title: Active site investigation of FeNC catalysts via in situ and operando Mössbauer spectroscopy
Creators: name: Ni, Lingmei
Publisher: name: UNSPECIFIED
Year: 2023

description: Proton exchange membrane fuel cells (PEMFCs) are a clean technology for the efficient conversion of chemical energy into electrical energy and are specifically promising for the decarbonization of heavy-duty vehicles. However, the drawback of PEMFCs is the high cost of Pt-based catalysts used for cathode and anode, which hinders their commercialization. To address this issue, the rapid developed FeNC catalysts holds great promise for replacing Pt-based catalysts for the oxygen reduction reaction (ORR) on the cathode. Until now, the newly developed FeNC catalysts show comparable ORR activities compared with Pt-based catalysts. However, the nature of the FeNC active sites is a challenging subject of research. For example, 1) The characterization results of the intrinsic active centers structures of FeNC catalysts by different techniques are still debatable. 2) The structures of the active sites obtained by ex situ and in situ conditions are different. 3) The role of FeNC active sites on the ORR reaction mechanism is still poorly understood. 57Fe Mössbauer Spectroscopy is a powerful technique for FeNC catalysts and therefore is used in this work for obtaining knowledge of iron sites with respect to structural composition, electronic states as well as the magnetic environment. Due to the discrepancy in active sites under in situ and ex situ conditions, this work aims to explore the structure of iron sites and identify their contributions to ORR by in situ and operando Mössbauer spectroscopy. Therefore, a new spectro-electrochemical cell was designed and subsequently some pre-tests were performed based on it to optimize the experimental conditions for in situ and operando Mössbauer measurements. The first work of this thesis is to identify changes in the structural motifs that are associated with an applied potential of the ORR via in situ Mössbauer spectroscopy on three differently prepared FeNC catalysts. The active sites change for the three catalysts are similar under electrochemical conditions independent of their preparation routes, which further show two transitions related to iron oxidation or spin state changes of relevant doublets. The second research work correlates the structural changes upon four different applied potentials with the ORR performance in terms of activity and selectivity to better verify the selectivity conclusions in the first paper. The third research work investigates the FeNx site under operando conditions by Mössbauer on a selected FeNC catalyst. Mössbauer signatures were explored at a fixed potential but varying oxygen gas flow and a new intermediate FeNx site D4 appeared. On the basis of spectroscopic and thermodynamical data, a pyrrolic N-coordination, i.e., FeN4C12, is found as a consistent model for the entire catalytic cycle. FeNC catalysts prepared by different methods, show very similar RT Mössbauer spectra, however, low-temperature (LT) Mössbauer showed clear differences between them. Their differences in ORR activity may be explained by the different composition of the small-sized metallic iron sites encapsulated in the carbon layer. However, the reaction mechanism of these small metal particles, i.e., how they synergize with the FeNx active site, is not fully understood. The results of this work overcome previous contradictions in the structural assignment of the active site and provide experimental and theoretical evidence for the structural assignment of an unknown intermediate in the ORR mechanism of the FeNC catalysts. This has significant effects on the rational design of highly active FeNC catalysts intended for commercialization in the future.