N, S codoped carbon matrix-encapsulated CoFe/Co0.2Fe0.8S heterostructure as a highly efficient and durable bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries

The realization of durable and efficient oxygen evolution reactions (OER) at large current densities and low overpotentials is of significant importance but remains a great challenge. In this study, a CoFe/Co0.2Fe0.8S@NS-CNTs/CC (CF/CFS@NS-CNTs/CC) heterogeneous structure was fabricated by isolating CoFe/Co0.2Fe0.8S (CF/CFS) particles locked in nitrogen/sulfur codoped carbon nanotubes (NS-CNTs). Appreciable oxygen evolution reaction activity and durability was achieved with an ultralow overpotential of 110 mV at 10 mA•cm-2. The operation was stable for 300 h at a current density of 500 mA•cm-2. The structure was then assembled into a zinc-air battery (ZAB), which delivered a high power density of 194 mW•cm-2, a specific capacity of 837.3 mAh•gZn-1, and stable operation for 788 h without obvious voltage attenuation and altered morphology. The electronic interactions were studied by X-ray photoelectron spectroscopy (XPS), which revealed that both the bimetal components and the synergistic effect at the interface stimulated the transfer of Co and Fe sites to higher chemical valence states. Theoretical calculations indicated that the synergistic effect of the bimetal components, build-in interfacial potential, and surface chemical reconstruction adjusted the Fermi level to optimize the thermodynamic formation of O* to OOH*, thus enhancing the intrinsic activity.