Electric Double-layer Capacitor - Materials

Materials

In general, EDLCs improve storage density through the use of a nanoporous material, typically activated charcoal, in place of the conventional insulating barrier. Activated charcoal is an extremely porous, "spongy" form of carbon with an extraordinarily high specific surface area — a common approximation is that 1 gram (a pencil-eraser-sized amount) has a surface area of roughly 250 m2 — about the size of a tennis court. It is typically a powder made up of extremely fine but very "rough" particles, which, in bulk, form a low-density heap with many holes. As the surface area of even a thin layer of such a material is many times greater than a traditional material like aluminum, many more charge carriers (ions or radicals from the electrolyte) can be stored in a given volume. As carbon is not a good insulator (vs. the excellent insulators used in conventional devices), in general EDLCs are limited to low potentials on the order of 2–3 V, and thus must be "stacked" (connected in series), just as conventional battery cells must be, to supply higher voltages.

Activated charcoal is not the "perfect" material for this application. The charge carriers are actually (in effect) quite large—especially when surrounded by molecules—and are often larger than the holes left in the charcoal, which are too small to accept them, limiting the storage.

As of 2010 virtually all commercial supercapacitors use powdered activated carbon made from coconut shells. Higher performance devices are available, at a significant cost increase, based on synthetic carbon precursors that are activated with potassium hydroxide (KOH).

Research in EDLCs focuses on improved materials that offer higher usable surface areas.

• Graphene has excellent surface area per unit of gravimetric or volumetric densities, is highly conductive and can now be produced in various labs, but is not available in production quantities. Specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current density of 1 A/g have been observed. These energy density values are comparable to that of the Nickel metal hydride battery. The device makes full utilization of the highest intrinsic surface capacitance and specific surface area of single-layer graphene by preparing curved graphene sheets that do not restack face-to-face. The curved shape enables the formation of mesopores accessible to and wettable by environmentally benign ionic liquids capable of operating at a voltage >4 V.
• Carbon nanotubes have excellent nanoporosity properties, allowing tiny spaces for the polymer to sit in the tube and act as a dielectric. Carbon nanotubes can store about the same charge as charcoal (which is almost pure carbon) per unit surface area but nanotubes can be arranged in a more regular pattern that exposes greater suitable surface area. The addition of carbon nanotubes in capacitors can greatly improve and enhance the performance of electric double-layer capacitors. Due to the high surface area and high conductivity of single-wall carbon nanotubes, the addition of these nanotubes allows optimization for these capacitors. Multi-walled carbon nanotubes have a presence of mesopores that allow for easy access of ions at the electrode/electrolyte interface. The thin walls of a carbon nanotube allow for high capacitance in an electric double-layer capacitor. By adding multi-walled nanotubes to these capacitors, the resistance of the electrodes can be decreased. The capacitor cells with multi-walled nanotube fibers had higher electron and electrolyte-ion conductivities as compared to cells that did not have these nanotubes. These nanotubes also improved the power capabilities of the capacitors.

• Some polymers (e.g. polyacenes and conducting polymers) have a redox (reduction-oxidation) storage mechanism along with a high surface area.

• Carbon aerogel provides extremely high surface area gravimetric densities of about 400–1000 m²/g. The electrodes of aerogel supercapacitors are a composite material usually made of non-woven paper made from carbon fibers and coated with organic aerogel, which then undergoes pyrolysis. The carbon fibers provide structural integrity and the aerogel provides the required large surface area. Small aerogel supercapacitors are being used as backup electricity storage in microelectronics. Aerogel capacitors can only work at a few volts; higher voltages ionize the carbon and damage the capacitor. Carbon aerogel capacitors have achieved 325 J/g (90 W·h/kg) energy density and 20 W/g power density.

• Solid activated carbon, also termed consolidated amorphous carbon (CAC). It can have a surface area exceeding 2800 m2/g and may be cheaper to produce than aerogel carbon.

• Tunable nanoporous carbon exhibits systematic pore size control. H₂ adsorption treatment can be used to increase the energy density by as much as 75% over what was commercially available as of 2005.

• Mineral-based carbon is a nonactivated carbon, synthesised from metal or metalloid carbides, e.g. SiC, TiC, Al₄C₃. The synthesised nanostructured porous carbon, often called Carbide Derived Carbon (CDC), has a surface area of about 400 m²/g to 2000 m²/g with a specific capacitance of up to 100 F/mL (in organic electrolyte). As of 2006 this material was used in a supercapacitor with a volume of 135 mL and 200 g weight having 1.6 kF capacitance. The energy density is more than 47 kJ/L at 2.85 V and power density of over 20 W/g.

• In August 2007 researchers combined a biodegradable paper battery with aligned carbon nanotubes, designed to function as both a lithium-ion battery and a supercapacitor (called bacitor). The device employed an ionic liquid, essentially a liquid salt, as the electrolyte. The paper sheets can be rolled, twisted, folded, or cut with no loss of integrity or efficiency, or stacked, like ordinary paper (or a voltaic pile), to boost total output. They can be made in a variety of sizes, from postage stamp to broadsheet. Their light weight and low cost make them attractive for portable electronics, aircraft, automobiles, and toys (such as model aircraft), while their ability to use electrolytes in blood make them potentially useful for medical devices such as pacemakers.

• Other teams are experimenting with custom materials made of activated polypyrrole, and nanotube-impregnated papers.