Three-dimensional homo-nanostructured MnO₂/nanographene membranes on a macroporous electrically conductive network for high performance supercapacitors

A three-step hydrothermal route was designed to fabricate three-dimensional (3D) homo-nanostructured MnO₂ (MnO₂-MnO₂)/nanographene membranes on a macroporous and electrically conductive network (MECN). The preparation technology, structure and morphology, and electrochemical properties of samples are determined systematically. The nanographene/MECN electrode with more defects as the active surface had been synthesized by hydrothermal carbonization. The in situ growth of δ-MnO₂ with a carbon-assisted reaction on the nanographene/MECN was strongly adhered to the substrate. The additional α-MnO₂ with a redox reaction enhanced the mass loading of MnO₂, developing the specific capacitance of the MnO₂-MnO₂/nanographene/MECN electrode. The materials are demonstrated as an electrode with a maximum capacitance of 4.5 F cm^-2 or 179 F cm^-3 (894 F g^-1) at 1 mA cm^-2 for 1 cm² samples and retaining over 83% after 20000 cycles in 1 M Na₂SO₄. The MnO₂-MnO₂/nanographene/MECNAC/Ni-foam supercapacitors with high volumetric energy densities exhibit the ideal performance of a supercapacitor (1 mW h cm^-3, 40.3 W h kg^-1, at 1000 W kg^-1), indicating a promising future for supercapacitors.