A Combined Probe-Molecule, Mossbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst

Nonprecious metal M-N-C (M = Fe or Co) catalysts that are effective for the oxygen-reduction reaction in polymer electrolyte fuel cells have been developed, but no consensus has yet been reached regarding the nature of the M sites in these heterogeneous catalysts that are responsible for the reaction with dioxygen (O-2). While multiple studies have developed correlations between Fe distributions in as-prepared catalysts and ORR activity, the direct identification of sites reactive toward O-2 or O-2-analogue molecules remains a significant challenge. In the present study, we demonstrate a new approach to identifying and characterizing potential Fe active sites in complex ORR catalysts that combines an effective probe molecule (NO(g)), Mossbauer spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) with density functional theory (DFT) calculations. Mossbauer spectroscopic studies demonstrate that NO(g) treatment of electrochemically reduced PANI-Fe-57-C leads to a selective reaction with only a subset of the Fe species present. Nuclear resonance vibrational spectroscopic studies identified new Fe-ligand vibrations associated with the site reactive toward NO(g). DFT calculations of the vibrational properties of a selection of previously proposed active-site structures suggest that graphene zigzag edge-hosted Fe-N structures may be responsible for the observed vibrational behavior with NO(g) probe molecules. Furthermore, such sites are likely also reactive to O-2, possibly serving as the ORR active sites in the synthesized materials.