New Polyethylene Based Anion Exchange Membranes (PE–AEMs) with High Ionic Conductivity

Abstract

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 (NR 3 + 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 x N(SiMe 3 ) 2 ], with or without styrenic diene (cross-linker), to form ethylene/C x N(SiMe 3 ) 2 copolymers and ethylene/C x N(SiMe 3 ) 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 x N(SiMe 3 ) 2 /diene case, further interconverting the silane-protected amino groups into the desired −NR 3 + Cl – groups (R: H, CH 3, and C 3 H 7 ) under solid state conditions. The resulting PE–NR 3 + Cl – and cross-linked x-PE–N(CH 3 ) 3 + Cl – membranes were systematically studied to understand how the PE structure (−NR 3 + 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(CH 3 ) 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.