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Re: supercaps

I don't claim to know any of the details as to how supercaps work, but I
thought that I would point out that the evidence that Ludwik K. presented for
his 1 F supercap having a nonlinear q vs. V relationship does not necessarily
indicate that a chemical reaction is going on. It is possible that the effect
is some kind of elasticity effect in the dielectric. As the capacitor charges
a huge electric field is developed in the dielectric which causes the charged
electrodes to attract each other electrostatically. This squeezes the
dielectric making it progressively thinner as the capacitor is charged. Thus
as the capacitor is charged its capacitance is increasing due to the thinner
electrode separation. Even without any chemical reactions going on the
energy stored in the capacitor is in three different forms: 1) electrostatic
energy of the E field between the electrodes, 2) the polarization energy in
the dielectric which microscopically and elastically separates bound charges
in the molecular dipoles, and 3) elastic mechanical energy from mechanically
compressing the dielectric as a whole. The first two energy reserviors lead
to a linear Q vs. V relationship, but the third one leads to a nonlinear one
which causes the dQ/dV slope to increase as V (and Q) increase(s) due to the
finite positive value of the dielectric's elastic modulus.

Short of actually asking the manufacturer how their supercaps work (or taking
one apart and doing a careful forensic autopsy on it using a battery of
analytical chemistry techniques) I'm not sure how to definitively find out
how they work. However, if I had such a black box capacitor and needed to
find out more about its operating principles, I would check into any
hysteresis effects that the capacitor may display. Differences in the charge/
discharge cycles would be expected to by time dependent if chemistry is
involved. Presumably it takes time for chemical reactions to occur in a
solid, and if the capacitor can be cycled on a comparable time scale the
capacitor would have to behave differently than at slower cycling frequencies
since the reactions wouldn't be able to keep up.

David Bowman