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New membranes based on high performance polymers for proton exchange membrane fuel cells PEMFC

Abstract : The current work is directed to production of a proton conducting membrane for a proton electrolyte membrane fuel cell (PEMFC) as a main goal. The originality and the challenge of the membrane elaboration lie in the multi-step procedure: starting with the synthesis of a simple unit – an ionic monomer, continuing with polymerization and overall estimation of the polymer performance at laboratory scale, and ending with production of the required material of industrial quantity and testing in real conditions. All the steps, except the last one, are explicitly studied. Firstly, much attention in the dissertation is paid to description of a protocol for production and purification of the ionic monomers. It is due to complexity of ionic interactions in a system ‘product-solvent' and due to the main requirement of high purity for a monomer that attentive synthesis and treatment of the monomers must be provided. In total three new monomers, bearing perfluorosulfonic acid chains, are reported. Then, a number of polymerization reactions with different commercial non-ionic monomers are proposed. Two main families of proton conducting ionomers are described: random poly(arylene ether)s (PAEs) and poly(arylene ether sulfone)s (PAESs), both random and block-copolymers. They are synthesized in series of different IEC in order to follow the impact of the ionic group to the properties of the material. Additionally, a new structure of the ionomer is proposed, where the block-copolymer contains a hydrophilic block, synthesized from two monomers, bearing perfluorosulfonic acid (PFSA) groups. It allows maximally approximating the superacid lateral groups along the polymer chain that, most probably, contributes to better organization and interaction between the ionic sites. For further characterization of the novel polymers, they are cast to membranes by casting-evaporation method from their solutions in dimethylacetamide (DMAc). The influence of production temperature is described briefly. The membranes of different series are compared between each other and to Nafion as a reference material. It is known that Nafion acquires its high performance due to: i) presence of superacid PFSA lateral groups, and ii) organization of polymer chains into well-separated proton-conductive (hydrophilic) and mechanically stable (hydrophobic) domains. However, production of this material comprises dangerous and expensive procedures of manipulation with fluorinated gases, since this ionomer contains a Teflon-type backbone. Moreover, transition temperature of the perfluorinated main chain is lower, than the required temperature of the ionomer functioning in a PEMFC. The novel ionomers are further characterized in terms of thermo-mechanical properties, stability, conductivity, bulk morphology. They exhibit: i) high transition temperatures, which allows utilization of these polymers at conditions of a PEMFC functioning; ii) phase separation phenomenon, which suggests the materials to have morphology with distinct domains for proton conduction, iii) highly organized structuring, which is rare to clearly evidence on aromatic materials; iv) high proton conductivity for several polymer series, which over-perform Nafion even at reduced humidity. Based on these results, some of the synthesized materials are considered to be promising in a PEMFC, but further study in real conditions must be provided. Additionally, the current work is pioneering in the way of production of the ionomers, therefore, more different series of polymers are previewed to be synthesized out of the ionic monomers, proposed here. Variety of the ionic monomers may be enlarged as well by changing the PFSA groups to perfluorosulfonimide ones or by searching for other fluorinated commercial materials that might be modified into monomers with two functional groups for polycondensation. Thus, the main objectives, set for the current work, are fulfilled.
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Olesia Danyliv. New membranes based on high performance polymers for proton exchange membrane fuel cells PEMFC. Other. Université Grenoble Alpes, 2015. English. ⟨NNT : 2015GREAI026⟩. ⟨tel-01823819⟩

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