MnO2 Nanosheets Grown on Nitrogen-Doped Hollow Carbon Shells as a High-Performance Electrode for Asymmetric Supercapacitors.

A hierarchical hollow hybrid composite, namely, MnO2 nanosheets grown on nitrogen-doped hollow carbon shells (NHCSs@MnO2 ), was synthesized by a facile in situ growth process followed by calcination. The composite has a high surface area (251 m(2) g(-1) ) and mesopores (4.5 nm in diameter), which can efficiently facilitate transport during electrochemical cycling. Owing to the synergistic effect of NHCSs and MnO2 , the composite shows a high specific capacitance of 306 F g(-1) , good rate capability, and an excellent cycling stability of 95.2 % after 5000 cycles at a high current density of 8 A g(-1) . More importantly, an asymmetric supercapacitor (ASC) assembled by using NHCSs@MnO2 and activated carbon as the positive and negative electrodes exhibits high specific capacitance (105.5 F g(-1) at 0.5 A g(-1) and 78.5 F g(-1) at 10 A g(-1) ) with excellent rate capability, achieves a maximum energy density of 43.9 Wh kg(-1) at a power density of 408 W kg(-1) , and has high stability, whereby the ASC retains 81.4 % of its initial capacitance at a current density of 5 A g(-1) after 4000 cycles. Therefore, the NHCSs@MnO2 electrode material is a promising candidate for future energy-storage systems.