Exploring N-alicyclic quaternary ammonium functional polymers as hydroxide exchange membranes

Sammanfattning: As the global power generation is shifted toward renewable and intermittent sources, the importance of fuel cells and other energy storage and conversion devices is continually increasing. However, many of these technologies are still under development and further research is necessary before they can be implemented. This is the case for the hydroxide exchange membrane fuel cell (HEMFC), which has been identified as an important next-generation conversion technology. One under-developed and critical component of this technology, which is currently hampering its application, is the hydroxide exchange membrane (HEM). It consists of cationic polymers that conduct hydroxide ions and is essential to the performance and durability of the fuel cell. During prolonged fuel cell operation current HEMs tend to degrade due to an incompatibility between the cationic polymers and the hydroxide ions. To overcome this, continued exploration of new polymer architectures and chemistries is needed and that is where this thesis work comes in. In this work, we have designed and synthesized different cationic polymers functionalized with N-alicyclic quaternary ammonium (QA) cations for use as HEMs. The overall focus of this thesis has been to explore the effect of different structural elements of the polymers, such as cation design and placement, on key properties of the HEMs. While extra emphasis was placed on the alkaline stability of the polymers, other ex-situ properties such as hydroxide conductivity, water uptake, morphology and thermal stability has also been studied. The scientific approach was constructed to highlight important structure-property relationships and gain other valuable insights that might aid the ongoing and future research on HEMs and HEMFCs.The investigated polymers include poly(diallylammonium hydroxide)s prepared in radical initiated cyclopolymerizations, poly(arylene piperidinium)s prepared in superacid promoted Friedel-Crafts polycondensations and polystyrene functionalized in hydroxyalkylation reactions. HEMs with high hydroxide conductivity (>100 mS cm-1) was obtained from all these polymers. A general finding observed while studying these materials was that the stability of N-alicyclic QA cations is very dependent on the conformational freedom of the ring. In contrast to studies on model compounds, when incorporated into a polymer the N-spirocyclic cation was consistently observed to be less stable than the monocyclic N,N-dimethylpiperidinium cation, and main pathway of degradation for both ions was via ring-opening β-hydrogen elimination.