AAWSAP DIRD, Ultracapacitors as Energy and Power Storage Devices, November 2010

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Initial ultracaps used aqueous electrolytes made with Ab(SO4)3 (aluminum sulfate),
H2SO4 (sulfuric acid), or KOH (potassium hydroxide) . While these electrolytes have
excellent ionic conductivity, the operating voltage is limited to 1.2 volts using carbon
electrodes. Organic electrolytes have lower ionic conductivity, but the higher dielectric
constant increases the nominal cell voltage up to as high as 3 volts. Carbon has been
used as a high-surface-area electrode material since the inception of the
electrochemical capacitor. It is still the material of choice for many ultracapacitors; one
of the primary reasons is the low cost of carbon materials. However, there are many
types of carbon that are available for use as an electrode material. In addition to
carbonaceous electrodes, metal oxides and conductive polymers are finding increasing
use in ultracap design. Advancements in the understanding of the electric double-layer
and ultracapacitor behavior have led to better materials utilization and, consequently,
improved devices.
Early electrochemical capacitors were rated at a few volts and had capacitance values
measured from less than one farad up to several farads. Today cells range in size from
small devices with exceptional pulse-power performance in the millifarad range up to
devices rated at several kilofarads. There are even some specialized ultracapacitor cells
now in production that have ratings of more than 100 kF. The technology is
experiencing increasingly broader use, replacing batteries in some cases and in others
complementing their performance. Ultracap technology has grown into an industry with
sales of several hundred million dollars per year that is poised for rapid growth in the
near term due to expansion of power quality needs and the emerging energy
management/conservation applications. 11
Advancements in ultracapacitors have led to numerous devices from an array of
manufacturers. Table 2 compares the various products on the market, showing voltage,
capacity, power density, and additional energy-storage characteristics. Ultracaps have
moved away from aqueous electrolytes and are typically organic electrolytes due to the
increased voltage performance. Electrodes vary from carbon/carbon systems to hybrid
systems using metal oxides or conductive polymers paired with a carbon electrode.
Packaging and sizes of ultracapacitors covers a large range as these are now used from
cellular communications and small electronics to power delivery and management for
seaport cranes. Ultracapacitor technology development is focusing on delivering better
energy density, and this is being approached by improved carbon electrodes, better
electrolytes, and alternative electrodes that provide pseudocapacitive behavior,
including battery-like electrodes. 12
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