Nov 26, 2010
Graphene supercapacitor breaks storage record
Researchers in the US have made a graphene-based supercapacitor that can store as much energy as nickel metal hydride batteries but which can be charged or discharged in just seconds or minutes. The new device provides a specific energy density of 85.6 Wh/kg by electrode weight at room temperature and 136 Wh/kg at 80 °C. These are the highest ever energy values reported for "electric double layer" supercapacitors based on nano-carbon materials.
Capacitors are devices that store electric charge. Supercapacitors, more accurately known as electric double-layer capacitors or electrochemical capacitors, can store much more charge thanks to the double layer formed at an electrolyte-electrode interface when voltage is applied.
The new device made by Bor Jang of Nanotek Instruments Inc. in Ohio and colleagues has electrodes made of graphene mixed with 5wt% Super P (an acetylene black that acts as a conductive additive) and 10wt% PTFE binder. The researchers coat the resulting slurry onto the surface of a current collector and assemble coin-sized capacitors in a glove box. The electrolyte-electrode interface is made of "Celguard-3501" and the electrolyte is a chemical called EMIMBF4.
The energy density values of the supercapacitor are comparable to that of nickel metal hydride batteries. "This new technology makes for an energy storage device that stores nearly as much energy as in a battery but which can be recharged in seconds or minutes," Jang told nanotechweb.org. "We believe that this is truly a breakthrough in energy technology."
The fast charging feature means that the device might be used to recharge mobile phones, digital cameras and micro-EVs, he adds.
The team, which includes scientists from Angstron Materials Inc. in Dayton, Ohio, and Dalian University of Technology in China, are now working hard to further improve the energy density of the device. "Our goal is to make a supercapacitor that stores as much energy as the best lithium-ion batteries (for the same weight) but which can still be recharged in less than two minutes," said Jang.
His team first discovered that graphene could be used as a supercapacitor electrode material in 2006. Since then, scientists around the world have made great strides in improving the specific capacitance of these electrodes but the devices still fall short of the theoretical values of 550 F/g.
"Despite the theoretically high specific surface area of single-layer graphene (which can reach up to 2.675 m2/g), a supercapacitance of 550 F/g has not been reached in a real device because the graphene sheets tend to re-stack together," explained Jang. "We are trying to overcome this problem by developing a strategy that prevents the graphene sheets from sticking to each other face-to-face. This can be achieved if curved graphene sheets are used instead of flat ones."
The work was reported in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org.