Preparation and Properties of Polyhedral Oligomeric Silsesquioxane (POSS)/ Blend Sulfonated Poly (Phenyl Sulfone) Membrane for PEM Fuel Cell Application

The performance of polymer fuel cells as one of the most popular cells is generally dependent on polymer membrane characteristics. To commercialize the technology, the development of a high performance and low cost membrane as an alternative to Nafion commercial membrane is the focus of researchers. In this paper, the preparation and evaluation of the mechanical properties as well as proton conductivity of nanocomposite membrane from poly (phenyl sulfone) blend and POSS nanoparticle has presented. To this end, two sulfonated poly (phenyl sulfone) with ۴۰ and ۵۰% sulfonation degree were synthesized by polycondensation. For this purpose, sulfonated monomer was first synthesized. After confirming the structure of synthesized polymers, the blending of two polymers was done through mixing a ۵۰/۵۰ weight ratio of components (as the critical composition). Then, glycidyl-functionalized POSS nanoparticle was introduced into the blend to improve the compatibility of the two components, the film-ability, and the stability. Considering the importance of mechanical stability as well as proton conductivity, which are the main features in the application of proton exchange membranes in cell media, these two properties were investigated in this stage. According to the attained results, the effect of glycidyl-functionalized POSS nanoparticle on film-ability was significant. The mechanical properties of the membrane including tensile strength and elongation at break showed acceptable values of ۴۰.۵ MPa and ۱۸.۵%, respectively. Also, the prepared membrane showed proton conductivity of ۵۸ mS/cm at ۸۰ °C in deionized water. Thus, the membrane was considered for further evaluations especially in cell media. Moreover, the preparation and testing of improved membranes of this type was regarded.Improving the stability of polymer membranes and reducing the cost has been the focus of researchers in energy conversion and storage area in recent years. Nafion commercial membrane as the most popular membrane has a high price despite the high performance and durability. Sulfonated aromatic hydrocarbon membranes have a low price as a substitute for Nafion. The obstacle of these membranes is the low stability and short lifespan, especially at high sulfonation degrees. Attaining a flexible membrane is also a challenge by using high amount of sulfonated groups. That is, increasing the amount of sulfonated groups in this type of membranes leads to excess swelling or dissolution in water and brittleness of the membrane in the dry state. Blending is intended to take advantages of the components and eliminate their disadvantages. However, the problem of high sulfonation content is still present in blend membranes. In addition, in the case of blend membranes, phase separation which cause the formation of a non-uniform film, especially at intermediate compositions is exist [۱-۳]. Using suitable nanomaterial would be useful to remove the issues [۴]. Thus, in this study, a ۵۰/۵۰ blend of two sulfonated poly (phenyl sulfone) with ۴۰ and ۵۰% sulfonation degree containing glycidyl functionalized-POSS was prepared. Then the mechanical behavior and proton conductivity were assessed for future study.For this purpose, sulfonated monomer was first synthesized. The synthesis of sulfonated monomer from difluorodiphenyl sulfone dihalide was performed via the electrophilic substitution reaction. After confirming the structure of the synthesized monomer using FT-IR and ۱H-NMR spectroscopies [۳], two poly (phenyl sulfone)s with ۴۰ and ۵۰% sulfonation degree were synthesized during the polycondensation in the presence of fluorinated chain extender [۳]. Then, for the preparation of nanocomposites, ۳% w/w of POSS nanoparticles was added into polymer structure by mechanical mixing and solution casting. As the nanoparticle was introduced into the polymer solution, the viscosity increased significantly. So, the solution was diluted with additional amounts of solvent. After obtaining a homogeneous solution, apolymer membrane with an average thickness of ۵۰-۶۰ μm was obtained by drying the cast solution atroom temperature for ۲۴ hours. The chemical structure of sulfonated polymer, nanoparticle, and the blendnanocomposite membrane are shown in Figure ۱. As can be seen, despite the preparation of a blendmembrane with a composition of ۵۰/۵۰, excellent film-ability of the membrane structure was obvious bygaining a flexible and uniform film. In the next step, proton conductivity and mechanical properties were studied as basic properties in theapplication of proton exchange membranes. As shown in Figure ۲, the nanocomposite blend membranehas a tensile strength of about ۴۰.۵ MPa and an elongation at break of about ۱۸.۴%. These values indicatethe applicability of this membrane in cell media. Proton conductivity values of ۱۷ and ۵۸ mS/cm at ۲۳ and۸۰ oC, respectively show the acceptable conductivity of this membrane in the presence of inertnanoparticles. By using two polymers with sulfonation degrees of ۴۰ and ۵۰%, proton conductivity andmechanical stability are brought together. Glycidyl-functionalized POSS nanoparticle also provideflexibility through silicon oxide bonds as well as aliphatic bonds. Moreover, due to the possible interactionof sulfonated groups in the polymer chain with epoxy bond, which would be investigated in detail, theproton conductivity is reduced. According to attained results in this stage, the preparation of membranes with different amounts ofnanoparticle as well as synthesizing highly sulfonated polymer to compensate the reduced conductivity isconsidered to reach an applicable substitution for Nafion. Characterization, properties measurements,performance and durability assessment in cell media would be also conducted in detail

کلیدواژه ها:

Proton exchange Membrane ، Blend Membrane ، Sulfonated poly (phenyl sulfone) ، POSS ، Tensile properties ، Proton conductivity.