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 57Fe Mössbauer spectroscopy and quantum chemistry. A Mössbauer signal newly emerging under operando conditions, D4, is correlated with the loss of other Mössbauer signatures (D2, D3a, D3b), implying a direct structural correspondence. Pyrrolic N-coordination, i.e., FeN4C12, 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.

Identification of the Catalytically Dominant Iron Environment in Iron- and Nitrogen-Doped Carbon Catalysts for the Oxygen Reduction Reaction.

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 CO2. 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 ORR‐active 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.

/pdf/time-resolved-potential-induced-changes-in-fe-n-c-catalysts-3tyfc365.pdf

Time‐Resolved Potential‐Induced Changes in Fe/N/C‐Catalysts Studied by In Situ Modulation Excitation X‐Ray Absorption Spectroscopy

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 Pd-electrocatalysts, previous studies disagree with regard to palladium’s ability to form a hydride phase (PdHx) under CO2 reduction reaction (CO2RR) conditions, as well as on the influence of this PdHx on its CO2RR-selectivity. These inconsistencies are partially related to the known poisoning of the Pd-surface during the CO2RR with adsorbed CO, whose precise influence on the formation of PdHx and corresponding effects on the CO2RR-mechanism and product selectivity remain poorly understood. With this motivation, herein we used an unsupported Pd-aerogel to investigate CO adsorption and PdHx formation at CO2RR potentials in electrolytes with different compositions (i.e., in the presence or absence of CO2 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 CO2 or bicarbonate. This finding was supported by in situ X-ray absorption spectroscopy (XAS) measurements that confirmed the complete formation of β-phase PdHx (with x ≈ 0.6) at all applied potentials under CO2RR conditions, thus shedding light on the contradictory results in this regard reported in previous operando XAS studies. 

Interplay between Surface-Adsorbed CO and Bulk Pd Hydride under CO2-Electroreduction Conditions

Non-noble metal catalysts (NNMCs) hold the potential to replace the expensive Pt-based materials currently used to speed up the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes, but they feature poor durability that inhibits their implementation in commercial PEMFCs. This performance decay is commonly ascribed to the operative demetallation of their ORR-active sites, the electro-oxidation of the carbonaceous matrix that hosts these active centers, and/or the chemical degradation of the ionomer, active sites, and/or carbon support by radicals derived from the H2O2 produced as an ORR by-product. However, little is known regarding the relative contributions of these mechanisms to the overall PEMFC performance loss. With this motivation, in this study, we combined four degradation protocols entailing different cathode gas feeds (i.e., air vs N2), potential hold values, and durations to decouple the relative impact of the above deactivation mechanisms to the overall performance decay. Our results indicate that H2O2-related instability does not depend on the operative voltage but only on the ORR charge. Moreover, the electro-oxidation of the carbon matrix at high potentials (which for the catalyst tested herein triggers at 0.7 V) seems to be more detrimental to the NNMCs' activity than the demetallation occurring at low potentials.

/pdf/decoupling-the-contributions-of-different-instability-2nbocwlp.pdf

Decoupling the Contributions of Different Instability Mechanisms to the PEMFC Performance Decay of Non-noble Metal O2-Reduction Catalysts.

The Role of Sulfur Doping in Determining the Performance of Fe/N-C Based Electrocatalysts for Oxygen Reduction Reactions

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 (Na2CO3) 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 fue...

Fe-Based O2-Reduction Catalysts Synthesized Using Na2CO3 as a Pore-Inducing Agent

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Fe-Based O2-Reduction Catalysts Synthesized Using Na2CO3 as a Pore-Inducing Agent