Aluminum electrolytic capacitors are polarized electrolytic capacitors whose anode electrode (+) is made of a pure aluminum foil with an etched surface. Because of their anodization principle, they can only be operated with DC voltage applied with the correct polarity. Operating the capacitor with wrong polarity or with AC voltage leads to a short circuit and can destroy the component. The exceptions is the bipolar aluminum electrolytic capacitor, which has a back-to-back configuration of two anodes in one case and can be used in AC applications.
The aluminum forms a vary thin insulating layer of aluminium oxide by anodization that acts as the dielectric of the capacitor. Aluminum electrolytic capacitors with non-solid electrolyte are the most inexpensive type and also those with widest range of sizes, capacitance and voltage values. The liquid electrolyte provides oxygen for re-forming or self-hearling of the dielectric oxide layer. However, it can evaporate through a temperature-dependent drying-out process, which causes electrical parameters to drift, limiting the service life time of the capacitors.