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

Abstract

Nonprecious metal M−N−C (M = Fe or Co) catalysts that are effective for the oxygen-reduction reaction in polymerelectrolyte 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− 57 Fe−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.

Elucidating the Structural Composition of an Fe–N–C Catalyst by Nuclear‐ and Electron‐Resonance Techniques

“…The modes between 250 cm −1 and 380 cm −1 are assigned to Fe−N pyr . A very dominant feature around 381 cm −1 and 395 cm −1 can be seen, which was not found by Kneebone et al . for their PANI‐based catalyst in any of the investigated states (reduced or NO‐treated).…”

“…Kneebone et al . used Mössbauer spectroscopy at 80 K and nuclear inelastic scattering (NIS) to characterize a PANI‐based Fe–N–C catalyst.…”

Electrochemical Evidence for Two Sub‐families of FeNxCy Moieties with Concentration‐Dependent Cyanide Poisoning

“…g . different ORR activities and Fe 2+/3+ redox potentials (Figure d)), and iii) electrochemical perturbation of FeN x C y moieties during reduction reaction require additional studies to address the reversible/irreversible Fe sites more clearly. We believe that the in situ observation of two different electrochemical behaviors of atomic FeN x C y sites is however still meaningful, and potentially guides rational design of next‐generation Fe−N−C catalysts.…”

Electrochemical estimation of active site density on a metal-free carbon-based catalyst

“…9 Besides, physical methods such as neutron activation and Mössbauer spectroscopy were also used to identify the possible ORR active centers in an Fe/N-C catalyst. [10][11][12][13] Recently, low temperature pulse CO chemisorption was applied to estimate the active site density of NPMCs, assuming the metal as the only ORR active center. 14 It was reported that, even at low temperatures (193 AE 5 K), CO does not adsorb on the nitrogen center of the catalyst.…”

“…In another report, NO (g) was used as the probe molecule to detect the iron center in Fe/N-C catalyst. 13 Thus, development of an electrochemical method to estimate the active site density of metal-free carbon and nitrogen-doped carbon is paramount.…”