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 (251m(2)g(-1)) and mesopores (4.5nm 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 306Fg(-1), good rate capability, and an excellent cycling stability of 95.2% after 5000 cycles at a high current density of 8Ag(-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 (105.5Fg(-1) at 0.5Ag(-1) and 78.5Fg(-1) at 10Ag(-1)) with excellent rate capability, achieves a maximum energy density of 43.9Whkg(-1) at a power density of 408Wkg(-1), and has high stability, whereby the ASC retains 81.4% of its initial capacitance at a current density of 5Ag(-1) after 4000 cycles. Therefore, the NHCSs@MnO2 electrode material is a promising candidate for future energy-storage systems.