Nitrogen can triple the energy storage capacity of carbon-based supercapacitors, researchers in China and the United States say, potentially helping make them competitive against some advanced batteries.
Supercapacitorscan capture and release energy much more quickly than batteries, but they usually can store less energy. Most supercapacitors in use today use carbon-based electrodes, because their high-surface area stores more charge. "We are able to make carbon a much better supercapacitor," says Fuqiang Huang, a material chemist at the Shanghai Institute of Ceramics.
The scientists began with a framework of porous silica and lined the pores with carbon. They next etched away the silica, leaving porous tubes 4 to 6 nanometers wide, each made of five or less layers of graphene-like carbon.
They then doped the carbon with nitrogen atoms. The nitrogen altered the otherwise inert carbon, helping chemical reactions occur within the supercapacitor without affecting its electric conductivity.
These changes enhanced the capacitor's ability to store energy by roughly threefold without reducing its ability to quickly charge and discharge. "It is as if we have broken the sound barrier," Huang says.
The scientist say that their devices could store 41 watt-hours per kilogram, comparable to lead-acid batteries.
"A bus can run on an 8 watt-hours per kilogram supercapacitor for 5 kilometers, then recharge for 30 seconds at the depot to run on the trip again,” says I-Wei Chen, a materials physicist at the University of Pennsylvania who also worked on the breakthrough. “This works in a small city or an airport, but there is obviously a lot to be desired," he says. "Our battery has five times the energy, so it can run 25 kilometers and still charge at the same speed. We are then talking about serious applications in a serious way in transportation."
The new supercapacitor does not store as much energy as lithium-ion batteries, which achieve 70 to 250 watt-hours per kilogram. However, the researchers say their supercapacitor beats them on power. The nitrogen supercapacitor can crank out 26 kilowatts per kilogram, while lithium-ion batteries are only capable of 0.2 to 1 kilowatts per kilogram.
The scientists are now investigating ways to create these supercapacitors in a scalable, robust, and inexpensive manner, Huang says. They are also experimenting with a variety of electrolytes to further improve the energy and power of these devices.
They detailed their findings this week in in the journal Science.
Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.