To get the highest possible capacitance we need to get the plates as close together as possible. Electrolytic capacitors use a special technique to achieve a plate spacing which is very small. They're made in two stages. First, fill the gap between the plates with an electrolyte.


Then ‘form’ the capacitor by applying a d.c. voltage. The electrolyte conducts, carrying charge between the plates. This stimulates a chemical reaction at one plate surface which forms a layer of insulating material. This layer is just a few molecules thick and, once it covers the surface, blocks any more charge flow. The result is a capacitor with an ultra-thin dielectric where one plate is actually the bulk of the electrolyte.

Electrolytic capacitors do have their disadvantages though...

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