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Growing MnO 2 nanofibers on graphitic hollow carbon spheres (GHCS) is conducted by refluxing GHCS in a KMnO 4 aqueous solution aimed to enhance the electrochemically active surface area of MnO 2 . The stoichiometric redox reaction between GHCS and MnO 4À yields GHCS-MnO 2 composites with controllable MnO 2 content. It is found that these ultrathin MnO 2 nanofibers are vertically grown on the external surface of the GHCS, yielding a composite electrode showing good electron transport, rapid ion penetration, fast and reversible Faradic reaction, and excellent rate performance when used as supercapacitor electrode materials. An asymmetric supercapacitor cell with GHCS-MnO 2 as the positive electrode and GHCS as the negative electrode can be reversibly charged/discharged at a cell voltage of 2.0 V in a 1.0 mol L À1 Na 2 SO 4 aqueous electrolyte, delivering an energy density of 22.1 Wh kg À1 and a power density of 7.0 kW kg À1 . The asymmetric supercapacitor exhibits an excellent electrochemical cycling stability with 99% initial capacitance and 90% coulombic efficiency remained after 1000 continuous cycles measured using the galvanostatic charge-discharge technique.

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