Dynamic Structures of the Active Sites in Iron-Based Catalysts during ORR

The prohibitive cost of platinum catalysts for accelerating the sluggish oxygen reduction reaction (ORR) severely hampers the commercialization of fuel cells. Despite recent progress in developing highly active non-precious catalysts (typically Fe−N_x−C) as promising alternatives to Pt, a less empirical synthesis approach is still missing due to the lack of fundamental understanding of the nature of the active sites, as well as their structural effects on ORR activity.

 Herein, the structural and electronic properties of iron meso-tetraphenylporphine chloride (FeTPPCl) pyrolyzed under various temperatures, as well as the state-of-the-art polyaniline (PANI)-derived catalyst [1] are probed using in situ x-ray absorption spectroscopy (XAS) in combination with ab initio calculations. Three Fe²⁺−N₄−like active sites with distinct Fe displacement are identified. All the active sites undergo Fe²⁺/Fe³⁺redox transition that triggers the adsorption of hydroxyl intermediate (OH*) from water activation, but they exhibit distinctly different Fe-N switching behavior during ORR, which accounts for their different ORR activities.

Acknowledgements:

The PANI-derived catalysts were prepared and provided by Gang Wu and Piotr Zelenay (Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, NM 87545). Use of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory (BNL), was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. This publication was made possible by the Center for Synchrotron Biosciences grant, P30-EB-009998, from the National Institute of Biomedical Imaging and Bioengineering (NBIB). Support from beamline personnel Dr. Erik Farquhar and Mark Chance (X3B) are gratefully acknowledged.

References

(1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443-447.