In one aspect, the present disclosure relates to an improved method of preparing concentrated MnO2 ink with increased efficiency and cost effectiveness. The method involves mixing KMnO4 solution with highly crystalline carbon particles (HCCPs) with average diameters less than 800 nm at 30-60° C. for at least 8 hours. The present disclosure further relates to a symmetric supercapacitor device comprising MnO2 coated electrodes and a solid state ionic liquid as electrolyte, as well as an interdigital transparent SC (IT-SC) device comprising aqueous MnO2 ink.