Researchers from Nanotek Instruments have developed a new graphene-based supercapacitor that can store as much energy as NiMH batteries, but charge and discharge in minutes or even seconds. The new device has a specific energy density of 85.6 Wh/kg at room temperature and 136 Wh/kg at 80 °C. These are the highest ever values for "electric double layer" supercapacitors based on carbon nanomaterials.

http://www.graphene-info.com/new-graphene-based-supercapacitor-can-store-much-energy-nimh-battery