A pyridinic Fe-N4 macrocycle models the active sites in Fe/N-doped carbon electrocatalysts

Travis Marshall-Roth, Nicole J. Libretto, Alexandra T. Wrobel, Kevin J. Anderton, Michael L. Pegis, Nathan D. Ricke, Troy Van Voorhis, Jeffrey T. Miller and Yogesh Surendranath ()
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Travis Marshall-Roth: Massachusetts Institute of Technology
Nicole J. Libretto: Purdue University
Alexandra T. Wrobel: Harvard University
Kevin J. Anderton: Harvard University
Michael L. Pegis: Massachusetts Institute of Technology
Nathan D. Ricke: Massachusetts Institute of Technology
Troy Van Voorhis: Massachusetts Institute of Technology
Jeffrey T. Miller: Purdue University
Yogesh Surendranath: Massachusetts Institute of Technology

Nature Communications, 2020, vol. 11, issue 1, 1-14

Abstract: 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.

Date: 2020
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DOI: 10.1038/s41467-020-18969-6

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