An electrolytic capacitor has two electrodes, one with a dielectric, that relies on electrostatic charge storage. A battery has electrodes that store energy faradaically in electrochemical reactions involving the electrodes and electrolyte. A hybrid capacitor is the combination of two electrodes, one that stores charge electrostatically, and the other storing charge faradaically.
Batteries have a big advantage in energy density over capacitors because chemical reactions are very energetic and can involve the bulk rather than just the surface of the electrodes. But batteries have many disadvantages, all related to electrochemistry. These include limited cycle life, low specific power, and reduced performance at low temperature. Another battery disadvantage, low cell voltage, means cells often must be connected in series. Active balancing is needed to prevent damage to a cell from over-voltage in series strings of more than a few cells.
Capacitors have unlimited cycle life and their specific power is much higher compared to batteries because charge storage does not depend on chemical reactions. Only its internal resistance limits the power of a capacitor. Resistance in electrolytic capacitors is mainly a function of electrolyte resistivity, and that varies with temperature.
A tantalum hybrid capacitor (US patent 5,369,547) has a negative electrode (cathode) comprised of ruthenium oxide which stores charge in a reversible reaction involving protons, RuO2 + 2H+ +2e↔-Ru(OH)2. The positive electrode (anode) is a porous tantalum pellet coated with tantalum oxide that serves as a dielectric and stores charge electrostatically. In contact with both electrodes is the highly conductive electrolyte, sulfuric acid. Porous separators between the two electrodes provide electrical isolation. The capacitor is hermetically sealed in a metal case. The capacitor is asymmetric and correct polarity must be observed.
Tantalum hybrid capacitors are available with voltage ratings up to 125 V. Most of the voltage drop in the cell occurs at the tantalum oxide dielectric, a thin insulating layer which thickness is proportional to the rated voltage. Since the hybrid capacitor is a series combination of two electrodes, the total capacitance is found by the familiar formula, 1/C = 1/Cc + 1/Ca. In the case of a hybrid capacitor, Cc >> Ca, so C approaches Ca. Because the electrodes are in series, the charge, Q on each is identical and follows the relationship, Q = CaVa = CcVc. The minimum cathode capacitance required to keep its voltage below the breakdown potential of the electrolyte is thus Cc = CaVa/Vc where Vc is set to 0.3V.
Though comparable in electrical performance to an aluminum electrolytic capacitor, the tantalum hybrid capacitor has several times higher energy density. The plot in Figure 2 shows room temperature energy versus power for a hybrid capacitor and an electrolytic capacitor of similar physical size. The devices have similar frequency response, topping out at a self-resonance frequency of a few kHz.
Hybrid capacitors can operate over the temperature range of –55° to 125°C. A high-temperature version, with a limit of 200°C is now available. The capacitance and resistance vary with temperature according to the properties of the electrolyte. The most pronounced difference is at low temperature where the resistance can be several times higher than at room temperature.
Tantalum capacitors are much more expensive to make than aluminum electrolytic capacitors. Accordingly, they tend to be specified wherever performance trumps cost. Hybrid capacitors are used extensively in high reliability applications where the high cost is justified by volume or weight considerations such as in airborne electronics. Here, the costs of additional space or weight over the operational life of an aircraft are a primary concern. The capacitors are used extensively by the military for power supplies in laser and high-powered radar.
Other applications take advantage of the long life and high reliability of hybrid capacitors. There are hybrid capacitors in daily use aboard the ISS and also in power supplies on the three retired space shuttles. Hybrid capacitors with an extended temperature range target applications in oil and gas exploration where temperatures at the bottom of a well can exceed 200°C. These capacitors must be rugged enough to survive the extreme shock and vibration environment near the operating drill point, and they must withstand the vapor pressure of the electrolyte without rupture or excessive swelling.
Hybrid capacitors combining a faradaic with an electrostatic electrode have a combination of characteristics that improve performance. Equipment designers face challenges to produce ever-higher performance electronics to meet the expectations of their customers. Hybrid capacitors are a key element because they have higher energy density and higher specific power than any electrolytic capacitor.