Development of redox deposition of birnessite-type MnO2 on activated carbon as high-performance electrode for hybrid supercapacitors

Birnessite-type MnO2/activated carbon nanocomposites have been synthesized by directly reducing KMnO4 with activated carbon in an aqueous solution. It is found that the morphologies of MnO2 grown on activated carbon can be tailored by varying the reaction ratio of activated carbon and KMnO4. An asymmetric supercapacitor with high energy density was fabricated by using MnO2/activated carbon (MnO2/AC) nanocomposite as positive electrode and activated carbon as negative electrode in 1 M Na2SO4 aqueous electrolyte. The asymmetric supercapacitor can be cycled reversibly in the cell voltage of 0–2 V, and delivers a specific capacitance of 50.6 F g−1 and a maximum energy density of 28.1 Wh kg−1 (based on the total mass of active electrode materials of 9.4 mg), which is much higher than that of MnO2/AC symmetric supercapacitor (9.7 Wh kg−1).