Status of High-Power Electrochemical Capacitors
presented at the International Seminar on Primary and Secondary Battery
Technologies and Applications, February 28, 1994
John R. Miller
Electrochemical capacitors (ECs) are
gaining acceptance in the electronics industry as system designers
become familiar with their attributes and benefits. In CMOS memory
backup applications, for instance, a one-Farad EC having a volume of
only one-half cubic inch can replace nickel cadmium or lithium
batteries and provide backup power for months. And in electric vehicle
applications, large ECs can load-level the power on the battery system
and thereby increase battery life and extend vehicle range.
The high volumetric capacitance density
of an EC (10 to 100 times greater than conventional capaci tors)
derives from using porous electrodes to create a large effective plate
area and from storing energy in the diffuse double layer. This double
layer, created naturally at a solid-electrolyte interface when voltage
is imposed, has a thickness of only 1 nrn, thus forming an extremely
small effective plate separation. In some ECs, stored energy is
substantially augmented by so-called pseudocapacitance effects,
occurring again at the solid-electrolyte interface.
Although the energy storage capability of
the double layer was recognized more than 100 years ago, it took the
development of low-current-draw volatile computer memories to create a
market for ECs.
ECs do not approach the energy density of
batteries. Nevertheless, they are extremely attractive power sources.
Compared with batteries, they require no maintenance, offer much higher
cycle-life, require a very simple charging circuit, experience no
memory effect, and are generally much safer. Physical rather than
chemical energy storage is the key reason for their safe operation and
extraordinarily high cycle-life.
Readily available EC products are limited
in size and power perforrnance, due primarily to their targeted memory
backup use. They are manufactured by several companies, primarily
Japanese, and have capacitance values of up to a few Farads, an
equivalent series resistance (ESR) of one to fifty ohms, and a working
voltage of 3 to 11 V.
Until recently, ECs suitable for
high-power applications have been unavailable. But interest in
automotive starting, lighting, and ignition(SLI) applications, as well
as in electric vehicle (EV) load-leveling, has stimulated product
development activities for such high-power devices. These efforts have
beenfocused primarily on development of ECs with improved frequency
response. The goal is to develop prod ucts that can be efficiently
charged and then discharged in the time specified for these high-rate
The following sections discuss properties
that are important for an EC in a high-power application. Particular
emphasis is given to the EV application. Ongoing R&D activities are
reported. Critical issues relating to the development of viable
high-power EC products are highlighted.