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

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 [A1-D1]m-[A1-D2]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.