all solid-state supercapacitor with phosphotungstic acid as the proton-conducting electrolyte
TRANSCRIPT
www.elsevier.com/locate/ssiSolid State Ionics 166 (2004) 61–67
All solid-state supercapacitor with phosphotungstic acid as the
proton-conducting electrolyte
Y.G. Wang, X.G. Zhang*
Institute of Applied Chemistry, Xinjiang University, Shengli Road 14, Urumqi 830046, PR China
Received 11 August 2003; received in revised form 29 October 2003; accepted 1 November 2003
Abstract
For the first time, the proton-conducting composite phosphotungstic acid PTA/Al2(SO4)3�18H2O was used as the electrolyte of symmetric
supercapacitor based on PANI. The optimum weight ratio of PTA/Al2(SO4)3�18H2O for using in this supercapacitor was also reported.
Electrochemical tests prove that the supercapacitor using this kind of composite as electrolyte has high capacitance performance. Its
capacitance is as high as 240 F/g at 6 mA. It was more important that it has long cycle life. After 1000 cycles, the attenuation of the
capacitance is less than 10% and the coulombic efficiency is still greater than 96%.
D 2003 Elsevier B.V. All rights reserved.
Keywords: PTA/Al2(SO4)3�18H2O; Supercapacitor; PANI
1. Introduction
Over the past few years, supercapacitors have ignited
significant worldwide investigation. The reason is that
supercapacitors are energy storage devices and have sev-
eral advantages compared to the secondary battery [1–4].
For example, their long cycle life, simple principle and
mode of construction, short charging time, safety, and high
power density are advantageous to the secondary battery.
Supercapacitors have been used as small-scale energy
storage devices in stationary electronics, such as memory
back-up devices and solar batteries with semi-permanent
charge–discharge cycle life. Now the high power density
of supercapacitors is leading to their application in various
other novel devices for load leveling, hybrid capacitor–
battery systems, cold-start assistance, and catalytic con-
verter preheating.
On the basis of electrode materials used and the charge
storage mechanisms, electrochemical supercapacitors are
classified as: (a) electrical double-layer capacitors (EDLCs)
which employ carbon or other similar materials as blocking
electrodes [2,5], and (b) redox supercapacitors in which
0167-2738/$ - see front matter D 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ssi.2003.11.001
* Corresponding author. Tel.: +86-991-858-2887; fax: +86-991-858-
2006.
E-mail address: [email protected] (X.G. Zhang).
electroactive materials such as insertion type compounds
(e.g. RuO2, NiO, etc.) or conducting polymers are employed
as electrodes [6–12]. With the characters of fast charge–
discharge cycles (in the order of 10–20 s) and long life cycle,
conducting polymers such as polyaniline (PANI) has been
the potential candidates for active electrode materials in
electrochemical supercapacitors. Moreover, conducting pol-
ymers offer both advantages of high charge capacity (com-
pared to activated carbons with high specific area) and low-
production costs (compared to noble metal oxides) which are
necessary conditions for an industrial development [13]. For
this reason, PANI is expected to be one of the most
promising materials for application of electrochemical
capacitors [14–16].
Because of high ionic conductivity, aqueous as well as
organic electrolyte solutions are used for assembly of super-
capacitors greatly. On the other hand, liquid electrolytes also
result in problems when handling and packaging the devi-
ces. Furthermore, the electrolyte of the device will leak if it
was used for a long time. For this reason, replacing liquid
electrolytes by solid electrolytes has become the focus of
present study of supercapacitor. Recently, many attempts
have been made to develop all solid-state supercapacitor
since this approach offers attractive features such as ease of
fabrication, rugged construction, no liquid leaks, long shelf-
life, miniaturization, safe, and a wide temperature range of
Fig. 1. Symmetric capacitor based on PANI.
Y.G. Wang, X.G. Zhang / Solid State Ionics 166 (2004) 61–6762
operation. Generally, the all solid-state supercapacitor was
realized by using solid polymer electrolyte or gel polymer
electrolyte.
Very recently, some of the well-known room tempera-
ture proton conductors with high ionic conductivity are
reported such as phosphotungstic acid (H3PW12O40�nH2O:
PTA) and phosphomolybdic acid (H3PMo12O40�nH2O:
PMA) [17]. However, the conductors tend to dehydrate
under low humidity or pelletization pressure. In order to
overcome this problem, composites of heteropolyacid
hydrates have been prepared by dispersing A12O3 or salt
hydrates like Al2(SO4)3�16H2O and ammonium paratung-
state ((NH4)10W12O41�2H2O: APT) since heterogeneous
doping or the formation of dispersed phase composites
has been found to be an efficient method for enhancing the
ionic conductivity with good mechanical properties [18–
20]. Recently, their application on battery has been
Fig. 2. CV curves of these supercapacitors using composite electrolyte with differe
PTA/Al2(SO4)3�18H2O are (a) 1:5, (b) 1:2, (c) 1:1 and (d) 4:1].
reported [21]. However, their application on the super-
capacitors based on polymer have not been studied.
In this paper, the symmetrical capacitors based on PANI
have been assembled in solid state. The proton-conducting
composite PTA/Al2(SO4)3�18H2O was used as electrolyte.
And their electrochemical capacitance performances were
studied by cyclic voltammetry, AC impedance and charge–
discharge tests. The effect of the weight ratio of PTA/
Al2(SO4)3�18H2O on capacitance performance was also
discussed.
2. Experiment
2.1. The preparation of PANI
The chemical oxidative polymerization of polyaniline
was carried out in a H2SO4 medium at pH 1 and using
0.5 M ammonium persulfate (added drop by drop) as an
oxidising agent. The reaction mixture was stirred for 2 h at 0
jC to ensure the completion of the reaction. The black green
product of the reaction was filtered and washed repeatedly
with distilled water and alcohol. The resulting polymer was
dried under vacuum at 50 jC.
2.2. The preparation of the composite electrolyte
PTA and Al2(SO4)3�18H2O (the weigh ratios of PTA/
Al2(SO4)3�18H2O are 1:5, 1:2, 1:1 and 4:1, respectively)
nt weight ratios of PTA/Al2(SO4)3�18H2O at scan rate [the weight ratios of
Fig. 3. CV curves of these supercapacitors using composite electrolyte with
different weight ratios of PTA/Al2(SO4)3�18H2O at scan rate 5 mV/s [the
weight ratios of PTA/Al2(SO4)3�18H2O are (a) 1:5, (b) 1:2, (c) 1:1 and (d)
4:1].
Fig. 4. The impedance plots of these supercapacitors using composite
electrolyte with different weight ratios of PTA/Al2(SO4)3�18H2O [the weight
ratios of PTA/Al2(SO4)3�18H2O are: (a) 1:5, (b) 1:2, (c) 1:1 and (d) 4:1].
Y.G. Wang, X.G. Zhang / Solid State Ionics 166 (2004) 61–67 63
were put in the mortar and with physical grinding method
for 20 min, then pellets of 1.2 cm diameter were made at a
pressure of 1�103 kg cm� 2.
2.3. The assembly of electrochemical capacitor
The PANI electrode was prepared according to the follow
steps. First, 85 wt.% PANI powder and 14 wt.% acetylene
black were intimately mixed. Then 1 wt.% PTFE solution
was added to the mixture as a binder. Lastly, the mixture
was pressed onto a graphite groove (which serves as a
current collector, surface is 1 cm2).
The assembly of the supercapacitor is shown in Fig. 1.
The electrodes (PANI) and composite electrolyte were
stacked together as a sandwich. The composite electrolyte
was also used as a spacer.
2.4. The electrochemical test
Cyclic voltammetry (CV) and AC impedance measure-
ments were carried out by the CHI660 electrochemical
workstation system. The Arbin BT2042 battery workstation
system was employed to perform charge–discharge test.
Table 1
The capacitance of these supercapacitors evaluated from impedance test
The weight ratios of
PTA/Al2(SO4)3�18H2O
in electrolyte
Capacitance
(F/g)
1:5 76.2
1:2 164.8
1:1 225.6
4:1 209.2
3. Results and discussion
3.1. Effect of the weight ratio of PTA/Al2(SO4)3�18H2O on
the capacitance performance
3.1.1. CV tests
Pseudocapacitance of a supercapacitor arises mainly from
the redox transitions of the electroactive species within the
electrode materials and the performance of a supercapacitor
is strongly dependent on the electrochemical characteristic of
the redox transitions within these materials. Therefore,
voltammetric response was employed to evaluate the elec-
trochemical characteristics of these supercapacitors.
The CV curves of these supercapacitors using compo-
sites electrolytes with different weight ratios of PTA/
Al2(SO4)3�18H2O (at different scan rates) were shown in
Fig. 2.
As seen in Fig. 2, in all these curves, there are two redox
peaks between 100 and 500 mV, which is according to the
structure transition of the PANI. At the same time, this result
indicates that the PANI electrodes have redox capacitance. In
addition, the cathodic peak shifts negatively as the scan rate
increases whereas the anodic peak shifts positively.
In order to further understand the effect of the weight
ratio of the PTA/Al2(SO4)3�18H2O on the capacitance , the
CV curves of these supercapacitors using composite electro-
lytes with different weight ratios of PTA/Al2(SO4)3�18H2O
were compared in Fig. 3 at scan rate 5 mV/s. As shown in
Fig. 3a,b,c and d, the voltammetric currents increased with
the growth of the weight ratio of PTA/Al2(SO4)3�18H2O.
According to this result, the redox capacitance of these
capacitors increases with the growth of the weight ratio of
PTA/Al2(SO4)3�18H2O. However, when the weight ratio of
PTA/Al2(SO4)3�18H2O was greater than 1:1, the voltam-
metric currents almost stop increasing.
3.1.2. AC impedance test
Impedance spectroscopy is a valuable tool not only to
determine the equivalent series resistance but also to study
the interfacial resistance and pseudocapacitance of the
Fig. 5. Charge–discharge curves of these supercapacitors using composite electrolyte with different weight ratios of PTA/Al2(SO4)3�18H2O are (a) 1:5, (b) 1:2,
(c) 1:1 and (d) 4:1.
Table 2
The capacitance of these supercapacitors evaluated from charge–discharge
curves
PTA/Al2(SO4)3�18H2O
in electrolyte
Capacitance
at 2 mA (F/g)
Capacitance
at 4 mA (F/g)
Capacitance
at 6 mA (F/g)
1:5 194.1 124.6 84.0
1:2 234.2 209.5 192.8
1:1 271.6 265.5 240.5
4:1 260.9 245.5 230.5
Y.G. Wang, X.G. Zhang / Solid State Ionics 166 (2004) 61–6764
electrode structure. In this technique, an alternating voltage
is applied to the electrode interface. The response is repre-
sented by the imaginary component (Z U). The capacitance of
the electrode structure is given by the equation Z U= 1/jxC.
In order to investigate the electrochemical behavior at the
electrode/electrolyte interface in detail, electrochemical im-
pedance measurement (at open circuit condition) were
carried out and typical result were shown in Fig. 4.
From a comparison of a, b, c and d in Fig. 4, several
features have to be mentioned. First, in Fig. 4 the high-
frequency semicircular region of every capacitor has a very
small radius of curvature, which means low resistance to
charge transfer process. This result can also indicate that this
kind of solid composite used as electrolyte of supercapacitor
has excellent performance. Second, from the point intersect-
ing with the real axis in the range of high frequency, the
internal resistances Ri of these supercapacitors is estimated to
6.2, 4.5, 2.1 and 2.2 V, respectively. It is obvious that with
growth of weight of PTA/Al2(SO4)3�18H2O, the internal
resistance Ri of these capacitors decrease. Furthermore, the
decrease is not continuous. When the weight ratio of PTA/
Al2(SO4)3�18H2O is greater than 1:1 , the internal resistance
Ri of these supercapacitors almost stop reducing. The reason
is that when the weight ratio of PTA/Al2(SO4)3�18H2O is
1:1, the conductivity of this kind of composites electrolyte
reach its maxima value [21]. Lastly, comparing these curves
in the low frequency region, it was found that with the
growth of concentration of PTA, the impedance plots of
these capacitors are more close to vertical line. It can also
indicate that with the increase of the concentration of PTA,
the capacitance performance of these capacitors become
better, which is attribute to that the degree of acidity of the
composites electrolyte increases with the growth of the
concentration of PTA. It is well known that higher degree
of acidity is advantageous to doping–undoping of PANI.
The specific capacitance of the supercapacitor can be
evaluated from impedance test according to the following
equation:
C ¼ 1
jxZWð1Þ
Cm ¼ C
m¼ 1
m� ðjxZWÞ ð2Þ
where Cm is the specific capacitance of the supercapacitor;
j =� 1; x = 2pf; f is the frequency; Z U is the imaginary part of
the impedance test; m is the mass of PANI in a capacitor. The
specific capacitances of these supercapacitors with different
Fig. 6. CV curves of graphite/proton conducting electrolyte/graphite. (A) At different scan rate; (B) after different cycle numbers (scan rate; 20 mV/s). (a) First
cycle, (b) 1000th cycle.
Y.G. Wang, X.G. Zhang / Solid State Ionics 166 (2004) 61–67 65
weight ratios of PTA/Al2(SO4)3�18H2O were calculated and
typical results were shown in Table 1.
3.1.3. Charge–discharge test
The charge–discharge behaviors of these supercapacitors
using composites electrolyte with different weight ratios of
PTA/Al2(SO4)3�18H2O were examined by chronopotenti-
ometry and typical results measured from 0 to 800 mV at
different currents are shown in Fig. 5.
In Fig. 5b, c and d, the E� t responses behaved as a
mirror-like during the charge–discharge process. It means
that a reversible oxidation occurs among the electrode
materials, i.e., the charge and discharge reversibly occur at
the electrode/electrolyte interface. However, this phenome-
non was not found in Fig. 5a which is due to its high internal
resistance Ri.
The specific capacitance of the supercapacitor can be
evaluated from charge–discharge test according to the
following equation:
C ¼ Q
DV¼ I � t
DVð3Þ
Cm ¼ C
m¼ I � t
DV � mð4Þ
where Cm is the specific capacitance of the supercapacitor; I
is charge–discharge current ; t is the time of discharge; DV
is the potential range. m is the mass of PANI in a capacitor.
Fig. 7. Cycle-life of the supercapacitor.
The capacitances of these supercapacitors evaluated from
charge–discharge curves were shown in Table 2.
Comparing Tables 1 and 2, it was found that the capaci-
tance evaluated from impedance test is smaller than capaci-
tance evaluated from charge–discharge test. However, the
change trend of capacitance in Tables 1 and 2 is same. In
Tables 1 and 2, the capacitances all increase with growth of
weight ratio of PTA/Al2(SO4)3�18H2O and approach their
maxima value when the weight ratio is 1:1.
From the results of CV tests, impedance tests and charge–
discharge tests, one can get an important conclusion that the
supercapacitor using composite electrolyte (the weight ratio
of PTA/Al2(SO4)3�18H2O is 1:1) have higher capacitance.
Therefore, the proton conducting electrolyte with the weigh
ratio of PTA/Al2(SO4)3�18H2O= 1:1 is the optimum electro-
lyte for application in supercapacitor. For this reason, in the
next section, the properties of the composite electrolyte with
the weigh ratio of PTA/Al2(SO4)3�18H2O= 1:1 was studied
in detail.
3.2. The stability of the composite electrolyte
In order to investigate the stability of this kind of proton
composite electrolyte within special potential window, a
capacitor (it has a sandwich structure graphite/proton con-
Fig. 8. The charge–discharge test of this supercapacitor at different cycle
numbers.
Table 3
The capacitance characters evaluated from charge–discharge tests
Cycle number Capacitance at
6 mA (F/g)
Coulombic
efficiency g (%)
First cycle 240.5 96.4
500th cycle 224.4 97.6
1000th cycle 218.5 98.9
Y.G. Wang, X.G. Zhang / Solid State Ionics 166 (2004) 61–6766
ducting electrolyte/graphite) was assembled in our experi-
ment and CV test was employed to examine the stability of
this kind electrolyte within certain potential window. The
CV curves of this capacitor based on graphite at different
scan rate were shown in Fig. 6A. In Fig. 6A, one can see
that within the certain potential window (from � 200 to 850
mV) there are no clear redox peaks in these curves. In
addition, compared to the data in Fig. 2, the voltammetric
currents almost can be neglected. This result proved that this
electrolyte don’t have redox reaction within this potential
window (from � 200 to 850 mV). On the other hand, with
increase of the scan rate, the shape of the curves does not
change. It also proves the stability of this kind of electrolyte.
Because one of the most important characters of super-
capacitor is rechargeable ability, the electrolyte of super-
capacitor must have long cycle life. In our experiment, the
CV test in the potential region between � 200 and 850 mV
for 1000 cycles was employed to investigate the cycle life of
the electrolyte and the typical result was shown in Fig. 6B.
As seen in Fig. 6B, the shape of the CV curve had changed.
Two very little redox peaks emerged between 0 and 300 mV,
voltammetric current is still very little and can be neglected.
This means that after 1000 cycles, although the electrolyte
generates obvious redox reaction, the voltammetric current
is too little to influence the redox capacitance of PANI. All
of results show that this kind of electrolyte is fit to be
applied in supercapacitors.
3.3. Cycle life test of the supercapacitor using composite
electrolyte
Since long cycle life is very important for supercapacitor,
the cycle charge–discharge test at current 6 mA was
employed to examine the service life of the supercapacitor.
The capacitance as a function of cycle-number was shown in
Fig. 7. As shown in Fig. 7, capacitance of the supercapacitor
almost kept on constant during 0–1000 cycles. This result
means that the supercapacitor using this composite electro-
lyte (the weight ratio of PTA/Al2(SO4)3�18H2O is 1:1) have a
long service-life ,which is very important for practical
application.
In order to know about its cycle life in detail, the charge–
discharge curves at different cycle numbers were compared
in Fig. 8. Charge–discharge characteristics of the super-
capacitor at different cycle numbers were evaluated galva-
nostatically.
The discharge capacitance per mass Cm was evaluated in
Eq. (4).
When the same current is used for charging and dis-
charging, the coulombic efficiency g can be evaluated from
the follow equation:
g ¼ tD
tC� 100% ð5Þ
where tD and tC are the time for galvanostatic discharging
and charging, respectively. The data evaluated from the
charge–discharge curves of the supercapacitor at different
cycle was shown in Table 3.
In Table 3, one can find that the capacitance value of
initial (1st cycle) is 240.5 F/g. After 1000 cycles, this value
decreases to 218.5 F/g. The attenuation of the capacitance is
less than 10%. Furthermore, coulombic efficiency gincreases with growth of cycle numbers. At 1000th, the
coulombic efficiency g almost approaches 99%, which
indicates that reversibility of charge–discharge increases.
4. Conclusion
In this paper, the proton-conducting composite electro-
lyte (PTA/Al2(SO4)3�18H2O) was prepared by physical
grinding method. Electrochemical tests prove that this type
of electrolyte has high stability within special potential
window and high conductivity. Furthermore, the super-
capacitor-based PANI using this composite as electrolyte
has high capacitance and long cycle life. It is more
important that the preparation of this kind of electrolyte is
very simple and abundant, which is important for practical
application.
Acknowledgements
This work was supported by XiBu ZhiGuang Science
Foundation of China.
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