Graphene-based supercapacitor offers energy density comparable to NiMH battery, but with rapid charge and discharge

Ragone plot of graphene supercapacitor. Credit: ACS, Liu et al. Click to enlarge.

Researchers from Nanotek Instruments and Angstron Materials have developed a graphene-based supercapacitor that exhibits a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C (all based on the total electrode weight), measured at a current density of 1 A/g. These values are comparable to those of NiMH batteries, the researchers note, but the new supercapacitor offers the ability to be charged or discharged within seconds or minutes. A paper on their work was published online in the ACS journal Nano Letters.

These are the highest energy density values ever reported with carbon electrodes without the pseudocapacitance contributions from a conducting polymer or metal oxide, the authors said, further stating that “We believe that this is truly a breakthrough in energy technology.”

The group, led by Bor Jang of Nanotek Instruments, reported in 2006 that graphene can be used as a supercapacitor electrode material. Despite a number of efforts to improve the specific capacitance of graphene-based electrodes, however, results fell sort of the theoretical capacitance of 550 F/g due to the high tendency for graphene sheets to re-stack together.

The team determined that the best strategy to achieve a high capacitance in such graphene-based electrodes is to use curved graphene sheets rather than flat sheets to prevent the sheets from sticking to one another face-to-face. The curved morphology enables the formation of mesopores accessible to and wettable by environmentally benign ionic liquids capable of operating at a voltage >4 V.

With the total electrode weight of a supercapacitor system being typically one-fourth to one-half of the total system weight, the system-level specific energy of graphene-based supercapacitors can exceed 21.4-42.8 Wh/kg, which is comparable to that of a modern nickel metal hydride battery used in a hybrid vehicle. This breakthrough energy storage device is made possible by the high intrinsic capacitance and the exceptionally high specific surface area that can be readily accessed and wetted by an ionic liquid electrolyte capable of operating at a high voltage.

—Liu et al.

Source: Green Car Congress