Counter-ions diffusion properties of silica embedded poly(perfluorosulfonic) acid membrane

To study the possibility of modifying diffusional-transport properties of the poly(perfluorosulfonic) acid (Nafion-117) membrane, the silica particles were incorporated in its matrix by in situ sol–gel method. The amount of silica incorporated in the membrane samples was varied by controlling the equilibrating time of the membrane sample in solution containing fixed proportion of precursor tetraethylorthosilicate (TEOS), methanol and water. The silica contents in the membrane samples were characterized by gravimetry, energy dispersive X-ray analysis (EDXRF) and transmission electron microscopy (TEM). Differential scanning calorimetry (DSC) studies showed that there were no significant difference in transition peak temperatures of both samples (Nafion-117 and Nafion/SiO₂) in same ionic form except that enthalpies were increased in Nafion/SiO₂ samples in all the ionic forms. The increase in enthalpy was due to higher water content in Nafion/SiO₂ as compared to Nafion-117 in given ionic form as enthalpies normalized with water content of the respective membrane were comparable. The ion-exchange capacity, water uptake capacity and ion-exchange isotherms of the Na⁺+Cs⁺ and Na⁺+Ba²⁺ binary systems in silica embedded membrane samples were experimentally measured using appropriate radiotracers, and compared with that of pristine Nafion-117 membrane. The specific conductivities of the membrane samples with and without silica were studied in Na⁺, Cs⁺ and Ba²⁺ ionic forms by electrochemical impedance spectroscopy (EIS). The self-diffusion coefficients of Na⁺, Cs⁺ and Ba²⁺ ions in the Nafion/SiO₂ were measured by the radiotracer method. It was observed from the experimentally measured specific conductivities and self-diffusion coefficients data that diffusion mobility of Cs⁺ and Ba²⁺ counter-ions were enhanced significantly after incorporation of SiO₂ in the Nafion-117 matrix. Contrary to this, diffusion mobility of Na⁺ counter-ion decreased in Nafion/SiO₂ sample. The selectivity of Cs⁺ and Ba²⁺ counter-ions was reduced in Nafion/SiO₂ sample with respect to pristine Nafion-117.