Proton‐Enriched Alginate–Graphene Hydrogel Microreactor for Enhanced Hydrogen Peroxide Photosynthesis

Efficient synthesis of H₂O₂ via photocatalytic oxygen reduction without sacrificial agents is challenging due to inadequate proton supply from water and difficulty in maintaining O-O bond during O₂ activation. Herein, we developed a straightforward strategy involving a proton-rich hydrogel cross-linked by metal ions [M(n)], which is designed to facilitate the selective production of H₂O₂ through proton relay and metal ion-assisted detachment of crucial intermediates. The hydrogel comprises CdS/graphene and alginate cross-linked by metal ions via O=C-O-M(n) bonds. Efficient O₂ reduction and hydrogenation occurred, benefitting from the collaboration between proton-rich alginate and the photocatalytically active CdS/graphene. Meanwhile, the O=C-O-M(n) bonds enhance the electron density of α-carbon sites on graphene, crucial for O₂ activation and *OOH intermediate detachment, preventing deeper O-O bond cleavage. The role of metal ions in promoting *OOH desorption was demonstrated through Lewis acidity-dependent activity, with Y(III) having the highest activity, followed by Lu(III), La(III), and Ca(II).