In this research, the hydrometallurgical synthesis of metallic cobalt nanoparticles from spent cobalt oxide in thermal energy storage cells was investigated with the aim of recovering and reusing cobalt. The leaching process of the sample containing Co₃O₄ was carried out using sulfuric acid, and the effects of solution temperature and reaction time on the cobalt recovery process were evaluated. The results showed that the maximum extraction efficiency of cobalt ions (with a concentration of approximately 9000 ppm) was achieved under optimum conditions consisting of a reaction temperature of 85 °C and a time of 240 minutes. Subsequently, cobalt nanoparticles were synthesized from the cobalt sulfate solution under controlled conditions in the presence of sodium borohydride (NaBH₄) as a reducing agent and polyvinylpyrrolidone (PVP) as a surface stabilizer in an alkaline medium. During the synthesis stage, the weight ratio of NaBH₄ to CoSO₄.7H₂O was investigated as a key parameter, and the best results were obtained at a sodium borohydride-to-cobalt sulfate weight ratio of 2 at a pH of 10.4. The leaching solution and nanoparticles were characterized using UV-Vis, XRD, DLS, and SEM analyses, and the average size of the synthesized cobalt nanoparticles under optimum conditions was reported to be 23 nm.