Durable poly(binaphthyl-co-p-terphenyl piperidinium)-based anion exchange membranes with dual side chains

Abstract

Building well-developed ion-conductive highways is highly desirable for anion exchange membranes (AEMs). Grafting side chain is a highly effective approach for constructing a well-defined phase-separated morphological structure and forming unblocked ion pathways in AEMs for fast ion transport. Fluorination of side chains can further enhance phase separation due to the superhydrophobic nature of fluorine groups. However, their electronic effect on the alkaline stability of side chains and membranes is rarely reported. Here, fluorine-containing and fluorine-free side chains are introduced into the polyaromatic backbone in proper configuration to investigate the impact of the fluorine terminal group on the stability of the side chains and membrane properties. The poly(binaphthyl-co-p-terphenyl piperidinium) AEM (QBNpTP) has the highest molecular weight and most dimensional stability due to its favorable backbone arrangement among ortho- and meta-terphenyl based AEMs. Importantly, by introducing both a fluorinated piperidinium side chain and a hexane chain into the p-terphenyl-based backbone, the prepared AEM (QBNpTP-QFC) presents an enhanced conductivity (150.6 mS cm⁻¹) and a constrained swelling at 80 °C. The electronic effect of fluorinated side chains is contemplated by experiments and simulations. The results demonstrate that the presence of strong electro-withdrawing fluorine groups weakens the electronic cloud of adjacent C atoms, increasing OH⁻ attack on the C atom and improving the stability of piperidinium cations. Hence QBNpTP-QFC possesses a robust alkaline stability at 80 °C (95.3% conductivity retention after testing in 2 M NaOH for 2160 h). An excellent peak power density of 1.44 W cm⁻² and a remarkable durability at 80 °C (4.5% voltage loss after 100 h) can be observed.