Novel Nafion composite membranes with mesoporous silica nanospheres as inorganic fillers

Novel Nafion composite proton exchange membranes are prepared using mesoporous MCM-41 silica nanospheres as inorganic fillers. The novelty of this study lies in the structural design of inorganic silica fillers: the nanosized and monodisperse spherical morphology of fillers facilitates the preparation of homogenous composite membranes, whilst the superior water adsorption of the mesostructure in fillers consigns enhanced water retention properties to the polymer membranes. Scanning electron microscopy images of the composite membranes indicate that well-dispersed silica nanospheres are embedded in the Nafion matrix, but a large amount of added fillers (3 wt.%) causes some agglomeration of the nanospheres. Compared with the Nafion cast membrane, the composite membranes offer improved thermal stability, enhanced water retention properties, and reduced methanol crossover. Despite the enhancement of water retention, the composite membranes still exhibit a proton conductivity reduction of 10–40% compared with pristine Nafion. This is likely due to the incorporation of much less conductive silica fillers than Nafion. The composite membrane containing 1 wt.% of fillers displays the best cell performance in direct methanol fuel cell tests; it gives a maximum power density of 21.8 mW cm⁻², i.e., ∼20% higher than the Nafion cast membrane. This is attributed to its similar conductivity to Nafion, and its markedly reduced methanol crossover, namely, ∼1.2 times lower.