The alkaline stability and fuel cell performance of poly(N-spirocyclic quaternary ammonium) ionenes as anion exchange membrane

Abstract A series of polymer based on spirocyclic quaternary ammonium (QA) cations having 5-/6- and 6-/6- membered rings were designed and prepared from tetrakis(bromomethyl) monomers to investigate the alkaline and fuel cell stability of spirocyclic ionenes. The alkaline stability testing of model compounds and DFT calculations showed that the spirocyclic QA having 5-/6-membered rings based on phenyl and biphenyl showed a better stability than that of the spirocyclic QA with naphthalene 6-/6-membered rings probably due to the strong electro-withdrawing effect of naphthalene rings. Although the high molecular weight polymer with naphthalene rings could not be obtained, the spirocyclic ionenes based on phenyl or biphenyl showed high molecular weight. Thus, the blending membrane with rigid NPBI have been fabricated to mitigate the water solubility of spirocyclic ionenes due to their high IEC values. The blending membrane showed a high ionic conductivity even at elevated temperature without any excessive swelling in spite of its high IEC value. Employing the spirocyclic ionenes as membrane, the H2/O2 single cell at 60 °C was firstly demonstrated. A peak power density of 135 mW cm−2 was achieved for PP80N20 membrane. Surprisingly, the fuel cell device durability provided a counterintuitive data that showed that spirocyclic ionenes with excellent alkaline stability were not superior in device function assessment. A rapid ring-opening degradation as confirmed by NMR technique for the spirocyclic ionenes was observed in device testing. These results that stability investigation gives us new directions for polymer and cations designs for highly durable devices.