Sub-1 μm Free-Standing Symmetric Membrane for Osmotic Separations

Desalination membranes have an asymmetric structure and thus experience severe internal concentration polarization (ICP) in osmotic separations, such as desalination and water purification. ICP substantially reduces the driving force for water permeation and thus hinders membrane performance. Here we report, for the first time, the design of sulfonated polybenzimidazole (SPBI) for the fabrication of symmetric, support-free, and self-standing osmotic membranes with exceptionally high mechanical robustness and desalination performance. Tunable desalination performance of SPBI membranes could be readily achieved by adjusting the membrane thicknesses to 250-650 nm. Owing to the ultrathin symmetric architecture and the presence of negatively charged sulfonate groups, the optimally designed membrane exhibited high water permeability and negligible salt passage. Such remarkable performance contributed to an outstanding water/Na₂SO₄ selectivity of 1882 L mol^-1 of our membrane, which is 5-fold higher than that of a commercial cellulose triacetate osmotic membrane. The identical physicochemical and transport properties of both sides of the symmetric membrane led to negligible ICP effects and thus high membrane performance. Our study provides a promising synthetic procedure for fabricating submicrometer osmotic membranes and highlights the significance of their symmetric structure for further development of membrane-based osmotic technologies.