Molecular Evidence for the Axial Coordination Effect of Atomic Iodine on Fe-N4 Sites in Oxygen Reduction Reaction

We present a molecular-scale investigation of the axial coordination effect of atomic iodine on Fe-N₄ sites in the oxygen reduction reaction (ORR) by electrochemical scanning tunneling microscopy (ECSTM). A well-defined model catalytic system with explicit and uniform iodine-coordinated Fe-N₄ sites was constructed facilely by the self-assembly of iron(II) phthalocyanine (FePc) on an I-modified Au(111) surface. The electrocatalytic activity of FePc for the ORR shows notable enhancement with axial iodine ligands. The modulation of the electronic structure of Fe sites to evoke a higher spin configuration by axial iodine was evidenced. The interaction strength between oxygen-containing species and active centers becomes weaker due to the presence of iodine ligands, and the reaction is thermodynamically preferable. Furthermore, the reaction dynamics of FePc on I/Au(111) were explicitly determined via in situ ECSTM potential pulse experiments. In contrast, axial atomic iodine was found inefficacious for improving the activity of Co-N₄ sites, and electron rearrangement was found to be marginal, demonstrating that adequate interactions between axial ligands and metal sites for optimizing electronic structures and catalytic behaviors are prerequisites for the impactful role of axial ligands.