Dual oxidation and sulfurization enabling hybrid Co/Co3O4@CoS in S/N-doped carbon matrix for bifunctional oxygen electrocatalysis and rechargeable Zn-air batteries

The development of high-performance non-precious bifunctional electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is urgently needed to realize the commercialization of rechargeable metal-air battery. Herein, we report a ZIF-67 derived hybrid Co-based carbon composite as bifunctional ORR/OER electrocatalyst exhibiting excellent performance and stability in a custom-built Zn-air battery. The catalyst was formed by pyrolytic carbonization of ZIF-67 followed by mild oxidation and sulfurization to a hybrid composite (Co/Co3O4@CoS-SNC) comprised of multiple active sites including Co-N4, Co-N/SC, C-N/S-C, Co3O4 and CoS, as well as the surface and edge defects in the carbon matrix, that act concertedly yielding a highly efficient bifunctional ORR/OER catalyst for reversible oxygen electrodes. The catalyst affords remarkably low reversible overpotentials and stability not only in a 3-electrode system, but also in a customconstructed Zn-air battery, exhibiting competitive performance against state-of-the-art bifunctional ORR/OER electrocatalysts reported to date and thus placing it among the best non-precious metal catalysts for reversible oxygen reactions.