In Situ Mössbauer and X-Ray Absorption Spectroscopy Studies of Atomically-Dispersed Fe-N-C Oxygen Reduction Reaction Catalysts

Abstract

The identity of the active site in iron-nitrogen-carbon polymer electrolyte fuel cell (PEFC) oxygen reduction reaction (ORR) electrocatalysts has been the subject of debate for decades. This is primarily because these catalysts are typically formed through high temperature treatment of iron-nitrogen-carbon-containing precursors, with the resulting material containing multiple iron species, and because common characterization techniques are not surface specific. For example, X-ray absorption and Mössbauer spectroscopies, two of the most widely used techniques for characterizing this class of catalysts, are bulk techniques. They, however, can be rendered surface specific by using probe molecules that adsorb/coordinate with the active site and perturb the spectral signatures of the absorbing atom. They can also be rendered surface specific by altering the oxidation state and coordination of the absorbing atom by changing the catalyst potential in an electrochemical environment. In this study we used both of these methods to probe the surface iron species in an Fe-N-C ORR catalyst. These studies were enabled by the recent development, at Los Alamos National Laboratory, of a highly active iron-zinc zeolitic imidazolate framework-derived catalyst with the vast majority of iron in atomically-dispersed sites, as determined by ex situ scanning transmission electron microscopy, Mössbauer spectroscopy, and X-ray absorption spectroscopy.1 These same techniques showed that the use of an 57Fe salt in the precursor, necessary for the in situ Mössbauer experiments, did not affect the catalyst’s morphology or Fe speciation. The results from the in situ X-ray and Mössbauer spectroscopy studies will be correlated to determine the change in the oxidation state and coordination of surface Fe sites as a function of catalyst potential. Results will be presented showing that the Fe sites are predominantly located on the surface of the catalyst and able to adsorb the probe molecule.1 Acknowledgements This work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under the auspices of the Electrocatalysis Consortium (ElectroCat). This research used the resources of the Advanced Photon Source (APS), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, also under contract DE-AC-02-06CH11357. References D.J. Myers, P. Zelenay, K.C. Neyerlin, and K.L. More, “ElectroCat (Electrocatalysis Consortium)”, 2018 Department of Energy Hydrogen and Fuel Cells Program Annual Merit Review and Peer Evaluation Meeting, Washington DC, June, 2018. (https://www.hydrogen.energy.gov/pdfs/review18/fc160_myers_2018_o.pdf)