Unraveling activity-durability trade-off in coated Fe-N-C catalysts for oxygen reduction via pyrolysis-duration-driven structural evolution

Fe-N-C catalysts stand out among precious-metal-free materials for the oxygen reduction reaction (ORR), yet their durability in acidic environments remains a critical challenge. The degradation pathways, particularly following high-temperature post-treatments, are not fully understood. In this study, we coated a benchmark Fe-N-C catalyst via chemical vapor deposition (CVD) using Zeolitic Imidazolate Framework-8 (ZIF-8), with dwell times ranging from 1 to 10 h at 1050 °C to tune the catalyst structure. During CVD process, a fraction of the Fe-N₄ sites was transformed into Fe nanoparticles due to the reducing environment created by volatile species derived from ZIF-8. This leads to decreased Fe-N₄ site density and turnover frequency, both contributing to decreased ORR activity. However, improved durability is observed, and can be explained in part by promoted graphitization catalyzed by Fe particles during pyrolysis. We reveal with online differential electrochemical mass spectrometry that CVD with ZIF-8 leads to strongly reduced carbon corrosion, an effect not observed when applying the same annealing treatment but without ZIF-8, supporting the deposition of a N-doped carbon coating in the former case. The study also reports the complex effect of CVD duration on activity, site density, turnover frequency and durability. Overall, these findings highlight the structural effects on ORR activity and durability of Fe-N-C catalysts, providing valuable insights for designing more robust Fe-N-C catalysts.