Polypyrrole-coated FeO nanotubes constructed from nanoneedles as high-performance anodes for aqueous asymmetric supercapacitors.

Asymmetric supercapacitors (ASCs) show promising potential for electrochemical energy storage applications. However, the energy density of ASCs is limited by the poor electrochemical performance of anodes. To achieve high-performance ASCs, herein, FeO nanotubes constructed from FeO nanoneedles were fabricated by employing MnO nanotubes as a self-sacrificing template, and then a layer of polypyrrole (PPy) was coated through an in situ chemical oxidative polymerization method to enhance their performance. The electrochemical tests indicate that the resultant PPy-coated FeO nanotubes (FeO@PPy) exhibit a high areal capacitance of 530 mF cm at 1 mA cm and good cycling stability, which are superior to those of the FeO nanotubes. The superior performance of the FeO@PPy nanotubes can be attributed to the synergistic effect between the PPy shell and FeO core, in which the conducting PPy shell not only works as a superhighway for charge transport, but also stabilizes the FeO nanotubes during charge-discharge processes. When the FeO@PPy nanotubes were assembled with MnO nanotubes, the as-assembled ASCs possess a high cell voltage of 2.0 V and deliver a high energy density of up to 51.2 Wh kg at a power density of 285.4 W kg with an excellent cycling stability (83.5% capacitance retention over 5000 cycles).