Investigating the Effect of the Coordination Environment and Bridging Atoms in Fe−N ₄ Macrocycles Applied to Alkaline Oxygen Reduction Reaction

Understanding and tuning the local coordination environment of Fe−N4 macrocyclic catalysts are essential for advancing non-precious metal oxygen reduction reaction (ORR) electrocatalysis. Here, we present a comprehensive experimental and theoretical investigation of three structurally distinct Fe macrocycles, iron(tetraphenylporphyrin) chloride (FeTPP), iron(II) phthalocyanine (FePc), and iron aza-bridged bis-1,10-phenanthroline hexaaza-macrocycle (Fe(Phen2N2)), to unravel how bridging atom identity and coordination geometry impact ORR activity in alkaline media. These measurements identified FePc/CNT as the most active catalyst, followed closely by Fe(Phen2N2)/CNT, with FeTPP/CNT exhibiting the lowest performance. Density functional theory simulations further demonstrated that shorter Fe−N bonds and more electronegative bridging atoms correlate with weaker *OH adsorption and higher theoretical limiting potentials. Axial coordination can alter the adsorption energetics of ORR intermediates, thereby enhancing ORR activity. Together, these results highlight the critical influence on the ORR mechanism of macrocycle bridging atoms, coordination symmetry, and axial ligation, providing molecular-level insights to guide the rational design of Fe−N4 catalysts for alkaline fuel cell applications.