Electrochemical studies of Ni(OH)₂, NiO, and Ni₃S₂ nanostructures on Ni-foam toward binder-free positive electrode for hybrid supercapacitor application

Self-supported, porous, and binder-free hexagonal nanosheets of Ni(OH)₂ [HS-Ni(OH)₂], hexagonal nanosheets of NiO [HS-NiO], and hexagonal-nanosheet/nanoporous-grain like Ni₃S₂ [HSNG-Ni₃S₂] were successfully grown on 3D Ni-foam at different stages of hydrothermal synthesis using Ni-foam as a precursor source for the cost-effective fabrication of positive electrode for hybrid supercapacitor (SC) application. Comparative analysis revealed that the HSNG-Ni₃S₂ exhibited a maximum areal capacitance of 1286 mF cm⁻² at 0.5 mA cm⁻², far more than the 217 mF cm⁻² of HS-NiO and 129 mF cm⁻² of HS-Ni(OH)₂, with remarkable capacitance retention of 97% for 5000 charge-discharge cycles. The porous binder-free electrode design, improved interfacial conductivity, and easy ionic diffusion are responsible for the remarkable performance of HSNG-Ni₃S₂. Furthermore, the aqueous alkaline hybrid SC assembled by HSNG-Ni₃S₂ as a positive electrode with activated carbon as a negative electrode delivered a maximum areal capacitance of 225.4 mF cm⁻² at 1 mA cm⁻² with remarkable stability up to 92.2% for 5000 charge-discharge cycles. This study presents insightful electrochemical properties of binder-free designed Ni-based Ni(OH)₂, NiO, and Ni₃S₂ electrodes for low-cost and environmental-friendly energy storage systems.