Harnessing ion resource recovery: Design of selective cation exchange membranes via a synergistic ionic control method

Effective recovery of valuable ion resources such as lithium is gaining increasing importance. Monovalent selective cation exchange membranes (MCEM) used in electrodialysis have shown great potential, but their development is hampered by the classical trade-offs related to membrane permeability, selectivity and energy efficiency. We reported a new route to design high performance MCEM, utilizing the synergy of ionic control (IC) through cation-π bonding of polydopamine (PDA) and alternating current (AC), which has not been reported before. A set of deliberately controlled membrane fabrication conditions were chosen, with different ionic control (i.e., no cation, K+, Li+) and with/without AC for systematic comparison. Combining with morphology & electrochemical measurements, the membrane with electro-assisted K+ control, i.e., PDAM-K+/AC, exhibited respective 72% and 51% higher Li+ and K+ transport rate, higher permselectivity of 9.54 (Li+/Mg2+) and 17.86 (K+/Mg2+), and 3.3-fold lower surface electrical resistance (SER) compared to other modified membranes (e.g., PDAM). Also, PDAM-K+/AC exhibited no sign of scaling, supported by its much increased limiting current density. The results supported the hypothesis of the synergistic effect between cation-π bonding and AC electro-deposition, which altered the polymerization dynamics and produced more ordered membrane structure. The control strategy showed a viable route to fabricate ion exchange membranes for imparting monovalent selectivity, reducing scaling and avoiding energy penalty.