Symmetric aqueous high voltage supercapacitors up to 3 V have been demonstrated using concentrated aqueous 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), namely, "water-in-imidazolium chloride", as working electrolytes, and graphene nanoplatelets-coated carbon paper as electrodes. Performance enhancement was further achieved either through adding redox species such as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (4hT) into the electrolytes (110 Wh kg-1 for a 20 m [BMIm]Cl/H2 O with 0.1 m 4hT) or by pre-inserting ClO4 - anions into the graphene platelets. Moreover, the newly studied aqueous electrolytes allow low-temperature operation at -20 °C and even at -32 °C, retaining competitive energy storage capability (maximum energy densities of 36 and 21 Wh kg-1 , respectively).
Keywords: electrolytes; energy storage; graphene; ionic liquids; supercapacitors.
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