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–N₄ 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–N₄ while nanoparticles with sizes >10 nm tend to coalesce and impede Fe–N₄ site formation. The work visualizes the pyrolysis process of Fe/N/C materials, providing theoretical guidance for the rational design of catalysts.

在追求廉价且有效的氧还原催化剂的过程中，Fe/N/C 体系成为一个有前途的候选者。然而，在热解条件下，起始材料向铁和氮掺杂碳催化剂的结构转变仍然很少被表征。在这里，我们通过采用不同的原位诊断技术探索 Fe 物种的演化并跟踪 Fe-N ₄位点的形成。原位加热显微镜揭示了 FeO _x纳米颗粒的初始形成以及随后在碳基质内的内部迁移，一旦 FeO _x完全还原，迁移就会停止。纳米颗粒的迁移和分解随后导致碳层重建。实验和理论分析揭示了 FeO _x的尺寸依赖性行为，其中低于 7 nm 的纳米颗粒容易释放 Fe 原子形成 Fe-N ₄ ，而尺寸 >10 nm 的纳米颗粒倾向于聚结并阻碍 Fe-N ₄位点形成。该工作可视化了Fe/N/C材料的热解过程，为催化剂的合理设计提供理论指导。