Effect of Shell Thickness on the Oxygen Evolution Activity of Core@shell Fe$_3$O$_4$@CoFe$_2$O$_4$ 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 Fe$_3$O$_4$@CoFe$_2$O$_4$ 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 Fe$_3$O$_4$@CoFe$_2$O$_4$ 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 in situ soft X-ray absorption spectroscopy (XAS). ... mehrXAS 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 CoFe$_2$O$_4$ 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 H$_2$ production.