<scp>PPY</scp> ‐ <scp>PEI</scp> Engineered Surface With Quaternized Crosslinking for Enhanced K ⁺ /Mg ²⁺ Selectivity in Cation Exchange Membranes

ABSTRACT The monovalent cation exchange membrane (MCEM) plays a crucial role in strategic resource recovery fields such as lithium/magnesium separation and seawater softening. This study proposes a two‐stage modification strategy of “nanoparticle deposition – quaternization cross‐linking”: first, deposit the Polypyrrole ‐Polyethyleneimine (PPY‐PEI) composite nanoparticles in situ on the commercial Cation Exchange Membrane (CEM) surface, and then undergo quaternization cross‐linking to prepare a high‐performance monovalent cation exchange membrane (MCEM). The rigid structure of Polypyrrole ‐Polyethyleneimine (PPY‐PEI) nanoparticles completely avoids the traditional problem of monomer penetration, and the rate of membrane pore blockage is significantly reduced. Electrified nanoparticles form gradient nanochannels to achieve dual mechanism synergy: The surface positive charge layer repels and blocks Mg 2+ through the Donnan effect; 2–5 nm mesoporous channels facilitate K + transport through size sieving. Subsequent quaternization crosslinking strengthened the modified layer and increased the positive membrane charge density, leading to further improvement in perm‐selectivity. The results show that the modified membrane achieves a breakthrough performance in the separation of K + /Mg 2+ : the selectivity reaches 6.9; the K + flux remains at 0.14/(m 2 ·s). Molecular dynamic simulation of the ion diffusion behavior confirmed that the presence of Polypyrrole ‐Polyethyleneimine (PPY‐PEI) can accelerate the diffusion rate of K + while limiting the transport of Mg 2+ , providing a theoretical basis for the performance prediction of selective ion exchange membranes.