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

Abstract

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 these 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.