Understanding Active Sites in Pyrolyzed Fe-N-C Catalysts for Fuel Cell Cathodes by Bridging Density Functional Theory Calculations and 57Fe Mossbauer Spectroscopy

Pyrolyzed Fe-N-C materials are promising platinum-group-metal-free catalysts for proton-exchange membrane fuel cell cathodes. However, the detailed structure, oxidation, and spin states of their active sites are still undetermined. Fe-57 Mossbauer spectroscopy has identified FeNx moieties as the most active sites, with their fingerprint being a doublet in low-temperature Mossbauer spectra. However, the interpretation of the doublets for such materials has lacked theoretical basis. Here, we applied density functional theory to calculate the quadrupole splitting energy of doublets (Delta E-QS) for a range of FeNx structures in different oxidation and spin states. The calculated and experimental values are then compared for a reference Fe-N-C catalyst, whereas further information on the Fe oxidation and spin states was obtained from electron paramagnetic resonance, superconducting quantum interference device, and Fe-57 Mossbauer spectroscopy under external magnetic field. The combined theoretical and experimental results identify the main presence of FeNx moieties in Fe(II) low-spin and Fe(III) high-spin states, whereas a minor fraction of sites could exist in the Fe(II) S = 1 state. From the analysis of the Fe-57 Mossbauer spectrum under the external magnetic field and the comparison of calculated and measured Delta E-QS values, we assign the experimental doublet D1 with a mean Delta E-QS value of around 0.9 mm.s(-1) to Fe(III)N4C12 moieties in high-spin state and the experimental doublet D2 with a mean Delta E-QS value of around 2.3 mm.s(-1) to Fe(II)N4C10 moieties in low and medium spin. These conclusions indicate that D1 corresponds to surface-exposed sites, whereas D2 may correspond either to bulk sites that are inaccessible to O-2 or to surface sites that bind O-2 weaker than D1.