3M PFSA ionomer membranes were doped with heteropoly acids (HPA) and the ionomer/HPA interaction characterized by attenuated total reflection FTIR, pulse field gradient spin echo NMR and small-angle X-ray scattering. The HPA doping was achieved by casting a mixed solution containing both components. Three Keggin-type HPAs were used in this work: 12- phosphotungstic acid, 12-silicotungstic acid, and 12- zincotungstic acid, at doping concentrations of 1 and 5wt %, respectively. FTIR spectra of the composite membranes clearly show the presence of the HPA and evidence for different interactions under different hydration states. Proton diffusion coefficients were determined by PFGSE NMR under dry and 100 % RH from room temperature to 130 {degree sign}C. SAXS patterns of the control PFSA and the HPA-doped PFSA membranes were recorded under dry and wet conditions. Scattering behavior for different samples and conditions are remarkably different, consistent with the incorporation of HPA and the swelling of the ionomer clusters under hydration.
Composite proton exchange membranes (PEMs) with a thermally cross-linked polymer backbone for high-temperature fuel cell applications were fabricated by casting tetrahydrofuran solution mixtures of tetraethoxysilane, a functional silane, a proton conductor, a molecular cross-linker (MXL), and an ethylenemethylacrylate copolymer with glycidyl methacrylate groups (PMG). The proton conductors used were commercial Kegginstructured silicotungstic acid (H 4 SiW 12 O 40 -26H 2 O, W12-STA) and lacunary W10-and W11-STA. The 3D cross-linked membranes were dense with in-situ formed SiO 2 nanoparticles and showed high thermal stability, high chemical resistance to the Fenton's reagent, and moderate mechanical strength and flexibility. The corresponding proton conductivity was affected by formulation, membrane quality, loading level of W12-STA, temperature, and relative humidity, and was in the 20-25 mS/cm range at 80 o C/100%RH with proton diffusion coefficients of 2.5-3.5 x 10 -6 cm/s peaked in the ~90 o -110 o C range for PEMs having a loading level of W12-STA >150 wt% (ratio to the weight sum of PMG and MXL).
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