The nature of the active site(s) of platinum group metal-free (PGM-free) electrocatalysts for the oxygen reduction reaction (ORR) remains a topic of intense debate. This is mainly due to the complex heterogeneous character produced during the high-temperature synthesis process required for the generation of electrocatalytically active materials. Quantum chemical modeling of possible active site structures has proposed heme-like moieties in which pyridinic nitrogen groups are coordinated to transition-metal center forming a Fe-N_x-type of structure (1-3). Nevertheless, unlike heme structures that easily poison with strongly bound molecules such as CO, PGM-free electrocatalysts have shown extreme resilience to a number of commonly known poisoning molecules, e.g., CO, H₂S, SO₂ (4, 5). Thus, the difficulty of developing a characterization technique based on molecular probes will not only rely on the identification of non-traditional probes but also on the successful recovery of electrocatalytic activity required to estimate active site density based on a proposed electrochemical reaction mechanism of poisoning and stripping. A molecular probe approach in which a particular molecule interacts specifically with the active site by forming a stable chemical adduct causing a decrease in electrocatalytic activity, and when removed fully recovers its initial activity, can be a powerful tool for the characterization and quantification of the active site(s). Development of such technique will not only provide experimental evidence about the nature of the active site(s), vital for the synthesis of next-generation PGM-free catalyst with more durable and increased concentration of active sites, but also will become a valuable tool for the further understanding of more complex processes such as reaction pathways and degradation mechanisms.

Due to the extraordinary tolerance of PGM-free electrocatalysts to a number of commonly used poisons, nitrogen-containing molecules: nitrite (NO₂^-) and nitric oxide (NO), are proposed as molecular probes as they have been reported to interact with Fe-heme structures in electrochemical environments (6). Furthermore, recent ex-situ studies reported by our group and collaborators showed that the NO probe molecule directly interacts with Fe_xN_y structures (7, 8). In this work, the poisoning extent of ORR activity in acidic electrolytes caused by the formed chemical adducts is investigated using electrochemical half-cell experiments. The specificity of the probe molecules was studied via X-ray spectroscopic techniques: XPS, EXAFS, and XANES, to better understand their interaction with Fe-centered Fe_xN_y moieties. Quantum chemical models of proposed active site interactions with probe molecules are used to determine possible binding motifs and relative binding energies of different sites/molecules. The combination of experimental and computational approaches in this work will yield a powerful suite of tools for interpreting the nature and density of active sites in PGM-free electrocatalysts.

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Electrocatalysis Consortium (ElectroCat).

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