<p>Nickel oxide (NiO) nanostructures are emerging as a promising electrode material to improve the supercapacitor performances due to their excellent electrical conductivity and sufficient redox active sites. In this study, we synthesize NiO by microwave-assisted method and calcinate under the different ambient such as (i) open air on hot-plate (named as NHA), (ii) in muffle-furnace under the air (NFA), and (iii) in tubular-furnace with nitrogen gas flow (NFN). The Field Emission Scanning Electron Microscopy shows that the calcination under the nitrogen ambient results in the formation of densely packed and stacked nanoflakes than the other two processes. The structural and electronic properties of NiO nanostructures are analyzed using the X-ray diffraction patterns and studied by Rietveld refinement and density functional theory calculations. The synthesized NiO coated electrode calcined under the nitrogen ambient (NFN) exhibits the higher specific capacity of 213 Cg⁻¹ and capacity retention of 98% after 2000 cycles than that of NFA (108 Cg⁻¹) and NHA (59 Cg⁻¹). The fabricated symmetric coin-cell supercapacitor device based on NFN electrode exhibits the energy density of 2.8 Wh/kg and power density of 171.2 W/kg with the capacity retention of 76.2% after 2000 cycles. These results show that NiO calcined under N₂ ambient (NFN electrode) will be very much useful to improve the supercapacitor performance and commercial applications.</p>