Understanding Pyrolyzed Fe-N-C Catalysts By Bridging Density Functional Theory Calculations and Mössbauer Spectroscopy
Authors: Atanassov Plamen , Matanovic Ivana , Mineva Tzonka , Sougrati Moulay-Tahar , Stievano Lorenzo , Jaouen Frederic 
Published: February 2020

Platinum group metal-free (PGM-free) catalysts for oxygen reduction are being pursued as a solution for scarcity and cost of catalysts utilizing platinum. Transition metal-nitrogen-carbon (M-N-C) materials are currently the most popular of PGM-free catalysts finding use in low temperature fuel cells. Iron-containing Fe-N-C are being synthesized lately in many variations ranging from atomically dispersed to metallic nano-particles containing types. Despite intense effort, much of the structure of these materials remains unknown and the interpretation of the activity and selectivity data rely on substantial hypothesis-based structural assumptions.

57Mössbauer spectroscopy is gaining significant importance in the experimental characterization of pyrolyzed Fe-N-C catalysts [1]. Isomer shifts and quadrupole splittings (QS), two main features of the 57Mössbauer spectra of Fe-bearing materials, vary systematically as a function of 57Fe oxidation, spin states, and coordination and can be used to accurately determine the local chemical environment of Fe ions in the Fe-N-C materials. All Fe-N-C catalysts have shown at least two clearlydistinct doublets in their Mössbauer spectra, often labeledas D1 and D2. The D1 doublet has an isomer shift value ranging from 0.33 to 0.4 mm/s and quadrupole splitting ranging from 0.7 to 1.2 mm/s, which is usually empirically attributedto Fe-N4defects with Fe2+in its low spin state. The assignment of the D2 doublet with isomer shifts ranging from 0.36 to 0.52 mm/s and with quadrupole splitting ranging from 2.4 to 2.7 mm/s is more ambiguous.It is attributedto Fe-N4 defects similar to iron phthalocyanine with Fe2+in its intermediate spin state or to a Fe-N2+2 defect in which Fe binds to four pyridinic nitrogen atoms from two adjacent nitrogen-doped graphene layers. However, the assignment of these doublets has been basedon experimental reports on carbon-supported FeN4macrocycles, which differs greatly from the case in which the Fe-Nxmoieties are integratedintothe graphene matrix. Therefore, there is a need for combining density functional theory (DFT) calculations of isomer shifts and quadrupole splittings on more realistic models, and compare them to those experimentally extracted with different fitting approaches from the Mössbauer spectraof real Fe-N-C materials.
The DFT theory has been applied previously to calculate 57Fe isomeric shifts and quadrupolesplitting for different Fe-containing molecules, but has not yet been appliedtothe Fe-Nxdefects incorporated in the graphene matrix. In this work, both cluster and periodic approacheswere used to calculate thequadrupolesplitting in Fe-N4defects that are identified as the active sites in the pyrolyzed Fe-N-C materials. Because of the well-documented dependence of the DFT computed QS on the exchange-correlation functionals and basis sets [2],the methodology has been first tested for benchmark systems, shown in the Figure. Extensive computations of QS in Fe-N4models with Fe in various coordinationsand with different oxidation/spin states have also been carried out. Furthermore, the DFT calculated values were compared with experimental data to interpret quadrupole doublets ubiquitous in the Mössbauer spectra of pyrolyzed Fe-N-C catalysts. This paper will bring to discussion both computational approaches and correlations of 57Mössbauerspectroscopy “fingerprints” with observed and hypothesized structural motifs of Fe-containing active sites in PGM-free electrocatalysts for oxygen reduction reaction.

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Figure 1