The latest news in energy storage has Drexel University with a new piece out that notes in a paper published in Science by John Chmiola doubling supercapacitors storage and then MIT Technology Review marking it up to triple. It’s a little amusing, yet Chmiola is on to something.
Chmiola idea is to 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. Chmiola and his colleagues had experience with CDC in powdered form so 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 the electron charge. Thus a new microfabrication-type technique.
The genius innovation here is in connecting up technologies in the use of “bulk” thin films. Chmiola explains, “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. The whole sandwich is then rolled up and put in a little soda can or plastic bag.” That’s just what most everyone else has been working on.
Chmiola and his 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.
The team uses a high-vacuum method called chemical vapor deposition to create thin films of metal carbides such as titanium carbide on the surface of a silicon wafer. The films are then chlorinated to remove the titanium, leaving behind a porous film of carbon. In each place where a titanium atom was, a small pore is left behind.
Chmiola’s advisor group leader is Yury Gogotsi, professor of materials science and engineering at Drexel University explains the film is like a molecular sponge, where the size of each pore is equal to the size of a single ion. This matching means that when used as the charge-storage material in an ultracapacitor, the carbon films can accumulate a large amount of total surface charge.
The Drexel researchers complete the device by adding metal electrodes to either surface to carry current into and out of the device and adding a liquid electrolyte to carry and dispense the charges. They found that the performance of the device is best when the carbon material is about 50 micrometers thick, about the same as the width of a human hair.
Thin film deposition solves some major issues. Conventional ultracapacitors are made from powdered activated carbon. But powders can’t be used to make large, thin films because they won’t stick to a surface. Other groups have developed printable thin-film ultracapacitors based on carbon nanotubes, a technology with lots of potential too.
Gogotsi says the Chmiola team’s devices can store more charge. Gogotsi notes that in theory there is no limit to the size of the films that could be made using these methods that are used by the solar industry and display industries to make panels as large as nine square meters. Because the carbon films are thin and can be made at temperatures as low as 200º C, it might be possible to integrate them with flexible plastic based electronics.
Even if the Drexel team’s work isn’t double or triple the current stage of ultracapacitor capacity they have solved the difficulty getting high enough total energy storage using practical fabrication methods using films.
This is important – Eestor seems to be fading, and the ultracapacitor field has trouble with for applications that require steady power over a long period, such as running a laptop or a motor.
A well-built ultracapacitor has a virtually unlimited lifetime, capacitors can live longer than any electronic device and some designs never need to be replaced. A steady long time period drain with near instant charge is electron charge storage nirvana. If they are cheap to make, and can match or better batteries in volume and weight the electron issue as energy storage, transport and use is over. It will be interesting to see how this develops.
The full Drexel team includes John Chmiola, Celine Largeot, Pierre-Louis Taberna, Patrice Simon, Yury Gogotsi for the Science paper entitled Monolithic Carbide-Derived Carbon Films for Micro-Supercapacitors.
Chmiola is on his way; he received a National Science Foundation IGERT and Graduate Research Fellowships for his Ph.D. studies, and is now a postdoctoral researcher in the Environmental Energy Technologies Division at Lawrence Berkeley National Laboratory. The effort is his second paper in Science magazine.
Now that this innovative design is out what others can do will be fascinating. The materials have already been licensed by Pennsylvania startup Y-Carbon.