Defective Poly(Heptazine Imide) Nanosheets for Efficient One‐Step Two‐Electron Photocatalytic O2 Reduction to Medical‐like H2O2

Poly(heptazine imide) (PHI) is a promising photocatalyst for H2O2 production; however, enhancing its specific surface area to expose internal active sites and understanding their roles in key mechanistic steps for the H2O2 synthesis remain challenging. Here, we utilized organic cations to exfoliate bulk PHI and fabricate PHI nanosheets for producing H2O2 at a rate of 27.35 mmol g‐1 h‐1 under simulated solar light irradiation, outperforming most of the reported carbon nitride‐based catalysts. Importantly, after 36 hours of cyclic accumulation reactions in a self‐created spiral flow reactor, the H2O2 concentration stabilized at 2.7 wt.%, close to medical sterilization levels. In‐situ spectroscopic characterizations and density functional theory calculations revealed that the exfoliation results in molecular reconfiguration of the PHI basal planes, forming the active sites to promote charge separation and electron localization. This new structure also creates midgap states, enabling direct H2O2 production via a one‐step two‐electron pathway, bypassing the superoxide radical pathway. Theoretical calculations suggested that the localized electron structure created by the active sites favors the protonation of adsorbed O2 and stabilizes the *OOH species, which converts to H2O2. This study elucidates underscores the importance of active‐site reconfiguration for efficient photocatalytic oxygen reduction reaction (ORR) pathways.