Modulating the Fe spin state in FeNC catalysts by Ru nanoparticles to facilitate the oxygen reduction reaction†

FeNC is a promising non-precious metal catalyst that can replace platinum-based catalysts in proton-exchange membrane fuel cells (PEMFCs) and zinc–air battery applications. The study utilized Fe-ZIF-8 as a precursor to improve the oxygen reduction reaction (ORR) activity of the catalyst. This was achieved by growing ruthenium particles of approximately 0.32 μm in situ using a physical milling method under a reducing gas atmosphere. This paper demonstrates that in situ grown ruthenium nanoparticles can alter the spin state of iron atoms from the high-spin state FeN₄(II)-(t⁴_2ge²_g) to the medium-spin state FeN₄(II)-(t⁵_2ge¹_g). This alteration changes the interactions of the π* antibonding orbitals of the oxygen and thus improves the ORR activity. The rotating ring-disk (RDE) electrode test resulted in a half-wave potential of (E_1/2) + 0.80 V vs. RHE in 0.1 M HClO₄, indicating remarkable ORR performance. Zinc–air battery tests showed a high peak power density of 148 mW cm⁻².