A supercapacitor is an electrochemical storage device that works like a large version of a common electrical capacitor. Supercapacitors are also known as ultracapacitors or electrochemical double-layer capacitors. Unlike batteries, supercapacitors store their energy in an electrostatic field rather than in chemical form.
Batteries are charged when they undergo an internal chemical reaction. They discharge, delivering the absorbed energy, when they reverse the chemical reaction. In contrast, when a supercapacitor is charged, there is no chemical reaction. Instead, the energy is stored as a charge or concentration of electrons on the surface of a material.
Supercapacitors are capable of very fast charges and discharges, and can typically be recharged hundreds of thousands of times, unlike conventional batteries which last for only a few hundred or thousand recharge cycles. But their power is available only for a very short duration, and their self-discharge rate is much higher than with batteries.
Common applications of supercapacitors include starting diesel trucks and railroad locomotives, and in electric/hybrid-electric vehicles for transient load leveling and capturing the energy used in braking. In power systems, they are most likely to be used as bridging power sources in uninterruptible power supplies, much like flywheels.