Space‐Confined Synthesis of Sulfonated Covalent Organic Framework‒Polymer Membranes for Enhanced Osmotic Energy Conversion

Osmotic energy, an infinite, clean energy source, can be efficiently harnessed through reverse electrodialysis using ion‐selective membranes. While polymeric membranes are excellent candidates due to their solution‐processability and scalability, their non‐uniform pore architecture and high resistance limit their power density output. Here, an in situ space‐confined synthesis strategy is proposed to fabricate sulfonated covalent organic frameworks within a sulfonated polymeric network, resulting in interconnected, well‐defined ion channels. This allows a maximum power density reaching up to 40.33 W m −2 under a 500‐fold salinity gradient and a real‐world power density of 14.84 W m −2 when extracting osmotic energy from natural seawater and river water. This study underscores the potential of space‐confined synthesis strategies in creating flexible and scalable ion‐selective membranes for efficient salinity gradient energy harvesting, marking a significant step toward their practical applications.