Nuclear Resonance Vibrational Spectroscopy and Mössbauer Spectroscopy Studies of Atomically Dispersed (AD)57fe-N-C Oxygen Reduction Reaction Catalysts for Polymer Electrolyte Fuel Cells

Abstract

Polymer electrolyte fuel cells (PEFCs) are a key technology to realize a hydrogen-based economy in the transportation sector, expected to significantly reduce greenhouse gas emission. Platinum (Pt) continues to be the electrocatalyst of choice for oxygen reduction reaction (ORR) in PEFCs, despite the fact that the scarcity, high cost, and monopolized global distribution of this metal severely impede its large-scale commercialization. As an alternative approach, substantial efforts have been made to move towards PGM-free catalysts, the best performing of which are heat-treated iron, nitrogen, carbon (Fe-N-C) materials. While promising, the Fe-N-C catalysts face significant challenges on the way to becoming viable. One of the challenges is the need for identifying the ORR active site as a prerequisite for further development of Fe-N-C catalysts. The nature of the active site has puzzled researchers for decades, with the very presence of Fe being often questioned in the highly acidic environment of the PEFCs. Demonstrating the presence of surface Fe and its coordination environment would thus represent a major step towards a better understanding of the origins of catalytic activity in PGM-free catalysts. In this work, we applied Fe-specific characterization techniques, nuclear resonance vibrational spectroscopy (NRVS) and Mössbauer spectroscopy (MS) for the purpose of detecting Fe on the surface of Fe-N-C catalysts. While NRVS and MS are bulk methods, the use of a differential technique, in which the spectrum for catalyst recorded without an Fe-specific surface probe is subtracted from the probe-treated material, allows for obtaining spectrum representative of the surface Fe only. In the case described in this presentation, we applied this differential technique using NO (an O2 analog) as a probe of the surface Fe. Then, by combining the NRVS and MS signatures with DFT modeling we were able to obtain electronic and structural information for the surface Fe sites. For the described approach to be successful it is critical for Fe in the catalyst to remain in a single chemical form, relevant to the ORR. A majority of PGM-free synthesis methods based on heat-treatment of a mixture of iron, nitrogen, and carbon yield highly heterogeneous materials, containing Fe in various chemical forms, including in particular Fe-rich nanoparticles. These nanoparticles contribute to the NRVS and MS signals, making detection of the ORR-relevant atomically dispersed surface Fe species difficult or altogether impossible. To address this challenge, LANL recently developed a nanoparticle-free, metal organic framework (MOF)-derived PGM-free ORR catalyst, (AD)Fe-N-C. For the purpose of NRVS and MS experiments, this catalyst was also fully enriched with the 57Fe isotope of iron. The (AD)57Fe-N-C catalyst was then treated with NO as a probe of surface Fe sites and characterized using differential NRVS and MS. In this presentation, we will summarize the results of NRVS and MS experiments that attest to the presence of Fe sites on the surface of the atomically dispersed Fe-N-C catalysts for ORR. Acknowledgements This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat). This research used resources of the Advanced Photon Source (APS) at sector 3, 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 DE-AC02-06CH11357.