Poly(fluorene alkylene) Anion Exchange Membranes with Pendant Spirocyclic and Bis-Spirocyclic Quaternary Ammonium Cations

Abstract

Anion exchange membranes (AEMs) functionalized with certain alicyclic quaternary ammonium cations show high alkaline stability, which is necessary for use in fuel cells and water electrolyzers. Here, we have prepared and studied a series of poly(fluorene alkylene)s incorporating N,N-dimethylpiperidinium (DMP) and 6-azoniaspiro[5.5]undecane (ASU) cations in spirocyclic and bis-spirocyclic arrangements, respectively. First, a new fluorene monomer carrying a piperidine ring attached in the C9 position was prepared in a cycloalkylation reaction with N-Boc-N,N-bis(2-chloroethyl)amine. After deprotection, this monomer was employed in superacid-mediated polyhydroxyalkylations with 2,2,2-trifluoroacetophenone (Ap) and 1,1,1-trifluoroacetone (Ac), respectively, to prepare piperidine-functional precursor polymers. Finally, the DMP and ASU cations were formed in quaternizations and cycloquaternizations of the piperidine groups in the polymers by using methyl iodide and 1,5-dibromopentane, respectively. Solvent-cast AEMs showed high thermal stability in combination with good mechanical properties and restricted water uptake as a result of the stiff polymer backbone and bulky cations. Still, the AEM prepared with Ac and carrying DMP cations reached high OH- conductivity, exceeding 80 mS cm-1 at 80 °C. At the same temperature, this AEM showed a high alkaline stability and retained more than 96% of the DMP cations after storage during 30 days in 1 M NaOH. Detailed NMR analysis revealed that ionic loss via Hofmann elimination dominated, but evidence of ring-opening and methyl substitution reactions was also found. Degradation of the ASU cation in the bis-spirocyclic arrangement occurred mainly through ring-opening reactions in the ring directly attached to the fluorene unit in the polymer backbone. These results provide valuable insights when it comes to molecular design of piperidinium-based cations in polycyclic arrangements toward alkali-stable and highly conductive AEMs.