17 August 2010—As we enter an era of ”smart” environments—buildings that know their structural weaknesses, military vests that detect toxic chemicals, rivers that monitor their own flood levels—engineers have turned their attention to a new class of energy-storage devices to power the sensors that will make our world smarter. Ultracapacitors, also called supercapacitors or electrochemical double-layer capacitors, can last for many more charge and discharge cycles and deliver energy faster than a chemical battery, although they store less energy.
Trying to make supercapacitors a bit more super, researchers in the United States and France have devised a snowflake-size device whose electrodes are coated with nanoscale carbon spheres. Thanks to its microscale electrodes and the unique onionlike layering of the carbon spheres, the device charges and discharges more quickly than a commercial ultracapacitor, the researchers report in the journal Nature Nanotechnology. Such a device might be ideal for powering wireless sensors that must transmit quick, powerful signals.
A typical ultracapacitor is made of two electrodes separated by a permeable insulator and filled with an ionic solution, or electrolyte. For decades, engineers have relied on activated carbon—think charcoal filled with holes—to coat the electrodes, making them porous and thus enabling the supercapacitor to hold a great deal more charge for its volume, although still not as much as a chemical battery. In recent years, new technologies have allowed materials engineers to ”nanosculpture the surface of materials to get a kind of behavior that nature doesn’t ordinarily do by itself,” says Joel Schindall, an electrical engineer at MIT who has created ultraporous ultracapacitors by coaxing carbon to grow in dense nanotube forests on the surface of their electrodes.
Instead of growing a forest, the researchers who designed the new ultracapacitor planted an onion patch. To create the carbon nano-onions, materials engineers at Drexel University, in Philadelphia, started with a shipment of nanosize diamonds produced from discarded military explosives. The decade-old technique of detonating the explosives in a closed chamber molds some of the carbon particles into tiny diamonds only 5 nanometers across—”about 10 million times smaller than an onion that you put in your salad,” says Yury Gogotsi, a materials engineer at Drexel, whose lab produced the nano-onions for the ultracapacitor project. ”Then the diamonds are just scraped off the wall.” Scraping, he clarifies, means washing the walls with water and filtering out the diamonds, which he and his colleagues then baked at 1800 C. The high heat ”transforms them into these carbon onions with, on average, five, six, seven…as many as 10 shells,” Gogotsi says.
Gogotsi and his colleagues then shipped the nano-onions to France, where systems engineer Magali Brunet and her colleagues at the University of Toulouse built the new miniaturized ultracapacitor on a silicon chip. They found that the device, although able to store less energy than an ultracapacitor made with activated carbon, could charge and discharge three orders of magnitude faster than an activated carbon ultracapacitor, at rates up to 200 volts per second.
The charge and discharge rate of an ultracapacitor is a measure of how powerful it is. The rate depends on how speedily the carbon coating on the electrodes can absorb and release ions in the electrolyte when a current passes through. Gogotsi says to think of the ion-absorption process like a bathing suit soaking up water. Activated carbon, with its intricate web of varying-size pores, behaves much like a slow-drying cotton suit; it takes a while for ions to migrate in and out. Onion-like spheres, on the other hand, act more like a modern synthetic suit, which wicks water to the surface, allowing it to evaporate much more quickly. ”Thanks to the fact that the material we used is created by layers like onions,” says Brunet, it’s easy for ions to get in and out of the electrodes by collecting on the outermost layer of the spheres. In addition, because the microsize electrodes on the device are positioned so closely, electrons can travel from one electrode to another very fast.
Despite the ability of the device to quickly store and release charge, the coating of carbon nano-onions on its electrodes provides less surface area on which ions can collect—520 square meters per gram compared to the 2000 m2 per gram offered by activated carbon. The result: more power, but less energy. An ultracapacitor with such characteristics might be useful to power a tiny heart-rate monitor that relays brief messages to a receiver, for instance. Or it could store small, quick bursts of energy harvested from the high-frequency vibrations of an airplane wing or from the sudden pressure of a footstep. An ultracapacitor made with activated carbon, on the other hand, works great for powering devices that need a lot of energy but can take a while to fire up, such as an LED light. But ”when you’re talking about pulses [of energy] happening on the millisecond scale, that’s when you’d want to have onion-like carbon,” Gogotsi says.
Schindall calls the carbon-onion approach an ”interesting idea with potential impact and importance.” But, he adds, it’s too early to tell which ultracapacitor designs will work best for future applications, particularly when scaled up to power larger devices.