Shallow Rate-Overpotential Scaling in Aqueous Molecular Oxygen Reduction Electrocatalysis Across a Family of Iron Macrocycles

Rate-overpotential scaling relationships have been employed widely to understand trends in oxygen reduction reaction (ORR) electrocatalysis by dissolved metal macrocycles in organic electrolytes. Similar scaling relationships remain unknown for surface-adsorbed ORR electrocatalysts in the acidic aqueous environments germane to proton-exchange membrane (PEM) fuel cells. Herein, we examine ORR catalysis in aqueous perchloric acid media for a structurally diverse array of iron macrocycle complexes adsorbed on Vulcan carbon black. The macrocycles encompass Fe–N4, Fe–N2N′2 and Fe–NxC4−x motifs bearing pyrrolic, pyridinic, and N-heterocyclic carbene (NHC) moieties in the primary ligation sphere, giving rise to a 530 mV range in Fe(III/II) redox potentials, EFe(III/II). Experimental Tafel data in the micropolarization regime were extrapolated to the EFe(III/II) to furnish estimated TOF values that span ~3 orders of magnitude across the family of compounds. Despite the structural diversity of this family of compounds, extrapolated TOF values correlate with Fe(III/II) redox potentials in a roughly log-linear fashion with a shallow scaling factor of approximately 180 mV/decade. These findings highlight that negative shifts in EFe(III/II) lead to diminishing returns in catalytic rate promotion and suggest that changes to the primary ligating environment in a macrocycle are insufficient to break fundamental rate-overpotential scaling relationships in aqueous ORR catalysis. Together these studies motivate the development of new higher-potential iron complexes that employ motifs beyond the equatorial ligation plane to enhance ORR catalysis.