Pérez-Pérez, Maritza

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    Novel sulfonated ether based block copolymer ionomers as proton exchange membranes for fuel cells applications
    (2017) Pérez-Pérez, Maritza; Suleiman-Rosado, David; College of Engineering; Padovani, Agnes M.; Curet Arana, María; Bogere, Moses N.; Department of Chemical Engineering; Hernandez, Carlos I.
    This investigation studied the morphology, hydration, and transport properties of different membranes for direct methanol fuel cell (DMFC) applications. First, sulfonated poly(ether ether ketone) (SPEEK) membranes were analyzed as function of eight counter-ions: Ba2+, Ca2+, Mg2+, Mn2+, Ni2+, Cu2+, Zn2+, K+. The results were used to understand the changes in the physical, chemical, and thermal properties upon counter-ion substitution and to explain the DMFC transport property results (e.g., proton conductivity and methanol permeability). Significant differences in their thermal, physical, and transport properties were observed for SPEEK when exchanged with the different counter-ions. Alternative transport mechanisms generated by the location of the counter-ions in the different ionic domains and the roles of water were discussed. Normalized selectivities (proton conductivity over methanol permeability divided by the respective values of Nafion®) were better than Nafion®, but with significant differences between the counter-ions studied, ranging from 3.8 for SPEEK-Mg2+ to 36.7 for SPEEK-Ca2+. Second, the effect of block composition on the properties of novel sulfonated ether based membranes was studied. A homopolymer and two block copolymers were synthesized using atom transfer radical polymerization (ATRP). The homopolymer poly(ethylene glycol phenyl ether methacrylate) (PEGPEM) was used as a bifunctional macroinitiator. Polystyrene (PS) was added to both sides of PEGPEM (A) with two different percentages of PS (B) (i.e. 18% and 31%). These copolymers, BAB 18, BAB 31 and the homopolymer A, were completely sulfonated (SA, SBAB 18 and SBAB 31), producing different water absorption values and transport properties for DMFC applications. The nanostructure and morphology of the casted membranes revealed that all six membranes exhibited a disordered phase-segregated morphology, which changed upon sulfonation into small- interconnected ionic domains. Normalized DMFC selectivities ranged from 1.16 for SBAB 31 to 15.30 for BAB 18, indicating that the performance of these materials can be comparable or better than NafionÆ. The outstanding results produced by some of these membranes suggest that chemistry (block nature and composition), morphology, and water content play a critical role in the transport mechanism of protons and methanol. Third, the effect of increasing the number of ether groups of proton conducting polymer membranes was studied as a function of block composition. Two homopolymers, six block copolymers and six random polymers were synthesized using ATRP. The homopolymer poly(2-ethoxyethyl methacrylate) (PEEM) (C) and 2-(2-methoxyethoxy) ethyl methacrylate (PMEEM) (D) were used as a bifunctional macroinitiator for the block copolymer. Styrene was used as a block copolymer and the random polymer, as well as the active site for the sulfonation. Three different sulfonation levels were synthesized for each different percentages of PS (B), (i.e. 8%, 18% and 30%). The membranes produced different water absorption, IEC, and methanol permeability values. Also for these membranes, the nanostructure and morphology of the casted membranes revealed a disordered phase-segregated morphology, which changed upon sulfonation into small-interconnected ionic domains. The results also showed that increasing the number of ether groups decreased the thermal stability of the membrane but increased their selectivity.