Enhanced electron delocalization in potassium poly(heptazine imide) triggered by indium sites and nitrogen defects promotes highly efficient H2O2 photosynthesis

• • The 10InPHI catalyst achieves a H2O2 production rate of 15.3 mmol g−1 h−1, which is among the highest reported for carbon nitride-based photocatalysts, indicating a significant improvement in photocatalytic efficiency for practical solar-driven H2O2 synthesis. • • The ionothermal synthesis using LiCl/KCl molten salts yields highly crystalline PHI with a large π-conjugated system, which enhances visible-light absorption and charge transfer, addressing the structural incompleteness of traditional thermally polymerized carbon nitride. • • The incorporation of In sites and nitrogen defects synergistically promotes electron delocalization, as evidenced by enhanced charge separation and transfer, leading to a two-step single-electron ORR pathway with high selectivity for H2O2 production. • • The presence of K+ ions in nitrogen cavities serves as interlayer electron channels, facilitating interlayer charge separation, while N defects induce asymmetric charge distribution, improving in-plane charge transfer, collectively boosting the photocatalytic performance.