Traditional Capacitors

Dielectric materials vary by cost and the capacitance needed for a specific application. Glass, ceramic, and mica papers are high quality, low capacitance dielectrics with extremely high breakdown resistance. Conversely, metal-ized polymer foils such as polystyrene, polyethylene terephthalate (PET), and Teflon (PTFE) are single-piece dielectric films that offer better capacitor performance. In recent years, polymer foils have come to dominate the static capacitor market because they have better stability at high temperatures, can be manufactured at lower cost, and age better than dielectric papers. 

Electrolyte materials require high ion mobility to provide ions to the double-layer quickly. Improved electrolyte performance also requires optimization of operating voltages, toxicity, corrosion, and safety. Separators must be electronically insulating to prevent short circuits between the two electrode layers, and allow high ionic mobility from the electrolyte to the electrode surface. The separator choices in ES design include microporous or non-woven polymers, glass, and cellulose derivatives. The most common ones are polymer separators that include polyolefins such as polypropylene (PP), polyethylene (PE), Teflon, PVdF, and PVC. 

The chosen polymer and production method vary based on the electrolyte used. 

Sealants must be non-conductive, prevent ion leakage between stacked cells, and resist corrosion and degradation. Sealant use depends on cell type sealants are commonly made of low melting temperature PE material or viscous fast setting polymers such as epoxies. Sealants do not directly impact performance they help control safety and moisture. A seal that fails can lead to short circuits within assembled cells. 

Overall, careful selection and matching of ES materials can minimize resistances, avoid short circuits, reduce safety issues, support high ion mobility, increase operating voltage, and enhance the charge storage capacity of future ES devices [3]. 

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EDLCs store energy within the variation of potential at the electrode/electrolyte interface. This variation of potential at a surface (or interface) is known as the electric double layer or, more traditionally, the Helmholtz layer. The thickness of the double layer depends on the size of the ions and the concentration of the electrolyte. For concentrated electrolytes, the thickness is on the order of 10 A, while the double layer is 1000 A for dilute electrolytes (5). In essence, this double layer is a nanoscale model of a traditional capacitor where ions of opposite charges are stored by electrostatic attraction between charged ions and the electrode surface. EDLCs use high surface area materials as the electrode and therefore can store much more charge (higher capacitance) compared to traditional capacitors. 

The terms supercapacitor and ultracapacitor are used to describe any double layer or redox capacitor with specific energy and specific power intermediate to batteries and traditional capacitors. Typically, ultracapacitor refers to a device comprised of two carbonaceous electrodes whereas supercapacitor refers to a similar device in which the two carbonaceous electrodes are catalyzed with metal oxides such as Ru02. This chapter will use the term supercapacitor to describe EAP-based capacitors, since that seems to be the most commonly used term for such materials. Another charge storage configuration uses an EAP electrode and a battery-type carbonaceous electrode in what is known as a hybrid device (however, outside of the EAP-based supercapacitor field, hybrid may refer to the combination of a battery electrode such as nickel hydroxide with a carbon electrode) [1]. 

In a traditional capacitor, energy is stored in the electric field between two oppositely charged plates separated by an insulator with a high dielectric constant. When we apply a potential, energy is stored on the opposite plates. In this type of capacitor capacitance is directly proportional to the surface area of each of the electrodes and is inversely proportional to the distance between the two electrodes as shown in the following equation ... 

Super-capacitors, which are capable of storing up to a hundred times more energy than traditional capacitors, act like batteries but without the handicap of equivalent series resistance (ESR). They are able to deliver high pulses of power with charge-up and discharge times which can be measured in seconds. Operating voltages tend to be between two and three volts. It is therefore necessary to... 

Metals used in traditional capacitors utilize highly conductive metals as electrode materials that can achieve very high power however, ESs commonly use carbons as the active electrode components. Not all carbon is sufficiently conductive to support high power operation. Graphitic planes are highly conductive but temperatures used in the activation processes are limited to prevent complete restructuring into nonporous graphite. 

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