Improving poly(arylene piperidinium) anion exchange membranes by monomer design

Energy conversion devices such as alkaline membrane fuel cells and water electrolyzers rely critically on durable anion exchange membranes (AEMs) with high hydroxide conductivity. In this context, poly(arylene piperidinium)s have emerged as one of the top candidate materials. Here, we report on the preparation and properties of poly(arylene alkylene piperidinium)s (PAAPs) with significantly higher alkaline stability than current state-of-the-art poly(arylene piperidinium)s derived from piperidone. A new piperidine trifluoromethyl ketone monomer (TFPip) was designed, synthesized and employed in superacid-mediated polyhydroxyalkylations with p- and m-terphenyl, biphenyl and fluorene, respectively. The pendant piperidine rings of the resulting polymers were then quaternized and cycloquaternized to form N,N-dimethylpiperidinium (DMP) and 6-azonia-spiro[5.5]undecane (ASU) cations, respectively. Polymers based on p- and m-terphenyl were cast into mechanically strong AEMs which reached OH^-
conductivities close to 80 and 180 mS cm^-1 at 20 and 80 °C, respectively. The AEMs also displayed an excellent resistance against OH^- attack. For example, AEMs carrying DMP cations showed a mere 14% ionic loss after storage in 5 M aq. NaOH at 90 °C during 20 days. In comparison, a corresponding benchmark poly(arylene piperidinium) AEM lost three times as many DMP cations (42%) under the same conditions. The results of the study demonstrate that the overall properties and alkaline stability of AEMs can be considerably improved by rational monomer design.