Effects of Conductivity Enhancement and Morphological Changes of Nickel Oxide on Supercapacitor Performance: A Review

The growing demand for high-performance energy storage devices has spotlighted nickel oxide (NiO) as a promising pseudocapacitive material for supercapacitor (SC) electrodes. However, its inherently low electrical conductivity (10⁻⁵–10⁻⁷ S cm⁻¹), attributed to its wide bandgap (3.6–4.0 eV), low carrier mobility (0.1–1 cm² V⁻¹ s⁻¹), and defect-induced electron scattering, hampers charge transport and electrochemical efficiency. This review examines the fundamental causes of NiO's poor conductivity and evaluates enhancement strategies such as metal doping (e.g., Co, Cu, Fe, Cr), composite formation, and morphological engineering. The relationship between conductivity improvements and morphological changes in SC performance metrics, such as specific capacitance, rate capability, and cyclic stability, is highlighted. For example, copper doping increases NiO's surface area from 5.68 to 14.54 m² g⁻¹, improving ion exchange pathways and raising. specific capacitance (Csp) from 647 to 1136 F g⁻¹. Similarly, a Ni₃S₂/NiO heterostructure alters the nanoflower morphology into a thorn-like honeycomb, achieving a Csp of 2077.12 F g⁻¹ compared to 348.21 F g⁻¹ for pure NiO. It also lowers charge transfer resistance from 11.5 Ω to 2.6 Ω and improves capacitance retention from 65% to 94% over 3000 cycles. These modifications significantly enhance NiO's electrochemical properties. The review concludes by outlining the current status, limitations, and future prospects of NiO-based materials in advancing SC electrode technology.