Supercapacitors can capture and release energy more quickly than batteries, but they usually store less energy. Most supercapacitors in use today use carbon-based electrodes, because their high-surface area stores more charge.
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 6nm wide, each made of five or fewer layers of graphene-like carbon.
The carbon was then doped with nitrogen atoms which 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 around three times without reducing its ability to quickly charge and discharge. "It is as if we have broken the sound barrier," said Fuqiang Huang, a material chemist at the Shanghai Institute of Ceramics.
The scientists say that their devices could store 41 watt-hours per kilogram (W·h/kg), comparable to lead-acid batteries.
"A bus can run on an 8W·h/kg supercapacitor for 5km, then recharge for 30s at the depot to run the trip again,” explained I-Wei Chen, a materials physicist at the University of Pennsylvania. “This works in a small city or an airport, but there is obviously a lot to be desired. Our battery has five times the energy, so it can run 25km and still charge at the same speed."
The new supercapacitor does not store as much energy as lithium-ion batteries, which achieve 70 to 250W·h/kg. However, the researchers say their supercapacitor beats them on power. The nitrogen supercapacitor is claimed to produce 26W·h/kg, while lithium-ion batteries are capable of 0.2 to 1W·h/kg.
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.
