Pyrolysis‐Engineered Fe/Mn‐ZIF Catalysts: Balancing Activity, Selectivity, and Stability for the Oxygen Reduction Reaction

ABSTRACT Metal oxide‐based catalysts for oxygen reduction reaction (ORR) have drawn great attention in recent years. However, it remains technically challenging for the precise regulation of the overall performance due to the trade‐off effect among the activity, selectivity, and stability of electrocatalysts. Herein, a balancing strategy to control the evolution of surface oxygen species of iron/manganese based zeolitic imidazolate framework (Fe/Mn‐ZIF) has been proposed by tuning the pyrolysis temperature and heating rates. In alkaline electrolyte, the Mn‐doped Fe 3 C nanoparticles (NPs) tightly encapsulated in nitrogen‐doped carbon nanotubes (FMC‐NC, 260–370°C at 10°C min −1 ) display direct 4e − pathway with higher ORR activity ( E 1/2 = 0.84 V), lower H 2 O 2 selectivity (4.73%), and excellent durability (88.0% after 6 h). The Mn‐doped Fe 3 C NPs encapsulated in larger‐sized nitrogen‐doped carbon nanotubes (FM‐NC, at 3°C min −1 ) shows a higher half‐wave potential ( E 1/2 = 0.87 V) but poorer stability (54.8% after 6 h), and FMC‐NC mixed with Fe‐doped MnOOH (FMO‐NC, 460–650°C at 10°C min −1 ) follows a 2e − ORR pathway with only a superior 2e − ORR selectivity (94.9%). The performance of FMC‐NC electrocatalyst surpasses most of the reported Fe‐based and carbon‐based electrocatalysts in 0.1 M KOH electrolyte. In situ heating electron microscopy analysis shows the evolution process of the surface/adsorbed oxygen and metals in different temperature ranges, which enables the effective control of the products via differential pyrolysis. Theoretical calculations and in situ spectra demonstrate that FMC‐NC sample has a strong metal‐support interaction confirming the optimal balance among activity, stability, and selectivity for ORR. This work not only provides an ORR electrocatalyst with outstanding performance but also proposes a balancing strategy to control the evolution of surface oxygen species.