<p>This study reports the successful preparation of cobalt-doped zinc sulphide (ZnS) nanoparticles using a simple and cost-effective co-precipitation technique, incorporating cobalt at 2 wt.% and 4 wt.% levels. The research investigates how cobalt doping influences the nanoscale properties of ZnS. Structural, optical, morphological, and electrochemical features were thoroughly examined using XRD, UV–Vis spectroscopy, FTIR, and cyclic voltammetry. Electrochemical testing in a 1&#xa0;mol L⁻¹ Na₂SO₄ solution revealed that the 2 wt.% Co–ZnS sample achieved the highest energy density of 14.17 Wh/kg, indicating strong charge storage ability. Higher cobalt levels led to reduced capacitance and energy density, likely due to increased resistance and lower surface activity. These results highlight the importance of optimizing doping levels to improve electrochemical performance. Furthermore, shifts in optical properties and reduced band gaps point to potential use in optoelectronic and photocatalytic fields. Overall, Co-doped ZnS nanoparticles show great promise for energy storage systems like supercapacitors, where high conductivity and efficient ion transport are essential.</p>