Synthesis of Core-Shell Structured MnO₂ Petal Nanosheet@Carbon Sphere Composites and Their Application as Supercapacitor Electrodes

A novel MnO₂ petal nanosheet@carbon sphere core-shell structure was successfully fabricated by adjusting the quantity of the KMnO₄ precursor employed during the in situ growth of MnO₂ on the surface of carbon spheres via a facile hydrothermal method. In the presence of low KMnO₄ contents, only MnOOH was generated. In contrast, upon increasing the quantity of KMnO₄, δ-MnO₂ nanorods and petal nanosheets were obtained, thereby allowing the formation of the core-shell structured δ-MnO₂ petal nanosheet@carbon sphere composites. However, beyond a certain point, further increases in the KMnO₄ content were unfavorable. Although, prepared MnO₂/carbon sphere composites of different morphologies can be used for supercapacitors (SCs) electrode materials, we found that the core-shell structured MnO₂ petal nanosheet@carbon sphere composites exhibited the optimal capacitance performances in all the composites. These composites exhibited an excellent specific capacitance of 231 F⋅g⁻¹ under a current density of 0.5 A⋅g⁻¹. Furthermore, they also demonstrated an impressive cycling stability. Indeed, after 1,000 cycles at 10 A⋅g⁻¹, the MnO₂ petal nanosheet@carbon spheres exhibited 96% of their initial capacitance in a 1 M Na₂SO₄ aqueous electrolyte. The synergistic effect between δ-MnO₂ and the porous carbon spheres in the unique core-shell structured is responsible for the excellent cycle life.