In the 1890s Charles Pollak found that an oxide layer on an aluminum anode was stable in a neutral or alkaline solution and was granted a patent in 1897 for a borax electrolyte aluminum capacitor.

More information: Zhen Pan et al, A High-Concentration Edge-Nitrogen-Doped Porous Carbon Anode via Template Free Strategy for High-Performance Potassium-Ion Hybrid Capacitors, Energy Material ...