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...

The cost reductions required for the large-scale commercialization of polymer electrolyte fuel cells (PEFCs) could be achieved by substituting state-of-the-art PEFC cathode catalysts based on platinum with more abundant and affordable materials. In this context, this work presents a new approach for synthesizing Fe-based oxygen reduction reaction (ORR) catalysts using sodium carbonate (Na 2 CO 3 ) as an inexpensive but effective pore-inducing agent offering microporosity control. By employing (scanning) transmission electron microscopy, a qualitative relation between the heat-treatment temperature and the formation of larger isolated Fe-based phases in particulate form was identified, mainly unveiling an effect of this variable on the Fe-speciation. Complementary bulk characterization, namely, X-ray absorption spectroscopy, on the other hand confirmed that the majority of the iron in the samples was present in single atomic sites. Electrochemical activity measurements in liquid environment as well as in a fuel cell demonstrate that the resulting materials display ORR-activities among the highest for this class of catalysts and synthesis conditions.

Palladium is an increasingly investigated electrocatalyst for the electrochemical reduction of carbon dioxide due to its unique ability to yield carbon monoxide or formate with large selectivities at high vs low overpotentials (i.e., ∼−0.5 to ∼−1.0 vs ∼−0.1 to ∼−0.4 V vs the reversible hydrogen electrode), respectively. While this behavior has been described multiple times on different Pdelectrocatalysts, previous studies disagree with regard to palladium's ability to form a hydride phase (PdH x ) under CO 2 reduction reaction (CO 2 RR) conditions, as well as on the influence of this PdH x on its CO 2 RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO 2 RR with adsorbed CO, whose precise influence on the formation of PdH x and corresponding effects on the CO 2 RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdH x formation at CO 2 RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO 2 and bicarbonate) and as a function of the applied potential and duration of the potential hold. The results of these electrochemical measurements unveiled the strong influence of surface-adsorbed CO on the formation rate of Pd-hydride, for which the final stoichiometry was nevertheless found to be independent of the presence of CO 2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of β-phase PdH x (with x ≈ 0.6) at all applied potentials under CO 2 RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies.

Pt-group metal (PGM)-free catalysts of the Me-N-C type based on abundant and inexpensive elements have gained importance in the field of oxygen reduction reaction (ORR) electrocatalysis due to their promising...