Abstract
Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; however, their active site structures remain poorly understood. A leading postulate is that the iron-containing active sites exist primarily in a pyridinic Fe-N4 ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen2N2)Fe, and compare its spectroscopic, electrochemical, and catalytic properties for ORR to a typical Fe-N-C material and prototypical pyrrolic iron macrocycles. N 1s XPS and XAS signatures for (phen2N2)Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen2N2)Fe has a relatively high Fe(III/II) potential with a correlated ORR onset potential within 150 mV of Fe-N-C. Unlike the pyrrolic macrocycles, (phen2N2)Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen2N2)Fe is a more effective model of Fe-N-C active sites relative to the pyrrolic iron macrocycles, thereby establishing a new molecular platform that can aid understanding of this important class of catalytic materials.
Highlights
Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; their active site structures remain poorly understood
Fe-N-C materials are typically prepared by the high-temperature pyrolysis of finely dispersed iron salts[16], porphyrins[17], or phthalocyanines[18] along with a metal-organic framework (MOF)[19] or carbon-based support[20]
A prototypical Fe-N-C material was synthesized by a combination of literature methods[19,48]
Summary
Iron- and nitrogen-doped carbon (Fe-N-C) materials are leading candidates to replace platinum catalysts for the oxygen reduction reaction (ORR) in fuel cells; their active site structures remain poorly understood. N 1 s peaks for (phen2N2)FeCl, (Pc)FeCl, and (OEP)FeCl (Supplementary Fig. 11) appear within 0.1–0.4 eV of the corresponding peaks for the μoxo compounds, indicating that the identity of the axial ligand does not impact the N 1s binding energy to the same extent as does changing the metal-coordinated nitrogen atoms from pyrrolic to pyridinic.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
