Molecular Weight-Driven Chain Entanglement in Fluorene-Based Anion Exchange Membranes for High-Performance CO ₂ Electrolysis

The hydration, mechanical, and ion-transport stability of anion exchange membranes (AEMs) govern the performance and durability of CO 2 electrolysis. However, conventional strategies focusing on backbone or cation design and cross-linking offer limited control over polymer network architecture. Here, we show that increasing monomer purity enables higher molecular weight, strengthening chain entanglement, and improving dimensional stability, ion selectivity, and durability. Fluorene-based ionomers, quaternary-ammonium-tethered poly[(fluorene alkylene)- co -(biphenylene alkylene)] (FLBN), exhibit high molecular weights ( M n >100 kDa) and low dispersities ( Đ < 3). The alkylene-spaced fluorene–biphenyl backbone promotes chain entanglement while maintaining mechanical rigidity. In CO 2 electrolyzers, FLBN-based AEMs deliver higher CO partial current densities than Sustainion and PiperION across all voltages, while maintaining stable CO Faradaic efficiency with negligible voltage increase during 300 h of operation. These results establish chain entanglement control as a key design principle for durable, high-performance AEMs.