Morphology-controlled synthesis of hierarchical mesoporous α-Ni(OH)2 microspheres for high-performance asymmetric supercapacitors

A simple morphology-controlled synthesis for hierarchical α-Ni(OH)2 microspheres has been developed. Three kinds of α-Ni(OH)2 microspheres with different structures were prepared by one-step surfactant-assisted solvothermal method, using water, ethanol and their mixture as solvent respectively. The effects of solvent on the structure, morphology and capacitive performance of α-Ni(OH)2 were investigated. The microstructure and chemical composition of as-prepared α-Ni(OH)2 were characterized by a variety of means, such as XRD, BET, XANES, SEM, TEM and HRTEM. Among the as-prepared samples, Ni(OH)2-c microspheres, obtained from the mixed solvent (ethanol solution), are composed of hierarchical flower-like nanosheets and possess the roughest surface and the largest specific surface area (101.3 m2 g−1). Electrochemical measurements reveal that Ni(OH)2-c is of the highest specific capacitance (1087.1 F g−1 at a current density of 1 A g−1) and excellent cycling stability (97.7% capacitance retention after 3000 charge-discharge cycles at a current density of 5 A g−1) in 6 M KOH electrolyte. Furthermore, an symmetric supercapacitor (Ni(OH)2//AC), using Ni(OH)2-c as positive electrode and activated carbon as negative electrode, delivers a high energy density 32.9 Wh kg−1 at power density 0.8 kW kg−1 under potential window 1.6 V. Additionally, Ni(OH)2//AC exhibits good cycling performance (98% capacitance retention after 3000 cycles at a current density of 5 A g−1) and nearly 100% Coulombic efficiency. The performances suggest its potential application in supercapacitors.