Synthesis and properties of novel HMS-based sulfonated poly(arylene ether sulfone)/silica nano-composite membranes for DMFC applications

Novel 4,4′-dihydroxy-α-methylstilbene (HMS)-based sulfonated poly(arylene ether sulfone) with sulfonic acid composition ranging from 10 to 40 mol% was synthesized via nucleophilic step polymerization of 4,4′-dihydroxy-α-methylstilbene, 4,4′-dichloro-3,3′-disulfonic acid diphenylsulfone and 4,4′-dichlorodiphenylsulfone and blended with silica sol to form organic/inorganic nano-composite membranes. The organic/inorganic nano-composite copolymers produced show a high glass transition temperature and thermal decomposition temperatures from 318 to 451 °C. The copolymers present appropriate toughness during the membrane process. The equilibrium water uptake and proton conductivity of the obtained organic/inorganic nano-composite membranes were measured as functions of temperature, degree of sulfonation and silica content. In general, the water uptake increased from 8 to 37 wt.%, and the proton conductivity of the organic/inorganic nano-composite membranes increased from 0.003 to 0.110 S cm⁻¹ as the degree of sulfonation increased from 10 to 40 mol%, the silica content increased from 3 to 10 wt.%, and the temperature increased from 30 to 80 °C. The single cell performance of the 40 mol% organic/inorganic nano-composite membrane with various silica contents ranged from 11 to 13 mW cm⁻² at 80 °C, and the power density was higher than Nafion^® 117. Including the thermal properties, swelling, conductivity and single cell performance, the nano-composite membranes are able to satisfy the requirements of proton exchange membranes for direct methanol fuel cells (DMFC).