Aluminum capacitors are a type of electrolytic capacitor that uses aluminum as its conducting surfaces. These capacitors were discovered as early as 1886, when Karol Pollack discovered that a very high capacitance could be generated between aluminum and the solution in which it was placed. The capacitor actually functioned because of the oxidation layer on the aluminum.
In a modern aluminum capacitor, the anode is made out of very pure aluminum. The dielectric is actually the oxide on the surface of the aluminum. There is paper between the two plates in the capacitor, containing an electrolytic fluid, that allows contact with the cathode.