Branched Anion-Conducting Poly(arylene alkylene)s for Alkaline Membrane Fuel Cells

Branched, bromopentyl-tethered poly(arylene alkylene)s were synthesized by Friedel–Crafts polyhydroxyalkylation of 7-bromo-1,1,1-trifluoroheptan-2-one and biphenyl with the addition of a small amount of trifunctional monomer (0.5–2.0 mol %), 1,3,5-triphenylbenzene. The subsequent conversion of bromomethyl group of bromopentyl-tethered precursors to quaternary ammonium (QA) to afford cationic ionomers, and these ionomers were then solution-cast into anion-conductive membranes. Compared to the linear, unbranched poly(arylene alkylene)-based AEM analogue, branched membranes exhibited suppressed water absorption, a low hydration number, and good swelling resistance, likely due to the improved chain entanglements as a result of branching. More importantly, although with an 11–38 wt % reduction in water absorption, the branched membranes showed comparable conductivities to the unbranched counterpart, which can be interpreted by the fact that the well-reserved phase-separated morphology and improved ion concentration took place after polymer branching. Specifically, at 80 °C, the conductivities of 131 and 63 mS cm–1 were recorded for branched membrane B-PBPA-1.0% DB in the OH– and Cl– forms, respectively. In addition to the suppressed water uptake and high ion conductivity, the branched membranes exhibited excellent alkaline durability. Over 97% conductivity retention and no structural degradation were observed for B-PBPA-1.0% DB after a 1000-h aging in 1 M NaOH at 80 °C. H2–O2 fuel cell performance was evaluated with the branched membrane B-PBPA-1.0% DB, and a peak power density of 690 mW cm–2 was achieved at 80 °C. The results of this study provide a new polymer structural modification for the fabrication of advanced ionomeric materials for electrochemical technology applications.