Microwave-assisted hydrothermal synthesis of αβ-Ni(OH)2 nanoflowers on nickel foam for ultra-stable electrodes of supercapacitors

Transition metal hydroxides (TMHs) are considered promising materials for supercapacitors because of their high theoretical capacitance and tunable morphology. Among TMHs, nickel hydroxide stands out for its multiple oxidation states, facilitating charge storage. This paper reports on a novel microwave-assisted hydrothermal technique employed to synthesize mixed-phase nickel hydroxide nanoflowers directly at the surface of nickel foam substrates with improved electrochemical stability compared to pure phases of nickel hydroxide. The analysis of nickel hydroxide in different phases alpha, beta and mixed-phase materials using physiochemical techniques like XRD, SEM, EDX, RAMAN, FT-IR and TGA is explained. The areal capacitance calculated from GCD tests for α-Ni(OH) 2 , β-Ni(OH) 2 and αβ-Ni(OH) 2 is estimated 7.35 F/cm 2 , 2.66 F/cm 2 and 5.12 F/cm 2 at a current density of 10 mAcm −2 . The mass-specific capacitance calculated from GCD tests for α-Ni(OH) 2 , β-Ni(OH) 2 and αβ-Ni(OH) 2 is estimated 2311 Fg −1 , 688 Fg −1 and 1511 Fg −1 at a current density of 10 mAcm −2 . The cyclic stability test revealed the superior cycling performance of nickel hydroxide in mixed-phase as the cathode of an ultralong supercapacitor with nearly 100 % capacitance retention after 5000 cycles at 50 mAcm −2 . The proposed synthesis technique advances the efficiency and control of nanostructure geometry, providing insights into the design for electrodes of extreme-performance supercapacitors. However, conventional synthesis methods are time-consuming and energy-intensive. These findings underscore the potential of mixed-phase nickel hydroxide as an outstanding electrode material with extended cyclic performance for supercapacitors, contributing to the improvement of energy storage technologies in the renewable energy landscape.