title: Homogenous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions.
abstract: Improved electrocatalysts for the oxygen reduction reaction (ORR) are critical for the advancement of fuel cell technologies. Herein, we report a series of 11 soluble iron porphyrin ORR electrocatalysts that possess turnover frequencies (TOFs) from 3 s-1to an unprecedented value of 2.2 × 106s-1. These TOFs correlate with the ORR overpotential, which can be modulated by changing theE1/2of the catalyst using different ancillary ligands, by changing the solvent and solution acidity, and by changing the catalyst's protonation state. The overpotential is well-defined for these homogeneous electrocatalysts by theE1/2of the catalyst and the proton activity of the solution. This is the first such correlation for homogeneous ORR electrocatalysis, and it demonstrates that the remarkably fast TOFs are a consequence of high overpotential. The correlation with overpotential is surprising since the turnover limiting steps involve oxygen binding and protonation, as opposed to turnover limiting electron transfer commonly found in Tafel analysis of heterogeneous ORR materials. Computational studies show that the free energies for oxygen binding to the catalyst and for protonation of the superoxide complex are in general linearly related to the catalystE1/2, and that this is the origin of the overpotential correlations. This analysis thus provides detailed understanding of the ORR barriers. The best catalysts involve partial decoupling of the influence of the second coordination sphere from the properties of the metal center, which is suggested as new molecular design strategy to avoid the limitations of the traditional scaling relationships for these catalysts.
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Pegis
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.
authors.forename: Bradley A
authors.initials: BA
authors.lastname: McKeown
authors.affiliation: Pacific Northwest National Laboratory (PNNL) , Richland, Washington 99352, United States.
authors.forename: Neeraj
authors.initials: N
authors.lastname: Kumar
authors.affiliation: Department of Chemistry, University of South Florida , Tampa, Florida 33620, United States.
authors.forename: Kai
authors.initials: K
authors.lastname: Lang
authors.affiliation: Department of Chemistry, University of Washington , Seattle, Washington 98195-1700, United States.
authors.forename: Derek J
authors.initials: DJ
authors.lastname: Wasylenko
authors.affiliation: Department of Chemistry, Merkert Chemistry Center, Boston College , Chestnut Hill, Massachusetts 02467, United States.
authors.forename: X Peter
authors.initials: XP
authors.lastname: Zhang
authors.affiliation: Pacific Northwest National Laboratory (PNNL) , Richland, Washington 99352, United States.
authors.forename: Simone
authors.initials: S
authors.lastname: Raugei
authors.affiliation: Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.
authors.forename: James M
authors.initials: JM
authors.lastname: Mayer