Abstract <p>Green synthesis of nanoparticles using plants has received significant attention because it is safe for human and for the environment. Cobalt hydroxide (Co(OH)₂) and cobalt oxide (Co₃O₄) nanostructures are synthesized in a reflux system using olive leaves extract as a surfactant and a reducing agent. The prepared nanoparticles are characterized by XRD, FTIR, UV–Visible Spectrophotometry, Zeta meter and SEM. The optical gap value of Co₃O₄ is 3.8 eV, which is justified by the nanometric size of the oxide particles observed by SEM (around 5 nm). Theynare deposited on graphite rods with spherical disks structure to form photoelectrodes. They show good capacitive properties with a capacitance of 206 g/F at a scan rate of 10 mV/s. Co₃O₄ capacitance values vary between 53.52–206.44 F/g for different sweep speeds from 10 mV/s up to 100&#xa0;mV/s. The specific capacitance of Co₃O₄ is greater at a lower scan rate (10 mV/s) and a constant decrease of specific capacitance is observed for a high scan rate (100 mV/s). Co₃O₄ nanoparticles present a faradic charge storage mode via oxidation and cobalt reduction.</p>