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Electrochemical Supercapacitors for Energy Storage and Delivery
1.3.1.3 Electrolytic Dielectrics and Their Capacitors
The use of electrolytes as dielectrics was initially developed by Charles
Pollak in 1886 as a result of his investigations of the anodization of metals.
However, electrolytic capacitors have faced numerous difficulties in applications primarily due to their low reliability that hindered their use despite
several patents filed for alternative designs. During World War II, the low
reliability of electrolyte capacitors was improved and their dependability
was increased in numerous applications. Key processes leading to their success were the etching and pre-anodizing of the metal foils prior to assembly. Etching can increase a surface 100 times the area of an unetched metal
foil and yields a larger capacitance. However, this kind of capacitor needs to
use less corrosive electrolyte solutions; otherwise the electrode will corrode,
causing performance deterioration under discharge conditions [6].
Electrolytic dielectrics include metal oxides, aqueous-based liquid electrolytes, and non-aqueous-based liquid electrolytes. The most commonly used
metal oxides are aluminum oxide, tantalum pentoxide, and niobium oxide.
A typical aqueous-based liquid electrolyte contains boric acid or sodium
borate in aqueous solution with various sugars or ethylene glycol to retard
evaporation. For example, there are three major types of water-based electrolytes for aluminum electrolytic capacitors: standard water-based (with 40%
to 70% water), those containing ethylene glycol (with less than 25% water),
and dipropyl ketone (with less than 25% water).
A typical non-aqueous-based liquid electrolyte is generally composed of a
weak acid, a salt derived from a weak acid, a solvent, an optional thickening
agent, and other additives. The electrolyte is usually soaked into an electrode
separator that serves as the dielectric. Weak acids are organic and include
glacial acetic acid, lactic acid, propionic acid, butyric acid, crotonic acid,
acrylic acid, phenol, and cresol. The salts are ammonium or metal salts of
organic acids, including ammonium acetate, ammonium citrate, aluminum
acetate, calcium lactate, and ammonium oxalate; or weak inorganic acids
such as sodium perborate and trisodium phosphate. Electrolyte solvents are
based on alkanolamines (monoethanolamine, diethanolamine, and triethanolamine) or polyols (diethylene glycol, and glycerol).
In practice, the two major types of electrolytic capacitors are (1) aluminum
electrolytic capacitors and (2) tantalum electrolytic capacitors. Aluminum
electrolytic capacitors are fabricated from two conducting aluminum foils,
one of which is coated with an insulating oxide layer and a paper spacer
soaked in electrolyte. The foil insulated by the oxide layer is the anode while
the liquid electrolyte and the second foil act as the cathode. Tantalum electrolytic capacitors are subdivided into wet and dry types based on whether their
counter electrodes are served by sulfuric acid or a manganese dioxide film.
Dry tantalum electrolytic capacitors possess a greater capacitance-to-volume
ratio relative to aluminum counterparts and are utilized in computer and
Electrochemical Supercapacitors for Energy Storage and Delivery
1.3.1.3 Electrolytic Dielectrics and Their Capacitors
The use of electrolytes as dielectrics was initially developed by Charles
Pollak in 1886 as a result of his investigations of the anodization of metals.
However, electrolytic capacitors have faced numerous difficulties in applications primarily due to their low reliability that hindered their use despite
several patents filed for alternative designs. During World War II, the low
reliability of electrolyte capacitors was improved and their dependability
was increased in numerous applications. Key processes leading to their success were the etching and pre-anodizing of the metal foils prior to assembly. Etching can increase a surface 100 times the area of an unetched metal
foil and yields a larger capacitance. However, this kind of capacitor needs to
use less corrosive electrolyte solutions; otherwise the electrode will corrode,
causing performance deterioration under discharge conditions [6].
Electrolytic dielectrics include metal oxides, aqueous-based liquid electrolytes, and non-aqueous-based liquid electrolytes. The most commonly used
metal oxides are aluminum oxide, tantalum pentoxide, and niobium oxide.
A typical aqueous-based liquid electrolyte contains boric acid or sodium
borate in aqueous solution with various sugars or ethylene glycol to retard
evaporation. For example, there are three major types of water-based electrolytes for aluminum electrolytic capacitors: standard water-based (with 40%
to 70% water), those containing ethylene glycol (with less than 25% water),
and dipropyl ketone (with less than 25% water).
A typical non-aqueous-based liquid electrolyte is generally composed of a
weak acid, a salt derived from a weak acid, a solvent, an optional thickening
agent, and other additives. The electrolyte is usually soaked into an electrode
separator that serves as the dielectric. Weak acids are organic and include
glacial acetic acid, lactic acid, propionic acid, butyric acid, crotonic acid,
acrylic acid, phenol, and cresol. The salts are ammonium or metal salts of
organic acids, including ammonium acetate, ammonium citrate, aluminum
acetate, calcium lactate, and ammonium oxalate; or weak inorganic acids
such as sodium perborate and trisodium phosphate. Electrolyte solvents are
based on alkanolamines (monoethanolamine, diethanolamine, and triethanolamine) or polyols (diethylene glycol, and glycerol).
In practice, the two major types of electrolytic capacitors are (1) aluminum
electrolytic capacitors and (2) tantalum electrolytic capacitors. Aluminum
electrolytic capacitors are fabricated from two conducting aluminum foils,
one of which is coated with an insulating oxide layer and a paper spacer
soaked in electrolyte. The foil insulated by the oxide layer is the anode while
the liquid electrolyte and the second foil act as the cathode. Tantalum electrolytic capacitors are subdivided into wet and dry types based on whether their
counter electrodes are served by sulfuric acid or a manganese dioxide film.
Dry tantalum electrolytic capacitors possess a greater capacitance-to-volume
ratio relative to aluminum counterparts and are utilized in computer and