Highly positively-charged membrane enabled by a competitive reaction for efficient Li⁺/Mg²⁺ separation

Advanced monovalent cation exchange membranes (MCEMs) have been applied to extract key monovalent cations (Li⁺ or Na⁺) from nature, but it is challenging to design high-performance MCEMs with low membrane area resistance and specific charge properties. Herein, a positively-charged Fe³⁺-bridged pyrogallic/polyethyleneimine membrane (Fe³⁺-bridged PY/PEIM) for efficient Li⁺/Mg²⁺ separation is fabricated through a covalent bond and coordination bond (CB/COB) competitive reaction. Various physicochemical characterizations elucidate that the formation of COBs reduces the reaction sites of PY and PEI molecules for Michael-addition and Schiff-base reactions, which reduces the thickness of the selective layers to reduce the membrane area resistance from 6.58 to 4.83 Ω·cm². A highly positively-charged selective layer caused by unreacted –NH₂ groups and Fe³⁺ makes the optimal Fe³⁺-bridged PY/PEIM exhibit a superior separation performance (perm-selectivity (Li⁺/Mg²⁺) of 45.57 with a Li⁺ flux of 4.61 × 10⁻⁸ mol·cm⁻²·s⁻¹). Moreover, the optimal Fe³⁺-bridged PY/PEIM also has excellent separation performance (high Li⁺ flux and perm-selectivity) and operational stability when treating three simulated solutions with different Mg/Li mass ratios and salinities. The new strategy for constructing a thin, defect-free and positively-charged selective layer through a CB/COB competitive reaction can provide new insights for designing high-performance MCEMs towards environmental remediation and energy harvesting.