Effect of coupling agents on the transport properties of sulfonated poly (ether ether ketone) based composite membrane

Development of low cost proton exchange membrane surges as a main research topic in order to replace expensive Nafion in the fuel cell industry. In this study, sulfonated poly (ether ether ketone) (SPEEK) was used as the base polymer for the purpose. Silicotungstic acid supported on silica (SiWA-SiO₂) was incorporated into SPEEK polymer matrix to improve the performance of composite membrane. 3-aminopropyltriethoxysilane (APTES) and carbonyldiimidazole (CDI) were used as coupling agents to increase the compatibility of the organic and inorganic phases in the composite membrane. The reaction mechanisms involve the interaction between imidazole group of CDI with sulfonic acid group of SPEEK polymer followed by substitution using APTES. Finally, inorganic silica will attach to the inorganic functionalities of APTES and complete the organic-inorganic bridge between the two phases. Stabilisation of SiWA-SiO₂ additives in the polymer matrix was greatly improved when APTES and CDI are added. It was found that SPEEK/SiO₂-SiWA exhibited smooth, homogenous and compact membrane structure. This could reduce the permeability of the membrane to methanol. Although the composite membrane had low water uptake, presence of ample amount of SiWA within the polymer matrix helped to enhance the proton conductivity of SPEEK composite membrane. It was reported that the overall selectivity is increased to 10.6 × 10⁴ S.s/cm³. (paper)