Ah, now you're getting more complicated.  I believe that ONE conductor is
the metal/semimetal mentioned (Al, carbon, etc)  The other is a liquid
dielectric (often not "very" liquid.)  The insulating layer is the oxide
film on aluminum (very thin), which explains the polarity sensitivity
(anodizing is done electrically in the first place, so I guess wrong polarity
would remove the film...)  I'm not sure what the insulator is in a carbon
based cap...

BillW