Phytosynthesis of Co(3)O(4) Nanoparticles as the High Energy Storage Material of an Activated Carbon/Co(3)O(4) Symmetric Supercapacitor Device with Excellent Cyclic Stability Based on a Na(2)SO(4) Aqueous Electrolyte

[Image: see text] The benign preparation of cobalt oxide nanoparticles (Co(3)O(4)-NPs) was performed using marine red algae extract (Grateloupia sparsa) as a simple, cost-effective, scalable, and one-pot hydrothermal technique. The nominated extract was employed as an environmental reductant and stabilizing agent. The resultant product showed the typical peak of Co(3)O(4)-NPs around 400 nm wavelength as ascertained by UV–vis spectroscopy. Size and morphological techniques combined with X-ray diffraction (XRD) showed the small size of Co(3)O(4)-NPs deformed in a spherical shape. The activated carbon (AC) electrode and Co(3)O(4)-NP electrode delivered a specific capacitance (C(sp)) of 125 and 182 F g(–1) at 1 A g(–1), respectively. The energy density of the AC and AC/Co(3)O(4) electrodes with a power density of 543.44 and 585 W kg(–1) was equal to 17.36 and 25.27 Wh kg(–1), respectively. The capacitance retention of designed electrodes was 99.2 and 99.5% after 3000 cycles. Additionally, a symmetric AC/Co(3)O(4)//AC/Co(3)O(4) supercapacitor device had a specific capacitance (C(sp)) of 125 F g(–1) and a high energy density of 55 Wh kg(–1) at a power density of 650 W kg(–1). Meanwhile, the symmetric device exhibited superior cyclic stability after 8000 cycles, with a capacitance retention of 93.75%. Overall, the adopted circular criteria, employed to use green technology to avoid noxious chemicals, make the AC/Co(3)O(4) nanocomposite an easily accessible electrode for energy storage applications.