Nuclear resonance vibration spectroscopy (NRVS) is a novel technique capable of probing vibrational modes of Fe atoms in samples enriched in the Mössbauer-active nucleus of ⁵⁷Fe. Compared to other vibrational spectroscopies, such as infrared or Raman, NRVS is not subject to the optical selection rules, affording full vibrational spectrum with limited spectroscopic noise. The NRVS intensity is directly related to the magnitude and direction of the motion, adding a unique quantitative capability to the spectral analysis. The technique provides insights into the dynamics of Fe atoms in the Fe-N-C catalysts for oxygen reduction reaction (ORR), which is not offered by other spectroscopic methods such as x-ray absorption spectroscopy (XAS), most commonly used for characterizing the atomic structure of Fe-sites in this class of platinum group metal-free (PGM-free) ORR catalysts.

For NRVS to be successful, it is critical for iron in the catalyst to remain in a single chemical form and to minimize contribution to the spectra from complex Fe compounds such as Fe-rich nanoparticles and clusters, routinely present in iron-based PGM-free catalysts. Since NRVS relies on the Mössbauer effect, a studied catalyst needs to be enriched in ⁵⁷Fe to maximize the signal-to-noise ratio. In this study, an (AD)Fe‑N-C catalyst, fully ⁵⁷Fe-enriched and containing exclusively atomically dispersed Fe sites, was synthesized from a metal organic framework (MOF) precursor.

As a heterogeneous electrocatalytic process, ORR is a surface reaction. Demonstrating the presence of surface Fe and providing its chemical characteristics thus represent a major step towards a better understanding of the origins of the catalytic activity of Fe-N-C catalysts. While fundamentally a bulk technique, NRVS can be made surface-specific when combined with molecules or ions capable of selectively interacting with Fe sites on the catalyst surface. The use of molecular or ionic surface probes such as nitric oxide (NO, an O₂ analog) or nitrite anion (NO₂^-) allows for the discrimination between the bulk iron and surface Fe species of interest to ORR electrocatalysis.

In this talk, we will summarize our NRVS study of the (AD) Fe-N-C ORR catalyst for oxygen reduction, which provides new and important information about the nature of Fe sites as the most likely active centers for oxygen reduction reaction on Fe-based ORR catalysts.

Acknowledgements

This research has been supported by DOE Fuel Cell Technologies Office through Electrocatalysis Consortium (ElectroCat). It used resources of the Advanced Photon Source (APS) at sector 3, a U.S. Department of Energy (DOE) Office of Science User Facility. Microscopy was performed as part of a user project supported by Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences, a DOE Office of Science User Facility.