Sustainable Synthesis of Defect-Rich NiO Nanoparticles from Seaweed for Enhanced Oxygen Evolution

Green synthesis has emerged as a sustainable alternative for preparing nanostructured electrocatalysts using renewable biological resources. In this work, NiO nanoparticles were synthesized using an ethanolic extract of Sargassum sp. as a biogenic mediator, and the influence of calcination temperature (300, 500, and 700 °C) on their physicochemical and electrocatalytic properties was systematically investigated. X-ray diffraction confirmed the formation of NiO cubic phase and showed an increase in crystallite size with increasing calcination temperature. FTIR, Raman, XPS, and EPR analyses revealed temperature-dependent changes in surface composition and defect chemistry, including mixed Ni²⁺/Ni³⁺ species and oxygen-vacancy-related centers. EPR signals at g ≈ 2.0005-2.0006 are assigned to F ⁺-type centers. Electrochemical measurements in 1.0 mol L^-1 KOH showed that NiO-300 exhibited the highest OER activity, requiring an overpotential of 352 mV to reach 10 mA cm^-2, compared with 368 and 356 mV for NiO-500 and NiO-700, respectively. Although NiO-300 displayed the lowest estimated electrochemically active surface area among the samples, it consistently showed the lowest charge-transfer resistance, indicating more favorable interfacial kinetics. Its enhanced performance is therefore associated with the combined effects of defect-related surface species, oxygen-vacancy-related centers, and improved charge-transfer dynamics. Chronopotentiometric measurements at 10 and 100 mA cm^-2 further demonstrated the excellent electrochemical stability of NiO-300, while post-OER XRD and Raman analyses confirmed the preservation of its crystalline framework after prolonged operation.