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 m²g^-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.