Experimental and Theoretical Insights into the Borohydride-Based Reduction-Induced Metal Interdiffusion in Fe-Oxide@NiCo2O4 for Enhanced Oxygen Evolution

The oxygen evolution reaction (OER) plays a key role in determining the performance of overall water splitting, while a core technological consideration is the development of cost-effective, efficient, and durable catalysts. Here, we demonstrate a robust reduced Fe-oxide@NiCo2O4 bilayered non-precious-metal oxide composite as a highly efficient OER catalyst in an alkaline medium. A bilayered oxide composite film with an interconnected nanoflake morphology (Fe2O3@NiCo2O4) is reduced in an aqueous NaBH4 solution, which results in a mosslike Fe3O4@NiCo2O4 (reduced Fe-oxide@NiCo2O4; rFNCO) nanostructured film with an enhanced electrochemical surface area. The rFNCO film demonstrates an outstanding OER activity with an extraordinary low overpotential of 189 mV at 10 mA cm-2 (246 mV at 100 mA cm-2) and a remarkably small Tafel slope of 32 mV dec-1. The film also shows excellent durability for more than 50 h of continuous operation, even at 100 mA cm-2. Furthermore, density functional theory calculations suggest that the unintentionally in situ doped Ni during the reduction reaction possibly improves the OER performance of the rFNCO catalyst shifting d-band centers of both Fe and Ni active sites.