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Fundamentals of Electrochemical Double-Layer Supercapacitors
batteries (Table 2.7). This can also be reflected by difference in ESRs. The ESR
of a battery is normally much higher than that of a supercapacitor due to differences in their electrode layer conductivities.
Third, the charge and discharge times of supercapacitors are much shorter
than the times for batteries. As discussed earlier, the physical charge separation and combination processes in a supercapacitor should be much faster
than the electrochemical oxidation–reduction steps in a battery.
Fourth, the life cycles of supercapacitors are much longer than those of batteries. In supercapacitors, the structure of the electrode does not change during each cycle, so theoretically a supercapacitor’s life cycle should be infinite.
In a battery, the structure of the electrode layer will slightly change for each
cycle due to side reactions that cause a gradual reduction in active electrode
material, leading to fast performance degradation. Therefore, supercapacitors have much longer cycle times than batteries (Table 2.7).
There is an intrinsic cell voltage increase or decrease with the charging or
discharging of a supercapacitor, while a battery has a constant cell voltage
during charge and discharge. Also, heat management is easier in supercapacitors, so they are much safer than batteries. To further explain supercapacitor technology, Table 2.8 summarizes the advantages and challenges of
supercapacitors and batteries.
Both devices will play roles in future energy storage and conversion technologies. Supercapacitors cannot yet replace batteries due to their low energy
densities; they are presently used in conjunction with batteries. It is expected
that the high energy densities, low power densities, and short life cycles of
batteries can be complemented by the high power densities, low energy densities, and long lives of supercapacitors if hybrid electrical energy storage
systems are developed. However, with extensive research and development
in new electrode materials and new hybrid technologies, the replacement
of batteries by hybrid supercapacitors is desired because of their longer life
cycles. For example, to improve the energy density of a double-layer supercapacitor, some hybrid strategies involving these supercapacitors and batteries
have been developed by introducing electrochemical reactive materials into
the carbon electrode layers. Further discussion is presented in Chapters 4
and 7.
2.9 Applications of Supercapacitors
Supercapacitors have several practical applications including:
Transportation — The most promising market for supercapacitors is
in the transportation industry. They can be used in automobiles by coupling them with other energy sources, particularly batteries. They can also
improve fuel efficiency by storing energy when an automobile is braking