Hybrid MnO₂/C nano-composites on a macroporous electrically conductive network for supercapacitor electrodes

A two-step hydrothermal process is designed to synthesize hybrid MnO₂/C nano-composites on a macroporous electrically conductive network (MECN) via a redox reaction in a 30 mM KMnO₄ solution with carbon microspheres. The microstructure, surface morphology, and electrochemical properties of the MnO₂/C coated on MECN are determined systematically. The MnO₂ nanoflakes, which are about 40-200 nm, are deposited regularly on the carbon microspheres coated on the MECN electrode. The MnO₂ nano-lamellas offer fast ion transport and adsorption-desorption to/from the MnO₂ surface, and the microporous carbon microspheres enhance the electrical storage and ion transfer. The in situ growth of MnO₂ on the carbon microspheres on the MECN substrate leads to a small contact resistance and short current transfer length. The materials are demonstrated to be excellent electrodes in supercapacitors boasting a high capacitance of 497 F g^-1 at 1 A g^-1 with a 1 cm² electrode and excellent long-term cycling stability over 5000 cycles in 1 M Na₂SO₄. The MnO₂/C/MECN||active carbon/Ni-foam asymmetrical supercapacitors (ASCs) deliver an energy density of 0.50 mW h cm^-3 (55.5 W h kg^-1 at a power density of 4000 W kg^-1) with 87.6% retention of the specific capacitance after 5000 cycles.