Mussel-Inspired Monovalent Selective Cation Exchange Membranes Containing Hydrophilic MIL53(Al) Framework for Enhanced Ion Flux

The surface properties and structure of a membrane play a significant role in the ion selectivity during electrodialysis. Recently, polydopamine (PDA) and its related nanomaterials have emerged as promising materials to develop composite membranes for higher separation requirements. Previous research has shown that the codeposition of PDA and polyethylenimine (PEI) triggered by Cu2+/H2O2 could facilitate the transport of H+ while rejecting multivalent ions though electrostatic effects. However, the enhanced H+ flux by acid-base pairs is not applicable in a Na+/Mg2+ system. Here we report a facile method to construct monovalent selective membranes through rapid codeposition of PDA/PEI and Mil(53)-Al, followed by cross-linking with trimesoyl chloride (TMC). The positive -NH2 allows us to reject multivalent cations, while porous Mil(53)-Al can accelerate the migration of Na+.The surface morphology and physicochemical properties of the as-prepared composite membranes were studied by SEM, AFM, and XPS analyses, and the electrochemical properties were evaluated by EIS and current-voltage curves. The results demonstrated that a robust skin layer was formed on the commercial cation exchange membrane substrate, endowing the ion-exchange membrane with an increased separation performance for multivalent ions. The monovalent selectivity and ion flux can be tuned by changing the ratio of Mil(53)-Al during the codeposition process. A mass ratio of 0.2-0.4% (w/v) for Mil(53)Al is the optimum protocol, yielding a membrane with a permselectivity of about 0.3 and an ion flux of about 22.0 and 0.6 mol- cm(-2).s(-1) for Na+ and Mg2+, respectively. At this condition, the PDA-coated membrane maintains a high monovalent selectivity with enhanced Na+ and Mg2+ flux in single salt solutions. This one-pot method to prepare PDA based membrane provides a new direction to prepare monovalent selective ion-exchange membranes.