Electrolytic Capacitor - Electrolyte

Electrolyte

The electrolyte is usually boric acid or sodium borate in aqueous solution, together with various sugars or ethylene glycol which are added to retard evaporation. Getting a suitable balance between chemical stability and low internal electrical resistance is not a simple matter; in fact, the exact compositions of high-performance electrolytes are closely guarded trade secrets. It took many years of painstaking research before reliable devices were developed. The electrolytic solvent has to have high dielectric constant, high dielectric strength, and low resistivity; a solute of ionic conductivity facilitators is mixed within.

Electrolytes may be toxic or corrosive. Working with the electrolyte requires safe working practice and appropriate protective equipment such as gloves and safety glasses. Some very old tantalum electrolytics, often called "Wet-slug", contain corrosive sulfuric acid; however, most of these are no longer in service due to corrosion.

There are three major types of water-based electrolytes for aluminium electrolytic capacitors: standard water-based (with 40-70% water), and those containing ethylene glycol or dipropyl ketone (both with less than 25% water). The water content helps lowering the equivalent series resistance, but can make the capacitor prone to generating gas, especially if the electrolyte formulation is faulty; this is a leading cause of capacitor plague, to which the high water content electrolytes are more susceptible. The lower voltage ratings (thinner oxide layer) and lower operating voltage (slower regeneration of oxide layer) are further aggravating factors.

There are a number of non-aqueous electrolytes, which use only a small amount of water. The electrolytes are generally composed of a weak acid, a salt of weak acid, and a solvent, and optional thickening agent and other additives. The electrolyte is usually soaked into an electrode separator. The weak acids are usually organic acid (glacial acetic acid, lactic acid, propionic acid, butyric acid, crotonic acid, acrylic acid, phenol, cresol, etc.) or boric acid. The salts employed are often ammonium or metal salts of organic acids (ammonium acetate, ammonium citrate, aluminium acetate, calcium lactate, ammonium oxalate, etc.) or weak inorganic acids (sodium perborate, trisodium phosphate, etc.). Solvent-based electrolytes may be based on alkanolamines (monoethanolamine, diethanolamine, triethanolamine,...) or polyols (diethylene glycol, glycerol, etc.).