Highly stable poly(p-quaterphenylene alkylene)-based anion exchange membranes

Abstract

To achieve highly stable anion conductive membranes, p-quaterphenylene moiety was copolymerized into the poly(arylene alkylene)-based ionomers with pendant pentyltrimethylammonium group to improve the swelling resistance and mechanical robustness but without solubility compromise. The copolymers, either bromomethyl terminated precursors or the subsequent alkyltrimethylammonium-tethered ionomers, were readily soluble as the quaterphenylene units controlled to less than 63 mol%, which make the solvent processing possible. Comparison to the poly(bipheylene alkylene)-based analogue, quaterphenylene-containing P(4PA-co-2PA) membranes showed lower water absorption and suppressed swelling. Specifically, at a given ion exchange capacity (IEC) of 2.0 mmol g‒1, quaterphenylene-free P2PA-73 showed the 29 wt% water uptake and 6.1% in-plane swelling at room temperature, while these two values decreased to 9 wt% and 0.6%, respectively, for P(4PA-co-2PA)-63 with 63 mol% of quaterphenylene units. Moreover, P(4PA-co-2PA) membrane also showed good alkaline stability, no detectable degradation was observed after a 1500-h aging in 1 M NaOH at 80 °C, and a 19.3% conductivity loss was recorded after a 720-h treatment in 5 M NaOH at 80 °C. P(4PA-co-2PA) membranes were further assembled to the H2–O2 fuel cell devices, and a 0.4 W cm‒2 peak power density was achieved for the cell prototype assembled from P(4PA-co-2PA)-47 AEM. Moreover, in-situ stability test showed that the P(4PA-co-2PA)-47-based cell can operated stably for 100 h with a low voltage decay rate of 0.82 mV h‒1. Overall, the results of this study provide a simple structural modification strategy that will guide the molecular design of poly(arylene alkylene)-based ionomeric materials with improved membrane properties.