Electrolytic Capacitors

Electrolytic capacitors are capacitors in which one or both of the "plates" is a non-metallic conductive substance, an electrolyte. Electrolytes have lower conductivity than metals, so are only used in capacitors when metallic plate is not practical, such as when the dielectric surface is fragile or rough in shape or when ionic current is required to maintain the dielectric integrity. The dielectric material of electrolytic capacitors is produced from the anode metal itself in what is known as the forming (or anodizing process. During this process, current flows from the anode metal – which must be a valve metal such as aluminum, niobium, tantalum, titanium, or silicon – through a conductive bath of a special forming electrolyte to the bath cathode. The flow of current causes an insulating metal oxide to grow out of and into the surface of the anode. The thickness, structure and composition of this insulating layer determine its dielectric strength. The applied potential between the anode metal and the bath cathode must be above the oxide breakdown voltage before significant current will flow. As current flows, the breakdown strength (formed voltage) and oxide thickness increase. 

The electrolysis reaction was investigated by Michael Faraday in the 1700's. A relationship between the charge flow through the system and the amount of product (in this case, metallic oxide) was found to exist. Faraday noted the relationship between gram-equivalents of product and charge transfer for all ideal (stoichiometric) electrolysis reactions in what is now known as Faraday's Law. Departures from this relationship exist for the process of oxide formation on anode metals, since some oxide can be grown chemically and thermally to reduce the electrical energy requirements in the formation process, which can cost several dollars per kilogram of anode produced. Also there can be some undesirable side reactions during the formation process which do not contribute to oxide formation. In the formation process, brittle metallic oxide is grown upon the metal foil, which is usually rough in shape. The anode metal is therefore in intimate contact with one side of the oxide dielectric. The electrolyte is used to make contact between the other side of the oxide and the cathode plate.

The advantage of electrolytic capacitors is the high capacitance per unit volume and per unit cost. The high capacitance arises from the high dielectric constant, the high breakdown field strength, the rough surface, and the extremely small, uniform thickness of the anodically formed metallic oxide. The reason that electrolytic capacitors have such uniform dielectric stress and can operate at such high field strength, within 80% of their breakdown strength, on the order of 1,000 volts/µm, is due to two reasons. First, the original anodization ("formation") process is performed at a fixed voltage, and the dielectric grows everywhere to whatever thickness is required to support that voltage. Second, once the foil is in a capacitor, the capacitor "fill" electrolyte continues the healing work of the original forming electrolyte, repairing and thickening the dielectric locally as required. This healing process is driven by the capacitor's dc leakage current, which is drawn whenever a dc voltage is applied to the capacitor, that is, whenever it is in operation. In fact, electrolytic capacitors often last longer when they are in continuous, mild use that when they are only charged up briefly every year or decade.

The disadvantage of electrolytic capacitors is the non-ideal, lossy characteristics which arise from the semiconductive oxide properties, double-layer effects from the electrolyte-oxide charge-space region, resistive losses from the high electrolyte resistivity, frequency response rolloff due to the roughness of the surface oxide, and finite capacitor life due to breakdown and degradation of the electrolyte. Some of these considerations will be discussed below in more detail from the standpoint of the aluminum electrolytic capacitor.

Also, the anodic oxide dielectric is polar, and so are the electrolytic capacitors (in contrast with the classical, electrostatic capacitors), that is the capacitors must be connected with the correct polarity as marked. Connecting with reverse voltage injects hydrogen ions through the oxide readily, causing high electrical conduction, heating and reduction of the anodic oxide film. Non-polar (or bi-polar) devices can be made by using two anodes instead of an anode and a cathode, or one could connect the positives or negatives of two identical device together, then the other two terminals would form a non-polar device.

Most electrolytic capacitors are constructed using aluminum electrodes, but tantalum and niobium is also used. Aluminum anode is the least expensive at $0.04 per gram. As such, it is used in large (even greater than one liter!) and small (tiny surface-mount) capacitors. Tantalum anode material is over $2.00 per gram but offers high stability, more capacitance (four times that of aluminum), lower resistance (up to 90% lower) per size. It is available as small units (typically less than 5 cm3) and surface mount. Niobium anodic powder is less than $1.00 per gram, much cheaper and more available than tantalum but still much more expensive than aluminum. Capacitance is much more than aluminum, nearly that of tantalum. It is a much newer technology than tantalum.

H.O. Siegmund invented the electrolytic capacitor in 1921. Julius Lilienfeld did much to develop electrolytic theory in the 1920's and 1930's. Cornell Dubilier was at this time the world's largest capacitor company, and did much to develop the technology of etching and anodizing.