Effect ofShell Thickness on the Oxygen EvolutionActivity of Core@shell Fe₃O₄@CoFe₂O₄ Nanoparticles

Hydrogen production via water splitting requires efficient electrocatalysts to reduce the overpotential of the anodic oxygen evolution reaction (OER) and cathodic hydrogen evolution reaction (HER). In this study, we investigated the influence of apparent shell thickness on the electrocatalytic activity of Fe₃O₄@CoFe₂O₄ core@shell nanoparticles, an efficient noble metal-free OER catalyst in alkaline media. Three different types of core@shell nanoparticles were synthesized by the seed-mediated crystal growth of cobalt ferrite on pristine magnetite nanoparticles. The synthesis conditions were adapted to modulate the shell structure. Importantly, all proposed core@shell structures showed excellent stability during electrochemical testing, which is important for eventual industrial applications. We showed that the electrocatalytic performance of Fe₃O₄@CoFe₂O₄ core@shell nanoparticles was significantly influenced by the shell structure. The cooperative redox mechanism proposed to be the origin of the activity enhancement in core@shell nanoparticles was investigated by using <i>in situ</i> soft X-ray absorption spectroscopy (XAS). XAS revealed that cooperative redox interactions occurred between Co(II) and Fe(II) residing in close proximity at the core/shell interface, hence requiring a thin and continuous CoFe₂O₄ shell. Overall, this study provides insights into the design of efficient core@shell nanocatalysts for the OER, offering a path toward improving the performance of earth-abundant transition metal-oxide (TMO) catalysts for sustainable H₂ production.