In lots of situations, the ideal energy storage device is not a battery, which stores lots of energy but can’t deliver it particularly quickly. Nor is it a supercapacitor, which has limited storage but delivers what it’s got quickly. Instead it would be something that could do both. Scientists in China and the United States recently took a big step toward that ideal component when they showed that nitrogen can triple the energy storage capacity of carbon-based supercapacitors, potentially making them competitive with some batteries in terms of the amount of energy stored.
Most supercapacitors in use today rely on carbon-based electrodes because their large 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.
Huang and his colleagues began with a framework of porous silica and lined the pores with carbon. They next etched away the silica, leaving porous tubes just 4 to 6 nanometers wide, each made of up to five 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 reactions enhanced the capacitor’s ability to store energy roughly threefold without reducing its ability to quickly charge and discharge. Their devices could store 41 watt-hours per kilogram—comparable to what lead-acid batteries can store.
“It is as if we have broken the sound barrier,” Huang says.
Other experts agree. “These results are a leap ahead in supercapacitor energy density,” says physicist Nunzio Motta at Queensland University of Technology, in Australia, who did not participate in this study.
I-Wei Chen, a materials physicist at the University of Pennsylvania, in Philadelphia, who also worked on the breakthrough, put it in perspective by theorizing what the device could do in electric transportation. “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 the trip again,” he says. “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 25 kilometers and still charge at the same speed. We are then talking about serious applications in a serious way in transportation.”
Materials chemist James Tour at Rice University, in Houston, who did not take part in this research, suggested that such advanced supercapacitors could also be useful for wearable devices. “Devices such as watches, glasses, and electronic skin do not favor big, heavy batteries, so a light, efficient supercapacitor could be a good alternative,” he says.
The new supercapacitor does not store as much energy as lithium-ion batteries, which achieve 70 to 250 Wh/kg. However, the researchers say this supercapacitor beats them on power, cranking out 26 kilowatts per kilogram, compared with lithium-ion batteries’ 0.2 to 1 kW/kg.
Huang and his colleagues are now investigating ways to create these supercapacitors in a scalable, robust, and inexpensive manner, he says. They are also experimenting with a variety of electrolytes to further improve these devices.
Future research could also aim to improve the porosity of the supercapacitors, and thus the amount of charge they can store, Queensland’s Motta says. “However, a further increase in the porosity might lead to a very fragile material,” he says, “and this could be a stumbling block.”