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Abstract
Electrochemical capacitors (EC) also called ‘supercapacitors’ or ‘ultracapacitors’ store the energy in the electric field
of the electrochemical double-layer. Use of high surface-area electrodes result in extremely large capacitance. Single
cell voltage of ECs is typically limited to 1–3 V depending on the electrolyte used. Small electrochemical capacitors
for low-voltage electronic applications have been commercially available for many years. Different applications
demanding large ECs with high voltage and improved energy and power density are under discussion. Fundamental
principles, performance, characteristics, present and future applications of electrochemical capacitors are presented in
this communication. © 2000 Elsevier Science Ltd. All rights reserved.
Introduction
Capacitors which store the energy within the electrochemical
double-layer at the electrode/electrolyte interface
are known under various names which are trade
marks or established colloquial names such as ‘doublelayer
capacitors’, ‘supercapacitors’, ‘ultracapacitors’,
‘power capacitors’, ‘gold capacitors’ or ‘power cache’.
‘Electrochemical double-layer capacitor’ is the name
that describes the fundamental charge storage principle
of such capacitors. However, due to the fact that there
are in general additional contributions to the capacitance
other than double layer effects, we will call these
capacitors electrochemical capacitors (EC) throughout
this paper.
Electrochemical capacitors have been known since
many years. First patents date back to 1957 where a
capacitor based on high surface area carbon was described
by Becker [1]. Later in 1969 first attempts to market such devices were undertaken by SOHIO [2].
However, only in the nineties electrochemical capacitors
became famous in the context of hybrid electric
vehicles. A DOE ultracapacitor development program
was initiated in 1989, and short term as well as long
term goals were defined for 1998–2003 and after 2003,
respectively [3]. The EC was supposed to boost the
battery or the fuel cell in the hybrid electric vehicle to
provide the necessary power for acceleration, and additionally
allow for recuperation of brake energy. Today
several companies such as Maxwell Technologies,
Siemens Matsushita (now EPCOS), NEC, Panasonic,
ELNA, TOKIN, and several others invest in electrochemical
capacitor development. The applications envisaged
are principally boost components supporting
batteries or replacing batteries primarily in electric vehicles.
In addition alternative applications of EC not
competing with batteries but with conventional capacitors
are coming up and show considerable market
potential. Such applications will also be discussed in
detail in the second part of the paper.
The reason why electrochemical capacitors were able
to raise considerable attention are visualized in Fig. 1 where typical energy storage and conversion devices are
presented in the so called ‘Ragone plot’ in terms of
their specific energy and specific power. Electrochemical
capacitors fill in the gap between batteries and conventional
capacitors such as electrolytic capacitors or
metallized film capacitors. In terms of specific energy as
well as in terms of specific power this gap covers several
orders of magnitude.
Batteries and low temperature fuel cells are typical
low power devices whereas conventional capacitors
may have a power density of \106 watts per dm3 at
very low energy density. Thus, electrochemical capacitors
may improve battery performance in terms of
power density or may improve capacitor performance
in terms of energy density when combined with the
respective device. In addition, electrochemical capacitors
are expected to have a much longer cycle life than
batteries because no or negligibly small chemical charge
transfer reactions are involved. A monograph volume
on electrochemical capacitors was recently published by
Conway [4].
In the following the basic principal of electrochemical
capacitors, the different types of ECs, some theoretical
considerations as to the performance of ECs, and some
applications will be discussed.
2. Principle of energy storage
Electrochemical capacitors store the electric energy in
an electrochemical double layer (Helmholtz Layer)
formed at a solid/electrolyte interface. Positive and
negative ionic charges within the electrolyte accumulate at the surface of the solid electrode and compensate for
the electronic charge at the electrode surface. The thickness
of the double layer depends on the concentration
of the electrolyte and on the size of the ions and is in
the order of 5–10 A, for concentrated electrolytes. The
double layer capacitance is about 10–20 mF/cm2 for a
smooth electrode in concentrated electrolyte solution
and can be estimated according to equation Eq. (1)
C/A=o*0or/d (1)
assuming a relative dielectric constant or of 10 for water
in the double layer [5]. d being the thickness of the
double-layer with surface area A. The corresponding
electric field in the electrochemical double layer is very
high and assumes values of up to 106 V/cm easily.
Compared to conventional capacitors where a total
capacitance of pF and mF is typical, the capacitance of
and the energy density stored in the electrochemical
double layer is rather high per se and the idea to build
a capacitor based on this effect is tempting.
In order to achieve a higher capacitance the electrode
surface area is additionally increased by using porous
electrodes with an extremely large internal effective
surface. Combination of two such electrodes gives an
electrochemical capacitor of rather high capacitance.
Fig. 2 shows a schematic diagram of an electrochemical
double-layer capacitor consisting of a single cell
with a high surface-area electrode material, which is
loaded with electrolyte. The electrodes are separated by
a porous separator, containing the same electrolyte as
the active material. The potential drop across the cell is
also shown in Fig. 2.
Metal oxides
The cyclic voltammogram of RuO2 (and also IrO2)
electrodes have an almost rectangular shape and exhibit
good capacitor behavior [11,12]. However, the shape of
the CV is not a consequence of pure double layer
charging, but of a sequence of redox reactions occurring
in the metallic oxide. The valence state of Ru may
change from III to VI within a potential window of
slightly \1 V. The ratio of surface charging to bulk
processes was nicely separated by Trasatti [11]. In
aqueous acid electrolytes the fundamental charge storage
process is proton insertion into the bulk material.
Very high specific capacitance of up to 750 F/g was
reported for RuO2 prepared at relatively low temperatures
[13]. Conducting metal oxides like RuO2 or IrO2
were the favored electrode materials in early EC s used
for space or military applications [14]. The high specific
capacitance in combination with low resistance resulted
in very high specific powers. These capacitors, however,
turned out to be too expensive. A rough calculation of
the capacitor cost showed that 90% of the cost resides
in the electrode material. In addition, these capacitor
materials are only suitable for aqueous electrolytes,
thus limiting the nominal cell voltage to 1 V.
Several attempts were undertaken to keep the advantage
of the material properties of such metal oxides at
reduced cost. The dilution of the costly noble metal by
forming perovskites was investigated by Guther et al.
[15]. Other forms of metal compounds such as nitrides
were investigated by Liu et al. [16]. However, these
materials are far from being commercially used in ECs.
3.1.3. Polymers
Polymeric materials, such as p- and n-dopable
poly(3-arylthiopene), p-doped poly(pyrrole), poly(3-
methylthiophene), or poly(1,5-diaminoanthraquinone) have been suggested by several authors [17–19] as
electrodes for electrochemical capacitors. The typical
cyclic voltammogram of a polymer however is in general
not of rectangular shape, as is expected for a
typical capacitor, but exhibits a current peak at the
respective redox potential of the polymer. In order to
be able to use one and the same electrode material on
both capacitor electrodes polymers with a cathodic and
an anodic redox process were utilized recently [19].
Using a polymeric material for electrochemical capacitor
electrodes gives rise to a debate as to whether
such devices should still be called capacitors or whether
they are better described as batteries. In terms of the
voltage transient during charge and discharge and with
respect to the CV they are batteries. Compared to
metallic oxides, however, the term capacitor is justified.
The difference being only that the metallic oxides exhibit
a series of redox potentials giving rise to an almost
rectangular CV while the polymer typically has only
one redox peak.
For such capacitors rather high energy density and
power density have been reported [19]. The long-term
stability during cycling, however, may be a problem.
Swelling and shrinking of electroactive polymers is well
known and may lead to degradation during cycling.
3.2. Electrolyte
Another criteria to classify different electrochemical
capacitors is the electrolyte used. Most of the presently
available capacitors use an organic electrolyte.
3.2.1. Organic
The advantage of an organic electrolyte is the higher
achievable voltage. According to Eq. (2) the square of
the unit-cell voltage determines the maximum stored
energy. Organic electrolytes allow for a unit cell voltage
above 2 V. Typically the cell float voltage is 2.3 V with
the possibility to increase the voltage for a short time to
2.7 V. The cell voltage is most probably limited by the
water content of the electrolyte. In order to achieve
higher voltage, some companies plan to go up to a float
voltage of 3.2 V, extreme purification procedures of
special electrolyte have to be applied and the corrosion
of the carbon electrodes has to be reduced by special
protective coatings [20]. However, similar problems
concerning the potential window of organic electrolyte
are known from Li-ion battery production and can be
overcome.
On the other hand organic electrolytes have a signifi-
cantly higher specific resistance. Compared to a concentrated
aqueous electrolyte the resistance increases by a
factor of at least 20, typically by a factor of 50. The
higher electrolyte resistance also affects the equivalent
distributed resistance of the porous layer and consequently
reduces the maximum usable power,