Benzene-Functionalized In-Plane Ordered Sodium Poly(Heptazine Imide) With Improved Electron Storage Capacity for Efficient Dark-State Photocatalytic Hydrogen Evolution.

Conventional photocatalytic hydrogen evolution systems face the challenge of solar intermittency, which limits their all-day operation. Additionally, bulk sodium poly(heptazine imide) (PHI-Na) suffers from intrinsic drawbacks, including low crystallinity, severe charge carrier recombination, and poor photoelectron storage capacity. To overcome these limitations, we develop an in-plane highly ordered PHI-Na-Mp photocatalyst through a synergistic strategy of surface-confined polymerization and in-situ benzene grafting. Benzene modification significantly enhances the π-conjugation, and visible-light absorption of PHI-Na. The optimized catalyst exhibits outstanding performance with a visible-light hydrogen evolution rate of 31.2 mmol·g-1 (52 times higher than pristine g-C3N4), an apparent quantum efficiency of 38.8% at 420 nm, a substantial dark-state hydrogen release of 690.8 µmol·g-1, and excellent cycling stability. Mechanistic studies reveal that the benzene moieties and the highly ordered structure synergistically suppress charge carrier recombination. Meanwhile, the heptazine units serve as electron reservoirs, enabling reversible photoelectron storage and release. This unique functionality allows continuous hydrogen production even in the absence of light, achieving dark-state photocatalytic operation. Our work provides experimental and theoretical support for designing efficient all-day photocatalytic hydrogen evolution systems.