Shallow Rate-Redox\nPotential Scaling in Aqueous Molecular\nOxygen Reduction Electrocatalysis Across a Family of Iron Macrocycles

Rate-overpotential scaling relationships have been employed\nwidely\nto understand trends in oxygen reduction reaction (ORR) electrocatalysis\nby dissolved metal macrocycles in organic electrolytes. Similar scaling\nrelationships remain unknown for surface-adsorbed ORR electrocatalysts\nin the acidic aqueous environments germane to proton-exchange membrane\n(PEM) fuel cells. Herein, we examine ORR catalysis in aqueous perchloric\nacid media for a structurally diverse array of iron macrocycle complexes\nadsorbed on Vulcan carbon black. The macrocycles encompass Fe–N<sub>4</sub>, Fe–N<sub>2</sub>N′<sub>2</sub> and Fe–N<sub><i>x</i></sub>C<sub>4–<i>x</i></sub> motifs\nbearing pyrrolic, pyridinic, and N-heterocyclic carbene (NHC) moieties\nin the primary ligation sphere, giving rise to a 670 mV range in Fe(III/II)\nredox potentials, <i>E</i><sub>Fe(III/II)</sub>. Experimental\nTafel data in the micropolarization regime were extrapolated to the <i>E</i><sub>Fe(III/II)</sub> to furnish estimated per-site-normalized\ncurrent density (<i>j</i><sub>per‑site</sub>) values\nthat span ∼4.6 orders of magnitude across the family of compounds.\nDespite the structural diversity of this family of compounds, extrapolated <i>j</i><sub>per‑site</sub> values correlate with the Fe(III/II)\nredox potentials in a roughly log-linear fashion with a shallow scaling\nfactor of approximately 145 mV/decade. These findings highlight the\nphenomenology that negative shifts in <i>E</i><sub>Fe(III/II)</sub> track with diminishing returns in catalytic rate promotion, regardless\nof the exact ORR mechanism, and suggest that changes to the primary\nligating environment in a macrocycle are insufficient to break fundamental\nrate-potential scaling relationships in aqueous ORR catalysis. Together\nthese studies motivate the further development of higher-potential\niron complexes that employ motifs beyond the equatorial ligation plane\nto enhance ORR catalysis.