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

Identifying active sites in FeNC catalysts for oxygen reduction reactions (ORR) and their alteration 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 their oxidation and spin states. Reversible and irreversible changes in these properties during the preparation of FeNC electrodes and their use in electrochemical cells are investigated by complementary spectroelectrochemical Mössbauer and EPR measurements. Microscopy of the as prepared FeNC materials reveals iron to be evenly distributed in isolated sites or few iron atoms containing sites. Mössbauer and EPR identify strongly and weakly magnetically coupled high-spin Fe(III) in oxygenic coordination environment or superparamagnetic clusters, high-spin Fe(II) six 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 amount of isolated and clustered iron are reduced 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 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. All except the intermediate-spin Fe(II) in square planar coordination were found to undergo significant changes during electrochemical treatment. The observed structural variability and in particular the reversible redox activity of iron oxide structures may indicate that they also contribute to the ORR activity of FeNCs.