Rice University researchers have created a solid-state, nanotube-based supercapacitor that combines the best qualities of high-energy batteries and fast-charging capacitors in a device suitable for extreme environments. Potential uses span on-chip nanocircuitry to entire power plants.
Standard capacitors that regulate flow or supply quick bursts of power can be discharged and recharged hundreds of thousands of times. Electric double-layer capacitors (EDLCs), or supercapacitors, are hybrids that hold hundreds of times more energy than a standard capacitor, like a battery, while retaining their fast charge/discharge capabilities.
Traditional EDLCs rely on liquid or gel-like electrolytes that can break down in very hot or cold conditions. In Rice's supercapacitor, a solid, nanoscale coat of oxide dielectric material replaces electrolytes entirely.
For the new device, the Rice team grew an array of 15-20 nanometer bundles of single-walled carbon nanotubes up to 50 microns long. Robert Hauge, a distinguished faculty fellow in chemistry, led the effort with former Rice graduate students Cary Pint, now a researcher at Intel, and Nolan Nicholas, now a researcher at Matric.
The array was then transferred to a copper electrode with thin layers of gold and titanium to aid adhesion and electrical stability. The nanotube bundles (the primary electrodes) were doped with sulfuric acid to enhance their conductive properties; then they were covered with thin coats of aluminum oxide (the dielectric layer) and aluminum-doped zinc oxide (the counterelectrode) through a process called atomic layer deposition (ALD). A top electrode of silver paint completed the circuit.
"Essentially, you get this metal/insulator/metal structure," said Pint. "No one's ever done this with such a high-aspect-ratio material and utilizing a process like ALD."
Hauge said the new supercapacitor is stable and scaleable. "All solid-state solutions to energy storage will be intimately integrated into many future devices, including flexible displays, bio-implants, many types of sensors and all electronic applications that benefit from fast charge and discharge rates," he said.
Pint said the supercapacitor holds a charge under high-frequency cycling and can be naturally integrated into materials. He envisioned an electric car body that is a battery, or a microrobot with an onboard, nontoxic power supply that can be injected for therapeutic purposes into a patient's bloodstream.