A pyridinic Fe-N 4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

• Published in: Nature communications Vol. 11; no. 1; p. 5283
• Main Authors: Marshall-Roth, Travis, Libretto, Nicole J, Wrobel, Alexandra T, Anderton, Kevin J, Pegis, Michael L, Ricke, Nathan D, Voorhis, Troy Van, Miller, Jeffrey T, Surendranath, Yogesh
• Format: Journal Article
• Language: English
• Published: England 19.10.2020
• ISSN: 2041-1723
• DOI: 10.1038/s41467-020-18969-6

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-N ligation environment, yet, molecular model catalysts generally feature pyrrolic coordination. Herein, we report a molecular pyridinic hexaazacyclophane macrocycle, (phen N )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 (phen N )Fe are remarkably similar to those of Fe-N-C. Electrochemical studies reveal that (phen N )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, (phen N )Fe displays excellent selectivity for four-electron ORR, comparable to Fe-N-C materials. The aggregate spectroscopic and electrochemical data demonstrate that (phen N )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.