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.

FeNC materials are considered as promising catalysts for fuel cell application and CO2 reduction. However, a general obstacle is their instability when stored at ambient conditions, typically this resulted in structural changes and loss of ORR activity as observed in half-cell measurements. So far little work was done to systematically explore the root-cause of this phenomenon and to correlate it with FC performance data. In this work, we investigated the degradation of two distinct FeNC catalysts (one initially pure and one impure) under various storage conditions to identify the impact of gas and temperature and to see to what extent the degradation depends on the purity of the pristine material. The catalysts are characterized by 57Fe Mossbauer spectroscopy, Xray diffraction, scanning transmission electron microscopy and X-ray absorption spectroscopy to follow structural changes as well as by rotating ring disk electrode experiments and fuel cell tests to check for performance changes. Our findings reveal that degradation occurs across all tested environments, albeit with varying degrees of severity. While inert storage under argon gas leads to Fe3C formation, storage under air and 80 degrees C causes further inorganic side phases to appear and had the strongest impact on performance. We identified iron carbide and iron(III)oxide as the main degradation products and assessed their influence on the oxygen reduction reaction (ORR) activity and selectivity. In this context, it was shown that the degradation of specific FeN4 sites leads to the inorganic phases mentioned above. Notably, our results indicate here that axial ligands play a critical role in influencing both site stability and ORR performance, highlighting their significance in the catalytic behavior of FeNC systems.

FeNC materials are a promising alternative to substitute platinum catalysts in proton exchange membrane fuel cells (PEMFCs) for electrocatalysis of the oxygen reduction reaction (ORR). However, it remains an active research challenge to improve their stability, based on the interconnection of the multiple degradation mechanisms such as carbon corrosion, active site (FeN4) demetalation, decrease of hydrophobicity, etc. In this study, we compare the impact of different purification strategies involving ball milling and forming gas (N2/H2) heat treatment followed by acid leaching designed to remove efficiently the remaining Fe-inorganic species after pyrolysis, which may promote the degradation of the catalyst and membrane during FC tests. Through a comprehensive structural characterization of the final composition by X-ray diffraction, X-ray absorption, and X-ray emission spectroscopies and carbon morphology (transmission electron microscopy and Raman spectroscopy) of the FeNCs, we achieved an efficient removal of Fe3C species, accompanied by distinct alterations to the carbon morphology, which prove to be crucial for the material's stability. Our results highlight the enhanced stability of the catalyst treated under N2/H2, which retained 82% of its initial current density while held at 0.5 V for 24 h in H2/Air during FC testing. With this study, we prove the importance of FeNC catalyst's purification and its benefit in fuel cell activity and stability.

FeNC catalysts are very promising candidates for the oxygen reduction reaction in proton exchange fuel cells (FCs). Their preparation is dominated by pyrolysis approaches, which however lead also to the formation of inorganic impurity species. While a subsequent removal of iron access is thus desirable, even after acid leaching (AL), some side phases still remain in the catalyst. Such species might leach out during operation and cause severe degradation by Fenton's reaction and subsequent formation of reactive oxygen species. However, it remains under debate if iron from the catalytically active FeN4 centers also contributes in this degradation mechanism. To get further insights, different strategies to improve the AL are compared with respect to structural (XAS, XES, and ICP) and morphological (TEM, Raman, and hydrophobicity) changes and their impact on the activity and more important stability of FeNC catalysts. In all cases, the iron content was significantly reduced. While the positive impact of purification on the performance in half-cell measurements is not obvious, almost doubling of the FC activity is observed, which goes hand in hand with an improved stability. Thus, this work underlines the importance of thorough ex situ leaching to avoid severe degradation.

Evaluation of the electrocatalyst performance data includes an electrode preparation step. Herein, we compare the structural composition of Fe-N-C materials, used to electrocatalyze the oxygen reduction reaction in proton-exchange membrane fuel cells, before and after catalyst layer preparation. The effects of this step on the electronic structure and local coordination of Fe were investigated by X-ray absorption (XAS) and emission spectroscopies (XES), for Fe-N-C materials prepared via different synthetic routes. This work underlines the importance of determining the Fe-N-C catalyst structure in the prepared electrode for further studies of the structure-activity-stability correlations.

Hydrogen peroxide is known to have a detrimental effect on the stability of FeNC catalysts, as previously concluded from the comparison of differently prepared FeNC catalysts. However, beside the release of hydrogen peroxide, the iron composition as well as the carbon morphology changes. Different iron species might cause varying degrees of demetallation (associated with Fenton reaction and reactive oxygen species), whereas the carbon morphology is characteristic of the ability to persist under oxidative conditions. Thus, the true effect of H2O2 is difficult to understand from the comparison of different FeNC catalysts. To overcome this, we explored the relation between H2O2 formation and FC performance for an FeNC catalyst and a series of catalysts obtained by modification of this catalyst with different precious group metal (PGM) nanoparticles (Pd, Ag, Ir, Au). At two catalyst loadings, we performed detailed electrochemical investigations to identify changes in the oxygen reduction reaction pathway induced by accelerated stress tests mimicking the load cycle conditions. Moreover, hydrogen peroxide oxidation and reduction experiments were used to identify changes in the kinetics before and after load cycles. The results were compared to FC activity and short stability tests. While all modified catalysts exhibited a higher degree of H2O2 formation, the addition of small quantities of PGM nanoparticles improved the stability in FC. The latter effect can be associated with an enhanced HPRR kinetic.

Proton exchange fuel cells (PEFCs) are a clean technology for efficient conversion of chemical into electrical energy and are specifically promising for the decarbonization of heavy duty vehicles [1]. Currently, the drawback of PEFCs is the high cost of Pt-based catalysts used for cathode and anode, which hinders their commercialization. [2] The rapid development of FeNCs holds promise for replacing Pt-based catalysts for the oxygen reduction reaction (ORR). The nature and characterization of the FeNC active sites is a challenging subject of research, and the exact structure of intrinsic active center for FeNC catalysts is still under debate. [3-6] 57 Fe Mössbauer Spectroscopy is powerful in obtaining knowledge of iron sites, with respect to structural composition, electronic states as well as magnetic environment [3,7-9]. To solve the debate, 57 Fe Mössbauer experiments were carried out under ex situ , in situ , or operando conditions to identify iron signatures and their changes induced by different conditions. On the basis of our in situ results of three differently prepared catalysts, two transitions between the oxygenated and deoxygenated state were found and assigned to sites involved in the direct and indirect ORR. [10-11] In order to gain an in-depth understanding of active sites operando conditions (thus during ORR) were performed for the FeNC catalyst that exhibited the strongest change during in situ testing. One iron signature (D4) gets exclusively formed under ORR conditions and its intensity scales with the ORR current. Together with density functional theory calculations the overall set of data enables us to make important conclusions on the ORR mechanism on FeNC catalysts. Literature: [1] M. K. Debe, Nature. 486, 2012, 43−51. [2] C. Sealy, Mater.Today.11, 2008, 65. [3] S. Wagner, H. Auerbach, et al. Angew. Chem. 131.31, 2019, 10596-10602. [4] A. Zitolo, V. Goellner, et al. Nat. Mater. 14.9, 2015, 937. [5] X.,Li, C. Cao, et al. Chem. 6, 2020, 3440–3454. [6] J. Li, M. T. Sougrati, et al. Nat. Catal. 4, 2021, 10–19. [7] U.I. Kramm, M. Lefèvre, et al. J. Am. Chem. Soc. 136, 2014, 978-985. [8] U.I. Kramm, J. Herranz, et al. Phys. Chem.Chem.Phys. 14, 2012,11673-11688. [9] U.I. Kramm, L. Ni, et al. Adv. Mater.31.31, 2019, 1805623. [10] L. Ni, C. Gallenkamp, et al. Adv. Energy Sustainability Res. 2, 2021, 2000064. [11] L. Ni, P. Theis,et al. Electrochim . Acta . 395, 2021, 139200.

The last decade of research on Fe-based O 2 -reduction single atom catalysts (SACs) has led to the development of SACs with an initial polymer electrolyte fuel cell (PEFC) performance close to that of Pt-based catalyst layers (CLs). However, these inexpensive materials generally suffer from a fast performance decay that has been ascribed to several mechanisms that can be simultaneously at play during device operation. These include (i) the demetallation of the SACs’ active sites; (ii) the potential-induced corrosion of the carbonaceous matrix that hosts these active centers; and/or (iii) the chemical degradation of the CL-ionomer, active sites and/or carbon support by radicals derived from the H 2 O 2 produced as an O 2 -reduction by-product. Unfortunately, little is known regarding the relative contributions of these mechanisms to the overall PEFC-performance loss and as a function of the operative conditions – a missing understanding of pivotal importance for the design of strategies to mitigate this instability. With this motivation, this contribution will start with a comparison between the device stability in the course of 30 min holds at various currents of two type of SACs featuring similar beginning-of-life PEFC-performance. Particular attention will be paid to the effect of the catalyst loading on the observed degradation, which will be linked to the operando mapping of the distribution of liquid water within these materials’ CLs based on neutron imaging. In a subsequent step, we will present the results derived from a stability protocol in which the combination of different cathode gas feeds (i.e., air vs. N 2 ) and potential hold durations allow decoupling the relative contributions of the above deactivation mechanisms to the overall performance decay. This requires a careful assessment of the kinetic, ohmic and mass transport overpotentials (and changes thereof in the course of the stability measurements) based on Tafel analyses and electrochemical impedance spectroscopy measurements. Moreover, these results are again complemented by an assessment of the protocol-induced changes in catalyst morphology and surface chemistry based on transmission electron microscopy and X-ray photoelectron spectroscopy measurements. In summary, this contribution will showcase our efforts to deconvolute the relative effects of various deactivation mechanisms to the overall PEFC-performance loss of SAC-CLs.

In this work, the effect of porphyrin loading and template size is varied systematically to study its impact on the oxygen reduction reaction (ORR) activity and selectivity as followed by rotating ring disc electrode experiments in both acidic and alkaline electrolytes. The structural composition and morphology are investigated by 57Fe Mössbauer spectroscopy, transmission electron microscopy, Raman spectroscopy and Brunauer–Emmett–Teller analysis. It is shown that with decreasing template size, specifically the ORR performance towards fuel cell application gets improved, while at constant area loading of the iron precursor (here expressed in number of porphyrin layers), the iron signature does not change much. Moreover, it is well illustrated that too large area loadings result in the formation of undesired side phases that also cause a decrease in the performance, specifically in acidic electrolyte. Thus, if the impact of morphology is the focus of research it is important to consider the area loading rather than its weight loading. At constant weight loading, beside morphology the structural composition can also change and impact the catalytic performance. This article is part of the theme issue ‘Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)’.

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 FeN4moieties 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 situ57Fe 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 FeN4moieties involved in the oxygen reduction reaction of FeNC catalysts.

FeNC catalysts are important substitutes for the oxygen reduction reaction (ORR) in fuel cells. This work reports on an in situ Mössbauer spectroelectrochemical study of a porphyrin-based catalyst. Activity and selectivity towards ORR were determined from rotating ring disc electrode (RRDE) experiments at different loadings in acidic electrolyte and accompanied by H2O2 oxidation reduction measurements in order to identify the contributions to the different ORR pathways as function of potential. The comparison to in situ 57Fe Mössbauer spectra enables an assignment of these contributions to the iron signatures. The results indicate that two different “onset potentials” for obtaining the deoxygenated state associated with two different iron environments can be identified and being associated with the selectivity data. Moreover, the in situ data enable the determination of mass-based site density and turn-over frequency data for ORR relevant conditions. As a consequence, this work sheds light on the oxygen reduction reaction mechanism involved in FeNC catalysts.

Fuel cells are regarded as environmentally friendly energy converters. This, in combination with national and global intentions of energy transition from fossil based resources to renewable ones, makes fuel cells a possible key technology regarding the future energy economy. Because of their advanced state of development, proton exchange membrane fuel cells (PEMFCs) are considered to be the most promising type of fuel cells in terms of commercial use in automotive propulsion. Nevertheless, the high requirements of precious platinum metal for the cathodic oxygen reduction reaction (ORR) is one major reason that still prevents a wide-spread use of this technology to this day. A promising alternative to platinum based catalysts are the so-called non-precious metal based materials. To this day, these catalysts already achieve high current density during PEMFC operation but still need major improvements regarding their durability. In this work, the preparation and characteristics of nanotube based non-precious metal catalysts for the ORR are presented. For the commonly applied platinum based ORR catalysts the positive impact on durability and activity of using carbon nanotubes (CNTs) as carbon support has already widely been proven [1-3]. Here, three different catalyst preparations are presented. First, utilization of commercially available CNTs together with Fe-phenathroline leads to catalyst where FeNxCy moieties are present in-between the CNTs. Secondly, a preceding surface modification step of the commercially available CNTs leads to a material with the active sides directly attached onto the CNTs surface. The third preparation route gives a material where the FeNxCy moieties are directly incorporated into the nanotubes wall. TEM pictures are presented in order to show the structural morphology of the resulting catalysts and the nature of the iron nitrogen active sides is studied via Mößbauer spectroscopy. ORR activity and durability of the resulting nanotube based materials in acidic electrolyte is investigated. PEMFC performance tests clearly show that the connection between active side and nanotube play a crucial role for the activity of the catalyst. Hence, a significantly enhanced performance is found for the catalyst with the active sides incorporated into the nanotube walls. References: [1] X. Wang, W. Li et al., Journal of Power Sources 158 (2006) 154-159. [2] F. Hasché, M. Oezaslan et al., Phys Chem Chem Phys 12 (2010) 15251-15258. [3] T. Maiyalagan, B. Viswanathan et al., Electrochem. Comm. 7 (2005) 905-912.