TABLE OF CONTENTS Supercapacitors (also termed electrochemical capacitors or ultracapacitors) store energy in the electric field of an electrochemical double layer. While their specific power is much higher than in batteries, their specific energy is substantially lower. As principal energy storage systems, these devices are being developed for power assist during acceleration and hill climbing, as well as for the recovery of braking energy [182], [124], [89]. Another possible application is in "mild" hybrids together with an integrated starter/alternator, as a low-voltage (42V) energy buffer that is also capable of high power recuperation [113], [200]. Supercapacitors are also potentially useful as secondary energy storage systems in HEVs, providing load-leveling power to electrochemical batteries which may be downsized [145]. Another advantage in this case would be the additional degree of freedom they add to the vehicle energy management, which allows for an optimization of the operating conditions of the main energy storage system.
A supercapacitor differs from conventional capacitors both in the materials of which it is made and in the physical processes involved. In a supercapacitor, the dielectric is an ion-conducting electrolyte interposed between conducting electrodes. The energy is stored by the charge separation taking place in the layers that separate the electrolyte and the electrodes. Since the voltage that can be applied is limited to a few volts by the physical characteristics of the electrolyte, the storage capacity is increased by raising the capacitance, i.e., increasing the surface and decreasing the thickness of the...
