Flexible supercapacitor based on MnO2 nanowalls vertically grown on ultrathin stainless-steel foils

Vertically aligned nanostructures are usually beneficial for ion diffusion during the charge/discharge process and thereby, are suitable to be electrodes for pseudo-supercapacitor applications. Herein, vertically aligned δ -MnO 2 nanowalls are successfully grown on the surface of ultrathin stainless-steel foils by a facile fabrication process that combines the conventional hydrothermal method with the polymer-assisted disposition method. The materials are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Due to the unique structural characteristics of δ -MnO 2 nanowalls, the electrodes exhibit excellent electrochemical capacitive performances. In particular, a quasi-solid-state asymmetric device exhibits a high energy density of 53.24 μWh/cm 2 at a power density of 451 μW/cm 2 . The device also exhibits outstanding cycling stability with a high capacitance retention (∼94.5 %) after 30,000 cycles.