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

Abstract 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).