Degradation of FeNC Electrocatalysts for Acidic and Alkaline Oxygen Reduction

Iron-nitrogen-carbon (FeNC) catalysts represent promising alternatives to platinum-group metals for the oxygen reduction reaction (ORR) in energy conversion technologies. However, their operational stability remains a critical challenge. In this study, we unravel the distinct degradation mechanisms and active-site behaviors of FeNC catalysts under acidic and alkaline ORR conditions. Intriguingly, catalysts subjected to electrochemical cycling in acidic media but tested in alkaline conditions exhibit nearly preserved ORR activity, revealing that degradation pathways differ fundamentally between the two environments. With the help of density functional theory calculations, we identify Fe-centered sites as the primary active centers in acidic media, whereas under alkaline conditions─where hydroxyl adsorption passivates Fe sites─neighboring carbon atoms adjacent to nitrogen become the dominant active sites. The Mössbauer spectroscopy results show that under acidic cycling, pyrrolic nitrogen-coordinated Fe (S1) sites suffer a significant loss while the pyridinic nitrogen-coordinated Fe (S2) sites retain high stability. In contrast, during alkaline cycling, Fe site losses are minimal, while the carbon support undergoes more severe corrosion. By selectively engineering the Fe sites and carbon support, we experimentally validated the conclusions on the active sites. This work provides critical insights into the site-dependent durability of FeNC catalysts and underscores the necessity of tailored catalyst design for stable and efficient ORR across diverse operating conditions.