|High surface area of graphene make supercapcitors possible|
With the high surface area of their electrodes and an extremely narrow gap between the electrodes, supercapacitors, also known as electric double-layer capacitors or electrochemical capacitors, can store a large amount of electrical charge in a tiny volume. The newly developed device has electrodes made graphene mixed with an acetylene black called Super P that acts as a conductive additive and a binder that holds it all together. The resulting slurry is coated onto the surface of a current collector and assembled in coin-sized capacitors. The electrolyte-electrode interface is made of "Celguard-3501" and the electrolyte is a chemical called EMIMBF4.
Specific energy density of the new capacitor (a measure of how much electricity can be stored per weight) has been measured at 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 degrees Celsius (176 F), which is comparable to Ni-mh batteries. These are the best values for electric double layer supercapacitors based on carbon nanomaterials recorded to date.
"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 said. "We believe that this is truly a breakthrough in energy technology."
The team, which includes scientists from Angstron Materials in the U.S. and Dalian University of Technology in China, is now trying 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," Jang said. "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. 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."