Operando X-ray absorption spectroscopy of Fe-N-C catalysts based on carbon black and biomass-derived support materials for the ORR

Iron nitrogen carbon (Fe-N-C) catalysts are among the most promising non-platinum group metal catalysts for the oxygen reduction reaction (ORR). Their activity and stability are considerably influenced by the structure of the C-support. New biochar materials offer native heteroatom doping, making them a promising precursor for Fe-N-C catalysts. In this study, we apply operando X-ray absorption spectroscopy at the Fe K-edge to characterize the atomic Fe-based active sites of a commercial Fe-N-C catalyst, a carbon black-based catalyst as well as a novel biomass-based Fe-N-C catalyst. We compare the density and the potential-dependent nature of the FeNx-type active sites during operation. Our results demonstrate that the novel biomass-based catalyst exhibits a higher active-site density compared to commercial and carbon black-based Fe-N-C catalysts. Moreover, dynamic detection of the Fe K-edge intensity during potential cycling reveals that their reversible iron redox potential is lower compared to that of conventional catalysts. Evaluation of the Fe K-edge shift as well as of the extended X-ray absorption fine structure (EXAFS) suggests hetero-atom doping and iron under-coordination as potential causes for the observed differences. These insights open the pathway to develop new optimization strategies for Fe-N-C catalysts based on biomass support materials.