Shallow Rate-Redox Potential 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 670 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 per-site-normalized
current density (jper‑site) values
that span ∼4.6 orders of magnitude across the family of compounds.
Despite the structural diversity of this family of compounds, extrapolated jper‑site values correlate with the Fe(III/II)
redox potentials in a roughly log-linear fashion with a shallow scaling
factor of approximately 145 mV/decade. These findings highlight the
phenomenology that negative shifts in EFe(III/II) track with diminishing returns in catalytic rate promotion, regardless
of the exact ORR mechanism, and suggest that changes to the primary
ligating environment in a macrocycle are insufficient to break fundamental
rate-potential scaling relationships in aqueous ORR catalysis. Together
these studies motivate the further development of higher-potential
iron complexes that employ motifs beyond the equatorial ligation plane
to enhance ORR catalysis.