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Electric double layer, Supercaps:

Device Construction & Distinguishing Traits:
Electric double layer capacitors (EDLCs) 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, EDLCs, 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, EDLCs, 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 EDLCs 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 EDLCs 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 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:

The chart below illustrates the voltage and capacitance ratings of EDLCs & supercapacitors in stock at Digi-Key at the time of writing. Note that the vertical scale has units of Farads, in contrast to units of microfarads found in similar charts.

Common Failure Mechanisms/ Critical design considerations:
Variations in technology among devices under the EDLC/supercap umbrella preclude detailed discussion of failure mechanisms and critical design considerations for the group as a whole. From an applications perspective however, it is sufficient to note that the concerns applicable to aluminum electrolytic capacitors transfer more or less directly to EDLCs and supercaps:
-They contain a liquid electrolyte solution that is subject to evaporation, and the Arrhenius rule of thumb predicting a halving of device longevity for each 10°C temperature increase holds. It should be noted that temperature ratings for many EDLCs/supercaps are relatively low and that self-heating effects can become significant in applications involving prolonged charge cycling. Also, many board-mounted devices will not tolerate reflow soldering processes, and may require special care during assembly as a result.
-They should not be operated above their rated voltage. So doing will cause failure through electrolyte loss and/or dielectric breakdown. This is particularly pertinent in the case of devices incorporating organic electrolytes, since materials released during a failure can prove to be quite toxic, as Dave has verified empirically. (After discussing the experiment with Ralph via the porcelain phone, Dave fortunately does remain among the living…)
-They exhibit significant dielectric absorption and changes in device characteristics as a function of temperature. Additionally, leakage current is often quite high in EDLCs/supercaps, particularly in compound devices composed of series-connected capacitors. Frequently, such devices require some form of circuitry to balance the voltage applied to each in order to avoid an overvoltage condition on any given cell caused by capacity or leakage current imbalances.
-Capacitors exhibit a linear relationship between state of charge and output voltage per the Q=C*V equation.  This differs from electrochemical cells which generally have a broad, more or less flat plateau in output voltage as a function of their state of charge. In many/most applications, this means that some form of power management circuitry will be needed to make full use of an EDLC/supercap's full capacity.

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