Flower-like NiO with a Hierarchical and Mesoporous Structure for Supercapacitors

Graphical Evolution of flower-like hierarchy : The growth process of hierarchical NiO is driven by surface free energy at the very beginning, and then re-crystallization according to Oster's curing mechanism. The hierarchical and mesoporous electroactive material is shown to be a promising alternative as an electrode material for supercapacitor applications. Hierarchical porous nickel oxide (NiO) has been synthesized through an ethanol-based hydrothermal method followed by a sintering process. The effects of hydrothermal duration on morphology, crystalline structure, and electrochemical performance are studied. With the prolongation of hydrothermal duration, the as-prepared NiOs appear in the form of scattered nanoflakes, then flower-like porous balls, and then solid microballs. In this period, poly-crystallization and re-crystallization are observed successively. It seems 120 minutes is suitable to form hierarchical NiO, 2 μm mesoporous balls composed of nanoflakes in all directions. The duration shows significant influence on charge-storage characteristics including reversibility, capacitance, and cyclic stability. The sample exhibits 678.4 F/g at 1 A/g discharge current density and maintains 94.8 % retention after 5000 cycles, suggesting a great prospect of the hierarchical NiO to be used as an electrode material for supercapacitors.