Micro-Supercapacitors to Power Electronic Devices
April 29, 2010 MSE alumnus Dr. John Chmiola ‘09 has published his second paper as first author in Science on April 23, 2010 (www.sciencemag.org – J. Chmiola, C. Largeot, P.-L. Taberna, P. Simon, and Y. Gogotsi, “Monolithic carbide-derived carbon films for micro-supercapacitors”).
The paper describes a new method of producing supercapacitors that doubles their performance over similar devices reported. Supercapacitors, also called electric double layer capacitors or ultracapacitors, store energy through reversible ion adsorption at high surface area electrodes, usually made of carbon, in contrast with batteries, which store electrical energy in chemical bonds in a bulk material. This difference allows supercapacitors to charge and discharge faster, recharge a near infinite number of times, and operate at a wider temperature range with a high efficiency. Supercapacitors are built of environmentally friendly materials, such as carbon, aluminum and polymers.
Chmiola and co-authors, Celine Largeot, Pierre-Louis Taberna, Patrice Simon, all from the Universite Paul Sabatier in Toulouse, France; and Dr. Yury Gogotsi (MSE), Chmiola’s PhD advisor, use an electrode material called carbide-derived carbon (CDC), in which metal atoms are etched from a metal carbide, such as titanium carbide TiC, to form a porous carbon with very high surface area and pore sized that matches precisely the electrolyte ion size. This material has very high capacitance and outperforms activated carbons that are currently used in supercapacitors and carbon nanotubes or graphene that are being explored for supercapacitor electrode applications.
Previous studies by this group used CDC in powdered form. The innovation reported here is the use of “bulk” thin films. The team took some cues from the microelectronics industry, starting with conductive TiC substrates, then etching a very thin electroactive layer (Ti-CDC) to store charge.
“In the traditional sandwiched construction, the electroactive materials that store the charge are loosely held together particles pressed onto some metal that transports electrons to and away from these materials and separated by some other material that keeps the individual electrodes from shorting to one another,” Chmiola said. “The whole sandwich is then rolled up and put in a little soda can or plastic bag.”
By using microfabrication-type techniques, Chmiola and colleagues avoided many of the pitfalls of the “sandwich” method, such as poor contact between electroactive particles in the electrode; large void space between the particles, which contributes significantly to mass and volume because it is filled with electrolyte, but does not store charge; and poor contact with the materials that carry electrons out of the electroactive materials and to the external circuitry.
Their technique led to a volumetric capacity twice as high as other micro- and macroscale supercapacitors reported so far. The report opens the possibility of integrating high-performance micro-supercapacitors into a variety of electronic devices.
Chmiola is now a post doctoral researcher in the Environmental Energy Technologies Division at Lawrence Berkeley National Laboratory.