Molecular engineering of atomically dispersed Fe-N4 and Cu-N4 dual-sites in carbon nitride nanotubes for rechargeable zinc-air batteries

Abstract

Metal-nitrogen-carbon (M-N-C) electrocatalysts have emerged as promising oxygen electrocatalysts with the excessive catalytically active M-Nx sites. However, M-Nx sites are not easy to be preserved at elevated temperature of pyrolysis step. Here, we show that a supercritical fluid with a fast reaction kinetics allows us to synthesize a high-purity carbon nitride nanotube filled with the iron and copper phthalocyanine nanorods as a bifunctional oxygen electrocatalyst. The well-preserved Fe-N4 and Cu-N4 sites inside of carbon nitride nanotubes are clearly observed by the systematic analysis. In addition, we investigate the synergistic effect of atomically dispersed Fe-N4 and Cu-N4 dual-atom catalysts inside the carbon nitride nanotube. The prepared sample exhibits the half-wave potential of 0.94 V for oxygen reduction reaction and the potential of 1.65 V at 10 mA cm-2 for oxygen evolution reaction. Further, we fabricate rechargeable zinc-air batteries with the dual-atomic catalyst, which show better bi-functional activities than the mixture of Pt/C and IrO2 under high depth of discharge (DOD) of -32.6% (12 h per cycle) for the zinc-air batteries. Finally, the in-situ X-ray absorption spectroscopy analysis during ORR and OER reactions revealed the catalytic origin of the FCN4-CNNT, providing a new insight into the development of efficient oxygen electrocatalysts.