Ultra-Thin Cation Exchange Membranes: Sulfonated Polyamide Thin-Film Composite Membranes with High Charge Density

Advancements in membrane technology are crucial for electrochemical separations, such as ion exchange, and pressure-driven processes, such as nanofiltration (NF). This study introduces high-charge-density sulfonated polyamide thin-film composite membranes fabricated via interfacial polymerization using disulfonated monomers, resulting in ultra-thin (∼50 nm) films that serve as nanofiltration (NF) membranes or cation exchange membranes (CEMs). Post-modifications enabled precise control over membrane chemistry, enhancing CEM properties such as ion exchange capacity, water uptake, and fixed charge concentration. The high charge density led to ion selectivity in NF via the Donnan exclusion mechanism, facilitating effective separation of monovalent and divalent ions. The incorporation of sulfonic acids within an ultra-thin polyamide matrix significantly reduced the resistance for ion and proton transport, enabling high in-plane conductivities (Na⁺: >80 mS cm^–1, H₃O⁺: >200 mS cm^–1) comparable to state-of-the-art polymer-based CEMs. Furthermore, the nanoscale thickness of these membranes dramatically enhanced ionic and proton conductance, achieving area conductance values 4 to 6 orders of magnitude higher than those of conventional thick CEMs. This enhancement is primarily attributed to the ultra-thin design of our sulfonated polyamide membrane, setting a new benchmark for the design and fabrication of highly conductive membranes, and laying the groundwork for future enhancements of ion conductive membranes for water purification and energy applications.