The role of nitrogen sources and hydrogen adsorption on the dynamic stability of Fe-N-C catalysts in oxygen reduction reaction

Fe-N-C catalysts are promising alternatives to Pt-based electrocatalysts for the oxygen reduction reaction (ORR) in various electrochemical applications. However, their practical implementation is impeded by their instability during prolonged operation. Various degradation mechanisms have been proposed, yet the real origin of the intrinsic instability of Fe-N-C structures under ORR operations is still disputed. Herein, we observed a new type of protonation mechanism based on advanced first-principles simulations and experimental characterizations. The results revealed strong evidence of pyrrolic-N protonation in pyrrolic-type FeN4, which plays a vital role for the low kinetic barrier of Fe leaching. Conversely, the pyridinic-type FeN4 prefers protonation at the Fe site, contributing to the higher barrier of Fe leaching and relatively higher stability. The facile pyrrolic-N protonation is verified by various spectroscopy characterizations in the Nafion-treated FePc molecule. Crucially, the presence of oxygen-containing intermediates at the Fe site can further work synergistically with N protonation to promote conversion of iron atoms (Fe-N4) into ferric oxide under working potentials, and the more positive the electrode potential, the lower the kinetic barrier of Fe leaching. These findings serve as a foundation for future research endeavors on the stability issues of Fe-N-C catalysts and advancing their application in sustainable energy conversion technologies. Combining dynamic solvation model with experiment, we found a protonation mechanism in acid ORR, causing Fe leaching in Fe-N-C. N-Protonation and oxygen intermediate adsorption at Fe convert FeN4 to ferric oxide, a previously unknown key process.