Sol-gel organic-inorganic acid blends based on styrene-ethylene-butylene-styrene block copolymer SEBS and (MPTMS-GPTMS) for PEMFCs

Many studies have been devoted to the development of new proton exchange membranes for low temperature fuel cells (PEMFCs) presenting a suitable dimensional stability, high ionic conductivity and low cost for a significant volume production. Among them, block copolymer ionomers based on styrenic thermoplastic elastomers bearing sulfonic acid groups are interesting and very economical membrane materials. The introduction of ionic groups into the polystyrene blocks causes significant changes in many physical properties, not only the emergence of ionic conductivity but also changes in hydrophilicity and mechanical strength, not detected in their non-ionic counterparts. Commercial SEBS (styrene-ethylene-butylene-styrene) triblock copolymer is obtained by hydrogenation of the thermoplastic elastomer of styrene and butadiene, eliminating the unsaturation of the butylene chain. After sulfonation, via electrophilic substitution, nanometer scale phase separated morphology of hydrophilic and hydrophobic domains is generated providing proton mobility paths in wet state which makes sulfonated SEBS (sSEBS) very attractive for its use as electrolyte in fuel cells. Unfortunately, a high degree of sulfonation is often required to achieve suitable proton conductivity which causes excessive swelling in water and loss of dimensional stability with moderate cell performance and durability. An interesting strategy to reduce water uptake and improve mechanical properties is the development of hybrid organic-inorganic membrane materials with covalent bonds between components, using the sol-gel method [1,2]. The inorganic component chosen is itself a hybrid (organic-inorganic) consisting of 3-mercaptopropyl trimethoxysilane (MPTMS) and (3-glycidoxypropyl) trimethoxysilane (GPTMS) in a molar ratio of 70/30 [3]. This system combines the polymerization of epoxide groups, the possibility of oxidation of mercapto groups to promote higher proton mobility and the inorganic polycondensation by sol-gel chemistry. The hybrid membranes were prepared by doctor blade from chloroform solutions of SEBS/(MPTMS-GPTMS) mixtures with different compositions (Table 1), followed by gelificación of the hybrid material with a thermal treatment, oxidation of mercapto groups and finally the sulfonation reaction of styrene groups of the copolymer. The effect of (MPTMS-GMTMS) proportion in the resulting hybrid membranes is studied in order to their future application as electrolytes in PEMFC. Thus, the s(SEBS-MPTMS-GPTMS) hybrid sol-gel membranes are compared with neat sSEBS and characterized in terms of swelling ratio and area increase in water, IEC, thermal stability, morphology, proton conductivity and performance in single H2/O2 fuel cells