Hybridization of Nafion membranes with an acid functionalized polysiloxane: effect of morphology on water sorption and proton conductivity

Polysiloxane-modified hybrid Nafion membranes were prepared by casting a mixture of Nafion solution and a precursor of acid functionalised polysiloxane based on tetraethoxysilane and a mercaptan-organoalkoxysilane. SEM and AFM analysis revealed that the functionalised polysiloxane was dispersed either as finely nanosized inclusions or as coarse domains depending on the rate of the solvent evaporation during the casting procedure. In particular the slower is the rate of solvent evaporation the more interpenetrated and homogenously dispersed at nanosized level is the polysiloxane inside the Nafion membrane. The hybridization process increases the thermal stability of the membranes of about 50°C relatively to the unmodified Nafion. SAXS analysis reveals that the hybrid membranes exhibited the typical morphology of Nafion consisting of distinct hydrophilic and hydrophobic domains. Water vapor sorption and proton conductivity were measured varying the temperature (up to 120°C) and the water activity conditions (from 0.1 to 0.8). The polysiloxane network always increases the water vapour uptake of the membranes and increases significantly the proton conductivity at higher temperature depending on the type of morphology developed by the manufacturing method. In particular hybrid membranes exhibiting nanosized polysiloxane dispersion show a proton conductivity which is up to one-and-half time higher than Nafion recast membrane at high temperature and low water content.