High‐Voltage and Low‐Temperature Aqueous Supercapacitor Enabled by “Water‐in‐Imidazolium Chloride” Electrolytes

Abstract 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/H 2 O with 0.1 m 4hT) or by pre‐inserting ClO 4 − 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).