A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts

التفاصيل البيبلوغرافية
العنوان: A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts
المؤلفون: Yogesh Surendranath, Jeffery T. Miller, Nicole J. Libretto, Travis Marshall-Roth, Alexandra T. Wrobel, Van Voorhis T, Kevin J. Anderton, Nathan D. Ricke
بيانات النشر: American Chemical Society (ACS), 2020.
سنة النشر: 2020
مصطلحات موضوعية: Bond length, Crystallography, X-ray absorption spectroscopy, biology, Chemistry, biology.protein, Active site, chemistry.chemical_element, Selectivity, Electrochemistry, Electrocatalyst, Platinum, Catalysis
الوصف: Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum in fuel cells, but their active site structures are poorly understood. A leading postulate is that iron active sites in this class of materials exist in an Fe-N4 pyridinic ligation environment. Yet, molecular Fe-based catalysts for the oxygen reduction reaction (ORR) generally feature pyrrolic coordination and pyridinic Fe-N4 catalysts are, to the best of our knowledge, non-existent. We report the synthesis and characterization of a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for oxygen reduction to a prototypical Fe-N-C material, as well as iron phthalocyanine, (Pc)Fe, and iron octaethylporphyrin, (OEP)Fe, prototypical pyrrolic iron macrocycles. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (Pc)Fe and (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of both (Pc)Fe and (OEP)Fe, and are remarkably similar to those of Fe-N-C with compressed Fe–N bond lengths of 1.97 Å in (phen2N2)Fe that are close to the average 1.94 Å length in Fe-N-C. Electrochemical studies establish that both (Pc)Fe and (phen2N2)Fe have relatively high Fe(III/II) potentials at ~0.6 V, ~300 mV positive of (OEP)Fe. The ORR onset potential is found to directly correlate with the Fe(III/II) potential leading to a ~300 mV positive shift in the onset of ORR for (Pc)Fe and (phen2N2)Fe relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe and (Pc)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe and (Pc)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with 2O2 production, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data establish (phen2N2)Fe as a pyridinic iron macrocycle that effectively models Fe-N-C active sites, thereby providing a rich molecular platform for understanding this important class of catalytic materials.
DOI: 10.26434/chemrxiv.10008545.v2
URL الوصول: https://explore.openaire.eu/search/publication?articleId=doi_dedup___::af3ffd93bd16be4b4b328e9b8f353beb
https://doi.org/10.26434/chemrxiv.10008545.v2
Rights: OPEN
رقم الانضمام: edsair.doi.dedup.....af3ffd93bd16be4b4b328e9b8f353beb
قاعدة البيانات: OpenAIRE
الوصف
DOI:10.26434/chemrxiv.10008545.v2