MnO2 nanoflakes/hierarchical porous carbon nanocomposites for high-performance supercapacitor electrodes

Abstract

A facile strategy is developed for the synthesis of MnO2 nanoflakes/hierarchical porous carbon spheres (HPCs) nanocomposites via a two-step redox process. The external MnO2 nanoflakes with thickness of ∼10 nm deposited on the surface of the HPCs result in the formation of hierarchical architecture of the composites, while the internal MnO2 layer stabilizes the interaction between MnO2 nanoflakes and HPCs. The resultant composites still retain porous structure after removal of mesoporous SiO2 template and exhibit relatively high specific surface area. The morphology control of the composites can be easily achieved by varying the initial content of Mn(NO3)2 and KMnO4. Electrochemical performance of the composites as supercapacitor electrode materials was evaluated by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy techniques. The MnO2 nanoflakes/HPCs composite with 75 wt% MnO2 possesses the highest specific capacitance at a high scan rate or current density (417.2 F g-1 at 20 mV s-1 and 326.9 F g-1 at 1 A g-1, respectively) and extraordinary cycling stability (slightly over 100% capacitance retention after 10000 cycles at a scan rate of 100 mV s-1), which are superior to other reported MnO2/carbon composites. The results suggest that rational design and synthesis of MnO2/porous carbon composite electrode materials with maximum electrochemical active sites is important to further improve their electrochemical performance.