Journal of The Electrochemical Society, 156 ͑10͒ A791-A795 ͑2009͒
A795
1
1
1
1
6.00
4.00
2.00
0.00
2
50
00
50
00
M2
M3
M4
M5
M6
2
1
Specific capacity
F/g MnO2)
8
.00
.00
(
1
6
5
0
4.00
2.00
0.00
6
5
0
4
pH
3
2
5
2
1
0
20
40
1
10
100
80
160
Scan rate (mV/s)
Pore diameter (nm)
Figure 6. Specific capacitance of MnO2 electrodes calculated from CV
against the scan rate.
Figure 7. The BJH adsorption dV/dD vs pore volume.
the electrode. However, as depicted in Fig. 5d, at pH 5 the MnO2
particles tend to agglomerate rather than form many particles. In
M5, a slower ionic transport occurs within the active material, or
pacity. With respect to Fig. 6, M5 and M6 have lower and higher
capacities, respectively. The study of physicochemical properties of
EMD samples has shown that besides water content and their crystal
structure, superior high power performance is linked to the morphol-
ogy, especially on porosity ͑specific surface area, pore volume, and
protons cannot diffuse freely across the thickness of the MnO par-
2
ticle.
12,13
Conclusion
pore-size distribution͒ of MnO2.
The porosity is a critical prop-
erty for high capacitance per volume and weight in supercapacitors.
Therefore, the porosity of samples is calculated by the BJH method,
and the results are presented in Fig. 7 and Table I.
In this paper, the crystalline structure, morphology, porosity, and
electrochemical properties of manganese dioxide have been studied
as a function of the electrolyte acidity ͑pH͒. The results show that
one of the biggest advantages of electrodeposition at fixed pH is the
ability to easily control the physicochemical properties and, conse-
The comparison of capacities for all samples indicates that the
capacity of M6 is much more than that of other samples ͑Fig. 6͒.
With respect to Fig. 7 and Table I, M6 shows a considerably larger
pore volume and mean diameter than the other samples. The average
pore diameter of the M6 material is 4 times larger than that of the
other samples. Consequently, the pore volume of M6 was
quently, the electrochemical performance of MnO . By SEM, the
2
particle shape and size and, consequently, the dimensions and dis-
tribution of the resulting pores of the MnO samples can be changed
2
and controlled with a variation in the pH value of the electrolyte.
3
−1
3
−1
0
.152 cm g , whereas it was in the range of 0.01–0.02 cm
g
The formation of oxygen-containing intermediate species ͑e.g.,
3
+
for other samples. A larger pore volume could facilitate the mass
transfer of cations, and it could lead to the augmentation of acces-
sible electrochemical active sites during charge/discharge cycling.
With respect to Fig. 6, Table I, and Fig. 7, it is understood that
M3, which has a lower porosity than M6, shows a capacity better
than those of other samples, except M6. The morphology of M3 also
consists of nanoholes ͑Fig. 4b and 5b͒. The improved performance
of M3 could be attributed to the enhanced accessible active site of
the material. Subsequently, even at high scan rates, a higher concen-
tration of working ions is still able to access the active regions of
MnO2 active sites, so a decrease in the pseudocapacitance takes
place to a much lesser extent.
MnOOH, ͓Mn͑OH ͒ ͔ , and Mn O ͒ and their stability at different
2 6 2 3
pH values lead to the production of excellent structures.
The different specific capacitances of manganese dioxide elec-
trodeposited at different pH values are attributed mainly to their
different water contents, porosities, and microstructures. The EMD,
which was electrodeposited at pH 6 ͑M6͒, performed well in elec-
trochemical capacitor applications. The large capacitance exhibited
by this sample arises from its morphology and porosity. The com-
parison of capacities for all samples indicates that the accessible
surface sites of M6 are much more than those of other samples.
Manganese dioxide prepared at pH 6 was highly porous. Large re-
gions of regular and homogeneous holes with diameters of about
1.5–2 m are formed under pH 6. The average primary particle size
is about 100 nm. This results from the higher stability of the passive
layer at pH 6.
Comparing M2 and M4, although the porosity of M2 is higher
than M4, the latter shows a higher capacity. Another important fac-
tor is the water content of the electrode material. It is well recog-
nized that the water content of manganese dioxide is one of the key
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2
regions in the electrode provide the kinetically facile sites needed
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1
1
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2
6
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In M5, the condition is different. This sample has lower porosity
and water content relative to other samples. Therefore, it is supposed
that M5 shows the least capacity. The limitation observed in the M5
electrode highlights the importance of the ionic diffusion throughout
1
1
͑