An in situ exploration of how Fe/N/C oxygen reduction catalysts evolve during synthesis under pyrolytic conditions
Shuhu Yin, H. Yi, Mengli Liu, Jian Yang, Shuangli Yang, Binwei Zhang, Long Chen, Xiaoyang Cheng, Huan Huang, Rui Huang, Yanxia Jiang, Hong‐Gang Liao, Shi‐Gang Sun
Abstract
Abstract In pursuing cheap and effective oxygen reduction catalysts, the Fe/N/C system emerges as a promising candidate. Nevertheless, the structural transformations of starting materials into Fe- and N-doped carbon catalysts remains poorly characterized under pyrolytic conditions. Here, we explore the evolution of Fe species and track the formation of Fe–N 4 site development by employing diverse in-situ diagnostic techniques. In-situ heating microscopy reveals the initial formation of FeO x nanoparticles and subsequent internal migration within the carbon matrix, which stops once FeO x is fully reduced. The migration and decomposition of nanoparticles then leads to carbon layer reconstruction. Experimental and theoretical analysis reveals size-dependent behavior of FeO x where nanoparticles below 7 nm readily release Fe atoms to form Fe–N 4 while nanoparticles with sizes >10 nm tend to coalesce and impede Fe–N 4 site formation. The work visualizes the pyrolysis process of Fe/N/C materials, providing theoretical guidance for the rational design of catalysts.