Core-shell-structured MnO2@carbon spheres and nitrogen-doped activated carbon for asymmetric supercapacitors with enhanced energy density

Asymmetric supercapacitors have potential applications in renewable-energy technology owing to their remarkable electrochemical properties. A high-voltage asymmetric supercapacitor was developed based on a core-shell-structured MnO2@carbon sphere composite (MnO2@CS) as the cathode, nitrogen-doped activated carbon as the anode and a neutral aqueous Na2SO4 solution as the electrolyte. MnO2@CS was successfully fabricated by hydrothermally growing MnO2 on the surface of carbon spheres. A nitrogen-containing benzoxazine resin was adopted as a precursor to produce in situ nitrogen-doped activated carbon. Such an aqueous electrolyte-based asymmetric supercapacitor can be cycled reversibly in the high-voltage region of 0-1.9 V and exhibits a superior energy density of 8 Wh kg(-1) at an ultrahigh power density of 9627 W kg(-1) owing to the matching of MnO2@CS and porous nitrogen-doped activated carbon. Moreover, the asymmetric supercapacitor presents acceptable cycling performance, with 74.4% retention after 1000 cycles at 1 A g(-1), and a charge-discharge efficiency of the electrode of almost 100%.