Fig. 1: Schematic illustration of the synthetic route and model of the 3D hierarchical MnO₂@GR.

The 3D hierarchical MnO₂@GR catalysts were synthesized via the processes of the complexation of MnSO₄, oxidation of KMnO₄, the crystal growth of MnO₂ under hydrothermal condition and self-assembly of 1D core–shell nanofiber.