Enhanced oxygen reduction with carbon-polyhedron-supported discrete cobalt-nitrogen sites for Zn-air batteries

Abstract

• A general strategy to construct discrete Co-N 4 sites confined in carbon polyhedron. • Porous carbon polyhedron with bumpy exterior boosts the exposure of active sites. • Zn acts as sacrificed “shield” to isolate the adjacent Co atoms from aggregating. • Co-N/ZIF-2 catalyst shows high ORR activity superior to commercial Pt/C. • Zn-air battery exhibits high power density of 260 mW cm −2 and stability in 200 h. The specific structure of metal atoms including coordination environment and dispersibility, together with carbon support architecture in metal-nitrogen-carbon (M−N−C) materials are of great importance for their capabilities to catalyze oxygen reduction reaction (ORR). Herein, we report a novel strategy to achieve highly exposed and atomically isolated Co-N 4 active sites within porous carbon polyhedron with bumpy exterior by pyrolysis of predesigned Zn-modified metal–organic frameworks (MOFs) together with tripolycyanmide, during which Zn is evaporated away at high temperature of 900 °C and leaves free vacancies for N to stabilize Co atoms. The bumpy exterior dramatically enlarges its specific surface area and improves the number of active sites. The local structure of single Co atoms coordinated with N is confirmed by combing spherical aberration correction electron microscopy and X-ray absorption fine structure analyses. Remarkably, the obtained Co-N 4 single sites exhibit a high ORR activity with a half-wave potential of 0.861 V that is superior to commercial Pt/C (0.819 V), as well as excellent performance and stability in Zn-air batteries with a peak power density of 260 mW cm −2 . Our work will endow the opportunities to propagate this approach to develop various single-metal-atom materials.