Protonic conductivity and viscoelastic behaviour of Nafion® membranes with periodic mesoporous organosilica fillers

• SO3H-functionalised mesoporous organosilicas (S-PMO) with high protonic conductivity.
• S-PMO as fillers for polymeric membranes with enhanced properties.
• Membrane swelling is reduced by absorption of water in rigid S-PMO porosity.
• Storage modulus is up to 15 times higher than Nafion.
• Protonic conductivity is up to 15 times higher than Nafion at 20% relative humidity.

Benzene-bridged periodic mesoporous organosilicas functionalised with sulfonic acid (S-Ph-PMO) are explored as fillers to improve the protonic conductivity and the viscoelastic properties of Nafion®. Homogeneous membranes with 5, 10 and 20 wt.% submicrometric S-Ph-PMO particles (roughly corresponding to 11, 20 and 36 vol.%) were obtained by control of the casting suspensions. The three composite membranes have acid loads and water uptake values similar to Nafion®. The storage modulus of the 20 wt.% S-Ph-PMO composite (0.2 GPa at 100 °C and 0.05 GPa at 140 °C) is 2.5–15 times higher than for pure Nafion® (respectively 0.08 and 0.005 GPa), denoting a positive effect of the fillers on the mechanical resistance of the membranes, also observed for lower filler fractions. The protonic conductivity of the composite membranes at 20% relative humidity (RH) and 40 °C is up to 1.5 orders of magnitude higher than for Nafion®. The magnitude of the effect decreases with increasing humidity, with the best composite attaining 0.03 S cm−1 at 120 °C/40% RH, 3 times more than Nafion®. All membranes have similar behaviour at 98% RH, showing a maximum of 0.2 S cm−1 at 94 °C, with the composites still showing slightly better performance. The results are discussed in terms of the effect of the fillers on reducing the internal swelling pressure and the activation energy for proton migration.