Coupling Light-Heat Utilization in Hydrogel-Encapsulated Ultrathin Defective Carbon Nitride Nanosheets for Scalable Continuous-Flow H2O2 Photosynthesis.

Efficient hydrogen peroxide (H2O2) photosynthesis in water is a highly desirable yet challenging route for sustainable energy and environmental applications, primarily hindered by poor charge separation and severe backward degradation. Herein, we develop a molten-salt-induced defect engineering on ultrathin carbon nitride (CNx) to enable highly efficient pure-water H2O2 photosynthesis via a dual-channel two-electron oxygen reduction reaction (ORR) and water oxidation reaction (WOR) pathway. The CNx catalyst delivers a record H2O2 production rate of 1100.9 µmol g-1 h-1 under pure water conditions without sacrificial agents. Mechanistically, transient photovoltage (TPV) and transient potential scan (TPS) quantitatively elucidate that defect sites expedite charge separation, mitigate deep trap states, and accelerate light-induced oxygen activation kinetics rather than merely enhancing oxygen adsorption. Crucially, we translate this system into a practical continuous-flow architecture by encapsulating CNx within a sodium alginate hydrogel matrix (CNx/SA). This hydrogel flow reactor sustains a stable H2O2 production of 180 µmol g-1 h-1 over 400 h. Furthermore, a multiphysics mathematical framework validates that the hydrogel microenvironment fundamentally suppresses degradation pathways and facilitates product desorption. This study provides a comprehensive paradigm integrating atomic-level catalyst design with macroscopic reactor engineering for scalable solar-driven chemical manufacturing.