Abstract
•Three phases of nickel hydroxide are prepared by a single-step sonochemical method.•Morphology and crystal structure of the product can be controlled.•Alfa-nickel hydroxide is calcined to NiO to exploit its high BET surface area.•The prepared NiO exhibits a high specific capacitance of 1033 F g−1.
Alfa-nickel hydroxide is prepared by a high-energy ultrasound (sonochemical) method that enables precise control over the reaction process. The prepared alfa-nickel hydroxide can be easily transformed into the beta phase of nickel hydroxide by adjusting the reaction time. BET analysis of the alfa-nickel hydroxide prepared in this method reveals a very high specific surface area (384 m2 g−1), extraordinarily high pore volume (0.77 cm3 g−1) and mesoporous structure. Scanning electron microscopy images show that the alfa-nickel hydroxide has a worm-like morphology, whereas the beta phase exhibits a petal-like morphology. To investigate the electrochemical properties of the nickel hydroxide, it is first calcined to nickel oxide at 250 °C, and then the capacitance of the resulting NiO sample is measured by cyclic voltammetry, galvanostatic charge-discharge measurements and electrochemical impedance spectroscopy. When alfa-nickel hydroxide is converted to NiO, the BET surface area increases slightly, due to dehydroxylation and calcination at a relatively low temperature. However, the pore volume remains nearly constant. The high specific capacitance of 955 F g−1 at the scan rate of 10 Ag−1 is obtained for the NiO sample. The presence of mesopores and the high specific surface area enhance ion transport during the charge and discharge processes to enable high energy storage.