Hierarchically Assembled Cobalt Oxynitride Nanorods and N-Doped Carbon Nanofibers for Efficient Bifunctional Oxygen Electrocatalysis with Exceptional Regenerative Efficiency

Oxygen-based electrocatalysis is an integral aspect of a clean and sustainable energy conversion/storage system. The development of economic bifunctional electrocatalysts with high activity and durability during reversible reactions remains a great challenge. The tailored porous structure and separately presented active sites for oxygen reduction and oxygen evolution reactions (ORR and OER) without mutual interference are most crucial for achieving desired bifunctional catalysts. Here, we report a hybrid composed of sheath-core cobalt oxynitride (CoO_x@CoN_y) nanorods grown perpendicularly on N-doped carbon nanofiber (NCNF). The brush-like CoO_x@CoN_y nanorods, composed of metallic Co₄N cores and oxidized surfaces, exhibit excellent OER activity (E = 1.69 V at 10 mA cm^-2) in an alkaline medium. Although pristine NCNF or CoO_x@CoN_y alone had poor catalytic activity in the ORR, the hybrid showed dramatically enhanced ORR performance (E = 0.78 V at -3 mA cm^-2). The experimental results coupled with a density functional theory (DFT) simulation confirmed that the broad surface area of the CoO_x@CoN_y nanorods with an oxidized skin layer boosts the catalytic OER, while the facile adsorption of ORR intermediates and a rapid interfacial charge transfer occur at the interface between the CoO_x@CoN_y nanorods and the electrically conductive NCNF. Furthermore, it was found that the independent catalytic active sites in the CoO_x@CoN_y/NCNF catalyst are continuously regenerated and sustained without mutual interference during the round-trip ORR/OER, affording stable operation of Zn-air batteries.