Structurally Cross-Linked Composite Proton Exchange Membranes

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 ethylene-methylacrylate copolymer with glycidyl methacrylate groups (PMG). The proton conductors used were commercial Keggin-structured silicotungstic acid (H4SiW12O40-26H2O, W12- STA) and lacunary W10- and W11-STA. The 3D cross-linked membranes were dense with in-situ formed SiO2 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 80oC/100%RH with proton diffusion coefficients of 2.5-3.5 x 10-6 cm/s peaked in the ~90o-110oC range for PEMs having a loading level of W12-STA >150 wt% (ratio to the weight sum of PMG and MXL).