Sulfonated SBA-15 mesoporous silica-incorporated sulfonated poly(phenylsulfone) composite membranes for low-humidity proton exchange membrane fuel cells: Anomalous behavior of humidity-dependent proton conductivity

Sulfonated SBA-15 mesoporous silica (SM-SiO2)-incorporated sulfonated poly(phenylsulfone) (SPPSU) composite membranes are fabricated for potential application in low-humidity proton exchange membrane fuel cells (PEMFCs). The SM-SiO2 particles are synthesized using tetraethoxy silane (TEOS) as a mechanical framework precursor, Pluronic 123 triblock copolymer as a mesopore-forming template, and mercaptopropyl trimethoxysilane (MPTMS) as a sulfonation agent. A distinctive feature of the SM-SiO2 particles is the long-range ordered 1-D skeleton of hexagonally aligned mesoporous cylindrical channels bearing sulfonic acid groups. Based on a comprehensive characterization of the SM-SiO2 particles, the effect of SM-SiO2 (as a functional filler) addition on the proton conductivity of the SPPSU composite membrane is examined as a function of temperature and relative humidity. An intriguing finding is that the proton conductivity of the SPPSU composite membrane exhibits a strong dependence on the relative humidity of measurement conditions. This anomalous behavior is further discussed with an in-depth consideration of the characteristics and dispersion state of SM-SiO2 particles, which affect the tortuous path for proton movement, water uptake, and state of water. Notably, at low-humidity conditions, the SM-SiO2 particles in the SPPSU composite membrane serve as an effective water reservoir to tightly retain water molecules and also as a supplementary proton conductor, whereas they behave as a barrier to proton transport at fully hydrated conditions.