Paramecium-like alpha-MnO2 hierarchical hollow structures with enhanced electrochemical capacitance prepared by a facile dopamine carbon-source assisted shell-swelling etching method

Enhancement of the electrochemical performances of electrode materials is a great challenge in the electrochemical capacitor field. Herein, a novel paramecium-like alpha-MnO2 hierarchical hollow structure used as an electrode material has been successfully prepared, which possesses an ultra-high specific surface area (285 m(2) g(-1)), uniform crystal orientation, and high specific capacitance (554.3 F g(-1) at 1 A g(-1)) and exhibits 97.9% capacitance retention after 5000 cycles. The earlier carbonaceous coating transformed from dopamine plays an important role in forming this novel paramecium-like morphology. The shells are constructed of ultra-thin alpha-MnO2 nanoflakes with highly uniform and preferentially exposed [001] crystal orientation. The high specific surface area increases the electrolyte-electrode contact area and the unique orientated structure effectively facilitates ion transportation. Quasi in situ electron energy-loss spectroscopy (EELS) analysis for both hierarchical hollow structures and commercial products at different charge-discharge stages indicates that Mn in our products can be almost completely restored to its original oxidation valence state due to a complete redox reaction, while there is still a fraction of Mn in the commercial product that cannot be restored, which causes the reduction of specific capacitance and retention. All the results suggest that this novel alpha-MnO2 hierarchical hollow structure with selective growth orientation might become an attractive type of electrode material.