PNNL

Abstract

Improvement of electrocatalysts for the oxygen reduction reaction (ORR) is critical for the advancement of fuel cell technologies. Herein, we report a series of eleven soluble iron porphyrin ORR electrocatalysts that possess turnover frequencies (TOFs) from 3 s-1 to an unprecedented 2.2 x 106 s-1. These TOFs correlate with the ORR overpotential, which can be changed by modulating the ancillary ligand, by varying the reaction conditions or by changing the catalyst’s protonation state. This is the first such correlation for homogeneous ORR electrocatalysis, and it demonstrates that the remarkably fast TOFs are a consequence of the high overpotential. Computational studies indicate that the correlation is analogous to the volcano plot analysis developed for heterogeneous ORR materials. This unique parallel between homo- and heterogeneous ORR electrocatalysts allows a fundamental understanding of intrinsic barriers associated with the ORR, which can aid the design of new catalytic systems that operate at low overpotential. This research was supported as part of the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences. Additional data is given in the Electronic Supporting Information.