Insights into the Alkaline Stability of Poly(arylene piperidinium)s

The development of anion-conducting polymers with excellent alkaline resistance is crucial for advancing reliable alkaline membrane electrochemical energy devices. Poly(arylene piperidinium)s are promising candidates for anion exchange membranes (AEMs), and various backbone configurations are being investigated to enhance their electrochemical performance. However, the impact of these modifications on the alkaline stability must first be taken into account. In this study, biphenylene, p-terphenylene, and p-quaterphenylene units with varying rigidity were employed as building blocks for poly(arylene piperidinium)s to systematically assess their effects on alkaline stability and other physicochemical properties. The results reveal a clear negative correlation between polymer backbone rigidity and alkaline durability, observed at both low and high hydration levels. Specifically, biphenylene-based PBPPip-67 exhibited a 3% loss of piperidinium moieties after 2000 h of alkaline hydrolysis in 1 M aq. NaOH, whereas PQPPip-100, based on quaterphenylene units, showed a 13% degradation. Additionally, during cell operation at a current density of 0.4 A cm^–2 at 60 °C for 100 h, PBPPip-67 exhibited half the piperidinium loss compared to PQPPip-100. Analysis of model compound degradation in a flame-sealed NMR tube revealed that rigid aryl substituents significantly promote β-H elimination of the piperidinium functionality, which is further rationalized by computational studies. This study provides new insights into the design strategies for poly(arylene piperidinium)-based AEMs and proposes structures for chemically and dimensionally stable membranes.