Piperidinium functionalized aryl ether-free polyaromatics as anion exchange membrane for water electrolysers: Performance and durability

The rapid progress development of anion exchange membranes (AEMs) enables their practical utilization in AEM alkaline electrolysers. Here, we reported a series of quaternized poly [(terphenyl piperidinium)-co-(oxindole terphenylylene)] (PTP) as AEMs materials for AEM electrolysers application, which were synthesized by superacid-catalyzed copolymerization of p-terphenyl, isatin and N-methyl-4-piperidone followed by quaternization methyl iodide. The aryl ether-free polyaromatics endow the PTP AEMs with alkaline-stable polymer backbone and excellent mechanical property with the tensile strength of 29.2–36.5 MPa. PTP-90 membrane (IEC = 2.52 mmol/g) showed the highest conductivity of 64.4 and 128.9 mS/cm at 20 and 80 °C, respectively. Ex situ alkaline stability in 1 M NaOH at 80 °C indicated 40% of loss in hydroxide conductivity after 934 h of testing for PTP-85 membrane, and NMR results of the aged membranes revealed the degradation of functional N,N-dimethylpiperidinium (DMP) cations in the PTP membranes via Hofmann elimination and nucleophilic substitution. More importantly, the membrane electrode assembly (MEA) with PTP membrane was utilized in an AEM alkaline electrolyser fed with 1 M NaOH. The MEA with highly conductive PTP-90 membrane demonstrated the performance of the 910 mA/cm2 at 2.2 V at 55 °C and 1000 mA/cm2 at 75 °C. Further cell durability tests showed that after operating at the 400 mA/cm2 for 120 h at 55 °C, significant chemical degradation was observed by post-cell analysis of PTP membrane using NMR spectroscopy, which was associated with the cationic group degradation via Hofmann elimination and nucleophilic substitution reaction. In contrast, no chemical degradation was found for PTP membrane even in ex situ 10 M NaOH at 55 °C for 200 h. The in-situ durability tests also suggest that the working environment of the electrolyzer may accelerate the degradation of cationic group in the AEM.