Improving long-term cation stability is crucial for adopting anion exchange membrane fuel cells as a commercially viable technology in clean energy conversion applications. To reliably identify the most stable cation structures, we analyzed the cation stability of various synthetically prepared quaternary ammonium organic molecules via a silver oxide ion exchange reaction. This method enabled us to compare the stability of various structures of quaternary ammonium hydroxide in pure form in water without excess alkaline solution or inconsistencies faced in the presence of polymer backbones. By quantitatively comparing cation degradation via NMR, we were able to identify three cation structures with greater cation stability than the most well-known benzyltrimethylammonium. In addition, we were able to elucidate byproduct formation and degradation mechanisms as a result of hydroxide attack. From this study we concluded that alkyl-substituted cations seem to impart greater stability than benzylic-substituted cations in the presence of hydroxide anions in aqueous solution.

Alkaline membrane fuel cells (AMFCs) show great potential as alternative energy conversion devices to acidic proton exchange membrane fuel cells (PEMFCs). Over the last decade, there has been significant progress in the development of alkaline-stable polyaromatic materials for membrane separators and ionomeric binders for AMFCs. However, the AMFC performance using polyaromatic ionomers is generally poor, ca. a peak power density of <400 mW cm⁻². Here, we report a rational design for polyaromatic ionomers which can minimize undesirable phenyl group interaction with hydrogen oxidation catalysts. The AMFC using a newly designed aryl ether-free poly(fluorene) ionomer exhibits a peak power density of 1.46 W cm⁻², which is approaching that of Nafion-based PEMFCs. This study further discusses the remaining challenges of high-performing AMFCs.