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

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most common type in use today and are rated at voltages in the range of 2.3 to 2.7 V/cell,
depending on manufacturer.
Operation at higher voltages offers distinct advantages for energy and power density, but
with some offsetting disadvantages. The dielectric constant of the organic solvent is less
than that of water; the double layer thickness (plate separation) is greater because of the
larger solvent molecules; the effective surface area of the electrode is somewhat
diminished because the larger ion sizes cannot penetrate all pores in the electrodes; and
the ionic conductivity of the electrolyte is much less than that of aqueous electrolytes,
particularly at low temperatures. Stable, long term operation at higher voltages requires
extremely pure materials: trace quantities of water in the electrolyte, for instance, can
create problems. Thus, the device must be packaged in such a way that water does not
enter the capacitor.
The net effect of using an organic electrolyte in the type II device is increased energy
density over type I. However, there often is a reduction in power performance over that
exhibited by the type I devices, even though each cell operates at higher voltage.
The type III design, referred to as asymmetric, is the most recent available. They are
comprised of two capacitors in series, one being an electrostatic capacitor and the other a
faradaic pseudocapacitor. The electrostatic capacitor is exactly like those used in the
symmetric type I and II devices. It consists of a high-surface-area electrode with double
layer charge storage. The faradaic-pseudocapacitor electrode relies on an electron charge
transfer reaction at the electrode-electrolyte interface to store energy. This is very similar
to an electrode in a rechargeable battery.
In this design the capacity of the faradaic-pseudocapacitor electrode is typically many
times greater than the capacitance of the double layer charge storage electrode. Thus the
depth of discharge of the faradaic-pseudocapacitor electrode is very small during
operation, allowing higher cycle life. Different asymmetric capacity ratios have been
built to tailor the capacitor for specific applications. Asymmetric electrochemical
capacitors have an important advantage of voltage self-balancing, which will be
discussed in the section on series connecting cells to create high-voltage systems. None
of the other types of capacitors offer this feature.
Comparison of the three product types is provided in Table 2.
Table 2 Comparison of functionality of electrochemical capacitor designs
Electrochemical
Capacitor Types
Type I Symmetric
/aqueous
Type II Symmetric
/organic
Type III Asymmetric
/aqueous
Energy Density Low to Moderate Moderate to High High to Very High
Power performance High High Low to High
Cycle life High High High
Self-discharge rate Low Low very Low
Low-Temp. performance Excellent Good to excellent Excellent
Packaging non-hermetic hermetic
Non-hermetic, resealable vent
valve
Voltage balance resistor/active resistor/active self limiting/active
Cell voltage < 1 V 2.3 - 2.7 V 1.4 - 1.6 V
Electrochemical Capacitors 11