Mapping Degradation of Iron-Nitrogen-Carbon Heterogeneous Molecular Catalysts with Electron-Donating/Withdrawing Substituents

Iron-nitrogen-carbon (Fe-N-C) single-atom catalysts are promising precious-metal-free catalysts for various essential reactions. However, poor stability is a roadblock to their practical applications. Recent studies suggested that introducing electron-donating or -withdrawing substituents near their catalytic active sites may improve their stability. However, standard M-N-C catalysts synthesized by high-temperature pyrolysis have inhomogeneous structures, making it challenging to understand their degradation mechanisms. Here, we use a series of heterogeneous molecular catalysts with well-defined structures as model Fe-N-C catalysts to map their degradation for oxygen reduction reaction in acidic electrolytes, relevant to M-N-C catalysts' applications in proton-exchange hydrogen fuel cells. Iron phthalocyanine molecules with different types of electron-donating and -withdrawing substituents (i.e., -H, -tBu, -NH2 at alpha-position, or beta-position, -NO2, and -F) are adsorbed on purified carbon nanotubes and exhibit varied oxygen reduction reaction (ORR) activity and stability in acidic electrolytes. Detailed characterizations identify five degradation paths and reveal the beneficial role of electron-withdrawing substituents, i.e., -F and -NO2, by quantifying the Fe distribution. We find that direct Fe leaching from Fe-N-4 sites plays a crucial role in early-stage degradation, and it can be significantly suppressed by -F and -NO2 substituents. The oxidative degradation becomes dominant with time, forming FeOx nanoclusters on a carbon nanotube substrate, which the electron-withdrawing substituents can partially alleviate. This work provides insights into the degradation of Fe-N-C single-atom catalysts, which can accelerate the development of robust catalysts for hydrogen fuel cells and beyond.