Proton-Conducting Sulfonated Aromatic Ionomers and Membranes by Chemical Modifications and Polycondensations

Abstract: Proton-exchange membrane fuel cells (PEMFC)s are increasingly regarded as promising environmentally benign power sources. Intensive development is today directed towards reducing the cost and increasing the durability of the fuel cell, as well as expanding the operational window for the PEMFC for a range of applications. The proton-conducting membrane is one of the key components in the PEMFC. The need for improved performance of the proton-conducting membrane has led to an extensive worldwide research, from which aromatic ionomers have emerged as promising candidates. One of the major challenges is to prepare functional high-performance proton-conducting membranes, with optimized properties. By concentrating the sulfonic acid groups, which facilitate the proton transport, to specific chain segments, the nanophase separation between hydrophilic and hydrophobic domains may be enhanced, which may provide membranes with balanced water sorption characteristics. In the present work, the sulfonic acid groups were concentrated to specific segments in either the polymer backbone or on pendant side chains. Based on the former approach, polysulfones with fully tetrasulfonated aryl-SO2-aryl-aryl-SO2-aryl segments were prepared by lithiation, reaction with sulfur dioxide, followed by oxidation of the resulting sulfinates. The possibility to fully tetrasulfonated these segments offer possibilities to prepare various aromatic copolymers and membranes with locally very high densities of hydrolytically stabile sulfonic acid groups. As a second approach to enhance the phase separation, the sulfonic acid groups were separated from the polymer backbone and were concentrated to side chains. PSUs carrying various mono-, di – and trisulfonated side chains were synthesized by chemical modification. Moreover, aromatic ionomer with various polymer backbones with pendant benzoyl side chains were synthesized by polycondensations. SAXS measurements showed that longer side chains and higher local acid concentrations, gave larger characteristic separation length between the ionic clusters accompanied with narrower distribution. Proton conductivity measurements showed that larger characteristic separation lengths resulted in higher proton conductivities. The ionic clustering of ionomers bearing sulfobenzoyl side chains was shown to be promoted by ionomers with flexible polymer backbones, which subsequently resulted in higher proton conductivity, but with the drawback of having lower thermal stability. The water uptake characteristics in the ionomers bearing sulfobenzoyl side chains was efficiently controlled by incorporating non-sulfonated comonomers.

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