Assembling carbon quantum dots to a layered carbon for high-density supercapacitor electrodes

It is found that carbon quantum dots (CQDs) self-assemble to a layer structure at ice crystals-water interface with freeze-drying. Such layers interconnect with each other, forming a free-standing CQD assembly, which has an interlayer distance of about 0.366 nm, due to the existence of curved carbon rings other than hexagons in the assembly. CQDs are fabricated by rupturing C60 by KOH activation with a production yield of similar to 15 wt.%. The CQDs obtained have an average height of 1.14 nm and an average lateral size of 7.48 nm, and are highly soluble in water. By packaging annealed CQD assembly to high density (1.23 g cm(-3)) electrodes in supercapacitors, a high volumetric capacitance of 157.4 F cm(-3) and a high areal capacitance of 0.66 F cm(-2) (normalized to the loading area of electrodes) are demonstrated in 6 M KOH aqueous electrolyte with a good rate capability.