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

Abstract

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.

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

“…In situ X-ray absorption spectroscopy experiments were carried out at the SuperXAS (X10DA) beamline of the Swiss Light Source (SLS). Therein, an improved version of our group’s in situ spectroelectrochemical flow cell previously used for multiple studies ,− was employed, as described in detail in Note S1 and shown in Figure S1. In brief, the main modifications relevant to the electrochemical and spectroscopic performance include the enlargement of the counter-electrode area (from ∼0.14 to ∼0.43 cm 2 , number 5 in Figure S1), the simplified assembly of the cell components attained by using alignment pins (10 in Figure S1), and the use of softer ethylene propylene diene monomer (EPDM) gaskets to ensure its leak-free sealing.…”

Potential Pitfalls in the Operando XAS Study of Oxygen Evolution Electrocatalysts

“…, for IrO 2 , ∼10 mg IrO 2 ·cm geom –2 ). On the other hand, recent progress in time-resolved fluorescence detection has opened the door to probe analyte electrodes with much lower loadings, − and thus, herein such measurements in fluorescence mode were additionally explored.…”

Insights into Heterogeneous Catalysts under Reaction Conditions by In Situ/Operando Electron Microscopy

“…ORR is a kinetically sluggish reaction at the cathode in a hydrogen fuel cell. [265][266][267][268][269][270] Mechanistically, there is host of factors influencing the activity and stability of various metals, alloys, and other catalysts for the ORR, including electronic d-band energy of the metals, lattice strain, surface composition, phase structure, size, faceting, etc. The design of low-cost, highly active, and robust catalysts has been the focal point of research and development.…”