Device Construction & Distinguishing Traits:
Electric double layer capacitors (ELDCs) and supercapacitors are a group of electrolytic-like devices characterized by extremely high capacitance per volume and low voltage ratings, typically no more than a few volts. Construction types and operating principles among these devices differ and are topics of ongoing R&D efforts, but common themes found among them are the use of electrode materials that offer extremely high surface area per volume (such as activated carbon, aerogels, etc.) and an absence of a conventional solid dielectric. In place of conventional ceramic, polymer, or metal oxide dielectrics as found in other capacitor types, ELDCs, supercapacitors, and similar devices by other names rely on various electrochemical, electrostatic, and charge transfer effects that provide extremely small charge separation distances; the distance by which the "plates" of the capacitor are separated is commonly measured in fractions of a nanometer.
For practical purposes, ELDCs, supercaps, and similar devices of a different name can be regarded as a sort of middle ground between traditional capacitors and secondary (rechargeable) cells. They have energy storage densities that are higher than traditional capacitors but lower than electrochemical cells, ESR values that are high by capacitor standards, but low by electrochemical cell standards, and a nearly-indefinite cycle life compared to chemical cells' cycle lives of only a few hundred to a few thousand cycles. As with electrochemical cells, several ELDCs can be integrated into a single package to yield a composite device with higher nominal voltage.
The combination of high ESR and poor linearity characteristics relative to other capacitor types renders ELDCs and supercaps unsuitable for most signal and high frequency (>kHz) applications, but they are quite useful for energy storage on human-scale time frames. Within this realm, there is a continuum of devices aimed intended for different applications. Smaller devices may have ESR values as high as a few hundred ohms, and are intended for applications such as memory and real-time clock backup supplies with uA-level current requirements. At the other end are devices with fractional-milliohm ESRs, intended for use in applications with currents into the hundreds of amps such as regenerative braking systems for vehicles.
Range of available capacitances & voltages: