Modulation of the Spin State of Atomic Fe-N₄ Sites with Interlayer-Adjacent Ir-N₄ for Superior ORR Activity

Development of efficient and durable oxygen reduction reaction (ORR) electrocatalysts is of great interest yet remains challenging. Herein, we predicted and screened a bilayer graphite carbon-supported Ir-N₄/Fe-N₄ catalyst with high ORR activity using density functional theory calculations. Subsequently, various bimetallic single atom supported on 3D ordered macroporous carbon were rationally designed and experimentally synthesized via a colloidal microsphere template-confined reaction method. As anticipated, the resulting Ir-N₄/Fe-N₄ bimetallic single-atom catalysts (IrFe-SACs) exhibit superior ORR activity and durability, reaching a half-wave potential of 0.928 V. The IrFe-SACs also demonstrate outstanding performance in Zn-air batteries, including a high discharge power density (314 mW cm⁻²) and excellent cycling stability (~ 1650 cycles over 550 h). Further experimental characterizations and theoretical analysis reveal that introducing interlayer-adjacent Ir-N₄ sites facilitates the transition of Fe-N₄ from a low-spin state to a medium-spin state, which optimizes the spin polarization of Fe 3d orbitals and enhances the non-localization of the Fe-O/OH molecular orbital, thereby significantly improving the ORR intrinsic activity and durability of atomic Fe-N₄ sites.