Ultrathin MnO2 nanofibers grown on graphitic carbon spheres as high-performance asymmetric supercapacitor electrodes

Abstract

Growing MnO₂ nanofibers on graphitic hollow carbon spheres (GHCS) is conducted by refluxing GHCS in a KMnO₄ aqueous solution aimed to enhance the electrochemically active surface area of MnO₂. The stoichiometric redox reaction between GHCS and MnO₄⁻ yields GHCS–MnO₂ composites with controllable MnO₂ content. It is found that these ultrathin MnO₂ 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–MnO₂ 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⁻¹ Na₂SO₄ aqueous electrolyte, delivering an energy density of 22.1 Wh kg⁻¹ and a power density of 7.0 kW kg⁻¹. 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.