Temperature-induced hierarchical Tremella-like and Pinecone-like NiO microspheres for high-performance supercapacitor electrode materials

Nickel oxide (NiO) microspheres with tremella-like and pinecone-like structures were facilely synthesized using the solvothermal method, followed by calcination treatment. NiO structures were controlled by adjusting the calcination temperature of the as-prepared nitrate precursor. NiO microspheres and nitrate precursors were investigated using a number of techniques. Tremella-like NiO (TN4-C1) microspheres obtained at 350 °C show higher surface areas, thinner secondary nanosheets, larger cavities, and smaller crystal sizes than pinecone-like NiO (PN4-C2 and PN4-C3) microspheres obtained at 600 and 850 °C, respectively. Electrochemical performances of TN4-C1, PN4-C2, and PN4-C3 were investigated using cyclic voltammetry and galvanostatic charging/discharging techniques. At 3 A g−1, TN4-C1 exhibits a maximum specific capacitance of 3242 F g−1, which is significantly higher than that of PN4-C2 (891 F g−1) and PN4-C3 (486 F g−1) and is close to its theoretical specific capacitance. This finding can be attributed to the superior morphology of TN4-C1 and its abundant structural water molecules, which ensure rapid ion diffusion. This study provides a facile and feasible method of preparing porous transition-metal-oxide-based electrode materials with specific morphology and hierarchical structure.