Transport Properties of Zeolite Na-X-Nafion Membranes: Effect of Zeolite Loadings and Particle Size

Na-X zeolites particles, synthesized in two size ranges, namely 200–300 nm and 30–100 nm, were used to prepare Nafion/Na-X zeolite composite membranes by recast method. The physical, chemical, and morphological properties of the zeolite powders and composite membranes were examined by XRD, N2 adsorption isotherms, FTIR, SEM, and SAXS analysis. Furthermore, the effect of zeolite particles size and loadings (i.e., 5 and 10% w/w) on the water, methanol, and proton transport properties was investigated. It has been found that the size of the Na-X zeolite particles plays a key role in the proton and methanol transport behavior since it rules the zeolite hydrophilic behavior, the morphology of polymer–filler interphase, and also the nature of water established in the composite membrane. The results show that the membranes loaded with a 5% w/w of submicron-sized Na-X zeolite exhibit a proton conductivity and selectivity significantly higher than Nafion. In particular the proton conductivity at 120 °C is around eight times and the selectivity at 25 °C is around 40% higher than those exhibited by recast Nafion.