High Mass Loading MnO₂ with Hierarchical Nanostructures for Supercapacitors

Metal oxides have attracted renewed interest as promising electrode materials for high energy density supercapacitors. However, the electrochemical performance of metal oxide materials deteriorates significantly with the increase of mass loading due to their moderate electronic and ionic conductivities. This limits their practical energy. Herein, we perform a morphology and phase-controlled electrodeposition of MnO₂ with ultrahigh mass loading of 10 mg cm^-2 on a carbon cloth substrate to achieve high overall capacitance without sacrificing the electrochemical performance. Under optimum conditions, a hierarchical nanostructured architecture was constructed by interconnection of primary two-dimensional ε-MnO₂ nanosheets and secondary one-dimensional α-MnO₂ nanorod arrays. The specific hetero-nanostructures ensure facile ionic and electric transport in the entire electrode and maintain the structure stability during cycling. The hierarchically structured MnO₂ electrode with high mass loading yields an outstanding areal capacitance of 3.04 F cm^-2 (or a specific capacitance of 304 F g^-1) at 3 mA cm^-2 and an excellent rate capability comparable to those of low mass loading MnO₂ electrodes. Finally, the aqueous and all-solid asymmetric supercapacitors (ASCs) assembled with our MnO₂ cathode exhibit extremely high volumetric energy densities (8.3 mWh cm^-3 at the power density of 0.28 W cm^-3 for aqueous ASC and 8.0 mWh cm^-3 at 0.65 W cm^-3 for all-solid ASC), superior to most state-of-the-art supercapacitors.