Organic Gradient Homojunction via D‐A Engineering Enables Photoelectric/Photothermal Dual‐Assisted Catalysis Toward Full Spectrum Light‐Coupled Low‐Temperature Seawater Batteries

Abstract Coupling solar into metal‐air batteries represents an appealing paradigm for storing intermittent solar energy and boosting device energy efficiency. Current solar‐coupled metal‐air systems rely on UV or visible light harvesting and suffer from inferior charge separation ability and limited solar utilization. Additionally, sunlight action behavior/mechanism in some useful scenarios (seawater electrolytes, low‐temperature) is underexplored. Herein, through gradient homojunction design via donor‐acceptor (D‐A) engineering, it exploits a novel full‐spectrum‐responsive polymer homojunction photoelectrode (PGH) for sunlight‐coupled seawater‐electrolyte‐based Zn/Na‐air batteries (Zn‐SWAB/Na‐SWAB) with boosted sunlight utilization and energy efficiency at lower temperatures. By stacking three pre‐designed analogous [A 1 ‐D 1 ] m ‐[A 1 ‐D 2 ] n copolymers with gradient energy‐levels and rich heterocycles, PGH integrates separate metal‐free active sites for oxygen reduction/evolution reaction (ORR/OER), efficient photothermal effect with full‐spectrum‐absorption, and superior photoelectric effect with high charge‐separation efficiency. Thus, PGH under simulated‐sunlight produces remarkably‐enhanced photocurrent up to 3.2 and 21.4 times during ORR/OER in near‐neutral electrolytes. This endows sunlight‐coupled PGH‐enabled Zn‐SWAB and Na‐SWAB with low voltage gaps of 0.08/0.25 V at room temperature, and 0.21/0.43 V at 0 °C – both of which surpass most reported room‐temperature results. Their energy efficiencies (84.6%/86.8%) at 0 °C even approach their room‐temperature counterparts (93.9%/92.3%). Mechanistic studies reveal photoelectric/photothermal dual‐promoted bidirectional oxygen catalysis responsible for intriguing performance.