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.
Highlights
Ion exchange membranes are semipermeable materials that can be used as electrolytes in many electrochemical devices, such as electrolyzers, redox flow batteries, and fuel cells.[1,2] In particular, anion exchange membrane fuel cells (AEMFCs) have emerged and gained attention as a clean and efficient energy conversion device
Considerable research has been focused on the development of anion exchange membranes (AEMs), which are composed of polymers tethered with cationic groups such as quaternary ammoniums (QAs)
A wide range of different polymers have so far been studied as backbones, e.g., polysulfones,[4] poly(phenylene)s,5 polystyrenes,[6,7] poly(ether ketone)s,8 and poly(phenylene oxide) (PPO).[9−11] Polymer backbones with aryl ether linkages are susceptible to cleavage, especially when these linkages are close to electron-withdrawing groups.[12,13]
Summary
Ion exchange membranes are semipermeable materials that can be used as electrolytes in many electrochemical devices, such as electrolyzers, redox flow batteries, and fuel cells.[1,2] In particular, anion exchange membrane fuel cells (AEMFCs) have emerged and gained attention as a clean and efficient energy conversion device. Azoniaspiro[5,5]undecane (ASU) cations possess high resistance against alkaline attack, even surpassing the trimethylammonium (TMA) benchmark.[27] The high stability of the DMP and ASU cations was attributed to the low ring strain of the six-membered rings as well as the constrained ring conformations which raise the activation energy of the ringopening elimination and substitution reactions, respectively.[27] Inspired by this finding, our group and others have studied a wide range of AEMs functionalized with piperidinium and Nspirocyclic QA cations.[20,28−30] For example, our group has designed, synthesized, and studied N-spirocyclic QA ionenes (spiro-ionene)s which were chemically and thermally stable, with only slight degradation after 336 h in 1 M KOD in D2O at 120 °C.31. Special attention was given to elucidate the degradation mechanism of the polycyclic cations under high-pH conditions
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