One embodiment of the current disclosure provides an ultracapacitor including at least one graphene-based electrode, an electrolyte containing an ionic liquid, and a dielectric separator dividing the ultracapacitor into two chambers, each chamber containing an electrode and a portion of the electrolyte. In another embodiment, the graphene has been expanded by exposure to microwave radiation then chemically activated. Another embodiment of the current disclosure provides an electrochemical energy storage device containing such an ultracapacitor. Still other embodiments of the disclosure relate to ionic liquids, some of which may be suitable for use in an ultracapacitor, methods of synthesizing such liquids, and methods of designing such liquids. Further embodiments relate to methods of using ultracapacitors, for example in automobiles, power grids, high-temperature applications, and other applications.
STATEMENT OF GOVERNMENT INTEREST
 At least portions of this invention were made using U.S. government funding provided by the National Science Foundation under grant number DMR-0907324 and by the Department of Energy under grant number ER46657. The U.S. government has certain rights in the invention.