A Pyridinic Fe-N4 Macrocycle Effectively Models the Active Sites in Fe/N-Doped Carbon Electrocatalysts
2019-10-23T18:31:06Z (GMT) by
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 and iron octaethylporphyrin, (OEP)Fe, a prototypical pyrrolic iron macrocycle. N 1s XPS signatures for coordinated N atoms in (phen2N2)Fe are positively shifted relative to (OEP)Fe, and overlay with those of Fe-N-C. Likewise, spectroscopic XAS signatures of (phen2N2)Fe are distinct from those of (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 similar to the average 1.94 Å length in Fe-N-C. Electrochemical data indicate that the iron center in (phen2N2)Fe is relatively electropositive, with an Fe(III)-OH/Fe(II)-OH2 potential at 0.59 V vs the reversible hydrogen electrode (RHE), ~300 mV positive of (OEP)Fe. This correlates with a 300 mV positive shift in the onset of ORR catalysis for (phen2N2)Fe with a corresponding 1400-fold increase in TOF relative to (OEP)Fe. Consequently, the ORR onset for (phen2N2)Fe is within 150 mV of Fe-N-C. Unlike (OEP)Fe, (phen2N2)Fe displays excellent selectivity for 4-electron ORR with <4% maximum H2O2 production, comparable to Fe-N-C materials. This study establishes a pyridinic iron macrocycle that effectively models Fe-N-C active sites and provides a rich platform for constructing high-performance Fe-based oxygen reduction catalysts.