(invited) Spectroscopic Techniques for the Identification of Fe and Co Coordination Chemistries in Fe(Co)-N-C Catalysts : Pristine Versus Degraded Catalysts
The initial metal coordination in Fe(Co)/N/C catalysts as well as coordination changes following accelerated degradation in PEMFC or accelerated ex situ degradation by H2O2 will be discussed, and compared to the parallel decrease in their ORR activity and other structural changes. In PEMFC, accelerated degradation was achieved by applying a set number of short potential steps up to 1.4 V vs. RHE, while the extent of ex situ H2O2 degradation was controlled through the amount of moles H2O2 contacted per gram of catalyst. The two different modes of degradation lead to two completely different set of changes in the metal contents, coordination and morphological changes in the catalysts.
Coordination of the Fe atoms was experimentally investigated with both 57Fe Mössbauer and X-ray absorption spectroscopy while that of the Co atoms was investigated solely with X-ray absorption spectroscopy. These techniques are sufficiently sensitive to investigate powders with small metal contents (down to 0.1 wt %) or even initial and degraded electrodes with metal loadings of ca 50-150 mg cm-2. Full exploitation of these techniques and interpretation of the spectra is possible only when the pristine Fe(Co)/N/C catalysts are free of metallic, oxide or carbide by-products that are believed to be ORR inactive. If one or several of these phases are present, the spectroscopic signal related to the ORR active site(s) is obscured, which complicates or impedes analysis of the spectra. Catalysts free of such crystalline phases were obtained either by optimizing the metal content or the synthesis, allowing the recording of clean initial spectra and the tracking of coordinative changes occurring upon degradation. Furthermore, ab initio modeling of the X-ray absorption near edge structure spectra was performed and compared to experimental spectra, leading to a more precise concept of the metal coordination in Fe(Co)/N/C pyrolyzed catalysts.