d-Orbital steered FeN4 moiety through N, S dual-site adjustation for zinc-air flow battery

The implementation of pristine covalent organic polymer (COP) with well-defined structure as air electrode may spark fresh vitality to rechargeable zinc-air flow batteries (ZAFBs), but it still remains challenges in synergistically regulating their electronic states and structural porosity for the great device performance. Here, we conquer these issues by exploiting N and S co-doped graphene with COP rich in metal-ligand nitrogen to synergistically construct an effective catalyst for oxygen reduction reaction (ORR). Among them, the N and S co-doped sites with high electronegativity properties alter the number of electron occupations in the d orbital of the iron centre and form electron-transfer bridges, thereby boosting the selectivity of the ORR-catalysed four-electron pathway. Meanwhile, the introduction of COP materials aids the formation of pore interstices in the graphene lamellae, which both adequately expose the active sites and facilitate the transport of reactive substances. Benefiting from the synergistic effect, as-prepared catalyst exhibits excellent half-wave potentials (E1/2 = 912 mV) and stability (merely 8.8% drop after a long-term durability test of 50000 s). Further, ZAFBs assembled with the N/SG@COP catalyst demonstrate exceptional power density (163.8 mW cm−2) and continuous charge and discharge for approximately 140 h at 10 mA cm−2, outperforming the noble-metal benchmarks.