Synthesis of Gemini Basic Ionic Liquids and Its Application for Anion Exchange Membranes Based on Pyridine Functionalized Poly(vinyl alcohol)

A new Gemini basic morpholine ionic liquid (IL, [Nbmd]OH) was synthesized with morphline, bromodecane and 1,4-dibromobutane via a two-step procedure. The structure of the new IL was characterized by H-1-NMR and FTIR. A series of anion exchange membranes (PVA-FP/[Nbmd]OH) were prepared by casting method with pyridine functionalized poly(vinyl alcohol) as the polymer matrix. The PVA-FP/[Nbmd]OH composite membranes were characterized in details by AC-impedance spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and tensile strength test. The results indicated that PVA-FP/[Nbmd]OH composite membranes have uniform morphology, and the introduction of pyridine groups enhance the thermal stability and alkali resistance of PVA matrix, due to the formation of the hyper conjugation structure between sigma bond (C- H bond in methyl group) and pi-bond conjugated region (pyridine group), therefore enlarging the conjugated region, decreasing the positive charge density of the cations. Meanwhile, [Nbmd]OH ionic liquid not only provided more cationic active sites but also reduced the crystalline of the composite membranes, resulting in an increase of OH- conductivity and an improvement of the mechanical properties. When the weight ratio of [Nbmd]OH to PVA was 2.5, the thermal decomposition temperature of the composite membrane was 75 degrees C higher than that of the pristine PVA membrane; the maximum OH- conductivity was found at about 4.42 x 10(-2) S.cm(-1) at 70 degrees C; no obvious decrease in OH- conductivity was observed for the composite membrane after immersing in 6 mol/L KOH solution at 80 degrees C. On the contrary, the OH- conductivity was improved to 1.6 times of the initial OH- conductivity after 400 h immersion time, showing an excellent alkali resistance stability. In addition, the methanol permeability of the composite membrane determined using 3 mol/L methanol solution at 30 degrees C was only about 2.5% - 5% of the commercial Nafion (R)-117 membrane under the same test conditions, indicating a promising potential use in alkaline direct methanol fuel cells.