Self‐Powered Green Hydrogen Production via Osmotic Energy Harvesting

Harnessing renewable energy for green hydrogen production is critical for decarbonization. An ideal, sustainable route involves self‐powered hydrogen production without additional energy input. Here, the osmotic energy between seawater and river water is used to continuously generate electricity to directly produce hydrogen. Efficient hydrogen production is successfully achieved by connecting the osmotic energy device composed of the polyamide acid PAA37 ion selective membrane and the water electrolysis device in series. The PAA37 membrane, featuring engineered sub‐nanometer channels, exhibits an ultra‐high cation transference number (t+ = 0.96). Targeting the critical challenge of scaling up osmotic power generation, the HLZ equation is introduced. It theoretically establishes that the decline in power density under large‐area conditions is primarily attributed to the electrode impedance within the low‐concentration zone. This finding offers a theoretical foundation for guiding the optimization of large‐scale device designs. Consequently, the PAA37 membrane achieves a power density of 6.0 W m−2 over a macroscopic area of 3.14 mm2 under a 50‐fold KCl. Furthermore, by stacking 110 RED units in series, a remarkable output voltage of 24.3 V is generated. By arranging this stack in series and parallel, the system successfully powers an electrolyzer for direct hydrogen production.