Rice University researchers led by Dr. Robert Hauge have created a solid-state, nanotube-based supercapacitor that can be deeply integrated into the manufacture of devices and that promises to combine the best qualities of high-energy batteries and fast-charging capacitors in a device suitable for extreme environments. A paper on their work is being published in the journal Carbon.
Traditional EDLCs (supercapacitors) rely on liquid or gel-like electrolytes that can break down in extreme conditions. In Rice’ supercapacitor, a solid nanoscale coat of oxide dielectric material replaces electrolytes entirely.
|“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.”|
|—Dr. Robert Hauge|
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. Hauge led the effort with former Rice graduate students Cary Pint, first author of the paper and now a researcher at Intel, and Nolan Nicholas, now a researcher at Matric.
A method developed at Rice University allows bundles of vertically aligned single-wall carbon nanotubes to be transferred intact to a conductive substrate. Metallic layers added via atomic layer deposition create a solid-state supercapacitor that can stand up to extreme environments.
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 (Al2O3, the dielectric layer) and aluminum-doped zinc oxide (Al-ZnO, 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. No one’s ever done this with such a high-aspect-ratio material and utilizing a process like ALD.—Cary Pint
Hauge said the new supercapacitor is stable and scalable. 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.
Experimental results yield values in agreement with those obtained through capacitive modeling using Al2O3 dielectric coatings (C > 20 mF/cm3), and the solidstate dielectric architecture enables the operation of these devices at substantially higher frequencies than conventional electrolyte-based capacitor designs. Furthermore, modeling of supercapacitor architectures utilizing other dielectric layers suggests the ability to achieve energy densities above 10 Wh/Kg while still exhibiting power densities comparable to conventional solid-state capacitor devices.
This device design efficiently converts the high surface area available in the conductive VA-SWNT electrode to space for energy storage while boasting a robust solidstate material framework that is versatile for use in a range of conditions not practical with current energy storage technology.—Pint et al.
Co-authors of the paper include former graduate student Nicholas; graduate student Zhengzong Sun; James Tour, the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science; and Howard Schmidt, adjunct assistant professor of chemical and biomolecular engineering, all of Rice. Other co-authors were Sheng Xu, a former graduate student at Harvard, and Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry at Harvard University, who developed the ALD process.
The research was supported by T.J. Wainerdi and Quantum Wired, in coordination with the Houston Area Research Council; the Office of Naval Research MURI program; the Wright Patterson Air Force Laboratory and the National Science Foundation.
Pint, C.L., Nicholas, N.W., Xu, S., Sun, Z., Tour, J.M., Schmidt, H.K., Gordon, R.G., Hauge, R.H. (2011) Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates, Carbon doi: 10.1016/j.carbon.2011.07.011