Hydroxide Conducting Membranes with Quaternary Ammonium Cations Tethered to Poly(arylene alkylene)s via Flexible Phenylpropyl Spacers

Abstract

Durable and high-performing anion exchange membranes (AEMs) are central components in alkaline water electrolyzers and fuel cells. Here, we have pursued a synthetic strategy where heteroatom-free polymer backbones are functionalized with alkali-stable dimethyl piperidinium (DMP) cations via flexible spacers. We achieved this design by synthesizing a trifluorobenzophenone monomer carrying a piperidine group in the 4-position via a straightforward Friedel-Crafts acylation of 4-(3-phenylpropyl)piperidine. Polyhydroxyalkylation of this ketone and p-terphenyl, followed by N-methylation, generated a DMP-functionalized poly(p-terphenyl alkylene) (PpT-DMP) with excellent alkaline stability, e.g., less than 7% ionic loss after treatment in 7 M aq. NaOH solution at 90 °C for 240 h. A high OH- conductivity, up to 104 mS cm-1 at 80 °C, was reached at an ion exchange capacity (IEC) of 1.79 mequiv g-1 and 53% water uptake. Atomic force microscopy (AFM) revealed a distinct microphase separation into ionic and nonionic nano-domains. The Friedel-Crafts acylation method proved very versatile and was also employed to synthesize a bromoalkylated trifluorobenzophenone monomer from 1-bromo-3-phenylpropane, which gives access to a diverse range of AEM materials. Poly(arylene alkylene)s based on this monomer and p-terphenyl and dimethylfluorene, respectively, were prepared in polyhydroxyalkylation and subsequently functionalized with alkyltrimethylammonium and N-methyl-N-alkyl piperidinium cations, respectively. Especially, AEMs based on the former material showed a remarkably high conductivity, up to 175 mS cm-1 at 80 °C. In summary, we present a straightforward approach to synthesize trifluorobenzophenone monomers substituted with 4-propylpiperidine and propyl bromide groups, respectively, which enables the preparation of various heteroatom-free poly(arylene alkylene)s tethered with alkali-stable cations for high-performance AEMs.