The effect of ultrasonication of polymer solutions on the performance of hybrid perfluorinated sulfonic acid membranes with SiO2 nanoparticles
Perfluorinated sulfonic acid (PFSA) polymer membranes are widely used as ion-conducting electrolytes in energy-conversion devices. The development of novel hybrid materials containing inorganic dopants offers the route towards optimization of the performance of the PFSA membranes. In this work, the effect of ultrasonic (US) treatment of the PFSA solutions in the presence of SiO2 nanoparticles on the characteristics of the cast hybrid Nafion+SiO2 membranes was studied for the first time. Upon ultrasonication of polymer solutions, the length of macromolecules is reduced, and the number of sulfo groups decreases. When polymer solutions are ultrasonicated in the presence of SiO2, they experience additional crosslinking due to the interaction of SiO2 with sulfo groups of the PFSA polymer. As a result, up to 20% of -SO3H groups appears to be excluded from the ion-exchange process, and the temperature corresponding to destabilization of ionic clusters is reduced. When a hydrophilic dopant is incorporated within pores, the overall water uptake of the hybrid membranes increases, and their proton conductivity is improved. Maximum conductivity (78.6 mS/cm) at 40°С in the contact with water is observed for the Nafion+1 wt% SiO2 membrane cast from polymer solutions upon the ultrasonication for 10 min. The membranes preserve their high conductivity at low relative humidity (4.1 mS/cm at 30°С, 30% RH), and this value is 1.7 times higher than that of the pristine Nafion membrane. Hydrogen permeability of the hybrid Nafion+SiO2 membranes appears to be lower than that of the Nafion membranes by 15%. Hence, US-assisted dispersion of dopant nanoparticles in the PFSA solutions allows preparation of hybrid membranes with improved transport characteristics.