Upgrading industrial electrolytic MnO₂ to highly active MnO/C supercapacitor electrode materials via composite-carbon-assisted solid-phase transition

Though numerous approaches have been proposed to synthesize high-activity MnO_x-based electrode materials, large-scale production still poses great challenges. Electrolysis serves as a mature, cost-effective route for large-scale synthesis of industrial-grade MnO₂, yet the obtained product generally delivers inferior electrochemical activity. Herein, a composite carbon-assisted solid-phase transformation strategy is proposed to synthesize high-activity manganese oxide electrode materials with electrolytic MnO₂ as the precursor. A bioorganic-inorganic composite carbon was designed to in-situ modify and restructure MnO₂ via gelatinization treatment. Hydrogen bonding enables uniform dispersion of MnO₂ within the 3D porous network of the carbon composite. This porous structure enables uniform and rapid heat transfer for MnO₂ particles during annealing. Furthermore, the synergistic effect of bioorganic and inorganic carbon effectively modulates the deoxygenation process of MnO₂, facilitating its solid-phase transformation into well-crystallized MnO along with surface structural evolution. It is found that graphene oxide facilitates MnO crystal growth, while starch suppresses its growth. The deoxidative phase transition temperature for MnO₂ converting to MnO falls to 600 °C. The prepared MnO composite achieves a specific capacitance of 331 F g⁻¹, which is 15 times higher than that of pristine MnO₂ (20.7 F g⁻¹). After activation, the MnO-based electrode further delivers a specific capacitance of 364 F g⁻¹. It also presents superior cycling stability, maintaining 97.4% of the initial capacitance after 5000 cycles at 5 A g⁻¹. This strategy offers a promising route to fabricate high-activity manganese oxide electrode materials via solid-phase transformation.