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Solid-State Supercapacitor Created with Carbon Nanotubes

Carbon nanotube illustration (National Science Foundation)

Carbon nanotube illustration (National Science Foundation)

Researchers at Rice University in Houston have developed a supercapacitor that can store large quantities of energy and charge quickly, and in a solid-state design made possible by the use of carbon nanotubes. Their findings appear online in the journal Carbon (paid subscription required).

Capacitors are devices that regulate flow or supply quick bursts of power, and can be discharged and recharged hundreds of thousands of times. Supercapacitors, also known as electric double-layer capacitors (EDLCs), can be quickly discharged and recharged as well, but hold hundreds of times more energy than a standard capacitor, like a battery.

Up to now, however, EDLCs needed liquid or gel-like electrolytes that are unstable, and can break down in very hot or cold conditions. The Rice team, from the lab of chemistry professor Robert Hauge, developed a supercapacitor with a solid, nanoscale coat of oxide dielectric material that replaces electrolytes entirely.

Rice graduate students in Hauge’s lab — Cary Pint and Nolan Nicholas (now both in industry) — had to overcome a key obstacle in creating a solid-state supercapacitor: finding enough room to give electrons more surface area to inhabit. They found the solution in carbon nanotubes, thin tubes of carbon atoms with unusual properties, including higher strength and greater electrical conductivity.

Nanotubes self-assemble into dense, aligned bundles, with each bundle of nanotubes 500 times longer than it is wide. A tiny chip may contain hundreds of thousands of these bundles. For the supercapacitor, the Rice chemists grew a single-walled carbon nanotube array of 15-20 nanometer bundles; 1 nanometer equals 1 billionth of a meter.

The team installed the nanotube array on a copper electrode with thin layers of gold and titanium. The conductivity of the nanotubes was then enhanced with sulfuric acid, and coated with aluminum oxide for the the dielectric layer, and aluminum-zinc oxide for the counterelectrode. The developers added a top electrode of silver paint to complete the circuit.

Hauge says the new supercapacitor is stable and scaleable. “All solid-state solutions to energy storage will be intimately integrated into many future devices,” notes Hauge, “including flexible displays, bio-implants, many types of sensors and all electronic applications that benefit from fast charge and discharge rates.”

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