Proton exchange fuel cells (PEFCs) are a promising technology for energy conversion, but are not yet commercialized due to highly expensive platinum as state-of-the-art catalyst for the cathodic oxygen reduction reaction (ORR). As low-cost materials with earth abundant metals, non-precious metal and nitrogen doped carbon catalysts (Me-N-C) have become a viable alternative for platinum. Encouraging results were especially achieved with the use of iron as the metal center.[1]

Me-N-C catalysts are obtained from pyrolysis of different metal, nitrogen and carbon precursors, which results in highly amorphous structures. The latter causes difficulties for the spectroscopic characterization of the active sites. Thus the exact chemical nature of the active center and the oxygen reduction reaction (ORR) mechanism are still being debated. [2]

So far, three sites were proposed as possible contributors to the ORR activity. In Mössbauer spectroscopy they give rise to the D1, D2 and D3 doublets. They are all assigned to some type of FeN₄ centers that are either integrated in graphene layers (D1: FeN₄) or situated between two graphene layers (D2 and D3: FeN₂₊₂). [3] While in the case of the D1 doublet, FeN₄ macrocycles seem to work well as a model system, in the case of the FeN₂₊₂ sites it is difficult to find suitable molecular model systems as references for the interpretation of the Mössbauer data. Furthermore, in the literature this site is often displayed as a flat structure. However, it remains unclear to what extent the graphene layers could possibly be tilted against each other.

In this contribution, we will use density functional theory calculations to determine structure-property relationships of FeN₂₊₂ site models with different Fe coordination environments – in particular the angle between adjacent graphene layers – with respect to relative energy, preferred spin state, Mössbauer spectroscopic parameters, and likely reaction intermediates for ORR.

[1] Szakacs, C.E. et al., Phys. Chem. Chem. Phys., 2014, 16, 13654.

[2] Kramm, U.I. et al., Adv. Mater., 2019, 1805623.

[3] Kramm, U.I. et al., Phys. Chem. Chem. Phys., 2012, 14, 11673–11688.