Homogenous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions

Authors

Michael L. Pegis, Yale University
Bradley A. McKeown, Yale University
Neeraj Kumar, Pacific Northwest National Laboratory (PNNL)
Kai Lang, University of South Florida
Derek J. Wasylenko, University of Washington
Peter X. Zhang, Boston College
Simone Raugei, Pacific Northwest National Laboratory (PNNL)
James M. Mayer, Yale University

Document Type

Article

Publication Date

2016

Digital Object Identifier (DOI)

https://doi.org/10.1021/acscentsci.6b00261

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^–1 to an unprecedented value of 2.2 × 10⁶ s^–1. These TOFs correlate with the ORR overpotential, which can be modulated by changing the E_1/2 of 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 the E_1/2 of 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 catalyst E_1/2, 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.

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Citation / Publisher Attribution

ACS Central Science, v. 2, issue 11, p. 850-856

This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Scholar Commons Citation

Pegis, Michael L.; McKeown, Bradley A.; Kumar, Neeraj; Lang, Kai; Wasylenko, Derek J.; Zhang, Peter X.; Raugei, Simone; and Mayer, James M., "Homogenous Electrocatalytic Oxygen Reduction Rates Correlate with Reaction Overpotential in Acidic Organic Solutions" (2016). Chemistry Faculty Publications. 112.
https://digitalcommons.usf.edu/chm_facpub/112