A polymer capacitor is a type of electrolytic capacitor with a solid ionic conductor rather than a liquid electrolyte. Capacitors are electric storage devices usually composed of two metallic conductors. Electrolytic capacitors replace one or both conductors with an electrolyte solution. In a polymer capacitor, the electrolyte is paper impregnated with organic semiconductor crystals instead of a liquid. The solid electrolyte enables a faster discharge rate and better response to current fluctuations.

The use of an electrolyte as a conductor allows electrolytic capacitors to hold a greater charge in a device of smaller volume. This makes them particularly suited for use in power supply filters, storing the charge needed to mitigate fluctuations in output. Using a solid electrolyte in the polymer capacitor enables a further reduction in size while increasing performance and durability.

In theory, a perfect capacitor would have an equivalent series resistance (ESR) rating of zero. There would be no electrical resistance from any of its components and none of the associated heat build up. A polymer capacitor typically has a lower ESR rating that remains stable over a greater temperature range than other electrolytic designs. Lower ESR ratings enable a faster response to greater transitions in current

Power supplies that have an alternating current (AC) input often experience a periodic fluctuation in direct current (DC) output called ripple. Some power supply systems, such as those used for computer motherboards, subject capacitors to a great deal of stress from ripple current. Polymer capacitors are far more resistant to this stress and have a longer operational life.

The metal conductor of a polymer capacitor is usually made from thin aluminum foil. An aluminum oxide dielectric is layered onto the foil by an electrochemical process called anodization. The foil with the dielectric layer forms the anode, or path of entry, for an electrical current. An electrolyte impregnated separator sheet and an uninsulated length of foil make up the cathode, or path exit, for an electrical current.

The effective surface area of the foil elements in an aluminum polymer capacitor is increased by etching before the aluminum oxide dielectric is generated. Paper impregnated with an organic semiconductor or a conductive polymer separates the anode and cathode foils. The layered elements are rolled into a coil, equipped with pin connectors and sealed in an aluminum sleeve.

Polymer capacitors are much more expensive than wet electrolytics. They are generally produced in small, low-voltage versions for use in devices such as cell phones, graphics cards and high-end server motherboards. Their specialized use makes finding polymer capacitors more difficult as relatively few consumer-oriented distributors carry them in stock.