This work addresses the importance of Mössbauer spectroscopy for the characterization of iron‐containing electrocatalysts. The most important aspects of electrocatalysis and Mössbauer spectroscopy are summarized. Next, Fe–N–C catalysts and important conclusions made by this technique on preparation, active site identification and degradation are summarized. Furthermore, recent highlights derived for other iron‐containing electrocatalysts are summarized.

FeNC catalysts are the most promising substitutes for Pt‐based catalysts for the oxygen reduction reaction in proton exchange fuel cells. However, it remains unclear which FeN4 moieties contribute to the reaction mechanism and in which way. The origin of this debate could lie in various preparation routes, and therefore the aim of this work is to identify whether the active site species differ in different preparation routes or not. To answer this question, three FeNC catalysts, related to the three main preparation routes, are prepared and thoroughly characterized. Three transitions A–C that are distinguished by a variation in the local environment of the deoxygenated state are defined. By in situ 57Fe Mössbauer spectroscopy, it can be shown that all three catalysts exhibit a common spectral change assigned to one of the transitions that constitutes the dominant contribution to the direct electroreduction of oxygen. Moreover, the change in selectivity can be attributed to the presence of a variation within additional species. Density functional theory calculations help to explain the observed trends and enable concrete suggestions on the nature of nitrogen coordination in the two FeN4 moieties involved in the oxygen reduction reaction of FeNC catalysts.

For large-scale utilization of fuel cells in a future hydrogen-based energy economy, affordable and environmentally benign catalysts are needed. Pyrolytically obtained metal-and nitrogen-doped carbon (MNC) catalysts are key contenders for this task. Their systematic improvement requires detailed knowledge of the active site composition and degradation mechanisms. In FeNC catalysts, the active site is an iron ion coordinated by nitrogen atoms embedded in an extended graphene sheet. Herein, we build an active site model from in situ and operando 57 Fe Mossbauer spectroscopy and quantum chemistry. A Mossbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mossbauer signatures (D2, D3a, D3b), implying a direct structural correspondence. Pyrrolic N-coordination, i.e., FeN 4 C 12 , is found as a spectroscopically and thermodynamically consistent model for the entire catalytic cycle, in contrast to pyridinic nitrogen coordination. These findings thus overcome the previously conflicting structural assignments for the active site and, moreover, identify and structurally assign a previously unknown intermediate in the oxygen reduction reaction at FeNC catalysts.

To advance the widespread implementation of electrochemical energy storage and conversion technologies, the development of inexpensive electrocatalysts is imperative. In this context, Fe/N/C-materials represent a promising alternative to the costly noble metals currently used to catalyze the oxygen reduction reaction (ORR), and also display encouraging activities for the reduction of CO 2 . Nevertheless, the application of these materials in commercial devices requires further improvements in their performance and stability that are currently hindered by a lack of understanding of the nature of their active sites and the associated catalytic mechanisms. With this motivation, herein the authors exploit the high sensitivity of modulation excitation X-ray absorption spectroscopy toward species undergoing potential-induced changes to elucidate the operando local geometry of the active sites in two sorts of Fe/N/C-catalysts. While the ligand environment of a part of both materials' sites appears to change from six-/five-to fourfold coordination upon potential decrease, they differ substantially when it comes to the geometry of the coordination sphere, with the more ORRactive material undergoing more pronounced restructuring. Furthermore, these time-resolved spectroscopic measurements yield unprecedented insights into the kinetics of Fe-based molecular sites' structural reorganization, identifying the oxidation of iron as a rate-limiting process for the less ORR-active catalyst.

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O 2 or CO 2 , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.Platinum-group metal (PGM-) free catalysts of the M/N/Ctype (whereby M corresponds to a 3d transition metal) hold great potential as inexpensive replacements for conventional, noble-metal-based materials. Originally developed as electrocatalysts for the oxygen reduction reaction (ORR, relevant to fuel cells [1,2] and metal-air batteries [3][4][5] ), these materials have recently been successfully employed in other catalytic processes, like CO 2 -electroreduction [6,7] or the oxidation of benzene to phenol. [8,9] Nevertheless, their commercial implementation requires further improvements in their activity and stability [1,10] that call for a better understanding of the reactions mechanism and their relation to the operando oxidation-, spin-state and orbital configuration of the Ncoordinated metal sites (M-N x ) regarded as their active centers. [11][12][13][14] These properties have been generally assessed using Mçssbauer [12,15,16] and X-ray absorption [17][18][19] spectroscopy (MS, XAS), which are highly sensitive techniques but also suffer from certain drawbacks. MS, on the one hand, allows distinguishing the chemical environment of the metal species present in iron-based M/N/C catalysts, which have been shown to be the most ORR-active among this material class; however, their heterogeneous composition results in complex spectra requiring a careful deconvolution. The latter is occasionally complemented by the assignment of spin-and oxidation-states to these deconvolution components, based on similarities between the Mçssbauer parameters of the Fe-N x sites in these Fe/N/C catalysts and those of compounds with resembling but better defined M-N 4 centers (like porphyrins or phthalocyanines). [12] However, the long-range structures and electronic properties of these reference compounds are likely to differ from those of the catalysts active sites, thus making a full conclusive analysis difficult. Furthermore, the combination of the low temporal resolution of MS and the low metal contents of M-N x sites (typically < 2 wt. %) in M/N/C catalysts le...