Reversibly Redox-Active Iron Oxide Structures in FeNC Catalysts Identified by Microscopy and Spectroelectrochemical EPR and Mössbauer Methods

Identifying active sites in FeNC catalysts for oxygen reduction reactions (ORR) and active site changes during preparation, storage, and electrochemical cycling are key challenges in the quest for improved catalysts. In this work, high-resolution transmission electron microscopy (TEM) is combined with ⁵⁷Fe Mössbauer and electron paramagnetic resonance (EPR) spectroscopies to investigate iron centers in high-performance FeNC catalysts with regard to their structure, coordination, and oxidation and spin states. Reversible and irreversible changes during storage, the preparation of FeNC electrodes, and their use in electrochemical cells are investigated by complementary spectroelectrochemical Mössbauer and EPR methods. Microscopy of the as-prepared FeNC materials reveals iron to be evenly distributed in isolated sites or a few atoms containing sites. Mössbauer and EPR identify weakly and strongly magnetically coupled high-spin Fe(III) in rhombically distorted octahedral coordination or superparamagnetic clusters, high-spin Fe(II) sixfold coordinated in iron oxides, and intermediate-spin Fe(II) in square planar coordination. Upon oxygen exposure, a notable oxidation state change from Fe(II) to Fe(III) is observed, the iron is less evenly distributed, and larger iron oxide nanoparticles are formed. It is noted that for this catalyst, before and after oxygen exposure, most of the iron is bound in iron oxide structures. Under the applied potential, Fe(III) is partially reduced to Fe(II) in clustered and isolated or weakly coupled sites. This change is mostly reversible, suggesting structural retention of the majority of the catalyst.