Even so, the charge storage ability of ordinary electrolytics is not nearly large enough to purify water. Patents to this technology date from the nineteen fifties or earlier, with Donald Boos of Carborundum Company prominent among the early inventors. This technology was subsequently licensed in Japan, where it was manufactured profitably for use in computer memory circuits and other applications that require a large amount of charge storage. More recently, large double layer capacitors have attracted interest in electric vehicle design. The electric double layer capacitor is made from alternating electrodes of activated carbon in a salt solution.
Activated carbon is conductive and has enormous surface area.
When a voltage is applied to this system, enormous capacitance results. The dielectric is a minutely thin layer of water that exists between the conductive surface and the absorbed ions As long as the charging voltage does not exceed the so called Nernst potential for electrochemistry to occur, ions in the salt solution absorb electrostatically to the charged surface. Some carbons provide as much as 150 Farads capacitance per gram. That is a big number. In contrast, film type capacitors are typically measured in pico Farads.