Degradation of Fe/N/C catalysts upon high polarization in acid medium.

A comprehensive study of the degradation of a highly active Fe/N/C catalyst in acid medium is reported. An accelerated aging protocol was applied in the temperature range of 20 to 80 °C. From fundamental rotating-disc electrode studies and polymer electrolyte membrane fuel cell investigations combined with identical-location electron microscopy and Mößbauer spectroscopy at various stages of degradation, important insights into the structural and chemical changes of the catalyst were obtained. Most importantly, the degradation is strongly enhanced at elevated temperature, which is correlated to (i) increased carbon-corrosion rate and (ii) parallel non-preferential dissolution of the FeNx-based active sites. The degradation not only leads to a decreased ORR kinetics over time but also induces significant charge- and mass-transport resistances due to the collapse of the electrode structure.