Toward Anhydrous Proton Conductivity Based on Imidazole Functionalized Mesoporous Silica/Nafion Composite Membranes

Highlights: • Imidazole-functionalized mesoporous silica/Nafion composite is formed. • Electrostatic interaction between ionic clusters leads to enhanced molecular rigidity and T_g. • Charge transfer resistance decreases with increase in temperature up to 130 °C. • The composite membrane exhibited considerable stability over 70 h at 130 °C. - Abstract: Although Nafion is regarded as the most preferred electrolyte membrane and often used as a benchmark for comparative evaluation of other electrolyte membranes, its wide spread for commercial PEM fuel cells is limited by the poor electrochemical properties at elevated temperatures and low relative humidity conditions. Herein, sol–gel synthesized mesoporous silica functionalized with a protogenic molecule (imidazole) is introduced into the Nafion matrix via a colloid mediated process. The formation of a stable colloid enables homogeneous dispersion of the silica-imidazole nanoparticles without aggregation. Under non-humidified conditions, the amphoteric and self-dissociative character of the tethered imidazole within the matrix functions as a transporting medium to facilitate proton conductivity. The structural and chemical phases are characterized, and qualitatively evaluated by XRD, TEM, FT-IR, TGA, and DMA. The results show that the average proton conductivity of the composite membrane with the optimal amount of functionalized nanoparticles increases progressively to 1.06 × 10⁻² S cm⁻¹ at 130 °C, corresponding to an activation energy of 6.95 kJ mol⁻¹ under non-humidified conditions. The mechanism governing the dynamics of proton conductivity and structural limitations as a function of temperature is discussed