High-Voltage and Low-Temperature Aqueous Supercapacitor Enabled by Water-in-Imidazolium Chloride Electrolytes

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/H2O 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 degrees C and even at -32 degrees C, retaining competitive energy storage capability (maximum energy densities of 36 and 21 Wh kg(-1), respectively).