Defective In 2 S 3 with Proton‐Enriched Interface Enable Sacrificial‐Agent‐Free Visible‐Light Photocatalytic H 2 O 2 Production

Photocatalytic oxygen reduction provides a sustainable route for hydrogen peroxide (H 2 O 2 ) production, but its practical implementation is impeded by ineffective O 2 activation and sluggish protonation kinetics. Here, a synergistic strategy combining defect engineering and interfacial microenvironment modulation is introduced to break these bottlenecks. Specifically, sulfur‐vacancy (S V )‐rich indium sulfide (In 2 S 3 ) grafted with polyethyleneimine (PEI) achieves a remarkable H 2 O 2 yield of 194.0 µ m h −1 under visible‐light in pure water, ≈33.4 times higher than that of pristine In 2 S 3 (5.8 µ m h −1 ). Experimental and theoretical results reveal that S V ‐induced low‐coordination In sites act as Lewis acid centers to stabilize end‐on O 2 adsorption and facilitate the generation of the critical •O 2 − intermediates. Surface‐grafted PEI substantially reduces the energy barrier for protonation of * •O 2 − to * OOH intermediates via an effective proton transfer process, favoring H 2 O 2 synthesis. By decoupling and optimizing O 2 activation and protonation dynamics, this work establishes a mechanism‐driven paradigm for rational design of high‐performance photocatalysts.