Regulating the Fe-spin state by Fe/Fe3C neighbored single Fe-N4 sites in defective carbon promotes the oxygen reduction activity

Non-noble iron-based single-atom catalysts (Fe-N-C) require more accessible active sites and rapid mass transportation, and spin state regulation of iron atoms is also key but challenging to synergistically improve the zinc-air battery (ZAB) performance. Thus, here we indicate that by pre-preparing a 3D nitrogen-doping carbon-sheet network from in situ gas-molecule cutting of bulk MOF and then adsorbing iron into defects and pores of this preformed matrix, we achieve a Fe-N-C catalyst with 14.78 wt.% iron existed as single-atom Fe-N4 sites neighboured Fe/Fe3C nanostructures. Electrons transfer to Fe3C/FeN4 from the adsorbed Fe atom and forms an electron-rich region around Fe3C, achieving the spin-state modulation of the d-band center of Fe atom in FeN4 through charges transfer. The calculation proves that d-band centers of spin-up/down for Fe in Fe-Fe3C/FeN4 are closer to the Fermi level than that of Fe3C/FeN4 (-1.51/-0.72 eV vs. -1.47/-0.65 eV), implying Fe site in Fe-Fe3C/FeN4 is more chemically active and is beneficial to the adsorption and reaction of O2 when involved in ORR. This catalyst delivers excellent ORR activity and the primary ZAB performance with the energy density (ED) of up to 1002 mWh g-1 Zn at 50 mA cm−2 and the maximum power density of 212 mW cm−2, especially in which it displays an excellent durability with 92.7% ED retention during a ∼150 h continuous discharge. These results highlight the significance of metal-atoms spin-state modulation by coupled active metal-species in hybrid catalysts for electrochemical energy devices.