New polyethylene based anion exchange membranes (PE-AEMs) with high ionic conductivity

Article abstract of DOI:10.1021/ma200836d

This paper discusses a new class of high performance polyethylene-based anion exchange membranes (PE-AEMs) that contain a wide concentration range of pendant (flexible) ammonium chloride (NR₃⁺Cl^-) groups and with or without a cross-linked PE matrix structure. The chemistry involves a metallocene-mediated polymerization of ethylene, silane-protected α,ω-amino-olefin [C_xN(SiMe₃)₂], with or without styrenic diene (cross-linker), to form ethylene/C _xN(SiMe₃)₂ copolymers and ethylene/C _xN(SiMe₃)₂/diene terpolymers, respectively. The resulting co- and ter-polymers were completely soluble in common organic solvents and were solution-casted into uniform films (thickness, 50-70 μm; without backing material) and then thermal cross-linked in ethylene/C _xN(SiMe₃)₂/diene case, further interconverting the silane-protected amino groups into the desired -NR₃ ⁺Cl^- groups (R: H, CH₃, and C₃H ₇) under solid state conditions. The resulting PE-NR₃ ⁺Cl^- and cross-linked x-PE-N(CH₃) ₃⁺Cl^- membranes were systematically studied to understand how the PE structure (-NR₃⁺Cl^- concentration, R group, cross-linking density, etc.) affects ionic conductivity, water uptake, film stability, and ion selectivity. For comparison, several commercially available AEMs were also examined. Evidently, an x-PE-N(CH ₃)₃⁺Cl^- membrane, with 28.1 mol % -N(CH₃)₃⁺Cl^- groups and 0.2 mol % cross-linkers, shows moderate water swelling and outperforms all commercial membranes with exceptionally high ionic conductivities of 119.6 mS/cm in 2 N HCl solution and 78.8 mS/cm in 2 N HCl-0.2N CuCl solution at room temperature.