For commercialization of proton-exchange fuel cells as green technology for automotive propulsion, it is important to replace the platinum-based oxygen reduction reaction (ORR) catalyst, that is considered as critical raw material, rare and expensive. FeNC catalysts are inexpensive substitutes but degrade during long-term operation. Part of the degradation is associated with the leaching of iron ions and related to it a Fenton’s reaction that can lead to severe degradation. Over the past couple of years, we developed a purified FeNC catalyst that outperforms our previous catalyst both in activity and stability. By operando synchrotron based Mössbauer spectroscopy (SMS) coupled with a FC, we would like to comparatively investigate both catalysts with respect to their structural changes during fuel cell operation by changing the voltage. The data will enable us to identify differences in both systems, as well as to determine degradation kinetics at different relevant cell voltages.