Defective Poly(heptazine imide) Nanosheets for Efficient One‐Step Two‐Electron Photocatalytic O ₂ Reduction to Medical‐Like H ₂ O ₂

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Poly(heptazine imide) (PHI) is a promising photocatalyst for hydrogen peroxide (H 2 O 2 ) production; however, enhancing its specific surface area to expose internal active sites and understanding their roles in key mechanistic steps for the H 2 O 2 synthesis remain challenging. Here, we utilized organic cations to exfoliate bulk PHI and fabricate PHI nanosheets for producing H 2 O 2 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 h of cyclic accumulation reactions in a self‐created spiral flow reactor, the H 2 O 2 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 H 2 O 2 production via a one‐step, two‐electron pathway, bypassing the superoxide radical pathway. Theoretical calculations suggest that the localized electronic structure created by the active sites favors the protonation of adsorbed O 2 and stabilizes the *OOH species, which converts to H 2 O 2 . This study elucidates and underscores the importance of active‐site reconfiguration for efficient photocatalytic oxygen reduction reaction (ORR) pathways.