Designed Formation of MnO2@NiO/NiMoO4 Nanowires@Nanosheets Hierarchical Structures with Enhanced Pseudocapacitive Properties.

Although the preparation of hierarchical structures of transition-metal oxides (TMOs) has been intensively studied in recent years, it is still a great challenge to synthesize hierarchical multicomponent TMOs. Herein, we report a versatile method to fabricate three-component TMOs, namely MnO₂@NiO/NiMoO₄ nanowires@nanosheets hierarchical porous composite structures (HPCSs). Through a combination of a chemical-solution-based route and subsequent calcination, the as-prepared MnOOH@NiMo precursor is topotactically transformed to MnO₂@NiO/NiMoO₄ HPCSs without notable structural variation. Ultrathin NiO/NiMoO₄ nanosheets become interconnected into a honeycomb analogue with plentiful mesopores. Comparative results demonstrate the vital role of hexamethylenetetramine (HMT), and the solvent system in the formation of the MnOOH@NiMo precursor. When examined as electrode materials for electrochemical capacitors, MnO₂@NiO/NiMoO₄ HPCSs, with an areal mass loading as high as 5 mg cm⁻², deliver a specific capacitance of 918 F g⁻¹ at a current density of 1.0 A g⁻¹ and maintain good cycling stability, which displays better electrochemical performance than electrodes composed of a single component. Note that a high-voltage asymmetric supercapacitor is configured with MnO₂@NiO/NiMoO₄ HPCSs (still as high as 2 mg cm⁻²) against activated carbon, and exhibits outstanding cycling stability with a high energy density of 26.5 Wh kg⁻¹ and a power density of 401 W kg⁻¹. These analytical and experimental results clearly confirm the advantages of distinctive 3D multicomponent hierarchical architectures for engineering high-performance electrochemical capacitors. [ABSTRACT FROM AUTHOR]