Size‐Dependent Performance of ZIF‐Encapsulated Precursors for Oxygen Reduction Reaction

The encapsulation‐pyrolysis serves as a flexible strategy for engineering high‐performance M─N─C oxygen reduction reaction (ORR) catalysts by tailoring metal precursor molecules. However, the dependence of ZIF‐encapsulated precursors size and oxygen reduction reaction performance remains elusive. Herein, zeolitic imidazolate framework‐8 (ZIF‐8) is employed as a model encapsulation matrix to confine iron‐based molecular precursors of varying dimensions. The correlation between guest molecular size and host pore aperture is quantified by the dimensionless ratio r p /r cd (r p : diameter of guest iron‐based molecules, r cd : cavity diameter of host ZIF‐8), elucidating the size‐dependent of ORR performance in encapsulation‐pyrolysis systems. When r p /r cd ≈1, Fe(Phen) 2 exhibites a spatial support effect, achieving an exceptional specific surface area of Fe(Phen) 2 ‐N/C (1537.65 m 2 ·g −1 ), 1.37 and 2.09 times than those of r p /r cd ≈0.13 or r p /r cd ≈1.22, which suffered from structural collapse or disintegration. Density functional theory calculations revealed that r p /r cd critically modulates the d ‐band center of Fe sites to achieve the regulation of ORR catalytic performance, where Fe(Phen) 2 ‐N/C with the optimal d ‐band center delivered a half‐wave potential of 0.917 V versus RHE. By tuning porous and electronic structure via precursor molecular size regulation, the catalytic performance in zinc–air battery exhibits a typical volcano‐type trend with increasing r p /r cd .