Pyrrolic FeN4 models for FeNC catalysts: the influence of planarity on electronic properties and Mössbauer parameters

A promising alternative for platinum-based electrocatalysts for the Oxygen Reduction Reaction are single-atom catalysts, particularly those based on iron, nitrogen and carbon. The active sites in FeNC catalysts are conceived of as individual FeN₄ centres embedded in a carbon matrix, often approximated by a planar sheet. While the coordination of FeN₄ centres via pyridinic nitrogen atoms, i.e. six-membered rings, does not break the symmetry of the graphene plane, coordination via pyrrolic, i.e. five membered rings, induces defects in the carbon matrix that can lift its planarity. Deviation from planarity is expected to influence the electronic properties of the FeN₄ centres. An open question is whether spectroscopic techniques can detect such differences. Among these, Mössbauer spectroscopy is of central importance to characterising FeNC catalyst materials. Since pyrrolic models have recently emerged as a spectroscopically and thermodynamically consistent model for FeNC active sites, we herein compare three different pyrrolic FeN₄ models proposed in the literature and discuss whether and how these pyrrolic centres can be discerned spectroscopically.