High catalytic activity of iron and cobalt-iron oxide nanoparticles synthesized via a green method for the oxygen-evolution reaction

Water oxidation, or the oxygen evolution reaction (OER), is the half-reaction that limits the efficiency of overall water splitting and represents a major bottleneck in the production of sustainable hydrogen fuel. Developing efficient, cost-effective, and environmentally benign OER catalysts is therefore critical for advancing green energy technologies. In this study, magnetite and cobalt–iron oxide nanoparticles with varying cobalt contents were synthesized via a green co-precipitation method using Seidlitzia rosmarinus plant extract, and their properties were compared with those of counterparts prepared by a conventional chemical route. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) revealed that green-synthesized samples exhibited a more amorphous character and contained hematite phases, while chemically synthesized materials were more crystalline and magnetite-rich. Field emission scanning electron microscopy (FESEM) and elemental mapping confirmed homogeneous cobalt distribution across all samples. Magnetic measurements via vibrating sample magnetometry (VSM) showed significantly reduced saturation magnetization in green-synthesized samples, attributed to cobalt substitution in the hematite structure, which induced paramagnetic behavior. Electrochemical characterization revealed superior OER performance for the green-synthesized cobalt–iron oxide nanoparticles, with the best sample (20 wt% Co) delivering a current density of 64 mA·cm⁻2 at 1.1 V vs. NHE and an overpotential of 275 mV at 10 mA·cm⁻2. The enhanced catalytic activity is ascribed to the amorphous microstructure, increased surface roughness, and synergistic electronic interactions between Fe and Co. These results underscore the potential of plant-extract-mediated green synthesis as a viable strategy for fabricating high-performance transition metal oxide catalysts for water oxidation and hydrogen production.