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

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 bi-functional 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.