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Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲
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Design, Fabrication, and Evaluation of a 1.5 F and 5 V
Prototype of Solid-State Electrochemical Supercapacitor
P. Staitiz and F. Lufrano
CNR-ITAE, Istituto di Tecnologie Avanzate per l’Energia ‘‘Nicola Giordano,’’ 98126 S. Lucia Messina, Italy
The scale-up from a small single cell to a larger stack prototype of a solid-state electrochemical supercapacitor based on polymer
electrolyte and activated carbon is demonstrated. The prototype is composed of five single cells stacked in series, has a nominal
capacitance of 1.5 F, and a maximum voltage of 5 V. The electrodes of the prototype were prepared using high-surface-area carbon
material 共Norit A Supra Eur兲 and Nafion ionomer. Nafion was used as the electrolyte membrane separator between the electrodes
of each single cell and as the binder/ion conductor in the electrodes. The fabricated prototype showed a higher series resistance
compared to that estimated in our previous study of a small size single supercapacitor. However, the prototype achieved a specific
capacitance of 114 F/g 共referred to the weight of active carbon materials for a single electrode兲, which is comparable to the specific
capacitance of the previously reported 4 cm2 single-cell supercapacitor. Moreover, an appreciable power density of 1.4 kW/L and
a RC-time constant of 0.3 s have been calculated for the device.
© 2005 The Electrochemical Society. 关DOI: 10.1149/1.1859614兴 All rights reserved.
Manuscript submitted July 12, 2004; revised manuscript received August 5, 2004. Available electronically January 31, 2005.
Electric double-layer capacitors 共EDLCs兲 or supercapacitors
共SCs兲 are promising energy storage devices being considered for
applications in areas such as electric vehicles, uninterruptible power
systems, and computer memory protection. Their main characteristic
is their excellent high-rate charging-discharging ability that makes
these devices, referring to this specific aspect, more efficient than
batteries and fuel cells. High-surface-area carbons are the active
materials commonly used for supercapacitor electrodes. The capacitance of the electric double layer of charges at the electrode/
electrolyte interface is proportional to the available surface area of
carbon composite electrodes. Other parameters such as electrolyte
accessibility in the carbon pores and electric conductivity of the
entire device are important figures of merit too.
A typical supercapacitor has two electrodes, made of highsurface-area activated carbon materials and, an aqueous or nonaqueous electrolyte impregnated in a porous separator stacked between
the two electrodes. An aqueous electrolyte is a low-cost and high
ionic conductivity component that provides a high supercapacitor
power density. A nonaqueous electrolyte provides a high achievable
voltage leading to a high supercapacitor energy density. Both types
of devices have been used in the EDLC manufacturing for pulse
power application or stable current operation systems, respectively.
This paper describes the performance of a carbon and Nafion
polymer-based supercapacitor developed for pulse power applications. Water is the solvating agent of ionic species and the charge
carrier in the solid polymer electrolyte; therefore, the device may be
considered an aqueous type of supercapacitor. Differently from commonly used acidic aqueous solutions, the acid electrolyte in solid
polymer form shows reduced corrosion of the auxiliary components
and no leakage of dangerous liquid from the device is possible.
Thus, this device has a high level of safety. Our recent research on
this type of supercapacitor, in single-cell configuration, achieved
interesting results in terms of electrochemical performance and
stability.1-5 From the point of view of electrochemical performance,
supercapacitor based on a Nafion polymer electrolyte and on electrodes containing a Norit activated carbon material gave a specific
capacitance as high as 130 F/g 共referred to the weight of activated
carbon material in the electrode兲.3 Moreover, the internal resistance
of the capacitor with Nafion electrolyte was comparable to that measured with a 1 M H2 SO4 solution.2,3 The solid polymer electrolyte
used in the experiments 共Nafion, from DuPont兲 is different than
other gel-like materials tested as supercapacitor electrolyte.6-9 The
latter includes materials obtained by mixing liquid electrolyte with
inert supports, for instance, phosphoric acid with nylon,6 phosphoric
acid-doped silica gel and polymers,7 perchloric acid-doped silica,8
z
E-mail: [email protected]
phosphoric acid and polyvinyl alcohol,9 and TEABF4 with propylene carbonate 共or ethylene carbonate兲 and poly共vinylidene
fluoride-hexafluoropropylene兲.10 Such gel-like electrolytes exhibited
insufficient mechanical strength, lower ion conductivity, and lower
dimension stability in solvents than solid polymer electrolyte. Solid
polymer electrolytes, like Nafion used in this work, consist of an
inert hydrophobic backbone, and highly hydrophilic terminal functional groups attached to the backbone are homogeneous materials
with outstanding mechanical strength, exhibiting high ionic conductivity in their highly hydrated form. The EDLC prototype presented
in this paper has been fabricated by stacking five cells and can be
charged and operated up to 5 V. The goal of our design and prototype fabrication is to highlight the problems arising from the
scale-up process from a single cell to a multicell stack as well as
from a smaller to a larger geometric area. The paper discusses and
attempts to explain the problems encountered in the scaling-up process. Moreover, the performance of the electrodes are compared to
that of electrodes previously studied in the small, 4 cm2 single cell.
Experimental
Preparation of electrodes.—The electrodes for the supercapacitor prototype were cut from a larger electrode prepared by a casting
method consisting of spreading onto a glass plate with a film applicator the ink containing the activated carbon, the Nafion ionomer
electrolyte, the graphite fibers, and the N,N-dimethylacetamide
共DMAc兲 solvent. The activated carbon powder 共Norit A Supra Eur兲
used in the ink preparation has been furnished by Norit Italia S.p.A.
共Ravenna, Italy兲 and had a surface area of 1500 m2/g. The graphite
fibers were cut 400-500 ␮m in length with a high-speed grinder
from a carbon fabric 共Avcarb 1071兲 furnished by Ballard Material
Products, Inc. 共MA, USA兲. The presence of graphite fibers in the
final composite electrode allowed the formation of an electrically
more conductive and mechanically stronger composite film. The
Nafion ionomer 共a DuPont product兲 intermixed with the carbon in
the electrode is acting as the binder of carbon materials 共powder and
fibers兲 and as the ion conductor. The composite electrode film was
obtained after drying the ink cast onto the glass plate at 70°C for 5-6
h. Further thermal treatments at 120°C for 1 h and at 160°C for 20
min were made to produce an electrode with adequate mechanical
strength and insolubility of polymer binder in water. Subsequently,
the electrode was rinsed several times in distilled water and then
chemically treated in 1 M H2 SO4 solution to obtain the material free
from contaminant ionic species eventually attracted by sulfonic
groups of Nafion during the preparation process. Further washing of
electrode in warm distilled water, until neutral pH, was made to
eliminate all the free sulfuric acid adsorbed in the porous structure.
The final electrode composition was 65 wt % of activated carbon
powder, 30 wt % Nafion, and 5 wt % graphite fibers. The carbon
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Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲
Figure 1. Schematic representation of the supercapacitor prototype.
loading was 8.2 ⫾ 0.9 mg/cm2 and the electrode thickness 150
⫾ 30 ␮m. The electrodes prepared with the casting technique
showed uniform distribution of the materials and exhibited excellent
mechanical strength.
Polymer electrolyte membrane.—The Nafion 115 membrane produced by DuPont was utilized as electrolytic separator between the
electrodes. The water-swelled membrane had an approximate thickness of 160 ␮m. It was purified by hydrogen peroxide at 3 wt % for
about 1 h at 70°C to remove organic impurities. After three-four
washings in pure water to remove H2 O2 and traces of soluble organic impurity, a treatment in 1 M sulfuric acid solution at 70-80°C
was carried out to exchange with protons the eventual metallic ionic
species attracted on the sulfonic groups of Nafion. The membrane
was finally rinsed several times in warm distilled water until the free
sulfuric acid was completely eliminated.
Membrane and electrodes assembly.—The membrane electrode
assemblies 共MEAs兲, which were inserted in the prototype, were preformed out the device by contacting face-to-face the membrane and
two electrodes. Each MEA was realized by a hot-pressing procedure
carried out at 100 kg/cm2 and 130°C for 10 min. The assembly, after
the hot-pressing process, was rehydrated by immersion in 1 M
H2 SO4 solution first and then rinsed in warm distilled water. All the
assemblies exhibit good mechanical characteristics, and they did
not need particular precaution in handling during the stacking of
prototype.
Preparation of the supercapacitor.—A supercapacitor prototype
was fabricated by stacking five well-humidified 16 cm2 MEAs. The
assemblies were separated by four bipolar plates of carbon fiber
paper 150 ␮m thick 共AvCarb P50T by Ballard Material Products兲.
Two end monopolar plates of the same carbon fiber paper interfaced
the external cells of stack with the metallic current collectors. These
latter were formed of titanium foils having a thickness of 250 ␮m
and were externally strengthened by thicker aluminum plates 共thickness 2 mm兲. The use of the metallic current collectors allowed electrical wire connection with testing equipment. The aluminum plate
helped the maintainance of titanium foil flatness also during and
after the sealing of the device. The prototype was fastened on three
sides with a ‘‘U’’-shaped iron that was electrically insulated from
the bottoms of the current collectors by a thin poly共tetrafluoroethylene兲 共PTFE兲 foil. The remaining side of the prototype was sealed
with silicon rubber. The so-formed case, containing the active elements of the supercapacitor, is completely sealed and thus the drying
of the Nafion electrolyte is not allowed. The weight and the volume
of the full supercapacitor device were 55 g and 22.5 cm3, respectively. The schematic view and the photograph of the named
ITAECAP-1 prototype are shown in Fig. 1 and 2, respectively.
Evaluation of the supercapacitor.—Cyclic voltammetry, dc galvanostatic charge/discharge tests, and ac impedance analysis were
carried out on the prototype to evaluate its electrochemical and conductivity performances.
Figure 2. The 1.5 F-5 V ITAECAP-1 prototype.
Cyclic voltammetry 共CV兲 and dc galvanostatic measurements
were carried out with AMEL equipment composed of a high-power
potentiostat model 2055, an integrator model 731, and a function
generator model 568. CV tests were carried in potentiodynamic
mode at prefixed voltage scan rates using a range from 100 to 400
mV s⫺1 and using a voltage range between 0 and 5 V. At least 100
cycles have been performed at each constant scan rate before collecting the results, which are shown and discussed in the next section of this paper.
Galvanostatic charge-discharge measurements were carried out
injecting currents of 50, 100, 150, and 200 mA and applying a
voltage varying from 0 to 5 V. For each of these currents, the supercapacitor is charged from 0 to 5 V and discharged from 5 to 0 V in
a continuous manner and for several cycles. The amount of charge
共coulomb兲 delivered by the capacitor during the discharge process
from 5 to 0 V was measured by an electrical integrator and used to
determine the capacitance of the prototype device.
The electrochemical impedance spectroscopy 共EIS兲 measurements were carried out on the discharged supercapacitor at ambient
temperature using a potentiostat 共PGSTAT30 Autolab/Eco chimie
NL兲 with a frequency response analyser 共FRA2兲 module interfaced
to a PC. An electrochemical impedance software 共by Autolab兲 was
used to carry out the impedance measurements between 10 MHz and
1 mHz. The amplitude of the sinusoidal voltage used in the tests was
10 mV.
Results and Discussion
CV measurements were carried out to evaluate the electrochemical stability of the electrodes and of the electrolyte by applying a
potential varying from 0 to 5 V in which the prototype operates.
Figure 3 shows CVs recorded at fixed voltage scan rates of 100, 200,
and 400 mV/s in the voltage range from 0 to 5 V. The absence of any
current peaks along the profile of the voltammograms is indicative
of pure capacitive behavior of the system, i.e., during the charging/
discharging phases in the range 0-5 V; therefore, only the storage of
electric charges occurs at the electrode/electrolyte interface. In other
words, faradaic processes 共reversible or irreversible redox reactions兲
are absent or negligible in the studied range of voltage; thus, the
system can be considered electrochemically stable. To obtain a preliminary evaluation of the capacitance of the prototype the values of
current collected during the CV tests were divided by the respective
voltage scan rates and reported as capacitance values in Fig. 4. The
figure shows the capacitance of supercapacitor as a function of the
voltage at the different voltage scan rates. The voltammograms
highlight a clear slight dependence of capacitance by the voltage
scan rate with a decrease of the former upon increasing the latter.
This observed variation could have different explanations, although
Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲
Figure 3. CVs of the ITAECAP-1 prototype at various voltage scan rates:
共a兲 100, 共b兲 200, and 共c兲 400 mV/s.
A619
Figure 5. Typical charge/discharge curve obtained at constant current of 200
mA.
two seem more probable. The first one could be that the internal
resistance of the device is somewhat high and at the highest voltage
scan rate, when highest current is delivered by the capacitor, a more
considerable voltage loss (⌬V) is originated. This voltage loss,
which has a resistive nature, is a parasitic process that subtracts part
of the capacitance of the device especially at higher current flows;
therefore, it should be maintained as low as possible. Unfortunately,
the voltage loss could never be totally eliminated. The second possible explanation is that the electric charges could have some difficulty to occupy homogeneously and rapidly all the available sites at
the interface electrode/electrolyte due to their limited rate of orientation and migration in the electrolyte 共low local ionic conductivity兲
contained in the narrow pores of electrodes that are farther from the
bulk of the electrolyte. Thus, because it is known that the capacitance of the supercapacitor is influenced by the electrical series resistance, this latter should be kept as low as possible in the device by
using appropriate building materials 共bipolar plates, current collectors兲, by improving the properties of electrode and electrolyte materials, and by decreasing the contact resistances. A more in-depth
analysis of the negative contributions to the ionic and electronic
resistances of the prototype performance are reported during the
discussion about the impedance behavior of device.
Figure 5 shows a dc galvanostatic charge/discharge curve of the
prototype at the current of ⫾200 mA as a function of time. The
charge amount that was discharged from the supercapacitor at each
prefixed electrical current was measured by a charge integrator and
used to calculate the capacitance of the device. Figure 6 plots discharge vs. time curves at different discharge currents. It is clear in
the figure that there is not proportionality between the increasing
currents and the decreasing discharge times, and thus higher capacitances are obtained at lower discharge currents. The dependence of
the capacitance of the device from the discharge current is also
evidenced in Fig. 7. A decrease of more than 20% of the capacitance
arises when the discharge current is increased by four times; thus,
the capacitance is 1.45 F at 50 mA and 1.12 F at 200 mA. This
variation of capacitance is comparable to that previously determined
on small size single-cell supercapacitors.3
The influence of series resistance on prototype performance has
been analyzed and discussed by taking into account the impedance
spectra. Figure 8 shows the Nyquist plot of the supercapacitor. At
high frequency, the supercapacitor behaves like a pure resistor with
a minimum of resistance of about 0.2 ⍀. A resistance of 0.64 ⍀ cm2
for the single cell has been calculated taking into account the geometric area of the electrodes 共16 cm2兲 and the number of cells 共five兲
contained in the device. This value, excessively high for this system,
is explained by the presence of some unexpected resistances, which
add to those of well-humidified membranes and electric conductive
components. We believe that these parassitic resistances are due to
excessive contact resistance between the interfaces of the components and/or to the distributed resistance of the composite electrodes. The former could be due to nonuniform or insufficient pres-
Figure 4. Capacitance vs. potential curves at various voltage scan rates as in
Fig. 3.
Figure 6. Discharge curves from 5 to 0 V at various currents: 共a兲 50, 共b兲
100, 共c兲 150, and 共d兲 200 mA.
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Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲
Figure 9. Capacitance as a function of frequency of the prototype.
Figure 7. Discharge capacitance of prototype as a function of current.
sure exerted by the external plates on the inner components and the
latter to an inappropriate pore size distribution in the carbon material. Referring to the latter aspect, Yoon et al.11 found that using a
carbon material with a well-ordered pore structure with uniform
mesopores of 2.3 nm, a 3.3 times smaller electric series resistance
共ESR兲 as compared to that observed with a conventional activated
carbon can be obtained. Notwithstanding the high resistance found
in our supercapacitor, the maximum deliverable power 共calculated
by P ⫽ V 2 /4R) of the device was 31 W, corresponding, when the
volume of the prototype is taken into account, to a maximum power
density of 1.4 kW/L.
The energy density calculated for 1.5 F device capacitance is
0.23 Wh/L. From the electric serial resistance 共R兲 and the capacitance 共C兲 of the device it is possible to calculate the time constant, ␶,
by the expression ␶ ⫽ RC. The time constant gives information on
the rate of charge and discharge process. The smaller the time constant the higher the charge and discharge rate. The RC time constant
of 0.3 s calculated for our device corresponds to a quick chargedischarge of the supercapacitor.
Figure 8. Nyquist plot of the prototype. The inset shows the high-frequency
region of impedance.
In the current literature, few examples of stacked-prototype of
supercapacitors have been reported, and this makes a comparison of
actual performance with that of our device difficult. Moreover, these
stacked prototypes use aqueous electrolyte solutions, while organic
electrolytes, which have low specific conductivity 共about 20 mS/
cm兲, are not considered suitable for the fabrication of multicell capacitors. The aqueous electrolytes, that show specific conductivity
as high as 800 mS/cm, are penalized by a lower working voltage
共1 V兲 compared to that of a nonaqueous electrolyte 共2.3-2.7 V兲.12
Thus, the higher voltage permits a much higher energy density to be
obtained with supercapacitors using organic electrolytes, while the
high ionic conductivity favors a higher power density of aqueous
supercapacitors. A prototype of multicell module was reported from
Hahn et al.12 based on electrodes made by activated glassy carbon.
Their 0.4 F-24 V device, which used a 3 M H2 SO4 solution as
electrolyte, was characterized by a maximum power density of 15
kW/L and by an energy density of about 0.09 Wh/L. In another
paper, the same group reported for an experimental 0.8 F-5 V bipolar glassy carbon capacitor, stable resistance and capacitance performance up to 100,000 cycles.13 Similarly, Kibi et al.14 showed that a
high-power electric double-layer capacitor of 470 F–15 V was realized with activated carbon electrodes and 30% solution of sulfuric
acid. The device had about 2.5 Wh/L and 2.3 kW/L maximum energy density and power density, respectively. Compared with the
results reported in the literature, lower energy and power density
have been obtained by our prototype. However, here we are showing
for the first time the design, fabrication, and the results for a solidstate supercapacitor in multicell module in which a Nafion 115
membrane is used as the electrolyte. We are remarking that the
development of solid-state supercapacitors could give the following
advantages: 共i兲 higher flexibility in cell design, (ii) reduced corrosion, (iii) increased safety in handling, and (i v ) probably, longer
lifetime.
In Fig. 9 the specific capacitance as a function of frequency 共f兲
from 1000 Hz to 5 mHz is shown for the device. The capacitance is
calculated from an imaginary component (Z ⬙ ) of impedance by the
expression C ⫽ ⫺1/(2␲ f Z ⬙ ). From the figure, it is evident that
there is a continuous increase of capacitance by decreasing the frequency and a plateau in capacitance is never achieved. This means
that the electrical signal is not yet reaching the deepest narrow pores
of carbon in the electrodes even at very low frequency 共5 mHz兲. A
maximum capacitance of 1.55 F is recorded at 5 mHz for the supercapacitor. From this value the energy of 19.4 J is calculated for the
supercapacitor, which corresponds to an energy density of about
0.23 Wh/L. Moreover, considering that the capacitance of the five
cell prototypes is 1.5 F, the capacitance for a unit area of a single
cell is 0.47 F/cm2. By this, taking into account the surface density of
carbon 共0.0082 g/cm2 for each electrode兲, a single cell capacitance
of 28.6 F/g is calculated. As normally the capacitances of superca-
Journal of The Electrochemical Society, 152 共3兲 A617-A621 共2005兲
pacitors are referred to the weight of the single electrode, this value
must be multiplied by a factor of four. Therefore, we obtain a capacitance value of 114 F/g, which is comparable to those previously
reported in the single-cell supercapacitor.3,4,15 This is an important
result because it demonstrates that the capacitance performance in
the scale-up from 4 cm2 single cell to the stack prototype is fully
maintained. Capacitances of 100–120 F/g are generally considered
typical values of well-designed practical supercapacitors using activated carbon materials and liquid electrolytes.16-18 The relatively
low energy density 共0.23 Wh/L兲 of our device is influenced by an
excessive volume of the external building components and by the
dead volume inside the case of the device. Furthermore, it is likely
that power and energy densities of this type of supercapacitor may
be further improved in the future by decreasing the resistance of the
full device and also by optimizing the carbon/Nafion loading, the
composition, and the structure of electrodes. The former aspect
could be improved by using adhesive paint to reduce contact resistance between the single cells and the bi- and monopolar plates19,20
and/or by balancing the pressure exerted on the single cells with
appropriate deformable bipolar plates, and the latter aspect, by using
electrodes with carbon having a more appropriate pore size distribution in order to increase the ion mobility and to have a faster charge
separation process at the electrode/electrolyte interface.
Conclusions
A multicell prototype of supercapacitor based on high-surfacearea activated carbon material and solid polymer electrolyte has
been successfully fabricated. The prototype, which was made by
stacking five single cells, is electrochemically stable in the range of
potential between 0 and 5 V. A maximum capacitance of 1.55 F and
a resistance of 0.2 ⍀ have been obtained for the device by impedance analysis. The electrical series resistance for the multicell prototype was higher than that measured in previous experiments in
single-cell capacitor, most likely due to larger contact resistances
between the different components and/or the not fully optimized
structure of electrodes. Nevertheless, an RC-time constant of 0.3 s is
obtained for the supercapacitor. Values of energy and power density
of 0.23 Wh/L and 1.4 kW/L, respectively, have been also calculated
for the prototype.
A621
A high specific capacitance of 114 F/g 共referred to weight of
active carbon materials of the single electrode兲 for the fabricated
prototype has been demonstrated, which is comparable to those previously reported for single-cell supercapacitors, confirming the optimal scale-up design of this multicell prototype.
Instituto di Tecnologie Avanzate per l’Energia assisted in meeting the
publication costs of this article.
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