Understanding the Superior ORR Activity of the Fe-N2+2 to That of the Fe-N4 Site

Despite recent progress made in the development of M–N_x–C catalysts, the exact nature of the active sites formed during pyrolysis, as well as the reaction mechanisms remains unclear. Herein, in situ x-ray absorption spectroscopy (XAS) measurements were performed on representative iron–based catalysts [1-3] to identify the nature of the active sites in these catalysts. XAS offers unique insights into electrode processes by providing simultaneous electronic and structural information on the electrode materials under actual in situ cell operating conditions [4]. In particular, Δμ technique, which has evolved into a powerful tool for elucidating surface adsorbed species in  electrocatalysis, was utilized to probe the local structure of the active sites and their interactions with reaction intermediates [3-5]. In addition, density function theory calculations were conducted to correlate the structure to the ORR activity.

We showed that the original macrocycle (denoted as Fe-N₄), in which the N is pyrolylic type can be differentiated from the active sites formed upon high temperature pyrolysis (denoted as Fe–N₂₊₂), in which the N is pyridinic type by using Δµ. Combined EXAFS and XANES showed that the central Fe in Fe–N₂₊₂ is off the N₄–plane and in high spin state, whereas the central Fe in Fe–N₄ is precisely in the N₄-plane and in intermediate spin state. Upon the adsorption of ORR intermediates at high potential region, the displacement, the oxidation state, and the spin state of the central Fe change in different ways between Fe–N₄ and Fe–N₂₊₂ sites. DFT calculations showed that this different Fe–N switching behavior accounts for the superior ORR activity of Fe–N₂₊₂ to Fe–N₄.

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.

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