Polarized Electronic Structure of Carbon Nitride Enhances Selective Triphase Continuous-Flow Photosynthesis of Hydrogen Peroxide

The H2O2 photosynthesis efficiency by carbon nitride (CN) via a two-electron oxygen reduction reaction (ORR) is limited by insufficient charge separation, sluggish O2 supply in aqueous solution, and difficulty in O2 adsorption and activation. Here, we modulated CN with sulfonic acid and cyano groups, which impart multiple favorable electronic properties for H2O2 photogeneration, including broadened light absorption, enhanced charge separation due to the polarized electronic structure, and improved O2 adsorption and activation. H2O2 photogeneration experiments showed that it delivered a 41-fold enhancement in H2O2 yield, reaching a value of 44.5 mmol g–1 h–1 with a high AQY of 36.5% at 400 nm and a high selectivity of >94%. Experiments and density functional theory (DFT) calculations revealed the preferential two-step 1e ORR mechanism. Further fabrication of a triphase flow photoreactor by immobilizing the functionalized CN on a gas diffusion layer achieved fast O2 supply and continuous-flow H2O2 photogeneration with a 4 times higher yield of 171 mmol m–2 in 6 h than in a two-phase system. As a proof-of-concept, continuous-flow H2O2 photogeneration was further coupled with organic synthesis exemplified by furfuryl alcohol photooxidation conversion. This work solves multiple important issues of CN in H2O2 photogeneration by combining electronic polarity modulation with a triphase continuous-flow photoreactor design.