Revisiting the Activity Descriptor for the Oxygen Reduction Reaction Catalyzed by Fe–N–C Catalysts

In supported metal catalysts, the support plays crucial roles in modulating the electronic structure of metal centers and the physicochemical properties of their surrounding microenvironments. Here, we report that microporous carbon supports derived from ZIF-8, a metal–organic framework (MOF), exert a pronounced influence on the intrinsic activity and selectivity of Fe–N–C catalysts in the oxygen reduction reaction (ORR). The average turnover frequency (TOF) of Fe–Nx sites monotonically increases with increasing surface basicity (quantified by a literature method) and a narrower C 1s X-ray photoelectron spectroscopy (XPS) peak, both of which reflect greater π-electron delocalization and a higher point of zero charge (PZC), thereby indicative of the enhanced electron-donating capability of the carbon support. The inverse relationship between C 1s peak width and TOF observed here contrasts with that reported for Fe–N–C catalysts derived from iron porphyrin complexes on conventional carbons, suggesting that MOF-derived carbons create distinct electrostatic environments for Fe–Nx sites located within their microporous structures and modulated by the π-electron system through thermal activation. Rapid microwave heating offers a strategy to tailor these local environments while preserving the microporous framework, in contrast to conventional furnace heating. Thermal activation at ≥1100 °C under a reductive atmosphere in a single-mode microwave reactor increases the surface basicity while preserving microporosity, simultaneously improving the intrinsic activity of Fe–Nx sites and suppressing H2O2 formation, leading to improved 4e– ORR selectivity.