Handbook of Energy Storage for Transmission or ... - W2agz.com

EPRI Proprietary Licensed Material
operate on an oxygen cycle just like sealed lead-acid or NiCd batteries. A second reason
is that the leakage current of this design has a well defined, fixed electrolyte
decomposition potential. So, it is very difficult to over-voltage a type III cell.
Cell operating voltage for a type I device is generally < 1 V. For type II devices, it
presently is 2.3 to 2.7 V and is expected to increase to perhaps 3.0 V after further
developments. Type III devices presently are comprised of a nickel oxyhydroxide
positive electrode mated with an activated carbon negative electrode. This system
operates at between 1.4 V and 1.6 V per cell, depending on the optimization of the
device. Type IV designs have voltages reported to exceed 4 V for some material
systems.
Asymmetric capacitor designs have led to higher energy densities and symmetric designs
usually have higher peak power. Today’s types I and II electrochemical capacitors are in
the 1 to 7 Wh/kg range. Commercial capacitors of the type III design are available with
energy densities of 10 Wh/kg. Energy densities as high as 19 Wh/kg are reported in
patent examples covering this technology. In comparison, lead-acid batteries have an
energy density in the range of 25 to 45 Wh/kg depending on design.
Electrochemical Capacitor Construction
The carbon electrodes used in both symmetric and asymmetric electrochemical capacitors
consist of a high-surface-area activated carbon having area on the order of 1000 m 2 /g or
more in particulate or cloth form. The carbon electrode is in contact with a current
collector. A material that prevents physical contact (shorts), but allows ion conduction,
separate the electrodes. One design for type II products utilizes particulate carbon in a
spiral-wound configuration. Such construction can be performed on a high-speed
winding machine, which introduces minimal labor content. While this construction lends
itself to a right-cylinder product, it can also form rectangular packaging. This form factor
is more desirable in some applications. Type III electrochemical capacitor cells are
constructed in a similar fashion to the type II product. The first commercial products
used a nickel-oxyhydroxide positive electrode with an activated carbon cloth negative
electrode.
The electrolyte of an electrochemical capacitor is an important constituent. Properties
most desired include high conductivity and high voltage stability. Little can be done to
change the conductivity and voltage characteristics of aqueous-based electrolytes used in
type I or type III products, but major improvements should be possible for type II
products. Higher-conductivity electrolyte yields increased power performance, and high
voltage stability allows stable operation at high voltage. These properties are important
for energy and power since each measure scales as the square of the voltage. Organic
electrolytes allow operation above two volts, the exact upper limit depending on the
solvent and salt, their levels of purity, the desired operating temperature, and component
design life.
The electrolyte in a type II capacitor is one of its more expensive constituents. It must
have low concentrations of water at the time of manufacture and over the life of the
product. This adds manufacturing costs in addition to material costs. Type II electrolytes
are generally comprised of an ammonium salt with a solvent such as propylene carbonate,
dimethyl-carbonate, or acetonitrile. At the present time, acetonitrile is the most popular
Electrochemical Capacitors 13