Indium Clusters With 6 s 2 Lone‐Pair Effect as Excitonic Electron Reservoirs for Direct 2e − Photocatalytic H 2 O 2 Photosynthesis

The intrinsic activity–selectivity trade‐off in photocatalytic 2e − oxygen reduction reaction (ORR) for H 2 O 2 photosynthesis originates from the kinetic dilemma between weak O 2 adsorption at single‐atom sites and unfavorable O–O cleavage at multimetal sites. Herein, we demonstrate that indium (In) clusters with a unique 6 s 2 lone‐pair effect can break this long‐standing single‐atom/cluster trade‐off by acting as excitonic electron reservoirs on carbon nitride. InSAC‐DCN delivers a remarkable H 2 O 2 generation rate of 1930.8 µmol∙g −1 ∙h −1 with selectivity exceeding 90%, far surpassing both pristine carbon nitride and In single‐atom counterparts. Combined fs‐TAS and in situ characterizations reveal that In clusters enable ultrafast exciton dissociation and instantaneously deliver electrons to the geometric Yeager‐type side‐on O 2 adsorption site, thereby favoring direct 2e − H 2 O 2 formation. DFT calculations elucidate lowered energy barriers for ∗OOH formation, suppressed O–O dissociation, and accelerated water dissociation. Meanwhile, the synergistic coupling between water oxidation and ORR establishes a self‐sustained proton‐feedback loop for enhanced overall efficiency. This work highlights that exciton engineering coupled with p ‐block metal clusters enables efficient and selective 2e − ORR beyond conventional single‐atom strategies to overcome the activity–selectivity trade‐off, establishing a distinct material‐design paradigm for high‐performance photocatalysts toward H 2 O 2 photosynthesis.