23 September 2010—The ultracapacitor—the battery’s quicker cousin—just got faster and may one day help make portable electronics a lot smaller and lighter, according to a group of researchers. John Miller, president of the electrochemical capacitor company JME, in Shaker Heights, Ohio, and his team reported the new ultracapacitor design this week in Science.
Ultracapacitors don’t store quite as much charge as batteries but can charge and discharge in seconds rather than the minutes batteries take. This combination of speed and energy supply makes them attractive for things like regenerative braking, where the ultracapacitors would have only seconds to recharge as a car comes to a stop. But sometimes a second is still too long: Using nanometer-scale fins of graphene, the researchers built an ultracapacitor that can charge in less than a millisecond. This agility, its designers say, means that the devices could replace the ubiquitous bulky capacitors that smooth out the ripples in power supplies to free up precious space in gadgets and computers.
The ultracapacitor’s secret weapon is its surface area: While batteries store charge chemically, capacitors store it electrostatically—in electric fields formed between conducting surfaces. The larger the surface area on these conducting surfaces, the more room there is for charge. Ultracapacitors achieve this by using tiny nanometer-scale pores, such as those found in activated carbon, and boost how much charge each pore can hold by filling them with an ionic solution.
For decades, the goal has been to increase the total amount of charge an ultracapacitor can store while retaining its small size. More energy storage means that the capacitors can work quickly in applications that demand more energy than traditional capacitors, and deliver that energy faster than batteries can.
The price of hoarding charge—cramming it into hard-to-access nanotube tangles and activated carbon pores—is that some of the nimbleness needed to do the things ordinary capacitors can do has been sacrificed, says Miller. "Many people are trying to make these more battery-like," he says. "What we’ve done is make them more capacitor-like." Miller and his team got some of that nimbleness back by redesigning the ultracapacitor’s electrodes.
One team member, Ron Outlaw, a material scientist at the College of William and Mary, in Williamsburg, Va., came up with an electrode consisting of up to 4 sheets of graphene—a one-atom-thick form of carbon with unusual electronic properties. The graphene was formed so that it stuck out vertically from a 10-nanometer-thick graphite base layer—what Miller describes as rows of 600-nm-tall "potato chips" standing on edge. It is much easier to get charge on and off chip surfaces, he says, than it is to get it off what he calls the "stacked potato chips" of earlier graphene ultracapacitors or off the "Swiss cheese–like" surface of activated carbon ultracapacitors. The earlier designs led to discharge "traffic jams."
Miller’s team, which also included Brian Holloway, a program manager at the Defense Advanced Research Projects Agency (DARPA), tested its graphene ultracapacitor in a filtering circuit, part of an AC rectifier. Many rectifiers leave a slight AC echo behind, called a "voltage ripple," and it’s the capacitor’s job to smooth it out. In order to do that, the capacitor needs to respond well at double the AC frequency—120 hertz in the United States.
Most commercial ultracapacitors fail at this filtering role at around 0.01 Hz, but when Miller’s team tested its ultracapacitor in such a 120-Hz filtering circuit, it did the job. That means the smaller ultracapacitors could replace the big electrolytic capacitors that do the filtering now. Miller estimates that a commercial version, operating at 2.5 volts, could be less that one-sixth the size of any other 120-Hz filtering technology.
"I think the work is exciting and marks an important advance that also looks reasonably likely to lead to improved methods for electrical energy storage while preserving good AC filtering performance," says Rod Ruoff, a professor of mechanical engineering at the University of Texas at Austin and cofounder of Graphene Energy, which is also developing graphene ultracapacitors.
Outlaw says this is only a first effort. He is already increasing the capacitance of the device by making the nanosheets more parallel and taller—attempting to find the ideal balance between creating more charge storage space and restricting the flow of ions in the electrolyte. Still, he says this original work is already a major advance. "We are approaching an order of magnitude reduction in size and weight, which means great benefits to the electronics in many industries such as NASA, airlines, and the military," he says.