<p>Polymeric carbon nitride (PCN) is identified as a promising photocatalyst for H₂O₂ production due to its visible-light response, low cost, and high selectivity of 2e⁻ oxygen reduction reaction (ORR). However, the H₂O₂ yield of carbon nitride is still restricted by narrow light absorption, low charge separation efficiency, and insufficient active sites. Herein, crystalline poly(heptazine imide) (PHI)-based carbon nitride with highly dispersed In sites and N defects was prepared through the ionothermal method using LiCl/KCl as molten salts. The large π-conjugated system and the existence of N defects greatly enhance the visible-light harvesting ability. The remaining K⁺ ions in the nitrogen cavities of PHI serve as interlayer electron channels, and the incorporation of N defects triggers asymmetric distribution of charges on the heptazine network, promoting interlayer and in-plane charge separation and transfer, respectively. The In sites accelerate charge transfer dynamics and act as active sites for ORR. The synergistic effect of metal modification and defect engineering boosts the electron delocalization within the photocatalyst and thus significantly improves the photocatalytic activity. The H₂O₂ production rate of 10InPHI reaches 15.3 mmol g⁻¹ h⁻¹ through a two-step single-electron ORR pathway, underscoring the great potential of modified carbon nitride materials in efficient H₂O₂ photosynthesis.</p>