In the current work, cationic polymers functionalized with N-alicyclic quaternary ammonium (QA) cations were synthesized and characterized as potential hydroxide exchange membranes (HEMs) for application in fuel cells. Three different polymers, namely poly(arylene ether sulfone), spiro-ionene and poly(arylene alkylene), were explored as polymer backbones for the HEMs. Various mono- and spirocyclic QA cations were incorporated either within the backbone, directly on the backbone or via spacers. The synthesis of these
polymers was a great challenge which was overcome with the help of numerous synthetic methods. Radical bromination, hydroboration and Suzuki coupling were employed when synthesizing the monomers. The backbone polymers were obtained by polyetherification, cyclo-polycondensation and super acid-mediated
polyhydroxyalkylation. The mono- and spirocyclic QA cations were incorporated by quaternization and cycloquaternization, respectively.
The HEMs were characterized with regard to morphology, hydroxide conductivity, water uptake, thermal and thermochemical stability, i.e., the key properties that determine the performance and durability of the HEMs as electrolytes for hydroxide exchange membrane fuel cells. Studying the change in the properties as different chemical and structural features of the HEMs varied gave valuable insights into the structure-property relationships of theses materials, paving the way for further development of high performance HEMs for FC applications.
In this work, HEMs based on poly(arylene alkylene) functionalized with dimethylpiperidinium via spacers were found to possess the most attractive combination of properties. They had high hydroxide conductivities (103-
146 mS cm-1) and still maintained a reasonable water uptake (73-103%). Most importantly, they were exceptionally stable under alkaline conditions at elevated temperatures, with less than 5% ionic loss after 720 h of storage in 2 M NaOH solution at 90 °C.