Asymmetric activated carbon-manganese dioxide capacitors in mild aqueous electrolytes containing alkaline-earth cations

Article abstract of DOI:10.1149/1.3106112

Manganese dioxide exhibits the ideal capacitive behavior in aqueous electrolytes containing alkaline-earth cations (Mg²⁺, Ca ²⁺, or Ba²⁺). A specific capacitance value as high as 325 F g-1 was obtained in 0.1 mol L-1 Mg (N

Full text of DOI:10.1149/1.3106112

Asymmetric Activated Carbon-Manganese Dioxide Capacitors in
Mild Aqueous Electrolytes Containing Alkaline-Earth Cations
Chengjun Xu, Hongda Du, Baohua Li, Feiyu Kang and Yuqun Zeng
J. Electrochem. Soc. 2009, Volume 156, Issue 6, Pages A435-A441.
doi: 10.1149/1.3106112
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2009 ECS - The Electrochemical Society
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Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
A435
0
013-4651/2009/156͑6͒/A435/7/$25.00 © The Electrochemical Society
Asymmetric Activated Carbon-Manganese Dioxide Capacitors
in Mild Aqueous Electrolytes Containing Alkaline-Earth
Cations
a,b
a
a
b,z
c
Chengjun Xu, Hongda Du, Baohua Li, Feiyu Kang, and Yuqun Zeng
a
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen City,
Guangdong Province 518055, China
b
Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua
University, Beijing 100084, China
c
Amperex Technology Limited, Dongguan City, Guangdong Province 523080, China
Manganese dioxide exhibits the ideal capacitive behavior in aqueous electrolytes containing alkaline-earth cations ͑Mg2+, Ca2+, or
2
+
−1
−1
−1
Ba ͒. A specific capacitance value as high as 325 F g was obtained in 0.1 mol L Mg͑NO ͒ electrolyte at 2 mV s . The ideal
3
2
capacitive behavior, high specific capacitance, good coulombic efficiency, and rate ability of manganese dioxides in aqueous
2
+
2+
2+
electrolytes containing bivalent cations ͑Mg , Ca , or Ba ͒ indicate that these alkaline-earth cations might be good alternatives
for the state-of-the-art univalent alkaline cations. Moreover, activated carbon/MnO2 asymmetric capacitors with 2 V operating
voltage were built up based on the aqueous electrolytes containing these alkaline-earth cations. The energy density of the
AC/MnO asymmetric capacitor with Ca cation at a current density of 0.3 A g was found to be 21 Wh Kg
2009 The Electrochemical Society. ͓DOI: 10.1149/1.3106112͔ All rights reserved.
2
+
−1
−1
.
2
©
Manuscript submitted December 22, 2008; revised manuscript received February 16, 2009. Published April 7, 2009.
The electrochemical capacitor ͑supercapacitor͒ is an attractive
device to be used solely or auxiliary as the power supply for electric
vehicles or electronic products. Due to the fast pace of expansion of
human activity, the need for supercapacitors with high energy and
power densities as well as low cost and environmental friendliness is
obvious and urgent. Because it does not need specific manufacturing
conditions, the solvent ͑water͒ is low cost and totally nontoxic, and
the aqueous electrolytes possess relatively high conductivity,
aqueous-based activated carbon ͑AC͒ symmetric supercapacitors are
the linear variation of the potential on the time at a constant current
charge or discharge, where the mechanism for energy storage is
based on the faradaic reactions between the electrode and the elec-
trolyte instead of the electrochemical double-layer mechanism.
During the last few years, various metal oxides have been proposed
1
for pseudo-capacitance purpose. Among them, RuO presents the
2
−
1
6,7
best performance ͑860 F g ͒ in H SO electrolyte. However, ru-
2
4
thenium is a noble metal, which limits its use on a large scale.
Another strategy is to increase the cell voltage to more than 1 V
by a smart asymmetric configuration. The so-called asymmetric or
hybrid configurations combining two kinds of electrode materials in
the same cell utilize the different potential windows of the different
electrode materials in the same electrolyte to obtain a high cell volt-
age. For the asymmetric or hybrid AC/metal oxide electrochemical
supercapacitors, one electrode stores charge through a reversible,
nonfaradaic reaction of ion adsorption/desorption on the surface of
an active carbon, and the other electrode utilizes a reversible redox
faradaic reaction in a transition metal oxide or a lithium-ion inter-
calated compound. AC/Ni͑OH͒ and AC/LiMn O aqueous cells
1
promising low cost devices for providing high power densities.
However, the energy density of such a device is relatively low due
to the limited cell voltage. The energy density ͑E͒ of a supercapaci-
tor is proportional to the capacitance ͑C͒ and the voltage ͑V͒ as
follows
2
E = CV /2
͓1͔
Due to the limitation of the water decomposition, the aqueous-based
carbon symmetric supercapacitor only works in a maximum window
of 1 V. According to Eq. 1, the relatively low energy density pre-
2
2
4
8
-11
1
are two typical representatives,
in which not only a much higher
cludes its use in the industrial scale.
voltage ͑more than 1.5 V͒ could be achieved but also a higher ca-
pacitance material was introduced to replace one AC electrode. Due
to the remarkable performance, more attention has been paid to
these AC/metal oxide aqueous supercapacitors. Recently, AC/
An efficient way to improve the cell voltage in terms of the
energy density is to use organic electrolytes with a wider electro-
1
chemical stability window than water. Organic electrolytes includ-
ing the combination of a solvent with different salts could enable the
manganese dioxide ͑MnO ͒ capacitors using aqueous electrolytes
maximum cell voltage to reach more than 3 V, a value three times
2
1
-3
than the maximum cell voltage of aqueous-based supercapacitors.
containing alkaline cations have become another hot spot in this area
due to the high operating voltage ͑more than 2 V͒ and nontoxicity
and low cost of manganese dioxides as well as the safety and non-
However, such improvements inevitably sacrifice the capacitance
and equivalent series resistance, which precludes it from easily
1,4,5
12-15
reaching-high power density.
The organic electrolytes also suffer
erosion of the mild electrolytes.
from highly toxic, flammable, or safety hazards. In addition, due to
a water-free fabrication environment and the high prices of the sol-
vents and salts, the manufacturing costs are also high for such
Manganese dioxides are promising electrode materials exhibiting
ideal capacitive behavior in aqueous mild electrolytes containing
+
+
+
alkaline cations ͑Li , Na , or K ͒ with a potential width ranging
1
12-15
devices. Therefore, it is clear that the aqueous-based supercapaci-
from 0.9 to 1.2 V.
The charge storage mechanism of manganese
tors would be more attractive from cost, safety, and environmentally
friendly points of view, but it is necessary to improve the energy
density dramatically. Based on Eq. 1, there are two obvious methods
to improve the energy density as well as the power density by using
large capacitance electrode materials and increasing cell voltage.
The low voltage could be partially compensated by the use of the
pseudo-capacitive materials with a high capacitance, such as
dioxides is based on the double injection and ejection of cations and
+
+
+
electrons, in which the cations ͑Li , Na , or K ͒ intercalate into
MnO lattice and correspondingly Mn͑IV͒ becomes Mn͑III͒ to bal-
2
1
6-22
ance the charge.
One univalent alkaline cation inserted into
MnO2 and one electron are stored. More recently, the polyvalent
cations intercalation process of MnO has been investigated in our
2
2
2
22
lab. Our previous work indicated that MnO showed ideal ca-
2
6
,7
RuO2. The so-called pseudo-capacitive properties of a material
were characterized with the rectangular cyclic voltammograms and
pacitive behavior in the aqueous solution containing bivalent cations
2
+
͑Ca ͒, and the specific capacitance has been improved dramatically
2
+
only through the use of polyvalent cations ͑Ca ͒ in place of tradi-
+
tional univalent cations ͑Na ͒. From the fundamental point of view,
z
E-mail: fykang@tsinghua.edu.cn
the polyvalent cations are advantageous because each polyvalent
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A436
Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
2
3
Table I. Ionic radii, ionic conductivity, and standard redox potential of alkaline and alkaline-earth cations.
Li+
Na+
K+
Mg2+
Ca2+
Ba2+
Size ͑Å͒
In crystals
In aqueous solution
0.69
6
1.02
4
1.38
3
0.66
8
0.99
5
1.34
6
Standard redox potential ͑V͒
−3.05
−2.71
−2.93
−2.38
−2.84
−2.92
cation intercalated into a host compound results in a concurrent
charge transfer of two or even three electrons to the host material.
Other alkaline earth cations ͑Mg2+ and Ba ͒ possess a similar elec-
tron structure and close ion size as well as a close standard redox
and a little 1-methy-2-pyrrolidinone ͑NMP͒ was added to get a
syrup. The syrup was cold rolled into thick films, and pieces of film
2+
2
5 mg in weight and 1 cm in size were then hot-pressed at 80°C
under 100 MPa on a titanium plate. The preparation process of the
+
+
23
potential with alkaline cations ͑Li and K ͒, as shown in Table I,
negative AC electrode was similar to the process of MnO electrode,
2
which is wildly used in the aqueous electrolytes for MnO superca-
except that the weight ratio of AC power to acetylene black to
binder was 8:1:1. Electrochemical tests were performed with an
Im6e ͑Zahner͒ electrochemical station. A piece of platinum gauze
and a saturated calomel electrode ͑SCE͒ were assembled as the
counter and reference electrode, respectively. A luggin capillary fac-
ing the working electrode at a distance of 2 mm was used to mini-
mize errors due to internal resistance ͑iR͒ drop in the electrolyte.
Impedance measurements of MnO2 electrodes were conducted at
0.4 V vs SCE by sweeping frequencies from 30 mHz to 50 KHz.
The measured impendence data were analyzed using Zview soft-
ware. In addition, the specific capacitance of acetylene black in all
2
pacitors. Therefore, Mg2+ and Ba ions may be the other good
2+
2
+
guest polyvalent cations for MnO2 supercapacitors just like Ca
ion. Thus, this work first aims to address the ideal capacitive behav-
ior of MnO in the aqueous systems containing alkaline earth cat-
2
2
+
2+
2+
ions ͑Mg , Ca , and Ba ͒. The ideal capacitive behavior, high
specific capacitance, high reversibility, and good rate ability indicate
2
+
2+
2+
that alkaline-earth cations ͑Mg , Ca , and Ba ͒ are promising
alternatives for the traditional alkaline cations.
Second, MnO only shows the ideal capacitive behavior in aque-
2
1
2-15
ous electrolytes within a potential window of 0.9 V.
Unfortu-
−
1
nately, this potential window is relatively smaller than organic-based
electrolytes was found to be negligible ͑less than 10 F g ͒.
capacitors, and consequently MnO -based capacitors deliver limited
power or energy densities. The best way to overcome this drawback
2
Asymmetric capacitor.— The capacitor test used the coin-cell as-
sembly consisting of MnO2 positive and AC negative electrodes.
is to utilize the asymmetric assemblies, in which MnO and AC are
2
The MnO /AC weight ratios were calculated by considering specific
2
used as positive and negative electrodes in the presence of aqueous
voltammetric charge and the appropriate potential window of posi-
tive and negative electrodes in different electrolytes. A glass paper
was used as the separator. The titanium foil ͑50 ␮m in thickness͒
2
+
2+
2+
electrolytes containing alkaline-earth cations ͑Mg , Ca , or Ba ͒.
This configuration also enables the operating voltage of the asym-
metric supercapacitors in mild electrolytes containing alkaline-earth
cations to reach 2 V. In addition, combined with the improved spe-
cific capacitance of the positive electrode due to the usage of
alkaline-earth cations, good performance of these asymmetric de-
vices was reported in this work.
was used as the current collector. All the electrolytes were bubbled
15
by N gas for 30 min to eliminate oxygen before assembly.
2
The real power density, Preal, and the real energy, Ereal, were
determined from the constant-current charge/discharge cycles as fol-
lows
Last, the size of the cation, the size of the hydrated cation, the
mobility of the cation, and the adsorption–desorption rate may have
a direct impact on the pseudo-capacitive behaviors of manganese
dioxide as well as the double-layer capacitive behavior of negative
AC. Therefore, the role of cation species on the electrochemical
properties of positive manganese dioxide and negative AC as well as
Preal = ⌬EI/m ͑W/Kg͒
͓2͔
where ⌬E = ͑Emax − Emin͒/2 with Emax the potential at the end of
charge and Emin at the end of discharge, I is the applied current ͑A͒,
and m is the total weight of positive and negative active material in
the electrode ͑kg͒
the performance of the asymmetric AC/MnO cells is pursued in
2
this work.
Ereal = Prealt ͑Wh/kg͒
͓3͔
Experimental
where t is the discharge time ͑h͒. Finally, the coulombic efficiency is
given by the discharge time/charge time ratio.
1
5
Preparation of positive MnO electrode.— A MnO sample was
2
2
prepared by a self-reacting microemulsion. A surfactant of 13.32 g
of sodium bis͑2-ethylhexyl͒ sulfosuccinate ͑AOT͒ was added in
Results and Discussion
3
00 mL iso-octane and stirred well to get an AOT/iso-octane solu-
−
1
Electrochemical properties of positive MnO electrode.— Cyc-
tion. Then, 32.4 mL of 0.1 mol L ͑M͒ KMnO aqueous solution
2
4
lic voltammetric tests of MnO electrode were measured in
was added and this solution was dispersed by ultrasound for 30 min
to prepare a dark brown precipitate. The product was separated,
washed copiously several times with distilled water and ethanol, and
2
1
−1
0
.1 mol L− Ca͑NO ͒ ͑pH 6.0͒ at 2 mV s with a wide potential
3 2
range to investigate the reversibility of MnO electrode by control-
2
24,25
ling the upper and lower potential limits. Figure 1 shows the cyclic
voltammograms ͑CVs͒ at different potential ranges. It can be seen
form curve 1 in Fig. 1 that for a potential more negative than about
dried at 80°C for 12 h. Our previous works
indicated that the
as-prepared MnO sample possesses a spherical morphology with an
2
individual particle size of around 4 nm. N adsorption and desorp-
2
0
.1 V, the irreversible faradaic processes are observed and might
tion measurement indicated that the S
sample is 145 m g . A commercial AC was used as the negative
electrode material.
value of the as-prepared
BET
2
−1
correspond to the reduction of surface manganese dioxide. When the
potential is more positive than 1.0 V, at ϳ1.05 V, an anodic wave is
detected. In addition, the onset of the water oxidation is also ob-
served at potentials more positive than ϳ1.3 V. As the potential is
controlled from 0.10 to 1.00 V vs SCE, the shape of the CV is a
nearly rectangular mirror image characteristic of the capacitive be-
havior, as shown in curve 2. The maximum and minimum values of
Single electrode measurements.— Electrodes were prepared by
mixing 70 wt % of MnO powder as active material with 20 wt %
2
acetylene black and 10 wt % polytetrafluoroethylene ͑PTFE͒. MnO2
powder ͑70 mg͒ and acetylene black ͑20 mg͒ were first mixed and
dispersed in ethanol by ultrasound for 30 min. Then, the ink was
dried at 80°C for 4 h to get a dark mixed powder, and 10 mg of
PTFE was added to get a paste. Then, the paste was dried at 80°C,
polarization potential of MnO electrode are controlled by the sur-
2
face reactions Mn͑IV͒ to Mn͑II͒ and Mn͑IV͒ to Mn͑VII͒, respec-
tively, which are irreversible because of the solubility of both Mn͑II͒
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Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
A437
Figure 1. CVs of MnO2 electrode in 0.1 mol L−1 Ca͑NO3͒2 at different
potential ranges.
Figure 3. Charge–discharge curves of MnO electrode in 0.1 mol L−1
2
−1
Mg͑NO ͒ at a current density of 4 A g
.
3
2
13,25
and Mn͑IV͒ in water.
Therefore, the potential must be controlled
The pseudo-capacitance of MnO electrode in mild electrolytes
containing alkaline-earth cations should be ascribed to the rapidly
reversible insertion–extraction reaction
2
in the region in which the reversible pseudo-capacitive behavior
occurs.
Cyclic voltammetric tests have been performed on MnO elec-
2
trodes between 0.10 and 1.00 V vs SCE in 0.1 mol L− ͑M͒
1
MnO2b + ␦C2+ + 2␦e ↔ C MnO
−
͓5͔
␦
2
Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒ electrolytes ͑pH 6.0͒. Figure
21,22
3
2
3 2
3 2
2+
2+
2+
where C is Mg , Ca , or Ba .
−
1
2
shows the CVs at a sweep rate of 10 mV s . These profiles are
It is known that the bigger the cation size, the more difficult it is
relatively rectangular in shape, which indicates the ideal capacitive
behavior. The specific capacitance ͑SC͒ of active material can be
estimated using half the integrated area of the CV curve to obtain
the charge ͑Q͒ and subsequently dividing the charge by the mass of
the active material ͑m͒ and the width of the potential window ͑⌬V͒
to squeeze and diffuse in the MnO matrix. From a vacancy point of
2
view, a higher storage capacity could be obtained as the guest cation
with a smaller size. Thus, the highest specific capacitance of MnO2
electrode in 0.1 M Mg͑NO3͒2 within the potential range from
0
.10 to 1.00 V vs SCE should ascribe to the smallest bare ions size
2
+
SC = Q/⌬Vm
͓4͔
of Mg , as shown in Table I.
Typical charge–discharge curves of MnO2 electrode in 0.1 M
The SC values in 0.1 M Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒ elec-
3
2
3 2
3 2
−1
Mg͑NO ͒ electrolyte at a current density of 4 A g are shown in
−
1
3 2
trolytes at a low sweep rate of 2 mV s are 325, 314, and
81 F g , respectively. These values were much higher than
94 F g in 0.2 M NaNO electrolyte ͑pH 6.0͒. It is shown that
Fig. 3. Although a sharp iR drop was found due to the poor conduc-
−
1
2
1
−
1
22
tivity of MnO as well as the high current density, there is a linear
2
3
variation of potential during charging and discharging processes,
which is another criterion for capacitance behavior of a material in
addition to exhibiting rectangular voltammograms. The rectangular-
like CV curves and the linear variation of potential on times during
charging and discharging processes from Fig. 2 and 3 indicate the
the capacitance of MnO2 has been improved dramatically only
through replacing the traditional univalent cation by the bivalent
alkaline-earth cations. This improvement method is the most effi-
cient, low cost, and convenient way.
ideal capacitive behavior of MnO in aqueous electrolytes contain-
2
ing alkaline-earth cations.
The electrodes were subjected to extended charge–discharge cy-
cling at a current density of 1 A g− in the electrolytes containing
alkaline-earth cations, and the result is shown in Fig. 4. No decrease
1
in SC was found after 200 cycles, which shows that MnO possesses
2
a good cycling property in the aqueous electrolyte containing biva-
2
+
2+
2+
lent cations ͑Mg , Ca , or Ba ͒. Moreover, for all electrolytes, the
coulombic efficiency was nearly 100% during charge–discharge cy-
cling tests, which indicates the high reversibility of MnO toward
2
cation intercalation in the potential range from 0.10 to 1.00 V vs
SCE. The ideal capacitive behavior, higher specific capacitance, and
good coulombic efficiency and rate ability of manganese dioxides in
2
+
2+
aqueous electrolytes containing bivalent cations ͑Mg , Ca , or
2
+
Ba ͒ indicate that these alkaline-earth cations might be good alter-
natives for the state-of-the-art univalent alkaline cations, which are
widely used for MnO supercapacitor applications.
2
The electrochemical impendence spectra were measured on the
electrodes in 0.1 M Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒ electro-
3
2
3 2
3 2
lytes at 0.4 V vs SCE. Figure 5 shows the Nyquist plots of the
experimental impendence data. All the measured impendence spec-
tra are similar in shape with a semicircle in the higher-frequency
region and a spark in the lower-frequency region. The measured
−
1
Figure 2. CVs of MnO2 electrodes in 0.1 mol L Mg͑NO ͒ , Ca͑NO ͒ ,
3
2
3 2
−
1
and Ba͑NO ͒ electrolytes at 10 mV s
.
3
2
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A438
Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
Table II. The calculated values of, C , R , and diffusion coeffi-
dl
ct
cient of MnO2 electrodes in the aqueous electrolytes containing
the listed cations.
Cation
species
Diffusion coefficient
͑ϫ10− cm s
14
2
−1
͒
Rs ͑⍀͒
Cdl ͑mF͒
Rct ͑⍀͒
Mg
Ca
Ba
2.10
1.50
2.26
0.14
0.30
0.16
13.36
9.10
11.48
10.81
8.70
7.41
2
+
2+
size of hydrated cations. The diffusion coefficient of Mg , Ca ,
2
+
−14
2
−1
and Ba are 10.81, 8.70, and 7.41 ϫ 10 cm s , respectively.
−
13
2
−1
These values are compared to 1.00 ϫ 10 cm s of the univalent
lithium ions and 3.05 ϫ 10− cm s of the trivalent yttrium ions
15
2
−1
2
7
in nanocrystalline transition-metal oxide ͑V O ͒.
2
5
Electrochemical properties of AC.— A commercial AC ͑Kuraray
Company, Japan͒ was used in this work. The S value is
BET
−1
Figure 4. ͑Color online͒ Cycle lives of MnO electrodes in all the electro-
lytes.
2
−1
3
2
1620 m g and the total pore volume is 0.8 cm g . In order to
obtain a high cell operating potential, the potential range of AC has
to be controlled elaborately. Because the minimum cutoff of MnO2
electrodes is 0.10 V vs SCE for all electrolytes, the positive polar-
ization cutoff of AC electrodes is set to be 0.10 V vs SCE. Figure 6a
shows the CV of AC in 0.1 M Ca͑NO ͒ ͑pH 6.0͒ at different nega-
impendence data were analyzed on the basis of the equivalent cir-
26
cuit, which is shown in Fig. 5. The model circuit consists of four
3
2
elements: the internal resistance ͑R ͒, the constant phase element
s
͑
Q͒ used in place of double-layer capacitance ͑C ͒, the charge-
dl
transfer resistance ͑R ͒, and the Warburg impendence ͑Z ͒. The
ct
w
internal resistance ͑R ͒ includes the bulk electrolyte solution resis-
s
tance, the intrinsic resistance of active material, and the electron-
transfer resistance at the current collector/electrode boundary. The
constant phase element ͑Q͒ is used in place of double-layer capaci-
tance ͑C ͒ at the electrode–electrolyte boundary because the non-
dl
ideal nature of capacitance arises due to the inhomogeneous nature
of the electrode. The charge-transfer resistance ͑R ͒ represents the
ct
kinetic resistance to charge transfer at the electrode–electrolyte
boundary or intrinsic charge-transfer resistance of the solution and
porous electrode. The Warburg impendence ͑Z ͒ associates with the
w
finite-length diffusion of cation in the solid particle.
The fitting results as well as the diffusion coefficient of alkaline-
earth cations in the as-prepared MnO particles are listed in Table II.
2
The value of Rct for 0.1 M Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒
3 2
3 2
3 2
electrolytes are 13.36, 9.10, and 11.48, respectively. It can be seen
from Tables I and II that Rct values decrease with the decrease in
Figure 6. CVs of AC electrodes: ͑a͒ in 0.1 mol L−1 Ca͑NO3͒2 at different
minimal cutoffs and ͑b͒ in all the electrolytes within the potential range from
−1.0 to 0.1 V vs SCE.
Figure 5. Nyquist plots of MnO2 electrode in 0.1 mol L−1 Mg͑NO ͒ ,
3
2
Ca͑NO ͒ , and Ba͑NO ͒ electrolytes. Insert shows the equivalent circuit.
3
2
3 2
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Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
A439
Figure 8. CVs of the asymmetric capacitors using different electrolytes at
−
1
2
mV s .
Figure 7. Schematic graph of the cell operating potential range using
−
1
0
.1 mol L Ca͑NO ͒ electrolytes.
3 2
q = C ϫ ⌬E ϫ m
and in order to get q = q , the mass balancing is
͓6͔
͓7͔
+
−
tive polarization cutoffs. The rectangular shape of the CV down to a
potential cutoff of −1.00 V with similar positive and negative cur-
rent proves a pure reversible capacitive behavior in this potential
range. As the potential became more negative ca. −1.20 V, the evo-
lution of water was also clearly observed. Figure 6b exhibits the
CVs of AC electrodes in all electrolytes at a scan rate of 2 mV s .
The mirrorlike CV indicates the double-layer capacitive behavior of
the AC electrodes in all electrodes. The SC values of AC in 0.1 M
m /m = C ϫ ⌬E /C ϫ ⌬E
+
+
−
−
−
+
On the basis of the specific capacitance values and potential ranges
found at pH 6.0 for the manganese oxide composite and AC, the
optimal mass ratios in all electrolytes between the electrodes is
listed in Table III.
−
1
Electrochemical performance of AC/MnO asymmetric capaci-
2
Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒ electrolytes calculated from
3
2
3 2
3 2
tors.— The asymmetric cells were built up according to the mass
ratio listed in Table III. The CVs of the asymmetric cells using
0.1 M Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒ electrolytes are shown
−
1
Eq. 2 are 72, 80, and 76 F g , respectively. The lowest SC of AC in
.1 M Mg͑NO ͒ electrolyte could be attributed to the largest hy-
drated ion size of Mg because the AC utilizes the electrochemical
double-layer mechanism, in which the anions and cations were
absorbed–desorbed on the surface of the inner pores.
0
3
2
3
2
3 2
3 2
2
+
−1
in Fig. 8 at 2 mV s in the potential range from 0 to 2 V. These
CVs are generally rectangular, which indicates that no other irre-
versible reactions occur in this voltage range, and a high voltage of
The above results provide a firm basis to build up the asymmetric
capacitors by the combination of MnO2 and AC in the aqueous
electrolytes containing alkaline-earth cations. The schematic graph
AC/MnO supercapacitors using the aqueous electrolytes containing
2
2+
2+
2+
the alkaline-earth cations ͑Mg , Ca , or Ba ͒ could be obtained.
The capacitance ͑SC͒ of the asymmetric supercapacitors can be
estimated using half the integrated area of the CV curve to obtain
the charge ͑Q͒ and subsequently dividing the charge by the total
mass of the positive and negative active materials ͑M͒ and the width
of the potential window ͑⌬V͒
of the cell operating potential range using 0.1 M Ca͑NO ͒ electro-
3
2
lytes is shown in Fig. 7. The potential range of positive MnO elec-
2
trode has to be controlled at 0.10 to 1.00 V vs SCE, in which the
reversible reaction occurs. The AC electrode shows a unique behav-
ior under negative polarization with a high hydrogen overpotential,
and the potential range of AC could be set to −1.00 to 0.10 V vs
SCE.
SC = Q/⌬VM
͓8͔
−1
At a low scan rate of 2 mV s , the SC values of the asymmetric
In order to obtain the maximal cell operating voltage, the loading
mass of positive and negative electrodes has to be balanced. For this
2+
2+
2+
cells with aqueous electrolytes containing Mg , Ca , or Ba cat-
−1
ions are 37, 32, and 29 F g , respectively. It is known that the
purpose, it must be considered that the charges at positive ͑q ͒ and
+
electrochemical performance of an asymmetric supercapacitor de-
pends on both negative and positive electrodes. The above single
electrode characterizations indicated that due to the largest bare ion
negative ͑q ͒ electrodes should be balanced to be equal. The charge
−
stored by each electrode depends on the specific capacitance ͑C͒,
the potential range for the charge/discharge process ͑⌬E͒, and the
2
+
−1
size of Ba cation, the SC value, 281 F g of the positive MnO in
2
1
3
−1
mass of the electrode ͑m͒ as in the following equation
0.1 M Ba͑NO ͒ electrolytes is 15 or 12% less than 325 F g in
3
2
Table III. Summaries of the properties of positive and negative electrodes in three electrolytes containing different cations.
Positive MnO electrode Negative AC electrode
Potential window ͑V͒
2
Cation species
SC ͑F g−1
͒
Potential window ͑V͒
SC ͑F g−1
͒
Mass ratio
Mg
Ca
Ba
325
314
281
0.9
0.9
0.9
72
80
76
1.1
1.1
1.1
3.69
3.17
3.03
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A440
Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
Figure 9. Charge–discharge curves of the asymmetric capacitors using dif-
Figure 10. Ragone plots of the asymmetric capacitors using different cat-
ions.
ferent electrolytes at 0.3 A g−
1
.
−
1
−
1
trolled from 0 to 2 V at a current density of 0.3 A g . The varia-
tions of specific capacitances of all the cells and the coulombic
efficiency of the AC/Mg͑NO ͒ /MnO cell during cycling are illus-
0
.1 M Mg͑NO ͒ electrolyte or 314 F g in 0.1 M Ca͑NO ͒ elec-
3
2
3
2
−
1
trolyte. Although the SC value ͑76 F g ͒ of negative AC electrode
in 0.1 M Ba͑NO3͒2 is higher than 72 F g−1 in 0.1 M Mg͑NO3͒2
3 2
2
trated in Fig. 11. All the asymmetric cells exhibited good cycle
ability in terms of capacitance retention and the coulombic effi-
ciency.
electrolyte, this slight enhancement on the negative electrodes could
not average the large gap between the SC values of the positive
MnO electrodes. Therefore, the cell with 0.1 M Ba͑NO ͒ electro-
2
3 2
2
+
lyte exhibited the lowest capacitance. Comparing the Mg cation
Conclusion
2
+
with Ca cation, despite that the positive SC value in 0.1 M
Mg͑NO3͒2 electrolyte is slightly higher than in 0.1 M Ca͑NO3͒2
electrolyte, there is the largest difference between positive and nega-
tive SC values when 0.1 M Mg͑NO ͒ electrolyte is used, which
Manganese dioxide was assessed to exhibit the ideal capacitive
behavior in aqueous electrolytes containing alkaline-earth cations
Mg , Ca , or Ba ͒. The ideal capacitive behavior, higher specific
capacitance, and good coulombic efficiency and rate ability of man-
ganese dioxides in aqueous electrolytes containing bivalent cations
Mg , Ca , or Ba ͒ indicated that these alkaline-earth cations
might be good alternatives for the state-of-the-art univalent alkaline
cations, which are widely used for MnO2 supercapacitor applica-
tions. The AC/MnO asymmetric capacitors with 2 V operating
voltage were built up based on the aqueous electrolytes containing
these alkaline-earth cations. The energy density of the AC/MnO
asymmetric capacitor with Ca cation at a current density of
.3 A g was found to be 21 Wh Kg , which is comparable to that
of the RuO capacitor or organic-based carbon symmetric capacitor.
2+
2+
2+
͑
3
2
means the largest total mass of both positive and negative active
materials, as shown in Table III. In addition, the smallest SC value
2+
2+
2+
͑
2
+
of negative electrode also prevented the AC/MnO cell with Mg
2
cation from reaching the highest capacitance. It seems that due to
the appropriate bare ion size and the smallest hydrated ion size
2
+
2+
2+
2+
2
among three alkaline-earth cations ͑Mg , Ca , and Ba ͒, Ca ion
is suitable for the asymmetric AC/MnO supercapacitor.
2
2
Figure 9 shows the charge–discharge curves of the asymmetric
2+
cells using 0.1 M Mg͑NO ͒ , Ca͑NO ͒ , and Ba͑NO ͒ electrolytes
−1
−1
3
2
3 2
3 2
0
−
1
at 0.3 A g ͑taking into account only the total weight of positive
and negative active materials͒. The potentials are nearly proportional
to the charge or discharge time for all the electrolytes containing
Mg , Ca , or Ba cations. It is seen from Fig. 8a and 9a that
during charging between 0 and 2 V, for all asymmetric cells using
aqueous electrolytes containing Mg , Ca , or Ba cations, irre-
versible faradaic processes such as dissolution of Mn and oxygen
or hydrogen evolution reaction seem to be avoided.
2
It was also found that due to the appropriate bare ion size and the
2
+
2+
2+
2
+
2+
2+
2
+
Figure 10 exhibits the Ragone plots of the three asymmetric cells
2
+
2+
2+
with the aqueous electrolytes containing Mg , Ca , or Ba cat-
ions. Because the packaging is not taken into account and all the cell
components are not yet optimized, these data only provide an esti-
mation of the performance of the capacitors. The Ragone plot data
clearly demonstrates that AC/MnO2 asymmetric systems in mild
aqueous electrolytes containing alkaline-earth cations can compete
2
,3
with the organic electrolyte-based carbon capacitors,
RuO2
1
,28
capacitors,
kaline cations,
or AC/MnO with aqueous electrolytes containing al-
2
1
2-15
although the SC value of the negative AC elec-
trode is much smaller than those in Ref. 12-15. For example, the real
energy density of AC/MnO using 0.1 M Ca͑NO ͒ electrolyte was
2
3 2
−
1
−1
2
1 Wh Kg at a power density of 300 W Kg for a cutoff voltage
of 0–2 V.
The cycling stability of all the asymmetric cells was tested over
000 cycles. The cutoff voltage of all the asymmetric cells is con-
Figure 11. Cycle lives of all the asymmetric capacitors using different cat-
ions.
5
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Journal of The Electrochemical Society, 156 ͑6͒ A435-A441 ͑2009͒
A441
2
+
on Double Layer Capacitors and Similar Energy Storage Devices, Florida Educa-
tional Seminars ͑1997͒.
smallest hydrated ion size among three alkaline-earth cations ͑Mg ,
2
+
2+
2+
Ca , and Ba ͒, Ca cation is suitable for the asymmetric
1
0. Y. Wang and Y. Xia, J. Electrochem. Soc., 153, A450 ͑2006͒.
AC/MnO supercapacitor.
2
11. Y. Wang and Y. Xia, J. Electrochem. Soc., 153, A1425 ͑2006͒.
1
2. M. S. Hong, S. H. Lee, and S. W. Kim, Electrochem. Solid-State Lett., 5, A227
2002͒.
3. V. Khomenko, E. Raymundo-Piňnero, and F. Bèguin, J. Power Sources, 153, 183
2006͒.
Acknowledgment
͑
1
This work was supported by the Natural Science Foundation of
China under grant no. 50632040 and no. 50802049.
͑
1
1
4. T. Brousse, M. Toupin, and D. Bélanger, J. Electrochem. Soc., 151, A614 ͑2004͒.
5. T. Brousse, P. L. Tabern, O. Crosnier, R. Dugas, P. Guillemet, Y. Scudeller, Y.
Zhou, F. Favier, D. Bélanger, and P. Simon, J. Power Sources, 173, 633 ͑2007͒.
6. S.-C. Pang, M. A. Anderson, and T. W. Chapman, J. Electrochem. Soc., 147, 444
Tsinghua University assisted in meeting the publication cost of this ar-
ticle.
1
͑
2000͒.
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