that the main part of the crystal structure of the EMD2 consists of
␥
-MnO and ␣-MnO [6].
2 2
4. Conclusion
It has been seen that the pH of electrolyte is decreasing dur-
ing the electrodeposition of electrolyte manganese dioxide. The
excess production of H+ due to the electro-oxidation of Mn to
MnO2 is responsible for this behavior. The bath pH effects on the
physico-chemical properties of the EMD samples are studied by
2+
electrodeposition at fixed pH. The mixtures of ␣/␥-MnO and ␤/␥-
2
MnO are obtained at pH 2 and pH 5, respectively. The morphology
2
of samples is affected by the formation of passive layers due to pres-
ence of oxygen containing intermediate Mn (III) species. The EMD2
displays better capacity and cycle life compared to the EMD5. This
is explained via the physico-chemical properties of the samples.
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Fig. 6. Cumulative capacities of the RAM batteries based on the EMD2 and the EMD5.
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the presence of fine distributed micro/nanoparticles of ␣-MnO2
within a ␥-MnO2 structure may be a major factor in evaluation
of the MnO2 stability for achieving a higher number of cycle life.