Symmetric aqueoush igh voltage supercapacitors up to 3V have been demonstrated using concentrated aqueous 1-butyl-3-methylimidazolium chloride ([BMIm]Cl), namely," water-inimidazolium chloride", as working electrolytes, andg raphene nanoplatelets-coated carbon paper as electrodes. Performance enhancement was furthera chieved either through adding redox speciess uch as 4-hydroxy-2,2,6,6-tetramethylpiperidin-1oxyl (4hT) into the electrolytes (110 Wh kg À1 for a2 0m [BMIm]Cl/H 2 Ow ith 0.1 m 4hT) or by pre-inserting ClO 4 À anions into the graphene platelets. Moreover,t he newly studied aqueous electrolytes allow low-temperature operation at À20 8C and even at À32 8C, retaining competitive energy storage capability (maximum energy densities of 36 and 21 Wh kg À1 ,r espectively).With ultrafastc harge-discharge rates through building up electricald ouble layers on porous electrodes with at ypical timescale of seconds, supercapacitors can generally realize high power densities of about 10 kW kg À1 and energy densities of about 5Whkg À1 . [1,2] They exhibit excellent cyclingr eversibility,w ithoutc ausingi nherent structural changes in electrodes and electrolytes. Thus,s upercapacitors are promising for rapid response in peak shavinga nd grid stabilization. [3,4] To facilitate their practical application,i ssues of low energy density,s afety, and high cost need to be solved. Besides the selection and optimization of electrode materials towards high availablesurface areas and porosities, [5][6][7][8][9][10] great efforts have been made to tailor the electrolyte formulation including the selection of types of solvents and electrolyte ions. [11][12][13] The amount of stored energy in supercapacitors can be increased by increasing the ion-accessible surfacea rea of the electrode materials. Another way to increase the energy density of supercapacitors is to increase the operating potential range of the system, as energy density is proportionaltot he square of the voltage.Organics olvents and room-temperature ionic liquidsa re commonlyu sed as workingm edia for high operating-voltage (> 2V)s upercapacitors. [14] However,t hey typically have poor ion conductivities, from about af ew mS cm À1 for ionic liquids to tens of mS cm À1 for conventionalo rganic electrolytes at room temperature. [15] By adding molecular solvents, such as water or acetonitrile, into ionic liquids, [16][17][18] the viscosity can be greatly reduced and the ion conductivity can be accordingly enhanced with as acrifice in the electrochemical stability window of the electrolytes. Water as an atural alternative to the flammable organic solvents or highly viscous room-temperaturei onic liquidsi si deal for use as aw orking medium for electrochemical devices with the merits of intrinsic safety and fast transport for ionic species. Aqueouse lectrolytes for supercapacitors couldenabletheir assembly and operation under atmosphericc onditions. However,i th as been al ongstanding challenge that water has an arrow electrochemical stability window (a sum of the thermo...