NH 3 ‐Guided Low‐Temperature Nanostructural Refinement Boosts Visible‐Light‐Driven H 2 O 2 Synthesis in Ionic Carbon Nitrides

Abstract Solar‐driven oxygen reduction on ionic carbon nitride frameworks presents a compelling strategy for sustainable hydrogen peroxide (H 2 O 2 ) production. Herein, a nanostructural engineering strategy is presented to tailor the morphology and defect chemistry of potassium poly(heptazine imide) (KPHI), enabling extended solar coverage and enhance photocatalytic performance. By incorporating NH 4 Cl into a molten KCl/LiCl eutectic medium, simultaneous nanoscale fragmentation of KPHI crystals and controlled introduction of cyano (–C≡N) defects are achieved. These molecular modifications induce n → π* electronic transitions, facilitate efficient charge separation, and accelerate oxygen reduction reaction kinetics. The optimal catalyst reaches an apparent quantum yield (AQY) of 49% at 410 nm and 5% at 525 nm without the need for cocatalysts, among the highest values reported for metal‐free photocatalyst systems. Transient absorption spectroscopy confirms preferential photoexcited electron localization at –C≡N sites, highlighting their key role in enhancing the charge carrier dynamics. Crucially, autogenous NH 3 pressure is harnessed from NH 4 Cl decomposition to unlock a low‐temperature (500 °C) KPHI variant that delivers analogous performance to its counterpart produced at 600 °C, offering a more sustainable synthetic route. This study elucidates the structure‐activity relationship in ionic carbon nitrides and provides a generalizable approach for controlling their morphology and defect characteristics.