Electrochemical capacitor technology for actuator applications

The development of a class of capacitors which use the interface between large surface area materials such as activated carbon to store charge is discussed. The key parameters of interest for the capacitor double layer (CDL) as an energy storage media are the value of the relative dielectric constant associated with the region in which the charge is stored, the thickness of the layer and the voltage at which the layer fails. These parameters are functions of the electrolyte species, and its concentration. This has led to the development of capacitors with capacitance of the order of 20-50 microfarads per real square centimeter of surface area. A second electrochemical capacitor of potential use in electrical actuation is based upon pseudocapacitance. Materials such as ruthenium oxide are capable of absorbing charge in the form of hydrogen ions (protons) into the surface in a reversible way. Both technologies allow capacitors to be built with a capacitance of several farads in a volume on the order of a “dime”. For pulsed electrical actuation, capacitor or a battery capacitor combination would appear to be particularly attractive. Electrochemical capacitor technology is a step in the right direction. This paper describes the near-term and the current state-of-the-art in CDL and pseudocapacitor technology for application to devices such as electric propulsion and electric actuation in terms of energy density, power density, availability and cost. Specific applications to NASA and Navy needs are presented