Friday, August 18, 2006
When it comes to powering laptops and hybrid cars, batteries get most of the attention. But these gadgets and myriad others also contain devices known as capacitors that provide quick bursts of energy. Capacitors can't store as much power as batteries, but the latest "supercapacitors" have started to close the gap. Now, their storage capabilities may be about to take another big jump.
In a report published online this week by Science (www.sciencemag.org/cgi/content/abstract/1132195), researchers from the United States and France report that by carefully controlling the nanoscale structure of a carbon-based supercapacitor, they've managed to increase the amount of electrical charges it can hold by about 50%. "It looks like they've got something significant there," says John Miller, a physicist who runs JME Inc., a supercapacitor materials evaluation company in Shaker Heights, Ohio. If this performance translates to commercial devices, it could help manufacturers create smaller and cheaper power packs for everything from cameras to cars, Miller says. First, however, researchers need to learn more about how it works.
Typically, a capacitor contains a pair of electrodes surrounded by an electrolyte. When a voltage is applied between the electrodes, oppositely charged ions in the electrolyte snuggle up to each electrode and remain there even when the applied voltage is turned off. When the two electrodes are connected by a wire, electrons flow from the negative electrode to balance the charges in the positive electrode and do work en route.
For many years, carbon has been the electrode material of choice for supercapacitors because it conducts electricity, is light, and can be formed into a meshlike structure that sops up ions like a sponge. The smaller the pores in the material, the larger its surface area and the more charge the capacitor can hold--at least up to a point. When ions move through an electrolyte, other molecules attracted to their charge normally encircle them like groupies mobbing a rock star. Researchers have long thought that if the pores in a carbon supercapacitor got too small--below about 1 billionth of a meter, or nanometer--the ion would not be able to squeeze through with its entourage, and thus the material's overall ability to store charge would drop. But because they had no way to carefully control the pore size throughout a large capacitor, they couldn't test this notion.
Figure 1 On demand. New supercapacitors store less charge than batteries but can supply it more quickly, making them ideal for hybrid cars.
CREDITS: (PHOTO) AP/ROB WIDDIS
Yury Gogotsi and his colleagues at Drexel University in Philadelphia, Pennsylvania, however, came up with a new way to do just that. They started with one of several commercially available compounds called a metal carbide, a mixture of a metal such as titanium and carbon. They then heated their material in a furnace while exposing it to chlorine gas. The gas reacted with the metal, forming volatile compounds that could easily be separated from the mixture, leaving behind carbon shot through with a continuous mesh of voids. By controlling the temperature and other conditions in their reactor, the researchers found they could tailor the holes in their carbon mesh to be a uniform size, between 0.6 and 2.25 nanometers across.
When Gogotsi and his students measured the charge-storing capabilities of the material, they got a shock. "We thought it would be useless" to study the smallest pores, Gogotsi says. But in powdered samples, their carbon with the 0.6-nanometer pores held 50% more charge than powders of standard supercapacitors. Gogotsi's group later teamed up with Patrice Simon, a leading supercapacitor expert at the University of Paul Sabatier in Toulouse, France, whose lab confirmed the results.
On a molecular level, it appeared that ions must be wiggling into the tiny pores, by either squeezing their entourage ions or perhaps abandoning them altogether. But how that could happen remains a puzzle, Miller says. In normal carbon supercapacitors, ions nestling up to an electrode form a layer about 1 nanometer thick. So if there is less space than that in the pores of the new material, it's not clear how they can get in. "That will be a bit controversial," Miller says. But both he and Gogotsi point out that thanks to the newfound control over pore size, researchers should quickly be able to figure out just what is going on.