Dual Electric Fields in Cyclooctatetrathiophene‐Based COF/ ZnIn ₂ S ₄ Z‐Scheme Heterojunction Boost Photocatalytic Seawater Hydrogen Evolution

The development of efficient photocatalysts for seawater-based photocatalytic hydrogen evolution (PHE) represents a crucial step toward sustainable green hydrogen production. While covalent organic frameworks (COFs) show great potential due to their structural tunability and optoelectronic properties, their practical application is limited by inherent challenges of low charge mobility and rapid carrier recombination. Herein, we present an innovative dual-electric-field strategy that synergistically combines intramolecular charge asymmetry with Z-scheme heterojunction engineering to overcome these limitations. Through precise molecular design, we developed two isomeric cyclooctatetrathiophene building blocks with distinct sulfur spatial arrangements, enabling the construction of COFs with controlled asymmetric charge distributions that generate strong intramolecular electric fields. These COFs were further integrated with ZnIn₂S₄ (ZIS) via in situ growth to form intimate Z-scheme heterojunctions, creating complementary interfacial electric fields. The resulting JUC-X@ZIS (X = 702, 703, or 704) systems exhibit remarkable charge separation efficiency, with the optimized JUC-704@ZIS (1:8) achieving an exceptional cocatalyst-free PHE rate of 9.46 mmol g^-1 h^-1 in seawater, outperforming most reported COF-based heterojunction photocatalysts. Combined experimental and theoretical analyses reveal that the spatial positioning of sulfur atoms governs both the charge asymmetry and dual electric field formation, providing new molecular-level insights for designing high-performance photocatalytic systems.